New Materials and Designs of Semiconductor Detectors New developments are driven by particle physics and applications in: • Medical & Synchrotron X-ray Imaging • Nuclear Medicine - -Ray Detection • Astronomy - X-ray Detection • Non-destructive testing Need to improve performance & reduce the dose. 1
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New Materials and Designs of Semiconductor Detectors New developments are driven by particle physics and applications in: Medical & Synchrotron X-ray Imaging.
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New Materials and Designsof Semiconductor DetectorsNew developments are driven by particle physicsand applications in:
• Medical & Synchrotron X-ray Imaging• Nuclear Medicine - -Ray Detection• Astronomy - X-ray Detection• Non-destructive testing
Risto Orava June 2002
Need to improve performance & reduce the dose.
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• Rad hard Si-detectors, Oxygenated Si• Crystalline Compound Semiconductors: CdTe, CdZnTe,...• High Purity Epitaxial Materials: SiC, GaAs,...• Polycrystalline CVD Materials: Diamond,...• Large Area Polycrystalline Materials: a-Si, a-Se, CdTe, HgI,...
For high performance detectors materialtechnologies are combined with deviceengineering and instrument design.
• Slicing, dicing• Chemical etching• Polishing• Metallization• Electrode deposition• Metal sputtering• Surface passivation• Contact technologies: Ohmic vs. blocking contacts• Uni-polar devices• Flip-chip bonding• 3D-structures
• Modality• -energies• Packaging• Operating environment: Temperature Radiation Electronic noise Mechanical stresses• Resolution• DQE• MTF• Frame rate• Fill factor
Material Technology Device Engineering Instrument Design
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I Material Technology
Need high purity, homogenous, defect-free material
High Z - small radiation length Xo for high QE (Xo = 716.4gcm-2A/[Z(Z+1)ln(287/Z)])
Large enough band gap - high resistivity (> 109 cm) and low leakage current for low noise
operation (high resistivity is achieved in high band gap materials with small intrinsic charge carrier concentrations and by controlling the extrinsic and intrinsic defects to pin Fermi-level near mid-gap)
Small enough band gap - small electron-hole ionization energy (< 5eV) (in general, need a minimum band gap of 1.5eV to control thermally generated currents and losses in energy resolution & noise. With sufficiently high - and stable - number of e-h pairs the S/N -ratio is high.
High intrinsic product - the carrier drift length, E (=carrier mobility, =carrier lifetime, E the applied electric field. Charge collection is determined by the fraction of detector thickness traversed by the photo- generated electrons and holes during the collection time. In the ideal case the carrier drift length would be much longer than the detector thickness for complete charge collection. This is possible for electrons but, most often, not for the holes. This broadens the photopeak and worsens the resolution.)
High purity, homogenous, no defects - good charge transport properties (low leakage currents, no conductive short circuits between the detector contacts - single crystals for avoiding grain boundaries and other extended defects)
High surface resistivity - low noise due to surface conductivity (the surfaces should be stable to prevent increased surface leakage currents with time, the electric field lines should not terminate at the non- contacted surfaces for complete charge collection and for preventing build-up of surface charges)
Material manufacturing - growth method vs. yield (stochiometry, ingot-to-ingot variations, doping, compensation, elimination of large defects, crystal size, quality control, cost)
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Why compound semiconductors?• Uniqueness of compound semiconductors
– Band gap engineering• Heterostructure devices
• Hg1-xCdxTe : -0.25 ~ 1.6 eV
• AlxGa1-xAs :
– AlAs : 2.16 eV, indirect– GaAs : 1.43 eV, direct
– Larger electron and/or hole mobility• Good for high speed (high frequency) devices
– Direct band gap materials• Optoelectronic devices (lasers, LED’s)
• Compound semiconductor processing– Cost
• Compound material growth is not cheap.– Difficulty of fabrication (example: GaAs,...)
• Doping– Some dopants are amphoteric. (Donor in the Ga site and acceptor in the As
site).• Oxidation
– Ge2O3 and As2O3 : oxidation rates are different.
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semiconductors
electronic semiconductors
mixed conductors
ionic conductors
intrinsic semiconductors
extrinsic semiconductors
n-typeextrinsic
p-typeextrinsic
Requirements for sensors:• band gap 1-6 eV• n- or p-type conduction• no ionic conduction• chemical and thermal stability• solubility of dopants in host lattice
covalentbonding
Semiconductors -classification1
Elemental and compound semiconductors arein everyday use.
