| Due to wide band gap of 3.37 eV and exciton binding energy of 60 meV at room temperature, ZnO has long been considered as a candidate material for ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes (LDs) and investigated intensively in recent years. P-type ZnO has been fabricated by doping of N, P, As etc, and Electroluminescence (EL) of p-n homojunction ZnO was also reported by many groups. Unfortunately, the EL emission does not come from the near band-edge emission of ZnO in a UV region, but the recombination emission of defect or donor-acceptor pair in a visible wavelength region.In order to realize UV EL, it is crucial to develop LEDs with active layers of ZnO based superlattices or quantum wells, which need suitable ZnO-based potential barrier material. Since MgO is a direct-gap semiconductor with a wide band gap of 7.8 eV and the ionic radii of Mg2+ (0.57 ?) and Zn2+ (0.60 ?) are quite close and may alloy with ZnO by substitution of Mg2+ for Zn2+. Mg-Zn-O alloy was proposed and has been investigated widely as a ZnO barrier layer. Hence, it is necessary to study the preparation, structure and properties of MgZnO alloy thin films.In this thesis, we investigate the structure and properties of MgZnO alloy thin films prepared on quartz substrates by radio-frequency reactive magnetron sputtering using Ar/N2,O2/N2,Ar/O2 as sputtering gases respectively. Mechanism of the variation of the structure and Mg concentration induced by change of the nitrogen partial pressure ratio of Ar/N2 sputtering gases was discussed based on thermodynamics in this work. The details are as follows:1. MgxZn1-xO films were prepared using mixture of nitrogen and argon as sputtering gases. It was found that Mg concentration, structures, electrical properties and band gaps of the films can be tuned by changing nitrogen partial pressure ratio of the Ar/N2 sputtering gases. The MgxZn1-xO film consists of wurtzite phase at the ratios from 0 to 50 %, mixture of wurtzite and cubic phases at the ratios of 78 to 83 % and cubic phase at 100 %. The Mg concentration increased linearly with increasing the ratio and substrate temperature. The Mg concentration increased with increasing the substrate temperature, which can be attributed to that Zn-related species have a higher vapor pressure than that of Mg-related species and can be easily desorbed at high growth temperature. The Mg concentration should be between 0.44 and 0.66 for occurrence of the phase segregation and between 0.68 and 0.84 for formation of single cubic Zn1-xMgxO. The band-gap increases from 3.64 eV at x=0.172 to 4.02 eV at x=0.44 for the wurtzite MgxZn1-xO and reaches 6.30 eV for cubic MgxZn1-xO with x=0.84, indicating that the MgxZn1-xO alloy is a suitable potential barrier layers of ZnO-based quantum well or superlattices.2. Mechanism of the variation of the structure and Mg concentration induced by change of the nitrogen partial pressure ratio of Ar/N2 sputtering gases was discussed based on reactive thermodynamics. The changes of the structures and band gaps with the nitrogen partial pressure ratio of Ar/N2 sputtering gases are due to variation of the Mg content with the RN2. While the variation of Mg concentration is mainly attributed to the loss of the O and Zn atoms, the former is induced by reaction between the N and O, and latter results from that some excessive Zn atoms are re-evaporated as they deposit on the substrate due to high substrate temperature and expelled from the chamber.3. MgxZn1-xO films were prepared using mixture of argon and oxygen as sputtering gases at various oxygen partial pressure ratio ranging from 0% to 100%. It is found that all the samples exhibit wurtzite phase. The MgxZn1-xO film is of wurtzite structure with (002) preferential orientation at the ratio of 0%, but polycrystalline films without preferential orientation at the ratios of 50 to 100%. And the MgxZn1-xO films at the ratios of 50 to 100% have the same Mg concentration. It can be attributed to the enough O source which make all the Mg and Zn can reactive with O to form MgxZn1-xO alloys with the same Mg concentration. This also leading to the MgxZn1-xO films at the ratios of 50 to 100% have the same band gap value of 3.564 eV.4. MgxZn1-xO films were prepared using mixture of nitrogen and oxygen as sputtering gases at various nitrogen partial pressure ratio ranging from 0% to 100%. It is found that the MgxZn1-xO films are wurtzite structure polycrystalline films without preferential orientation at the ratios of 0 to 83%, but of cubic structure with (200) preferential orientation at the ratio of 100%. And the MgxZn1-xO films at the ratios of 50 to 83% have the same band gap value of 3.63 eV, which is lager than the value of 3.564 eV of the sample deposited in pure oxygen ambient. When the film deposited in pure nitrogen ambient, its band gap reaches 6.30 eV.5. The films deposited with O2 are randomly oriented films. The deterioration in alignment originates from the low energy of deposition flux. During the process of sputtered atoms of Mg, Zn and O deposited, the Mg and Zn will combine with O-related species of plasma, which leading to the decrease of the deposition flux energy. So the deposited atoms do not have sufficient time to undergo surface diffusion to thermodynamically stable sites before being covered by the next layer of atoms. As a result, the films deposited with O2 do not show preferred orientation.6. In Ar/N2 ambient, the annealed film grown at RN2=0 behaves high resistivity, while the annealed films grown at 50 %≤RN2≤100 % show p-type conduction. In O2/N2 ambient, the annealed films grown at 83 %≤R'N2≤100 % behaves p-type conduction, while the annealed films grown at 0%≤R'N2≤78% show n-type conduction. And MgxZn1-xO films grown using mixture of Ar and O2, and no p-type MgxZn1-xO was obtained after annealed at 600℃. So, we deduce that some N atoms substitute for O atoms in the wurtzite or cubic MgxZn1-xO to form shallow acceptors during the growth process. These acceptors overcome compensation of other possible native donor defects and make the MgxZn1-xO conduct in p-type. |
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