Efficiency spectrum of a CdTe X‐ and γ‐ray detector with a Schottky diode

Abstract

A study on the efficiency spectrum of CdTe X-and γ-ray detectors utilizing Schottky diodes for different material parameters and diode structures is reported. Special attention is paid to the effect of deep levels and compensation on space-charge region width. It is shown that charge collection in the neutral region of the detector considerably contributes to the device efficiency. Calculations also show that the efficiency of a stacked detector can be higher than that of a bulk detector with ohmic contacts.1 Introduction The effective operation of X-and γ-ray detectors in a spectrometric mode implies full collection of charge generated by the absorbed high-energy photon. It is known that the carrier lifetime in the purest and most perfect CdTe crystals does not exceed several microseconds. Therefore, in order to prevent capture or recombination of carriers, a bias voltage 100-400 V has to be applied to a several millimeters thick semiconductor chip. The high bias voltage imposes another critical requirement to the detector material: the conductivity of the CdTe crystal should be semi-intrinsic, otherwise a considerable dark current will deteriorate the energy resolution of the device.In the late 1990s, the previously proposed idea of using a diode structure in X-and γ-ray detectors [1-4] has undergone further elaboration. The results presented in a number of publications by T. Takahashi et al. [5][6][7] indicated a considerable improvement of a CdTe detector characteristic as a result of employment of an In/p-CdTe Schottky diode. This achievement allowed for manufacturing of high-energy photon CdTe detectors without implementation of any electronic schemes for signal processing [8]. Nevertheless, a number of questions on the Schottky barrier role and material requirements still remain unclarified. The present paper analyzes spectral response of a CdTe detector utilizing a Schottky diode.

Concentration of uncompensated impurities as a key parameter of CdTe and CdZnTe crystals for Schottky diode x\ssty{/}γ-ray detectors

“…If we ignore the surface recombination at the back crystal surface, the expression for the diffusion component of detection efficiency of a Schottky diode due to the photogeneration of electron-hole pairs outside the SCR can be simplified in comparison with equation (10). In this case, the solution of the continuity equation leads to the following expression for the diffusion component of the detection efficiency [21]:…”

CdTe X/γ-ray Detectors with Different Contact Materials

“…An additional confirmation of this assumption is provided by the comparison of the experimental I-V characteristics with the calculation results obtained in the framework of the Sah-Noyce-Shockley theory of generation-recombination of carriers, adapted to a metal-semiconductor (Schottky) contact (Figure 5). According to the theory [41], the generation current I g can be found by integration of the generation rate U(x) throughout the entire SCR as shown in [17,[21][22][23][24][25][42][43][44][45][46]:…”

“…An additional confirmation of this assumption is provided by the comparison of the experimental I-V characteristics with the calculation results obtained in the framework of the Sah–Noyce–Shockley theory of generation–recombination of carriers, adapted to a metal-semiconductor (Schottky) contact ( Figure 5 ). According to the theory [ 41 ], the generation current I g can be found by integration of the generation rate U ( x ) throughout the entire SCR as shown in [ 17 , 21 , 22 , 23 , 24 , 25 , 42 , 43 , 44 , 45 , 46 ]: where A is the diode area, q is the electron charge, W is the width of the SCR, n i = ( N c N v ) 1/2 exp(− E g /2 kT ) is the intrinsic carrier concentration, N c = 2( m n kT /2π ħ 2 ) 3/2 and N v = 2( m p kT /2π ħ 2 ) 3/2 are the effective state densities in the conduction and valence bands, respectively, then m n and m p are the effective masses of electrons and holes, respectively, τ p0 and τ n0 are the effective lifetimes of holes and electrons in the SCR, x is the coordinate where an electron–hole pair is generated, and k is the Boltzmann constant. The values n 1 and p 1 are equal to the equilibrium concentrations of electrons and holes, respectively, under the condition that the Fermi level in the semiconductor coincides with the generation–recombination level with the ionization energy E t (calculated from the top of the valence band).…”

Mechanisms of Charge Transport and Photoelectric Conversion in CdTe-Based X- and Gamma-Ray Detectors

“…The total detection efficiency for a detector utilizing a Schottky diode is the sum of the drift and diffusion components [27,28]. As shown in Figure 7a, the contribution of the diffusion component to the total efficiency of the detector is quite important at τ p = 10 −6 s and in the case of high-energy photons (i.e., at lower absorption coefficients) it is dominant.…”

“…The use of a stacked CdTe detector with a Schottky diode, which is already practiced, can significantly improve the detecting efficiency of the device especially in the high-energy range of the spectrum. In the energy range ~100 keV the efficiency of a stacked detector is greater than that of a single layer detector [28]. The highly developed technology of the deposition of polycrystalline CdTe layers of large area with a surface-barrier structure in solar cells can be adapted to the fabrication of flat-panel X-ray image detectors.…”