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Single nanowire AlN/GaN double barrier resonant tunneling diodes with bipolar tunneling at room and cryogenic temperaturesYe Shao , Santino D. Carnevale , A. T. M. G. Sarwar , Roberto C. Myers , and Wu Lu

Citation: Journal of Vacuum Science & Technology B 31 , 06FA03 (2013); doi: 10.1116/1.4829432 View online: http://dx.doi.org/10.1116/1.4829432 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/31/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing

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Single nanowire AlN/GaN double barrier resonant tunneling diodeswith bipolar tunneling at room and cryogenic temperatures

Ye Shao, Santino D. Carnevale, and A. T. M. G. Sarwar Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210

Roberto C. Myers a)

Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210

and Department of Electrical and Computer Engineering, The Ohio State University, Columbus,Ohio 43210

Wu Lu b)

Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210

(Received 22 June 2013; accepted 25 October 2013; published 12 November 2013)

III-N semiconductor resonant tunneling diodes (RTDs) have attracted great research interestbecause of their potential high speed performance. Thin lm III-N RTDs are challenging due tohigh dislocation densities resulted from large lattice and thermal expansion coefcient mismatchesto substrates. Here the authors present the growth and fabrication of AlN/GaN double barrier nanowire RTDs. The AlN/GaN double barrier nanowire RTDs show clear negative differentialresistance with an onset voltage between 3.5 V and 4.5 V at both room and cryogenic temperatures.The bipolar tunneling and temperature dependent device performance suggest that the electron

transport of these devices is based on resonant tunneling. VC 2013 American Vacuum Society .[http://dx.doi.org/10.1116/1.4829432 ]

I. INTRODUCTIONBecause of the fast tunneling process or extremely high

switching speed (in terahertz range), resonant tunnelingdevices have attracted considerable interest for a variety of potential applications including high-resolution radar, imag-ing systems for low visibility envi r onments, wide-bandsecure communications systems, etc. 1 – 4 III-nitride (III-N)wide band gap semiconductors are promising materials for

resonant tunneling diodes (RTDs) because of their uniquematerial properties such as large conduction band offset (i.e.,2.1 eV between AlN and GaN), and excellent thermal sta-

bility. However, III-N thin lms always suffer from alarge density of threading dislocations (typically10 8 –109 cm 2) since they are grown on non-native latti-ce-mismatched substrates such as sapphire, SiC, or Si.As a result, previously reported planar tunneling devicesexhibited strong hysteresis and no negative differentialresistance (NDR) peak when scanning backwards (i .e.,from positive voltage to 0 V) or at low temperatures. 5 – 8

This has been attributed to trap-assisted tunneling rather than resonance tunneling. The planar devices may also

suffer from interface roughness and island scattering,which causes the degradation of peak to valley currentratio (PVCR) after initial scans. 9,10

Recently, III-nitride nanowires (NWs) have emergedas an alternative choice for high performance RTDs. Thisis because large surface-to-volume ratio and small crosssections allow NWs to accommodate much higher latticemismatch with an efcient elastic strain relaxation, therebypreventing the formation of dislocations during epitaxial

growth. Because of these advantages, III-N NWs have beenconsidered as a promising candidate of next generationnanoscale electronic and optoelectronic devices. In general,NDR features observed in III-N semiconductor N W devicescan be attributed to three dif ferent mechanisms: 11 (a) trap-assisted inelastic tunneling; 12 (b) tunnelin g through potentialbarriers b etwe en NWs in a network; 13 (c) intervalleyscattering. 14 – 17 The trap-assisted inelastic tunneling relatedNDR is commonly hysteretic, degrades after repeated scans,and disappears at low temperatures. 5 – 7,18 Thus, technicallyrobust RTDs require resonant tunneling through the barriersruling out any trap-assisted tunneling transport. So far, manyexperimental studies have been focused on AlN/GaN doublebarrier NW based RTDs to pursue reliable and reproducibleNDR with high PVCR and tunneling current density. 15 – 17

In this Letter, we report on axial catalyst freeAlN/GaN double barrier single NW RTDs grown byplasma-assisted molecular beam epitaxy (PAMBE). Thehigh quality of Al N /GaN interfaces makes ideal resonanttunneling possible. 11 These NW RTDs show clear NDRat both room and cryogenic temperatures and tunneling

under a bipolar bias.

