Chế tạo và nghiên cứu một số tính chất của các màng mỏng bán dẫn dung dịch rắn a2b6

Bang phuang phap nhilu loan dien tu cd the tinh dupe vung nang lupng cua mang luc giac tuf viing nang lupng cua mang lap phuang.. Trong qua trinh nay dien tir tiit miic nang lupng tap ch

BO GIAO DUG VA OAO TAO DAI HOC QUOC GIA HA N O I

TRLTCJNG DAI HOC KHOA HOC TlTNHIEN

-oOo-Le Thi Thanh Birth

Che tao va nghien cihi mot so tinh chat cua cac mang

mong ban dSn dung dich ran A B

Chuyen nganh: Vat ly Dien tijf

J LUAN AN PHO TIEN SI KHOA HOC TOAN - LY

M U C L U C

Mord^u 1 Chirang 1 CAC HOP CHAT BAN DAN ZnS, CdS vA CAC DUNG DICH RAN

BAN DAN Zn,Cd,.,S 5

1.1 Cft'u true dnh thi cua ZnS va CdS 5

1.1.1 CS^u tnic mang lap phuong gia kem 5

1.1.2 C^u tnic mang luc gidc Wurtzite 6

1.1.3 Mang tinh the ZnS va CdS 8

1.2 Ca'u tnic vising nSng lupng cua ZnS va CdS 10

1.2.1 Ca'u tnic viing nang lupng ciia mang I^p phuong 10

1.2.2 Ca'u true virng nang lupng cua mang luc giac Wurtzite 12

1.2.3 Viing nang lupng cua ZnS va CdS 13

1.3 Tinh ch^t h^p thu ciia ban dSn 15

1.3.1 Khai niem chung 15

1.3.2 Ha'p thu chuyin ddi thang 17

1.3.3 Ha'p thu chuyin ddi nghieng 18

1.4 Cac cach^'phat huynh quang trong ban dan 20

1.4.1 Khai niem chung 20

1.4.2 Mo hinh tpa dp cau hinh 21

1.4.3 Tai hop viing - viing 25

1.4.4 Tai hop buc xa qua cac trang thai exciton 28

1.4.5 Chuyin ddi vung - tap ch^t 33

1.4.6 Chuyin ddi done - axepto 35

1.5 Mdt s6 phuong phap ch^'tao tinh thi va mang dung dich thi 37

1.5.1 Cdc phuong phap klo dan tinh the 37

1.5.2 Cac phuong phap che tao mang dung dich ran 38

1.6 T6ng quan v^ dung dich ran Zn^^Cdi.j^S 39

1.7 K6t luan 41 Chirang 2 KY THUAT THl/C NGHIEM 43

2.1 He tao mang 43

2.1.1 L6 44 2.1.2 Bpgamau 44

2.1.3 He thdng vdi phun, khi nen 45

Chuong3 CHE TAO VA NGHIEN CLTU CAU TRUC CUA M A N G Zn^Cd,.,S 53

3.1 Tao mang Zn^^Cdj.^S bang phuong phap phun dung dich len de'ndng 53

3.2 Anh hudng cua nhiet dd d^d^n ch^t lupng m^ng 55

3.3 Ch^d6 u mang 60 3.4 Che^dOphatap 61

3.4.1 Phuong phap nhiing mang vao dung dich mudi tapcha'trdi ii d nhiet dp cao 61

3.4.2 Phuong phap hda dung dich mudi tap chat cung vao dung dich phun nen 63

3.5 Tao dien cue trong sudt Sn02 va l^m dien cue ohmic len mang Zn^^Cdi.^^S 66

3.6 Che'tao mau Zn^Cd^.^S bang phuong phap diing Id vi song 68

3.7 Ph6 nhilu xa tia X ciia mtlng Zn^Cdi.,S 70

3.8 Anh bfe mat cac mang Zn^Cd^^^S chup bang kinh hiln vi dien tu 73

3.9 K a luan 75 Chirang 4 TINH CHAT QUANG vA HUYNH QUANG CUA M A N G Zn.Cdi , S

KHONG PHA TAP 78

4.1 Tfnh cha't quang cua mang Zn^Cd].xS 78

4.1.1 Ph6 hSp thu cua mang Zn.Cdi.^S 78

4.1.2 X^c dinh d6 r6ng viing ca'm cua cac mang Zn^^Cdi.^S 80

4.2 Huynh quang ciia cdc mang Zn^Cdi.^S khdng pha tap 82

4.2.1 Dang phd huynh quang 83

4.2.2 Su phu thudc ciia dinh ph6 huynh quang theo thanh phan x 86

4.2.3 Pho huynh quang phan giai thai gian, su phu thupc ciia pho huynh quang

v^o cudng dd kich thich Thdi gian sdng huynh quang 87

4.2.4 Anh hudng ciia che'dd li nhiet len huynh quang ciia mang Zn^Cdj.^S 91

4.2.5 Su phu thudc nhiet dd cua ph6 huynh quang cua cac mang Zn^Cdj.^S 92

4.2.6 Co che' biic xa cap dono - axepto trong phd huynh quang cua mang

Zn^Cdi.^S khdng pha tap 94

4.2.7 Hien tupng quang hda ciia mang Zn^Cd,,^S 99

4.3 K^ luan 105 Chirang 5 HUYNH QUANG CUA CAC M A N G ZnxCd,.xS PHA TAP Cu VA Mn 106

5.1 Phd huynh quang ciia m ^ g Zn^Cdi^^S : Cu 106

5.1.1 Dang pho huynh quang Anh hudng cua sir pha lap ddng len pho huynh quang 106

5.1.2 Ph6 huynh quang cua mang Zn^Cdj ^SiCu vdi cac thdng sd thanh phan x khac

nhau = 109

5.1.3 Ph6 huynh quang cua mang Zn^^Cdi.^S : Cu khi kich thfch lien tuc

vdi cac cudng dp khac nhau va a cac nhiel do khac nhau 111

5.1.4 Ca che' biic xa ciia dai sdng dai trong phd huynh quang cua mang

Zn^Cd,.^S:Cu 113

5.1.5 Tach ph6 huynh quang 114

5.2 Phd huynh quang ciia mang Zn^Cdi^^S : Mn 117

5.2.1 Dang phd huynh quang Anh hudng ciia su pha tap len ph6 huynh quang 117

5.2.2 Pho huynh quang ciia mang Zn^Cdi.^S : Mn vdi cac thong sd thanh phan

Cac cong trinh khoa hoc lien quan den luan an 129

Danh muc tai lieu tham khao 131

D A N H SACH CAC K t HIEU StTDLNG TRONG LUAN AN

- tie't dien ha'p thu ciia tam i

- hang sd dien moi ciia cha't ban dSn

- budc sdng anh sang

- khdi lupng rat gpn ciia exciton

- ndng dp tap cha't

- thdi gian sdng huynh quang ciia mang

- dd day ciia mang

- nang lupng photon

- vecta sdng ciia dien tii

- khdi luomg hicu dun^ cua dien tix

- khdi lupng hieu dung ciia 16 trong

- mat dp trang thai i

- vecta sdng cua phonon

- thanh phan Zn trong mang

- dono - axepto

- nang lupng ion hda axepto

- nang lupng ion hda dono

- nang lupng dinh huynh quang

- nang lupng viing ca'm

- nang lupng dien tir d trang thai i

- nang lupng phonon

- ban dp rpng dai pho

- xac sua't chuyen ddi biic xa cap D-A

M O D A U

Hap chat ban dan A " B ^ la ddi tupng nghien cuti ciia rat nhieu cong trinh Cac hop chat A B cd nhieu dac tinh ca ban dang quy nhu dp rpng viing cam Idn, chuyen miic thing

