Physics Department Yarmouk University Irbid Jordan Solid State Detectors By : Ahmad.Waleed.Tawalbeh Phys. 641 Nuclear Physics 1 First Semester 2010/2011 Instructor : Nidal.Ershidat

Abstract : * Here we want to study some types of " solid state detectors ", like : Lithium-drifted silicon detectors, semiconductor materials other than silicon and germanium and avalanche detectors. * Solid detectors have the higher densities that give reasonable absorption probabilities for detectors of reasonable size. * Ionization produces electron-hole pairs which drift to opposite electrodes.

* Solid media, especially semiconductors, have clear advantages ( signal speed, low ionization energy, mechanical advantages ). * And semiconductor have advantages (small band gap (energy gap between valence and conduction bands), i.e. small energy deposition will move electrons to conduction band, leaving holes behind ). * Also ( Large band gap: insulator. No band gap: conductor).

Table of Content : I. Introduction. II. Lithium – drifted Silicon Detectors. III. semiconductor materials other silicon or germanium. IV. Avalanche Detectors. Conclusions. References.

I. Introduction : Solid-state detector, also called Semiconductor Radiation Detector, radiation detector in which a semiconductor material such as a silicon or germanium crystal constitutes the detecting medium. One such device consists of a p-n junction across which a pulse of current develops when a particle of ionizing radiation traverses it.

In a different device, the absorption of ionizing radiation generates pairs of charge carriers (electrons and electron-deficient sites called holes) in a block of semiconducting material ; the migration of these carriers under the influence of a voltage maintained between the opposite faces of the block constitutes a pulse of current. The pulses created in this way are amplified, recorded, and analyzed to determine the energy, or identity of the incident-charged particles.

The ions themselves do not move in a solid ; instead, the electronic vacancy or " hole " is filled by successive electron transfers from one atom to the next, so that the " hole " appears to travel ).

II. Lithium – drifted Silicon Detectors : In a Si(Li) detector incident x-ray photons are absorbed photo electrically, leaving Si atoms ionized principally in the K shell. These ions emit either an Auger electron or a Si K x-ray photon. Auger electrons have a range of less than 1 pm, and are very unlikely to escape from the detector, but Si K x-rays have a 10 % probability of penetrating 30 pm in silicon, and therefore a significant fraction may escape.

If escape occurs the pulse recorded is equivalent to that produced by an x-ray photon of energy (E i - E si ), where E i and E si are the energies of the incident radiation and Si K x rays respectively. An " internal fluorescence " Si K peak can be produced by the inverse of the escape peak mechanism, when primary ionization and absorption of the photoelectron take place in the 'dead layer', and the Si K photon enters the active region of the detector.

1. Pure silicon won’t do : In pure silicon, many more free charge carriers than those produced by a charged particle, so electron-hole pairs quickly recombine. Want to “deplete” detector of free charge carriers, done by making a p-n junction (diode). * n-type: add impurities of donor ions, e.g. phosphorus. Produce energy levels close to conduction band. * p-type: add impurities of acceptor ions (e.g. boron). Produce energy levels close to valence band.

pn junction *Migration of carriers across junction, leave region depleted of free charge carriers *Depleted region contains a fixed electric field *electron-hole pair will not recombine, but drift along field lines *Can artificially increase region by applying a “reverse bias voltage” pn

pn junction p n V bias *Migration of carriers across junction, leave region depleted of free charge carriers *Depleted region contains a fixed electric field *electron-hole pair will not recombine, but drift along field lines *Can artificially increase region by applying a “reverse bias voltage” reversed bias junction (bias voltage about 100 V) apply negative voltage to n-side -> attract holes apply positive voltage to p-side -> attract electrons -> increase depletion zone

These consist essentially of a 3-5 mm thick silicon junction type p-i-n diode with a bias of V across it. The lithium-drifted centre part forms the non-conducting i- layer. When an x-ray photon passes through, it causes a swarm of electron-hole pairs to form, and this causes a voltage pulse. To obtain sufficiently low conductivity, the detector must be maintained at low temperature, and liquid- nitrogen must be used for the best resolution. With some loss of resolution, the much more convenient Peltier cooling can be employed

Classification of Conductivity

The probability that an electron occupies a certain energy level is given by the Fermi- Dirac-Distribution: f e (E) = 1 / ( exp^((E – E f )/KT) + 1 ). and for holes : f h (E) = 1 - f e (E) = 1 / ( exp^((E f – E)/KT) + 1 ). For intrinsic semiconductors (e and h concentration equal) : E f = E gap / 2.

3. Energy Loss ( Photons ) : Energy loss, dependent on energy, mainly due to : photo effect compton effect pair production

4. Electron spectroscopy : Electron spectroscopy is an analytical technique to study the electronic structure and its dynamics in atoms and molecules. Detecting photoelectrons that are ejected by x- rays is called X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA).

