ZnO nanowire growth and devices

ZnO nanowire growth and devices
Y .W.Heo a ,*,D.P.Norton a ,L.C.Tien a ,Y .Kwon a ,B.S.Kang b ,F.Ren b ,S.J.Pearton a ,Roche c
a Department of Materials Science and Engineering,University of Florida,Gainesville,FL 32611,USA
b
Department of Chemical Engineering,University of Florida,Gainesville,FL 32611,USA
c Raytheon,Waltham,MA 02451,USA Accepte
d 23September 2004
Available online 25December 2004
Abstract
The large surface area of ZnO nanorods makes them attractive for gas and chemical sensing,and the ability to control their nucleation sites makes them candidates for micro-lasers or memory arrays.In addition,they might be doped with transition metal (TM)ions to make spin-polarized light sources.To date,most of the work on ZnO nanostructures has focused on the synthesis methods and there have been only a few reports of the electrical characteristics.We review fabrication methods for obtaining device functionality from single ZnO nanorods.A key aspect is the use of sonication to facilitate transfer of the nanorods from the initial substrate on which they are grown to another substrate for device fabrication.Examples of devices fabricated using this method are briefly described,including metal-oxide semiconductor field effect depletion-mode transistors with good saturation behavior,a threshold voltage of $À3V and a maximum transconductance of order 0.3mS/mm and Pt Schottky diodes with excellent ideality factors of 1.1at 258C and very low (1.5Â10À10A,equivalent to 2.35A cm À2,at À10V)reverse currents.The photoresponse showed only a minor component with long decay times (tens of seconds)thought to originate from surface states.These results show the ability to manipulate the electron transport in nanoscale ZnO devices.
#2004Elsevier B.V .All rights reserved.
Keywords:Nanowires;Nanorods;ZnO;Bandgap
1.Introduction
In recent years,significant interest has emerged in the synthesis of nanoscale materials [1].One of the most attractive classes of materials for functional nanodevices are semiconductors.Various means have been reported for the synthesis of semiconducting nanowires and nanorods [2–4].Much effort has focused on catalysis-driven bulk synthesis of nanomaterials using approaches that are neither substrate site specific nor compatible with most planar device platforms.Nevertheless,nanodevice functionality has been demonstrated with these materials in the form of electric field-effect switching
[5],single electron transistors [6],biological and chemical sensing [7],and luminescence [8]for one-dimensional (1-D)semiconducting structures.Included in the semiconductors of interest are semi-conducting oxides [9–12].Of these,zinc oxide is particularly interesting for nanodevice applications.ZnO is an n -type,direct bandgap semiconductor with E g =3.35eV [13,14]
.
Materials Science and Engineering R 47(2004)1–47
*Corresponding author.Tel.:+13528461091;fax:+13528461182.
E-mail address:ywheo@mse.ufl.edu (Y .W.Heo).
0927-796X/$–see front matter #2004Elsevier B.V .All rights reserved.
doi:10.1016/j.mser.2004.09.001
2Y.W.Heo et al./Materials Science and Engineering R47(2004)1–47
ZnO has been effectively used as a gas sensor material based on the near-surface modification of charge distribution with certain surface-absorbed species[15].ZnO nanorods would provide
significant enhancement in sensitivity due to high surface-to-volume ratio.ZnO is also piezoelectric,
and is used in surface acoustic wave devices[16].Huang et al.have reported the site-specific
nucleation and growth of ZnO nanorods on deposited Au catalyst using a high-temperature vapor
transport process[8].As with any semiconductor,1-D ZnO nanostructures provide an attractive
candidate system for fundamental quantization and low-dimensional transport studies[17–19].
A large variety of ZnO one-dimensional structures have been demonstrated[20–47].The large
surface area of the nanorods and bio-safe characteristics of ZnO makes them attractive for gas and
chemical sensing and biomedical applications,and the ability to control their nucleation sites makes
them candidates for micro-lasers or memory arrays.To date,most of the work on ZnO nanostructures
has focused on the synthesis methods.There have been only a few reports of the electrical
characteristics[20–24].The initial reports show a pronounced sensitivity of the nanowire conductivity
to ultraviolet(UV)illumination and the presence of oxygen in the measurement ambient.There is
strong interest in developing solid-state ozone and hydrogen gas sensors for use in both industry and
domestic applications.Ideal sensors have the ability to discriminate between different gases and arrays
that contain different metal oxides(e.g.SnO2,ZnO,CuO,WO3)on the same chip can be used to obtain
this result.The fact that ZnO can be grown at low temperatures on cheap substrates such as glass also
makes it attractive for transparent electronics.
