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 -  -Ray Detection Astronomy - X-ray Detection Non-destructive testing Risto Orava June 2002 Need to improve performance & reduce the dose. 1

2. 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 material technologies are combined with device engineering 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 TechnologyDevice EngineeringInstrument Design 1

3. I Material Technology Need high purity, homogenous, defect-free material High Z- small radiation length X o for high QE (X o = 716.4gcm -2 A/[Z(Z+1)ln(287/Z)]) Large enough band gap- high resistivity (> 10 9  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) 1

4. Why compound semiconductors? Uniqueness of compound semiconductors –Band gap engineering Heterostructure devices Hg 1-x Cd x Te : -0.25 ~ 1.6 eV Al x Ga 1-x As : –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 –Ge 2 O 3 and As 2 O 3 : oxidation rates are different. 1

5. 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 Semiconductors -classification 1

6. Elemental and compound semiconductors are in everyday use. Elementary semiconductorsSi, Ge IV CompoundsSiC, SiGe Binary III-V CompoundsAlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb Binary II-VI CompoundsZnS, ZnSe, ZnTe, CdS, CdSe, CdTe Sirectifiers, transistors, IC’s Geearly transistors and diodes Compoundshigh-speed devices, light absorption applications GaAs, GaPLED’s ZnSfluorescent - TV screens InSb, CdSe, PbTe, HgCdTelight detectors Si, GeIR and ionizing radiation detectors GaAs, InPmicrowaves (the Gunn diode) GaAs, AlGaAs,...semiconductor lasers IIIIIIVVVIVII BeBCNOF MgAlSiPSCl Ca ZnGaGeAsSeBr Sr CdInSnSbTeI p-typen-type dopants for Si and Ge 1

7. Elemental Compound semiconductorsno. of electrons IV-IV bonding III-V bondingII-VI bondingper unit C 6 SiC 10 SiAlP 14 GeSiAlAs, GaPZnS 23 GeAlSb,GaAs,InPZnSe,CdS 32 GaSb, InAsZnTe, CdSe,HgS 41 SnInSbCdTe,HgSe 50 HgTe 66 atomic bonding forces become more ionic Elemental & Compound Semiconductors 1

8. 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: 1

9. 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 SolidsPolycrystalline Amorphous: No periodic structure 1

10. Se 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

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

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. Material Properties at Room Temperature (295K) X o (cm)  (g/cm 3 ) E g (eV)  (  cm)  e  e (cm 2 /V)  h  h (cm 2 /V) Diamond(IV) 12 3.51 5.5 >10 11 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 10 12 1.5  10 -9 1.4  10 -7 Si(IV) 9.4 2.33 1.12 <10 4 0.4 0.2 Compound semiconductor properties - Elemental 1 Structure e/h-mobility e/h-lifetime growth availability/ cm 2 /V  s yield Diamond diamond 2800/130-2010 Ge diamond 3900/190 Se monoclinic Si diamond 1600/430 Intrinsic Dielectric W e-h pairs carrier constant (eV) per 0.3%X o density (cm -3 ) Diamond 5.7 13 7200 Ge 16 2.9 16000 Se Si 6.68  10 9 11.9 3.6 26000 Se Ge Si

14. Compound semiconductor properties - Binary II-VI Material Properties at Room Temperature (295K) X o (cm)  (g/cm 3 ) E g (eV)  (  cm)  e  e (cm 2 /V)  h  h (cm 2 /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 10 9 3.3  10 -3 2.2  10 -4 Hg(II)I 2 () 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 1

15. Material Properties at Room Temperature (295K) Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm 2 /V  s 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 HgI 2 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 Compound semiconductor properties - Binary II-VI

16. Material Yield of e-h pairs/0.3%X o at Room Temperature (295K) X o (cm) Intrinsic Dielectric W e-h pairs carrier constant (eV) per 0.3%X o density (cm -3 ) Cd(II)S Cd(II)Se 10.2 Cd(II)Te 1.5 10.2 4.4 6600 HgI 2 4.2 4.2 HgS HgSe HgTe ZnS 8.9 ZnSe 9.1 ZnTe 7.4 1

17. Material Properties at Room Temperature (295K) X o (cm)  (g/cm 3 ) E g (eV)  (  cm)  e  e (cm 2 /V)  h  h (cm 2 /V) Al(III)As(V) 3.717 2.153 Al 0.5 (III)Ga 0.5 (V) x.y 5.85 1.44 >10 5 3.3  10 -3 2.2  10 -4 Al(III)N(V)* x.y 3.285/3.255 /6.2 10 11 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 10 7 8  10 -3 4  10 -6 Ga(III)N(V)* x.y 6.10/6.095 3.24/3.44 >10 11 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 Compound semiconductor properties - Binary III-V 1

18. Material Properties at Room Temperature (295K) Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm 2 /V  s yield AlAs 75-294/ Al 0.5 Ga 0.5 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/ GaSb4000-5000/680-1000 InN* 3200/ InP 4000-5000/150-600 InSb 70000-100000/500-1700 1

