Chế tạo, nghiên cứu các màng vật liệu bán dẫn quang điện tử và triển khai khả năng ứng dụng

Cac ket qua dat duac gom: • Da tao duoc cac mang ZnO bang cac phmrng phap khac nhau nhu phun trnh dien, sol-gel, phiin xa Kalot.. Do nhiet do nong chay ciia ZnO rat cao va la vat lieu t

DAI HOC QUOC GIA HA NOI TRUONG DAI HOC KHOA HOC TUNHIEN

DAI HOC QUOC GIA HA NOI TRUiNCi DAI HOC KHOA HOC TUNHIEN

1 en cle tai

C H £ ' TAO, NGHIEN CUU CAC MAN(i VAT LIEU I5AN D A N QUANG DIEN TLT VA TRIEN KHAI KHA NANCJ UN(; DUNG

MA SO: QG 01-03

CHU TRI DE TAI: PGS IS NGUYEN THI THUC HIEN

Cac can bo tham gia:

6 TS Le Thi Tlianh Binh

7 TS Pham Nguyen Hai

8 TS LeVanVu

9 TS Hoang Nam Nhai

10 CN Tran VTnh Tilling

1 L CN Ngo Xuan Dai

12 CN Nguyen Tlianh Binh

13 CN Le Duy Khanh

HA N O I - 2 0 0 3

Bao cao torn tdt

a Ten de tai: Che tao, nghien curu cac mang vat lieu ban dan quang dien t

va trien khai kha nang utig dung

M 5 s o : QG.Ol - 0 3

b Chu tri de tai: PGS TS NguySn Thi Thuc Hien

c Cac can bo tham gia:

19.TS.Le Thi Thanh Binh

20.TS Pham Nguyen Hai

21.TS Le Van Vu

22.TS Hoang Nam Nhai

23 CN Tran VTnh TliSng

24 CN Ngo Xuan Dai

25 CN NguySn Thanh Binh

26 CN Le Duy Khanh

d Muc lieu va noi dung nghien cu'u (iheo dang kf)

• Che tao cac mang mong don 16p va da 16p nhu ZnO, SnO^, CdS ^ CdTe bang cac cong nghe khac nhau de lim ra cong nghe loi u"u nhat

• Nghien cihi cac qua Irinh cong nghe cung nhu cac lmh chai vai Iy ci cac mang da che lao

• Tim hieu klia nang utig dung cac mang lix)ng vice che lao cac cam bie cac dung cu quang dien tu

e Cac ket qua dai du'Oc

De tai da lap trung nghien cuu vai lieu ZnO, day cung la chat ban dan dai duac the gidi tap trung nghien cuu rat manh Ngoai ra, cCing da bai dau nghi(

ciru vat lieu CdTe Tuy nhien de tai chi neu cac ket qua dat duyc doi vai vat lie

ZnO Cac ket qua dat duac gom:

• Da tao duoc cac mang ZnO bang cac phmrng phap khac nhau nhu phun trnh dien, sol-gel, phiin xa Kalot Da nghien cuu va lim ra con nghe, che do loi uu

• Da nghien cuu cac linh chat vat ly cua cac mang che lao duOc nhu cau true tinh the, tinh chat quang, tinh chat dien, ti^r

• Da tien hanh pha tap AI, Ga, In, Eu, Mn vao ZnO va nghien cu*u ca tfnh chat vat ly ciia cac mang co pha tap Da nghien ciru ban chi ciia cac tai hap exciton lrong ZnO

• Buac dau nghien ciru ve Spinlronics bang vice nghien cUu cac ba dSn pha li;r loang ZnO:Mn

• Nghien cii'u klia nang li'ng dung ZnO lam chat xiic Uic, lar photodiode, dan S(')ng am be mai

• Xay dung duac he do pho truyen qua va tao duoc he nhiing mfiu ch phep dieu khien duac toe do nhiing liJ 0,8 cm/phiil den 7 cm/phiit

• Hu6ng dan nhieu sinh vien nghien cuu klioa hoc:

14 (da va dang huong dan) khoa luan Inl nghiep Dai ho nganh Khoa hoc Vat lieu va chuyen nganh Vat ly Chat ran

3 Cao hoc (mot hoc vien da bao ve luan van )

1 Nghien cii'u sinh (dang lam)

• Co 10 bao cao lai cac hoi nghi Khoa hoc, lrong do ca 10 bai de viet bang lieng Anh Gom:

5 bao cao tai hoi ihao Viet Due, Hue 2/2002

2 bao cao tai hoi nghi Quang hoc quang pho Toan quoc,Nh Trang S/2()02

2 bao cao tai hoi nghi khoa hoc Trucmg DHKHTN 11/2002 (Da duac nhan dang a tap chf Khoa h(.)c DHQGHN)

1 bai bao o lap chf Khoa hoc DHQGHN

f Tinh hinh kinh phf ciia de lai

Kfnh phf duac cap: 60 trieu (2001: 30 trieu; 2002: 30 Irieu)

Cac khoan chi kinh phf chfnh:

1.12 Hoi nghi, Semina, chi kliac :

1.14 Thue khoan chuyen mon :

1.19 Vat tu, lK)a chat, dung cu, ihiel bi.:

ll.8K0.000d 21.900.000 d 26.499.000 d

Da CO du chung tir quyei loan voi Phong tai vu, ban Ke hoach lai chfn DHQGHN

(/'•

/\/^U£j^^ T^ / % ^ 5

Ccy quan chii Iri de lai

1 Assoc Prof Dr Nguyen Ngoc Long

2 Assoc Prof Dr Le Hong Ha

3 Assoc Prof Dr Ta Dinh Canh

4 Assoc Prof Dr Pham Quoc Trieu

5 Assoc Prof Dr Nguyen Hanh

6 Dr Le Thi Thanh Binh

7 Dr Pham Nguyen Hai

8 Dr L e V a n V u

9 Dr Hoang Nam Nhat

10 BSc Tran Vinh Tliang

11 BSc Ngo Xuan Dai

12 BSc Nguyen Thanh Binh

13 BSc Le Duy Khanh

d Purpose and contents of investigation project

• Preparation of single and multilayer films such as ZnO, Sn02, CdJ" and CdTe by different methods in order to find the best fo application

• Investigation of the technology as well as the physical propertie:

of produced films

• Investigation of ability for application in sensors, opto-electronic! devices

e The main results

The project has been concentrated on ZnO material.The results are:

• Single and multilayer ZnO films were prepared by differer methods such as sol-gel, electrostatic spray pyrolysis, rf magnetro sputtering

• Investigation structural, electricity, optical and magneticc properties

• Doping ZnO by Al, Ga, In, Eu, Mn

• Investigation of dilutted magnetic semiconductors such as ZnO:Mn

• Investigation of possibility of application ZnO for photocatalyst an ZnO/Si junction for photodiode

• Building a absorption apparatus and a device for dipping of th withdrawal speed in the ranger 0.8 to 7.0 cm/min

• 10 publications have been presented at scientific conferences a followings:

5 reports in Proceedings of the fifth Vietnamese - German seminar o Physics and Engineering, Hue March 2002

2 reports in Proceedings of the 3*^ National conferences on Optics an Spectroscopy, Nha Trang August 2002

2 reports in Proceedings of the conference of Hanoi University c Science, VNU (to be accepted to Journal of Science, VNU Hanoi Hanoi, November 2002

I paper in Journal of Science, VNU Hanoi

Have been instructed many students in scientific reseachs:

14 (done and being done) university graduate theses on specialities c Materials Science and Solid state Physics

3 Msc theses (One has been done)

1 PliD thesis (being done)

Bao cao chuyen mon

De tai: Che tao, nghien ciru cac mang vat lieu ban dan quang dien tir va Iriei

khai kha nang ung dung

Muc luc 7

M a dau 8

Chuang I: Trmg quan ve vat lieu ZnO 10

1.1 Mot so tfnh chat vat ly 10

1.1.1 Cau Iriic tinh the 10

1.1.2 Tfnh chat dien 11

1.1.3 Tfnh chat quang 11

1.1.4 Tfnh chat tir 12

1.2 Kha nang ung dung cua ZnO 12

Chuang II: Cac phuang phap che tao mang mong 14

2.1 Gi6i thieu chung 14

2.2 Phuang phap phun iTnh dicn 14

2.3 Phuang phap phiin xa Kalot (sputtering) 14

2.4 Phuang phap Sol-gel 15

Chuang III: Cac ket qua va thao luan 16

3.1 Che tao mang ZnO va nghien ciiu cac linh chat vai ly 16

3.2 Che tao cac mang ZnO pha tap 18

3.2.1 ZnO pha cac lap chat lao do dan loai n 18

3.2.2 ZnO pha lap dat hiem 19

3.2.3 ZnO pha kim loai chuyen tiep 20

3.3 Nghien ciJu ly thuyet ve phuang phap DLTS 20

3.4 Trien kliai klia nang ung dung ciia cac mang ZnO 21

Ket luan ^ 22

Tai lieu tham kliao 23

Phu luc

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M o dau

Hien nay, tren the giai, cac hap chat ban dan Ihuoc nhom A^^B^^ danj duac quan tam nghien cufu manh me Trong cac ban dSn do, ZnO duac xem nhi

mot ''hot topics" do co nhieu tfnh chat quf bau nhu do rong vising cam Win (cd 3,^

eV a nhiet do phong), nang luang lien ket exciton tu do lc3fn (60 meV), do ber viJng, do on dinh, nhiet do nong chay cao, klii tao ban dan pha lir loang co tfnl sat ttr a nhiet do phong va cao han No hua hen nhieu trien vong sir dung lan cac linh kien quang dien tii' nhu man hinh hien thi, bo loc song am, laser, diode phat quang, dien cue trong suot, spinlronics

Do nhiet do nong chay ciia ZnO rat cao va la vat lieu thang hoa rat de b tach pha nen no thudng duac che lao dudi dang mang mdng bang cac phucmj phap nhu phiin xa Katot 11 ], epilaxy bang chi^im phan lir | 2 | , phun iTnh dicn |31 sol-gel [4]

