Experimental study of negative photoconductivity inn-PbTe(Ga) epitaxial films

Abstract

We report on low-temperature photoconductivity ͑PC͒ in n-PbTe(Ga) epitaxial films prepared by the hotwall technique on ͗111͘-BaF 2 substrates. Variation of the substrate temperature allowed us to change the resistivity of the films from 10 8 down to 10 Ϫ2 ⍀ cm at 4.2 K. The resistivity reduction is associated with a slight excess of Ga concentration, disturbing the Fermi level pinning within the energy gap of n-PbTe(Ga). PC has been measured under continuous and pulse illumination in the temperature range 4.2-300 K. For films of low resistivity, the photoresponse is composed of negative and positive parts. Recombination processes for both effects are characterized by nonexponential kinetics depending on the illumination pulse duration and intensity. Analysis of the PC transient proves that the negative photoconductivity cannot be explained in terms of nonequilibrium charge carriers spatial separation of due to band modulation. Experimental results are interpreted assuming the mixed valence of Ga in lead telluride and the formation of centers with a negative correlation energy. Specifics of the PC process is determined by the energy levels attributed to donor Ga III , acceptor Ga I , and neutral Ga II states with respect to the crystal surrounding. The energy level corresponding to the metastable state Ga II is supposed to occur above the conduction band bottom, providing fast recombination rates for the negative PC. The superposition of negative and positive PC's is considered to be dependent on the ratio of the densities of states corresponding to the donor and acceptor impurity centers.

Anomalous photoconductivity in gamma In2Se3

“…This type of anomalous behavior was observed in several other materials such as GaAs, 6,7 Au:Ge alloy, 8,9 cobalt doped Si, 10 Cd 1−x Fe x Se, 11 n-PbTe, 12 and in ␣-Se. It may be due to the reduction of mobile carriers, ͑electrons in n type͒ in the conduction band and simultaneously destruction of minority carriers in valance band leading to the decrease of photoconductivity, lower than the dark conductivity.…”

Anomalous Photoelectric Effect of a Polycrystalline Topological Insulator Film

“…However, an instantaneous positive resistance effect cannot be observed due to the long relaxation times, especially during the recovery process after the illumination is removed, because an extremely long-lived tail is common in materials that demonstrate a heat-induced negative photocurrent. Moreover, there is no saturated, steady photocurrent state that arises with increasing illumination time2930. By contrast, the photoresponse of our prepared Bi 2 Te 3 film is quite different: after an initial rapid decline, a steady-state photocurrent is quickly attained, and a nearly symmetrical relaxation to the dark conduction state follows once the illumination is removed.…”

Graphene‐Skeleton Heat‐Coordinated and Nanoamorphous‐Surface‐State Controlled Pseudo‐Negative‐Photoconductivity of Tiny SnO2 Nanoparticles

“…Then, the optical–electric conversion of the composite was studied and a graphene‐improved PsdNPC effect was observed. This PsdNPC is different from the ordinary NPC with small η ( η is defined as η = I dark / I light, usually 1 < η < 2) . In this investigation, the new NPC effect due to light heating has a large η , and can serve in optoelectronic devices by combining with the light exciting effect, as shown in the schematic in Figure a.…”