Semiconductor Scintillator and 3D I ntegration Serge Luryi ECE - PowerPoint PPT Presentation

Semiconductor Scintillator and 3D I ntegration Serge Luryi ECE Department and Sensor CAT Radiation Detection for Homeland Security: • Isotope identification spectroscopic energy resolution • Direction to source angular resolution Apr 24, 2007 Physics Colloquium 1

X-ray ( γ -ray) attenuation absorption length 1 mm 1 cm 1 dm Apr 24, 2007 Physics Colloquium 2

Compton Scattering γ ′ θ E 0 e = 2 m e c 511 KeV γ L 1 = − L E E = 137 ′ E 662 KeV (Cs ) γ γ γ θ 1 L 2 kinematics (Compton): L 3 90 2 2 120 60 m c m c θ 2 2.0 θ = + − cos 1 e e 1.5 σ(θ) θ 3 E E 150 30 1.0 ′ γ γ 0.5 dynamics (Klein-Nishina): 0.0 0.0 180 0 0.5 2 ⎛ ⎞ ⎛ ⎞ 1.0 E E E 210 330 ⎜ ⎟ ⎜ ⎟ ′ ′ γ γ γ σ θ = σ + − θ 2 ( ) sin ( ) 1.5 ⎜ ⎟ ⎜ ⎟ 0 E E E ⎝ ⎠ ⎝ ⎠ γ γ γ ′ 2.0 240 300 270 Apr 24, 2007 Physics Colloquium 3

Diodes and scintillators Si or Ge pin diode NaI scintillator E C Thallium levels cm h ν = 3 eV E G = 7 eV � > 100 nS � > 200 nS � 77K � 38,000 ph/MeV � > 10 kV E V � up to 300,000 e-h/MeV Apr 24, 2007 Physics Colloquium 4

Gamma spectroscopy Apr 24, 2007 Physics Colloquium 5

Main I dea * E F • Radiative decay time ≈ 1 ns E C • Photon re-absorption suppressed due to high Fermi level InP or GaAs h ν (Burstein shift) • Expected light yield ≈ 100% E V • Absolute yield ≈ 240,000 ph/MeV material transparent to its own fundamental light emission photons are delivered to the surface from deep inside the semiconductor *A. Kastalsky, S. Luryi, B. Spivak, Nucl. Instr. and Methods in Phys. Research A 565 , pp. 650-656 (2006) Apr 24, 2007 Physics Colloquium 6

Burstein shift Absorption (10 4 cm -1 ) Emission (a. u.) 1.0 4.0 InP, 300K λ ≈ λ E F / kT e 0.9 3.5 0 undoped 0.8 3.0 0.6 2.5 absorption mean free path λ 0.5 2.0 is exponentially enhanced compared to λ 0 ≈ 1 µm 0.4 1.5 0.3 1.0 n =3.7 · 10 18 cm -3 0.1 0.5 0.0 0.0 1.3 1.4 1.5 1.6 1.7 1.8 Energy (eV) Apr 24, 2007 Physics Colloquium 7

Comparison of I nP scintillator with activated crystalline NaI NaI InP E C E C Thallium levels h ν = 1.3 eV E G = 1.3 eV h ν = 3 eV E G = 7 eV E V 33% E ν h γ 12% × E V 3 E E γ G Apr 24, 2007 Physics Colloquium 8

Response time Radiative recombination E F p = = R Bnp E C τ rad Nonradiative recombination τ rad ≈ 1 ns τ nr ≈ 100 ns p ( Auger ) = ≡ 2 R Cn p nonrad τ E V nr Radiative efficiency 99% with response time : τ rad Apr 24, 2007 Physics Colloquium 9

Free-carrier absorption InP (at 0.92 µm) E F ∆ E 10 150 K Absorption length (mm) 200 E G Free-carrier 250 300 K • Room temperature optimum ≈ 1mm with re-emission several mm 1 (explained on next viewgraph) • Standard GaAs or InP 18 19 10 10 wafer thickness 0.5 mm -3 ) Concentration (cm • Stack up layered systems Apr 24, 2007 Physics Colloquium 10

Diffusive propagation of light N after every interband absorption event ∑ = λ h the photon is regenerated (in a random j = j 1 direction) with probability 99% ∑ ∑ = λ + λ ⋅ λ 2 2 h j i j ≠ j i j = λ 2 2 h N λ j = λ h N λ h N We can take 1 cm thick slab λ and still collect over 80% 1 each re-emission introduces delay τ rad ≈ 1 nS Apr 24, 2007 Physics Colloquium 11

Heterostructures ~ E C1 λ = D λ × E F ≈ D 100 (Duty cycle) E C2 • virtually unlimited absorption length …. • free-carrier absorption also suppressed • tough to make, however ! Emitted light E V2 E V1 Apr 24, 2007 Physics Colloquium 12

Epitaxial detector enables 3D integration 2 µm pin InGaAsP photodiode E G = 1.25 eV n + InP 0.5 mm E G = 1.35 eV scintillator Apr 24, 2007 Physics Colloquium 13

back to DHS applications • Scintillator with a “semiconductor” resolution: excellent isotope discrimination high spectroscopic resolution results from 240,000 photons per MeV and nearly 100% photoelectric conversion at epitaxial photodetector expected Fano factor F=0.1 • Exceptional sensitivity results from virtually unlimited detector thickness by 3D integration: increased stand-off distance • Room temperature operation: moderate cost • Ultrafast response (1 ns): rapidly moving targets • Pixellated and layered photodetector system: directional capability the technique provides both angular resolution and isotope identification Apr 24, 2007 Physics Colloquium 14