Vice sU dung phu'o'ng phap nao de lao mang liiy ihudc vao dieu kien lliie

bi cho phep cQng nhu liiy ihuoc vao muc dfch su dung Phutmg phap epilax> chiim phan tu cho cac mang cd pham chat rat cao, cd the lao cac mang mdng c( vai nguyen tu sir dung trong cac cau triic nanomel nhung nd ddi hdi thiet bi ra

dat tien klidng the cd duac d Vict Nam Chfnh vi vay chiing ldi da lieii hanh hxi

mang ZnO bang mot sd phuctng phap tUcmg ddi re lien, de dung he nhu phun iTnl dien, Sol-gel Tuy nhien, de lao duoc mSu cd pham chat cao hem, de dang h(/i cho viec nghien ciiu cac linh chat vat ly, chiing ldi cung da tien hanh lao man^ ZnO bang phuang phap phiin xa Kalot

Viec ciing tao mang ZnO bang nhirng phuang phap khac nhau cho phep s( sanh chat luong, kha nang ung dung ciia mdi loai Tuy nhien, vdi mdi phucmj phap deu phai tim duac quy lrinh dam bao tfnh on dinh, lap lai cao Vi vay, ca< phuang phap nghien cuu, cac thiet bi do phai chfnh xac va cd do lin cay cao D( dat duac cac muc lieu tren, de tai da sii* dung cac phucmg phap do, cac he do che

do tin cay cao cd san tai Trung iam Khoa hoc Vai lieu thudc khoa Vai ly, Irudnj Dai hoc Khoa hoc Tu nhien

Ket qua ciia de tai la da tao duac cac mang ZnO vdi do lap lai cao, da ph; tap Ga, In, Al, Eu, Mn vao cac mang va nghien cuu cac tfnh chat vai ly cii; chiing Ngoai ra, de tai da bat dau nghien cuu Irien khai kha nang ung dung cu; cac mang che tao duac nhu lam photodiode, bd loc sdng am Ngoai vie^ nghien ciixi bang thuc nghiem, de lai cung da lien hanh nghicMi ciiu ly thuyet vi phuang phap do DLTS lam lien de c(y sd lot cho vice sir dung phmmg phap d( nay

Vdi kinh phf de lai duac ho tra va tan dung mot sd ihicl bi cd san, chiinj tdi da xay dung duac he do pho Iruyen qua va mdi ihiel bi nhiing mang V(')i loc d< riit mang nam trong dai ti^r 0,8 cm/phiil dcii 7.0 cm/pluil

8

Ngoai ra, de tai da gdp phan dao lao nhieu cii' nhan vat ly chat ran va kh( hoc vat lieu ciing nhu cac sinh vien nghien cuu khoa hoc d cac nam dudi, cac h( vien cao hoc va nghien ciJu sinh

Day la moi de lai tap hop duac hau het cac can bd lam ve vat lieu ban di

trong Khoa Vat ly Mac dii kinh phf cd han cho mdi lap the ddng can bo nghit curu nhung dd la de tai dau lien duac ihao luan va dai muc lieu lien Uii lap h( can bo trien khai nhiing hudng phat trien ciia Khoa Vai ly va lien kcl nghien ci vdi cac ca quan khac

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Chmmg I

.A,- ^

TONG QUAN VE VAT LIEU ZnO

l.L Mot so tinh chat vat ly cua ZnO

LLL Cau true tinh the

Hap chat ban dan ZnO cd cau Iriic linh ihe luc giac wurl/ile Moi nguyen I

Zn lien ket vdi 4 nguyen tu oxy nam d 4 dinh lan can Hi* dien (hinh 1)

Nguyen tu Zn Nguyen tu Zn

Hinh 1 Cdu true Iuc gidc wurzite cua ZnO

Hang sd mang trong cau triic nay la a= 3,24265 A va c= 5,1948 A

Ngoai cau triic wurzile, lrong nhu'ng dieu kien dac bicl, linh the ZnO cdn c the ton lai d trang thai cau true luc phucmg gia kem (cV nhiel dc) lal cao) hoac ci

Iriic gidng linh \h& Nalri Ciorua NaCl (cV ap suai rat cao)

Khi tao mang ZnO Irt^n de, neu true c vudng gdc ven be mai dc ihi mat Ircn •

mat dudi ciia mang cd the la mat (oooi) hoac (OOOI) lii'c \l\ mat ciia cac ion ^

hoac oxy

0

Lien ket cua mang tinh the ZnO vii'a la lien kei ion vii'a la lien kei ddng hda tri

LL2- Tinli chat dien

Khi che tao, ZnO ludn cd do dan loai n vdi cac donor la cac sai hdng tu

nhien gay nen bdi mit kliuyel oxy va nguyen tii* kem dien ke Tuy nhien, cac mang ZnO khdng pha tap thudng khdng ben vii'ng va cd dc) dan klidng cao Vi

vay, ZnO ductc lam tang do dan va dc) on dinh bang each pha cac tap nhu Al, Ga,

In Nong do tap trong ZnO cd the len dc^'n 1()~*^ cm"^ vi vay ven dc) rc)ng viing cam Idn (3,3 eV d nhiet dc) phdng) nd rat thfch hcvp diing lam cac dicn cUc li^ong suCil cho man hinh hic?n thi, pin mat Uxri va ti"ong lUcmg lai se thay ihe cho mang ITO (Sn02:In) Iruyen thc)ng nhung gia ihanh kha dai

Can day da cc') mc)l sc) cc^)ng trinh | 5 | cc^)ng bc^i vc vice lao dc) dan loai p cho ZnO bang each ddng pha lap Ga va N Tuy nhien vice lao dirc/c ban dan ZnO loai

p cdn la mc)t van de can dau lu nghien cUu nhic^u hem nii'a

L L 3 Tinh chat quang

ZnO cd do rdng viing cam 1cm, chuyti'n mii'c ihang va dac bicit la cd nang luang lien ket exciton 1cm (60 meV) hem han GaN ( 20 meV) la mc)l vat lic^u ciing dang duc^c quan tam nhieu Chfnh vi the, ZnO la me)l vat lieiu hua he:n sir dung lam cac linh kien quang diein lU, dac biet la cac linh kien phat quang vi;ing budc sdng ngan

Thdng ihuimg dc)i ven mang die}n cUc lrong suc)l, dc) Iruyen e[ua ciia ZnO lrong viing anh sang nhin thay cd Ihe len deii 90% vi vay su mat mat anh sang rat nhd

Phei quang huynh quang ciia ZnO a nhiel dc) phc')ng ihinrng cd nhung dai sau:

Dai phai xa d vimg u'r ngoai gan bc'y hap ihu, cd dinh nam cV khoang 380nm

li'ng vdi cac lai he)p exciU)n [6] Ti^iy Iheo phucmg phap che lao, chai luemg ciia mau, nd cd lh(5 la excilon lir do hoac cxcilon licMi ke't, exciU)n Ironi^ Iranu thai plasma hoac chewing chap nhieu loai lai hc^p Dac diem ciia dai nay la khdng de)i xung, chan sdng keo dai, khi lang cUe'mg de") kfch thfch ihi dinh dich ve phfa budc sdng dai

Dai pho tir 390nm den 410 nm lue^)n tdn lai \6i moi loai mau Dai lai hctj

tap chat nay bien mat khi nhiet de) lem hem 77 K va vi trf dinh phci khdng dcii thee nhiet do va ban chat ciia nd la tai he;p cap donor - acceplor [7|

Viing xanh

Dinh pho huynh quang quanh huac sdng 500nm cd dac diem la rat rc)ng

tii Cho den nay, cd nhieu quan diem kliac nhau ve dinh tai he;p nay Cd tae

gia cho rang tai hap nay lien quan {6'\ tap hydro hoac sai hdng do Zn nan dien ke, cd tac gia lai cho rang ban chat la do tai hetp gan va\ oxy dien kc [8,9,10] Tuy nhien, nhieu cdng lrinh nghieing ve gia thuye'l dd la tai ha\

lien quan den sai hdng lU nhien do mil khuyei'l oxy 11 1 | Cho deii nay nguein gc)c ciia dinh nay vAn cdn ductc ihao luan

Viing da cam

Ban chat ciia dai pliei lai 620 nm nay la do lrong mang linh the ZnO idn Ui

cac mil khuycSf ciia Zn hay cac ion O a vi Irf dien ke tao ihanh cap donor

-acceplor [12] Neu lrong ZnO tdn lai tap chai la cac kim loai kicSm (Li, Na thi dai nay se lach thanh vang va cam

Viing dd

Dinh chfnh cV 663 nm, ngoai ra cc)n cac lap lai phonon ciia nc) Biin chai cii dai nay la do Fc^"^ hoac Li"^ ce') lrong cac hda chai ban dau du'etc sir dung d tao mau

LL4 Tinh chat tir

Khi ZnO duac pha cac kim loai chuyen tiep nhu Mn, Co, CV, Fe ihi n

tao nen ban dSn pha tii' Ket qua la lii* ban dan khdng cd lir tfnh, do luo'ng lac cii

Ccic ion lir vdi cac hat tai dicin ciia ban dan ma [it tfnh xiuYl hien Tfnh chai na duetc su* dung a spinlronics la mc)l huemg mei'i ket hap giiJa ban dan va lii

L2 Kha nang iirng dung ciia ZnO

Be)l va mang ZnO la nhung vat lieu lal cpian lrong lrong cdng nghe gc)m v cdng nghe mang mdng

12

Bdt ZnO pha tap thich hc)[) due^c sir dung lam chai c|uang LIAU lii>ug cong

nghe chup anh dien Bot ZnO d dang ge)m ducfc diing lam varislor, cam bieii khf

chay

Mang mdng ZnO dinh huemg cao ngoai vice sir dung lam dicn cu'c liong

sue)t nhu da neu a trejn, nci cc^n cei ifnh ap dien manh difoc dimg lrong cac ihiei bi

semg am be mat, scMig am khc)i, ihici bi tpiang am hoc, cac he ihdng vi co' dicjn

Mang ZnO pha tap Al, Ga, In cd de) phan xa hdng ngoai lc)l nen duo'c dung phii len cua kfnh nha d Cdn khi pha cac kim loai quy nhu Pd Pl nd du'evc ung dung lam cam nhan khf chay

Hien nay, cac hat kfch thue^c nano ZnO dang duc^c lap trung nghitjn ciru rat manh vi nc5 md ra mc)l trien vong cho the he cac linh kien men vei'i nhu'ng ifnh chat hoan loan kliac so vc^i mau klic)i

Cac chuyen tiep di ihc [ix ZnO dirc;c dicing lam cac linh kien quang dien lU chat lucmg cao hoat de)ng a viing tu ngoai va lam pin mat treVi

