Microcrystalline Silicon Germanium Research Articles

Reactive Force-Field Molecular Dynamics Study of SiGe Thin Film Growth in Plasma Enhanced Chemical Vapor Deposition Processes

SiGe thin films, such as hydrogenated amorphous silicon germanium (a-SiGe:H) and hydrogenated microcrystalline silicon germanium (μc-SiGe:H), are formed by plasma enhanced chemical vapor deposition (PECVD) methods from SiH4, GeH4, and H2. Thin film properties such as surface morphology, hydrogen content, and crystal structure are greatly affected by process parameters such as a substrate temperature, gas pressure, and gas composition. For example, SiH3 and GeH3 are dominant species when the SiGe thin film with low defect is deposited under low power density conditions. However, the structural defects are formed by SiH and SiH2 that are highly reactive species under high power density conditions. Although the density and flux of these highly reactive species are much lower than those of the dominant species, the electrical properties of the device are greatly affected by a few structural defects. Therefore, to understand the influence of the composition of each precursor on the thin film properties is important. The purpose of the present study is to clarify the influence of the composition of multiple precursors such as SiH3, GeH3, and H under different substrate temperatures on the surface morphology, hydrogen content, and crystal structure.We performed reactive force-field molecular dynamics (ReaxFF MD) simulations. An existing parameter set was modified to match the dissociation energies in gaseous species because bond formation and breaking are particularly important in this study. The substrate was Si (100)– (2×1), and the size of the simulation box was 30.72 Å× 26.88 Å× 45.00 Å. A periodic boundary condition was applied in the x and y directions, and a fixed boundary condition was applied in the z direction. The first bottom layer of the substrate was fixed to prevent vibrations of the substrate up and down. The second and third layers were controlled constantly at desired temperature by the Berendsen thermostat. The temperature was not controlled on the fourth and subsequent layers to prevent direct influences of changing the velocity of atom due to the temperature control on the surface reaction. The substrate temperature was set at 800 K, 1000 K, 1300 K, and 1600 K. 2000 SiH3 molecules with the velocity corresponding to 1300 K in the direction towards the substrate were deposited one by one onto the substrate surface every 7.5 ps. The initial position of the gaseous species was set at 10 Å above the top of substrate Si atoms, and the in-plane (x-y) position were set randomly. The Velocity-Verlet algorithm was used for time integrations corresponding to 0.25 fs time step. A pre-thermal annealing for 7.5 ps and a post-thermal annealing for 60 ps were performed before and after the deposition to fully equilibrate the substrate and deposited thin films.Figure shows front views of the final configurations and results of analysis when the substrate temperature is 800 K, 1000 K, 1300 K, and 1600 K. The yellow, blue, and red represent substrate Si atoms, added Si atoms, and added H atoms, respectively. Gray shown in the figure are gas-solid surfaces. The surface construction algorithm was used to calculate a surface area that is closely related to the surface morphology. The hydrogen content was calculated using the select type algorithm. The structure identification algorithm for the crystal structure was used to characterize the structure of simulated films. These analyses were carried out by the Open visualization tool (OVITO). The surface area decreases as the substrate temperature increases, indicating that the formation of smoother surface at high temperature. The diffusivity of a SiH3 molecule that reached to the surface increases as the substrate temperature increases, and the atomic scale valleys are filled. This tendency is in good agreement with relevant studies. The hydrogen content was calculated by dividing the number of added H atoms by the total number of atoms existing in the system. The hydrogen content decreases as the substrate temperature increases because a hydrogen desorption is promoted as the substrate temperature increases. Figure 1

Effect of Alloy Composition on the Optoelectronic Properties of Hydrogenated Microcrystalline Silicon-Germanium Films Deposited with Various Hydrogen Dilution

Hydrogenated microcrystalline silicon-germanium (μc-Si1-xGex: H) films were fabricated by plasma-enhanced chemical vapor deposition. The influence of hydrogen dilution on the optoelectronic and crystal structural properties of μc-Si1-xGex: H films were investigated upon alloy composition. Under the relatively low hydrogen dilution condition, the Si atoms of films are easier to crystallize than the Ge atoms, and the dark and photo-conductivity decreased with the Ge concentration because of the reduced crystalline volume fraction of the Si atoms (X Si-Si ) and the increased Ge dangling bond density. However, under higher hydrogen dilution condition, crystallization of the Si atoms decreased while crystallization of the Ge atoms increased with Ge incorporation, and the optoelectronic properties were strongly influenced by the defect density and crystallization of Ge atoms in films.

