Optical properties of Si-Ge superlattices and wide band gap II-VI superlattices

Citation

Rajakarunanayake, Yasantha Nirmal (1991) Optical properties of Si-Ge superlattices and wide band gap II-VI superlattices. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/b43v-6930. https://resolver.caltech.edu/CaltechETD:etd-07122007-074702

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.

This thesis presents the investigation of semiconductor heterostructures for optoelectronic applications, with particular emphasis on band alignment considerations, strain effects, band structure calculations and characterization by optical spectroscopy. The purpose of the work described here is two-fold. The first part of this thesis is concerned with the study of novel optoelectronic properties exhibited by Si/Ge superlattices both in the near infrared (interband transitions) and far infrared (intersubband transitions) energy ranges. The second part of this thesis is concerned with establishing the merits of II-VI semiconductor heterostructures for producing visible light emitters, and investigating techniques to improve the dopability of II-VI semiconductors.

In the first part of this thesis we investigate the merits of Si/Ge superlattices for optical applications. Although Si and Ge are indirect band gap materials, Si/Ge superlattices can exhibit a direct band gap for certain layer thickness combinations. In Chapter 2, we show that the optical absorption/emission strengths for interband transitions in Si/Ge superlattices can be enhanced by six orders of magnitude over pure Si or Ge. However, these numbers are still three to four orders of magnitude lower than the optical absorption/emission strengths of direct band gap materials such as GaAs. These results are based on a full zone [...] formalism that we developed specifically to study the band structure of Si/Ge superlattices. In Chapter 3, we investigate the intersubband absorption coefficients in doped Si/Ge superlattices. Intersubband transitions in these superlattices make them interesting candidates for long-wavelength infrared detectors. Such infrared detectors are analogous to extrinsic Si detectors, with the additional advantage of tunability of the peak absorption wavelength. The intersubband absorption strengths of Si/Ge superlattices reported in this thesis are comparable to those for AlxGa1-xAs/GaAs superlattices, with the additional benefits of the ability to detect normally incident light, and compatibility with the fabrication and processing technology of Si electronics.

In the second part of this thesis, we describe investigations of II-VI semiconductor heterostructures for visible light emitter applications. The wide band gap II-VI semiconductors are ideally suited for visible optoelectronics by virtue of their direct band gaps in the blue/green region of the spectrum. However, difficulties associated with doping these materials have severely limited their applications. Low-temperature, epitaxial-growth techniques such as molecular beam epitaxy have opened up new approaches for II-VI materials that show potential for overcoming some of these problems. In Chapter 4, we investigate minority carrier injection in II-VI semiconductors using heterojunctions. We also perform band structure calculations on II-VI strained layer superlattices to investigate the role of strain on the heterojunction band alignments. We experimentally determine the band offsets for CdTe/ZnTe and ZnSe/ZnTe heterojunctions using optical techniques, and remark on the merits of these heterojunctions for carrier injection. We theoretically extended our conclusions to II-VI quaternary alloys and show that there is great promise for visible light-emitter applications within quaternary heterostructures. In Chapter 5, we analyze the role of external electric fields applied during growth in suppressing self-compensation in II-VI semiconductors. This is a novel approach to achieve and control metastability in semiconductors. Our results indicate that II-VI doping efficiencies can be dramatically improved if substantial electric fields are applied during growth.

Thesis Files