13

Chucmg II

CAC PHU()N(i PHAP CHE TAO MANG M()N(i

2.L Gioi thieu chung

Hic^n nay treSn ihe gicifi dang phat trien rai nhieu phucmg phap che lao mang mdng, tii* nhirng ky thuat hien dai nhu epitaxy chiim phan lir (MBE), lang dong xung laser (PLD), phiin xa KaU)l (spullering), lang dong ho'i hda hoc (CVD) dc}'n cac ky thuat tucmg de)i dcm gian nhu phun dung dich tren de ndng, tang dong hda hoc, Sol-gel Mdi phmmg phap deu cd cac uu diem va nhirc;c die}'m ric}ng liiy

thuoc vao muc dfch nghien ciru, ung dung cung nhu loai vai hou lao nen mang Trong chU(]^ng nay se trinh bay chi liet hem ve^ cac phucmg phap da duac de lai sir

dung de che lao mang ZnO pha lap va khc')ng pha lap dicing cho muc dfch nghie}n cuu, li'ng dung phii he)p ven hoan canh va dieu kien ciia Vieil Nam

2.2 Phuong phap phun tinh dien

Phuang phap nay duoc ihuc hicn ducn Uic dung ciia dien Iru'cVng Dien tm'dng tir ngudn cao the (klioang 20 kV) lam cho dung dich cd chii*a lien chat ciia vat lie^u can tao bi ban xudng de ndng ducn dang sucmg mii va ngay sau dd lang dc)ng tren deS'va tao Ihanh mang

PhUemg phap nay cei uu ehem la dcm gian, de ihifc hien, chi phf rti, mang ihu duac cd do ddng deu cao, dicn phu rc)ng

Vdi ZnO, khi sir dung phucmg phap nay cd Ihcj diing mue)i kein acelal hc^a

tan trong dung mdi la rirc)u isopropanol va nUe'Vc cat Mang duac lao tren de kfnh

vdi nhiet dc) de nam lrong khoang 300 - 40(rC

2.3 Phuong phap phiin xa Katot (sputtering)

Day la phucmg phap lao mang mdng tu'cmg ddi hicn dai Mang due;c lao

lrong budng chan khdng cao (cd 10^' Torr) Ca scV ciia phucmg phap nay dua lien hieu irng ban pha ciia cac hat lai cd nang lUcmg cao yCi'\ cac nguyen lu ciia chat

ran can tao mang lam bia (bia dirc;c nc)i ven KaU)l) lam bat cac nguyen lii* bia

hUcmg ten dc (dc duac ne)i vci'i anot) rdi lang dong lao ihanh mang Cac nguyen lir diing de ban pha bia thUcVng la khf iva Argon vei'i bia la gc)m duoc lao lir bc)l ZnO

nung then k(}'t Che dc) lao mang nay liei kiem duoc nang lifo'ng, cac hai ldi dc cci

nang lUcmg cao do de') mang co cluil luo'ng ldi, dd chac dac va ifnh dinh huemg

cao Tuy nhien gia thanli ciia san pham lao hang phucmg phap nay luong ddi cao

4

2.4 Phircmg phap Sol-gel

Phuctng phap Sol-gel da cd ti^r hem 30 nam nay, luy nhien hicn nay nd vfin

dang duac chii y ap dung rdng rai vi nhii'ng uu [he rieng nhu cho phep tao duac ca

mau khdi, mSu mang va bc)l ven quy lrinh khdng phUc tap va liei kie:m nang luang

Ban chat ciia phUemg phap Sol-gel la dua Iren phan iing ihiiy phan va ngUng tu ciia cac lien chat (precursor) bang each dieu chinh lc)c dc) ciia hai phan ling tren de dat duac vat lic^u mong mudn Tir dung dich (sol) bao gdm cac chat dua vao phan iing ductc hda lan ven nhau qua phan iing thiiy phan va ngung lu ta

thu dugc gel Sau dd gel xe)p nay duc)c dua xii* ly de lao ra cac chat ran

Dua vao vat lieu ban dau di sii* dung cho qua lrinh tcSng ho'p ngUc^n la cd ihe

chia phucfng phap Sol-gel thanh 3 dang chfnh:

-f Sol-gel di lir ihiiy phan mue)i lrong dung dich nUeic + Sol-gel di tu thiiy phan phUc chai

+ Sol-gel di lir thuy phan alkoxide

Trong ba each tren, phuo'ng phap thiiy phan alkoxide cho chai lue^ng san pham tc)t nhai tuy nhicjn gia ihanh ciia nguyen vai licjii rai cao

Phucmg phap Ihiiy phan mudi cung thudng duc^c chon vi cac mudi duo'c sir dung la rat re so vclfi cac phucmg phap khac Tuy nhien, phucmg phap nay khd

dieu chinh de hat tinh thcS' c6 kfch ihuac nano

Khi tao mang bang phucmg phap Sol-gel ihuc'mg dicing 3 each: nhiing de}' vao dung dich sol (dip-coaling) va keSo len ven cac tdc do kcSo phu heyp hoac gid dung dich trcin dcS' va quay li tam (spin-coaling) va thao (diaining) hoac phun (spraying)

Phucmg phap Sol-gel cd cac Uu diem sau:

+ Ducrc thuc hien c'^ nhiel eV nhiel de) thap, giam due)c sir bay hen ciia cac ihanh phan do dd tieS't kiem nang lUcmg nhiet, giam d nhiem mdi trudng vi chat dc)c

+ Cd the tron lan d quy md nguyen lir Khdng lc)n nang lUemg nghien lre)n

-I- Cho san pham ce') dc) linh khiejl, ddng nhai cao, be mai lieng Ici'n, hoai ifnh cao,

cd the bao phu 2 mat cua cac vat cd hinh dang bat ky

-f Dieu khien dugc hau het cac giai doan Icing hevp san pham

+ Hda chat cd the sir dung nhi(5u lan, lao ra fl chai ihai

Nhucyc diem ciia phucmg phap nay la gia thanh cc^n lUemg ddi cao va khd lao duac

mang day

15

Tai khoa Vat ly trUcmg Dai hoc Khoa hoc Tu nhien, phuo'ng phap Sol-gel

dugc su* dung lan dau tieSn vao dau nam 2001 lai phdng thf nghieim Vat ly ban dan cua Khoa Cc^ng lrinh tao mang ZnO dau tiein dugc dUa bao cao lai he)i nghi vai

ly chat ran U)an quc)c lan ihii' 3, 8-2001 lai Nha Trang [12 ] Tuy nhicjn ihc^'i gian

dd chat lue:^ng ciia mang chua cao, chua tao duetc mang da 16fp, dinh huc'mg ciia mang chua tdt Muc ticju ciia de lai la phai lao due)c mang da Ic'tp va sir dinh hucfng phai cao Sau qua trinh cai tien, thay ddi cdng nghe:, cac mang da le'yp cd sir dinh huc'yng cao da due)c che}' lao

Ket qua nay ciia de tai da duc;^c bao cao va dua vao ky yeu ciia he)i ihao Viet-Diic ve Vat ly va Ky ihual lai Hue}' thang 3/2002 113j Tren hinh 2 la gian dd nlii(}u xa tia X ciia cac manu che lao dUo'c

/////// 2 Gicln do nhieu xa lia Xcua mdiii^ ZnO ehe lao hdng phu'o'ng phdp Sol-gel

Nhin gian do, dc ihay rfuig chi xuAi hicn dinh nhieu x;i (002) Dicu dn

chirng td cac hat linh the}' cd dinh huemg sao cho iriic c vudng gcic ven he mai de

Ngoai su dinh hUcmg cao, cac mang ZnO che lao bang phucyng phap gel ciing cd do truyen qua kha cao, khoang 80% 113| Tfnh chat quang ciia cac

Sol-16

mang che tao duac da duac nghien curu thdng qua phc^ quang huynh quang (PL)

Nhu da neu d phan 2.1, ph6 PL ciia mang ZnO a nhiel do phdng thuc'mg cd hai

dai: dai nang lue^fng ung vc'n qua lrinh lai hctp exciton quanh buci'c sdng 380 nm va

dai tai he;p cd dinh quanh 500 nm ducifc gan cho tai hgp qua V<\m sau iing ven mil khuyet oxi Phci PL ciia cac mang che lao dugc cho iha'y a nhiel dc) phdng, cuc'yng

do dinh exciton la'n at cUcmg de) dinh 500 nm Dieu dci chirng id sir kei linh va chat luang ciia mang te)l

Vc^i cac mang da tao duoc, chiing tdi da tim hieu ve ca che}' ciia hai dai lai hap ndi tren Ket qua do ph6 PL d viing nhiel de) ihap [ 13 | cho ihay cUc^'ng dc) dai

PL gan bd viing tang khi nhi(}l do li mang tang Dieu nay chii'ng id sir kcl linh ciia mang tang vc'n nhiel dc) Da lim ra nhic}l dc) li le)l nhai cho mang la 500 "C Vi mang ZnO duc)c phii trc}n de ihiiy linh nen khi nhieil dc) li cao ho'n 500 "C, chai luang ciia mang kem di Mc)l lrong cac nguye}n nhan la do sir cong venh ciia de kfnh

Cac ket qua do PL d nhiet dc) ihap cho phep doan nhan excilon la lie}n kcl

vcifi cac tap chat (excilon lien ket) va dinh huynh quang quan sal diro'c d nhie}l de)

phdng la thdng qua lai ho'p ciia exciU)n hen kc}'l veifi lap accepU)r Irung hda 113]

Ngoai dinh PL ciia exciton, chiing tcM ciing quan tam lej^i dinh 500 nm Ve dinh nay, cd nhieu quan diem ddng nhat khac nhau Cd tac gia cho rang lai he;p nay lien quan Ic^i tap Cu [ 14], lap hydro f 10] sai hdng do Zn nam dic5n ke 115]

Tuy nhien, nhie}u Uic gia nghicMig vc sir hen quan Icii sai hc')ng lU nhien ciia mil

khuyet oxy [11]- Chiing ldi da li(}'n hanh xem xcl sU anh huc'mg dcii cu\mg dc) PL cua dinh nay va thu duc)c kcl qua | I 3 | la ven lal ca cac mau du'oc li lai lrong mdi trudng oxy, cudng do ciia dai 500 nm deu lang len Ke}'l qua nay Irai vcii gia lhi(}'l tai hap lien quan tci'i mil khuye}'l oxi vi khi li mau li'ong mdi iruc^ng oxy, ndng dc) mil khuyet oxy phai giam Chiing ldi cho rang Uii hc)p nay lien quan manh ven cac tam be mat hoac bien hai vi cUc'mg dc) dinh 500 nm giam manh khi do mau trong chan khdng cao