Study of the effect of the deposition RF power on the characteristics of microcrystalline Silicon-Germanium thin films produced by PECVD

ABSTRACTHydrogenated microcrystalline Silicon-Germanium (μc-SiGe:H) thin films were deposited using the Plasma Enhanced Chemical Vapor Deposition (PECVD) technique from a gas mixture of SiH4, GeH4, H2 and Ar at a substrate temperature of 200 ° C. The films were deposited at a pressure of 1.5 Torr, while the RF power was varied in the range of 20 W to 35 W. Structural, optical and electrical characterization was performed in the films, Fourier Transform Infrared Spectroscopy (FTIR) was performed in order to analyze the hydrogen bonding of silicon and germanium, while Raman spectroscopy was used in order to analyze the crystallinity of the films. Through the optical and electrical characterization of the films, parameters such as the optical band gap (Eg) and the activation energy (EA) were obtained, respectively. The conductivity of the films changed up two to orders of magnitude from dark conditions to illumination AM 1.5. Finally, the correlation between deposition RF power and the film properties is presented.

Effect of Ge atoms on crystal structure and optoelectronic properties of hydrogenated Si–Ge films

Optoelectronic and structural properties of hydrogenated microcrystalline silicon–germanium ([Formula: see text]c-Si[Formula: see text]Ge[Formula: see text]:H) alloys prepared by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) were investigated. When the Ge atoms were predominantly incorporated in amorphous matrix, the dark and photo-conductivity decreased due to the reduced crystalline volume fraction of the Si atoms (X[Formula: see text]) and the increased Ge dangling bond density. The photosensitivity decreased monotonously with Ge incorporation under higher hydrogen dilution condition, which was attributed to the increase in both crystallization of Ge and the defect density.

Non-uniform distribution in µc-Si1−xGex:H and its influence on thin film and device performance

In this work, hydrogenated microcrystalline silicon germanium (µc-Si1−xGex:H) thin films were fabricated by plasma-enhanced chemical vapor deposition (PECVD) and developed as the infrared absorber for thin film silicon solar cells. Three kinds of the non-uniform distribution in µc-Si1−xGex:H thin films including: “the non-uniform distribution of Si and Ge”, the non-uniform distribution of crystallization”, and “the non-uniform distribution of H”, and how these affect the structural, optical and photoelectric properties of µc-Si1−xGex:H thin films have been explored. The results show that the good film quality of µc-Si1−xGex:H is associated with the low crystalline volume fraction and microstructure factor. The band gap of µc-Si1−xGex:H is determined by the proportion of the Ge-related crystalline networks. In addition, it is suggested that the deterioration of the photosensitivity of µc-Si1−xGex:H is mainly due to the increase of the Ge clusters with higher micro-void and defect density. Furthermore, by using µc-Si1−xGex:H bottom sub-cells, the comparable efficiency can be realized under the thin thickness condition. An efficiency of 11.35% in an a-Si:H/a-Si0.6Ge0.4:H/µc-Si0.5Ge0.5:H triple junction structure with total cell thickness as thin as 1200nm was obtained. It is believed that the µc-Si1−xGex:H thin films can be a better candidate for effective infrared absorber by further improving its microstructure uniformity.

