The authors present a study on the effects of design parameters on the performance of terahertz quantum-cascade lasers by using a Monte Carlo method, including essential scattering mechanisms and the hot-phonon effect. Three design parameters, i.e. injection and extraction barrier widths and doping concentration, are investigated. In the range of the three design parameters we used, simulated results show that the current density increases monotonically with the decrease of barrier widths and the increase of doping concentration. The calculated gain is more sensitively dependent on the injection barrier width and doping concentration. Thicker extraction barriers, i.e. from 39 to 48 Å, are acceptable.

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We report the electrical and the optical characteristics of 2 THz quantum-cascade lasers with a similar four-well resonant-phonon design. A Monte Carlo simulation, employed to evaluate the temperature performance of the device, indicates that the degradation of material gain with increasing temperature is attributed to the rapid decrease in the lifetime of the upper lasing level and the relatively stable lifetime of the lower lasing level. Because of the broadening effect of the temperature dependent gain profile, our calculations overestimate the peak gain and subsequently overrate the maximum operating temperature. Under a linear approximation condition, the deduced maximum operating temperature is in good agreement with experiment. Both experiment and simulation show that the lasing frequency is insensitive to temperature.

We have performed current-voltage (I-V) measurements on a terahertz quantum-well photodetector (QWP) at different temperatures and employed an emission-capture model to simulate the I-V curves. A temperature-dependent vertical electron drift mobility has been used to fit the curves from 7 K to 20 K. Photocurrents caused by 300 K background radiation have also been measured at different temperatures and a background-limited infrared performance (blip) temperature of 12 K for this terahertz detector has been determined. The current-temperature (I-T) curves derived from the measured dark I-V curves indicate that the thermionic emission process is the major mechanism for dark current in this terahertz detector.

Nanostructures made of semiconductors, such as quantum wells and quantum dots (QD), are well known, and some have been incorporated in practical devices. Here we focus on novel structures made of QDs and related devices for terahertz (THz) generation. Their potential advantages, such as low threshold current density, high characteristic temperature, increased differential gain, etc, make QDs promising candidates for light emitting applications in the THz region. Our idea of using resonant tunneling through QDs is presented, and initial results on devices consisting of self-assembled InAs QDs in an undoped GaAs matrix, with a design incorporating a GaInNAs/GaAs short period superlattice, are discussed. Moreover, shallow impurities are also being explored for possible THz emission: the idea is based on the tunneling through bound states of individual donor or acceptor impurities in the quantum well. Initial results on devices having an AlGaAs/GaAs double-barrier resonant tunneling structure are discussed.

This letter demonstrates an application of femtosecond laser ablation to eliminate the effect of crystallographic defects on optoelectronic devices. These defects can electrically shunt the devices and, in some applications, it is of great importance to circumvent the limitations due to these defects. On a large-area light-emitting diode, we show that laser ablation is an ideal treatment technique to eliminate the effect of defects. We found that laser surgery on light-emitting diodes is an easy technique to implement. It allows an in situ repair of defective areas.

We explore the possibility of far-infrared and terahee photovoltaic detection based on a quantum ballistic channel with an abrupt narrow end. By calculating the photoinduced intersubband transition in the channel and the transfer op!i~ized Snlit-nate ' " I ?-.structure with maximum electron transfer efficiency. The detector responsivity, operating temperature for background-limited performance and noise equivalent power are evaluated. These detectors may be used in applications such a s spectroscopy, infrared astronomy and space and environmental sensing.efficieficy of e!e&on+ oyer !he narrow end, we propose introductionEfficient far-infrared (FIR) and terahertz (THz) photon radiation in this wavelength (30-300 pm) and frequency (1-10 THz) region include bolometers, Schottky diodes and superconductor-insulator-superconductor (SIS) diodes. Bolometem are suited for some laboratory uses but are too slow in their response time fnr many app!icarinns. 2 1 s highest frequency of Schottky and SIS diodes is limited by their capacitances. It is difficult to fabricate these devices with sufficiently small capacitances for multi-THz applications. There is also a tunable detector based on the quantum Hall effect [l] in a two-dimensional electron gas (ZDEG), which requires a strong magnetic field. Recently, several authors investigated the possibility of FIR and

The authors present a simulation and experimental study on the effect of phonon extraction level separation on the performance of GaAs-based three-well resonant-phonon terahertz quantum-cascade lasers (QCLs). The phonon extraction level separation is varied from 30 to 42 meV. Because of the efficient longitudinal-optical phonon scattering, the 36 meV QCL shows the largest gain, the best temperature performance and the highest output power. As for the lower (30 meV) or higher (42 meV) energy separation QCLs, the electron-longitudinal-optical phonon interaction still works by involving a transfer of in-plane momentum. The measured lasing characteristics are in qualitative agreement with simulation.