T. Frey

Phase separation suppression in InGaN epitaxial layers due to biaxial strain

In this letter, we show that external biaxial strain suppress spinodal phase separation in thin InGaN epitaxial layers pseudomorphically grown on thick unstrained cubic ~c! GaN~001! buffer layers. The InGaN films are terminated by a top GaN layer forming GaN/InGaN/GaN double heterostructures. By monitoring the alloy composition and thickness for a fixed growth temperature, we control the presence of biaxial strain induced by the rigid GaN buffer in the InGaN layers. We start by first showing from ab initio calculations of the alloy free energy taking strain into account that the biaxial strain is expected to induce a suppression of the miscibility gap leading to a single homogeneous phase for the InGaN alloys. We use high resolution x-ray diffraction ~HRXRD! reciprocal space maps to select the strained layers. We have shown recently that micro-Raman is an accurate tool to observe separate phases in InGaN epitaxial layers. 4,8 Micro-Raman spectroscopy measurements are also used in this work to demonstrate conclusively the suppression of the spinodal phase separation process in strained quantum wells. The c-GaN/In x Ga 12x N/GaN double heterostructures were grown on GaAs~001! substrates by molecular-beam epitaxy using a rf plasma nitrogen source. The GaN buffer layers were grown at T5720 °C with thicknesses of about 400 nm. The c-InGaN films were deposited at lower growth temperatures of 600 °C. The films were deposited at growth rates of 40 nm/h. The GaN cap layers, of about 30 nm thick, were grown at low temperatures of about 600 °C in order to reduce In desorption and interdiffusion. The growth front was continuously monitored by reflection high-energy electron diffraction and the diffraction patterns exhibited a cubic symmetry along all major azimuths.

STRUCTURAL PROPERTIES AND RAMAN MODES OF ZINC BLENDE INN EPITAXIAL LAYERS

We report on x-ray diffraction and micro-Raman scattering studies on zinc blende InN epitaxial films. The samples were grown by molecular beam epitaxy on GaAs(001) substrates using a InAs layer as a buffer. The transverse-optical (TO) and longitudinal-optical phonon frequencies at Γ of c-InN are determined and compared to the corresponding values for c-GaN. Ab initio self-consistent calculations are carried out for the c-InN and c-GaN lattice parameters and TO phonon frequencies. A good agreement between theory and experiment is found.

Refractive index and gap energy of cubic InxGa1−xN

Spectroscopic ellipsometry studies have been carried out in the energy range from 1.5 to 4.0 eV in order to determine the complex refractive indices for cubic InGaN layers with various In contents. The films were grown by molecular-beam epitaxy on GaAs(001) substrates. By studying GaN films, we prove that for the analysis of optical data, a parametric dielectric function model can be used. Its application to the InGaN layers yields, in addition, the composition dependence of the average fundamental absorption edge at room temperature. From the latter, a bowing parameter of 1.4 eV is deduced.

Photoluminescence from quantum dots in cubic GaN/InGaN/GaN double heterostructures

We have measured photoluminescence spectra of molecular-beam-epitaxy-grown cubic GaN/InxGa1−xN/GaN double heterostructures with x between 0.09 and 0.33. We observe a luminescence peak at about 2.3–2.4 eV which is almost independent of the InGaN layer composition. High-resolution x-ray diffraction measurements revealed a pseudomorphic In-rich phase with x=0.56±0.02 embedded in the InGaN layers. Including strain effects we calculate a gap energy Eg=2.13 eV of this phase. In cubic InGaN, spontaneous polarization and strain-induced piezoelectric fields are negligible. Therefore, the observed difference between the luminescence energy and the gap of the In-rich phase is assumed to be due to the localization of excitons at quantum-dot-like structures with a size of about 15 nm.

Evidence of phase separation in cubic InxGa1−xN epitaxial layers by resonant Raman scattering

Phase separation effects in cubic InxGa1−xN epitaxial layers were investigated by means of resonant Raman scattering. The alloy epilayers were grown by radio-frequency plasma-assisted molecular beam epitaxy on GaAs (001) substrates. The results, which are confirmed by x-ray diffractometry (XRD) experiments, show the presence of In-rich inclusions in c-InGaN layers with x=0.19 and 0.33. In-rich inclusions were also found by XRD in a lower In-content layer with x=0.07. Compositional inhomogeneity of about 10% was observed through selective resonances of localized regions in the In-rich separated inclusions. We find that the In-rich separated phase has nearly the same composition in all analyzed samples (x≅0.8).

