Electronic defects of III-V compound semiconductor materials grown on metamorphic silicon-germanium substrates for photovoltaic applications

The use of step-graded SiGe buffers to accommodate the lattice constant difference between Si and III-V materials is an extremely promising approach to achieve monolithically integrated III-V optoelectronic device technology on Si wafers. The potential of this technology has already been demonstrated by the integration of numerous high performance GaAs-InGaP based devices on Si substrates. As a result, there is now a great interest in knowing the basic properties of residual defects within the III-V/SiGe materials and devices since their understanding is fundamental to maintain the progress achieved to date. The focus of this research is thus the issues of generation, identification and electronic (deep level) properties of residual defects present III-V/SiGe materials and heterostructures of interest for photovoltaic applications.In this dissertation "grown in" defects present within GaAs and InGaP-based layers and devices grown on SiGe/Si substrates are analyzed, generating a fundamental understanding on defect introduction in lattice mismatch III-V/SiGe heteroepitaxy. The role of the SiGe substrate is analyzed by comparing deep level properties with identical structures grown on GaAs substrates. For all cases, the substitution of the substrate does not introduce additional deep levels in the bandgap of the material. An increase in concentration for the as-grown deep levels is observed for samples grown on SiGe substrates indicating the influence of the dislocation network in the incorporation of native defects.In addition, and from the point of view of the technology, the effect of radiation damage in III-V/SiGe PV structures is analyzed, in particular detection and identification of "ambient-generated" defects that result from the application of these materials and photovoltaic devices operating in the space environment. In this dissertation the first radiation study for III-V/SiGe structures was performed. Results indicated that for single junction GaAs structures, the primary impact of the SiGe/Si substrate was to improve the radiation-tolerance of these devices, in particular for n+p GaAs diodes. While DLTS results showed generally lower radiation-induced trap concentrations for both n-type and p-type GaAs grown on SiGe compared to growth on conventional substrates, the reduction was far more dramatic for p-type GaAs showing a reduction in trap introduction rate by as much as &sim 30% for hole traps in p-type GaAs on SiGe/Si. The improved radiation-tolerance for GaAs grown on SiGe/Si is attributed to interactions between radiation-induced point defects and residual dislocations present in the metamorphic samples and in particular to the more mobile nature of radiation-induced point defect complexes that are introduced in p-type GaAs as opposed to n-type GaAs, which allows them to diffuse to the dislocation network where they may reconfigure. This result was correlated with GaAs/SiGe solar cells where at the end of the radiation cycle the values of fundamental parameters like maximum output power or efficiency were higher for the samples grown on SiGe/Si substrates with an overall lower degradation for the n+p cells.For InGaP/SiGe single junction devices a combination of DLTS and DLOS allowed the inspection for the totality of the bandgap otherwise not feasible. Results for as-grown and under proton radiation conditions confirmed that even within these wider bandgap structures the low dislocation network does not introduce additional deep levels. In addition, first as-grown and radiation studies were performed within the subcells components of InGaP/GaAs/SiGe dual junction structures, revealing that the actual structure, dual versus single, does not influence deep level incorporation and indicating that the origin of the levels is intrinsic from the InGaP layer and not a consequence of the structure or the substrate. Introduction rate studies for p+n polarity showed no significant differences between InGaP/SiGe and InGaP/GaAs devices possibly due to the individual nature of the introduced defects.The results obtained during the course of this research demonstrate that substitution of conventional GaAs or Ge substrates by SiGe for space PV applications truly conveys advantages regarding not only issues of weight and cost but also performance which makes them ideal for long lasting space missions.