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Defects

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Defects

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  1. Defects • maria.berdova@aalto.fi Maria Berdova Postgraduate Course in Electron Physics I

  2. Types of Defects • Generation-Recombination Statistics • Mathematical Description • Detection Methods Outline Postgraduate Course in Electron Physics I

  3. 3) Vacancy 1) Foreign interstitial (e.g. Oxygen in silicon) 4) Self interstitial 2) Foreign substitutional (like dopant atom) 5) Stacking fault Types of defects 6) Edge dislocation 7) Precipitate Postgraduate Course in Electron Physics I

  4. Vacancy http://open.jorum.ac.uk/xmlui/handle/123456789/5649 Defects Stacking fault Postgraduate Course in Electron Physics I Interstitial

  5. Degradation of gate integrity • Degradation of the device (at high stress point and in junction space charge region) Metallic impurities Postgraduate Course in Electron Physics I

  6. Effect of contamination Fe in Si, and Cu in Si Postgraduate Course in Electron Physics I

  7. Shallow defects • Energy levels close to the valence or conduction band • Acting as dopants • Deep defects • Energy level away from the band edges • Short range part of the potential determines energy level • Normally non-wanted defects • E  ~ 150 meV(from the conduction band or valence band edges) Defects Postgraduate Course in Electron Physics I

  8. Traps or G-R centers • Deep level impurities (metal impurities, crystal imperfections) Trapping Trapping Generation-Recombination Statistics Postgraduate Course in Electron Physics I G R

  9. G-R center is occupied by hole or by electron, which are recombined or generated Time dependence of electron or hole density (electron/hole time rate of change due to G-R mechanisms) Mathematical Description Center occupancy rate thermal velocity Postgraduate Course in Electron Physics I electron capture cross-section of the G-R center

  10. Solution nT(0) is the density of G-R centers occupied by electrons at t = 0 Mathematical Description the steady-state density n-type substrate

  11. a) nT = NT Capture dominates emission b) t G-R centers are initially occupied by electrons electrons are emitted from G-R centers Schottky diode Near the edge of scr the mobile electron density tails of from qnr to scr – captures compete with emissions Postgraduate Course in Electron Physics I

  12. from zero bias to reverse bias Emission period Capture period from reverse bias to zero bias Mathematical Description Postgraduate Course in Electron Physics I

  13. Capacitance of the Schottky diode • Nscr - ionized impurity density in the SCR • time dependence of nT (t ) or pT (t ) Capacitance measurements capacitance at t = 0 and t = ∞ time – varying capacitance Postgraduate Course in Electron Physics I

  14. the steady-state density Plot 1/C2vs V Capacitance measurements S (t) – slope Postgraduate Course in Electron Physics I

  15. Transient Measurements time-varying W is detected as time-varying capacitance C0 is the capacitance of a device with no deep-level impurities at reverse bias -V Capacitance measurements Postgraduate Course in Electron Physics I

  16. Emission—Majority Carriers: • During the reverse bias pulse, majority carriers are emitted as a function of time Capacitance measurements As majority carriers are emitted from the traps , W decreases and C increases until steady state is attained Postgraduate Course in Electron Physics I

  17. Reverse biased capacitance change The capacitance increases with time for majority carrier emission whether the substrate is n- or p-type and whether the impurities are donors or acceptors. Capacitance measurements Intercept on theln-axis givesln[nT(0)Co/2ND] Postgraduate Course in Electron Physics I

  18. During the forward-bias phase, holes are injected into the n-substrate and capture dominates emission. (p+n junction) Lower half of the band gap pulses minority carrier forward bias Emission minority carriers charge changes from neutral to negative reverse bias Postgraduate Course in Electron Physics I

  19. The density of traps able to capture majority carriers tf is ”filling” time tf>>τc tf<<τc Capture—Majority Carriers 1. Reverse bias 2. Zero bias Postgraduate Course in Electron Physics I

  20. The reverse-bias capacitance depends on the filling pulse width τccan be determined by varying tf Capture—Majority Carriers Postgraduate Course in Electron Physics I

  21. ln(∆Cc) versustfhas a slope of 1/τc = σnvthn Capture—Majority Carriers an intercept on ln(∆Cc) axis of ln{[NT − nT (0)]C0/2ND} obtained by varying the capture pulse width during the capacitance transient measurement Postgraduate Course in Electron Physics I

  22. The carriers emitted from traps can be detected as a capacitance, a charge, or a current. The integral of the I -t curve representsthe total charge emitted by the traps. high temperatures is short is high time constant current low temperatures increases decreases Area under I -t curveremains constant Current Measurements C-t measurements at low temp & I-t measurements at high temp time constant data Postgraduate Course in Electron Physics I

  23. Emission current Displacement current Junction leakage current I1 Current Measurements Postgraduate Course in Electron Physics I

  24. Leakage current might be sufficiently high • The instrumentation must handle the large current transients during the pulse • The amplifier should be non-saturable, or the large circuit transients must be eliminated from the current transient of interest • No distinction between majority and minority carrier emission Drawbacks of Current Measurements Postgraduate Course in Electron Physics I

  25. When difficult to make capacitance measurements • Low capacitance of small-geometry MOSFETs • When possible to detect the presence of deep-level impurities by pulsing the gate voltage and monitoringthe drain current as a function of time • In devices in which the channel can be totally depleted Current Measurements is applied Postgraduate Course in Electron Physics I Drain current ID and gate capacitance CG transients of a 100 μm × 150 μm gate MESFET.

