P. Das

Nondestructive evaluation of the semiconductor interface states' density using the transverse acoustoelectric voltage

Abstract Nondestructive determination of the interface states density (Dit) is presented. The measurement technique, utilizing the surface acoustic waves, is applied to silicon/thermal oxide structure. The detected signal is the transverse acoustoelectric voltage amplitude (TAV) which is monitored as a function of the applied bias voltage (TAV-V). Dit is measured by comparing the theoretical and experimental TAV-V curves. The calculation procedure of the Dit and the experimental example are presented.

Determination of semiconductor surface properties using surface acoustic waves

Surface properties of semiconductors may be determined by measuring the change in attenuation constant, acoustoelectric voltage, and convolution output of a semiconductor on a lithium niobate acoustic surface wave convolver structure. The change is caused by the application of a dc pulse voltage. Experimental results are presented for both p‐ and n‐type silicon. The method is contactless, sensitive, and requires no sample preparation except polishing of one side. Also it is possible to study the variations of the semiconductor surface states along the direction of propagation of the surface wave by proper modification of this method.

Acoustoelectric effects in superlattices using a separate‐medium structure

The interaction of the surface acoustic wave (SAW) with the GaAs/AlAs superlattice is investigated using a separate‐medium convolver structure. The magnitude and polarity of the acoustoelectric voltages exhibit strong temperature and SAW frequency dependences, a phenomenon that is not observed in homogeneous semiconductors. Possible SAW‐superlattice interaction models that tentatively explain the observed data are discussed.

Real‐time convolution using acousto‐optic diffraction from surface waves

Convolution of two signals has been obtained in real time using the efficient diffraction of laser light from acoustic surface waves. Two pulse‐modulated 45‐MHz surface waves are launched from the two ends of a surface wave delay line. Laser light enters the delay line from one side, interacts with two oppositely directed parallel acoustic beams along their widths, and exits from the other side. The diffracted light, after an optical Fourier transformation, is incident on a photodiode which produces an electrical output containing the desired convolution signal. With the present 5‐μsec interaction length and 9‐MHz transducer bandwidth, the time‐bandwidth product is 45.

Transient type-inversion of semi-insulating GaAs under strong illumination at 140 K detected by the nondestructive surface-acoustic-wave technique

A nondestructive surface-acoustic-wave (SAW) technique is used to study the behavior of deep trap levels in semi-insulating, Cr-doped GaAs. The importance of these levels in producing high-quality, semi-insulating GaAs wafers is well recognized. The temperature dependence of the free energy of these levels as well as their trapping behavior at different temperatures has been the subject of increasing interest in the past few years. The SAW technique is extremely sensitive in detecting conductivity variations at the surface of the high-resistivity materials and also yields information about surface-conductivity type. At 140 K, high injection level is achieved by illuminating the wafer with a 100 W tungsten lamp. Under the illumination, the surface conductivity is n-type. When the illumination is turned off, the surface becomes p-type for a duration of 1 s. This duration is a function of light intensity and temperature. The type inversion can be attributed to the large-scale capture of electrons by the deep traps, resulting in momentary increase of the effective hole concentration.

Quenching and enhancement of the exciton and subband‐gap absorption in GaAs:Cr using two‐beam transverse acoustoelectric voltage spectroscopy

Transverse acoustoelectric voltage (TAV) spectroscopy of GaAs:Cr samples is performed. Samples are illuminated by two monochromatic beams. The characteristic bound exciton peak is observed at temperatures below 200u2009°K. At lower temperatures a maximum around 1.3 eV (energy of the incident beam) can also be observed. The excitonic peak can be enhanced or quenched upon shining a 1.38‐ or 1‐eV bias light respectively. The 1.3‐eV peak of the TAV spectrum can be completely quenched by illuminating the sample with the 1‐eV bias light, whereas the 1.38‐eV bias light does not have an appreciable effect on this peak. The theoretical model suggests the presence of a band of donor level at about 1.3 eV below the conduction band and a band of acceptor level at 1 eV above the valence band.

Microwave hot electron effects in semiconductor quantized inversion layers

Abstract Hot electron microwave conductivity of quantized inversion layers in semiconductor surfaces has been calculated based on a displaced Maxwellian approximation for the electron distribution function. The calculations performed take into account repopulation of carriers among the various subbands. The effects of the energy, momentum and intervalley population exchange rates due to scattering by acoustical and intervalley phonons are included in the derivations. The results are applied to silicon where the electron energy and momentum losses by intervalley phonons in both the zero-order coupled and first-order coupled cases are important. Numerical computations for the microwave conductivity of silicon are presented as a function of bias electric field and frequency. It is found that significant changes in the conductivity contribution for a fixed bias field occur at frequencies on the order of the intervalley repopulation rate. Though no experimental work in this frequency range has been performed, it should be easily observable.

Electrical properties of semiconductor‐electrolyte (CdS‐NiCl2) using surface acoustic wave techniques

Spectroscopy of the cadmium sulphide–nickel chloride interface has been performed by measuring the acoustoelectric voltage induced by a SAW‐delay line. Observation of transitions at different radiation wavelengths is attributed to the presence of electronic levels at the interface.

Determination of energy band and surface‐state locations in GaAs using the separated‐medium surface‐acoustoelectric effect

The wavelength dependence of the peak transverse acoustoelectric voltage and the surface acoustic wave attenuation determine the energy band and the locations of the surface states in the energy gap of GaAs. The dependence is explained by direct optical transition between the bands and optical transition from surface states. This method needs no contacts to the semiconductor surface, is simple, very sensitive, and accurate.

Anomalous acoustoelectric effect in semiconductor layered structures using separated medium configuration

Abstract An anomalous acoustoelectric effect is observed in semiconductor layered structures and bulk semiconductors due to semiconductor surface conditions. We report preliminary results of this effect in semiconductors using the nondestructive surface acoustic wave (SAW) technique. The magnitude and polarity of the acoustoelectric voltages in GaAs/AlAs superlattices exhibit strong SAW frequency dependencies, a phenomenon that is not observed in bulk semiconductors. The anomalous acoustoelectric voltage (AAV) is detected in high electron mobility transistor (HEMT) and also bulk semiconductors as a function of bias voltage.