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Abstract
Gallium nitride (GaN) films on high-thermal-conductivity substrates have attracted considerable attention for their applications in high-power light-emitting diodes and electronic devices. Herein, a 2-inch 8-µm-thick thermostable GaN/Mo template with Ga-face was fabricated via two consecutive layer transfer technique. The full-widths at half-maximum for the x-ray rocking curves of GaN (002) and (102) plane were 314 and 325 arcsec, respectively. Atomic force microscopy revealed that the surface had step-and-terrace structures with a root-mean-square value of 0.397 nm. Five periods of In0.15Ga0.85N/GaN multiple-quantum-wells and Mg-doped p-type GaN layers were regrown on the GaN/Mo template, which exhibited blue light emission without distinct degradation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
GaN is among the most promising materials for high-frequency high-power electronic devices and optoelectronic applications, including light-emitting diodes (LEDs) and high electron mobility transistors (HEMTs) [1–5]. GaN-based devices are typically fabricated on sapphire substrates [6]. However, owing to the low thermal conductivity of sapphire, these devices normally suffer from poor heat dissipation, and thereby short lifetime and poor performance. To address these issues and expand the application of GaN-based devices, significant efforts have been invested to manufacture vertical-structure devices on alternative high-thermal-conductivity substrates [7]. Generally, GaN film on metallic substrate could improve heat dissipation, conductivity and reduce cost.
However, the direct deposition of GaN on metallic materials is limited by poor epitaxial uniformity, crystalline properties, and serious interfacial reaction [8–10]. Alternatively, layer transfer techniques with wafer bonding and substrate lift-off are promising for the integration of high-quality GaN layers with metallic substrates. Currently, the low thermal stability of these hybrid wafers limits the application of GaN in high-temperature high-power devices [11,12]. Some researchers have demonstrated the robust wafer bonding of GaN and Si wafers using a SiO2 interlayer to improve thermal stability, however, the low electrical conductivity of SiO2 is unsuitable for fabricating vertical devices [10]. We have previously reported 2-inch N-face GaN/Mo template fabricated by metal bonding and laser lift-off processes, whereby we discovered that the thermal stability of the N-face transferred GaN template was substantially improved by heat and pressure treatment with the same bonding equipment [13]. However, such technique could not be applied for practical device application, as the N-face GaN surface was generally not suitable for the subsequent GaN epitaxy [14,15]. In our follow-up study, we also found that based on the resulting N-face GaN template, the crystal quality after the further epitaxy of MQWs and p-GaN was extremely poor.
In this work, a 2-inch Ga-face transferred GaN/Mo template with small full-widths at half-maximum (FWHM) values of 314 and 325 arcsec for GaN (002) and (102), respectively, was obtained via two consecutive layer transfer for the first time, which is more suitable for the further epitaxy of InGaN/GaN MQWs and p-GaN. The transferred GaN/Mo template provides a route to manufacturing high-power GaN-based devices on high-thermal-conductivity substrates.