  26. Switch S is closed to discharge the feedback capacitor CF At t = 0 the diode is reverse biased Sis opened Charge Measurements Current through the diode Postgraduate Course in Electron Physics I

  27. With the input current into the op-amp approximately zero, the diode current must flow through the RFCFfeedback circuit, giving the output voltage Choosing the feedback network such that tF>>τe Charge transientMeasurements Postgraduate Course in Electron Physics I

  28. The measurements use a two stage carrier capture and emissionprocess • Quantitative (deduce absolute concentrations of electrically active defects) • Sensitive (In 20 Ω-cm silicon detection of 1010cm-3 electrically active defects) • Trap Energy Level • Carrier Capture and Emission Rates • Trap density • Spatial Distribution of Defects Deep-level Transient Spectroscopy (DLTS) Postgraduate Course in Electron Physics I

  29. Pulse applied to change occupancy of deep states • Pulse from reverse to zero for majority carrier traps • Into forward bias to inject minority carriers capacitance changes as carriers are emitted from states (can also use current) • ƒ Rate depends on temperature and binding energy Deep-level Transient Spectroscopy (DLTS) Postgraduate Course in Electron Physics I J. Appl. Phys. 45, 3023 (1974)

  30. Conventional DLTS Rate window concept to deep level impurity characterization Conventional DLTS varies the temperature and produces a peak when the emission rate matches a ‘standard’ rate (the rate window) determined by the positions of t1 and t2 http://www.ph.unimelb.edu.au/~part3/notes/dlts02.pdf The magnitude of the peak ΔC gives the concentration of deep states: Deep-level Transient Spectroscopy (DLTS) signal changes as a function of temperature when a single trap is present Postgraduate Course in Electron Physics I

  31. ‘Tutorial Day: DLTS’ 16th July 2011 Tony Peaker Deep-level Transient Spectroscopy (DLTS) By repeating the temperature scan with different settings of t1 and t2 the system filters out different rates (rate windows) and so each Tmax corresponds to the temperature at which the trap emits carriers at that rate window. So by making an Arrhenius plot (plotting log en vs. 1/T) it is possible to determine the energy of the state from the slope Postgraduate Course in Electron Physics I

  32. Xnis an entropy factor • plot log (en/T 2) vs 1/T en/T 2 • slope gives -Ea • intercept A is used to obtain Xnσn(∞) Arrhenius plot of emission rates Postgraduate Course in Electron Physics I

  33. Advantages Disadvantages DLTS: Conclusion • Highly sensitive - Defect concentrations to 1010 cm-3 • Requires electrically active defects • Contact less, non-destructive relatively easy measurement • Levels identification requires comparison with other techniques • Identification of impurities is not always straightforward • Inability to characterize high resistivity substrates (capacitance transient)

  34. Capacitance steps or current peaks are observed as traps emit their carriers Appl. Phys. Lett. 22, 384 (1973) The trap density is from the area under the TSC curve or from the step height of the TSCAP curve Thermally stimulated capacitance and current From zero bias to reverse bias Postgraduate Course in Electron Physics I

  35. The spectroscopy of gamma (γ ) rays emerging from the annihilation of positrons and electrons • positron wave-function can be localized in the attractive potential of a defect • annihilation parameters change in the localized state (e.g. positron lifetime increases in a vacancy) • lifetime is measured as time difference between appearance of start and stop quanta • defect identification and quantification possible Positron annihilation spectroscopy Postgraduate Course in Electron Physics I AMERICAN JOURNAL OF UNDERGRADUATE RESEARCH, VOL. 2, NO. 3 (2003)

  36. Positron annihilation spectroscopy - Positron lifetime is measured as time difference between 1.27 MeV quantum (β+ decay) and 0.511 MeV quanta (annihilation process) - PM…photomultiplier; SCA…single channel analyzer (constant-fraction type); TAC…time to amplitude converter; MCA… multi channel analyzer Postgraduate Course in Electron Physics I

  37. Positron annihilation spectroscopy ReinhardKrause-Rehberg, Martin-Luther-University Halle-Wittenberg, Germany Postgraduate Course in Electron Physics I (Polity et al., 1997)

  38. Thank you Postgraduate Course in Electron Physics I