De tim each tang cUc'mg dc) huynh quang ciia cac mau, chiing ldi da tic}'n hanh xu* ly mau lrong ludng khf hydro va cho an mdn be mai 116] K(}'l qua cho thay klii an mdn be^ mat mang bang axil HCl (PH = 3) cuc'mg dc) PL tang l(}n 2 lan Khi li mau Irc^ng H2 cUcmg de) ciia dinh tai hctp excilon tang len Tren hinh 7

va 8 [16] la dd thi phci PL trucl^c va sau khi u H2, an mdn

Qua ket qua li H2 va an mdn be mat bang HCl chiing ldi cho lang vai lid ciia hydro rat quan li'ong Nd lam thu dc)ng hda nhu'ng Uim oxy nam d bien hai

NhiJng tam nay ddng vai Ird cac acceplor va lam giam cUe^mg dd huynh quang

Nhirng k(}'l qua Iren diing ca cho cac mang do'n lei'p cung nlur cac mang ZnO da Icyp Chiing ldi da licMi hanh phii chdng nhieu Icip ZnO bang 2 phuong phcip: chdng Ic'yp say va dicing Ic'yp u so" be) |16,17| Cac Uii lieu cluing ldi ihaiii khao duac deu lao bang phucmg phap li so' be) nhung ke't qua ciia cluing ldi cho thay vice clic'')iig Ic'ifp say da cho cac mang cc) cUc^ng dc) PL cao hon

Ngoai ra, chiing tdi cung da tic}'n hanh tao cac mang ZnO bang cac phUcmg phap khac nhu bcic bay Zn trong chan khdng sau dd li nhiet lrong khdng khf, lao bang phuang phap phun trnh die}n, spullering Cac mang tao bang phuenig phap bdc bay Zn trong chan khdng cd ifnh dinh hUemg khdng cao, cUc^'ng de) dai PL

exciton nhd [18] nen khc)ng duc;c lrinh bay d day Cac mang che lao bang phUo'ng phap phun lmh dien va spullering se dugc trinh bay trong phan ZnO pha tap

3.2 Che tao mang ZnO pha tap

3.2.1 Pha cac tap chat tao dp dan loai n

Ta biet rang, mang ZnO due)c che tao ludn mang tfnh dan dic}n loai n Tuy

nhien cac mang ZnO nay khd cd Ifnh Cm dinh cao va ndng dc) hai lai thap lam

mang cd dien trd 1cm Mue)n tao duc^c mang ZnO cd tfnh cin dinh cao va dac biet

la cd dien trd nhd lam dien cue phai pha tap

Chiing tc')i da tien hanh pha tap In vao mang ZnO che}' lao bang phUcmg phap phun iTnh di(}n [19,20], pha Ga vao mang ZnO che}' lao bang phucmg phap Sol-gel [21] va pha Al vao mang che tao bang phucmg phap spullering |22] Vei'i phuonng phap Sol-gel, Ga duc;c pha vao ZnO bang each dua them Ga(NO J^ 9H2O vao dung djch lao ZnO [21] Qua khao sal sir ihay dcii dien IreV mau ZnO:Ga len nong do tap Ga va nhiet do li man chiing ldi thay rang cac mang cho dien IrcV nhd nha't khi nhie}t do u la 5(){)''C va ndng de) lap Ga la 1 al%

Ket qua tren cung duofc Ihci' hie}n lrc}n gian dd nhie}u xa lia X Vci'i ndng dc)

Ga 1 at% mang cd dinh hueing le)l hem va cUcmg dc) dinh nhi(}u xa cung lem ho'n Ngoai su* phu thudc lrc}n, dien IrcV suat ciia mang cung phu lhuc)c vao sc) Icip chdng cua mang Ket qua cho thay dien IreV suat ciia mang giam khi sc) le':fp lang va dat

cue tieSu klii sc) Icifp la 10 Tren 10 lap die}n IrcV suat lai tang

Nhu'ng cdng lrinh gan day cho ihay, mang ZnO pha lap cd diejn IreV suat

nhd klii mSu duac u trong hdn hc)p khf nha va hydro Chiing ldi da lie}'n hanh li

mang ZnO:Ga trong khf hydro Ke't qua cho ihay dien tre'y ciia mang giam di 2 bac va bao hda klii then gian li due;c 3 gid d nhiel do 450 ''C

Ven cac qua lrinh xii* ly Iren chiing ldi da thu dire)c ki}'l cpia dicn IreV ciia mang li trong hydro, cd 10 IcVp, ndng dc) Ga 1 al% la 4 10 ~ Qcm Gia tri nay ce)n cao so vei'i mang che}' lao bang phu'o'ng phap spullering nhirng ihap ho'n so vcii mang ZnO:In chei' lao bang phiremg phap pluin iTnh dicn va dii dc lani dicn cue

lrong suc)l vi do truyen c[ua ciia mang > X(Y/(i

Vie}c pha lap In vao mang ZnO che lao bang phUong phap phun iTnh dicn duo^c thuc hien bang each dua acelale indium vao dung dich lao ZnO | I9| Ndng

dd In nam lrong khoang 3 - 5^/(' Kcl qua cho Ihay, dicn Irc'y mai ciia mang giam lir

20 M Q xudng 20 k Q khi pha 3 al% In Kei qua do EDS cho ihay In da vao duc^c mang vc'Vi ii(5ng dc) ho'p ly nhu'ng dicn Ird van lein so vdi cac mang pha bang cac

S

phuang phap khac Chiing ldi cho rang nguyen nhan ciia nd la do cac ion In chii yeu nam c^ bien hat va mang kha xc)p nhu anh SEM da cho ihay

Mac dii viec pha tap In chua dat duerc dieu kie}n lam die}n cue lrong sue)l nhung ke't qua do phei PL ciia cac mang ZnO:In che}' lao bang phuo'ng phap phun

lmh dien d nhict do thap cho phep chiing tdi khang dinh lai ca che}' lai ho'p exciU)n

bang each " lit " cac gia tri Ihiic nghiem ven cdng ihirc ly ihuyei | 2 0 | Kei qua so sanh ly thuyc}'t va thuc nghiem cho phep ke}'l luan rang 3 dinh Uii hc^p exciU)n tuang ling vdi exciU)n lien kcl veii donor ion hda |D'',x|, exciU)n lien ke}'t ven acceptor trung hda [A'\ x] va lap lai phonon ciia nd

Trc^ng cac tap chat pha vao ZnO, lap Al cd gia thanh ihap nhat Tuy nhien, dcM vcVi phucfng phap Sol-gel va phun tinh dic}n chiing ldi ihay rang vie}c pha lap

Al klidng thu duc;c kcl qua tc)l bang In va Ga Nguyen nhan cd ihe do vice lao ra ALO^ trong qua trinh li mau NgUctc lai, vie}c pha Al vao ZnO bang phu'emg phap spullering cho ket qua te')l hon rai nhieu Chiing ldi da ur lao bia ZnO:AI iheo ndng do tap mong mucin va kcl qua cac mang Ihu due;c cho ihay vdi phucmg phap phiin xa rf magnhetron mang ZnO ihda man yeu cau ciia me)l dien cue lrong sueit

la dinh liUe'mg cao, dien trd suai nhd 8,7 10"' Qcm d(}'n 1,8 10"' Qcm, ndng de) hat tai nam tre:)ng klioang (0,2 - 3,1) l ( P c m " \ dc) Iruyejn qua trung binh dat d(}'n 89% ve'Vi mang cd chie!u day 1,9 pm [22]

3.2.2 ZnO pha tap dat hiem

De ung dung sir phat quang lrong cac linh kien quang dicn lii, cae lap chai dat hiem nhu Eu, Er, Dy thuc'mg duc^c pha vao ZnO Tlidng ihue^ng, khi pha lap Eu vao ZnO ngmyi la phai ddng pha lap Li de dua diro'c Eu vao mang, hoac sir dung EUCI3 Chiing ldi da sir dung phuong phap Sol-gel de lao mang ZnO:Eu

bang each lhe}m Eu^O^ -\- HCl vao dung dich de}' du'o'c ndng de) Eu I al% | 2 3 | Ke}i qua cho thay pho PL xual hien 5 vach hep d 577, 590, 615, 650 va 690 nm Nhii'ng dinh nay khdng phai la nhung dinh hen quan den cac Uii hop gan veii sai

being ric}ng ciia ZnO ma la nhu'ng chuyen muc bcjn lrong ciia Icip 41' lrong cac ion

Eu^^"": ^D^ (j = 0,1) -^ ^Fj (j = 0,1, ,6) Su xual hi(}n cac dinh huynh quang nay

chung id sir cd mat ciia Eu lrong mang ZnO Vice pha lap Eu'"^ va cac ion dai

hiem klicic vao ZnO da dugc nhieu cdng lrinh cdng be) Tuy nhien, cci ihe ihay

rang Eu khd khuye}'ch lan vao lrong cac hai linh ihe ZnO vi ban kfnh ciia chiinj

km va cac ion Eu-'"" lai lich dicn Tir IrUcVc len nay cei 2 md hinh vc sir phai quanj ciia Eu trong ZnO che lao cV nhiel dc) cao la: cac ion Eu'^ nam chii yeu ci bien hai

va huynh quang ciia cac ion Eu^"" chi cd ihe xay ra khi ddng kfch hoai cac ion Li"" hoac N^"" dirc)c dUa ihcMn vao ZnO d 900 "C hoac 1200 "C va lao ncMi su Iruyen nang luemg tii* ban dan cho cac ion Eu'* Gia Unci llur hai cho rang huynh quaii^ ciia Eu-'"" d ZnO:EuCl3 leing hop eV I 100 X la do EuOCI Mau cluing ldi che lao khdng pha them Li, hon nua nhiejl de^) li mau chi 500 "C vi vay md hinh c(i Li nhu

19

t^

cac tac gia khac neu d tren khd xay ra Tii pho EDS chung ldi thay cd xuat hien

Cl vi vay cd the xay sir phat xa ciia mSu cd nguein gcic ciia EuOCl Tuy nhien mau cua chiing toi lai chi u d 500 ^^C va anh sang kfch thfch cd nang lucmg nhd han do rong viing cam vi vay cd the EuClO nam ei cac bien hat