Dp rpng viing cam Idn ciia cac hop chat A B cho phep md rpng gidi ban ufng dung cua cac linh kien quang dien tuf sang mien nhin thay va cue tim gan

Viing cam ciia cac hop chat A B deu la thang, ddng thdi cac chuyen miic phat quang gay ra bdi cac tam sau cd xac suat Idn, nen cd the by vpng cac chuyin ddi dien tu vdi hieu suat lupng tu cao Vi du ddi vdi ZnS, hieu suat lupng tu phat quang

c d t h e d a t g a n 100% [15]

Ngay nay, tit cac hop chat A B , ngudi ta da che tao dupe cac bpt huynh quang

hieu suat cao, cac quang trd nhay trong viing nhin thay

Trong cac hpp chat A B , ZnS cd dp rpng viing cam Idn ban ca (Eg = 3,7eV) Cung vi the ma ZnS cd limg dung thuc te rpng rai Bpt huynh quang ZnS dupe sii dung trong cac tu dien huynh quang, cac man Rangen, man cua cac dng phdng dien

tu Ngudi ta da che tao dupe nhieu loai photodiot tren ca sd chuyen tiep p - n cua ZnS Suat quang dien dpng (photo emf) <5 cua chuyen tiep p - n tren tinh the ZnS thudng dat tdi 2,5 V [15]

Ben canh ZnS, CdS (Eg = 2,4 eV) cung la mot trong cac vat lieu quan trpng nhat thudng dupe sir dung trong cac luxmeter va cac he thdng pin mat trdi Ddi vdi cac he thdng pin mat trdi, mang mong CdS thudng dupe dting lam vat lieu cUa sd trong cac chuyen tiep di the vdi cac ban dSn boat quang nhu Cu^S [47], CdTe [36] Khong chi cac hop chat A B , ma ca cac dung dich ran ban dfin tren ca sd cac hap chat nay cung dupe nghien ciin mot each rpng rai va cd nhieu ling dung trong thuc te

Dp rpng viing cam cua dung dich ran A B cd the thay ddi dupe tiiy theo thanh phan cua hop chat trong dung dich ran Dieu dd tao ra kha nang che tao nhiJng ngudn phat quang va cac dau thu quang lam viec trong viing pho trai rpng tir viing hong ngoai tdi viing kha kien

Mang Zn^Cdi^S la mot trong cac mang dung dich ran A B dupe quan tam nghien cuti nhieu [13, 83, 74, 21, 47] Dp rpng viing cam cua Zn^Cdi.^S cd the thay

doi lien tuc th 2,4 eV den 3,7 eV Mang Zn^Cdi.^S hiia hen nhieu trien vpng suf

dung trong cong nghiep che tao cac linh kien quang dien tir nhu cac pin mat trdi, cac diot phat quang, cac quang trd

Nghien cufu cac tinh chsft quang va huynh quang cua mang Zn^Cdi.xS cho den nay vin la de tai hap dan cac nha vat ly ban dan Cac ket qua nghien ciiti nay cd the cung cap nhuHg thdng tin ve ban chat cua nhiitig tam tap chat hoac ve cau tnic vi

md cua cac tam huynh quang, ca che biic xa huynh quang

De nghien ciiru cac tinh chat quang va huynh quang cua ban dan, ngudi ta thudng sir dung cac phuong phap pho hap thu va pho huynh quang Viec su dung cac phuang phap huynh quang cho phep don gian hda va lam giam gia thanh he thdng do ludng kiem tra trong san xuat, bdi vi viec kiem tra cac dac tnmg huynh quang cd the thay the viec do dac hang loat cac dac tnmg dien, quang dien cua vat lieu Trong sd cac phuong phap pho huynh quang thi quang huynh quang la phuong phap dupe pho bien rpng rai nhat Dd la phuong phap nghien culi nhay nhat ddi vdi nhutig vat lieu cd ndng dp tam tap thap Phuang phap nay con cd iru diem la khdng can tao dien ctrc len ban dan, phep do cd the thuc hien nhanh, dan gian va chfnh xac

ma khdng ddi hoi cac thiet bi phiic tap [31]

Mac dii tinh chat quang va huynh quang cua mang Zn^Cdi^S da dupe mot sd tac gia nghien ciiti, nhung ban chat cua cac dai pho, ca che phat huynh quang cua cac tam vSn con la nhihig van de chua dupe lam sang to

Tinh chat quang va huynh quang cua mang Zn^Cdi.xS che tao bang phuang phap phun dung dich len de ndng Ian dau tien dupe nghien ciiti va cong bd trong luan an nay

Luan an dupe hoan thanh trong 5 nam (1992-1996) tai phdng thi nghiem cua bp mon Vat ly Dai cuang, trudng Dai hpc Khoa hoc Tu nhien Luan an dupe mang ten

"Che tao va nghien ciiti mot sd tinh chat cua cac mang mong ban din dung dich ran

A"B^"

Muc dich cua luan an nham:

1- Che tao mang dung dich ran ban dan bang phuang phap phun dung dich 2- Pha tap vao mang

3- Nghien ciiru tinh chat quang va huynh quang ciia mang ZUxCd^.^S

4- Nghien ciiu tinh chat huynh quang cua mang Zn^Cdi.xS pha tap Cu va pha tap Mn

Phuang phap nghien ciiu cua luan an la phuang phap thuc nghiem:

1- Cac mang Zn^Cd^.^S dupe che tao tren he phun dung dich do chung tdi tu che tao

2- Cac phuang phap phan tich bang ky thuat Vi phan tich tia X, nhilu xa tia X, kinh hien vi dien tuf da dupe su dung

3- Cac pho truyen qua va huynh quang cua cac mang Zn^Cdi.^S dupe khao sat vdi cac thanh phan x thay ddi, ndng dp tap chat thay ddi, trong mien nhiet dp 77K -300K Mot sd pho dupe do tai 2K va 4K

4- Cac pho phan giai thdi gian cua mang Zn^Cd^.^S dupe do d 77K va 300K Cac ket qua chinh cua luan an la:

1- Da xay dung va hoan thien cac he tao mau, lam dien cite va cac he do pho truyen qua va huynh quang

2- Bang phuang phap phun dung dich len de ndng da che tao dupe mang da tinh the dung dich ran ZuxCdi.^S cd dp rpng viing cam thay ddi tiir 2,4 eV den 3,5 eV va

cd hang sd mang phu hop vdi hang so mang cua don tinh the hoac mang Zn^Cdi.^S che tao bang cac phuong phap khac

3- Lan dau tien nghien cuu mot each he thdng pho huynh quang ciia mang Zn^Cdi.xS che tao bang phuang phap phun dung dich, khdng pha tap va cd pha tap

