Influence of Filament and Substrate Temperatures on Structural and Optoelectronic Properties of Narrow Gap a-SiGe:H Alloys Deposited by Hot-Wire CVD

2003 ◽  
Vol 762 ◽  
Author(s):  
Yueqin Xu ◽  
Brent P. Nelson ◽  
D.L. Williamson ◽  
Lynn M. Gedvilas ◽  
Robert C. Reedy
Keyword(s):  
Hydrogen Content ◽  
Silicon Germanium ◽  
Chemical Vapor ◽  
Dark Conductivity ◽  
Hot Wire ◽  
Standard Temperature ◽  
X Ray Scattering ◽  
Deposition Parameters ◽  
Amorphous Silicon Germanium ◽  
Structural And Optoelectronic Properties

AbstractWe have found that narrow-bandgap—1.25 < Tauc Gap < 1.50 eV—amorphous silicon germanium (a-SiGe:H) alloys grown by hot-wire chemical vapor deposition (hot-wire CVD) can be improved by lowering both substrate and filament temperatures. We systematically study films deposited using a one-tungsten filament, decreasing filament temperature (Tf) from our standard temperature of 2150° down to 1750°C, and fixing all other deposition parameters. By decreasing Tfat the fixed substrate temperature (Ts) of 180°C, the Ge-H bonding increases, whereas the Si-H2bonding is eliminated. Films with higher Ge-H bonding and less Si-H2have improved photoconductivity. For the series of films deposited using the same germane gas fraction at 35%, the energy where the optical absorption is 1x104(E04) drops from 1.54 to 1.41 eV with decreasing Tf. This is mainly due to the combination of an increasing Ge solid fraction (x) in the film, and an improved homogeneity and compactness due to significant reduction of microvoids, which was confirmed by small angle X-ray scattering (SAXS). We also studied a series of films grown by decreasing the Tsfrom our previous standard temperature of 350°C down to 125°C, fixing all other deposition parameters including Tfat 1800°C. By decreasing Ts, both the total hydrogen content (CH) and the Ge-H bonding increased, but the Si-H2bonding is not measurable in the Tsrange of 180°-300°C. The E04 increases from 1.40 to 1.51 eV as Tsdecreased from 350° to 125°C, mainly due to the increased total hydrogen content (CH). At the same time, the photo-to-dark conductivity ratio increases almost three orders of magnitude over this range of Ts.

Hydrogenated Amorphous Silicon Germanium Alloys Grown by the Hot-Wire Chemical Vapor Deposition Technique

1998 ◽  
Vol 507 ◽  
Author(s):  
Brent P. Nelson ◽  
Yueqin Xu ◽  
D.L. Williamson ◽  
Bolko Von Roedern ◽  
Alice Mason ◽  
...  
Keyword(s):  
Chemical Vapor Deposition ◽  
Vapor Deposition ◽  
Silicon Germanium ◽  
Hydrogenated Amorphous Silicon ◽  
Chemical Vapor ◽  
Hot Wire ◽  
Germanium Content ◽  
Amorphous Silicon Germanium ◽  
Hydrogenated Amorphous ◽  
Gas Ratio

ABSTRACTWe successfully grow high-quality hydrogenated amorphous-silicon-germanium alloys (a-SiGe:H) by the hot-wire chemical-vapor deposition (HWCVD) technique using silane and germane gas mixtures. These alloys display electronic properties as good as those grown by the plasma-enhanced chemical-vapor deposition (PECVD) technique, when comparing materials with the same optical bandgaps. However, we grow materials with good electrical properties at high deposition rates—up to 40 Å/s, compared to 1–4 Å/s for PECVD materials. Our alloys exhibit similar trends with increasing Ge content to alloys grown by PECVD. The defect density, the dark conductivity, and the degree of nanostructural heterogeneity (as measured by small-angle X-ray scattering) all increase with increasing germanium content in the alloy. The nanostructural heterogeneity displays a sharp transition between 9 at.% and 14 at.% germanium. PECVD- grown a-SiGe:H alloys exhibit a similar transition at 20 at.% Ge. The photoconductivity and the ambipolar diffusion length of the alloys decrease with increasing germanium content. For a fixed silane-to-germane gas ratio, all material properties improve substantially when increasing substrate temperature (Tsub) from 220°C to 375°C. Increasing Tsub also narrows the optical bandgap and lowers the hydrogen content in the alloys for the same germane-to-silane gas ratio.

