Rad.det.munich 07252011
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Rad.det.munich 07252011

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this talk reviews radiation detection physics with emphasis on high resolution gamma ray detectors using semiconducors

this talk reviews radiation detection physics with emphasis on high resolution gamma ray detectors using semiconducors

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Rad.det.munich 07252011 Rad.det.munich 07252011 Presentation Transcript

  • SEMICONDUCTOR RADIATION DETECTORS
    Eugene E. Haller
    University of California and
    LBNL, Berkeley, CA 94720
    Fourth Summer School in Radiation Detection and Measurements, July 24-30, 2011
    Munich, Germany
  • 7/19/11
    E. E. Haller
    2
    General References
    • "Radiation Detection and Measurement," (4th edition), G.F. Knoll (J. Wiley & Sons, 2010) (IEEE-Short Course Textbook); pp. 353-503.
    • "Nuclear Radiation Detector Materials," eds. E.E. Haller, H.W. Kraner and W.A. Higginbotham, Mat. Res. Soc. Proc. Vol. 16 (North Holland, 1983).
    • "Detector Materials: Germanium and Silicon," E.E. Haller, IEEE Trans. Nucl. Sci. NS-29, No. 3, 1109-18 (1982).
    • "Nuclear Radiation Detectors," E.E. Haller and F.S. Goulding, in Handbook on Semiconductors, 2nd edition, ed. C. Hilsum (Elsevier North Holland, 1993), pp. 937-63.
  • 7/19/11
    E. E. Haller
    3
    Table of Contents(see chapters 11-13 in the course text)
    • The Ionization Chamber
    • Semiconductor Properties
    • Interaction of Radiation with Semiconductors
    • P-N-Junctions
    • Detector Materials: Ge, Si, (GaAs)
    • Room Temperature and High Z Materials: CdTe, CdZnTe,
    HgI2, GaAs, AlSb, TlBr, diamond
    • Conclusions
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    4
    NaIscintillatorsversus Ge pin diodes
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    5
    J. Q. Philippot, IEEE NS 17(3), 446 (1970)
  • 7/19/11
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    6
    The Ionization Chamber: Displacement
    Current and Charge Collection
    Applies to:
    • reverse biased p-n or p-i-n junction,
    • resistive detectors,
    • gas ionization chambers,
    • proportional counters, etc.
    f
    g
  • 7/19/11
    E. E. Haller
    7
    The Ionization Chamber:Operating Principle
    The field lines of a charge in volume V end on the electrodes and "influence" an electric charge. The relative quantities of charge influenced on each electrode depends only on the geometry. When the charge moves (in an externally applied field), the relative quantities of influenced charge change, forming a displacement current. When the charge stops moving, the displacement current stops flowing. Charge may stop flowing because it gets "trapped" at a defect center or gets "collected" at a contact. In a typical p-n junction, charge ideally flows to one of the electrodes. In a resistive detector, charge arriving at one electrode may lead to the injection of a new charge at the opposite electrode.
    We define the "photoconductive" gain:
    Note: For a high resolution detector, a fixed, constant gain is required (for p-n junctions: G = 1.00; though there are ways to compensate incomplete charge collection)
    = average distance traveled by a free charge
    L = inter electrode distance
  • 7/19/11
    E. E. Haller
    8
    SEMICONDUCTOR PROPERTIES
    Diamond Crystal Lattice: [100] Direction
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    9
    [110] Direction
  • 7/19/11
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    10
    [111] Direction
  • 7/19/11
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    11
    Silicon
    Real space structure,
    [100] projection
    (The sign = represents the two electrons in each bond.)
    Electronic band structure
    (CB = conduction band,
    VB = valence band)
  • 7/19/11
    E. E. Haller
    12
    Doped
    Silicon
    Donor doping (e.g., phosphorus on Si sites) provides extra electrons which can move into the conduction band or compensate an acceptor;
    Acceptor doping (e.g., aluminum on Si sites) creates holes which can move into the valence band or compensate a donor
  • 7/19/11
    E. E. Haller
    13
    Radiation detector performance is affected by temperature in several ways. Two important ones are:
    A. Generation of free electrons and holes by thermal
    ionization across the bandgap. This affects the bias
    current I (also called "leakage" or "dark" current) through
    the detector.
