Shigekazu Izumi

Selective wet etching for highly uniform GaAs/Al0.15Ga0.85As heterostructure field effect transistors

We have developed a novel selective citric acid-based etchant used for GaAs over AlGaAs, which consists of citric acid, NH4OH, and H2O2. The etching rate ratio of GaAs to Al0.15Ga0.85As was as high as 80 by optimizing the pH and citric acid/H2O2 ratio. The etch stop mechanism was investigated using x-ray photoelectron spectroscopy. The etching is stopped at the AlGaAs surface due to Al2O3 formation, whose density is higher than that on a nonselectively etched surface. This selective citric acid-based etchant was applied to the fabrication of GaAs/AlGaAs heterostructure field effect transistors (HFETs). The threshold voltages of the HFETs exhibit excellent uniformity (the standard deviation of 32 mV across the 3 in. wafer), demonstrating the applicability of this selective etchant to the gate recess process.

Less than 10 defects/cm2 · μm in molecular beam epitaxy grown GaAs by arsenic cracking

Abstract Drastic reduction of irregular defect density in molecular beam epitaxy (MBE) grown GaAs is obtained by using a novel arsenic Knudsen effusion cell with a cracking furnace. A surface defect density of less than 10 cm −2 is routinely achieved for continuously grown three 3-inch diameter, 1.7 μm thick, metal-semiconductor-field effect transistor (MESFET) structures when the cracking is carried out at about 700°C, which is the critical temperature for the conduction type change, and with the group III Knudsen cell only heated near the orifice of the crucible (top heat cell). These optimized cracking conditions also lead to successful mass production of some microwave devices with not only ultra-low defect density but suitable electrical performances.

Chemical beam epitaxial growth of Si‐doped GaAs and InP by using silicon tetraiodide

Silicon tetraiodide (SiI4), which has a very weak Si–I bond strength (70 kcal/mol), is successfully employed as a novel Si dopant in the chemical beam epitaxy of GaAs and InP. No precracking is necessary before supplying SiI4 with He carrier gas. High electrical quality is ascertained for both GaAs and InP with linear Si doping controllability in the range from 2×1016 to 6×1018 cm−3 with a uniformity of less than 2% within a 3‐in.‐diam area. The electron mobility in a GaAs with a carrier concentration of 1×1017 cm−3 is 4400 cm2/V s and that in InP with a carrier concentration of 4×1017 cm−3 is 2400 cm2/V s, respectively. Abrupt interfaces and precise on‐off controllability without any memory effect is also confirmed by secondary‐ion‐mass, spectroscopy measurements. The electrical activation ratio of Si in SiI4 for both GaAs and InP is almost 100% in the range studied here. These versatile features suggest that SiI4 is a promising candidate as a Si dopant source for chemical beam epitaxy growth.

Study on the accumulated impurities at the epilayer/substrate interface and their influence on the leakage current of metal-semiconductor-field effect transistors

Carrier impurities accumulated at the interface of a molecular beam epitaxial (MBE) grown GaAs layer-substrate have been studied in connection with substrate surface preparation just prior to MBE growth. Although the accumulated impurity density of silicon (higher than 7 × 1017 cm-3) at the interface is the same for both H2SO4- and HF-treated samples, the oxygen level is much higher for the H2SO4-treated (higher than 1 × 1019 cm-3) than for the HF-treated sample (less than 2 × 1018 cm-1). High density of carriers (higher than 7 × 1017 cm-3 is found to be accumulated at the interface only for the HF-treated sample. Further, absorbances at 1080 and 1260 cm-1 in FTIR spectra, which correspond to Si-O bonds, are observed only for the H2SO4-treated sample. The accumulated carrier concentration obtained in the HF-treated sample is thought to originate from the accumulated silicon which is not inactivated by making Si-O bonds. The accumulated impurities depend upon the initial surface condition of GaAs substrate before growth. The arsenic passivation layer has a good effect on reducing the accumulated impurities, which causes the carrier accumulation.

