GaN Tunnel Junction Nanowire White-Light

Sep 18, 2015 - (a) Light-current (L−I) characteristics of LEDs A, B, and C. The inset shows an optical image of LED C. (b) Relative external quantum...
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Alternating-current InGaN/GaN tunnel junction nanowire white-light emitting diodes Sharif Md. Sadaf, Yong-Ho Ra, Hieu Pham Trung Nguyen, Mehrdad Djavid, and Zetian Mi Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02515 • Publication Date (Web): 18 Sep 2015 Downloaded from http://pubs.acs.org on September 21, 2015

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Alternating-current InGaN/GaN tunnel junction nanowire

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white-light emitting diodes

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S. M. Sadaf, Y.-H. Ra, H. P. T. Nguyen, M. Djavid, and Z. Mi*

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Department of Electrical and Computer Engineering, McGill University, 3480 University Street,

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Montreal, Quebec H3A 0E9, Canada

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*

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

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ABSTRACT

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The current LED lighting technology relies on the use of a driver to convert alternating current

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(AC) to low-voltage direct current (DC) power, a resistive p-GaN contact layer to inject positive

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charge carriers (holes) for blue light emission, and rare-earth doped phosphors to down-convert blue

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photons into green/red light, which have been identified as some of the major factors limiting the

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device efficiency, light quality, and cost. Here, we show that multiple-active region phosphor-free

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InGaN nanowire white LEDs connected through a polarization engineered tunnel junction can

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fundamentally address the afore-described challenges. Such a p-GaN contact-free LED offers the

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benefit of carrier regeneration, leading to enhanced light intensity and reduced efficiency droop.

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Moreover, through the monolithic integration of p-GaN up and p-GaN down nanowire LED

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structures on the same substrate, we have demonstrated, for the first time, AC operated LEDs on a Si

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platform, which can operate efficiently in both polarity (positive and negative) of applied voltage.

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Key words: Nanowire, quantum dot, GaN, light emitting diode, tunnel junction, AC LED

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Compared to the conventional inefficient incandescent and fluorescent lighting technologies,

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LED light bulbs can, in principle, operate at an efficiency level of 100%. The current LED lighting

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technology, however, is not even close to reach this limit, which has been limited by several factors.

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First, the current LED lamps still rely on the use of phosphors to down-convert blue light into green

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and red light. Associated with this down-conversion process is an energy loss of ~30%, or more.

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Second, the performance of GaN-based LEDs has been limited by the inefficient current conduction

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of p-GaN, which typically has a resistance ~ 100 times higher than that of n-GaN1, leading to poor

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current spreading2, 3, reduced efficiency, and efficiency droop. Third, unlike conventional light bulbs,

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LEDs are low voltage devices and cannot operate on an alternating current voltage. As a consequence,

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an electrical circuit is required to convert AC power to low-voltage DC power (typically 2-4V)4, 5.

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Such a driver adds a significant level of complexity, cost, and efficiency loss to the LED devices and

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systems.

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These critical challenges can be potentially addressed by employing the scheme of tunnel

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junction and the integration with nanowire LED structures. Since the first demonstration by Esaki in

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19586, tunnel diodes and their integration with a vast number of electronic and photonic devices have

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been extensively studied. With the use of tunnel junction, the resistive p-GaN contact layer can be

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replaced by an n-GaN contact, leading to significantly reduced device resistance3, 7-10, voltage loss,

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and heating effect11, 12. Tunnel junction also enables the stacking of multiple p-n junctions/LEDs7, 11,

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13-16

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operation can be achieved at low injection current, leading to enhanced efficiency18 and reduced

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efficiency

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GaN/InGaN/GaN1, 2, 21-25, and GaN/GdN/GaN11, 26 tunnel junctions have been implemented in GaN-

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based LED structures. However, the tunneling probability has been severely limited by the difficulty

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in creating a highly doped p-region.8,

, providing the unique opportunity of repeated carrier usage11,

droop18,

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.

