PARTICLE PHYSICS Semiconductor particle detectors Pierpaolo Palestri DPIA

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PARTICLE PHYSICS 粒子物 Semiconductor particle detectors Pierpaolo Palestri DPIA, University of Udine (palestri@uniud. it)

PARTICLE PHYSICS 粒子物 Semiconductor particle detectors Pierpaolo Palestri DPIA, University of Udine (palestri@uniud. it) Thanks to: M. Cobal, A. Cristofoli, G. Dalla Betta, A. Dalla Costa, M. P. Giordani, G. Giugliarelli, A. Micelli, A. Pilotto, L. Selmi, A. Vacchi 1

PARTICLE PHYSICS 粒子物 25 m Detectors in high-energy physic 7000 t 46 m ATLAS

PARTICLE PHYSICS 粒子物 25 m Detectors in high-energy physic 7000 t 46 m ATLAS detector • 3 Layers: B-Layer, 286 modules Layer-1, 494 modules Layer-2 676 modules • 6 disks: 144 modules Pixel detector 35 cm Pixel detector module 1, 3 m 2

PARTICLE PHYSICS 粒子物 What a detector does • the incoming particle generates free carriers

PARTICLE PHYSICS 粒子物 What a detector does • the incoming particle generates free carriers inside a semiconductor • the electric field inside the device separates the charges and creates a current at the terminals • the electrical signal is amplified by an electronic read-out 3

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits for detectors Effect of radiation on performance From planar to 3 D structures 4

PARTICLE PHYSICS 粒子物 Semiconductors (1) • group IV: Si, Ge • III-V compounds (Ga.

PARTICLE PHYSICS 粒子物 Semiconductors (1) • group IV: Si, Ge • III-V compounds (Ga. As, In. P, In. As, Ga. N…) • Alloys (Si 1 -x. Gex, In 1 -x. Gax. As) • Cd. Te. Se, Zn. Te. Se, Cd. Zn. Te • behave as conductive or insulating materials by adding impurities a/o changing the device bias 5

PARTICLE PHYSICS 粒子物 Semiconductors (2) • crystal structure of silicon 8 electrons in the

PARTICLE PHYSICS 粒子物 Semiconductors (2) • crystal structure of silicon 8 electrons in the outer shell simplified 2 D view 6

PARTICLE PHYSICS 粒子物 Free carriers: electrons and holes one electron can break the bond

PARTICLE PHYSICS 粒子物 Free carriers: electrons and holes one electron can break the bond and become a free carrier (negative charge) the hole that is left can move too: free carrier with positive charge 7

PARTICLE PHYSICS 粒子物 Energy bands single atom: energy levels crystal: energy bands free electrons

PARTICLE PHYSICS 粒子物 Energy bands single atom: energy levels crystal: energy bands free electrons in the outer shell 8

PARTICLE PHYSICS 粒子物 Doping (1) Si Si As Si Si B Si Si doping

PARTICLE PHYSICS 粒子物 Doping (1) Si Si As Si Si B Si Si doping with donors: As has 5 electrons in the outer shell one is not needed for bonding and becomes a free electron doping with acceptors: B has 3 electrons in the outer shell and this results in a hole that can move in the crystal 9

PARTICLE PHYSICS 粒子物 Doping (2) Conduction Band Donor Level Acceptor Level Valence Band Ed

PARTICLE PHYSICS 粒子物 Doping (2) Conduction Band Donor Level Acceptor Level Valence Band Ed electron from the donor easily jumps to the conduction band Ec Donor ionization energy Acceptor ionization energy Ea Ev electron in the valence band easily captured by the acceptor a hole is left in the valence band 10

PARTICLE PHYSICS 粒子物 Doping (3) It can be shown that free electron concentration with

PARTICLE PHYSICS 粒子物 Doping (3) It can be shown that free electron concentration with hole concentration intrinsic density ( 1010 cm-3 in Si@room temperature) Charge neutrality: Example: Na=1016 cm-3 , Nd=0 p=1016 cm-3 , n=104 cm-3 Note: density of atoms in Si is 5 1022 cm-3 11

