Amorphous Silicon-germanium Alloys Research Articles

Ultrafast Dynamics of Photoexcitations in HWCVD Hydrogenated Amorphous Silicon Alloys

ABSTRACTWe present femtosecond studies of carrier dynamics in hydrogenated amorphous silicon-germanium alloys grown by the recently developed hot-wire assisted chemical vapor deposition (HWCVD) technique, which is promising for producing high-quality device-grade materials. We have used wavelength-resolved femtosecond pump-probe techniques, in which an intense pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in optical properties as a function of time delay following the pump pulse, to study the dynamics of photoexcitations in these materials. Femtosecond dynamics measurements have been carried out on thin film samples under experimental conditions with varying sensitivity to carriers in extended states or in band tail states. The relaxation dynamics of carriers associated with extended states show a strong dependence on excitation density and follow a bimolecular recombination law, consistent with a number of earlier studies on related amorphous materials. In contrast, measurements involving carriers excited directly into band tail states reveal significantly altered dynamics, characterized by a marked deviation from simple bimolecular recombination at short times.

A Critical Test of Defect Creation Models in Hydrogenated Amorphous Silicon Alloys

ABSTRACTUsing the modulated photocurrent method we studied the deep defect creation and annealing kinetics of amorphous silicon-germanium alloys with Ge fractions below 10at.%. The modulated photocurrent spectroscopy clearly discloses the existence of two distinct bands of majority carrier traps in these alloys. The bands were identified as neutral Si dangling bonds and neutral Ge dangling bonds. Our studies show clearly that the Si and Ge defects directly compete with each other during annealing, implying a global reconfiguration mechanism. The creation kinetics reveal the usual t1/3 illumination time dependence for the total deep defect density. However, the individual densities of Si and Ge defects have different time dependencies. The details of the creation and annealing kinetics of Ge and Si defects are used to test predictions of certain defect creation models.

Composition Dependence of Inhomogeneous Hydrogen Bonding Structures in a-SiGe:H

Local hydrogen bonding configurations in hydrogenated amorphous silicon germanium (a-SiGe:H) alloys fabricated by rf plasma chemical vapor deposition were systematically investigated by means of IR absorption spectroscopy with relating to the compositions. The H bonding configurations around Si in a-SiGe:H are quantitatively equivalent to those in a-Si:H. It was suggested that the deteriorating alloying property is partly due to the increase in the Si–H2 content with increasing H content bonding to Si per Si atom (CH(Si)). The CH(Si) has positive correlations to both the H content (CH) and Ge content (CGe), while the H content bonding to Ge per Ge atom (CH(Ge)) is almost constant at 2–4 at.%. As a result, the preference of H attachment to Si over Ge has positive correlations to both CH and CGe. The origin of the composition dependence of the preferential attachment is attributable to the H-elimination kinetics during film growth.

Use of A GAS Jet Technique to Prepare Microcrystalline Silicon Based Solar Cells at High I-Layer Deposition Rates

AbstractUsing a Gas Jet thin film deposition technique, microcrystalline silicon (μc-Si) materials were prepared at rates as high as 15-20 Å/s. The technique involves the use of a gas jet flow that is subjected to a high intensity microwave source. The quality of the material has been optimized through the variation of a number of deposition conditions including the substrate temperature, the gas flows, and the applied microwave power. The best films were made using deposition rates near 16 Å/s. These materials have been used as i-layers for red light absorbing, nip single-junction solar cells. Using a 610nm cutoff filter which only allows red light to strike the device, pre-light soaked currents as high as 10 mA/cm2 and 2.2-2.3% red-light pre-light soaked peak power outputs have been obtained for cells with i-layer thicknesses near 1 micron. This compares with currents of 10-11 mA/cm2 and 4% initial red-light peak power outputs obtained for high efficiency amorphous silicon germanium alloy (a-SiGe:H) devices. The AM1.5 white light efficiencies for these microcrystalline cells are 5.9-6.0%. While the efficiencies for the a-SiGe:H cells degrade by 15-20% after long term light exposure, the efficiencies for the microcrystalline cells before and after prolonged light exposure are similar, within measurement error. Considering these results, the Gas Jet deposition method is a promising technique for the deposition of μc-Si solar cells due to the ability to achieve reasonable stable efficiencies for cells at i-layer deposition rates (16 Å/s) which make large-scale production economically feasible.