II. EXPERIMENT

In our work, an axial GaN/AlN double barrier NW struc-ture was grown using a Veeco 930 radio frequency PAMBEon n-Si (111) substrates. The NWs have a symmetrical struc-ture consisting of 500 nm n-GaN/1.5 nm i-AlN/2.5 nmi-GaN/1.5nm i-AlN/500nm n-GaN along the axial c-axispreferential growth d irection with a majority of the wiresexhibiting N-polarity. 18 As discussed below, a fraction of each layer also deposits radially. The 500 nm n-GaN wasdoped by Si at a doping level of 1 1019 cm 3 for

a) Electronic mail: myers.1079@osu.edub) Electronic mail: lu.173@osu.edu

06FA03-1 J. Vac. Sci. Technol. B 31(6), Nov/Dec 2013 2166-2746/2013/31(6)/06FA03/5/$30.00 VC 2013 American Vacuum Society 06FA03-1

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electrical contacts. We have developed a tw o-step growthmethod for catalyst free III-N NW growth. 15 First, GaNnanoislands were nucleated at a substrate temperature of 720 C until the desired areal density was achieved. Thenthe substrate temperature was ramped up to 790 C to growthe rest of the NW heterostructure. The elevated substratetemperature inhibits nucleation of new islands, but alreadynucleated nanoislands continue to grow as NWs, thus main-taining the same density achieved in the rst growth step.The Z-contrast atomic resolution scanning tunneling electronmicroscope images of both as-grown vertical and coaxialAlN/GaN heterostructures with the s ame two-step growthmethod have been reported elsewhere. 15 ,19 Thus, a clear ab-rupt AlN/GaN heterostructure interface without any threaddefects can be expected here.

For device fabrication, the as-grown GaN/AlN doublebarrier NWs were removed from the original Si substrate bysonication in ethanol, then spin-coated on a foreign siliconsubstrate with a 100 nm thick, thermally grown SiO 2 layer.To make sure there is good chance to get working RTDs, a

500nm electrodes gap as shown in Fig. 1(c) was designedand dened by electron beam lithography using a VistecEBPG 5000 system at 50 keV and a beam current of 20 nAwith proximity correction implemented. A PMMA(950 k)/PMMA-MAA bilayer resist scheme was used anddeveloped in methyl isobutyl ketone:isopropyl alcohol(IPA) ¼ 1:2 for 30 s and rinsed in IPA to generate undercutsfor lift off. Ti/Al/Ti/Au metal contacts were deposited byelectron beam evaporation and lifted-off to form electrodes.

Room temperature measurements were performed on aKarl Suss probe station using an Agilent 4156c semiconduc-tor device parameter analyzer. The temperature dependentI–V measurements were performed on a cryogenic probe sta-

tion (TTP4 Probe Station, Lake shore Cryotronics, Inc) from77 to 256 K.

III. RESULTS AND DISCUSSIONA cross section SEM image of as-grown NWs is shown

in Fig. 1(a) . It shows most of the NWs are wedge-shapedwith a slim root and sturdy head, indicating that NW growthtakes place not only axially but also radially. All NWs have

lengths around 1 l m and diameters between 150 and200 nm.

The device structure sketch is shown in Fig. 1(b) . As aworking device, a single NW needs to be in good contactwith two electrodes [shown in Fig. 1(c) ]. So far, differenttheoretical methods have been developed for quantum trans-port simulations in AlN/GaN double barrier heterostruc-ture. 20 To better understand the tunneling mechanism, aone-dimensional self-consistent Schrodinger Poisson model,coupled spontaneous and piezoelectric polarization effects,was performed using software SILVACO ATLAS , to simulate theband diagram of AlN/GaN double barrier heterostructure.Figure 2 shows the conduction band edge (E c), valence bandedge (E v), and rst two quasi-bound state energy levels withtheir corresponding wave functions in the quantum wellalong the c-axis growth direction. The simulated rst twoquasi-bound state energy levels are 0.492eV and 1.023 eVabove the Fermi level, respectively. At low biases or close toequilibrium, both quasi-bound states are above the Fermilevel (E f ); thus, no tunneling is likely to happen. Most car-

riers are blocked by the AlN double barriers. At a certainapplied bias, the electron energy in the GaN region alignswith the rst bound state in the GaN well between the twothin AlN barriers resulting tunneling resonance. Since theNWs are made of polar crystalline materials (GaN and AlN),the biasing conditions for tunneling at forward and reverseare different due to the asymmetrical band structure causedby the strong polarization effects.

