Terahertz Optoelectronic Properties Of Monolayer Hexagonal Boron Nitride

Hexagonal boron nitride(hBN)has emerged as realm of great interest for advanced electronic and optoelectronic devices due to its distinguishable characteristics such as high chemical and thermal stability,mechanical strength,low dielectric constant and near zero polarization.Whereas,Monolayer(ML)hexagonal boron nitride(hBN)is an important material in the fabrication of deep ultraviolet optoelectronic devices,power devices and van der Waals heterojunctions in combination with other two-dimensional(2D)electronic systems such as graphene and ML MoS2.The optical properties of ML hBN can be analyzed by using Terahertz(THz)time-domain spectroscopy(TDS)technique.THz TDS is a powerful optical technique in characterizing and studying the optoelectronic properties of electronic materials and devices.In this research work optoelectronic and valleytronic properties of ML hBN on different substrates have been analyzed via THz TDS.Appropriate substrate for ML hBN based optoelectronic devices has been proposed and also optical Hall effect is observed without external magnetic field which proves ML hBN a valleytronic material.The comparative study of the basic optoelectronic properties of low resistance ML hBN placed on different substrates such as SiO2/Si,quartz,PET and sapphire has been performed in this work.The measurement is carried out by using terahertz(THz)time-domain spectroscopy(TDS)in temperature regime from 80 K to 280 K.The real and imaginary parts of the optical conductivity were obtained experimentally for low resis-tance ML hBN on different substrates and could be fit well by using the Drude-Smith formula.Thus,the key sample and material parameters(e.g.,the electronic relaxation time or mobility,the carrier density,the electronic localization factor,etc.)of ML hBN have been determined.The effect of temperature on these parameters is also examined and analyzed.The results obtained from this study enable us to suggest the appropriate substrate for ML hBN based electronic and optoelectronic devices.This work is rele-vant to the application of newly developed 2D electronic system as advanced electronic and optoelectronic materials.Furthermore,we demonstrate for the first time,to the best of our knowledge,that the optical Hall effect(OHE)can be observed in p-type monolayer(ML)hexagonal boron nitride(hBN)on fused silica substrate by applying linearly polarized terahertz(THz)irradiation.When ML hBN is placed on fused silica,in which the incident pulsed THz field can create the local and transient electromagnetic(EM)dipoles,the proximity-induced interactions can be presented.The Rashba spin-orbit coupling(SOC)can be enhanced and the in-plane spin component can be induced,along with the lifting of the valley degeneracy.Thus,in the presence of linearly polarized THz radiation,the nonzero transverse optical conductivity(or Hall conductivity)can be observed.We measure the THz transmission through ML hBN/fused silica in the temperature range from 80 to 280 K by using THz time-domain spectroscopy(TDS)in combination with the optical polarization examination.The Faraday ellipticity and rotation angle,together with the complex longitudinal and transverse conductivities,are obtained.The temper-ature dependence of these quantities are examined.The results obtained from this work indicate that ML hBN is a valleytronic material and the proximity-induced interactions can lead to the observation of OHE in the absence of an external magnetic field.Valleytronic properties and key sample parameters of ML hBN on different sub-strates have been examined and analyzed via THz TDS.Temperature dependent study of these characteristics is also investigated.Based on this study,suggestions have been made for the use of ML hBN in electronic and optoelectronic devices.