More than 50 CZT detector samples were tested during this work. The aim of this characteri-zation was the study of the quality of the material grown at IMEM-CNR institute. During the PhD work several ingots were grown and characterized. Some of the measurements that will be shown in the following pages were made in collaboration with PhD Laura Marchini; my work follows her work on CZT detector characterizations.
5.7.1 Current-Voltage characteristic
The first characterization for detectors is the I-V characteristic. This because, as discussed in the previous chapter, the resistivity of the material, the quality of the contacts and surfaces can be obtained at once from a simple I-V measurement. Some examples of typical IV characteristics for planar detector with gold electroless contacts are shown in fig.5.7 (a) and fig. 5.8. As shown in these figures, gold electroless contacts on CZT grown at IMEM-CNR institute have a double diode back-to-back behaviour.
In fig.5.7 the low-voltage linear IV characteristics and the linear fits are shown. In table 5.1 the resistivity of several samples is shown. For different ingots is possible to note a small difference in the value of the resistivity which is, however, always in the order of 1010⌦cm(i.e. the typical value reported also in literature for CZT material).
The I-V characteristic is also important for the spectroscopic performance of the detector, because small dark current produces less noise inside the system and a better energy resolution can be achieved.
5.7.2 Photo-Induced Current Transient Spectroscopy
Photo-Induced Current Transient Spectroscopy (PICTS) was performed on the boron-encapsulated CZT grown at IMEM- CNR institute in order to investigate the electronic levels in the forbidden gap. These measurements were carried out at University of Bologna by prof.ssa Anna Cavallini.
PICTS technique can identify deep levels, against strong background signals, and it is a strong
Figure 5.7: a)I-V characteristic of CZT samples of the ingot 042. b) and c)Low voltage linear I-V characteristic and the linear fit.
Figure 5.8: a) I-V characteristic of several samples of ingot 040 with gold electroless contacts.
b)I-V characteristic of two samples with gold electroless contacts.
technique for characterizing the CZT material. In particular a comparison between “standard CZT” grown with standard Vertical Bridgman (VB) and CZT growth with Encapsulated Vertical Bridgman (EVB) was carried out. The PICTS spectra of these different materials are shown in figure 5.9 (a).
From comparison of PICTS spectra it is possible to note that the response of the two materials is quite different. Each peak ( Z1 to Z6 ) in the spectra reported in fig.5.9 (a) corresponds to a type of level inside the band gap. From these spectra the type and the concentration of these levels can be extrapolated.
The most evident result is that in EVB detectors less type of levels can be found and the density of traps emitting in the range [110–320] K is lower, as shown in the histogram of fig. 5.9 (b).
This measurement confirm that with the Boron Encapsulated Vertical Bridgman technique ma-terial with low types of level in lower concentration (in comparison to the standard Vertical Bridgman method) can be grown. These concentrations of traps are also in agreement with the values that can be found in literature[48]. This measurement confirms that with the Boron
Sample Thickness (mm) Resistivity (Wcm) Sample Thickness (mm) Resistivity (Wcm)
035-01-a 2.66 2.5·1010⌦cm 040-01-d 2.6 2.2·1010⌦cm
035-01-b 2.68 2.5·1010⌦cm 040-01-e 2.6 2.46·1010⌦cm
035-02-e 2.62 6.5·1010⌦cm 040-01-a 2.6 2.48·1010⌦cm
035-02-d 2.64 2.06·1010⌦cm 040-01-b 2.6 1.91·1010⌦cm
036-04-a 1.04 4.2·1010⌦cm 040-01-c 2.6 2.01·1010⌦cm
036-04-b 1.01 1.72·1010⌦cm 042-02-b 2.5 2.83·1010⌦cm
036-05-b 2.14 3.19·1010⌦cm 042-01-b 5 2·1010⌦cm
036-01b-a 2.15 3.92·1010⌦cm 042-01-c 5 2.32·1010⌦cm
Table 5.1: Resistivity of some CZT samples measured during this work.
Figure 5.9: a)Typical PICTS spectra of VB (red line) and EVB (dotted blue line) detectors.
b)Histogram of the density of the traps in VB (red) and EVB (blu) detectors[47].
