O. Zsebök

The effect of the first GaN monolayer on the nitridation damage of molecular beam epitaxy grown GaN on GaAs(001)

The initial molecular beam epitaxy growth of GaN on GaAs(001) was studied by real-time monitoring of the (3×3) surface reconstruction and its transition to an unreconstructed (1×1). Various growth conditions were established by variation of the V/III ratio, i.e., the Ga flux. We characterized the effect of the first two strained GaN monolayers: a N-terminated GaN (3×3) monolayer and a second unreconstructed (1×1) monolayer. A series of samples were grown under N-rich, Ga-rich, and near-stoichiometric growth conditions. The resulting morphology of the interface region was analyzed by high-resolution scanning electron microscopy, Auger-electron spectroscopy, and double crystal x-ray diffractometry. The N-rich and Ga-rich conditions resulted in extensive defect formation due to the nitridation damage of the GaAs substrate. The extent of this was found to be determined by the properties of the first GaN monolayer. The surface roughness under optimum growth conditions could be as low as ∼20 nm, defined by nano...

The Formation of Nitridation Damage during the Growth of GaN on GaAs(001)

We report a correlation between the surface reconstruction transition during the initial phase of the molecular beam epitaxy growth of GaN on GaAs(001) and the nitridation damage determined final layer morphology. In order to study the formation of the nitrogen damage under a wide range of growth conditions, a series of layers were grown by changing the V/III-ratio through the Ga-flux, while keeping all other growth parameters constant. The results, describing the strong effect of the first N-terminated GaN (3×3) monolayer, were summarised in defect formation models for N-rich and Ga-rich conditions, defining the means of near-stoichiometric growth.

Surface Morphology of MBE-grown GaN on GaAs(001) as Function of the N/Ga-ratio

Molecular beam epitaxy growth utilising an RF-plasma nitrogen source was used to study surface reconstruction and surface morphology of GaN on GaAs (001) at 580 °C. While both the nitrogen flow and plasma excitation power were constant, the grown layers were characterised as a function of Ga-flux. In the initial growth stage a (3×3) surface reconstruction was observed. This surface periodicity only lasted up to a maximum thickness of 2.5 ML, followed by a transition to the unreconstructed surface. Samples grown under N-rich, Ga-rich and stoichiometric conditions were characterised by high-resolution scanning electron microscopy and atomic force microscopy. We found that the smoothest surfaces were provided by the N/Ga-ratio giving the thickest layer at the (3×3)=>(1×1) transition. The defect formation at the GaN/GaAs interface also depended on the N/Ga-flux ratio.

Surface Morphology and Structure of GaN x As 1−x

GaN x As 1−x -layers were grown by solid source molecular beam epitaxy using N 2 and excitation by RF-plasma source. The average nitrogen concentration, x, determined by secondary ion mass spectrometry, ranges from isovalent nitrogen doping in GaAs up to GaN. X-ray diffraction revealed two peaks, close to the ones for GaAs(002) and the GaN(002) diffraction, respectively. The position of both peaks moved slightly as a function of nitrogen content. Scanning electron microscopy indicated a rough surface structure with improved smoothing for low and high nitrogen compositions. The rough surface is partly due to crystallite formation when mixing arsenic and nitrogen as a result of phase separation between the GaAs and GaN. In ternary films with very high nitrogen composition there were structural features with a different contrast. These were crystallites that arose at the interface and grew up to the surface.

Influence of N/Ga-flux ratio on optical properties and surface morphology of GaN grown on sapphire(0001) by MBE

Abstract We studied the correlation between the low-temperature optical properties and the surface morphology of molecular beam epitaxy (MBE) grown GaN on sapphire(0001). The samples were grown under Ga-rich conditions, with the Ga-flux as a parameter, and with all other growth parameters constant. High-resolution scanning electron microscopy (SEM) provided a measure of the surface morphology, while the optical properties were characterised by low-temperature photoluminescence. These spectra were dominated by the exciton bound to neutral donor transition, at 3.472–3.474 eV, indicating fairly strain relaxed layers. This peak width is increased when the surface morphology improved. Our results also showed a clear correlation between the optical properties and the purity of the nitrogen source, as improved oxygen purification improved the photoluminescence linewidth from 41 to 20 meV.

