Optoelectronic characterization of hyperdoped silicon thin films

Abstract

By controlling the surface illumination conditions the locations within devices where photocarriers are generated can be manipulated. This level of control allows for the investigation of carrier transport behavior within the films. Two primary techniques are employed to control illumination conditions. The first is Light Beam Induced Current surface mapping. This technique provided experimental confirmation of sub-band photocurrent response within gold hyperdoped silicon. This technique also allowed for the electronic characterization of the internal gain demonstrated by single crystal sulfur hyperdoped material. The electronic properties of this gain are most effectively described with a trapped carrier barrier height lowering mechanism.

Pulsed laser melting has been shown to be capable of producing crystalline Silicon (Si) films which contain dopant concentrations as high as one atom % (of order 1x1020 atoms/cm2). These materials have demonstrated very high absorption coefficients both above and below the Si bandgap. In some cases these devices have also demonstrated strong internal gain. This thesis explores a series of characterization techniques to provide insight into the optoelectronic properties of devices which employ these highly doped thin films. These techniques provide a methodology which can be expanded into future analysis of novel semiconductor films which may contain large concentration gradients or demonstrate internal gain. Through a combination of optoelectronic techniques insight into device transport behavior can guide the design of future devices incorporating hyperdoped films.

The second technique investigates the spectral dependence of photoresponse. Hyperdoped materials contain large concentration gradients which can also have absorption coefficients which vary strongly with wavelength. Varying the wavelength of illumination will significantly impact the distribution of generated carriers within the sample, which will significantly impact carrier extraction probabilities. Modeling of this technique is able to describe the relative spectral response of sulfur hyperdoped diodes with a range of layer thicknesses. By assuming low extraction rates from carriers generated within hyperdoped regions, spectral response can be fit to a simple absorption model. The spectral dependence of photoresponse also provides a lower limit of 10-8 cm2/V on the effective mobility-lifetime product for the extraction of holes generated within the hyperdoped layer of a film deposited on silicon on insulator substrate. The cross-section for recombination at these concentrations appears to be much lower than in the case of isolated sulfur impurities.