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Postgraduate Course in Electron Physics I
1
Maria Berdova
Defects
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Postgraduate Course in Electron Physics I
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*Outline
*Types of Defects
*Generation-Recombination Statistics
*Mathematical Description
*Detection Methods
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Postgraduate Course in Electron Physics I
Types of defects1) Foreign interstitial (e.g. Oxygen in silicon)
2) Foreign substitutional (like dopant atom)
3) Vacancy
4) Self interstitial
5) Stacking fault
6) Edge dislocation7) Precipitate
3
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Postgraduate Course in Electron Physics I
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Defects Vacancy
Interstitial
Stacking fault
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Postgraduate Course in Electron Physics I
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Metallic impurities
*Degradation of gate integrity
*Degradation of the device (at high stress point and in junction space charge region)
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Postgraduate Course in Electron Physics I
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Effect of contamination
Fe in Si, and Cu in Si
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Postgraduate Course in Electron Physics I
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*Defects*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)
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Postgraduate Course in Electron Physics I
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Generation-Recombination Statistics
*Traps or G-R centers
*Deep level impurities
(metal impurities, crystal
imperfections)
R G
Trapping Trapping
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Postgraduate Course in Electron Physics I
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Mathematical Description
(electron/hole time rate of change due to G-R mechanisms)
G-R center is occupied by hole or by electron, which are recombined or generated
Time dependence of electron or hole density
thermal velocity
electron capture cross-sectionof the G-R center
Center occupancy rate
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Mathematical Description
nT(0) is the density of G-R centers occupied by electrons at t = 0
the steady-state density
n-type substrate
Solution
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Postgraduate Course in Electron Physics I
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Schottky diodea) nT = NT
Capture dominates emission
b) t
G-R centers are
initially occupied by electrons
electrons are
emitted from G-R centersNear the edge of scr the mobile electron density tails of from qnr to scr – captures compete with emissions
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Postgraduate Course in Electron Physics I
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Mathematical Description
Emission period
Capture period
from reverse bias to zero bias
from zero bias to reverse bias
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Postgraduate Course in Electron Physics I
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Capacitance measurements
• Nscr - ionized impurity density in the SCR
• time dependence of nT (t ) or pT (t )
Capacitance of the Schottky diode
capacitance at t = 0 and t = ∞time – varying capacitance
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Postgraduate Course in Electron Physics I
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Capacitance measurementsthe steady-state density
S (t) – slope
Plot 1/C2 vs V
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Postgraduate Course in Electron Physics I
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Capacitance measurementsTransient Measurements
C0 is the capacitance of a device with no deep-level impurities at reverse bias -V
time-varying W is detected as time-varying capacitance
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Capacitance measurementsEmission—Majority Carriers:
*During the reverse bias pulse, majority carriers are emitted as a function of time
As majority carriers are emitted from the traps , W decreases and C increases until steady state is attained
Postgraduate Course in Electron Physics I
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Postgraduate Course in Electron Physics I
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*Capacitance measurements*Reverse biased capacitance change
Intercept on the ln-axis gives ln[nT(0)Co/2ND]
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.
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Postgraduate Course in Electron Physics I
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Emission minority carriersDuring the forward-bias phase, holes are injected into the n-
substrate andcapture dominates emission.
forward bias
reverse bias charge changes from neutral to negative
(p+n junction)
Lower half of the band gap pulses minority carrier
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Postgraduate Course in Electron Physics I
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Capture—Majority Carriers
The density of traps able to capture majority carriers
tf is ”filling” time tf>>τc
tf <<τc
1. Reverse bias
2. Zero bias
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Postgraduate Course in Electron Physics I
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Capture—Majority Carriers
τc can be determined by varying tf
The reverse-bias capacitance depends on the filling pulse width
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Postgraduate Course in Electron Physics I
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Capture—Majority Carriers
ln(∆Cc) versus tf has a slope of 1/τc = σnvthn
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
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Postgraduate Course in Electron Physics I
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Current MeasurementsThe carriers emitted from traps can be detected as a capacitance, a
charge, or a current.The integral of the I -t curve represents the total charge emitted by the traps.high temperatures
low temperatures
time constant
increases
is short
decreases
current
is high
Area under I -t curve remains constant
C-t measurements at low temp& I-t measurements at high temp
time constant data
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Postgraduate Course in Electron Physics I
Current Measurements
Emission current
Displacement current
Junction leakage current I1
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Postgraduate Course in Electron Physics I
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Drawbacks of Current Measurements• 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
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Postgraduate Course in Electron Physics I
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Current Measurements is applied
Drain current ID and gate capacitance CG transients of a 100 μm × 150 μm gate MESFET.
• 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 monitoring the drain current as a function
of time
• In devices in which the channel can be totally depleted
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Postgraduate Course in Electron Physics I
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Charge Measurements
Switch S is closed to discharge the feedback capacitor CF
At t = 0 the diode isreverse biasedS is opened
Current through the diode
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Postgraduate Course in Electron Physics I
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Charge transient Measurements
With the input current into the op-amp approximately zero, the diode current must flow through the RFCF feedback circuit, giving the output voltage
Choosing the feedback network such that tF>>τe
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Postgraduate Course in Electron Physics I
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Deep-level Transient Spectroscopy (DLTS)
The measurements use a two stage carrier capture and emission process
*Quantitative (deduce absolute concentrations of electrically active defects)
*Sensitive (In 20 Ω-cm silicon detection of 1010 cm-3 electrically active defects)
• Trap Energy Level• Carrier Capture and Emission Rates• Trap density• Spatial Distribution of Defects
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Postgraduate Course in Electron Physics I
Deep-level Transient Spectroscopy (DLTS)
*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)
* Rate depends on temperature and binding �energy
J. Appl. Phys. 45, 3023 (1974)29
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Postgraduate Course in Electron Physics I
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Deep-level Transient Spectroscopy (DLTS)
The magnitude of the peak ΔC gives the concentration of deep states:
Conventional DLTSRate window concept to deep level impurity characterizationConventional 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
signal changes as a function of temperature when a single trap is present
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Postgraduate Course in Electron Physics I
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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
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Postgraduate Course in Electron Physics I
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Arrhenius plot of emission rates
Xn is an entropy factor
• plot log (en /T 2) vs 1/T e
n /
T 2
• slope gives -Ea
• intercept A is used to obtain Xnσn(∞)
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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)
Advantages Disadvantages
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34Postgraduate Course in Electron Physics I
Thermally stimulated capacitance and current
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Capacitance steps or current peaks are observed as traps emit their carriers
The trap density is from the area under the TSC curve or from the step height of the TSCAP curve
From zero bias to reverse bias
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Postgraduate Course in Electron Physics I
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Positron annihilation spectroscopy
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
AMERICAN JOURNAL OF UNDERGRADUATE RESEARCH, VOL. 2, NO. 3 (2003)
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Postgraduate Course in Electron Physics I
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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
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Postgraduate Course in Electron Physics I
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Positron annihilation spectroscopy
(Polity et al., 1997)
Reinhard Krause-Rehberg, Martin-Luther-University Halle-Wittenberg, Germany
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Postgraduate Course in Electron Physics I
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*Thank you