Recently, we reported a simulation of spectroscopic performances of CdTe detectors by a Monte Carlo program ‘ISIDE’. The program was demonstrated to be extremely useful both for an exact simulation of the spectral response of CdTe to γy-rays and for a detailed understanding of the effect of physical and electronic parameters on the experimental performances. In the present work, the attention is focused on the various aspects of CdTe response to 57Co γ-rays, in an energy interval which is extremely interesting for practical applications of CdTe spectroscopy. Spectra are presented for different experimental conditions, particularly as far as electronics set up is concerned, in correlation with the relevant rise time distributions and with spatial distribution of interaction points in the detector. It is proved that in order to account for the rise times distributions as compared with the experimental ones, one must assume the presence of a dead region on the front of the detector. Moreover, a clear indication of what is needed in order to reach the best energy resolution in realistic conditions is given.

We report on electrical and optical properties of vertical Bridgman grown Cl-doped CdTe including the ternary compositions Cd0.9Zn0.1Te and CdTe0 9Se0.1 with respect to application as a radiation detector. Based on Hall effect measurements, photoinduced current spectroscopy (PICTS) and photoluminescence we infer that high resistive material with good performance is controlled by deep level defects. The resistivity is calculated as a function of the shallow acceptor concentration (Cl-A-centers) with the conclusion that a deep donor state at mid gap must be present.

Planar CdTe detectors with different performances for gamma radiation detection have been investigated. The detector internal electric field was analyzed by means of time-of-flight experiments. The correlation of the results with the current-voltage characteristics of the samples, their risetime distribution for different gamma irradiation and, eventually, the shape of detected gamma spectra led us to a detector model based on the theory of space charge limited current.

Using this model, we try to explain operation and failure of trapping loss correction methods for the different detector groups.

A new circuital configuration for the charge amplifier is presented. By means of a double feedback loop, the input field-effect transistor can operate with its gate junction sligtly forward biased, collecting the detector current and discharging the feedback capacitor. The feedback resistor is so avoided and no resetting device or circuit is required for the preamplifier operation. The noise is limited by the input transistor, an equivalent noise charge of 19.5 r.m.s. electrons has been measured at room temperature by employing a commercial JFET.

After reviewing the noise limits in room temperature preamplifiers based on discrete elements, the paper discusses some results obtained with monolithic circuits and the perspectives opened-up by active devices directly integrated on the chip of a silicon detector.

Current-voltage characteristics of high resistivity CdTe, with consideration to Schottky barrier height at the contact and defect levels in the band gap were investigated by experimental and numerical simulation methods. From the electron injection investigation, the density of electron traps corresponded to the doped Cl concentration in the crystal. Numerical simulation of current-voltage characteristics was made based on Schockley Read Hall (SRH) statistics. In the low bias region, calculated characteristics were in fairly close agreement with the experimental results for Pt/CdTe/Pt diode. The electric field was not uniform along the biased direction and an almost neutral region was present in the vicinity of anode contact even when the bias was applied.

Deep levels have a great influence on the recombination behavior of the free carriers in semiconductors. For several years PICTS has been used to investigate the deep levels in high resistivity material such as GaAs or CdTe used in detector applications. An important feature of the PICTS measurements is the analysis of the current transients after pulse excitation. We propose using a new method based on Tikhonov regularization. This method was implemented in the program FTIKREG (Fast Tikhonov Regularization) by one of the authors. The superior resolution of the regularization method in comparison to conventional techniques is shown using simulated data. Moreover, the method is applied to investigate deep levels in CdTe:Cl, SI-GaAs and GaAs:Cr samples used for room temperature radiation detectors. A relation between deep level properties and detector performance is proposed.

Temperature measurements in a crystal growth ampoule of CdTe (Cadmium Telluride) are very difficult. Mathematical simulation is a very powerful tool to predict the behavior and temperature distribution during crystal growth. We applied this technique to the bulk growth of large single crystals of CdTe. The simulation revealed that for CdTe, it is possible to create a convex interface shape during growth, necessary for single crystal growth. The controlling parameter is the temperature profile along the ampoule. The temperature gradient is rather meaningless, as we can show with our simulations.

