- Email: [email protected]
GaAs goes nuclear
m DEVICE FEATURE m
GaAs Goes Nuclear Readers of III-Vs Review may be surprised to hear of some of the more interesting uses to which GaAs is being applied. One of these is as high energy nuclear particle detectors for the Large Hadron Collider (LHC) being constructed at CERN in France. he LHC experiment at CERN (Centre for European Nuclear Research) which is designed to detect the Higgs Boson, the final block (apparently!) to our understanding of why the universe contains mass, requires detectors to determine whether an interaction has taken place and some of these are placed at the vertex of the collision region. The high flux of neutrons produced in this region produces radiation damage in any solids nearby and this means that the detectors must be made of material which is relatively insensitive to radiation damage. Silicon detectors, although of extremely high efficiency, would last for only a year under these conditions: GaAs detectors are expected to last for much longer.
of work for the lucky suppliers and it follows that there is keen competition amongst the collaborators in this European programme to produce the most effective cheapest and reliable detectors. The performance of this type of detector is often described in terms of the Charge Collection Efficiency (CCE) which is the ratio of the charge detected to the charge produced by the incoming nuclear particle. The state of the art CCE presented in the meeting was about 85%, but this was only obtained from detectors fabricated on thinned and, therefore, very fragile substrates. When thicker, commercial substrates are used, the efficiency is markedly reduced and this is apparently a result of a non-uniform electric field, carrier recombination and possibly trapping.
GaAs must be thick
Workshop
The detectors that we are discussing are the solid state equivalent to ionisation detectors where the incoming particle produces electron-hole pairs and these are swept out separately under an electric field to produce a current in the external circuit. The GaAs must be relatively thick to produce a measurable number of mobile carriers and nonconducting in order to reduce background leakage currents. What better material can be found than bulk semiinsulating GaAs substrates which can be obtained, off-the-shelf, at reasonable cost? Typically each detector has an ohmic and Schottky contact and is reverse biased. The total area of the detectors is of the order of 10 m 2. In order to achieve good spatial resolution they are fabricated into individual strip detectors and there will be more than a million of these single detectors in the entire array. Of course, the growth of the material and the fabrication of these detector elements represents a sizeable amount
All aspects of the fabrication and testing of these detectors and associated electronics were discussed at the 2nd International Gallium Arsenide Workshop held at Jostal, near Freiburg, Germany, from 7 to 10 March. Do not let the rather general conference title confuse you, dear reader! The conference was dedicated to GaAs radiation detectors and was by invitation only? The UMIST Group is involved in a rather peripheral manner in as much as all the other collaborating laboratories are basically nuclear physics laboratories and are funded accordingly. We operate very much as a consultancy to the UK universities involved in this collaboration. For further information and for names of collaborators in this exciting work, please contact me at the address below. • Contact: Mike Brozel, Centre for Electronic Materials, UMIST, PO Box 88, Manchester, M60 1QD, UK. Tel/fax: [44] (0)61 200 4704/4770.
T
Vacancies for positrons? Most readers of III-Vs Review may be vaguely aware of the importance of point defects in materials like GaAs! The most important of these are vacancies which are particularly difficult to investigate in these materials because atomic probes such as electron paramagnetic resonance do not work well. One technique that has been remarkably successful, especially in the case of GaAs, is positron annihilation (PA). Positrons are the anti-particles of electrons and they annihilate to produce little more than a couple of high energy photons. These photons can be detected externally using photomultiplier tubes, for instance. If positrons are introduced into a material such as GaAs, they find themselves surrounded by huge numbers of electrons and are destroyed within a couple of hundred picoseconds. However, if they can be trapped by vacancies, they are also in regions where the electron concentrations are low and therefore they last for a longer period. By measuring the lifetimes of positrons introduced into solids, considerable information can be derived regarding the identification and concentrations of vacancies. In GaAs for instance, both vacancies can be detected and the UMIST Group has been performing collaborative work with the Saclay Group headed by Dr C Corbel*in correlating electrical and optical parameters with vacancies identified by PA. At a European Physical Society Industrial Workshop, EIW-12, held in the Netherlands from 10 to 12 March 1994, the industrial applications of PA were discussed Continued on page 57
0961-1290/94/$7.00 © 1994, Elsevier Science Ltd.
