V. N. Jmerik

Mie Resonances, Infrared Emission, and the Band Gap of InN

Mie resonances due to scattering or absorption of light in InN-containing clusters of metallic In may have been erroneously interpreted as the infrared band gap absorption in tens of papers. Here we show by direct thermally detected optical absorption measurements that the true band gap of InN is markedly wider than the currently accepted 0.7 eV. Microcathodoluminescence studies complemented by the imaging of metallic In have shown that bright infrared emission at 0.7-0.8 eV arises in a close vicinity of In inclusions and is likely associated with surface states at the metal/InN interfaces.

AlGaN-based quantum-well heterostructures for deep ultraviolet light-emitting diodes grown by submonolayer discrete plasma-assisted molecular-beam epitaxy

Features of plasma-assisted molecular-beam epitaxy of AlGaN compounds at relatively low temperatures of the substrate (no higher than 740°C) and various stoichiometric conditions for growth of the nitrogen- and metal-enriched layers are studied. Discrete submonolayer epitaxy for formation of quantum wells and n-type blocking layers without varying the fluxes of components was used for the first time in the case of molecular- beam epitaxy with plasma activation of nitrogen for the nanostructures with the AlxGa1 − xN/AlyGa1 − yN quantum wells. Structural and optical properties of the AlxGa1 − xN layers in the entire range of compositions (x = 0–1) and nanostructures based on these layers are studied; these studies indicate that there is photoluminescence at room temperature with minimum wavelength of 230 nm. Based on the analysis of the photoluminescence spectra for bulk layers and nanoheterostructures and their temperature dependences, it is concluded that there are localized states in quantum wells. Using the metal-enriched layers grown on the c-Al2O3 substrates, heterostructures for light-emitting diodes with AlxGa1 − xN/AlyGa1 − yN quantum wells (x = 0.4–0.5, y = x + 0.15) were obtained and demonstrated electroluminescence in the ultraviolet region of the spectrum at the wavelength of 320 nm.

Solar-blind UV photocathodes based on AlGaN heterostructures with a 300- to 330-nm spectral sensitivity threshold

Solar-blind UV photodetectors based on photocathodes are among the important applications of heterostructures based on group III metal nitride semiconductors. Related investigations are most frequently devoted to photocathodes with p-GaN active regions characterized by a long-wavelength sensitivity threshold at 360 nm. Since the detected radiation is mostly concentrated in the spectral range below 240–290 nm, corresponding displacement of the long-wavelength sensitivity threshold of photodetectors by using photocathodes with p-AlGaN active regions is a topical task. We present preliminary results on manufacturing photocathodes with a p-AlxGa1 − xN (x = 0.1 and 0.3) active region (possessing a long-wavelength sensitivity threshold at 330 and 300 nm, respectively).

Terahertz electroluminescence of surface plasmons from nanostructured InN layers

The terahertz (THz) emission with the maximum at the 3–5 THz is observed under electrical pumping from InN epilayers, where structural imperfections such as nanocolumns, pores, and In clusters are situated with a certain periodicity. This radiation is ascribed to the spontaneous emission of surface plasmon polariton like waves guided in thin surface/interior layers and coupled to electromagnetic field at these imperfections. These waves are generated thermally under applied electric field. The registered THz spectra and power dependences are well consistent with this plasmon-related radiation mechanism.

Quantum-confined stark effect and localization of charge carriers in Al0.3Ga0.7N/Al0.4Ga0.6N quantum wells with different morphologies

The electric fields in Al0.3Ga0.7N/Al0.4Ga0.6N quantum wells are estimated. The quantum wells are grown by plasma-assisted molecular-beam epitaxy with plasma activation of nitrogen. The three-dimensional and planar modes of buffer layer growth are used. The transition to the three-dimensional mode of growth yields a substantial increase in the photoluminescence intensity of the quantum wells and a shift of the photoluminescence line to shorter wavelengths. These effects are attributed to the fact that, because of the extra three-dimensional localization of charge carriers in the quantum-well layer, the quantum-confined Stark effect relaxes. The effect of localization is supposedly due to spontaneous composition fluctuations formed in the AlGaN alloy and enhanced by the three-dimensional growth.

Controlling active nitrogen flux in plasma-assisted molecular beam epitaxy of group III nitrides

The intensity of the flux of activated nitrogen from an RF inductively coupled discharge source for the plasma-assisted molecular beam epitaxy (PAMBE) of group III nitrides (A3N) can be linearly controlled using a modified output diaphragm design and increased nitrogen supply (∼5 sccm). This source provides a linear variation of the maximum A3N growth rate from 0.2 to 0.8 μm/h for the RF power controlled between 110 and 200 W, respectively. The use of excited nitrogen molecules favorably influences the growth of GaN and InN epilayers, which are characterized by a perfect structure and high optical quality.

Phonon-assisted exciton luminescence in GaN layers grown by MBE and chloride-hydride VPE

Optical properties of GaN layers grown by the molecular-beam epitaxy (MBE) and chloride-hydride vapor-phase epitaxy (CHVPE) have been studied, and the quality of two types of samples has been compared. The photoluminescence spectra have an excitonic nature for both types of layers. To determine precisely the nature of exciton transitions, the reflectance spectra were studied. A key point was the investigation of phonon-assisted exciton luminescence, which provides information on the density distribution of the exciton states. Temperature dependences of the exciton transition energy and the ratio of intensities of one-and two-phonon replicas were studied. The high quality of both types of layers has been confirmed, though the concentration of acceptors in MBE-grown samples is higher than in CHVPE samples.

Nanometric-scale fluctuations of intrinsic electric fields in GaN/AlGaN quantum wells with inversion domains

Strain and electric field fluctuations in regions of different polarities in GaN/AlGaN quantum well (QW) structures of dominant N-polarity with inversion domains (IDs) split the photoluminescence (PL) emission into two bands. Micro-PL and time-resolved PL studies reveal strong inhomogeneity of the array of the IDs, where essential parameters, such as strain, electric fields, and sizes are fluctuating quantities. We demonstrate also that the ID formation decreases the intrinsic electric field magnitudes.

Coaxial rf-magnetron nitrogen activator for GaN MBE growth

Abstract Novel compact coaxial magnetron nitrogen activator with a radio frequency (rf) capacitively-coupled discharge has been used for the first time for GaN molecular beam epitaxial growth on different substrates, including GaAs(113). Optical emission spectra of the discharge have been studied as a function of nitrogen flow rate, rf power, and magnetic field, focusing on first negative (391 nm) and second positive (380 nm) lines associated with nitrogen molecular ions and excited molecules, respectively. Optimization of the activator parameters and distance of the discharge zone from the substrate resulted in GaN growth rate as high as 0.5 μm h −1 at a 350 l s −1 pumping speed.

RHEED monitoring of elastic stresses during MBE growth of group III nitride heterostructures

Results of statistical analysis of the patterns of reflection high-energy electron diffraction (RHEED) measured during molecular beam epitaxy (MBE) of heterostructures of wide-bandgap (Al, Ga, In)N compounds are presented, which can be used for determining the lateral lattice constant (a) of the epitaxial layers. It is established that the error of determination can vary from a minimum of 0.17% up to several percent, depending on the contrast of RHEED patterns, which is determined by the stoichiometry of MBE growth. It is demonstrated that the RHEED data can reveal relaxation of elastic stresses during the growth of short-period {GaN(4 nm)/AlN(6nm)}30/AlN superlattices.