Investigation on Sb Doping p-ZnO and Zn1-x MgxO Epitaxial Films, ZnMgO/ZnO MQWs Grown on Si Substrates
|Course||Materials Physics and Chemistry|
|Keywords||pulsed laser deposition p-type ZnO thin film Sb doping doping mechanism Zn1-xMgxO thin films Lu2O3 buffer layer epitaxial growth ZnO/Zn1-xMgxO multiple quantum wells|
As a novel II-VI compound semiconductor, ZnO is considered to be a potential candidate for short wavelength optoelectronic devices, such as blue-ultraviolet light-emitting diodes (LEDs) and laser diodes (LDs), due to its wide direct band gap of 3.37 eV and large room-temperature exciton binding energy of 60 meV. This large exciton binding energy in ZnO allows for efficient UV excitonic emission at room temperature (RT) or higher temperatures.In order to realize the application of ZnO in optoelectronic devices, two imperative issues including doping engineering and band gap engineering must be solved. Due to the asymmetric doping limitations, the realization of p-type ZnO has proven difficult. Therefore, the fabrication of reliable, stable and reproducible p-type ZnO thin films remains a major bottleneck limiting the development of ZnO-based devices. Another important step in order to design high-performance ZnO-based devices is the realization of band gap engineering to create barrier layers and quantum wells in device heterostructures. The ternary alloy semiconductor, Zn1-xMgxO, is considered to have larger fundamental band gap energy than ZnO, making it an appropriate candidate as barrier material for ZnO quantum wells. The ability to grow high quality epitaxial Zn1-xMgxO thin films and ZnO/Zn1-xMgxO multiple quantum wells (MQWs) may thus be very useful. In this regard, our work mainly concerns about both issues. Firstly, we focuse on growth and characterization of Sb-doped p-type ZnO thin films, in an attempt to reveal the p-type doping mechanism. Secondly, we investigate Zn1-xMgxO thin films and ZnO/Zn1-xMgxO MQWs grown on Si substrates using intervening epitaxial Lu2O3 buffer layers. The work includes:1. Sb-doped p-type ZnO thin films are prepared by pulsed laser deposition (PLD). The effects of growth temperature, oxygen pressure and Sb content on properties of Sb-doped ZnO are discussed. The optimized gowth parameters about growth temperature, oxygen pressure and Sb content are found to be 550℃,1 Pa and 2 at.%, respectively. The optimal p-type conduction is obtained under the above parameters on quartz substrates with a resistivity of 2.21Ω·cm, a carrier concentration of 2.3×1018 cm-3, and a Hall mobility of 1.23 cm2/V·s.2. Light Sb-doped p-type ZnO thin films are grown by O2 plasma-assisted PLD. We investigate optical properties of light Sb-doped p-type ZnO films. O2 plasma helps supply oxygen-rich growth conditions. Two acceptor states, with the acceptor level of～161 and～336 meV, are identified by well-resolved photoluminescence (PL) spectra. The deep acceptor is assigned to Zn vacancy (Vzn), which emerges with oxygen-rich growth. The shallow acceptor is related to Sb doping. Based on the model for large-sized-mismatched group-V dopant in ZnO by first-principles calculations, the origin of p-type behavior in Sb-doped p-type ZnO has been ascribed to the formation of SbZn-2VZn complex.3. Sb-doped p-type Zn1-xMgxO thin films are obtained by PLD. The effects of Mg content on properties of Sb-doped Zn1-xMgxO are discussed. A proper Mg content plays a key role in realizing p-type Zn1-xMgxO.4. Zn1-xMgxO thin films are epitaxially grown on Si (111) substrates using intervening epitaxial Lu2O3 buffer layers by PLD. Lu2O3 buffer layer on Si substrate is essential to the Zn1-xMgxO epitaxial growth. Good structural, optical and electrical qualities are revealed. In addition, a new growth method is proposed to solve the problem of cracks in thicker Zn1-xMgxO thin films. Cracks can be effectively controlled by applying periodic Zn1-xMgxO/LT-Zn1-xMgxO growth.5. We discuss the effect of conduction-band edge shift induced by alloying ZnO with Mg. Temperature-dependent Hall and temperature-dependent PL measurements are used to characterize the epitaxial Zn1-xMgxO thin films grown on Si (111) using intervening epitaxial Lu2O3 buffer layers. We prove that the addition of Mg in ZnO shifts the conduction-band edge to higher energy, thus increasing the activation energy of the defect donor states and reducing the n-type background carrier concentration, which results the realization p-type easily.6. ZnO/Zn0.9Mg0.1O MQWs are fabricated by PLD on Si (111) substrates using intervening epitaxial Lu2O3 buffer layers. In ZnO/Zn0.9Mg0.1O MQWs, the luminescence is dominated by localized exciton (LE) emission. Also, the LE emission in ZnO/Zn0.9Mg0.1O MQWs shows a systematic blueshift with decreasing well width, which is consistent with the quantum size effect.In addition, we investigate the optical properties of ZnO/Zn0.9Mg0.1O MQWs with well width of～5 nm. The LE emission from the MQWs shows a blueshift compared with the free exciton (FX) energy of epitaxial ZnO film. Such blueshift (～13 meV) is believed to be the results of quantum confinement effect and quantum confinement stark effect. Moreover, The LE emission from the MQWs decays more slowly than exciton emission from ZnO. The efficient excitonic emission at RT or higher temperatures is desirable for LD and LED applications.