1.4829432.pdf

Single nanowire AlN/GaN double barrier resonant tunneling diodes with bipolartunneling 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 Articles you may be interested in Coaxial nanowire resonant tunneling diodes from non-polar AlN/GaN on silicon Appl. Phys. Lett. 100, 142115 (2012); 10.1063/1.3701586 Investigation of the negative differential resistance reproducibility in AlN/GaN double-barrier resonant tunnellingdiodes Appl. Phys. Lett. 99, 182109 (2011); 10.1063/1.3659468 Reliability in room-temperature negative differential resistance characteristics of low-aluminum contentAlGaN/GaN double-barrier resonant tunneling diodes Appl. Phys. Lett. 97, 181109 (2010); 10.1063/1.3515418 AlN/GaN double-barrier resonant tunneling diodes grown by metal-organic chemical vapor deposition Appl. Phys. Lett. 96, 042103 (2010); 10.1063/1.3294633 AlN/GaN double-barrier resonant tunneling diodes grown by rf-plasma-assisted molecular-beam epitaxy Appl. Phys. Lett. 81, 1729 (2002); 10.1063/1.1501157

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

Roberto C. Myersa)

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

Wu Lub)

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 interest

because of their potential high speed performance. Thin film III-N RTDs are challenging due to

high dislocation densities resulted from large lattice and thermal expansion coefficient mismatches

to 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 differential

resistance 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. INTRODUCTION

Because of the fast tunneling process or extremely high

switching speed (in terahertz range), resonant tunneling

devices have attracted considerable interest for a variety of

potential applications including high-resolution radar, imag-

ing systems for low visibility environments, wide-band

secure communications systems, etc.14 III-nitride (III-N)

wide band gap semiconductors are promising materials for

resonant tunneling diodes (RTDs) because of their unique

material 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 films always suffer from a

large density of threading dislocations (typically

108109 cm2) since they are grown on non-native latti-

ce-mismatched substrates such as sapphire, SiC, or Si.

As a result, previously reported planar tunneling devices

exhibited strong hysteresis and no negative differential

resistance (NDR) peak when scanning backwards (i.e.,

from positive voltage to 0 V) or at low temperatures.58

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 current

ratio (PVCR) after initial scans.9,10

Recently, III-nitride nanowires (NWs) have emerged

as an alternative choice for high performance RTDs. This

is because large surface-to-volume ratio and small cross

sections allow NWs to accommodate much higher lattice

mismatch with an efficient elastic strain relaxation, thereby

preventing the formation of dislocations during epitaxial

growth. Because of these advantages, III-N NWs have been

considered as a promising candidate of next generation

nanoscale electronic and optoelectronic devices. In general,

NDR features observed in III-N semiconductor NW devices

can be attributed to three different mechanisms:11 (a) trap-

assisted inelastic tunneling;12 (b) tunneling through potential

barriers between NWs in a network;13 (c) intervalley

scattering.1417 The trap-assisted inelastic tunneling related

NDR is commonly hysteretic, degrades after repeated scans,

and disappears at low temperatures.57,18 Thus, technically

robust RTDs require resonant tunneling through the barriers

ruling out any trap-assisted tunneling transport. So far, many

experimental studies have been focused on AlN/GaN double

barrier NW based RTDs to pursue reliable and reproducible

NDR with high PVCR and tunneling current density.1517

In this Letter, we report on axial catalyst free

AlN/GaN double barrier single NW RTDs grown by

plasma-assisted molecular beam epitaxy (PAMBE). The

high quality of AlN/GaN interfaces makes ideal resonant

tunneling possible.11 These NW RTDs show clear NDR

at 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 PAMBE

on n-Si (111) substrates. The NWs have a symmetrical struc-

ture consisting of 500 nm n-GaN/1.5 nm i-AlN/2.5 nm

i-GaN/1.5 nm i-AlN/500 nm n-GaN along the axial c-axis

preferential growth direction with a majority of the wires

exhibiting N-polarity.18 As discussed below, a fraction of

each layer also deposits radially. The 500 nm n-GaN was

doped by Si at a doping level of 1 1019 cm3 fora)Electronic mail: [email protected])Electronic mail: [email protected]

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 two-step growth

method for catalyst free III-N NW growth.15 First, GaN

nanoislands 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 substrate

temperature inhibits nucleation of new islands, but already

nucleated nanoislands continue to grow as NWs, thus main-

taining the same density achieved in the first growth step.

The Z-contrast atomic resolution scanning tunneling electron

microscope images of both as-grown vertical and coaxial

AlN/GaN heterostructures with the same two-step growth

method have been reported elsewhere.15,19 Thus, a clear ab-

rupt AlN/GaN heterostructure interface without any thread

defects can be expected here.