In this review,we report on the synthesis of both cored and radial heterostructure nanorods and simple,reproducible methods for nanorod device fabrication and give some examples of device
functionality.The ability to control the synthesis of high quality ZnO nanowires leads to potential
applications in UV photodetection,gas sensing and transparent electronics.
2.ZnO nanorod synthesis
Previous effort in the synthesis of ZnO nanowires and nanorods have employed vapor-phase transport via a vapor–liquid–solid(v–l–s)mechanism[46,47],gas reactions[48]and oxidation of
metal in the pores of anodic alumina membranes[49,50].While these materials provide interesting
systems for investigating fundamental properties or for exploring device concepts via single prototype
device construction,the ability to synthesize nanorods at arbitrary locations at moderate temperatures
is needed for nanodevice integration.This requires site-specific nucleation of nanorods,as well as a
growth process that remains site specific and is compatible with the device platform of interest.It
would be advantageous to achieve nanorod growth from aflux source that could be controlled at the
atomic level,thus enabling compositional modulation along the rod length.
In this section,we report on the site-selective growth of ZnO nanorods using a catalysis-driven molecular beam epitaxy(MBE)method.Low temperature MBE conditions are identified so that ZnO
nucleation and growth occurs only on the deposited metal catalyst.With this approach,site specific,
single crystal ZnO nanorod growth is achieved with nanorod diameters as small as15nm.
The growth experiments were performed using a conventional MBE system.The background base pressure of the growth chamber was$5Â10À8mbar.An ozone/oxygen mixture was used as the
oxidizing source.The nitrogen-free plasma discharge ozone generator yielded an O3/O2ratio on the
order of1–3%.No effort was made to separate the molecular oxygen from the ozone.The cationflux
was provided by a Knudsen effusion cell using high purity(99.9999%)Zn metal as the source.Cation
and O2/O3partial pressure was determined via a nude ionization gauge that was placed at the substrate
position prior to growth.The beam pressure of O3/O2mixture was varied between5Â10À6and
5Â10À4mbar,controlled by a leak valve between the ozone generator and the chamber.The Zn pressure was varied between5Â10À7and4Â10À6mbar.The substrates were Si wafers with native SiO2layer terminating the surface.
Site-selective nucleation and growth of ZnO was achieved by coating Si substrates with Ag islands.For thick Ag,a continuous ZnOfilm could be deposited.For nominal Agfilm thicknesses of 20–200A˚,discontinuous Ag islands are realized.On these small metal islands,ZnO nanorods were observed to grow.Efforts to deposit ZnO on Ag-free SiO2/Si surface area under a variety of growth conditions proved unsuccessful.Zn metal deposition could be achieved at substrate temperatures of 25–1008C,but with no ZnO formation for a wide range of O2/O3partial pressure.Higher substrate temperatures yield no deposition as the Zn metal vapor pressure rises quickly at moderate tempera-tures.Typical growth times for ZnO on the Ag-coated silicon was2h with growth temperatures ranging from T g=300to5008C.After growth,the samples were evaluated by X-ray diffraction, scanning electron microscopy(SEM),transmission electron microscopy(TEM),and photolumines-cence(PL).
Fig.1shows a scanning electron microscopy image of ZnO nanorods grown on a Si wafer that was coated with a nominally10nm thick layer of Ag.The Ag was deposited using e-beam evaporation.The images are for ZnO nanorods grown at4008C with a Zn pressure of2Â10À6Torr Torr and an oxygen/ozone pressure of5Â10À4Torr.Under these conditions,ZnO deposition was observed only on the Ag with no growth on regions of the SiO2-terminated Si surface that was devoid of Ag.A dense entangled collection of ZnO nanorods is observed to grow from the surface.Both cylindrical nanorods and faceted whiskers can be observed in the forest of ZnO nanostructures grown at4008C.At higher temperatures,only nanorods are observed.In many cases,the length of ZnO nanorods is in excess of2m m.Note also that multiple nanorods are observed to nucleate from the relatively large Ag islands.As such,it does not appear that the diameter of the nanorods is determined by the initial radii of the Ag islands.X-ray diffraction of the deposited materials confirms that the Y.W.Heo et al./Materials Science and Engineering R47(2004)1–47
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Fig.1.SEM image of ZnO nanorods nucleated on Ag-coated Si/SiO2substrate.
