This content has been downloaded from IOPscience. Please scroll down to see the full text. Download details: IP Address: 161.45.205.103 This content was downloaded on 16/08/2014 at 03:27 Please note that terms and conditions apply. Evidence of space charge regions within semiconductor nanowires from Kelvin probe force microscopy View the table of contents for this issue, or go to the journal homepage for more 2009 Nanotechnology 20 465705 (http://iopscience.iop.org/0957-4484/20/46/465705) Home Search Collections Journals About Contact us My IOPscience
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Evidence of space charge regions within semiconductor nanowires from Kelvin probe force
microscopy
View the table of contents for this issue, or go to the journal homepage for more
Evidence of space charge regions withinsemiconductor nanowires from Kelvinprobe force microscopyAngela C Narvaez, Thalita Chiaramonte, Klaus O Vicaro,Joao H Clerici and Monica A Cotta
Departamento de Fısica Aplicada, Instituto de Fısica Gleb Wataghin, UNICAMP CP 6165,CEP 13083-970, Campinas, SP, Brazil
Received 2 August 2009, in final form 28 September 2009Published 21 October 2009Online at stacks.iop.org/Nano/20/465705
AbstractWe have studied the equilibrium electrostatic profile of III–V semiconductor nanowires usingKelvin probe force microscopy. Qualitative agreement of the measured surface potential levelsand expected Fermi level variation for pure InP and InAs nanowires is obtained from electricalimages with spatial resolution as low as 10 nm. Surface potential mapping for pure andheterostructured nanowires suggests the existence of charge transfer mechanisms and theformation of a metal–semiconductor electrical contact at the nanowire apex.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Quasi-one-dimensional systems such as semiconductornanowires (NWs) are considered to be one of the main possiblebuilding blocks for nanoscale electronic and optoelectronicdevices [1–4]. Several materials have been employedfor the studies carried out so far; among those, III–Vcompounds constitute one of the main choices due totheir well developed applications in optoelectronics [2, 3].From the possible options, InP and InAs NWs have beenextensively investigated due to the large carrier mobilitiesand smaller surface recombination rates of these materialswhen compared to GaAs. In particular, InAs nanowire-basedfield-effect transistors (FETs) with electron mobilities upto 16 000 cm2 V−1 s−1 have been demonstrated [5]. Newarchitectures for transistor implementation have also beenachieved with InAs [6, 7].
However, the performance of these devices can be deeplyaffected by variations in nanowire characteristics. This isan important issue of concern when large scale integrationis considered; statistical variability is today one of themain bottlenecks for silicon-based microelectronics, alreadyworking at the nanoscale [8]. Spatially resolved techniquescan provide information on nanowire characteristics that isoften unobtainable using conventional macroscopic electrical
measurements. If this information is properly correlatedto structural characteristics, new and unexpected materialproperties can be achieved [9].
In the last few years, electronic properties of InAsNWs have been probed at the nanoscale using scanningprobe microscopy (SPM) techniques [10–12]. These resultsindicate that the electrical response of single nanowires is notnecessarily homogeneous; indeed, the presence of crystallinedefects and the nanometer-sized metal nanoparticle (NP) atthe NW apex represent non-uniformities from the materialspoint of view which could be reflected in the electroniccharacteristics of single nanowires. This point is particularlyimportant for heterostructures fabricated from binary materialssince space charge regions can build up within the nanowiredue to the electrostatic profile and the presence of interface orsurface states.
