Spectroscopic ellipsometry (SE) together with the optical transmission method is successfully used to determine the refractive index n and absorption coefficient α of undoped gallium nitride film over the spectral range of 0.78–4.77 eV of photon energy. The SE measurement is carried out at angle of incidence of 60° over the 1.5–4.77 eV energy range and optical transmission measurement over the 0.78–3.55 eV energy range. The refractive index n and absorption coefficient α obtained by both methods show unique results in the overlap wavelength region. Refractive index n is found to follow the Sellmeir dispersion relationship n2(λ)=2.272+304.72/(λ2−294.02) below the fundamental band edge. A free excitonic structure at the band is clearly observed at room temperature, with the transmission energy of free exciton at 3.44 eV, which is in reasonable agreement with the reported results.
A promising passivation method for GaAs solar cell grown on Si substrate (GaAs/Si solar cell) by phosphine-added hydrogen (PH3/H2) plasma exposure has been envisaged. The defect-hydrogenation and the surface-phosphidization effects of GaAs/Si solar cell are realized simultaneously by this single passivation process. Consequently, surface recombination states are reduced and the minority carrier lifetime is increased, resulting in a significant reduction in saturation current density (J0) of the GaAs/Si p–n junction. High open-circuit voltage (0.93 V) and fill factor (80.9%) are obtained for the PH3 plasma exposed GaAs/Si solar cells. As a result, the conversion efficiency is increased from 15.9% to 18.6%. This approach provides a simple and effective method to improve the photovoltaic properties of GaAs/Si solar cell.
The phosphidization effect on dislocations in GaAs grown on Si substrate (GaAs/Si) has been investigated. It was found that the high density of dislocations in GaAs/Si heteroepitaxial layers largely enhanced the diffusion of phosphorus (P) atoms during the phosphine (PH3) plasma exposure. The incorporated P atoms strongly passivated the electrical states of residual dislocations in GaAs/Si solar cell. As a result, the PH3 plasma exposure largely increased the open circuit voltage (Voc) and the efficiency of GaAs/Si solar cell.
Hydrogen (H) plasma passivation effects on GaAs grown on Si substrates (GaAs on Si) are investigated in detail. H plasma exposure effectively passivates both the shallow and deep defects in GaAs on Si, which improves both the electrical and optical properties. It was found that the minority carrier lifetime is increased and the deep level concentration is decreased by the H plasma exposure. In addition, after H plasma exposure, room temperature photoluminescence (PL) for Al 0.3 Ga 0.7 As/GaAs multiple-quantum-well (MQW) on Si is enhanced with a decrease in the spectral width.
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