Developments of c-Si1-xGex:H thin films as near-infrared absorber for thin film silicon solar cells

Hydrogenated microcrystalline silicon germanium (c-Si1-xGex:H) thin films have been developed as alternative bottom sub-cell absorbers for multi-junction thin film silicon solar cells due to their narrower band-gaps and higher absorption coefficients than conventional hydrogenated microcrystalline silicon (c-Si:H) thin films. However, since the structure complexity was increased a lot by Ge incorporation, the influences of c-Si1-xGex:H film properties on Ge composition have not been understood yet. In this work, c-Si1-xGex:H thin films with various Ge content and similar crystalline volume fraction are fabricated by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD). The evolutions of c-Si1-xGex:H material properties by Ge incorporation are characterized by X-ray fluorescence spectrometry, Raman spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, absorption coefficient spectrum, and conductivity measurement. The results show that the properties of c-Si1-xGex:H thin films are strongly determined by Ge content. With the increase of Ge content, the absorption coefficient, (111) grain size, microstructure factor, and dark conductivity of c-Si1-xGex:H thin films increase, while the H content, (220) grain size, and photosensitivity of c-Si1-xGex:H thin film decrease. Then, c-Si1-xGex:H is used as the intrinsic layer in the single junction solar cells. The performances of c-Si1-xGex:H solar cells with different Ge content and two types of transparent conductive oxide (SnO2 and ZnO) substrates are systematically studied. The results indicate that although c-Si1-xGex:H thin films become more defective and less compact with Ge incorporation, c-Si1-xGex:H solar cells exhibit a significant improvement in near-infrared response, especially under the circumstances of thin cell thickness and inefficient light trapping structure. Meanwhile, by using ZnO substrates, initial efficiencies of 7.15% (Jsc=22.6 mA/cm2, Voc=0.494 V, FF=64.0%) and 7.01% (Jsc=23.3 mA/cm2, Voc=0.482 V, FF=62.4%) are achieved by c-Si0.9Ge0.1:H solar cell and c-Si0.73Ge0.27:H solar cell, respectively. Furthermore, the c-Si0.73Ge0.27:H solar cell is used as the bottom sub-cell of the double-junction solar cell, and a Jsc.bottom of 12.30 mA/cm2 can be obtained with the bottom sub-cell thickness as thin as 800 nm, which is even higher than that of c-Si:H bottom sub-cell with 1700 nm thickness. Finally, an initial efficiency of 10.28% is achieved in an a-Si:H/c-Si0.73Ge0.27:H double junction cell structure. It is demonstrated that by using the c-Si1-xGex:H solar cell as the bottom sub-cell in multi-junction thin film silicon solar cells, a higher tandem cell performance can be achieved with a thin thickness, which has a great potential for cost-effective photovoltaics.

Simulation, fabrication, and application of transparent conductive Mo-doped ZnO film in a solar cell

In this study, Mo-doped ZnO (MZO) films were investigated using first-principles calculations based on density functional theory and magnetron sputtering technology. The theoretical results predicted that after Mo was incorporated into ZnO, the MZO film would present n-type metallic properties and the optical band gap would widen, which predicts that MZO films possess favorable electrical and optical properties. An MZO film with high conductivity and a wide spectral range was fabricated at various substrate temperatures (TS) using pulsed direct-current magnetron sputtering. From 400 to 1200nm, a lower resistivity of 7.68×10−4Ωcm and a higher average transmittance (exceeding 80%) were observed when the MZO film was deposited at the optimal TS of 280°C. Thin-film solar cells consisting of hydrogenated microcrystalline silicon germanium fabricated on textured MZO films demonstrated a strong enhancement of 7.24% in the conversion efficiency because of their improved short-circuit current density (Jsc) and open-circuit voltage (Voc) compared with a commercial Al-doped ZnO film applied to a reference solar cell.

Band gap grading in microcrystalline silicon germanium thin film solar cells

In this work, hydrogenated microcrystalline silicon germanium (μc-SiGe:H) thin film solar cells with a novel band gap grading profile have been designed. By comparing different profile types (normal profile, reverse profile and no profile), the normal profile was formed in sequence by the superposition of a high Ge content layer, a Ge content grading layer and a μc-Si:H layer has been proposed. This structure exhibits higher short-circuit current density (Jsc) than conventional cell design with the similarly Ge content owing to the enhancement of the infrared response. Finally, an initial efficiency of 6.53% was achieved by μc-SiGe:H solar cell with this novel cell structure. The results have demonstrated a great potential of the μc-SiGe:H solar cells as the infrared absorber in multi-junction silicon based thin film solar cells.

