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.

Elementary semiconductors Si, GeIV Compounds SiC, SiGeBinary III-V Compounds AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSbBinary II-VI Compounds ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe

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)

Diamond(IV) 12 3.51 5.5 >1011 210-3 <1.610-3

Ge(IV) 2.3 5.32 0.66 50 0.8 0.8

Se(VI) x.y 4.82 2.3 1012 1.510-9 1.410-7

Si(IV) 9.4 2.33 1.12 <104 0.4 0.2

Compound semiconductor properties - Elemental1

Structure e/h-mobility e/h-lifetime growth availability/ cm2/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%Xo

density (cm-3) Diamond 5.7 13 7200

Ge 16 2.9 16000

Se

Si 6.68109 11.9 3.6 26000

Se

Ge

Si

Compound semiconductor properties - Binary II-VIMaterial Properties at Room Temperature (295K)

Xo(cm) (g/cm3) Eg(eV) (cm) ee(cm2/V) hh(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 3.310-3 2.210-4

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

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Material Properties at Room Temperature (295K) Dopant Structure e/h-mobility e/h-lifetime growth availability/

cm2/V s yieldCd(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

Compound semiconductor properties - Binary II-VI

Compound semiconductor properties - Binary II-VIMaterial Yield of e-h pairs/0.3%Xo at Room Temperature (295K)

Xo(cm) Intrinsic Dielectric 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

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Material Properties at Room Temperature (295K)

Xo(cm) (g/cm3) Eg(eV) (cm) ee(cm2/V) hh(cm2/V)

Al(III)As(V) 3.717 2.153

Al0.5(III)Ga0.5(V) x.y 5.85 1.44 >105 3.310-3 2.210-4

Al(III)N(V)* x.y 3.285/3.255 /6.2 1011 1.010-3 510-4

Al(III)P(V) 2.45

Al(III)Sb(V) 4.29 1.615

Ga(III)As(V) 2.3 5.318 1.424 107 810-3 410-6

Ga(III)N(V)* x.y 6.10/6.095 3.24/3.44 >1011 210-3 <1.610-3

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

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Compound semiconductor properties - Binary III-VMaterial Properties at Room Temperature (295K)

Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm2/V s yieldAlAs 75-294/

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

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Compound semiconductor properties - Binary III-VMaterial Yield of e-h pairs/0.3%Xo at Room Temperature (295K)

Xo(cm) Intrinsic Dielectric 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 2.1106 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

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Material Properties at Room Temperature (295K) Xo(cm) (g/cm3) Eg(eV) (cm) ee(cm2/V) hh(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-xAs 0.36+1.064x

GaxIn1-xSb 0.172+0.139x+0.415x2

GaxIn1-xP 1.351+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-x 0.360+0.891x+0.101x2

CdZn0.1Te 49.1 5.78 1.57 21010 410-3 (0.2-5.0)10-5

Sl-GaAs 5.32 10 -5 10-6

Compound semiconductor properties - ternary

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Compound semiconductor properties - ternaryMaterial Properties at Room Temperature (295K)

Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm2/V s yieldAlxGa1-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

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Compound semiconductor properties - ternaryMaterial Yield of e-h pairs/0.3%Xo at Room Temperature (295K)

Xo(cm) Intrinsic Dielectric 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

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Material Properties at Room Temperature (295K)

Xo(cm) (g/cm3) Eg(eV) (cm) ee(cm2/V) hh(cm2/V)

a-Se 4.3 2.3 1012 510-9 1.410-7

a-Si 2.3 1.8 1012 6.810-8 210-8

Compound semiconductor properties - amorphous

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Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm2/V

s yield a-Se 0.13/0.007

a-Si 1/0.1 Xo(cm) Intrinsic Dielectric W e-h pairs carrier constant (eV) per 0.3%Xo

density (cm-3)

a-Se 6.6

a-Si 11.7

Material Properties at Room Temperature (295K) Xo(cm) (g/cm3) Eg(eV) (cm) ee(cm2/V)

hh(cm2/V)

Pb(II)I2() 6.2 2.3 1012 810-6

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

Compound semiconductor properties - other

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Dopant Structure e/h-mobility e/h-lifetime growth availability/ cm2/V s yield

PbI2 hexag.crystal 8/2

SiC** 200/20(800-400/320-90) Tl(I)Br* cubic 30/7 Xo(cm) Intrinsic Dielectric 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*

Compound semiconductor properties1

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

)

III-V Nitrides 1

Band Gap vs. e-h pair energy

y = 1.8129x + 1.6948

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

Band Gap (eV)

e-h

cre

atio

n e

ner

gy

(eV

)

Ge

Si GaAs

CdTe

Compound semiconductor properties1

Band Gap vs. e-h pair energy

y = 2.8911x - 1.8306

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Band Gap (eV)

e-h

cre

atio

n e

ner

gy

(eV

)

GaSeHgI2 PbI2

TlBr

Compound semiconductor properties1

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.

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Detector configuration is optimized for optimumperformance for a given application.

Single element planarstructure

Co-planar grid structure

Pixel detector structure-small pixel effect.

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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.

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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