X-ray escape peak variations in diodes made from doubly travelling solvent grown p-type CdTe

Abstract

2014 The dependence of the escape peak height on the applied diode voltage was measured at diodes made from doubly travelling solvent grown CdTe. The crystal was In-doped with a concentration of 2.1 1016 cm-3 and p-type with a resistivity of 4 x 106 03A9.cm. The escape peak height saturates at higher voltages. The theoretical dependence of the X-ray escape probability on space charge layer depth was derived. A method for evaluating the experimental curve according to the thoeretical correlation was developped : it yields the actual space charge layer depth and the space charge density. REVUE DE PHYSIQUE APPLIQUEE TOME 12, FEVRIER 1977, PAGE The X-ray escape peak is a well-known phenomenon in CdTe gamma detectors ; it is much more pronounced than, for instance, in silicon. The reason is the marked increase in the X-ray fluorescence coefficient co with the atomic number Z of the absorbing element. For CdTe whose average Z is 50, OJK amounts to about 0.85, for Si it is 0.04, for Ge and GaAs 0.54 (Fig. 1). FIG. 1. The K-shell X-ray fluorescence yield as a function of the atomic number Z (from Bambynek, [4]). The process responsible for the occurrence of the escape peak in the gamma spectra of a CdTe detector is described (Fig. 2) in the following for a detector structure with a space charge layer : in each energy transfer from the gamma quantum to a K-shell electron, a K-X-ray quantum is formed simultaneous to the photoelectron. As shown in the diagram at the right-hand side of figure 2 the absorption length curves of photoelectrons and photons are rather different. The K-X-rays generated have larger absorption lengths than the electrons and hence will escape from the space charge layer to the outside or to the field-free bulk region with higher probability than the photoelectrons. The result is the well-known gamma spectrum with the full energy photopeak and when going to lower energies the escape peak at (E1 -. EK) and finally the K-X-ray line. This X-ray peak is either due to X-ray fluorescence quanta coming from the bulk CdTe and absorbed in the space charge layer or due to events of electron escape from the space charge layer at simultaneous absorption of the X-ray quantum arising from the same y-quantum. The relative peak heights in such spectra are changed when the space charge depth is modified by reverse voltage variations. Figure 3 shows two spectra taken at the lowest (5 V) and the highest voltage (200 V) with an In-doped doubly travelling solvent grown CdTe crystal. This material was grown with zone temperature of about 800 °C at a speed of 4.3 mm/d. In was added to the Te-zone prior to the second growth in form of a Cd-0.5 0/00 In-alloy resulting in a 3.5 x 101’ cm-3 concentration of In in Te. According to Zanio [8], who gives a segregation coefficient for 880 °C growth Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01977001202029300