Enhancement of Spectral Response inμc-Si1-xGex:H Thin-Film Solar Cells with a-Si:H/μc-Si:H P-Type Window Layers

The hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline silicon (μc-Si:H) double p-type window layer has been developed and applied for improving microcrystalline silicon-germanium p-i-n single-junction thin-film solar cells deposited on textured SnO2:F-coated glass substrates. The substrates of SnO2:F, SnO2:F/μc-Si:H(p), and SnO2:F/a-Si:H(p) were exposed to H2plasma to investigate the property change. Our results showed that capping a thin layer of a-Si:H(p) on SnO2:F can minimize the Sn reduction during the deposition process which had H2-containing plasma. Optical measurement has also revealed that a-Si:H(p) capped SnO2:F glass had a higher optical transmittance. When the 20 nmμc-Si:H(p) layer was replaced by a 3 nm a-Si:H(p)/17 nmμc-Si:H(p) double window layer in the cell, the conversion efficiency (η) and the short-circuit current density (JSC) were increased by 16.6% and 16.4%, respectively. Compared to the standard cell with the 20 nmμc-Si:H(p) window layer, an improved conversion efficiency of 6.19% can be obtained for the cell having a-Si:H(p)/μc-Si:H(p) window layer, withVOC = 490 mV,JSC = 19.50 mA/cm2, and FF = 64.83%.

Effect of oxygen doping in microcrystalline SiGe p-i-n solar cells

The effect of doping with oxygen the microcrystalline silicon-germanium absorber layer of single-junction p-i-n solar cells has been studied. In parallel, the absorber layer quality was measured by depositing absorber layers directly on glass and measuring their electrical properties. By doping the absorber layer with the optimum oxygen concentration (about 1.4×1019 cm−3), an increase in short-circuit current density of almost 4 mA/cm2 was achieved in 3 μm thick p-i-n solar cell. This effect is attributed to the oxygen doping compensating the space charges caused by the germanium dangling bonds rather than the direct defect passivation, as no drastic change in layer quality was measured.

Optical absorption enhancement of μc-SiGe:H films deposited via high pressure and high power

Hydrogenated microcrystalline silicon-germanium (μc-SiGe:H) films are fabricated by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD). The optical absorption coefficient and the photosensitivity of the μc-SiGe:H films increase dramatically by increasing the plasma power and deposition pressure simultaneously. Additionally, the microstructural properties of the μc-SiGe:H films are also studied. By combining Raman, Fourier transform infrared (FTIR) and X-ray fluoroscopy (XRF) measurements, it is shown that the Ge-bonding configuration and compactability of the μc-SiGe:H thin films play a crucial role in enhancing the optical absorption and optimizing the quality of the films via a significant reduction in the defect density.

Microcrystalline silicon–germanium solar cells with spectral sensitivities extending into 1300 nm

High germanium (Ge) content hydrogenated microcrystalline silicon–germanium alloy (μc-Si1−xGex:H) has been prepared by conventional plasma enhanced chemical vapor deposition. The influence of the bombardment of energetic ions on the structural and electrical properties of high Ge content μc-Si1−xGex:H films was studied by Raman, transmission electron microscopy and conductivity measurements. Under low pressure and high levels of ion bombardment conditions, a mitigation of the rapid increase of Ge incorporation and a significant improvement of the homogeneous distribution of the crystal grains for high Ge content μc-Si1−xGex:H films were achieved. Consequently, an enhancement in the quantum efficiencies over 800nm wavelengths and a spectral response extending to 1300nm for μc-Si1−xGex:H solar cells were achieved by using an intrinsic layer with x=0.77.

Influences of electrode separation on structural properties of μc-Si1-xGex：H thin films

Hydrogenated microcrystalline silicon germanium (μc-Si1-xGex:H) thin films have been prepared by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) using a mixture of SiH4 and GeH4 as the reactive gases. Effects of electrode separation on the structural properties of μc-Si1-xGex:H thin films have been investigated. Results show that reduction of the electrode separation can increase the Ge content in the films. Moreover, μc-Si1-xGex:H thin film deposited at a lower electrode separation of 7 mm possesses not only a stronger (220) orientation and a larger grain size, but also a lower microstructural factor. Then, the decomposition characteristics of the reactive gases are analyzed according to the variation of the structural properties of the μc-Si1-xGex:H thin films. It is found that the increase of the Ge content is due to the decrease of the SiH4 decomposition rate in the plasma. While the better film quality obtained at the lower electrode separation is attributed to the enhancement of the diffusibility of the Ge precursors caused by improving the proportion of GeH3 radicals

