DLTS

Hussein Ayedh
PhD Studet
Department of Physics
OUTLINE
Introduction
Semiconductors Basics
DLTS Theory
DLTS Requirements
Example
Summary
Introduction
• Energetically "deep“ trapping levels in semiconductor space charge
region affect semiconductor performance
- Shortening of carrier life-time
- Enhanced recombination of minority carriers
- Output power limitations etc.
• Characterization of impurities and traps is essential to analyze the
performance of semiconductor
• DLTS was Introduced by D. V. Lang in 1974
• It is commonly Used for characterizing point defects in semiconductors
Introduction
• DLTS Principle
– Emission of trapped charge carriers change the depletion capacitance
of a pn-junction or Schottky diode.
– The transient measurement provides information on the defect levels
in the band gap.
• Electrical properties of defects:
– Energy position in band gap
– Capture cross section
– Concentration of defects
• Very sensitive: It can detect traps down to 108 cm-3 in very good samples
• Non destructive
Semiconductor Basics
 pn- junction
p
n
Ec
Ef
Ev
Ec
Ef
Ev
W
-- -- ++
p - - ++
++ n
- - ++
F
qV0
A
C
W
Ec
Ev
• Connection of p- and n-type regions:
– Diffusion of charge carriers into the opposite regions.
–This will give rise to an electric field across the depletion region (W),
with a capacitance C
– No free charge carriers in W as the field will sweep them across the
junction
Equilibrium
Forward Bias
Reverse Bias
VR
VF
W
-- -- ++
p - - ++
n
- - ++
++
F
qV0
p
W
-- +
- +
+
- +
F
n
p
W
-- -- -- +++
+++
n
-- -- -- +++
+++
F
q(V0 – VF)
Ec
q(V0 + VR)
Ev
• The SCR width (W) Changes with applied voltage and doping concentration
–High doping
small W
–Low doping
large W
• The depletion region width (W) will extend mostly into low-doped material in
order to keep charge balance
Point Defects
- Substitution impurity: extra impurity atom in an origin position
- Vacancy: missing atom at a certain crystal lattice position
- Interstitial impurity atom: extra impurity atom in an interstitial position
- Self-interstitial atom: extra atom in an interstitial position;
Introduce energy level in the band
structure
•Shallow level
–Close to the edges of the bandgap
–Use mainly as a dopant
•Deep level
–Close to the middle of the bandgap
–Act as generation/recombination or
trap center.
http://cnx.org/content/m16927/latest/
Capture & Emission Processes
Deep levels in the band gap act as
- Recombination centers: can interact with both edges of bandgap cn= cp .
- Electron traps: If they mostly interact with the conduction band cn
- Hole traps: If they mostly interact with the valence band cn ≪ cp .
Thermal (electron) emission rate:
en 
 n vn N c
𝑣𝑛 ∝
g
  ( EC  Et ) 
exp 

kT


1
𝑇2
𝑁𝑐 ∝
Conduction Band
cn
en
Electron Trap center
cn
3
𝑇2
 EC  Et 
en   n  T exp  

kT 

2
≫ cp .
Recombination center
cp
cp
ep
Valence Band
Hole Trap Center
DLTS Theory
V
Principle of measurement
Vp
• Diode kept at fixed reverse bias.
t
• Filling Pulse to fill the traps.
• Return to the reverse bias:
Vr
- Change of the W
- Emission of charge carriers changes
C
the capacitance of the depletion region
as a function of time
𝐶 𝑡 = 𝐶𝑟𝑏 − ∆𝐶𝑜 𝑒𝑥𝑝 −𝑒𝑛 𝑡
• Repeated through a temperature scan
C(rb)
DC
0
t
DLTS Theory
DLTS Measurement:
(A) Equilibrium state
(B) Filling pulse
(C) Return to the
reverse bias with change
in the capacitance
(D) Emission case.
DLTS Theory
•Built of the DLTS spectra
C(t2) – C(t1) vs Temperature
C(t2) – C(t1) max at certain T
- The trap concentration can be
deduced from the maximum
amplitude of the transient
𝑁𝑇
∆𝐶𝑜 =
𝐶𝑟𝑏
2𝑁𝑑
Temperature (K)
nT (t )  NT exp( ent )
Capacitance Transient (pF)
-The carrier concentration of the
traps is changed exponentially
t1
t2 Time (s)
Build of the DLTS spectra
– DLTS transient is multiplied by a weighting function W(t)
𝑆=
𝐶𝑟𝑏 𝑁𝑇
∆𝐶 𝑡 𝑊 𝑡 𝑑𝑡 =
2𝑁𝑑
exp −𝑒𝑛 𝑡 𝑊 𝑡 𝑑𝑡
– Lock-in weighting function: -1,1
DLTS Spectra
Capacitance Transient (pF)
We “record” the transient for each temperature, then create the spectra
after the measurement
1
-1
Time (s)
Temperature (K)
Build of the DLTS spectra
– Lock-in gives a good signal to noise ratio But wide spectra
– Use of different weighting function in order to separate close defect levels
Like GS4 and GS6.
Lock-in: high SNR, wide peak
GS4: low SNR, narrow peak
(DLTS) Presentation, MENA9510 Course,2013.
D. Åberg, PhD thesis, KTH (2001)
Extraction of defect properties
– By varying the length of the “rate-window”, the peak is shifted in temperature
– Emission rate (en) of each window can be numerically calculated.
Capacitance Transient (pF)
– A set of Tmax and en for all DLTS spectra will be obtained.
T3, en3 T2, en2 T1, en1
W1 W2 W3
Time (s)
Temperature (K)
Extraction of defect properties
- The emission rate
 E  Et 
en   n  T exp   C

