Effects of Gold Diffusion on n-Type Doping ofGaAs NanowiresMichael J. Tambe, Shenqiang Ren, and Silvija Gradecak*

Department of Materials Science and Engineering, Massachusetts Institute of Technology,Cambridge, Massachusetts 02139, United States

ABSTRACT The deposition of n-GaAs shells is explored as a method of n-type doping in GaAs nanowires grown by the Au-mediatedmetal-organic chemical vapor deposition. Core-shell GaAs/n-GaAs nanowires exhibit an unintended rectifying behavior that isattributed to the Au diffusion during the shell deposition based on studies using energy dispersive X-ray spectroscopy, current-voltage, capacitance-voltage, and Kelvin probe force measurements. Removing the gold prior to n-type shell deposition results inthe realization of n-type GaAs nanowires without rectification. We directly correlate the presence of gold impurities to nanowireelectrical properties and provide an insight into the role of seed particles on the properties of nanowires and nanowire heterostructures.

KEYWORDS Nanowires, doping, gallium arsenide, gold

Semiconductor nanowires are being pursued as a high-performanceplatformforelectronic1,2andphotonic1,3,4

devices due to their potential to enable integration ofIII-V materials on silicon5 and one-dimensional confine-ment.6 Proof-of-concept transistors,7-9 light-emitting di-odes,10 and lasers11,12 have been demonstrated using III-Vnanowires grown by the metal particle-mediated vapor-liquid-solid (VLS) growth mechanism.13 Among III-V ma-terials, GaAs is of significant interest for device applica-tions because it possesses an intrinsically high electronmobility,14 and the two-dimensional conduction in GaAs/AlGaAs modulation doped FETs has demonstrated enhance-ment of carrier mobility over bulk systems15 at the highcarrier concentrations needed for commercial electronics.To push these concepts to nanowire devices, morphologicaland compositional control has been achieved in Au-medi-ated growth of GaAs nanowire arrays16 and GaAs-basedaxial17 and radial18 nanowire heterostructures.

In addition to the compositional control, the realizationof nanowire devices will require controlled n- and p-typedoping. Direct delivery and incorporation of dopant precur-sors through metal particles during VLS growth has led torealization of n- and p-type doping in silicon,19-21 germa-nium,22,23 and indium phosphide24 nanowires, but dopingGaAs nanowires has proved significantly more challenging.Co-introduction of a zinc precursor during VLS growth hasyielded controllable p-type doping,25 but no controllablen-type doping in GaAs nanowires has been achieved so far.We note that nominally n-type GaAs nanowires grown usingAu-assisted molecular beam epitaxy (MBE) growth at 550°C were reported as building blocks for hybrid organic-inorganic solar cells,26 but the exact electrical behavior and

doping mechanism were not discussed. As an alternative todirect incorporation of dopant atoms through the metalparticle, controlled nanowire doping can be achieved by thedeposition of a doped epitaxial shell after the VLS growth ofthe nanowire core. Indeed, deposition of a doped InP shellhas been demonstrated to be an effective method of p-doping in InAs nanowires,27 which is significant becauseboth InAs and GaAs are well-documented to experienceFermi level pinning.28 More broadly, the ability to deposit adoped shell is essential to the development of devices basedon radial nanowire heterostructures.10,29

In this Letter, the deposition of a Si-doped epitaxial GaAsshell on GaAs nanowires (hereafter referred to as GaAs/n-GaAs nanowires) grown by the Au-mediated metal-organicchemical vapor deposition (MOCVD) was explored as amethod of n-type doping. As-grown GaAs/n-GaAs nanowiresdemonstrated an unintended rectifying current-voltage(I-V) behavior, which was investigated by a combination ofI-V and capacitance-voltage (C-V) measurements andattributed to the diffusion of Au into the GaAs nanowireduring the shell deposition step. Based on this knowledge,nonrectifying n-type GaAs/n-GaAs nanowires were realizedby removing the Au seed nanoparticle prior to shell deposi-tion. We present a general method of n-type doping in bothGaAs nanowires and nanowire radial heterostructures andfurther the knowledge on the effects of seed particles duringVLS growth.

