A. J. Vickers

Transient response of photodetectors

Calculations by drift diffusion and Monte Carlo methods and experimental data on the transient response of metal–semiconductor–metal photodetectors are reported. We have shown how the interplay of carrier and displacement currents, inhomogeneities in field and charge distributions, and hot electron effects determines observed structure in the transient response. In particular, we have demonstrated the role played by the regions under the contacts that are not illuminated in forming peaks in the transient response. We have also demonstrated that the peaks in the transient response need not be attributed to velocity overshoot and the scaling of detector response with contact separation has been studied.

Fabry-Pérot enhancement of external electro-optic sampling

This paper describes the Fabry–Pe´rot enhancement of external electro-optic sampling. Previous reports of Fabry–Pe´rot enhancement have been exclusively on continuous wave systems. Using a picosecond dye laser and a standard electro-optic probe tip an increase is shown both in the sensitivity and the responsivity of the picosecond probing system. The results show that for a picosecond system considerable advantage can be gained by use of the Fabry–Pe´rot effect.

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.

Molecular beam epitaxy growth and properties of GaAs/(AlGa)As p-type heterostructures on (100), (011), (111)B, (211)B, (311)B, and (311)A oriented GaAs

We report on a series of Be-doped GaAs/AlGaAs two-dimensional hole gas (2DHG) structures grown on (110), (111)B, (211)B and (311)B oriented substrates and compare their properties with high-mobility samples grown on (311)A using Si doping. The samples were prepared and grown under the same conditions so as to render them comparable. They are found to have mobilities which are strongly anisotropic within the plane. The highest mobility is found on the (110) surface with ∼100,000 cm 2 V -1 s -1 , while the (211) surface gave the lowest values ∼10,000 cm 2 V -1 s -1 . However, the later samples are found to have quantum Hall effect critical currents of >70 μA: an exceptionally high value for a hole gas which makes them suitable for metrology. All the samples show strong low-field positive magnetoresistance with resistance increases of up to 30% at magnetic fields of only 0.1 T. The presence of this feature on all the different planes shows that it does not depend upon the details of the band structure. It is identified with the lifting of the degeneracy of the spin sub-bands by the asymmetrical potential giving rise to a classical two-band magnetoresistance

Study of a backgated metal‐semiconductor‐metal photodetector

In this letter we outline our results on a new type of high‐speed photodetector [E. Gregor et al., Appl. Phys. Lett. 65, 2223 (1994)]. The device is based on the hybridization of a metal‐semiconductor‐metal photodetector and a p‐i‐n photodiode. The advantage of the device, which operates in the transit‐time limited regime, is that it removes the hole current from the high‐speed circuit through a third contact, and hence, increases the response speed of the device. In contrast to the previously published work [E. Gregor et al., Appl. Phys. Lett. 65, 2223 (1994)] we have used an excitation pulse that is much faster than the device response in order to fully investigate the effect of the third contact. We observe significantly more effect on the response once the third contact is connected with a subsequent increase of device response speed with increasing application of bias to this third contact.

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.

THz spectroscopic studies of biomolecules

Terahertz (THz) spectroscopy is playing an increasingly bigger role in biomedical science, spear headed by terahertz time domain spectroscopy (THz-TDS) [1]. Terahertz spectroscopy has given us insights into the low frequency behaviour of biomolecules such as DNA and proteins [2-4]. Computational simulated models of biomolecules are improving and now provide comparison data [5].

A proposed semiconductor laser pump-probe source

A model is presented which determines the output from a semiconductor laser as a function of the direct current (d.c.) and radio frequency (r.f.) current applied to the laser. It is shown that by adjusting these two components the output optical pulse from a laser can be shifted in time without change to the pulse shape or height. Experimental results are presented which support the model. It is suggested how these results can be used to produce a semiconductor pump-probe source, which is essential for the development of optical probing.