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Acoustoelectric instability in acoustically amplifying CdS
Başlık:
Acoustoelectric instability in acoustically amplifying CdS
Yazar:
Twomey, John, author.
ISBN:
9780438052598
Yazar Ek Girişi:
Fiziksel Tanımlama:
1 electronic resource (164 pages)
Genel Not:
Source: Dissertation Abstracts International, Volume: 76-08C.
Advisors: J. Lamb.
Özet:
The acoustoelectric instability occurring in thin single crystals of acoustically amplifying piezoelectric cadmium sulphide has been investigated both experimentally and theoretically. The r.f. oscillation was studied analytically by examining the occurrence of negative real values of the steady state electrical input impedance using a linear full mode analysis. Electrical loading was found to have no significant effect on the frequency of the oscillation or on the acoustic behaviour. When freely vibrating, the frequency of the oscillation was determined by the condition that there were an integral number of acoustic wavelengths in a round-trip, Negative resistance was obtained even when the acoustic gain in the forward direction was slightly less than the attenuation in the reverse direction. The impedance v/as strongly resonant and negative resistance occurred at both the even and odd acoustic normal modes of the platelet. Coupling to the even modes was caused by the asymmetric dispersion. This assymmetry was due to the presence of the drift field. The highest attainable frequency v/as a function of carrier diffusion and increased as the drift mobility decreased. For longitudinal waves in ZnO, oscillations v/ere predicted at frequencies as high as X-band. The effect of acoustic loading on the frequency and the relative coupling at the odd and even modes was also investigated. The frequency changes due to mechanical loading were found to be directly proportional to the change in the phase angle of the stress reflection coefficient, and to the frequency of the fundamental resonant mode of the platelet. The diffusion and penetration of vapour diffused transparent indium contacts, into the CdS platelet oscillator, were studied by means of an electron probe micro-analyser. One of the dominant diffusion mechanisms was the displacement of cadmium by indium. The measured and estimated penetration depths were less than 1 micron for a maximum diffusion time of one hour, at diffusion temperature of 520°C. The crystallographic orientation dependence of the d.c. voltage-current characteristics, the amplitude, of the r.f. output and the field dependence of the frequency were studied in CdS oscillators. The piezoelectric coupling (K2 ) for one orientation was eight times the coupling for the other. In the weak coupling oscillator, the field dependence of the frequency was much less than expected from linear theory. This behaviour can be utilised in the construction of highly stable oscillators. At higher drift fields, a negative dispersion was observed in which the frequency decreased with increasing field. This was contrary to linear predictions and was attributed to the variation in the temperature of the crystal and also to non-linear processes associated with the interaction between the electrons and the acoustic waves. The conductivity dependence of the lowest drift field at which an oscillation was observed and the corresponding threshold frequency for pure mode high coupling freely vibrating CdS shear oscillators were in quantitative agreement with linear theory. At higher drift fields, the output spectrum exhibited strong harmonic generation and was dominated by non-linear processes. The most significant measurements were carried out on CdS shear oscillators for the purpose of investigating the effects of mechanical loading on their electrical response. The loading was produced by evaporating either copper or indium onto the oscillator. Copper films produced essentially a reactive loading, while there was considerable scattering associated with reflection from indium films. The frequency shift of the oscillator produced by copper loading was measured and compared with theory. The results suggest that the frequency spectrum was primarily controlled by the lowest observed frequency, and that the spectrum did not contain any directly coupled sub-harmonic components. The lowest frequency was a normal mode of the platelet, and its electrical output was generally not the strongest in the spectrum. The influence of loading, upon the electrical amplitudes in the r. f. spectrum, the threshold and cut-off voltages, and the shape of the d.c. current-voltage characteristic,was examined. It was found that indium layers, a few microns thick, could prevent oscillation with corresponding reversion to ohmic V-I characteristics. For thinner layers, the oscillator output spectrum was confined to smaller numbers of components with increased amplitudes. The acoustoelectric instability occurring in thin single crystals of acoustically amplifying piezoelectric cadmium sulphide has been investigated both experimentally and theoretically. The r.f oscillation was studied analytically by examining the occurrence of negative real values of the steady state electrical input Impedance using a linear full mode analysis. Electrical loading was found to have no significant effect on the frequency of the oscillation or on the acoustic behaviour. When freely vibrating, the frequency of the oscillation determined by the condition that there were on integral number of acoustic wavelengths in a round-trip. Negative resistance was obtained even when the acoustic gain in the forward direction was slightly less than the attenuation in the reverse direction. The impedance was strongly resonant and negative resistance occurred at both the even and odd acoustic normal modes of the platelet. Coupling to the even modes was caused by the assymmetroc dispersion. This asymmetry was due to the presence of the drift field. The highest attainable frequency v/as a function of carrier diffusion and increased as the drift mobility decreased. For longitudinal waves in ZnO, oscillations v/ere predicted at frequencies as high as X-band. The effect of acoustic loading on the frequency and the relative coupling at the odd and even modes was also investigated. The frequency changes due to mechanical loading were found to be directly proportional to the change in the phase angle of the stress reflection coefficient, and to the frequency of the fundamental resonant mode of the platelet. The diffusion and penetration of vapour diffused transparent indium contacts, into the CdS platelet oscillator, were studied by means of an electron probe micro-analyser. One of the dominant diffusion mechanisms was the displacement of cadmium by indium. The measured and estimated penetration depths were less than 1 micron for a maximum diffusion time of one hour, at diffusion temperature of 520°C. The crystallographic orientation dependence of the d.c. voltage-current characteristics, the amplitude of the r.f. output and the field dependence of the frequency were studied in CdS oscillators. The piezoelectric coupling (K2) for one orientation was eight times the coupling for the other. In the weak coupling oscillator, the field dependence of the frequency was much less than expected from linear theory. This behaviour can be utilised in the construction of highly stable oscillators. At higher drift fields, a negative dispersion was observed in which the frequency decreased with increasing field. This was contrary to linear predictions and was attributed to the variation in the temperature of the crystal and also to non-linear processes associated with the interaction between the electrons and the acoustic waves. The conductivity dependence of the lowest drift field at "which an oscillation was observed and the corresponding threshold frequency for pure mode high coupling freely vibrating CdS shear oscillators were in quantitative agreement width linear theory. At higher drift fields, the output spectrum exhibited strong harmonic generation and was dominated by non-linear processes. The most significant measurements wore carried out on CdS shear oscillators for the purpose of investigating the effects of mechanical loading on their electrical response. The loading was produced by evaporating cither copper or indium onto the oscillator. Copper films produced essentially a 'reactive loading, while there was considerable scattering associated with reflection from indium films. The frequency shift of the oscillator produced by copper loading was measured and compared with theory. The results suggest that the frequency spectrum was primarily controlled by the lowest observed frequency, and that the spectrum did not contain any directly coupled sub-harmonic components.
The lowest frequency was a normal mode of the platelet, and its electrical' output was generally not the strongest in the spectrum. The influence of loading, upon the electrical amplitudes in the r.f. spectrum, the threshold and cut-off voltages, and the shape of the d.c. current-voltage characteristic, was examined. It was found that indium layers, a few microns thick, could prevent oscillation with corresponding reversion to ohmic V-I characteristics. For thinner layers, the oscillator output spectrum was confined to smaller numbers of components with increased amplitudes.
Notlar:
School code: 0547
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