PHYS 352

Radiation Detectors III: Semiconductor Detectors

Semiconductor Radiation Detectors

• why choose one over a gas counter or a scintillation detector?

• improved energy resolution

• compact size

• pixelated detectors/readout are possible thus allowing for position resolution (down to microns)

How It Works

• conduction in a semiconductor depends on charge carrier population in the conduction band

• intrinsic semiconductor carrier concentration ni:

• where Ns is the density of available states (depends on the material)

• note: Ns is also a function of T, goes as T3/2

• in silicon at room temperature ni = 1.5 × 1010 cm–3

• in germanium at room temperature ni = 2.5 × 1010 cm–3

• in comparison, silicon has ~1022 atoms/cm3; only 1 in 1012 atoms has liberated charge

• ionizing radiation creates electron-hole pairs in the semiconductor

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 30

Signal Formation in Silicon

• Conduction band really empty only at T=0

• Distribution according Fermi-Dirac Statistics

• Number of electrons in conduction band at room temp.:

!Ratio of electrons in conduction band 10-12

(Silicon ~5x1022 Atoms/cm3)

• A volume of intrinsic Si of 1cm x 1cm x 300µm contains ~4.5x108 free charge carriers at RT compared to only 2.3x104 electron-hole pairs for a MIP.

!To detect this signal, the number of free charge carriers has to be reduced drastically.Possibilities are:

• cooling

• pn-junction in reverse bias

conduction band

valence band

ni = Ns exp −Eg

2kBT⎛⎝⎜

⎞⎠⎟

Electron-Hole Pairs (How It Works cont’d)

• average energy deposit needed to create 1 e-h pair ε

• note: band gap Eg in Si is 1.1 eV and 0.7 eV in Ge; hence most of the energy needed to create an electron-hole pair goes to lattice vibrations (phonons)

• still, this is a small number compared to ε =~30 eV in a gas counter, and scintillation photons ~100 eV per photon

• hence the number of quanta available to count (that’s supposed to be proportional to the energy deposited) is much larger and 1/sqrt(N) is going to be smaller → better energy resolution

Si at 300 K 3.62 eV

Si at 77K 3.81 eV

Ge at 77 K 2.96 eV

How It Works

• conduction in a semiconductor depends on charge carrier population in the conduction band

• intrinsic semiconductor carrier concentration ni:

• where Ns is the density of available states (depends on the material)

• note: Ns is a function of T, goes as T3/2

• in silicon at room temperature ni = 1.5 × 1010 cm–3

• in germanium at room temperature ni = 2.5 × 1010 cm–3

• in comparison, silicon has ~1022 atoms/cm3; only 1 in 1012 atoms has liberated charge

• ionizing radiation creates electron-hole pairs in the semiconductor

• in comparison, the number of e-h pairs produced is order 106 cm–3

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 30

Signal Formation in Silicon

• Conduction band really empty only at T=0

• Distribution according Fermi-Dirac Statistics

• Number of electrons in conduction band at room temp.:

!Ratio of electrons in conduction band 10-12

(Silicon ~5x1022 Atoms/cm3)

• A volume of intrinsic Si of 1cm x 1cm x 300µm contains ~4.5x108 free charge carriers at RT compared to only 2.3x104 electron-hole pairs for a MIP.

!To detect this signal, the number of free charge carriers has to be reduced drastically.Possibilities are:

• cooling

• pn-junction in reverse bias

conduction band

valence band

ni = Ns exp −Eg

2kBT⎛⎝⎜

⎞⎠⎟

Operating a Semiconductor Detector

• since the amount of charge produced by a radiation/particle interaction is small compared to the intrinsic carrier concentration at room temperature, you do two things (to reduce the intrinsic charge carriers so the signal from the interaction is relatively larger)

• operate at lower temperatures (liquid nitrogen 77 K)

• operate a junction device at reverse bias

p-n Junction Semiconductor Detector

• at the junction, free electrons diffuse from n to p leaving behind positive ions and free holes diffuse from p to n, leaving behind negative ions

• happens until electric field opposes the diffusion (equilibrium condition)

• applying reverse bias increases thickness of depletion region and the E-field

• e-h pair created in the depletion region drifts because of high E-field, thus preventing recombination

E-field in depletion region

+ –V

Germanium Detectors

• Ge chosen over Si for detecting gamma rays

• higher Z

• Fano factor is ~0.1 for Ge, ε = 2.96 eV

• what’s the expected energy resolution of 1.33 MeV gamma ray from 60Co?

