Influence of oxygen partial pressure on the quality of nanowires for energetic photon detection applications
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Influence of oxygen partial pressure on the quality of nanowires for energetic photon detection applications Y. Zhang1 and S. S. Mao1,2 1 Lawrence Berkeley National Laboratory 2 Department of Mechanical Engineering University of California at Berkeley Berkeley, CA 94720, U.S.A. Email: [email protected] ABSTRACT ZnO based semiconductors have emerged as promising materials for energetic photon detection. Aimed to improve photon detection efficiency, we have successfully grown ZnO nanowires based on vapor- liquid-solid approach. In this paper, we examine the influence of oxygen partial pressure on the quality ZnO nanowires. Optical properties of the nanowires grown under different oxygen environment were characterized, and we found that low oxygen partial pressure can result in large concentration of defect states, as reflected by increased visible emission and an elevated threshold for stimulated emission. INTRODUCTION There has been a significant amount of recent progress in studying direct band gap semiconductor materials (e.g., ZnO) for energetic photon (e.g., EUV and X-ray) detection, e.g., as scintillators1,2. Radiative transitions may arise from band-to-band or exciton recombination. Unlike commonly used ionic scintillators in which transitions are spin forbidden, these semiconductor materials are covalently bonded, so the wave function of the hole is a combination of many ionic wave functions that can effectively mix different quantum numbers, and the transitions are spin/parity allowed. As a consequence, the radiative lifetime may be short, typically a nanosecond or less. In fact, fast luminescence has been observed in un-doped ZnO as well as CuI, HgI 2 and PbI2 . The application of nanostructured semiconductors for energetic photon detection has the potentia l to increase radiative efficiency due to the nature of carrier confinement in the nanometer scale. One-dimensional semiconductor nanowires represents a new family of promising energetic photon detection materials. Various types of semiconductor nanowires, including Si, Ge, GaAs, and GaN have been successfully synthesized3 , and much recent effort has been devoted to ZnO nanowires for photonic applications after the successful demonstration of room temperature ZnO nanowire nanolaser4,5. The band-edge recombination radiation is efficient in ZnO nanowires in the absence of native defects. However, typical ZnO crystals may contain numerous types of defects and impurities that yield nonradiative recombination centers. Consequently, the radiative efficiency and decay time can deviate significantly from a defect-free material.
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Clearly, radiative transition in ZnO nanowires strongly depends on material’s crystalline quality. It is therefore the objective of this paper to investigate the nanowire growth condition, particularly the oxygen partial pressure, on the crystalline quality of ZnO nanowires. EXPERIMENTS AND RESULTS A classical vapor- liquid-solid (VLS) model6 can be applied to explain onedimensional nanowire growth, for which metal catalys
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