For device fabrication, the as-grown GaN/AlN double

barrier NWs were removed from the original Si substrate by

sonication in ethanol, then spin-coated on a foreign silicon

substrate with a 100 nm thick, thermally grown SiO2 layer.

To make sure there is good chance to get working RTDs, a

500 nm electrodes gap as shown in Fig. 1(c) was designed

and defined by electron beam lithography using a Vistec

EBPG 5000 system at 50 keV and a beam current of 20 nA

with proximity correction implemented. A PMMA

(950 k)/PMMA-MAA bilayer resist scheme was used and

developed in methyl isobutyl ketone:isopropyl alcohol

(IPA) 1:2 for 30 s and rinsed in IPA to generate undercutsfor lift off. Ti/Al/Ti/Au metal contacts were deposited by

electron beam evaporation and lifted-off to form electrodes.

Room temperature measurements were performed on a

Karl Suss probe station using an Agilent 4156c semiconduc-

tor device parameter analyzer. The temperature dependent

IV measurements were performed on a cryogenic probe sta-

tion (TTP4 Probe Station, Lake shore Cryotronics, Inc) from

77 to 256 K.

III. RESULTS AND DISCUSSION

A cross section SEM image of as-grown NWs is shown

in Fig. 1(a). It shows most of the NWs are wedge-shaped

with a slim root and sturdy head, indicating that NW growth

takes place not only axially but also radially. All NWs have

lengths around 1 lm and diameters between 150 and200 nm.

The device structure sketch is shown in Fig. 1(b). As a

working device, a single NW needs to be in good contact

with two electrodes [shown in Fig. 1(c)]. So far, different

theoretical methods have been developed for quantum trans-

port simulations in AlN/GaN double barrier heterostruc-

ture.20 To better understand the tunneling mechanism, a

one-dimensional self-consistent Schrodinger Poisson model,

coupled spontaneous and piezoelectric polarization effects,

was performed using software SILVACO ATLAS, to simulate the

band diagram of AlN/GaN double barrier heterostructure.

Figure 2 shows the conduction band edge (Ec), valence band

edge (Ev), and first two quasi-bound state energy levels with

their corresponding wave functions in the quantum well

along the c-axis growth direction. The simulated first two

quasi-bound state energy levels are 0.492 eV and 1.023 eV

above the Fermi level, respectively. At low biases or close to

equilibrium, both quasi-bound states are above the Fermi

level (Ef); thus, no tunneling is likely to happen. Most car-

riers are blocked by the AlN double barriers. At a certain

applied bias, the electron energy in the GaN region aligns

with the first bound state in the GaN well between the two

thin AlN barriers resulting tunneling resonance. Since the

NWs are made of polar crystalline materials (GaN and AlN),

the biasing conditions for tunneling at forward and reverse

are different due to the asymmetrical band structure caused

by the strong polarization effects.

The room temperature IV characteristics of RTDs are

shown in Fig. 3(a). These RTDs show clear NDR with an

onset voltage between 3.5 V and 4.5 V as shown in the

zoom-in views of insets in Fig. 3(a). It is important to note

that 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 voltage

scans take time to be detrapped again. Before detrapping is

completed, traps are occupied; thus, no more trap-assisted

inelastic tunneling can happen during the positive voltage

scan. Thus, the odd (bipolar) symmetry of the IVs 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 lm and diame-ters between 150 and 200 nm. (b) Schematic single NW RTD design. The NWs are transferred to a foreign Si substrate with a 100 nm thick SiO2. Ti/Al/Ti/Au

metal contacts with a 500 nm gap are defined by e-beam lithography and deposited by electron beam evaporation to form two electrodes. (c) The scanning

electron microscopy image of a working RTD device. It clearly shows a single NW cross the two electrodes as designed. The transferred NWs have lengths

around 1 lm and diameters between 150 and 200 nm.

06FA03-2 Shao et al.: Single nanowire AlN/GaN double barrier RTDs 06FA03-2

J. Vac. Sci. Technol. B, Vol. 31, No. 6, Nov/Dec 2013


evidence of resonant tunneling rather than trap-assistant

inelastic tunneling. It is worth noting that most of the RTDs

show symmetrical nonlinear IV characteristics at room

temperature. We believe this is due to the existence of a

Schottky barrier between the metal contacts and NWs. The

exponentially 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 K

with onset voltage all around 4.5 V [Fig. 3(b)]. It is also

noted that the NDR onset voltage has a clear positive shift

when temperature is decreased. This is because the decrease

of carrier density at low temperature pushes the Fermi level

farther below the first quasi-bound state in the quantum well.

