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COMPARATIVE STUDY OF THE STRUCTURE OF AMORPHOUS Ge AND AMORPHOUS III-V
COMPOUNDS
A. Gheorghiu, K. Driss-Khodja, Serge Fisson, M. Thèye, J. Dixmier
To cite this version:
A. Gheorghiu, K. Driss-Khodja, Serge Fisson, M. Thèye, J. Dixmier. COMPARATIVE STUDY OF
THE STRUCTURE OF AMORPHOUS Ge AND AMORPHOUS III-V COMPOUNDS. Journal de
Physique Colloques, 1985, 46 (C8), pp.C8-545-C8-549. �10.1051/jphyscol:1985886�. �jpa-00225239�
COMPARATIVE STUDY OF THE STRUCTURE OF AMORPHOUS Ge AND AMORPHOUS III — V COMPOUNDS
A. Gheorghiu, K. Driss-Khodja, S. Fisson, M.L. Thèye and J . Dixmier
Laboratoire d'Optique des Solides, U.A. CNRS 781, Université Pierre et Marie Curie, 4 place Jussieu, 752S30 Paris Cedex 05, France
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Laboratoire de Physique des Solides, CNRS-Bellevue, 1, place Aristide Briand, 92195 Meudon Principal Cedex, France
Résumé - La structure de couches minces de Ge et GaAs amorphes préparées par évaporation, non recuites et recuites, est étudiée par diffraction électronique. Les fonctions d'interférence ainsi que les fonctions de distribution radiale sont comparées aux prédictions de deux modèles de réseau continu aléatoire comprenant ou non des anneaux à nombre impair d'atomes, avant et après relaxation. On confirme que les composés tétracoordonnés amorphes doivent contenir une proportion négligeable d'anneaux impairs.
Abstract - The structure of evaporated amorphous Ge and flash-evaporated amorphous GaAs films, both as-deposited and annealed, is investigated by careful electron diffraction experiments. Both the interference functions and the radial distribution functions are compared to the predictions of two unrelaxed and relaxed continuous random network models with and without odd-membered rings. It is confirmed that amorphous tetracoordinated compounds must contain a negligible proportion of odd- membered rings.
1. Introduction
It has early been shown by diffraction studies that the structures of all the amorphous tetracoordinated semiconductors are very similar (1,2), and are well accounted for by continuous random network (CRN) models (3-6). These models can be separated into two classes, according as they contain some proportion of odd-membered rings (3-5), or only even-membered rings (6). The second class appears to be more appropriate to the description of the structure of the amorphous compounds, since odd-membered rings would necessarily introduce bonds between like atoms (wrong bonds), which are not energetically favored in the case of partially ionic bonding, in an otherwise chemically ordered network. The differences between the radial distribution functions computed for the two classes of models are however rather small and show up essentially at large r values, where the experimental spectra deduced from diffraction data do not often present sufficient accuracy. Different studies have nevertheless shown that the experimental radial distribution function of a-Ge is better reproduced by CRN models containing an appreciable proportion of odd-membered rings (7,8). On the other hand, careful comparative X-ray diffraction experiments have revealed differences between the radial distribution functions of sputtered a-Ge and a-GaAs; which have been interpreted as due to differences in the dihedral angle distribution, i.e. to a higher probability of the staggered configuration in a-GaAs, implying a smaller proportion of odd-membered rings (9).
In order to gain more information on this problem, it seemed interesting to consider, not
only the radial distribution functions, but also the interference functions which are more
directly related to the diffraction data. We have recently proposed a simple criterion, based
on the relative positions k
i/k
1of the peaks of the interference functions, allowing to
discriminate between the two classes of CRN models, and we have shown that the experimental
data for a-Si and several a-III-V compounds were respectively in better agreement with the
predictions of the models with and without odd-membered rings (10). The aim of the present
paper is to check this conclusion by a detailed comparison between both the interference
functions and the radial distribution functions computed for two CRN models belonging to
the two different classes, and determined from electron diffraction experiments performed
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985886
C8-546 JOURNAL DE PHYSIQUE
on amorphous Ge and amorphous GaAs under identical conditions. Since t h e interatomic distance is about t h e s a m e in both materials, such experiments a r e expected t o allow a reliable study of t h e structural differences which may exist between amorphous elemental and compound tetracoordinated semiconductors.
