Hydrogenated Amorphous Silicon Germanium Research Articles

The Electrical Characterization of Hydrogenated Amorphous Silicon Germanium Film Used in Un-Cooled Micro-Bolometer

In this paper, the deposition and the electrical characterization of hydrogenated amorphous silicon germanium (a-SixGey:H) thin films were performed by plasma enhanced chemical vapour deposition (PECVD) at low temperature with different flow ratios of SiH4/GeH4. The temperature coefficient of resistance (TCR) and temperature dependence of conductivity were measured to study the influence of deposition parameter. The resistance uniformity were also investigated. The result showed that the film presented high TCR values of around 3.5%K-1and moderate conductivity value of 1.47×10-3(Ω•cm)-1respectively at room temperature, while the non-uniformity below 5% which indicated the high resistance uniformity in films.

Reduction in Photocurrent Loss and Improvement in Performance of Single Junction Solar Cell Due to Multistep Grading of Hydrogenated Amorphous Silicon Germanium Active Layer

Single junction solar cells were fabricated with intrinsic hydrogenated amorphous silicon germanium (a-SiGe:H) as the active layer, that shows a 10% photovoltaic conversion efficiency. The a-SiGe:H active layer of the solar cells of type-A had constant band gap materials while that of type-B had a four step graded band gap by composition gradient (CG). The cells with composition gradient show an enhancement in fill factor and open circuit voltage (V oc) by 5% and 20 mV respectively, with respect to a cell without the graded band gap active layer. Such an enhanced device performance is attributed to reduction in recombination loss of photo-generated electron hole pairs. The effect of this photo-current loss was investigated in the electrical bias dependent external quantum efficiencies (EQE), illumination dependent current-voltage measurements and dark current-voltage characteristics. The device parameters like reverse saturation current density (J o), series resistance (R s) and diode quality factor (n) were also estimated. In comparison to the cell without the composition gradient, the EQE shows a reduced recombination loss across the whole wavelength range for the cell with the CG. Furthermore, the introduction of the CG results in a significantly increased shunt resistance from 720 to 1200 Ω.cm 2. The estimated n values of the cells under dark operating condition, decreases from 1.8 to 1.7 along with J o from 3 × 10 −7 down to 4.5 × 10 −8 A/cm 2 with CG, while the same parameters decreased from 3.73 to 3.06, 2.96 × 10 −6 to 3.05 × 10 −7 A/cm 2 respectively under AM1.5G insolation.

Hydrogenated Amorphous Silicon Germanium Active Layer for Top Cell of a Multi Junction Cell Structure.

Intrinsic hydrogenated amorphous silicon-germanium (a-SiGe:H) alloy is generally used in the bottom cell because of its low band gap. The a-SiGe:H has a higher photo conductivity in comparison to the a-Si:H; thus, it is expected that the a-SiGe:H can show better short circuit current density than that of the a-Si:H based solar cell. Therefore, we optimized a-SiGe:H active layer that can be a suitable choice for the front cell of a multi junction.solar cell. Furthermore, we carried out a comparative study of the solar cells that have a-SiGe:H and a-Si:H as respective active layers. The a-SiGe:H based solar cells show higher short circuit current density, while the a-Si:H based cells show higheropen circuit voltage. The current-voltage characteristics of these cells are as follows: (a) V(oc) = 770 mV, J(sc) = 15.0 mA/cm2, FF = 64.5%, and η = 7.47% for a-SiGe:H based cell; and (b) V(oc) = 826 mV, J(sc) = 13.63 mA/cm2, FF = 72.0%, and η = 8.1% for a-Si:H based cell.

Modulation Doped Hydrogenated Amorphous Silicon Germanium Superlattice Contacts and Application to Optoelectronic Devices

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Graded Layer Modification for High Efficiency Hydrogenated Amorphous Silicon–Germanium Solar Cells

Hydrogenated amorphous silicon–germanium (a-SiGe:H) solar cells are fabricated with different thicknesses of the i/n graded layer and profiling shapes for appropriate band gap profiling. Comparison of the solar parameters between the U-shape profiling and the exponential shape (E-shape) profiling has been carried out at the same total thickness. In the U-shape profiling, as the thickness of the i/n graded layer increase, the fill factor (FF) and open circuit voltage (Voc) of p–i–n single-junction a-SiGe:H solar cells increase, but the short circuit current (Jsc) of cells decreases. In the E-shape profiling, the Jsc of the a-SiGe:H cell is enhanced without significant losses in Voc. For further analysis, a modified E-shape profiling is incorporated in a-Si:H/a-SiGe:H double-junction cells, which has resulted in the improvement of Voc and FF of double-junction cells to 1.67 V and 0.753, respectively, without significant reduction in JscSiGeQE, 12.58 mA/cm2.