GaAs/n-GaAs nanowires were grown by atmosphericpressure MOCVD using the procedure illustrated in Figure1. Vertical GaAs nanowires were first grown at 420 °C, asdescribed in our previous work,18 followed by the depositionof a Si-doped shell at 750 °C. During 5.0 min shell depositionthe flow rates of arsine, trimethyl gallium, and silane wereheld constant at 47, 0.23, and 0.088 sccm, respectively. Themorphologies of nanowires were studied using a JEOL6320FV high-resolution scanning electron microscope (SEM).

* Corresponding author. E-mail: gradecak@mit.edu.Received for review: 7/23/2010Published on Web: 10/12/2010

pubs.acs.org/NanoLett

© 2010 American Chemical Society 4584 DOI: 10.1021/nl102594e | Nano Lett. 2010, 10, 4584–4589

The chemical compositions of individual nanowires werecharacterized using a JEOL 2010F field emission transmis-sion electron microscope (TEM) operating at 200 keV andequipped with an Inca XSight energy dispersive X-ray spec-troscopy (EDS) detector. The electrical properties of indi-vidual nanowires were probed by depositing nanowires ontoan n++-Si substrate covered with a 200 nm thick SiO2 layer.Metal contacts were defined using electron beam lithographyfollowed by electron beam evaporation of a Ni/Ge/Au/Ge/Au(25/25/150/25/150 nm) multilayer and by 30 s rapid thermalannealing in N2 at 420 °C. C-V measurements were con-ducted at an oscillation frequency of 1kHz and an oscillationvoltage of 500 mV. Kelvin probe force microscopy (KPFM)samples were prepared by depositing nanowires onto then++-Si/SiO2 substrates followed by a 20 min oxygen plasmacleaning. KPFM measurements were conducted in a Dimen-sion 3100 SPM with a Nanoscope IV Kelvin controller. Allmeasurements were performed at ambient atmosphere innoncontact mode with a Pt-coated Si tip of <25 nm radius,oscillating 50 nm above the sample surface. The topographyand surface potential were detected simultaneously using thebias modulation technique with an alternating current biasvoltage of (0.5 V. The Kelvin signal was measured at theresonance frequency of the tip (68.75 kHz), and the topog-raphy was measured approximately 2 kHz off resonance.

First, undoped VLS-grown nanowire cores were grownwithout the shell (growth ceased at the point indicated by astar in Figure 1), and their electrical properties were mea-sured by the transmission-line method (TLM)30 (Figure 2a).The measured resistances exhibited a linear dependence

with respect to the contact separation, l, with the contactresistance, RC, of 800 MΩ. The conductivity of 5.2 × 10-5

(Ω-cm)-1 was calculated from the TLM measurementsassuming a hexagonal nanowire cross-section accordingto eq 1:

Here, d is the nanowire diameter as measured by SEM, Ais the nanowire cross-section surface, R/l is the resistanceper length, as determined by a linear fitting of theresistance versus length data, e is the charge of singleelectron, µ is the carrier mobility, and N is the chargecarrier density. Assuming a carrier mobility of 2000 cm2/V·s, the carrier concentrations of undoped nanowireswere found to be in the range of 1010-1012 cm-3. Thesevalues are between two and four orders of magnitudehigher than the intrinsic carrier concentration in GaAs14

but at least four orders of magnitude too low for functionaldevices.30

Next, TLM measurements were performed on GaAs/n-GaAs core-shell nanowires grown as described in Figure 1.Interestingly, we found that the electrical properties of theGaAs/n-GaAs nanowires vary along the length of the nano-wire and that this variation is correlated with the presence

FIGURE 2. Electrical characterization of GaAs/n-GaAs nanowiresbefore and after the shell deposition. (a) I-V data of undoped VLS-grown GaAs nanowires and SEM image (inset) of contacted nanowireshowing ohmic conduction with a carrier concentration of 1010-1012

cm-3. (b) I-V data and SEM image (inset) of GaAs/n-GaAs nanowireshowing rectifying behavior near the Au seed nanoparticle (contactsA-B and A-C) with turn on voltage of approximately 0.6 V andohmic behavior away from the Au seed nanoparticle (contacts B-C)with carrier concentration of 8 × 1017 cm-3.