PARTICLE PHYSICS 粒子物 e-h generation by radiation electron Ec photons Eg photon energy: h

PARTICLE PHYSICS 粒子物 e-h generation by radiation electron Ec photons Eg photon energy: h v > E g Ev hole • the photon promotes an electron in the conduction band • a hole is left in the valence band • if h >> Eg: number of pairs h / Eg • The same happens with many types of particles, not only photons • To measure the generated e-h pairs we need to separate electrons and holes p-n junction 12

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits for detectors Effect of radiation on performance From planar to 3 D structures 13

PARTICLE PHYSICS 粒子物 Structure N-region: • donor doping; • plenty of free electrons •

PARTICLE PHYSICS 粒子物 Structure N-region: • donor doping; • plenty of free electrons • balanced by fixed positive charge – V + I N P I V Reverse bias P-region: • acceptor doping; • plenty of free holes • balanced by fixed negative charge Forward bias region where the detectors operate 14

PARTICLE PHYSICS 粒子物 Charge profile (1) at the border between the N and P

PARTICLE PHYSICS 粒子物 Charge profile (1) at the border between the N and P region we have very few free carriers N neutral N-region: • donor doping; • plenty of free electrons • balanced by fixed positive charge P depletion region: only fixed charges neutral P-region: • acceptor doping; • plenty of free holes • balanced by fixed negative charge 15

PARTICLE PHYSICS 粒子物 Charge profile (2) N n=ND p 0 =0 neutral N-region: •

PARTICLE PHYSICS 粒子物 Charge profile (2) N n=ND p 0 =0 neutral N-region: • donor doping; • plenty of free electrons • balanced by fixed positive charge P n 0 p 0 =q. ND =-q. NA depletion region: only fixed charges p=NA n 0 =0 neutral P-region: • acceptor doping; • plenty of free holes • balanced by fixed negative charge 16

PARTICLE PHYSICS 粒子物 Poisson equation Gauss’s Law: a ar e s: permittivity (~12 o

PARTICLE PHYSICS 粒子物 Poisson equation Gauss’s Law: a ar e s: permittivity (~12 o for Si) : charge density (C/cm 3) E(x) A E(x + Dx) Dx x Poisson’s equation 17

PARTICLE PHYSICS 粒子物 Electric field profile (1) N P =0 =q. ND =-q. NA

PARTICLE PHYSICS 粒子物 Electric field profile (1) N P =0 =q. ND =-q. NA =0 E d. E/dx=q. ND/ S d. E/dx=-q. NA / S x Area=applied potential + built-in different chemical potential of N and P region ( 1 V) 18

PARTICLE PHYSICS 粒子物 Electric field profile (2) E Area=applied potential + built-in N P

PARTICLE PHYSICS 粒子物 Electric field profile (2) E Area=applied potential + built-in N P x • positive electric field electrons (holes) generated by radiation go to the N (P) region 19

PARTICLE PHYSICS 粒子物 Electric field profile (3) N+ N P+ W E d. E/dx=

PARTICLE PHYSICS 粒子物 Electric field profile (3) N+ N P+ W E d. E/dx= q. ND / S d. E/dx=-q. NA+ / S d. E/dx=-q. NA / S x • insertion of a low-doped region: trapezoidal electric field profile same “area” with lower peak electric field 20

PARTICLE PHYSICS 粒子物 Electric field profile (4) • doping in the P+ and N+

PARTICLE PHYSICS 粒子物 Electric field profile (4) • doping in the P+ and N+ regions is large so that: W E d. E/dx=-q. NA / S x • this however requires: to fully deplete the low doping region 21

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits for detectors Effect of radiation on performance From planar to 3 D structures 22

PARTICLE PHYSICS 粒子物 Spatial resolution • the structure needs to be split in stripes

PARTICLE PHYSICS 粒子物 Spatial resolution • the structure needs to be split in stripes or pixels stripes pixels cross section 23

PARTICLE PHYSICS 粒子物 Collected charge • the longer the particle travels in the detector

