A. J. Kent

Growth and characterization of free-standing zinc-blende (cubic) GaN layers and substrates

In this paper, we describe bulk, free-standing, zinc-blende (cubic) GaN wafers grown by plasma-assisted molecular beam epitaxy. We have grown GaN layers of up to 60 ?m in thickness. We present the data from characterization measurements that confirm the cubic nature of the GaN crystals and show that the fraction of the material that is hexagonal in nature is not more than about 10% in the best thick samples. Cubic (0?0?1) GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0?0?0?1) c-axis wurtzite GaN. Therefore, the free-standing GaN wafers we have grown would make ideal lattice-matched substrates for the growth of cubic GaN-based structures for blue and ultraviolet optoelectronic devices, and high-power and high-frequency electronic applications.

Energy relaxation by hot electrons in n-GaN epilayers

The energy relaxation rate for hot electrons in n-type GaN epilayers has been measured over the temperature range 1.5–300 K. Several samples grown by molecular-beam epitaxy and having different electron concentrations have been studied. At low electron temperatures (Te<20 K), the energy relaxation is via acoustic phonon emission. The magnitude and temperature dependence of the energy relaxation are found to be in good agreement with theoretical calculations using appropriate values of the deformation potential and piezoelectric coupling constants and ignoring screening. For Te⩾70 K, the dominant mechanism of energy loss is optic phonon emission. For the several samples studied, consistent values of the optic phonon energy and electron-optic phonon relaxation time, 90±4 meV and 5–10 fs, respectively, are measured. The energy agrees well with values obtained by other methods and the relaxation time is consistent with theoretical calculations of the Frohlich interaction and indicate that hot phonon effects a...

Dynamics of a vertical cavity quantum cascade phonon laser structure

Driven primarily by scientific curiosity, but also by the potential applications of intense sources of coherent sound, researchers have targeted the phonon laser (saser) since the invention of the optical laser over 50 years ago. Here we fabricate a vertical cavity structure designed to operate as a saser oscillator device at a frequency of 325 GHz. It is based on a semiconductor superlattice gain medium, inside a multimode cavity between two acoustic Bragg reflectors. We measure the acoustic output of the device as a function of time after applying electrical pumping. The emission builds in intensity reaching a steady state on a timescale of order 0.1 μs. We show that the results are consistent with a model of the dynamics of a saser cavity exactly analogous to the models used for describing laser dynamics. We also obtain estimates for the gain coefficient, steady-state acoustic power output and efficiency of the device.

Coherent elastic waves in a one-dimensional polymer hypersonic crystal

Using the methods of picosecond acoustics, we inject high amplitude hypersonic wavepackets into a polymer superlattice and optically detect the propagating coherent elastic waves. The spectrum of the optically detected signal shows the elastic modes typical for folded phonon dispersion curves. The experimental results and related modeling show the feasibility of using polymer one-dimensional hypersonic crystals as acoustic devices in the gigahertz frequency range.

Determination of relative internal quantum efficiency in InGaN/GaN quantum wells

We have investigated the relative quantum efficiency in a series of InGaN∕GaN single quantum wells with differing indium concentration. The results of measurements involving direct detection of phonons emitted as a result of nonradiative recombination and carrier energy relaxation are compared with time-resolved photoluminescence studies. Using these complementary techniques we have extracted the low-temperature internal quantum efficiency of the recombination and observed the effect of free-carrier screening on the radiative and nonradiative processes in the quantum well samples. All the samples exhibit high quantum efficiency, with the maximum being observed in the 10% indium sample. In addition, we observe the appearance of a delayed phonon signal, which we correlate to the measured quantum efficiency of the samples.

Generation and propagation of monochromatic acoustic phonons in gallium arsenide

We have generated coherent zone-folded longitudinal acoustic (LA) phonons by femtosecond pulse-laser excitation of a gallium arsenide/aluminum arsenide superlattice. Using the filter effect due to frequency-dependent scattering in the gallium arsenide substrate, we have determined the frequency spectrum of the LA phonons that leak out of the superlattice and propagate to a bolometer on the opposite face of the gallium arsenide substrate. We show that the phonon spectrum has a strong monochromatic component centered on ν=sLA/dSL, where dSL is the superlattice period and sLA is the phonon speed. We propose that such phonons may be used for phonon spectroscopy and in a range of “phonon optics” applications.

Time-resolved photoluminescence of InAs quantum dots in a GaAs quantum well

We study the time-resolved photoluminescence emission of InAs self-assembled quantum dots (QDs) incorporated in a GaAs/(AlGa)As quantum well. We show that the quantum well confinement affects the decay time of the dot photoluminescence. In addition, we use the strong dependence of the decay time on excitation energy and temperature to shed light on carrier relaxation mechanisms in QDs.

Changeover from acoustic to optic mode phonon emission by a hot two-dimensional electron gas in the gallium arsenide/aluminium gallium arsenide heterojunction

The authors have used heat pulse techniques to study the energy relaxation of a hot two-dimensional electron gas (2 DEG) in a GaAs/AlGaAs heterojunction. The 2 DEG was heated by applying short ( approximately=100 ns) electrical pulses to the drain-source contacts of the device. The electrons lost energy by emitting phonons which were detected by a CdS bolometer on the opposite side of the GaAs substrate. A change in the nature of the phonon signal occurring at an excitation level of about 5 pW per electron indicated a change in the phonon emission process. The corresponding electron temperature, Te, at which optic phonon emission is expected to become the dominant energy relaxation process was estimated to be about 60 K. At powers well below the change-over, the authors found that the energy loss rate per electron, Pe, due to acoustic phonon emission is proportional to Te3. At much higher powers, Pe varies as exp(-h(cross) omega LO/kTe), where h(cross) omega LO is the longitudinal optic phonon energy. They obtained a value of 3.3 ps for the electron-optic phonon scattering time, which is consistent with the range of values found in the literature.

Molecular beam epitaxy as a method for the growth of freestanding zinc-blende (cubic) GaN layers and substrates

The authors have studied the growth of bulk, freestanding zinc-blende (cubic) GaN layers by plasma-assisted molecular beam epitaxy (PA-MBE). They have established that the best structural properties of freestanding zinc-blende GaN can be achieved with initiation under Ga-rich conditions but without Ga droplet formation. It is difficult to initiate the growth of zinc-blende GaN, but it is even more difficult to sustain the growth of the pure zinc-blende polytype in thick layers without any wurtzite inclusions. In order to grow high quality freestanding cubic GaN layers, it is necessary to maintain the same growth conditions for about 1week. The best quality zinc-blende phase GaN was achieved in the first 10μm of the GaN layers. The authors have produced zinc-blende GaN substrates from our thick bulk GaN layers and they used the side previously attached to the GaAs substrate as the episide of these zinc-blende GaN substrates. They have demonstrated the scalability of the process by growing zinc-blende GaN l...

Current-voltage characteristics of zinc-blende (cubic) Al0.3Ga0.7N/GaN double barrier resonant tunneling diodes

Measurements of the current-voltage characteristics of zinc-blende (cubic) Al0.3Ga0.7N/GaN, double barrier resonant tunneling diodes are presented. Clear and reproducible negative differential resistance effects are observed, with room temperature peak-to-valley ratios up to 4 and peak currents up to about 1000 A cm−2.