    This current is a source of electrical noise, in its simplest
    form "shot" noise ( df= 2eIdf; e = charge of the electron,
    f = frequency)
    B. Trapping and de-trapping of the free charge carriers
    at defect or doping related energy levels in the bandgap.
    Temperature Effects
  • 7/19/11
    E. E. Haller
    14
    A. Free Carrier Concentration
    At finite temperatures a very small number of bonds in a crystal is broken. For each broken bond there exists a mobile electron and a mobile hole. The concentration of these electrons (n) and holes (p) is called intrinsic carrier concentration ni:
    ni2= NcNv exp (-Egap/kBT)= np (Law of mass action)
    Nc = conduction band effective density of states
    Nv = valence band effective density of states
    kB = Boltzmann's constant = 8.65 × 10-5eV/K
    Egap = energy gap (Si: 1.1 eV, Ge: 0.7 eV, GaAs = 1.42 eV, CdTe: 1.47 eV,
    Hgl2: 2.13 eV, AlSb: 1.58 eV)
    ni (Si, 300K) = 2 x 1010 cm-3
    ni (Ge, 300K) = 3 x 1013 cm-3
    ni (GaAs, 300K) = 107 cm-3
  • 7/19/11
    E. E. Haller
    15
    Thermal Carrier Generation
    The thermal generation of electrons and holes gives rise to the reverse (also called "leakage" or "dark") current. This current constitutes a source of electronic noise.
    Consequences
    • Because of the small bandgap (Eg=0.7 eV), Ge diodes must be cooled in order to achieve sufficiently low reverse currents.
    • Si diodes (Eg=1.2 eV) have to be cooled only for high resolution applications.
    • CdTe, CdZnTe, GaAs, AlSb, ThBr, PbOand Hgl2 should not require cooling.
  • 7/19/11
    E. E. Haller
    16
    Arrhenius plot
    (log conc. versus 1/T) of the intrinsic carrier concentration of the major semiconductors
  • 7/19/11
    E. E. Haller
    17
    B. Charge Trapping
    Electrons and holes are also created in ionization events caused by radiation. In order to be detected they have to traverse the detector crystal all the way to their respective electrodes. On the way they encounter shallow and deep impurities, some of which may be charged. Trapping and release of charge from deep impurities is described as follows:
    emission rate e (per level) of a carrier to the nearest band:
    e =  <v> Nband exp (-E/kBT)
    with
    = carrier capture cross-section (cm2)
    <v> = average thermal velocity = (3kT/m*)1/2
    Nband= effective density of states
    = 2 (2πm*kT/h2)3/2
    kB = 8.65 × 10-5eV/K
    E = binding energy (depth!) of a particular level
  • 7/19/11
    E. E. Haller
    18
    B. Charge Trapping (cont.)
    Shallow dopant levels in Si (E ~ 45 meV) or Ge (E ~ 10 meV) do not trap charge for any significant length of time at temperatures above 77 K (liquid nitrogen). Their binding energy is too small for trapping.
    However, already very small concentrations of deep levels (E > 100 meV) trap charge effectively and do not release it within the collection time. The fluctuations in this charge loss lead to asymmetric peaks in the energy spectrum.
  • 7/19/11
    E. E. Haller
    19
    Deep Levels <—> Trapping
    1: trapping of an electron by a deep level:
    2: recombination of the trapped electron with a free hole or
    3: detrapping after a time t.
  • 7/19/11
    E. E. Haller
    20
    Electric Charge Transport
    At low electric fields the carrier velocity is proportional to the electric field:
    The mobility µ rises with decreasing temperature because the phonon density drops:
    At high electric fields velocity saturationoccurs. The carriers emit phonons* at a rapidly increasing rate and the velocity becomes almost constant. (See figures 11-2 on pages 341 and 342 in G. Knoll’s textbook). It is interesting to note that the saturation velocity for most semiconductors lies around 107cm/s.
