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Defects. maria.berdova@aalto.fi. Maria Berdova. Types of Defects Generation-Recombination Statistics Mathematical Description Detection Methods. Outline. 3) Vacancy. 1) Foreign interstitial (e.g. Oxygen in silicon). 4) Self interstitial. 2) Foreign substitutional (like dopant atom).

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defects

Defects

  • maria.berdova@aalto.fi

Maria Berdova

Postgraduate Course in Electron Physics I

outline

Types of Defects

  • Generation-Recombination Statistics
  • Mathematical Description
  • Detection Methods
Outline

Postgraduate Course in Electron Physics I

types of defects

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

defects1

Vacancy

http://open.jorum.ac.uk/xmlui/handle/123456789/5649

Defects

Stacking fault

Postgraduate Course in Electron Physics I

Interstitial

metallic impurities

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

effect of contamination
Effect of contamination

Fe in Si, and Cu in Si

Postgraduate Course in Electron Physics I

defects2

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

generation recombination statistics

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

mathematical description

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

mathematical description1

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

schottky diode

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

mathematical description2

from zero bias to reverse bias

Emission period

Capture period

from reverse bias to zero bias

Mathematical Description

Postgraduate Course in Electron Physics I

capacitance measurements

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

capacitance measurements1

the steady-state density

Plot 1/C2vs V

Capacitance measurements

S (t) – slope

Postgraduate Course in Electron Physics I

capacitance measurements2

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

capacitance measurements3

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

capacitance measurements4

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

emission minority carriers

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

capture majority carriers

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

capture majority carriers1

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

capture majority carriers2

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

current measurements

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

current measurements1

Emission current

Displacement current

Junction leakage current I1

Current Measurements

Postgraduate Course in Electron Physics I

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
Drawbacks of Current Measurements

Postgraduate Course in Electron Physics I

current measurements is applied

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.

charge measurements

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

charge transient measurements

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

deep level transient spectroscopy dlts

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

deep level transient spectroscopy dlts1

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)

deep level transient spectroscopy dlts2

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

deep level transient spectroscopy dlts3

‘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

arrhenius plot of emission rates

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

dlts conclusion

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)
thermally stimulated capacitance and current

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

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
Positron annihilation spectroscopy

Postgraduate Course in Electron Physics I

AMERICAN JOURNAL OF UNDERGRADUATE RESEARCH, VOL. 2, NO. 3 (2003)

positron annihilation spectroscopy1
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

positron annihilation spectroscopy2
Positron annihilation spectroscopy

ReinhardKrause-Rehberg,

Martin-Luther-University Halle-Wittenberg, Germany

Postgraduate Course in Electron Physics I

(Polity et al., 1997)

slide38

Thank you

Postgraduate Course in Electron Physics I