Phonon and Photon Assisted Tunneling Out of Shallow Impurities in Silicon

The effect of thermal phonons and infrared photons on the depletion of trapped electrons in shallow impurities in silicon has been investigated in the presence of an applied electric field. The impurity levels which have been studied are in the i-region of a p-i-n diode. The experiment consists of a three phase cycle. During the first phase charge is injected into traps by forward biasing the p-i-n diode. During the second phase the sample is exposed to effects which depopulate levels. The remaining charge is ejected via field ionization in the third phase. The ejected charge is detected in a specially developed electronic system. Thermal ionization of phosphorus impurities has been found to be dominated by phonon-assisted tunnelling at temperatures of less than 15 Kelvin. At higher temperatures classical ionization (Frenkel - Poole effect) is more important. In order to compare theory with experiment, a computer simulation has been carried out, which takes into account the non -uniform field distribution due to the space charge of ionized donor centers. Possible implications for low temperature operation of semiconductor devices are being discussed. Photoionization of shallow impurities in the presence of an applied electric field is aided by photon-assisted tunnelling. Excitation into resonance excited states yields cross-section maxima. 2 such states have been found and identified. A new type of integrating far infrared detector has been considered. This detector integrates infrared photons by depletion of stored charge and exhibits very low dark currents, which make it ideal for long integration times. This detector is based on extrinsic silicon and the range of wavelength to be covered can be chosen by doping with proper impurities. Its subthreshold response can be modulated by an applied electric field.