Behaviors Of Impurities And P-Type Doping In Zinc Oxide

Since the observation of room-temperature lasing phenomenon in zinc oxide (ZnO) in 1997, the world-wide research, focusing on the optical and electrical properties of ZnO related semiconductors, has been booming for nearly 15 yrs. Currently the boom continues, yet with decreased enthusiasm, mostly due to the mono-polar doping trait of ZnO, i.e., the lack of stable, controllable and reproducible p-type materials. It is well known that both high-quality n-type and p-type materials are indispensable for the application of a specific semiconductor. However, the mono-polar doping trend usually occurs in wide band-gap semiconductor materials, which critically hinders their attractive optical and electrical properties. Recently, researchers stopped fabricating ZnO based optoelectronic devices blindly, but concentrated more on the investigation of intrinsic physical properties of ZnO.Nitrogen, regarded as a best candidate for substituting acceptors, is widely employed in the exploration of p-type doping in ZnO. Various compensation mechanisms, which block the realization of effective p-type doping in N-doped ZnO fabricated by metal-organic chemical vapor deposition (MOCVD) technique, have been investigated in present dissertation. The compensation behaviors and suppression mechanisms of these impurities have also been studied via systematical experiments. Some feasible routes to realize p-type conductivity in N-doped ZnO using MOCVD technique have been proposed and preliminarily validated. Detailed results are listed below:1. The evolution of the structures and properties of the films during each growth process has been investigated in the standard technique of fabricating high-quality undoped ZnO films on sapphire substrate, revealing the enhanced in-diffusion phenomenon of Al elements from substrate induced by the duration of high-temperature process and growth. In each growth step (low-temperature buffer growth, high-temperature thermal process of buffer layer, and the high-temperature grown process of epilayer), the electron concentration and the content of Al element shows 1-on-1 correspondence which demonstrates the fact for the first time that Al elements are the decisive origin of the background electron concentration in the high-temperature grown undoped ZnO films. This study shows that the high-temperature thermal processing, which is beneficial to improving the crystal quality of the epilayer grown on sapphire substrate, should be optimized and well-controlled. Introducing the thick low-temperature buffer layer would prevent Al elements in the substrate from diffusing into the high-quality epilayers.2. The form, structure and properties of carbon, mainly unintentionally incorporated in ZnO films, esp. N-doped ones, fabricated by MOCVD technique have been extensively studied. Observed directly by transmission electron microscopy, the unintentionally doped carbon would usually form graphitic nano-clusters along grain boundaries, resulting in high-impurity-density and high conductive regions, which is believed to be the main cause of the inhomogeneous conductivity in N-doped ZnO. The hydrocarbon species in metal-organic (MO) precursors are considered as the main origin of unintentionally doped carbon in ZnO films. The suppressing mechanism of unintentionally doped carbon has also been investigated. It is found that the substrate temperature plays a dominant role in decreasing the content of carbon in films, which is also affected by the employment of different reaction gases, e.g., oxygen source.3. The compensation behaviors of impurities in N-doped ZnO films, esp. the formation and suppression of (NN)o and (NC)o species, which are two main complexes of C and N in film, have been investigated. In films grown under quasi-equilibrium technique, such as MOCVD, the formation energies of these complex defects, acting as donors, are relatively low while the one of the desired No acceptors is relatively high, resulting in the difficulty of the realization of p-type conductivity in N-doped ZnO. In the exploration process of modulating N2O/O2 ratio to grow ZnO films, it is found that (NN)o complexes could be completely suppressed when growing N-doped ZnO films using a specific N2O/O2 ratio which is consistent with the results from first-principle calculation that the formation energy of (NN)o is much larger in O-rich condition than in Zn-rich one. However, it is really hard to suppress the (NC)o complexes no matter how the growth parameters are modulated, according with the first-principle calculation indicated fact that the formation energy of (CN)o is ultra-low, which critically affects the p-type efficiency and stability in N-doped ZnO.4. The growth and properties of N-doped ZnO fabricated on high-quality ZnO template have been studied. The surface and cross-section morphologies improve greatly with invariant even decreased concentration of intrinsic defects and the N doping remains effective regardless of the improvement of crystal quality. Hall-effect measurement shows no p-type conductivity because of the highly conductive layer existing in the template, yet implies obvious contribution from No acceptors. This study indicates that the N doping behaviors show weak dependence on the crystal quality of film and type of substrate, which provides valuable reference to the epitaxy of N-doped ZnO on single-crystal ZnO substrate of higher quality. It is also an effective solution to the contradiction between doping efficiency and crystal quality of epilayer.5. An acceptor-isovalent codoping technique has been developed. The influence of Te/N codoping on the properties of N-doped ZnO films has been investigated, and an effective pathway to enhance the p-type conductivity in N-doped ZnO is discovered. Analyzed from the experimental data, it is found that the codoping with Te and the introduction of diisopropyltelluride MO precursors are beneficial to the enhancement of N solubility, the suppression of incorporation rate of unintentionally doped carbon, and the decrease of ionization energy of No acceptors. Hall-effect measurement shows that the hole conductive behaviors and crystal quality of films can be definitely enhanced via Te/N codoping technique, which is a feasible route to realize p-type conductivity in ZnO.