    _______________________________
    * Lattice vibrations (quantized E = hw)
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    21
    The Asymmetric Planar N+-P-Junction (parallel plate capacitor)
    Ionized positively charged donors
    Ionized negatively
    charged acceptors
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    22
    The ionized shallow impurity levels constitute a space charge e |NA - ND|. Poisson's equation relates the potential  to the charge:
    2 = -e |NA - ND| /o
     = dielectric constant, o = permittivity of vacuum
    In one dimension:
    ∂2/∂x2 = -e |NA - ND| /o
    integrating twice leads to:
     = d2 |NA - ND| =d2|NA - ND| C
    CSi = 7.72  10-8Vcm
    CGe = 5.64  10-8Vcm
    with:
    d = width of the depletion layer
    Planar N+-P-Junction
  • 7/19/11
    E. E. Haller
    23
    Planar N+-P-Junction
    For Ge junctions we find:
    V = d2 | NA - ND | x 5.64 x 10-8 (Vcm)
    and for Si:
    V = d2 | NA - ND | x 7.72 x 10-8 (Vcm)
    with
    V = applied voltage (V)
    d = depletion depth (cm)
    | NA - ND | = net-impurity concentration (cm-3)
    EXAMPLE: Planar P-I-N Detector
    d = 2 cm, V = 3000V
    What is | NA - ND | for full depletion?
    Ge: 1.33 x 1010 cm-3
    Si: 9.7 x 109 cm-3
    }
    These are extremely small concentrations
    compared to ~ 4 x 1022 Ge or Si per cm3!
  • 7/19/11
    E. E. Haller
    24
    Ultra-pure Ge Crystal Growth
    Ultra-purity (~ 1 residual impurity in 1012 cm-3 Ge atoms!) requires highly specialized materials: e.g. Ultra pure silica for the crucible, ultra pure graphite for RF coupling, Ultra pure H2 atmosphere and ultrapure chemicals.
  • 7/19/11
    E. E. Haller
    25
    Ge seed
    An ultra-pure
    Germanium single
    crystal is being “pulled” from a melt contained
    in a silica crucible at 936oC. The atmosphere is pure Hydrogen. Heat is supplied by the water cooled radiofrequency (RF) coil surrounding the silica envelope. This bulk crystal growth technique carries the name of it’s inventor, “Jan Czochralski.”
    Ge single crystal
    Ge melt
    Silica crucible
    RF coil
  • 7/19/11
    E. E. Haller
    26
    The Floating Zone (FZ) crystal growth process is used for ultra pure silicon up to 6” in diameter. No crucible is used. The ambient is typically nitrogen but vacuum and argon have been used.
  • 7/19/11
    E. E. Haller
    27
    Poly Si feed rod
    One turn RF coil
    Si single crystal
    Silicon Floating Zone (FZ) crystal with 10 cm diameter is being pulled. The single turn RF heating coil creates the liquid “floating zone” between the lower part (single crystal) and the upper polycrystalline section.
  • 7/19/11
    E. E. Haller
    28
    Determining Ultra-Purity
    Electrical conductivity is an accurate measure of the net-impurity concentration. The intrinsic conduction can be “turned-off” through cooling to liquid nitrogen temperature (77K).
  • 7/19/11
    E. E. Haller
    29
    Dopant Impurity Profiles
    Typical impurity profile of an ultra-pure Ge crystal. The acceptor aluminum (Al, dashed line) does not segregate in silica grown Ge, leading to a constant concentration. The donor phosphorus segregates (P, dotted line). In our particular example, the phosphorus concentration equals the aluminum concentration at 80% of the melt frozen and exceeds it beyond. In the Al dominated part, the crystal is p-type, in the P dominated type, the crystal is n-type. All ultra-pure Ge single crystals contain about 1014cm-3 H, O and C, inert impurities. Special thermal treatment can lead to the formation of shallow acceptors [A(Si,H) and A(C,H)], and donors (D(O,H). These trigonaldopants are not thermally stable and disappear at ~160oC.
