HAL Id: jpa-00220820
https://hal.archives-ouvertes.fr/jpa-00220820
Submitted on 1 Jan 1981
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
ELECTRONIC PROPERTIES OF
FLASH-EVAPORATED AMORPHOUS GaSb FILMS
A. Gheorghiu, T. Rappeneau, J. Dupin, M. Theye
To cite this version:
A. Gheorghiu, T. Rappeneau, J. Dupin, M. Theye. ELECTRONIC PROPERTIES OF FLASH-
EVAPORATED AMORPHOUS GaSb FILMS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-
881-C4-884. �10.1051/jphyscol:19814193�. �jpa-00220820�
ELECTRONIC PROPERTIES OF FLASH-EVAPORATED AMORPHOUS
GaSbFILMS
A. Gheorghiu, T. Rappeneau, J.P.
up in*
and M.L. TheyeLaboratoire drOptique des SoZides, Equipe de Recherche associge au C.N.R.S.
N0462, Universite' Pierre e t Marie Curie, 4 place Jussieu, 75230 Paris Cedex 05, France
*
Ddpartement de Physique des Mate'riam, Laboratoire associe' au C. N. R. S. NO1 72, Universi te' CZaude Bernard, Lyon I, 69621 ViZZeurbanne, FranceAbstract.
-
The optical and transport properties of flash-evaporated amorphous GaSb thxn films are described as a function of preparation conditions. The results for nearly-stoichiometric samples are discussed in relation with the presence of defects. The influence of Sb excess is also considered.1. Introduction.
-
GaSb is one of the less ionic 111-V compound semiconductors : f. = 0.261 according to Phillips' scale (1). This may favour the existence of cftemical disorder in amorphous GaSb samples, especially if they are deposited from the vapour. Wrong bonds (bonds between like-atoms) are expected to produce detecta- ble effects, in particular on the optical properties, since the Sb-Sb covalent bonding is weaker than the Ga-Sb heteropolar bonding. We present here the results of optical and transport measurements performed on flash-evaporated amorphous GaSb thin films as a function of preparation conditions. Attention is focused on nearly- stoichiometric (N.S.) samples but the influence of Sb excess is also considered.2. Sample preparation and characterization.
-
Thin (200-1000 A) films of amorphous GaSb are deposited under ultra-high vacuum by flash-evaporation of crystalline powder onto supersmooth silica substrates maintained at different temperatures between 100 and 390 K. They can be annealed in situ under control by resistance measurements. The crystallization temperature ranges from 430 to 470 K. The average film composition is determined by a-particle back-scattering with a few % accuracy.Nearly stoichiometric samples (within a few %) have been obtained with powder grain size in the 125-160 pm range, and crucible temperatures of the order of 1500' C.
Higher crucible temperatures result in films with increasing Sb excess.
The film thickness is determined with 1 % accuracy by an X-ray iqterference technique (2). The complex dielectric function E = + i~ = (n + ik) is deduced from reflectance and transmittance measurements performed In air between 2 0.5 and 6.2 eV, using exact thin film formula (3) ; a Kramers-Kronig analysis of reflectance data is also used for opaque films. The electrical d.c. conductivity is measured in situ versus temperature with a Keithley electrometer.
The film structure is investigated by electron diffraction and microscopy. For N.S. samples, the diffraction diagrams are typical of tetrahedrally-coordinated amorphous semiconductors ( 4 ) , but the first diffuse ring is slightly shifted with respect to the crystalline (111) peak. Increasing the.Sb content produces a gradual modification of these diagrams ; there is no evidence for microcrystallites of metallic Sb. These data will be analysed in order to characterize the changes in average local structure.
