Optoelectronic devices with 2D materials

Abstract

This thesis reports various optoelectronic properties of Transition Metal Dichalcogenides (TMDs) such as MoS2, WS2 WSe2 and MoSe2, Photoluminescence (PL) spectra of these materials were investigated in detail using linearly polarized laser beam as the excitation source. The PL spectrum shows few broad peaks and a large number of sharp Raman-peaks, close to the excitation wavelength. It is seen that the peaks close to the excitation wavelength are sensitive to the relative orientation of the plane of polarization of the laser light, wherein the change in the intensity of such peaks is as high as 40% as the plane of polarization is changed from vertical to horizontal. From the analysis of PL spectra, the temperature of the charge carriers in the TMD films has been estimated. Photoconductivity measurements on these materials indicated that MoS2 has the highest electrical conductivity. Hence, the photo-response of MoS2 based heterojunction photo-detectors was investigated extensively. To study the photoresponse from MoS2 devices, two terminal devices were fabricated using nanofabrication techniques. Devices of uniform channel thickness and varying channel thickness (heterojunction) were fabricated. Photocurrent mapping, over the device surface, was done by scanning a laser beam across the device together with the electrical measurements. In devices with uniform channel thickness, the photocurrent is seen to be maximum when the laser beam is incident on the edge of the source electrode (metal) of the device. This suggests injection of hot electrons from metal to MoS2 film thereby generating the photocurrent. Heterojunction devices showed maximum current generation when the laser beam was incident at the junction separating mono-layer and multi-layer zones of the MoS2 film. It is also seen to depend on the magnitude of the bias voltage which may be understood on the basis of band offset at the junction. In this context, various other photocurrent generation mechanisms have been considered to understand the device performance. Heterojunction devices were made by combining a bi-layer or fewlayer or many layer MoS2 along with a mono-layer MoS2. Photo-response measurements on these devices revealed that the speed of photo detection was highest in devices having multi-layers. Response time up to 26ms was obtained for such devices as against a few sec for the device made using mono-layer alone. The mechanism of charge transfer from metal electrode to the MoS2 device was also studied. The amount of charge transfer at the metalMoS2 junction was studied using Kelvin Probe Force Microscopy for gold and nickel metals as electrodes. At higher gate bias, these devices show nearly zero potential barrier, wherein the contact resistance is limited by the air gap between metal and 2D layer. Basedon these investigations a new model has been proposed for the barrier formation at metalTMDs junction and thereby understand the nature of contact resistance. These studies have been exploited for proposing a better structure for photodetectors based on TMDs.