Hisao Hashimoto

GaAs cleaning with a hydrogen radical beam gun in an ultrahigh‐vacuum system

GaAs surface cleaning prior to molecular‐beam epitaxy (MBE) using a new hydrogen radical beam produced by an electron cyclotron resonance plasma in an ultrahigh‐vacuum system is investigated. Residual oxygen and carbon on the GaAs surface after cleaning are monitored in situ with Auger electron spectroscopy. Oxygen and carbon, which are hard to remove by conventional thermal cleaning, can be removed by hydrogen radical (H*) beam irradiation at a substrate temperature of <400 °C. It is verified that the plasma‐dissociated radicals are much more reactive in the gas phase–solid phase surface chemical interaction than the nondischarged molecules. Ga and As stoichiometry is kept after H* beam cleaning. Good crystallinity is obtained according to reflection high‐energy elecron diffraction. By carrier concentration measurement at the interface between the cleaned surface and the in situ MBE‐regrown layer, it is found that H* beam cleaning reduces the interface state concentration. This cleaning technique makes c...

Cleaning of MBE GaAs Substrates by Hydrogen Radical Beam Irradiation

Hydrogen radical beam cleaning of substrates for GaAs MBE growth has been studied. The cleaning effects of carbon and oxygen contaminated by exposure to air and a lower-grade vacuum were investigated using Auger electron spectroscopy (AES), capacitance-voltage (C-V) carrier profiling, and secondary ion mass spectrometry (SIMS). AES signals of contaminants decreased to the detection limit with hydrogen radical irradiation. Carrier depletion around the growth interface was also reduced with decreasing carbon density.

AlN capped annealing of Si implanted semi‐insulating GaAs

AlN capped annealing can be used successfully for thin n‐layer formation to minimize the thermal conversion effects. Si ions were implanted into Cr‐doped semi‐insulating GaAs substrates with 145 keV, to a dose of 2.40×1012 cm−2. Subsequently, the samples were annealed at 850 °C, 20 min with reactive sputtered AlN cap. The carrier profiles fit very well to the theoretical one. No thermal pits, cracks, and peeling‐off were observed even on 1.2‐μm‐thick AlN cap after up to 1000 °C annealing.

Chemically enhanced focused ion beam etching of deep grooves and laser-mirror facets in GaAs under Cl2 gas irradiation using a fine nozzle

Chlorine‐enhanced GaAs maskless etching has been performed with a novel focused ion beam etching (FIBE) system. The system is composed of an air‐locked ultrahigh vacuum chamber, a 30‐keV Ga+ FIB column, and a fine nozzle. The nozzle irradiates a high‐density Cl2 flux on a desired, small area of the sample while retaining a sufficiently low surrounding gas pressure for stable Ga+ FIB emission. Condensed Ga residues, appearing on the etched surface with no Cl2 gas, could be suppressed under Cl2 gas irradiation. Highly chemically enhanced sputtering yields (up to 50 GaAs molecules per incident ion) were obtained by selecting the optimum relationship between scanning time and Cl2 gas pressure. At the maximum yield, a deep groove (about 6 μm) with a smooth surface was obtained by line‐scanning FIBE. The etching was applied to laser‐mirror formation of an AlGaAs laser. A vertical mirror facet, fabricated in advance by a reactive ion beam etching, was trimmed about one micron thick by line‐scanning FIBE. Light o...

Maskless ion beam writing of precise doping patterns with Be and Si for molecular beam epitaxially grown multilayer GaAs

We have developed a computer‐controlled ion beam writing system with 14‐bit resolution to operate a 100 kV maskless ion implanter combined with an MBE growth chamber for patterned impurity‐doped GaAs/AlGaAs multilayer growth. Performances of various functions of this new computer‐controlled, crystal growth system have been tested. (1) Ion species (Si++, Si+, Be++, Be+) are rapidly selected with minimum astigmatism. (2) Depth impurity doping geometry can easily be controlled by automatically selecting ion charges or semiautomatically changing accelerating voltage. (3) Ion beam diameter and position are precisely monitored and controlled. (4) Ion beam mark detection can be performed with submicron resolution over double layers. (5) Ion beam writing for maskless implantation can be done in arbitrary shapes over the 400 μm square field with assigned impurity, ion dose, and energy, while monitoring ion current and beam diameter. Using this system, we have demonstrated arbitrary pattern implantation and GaAs gr...