Pago 18 ( ~ l V o l
7 No 3
GaAs Goes Nuclear Readers of III-Vs Review may be surprised to hear of some of the more interesting uses to which GaAs is being applied. One of these is as high energy nuclear particle detectors for the Large Hadron Collider (LHC) being constructed at CERN in France. he LHC experiment at CERN (Centre for European Nuclear Research) which is designed to detect the Higgs Boson, the final block (apparently!) to our understanding of why the universe contains mass, requires detectors to determine whether an interaction has taken place and some of these are placed at the vertex of the collision region. The high flux of neutrons produced in this region produces radiation damage in any solids nearby and this means that the detectors must be made of material which is relatively insensitive to radiation damage. Silicon detectors, although of extremely high efficiency, would last for only a year under these conditions: GaAs detectors are expected to last for much longer.
of work for the lucky suppliers and it follows that there is keen competition amongst the collaborators in this European programme to produce the most effective cheapest and reliable detectors. The performance of this type of detector is often described in terms of the Charge Collection Efficiency (CCE) which is the ratio of the charge detected to the charge produced by the incoming nuclear particle. The state of the art CCE presented in the meeting was about 85%, but this was only obtained from detectors fabricated on thinned and, therefore, very fragile substrates. When thicker, commercial substrates are used, the efficiency is markedly reduced and this is apparently a result of a non-uniform electric field, carrier recombination and possibly trapping.
GaAs must be thick
Workshop
The detectors that we are discussing are the solid state equivalent to ionisation detectors where the incoming particle produces electron-hole pairs and these are swept out separately under an electric field to produce a current in the external circuit. The GaAs must be relatively thick to produce a measurable number of mobile carriers and nonconducting in order to reduce background leakage currents. What better material can be found than bulk semiinsulating GaAs substrates which can be obtained, off-the-shelf, at reasonable cost? Typically each detector has an ohmic and Schottky contact and is reverse biased. The total area of the detectors is of the order of 10 m 2. In order to achieve good spatial resolution they are fabricated into individual strip detectors and there will be more than a million of these single detectors in the entire array. Of course, the growth of the material and the fabrication of these detector elements represents a sizeable amount
All aspects of the fabrication and testing of these detectors and associated electronics were discussed at the 2nd International Gallium Arsenide Workshop held at Jostal, near Freiburg, Germany, from 7 to 10 March. Do not let the rather general conference title confuse you, dear reader! The conference was dedicated to GaAs radiation detectors and was by invitation only? The UMIST Group is involved in a rather peripheral manner in as much as all the other collaborating laboratories are basically nuclear physics laboratories and are funded accordingly. We operate very much as a consultancy to the UK universities involved in this collaboration. For further information and for names of collaborators in this exciting work, please contact me at the address below. • Contact: Mike Brozel, Centre for Electronic Materials, UMIST, PO Box 88, Manchester, M60 1QD, UK. Tel/fax: [44] (0)61 200 4704/4770.
T
Vacancies for positrons? Most readers of III-Vs Review may be vaguely aware of the importance of point defects in materials like GaAs! The most important of these are vacancies which are particularly difficult to investigate in these materials because atomic probes such as electron paramagnetic resonance do not work well. One technique that has been remarkably successful, especially in the case of GaAs, is positron annihilation (PA). Positrons are the anti-particles of electrons and they annihilate to produce little more than a couple of high energy photons. These photons can be detected externally using photomultiplier tubes, for instance. If positrons are introduced into a material such as GaAs, they find themselves surrounded by huge numbers of electrons and are destroyed within a couple of hundred picoseconds. However, if they can be trapped by vacancies, they are also in regions where the electron concentrations are low and therefore they last for a longer period. By measuring the lifetimes of positrons introduced into solids, considerable information can be derived regarding the identification and concentrations of vacancies. In GaAs for instance, both vacancies can be detected and the UMIST Group has been performing collaborative work with the Saclay Group headed by Dr C Corbel*in correlating electrical and optical parameters with vacancies identified by PA. At a European Physical Society Industrial Workshop, EIW-12, held in the Netherlands from 10 to 12 March 1994, the industrial applications of PA were discussed Continued on page 57
0961-1290/94/$7.00 © 1994, Elsevier Science Ltd.
Pago 18 ( ~ l V o l
7 No 3