kT 

𝑙𝑛
2
Arrhenius plot of 𝑙𝑛
𝑒𝑛
𝑇2
𝑒𝑛
𝐸𝑐 − 𝐸𝑇
=
𝑙𝑛
𝛽𝜎
−
𝑛
𝑇2
𝑘𝑇
against 1/𝑇
𝐸𝑐 = 0.67 𝑒𝑉
𝜎𝑛 = 4 ∗ 10−14 𝑐𝑚2
DLTS Requirement
•Samples
– Rectifying junction (Schottky or pn-junction)
– Junction capacitance 1-1000pF, (100pF-range most ideal)
– Trap concentration less than 10-15% of doping
DC  Crb or NT  Nd
– Low leakage current and low conductance
Lower limit for detectable trap concentrations:
Depends on the sensitivity of the C-bridge and S/N ratio
E.g. for DC0,min ≈ 5 fF, CR ≈ 50 pF  (Nt/Ns)min ≈ 2(DC0,min/CR) ≈ 2·10-4
DLTS Requirement
Required Equipment
– Capacitance meter
– Pulse generator
– Temperature controller
• Liquid Nitrogen
• Helium cryostat
• Heater
– Temperature Reader
(DLTS) Presentation, MENA9510 Course,2013.
Equipment at MiNaLab
 Two setups in temperature range 15K-300K
 One setup in temperature range 77K-400K
 One setup in temperature range 77K-600K
Example
DLTS measurements
PN-diodes (Si)
Irradiated by protons
with dose
2 ∗ 1010 𝑐𝑚−2
Defect Identity
Energy Position
Capture Cross Section
VO [78K]
𝐸𝐶 − 𝐸𝑡 = 0.17 𝑒𝑉
𝜎 = 7.2 ∗ 10−15 𝑐𝑚2
V2(=/−) [114K]
𝐸𝐶 − 𝐸𝑡 = 0.23 𝑒𝑉
𝜎 = 3.5 ∗ 10−15 𝑐𝑚2
V2(−/0) [196K]
𝐸𝐶 − 𝐸𝑡 = 0.41 𝑒𝑉
𝜎 = 2 ∗ 10−15 𝑐𝑚2
Examples
Window 6 of three PNdiodes are irradiated
by protons at different
doses:
- Sample #1
𝟑 ∗ 𝟏𝟎𝟗 𝒄𝒎−𝟐
-
Sample #2
𝟔 ∗ 𝟏𝟎𝟗 𝒄𝒎−𝟐
-
Sample #3
𝟐 ∗ 𝟏𝟎𝟏𝟎 𝒄𝒎−𝟐
Defect Identity
𝐍𝐭 /𝐍𝐝 in Sample #1
𝐍𝐭 /𝐍𝐝 in Sample #2
𝐍𝐭 /𝐍𝐝 in Sample #3
VO [78K]
0.0053
0.012
0.042
V2(=/−) [114K]
0.0014
0.0034
0.012
V2(−/0) [196K]
0.0020
0.0045
0.017
Summary
• Deep Level Transient spectroscopy
– Characterization of electrically active defects
• Energy position in band gap
• Capture cross section
• Concentration of defects with accuracy up to (~108cm-3)
– No information about the chemical composition.
• Signal is obtained by filling pulse in applied bias, and observing a
transient decay of trapped charge carriers in the depletion region.
• Requirement:
– DLTS requires rectifying junction with capacitance in 1-1000pF range
– Low leakage current is important to get good measurements
– Trap concentrations between 0.0001 - 0.2 of doping concentrations
REFERENCES
[1] D.V.Lang, JAP, 45, 7, 1974.
[2] P. Blood and J. W. Orton, The Electrical characterization of Semiconductors:
Majority Carriers and Electron States (ACADIMIC PRESS, USA, 1992).
[3] W. E. Meyer, Digital DLTS Studies on radiation induced defects in Si, GaAs
and GaN, PhD dissertation, University of Pretoria, 2007.
[4] F. D. Auret and P. N. K. Deenapanray, Deep Level Transient Spectroscopy
of Defects in High-Energy Light-Particle Irradiated Si, Solid State and Materials
Sciences, 29:1–44, 2004.
[5] Deep level transient spectroscopy (DLTS) Presentation, Advanced
Characterization Methods Course MENA9510, 2013.
[6] Dieter K. Schroder: Semiconductor Material and Device Characterization,
John Wiley & Sons, 2006.

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