We have addressed this question using Kelvin probe forcemicroscopy (KPFM) on pure InP, InAs and heterostructured(HT) InP/InAs/InP NWs. KPFM [13] allows topographyand surface potential (SP) imaging of the sample, while stillkeeping the spatial resolution in the nanometer range. Thecontact potential between tip and sample is defined by thedifference between their work functions, �φ [13–19]. Byapplying external bias voltages and compensating the contactpotential, KPFM is able to cancel this particular electrostatic
interaction between tip and sample thus making possible thesimultaneous acquisition of topography and SP images, SP =�φ/e, where e is the electron charge. In spite of limitationsdue to tip–sample size effects [14–17], a qualitative analysiscan be carried out. Our results show a variation of SP alongtapered nanowires associated both with limitations due to tipsize and to possible charge transfer processes along the NWs.By changing the sample structure we were able to detecta space charge region confined to the vicinity of the NP,suggesting the formation of an electrical contact to the metallicNP at the semiconductor NW apex.
2. Experimental details
InP, InAs and InP/InAs/InP nanowires were grown by chemicalbeam epitaxy using Au nanoparticles (25–30 nm in diameter)as catalyzers for vapor–liquid–solid (VLS) growth; detailsof the growth procedure are provided elsewhere [20]. Highresolution transmission electron microscopy (HRTEM) ofsamples used in this work shows that InP and InAs NWs growin the wurtzite (WZ) structure in the [0001] direction (along thec axis) for both temperatures used [20]. The NWs were thentransferred by mechanical abrasion to the reference substratewhere the electrical measurements were carried out. Two typesof surfaces were considered for that purpose: either a 100 nm-thick Pt film on top of a Si substrate or a 4 μm-thick, n-type,zincblende (ZB) InP film grown on top of a semi-insulatingInP substrate, both electrically contacted through the top layer.These reference surfaces were selected in order to compare SPimages for different NWs; the relatively inert Pt surface was themetal chosen to provide electrical contact and an equipotentialsurface to the deposited NW, according to previously reportedresults [21].
Topography and surface potential images were acquiredsimultaneously using an Agilent 5500 with a three-lock-inamplifier in the amplitude modulation KPFM [13]. For themeasurements, VAC bias at frequencies in the range 10–15 kHzplus a DC bias, VDC, were applied between tip and sample. Wehave used tips with different shapes and coatings (Pt, Cr–Au,TiN and W2C); the particular data shown in this work wereacquired using conical tips mounted on triangular cantilevers(resonance frequency ∼100 kHz, typical uncoated tip radius∼10 nm and full tip cone angle <40◦) coated with eitherW2C or TiN, as well as similar tips on rectangular cantilevers(resonance frequency ∼ 160 kHz) coated with TiPt. Tipscoated with W2C or TiN provided slightly more stable imagesand longer lifetimes. The detector signal amplitude at the VAC
frequency should be proportional to ∂C∂z (VDC − �φ/e)VAC,
where C and z are the capacitance and distance between tip andsample, respectively [16]. SP images thus show a map of theVDC values necessary to minimize this signal amplitude at eachmeasured point. In order to check for capacitance variations,we have acquired curves of this signal amplitude as a functionof VDC at selected spots on the surface. All measurementswere carried out under dry (pure N2) atmosphere unless statedotherwise.
Figure 1. Topography (a) and surface potential (b) images(VAC ∼ 1.0–1.5 V at 10 kHz) for WZ InP nanowires on top of the ZBInP film used as substrate; (c) SP∗ profile as a function of nanowirediameter.
3. Experimental results
Figure 1 shows the topography and SP images for WZ InPNWs deposited on the ZB InP substrate. The differencebetween SP for the NW and the SP from the reference substrateis henceforth referred to as SP∗. By analyzing SP∗ we couldeliminate the tip work function from our analysis and thusprevent artifacts from tip wear and contamination. Indeed SP∗values have shown no strong dependence on the tip coatingmaterial or the cantilever used for the range of tip–surfacedistances used for this work (below 50 nm). SP∗ values areshown in the profile as a function of the NW diameter, hereafterconsidered as the height at the corresponding point in thesimultaneously acquired topography image. We can notice thatthe NWs are tapered; a height variation is observed along the
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Figure 2. Typical topography and surface potential images for InP ((a), (b)) and InAs ((c), (d)) nanowires on Pt surfaces. Particles on thesurface originate from the abrasion transfer process and are expected to present either semiconductor or metallic characteristics.