We will present some crystals grown under good and bad thermal conditions.

Two complementary techniques are used to study the electrical transport properties related to the use of diamonds as materials for ionizing radiation detectors. Transient photoconductivity using soft x-rays is used to probe the first few microns of the material, while ionizing particle-excited conductivity is used to probe the entire bulk of the material (1 millimeter). Both techniques measure the mean drift distance of free carriers, or the collection distance d. In addition, transient photoconductivity is able to extract the lifetimes and mobilities of the excited carriers. The collection distance measured by the two methods are in agreement, suggesting the material is homogeneous. At an applied field of 10 kV/cm, d is 25 to 30 microns, and, up to a field of 25 kV/cm, d has not saturated. The lifetime varies between 100 and 600 ps, and the mobility varies between 1000 and 4000 cm2/V-s, the range due to natural variations from sample to sample. The primary defects limiting the lifetime are believed to be nitrogen impurities and dislocations.

A novel method of implanting dopant material in diamond using low energy ions during growth is described. In this method, relatively low energy (∼10 KeV) dopant ions are directed through an aperture into a hot filament chemical vapor deposition growth chamber and onto the growing diamond sample. Collisions with the gas molecules in the growth chamber (∼10 Torr of a 99.5% H2 - 0.5% CH4 gas mixture) partially neutralize the ion beam and slow the dopant atoms down to a few hundred electron volts before striking the growing diamond crystal. The residual energy is large enough to embed the dopant atoms a few layers deep in the crystal but not large enough to cause significant lattice damage. Continued doping during growth yields uniformly doped material throughout the implanted region. Results for sodium, rubidium, and phosphorus atom doping are presented with sodium found to be a p-type dopant and phosphorus a deep n-type dopant.

The electrical properties associated with carrier mobility, μ, and lifetime, τ, have been investigated for the chemical vapor deposited (CVD) diamond films using charged particle-induced conductivity and time resolved transient photo-induced conductivity. The collection distance, d, the average distance which electron and hole depart when driven by an applied electric field E, was measured by both methods. The collection distance is related to the carrier mobility and lifetime by d = μEτ Our measurements show that the collection distance increases linearly with sample thickness for CVD diamond films. The collection distance at the growth side of the CVD diamond film is comparable to that of single crystal natural type IIa diamond; at the substrate side of the film, the collection distance is near zero. No saturation of the collection distance is observed for film thickness up to 500 microns.

Type IIa diamonds present photoconductive properties as well in X-ray detection range, as in near UV, visible, IR light range, in spite of the large bandgap of the material. Experimental results in these four fields, time measurements in picosecond and subpicosecond ranges are presented.

Raman and photoluminescence spectroscopy was used to investigate radiation damage by 5 MeV He+ ions at room temperature in prototype CVD-diamond detector material. Amorphization of the diamond is not observed at fluences up to 8×1015 cm-2. A threshold behavior is seen in the formation of the H3 vacancy-dinitrogen color center. The defect is not observed for fluences in the range 1.6×1012 to 1.6×1013 cm-2; a linear behavior increase in H3 intensity is observed over the range 1.6×1014 to 1.6×1015 cm-2. The formation mechanism of the H3 color center under He+ irradiation involves a self-annealing effect that allows vacancies to diffuse to, and complex with, nitrogen complexes.

Preliminary results on the response of type Ib and IIa diamond photodetectors to fast laser pulse exposures at 265 and 530 nm are presented. The influence of the applied bias, the laser wavelengths and the light intensity on the detector sensitivity is studied. Also, recent measurements with 1.25 MeV gamma ray pulses are reported.

A method of radiation dosimetry based upon the radiophotoluminescence (RPL) of synthetic diamond crystals is presented. When the RPL is generated by a stimulation wavelength of 296 nm a linear response vs radiation dose of up to 20 kGy can be obtained with selected crystals. The response is very dependent on the chemistry of the synthesized specimen.