2. Experiment
The samples a r e evaporated amorphous G e films and flash-evaporated nearly-stoichiometric (xGa = 50 + 2 at.%) amorphous GaAs films (11) deposited under high vacuum (=: Torr) onto glass substrates maintained a t room temperature. The a-GaAs films were investigated before and a f t e r annealing a t 200°C. All samples had t h e s a m e thickness, chosen sufficiently small (240-250 A) t o minimize multiple scattering effects.
The electron diffraction experiments were performed inside a Philips E M 300 electron microscope (accelerating voltage of 100 keV) without velocity filter, on pieces of t h e films stripped off their substrates with collodion and collected on microgrids a f t e r collodion dissolving. The diffraction patterns w e r e recorded photographically and analyzed with a Joyce microdensitometer. The k s c a l e was calibrated by using t h e diffraction peaks of a very thin Au layer ( r 50 A) deposited on one of t h e samples. The inelastically s c a t t e r e d intensity was estimated from t h e back ground of t h e diffraction diagrams of t h e s a m e film a f t e r complete thermal crystallization and subtracted from t h e data, a s explained before (12). The experiments were performed under identical conditions f o r a l l samples. Reliable d a t a were obtained up t o k u 1 5 kl, although the uncertainties were larger above 10 The d a t a t r e a t m e n t procedure was t h e s a m e a s the one described previously (10).
3. Predictions of t h e models
Two models have been considered
:a 519 a t o m s model (4) relaxed by Steinhardt e t al. (131, containing 0.38 five-fold, 0.91 six-fold and 1.04 seven-fold rings per a t o m (model I), and a 238 a t o m s model containing only six-fold rings (6), unrelaxed and relaxed by Mosseri by a procedure similar t o t h a t used by Steinhardt e t al. (model 11). Both models were scaled t o a 2.35 .8 bond length (corresponding t o Si). The interference function was f i r s t computed from t h e coordinates of t h e models, using t h e Debye formula f o r powder diagrams; t h e statistical distribution function was then deduced by Fourier transform.
Fig.1 - Reduced interference functions i(k) (a) and radial distribution functions J(r) (b) computed
for t h e relaxed model I (dashed lines) and t h e unrelaxed model I1 (continuous lines).
i(k) spectrum when going from model I t o model I1 a r e , a p a r t from t h e changes in t h e shape and location of t h e first peak, a systematic shift of t h e following peaks t o lower k values and a narrowing of the third structure accompanied by a smoothing of t h e secondary maximum i n t o a m e r e shoulder. As for t h e J ( r ) spectra, significant differences between t h e two models appear only beyond t h e t w o f i r s t peaks, t h e positions of which a r e essentially determined by t h e t e t r a h e d r a l bonding configuration. In t h e model I1 J(r) spectrum, t h e high-r edge of t h e second peak is broadened, t h e minimum between t h e second and t h e third peak i s deeper and t h e third peak is shifted t o higher r values. I t has been shown t h a t this region is determined by three-bond connected neighbours, and is therefore particularly sensitive t o t h e dihedral angle distribution (14,15). The modifications from model I t o model I1 can be explained by a n increase of t h e probability of t h e staggered configuration (14). One c a n further notice t h a t , for t h e model 11, t h e fourth peak of t h e J(r) spectrum is shifted t o higher r values and t h a t t h e shoulder on i t s low-r edge has disappeared. Well-defined oscillations can also be detected up t o larger r values, indicating a higher degree of longer-range order imposed by t h e restriction t o six-fold rings (8).
4. Experimental results
Figure 2 presents t h e reduced interference functions i(k) for as-deposited a-Ge and a-GaAs.
