The structure of GaN layers grown on SiC and sapphire by molecular beam epitaxy

A comparative study of the defects at the interfaces and inside the layers was carried out in GaN/AlN epitaxial layers on SiC and sapphire. Whereas surface cleaning of the sapphire substrates is rather standardised now, the SiC substrates cleaning is still to optimise conditions, as the high densities of defects inside the epitaxial layers cannot be explained solely by the 3.54% lattice mismatch. The investigated specimens were grown by molecular beam epitaxy (MBE), either assisted by electron cyclotron resonance or an NH₃ gas source system to provide atomic nitrogen. Assuming that MBE is a growth technique more or less close to equilibrium, the observed defects are interpreted and a growth mechanism, for GaN layers on the stepped (0001) SiC and sapphire surfaces, is proposed.     

Structural and optical properties of GaN films grown on GaAs substrates by molecular beam epitaxy

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. X-ray diffraction and transmission electron microscopy indicate that this method promotes prismatic growth of c-oriented α-GaN. Photoluminescence studies show that the emission from cubic β-GaN inclusions dominates the spectrum.     

InAs/GaAs nanostructures grown on patterned Si(001) by molecular beam epitaxy

The potential benefit from the combination of the optoelectronic and electronic functionality of III-V semiconductors with silicon technology is one of the most desired outcomes to date. Here we have systematically investigated the optical properties of InAs quantum structure embedded in GaAs grown on patterned sub-micron and nanosize holes on Si(001). III-V material tends to accumulate in the patterned sub-micron holes and a material depletion region is observed around holes when GaAs/InAs/GaAs is deposited directly on patterned Si(001). By use of a 60?nm SiO(2) layer and patterning sub-micron and nanosize holes through the oxide layer to the substrate, we demonstrate that high optical quality InAs nanostructures, both quantum dots and quantum wells, formed by a two-monolayer InAs layer embedded in GaAs can be epitaxially grown on Si(001). We also report the power-dependent and temperature-dependent photoluminescence spectra of these structures. The results show that hole diameter (sub-micron versus nanosize) has a strong effect on the structural and optical properties of GaAs/InAs/GaAs nanostructures.     

Epitaxial growth of ZnO layers on (111) GaAs substrates by laser molecular beam epitaxy

ZnO layers were grown on (111) GaAs substrates by laser molecular epitaxy at substrate temperatures between 200 and 550 °C. X-ray diffraction analysis revealed that c-axis of ZnO epilayer with a wurtzite structure is perpendicular to the substrate surface. X-ray rocking curves and Raman spectroscopy showed that the crystal quality of ZnO epilayers depends on the substrate temperature during the growth. Strong near-band-edge emission in the UV region without any deep-level emissions was observed from the ZnO epilayers at room temperature. The results indicate that laser molecular beam epitaxy is a promising growth method for obtaining high-quality ZnO layers on (111) GaAs substrates.     

GaAsN/GaAs and InGaAsN/GaAs heterostructures grown by molecular beam epitaxy

Molecular beam epitaxy was used to fabricate GaAsN/GaAs and InGaAsN/GaAs heterostructures, and the influence of the growth regimes on their characteristics was studied. It is shown that implantation of nitrogen causes a substantial long-wavelength shift of the radiation. The possibility of obtaining 1.4 μm radiation at room temperature was demonstrated using In_0.28Ga_0.72As_0.97N_0.03/GaAs quantum wells. Pis’ma Zh. Tekh. Fiz. 24, 81–87 (December 12, 1998)     

Structural characterization of molecular beam epitaxy grown ZnSe-based layers on GaAs substrates for blue–green laser diodes

