Amorphous Silicon-germanium Alloys Research Articles

Structural, optical and electrical properties of hydrogenated amorphous silicon germanium (a-Si 1−xGe x) deposited by DC magnetron sputtering at high rate

Hydrogenated amorphous silicon germanium alloys (a-Si 1− x Ge x :H) thin films are prepared using DC magnetron sputtering method. The films are deposited under optimized preparation conditions of hydrogen and argon partial pressures at substrate temperature of 150 °C. The relative germanium concentration x was varied between 0 and 0.6. Their structural, optical and electrical properties are investigated. The structural properties of the films are analyzed from the infrared absorption. The hydrogen bond concentration in the films is then deduced using the absorption peaks of stretching mode. The optical band gap E g of the films determined from the optical transmission measurements decreases uniformly from 2.1 to 1.35 eV when x varies from 0 to 0.6. The conductivity and the photoconductivity measurements are reported. We have observed the increasing of both dark and photo-conductivity when germanium content x increases. All the investigated films properties show a coherent variation with the germanium content.

Superior structural and electronic properties for amorphous silicon–germanium alloys deposited by a low temperature hot wire chemical vapor deposition process

Very good electronic properties of hot-wire CVD a-Si,Ge:H alloys have been established by junction capacitance methods. The samples were deposited using a tantalum filament maintained at about 1800°C instead of the usual 2000°C tungsten filament process. Urbach energies below 45meV were found, as well as annealed defect densities below 1016cm−3, for Ge fractions up to 30at.%. However, samples with 1019cm−3 levels of oxygen exhibited much broader Urbach energies and higher defect densities. Light induced degradation was examined in detail for one a-Si,Ge:H alloy sample and compared to the behavior of PECVD grown a-Si:H alloys of similar optical gap.

Temperature induced differences in the nanostructure of hot-wire deposited silicon-germanium alloys analyzed by anomalous small-angle x-ray scattering

The nanostructure of hydrogenated amorphous silicon-germanium alloys, a-Si1−xGex:H (x=0.62–0.70), prepared by the hot-wire deposition technique applying different substrate and filament temperatures was analyzed by anomalous small-angle x-ray scattering experiments. The pure-resonant scattering contribution, which is related to the structural distribution of the Ge component in the alloy, was separated from the total small-angle scattering for one sample series. For all alloys the Ge component was found to be inhomogeneously distributed. The shape of the pure-resonant and the mixed-resonant scattering curves reveal significant differences indicating the presence of a third phase, probably hydrogen clusters and/or voids. The thin films showed improved microstructure when lowering the filament temperature to 1800°C. Additional improvement was achieved by optimizing the substrate temperature (between 260 and 305°C) resulting in suggested mass fractal structures of Ge with the fractal dimension p<1.6 and a size of about 40nm. The nature of the microstructural changes induced by changes in filament temperature compared to those induced by the changes in substrate temperature were clearly different. The improved microstructure of the alloys could be correlated with improved optoelectronic properties of the material.

The Effect of Oxygen Contamination on the Electronic Properties of Hot-Wire CVD Amorphous Silicon Germanium Alloys

AbstractA series of four a Si,Ge:H alloy samples with Ge fractions near 30 at.% were deposited by hot-wire CVD (HWCVD) using a Ta filament maintained at 1800oC. During film growth, the level of oxygen contamination was varied from less than 1019 cm−3 to roughly 5 × 1020 cm−3 using a controlled air-leak. The electronic properties of these films were then characterized using transient photocapacitance (TPC) and transient photocurrent (TPI) spectroscopy, as well as the drive-level capacitance profiling (DLCP) techniques. We observed an unexpected systematic improvement of the electronic properties of these HWCVD a Si,Ge:H with increasing oxygen impurity level, which was reflected by a decrease in the deduced Urbach energies. Comparing these with films co-deposited on stainless-steel versus p+ c-Si substrates, we found significantly better electronic properties in the latter case. Comparisons of the TPC and TPI spectra indicated a very high level of hole collection, consistent with these narrow bandtail distributions.

