David W. Weyburne

Inversion domains in AlN grown on (0001) sapphire

Al-polarity inversion domains formed during AlN layer growth on (0001) sapphire were identified using transmission electron microscopy (TEM). They resemble columnar inversion domains reported for GaN films grown on (0001) sapphire. However, for AlN, these columns have a V-like shape with boundaries that deviate by 2 {+-} 0.5{sup o} from the c-axis. TEM identification of these defects agrees with the post-growth surface morphology as well as with the microstructure revealed by etching in hot aqueous KOH.

Correlation between optoelectronic and structural properties and epilayer thickness of AlN

AlN epilayers were grown by metal organic chemical vapor deposition on sapphire substrates. X-ray diffraction measurements revealed that the threading dislocation (TD) density, in particular, the edge TD density, decreases considerably with increasing the epilayer thickness. Photoluminescence results showed that the intensity ratio of the band edge emission to the defect related emission increases linearly with increasing the epilayer thickness. Moreover, the dark current of the fabricated AlN metal-semiconductor-metal deep ultraviolet (DUV) photodetectors decreases drastically with the AlN epilayer thickness. The results suggested that one effective way for attaining DUV optoelectronic devices with improved performance is to increase the thickness of the AlN epilayer template, which results in the reduction of the TD density.

Optical parametric generation of a mid-infrared continuum in orientation-patterned GaAs

We have generated an ultrabroad mid-infrared continuum by using single-pass optical parametric generation (OPG) in orientation-patterned GaAs (OP-GaAs). The spectrum spans more than an octave, from 4.5 to 10.7 microm, measured 20 dB down from the peak. The 17.5 mm long, 0.5 mm thick, all-epitaxially-grown OP-GaAs sample with a 166.6-microm quasi-phase-matching period was pumped with 3.1-3.3 microm wavelength, 1 ps pulses up to 2 microJ in energy. The OPG threshold was observed at 55 nJ pump energy with the pump polarized along the [111] crystal direction. The slope efficiency near threshold was 51%, and the external conversion efficiency was as high as 15%.

GaAs optical parametric oscillator with circularly polarized and depolarized pump.

We demonstrate an optical parametric oscillator (OPO) based on GaAs pumped with linearly polarized and circularly polarized light and show that the relative OPO thresholds agree with theoretical expectations. For the circularly polarized pump, the threshold was as low as for the [111]-linearly polarized pump case. The pump was also passed through a Lyot depolarizer to produce pseudo-depolarized light, and the OPO threshold in this case was only 22% higher than that for [001]-linearly polarized pump.

Two-step Process for the Metalorganic Chemical Vapor Deposition Growth of High Quality AlN Films on Sapphire

Epitaxial AlN films were deposited on (0001) sapphire using a new metalorganic chemical vapor deposition process in which optimal substrate nitridation is combined with modulated ammonia flow and a growth pressure reduction from 150 torr to 40 torr after the first stage of growth. A significant improvement in the full width half maximum (FWHM) of the rocking curves was obtained. The best layers had FWHM of 330 arc sec/650 arc sec for the (0002)/(1012) reflections, respectively.

Investigation of near interface properties in semi‐insulating InP substrates with epitaxial grown InGaAs and InAlAs by photoreflectance

An extensive photoreflectance (PR) study has been done on a series of undoped and n‐type, InGaAs and InAlAs molecular beam epitaxy (MBE) grown layers with different In mole fractions, and epilayer thicknesses on Fe‐doped semi‐insulating (SI)‐InP substrates. Three substrate features were observed in the spectra. From investigations of the temperature dependence, time constant dependence, and an additional cw light beam intensity dependence, they were identified as an excitonic transition from the substrate, a free electron transition near the interface which gives a Franz–Keldysh oscillation, and a transition from the spin–orbit split‐off valence band. The Franz–Keldysh effect indicates that a temperature dependent built‐in electric field is formed near the interface. The dependence of the field on doping concentration, strain, or epilayer composition (band gap) was insignificant. The PR signal from a SI‐InP wafer after a pre‐MBE‐growth heating was found to be strongly enhanced over that from an untreated ...

Design and operating considerations for a water-cooled close-spaced reactant injector in a production scale MOCVD reactor

Abstract In a previous paper, we described experimental results of the growth of a wide range of III–V materials using a water-cooled close-spaced injector that permits organometallic and hydride reactants to be injected ∼ 1 cm above the hot wafer. In this paper, we describe important design and operating considerations for this new metalorganic chemical vapor deposition (MOCVD) injector, including construction details, deposition efficiency and process window considerations. Our theoretical modeling indicates that the gas flow patterns are very different from those in a typical rotating disk reactor. By making analytical approximations for the velocity and temperature profiles, we have developed a flow stability parameter and find that the thermal stability depends on the square of the injector-to-substrate gap.

Optimized Coalescence Method for the Metalorganic Chemical Vapor Deposition (MOCVD) Growth of High Quality Al-Polarity AlN Films on Sapphire

Epitaxial Al-polarity AlN films were deposited on (0001) sapphire using a metalorganic chemical vapor deposition process involving a modulated ammonia/trimethyl aluminum flow, a pressure reduction from 150 Torr to 40 Torr after the first stage of growth, and growing the first ~0.1 µm of AlN at a moderate V/III ratio, followed by a switch to low V/III for the remainder of the growth. Smooth, flat epitaxial films with a full width half maximum of the X-ray rocking curves as low as 177 arcseconds/650 arcseconds for the (0002)/(1012) reflections were obtained.

Determination of alloy composition and residual stress for AlxGa1-xN/GaN epitaxial films

The alloy composition and the residual stress in Al x Ga 1-x N on GaN epi-layers can be estimated by measuring both the a-axis and c-axis lattice constants using HRXRD. Using linear elastic theory, we derive the alloy composition of a biaxial strained layer from a simple equation relating the measured c-axis and a-axis lattice constants, the lattice constants for unstrained Al x Ga 1-x N (assuming Vegards law), and the ratio of elastic stiffness constants. The method was used to calculate the composition and strain, as well as error estimates, for a set of Al x Ga 1-x N layers grown by MOCVD with 0.23 ≤ x ≤ 0.84.

Characterization of HVPE-grown thick GaAs structures for IR and THz generation

In a previous paper, we described a method for growing thick epitaxial layers of GaAs on orientation-patterned wafers by low pressure hydride vapor phase epitaxy. The low pressure method allows for rapid growth at rates well above 100 mum/hr and layers up to 1 mm thick have been successfully produced. In this paper we present characterization of these layers by optical microscopy, Hall measurement, and cathodoluminescence imaging. We demonstrate growth of low free carrier concentration, mm-thick orientation-patterned GaAs for efficient nonlinear optical conversion