Effect of the Surface Roughness on the Impedance Spectrum of a Tellurium Electrode

Abstract

At present, tellurium as a semiconductor is widely used in the radiation and infrared detectors; in structures with unipolar (hole) conduction; and in many practically important binary, ternary, and zero-gap semiconductors [1][2][3][4]. Physical properties of tellurium are completely determined by its defect structure. The density of bulk defects in tellurium can reach ~ 10 18 cm -3 . They are of an acceptor nature, and tellurium remains a semiconductor with unipolar (hole) conduction no matter the dopants [5][6][7].Tellurium is of interest also from a chemical viewpoint. It exhibits metallic and nonmetallic properties almost to the same extent. Its chemical and electrochemical properties are well studied [8][9][10][11][12][13][14], whereas information about its interface properties is scanty.Only several studies concerning properties of the electrical double layer (EDL) of tellurium in contact with electrolytes were published [15][16][17][18]. Certainly, this indicates that the tellurium behavior in electrochemical processes is complex and difficult to investigate. Measurements of impedance Z , customarily used to investigate electrochemical processes, in these cases show Z to depend on the working frequency ( ω = 2 π f ). The presence in an electrochemical circuit of a constant phase element (CPE), which involves the relaxation processes in EDL, including processes in the space charge region on the semiconductor surface; adsorption processes; and the surface roughness are considered possible reasons for the Z ( f ) dependence [19][20][21][22]. It should be noted that CPE is a universal method for simulating impedance of a wide class of electrochemical systems. Formally, the CPE impedance is related to the frequency as follows:, where A is a proportionality factor, j is the imaginary unit, and n (0 < n ≤ 1) is determined by the fractal dimension of the surface.Possible roughness of an interface is a factor that frequently makes an analysis of impedance data more difficult. Therefore, here the effect of the tellurium electrode surface treatment on the spectrum of the linear impedance is studied.All impedance measurements were done on specially selected tellurium single crystals grown by the Czochralski method and subjected to repeated zone refining [6]. The initial specimens were rectangular chips of average size 15 by 4 by 4 mm cut out of a single-crystal ingot. Firstly, using rough grinding, a specimen was made cylindrical, which is important for impedance measurements. As a result, macroroughness appeared on the tellurium surface, because tellurium is very plastic material (during mechanical treatment, its structure is readily disturbed, leading to a high density of dislocations and recombination centers [6,23,24]). Then, tellurium was polished to mirror finish. The specimen prepared in this manner (cylindrical electrode) was inserted in a polytetrafluoroethylene holder. A low-resistance ohmic contact for it was prepared by fusing platinum wire to the specimen's end face insulated from solution. After successi...