Preparation And Characteristics Of ZnO-Based Semiconducting And Optoelectronic Materials

Zinc oxide (ZnO) is a novel II-VI compound semiconductor with a hexagonal wurtzite structure. It is grown usually along the (002) orientation due to its lower surface free energy for the (002) plane. ZnO is a unique material that exhibits optoelectronic, piezoelectric, and ferromagnetic multiple properties, as well as its versatile nanostructures. In particular, it is a potential candidate for applications in short-wavelength optoelectronic devices, including light emitting diodes (LEDs) and laser diodes (LDs), due to its direct wide-bandgap (3.37 eV) and high exciton binding energy (60 meV, cf. 25 meV for GaN), which will favor efficient excitonic emission processes at room temperature. In low-dimensional ZnO nanostructures many significant exciton effects may be expected due to quantum confinement effects, and so the improvement in device performance can be predicted, especially for the three-dimensional confined system of ZnO quantum dots (QDs).To realize these device applications, an imperative issue is to fabricate both high-quality p- and n-type ZnO and ZnO-based homojunctions. However, like most wide bandgap semiconductors, ZnO has the "asymmetric doping" limitation. It can be an easily doped high-quality n-type, but it is difficult to dope p-type. So the key issue in exploiting ZnO, as in the case of GaN, is in achieving p-type doping. In this work, we mainly concern about this issue, and moreover this work is also expended from it.Two deposition techniques, dc reactive magnetron sputtering (DCMRS) and solid source chemical vapor deposition (SSCVD), were used to fabricate ZnO materials. By using DCRMS, intrinsic, n-, and p- type ZnO microcrystallite thin films and ZnO-based p-n junctions were successfully fabricated with acceptable properties. The SSCVD system was also used originally to realize p-type ZnO, but the result was not good, so this method was then applied to investigate nanoscale ZnO, such as ZnO nanoparticle films and ZnO QDs. The structural, morphonological, optical, electrical, and compositional characteristics for all the ZnO materials obtained here were investigated and studied detailedly and profoundly.1. ZnO microcrystallite thin films fabricated by DCMRSIntrinsic ZnO (i-ZnO), Al doped n-type ZnO (n-ZnO:Al, ρ=～10^-4 Ωcra), N doped p-type ZnO (p-ZnO:N, p= -10¹ Ωcm) and N-Al codoped p-type ZnO (p-ZnO:(N,Al), p= -10° Ωcm) microcrystallite thin films were well prepared by dcreactive magnetron sputtering. Then a round ZnO system has been developed. In particular, p-type ZnO was studied in detail and good results were obtained. The resistivity can be lowed to 2.64 Qcm for the p-ZnO:(N,Al) films and the p-type conductivity is reproducible and stable.All the films were of good crystal quality with high (002) orientation and closed-packed columnar grains. They also have good optical quality with high transmittance about 90% in the visible region and prominent UV emission at room temperature.In the as-grown intrinsic ZnO films, the compressive in-plane stress was formed due to the presence of Znj in ZnO. In n-ZnO:Al films, the stress decreased due to that Al has a smaller ionic size compared with Zn. On the contrary, the stress increased in p-ZnO:N films due to the larger ionic size of N compared with O. Interestingly, in p-ZnO:(N,Al) films, the stress was low because of the formation of N-Al-N complex, which tuned down the stress induced in ZnO due to the N incorporation. This had an evident effect in stabilizing the properties of ZnO:(N,Al) films.For doped ZnO films, ionized impurity scattering is the main scattering mechanism. In addition, a new scattering mechanism, intra-grain cluster scattering, was proposed to explain the rapid decrease in carrier mobility in a relatively high Al content in ZnO;when the Al content was too high, the second phase appeared around the grain boundary. For p-type ZnO, including ZnO:N and ZnO:(N,Al), the neutral impurity scattering and intra-grain cluster scattering were also somewhat important. They should be both emphasized in our research.Compared with intrinsic ZnO, the bandgap showed an evident blueshift in n-ZnO:Al, which is believed to be the result of the Burstein-Moss shift. For the p-ZnO:N or p-ZnO:(N,Al) films, the bandgap commonly showed redshift, which may be due to a merging of the acceptor impurity band with the valance band.An annealing model, including both the structural and stress aspects, for i-ZnO films was proposed, in which Zn;and Vo played an important role. The quality of n-ZnO:Al films was improved by the proper incorporation of Al. A H-enhanced N doping mechanism was proposed to realize p-type conductivity in N-doped ZnO films. In particular, a N-Al codoping technique was intensively proposed as an effective approach to achieve p-type ZnO. The presence of Al in ZnO can engineer a both comfortable and stable local chemical environment, which will greatly enhance the N incorporation and is responsible for the stable p-type behavior in ZnO:(N,Al).By using this round ZnO system, ZnO-based p-n heterojunctions andhomojunctions were fabricated. In-Sn alloys were used as the contact spots on both layers to form good ohmic behavior. In particular, the p-ZnO:(N,Al)/n-ZnO:Al homojunctions were successfully achieved, exhibiting apparently electrical rectification behavior of a typical p-n junction. The turn-on voltage appeared at about 2 V under forward-biased voltage, and the reverse breakdown voltage was about 4 V. The ideal factor r\ was about 8. The results were acceptable and stable.2. ZnO materials fabricated by SSCVDBy using SSCVD, the p-type ZnO films were also realized by doping N with the resistivity lowed to -101 Qcm. The p-ZnO:N films had (100) and (110) mixed orientation with a reticular structure. Although the film quality was not good, it was from this trial that the new growth technique of SSCVD has been well developed.Through optimizing the deposition process, ZnO films with a good (100) orientation were achieved on a selected substrate at a non-equilibrium state. It is a novel growth orientation for ZnO, which has never been reported in previous literatures. The (100) oriented ZnO films were composed of nanoscal particles with the particle size about 30 ran. The optical bandgap was about 3.31 eV, markedly higher than the value of 3.22 V for the microcrystallite thin films (>60 nm for grain size) grown by DCRMS, which may be due to the quantum-confined effects.By further designing the technology of SSCVD, ZnO QDs were successfully obtained. A vapor-liquid-solid (VLS) mechanism was proposed to illuminate the deposition process of ZnO QDs. The ZnO QDs size was about 16 nm, and the dot density was about 109 cm"2. The bandgap of ZnO QDs, derived from the optical adsorption spectra, was about 3.40 eV, indicating the evident quantum-confined effects. The UV emission (3.32 eV) was strong and predominant in the room-temperature PL spectra for the as-grown ZnO QDs without any surface modification or annealing treatment, which was the advantage for the ZnO QDs obtained here.