High-gain photoconductivity in semiconducting InN nanowires

Article abstract of DOI:10.1063/1.3242023

We report on the photoconductivity study of the individual infrared-absorbing indium nitride (InN) nanowires. Temperature-dependent dark conductivity measurement indicates the semiconducting transport behavior of these InN nanowires. An enhanced photosens

Full text of DOI:10.1063/1.3242023

High-gain photoconductivity in semiconducting InN nanowires
Reui-San Chen, Tsang-Ho Yang, Hsin-Yi Chen, Li-Chyong Chen, Kuei-Hsien Chen, Ying-Jay Yang, Chun-Hsi
Su, and Chii-Ruey Lin
Citation: Applied Physics Letters 95, 162112 (2009); doi: 10.1063/1.3242023
View online: http://dx.doi.org/10.1063/1.3242023
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APPLIED PHYSICS LETTERS 95, 162112 ͑2009͒
High-gain photoconductivity in semiconducting InN nanowires
Reui-San Chen,1 Tsang-Ho Yang,2,3 Hsin-Yi Chen,4 Li-Chyong Chen,3,a͒
Kuei-Hsien Chen,1,3,a͒ Ying-Jay Yang,4 Chun-Hsi Su,2 and Chii-Ruey Lin2
1Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
2Graduate Institute of Mechanical and Electrical Engineering, National Taipei University of Technology,
Taipei 10608, Taiwan
3Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
4Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan
͑Received 7 April 2009; accepted 11 September 2009; published online 23 October 2009͒
We report on the photoconductivity study of the individual infrared-absorbing indium nitride ͑InN͒
nanowires. Temperature-dependent dark conductivity measurement indicates the semiconducting
transport behavior of these InN nanowires. An enhanced photosensitivity from 0.3 to 14 is observed
by lowering the temperature from 300 to 10 K. A calculated ultrahigh photoconductive gain at
around 8ϫ107 at room temperature is obtained from the low-bandgap nitride nanowire under
808 nm excitation. © 2009 American Institute of Physics. ͓doi:10.1063/1.3242023͔
Indium nitride ͑InN͒ with the lowest direct bandgap
͑ϳ0.6 eV͒ among III-nitride semiconductors has gained
substantial attention due to its potential for the development
of new-generation infrared ͑IR͒ optoelectronic devices. The
notes yttrium aluminum garnet͒ laser as excitation source.
The collected PL light was dispersed through a 0.5 m mono-
chromator equipped with a 300 g/mm grating and was de-
tected by an extended InGaAs detector. The temperature-
dependent dark- and photo-current measurements of the InN
NW devices were carried out on a cryogenic probe station
͑LakeShore Cryotronics model TTP4͒. A semiconductor
characterization system ͑Keithley model 4200-SCS͒ was uti-
lized to source the dc bias and measure the current. The
808 nm diode laser was used as the excitation source in the
PC experiment.
*
low electron effective mass ͑m ϳ0.06me͒ also enables InN
to be a candidate material for high-speed electronics.1,2 Re-
cently, an intrinsic surface conduction due to the presence of
resonant surface states in InN is also observed,3 which in-
spires the interest in fundamental researches4,5 and related
applications such as terahertz emission,6 ion sensing,7 and
field emission.8
Figure 1͑a͒ depicts the FESEM image of the ensemble of
the as-grown InN NWs. The average diameters ͑d͒ and
lengths ͑l͒ are 20–80 nm and 8–12 ␮m, respectively. The
single crystalline structure and the ͗110͘ long-axis orienta-
tion are confirmed by the TEM and its corresponding SAED
pattern ͓Fig. 1͑b͔͒. Figure 1͑c͒ illustrates the PL spectrum of
the as-grown InN NWs at the temperature of 20 K. Using
Owing to these interesting bulk and surface nature of
InN, a numbers of effort have also been extended to the
electrical9–14 and optical properties15,16 of its nanowire ͑NW͒
with size-confined and quasi-one-dimensional ͑quasi-1D͒
nanostructure. Electronic transport12–14 and IR photolumines-
15,16
cence ͑PL͒
have been proven of being dominated or in-
fluenced by the electron accumulation layer on surface.