3. Properties of nearly stoichiometric (N.S.) films.
-
Figure 1 presents the E andE, spectra for N.S. amorphous GaSb films deposited at 100 K and annealed at 306 and 420 K ; the results for crystalline GaSb (5) are. given for comparison. The amorphous data look roughly similar to those previously reported for flash-evaporated samples
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814193
JOURNAL DE PHYSIQUE
20
15
&- 10
5
a 5 1 2 3 4 5 6
hw(eV)
(b:
: (a) Real ( E ~ ) and (b) imaginary w a r t s of the dielectric constant
versus energyliw for 2 NS amorphous GaSb
(3) films deposited at 100 K and annealed at 300 (1) and 420 K (2). Curves 3 refer to crystalline GaSb (ref. 5).
( 6 ) , but in our case the E~ maximum is shifted to higher energies and the E, values
in the infrared are lower,
The effects of increasing the substrate temperature during deposition and of annealing are qualitatively similar to those already observed for amorphous Ge (7) and for the other amorphous Ga compounds (8). One can see in figure 1 that, even after annealing, the values in the infrared remain significantly higher than the corresponding crystalline values (5, 9). This indicates an appreciable decrease of the Penn gap, thus of the average bonding strength, which is difficult to explain by structural disorder and defects of the dangling-bond-type only, and suggests the presence of a certain proportion of wrong bonds.
2 The optical gap Eo determined through the empirical law : (%u)2.c2a(~w-~~
,
is equal to 0.64 eV for NS films deposited or annealed at room temperature, in agreement with previous results on flash-evaporated films (10). It increases to 0.74 eV after high-temperature annealing, i.e. a value very close to the crystalline minimum gap. Moderate absorption tails are observed below Eo. The slope of the absorption edge is only slightly increased by annealing (figure 2).
Fig. 2 : Absorption coefficient a = 4 n k l A versus energy4iw for the same amorphous films as in fig. 1 ; E indicates the optical gap for films gnnealed at 420 K.
The error bars correspond to AR =
+_
0.002,AT =
_+
0.003.can be f i t t e d w i t h a sum of two e x p o n e n t i a l s : a = Do exp (-E,,/kT) + o: exp
(-Eb
/ k ~ ),
-6
-
w i t h Eo 0.55 eV
,
oo 2 4.
lo5 d l c;'E i ; 0.31 eV
,
o t = 90 a-' c,' -8-
This i n d i c a t e s two p a r a l l e l t r a n s p o r t mechanisms.
Above room t e m p e r a t u r e , conduction o c c u r s i n
2 -
IO'/T(KI
extended s t a t e s beyond t h e m o b i l i t y edge. I n c o n t r a d i c t i o n w i t h t h e c a s e of amorphous Ge(l1)
o r amorphous GaAs
(a),
t h e a c t i v a t i o n energy-
F i g . 3 : Conductivity 0 v e r - i s s i g n i f i c a n t l y l a r g e r t h a n h a l f t h e o p t i c a l s u e r e c i p r o c a l temperature gap (0.55 eV compared t o 0.37 eV). This means, T f o r t h e same f i l m s a s i n e i t h e r a s t r o n g temperature dependence of t h e f i g s . 1 and 2 .gap w i d t h , o r a s h i f t of t h e Fermi l e v e l due t o l o c a l i z e d s t a t e s r e l a t e d t o some s p e c i f i c d e f e c t s . Below room temperature, t h e observed
a c t i v a t e d behaviour, a g a i n i n c o n t r a d i c t i o n w i t h amorphous Ge o r GaAs, s u g g e s t s t r a n s p o r t by hopping f o l l o w i n g e x c i t a t i o n i n t a i l s t a t e s a t t h e band edge. It i s remarkable t h a t v a r i a b l e - r a n g e hopping conduction i n s t a t e s n e a r t h e Fermi l e v e l i s n o t observed, e i t h e r b e f o r e o r a f t e r a n n e a l i n g , although numerous d e f e c t s a r e obviously p r e s e n t i n t h e f i l m s .
