A. Straw

High-frequency current oscillations in GaAs/AlGaAs single quantum wells

High-frequency current oscillations have been observed in highly doped single quantum well structures of GaAs/AlGaAs. Several samples of the same structure, but aligned along different crystallographic directions, have been studied. The field, temperature and time dependences of these oscillations have been investigated. Various mechanisms associated with current oscillations are discussed including NDR, trapping and dual streaming, but as yet no single mechanism fully describes the observations. In some samples, the period doubling was observed, which has been related by others with routes to chaos.

Hot-electron relaxation via the emission of GaAs optical modes and AlAs interface modes in GaAs/AlAs multi-quantum wells

We demonstrate via hot-electron photoluminescence and high-temperature mobility measurements the importance of the AlAs interface mode in the energy relaxation of electrons in GaAs/AlAs multi-quantum wells. A corresponding investigation of a similar GaAs/Al0.24Ga0.76As system illustrates that this is not the case for AlGaAs barrier devices where GaAs modes are the dominant energy relaxation process. The importance of the AlAs interface mode is not simply related to its intrinsic scattering rate but also to its shorter lifetime (compared with GaAs modes). Hot-phonon effects are therefore crucial to a full understanding of the experimental data.

Tunable wavelength hot electron light emitter

We demonstrate the operation of a surface emitting light emitting diode. The wavelength of the emitted light can be tuned with the applied voltage. The device is based on a p‐GaAs and n‐Ga1−xAlxAs heterojunction containing an inversion layer in the p side and, GaAs quantum wells in the n side, and, is referred to as HELLISH‐II (hot electron light emitting and lasing in semiconductor heterojunction). The device utilizes hot electron longitudinal transport and, therefore, light emission is independent of the polarity of the applied voltage.

Energy relaxation in GaInAs/AlInAs heterojunctions and GaAs/AlGaAs multiple quantum wells

We have studied electron energy relaxation in GaInAs/AlInAs heterojunctions and GaAs/AlGaAs multiple quantum wells using mobility measurements as a function of electric field and temperature, in the range 3K to 300K. The results in the range 3 to 20K show a power loss rate which is dependent on (Te − Tl), suggesting that the energy relaxation occurs through acoustic phonon scattering. At electron temperatures greater than 20K, the experimental results are modelled using a standard expression for polar optical phonons. This modelling yields 30meV and 31meV for the polar optical phonon energy in GaAs and InGaAs respectively.

Acoustic and optic phonon energy relaxation in non-degenerate GaAs/AlGaAs multiple quantum wells

Abstract Energy relaxation in two dimensional systems has recently been dominated by discussions concerning hot phonons associated with energy relaxation by optical phonons. We present results on energy relaxation in non-degenerate quantum wells in the acoustic and optic phonon regions. In the acoustic phonon region, below about 30K, we experimentally determine the power loss per carrier as a function of electron temperature, using the mobility field, mobility-temperature thermometric technique. We find that although the form of the function is the same as the current model, that is it is proportional to ( T e − T L ), the magnitude is not. In the light of this the current model is discussed and possible reasons for the anomaly suggested. In the region above 30K we find that our power loss per carrier data agrees favourably with data obtained using the hot electron photo-luminescence technique. These experimental results are fitted with a standard expression for energy loss via polar optical phonon scattering. This fitting yields 35.5 meV for the polar optical phonon energy and 245 fs for the phonon emission time.

Energy relaxation in GaAs/AlGaAs two-dimensional structures

Results on energy relaxation in low carrier concentration two-dimensional structures over the lattice temperatures, Tl, of 3 K to 150 K are presented. The power loss per carrier as a function of electron temperature, Te, was experimentally determined using the mobility-field, mobility-temperature thermometric technique. In the acoustic phonon regime, below about Tl=20 K, the power loss per carrier was found to be proportional to Te-Tl. Theoretical calculations in this temperature regime predict a similar form but do not agree in magnitude. Above Tl=40 K the power loss per carrier was modelled using an optic phonon scattering model, which includes the effects of hot phonons, and which predicts a phonon lifetime of 11 ps+or-1 ps. Between these two regimes, for the multi-quantum well, the experimental power loss was found to be greater than the theoretical power loss. This discrepancy has been explained by invoking scattering due to optic phonon-plasmon coupling. Using the optic phonon-plasmon coupled mode energy and the scattering time as fitting parameters values of 10 meV for the energy of the coupled mode and 400 ps for the scattering time were obtained.

