Optical studies of ZnO grown by metalorganic chemical vapour deposition

The controlled n-type and p-type doping of ZnO is an ongoing and challenging field of study which needs to be resolved before this material can fulfill its great promise as an optoelectronic material. Metalorganic chemical vapour deposition (MOCVD) is a process that has been extensively investigated for ZnO growth. However, there have been very few reports of high resolution photoluminescence (PL) spectroscopy for MOCVD grown ZnO. In this thesis, strong donor bound exciton transitions with linewidths as low as 0.2meV were observed in a 4.2K low temperature PL spectrum of nominally undoped ZnO epilayers grown with dimethylzinc and nitrous oxide at 800°C on c-plane sapphire substrates by MOCVD. Intentional n-type doping of ZnO by MOCVD with group III dopants such as In and Al at high growth temperature has been studied. The addition of controlled amounts of In and Al precursors allows us to unambiguously observe donor bound exciton emission from these impurities which results in identification of the dominant residual donor in nominally undoped ZnO to be Ga and to infer the residual compressive strain in the layers due to lattice mismatch between ZnO and the sapphire substrate. PL measurements on ZnO epilayers annealed in oxygen at 900 - 1000°C show that group III dopants are quite mobile at temperatures above 800°C. Raman scattering spectroscopy measurements were performed to investigate the vibrational properties of MOCVD grown ZnO. Strong and broad vibrational modes due to sp2 graphitic carbon clusters were found in the Raman spectrum of low growth temperature ZnO epilayers at around 1350cm-1 and 1600cm-1. At high growth temperatures (>700°C), the graphitic modes disappear and the spectrum is dominated by ZnO lattice phonons.