P. Hawker

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...

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.

Observation of coherent zone-folded acoustic phonons generated by Raman scattering in a superlattice

We have used pulse time-of-flight techniques to examine the phonon emission from an optically excited GaAs/AlAs superlattice structure. For laser excitation wavelengths shorter than 767 nm (the energy of E1HH1 transition), we detect a significant longitudinal acoustic phonon component directed in a narrow beam normal to the structure. Under identical excitation conditions, generation of coherent longitudinal acoustic phonons has previously been observed in this structure. We suggest that the excitation wavelength and angular characteristics of the longitudinal acoustic emission is consistent with those of propagating modes produced as coherent phonons “leak” from the superlattice structure.

Energy relaxation by photoexcited carriers in the InAs/GaAs quantum-dot system: Bolometric detection of strong acoustic-phonon emission

We have used bolometric detection to observe directly the phonons emitted by photoexcited carriers in the InAs/GaAs self-organized quantum-dot system. We find that about 74% of the energy lost by carriers in the InAs dots and wetting layer is via emission of low-frequency acoustic phonons and argue that this is facilitated by Auger scattering.

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.

Magnetophonon resonance effects in the phonon emission by a hot two-dimensional electron gas in a GaAs-AlGaAs heterojunction

The dominant energy relaxation process for hot electrons (Te>50 K) in GaAs-AlGaAs heterojunctions is generally believed to be through the emission of optical phonons. In magnetic fields satisfying the condition nh omega c=h omega LO the energy relaxation rate is expected to be enhanced by magnetophonon resonance. The authors have heated the two-dimensional electron gas (2DEG) in a single GaAs-AlGaAs heterojunction by applying pulsed electric fields in the region of 104 V m-1 across the conducting channel. The optical phonons emitted rapidly decay into acoustic modes which they observe using CdS bolometers. Peaks in the amplitude of the detected phonon pulse as a function of magnetic field are observed which the authors attribute to magnetophonon resonant emission. The positions of the peaks are consistent with the LO phonon frequency in bulk GaAs.

Magnetic-Field Dependence of the Phonon-Scattering and Phonon Emission by A 2Deg in A Si Mosfet

Abstract Measurements of phonon scattering by a 2DEG in a Si MOSFET show quantum oscillations as the Fermi levels is moved through the Landau level spectrum. Phonon emission by a hot 2DEG shows an increasing proportion of cyclotron emission at high fields. The emission is concentrated more closely normal to the 2DEG as the field is increased.

Heat-pulse study of the phonon emission by hot two-dimensional holes in a gallium arsenide heterojunction

The process of energy relaxation by phonon emission of hot two-dimensional (2D) holes in a GaAs/AlGaAs heterojunction has been studied using the heat-pulse technique. The holes were heated above the substrate temperature by applying short current pulses to a device, and the emitted phonons were detected by superconducting bolometers on the opposite side of the GaAs substrate. At the same time, by making measurements of the device resistance during the excitation pulse, the hole temperature was determined as a function of the power dissipated. In contrast to earlier measurements on 2D electron devices, significant longitudinal mode phonon emission was seen as well as the dominant transverse mode. Theoretical calculations of the temperature, mode and angular dependence of the acoustic phonon emission by hot holes were made. The model was based on the theory of acoustic phonon emission by 2D electrons, with the appropriate change of parameters such as the effective mass and the deformation potential constant. Detailed comparison is made between the experimental and theoretical results. We obtained a value for the valence band deformation potential, . It was found that at hole temperatures above 55 K optic phonon emission takes over as the dominant energy loss process. The changeover occurs at a power dissipation of 25 pW/hole, which is about ten times greater than for a 2D electron system of similar areal density, and is attributable to the difference in effective masses of the holes and electrons. We determined the hole - optic phonon scattering time, . The angular dependence of the phonon emission has also been investigated using a spatially resolving superconducting bolometer. Evidence of a predicted abrupt cut-off of the emission at small angles, close to the normal to the 2D carrier gas, is observed for the first time.

The Effect of a Magnetic Field on the Phonon Emission from a Hot 2-DEG in the Inversion Layer of a Silicon MOSFET

Phonon emission by a heated two dimensional electron gas in a magnetic field has been studied using heat pulse techniques. The 2-DEG was heated by applying electrical pulses to the MOSFET source-drain electrodes at constant gate voltage and the heat pulses detected by cadmium sulphide bolometers. Using constant power input to the 2-DEG the detected signal was found to decrease by up to 20% in a field of 7T. In a magnetic field phonons are emitted at the cyclotron frequency. At high fields the cyclotron phonons are severely attenuated by isotope scattering in the Si substrate.

Classical Edge Magnetoplasmon in a GaAs/AlGaAs Two-Dimensional Electron System

We have studied the edge magnetoplasmon (EMP) in a GaAs/AlGaAs two-dimensional electron gas (2DEG) system under conditions where the quantum Hall effect (QHE) is not observed; we call this a classical EMP. The classical EMPs are excited in a 2DEG disk by radio frequency electric field excitation (≤1 GHz) in a high magnetic field B (≤5 T) with the 2DEG sheet carrier density n s between 2×10 15 m -2 and 1×10 16 m -2 . The resonant frequency is found to be almost proportional to n s and inversely proportional to B , which agrees quantitatively with the theory proposed by Volkov and Mikhailov, Sov. Phys.-JETP 67 (1988) 1639 by assuming the effective dielectric constant \(\hat{\varepsilon }=(1+\varepsilon _{\rm GaAs})/2\) where e GaAs is appropriate to the GaAs. A comparison of the experimental results with the calculations revealed that the electron mobility plays an important role in the definition of the EMP frequency.