Semiconductor Tunnel Junction

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Monolithically Integrated Metal/Semiconductor Tunnel Junction Nanowire Light Emitting Diodes Sharif Md. Sadaf, Yong-Ho Ra, Thomas Szkopek, and Zetian Mi Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04215 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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Monolithically Integrated Metal/Semiconductor Tunnel

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Junction Nanowire Light Emitting Diodes

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S. M. Sadaf, Y. H. Ra, T. Szkopek, and Z. Mi*

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Department of Electrical and Computer Engineering, McGill University

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3480 University Street, Montreal, Quebec H3A 0E9, Canada

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*

: E-mail: [email protected]; Phone: 1 514 398 7114

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ACS Paragon Plus Environment

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ABSTRACT

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We have demonstrated, for the first time, an n++-GaN/Al/p++-GaN backward diode, wherein an

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epitaxial Al layer serves as the tunnel junction. The resulting p-contact free InGaN/GaN nanowire light

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emitting diodes (LEDs) exhibited a low turn-on voltage (~ 2.9 V), reduced resistance, and enhanced

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power, compared to nanowire LEDs without the use of Al tunnel junction or with the incorporation of

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an n++-GaN/p++-GaN tunnel junction. This unique Al tunnel junction overcomes some of the critical

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issues related to conventional GaN-based tunnel junction designs, including stress relaxation, wide

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depletion region, and light absorption, and holds tremendous promise for realizing low resistivity, high

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brightness III-nitride nanowire LEDs in the visible and deep ultraviolet spectral range. Moreover, the

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demonstration of monolithic integration of metal and semiconductor nanowire heterojunctions provides

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a seamless platform for realizing a broad range of multi-functional nanoscale electronic and photonic

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devices.

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KEYWORDS: nanowire, quantum dot, tunnel junction, GaN, light emitting diode, molecular beam

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epitaxy

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The scheme of tunnel junction has been employed in a broad range of electronic and optoelectronic

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devices. However, its application in wide bandgap devices, such as GaN and AlN based light emitting

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diodes (LEDs) and lasers has been limited. To date, it has remained challenging to form an efficient

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tunnel junction using GaN-based materials.1 Illustrated in Figure 1a is the schematic energy band

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diagram of a conventional n++-GaN/p++-GaN tunnel junction. The inefficient p-type doping in GaN and

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AlN, due to the low acceptor ionization efficiency,2 leads to a large depletion region width (wide

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tunnel barrier thickness) that inhibits efficient inter-band tunneling. In this context, the inherent

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spontaneous and piezoelectric polarization of wurtzite GaN-based heterostructures has been exploited

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to enable efficient inter-band tunneling. Several tunnel junction designs, including GaN/AlN/GaN,3, 4

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GaN/InGaN/GaN,5-11

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demonstrated. Although promising results have been achieved using these tunnel junctions in planar

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devices, it has remained challenging to incorporate these designs in emerging nanowire structures. Due

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to the inherent strain relaxation, polarization-induced sheet charge at the heterointerface of nanowire

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structures is significantly reduced. Consequently, polarization engineered tunnel junctions may exhibit

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a higher voltage drop in nanowire-based devices.9 In addition, the successful incorporation of such

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tunnel junctions by and large depends on the crystal polarity (N-face or Ga-face)8,

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reduction in tunnel barrier width over conventional tunnel junctions, the afore-described AlN/GaN and

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InGaN/GaN based tunnel junctions still suffer from comparatively low inter-band tunneling conduction

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as well as optical absorption loss.

AlGaN/InGaN,12,

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AlGaN/GaN,14,

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and GaN/InGaN9,

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have been

Despite the

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Alternatively, tunnel junction devices that incorporate rare earth materials GdN18 or semi metallic

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MnAs19 and ErAs20, 21 nanoparticles have been demonstrated under forward and reverse bias, wherein

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tunneling is enhanced by the presence of mid-gap states. In this context, we have proposed and

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demonstrated, for the first time, a monolithically integrated metal/semiconductor nanowire tunnel

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junction LED, wherein the tunnel junction consists of n++-GaN/Al/p++-GaN, schematically shown in

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Figure 1b. With a work function of 4.08 eV, Al metal can form an ohmic contact to n-GaN.22 The


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presence of defects at the Al/p++-GaN interface, partly due to the very high Mg-doping (NA~1×1020 cm-

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Al in a similar manner to conventional trap-assisted tunneling.26 As such, it is expected that the

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epitaxial Al/p++-GaN can exhibit quasi-ohmic contact characteristics.27, 28 Consequently, the effective

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tunneling barrier width is minimized, thereby enabling efficient inter-band conduction from p-GaN to

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n-GaN through the Al interconnect. The back-to-back Al quasi-ohmic/ohmic contacts to the p++ and

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n++-GaN layers eliminate the need for polarization engineering at the interface to shrink the depletion

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region for efficient tunneling. With the use of molecular beam epitaxy (MBE), we have demonstrated

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the monolithic integration of nearly defect-free GaN/Al/GaN tunnel junction LED heterostructures.

