Preparation And Properties Of P-type Zinc Oxide Thin Films

Zinc oxide (ZnO) is a II-VI semiconductor with a wide band-gap of3.37 eV and a hexagonal wurtzite structure. Due to its large excitonbinding energy, 60 meV (much larger than the room temperature thermalenergy, 25meV), it can be considered as a prime candidate for ultraviolet(UV) light emitting diodes (LED) in both room and high temperature. Thegrowth temperature of ZnO is only a half value of that of GaN, whichreduces the atomic diffusion between the film and substrate when grow inhigh temperature. This makes it a promising candidate after GaAs for UVlaser diodes and UV LED etc. Interest of ZnO as a promising material foroptoelectronic devices has intensified since the first reported in 1997 onthe room-temperature ultraviolet (UV) laser emission using poly-crystalliteZnO thin films. In 2004, the success in ZnO p-n homojunctionelectroluminescenc has opened a door for material scientists on studyingp-type ZnO.Recently many groups tried to grow p-type ZnO. Some groups havereported successfully fabricating p-type ZnO:N, which is reasonablebecause nitrogen has a similar ionic radius as oxygen and is easilysubstitutable. Unfortunately, reliably obtaining p-type ZnO is stillproblematic. Moreover, the low solubility and the deep acceptor levels ofthe dopants may yield high resistivity, low carrier concentrations andunstable, making p-ZnO even harder to fabricate. There is a still long wayto go to make p-type ZnO practicality. The key to obtain the high qualitydevice is producible stable low resistivity ZnO.The study of ZnO is a hot and hard problem. To explore it, the details ofthe series studies have been exhibited successively. We focus on thefabrication, structures, and optoelectronic properties of p-type ZnO, bothun-doped and N-doped. The films were prepared on quartz substrates by r.f.magnetron sputtering method using ceramic ZnO target at 2400C in Ar forun-doped while N2 for N-doped. The structural, optical and electricalproperties of the ZnO thin films was systematically studied through X-raydiffraction (XRD), photoluminescence (PL) measurement, X-rayphotoelectron spectroscopy (XPS) and Hall Effect measurement, withcomparison with those of as-grown samples.The Un-doped ZnO films were polycrystalline in nature withpreferred (002) orientation. After grown, a vacuum annealing process hasbeen employed. The greatly improved crystallization property of the ZnOfilms has been found during the vacuum annealing due to release of theresidual stress. The electrical and optical properties were also improvedand the p-type was realized after annealed at 5900C. The concentration ofintrinsic defects was determined to be changeable while annealing. Thoseannealed below 500 C were dominated by zinc interstitials (Zni) andoxygen vacancies (Vo) donor defects, which result in n-tpye ZnO. With thehigher annealing temperature, zinc interstitials (Zni) and oxygen vacancies(Vo) donor defects decrease, and the increased zinc vacancies (V°Zn)accepter defects will fight over the donor defects Zni, leave us p-type.When the temperature keeps going up, the concentrations of Vo willincrease dramatically. The Vo together with the Zni exert a compensationfor VZn and turn the sample back to n-type. This explained why the n-typeat 4300C, n-p mixed type at 5000C and n-type again at 6000C.Un-doped p-type ZnO was unstable. Light can induce it turn back ton-type due to excitation of the electrons from valence band (VB) toconduction band (CB). The so called photoelectrons in CB equal to the socalled photoholes left in the VB. Equal they are, however, the Hall Effect ishard to detect the holes for weak p-type due to the much higher mobility ofthe electron than the holes.Great effect of the substrate temperature on the film quality andelectrical properties have also been predicted. The effect of substratetemperature on structural, optical and electrical properties of the un-dopedZnO thin films was systematically studied at the temperatures 1500C,2000C, 3000C, 4000C and 5000C. The un-doped p-type ZnO was producedat 5000C.The polycrystalline ZnO:N films were also preferred (002) orientationin nature. The impurities have a natural tendency to pair up to be N2molecules, which form a shallow donor defects (N2)O together with theother main form, acceptor defects No. The un-annealed ZnO:N is perfectinsulator and orange color with larger (about 1.9% ) crystal constantscompared to pure ZnO due to substitute N2 for O atom and the internalstress. With increasing the annealing temperature, the color of the ZnO:Nturns weaker and the crystal constants decrease due to the outwardsdiffusion of N2 and release of the stress. When N2 diffuse out, No acceptordefects are left inside. In the same time, more and more VZn were formed.These two effects act together to yield p-type ZnO. If the sample wereannealed in O2 atmosphere, oxygen will substitute for the nitrogen due tohigher reactivity, which reduces the acceptor concentration, resulting instable n-type ZnO. When the temperature goes too high, a large amount ofOZn defects will occur, which can be detected from the intense green lightpeak in PL spectra.