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Abstract
Discovering ways to increase the LED light extraction efficiency (LEE) should help create the largest performance improvement in the power of UV AlGaN LEDs. Employing surface roughening to increase the LEE of typical AlGaN UV LEDs is challenging and not well understood, yet it can be achieved easily in AlGaN LEDs grown on SiC. We fabricate thin-film UV LEDs (~294-310 nm) grown on SiC—with reflective contacts and roughened emission surface—to study and optimize KOH roughening of N-face AlN on the LEE as a function of roughened AlN pyramid size and KOH solution temperature. The LEE increased the most (2X) when the average AlN pyramid base diagonals (d) were comparable to the electroluminescence (EL) wavelength in the AlN layer (d ~λEL; 42–52 pyramids/µm2), but the LEE enhancement diminished when d was much larger than λEL (d ~5.5λEL; 2–3 pyramids/µm2). The UV LEDs had a 10 nm p-GaN contact layer, and the forward voltage was ~6 V at ~8 A/cm2, with a voltage efficiency (VE) of ~70%. The VE of the LEDs did not change after KOH roughening. This work suggests important implications to increase the LEE of AlGaN LEDs.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultraviolet light-emitting diodes (UV LEDs) have a wide range of potential applications including: curing systems, novel water- and surface-disinfection systems, DNA and protein assays, instrumentation, and sterilization of critical-care patients rooms [1–4]. AlGaN UV LEDs are expected to replace mercury lamps because UV LEDs are mercury-free, smaller, have higher disinfection efficacy, operate at a lower voltage, have simpler driving circuits, provide higher brightness, and can achieve higher power densities. Although the power conversion efficiency (PCE) (also known as “wall-plug-efficiency”) of typical, low-pressure mercury lamps is 20–30%, the surface power density of these cylindrical lamps is very low (~0.6 mW/mm2) [4]; by comparison, a UV LED with 0.01% PCE can deliver more than 0.6 mW/mm2 at typical operating currents; at 1% PCE, it can deliver more than 60 mW/mm2; at 10% PCE, the surface power density of UV LEDs is 1000-fold higher than that of low-pressure mercury lamps. However, the PCE of commercial UV LEDs still needs to improve beyond a single-digit percentage to improve LEDs’ reliability, increase their lifetime, and simplify heat sinking.
AlGaN LEDs’ power efficiency is limited by light extraction efficiency (LEE) [5–13]. Higher LEE of deep UV LEDs might be realized by focusing on developing an AlGaN thin-film flip chip (TFFC) structure with optimized roughened surfaces and high effective p-contact reflectivity. One advantage of growing AlGaN UV LEDs on SiC is the processing of LEDs into thin-film architectures [14–16]. Moreover, AlN can be grown on SiC with low threading dislocation density (TDD < 109 cm−2) using metal-organic chemical vapor deposition (MOCVD) [17,18] and molecular beam epitaxy (MBE) [19–22]. The TFFC geometry results in higher LEE of oblique TM emissions from the AlGaN quantum well than bulk flip-chip (FC) LEDs [8,23–25], as well as higher surface brightness than bulk FC UV LEDs [26]. For example, Ryu et al.’s. FDTD calculations [8] indicate that the LEE of TM emission in roughened thin-film LEDs is approximately six times higher than in bulk AlGaN FC-LEDs. Furthermore, the lateral TFFC LEDs avoid light losses through top n-contacts (in vertical injection n-contact TFFC) and wire-bonding wires.
N-face GaN roughening in aqueous KOH is very slow (without above-bandgap assistance, etch rate is less than 3 nm/min in 1 M KOH at room temperature) [27]. Guo et al. used photoelectrochemical (PEC) etching to accelerate GaN roughening in KOH by ~100X. Thin-film LEDs architecture is widely used in the blue and infrared LED literature; however, this technique has been largely underutilized in AlGaN UV LED research. This is partly because it is harder to make AlGaN thin-film LEDs [28,29], but it is also challenging to PEC etch N-face AlN in KOH at 210 nm because of the low density of the light source and absorption by air and water. However, at room temperature, and without above bandgap-assistance, N-face AlN etches in KOH at 600-1000 nm/min, which is similar to the etch rate of PEC etching of N-face GaN in KOH, and is sufficiently faster than the Al etch rate in dilute aqueous KOH (50-100 nm/min; Al is necessary to fabricate a reflective p-contact).