Hot Electron Light Emitting Semiconductor Heterojunction Devices (Hellish) — Type — 1 and Type — 2

One of the draw-backs of the conventional light emitters appears to be that the light emission is confined to a small region of the facets of the devices1. Thus, the compatibility in generic integration technology remains a problem. The research on simple devices that emit light from the surface with good control of wavelength tunability, and which can be fabricated in large scale 2- dimensional arrays has been largely stimulated by potential applications in optical signal processing. One possible candidate for such a simple functional device is the light emitting charge injection transistor (CHINT) 2. Another light emitter, HELLISH-1 (Hot Electron Light Emission and Lasing in Semiconductor Heterostructures) has been proposed by us3–5. In this paper we present a novel surface emitting device, HELLISH-2 and demonstrate its operation with a simple model. We also report the results of our recent studies on a heavily p-n doped HELLISH-1 device.

Energy and momentum relaxation of hot electrons in GaAs/AlxGa1-xAs quantum wells : effect of hot phonon lifetime

The experimental results of hot electron energy and momentum relaxation rates in GaAs/AlxGa1-xAs multiple quantum wells, grown by molecular beam epitaxy (MBE) are presented. The energy relaxation rates are measured at lattice temperatures of T=4.2 K and T=77 K and up to a maximum electron temperature of Te=270 K. The results are compared with a theoretical model involving non-equilibrium interface phonon production. The experimental energy relaxation time of tau eff approximately=0.55 ps is shown to be in disagreement with the calculations. This discrepancy is attributed to the uncertainty in the hot phonon lifetime. The authors also present the results of high field, parallel transport, drift velocity measurements. The results indicate a drift velocity saturation at Vd=7*106 cm s-1 at 300 K and Vd=1.5*107 cm s-1 at 77 K. The saturation is followed by current instabilities.

Electric field profiling in 2D semiconductors exhibiting electrical instabilities

Two novel techniques-field contrast with a scanning electron microscope and electro-optic probing-have been employed to image the formation, and the resulting transients, of high-field domains in GaAs/AlxGa1-xAs quantum wells. The field contrast mode of a scanning electron microscope involves the analysis of the energy of secondary electrons emitted from the surface of the specimen, the energies of which depend on the local electrostatic fields present at or near the surface, whilst the electro-optic probing technique makes use of the electric-field-induced birefringence that occurs in non-centrosymmetric crystals, such as GaAs. Both techniques are therefore sensitive to changes in the local electric fields within the samples. In this work both techniques were used to profile the electric field distribution along the GaAs quantum well samples where negative differential resistance, and the associated current instabilities, have been induced at room temperature. Unlike the case of bulk material, where high-field domains propagate along the samples (Gunn domains), in 2D GaAs the high-field domains are found to be either static or annihilate before reaching the anode. A model based on a lateral dissipative mechanism is proposed and the merits of the two experimental techniques are discussed in terms of time and spatial resolution.

Self-generated nonlinear oscillations in multilayer semiconductor heterostructures

Nonlinear charge transport parallel to the layers of modulation-doped GaAs/AlxGa1-xAs heterostructures is studied theoretically and experimentally. In the field regime of about 2 kV cm-1 we find DC-induced current oscillations associated with N-shaped negative differential resistance. We develop a dynamic model based on real space transfer of hot electrons from the undoped high-mobility GaAs layers to the adjacent n-doped low-mobility AlxGa1-xAs layers. In particular, we extend previous models to multilayer structures and investigate the dependence of the self-generated oscillations upon circuit conditions and the lattice temperature in the range TL=77-200 K. In the light of the experimental results the theoretical predictions are analysed and discussed.