Various

design

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schemes,

including

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. As such, high power

GaN/Al(Ga)N/GaN8-10,

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,

Recently, it has been demonstrated that dopant 4

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incorporation can be significantly enhanced in nanowire structures, due to the much reduced

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formation energy in the near-surface region27-29. Moreover, compared to conventional GaN quantum

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well devices, GaN nanowire LEDs can exhibit significantly reduced dislocation densities and

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polarization fields.30-35 Nanowire LEDs with tunable, multi-color emission have been demonstrated,

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which can be epitaxially grown on extremely low cost, large area Si substrates.31, 36-43 In spite of these

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intensive studies, the incorporation of tunnel junction in nanowire LEDs has not been reported to our

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knowledge.

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In this context, we have investigated the incorporation of GaN/InGaN/GaN polarization-

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enhanced tunnel junction in nearly defect-free InGaN/GaN phosphor-free nanowire white LED

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heterostructures. We have demonstrated, for the first time, p-contact free InGaN/GaN nanowire

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LEDs. With the use of tunnel junction interconnect, we have shown that multi-junction phosphor-free

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nanowire LEDs can exhibit improved light intensity and reduced efficiency droop. Due to the

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repeated carrier usage, such devices operate at low current and high voltage. Moreover, through the

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monolithic integration of p-GaN up and p-GaN down nanowire LED structures on the same substrate,

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we have demonstrated, for the first time, AC LEDs on a Si platform, which can operate efficiently in

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both positive and negative polarity of applied voltage. This work offers a new avenue for realizing

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high efficiency LEDs that can operate under a large range of voltage biasing conditions and can

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possibly eliminate, or greatly simplify the electrical driver required in today’s LED lighting systems.

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Schematically shown in Figure 1a, we have first studied four types of nanowire LEDs,

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including a conventional InGaN/GaN dot-in-a-wire p-GaN up LED (LED A), single active region

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(SAR) (n=1) tunnel junction dot-in-a-wire p-GaN down LED (LED B), and multiple-active region

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(MAR) (n=3) tunnel junction dot-in-a-wire LEDs (LEDs C and D). The device active region consists

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of multiple InGaN/GaN quantum dots. Each InGaN quantum dot has a height of ~3 nm and is capped

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by ~3 nm GaN layer. Self-organized InGaN/GaN dot-in-a-wire LED heterostructures were grown on 5 ACS Paragon Plus Environment

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n-Si (111) substrates by radio frequency plasma-assisted molecular beam epitaxy (MBE) under

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nitrogen rich conditions. The substrate surface oxide was desorbed in situ at 770 oC. The growth

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conditions for Si-doped GaN nanowires included a growth temperature of 750 oC, nitrogen flow rate

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of 1.0 standard cubic centimeter per minute (sccm), forward plasma power of 350 W, and Ga beam

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equivalent pressure of 6×10-8 Torr. The InGaN quantum dots were grown at relatively low

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temperatures (~650 oC) to enhance the In incorporation into the dots. Each quantum dot layer was

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subsequently capped by a GaN layer of ~3 nm. In this experiment, 10 InGaN/GaN quantum dots were

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incorporated in each GaN nanowire for SAR devices, and 5 InGaN/GaN quantum dots were

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incorporated for MAR tunnel junction devices. During the growth of tunnel junction region, the

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substrate temperature was reduced to 650 oC. Each active region of LED C is nearly identical to that

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of LEDs A and B, with peak emission at ~ 540 nm. The multiple active regions of LED D are

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designed to emit at blue (~ 450 nm), green (~ 550 nm), and red (~ 620 nm) spectral range,

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respectively, thereby leading to phosphor-free white light emission. In order to compare the intrinsic

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performance of these devices, no electron blocking layer was incorporated in the device active region.

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The GaN/InGaN/GaN tunnel junction consists of ~12 nm Si-doped GaN (ND~5×1019 cm-3), 3 nm

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InGaN with In concentration of ~32% and 20 nm Mg-doped GaN (NA~1×1020 cm-3). The doping

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together with In concentration and thickness were carefully optimized to maximize tunneling

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efficiency. The large area nanowire LED device is schematically shown in Figure 1b.

