Prof. Arie Ruzin (D. Sc.)
Department of Physical Electronics
Faculty of Engineering
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Numerous scientific, medical, technological, and environmental applications and systems require particle detection. Such particles could be photons, neutrons, protons, etc. Among the most known application one finds all forms of medical imaging (Computerized Tomography, Mammography, Nuclear Medicine, etc.), mechanical non destructive testing (imaging), security baggage imaging (at the airport for instance), particle and nuclear physics experiments, and so on.
In order to be detected the particles
have to interact with the detectors, which means that
the detectors should be tailored to the desired interaction mechanism/s of the
specific particle. In the case of
medical imaging mainly high energy photons have to be detected (~10 keV to ~0.5
MeV), for which the most probable interactions are the photoelectric effect and
Beside generating the free carriers which enable the detection operation, the particles could introduce lattice damage by dislocating one or more absorbing atoms. The effect is caused by Non-Ionizing Energy Loss (NIEL) mainly in the case of non-zero resting-mass particles, but also for particularly high energy photons, etc. The defects caused by the incident particles can be electrically active, introducing carrier traps and generation-recombination centers. In this case the detector's performance could deteriorate with time (as the accumulated damage increases). Understanding of the defects is the first and vital step on the way to designing of radiation tolerant detectors. One of the possible ways to minimize the effect of the radiation induced damage is by introducing defect engineering. Namely, by creating certain defects in the semiconductor material of the detector one could neutralize the electrical activity of the radiation-induced defects.
Compound II-VI semiconductor detectors for X- and Gamma-rays
The wide band-gap Cd1-xZnxTe semiconductors have very beneficial properties for Gamma-ray detector application. The forbidden band gap of the ternary compound semiconductor depends on the Zn content, x: Eg(x)@Eg,CdTe×(1-x)+ x×Eg,ZnTe - x×b× (1-x), where b is the bowing factor (firstname.lastname@example.org). The average ionization energy of CdZnTe with low Zn content is ~4.4 eV (the average energy required to generate one electron-hole pair). The electron and hole mobilities are about 1000 and 100 [cm2/V×sec], respectively. We study macroscopic as well as microscopic properties of these semiconductors and related devices. We use device characterization such as I-V(T), Noise PSD, TCT (Transient Current Technique), X-ray spectroscopy, etc., to evaluate the overall properties and performance of the resulting device. We use TSC (Thermally Stimulated Current) and Laplace-DLTS setup combined with optical injection (PITS) to characterize the deep levels in the band-gap, and AFM based measurements to study local electrical activity.
Silicon detectors for MIPs (Minimum Ionizing Particles), X- and Gamma-rays
Silicon is the most studied and probably best understood material on the planet, but unfortunately it is not suitable for many of the imaging applications, because of the low atomic number (14). In other words, many of the high energy photons simply do not interact with the detector's volume and therefore are not detected. However, silicon detectors are useable in some photon applications, when low efficiency detection is acceptable or when "soft x-rays" (photons with energy range up to ~30 keV) have to be measured. In the case of relativistic, Minimum Ionizing Particles (MIPs), such as fast electrons, protons, pions, etc., the ionization signal is not strongly dependent on the atomic number of the detector's media (compared to photons). In such application silicon detectors have an advantage of low cost, availability of large area detectors, advanced technology (developed mostly for VLSI), etc.
However, the silicon used for these application has to be of high resistivity (usually grown by Floating Zone method), while the VLSI technology uses low resistivity silicon, grown by Czochralski method. The high resistivity silicon and its processing were not studied as extensively as those of the low resistivity material. We study some aspects of the dependence of device technology and geometry on performance and yield.