There have been a few attempts at creating TFFC LEDs with laser liftoff of the buffer layer [28–34]. However, KOH roughening as a means to improve LEE in UV-C LEDs, have largely been overlooked. For example, researchers have yet to demonstrate enhanced LEE by KOH roughening in AlGaN LEDs grown homoepitaxially on AlN substrates [10,35,36]. Two novel studies showed roughening of the N-polar surface but it resulted in either device degradation, emergence of a parasitic peak, and current leakage in a 325 nm LEDs [29] or it resulted in small improvements in LEE in a 343 nm LEDs [28]. We recently reported on the optimization of a highly selective SF6 plasma etch to remove the SiC substrate and expose a pristine AlN, and we demonstrated TFFC deep UV LEDs (278 nm LEDs) [16]. However, we did not study the impact of KOH temperature on LED LEE and pyramid density and size. This topic have important implications for optimizing the LEE of both: AlGaN TFFC LEDs and AlGaN LEDs grown on AlN and sapphire substrates.
In this paper, we studied the impact of the dilute KOH temperature on the AlN surface hexagonal pyramid dimensions, density and the LED LEE enhancement after KOH roughening. Rigorous ray tracing simulations predicted that the impact of submicron pyramids dimensions on LEE was negligible [37]; however, our results show that the LEE increased the most (2X) when the average AlN pyramids base diagonal (d) was comparable to the electroluminescence (EL) wavelength inside AlN (d ~λEL), but the LEE enhancement diminished when d was much larger than λEL (d ~5.5λEL). The LEE of the TFFC LEDs (294-310 nm) increased after KOH roughening without a penalty to the current-voltage (I-V) characteristics or voltage efficiency (VE).
2. Experimental and device description
AlGaN LEDs were grown by MOCVD (TNSC SR-4000HT MOCVD) on the Si-face of a c-plane (0001) SiC substrate. The atomic force microscopy (AFM) of the SiC substrate (SiCrystal Ag) is shown in Fig. 1(a) and the RMS roughness was ~1 nm. The MOCVD precursors were ammonia, trimethylgallium, and trimethylaluminum. Disilane and bis(cyclopentadienyl) magnesium (Cp2Mg) were used for AlGaN n- and p-doping, respectively.
Figure 1(b) shows the LED structure, which consisted of six layers: an AlN buffer layer (700 nm), n- Al0.49Ga00.51N (1200 nm grown at 1175 °C), a 3x-MQWs (grown at 1175 °C): 3x (Al0.49Ga0.51N/Al0.36Ga0.64N), Al0.45Ga00.55N:Mg (70 nm grown at 1050 °C), and p-GaN:Mg (10 nm; AFM is shown in Fig. 2). The AlN buffer layer was grown on SiC substrates (70 nm at 1100 °C/600 nm at 1200 °C); further details are reported elsewhere [17,38].
After the growth, the LED was annealed at 900 °C for 3 min under N2 to activate the Mg-doped p-GaN and AlGaN:Mg. The samples were then dipped in boiling aqua regia (HCl:HNO3 (3:1)) at 120 °C for 3 × 10 min to remove any oxide that formed on top of the p+-GaN. The LEDs’ mesas were etched with RIE (BCl3/SiCl4) to access the n-AlGaN layer. The samples were then dipped in DI water for 1 min and in HF for 30 sec to remove etch-residue. The n-contacts were deposited by electron beam evaporation (Ti/Al/Ni/Au) (10/100/100/300 nm) and annealed at 850 °C under a nitrogen flow for 30 sec, resulting in ohmic contacts. After annealing, Ti/Au (10/700 nm) was deposited on the n-contacts to provide soft and un-alloyed Au for bonding. Circular transmission line model (CTLM) measurements were performed to estimate the n-contacts’ specific resistivity. The p-contacts (0.0129 mm2) were deposited by electron beam (Ni/Al/Ni/Au) (2/100/100/1000 nm). Further details of the LED structure are given in Table 1. The n- and p-type contacts were aligned and bonded to a new carrier substrate by Au-Au thermocompression bonding; the new carrier substrate was prepared by patterning n- and p-pads (Ti/Au, 20/1000 nm) on a highly thermally conductive substrate (n-type SiC substrate covered with 100 nm of low-stress SiNx). After the bonding, the SiC growth substrate was thinned to 80 µm and subsequently removed by a highly selective SF6 plasma etch. The details of the thinning and the highly selective SF6 etch of SiC over AlN are described elsewhere [16].
