Preparation And Characterizations Of Polar And Non-polar MgZnO Alloy Thin Films Grown By PLD

Band gap engineering of ZnO,which is of great importance towards designing efficient ZnO-based heterojunction,supper lattices and quantum structures and to extend its usable application in optoelectronics,has been a hot area of research for scientists in the world.While replacing cations in ZnO by isoelectronic elements(replacing zinc by Mg and Cd)and anion elements(replacing oxygen by sulfur,selenium or tellurium)are two ways in the band gap engineering of ZnO.Because of the similarity of ionic radius between Zn2+(0.60 A)and Mg2+(0.57 A),it has less influences on the lattice constant and the lattice distortion when the Zn2+ was replaced by Mg2+.As well,the doping of Mg can significantly adjust the band structure of ZnO.However,the different crystal structures that the MgO is the rock salt structure and the ZnO is the hexagonal wurtzite structure makes a limited solubility of the MgO in ZnO.Therefore,in order to extend its application in photoelectric devices,extending the solubility limit of MgO in ZnO has been attracted much attention.In this thesis,the MgxZn1-xO thin films were grown by using pulsed laser deposition(PLD)method with the home-made MgxZn1-xO ceramic as the target and the high purity O2 as the reactive gas.We systematically studied the properties of the MgxZn1-xO thin films deposited at the different O2 partial pressures and the deposition temperatures.In order to study the influence of the polar and nonpolar substrates on the properties of MgxZn1-xO thin films,the MgxZn1-xO thin films with different Mg content were grown on the c-and r-sapphire substrates,respectively.The main contents and results are shown as the following:Firstly,we obtained high-quality epitaxial MgxZn1-xO thin films by using PLD method at the temperature of 750 ?.The home-made Mg0.08Zn0.92O ceramic was used as the target,and the oxygen partial pressure was changed from 0 to 30 Pa.Effects of different oxygen partial pressure on the surface morphology,crystal structure,composition and optical properties of the films were systematically studied.The results show that the sample with the oxygen partial pressure of 0.5 Pa shows the highly preferred c-axis orientation and the best crystalline quality,and the transmittance of the films is over 90%.Secondly,high-quality MgxZn1-xO thin films with single phase wurtizite structure were successfully prepared at different substrate temperatures on the c-sapphire substrates.The oxygen partial pressure was fixed at 0.5 Pa,and the home-made Mg0.08Zn0.92O ceramic was used as the target.The results show that c-plane orientated MgxZn1-xO thin films with the best crystalline quality was obtained at a substrate temperature of 700 ?,and with increasing the deposition temperature from 300 to 700 ?,the Mg content in the MgxZn1-xO increased from 17.7 at.%to 34.5 at.%,and the band gap of the MgxZn1-xO thin films changed from 3.14 to 3.64 eV.Finally,we obtained a series of MgxZn1-xO thin films with different Mg contents on the c-and r-sapphire substrates,respectively,by using the home-made MgZnO ceramics with different Mg contents as the targets.We studied the influence of Mg content on the surface morphology,phase structure and the optical properties,and we also compared the difference between the MgxZn1-xO thin films grew on the polar and nopolar sapphire substrates.The results showed that the single-phase MgxZn1-xO thin films with wurtzite structrue were achieved in composition ranges of x ? 0.575 on the c-sapphire substrates and x ? 0.329 on the r-sapphire substrates.The phase separation of wurtzite ZnO and rock salt MgO was found in MgxZn1-xO ternary alloy with x=0.583 grown on the r-sapphire substrate.