Synthetic diamonds with a nitrogen content less than 100ppm may be used as radiation dosimeters in a conduction counting mode, and are especially useful in medical applications. Crystal imperfections, revealed by X-ray diffraction topography, were found to affect counting performance. The best quality diamond gave the highest photocurrent (500nA at 50 V mm−1 and 2.75 Gy min−l). Diamonds containing dislocations had lower photocurrents but had the advantage of shorter settling times (seconds rather than minutes). Placing contacts on two opposite cube {100} faces gave a higher photocurrent than on a pair of octahedral {111} faces. Higher photocurrents were also achieved when the majority of dislocations were perpendicular rather than parallel to the electric field. Some recommendations for selecting synthetic diamonds for dosimeters are given.

Single-crystal type IIa diamonds were synthesized by applying high temperatures (1200°;C) and high pressures (52000 atm) to powdered polycrystalline diamond grown using conventional CVD techniques. These samples were isotopically pure, consisting of approximately 99.93% 12C, compared to roughly 98.96% in natural IIa diamonds. In addition, the dislocation density of the synthetic samples is significantly lower than in natural IIa diamonds, as indicated by birefringence measurements. Electrical properties of these samples were measured using transient photoconductivity, where a 3 ps pulse of ultraviolet light (6.1 eV) was used to excite free electron-hole pairs. Compared to natural IIa diamonds, the lifetime at low fields (200 V/cm) and low excitation densities (1015 cm-3) in the synthetic sample was significantly longer (1 ns in the synthetic sample vs. 200 to 300 ps in natural diamond). At higher fields, a much longer decay component, exceeding 10 ns, was observed in the synthetic sample. Combined electron and hole mobilities were around 2500 cm2/V-s in the synthetic diamond, compared to 3000 to 4000 cm2/V-s in the best natural samples. At a field of 2 kV/cm, the drift distance in the synthetic sample was over 50 μm, considerably longer than that of natural diamond (10 μm). This is due primarily to the much longer carrier lifetimes. The longer lifetimes in the synthetic sample demonstrate that the properties of the best natural diamonds can be exceeded and are encouraging for the development of sensitive diamond radiation detectors. These longer lifetimes are likely due to the higher quality and lower defect density in the synthetic samples, rather than the isotopic purity.

The sub-nanosecond electrical transients induced by 5-MeV He+ and 10-MeV Si3+ ions have been measured in single-crystal, natural type Ila diamonds. The detectors were fabricated into conductivity modulated devices and were incorporated into 50-Ω high bandwidth transmission line structures. The electrical signals were recorded with a system based on a 70 GHz random sampling oscilloscope with the total recording rise time of 18.6 ±:0.6 ps.

Signal rise times are less than 70 ps and fall times are less than 200 ps for electric fields in the range 3.8x104 - 1×105 V/cm. The plasma time appears to play a key role in defining the initial stages of the charge transport because signal rise times are much greater than the recording system rise time, especially with the Si-ion excitation. Furthermore, incomplete charge collection is quite severe even at the highest applied electric fields due to the dominance of carrier trapping/recombination at the defect sites.

Band-edge photoresponse measurements on simple diamond metal-semiconductor-metal (MSM) devices can give information about the quality of the diamond material and its utility as a detector. We have fabricated several such devices on a variety of substrates and measured their response between 700 and 120nm with special emphasis around the band edge near 220nm. Steady state and transient response have been measured as a function of bias conditions. Transient response in this case refers to initial overshoot, undershoot, and increased dark current in response to a chopped vacuum ultraviolet (VUV) light signal for times on the order of seconds. We compare steady state quantum efficiency results to a simple model, discuss the characteristics of response versus applied voltage, and examine relationships between response and substrate properties.

In many instances, the ability of scientists and engineers to make nuclear radiation measurements is only limited by the properties of available radiation detectors. Because of this, research into new semiconductor materials for radiation detection has been, and continues to be, a very active field. This article reviews recent research on several promising “new” materials.