2. Sample structures, fabrication procedures, and characterization methods
The entire layer transfer technique, including metal bonding and laser-lift-off, is illustrated in Fig. 1. Initially, a 4.5- or 8-µm-thick n-type intrinsic GaN layer was grown on a 2-inch c-plane sapphire substrate via metal organic chemical vapor deposition (MOCVD). The GaN film on sapphire (GaN/Al2O3) template was bonded to another temporary sapphire substrate with ultraviolet curing adhesive [(UV 8102; Fig. 1(a)]. Various kinds of ultraviolet curing adhesive and sapphire substrates with different chamfer angle was applied to control the flow process of the molten adhesive during bonding process while MUSASHI SM300DSS automatic dispensing system was used. After hundreds of experiments, 100 µl UV 8102 adhesive for each 2-inch wafer was selected to achieve the best adhesive thickness uniformity and accordingly surface roughness of fabricated GaN/Mo template. Subsequently, a solid-state laser with a wavelength of 355 nm, energy density of 450 mJ/cm2, and irradiation diameter of 30 µm was used to separate the GaN film from the sapphire substrate to form the N-face GaN/Al2O3 template [Fig. 1(b)]. The residual metallic Ga droplets on top of the transferred GaN epilayer were etched away using a dilute HCl solution (10 vol%). In order to realize the high quality MQWs/p-GaN epitaxy, the second layer transfer process was performed to obtain the Ga-face GaN template. To ensure a good thermal match, a high-purity polycrystalline Mo wafer was selected as the carrier substrate. The N-face GaN/Al2O3 template and Mo substrate were cleaned with acetone and isopropanol. Metallized 50-nm Ti and 300-nmPd bonding layers were sequentially deposited on top of the transferred N-face GaN/Al2O3 template and Mo substrate, respectively, via DC magnetron sputtering. A high temperature diffusion furnace [Fig. 1(c)] was used to achieve GaN-Mo bonding. The bonding temperature and pressure were 900 °C and 4000 kgf, respectively. The bonding time was 240 min. During this process, the temporary sapphire substrate automatically dropped down because of the high temperature during bonding [Fig. 1(d)]. An O2-based etching process using an inductively coupled plasma (ICP) etcher was then performed with 100 W ICP power and 40 W RF bias power to remove UV residue on the surface of the related Ga-face GaN/Mo template.
After the layer transfer process and testing, the 8-µm-thick Ga-face GaN/Mo template was utilized to regrow a full LED structure on top of it. Five periods of In0.15Ga0.85N (3 nm)/GaN (10 nm) multiple quantum wells (MQWs) were regrown on the transferred 8-µm-thick Ga-face GaN/Mo template and topped by a 0.2-μm-thick Mg-doped p-type GaN layer via MOCVD over the temperature range 850–990 °C [Figs. 1(e, 1(f)]. The surface morphology was investigated by optical microscopy and atomic force microscopy (AFM). The overall crystalline nature of the GaN materials and structural properties of the GaN films were characterized by X-ray diffraction (XRD) using a Bruker D8 diffractometer. Secondary ion mass spectrometry (SIMS) was performed with a CAMECA IMS-4F using cesium ion bombardment to obtain concentration-depth profiles. The profiles were quantified by analyzing ion-implanted GaN standards. Detection limits due to the background of GaN for each species were 1 × 1014 cm-3, 1 × 1016 cm-3, and 1 × 1016 cm-3 for Ti, Pd, and Mo, respectively.
3. Results and discussions
To satisfy the thermal requirement during layer transfer and subsequent MQWs/p-GaN epitaxy, metallic materials with high melting points and well-matched coefficients of thermal expansion (CTE) should be used as the bonding medium and carrier substrate. We measured the CTE versus temperature for several materials, including GaN, Al2O3, Mo [13] and W-Cu alloy (Fig. 2). The thermal match between GaN and high-purity polycrystalline Mo substrate was good, especially below 600 °C, and therefore beneficial for controlling thermal stress during layer transfer and further MQWs/p-GaN epitaxy. Moreover, high-melting-points refractory metals Ti and Pd were used as the bonding medium, in which the Ti and Pd layers served as the adhesive barrier and bonding layers, respectively. Because the appropriate thickness of Ti layer will react with GaN during the high-temperature bonding process, which could form titanium nitride-gallium to increase the bonding strength, while Pd is selected due to its good ductility under low element diffusion and is conducive to reducing the background concentration of metal ion diffusion for GaN. It lay the foundation of better performance in the later stage.
Fig. 2. The thermal expansion coefficient versus temperature curves for GaN, Al2O3, Mo and W-Cu alloy.
By performing two consecutive layer transfer processes, we obtained 2-inch Ga-face GaN/Mo templates with different epilayer thicknesses, which are more suitable for GaN homoepitaxy than N-face GaN/Mo templates formed with only one layer transfer process. During the second bonding process, the temporary sapphire substrate automatically dropped down and the UV 8102 adhesive was carbonized due to the high temperature during metal bonding process [Fig. 1(d)]. After conducting O2 plasma etching with ICP to oxidize and remove the residual UV adhesive on the surface of the related Ga-face GaN/Mo template, the 2-inch GaN/Mo templates with 4.5- and 8-µm-thick GaN epilayers [Figs. 3(b) and 3(d), respectively] exhibited no cracks or wrinkles, as observed by scanning-acoustic and optical microscopies, which suggested that the prepared GaN composite substrate has less residual stress and Lower curvature.
