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New Materials and Designs of Semiconductor Detectors

1. New Materials and Designs of Semiconductor Detectors. New developments are driven by particle physics and applications in:. Medical & Synchrotron X-ray Imaging Nuclear Medicine - g -Ray Detection Astronomy - X-ray Detection Non-destructive testing.

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New Materials and Designs of Semiconductor Detectors

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  1. 1 New Materials and Designs of Semiconductor Detectors New developments are driven by particle physics and applications in: • Medical & Synchrotron X-ray Imaging • Nuclear Medicine - g-Ray Detection • Astronomy - X-ray Detection • Non-destructive testing Need to improve performance & reduce the dose. Risto Orava June 2002

  2. For high performance detectors material technologies are combined with device engineering and instrument design. 1 Material Technology Device Engineering Instrument Design • 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,... • 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 • g-energies • Packaging • Operating environment: • Temperature • Radiation • Electronic noise • Mechanical stresses • Resolution • DQE • MTF • Frame rate • Fill factor

  3. I Material Technology 1 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 mt product - the carrier drift length, mtE (m=carrier mobility, t=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)

  4. 1 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.

  5. 1 Semiconductors -classification semiconductors electronic semiconductors mixed conductors ionic conductors intrinsic semiconductors extrinsic semiconductors n-type extrinsic p-type extrinsic • 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 covalent bonding

  6. Elemental and compound semiconductors are in everyday use. 1 II III IV V VI VII Be B C N O F Mg AlSiP S Cl Ca Zn GaGeAs Se Br Sr Cd In Sn Sb Te I p-type n-type dopants for Si and Ge Elementary semiconductors Si, Ge IV Compounds SiC, SiGe Binary III-V Compounds AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb Binary II-VI Compounds ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe Si rectifiers, transistors, IC’s Ge early transistors and diodes Compounds high-speed devices, light absorption applications GaAs, GaP LED’s ZnS fluorescent - TV screens InSb, CdSe, PbTe, HgCdTe light detectors Si, Ge IR and ionizing radiation detectors GaAs, InP microwaves (the Gunn diode) GaAs, AlGaAs,... semiconductor lasers

  7. 1 Elemental & Compound Semiconductors Elemental Compound semiconductors no. of electrons IV-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

  8. 1 Elemental and compound semiconductors have crystalline, polycrystalline or amorphous structure. Crystalline Solids: Atoms are arranged in a periodic fashion Amorphous solids: No periodic structure at all Polycrystalline: Many small regions of single-crystal material Lattice: The periodic arrangement of atoms in a crystal Basic Lattice: simple cubic, body-centered cubic, face-centered cubic Miller 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:

  9. 1 Crystalline Solids Polycrystalline Silicon is the most widespread semi- conductor used for digital electronics. Si is cheap, abundant, structurally robust and environmentally harmless. Gallium Arsenide (GaAs) has a zinc-blend structure, which is a superstructure of the diamond structures. Amorphous: No periodic structure

  10. 1 • 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 Se

  11. Detector Structure 1 E electron conduction band - h Reverse biased! Band gap Electron-hole generation + valence band hole Si: Eg = 1.1 eV, c= 1130 nm Simple detector: conductivity increase of semiconductor when illuminated. P-I-N photo-detector: low dark current, quick response.

  12. Zinc Blende Semiconductors • 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

  13. 1 Compound semiconductor properties - Elemental Material Properties at Room Temperature (295K) Xo(cm) r(g/cm3) Eg(eV) r(cm) mete(cm2/V) mhth(cm2/V) Diamond(IV) 12 3.51 5.5 >1011 210-3 <1.610-3 Ge(IV) 2.3 5.32 0.66 50 0.8 0.8 Se(VI) x.y 4.82 2.3 1012 1.510-9 1.410-7 Si(IV) 9.4 2.33 1.12 <104 0.4 0.2 Ge Structure e/h-mobility e/h-lifetime growth availability/ cm2/V ms yield Diamond diamond 2800/130-2010 Ge diamond 3900/190 Se monoclinic Si diamond 1600/430 Se Intrinsic Dielectric W e-h pairs carrier constant (eV) per 0.3%Xo density (cm-3) Diamond 5.7 13 7200 Ge 16 2.9 16000 Se Si 6.68109 11.9 3.6 26000 Si