19. Compound semiconductor properties - Binary III-V Material Yield of e-h pairs/0.3%X o at Room Temperature (295K) X o (cm) Intrinsic Dielectric W e-h pairs carrier constant (eV) per 0.3%X o density (cm -3 ) AlAs Al 0.5 Ga 0.5 AlN x.y 3.285/3.255 4.6-8.5/9.14 AlP AlSb CdS GaAs 2.3 2.1  10 6 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 1

20. Material Properties at Room Temperature (295K) X o (cm)  (g/cm 3 ) E g (eV)  (  cm)  e  e (cm 2 /V)  h  h (cm 2 /V) Al x Ga 1-x As 1.424+1.247x Al x Ga 1-x Sb 0.76+1.129x+0.368x 2 Al x In 1-x As 0.360+2.012+0.698x 2 Al x In 1-x P 1.351+2.23x Al x In 1-x Sb 0.172+1.621x+0.43x 2 GaAs x Sb 1-x 0.726-0.502x+1.2x 2 Ga x In 1-x As 0.36+1.064x Ga x In 1-x Sb 0.172+0.139x+0.415x 2 Ga x In 1-x P 1.351+0.643x+0.786x 2 GaP x As 1-x 1.42+1.150x+0.176x 2 InAs x Sb 1-x 0.18-0.41x+0.58x 2 In x Ga 1-x N 3.44-3.0x InP x As 1-x 0.360+0.891x+0.101x 2 CdZn 0.1 Te 49.1 5.78 1.57 2  10 10 4  10 -3 (0.2-5.0)  10 -5 Sl-GaAs 5.32 10 -5 10 -6 Compound semiconductor properties - ternary 1

21. Material Properties at Room Temperature (295K) Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm 2 /V  s yield Al x Ga 1-x As Al x Ga 1-x Sb Al x In 1-x As Al x In 1-x P Al x In 1-x Sb GaAs x Sb 1-x Ga x In 1-x As Ga x In 1-x Sb Ga x In 1-x P GaP x As 1-x InAs x Sb 1-x In x Ga 1-x N InP x As 1-x CdZn 0.1 Te - large poly 1000/50 1.0/1.0 HPB OK? Sl-GaAs 1

22. Compound semiconductor properties - ternary Material Yield of e-h pairs/0.3%X o at Room Temperature (295K) X o (cm) Intrinsic Dielectric W e-h pairs carrier constant (eV) per 0.3%X o density (cm -3 ) Al x Ga 1-x As Al x Ga 1-x Sb Al x In 1-x As Al x In 1-x P Al x In 1-x Sb GaAs x Sb 1-x Ga x In 1-x As Ga x In 1-x Sb Ga x In 1-x P GaP x As 1-x InAs x Sb 1-x In x Ga 1-x N InP x As 1-x CdZn 0.1 Te x.y 11 4.7 Sl-GaAs 1

23. Material Properties at Room Temperature (295K) X o (cm)  (g/cm 3 ) E g (eV)  (  cm)  e  e (cm 2 /V)  h  h (cm 2 /V) a-Se 4.3 2.3 10 12 5  10 -9 1.4  10 -7 a-Si 2.3 1.8 10 12 6.8  10 -8 2  10 -8 Compound semiconductor properties - amorphous 1 Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm 2 /V  s yield a-Se 0.13/0.007 a-Si 1/0.1 X o (cm) Intrinsic Dielectric W e-h pairs carrier constant (eV) per 0.3%X o density (cm -3 ) a-Se 6.6 a-Si 11.7

24. Material Properties at Room Temperature (295K) X o (cm)  (g/cm 3 ) E g (eV)  (  cm)  e  e (cm 2 /V)  h  h (cm 2 /V) Pb(II)I 2 () 6.22.3 10 12 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 10 11 10 -4 10 -5 Compound semiconductor properties - other 1 Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm 2 /V  s yield PbI 2 hexag.crystal 8/2 SiC** 200/20(800-400/320-90) Tl(I)Br* cubic 30/7 X o (cm) Intrinsic Dielectric W e-h pairs carrier constant (eV) per 0.3%X o density (cm -3 ) PbI 2 SiC** 8.1 <10 10 9.7 15900 Tl(I)Br*

25. Compound semiconductor properties 1

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

29. III-V Nitrides 1

30. Ge SiGaAs CdTe Compound semiconductor properties 1

31. GaSe HgI 2 PbI 2 TlBr Compound semiconductor properties 1

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

33. 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. 1

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

35. 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 Cd 0.9 Zn 0.1 Te 11-13 >90 0.7 15-30 100 Bench Marks in Instrument Design 1 Resolution, Detective Quantum Efficiency (DQE), Modular Transfer Function (MTF), Frame rate and Fill Factor constitute the bench marks for instrument design