We analyze the optical properties of zincblende gallium-nitride in the infrared and ultraviolet spectral range (≈27 meV-6.5 eV) experimentally by spectroscopic ellipsometry and provide a quantitative description of these results by k • p perturbation theory. Free-electron concentrations above 10 20 cm −3 are achieved by introducing germanium as a donor. We determine the dielectric function as well as band filling effects like the Burstein-Moss shift and band gap renormalization. The Kane model for the band structure of semiconductors near the-point allows to calculate the effective electron mass and to determine the nonparabolicity of the conduction band. At the same time, these results can be used to derive the free-electron concentration all-optically. The combination of Kane's model, Burstein-Moss shift, and band-gap renormalization can be used to expertly describe the measured transition energies up to ≈3.7 eV dependent on the carrier concentration, yielding an averaged hole mass of ≈0.61 m e for the contributing valence bands.
Selective area growth of cubic gallium nitride is investigated in a plasma assisted molecular beam epitaxy setup. 380 μm thick silicon (001) and 10 μm thick 3C-silicon carbide (001), grown on 500 μm silicon (001), were used as substrates and structured with silicon dioxide masks. Selective area growth on silicon and 3C-silicon carbide was tested for both thermal and plasma deposited oxides. Multiple growth series showed that gallium nitride coverage of silicon dioxide vanished at growth temperatures of 870 °C for silicon substrates and at a surface temperature of 930 °C for 3C-silicon carbide substrates. Whereas gallium nitride is grown in its hexagonal form on silicon substrates, phase pure cubic gallium nitride could selectively be grown on the 3C-silicon carbide template. The cubic phase is verified by high resolution x-ray diffraction and low temperature photoluminescence measurements. The photoluminescence measurements prove that gallium nitride condensed selectively on the 3C-silicon carbide surfaces uncovered by silicon dioxide.
An investigation of different n‐type doped zincblende gallium nitride thin films measured by photoluminescence from 7 K to room temperature is presented. The spectra change with increasing free‐carrier concentration due to many‐body effects such as the Burstein–Moss shift and band‐gap renormalization. The samples are grown by molecular beam epitaxy on a 3C‐SiC/Si (001) substrate, and a free‐carrier concentration above 1020 cm−3 is achieved by introducing germanium as a donor. The analysis of the measured spectra by a line‐shape fit yields different transition processes for different doping concentrations and temperatures, such as a band–band transition and a band–acceptor transition. The conduction band dispersion of Kane's model is perfectly suited to explain the experimental data quantitatively.
We present a quantitative description of the change in optical properties of zincblende aluminium-gallium-nitride thin films dependent on the free-carrier concentration due to band filling and renormalization effects. Free-electron concentrations above 1020 cm−3 in GaN are achieved by introducing germanium as a donor. Spectroscopic ellipsometry in the infrared and ultraviolet spectral range yields the dielectric function (DF). The plasmon contribution for the infrared part of the DF allows to determine the free-electron concentration all-optically. Furthermore, by utilizing the Kane model for the band structure of semiconductors near the Γ-point of the Brillouin zone as well as taking into account Burstein-Moss-shift and band-gap renormalization, measured transition energies are efficiently described.
Time-dependent femtosecond pump-probe spectroscopic ellipsometry studies on zincblende gallium-nitride (zb-GaN) are performed and analyzed between 2.9-3.7eV. An ultra-fast change of the absorption onset (3.23eV for zb-GaN) is observed by investigating the imaginary part of the dielectric function. The 266nm (4.66eV) pump pulses induce a large free-carrier concentration up to 4 × 10 20 cm −3 , influencing the transition energy between conduction and valence bands due to many-body effects, like band filling and band gap renormalization, up to ≈500meV. Additionally, the absorption of the pump-beam creates a free-carrier profile within the 605nm zb-GaN layer. This leads to varying optical properties from sample surface to substrate, which are taken into account by grading analysis for an accurate description of the experimental data. A temporal resolution of 100fs allows in-depth investigations of occurring ultra-fast relaxation and recombination processes. We provide a quantitative description of the free-carrier concentration and absorption onset at the sample surface as a function of relaxation, recombination, and diffusion yielding a characteristic relaxation time of 0.19ps and a recombination time of 26.1ps.
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