A. S. Usikov

Room-temperature photopumped InGaN/GaN/AlGaN vertical-cavity surface-emitting laser

We report photopumped room-temperature surface-mode lasing at 401 nm in a InGaAlN vertical-cavity surface-emitting laser grown on a sapphire substrate using metal–organic vapor-phase epitaxy. A 2λ cavity was formed by a quarter-wave Al0.15Ga0.85N/GaN distributed Bragg reflector on the one side of the active layer and a GaN–air interface on the other. A multilayer structure composed of 12-fold-stacked ultrathin InGaN insertions in a GaN matrix served as an active layer providing ultrahigh material gain and making possible vertical lasing without use of the upper Bragg reflector.

Influence of metalorganic chemical vapor deposition growth conditions on In-rich nanoislands formation in InGaN/GaN structures

The influence of different growth conditions on the In distribution in ultrathin InGaN insertions in a GaN matrix is investigated by high-resolution transmission electron microscopy and an appropriate image evaluation technique. It is demonstrated that the indium distribution represents dense arrays of In-rich nanodomains inserted in a layer with a lower indium concentration. The sizes of the In-rich regions are about 4–5 nm at a growth temperature of 720u200a°C. Increasing the growth temperature leads to a strong decrease in the of nanoisland density and, also, a moderate decrease in their lateral size. Increasing the trimethylindium/trimethylgallium ratio strongly increases the density of the islands, but the lateral size remains weakly effected. The observations are in agreement with a thermodynamic model of island formation including entropy effects.

Point defects in gamma-irradiated n-GaN

Radiation-induced point defects and their annealing in silicon-doped n-GaN have been investigated by means of Hall effect measurements and Raman spectroscopy. Correlated compensation effects due to simultaneous introduction of donor and acceptor centres are observed in irradiated n-GaN. The defect production rate is dependent on the dopant concentration. This means that the model of all native defects immobile at room temperature is not true. The behaviour of radiation-induced defects upon heating is complicated, exhibiting two prominent stages of reverse annealing. The presence of radiation defects is still observable after annealing to T = 750 °C.

Deep hole traps in undoped n-GaN films grown by hydride vapor phase epitaxy

Deep hole traps were studied in bulk free-standing GaN crystals and in thinner (10–20u2009μm) GaN films prepared by hydride vapor phase epitaxy (HVPE) on sapphire. Six hole traps in different combinations were detected in these crystals, H1 (activation energy 0.92–0.94u2009eV), H2 (0.55u2009eV), H3 (0.65–0.7u2009eV), H4 (0.85–0.9u2009eV), H5 (1.1–1.2u2009eV), and H6 (0.95–1.05u2009eV). The dominant traps in all samples were the H5 and H6 traps that were attributed, respectively, to gallium vacancy complexes with oxygen (VGa-O) and substitutional carbon related centers. We associate the H5 hole traps with the red luminescence bands, the H4 hole traps with the green luminescence bands, and the H6 hole traps with the yellow luminescence bands often observed in HVPE GaN. These attributions are based on the low energy thresholds of the deep traps optical excitation spectra and the depth of the respective trap levels.

Radiation-induced defects in n-type GaN and InN

The electrical properties of the n-GaN and n-InN, subjected to proton irradiation, are studied. The irradiation of the n-InN results in an increasing concentration of charge carriers, whereas strong compensation effects take place in the proton-irradiated n-GaN. The annealing behavior of the radiation-induced defects in both materials is discussed briefly.

Surface-mode lasing from stacked InGaN insertions in a GaN matrix

We report surface-mode lasing in a structure with 12-fold stacked InGaN insertions in a GaN matrix without using of Bragg mirrors. At high excitation densities, one of the modes of the Fabry–Perot cavity formed by the GaN sapphire and the GaN air interfaces, shows a strong superlinear increase in intensity with excitation density rise. The possibility to reach surface lasing in a very low finesse microcavity is due to the ultrahigh material gain of the InGaN insertions. The strong modulation of the absorption-gain spectrum with increase in the excitation density results in a pronounced energy shift of the cavity modes. We found that the threshold excitation density is weakly affected by temperature up to 110 K, while increases at higher temperatures. This behavior is attributed to thermal evaporation of carriers from InN-rich nanodomains and is typical for quantum dot lasers.