Fig. 3. SIMS measurements of the transferred Ga-face GaN/Mo templates a) 4.5-µm-thick and c) 8-µm-thick; photographs of the transferred GaN/Mo templates b) 4.5-µm-thick and d) 8-µm-thick.
SIMS depth profile analyses were performed on the transferred GaN/Mo templates with a 4.5- or 8-µm-thick GaN epilayer. The impurities related to Ti, Pd, and Mo were more readily incorporated into GaN films in the 4.5-µm-thick GaN/Mo template [Fig. 3(a)], however, they were near or below the detection limits in the top 5 µm of the 8-µm-thick GaN/Mo template [Fig. 3(c)]. As well known, the diffusion of impurities mainly correlates with temperature and pressure during bonding process. However, another important issue can’t be ignored, i.e. the integrality and tolerance of material during bonding process. We believed that the tolerance of 8-µm-thick GaN is much higher than that of 4.5-µm-thick GaN. Therefore, some micro-crack and other type of damage may appear in relatively thinner GaN layer (4.5-µm-thick GaN) at 900 °C and 4000 kgf during bonding process and thus diffusion of the impurities into GaN will be much easier through such channels. In this case, some GaN compound may be decomposed to some extent and thus Ga atom may accumulate in the form of metal droplet or form different types of alloy with other metal. The variation tendency of Ga concentration for the transferred 4.5-µm-thick GaN sample, which decreases at first but increases after the thickness of 3 um, could be mainly attribute to the resulting metal droplet or different types of GaN alloy with other metal. The impurities diffusion wasn’t observed in the top surface of the transferred 8-μm-thick GaN/Mo template which could be attributed to the fact that the metal impurities couldn’t penetrate the 8-um-thick GaN layer under the bonding temperature of 900 °C and bonding time of 240 min. This implied that the metal diffusion, which could substantially degrade the properties of the further MQWs/p-GaN epitaxy and the fabricated LED device [9,16,17], was effectively blocked by the relatively thick GaN epilayer. The critical GaN thickness for blocking metal diffusion is about 4 μm. In order to reduce the metal diffusion and unwanted interface reactions during the high-temperature metal bonding and further MQWs/p-GaN epitaxy process, 8-µm-thick GaN epilayer was selected in this work and mentioned as the transferred GaN/Mo template in the following paragraph.
Figures 4(a) and 4(b) show the typical X-ray rocking curves of GaN (002) and (102) for the GaN/Mo template. The FWHM values of GaN (002) and (102) planes for the transferred 8-µm-thick GaN/Mo template were 314 and 325 arcsec, respectively, which were slightly larger than those for the (002) and (102) planes of the as-grown GaN/Al2O3 template (272 and 305 arcsec, respectively), demonstrating that the GaN epilayer was successfully transferred to Mo substrate with intact wurtzite structure [11,18]. The broadening of the (002) and (102) FWHMs after layer transfer may be attributed to the relatively large XRD spot, integrating a contribution from a loosely bonded section of GaN on the sample surface [11], as well as the possible degradation of crystal quality during bonding process and small strains in the c-axis. Optical microscopy showed a smooth surface was obtained. AFM revealed the surface had step-and-terrace structures with a RMS value of 0.397 nm, which were similar to the results for the as-grown GaN surface on sapphire substrate [ Figs. 5(a) and 5(b)]. Moreover, it was noted that the width of step-and-terrace structures was broadened after the two consecutive layer transfer technique which comes from the common action of many factors and could be mainly attributed to the high pressure during the metal bonding process.
Fig. 4. Rocking curves of a) (002) plane and b) (102) plane for the as-grown GaN/Al2O3 template, transferred GaN/Mo template, and transferred GaN/Mo template after MQWs/p-GaN regrowth.