  14. 1 Compound semiconductor properties - Binary II-VI Material Properties at Room Temperature (295K) Xo(cm) r(g/cm3) Eg(eV) r(cm) mete(cm2/V) mhth(cm2/V) Cd(II)S(VI) 2.1 4.87 2.5 Cd(II)Se(VI) 5.655 1.751 Cd(II)Te(VI) 1.5 5.86 1.475 109 3.310-3 2.210-4 Hg(II)I2()1.2 6.40 2.13 Hg(II)S(VI) 7.72 Hg(II)Se(VI) 8.22 Hg(II)Te(VI) 8.12 Zn(II)S(VI) 4.11 3.68-3.911 Zn(II)Se(VI) 5.26 2.822 Zn(II)Te(VI) 5.65 2.394

  15. Compound semiconductor properties - Binary II-VI Material Properties at Room Temperature (295K) Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm2/V ms yield Cd(II)S wurzite 340/340 Cd(II)Se wurzite 650/10 Cd(II)Te Cl zincblende 1050/100 2.0/2.0 THM critical HgI2 50-65/ HgS zincblende 10-30/10-30 HgSe zincblende 1.5/ HgTe zincblende 35/ ZnS* 165/5(?/100-800) ZnSe 500/30 ZnTe 330-530/100-900

  16. 1 Compound semiconductor properties - Binary II-VI Material Yield of e-h pairs/0.3%Xo at Room Temperature (295K) Xo(cm) IntrinsicDielectric W e-h pairs carrier constant (eV) per 0.3%Xo density (cm-3) Cd(II)S Cd(II)Se 10.2 Cd(II)Te 1.5 10.2 4.4 6600 HgI2 4.2 4.2 HgS HgSe HgTe ZnS 8.9 ZnSe 9.1 ZnTe 7.4

  17. 1 Compound semiconductor properties - Binary III-V Material Properties at Room Temperature (295K) Xo(cm) r(g/cm3) Eg(eV) r(cm) mete(cm2/V) mhth(cm2/V) Al(III)As(V)3.7172.153 Al0.5(III)Ga0.5(V)x.y 5.85 1.44 >105 3.310-3 2.210-4 Al(III)N(V)* x.y 3.285/3.255 /6.2 1011 1.010-3 510-4 Al(III)P(V) 2.45 Al(III)Sb(V) 4.29 1.615 Ga(III)As(V) 2.3 5.318 1.424 107 810-3 410-6 Ga(III)N(V)* x.y 6.10/6.095 3.24/3.44 >1011 210-3 <1.610-3 Ga(III)P(V) 3.5 4.129 2.272 Ga(III)Sb(V) 5.63 0.75 In(III)As(V) In(III)N(V)* 6.93/6.81 /1.89-2.00 In(III)P(V) In(III)Sb(V) 5.80 0.17

  18. 1 Compound semiconductor properties - Binary III-V Material Properties at Room Temperature (295K) Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm2/V ms yield AlAs75-294/ Al0.5 Ga0.5 AlN 300/14 AlP 80/ AlSb 200-900/200-400 CdS 250-300/15? GaAs 9200/400 GaN 1000-1350/100-350 GaP 300-400/ GaSb 4000-5000/680-1000 InN* 3200/ InP 4000-5000/150-600 InSb 70000-100000/500-1700

  19. 1 Compound semiconductor properties - Binary III-V Material Yield of e-h pairs/0.3%Xo at Room Temperature (295K) Xo(cm) IntrinsicDielectric W e-h pairs carrier constant (eV) per 0.3%Xo density (cm-3) AlAs Al0.5 Ga0.5 AlN x.y 3.285/3.255 4.6-8.5/9.14 AlP AlSb CdS GaAs 2.3 2.1106 12.5 4.3 11000 GaN 5.35-8.9/9.5-10.4 GaP 3.5 11 5200 GaSb InN* 8.4-15.3 InP 2.1 13 4.2 8900 InSb

