D. Korakakis

LONG RANGE ORDER IN ALXGA1-XN FILMS GROWN BY MOLECULAR BEAM EPITAXY

The first observation of atomic long range ordering in AlxGa1−xN thin films grown by electron cyclotron resonance assisted molecular beam epitaxy on sapphire and 6H-SiC substrates is reported. The phenomenon was investigated by studying the superlattice peaks (0001), (0003), and (0005) using x-ray diffraction. The relative intensity of these peaks was found to be largest for Al content in the 30%–50% range in qualitative agreement with expectations for an ordered structure of ideal Al0.5Ga0.5N stoichiometry. The average size of the ordered domains in the films was found to be within a factor of 4 of the films’ thicknesses. The degree of ordering depends on the III/V flux ratio and exhibits a weaker dependence on Si doping.

PHOTOCONDUCTIVE DETECTORS BASED ON PARTIALLY ORDERED ALXGA1-XN ALLOYS GROWN BY MOLECULAR BEAM EPITAXY

Photoconductive detectors based on partially ordered AlxGa1−xN alloys with AlN mole fractions up to 45% were fabricated and evaluated. The degree of ordering in these alloys was found to increase with the AlN mole fraction and it has a maximum value at about 50%. The resistivity of the AlxGa1−xN films was found to increase from 10 to 108 Ω cm by increasing the Al content in the films. Correspondingly, the mobility-lifetime (μτ) product, which was determined by measuring the photoconductive gain, was found to decrease from 10−2 to 10−5 cm2/V. These high values of the μτ product at the high AlN mole fraction are attributed to spatial separation and indirect recombination of the photogenerated electron hole pairs, due to band-gap misalignment of the ordered and disordered domains in these films.

MBE growth and doping of III–V nitrides

We report on the growth and doping of GaN by molecular-beam epitaxy on the c-plane of sapphire. We find that the steps of nitridation and low-temperature buffer have a significant effect on the structure, microstructure, defects and opto-electronic properties of the grown GaN films. The electron mobility in Si-doped GaN films was found to be controlled both by the density of ionized impurities and the density of dislocations. This result is consistent with the model which assumes that dislocations introduce acceptor centers. Films were doped p-type with Mg with carrier concentration up to 6 x 10 18 cm -3 , but relatively low hole mobilities (0.3 cm 2 /V s). These low mobilities can be improved by thermal annealing, a result attributed to the removal of static disorder.

X-ray characterization of GaN/AlGaN multiple quantum wells for ultraviolet laser diodes

GaN/Al0.20Ga0.80N (50 A/50 A) multiple quantum wells (MQW) with 15 periods were grown on (0001) sapphire substrates by molecular beam epitaxy and evaluated by x-ray diffraction. To simulate an ultraviolet laser diode structure, the substrate was coated first with n-GaN as the bottom contact layer and n-Al0.25Ga0.75N as the corresponding cladding layer. The crystal structure of this system was investigated by studying the reciprocal lattice map of off-axis diffraction peaks as well as the θ–2θ pattern around the (0002) reflection. The MQW was found to be coherent and has the a-lattice parameter of the underlying Al0.25Ga0.75N. The good agreement between experimental and theoretical data in the relative intensity of up to third-order satellite peaks supports that the interfaces of the MQW are abrupt, and thus, interdiffusion of Ga and Al atoms at the growth temperature was negligible.

Soft x-ray emission studies of the bulk electronic structure of AlN, GaN, and Al0.5Ga0.5N

The electronic structure of wurtzite GaN, Al0.5Ga0.5N, and AlN has been studied using synchrotron radiation excited soft x-ray emission spectroscopy. In particular, the elementally resolved partial densities of states has been measured and found to agree well with calculations. The shift in energy of the valence band maximum as x varies from 0 to 1 in AlxGa1−xN was measured by recording N K-emission spectra, and found to be linear. Furthermore, N K-emission spectra revealed resonantlike hybridization of N 2p and Ga 3d states at 19 eV below the GaN valence band maximum. The spectral intensity of this feature is proportional to Ga content.

Growth and doping of GaN directly on 6H-SiC by MBE

The authors report on methods for the growth of GaN by MBE directly on 6H-SiC substrates. The films were doped p-type by the incorporation of Mg and the samples were characterized by studying their structure and morphology by RHEED, XRD and SEM and their recombination properties by photoluminescence measurements. The undoped films were found to be atomically smooth with 2 x 2 surface reconstruction and have an x-ray rocking curve with a FWHM of 3.5 arcmin. The photoluminescence spectra indicate that recombination is dominated by transition across the gap. The p-type doped films have a rocking curve with FWHM of 6.5 arcmin, and the majority of recombination occurs through D-A transitions at 3.26 eV.

Metal Contacts to n- Al X Ga 1-x N

In this paper we report on the formation of ohmic contacts to n- Al x Ga 1-x N alloys. The films were produced by plasma-assisted MBE and doped n- type with silicon at doping levels between 10 18 to 10 19 cm -3 . Contacts were formed by sequential deposition of 200 A of Ti and 2000 A of Al and the contact resistivities were determined from TLM measurements. For low Al- content (x -4 to 10 -5 cm 2 . The contacts become progressively non-ohmic at Al concentrations greater than 10%. There results are consistent with the Schottky limit being applicable to these alloys and thus the Ti/Al contact forms Schottky barriers with higher barrier height as the conduction band of the alloy moves towards the vacuum level.

Disorder induced IR anomaly in hexagonal AlGaN short-period superlattices and alloys

The authors report an experimental (infrared reflectance spectroscopy) and theoretical study of the polar optical phonons in hexagonal ternary nitride compounds: AlN{sub m}/GaN{sub n} (n = 2--8, m = 4, 8) superlattices (SL) and spontaneously ordered Al{sub x}Ga{sub 1{minus}x}N (x = 0.08--0.55) alloys. In infrared (IR) reflectivity spectra they revealed two modes having strong LO-TO splitting (20--150 cm{sup {minus}1}), and several modes, having a small (1--3 cm{sup {minus}1}) LO-TO splitting. All modes have a very high damping parameter {ge} 20 cm{sup {minus}1}. The unusual observation is the negative value of the oscillator strength for the weak IR mode at {approximately} 690 cm{sup {minus}1}, suggesting possible lattice instability, consistent with high damping observed. They found from lattice dynamical calculations that weak IR active modes correspond to modes localized at GaN-AlN interfaces. The analysis has shown that an anomalous mode is induced by the disorder effects and arises due to strong overlapping of the LO-TO phonon branches of the bulk GaN and AlN. In SL samples the anomalous mode corresponds to phonons localized on interface inhomogeneities.