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Abstract
We demonstrated the growth of crack-free high-quality GaN-based UV vertical LEDs (VLEDs) (λ = 365 nm) on 6-inch sapphire substrates by using an ex-situ sputtered AlN nucleation layer (NL) and compared their performance with that of UV VLEDs with an in situ low temperature (LT) AlGaN NL. The X-ray diffraction (XRD) results showed that the ex-situ AlN sample contained lower densities of screw-type and edge-type threading dislocations than the in situ AlGaN NL sample. The micro-Raman results revealed that the ex-situ AlN sample was under more compressive stress than the in situ AlGaN sample. As the current was increased, the electroluminescence peaks of both of the samples blue-shifted, reached a minimum wavelength at 1000 mA, and then slightly red-shifted. Packaged VLEDs with the ex-situ AlN NL yielded 6.5% higher light output power at 500 mA than that with the in situ AlGaN NL. The maximum EQEs of the VLED with the in situ AlGaN and ex-situ AlN NLs were 43.7% and 48.2%, respectively. Based on the XRD and Raman results, the improved light output power of the ex-situ AlN sample is attributed to the lower density of TDs.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Group III-nitride-based ultraviolet light-emitting diodes (UV LEDs) are of technological importance because of their applications in sterilization, disinfection, water and air purification, biochemistry, and solid-state lighting [1–4]. Because homoepitaxial substrates are commercially not available, the III-nitride-based UV LEDs are grown on sapphire substrate using a metal-organic chemical vapor deposition (MOCVD) system [4]. Incidentally, there is a huge difference in the lattice parameters and the thermal expansion coefficients between III-nitride layers and sapphire substrates. These structural discrepancies cause the generation of a high density of threading dislocations (TDs) during the growth of nitride layers [5]. A high density of TDs significantly reduces the internal quantum efficiency (IQE) of III-nitride-based LEDs by producing non-radiative recombination centers [6,7], resulting in the lower performance of UV LEDs. Note that the power efficiency of UV LEDs is more markedly affected by the presence of TDs compared with blue LEDs because they contain less In [1,8]. Thus, a low temperature (LT) GaN or AlN nucleation layer (NL) is even more important before growing the high-temperature (HT) GaN epilayers [9,10]. Moreover, a high-temperature AlN NL was also used to reduce the TD density [11–13]. For instance, Wang et al. [13], characterizing the performance of 310–340nm UV LEDs, reported that use of buffer layers consisting of a thin GaN interlayer and a HT AlN buffer layer could be effective in growing UV LEDs emitting at wavelengths down to 300 nm. In addition, a rocksalt-structured CrN layer was employed as a novel buffer for the growth of GaN layers on c-sapphire substrate [14,15]. Lee et al. [15], investigating the effect of molecular-beam-epitaxy (MBE)-grown CrN NL on the structural property of GaN layers, found that N-polarity GaN epilayers grown on the CrN buffer layer showed remarkably better crystalline quality than those grown on conventional LT MBE-GaN buffer.
Recently, in a way of enhancing the crystalline quality and performance of GaN-based LEDs, ex-situ sputtered AlN NL was introduced [8,16–20]. For example, Yen et al. [8], investigating the effect of a sputtered AlN NL on the crystal quality, electrical, and optical characteristics of GaN-based LEDs (λ = 450 nm), reported that the use of a sputtered AlN NL reduced the (002) and (102) X-ray rocking curve widths of the GaN layer from 318.0 to 201.1 and 412.5 to 225.0 arcsec, respectively. Consequently, LEDs with the sputtered AlN NL showed 5.73% higher output power at 20 mA than ones with the in situ AlN NL. Chen et al. [17] also investigated the effect of ex-situ sputtered AlN NL on the optical performance of blue GaN-based LEDs and found that LEDs with the sputtered AlN NL yielded 3.7% higher light output and lower reverse leakage current than LEDs with in situ LT GaN NL. Chiu et al. [18], investigated the output power of GaN-based UV (380 nm) LEDs with sputtered AlN NL on patterned sapphire substrate (PSS) by atmospheric pressure-MOCVD and observed that undoped GaN layers with conventional GaN NL and AlN NL-PSS contained TD densities of 6 × 107 cm−2 and 2.5 × 107 cm−2, respectively. Accordingly, LEDs with AlN NL produced 30% higher output power at 20 mA than conventional ones. Hu et al. [19] compared the performance of GaN-based UV (375 nm) LEDs grown on PSS with in situ LT GaN/AlGaN NL and ex-situ sputtered AlN NL. It was found that the TD density in UV LED with AlN NL was lower than those in UV LED with sputtered AlGaN and GaN NLs. The output power of UV LED with AlGaN NL was 18.2% higher than that of UV LED with GaN NL, which was ascribed to a reduced absorption of 375 nm UV light in the AlGaN NL. On the one hand, the use of a sputtered AlN NL resulted in further 11.3% improvement in the output power, which was attributed to reduced TD density in InGaN/AlInGaN active region. The optimal thickness of AlN NL was reported to be 15 nm. In this study, we also employed an ex-situ sputtered AlN NL to grow high-quality GaN-based UV vertical LEDs (λ = 365 nm) on 6-inch sapphire substrate using MOCVD and compared their performance with those of UV vertical LEDs with an in situ LT AlGaN NL. We first report the output performance of mass-produced crack-free UV-LEDs grown on 6-inch sapphire substrate and the mass-production of 6-inch wafer-level vertical UV-LEDs by using laser lift-off (LLO) process. Furthermore, to the best of our knowledge, our EQE is the highest value reported so far for the mass-produced UV LEDs (λ = 365 nm).