GaAs Growth Using an MBE System Connected with a 100 kV UHV Maskless Ion Implanter

A new molecular beam epitaxy (MBE) system coupled with a 100 kV maskless ion implanter via an ultrahigh vacuum (UHV) sample transfer module was constructed. This system can grow epitaxial layers with ion beam pattern-implantation without exposing a sample surface to the outer atmosphere. Buried Be implanted layers in MBE grown GaAs were fabricated using this apparatus. Because of the contamination-free UHV growth process, the photoluminescent intensity depth profile of the grown crystal showed no degradation at the interface where the MBE growth was interrupted for the ion implantation process.

Extremely High Mobility of Two-Dimensional Electron Gas in Selectively Doped GaAs/N-AlGaAs Heterojunction Structures Grown by MBE

Selectively Si-doped GaAs/N-AlGaAs heterojunction structures have been grown by MBE. Two-dimensional electron gas accumulating at the interface of the heterojunction showed mobilities as high as 69,000 cm2/Vs at 77 K and 100,000 cm2/Vs at 4.2 K, with a sheet electron concentration of 5.5×1011 cm-2, which are higher than any reported so far. An enhancementmode high electron mobility transistor (E-HEMT), which was first fabricated from the heterojunction material, showed a field effect mobility of 49,300 cm2/Vs at 77 K, suggesting that this heterojunction material has potential for application to low-power, high-speed integrated circuits.

Focused Si Ion Implantation in GaAs

A 160-keV, submicron-focused Si ion implantation in MBE-GaAs was made using a 100 kV maskless implanter with a Au–Si–Be alloy ion source. Obtained Raman spectra indicated that compared with the implantation current densities of an unfocused ion beam, the higher current density (about 104 times greater) of the focused beam resulted in less implantation-induced and residual (after annealing up to 500°C) damage. Moreover, 850°C annealing led to a higher electrical activity of focused implanted Si-ions but almost the same optical quality of conventional implantation.

Arsenic Pressure Dependence of Interdiffusion of AlGaAs/GaAs Interface in Quantum Well

Arsenic pressure dependence of interdiffusion of AlGaAs/GaAs hetero-interface at a high temperature has been studied by masuring the wavelength shift of photoluminescence in a multi quantum well. It was found that the interdiffusion at the temperature of 850°C was minimized under arsenic pressure of 100 Torr and it was enhanced both under the lower and higher arsenic pressure. Also the degradation of photoluminescence intensity was observed only under the higher arsenic pressure. These properties are considered to be caused by excess vacancies of gallium and aluminum, and their assosiated defects.

Application of focused ion beam technology to maskless ion implantation in a molecular beam epitaxy grown GaAs or AlGaAs epitaxial layer for three‐dimensional pattern doping crystal growth

We report on a new microfabrication process in UHV, implemented with a focused ion beam implanter (FIBI)–molecular beam epitaxy (MBE) crystal growth system with a computer controlled ion beam writing system. By using this combination process technology, GaAs/AlGaAs multilayer pattern doping structures are fabricated by iteration of growing a molecular beam epitaxy (MBE) GaAs or AlGaAs layer followed by implanting focused ion beams, arbitrarily choosing such impurity ions as Be ( p‐type) or Si (n‐type). This process technology is considered as a basis for integrating optical or electrical devices three dimensionally. An application of this technology to a new structure for an AlGaAs/GaAs DH laser is also described.