New Structures for AlGaAs Lasers and Avalanche Photodetectors

Citation

Blauvelt, Henry A. (1983) New Structures for AlGaAs Lasers and Avalanche Photodetectors. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/DJMZ-S785. https://resolver.caltech.edu/CaltechETD:etd-09072006-080636

Abstract

This thesis describes the fabrication and the properties of five new semiconductor laser diode structures. All of these devices were grown from the GaAs-AlGaAs ternary system using the liquid phase epitaxial technique. In addition, a new low noise avalanche photodetector is proposed.

The first example is a new technique for fabricating cleaved mirrors without cleaving through the substrate. This technique, called micro-cleavage, has potential applications for both opto-electronic integrated circuits and for the fabrication of short cavity length lasers. In this technique, cantilevers are formed by a sequence of etching steps. These cantilevers are subsequently cleaved using ultrasonic vibrations.

Three devices related to high power single mode lasers are described. The first of these is the large optical cavity buried heterostructure window laser. The output power of semiconductor lasers, particularly during pulsed operation is limited by catastrophic mirror damage which occurs at power densities above a pulse width dependent damage threshold. The damage occurs due to local heating up to the melting point of the active region in the vicinity of the cleaved mirror facets. However, catastrophic mirror damage can be avoided by isolating the active layer from the cleaved mirrors, as is done in these window lasers. The second device related to high power that is described is the Inverted Strip Buried Heterostructure laser. These lasers combine many of the best features of both the buried optical guide lasers and the strip buried heterostructure that have been previously developed elsewhere. The inverted strip buried heterostructure lasers have significantly better beam quality than buried optical guide lasers and can be operated in the fundamental spatial mode for larger emitting areas (and therefore greater output power). The third device related to high power lasers is a variation of a buried heterostructure laser in which the injected current is confined to a narrow section in the center of the active layer. The optical gain is therefore also confined to a narrow section in the center of the active layer. By doing so the fundamental mode is much better matched to the optical gain than the higher order spatial modes. The result is that fundamental mode operation is possible for buried heterostructure lasers with active layer widths up to 8 µm. When the current is injected uniformly into the active layer, fundamental mode operation is possible only for active layer widths less than 2 µm. In addition to the descriptions of these devices a theoretical chapter on high power single mode lasers is included.

The final laser structure that is described is a single liquid phase epitaxial growth laser structure in which the current is restricted to flow between two narrow stripes located above and below the active layer. This structure, which is fabricated using a meltback-growth technique allows the current injection to be restricted to a very narrow section of the active layer, which results in several interesting properties which are described and explained using a simple model.

The final subject of this thesis is a multilayer avalanche photodetector (APD) which has been proposed for low noise applications. The noise generated by an APD is dependent on the statistics of the carrier multiplication process, since positive feedback effects, which exist when both electrons and holes produce secondary pairs, can greatly amplify any current fluctuations. Significantly more noise is generated if the electron and hole ionization rates (α, β) are equal than if only one carrier produces secondary pairs. The multilayer structure described and analyzed in this chapter is expected to have impact ionization which is dominated by electrons and therefore would be of importance for low noise applications.

Thesis Files