Cu, Mn Tim ra quy luat phu thudc ciia nang lupng dinh huynh quang cac mang vao thanh phan x, nhiet dp T va ndng dp tap chat t] trong mang

4- Lan dau tien cac pho huynh quang phan giai thdi gian cua mang Zn^Cdi.^S

va su phu thupc cua pho huynh quang vao cudng dp kich thfch dupe nghien ciiti Cac ket qua thuc nghiem dupe giai thich thoa dang, neu gia thiet ca che phat huynh quang cua mang la su tai hpp cua cap dono - axepto

Ngoai phan md dau, ket luan va danh muc tai lieu tham khao, luan an dupe trinh bay trong 5 chuong:

- Chuang 1: Cac hop chat ban dan ZnS, CdS va dung dich ran ban dan Zn^Cd^.^S

- Chuang 2: Ky thuat thirc nghiem

- Chuang 3: Che tao va nghien ciiu cau tnic cua mang Zn^Cdi.^S

- Chuang 4: Tinh chat quang va huynh quang cua mang Zn^Cdi.^S khdng pha tap

- Chuang 5: Huynh quang cua mang Zn^Cdi.^S pha tap Cu va Mn

Chuang 1 CAC HOP CHAT BAN DAN ZnS, CdS

VA CAC DUNG DICH RAN BAN D A N ZoxCdi XS

1.1, Cau true tinh the cua Z n S va C d S

1.1.1 Cau true mang lap phirong gia Rem [6]

gia kem chiia hai loai nguyen tut khac nhau Ddi xiing diem d cac vi tri |a| va |c| tuong utng vdi 43m Mot nguyen tut bat ky dupe bao bpc bdi 4 nguyen tu khac loai: Mdi nguyen tu A " ( B ^ ) dupe bao quanh bdi 4 nguyen tu B (A ) nam d dinh ciia tii dien tren khoang each — j — , vdi a la thdng sd cua mang lap phuang

Mdi nguyen tut A (B ) con dupe bao bpc bdi 12 nguyen tu ciing loai, chiing la

fl

lan can bac 2, nam tren khoang each Sau nguyen tu trong sd dd nam d dinh cua luc giac, tren ciing mot mat phang vdi nguyen tir ban dau Sau nguyen tir con lai tao thanh phan lang tru gdm 3 nguyen tut nam d mat phang cao ban va 3 nguyen tu nam d mat phang thap ban mat phang luc giac ke tren

Dac tinh quan trpng ciia cau tnic nay la khdng ton tai tam ddi xiing hay tam dao Cac Idp A " - B ^ dupe dinh hudng theo true [111] Do dd tinh the A " B ^ ' lap phuang gia kem cd tinh di hudng Cac mat ddi xufng nhau (h k 1) va (h k 1 ), cac hudng ngupc nhau [h k I] va [h k 1 ] cd the cd tinh chat vat ly khac nhau

1.1.2 Cau true mang luc giac Wurtzite [6]

Nhdm ddi xiing khdng gian cua mang tinh the nay la C'^gy - Pdamc Mdi d nguyen to chiia hai phan tu A B Trong dd hai nguyen tii A " nam d vi tri: 0, 0, 0

va 1/3, 2/3 , 1/3 va hai nguyen tu B ^ nam d vi tri : 0, 0, u va 1/3, 2/3, 1/3 + u Vdi

Hinh 2 Ca'u true mang tinh the luc giac Wurtzite [6]

nguyen tut A° @ nguyen tir B VI

- 6 nguyen tut d dinh luc giac, trong ciing mot mat phang vdi nguyen tuf ban dau

va each nguyen tut ban dau mot khoang a

- 6 nguyen tii khac d dinh lang tru tam giac, each nguyen tuf ban dau mot khoang [1/3 a^ + 1/4 c^]'^l Neu - = 2V2

1,6330 va u = 3/8 thi cac nguyen tu lan

a V3

can bac mot nam chfnh xac d dinh tii dien va 12 nguyen tut lan can bac hai deu nam each nguyen tut ban dau mot khoang bang nhau

IlnVI Tinh the A B cau tnic luc giac khdng cd tam ddi xiing vi the trong mang ton tai true phan cue song song vdi hudng [0 0 0 1]

Ddi vdi mot chat, giiia cau tnic luc giac va cau tnic lap phuong ton tai mot mdi lien he chat che Cac hang sd mang cua 6 nguyen to luc giac hen he vdi hang sd mang d nguydn to lap phuang bang cong thiic sau

^h = 2^K Ch = 3-V3a,

trong do: a^ va c^ la hang sd mang luc giac

a^ la hang sd mang lap phuang

Vi tri tuang ddi cua nguyen tuf trong cac mang lap phuang va luc giac gan gidng nhau Su bao bpc cac nguyen tuf A (hay B ) bdi nhung nguyen tuf lan can bac hai trong ca 2 loai mang cung nhu nhau Su khac nhau ve tpa dp cac nguyen tuf the hien d chd cau tnic luc giac dac tnmg bdi phan lang tru De phat hien dupe su khac nhau trong cac cau tnic nay can phai xet den cac nguyen tir lan can bac 3

1.1.3 Mang tinh the ZnS va CdS [6]

1.1.3.1 ZnS

Tinh the ZnS thudng cd cau tnic lap phuong gia kem va luc giac Wurtzite Cau tnic luc giac cua ZnS on dinh d nhiet dp cao Su chuyen pha tijf luc giac sang lap phuang chi xay ra d nhiet dp 1020*^C - 1150*^C Cac hang sd mang cua ZnS phu thupc vao dp hoan thien ciia mang tinh the Su ton tai ciia tap chat ciing gay nen sai khac ve hang sd mang so vdi tinh toan ly thuyet Oxy thudng la tap chat trong ZnS tinh the luc giac Nhung sai hong trong tinh the luc giac ZnS cd the tao ra mot viing nhd cd cau tnic lap phuang nam trong tinh the luc giac

Tinh the ZnS ket tinh trong cac dieu kien khac nhau cd the tao ra cac dang cau tnic khac nhau Dd la cac bien the ciia cau tnic lap phuang va cau tnic luc giac

1.1.3.2 CdS

Tinh the CdS ciing ton tai d cau tnic lap phuang gia kem va luc giac Wurtzite Hang sd mang cua cac tinh thi thuc te phii hpp tot vdi cac tinh toan ly thuyet Tuy thupc vao dieu kien cong nghe, cd the nhan dupe cac tinh the CdS lap phuang va luc giac mau do hoac mau vang Cac thdng sd mang cua CdS va ZnS vdi cac cau true lap phuong va luc giac dupe thdng bao trong bang 1

Hop chat

ZnS ZnS CdS CdS

Hang so mang

Lap phuong gid kem a (A )

Trong mien chuyen tiep tii mang nay sang mang khac thi mang tinh the dung dich ran cd the d dang hdn hop hai pha hoac cau tnic hdn don vdi cac gia tri sai hong Idn

Tac gia [14] nghien cuti su chuyin pha trong he Zn^Cd^.^S da chi ra rang str chuyen pha trong mang tinh the lap phuang gia kem sang Wurtzite luc giac cd the xay ra theo trinh tu 3C - lOH - 6H - 4H - 2H ddi vdi nhung thanh phan 0 < x < 0,062