Multiwafer gas source molecular beam epitaxial system for production technology

High throughput epitaxial wafer production is demonstrated by using a newly designed multiwafer gas source molecular beam epitaxy apparatus. The actual application data show excellent results of uniformity, cost performance, and material performance through practical mass production operation. Electron mobility as high as 124 000 cm2/V s is obtained at 77 K for a 7-μm-thick GaAs layer with a carrier concentration of 7.7×1013 cm−3. A typical surface defect density of 25 cm−2 is achieved for continuously grown 1.7-μm-thick metal–semiconductor field effect transistor (MESFET) structures. The uniformity of sheet resistance in n-GaAs and AlAs mole fractions in AlGaAs is less than 2.0% (1.5% and 0.27%, respectively) over a 27 cm diameter area. A quantitative throughput number for a typical growth of a MESFET structure is four 4 in. or seven 3 in. diameter wafers per 2.5 h in a continuous process flow.

Low damage etching of InGaAs/AlGaAs by the electron cyclotron resonance plasma with Cl2/He mixture for heterojunction bipolar transistors

Low damage InGaAs/AlGaAs etching was realized by electron cyclotron resonance (ECR) plasma with a Cl2/He mixture for heterojunction bipolar transistors (HBTs) emitter mesa formation. By optimizing the etching pressure and the Cl2/He ratio, n‐InGaAs cap layer and n‐AlGaAs emitter layer are successively etched and the smooth surface morphology was obtained. An optical emission measurement reveals that the enhancement of the ionized Cl etching plays the essential role for etching of InGaAs. Raman scattering spectra and the base contact resistance measurements indicate that the etching induced damage is extremely low. Moreover, it was found that the etching rate of p‐AlGaAs base layer decreased down to two‐thirds of that for n‐AlGaAs emitter layer under the optimized etching condition. These results demonstrate the potentiality of ECR plasma etching with Cl2/He discharge providing degradation‐free dry etching for AlGaAs/GaAs HBTs.

Selective area chemical beam epitaxial regrowth of Si-doped GaAs by using silicon tetraiodide for field effect transistor application

Selective area regrowth of silicon-doped GaAs has been successfully achieved by chemical beam epitaxy (CBE) on dry etched trench structures. Abrupt doping-interface and excellent doping controllability have also been achieved by using a novel silicon dopant source of silicon tetraiodide (SiI4). Metal-semiconductor field effect transistor (MESFET) and heterostructure field effect transistor (HFET) fabricated in this way with Ni/AuGe/Au alloyed contact contact metals on the Si-doped GaAs regrown layer reveal contact resistances as low as 3 × 10− 7 Ω cm2 and perfect selectivity there being no polycrystalline growth on the dielectric mask film.

Quantitative correlation between oxygen impurity and current gain β of AlGaAs/GaAs heterojunction bipolar transistors grown by molecular beam epitaxy

The experimental correlation between current gain β of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) and oxygen in the n-AlGaAs emitter layer is discussed quantitatively. It has been observed that β decreases monotonically with an increase of the oxygen concentration in the n-AlGaAs layer. It has been found that the growth rate is one of the dominant factors which determine the crystal quality including the oxygen concentration, and that the optimum growth rate for the AlGaAs/GaAs HBT structure is around 0.7 μm/h under the constant growth temperature used. It has been thought that the variation of β is mainly ascribed to the recombination center at the emitter/base junction due to the oxygen impurity. The value of 3×10 17 cm -3 is consequently obtained as the upper limit of the oxygen concentration for high and stable β by extrapolating the linear relation between the oxygen concentration and 1/β Brs (the reciprocal of the recombination component of β at the emitter/base junction)

Evaluation of molecular beam epitaxially grown AlGaAs/GaAs heterojunctions for bipolar transistor with InGaAs emitter contact layer

Molecular beam epitaxially grown AlGaAs/GaAs heterojunctions were characterized by isothermal capacitance transient spectroscopy to study the performance of bipolar transistors with lattice‐mismatched InGaAs emitter contact layer. A deep level around 0.48 eV is found to be a recombination center in the N‐AlGaAs/p+‐GaAs junction which might be induced by oxygen. Anomalous signals are also observed under an isothermal condition where the edge of the depletion layer reaches the graded InGaAs/AlGaAs heterointerface. Two electron traps with activation energies of 0.26 and 0.62 eV are identified as dominant factors.