  20. Compound semiconductor properties - ternary 1 Material Properties at Room Temperature (295K) Xo(cm) r(g/cm3) Eg(eV) r(cm) mete(cm2/V) mhth(cm2/V) AlxGa1-xAs 1.424+1.247x AlxGa1-xSb 0.76+1.129x+0.368x2 AlxIn1-xAs 0.360+2.012+0.698x2 AlxIn 1-x P 1.351+2.23x AlxIn 1-x Sb 0.172+1.621x+0.43x2 GaAsxSb1-x 0.726-0.502x+1.2x2 GaxIn1-xAs0.36+1.064x GaxIn1-xSb 0.172+0.139x+0.415x2 GaxIn1-xP1.351+0.643x+0.786x2 GaPxAs1-x 1.42+1.150x+0.176x2 InAsxSb1-x 0.18-0.41x+0.58x2 InxGa1-xN 3.44-3.0x InPxAs1-x0.360+0.891x+0.101x2 CdZn0.1Te 49.1 5.78 1.57 21010 410-3 (0.2-5.0)10-5 Sl-GaAs 5.32 10-5 10-6

  21. 1 Compound semiconductor properties - ternary Material Properties at Room Temperature (295K) Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm2/V ms yield AlxGa1-xAs AlxGa1-xSb AlxIn1-xAs AlxIn 1-x P AlxIn 1-x Sb GaAsxSb1-x GaxIn1-xAs GaxIn1-xSb GaxIn1-xP GaPxAs1-x InAsxSb1-x InxGa1-xN InPxAs1-x CdZn0.1Te - large poly 1000/50 1.0/1.0 HPB OK? Sl-GaAs

  22. Compound semiconductor properties - ternary 1 Material Yield of e-h pairs/0.3%Xo at Room Temperature (295K) Xo(cm) IntrinsicDielectric W e-h pairs carrier constant (eV) per 0.3%Xo density (cm-3) AlxGa1-xAs AlxGa1-xSb AlxIn1-xAs AlxIn 1-x P AlxIn 1-x Sb GaAsxSb1-x GaxIn1-xAs GaxIn1-xSb GaxIn1-xP GaPxAs1-x InAsxSb1-x InxGa1-xN InPxAs1-x CdZn0.1Te x.y 11 4.7 Sl-GaAs

  23. 1 Compound semiconductor properties - amorphous Material Properties at Room Temperature (295K) Xo(cm) r(g/cm3) Eg(eV) r(cm) mete(cm2/V) mhth(cm2/V) a-Se 4.3 2.3 1012 510-9 1.410-7 a-Si 2.3 1.8 1012 6.810-8 210-8 Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm2/V ms yield a-Se 0.13/0.007 a-Si 1/0.1 Xo(cm) IntrinsicDielectric W e-h pairs carrier constant (eV) per 0.3%Xo density (cm-3) a-Se 6.6 a-Si 11.7

  24. 1 Compound semiconductor properties - other Material Properties at Room Temperature (295K) Xo(cm) r(g/cm3) Eg(eV) r(cm) mete(cm2/V) mhth(cm2/V) Pb(II)I2()6.2 2.3 1012 810-6 Si(IV)C(IV)** 8.1 3.21 2.36-3.23 Tl(I)Br(VII)* 81/35 7.5 2.7 1011 10-4 10-5 Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm2/V ms yield PbI2 hexag.crystal 8/2 SiC** 200/20(800-400/320-90) Tl(I)Br* cubic 30/7 Xo(cm) IntrinsicDielectric W e-h pairs carrier constant (eV) per 0.3%Xo density (cm-3) PbI2 SiC** 8.1 <1010 9.7 15900 Tl(I)Br*

  25. Compound semiconductor properties 1

  26. Antimonide-Based Compound Semiconductors(6.1 Angstrom Compounds) 3 Band Gap (eV) 2 1 0 5.4 5.6 5.8 6.0 6.2 6.4 6.6 Lattice Constant (Å)

  27. 1 III-V Nitrides

  28. 1 Compound semiconductor properties CdTe Si GaAs Ge

  29. 1 Compound semiconductor properties TlBr HgI2 PbI2 GaSe

  30. 1 II Device Engineering Device engineering facilitates efficient, robust and stable sensor operation. • 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

  31. 1 Detector configuration is optimized for optimum performance for a given application. Single element planar structure Co-planar grid structure Pixel detector structure -small pixel effect.

  32. 1 III Instrument Design Instrument design aims at optimal use of the sensor technology in different applications. • Modality • g-energies • Packaging • Operating environment: Temperature, Radiation, • Electronic noise, Mechanical • stresses • Resolution • DQE • MTF • Frame rate • Fill factor

  33. 1 Bench Marks in Instrument Design Resolution, Detective Quantum Efficiency (DQE), Modular Transfer Function (MTF), Frame rate and Fill Factor constitute the bench marks for instrument design 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

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