material is ZnO.X-ray diffraction patterns taken along the surface normal,indicating only ZnO peaks.The diffraction pattern for the material grown at 4008C is consistent with randomly oriented polycrystalline material,although selected area electron diffraction (discussed below)indicates a preferred c -axis orientation of individual nanorods along the long axis.A preferred (002)orientation seen for nanorods obtained at 5008C indicates a more vertically aligned growth at this temperature.
The most intriguing structures are those that result from isolated Ag nanoparticles.In depositing the Ag catalyst films,certain regions of the SiO 2/Si surface were shadowed from deposition,leading to a gradient in Ag thickness,Ag nanoparticle coverage,and average nanoparticle diameter.Within these areas,isolated Ag nanoparticles could be located,thus allowing direct imaging of nanorod formation from individual Ag islands.Clusters of ZnO nanorods were observed to nucleate from these isolated Ag islands.Fig.2shows field-emission SEM images of ZnO nanorod clusters,including a high-resolution image of a single nanorod.Energy-dispersive spectrometry was used to determine the nanorod composition (ZnO)in addition to con firming the absence of ZnO on regions of the substrate surface that are devoid of Ag.In order to acquire these images,the sample was coated with a thin layer of carbon to avoid charging effects.From the high-resolution image,the nanorod cross-section appears to be cylindrical,although any faceting of the side walls would be obscured by the carbon coating.The thickness of the nanorod shown in Fig.2is on the order of 30nm,although the carbon coating may exaggerate this thickness.
In addition to SEM,the nanorods were examined using transmission electron microscopy.Fig.3shows a transmission electron microscopy image of an individual ZnO nanorod.The rod was imaged from a cross-sectioned sample.The nanorod shown in Fig.3was not carbon coated.An estimate of the rod thickness is 20nm.Selected area diffraction (SAD)of nanorod specimens indicates that the rods are single crystal ZnO,with the c -axis oriented along the long axis of the rod.Also evident in the image is a small particle embedded at the tip of the rod.This is similar to what is observed for other nanorod synthesis that is driven by a catalytic reaction,where catalyst particles become suspended on the nanorod tip [51,52].Evidence for termination of the ZnO nanorods tips with catalyst particles is also observed in field-emission SEM images.Local energy-dispersive spectrometry (EDS)measurements indicate that the terminating particle is Ag,although more characterization is needed in order to con firm this.
The mechanism for nanorod growth is catalysis driven,and appears to be related to the vapor –liquid –solid model reported for the nanorod synthesis of other materials.ZnO nanoparticle formation 4Y.W.Heo et al./Materials Science and Engineering R 47(2004)1–
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Fig.2.Deterministic growth of ZnO nanorod clusters formed via catalytically driven MBE;nanorod diameter is 20–30nm.
via the internal oxidation of Zn in Ag/Zn alloys has previously been reported [53].In these studies,oxygen is diffused into an Ag/Zn alloy,with nanoscale ZnO precipitates forming in the bulk of the sample.For the present case of nanorod formation,the reaction between ozone/oxygen flux and the Ag islands appears to result in surface and subsurface oxygen diffusion in the metal island,perhaps involving the intermediate formation of Ag 2O.Zn atoms impinging on the Ag island surface then diffuse either on the surface or in the bulk of the island,where they react with the Ag 2O to form ZnO.The solid solubility of Zn in Ag is on the order of 25wt.%for the temperatures considered in these experiments.Zn addition to Ag signi ficantly suppresses the melting point to 7108C at 25wt.%Zn.Note that the melting point of Zn is rather low (4208C).Based on these arguments,one might anticipate rather high diffusion rates for Zn in Ag for the temperatures considered.Note that the temperatures at which vapor –liquid –solid growth was previously reported for ZnO are signi ficantly higher than that used with the present work.It should also be noted that the addition of Ag during the growth of complex oxide thin film has been reported to be effective in enhancing the oxidation process for various oxide thin-film compounds [54].