Development of Hydrogenated Microcrystalline Silicon-Germanium Alloys for Improving Long-Wavelength Absorption in Si-Based Thin-Film Solar Cells

Hydrogenated microcrystalline silicon-germanium (μc-Si1-xGex:H) alloys were developed for application in Si-based thin-film solar cells. The effects of thegermane concentration(RGeH4)and thehydrogen ratio(RH2)on theμc-Si1-xGex:H alloys and the corresponding single-junction thin-film solar cells were studied. The behaviors of Ge incorporation in a-Si1-xGex:H andμc-Si1-xGex:H were also compared. Similar to a-Si1-xGex:H, the preferential Ge incorporation was observed inμc-Si1-xGex:H. Moreover, a higherRH2significantly promoted Ge incorporation for a-Si1-xGex:H, while the Ge content was not affected byRH2inμc-Si1-xGex:H growth. Furthermore, to eliminate the crystallization effect, the 0.9 μm thick absorbers with a similar crystalline volume fraction were applied. With the increasingRGeH4, the accompanied increase in Ge content ofμc-Si1-xGex:H narrowed the bandgap and markedly enhanced the long-wavelength absorption. However, the bias-dependent EQE measurement revealed that too much Ge incorporation in absorber deteriorated carrier collection and cell performance. With the optimization ofRH2andRGeH4, the single-junctionμc-Si1-xGex:H cell achieved an efficiency of 5.48%, corresponding to the crystalline volume fraction of 50.5% and Ge content of 13.2 at.%. Compared toμc-Si:H cell, the external quantum efficiency at 800 nm had a relative increase by 33.1%.

Microcrystalline SiGe Absorber Layers in Thin-film Silicon Solar Cells

We report on physical properties of microcrystalline silicon-germanium (μc-SiGe:H) absorber layers for the use as a bottom structure in silicon based multijunction thin-film solar cells. Due to incorporation of Ge the absorption of the film is enhanced compared to pure μc-Si:H films. This provides the opportunity to significantly reduce the absorber layer thickness. The experiments were carried out in a 13.56MHz PECVD reactor using germane, silane and hydrogen as process gases. Single layers were characterized for their optical and electrical properties. Results from single and multijunction solar cells using a μc- SiGe:H absorbers will be shown. In tandem solar cells a reduction of about 60% of the absorber layer thickness could be reached by using SiGe alloys compared to pristine silicon tandem cells.

Hydrogenated microcrystalline silicon germanium as bottom sub-cell absorber for triple junction solar cell

Hydrogenated microcrystalline silicon germanium (µc-Si1–xGex:H), with the advantage of its narrower variable band gap and higher absorption coefficient over the conventional hydrogenated microcrystalline silicon (µc-Si:H), has been implemented as the bottom sub-cell absorber of the triple junction solar cells. By replacing µc-Si:H i-layer with µc-Si0.91Ge0.09:H i-layer in the triple junction solar cell, the bottom sub-cell thickness (Dbottom) could be reduced by almost a half, meanwhile a higher efficiency was attained. As a result, an initial efficiency of 12.02% in an a-Si:H/a-Si0.6Ge0.4:H/µc-Si0.91Ge0.09:H triple junction structure with a total cell thickness as small as 1800nm was achieved. It is demonstrated that the triple junction solar cell incorporating µc-Si1−xGex:H bottom sub-cell with high efficiency and a relatively low thickness has a high potential for cost-effective photovoltaic applications.

