Molecular Beam Epitaxy Growth and Characterization of Germanium‐Doped Cubic Al_xGa_1−xN

Abstract

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.

Optical evidence of many-body effects in the zincblende Al x Ga 1−x N alloy system

“…The samples were previously investigated by high-resolution x-ray diffraction (HRXRD) and reflection measurements to determine the Alconcentration and the AlGaN-layer thickness, respectively. A more detailed description of this as well as the growth process and substrate properties can be found elsewhere [19][20][21][22][23][24].…”

“…Here, two different sample series are presented. The samples of series A were grown with different Al-concentrations, between ≈10% and ≈70%, while the samples of series B hold very similar Al-concentrations of ≈25%, but offer different doping concentrations up to n ≈ 10 20 cm −3 [21]. Furthermore, GaN and AlN reference samples are present in series A.…”

The infuence of compressive lattice deformations on the zone-center energy band properties of zincblende GaN and InN. Hybrid density functional results

“…The natural appearance of these compounds corresponds to a crystalline structure of the wurtzite (WZ, hexagonal) type, which reveals as their lowest-enthalpy allotropic form. However, the zincblende (ZB, cubic) phase is only slightly higher in energy, and it has been possible to grow it in the form of thin films with suitable thermodynamical stability [4][5][6][7][8][9][10][11][12][13]. Contrary to the wurtzite phase, the zincblende -as a centrosymmetric material-lacks internal electric polarizations in the unit cell; which has been named as the source of emission efficiency loss when using WZ III-V nitrides as core elements of light-emitting diodes and semiconductor lasers (see [14,15] and references therein).…”