In addition to examining the structure with microscopy and X-ray diffraction,the optical properties of the nanorods were examined using photoluminescence.A He –Cd (325nm)laser was used as the excitation source.The room temperature luminescence reveals a robust near band edge emission peak located at 375nm indicating that that rods are highly crystalline.This is consistent with luminescence reported for near-band edge emission in epitaxial films [29,30]and larger diameter ZnO nanorods [55].A broad,but weak,green emission peak is also observed at $520nm that is typically associated with trap-state emission attributed to singly ionized oxygen vacancies in ZnO [56].Y.W.Heo et al./Materials Science and Engineering R 47(2004)1–47
5Fig.3.TEM and selected area diffraction image of a single crystal ZnO nanorod.
6Y.W.Heo et al./Materials Science and Engineering R47(2004)1–47 The emission is the result of the radiative recombination of photogenerated holes with electrons
occupying the oxygen vacancy.Similar results have been observed for ZnO nanorods formed via vapor
transport.Enhancement in the green emission in nanorods as compared to bulk may be attributed to a
higher density of vacancies in the rods.This may be due to the higher surface area to volume ratio for
nanorods as compared to bulk.
3.Structure and optical properties of cored wurtzite(Zn,Mg)O heteroepitaxial nanowires
Nanowire growth has been reported using several techniques[57,58],and has included numerous semiconductors[59,60],including the oxides Ga2O3[61],In2O3[62],SnO2[63],and ZnO[64].
Despite significant progress,major challenges in the manipulation of nanowire materials remain.The
fabrication of integrated systems using nanowire material requires the site-specific growth or
placement of nanowires on relevant device platforms.In addition,the formation of complex,
multi-component structures and interfaces are needed for low-dimensional structures and electronic
devices.In thin-film semiconductor research,the formation of heteroepitaxial interfaces has proven to
be useful in the development of numerous device concepts,as well as in the investigation of low-
dimensional phenomena[65].Unfortunately,such heterostructures have rarely been realized outside
of the conventional2-D planar thin-film geometry[66].Nevertheless,the synthesis of1-D linear
heterostructures is scientifically interesting and potentially useful,particularly if a technique is
employed that allows for spatial selectivity in nanowire placement.Addressing these challenges could
prove useful in realizing integrated device functionality involving semiconducting nanowires for a
number of applications,including nanoscale electricfield-effect transistors[67],single electron
transistors[68],biological and chemical sensors[69],electron emitters[70],optical emitters and
detectors[69,70].
In this section,the properties of1-D heteroepitaxial structures are described.In particular,the structural and optical properties of cored(Zn,Mg)O nanowires,formed via self-assembled bimodal
growth,are discussed.ZnO is among the more interesting and important semiconducting oxides[71].
ZnO is an n-type,direct bandgap semiconductor with E g=3.35eV.Electron doping via defects
originates from Zn interstitials in the ZnO lattice.The intrinsic defect levels that lead to n-type doping
lie0.05eV below the conduction band.The room temperature Hall mobility in ZnO single crystals is
among the highest for the oxide semiconductors,on the order of200cm2VÀ1sÀ1.The exciton
binding energy for ZnO is on the order of60meV,yielding efficient luminescence at room
temperature.The synthesis of ZnO nanowires and nanorods has been demonstrated using vapor-
phase transport via a vapor–liquid–solid mechanism[72],gas reactions[73],and oxidation of metal in
the pores of anodic alumina membranes[74].Room temperature ultraviolet lasing via optical pumping
has been demonstrated with ZnO nanorods on deposited Au catalyst using a high-temperature vapor
transport process[70].Recently,we reported on catalyst-driven molecular beam epitaxy of ZnO
nanorods[75].The process is site specific,as single crystal ZnO nanorod growth is realized via
nucleation on Agfilms or islands that are deposited on a SiO2-terminated Si substrate surface.Growth
occurs at relatively low substrate temperatures,on the order of300–5008C,making it amenable to
integration on numerous device platforms.With this approach,nanorod placement can be predefined
via location of metal catalyst islands or particles.
