Single crystal CVD (scCVD) diamond is an attractive material for particle detection in high energy physics for its good time resolution and reported outstanding radiation tolerance. In addition to ...direct signal loss via charge carrier trapping, polarization effect, caused by non-homogeneous filling of trap defects, is a known cause of signal degradation in irradiated scCVD diamond. This phenomenon was studied by intentionally polarizing irradiated diamonds. Even the relatively lightly irradiated (1014 protons/cm2) diamonds exhibited strong enough polarization to collapse the electric field with moderate rate of 5 MeV alpha particles. The transient current measurements were reproduced with TCAD simulations. The hypothesis that the polarization is caused by single neutral defect type in the bulk, was tested using two generic models. Neither one has a satisfactory agreement with the measurement data, which indicates that trapping at the interfaces play a significant role in space charge polarization.
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•Space charge polarization was studied with irradiated single crystal diamond sensors.•Full collapse of the electric field was observed for all irradiated samples.•To remove polarization in low fluence samples switching bias voltage on-off effective•Polarization very likely cannot be explained by trapping in the bulk only.
Abstract
To study the impact of various defects associated close to the surface layer of CdTe material, we use scanning laser Transient Current Technique. This gives us an overview of different ...compositional inhomogeneities, such as dislocations, grain boundaries, and tellurium inclusions. Particularly, reconstructed high resolution spatial images provide a map of different electrically active defects. Each spatial point contains a recording of a current pulse, from which shape we calculate drift times and total collected charge. Charge mobility and charge loss are extracted from current pulses and show the effects of charge trapping and polarization. In addition, we investigate the impact of the ALD alumina-CdTe interface and negative fixed charge trapping using both passivated and non-passivated CdTe crystals.
Abstract
The high-luminosity operation of the Tracker in the Compact Muon Solenid (CMS) detector at the Large Hadron Collider (LHC) experiment calls for the development of silicon-based sensors. This ...involves implementation of AC-coupling to micro-scale pixel sensor areas to provide enhanced isolation of radiation-induced leakage currents. The motivation of this study is the development of AC-pixel sensors with negative oxides (such as aluminium oxide — Al
2
O
3
and hafnium oxide — HfO
2
) as field insulators that possess good dielectric strength and provide radiation hardness. Thin films of Al
2
O
3
and HfO
2
grown by atomic layer deposition (ALD) method were used as dielectrics for capacitive coupling. A comparison study based on dielectric material used in MOS capacitors indicate HfO
2
as a better candidate since it provides higher sensitivity (where, the term sensitivity is defined as the ratio of the change in flat-band voltage to dose) to negative charge accumulation with gamma irradiation. Further, space charge sign inversion was observed for sensors processed on high resistivity p-type Magnetic Czochralski silicon (MCz-Si) substrates that were irradiated with gamma rays up to a dose of 1 MGy. The inter-pixel resistance values of heavily gamma irradiated AC-coupled pixel sensors suggest that high-
K
negative oxides as field insulators provide a good electrical isolation between the pixels.
While Cadmium Telluride (CdTe) excels in terms of photon radiation absorption properties and outperforms silicon (Si) in this respect, the crystal growth, characterization and processing into a ...radiation detector is much more complicated. Additionally, large concentrations of extended crystallographic defects, such as grain boundaries, twins, and tellurium (Te) inclusions, vary from crystal to crystal and can reduce the spectroscopic performance of the processed detector. A quality assessment of the material prior to the complex fabrication process is therefore crucial. To locate the Te-defects, we scan the crystals with infrared microscopy (IRM) in different layers, obtaining a 3D view of the defect distribution. This provides us with important information on the defect density and locations of Te inclusions, and thus a handle to assess the quality of the material. For the classification of defects in the large amount of IRM image data, a convolutional neural network is employed. From the post-processed and analysed IRM data, 3D defect maps of the CdTe crystals are created, which make different patterns of defect agglomerations inside the crystals visible. In total, more than 100 crystals were scanned with the current IRM setup. In this paper, we compare two crystal batches, each consisting of 12 samples. We find significant differences in the defect distributions of the crystals.