1.2 Cau true vung nang luong cua ZnS va CdS [6]

1.2.1 Cau true vung nang lupng cua mang lap phuong

Mang tinh the lap phuang gia kem cd ddi xutig tinh tien cua mang lap phuang tam mat Cac vecta tinh tien ciia mang la:

^1 = ^ a(l 1 0)

^2 = | a ( 1 0 1)

t 3 = | a ( 0 11)

trong do: a la canh ciia mang lap phuOng

Mang dao la mang lap phuang tam khdi, cd cac vecta ca sd la:

X i = 2 7 i a - \ l , l , - l ) X2-27ra-Vl,-l, 1)

Bang mot sd phuang phap nhu phuang phap gia the, phuang phap sdng phang true giao, ngudi ta da tinh toan dupe cac viing nang lupng cua cac hop chat A B Tat ca cac hpp chat A B deu cd viing cam thang Dp rpng viing cam cua cac hpp chat A B giam khi nguyen tuf lupng tang Ddi vdi mang kim cuang trong khdng gian vecta sdng, tai k = 0, trang thai nang lupng cao nhat cua viing hda tri ling vdi r25 suy bien bac 3

Ddi vdi mang lap phuang gia kem, trang thai cua r25- chuyen thanh Ti^y Neu

tinh den tuang tac spin quy dao thi trang thai ri5v tai k = 0 se suy bien bac 6 thanh trang thai Tg suy bien bac 4 va trang thai Fy suy bien bac 2 Su suy bien tai k = 0 duac bieu dien tren hinh 4

(0,0,0)

Hinh 4 Sa do vung nang lupng cua mang tinh the lap phuang gia kem

tai lan can^ = 0 [6]

Do mang lap phuang gia kem khdng cd ddi xiing dao nen cue dai cua viing hda tri hai lech khdi vi tri k = 0 va lam mat di su suy bien viing cac Id trong nang Vj va

16 trong nhe V2

1.2.2 Cau true viing nang lupng cua mang luc giac Wurtzite

Mang tinh the Wurtzite cau tao tii hai mang luc giac long vao nhau Mot mang chiia anion, mang con lai chiia cation

Cac vecta tinh tien ca sd ddi vdi mang Wurtzite la:

t i = | a ( l , - V 3 , 0 )

^ 2 - ^ a ( l , V 3 , 0 )

?3 = c ( 0 , 0 , 1) Mang dao cung cd cau tnic luc giac vdi cac vecta tinh tien ca sd la:

2=27ia-Vl,-V3,0)

X2 = 2 ^ - ^ 1 , 7 3 , 0 ) X3 = 27ic"V0,0, 1) Viing Brillouin la mot khdi bat dien bieu diln tren hinh 5

Do cau tnic tinh thi ciia mang lap phuang va mang luc giac cd khac nhau nen the nang tac dung len dien tir trong hai loai tinh the khac nhau Tuy nhien ddi vdi

cung mot chat, khoang each giua cac nguyen tit trong hai loai mang tinh the bang

nhau Lien ket hda hpc ciia cac nguyen tut trong hai loai mang tinh the cung nhu nhau Chi cd su khac nhau cua trudng tinh the va vhng Brillouin gay ra su khac biet trong the nang tac dung len dien tu Bang phuang phap nhilu loan dien tu cd the tinh dupe vung nang lupng cua mang luc giac tuf viing nang lupng cua mang lap phuang Sa do vung dfin va viing hda tri cua A B vdi mang tinh the luc giac dupe dan tren hinh 6

So vdi sa dd viing cua mang lap phuang, ta thay rang do anh hudng cua nhilu

loan trudng tinh the ma miic Fg (J = 3/2) va F7 (J = 1/2) cua viing hda tri mang lap phuang bi tach lam 3 md'c F9(A), F7(B) va F7(C) trong mang luc giac

K

H

Hinh 5 Vung Brillouin ddi vdi mang tinh the Wurtzite [6]

Hinh 6 Cau true vung nang lupng ciia mang Wurtzite tai lan can k = 0 [6]

1,2.3 Vung nang lupng ciia ZnS va CdS

Tinh the ZnS va CdS cd the ton tai dudi dang lap phuong gia kem, luc giac Wurtzite hoac bien the cua hai dang nay Tiiy vao dang cu the cua mang tinh the ma cau tnic viing cua ZnS va CdS cd tinh ddi xiing khac nhau Tuy nhien, tat ca cac

dang tinh the ZnS va CdS deu cd dac diem chung la cd cau tnic viing thang: cue dai tuyet ddi cua viing hda tri va cue tieu tuyet ddi cua viing dan cung nam tai gia tri

k = 0, tiic la d tam ciia viing Brillouin

Hinh 8 Sa dd vung nang lupng cua tinh the lap phuang CdS [14]

Tren hinh 7 va hinh 8 la sa dd viing nang lupng cua ZnS va CdS dudi dang tinh the lap phuang Khe nang lupng giua cac diem ddi xiing trong viing nang lupng ciia

ZnS va CdS cd the tham khao trong [14] Khdi lupng hieu dung cua dien tu d Ian can day viing din va cua Id trong d lan can dinh vung hda tri ciia tinh thi ZnS va CdS theo tinh toan ly thuyet va thuc nghiem cd the tham khao trong [6]

1.3 Tinh chat hap thu ciia ban d i n

U l Khai niem chung [24,17]

Neu chieu mot chum sang len mot ban mong ban din thi mot phan anh sang bi phan xa lai tren be mat ban dSn, mot phan anh sang se di xuyen qua ban dSn, phan con lai hoac bi tan xa, hoac bi hap thu trong ban dan Nang lupng hap thu dupe chuyen thanh cac dang nang lupng khac: npi nang, quang nang (huynh quang), dien nang V.V Tuang tac cua anh sang vdi ban dSn dupe xac dinh bdi hai yeu to:

- Tinh chat cua anh sang: thanh phan pho, su phan cue, cudng dp, hudng truyen

- Tinh chat cua ban dan: cau tnic viing, cau tnic tinh the v.v

Ngoai ra, cac dieu kien ben ngoai nhiT: nhiet dp, ap suat, dien tnrdng, tii trudng ciing anh hudng Idn den qua trinh hap thu ciia ban din

Trong qua trinh hap thu, photon vdi mot nang lupng xac dinh, se kich thich dien tur tiit trang thai nang lupng thap ban len trang thai nang lupng cao han

He sd dac trung cho qua trinh hap thu la he sd hap thu a Neu anh sang cudng

dp IQ, fan sd VQ chieu den ban mong ban dSn cd chieu day d, thi anh sang sau khi ra khdi mlu se cd cudng dp I tuan theo dinh luat Bouguer - Lambert:

I(Vo) = Io(Vo)exp(-ad) (1)

trong dd a la he sd hap thu cua ban dSn d tan sd VQ cd thii nguyen c m ' \ Nd cho

biet dp day tuong ling cua chat ban din ma khi anh sang truyen qua cudng dp se bi

giam di e Fan

a la ham sd ciia fan sd anh sang va phu thudc vao ban chat cua chat ban din He

so hap Ihu (y l^ vdi: mai dp cac iam ha'p Ihu va (iel ditn han ihu ciia chung:

a = crN (2)

