Investigation of single indium phosphide nanowires and cadmium sulfide nanosheets by using photocurrent and transport spectroscopy

In this work we use photocurrent and transport spectroscopes to invenstigate an optoelectronic properties of single InP nanowire and single CdS nanosheet both at room temperature and low temperature. Photolithography is used to fabricate the Ti/Al metal contact pads separated by 3-5 microns on several InP nanowires and CdS nanosheet devices. Back to back metal semiconductor metal (MSM) contacts are modeled based on thermionic emission theory and thermionic field emission theory .These are used to explain the dark I-V characteristics of these nanostructures which enables measurement of the important intrinsic properties including donor density, barrier heights and electrical conductivity of nanostructures. We have used photocurrent spectroscopy of single InP nanowires at room temperature and low temperature to study nanowires having either zincblende(ZB) or wurtzite (WZ) crystal structures. Current-voltage (I-V) photocurrent characteristics of a nanowire are obtained by broad illumination of the device from a Ti-Sapphire laser with energies ranging from 1.30eV to 1.55eV.The photocurrent at a given bias voltage is plotted as a function of photon energy to determine the band edge of given semiconductor nanowire. The photocurrent drops exponentially below the band edge reflecting Urbach's rule. We find that the energy band gap of wurzite (WZ) phase nanowire is about 70meV greater than the zincblende (ZB) phase nanowires at room temperature is in agreement with the recent photoluminescence (PL) measurements. It shows that the photocurrent spectroscopy can clearly distinguish ZB and WZ structures. Since photocurrent measurement needs lot less power of light and can be used even at room temperature, photocurrent spectroscopy tool can become a very cheap and rapid means of characterizing the nanostructures such as nanowires, nanotubes and nanosheets.;At low temperature we observe an exciton like peak in photoresponse of ZB InP nanowire and find the binding energy of exciton is ∼5meV at 10K, in agreement with the recent PL experiments. The scanning photocurrent microscopy (SPCM) of single InP nanowire and single CdS nanosheet shows that the peak photocurrent always appears at the reverse (negative) contact of the MSM nanowire device revealing that the photoresponse has strong spatial inhomogeneity. We present two different charge carrier mechanisms in nanowires in terms of the mobility of the charge carriers and space charge limited current (SCLC). The polarization measurement of single CdS nanosheet shows that the c-axis is almost perpendicular to the long axis of the nanosheet.