-1 Introduction
1D SnO2 NWs are known to have many unique electronic and optical properties ue to the high surface-to-volume ratio to make excellent nanodevices. Because of its igh guantum efficiency in ultraviolet (UV) region, it is extensively used in visible-blind photodetectors.6 Add ilms and NWs have also been used in active pollutant gas sensors based n the measurement of resistance, such as H2, CO, and NO2 etc.7-9 For both, the mo odel to explain the resistance
change i n from
miconductor metal oxide, and react with chemical gas, and then the conductivity is premise has been adapted to explain the photoresponse of metal oxid
3
d h
itionally, SnO2 thin f o
st quoted m
s that the adsorbed oxygen on the surface will extract the electro se
altered. Even this
es, such as ZnO and SnO2;10,11 however, so far, a quantitative description of the
photoresponse gain of SnO2 NWs is still lacking, and the underneath mechanism of the photoresponse has not been well established. In this letter, we point out that the mechanism responsible for the photocurrent (PC) in SnO2 NWs includes oxygen-related hole-trap states at the surface of NW as well as band-bending induced by surface electric field.
3-2 Results and Discussion
SnO2 is a direct band-gap semiconductor with good optical adsorption in UV region as shown in Fig. 3-1. On illumination with photon energy larger than the energy band gap (~3.6 eV), the conductivity of the sample increases drastically due to photoexcited electron-hole pairs. Figure 3-2 shows the current of the sample with a bias of 0.1 V and under the illumination of a He-Cd laser working at 325 nm with different excitation intensity. The gain (Γ) of the photoresponse could be obtained by expression,13
where is the current difference between photocurrent and dark current, q is the electron charge, hνis the photon energy of incident light,
Δi
P is the power of photon that the NW has absorbed, which means P=I×l×d . I is the exciting intensity
understand the mechanism behind the large photoresponse gain.
14,15
gain and the illumination intensity is very useful to reveal the mechanism responsible for photocurrent. There are 2 types of PC can be observed with “short rise time” and
“long rise time” in the graph. The current is recorded in an interval of 0.5 s, and can still measure a step PC with short rise time (
respectively. η is the quantum efficiency which is set to be 1 for simplicity. Quite surprisingly, the calculated gain for the SnO2 NW can be as high as 8000, which implies that SnO2 NWs can serve as a highly sensitive photodetector. It will be very interesting to
According to the reports of Binet et al and Muñoz et al, the relation between the
τ ) smaller than 0.5 s (even smaller than 1
0.01 s), so we cannot measure the correct rise e. And the focus is on the “long rise time” PC.
As shown in Fig. 3-3(a), for the excitati tensity below the critical value of 2 W/m2 , Γ does not depend on the intensity which means Γ remains constant. This behavior can be understood in term of oxygen-related hole-trap states at the surface of NW.14,16 When SnO2 NWs are exposed in air and adsorb oxygen molecules at their surface, the adsorbed oxygen molecules will capture electrons from the NWs and become negatively charged.16 It can be described by the expression
2
UV light illumination, the photoinduced holes will migrate to the surface by the surface electric field and discharge negative-charged oxygen molecules, as des
After the
cribed the expression,16
[h+ +O2− →O2]. (3.3)
Accordingly, the neutralized oxygen molecules are photodesorbed from the surface . Therefore, the presence of hole-trap states prolongs the photoinduced electron lifetime, and hence the large value of gain can be understood. Indeed, Γ is defined as τμΦ l2, where τ is the carrier lifetime, μis the carrier mobility, Φis applied
en the
/m2), oxygen-related hole-trap states will be filled and c
voltage. Wh excitation intensity is low, τ remains unchanged. Therefore, Γ does not depend on the excitation intensity. The oxygen-related hole-trap mechanism results in photoconductive gain as high as 104. Nevertheless, once the excitation intensity exceeding the critical intensity (2 W
hange the electron-hole recombination behavior, and then lower down the gain. Under this circumstance, Γ versus intensity follows an inverse
power law [Γ I ]. As shown in Fig. 3-3(a), the value of κ is about 0.81 fort the ∝ −κ intensity beyond 2 W/m2, which is much larger than the exponent of κ=0.5 dom by the trap mechanism. Thus, this result suggests a non-trap mechanism in SnO
inated
2 NWs. The mechanism responsible for the PC observed here may be attributed to the modulation of surface space charge region according to the
l-known
s upon, the recombination probability of electrons and holes reduces, nd consequently the lifetime of conducting electrons increases. At the intensity of simulation of Muñoz et al and Garrido et al.15,17 It predicts a power law dependence for the relationship between gain and excitation intensity with the value of κ in the range 0.5-0.9.