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Shown in Figure 1c, electrons from the valence band of p-GaN tunnel into conduction band of

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n-GaN, and, as such, holes are injected into p-GaN and also into the device active region (quantum

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dots). The use of tunnel junction increases the concentration of holes in the p-GaN, thereby

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minimizing the restriction of low hole injection efficiency in wide bandgap nitride materials. The

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injected holes recombine with injected electrons from n-GaN in the active region to give rise to

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efficient photon emission. This process repeats itself through the rest of the tunnel junctions and 6 ACS Paragon Plus Environment

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stacked active regions. Consequently, a single electron injection can lead to multiple photon emission

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in LEDs C and D, due to the repeated carrier regeneration in each tunnel junction. The output power,

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energy band diagram, and electron and hole distributions of the tunnel junction integrated dot-in-a-

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wire LEDs were numerically calculated using the APSYS simulation software (see Supplementary

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Figure S1).

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The nanowires are of wurtzite crystal structure and possess N-polarity44-46. The MAR tunnel

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junction LEDs were epitaxially grown by stacking the multiple active regions with tunnel junction

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interconnects. The dot-in-a-wire LEDs exhibit excellent structural properties. Shown in Figure 2a is

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the scanning electron microscopy (SEM) image of the nanowire structures for LED C. The nanowire

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diameters are in the range of 150 nm (See Supplementary Figure S2 for SEM images of LED A and

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LED B). A 405 nm laser was used as the excitation source for the photoluminescence (PL)

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measurement of the nanowire heterostructures. Figure 2b illustrates the normalized PL spectra of

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LED C measured at room temperature, which is nearly identical to that measured from LEDs A and

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B. The peak at ~550 nm is related to the emission from the quantum dot active region; there are no

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noticeable additional peaks from the tunnel junctions. PL spectrum of LED D is also shown in the

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figure for comparison. The broad spectral linewidth (white-light emission) of LED D is due to the

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stacking of multiple active regions with different emission colors (indicated by the arrows in Figure

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2b).

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Structural properties of the tunnel junction LED heterostructures were characterized by

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scanning transmission electron microscopy (STEM), high angle annular dark field (HAADF) and

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energy dispersive X-ray spectrometry (EDXS) analysis. JEOL JEM-2100F equipped with a field

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

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For STEM and STEM-HAADF imaging, the same equipment with a cold field emission emitter

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operated at 200 kV and with an electron beam diameter of approximately 0.1 nm was used. The Ga 7 ACS Paragon Plus Environment

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and In Lα lines were used for the EDXS microanalysis. Figure 2c shows the HAADF of the MAR

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tunnel junction nanowire structure (LED C). The presence of multiple quantum dot active regions can

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be clearly identified. No noticeable extended defects or misfit dislocations were observed. It is seen

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that multiple InGaN/GaN quantum dots are positioned in the center of the nanowires, due to the

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strain-induced self-organization36,44. The InGaN thickness is ~ 3 nm in each tunnel junction, shown in

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Figure 2d. To further confirm compositional variations of the tunnel junction and active region,

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EDXS analysis was performed. The signal variations of Ga, In, and N across different tunnel

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junctions and the quantum dot active region along the growth direction of LED C are displayed in the

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lower panel of Figure 2d. Clear In peaks reveals the existence of the InGaN layer in each tunnel

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junction. Also we observed regular peaks and dips of In and Ga throughout the quantum dot region.

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The EDXS elemental mapping image of the tunnel junction region, illustrated in Figure 2e, further

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provides unambiguous evidence for the presence of GaN/InGaN/GaN tunnel junction.

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The nanowire LED fabrication process included the following steps. First, a polyimide resist

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layer was spin-coated to fully cover the nanowires, followed by O2 plasma etching to expose the

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nanowire top surface. Thin Ni (8 nm)/Au (8 nm) and Ti (20 nm)/Au (120nm) metal layers were then

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deposited on the nanowire surface and the backside of the Si substrates to serve as p- and n-metal

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contacts, respectively. For the tunnel junction LEDs, thin Ti (8 nm)/Au (8 nm) was deposited on the

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nanowire top surface to serve as the n-metal contact. Subsequently, a 150 nm indium tin oxide (ITO)

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layer was deposited to serve as a transparent electrode and current spreading layer. The fabricated

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devices with metal contacts were annealed at ~500 oC for 1 min in nitrogen ambient, and the

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complete devices with ITO contacts were annealed at 300 oC for 1 hour in vacuum.

