Electric Instability in n-CdTe:In Layers with S-Shaped Voltage-Current Characteristics

Abstract

A switching of the S-type in the 20-200 µm thick polycrystalline n-CdTe:In layers with resistance of 10 3 -10 6 Ω·cm is studied. The electric instability in the layers is found to be due to the electron-thermal breakdown mechanism. The dependence of the switching threshold parameters on the intensity of exposure can be used for fabrication of infrared-radiation controlled electric switches on the basis of n-CdTe:In layers.А 2 В 6 semiconductor compounds find application in production of solar cells, radiation detectors, IR-devices, etc. To extend the field of application, new principles of operation of these devices should be found. As known [1][2][3][4], the CdTe bulk crystals and thin films can switch on from a high-resistance state to a low-resistance one, that is, they have S-shaped voltage-current characteristics (VCC). However, the switching mechanism in GdTe is not completely understood.In this work, the electric switching of the S-type in the 20-200 µm thick polycrystalline n-CdTe:In layers with resistance of 10 3 -10 6 Ω·cm is studied. The layers were produced on glass substrates by a vacuum deposition technique in the quasi-closed volume and doped with donors using co-evaporation of an indium dopant.The indium films deposited on free surfaces of the layers with the gap of 1 mm by a vacuum condensation technique served as non-rectifying contacts to the n-CdTe:In layers. The voltage-current characteristics of the In-n-CdTe:In-In samples were studied in vacuum at room temperature in a static regime. The switching kinetics was studied using a square-wave generator and an oscillograph. Use was made of the pulses with duration 0.5-65 µs, frequency 7 Hz -1 kHz, and amplitude 200 V.All In-n-CdTe:In-In samples under study have symmetric voltage-current characteristics where the second turning point of the transition to the positive differential resistance is absent in the range of high currents. The switching to the lowresistance state occurs by a jump. The switching time is about 10 -3 s. Figure 1 shows that each subsequent switching of the S-type in In-n-CdTe:In-In samples occurs at lower voltages (curves 1, 2). The transition to a steady state is observed upon 4-5 switching cycles (curve 3). In a steady state, the threshold switching parameters (voltage U p and current I p ) remain unaffected from cycle to cycle. The reverse trend of the voltage-current characteristic mismatches the direct one (cf. curves 3 and 4).The voltage-current characteristics of In-n-CdTe:In-In samples measured at U < 15-35 V are linear. With further increase in U, this dependence becomes superlinear with the following transformation to the exponential form I = U/R 0 ·exp[U/U 0 ], which remains unchanged up to switching voltage. Here R 0 и U 0 are the parameters having dimension of the resistance and voltage, respectively.The absence of the second turning point in the voltage-current characteristic in the range of high currents is typical of a thermal mechanism of electric instability [5]. The thermal mechanism of switching of the S-ty...