GaN-based blue light-emitting diodes grown and fabricated on patterned sapphire substrates by metalorganic vapor-phase epitaxy

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1 Journal of Crystal Growth 272 (2004) GaN-based blue light-emitting diodes grown and fabricated on patterned sapphire substrates by metalorganic vapor-phase epitaxy Z.H. Feng, Y.D. Qi, Z.D. Lu, Kei May Lau Photonics Technology Center, Department of Electrical and Electronic Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Available online 19 October 2004 Abstract GaN-based blue light-emitting diodes (LEDs) with InGaN multi-quantum-wells were grown on patterned sapphire substrates by metalorganicvapor-phase epitaxy (MOVPE). The patterned substrates with 60-nm-deep parallel grooves of different dimensions along /11 20S and /10 10S orientations were etched by BCl 3 /Cl 2 -based inductively coupled plasma reactive ion etching (ICP-RIE). Enhancement of the electroluminescence (EL) peak at 465 nm from LEDs grown on these grooves has been observed, in comparison with LEDs grown on unpatterned sapphire substrates. A maximum power increase of 25% was measured for LEDs grown on patterned substrate with 2 mm (ridge) 4 mm (trench) grooves along the /10 10S orientation at a forward current of 20 ma at room temperature. Stronger light emission in the trench regions of the stripes at very low drive currents has been observed using optical microscopy. Several possible mechanisms are proposed to interpret this groove-induced EL enhancement. r 2004 Elsevier B.V. All rights reserved. PACS: Gh; Cf; z Keywords: A1. Etching; A3. Metalorganic vapor phase epitaxy; B1. Nitrides; B1. Patterned sapphire substrate; B2. Semiconducting III V materials; B3. Light-emitting diodes 1. Introduction With the recent commercial success of high brightness InGaN/GaN-based light-emitting Corresponding author. Tel.: ; fax: address: eekmlau@ust.hk (K.M. Lau). diodes (LEDs) the need of even higher power and higher efficiency blue devices is imminent for the wide spread use of illumination applications of LEDs [1]. Many different approaches have been attempted to improve the overall device performance, including continuous efforts on better hetero-epitaxy on inexpensive sapphire substrates. Up to now, most III-nitride-based LEDs are /$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi: /j.jcrysgro

2 328 Z.H. Feng et al. / Journal of Crystal Growth 272 (2004) grown and fabricated on planar sapphire substrates, which results in relatively high-density dislocations in the order of cm 2 and the device performance is compromised [2,3]. The growth of GaN with low-density dislocation was a major effort for the fabrication of high-efficiency LEDs and reliable laser diodes (LDs) [4]. Various techniques have been proposed for this purpose. The epitaxial lateral overgrowth (ELOG) with a strip-type mask pattern on GaN is used most commonly and it successfully decreased the threading dislocation densities to the 10 6 cm 2 range [5,6]. Although this technique improves the crystallinity of the overgrown layer, it inherently suffers from the inevitable growth interruption in which SiN x or SiO 2 mask is deposited after the growth of a GaN layer of 1 2 mm-thick. Additional mask-related drawbacks of this technique include the possibility of impurity contamination and the induced strain in the subsequent growing layer [7]. The maskless, growth interruption-free and single-step overgrowth technique is desirable for reducing the dislocation density to improve the performance of optoelectronic devices. Low-dislocation-density GaN from single cantilever overgrowth on deep trenched sapphire substrates has been reported earlier [8]. Enhancement of the external efficiency in InGaN-based LEDs grown on patterned sapphire substrates has been achieved based on the effect of the optical reflection from the side edge of the etched sapphire [9]. Chang et al. also reported enhancement of electroluminescence (EL) intensity from LEDs grown on a shallow trenched sapphire substrate [10]. However, the dependence of EL on the pattern characteristics has not been examined in detail. In this paper, we have successfully grown GaNbased blue LEDs on patterned sapphire substrates using single-step growth with no mask by MOVPE. The effects of trench dimensions and orientations on the EL characteristics were investigated. 2. Experimental procedure For the preparation of patterned sapphire substrates, we employed a mixed gas of BCl 3 and Cl 2 in an ICP-RIE system to etch 60-nmdeep parallel grooves on ( ) sapphire substrate with a PECVD-deposited SiO 2 mask. The etch rate was approximately 90 nm/min and the selectivity of oxide to sapphire is close to 2. The roughness of the etched surfaces was about 0.28 nm, as determined by AFM measurements. Grooves with three different dimensions of 1 mm 3 mm, 2 mm 4 mm, and 4 mm 6 mm (widths of ridge and trench) along two crystallographic orientations, /1120S sapphire and /1010S sapphire, were included in the patterns. The same LED structure has been grown on patterned and planar sapphire substrates for comparison by low-pressure (200 mbar) MOVPE in an Aixtron 2000HT system. Trimethylgallium (TMGa), trimethylindium (TMIn) and ammonia (NH 3 ) were used as the Ga, In and N precursors, respectively, while biscyclopentadienyl magnesium (Cg 2 Mg) and silane (SiH 4 ) were used as the p- and n-type doping sources. Following a GaN nucleation layer grown at 550 1C a2-mm-thick Sidoped GaN bulk layer is grown at C. The active region consists of five periods of InGaN/ GaN MQW structures with Si doping in GaN barriers. In situ monitoring of the LED growth was done by measuring the reflectivity of the growing wafer with a 600-nm-wavelength light source. From the simulation of high-resolution X-ray ( ) diffraction, the well and barrier thicknesses were 2.5 and 9.6 nm, respectively, and the indium composition in the well is approximately 19.2%. Finally, a thin undoped spacer and an Mg-doped GaN cap layer were grown on top. After thermal activation of p-type GaN at 800 1C inann 2 ambient for 20 s by rapid thermal annealing (RTA), 300 mm 300 mm LED mesas were patterned by standard photolithography and etched down to the n-type GaN by ICP. Ni/Au was deposited on the p-type GaN surface for the p-electrode, and Ti/Al/Ni/Au was deposited on the exposed n-gan surface for n-electrode. Optical properties of the LED dies were measured using an ocean optics LED characterization system with a small integrated sphere detector.