Simulation of Hydrogenated Amorphous Silicon Germanium Alloys for Bandgap Grading

AbstractComputer simulations are reported of hydrogenated amorphous silicon germanium (a-SiGe:H) layers that make up the graded part of the intrinsic layer near the interfaces of a-SiGe:H solar cells. Therefore the graded part is approached with a ‘staircase’ bandgap profile, consisting of three layers within which the material properties are constant. Calibrated model parameters are obtained by matching simulation results of material properties of intrinsic a-SiGe:H single layers to measurements. Using the obtained model parameter sets subsequent simulations of p-i-n devices with intrinsic material similar to the single layers are matched to measured current-voltage characteristics. The changes in parameter values are evaluated as a function of optical gap.

Composition dependence of surface morphology of ultrathin a-SiGe:H alloys studying by atomic force microscopy

Atomic force microscopy (AFM) has been employed to observe the top surface morphology of hydrogenated amorphous silicon–germanium alloys at the initial stage of deposition. It shows that the different growth mode is dominant for a sample with low and high germanium content, which strongly influences the structures and the photoelectric properties of the corresponding bulk films. The samples with hydrogen plasma annealing are also investigated by AFM in order to understand the effect of hydrogen annealing on the structure of a-SiGe:H films.

Nanostructured Ge distribution in a-SiGe : H alloys from anomalous small-angle X-ray scattering studies

The nanostructures of hydrogenated amorphous silicon germanium alloys, a-Si1−xGex:H (x∼0.5 to 0.7), prepared by different plasma enhanced chemical vapor deposition techniques, are examined by anomalous small-angle X-ray scattering (ASAXS). Clear evidence of inhomogeneous distributions of Ge are found with correlation lengths of 1.0 to 1.4nm. This non-uniformity is removed by enhanced ion bombardment during growth and it is reduced by hydrogen dilution of the plasma. These more homogeneous materials result in improved photovoltaic and opto-electronic performance. The anisotropic ASAXS from the Ge is consistent with Ge enrichment along columnar-like nanostructures.

Structural, optical, and electrical properties of hydrogenated amorphous silicon germanium alloys

Hydrogenated amorphous silicon germanium alloys (a-SiGe:H) have been prepared by rf glow discharge of silane, germane, and hydrogen gas mixture at substrate temperature of 200 and 250 °C. The structural properties of the films have been investigated by infrared, Raman, and secondary ion mass spectroscopy. It is found that there is a preferential incorporation of germanium into the film relative to silicon and the films with high germane gas phase composition Xg>0.4 tend to oxidize in atmosphere. Besides, polysilane is enhanced in the films with low germane gas phase composition. The electrical properties including dark, photo conductivities, and conduction activation energy are measured. As for the optical properties, optical transmission is adopted to determine the optical gap while photoluminescence spectra together with temperature variation are used to study the band tail states of the films. By applying Brodsky’s quantum well model, the various optical and electrical properties could be explained successfully.

Evidence for charged defects in intrinsic glow-discharge hydrogenated amorphous-silicon–germanium alloys

We have applied drive-level capacitance profiling, transient photocapacitance, and junction transient photocurrent measurements to characterize the defect-state distribution for a set of device-quality glow-discharge a-Si,Ge:H films. The combination of the latter two methods can distinguish majority- from minority-carrier optical transitions. Comparing the optical spectra of intrinsic samples with those in p-type and n-type samples, we have concluded that significant densities of positively and negatively charged deep defects exist in intrinsic glow-discharge a-Si,Ge:H alloys. Our measurements also indicate how the density of these charged defects increase upon light-induced degradation and how they affect carrier recombination processes. {copyright} {ital 1998} {ital The American Physical Society}

Gap-State Distributions in Hydrogenated Amorphous Silicon-Germanium Evaluated Using Capacitance–Voltage Method

The density of gap states in hydrogenated amorphous silicon-germanium (a-SiGe:H) alloys deposited at temperatures of 150–250°C has been evaluated by using a capacitance–voltage (C–V ) method. It was found that the gap-state distribution for a-SiGe:H deposited at 150°C has a broad bump around 0.6 eV below the conduction band edge Ec which is associated with Ge dangling bonds. In contrast, a 250°C-deposited film exhibits a peak density of 8×1018 cm-3·eV-1 around 0.75 eV below the conduction band edge Ec, which might originate from Ge and/or Si dangling bond states. After AM1 (200 mW/cm2) light soaking, the gap-state density above the midgap is increased for films deposited below 200°C, while at 250°C deposition, defect states near the midgap are increased.

