James H. Edgar

Raman scattering studies on single-crystalline bulk AlN under high pressures

We report on the Raman analysis of wurtzite single-crystalline bulk AlN under hydrostatic pressures up to 10 GPa. The pressure dependence of the AlN phonon frequencies was investigated. Mode Gruneisen parameters of 1.39, 1.57, 1.71, 0.93, and 1.26 were determined for the A1 (TO), E1 (TO), E2 (high), A1 (LO), and the quasi-longitudinal optical phonons, respectively. Recent theoretical calculations underestimate the pressure-induced frequency shift of the AlN phonons by about 20%–30%. Mode Gruneisen parameters of AlN were compared to those of GaN.

Phonon lifetimes in bulk AlN and their temperature dependence

We report on the Raman analysis of the phonon lifetimes of the A1(LO) (longitudinal optical) and E2(high) phonons in bulk AlN crystals and their temperature dependence from 10 to 1275 K. Our experimental results show that amongst the various possible decay channels, the A1(LO) phonons decay primarily into two phonons of equal energy (Klemens model), most likely longitudinal-acoustic phonons, whereas the E2(high) phonon decays asymmetrically into a high-energy and a low-energy phonon. Possible decay channels of the E2(high) phonon have been shown to include combinations of E2(low) and acoustic phonons. Phonon lifetimes of the A1(LO) phonon and the E2(high) phonon of 0.75 and 2.9 ps, respectively, were measured at 10 K.

Self-Assembled Monolayers of Alkylphosphonic Acid on GaN Substrates

In this paper we describe the formation and characterization of self-assembled monolayers of octadecylphosphonic acid (ODPA) on epitaxial (0001) GaN films on sapphire. By immersing the substrate in its toluene solution, ODPA strongly adsorbed onto UV/O 3-treated GaN to give a hydrophobic surface. Spectroscopic ellipsometry verified the formation of a well-packed monolayer of ODPA on the GaN substrate. In contrast, adsorption of other primarily substituted hydrocarbons (C n H 2 n+1 X; n = 16-18; X = -COOH, -NH 2, -SH, and -OH) offered less hydrophobic surfaces, reflecting their weaker interaction with the GaN substrate surfaces. A UV/O 3-treated N-polar GaN had a high affinity to the -COOH group in addition to ODPA, possibly reflecting the basic properties of the surface. These observations suggested that the molecular adsorption was primarily based on hydrogen bond interactions between the surface oxide layer on the GaN substrate and the polar functional groups of the molecules. The as-prepared ODPA monolayers were desorbed from the GaN substrates by soaking in an aqueous solution, particularly in a basic solution. However, ODPA monolayers heated at 160 degrees C exhibited suppressed desorption in acidic and neutral aqueous solution maybe due to covalent bond formation between ODPA and the surface. X-ray photoelectron spectroscopy provided insight into the effect of the UV/O 3 treatment on the surface composition of the GaN substrate and also the ODPA monolayer formation. These results demonstrate that the surface of a GaN substrate can be tailored with organic molecules having an alkylphosphonic acid moiety for future sensor and device applications.

Temperature Dependence of the Phonons of Bulk AlN

Micro-Raman scattering measurements were performed on bulk wurtzite AlN crystals over a temperature range from 10 K to 1275 K. The temperature dependence of the frequency of the AlN phonons follows an empirical relationship previously introduced for diamond. A temperature-induced frequency shift of the E2 phonon of -(2.22±0.02)×10-2 cm-1/K was determined for high temperatures, which is similar to values reported for bulk GaN. The results provide the basis for the non-invasive monitoring of the temperature of (Al,Ga)N by Raman scattering.

Transport effects in the sublimation growth of aluminum nitride

In this paper, a detailed two-dimensional model accommodating the thermal convection and Stefan flow is formulated for AlN sublimation growth. Some assumptions are applied in the model and inspected by the experimental data. A detailed numerical investigation is carried out for the transport effect on the sublimation growth. Theoretical predictions agree with the experimental data at pressure above 100 Torr and current operating temperature (seed temperature) ranging from 1700 to 1900°C. The activation energy of the growth rate is 681 KJ/mol close to the sublimation heat of aluminum nitride. Nevertheless, at low pressure less than 100 Torr, a combination of surface crystallization model is necessary to describe the process kinetics correctly. Meanwhile, a detailed heat transfer model for furnace temperature gradient is required for more accurate prediction of growth rate.

Two-dimensional excitons in three-dimensional hexagonal boron nitride

The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8 A and 740 meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54mo, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.

X‐ray double crystal characterization of single crystal epitaxial aluminum nitride thin films on sapphire, silicon carbide and silicon substrates

A detailed double crystal x‐ray diffractometry study of epitaxial AlN thin films grown on sapphire, silicon and silicon carbide substrates was carried out to compare the structure, residual stress and defect concentration in these thin films. The structure of AlN is wurtzite with a small distortion in lattice parameters. This results in a small residual stress of the order of 109 dynes/cm2 in the film and can be accounted for from the difference in thermal expansion coefficients between the film and substrate. Both the x‐ray and transmission electron microscopy measurements indicate a low defect density in the AlN thin film grown on 6H‐SiC substrate which could be attributed to the small difference in lattice parameters between AlN and 6H‐SiC.

Photoluminescence properties of AlN homoepilayers with different orientations

AlN homoepilayers and heteroepilayers were grown on polar c-plane and nonpolar a-plane and m-plane orientations of AlN bulk and sapphire substrates by metal organic chemical vapor deposition. A systematic comparative study of photoluminescence properties of these samples revealed that all AlN homoepilayers (c, a and m planes) were strain free with an identical band gap of about 6.099 (6.035)eV at 10 (300)K, which is about 42meV below the band gap of c-plane AlN heteroepilayers grown on sapphire. Also, nonpolar a-plane homoepilayers have the highest emission intensity over all other types of epilayers. We believe that a-plane AlN homoepilayers have the potential to provide orders of magnitude improvement in the performance of new generation deep UV photonic devices.

The origin of 2.78 eV emission and yellow coloration in bulk AlN substrates

The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light with wavelengths shorter than that of yellow. This yellow impurity limits UV transparency and hence restricts the applications of AlN substrates for deep UV optoelectronic devices. Here, the optical properties of AlN epilayers, polycrystalline AlN, and bulk AlN single crystals have been investigated using photoluminescence (PL) spectroscopy to address the origin of this yellow appearance. An emission band with a linewidth of ∼0.3 eV (at 10 K) was observed at ∼2.78 eV. We propose that the origin of the yellow color in bulk AlN is due to a band-to-impurity absorption involving the excitation of electrons from the valence band to the doubly negative charged state, (VAl2−), of isolated aluminum vacancies, (VAl)3−/2− described by VAl2−+hν=VAl3−+h+. In such a context, the reverse process is responsible for the 2.78 eV PL emission.

Raman scattering studies on single-crystalline bulk AlN: temperature and pressure dependence of the AlN phonon modes

Abstract We report on the Raman analysis of single-crystalline bulk AlN. AlN phonon modes were investigated as a function of temperature and hydrostatic pressure. Phonon decay channels were studied via the AlN Raman linewidth. Mode Gruneisen parameters describing the low-pressure behavior of the AlN phonon modes were determined and used to estimate hydrostatic stress in amber discolored AlN substrates.