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Figure 3a shows typical current-voltage (I-V) characteristics of the three sets of devices,

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including conventional LED A, SAR tunnel junction LED B, and MAR (n=3) tunnel junction LED

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(LED C). The mesa size of the measured device is 500 × 500 µm2. The turn-on voltages of the 8 ACS Paragon Plus Environment

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devices approximately scale with the number of active regions (n). The measured voltage drop at 20

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mA for LEDs A, B and C are 5.5 V, 4.9 V, and 16 V, respectively. We also modeled these tunnel

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junction integrated SAR and MAR dot-in-a-wire LEDs. The simulation results are consistent with the

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experimental results (Supplementary Figure S4a). Figure 3b shows the temperature-dependent I-V

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curves for the multiple-stacked devices (LED C). There are no significant changes in the I-V

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characteristics, further confirming that the tunnel junction interconnect functions well even at low

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temperature. It is also worthwhile mentioning that the relatively high turn on voltage for the

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presented single junction nanowire LEDs is partly related to the non-uniform current injection due to

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variations of the height of the spontaneously formed nanowire arrays as well as the presence of a

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SiNx layer at the nanowire-Si interface.47-50 Compared to LED A, LED B shows reduced turn on

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voltage, due to the efficient hole injection with the incorporation of tunnel junction and lower contact

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resistance for n-GaN.

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Figure 4a shows the light intensity vs. current characteristics of the devices.

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Electroluminescence (EL) spectra of the LEDs are shown in Supplementary Figure S3. An optical

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image of the device is illustrated in the inset of Figure 4a, showing green light emission. Compared to

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the reference LED A, SAR tunnel junction dot-in-a-wire LED B shows higher light intensity owing

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to the better current spreading through the low resistant n-GaN contact, and also the improved hole

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injection inside the dots19, 21. MAR tunnel junction dot-in-a-wire devices (LED C) shows significantly

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enhanced light intensity compared to the SAR tunnel junction devices (LED B), due to the repeated

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carrier regeneration at each tunnel junction and the resulting multiple opportunities for radiative

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recombination. These measurements are consistent with the simulated results (Supplementary Figure

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S4b). Figure 4b shows the external quantum efficiency (EQE) trend of the measured devices under

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pulsed biasing conditions (10% duty cycle). All of the devices show efficiency droop at high

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injection current. It is worthwhile mentioning that no electron blocking layers were incorporated in 9 ACS Paragon Plus Environment

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the device active regions.

The intriguing part of the result is that, for LED C, the current

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corresponding to the maximum efficiency point is almost identical to LED A and LED B, suggesting

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that MAR tunnel junction LED C can be operated at a higher output power while maintaining the

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same level of efficiency loss of the conventional LEDs. This can be better illustrated in the variations

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of EQE vs. input power, shown in the inset of Figure 4b. It is seen that efficiency droop occurs at

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higher input power for LED C, compared to LEDs A and B. An output power of a few mW was

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measured for the MAR tunnel junction LED C under an injection current of 350 mA. Recent studies

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have shown that the output power of axial nanowire LEDs can be drastically enhanced by

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incorporating a large band gap AlGaN shell to minimize nonradiative surface recombination and

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increase carrier injection efficiency.37, 51

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In general, stacking multiple quantum wells/dots in planar structures is not a suitable route to

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realize low current, high voltage operation since it also significantly increases the densities of defects

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and dislocations. Such issues can be fundamentally addressed in tunnel junction nanowire LED

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structures, as demonstrated in this work. Moreover, such MAR tunnel junction nanowire LEDs can

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be designed to operate in a broad wavelength range, leading to phosphor-free white light emission. In

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LED D, the three interconnected active regions consist of blue, green, and red-emitting quantum dots,

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respectively, shown in Figure 2b. The HAADF TEM image and EDXS line scan are shown in

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Supplementary Figure S5. Figure 5a shows the EL spectra. Three distinct peaks positioned at around

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445 nm, 570 nm, and 625 nm can be clearly measured. The resulting strong, nearly white-light

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emission is shown in the inset of Figure 5a. No noticeable wavelength shift was measured with

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increasing injection current. SEM image, I-V and L-I characteristics are shown in Supplementary

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Figure S6. The device emission characteristics are illustrated in the CIE diagram, shown in Figure 5b.