Tiet dien hap thu ciia tam phu thupc vao nang lupng photon va ban chat ciia tam hap thu N^'u trong cha't ban din cd i loai tam ha'p thu vdi cac ban cha't khac nhau thi:

trong ao: a^ - ne sd nap tnu cua tam i

Nj- ndng dp tam i Q: - tiet dien ha'n thu cua tam i

Tiiy thupc vao ban chat ciia cac qua trinh hip thu, ngudi ta phan loai nhung dang h i " thu chfnh nhu sau:

1- Hip thu CO ban (rieng) iam chuyen dien tii tir viing hda tri len viing din Hip thu ca ban xay ra khi hv > Eg

2 - Hip thu tap chit xiy ra do hien tupng ion hda cac nguy6n tir tap chit Trong qua trinh nay dien tir tiit miic nang lupng tap chit dupe chuyin ddn viing din hoac dien tu tir viing hda tri dupe chuyen len miic nang lupng tap chat

3 - Hip thu bdi cac hat tai dien tu do xay ra khi cac hat tai nay chuyen dpng dudi tac dung ciia dien trudng sdng anh sang

4- Hip thu bdi dao dpng mang xiy ra khi anh sang tuang tac vdi dao dpng

mang lam thay ddi sd phonon quang hpc

5 - Hip thu exciton xay ra ddng thai vdi su hinh thanh cap dien tu - Id trong Hen

ket (exciton)

I

I

6 - Hap thu plasma do tap hop cac dien tu va Id trong tu do gay ra

He sd hap thu dac tnmg cho kha nang hap thu photon cd nang lupng hv cua he dien tu cd mat dp trang thai ban dau U; de chuyen tdi trang thai cao ban cd mat dp trang thai nf Neu xac suit chuyin rdi tuf trang thai i sang f la pif thi he so hap thu se la:

a( hv) = ASP.jn.nj (4)

trong do: A - he sd ty le

Z - lay tdng theo tat ca cac chuyin ddi dupe phep

Khi tinh toan ngudi ta thudng gia thiet rang tat ca cac trang thai nang lupng thap deu dupe lap day cac dien tu, con cac trang thai nang lupng cao deu trong Thuc te dieu kien nay thda man ddi vdi ban din khdng pha tap d nhiet dp rit thap (gan 0 K)

1.3.2 Hap thu chuyen doi thang [30]

Su chuyen ddi dien tuf giua cac trang thai tuan theo dinh luat bao toan xung lupng va dinh luat bao toan nang lupng:

ki + k = kf

Ei + hv = E,

(5) (6)

trong do: k^, kf - vecta sdng ciia dien tut d trang thai i, f

hk - xung lupng photon

Ej, Ef - nang lupng ciia dien tii d trang thai i, f

hv - nang lupng ciia photon

Vi ki, k f » k nen dieu kien (5) trd thanh

(7) chinh la dieu kien Ipc Ipa cho cac chuyen mufc thing Trong trudng hpp nay

he sd hap thu :

Hinh 9 Chuyen ddi thang trong qua trinh hap thu cua ban din [30]

Chuyen ddi thang cd the xay ra khi day viing din va dinh viing hda tri cua ban din nam d cung mot gii tri k trong khdng gian vecta sdng Hinh 9 la sa do chuyen ddi thing trong hap thu ciia ban dan

1.3.3 Hap thu chuyen ddi nghieng [27]

Ddi vdi nhiing ban din cd viing cam khdng thing, trong qua trinh chuyen ddi, nang lupng va vecta sdng ciia dien tut deu bi thay ddi Chuyin ddi nay phii thuc hien theo hai budc va cd su tham gia cua phonon Mac dii trong tinh the ton tai rat nhieu phonon khic nhau, nhung cac phonon khac tham gia trong cac chuyen miic quang hpc chi gdm cac phonon dap ling cac dieu kien:

k + q = kf Trong qua trinh dd, nang lupng se dupe bio toan:

(9)

hVa = Ef - Ej - Ep hVe = Ef - E, + Ep

(10) (11)

voi: Ep - nang lupng phonon

hva - nang lupng photon ling vdi trudng hop hap thu phonon hVe - nang lupng photon ling vdi trudng hop biic xa phonon Hinh 10 bieu diln sa do chuyen miic nghieng trong qui trinh hap thu Ei(ki) va Ef{kf) tuang ling la trang thai dau va trang thai cudi cua dien tuf trong chuyen ddi Chuyen ddi tii trang thai i sang trang thai f cd the xay ra theo hai budc, qua hai trang

7

> k

Hinh 10 Chuyen ddi nghieng trong qua trinh hip thu cua ban din [27]

thai trung gian m hoac m' Xet trudng hpp chuyen ddi qua trang thai trung gian m nam trong viing dan Dau tien, dien tuf chuyen miic thing Ei(ki) den Ejj^(k,) bao toan vecta sdng k, Sau dd d budc thu" hai, dien tut tuong tie vdi phonon cd vecta sdng

q = kf - kj va liic nay dien tii chuyen den trang thai Ef(kf) Tuang tu nhu vay, dien tut

cd the chuyin ddi thdng qua trang thai trung gian m ' trong vung hda tri Budc 1 dien tuf tuang tic vdi phonon cd vecta sdng q^va ddi den trang thai Ej^ (kf) Budc hai

dien tut chuyen miic thing len Ef(kf) trong viing din, bio toan vecta sdng kf

1.4 Cac c a che phat huynh quang trong ban dkn

1.4.1 Khai niem chung [30],[6]

Khi hap thu nang lupng, chat ban din se d vao trang thai kich thfch Cac cap hat tai dien - dien tuf va Id trong - dupe hinh thanh Qua trinh tai hop trong bin din cd bin chat ngupc vdi qua trinh hap thu Qua trinh tii hpp lam bien mat cac hat tai dien trong ban din Qua trinh nay cd the xay ra kem theo biic xa hay khdng biic xa photon Trong tnrdng hop tai hop khong kem theo biic xa , tat ca nang lupng dupe giai phdng ra dupe truyen cho dao dpng mang (phonon) truyen cho hat tai dien tu

do thii ba (tai hpp Auger) hoac dupe diing de kich thich cac dao ddng plasma trong chit ban din (tai hop plasma)

Trong trudng hop tai hpp biic xa, tit ca hoac mot phan nang lupng dupe giii phdng dupe phit ra dudi dang lupng tuf anh sang Q u i trinh tii hop biic xa anh sang dupe gpi la huynh quang

Dieu kien chu yeu de xuat hien huynh quang la ban din phii d trang thii khong can bang Tiiy thupc vao phuong phap kich thich chat ban din, ngudi ta nhan dupe cac dang huynh quang khac nhau nhu:

- Quang huynh quang xuat hien khi chieu vao chat bin dan anh sang cd budc sdng thich hop

- Dien huynh quang xuat hien khi kich thich ban dan bang dien trudng hoac ddng dien qua cac Idp tiep xiic di thi hay chuyen tiep p - n

- Huynh quang catot (cathode) xuat hien khi chieu chiim tia dien tut cd nang Itfpng cao