The room temperature I–V characteristics of RTDs areshown in Fig. 3(a) . These RTDs show clear NDR with anonset voltage between 3.5 V and 4.5 V as shown in thezoom-in views of insets in Fig. 3(a) . It is important to notethat for most RTDs, the NDR feature appears during bipolar

voltage scan (i.e., scanning all the way from negative to pos-itive voltage) with a similar absolute onset voltage. Usually,in the case of trap-assisted inelastic tunneling based NDR,the NDR feature itself is absent during bipolar voltage scan.This is because the trapped electrons during negative voltagescans take time to be detrapped again. Before detrapping iscompleted, traps are occupied; thus, no more trap-assistedinelastic tunneling can happen during the positive voltagescan. Thus, the odd (bipolar) symmetry of the I–Vs is strong

FIG. 1. (Color online) (a) Cross-section scanning electron microscopy images of an as-grown NW sample. Most of NWs have lengths around 1 l m and diame-ters between 150 and 200nm. (b) Schematic single NW RTD design. The NWs are transferred to a foreign Si substrate with a 100 nm thick SiO 2. Ti/Al/Ti/Aumetal contacts with a 500 nm gap are dened by e-beam lithography and deposited by electron beam evaporation to form two electrodes. (c) The scanningelectron microscopy image of a working RTD device. It clearly shows a single NW cross the two electrodes as designed. The transferred NWs have lengthsaround 1 l m and diameters between 150 and 200 nm.

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evidence of resonant tunneling rather than trap-assistantinelastic tunneling. It is worth noting that most of the RTDsshow symmetrical nonlinear I–V characteristics at roomtemperature. We believe this is due to the existence of aSchottky barrier between the metal contacts and NWs. Theexponentially increasing current before the NDR onset volt-age is a result of thermionic emission.

In addition, cryogenic measurements were also per-

formed. Our RTDs show clear NDR features down to 77 Kwith onset voltage all around 4.5V [Fig. 3(b) ]. It is alsonoted that the NDR onset voltage has a clear positive shiftwhen temperature is decreased. This is because the decreaseof carrier density at low temperature pushes the Fermi levelfarther below the rst quasi-bound state in the quantum well.Therefore, a higher bias is needed to bend the conductionband enough to achieve energy level resonance leading totunneling. In the case of trap-assisted inelastic tunneling,electrons would not have enough activation energy to ll uptraps at low temperatures, and NDR could not occur. Thus,the RTD operation at cryogenic temperatures also supportsresonant tunneling transport mechanism.

However, in a small number of cases, RTDs show multi-ple NDR features during a single voltage scan [Fig. 3(a) ].Besides the onset voltage between 3.5V and 4.5 V as mostRTDs have, some RTDs also show other NDR features atlower bias. We also have observed NDR features that appear only during the rst few scans in some devices. In these devi-ces, the PVCR, though high at the beginning, becomes com-pressed until disappearing completely in subsequent scans.Figure 4(a) shows the comparison of I–V characteristics of afresh device and after it is aged. The current difference beforethe onset of NDR between the two measurements is simplythe tunneling current. The exact mechanism of device

degradation after electrical stressing is unknown and under investigation. This is also in debate for thin lm GaN RTDs,but it is widely accepted that the device degradation is aresult of high density threading dislocations in epitaxial thinlms d ue to the large lattice mismatch between GaN and sub-strates. 9 For nanowire RTDs, we attribute the device degrada-tion to the radial growth of long nanowires. Our hypothesis isthat such unexpected NDR features may be associated withthe three-dimensional heterostructure contained within theNW. Though the preferential growth direction is in the c-axis, in the case of AlN, however, growth occurs along boththe axial and radial direction due to the minimal adatom dif-fusion length of Al at the growth temperatures used. 18 ,19 Asdescribed above, the wedge shaped NW indicates that growthoccurs not only along the axial direction, but also radially.Catalyst-free III-nitrides grown by PAMBE are well knownto exhibit radial growth. Previously, we found that GaNradial growth is suppressed using the two-step growth methoddescribed above. However, the original studies were connedto shorter nanowires that were at most 200 nm in length. It islikely that for the long growth times needed to form 1 l m

FIG. 2. (Color online) Simulated band diagram of designed AlN/GaN doublebarrier hereostructure. It also includes the rst two quasi-bound states andelectron wavefuctions within the AlN/GaN quantum well in the conductionband at equilibrium.