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Unique to the Al-based tunnel junction is that the Al layer, with appropriate thickness, can serve as a

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mirror to reflect light emitted from the active region29, 30, given its high reflectivity (~ 90%) in the

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visible and UV spectral range, which is in direct contrast to the light absorption induced by polarization

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engineered tunnel junctions in previous reports.6, 9 We have further shown that the incorporation of

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such tunnel junction in InGaN/GaN dot-in-a-wire LEDs can lead to significantly improved light output

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power and lower operation voltage, compared to identical nanowire devices without the use of tunnel

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junctions or with the incorporation of n++-GaN/p++-GaN tunnel junctions.

),

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results in deep energy levels, which can significantly enhance carrier transport from p-GaN to

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In this work, we have investigated three different types of InGaN/GaN nanowire LED structures,

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including a) n++-GaN/Al/p++-GaN tunnel junction LEDs (LED A), b) n++-GaN/p++-GaN tunnel junction

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LEDs (LED B), and c) conventional nanowire LEDs without the use of any tunnel junction (LED C).

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Schematics for LED A, LED B, and LED C are shown in Figure 2a. The tunnel junction of LED A

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consists of n++-GaN (7 nm), Al (2 nm), and p++-GaN (10 nm). The tunnel junction of LED B is

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identical to that of A but without the incorporation of the Al layer. The active regions of LEDs A, B,

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and C consist of ten self-organized InGaN (3 nm)/GaN (3 nm) quantum dots.31-33 Each quantum dot

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layer is modulation doped p-type to enhance the hole injection and transport in the device active

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region.32, 33 In order to examine the intrinsic properties of the tunnel junction dot-in-a-wire LEDs, no


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AlGaN electron blocking layers were incorporated. All the LED structures were grown by plasma-

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assisted MBE on n-Si (111) substrate under nitrogen-rich conditions without using any external metal

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catalyst.34, 35 Prior to the growth, native oxide on the Si substrate was removed by hydrofluoric acid

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(10%), and further in situ desorbed at ~770 °C. The N2 flow rate was kept at 1.0 standard cubic

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centimeter per minute (sccm) with a forward plasma power of ~350 W during the growth. The substrate

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temperature was ~ 780 °C for n-GaN and 750 °C for p-GaN segments. Doping concentration and

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degeneracy in the tunnel junction were controlled by the Si (n-doping) and Mg (p-doping) effusion cell

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temperatures. The Al layer was grown at ~ 450 °C and was subsequently capped with a thin layer of

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Ga. In this process, the nitrogen plasma was turned off to avoid the formation of AlN. The substrate

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temperature was then increased to 650 °C for the growth of p++-GaN (10 nm). Such optimum growth

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conditions were obtained based on extensive studies of the LED performance by varying the Al

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thickness (~ 1 to 6 nm) and growth temperature (~ 300 to 650 °C) and by changing the Si and Mg-

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doping concentrations. It was observed that a high quality pure Al metal layer could be grown in situ

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on GaN nanowires without any metal agglomeration and void formation. Moreover, detailed structural

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characterization (to be described later) further confirmed that defect-free nanowires could be grown

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directly on an epitaxial Al layer.36

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The three nanowire LED structures exhibited nearly identical photoluminescence (PL)

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characteristics. Shown in Figure 2b is the PL spectrum of LED A measured at room temperature with a

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405 nm laser excitation. A single PL emission peak at ~534 nm corresponds to emission from the

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quantum dot active region. Inhomogeneous broadening seen in the PL emission is largely due to In

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compositional variations inside the quantum dots. Structural properties of nanowire LEDs were

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characterized by field emission scanning electron microscopy (SEM). Shown in Figure 2c is a 45° tilted

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image of the Al-based tunnel junction nanowire structure (LED A).The nanowires are vertically aligned

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on the substrate, with diameters and densities in the range of 40 to 100 nm and 1×1010 cm-2,

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respectively.


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Scanning transmission electron microscopy (STEM) and high resolution transmission electron

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microscopy (HR-TEM) studies were further performed to characterize the tunnel junction thickness and

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composition. The nanowires were first dispersed on a Cu grid. A JEOL JEM-2100F equipped with a

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field emission gun with an accelerating voltage of 200 kV was used to obtain bright-field TEM images.