2. Experimental procedure
UV LEDs were grown on 6-inch sapphire substrates by MOCVD. The LED structures included a 20 nm-thick in situ AlGaN nucleation layer (NL) deposited with MOCVD or a 20 nm-thick ex-situ AlN NL deposited by physical vapor deposition (PVD), a 2 μm-thick undoped GaN layer, a 2 μm-thick n-Al0.07Ga0.93N layer (1 × 1019 cm−3), an AlGaN/InGaN multiple quantum well (MQW), an 80 nm-thick Mg-doped Al0.3Ga0.7N electron blocking layer (2 × 1020 cm−3), and a p-GaN layer (1 × 1019 cm−3) that was in situ activated at 700 °C for 5 min in a N2 stream within the MOCVD chamber. Detailed fabrication processes of vertical LEDs (VLEDs) (1100 × 1100 μm2) were published elsewhere [21]. A Ag p-reflector was deposited by radio frequency magnetron sputter. A bonding metal alloy consisting of Au, Sn, and Cu was then deposited on the reflector by a dual electron-beam system. After completing LED structures, the whole 6-inch wafer was bonded to a metal alloy wafer by thermal compression at 300 °C. A laser lift-off (LLO) process was carried out with a KrF excimer laser to separate the LED structure from the sapphire substrate. This caused an undoped GaN to be exposed to air. The undoped GaN was etched down to the n-AlGaN layer using an inductively coupled plasma (ICP) etcher, followed by wet chemical etching. A heated KOH solution was then used to roughen the n-AlGaN surface. Finally, square mesa structures were fabricated with an ICP process for device isolation. Wafer level GaN-based VLEDs with n- and p-contact pad metals were successfully fabricated without any cracks, as shown in Fig. 1. To investigate the electrical, optical and structural characteristics, the wafer-level VLED samples were cut into chips and encapsulated into standard LED lamps. For the characterization of VLED chips, current-voltage (I–V) characteristics were examined with an Agilent B1505A system. The light output power-current relations were examined using an integrating sphere (Instrument Systems GmbH. ISP 500) with Spectroscope (CAS 140CT) and Sourcemeter (Keithley KE2601A). Raman scattering examination was made to characterize the strain state of GaN layers. The crystalline quality of samples was characterized with X-ray diffraction (XRD) rocking curve measurement.
Fig. 1 (a) Optical micrograph of crack-free GaN-based VLEDs fabricated on a 6-inch metal supporter. (b) A VLED chip and (c) an emission image obtained at 2 mA are shown right.
3. Results and discussion
To characterize the crystalline quality of the GaN layer in the VLEDs, full widths at half-maximum (FWHMs) of XRD rocking curves for the (002) and (102) planes were estimated (Figs. 2a and 2b), which denote the presence of screw-type and edge-type threading dislocations (TDs) [22–24]. The FWHMs of (002) and (102) XRD spectra were estimated to be 151 and 241 arcsec for the in situ AlGaN NL sample, and 87 and 172 arcsec for the ex-situ AlN NL sample, respectively. The TD density can be assessed from the FWHMs of X-ray rocking curve ω-scan (002) and (102) diffractions using the relation, N = β2/4.35|b|2, where N is the dislocation density, β is the FWHM of the X-ray rocking curve, and |b| is the magnitude of the Burgers vector [25]. According to the Eq. (1), the densities of screw-type and edge-type TDs were measured to be 4.58 × 107 and 3.07 × 108 cm−2 in the in situ AlGaN NL sample, respectively, and 1.52 × 107 and 1.57 × 108 cm−2 in the ex-situ AlN samples.
Fig. 2 XRD rocking curves for (a) the (002) and (b) (102) planes of in situ AlGaN and ex-situ AlN NL samples.
To characterize the strain distribution in the in situ AlGaN and ex-situ AlN samples, micro-Raman examinations were performed, Fig. 3(a). The strain in the samples causes changes in the Raman spectrum, by which the relaxation of the samples can be determined [26]. For the in situ AlGaN sample, E2(high) peak is observed at 572.32 cm−1 in the Raman spectrum, while for the ex-situ AlN sample, E2(high) peak appears at 571.34 cm−1, Fig. 3(b). The Raman peak at 571.34 cm−1 is indicative of the presence of a high compressive strain in the the ex-situ AlN sample, compared with the in situ AlGaN sample [27]. On the other hand, the Raman peak at 572.32 cm−1 implies the relaxation of strain in the in situ AlGaN sample [28].
Fig. 3 (a) Raman spectra obtained from the in situ AlGaN and ex-situ AlN NL samples. (b) Enlarged E2 (high) spectra.
