Room Temperature Semiconductor Detectors. 2 Tutorial Presented at Alabama A&M University Lodewijk...
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Transcript of Room Temperature Semiconductor Detectors. 2 Tutorial Presented at Alabama A&M University Lodewijk...
Room Temperature Semiconductor Detectors
2
Room Temperature Semiconductor Detectors
Tutorial Presented at Alabama A&M University
Lodewijk van den Berg
Constellation Technology Corporation
With the cooperation of
Alexsey Bolotnikov
Brookhaven National Institute Laboratory
Normal, AL, Thursday 13 July, 2006
3
Room Temperature Semiconductor Detectors
What are semiconductor Detectors, and what are the Requirements?
• Solid semiconductor material
• Chemically stable and preferably inert to atmospheric conditions
• Able to absorb high energy nuclear radiation without being destroyed
• Able to convert absorbed radiation photons into electronic charges of
• The amount of electronic charges created should be a linear function of the
energy of the radiation
• Should have high quality single crystalline structure
• Should have very small levels of impurities ( < 10 ppm total)
4
Principles of Operation
• A piece of semiconductor material is cut to the desired dimensions and
electrodes are deposited on opposite sides.
• The electrode material is usually a noble metal, e.g. Au, Pd, Pt, Ag, but
sometimes common metals and conductive organic polymers are used.
• Contact wires are attached to the electrodes to connect the detector to a
high voltage power supply on one side and a signal processing system on
the other side.
• The charges created by the radiation are driven by the bias toward the
electrodes and are counted and processed by the electronic system.
• Charges may not be able to travel the whole distance from their point of
origin to the respective electrodes because of trapping at material defects.
5
Charge Transport Properties
• The movement of the electronic charges to the contacts can be described by
the mobility and the trapping time (also called lifetime).
• The mobility µ with dimensions cm2/Vs (or cm/sec per V/cm) determines the
velocity with which the charge moves in the lattice under the force of the
field E applied to the detector.
• The trapping time in seconds represents the probability that a charge is
trapped at a crystalline defect during the time that the charge travels through
the detector.
• The drift length (cm) = E is the average distance a charge can travel
before it is trapped.
6
Charge Transport and Collection
• The collection of charges is described in the most extensive way by the
Hecht equation, which considers all the possible trapping mechanisms.
• Basically it can be expressed for one type of charge (electrons or holes) :
Q = Q0 x e – d/
where Q is the amount of charge collected,
Q0 is the amount of charge generated
d is the thickness of the detector.
• One can see from the equation that in order to collect 98 % of the charges
generated, the value of the drift length should be > 5d.
• This condition is often difficult to meet in semiconductor detectors;
therefore values of > 2d are often accepted.
7
Charge Collection and Analysis System
Gate Generator
Gate
Energy
Gated IntegratorShaping Amp
Fast Pickoff
Gain
HV
HV Filter
Preamplifier
Daughterboard
Hg I2 Detector
Multi Channel
Analyzer (MCA)0 100 200 300 400 500 600 700 8000
2k
4k
6k
8k
10k
Cs-137
Channels (Energy)
Counts
8
Charge Collection and Analysis System (continued)
• The schematic on the previous page shows the following features:
• The high voltage applied to the detector makes the charges generated by the
absorbed radiation move to the respective electrodes.
• The side of the detector from which the signal is taken is in most cases held at
a neutral bias.
• Charges which are collected at the opposite electrode create an image charge
on the signal electrode, so that essentially all charges are accounted for.
• The charge pulse (current) is processed by the preamplifier containing a Field
Effect Transistor (FET) which removes a large part of the continuous current
flowing through the preamplifier and highlights the signal.
9
Charge Collection and Analysis System (continued)
• The charges enter the shaping amplifier which amplifies the current and also
determines the time (in microseconds) during which charges will be collected.
• The charges from the shaping amplifier are processed by the Multi-Channel
Analyzer. The MCA reads the amount of charge in the pulse delivered by the
shaping amplifier. It sets up a number of bins (or channels) over which it
distributes the number of times a certain charge is received.
• This information can be displayed on a PC and is called a spectrum.
• Ideally the spectrum of a monochromatic test source should be a sharp peak
distributed over a few channels. The broadening of the peak is caused by
incomplete charge collection, Compton scattering and other effects in the
crystal which cause charges to be lost or not generated.
10
Materials
• Many materials have been investigated in the past. In the following table a
selection is made using criteria which will help to focus this discussion.
• High density of a material improves the absorption of the radiation.
• The type of interaction needs to be considered. Materials containing high Z
element(s) have a higher full-energy peak efficiency. This means that the
energy of the radiation is more efficiently converted into the maximum
number of counts possible. This relationship is related to Z3.
• Higher Z materials also have lower Compton Scattering, which is a loss of
energy of the absorbed radiation photon by means of elastic scattering.
