Mat. Res. Soc. Symp. Proc. Vol. 743 2003 Materials Research Society L6.28.1 Efficient GaN-based Micro-LED Arrays H.W. Choi, C.W. Jeon, M.D. Dawson, P.R. Edwards 1 and R.W. Martin 1 Institute of Photonics, University of Strathclyde, Glasgow G4 0NW, UK 1 Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK ABSTRACT Highly efficient, two-dimensional arrays of parallel-addressed InGaN blue micro- LEDs with individual element diameters of 8, 12 and 20µm have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient stepheight coverage for contact metallisation, a novel scheme involving the etching of slopedsidewalls has been developed. The devices have I-V characteristics similar to those of broadarea reference LEDs fabricated from the same wafer, and give superior (3mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-led arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction. INTRODUCTION As GaN-based light-emitting diode (LED) technology grows in maturity, the focus of many research groups has shifted towards the fabrication of higher power and higher efficiency LEDs. The improvement of output power to date from these devices has been achieved via a number of techniques, including optimisation of epitaxy and processing [1], improved current spreading [2] or through resonant cavity structures [3]. The overall performance of such LEDs is, in addition, strongly dependent on the extraction efficiency of the devices. Due to total internal reflections occurring at the LED-ambient interface, as much as several tens percent of the light emitted from the active region may be confined within the device. In the case of InGaN/GaN LEDs, the novel approaches adopted to allow more light to be extracted include formation by etching of microdisks within the LEDs, to increase the overall surface area [4]. In this manner, apart from benefitting from an enhanced level of light extraction, an enhancement of quantum efficiency was also reported, attributed to micro-size effects as well as to a more efficient usage of the injected current. We investigate here further development of the pattern-etching approach, where arrays of isolated pillar-like micro-size LEDs are formed, sharing a common broad-area metallisation. We use the term parallel-addressed micro-leds for this type of device. They offer enhancement in surface area to volume ratio, and ready flexibility in number and size of emitting elements. We report on the fabrication of these devices and on their performance compared to conventional broad-area LED s fabricated from the same wafer. The major issue involved in the fabrication is the interconnection of each individual element, via metallization, as a result of the non-planar device topology. In order to overcome this difficulty involving metal step-height coverage, the sidewalls of the micro-leds are made to be non-vertical.
L6.28.2 Table I. Summary of device information and characteristics Elemental diameter (µm) Chip area (µm 2 ) 8 240000 12 240000 20 240000 Number of elements 625 (25x25) 400 (20x20) 256 (16x16) Total active area (µm 2 ) Turn-on voltage (Volts) Operation voltage @20mA 31416 7.62 7.48 45239 4.64 4.60 80425 3.12 3.49 Broad Area 240000 1 160000 3.42 3.80 DEVICE FABRICATION The micro- and broad area- devices are fabricated on an LED wafer grown on the c- plane of a sapphire substrate. The LED structure consists of a 25 nm GaN buffer layer, 3.2 µm of Si-doped GaN, a 3-period InGaN/GaN multi-quantum well (MQW) for emission at ~470nm, topped with a 0.25 µm Mg-doped GaN epi-layer. Activation of the Mg dopant was carried out by rapid thermal annealing (RTA) at 950 o C for 30sec in a N 2 ambient. Processing of the devices begins with the formation of mesa structures using inductively-coupled plasma (ICP) dry etching. The plasma comprised 30 sccm of Cl 2 and 10 sccm of Ar at a process temperature and pressure of 25 o C and 20mTorr, respectively. Operating parameters were 400W of ICP power and 200W of RIE power, yielding a dc bias voltage of 650V and an etch rate of more than 0.5 µm/min. An additional masking step was needed to form the pillar-like structure of the individual micro-led elements, whose diameters ranged from 8 to 20µm, respectively, for a series of processed samples. In order to achieve non-vertical sidewalls, an anisotropic etch recipe has been employed. The sidewalls can be made to have an inclination of 30 o to the vertical, which allows for conformal metal step-coverage of up to 3µm. Prior to metal deposition, a 40nm-thick SiO 2 layer was deposited onto the sample by electron-beam deposition, which acts as an isolation layer. Finally, the metal layers, including the spreading and pad layers, were deposited by electron-beam evaporation patterned by a lift-off procedure. The choice of metal is Ti/Al (20/200nm) for the n-contact pad and Ni/Au (30/30nm for spreading, 20/200nm for pad) for the p-contact. A premetallisation HCl treatment was applied, and the contacts were alloyed by rapid thermal annealing in air for 5 min at 550 o C. The I-V characteristics of the devices were measured with an HP4145 parametric analyser, whilst the room-temperature electroluminescence (EL) data were collected with a coupled spectrometer and cooled charge coupled device (CCD) detection system (0.4nm resolution). The light output power measurements were performed using a calibrated power meter with the Si detector (detector area 3x3mm 2 ) approximately 8mm above the device, collecting light emitted in the forward direction. An electron microprobe system was used for cathodoluminescence studies. RESULTS AND DISCUSSION Figure 1 shows optical microscope images of operating device arrays of individual element size 8µm, 12µm and20µm, respectively (Fig.1 (a)-(c)), and also a
L6.28.3 (a) (b) (c) (d) Figure 1. Optical microscope images of operating arrays of (a) 8µm, (b) 12µm and(c)20µm element diameter, and (d) optical image of cathodoluminescence under broad beam excitation of a cluster of four 12µm micro-led elements. optical image, referred to later, of a cluster of four of the 12µm devices excited under broadarea electron beam excitation (Fig.1(d)). The I-V curves (Figure 2) for the micro-led arrays and reference broad-area LEDs show that the turn-on and operating voltages increase as the size of the micro-leds are reduced. These values are summarized in Table I, as are the respective element diameters and quantities, overall areas of the respective chips and their respective total active areas. As the current spreading layer in each case covers the entire micro-led array, a smaller element will also have a smaller contact area. The contact resistance is sensitive to the actual contact area [5], which is the most likely explanation of higher voltage drops developed across the p-contact as the dimensions of the micro-leds are scaled down. Figure 2. I-V characteristics of micro-leds compared to a macro-led.
