A. Many

Quantum properties of strong accumulation layers on ZnO surfaces

Abstract Extremely strong accumulation layers with surface electron densities ΔN approaching 1014 cm−1 have been achieved on ZnO surfaces in contact with an electrolyte. Quantization effects, which are very pronounced in such narrow (≳10 A) layers, are studied by measurements of ΔN versus surface barrier height Vs. Comparison of the results with self-consistent calculations shows very good agreement up to ΔN = 2 × 1013 cm−2. Deviations observed at higher ΔN are probably associated with the huge electric fields (∼107 V/cm) experienced by the surface electrons.

ELS studies of quantized accumulation layers on ZnO surfaces

Abstract Electron energy loss spectroscopy studies of clean ZnO surfaces under extreme accumulation layer conditions are presented. The experimental results are analyzed in terms of excitations due to coupled surface phonons and two-dimensional surface plasmons. The fair agreement between the experiment and theory strongly suggests that part of the losses are due to two-dimensional surface plasmons.

Resonant enhancement and cancellation of Brillouin scattering from CdS

Abstract The dispersion spectrum of Brillouin scattering from acoustoelectrically amplified flux domains in CdS is studied in the photon-energy range 1.75–2.38 eV. The dispersion curve shows resonant enhancement and cancellation and is in good agreement with Loudons theory.

FREQUENCY SHIFTS DURING GROWTH OF AMPLIFIED ACOUSTIC FLUX IN CdS

A simple experimental procedure based on measurements of the acoustoelectric after‐current is described for estimating the frequency of maximum intensity of the amplified acoustic flux at various stages of its buildup. In CdS, the rapid buildup in flux leading to current saturation is followed by a slower growth which continues until the flux domain leaves the filament. The frequency of maximum intensity coincides initially with the frequency of maximum gain and decreases continuously as the domain propagates down the filament, reaching in a typical case a value ten times smaller.

On the mechanism of acoustic gain under large signal conditions

Abstract The two possible models suggested by the extension of the Hutson-White theory to large signal conditions are analyzed for conducting materials, and the conclusions of each model are expressed in a form amenable to direct experiment test.

Extreme accumulation layers on ZnO surfaces due to He+ ions☆

Abstract Accumulation layers with excess surface electron densities of up to 1014 cm−2 have been produced on the (001) face of ZnO single crystals by an electrical discharge in helium atmosphere. Hall effect measurements show that at these extreme surface electron densities both the electron density and the mobility are temperature independent from room temperature down to 1.6 K.

On the modes of acoustoelectric gain in n-InSb

Abstract The mode I and II of the acoustoelectric effect in n-InSb are attributed to the two regimes in which the effective electron mean-free-path is, respectively, large and small compared to the acoustic wavelength. Evidence is presented in support for this contention.

Two-dimensional plasma oscillation on ZnO surfaces

Abstract Angle resolved, low energy electron spectroscopy is used to study collective excitations on accumulaton layers on ZnO. As the electron density in the accumulation layer is increased a prominent loss peak is observed which, at specular reflection angles, shifts from ~ 67 meV in the absence of surface electrons to ~320 meV in strong accumulation layers. This behavior is well accounted for theoretically in terms of scattering by two-dimensional plasmon-like collective surface excitations. A shift in energy of the loss peak position as a function of δθ, the deviation angle from specular reflection, was also measured.

Very strong accumulation layers on ZnO surfaces

Abstract Extremely strong accumulation layers with surface electron densities Δ N approaching 10 14 cm −2 have been achieved on ZnO surfaces in contact with an electrolyte. Experimental results of Δ N versus surface barrier height V S are compared with self-consistent calculations.

The Acoustoelectric Effect

The acoustoelectric effect in piezoelectric semiconductors encompasses the various phenomena arising from the interaction between the rather strong electric field associated in these materials with acoustic waves and the electrons or holes present in the crystal. In 1960 Nine1 observed a large conductivity-sensitive ultrasonic attenuation in photoconducting CdS crystals. Soon after, Hutson et al.2 measured the ultrasonic attenuation in CdS under an externaliy applied electric field. They found that when the electron drift velocity exceeds the sound wave velocity, amplification rather than attenuation of the input acoustic signal takes place. Simultaneously with these experiments Hutson and White3 developed a classical small-signal theory of the acoustoelectric effect, which accounted quite well for the dependence of the observed gain (or attenuation) constant on applied field, sample conductivity and acoustic-wave frequency.