Electrical and Computer Engineering ETDs

Publication Date

5-5-1977

Abstract

Experimental damage thresholds for Si, Ge, GaAs, GaAsxP1-x and GaP have been determined at 10.6 µm and 1.06 µm laser wavelengths. For CW CO2 laser damage, all the thresholds are of the order of a few kW/cm2 for an irradiation time of about 0.10 second. At 1.06 µm wavelength and for nanosecond pulses, the damage thresholds are of the order of 109 W/cm2. The damage thresholds can be explained by assuming that the anomalous absorption takes place through the parametric instability of the type formulated by DuBois and Goldman. The plasma concentration required for the onset of this instability at 10.6 µm is easily achieved by photon induced excitation of the valence electrons to the conduction band through a continuous distribution of Shockley surface states. At 1.06 µm, the required plasma concentration in case of Germanium and Silicon is produced by band to band transitions and by Zener tunneling in case of GaAs, GaAsxP1-x and GaP. In practice, the damage occurs both at the surface of the semiconductor and within the bulk, creating defects that cause carrier removal and mobility degradation. The defects so created are more active at lower temperatures than at room temperature as seen from the experimentally measured Hall mobility versus temperature curves before and after laser damage. Carrier removal and mobility degradation curves appear qualitatively to be similar to those observed for electron, neutron and γ-ray damage. These changes in electrical properties can be explained applying the model of James and Lark-Horovitz. A study of Faraday rotation in p-type Hg1-xCdxTe is conducted. The observed Verdet constant and absorption coefficient at 10.6 µm match rather well with the theoretically calculated results. It is shown that the absorption is almost wholly due to free carriers. In this material, the rotation due to free carriers is in a direction opposite to the direction of the interband Faraday rotation. The intrinsic crystals of this material will be capable of providing a figure of merit two to three orders higher than Germanium because of the lower band gaps of Hg1-xCdxTe. Hence, when pure crystals of Hg1-xCdxTe become available, they will be excellent candidates for use as Faraday rotator isolators in high power CO2 lasers employed in laser fusion programs.

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Wayne Willis Grannemann

Second Committee Member

William Jackson Byatt

Third Committee Member

Harold Dean Southward

Fourth Committee Member

Edward Demah Graham Jr.

Download

Share

COinS