E. Sakalauskas

Dielectric function and optical properties of Al-rich AlInN alloys pseudomorphically grown on GaN

A detailed discussion of the optical properties of Al-rich Al1−xInxN alloy films is presented. The (0u20090u20090u20091)-oriented layers with In contents between x = 0.143 and x = 0.242 were grown by metal-organic vapour phase epitaxy on thick GaN buffers. Sapphire or Si(1u20091u20091) served as the substrate. High-resolution x-ray diffraction revealed pseudomorphic growth of the nearly lattice-matched alloys; the data analysis yielded the composition as well as the in-plain strain. The complex dielectric function (DF) between 1 and 10u2009eV was determined from spectroscopic ellipsometry measurements. The sharp onset of the imaginary part of the DF defines the direct absorption edge, while clearly visible features in the high-photon energy range of the DF, attributed to critical points (CPs) of the band structure, indicate promising crystalline quality of the AlInN layers. It is demonstrated that the experimental data can be well reproduced by an analytical DF model. The extracted characteristic transition energies are used to determine the bowing parameters for all CPs of the band structure. In particular, strain and the high exciton binding energies for the Al-rich alloys are taken into account in order to assess the splitting between the valence band with symmetry and the conduction band at the centre of the Brillouin zone. Finally, the compositional dependence of the high-frequency dielectric constants is reported.

Dielectric function and optical properties of quaternary AlInGaN alloys

The optical properties of quaternary Alx Iny Ga1-x-yN alloy films with 0.16< x<0.64 and 0.02< y<0.13 are presented. The (0001)-oriented AlInGaN layers were grown by metal-organic vapor phase epitaxy on thick GaN/sapphire templates. High-resolution x-ray diffraction measurements revealed the pseudomorphic growth of the AlInGaN films on the GaN buffer. Rutherford backscattering and wavelength-dispersive x-ray spectroscopy analysis were used in order to determine the composition of the alloys. The ordinary dielectric function (DF) of the AlInGaN samples was determined in the range of 1–10 eV by spectroscopic ellipsometry (SE) at room temperature (synchrotron radiation: BESSY II). The sharp onset of the imaginary part of the DF defines the direct absorption edge of the alloys. At higher photon energies, pronounced peaks are observed in the DF indicating a promising optical quality of the material. These features are correlated to the critical points of the band structure (van Hove singularities). An analytica...

N-type conductivity and properties of carbon-doped InN(0001) films grown by molecular beam epitaxy

In this work, we have analyzed the effect of intentional carbon doping on molecular beam epitaxy grown In-polar InN epilayers using carbon bromide (CBr4) as dopant source. Hall effect measurements, high resolution X-ray diffraction, atomic force microscopy, transmission electron microscopy, secondary ion mass spectrometry, spectroscopic ellipsometry, as well as X-ray photoelectron spectroscopy were employed to characterize the influence of different dopant concentrations on the electrical, optical, crystallographic, morphological, and electronic properties of InN. It was found that the electron concentration increases linearly with the incorporation of carbon pointing towards the effect of n-type doping and that incorporated C impurities reduce the electron mobility within the InN films. This correlation is further reflected in associated properties such as the onset of optical absorption, the plasmon frequency, the effective electron mass and the position of the bulk and surface Fermi level. Furthermore,...

Luminescence from two-dimensional electron gases in InAlN/GaN heterostructures with different In content

The luminescence properties of InxAl1−xN/GaN heterostructures are investigated systematically as a function of the In content (xu2009=u20090.067u2009−u20090.208). The recombination between electrons confined in the two-dimensional electron gas and free holes in the GaN template is identified and analyzed. We find a systematic shift of the recombination with increasing In content from about 80u2009meV to only few meV below the GaN exciton emission. These results are compared with model calculations and can be attributed to the changing band profile and originating from the polarization gradient between InAlN and GaN.

Injection-Activated Defect-Governed Recombination Rate in InN

Excess carrier dynamics was investigated by free-carrier absorption and light-induced transient grating techniques in InN layers with residual electron density varying from n0=1.4×1018 to 4.7×1018 cm-3 in a wide excitation range (up to 1020 cm-3). Carrier lifetime τ decreased with injected carrier density ΔN≥n0 and followed the same inverse relationship as on residual electron density τ∝[B(n0+ΔN)]-1, thus confirming defect-related recombination mechanism. Its nonradiative origin was verified by τ(T) measurements and ascribed to injection-enhanced nonlinear recombination via defect-assisted Auger recombination with CTAAR= B/NT=(4.5±2)×10-28 cm6/s, assuming the defect density NT being equal to electron density. Oxygen or hydrogen impurities are proposed as possible candidates for traps assisting in Auger process.

Systematic Optical Characterization of Two-Dimensional Electron Gases in InAlN/GaN-Based Heterostructures with Different In Content

Model calculations have been performed to study systematically the formation of a two-dimensional electron gas (2DEG). The results are used for analyzing the photoluminescence properties of corresponding InAlN/GaN heterostructures (HS) for various In concentrations (x = 6.7–20.8%). We found a luminescence peak, clearly dependent on the In content, that is attributed to the recombination between electrons in the 2DEG at the second level (En=2) and photoexcited holes in the GaN buffer. The results can be understood with the changing band profile attributed to the different polarization gradient between InAlN and GaN.