FIGURE 1. Schematic of GaAs/n-GaAs core-shell nanowire growthprocess (top) starting with gold nanoparticle deposition and anneal-ing, followed by VLS growth and shell deposition. The MOCVDgrowth profile for these three steps (bottom) is shown with thetemperature plotted on the left axis and the flow of silane (SiH4),arsine (AsH3), and trimethyl gallium (TMGa) precursors plotted onthe right axis. The yellow star indicates the end of the VLS growthand the beginning of the shell deposition.

σ ) 1R/l

× 1A

) 1R/l

× 8

3d2√3) eµN (1)

© 2010 American Chemical Society 4585 DOI: 10.1021/nl102594e | Nano Lett. 2010, 10, 4584-–4589

of the gold nanoparticle (Figure 2b). Away from the Aunanoparticle (between contacts B and C in Figure 2b), thenanowire exhibited ohmic conduction, and the carrier con-centration was measured to be 8 × 1017 cm-3. In contrast,rectification behavior with a built-in voltage around 0.6 Vwas observed near the Au nanoparticle. As shown in Figure2b, similar rectification was measured in all measurementsthat involved contact A, i.e., the contact at the nanowire endcontaining the Au nanoparticle. More than 20 GaAs/n-GaAsnanowires were measured, and all nanowires exhibitedsimilar rectifying behavior with a turn-on voltage of ap-proximately 0.6 V and a directionality determined by theposition of the Au nanoparticle, indicating that this rectifyingbehavior was representative.

To confirm that the observed rectification resulted fromthe electronic properties of the nanowires, potential experi-mental artifacts were considered and demonstrated not tobe the cause of the observed rectifying behavior. First, a thinfilm of n-GaAs was deposited on a GaAs (100)B substrate asa control, and the resulting n-GaAs film exhibited a donorconcentration of 3 × 1017 cm-3 (see Supporting Informa-tion), indicating that the MOCVD reactor growth conditionswere not causing the observed rectification. Next, the pres-ence of a Schottky contact was ruled out because all threecontacts in Figure 2b, including the ones not showing therectifying behavior, are identical, and the measured built-involtage of 0.6 V is lower than the Au-GaAs barrier height of0.9 V.31 Finally, the gradual decrease of the nanowire shellthickness close to the Au particle was also dismissed as apossible cause for the observed rectifying behavior becausecontact A was designed to extend far enough past the Aunanoparticle to contact the shell at its largest thickness.Taken together, these data indicate that the electrical prop-erties and the effective doping of the nanowire vary nearthe Au seed nanoparticle.

To explore why this rectifying behavior correlates withthe position of the Au nanoparticle, the compositional profilenear the Au-GaAs interface was measured with EDS (Figure3a). The EDS line scan of GaAs/n-GaAs nanowires close tothe Au particle shows a distinct diffusion tail with Au signaldetectable as far as 90 nm away from the Au-GaAs inter-face. This distance is too large to be caused by beam-broadening effects or other experimental artifacts. Forexample, going from the nanowire into the Au particle, thearsenic signal falls steeply at the Au-GaAs interface, whereasthe gallium signal extends the entire length of the Aunanoparticle (Figure 3a). This observation is consistent witha previous report that gallium (soluble in gold) diffuses intothe gold nanoparticle, while arsenic (not soluble in gold) doesnot diffuse into the gold nanoparticle upon annealing.32