Table 1. Summary of the structure of the UV LEDs (294-310 nm)
A schematic cross-section image of a TFFC UV LED with surface roughening is shown in Fig. 3. Figure 4(a) shows a micrograph of the LED’s n-contact (0.019 mm2) and p-contact (0.013 mm2) before FC bonding. Figure 4(b) shows a micrograph of the TFFC LED after complete SiC substrate removal. Figure 4(c) shows an SEM image of a packaged TFFC UV LED. The freestanding thin-film AlN/n-AlGaN surrounding the LED was under tensile stress (concave-up thin-film). The excess suspended thin-film does not impact the operation of the LED and can be removed via an optional “haircut” lithography by etching a larger and deeper mesa around the LEDs mesa (into ~70% of the AlN thickness)—before FC bonding.
Fig. 3 Schematic cross-section of TFFC UV LED with surface roughening (not to scale with the actual LED dimensions).
Fig. 4 (a) A micrograph of the LED’s n-contact (0.019 mm2) and p-contact (0.013 mm2) before FC bonding. (b) A micrograph of a thin-film flip-chip (TFFC) TFFC UV LED after substrate removal. (c) SEM image of a packaged UV AlGaN TFFC LED (294-310 nm).
We packaged the TFFC UV LEDs with TO-39 headers and measured them in a small integrating sphere (ISP 75, Instrument Systems GmbH) using a spectrophotometer (MAS 40 Mini-Array Spectrometer, Instrument Systems GmbH). The packaged LEDs were roughened in dilute KOH solution (0.25 M) at three temperatures (3.5 °C, 25 °C, 75 °C), and we observed the impact of KOH solution temperature on the pyramid dimensions and the LEE of the LEDs (refer to Table 2).
Table 2. Summary for TFFC LED LEE enhancement after KOH roughening at different temperature. The LEE enhancement depends on pyramid density, dimensions and AlN/AlGaN etch depth.
3. Results and discussion
To study the impact of KOH solution temperature on LEE and AlN pyramid dimensions, we tested several LEDs at three temperatures (3.5 °C, 25 °C, and 75 °C). Larger pyramids resulted in less LEE than smaller pyramids when the pyramid diagonal was comparable to the emitted light wavelength in AlN (refer to Table 2 and Fig. 5). After KOH roughening, the light output and the LEE increased by 1.8X at 3.5 °C (refer to Fig. 5(a)), 2X at 25 °C (refer to Fig. 5(b)), and 1.15X at 75 °C (refer to Fig. 5(c)).
Fig. 5 L-I curves for UV TFFC LEDs shows the impact of KOH (~0.25 mol/L) roughening on TFFC UV LED (~300 nm) light power, before and after roughening. The LEE enhancement after KOH roughening is shown in blue stars on the right ordinate axis. (a) At 3.5 °C KOH temperature, the LEE enhancement after KOH roughening was ~1.8X. (b) At 25 °C KOH temperature, the LEE enhancement after KOH roughening was ~2X. (c) At 75 °C KOH temperature, the LEE enhancement after KOH roughening was ~1.15X.
In every LED we tested, in this study and elsewhere with thicker AlN [3], the marginal increase in the LEE enhancement reduced as the AlN etch depth (time) increased—well before etching most of the AlN. For example, Fig. 5a shows that the LEE enhancement was ~1.8X after KOH roughening for 88 sec and beyond (we show additional data at 119 sec and 129 sec). In Fig. 5b, the LEE enhancement after KOH roughening for 15 sec and beyond (we show additional data at 20 sec). We present data after 26 sec of etching to show an example of what could happen if KOH started etching into the n-AlGaN, affecting its sheet resistance, as well as, etching into the active region and making leakage pathways.
KOH etching formed random hexagonal pyramids that roughened the N-face AlN surface, leading to increased LEE due to favorable scattering-geometry for light extraction [37] and a reduction in the effective refractive index of AlN [39]. The hexagonal pyramids are defined by {} [40] due to the etching rate ratio between these planes and the other AlN crystal planes facets (refer to Fig. 6).