However, so far, the study of photoconductivity ͑PC͒, which
is the most crucial issue for the development of IR detectors
using InN and its nanostructures, has hardly been investi-
gated yet.17 The extremely degenerate nature with high in-
trinsic carrier concentration could partially suppress the pho-
toresponse in InN.14 Here, we report on the efficient PC
performance in the nonmetallic InN NWs upon 808 nm IR
irradiation. The temperature-sensitive photosensitivity and
very high photocurrent gain induced by long carrier lifetime
are presented and discussed.
The InN NWs used for this study were grown at 480 °C
by metalorganic chemical vapor deposition ͑MOCVD͒ using
trimethylindium and ammonia ͑NH3͒ source reagents and
gold ͑Au͒ catalyst. The morphologies, structures, and long-
axial orientations of the InN NWs grown in this way were
characterized by field-emission scanning electron micros-
copy ͑FESEM͒ ͑JEOL 6700͒, transmission electron micros-
copy ͑TEM͒ ͑JEOL JEM-4000EX͒, and selected-area elec-
tron diffractometry ͑SAED͒, respectively. PL measurements
were performed at 20 K using 532 nm Nd+:YAG ͑YAG de-
FIG. 1. ͑Color online͒ ͑a͒ The FESEM image of the ensemble of the as-
grown InN NWs. ͑b͒ The TEM image and its corresponding SAED pattern
͑the inset͒ of a single InN NW. ͑c͒ A typical PL spectrum measured at 20 K
of the ensemble of the as-grown InN NWs. ͑d͒ The selected id-V curves of
a single InN NW device with diameter of 35Ϯ5 nm measured at different
temperature in vacuum. Inset in ͑d͒ shows the typical FESEM image of a
single InN NW device with two Pt contacts fabricated by FIB deposition.
a͒
Authors to whom correspondence should be addressed. Electronic ad-
dresses: chenlc@ntu.edu.tw and chenkh@pub.iams.sinica.edu.tw.
0003-6951/2009/95͑16͒/162112/3/$25.00
95, 162112-1
© 2009 American Institute of Physics
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162112-2
Chen et al.
Appl. Phys. Lett. 95, 162112 ͑2009͒
FIG. 3. ͑Color online͒ The temperature-dependent carrier lifetime and its
corresponding photocurrent gain of the single InN NW in the range of
10–300 K in vacuum.
ing. Since narrow-bandgap semiconductors frequently suffer
from high thermal current, the preliminary sensitivity of InN
NWs is already compatible with other photoconductor-type
IR detectors based on 1D nanostructure, such as germanium
͑Ge͒ NW,19 indium phosphide ͑InP͒ NW,20 and carbon
nanotube.21
Upon fitting the rise times of ip response curve in Fig.
2͑b͒, the carrier lifetime ͑␶͒ at different temperatures can be
obtained and plotted as shown in the inset of Fig. 3. It is
noted that the very long ␶ of the order of tens of seconds is
obtained in the InN NWs. The ␶ values show a 2.5 times
increase from 15 to 38 s as the temperature decreases from
300 to 10 K. This result corroborated with the trend of ip
versus T ͓Fig. 2͑a͔͒ suggests that the ip is dominated by the
long ␶ in this nitride NW. Compared to normal photoconduc-
tors with ␶ in the time scales of microseconds to millisec-
onds, the ␶ of InN NWs in tens of seconds implies several
decades higher ip generation efficiency and could originate
from a different mechanism. To investigate the transport
property of photocarriers, the photocurrent gain ͑⌫͒, a factor
determining the electronic transport and carrier collection ef-
ficiency during the PC process, is estimated. The physical
meaning of ⌫ relates to the number of electrons circulating
through the photoconductor per absorbed photon and per unit
time. Accordingly, ⌫ is defined as22,23
FIG. 2. ͑Color online͒ ͑a͒ The temperature-dependent dark and photocur-
rents of the single InN NW in the range of 10–300 K. ͑b͒ The selected
photocurrent response to the 808 nm excitation of the single InN NW at
different temperatures in vacuum. Inset in ͑b͒ shows the temperature-
dependent photosensitivity of the single InN NW in the range of 10–300 K.
