,VNU JOURNAL OF SCIENCE. M alhem atics - Physics, T.xx, N03AP. 2004
S 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 s y n th e s iz e d b y D C s p u tte rin g te ch n iq u e . T h e crystal s tru c tu re o f s a m p le s w as 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 o p tim a l g ro w th p ro c e s s , w e h a v e ob ta in ed th e sing le p h a s e A IM n N film s w ith 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 g a p w e re fo u n d to d e c re a s e w ith incre a sin g M n con cen tratio n. T h e v a lu e s of 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
10 6
S y n t h e s i s a n d p r o p e r tie s o f f e r r o m a g n e tic M n -doped. A I N fi l m s 1 0 7
c 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 behavior with M n concentration exceeding 5 %.
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
108 P h a m H o n g Q u a n g , N g u y e n H u y S in h ,..
T 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
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