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

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.

The Electronic Structure, Metastability and Transport Properties of Optimized Amorphous Silicon-Germanium Alloys

ABSTRACTWe have carried out a comprehensive study of the electronic properties of two series of optimized a-Si1−xGex:H alloys fabricated at United Solar Systems Corporation (“Uni-Solar”) and Harvard University, encompassing the composition range 0.2 ≤ × ≤ 1.0. Both series of samples exhibit deep defect densities that obey quite accurately the spontaneous bond-breaking model proposed by M. Stutzmann which, by considering how the defect formation energy varies with the position of Fermi energy, we have been able to extend to doped samples.We have also extended our studies to include measurements of ambipolar diffusion lengths and the effects of light-induced degradation, and thus have been able to demonstrate a direct relation between these transport properties and the measured defect levels before and after degradation.

Triple-junction amorphous silicon alloy solar cell with 14.6% initial and 13.0% stable conversion efficiencies

We have achieved 14.6% initial and 13.0% stable conversion efficiencies using an amorphous silicon-based alloy in a spectrum-splitting, triple-junction structure. These efficiencies have been confirmed independently by the National Renewable Energy Laboratory. Key factors leading to this major advance include improvements made in the low band-gap amorphous silicon–germanium alloy cell, the pn tunnel junction between the component cells, and the top conducting oxide.

Optimization of light-induced metastable changes in hydrogenated amorphous silicon-germanium alloys

Abstract Light-induced degradation of hydrogenated amorphous silicon-germanium (a-Si x Ge1-x: H) alloys, prepared in an ultrahigh-vacuum (about 10−10 Torr) plasma-enhanced chemical vapour deposition system having load-lock facility, has been studied. Metastable changes under prolonged light soaking can be controlled by varying the different deposition parameters such as the rf power density and the flow rates of diluent and sources gases. Appropriate choice of deposition parameters can reduce the bonded hydrogen content, microvoid fraction and heterogeneity of a-Six Ge1-x: H alloys and hence increase the stability of the materials. Light-induced degradation of electronic properties of a-SixGe1-x: H alloy films can be correlated with the p-i-n a-SixGe1-x, solar cells to some extent. However, discrepancies between the degradation of material and cell have also been observed. After light soaking for 100 h, a-SixGe1-x: H films, prepared at Hf= 30 sccm and Pw = 30 mW cm−2 degrades slowly while the internal qua...

Low defect density amorphous silicon germanium alloy (1.5 eV) deposited at high growth rate under helium dilution in RF-PECVD method

Optimized deposition parameters for helium diluted a-SiGe: H material differ from those for hydrogen diluted a-SiGe: H material with the same optical gap (∼ 1.5 eV). Deposition rate of optimized helium diluted film is 3.3 times greater than that of hydrogen diluted film, however the photoconductivities of both type of materials are the same. Defect density (6.72 ± 1 × 10 15 cm −3 eV −1) and Urbach energy (46 ± 1.5 meV) of helium diluted optimized film is less than corresponding values (8.45 ± 1 × 10 15 cm −3 eV −1, 49 ± 1.5 meV) of hydrogen diluted alloy materials. Schottky barrier solar cells have been fabricated using both optimized materials. Blue response is greater for the optimized helium diluted a-SiGe:H material than that of optimized hydrogen diluted alloy film. Red response is also greater in the first case in comparison to that in the second case.