Tuy nhien hau nhu khdng phu thudc vao phuang phap kich thfch, qua trinh tii

hop bufc xa trong cac chat ban din thuc hien qua cic ca che tai hop ca bin sau (hinh 11):

a Tai hop cua cac dien tu va Id trong tu do (chuyen ddi vung - viing)

b Tai hop qua cac trang thai exciton tu do va exciton lien ket

c Tai hpp cua cac hat tai dien tu do vdi cac hat tii dien dinh xdr tren cac tam tap chit (chuyen ddi vung - tap chat)

d Tai hop cac hat tii dien dinh xii tren cac tam tap chat (chuyen ddi giua cic tam tap chit hoac chuyen ddi cap dono - axepto)

Hinh II Cac chuyen ddi tai hpp biic xa ca ban trong vung ban dan [61]

C ^ V: Tai hpp viing - vung; E —> V: Tai hpp exciton; D —> V: Tai hop dono - 16 trong

vung hda tri; C ^ A: Tai hop dien tii tu do - axepto; DD -^ V: Tai hpp dono sau - 15

trong vung hda tri; C —> DA: Tai hop dien tuf tu do - axepto sau; D —> A: Tai hpp cap dono

- axepto

1.4.2 Mo hinh toa do cau hinh

De giai thfch cac qua trinh huynh quang cd su tham gia cua cac tam sau, ngudi

ta thudng su dung md hinh tpa dp cau hinh trong dd cd tinh den su tuang tie manh

vdi nhieu phonon Cac ca sd tinh toan ly thuyet mat dp trang thai, xac suit chuyen doi dien tii, tiet dien hip thu, phan bd nang lupng biic xa dua tren md hinh tpa dp ciu hinh da dupe D Curie trinh bay trong [25]

Tren hinh 12 la sa do ciu hinh bieu diln nang lupng cua trang thai ca ban va

trang thai kich thich diu ti6n ciia nguyen tii theo vi tri ciia nguyen tu Nguyen hi d day cd the la nguyen tu tap chit hoac nguyen tut cua mang tmh the O trang thai co ban, cue tilu nang lupng A ting vdi vi tri can bang ciia nguydn tii (dupe chpn la gdc tpa dp 0) O trang thai kich thich cue tieu nang lupng C nam d tpa dp khac ciia

nguyen tu

Nhu vay, khi di6n tir bi kich thich tir trang thai ca ban \6n trang thai kich thich

thi loan bp he nhich sang trang thai C Dieu dd cd nghia la nguyen tut da chuyen ddi mdt doan Ax Trong qua trinh iy, di^n tu dupe kich thfch se mit mdt phan nang lupng nhd dl cho nguyen tii thuc hi6n chuyen ddi dd, phan nang lupng nay dupe giii phdng bang each biic xa phonon Khi nguyen tii trd v6 trang thai co bin (dilm

D), dl cho ca ht chuyin d^'n vi tri nang lupng thip nhit d dilm A, nguyen tii c^an

phii thuc hien chuyen ddi Ax Su dich chuyen D -^ A cQng xay ra bang each biic

Xa phonon

Nang lupng biic xa trong chuyen miic C —> D nhd ban nang lupng hip thu trong chuyen miic A -> B Hieu hai nang lupng nay gpi la "dd dich chuyin Frank -Condon"

Ky hieu Eo la hieu giua cac nang lupng cue tieu cua dudng cong ciu hinh ling vdi trang thai kich thich va ling vdi trang thai ca ban ciia dien tu Vi tri cue dai cua dai pbd biic xa va hap thu se dupe xac dinh bdi cac he Ihiic:

hv.i, = E, + S,hv, (13) trong dd hv^ va hv la nang lupng phonon d trang thai kich thfch va trang thai co ban

Sa va Se la so phonon trung binh phat ra trong khi hap thu va birc xa

Sir dung md hinh tpa dp ciu hinh, nguyen tic Frank - Condon va mot sd gia thiet gan diing, cd the tfnh dupe pho huynh quang cd dang Gauss, bieu diln bdi ham

a - thiet dien hip thu

Dai luong a !i6n he vdfi binh phucmg ban do rong dai pho W qua cong thiii

Mdt trong nhung ket qua quan trpng ciia vi6c ap dung md hinh tpa dp ciu

hinh la tinh dupe su phu thupc cua ban dp rpng dai phd vao nhiet dp |6J:

trong do: W(o) - ban dp rpng tai T = 0 K

Ve - tan sd dao dpng mang

Ct n\^rtno nht'i^f rtA po<-\ /OV-'T' "^ 1I\F \ flii KiAn tVii'm ( 1 f\\ trrt fV\rtr\)r\*

Vv i i i i L i A i g I m A ^ i \x\J \^ix\j y^.'iv 1 ::_ i i v ^ y LULL L/IV^LI vi.L\A\^ \i.\j) vi\j LiK^iili

7kT

\V(T) - V/(o)( ^ ) ' ' ' ' (17)

Cong ihiic (17) cd nghia la a nhiet dd cao, W(T) thay ddi tuyen ii'nh theo T

Mo hinh tpa dp ciu hinh dac biet thfch hpp dl giai thich cac tfnh chit ciia cac tam tap sau cd miic dp dinh xir cao Md hinh nay cung da dupe irng dung de mo ti cac tam tap chit kem dinh xii: ban trong mdt loat cac chit ban din nhu ZnS, CdS,

Gap, GaAs Cac ket qui cho thiy phan idn cac dai phd deu cd ban dp rdng phu thudc vao nhiet dp nhu c6ng thiie (16) Tuy nhien, ddi vdi tinh thi ZnS, nhu Shionoya [78J da nhan xet, de giai thich sir phu thupc vao nhiet dp thi cong thiic

h v 1 n

O day B la hang sd tim dupe bang each ngoai suy cac gia tri d nhiet dp cao khi

T - > 0 K

Ddi vdi cac tam rit ndng, ngudi ta thudng su dung md hinh do neKapB de

xudng, trong do true tiep khio sat tuang tic giufa cac trang thai di6n tu ciia tam tap

chit va trang thai dao ddng phonon ciia mang tinh the ca sd Tuy nhien ket qui nhan dupe khi tfnh toan dua tren hai md hinh ndi tren thudng gidng nhau

Bay gid ta se xet ky ban ve cac ca che phit huynh quang

1-4.3 Tai hpp vung - viing [30]

1.4.3.1 Tdi hop chuyen doi thdng

Khi chieu anh sang cd nang lupng hv > Eg vao chat ban dan, q u i trinh hip thu photon din den chat ban din d trang thai kfch thfch, nghia la trong ban din xuat hien cac cap hat tai dien khong can bang Nhimg hat tii dien tu do viia hinh thanh se tan xa tren mang tinh thi va sau khoing thdi gian hoi phuc (khoing 10 - 10" s) dien tir se roi xudng diy viing dan, con Id trong se len dinh ciia viing hda tri

Qua trinh tai hop biic xa true tiep giiia dien tut va Id trong tu do cd kha nang xay

ra nhfeu nhit neu sau qui trinh bdi phuc, dien tut va Id trong cd vecta sdng nhu nhau Dieu nay d l dang thuc hien neu cue tilu cua vung din va cue dai cua viing hda tri nam d ciing mot vecta sdng, ching ban tai k = 0, nhu trong bin din cd ciu tnic viing thing (hinh 13)

