The exponential decrease of optical ban d gap in Mn- doped is strongly correlated with m agnetic behavior.. R.TKi vs T plot und fitting curve for Al|JS(1Mn„ UN film[r]
Trang 1S Y N T H E S IS A ND P R O P E R T IE S O F FE R R O M A G N E T IC
M n -D O P E D AIN F ILM S
P h a m H o n g Q u a n g '-2 , N g u y e n H u y S in h ', N g u y e n H u u D u e 1 a n d S e o n g C ho Yu*
'C ryogenic Laboratory, F aculty o f Physics, College o f N atural, V N U
2 D epartm ent o f Physics, C hungbuk N ational University, South Korea
A b s tr a c t: Mn-doped AIN semiconductor (AIMnN) films are a p ro m is in g m ate ria l
fo r s p in tro n ic s b e c a u s e o f h ig h te m p erature fe rro m a g n e tism A IM n N film s w ere
c h a ra c te riz e d b y X R D T h e c o n ce n tra tio n o f M n w a s de te rm in e d by E D S U sing
th e M n c o n te n t u p to 13.6 a t % The s aturated m a g n e tiza tio n an d e n e rg y band
a c tiv a tio n e n e rg y w e re d e riv e d from th e te m p erature d e p e n d e n c e o f re sista n ce ,
w h ic h e x h ib its c le a rly a se m ic o n d u c to r cha racte ristic
1 I n t r o d u c t io n
T he com bination of several discoveries in m agnetic sem iconductors h a s opened up the possibility of a new re search field th a t is so-called spintronics A s u b set of th is field involves diluted m agnetic sem iconductors (DMS) The m ain focus of th e DMS a re a is to effectively incorporate m agnetic ions in to a sem iconductor lattice and create a ferrom agnetic m aterial Since Dietl e t al [1] predicted t h a t cubic GaN doped with 5 a t % of M n should exhibit a
C urie te m p e ra tu re (Tc) exceeding room tem perature, a num ber of w orks have focused on wide ban d gap sem iconductors a s being the m ost prom ising ones for achieving high Tc Among them , th e w orks on M n- o r Cr-doped AIN films [2,3] based on AIN sem iconductor with th e b an d gap of 6.2 eV a r e of partic u la r im portant Room te m p e ra tu re m agnetism h as been reported for M n-doped film s grown by m olecular beam epitaxy [2]
T his p ap e r p re se n ts th e synthesis of Mn-doped AIN films usin g reactive DC
sp u tte rin g technique, along w ith th e effect of M n concentration on th e properties such as crystal s tru c tu re , m agnetization, optical ban d gap, an d resistivity
2 E x p e r i m e n t s
S am ples of com position A l|.xM nxN, w here X is the atom ic fraction of M n substituted for Al, w ere deposited sim ultaneously on q u artz s u b strates in a reactive DC m agnetron
s p u tte rin g system T he com posite ta rg e t includes a high p urity (99.999%) alum inum disk and a n u m b er o f s q u a re M n pieces 5x5 mm, which w ere placed sym m etrically on the surface of th e A1 disk A n investigation of deposition process has been done on undoped films T he deposition conditions w ere optimized and fixed a s following: th e partia l pressure
of A r an d N., gas m ixture (67% Ar, 33% N,) w as 6 mTorr, the DC voltage w as 240V, the DC
Trang 2c u rre n t w as 330mA, th e s u b s tra te tem p eratu re d u rin g deposition w as 300°c The Mn contents w ere controlled by varying th e num ber of Mn pieces T he XRD m ea su rem en ts were carried o u t in grazing mode using Cu-K„ radiation T h e m ag n e tiz atio n of th e films
w as m easured by a SQUID m agnetom eter T he band gaps w ere o b tained from optical absorption m ea surem ent over th e w avelength from 200 to 900 nm in tra n s m itta n c e mode
3 R e s u lts a n d d is c u s s io n
The atom ic concentrations of Mn determ ined by EDS w e re from X = 0.00 to 0.136, corresponding to th e num ber of Mn pieces varying from 0.0 to 3.0
Fig 1 show s XRD p a tte rn for one p artic u la r 0.40 ụm thick film w ith X = 0.075 All peaks are identified with th e hexagonal AIN stru ctu re T he X-ray d a ta provide evidence
th a t th e p re sen t M n-doped AIN film s are single phase The second p h ase s w ere found by X-
ra y for the film s with Mn content exceeding 0.136
