X-ray Semiconductor detectors I

15th March 2012

Semiconductor detector basics

• Working principle of semiconductor detectors relays on ionization due to the incident photons (or other particles) on detector material

• Absorbed photons excite valence band electrons into conduction band, creating thus electron-hole pairs

• The number of pairs created

is related to the total ener-

gy absorbed, so that detec-

tors can be used to in spec-

troscopy

• Semiconductor detectors

are also suitable for ima-

ging observations IXO WFI instrument Si semiconductor detectors

Co

urtesy o

f Nasa

• Semiconductor materials are made of (usually group IV), which are neither insulators nor conductors.

• Doping the material with atoms that have an extra valence electron results an n-type semiconductor, where the surplus electrons work as negative charge carriers.

• P-type semiconductors are produced by doping material with 3 valence electron atoms, so that the missing electrons form holes that work as positive charge carriers.

• When strips of the two semiconductor materials are joined, a p-n junction i.e. diode is formed. In p-n junction, the positive and negative charge carriers are situated near the opposite edges of the material and in between a neutral region, a depletion zone is formed.

• The depletion zone work as active region in semiconductor detectors. By introducing a reverse bias voltage over the semiconductor chip, charge carriers can be enforced closer to the edges and accordingly the active region can be further enlarged.

• Reverse bias voltage also creates an electric field across the detector, which forces negative and positive charge carriers formed by ionization to drift opposite directions within the depletion zone. (Charge carrier distribution of p-n junction induce a similar but weaker field, so it works as detector even without biasing)

• Drifting charge carriers’ electric signal can be measured by electrodes placed on p/n -sides of the detector

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d

Signal formation

• As photon is absorbed in depletion zone, absorbed energy generates electron/hole pairs. Required energy to create one such pair in typical semiconductor materials varies between 1...5 eV (e.g. for Si εi = 3.62 eV)

→ therefore one X-ray photon creates a cl ~ 103...4 charge carrier pairs

• Part of the absorbed photons energy goes into material lattice vibrations, i.e. to phonons. The amount of energy that goes into phonon excitations usually exceeds that of going into ionization.

the Fano factor

1/ 2 1/ 2

0 1

Q

x iQ

i i i

N F

Q Q

E E

E E

FN

The fluctuations of charge carrier pair vs. phonon creation is described by Fano the factor (derivation at lecture). The variance σQ of number of charge carrier pairs NQ created:

where F is the Fano factor, E0 absorbed energy, Ex and Ei average phonon and ion pair excitation energies, and εi average energy amount required to produce one charge pair.

Example:

For Si: Ex = 0.04 eV, Ei = 1.1 eV, εi = 3.6 eV

Fano factor defines the intrinsic energy resolution of detector:

where factor 2.355 is for transformation into FWHM units.

0

2.355

2.355

i Q

i

E FN

E FE

0.04 3.61 0.08

1.1 1.1F

0, Q

i

EN

Energy resolution

Intrinsic resolution for Si detector plotted from 1 to 10 keV. Energy resolution scales as . E E

Comparison of Si, GaAs and CdTe as semiconductor detectors

Nuclear Instruments and Methods in Physics Research A 460 (2001) 159–164

Quantum efficiency

Detector quantum efficiency depends on the following factors:

1. Filter materials and respective thicknesses

2. Detector material and respective thickness

3. Photon energy

Si 0.5 mm detector

Pl 0.25 μm filter

Al 0.06 cathode + filter

Si dead layer 0.1 nm

Be 25 μm filter

Charge collection

When the charge pairs are

created in the depletion zone,

they couple with the electrodes.

Coupling gets stronger as

charge carriers drift closer to

the electrodes giving rise to

electric current.

H. Sp

ieler: Rad

iation

detecto

rs and

signal p

rocessin

g

ct

totQQ

t

Example of spectroscopic system that can be used in spectroscopic measurements and signal shape in each state of the readout electronic chain.

The electric current from single event in detector is transformed into semi-gaussian voltage pulse by the electronics, of which amplitude is related to the total number of generated charge carriers. ADC converts analog input pulses into digital numbers by the amplitudes of the pulses, and sends the information to computer.

leak

CVI

t

offset

Sc

ch

EE G E

N channels E chN

55Fe Kα

Electric signal from detector is includes several noise sources, such as current, voltage and thermal noise. Noise translates into worsen performance of detector so that for measured energy resolution:

Thermal noise can be reduced e.g. by peltier cooling, which can be used to

provide measurement temperatures of ~ -20 °C GaAs detector energy resolution has a minimum around 180 K

2 2 2

02.355 i noiseE FE FWHM

Curves from leakage current measurements with a set of CdTe pixel detectors. High leakage current worsens detector energy resolution by introducing more noise. In addition, in transistor reset amplifiers high leakage current adds relative portion of reset time to the signal integration time.

In space science instruments, feedback resistor preamplifier type has been commonly used. In this case the a signal for linear amplifier consists of flat background level due to the leakage current, with peaks of interaction pulses. In this case, the growing leakage current background occupies a portion of the total voltage range that can be handled by the linear amplifier.

Detector use in space environment

• Typical solar proton models give few-MeV proton fluxes exceeding 1010 p/cm2 per year at 1AU

• High energy particles induce damage in lattice structure, which worsens the performance of the detector: e.g. Increase of leakage current and decrease of charge collection efficiency

• Different methods to minimize irradiation damages can include shielding, using shutter, choose of detector material and annealing

Radiation hardness

Radiation hardness measurements for four

CdTe detectors before and after four 22

MeV irradiation doses of 109, 1010, 1011

and 1012 cm-2, respectively.

Mobility-lifetime values for electrons (μτ)e

relate to the electron trap density in detector

depletion zone, which defines the lifetime of

the charge carriers (electrons) in the semi-

conductor material.

Left: CdTe charge collerction efficiency as function of radiation dose. When the pulse rise time of the signal approaches the time constant of the linear amplifier, part of the information of ionization energy is lost. Therefore the peaks shift towards lower channels in spectra according the Hecht relation: Right: Example of leakage current growth as function of irradiation dose for Si detector.

2

ed V

centroidN Ae

Left: Log-log diagram of (μτ)e product and non-ionizing energy loss with pre-irradiated value as comparison. Right: Detector performance can be restored in some extent with annealing. The baking temperature was +100 °C.

Literature

• Helmuth Spieler: radiation detectors and signal processing

• Glenn Knoll: Radiation detection and measurement