Gaussian fitting, the PL spectrum can be fitted with a single
peak centering at ϳ0.707 eV and broadening of ϳ102 meV.
This IR-emission peak position is also consistent with the
band-to-band emission observed from the high-quality
nanorods15 and epitaxial films.18
Two-terminal single InN NW samples using platinum
͑Pt͒ metal contacts, as shown in the inset of Fig. 1͑d͒, were
fabricated by focused-ion-beam ͑FIB͒ deposition on the in-
sulating SiNx͑500 nm͒/Si substrates. Figure 1͑d͒ illustrates
the dark current ͑id͒ to bias ͑V͒ measurements at different
temperatures ͑T͒ in vacuum for the NW with d=35Ϯ5 nm.
The linear id-V curves in the temperature range of 10–300 K
reveal good Ohmic contact property of this single wire de-
vice. Moreover, at a constant bias of 0.1 V, the id decreases
over 1 decade as the temperature lowers from 300 to 10 K
͓Fig. 2͑a͔͒, indicating the semiconducting nature of the InN
NW. The room-temperature conductivity estimated from the
id-V measurement is around 10 ⍀−1 cm−1.
The PC of the single InN NW was characterized subse-
quently using the 808 nm excitation source. Figure 2͑b͒ de-
picts the selected photocurrent ͑ip͒ responses at the bias of
0.1 V at different temperatures in vacuum, in which the
background id currents have been subtracted. The ip value
shows a continuous increase as the temperature lowers,
which is opposite to the trend of id, as shown in Fig. 2͑a͒.
The effective reduction in thermal current and the enhance-
ment of ip response at low temperature both benefit the pho-
tosensitivity ͑S͒ to the IR detection of InN NWs. Photosen-
sitivity, defined as the ratio of photocurrent to dark current,
i.e., S=ip/id, is the parameter electrically determining the
signal-to-noise ratio for the operation of photodetectors. The
inset in Fig. 2͑b͒ depicts the increase in S over one order of
magnitude from 0.3 to 14 in this investigated temperature
range, indicating the good device performance for IR sens-
␶
⌫ =
,
͑1͒
t
where t is the transit time of carrier between electrodes.
Since t=l/ , where , the carrier drift velocity, is the product
v
v
of mobility ͑␮͒ and applied electric field ͑F͒, i.e., =␮F, ⌫
v
can be rewritten as23
F
⌫ = ␶␮
.
͑2͒
l
According to our field-effect measurement by directly
applying gate voltage from the bottom of the single NW
devices, the ␮ values of the InN NWs are around
2Ϯ1 cm2/V s. Thus, the ⌫ values are estimated as ͑8Ϯ4͒
ϫ107 at room temperature for ␶=15 s, F=5ϫ102 V/cm,
and l=2 ␮m. Its temperature dependence is also illustrated
in Fig. 3. So far, no ⌫ value has been defined from the InN
system. As ⌫ value depends on the experimental parameters,
comparison must be made in the same condition. However,
overall, this value of InN NWs is several orders of magni-
tude higher than those ͑⌫Ͻ104͒ of different types of
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162112-3
Chen et al.
Appl. Phys. Lett. 95, 162112 ͑2009͒
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served in the wide-bandgap semiconducting NWs of GaN29
and ZnO.30 Soci et al. reported a maximal ⌫ value of 2
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