Yozo Tokumaru

Precise determination of lattice parameter and thermal expansion coefficient of silicon between 300 and 1500 K

The lattice parameter of high‐purity silicon is measured as a function of temperature between 300 and 1500 K, and the linear thermal expansion coefficient is accurately determined. Precise measurements are made by the high‐temperature attachment for Bond’s x‐ray method to a few parts per million. It is found that the temperature dependence of the linear thermal expansion coefficient α(t) is empirically given by α(t)=(3.725{1−exp[−5.88×10−3{(t−124)} +5.548×10−4t)×10−6 (K−1), where t is the absolute temperature ranging from 120 to 1500 K. It is shown that the lattice parameter in the above temperature range can be calculated using α(t) and the lattice parameter at 273.2 K (0.5430741 nm). Measured values of the lattice parameter and the thermal expansion coefficient for high‐purity float‐zoned (100 kΩ cm) and Czochralski‐grown (30 Ω cm) single crystals are uniformly distributed within ±1×10−5 nm and ±2×10−7 K−1 with respect to the values obtained from the above empirical formula.

Isothermal Capacitance Transient Spectroscopy for Determination of Deep Level Parameters

A new measurement method for deep levels in semiconductors is proposed, by which the measurement of the transient change of capacitance is performed under an isothermal condition (Isothermal Capacitance Transient Spectroscopy). The method allows us to construct a precise measurement and analysis system by a programmable calculator. Computer simulation and experiment by the method in the case of Au-doped Si are demonstrated. It is shown that the method is one of useful tools for spectroscopic analysis of deep levels in semiconductors.

Deep Levels Associated with Nitrogen in Silicon

Two deep levels at Ec-0.19 eV and Ec-0.28 eV are found in nitrogen-doped silicon, in which nitrogen is doped during crystal growth by the float-zone method. The Ec-0.19 eV level has an electron capture cross section of 8×10-17 cm2 and the Ec-0.28 eV level has that of 5×10-16 cm2. The concentrations of the former and latter levels are about 0.1 and 0.01% of the doped nitrogen concentration (~1015 cm-3), respectively.

Determination of the Density of State Distribution of a-Si:H by Isothermal Capacitance Transient Spectroscopy

An ICTS (Isothermal Capacitance Transient Spectroscopy) theory for a system of continuously-distributed gap states is given and experimental data by the ICTS applied to P-doped a-Si:H Schottky barrier diode are presented. It is shown that the ICTS is a useful tool for the study of gap states of a-Si:H whose material parameters are strongly temperature-dependent.

Properties of Silicon Doped with Nickel

Measurements of the temperature dependence of the resistivity and Hall coefficient of nickel-doped silicon indicate that nickel acts as an acceptor impurity in silicon, introducing two acceptor levels, namely, 0.35±0.03 eV from the conduction band and 0.23±0.03 eV from the valence band. Precipitation of nickel from supersaturated solution in silicon has also been studied. The results can be expressed nearly as an exponential decay with time of the unprecipitated fraction of nickel. Precipitate particles of nickel in heat treated samples are observed by infrared microscopic inspection, but typical cylindrical precipitates are not observed. Therefore, according to Hams theory the particles are most likely of finite dimension at the start of the precipitationmeasurements. This is probably attributable to the fairly long cooling time, about 40 seconds, after nickel diffusion. Thus, it is expected that nickel atoms have fairly precipitated in the course of the cooling. The nuclei for precipitation of nickel may be considered to be vacancies generated at dislocations.

Dependence of lattice parameter on elastic strain and composition in undoped Czochralski grown GaAs

By precise measurement of the spatial variation in the lattice parameters of {001} undoped Czochralski grown gallium arsenide wafers, it was found that the lattice parameters are strongly dependent upon elastic strain and not so much on the melt composition. Although a remarkable spatial variation in the lattice parameters of ±5×10−6 nm from the average value was observed in a wafer specimen, the variation was considerably reduced to ±2×10−6 nm by dividing the wafer into small specimens. The lattice parameter variation was smaller than 2×10−6 nm due to a change in the As composition ratio from 0.42 to 0.52 in the melt.

Reversible photoinduced modification of electron‐capture cross section at localized states in a‐Si:H

A reversible photoinduced modification of the electron‐capture cross section at localized states in P‐doped a‐Si:H has been directly observed, being associated with the Staebler–Wronski effect. Isothermal capacitance transient spectroscopy measurement shows that the electron‐capture cross section at the gap states located at around 0.5 eV below the conduction‐band mobility edge (Ec) increases after the band‐gap illumination and is nearly restored to its original value by thermal annealing below 200 °C. It has been suggested that a metastable state related with the dangling bond is created through the multiphonon emission when excess carriers recombine.

A Modulated DLTS Method for Large Signal Analysis (C2-DLTS)

For the analysis of deep-level parameters in semiconductors, a new DLTS (Deep-Level Transient Spectroscopy) method has been proposed, in which a squaring device for the capacitance-signal processing is used with the conventional DLTS system. This method (C2-DLTS) is simple and is especially effective in the precise analysis of large signals due to deep-level impurities and defects.

Current Oscillations and High-Field Domains in Photoexcited High-Resistivity GaAs

Current oscillations (10 kHz~102 kHz) due to the periodic generation of a moving high-field domain have been observed in high-resistivity GaAs (~3×103 ohm-cm in the dark) under illumination at room temperature. It has been found that threshold field for the oscillation decreases with increasing illumination level and lies in the range from 650 V/cm to 2100 V/cm. Properties of the high-field domain have been investigated by a light probe and a contact probe technique. The domain velocity is 103 cm/sec~104 cm/sec. Width and field of the domain are of the order of 10-2 cm and 104 V/cm, respectively. By shifting a light spot position along the specimen to change the distance that the domain transits, the oscillation frequency can be varied continuously. The origin of the oscillations can be explained with field-enhanced electron trapping at deep levels.

Low-Frequency Photocurrent Oscillations and Trapping Levels in High-Resistivity GaAs Doped with Oxygen

Low-frequency (0.1–1000 Hz) photocurrent oscillations together with photo and dark conductivity in oxygen-doped n-GaAs having room temperature resistivity of 3400 ohm-cm are investigated at or below room temperature. It is shown that the behavior of photoconductivity at low temperatures can be explained by considering three deep levels (about 0.49, 0.73, and 1.3 eV below the conduction band) and that the current oscillations are associated with the 0.73 eV level. The origin of the current oscillations attributable to the field-dependent trapping effect is discussed briefly.