Karl Woodbridge

Rheed studies of heterojunction and quantum well formation during MBE growth-from multiple scattering to band offsets

Abstract The basic concepts and first-order growth model derived from the RHEED intensity oscillation technique are described and the limitations imposed by the experimentally demonstrated multiple-scattering nature of the diffraction process are indicated. Despite these restrictions the value of the technique is illustrated in relation to growth mechanism studies, heterojunction and quantum well interface formation and as a process control monitor.

Computer modeling of the electric field dependent absorption spectrum of multiple quantum well material

The authors present a simple computer model for the electric field dependence of the absorption of semiconductor multiple-quantum-well (MQW) structure that will be used to optimize the performance of MQW modulators. This model has been compared to absorption spectra derived from photocurrent measurements on a GaAs/(GaAl)As MQW p-i-n diode and it has been found that the well-established Stark shifts of the exciton and subband continua energies are significantly overestimated. This might be linked to uncertainty in knowing the electric fields over the wells; and if a drop of 1.4 V somewhere in the device is assumed, a much better match can be achieved between the theoretical and experimental shifts. Given this improved match in the shifts the reduction in the oscillator strengths and the broadening are modeled very well. It is concluded that the model is likely to prove a useful tool for optimizing electroabsorption modulator design. >

Through-the-Wall Sensing of Personnel Using Passive Bistatic WiFi Radar at Standoff Distances

In this paper, we investigate the feasibility of uncooperatively and covertly detecting people moving behind walls using passive bistatic WiFi radar at standoff distances. A series of experiments was conducted which involved personnel targets moving inside a building within the coverage area of a WiFi access point. These targets were monitored from outside the building using a 2.4-GHz passive multistatic receiver, and the data were processed offline to yield range and Doppler information. The results presented show the first through-the-wall (TTW) detections of moving personnel using passive WiFi radar. The measured Doppler shifts agree with those predicted by bistatic theory. Further analysis of the data revealed that the system is limited by the signal-to-interference ratio (SIR), and not the signal-to-noise ratio. We have also shown that a new interference suppression technique based on the CLEAN algorithm can improve the SIR by approximately 19 dB. These encouraging initial findings demonstrate the potential for using passive WiFi radar as a low-cost TTW detection sensor with widespread applicability.

Effects of prelayers on minority‐carrier lifetime in GaAs/AlGaAs double heterostructures grown by molecular beam epitaxy

From room‐temperature measurements of the photoluminescence decay time, values for the sum of the interface recombination velocities in a series of GaAs/AlGaAs double heterostructures have been obtained. The effects of various combinations of prelayers of GaAs and AlGaAs have been investigated and we show that it is possible to reduce the sum of the interface recombination velocities to as low as 60 cm/s.

Delta-doping of GaAs and Al0.33Ga0.67As with Sn, Si and Be: a comparative study

Abstract We have performed a study of the contributions of segregation, diffusion and aggregation to the broadening of delta-doped planes of Sn, Si and Be in GaAs and Al 0.33 Ga 0.67 As. Sn planes are severely broadened by all three processes, but sharp spikes of Si and Be can be obtained in both host materials for sheet densities below 10 13 cm −2 and growth temperatures of 500°C or less. At higher temperatures or densities, segregation or concentration-dependent rapid diffusion may occur, causing significant spreading even during growth. Co-deposition of Si and Be dramatically reduces this broadening, and various mechanisms are considered to explain these effects.

Ambiguity Function Analysis of Wireless LAN Transmissions for Passive Radar

Wireless transmission is becoming an increasingly widely available source of transmissions for passive radar detection. In this paper, we present a detailed analysis of the ambiguity function (AF) of a range of typical IEEE 802.11 signals obtained during a series of experimental trials. Theoretical analysis has been used to identify the average properties of basic signal types in terms of resolution and sidelobe levels in both the range and Doppler domains. The theoretical model of a range of typical 802.11 transmissions has been verified and range and Doppler resolutions have been investigated for a range of transmission types. It has been found that using Doppler with a suitable integration time can enable detection of typical personnel targets. A number of issues relating to the use of these transmissions have been identified during this study.

Developments in target micro-Doppler signatures analysis: radar imaging, ultrasound and through-the-wall radar

Target motions, other than the main bulk translation of the target, induce Doppler modulations around the main Doppler shift that form what is commonly called a target micro-Doppler signature. Radar micro-Doppler signatures are generally both target and action specific and hence can be used to classify and recognise targets as well as to identify possible threats. In recent years, research into the use of micro-Doppler signatures for target classification to address many defence and security challenges has been of increasing interest. In this article, we present a review of the work published in the last 10 years on emerging applications of radar target analysis using micro-Doppler signatures. Specifically we review micro-Doppler target signatures in bistatic SAR and ISAR, through-the-wall radar and ultrasound radar. This article has been compiled to provide radar practitioners with a unique reference source covering the latest developments in micro-Doppler analysis, extraction and mitigation techniques. The article shows that this research area is highly active and fast moving and demonstrates that micro-Doppler techniques can provide important solutions to many radar target classification challenges.

Evaluation of WiFi beacon transmissions for wireless based passive radar

Wireless transmissions are a potentially powerful and widely available source of transmissions for passive radar detection. In this work we have carried out a detailed study on the use of IEEE 802.11 (WiFi) transmissions in a passive radar system. The WiFi transmission sequence has been found to be complex and dependent on the user environment but is dominated by direct sequence spread spectrum (DSSS) and orthogonal frequency division multiplexing (OFDM) signals. An ambiguity function analysis of the DSSS based WiFi beacon signal has been carried out followed by field measurements using a wireless based passive radar system. Range and Doppler characterization of this system is reported and compared with the theoretical predictions. Detection of moving human targets has been achieved for the first time using 802.11 transmissions. This work shows that this technique has considerable promise for a low cost and widely deployable detection and tracking system.

Influence of the barriers on the temperature dependence of threshold current in GaAs/AlGaAs quantum well lasers

Using window devices, light emission has been observed from the barrier regions of lasers with 25-A-wide quantum wells. From measurements of threshold current as a function of temperature on devices grown by molecular-beam epitaxy using different Al cells for the barriers, the strong influence of nonradiative barrier recombination processes on the threshold current has been demonstrated. Further measurements of threshold current as a function of hydrostatic pressure show that recombination from the L and X conduction-band minima makes an important contribution to the current. The calculations show how the temperature dependence of threshold depend on factors such as cavity length and the number of quantum wells. >

Passive bistatic WiMAX radar for marine surveillance

In this study we investigate the feasibility of using passive bistatic radar to exploit WiMAX communication signals as transmitters of opportunity for marine surveillance. The range and Doppler characteristics of such a system are first assessed through ambiguity function analysis on a typical set of WiMAX data transmission waveforms. Theoretical simulations are then used to investigate the ability of passive WiMAX radar for detecting marine vessels within three busy shipping areas in the UK, and in the presence of noise and interference signals. The initial analysis shows a range resolution of 5.6 m is achievable. Additionally, the simulations demonstrate specific scenarios where WiMAX radar may be used as a low cost surveillance device for detecting both small and large marine vessels in port areas and open waters.