GaN and ZnO are promising semiconductor materials that exhibit many outstanding physical and chemical properties. Recently, one-dimensional nanowires (NWs) of these materials have also received much attention due to their significant potential for optoelectronic nano-devices. To this end, many experimental and theoretical studies have been carried out to clarify structural and electronic properties of GaN and ZnO NWs: Transmission electron microscopy (TEM) observations reveal that they grow along the [0001] direction and photoluminescence (PL) measurements indicate a blue-shift in the light emission with deceasing diameter of the nanowire. [1] Further, theoretical calculations have revealed that they always take the wurtzite structure while other compound semiconductor NWs exhibit polytypes. [2] Besides these findings, clarifying electronic and optical properties for small-sized NWs are still unclear. Here, we investigate electronic structures and optical properties of GaN and ZnO [0001] NWs using the highly precise full-potential linearized augmented plane wave (FLAPW) method. [3] We consider both unpassivated and passivated NWs in order to clarify effects of surface electronic states on the electronic structures. Our results demonstrate that the band gap energy of both the unpassivated and passivated NWs (~3.4 and ~2.4 eV for GaN and ZnO, respectively) becomes large compared with the calculated bulk energy gap (1.7 and 0.7 eV for GaN and ZnO, respectively) due to quantum confinement effects, although surface states crucially affect the electronic structure. Moreover, the calculated imaginary part of their dielectric functions exhibit strong anisotropy and there are several side peaks near the absorption edge caused by valence electronic states around the highest-occupied band involved in the large dipole matrix elements. These results thus provide a firm theoretical framework to predict microscopic properties of semiconductor NWs. Work supported by the NSF through the Materials Research Center of Northwestern University.
References: [1]H. T. Ng et al., Appl. Phys. Lett. 82, 2023 (2003).[2]T. Akiyama et al., Jpn. J. Appl. Phys. 45, L275 (2006).[3]E. Wimmeret, H. Krakauer, M. Weinert and A.J. Freeman, Phys Rev B 24, 864 (1981).
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