L6.28.4 (a) (b) Figure 3. (a) L-I plots of micro-led devices, and (b) active-area-normalized L-I plots for comparison between the external efficiencies of micro-leds with different dimensions. The optical output power characteristics of the micro-leds (measured in the forward direction, and note that the chips are unencapsulated) as a function of current are plotted in Figure 3(a). In passing, we note that the gradient of the broad-area LED plot decreases at a slightly faster rate than the micro-leds at higher injection currents, implying that the greater surface area of micro-leds offers some improvement in heat dissipation. While the different devices appear to emit similar amounts of light, their total active emission area varies greatly (Table 1). For comparison purposes, the light output powers are normalized to their respective total active areas, and are re-plotted in Figure 3(b). The effect of scaling down the size of the individual elements becomes immediately apparent, with the highest power density of emission coming from the smallest devices, despite their suffering some degradation in electrical characteristics. Figure 4. EL spectra of the respective devices operated at 10mA.
L6.28.5 The light output of the devices is seen to be a strong function of the pillar size. In the recent literature, the mechanism of such enhancements in related structures has been attributed to various sources. Dai et al. [6] proposed an enhancement of internal quantum efficiency in InGaN micro-leds due to partial strain relaxation in the microstructures. However, in a Raman study carried out by F. Demangeot et al. [7] on reactive-ion etched (non-device) GaN pillars, the observation of strain relief was shown to be limited to submicron structures, which are of smaller scale than the microstructures in [6] and those considered here. Thus, an increase in light extraction efficiency appears to be the more likely cause. However, the exact mechanism is unclear from the earlier reports, particularly as the increased light output emitted in the forward direction is unaccounted for. A detailed investigations of this increase in efficiency will be reported elsewhere [8]. In short, we attribute a major cause as being the scattering of in-plane propagating light at the etched sidewalls into the forward direction. Strong evidence for this is provided by optical images of cathodoluminescence taken under broad-area e-beam excitation (Fig.1(d)), which show a bright ring of light at the periphery of each element. A contribution to this scattering may be attributed to resonant modes [9,10] supported by the micro-disk geometry, whose presence is inferred from our analysis [8] of the substructure of the electroluminescence spectral data (Fig.4) and our previous work [11]. Further contributions to the increase in extraction efficiency include reduced absorption in the semiconductor material. Since photons in micro-leds may travel shorter distances before escaping from the mesa, the likelihood of (re)absorption is also reduced. The absorption coefficient of GaN at 470nm is reported [12] to be approximately 10 3 cm -1, which correspond to an absorption length of ~10µm. Consequently, we can expect lower absorption in micro-leds, particularly those with radius of less than 10µm. CONCLUSION In summary, high-performance InGaN parallel-addressed micro-led arrays have been fabricated and characterised. The basis of the fabrication procedure is the formation of sloped sidewalls, which allows the individual elements of the device to be straightforwardly interconnected (for addressing in parallel) via metallisation. Whilst the I-V characteristics of the micro-leds suffer minor degradation as they are scaled down in size, the devices offer superior extraction of output power in the forward direction as a result of increased surface area and surface scattering. REFERENCES 1. S.Nakamura,M.Senoh,S.Nagahama,N.Iwasa,T.Yamada,T.Matsushita,H. Kiyyoku and Y. Sugimoto, InGaN-based multi-quantum-well-structure laser diodes, Jpn. J. Appl. Phys. 35, pp.l74-l76, 1996. 2. S.R. Jeon, Y.H. Song, H.J. Jang, G.Y. Yang, S.W. Hwang and S.J. Son, Lateral current spreading in GaN-based light-emitting diodes utilizing tunnel contact junctions, Appl. Phys Lett. 78, pp.3265-3267, 2001. 3. M. Diagne, Y. He, E. Makarona, A.V. Nurmikko, J. Han, K.E. Waldrip, J.J. Figiel, T. Takeuchi and M. Krames, Vertical cavity violet light emitting diode incorporating an aluminum gallium nitride distributed Bragg mirror and a tunnel junction, Appl. Phys. Lett. 79, pp.3720-3722, 2001.
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