Surface Roughening Transition in Si1-xGex:H Thin Films

AbstractIn this study, the amorphous-phase roughening transition thickness has been determined as a function of process variables in plasma-enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous silicon-germanium alloys (a Si1-xGex:H). Among the process variables include the H2-dilution gas flow ratio, the alloying flow ratio, the electrode configuration (anode vs. cathode), and the He-dilution ratio. One clear feature of this study is a maximum in the amorphous roughening transition thickness (and hence surface stability) at a H2-dilution ratio just below the transition from amorphous to mixed-phase (amorphous+microcrystalline) (a+μc) growth. A second feature for high Ge content films is a significant increase in the roughening transition thickness for cathode PECVD (with a self-bias of ~ −20 V) relative to anode PECVD. Additional features of interest involve suppression of the mixed-phase transition to (a+μc) for (i) alloying with Ge, (ii) cathodic substrate biasing, and (iii) diluting the gas with He. The close correlation of high surface stability with enhanced short-range order and overall electronic performance has led to a simple model for the transition in terms of a competition between roughening due to the atomic size and smoothening due to precursor surface diffusion. It is proposed that diffusing precursors are immobilized by surface defects (or by other diffusing precursors), and the pre-existing (or resulting) defects are ultimately incorporated in the bulk.

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

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.

Photoinduced structural instability in a hydrogenated amorphous silicon-germanium alloy with high structural inhomogeneity

We studied photoinduced structural instability around the $\mathrm{Si}\penalty1000-\hskip0pt\mathrm{H}$ bond in hydrogenated amorphous silicon-germanium alloy $(a\text{\ensuremath{-}}\mathrm{SiGe}\text{:}\mathrm{H})$ using modulated-infrared (IR) absorption, which is a real-time change in the absorption strength of the $\mathrm{Si}\penalty1000-\hskip0pt\mathrm{H}$ stretching vibration following a modulated band-gap excitation. We found in the modulated IR-absorption spectrum, that a band expected around $2000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, based on the absorption-coefficient spectrum of the $\mathrm{Si}\penalty1000-\hskip0pt\mathrm{H}$ stretching vibration, is missing. An intense modulated IR-absorption band is present at about $2000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in the spectra of materials such as undoped $a\text{\ensuremath{-}}\mathrm{Si}\text{:}\mathrm{H}$ and its alloys with N or O, which are characterized by a continuously connected, disordered network with a built-in local strain. Thus, the absence of the $2000\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ spectral component is interpreted as a sign of the absence of structural instability in the vicinity of the isolated $\mathrm{Si}\penalty1000-\hskip0pt\mathrm{H}$ bond in $a\text{\ensuremath{-}}\mathrm{SiGe}\text{:}\mathrm{H}$ that has a fragmented network structure. The modulated IR-absorption band that is still observed in $a\text{\ensuremath{-}}\mathrm{SiGe}\text{:}\mathrm{H}$ is found to be closely related with the $2030\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ constituent component of the IR-absorption band of the $\mathrm{Si}\penalty1000-\hskip0pt\mathrm{H}$ stretching mode in this alloy. Studies on the $2030\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}1}$ component present in the IR-absorption band of the $\mathrm{Si}\penalty1000-\hskip0pt\mathrm{H}$ stretching mode in polymorph- and microcrystalline-Si make us infer that structural instability in $a\text{\ensuremath{-}}\mathrm{SiGe}\text{:}\mathrm{H}$ takes place in the vicinity of the H-induced platelets.