ZnSe films and fully developed p-on-n laser structures, including CdZnSe-active and ZnSSe-guiding layers were grown by molecular beam epitaxy (MBE) on lattice matched p-GaAs, p-AlGaAs and p-GaInP buffer layers. The structural characteristics of these layers were studied by combined cross-section and planar view transmission electron microscopy (TEM). The defect density of the ZnS_xSe_1−x epilayers was shown to be very low, <10⁵ cm⁻². However, on their interfaces with the GaAs substrate, a high density of small dislocation loops and clusters of the order of 3×10¹⁰ cm⁻² was observed. In situ TEM experiments revealed that dislocations and stacking faults (SFs) were generated under the electron beam influence. From them, the perfect dislocations were confined at the ZnSe/GaAs interface, while the SFs propagated to the ZnSe overgrowth or the GaAs substrate, having one of their partial dislocations at the interface. The generation of dislocations under the electron beam was not related to radiation damage but to thermal strain, which was developed by the heating effect due to differential thermal dilatation. Defects around the active zone of fully developed p-on-n laser structures were also studied. The nature of such defects was defined by Burgers vector determination experiments. The critical role of growth variations, such as compositional changes resulting in strain, in the MBE process of II–VI materials was demonstrated. The destructive role of the defected guiding layers in the laser structure was shown.     

Structural characterization of high temperature AlN intermediate layer in GaN grown by molecular beam epitaxy

Transmission electron microscopy has been used to study the structural quality of GaN grown on sapphire by plasma assisted molecular beam epitaxy using high temperature AlN intermediate layers with different thicknesses. The introduction of an AlN intermediate layer with an optimum thickness is observed to minimize the density of dislocations reaching the overgrown GaN surface. In this sample, the measured threading dislocation density reaching the surface of 1×10¹⁰ cm⁻² resulted to be seven times lower than that of a reference sample, without any AlN interlayer. The bending at the GaN/AlN interface and following interactions between dislocations have been observed in cross-sectional transmission electron micrographs. This fact explains the decrease of dislocation density reaching the GaN surface.     

Structural characterization of InAs quantum dot chains grown by molecular beam epitaxy on nanoimprint lithography patterned GaAs(100)

We combine nanoimprint lithography and molecular beam epitaxy for the site-controlled growth of InAs quantum dot chains on GaAs(100) substrates. We study the influence of quantum dot growth temperature and regrowth buffer thickness on the formation of the quantum dot chains. In particular, we show that by carefully tuning the growth conditions we can achieve equal quantum dot densities and photoluminescence ground state peak wavelengths for quantum dot chains grown on patterns oriented along the [011], [01 ?1], [011] and [001] directions. Furthermore, we identify the crystal facets that form the sidewalls of the grooves in the differently oriented patterns after capping and show that the existence of (411)A sidewalls causes reduction of the QD density as well as sidewall roughening.     

Wafer-level photocatalytic water splitting on GaN nanowire arrays grown by molecular beam epitaxy

We report on the achievement of wafer-level photocatalytic overall water splitting on GaN nanowires grown by molecular beam epitaxy with the incorporation of Rh/Cr(2)O(3) core-shell nanostructures acting as cocatalysts, through which H(2) evolution is promoted by the noble metal core (Rh) while the water forming back reaction over Rh is effectively prevented by the Cr(2)O(3) shell O(2) diffusion barrier. The decomposition of pure water into H(2) and O(2) by GaN nanowires is confirmed to be a highly stable photocatalytic process, with the turnover number per unit time well exceeding the value of any previously reported GaN powder samples.     

Self-assembled flower-like nanostructures of InN and GaN grown by plasma-assisted molecular beam epitaxy

Nanosized hexagonal InN flower-like structures were fabricated by droplet epitaxy on GaN/Si(111) and GaN flower-like nanostructure fabricated directly on Si(111) substrate using radio frequency plasma-assisted molecular beam epitaxy. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the crystallinity and morphology of the nanostructures. Moreover, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to investigate the chemical compositions and optical properties of nano-flowers, respectively. Activation energy of free exciton transitions in GaN nano-flowers was derived to be ∼28.5 meV from the temperature dependent PL studies. The formation process of nano-flowers is investigated and a qualitative mechanism is proposed.     