  • E. E. Haller
    30
    Charge Collection in Real Time in a 1 mm thick Si p-n-Diode
    Courtesy J. Fink et al., NIM A 560, 435-443 (2006)
    n+
    P+
    α
    α
    n-Si
    +

    min E-field max
    7/19/11
  • E. E. Haller
    31
    Charge Collection in Real Time in a 1 mm thick Si p-n-Diode
    Courtesy J. Fink et al., NIM A 560, 435-443 (2006)
    α
    α
    n-Si
    +

    (b)
    min E-field max
    7/19/11
  • 7/19/11
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    32
    The n+-i-p+ Diode Detector
    or
  • 7/19/11
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    33
    Free Carrier Velocity as a
    Function of the Electric Field
  • 7/19/11
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    34
    High-Purity Ge Detector Structures
    Planar high-purity germanium (hpGe) radiation detector
    After G. S. Hubbard, MRS Proc. Vol. 16, 161 (1983)
    Coaxial structure hpGe radiation detectors. Device (a) has standard electrode geometry (SEGe); detector (b) has been fabricated with reverse electrode geometry (REGe)
  • 7/19/11
    E. E. Haller
    35
    Position sensitive orthogonal-strip hp Ge detector with 64 strip electrodes on each side fabricated using amorphous-Ge contacts. Strip pitch is 1.25 mm. The active volume of the detector is 9 cm X 9 cm X 2 cm. (Courtesy P.N. Luke, LBNL)
  • Is this the end of the story ? No, because ultra-pure Ge detectors offer the highest resolution gamma ray spectroscopy only when they are cooled. This requirement causes serious problems in certain applications especially in security surveying situations. There is a need for room temperature medium resolution detectors. Many groups have tested many materials but only a very small number look promising.
    What do we need:
    - High Z material which can stop gamma rays efficiently
    - A bandgap larger than ~1.7 eV so no cooling is required
    - A bandgap < 2.5 eV to keep the number of electron-hole pairs large for good resolution
    - High purity to minimize the trap concentration
    - large µτ (mu-tau) products for electron and holes (µ = mobility, τ = free carrier lifetime)
    7/19/11
    36
    E. E. Haller
  • 7/19/11
    E. E. Haller
    37
    Properties of Compound Semiconductors
    [Courtesy M.R. Squillanteet al., MRS Proc. Vol. 302, 326 (1993)]
  • Lattice Const. and Bandgapsof Important Semiconductors
    7/19/11
    38
    E. E. Haller
  • What do we find:
    - there is no material matching all the above conditions
    - there are a few materials with at least one free carrier exhibiting good µτ (mu-tau) products
    The challenge:
    - Obtain good energy resolution with single carrier collection
    - Form a
    High Resistivity Detector
    instead of a p-n junction
    7/19/11
    39
    E. E. Haller
  • E. E. Haller
    40
    Why do we want high resistivity?
    No Radiation, No Signal: I=0
    -
    +
    High Bias
    In presence of radiation, strong signal: I
    +
    -
    High Bias
    7/19/11
    -
    +
  • 41
    The “Right Kind” of Resistivity
    These should be high!
    These should be low!
    Three options for realizing low free carrier concentration:
    Achieve extremely high purity
    Materials: Only FZ- Si and high purity Ge
    2) Deep level compensation
    Materials: GaAs,CZT and most other resistive material candidates
    3) Shallow level compensation
    Materials:Ge:Ga,Li; TlBr(?)
    7/19/11
    E. E. Haller
  • 42
    The “Right Kind” of Resistivity
    The Fermi level must be in the middle of the band gap for the highest resistivity possible at room temperature.
