STUDY OF ULTRA-THIN ZINC OXIDE EPILAYER GROWTH AND UV DETECTION PROPERTIES

摘要

ZnO is a wide bandgap (3.4 eV) II-VI semiconductor with large exciton binding energy (60 meV), and holds a strong potential for light emitting/detecting or nonlinear optical devices in the UV range. Essential to development of such devices is establishment of proper methods to grow/synthesize high quality materials and structures whose properties (electrical, optical, etc.) can be tailored to specific device application. Ultra-thin (nanometer-scale) ZnO films, for example, are of particular interest, due to the device potential involving the quantum confinement effects. In this study, we have investigated the early-stage growth mode of ZnO on sapphire. The evolution of structural, morphological, and electrical properties was characterized with 2 to 20-nm-thick ZnO films grown at 700 oC with radio-frequency magnetron sputtering. X-ray diffraction results show that ZnO initially grows highly strained and epitaxial to substrate with negligible degree of mosaicity for up to ~5 nm thickness, despite the occurrence of partial strain-relaxation which indicates an incommensurate growth involving misfit dislocations. Then the mosaicity (out-of-plane tilt) develops as film thickness increases to around 10 nm. Both the atomic force microscopy (AFM) and resistivity measurement results suggest that ZnO grows as mostly discontinuous (electrically and physically) three-dimensional (3D) nano-islands at 2 to 5 nm thickness, and then the islands coalesce/merge and become connected, fully covering the substrate surface at 5 to 10 nm. The optoelectronic properties of nanometer-thickness films are often dominated by the surface-mediated phenomena due to the large surface/volume ratio. It is well known that ZnO exhibits a strong chemisorption behavior through surface. While this phenomenon could be beneficial to some applications (such as chemical/gas sensing), it would also be desirable to control/alleviate this phenomenon in order to observe the effects originating from the dimensional and size confinement of intrinsic materials. We have investigated an oxygen-plasma treatment as a possible means of modifying/controlling the surface properties of ultra-thin (~20-nm-thick) ZnO epitaxial films. Oxygen plasma treatment is found to dramatically enhance the UV detection properties of ZnO, reducing the decay time constant and increasing the on/off ratio. Thus, for the first time, we have developed and demonstrated high speed, high reponsivity UV photodetectors with extremely low dark current using a single layer of nanometer-thick ZnO.A model, based on modulation mechanism of the conductive volume and carriers, has been developed to explain the power dependence of the UV responsivity of ZnO photodetectors. In this model, the photocurrent decay process is analyzed with oxygen chemisorption and thermionic theory. The results suggest that the plasma treatment reduces the oxygen vacancy concentration at the surface and in the near-surface bulk of ZnO, which in turn reduces the surface band bending and therefore the chemisorption effects. Oxygen plasma treatment is considered an effective way of making nanometer-scale ZnO viable for high performance UV optoelectronic devices. The effects observed in this study are also expected to be observable in other low-dimensional structures of ZnO, such as quantum dots, nano wires and ribbons.