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Title: A theoretical study of non-equilibrium photoexcited carriers in semiconductors
Author: Barker, John Reginald
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1969
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A theoretical study is made of the physics of photoexcited carriers in one of the bands of a semiconductor. The emphasis is on photoexcited hot carrier phenomena, for which the mean carrier energy deviates significantly from the thermal equilibrium value in the steady state. Very little previous theoretical work in this area has been reported. Two situations are analysed. The first is an investigation of hot photoexcited carriers in germanium and silicon at low temperatures. The carriers are excited into the band by a model black-body excitation spectrum having a mean energy in excess of the thermal energy. Full account is taken of the interaction of the carriers with impurities and phonons, and recombination is asswned to occur via a cascade mechanism. Significant carrier heating is found for trapping densities of the order 1016 cm-3 at lattice temperatures below about 300K. The steady-state carrier distribution functions are derived numerically from the Boltzmann equation in the absence of external fields. The low field transport and trapping parameters are then derived by a perturbation theory. The assumption of a linear response to applied fields is checked by an adaptation of tileHonte Carlo technique first employed by Kurosawa (1966) and Boardman et al (1968) in high field studies of semiconductors. The technique is extensively modified to suit our problem; in particular the concert of the self-scattering device is enlarged. The theory shows good agreement with the experimentally measured (Rollin and Rowell 1960) temperature variation of the Hall mobility of photoexcited holes in germanium. Agreement is also obtained with an experimental curve for the temperature dependence of the capture cross section for electrons in silicon. experimentally a 'cut-off' is found in the temperature dependence below Tile hot carrier model explains this phenomenon in terms of an anomalous temperature dependent Hall number which arises from the severe non- Maxwellian heating of the carriers. However, not all the experimental results can be explained this way and a tentative alternative mecnanism is suggested. The second situation analysed involves monochromatic photoexcitation leading to the oscillatory photoconductivity effect in many polar semiconductors. Considerable controversy has existed previously as to the origin of this effect. The distribution functions and photoconductivity are studied as a function of photon frequency and electric field strength on the basis of an analytical model and detailed Monte Carlo calculations. Good agreement is found with experiment as regards tile field dependence of the overall spectral response, confirming the assumptions of an earlier approximate analytical approach (Stocker and Kaplan 1966). For certain photon frequencies the photoexcited carriers can theoretically exhibit both total and differential negative mobility for certain ranges of applied electric field, confirming a previous approximate theory (Stocker 1967). although tile effect nas not been observed experimentally. This leads to a non-uniform field distribution in the semiconductor and the possibility of spacecharge instabilities. The possibility of steady-state negative photoconductivity is investigated with particular reference to the spatial distribution of electric field and the stability of the carrier system. The evolution and form of the instabilities and steady-states are evaluated numerically. Full account is taken of the electron and trap dynamics. It is shown that the total negative resistance state is unstable in the presence of injecting contacts. Instead either a non-uniform field distribution showing bulk positive resistance is established or there occur propagating instabilities leading to positive current oscillations.
Supervisor: Not available Sponsor: Ministry of Defence (Navy)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics