Structural, Electrical and Optical Properties of p-Type ZnO Epitaxial Films.

[摘要] With a direct band gap energy of 3.37 eV at room temperature and an exciton binding energy of 59 meV, ZnO is a promising material for short-wavelength optoelectronics.The realization of ZnO ¬p-n homojunction devices requires to fabricate stable p-type material with a hole concentration in the 1017-1018 cm-3 range.This thesis addresses the relationships between the microstructure and the optoelectronic properties of acceptor-doped ZnO films fabricated by pulsed laser deposition (PLD). The acceptor dopants studied are nitrogen and phosphorus. The effects of doping, substrate and processing conditions on the film optoelectronic properties are investigated. ZnO epilayers doped uniformly with 1020 nitrogen at./cm3 can be fabricated by ablation of a Zn-rich Zn3N2 target in O2 atmosphere. When the growth temperature exceeds 300 °C, the nitrogen solubility falls rapidly. Films deposited at 300 °C and heat-treated in O2 are p-type with p~1017 cm-3. The moderate level of hole conduction is explained by the kinetics of PLD. Two doping-induced acceptor levels at 105 meV and 280 meV are identified. The effect of nitrogen doping on the film microstructure is studied.Heavy doping with phosphorus (1020 at./cm3) is necessary to achieve p-type conductivity with p~1017 cm-3. The dominant crystal defects are interstitial dislocation loops and partial dislocations (1011-1013 cm-2). These defects play a key role in the formation of shallow acceptors. Their interaction with phosphorus is discussed and a formation mechanism for the PZn-2VZn acceptor complex is proposed. The acceptor activation energy is estimated to be 120-150 meV. These layers have a weak photoluminescence yield. With the use of an oxygen plasma during growth, the luminescence yield can be improved and a violet band, assigned to a transition between a shallow donor and a 340 meV-deep acceptor, appears. The acceptor may be the zinc vacancy. Finally, issues of homoepitaxy, p-n junction fabrication and electroluminescence are discussed. Non-radiative recombination induced by extended defects is a severe limit to device operation.The scientific merit of this work is to understand the effects of doping and microstructure on the electronic transport and photoluminescence properties of p-type ZnO epilayers and p-n homojunctions.

[发布机构] University of Michigan

[效力级别] P-type Doping

[关键词] ZnO;P-type Doping;Defects;Microstructure;Electrical Properties;Optical Properties;Materials Science and Engineering;Engineering;Science;Materials Science and Engineering