• N = 1.33×106 / 2.96 = 449324

• ΔE is thus ~1.5 keV at 1.33 MeV

σ = FN = 44932.4 212ΔEFWHM

E=2.354σN

=2.354(212)449324

= 0.11%

Germanium Detector History

• in 1963 George Ewan, now Queen’s Professor Emeritus in Physics, developed (together with A. Tavendale) high resolution gamma ray spectroscopy using lithium-drifted germanium detectors while they were at Chalk River

• back then “large” germanium crystals weren’t able to be grown pure enough; impurities became charge trapping sites and prevented large gamma ray detectors from being made → lithium drifting overcame this problem

• Li is an electron donor and makes p-type semiconductor “intrinsic” with very few free charge carriers → making a thick depletion region

thick enough intrinsic, depleted region to absorb gamma rays

George Ewan

Germanium Detector History cont’d

• the development of Ge(Li) detectors revolutionized gamma ray spectroscopy

• drawback: Li is mobile at room temperature (that’s why it is easy to drift into the germanium; usually prepared at higher temperatures)

• once the detector is made, it has to be cooled and stored, forever, at liquid nitrogen temperatures to prevent the lithium from diffusing around and ruining the detector

• modern day Ge detectors are also known as HPGe (high purity germanium); crystal growing/purification techniques have been developed to grow large Ge detectors with low enough impurities that doesn’t hinder charge collection

• all Ge detectors have to be operated at liquid nitrogen temperatures so that thermal charge carriers have a low concentration

• whereas the old ones also had to be stored, transported, whatever, always cooled by liquid nitrogen

Charged Particle Detection with Semiconductors

• for gamma rays, germanium is preferred

• for charged particles (alphas, electrons) you can use silicon

• silicon is used since germanium doesn’t have that big of an advantage (remember: and Z/A is roughly the same for all materials)

• and if detecting alpha particles, well you don’t need to worry about having enough stopping power

• more important is the dead layer...

• how do you make a p-n junction detector?

1ρdEdx

∝ZA

Diffused Junction and Ion-Implanted

• diffuse n-type dopant into one end of a p-type material; leaves a ~micron thick dead layer the radiation has to pass through before it reaches the junction (intrinsic) region

• ion-implanted junctions: ion beam implants the dopants into the substrate with high degree of control of beam energy

• can have entrance dead layer only 10’s of nm thick!

• must anneal detector after bombarding with ions

dark blue: dead layer

p-type Si

arsenic ionsmake n-type

Surface Barrier Detectors (Silicon)

• n-type silicon with metallic gold form a surface barrier junction (also known as a Schottky barrier junction)

• contact potential created

• depletion zone extends entirely into the semiconductor

• properties of this junction are similar to p-n junction

• very thin gold contact layer (about 200Å thick or ~40 μg/cm2) is the dead layer

• depending on the bias, can make the entire thickness of the silicon depleted and hence sensitive

• could be useful to measure energy deposited by charged particles going all the way through the detector

Silicon Detectors in Particle Physics Experiments

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 59

Strip Detectors

Back side

n-Type silicon easier to produce.

Segmentation by implanting of strips with opposite doping.

Voltage needs to be high enough to completely deplete the high resistivitysilicon. CMS: ~100 V

Readout typically via capacitive coupling.

Schematic view of CMS Si-Sensor.

• used for particle tracking with μm position resolution (especially important near the particle collision point)

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 74

Pixels 150x150 µm

Each pixel is bump-bonded to a readout pixel

Making use of the large Lorentz-angle for electrons (barrel). Lorentz-angle: drift angle for charge carriers in magnetic field.

!Charge spreads over several pixel.

!Spatial resolution 10, 15 µm in !, z

The CMS-Pixel Detector III

(n-type)

silicon strip detector

silicon pixel detector

Silicon Detectors in Particle Physics Experiments

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 59

Strip Detectors

Back side

n-Type silicon easier to produce.

Segmentation by implanting of strips with opposite doping.

Voltage needs to be high enough to completely deplete the high resistivitysilicon. CMS: ~100 V

Readout typically via capacitive coupling.

Schematic view of CMS Si-Sensor.

• used for particle tracking with μm position resolution (especially important near the particle collision point)

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 74

Pixels 150x150 µm

Each pixel is bump-bonded to a readout pixel

Making use of the large Lorentz-angle for electrons (barrel). Lorentz-angle: drift angle for charge carriers in magnetic field.