It is wel for SnO2 NWs with surface defects, usually oxygen defects, esulting in an n-type semiconductor.18 Owing to the defects at the surface, the pward band-bending exists and forms a low-conductivity depletion layer at the urface referred to space charge region. Because NWs have much higher urface-to-volume ratio, the surface of NWs influences the conductivity more rastically. Once electron-hole pairs are photogenerated, holes drift to the surface eadily surface electric field, leaving unpaired electrons inside, thus being spatially eperated.18 There
a
100 W/m2 as shown in Fig. 3-3(b), the PC contributed from surface band-bending is 81% and only 19% is from hole-trap effect.
However, the κ of “short rise time” PC is 0.5 as shown in Fig.3-4, which is believed that the PC is induced by the intrinsic recombination mechanism from bulk, and is not the focus here.
ith an intensity of 100 W/m2, the PC is reatly enhanced by up to one order of magnitude compared with that measured in air.
Besi
ily understood based on the reduction of adsorbed oxygen olecules on the NW surface. With the reduction of adsorbed oxygen molecules, the Let us now examine the PC response in vacuum condition. As shown in Fig. 3-5, when the SnO2 NW is placed in a vacuum of 10-6 torr for 30 min, the dark current is a little higher than that in air which can be explained by the existence of the oxygen adsorption. However, under the illumination w
g
des, both of the PC rise and decay times exhibit a much slower process. Quite remarkably, the gain now reaches an extremely high value of one hundred thousand.
This behavior can be eas m
photoexcited electrons can now enjoy a longer lifetime, therefore all the rise and decay times are enhanced. As shown in Fig. 3-6(a), the high gains can be also obtained under different intensity, and in Fig. 3-6(b), the exponent κ is changed to 0.7, which implies the difference of surface band-bending in different ambient surrounding.
The SnO2 NW device is also studied under green laser illumination (532nm) with light intensity 40000 W/m2, and the photoresponse in air was given in Fig. 3-7. As for the SnO2 NW device, the current increases from 12.5 nA to 15nA on green light
owers the band-bending. Both of them give rise to the increase of onductance.
illumination. As oxygen molecules are adsorbed on the surface of the SnO2 NW, they capture electrons from the NW and form negatively charged ions. When light with energy below the band gap is introduced, the electrons captured at the oxygen molecules are photoexcied to the conduction band, which enhances the electron density and l
c
Fig. 3-1: I-V characteristics of SnO2 nanowire in ambient air.
0 4000 8000 12000 16000
3.0x10-8 6.0x10-8 9.0x10-8 1.2x10-7 1.5x10-7
Cu rren t ( A)
Time (s)
Fig. 3-2: Photocurrent of SnO2 nanowire with a bias 0.1V under different excitation
-0.4 -0.2 0.0 0.2 0.4
-1x10-7 -5x10-8 0 5x10-8 1x10-7
Current (A)
Bias (V)
10μm
(a)
Fig. 3-3: (a) Photoconduction gain of SnO2 nanowire as a function of illum
Ga in (a .u.)
Intensity ( W/m
2)
κ ~0.81
1 10 100
10-8
intensity. (b) Photocurrent of a SnO2 nanowire as a function of excitation intensity 10-7
1 10 100 103
104
Short rise time PC gain
Intensity
(
W/m2)
K~0.5
Fig. 3-4: Photoconduction gain of “short rise time” photocurrent versus different excitation intensity.
Fig. 3-5: Photocurrent of a SnO2 nanowire in air and in vacuum under the illumination of He-Cd laser with wavelength 325nm and excitation intensity of 100 W/m2 in
0 2000 4000 6000 8000 10000 12000 14000 0.0