3 Z.H. Feng et al. / Journal of Crystal Growth 272 (2004) Results and discussion Fig. 1 gives a comparison of the optical reflectivity traces of GaN growth on a patterned (2 mm 4 mm) sapphire substrate (solid line) and on an unpatterned sapphire substrate (dotted line). An overall intensity reduction and a smaller oscillation amplitude of the optical reflectivity during the desorption and GaN growth on patterned substrate was obvious, suggesting some light scattering by the grooves. However, the difference of the reflectivity oscillation mean between GaN bulk growth and GaN nucleation growth on patterned substrate is much larger than that on unpatterned substrate, which indicates that a smoother surface is formed during the early stage of GaN bulk growth on patterned substrate [11]. During the initial stage of GaN bulk growth, a section of slower increase of the optical reflectivity was observed. The reflectivity oscillation is from the optical interference between the growing layer and the substrate, which depends on the thickness of the deposited epilayer [11]. The relatively slow increase of the reflectivity implies that more lateral growth and less vertical growth occurs at this stage, a growth process which is similar to islands coalescence. After that, a smooth growing surface should be achieved indicated by the oscillation of the optical reflectivity. Reflectivity unpatterned substrate patterned subs trate Time [s] lateral coalescence Fig. 1. In situ optical reflectivity measurements for GaN growth on 2 mm 4 mm periodical patterned sapphire substrate (solid line) and unpatterned sapphire substrate (dot line). The surface morphologies of LED wafers grown on patterned and unpatterned substrates were characterized by atomic force microscope (AFM) as shown in Fig. 2(a) and (b). No significant difference was observed from these two AFM images. Both have the similar terrace-like structure with a roughness of approximately and nm. Same structural quality of the MQW on patterned and unpatterned substrates was also observed based on the high-resolution X-ray ( ) diffraction results. The output power of these LED dies without thinning and packaging was measured using the integrated sphere detector from the top of the die. Compared with the EL emission of the same wavelength around 465 nm from LEDs on unpatterned substrates, the enhancement of optical output for dies on patterned substrates at a forward current of 20 ma was clearly observed, on both groove dimensions along /10 10S sapphire and /11 20S sapphire, as shown in Table 1. From these results, the EL intensity is also somewhat sensitive to the trench orientation. The conventional ELOG under various conditions showed that the lateral growth rates of /10 10S oriented stripes were much faster than /11 20S oriented stripes [12]. In our study, EL intensity was about 25% and 21% larger when the LEDs were fabricated on the patterned substrate with 2 mm 4 mm periodical grooves along /10 10S sapphire and /10 20S sapphire directions, respectively. Fig. 3 shows a comparison of room temperature EL spectra at a forward current of 20 ma, from an LED die on a patterned substrate with 2 mm 4 mm dimension along /10 10S sapphire, and that on unpatterned substrate. There is no shift of the EL peak position, both at 465 nm, except that the intensity on the patterned substrate was about 25% larger than that on unpatterned substrate. The output power vs. drive current (L I) of these two blue LEDs is also shown in Fig. 4. The one on patterned substrate (open circle) has a larger slope, i.e., higher efficiency. Both LEDs are quite linear up to near 100 ma due to the lack of proper thermal dissipation on unthinned sapphire substrates.