Plasma-assisted molecular beam epitaxy of ZnO on in-situ grown GaN/4H-SiC buffer layers

Plasma-assisted molecular beam epitaxy (MBE) was used to grow ZnO(0001) layers on GaN(0001)/4H-SiC buffer layers deposited in the same growth chamber equipped with both N- and O-plasma sources. The GaN buffer layers were grown immediately before initiating the growth of ZnO. Using a substrate temperature of 440°C–445°C and an O₂ flow rate of 2.0–2.5 sccm, we obtained ZnO layers with smooth surfaces having a root-mean-square roughness of 0.3 nm and a peak-to-valley distance of 3 nm shown by AFM. The FWHM for X-ray rocking curves recorded across the ZnO(0002) and ZnO(101ˉ5) reflections were 200 and 950 arcsec, respectively. These values showed that the mosaicity (tilt and twist) of the ZnO film was comparable to corresponding values of the underlying GaN buffer. It was found that a substrate temperature >450°C and a high Zn-flux always resulted in a rough ZnO surface morphology. Reciprocal space maps showed that the in-plane relaxation of the GaN and ZnO layers was 82.3% and 73.0%, respectively and the relaxation occurred abruptly during the growth. Room-temperature Hall-effect measurements showed that the layers were intrinsically n-type with an electron concentration of 10¹⁹ cm^–3 and a Hall mobility of 50 cm²·V^–1·s^–1.     

Structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm Si (111) by ammonia molecular beam epitaxy

The structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm diameter Si (111) substrate by ammonia molecular beam epitaxy have been reported. High resolution X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy and secondary ion mass spectroscopy have been used to study the influence of AlN thickness and AlGaN growth temperature on the quality of GaN. GaN grown on thicker AlN showed reduced dislocation density and lesser tensile strain. Three-dimensional growth regime was observed for GaN grown at lower AlGaN growth temperature while higher AlGaN growth temperature resulted in two-dimensional growth mode. The dislocation bending and looping at the AlGaN/AlN interface was found to have significant influence on the dislocation density and strain in the GaN layer. The evolution and interaction of threading dislocations play a major role in determining the quality and the strain states of GaN.     

Growth of cubic GaN by molecular-beam epitaxy on porous GaAs substrates

It is shown that GaN layers can be grown on (100)-and (111)-oriented porous single-crystal GaAs substrates by molecular-beam epitaxy with plasma activation of the nitrogen by an rf electron cyclotron resonance discharge. The resulting undoped epitaxial layers possessed ntype conductivity with a carrier concentration ∼10¹⁸. Data obtained by scanning electron microscopy and cathodoluminescence indicate that at thicknesses ∼2000 Å, continuous layers of the cubic GaN modification are obtained regardless of the substrate orientation. Pis’ma Zh. Tekh. Fiz. 25, 3–9 (January 12, 1999)     

Effect of cleaving environment on the growth of ZnSe on the GaAs (1 1 0) surface by molecular beam epitaxy

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Structural perfection of heteroepitaxial silicon layers grown on sapphire by sublimation-source molecular beam epitaxy

Structurally perfect, single-crystal silicon layers have been grown on \((1\overline 1 02)\) sapphire by sublimation-source molecular beam epitaxy. Electron and x-ray diffraction data demonstrate that silicon-on-sapphire epitaxy occurs at substrate temperatures from 550 to 850°C. As the thickness of the layers decreases from 1.0 to 0.2 μm, their structural perfection degrades. In the layers grown at 600°C, the density of nucleation sites in the initial stages of growth is ? 5 × 10⁹ cm^?2.     

Ultraviolet photodetectors with MgZnO nanowall networks grown by molecular beam epitaxy on Si(1 1 1) substrates

Mg_0.05Zn_0.95O nanowall networks ultraviolet (UV) photodetector was fabricated on Si(1 1 1) by plasma-assisted molecular beam epitaxy. Based on the Mg_0.05Zn_0.95O nanowall networks, planar geometry photoconductive type metal-semiconductor-metal photodetector was fabricated. At 5 V bias, the peak responsivity of 24.65 A/W was achieved at 352 nm, corresponding to an external quantum efficiency of ∼8490%. Such high external quantum efficiency was attributed to the photoconductive gain, which can be explained by the presence of oxygen-related hole-trap states at the nanowall surface. The response time of 25 ms was determined by the measurements of photocurrent versus modulation frequency.