Graded Layer Modification for High Efficiency Hydrogenated Amorphous Silicon–Germanium Solar Cells

Hydrogenated amorphous silicon–germanium (a-SiGe:H) solar cells are fabricated with different thicknesses of the i/n graded layer and profiling shapes for appropriate band gap profiling. Comparison of the solar parameters between the U-shape profiling and the exponential shape (E-shape) profiling has been carried out at the same total thickness. In the U-shape profiling, as the thickness of the i/n graded layer increase, the fill factor (FF) and open circuit voltage (Voc) of p–i–n single-junction a-SiGe:H solar cells increase, but the short circuit current (Jsc) of cells decreases. In the E-shape profiling, the Jsc of the a-SiGe:H cell is enhanced without significant losses in Voc. For further analysis, a modified E-shape profiling is incorporated in a-Si:H/a-SiGe:H double-junction cells, which has resulted in the improvement of Voc and FF of double-junction cells to 1.67 V and 0.753, respectively, without significant reduction in JscSiGeQE, 12.58 mA/cm2.

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

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.

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.

Investigation of the Material Properties of Hydrogenated Amorphous Silicon and Hydrogenated Amorphous Silicon–Germanium for Thin Film Photovoltaic Cells

A photovoltaic device with hydrogenated amorphous silicon (a-Si:H) and silicon―germanium (a-SiGe:H) in its intrinsic layer deposited using plasma-enhanced chemical vapor deposition (PECVD) was characterized, and the material properties of the intrinsic layer was investigated. An energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and atomic force microscope (AFM) were utilized to study the material composition, crystallinity and surface roughness layers in the intrinsic region, respectively. The surface was amorphous and smooth in a small scan area. We also measured the J―V characteristic of the photovoltaic cell using a solar simulator under standard AM 1.5G conditions, and the efficiency was more than 1%. The results demonstrated the feasibility of the photovoltaic cell.

The I-V Characteristics and Optical Properties of Hydrogenated Amorphous Silicon Germanium p-i-n Solar Cells

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High-Rate Deposition of High-Quality Hydrogenated Amorphous Silicon Germanium Using Very High-Frequency Plasma-Enhanced Chemical Vapor Deposition with a High Hydrogen Dilution

Ways of improving the deposition rate of hydrogenated amorphous silicon germanium (a-SiGe:H) while maintaining its film quality, using the very high-frequency plasma-enhanced chemical vapor deposition (VHF-CVD) technique, were investigated. It was concluded that the VHF-CVD technique with a high hydrogen dilution of silane and germane is suitable for high-quality a-SiGe:H fabrication at a relatively high deposition rate. The reason why the high deposition rate is compatible with high quality is related to the high density of hydrogen radicals, which are proper to the VHF-CVD technique. The low plasma potential of VHF plasma contributes to high-quality a-SiGe:H film deposition by suppressing ion bombardment and excessive penetration of hydrogen atoms into the film. In device fabrication, the VHF-CVD technique is also useful because a high deposition rate with low ion bombardment may suppress negative effects to the underlying layers during deposition of the a-SiGe:H photovoltaic layer. This investigation showed that almost the same initial and stabilized efficiency for an a-Si/a-SiGe tandem solar cell was obtained at a five times higher deposition rate for the a-SiGe:H photovoltaic layer than those in conventional RF-CVD methods using the same reaction chamber.

Femtosecond Far-Infrared Studies of Carrier Dynamics in Hydrogenated Amorphous Silicon and Silicon-Germanium Alloys

AbstractWe present femtosecond time-resolved studies of the photoexcited carrier response in the far-infrared spectral range in PECVD a-Si:H and a-SiGe:H thin films. The experiments are carried out using an optical pump / terahertz (THz) probe technique, in which a femtosecond pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in the far-infrared optical properties as a function of time delay following the excitation. These measurements are sensitive to carrier processes at low energy, corresponding to a range of approximately 1 - 10 meV, a key energy scale in these materials. We find that the observed photoexcited carrier dynamics are consistent with trapping of carriers into band tail states on a picosecond time scale.

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.