Hence, the behavior of the gallium and arsenic profilesindicates that the EDS signal is accurately measuring the Aucompositional profile. The same EDS analysis was per-formed on VLS-grown GaAs nanowires (no shell deposition)to identify during which stage of the growth process the

observed Au diffusion occurs. A comparison of the goldprofile in nanowires before and after n-GaAs shell deposition(Figure 3b) reveals that the Au EDS signal of the core-shellnanowire extends five times as deep into the nanowire asthat of the undoped nanowire. This result indicates that golddiffuses during the shell deposition stage, though it shouldbe noted that the measured distance of 90 nm is less thanthe true penetration depth because of the detection limit ofthe EDS technique (≈1%). Taken together, these resultsindicate that the diffusion occurred during the shell deposi-tion and was responsible for the rectifying behavior ob-served in Figure 2b.

Based on these results, GaAs/n-GaAs nanowires weregrown by removing the Au nanoparticle using triiodide[KI-I2 based, transene-type trifluoroacetic acid (TFA)] solu-tion after the VLS nanowire growth and before n-GaAs shelldeposition (indicated as a star in Figure 1). Utilizing TFAetchant to remove the Au seed nanoparticles has beenshown in Ge nanowires33 and proven compatible with theshell deposition process.34 The growth substrate was re-moved from the growth chamber after the VLS growth (pointindicated by a star in Figure 1), immersed in commercialTFA etchant for 5 s, rinsed in deionized water for 30 s, blowndry in nitrogen, and then reinserted in the growth chamberfor shell deposition. The I-V behavior of a representativeGaAs/n-GaAs nanowire produced using this procedure isshown in Figure 4a (the nanowires were contacted using thesame three-point scheme as discussed above). Interestingly,

FIGURE 3. (a) EDS line profiles for Ga, As, and Au at the Au-GaAsinterface. Inset is the corresponding dark-field scanning transmis-sion electron microscopy image showing position and direction ofthe line scan (green dashed arrow). (b) Comparison of Au EDS linescans in GaAs/n-GaAs vs undoped GaAs nanowire. The position wasdefined relative to the Au-GaAs interface, and the signals werenormalized to their maximum Au count at the Au-GaAs interfacein both figures (a) and (b).

© 2010 American Chemical Society 4586 DOI: 10.1021/nl102594e | Nano Lett. 2010, 10, 4584-–4589

as-prepared nanowires exhibit linear ohmic behavior acrossall contacts, including the ones at both nanowire ends. TLMconductivity measurements showed a linear dependence ofresistance on contact separation with RC ) 2.2 kΩ and R/l) 6.4 kΩ/m, which corresponds to an observed conductivity30 (Ω·cm)-1 and a carrier concentration of 1017 cm-3. Theelectronic transport properties of the nanowire were mea-sured by applying a backgate voltage on the bottom of theSi/SiO2 grid and measuring the effects on the net conductiv-ity, as illustrated in Figure 4b. The change in source-draincurrent with gate voltage, dIDS/dVGS, was consistent withn-type conductivity and measured to be 5.6 × 10-2 µA/V atan applied voltage of 1.0 V.

The EDS data and the Au-removal experiment confirmthat the rectification observed in Figure 2b is caused by Authat diffuses from the seed nanoparticle during shell deposi-tion at 750 °C and becomes electrically active in GaAs.Moreover, these data demonstrate that uniform n-typedoping can be achieved by removing the Au seed nanopar-ticle prior to n-GaAs shell deposition. While there have beenreports seeking to measure the concentration of Au in VLS-grown Si35,36 and InAs37 nanowires, there are no reportsdirectly correlating the presence of Au to the electronicproperties of nanowires. Therefore, to get insight into effectsof Au diffusion on the electrical properties of GaAs nanow-ires, a combination of I-V measurement and C-V profilingon the nanowire shown in Figure 2b was performed (Figure5). The analyses (see Supporting Information) suggest thepresence of two distinct diode regions between contacts A

