Amorphous Silicon-germanium Alloys Research Articles

Nanostructured Silicon Oxide Dual-Function Layer in Amorphous Silicon Based Solar Cells

ABSTRACTWe report the results of using n-type hydrogenated nanocrystalline silicon oxide alloy (nc-SiOx:H) in hydrogenated nanocrystalline silicon (nc-Si:H) and amorphous silicon germanium alloy (a-SiGe:H) single-junction solar cells. We used VHF glow discharge to deposit nc-SiOx:H layers on various substrates for material characterizations. We also used VHF glow discharge to deposit the intrinsic layer in nc-Si:H solar cells. RF glow discharge was used for the deposition of the doped layers and the intrinsic layer in a-SiGe:H solar cells. Various substrates such as stainless steel (SS), Ag coated SS, and ZnO/Ag coated SS were used for different cell structures. We found that by using nc-SiOx:H to replace the ZnO and the a-Si:H n-layer in nc-Si:H solar cells, the cell structure is greatly simplified, while the cell performances remain nearly identical to those made using the conventional n-i-p structure on standard ZnO/Ag BR’s. Solar cells with nc-SiOx:H as the n layer directly deposited on textured Ag show similar quantum efficiency (QE) as the n-i-p cells on ZnO/Ag BRs. In both cases, QE is higher than that in the n-i-p cells made directly on Ag coated SS. This effect is probably caused by the shift of surface plasmon-polariton resonance frequency due to the difference in index of refraction of ZnO, nc-SiOx:H, and Si.

Properties of Hydrogenated Amorphous Silicon-Germanium Alloys Deposited by Dual Target Reactive Magnetron Sputtering

ABSTRACTHydrogenated amorphous silicon-germanium alloy thin films (a-Si1-xGex:H) were deposited using reactive magnetron sputtering. Dual targets of silicon and germanium were sputtered in an argon + hydrogen atmosphere using rf excitation. Films with x = 0.4 were deposited as a function of substrate temperature and hydrogen partial pressure, and were evaluated by dark and photoconductivity, infrared absorption, and optical transmission. Photosensitivity reached a maximum value of about 5000 between 150 and 200 °C. Using the stretching modes in the region of 2000 cm-1, the hydrogen bonding was characterized in terms of the preferential attachment ratio (PA), which represents the ratio of H bonded to silicon to that bonded to germanium. The PA shows a systematic increase with increasing temperature, independent of hydrogen partial pressure. The interplay between thermodynamic and kinetics effects in determining PA and film quality will be discussed.

Characteristics of Hydrogenated Amorphous Silicon Germanium Thin Film Solar Cells by GeH4 Flow Rate

Hydrogenated amorphous silicon germanium alloys thin films had been grown by PECVD method. This article shows characteristics of hydrogenated amorphous silicon germanium thin film solar cells with GeH4 flow rates. The optical bandgap of hydrogenated amorphous silicon germanium alloy decreased from 1.78 eV - 1.54 eV with increasing in GeH4 flow rate from 0 sccm – 100sccm. The various values of Voc, Jsc, FF, and conversion efficiency were measured by the solar simulator. These results show that the conversion efficiency of the a-SiGe:H solar cells was very high when the GeH4 flow rate was 60 sccm.

A Novel Amorphous SiGe Material Used in CMOS Device

In this paper, we propose a novel material- amorphous silicon germanium(a-SiGe). The a-SiGe film was formed by PECVD at a low temperature and a low frequency. By adjusting the fraction x of Ge in Si1-xGex, optimal SiGe bandgap was achieved. We used amorphous silicon germanium alloy as MOSFET source/drain. The parameter of MOSFET shows that, as the fraction increases, the drain-to-source breakdown voltage increases. With reduction of the minority carrier inject ratio, the current gain β of parasitic BJT in MOSFET was reduced greatly, which eliminates the limit of the breakdown voltage of the device.

Microstructural analysis of nanostructured amorphous silicon–germanium alloys: Numerical modeling

A detailed microstructural analysis of amorphous silicon–germanium alloys with germanium fraction ranging from 0.1 to 0.5 is performed by means of a numerical modeling technique. By substituting Ge atoms for Si atoms in nanoporous paracrystalline network of amorphous silicon, several amorphous silicon–germanium structures have been generated then relaxed. The main aim of our work is to study the effect of compositional heterogeneities on the structural properties of amorphous silicon–germanium alloys in comparison with the standard case, that of a homogeneous random distribution of the atoms. In the present work we envisage the two-phase amorphous silicon–germanium model proposed by Goerigk and Williamson to interpret their anomalous small-angle X-ray scattering measurements; it consists on a mixture of Ge-rich and Ge-poor domains at the nanoscale. The microstructure of our structural models is analyzed by examining the macroscopic mass density, the X-ray diffraction intensity, the radial distribution functions, the bond lengths and the coordination numbers within the first coordination shell of Si and Ge atoms.