Photopumped InGaN/GaN/AlGaN Vertical Cavity Surface Emitting Laser Operating at Room Temperature

Room temperature operation in the wavelength range of 401 to 415 nm has been successfully realized in InGaN/GaN/AlGaN vertical cavity surface emitting lasers (VCSELs) under photoexcitation. The VCSELs are grown by metal-organic vapor phase deposition and composed of a 2l vertical cavity including twelvefold stacked multiple InGaN insertions in a GaN matrix grown on top of a quarter-wave strain-compensated Al0.15Ga0.85N/GaN distributed Bragg reflector.

Quantum dots formed by ultrathin insertions in wide-gap matrices

We report on experimental and theoretical studies on a new type of quantum-dot (QD) structures obtained using ultrathin, i.e. below the critical thickness for 2D‐3D transition, strained narrow gap insertions in wide bandgap matrices. We concentrate on submonolayer (SML) or slightly above 1 ML CdSe insertions in a wide-gap II‐VI matrices and give the first results on ultrathin InGaN insertions in a GaN matrix. A discussion on detailed comparison of our original results with the results of other authors is presented. The formation of dense arrays (up to 10 12 cm 22 ) of nanoscale two-dimensional (2D) islands is revealed in processed high-resolution transmission electron microscopy images. In the case of stacked sheets of SML insertions, the islands in the neighboring sheets are formed predominantly in correlated or anticorrelated way for thinner and thicker spacer layers, respectively. Different polarization of photoluminescence (PL) emission recorded in edge geometry for vertically-uncoupled and coupled QDs confirms the QD nature of excitons. By monitoring of sharp lines due to single QDs using cathodoluminescence the 3D confinement is manifested. We demonstrate significant squeezing of the QD exciton wavefunction in the lateral direction using magneto-optical experiments. We point to complete suppression of lateral motion of excitons bound to islands in case of wide-gap (ZnMgSSe) matrices, as follows from PL excitation studies. A resonant (0-phonon) lasing is observed in ultrathin CdSe insertions and proves the lifting of the k-selection rule for QD excitons. We show that lack of exciton screening in QDs up to high excitation densities enables strong resonant modulation of the refractive index in stacked ultrathin insertions and allows realization of resonant (excitonic) waveguiding and lasing. This enables the realization of a new type of heterostructure laser operating without external optical confinement by layers having lower average refractive indices. Ultrahigh QD excitonic gain in dense arrays of stacked QDs allows a new type of surface-emitting laser. q 2000 Elsevier Science S.A. All rights reserved.

Hydride vapor phase GaN films with reduced density of residual electrons and deep traps

Electrical properties and deep electron and hole traps spectra are compared for undoped n-GaN films grown by hydride vapor phase epitaxy (HVPE) in the regular process (standard HVPE samples) and in HVPE process optimized for decreasing the concentration of residual donor impurities (improved HVPE samples). It is shown that the residual donor density can be reduced by optimization from ∼1017u2009cm−3 to (2–5)u2009×u20091014u2009cm−3. The density of deep hole traps and deep electron traps decreases with decreased donor density, so that the concentration of deep hole traps in the improved samples is reduced to ∼5u2009×u20091013u2009cm−3 versus 2.9u2009×u20091016u2009cm−3 in the standard samples, with a similar decrease in the electron traps concentration.

A new approach to analysis of mosaic structure peculiarities of gallium nitride epilayers

Abstract The successful results of multifractal analysis application to a quantitative description of mosaic structure peculiarities, which are typical for GaN epitaxial layer with hexagonal modification grown on (0xa00xa00xa01) sapphire substrates, have been obtained. A linear dependence of mobility on the multifractal parameters of surface topology of the mosaic structure (the self-organization degree and the disruption of the local symmetry) has been observed in GaN layers.