3.2.3 ZnO pha kim loai chuyen tiep

Ban dan pha loang lii* dang duefc quan Uim rai manh nhc'y cei ihc kcl he^p giiia ban dan khdng cd tii ifnh va lii' ifnh ciia cac ion kim loai chuyen licp khi duac pha vao ban dan chii Day la mdi hucmg manh lrong linh vuc spinlronics

De nghien cuu ban dan pha lii' loang chiing ldi da lien hanh pha lap Mn vao

ZnO che tao bang phucmg phap y.[\ magnhetron spullering Bia due;c lao lii be)l MnO^ va ZnO va nung iheu kei d 1300 "C lrong khdng khf lrong 3 gid Ndng de)

Mn chay ti^O den 0,36 [241

Ket qua cho thay khi pha Mn cUe'mg dc) dinh nhieu xa lia X lem hem, ban do rdng he^p hem so vcjfi ZnO tao citing dieju kie}n Dieu nay chung id khi pha Mn sir ket tinh ciia mang tdi hem va kfch thucic hat tang ICMI. Ngoai ra, cac hang sd mang

c va a tang tuyen tfnh vcii su* tang ncing de) pha lap Mn d(}'n tan x= 0,36 Dieu nay cliii*ng td cac ion Mn da nam ei mil mang thay the cho ion Zn

Phci Iruyen qua ciia cac mang cho iha'y bc'y hap thu co' ban dich ve phfa nang lucmg cao klii ndng dc) Mn lang Kei qiia ifnh U)an dc) re)ng vising cam Eg cho ihay Eg tang tuyen tfnh ve'ti x

Dudng cong tii' tr^ do duci'c ci nhiel dc) phdng cho ihay mang cd Ifnh sal lu

va lire kliang tii* klioang 44 Oe

Gan day chiing tdi da lien hanh pha Co vao ZnO va da ihu diroc kcl cpui uil han ra't nhieu so vdi pha Mn [271

3.3 Nghien cuu ly thuyet ve pho qua dp tam sau

Phci qua do iam sau (DLTS) la mc)l cdng cu huu hie}u de nghien ciiu ve iam

sau Tuy nhie}n, vic}c hi(}'u rd ban chat ciia phUemg phap qua dc) tam sau nham khai thac het nhiing khfa canh ciia nd, ung dung vao vice nghien cuu cac chat ban dan

la me)t van de can dat ra

Trong mdt cdng trinh dang d ky ye'u hc)i ihao Vict- Diic | 2 5 | chiing ldi da lrinh bay mdt phUemg phap lach nhieu ngau nhien khdi ifn hic}u dicn dung lrong phe}p do DLTS Phuo'ng phap dicing ke nay cho pht}p lach nhieu khdi ifn hieu khi

ly sci tfn hie}u lr(}n nhieu giam xueing dueii 10 Kcl qua cho ihay, bang phu'emg

phap nay cd [he khai ihac IhcMTi Ihdng lin, nhu Uich duo'c nhung dinh qua gan

nhau ma phucmg phap chi diing lock-in khdng the}' ihuc hien duoc

20

Ket qua kiem tra tren mau CdTe cho thay, binh thuc'mg chi cd the quan sal duac 3 dinh DLTS nhung bang phucmg phap lach nhifiu da quan sal thay 5 dinh [25]

Nge^ai phucmg phap Uich nhieu da neu, me)t phUcmg phap phan tfch dinh lucmg cac muc nang lucnig lal gan nhau lrong phd DL1\S cfing da duo'c de xuai [26]

3.4 Trien khai kha nang urng dung ciia cac mang ZnO

Veii cac mang ZnO che}' tao dugc chiing ldi da lrie}'n khai lim hieu cac ap

dung thuc licMi va cei cac kcl qua sau:

- Mang ZnO:Al che}' lao bang phUemg phap r.F magnhetron da cd [he dimg lam

dien cue trong sudl Iruyen qua ihay cho ITO Mang ZnO:Ga che lao bang phucmg phap Sol-gel cung cd the}' diing lam dien ciic trong sueit luy khdng Icil bang mang che}' tao bang phUcmg phap phiin xa

- Da lien hanh thii* nghiem xiic lac lre}n mang ZnO che}' lao bang jihucmg phap Sol-gel, ket qiia cho tha'y hieu iing xiic tac xay ra nhung vi mang mdng nen tfn

hieu cdn nhd Day chfnh la nhuc^c diein khdng lao dugc mang day ciia phuemg phap Sol-gel Trong lucmg Iai cluing ldi se cci gang lim each lao duyic mang

day hem de lan dung tfnh chai xcip, dc dang cho hieu iing xiic Uic ciia mang che tao bang phUcmg phap Sol-gel

- Da tien hanh tao mang tiep xiic ZnO/si loai n bang phu'o'ng phap phiin xa r.l" magnhetron Ket qiia do phei quang dan va ihe'n gian dap cho ihay diode ZnO/Si-n cd the sir dung lam pholodiodc ven mien nhay quang nam lrong khoang 1 ).Lm va ihc^'i gian dap ngan.Tren hinh 3 la phd quang dan ciia diode

- Da bucic dau nghien ciru hie}u ung ap dicn ciia mang ZnO che lao bang

phucmg phap phiin xa iA\ magnhelron

KET LUAN

Sau thdi gian hai nam ihu'c hien, de tai da thu due)c nhiing kcl ciiia sau

• Da tao ducjfc cac mang ZnO, ZnO pha lap Ga, Al, In, Eu, Mn bang cac

phuemg phap Sol-gel, phun iTnh dicn va \'A\ magnhelron s[nillering veii tpiy

trinh on dinh

• Cac mang che lao due^c cci tfnh dinh hUcing cao, true c vudng geic veii he

mat de, dc) truye^n qua dat > 80% lrong viing nhin thay

Mang ZnO che lao bang phirctng phap phun iTnh dien cd de) xcip cao ihfch hcTfp cho vice sir dung lam sensor nhay khf, xiic Uic Vice pha lap In vao ZnO bang phuemg phap nay chUa dat yeu cau sir dung lam die}n ciic trong sueit

Cac mang che lao bang phuo'ng phap Sol-gel cei Ifnh lap lai cao, dn dinh Khi pha tap Ga dicn Ird suat la 10'~ Qcm vdi ndng de) Ga lal% dai chat luang tdt nhai va cd the sii dung lam die:n cUc lrong sueil Khi pha Eu,

da thu dugc phci huynh quang lUemg ung veii chuyen miic ciia Eu liong nen

tinh the ZnO

Mang ZnO:AI che}' tao bang phuemg phap phiin xa r.f magnhelron

cho kei qua rai kit lrong vi(}c sir dung lam dien cue lrong sueil veii dien IreV

suat 10"^ Qcm va de) truyeSn qua > 80%

• Da bucVc dau nghi(}n cii'u ITnh vUc spinlronics bang each lao ban dan pha loang tii tren ce:^ seV pha Mn va ZnO Da nghie}n ciiu cac ifnh chai dicn, li^r, quang ciia chiing

• Da nghien cii'u ly lhuye}'l ve phUcmg phap DLTS lao dieu kien di sau lim hieu khai ihac sir dung phuevng phap hicn dai nay

• Da xay dung he do phci hap thu, quang dan va he: nhiing mang

• Da nghien cii'u trien khai lilig dung cac mang che lao duo'c lam diejn cUc lrong sudt truyen qua, lam phe)U)die)de

Nhiing ke}'t c^ua lr(}n cho Ihay vi(}c Irien khai ung dung cac mang da che lao

ce^n bi han che so veii vice nghien ciiu cac linh chai vai ly cua chiing Nguyen nhan ciia sir han che Uvn la di) ihai gian chua dii dai dc cci ihe vira nghie}n ciru

quy lrinh lao mau vira nghien ciiu cac ifnh chai vai ly vira liicn khai phan ung dung cho that hoan chinh Mai khac, cd nhung khd khan khac nam lrong quy

lrinh cdng nghe vf du vice lao dugc mang day Ircn 1 |.im de nghien cuu kha nang

xiic tac la klici klian Ngoai ra, kinh phf duo'c cap chua dap li'ng dii de Iricjn khai

tdt phan iing dung, lrong lUo'ng, lai neu dugc sir iing he) ciia lanh dao chung ldi

22

hy vong ring viec nghien cufu licp tuc hiring vai lieu mang mong nhom II-VI se clal duoc nhirng kei qua toi hem, dac biet la huong ung dung

Cuoi cijng cac can bo tham gia de lai xin cam ofn cac cap lanh dao cua DHQGHN, DHKHTN da lao dieu kien de de lai c(') liie hoan ihanh, dai kcl qua tot

Tai lieu tliam khao

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technology, 1996

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[5] D Bao, H.Gu, A Kuang, Tliin Solid Films 312 (2002) 37

[6] J F Muth el.al J Appl Phys S5 (1999), 78K4

[71 T.V Bulkhuzi el.al J Lumines 90 (2000), 223

[8j D Haln el.al Phys Cond Mailer 3 (1995) 311

[91 C.M Mo el.al J Appl Phys 85 (1998) 4389

[101 Y.R Ruy, S Zhu el.al, 1 Appl Phys 88 (2000) 201

[111 K Vanheusden el.al .1 Appl Phys 79 (1996), 7983

[121 K.Q.Dal,P.Q.Bao,N.T.TluicHien Nhiing van de hien dai cua vai ly chai ran, luyen lap cac bao cao lai hoi nghi Vat ly chat ran loan quoc lan ihlr 3,Nha Trang 8/2001,831-835

[13] N.T.T Hien el.al, Proceeding ol the Fifth Vielnamese-Gennan on Physics and Egineering, Hue, (2002), 232-235

[14] R Dingle, Phys Rev Lett 23, 579 (1969)

[15] Y.K Park el.al, Appl Phys Lell 72 (1998), 668

[16] N.X Dai, N.T.T Hien, RN Hai, Proceeding ol" the 3"' National Conlerence

on Optics and Spectroscopy Nha Trang August 1 1-15"' 2002

[17] Khoa luan lot nghiep - Ngo Xuan Dai 6-2002

[18] T.D Canh, N.N Long, N.D Phmmg L.T.T Binh, Ky yeu hen nghi Vat ly toan quoc lan thu 5 Ha Noi 3 2001