Si rectifiers, transistors, IC’sGe early transistors and diodesCompounds high-speed devices, light absorption applicationsGaAs, GaP LED’sZnS fluorescent - TV screensInSb, CdSe, PbTe, HgCdTe light detectorsSi, Ge IR and ionizing radiation detectorsGaAs, InP microwaves (the Gunn diode)GaAs, AlGaAs,... semiconductor lasers
II III IV V VI VIIBe B C N O FMg Al Si P S ClCa Zn Ga Ge As Se BrSr Cd In Sn Sb Te I
p-type n-type
dopants for Si and Ge
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Elemental Compound semiconductors no. of electronsIV-IV bonding III-V bonding II-VI bonding per unit
C 6 SiC 10 Si AlP 14 GeSi AlAs, GaP ZnS 23 Ge AlSb,GaAs,InP ZnSe,CdS 32
GaSb, InAs ZnTe, CdSe,HgS 41 Sn InSb CdTe,HgSe 50
HgTe 66
atomic bonding forces become more ionic
Elemental & Compound Semiconductors
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Elemental and compound semiconductors havecrystalline, polycrystalline or amorphous structure.Crystalline Solids: Atoms are arranged in a periodic fashionAmorphous solids: No periodic structure at allPolycrystalline: Many small regions of single-crystal material
Lattice: The periodic arrangement of atoms in a crystalBasic Lattice: simple cubic, body-centered cubic, face-centered
cubicMiller Indices: The smallest set of integers (h,l,m) proportional
to (1/a, 1/b,1/c)Crystal Growth: Czochralski Si, Floating-Zone Si, High Pressure Bridgman (HPB), Travelling Heater Method (THM), Modified Markov Techique (MMT)...Epitaxy:
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Gallium Arsenide (GaAs) has a zinc-blend structure, which is a superstructure of the diamond structures.
Silicon is the most widespread semi- conductor used for digital electronics.
Si is cheap, abundant, structurally robust and environmentally harmless.
Crystalline Solids Polycrystalline
Amorphous: No periodic structure
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Se
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• Lattice symmetry is essential: atomic shells electron energy bands Energy gap between valence and conduction bands.
• Dope material with nearby valence atoms: • donor atoms n-type• acceptor atoms p-type
• Dopants provide shallow doping levels (normally ionized at room temperature)•conduction band occupied at room temperature•NB strong T dependence
• Two basic devices: p-n diode, MOS capacitor
Detector Structure
conduction band Bandgap
+
-
electron
valence band
Si: Eg = 1.1 eV, c= 1130 nm
hole
hElectron-hole generation
E
Simple detector: conductivityincrease of semiconductor when illuminated. P-I-N photo-detector: low dark
current, quick response.
Reverse biased!
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Zinc Blende Semiconductors
Similar shading indicates complementary pairs that preserve the total valence electron count for AZ stoichiometry. In the zinc blende structure each AZ atom is four coordinate.
F
I
Pb AtAu
B
Al
Ga
In
Tl
C
Si
Ge
Sn
N
P
As
Sb
Bi
O
S
Se
Te
Po
Cl
BrCu
Ag
Zn
Cd
Hg
11 12
13 14 15 16 17
• sphalerite (ZnS) structure: like diamond only involving two different types of atoms
• note no atom of an element is bonded to another of the same element
Material Properties at Room Temperature (295K) Xo(cm) (g/cm3) Eg(eV) (cm) ee(cm2/V) hh(cm2/V)
Instrument design aims at optimal use of the sensor technology in different applications.
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Material Resolution DQE MTF Frame Rate Fill Factor (line-pairs/mm) (%) (5lp/mm) (frames/sec) (%)
a-Se 2.5-4 10-70 0.2-15 57-86
a-Si 2.5-4 10-70 0.3-0.4 0.2-15 57-80
Cd0.9Zn0.1Te 11-13 >90 0.7 15-30 100
Bench Marks in Instrument Design
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Resolution, Detective Quantum Efficiency (DQE), Modular Transfer Function (MTF), Frame rate and Fill Factor constitute the bench marks for instrument design