Photoconductivity and electronic transport in III-Nitrides nanowires

• Main Authors: Hsin-Yi Chen, 陳新鎰
• Other Authors: Ying-Jay Yang
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/47002220552644564777

博士 === 臺灣大學 === 電子工程學研究所 === 98 === In this thesis, we have successfully fabricated three different kinds (GaN、InN、AlN) of single nanowire device as phootodetectors. We measured the photocurrent response of nanowires to discuss the photoconductivity and electronic transport of nanowires under the photo excitation at different wavelength. For material growth, the GaN and AlN nanowires were grown by chemical vapor deposition and InN nanowires were grown by metal organic chemical vapor deposition (MOCVD). For device fabrication, e-beam lithography was utilized to make single GaN nanowire device and focus ion-beam technique was applied to fabricate single InN nanowire device. We made use of different experimental parameters to reach ohmic contact between nanowires and contact metal. For GaN nanowires, we first fabricated the GaN bridge structure as photodetector and found the devices have very high photocurrent responsivity. Meanwhile, we discover the calculated photocurrent gain of single GaN nanowire was three to four orders magnitude higher than its bulk or thin film counterparts. It suggests that single GaN nanowire has potential for being high photo-detection efficiency detector. The intensity-dependent gain study showed that the gain value is very sensitive to the excitation intensity following an inverse power law and no gain saturation was observed in this investigated intensity range from 0.75 to 250 W/m2. This behavior strongly suggested a surface-dominant rather than trap-dominant high gain mechanism in this one-dimensional nanostructure. In addition, we measured the photo response and conductivity in different size of nanowires.The result shows an obvious size effect while the nanowire diameter is smaller than a critical diameter of 30-40nm. The critical diameter of m-axial CVD-grown nanowire is much smaller than the reported value of c-axial MBE-grown nanowire (80-100nm). It indicates that we can make use of more m-axial CVD-grown nanowire as high-gain photodetector. We also propose the band diagram of different size to elucidate the sizedependent effect of GaN nanowire. For InN nanowires, we report on the photoconductivity study of the individual infraredabsorbing InN nanowires. Temperature-dependent dark conductivity result indicates the semiconducting transport behavior of these InN nanowires. A measured conductivity of 10 Ω-1cm-1 is much lower than the previous reported value (>1000 Ω-1cm-1). The photo sensitivity of 0.3 and calculated ultrahigh photoconductive gain of around 107 at room temperature are obtained under 808 nm excitation. Furthermore, our studies suggest that the photocurrent in InN NWs is sensitive to the oxygen environment and its PC could be surface dominant and follows a similar mechanism of molecular sensitization. The excitation of electron from surface state created by foreign oxygen molecule could give rise to a similar effect as interband excitation since the lifetime of photoelectron is also determined by the readsorption rate of oxygen. For AlN nanowires, photoconductivity of individual AlN nanowires has been characterized using different subband gap excitation sources. It is interesting that both positive photocurrent under 1.53 and 2.33 eV excitations and negative photocurrent under 3.06 and 3.81 eV excitations are observed from the wide band gap nitride nanowires. The negative photoconductivity, which is attributed to the presence of electron trap and recombination center in the bulk of AlN, is capable to be inversed by a strong positive photoconductive mechanism of surface while changes the ambience from the atmosphere to the vacuum. An oxygen molecular sensitization effect is proposed to explain the enhancement of positive photocurrent and the inversion of negative photoresponse in the vacuum.