NW length. SP∗ profiles also vary with NW diameter althougha saturation level, below 100 mV, is reached for diameterslarger than 90–100 nm. The ZB InP film is a reference surfacefor the n-type doped material [22]. Band gap energies for WZInP are approximately 80 meV [23] larger than for ZB InP; thisvalue is very close to the SP∗ saturation levels obtained fromour images (figure 1(c)). Thus we expect the WZ NWs to ben-type as well.
The profile in figure 1 also shows a steady drop of SP∗for NW diameters smaller than ∼60 nm. This effect can beexpected when the tip and the object in the image are similarin size [14–17]. However, if the reduced object SP levels wereattributed to tip size effects, we would expect the minimumSP values to be similar to the substrate, since the tip wouldbe integrating the material around the NW into the electricalimage [14]. Figure 1 shows two features in the SP image whichdo not concur with this picture, suggesting that this is not theonly effect taking place during measurement. One of them isthe bright edge around the NWs which we assume to resultfrom the electric field between the NW and the slightly dopedInP film used as substrate [24, 25]. Moreover, several NWsshow SP values below that of the neighboring substrate, atthe regions with smaller diameter, and closer to the NW apex.These two results altogether indicate that spatial resolution inour SP images can be as small as 10 nm and that KPFM isstill able to probe the NW electrostatic characteristics. Themeasured SP∗ values then indicate that the semiconductor workfunction increases as the NW diameter drops. Larger band gapenergies (and thus smaller work functions) are expected whensemiconductor NW diameters are reduced below ∼20 nm dueto quantum size effects [26–28]; nevertheless, most of ourNWs have diameters larger than that due to the NP used forgrowth. If such size effects are neglected and residual dopinglevels are maintained, we must assume that the drop in SP∗(or larger work function) is associated with a more intrinsicelectronic character of the semiconductor.
The total SP variation in the images could be enhancedwhen Pt was used as a reference surface. This also providedan actual equipotential surface for measuring the NWs, whichshould prevent the SP edge instabilities in figure 1; the Pt filmwas grounded during all measurements, thus keeping the NWsat constant potential [21]. This is indeed observed in figure 2which shows topography and homogeneous SP images for bothInP (figures 2(a) and (b)) and InAs (figures 2(c) and (d)) wires.However, the SP levels for InP NWs with regard to Pt arewell below those observed for the bulk. The larger reductionin SP∗ as compared to the bulk value and also to the resultsshown in figure 1 is attributed to larger capacitance effects [14]which make tip size effects more pronounced. Since thetip is also actually integrating within the electrical signal thecapacitive interaction with the substrate around the NW, alarger difference in capacitance between substrate and NWshould result in a different SP averaging—and consequentlydifferent SP∗ values—in each case. In fact, figure 3(a) showsthe amplitude signal (at the same frequency as the applied bias,VAC) versus VDC acquired both on the Pt surface and on the InPNW. The difference in slopes between the two curves indicateslarge changes in the capacitance derivative as we move fromone material to the other. Larger noise levels and asymmetryin the curves also show up at the intermediate region. Thisdifference in slope is much less pronounced when the samecurves are similarly acquired both on the InP NW and the InPsubstrate as shown in the inset of figure 3(a). As a result, non-local tip effects on SP∗ levels (or SP∗ range compression inthis case) for NWs on Pt are expected to be larger than for InPsubstrates, in accordance with our data.