PARTICLE PHYSICS 粒子物 Collected charge • the longer the particle travels in the detector the larger the V induced charge • collected charge proportional to W A where A is the pixel area. • large W is required, • but the condition for depletion implies using high bias and very low doping levels (high purity crystals) W 24

PARTICLE PHYSICS 粒子物 Dark current • depletion width area generation time 25

PARTICLE PHYSICS 粒子物 Dark current • depletion width area generation time 25

PARTICLE PHYSICS 粒子物 Breakdown voltage (1) high electric field results in unwanted e-h pair

PARTICLE PHYSICS 粒子物 Breakdown voltage (1) high electric field results in unwanted e-h pair generation by impact ionization a free electron hits an electron in the valence band obtaining an additional free electron + a hole the same process can be initiated by a hole the dark current increases significantly 26

PARTICLE PHYSICS 粒子物 Breakdown voltage (2) • reduce peak electric field as much as

PARTICLE PHYSICS 粒子物 Breakdown voltage (2) • reduce peak electric field as much as possible low doping d. E/dx=-q. NA / S E W x • avoid breakdown at the borders guard rings 27

PARTICLE PHYSICS 粒子物 Breakdown voltage (3) multiplication by II can be exploited as internal

PARTICLE PHYSICS 粒子物 Breakdown voltage (3) multiplication by II can be exploited as internal signal amplification (Avalanche Photo Diodes) 28

PARTICLE PHYSICS 粒子物 Speed W • generated e-h pairs need to reach the highdoping

PARTICLE PHYSICS 粒子物 Speed W • generated e-h pairs need to reach the highdoping regions carrier velocity 107 cm/s 29

PARTICLE PHYSICS 粒子物 Capacitance • The electronic read-out converts the generated charge into a

PARTICLE PHYSICS 粒子物 Capacitance • The electronic read-out converts the generated charge into a signal • it adds noise (unwanted random signal) during the process • this noise is amplified by the detector capacitance • C should be as small as possible W area of the pixel 30

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits for detectors Effect of radiation on performance From planar to 3 D structures 31

PARTICLE PHYSICS 粒子物 Effective doping radiation experienced during the experiments creates defects that act

PARTICLE PHYSICS 粒子物 Effective doping radiation experienced during the experiments creates defects that act as additional (unwanted) doping higher and higher biases are needed during the detector lifetime to deplete it, since: 32

PARTICLE PHYSICS 粒子物 Increased dark current radiation experienced during the experiments creates defects assisting

PARTICLE PHYSICS 粒子物 Increased dark current radiation experienced during the experiments creates defects assisting unwanted e-h generation the dark current increases reduced when defect density increases 33

PARTICLE PHYSICS 粒子物 Charge in the oxide • radiation experienced during the experiments creates

PARTICLE PHYSICS 粒子物 Charge in the oxide • radiation experienced during the experiments creates positive charges in the Si. O 2 layer on top of the structure • free electrons are induce inside Si and shorts the pixels • additional p-doped regions to maintain isolation Si. O 2 34

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits

PARTICLE PHYSICS 粒子物 Outline • • • Semiconductors p-n junction Main figures of merits for detectors Effect of radiation on performance From planar to 3 D structures 35

PARTICLE PHYSICS 粒子物 3 D detectors (1) W W d • planar structure: trade-off

PARTICLE PHYSICS 粒子物 3 D detectors (1) W W d • planar structure: trade-off between large W (for collection efficiency) and small W (for high speed and low depletion voltage) • 3 D detector: large W for high collection efficiency, small d for 36 high speed and low depletion voltage

PARTICLE PHYSICS 粒子物 3 D detectors (2) • sample performance for 3 D and

PARTICLE PHYSICS 粒子物 3 D detectors (2) • sample performance for 3 D and planar detectors • vertical electrodes are more difficult to fabricate • p-spray/p-stop needed for isolation after irradiation 3 D Planar Depletion voltage <10 V 70 V Edge sensitivity < 5 μm 500μm Charge 1 MIP (300 mm) 24000 e- Capacitance 30 -50 f. F 20 f. F Collection distance 50μm 300μm Speed 1 -2 ns 10 -20 ns • 3 D are faster and with lower depletion voltage • higher capacitance 37