M. Missous

High resistivity annealed low-temperature GaAs with 100 fs lifetimes

We demonstrate the critical effect of postgrowth annealing temperature on the properties of low-temperature-grown GaAs. By using annealing temperatures substantially below the 500–600 °C commonly reported, GaAs with high resistivity and with carrier lifetimes as short as 100 fs can be routinely obtained. We discuss the optimum, but different, anneal conditions required for terahertz photoconductive emitters and detectors, and illustrate their use in a continuous-wave system.

Terahertz pulsed imaging with 1.06 μm laser excitation

We report the operation of a pulsed terahertz (THz) imaging system that uses a 120 fs pulse-width laser operating at 1.06 μm, and photoconductive InGaAs emitters and detectors. THz images are presented of biological and nonbiological specimens. The signal-to-noise ratio and frequency range of this system are compared to those obtained from existing pulsed imaging systems based on 0.8 μm excitation.

Room-temperature operations of memory devices based on self-assembled InAs quantum dot structures

Memory devices have been fabricated in high-electron-mobility transistors with embedded InAs quantum dots (QDs). We show that memory operations can be fully controlled by gate biases at room temperature, without the need for light excitations to erase memory states. Real-time measurements indicate a charge retention time of a few minutes. Neither such retention time nor the self-consistent simulations can justify the picture that the memory effect is due to charging/discharging of intrinsic QD states. Experiments at a series of gate biases point to the presence of deep levels coexisting in the QD layer(s), which are responsible for the memory effect.

Resonant dipole antennas for continuous-wave terahertz photomixers

We present measurements and numerical simulations for the performance of resonant dipole antennas used in continuous-wave terahertz photomixers. We demonstrate that center-feed dipoles, with inductively choked electrical feeds, are a good choice for a resonant emitter design, and outperform broadband spiral antennas by a factor of five at a resonant frequency of 0.4THz. Excellent agreement is found between experiment and the predictions of finite element simulations. Furthermore, we show that the dipoles can outperform spirals across frequency bands of over 0.5THz, opening the way for their use in terahertz spectroscopy.

Highly resistive annealed low-temperature-grown InGaAs with sub-500 fs carrier lifetimes

We have optimized low-temperature-grown In0.3Ga0.7As for use in ultrafast photoconductive devices. Using low temperature ex situ annealing techniques, we have produced a photoconductive material that is highly resistive (∼104Ωcm), has sub-500fs carrier trapping lifetimes, and is matched to 1.06μm laser excitation.We have optimized low-temperature-grown In0.3Ga0.7As for use in ultrafast photoconductive devices. Using low temperature ex situ annealing techniques, we have produced a photoconductive material that is highly resistive (∼104Ωcm), has sub-500fs carrier trapping lifetimes, and is matched to 1.06μm laser excitation.

All-optoelectronic terahertz system using low-temperature-grown InGaAs photomixers

We demonstrate, for the first time, an all-optoelectronic continuous-wave terahertz photomixing system that uses low-temperature grown InGaAs devices both for emitters and coherent homodyne detectors. The system is compatible with fiber-optic excitation wavelengths, and we compare the performance to the more common LT-GaAs photomixers.

Analysis of thin AlN carrier exclusion layers in AlGaN/GaN microwave heterojunction field-effect transistors

We present a study of the effect of the growth of a thin AlN exclusion layer between the AlGaN barrier layer and GaN buffer layer in microwave heterojunction field-effect transistor structures. A dramatic improvement in carrier drift mobility is observed and we present evidence from electronic structure calculations and capacitance–voltage experiments that this improvement is associated with reduced alloy scattering. However, no significant benefit is seen at low carrier concentrations. Reduced electron trapping in the AlGaN is an additional benefit.

Evidence for donor‐gallium vacancy pairs in silicon doped GaAs grown by molecular beam epitaxy at low temperatures

Infrared absorption due to localized vibrational modes of silicon impurities in heavily doped GaAs grown by molecular beam epitaxy at low temperatures has been measured. Almost all Si atoms were found to occupy Ga sites. Second neighbor pairs SiGa‐YGa were also detected in a concentration of ∼2×1018 cm−3 and it was deduced that Y was a gallium vacancy (VGa). These results provide the first direct evidence for the presence of a high concentration of gallium vacancies in GaAs grown at low temperatures.

Multiple-frequency terahertz pulsed sensing of dielectric films

We report on a technique for the on-chip sensing of dielectric materials in the terahertz frequency range. We show that an array of band-stop filters, excited by integrated ultrafast photoconductive switches, can be used to sense dielectric loading at a number of distinct lithographically defined locations on a chip simultaneously, each location sensing a different terahertz frequency. This technique is expected to have a range of future applications in the field of on-chip terahertz systems, such as the analysis of deoxyribonucleic acid and other molecular films.

Low‐temperature molecular beam epitaxy of gallium arsenide

Device quality GaAs was grown at substrate temperatures as low as 430 °C, by decreasing the growth rate to 0.2 μm/h and by using dimeric arsenic. Precise doping control down to 1×1016 cm−3 has been easily achieved and Hall mobilities of up to 66% of the best molecular beam epitaxy values have been measured. By contrast, the use of As4 under the same growth conditions led to films with poor electrical and optical properties. These results add further evidence to the superiority of As2 in reducing the concentration of compensation deep levels, and provide the first conclusive evidence of its significant role in the low‐temperature growth of high quality GaAs.