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 if to investigate “grown in” defects present within GaAs and InGaP-based layers and devices grown on SiGe/Si 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. Results showed that the presence of dislocations do not introduced additional deep levels.
In addition, and from the point of view of the technology, the effect of radiation damage in III-V/SiGe PV structures is analyzed, specifically 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. 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, specifically 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 observed and correlated at a device level by studying the degradation in GaAs/SiGe solar cells.
For InGaP/SiGe single junction devices a combination of DLTS/ DLOS allowed the inspection for the totality of the bandgap otherwise not feasible. Results for as-grown and under radiation conditions confirmed that within these wider bandgap structures the dislocation network does not introduce additional deep levels. In addition, first as-grown and radiation studies were performed within the sub-cells 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.
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.