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Description
Single-crystal semi-insulating CdTe and Cd₁₋ₓZnₓTe (x=0.1) materials are highly promising for high-efficiency X- and γ-ray detectors operating at room temperature. Utilizing diode-type potential barriers (p-n junctions or Schottky barriers) significantly reduces dark current and noise levels compared to homogeneous detectors, which is critical for photon-counting and spectrometry modes. However, classical diffusion p-n junctions in p-CdTe:Cl are strictly limited by thermal constraints and self-compensation. Laser-induced doping with indium (In) as a donor impurity offers an efficient alternative for barrier formation.
Experimental samples (6x6x0.5 mm) were prepared from semi-insulating p-CdTe:Cl wafers (ρ ≈ 2·10⁹ Ω·cm). After abrasive wire sawing, the damaged surface layers were removed via chemical-mechanical polishing with bromine-containing etching compositions. A thin Au film (0.8 μm, 5x5 mm²) was deposited onto one side. Thermal annealing of the Au/p-CdTe:Cl structure in a pure hydrogen atmosphere (T ≈ 280°C, 8–10 min) ensured an ohmic contact and surface stoichiometrization. A thin In film (0.5 μm, 5x5 mm²) was deposited onto the opposite surface. Modification was performed by irradiating the In film with a single nanosecond pulse from a Nd:YAG laser at an energy density of ≈0.11 J/cm². Subsequently, the remaining In was dissolved in hydrochloric acid, and contact pads (Ø 0.8 mm) with lead wires were deposited at the center. Electrical and photoelectric properties were studied at T ≈ 293 K. During spectral photoconductivity measurements in the photovoltaic mode, the In contact pads were shaded.
The current-voltage (I-V) characteristics of the fabricated structures exhibit a distinct diode-like behavior, with forward bias corresponding to a negative voltage polarity at the In contact. The reverse I-V branch lacks a dark current saturation region and shows smooth pre-breakdown behavior, indicating a negligible diffusion component and a predominant role of generation and surface current components. Under forward bias voltages U ≥ 6 V, the I-V curve linearizes in semi-logarithmic coordinates, confirming an exponential current behavior under high carrier injection into the i-region, consistent with n-i-p junction theory. The spectral photosensitivity under illumination from the shaded In contact side (0.40–0.86 μm) maintains a constant photoresponse sign (negative polarity at the In contact), verifying the presence of a single potential barrier. The maximum saturation open-circuit photovoltage under a 250 W quartz-halogen lamp was 90±5 mV at T ≈ 293 K. Etching the near-surface CdTe:Cl region on the In contact side to a depth of ≈0.12 μm resulted in the complete disappearance of the photovoltaic effect and a fundamental alteration of the I-V characteristics.
The experimental results are explained by a laser-pulse-induced conduction-type inversion in the thin near-surface region of p-CdTe:Cl due to In donor implantation, forming a sharp n-p junction. Based on the data, the parameters of the equilibrium energy band diagram for these CdTe:Cl n-p junctions are discussed.