F. W. Smith

New MBE buffer used to eliminate backgating in GaAs MESFETs

The buffer is grown by molecular beam epitaxy (MBE) at low substrate temperatures (150-300 degrees C) using Ga and As/sub 4/ beam fluxes. It is highly resistive, optically inactive, and crystalline, and high-quality GaAs active layers can be grown on top of the buffer. MESFETs fabricated in active layers grown on top of this new buffer show improved output resistance and breakdown voltages; the DC and RF characteristics are otherwise comparable to MESFETs fabricated by alternative means and with other buffer layers.<<ETX>>

Structural properties of As-rich GaAs grown by molecular beam epitaxy at low temperatures

GaAs layers grown by molecular beam epitaxy (MBE) at substrate temperatures between 200 and 300 °C were studied using transmission electron microscopy (TEM), x‐ray diffraction, and electron paramagnetic resonance (EPR) techniques. High‐resolution TEM cross‐sectional images showed a high degree of crystalline perfection of these layers. For a layer grown at 200 °C and unannealed, x‐ray diffraction revealed a 0.1% increase in the lattice parameter in comparison with bulk GaAs. For the same layer, EPR detected arsenic antisite defects with a concentration as high as 5×1018 cm−3. This is the first observation of antisite defects in MBE‐grown GaAs. These results are related to off‐stoichiometric, strongly As‐rich growth, possible only at such low temperatures. These findings are of relevance to the specific electrical properties of low‐temperature MBE‐grown GaAs layers.

Subpicosecond carrier lifetime in GaAs grown by molecular beam epitaxy at low temperatures

Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time‐resolved‐reflectance techniques, a sub‐picosecond (<0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at ∼200°C and annealed at 600 °C. With the same material as a photoconductive switch we have measured electrical pulses with a full‐width at half‐maximum of 0.6 ps using the technique of electro‐optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of ∼120–150 cm2/V s. GaAs grown by MBE at 200 °C and annealed at 600 °C is also semi‐insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of subpicosecond photoconductive applications.

Picosecond GaAs‐based photoconductive optoelectronic detectors

A novel material deposited by molecular beam epitaxy at low substrate temperatures using Ga and As4 beam fluxes has been used as the active layer for a high‐speed photoconductive optoelectronic switch. The high‐speed photoconductive performance of the material was assessed by fabricating two devices: an Auston switch and a photoconductive‐gap switch with a coplanar transmission line. In a coplanar transmission line configuration, the speed of response is 1.6 ps (full width at half maximum) and the response is 10 to 100 times greater than that of conventional photoconductive switches. Since the material is compatible with GaAs discrete device and integrated circuit technologies, this photoconductive switch may find extensive applications for high‐speed device and circuit testing.

375‐GHz‐bandwidth photoconductive detector

We report the development of a new, integrable photoconductive detector, based on low‐temperature‐grown GaAs, that has a response time of 1.2 ps and a 3‐dB bandwidth of 375 GHz. The responsivity is 0.1 A/W. Signal amplitudes up to 6 V can be produced with virtually no degradation in response time.

Breakdown of crystallinity in low‐temperature‐grown GaAs layers

A systematic study of the change in structural quality of as‐grown GaAs layers deposited at temperatures between 180 and 210 °C by molecular beam epitaxy was performed using transmission electron microscopy, double‐crystal x‐ray rocking curves, and particle‐induced x‐ray emission. We found that the crystal quality was correlated strongly with growth temperature near 200 °C. The lattice parameter and the amount of As incorporated in the layer were observed to increase at lower growth temperatures. After exceeding a certain growth‐temperature‐dependent layer thickness, large densities of pyramidal‐type defects are formed, which at lowest growth temperature result in the breakdown of crystallinity and in columnar polycrystalline growth. The lattice expansion is ascribed to the excess As in the layers. The mechanisms of breakdown of crystallinity are discussed.

VA-2 New MBE buffer used to eliminate backgating in GaAs MESFET's

A new buffer layer bas been developed that eliminates backgating between MESFETs fabricated in active layers grown upon it. The new buffer is grown by molecular beam epitaxy (MBE) at low substrate temperatures (150-300°C) using Ga and As, beam fluxes. It is highly resistive, optically inactive, and crystalline, and high-quality GaAs active layers can be grown on top of the new buffer. MESFETs fabricated in active layers grown on top of this new buffer show improved output resistance and breakdown voltages; the dc and RF Characteristics are otherwise comparable to MESFETs fabricated by alternative means and with other buffer layers.

High-power-density GaAs MISFETs with a low-temperature-grown epitaxial layer as the insulator

A GaAs layer grown by molecular beam epitaxy at 200 degrees C is used as the gate insulator for GaAs MISFETs. The gate reverse breakdown and forward turn-on voltages, are improved substantially by using the high-resistivity GaAs layer between the gate metal and the conducting channel. It is shown that a reverse bias of 42 V or forward bias of 9,3 V is needed to reach a gate current of 1 mA/mm of gate width. A MISFET having a gate of 1.5*600 mu m delivers an output power of 940 mW (1.57-W/mm power density) with 4.4-dB gain and 27.3% power added efficiency at 1.1 GHz. This is the highest power density reported for GaAs-based FETs.<<ETX>>

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.

High-voltage picosecond photoconductor switch based on low-temperature-grown GaAs

A GaAs material grown by molecular beam epitaxy at a low substrate temperature was used to fabricate a photoconductor switch that produces 6-V picosecond electrical pulses. The pulses were produced on a microwave coplanar-strip transmission line lithographically patterned on the low-temperature (LT) GaAs. A 150-fs laser pulse was used to generate carriers in the LT GaAs gap between the metal strips, partially shorting a high DC voltage placed across the lines. The 6-V magnitude of the electrical pulses obtained is believed to be limited by the laser pulse power and not by the properties of the LT GaAs. Experiments were also performed on a picosecond photoconductor switch fabricated on a conventional ion-damaged silicon-on-sapphire substrate. Although comparable pulse durations were obtained, the highest pulse voltage achieved with the latter device was 0.6 V. >