J.V. Thordson

Properties of molecular-beam epitaxy-grown GaNAs from optical spectroscopy

GaNxAs1−x layers with different nitrogen concentrations x grown on (001)GaAs substrates by molecular-beam epitaxy have been studied by photoluminescence, optical absorption, and Raman spectroscopy. The content of nitrogen in the layers was determined by x-ray diffraction and secondary-ion-mass spectrometry. The samples can be classified in three categories with respect to the concentration of N: with doping nitrogen concentration, with average content of N less than 0.3, and with x close to 1. From optical measurements and from analysis of x-ray diffraction spectra, different phases are observed in the GaNxAs1−x layers: GaAs, GaN, and the solid ternary solution GaNxAs1−x. In Raman spectra both GaAs-like and GaN-like optical phonons are observed. We have estimated the fundamental band-gap energy in the GaNxAs1−x alloy with low nitrogen concentration up to x=0.04 from absorption measurements, and in GaNxAs1−x with high nitrogen concentration x>0.96 from photoluminescence spectra. Fitting of the experimental...

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...

Inverse parabolic quantum well and its quantum‐confined Stark effect

An inverse parabolic quantum well was successfully grown by molecular‐beam epitaxy using a digital compositional grading superlattice composed of Al0.36Ga0.64As/GaAs. The photoluminescence and photocurrent measurements for this structure gave a good agreement between experimental and theoretical results. Large Stark shift and amplitude reduction of 1e‐1hh exciton resonance under applied electric field were found in the photoluminescence spectra, which are substantially larger than the conventional square quantum well. These properties benefited from the concept of local‐to‐global state transitions.

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.

Influence of cap layer thickness on optical quality in In0.2Ga0.8As/GaAs single quantum wells

Influences of GaAs cap layer thickness on residual strain in partially relaxed, 25-nm-thick In0.2Ga0.8As/GaAs single quantum wells have been investigated by photoluminescence and photoreflectance at 77 K. It was found that the residual strain increased and the optical quality improved with increasing cap layer thickness. Therefore, both quantum well and cap layer thicknesses determine the optical quality in lattice-mismatched semiconductor heterostructures.

Spiral growth of GaAs by molecular beam epitaxy

Steps of monolayer height (0.28 nm) were observed by atomic force microscopy on a GaAs surface grown by molecular beam epitaxy. The monolayer terrace width between steps was found to be as large as 1000 nm in some areas. Surface reconstruction affects the surface diffusion process during growth and the shape of the step edges. Growth spirals were observed. Spirals originate from screw dislocations. The growth mechanism is according to the Burton–Cabrera–Frank theory.

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.

THE ORDERED-DISORDERED TRANSITION IN SI DELTA-DOPED GAAS

We report results for Si layers embedded in GaAs, extending from the delta‐doped (δ‐doped) range up to 6 monolayers (MLs) derived by means of variable temperature resistivity and Hall effect measurements, secondary ion mass spectrometry (SIMS), and high resolution x‐ray diffractometry (HRXRD) techniques. The conductivity transition from free carrier transport in ordered δ‐layers (<1 ML) to strongly localized two‐dimensional variable range hopping (2D‐VRH) transport under potential fluctuation disordered conditions (≳4 ML) is clearly observed. This observation is in good agreement with the SIMS and HRXRD data. Results from the intermediate case with 2–3 MLs are also discussed.

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.

Growth and characterization of GaAs/Si/GaAs heterostructures

Abstract GaAs/Si/GaAs heterostructures ranging from ∼ 2% of a monolayer (ML) up to 6 monolayers were grown by molecular beam epitaxy at 500°C, using the dopant furnace for the silicon evaporation. The samples were characterized by Hall effect measurements, X-ray diffraction, and secondary ion mass spectroscopy. A reduced carrier concentration with doping in the 10 13 cm −2 region is discussed. A strong decrease in electron mobility at 1 monolayer is a result of the transition from a doped layer to a GaAs/Si/GaAs heterostructure. The relaxation of the structure is commenced between 3 and 4 monolayers of silicon. Secondary ion mass spectroscopy indicated no or negligible Si diffusion in the strained case up to 3 MLs while a clear peak broadening was seen in the relaxed case.