Effects of Compositional Disorder on Ultrafast Photocarrier Dynamics in a-Si1-xGex:H
Josef
2
Felver ,
Susan
2
Dexheimer
University, Middletown, CT,
2 Department
of Physics and Astronomy, Washington State University, Pullman, WA
ABSTRACT
Amorphous semiconductors have been of substantial
physical interest for many years due to their potential for
solar energy generation as well as their status as a
prototypical material for investigating the effects of disorder
on electronic properties in semiconductors. Silicongermanium alloys (a-Si1-xGex:H) are of particular interest
due to their tunable band gap and ability to be
compositionally tailored to contain varying degrees of
disorder. Systematically altering the Ge content allows for
an investigation of the effects of compositional disorder on
electronic properties. Time-resolved optical absorption
measurements were carried out on a-Si1-xGex:H using
optical pulses 35 fs in duration centered at 800 nm
generated by an amplified Ti:S laser system. Photoinduced
carrier absorptions were measured at various pump
fluences and correlated to photocarrier densities. The timeresolved responses in these a-Si1-xGex:H materials are
consistent with diffusion-limited bimolecular recombination,
with decreased mobility in more disordered materials.
EXPERIMENT
Time-resolved differential transmittance measurements
were carried out using the pump-probe technique.
Optical pulses 35 fs in duration were generated by a
Ti:sapphire laser system operating at a repetition rate of
1 kHz. Pump pulses centered at 800 nm were used to
excite the a-Si1-xGex:H, and probe pulses generated
from a white light continuum detected the decay
dynamics of the initially excited carriers. A reference
beam with a lockin amplifier was used for noise
reduction and normalization.
Nonexponential Carrier Relaxation (a-Si .9Ge.1:H) - 880 nm Probe
a)
0.12
(b) Amorphous Semiconductors
19
-3
19
-3
17W - n0=3.8x10 cm
19
7W - n0=1.4x10 cm
0.08
-3
0.06
X = .1
0.04
K = 6.2x10-9 s*cm-3
0.02
0.00
0
10
100
200
300
Delay Time (ps)
Nonexponential Carrier Relaxation (a-Si .8Ge.2:H) - 880 nm Probe
b)
0.12
19
-3
19
-3
19
-3
19
-3
27W - n0=3.8x10 cm
Setup for the Pump-Probe Experiment
21W - n0=3.0x10 cm
0.10
(1 Khz)
(Probe Wavelength)
15W - n0=2.1x10 cm
0.08
10W - n0=1.4x10 cm
0.06
X = .2
0.04
K = 5.5x10-9 s*cm-3
0.02
a-Si1-xGex:H
0.00
10
INTRODUCTION
Density of States (DOS) as a function of energy
(a) Crystalline Semiconductors
-3
24W - n0=5.7x10 cm
0.10
0
100
200
300
Delay Time (ps)
Nonexponential Carrier Relaxation (a-Si .7Ge.3:H) - 880 nm Probe
c)
0.12
19
-3
19
-3
17W - n0=4.0x10 cm
11W - n0=2.6x10 cm
0.10
(1 Khz)
19
Induced Absorbance -T/T
The loss of long-range crystalline order in a
semiconductor lattice profoundly affects the nature and
dynamics of the generated photocarriers. In amorphous
semiconductors, the random potential fluctuations
associated with the disorder result in the localization of
low-energy carriers, substantially reducing the effective
carrier mobility. Multiple trapping of carriers into localized
states (the Urbach Tail) with a distribution of binding
energies, as well as hopping transport between localized
sites at a distribution of distances, can result in dispersive
transport, in which photogenerated carriers exhibit a
time-dependent mobility. Alloys with increasing Ge
content possess a higher degree of disorder.
20
36W - n0=1.1x10 cm
Induced Absorbance -T/T
1 Wesleyan
Jason
2
Mance ,
Induced Absorbance -T/T
Nick
1
Jackson ,
6W - n0=1.4x10 cm
18
-3
3 W - n0=7.1x10 cm
0.08
-3
0.06
X = .3
0.04
K = 3.7x10-9 s*cm-3
0.02
0.00
RESULTS
0
10
100
200
300
Delay Time (ps)
Data can be accurately fit to a simple bimolecular
recombination model:
dn
2
 kn
dt
(1)
CONCLUSIONS
Where k is proportional to a time-dependent
mobility defined by
 (t )   0 ( 0t )
Time-resolved induced absorbance response for a-Si.1-xGe.x:H
probed at various initial excitation densities. Solid black lines
represent the original data and colored solid lines represent fits
to model given by Eqn. (3). (Note Scale Break at 15 ps)
 1
(2)
Yielding:
n0
nt  

cn
0

1  n0 k 0 ( 0t )
The measured photoinduced absorbance is directly
proportional to the carrier population, with a small
offset due to lattice heating.
Preliminary results give bimolecular recombination
constants that decrease systematically with higher
Ge content, reflecting decreasing mobility with
increasing disorder. Future work will correlate these
results with time-resolved THz measurements on
these materials.
ACKNOWLEDGMENTS
(3)
This work was supported by the NSF under grant
DMR0706407.
We thank Brent Nelson at NREL for growing the aSiGe:H thin film samples.