| Thin-film silicon (Si) solar cells have received wide attention due to their potential for large areas and low-cost manufacturing. In attempt to further improve the efficiency and stability, multi-junction thin film silicon solar cells have been tested a lot and are being developed rapidly. Hydrogenated microcrystalline silicon germanium (μc-Si1-xGex:H), which exhibits narrower variable band gap and higher absorption coefficient, has attracted much attention for the use as the bottom sub-cell i-layer of the multi-junction solar cell. However, it has become evident that the random distribution of Ge and Si atoms in the mixed-phase μc-Si1-xGex:H alloy materials would contain amorphous phase, crystalline phase, grain boundaries and voids, as well as fractions of Si-Si, Si-Ge, Ge-Ge and (Si(Ge)-Hn)n bonding structures, which brings difficulties in characterizing the microscopic bonding structures and optimizing the film properties of μc-Si1-xGex:H alloys.In recent years, optimization of the deposition conditions has been tried and so far has not resulted in satisfactory improvements.In this thesis, radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) is used for the preparation of μc-Si1-xGex:H thin films. The electrical and optical absorption properties of the μc-Si1-xGex:H thin films were investigated and discussed in detail.The main content of this work are as follows:First, based upon the Ge content (0～100%) and crystalline volume fraction (Si-Ge,Si-Ge, Ge-Ge), which are the two basic structural characteristics of μc-Si1-xGex:H thin films, the microscopic structural properties and their influence on the electrical properties were investigated in detail.1)It was found that the Ge-related bonds (Si-Ge and Ge-Ge) mainly embedded in amorphous matrix when Ge content was lower than60%. The influence of Ge content and crystalline volume fraction on the electrical properties of μc-Si1-xGex:H films can be concluded as:i. The crystalline volume fraction and dark conductivities of μc-Si1-xGex:H films decrease with the increase of Ge content,while the photo conductivities decrease continuously and then increase. Thus, the maximum value for the photosensitivity of μc-Si1-xGex:H films can be obtained; ii. The dark and photo conductivities increase with the crystalline volume fraction while keeping the Ge content unchangeably. The maximum value for the photosensitivity of μc-Si1-xGex:H films can be obtained by varying the Si crystalline volume fraction (XSi-Si)at45～60%; iii. The photo conductivity and photosensitivity decrease with the increase of Ge content while keeping the crystalline volume fraction unchangeably. Thus, a monotonic decrease of carrier transport properties and a disappearance of photosensitivity for the μc-Si1-xGex:H with Ge content larger than70%were generally found.iv. The total crystalline volume fraction of μc-Si1-xGe:H films increase with Ge content while decreasing the electrode separation. Thus, the structure of μc-Si1-xGex:H films became loose due to the low H content and high value of the micro-structure factor (R).2) On the other hand, the increase of H dilution ratios was not only beneficial to form the ordered Si-Ge and Ge-Ge bonds, but also can enhance the total crystalline volume fraction of the μc-Si1-xGe:H films when Ge content was higher than60%.. Unfortunately, the photosensitivity of μc-Si1-xGex:H films decreases dramatically with the increase of grain size, which is considered as the results of large density of defects at grain boundaries.Based on the previous results, we argue that there is a complex, relationship between the Ge content, crystalline volume fraction and photosensitivity for the μc-Si1-xGex:H alloy materials. The photosensitivity, which is the most important parameter for the electrical properties of μc-Si1-xGex:H films, is determined simultaneously by the Ge content and crystalline volume fraction.Second, fundamental parameters suitable to prepare μc-Si1-xGex:H thin films with excellent optical absorption properties were determined. It was found that the absorption coefficient of μc-Si1-xGex:H films increase slightly with the increase of the substrate temperature. The absorption coefficient monotonically increases with decreasing the electrode separation and increasing H dilution ratio, which is mainly due to the high Ge content and crystalline volume fraction. The absorption coefficient increase continuously and then decrease with the total gas flow. At low total gas flows, the powder formation in the plasma space could lead to the incorporation of particles into the growing film due to the long gas residence time, which could adversely affect the optical absorption properties of μc-Si1-xGex:H films. On the other hand, the SiGe:H films transform into amorphous phase at high total gas flows conditions, which also leads to the poor optical absorption for μc-Si1-xGex:H films.The variation of absorption coefficient showing the same trend when increasing the glow discharge power and decreasing the gas pressure. Thus, the maximum value for the absorption coefficient of μc-Si1-xGex:H films can be obtained at the lowest defect density conditions. At low power and high pressure conditions, the crystalline volume fraction decreases with the electron temperature in the plasma, which leads to the poor optical absorption for μc-Si1-xGex:H films. On the contrary, at high power and low pressure conditions, the defect density increases with the ion bombardment effects, which also lead to the low optical absorption. These results provide convincing evidence that the so-called high-pressure depletion regime, under which the increase of the total gas pressure is used to reduce the ion bombardment, enhance the consumption of silane in the discharge and maintain a high hydrogen flux towards the surfaces. Thus, the optical absorption for μc-Si1-xGex:H films was significantly enhanced.The microstructure parameter of device-quality μc-Si1-xGex:H films for Ge content at lower than25%and25～45%can be respectively concluded as:1) Ge content at lower than25%, dark conductivity at10-8～10-7S·cm-1, photosensitivity at1000-1500, absorption coefficient at1000nm wavelength higher than103cm-1, the crystalline volume fraction of Si-Si bonds at45～60%, Hydrogen content at6-8%, grain size at15-20nm with (220) preferred orientation, micro-structure factor lower than0.3,Ge-H priority factor higher than0.5;2) Ge content at25～45%, dark conductivity at10－8～10-6S·cm-1, photosensitivity at500～700, absorption coefficient at1000nm wavelength higher than5×103cm-1, the crystalline volume fraction of Si-Si bonds at40～55%, Hydrogen content at5～7%, grain size at15～25nm with (220) preferred orientation, micro-structure factor lower than0.45, Ge-H priority factor higher than0.2;Third, in order to take full advantage of high Ge content μc-Si1-xGex:H films in expanding the spectral response range and enhancing the long wavelength absorption coefficient, the combination of hydrogen and helium dilutions was introduced in improving the electrical properties of μc-Si1-xGex:H films with Ge content higher than 40%. The ’penning ionization effect’ for helium in the plasma provides additional momentum and energy for the growth of the film. The optimized He/H2ratio will promote the formation of ordered Si-Ge and Ge-Ge bonds. Thus, high quality μc-Si1-xGex:H films with (220) crystal plane preferred orientation, compact microstructure and low defect densities can be obtained. |
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