Effects of Size and Surface Molecules on Photoconductivity of Individual GaN Nanowire

• Main Authors: Fu-Chieh Chang, 張富杰
• Other Authors: Ying-Jay Yang
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/65521500272238436325

碩士 === 國立臺灣大學 === 電子工程學研究所 === 96 === Size effect on the dark- and photo- conductivities (PC) in the individual m-axial GaN nanowires grown by thermal chemical vapor deposition (Thermal CVD) have been studied. The dark conductivity shows a rapid drop from a constant level of 150 Ohm-1cm-1 to 6 Ohm-1cm-1 as diameter decreases from 135 nm via a critical value of 35nm to 20 nm. Similar size dependence is also observed from the PC measurement. The calculated products of carrier lifetime and mobility show a near two orders of magnitude drop at diameters below a consistent critical diameter (dcrt) of 40 nm. These results show that nanowires are fully depleted below dcrt due to the electron capture of surface states. The surface-dominant transport properties of GaN nanowires are further confirmed by the power- environment- and temperature- dependent PC measurements. The effective barrier height induced by surface band-bending has been estimated to be about 97 meV. In addition, an anomalous negative PC has been observed under 325 nm UV illumination from the GaN nanowires with smaller diameters of 10-20 nm grown by low-temperature (LT) metal organic chemical vapor deposition (MOCVD). Positive PC can be also detected by below bandgap excitation using 532 and 808 nm light sources. It is noted that either negative or positive PC are very sensitive to oxygen ambient. The coexistence of surface electron trapping states mediated by foreign oxygen and the recombination center/acceptor level are suggested to explain the variety of PC in the LT-grown GaN nanowires.