Effect of Ge Incorporation on Hydrogenated Amorphous Silicon Germanium Thin Films Prepared by Plasma Enhanced Chemical Vapor Deposition

2012 ◽  
Vol 569 ◽  
pp. 27-30
Author(s):  
Bao Jun Yan ◽  
Lei Zhao ◽  
Ben Ding Zhao ◽  
Jing Wei Chen ◽  
Hong Wei Diao ◽  
...  
Keyword(s):  
Thin Films ◽  
Chemical Vapor Deposition ◽  
Amorphous Silicon ◽  
Vapor Deposition ◽  
Silicon Germanium ◽  
Hydrogenated Amorphous Silicon ◽  
Chemical Vapor ◽  
Dark Conductivity ◽  
Amorphous Silicon Germanium ◽  
Hydrogenated Amorphous

Hydrogenated amorphous silicon germanium thin films (a-SiGe:H) were prepared via plasma enhanced chemical vapor deposition (PECVD). By adjusting the flow rate of GeH4, a-SiGe:H thin films with narrow bandgap (Eg) were fabricated with high Ge incorporation. It was found that although narrow Eg was obtained, high Ge incorporation resulted in a great reduction of the thin film photosensitivity. This degradation was attributed to the increase of polysilane-(SiH2)n, which indicated a loose and disordered microstructure, in the films by systematically investigating the optical, optoelectronic and microstructure properties of the prepared a-SiGe:H thin films via transmission, photo/dark conductivity, Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR) measurements. Such investigation provided a helpful guide for further preparing narrow Eg a-SiGe:H materials with good optoelectronic properties.

High-Growth Rate a-Si:H Deposited by Hot-Wire CVD

1994 ◽  
Vol 336 ◽  
Author(s):  
P. Brogueira ◽  
V. Chu ◽  
J.P. Conde
Keyword(s):  
Flow Rate ◽  
Chemical Vapor ◽  
High Growth ◽  
Dark Conductivity ◽  
High Growth Rate ◽  
Hot Wire ◽  
Tungsten Filament ◽  
Hydrogen Flow ◽  
Deposition Parameters ◽  
Pressure Regime

ABSTRACTWe present a study of the optoelectronic and structural properties of a-Si:H deposited by Hot-Wire chemical vapor deposition (HW-CVD) from SiH4 and H2 at “Medium” (Tfil ≃ 1500°C) and “high” (Tfil ≃ 1900 °C) filament temperatures. For each tungsten filament temperature regime, the following deposition parameters are varied: (i) pressure (p ∼ 10−2 — 0.5 Torr); (ii) substrate temperature (Tsub ∼ 180 — 270 °C); (iii) silane flow rate (FsiH4 ∼ 1 — 10 ccm) and (iv) hydrogen flow rate (FH2 ∼ 0 — 10 seem). Films deposited at Tfil ∼ 1900 °C in a low pressure regime (p ∼ 2.7 × 10−2Torr) using flows of 5 sccm for both H2 and SiH4 had high deposition rates (rd ∼ 8 Ås−1). These films showed an optical bandgap, E9Tauc ≃ 1.7 eV, a dark conductivity σd ∼ 10−8Scm−1 with an activation energy Eα,σd ≃ 0.8 eV, and photoconductivity σph ≥ 10−5Scm−1 (σph ∼ 10−5). Films deposited at Tju = 1500 °C and p ≃ 0.3 Torr, showed 1.7 < E9Tauc < 2 eV, 10−5 < σd < 10−3Scm−1, 0.2 < Eα,σ d < 0.5 eV and σph/Σd < 102. For the same Tfit and p ∼ 3 × 10−2 — 0.1 Torr, the films show 1.7 < E9Tauc < 2 eV, 10−3 < Σd < 10−1Scm−1 and σph/σd < 1. Films deposited using molybdenum and rhenium filaments at Tfil ≃ 1900 °C show E9Tauc ≃ 1.7 eV and σd ∼ σph ∼ 10−7Scm−1

Dielectric Functions of a-Si1-xGex:H versus Ge Content, Temperature, and Processing: Advances in Optical Function Parameterization

2006 ◽  
Vol 910 ◽  
Author(s):  
Nikolas J. Podraza ◽  
Christopher R. Wronski ◽  
Mark W. Horn ◽  
Robert W. Collins
Keyword(s):  
Real Time ◽  
Surface Diffusion ◽  
Silicon Germanium ◽  
Chemical Vapor ◽  
Real Time Control ◽  
Time Control ◽  
Deposition Parameters ◽  
Short Range Ordering ◽  
Diffusion Characteristics ◽  
Amorphous Silicon Germanium

AbstractWe have applied an advanced model to analyze the dielectric functions e = e1 + ie2 of amorphous silicon-germanium alloys (a-Si1-xGex:H) (i) as a function of alloy content × by varying the flow ratio G = [GeH4]/{[SiH4]+[GeH4]} in plasma-enhanced chemical vapor deposition (PECVD), and (ii) for the first time as a function of the measurement temperature Tm by cooling the newly-deposited film. All e spectra (1.5 − 4.5 eV) have been measured by spectroscopic ellipsometry (SE) either in real time during deposition or in situ post-deposition in order to avoid surface contamination. From the resulting extensive database, the optical properties of the alloys can be predicted for any value × and Tm within the ranges of the database. Such a capability is expected to be useful, for example, in real time control of optical gap in the PECVD process and in predicting the quantum efficiency of multijunction a-Si:H-based solar cells versus operating temperature. The effect on the database of other deposition parameters such as the electrode configuration and the H2-dilution ratio R = [H2]/{[SiH4]+ [GeH4]} have also been explored. The latter two studies provide useful insights into materials properties that can be extracted from a single spectroscopic measurement performed in real time during PECVD. For example, the energy width of the resonance in e correlates closely with the precursor surface diffusion characteristics observed throughout growth -- both determined from real time SE. This result indicates that short-range ordering in the film is improved when surface diffusion is promoted.