Encapsulated Vertical Bridgman technique provides a material with a lower number and lower concentration of levels than Vertical Bridgman method. The revealed concentrations of traps are in agreement with the values that can be found in literature. Furthermore, these results also indicate that most of the boron atoms that contaminate the CZT grown with EVB method, are however electrically inactive and do not affect the transport properties of the material.
5.7.3 CZT Spectroscopy
The detector performance is usually qualified by the photopeak resolution and the peak to valley ratio of the photopeak. These quantities depend on several experimental parameters that can be set during the measurement (as discussed in the spectroscopy setup section).
In CZT detectors the resistivity is higher with respect to CdTe and hence the standard bias applied to the device is higher with respect to the one used for CdTe detectors. 100 V/mm are commonly accepted as the minimum bias to have a sufficient electric field to drift charge carriers (when the CCE is close to 100% ). During the testing of IMEM devices several param-eters were studied in order to obtain the best resolution in the acquired spectrum. The optimal shaping time of the amplifier and voltage were deeply analysed for several of the studied sam-ples. In general the behaviour of IMEM samples was found to be very reproducible. A strong
homogeneity of characteristics was found among the different ingots and in particular among the samples cut from the same ingot. The acquisition method was set up in order to have the best reproducibility in the measurement. The spectra were all acquired without the collimation of the sources and by keeping constant the distance between the source and the measuring box.
The spectra were acquired one hour after the biasing, so that sample internal electronic con-figuration was stable after a relatively long stay in the dark and under polarization condition.
The first test discriminates the samples with better qualities: if the241Amphotopeak (59 KeV) was resolved at 100 V per mm, further investigations were done to find the optimal configura-tion. The samples were biased at different voltages and with different sources; then the optimal bias is chosen by looking at the FWHM of the characteristics photopeak of every radiation source.
Figure 5.10: Response of sample 040-01-a with the 241Am source at different biases.
In fig.5.10 it is possible to see that the photopeak position changes with the applied bias due to the increase in the charge collection. On the contrary, the increase of bias can lead also to an increase of dark current and a compromise must be find in order to achieve the best energy resolution and the highest CCE (peak position close to the theoretical position in energy). In fig.5.11 the response spectra of two samples with 241Am source are shown. These spectra were taken in the best condition and the FWHM and the resolution can be calculated from them. In particular an energy resolution of 6% for the 040-01-f and 6.2% for 040-01-a can be extracted.
Moreover, for both spectra also a large number of peaks are solved in the lower energy range. In fig.5.12 several emission peak of the241Am are solved and labelled. Also the two escape peaks of Cd and Te are clear visible.
Figure 5.11: The spectral response of two samples with the241Am source in PTF configuration.
Figure 5.12: Response of the 040-01-a sample with labelled peaks.
Figure 5.13: Response of the sample 033-07-b with the 57Co source.
For simple planar detector, upon increasing of the gamma photon energy, the effect of hole trapping starts to be dominant and detector resolution decreases. In figure 5.13 the response of the sample 033-07-b detector in PTF configuration with 57Co is shown. In this spectrum only three main peaks are solved by the detector and the energy resolution of 9% at 122 KeV can be calculated.
Figure 5.14: Response of the 033-07-b detector with 109Cd source.
In fig.5.14 the response of the detector 033-07-b with 109Cd is shown, in particular on the right figure the 88 KeV photopeak is shown with 8.4% energy resolution.
In 57Co and 109Cd spectra the effect of trapping is clearly visible in the lower energy part of the photopeak. Increasing the energy of the incident photons the effect of trapping starts to be dominant and at 660 KeV, for example, the photopeak cannot be solved by the detector.
For the material characterization point of view this means that in the CZT material grown at IMEM-CNR institute the transport properties of holes are very low and the quantity µh⌧hE is always smaller than detector thickness. Hole transport properties, indeed, cannot be measured by standard technique in CZT grown at IMEM-CNR institute because of the strong trapping. On the contrary, electrons transport properties are good and in agreement with the values reported in literature. Single charge devices (such small pixel, stripes and coplanar grid) must be fabricated in order to increase the energy resolution of of IMEM-CNR CZT detectors.