The heteroepitaxial cored nanostructures described here are based on the(Zn,Mg)O alloy system, and were synthesized using the catalysis-driven molecular beam epitaxy method.Details of the growth
experiments are reported elsewhere.An ozone/oxygen mixture was used as the oxidizing source.The
cationflux was provided by Knudsen effusion cells using high purity(99.9999%)Zn metal and Mg
(99.95%)as the source materials.The substrates were Si wafers with native SiO 2terminating the surface.No effort was made to remove the native oxide or to terminate the surface with hydrogen.
Site-selective nucleation and growth of cored nanorods was achieved by coating Si substrates with Ag islands.For a nominal Ag film thickness of 20A
˚,discontinuous Ag islands are realized.On these small metal catalyst islands,(Zn,Mg)O nanorods were observed to grow.Fig.4shows an FE-SEM micrograph of these nanorods,indicating a length approaching 1m m.The growth temperature was 4008C.Energy-dispersive spectrometry measurement was performed on a single nanowire under the Transmission electron microscopy.EDS con firmed the presence of Mg in the nanorod,as seen in Fig.5.Typical growth times for (Zn,Mg)O on the Ag-coated silicon was 2h with growth temperatures ranging from T g =300–5008C.The site speci ficity for nanowire growth using this technique is evident
Y.W.Heo et al./Materials Science and Engineering R 47(2004)1–47
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Fig.4.Field-emission scanning electron microscopy image of cored (Zn,Mg)O nanorods grown on Ag-coated Si.The conditions for growth were T g :4008C;Zn pressure:3Â10À6mbar,Mg pressure:4Â10À7mbar,O 2/O 3pressure:5Â10À4mbar.The Mg source was shuttered with a 60s open/60s closed
cycle.
Fig.5.Energy dispersive spectrometry data for (Zn,Mg)O nanorods.
in Fig.6,showing FE-SEM images of (Zn,Mg)O nanorods on a Ag-patterned substrate grown in a Zn pressure of 3Â10À6mbar,a Mg pressure of 4Â10À7mbar,and an O 2/O 3pressure of 5Â10À6mbar.(Zn,Mg)O nanorods form only on the Ag-coated regions.The potential for growing single nanorods on selected locations is exempli fied in Fig.7,where single ZnO nanorods are nucleated on Ag nanoparticles dispersed on a SiO 2-terminated Si surface.In order to acquire these images,the sample was coated with a thin layer of carbon to avoid charging effects.From the high-resolution image,the 8Y.W.Heo et al./Materials Science and Engineering R 47(2004)1–
47
Fig.6.Field-emission scanning electron microscopy image of cored (Zn,Mg)O nanorods grown on a patterned Ag-coated Si substrate.The conditions for growth were T g :4008C;Zn pressure:3Â10À6mbar;Mg pressure:4Â10À7mbar;O 2/O 3pressure:5Â10À4mbar.Note that (Zn,Mg)O nanorod nucleation occurs only on the catalyst-coated
regions.
Fig.7.Field-emission scanning electron microscopy image of individual ZnO nanorods grown on Ag nanoparticles dispersed on the Si substrate.
nanorod cross-section appears to be cylindrical,although any faceting of the side walls may be obscured by the carbon coating.The thickness of the nanorods shown is on the order of30nm, although the carbon coating may exaggerate this thickness.
The formation of the(Zn,Mg)O nanorods includes the v–1–s mechanism described earlier, although heteroepitaxial growth occurs as well as will be seen.Fig.8shows a Z-contrast scanning transmission electron microscopy(Z-STEM)image of an individual(Zn,Mg)O nanorod grown at Y.W.Heo et al./Materials Science and Engineering R47(2004)1–47
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Fig.8.Z-contrast scanning transmission electron microscopy image(a)of a(Zn,Mg)O nanorod with(b)a Ag catalyst particle at the rod tip.
10Y.W.Heo et al./Materials Science and Engineering R47(2004)1–47 4008C,with a Zn pressure of3Â10À6mbar,a Mg pressure of4Â10À7mbar,and an O2/O3pressure
of5Â10À4mbar.The Mgflux was cycled on and off every60s,which was inconsequential to the
nanorod structure.Evident in the image is a small particle embedded at the tip of the rod.This is
similar to what is observed for other nanorod synthesis that is driven by a catalytic reaction,where
catalyst particles become suspended on the nanorod tip.The diameter of the catalyst particle is$6nm.