Fig. 6 The effect of KOH temperature on AlN pyramid densities, average diagonal length (d) and the LEE. Surface roughening by KOH increased LEE. The LEE enhancement was ~2X at 25 °C and ~1.15X at 75 °C. The SEM images show that the KOH-etched surface has random hexagonal pyramids bound by {} facets. The relative LEE enhancement was limited to 2X because of: p-contact reflectivity (Ni/Al/Ni/Au; 2/100/100/1000 nm) and p-GaN thickness (10 nm).
Figure 6 shows the effect of dilute KOH temperature on AlN hexagonal pyramid densities, average base diagonal length (d), and the LEE. The pyramids density was ~50 pyramids/µm2 at 25 °C which was ~20X larger than the pyramid density at (~75 °C). This result suggests that the pyramids (etching hillocks) are not controlled solely by threading dislocations. Moreover, the KOH solution temperature impacts the ratio between the net lateral and vertical etch rates which affect the pyramids density and height.
The LEE was enhanced by 2X by roughening in KOH at room temperature (~25 °C) with a pyramid density of 42–52 pyramids/µm2 (d ~190–170 nm ~λEL), but only by 1.1X at 75 °C with a pyramid density of around 2–3 pyramids/µm2 (d ~880–715 nm ~5.5λEL)—as shown in Fig. 6. The LEE was larger when the pyramid diagonal was comparable to the emitted light wavelength in AlN (λEL ~142 nm if the refractive index of AlN at 297 nm is 2.1) than when λEL << d. These results suggest that LEE-enhancing structures fabricated by dry etching, need to be comparable to the emitted UV light wavelength. Inoue et al. [36] demonstrated a 2X relative enhancement in bulk FC UV LEDs by fabricating submicron circular AlN cones on the back side of the AlN substrate using dry etching and nanoimprint lithography, and we observe that the optimal structure dimensions in Inoue et al. [36] (submicron cones with d ~240 nm) were also comparable to the emitted UV light wavelength in AlN.
Although ray tracing simulations for TE polarized emission indicate that larger pyramids are as efficient for light extraction as smaller pyramids [37], TM polarized emission is emitted in-plane, and the LEE could diminish due to poor light extraction at high incident angles. Moreover, ray tracing simulations do not account for the wave nature of light at subwavelength dimensions [41] and coherence effects, such as diffraction and interference [41–43].
There are two main reasons for the light extraction being limited to 2X in this paper was: p+-GaN absorption and p-contact absorption. The p-contact (2/100/100/1000 nm) reflectivity was ~60% in the 290-300 nm range. Moreover, GaN has an absorption coefficient of ~1.5 × 105 cm−1 at 275 nm [44], and p+-GaN absorption could be even higher. An absorption coefficient of 1.5 × 105–2.0 × 105 cm−1 in a total of 20 nm (2 × 10 nm) of p-GaN will cause 26–33% of absorption losses per pass. With thinner p-GaN (5 nm) and more reflective p-contact (76% at 275 nm), we reported elsewhere [16] higher relative enhancement in LEDs LEE after KOH roughening.
The I-V characteristics of the LEDs did not change before, or during, roughening, as shown in Fig. 7(a). Roughening N-face AlN is more advantageous than roughening of n-AlGaN [29] and led to increased gains in LEE without impacting the LEDs VE or I-V characteristics. Moreover, AlN has a lower refractive index than Al0.6Ga0.4N, which enables AlN pyramids to extract more light than Al0.6Ga0.4N pyramids. Also, the sheet resistance of the n-AlGaN layer did not increase nor caused a leakage. However, if the LEDs were over-etched, the LEDs became leaky diodes as the KOH etched through the AlN, AlGaN, and into the active region. With increasing forward currents, a slight redshift of the electroluminescence (EL) peak wavelength was observed (data not shown), a redshift by ~1 nm over 200 A/cm2 to 400 A/cm2. This result indicates good thermal performance and uniform Au-Au bonding at 300 °C.
