Understanding defects in germanium and silicon for optoelectronic energy conversion

Abstract

This thesis explores bulk and interface defects in germanium (Ge) and silicon (Si) with a focus on understanding the impact defect related bandgap states will have on optoelectronic applications. Optoelectronic devices are minority carrier devices and are particularly sensitive to defect states which can drastically reduce carrier lifetimes in small concentrations. We performed a study of defect states in Sb-doped germanium by generation of defects via irradiation followed by subsequent characterization of electronic properties via deep-level transient spectroscopy (DLTS). Cobalt-60 gamma rays were used to generate isolated vacancies and interstitials which diffuse and react with impurities in the material to form four defect states (E₃₇, E₃₀, E₂₂, and E₂₁) in the upper half of the bandgap. Irradiations at 77 K and 300 K as well as isothermal anneals were performed to characterize the relationships between the four observable defects. E₃₇ is assigned to the Sb donor-vacancy associate (E-center) and is the only vacancy containing defect giving an estimate of 2 x 10¹¹ cm-³ Mrad-¹ for the uncorrelated vacancy-interstitial pair introduction rate. E₃₇ decays by dissociation and vacancy diffusion to a sink present in a concentration of 10¹² cm-³. The remaining three defect states are interstitial associates and transform among one another. Conversion ratios between E₂₂, E₂₁, and E₃₀ indicate that E₂₂ likely contains two interstitials. The formation behavior of E₂₂ after irradiation in liquid nitrogen indicates that E₃₀ is required for formation of E₂₂. Eight defect states previously unseen after gamma irradiation were observed and characterized after irradiation by alpha and neutron sources. Their absence after gamma irradiation indicates that defect formation requires collision cascades. We demonstrate electrically pumped lasing from Ge epitaxially grown on Si. Lasing is observed over a ~200nm bandwidth showing that this system holds promise for low-cost on-chip communications applications via silicon microphotonics. The observed large threshold currents are determined to be largely a result of recombination due to threading dislocations. We estimate that recombination by threading dislocations becomes negligible when threading dislocation density is </~ 4 10⁶ cm-². We developed a process for incorporation of colloidal quantum dots (QD) into a chalcogenide glass (ChG) matrix via solution based processing in common solvents. Observation of photoluminescence (PL) comparable to QD/polymethyl methacrylate (PMMA) films shows potential for this material to form the basis for low cost light sources which can be integrated with ChG microphotonic systems. We investigated the impact of surface recombination on the benefit of combining a singlet fission material (tetracene) with a Si solar cell. Our simulations show that for efficiency gains, surface recombination velocity (SRV) for the tetracene/silicon interface must be less than 10⁴ cm s-¹. Characterization via radio frequency photoconductivity decay (RFPCD) measurements show that tetracene does not provide a sufficient level of passivation thus requiring another material which passivates the interface. Using thin films fabricated by atomic layer deposition (ALD), we showed the first direct evidence of triplet energy transfer to Si via magnetic field effect (MFE) PL measurements.