Correlation of growth temperature, photoluminescence, and resistivity in GaNnanowiresA. Alec Talin, George T. Wang, Elaine Lai, and Richard J. Anderson Citation: Applied Physics Letters 92, 093105 (2008); doi: 10.1063/1.2889941 View online: http://dx.doi.org/10.1063/1.2889941 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/9?ver=pdfcov Published by the AIP Publishing

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Correlation of growth temperature, photoluminescence, and resistivityin GaN nanowires

A. Alec Talin,1,2,a� George T. Wang,3 Elaine Lai,1 and Richard J. Anderson1

1Sandia National Laboratories, Livermore, California 94550, USA2Center for Integrated Nanotechnologies, Albuquerque, New Mexico 87185, USA3Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

�Received 27 December 2007; accepted 8 February 2008; published online 3 March 2008�

The photoluminescence and electrical transport of GaN nanowires grown by metal catalyzedmetal-organic chemical vapor deposition were investigated as a function of substrate temperatureduring growth. As the growth temperature increased from 800 to 900 °C, the electrical conductionmechanism changed from space-charge limited to ohmic transport, the nanowire resistivity droppedfrom �107 to �10−3 � cm, and the band edge luminescence increased by more than two orders ofmagnitude. A strong correlation between the resistivity and the fraction of band edge luminescencefor individual nanowires was observed. © 2008 American Institute of Physics.�DOI: 10.1063/1.2889941�

Group III-N nanowires grown by metal nanoparticlecatalyzed chemical vapor deposition �CVD� have recentlyattracted considerable interest, motivated by the desirableoptoelectronic properties of this material system, the highcrystalline quality of the nanowires, and the potential of in-tegrating these quasi-one-dimensional nanostructures withSi.1 Particularly attractive device applications include verti-cally oriented nanoscale laser diodes and high electron mo-bility transistors.2,3 The growth mechanism which results inthe GaN nanowires is vapor-liquid-solid �VLS� or vapor-solid-solid epitaxy �VSS�, whereby gas phase Ga and N pre-cursors dissolve in transition metal catalyst such as Ni, andprecipitate as single-crystalline nanowires. Numerous trans-mission electron microscopy �TEM� studies have shown thatVLS derived GaN nanowires, grown under a variety of con-ditions �i.e., precursors, substrate temperature, etc.�, havehigh crystalline quality with low dislocation density, thoughstacking faults are frequently observed.4,5 Unlike extendeddefects, characterization of point defects such as impurities,vacancies, and substitutions is far more challenging. How-ever, in nanowires, the impact of point defects may, in fact,be more pronounced, since dislocations in bulk or thin filmGaN act as sinks or getters for point defects.6 Furthermore,conditions that favor nanowire growth, including substratetemperature �Tsub� and III-V precursor ratio, depart signifi-cantly from the conditions typically employed for devicequality thin film GaN growth, and may further increase theconcentration of point defects.4

In this letter, we describe the results of electrical andoptical characterization of GaN nanowires grown by metalcatalyzed metal-organic CVD �MOCVD� where only onegrowth condition, the substrate temperature, is varied from800 to 900 °C. We use a simple platform that was specifi-cally designed for combined electrical and optical character-ization of individual nanowires �as compared to ensembles�.Our results show that for a 100° increase in substrate tem-perature �Tsub� during growth, the average nanowire resistiv-ity decreases by nine orders of magnitude, while band edgeluminescence �BEL� intensity increases by more than three

orders of magnitude. Furthermore, we show that for the en-tire set of nanowires investigated here, the resistivity andphotoluminescence �PL� are strongly correlated. Comparisonof our results with previous work on GaN nanowires grownunder similar conditions but without the use of carbon-bearing precursors strongly suggests that residual carbon in-corporated during growth is responsible for the effects ob-served here.

The growth of GaN nanowires used in this study wasdescribed in detail previously.4 Nickel catalyst in the form ofnickel nitrate hexahydrate was diluted in ethanol and depos-

ited dropwise onto a 2 in. �11̄02� oriented sapphire substrate.Nanowire growth was carried out in a home-built MOCVDrotating-disk cold-wall reactor with trimethyl gallium andammonia precursor with the substrate held at 800, 850, or900 °C. Extensive TEM analysis indicated that these nano-

wires have primarily a �112̄0� orientation, are single crystal-line, and are generally free from dislocations.4

Electrical contacts to individual nanowires werefabricated by first spin casting a nanowire suspensionin ethanol onto degenerately doped Si wafers with 100 nmthick thermal oxide. Next, arrays of interdigitated, individu-ally electrodes were defined on the substrate using opticallithography, followed by an O2 plasma etch to remove re-sidual photoresist, electron beam evaporation of Ti /Au�10 nm /100 nm� metallization, lift-off, and finally an annealat 600 °C for �5 min in a vacuum furnace. This approachdoes not require direct-write electron beam lithography andgenerally results in tens of individual nanowire devices perwafer.

