Electronic Properties of Amorphous SixGe1−x:H Films

Hauschildt, D.; Fischer, R.; Fuhs, W.

doi:10.1002/pssb.2221020214

Cited by 66 publications

(8 citation statements)

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“…Additionally, the data of Fig. 11 are nearly identical to the ⌬E pl vs x reported by Hauschildt et al 4 for anodic rf glow discharge a-Si 1Ϫx Ge x :H. A decrease of ⌬E pl with x is what would be expected for Stokes shift broadening.…”

Section: Photoluminescence (Pl)supporting

confidence: 83%

“…1-3 for recent reviews͒. Many deposition processes have been explored, [4][5][6][7][8][9][10][11][12][13][14] and, regardless of the deposition process, it has generally been found that the optoelectronic properties of the alloys deteriorate significantly and monotonically from a-Si:H to a-Ge:H. Corresponding to this deterioration, the alloy microstructure has also been observed to change from a homogeneous structure ͑as viewed with an electron microscope͒ in device-quality a-Si:H to a heterogeneous columnarlike structure in a-Ge:H. 5,[15][16][17][18][19] Although moderate improvements have been obtained by the careful use of hydrogen ͑or helium͒ dilution during glow discharge chemical vapor deposition ͑CVD͒, 20-23 by use of fluorinated gases, 24 by use of a triode-type glow discharge system, 21,25 by increasing ion bombardment through the application of a dc bias, 26 and most recently by the use of very low power discharges, 27 the trend of deterioration with increasing germanium content has persisted and the properties relevant for use in a device have remained very inferior to a-Si:H for most of the alloy series.…”

Section: Introductionmentioning

confidence: 99%

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High performance glow discharge a-Si1−xGex:H of large x

Wickboldt

Pang

Paul

et al. 1997

Journal of Applied Physics

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Radio frequency glow discharge chemical vapor deposition has been used to deposit thin films of a-Si1−xGex:H which possess optoelectronic properties that are greatly improved over any yet reported in the range of x⩾0.6. These films were deposited on the cathode (cathodic deposition) of an rf discharge. Their properties are assessed using a large variety of measurements and by comparison to the properties of alloys conventionally prepared on the anode (anodic deposition). Steady state photoconductivity measurements yield a quantum-efficiency-mobility-lifetime product, ημτ, of (1–3)×10−7 cm2 V−1 for 1.00⩾x⩾0.75 and (6–10)×10−8 cm2 V−1 for 0.75⩾x⩾0.50, and photocarrier grating measurements yield ambipolar diffusion lengths several times greater than previously obtained for alloys of large x. It is confirmed that the improvements in phototransport are not due to a shift in the Fermi level. In fact, results of recent measurements on lightly doped samples strongly suggest that for these cathodic alloys neither photocarrier is dominant [(μτ)e≈(μτ)h]. The improvements are attributed in large part to the reduction of long range structural heterogeneity observed in x-ray scattering and electron microscopy, and partly to the reduction in midgap state density. In spite of the superior properties, an assessment of the data of the cathodic alloys suggests that alloying introduces mechanisms detrimental to transport which are not present in a-Si:H or a-Ge:H. The Urbach tail width is 42±2 meV for cathodic a-Ge:H and 45±2 meV for cathodic a-Si1−xGex:H and is constant with x. From differences in the band edges and tails we infer that the atomic bond ordering is different between the cathodic and anodic alloys. For a given composition the cathodic alloys have roughly an order of magnitude lower midgap state density than do the anodic alloys, and both midgap densities increase exponentially with x, consistent with defect creation models from which the lower midgap density can be attributed to a larger band gap and decreased valence band tail width. A photoluminescence peak is observed with an intensity roughly an order of magnitude greater than for the anodic alloys, and a significantly different peak energy. Section VII E provides an overview of the results and conclusions. The improved properties of these alloys have significant implications for current and future device applications.

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Section: Photoluminescence (Pl)supporting

confidence: 83%

Section: Introductionmentioning

confidence: 99%

High performance glow discharge a-Si1−xGex:H of large x

Wickboldt

Pang

Paul

et al. 1997

Journal of Applied Physics

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“…As a consequence the electronic properties of a-SixGel-x: H alloys are greatly changed when x decreases. This has been observed in both sputtered [8] and GD [9] alloys. In particular a decrease of the photoconductivity efficiency by four orders of magnitude has been measured when x is decreased from 1 to 0.1 [9].…”

Section: Introductionsupporting

confidence: 54%

Electronic properties of hydrogenated amorphous silicon-germanium alloys

et al. 1983

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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.

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“…The relatively poor photoelectronic properties of a-SiGe:H alloys as compared with those for a-Si:H have limited its use in actual photovoltaic devices and led to fundamental studies of this intrinsic material [1,2]. It has been concluded from several investigations that these deteriorations in the photoelectronic properties are a result of changes in the atomic scale structure, which include larger numbers of dangling bonds [3] and increased dihydride bonding [1] as well as larger scale structural changes such as column formation [1], as the germanium content, x, is increased.…”

Section: Introductionmentioning

confidence: 99%

Small-Angle X-Ray Scattering Studies of Glow-Discharge-Produced a-SiGe:H Alloys

et al. 1992

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Electron density fluctuations associated with microstructural features on a scale from about 1 to 25 nm in glow-discharge-deposited a-SilxGex:H films were studied by the technique of small-angle x-ray scattering (SAXS). Films prepared in four different deposition systems (in different laboratories) have been characterized and a general increase in the SAXS signal with increasing x is observed. Density deficiencies determined from film flotation measurements lead to the correlation of the increased scattering intensities with increases in the volume fractions ofmicrovoids. Modeling ofthe data yields void size distributions that demonstrate significantly more of the larger voids (2 to 6 nm) than found at x--0 (around I nm). For the alloys with x>0.4, the scattering at the smallest angles was observed to decrease substantially upon tilting of the sample relative to the x-ray beam. This result contrasts with the small or no changes in SAXS upon tilting device-quality x--0 films. This anisotropic scattering associated with the tilting experiments has been modeled with distributions of ellipsoidal microvoids that are preferentially oriented with their major axes normal to the film plane. This latter result is consistent with a columnar-like microstructure. However, one film with x=0.37 shows no evidence for such microstructure.

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Electronic Properties of Amorphous Si_xGe_1−x:H Films

Cited by 66 publications

References 6 publications

High performance glow discharge a-Si1−xGex:H of large x

High performance glow discharge a-Si1−xGex:H of large x

Electronic properties of hydrogenated amorphous silicon-germanium alloys

Small-Angle X-Ray Scattering Studies of Glow-Discharge-Produced a-SiGe:H Alloys

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