119] N.N Long, L.T.T Binh N.T Binh L.D Khanh T.V Tliang Proccednigs ol' the Fifth Vietnamese-German on Physics and Egineering, Hue, (2002)

|2()| L.T.T Binh, N.T Binh, L.D Khanh N.L Long, to be published in lournal

[23] P.N Hai, N.C Thanh, N.T.T Hien, P.Q Tlianh, K.Q Dal and P.T Nga, Proceedings of the Filth Vietnamese-Gcnnan Seminar on Physics and Engineering, Hue 2 March 2002, 228

[24] N.D Phuong, T.D Canh and N.N Long, Proceedings oi' the 3'' National

conrerence on Optics and Speclroscopy, Nha Trang, Augusl 1 1-13''' , 2002

1251 Hoang Nam Nhai and Pham Quoc Trieu, Proceeding oi ihe Fil ill

Vietnamese-German Seminar on Physics and Engineering Hue, 2 March 2002,115

[26] Hoang Nam Nhai and Pham Quoc Trieu, Proceeding of ihc Filth Vietnamese-German Seminar on Physics and Engineering Hue, 2 March 2002 [27] N.T.Thuc Hien, Ngo Xuan Dai, Nguyen Anh Tuan, sc dang lrong ky yeu hc)i nghi Vat ly chat ran U)an quc)c lan thii tu,Nui Ce)c, Tliai Nguyen, 1 1.2003

24

PHU LUC

25

Physics & Engineering

in Evolution

Proceedings of the Fifth Vietnamese-German Seminar

on Physics and Engineering

Hue, Vietnam 25 February - 02 March, 2002

Edited by

Prof Dr Do Tran Cat

Prof Dr Nguyen Phu Thuy

Prof Dr Vo Thach Son

Prof Dr Annemarie Pucci Prof Dr Peter Paufler

Prof Dr Michael Hietschold

Institute of Engineering Physics - Hanoi University of Technology

Hanoi-2002

Peak effect of erilieal current density Jc of the NdBa:Cii^07-d polycrystalline

superconductors 181

Duong Cong Hicp and Lc Van Hong

GMR effect of Rapidly Solidified Co-Cu alloys 1 85

N.li Nghi, N.iy Nlian, C V Thang, N.X IMnic, N.M llong, N.H Due

N.A Tuan, B.X Chien and L.M Hoa

HR TEM study of inagnetic composite materials 190

N.H Nghi, D.N Uan, N.X Chien, N.M Hong, S Schulze, M Hietschold and LT Hung

Influence of partly replacing La hy other rare-earth elements on the magnetic and

eleelrochemical properties of LaNi^ 194

Bui Thi Hang, Luu Tuan Tai, Le Xuan Que and Mai Due Hanh

Ab-initio calculations of structural and lattice-dynamical of semiconductors 198

Nguyen Ngoc Trung, V.E Van Doren and Vo Thach Son

Investigation of local structure of the amorphous alloys Fe^Bioo-x 204

P.N Nguyen, N.V Hong, L.T Vmh, N.P Hai, D.H.N Anh, N.Q Long and P.K Hung

Computer sinnilation of impurity diffusion in irons 208

P.K Hung, N.V Hong, L.T Vmh, N.Q Long, N.P Hai and N.H Hung

Sinnilation of diffusion in c(uu)nical system 212

L.T Vmh, N.V Hong, P.N Nguyen, N.Q Long, L.V Huy and P.K Hung

Structural, optical and electrical properties of sputtered ZnO films 2 1 6

Nguyen Duy IMuiong, Ta Dmh Canh, Nguyen Ngoc Long and Phung Quoc Thanh

Gas sensing properties of Fe-f^t doped TiO: thin films 220

Nguyen Due Chien, Dang Thi Thanh Le and Nguyen Van Quy

High transmittance- low resistivity ZnO: Gajilms prepared by rf magnetron sputtering 224

Ta Dinh Canh, Nguyen Duy Phuong, Nguyen Ngoc Long and Nguyen Sy Hai

Optical investigation ofPu-doped ZnO thin flms hv sol-gel deposition 228

Pham Nguyen Hai, Nguyen Chi Thanh, Nguyen Thi Thuc Hien, Phung Quoc

Thanh, Khuc Quang Dat and Pham Thu Nga

Sol-gel preparation of highly-oriented ZnO thin films 232

Nguyen Thi Thuc Hien, Nguyen Chi Thanh, Pham Nguyen Hai, Hoang Nam

Nhat, Khuc Quang Dat and Phung Quoc Bao

Surface anisotrocopv contribution to giant magnetic in coercivity FeyCunn granular films 236

N.H Due, N.H Nghi, N.A Tuan, N.A Tuan and A Fnidiki

Preparation of zinc oxide thin films hv electrostatic spray pyrolysis 240

Nguyen Ngoc Long, Lc Thi Thanh Bmh, Nguyen Thanh Binh, Le Duy Khanh

and Tran Vinh Thang

Electrochemical properties of'natural passn'c film formed on 3I6L stainless steel

immersed in 0.9"o NaCl solution 244

Le Xuan Que

ix

Proceedings of the Fifth Vietnamese-German Seminar on Physics and Engineering, Hue, 25 February - 02 Mar, 2002

Sol-gel preparation of highly-oriented ZnO thin films

Nguyen Thi Thuc Hien, Nguyen Chi Thanh, Pham Nguyen Hai Hoang Nam Nhat, Khuc Quang Dat and Phung Quoc Bao

Faculty of Physics, Hanoi National University

334 Nguyen Trai Road Hanoi, Vietnam C-axis oriented zinc oxide thin tllms were prepared on glass substrates by the sol-gel

method using zinc acetate as a starting material A homogeneous and stable cclution was

obtained by dissolving zinc acetate in a solution ofethylene glycol and n-propanol ZnO thin

films with the preferred c-axis orientation were obtained after heat treatments at the

temperature starting from 2 5 0 T Decomposition and crystallization behavior of deposited

solid films during heal treatment were investigated by differential ••:^anning calorimeter

(DSC), X-ray diffraction (XRD), optical transmittance -nd photoluminescence

measurements

L Introduction

Recently, ZnO films have been widely investigated because they have notable properties such as it's wide band-gap of 3.3 eV at room temperature, large exciton binding energy (60 meV), high melting temperature (1975"C), thermal stability and higher chemical stability than other II-VI and III-V group semiconductors, e.g ZnSe, GaN Besides, they have strong an ultraviolet (UV) emission that can be used as the UV or blue emitting material

ZnO thin films have been prepared by a number of different techniques such as sputtering [1], pulsed laser deposition (PLB) [2], spray pyrolysis [3], etc Very recently the sol-gel process has been extensively used for fabricating ZnO films [4,5]

In general, the metal alkoxides are used as raw materials for the sol-gel process but they are very expensive Therefore metal salts are used to replace them

In this paper we report that it is possible to grow pure ZnO film of high quality by the sol-gel chemical deposition method with the initial precursor as zinc acetate We also present an investigation of the low-temperature near-band-edge photoluminescence (PL) of ZnO films because it can provide valuable information on the quality and purity of the material Band-edge PL in polycrystalline ZnO prepared by the sol-gel method has not received as much attention as by other methods e.g laser molecular-beam epitaxy, sputtering, etc We also discuss the origin of the green band emission of ZnO films

2 Experimental

Zinc acetate Zn(CH3COO)2.2H20 was dissolved in ethylene glycol and n-propanol The mixture was heated at 60"C for 90 min Triethyl amine was added to the transparent solution and the mixture was stirred vigorously to eliminate turbidity

Tlie sol-gel coating was prepared by the dipping procedure and was made a day after the solution was prepared Pnor to use the glass substrates were degreased in an acetone ultrasonic bath After each dipping, tlie film was pre-fired at 150'^C in air atmosphere for 30 min The withdrawal speed

232

is 4 cm/min The films were annealed at 100, 250, 300, 400, 500°C also in open air for 5 hours Some films were cut into two pieces and one piece was annealed again at oxygen flow for 2.5 hours

The ZnO layers were characterized by a Bruker D5005 X-ray diffractometer and DSC STD 2960 equipment, Shimazu UV-3 101/DC spectrophotometer and Jobin-Yvon FL 3-22 spectrofiuorometer were used for the optical characterization

3 Results and discussion

X-ray diffraction analysis (XRD) was

used to investigate the physico-chemical

processes involved in the sol-gel deposition

of ZnO Fig 1 shows the XRD pattern of

the films annealed at 100 and 250''C For

the film annealed at 250''C only the (002)

peak is produced This means the film had

preferential orientation along the (002)

plane, i.e c-axis is perpendicular to the

plane of the substrate A very important

result is that the preferential orientation of

the crystallines is not lost with the

annealing The crystal staictiire corresponds

to the lattice constants a = 3.2454 A

at lOOT(l) and250°C(2)

An endothermic peak around 100°C indicates the liberation of the organic molecules It makes a loss of mass The range from 200°C to 310'^C is probably due to the crystallization of material This can be seen from the X-ray spectra in Fig 1 A stronger exothermic peak at about 380°C is attributed to the combustion of the remaining organic material The mass remains constant at temperatures above 4 0 0 T The above results obtained by TOA, DSC, XRD indicate that the transformation of the amorphous to the crystalline state starts at temperatures between 200-250'C and is completed between 450-500T

Fig.2:DSC-DTA grapiiic of ZnO film Fig.3:0ptical transmittance spectrum of

the nim heated at 500°C

The optical transmittance spectra of the ZnO films which were prepared by two dipping and annealed at 500''C is shown in Fig.3 The strong and sharp absorpfion at wavelengths less than 380 nm corresponds to an electronic transition beyond the band-gap, 3.34 eV of the crystalline ZnO, which is very close to the intrinsic band-gap of ZnO

233

The low-temperature PL measurements were carried out at the rangq ft-om 1 IK to room temperature As the annealing temperature increases the PL peak intensity increases and the peak become sharper The PL spectra in UV and visible range of the film annealed at 500°C are shown in Fig 4 and Fig 5 The excitation wavelength was 335 nm It is seen that the visible luminescence (green and yellow) which are usually observed at the ZnO film prepared by the sol-gel process, is very small in compare with the near-band-gap PL

1

c CL)

0.0

Wavelength (nm)

Fig 5: PL spectra at 1 50 IC of ZnO film annealed in air at 500°C

It is also seen from Fig 4 that due to an overlapping of several lines, the band-gap PL are broadened and it is difficult to define an individual half-width for each band

near-At 1 IK there were three bands, two sharp and one broadened with 368 nm (labeled by II), 373 nm (LJ and 392 nm (Io), respectively

