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Cadmium Zinc Oxide Based Optoelectronics Materials and Devices

  • Author(s): Li, Lin
  • Advisor(s): Liu, Jianlin
  • et al.
Abstract

Cadmium Znic Oxide (CdZnO) based optoelectronics materials and devices are studied and discussed in this dissertation. CdZnO thin films with various Cd:Zn mol ratios were grown by molecular-beam epitaxy (MBE) on silicon (Si) and c-plane sapphire substrates, respectively. Room temperature (RT) photoluminescence (PL) showed the peak position of the near band edge (NBE) emission of the CdZnO thin films shifted from 3.30eV to 2.59eV on Si (100) substrates, while it shifted from 3.26eV to 2.40eV on c-plane sapphire substrates. The color of the RT PL emission of the CdZnO thin films covers from ultraviolet (UV) to blue region on Si (100) substrates, and to green region on c-plane sapphire substrates. The NBE emission peak energy was estimated to be the band gap energy of the CdZnO thin films at RT. Energy-dispersive X-ray spectroscopy (EDS) study of the CdZnO thin films showed Cd:Zn ratio increased with the decrease of the band gap energy of the CdZnO. X-ray diffraction (XRD) studies showed the crystal structure of the CdZnO thin films changed from the pure wurtzite (wz) structure to mixture of wz and rocksalt (rs) structure, and finally to pure rs structure with the increases of Cd:Zn ratio. Temperature dependent PL measurements were performed on three typical CdZnO samples having pure wz, pure rs, and wz-rs mixture structures, respectively. The temperature dependence of the CdZnO bandgap change was investigated and analyzed based on the empirical Varshni and Bose-Einstein fitting on the NBE PL peak positions. The temperature dependence of the integral PL intensity in the CdZnO samples was also fitted, where fitting equation with hopping term showed a closer fit to the experimental data. The possible hopping processes in the CdZnO thin films may be related to the band tail states due to alloying effect and nonuniformity of Cd distribution in the CdZnO samples. Rapid thermal annealing (RTA) at different temperature was performed on in situ annealed CdZnO thin films to study the thermal stability of the CdZnO. Pure wz CdZnO showed insignificant NBE PL peak shift after RTA, while mixture structure CdZnO showed evident blue shifts due to phase change after annealing, indicating the rs phase CdZnO changes to wz phase CdZnO during RTA process. On the other hand, CdZnO thin films without in situ annealing showed dramatic NBE emission peak energy increase with increasing RTA temperature. RT PL and temperature dependent PL showed phase separations in the samples after the annealing process. Secondary ion mass spectroscopy (SIMS) measurements showed redistribution of Cd in the as-annealed sample, which is believed to be the reason of PL peaks shift. ZnO heterojunction samples with CdZnO active layers were grown on p-type Si (100) substrate and n-type Si (100) substrate, with structures of n-ZnO/i-CdZnO/p-Si and p-ZnO/i-CdZnO/n-ZnO/n-Si, respectively. Light emitting diodes (LEDs) fabricated from both samples showed typical diode rectifying I-V curves. Blue and cyan color emissions were observed under forward bias at RT. RT PL measurements at different thicknesses on the sample showed the existence of CdZnO active layer. Temperature dependent electroluminescence (EL) showed that the change of EL peak energy agree with Varshni equation, while RT PL verified that the EL emission come from the CdZnO layer. Devices with metal-insulator-semiconductor structure and ZnO p-n junction with CdZnO quantum well structure were grown and fabricated respectively. Random laser emissions were observed from both of the devices under forward bias. Closely packed column structures are believed to serve as random resonance cavities for the laser emissions.

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