PL spectra for individual nanowires were collected usinga home-built system, with a He–Cd laser focused though a32� reflective objective, and a charge coupled device �CCD�camera/spectrometer �OceanOptics™� combination coupledto the electrical probe station microscope. The resulting laserspot incident on the sample had a diameter of �10 �m and atotal cw power of �2 mW. Note that this power densitycannot be directly compared with bulk GaN experiments,since in our case the nanowire diameter is much smaller thanthe spot size. A Semrock™ band pass filter was used toeliminate the reflected laser light from reaching the CCDa�Electronic mail: aatalin@sandia.gov.

APPLIED PHYSICS LETTERS 92, 093105 �2008�

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camera or the spectrometer. Current-voltage �I-V� measure-ments were performed using a computer controlled Keithley237 source-measure unit.

PL spectra collected from three different GaN nanowiresgrown at 800, 850, and 900 °C are shown on a log scale inFigs. 1�a�–1�c�, respectively. Also included are insets of theluminescence images of the three nanowires. The PL spec-trum for the nanowire grown at 800 °C contains a broadyellow luminescence �YL� band centered at �2.3 eV, andonly a shoulder at �3.4 eV, where BEL is usually observedat for GaN at room temperature. The sharp lines in Fig. 1�a�above 3 eV are part of the background and originate from theHe–Cd laser. However, as Tsub is increased to 850 °C, theBEL peak grows by approximately ten times, and increases afactor of approximately thirty for Tsub of 900 °C. In com-parison, the broad YL band remains approximately un-changed. The undulations in the YL band are due to opticalinterference of the Semrock™ filter, used to remove the re-flected laser light, for wavelength above 360 nm.

I-V characteristics for the same three nanowire speci-mens are shown in Figs. 1�d�–1�f�, together with the scan-ning electron microscope images of the nanowire specimens.Additional I-V characteristics for nanowires grown at allthree temperatures are displayed on a log-log plot in Fig. 2,where we show the less resistive portion for those specimens

with nonsymmetrical I-V curves �i.e., the negative portionfor I-V in Fig. 1�e��. The log-log plot reveals a power lawbehavior where I�Vn, with n�1 �i.e., Ohmic behavior� fornanowires grown at 900 °C and n�2 for nanowires grownat 800 and 850 °C.

The lower BEL intensity observed with lower Tsub sug-gests that these nanowires have a low free electron concen-tration, and that the extra carriers created upon photon ab-sorption recombine nonradiatively at defect sites. However,the concentration of the defects responsible for YL remainsessentially unchanged, as evident by the relative invariancein the YL band with Tsub. According to Reshchikov andMorkoc,7 the broad YL band with peaks at �2.2 and�2.5 eV originates from two charge states of the same de-fect, a complex involving a gallium vacancy VGa and a shal-low oxygen donor on a N site ON.

The I�V2 power law dependence observed for nano-wires grown at 800 and 850 °C indicates that electricaltransport is dominated by space-charge limited conduction�SCLC�.8 SCLC in Si and compound semiconductor nano-wires has recently been reported by several groups.9,10 SCLCarises when the carriers injected from the contacts exceed thethermal free carrier concentration in the specimen, and istypical of insulating materials where transport is not limitedby contact resistance.8 In order to estimate the nanowire re-sistivity, we fit the I-V characteristics to I=�Vn, where � is aconstant, and then evaluate the resistivity at V=1 V using theexpression �nanowire=A /L�dI /dV�V=1

−1 , where A is the nano-wire cross sectional area measured in the middle and L is thenanowire length spanning between the two metal electrodes.In Fig. 3, the nanowire resistivity is plotted versus the inte-grated BEL intensity normalized by the total integrated lu-minescence intensity, i.e., BEL / �BEL+YL�. The data werefitted to a power law function which yielded the best fit of�=4.7�10−4�BEL /BEL+YL�−6.4, and where the three datapoints corresponding to the highest BEL / �BEL+YL� valueswere not included in the mathematical fit. This apparentchange in the resistivity-PL relationship at the higher BELintensity is possibly due to other factors, such as impurityand phonon scattering, playing a stronger role in determiningthe electrical transport once the free carriers are no longercompensated by the traps present in those nanowires with thelower BEL intensity.