Radiation hardness of semiconductor detectors and VLSI electronics
Electronic systems operated in harsh radiation environments sustain significant radiation-induced damage. Some of the 'mechanical' defects in the lattice are electrically active and therefore may alter system's operation. Such harsh radiation environment exists in space, in nuclear reactors, in high-energy physics experiments, etc. Often the same particles that the sensors were built to detect introduce lattice damage into the sensors, making them un-usable. Another aspect of radiation damage has to do with 'single event upsets'. Even if the incoming particle does not introduce permanent lattice damage, it still may generate electron-hole pairs, which may yield a false electronic signal.
We study radiation-induced damage in semiconductors and semiconductor devices. Our group participates in the RD-50 Collaboration at CERN (European center for nuclear and particle Physics). The aim of the collaboration is to advance the understanding of the nature of the defects introduced by various particles to detectors, and to find ways to improve the radiations hardness (tolerance) of the detectors.
Silicon-germanium has a great potential to replace silicon detectors in many soft X-ray applications. The atomic number of germanium is significantly higher than that of silicon (32 compared to 14), therefore even a small fraction of germanium atoms should improve significantly the absorption efficiency. The main problem is that for gamma photon detection thick detectors must be used, therefore the epitaxial layer technology used in fast VLSI devices could not be applied directly to the detector realization. Bulk SiGe is rather difficult to grow due to Ge segregation. Silicon-germanium grown by Float Zone technique is only available at this time in small ingots (~1 cm diameter). Czochralski grown Si1-xGex can be grown in larger ingots, however the resistivity is rather low, thus the active region of the detectors is very limited. To overcome the resistivity issue we try diffusing and drifting compensating lithium atoms.
We study the electrical properties of currently available SiGe bulk semiconductors (with and without lithium diffusion) and devices by means of "dark" properties characterization, as well as by Laplace DLTS and other techniques.
Indium-Antimonite semiconductor has high carrier mobility and bandgap suitable for infra red absorption. The binary semiconductor is often used for IR detectors. We investigate the bulk material and passivation layer for this semiconductor .
When the size of devices/structures is reduced below a certain threshold, classical (textbook) assumptions and approximations no longer apply. Nano-contacts are interesting subjects for research for two main reasons: (i) with the down-scaling of VLSI technology nano size contacts become feasible and thus important to understand. ; (ii) Less averaging effect often yields better understanding. Semiconductors in general and "exotic" semiconductors in particular can not be grown without defects, thus when making large contacts one includes various material "abnormalities" in the overall behavior. The smaller the contact size, the less averaging there is.
However, with size reduction come increased edge effects, high electric field, etc.
Atomic Force Microscopy (AFM)
Atomic force microscopy is a part of a larger family called Scanning Probe Microscopy (SPM). Contrary to the optical microscopy (limited by the visible wavelength), in scanning probe microscopes the surface of the sample is scanned by a tiny probe with the signal recorded at each point to produce an image. In its origin AFM was used mainly for topography characterization using Van der Waals forces between the surface and the tip of AFM. Later on many applications were developed on the AFM platform for studies of other material properties: electrical, mechanical, magnetic, etc. For electrical characterization we use Contact Potential Difference (CPD), Electrostatic Force Microscopy (EFM), Scanning Capacitance Microscopy (SCM), Tunneling current (TUNA), Scanning Spreading Resistance Microscopy (SSRM). For subtle contacts and surfaces Torsion mode is used.
M. Sc. and D. Sc. students are wanted
In the following fields of research: CdZnTe detectors for x- and gamma-rays, silicon detectors for soft x-rays, computer simulations of various detectors (with existing and self-written software), Atomic Force Microscopy (AFM) measurements of various detectors, radiation damage to detectors and VLSI electronics, VLSI design and testing of front-end electronic circuits (fabrication will be performed elsewhere).
B. Sc. Students for interesting projects are wanted
A variety of interesting projects is offered to fine students: Design and LabView programming of automatic electronic measurements; Design, simulation and testing of analog and digital VLSI circuits; Computer simulation of novel semiconductor devices by a commercial simulation software; Hardware and software design for student laboratory upgrades, etc.
If you have comments or suggestions, email me at email@example.com