Electronic properties of hydrogenated amorphous silicon-germanium alloys

Abstract

2014 On décrit les propriétés électroniques de quelques alliages amorphes de silicium et de germanium hydrogénés a-SixGe1-x: H riches en silicium (x > 0,6). Tout d'abord nous montrons que ces alliages subissent des effets photo-induits du type de ceux décrits par Staebler et Wronski pour a-Si:H. Les propriétés électroniques sont alors étudiées en fonction de la concentration en germanium et de l'histoire photothermique des films. Les changements observés de la conductivité entre l'état recuit et l'état irradié sont interprétés en fonction des variations du coefficient de température du niveau de Fermi. Nous montrons que le rendement de photoconduction reste proche de celui de a-Si:H pour 1 > x ~ 0,9 puis décroît fortement quand la teneur en germanium devient plus élevée: il est environ 104 fois plus faible pour l'alliage Si0,62Ge0,38 que pour a-Si:H. Cette degradation des propriétés photoconductrices est interprétée par une augmentation de la densité d'états dans la bande interdite liée à l'incorporation de germanium. Cette conclusion repose sur l'étude de la région exponentielle du front d'absorption et sur les résultats obtenus lors de la mesure des temps de réponse de la photoconduction. Ces derniers sont interprétés dans le cadre du modèle de Rose sur la cinétique de piégeage et de recombinaison. On a trouvé que pour x ~ 0,6 la densité d'états à ~ 0,4-0,5 eV en dessous du seuil de mobilité est de 7 x 1017 eV-1 cm-3 alors qu'elle est seulement de 2,4 x 1016 eV-1 cm-3 pour x = 0,97. Abstract. 2014 The electronic properties of some binary hydrogenated amorphous silicon-germanium alloys a-SixGe1-x: H in the silicon rich region (x > 0.6) are investigated. Experimental evidence is presented of photoinduced effects similar to those described in a-Si:H (Staebler-Wronski effect). The electronic properties are then studied from the dual point of view of the germanium content dependence and of the photo and thermal histories of the films. The dark conductivity changes between the annealed state and the light-soaked state are interpreted in terms of the variation of the temperature coefficient of the Fermi level. The photoconductivity efficiency is shown to remain close to that of a-Si:H for 1 > x ~ 0.9 and to strongly decrease when the germanium content is further increased : the photoresponse of the Si0.62Ge0.38 alloy is 104 times smaller than that of a-Si:H. This deterioration of the photoconductive properties is explained in terms of the increase of the density of gap states following Ge substitution. This conclusion is based on the study of the width of the exponential absorption edge and on the results of photoconductivity time response studies. The latter data are interpreted by means of the model of Rose of trapping and recombination kinetics and it is found that for x ~ 0.6 the density of states at 0.4-0.5 eV below the mobility edge is 7 x 1017 eV-1 cm-3 as compared to 2.4 x 1016 eV-1 cm-3 for x = 0.97.

The effect of variation in hydrogen dilution and RF power density on the properties of a-SiGe:H and related solar cells

“…Among several limitations, poor electronic quality of amorphous silicon germanium (a-SiGe:H) layers in multijunction cells limits the performance of solar cells [1,2]. However, main advantages of this material are that (i) its optical gas can be easily tuned to match the wide range (1.7-1.2 eV) of the solar radiation spectrum [3] and (ii) light-induced degradation of such materials is less than that of normal bandgap a-Si:H materials [4]. Before 1986, several endeavours [3,[5][6][7][8] failed to develop device grade a-SiGe:H alloy films by discharging silane (SiH 4 ) and germane (GeH 4 ) gas mixtures in diode/triode glow discharge systems.…”

Reduction of light-induced metastable changes in a-SiGe:H prepared by using helium dilution: comparison of metastability of helium- and hydrogen-diluted a-SiGe:H alloys

“…Thus potential fluctuation due to inhomogeneity within helium-diluted a-SiGe:H films will be substantially less than that in hydrogen-diluted films. According to the model of Branz and Silver [1] helium-diluted alloys are more stable than hydrogen-diluted films under similar light-soaking conditions.…”

“…Moreover, it has been observed experimentally that the nature of metastable changes under prolonged illumination of amorphous silicon alloy materials, such as amorphous silicon germanium (a-SiGe:H), is rather complicated. Light-induced degradation of a-SiGe:H is significantly lower than that of a-Si:H [1,2], but the initial electronic properties of a-SiGe:H in the range of optical gap 1.7 to 1.5 eV are more of less the same as those of a-Si:H. There is endeavour to explain these experimental findings by thermodynamic arguments. From defect pool calculations which are primarily based on thermodynamic relations for equilibrium and non-equilibrium states, defect generation kinetics and saturated carrier density depend on the equilibrium charge carrier density which is a function of the optical gap of the materials [3,4].…”

The effects of native and light induced defects on optoelectronic properties of hydrogenated amorphous silicon–germanium (a-SiGe:H) alloy thin films

“…Contrarily, the density of deep defects in the bandgap increases with increased Ge alloying which finally degrades the lifetime of photogenerated carriers [3][4][5]. Even though earlier studies reported negligible Staebler-Wronski effect (SWE) in a-SiGe:H mainly due to poor material quality available at those times [6,7], high quality alloy films prepared after improved deposition systems exhibited a substantial SWE under light soaking similar to that intrinsic to pure a-Si:H [8][9][10][11][12][13][14][15][16]. For this reason, investigations of the light induced defect creation as well as native defects in the alloy as a function of Ge content has become an important issue to be understood and resolved both scientifically and technologically.…”