Note that,at the nanorod tip,the contrast in the Z-STEM image is relatively uniform,indicating
uniform cation distribution.However,the rod diameter is also tapered along the length,being thicker
at the base than on the tip,with an average diameter on the order of10nm.
As reported elsewhere,a radial segregation of the Zn and Mg occurs during growth[76].A Zn-rich core surrounded by a Mg-rich sheath is observed as seen in Fig.9.As discussed elsewhere,in
bulk material,the solubility of Mg in ZnO is relatively low,on the order of4at%[77].In contrast,
Mg content as high as Zn0.67Mg0.33O has been reported to be metastable in the wurtzite structure for
epitaxial thinfilms.For this composition,the bandgap of ZnO can be increased to$3.8eV.For
(Zn,Mg)O nanorod growth,it appears that both(hence bimodal)growth modes are relevant,but for
different regions in the rod.Under low temperature MBE growth conditions,a solubility-driven
segregation occurs during the catalyst-driven core formation,with the core composition determined
by bulk solid solubility.Subsequently,an epitaxial sheath grows with Mg content and crystal
structure determined by epitaxial stabilization.The net result is the growth of(Zn,Mg)O nanorods
that are not uniform in composition across the diameter,but distinctly cored.Fig.9shows a high-
resolution Z-STEM image of a nanorod grown under the conditions described.The lattice image for
the nanorod specimen indicates that the rod is crystalline with the wurtzite crystal structure
maintained throughout the cross-section.The c-axis is oriented along the long axis of the rod.
The higher contrast for the center core region clearly indicates a higher cation atomic mass.The
structures consist of a zinc-rich Zn1–x Mg x O core(small x)surrounded by a Zn1Ày Mg y O(large y)
sheath containing higher Mg content.
While the nanorod imaged in Fig.9is crystalline across the entire cross-section,other rods exhibit sheath properties that vary along the length.In particular,consider the nanorod shown in Fig.10.In
this case,the core and sheath are both crystalline in one region of the rod.However,as one proceeds
along the length,the crystallinity changes.In particular,for the rod considered,the sheath region
becomes either polycrystalline or amorphous as one approaches the rod tip.Still further down the
nanowire,the image suggests a lack of crystallinity for both the core and sheath,although the lack of
crystallinity in the sheath may effectively obscure imaging of the core region.This change in
crystallinity change along the length of the rod may reflect the fact the temperature gradient will
develop along the nanowire length during growth.This occurs since the substrate is the source of heat
during nanorod formation.Sheath material deposited on the tip of longer rods during the latter part of
the synthesis process will do so at a lower local temperature than the material closer to the substrate.
In order to assess crystalline quality and investigate possible quantization effects,the optical properties of the cored nanorods were examined using photoluminescene.Spectra were taken over the
temperature range6–300K.A He–Cd(325nm)laser was used as the excitation source.For the low
temperature measurements,the sample was cooled using either a heliumflow or closed cycle cryostat.
Fig.11shows the photoluminescence spectra taken at various temperatures for the cored nanorod
specimens.For ZnO rods(no Mg),the photoluminescene results are consistent with luminescence
reported for near band edge emission in crystals[78],epitaxialfilms[79],and larger diameter ZnO
nanorods[80].The free exciton emission dominates luminescence,with a room temperature peak at
3.30eV.At room temperature,the spectra for the cored(Zn,Mg)O nanorods is also dominated by the
free exciton luminescence.However,the peak in luminescence at room temperature is at3.35eV,
which is blueshifted relative to that seen in pure ZnO(peak at3.30eV).As the temperature is
decreased,the free exciton emission shifts to higher energy due to the temperature dependence of the bandgap.At the lower temperatures,the donor-bound exciton (D 0,X)dominates luminescence [79].Also seen at low temperatures is a peak at 3.35eV ,which is assigned as the corresponding longitudinal optical phonon replica (D 0,X)-LO.At lower energies,at least four relatively weak peaks are also observed in the visible spectrum,indicated by arrows in the figure.Similar peaks have been reported elsewhere for bulk and thin-film ZnO materials [78,79].
Fig.9.Z-contrast scanning transmission electron microscopy images of cored (Zn,Mg)O nanorods.The high contrast of the core (a)indicates a signi ficantly higher Zn content for the core relative to the sheath.The higher resolution image (b)of the core indicates the wurtzite structure for the core and sheath material.。