氮化鎵奈米線的光電導之尺寸及表面分子效應

Abstract

本論文主要探討以熱化學氣相沉積法所成長之m軸單一氮化鎵奈米線的暗電導及光電導之尺寸效應。當直徑從135 nm經臨限直徑35nm至20 nm，氮化鎵奈米線之暗電導有著明顯的下降，其值從150 Ohm-1cm-1降至6 Ohm-1cm-1;其光電導也觀察到類似的尺寸效應，當直徑小於臨限值40 nm時，載子生命週期與遷移率之乘積會下降約兩個數量級。這些結果顯示出當直徑小於臨限直徑時，氮化鎵奈米線會因表面態補捉電子而完全空乏。而從調節激發光源功率、環境及溫度之光電導實驗使我們更深入了解由表面所主導之電傳導特性。因表面能帶彎曲所造成之位障由實驗結果估計約97 meV。 另外，以低溫有機金屬化學氣相沉積法所成長，直徑範圍為10-20 nm之氮化鎵奈米線在325 nm UV雷射激發下顯示出異常的負光導，而使用低於能隙之532 及 808 nm雷射激發時則呈現正光導;值得注意的是不論是正光導還是負光導都對周圍的氧氣相當敏感。最後我們提出與周圍氧氣有關之表面電子補捉態及復合中心/受體級來解釋低溫成長之氮化鎵奈米線其光電導之變化。

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.