Trong qua trinh chuyen ddi biic xa, cic dinh luat bao toan nang lupng va xung lupng phii ddng thdi thoa man:

hv = E , - E , (19)

t = t,~% = 0 (20)

CJ day, Ec, Ey - la nang lupng viJi^, k^ la vecta sdng ciia dien tu d vung din va cua Id trong d vung hda tri

k - la vecta sdng cua photon

Toe dp tai hap biic xa da dupe Van Roosbroeck va Shockley [77] tfnh lan dau dua tren nguyen tie can bang chi tiet:

R = Snk'Vran'u

<lu (21)

kT a(hv) - he sd hap thu

n - chiet suit cua chat bin din Diing bilu thiic nay (21) cd the tfnh dupe dang phd huynh quang d mot nhiet dp

T cho trudc, neu biet dupe he sd hip thu a(hv) Ching ban, trong trudng hop cic vung nang lupng cd dang parabol va bin din khdng suy bien, phd huynh quang cd dang [73]:

h v - E

I(hv) -1, exp

vdi lo = V (hv - Ep)

Tii cong thiic (22) ta suy ra tai hop biic xa cac hat tii dien tut tu do phii cd mdt

nguong d phfa nang lupng thip tai hv = Eg Khi nhiet dp tang hoac cudng dp kfch

thfch tang cac trang thai nang lupng cao ban trong viing din se bi dien ttr lap day, tao dieu ki6n cho pho huynh quang md rpng va cue dai cua nd dich ve phfa nang Itrpng cao

Nhu vay tai hpp thing viing - viing dac trung bdi sir md rpng phd ve phfa nang lupng cao khi nhiet dp tang, trong khi dd phfa nang lupng thap phd bi chan d nguong hv = Eg

1.43.2 Tdi hop chuyen ddi nghieng

Trong cic chit ban din cd cau tnic vung nghieng (hinh 14) tai hpp giua dien tuf

va Id trong chi cd the xiy ra vdi su tham gia cua phonon, nghia la qua trinh biic xa photon cd kem theo qua trinh hip thu hoac biic xa phonon, sao cho cac dinh luat bio toan xung lupng va nang lupng ddng thdi thda man:

I^p ~ l^c ~ l^v

hv = Ec - Ev ± Ep

d day, kp - vecta sdng cua phonon

Ep - nang lupng cua phonon

E k

(23) (24)

Hinh 14 Chuyen ddi nghieng trong tai hop biic xa virng - viing [30]

Qua trinh biic xa phonon bdi cac dien tut nam d trang thii nang lupng cao cd xac suit Idn Vi the trong chuyin ddi nghieng thudng kem theo biic xa phonon Qua trinh hap thu phonon hau nhu khdng ton tai trong tai hpp chuyen ddi nghieng, bdi vi

sd phonon cd kha nang bi hap thu ft va giim nhanh khi nhiet dp giam

Ddi vdi bin din khdng suy bien cd viing nang lupng dang parabol, phd huynh quang ling vdi chuyen ddi nghieng giira cac viing cd the bieu diln bang cong thiic [73]

- Dai sdng dai do chuyin ddi nghieng

- Dii sdng ngan do chuyen ddi thing

1.4.4 Tai hpp bufc xa qua cac trang thai exciton

Cic cap dien tut - Id trong lien ket vdi nhau nhu vay dupe gpi la exciton Nhu

vay exciton la mdt trang thai kich thfch trung hda ve dien cua mang tinh the Exciton cd the chuyen dpng trong tinh the Trong qua trinh tai hpp exciton giii phdng nang lupng Nhung vi exciton trung hda ve dien, nd khdng ddng vai trd true tiep nao trong dp din dien cua ban din [23]

Nhiing tfnh chat ca ban ciia exciton cd the dupe suy ra tir mot md hinh dOn gian

[34] ma Frenkel gii thiet Ian dau nam 1931 Trong md hinh nay dien tuf va Id trong

dupe coi nhu nhiing chuan hat cd dien tfch bang dan vi va trai dau, cd nang lupng

tuong tie bang (s la hang sd dien mdi ciia chat bin din) va khdi lupng hieu

o l

dung bang m*c (doi vdi dien tur 6 day viing din) va m*h (ddi vdi Id trong d dinh viing hda tri) Bang cich giai phuang trinh Schrodinger ddi vdi trudng hop cic viing nang lupng cd dang parabol va cd cue tri tai k = 0, cd the tim dupe nang lupng ciia cap difn tii -1§ trong lien ket, tiic la nang lupng cua exciton:

Sd hang thii 3 trong ve phii (26) bilu diln trang thai kfch thfch gian doan

n = 1,2,3 gidng nhu trang thii kfch thfch cua nguyen tut hydro cd khdi lupng nit gpn |a va nam trong mdi trudng lien tuc cd hang sd dien mdi s

Neu cac chuyin doi quang hpc thing xay ra tai k = 0 (hoac trong trudng hpp tdng quit tai ciing mot gia tri k) thi nang lupng cua exciton se dupe bieu diln bang

Sa dd cac miic nang lupng cua exciton va chuyen ddi biic xa cd the xiy ra, dupe

chi ra tren hinh 15

E Nhiing day vach hep vdi nang luang hv = E^ ^ tuang tu nhu phd

g n

- ^

Hinh 15 Cac miic nang lupng cua exciton va cac chuyen ddi biic xa [6]

nguyen tir hydro da dupe quan sat thay trong phd hap thu, ciing nhu trong phd huynh quang ddi vdi mot loat tinh thi ban din [29]

Trong tnrdng hop ban din vdi viing cam khdng thing, de bio toan nang Iirpng,

tii hap exciton phii kem theo biic xa phonon Khi dd nang lupng photon phat ra:

hv = E g - E , - E p (29) Vdi Ep - nang lupng phonon

Md hinh tpa dp ciu hinh cho thay, ngay ca cac chuyen ddi thing cung cd the biic xa phonon Nhu vay pho biic xa exciton cd the lap lai tai mot sd vi tri nang lupng thap han sao cho:

hv = Eg - Ex - mEp (30)

m - Sd phonon quang hpc phat ra trong tii hop exciton Khi m cang Idn xac suat cang nhd va cudng dp anh sing biic xa cang yeu Hinh 16 la sa do chuyen ddi tai hpfp biic xa exciton cd su tham gia cua phonon

Cac tic gii [57,19] da chi ra rang tii hop biic xa qua cac trang thai exciton thudng xay ra vdi xac suit Idn ban tii hop true tiep viing - viing, ban niia tai hpp exciton thudng manh ban tii hop giiia cac viing khdng nhiing d nhiet dp thap, ma d ngay c i nhiet dp khi kT bang hoac Idn ban nang lupng lien ket cua exciton

b

Hinh 16 Sa do chuyen ddi tai hop biic xa exciton co su tham gia cua phonon [30]

a - Biic xa 1 phonon b - Biic xa 2 phonon

1.4.4.2 Exciton lien ket

Neu trong chit ban din cd tap chit thi exciton cd thi ciing vdi nguyen tii tap

chit tao thanh nhiing to hap exciton - tap chit hay con gpi la exciton lien k^t O

nhidt dp thip, tdc dd tii hpp qua cac exciton lien ket thufdng Idn ban tdc dp tai hop

qua cac exciton tu do [29]