Microstructural analysis of atomistic models of Si-rich amorphous silicon–germanium alloys

A detailed microstructural analysis of amorphous silicon–germanium alloys with Ge fraction ranging from 0.1 to 0.5 is performed by means of a numerical modeling technique. By substituting Ge atoms for Si atoms in the nanoporous paracrystalline network of amorphous silicon, several amorphous silicon–germanium structures have been generated. Our main aim in the present work is to study the effect of compositional heterogeneities on the structural properties of amorphous silicon–germanium alloys in comparison with the standard case, that of a homogeneous random distribution of the atoms. We have limited ourselves here to the borderline case, that of segregation of Ge atoms at the nanoscale. The microstructure of our structural models is analyzed by examining the macroscopic mass density, the intensity of X-ray diffraction, the pair distribution functions, the bond lengths and the coordination numbers within the first coordination shell of Si and Ge atoms. Our structural models account for the experimentally derived mass densities regardless of the Ge distribution pattern. They also account for the intense small-angle X-ray scattering observed for some amorphous silicon–germanium samples. The short-range compositional disorder, reflected in the bond lengths and the coordination numbers within the first coordination shell of Ge atoms, is found to be very sensitive to how these atoms are arranged in the alloys.

Method to Determine the Collection Length in Field-Driven a-Si1-xGex:H Solar Cells

Abstract Amorphous silicon germanium alloys are widely used as a p-i-n bottom cell for amorphous silicon based tandem solar cells due to its small and tailored band gap. Decreasing the band gap is achieved by increasing the germanium atomic content. Increasing the germanium content reduces the material electronic quality and hence decreases the collection probability of the generated photocarriers. The determination of the collection distance in p-n solar cells is determined from the internal quantum efficiency because the minority carrier diffusion dominates. This method is not valid for p-i-n structures because the carrier drift dominates. This contribution presents a simple method to determine the collection distance for drift-driven solar cells from the measured quantum efficiency. The method is self checked by reproducing i-layer thickness beside the collection distance. We show that increasing the hydrogen dilution from 20% to 25% increases the collection length from 146 nm to 194 nm. The method estimates the original absorber thickness with error

The Optical Properties of a-Si:H/a-SiGex:H Superlattice Structure to Apply Intrinsic Layer in Solar Cell

The hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon-germanium (a-SiGe:H) alloys have been prepared by the plasma enhanced chemical vapor deposition (PECVD) method. The films were grown on corning #1737 glass from the gas mixture of silane and germane. Superlattice thin films consisted of a-Si:H/a-SiGe:H multilayer structure. The UV-VIS Spectrometer analysis of a-SiGex:H alloys and superlattices thin films showed that the bandgap absorption spectra of a-SiGex:H alloys shift to lower photon energy with an increase in the GeH4 flow rate [0–300 sccm] and various a-Si:H [2−10 nm]/a-SiGex:H [2 nm] thickness. The optical bandgap had decreased by between 1.24∼1.71 eV in regards to a-SiGe:H and superlattice thin films.

Preface: Phys. Status Solidi C 7/3‐4

Preface: Phys. Status Solidi C 7/3‐4

An overview of uncooled infrared sensors technology based on amorphous silicon and silicon germanium alloys

AbstractAt the present time there are commercially available large un‐cooled micro‐bolometer arrays (as large as 1024×768 pixels) for a variety of thermal imaging applications. Different thermo‐sensing materials have been employed as thermo sensing elements as Vanadium Oxide (VOx), metals, and amorphous and polycrystalline semiconductors. Those materials present good characteristics but also have some disadvantages. As a consequence none of the commercially available arrays contain optimum pixels with an optimum thermo‐sensing material. This paper reviews the development of the un‐cooled bolometer technology and the research achievements on this area, with special attention on the key factors that would lead to improve the pixels performance characteristics. The work considers the R&D of microbolometer arrays and the integration with MEMS and IC technologies. A comparative study with the state of the art and data reported in literature is presented. Finally, further directions of uncooled bolometer based in thin films materials are also discussed in this paper. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Combination of optical and X-ray techniques in the study of amorphous semiconductors under high pressure: an upgrade setup for combined XAS and XRD measurements

X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are complementary techniques whose combination is a powerful tool of investigation of matter under extreme conditions (high pressure, high temperature). The standard setup at the ODE beamline (Soleil Synchrotron) has been modified in order to allow the performance of simultaneous XAS/XRD measurements on the same sample. These techniques, together with Raman spectroscopy measurements, were applied to the study of pressure-induced phase transitions of amorphous silicon-germanium alloy (a-Si x Ge1−x , with x=0.75) using a membrane diamond anvil cell as the pressure device.