Crystallization behavior of microcrystalline silicon germanium

Hydrogenated microcrystalline silicon germanium (μc-Si1−xGex:H) thin films were evaluated as a bottom cell absorber for multi-junction solar cells based on Si thin films. The crystallization behavior of μc-Si1−xGex:H was examined. Raman spectroscopy and grazing angle X-ray diffraction showed that the crystallinity of the μc-Si1−xGex:H films decreased with increasing Ge content. Fourier transform infrared spectroscopy revealed an increase in the intensity of the GeH and GeH2 bonds in μc-Si1−xGex:H due to the increased amorphous phase. The rapid GeH4 decomposition rate and preferential etching of Si bonds led to a decrease in crystallinity and an extremely high Ge content in the μc-Si1−xGex:H films. The dark conductivity of μc-Si1−xGex:H was affected by the internal crystallinity and an excessively high crystallinity deteriorated the electrical properties. The photo response increased from 102 to 103 with increasing Ge content. This study presents a comprehensive evaluation of the crystallization behavior of μc-Si1−xGex:H films for solar cell absorber applications.

Effects of seed layer on the performance of microcrystalline silicon germanium solar cells

Using plasma enhanced chemical vapor deposition (PECVD) at 13.56 MHz, a seed layer is fabricated at the initial growth stage of the hydrogenated microcrystalline silicon germanium (μc-Si1−xGex:H) i-layer. The effects of seeding processes on the growth of μc-Si1−xGex:H i-layers and the performance of μc-Si1−xGex:H p—i—n single junction solar cells are investigated. By applying this seeding method, the μc-Si1−xGex:H solar cell shows a significant improvement in short circuit current density (Jsc) and fill factor (FF) with an acceptable performance of blue response as a μc-Si:H solar cell even when the Ge content x increases up to 0.3. Finally, an improved efficiency of 7.05% is achieved for the μc-Si0.7Ge0.3:H solar cell.

Preferential etching of Si–Si bond in the microcrystalline silicon germanium

Hydrogenated microcrystalline silicon germanium (μc-Si1−xGex:H) films were investigated as a bottom cell absorber in multi-junction solar cells. μc-Si1−xGex:H films were prepared using very high frequency (VHF, 60 MHz) plasma enhanced chemical vapor deposition (PECVD) systems working pressure of about 1.5 Torr. The precursor flow rates were carefully controlled to determine the phase transition point and to improve the crystallinity of μc-Si1−xGex:H. A relatively high plasma power was necessary to have the high hydrogen (H2) dilution. Raman spectroscopy study showed transition steps from amorphous to microstructure morphology as hydrogen dilution increasing. Crystallite Si–Ge and Ge–Ge bonds were occurred at relatively higher H2 dilution compare to crystallite Si–Si bond. The rapidly increased Ge content as increasing the H2 dilution is believed mainly due to the different decomposition rate of silane (SiH4) and germane (GeH4). The other reason of high Ge content even at the low GeH4 precursor flow rate is probably due to the preferential etching of silicon atom by H2. The preferential etching of Si–H possibly occurred in very highly concentrated H2 plasma due to the preferential attachment of Si–H. The compositions of μc-Si1−xGex:H films measured using RBS were Si0.83Ge0.17, Si0.67Ge0.33 and Si0.59Ge0.41 at H2/SiH4 flow rate of 60, 80 and 100, respectively. μc-Si1−xGex:H films showed the dark (σd) and photo conductivity (σp) of about 10−7 and 10−5 S/cm, respectively and photo response (σp/σd) was about 102. This study will present the comprehensive evaluation of crystallization behavior of μc-Si1−xGex:H films.

Active roles of helium in the growth of hydrogenated microcrystalline silicon germanium thin films

A combination of hydrogen and helium dilutions was introduced when the microcrystalline silicon germanium (μc-SiGe:H) thin films were prepared by very high frequency plasma enhanced chemical vapor deposition on a low-temperature substrate. An optimum helium flow rate was found to achieve the structural uniformity in the growth direction, while Ge content was found to nearly keep constant with varying flow rates of helium. An abundance of atomic H was detected in plasma due to the attendance of helium and no obvious photosensitivity deterioration was observed on the thin film with a high crystalline volume fraction. The active roles of helium were identified by analyzing the mechanism in the plasma, where both metastable Hem⁎ and He+ can accelerate the diffusion of Ge related radicals and passivation of the dangling bonds on the growth surface, respectively. These phenomena have been revealed by experimental results. Therefore, a combination of hydrogen and helium dilutions can improve the structure of the μc-SiGe:H thin films with little degradation of photo-electronic properties.