and B and one diode region between contacts B and C. Thefirst diode region between contacts A and B, referred to asdiode 1 and indicated as the blue dashed line in Figure 5aand b, is dominant from 0 < V < 0.20 V. From the I-V data,diode 1 can be modeled with a leakage current I1 ) 1.2 ×10-4 µA and an ideality factor n1 ) 1.4. The same diode wasalso observed in the C-V data (Figure 5b, inset) with ameasured built-in voltage of 0.21 ( 0.01 V. The seconddiode region between contacts A and B, referred to as diode2 and indicated as the red dashed line in Figure 5a and b,appears dominant in the range of 0.25 V < V < 0.45 V witha leakage current I2 ) 1.8 × 10-3 µA, an ideality factor n2 )2.4, and a measured built-in voltage of 0.58 ( 0.02 V.Finally, diode 3 was observed between contact B and C hadthe same ideality factor (1.4) as the diode 1 with a similarbuilt-in voltage of ≈0.2 V and a leakage current of 2.6 × 10-3

µA, 20 times that of diode 1.Since the observed built-in voltage of 0.58 V correspond-

ing to diode 2 is not consistent with an Au-GaAs Schottkycontact (0.9 V),31 we hypothesize that the observed rectify-ing behavior results from a conduction band energy barrierwithin the nanowire caused by a midgap Fermi level pinnednear the Au nanoparticle (see Supporting Information). Inthis model, diode 2 results from a conduction band energybarrier caused by the difference between the conductionband of the GaAs under contact A and contact B, whilediodes 1 and 3 result from the annealed NiGeAu contactswith similar ideality factors and built-in voltages. It is well-documented that annealed NiGeAu contacts are thermionic

FIGURE 4. Electrical characterization of GaAs/n-GaAs nanowirewithout Au seed nanoparticle. (a) I-V data and a SEM micrograph(inset) from a representative nanowire showing homogeneous dop-ing along the entire length of the nanowire. (b) Source-drain currentas a function of the gate bias at a source-drain bias of 1.0 V from arepresentative nanowire. The positive slope dIDS/dVGS is indicativeof n-type conduction. Inset is a schematic of the gating configurationshowing applied voltage on the bottom of n++-Si/SiO2 (200 nm)substrate.

FIGURE 5. (a) I-V behavior of the nanowire shown in Figure 2bbetween contacts A-B and B-C with diode-like regions noted bydiodes 1-3 (defined in the text). (b) C-V profile of the samenanowire between contacts A-B in two different voltage regionswith diode-like regions noted by diodes 1-2 (defined in the text).

© 2010 American Chemical Society 4587 DOI: 10.1021/nl102594e | Nano Lett. 2010, 10, 4584-–4589

contacts with an energy barrier width that decreases whenthe donor level of the GaAs increases.28 Hence, the de-creased leakage current of contact A (diode 1) versus con-tacts B and C (diode 3) would indicate that the Fermi levelmust be further from the conduction band under contact Athan under contacts B and C. To test this model, KPFMmeasurements were performed (Figure 6). The KPFM resultssupport this model and show the presence of a diode-likeenergy barrier along the length of the nanowire with apotential difference ∆VK of 60 ( 30 mV located at 1.2 ( 0.2µm from the nanowire end. This measured value is compa-rable to a reported Kelvin potential difference of 85 ( 5 mVacross a p-i junction in a GaAs nanowire, where the differ-ence in bulk Fermi level was estimated to be 0.7 V.38 Itshould be noted that rectifying contacts have been docu-mented to create multiple diodes with different idealityfactors30 and that the Kelvin potential was performed inambient atmosphere, which may affect the value of themeasured voltage.39 Nevertheless, the qualitative agreementbetween the I-V, C-V, and KPFM data on multiple samplesstrongly supports the idea of the conduction band energybarrier within the nanowire being the most probable expla-nation of the observed rectification.

