Room Temperature Semiconductor Detectors

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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

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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)

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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.

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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.

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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.

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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

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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.

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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.

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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

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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

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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.

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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.

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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

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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)

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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.

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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.

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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.

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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.

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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.