D. V. Nechaev

Suppression of the quantum-confined Stark effect in AlxGa1−xN/AlyGa1−yN corrugated quantum wells

We report comparative studies of 6-nm-thick AlxGa1−xN/AlyGa1−yN pyroelectric quantum wells (QWs) grown by plasma-assisted molecular beam epitaxy on c-sapphire substrates with a thick AlN buffer deposited under different growth conditions. The Al-rich growth conditions result in a 2D growth mode and formation of a planar QW, whereas the N-rich conditions lead to a 3D growth mode and formation of a QW corrugated on the size scale of 200–300 nm. Time-resolved photoluminescence (PL) measurements reveal a strong quantum-confined Stark effect in the planar QW, manifested by a long PL lifetime and a red shift of the PL line. In the corrugated QW, the emission line emerges 200 meV higher in energy, the low-temperature PL lifetime is 40 times shorter, and the PL intensity is stronger (∼4 times at 4.5 K and ∼60 times at 300 K). The improved emission properties are explained by suppression of the quantum-confined Stark effect due to the reduction of the built-in electric field within the QW planes, which are not nor...

Site-controlled GaN nanocolumns with InGaN insertions grown by MBE

The site-controlled plasma-assisted molecular beam epitaxy (PA MBE) has been developed to fabricate the regular array of GaN nanocolumns (NCs) with InGaN insertions on micro-cone patterned sapphire ...

Metal-Semiconductor Nanoheterostructures with an AlGaN Quantum Well and In Situ Formed Surface Al Nanoislands

We report on fabrication and studies of composite heterostuctures consisting of an Al0.55Ga0.45N/Al0.8Ga0.2N quantum well and surface Al nanoislands, grown by plasma-assisted molecularbeam epitaxy on c-sapphire substrates. The influence of a substrate temperature varied between 320 and 700ºC on the size and density of the deposited Al nanoislands is evaluated. The effect of Al nanoislands on decay kinetics of the quantum well middle-ultraviolet photoluminescence has been investigated by time resolved photoluminescence. The samples with the maximum density of Al nanoislands of 108 cm–2 and lateral dimensions in the range of 100–500 nm demonstrated shortening of the photoluminescence lifetime, induced by interaction of the emitting quantum well and the plasmonic metal particles.

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.

Kinetics of Metal-Rich PA Molecular Beam Epitaxy of AlGaN Heterostructures for Mid-UV Photonics

Abstract Main peculiarities of Plasma-Assisted MBE (PA MBE) of atomically smooth and droplet-free AlGaN binaries and ternary layers with a variable compositional inhomogeneity at the controlled III-rich stoichiometric conditions are described. A thorough analysis of AlGaN PA MBE growth kinetics and thermodynamics at both continuous and pulsed growth methods as well as at varied stoichiometric conditions and growth temperatures is used to elucidate specific features of this technology. The structural and optical properties of the AlGaN epilayers are discussed. We describe a novel precise technique for fabrication of Al x Ga 1- x N/Al y Ga 1- y N QW structures by submonolayer digital alloying epitaxy with an in situ control of the QW thickness and composition at atomic resolution. The main results on PA MPE of SQW and MQW AlGaN-based heterostructures for the optically and electron-beam-pumped mid-UV emitters ( λ

HAADF STEM and PL Characterization of Monolayer-Thick GaN/(Al,Ga)N Quantum Wells for Deep UV Optoelectronics Applications

A few monolayer-thick (ML) GaN quantum well (QW) structures are promising for high-power optoelectronic applications in mid-UV wavelength ranges [1-2]. High Resolution Transmission Electron Microscopy (HRTEM) and especially High Angle Annual Dark Field (HAADF) Transmission Electron Microscopy (STEM) appear to be a key metrology to establish a relationship between atomic structure and optical properties of nanostructures. TEM/STEM also provides an effective pathway for optimization of crystal growth of ML thick nanostructures [1]. Here we report TEM analysis of 1-5 ML thick GaN QWs in conjunction with optical research of peculiarities of their electronic band structure.

Stress in (Al, Ga)N heterostructures grown on 6H-SiC and Si substrates byplasma-assisted molecular beam epitaxy

The paper describes experimental results on low temperature plasma-assisted molecular beam epitaxy of GaN/AlN heterostructures on both 6H-SiC and Si(111) substrates. We demonstrate that application of migration enhanced epitaxy and metal-modulated epitaxy for growth of AlN nucleation and buffer layers lowers the screw and edge(total)threading dislocation (TD) densities down to 1.7108 and 2109 cm-2, respectively, in a 2.8-μm-thick GaN buffer layer grown atop of AlN/6H-SiC. The screw and total TD densities of 1.2109 and 7.4109 cm-2, respectively, were achieved in a 1-μm-thickGaN/AlNheterostructure on Si(111). Stress generation and relaxation in GaN/AlN heterostructures were investigated by using multi-beam optical stress sensor (MOSS) to achieve zero substrate curvature at room temperature. It is demonstrated that a 1-μm-thick GaN/AlN buffer layer grown by PA MBE provides planar substrate morphology in the case of growth on Si substrates whereas 5-μm-thick GaN buffer layers have to be used to achieve the same when growing on 6H-SiC substrates.

Coexistence of type-I and type-II band line-ups in 1-2 monolayer thick GaN/AlN single quantum wells

GaN/AlN quantum wells (QWs) with varied nominal thickness of 0.5-4 monolayers have been studied by time-resolved photoluminescence (PL) spectroscopy. The structures demonstrate an emission peak with the thickness-dependent wavelength in the range 225-320 nm. The observed temporal behavior of PL between 225 and 280 nm can be described as a superposition of fast and slow decaying components with characteristic decay time constants of the order of 0.1-0.7 ns and 7-30 ns, respectively. The fast PL component with the decay time smaller than 1 ns dominates in the thicker GaN insertions and tends to vanish in the thinnest ones, where the slow PL component becomes progressively longer. These observations imply formation in the GaN/AlN monolayer-thick layers of an inhomogeneous excitonic system involving both direct and indirect in space excitons.

Control of stress and threading dislocation density in the thick GaN/AlN buffer layers grown on Si (111) substrates by low- temperature MBE

We report on successful growth by plasma-assisted molecular beam epitaxy on a Si(111) substrate crack-free GaN/AlN buffer layers with a thickness more than 1 μm. The layers fabricated at relatively low growth temperature of 780°C have at room temperature the residual compressive stress of -97 MPa. Intrinsic stress evolution during the GaN growth was monitored in situ with a multi-beam optical system. Strong dependence of a stress relaxation ratio in the growing layer vs growth temperature was observed. The best-quality crack-free layers with TDs density of ~109 cm-2 and roughly zero bowing were obtained in the sample with sharp 2D-GaN/2D-AlN interface.

Enhanced photoluminescence efficiency in AlGaN quantum wells with gradient-composition AlGaN barriers

We present photoluminescence studies of AIxGa1-xN/AlyGa1-yN (y = x+0.3) quantum well (QW) heterostructures with graded AI content in barrier layers, emitting in the range 285-315 nm. The best-established internal quantum efficiency of the QW emission is as high as 81% at 300 K, owing to enhanced activation energy of charge carriers and exciton binding energy in the QW heterostructure with optimized design.