Yuichi Ide

Direct Observation of Species Liberated from GaAs Native Oxides during Atomic Hydrogen Cleaning

A real-time mass spectroscopic observation of liberated species was carried out to investigate the mechanism of atomic-hydrogen-( H•)-induced deoxidation of GaAs native oxides. Atomic hydrogen treatment at 410°C caused, initially, the liberation of molecular arsenic ( As2/As4), resulting in the removal of As oxides, which was then followed by the liberation of Ga2O, leading to complete deoxidation. These results indicate that the deoxidation proceeds through two stages. The main chemical reactions are As2Ox +2 x H•→ x H2O+ As2/(1/2As4) in the first stage and Ga2O3+ 4H•→2H2O+ Ga2O in the second stage.

Anomalous behaviors observed in the isothermal desorption of GaAs surface oxides

Abstract Isothermal desorption of oxide layers formed on GaAs(001) surfaces of arsenic-rich c(4 × 4) reconstructions was observed at about 540°C using a quadrupole mass spectrometer. Ga2O and molecular arsenic desorbed in parallel. After long induction periods of the order of 10 min, their desorption rates increased exponentially, and then decreased steeply due to the completion of oxide removal. The behaviors observed in the final stage of oxide desorption suggest that the removal of the oxide layer proceeds through formation of voids, and that the oxide desorption rate is proportional to the area of the voids.

Role of Ga2O in the removal of GaAs surface oxides induced by atomic hydrogen

The role of Ga2O in the removal of GaAs surface oxides induced by atomic hydrogen (H⋅) has been studied using temperature‐programed desorption (TPD), x‐ray photoelectron spectroscopy, and low‐energy electron diffraction (LEED). GaAs(001) substrates with ‘‘Ga2O3‐like’’ surface oxides formed in vacuo, as well as those ‘‘as‐loaded’’ from air, were treated by thermally generated H⋅ at substrate temperatures ranging from 210 to 410 °C. Formation of Ga2O‐like oxides was indicated by TPD performed after H⋅ treatment. Above the onset temperature of Ga2O desorption, which was 350–400 °C, oxide removal was rapid. Below the onset temperature, oxide removal was slow and incomplete, suggesting that Ga2O‐like oxides act as a mask against H⋅ irradiation. When treated by H⋅ at 210 °C, the amount of Ga2O‐like oxides formed did not increase with the treatment time. Moreover, prolonged H⋅ treatment at 210 °C resulted in a Ga‐rich LEED pattern. We attribute this to a further reaction of H⋅ with Ga2O, resulting in Ga and water. In practice, H⋅ treatment at temperatures above the Ga2O desorption onset is desirable for rapid and complete oxide removal, which results in a stoichiometric surface.

Interaction of atomic hydrogen with GaAs (001) surface oxide: volatile Ga-oxide formation

The effect of an atomic-hydrogen (H ·) treatment on Ga2O3-like oxide on GaAs (001) was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Samples treated with H · at 210 and 300°C caused a new type of Ga2O desorption starting at around 400°C in TPD, indicating the formation of a Ga2O-like oxide which is more volatile than the initial Ga2O3-like oxide. XPS showed that, although oxide removal by H · proceeded fast at 405°C, at 210°C the deoxidation became retarded after an initial decrease of the oxide. This suggests that desorption of the resulting Ga2O-like oxide is essential for the smooth progress of deoxidation.

High-mobility two-dimensional electron gas in modulation-doped InAlAs/InGaAs heterostructures

Abstract The two-dimensional electron gas (2DEG) mobilities in MBE-grown modulation-doped InAlAs/InGaAs heterostructures have been studied. For the MBE growth, the flux intensity transients were minimized by adopting specially designed cell-shutters, which ensured highly abrupt heterointerfaces. A low temperature 2DEG mobility as high as 110000 cm2/V s has been achieved with an optimized layer structure. The 2DEG mobilities are discussed in relation with relevant scattering mechanisms.