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The correlated color temperature is ∼ 3000 K. The x and y values stay nearly constant in the range of

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0.47 and 0.49 for injection currents varying from 20 mA to 250 mA. 10 ACS Paragon Plus Environment

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Schematically shown in Figure 6, we have further demonstrated, for the first time, AC LEDs

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consisting of p-GaN up and p-GaN down tunnel junction dot-in-a-wire structures monolithically

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grown on the same Si chip using the technique of selective area growth. This AC operated nanowire

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LED array emits green light at positive and negative polarity of AC voltage. For the selective area

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growth of AC nanowire LEDs, a 100 nm SiO2 was first deposited on Si substrate by plasma enhanced

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chemical vapor deposition (PECVD). Various openings with sizes from 300 × 300 µm2 to 50 × 50

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µm2 were then defined on SiO2/Si substrate by standard photolithography and wet etching process.

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Conventional p-GaN up InGaN/GaN nanowire LEDs was first grown. Subsequently, the sample was

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soaked in BOE solution to selectively remove SiO2 and also nanowires on SiO2. Prior to the second

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growth step, a SiO2 layer was deposited onto the previously grown p-GaN up nanowire LEDs to

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protect these nanowires. The second reverse polarity p-GaN down nanowire LEDs were then

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selectively grown on the opening areas. Tunnel junction connects the p-GaN layer with the n-type Si

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substrate, thereby enabling reverse polarity LED operation. The selective area growth process is

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illustrated in Figures 6a, b, and c. SEM images of the nanowire devices grown on Si (111) substrates

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are also shown in Figure 6d (also Supplementary Figure S7 for SEM images of p-GaN up and p-GaN

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down AC LED devices). The electrical measurements were done with a frequency of 20-60 Hz at a

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peak to peak voltage of 10 V using an Agilent 6812B AC power source. Shown in Figure 6e, both

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devices emit green color under AC operation (also see the Supplementary Video). The presence of

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discrete electroluminescence spots is directly related to the non-uniform current injection, due to

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variations of nanowire height in the two-step selective area growth process. Highly uniform light

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emission can be achieved by further optimizing the growth and fabrication conditions. The flickering

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effect can be minimized by using nanowire LED arrays with nearly identical performance and by

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operating at relatively high frequency. At a frequency of 60 Hz, these devices are almost flicker free,

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and are suited for practical applications. These devices can also be operated independently by DC

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bias as shown in Supplementary Figure S8.

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In summary, we have developed tunnel junction nanowire LEDs that can eliminate the use of

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resistive p-GaN contact layers, leading to reduced voltage loss and enhanced hole injection.

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Moreover, by using tunnel junction interconnect, we have demonstrated multiple-active-region

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nanowire LEDs with significantly enhanced light intensity. We have also realized AC operated

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nanowire LEDs on a Si platform. Compared to the current quantum well LEDs, the demonstrated

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tunnel junction nanowire LED technology enables phosphor-free white emission and reduced

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efficiency droop. Moreover, it offers extreme flexibility in the operation voltage and can completely

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eliminate the use of an AC/DC converter required in conventional LED lighting technologies, thereby

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leading to reduced cost and further enhanced efficiency.

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ACKNOWLEDGMENT

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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. The authors would like to express special thanks to Dr. A.

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Connie and Mr. J. Kang for CIE and AC LED measurements and also many useful discussions over

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the course of this study. We would like to acknowledge CMC Microsystems for the provision of

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products and services that facilitated this research, including Crosslight simulation package,

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fabrication fund assistance using the facilities of McGill Nanotools Microfab.