4. I n f l u e n c e of Sb excess.
-
As t h e f i l m Sb c o n t e n t i s i n c r e a s e d , t h e o p t i c a l and t r a n s p o r t p r o p e r t i e s a r e g r a d u a l l y modified. For a given composition, a m a t e r i a l w i t h well-defined p r o p e r t i e s i s o b t a i n e d , and i t undergoes s i m i l a r a n n e a l i n g e f f e c t sa s t h e n e a r l y s t o i c h i o m e t r i c samples. For t h e same d e p o s i t i o n c o n d i t i o n s and i n c r e a s i n g Sb e x c e s s , t h e E v a l u e s i n t h e i n f r a r e d i n c r e a s e and t h e E spectrum i s s h i f t e d t o lower e n e r g i e s , khe o p t i c a l gap d e c r e a s e s ( f i g u r e 4 ) . The a z s o r p t i o n
t a i l s a r e n o t s i g n i f i c a n t l y enhanced. The room temperature c o n d u c t i v i t y i s f i r s t l i t t l e a f f e c t e d , t h e n i n c r e a s e s . The o behaviour below roo empera- t u r e i s modified and t e n d s t o f o l l o w a T-TiZ law f o r non-annealed f i l m s w i t h h i g h Sb c o n t e n t (Ga/Sb 1 0.5) ; f o r annealed f i l m s , an a c t i v a t e d behaviour i s no more observed. Above room temperatu- r e , conduction i s s t i l l a c t i v a t e d , w i t h an a c t i v a - t i o n energy remaining l a r g e r t h a n h a l f t h e o p t i c a l gap.
F i g . 4 : P l o t s of v e r s u s energy ?iW showing t h e d e c r e a s e of t h e o p t i c a l gap with i n c r e a s i n g Sb e x c e s s :
(.) Ga0.44Sb0.56
'
(.) Ga0.31Sb0,69'
JOURNAL DE PHYSIQUE
5. Conclusion.
-
In spite of an overall similarity, the properties of flash- evaporated amorphous GaSb films present significant differences with those of the other amorphous Ga compounds. The observed large decrease of average bonding strength with respect to the crystal suggests the presence of a certain proportion of wrong bonds. The defects, both structural and chemical, do not give rise to large absorption tails and to conduction by variable-range hopping in localized states near the Fermi level. More experiments are under way in order to determine the nature of these defects. The gradual changes of the properties due to increasing Sb excess suggest that Sb in excess is incorporated into the network in a self- compensated or alloying mode (12).References
PHILLIPS, J.C., Bonds and bands in semiconductors (Academic Press, New-York) 1913.
CROCE, P., NEVOT, L. and PARDO, B., Nouv. Rev. Opt. Appl. 3 (1972) 37.
ABELES, F. and THEYE, M.L., Surface Sc. 5 (1966) 325.
SHEVCHIK, N.J. and PAUL, W., J. on-Crystalline Solids 13 (1973174) 1.
DIVRECHY, A., ROBIN-KANDARE, S. and ROBIN, J., J. Phys. Chem. Solids 34 (1973) 413.
STUKE, J. and ZIMMERER, G., Phys. Stat. Sol. (b) 49 (1972) 513.
THEYE, M.L., Opt. Commun. 2 (1970) 329 ; Mat. Res. Bull. 6 (1971) 103.
GHEORGHIU, A. and THEYE. M.L., in Proc. 7th Int. Conf. on Amorphous and Liquid Semiconductors, edited by W. SPEAR (Stevenson, Dundee) 1977, p. 462 ; J. Non- crystalline Solids 35/36 (1980) 397 ; Phil. Mag. B (1981) to be published.
EDWARDS, D.F. and HAYNE, G.S., J. Opt. Soc. Am. 49 (1959) 414.
NAIDU, B.S., SHARMA, A.K., SASTRY, D.V.K., SYAMALAMBA, Y. and JAYARAMA REDDY, P., J. Non-Crystalline Solids 42 (1980) 637.
THEYE, M.L., GHEORGHIU, A., RAPPENEAU, T. and LEWIS, A., J. Physique 41 (1980) 1173.
ROBERTSON, J., Phil. Mag. B, to be published.