Electrical characterization of very-low energy (0–30 eV) Cl-radical/ion-beam-etching induced damage using two-dimensional electron gas heterostructures

Damage induced by very low energy (30 eV) Cl reactive ion beam etching (RIBE) and radical etching (RE) has been electrically characterized. GaAs/n-AlGaAs two-dimensional electron gas heterostructures were used as damage sensitive probes. Sheet carrier concentrations and Hall mobilities were measured at 77 K, under dark as well as illuminated conditions. By carefully designing the sample structure, the damaged layer thickness could be estimated by comparing dark and illuminated data. In case of RE, no degradation was detected at depths as shallow as 25 nm from the etched surface. For 30 eV-RIBE, the damage was detected but found to be reasonably small (38% decrease in electron mobility) and shallow (<50 nm). Electrical and optical damage are compared briefly.

Characterization of GaAs-(001) Surface Photo-Oxide Formed by Visible-Light Irradiation

The effects of visible-light irradiation (70-280 mW/cm2) on oxidation of a GaAs (001) surface under an oxygen atmosphere (3.6×101 and 8.6×104 Pa) were investigated by temperature programmed desorption and X-ray photoelectron spectroscopy (XPS). XPS showed that irradiation at an intensity of 140 mW/cm2 increased the amount of surface oxygen by about 15% over dark oxidation of the corresponding oxygen exposure. Under 540°C isothermal conditions, the oxide desorption rate was initially low, but reached its maximum level with a time delay, showing the existence of expanding voids in the oxide layer during desorption. Light irradiation, as well as higher oxygen exposure, decreased the initial desorption rate, and increased the time delay by up to 6.4 min. This is interpreted as indicating that irradiation reduces the density of the weak parts in the oxide layer through an increase in the oxide thickness.

GaAs Surface Cleaning/Etching Using Plasma-Dissociated Cl Radical

Cl radical etching (RE) of GaAs, a previously evaluated dry etching method with very low damage which is suitable for nanometer-scale fabrication, damage removal and surface cleaning, is investigated in comparison with Cl2 etching. At room temperature, etching conditions with a higher chiorine pressure of about 10-3 Torr provide no significant etching rate, whereas etching conditions at a lower chlorine pressure of (4±1)×10-5 Torr provide a moderately slow etching rate (40 A/min) and precise control of etching depth, both of which are useful for shallow etching of nanometer-scale structures. Effective cleaning of GaAs native oxide and carbon contaminants by the Cl radicals is clearly demonstrated, but this cleaning does not occur when using Cl2 molecules. When the GaAs surface is not covered with such contaminants, both the Cl-RE and the Cl2 etching progress. The roughness of the Cl-radical-etched surface is as low as 50 A after etching to a depth of 2640 A. After heat treatment of the Cl-radical-etched sample at 400°C, an atomically ordered and stoichiometric GaAs surface is obtained.

Electron Beam Excited GaAs Maskless Etching Using C12 Nozzle Installed FIB/EB Combined System

We have developed a new fine-beam assisted GaAs maskless etching system capable of nanofabrication; a focused ion beam (FIB) and electron beam (EB) combined etching system with a reactive gas nozzle. In this FIB/EB combined system, EB excited GaAs etching was successfully performed by irradiating Cl2 gas on a temperature-controlled substrate. 5KeV EB was raster-scanned in a 100pm X 20pm rectangular pattern on a GaAs surface. With special care to remove the native oxide layer, spatially selective etching was also confirmed on a cleaned GaAs surface by controlling the Cl2 pressure.

Structural and optical studies on molecular beam epitaxially grown (InAs)m(AlAs)m and (InAs)m(GaAs)m short-period strained layer superlattices

Abstract Binary consisting (InAs) m (AlAs) m and (InAs) m (GaAs) m short-period strained layer superlattice InP substrates were grown and their structural and optical properties were examined. Mirror-like surface morphologies were obtained as long as their total thicknesses were less than a critical value. X-ray diffraction measurements showed the formation of atomically ordered periodic superstructure. Photoluminescence spectra measured at 77 K revealed good cry stalline quality in spite of about 7% lattice mismatch between the binary layers. The peak energy changes greatly on (InAs) m (IlAs) m according to the layer thickness, while it varies by a small amount on (InAs) m (GaAs) m structures, which reflects to the layer band gap discontinuities between InAs and AlAs. These structures can be substituted for InAlAs and InGaAs alloys.