Electronic Properties of Improved Amorphous Silicon-Germanium Alloys Deposited by a Low Temperature Hot Wire Chemical Vapor Deposition Process

2005 ◽  
Vol 862 ◽  
Author(s):  
Shouvik Datta ◽  
J. David Cohen ◽  
Yueqin Xu ◽  
A. H. Mahan
Keyword(s):  
Chemical Vapor Deposition ◽  
Electronic Properties ◽  
Vapor Deposition ◽  
Silicon Germanium ◽  
Chemical Vapor ◽  
Deposition Process ◽  
Alloy Sample ◽  
Hot Wire ◽  
Novel Material ◽  
Amorphous Silicon Germanium

AbstractWe report novel material properties of a series of a-Si,Ge:H alloys grown by hot-wire chemical vapor deposition under low filament temperature (˜1800°C) and low substrate temperature (˜200-300°C). These alloys exhibit significantly improved electronic properties including low defect densities and sharp band tails (Urbach energies ≤ 45meV even for Ge fractions as high as 47at.%). On the other hand, comparisons of the transient photocapacitance and transient photocurrent spectra do not indicate very efficient hole collection in these materials. We found two distinct regimes of light-induced degradation in the alloy sample with 29at.% Ge fraction, possibly corresponding to the light induced increase of Ge and Si dangling bonds, respectively.

Two-Step Capacitance Transients From an Oxygen Impurity Defect

2008 ◽  
Vol 1066 ◽  
Author(s):  
Shouvik Datta ◽  
J. David Cohen ◽  
Yueqin Xu ◽  
Howard M. Branz
Keyword(s):  
Electron Emission ◽  
Vapor Deposition ◽  
Silicon Germanium ◽  
Chemical Vapor ◽  
Oxygen Impurity ◽  
Pulse Time ◽  
Hot Wire ◽  
Impurity Defect ◽  
Amorphous Silicon Germanium ◽  
Hydrogenated Amorphous

ABSTRACTThis paper describes the study of an electron-trapping defect which underwent significant configurational relaxation in oxygen contaminated hydrogenated amorphous silicon-germanium (a-Si,Ge:H) alloys grown by hot-wire chemical vapor deposition. An unusual two-step electron emission from this relaxed defect is studied using junction-capacitance-based measurements. In this work, we monitor the recovery of the relaxed defect after filling it by photoexcited electrons and also by electrons injected with a voltage filling pulse. The dependence of the transient shape on filling pulse time is described. We have also performed experiments which clearly demonstrate that this is a bulk defect and exclude contributions from any additional blocking junctions.

scholarly journals High Quality Amorphous Silicon Germanium Alloy Solar Cells Made by Hot-wire CVD at 10 Å/s

2001 ◽  
Vol 664 ◽  
Author(s):  
Qi Wang ◽  
Eugene Iwaniczko ◽  
Jeffrey Yang ◽  
Kenneth Lord ◽  
Subhendu Guha
Keyword(s):  
Solar Cells ◽  
Amorphous Silicon ◽  
Silicon Germanium ◽  
Chemical Vapor ◽  
Hot Wire ◽  
High Quality ◽  
Area Efficiency ◽  
Back Reflectors ◽  
Double Junction ◽  
Amorphous Silicon Germanium

ABSTRACTHigh quality amorphous silicon germanium (a-SiGe:H) alloys have been obtained using the hot wire chemical vapor deposition (HWCVD) from a gas mixture of SiH4, GeH4, and H2 at a deposition rate of ∼10 Å/s. Solar cells in a SS/n-i-p/ITO configuration are evaluated in which the n- and i-layers are deposited by HWCVD at NREL and the microcrystalline p-layer by conventional RF glow discharge in a separate reactor by United Solar. Effects of hydrogen dilution and step-wise bandgap profile have been studied and optimized. The best cell has an average optical bandgap of 1.6 eV and incorporates multi-bandgap steps where the narrow-most bandgap is near the p-i interface. J-V characteristics are measured under AM 1.5 illumination with a λ>530 nm filter. The best initial power output obtained exceeds 4 mW/cm2, which is usually used as an indicator for a good quality middle-gap cell. Double-junction cells are made on textured Ag/ZnO back reflectors. The bottom cell uses the optimized a-SiGe:H alloy cell by HWCVD, and the top cell uses an optimized a-Si:H cell near the amorphous-to-microcrystalline transition by PECVD at ∼1 Å/s. The best double-junction cell made to date exhibits an initial AM 1.5 active-area efficiency of 11.7%, and a stable efficiency after 1000 hours of one sun light soaking of 9.6%.

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