Kouji Mochizuki

New approach to the atomic layer epitaxy of GaAs using a fast gas stream

A new growth method has been developed for the atomic layer epitaxy of GaAs. The gas system was based on a conventional metalorganic vapor phase epitaxial system but the decomposition of methylgallium was suppressed in the stagnant layer by using a fast pulsed gas stream from a jet nozzle. The method enabled us to grow high purity epitaxial layers with a clear self‐limiting mechanism even at 560 °C. The variations in the growth rate with respect to various growth parameters were explained by the rate equations based on the selective adsorption of methylgallium on surface As atoms. The decomposition rate of methylgallium on the surface had an activation energy of 42 kcal/mole from 440 to 560 °C.

A high-resolution, linear resistance-to-frequency converter

A resistance-to-frequency converter consisting of a Wheatstone bridge followed by an integrator and a comparator is described. In concept the circuit represents a relaxation oscillator whose frequency changes linearly with a resistance change detected by the bridge. Analyses show that a resolution better than 0.05% is possible with the simple configuration, and an excellent linearity is maintained over the wide resistance change by using a simple compensation method. The converter is therefore suited as a signal conditioner of a resistive sensor. Experimental results are included to demonstrate its performance.

An interface circuit for high-accuracy signal processing of differential-capacitance transducers

For high-accuracy signal processing of differential-capacitance transducers, an interface circuit is developed. The architecture is based on the idea that the ratio of one of the transducer capacitances to its total capacitance represents the offset binary equivalent of the physical quantity under measurement. An op amp-based capacitance-to-voltage converter is commonly used for capacitance detection and an analog-to-digital converter is used for the ratiometric operation. A circuit analysis shows that the interface can detect the capacitance change as small as 0.01% of the total capacitance. Experimental results are also given to confirm the analysis.

In situ x‐ray photoelectron spectroscopic study of GaAs grown by atomic layer epitaxy

We carried out an in situ investigation of GaAs grown by atomic layer epitaxy, using an x‐ray photoelectron spectroscopy system combined with a metalorganic vapor phase epitaxial growth chamber, where Ga(CH3)3 and AsH3 were the source gases. It has been proved that Ga(CH3)n molecules (where n=1 or 2) are decomposed into Ga atoms after being adsorbed on the GaAs surface around 500 °C. This means that the self‐limited adsorption of Ga in the atomic layer epitaxy of GaAs can be achieved on the surface where the Ga adsorbate is atomic Ga.

A relaxation-oscillator-based interface for high-accuracy ratiometric signal processing of differential-capacitance transducers

For high-accuracy signal processing of differential-capacitance transducers, an interface circuitry is developed based on a relaxation oscillator. Two capacitors of the transducer are multiplexed by diode switches to form the op-amp-based integrator. The duty ratio of the oscillator output then measures the ratio of two capacitances. A circuit analysis shows that the interface can detect the capacitance change as small as 0.01% of the total capacitance. Experimental results are also given to confirm the analysis.

Carbon incorporation in GaAs layer grown by atomic layer epitaxy

Abstract The mechanism by which carbon is incorporated into GaAs layers by atomic layer epitaxy using trimethylgallium or triethylgallium was investigated. The carbon density varied from 1×10 13 to 8×10 18 cm −3 with the trimethylgallium (or triethylgallium) and arsine pulse durations and mole fractions. It was also observed that the carbon incorporation drastically changed at the pulse duration and mole fraction where the growth rate per gas cycle started to saturate to one monolayer (0.283 nm/cycle for a (100) substrate). The results were explained by the selective adsorption of carbon on surface gallium, the reaction of methygallium with arsine, and the exchange interaction between arsenic and carbon atoms. Even when the trimethylgallium source was used, the epitaxial layers grown under the optimized growth conditions exhibited an electron concentration of 1×10 14 cm −3 and a mobility of 80 000 cm 2 /V·s at 77 K, a photoluminescence spectrum with several sharp excitonic lines at the band gap energy, and an extremely low level of carbon related peaks.

A high-accuracy, high-speed signal processing circuit of differential-capacitance transducers

For high-accuracy signal processing of differential-capacitance transducers, an interface circuitry is developed based on a relaxation oscillator. The interface consists of an integrator, a differentiator, and a comparator, and it uses two capacitors of the transducer-one for the integration and the other for the differentiation. This configuration allows the ratiometric operation in the amplitude domain and provides a square wave whose amplitude is proportional to the ratio of the capacitance difference between the two transducer capacitors to their sum. A circuit analysis shows that the interface can detect the capacitance change as small as 0.1% of the total capacitance in 10 /spl mu/s. Experimental results are also given to confirm the analysis.

A linear resistance-to-frequency converter

A raktance-to-frequency converter consisting of a Wheatstone bridge jfbbved by an integrator and a Comparator is described. It is basically a relaxation oscillator whose frequency changes linearly with a resistance change detected by the bridge. Analyses show that a resolution highear than 0.05% is possible with the simple coilfiguration and an excellent linearity is kept over the wide resistance change by a simple compensatioii method. fit. converter is therefore best suited for un interface oj a resistive sensoK Experimental results are also given to demonstrate its performance

In situ X-ray photoemission spectroscopy for atomic layer epitaxy of InP and GaAs

Abstract In situ observations of InP and GaAs grown by atomic layer epitaxy have been made, using and X-ray photoemission spectroscopy system combined with a metalorganic vapor phase epitaxial growth chamber, where In(CH 3 ) 3 , PH 3 , Ga(CH 3 ) 3 and AsH 3 were the source gases. It has been proved that Me(CH 3 ) n precursors (where Me=In or Ga and n =1, 2 or 3) decompose into Me atoms after being adsorbed on the InP and GaAs surfaces. This means that self-limiting adsorption of In and Ga can be achieved on a surface where the Me adsorbate is atomic In and Ga.