Abstract
Cadmium telluride (CdTe) is a high-
Z
material with excellent photon radiation absorption properties, making it a promising material to include in radiation detection technologies. However, ...the brittleness of CdTe crystals as well as their varying concentration of defects necessitate a thorough quality assessment before the complex detector processing procedure. We present our quality assessment of CdTe as a detector material for multispectral medical imaging, a research which is conducted as part of the Consortium Project Multispectral Photon-counting for Medical Imaging and Beam characterization (MPMIB). The aim of the project is to develop novel CdTe detectors and obtain spectrum-per-pixel information that make the distinction between different radiation types and tissues possible. To evaluate the defect density inside the crystals — which can deteriorate the detector performance — we employ infrared microscopy (IRM). Posterior data analysis allows us to visualise the defect distributions as 3D defect maps. Additionally, we investigate front and backside differences of the material with current-voltage (IV) measurements to determine the preferred surface for the pixelisation of the crystal, and perform test measurements with the prototypes to provide feedback for further processing. We present the different parts of our quality assessment chain and will close with first experimental results obtained with one of our prototype photon-counting detectors in a small tomographic setup.
Cadmium telluride is a favorable material for X-ray detection as it has an outstanding characteristic for room temperature operation. It is a high-Z material with excellent photon radiation ...absorption properties. However, CdTe single crystals may include a large number of extended crystallographic defects, such as grain boundaries, twins, and tellurium (Te) inclusions, which can have an impact on detector performance. A Technology Computer Aided Design (TCAD) local defect model has been developed to investigate the effects of local defects on charge collection efficiency (CCE). We studied a 1 mm thick Schottky-type CdTe radiation detector with transient-current technique by using a red laser at room temperature. By raster scanning the detector surface we were able to study signal shaping within the bulk, and to locate surface defects by observing their impact on the CCE. In this paper we present our TCAD model with localized defect, and compare the simulation results to TCT measurements. In the model an inclusion with a diameter of 10 μm was assumed. The center of the defect was positioned at 6 μm distance from the surface. We show that the defect has a notable effect on current transients, which in turn affect the CCE of the CdTe detector. The simulated charge collection at the position of the defect decreases by 80 % in comparison to the defect-free case. The simulations show that the defects give a characteristic shape to TCT signal. This can further be used to detect defects in CdTe detectors and to estimate the overall defect density in the material.
Abstract
Aluminium oxide (Al
2
O
3
) has been proposed as an alternative to thermal silicon dioxide (SiO
2
) as field insulator and surface passivation for silicon detectors, where it could ...substitute p-stop/p-spray insulation implants between pixels due to its negative oxide charge, and enable capacitive coupling of segments by means of its higher dielectric constant.
Al
2
O
3
is commonly grown by atomic layer deposition (ALD), which allows the deposition of thin layers with excellent precision.
In this work, we report the electrical characterization of single pad detectors (diodes) and MOS capacitors fabricated on magnetic Czochralski silicon substrates and using Al
2
O
3
as field insulator. Devices are studied by capacitance-voltage, current-voltage, and transient current technique measurements. We evaluate the influence of the oxygen precursors in the ALD process, as well as the effect of gamma irradiation, on the properties of these devices. We observe that leakage currents in diodes before the onset of breakdown are low for all studied ALD processes. Charge collection as measured by transient current technique (TCT) is also independent of the choice of oxygen precursor. The Al
2
O
3
films deposited with O
3
possess a higher negative oxide charge than films deposited by H
2
O, However, in diodes a higher oxide charge is linked to earlier breakdown, as has been predicted by simulation studies. A combination of H
2
and O
3
precursors results in a good compromise between the beneficial properties provided by the respective individual precursors.
