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 Al _x Ga_{1 ? x} N/Al _y Ga_{1 ? y} N quantum wells. Structural and optical properties of the Al _x Ga_{1 ? x} N 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 Al _x Ga_{1 ? x} N/Al _y Ga_{1 ? y} N 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.     

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.     

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.     

Features of the spatial distribution of indium in InGaN epitaxial layers grown by plasma-assisted molecular beam epitaxy

The processes leading to the formation of a spatially nonuniform distribution of indium in the In _x Ga_{1 ? x} N layers with x = 0?0.6 grown by molecular beam epitaxy with plasma activation of nitrogen at relatively low growth temperatures (590–630°C) are studied. It is found that at low values of x < 0.1, the growth proceeds pseudomorphically at least to a thickness of 70 nm, and these layers are characterized by a high uniformity of the In distribution, which confirms their thermodynamic stability. Upon increasing x up to ～0.3, the signs of a nonuniform In distribution are observed, which is associated with the stress relaxation facilitating the development of phase separation. It is shown that layers with In distribution of lower uniformity feature more intense photoluminescence in the energy range 2.0–2.5 eV. For the layers with x = 0.6, complete phase separation is observed with the formation of several phases with a wide range of compositions, including the region in the vicinity of the InN binary compound.     

Structural and Dynamical Properties of Short-Period GaN/AlN Superlattices: Experiment and Theory

Semiconductors - We report the results of systematic experimental and theoretical studies of structural and dynamical properties of short-period GaN/AlN superlattices. The multilayer structures...     

Stress generation and relaxation in (Al,Ga)N/6<Emphasis Type="Italic">H</Emphasis>-SiC heterostructure grown by plasma-assisted molecular-beam epitaxy

In situ stress generation and relaxation in Al_0.25Ga_0.75N/GaN/AlN heterostructure with an overall thickness exceeding 3 μm in the process of its growth on a 6H-SiC substrate by low-temperature plasma-assisted molecular-beam epitaxy at substrate temperatures ranging from 690 to 740°C was studied. At room temperature, AlN and GaN layers revealed residual compressive stresses of–2.3 and–0.1 GPa, respectively. This made it possible to avoid cracking during postgrowth cooling of the structure.     

AlGaN Nanostructures with Extremely High Room-Temperature Internal Quantum Efficiency of Emission Below 300 nm

We present theoretical optimization of the design of a quantum well (QW) heterostructure based on AlGaN alloys, aimed at achievement of the maximum possible internal quantum efficiency of emission in the mid-ultraviolet spectral range below 300 nm at room temperature. A sample with optimized parameters was fabricated by plasma-assisted molecular beam epitaxy using the submonolayer digital alloying technique for QW formation. High-angle annular dark-field scanning transmission electron microscopy confirmed strong compositional disordering of the thus-fabricated QW, which presumably facilitates lateral localization of charge carriers in the QW plane. Stress evolution in the heterostructure was monitored in real time during growth using a multibeam optical stress sensor intended for measurements of substrate curvature. Time-resolved photoluminescence spectroscopy confirmed that radiative recombination in the fabricated sample dominated in the whole temperature range up to 300 K. This leads to record weak temperature-induced quenching of the QW emission intensity, which at 300 K does not exceed 20% of the low-temperature value.     

Site-Controlled Growth of GaN Nanorods with Inserted InGaN Quantum Wells on μ-Cone Patterned Sapphire Substrates by Plasma-Assisted MBE

We report on a new approach to fabricate regular arrays of GaN nanorods (NRs) with InGaN QWs by plasma-assisted molecular-beam epitaxy (PA MBE) on micro-cone patterned sapphire substrates (μ-CPSSs). A two-stage PA MBE fabrication process of GaN NRs has been developed, starting with a high temperature nucleation layer growth at metal-rich conditions to aggregate selectively GaN nucleus on c-oriented areas of the μ-CPSSs and followed by growth of 1-μm-thick GaN NRs at strongly nitrogen-rich conditions exactly on the cone tips. These results are explained by energetically favorable GaN growth on the (000-) oriented sapphire surface. Both micro-photoluminescence and micro-cathodoluminescence confirm the formation of regular array of optically and spectrally isolated NRs without usage of any nanolithography.

     

Specific features of the cathodoluminescence spectra of AlInGaN QWs,caused by the influence of phase separation and internal electric fields

The specific features of the cathodoluminescence (CL) spectra in AlInGaN heterostructures, caused by the influence of phase separation and internal electric fields, observed at varied CL excitation density, are studied. It is shown that the evolution of the CL spectrum and the variation in the spectral position of emission lines of nanoscale layers with current density in the primary electron beam makes it possible to identify the occurrence of phase separation in the layer and, in the absence of this separation, to estimate the electric-field strength in the active region of the structure.