Fig 2, shows m agnetization versus field d ata
for 0.4 Jim thick with X = 0.136 The values of
sa tu ra te d m agnetization indicate th a t 20% of Mn is
m agnetically active in AIN sam ples com pared with
4|V M n atom [4j We found th a t Ms a t room
tem p eratu re decreased w ith increasing M n
concentration, nam ely, 9.6, 8.3 an d 7.7 em u/cm 3 for
X = 0.045, 0.07Õ an d 0.136, respectively The origin
of th is behavior is unclear I t m ay re su lt from the
com pensation by in te rs titia l Mn donors and/or from
changes in th e local spin configurations [5]
The u v optical absorption was perform ed on
the films grown on q u artz su b s tra te s with the
typical thickness of 400 nm In se t in Fig 3 shows a 2
vs hv plots (a is absorption coefficient) for a
Al(i.95sMn00<(iN film s obtained by Tsuc’s m ethod The
band gap w as d eterm ined by extrapolating the
lin ear portion of th e absorption edge to zero value
The band gap w ith M n concentration is show n in
m ain graph of Fig 3 It is clear th a t th e band gap
decreases exponentially w ith increasing Mn concentration T hese b an d gaps correspond to
th e tran sitio n from th e top of th e im purity band to th e bottom of th e conduction band We are un aw are of an electronic s tru c tu re calculation for tran sition-m etal-doped AIN system s Some recent re p o rts have theoretically produced th e electronic s tru c tu re of tran sitio n -
m etal-doped GaN, w hich is ra th e r related to o u r cases [4,6], According to S an y al e t al [6], the system behaviors like a ferrom agnetic m etal with a re duced m agnetic m om ent a t Mn
concentration of 5 % In th is point of view, th e exponential d ecrease of optical b and gap in
Mn-doped is re la ted w ith band s tru ctu re change an d strongly co rrela te d w ith m agnetic
2 0 4 0 M SO
F ig l X-ray diffraction (xkỉ)) data for the 0.4 nm thick Al0»2jCr00T5N films
H (kOe)
Fig 2: Magnetization M versus static
external magnetic field H measured at
10 K and 300 K for the 0.40 nm thick film The field was applied in plane
Trang 3T ra n s p o rt p ro p e rty w a s m ea su red by
four-probe m ethod Fig 4 show s the
te m p e ra tu re dependence of re sistan c e, R(T),
for th e 13.6 % M n-doped AIN film R(T) plots
follow th e expression R = R„Tl/2exp(En/k BT),
which describes th e tr a n s p o r t p ro perty in a
sem iconductor m ateria l By fittin g th e R(T)
plot, we could o b tain th e valu e of activation
energy E„ = 145 meV F u rth e r s tu d ie s have
pointed o u t th a t tr a n s p o r t ch a rac teriza tio n
depends n o t only on th e M n concentration but
also on th e cry sta lin ity o f sam ples
Fig.3 Optical band gap as a function of Mn concentration for Al,.,Mn,N (x = 0.0 - 0.136) films Inset is a 2 vs photon energy plots at
X = 0.045 to get optical band gap
In sum m ary, we have obtained the
single-phase A l|.vM nxN film s w ith th e Mn
concentration up to a valu e of X = 0.136 The
sa tu ra te d m ag netization a n d band gap
decreased w ith in cre asin g M n concentration
The exponential decrease of optical ban d gap
in Mn- doped is strongly correlated with
m agnetic behavior T e m p e ra tu re dependence
of re sistan c e exhibits clearly a sem iconductor characteristic,
Fig.4 R.TKi vs T plot und fitting curve for Al|JS(1Mn„ UN film
Acknow ledgem ents T h e w ork a t Hanoi U niversity w as supported by the V ietnam ese-Italy
cooperation program (8BS3) T he work in K orea w as supported by th e K orea Research
F oundation G ra n t (K RF-2003-005-C00018)
R e fe re n c e s
1 T Dietl, H Ohno, F M atsukura, J Cibert, and D Ferrand, Science 287(2002) 1019.
2 R Frazier, G T haler M Overberg, B Gila, c R Abernathy, and s J Pearton Appl Phys Lett 83(2003) 1758.
3 S G Yang, A B Pakhomov, s T Hung, and c Y Wong, Appl Phys Lett 81(2002) 2418.
4 M ark van Schilfgaarde and o N Mryasov Phys Rev B, 63(2001) 233205-1.
5 J M asek and F M ark Acta Phys Polon A 100(2001) 319.
6 B Sanyal, o Bengone, and s Mirbt Phys Rev B, 68(2003) 20.