Low temperature silicon epitaxy : defects and electronic properties (2003)

The work investigates the electronic properties of thin epitaxial silicon films and their suitability for microelectronic and photovoltaic applications. The films are grown by ion-assisted deposition (IAD), a molecular beam epitaxy (MBE) method that uses a small fraction of accelerated Si+ ions in the molecular beam, allowing for additional kinetic energy transfer to the substrate during low temperature epitaxy. This work concentrates on films grown at low deposition temperatures Tdep in the range of Tdep = 450°C to 750°C with deposition rates rdep in the range of rdep = 0.1 to 0.5 µm/min. As substrate materials, either monocrystalline (100)-, (111)-, (110)-, and (113)-oriented Si-wafers or block-cast polycrystalline Si-wafers are used. This work shows that the structural and electronic properties of epitaxial films deposited at low temperatures depend significantly on the substrate orientation. The number of extended defects in (100)-oriented films, i.e. dislocations and stacking faults, is significantly lower than in non-(100)-oriented films. The etch pit density nep, as deduced by anisotropic defect etching, is below nep = 1 x 10 3 cm -2 , for (100)-oriented films, independent of deposition temperature and rate. This low number of extended defects ensures that the electronic properties of (100)-oriented films are dominated by point defects. Photoluminescence and deep level transient spectroscopy (DLTS) serve to characterize defects in the (100)-oriented films. A broad defect luminescence band, located at photon energies around hnu = 0.8 eV, appears in all films deposited at Tdep = 460°C. When accelerated silicon ions are used to deposit the films, additional defect peaks appear at hnu = 0.767 eV and below. These defects are correlated to thermal donors, that are typically observed in oxygen rich silicon after thermal treatment at 450°C. Several broad defect bands in the band gap are identified by DLTS-measurements, the most prominent at trap levels Et = 0.2 eV and 0.25 eV above the valence band. The defect density is of the order of 1 x 10 13 cm -3, and shows a minimum for rdep = 0.3 µm/min. For deposition temperatures Tdep > 550°C, no defects are observed with either photoluminescence or DLTS, but the minority carrier diffusion length of the films increases with Tdep. The use of the minority carrier diffusion length as a sensitive measure for the density of electrically active defects reveals an exponential decay of the defect density with rising deposition temperature. Ion-bombardment with Si+-ions during deposition at low temperatures has an important influence on the electronic properties of the films: At Tdep = 460°C and 650°C, the use of accelerated silicon ions in ion-assisted deposition leads to an increase of the minority carrier diffusion length L for moderate acceleration voltages up to 100 V. At higher deposition temperatures, ion-bombardment did not result in a measurable difference of the electronic properties: Thin film solar cells, deposited at Tdep = 750°C with and without accelerated silicon ions showed identical conversion efficiencies of 13.8 %. Despite the variety of defects detected in low temperature epitaxial films, Photoluminescence and DLTS did not allow to identify the dominant recombination mechanism that is responsible for the poor photovoltaic properties of the films deposited at Tdep < 650°C. Therefore, a new lifetime spectroscopy method is developed in this work, that is compatible with thin films and fully processed devices: Temperature dependent quantum efficiency (TQE). Using the TQE method for analysis of thin film solar cells deposited by IAD at Tdep = 460°C and 510°C revealed the presence of two dominant defect centers, active at different temperatures. Applying a multilevel model for the lifetime to the TQE data allows for the identification of a defect center with an activation energy Ea = 0.2 eV as the dominant recombination center at room temperature and a center with Ea = 0.07 eV being active at temperatures below 150 K. The TQE results are in good agreement with DLTS experiments, where comparable defect levels are determined in the same films. Growth on non-(100)-oriented substrates, such as Si(111), Si(110), and Si(113), is dominated by the formation of high densities of extended defects, in particular stacking faults, resulting in significantly lower electronic quality of the films. Light beam induced current investigations of films deposited on polycrystalline substrates with randomly oriented grains show highly differing electronic quality of the grains. As a consequence, this work classifies the suitability of surface orientations for epitaxy according to the resulting electronic quality of the deposited films as follows: Type A)(100)-oriented surfaces result in the highest electronic quality. Type B) singular (stable) surfaces (e.g. (111), (110)) result in medium electronic quality. Type C) facetted surfaces result in the poorest structural and electronic quality. This work gives a detailed analysis of defects in low temperature epitaxial films with their dependence on deposition temperature, deposition rate, and substrate orientation, allowing for a profound judgement of the possibilities and restrictions of low temperature epitaxial films for photovoltaic and microelectronic applications. In most cases, the high number of extended defects and the inferior electronic properties will exclude deposition on non-(100)-oriented substrates. Especially in the case of photovoltaic devices, only epitaxy on (100)-oriented substrates at deposition temperatures above 650°C results in sufficiently high minority carrier diffusion lengths for effective thin film solar cells.