The i(k) spectrum of annealed a-GaAs is identical t o t h a t of as-deposited a-GaAs within experimental uncertainties, except for a n increase of t h e contrast of t h e two first maxima and minima. When going from a-Ge t o a-GaAs, one c a n s e e t h a t , if t h e position and shape of t h e first peak a r e not appreciably modified, all t h e following maxima a r e shifted t o lower k values. The ki/kl ratios, determined with 1 % accuracy, a r e given in table I and must be compared t o t h e ratios computed f o r t h e models, reported in (10). The a-GaAs values a r e in good a g r e e m e n t with those of t h e unrelaxed model 11, confirming previous results (10);
n o change is observed a f t e r annealing. As for t h e a-Ge values, t h e y a r e definitely larger than t h e a-GaAs values, but i t is difficult t o decide whether t h e y a r e b e t t e r reproduced by t h e unrelaxed model I, which, on t h e analogy of t h e results obtained for model 11, should give ratios smaller than those computed a f t e r relaxation, o r by t h e relaxed model 11. I t must however be emphasized t h a t t h e ki/kl ratios determined in a previous study on evaporated a-Ge films deposited a t lower r a t e s (1-2 a / s e c instead of 25-50 A/sec) (12) (they a r e indicated in parentheses in table I) were very close t o those of t h e relaxed model I. This indicates t h a t t h e deposition conditions may strongly influence t h e film s t r u c t u r e . One c a n also notice t h a t t h e third peak of t h e a-Ge i(k) spectrum exhibits a broad shoulder instead of t h e secondary maximum predicted by t h e model I. The modifications of this peak when going t o a-GaAs a r e however very similar t o those observed in figure l a between t h e model I and t h e model 11.
Fig.2 - Experimental reduced interference functions i(k) f o r as-deposited a-Ge (empty circles)
and a-GaAs (full circles)
C8-548 JOURNAL DE PHYSIQUE
Table I - Experimental values of t h e ratios ki/kl of t h e positions of the interference function maxima and of t h e positions r i of t h e radial distribution function peaks for as-deposited a- Ge (the ki/kl ratios of ref. 1 2 a r e indicated in parentheses) and as-deposited and annealed a-GaAs.
a-Ge (as-deposited)
a-GaAs (as-deposited)
a-GaAs
Figure 3 presents t h e radial distribution functions J ( r ) f o r as-deposited a-Ge and a-GaAs.
Here again t h e r e is very l i t t l e difference between as-deposited and annealed a-GaAs, except in t h e region between t h e second and third peaks, a s w e will see. Five well-defined structures can be ~ b s e r v e d on t h e J ( r ) spectra, but t h e l a s t one is a f f e c t e d by large uncertainties. As indicated in table I, t h e average first-neighbour distance r l is very close t o t h e crystalline value (2.45 A) for a-Ge, but appreciably larger for a-GaAs. This l a s t result is in contradiction with those of EXAFS measurements on thicker flash-evaporated films (16) and of X-ray diffraction experiments o n sputtered films (1). The position of t h e second peak r 2 corresponds in both cases t o a n average bond angle very close t o t h e regular tetrahedron value (r2/rl
1.63). Significant differences between t h e J(r) s p e c t r a of t h e two materials appear for r values larger than r2. When comparing a-GaAs t o a-Ge, one observes, a t l e a s t qualitatively, the s a m e modifications a s when comparing t h e model I1 t o t h e model I (figure l b ) : t h e second peak becomes more asymmetric and t h e following minimum is deeper, t h e third peak shifts t o higher r values and has less contrast, t h e fourth peak also s h i f t s and changes i t s shape.
Fig.3 - Experimental radial distribution functions J(r) f o r as-deposited a-Ge (full circles) and a-GaAs
-.(empty circles); t h e d a t a f o r annealed a-GaAs a r e indicated by t h e dashed line.
k2/kl
1.74 ( 1.82 )
1.68
1.68
t
kq/kl
4.23 ( 4.35 )
4.02
4.02
i
