Time resolved microphotoluminescence studies of single InP nanowires grown by low pressure metal organic chemical vapor deposition

Article abstract of DOI:10.1063/1.2776358

The authors report optical studies of InP nanowires (NWs) grown by metal organic chemical vapor deposition. By means of low temperature microphotoluminescence experiments, the authors determined the optical properties of as-grown NWs. The emission of individual NWs is characterized by small linewidths as low as 2.3 meV. Blueshifts of the NW emission energy between 25 and 56 meV with respect to bulk InP are related to radial carrier confinement in nanowires with diameters between 15 and 50 nm. Time resolved investigations reveal a low surface recombination velocity of 6× 102 cms and indicate thermally activated nonradiative surface recombination above approximately 20 K.

Full text of DOI:10.1063/1.2776358

Time resolved microphotoluminescence studies of single InP nanowires grown by low
pressure metal organic chemical vapor deposition
S. Reitzenstein, S. Münch, C. Hofmann, A. Forchel, S. Crankshaw, L. C. Chuang, M. Moewe, and C. Chang-
Hasnain
Citation: Applied Physics Letters 91, 091103 (2007); doi: 10.1063/1.2776358
View online: http://dx.doi.org/10.1063/1.2776358
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/9?ver=pdfcov
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APPLIED PHYSICS LETTERS 91, 091103 ͑2007͒
Time resolved microphotoluminescence studies of single InP nanowires
grown by low pressure metal organic chemical vapor deposition
S. Reitzenstein,a͒ S. Münch, C. Hofmann, and A. Forchel
Technische Physik, Physikalisches Institut, Universität Würzburg, Am Hubland,
D-97074 Würzburg, Germany
S. Crankshaw
Applied Science and Technology Group, University of California at Berkeley, Berkeley, California 94720
L. C. Chuang, M. Moewe, and C. Chang-Hasnain
Department of Electrical Engineering and Computer Sciences, University of California at Berkeley,
Berkeley, California 94720
͑Received 26 June 2007; accepted 6 August 2007; published online 27 August 2007͒
The authors report optical studies of InP nanowires ͑NWs͒ grown by metal organic chemical vapor
deposition. By means of low temperature microphotoluminescence experiments, the authors
determined the optical properties of as-grown NWs. The emission of individual NWs is
characterized by small linewidths as low as 2.3 meV. Blueshifts of the NW emission energy
between 25 and 56 meV with respect to bulk InP are related to radial carrier confinement in
nanowires with diameters between 15 and 50 nm. Time resolved investigations reveal a low surface
recombination velocity of 6ϫ102 cm/s and indicate thermally activated nonradiative surface
recombination above approximately 20 K. © 2007 American Institute of Physics.
͓DOI: 10.1063/1.2776358͔
The study of semiconductor nanowires ͑NWs͒ has been
a field of growing interest in recent years. High quality semi-
conductor nanowires are reported based on material systems
such as GaAs, GaN, or InP using various growth methods.1–4
Optimization of catalyzed patterning and growth control en-
abled the growth of NWs with controlled size and composi-
tion, which is crucial for the application of NWs in nanoelec-
tronic or nanophotonic devices.5–7 Nanowires exhibit
interesting electronic and optical properties due to quantum
confinement effects. However, the large surface to volume
ratio of these quasi-one-dimensional systems makes them
sensitive to surface states and may result in a low lumines-
cence quantum yield in photonic wire structures. Nonradia-
tive recombination due to surface states is most severe in
thin wires, which leads to a major trade-off between quantum
efficiency and strong quantum confinement effects. For in-
stance, detrimental nonradiative processes are observed for
GaAs NWs, which thus require effective surface
passivation.8,9 In contrast to GaAs NWs, InP NWs have
shown to be good candidates for optoelectronic devices with
both high quantum efficiency and strong quantum
confinement.10
In the present work, we performed optical studies on
individual InP NWs in order to assess important properties
such as confinement energies and nonradiative surface re-
combination relevant for nanowire based devices. In particu-
lar, photoluminescence experiments reveal ultralow emission
linewidths and a very low surface recombination velocity of
high quality, as-grown InP NWs. The NWs were grown on
B-doped ͑100͒ Si substrates by low pressure metal organic
chemical vapor deposition ͑MOCVD͒. After cleaning the Si
substrate, 10 Å Au was evaporated as the vapor-liquid-solid
catalyst. The substrate was annealed for 3 min at 640 °C in
the MOCVD reactor to form the Au catalyst droplets. For the
subsequent 10 min nanowire growth, the reactor temperature
was reduced to 460 °C, using tertiarybutylphosphine and tri-
methylindium with a V/III ratio of 60 as the metal-organic
sources. To characterize the NWs, spatially and temporally
resolved microphotoluminescence ͑␮PL͒ studies were per-
formed on as-grown, individual InP nanowires. For
continuous-wave experiments, the NWs were excited by an
argon-ion laser at 514 nm. The laser beam was focused by a
microscope objective with a numerical aperture of 0.4 to a
spot size of about 3 ␮m on the sample. A computer-
controlled x-y-z-piezostage with an accuracy of 0.1 ␮m in
each direction allowed a precise addressing of single NWs.
The photoluminescence ͑PL͒ was collected by the same mi-
croscope objective, dispersed by a 0.55 m monochromator,
and detected by a nitrogen cooled charge coupled device. For
time resolved measurements, a mode locked Ti:sapphire la-
ser tuned to 790 nm providing pulses with a width of about
150 fs at a repetition rate of 82 MHz was used. The decay
curves were recorded by a photon-multiplier tube in combi-
