Effect of the concentration of impurities: determining the space charge region thickness on detection properties of Cr/CdTe/Au Schottky diode detectors

The electrical and detection properties of the X/γ-ray detectors, fabricated as Cr/CdTe/Au diode structures using semiinsulating p-CdTe single crystals with different concentrations of uncompensated impurities (N ~ 4 × 1010 cm-3, 1 × 1011 cm-3, 2 × 1011 cm-3 and 2 × 1012 cm-3), which determines the width of the space-charge region (SCR), were studied. All the diode-structured detectors were fabricated by the same elaborated technique with vacuum deposition of a Cr electrode (rectifying Schottky contact) and Au electrode (near Ohmic contact) onto the B- and A-face of CdTe(111) crystals pre-treated with Ar-ion bombardment. The dark I-V characteristics of the Cr/CdTe/Au Schottky diodes evidenced steep rectification properties that allowed applying high reverse bias voltage (V = 1-1.5 kV) under moderately low dark currents (I = 3-10 nA at rectifying contact area of 10 mm2 and T = 300 K). The charge transport mechanisms in the Schottky diodes were analyzed and attributed to generation currents in the SCR at V < 100 V, charge transport in the conditions when the SCR width exceeded the crystal thickness at elevated V, and currents limited by space charge at higher V. The as Cr/CdTe/Au Schottky-diode detectors, fabricated using CdTe with N ≤ 5×1010 cm-3, demonstrated the highest detection efficiency and energy resolution (0.6-1.0 %@662 keV). With increasing N, these characteristics deteriorated. Dependences of detection efficiency on N were calculated for different applied bias voltages V.