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Title: Structural and electronic studies of a-SiGe:H alloys

Abstract

This report describes work to produce alloys of a-Si[sub 1-x]Ge[sub x]:H of improved photoelectronic quality by plasma-enhanced chemical vapor deposition (PECVD). The goal was to discover optimum preparation conditions for the end-component, a-Ge:H, to establish whether modification of the usual practice of starting from a-Si:H preparation conditions was advisable. Such modification, found to be necessary, gave films of a-Ge:H with efficiency-mobility-lifetime products ([eta][mu][tau]) 10[sup 2] to 10[sup 3] higher than were earlier available, in homogeneous environmentally stable material. Both a-Ge:H and a-Si[sub 1-x]Ge[sub x]:H of large x were studied in detail. Alloy material was shown to have [eta][mu][tau] 10[sup 2] larger than found earlier. However, just as the [eta][mu][tau] of a-Si:H decreases when Ge is added, so also the [eta][mu][tau] of these alloys with Si addition. By contrast, the ambipolar diffusion lengths, L[sub o] which are governed by the hole mobility, vary by only a factor of two over the whole alloy series. Using the experimental finding of a small valence band offset between a-Si:H and a-Ge:H compositional fluctuations on a 10-mm scale are suggested to explain the behavior of [eta][mu][tau] and L[sub o] The implications for eventual improvement of the alloys are profound, but require direct experimental tests ofmore » the postulated compositional fluctuations.« less

Authors:
 [1]
  1. (Harvard Univ., Cambridge, MA (United States))
Publication Date:
Research Org.:
National Renewable Energy Lab., Golden, CO (United States); Harvard Univ., Cambridge, MA (United States)
Sponsoring Org.:
DOE; USDOE, Washington, DC (United States)
OSTI Identifier:
6633607
Alternate Identifier(s):
OSTI ID: 6633607; Legacy ID: DE93010021
Report Number(s):
NREL/TP-411-5457
ON: DE93010021
DOE Contract Number:
AC02-83CH10093
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; GERMANIUM ALLOYS; CHEMICAL VAPOR DEPOSITION; SILICON ALLOYS; SILICON SOLAR CELLS; FABRICATION; AMORPHOUS STATE; ELECTRICAL PROPERTIES; GLOW DISCHARGES; INTERMETALLIC COMPOUNDS; MICROSTRUCTURE; PROGRESS REPORT; SILICON; ALLOYS; CHEMICAL COATING; CRYSTAL STRUCTURE; DEPOSITION; DIRECT ENERGY CONVERTERS; DOCUMENT TYPES; ELECTRIC DISCHARGES; ELEMENTS; EQUIPMENT; PHOTOELECTRIC CELLS; PHOTOVOLTAIC CELLS; PHYSICAL PROPERTIES; SEMIMETALS; SOLAR CELLS; SOLAR EQUIPMENT; SURFACE COATING 140501* -- Solar Energy Conversion-- Photovoltaic Conversion; 360101 -- Metals & Alloys-- Preparation & Fabrication; 360104 -- Metals & Alloys-- Physical Properties

Citation Formats

Paul, W. Structural and electronic studies of a-SiGe:H alloys. United States: N. p., 1993. Web. doi:10.2172/6633607.
Paul, W. Structural and electronic studies of a-SiGe:H alloys. United States. doi:10.2172/6633607.
Paul, W. Thu . "Structural and electronic studies of a-SiGe:H alloys". United States. doi:10.2172/6633607. https://www.osti.gov/servlets/purl/6633607.
@article{osti_6633607,
title = {Structural and electronic studies of a-SiGe:H alloys},
author = {Paul, W.},
abstractNote = {This report describes work to produce alloys of a-Si[sub 1-x]Ge[sub x]:H of improved photoelectronic quality by plasma-enhanced chemical vapor deposition (PECVD). The goal was to discover optimum preparation conditions for the end-component, a-Ge:H, to establish whether modification of the usual practice of starting from a-Si:H preparation conditions was advisable. Such modification, found to be necessary, gave films of a-Ge:H with efficiency-mobility-lifetime products ([eta][mu][tau]) 10[sup 2] to 10[sup 3] higher than were earlier available, in homogeneous environmentally stable material. Both a-Ge:H and a-Si[sub 1-x]Ge[sub x]:H of large x were studied in detail. Alloy material was shown to have [eta][mu][tau] 10[sup 2] larger than found earlier. However, just as the [eta][mu][tau] of a-Si:H decreases when Ge is added, so also the [eta][mu][tau] of these alloys with Si addition. By contrast, the ambipolar diffusion lengths, L[sub o] which are governed by the hole mobility, vary by only a factor of two over the whole alloy series. Using the experimental finding of a small valence band offset between a-Si:H and a-Ge:H compositional fluctuations on a 10-mm scale are suggested to explain the behavior of [eta][mu][tau] and L[sub o] The implications for eventual improvement of the alloys are profound, but require direct experimental tests of the postulated compositional fluctuations.},
doi = {10.2172/6633607},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Apr 01 00:00:00 EST 1993},
month = {Thu Apr 01 00:00:00 EST 1993}
}

