Polarity inversion of aluminum nitride by direct wafer bonding

A novel fabrication process based on direct bonding technologies is proposed and demonstrated to achieve polarity inversion in AlN. High-angle annular dark-field scanning transmission electron microscopy observation clearly showed an atomically flat bonding interface and an abrupt transition from Al polarity (+c) to N polarity (−c) through a single monolayer. This ideal polarity inversion of III–nitride materials is expected to provide new insight into heteropolar device applications.

T he aluminum gallium nitride (AlGaN) system has been used to boost the development of optoelectronics and electronics, as represented by deep-ultraviolet (DUV) light-emitting diodes 1,2) and high-electron-mobility transistors (HEMTs). 3,4) In indium gallium nitride laser diodes (LDs) [5][6][7] and vertical AlGaN=GaN HEMTs, 8,9) freestanding GaN substrates with low threading dislocation densities (TDDs) strongly support device performance, including lifetime and reliability. However, the free-standing AlN substrate remains expensive, and epitaxial growth of high-crystallinity AlN on sapphire substrates is not yet well established. Therefore, the paucity of inexpensive and highcrystallinity AlN substrates inhibits further improvement in device performance. Our group recently proposed novel fabrication of high-crystallinity AlN templates on sapphire by combining sputter deposition of an AlN layer with a thickness of 170-340 nm with subsequent post-growth annealing at temperatures reaching 1700°C. 10) A key finding is that even a thickness of 340 nm sufficiently reduces the TDDs to on the order of 10 8 cm −2 . Namely, this method can lower the production cost because few-micrometer-thick AlN buffer layers currently need to be grown by metal organic vapor phase epitaxy (MOVPE) to reduce the TDDs. X-ray rocking curve (XRC) measurements of the ð10 12Þ plane showed marked improvement in the full width at half-maximum (FWHM), where a value of 287 arcsec was successfully achieved, compared with 6031 arcsec before annealing. Although the detailed mechanism of the improvement in crystallinity remains unclear, high-temperature annealing is assumed to strongly enhance the coalescence of sputtered AlN (SPT-AlN) grains through a solid-phase reaction.
Direct wafer bonding (DWB), which plays an essential role in, for example, the packaging of micro-electro-mechanical systems 11) and the fabrication of silicon-on-insulator wafers, 12) is also based on a solid-phase reaction at the wafer surface in order to achieve an adhesion strength comparable to that of the bulk material. For common Si=Si hydrophilic bonding, an annealing temperature of 800-1000°C is required to form strong covalent bonds at the Si=Si interfaces. 13) Recent progress using plasma and an ion beam for surface treatment enabled low-temperature bonding at temperatures as low as room temperature (∼300°C), which paved the way for novel device applications such as back-illuminated complementary metal oxide semiconductor sensors, 14) multijunction solar cells, 15) and III-V=Si LDs for Si photonics. 16) This similarity in the mechanism was taken into account when high-temperature annealing by face-to-face stacking was applied for direct bonding of SPT-AlN. Polarity inversion generated at the bonding interface is of considerable interest with respect to the epitaxial growth of N-polar materials, [17][18][19] quasi-phase matching (QPM) for second harmonic generation (SHG), [20][21][22] and two-dimensional electron gases induced at the heteropolar interface. 23,24) In particular, transverse-mode QPM-SHG using direct bonding is expected to overcome the difficulty in longitudinal-mode QPM-SHG in the DUV region, because a grating periodicity of less than 1 µm and the roughness of the N-polar surface are still critical issues in the fabrication of periodically poled AlN waveguides. 21,22) In this paper, we report the novel fabrication and crystallographic evaluation of polarity-inverted AlN based on DWB (Fig. 1). To the best of our knowledge, this is the first demonstration of III=N polarity inversion through a single monolayer. Although much effort has been directed toward intentional polarity control based on epitaxial growth by MOVPE and molecular beam epitaxy, strict optimization of the growth conditions is still necessary to achieve a smooth morphology and low defect densities. As a completely different approach, our proposal using DWB provides abrupt polarity inversion without the need to insert intermediate layers or amorphous layers. A conceptual diagram of the fundamental wave E ω (λ = 532 nm, TM 00 mode) and second harmonic wave E 2ω (λ = 266 nm, TM 01 mode) in the polarity-inverted AlN waveguide are shown in Fig. 1 as an application of transverse-mode QPM-SHG. The blue and red sections indicate positive and negative amplitudes of the electric field, respectively. The abrupt polarity inversion is indeed appropriate for the SHG device because overlapping of the electric field and the interlayer can cause optical absorption or scattering. Therefore, the proposed approach is expected to provide new insight into previous studies and into device applications in the future. Fabrication started with the preparation of a pair of 2 in. AlN samples by sputtering an AlN target onto vicinal c-plane sapphire substrates. The sputtering conditions were an RF power of 700 W, a chamber temperature of 600°C, and an Ar=N 2 ratio of 1=4. Subsequently, the +c-oriented SPT-AlN wafers were stacked face-to-face and annealed at 1700°C for 3 h under N 2 atmosphere. The sapphire substrates were orientated such that the ð11 20Þ planes were aligned in parallel. The annealing process simultaneously improved the crystallinity and the DWB of SPT-AlN. Subsequently, one side of the sapphire substrate was separated by blade insertion to measure the crystallographic characteristics of the bonded AlN. Cracks and unintentional separation were not seen across the 2 in. wafer either after annealing or after sapphire removal.
