Time response of photocurrent at both room temperature and low tempera-ture suggests that the trap states play an important role in the photoelectrical process.. Further investigations a
Trang 1N A N O E X P R E S S
Temperature Dependence of Photoelectrical Properties of Single
Selenium Nanowires
Zhi-Min Liao•Chong Hou•Li-Ping Liu•
Da-Peng Yu
Received: 22 February 2010 / Accepted: 16 March 2010 / Published online: 28 March 2010
Ó The Author(s) 2010 This article is published with open access at Springerlink.com
Abstract Influence of temperature on photoconductivity
of single Se nanowires has been studied Time response of
photocurrent at both room temperature and low
tempera-ture suggests that the trap states play an important role in
the photoelectrical process Further investigations about
light intensity dependence on photocurrent at different
temperatures reveal that the trap states significantly affect
the carrier generation and recombination This work may
be valuable for improving the device optoelectronic
per-formances by understanding the photoelectrical properties
Keywords Se nanowires Trap states
Photoconductivity Temperature effects
Semiconductor nanowires have great potential applications
as the building blocks for nanoscale electronic and
opto-electronic devices Selenium (Se) is an important
semi-conductor, and Se nanowires have attracted enormous
attentions due to their unique physical properties, such as
high photoconductivity, large piezoelectric, and
thermo-electric effects [1 14] In particular, photodetectors and
photoelectrical switchers based on individual Se nanowires
have been fabricated [10–14] In order to further improve
the performance of the Se nanowires optoelectronic devi-ces, well understanding of the photoelectrical properties is desirable Measurement of the temperature dependence of photoconductivity is an efficient method to study the photoelectrical properties because it can yield more infor-mation about the carrier generation and recombination However, the detailed temperature effects on photocon-ductivity of single Se nanowires are not yet reported In this work, we study the photoconductivity of single Se nanowires by measuring the time response on photocurrent and the light intensity dependence on photocurrent at dif-ferent temperatures It is found that the trap states are sensitive to the temperature
The Se nanowires were grown by solution method as described previously [9] Briefly, solid Se spheres were first fabricated by the dismutation of Na2SeSO3solution And then, the Se spheres were dispersed in ethanol to finally form the solid Se nanowires Field-emission scanning electron microscopy (SEM, FEI DB 235) was used to image the Se nanowires The SEM image shown in Fig.1 reveals the nanowires having lengths of about several micrometers with dendritic structures The Se nanowires were characterized through Raman spectroscopy and pho-toluminescence (PL) spectroscopy using a Renishaw inVia Raman-PL microscope with a 514 nm laser excitation Figure1b shows the Raman spectra taken on the Se nanowires The sole peak is centered at 238 cm-1 sug-gesting that the selenium nanowires are with the trigonal phase [10] A typical PL spectrum of the Se nanowires is shown in Fig 1c The PL spectrum is dominated by a peak centered at 706 nm corresponding to 1.76 eV, which is well consistent with the band-gap of trigonal-Se [10]
In order to perform the measurement of photoelectrical properties of individual Se nanowires, the Se nanowires were mechanically transferred onto the SiO2(500 nm)/Si
Z.-M Liao ( &) C Hou D.-P Yu (&)
State Key Laboratory for Mesoscopic Physics, Department
of Physics, Peking University, Beijing 100871,
People’s Republic of China
e-mail: liaozm@pku.edu.cn
D.-P Yu
e-mail: yudp@pku.edu.cn
L.-P Liu
Department of Chemistry, TsingHua University, Beijing,
People’s Republic of China
DOI 10.1007/s11671-010-9585-2
Trang 2substrate Ti/Au (10/70 nm) electrodes contacting onto
individual Se nanowires were fabricated by electron beam
lithography, metal sputtering deposition, and lift-off SEM
image of the fabricated two-terminal nanowire device is
shown in Fig.1d Conductance measurements were carried
out using a Keithley 4200 Semiconductor Characterization
System and the samples were placed on a Janis
micro-cryostat under vacuum (*10-6Torr) For
photoconduc-tivity measurements, illumination was provided by an Ar
ion laser with 514 nm wavelength guided by a Renishaw
Raman microscope
Figure2a shows the time course of the photocurrent
measured at 300 K and with constant 0.1 V bias voltage as
the laser illumination was turned on and off The
photo-current rapidly increased upon light exposure and then
reduced to a constant, as seen the sharp peaks in Fig.2a
When illumination was removed, the current quickly
jumped down to the level of initial dark current value The
photocurrent–time characterization suggest the existence of
trap states in the Se nanowires Our previous work also
indicated that the surface-absorbed oxygen molecules on
the Se nanowire can capture electrons and induce the
p-type conductivity [14] At room temperatures and dark
conditions, the trap states are mostly not occupied due to
the thermal activation At the beginning of the illumination,
a mass of electron–hole pairs are generated resulting in the
sudden jump of current in Fig.2a The photo-generated
electron–hole pairs upset the original carrier balance and
will fill in the trap states The filling of the trap states leads
to the decay of the photocurrent in Fig.2a The rebalance