Nevertheless, important material information is present inthe images. The SP∗ profiles averaged over several NWs areshown in figure 3(b). The SP∗ saturation level for InAs islower than for InP as expected from the corresponding bandgap variation. Again, for both NWs, SP∗ drops continuouslyfor diameters smaller than ∼50 nm, although SP values for the
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Nanotechnology 20 (2009) 465705 A C Narvaez et al
Figure 3. (a) SPM detector signal amplitude at the AC biasfrequency as a function of applied DC bias between tip and sample.The different curves are acquired with the tip positioned either at thePt surface or on the nanowire, as indicated in the figure—curvesacquired close to the NW edge are shown as the intermediate region.The inset shows the same curves (keeping the same y-scale) for theInP NW on the InP surface obtained with a tip coated with differentmaterial; the difference in slope between the two curves in each plotprovides an estimate for the capacitance effects in the correspondingSP image. (b) SP∗ as a function of NW diameter for both InP andInAs NWs on Pt surfaces. The results shown here correspond to theaverage of 10 different NWs for each material.
NW are still above those from the Pt substrate. This couldalso be an effect of the larger surface to volume ratio—andconsequently larger density of surface states—for the thinnersections of the NW. The variation in SP images could thenbe attributed to a carrier-depleted semiconductor volume at theregion closer to the NW apex—which shows a larger effectivework function (or smaller SP). This hypothesis is supported bySP images acquired under different humidity levels in the SPMchamber. The observed InP SP∗ saturation levels decreasedwith increasing humidity for the same NW—in a reversibleeffect—due to the different surface charge density in eachcase1. This interpretation is also sustained by first principlecalculations for InP NW surfaces passivated with OH radicalswhich show a significant band gap reduction [29].
If we examine the SP profiles in our samples, we canobserve that the length of the region where SP drops can reachup to a few microns for very long (∼10 μm) NWs; it is alsocorrelated to the variation of NW diameter. However, if there
1 The bias values used here were below oxidation levels; no oxide thicknessvariation during measurements could be noticed in topography images.
is an actual variation of the work function as discussed, chargetransfer is expected to occur along the NW to compensate forthe increasingly larger surface state densities close to the NWapex, thus creating an extended depletion region within theNW. Unfortunately, we cannot decouple this physical processfrom the intrinsic artifact created by the limited size of the tipsince both the surface/volume ratio and tip size effects increaseas the NW diameter drops. In our samples this artifact ismainly due to the tapered shape of the NW which implies thatthe substrate contribution to the measured SP∗ varies as the tipmoves towards the NW apex.
Quantitative KPFM analysis has been carried out bysimulating the tip–sample electrical interaction as a function oftip shape and size [15–17, 30] or by estimating an effective areaof interaction to deconvolute the local SP [31, 32]. Nony et al[33] have studied both amplitude and frequency modulationKPFM; these authors argue that SP values measured with thesetwo methods are expected to differ. Moreover, the measuredeffective values are convoluted by the geometry of the tip andactual quantitative numbers are unlikely to be derivable [33].Most of these quantitative studies, however, were carried outon ultrahigh vacuum conditions, using very thin films or small(<5 nm) nano-objects as samples. In those cases, modelingtip–sample interaction to extract quantitative SP data is a morestraightforward task. For larger objects, quantitative analysisis usually carried out when large potential variations arepresent [30, 34]. In our case, we have primarily chosen KPFManalysis to study the electrostatic characteristic of the NWs andthe main physical processes involved. In this sense, a statisticalanalysis of the NWs (figure 3(b)), carried out with differenttips, not only provided an average electrostatic behavior ofthese nano-objects, but also established the reliability of ourKPFM measurements. However, in order to discriminatethe relative importance of each physical process on theelectrostatic behavior, a quantitative KPFM analysis shouldhave to consider not only tip size and shape effects but thosedue to both surface and volume processes on semiconductormaterials as well; uncertainties on tip geometry as well assemiconductor carrier density and distribution along the NWwould only make this calculation more complex.
