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.

We introduce germanium as an alternative to silicon for n‐type doping of cubic gallium nitride. Layers with electron concentrations up to 3.7 × 1020 cm−3 were grown by plasma‐assisted molecular beam epitaxy on 3C‐SiC substrates. Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) measurements were performed to verify the incorporation of Ge into our layers. The incorporation of Ge is in good agreement with the trend of the Ge vapour pressure curve. For the highest doped sample a drop of the incorporation efficiency is observed. A reduction of the growth rate is noticed for high Ge fluxes. Additionally, a sample comprising an alternating pattern of Ge‐doped and not intentionally doped interlayers was grown. In the recorded TOF‐SIMS depth profile we observe that in doped regions the Ge concentration increases along the growth direction. A gradually decreasing amount of Ge is incorporated into each overlying not intentionally doped interlayer. We suppose these observations are due to segregation effects and a resulting accumulation of Ge at the sample surface during growth.

We present a study of germanium as an alternative to silicon for n-type doping of cubic GaN. We find that Ge is a well-suited donor impurity. Our layers were grown by plasma-assisted molecular beam epitaxy on 3C-SiC/Si (001) substrates. Germanium-doped layers were fabricated with donor concentrations ranging over several orders of magnitude up to 3.7 × 10 20 cm −3 . For comparison, silicon-doped layers with donor concentrations of up to 3.8 × 10 19 cm −3 were also grown. Incorporation of germanium into the cubic GaN layers was verified by time-of-flight secondary ion mass spectrometry. The crystalline quality of our layers was analyzed using high-resolution x-ray diffraction. Germanium-as well as silicon-doped layers with donor concentrations above 10 19 cm −3 exhibited an increase of the dislocation density with increasing dopant concentration. The surface topography of our layers was investigated by atomic force microscopy. Comparable values for the surface roughness were measured for germanium-as well as silicon-doped layers. Optical properties were investigated by photoluminescence spectroscopy at 13 K. Doping with silicon resulted in a spectrally slightly narrower luminescence than doping with germanium. Donor concentrations and carrier mobilities were determined by Hall effect measurements at room temperature and we observe 20% higher electron mobilities for Ge-doping compared to Si-doping in the case of high dopant concentrations.Published under license by AIP Publishing. https://doi.

We report on recent doping experiments of cubic GaN epilayers by Ge and investigate in detail the optical properties by photoluminescence spectroscopy. Plasma-assisted molecular beam epitaxy was used to deposit Ge-doped cubic GaN layers with nominal thicknesses of 600 nm on 3C-SiC(001)/Si(001) substrates. The Ge doping level could be varied by around six orders of magnitude by changing the Ge effusion cell temperature. A maximum free carrier concentration of 3.7×1020 cm-3 was measured in the GaN layers via Hall-effect at room temperature. Low temperature photoluminescence (PL) showed a clear shift of the donor-acceptor emission to higher energies with increasing Ge-doping. Above a Ge concentration of ∼ 2x1018cm-3 the near band edge lines merge to one broad band. From temperature dependent measurements of the observed excitonic and donor-acceptor transitions a donor-energy of ∼ 36 meV could be estimated for Ge.

In cubic (c‐)GaN Ge has emerged as a promising alternative to Si for n‐type doping, offering the advantage of slightly improved electrical properties. Herein, a study on Ge doping of the ternary alloy c‐AlxGa1−xN is presented. Ge‐doped c‐AlxGa1−xN layers are grown by plasma‐assisted molecular beam epitaxy. In two sample series, both the Al mole fraction x and the doping level are varied. The incorporation of Ge is verified by time‐of‐flight secondary ion mass spectrometry. Ge incorporation and donor concentrations rise exponentially with increasing Ge cell temperature. A maximum donor concentration of 1.4 × 1020 cm−3 is achieved. While the incorporation of Ge is almost independent of x, incorporation of O, which acts as an unintentional donor, increases for higher x. Dislocation densities start increasing when doping levels of around 3 × 1019 cm−3 are exceeded. Also photoluminescence intensities begin to drop at these high doping levels. Optical emission of layers with x > 0.25 is found to originate from a defect level 0.9 eV below the indirect bandgap, which is not related to Ge. In the investigated range 0 ≤ x ≤ 0.6, Ge is a suitable donor in c‐AlxGa1−xN up to the low 1019 cm−3 range.