Deep Level Transient Spectroscopy

2. Capacitance techniques
Do What?
Measure and record transients (capacitance, voltage, current) of the space charge area Extract Aj, Sj and other related parameters from transient in a form
DC
(a)
(b)
(c)
Transient
Exponential and nonexponential transients
Single deep level exponential transient Level with microscopic structure or multi-levels Transients
Deep levels
Complicated origin, impurities of larger different size
Functions
Intentionally added to make the semiconductor more conductive
Ns 10 20 cm -3 Usually within 0.1eV to band edges Readily thermally ionized few
dnD (cn n e p )( N D nD ) (c p p en )nD dt
schematic diagram of the kinetics process for deep donor.
dn D c n nN D ( c n n en ) n D dt
ep en
c p cn
Emission
n=0
nD (t ) N D exp(en t )
en cn Nc exp(DGe / kT)
en AT exp( Ee / kT )
2
Ee = D H + Ec
Arrhenius plot
en AT 2 exp( Ee / kT ) ln en
Single level
Multi-levels
nonexponential transient
Time
Rate window technique in classical DLTS
Capacitance transient
DLTS signal
DC
t1 t2 time
Rate window
temperature
History
• Capacitance method: C.T. Sah et al. Solid State Electronics. 13 (1970) 759 • DLTS. D. V. Lang, in “Deep centers in semiconductor” edited by S. T. Pantelides, (Gorden and Breach, New York, 1986). P489
Where Ns, ND, nD, C, VD and V are concentration of space charge, total concentration of the deep centers, concentration of deep centers occupied by electron, built-in voltage and bias voltage respectively. ND is usually very large for composition of deep levels, the capacitance transient is obviously non-exponential under constant bias.
2 0 (VD V ) e
C
0 A
w
0A [ N s N D nD (t )](e0 A ) 2 2[ N D nD (t )] C
2 [ N D nD (t )]
Under constant capacitance condition
DV (t ) DV (0) exp(ent ) DV (t ) DV (0)[1 exp(cnnt)]
1 kT
Ee b
-4 -6
1
log ( e / T2) (s -1K -2)
-8
2
-10 -12 -14 -16 -18 7 8 9
3
n
4
x = 0.26 DX ( Sn )
5
6
10
1000/T ( K -1)
Capture
nD (0) 0
cn nN D ( c n n en ) t nD (t ) [1 e ] cn n en
localized Dis-localized Electrical/optical properties Identifications Fine structures, etc
r-space localization K-space localization Main concerning research
Outline
1.Deep centers/levels in semiconductors
2.Capacitance techniques
3.DLTS
4.DFT DLTS
place Defect Spectroscopy 6.References
1. Deep centers/levels in semiconductors
f (t , T ) sj
1 j (T )
A e
j 1 j
n
s jt
A0
Where Aj is usually related to state density or carrier concentration, and Sj is usually related to energy levels position in bandgap, carrier emission rates, capture rate, respectively. Study the optical/electrical properties of materials
An introduction of
Deep Level Transient Spectroscopy (DLTS)
About us DLTS in the Dept of Physics, XMU
From DLTS to Laplace Defect Spectroscopy (LDS) 黄启圣、康俊勇、吴正云、詹华瀚
Rate Window
Temperature scan
Transients t1 t2
DLTS signal
Temperature
One peak for one deep level.
DLTS peak to Arrhenius plots
log(en;p)
en1(T)
en2(T)
e0
Different deep levels are leading to different peaks Peak condition:
Schematic diagram of band structures
Ec Eds Edd Ef Conduct band
Mid-band gap
Ead Eas Ev valance band
Shallow levels and deep levels
Shallow levels
Origin sources Impurities: approximately the same size
Shallow levels Deep levels
Characteristic properties
E = E(k) concentration n/p effective mass m*
Activation energy ET: related to the position of the level in the bandgap concentration NT capture cross section
Experimental system
Schematically diagram of CC-DLTS and CC voltage transient measuremettky Barrier
Barrier height
b
e(VD VR )
C Es Ef Ed V
Metal

w
N-type semiconductor
Filling pulse (a)
(b)
(c)
• Transient can be obtained via apply pulse on the sample
Concentrations Energy/state position
Stability Wave functions: Taylor series extension with plane wave functions (k)
Continue of
Shallow levels and deep levels
Dis-localized localized
theory
experiments
Hydrogen-like models …
Hall etc, a lot
No general model
Most of techniques for shallow levels can not be applied. Capacitance technique Photoluminescence (PL) Scanning probe microscopy (SPM), etc
t ln 2 t1 e0 t 2 t1
DLTS
T T T1 T2 DC2
DC1
peak height DC is proportional to trap concentration
By choosing t1 and t2 a "rate window" [s-1] is selected, in which the emission rate has to fall for a DLTS peak to appear (D.V. Lang 1974)
Carrier traps, generation recombination centers if they are new to midbandgap
Nt Ns Recombination centers/traps metastable state Many or even all Activation energy ET: related to the position of the level in the bandgap concentration NT capture cross section
cn () n exp(Ec / kT)
cn BT exp(Ec / kT )
1 2
Constant capacitance technique
N s N D nD (t ) 0 x w e Ns w x w 0 w x
Space charge area in a PN junction
P
N
The energy band diagram of space charge area under reverse-bias condition.
Transients of thermal electron emission process of deep centers
3. Deep Level Transient Spectroscopy (DLTS)
Samples
• Sample is often made into Schottky Barrier or Diode • It can be regarded as a Capacitance changeable under different BIAS.