Therefore, a higher bias is needed to bend the conduction

band enough to achieve energy level resonance leading to

tunneling. In the case of trap-assisted inelastic tunneling,

electrons would not have enough activation energy to fill up

traps at low temperatures, and NDR could not occur. Thus,

the RTD operation at cryogenic temperatures also supports

resonant 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 3.5 V and 4.5 V as most

RTDs have, some RTDs also show other NDR features at

lower bias. We also have observed NDR features that appear

only during the first 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 IV characteristics of a

fresh device and after it is aged. The current difference before

the onset of NDR between the two measurements is simply

the tunneling current. The exact mechanism of device

degradation after electrical stressing is unknown and under

investigation. This is also in debate for thin film GaN RTDs,

but it is widely accepted that the device degradation is a

result of high density threading dislocations in epitaxial thin

films due 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 is

that such unexpected NDR features may be associated with

the three-dimensional heterostructure contained within the

NW. Though the preferential growth direction is in the c-

axis, in the case of AlN, however, growth occurs along both

the axial and radial direction due to the minimal adatom dif-

fusion length of Al at the growth temperatures used.18,19 As

described above, the wedge shaped NW indicates that growth

occurs not only along the axial direction, but also radially.

Catalyst-free III-nitrides grown by PAMBE are well known

to exhibit radial growth. Previously, we found that GaN

radial growth is suppressed using the two-step growth method

described above. However, the original studies were confined

to shorter nanowires that were at most 200 nm in length. It is

likely that for the long growth times needed to form 1 lm

FIG. 2. (Color online) Simulated band diagram of designed AlN/GaN double

barrier hereostructure. It also includes the first two quasi-bound states and

electron wavefuctions within the AlN/GaN quantum well in the conduction

band at equilibrium.

FIG. 3. (Color online) (a) Room temperature IV characteristics of NW

AlN/GaN double barrier RTDs. The NDR feature appears during bipolar

voltage scan. (b) Cryogenic IV characteristics of NW AlN/GaN double bar-

rier RTDs. It shows a positive NDR onset voltage shift when temperature

goes down. Zoom-in views of NDR features are shown in the insets.


JVST B - Microelectronics and Nanometer Structures


long wires, radial growth of GaN must be occurring, which

differs from recent observation of Fernandez-Garrido.21 The

resulting three-dimensional heterostructure is schematically

drawn in Fig. 4(b). The growth rate ratio between axial and

radial growth is over 10. In other words, 1.5 nm thick axial

AlN barrier growth results a core shell growth of AlN in a

range of few angstroms or less, especially at the foot of nano-

wires. We propose that the device degradation is due to the

alloy scattering or island scattering taking place at the bottom

surface of nanowires in the electron transport process due to

the small amount of Al deposition resulted from the Al ada-

tom diffusion.

We modeled the leakage current [thermionic emission

current, blue circles in Fig. 4(a)] in an aged device after the

NDR feature disappears, as a pair of back to back Schottky

diodes with a shunt resistor in parallel. In the equivalent cir-

cuit shown in the inset of Fig. 4(a), once the lateral voltage

drop is established at either side, the current will flow as like

in a reversely biased metal-n-GaN Schottky diode dominated

by the energy barrier. As shown in Fig. 4(a), the fitted cur-

rent is the sum of reverse saturation current, the barrier low-

ering effect under an electrical field, and the shunt current.

The fitted energy barrier is 0.25 eV and the shunt conduct-

ance Gp is 2.68 106 S. The modeled current has a verygood agreement with the experimental results. Since the

thermionic 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, but

the shunt resistance is essentially the same. This is not unrea-

sonable because the shunt path current should be going along

the path of Al poor regions or domains on the nanowire sur-

face. To verify this hypothesis, current research efforts are

being directed to develop an etching process to etch the AlN

sidewalls before device processing.

IV. SUMMARY AND CONCLUSIONS

In summary, we have demonstrated AlN/GaN double bar-

rier single NW based RTDs. IV characteristics show clear

NDR at both room temperature and cryogenic temperatures

with an onset voltage between 3.5 V and 4.5 V. NDR fea-

tures at bipolar voltage biases and at cryogenic temperatures

suggest that the electron transport in these devices is reso-

nant tunneling rather than trap-assisted tunneling. The device

degradation observed in some devices is likely caused by the

radial growth of AlN during the long growth time of nano-

wires due to the minimal adatom diffusion length of Al at

the growth temperatures.

ACKNOWLEDGMENTS

This work was partially supported by National Science

Foundation and Office of Naval Research.

1K. B. Cooper, R. J. Dengler, N. Llombart, A. Talukder, A. V. Panangadan,

C. S. Peay, I. Mehdi, and P. H. Siegel, Proc. SPIE 7671, 76710 (2010).2N. Orihashi, S. Suzuki, and M. Asada, Appl. Phys. Lett. 87, 233501(2005).

