Kevin Doyle

Date of Graduation

2013

Document Type

Dissertation/Thesis

Abstract

Hg1-xCd xSe grown on nearly lattice-matched GaSb substrates could serve as a new basis for infrared detector development. The preparation of the GaSb substrate surfaces and the growth of ZnTe1-xSex buffer layers via molecular beam epitaxy were investigated. ZnTe and ZnTe1- xSex layers were grown on GaSb substrates prepared with atomic hydrogen cleaning. The lattice constant of ZnTe1-xSex was tuned by controlling the ratio of Se/Te beam equivalent pressures, and ZnTe1- xSex was found to be lattice-matched to GaSb for x=0.01. Confocal photoluminescence measurements indicated that ZnTe0.99Se0.01 layers grown on GaSb have dislocation densities ∼7x104 cm-2, indicating that ZnTe0.99Se0.01/GaSb provides a high quality substrate with low dislocation densities for Hg1-xCd xSe growth. In parallel with the ZnTe1-xSe x/GaSb substrate development, the growth of Hg1- xCdxSe was studied via molecular beam epitaxy on GaSb substrates and Si substrates with ZnTe buffer layers. Growth rate, composition, and surface quality were evaluated for different growth parameters. Two sources of Se were used, an effusion cell loaded with 5N purity source material that produced a predominantly Se6 flux, and a disassociation source loaded with 6N purity source material that could produce either a predominantly Se2 or a predominantly Se6 flux. For a given substrate temperature and Hg overpressure, the growth rate was controlled by the Se flux and the x-value was controlled by the Cd/Se flux ratio. Growths under Hg-deficient conditions produced “needle” and “diamond”-shaped defects. The optimal substrate temperature was found to be 90- 110°C for growths performed with a predominantly Se 6 flux. from the effusion cell and a standard Hg flux of 2.5x10 -4 Torr. Previous studies of nominally undoped Hg1-xCd xSe samples have reported large background electron concentrations ranging from 1017-1018 cm-3 at temperatures as low as 4K. In the study reported here, the use of Se source material with 6N purity instead of 5N reduced the electron concentration in Hg1-xCdxSe by an order of magnitude, suggesting contaminants in the Se source material are a significant source of the background electrons. Certain anneals can alter the electron concentration of Hg1-xCd xSe, suggesting the presence of native point defects as well as background impurities. Positron annihilation spectroscopy measurements strongly suggest the presence of p-type mercury vacancies in Hg1- xCdxSe samples both as-grown and after annealing under an Se overpressure. Temperature-dependent Hall measurements of annealed samples suggest two donor energy levels: one in the bandgap with ionization energy of ∼40 meV that produces an electron concentration of ∼8x10 15 cm-3 at 300K, and one in the conduction band with a concentration of ∼2x1016 cm-3. The former could originate from n-type Se vacancies, while the latter is most likely impurities from the Se source material.

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