R. J. Hauenstein

PRESSURE-DEPENDENT PHOTOLUMINESCENCE STUDY OF WURTZITE GAN

Low‐temperature photoluminescence (PL) in single‐crystal GaN films grown on sapphire substrates by metalorganic chemical vapor deposition has been studied as a function of applied hydrostatic pressure using the diamond‐anvil‐cell technique. The PL spectra of the GaN at atmospheric pressure were dominated by two sharp, strong, near‐band‐edge exciton luminescence lines and a broad emission band in the yellow spectral region. The exciton emission lines were found to shift almost linearly toward higher energy with increasing pressure. While the yellow emission band showed a similar blue shift behavior under applied pressure, a relatively strong sublinear pressure dependence was observed. By examining the pressure dependence of the exciton emission structures, the pressure coefficient of the direct Γ band gap in the wurtzite GaN was determined. The value of the hydrostatic deformation potential of the band gap has also been deduced from the experimental results.

Schottky‐based band lineups for refractory semiconductors

An overview is presented of band alignments for small-lattice parameter, refractory semiconductors. The band alignments are estimated empirically through the use of available Schottky barrier height data, and are compared to theoretically predicted values. Results for tetrahedrally bonded semiconductors with lattice constant values in the range from C through ZnSe are presented. Based on the estimated band alignments and the recently demonstrated p-type dopability of GaN, we propose three novel heterojunction schemes which seek to address inherent difficulties in doping or electrical contact to wide-gap semiconductors such as ZnO, ZnSe, and ZnS.

REFLECTION HIGH ENERGY ELECTRON DIFFRACTION STUDY OF NITROGEN PLASMA INTERACTIONS WITH A GAAS (100) SURFACE

Effect of a nitrogen electron‐cyclotron‐resonance (ECR) microwave plasma on near‐surface composition, crystal structure, and morphology of the As‐stabilized GaAs (100) surface is investigated with the use of digitally image‐processed in situ reflection high energy electron diffraction. Nitridation is performed on molecular beam epitaxially (MBE) grown GaAs surfaces near 600 °C under typical conditions for ECR microwave plasma‐assisted MBE growth of GaN films on GaAs. Brief plasma exposures (≊3–5 s) are shown to result in a specular, coherently strained, relatively stable, GaN film approximately one monolayer in thickness, which can be commensurately overgrown with GaAs while longer exposures (up to 1 min) result in incommensurate zincblende epitaxial GaN island structures. Specular and nonspecular film formations are explained in terms of N‐for‐As surface and subsurface anion exchange reactions, respectively. Commensurate growth of ultrathin buried GaN layers in GaAs is achieved.

ATOMIC-SCALE STRUCTURE AND ELECTRONIC PROPERTIES OF GAN/GAAS SUPERLATTICES

We have investigated the atomic‐scale structure and electronic properties of GaN/GaAs superlattices produced by nitridation of a molecular beam epitaxially grown GaAs surface. Using cross‐sectional scanning tunneling microscopy (STM) and spectroscopy, we show that the nitrided layers are laterally inhomogeneous, consisting of groups of atomic‐scale defects and larger clusters. Analysis of x‐ray diffraction data in terms of fractional area of clusters (determined by STM), reveals a cluster lattice constant similar to bulk GaN. In addition, tunneling spectroscopy on the defects indicates a conduction band state associated with an acceptor level of NAs in GaAs. Therefore, we identify the clusters and defects as nearly pure GaN and NAs, respectively. Together, the results reveal phase segregation in these arsenide/nitride structures, in agreement with the large miscibility gap predicted for GaAsN.

High resolution x-ray analysis of pseudomorphic InGaN/GaN multiple quantum wells: Influence of Si doping concentration

We report the structural properties of InGaN/GaN/AlGaN multiple quantum wells (MQWs) by means of two-dimensional reciprocal space mapping (RSM) of high resolution x-ray diffraction. The influence of Si doping in GaN barriers on the characteristics has been studied for 12-period MQWs grown by metalorganic chemical vapor deposition, which have different Si doping concentrations in the GaN barriers ranging from 1×1017 to 3×1019 cm−3. Information on the structural quality of these MQWs was extracted from the linewidth broadening of the higher-order superlattice satellite peaks, as well as from the presence of Pendellosung oscillations. The measured diffraction curves were modeled using kinematic diffraction theory. From the symmetric and asymmetric RSMs around (0002), (0004) and (1124) reflections, we found that the InGaN/GaN/AlGaN MQWs are grown coherently on the GaN base layer. Better interface properties are achieved with Si doping. Our results indicate that Si doping in the GaN barriers affects the inter...