3M. Feiginov, C. Sydlo, O. Cojocari, and P. Meissner, Appl. Phys. Lett. 99,233506 (2011).

4S. Suzuki, M. Asada, A. Teranishi, H. Sugiyama, and H. Yokoyama, Appl.

Phys. Lett. 97, 242102 (2010).5C. T. Foxon et al., Phys. Status Solidi C 0, 2389 (2003).6S. Golka, C. Pflugl, W. Schrenk, G. Strasser, C. Skierbiszewski, M.

Siekacz, I. Grzegory, and S. Porowski, Appl. Phys. Lett. 88, 172106(2006).

7K. Kishino and A. Kikuchi, Phys. Status Solidi 190, 23 (2002).8S. Sakr, E. Warde, M. Tchernycheva, L. Rigutti, N. Isac, and F. H. Julien,

Appl. Phys. Lett. 99, 142103 (2011).9C. Bayram, Z. Vashaei, and M. Razeghi, Appl. Phys. Lett. 96, 042103(2010).

10A. E. Belyaev, C. T. Foxon, S. V. Novikov, O. Makarovsky, L. Eaves, M.

J. Kappers, and C. J. Humphreys, Appl. Phys. Lett. 83, 3626 (2003).11A. Dhabal, D. S. Chander, J. Ramkumar, and S. Dhamodaran, Micro Nano

Lett. 6, 280 (2011).12W. Chu, H. Chiang, C. Liu, Y. Lai, K. Hsu, and H. Chung, Appl. Phys.

Lett. 94, 182101 (2009).13B. Chitara, D. S. I. Jebakumar, C. N. R. Rao, and S. B. Krupanidhi,

Nanotechnology 20, 405205 (2009).14M. Dragoman et al., Appl. Phys. Lett. 96, 053116 (2010).15S. D. Carnevale, C. Marginean, P. J. Phillips, T. F. Kent, A. T. M. G.

Sarwar, M. J. Mills, and R. C. Myers, Appl. Phys. Lett. 100, 142115(2012).

16R. Songmuang, G. Katsaros, E. Monroy, P. Spathis, C. Bougerol, M.

Mongillo, and S. De Franceschi, Nano Lett. 10, 3545 (2010).

FIG. 4. (Color online) (a) Comparison of IV characteristics of a fresh device and after NDR feature disappears. The inset is an equivalent circuit for modeling

of thermionic emission current. The modeled thermionic emission current is a sum of reverse saturation current with the barrier lowering effect included and

the 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).


J. Vac. Sci. Technol. B, Vol. 31, No. 6, Nov/Dec 2013


http://dx.doi.org/10.1117/12.850395http://dx.doi.org/10.1063/1.2139850http://dx.doi.org/10.1063/1.3667191http://dx.doi.org/10.1063/1.3525834http://dx.doi.org/10.1063/1.3525834http://dx.doi.org/10.1002/pssc.200303376http://dx.doi.org/10.1063/1.2199445http://dx.doi.org/10.1002/1521-396X(200203)190:1%3C23::AID-PSSA23%3E3.0.CO;2-Khttp://dx.doi.org/10.1063/1.3645011http://dx.doi.org/10.1063/1.3294633http://dx.doi.org/10.1063/1.1622987http://dx.doi.org/10.1049/mnl.2011.0043http://dx.doi.org/10.1049/mnl.2011.0043http://dx.doi.org/10.1063/1.3130728http://dx.doi.org/10.1063/1.3130728http://dx.doi.org/10.1088/0957-4484/20/40/405205http://dx.doi.org/10.1063/1.3309670http://dx.doi.org/10.1063/1.3701586http://dx.doi.org/10.1021/nl1017578

17L. Rigutti et al., Nanotechnology 21, 425206 (2010).18S. D. Carnevale, T. F. Kent, P. J. Phillips, A. T. M. G. Sarwar, C. Selcu,

R. F. Kile, and R. C. Myers, Nano Lett. 13, 3029 (2013).19S. D. Carnevale, J. Yang, P. J. Phillips, M. J. Mills, and R. C. Myers,

Nano Lett. 11, 866 (2011).

20E. Warde, S. Sakr, M. Tchernycheva, and F. Julien, J. Electron. Mater. 41,965 (2012).

21S. Fernandez-Garrido, V. M. Kaganer, K. K. Sabelfeld, T. Gotschke, J.Grandal, E. Calleja, L. Geelhaar, and O. Brandt, Nano Lett. 13, 3274(2013).


JVST B - Microelectronics and Nanometer Structures


http://dx.doi.org/10.1088/0957-4484/21/42/425206http://dx.doi.org/10.1021/nl400200ghttp://dx.doi.org/10.1021/nl104265uhttp://dx.doi.org/10.1007/s11664-012-1920-1http://dx.doi.org/10.1021/nl401483e

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