A cross section of a bonded sample was observed by highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM), as shown in Fig. 2(a). A SPT-AlN sample with a thickness of 200 nm was used for the bonding experiment. At the AlN=sapphire interface, unintentional polarity inversion over a thickness of 20 nm was found to take place, the formation of which could probably be attributed to a rhombohedral Al x O y N z layer. 25) A previous study reported that −c AlN is inverted to +c AlN through a rhombohedral Al x O y N z layer when a low-temperature AlN buffer (T g = 580°C) is grown on a nitride sapphire substrate. 25) In an examination of the AlN surface from which the sapphire substrate was removed, separation was found to take place in the middle of the AlN, 15 nm from the sapphire substrate. The dark line appearing at the −c=+c interface is attributed to the external stress resulting from blade insertion [ Fig. 2(b)]. As shown in Fig. 2(c), a 5 × 5 µm 2 atomic force microscopy (AFM) image depicts a smoother surface than expected even after the forcible separation. The estimated root mean square (RMS) roughness is 0.90 nm.
The polarity inversion was evaluated by observing the Al and N atoms with a magnification of 15,000,000×, which made it possible to see abrupt polarity inversion through a single monolayer at the bonding interface [ Fig. 2(d)]. Additionally, an atomically flat interface and regular atomic arrangement were observed across the measured sample. This abrupt inversion interface without the presence of an intermediate layer is beneficial for optoelectronics applica-tions owing to the resulting suppression of optical absorption due to the band tailing effect.
The atomic distance of Al extracted from the STEM image is 2.8 Å at the bonding interface and 2.5 Å at a position nine monolayers from the interface. The 12% larger lattice constant at the interface implies the existence of impurities. Additionally, the depth profile obtained by X-ray photoelectron spectroscopy showed that 11 atom % oxygen was detected at the bonding interface, whereas approximately 1 atom % was detected in the film. On the basis of these experimental findings, a structural model in which +c AlN and −c AlN are mediated by oxygen atoms is proposed on the basis of first-principle calculation and overlapped with the STEM image [ Fig. 2(d)]. First-principle calculations of a (2 × 1) unit cell were carried out by the plane-wave pseudopotential approach using the generalized gradient approximation. 26) To satisfy the electron counting rule, 27) three nitrogen atoms in a unit cell were replaced by oxygen atoms. The proposed model successfully explains the entire atomic arrangement. Additionally, an 18% expansion in atomic distance is expected from this model, which agrees with the experimentally observed expansion. Therefore, the presence of oxygen is estimated to play an essential role in this bonding process, as it does in Si=Si hydrophilic bonding. 13) The crystallinity of the bonded sample was evaluated using the FWHM of the XRCs, as shown in Fig. 3, where the (0002) and ð10 12Þ XRCs of the bonded sample are compared with those of typical as-grown SPT-AlN with a thickness of 200 nm. X-ray diffraction measurement was carried out using an asymmetric Ge(220) monochromator and Cu Kα 1 radiation (0.154 nm). The bonded sample exhibited FWHMs of 137 arcsec in the (0002) plane and 302 arcsec in the ð10 12Þ plane, whereas those of the as-grown SPT-AlN were 64 and 6900 arcsec, respectively. For the (0002) plane, only the sharp peak remained after annealing, and the broad peak vanished. On the other hand, for the ð10 12Þ plane, the FWHM decreased markedly, by a factor of 23. Significant improvements in crystallinity are currently understood on the assumption that the coalescence of small grains is enhanced by a solid-phase reaction, and the TDDs are decreased to the order of 10 8 cm −2 through the elimination of grain boundaries. 