nation with single photon counting electronics after spectral
filtering with a 0.55 m monochromator.
The structural quality of individual as-grown InP NWs
was analyzed by transmission electron microscopy ͑TEM͒
imaging and diffraction analysis. A TEM image of a NW
with a diameter of about 35 nm is shown in Fig. 1͑a͒. From
the electron diffraction ͑not shown͒ of the nanowires, we
conclude that the NWs crystallized in the wurtzite structure
with the nanowire axis in the ͓0001͔ direction. For studies on
individual NWs, sample positions with a low surface density
of approximately 1 wire/␮m2 were chosen. A typical ␮PL
spectrum of individual NWs recorded at 10 K with an exci-
tation power of 2 ␮W is depicted in Fig. 1͑b͒. The spectrum
shows two NW emission peaks at 1.452 and 1.460 eV, re-
spectively. We observe remarkably small linewidths of 4.7
and 2.3 meV, which are significantly narrower than reported
a͒
Electronic mail: stephan.reitzenstein@physik.uni-wuerzburg.de
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091103-2
Reitzenstein et al.
Appl. Phys. Lett. 91, 091103 ͑2007͒
TABLE I. Emission energy, radial confinement energy, and calculated di-
ameter of nanowires NW1–NW5.
Emission energy
Confinement energy
Diameter
Peak No.
͑eV͒
͑meV͒
͑nm͒
NW1
NW2
NW3
NW4
NW5
1.445
1.452
1.460
1.464
1.476
25
32
40
44
56
50
34
23
20
15
By changing the excitation spot position in a micrometer
range, the relative contribution of the components can be
varied strongly, indicating that each feature has a different
spatial origin, i.e., is due to a different NW. The intensity of
all lines depends to a good approximation linearly on the
excitation power. This demonstrates that the lines are due to
ground state transitions in NWs of different diameters.
We relate the different peak emission energies of NW1–
NW5 to size dependent radial confinement energies of the
respective wires. Lacking consistent experimental data on
the bandgap of wurtzite InP,14,15 we estimate this size depen-
dence using zinc-blende InP parameters ͑Eg=1.42 eV͒. In
this case, the emission peaks of NW1–NW5 show confine-
ment induced blueshifts of about 25 to about 56 meV. In
order to explore the size dependence of the nanowire lumi-
nescence quantitatively, we applied an effective mass model
for electrons and holes confined in a cylindrical potential.11
Using the InP effective masses of electrons and holes and the
reduced effective mass, we obtain estimates for the wire di-
ameters ranging from 50 to 15 nm. The values agree well
with the results of TEM and scanning electron microscopy
investigations, which show typical wire sizes in the calcu-
lated range of diameters. The results for the different wires
are summarized in Table I.
FIG. 1. ͑Color online͒ ͑a͒ TEM image of an InP NW with a diameter of
about 35 nm, Inset: zoom in of the upper part of the NW. ͑b͒ ␮PL spectrum
of two nanowires with emission energies of 1.452 eV ͑labeled A͒ and
1.460 eV ͑labeled B͒, respectively ͑P=2 ␮W, T=10 K͒. Intensity maps
showing the spatial distribution of the integrated intensity of nanowire A
͓panel ͑c͔͒ and B ͓panel ͑d͔͒, respectively.
previously4,11 and indicate a low density of surface states for
the as-grown NWs.12,13 The spatial distribution of the PL
intensity was investigated by taking PL spectra at different
positions over a scan area of about 50ϫ50 ␮m2 ͑P=2 ␮W,
T=10 K͒. Corresponding surface maps of the integrated in-
tensity associated with peak A at 1.452 eV and peak B at
1.460 eV are displayed in Figs. 1͑c͒ and 1͑d͒. It is clearly
seen that the PL originates from distinct positions associated
with two wires centered at ͑arbitrary͒ positions
͑x,y͒=͑22,17 ␮m͒ and ͑x,y͒=͑24,23 ␮m͒, respectively.
We would like to point out that no signatures of optical tran-
sitions due to Au from the catalyst droplets have been ob-
served in the PL spectra.
The optical properties of the NWs were investigated in
more detail by time resolved measurements. These types of
measurements reveal important information on the free car-
rier dynamics which in turn allows one to estimate the con-
tribution of radiative and nonradiative channels on the free
carrier decay. Since surface states usually introduce nonradi-
ative recombination centers, time resolved investigations are
a direct tool to assess the surface quality of the NWs. In
order to gain information on the surface properties of the
present NWs, we performed time resolved studies on the
emission associated with NW4 at 1.463 eV as it could be
well separated from the other peaks. The temporal evolution
of the luminescence from NW4 is depicted in Figs. 3͑a͒ and
3͑b͒ for a temperature of 4 and 27 K, respectively ͑spectral
window for time resolved studies: 1.461–1.465 eV͒. At 4 K,
we observe a monoexponential dynamics associated with a
decay time of ␶=2.6 ns. This long decay time together with
the monoexponential decay reflect again the high surface
quality of the as-grown NWs and indicate that nonradiative
surface recombination is negligible at low temperature. In
particular, the decay time of as-grown NWs investigated in
the present work is much higher in comparison to 0.2 ns
reported for as-grown InP NWs on InP substrate and are
comparable to decay times after HF passivation of these
catalyst-free grown NWs.12 The monoexponential decay
characteristic is sustained up to about 20 K above which a
The emission of a few InP NWs recorded at 10 K with
an excitation power of 15 ␮W at a position with a slightly
higher surface density is displayed in Fig. 2. The emission
consists of a group of five features denoted as NW1–NW5.
FIG. 2. ␮PL spectrum of individual nanowires ͑NW1–NW5͒ recorded at an
excitation power of 15 ␮W at 10 K.
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transition to biexponential free carrier decay evolves. As
On: Sun, 21 Dec 2014 22:40:25