Influence of the a-SiGe:H thickness on the conduction mechanisms of n-amorphous-SiGe:H∕p-crystalline-Si heterojunction diodes

We have fabricated, electrically characterized and simulated n-type hydrogenated amorphous silicon germanium alloys on p-type crystalline-silicon heterojunction diodes with three different a-SiGe:H layer thicknesses: 37, 86, and 200nm. The capacitance–voltage results confirm the existence of abrupt heterojunctions. The conduction and valence-band discontinuities of the heterojunctions and the electron affinity of the n-type a-SiGe:H films were obtained. The conduction mechanisms were determined by analyzing the temperature dependence of the current–voltage characteristics. The results show that at low forward bias (V<0.45V) the diodes with thinner amorphous layers (37 and 86nm) are dominated by recombination in the a-SiGe:H depletion region, whereas the thicker diode (200nm) is dominated by multistep tunneling through depletion region. In addition, at high forward bias (V>0.45V) the space-charge limited effect becomes the main transport mechanism in all the measured devices. The increase in the amorphous layer thickness also causes an increase in the leakage reverse current. Numerical simulations support the proposed transport mechanisms.

Amorphous and nanocrystalline silicon-based multi-junction solar cells

The status of multi-junction solar cells using amorphous silicon and amorphous silicon germanium alloys is discussed vis-a-vis those using nanocrystalline silicon alloy in the bottom cell. We have achieved an initial active area (0.25 cm 2) efficiency of 14.6% for a-Si:H/a-SiGe:H/a-SiGe:H cells and have built a roll-to-roll continuous deposition production plant with an annual capacity of 30 MW using this structure. We have also explored using nanocrystalline silicon (nc-Si:H) as the bottom cell of the triple-junction structure. Using hydrogen profiling for the optimization of the bottom cell and appropriate current matching of the component cells, we have achieved an initial active area efficiency of 14.6% using a-Si:H/a-SiGe:H/nc-Si:H structure.

Improved Back Reflector for High Efficiency Hydrogenated Amorphous and Nanocrystalline Silicon Based Solar Cells

AbstractAg/ZnO back reflectors (BR) on specular stainless steel substrates are optimized for hydrogenated amorphous silicon germanium alloy (a-SiGe:H) and nanocrystalline silicon (nc-Si:H) solar cells. The BRs are deposited using a sputtering method. The texture of the Ag and ZnO layers is controlled by deposition parameters as well as chemical etching with diluted HCl. The surface morphology is investigated by atomic force microscopy. The scattered light intensity from a He-Ne laser, which illuminates the sample surface perpendicularly, is measured at different angles. Finally, a-SiGe:H and nc-Si:H solar cells are deposited on the BR substrates prepared under various conditions. For a-SiGe:H bottom cells, the improved BR with large micro-features leads to an enhanced open-circuit voltage. For the nc-Si:H solar cells, large micro-features on the improved BR eliminate interference fringes otherwise observed in the quantum efficiency measurement and result in high short circuit current density. The result is consistent with an enhanced scattered light intensity. Hence, the cell performance was improved. We also deposited a-Si:H/a-SiGe:H/nc-Si:H triple-junction cells on the optimized BR and achieved a high initial active-area efficiency of 14.6%.

Determination of the Trap State Density Differences in Hydrogenated Amorphous Silicon-Germanium Alloys

Time-resolved photo- and thermoelectric effects (TTE) were used to determine simultaneously trap levels and trap state density differences in amorphous (a-SiGe:H) samples. In particular, the trap state density differences are obtained from the decay of the ambipolar charge distribution (i.e., stage II of the TTE transients). This type of spectroscopy has been applied for the first time to a-SiGe:H samples, and indeed trap states that seem to relate to concentration fluctuations, that is, Si(Ge) and Ge(Si) clusters, are observed.

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

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.

Structural Relaxation of Amorphous Silicon-Germanium Alloys: Molecular-Dynamics Study

Thermally-induced structural relaxation of amorphous silicon-germanium (a-Si1-xGex) alloys has been examined using molecular-dynamics (MD) simulations with the Tersoff interatomic potential. The a-Si1-xGex networks were successfully observed in the present MD simulations. Major changes in the topological short-range order upon annealing occurred within the second coordination shell, while the chemical short-range order was not altered. It was found that the decrease in potential energy associated with the change in the standard deviation of bond angles becomes large as the Ge composition increases. This result was consistent with the heat release due to structural relaxation, which was observed experimentally. We also examined differences in the phonon densities of states between the as-quenched and annealed a-Si1-xGex alloys.