Fig. 5. AFM images of a) the as-grown GaN/Al2O3 template, b) the transferred GaN/Mo template and c) the transferred GaN/Mo template after MQWs/p-GaN regrowth.
Raman scattering has been extensively used to study the stress of GaN, which is an important factor that alters the energy band structure and the phonons vibration. Figure 6. demonstrated the Raman spectra of the 8-µm-thick GaN/Al2O3 template and GaN/Mo template. The E2 (high) phonon mode of Raman spectrum is sensitive to the amount of strain and has been widely used in the characterization of GaN to quantify stress by the following equation:
where $\delta $ is the residual stress and $\Delta \omega $ is the E2 (high) phonon peak shift [18,19]. It has been reported that the stress-free GaN E2 (high) Raman mode is located at 568.0 cm-1. The E2 (high) phonon frequency for 8-µm-thick GaN/sapphire template and 8-µm-thick GaN/Mo template are 569.07 cm-1 and 567.92 cm-1, respectively. It can be calculated that the compressive stress existing in the GaN/Al2O3 template is approximately 0.25 GPa, while the tensile stress in the transferred Ga-face GaN/Mo template is 0.019 GPa. It has been reported the compressive stress in the GaN/Al2O3 template was caused by the thermal expansion coefficient mismatch and cooling process of Al2O3 substrate after MOCVD epitaxy [20]. These results indicated that the residual stress in the GaN layer was reduced by an order of magnitude and changed from the compressive stress to the tensile stress after transferring GaN layer from Al2O3 substrate to Mo substrate, implying that the tensile stress of 0.019 GPa caused by Mo substrate and bonding medium in the layer transfer process remains in the GaN layer. The extended distance between E2 (high) and A1(LO) could be mainly attributed to the combined effect of the variation of the residual stress and the change of build-in electric field in the GaN layer [21,22]. It was proved that the designed Ti/Pd bonding layer has good elastic contact with GaN epitaxial layer. In short, the residual stress existing in the GaN layer was mostly relieved by the two consecutive layer transfer technique.After the regrowth of MQWs and p-GaN layers on the transferred 8-µm-thick GaN/Mo template, XRD was performed to investigate the crystalline quality of the epilayer [Figs. 4(a) and 4(b)]. The FWHMs for the rocking curves of GaN (002) and (102) planes of this LED structure were 380 and 401 arcsec, respectively, which are comparable to those on conventional substrates, such as Al2O3 or Si. A smooth surface was obtained after epitaxy [Fig. 5(c)], indicating that GaN growth is two-dimensional. The LED structure on the transferred GaN/Mo template exhibited blue light emission without observable degradation (Fig. 7). Using the transferred GaN/Mo template enables the production of epitaxial GaN films on Mo substrate without interfacial reactions, thereby facilitating the fabrication of large-area high-power GaN-based optoelectronic devices and power devices.
4. Conclusions
A thermostable 2-inch transferred GaN/Mo template was fabricated via two consecutive layer transfer processes combining medium bonding and laser lift-off. AFM revealed that the surface had step-and-terrace structures with an RMS value of 0.397 nm. No metal diffusion was detected with SIMS in the top 5 µm of GaN layer before or after MQWs/p-GaN epitaxy, implying that the metal impurities couldn’t penetrate the 8-um-thick GaN layer and the interfacial reaction between GaN and the Mo substrate was suppressed, allowing the regrowth of high-quality InGaN/GaN MQWs and p-GaN films on the transferred GaN/Mo template. The resulting GaN/Mo template could be used for further epitaxial growth of high-quality GaN and direct fabrication of large-scale GaN-based optoelectronics and power devices.
Funding
National Natural Science Foundation of China (61704004); Guangdong Basic and Applied Basic Research Foundation (2019B1515120091); Research and Development Project in key area of Guangdong Province (2020B090922001); Guangdong Financial Work ([2015] 639); Innovation center for Wide band-gap Semiconductor and Device of Guangdong Province (Sponsored by Department of industry and information technology of Guangdong Province).
Disclosures
The authors declare no conflicts of interest.
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