of the carrier concentration makes the photocurrent toward saturation Figure2a also shows that the current is con-tinued to increase slightly after the illumination was turned off, which is attributed to the fact that the carriers are returned slowly from the trap states with the assistant of thermal activation
Figure2b shows the time course of photocurrent mea-sured at 100 K and 0.2 V bias The photocurrent has a quick response to the laser illumination switching No transient decay of photocurrent was observed after the device was exposed under illumination At low tempera-tures and at dark, the carriers captured in the trap states are almost frozen Under illumination, the carriers generated from the trap states and the bandgap contribute to the photoconductivity The dynamic balance between the car-rier generation and recombination results in the relatively steady photocurrent but with some noise After turning off the illumination, the current declines quickly, and then the dark current slowly decreases with a long relaxation time,
as shown in Fig.2b The relaxation of the dark current is due to the recapture of the carrier by the trap states Figure3a shows the time response of a single Se nanowire device at different excitation intensities measured
at 10 K and 0.5 V bias voltage As the illumination intensity was varied from 0 to 2.3 9 104mW/mm2, the photocurrent was increased from 0.03 to 22 nA The light
Raman shift (cm-1)
(b)
Wavelength (nm)
(c) (a)
(d)
Fig 1 a SEM image of the Se
nanowires b Raman spectrum
of the Se nanowires
corresponding to trigonal Se
structure c PL spectrum of the
Se nanowires showing 1.76 eV
(706 nm) band-gap of trigonal
Se d SEM of single Se
nanowire two-terminal device
Trang 3intensity dependences of the photocurrent at different
temperatures are shown in Fig.3b with log–log scales It is
interesting to notice the photocurrent toward the similar
values at high light intensities for the different
tempera-tures At high illumination intensities, the number of
photogenerated carrier is overwhelmingly larger than the
thermal activated carriers Therefore, the current is
gov-erned by the light intensity but not the temperature The
dependence of photocurrent (Iph) on laser intensity (P) can
be well fitted by a power law, Iphµ Pa, where exponent a
can help to reveal the dynamics of carrier generation and
recombination Fitting the power law dependence to the
experimental data gives a = 0.64, 0.49, and 0.07 at
tem-peratures of 10, 200, and 300 K, respectively The power
law dependence can be further analyzed by inspecting the
variation of the density of free carriers (N) in the nanowire
[15]
dN
dt ¼ F C N þ Ntrap
where, F is the photon absorption rate and is proportional
to the illumination intensity P, C is the probability of a
charge to be captured, and Ntrap is the density of trapped
carriers Under steady-state conditions, (dN/dt) = 0 and we
can obtain that
Nþ Ntrap
Assuming the photocurrent Iph proportional to the free carrier density N and considering two extreme conditions,
if Ntrap[[ N, then Nµ P and thus Iphµ P; but if
Ntrap\\ N, then Nµ P0.5and thus Iphµ P0.5 The expo-nent a = 1 and a = 0.5 are corresponding to the mono-molecular recombination and bimono-molecular recombination, respectively [15] For our experimental results, at 10 K,
a = 0.64, a signature of existence of both monomolecular recombination and bimolecular recombination At low temperatures, the free carriers are not thermally activated, and therefore, the response is dominated by the trap states
At elevated temperature of 200 K, a = 0.49, which is characteristic of bimolecular recombination The free car-riers increase as a result of increased thermal activation, which induces the transition from monomolecular to bimolecular recombination However, the exponent a of the photocurrent dependence on the excitation intensity changes to 0.07 at 300 K, which may be due to the fact that the trap states become recombination centers At 300 K, the trap states are thermally activated and act as recombi-nation centers under illumirecombi-nation, leading to the weak light intensity dependence of photocurrent
In summary, the temperature effects on photoelectrical properties of single Se nanowires have been investigated
0.2
0.6
1.0
1.4
Time (S)
300 K 0.1 V Bias
(a)
0.0
0.2
0.4
0.6
Time (S)
100 K 0.2 V Bias
(b)
Fig 2 Photocurrent versus time curves a at 300 K and 0.1 V bias
voltage, b at 100 K and 0.2 V bias
0 5 10 15 20 25
Time (S)
10 K 0.5 V Bias
10 100 1000 5000
18500
mW/mm2
(a)
10 0
10 1
10 2
10 3
10 4
10-10
10-9
10-8
300 K
200 K
10 K
Laser Intensity (mW/mm2)
0.5 V Bias
(b)
Fig 3 a Photoelectrical response of a single Se nanowire device under laser illumination of varying intensities at 10 K and 0.5 V bias.
b Photocurrent as a function of illumination intensity for 514 nm laser excitation measured at 0.5 V bias and at different temperatures
Trang 4The light intensity dependence of photocurrent suggests
that the Se nanowire photoconductivity is dominated by
trap states at low temperatures, while a weak dependence
of photocurrent on incident light intensity was observed at
room temperature
Acknowledgments We thank Prof Yadong Li of Tsinghua
Uni-versity for supplying the Se nanowires This work was supported by
NSFC (No 10804002), MOST (Nos 2007CB936202, 2009CB62
3703), and the Research Fund for the Doctoral Program of Higher
Education (RFDP).
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which
per-mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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