Detecting electrons that are ejected from higher orbitals to conserve energy during electron transitions is called Auger electron spectroscopy (AES). Experimental applications include high-resolution measurements on the intensity and angular distributions of emitted electrons as well as on the total and partial ion yields. Ejected electrons can escape only from a depth of approximately 3 nanometers or less, making electron spectroscopy most useful to study surfaces of solid materials.

III.semiconductor materials other silicon or germanium : This study will concentrate on cadmium zinc telluride (CZT) and cadmium telluride (CdTe) material systems which continue to dominate compound semiconductor radiation detectors. A range of other materials which continue to improve in quality will be discussed, including gallium arsenide (GaAs), indium phosphide (InP), mercuric iodide (HgI 2 ) and thallium bromide (TlBr).

A further group of low Z radiation-hard materials such as diamond and silicon carbide (SiC), which are of interest for use in future high luminosity particle physics experiments.

1. cadmium Telluride ( CdTe ) and ( CZT ) Detectors : Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) are very attractive semiconductor materials for gamma-ray detection. The high density (_ 5.8 g/cm 3 ) and the high atomic number of the materials (ZCd=48, ZTe=52) give a high detection efficiency for gamma-rays.

In contrast to CZT, CdTe material tends to have better monocrystalline uniformity, although with reduced resistivity and greater sensitivity to charge polarisation effects. 2. Mercuric Iodide ( HgI 2 ) Detectors : The development of mercuric iodide (HgI 2 ) as a solid state radiation detector material is motivated by the need for compact spectrometer grade detectors, which operate at ambient temperatures.

Among candidates for solid state detectors, mercuric iodide possesses the highest intrinsic resistivity with values ranging from 1012 to 1013 Ohm.cm. It can therefore sustain high bias voltages while still generating low leakage currents. HgI 2 potentially offer excellent detection efficiency at higher photon energies, with Z combinations of 81:35 and 80:53, respectively.

3. Other detectors : Other wide band gap compound semiconductor materials continue to be of interest for ionizing radiation detection. Intermediate-Z materials such as GaAs and InP are being developed principally for X-ray imaging applications. GaAs and InP offer photon attenuation coefficients between that of Si and CZT/CdTe. GaAs in particular has relatively mature contact technologies and has the potential for devices with integrated microelectronic.

IV. Avalanche Detectors : The concept of the avalanche drift diode was motivated by the shortcomings of the ‘‘silicon photo multiplier’’, a device consisting of many small area avalanche diodes working in the limited Geiger mode. Each of these micro-cells provides a standard pulse when an avalanche is initiated by one (or several) electron(s).

A new avalanche detector concept is presented that promises very high (close to 100%) quantum efficiency. Integrating the structure into the centre of a drift diode one obtains a large area device that focuses the photoelectron onto a small ‘‘point-like’’ avalanche region.

Uses and Applications for semiconductors Detectors : Good detection efficiency and high energy- resolution make Cadmium Zinc Telluride (CdZnTe) and Cadmium Telluride (CdTe) detectors attractive in many room temperature X-ray and gamma-ray detection applications such as medical and industrial imaging, industrial gauging and non- destructive testing, security and monitoring, nuclear safeguards and non-proliferation, and astrophysics.

Advancement of the crystal growth and device fabrication technologies and the reduction of bulk, interface and surface defects in the devices are crucial for the widespread practical deployment of Cd 1−x Zn x Te-based detector technology.

Here we review the effects of bulk, interface and surface defects on charge transport, charge transport uniformity and device performance and the progress in the crystal growth and device fabrication technologies aiming at reducing the concentration of harmful defects and improving Cd 1−x Zn x Te detector performance.

Compound semiconductors detectors combined with the rapid development of radiation hard electronics will lead to full systems that will operate at elevated temperatures and in radiation environments which would be unsuitable for most currently available spectroscopic detector systems.

Future development of compound semiconductors detectors will lead, in principle, to X-ray imaging spectroscopic arrays capable of meeting the most stringent demands of future space missions and industrial applications in harsh terrestrial environments.

Conclusions : Advantages Si: large yield in generated charge carriers, fine segmentation, radiation tolerant, mechanically stable, … Solid state detectors play a central role in modern high energy and photon physics. The most significant recent advances in compound semiconductor radiation detectors have been reviewed, covering both intermediate and high Z materials. Imaging detector development using compound semiconductors.

Working principle (general) diode in reverse bias (p-n junction).

Important Advances in Tracking * Multi-wire Proportional Chambers (1968) and drift chambers (electronic recording of tracks) Resolution: mm-100  m * Vertex Drift chambers (~1981) Vertexing for lifetime of long-lived particles Resolution 50  m, 0.1channels/cm 2 * Silicon Microstrip detectors (>1983) Res. <10  m, 100 channels/cm 2 measure drift time to sense wires, to improve position measurement See L.Gibbon’s seminar next week