Zhonghai Yu

Date of Graduation

1996

Document Type

Dissertation/Thesis

Abstract

Both compensation and hydrogen-related phenomenon in ZnSe grown by MBE have been systematically investigated in this study. Atomic hydrogen is demonstrated to effectively clean GaAs substrates for subsequent growth of ZnSe by molecular beam epitaxy. Significant reduction of micro-defects in both undoped and nitrogen doped ZnSe is achieved using atomic hydrogen cleaning of GaAs substrate prior to ZnSe growth without any other treatment. The density of stacking faults in ZnSe is less than {dollar}\\rm1\imes10\\sp4\\ cm\\sp{lcub}-2{rcub}{dollar} in layers grown with atomic hydrogen cleaning, while it is greater than {dollar}\\rm1\imes10\\sp7\\ cm\\sp{lcub}-2{rcub}{dollar} in layers grown with conventional thermal cleaning. Optical microscopy is shown to be a useful technique to image non-radiative defects such as stacking faults. Low-temperature photoluminescence (PL) spectra of undoped ZnSe are dominated by excitonic transitions for the low dislocation density samples in contrast to the high level of defect-related emission from high defect density samples. Similar PL and Hall measurements for nitrogen-doped ZnSe layers with comparable nitrogen concentrations grown with both cleaning procedures indicate that compensation of nitrogen doping in ZnSe is not affected by the presence of micro-defects. Hydrogen is incorporated into ZnSe during MBE growth using a thermally-cracked atomic hydrogen source. It is found that the hydrogen incorporation is strongly nitrogen-related, and the incorporation is significant only when both atomic hydrogen and atomic nitrogen are present during growth. The observation of vibrational absorption bands using Fourier transform infrared spectroscopy in ZnSe:N,H samples indicates the presence of N-H bonding in these layers. Absorption bands occurring at 3193 cm{dollar}\\sp{lcub}-1{rcub}{dollar} and 783 cm{dollar}\\sp{lcub}-1{rcub}{dollar} correspond to the stretching mode and wagging mode of the N-H bond. The isotope shift due to exchanging hydrogen with deuterium is also observed, resulting in an absorption band at 2368 cm{dollar}\\sp{lcub}-1{rcub}.{dollar} This is in excellent agreement with the predictions of the harmonic oscillator approximation. The low incorporation of molecular hydrogen in nitrogen-doped ZnSe grown by MBE implies that the hydrogen is not the source of nitrogen compensation in ZnSe grown by conventional MBE technique. A substitutional nitrogen absorption band is observed for the time in as-grown ZnSe materials. The integrated absorption for the substitutional nitrogen signal does not linearly increase with the nitrogen concentration in the layer as measured by SIMS. The discrepancy implies that a very large amount of incorporated nitrogen is either not in the substitutional site or is in another charge state which does not contribute to the p-type doping in ZnSe. A Se vacancy signal is observed by electron paramagnetic resonance measurement in both undoped and nitrogen-doped ZnSe. The Se vacancies are estimated to be in the range of {dollar}10\\sp{lcub}17{rcub}{dollar} to {dollar}10\\sp{lcub}18{rcub}{dollar} cm{dollar}\\sp{lcub}-3{rcub}.{dollar} PL result indicates that a donor at a level of as deep as 0.3 eV possibly exists in heavily nitrogen-doped ZnSe. It is suggested that the nitrogen compensation in ZnSe can be from Se vacancy-related defects at doping levels lower than {dollar}10\\sp{lcub}18{rcub}{dollar} cm{dollar}\\sp{lcub}-3{rcub},{dollar} while it can be limited by the nitrogen solubility at levels greater than {dollar}\\rm10\\sp{lcub}18{rcub}\\ cm\\sp{lcub}-3{rcub}.{dollar}.

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