Variation in atomistic structure due to annealing at diamond/silicon heterointerfaces fabricated by surface activated bonding

Chemical composition around diamond/silicon heterointerfaces fabricated by surface activated bonding (SAB) at room temperature is examined by energy-dispersive X-ray spectroscopy under scanning transmission electron microscopy. Iron impurities segregate just on the bonding interfaces, while oxygen impurities segregate off the bonding interfaces in the silicon side by 3–4 nm. Oxygen atoms would segregate so as to avoid the amorphous compound with silicon and carbon atoms, self-organized at the bonding interfaces in the SAB process. When the bonding interfaces are annealed at 1000 °C, the amorphous compound converts into cubic silicon carbide (c-SiC), and nano-voids 5–15 nm in size are formed at the region between silicon and c-SiC, at which the oxygen density is high before annealing. The nano-voids can act as the gettering sites in which metal impurities are preferentially agglomerated, and the impurity gettering would help to improve the electronic properties of the bonding interfaces by annealing.


Introduction
Diamond crystals have superior physical properties towards high power, high frequency, and low-loss electronic devices, 1) for which the figures of merit are extremely high. 2) They have a wide bandgap, high carrier mobility, 3) high saturation velocity, 4) and the highest electrical breakdown field strength, 5) that are suitable for power electronics. 6,7) Also, they have high thermal conductivity, 8) that are suitable for superior heat spreaders of power devices for the next generation applications beyond 5G, 9) as demonstrated in heterointerfaces between diamond and gallium nitride (GaN) 10,11) or silicon (Si). 12,13) Although high-quality monocrystalline diamond wafers are commercially available, 14,15) their surface size is rather small (<15 × 15 mm 2 ) and they are still expensive for device manufacturing. A possible solution is bonding diamond chips with an inexpensive Si wafer, which would be effective for developing electronics applications. 16,17) In order to design high-performance power devices, the thermal stability of diamond/Si bonding interfaces is an important issue, because the interfaces are exposed to high temperatures during device fabrication and operation processes. Diamond/Si bonding interfaces with high crystallinity, free from adhesive intermediate layers, can be fabricated by fusion bonding at elevated temperatures above 1150°C 18) and by hydrophilic bonding at about 200°C . 19,20) However, the interfaces would be cracked at elevated temperatures due to a large thermal expansion coefficient mismatch, and therefore mechanical and thermal properties of the interfaces would be degraded via the structural modification. Similar interfaces can be fabricated using adhesive metallic layers, 21) that could suppress the structural modification at elevated temperatures. However, the metallic layers can act as an impurity source, and the electronic properties of the interfaces could be degraded via the migration of metallic impurities at elevated temperatures.
Recently, it is shown that diamond/Si heterointerfaces fabricated by surface activated bonding (SAB) at room temperature (RT) are not cracked even after 1000°C annealing, 17) via some kind of stress relaxation. 22) This mechanical stability against thermal processes is attributed to a composition-graded amorphous layer with carbon and silicon atoms self-organized in the bonding process. 23) According to the model, the amorphous compound is crystallized forming cubic silicon carbide (c-SiC) during 1000°C annealing, and the crystallized layer would suppress the concentration of thermal stresses at the interface and consequently keep the crack-free interface in thermal processes. 23) Meanwhile, impacts of compositional modification around the interfaces by annealing, including impurity diffusion, on electronic properties are not fully examined, even though the diamond devices fabricated on the interfaces are operated well. 16) In the present work, we have examined the chemical composition around the interfaces and discussed how the impacts of impurity atoms can be suppressed.

