Search

GaN/AlxGa1−xN quantum wells were grown by molecular beam epitaxy on high quality bulk (0001) GaN substrates. The quantum well thickness was set in the 6–8 nm range to favor the photoluminescence emission of indirect excitons. Indeed, such excitons are known to be spatially indirect due to the presence of the internal electric field which spatially separates the electron and hole wave functions. The growth conditions were optimized in view of minimizing the photoluminescence peak broadening. In particular, the impact of growth temperature (up to 900 °C) on the surface morphology, structural, and photoluminescence properties was studied. The diffusion of indirect excitons on the scale of tens of micrometers was measured with a micro-photoluminescence setup equipped with a spatially resolved detection. A dedicated model and its analysis allow us to extract from these measurements the exciton diffusion constant and to conclude on the optimum growth conditions for the GaN/AlxGa1−xN quantum well structures suited for studies of quantum collective effects in indirect exciton liquids. [ABSTRACT FROM AUTHOR]

We present Al2O3-ZnAl2O4-ZnO nanostructure, which could be a prominent candidate for optoelectronics, mechanical and sensing applications. While ZnO and ZnAl2O4 composites are mostly synthesized by sol-gel technique, we propose a solid-vapor growth mechanism. To produce Al2O3-ZnAl2O4-ZnO nanostructure, we conduct ZnO:C powder heating resulting in ZnO nanowires (NWs) growth on sapphire substrate and ZnAl2O4 spinel layer at the interface. The nanostructure was examined with Scanning Electron Microscopy (SEM) method. Focused Ion Beam (FIB) technique enabled us to prepare a lamella for Transmission Electron Microscopy (TEM) imaging. TEM examination revealed high crystallographic quality of both spinel and NW structure. Epitaxial relationships of Al2O3-ZnAl2O4 and ZnAl2O4-ZnO are given. [ABSTRACT FROM AUTHOR]

The bandgaps of short period ZnO/(Zn)MgO superlattices deposited on c-, m-, and a-ZnO substrates were examined both theoretically and experimentally. Ab initio calculations showed that the bandgaps of c-oriented polar superlattices are smaller than those of nonpolar ones; however, this is mainly due to different geometric configurations, because the influence of internal electric fields existing in polar superlattices is not very significant. The calculations revealed that for 5–6 MgO monolayers in the barriers, the bandgap values become independent of the barrier thickness, which suggests that such superlattices can be treated as sets of isolated ZnO wells. In the experimental part of this work, it is demonstrated that short period ZnO/MgO and ZnO/ZnMgO superlattices can be grown successfully on differently oriented crystalline bulk ZnO substrates using molecular beam epitaxy. The bandgaps of the superlattices were determined from low temperature photoluminescence measurements. It is shown that they agree well with the theoretical results. [ABSTRACT FROM AUTHOR]

Structural and electronic properties of MgZnO and BeMgZnO alloys are studied by the ab-initio Density Functional Theory method. Large band gap bowings are found for both kinds of alloys. The total energies as functions of the lattice constants are calculated and used to determine the ranges of composition in which the alloys are stable in the wurtzite structure. It is shown that the addition of 6% of Be can already help in stabilization of the MgZnO alloy in the wurtzite structure. The band gap can reach 7 eV for the wurtzite BexMg0.5Zn0.5-xO alloys with x approaching 0.5 and about 5.0 eV for Be0.125MgxZn0.875-xO type alloys for x approaching 0.6. Varying the alloy composition according to the presented stabilization diagram showing ranges of the x, y, for which BexMgyZn1-x-yO is stable in the wurtzite phase, one may tune band gaps over a wide spectral range, which provides flexibility in band gap engineering. [ABSTRACT FROM AUTHOR]

Optical-absorption analysis of energy gap in bulk single crystals of GaN and an epitaxial layer grown on a sapphire substrate indicates temperature dependence of the absorption edge and the importance of free-electron concentrations to absorption edge positions. The different free-electron concentrations of bulk single crystals and the epitaxial layer lead to their varying negative temperature coefficients. Variation of absorption edge and temperature dependence of the absorption edge in various GaN samples are explained by the Burstein- Moss effect. The strong nonlinear shift of energy gap with temperature, noted even at high temperatures, is due to the high Debye temperature of GaN crystals.

The implantation of zirconium ions in the epitaxial film of gallium nitrate through annealing method resulted in its optical activation. Gallium nitrate decomposition was avoided by applying pressure which also helped in increasing the annealing temperature of up to 1550 degrees celsius versus a limit of 1000 to 1100 degrees at ambient pressure. The annealing method over 1350 degrees celsius maximized the zirconium-acceptor-related photoluminescence intensity.

We investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320 K range. We first show that the effective lifetime of QW excitons τ increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing τ depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs. [ABSTRACT FROM AUTHOR]

High quality GaN/AlGaN multiquantum well (QW) structures were grown by ammonia molecular beam epitaxy along the (0001) polar and [formula] nonpolar directions. Each sample contains three QWs with thicknesses of 2, 3, and 4 nm as well as 10 nm Al0.30Ga0.70N barriers. The measured photoluminescence (PL) spectrum consists of three peaks originating from the radiative recombination of excitons in individual QWs. In the nonpolar sample, the energy positions (EPL) of the observed peaks are separated because of the quantum confinement effect, whereas in the polar sample an additional redshift is induced by the quantum confined Stark effect. The dependence of EPL on QW width was used to estimate the built-in electric field magnitude in the latter sample to be about 2 MV/cm. Hydrostatic pressure studies of the PL in both samples gave qualitatively different results. In the polar sample, the pressure shift of EPL, dEPL/dp decreases significantly with QW width. The important finding is derived from the observation of a QW width independent dEPL/dp in the nonpolar sample. It shows that for GaN/Al0.30Ga0.70N, the quantum confinement remains practically independent of the applied hydrostatic pressure. This result reveals that in the polar sample, the variation in dEPL/dp with the QW width is due to the pressure-induced increase in the built-in electric field Fint. Thus, a more quantitative analysis of the latter effect becomes justified. We found that the Fint increases with pressure with a rate of about 80 kV(cm GPa)-1. [ABSTRACT FROM AUTHOR]

We investigated the relation between structural properties and carrier recombination processes in InGaN/GaN multiple quantum well (MQW) structures with quantum well widths of 3 and 9 nm, grown by metal-organic chemical-vapor deposition on bulk GaN crystals. Quantum barriers of the samples are heavily n-type doped in order to effectively screen the large polarization-induced electric fields which commonly occur in hexagonal InGaN/GaN quantum structures. High thermal stability in these structures, reflected by strong photoluminescence (PL) even above 400 K, is attributed to a combination of low-dislocation densities and potential profile fluctuations in the InGaN/GaN quantum wells. The role of potential profile fluctuations is further investigated by time-resolved photoluminescence and cathodoluminescence (CL) mapping. Comparison of both samples shows that the sample with 3-nm-wide QWs exhibits (i) a larger width of the PL peak in the temperature range of 8-420 K, (ii) a higher amplitude of potential profile fluctuations as measured by CL mapping, and (iii) higher radiative and nonradiative PL recombination times. At the same time a much weaker drop of PL intensity with temperature is recorded on the sample with 9-nm-wide QWs. Our results show that, contrary to intuitive expectation, a decrease of the potential profile fluctuation amplitude can be helpful in enhancing the radiative recombination efficiency, particularly at high temperatures. [ABSTRACT FROM AUTHOR]

Beryllium is a promising dopant for obtaining an efficient p-type conductivity of GaN. A high-pressure study of the donor-acceptor-pair (DAP) photoluminescence line at 3.39 eV in Be-doped GaN samples is reported, together with ab initio calculations of the electronic structure of GaN:Be under pressure. The analysis confirms the formation of a Be-acceptor state which is involved in the DAP photoluminescence. Different properties of the Mg- and the Be-acceptor states in GaN are found. The Be level is located ∼100–140 meV lower in the band gap than that of Mg, and its pressure coefficient is lower by approximately 3–4 meV/GPa. [ABSTRACT FROM AUTHOR]

X-ray diffractometry helps determine the lattice constants of two types of gallium nitride samples, bulk monocrystal grown at pressure of 15 kbar and epitaxial layer growth on a sapphire substrate, at temperatures 294-753 K. The presence of a smaller lattice constant in the epitaxial layer in a direction parallel to the interface plane is due to an increased thermal expansion of the sapphire compared to GaN layer. The thermal strain is significant only below 500 K. The lattice mismatch decreases with increasing temperature, diminishing to zero at about 800 K.

Contactless electroreflectance (CER) has been applied to study optical transitions between the ground and excited states in polar and a-plane nonpolar 2 nm wide GaN/Al0.12Ga0.88N quantum well (QW) structures. In addition to the fundamental transition, CER features related to optical transitions between excited states were clearly observed for the polar QW structure whereas such features were not observed for the nonpolar QW structure. This experimental result clearly shows that the polarization-related electric field leads to a quantum confinement of some extra states in the polar QW system. Such states are not confined in the nonpolar QW and, therefore, optical transitions between them are not detected, i.e., only the fundamental transition is observed in CER spectrum of the nonpolar QW. [ABSTRACT FROM AUTHOR]