The 368 nm peak has the full width at half maximum (FWHM) of 19 meV and was broadened with increased temperatures and the peak position was shifted a Httle to longer wavelength The behavior of 373 nm peak is similar When temperature increases over 50

K, as the intensity of U quickly decreases a new peak of 382 nm (I3) appears Up to 150 K

the peak h becomes smaller so that two peaks at 391 nm (L) and 401.5 nm (I5) can be seen

234

clearly The Ii peak is disappeared from 150 K As the temperature increases there were only I2,13 peaks and at room temperature one cannot distinguish these two peaks

To our knowledge there was no report on the origin of the near-band-edge PL of the film prepared by the sol-gel process Compared the results with the one obtained for samples prepared by other methods [6-9] we can say that the Ii is the near-band-edge

emission of exciton bound to donor (DX) The L peak 54 meV far from the U may be due

to electron transition of exciton bound to neutral acceptor I3 peak is 70 meV separated from I2 and is its phonon-replica It is difficult to idenfify I4.15 and I^ transitions because of

their overlap Nevertheless, we supposed U is related to donor-acceptor pairs because the

PL peak broadness is due to the energy dependence on donor-acceptor distance

Up to now the green emission has been accepted as associated with oxygen vacancies but several reports [6,10] announced that it is not the case In our measurements, the intensity of the green emission increases for all films annealed in oxygen flow The result enables us to argue that the green emission may not be associated with oxygen vacancies or it is relate to interface traps

4 Conclusions

Zinc oxide films with good optical and structural properties were growth by the sol-gel technique using the non-alkoxide The films have good orientation, there is not red emission and the green emission is small in compare with the band-edge one The films quality could be comparable to that of the films prepared by more complicated and expensive methods

Acknowledgements

The authors would like to thank the Center for Materials Science of the Faculty of Physics (Hanoi University of Science) for using the equipment at the Center, and Dr Nguyen Huy Sinh for the arrangement of oxygen flow heat treatment

This work was supported by the national projects QG01.03 and 42130L

[3] S.A Studenkin, N Golego and M CociveraJ.AppI Phys 83 (1998)2104

[4] S Fujihara, C Sasaki, T Kimura Appl surface Science 180(2001) 341

[5] D Bao, H Gu, A Kuang, Thin Solid Films 3 1 2 (2000) 37

[6] Y.R Ryu, S Zhu,J.D Budai, H R Chandrasekhar, P F Miceli , H.W White , J Appl Phys 88 (2000)

201

[7] B.P Zhang, Y Segawa, K Wakatsuki, Y Kashiwaba K Haga, Appl Phys Lett 79(2001) 3953

[8] S.A Studenikin, Michael Cocivera W Kellner and H Pascher , J Lumines.90 (2000) 223

[9] T.V Butkhuzi,T.G Chehdze, A.N Georgobiam, D.L Jashiashvili, T.G Khulordava

and B.E.Tsekvava, Phys Rev B58 (1998) 10692

[10] Bixia Lin, Zhuxi Fu, Yunbo Jia and Guihong Liao, J.Elcctrochcm Soci 148 (2001) GI 10

235

Proceedings of the Fifth Vietnamese-German Seminar on Physics and Engineering, Hue, 25 February - 02 March, 2002

P r e p a r a t i o n o f z i n c o x i d e t h i n films b y e l e c t r o s t a t i c s p r a y p y r o l y s i s

Nguyen Ngoc Long, Le Thi Thanh Binh, Nguyen Thanh Binh

Le Duy Khanh and Tran Vinh Thang Faculty of Physics College of Science, Hanoi National University

334 Nguyen Trai, Ha Noi Viet Nam Electrostatic spray pyrolysis (ESP) technique has been used for thm films deposition of

indium-doped zinc oxide, whicli is used for optoelectronic applications The opHcal and

electrical properties of undoped ZnO tlims and In-doped ZnO films prepared by ESP

technique were studied The energy gap o\' ZnO lllms evaluated from optical absorption

measurement was 3.4eV The photoluminescence spectra were in\ estigated in the

temperature range from I 1 K to 30()K

1 Introduction

Zinc oxide is a technologically important material Zinc oxide thin films have been widely studied because they can be used in many applications such as gas sensors, conducting and transparent window layers in semiconductor solar cells, electro- and photoluminescent devices

Generally ZnO films have IK'CII ohuiincd hy one ni' the (ollowmg mclhods: sputtering

[1], reactive thermal evaporation f2j, chcniical vapor deposition [3], spray pyrolysis [4] and electrostatic spray [5J Amoiig those methods, electrostatic spray deposition is a convenient low cost and simple process for preparation of ZnO films There are several reports about photolummescense property of ZnO films deposited by spray pyrolysis from zinc acetate solution in isopropyl alcohol [6][7]

In this paper, wc describe the investigation of the ZnO and ZnO:ln films also prepared from zinc acetate solution in isopropyl alcohol, but by electrostatic spray deposition

2 Experiment

Zinc acetate [Zn(CH3COO)2.2H20] was dissolved in a mixed solvent consisting

of double distilled water and isopropyl alcohol in proportion 2:3 A solution of 0.2M zinc acetate was prepared A few drop of chlorine acid were added to the solution to dissolve some precipitates of zinc hydroxide resulting from the reaction

of isopropyl alcohol with zinc acetate A suitable amount of indium acetate was put into the solution for preparation of In doped zinc oxide films The doping ratio for the films was In/ZnO = 3 - 5 at %

The sample preparation apparatus similar to that in [8] has been describe elsewhere [9]

A hieh DC voltage fixed at ISkV was applied between a substrate and a metal capillary nozzle The distance from the nozzle to the substrate was kept at 25cm Substrates were glass plates 20x20x0.1mm m si/c The substrate temperature T, during deposition could be varied from 220"C to 400"C

The structure and lattice parameters of the films were analyzed by SIMLNS D5()()5 diffractometer The surface moiphology of the films was examined by means of scanning

240

electron microscopy using JEOL 5410VL microscope The microprobe spectra were taken The photoluminescence spectra were recorded by a spectrometer FL3-22 The optical transmittance spectra were recorded by a spectrophotometer UV-3101 PC

3 Results and discussions

3.1 Structural properties

Figure 1 shows typical X-ray diffraction pattems of no and ZnO:In films deposited at 350^C substrate temperature The XRD pattems clearly show the polycrystalline nature and hexagonal structure of ZnO and ZnO:In films From the values of plan spacing d measured

V M > > « * « E U E N 3 0500) - Uanc ZrO- iftJ-* - « 0 C

Figure 2 shows the micrograph of the ZnO:In film The average grain size is around l)-tm Numerous particles of diameter in the range 5-lOfim are present on the film surface The larger structures are attributed to variation in growth rates that result from

irregularities in the substrate surface

Figure 3 shows the element peaks for the microprobe test of the ZnO:In films The microprobe analysis of the films indicated that for all films the oxygen content is less than that

of stoichiometric crystal From the microprobe analyses the atomic In: ZnO ratio was calculated It is shown that the ratio is lower than that in the initial spray solution Tab.l shows the film compositions

Table 1 The film compositions (at%)

In in solution Zn 0 in

3.2 Electrical and optical properties

The undoped ZnO films show high

sheet resistance The sheet resistance of

a square of ZnO films was higher than

20MQ The doping of indium

remarkably decreased the sheet

resistance of ZnO films However, the

sheet resistance of the film doping with

3at% In is still high, it was lOkQ

Figure 4 shows the typical absorption

spectrum of ZnO:In film The absorption

edge was calculated from the spectrum

It was found that Eg = 3.4eV

an exciton bound to neutral donor [10] About the [A X] nature of the peak as 377nm there are different opinions This peak can be assigned to a neutral acceptor bound exciton The peak at 385.4nm can be attributed to the phonon replica of the peak at 377nm because the AE ^ 72meV is equated with the optical phonon energy of ZnO (ELo^72meV)[10]

370

350 400 450

Waveiength(nm) Fig 5: The photoluminescence spectrum of

ZnO:ln (5%) T,=350"C /.,,=300nm

ilK(a), 30K(b), 50K(c}, 1 50K(d}

500

Wavelenfith(nm) Fig 6: The photoluminescence spectrum of ZnO:in (5%) T,=350'C >,,,=365nm at 11 K

Figure 6 shows the second luminescence band It is a broad green band centered at 502nm This green photoluminescent emission may be assigned to the presence of oxygen vacancies in the ZnO film [11]

242

Figure 7 shows the third luminescence band

-the red band This red band had a maximum at

636nm and another at 668nm, The red emission

was attributed to the donor - acceptor pair

recombination mechanism [9]

It should be mentioned that the intensities of

the red and the green bands of ZnO:In film are

much less than that of undoped ZnO films It may

be explained that indium dopants maked the film

more conductive but at the same time it reduced

vacancy concentration which can be take part in

emission process The result from Tab 1 also

supported this opinion

4 Conclusions

Wavelcngth{nm)

Fig 7: The photoluminescence spectrum of

ZnO:In (5%) T,=350*^C, A.e,=365nm llK(a), lOOK(b), 250K(c)

Polycrystalline ZnO and ZnO:Ifilmshave been prepared by electrostatic spray pyrolysis technique The X-ray analysis revealed that the crystalline of the films was the hexagonal wurtzite with lattice parameters a = 3.255 A'\ C = 5.217 A'\

The doping with indium decreased the sheet resistance of ZnO films

The energy gap Eg of the films is 3.4eV

Photoluminescence of the films was measured in the temperature range from 1 IK to 300K Three emission bands related to exciton, oxygen vacancy and donor- acceptor pair were observed

Aknowlegments

This work is supported by the Project VNU QTOl-02

The authors would like to thank Mr Le Van Vu for X-ray measurements and Mr Phung Quoc Thanh for taking the SEM micrographs

References

[1] W.-C shihand M.-S Wu, J Cryst Growth 137, 319(1994)

[2] Y Sato and S Sati, Thin solid films 281,445 (1996)

[3] Y Natsume, H Sakata, T Hirayama, and H Yanagida, J Appl Phys 72,4203 (1992)

[4] S.A Studenikin, N.Golego, and M Cocivera, J Appl Phys 83,2104(1998)

[5]S,A Studenikin, N.Golego, and M Cocivera, J Appl Phys 84,2287(1998)