Technical Report:

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  • This report describes work to produce alloys of a-Si{sub 1-x}Ge{sub x}:H of improved photoelectronic quality by plasma-enhanced chemical vapor deposition (PECVD). The goal was to discover optimum preparation conditions for the end-component, a-Ge:H, to establish whether modification of the usual practice of starting from a-Si:H preparation conditions was advisable. Such modification, found to be necessary, gave films of a-Ge:H with efficiency-mobility-lifetime products ({eta}{mu}{tau}) 10{sup 2} to 10{sup 3} higher than were earlier available, in homogeneous environmentally stable material. Both a-Ge:H and a-Si{sub 1-x}Ge{sub x}:H of large x were studied in detail. Alloy material was shown to have {eta}{mu}{tau} 10{sup 2}more » larger than found earlier. However, just as the {eta}{mu}{tau} of a-Si:H decreases when Ge is added, so also the {eta}{mu}{tau} of these alloys with Si addition. By contrast, the ambipolar diffusion lengths, L{sub o} which are governed by the hole mobility, vary by only a factor of two over the whole alloy series. Using the experimental finding of a small valence band offset between a-Si:H and a-Ge:H compositional fluctuations on a 10-mm scale are suggested to explain the behavior of {eta}{mu}{tau} and L{sub o} The implications for eventual improvement of the alloys are profound, but require direct experimental tests of the postulated compositional fluctuations.« less
  • This report describes work on the growth of a-Si:H and a-(Si,Ge):H materials and devices using well-controlled growth techniques. The a-Si:H materials were grown at higher temperatures (300{degrees}-375{degrees}C) using electron-cyclotron-resonance (ECR) plasma techniques with a remote H beam. These films have excellent electronic quality and show significant improvements in stability compared with glow-discharge-produced a-Si:H materials. Several problems were encountered during the fabrication of devices in these materials, and we were able to overcome them by a systematic work on buffer layers in these cells. We also studied alternative designs for improving the stability of a-Si:H cells and produced graded-gap a-Si:H cellsmore » using glow-discharge that are more stable than comparable standard, ungraded glow discharge devices. Finally, systematic work was done to produce good-quality a-(Si,Ge):H films, using triode radio frequency (RF) glow-discharge with ion bombardment during growth. Diagnostic devices were made using these films, and the properties of the material, such as Urbach energies and hole mobility-lifetime products, were measured in these devices. We found a systematic increase in the Urbach energies, and a corresponding decrease in the hole and electron {mu}{tau} products, as the Ge content of the alloys increases.« less
  • This report describes a research program to investigate hydrogenated amorphous silicon-germanium alloys (a-Sil-xGex:H) for solar-cell applications. Specifically, studies were carried out to determine why these low-band-gap alloys exhibit photo-electronic properties inferior to those of hydrogenated amorphous silicon (a-Si:H). Two contributors to this phenomenon were established: (1) An amorphous semiconductor of smaller band gap than a-Si:H, but possessing tails to the conduction band and valence band densities of states of the same extent as in a-Si:H, and also possessing at least the same density of defect-related states, was bound to have shorter electron and hole lifetimes. (2) The structure of alloymore » films (whether with smaller or larger band gaps than that of a a-Si:H), exhibited considerable inhomogeneity; this led to an even greater reduction in the quantum-efficiency mobility lifetime product. The extensive studies confirmed that a-Ge:H is much poorer than a-Si:H from a photoelectronic standpoint.« less
  • The report contains a detailed description of the experimental complexities encountered in developing scanning tunneling microscope (STM) probing of atomic structure on the surface of freshly-grown hydrogenated-amorphous semiconductors. It also contains a speculative microscopic film-growth model that explains differences between the disorder in CVD grown a-Ge:H versus a-Si:H films. This model is derived from prior results obtained in the chemical analysis of GeH{sub 4} plasmas, combined with surface reaction and thermodynamic considerations. The neutral radical fragments of silane, disilane and germane dissociation in discharges, which dominate the vapor and film-growth reactions, have been deduced from detailed analysis of prior datamore » and are reported. 4 refs., 7 figs.« less
  • Occurrence of various phases in binary transition-metal alloys; axial ratio in Hcp binary transition-metal alloys, c/a; electronic specific heat coefficient, experimental, results, electronic specific heat, Debye temperature, superconductivity; magnetic susceptibility, experimental, results, transition-metal susceptibility, relationship between gamma chi; hardness of transition-metal alloys, alloys measured, alloys of 5D transition metals, other transition-metal alloys, results, alloys of 5D transition metals, other transition-metal alloys.