I. Ohbu

Defects in low‐temperature‐grown GaAs annealed at 800 °C

GaAs grown by molecular beam epitaxy (MBE) at 300 °C is annealed at 800 °C and optical properties are studied using photoluminescence spectroscopy (PL) and infrared‐absorption spectroscopy. Three kinds of defects are observed. One of them is attributed to gallium vacancies with an energy level at 0.3 eV above the valence‐band edge. The concentration of gallium vacancies is increased by the high‐temperature annealing. The GaAs can render inactive free electrons of 1.3×1018 cm−3 even after annealing at 800 °C for 10 min.

Ultrafast characterization of an in‐plane gate transistor integrated with photoconductive switches

An in‐plane gate field‐effect transistor is characterized by ultrafast electro‐optic sampling. The transistor is monolithically integrated with photoconductive switches in coplanar waveguide and <0.5 ps measurement time resolution is achieved. The gate‐drain capacitance of the transistor is obtained as 1.8 fF at zero drain voltage from displacement current transients. The gate‐drain capacitance is dominated by parasitic capacitance and the intrinsic gate‐drain capacitance is estimated as less than 0.2 fF.

Monolithically-integrated optoelectronic circuit for ultrafast sampling of a dual-gate field-effect transistor

An integrated optoelectronic circuit for ultrafast sampling of multi-terminal devices is described. This is achieved using optimized photoconductive switches fabricated from low-temperature-grown GaAs, monolithic integration of the device with the sampling circuit, control of the electromagnetic modes propagating on the coplanar waveguide using microfabricated airbridges, and discrimination of guided and freely-propagating modes using a novel electrooptic sampling method. As an example, the scattering parameters associated with the propagation of a picosecond pulse through one of the gates of a dual-gate heterojunction field-effect transistor are obtained at frequencies up to 300 GHz. The inter-gate capacitance is determined by measuring the electromagnetic transient coupled between the gates.

Effects of indium doping on crystalline qualities of GaAs on Si by molecular beam epitaxy

The effects of In doping on crystalline qualities are demonstrated in the heteroepitaxy of GaAs on Si grown by molecular beam epitaxy. The etch pit density of the GaAs layers doped with In at 8×1017 cm−3 decreased by a factor of 7 compared with undoped GaAs layers. Dark regions observed in electron beam induced current images became small by In doping. The improvement of the crystalline qualities was also verified by Raman spectroscopy.

Ultrafast characterization of in-plane-gate field-effect transistors: parasitics in laterally gated transistors

In-plane-gate field-effect transistors are probed by femtosecond electrooptic sampling. Ultrafast response of the transistors is dominated by a displacement current induced by parasitic gate-drain capacitance. Intrinsic and parasitic gate-drain capacitances of various transistor structures are obtained from displacement-current characteristics and are in quantitative agreement with the calculation of planar capacitances. Intrinsic gate-drain capacitances are in the order of 100 aF, while parasitic gate-drain capacitances are between 1.7 and 4.8 fF, more than ten times that of intrinsic gate-drain capacitances. Reduction in parasitic capacitance by a factor of two is achieved by means of grounded shields and is confirmed by calculation. The grounded-shields screen parasitic electric fields and transform parasitic coupling into a part of the waveguide coupling. This reduction in parasitic capacitance is the first demonstration that the parasitic field effect is controlled artificially by nanometre-scale device technology.

The electron mobility transition in n-GaAs heavily doped channel

Low-field electron transport properties in heavily doped n-GaAs channels at 300 K are investigated by Monte Carlo simulation. The matrix element for electron-impurity scattering is obtained from the Fourier-transformed Coulomb potential accounting for the screening effects of the two-dimensional electron gas. The effects of the screening and the impurity profile are analysed. Electron mobility is calculated for several values of sheet electron density and n-GaAs thickness. For low sheet electron density, the calculated mobility increases with the sheet electron density and is almost independent of n-GaAs thickness. For high sheet electron density, the mobility approaches a constant value and becomes independent of sheet electron density. These findings are explained for the first time as a transition from a two- to three-dimensional nature and are well confirmed by our experiments.

Time dependence of the surface Fermi level of GaAs in atmosphere

This letter reports the time dependence of the surface Fermi level of GaAs grown by molecular‐beam epitaxy and then exposed to atmosphere. The sheet resistance of sample structures for field effect transistors alternately increased, decreased, increased, and decreased to become nearly constant after about 500 h. These changes correspond to the surface Fermi level varying between 0.3 and 0.7 eV and finally settling 0.7 eV above the valence band maximum. Comparison between annealed and unannealed samples with low‐temperature‐grown GaAs layers showed that the pinning of the surface Fermi level at 0.7 eV above the valence band maximum is caused by arsenic antisite defects. The result supports the advanced unified defect model.