!Charge spreads over several pixel.

!Spatial resolution 10, 15 µm in !, z

The CMS-Pixel Detector III

(n-type)

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 68

The CMS Inner Barrel Detector under Construction

silicon strip detector

silicon pixel detector

Silicon Detectors in Particle Physics Experiments

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 59

Strip Detectors

Back side

n-Type silicon easier to produce.

Segmentation by implanting of strips with opposite doping.

Voltage needs to be high enough to completely deplete the high resistivitysilicon. CMS: ~100 V

Readout typically via capacitive coupling.

Schematic view of CMS Si-Sensor.

• used for particle tracking with μm position resolution (especially important near the particle collision point)

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 74

Pixels 150x150 µm

Each pixel is bump-bonded to a readout pixel

Making use of the large Lorentz-angle for electrons (barrel). Lorentz-angle: drift angle for charge carriers in magnetic field.

!Charge spreads over several pixel.

!Spatial resolution 10, 15 µm in !, z

The CMS-Pixel Detector III

(n-type)

!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 68

The CMS Inner Barrel Detector under Construction

silicon strip detector

silicon pixel detector!"#$%&'()*+,!-.$/,!"0"#,1#"#)"-'2 70

Insertion of the CMS Tracker

Diagnostic X-ray Panel

• what technology is inside?

• or how would you build one?

• let’s make this an engineering design problem

• what are the “core” challenges behind making an X-ray imaging panel?

• X-rays are penetrating so prefer thick, high Z, high density material

• need pixelation (finer the better)

• easy to do either or both of the above but becomes challenging for a large area

Diagnostic X-ray Panel

• what technology is inside?

• or how would you build one?

• let’s make this an engineering design problem

• what are the “core” challenges behind making an X-ray imaging panel?

• X-rays are penetrating so prefer thick, high Z, high density material

• need pixelation (finer the better)

• easy to do either or both of the above but becomes challenging for a large area

• CCD, like a giant digital camera?

• can’t make them that big though

• array of individual silicon surface barrier detectors?

• each one is kind of expensive, and the pixelation would be coarse

• array of individual scintillators?

• each one is expensive, pixelation would be coarse

• gas counter multiwire proportional chamber?

• need a conversion layer (metal) that converts the X-rays to photoelectrons (via the photoelectric effect)

• has adequate position resolution (down to tens of microns) but can’t be readout pixel by pixel to get an image

Brainstorm

• CCD, like a giant digital camera?

• can’t make them that big though

• array of individual silicon surface barrier detectors?

• each one is kind of expensive, and the pixelation would be coarse

• array of individual scintillators?

• each one is expensive, pixelation would be coarse

• gas counter multiwire proportional chamber?

• need a conversion layer (metal) that converts the X-rays to photoelectrons (via the photoelectric effect)

• has adequate position resolution (down to tens of microns) but can’t be readout pixel by pixel to get an image

Brainstorm

R&D to Look at Existing Designs

• TFT (like what controls an LCD display panel) controlled array of amorphous silicon photodiodes

• or CMOS image sensor

• scintillator typically deposited (thick) layer of CsI

The fact that the recording devices attached to an IIT are not required with FPDs is a result of the multi-mode capability inherent in flat paneltechnology. From an electrical point of view, the array architecture andreadout are very similar to those used for dynamic random access memory(DRAM). Accessing sections or regions-of-interest (ROIs) in the array isonly a matter of addressing the proper columns and rows. As with DRAM,the signal is stored as a charge packet, which makes summing the datafrom more than one pixel a simple matter of combining the charge packets.Reading out a 2x2 neighborhood of pixels as one super-pixel is easily done

by combining the signalsfrom neighboring pixelsat the front-end chargeintegrating amplifiers.Pixel binning offers twoimportant opportunitiesfor trade-offs. The firsttrade-off is sensitivity,since the super pixelwill see more X-rayphotons and so havehigher signal-to-noiseratio (SNR). Very oftenthe maximum digitaldata conversion rate ofthe panel is limited to afixed value. Binningalso can reduce theoverall matrix size, thusallowing higher framerates. For example, a1024x1024 imagercapable of 30fps canalso be read out as512x512 super pixels at60fps or higher. Theflat panel imager shown

in Figure 7 typically has three modes: full field, full resolution at 7.5fpsused for DSA and radiography; full field, half resolution at 30fps used forfluoroscopy and cine; and ! field of view, full resolution at 30fps which isused for fluoroscopic zoom mode. Since these modes are defined only bythe method of

8

Figure 7 - This is experimental R&F equipment used by Hitachiin a clinical evaluation of flat panel technology. Here a 12”x16”(active area) FPD is mounted on the side of a 12” ImageIntensifier Tube (IIT). With this system, either the FPD or IITcould be rotated into place, facilitating straightforward comparisonimages.

here are both types!

