N. M. Stanton

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...

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.

Energy relaxation by hot 2D electrons in AlGaN/GaN heterostructures: the influence of strong impurity and defect scattering

We report measurements of the energy loss rates and scattering times for two-dimensional electrons in a series of AlGaN/GaN heterostructures grown on different substrate materials. It was found that the device grown on a substrate having the largest lattice mismatch to GaN was within the dirty limit where strong carrier scattering gives rise to an enhanced energy loss rate, which is proportional to T4e at low electron temperatures, Te < 10 K. This is consistent with the measurements of the scattering times and shows that strong scattering by impurities and defects can play an important role in the energy relaxation of some GaN-based devices at low temperatures.

Generation and detection of terahertz coherent transverse-polarized acoustic phonons by ultrafast optical excitation of GaAs∕AlAs superlattices

We have used femtosecond pump-probe techniques to generate and detect coherent transverse and quasitransverse polarized acoustic phonons in GaAs∕AlAs superlattices. Direct generation of transverse phonons is achieved using superlattices grown on the low-symmetry, (311) and (211), crystal planes. The frequency of the generated phonons is determined by the superlattice period and is in the region of 0.4THz. The dependence of the mode structure on the pump polarization suggests that a Raman scattering process is responsible for coherent phonon generation. Using bolometers on the back surface of the substrate, we show that the transverse phonons leak out of the superlattice and propagate over macroscopic distances at low temperature.

Photoenhanced wet chemical etching of MBE grown gallium nitride

Abstract Room-temperature photoenhanced wet chemical etching of MBE grown GaN has been studied during the processing of devices for a physics research programme. The process uses a 0.5 M KOH solution with illumination provided by a mercury arc lamp emitting at 365 nm. The maximum etch rate achieved was ∼45 nm min −1 , much lower than that for MOCVD material etched under similar conditions. Surface morphologies vary significantly for wafers etched under nominally identical conditions, from surfaces with whiskers to highly crystallographic faces.

Influence of internal fields on radiative and nonradiative processes in AlN/GaN superlattices

We have investigated the effect of the internal electric fields on radiative and nonradiative processes in a series of AlN/GaN superlattices. 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 observed directly the effect of free carrier screening on the radiative and nonradiative recombination processes in the superlattice samples. We find that at high excitation power, photoinjected carriers screen the strong internal fields, resulting in an enhanced radiative recombination. As the carriers recombine, the descreening effect results in an increase in recombination via nonradiative processes, which we observe directly as a delayed phonon signal in the time of flight measurements.

Energy Relaxation by Warm Two‐Dimensional Electrons in a GaN/AlGaN Heterostructure

The rate of energy loss per electron, P e , by a two-dimensional electron gas in an GaN/AIGaN heterostructure has been measured as a function of electron temperature, T e , in the range 0.4-35 K. A combination of zero and high magnetic field electrical transport measurements were used to determine T e as a function of the power dissipated in the device. It was found that P e T n e , with n 5 at the lowest temperatures, T e « 2 K, while for higher temperatures, T e > 10 K, n → 1. The experimental results are compared with numerical calculations of the energy relaxation rate. In the range of temperatures studied, emission of piezoelectrically coupled acoustic phonons was found to be the dominant energy relaxation mechanism.

Generation of terahertz monochromatic acoustic phonon pulses by femtosecond optical excitation of a gallium nitride/aluminium nitride superlattice

We describe the generation of pulses of monochromatic acoustic phonons by ultrafast laser excitation of a gallium nitride/aluminium nitride superlattice (SL). The SL response to the optical excitation was studied using femtosecond pump-probe measurements. Superconducting bolometers were used to detect the phonons that leak out of the SL and propagate across the substrate. We see that, for excitation wavelengths around 430 nm, a pronounced enhancement in the longitudinal acoustic phonon signal occurs. The results are consistent with similar measurements using GaAs∕AlAs SL’s, where propagating monochromatic phonons were generated under resonant photoexcitation [Appl. Phys. Lett. 81, 3497 (2002)].