• Materials with a small electronic band-gap have low resistivity. This causes
a high leakage current and a noisy spectrum when bias is applied
11
Materials (cont’d)
Material Band Gap eV (RT)
Resist.(25oC).cm
Density
g/cm3
Atomicnumber Z
(e)
cm2/V
(h)
cm2/V
Remarks
Ge 0.67 50 5.33 32 > 1 > 1Very low bandgapNeeds cooling to LN Temp.Very high resolution
Si 1.12 Up to 104
2.33 14 > 1 ~ 1Needs coolingVery low efficiency(low Z, low density). X-rays
GaAs 1.43 107 5.32 31,33 8x10-5 4x10-6
Mid-rangeBetter with cooling
CdTe 1.44 109 6.2 48,52 3.3x10-3 2x10-4
Mid-range.Cooling with Peltier cooler.
CdZnTe 1.5 – 2.2
> 1010 ~ 6 48,30,52 1x10-3 6x10-6
High resistivityLow transport properties for holes
HgI2 2.13 1013 6.3 80,53 2x10-4 4x10-5
Very high resistivityMarginal transport properties
12
Comments on Materials Table
• The resistivities for the different materials given in the previous table have
been calculated on the basis of their measured band gap.
• In reality, the actual crystals grown contain many defects which usually
lowers the resistivity by as much as two or three orders of magnitude.
• The standard for all nuclear research and measurements is still LN2
cooled high purity Ge (HPGE), but cooling is not readily available
for field applications.
• The only two solid state detector materials presently available for ambient
temperature applications are CdZnTe and HgI2 .
• This discussion will continue with the description of some properties and
applications of CZT. A separate presentation will discuss HgI2.
13
Room Temperature DetectorsCZT
Review of CZT Properties
Resistivity after doping 1010 Ohm.cm
Electron Mobility 1350 cm2/Vsec
Hole Mobility 120 cm2/Vsec
Electron Mu-Tau 1x10-3 cm2/V
Hole Mu-Tau 6x10-6 cm2/V
• Crystal can be grown in large sizes (2 inch diameter and 10 inches long or
larger), but contain inclusions and segregations.
• The low values of the mu-tau for holes makes it impossible to make planar
detectors with large thicknesses.
14
Room Temperature DetectorsCZT Bulk Leakage Currents
(Brookhaven National Laboratory)
Fitting results:
3x109 Ohm-cm – Imarad5x1010 Ohm-cm – eV-Products 3x1010 Ohm-cm – Yinnel Tech
Apparent bulk resistivity
~V1/2
measurements of
bulk resistivity
values ~1010 Ohm.cm
15
Room Temperature DetectorsCZT
• The low mu-tau product of the holes in CZT combined with the relatively low
resistivity makes it impossible to make planar detectors. Thicker detectors can
be used by using single charge collection of the electrons only. Three detector
configurations have been developed to make this possible.
• Coplanar Grid detectors, where the anode is formed by two interwoven anode
grids. One grid serves as the signal anode, and the other is a steering grid used
to drive the electrons to the anode grid. (P. Luke at U.C. Berkeley)
• Pixellated anodes where each pixel is connected to analysis channel of a
CMOS based ASIC. (Zhong He and coworkers at Univ. of Michigan)
• Frish Grid detectors where the long narrow body of the detector is wrapped
in a teflon/conductor as the cathode. ( McGregor, KSU, and Bolotnikov, BNL)
16
Room Temperature DetectorsCZT
Co-planar Grid Detectors
The bottom of the detector has a
solid contact and is the cathode.
The top has two parallel grids; one
serves the anode and the other as a
steering grid.
Dimensions of the detector:
1 cm x 1 cm x 1 cm.
17
Room Temperature DetectorsCZT
Pixellated Detector
The back of the detector is a solid
cathode contact.
The front has an array of small pixel
contacts as sketched in the lower figure.
The anode pixels are connected to an
ASIC as shown.
Dimensions of the detector:
1 cm x 1 cm x 7 mm.
18
Room Temperature DetectorsCZT
Frisch Grid Detectors
The top figure shows the bare detector.
The bottom figure shows the teflon
wrap and the conductive shield which
acts as the cathode.
The top surface has the anode contact.
Dimensions of the detectors:
6 mm x 6 mm x 3 mm
up to
15 mm x 10 mm x 10 mm.
19
Room Temperature DetectorsCZT
Cs-137 Spectrum (BNL)
Energy resolution: 1.1% (Frisch-ring) vs. 0.9% (3D device)
Resolution is limited by material non-uniformities!
Spectrum of Cs-137 with
detector 5 mm thick.
Resolution 1.1% FWHM.
Single charge collection.
20
Room Temperature Semiconductor DetectorsCZT and HgI2
Summary
• Room temperature semiconductor detectors are able to provide nuclear
spectra with a resolution adequate for many applications.
• Since they are solid state devices, they are very rugged and are suitable for
devices used in the field (no glass components).
• The signal output is stable with temperature and does not drift.
• Improvements in the material are needed, especially in the crystal growth.
• Specifically, the single crystal material needs to be more homogeneous with
respect to its electronic properties, and segregation and inclusions need to be
minimized. In this way, detector bodies with larger volumes will become
available, so that efficiency can be maximized.