Fig. 2
Voltage-controlled d11-SHG signal and mechanism in CdS nanobelt. a d11-SHG signal as a function of applied voltage under three excitation wavelengths, 1018, 1028, and 1043 nm. Each curve includes three different regions, VDS < Vt, Vt < VDS < Vs and VDS > Vs. Here Vt is the turn-on voltage or threshold voltage while Vs represents the saturation voltage where SHG signal begins to saturate. The d11-SHG response when Vt < VDS < Vs is fitted by using equation 1 and Supplementary Equation 19 in Supplementary Note 4. b I–V characteristics of the device in the dark and under excitation at 1018 and 1043 nm. Vc stands for the saturation current–voltage. c Schematic of the band structure with donors (ND) and deep deep-level acceptors (NA). Electrons (represented by blue dots) can be excited from the valence band to the accepter traps via a strong field (i.e., field-induced ionization of acceptor traps), to generate holes (represented by red dots), which can then recombine with electrons in the conduction band and also take part in conduction. γ is the transition rate driven by the electric field. d Schematic of the electric field profile using the Schottky high-field domain model. Fc represents the critical field at which field-induced ionization takes place efficiently when the applied bias VDS equals the critical bias voltage, Vc, \(h = qN_{\rm D}/(\varepsilon _{\rm r}\varepsilon _0)\) and \(x_{\rm d} = F_{\rm c}/h\). Please note that this is a simplified model by assuming the net space charge density (ρ) is changed from qND to be zero at the critical electric field