Investigation of Hydrogenated Amorphous Silicon Germanium Fabricated under High Hydrogen Dilution and Low Deposition Temperature Conditions for Stable Solar Cells

The effects of hydrogen dilution of up to 54:1 (=H2:SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) were investigated while keeping the optical gap (Eopt) constant. It was found that deterioration of the film properties of a-SiGe:H due to a decrease in substrate temperature (Ts) can be compensated by the high hydrogen dilution method. As Ts decreases from 230°C to 180°C, the high photoconductivity [∼1×10-5 (Ω·cm)-1] and low silicon dihydride content (∼1 at.%) of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties deteriorate with the conventional low hydrogen dilution ratio of 2.5:1. Probably, hydrogen radicals supply the energy required for the surface reaction during a-SiGe:H deposition which is lost when Ts is decreased. This tendency is useful for solar cell fabrication, especially for superstrate-type a-Si/a-SiGe tandem solar cells, because the decrease in the deposition temperature of a-SiGe:H for the bottom photovoltaic layer can reduce damage to the underlying layers caused by a high deposition temperature. As a result of applying this technique to the fabrication process of an a-Si/a-SiGe stacked solar cell submodule (area: 1200 cm2), the world's highest stabilized efficiency of 9.5% (light-soaked and measured at JQA) was achieved.

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}

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.

Hydrogenated Amorphous Silicon Germanium Alloy for Stable Solar Cells

ABSTRACTThe film properties and solar cell performance of a-SiGe:H samples with the same optical gap and different combinations of hydrogen content (CH) and germanium content (CGe) have been compared. The optimum composition for the initial properties, such as the tail characteristic energy, defect density and conversion efficiency of the solar cell, was determined, and the differences could be explained by the difference in H bonding configuration. The degradation ratio of the conversion efficiency becomes larger in higher CH samples. This suggests that hydrogen or Si-H2 participates in light-induced degradation. As a result, the optimum CH for an efficient solar cell is believed to shift to the lower CH region after light soaking. Based on these findings, the stabilized conversion efficiency of 3.3% under red light (γ>650nm) for an a-SiGe:H single-junction solar cell (1cm2) and 10.6% under lsun light for an a-Si/a-SiGe double-junction stacked solar cell (1cm2) have been achieved. The degradation ratio is only 8.6% for the double-junction solar cell.

Film Property Control of Hydrogenated Amorphous Silicon Germanium for Solar Cells

The optoelectric properties of a-SiGe:H alloys, deposited by the plasma chemical vapor deposition (plasma-CVD) method, were investigated with precise measurement of their germanium content (CGe) and hydrogen content (CH). This investigation revealed that the optical gap of a-SiGe:H alloys can be approximated by a linear function of CH and CGe and various combinations of CH and CGe resulted in identical optical gaps. For each optical gap, the optimum composition for the lowest defect density was derived by comparison with the subgap absorptions measured by the constant photocurrent method (CPM). Based on these, the highest conversion efficiency of 3.7% under red light illumination (>650 nm) for a 1 cm2 a-SiGe single-junction solar cell was achieved.

Thermal Equilibration Between Band Tail and Near Surface Defect States in Hydrogenated Amorphous Silicon and Silicon-Germanium Alloys

ABSTRACTWe employ total yield photoelectron spectroscopy to measure the density of occupied states at the clean a-SixGe1_x:H alloy surface. The near surface defect states are observed to lie at 0.57 eV above the valence band edge with a density of 4×l017 cm−3, independent of Ge content. The valence band tail characteristic energy is also measured to be independent of alloy composition with an average value of 54 meV. We demonstrate that thermodynamic equilibrium at the surface between weak bonds (forming the valence band tail) and the dangling bonds provides an excellent description of the experimental data and explains why the surface state density in a-Si:H cannot be lowered below the 1011 to 1012 cm−2 range.

Band Tails and Thermal Disorder in Doped and Undoped Hydrogenated Amorphous Silicon and Silicon-Germanium Alloys

ABSTRACTThe energy distribution and temperature dependence of the conduction and valence band tail density of states in a-Si:H and a-Si,Ge:H alloys is determined via total yield photoelectron spectroscopy. All films are observed to possess purely exponential conduction and valence band tail densities of states; however, the characteristic energy of the conduction band tail increases much more rapidly with temperature in the range of 300K to 550K than that of the valence band tail. This indicates that over that temperature range the conduction band tail is considerably more susceptible to thermal disorder than to structural disorder whereas the reverse holds for the valence band tail.