An increase in the radiation levels during the high-luminosity operation of the Large Hadron Collider calls for the development of silicon-based pixel detectors that are used for particle tracking ...and vertex reconstruction. Unlike the conventionally used conductively coupled (DC-coupled) detectors that are prone to an increment in leakage currents due to radiation, capacitively coupled (AC-coupled) detectors are anticipated to be in operation in future collider experiments suitable for tracking purposes. The implementation of AC-coupling to micro-scale pixel sensor areas enables one to provide an enhanced isolation of radiation-induced leakage currents. The motivation of this study is the development of new generation capacitively coupled (AC-coupled) pixel sensors with coupling insulators having good dielectric strength and radiation hardness simultaneously. The AC-coupling insulator thin films were aluminum oxide (Al
2
O
3
) and hafnium oxide (HfO
2
) grown by the atomic layer deposition (ALD) method. A comparison study was performed based on the dielectric material used in MOS, MOSFET, and AC-coupled pixel prototypes processed on high resistivity p-type Magnetic Czochralski silicon (MCz-Si) substrates. Post-irradiation studies with 10 MeV protons up to a fluence of 10
15
protons/cm
2
suggest HfO
2
to be a better candidate as it provides higher sensitivity with negative charge accumulation on irradiation. Furthermore, even though the nature of the dielectric does not affect the electric field within the AC-coupled pixel sensor, samples with HfO
2
are comparatively less susceptible to undergo an early breakdown due to irradiation. Edge-transient current technique (e-TCT) measurements show a prominent double-junction effect as expected in heavily irradiated p-type detectors, in accordance with the simulation studies.
The Silicon Drift Detectors (SDDs) have replaced simple diodes in demanding X-ray fluorescence applications like in element analysers capable of detecting light elements. The reason for this is that ...with similar collection area the SDDs have a much smaller output capacitance than diodes due to a much smaller anode size. Thus the SDDs provide much better Signal to Noise Ratio (SNR) at smaller signal levels than diodes. The small capacitance in SDDs is achieved by placing concentric rings around a miniature sized anode. These rings are biased such that inside the SDD's fully depleted bulk a radial electric field component is established guiding signal charges towards the anode. Problems complicating the design of SDDs are positive oxide charge and interface dark noise. The latter is caused when leakage current generated at depleted interfaces mixes with the signal charge. It has been shown previously that by utilizing a chain of resistors connected to SDD's p+ drift rings and intermediate n+ rings both of these problems can be solved but the resistor chain arrangement requires an additional process step, which may not be standardly available. The interface generated dark noise and the requirement for a resistor chain can be removed by implementing suitable gaps in the p+ rings or with a resistive spiral as well as by implementing an additional anode for the collection of interface leakage current. Such SDDs are, however, vulnerable to accumulation of positive oxide charge complicating the manufacturing and likely reducing the effective lifetime of the detector.We present an SDD design comprising a novel ring arrangement preventing the formation of interface dark noise, being resistant to positive oxide charge, and removing the need for a resistor chain. In this work the design and operation principle of the proposed SDD is presented. The operation of the proposed SDD has been evaluated on TCAD with cylindrically symmetric 3D process and device simulations.
We report on the fabrication of capacitively (AC) coupled n+-in-p pixel detectors on magnetic Czochralski silicon substrates. In our devices, we employ a layer of aluminium oxide (Al2O3) grown by ...atomic layer deposition (ALD) as dielectric and field insulator, instead of the commonly used silicon dioxide (SiO2). As shown in earlier research, Al2O3 thin films exhibit high negative oxide charge, and can thus serve as a substitute for p-stop/p-spray insulation implants between pixels. In addition, they provide far higher capacitance densities than SiO2 due to their high dielectric constant, permitting more efficient capacitive coupling of pixels. Furthermore, metallic titanium nitride (TiN) bias resistors are presented as an alternative to punch-through or poly-Si resistors.
Devices obtained by the above mentioned process are characterized by capacitance–voltage and current–voltage measurements, and by 2 MeV proton microprobe. Results show the expected high negative charge of the Al2O3 dielectric, uniform charge collection efficiency over large areas of pixels, and acceptable leakage current densities.
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