INCORPORATION AND OPTICAL PROPERTIES OF MAGNESIUM IN CUBIC GAN EPILAYERS GROWN BY MOLECULAR BEAM EPITAXY

The incorporation and optical properties of Mg in cubic GaN (c-GaN) epilayers grown by rf plasma-assisted molecular beam epitaxy on (100) GaAs are investigated by secondary ion mass spectroscopy and low-temperature photoluminescence (PL). By varying the Mg flux by more than four orders of magnitude, the incorporation of Mg saturates at high Mg flux and is limited to a value of about 5×1018 cm−3 due to the high volatility of Mg at growth temperature. In addition, we observe an accumulation of Mg at the GaN/GaAs interface due to a diffusion of Mg to the GaAs substrate. Low-temperature PL spectra reveal several well-separated lines. Besides a shallow acceptor level at EA≅0.230 eV, additional Mg-related deep defect levels indicate an incorporation of Mg at off-gallium sites or as complexes.

RAMAN PHONON MODES OF ZINC BLENDE INXGA1-XN ALLOY EPITAXIAL LAYERS

Transverse-optical (TO) and longitudinal-optical (LO) phonons of zinc blende InxGa1−xN (0⩽x⩽0.31) layers are observed through first-order micro-Raman scattering experiments. The samples are grown by molecular-beam epitaxy on GaAs (001) substrates, and x-ray diffraction measurements are performed to determine the epilayer alloy composition. Both the TO and LO phonons exhibit a one-mode-type behavior, and their frequencies display a linear dependence on the composition. The Raman data reported here are used to predict the A1 (TO) and E1 (TO) phonon frequencies of the hexagonal InxGa1−xN alloy.

Cubic GaN epilayers grown by molecular beam epitaxy on thin β-SiC/Si (001) substrates

The molecular beam epitaxy of cubic GaN on Si(001) substrates, which were covered by a 4 nm thick β-SiC layer, is reported. The structural and optical properties of the cubic GaN epilayers were studied by transmission electron microscopy, high-resolution x-ray diffraction, and low-temperature photoluminescence measurements. We find clear evidence for the growth of cubic GaN layers almost free of hexagonal inclusions. The density of extended defects and the near band edge photoluminescence of the cubic GaN layers grown at substrate temperatures of 835 °C is comparable to that of high quality cubic GaN epilayers grown by molecular beam epitaxy on GaAs (001) substrates.

Phase separation and gap bowing in zinc-blende InGaN, InAlN, BGaN, and BAlN alloy layers

Abstract We present first-principles calculations of the thermodynamic and electronic properties of the zinc-blende ternary In x Ga 1− x N, In x Al 1− x N, B x Ga 1− x N, and B x Al 1− x N alloys. They are based on a generalized quasi-chemical approximation and a pseudopotential-plane-wave method. T – x phase diagrams for the alloys are obtained. We show that due to the large difference in interatomic distances between the binary compounds a significant phase miscibility gap for the alloys is found. In particular for the In x Ga 1− x N alloy, we show also experimental results obtained from X-ray and resonant Raman scattering measurements, which indicate the presence of an In-rich phase with x ≈0.8. For the boron-containing alloy layers we found a very high value for the critical temperature for miscibility, ∼9000 K , providing an explanation for the difficulties encountered to grow these materials with higher boron content. The influence of a biaxial strain on phase diagrams, energy gaps and gap bowing of these alloys is also discussed.

Inter- and Intraband Transitions in Cubic Nitride Quantum Wells

In this work we analyze the luminescence emissions from selected isolated GaN/InGaN quantum wells comparing measured and theoretical photoluminescence (PL) spectra. The calculations are performed within the k.p method by means of an 8 x 8 Kane Hamiltonian, generalized to treat different materials. Strain effects due to the large lattice mismatch between InN and GaN are taken into account. From the direct comparison with experimental results, we found evidence for transitions involving confined levels which, besides those related to quantum dots, may be ascribed to the first electron-heavy-hole transition in the quantum wells. Since the studies of optical properties of quantum wells based on cubic nitrides are at an early stage, the results reported here will provide guidelines for the interpretation of forthcoming experiments.