Experimental methods
A diamond/Si heterointerface was fabricated by SAB with a square wafer (4 mm × 4 mm × 0.65 mm in size) of high-pressure, high-temperature synthetic Ib type (100) monocrystalline diamond and a rectangular wafer (12 mm × 10 mm × 0.52 mm) of (100) n-Si (2.6 × 10 16 mm −3 ). 17) The wafers were activated at RT in a high vacuum (below 5 × 10 −5 Pa) using an argon (Ar) atom beam with a current of 1.65 mA at an applied voltage of 1.6 kV. The activated wafers were then pressed each other for bonding immediately after the activation process. The exact location of the bonding interface was determined with iron (Fe) impurities introduced intentionally in the surface activation process. 24) A part of the bonding interface was annealed at 1000°C for 12 h in a nitrogen gas ambient. 16) Specimens with the interface for scanning transmission electron microscopy (STEM) were fabricated using the following steps. A thick foil (more than a few micrometers thick) with the diamond/Si heterointerface was cut out by using a conventional focused ion beam (FIB) system, equipped with a high-resolution scanning electron microscope (SEM) (FEI, Helios NanoLab600i), 25) and mounted on a lift-out grid for STEM. The surface normal for Si side was 〈110〉, while that for the diamond side was 〈100〉. Then, the foil was thinned by FIB milling operated at low temperatures (LT-FIB), since the conventional FIB milling operated at RT (RT-FIB) easily introduce structural defects at SAB-fabricated interfaces. 26,27) The foil was thinned to about 100 nm thick by LT-FIB milling with a cold stage operated at −150°C (IZUMI-TECH, IZU-TSCS004) to suppress the structural modification in the FIB processes. 24,[27][28][29] The chemical composition around the interface was examined by energy-dispersive X-ray spectroscopy (EDX) under STEM, using a JEOL JEM-ARM200F analytical microscope. The impurity detection limit was about 0.1 at%. The atomic arrangement around the interface was examined by high-angle annular dark-field (HAADF-) STEM using a JEM-ARM200F microscope with an atomic resolution of about 0.12 nm. , and no other impurity atom is detected. It is known that Fe atoms are introduced just on the activated surfaces during the surface activation process, and they slightly diffuse during the bonding process, presumably via the transient enhanced effect. 24) The half-width at half-maximum (HWHM) of the Fe distribution, estimated to be 1 nm, is independent of the specimen thickness, even though the spatial resolution of STEM-EDX would depend on the specimen thickness via the spread of the electron beam. This suggests that the spatial resolution of our STEM-EDX is less than 1 nm, and the estimated resolution is consistent with the previous report. 24) As indicated with the green curve in Fig. 1(f), Fe atoms can diffuse by 2-3 nm in the Si side from the bonding interface. This diffusion length is almost the same as the diffusion length estimated in the SAB-fabricated Si/GaAs 24) and Si/Si 27) interfaces, suggesting the same diffusion mechanism in the bonding process. Similarly, the density of Ar atoms, which are inevitably introduced during the surface activation process, is maximum at the bonding interface, and the Ar density profile across the interface is almost the same morphology as the Fe density profile [the pink curve in Fig. 1(f)]. These results would support the transient enhancement diffusion model that the diffusion lengths of those impurities are dominated by the diffusivity of the point defects assisting the impurity diffusion. 29) On the other hand, the oxygen density peaks of the bonding interface in the Si side by 3-4 nm [the yellow curve in Fig. 1(f)]. Since oxygen atoms can be observed in any SAB-fabricated interfaces, they would be introduced from the residual gas molecules containing oxygen, such as water and oxygen molecules, in the vacuum chamber. At similar SABfabricated interfaces such as GaAs/Si, 32) diamond/GaN, 33) diamond/aluminum, 34) and diamond/copper, 25) the oxygen density peaks just on the bonding interfaces. Meanwhile, the oxygen density at the SAB-fabricated diamond/Si interface seems to have a negative correlation with the excess number of carbon atoms in the Si side [the blue curve in Fig. 1(f)], that are related to an amorphous layer with carbon and Si atoms self-organized in the SAB process. 23) Since the solubility of oxygen in silicon carbide is rather low, 35) oxygen atoms would be kicked out from the bonding interface via the self-organization of the amorphous compound with carbon and Si atoms.

