J. R. Leite

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.

Lattice parameter and energy band gap of cubic AlxGayIn1−x−yN quaternary alloys

First-principles total energy calculations, combined with a generalized quasichemical approach to disorder and compositional effects, are used to obtain the lattice parameter and the energy band gap of cubic AlxGayIn1−x−yN quaternary alloys. It is found that the lattice parameter a(x,y) fulfills a Vegard’s-like law; that is, it shows a linear dependence on the alloy contents x and y. The range of compositions for which the alloy is lattice-matched to GaN is obtained. The energy band gap Eg(x,y) of the quaternary alloy deviates from a planar behavior displaying a two-dimensional gap bowing in the x–y plane. Analytical expressions that fit the calculated a(x,y) and Eg(x,y) surfaces are derived in order to provide ready access to the lattice parameter and energy band gap of the alloy for the entire range of compositions. The results are compared with data for the wurtzite phase alloys.

InAs/GaAs quantum dots optically active at 1.5 μm

InAs quantum dots grown by molecular-beam epitaxy on GaAs substrates are demonstrated to be suitable structures to achieve an optical emission in the 1.3–1.5-μm range. Their tuning towards such long wavelengths was made possible by combining an extreme reduction of the InAs growth rate and a fast growth of the GaAs cap layer at low temperature. Our results create perspectives for the fabrication of GaAs-based devices operating in the most important telecommunications window.

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).

Room temperature ferromagnetism in cubic GaN epilayers implanted with Mn + ions

Mn ions were implanted in p-type cubic GaN at doses from 0.6 to 2.4×1016cm−2 at 200 keV energy. A 200-nm-thick epitaxial layer, grown by molecular beam epitaxy on GaAs(001) substrate, is used for the Mn implantation. The Mn implanted samples were subjected to an annealing at 950 °C for 1–5 min. The structural quality of the samples was investigated by high resolution x-ray diffraction and Raman spectroscopy. The annealing procedure leads to a significant increasing of the crystalline quality of the samples. Hysteresis loops were observed for all cubic GaMnN annealed samples and ferromagnetism was detected up to room temperature.

Influence of the temperature and excitation power on the optical properties of InGaAs/GaAs quantum wells grown on vicinal GaAs(001) surfaces

Photoluminescence experiments were performed as a function of temperature and excitation intensity in order to investigate the optical properties of In0.1Ga0.9As/GaAs quantum wells grown on vicinal GaAs(001) substrates with different miscut angles. The misorientation of the surface played an important role and influenced the intensity, efficiency, energy, and full width at half maximum of the optical emission, as well as the segregation of indium atoms. It is shown that at high temperature the optical properties of InGaAs quantum wells grown on vicinal substrates are slightly inferior to ones of the same structure grown a nominal surface because of the faster escape of the carriers.

Temperature dependence of optical transitions in AlGaAs

AlGaAs structures with different aluminum concentration (x=0.0, 0.17, 0.30, and 0.40) were characterized by photoluminescence and photoreflectance techniques. The temperature dependence of optical transitions in the temperature ranging from 2 to 300 K were investigated. Y. P. Varshni [Physica (Utrecht) 34, 194 (1967)], L. Vina et al. [Phys. Rev. B 30, 1979 (1984)], and R. Passler [Phys. Status Solidi B 200, 155 (1997)] models were used to fit the experimental points. The Passler model gave the best adjustment to the experimental points. The tree models showed that the empirical parameters obtained through the adjustment of the experimental data in the three different models are aluminum composition dependent in the ternary alloy.

Sensitivity-enhanced reflection Z-scan by oblique incidence of a polarized beam

We demonstrate a new experimental method which allows the measurement of the nonlinear optic index in absorptive media with great sensitivity. In this technique the reflection from a polarized Gaussian laser beam close to the Brewster angle is measured. A sensitivity enhancement factor of 30 with respect to other techniques is observed for an optical crystal, and higher values are possible to be obtained.

Hole confinement effects on multiple Si δ doping in GaAs

The observation of quantum‐confined optical transitions in multiple δ doping in GaAs, grown by molecular beam epitaxy, is reported. Doping efficiency and carrier confinement are investigated by Hall and photoluminescence measurements. Hall measurement results for multiple δ‐doped samples show a dramatic enhancement of carrier concentrations compared to the uniform doping case. From photoluminescence spectra we observed that the cutoff energy is significantly affected by the spacing between the dopant sheets. The strong localization of confined photoexcited holes in the spacing layers of these structures plays a fundamental role in the interpretation of the optical data.