10) Residual stress is problematic because cracking in the epilayer and sapphire substrate results in a low yield ratio. Using 2θ-ω scans of the (0002) and ð10 12Þ planes, the c-axis lattice constant was found to be 4.990 Å, and the a-axis lattice constant was found to be 3.098 Å. The out-of-plane strain ε zz is given by ε zz = (c − c 0 )=c 0 , where c is the measured lattice constant, and c 0 = 4.982 Å is the lattice constant of free-standing AlN. 28) The in-plane strain ε xx is similarly given by ε xx = (a − a 0 )=a 0 , where a 0 = 3.112 Å. Thus, it is found that the strain along the c-axis, ε zz , is 0.16%, and the strain along the a-axis, ε xx , is −0.45%. The in-plain biaxial stress σ xx is theoretically given by where C ij is the stiffness constant, which has values of C 11 + C 12 = 538 GPa, C 13 = 113 GPa, and C 33 = 370 GPa. 29) Hence, σ xx is estimated to be −2.11 GPa, which indicates that the induced in-plane compressive stress is quite high. However, these values are close to those in a previous work on annealing of SPT-AlN performed without the intention of obtaining DWB. 10) Thus, no additional stress caused by DWB or sapphire separation is recognized. According to Ref. 30, it has been reported that screw dislocations occur owing to misalignment of the azimuthal angle φ. 30) Here, a φ scan of the AlN ð10 12Þ plane indicates a misalignment angle of 0.87°. This value can be decreased by precisely adjusting the orientation flat before the DWB process.
A comprehensive evaluation of the bonded sample was carried out using Raman spectroscopy under the zðx; ÀÞ z backscattering geometry with a 532 nm excitation light source, as shown in Fig. 4. The Raman shift of the E 2 (high) phonon mode shows a peak intensity of 664.2 cm −1 , suggesting that in-plane compressive stress is induced compared with free-standing AlN (peak value of 657.4 cm −1 ). 31) An FWHM of 6.1 cm −1 indicates high crystallinity even after DWB and sapphire separation. Additionally, the A 1 (LO) peak clearly observed at 893.5 cm −1 indicates a low residual carrier density of less than 10 17 cm −3 . 32) Note that the peaks located at 577 and 751 cm −1 resulted from the E g (int) phonon modes of the sapphire substrate. 33) Here we discuss the mechanism of polarity inversion in the DWB process. The experiment revealed that, surprisingly, no wafer cleaning, surface treatment, or load application was required for successful bonding. This simple and powerful process can be understood as follows. High-temperature annealing at 1700°C strongly enhances the solid-phase reaction of AlN grains, leading to the generation of covalent bonds even at the bonding interface. Practical exploitation of this mechanism would require careful control of the wafer bowing and surface roughness. The SPT-AlN samples for the bonding experiment showed a concave curvature of 12 km −1 and an RMS surface roughness of 0.26 nm. Regarding the wafer bowing, concave SPT-AlN is expected to expand thermally during the annealing process and consequently become flat. Regarding the surface roughness, the RMS value was improved from the previously reported value of 0.47 to 0.26 nm. 10) Owing to the smoother surface, further adherence resulting from the van der Waals strength can proceed. Finally, covalent bonds form at the adhered interface by the solid-phase reaction, and DWB of SPT-AlN is accomplished. Therefore, the key components of the proposed DWB process are considered to be a solid-phase reaction, spontaneous contact, and a smooth surface. In our experiments, the DWB process did not occur with the already annealed SPT-AlN, which showed a rougher surface (0.40-0.54 nm) and larger curvature (21-44 km −1 ). Although we have not performed comprehensive experiments with AlN samples grown via MOVPE, the DWB process is probably possible if a smooth surface, small curvature, and uniform film thickness are achieved simultaneously.
In conclusion, a novel fabrication process based on direct bonding technologies was proposed and demonstrated to achieve polarity inversion in AlN. Face-to-face annealing of SPT-AlN samples at 1700°C enabled the generation 12Þ planes.
of covalent bonds at the wafer surfaces. HAADF-STEM observation of the bonding interface revealed an abrupt transition from Al polarity to N polarity through a single monolayer. Additionally, an atomically flat interface and regular atomic arrangement were observed across the measured sample. The bonded sample exhibited XRC FWHMs of 137 arcsec in the (0002) plane and 302 arcsec in the ð10 12Þ plane. No additional stress caused by direct bonding was recognized. Such ideal polarity inversion of III-nitride materials is expected to provide new insight into heteropolar device applications.