We have thus chosen a different approach to this problem,by designing a more adequate NW which minimizes tip sizeeffects. Heterostructured InP/InAs/InP NWs were grownat slightly higher temperatures. Under these conditions,the material still preserves the WZ structure but presents aless tapered shape and sharper interfaces between InP andInAs [20]. Moreover, the insertion of the InAs segment slightlyincreases the NW diameter—acting as a marker—and alsoprevents charge transfer along the NW if a carrier-depletedregion is formed at the narrower regions. The InP and InAssegments are rather long (∼μm) to prevent quantum sizeeffects regarding charge accumulation at the InAs region, forexample. Another important structural feature in this type ofsample is the presence of a neck region (∼5 nm long), close toNP, where an InAsP (As up to 20 at.%) alloy is formed duringsample cool down [20].
Figure 4 shows topography and SP images for the wholeHT NW (figures 4(a) and (b)) as well as a zoom on its apex
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Figure 4. Topography ((a), (c)) and surface potential ((b), (d)) images for the heterostructured (InP/InAs/InP) nanowire (VAC ∼ 1.0–1.5 V at10 kHz). Labels A, B and C in (a) indicate the approximate regions for bottom and top InP (A and C, respectively) and InAs (B) segments.
(figures 4(c) and (d)). The shapes in the topography imagesuggest that both the NP used for the growth and the neckregion can be observed. A striking feature in figure 4(d)is the sharp drop in SP levels close to the neck region, ascompared to the pure NWs. SP∗ values as a function ofNW diameter, obtained for the three segments correspondingto bottom InP, InAs and top InP sections of the NW (A,B and C, respectively), are shown in figure 5. Horizontallines in figure 5(a) show the maximum, minimum (dashedlines) and average (solid line) SP∗ values observed in all ourmeasurements for both pure InP and InAs NWs. At the bottomInP segment (A region), SP∗ values corresponding to pureInP levels can be observed even for diameters smaller than30 nm (at the region where the NW broke during transfer tothe reference substrate). As the diameter of the NW enlarges (Bregion), a signature of the InAs segment insertion, SP∗ dropscloser to the maximum values observed for pure InAs. TheC region of the NW is shown in figure 5(b): SP∗ decreasesagain, reaching very low values close to the NW extremity.Moreover, at the NW apex, SP∗ drops to values slightly belowthat of the neighboring Pt surface, indicating the presence ofthe metallic NP, which contains both Au and In according toenergy dispersive spectroscopy measurements [20].
Thus the monotonic SP∗ drop with diameter observedbefore is no longer present for the whole HT NW sincediameter variation is not as pronounced in this case (a smallSP∗ drop is still noticeable at the thinner sections of the bottomInP segment, region A in figure 5(a)). The comparatively largerSP∗ for InAs can result from residual charge transfer along theNW and charge accumulation at the InAs segment due to itssmaller band gap, thus increasing SP∗. An important featureobserved in figure 5 is the different SP∗ levels for the twoInP segments (A and C regions); the previous trend— SP∗drop with diameter—is present only for data from the top InPsegment (C region, figure 5(b)). Moreover, a more abrupt dropin SP∗ values (from 140 down to ∼60–70 mV) occurs at a
region ∼200 nm away from the position of the NP, assumedaccording to the image shown in figure 4(d).
These results altogether point to the existence of awell defined depletion region within the NW, closer to itsapex, which is neither diameter dependent nor a tip-inducedartifact. The NW apex is associated with the presence ofthe metallic NP; thus a depletion region should be formeddue to the metal–semiconductor junction. In this case, underequilibrium conditions, the semiconductor Fermi level variesto accommodate the difference in work function between thetwo materials as well as the surface state density. For III–V semiconductors, however, pinning of the Fermi level isusually observed so that Schottky barrier heights for thesesemiconductor compounds do not significantly vary with themetal used for the junction [35]. For Au on InP (110), forexample, the position of the Fermi level is roughly 0.5 eVbelow the conduction band, even when less than 0.5 ML of Auis deposited on the InP surface [36]. Surface states induced bythe presence or deposition of the metal on the semiconductorare assumed to be at the origin of this behavior [35, 36]. Inthe particular case of the NWs the final metal–semiconductorinterface is formed during sample cool down. Interfacestates should then be more related to point or clusteredstructural defects than to the influence of impurities; in fact,HRTEM images show that the NP/NW interfaces are usuallynot as well defined as the NW or NP materials (figure 6).Moreover, contrarily to planar semiconductor structures, thefinal electronic configuration and extension of the space chargeregion will depend not only on the NW doping levels but onsurface states associated with the surrounding NW oxidizedsurfaces as well [29].