1000°C annealed interfaces
When the bonding interface is annealed at 1000°C, spherical defects about 5-15 nm in size are formed in the Si side [ Fig. 2(a)]. The defects locate inside the Si matrix just on the interface between Si and c-SiC [ Fig. 2(b)], introduced via the crystallization of the amorphous compound with carbon and  At bright spherical defects in HAADF-STEM images, Fe atoms are observed and the number density of Si atoms is slightly decreased [see Figs. 2(b), 2(c), and 2(e)]. No Fe agglomerate is observed at the region free from nano-voids, within the detection limit of our STEM-EDS [ Fig. 2(e)]. These results suggest that Fe atoms would agglomerate inside nano-voids and/or Fe agglomerates are nucleated nearby nano-voids. Therefore, nano-voids would act as gettering sites for Fe atoms. It is shown that electronic properties of SAB-fabricated Si/diamond heterojunction diodes, such as ideality factor, reverse-bias current, and barrier height at Si/ diamond bonding interfaces, are improved by post-bonding annealing, presumably via the reduction of interface states that are formed during the SAB processes. 36) Isolated metal atoms including Fe and point defects, introduced during the surface activation process, can induce defect levels around the bonding interfaces. Meanwhile, metal agglomerates with a moderate size do not affect the electronic properties so much. 37) Gettering of metallic impurities including Fe into nano-voids, as well as the recovery of crystallinity around the bonding interface, 23) would help to reduce the interface states.
Oxygen atoms also impact the electronic properties when they form precipitates (such as oxides) and defect clusters (such as oxygen-vacancy agglomerates) by annealing, while they are electronically inactive when they are isolated. The impacts would be small at the annealed diamond/Si interfaces, since oxygen atoms do not segregate around the interfaces including nano-voids [ Fig. 2(f)]. Figure 2(g) shows that Ar atoms can segregate at nanovoids, as well as at the c-SiC layer, in which the crystallinity is still decreased. 23) Similar Ar segregation into a damaged layer at a SAB-fabricated interface is reported. 38) It is hypothesized that inert Ar atoms would segregate so as to fill the vacant spaces at nano-voids and at the vacant defects introduced during the surface activation process. Agglomerates of inert Ar atoms would not impact the electronic properties, while they may impact the thermal properties of the interfaces.
Finally, we briefly discuss the formation process of nanovoids. Similar nano-voids acting as the gettering sites for Fe and Ar atoms are formed at the Si/Si homointerfaces fabricated by SAB with the same bonding condition (Fig. 3). Unlike at the diamond/Si heterointerfaces, the nano-voids are formed just on the bonding interface, at which the oxygen density is maximum. The results in Figs. 2 and 3 suggest that nano-voids are formed at the region in which a number of oxygen atoms exist. It is hypothesized that oxygen would assist the void formation, by producing joint vacancy-oxygen agglomerates (oxide particles) and by trapping vacancies into VO 2 clusters, as proposed in Czochralski-grown Si ingots. 39) Moreover, nano-voids at the diamond/Si interfaces (5-15 nm in size) are much larger in comparison with the Si/Si interfaces (less than 5 nm in size). It is known that voids are self-organized at the Si/SiC interfaces during the carbonization of Si crystals, 40) and the volume of the voids per unit area is proportional to the oxygen density. 41) Although the formation mechanism is still controversial, the void formation would be correlated with the growth of c-SiC and oxygen impurities.

Conclusions
Chemical composition around SAB-fabricated diamond/Si heterointerfaces is examined by STEM-EDS combined with LT-FIB. Fe and Ar impurities segregate just on the bonding interfaces, while oxygen impurities segregate off the bonding interfaces in the Si side by 3-4 nm, so as to avoid the amorphous compound with carbon and Si atoms introduced at the bonding interfaces. After 1000°C annealing, nanovoids are formed at the region where the oxygen density is high before bonding. They would act as the gettering sites for metal impurities, and the impurity gettering would help to improve the electronic properties of the interfaces by annealing.