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SUPPORTING INFORMATION

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Simulation of tunnel junction nanowire LED devices, simulated I-V and L-I curves of tunnel junction

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nanowire LED devices, Figures S1−S8, supporting video. This material is available free of charge

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via the Internet at http://pubs.acs.org.

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COMPETING FINANCIAL INTEREST

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The authors declare no competing financial interests.

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Onushkin, G. A.; Lee, Y. J.; Yang, J. J.; Kim, H. K.; Son, J. K.; Park, G. H.; Park, Y. IEEE

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Zhang, K.; Liang, H.; Liu, Y.; Shen, R.; Guo, W.; Wang, D.; Xia, X.; Tao, P.; Yang, C.; Luo,

Zhang, K.; Liang, H.; Shen, R.; Wang, D.; Tao, P.; Liu, Y.; Xia, X.; Luo, Y.; Du, G. Applied

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FIGURE CAPTIONS

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Figure 1. Schematic and energy band diagram of tunnel junction (TJ) integrated dot-in-a-wire

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LEDs. (a) Schematic illustration of the dot-in-a-wire LEDs, including LEDs A, B, C and

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D. (b) Schematic illustration of the fabricated large area nanowire LEDs. (c) Simulated

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energy band diagram of GaN/InGaN/GaN tunnel junction showing carrier regeneration

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and injection process under reverse bias.

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Figure 2. Structural characterization by SEM, TEM and photoluminescence. (a) A 45˚ tilted SEM

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image of MAR TJ dot-in-a-wire LED. The scale bar denotes 1 µm. (b) Room temperature

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photoluminescence spectra of LED C and LED D. (c) HR-TEM of MAR TJ dot-in-a-wire

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LED structure. (d) High angle annular dark field (HAADF) image showing the MAR

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tunnel junction dot-in-a-wire LED structure with the presence of tunnel junctions and

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quantum dot active regions. EDXS line profile along c-axis (growth direction) showing In

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peaks in the tunnel junctions and Ga dips and In peaks in dot region (lower panel). (e)

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EDXS elemental mapping image of tunnel junction region showing the In, Ga, and N

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variations. Scale bar represents 30 nm.

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Figure 3. Current-voltage (I-V) characteristics of tunnel junction dot-in-a-wire LEDs. (a) I-V

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characteristics of LEDs A, B, and C measured at room temperature. (b) Temperature-

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dependent I-V characteristics of LED C.

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Figure 4. Electroluminescence characteristics of tunnel junction dot-in-a-wire LEDs. (a) Light-

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current (L-I) characteristics of LEDs A, B, and C. (b) Relative external quantum

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efficiency (EQE) of LEDs A, B, and C. The inset shows the relative EQE comparison for

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the three LEDs as a function of input power measured at room temperature.

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Figure 5. Phosphor-free white light emission of tunnel junction MAR dot-in-a-wire LEDs. (a)

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Electroluminescence spectra LED D measured under different injection currents. The 18 ACS Paragon Plus Environment

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inset shows the device optical image. (b) Correlated color temperature (CCT) properties

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of the device.

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Figure 6. Alternating current tunnel junction dot-in-a-wire LED arrays. (a) Two-step selective area

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growth of p-GaN up and p-GaN down AC nanowire LEDs on Si substrate. p-Up

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nanowire LED arrays were first grown on the opening areas of SiOx coated Si substrate.

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Then the SiOx and the nanowires on top were selectively removed using chemical

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etching. The p-up nanowire LED structures were then covered with SiOx and additional

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opening areas were created prior to the growth of the p-down nanowire LED structures.

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Subsequently, the SiOx and the nanowires on top were selectively etched. This leads to

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the formation of p-up and p-down nanowire LED arrays on the same Si chip. (b), (c)

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Device schematics. (d) 45° tilted SEM images of as grown p-GaN up and p-GaN down

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nanowire LED structures. (e) Optical image of green light emitting nanowire LED arrays

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on Si under AC biasing conditions.

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Figure 2.

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Figure 6.

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