Dac dilm cua exciton lidn k^t la khdng dich chuyin tuf do trong mang tinh

thi, ma dinh xii gan tam tap chat, vi the cic vach phd huynh quang do tai hop qua exciton li6n ket thudng rat hep, dp rpng ciia chiing nho ban kT va hau nhu khdng phu thudc vao nhiet dd

Hifu sd gifla vi tri nang lufpng ciia exciton tu do va exciton lidn ket se la nang

lupng lidn ket ciia to hop exciton - tap chit Mdi li6n he giufa cudng dd phd cua exciton lien k6t va exciton tu do cd thi cho biet cac thdng tin v^ tap chat trong miu

Cac phep tfnh ciia Hopfield [59] da chiing to rang nang lupng lidn ket ciia exciton vdi tap chit phu thudc vao ty sd khdi lupng hifu dung cua di^n tuf va 16

trdng ching ban nhu nang lupng lidn ket ciia exciton vdi dono trung hda cd gia tri:

trong dd E^ la nang lupng ion hda dono

Thuc nghidm cho thay ddi vdi Si: Eex-o = 0,1 Ep

Ddi vdi cac hprp chit A " B ^ : Eex-o = 0,1 E^

Ddi vdi cac hop chit A"^B^: E^X-D = 0,33 Ep

Trong chit ban din cd cau tnic vung nghi6ng, cd chiia tap chit, dl bao toan

xung lupfng, dien tut khdng nhiing cd thi trao ddi xung lupng vdi phonon mang tinh

the, ma con cd thi truyen xung lupng cho nguyen tu tap chit

1,4.5 Chuyen ddi visng - tap chat

Cac tam dono va axepto trong ban din tap chit ciing cd the trd thanh cac tam

tai hpp biic xa

Xet trudng hpp ban din loai p Sau khi hip thu photon, cac cap hat tai dien tu

do dupe hinh thanh trong ban din Cac Id trdng tu do dich chuyen den gan tam

axepto A, hi bit tren tam nay Qua trinh tai hpp buc xa se xiy ra giira dien tii tu do

trong viing din va Id trdng dinh xii tren axepto

Tuang tu nhu vay, qua trinh tai hop biic xa giua di^n tu dinh xii tren dono va id

trdng tu do trong viing hda tri cd the xiy ra trong ban din loai n Cac qua trinh tai

hop do tuang ling vdi chuyen ddi D -^ V va C -> A trong hinh 11

Vi mot trong hai hat tii dien tham gia vao qua tnnh tai hpp, chuyen dpng tu do,

n6n phd huynh quang trong trudng hpp nay phai cd dang Maxwell - Boltzmann

NhCmg phep tfnh cua Eagles [67] cho thiy ddi vdi chuyin ddi biic xa thing tir viing

din xudng miic nang lupng co ban cua axepto ndng, dang phd cd thi bilu diln bang

Phd cd cue dai tai: hv^ax = Eg - Eg + ^ k T (32)

Mat khac vdi gia thiet xac suit chuyen ddi biic xa viing - tap chit phu thudc vao

van tdc ciia dien tii trong vung din, tac gii [50] da dua ra bieu ihiic cho tdc dp

tai hpp biic xa viing - tap chit:

r(E,) - r%exp( A ) (33)

vdi Eic = hv - Eg + Ea

Tdc dd phat xa dat cue dai khi:

hv„,ax = Eg-Ea + kT (34)

Qua trinh chuyin ddi biic xa viing - tap chat thudng xiy ra vdi sU tham gia ciia

phonon Trong trudng hop tam tap chit ndng (nang lupng ion hda nhd), quy dao

dien tii trai rpng trdn mot khoang each rat Idn so vdi hing sd mang, do dd didn tuf se

tuong tie vdi phonon cua mang tinh thi ca sd, chu* khdng tuong tic vdi cac dao

dpng dia phuang Cudng dp ciia vach huynh quang xuit hien do cac ket qua tuong

tac cua cac dien tuf vdi mdt loai phonon xac dinh boi cdng thiic [6]:

In = I o ^ (n = 0,1, 2,3 ) (35)

vdi In - cudng dp vach xuit hi6n khi biic xa ddng thdi photon va n phonon

lo - cudng dp vach xuit hidn do chuyin ddi t h u ^ tiiy dien tii (khdng co

phonon tham gia)

N - la sd phonon trung binh phat ra trong qua trinh chuyen di6n tir,

dd la mdt dai lupng dac trung cho cudng dp tuong tic cua phonon vdi didn tuf trong

chuyen doi cho trudc

Vdi ban din ma tam tap chit cang sau (lupng ion hda cang Idn) thi tap chit lien

ket vdi mang tinh thi ca sd cang manh, do dd xac suit tUOng tic vdi phonon cang

Idn, k^t qua la N cang Idn Trudng hap tam tap chit li6n k^t rit manh vdi mang tinh

thi, di6n tuf chi cd the tuong tac vdi cac dao dpng dia phuang Khi dd dl giai thich

cac tfnh chat ca ban ciia phd hip thu va phd huynh quang ngudi ta thudng sii dung

md hinh toa dd cau hinh

1.4.6 Chuyen ddi dono - axepto

Nhutig chuyin ddi quang hpc vdi su tham gia cua cap tam tap chit dono

-axepto da dupe Williams va Prener [75] khao sat lin diu tien khi nghien cihi huynh quang ciia tinh thi ZnS cd chiia dono (Ga hoac In) va axepto (Cu hoac Ag) Dai biic

xa sdng dai cd ph6 phu thupc vao ban chit hda hpc cua dono va axepto da dupe cac

tac gia tran giai thfch la do chuyen ddi biic xa giiia cac trang thai ca ban ciia dono

va axepto

Suf dung md hinh tuong tie coulomb giiia dien tuf va Id trdng bi bat trdn cap dono - axepto (D-A), nang lupng trong chuyin ddi D-A dupe bilu diln qua cong thiic [84]:

hvD.A(r) = Eg - (ED + EA) + ~ : (36)

trong do: ED, E ^ nang lupng ion hda dono va axepto

r - khoang each gifia dono va axepto

s - hang sd dien mdi ciia chit ban din

Tii bieu thiic (36) suy ra nang lupng cua photon phat ra phu thupc manh vao

khoang each r gifia dono va axepto Vi trong mang tinh the r chi cd thi nhin cac gia

tri gian doan, n6n nang luong photon se cd gia tri gian doan, nghia la phd huynh quang cap D-A se gdm cac vach hep diing each nhau mdt khoang nang lupng A(hv)

nao dd

Tuy nhi6n ddi vdi nhiing cap D-A cd khoang each r rit Idn sao cho Ar « r thi

khoang each nang lupng A(hv) se rit nho Neu A(hv) nhd hon dp rpng cua vach thi cac vach se nhip lai vdi nhau thanh mot dai pho g ^ nhu li6n tuc

Nhu vay phd huynh quang lien quan den cap D-A se cd dang rat dac biet: Dang

phd huynh quang gdm mdt sd Idn vach hep hdi tu dan d^n dai pho lien tuc nam d