FIG. 3. (Color online) (a) Room temperature I–V characteristics of NWAlN/GaN double barrier RTDs. The NDR feature appears during bipolar voltage scan. (b) Cryogenic I–V characteristics of NW AlN/GaN double bar-rier RTDs. It shows a positive NDR onset voltage shift when temperaturegoes down. Zoom-in views of NDR features are shown in the insets.

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long wires, radial growth of GaN must be occurring, whichdiffers from recent observation of Fernandez-Garrido. 21 Theresulting three-dimensional heterostructure is schematicallydrawn in Fig. 4(b) . The growth rate ratio between axial andradial growth is over 10. In other words, 1.5 nm thick axialAlN barrier growth results a core shell growth of AlN in arange of few angstroms or less, especially at the foot of nano-wires. We propose that the device degradation is due to thealloy scattering or island scattering taking place at the bottomsurface of nanowires in the electron transport process due tothe small amount of Al deposition resulted from the Al ada-tom diffusion.

We modeled the leakage current [thermionic emissioncurrent, blue circles in Fig. 4(a) ] in an aged device after theNDR feature disappears, as a pair of back to back Schottkydiodes with a shunt resistor in parallel. In the equivalent cir-cuit shown in the inset of Fig. 4(a) , once the lateral voltagedrop is established at either side, the current will ow as likein a reversely biased metal-n-GaN Schottky diode dominatedby the energy barrier. As shown in Fig. 4(a) , the tted cur-rent is the sum of reverse saturation current, the barrier low-ering effect under an electrical eld, and the shunt current.The tted energy barrier is 0.25 eV and the shunt conduct-ance G p is 2.68 10 6 S. The modeled current has a verygood agreement with the experimental results. Since thethermionic emission current is essentially the same after NDR disappears as shown in Fig. 3(a) , it is suggested that

the radial growth is likely the reason to cause device degra-dation, e.g., NDR disappears after electrical stressing, butthe shunt resistance is essentially the same. This is not unrea-sonable because the shunt path current should be going alongthe path of Al poor regions or domains on the nanowire sur-face. To verify this hypothesis, current research efforts arebeing directed to develop an etching process to etch the AlNsidewalls before device processing.

IV. SUMMARY AND CONCLUSIONS

In summary, we have demonstrated AlN/GaN double bar-rier single NW based RTDs. I–V characteristics show clear

NDR at both room temperature and cryogenic temperatureswith an onset voltage between 3.5 V and 4.5 V. NDR fea-tures at bipolar voltage biases and at cryogenic temperaturessuggest that the electron transport in these devices is reso-nant tunneling rather than trap-assisted tunneling. The devicedegradation observed in some devices is likely caused by theradial growth of AlN during the long growth time of nano-wires due to the minimal adatom diffusion length of Al atthe growth temperatures.

ACKNOWLEDGMENTS

This work was partially supported by National ScienceFoundation and Ofce of Naval Research.

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FIG. 4. (Color online) (a) Comparison of I–V characteristics of a fresh device and after NDR feature disappears. The inset is an equivalent circuit for modelingof thermionic emission current. The modeled thermionic emission current is a sum of reverse saturation current with the barrier lowering effect included andthe shunt path current. (b) Schematic NW growth process. NW growth is not only axially but also radially. During the two AlN barriers growth, two layers of AlN side walls also formed. The overall grown NW is wedge shaped. (c) Two types of electron transport for: (1) transport along the center of NW and tunnel-ing cross AlN/GaN double barrier. Thus, resonant tunneling is expected (left); (2) Thermionic emission current along the surface of NW and over the AlN bar-riers (right).

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