For the pure NWs analyzed, we can similarly expect vari-ations in SP∗ levels associated with the metal/semiconductorjunction but with a larger spatial extension of the space chargeregion. In most cases, however, this observation is hindered
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Figure 5. SP∗ as a function of diameter for the heterostructurednanowire shown in figure 4, obtained from several SP images alongthe NW: (a) A and B regions; horizontal lines indicate the minimum,maximum (dashed lines) and average (solid line) SP∗ values for bothInP and InAs as indicated. (b) C region of the NW, with dashed linesindicating the average SP∗ values in the A, B and substrate regions.The error bars reflect the average over five profiles at each chosenpoint on the NW. The HRTEM image in the inset shows the NP andthe neck region of a HT NW.
due to tip–sample size effects. For the HT NW, there are twomain reasons for the localization of the space charge regioncloser to the NP, at the NW apex. One of them is the presenceof the InAs segment which alters the electrostatic profile andinhibits charge transfer along most of the NW length. Anotherreason is the short InAsP section close to the NP/NW interfacewhich is highlighted in the inset of figure 5(b). The introduc-tion of this ternary layer, however, cannot solely explain the ob-served SP∗ behavior, considering its average composition andwidth [20]. Nevertheless, the introduction of this lower bandgap material can better accommodate the different Fermi levelsand provide electrical contact formation in the HT NW with asmaller extension of the space charge region.
It is important to comment that we were not able to clearlyidentify a space charge region across the InP/InAs or InAs/InPinterfaces; this region should be expected if charge transfermechanisms and carrier accumulation at the InAs segmentare assumed, even though the interfaces are abrupt from themetallurgical point of view [20]. The intrinsic noise levels inour SP images make it extremely difficult to observe a SP∗variation within the ∼50 mV range between InAs and InP
Figure 6. HRTEM image of an InP nanowire grown at 420 ◦Cshowing the region of the InP/Au NP interface. Electron diffractionshows that the NW presents hexagonal WZ structure; growth occursin the [0001] direction along the c axis [20].
levels shown in figure 5. We have observed that larger tipdiameters could improve noise levels but at the cost of losingspatial resolution in both topography and SP images.
4. Conclusions
In summary, our KPFM results show that SP mapping in NWsprovides electrostatic information associated with a material’scomposition and band gap profiles within the structure. Bycomparing results for pure and HT NWs we suggest theexistence of charge transfer mechanisms and electrical contactformation between the NP and the semiconductor. Moreover,the results presented in figures 4 and 5 show that metallurgicaland electrical interfaces in a HT NW can be quite differentwhen observed with spatially resolved techniques. Althoughdevices based on a single NW can be polarized and thus keepthe electronic structure and Fermi levels within a desirablerange, we show that simple geometry fluctuations, even forpure NWs, could be at the origin of variations in the electronicprofile such as the transconductance asymmetry reported byZhou et al [11]. These results may be particularly importantwhere nanowire-based sensor applications are concerned sincethese devices are strongly based on conductance variationsmediated through surface interactions.
Acknowledgments
We are indebted to Professor Peter A Schulz for fruitfuldiscussions and A A G von Zuben for technical assistance insample preparation. This work was supported by the Brazilianagencies FAPESP and CNPq. HRTEM was carried out atthe electron microscopy facilities of the Brazilian SynchrotronLight Laboratory. ACN and TC acknowledge scholarshipsfrom CNPq and FAPESP, respectively.
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