Influence of Hydrogen on the Germanium Incorporation in a-Si1-xGex:H for Thin-film Solar Cell Application

AbstractIn this work, we examined the Ge incorporation and the accompanied defect formation during PECVD deposition of hydrogenated amorphous silicon-germanium alloys (a-Si1-xGex:H). In particular, we studied the effect of hydrogen on film growth, defect formation, Ge and Si incorporation efficiencies, and the H-bonding configuration. Our results indicate that hydrogen has a strong effect on improving the a-Si1-xGex:H film quality and the Ge incorporation in a-Si1-xGex:H. With adequate hydrogen dilution, the a-Si1-xGex:H thin-film quality significantly improved. However, excessive hydrogen dilution degraded the film properties. A number of analytical tools were employed, including FTIR, XPS, UV-Visible spectroscopy, photoconductivity, etc. The a-Si1-xGex:H material having 24% Ge content and a bangap of 1.61ev produced the solar cell with a conversion efficiency of 7.07%.

Synthesis of amorphous Si–Ge alloys using microwave energy

Amorphous silicon–germanium alloys with 70% Si and 30% Ge have been formed by rapid heating in a pure E-field in a single mode 2.45 GHz microwave cavity. The synthesis of amorphous Si–Ge alloy by all conventional processing is difficult. This polarized microwave process provides another striking example of its unique abilities: producing bulk amorphous Si–Ge alloys. The microwave fabricated amorphous Si0.7Ge0.3 alloy composition exhibited relatively high Seebeck coefficient with high potential of its application in high efficient thermoelectric materials.

A direct correlation between film structure and solar cell efficiency for HWCVD amorphous silicon germanium alloys

The film structure and H bonding of high deposition rate a-SiGe:H i-layers, deposited by HWCVD and containing ~ 40 at.% Ge, have been investigated using deposition conditions which replicate those used in n–i–p solar cell devices. Increasing the germane source gas depletion in HWCVD causes not only a decrease in solar cell efficiency from 8.64% to less than 7.0%, but also an increase in both the i-layer H preferential attachment ratio (PA) and the film microstructure fraction ( R⁎). Measurements of the XRD medium range order over a wide range of germane depletion indicate that this order is already optimum for the HWCVD i-layers, suggesting that energetic bombardment of a-SiGe:H films may not always be necessary to achieve well ordered films. Preliminary structural comparisons are also made between HWCVD and PECVD device layers.

Light trapping in hydrogenated amorphous and nano-crystalline silicon thin film solar cells

AbstractLight trapping effect in hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) and nano-crystalline silicon (nc-Si:H) thin film solar cells deposited on stainless steel substrates with various back reflectors is reviewed. Structural and optical properties of the Ag/ZnO back reflectors are systematically characterized and correlated to solar cell performance, especially the enhancement in photocurrent. The light trapping method used in our current production lines employing an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure consists of a bi-layer of Al/ZnO back reflector with relatively thin Al and ZnO layers. Such Al/ZnO back reflectors enhance the short-circuit current density, Jsc, by ˜20% compared to bare stainless steel. In the laboratory, we use Ag/ZnO back reflector for higher Jsc and efficiency. The gain in Jsc is about ˜30% for an a-SiGe:H single-junction cell used in the bottom cell of a multi-junction structure. In recent years, we have also worked on the optimization of Ag/ZnO back reflectors for nano-crystalline silicon (nc-Si:H) solar cells. We have carried out a systematic study on the effect of texture for Ag and ZnO. We found that for a thin ZnO layer, a textured Ag layer is necessary to increase Jsc, even though the parasitic loss is higher at the Ag and ZnO interface due to the textured Ag. However, a flat Ag can be used for a thick ZnO to reduce the parasitic loss, while the light scattering is provided by the textured ZnO. The gain in Jsc for nc-Si:H solar cells on Ag/ZnO back reflectors is in the range of ˜60-75% compared to cells deposited on bare stainless steel, which is much larger than the enhancement observed for a-SiGe:H cells. The highest total current density achieved in an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure on Ag/ZnO back reflector is 28.6 mA/cm2, while it is 26.9 mA/cm2 for a high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cell.