04/02885 Competitive solar heat engines: Valdès, L.-C. Renewable Energy, 2004, 29, (11), 1825–1842

Hydrogenated amorphous silicon-germanium alloy films have been prepared by rf glow discharge decomposition of a mixture of silane, germane and hydrogen varying the germane concentration in the gas mixture and the substrate temperature. The opto-electronic properties of a-SiGe: H films deteriorate sharply as the germane concentration is increased in the source gas mixture. This deterioration is attributed to an increase in defect density with Ge incorporation. The substrate temperature, Ts, was varied in the range 100–350°C. Due to the healing effect on the amorphous network at higher temperature, the opto-electronic properties of a-SiGe: H films improve up to Ts ∼ 250°C. At higher Ts, the properties deteriorate due to the breaking of weak GeH bonds. Infrared vibrational spectra show formation of polyhydrides and higher Ge concentration at lower Ts. The increase in density of states with higher Ge content is shown by ESR data.

IR bolometers based on amorphous silicon germanium alloys

In this work, we report the fabrication and characterization of bolometers based on amorphous silicon germanium alloys (a-Si 1 − x Ge x :H,F). The fabrication of microbolometers with thermally isolated a-SiGe thin film as sensing layer, is described. Membrane-supported and bridge-supported detectors have been fabricated. Sensing layer is deposited by low frequency plasma enhanced chemical vapor deposition technique. Since this deposition is carried out at relative low temperature T D=573 K, the read-out IC fabricated on silicon substrate is not affected, providing compatibility with silicon IC technologies. Responsivity and detectivity were measured under illumination at different bias voltages. The thermal time constant, the thermal conductance and current noise are determined by means of the voltage–time curves. Responsivity and detectivity are observed to depend on the physical properties of the materials forming the device and on the device geometry. Obtained results of pixel resistance, thermal resistance and responsivity demonstrate a-SiGe:H,F like a material suitable for realizing high-performance microbolometers.

Devices Fabrication with Narrow-Bandgap a-SiGe:H Alloys Deposited by HWCVD

ABSTRACTWe incorporate narrow-gap amorphous silicon germanium (a-SiGe:H) alloys grown by hot-wire chemical vapor deposition (HWCVD) into single-junction n-i-p solar cells, and improve both fill factor (FF) and open-circuit voltage (Voc) by bandgap grading. The Tauc bandgap (ET) of the a-SiGe:H is as low as about 1.25 eV. Previously [1], we obtained a short-circuit current density (Jsc) up to 20 mA/cm2 in an n-i-p device incorporating an ungraded 120-nm i-layer of 1.25-eV a-SiGe:H. However, without buffer layers or bandgap profiling, the fill factor was only 38%, likely due to an abrupt bandgap transition and poor hole collection. To overcome these problems, we have used composition bandgap profiling throughout the i-layer and improved both Voc and FF significantly without any Jsc loss. The solar cell efficiency is improved from 3.55% to 5.85% and Voc rises from 0.475 to 0.550 eV. This improved single-junction a-SiGe:H solar cell has a quantum efficiency of about 48% at l=800 nm and about 15% at l=900 nm. We present details of the bandgap profiling and its effect on device performance.