R&D to Look at Existing Designs

• TFT (like what controls an LCD display panel) controlled array of amorphous silicon photodiodes

• or CMOS image sensor

• scintillator typically deposited (thick) layer of CsI

The fact that the recording devices attached to an IIT are not required with FPDs is a result of the multi-mode capability inherent in flat paneltechnology. From an electrical point of view, the array architecture andreadout are very similar to those used for dynamic random access memory(DRAM). Accessing sections or regions-of-interest (ROIs) in the array isonly a matter of addressing the proper columns and rows. As with DRAM,the signal is stored as a charge packet, which makes summing the datafrom more than one pixel a simple matter of combining the charge packets.Reading out a 2x2 neighborhood of pixels as one super-pixel is easily done

by combining the signalsfrom neighboring pixelsat the front-end chargeintegrating amplifiers.Pixel binning offers twoimportant opportunitiesfor trade-offs. The firsttrade-off is sensitivity,since the super pixelwill see more X-rayphotons and so havehigher signal-to-noiseratio (SNR). Very oftenthe maximum digitaldata conversion rate ofthe panel is limited to afixed value. Binningalso can reduce theoverall matrix size, thusallowing higher framerates. For example, a1024x1024 imagercapable of 30fps canalso be read out as512x512 super pixels at60fps or higher. Theflat panel imager shown

in Figure 7 typically has three modes: full field, full resolution at 7.5fpsused for DSA and radiography; full field, half resolution at 30fps used forfluoroscopy and cine; and ! field of view, full resolution at 30fps which isused for fluoroscopic zoom mode. Since these modes are defined only bythe method of

8

Figure 7 - This is experimental R&F equipment used by Hitachiin a clinical evaluation of flat panel technology. Here a 12”x16”(active area) FPD is mounted on the side of a 12” ImageIntensifier Tube (IIT). With this system, either the FPD or IITcould be rotated into place, facilitating straightforward comparisonimages.

here are both types!

given dose. Furthermore, the thickness of the CsI can be greater than thatof a rare earth screen because when CsI is deposited on the array it growsin a columnar structure. The columns tend to act as light pipes, reducingthe amount of light spreading in the scintillator. So, for example, a 600µmCsI layer can have resolution similar to a 300µm thick rare earth screen.These screens such as gadolinium oxysulfide have the advantage of muchlower cost and greater flexibility in that the screen can easily be changedto match the resolution requirements of the application.

The light generated by thescintillator is absorbed by thephotodiodes in the array, cre-ating electrons which arestored on the capacitance ofthe photodiode itself. Thepeak light absorption effi-ciency of the photodiodes isin the green spectrum, at550nm wavelength. Bothgadolinium oxysulfide andthallium doped cesium iodide,CsI(Tl), produce their peaklight output at this frequency.The amorphous- silicon pho-todiodes are typically the “n-i-p” type. In other words, thelayers in the photodiode con-sist of an electron-rich layerat the bottom, an intrinsic orundoped layer in the middle,and a hole-rich (positivelycharged) layer at the top. This type of amorphous-silicon photodiode hasthe advantages of low dark current and a capacitance that is independent ofthe accumulated signal. The thermally generated dark current intrinsic tothe photodiodes is always working to charge the diode. If the photodiodeshave large amounts of dark current, much of the diode’s signal capacitywill be filled up by charge with no signal information. Compared to crystalline silicon photodiodes like those used in CMOS imagers, the darkcurrent in amorphous silicon photodiodes is orders of magnitude less. Soit is not unusual for amorphous silicon flat panel arrays to be capable ofmore than ten second integration times at room temperature. The fact thatthe diode capacitance is independent of signal helps make the detectionsystem linear.

5

Figure 4 - The relative light spreading of columnar CsIversus an amorphous phosphor screen. (Photographcourtesy of Hamamatsu Photonics.)