Intrinsic exciton transitions in GaN

Intrinsic excitonic transitions in GaN have been studied using a variety of spectroscopic measurements. Sharp spectral structures associated with intrinsic free excitons could be observed in photoluminescence, reflection, and absorption spectra. The energy positions of excitonic transitions in GaN epitaxial layers were found to be influenced by the residual strain resulting from lattice-parameter and thermal-expansion mismatches between the epilayers and the substrates. The values of the four principal deformation potentials of wurtzite GaN were derived by using the strain tensor components determined by x-ray measurements. The observation of spectral features involving the emission of LO phonons in absorption and photoluminescence excitation spectra at energies above exciton resonances indicate that a phonon-assisted indirect excitation process, which simultaneously generates a free exciton and a LO phonon, is a very significant and efficient process in GaN. The lifetime of the free excitons is found to ...

Photoluminescence of zinc‐blende GaN under hydrostatic pressure

Photoluminescence spectra of cubic GaN grown on a GaAs substrate by molecular beam epitaxy have been studied as a function of hydrostatic pressure at 10 K. The spectra are abundant in emission structures arising from a variety of radiative recombination processes, such as free‐electron–bound‐hole and donor‐acceptor pair transitions. These emission peaks shift to higher energy with increasing pressure, providing a measure of the pressure coefficient of the band gap of cubic GaN. In addition, a spectral feature, which is superimposed on the other emission peaks and not observable at atmospheric pressure, becomes gradually resolvable as pressure increases. The difference of pressure dependence of this emission from the others suggests that it is associated with a deep center.

Kinetic modeling of microscopic processes during electron cyclotron resonance microwave plasma-assisted molecular beam epitaxial growth of GaN/GaAs-based heterostructures

Microscopic growth processes associated with GaN/GaAs molecular beam epitaxy (MBE) are examined through the introduction of a first-order kinetic model. The model is applied to the electron cyclotron resonance microwave plasma-assisted MBE (ECR-MBE) growth of a set of delta-GaNyAs1–y/GaAs strained-layer superlattices that consist of nitrided GaAs monolayers separated by GaAs spacers, and that exhibit a strong decrease of y with increasing T over the range 540–580 °C. This y(T) dependence is quantitatively explained in terms of microscopic anion exchange, and thermally activated N surface-desorption and surface-segregation processes. N surface segregation is found to be significant during GaAs overgrowth of GaNyAs1–y layers at typical GaN ECR-MBE growth temperatures, with an estimated activation energy Es ~ 0.9 eV. The observed y(T) dependence is shown to result from a combination of N surface segregation/desorption processes.

Strain, morphological, and growth-mode changes in AlGaN single layers at high AlN mole fraction

We report on morphological and residual-strain characteristics of high-AlN-mole-fraction N-polar AlxGa1−xN epilayers on sapphire. Nominally relaxed, thick single-alloy layers in the compositional range 0.4<x<1 were grown by plasma-assisted molecular beam epitaxy (PA-MBE) and characterized structurally and optically. High-resolution X-ray diffraction using an extension of the Bond method was employed to examine residual strains while film morphologies were examined directly with the use of atomic force microscopy. Under nominally identical PA-MBE growth conditions apart from Al flux, a continuous change in growth characteristics as a function of Al flux (alloy composition) is observed. For one particular value of Al flux (corresponding to x≅0.65), a maximum in growth rate and minimum in surface roughness are found whereas at a somewhat greater flux value (corresponding to x=0.86) a minimum in stress (lateral and vertical) is obtained. The observed growth-mode phenomenology suggests an approach for improving PA-MBE growth of high-AlN-mole-fraction layers of certain AlGaN/GaN structures such as the distributed Bragg reflector. Finally, optical transmission experiments lead to a bandgap bowing parameter in the large-x region of AlxGa1−xN of b≅0.75.

Effect of substrate temperature and V/III flux ratio on In incorporation for InGaN/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy

Reflection high-energy electron diffraction (RHEED) and laterally spatially resolved high resolution x-ray diffraction (HRXRD) have been used to identify and characterize rf plasma-assisted molecular-beam epitaxial growth factors which strongly affect the efficiency of In incorporation into InxGa1−xN epitaxial materials. HRXRD results for InxGa1−xN/GaN superlattices reveal a particularly strong dependence of average alloy composition x upon both substrate growth temperature and incident V/III flux ratio. For fixed flux ratio, results reveal a strong thermally activated behavior, with over an order-of-magnitude decrease in x with increasing growth temperature within the narrow range 590–670 °C. Within this same range, a further strong dependence upon V/III flux ratio is observed. The decreased In incorporation at elevated substrate temperatures is tentatively attributed to In surface-segregation and desorption processes. RHEED observations support this segregation/desorption interpretation to account for...