3D Pixellation Upon analog-to-digital 2D pixel Flip Chip conversion each unit reports Silicon Photosensitive Circuitry not a 1 ns pulse but an Layer information-carrying signal: γ - Detection Semiconductor • where ionization occurred Slab Data & • time of the event Power Buses Stack of Detector • amplitude of the event Slabs Apr 24, 2007 Physics Colloquium 15

Photodetection Matrix m × n detector array + V BIAS Photo m + n # of data lines Detector m and converters I PD “Y” 0 B VDD u Data s Converter A m 0 2-D CPU n A m 0 0 “X” Bus GND Position + Intensity Apr 24, 2007 Physics Colloquium 16

Partition between I nP and Si V CC I B flip-chip I C = β I B pin photodiode R C n + scintillator A Address Apr 24, 2007 Physics Colloquium 17

Compton “telescope” cos θ = + − − − 1 1 1 E E kinematics: − i i 1 i 2 = = − L E E m e c 511 keV) (in units of − i i 1 i E 0 Having identified first three interactions (in the correct order) we find the energy L 1 of the incident photon: ˆ n θ 1 0 L 2 ⎛ ⎞ L 1 4 L ⎜ ⎟ = + + + 2 2 2 E L L ⎜ ⎟ L 3 0 1 2 − θ 2 2 1 cos ⎝ ⎠ θ 2 2 ˆ n θ 3 1 ⋅ ˆ 0 ˆ n n Also the directional cosine 1 about the measured direction n 1 i.e., the object is placed on a cone Apr 24, 2007 Physics Colloquium 18

Direction to source dynamics (Klein-Nishina formula): 2 ⎛ ⎞ ⎛ ⎞ r E E E cluster of n ⎜ ⎟ ⎜ ⎟ σ θ = σ + − θ − 2 ( ) i i i 1 sin ( ) ρ ⎜ ⎟ ⎜ ⎟ i 0 i E E E ⎝ ⎠ ⎝ ⎠ interactions − − i 1 i 1 i 1 90 anisotropic scattering C enter 120 60 2.0 cross-section 1.5 σ(θ) of mass 150 30 1.0 662 keV : 0.5 0.0 0.0 180 0 r 0.5 δ N 1 r r v ( ) ∑ n 1.0 ρ ≡ ρ = − ρ + ( j ) np 1 210 330 1 1 n 1.5 n N N = j 1 n n 2.0 240 300 270 Apr 24, 2007 Physics Colloquium 19

GaAs versus I nP • GaAs pros • Higher radiative recombination coefficient, while nonradiative similar ≈ ≈ B 7 B C C GaAs InP GaAs InP • Higher bandgap (good for low-noise photodetection at room temperature) • Lower cost, mature electronics • GaAs cons • Lighter weight elements • Epitaxial detector questionable Apr 24, 2007 Physics Colloquium 20

X-ray ( γ -ray) attenuation 100000 Element Z -1 ) Attenuation Coefficients (cm 10000 CdTe Si 14×2 InP 1000 Ge Ga/As 31/33 Si 100 Ge 32×2 10 In/P 49/15 1 Cd/Te 48/52 0.1 1E-3 0.01 0.1 1 10 h ν (MeV) Apr 24, 2007 Physics Colloquium 21

GaAs versus I nP epitaxial photodetector issues 2 µm pin photodiode E G = 1.25 eV E G = 1.35 eV InGaAsP (In)GaAs-N n + scintillator 0.5 mm E G = 1.35 eV E G = 1.45 eV InP GaAs U. Michigan (Rachel Goldman) collaboration: dilute nitride epitaxial photodiodes on GaAs Apr 24, 2007 Physics Colloquium 22

Dilute Nitride Due to their high electronegativity and small size, atoms of nitrogen, when added in small amount (atomic %) to III-V compounds, dramatically reduce the material bandgap. GaN see, e.g. “Dilute Nitride Semiconductors”, ed. by M. Henini, Elsevier (2005) GaAs Apr 24, 2007 Physics Colloquium 23

Summary � The basic issue for any semiconductor scintillator semiconductor is not transparent to the radiation it produces � We have resolved this issue Moss-Burstein shift in a direct-gap, low-effective-mass semiconductor, such as InP or GaAs, makes it transparent to its own radiation, permits extracting photons from as deep as several millimeters from the detector surface � New type of scintillator is proposed, advantageous for: multiple applications, including 3D pixellation for: radiation spectrometry and SNM identification rapid determination of direction to source Apr 24, 2007 Physics Colloquium 24

Cooperative Agreement 2007-DN-077-ER0005 (started March 1, 2007) “Semiconductor high-energy radiation detector with excellent isotope identification and directional capability” SB Team: PI’s Other SB participants U. Michigan (Ann Arbor) Serge Luryi Michael Gouzman Rachel Goldman Alex Kastalsky Oleg Semenov Brookhaven National Lab Nadia Lifshitz Peter Shkolnikov Aleksey Bolotnikov Milutin Stanacevic Apr 24, 2007 Physics Colloquium 25