    The 3 ways to do this:
    3. Shallow Level Compensation*
    2. Deep Level Compensation
    1. Ultra Purity
    CB
    EF
    VB
    7/19/11
    E. E. Haller
    * The idea of shallow level compensation or passivation is attractive because deep levels frequently negatively affect carrier mobilities. This mechanism provides a way to achieve low carrier concentrations without affecting mobilities. However, its realization with Li drifting of p-type Ge or Si is thermally unstable. This lead to the development of ultra-pure Ge
  • 43
    Cadmium Zinc Telluride (CZT)
    • Current leading room temperature material – In wide exploitation across several fields. Leading Manufacturer: Redlen Technologies
    • Energy Resolution of ~1%* obtained, close to that of Ge.
    • Deeply Researched – Growth processes improved over several decades, electronic properties well studied, resistivity understood
    Principle Issues Still Facing CZT
    E. E. Haller
    7/19/11
    • Improving yield of detector grade material from crystal growth
    • Pushing the energy resolution limit while simplifying electronics
    • Minimization of Tellurium inclusions
  • 44
    Thallium Bromide (TlBr)
    • Higher Z than CZT: Tl: 81, Br: 35 vsCd: 48, Te: 52
    • Higher Density than CZT: 7.56 g/cm3 vs 5.29 g/cm3
    • ~Higher Resistivity than CZT: 1010-1011 ohm-cm
    • Band Gap: 2.68 eV, Indirect; 3.04 eV, Direct
    • Transport Properties comparable to CZT
    Electronic transport properties dramatically improved in last decade by RMD!
    TlBr detector, courtesy of RMD
    E. E. Haller
    7/19/11
  • 7/19/11
    E. E. Haller
    45
    The “Frisch Grid”: An Old Idea with Great Relevance and Promise
    • Gas ionization chambers suffer from the same problem as many semiconductor detectors:  of one charge species (ions) is significantly
    lower than of the other charge species (electrons).
    • Solution: the "Frisch Grid" (FG)
  • 7/19/11
    E. E. Haller
    46
    Reincarnation of the Frisch Grid as two Sets of Interdigitated Contacts A and B
    P.N. Luke, Appl. Phys. Lett. 65(22) 2884 (1994)
  • 7/19/11
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    47
    Cd0.8Zn0.2Te (5 x 5 x 5 mm3)
    Courtesy P.N. Luke, LBNL
  • 7/19/11
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    48
    Courtesy P.N. Luke, LBNL
  • 7/19/11
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    49
    Sub 1% Resolution CZT Detector
  • 7/19/11
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    50
    CZT CPG vs Ge
    CZT coplanar-grid detector – 1 cm3
    (cooled to -30 °C)
    Peak/Compton=16.2
    Ge detector – 3 cm3
    Peak/Compton=11.4
    Courtesy P.N. Luke, LBNL
  • 7/19/11
    E. E. Haller
    51
    Conclusions
    • The ideal semiconductor material for all radiation detection applications does not exist but it can be approached closely.
    • Certain material property requirements and application requirements are incompatible (e.g. bandgap: large for low leakage current but small for small energy per e/h pair).
    • Si & Ge are the high resolution spectrometer materials. Detectors exhibit excellent stability, good efficiency, timing, etc.
    • Thin epitaxial films (100-150 µm) of high-purity GaAs (|NA-ND|< 1012 cm-3) have been grown by the LPE technique and early spectrometer results look promising. In contrast, semi-insulating (SI) GaAs contains very large concentrations of deep traps which make the material highly resistive and lead to extreme charge trapping.
  • 7/19/11
    E. E. Haller
    52
    Conclusions, cont.
    • CdTe, CdZnTe, ThBr& HgI2 are room temperature materials. Low energy X rays can be detected with good resolution. Medium and high energy photons ( rays) still pose problems. Trapping and poor hole transport seem to be fundamental and/or related to material inhomogeneitiesand defects.
    • Single-polarity charge sensing using coplanar electrodes (an analog of the Frisch grid in gas proportional counters) looks very promising for semiconductors with good collection of at least one type of charge carrier (typically electrons; see: P.N. Luke, Appl. Phys. Lett. 65, 2884 (1994) and more recent publications).
  • 7/19/11
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    53
    Thanks for Your Attention