Study Of Space Charge Layer And Electrical Properties Of GaN Nanowires

Ga N nanomaterials are used as core components of nano-functional devices;their characteristics of space charge layer udner external electric field and electrical performance are essential for the design and optimization of semiconductor devices.We applied in-situ electron holography(EH),combined with selected-area electron diffraction(SAED),high-resolution TEM(HRTEM)to investigate the depletion layer of the Ga N-Au heterojunction under different bias conditions,roles of strain in the space charge in the single Ga N nanowire and the space charge(SC)distribution near the nanowire.The main points and conclusions are summarized as follows:1.The microstructures of Ga N nanowires were studied systematically.Three kinds of different microstructures of Ga N nanowires were found,including defective-less single crystal nanowires and defective nanowires with fault and twins.The distribution of the SC layer of the Ga N-Au heterojunction under external bias was investigated by in-situ EH.The SC distributions in 0V,positive and negative bias were obtained by analyzing the phase diagram reconstructed from the electron hologram.The maximum and minimum space charge layer width is 91 nm and 68 nm,respectively,which is in good agreement with the theoretical analysis.By analyzing the relationship between the SC layer and the bias voltage,a doping concentration about 4.3×106cm-3 of Ga N nanowires was obtained.2.The effect of strain on the distribution of the SC layer on a single Ga N nanowire were studied by in situ off-axis EH,SAED and HRTEM.Based on the phase image reconstructed from the complex hologram,the width of SC layer in a strained Ga N NW is significantly reduced to about 60 nm,comparing to the 85 nm of the unstrained NW,demonstrating modification of the SC layer in the strained NW at the M-S contact.The first principles calculations showed that the stress reduces the Fermi energy gap between Ga N and Au,and finally reduces the band gap of Ga N nanowires.3.The effect of the strain on the charges distribution at the surface of a Ga N semiconductor nanowire(NW)has been investigated inside TEM by in-situ off-axis EH.The potential distributions induced by charge redistribution in the two vacuum sides of the bent NW have been examined respectively,and the opposite nature of the bounded charges on the outer and inner surfaces of the bent NW was identified.4.The relationship between the I-V curves of Ga N nanowires and the thermal effect,the electric breakdown and the piezoelectric effect was investigated by in situ electrical test inside a TEM.With the increase of measurement time,the current increased gradually,and the I-V curve from the initial asymmetry becomes nearly symmetrical.The pressure is applied to the Ga N nanowires and the current is reduced.When the nanowires are insistently measured with a relative low voltage,they can also be broke down.Pressure is applied to the nanowires,and the current becomes smaller obviously due to the piezoelectric effect.The smaller diameter Ga N nanowires are likely to directly melt after the power supply.The results show that temperature,stress and electric breakdown are the important factors that lead to the change of I-V curve.The effects of defects on the mechanical and electrical properties of Ga N nanowires were studied.