Based on these results the magnitude of the conductionband energy barrier can be estimated. Taking 0.1 V40 as aconservative estimate of the maximum voltage drop acrossthe contacts and the built-in voltage of diode 2 (0.58 V), theconduction band energy barrier is estimated to be 0.53 (0.05 eV. Since the GaAs is n-type under contact B, the Fermilevel of the GaAs under contact B is approximately 1.42 Vabove the valence band, the approximate level of the silicondonor state in GaAs.30 Since the Fermi level under contactA is 0.53 ( 0.05 eV lower than the Fermi level under contactB, the Fermi level under contact A can be estimated as 0.89( 0.05 eV above the valence band, indicating the presenceof midgap defect states at that energy. Here, we discusspossible origins of these midgap defect states in our GaAsnanowires. Gold has been reported to create an acceptor atEV + 0.04 eV and a deep trap at EV + 0.40 eV in GaAs,41

however the properties of all electronically active gold-related defects in GaAs are not fully understood. The exist-ence of an Au-Ga complex state at EV + 1.07 eV has beenproposed,42 and other midgap states related to Au may exist.Another possibility is that the Au diffusion is indirectlycreating midgap states in the range of 0.89 ( 0.05 eV abovethe valence band. If the diffusion current of Au into the GaAsnanowire is matched by the diffusion of gallium into thegold, then the density of Ga vacancies (E3, located at EV +1.10 eV)43 and As antisite defects (EL2, located at EV + 0.83eV)44 would be greatly increased. Furthermore, if the dif-fused gold creates a small degenerately doped p-type region,then the observed midgap states could be intrinsic GaAsdefect states located in the diode depletion region. Forexample, it has been reported that GaAs possesses midgapsurface states in the range of 0.7-0.9 eV above the valenceband.28 Precise determination of the density and origin ofthe midgap states would require deep level transient spec-troscopy (DLTS)45 studies performed on shell-doped nanow-ires grown under various growth conditions. Moreover, theextent and the spatial uniformity of the diffused gold withinthe nanowire would need to be precisely measured todetermine if the extent of the pinned segment exceeds theextent of the Au diffusion. Future work will focus on moreprecise measurement of electrical properties and impurityconcentrations using local electron atom probe microscopyand DLTS.

In summary, a combination of I-V and EDS measure-ments determined that the presence of Au has an adverseeffect on n-type doping in GaAs nanowires. A model of Fermilevel pinning was proposed as a source of the rectifyingbehavior observed in GaAs/n-GaAs core-shell nanowiresgrown without the gold removal based on a combination ofI-V, C-V, and KPFM measurements. By removing the goldprior to depositing a doped shell, n-type doping was suc-cessfully realized in a GaAs nanowire system for the firsttime. Since gold is often used as a material of choice fornanowire seeding in a variety of materials systems, itseffects on electrical properties of nanowires and nanowireheterostructures should be carefully addressed. Therefore,our results represent a significant advancement in theunderstanding of Au incorporation and the doping in nano-wire systems and should further the development of func-tional devices based on nanowire heterostructures.

Acknowledgment. This work was supported by NSFCAREER award DMR-0745555. We acknowledge access tothe MIT’s Microsystems Technology Laboratories and to theShared Experimental Facilities provided by MIT’s Center forMaterials Science Engineering (supported in part by theMRSEC Program of National Science Foundation underaward number DMR-0213282). The authors would like tothank Bin Lu for assitance with C-V measurements aswell as Prof. E. Fitzgerald and Prof. T. Palacios for helpfuldiscussions.

FIGURE 6. SEM image (top) of a GaAs/n-GaAs nanowire with the goldparticle present and the corresponding Kelvin voltage and heightprofiles (bottom) measured along the dashed green line. Kelvinvoltage and position are shown with respect to position of the Aunanoparticle.

© 2010 American Chemical Society 4588 DOI: 10.1021/nl102594e | Nano Lett. 2010, 10, 4584-–4589

Supporting Information Available. Electrical character-ization of GaAs/n-GaAs thin films, detailed description of thenanowire diode analysis, and a proposed model of rectifyingGaAs/n-GaAs nanowires without gold etching. This materialis available free of charge via the Internet at http://pubs.acs.org.

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