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For STEM and high angle annular dark field (HAADF) imaging, the same equipment with a cold field

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emission emitter operated at 200 kV and with an electron beam diameter of approximately 0.1 nm was

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used. Illustrated in Figure 3a is the STEM image of a tunnel junction nanowire structure, wherein the

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different segments are identified. It is seen that InGaN/GaN quantum dots are positioned in the center

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of the nanowires due to the strain-induced self-organization. Also, no noticeable stacking faults and

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threading dislocations were observed. Energy dispersive X-ray spectroscopy (EDXS) analysis was

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performed along the InGaN/GaN quantum dot active region. The HAADF image and EDXS analysis of

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the quantum dot active region is further illustrated in Figure 3b, showing the signal variations of Ga

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and In (dips and peaks) across the active region (quantum dots) along the growth direction (c-axis). The

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HR-TEM image of the tunnel junction is illustrated in Figure 3c. It is seen that the thickness of the Al

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layer is ~2 nm. EDXS analysis was further performed to study the compositional variations of the n++-

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GaN/Al/p++-GaN tunnel junction. An EDXS point profile taken in the vicinity of the Al-layer provided

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unambiguous evidence for the presence of Al. The thin Al layer is surrounded by relatively thick p- and

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n-GaN. Even though the beam was directed toward the Al-layer, Ga and N signals were also detected

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from the surrounding GaN layers due to the enlarged beam size. It is worthwhile mentioning that due to

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the high atomic number (atom density) of Ga compared to Al, EDXS analysis gives greater sensitivity

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to Ga. Point profile measurement away from the Al-layer (p-GaN region) shows no measureable trace

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of Al (Figure 3c).

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During the LED fabrication process, the nanowire arrays were first planarized using a polyimide

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resist layer, followed by contact metallization and thermal annealing. For LEDs A and B, Ti/Au (8

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nm/8 nm) and Ti/Au (20 nm/120 nm) layers were deposited on the nanowire surface and the backside


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of the Si substrate as the top and back metal contact layers, respectively. For LED C, the top metal

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contact consists of Ni (8 nm)/Au (8 nm). 120 nm indium tin oxide (ITO) was subsequently deposited

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on the top surface to serve as a transparent current spreading layer (see Supplementary Figure S1 for

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the step-by-step fabrication process of nanowire LEDs). Details about the nanowire LED fabrication

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process were also described elsewhere.33, 37, 38 Current-voltage characteristics of the nanowire LEDs

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were measured under continuous wave biasing conditions at room temperature. During the

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measurements, a negative bias was applied on the top surface for n-GaN up LEDs (LEDs A and B). As

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such, the LED was forward biased and the tunnel junction was reverse biased. Conversely, a positive

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bias was applied on the top surface for p-GaN up device (LED C, without the use of tunnel junction).

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Shown in Figure 4, the Al tunnel junction device (LED A) shows clear rectifying characteristics with a

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sharp turn-on voltage of ~2.9 V. The device areal size is 500 µm × 500 µm, and the nanowire filling

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factor is ~ 30%. The device specific resistivity estimated from the linear portion of the forward

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characteristics is ~4×10-3 Ωˑcm2 at 400 A/cm2 (see Supplementary Figure S2). Both the turn-on voltage

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and resistance is much smaller than that of the conventional nanowire LEDs grown and fabricated

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under similar conditions but without the use of tunnel junction (LED C). For comparison, the n++-

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GaN/p++-GaN tunnel junction LED (LED B) showed significantly larger turn-on voltage ~5.5 V and

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higher resistivity ~5×10-2 Ω.cm2, due to the wide depletion region width formed between n++-GaN and

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p++-GaN regions and the resulting low tunneling efficiency. It is also worthwhile mentioning that the

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specific resistivity of 4×10-3 Ωˑcm2 for LED A includes not only the Al tunnel junction resistance, but

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also the contact resistance and series resistance of the p-and n-GaN layers. In addition, the device

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resistance is further limited by the presence of a SiNx layer at the Si and GaN nanowire interface39-41

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and nonuniform contact to the nanowire arrays due to variations of nanowire heights.42 Taking these

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factors into account, the specific resistivity for the Al tunnel junction is estimated to be ~ 1×10-3 Ωˑcm2,

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or lower (Supplementary Note 1). Specific resistivity values in the range of 10-4 to 10-2 Ωˑcm2 have

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been previously reported in GaN-based planar tunnel junction devices

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Supplementary Table S1). Given the identical design, growth and fabrication processes for the three ACS Paragon Plus Environment

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(see

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LEDs, the significantly reduced turn-on voltage and resistance of LED A provides unambiguous

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evidence that the n++-GaN/Al/p++-GaN can serve as a low resistivity tunnel junction.