4 330 Z.H. Feng et al. / Journal of Crystal Growth 272 (2004) Fig. 2. AFM images of the surfaces of LEDs: (a) on unpatterned sapphire substrates, (b) on patterned sapphire substrate. The surface roughnesses of these two films are and nm, respectively. Table 1 Dependence of the room temperature output power increase at 20 ma drive current on different groove dimensions along / 10 10S sapphire and /11 20S sapphire orientations Pattern dimension (mm) Increase ratio /11 20S sapphire (%) Increase ratio / 10 10S sapphire (%) Out Power (mw) 5 patterned substrate unpatterned substrate Forward Current (ma) unpatterned substrate patterned substrate Fig. 4. The output power as a function of drive current of the blue LEDs on patterned sapphire substrate (open circle) and unpatterned sapphire substrate (solid circle). EL Intensity(a.u.) I=20 ma Wavelength (nm) Fig. 3. Typical room temperature EL spectra of blue LEDs grown on 2 mm 4 mm periodical patterned sapphire substrate (open circle) and unpatterned sapphire substrate (solid circle) at a forward current of 20 ma. Fig. 5(a d) shows the optical microscope images of these two blue LEDs operated at low drive currents, 0.4 and 1.2 ma. It can be seen that there are more light-emitting dots on patterned substrate than that on unpatterned substrate at the same low current of 0.4 ma. At a slightly larger drive current, it is more obvious that almost all of the light-emitting dots locate in the trench region, indicating stronger EL efficiency in this region. Only random distribution of light-emitting dot is observed from LED on unpatterned substrate in spite of large drive current. The EL enhancement mechanism is not clear at this point. However, we can eliminate the optical

5 Z.H. Feng et al. / Journal of Crystal Growth 272 (2004) (a) (b) (c) (d) Fig. 5. Optical microscope images of blue LEDs operated at 0.4 and 1.2 ma (a) (c) on patterned sapphire substrate (b) (d) on unpatterned sapphire substrate. diffraction from the patterned sapphire because of the non-coherent properties of light emitted from LEDs. One possible explanation is that homoepitaxial lateral growth may have been initiated from the ridge region to cover the trench region at the initial stage of GaN bulk growth, which leads to high crystalline quality over the trench region. So there is less non-radiative recombination and more light emission in this region [13] resulting in overall enhanced output power and efficiency from LEDs on patterned substrate. The other possible reason is that a different strain in the ridge and the trench regions [14,15] may cause fluctuation of indium composition or phase-separated In-rich clusters in MQWs. Higher indium composition or In-rich clusters of high density could thus be achieved in the trench region, where carrier localization effect is stronger and the EL efficiency is higher. In addition, the enhancement may be explained as a consequence of the improved extraction efficiency of emission light from the MQWs grown on patterned substrate compared to planar substrate due to the effect of optical back scattering from the side edge of the etched sapphire. In summary, we have successfully grown GaNbased blue LEDs on patterned sapphire substrates using MOVPE without mask and growth interruption. Homoepitaxial lateral growth may have occurred at the initial stage of GaN bulk growthbased on in situ optical reflectivity measurement. Enhancement of output power and efficiency from LEDs on sapphire substrates with patterns of different dimensions along /1010S sapphire and /1020S sapphire orientations has been achieved. Stronger light emission in the trench regions was observed at low drive current indicating that better crystalline quality material may have been initiated in this region. Acknowledgements The authors thank Mr. S.Y. Zhang and Mr. Q.B. Wu for LEDs Processing, and Fangda Guoke Optronics Technical Limited Company, Shenzhen, China, for EL mapping measurements. This work was supported by a grant (ITS239/00) from the Innovation and Technology Commission of Hong

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