K. B. Nichols

Photomixing up to 3.8 THz in low‐temperature‐grown GaAs

Low‐temperature‐grown (LTG) GaAs is used as an optical‐heterodyne converter or photomixer, to generate coherent continuous‐wave output radiation from microwave frequencies up to 3.8 THz. The photomixer consists of an epitaxial layer of LTG GaAs with interdigitated electrodes fabricated on the top surface. Terahertz photocurrents are generated in the gaps between the electrodes, and power is radiated into free space through a three‐turn self‐complementary spiral antenna. In a photomixer having a 0.27‐ps electron‐hole lifetime and small electrode capacitance, the output power is practically flat up to about 300 GHz and then rolls off at a rate of approximately 12 dB/oct.

Terahertz photomixing with diode lasers in low-temperature-grown GaAs

Recent optical heterodyne measurements with distributed‐Bragg‐reflector diode‐laser pumps demonstrate that low‐temperature‐grown (LTG) GaAs photomixers will be useful in a compact all‐solid‐state terahertz source. Electrical 3 dB bandwidths as large as 650 GHz are measured in mixers with low electrode capacitance. These bandwidths appear to be independent of pump‐laser wavelength over the range 780–850 nm. Shorter wavelength pumping results in a significant reduction of the bandwidth. The best LTG‐GaAs photomixers are used to generate coherent continuous‐wave output radiation at frequencies up to 5 THz.

Milliwatt output levels and superquadratic bias dependence in a low‐temperature‐grown GaAs photomixer

A cw output power up to 0.8 mW is obtained from a low‐temperature‐grown (LTG) GaAs, 0.3 μm gap, interdigitated‐electrode photomixer operating at room temperature and pumped by two modes of a Ti:Al2O3 laser separated in frequency by 0.2 GHz. The output power and associated optical‐to‐electrical conversion efficiency of 1% represent more than a sixfold increase over previous LTG‐GaAs photomixer results obtained at room temperature. A separate LTG‐GaAs photomixer having 0.6 μm gaps generated up to 0.1 mW at room temperature and up to 4 mW at 77 K. Low‐temperature operation is beneficial because it reduces the possibility of thermal burnout and it accentuates a nearly quartic dependence of output power on bias voltage at high bias. The quartic dependence is explained by space‐charge effects which result from the application of a very high electric field in the presence of recombination‐limited transport. These conditions yield a photocurrent‐voltage characteristic that is very similar in form to the well‐known Mott–Gurney square‐law current in trap‐free solids.

Investigation of ultrashort photocarrier relaxation times in low-temperature-grown GaAs

Photocarrier relaxation times τr in low-temperature-grown (LTG) GaAs have been determined with time-resolved reflectance measurements. Measured τr values are extremely sensitive to the substrate temperature during LTG GaAs growth and postgrowth anneal. Photogenerated-electron relaxation times as short as 90 fs are found for LTG GaAs grown at temperatures near 200 °C and annealed at temperatures below 580 °C. We report the results of a systematic investigation of the dependence of τr on growth temperatures between 180 and 260 °C and anneal temperatures between 480 and 620 °C.

Terahertz measurements of resonant planar antennas coupled to low‐temperature‐grown GaAs photomixers

Resonant slot and dipole antennas coupled to low‐temperature‐grown GaAs photomixers have been fabricated and tested at terahertz operating frequencies. Enhanced output power is seen from the resonant structures compared to mixers coupled to broadband self‐complementary spiral antennas. Driving point impedances as high as 300 Ω are attained at the resonant frequencies. These devices will be useful as fixed frequency local oscillators for submillimeter heterodyne receivers.

The Role of Impurities in Hydride Vapor Phase Epitaxially Grown Gallium Nitride

Gallium nitride (GaN) films grown by hydride vapor phase epitaxy on a variety of substrates have been investigated to study what role silicon and oxygen impurities play in determining the residual donor levels found in these films. Secondary ion mass spectroscopy analysis has been performed on these films and impurity levels have been normalized to ion implanted calibration standards. While oxygen appears to be a predominate impurity in all of the films, in many of them the sum of silicon and oxygen levels is insufficient to account for the donor concentration determined by Hall measurements. This suggests that either another impurity or a native defect is at least partly responsible for the autodoping of GaN. Additionally, the variation of impurity and carrier concentration with surface orientation and/or nucleation density suggests either a crystallographic or defect-related incorporation mechanism.

Optical heterodyne detection and microwave rectification up to 26 GHz using quantum well infrared photodetectors

We have demonstrated heterodyne detection up to an intermediate frequency of 26.5 GHz using quantum well infrared photodetectors. A CO/sub 2/ laser and a lead-salt tunable diode laser were used as the infrared sources. Heterodyne detection experiments measure the high frequency behavior of photoexcited electrons and their transport properties. We have also carried out microwave rectification experiments which measure the high frequency behavior associated with the dark-current electron-transport processes.

High-performance 0.15-/spl mu/m-gate-length pHEMTs enhanced with a low-temperature-grown GaAs buffer

An improved GaAs power pHEMT is presented. The device utilizes a low-temperature-grown (LTG) GaAs buffer layer instead of the conventional-buffer layers commonly used by pHEMT manufacturers. When contrasted with identical devices using a conventional buffer, these LTG-buffered pHEMTs have shown a 45% increase in channel breakdown voltage, a 12% increase in power output, and a record 63% power-added efficiency at 20 GHz.<<ETX>>

Optical-heterodyne generation in low-temperature-grown GaAs up to 1.2 THz

Low-temperature-grown, non-stoichiometric GaAs is used as an optical mixer to generate coherent output radiation up to a frequency of 1.2 THz. The mixer structure consists of an epitaxial layer of the LTG GaAs material with submicron interdigitated electrodes fabricated on the top surface. Terahertz photocurrents are generated in the gaps between the electrodes and power is radiated by coupling these currents efficiently into a self-complementary spiral antenna. The experimental roll-off in photomixer output power is explained by two time constants - one for the electron-hole recombination time of 0.35 ps and the other for the photomixer-antenna RC time constant of 0.62 ps. The photomixer demonstrates the capability to generate continuous-wave radiation in a spectral region where tunable coherent radiation has been lacking.

Monolithic optoelectronic transistor: A new smart‐pixel device

A new optical switching and logic device, the monolithic optoelectronic transistor (MOET), is demonstrated. The MOET is an integrated circuit consisting of a p‐i‐n photodiode, a resonant tunneling diode, a multiple‐quantum‐well modulator, and a field‐effect transistor. The device can function as an optical inverter or NOR gate. Present devices switch at an input optical power of 12.5 μW and have a large‐signal optical gain exceeding ten. The advantages of the MOET include abrupt switching thresholds, saturated ‘‘on’’ and ‘‘off’’ states, and nonlatching operation.