アモルファスＳｉ１−ｘＧｅｘ合金構造の分子動力学シミュレーション

Although extensive studies have been carried out on structural analyses of amorphous silicon-germanium (a-Si1-xGex) alloys, there are still controversies concerning short-range order in a-Si1-xGex alloys. In this study, we examined amorphous structures of Si1-xGex alloys by molecular-dynamics (MD) simulations using the Tersoff interatomic potential. Amorphous networks were prepared by rapid quenching from liquid Si1-xGex alloys. Computer-generated atomic configurations consisted of tetrahedral networks with complete chemical disorder in the first coordination shell. The bond lengths of Si-Si, Si-Ge and Ge-Ge pairs slightly increased with the Ge composition, and a weaker composition dependence was observed in the bond lengths of Si-Si and Si-Ge pairs than that for Ge-Ge pair. These results were in good agreement with those obtained experimentally, suggesting that the MD calculations based on the Tersoff potential is useful for the structural analysis of a-Si1-xGex alloys. It was confirmed that the Ge-Ge bond length and bond angle surrounding Ge atoms are more distorted. This is attributed to the difference between Si and Ge in the bond-stretching and in the bond-bending forces.

Measurement of the Electric Potential on Amorphous Silicon and Amorphous Silicon Germanium Alloy Thin-Film Solar Cells by Scanning Kelvin Probe Microscopy

ABSTRACTWe report on direct measurements of surface potentials on cross sections of a-Si:H and a-SiGe:H n-i-p solar cells using scanning Kelvin probe microscopy. External bias voltage (Vb)induced changes in the electric field distributions in the i layer were further deduced by taking the derivative of the Vb-induced potential changes. This procedure avoids the effect of surface charges or surface Fermi-level pinning on the potential measurement. We found that the electric field does not distribute uniformly through the i layer of a-Si:H cells, but it is stronger in the regions near the n and p layers than in the middle of the i layer. The non-uniformity is reduced by incorporating buffer layers at the n/i and i/p interfaces in the a-Si:H solar cells. For a-SiGe:H solar cells, the electric field at the p side of the i layer is much stronger than at the n side and the middle. The non-uniformity becomes more severe when a profiled Ge content is incorporated with a high Ge content on the p side. We speculate that the increase in defect density with increasing of Ge content causes charge accumulation at the i/p interface.

Sensitivity of amorphous silicon-germanium solar cells to oxygen impurity atoms

The performance of thin-film solar cells based on amorphous silicon germanium alloys (a-SiGe:H) are shown to be relatively sensitive to the contamination level of oxygen and/or nitrogen impurity atoms. Both a-SiGe single-junction solar cells and amorphous silicon (a-Si:H)/a-SiGe:H tandem solar cells were fabricated using a calibrated leak during deposition of individual layers. After light soaking, the tandem cells with a-SiGe layers deposited with an air leak, and observed to incorporate ∼4 (2.3)×1019 cm−3 oxygen (nitrogen) atoms, have significantly (10%) lower performance. The efficiency of a-SiGe:H single junction cells fabricated with varying air leak rates are found to improve systematically by ∼20% as the incorporated oxygen (nitrogen) concentration decreased by a factor of ∼3 (23) down to 1.3 (0.1)×1019 cm−3.

Improving narrow bandgap a-SiGe:H alloys grown by hot-wire chemical vapor deposition

We have improved the electronic properties of narrow-bandgap (Tauc gap below 1.5 eV) amorphous-silicon germanium alloys (a-SiGe:H) grown by hot-wire chemical vapor deposition (HWCVD) by lowering the substrate temperature and deposition rate. Prior to this work, we were unable to grow a-SiGe:H alloys with bandgaps below 1.5 eV that had photo-to-dark conductivity ratios comparable with our plasma-enhanced CVD (PECVD) grown materials [B.P. Nelson et al., Mater. Res. Soc. Symp. 507 (1998) 447]. Decreasing the filament diameter from our standard configuration of 0.5 mm to 0.38 or 0.25 mm provides first big improvements in the photoresponse of these alloys. Lowering the substrate temperature from our previous optimal temperatures ( T sub starting at 435 °C) to at 250 °C provides additional photo-to-dark conductivity ratio increasing by two orders of magnitude for growth conditions containing 20–30% GeH 4 in the gas phase (relative to the total GeH 4+SiH 4 flow).