Development of MgZnO Thin Films for Optoelectronic Devices
MgxZn1-xO thin film was developed to realize optoelectronic devices in the ultraviolet region. The alloy was grown without any phase separation and characterized with different methods, such as, photoluminescence, absorption, x-ray diffraction etc. to confirm the structural properties. P-type doping of MgxZn1-xO thin film was investigated. Antimony (Sb) and nitrogen (N) were used as the p-type dopant. It was found that incorporation of Sb degrades the optical quality of the film. On the other hand, N-doped p-type MgxZn1-xO thin film was demonstrated without any degradation of the optical quality. Temperature dependent photoluminescence of nitrogen doped p-type MgxZn1-xO nano crystalline thin films grown on c-plane sapphire substrate by rf plasma assisted molecular beam epitaxy were examined. P-type behavior was confirmed by both Hall effect and Seebeck measurements. However, structural defect related bound excitonic emission peak was distinguished in the low temperature photoluminescence spectra. Also, typical `S shape' behavior of energy position versus temperature is observed due to polarization induced internal field. Nitrogen related acceptor ionization energy was calculated to be ~180-200meV.
N-doped p-type Mg0.12Zn0.88O film was utilized in a p-n junction based random lasing device. A heterostructure device consisting of nitrogen-doped Mg0.12Zn0.88O and gallium-doped ZnO thin films was grown on c-plane sapphire substrate. Current-voltage and photocurrent characteristics indicated the formation of a p-n junction. Random lasing behavior with lasing modes centered at 356 nm was observed. A low threshold current of 6 mA was determined and an output power of 34 nW was measured at an injection current of 8 mA. The film consist of columnar structures with much air gaps, which assisted in light scattering to achieve necessary gain for random lasing. N-doped resistive Mg0.12Zn0.88O and ZnO films were also employed in metal-semiconductor-metal planar devices to realize random lasing in the ultraviolet region. Asymmetric Ni/Au and Ti/Au Schottky contacts and symmetric Ni/Au contacts were deposited on the thin film to form metal-semiconductor-metal laser devices. Current-voltage, photocurrent, and electroluminescence characterizations were performed. Evident random lasing with a threshold current of ~36 mA was demonstrated only from the Mg0.12Zn0.88O:N based asymmetric MSM device. Random lasing peaks were mostly distributed between 340-360 nm and an output power of 15 nW was measured at 43 mA injection current from the device. The electron affinity difference between the contact metal and N-doped resistive layer played an important role for electron and hole injection and subsequent stimulated random lasing.