Structural and optoelectronic properties of silicon germanium alloy thin films deposited by pulsed radio frequency plasma enhanced chemical vapor deposition

Hydrogenated amorphous silicon germanium (a-SiGe:H) alloy films are still under study to improve their incorporation into tandem solar cells. In this paper we have investigated the quality of films deposited in a radio frequency powered plasma enhanced chemical vapor deposition unit. Two series of samples were prepared either from a mixture of silane and germane diluted into hydrogen or from the same mixture to which a small amount of argon was added. The applied rf power at 13.56 MHz was either continuous or modulated by a square wave pulse at a frequency of 1356 kHz. Different films were deposited in the two series controlling the time the plasma was “on.” The ratio of the on time to the total period of the modulation, the duty cycle, was varied between 100% (continuous mode) and 50%. The plasma during deposition as well as structural and optoelectronic properties of the resulting films was studied with several techniques. We found an optimum in the transport properties for a duty cycle of 75%: an ambipolar diffusion length of the order of 100 nm for a material presenting a bandgap of 1.44 eV. To explain this promising result, we present a discussion on the growth mechanisms of such layers linking the structural results to the transport data.

Junction capacitance study of an oxygen impurity defect exhibiting configuration relaxation in amorphous silicon–germanium alloys deposited by hot-wire CVD

We report the observation of light induced electron capture in oxygen contaminated (∼5×1020cm−3) hydrogenated amorphous silicon–germanium alloys grown by hot-wire chemical vapor deposition (HWCVD). By examining the time evolution of dark capacitance after 1.2eV photoexcitation, we are able to estimate the free energy barrier (⩾0.8eV) for the release of electrons into the conduction band. Such a large thermal barrier, for a defect whose optical threshold is centered (∼1.35eV) so close to the band-gap (1.5eV), indicates significant configurational relaxation once the oxygen impurity state is occupied with photoexcited electrons.

High diffusion length silicon germanium alloy thin films deposited by pulsed rf PECVD method

Hydrogenated amorphous silicon germanium (a-Si1−xGex:H) alloy thin films were deposited in a rf (13.56MHz) powered plasma CVD system from a mixture of silane, germane and hydrogen. The rf power was pulsed at a frequency of 1356Hz with the duty cycle (Q) ranging from 50% to 100%. The Ge content in the films, found in the range 40–45%, was deduced from Raman spectra. The band gaps Eg of the films were in between 1.44 and 1.48eV. We find a good correlation between the ambipolar diffusion length (Ld) and the microstructure factor (R∗) for different duty cycles. At Q=75% the R∗ value is lowest (0.14) which correlates well with the maximum value of Ld (100nm), the highest Ld value reported for a-Si1−xGex:H films with Eg = 1.44eV. It was also found to be very stable under light soaking. Powder incorporation within the films was also studied. Quality of the films is determined by two opposing effects viz. size of the powders incorporated into the films from the plasma and the bombardment of energetic ions and neutrals. Best transport properties were found at an optimum duty cycle of 75%.

Two-Step Capacitance Transients From an Oxygen Impurity Defect

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.

Dependence of the Electronic Properties of Hot-Wire CVD Amorphous Silicon-Germanium Alloys on Oxygen Impurity Levels

AbstractWe report the effects of intentionally introducing up to ∼ 5×1020/cm3 oxygen impurities into hydrogenated amorphous silicon-germanium alloys (of roughly 30at.% Ge) grown by the hot-wire chemical vapor deposition (HWCVD) method. Deep defect densities determined by drive-level capacitance profiling (DLCP) indicated a modest increase with increasing oxygen content (up to a factor of 3 at the highest oxygen level). Transient photocapacitance (TPC) spectra indicated a clear spectral signature for an optical transition between the valence band and an additional defect level which is attributed to oxygen impurities. The oxygen impurity related defect transition has an optical threshold around 1.4eV above the valence band and also results in a negative contribution to the TPC signal. This initially led us to believe that the bandtail for the higher oxygen samples was much narrower than it actually is. Surprisingly, this additional oxygen related defect level appears to have only a very minor effect upon the estimated minority carrier collection fraction. The effects of light-induced degradation upon some of these oxygen contaminated samples were also examined. We infer the existence of a significant thermal barrier to explain the observed spectral signatures of this oxygen impurity defect.