Figure 4(a) displays the peak wavelengths of the electroluminescence spectra of packaged UV VLEDs with the in situ AlGaN and ex-situ AlN NLs as a function of injection current. As the current increases from 10 to 1500 mA, both of the samples experience a shift of the peak wavelength. At 10 mA, the in situ AlGaN and ex-situ AlN samples give a wavelength of 370.89 and 370.02 nm, respectively. As the current increases, the peak of the in situ AlGaN sample blue-shifts to a minimum wavelength of 365.77 nm at 1000 mA and then slightly red-shifts to 366.11 nm. For the ex-situ AlN sample, the peak also blue-shifts to a minimum wavelength of 365.79 nm at 1000 mA and then red-shifts. The Raman results indicate that unlike the in situ AlGaN sample, the ex-situ AlN sample is under compressive stress. Thus, the in situ AlGaN sample is relatively relaxed, resulting in more In segregation. This causes the generation of deeper potential fluctuation, namely, energy gap fluctuations, and so the in situ AlGaN gives longer wavelength than the ex-situ AlN sample. Figure 4(b) exhibits the FWHM of the peak wavelengths of packaged UV VLEDs with the in situ AlGaN and ex-situ AlN NLs. Across the whole current range, the ex-situ AlN sample give narrower FWHM than the in situ AlGaN sample. This could be attributed to the better crystallinity and moreuniform In distribution of the ex-situ AlN sample (due to compressive stress [29]). As the current increases, the FWHMs of both the samples become narrower. This may be due to the dominant band filling and screening effects [30–32]. Figure 4(c) displays the EL spectra measured at an injection current of 500 mA, where the emission peak wavelength for both samples is ~365 nm. Note that the ex-situ AlN sample exhibits somewhat higher EL intensity than the in situ AlGaN sample.
Fig. 4 (a) The peak wavelengths and (b) FWHMs of the EL spectra of LEDs with the in situ AlGaN and ex-situ AlN NLs as a function of injection current. (c) The EL spectra measured at an injection current of 500 mA.
Figure 5(a) exhibits the light output-current power-drive current characteristics of packaged UV VLEDs fabricated with the in situ AlGaN and ex-situ AlN NLs. For both of the samples, the light output power is increased with increasing drive current up to 1000 mA. It is noted that the VLED with the ex-situ AlN NL yields higher light output power than that with the in situ AlGaN NL. For example, the VLED with the ex-situ AlN NL shows 9.8% higher light output power at 350 mA than that with the in situ AlGaN NL. For example, the light output powers obtained from the VLEDs with the in situ AlGaN and ex-situ AlN NLs are 718.84 and 765.46 mW at 500 mA, respectively. Based on the XRD structural results, the improved light output of the ex-situ AlN sample can be attributed to the reduced density of TDs rather than the improved interface quality of the MQWs. The external quantum efficiency (EQE) of the packaged UV VLEDs with the in situ AlGaN and ex-situ AlN NL was examined as a function of the forward current, as displayed in Fig. 5(b). Both of the VLEDs suffer from efficiency droop, but the VLED with the ex-situ AlN NLs produces higher EQE than that with the in situ AlGaN NL across the whole current range. The maximum EQEs of the VLED with the in situ AlGaN and ex-situ AlN NLs are 43.7% and 48.2%, respectively. Note that the VLED with the ex-situ AlN NL gives maximum EQE at lower current than that with the in situ AlGaN NL. This may be explained by the smaller A factor in the ABC model as a result of the better crystallinity [33]. On the assumption of the same light extraction efficiency (LEE),the improved EQE for the VLEDs with the ex-situ AlN NL can be associated with the reduction in the density of TDs in the MQWs.
Fig. 5 (a) The light output-current power-drive current characteristics and (b) EQE of fully packaged UV LEDs fabricated with the in situ AlGaN and ex-situ AlN NLs. (b) The EQE of packaged VLEDs with the in situ AlGaN and ex-situ AlN NLs as a function of the forward current.
4. Conclusion
We investigated the output performance of crack-free GaN-based UV VLEDs grown on 6-inch sapphire substrate using ex-situ sputtered AlN NL and compared them with that of UV VLEDs with in situ AlGaN NL. The ex-situ AlN sample showed better crystallinity with lower TDs than the in situ AlGaN NL sample. Both the VLED samples had the same widths of QWs and barrier layers. The micro-Raman results exhibited that the in situ AlGaN NL sample was relaxed, but the ex-situ AlN NL sample was under compressive stress. With increasing current, both of the ex-situ and in situ samples underwent a shift of the EL peaks. Packaged VLEDs with the ex-situ AlN NL exhibited higher light output power and EQE than that with the in situ AlGaN NL. The higher light output performance was attributed to the improved crystallinity. These results together with a high EQE of 48.2% show that the use of ex-situ sputtered AlN NL is a highly promising growth parameter for the fabrication of high efficiency UV VLEDs.
Acknowledgments
This work was supported by LG Innotek Co., Ltd. and the Global Research Laboratory (GRL) program through the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT (NRF-2017K1A1A2013160).
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