The strong decrease in resistivity and increase in BEL asa function of Tsub in our study is unlikely due to extendeddefects, as TEM analysis revealed no appreciable differencein dislocation density for nanowires grown at different Tsub.

FIG. 1. �Color online� Photoluminescence spectra collected at room tem-perature using He–Cd laser with a spot size of 10 �m in diameter and apower of 2 mW for GaN nanowires grown at �a� 800 °C, �b� 850 °C, and�c� 900 °C. ��d�–�f�� I-V characteristics for the same three nanowires.

FIG. 2. �Color online� A sample of I-V curves collected for nanowiresgrown at different substrate temperatures. The green dotted lines indicateI�V1 and I�V2 relationships.

FIG. 3. �Color online� Nanowire resistivity plotted vs the normalized BELintensity. The last three data points were not included in the data fit.

093105-2 Talin et al. Appl. Phys. Lett. 92, 093105 �2008�

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A similar conclusion with respect to lack of PL dependenceon dislocation density was also reached by Sun et al. whostudied bulk GaN films with widely different dislocationdensities.11

Besides bulk point defects, surface states can also de-plete the free carriers within a nanowire owing to the highsurface to volume ratio.12,13 To address this possibility, wehave measured the resistance of a set of nanowires grown at900 °C with diameters ranging from �100 to �750 nm.These results are shown in Fig. 4, where the two electroderesistances are divided by the nanowire length, and are plot-ted versus the nanowire diameter. A sharp increase in resis-tance is indeed observed for diameter below �150 nm, sug-gesting that band bending due to surface states completelydepletes those nanowires. This result is in agreement with theprevious experiments of Calarco et al., as well as with arough estimation of the depletion width assuming planarcontacts and assuming surface energy level at 0.5 eV belowthe conduction band minimum, and an unintentional n-typedoping of 1017 /cm3.12 The BEL intensity of these thinner,900 °C grown nanowires is lower compared to those withlarger diameters, but this effect was not investigated system-atically in this study and will be addressed in the future. Todiminish the effects of surface depletion on the correlation ofTsub with PL and resistivity, only nanowires with diametersof �200 nm were used to compile the data shown in Figs. 2and 3.

Previously, Stern et al. investigated electrical transport inGaN nanowires grown under similar conditions, but withoutthe use of metal-organic precursors for the Ga source.14 Un-like in the present case, Stern et al. observed that the carrierconcentration was �1019 /cm3 with little dependence on thesubstrate temperature in the range of 800–1100 °C. Com-parison of our results with those of Stern et al. suggests thatpossible carbon incorporation during growth from the tri-methylgallium precursor is responsible for the low BEL in-

tensity and the high resistivity observed at the lower growthtemperatures. Bulk GaN films doped with carbon at1017 /cm3 are indeed insulating, with the C substituting nitro-gen acting as either shallow or moderately deep acceptor.7

Furthermore, while bulk GaN specimens with high carboncontent have been reported to have strong YL, a clear corre-lation between carbon concentration and YL intensity wasnot observed.7

In conclusion, we have shown that substrate temperaturecan substantially affect the luminescence and resistivity ofmetal catalyzed MOCVD grown GaN nanowires, and that anincrease from 850 to 900 °C can results in a factor of �30increase in BEL intensity, and a factor of �104 decrease inresistivity. We have also shown that these changes are notrelated to YL intensity, nor do they originate from surfacerelated effects. Finally, by comparison with previous work onGaN nanowires grown without carbon-bearing precursors,we suggest that carbon incorporation at the lower Tsub isresponsible for the observations reported in this study.

This work was performed, in part, at the Center for In-tegrated Nanotechnologies, a U.S. Department of Energy,Office of Basic Energy Sciences user facility. Sandia Na-tional Laboratories is a multiprogram laboratory operated bySandia Corporation, a Lockheed-Martin Company, for theU.S. Department of Energy under Contract No. DE-AC04-94AL85000.

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FIG. 4. Nanowire resistance divided by the length vs the nanowire widthmeasured midway between the two metal electrodes.

093105-3 Talin et al. Appl. Phys. Lett. 92, 093105 �2008�

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