High-gain p-i-n infrared photosensors with Bragg reflectors on amorphous silicon–germanium alloy

An amorphous silicon–germanium alloy (α-SiGe:H) infrared photosensor with a Bragg reflector to obtain high optical gain and responsivity is demonstrated. The Bragg reflector containing α-Si:H and α-SiGe:H layers was grown on the top of p-i-n structure with an amorphous silicon/germanium alloy. All of the amorphous silicon and amorphous silicon–germanium layers were deposited by a low-temperature plasma enhanced chemical vapor phase deposition system. The experimental results of the new structures exhibit a much superior performance to that of conventional p-i-n avalanche photosensor structures. That is, the structure with a Bragg reflector shows a significant improvement in optical gain from 80 to 328 under the incident optical power of 1 μW, and the full width at half maximum can be reduced from 250 to 150 nm.

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

Light-induced metastable defect creation has been studied in amorphous silicon germanium alloy (a-SiGe:H) materials prepared by using helium or hydrogen dilution in plasma enhanced chemical vapour deposition. From the study of comparison of light-induced degradation in electronic properties and defect density of helium-diluted alloy films with those of hydrogen-diluted a-SiGe:H, it has been revealed that not only the optical gap but also the microstructure of alloy films is responsible for light-induced degradation. Since helium-diluted alloy materials have a compact network and improved microstructure, they are more stable than hydrogen-diluted a-SiGe:H with the same optical gap. The temperature dependence of the mobility - lifetime product in the annealed state and the light-soaked state also shows that the creation of light-induced recombination centres in helium-diluted a-SiGe:H is lower than that in hydrogen-diluted alloy films.

An Amorphous Silicon Alloy Triple-Junction Solar Cell with 14.6% Initial and 13.0% Stable Efficiencies

ABSTRACTAn initial conversion efficiency of 14.6% has been achieved using amorphous silicon-based alloy in a spectrum splitting triple-junction structure. After 1000 hours of indoor one-sun light soaking at 50 °C, the stabilized efficiency is 13.0%. Both efficiencies are the highest reported to date for amorphous silicon alloy solar cells and have been independently confirmed by the National Renewable Energy Laboratory. The device was deposited onto a stainless steel substrate coated with textured silver/zinc oxide back reflector. The bottom and middle cells use amorphous silicon-germanium alloys, employing high hydrogen dilution in the gas mixture and bandgap profiling in the cell design. The top cell uses amorphous silicon alloy with high hydrogen dilution. Key factors leading to the achievement include a) improvement of the bottom cell that exhibits an AM1.5 efficiency of 10.4% and quantum efficiency of 45% at 850 nm; b) improvement of the tunnel junctions between the component cells by incorporating a novel multilayered structure with microcrystalline p and n layers; and c) improvement of transparent conductive oxide for enhancing the short wavelength response of the top cell.

In-Situ Monitoring of Surface Hydrogen on the a-SiGe:H Films

ABSTRACTThe bonded hydrogen on the growing surface of hydrogenated amorphous silicon germanium (a-SiGe:H) alloy films has been investigated by use of infrared reflection absorption spectroscopy (IR-RAS). When the alloy films are Si-rich, the surface hydrogen bonded to Si atoms is found to behave in a similar way to those on the hydrogenated amorphous silicon (a-Si:H) films. This means that the thermal desorption stability of surface Si hydride species is not significantly affected by the coexistence of a small amount (typically 20 at.%) of Ge. On the contrary, the desorption behavior of surface hydrogen depends on the alloy composition when the a-SiGe:H films are Ge-rich. A surface reaction scheme is provided in an attempt to explain this series of behavior in surface hydrogen on the a-SiGe:H films.

An integrated PIN/MISS OEIC for high current photoreceiver applications

A new PIN/MISS photoreceiver with very high output current has been developed by using the combination of an amorphous silicon germanium alloy PIN photodiode and a metal-insulator-semiconductor switch (MISS) device. The developed photoreceiver uses a PIN photodiode as the light absorption structure and light wavelength selector and the MISS device as an actuator to drive an electronic load. Based on the experimental results, the photoreceiver yields a very high output current of 3.2 mA at a voltage bias of 6 V under an incident light power of P/sub in/=100 /spl mu/W, and has a rise time of 465 /spl mu/s with a load resistance of R=1 k/spl Omega/. The peak response wavelength of the PIN photodiode is at 905 nm, i.e., infrared light. Thus the high output current PIN/MISS photoreceiver is a good candidate for some specific applications.