Effects of small amounts of Al in GaN grown on sapphire (0001) by molecular beam epitaxy

A series of 0.7 μm thick GaN layers have been grown by solid-source rf-plasma assisted molecular beam epitaxy on sapphire (0001) substrates with the addition of 0.10%–3.30% Al. The Al concentration was determined by secondary ion-mass spectrometry and Auger-electron spectroscopy, while the layer quality was assessed by photoluminescence, Hall effect measurement, and high-resolution scanning electron microscopy. Microscopy revealed a surface roughness varying with Al content. The smallest surface roughness was obtained at 0.10% and 3.30% Al. Low-temperature photoluminescence revealed dominating peaks attributed to the neutral donor-bound exciton. Its energy increased slightly with Al concentration, which established a correlation between the Al concentration and the band gap. The surface morphology, the corresponding optical and electrical properties, showed a clear improvement of the GaN layer quality for the range of 0.10%–0.17% Al content.

Nanocrystals at MBE-grown GaN/GaAs(001) interfaces

Abstract Molecular beam epitaxy (MBE) growth utilising an RF-plasma nitrogen source was used to study surface reconstruction and the effects of nitridation damage on the surface and interface morphology of GaN on GaAs(001) at 580°C. Keeping both the N-flow and plasma excitation power constant, the grown layers were studied with the Ga-flux as a parameter. In the initial growth stage, a (3×3) surface reconstruction was observed. Samples grown under N-rich, Ga-rich and stoichiometric conditions were characterised by high-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) to reveal the surface morphology. The nitridation damage was characterised by Auger-electron spectroscopy (AES), X-ray diffraction (XRD) spectroscopy and high-resolution SEM, which revealed separated GaN and GaAs phases at interfaces deeply in the substrate region, bordered by {101} and {111} facets. The defect formation at the GaN/GaAs interface depended on the N/Ga-flux ratio.

Characterisation of surface morphological defects in MBE-grown GaN0.1As0.9 layers on GaAs

Abstract Layers of GaN 0.1 As 0.9 were grown by molecular beam epitaxy (MBE) on GaAs(001). By high-resolution scanning electron microscopy (SEM), 3–4 μm surface defects were observed in 4-μm-thick layers. Their origin is a combination of nitrogen reaction with the substrate and the lattice mismatch. These defects had a conical shape and their walls consisted of GaN nano-crystallites with solved arsenic as detected by Auger-electron spectroscopy (AES).

Surface morphology and compositional variations in molecular beam epitaxy grown GaNxAs1−x alloys

Abstract GaN x As 1−x -layers were grown by molecular beam epitaxy with a nitrogen concentration ranging from low N-doping concentration in GaAs up to GaN. The average nitrogen concentration was assessed by secondary ion mass spectrometry and energy dispersive X-ray spectrometry. X-ray diffraction measurements provided two peaks from the grown layer, one arising from a lattice constant close to the GaAs(004) and one close to the GaN(004) diffraction, due to phase-separation. Scanning electron microscopy indicated a rough surface structure with improved smoothing for low and high nitrogen concentrations. Crystallite features were observed on the rough surfaces and characterised by Auger-electron spectroscopy.

The Effect of Al in Plasma-Assisted MBE-Grown GaN

We have grown GaN, with addition of a 0.10 to 0.33 % Al, on sapphire(0001) substrates by solid-source RF-plasma assisted MBE. The Al-concentration was determined by secondary ion-mass spectrometry and Auger-electron spectroscopy, while the layer quality was assessed by photoluminescence and high-resolution scanning electron microscopy. Microscopy revealed a meandering pattern and a surface roughness varying with Al-content. The smallest surface roughness was obtained at 0.10 % Al. Photoluminescence revealed two main peaks attributed to the neutral donor-bound exciton. Its energy increased slightly with Al-concentration, which established a correlation between the Al-concentration and the band gap.