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The Al tunnel junction LED also showed significantly improved light intensity compared to the

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conventional nanowire device (LED C) and n++-GaN/p++-GaN tunnel junction device (LED B),

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illustrated in Figure 5a. These devices were measured under pulsed bias (10% duty cycle) to minimize

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junction heating effect. The significantly improved light intensity is largely due to the efficient tunnel

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injection of holes into the active region. The output spectra of LED A measured from 30 mA to 250

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mA are shown in Figure 5b. An optical micrograph of the device is shown in the inset of Figure 5b.

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Output spectra of LED B and LED C are also shown in Supplementary Information (see

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Supplementary Figure S3). The measured external quantum efficiency (EQE) is also shown in

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Supplementary Information (see Supplementary Figure S4). There was no noticeable shift in the peak

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position with increasing current. Such highly stable emission characteristics are a direct consequence of

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the highly efficient and uniform hole injection in the quantum dot active region. It is further expected

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that, by suppressing nonradiative surface recombination with the incorporation of core-shell quantum

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dot active regions35, 45, 46, the Al tunnel junction nanowire LEDs can lead to high power operation.

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In summary, we have demonstrated low resistance Al tunnel junction integrated dot-in-a-wire

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LEDs, enabling p-contact free devices with significantly improved hole injection efficiency. Compared

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to previously reported polarization engineered tunnel junctions, the presented Al tunnel junction

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completely eliminates the use of either a low band gap InGaN or a large bandgap Al(Ga)N layer in the

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tunnel junction design that often leads to undesired optical absorption and/or high voltage loss. Such an

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Al tunnel junction may also be implemented in either N-face or Ga-face III-nitride quantum well and

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nanowire LEDs. It also holds tremendous promise for applications in the emerging non-polar and semi-

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polar GaN optoelectronic devices. Moreover, the seamless integration of defect-free nanowire

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structures with various metal layers offers a unique approach for achieving high performance nanoscale

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electronic and photonic devices that were not previously possible. ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS

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This work was supported by the Natural Sciences and Engineering Research Council of Canada

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(NSERC). Part of the work was performed in the Micro-fabrication Facility at McGill University.

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Electron microscopy images and analysis were carried out at Facility for Electron Microscopy

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Research (FEMR), McGill University and at Ecole Polytechnique de Montreal. We wish to thank Jean

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Phillipe Masse for his help with the TEM studies. We also wish to thank Mr. S. Hossain in the

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Department of Mining and Materials Engineering at McGill Univ. for useful discussions. We would

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like to acknowledge CMC Microsystems for the provision of products and services that facilitated this

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research, including Crosslight simulation package and fabrication fund assistance using the facilities of

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McGill Nanotools Microfab.

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Supporting Information: This material is available free of charge via the Internet at

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http://pubs.acs.org. Additional information on the device fabrication process, specific resistance-

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current density, specific resistance summary of previous reports, electroluminescence spectra, relative

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external quantum efficiency, and device resistivity.

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FIGURE CAPTIONS

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Figure 1

n++-GaN/Al/p++-GaN tunnel junction structures.

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Schematic energy band diagram of (a) conventional p++/n++ GaN tunnel junction and (b)

Figure 2

(a) Schematic diagram of Al tunnel junction (TJ) dot-in-a-wire LED (LED A), n++-

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GaN/p++-GaN tunnel junction LED (LED B), conventional nanowire LEDs without the use

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of any tunnel junction (LED C) grown on a Si substrate. (b) PL spectra of Al TJ dot-in-a-

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wire LEDs (LED A) measured at room temperature. (c) An SEM image of Al-interconnect

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TJ dot-in-a-wire LEDs (LED A) taken with a 45° angle.

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Figure 3

(a) STEM image of a single Al tunnel nanowire LED structure (LED A). (b) HAADF (left)

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and EDXS line profile (right) across the InGaN/GaN quantum dots. (c) HR-TEM image of

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the Al tunnel junction (left) and EDXS point profiles (right) of the tunnel junction region

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and p-GaN region.

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Figure 4

I-V characteristics of the Al tunnel junction dot-in-a-wire LED (LED A), n++-GaN/p++-GaN

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tunnel junction LED (LED B), and conventional nanowire LED without the use of any

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tunnel junction (LED C).

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Figure 5

(a) L-I characteristics of the Al tunnel junction (TJ) dot-in-a-wire LED (LED A), n++-

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GaN/p++-GaN tunnel junction LED (LED B), and conventional nanowire LED without the

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use of any tunnel junction (LED C). (b) Electroluminescence (EL) spectra of the Al tunnel

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junction dot-in-a-wire LED (LED A) under pulsed biasing condition (10% duty cycle).

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Inset: the optical micrograph of the Al tunnel junction dot-in-a-wire LED (LED A) showing

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strong green light emission.

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Semiconductor Tunnel Junction

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