091103-3
Reitzenstein et al.
Appl. Phys. Lett. 91, 091103 ͑2007͒
of the fast decay component at higher temperatures is asso-
ciated with a strong decrease of PL intensity which is shown
in the Arrhenius plot in Fig. 3͑c͒. The temperature depen-
dence of the emission intensity allows us to estimate the
characteristic activation energy and yields a band bending of
␸=2.8 meV.
In summary, spatially and temporally resolved ␮PL
spectroscopies have been used to address physical properties
of single InP nanowires. We observed a size dependent blue-
shift of up to 56 meV for the nanowire emission energy with
respect to InP bulk band gap emission and narrow linewidths
as low as 2.3 meV which indicates a nonradiative surface
recombination in the as-grown nanowires. This interpretation
is confirmed by temperature and time resolved investigations
which reveal a low surface recombination velocity of about
6ϫ102 cm/s and point to thermally activated nonradiative
surface recombination above approximately 20 K.
FIG. 3. ͑Color online͒ Photoluminescence decay of NW4 emission taken
after spectral filtering ͑1.461–1.465 eV͒ at ͑a͒ 4 K and ͑b͒ 27 K, respec-
tively. ͑c͒ Arrhenius plot showing the PL intensity of NW4 emission vs
reciprocal temperature.
This work was partly supported by the European Com-
mission through the IST Project Node and the State of Ba-
varia. Support is also acknowledged from DARPA HR0011-
04-1-0040 ͑CONSRT͒ and HP-CITRIS grants, with author
fellowship support from the NSF Graduate Research Fellow-
ship Program to one of the authors ͑S.C.͒, and NSF-IGERT
Program to two of the authors ͑L.C.C. and M.M.͒.
shown in Fig. 3͑b͒ for T=27 K the biexponential is associ-
ated with a fast decay shortly after the laser pulse and a
slower decay at longer delay times. We relate the fast decay
with a time constant of ␶1=1.1 ns to nonradiative surface
recombination while the longer component with ␶2=2.6 ns
observed also for T=4 K ͓c.f. Fig. 3͑a͔͒ is attributed to ra-
diative recombination in the NWs.12
In order to describe the time resolved data more quanti-
tatively, we take into account a model introduced to describe
carrier decay in GaAs NWs.16 In this model, the carrier de-
cay rate depends via
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͑2͒
B
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͑
͒
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