Fig. 7 (a) J-V curve of a 294–295 nm TFFC LED under DC operation shows the impact of KOH (at 25 °C) roughening on J-V characteristics. (b) LI curve shows the impact of (0.25 M) KOH roughening (at 25 °C) on TFFC LED light power, before roughening. (c) Voltage efficiency (VE) of a 297 nm LED.
The resistance for a typical UV LED was ~3 mΩ cm−2 (24 Ω), and the resistance was dominated by the p-contact resistance (~11.6 Ω), n-contact resistance (~2.6 Ω), and p-AlGaN series resistance (~10 Ω). Because the n- and p- contacts area of the LEDs were relatively small (refer to Fig. 3(a)), their excess voltage contribution to the LEDs was high (11% from the n-contact resistance, 48% from the p-contact resistance, and 41% from the AlGaN:Mg resistance; refer to Fig. 8). The n-contacts were ohmic, with specific contact resistance of 5 × 10−4 Ω cm2. Replacing the Ti-based n-contacts with V-based n-contacts [45,46] will make n-contact resistance contribution to voltage negligible. The p-contact specific resistance the ultrathin 10 nm p-GaN (AFM showed 3D island growth, and is shown in Fig. 2) was estimated to be 1.5 × 10−3 Ω cm2. Increasing the LED and p-contact area will reduce the p-contact resistance contribution.
Fig. 8 Estimate of series resistance (excess voltage) contribution to TFFC LED (p-contact area is 0.013 mm2). Current spreading in the n-AlGaN layer was uniform.
The series resistance of the LED limits the VE, which is defined as the ratio between the photon energy and the potential energy of injected electrons, which corresponds to the ratio between the PCE and EQE. The VE for state-of-the-art UV LEDs (275 nm) ranges from 47%–70% at 20 mA [47,48], whereas for the best commercial blue LEDs, VE approaches 95% [49] and 99% [50]. Figure 7(a) shows LEDs with a forward voltage of ~6 V at 7.8 A/cm2 (1 mA), and Fig. 7(b) shows that the VE of 56% at 45 A/cm2; the VE was ~70% at 20 A/cm2. As a result of the LED series resistance, the voltage drop at higher current density was high; for example, in Fig. 7(a), 6.4 V at 20 A/cm2. There is a tradeoff between the VE, which characterizes voltage losses in contacts and LED layers, and LEE, which characterizes light losses in contacts and LED layers, as it is challenging to achieve low contact resistance with reflective contacts; however, we show that the TFFC LED approach can result in good VE and LEE.
Although we conducted the demonstration on LEDs of ~0.013 mm2, the LED area is scalable to larger LED areas (> 1 mm2) [51] that have lower p-contact resistance. The TFFC LED luminous flux directly scales with the thin-film LEDs’ emitting area [52].
4. Summary
KOH roughening of AlN in AlGaN TFFC LEDs increased the LEE due to favorable geometry for light extraction by scattering and the reduction in the effective refractive index of AlN. We inspected the impact of dilute KOH temperature on roughening the N-face AlN surface of LEDs, by quantifying the roughening hexagonal pyramid density and dimensions. We observed that the LED LEE increased more when the average pyramid diagonals (d) was comparable to λEL in AlN than when d was much larger than λEL (d ~5.5λEL). The highest increase in LEE of the TFFC LEDs (p-contact reflectivity ~60%) was ~2X after the KOH roughening at room temperature (25 °C)—without affecting the VE. The LEDs’ forward voltage was ~6 V at ~8 A/cm2, with a VE ~70%. This work has important implications for increasing the LEE of AlGaN LEDs grown homoepitaxially on bulk AlN substrates, or for any AlGaN UV LEDs that are processed into thin-film LEDs.
Funding
King Abdulaziz City for Science and Technology (KACST) Technology Innovations Center (TIC) program; KACST-KAUST-UCSB Solid State Lighting Program; Solid State Lighting and Energy Electronics Center (SSLEEC) at UCSB; UCSB-Collaborative Research in Engineering, Science and Technology (CREST) Malaysia project; NSF NNIN network (ECS-0335765); NSF MRSEC Program (1650114); National Science Foundation Graduate Research Fellowship Program (1650114).
Acknowledgments
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and may not reflect the views of the National Science Foundation. The authors would like to thank Claude Weisbuch for useful discussions and the cleanroom staff at UCSB nanofabrication facility.
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