Abstract
The single crystals were grown from preliminarily synthesized polycrystals by the method of chemical transport reactions in a closed volume with iodine used as a carrier. The influence of doping CdIn2S4 single crystals by gold (3 mol %) on their X-ray dosimetric parameters is studied. It is found that the X-ray sensitivity coefficients of crystals increase 6–8 times compared with undoped CdIn2S4 at effective radiation hardness keV and dose rate R/min. Moreover, the persistence of the crystal characteristics completely disappears and the supple voltage of a roentgen detector decreases threefold. The dependence of the steady X-ray-induced current in on the X-ray dose is described by linear law. The studied crystals have a rather high room-temperature X-ray sensitivity ( (A·min)/(R·V)) and are attractive as materials for X-ray detectors.
1. Introduction
Single crystals of the CdIn2S4 compound belong to the class of wide-band-gap semiconductors [1] with a high specific resistance on the level of Ohm·cm. The band gap of CdIn2S4 thiospinel is indirect, and values between 2.1 and 2.4 eV (between 2.5 and 2.7 eV for direct gap) have been reported by different authors [2]. According to [3, 4], the band gap of CdIn2S4 at room temperature is 2.62 eV. CdIn2S4 is a highly photosensitive semiconductor in the visible range of spectrum and may be used as active material for creation of solar cells and various optoelectronic devices [5–9]. Of interest is also the sensitivity of this material to X-rays. With time, more and more X-ray sensitive materials attract the attention of designers of X-ray detectors. The rather large values of the effective atomic number and the energy gap make CdIn2S4 a suitable material for the fabrication of X-ray detectors, which do not require being cooled. In our previous work [10], we reported the X-ray dosimetric properties of CdIn2S4 single crystals. It was shown that the X-ray sensitivity coefficient of CdIn2S4 is sufficiently high and ranged from 2.4·10−10 to 2.4·10−9 (A·min)/(R·V) at effective radiation hardness –50 keV and dose rate –78.05 R/min. But experimental data demonstrate that the photocurrent-dose curves of CdIn2S4 single crystals have some time lag: after X-rays are turned off, it takes several minutes for the current through CdIn2S4 to reach its dark level. Doping of CdIn2S4 single crystals with metals makes it possible to vary their roentgen dosimetric properties. Earlier, we reported the X-ray dosimetric properties of CdIn2S4 single crystals doped with Cu [11] and Fe [12]. For example, it was shown that the doping of CdIn2S4 single crystals with copper and iron substantially increases their coefficients of X-ray conductivity () and completely removes the inertia of X-ray-ampere characteristics.
It must be noted that in such kind of doped sulfide crystals a cationic disordering plays a principal role [13]. This in turn has a significant effect on the physical properties of these objects.
The aim of this work was to study the effect of doping CdIn2S4 single crystals with gold on their X-ray dosimetric characteristics. Therefore, the fabrication of CdIn2S4Au crystals and the experimental studies of their X-ray conductivity at room temperature became our priority direction.
2. Experiment
The gold-doped (3 mol%) CdIn2S4Au compound was prepared using the method of high-temperature synthesis by alloying high-purity (no lower than 99.999%) constituents in an evacuated quartz ampoule. CdIn2S4Au crystals were grown from synthesized pellets by the chemical transport technique with iodine as a carrier gas. Crystal thus obtained had an octahedral shape with clear-cut faceting and a high optical transparency. X-ray studies showed that CdIn2S4Au crystals have a normal-spinel-like cubic structure and their dark specific conductivity is ·10−7 Ohm−1·cm−1 at K.
Ohmic contacts to CdIn2S4Au samples were made by firing indium into the end faces. The contact spacing, which was exposed to X-ray radiation, was 0.1 cm.
A URS X-ray setup with a BSV-2 tube (Cu radiation) was used as an X-ray source. The X-ray intensity was controlled by varying the current in the tube at each value of the applied accelerating voltage (). The absolute X-ray dose was measured with a DRGZ-O2 X-ray dosimeter. The sample to be examined was placed in a light-tight X-ray chamber. An X-ray-induced change in the current through the sample was detected by U5-9 electrometric amplifier using a low-value load resistor. During the measurements, the effective radiation hardness was –50 keV and interval of dose rate –78.05 R/min. All measurements were taken at K.