[6] A Ortiz, C Falcony, J.A Hernandez, M Garcia, and J C Alonsa, Thin solid Films 293, 103 (1999) [7] C Falcony, A Ortiz, M Garcia, and J S Helmam J Appl Phys 63, 2378 (1988)

[8] C K Ryu and Kim, Appl Phys Lett 6 7 ( 2 2 ) 3 3 3 7 ( 1 9 9 5 )

[9] Le Thi Thanh Binh, Nguyen Thanh Binh, Nguyen Hung Cuong, Le Hong Ha, Nguyen Ngoc Long Proceeding of the 3'*^ National Conference on Solid State Physics , (Nha Trang, Viet Nam, August 2001) [10] S A Studenikin, Michael Cocivera, W Kellner, H Pascher, J of Luminescence 91, 223, (2000)

[1 1] F H Letter, H R Alves, A Hofstaetter, D M Hofmann and B K Meyer, Phys Stat Sol (b) 222, R5(2001)

R4-243

Proceedings of the Fifth Vietnamese-German Seminar on Physics and Engineering, Hue 25 February - 02 March, 2002

Optical investigation of Eu-doped ZnO thin films by sol-gel deposition

Pham Nguyen Hai', Nguyen Chi Thanh', Nguyen Thi Thuc Hien' Phung Quoc Thanh', Khuc Quang Dat' and Pham Thu Nga^

"* Faculty of Physics, Hanoi National University

334 Nguyen Trai Road, Hanoi, Vietnam Institute of Materials Science, NCST Hoang Quoc Viet Road, Hanoi, Vietnam

In this report, Eu-doped ZnO thin films on standard microscope glass substrates were prepared

by the sol-gel method, using Zn(CH3COO)2 and EU2O3 as the precursor and the dopant source,

respectively The deposited ZnO films have the preferred (002) orientation aftoi annealing in air

at temperature of 250''C or higher The presence of Eu'"* ions in ZnO thin dims was clarified

from photoluminescence and energy dispersive X-ray measureme-.iS The optical and

crystallographic properties of Eu-doped ZnO thin films under heating conditions were discussed

to study Eu^"* ions in these highly-oriented ZnO thin films

1 Introduction

4f rare- earth (RE) ions, like Eu"^^ and Er"'^, in ZnO and others semiconductors have

received much attention because of their interesting physical properties and their high potentials in opto-electronic applications [1-3] However, only sintered polycrystalline ZnO co-doped with Eu^"*" and Li^, or sintered ZnO:EuCl3 phosphors were intensively studied [1,2] One may note that ZnO thin films can be prepared by various deposition techniques, such as chemical-vapor deposition, sputtering, sol-gel and pulsed laser deposition, all having well-textured c-axis films (without in-plane alignment) on various substrates [4] The aim of this study is to characterize the optical and crystallographic properties of Eu-doped ZnO thin films prepared by a sol-gel method

2 Experiments

Zn(CH3COO)2-2H20 was dissolved in ethylene glycol (C2H6O2) as the starting solution Propan-2-ol (C3H8O) and triethyl amine ((C2H5)3N) were also mixed to the solution All chemical substances are Merck and Prolabo products Eu dopant was then added using europium oxide (EU2O3) in the HCl media as the source (I at %), The sol solution was aged at 70'C for few hours and appeared stable Thin films were formed by slowly dipping clean standard microscope glass substrates into the sol solution, followed by heating at lOO^C for few hours to dry precursor films Heat treatments of thin films at temperature from 200 to 500°C were then carried out in air for 5h using a controlled fumace with a he^t ramp of 5"C/min Deposited tllms were investigated in the following measurements: /) Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) in order to clarify the processes carried out during ageing and annealing (TA Instruments SDT 2960); //) X-ray diffraction (XRD) using a Cu K^ radiation of 1.54056 A (Bruker D5005

diffractometer) for the crystalline orientation; iii) Energy dispersive X-ray spectroscopy

(Oxford ISIS 300 system attached in a SEM, JEOL JSM 5410 LV) to verify chemical

constituents and residuals of deposited films, before and after annealing; iv)

228

.IWl

' I

Photoluminescence (PL) spectroscopy performed at room temperature (Jobin Yvon FL 22) for studying the optical properties Single wavelengths /l^.^c from a Xe lamp for above

3-or below ZnO bandgap excitations were selectively used

3 Results and discussion

Fig 1 shows the DTA-TGA

data recorded for the aged gels at

temperature up to 700"^ C in air

The liberation of organic

molecules started at 200''C, and

the crystallization in-the deposited

films is partly carried out at this

temperature and higher An

exothermic peak at about 381°C is

attributed to the combustion of the

remaining organic material The

mass remains unchanged after

400^^0, indicating the complete

formation of the zinc oxide phase

In EDS measurements all

vx •JOO

1.«»«n

7 0 0

^ VI to 1*

FM^ 1: DT.^-TGA spectra for Eu-doped ZnO

constituents of a substance, having the atomic percentage higher than 0.1%, are detected The chemical nature of the constituents is revealed, based on their EDS peak position(s) within the energy resolution limit of 3 eV Fig 2 presents the EDS spectra measured in Eu-doped ZnO thin films treated at 250^^0 The signal related to Zn, O and Eu are clearly visible EDS peaks of Si, K and Ti are from glass substrates as concluded when monitoring only glass substrates The signal of C and Cl result from organics, which is burnt out in samples heated at temperature above 400^^0 and in agreement with the DTA-TGA analysis EDS data also confirms that the Eu element is about 1 at % in all samples It indicates the amount of Eu atoms remains unchanged regardless of the annealing conditions

In general, pure ZnO polycrystallines have three XRD peaks reflecting from the (100), (002) and (101) plane when monitoring at 30''<2^<44" [1-4] In this study, no XRD could

cps

20 Energy (keV)

Fig 2: EDS spectra of Eu-doped ZnO ihin films after annealing at 250"C for 5h

be detected in the samples annealed below 250^^0 However, by increasing temperature in the heat treatment above 250''C, only one XRD peak at 34.42"^ was observed, which

229

corresponds to the reflection of the (002) plane of ZnO Fig 3 shows XRD pattems of thin films after heat treatments at 100 and 250''C This observation means that Eu-doped ZnO thin film has been partly crystallised in a hexagonal wurtzite form after annealing at temperature of 250°C or higher It also indicates that the obtained films are well-textured in the c-axis orientation The ZnO lattice constants derived from the XRD calculation are similar to the reported standard ones [1,2,4], confirming that the Eu dopant gave rise no

distortion in the lattice of ZnO polycrystallines The average crystalline size t can be

evaluated from the XRD data using Scheirer's formula:

/ = 0.9*A/(5*cos2^

here: A, 26*and B denote for the X-ray radiation (1.54056 A), the Bragg angle (34.42"") and

the FWHM value of the XRD peak (0.45°), respectively The average crystalline size is small as about 20 nm

Fig 3: XRD spectra of Eu-doped ZnO films after

annealing at (a) 1 OO^C and (b) 250^C for 5h

Fig 4: PL spectra observed in Eu-doped ZnO thin

films heated at 250 C for 5h /l^.,c = 300 nm (left part) and Aexc ~ ^65 nm (right part)

In the PL investigation of undoped ZnO, three self-activated recombination defects having broad PL signals at 380, 510 and 620 nm are often observed at room temperature The first one is associated to free excitons in ZnO [1,2,4,5] while the others, depending on their crystal imperfection, are attributed to native defects of oxygen vacancy (VQ) [5,6] and zinc vacancy (Vzn) or oxygen interstitials (Oj) [6,7], respectively Typical PL spectra observed in Eu-doped ZnO thin films, after annealing at 250°C, are illustrated in Fig 4 (Two excitations were used in order to prevent a second-order effect from the excitation source to the signal due to the limit of the PL spectroscopy in this study) Two broad and overlapped PL bands are centered at 380 and 500 nm, indicating the existence of free excitons and VQ defects There is no PL center at 620 nm However, five groups of much narrower PL peaks at 577, 590, 615, 650 and 690 nm were monitored These peaks are not related to any native defects of ZnO Instead, they are very well marched to the specific PL spectra of Eu " ions, due to the Dj (j==0, F, (j=0,l, 6) transitions of the 4/-electrons,

in ZnO powders or other semiconductors as published in the literature [1] The observation

of these sharp red-emissions is a clear evidence for the presence of the Eu dopan in ZnO

thin films prepared in this work Increasing annealed temperature up to 50(fC the intensity

of all PL signals was reduced

230

Eu ^ and other RE"^^ in single crystal or polycrystalline ZnO have been investigated in many works [1,2] However, it is difficult to find that the Eu dopant could be diffused efficiently inside the ZnO grains, due to its large size and its charge So far there are two proposed models to explain Eu^^ photo-emissions in ZnO prepared at high temperature

Bachir et al [1] concluded that Eu ^ ions remain located mainly outside the ZnO grains, at

the grain boundaries, and as agregates The luminescence of Eu'^ ions can only be photoexcited when a coactivator, like Li"^ or N "^ ions, were added to the polycrystalline sintered ZnO at 900°C or 1200°C, which induce an energy transfer from the semiconductor

to the Eu^"^ ions Park et al [2] observed the characteristic Eu"*^ photoluminescence in sintered ZnO:EuCl3 phosphors at 1 lOO'^C, and suggested that Eu ^ ions exist in the host

lattice as EuOCI complexes In this study, since the Li dopant was not used intentionally or detected as an impurity and the existence of Cl as visible from EDS spectra, it is possible

to suggest that the EuOCI complex was formed and responsible for sharp red-emissions However, because of their observation under ZnO below bandgap excitations and thin solid ZnO films were annealed at maximum temperature 500°C, the EuOCI complex seems

to be located just at the ZnO grain boundaries More works need to be carried out to confirm this

4 Conclusion

In summary, Eu-doped ZnO thin films were prepared by the sol-gel method These films were transparent and investigated by DTA-TGA, XRD, EDS and PL techniques The ZnO crystallization partly started at 250''C, and all organics were removed at 400°C After heat treatments from 250''C, thin solid films were highly c-axis oriented PL measurements revealed the specific red emissions of Eu"^' ions, beside the broad bands of free excitons and Vo recombination defects in ZnO

This work was financially supported by QG 01.03 and 421.301 projects

[4] Y F Lu, H Q Ni, Z H Mai and Z M Ren, J Appl Phys 88, 498 (2000)

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