3. Experimental Results
The X-ray conductivity coefficient characterizing the X-ray sensitivity of crystals is defined as a relative X-ray-induced change in the conductivity per unit dose rate:where is the conductivity of a crystal subjected to X-ray radiation with dose rate and is the dark conductivity at 300 K.
The X-ray sensitivity coefficient was determined by the formulawhere is the current through the sample subjected to X-ray radiation with dose rate , is the dark current, and is the applied voltage.
Experimental values of X-ray conductivity coefficients obtained for CdIn2S4Au crystals at different values of accelerating potential Va across the tube and dose rates are listed in Table 1. Associated values of for undoped CdIn2S4 single crystal are also given in Table 1 for comparison. The values of for CdIn2S4Au are seen to far exceed for CdIn2S4.
Figure 1 plots X-ray sensitivity coefficient calculated by formula (2) versus the X-ray dose rate for the CdIn2S4Au crystal at K and V. It is seen that X-ray sensitivity of the CdIn2S4Au crystal varies between 2.0·10−9 and 1.5·10−8 (A·min)/(V·R). These values of for CdIn2S4Au exceed by 6–8 times for undoped CdIn2S4 [10]. It must be noted that X-ray sensitivity coefficients of studied CdIn2S4Au crystals exceed also values of for CdIn2S4 single crystals doped with Cu [11] and Fe [12]. These values of for CdIn2S4 single crystals undoped and doped with Fe, Cu, and Au are listed in Table 2 for comparison.
From experimental data (Figure 1), it follows that the dependence for CdIn2S4Au at low dose rates is an increasing function (curve 1, Figure 1). Curves 2–6 first increase with the dose rate and then decrease starting from certain ; at R/min, X-ray sensitivity coefficient becomes almost independent of .
Figure 2 shows the roentgen-ampere characteristics of the CdIn2S4Au crystal at different radiation hardness. With an increase in , the roentgen current through the sample decreases whatever the dose rate is.
Figure 3 plots the roentgen current versus the radiation hardness for CdIn2S4Au at R/min. When rises from 30 to 50 keV, linearly drops.
The roentgen-ampere characteristics of the CdIn2S4 crystal for all values of (except for the initial points) and were almost linear; that is,
Linear dosimetric characteristics are most suitable for practical use.
Figure 4 illustrates dose dependence of resistance of the CdIn2S4Au crystal at various radiation hardness values. Dark resistance of studied sample was equal to 10 MOhm. As it is seen from Figure 4 at all radiation hardness values, the CdIn2S4Au resistance drops when rises from 0.75 to 78.05 R/min. For example, at keV, the value of the CdIn2S4Au resistance decreases from 10 to 2.2 MOhm.
Earlier [10], when studying the X-ray dosimetric characteristics of undoped CdIn2S4 single crystals, we found that when X-ray radiation is switched off, the dark current reaches a steady-state value within 5-6 min rather than at once. Doped CdIn2S4Au crystals compare favorably with undoped ones in that the roentgen current in them does not relax with time. When X-ray radiation is switched off, the dark current is established almost at once. In addition, the supply voltage of a CdIn2S4 X-ray detector is 24 V/cm, while 8 V/cm is sufficient for a CdIn2S4Au detector. The values of supply voltage for CdIn2S4 crystals undoped and doped with Fe, Cu, and Au are listed in Table 2. It must be noted that roentgen dosimetric characteristics of studied CdIn2S4Au single crystals were well reproduced.
4. Conclusions
The gold-doped (3 mol%) CdIn2S4Au compound was prepared by high-temperature synthesis. CdIn2S4Au single crystals were grown from synthesized pellets by the chemical transport technique with iodine as a carrier gas. X-ray studies showed that CdIn2S4Au crystals have a normal-spinel-like cubic structure. Comparative analysis shows that values of X-ray conductivity coefficient () and X-ray sensitivity coefficient () for CdIn2S4Au far exceed and for CdIn2S4. The roentgen current in CdIn2S4Au crystals does not relax with time, and their roentgen-ampere characteristics are linear. Thus, it can be concluded that gold-doped (3 mol%) CdIn2S4 single crystals are highly sensitive to X-rays and can be used for fabrication of low-power fast-response X-ray detectors, which do not require cooling.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.