1. Introduction

The wafer bonding technique, in which semiconductor junctions with different crystal structures or lattice constants can be formed by using this method, is playing an important role in micro electromechanical systems (MEMS)^{[1, 2]}, photoelectric devices^{[3, 4]}, and high-performance silicon-on-insulator (SOI) wafers^{[5, 6]}. At present, many methods, such as conventional hydrophilic bonding, HF-treated hydrophobic bonding, surface-activated bonding (SAB), and plasma-assisted bonding, have been applied to form Si-based junctions.

Compared to conventional hydrophobic and hydrophilic wafer bonding, the plasma-assisted method can achieve a strong bonding strength at low temperature. Nevertheless, because of the hydrophilic reaction during post annealing, an oxidation layer was formed at the bonded interface^[7]. The existence of the oxidation layer greatly limits the carrier transport across the bonded interface. It was reported that the I–V curves of the p-Si/p-Si junctions based on the plasma treatment technique shows nonlinear characteristics, and large resistance exists at the bonded interface by reason of the fact that the oxide interlayer is regarded as a potential barrier at the bonded interface^[8]. The junction resistance of the bonded wafers based on the traditional hydrophilic methods is also at least three orders of magnitude larger than that of the bare Si substrate^[9].

Refs. [10, 11] reported that the linear I–V characteristic of the Si bonded wafers can be achieved by a high-vacuum activation method^[10] or hydrophobic bonding^[11]. The high-vacuum activation method can form an oxide-free bonded interface after the surface has been cleaned by Ar⁺ bombardment and wafer bonding in a high vacuum. However, an amorphous-like layer appears around the bonded interface and is recrystallized after thermal annealing at 600 °C^[12]. In addition, the cost of the ultra-high vacuum cannot be ignored.

The hydrophobic direct bonding can also produce an oxide-free bonded interface due to the fact that the hydrophobic reaction cannot produce the oxide interlayer at the Si/Si interface^[11]. It is reported that the ohmic Si/Si junctions^[13] and semi-ohmic p⁺-Si/n⁺-Si reverse biased tunnel diodes^{[14, 15]} have been fabricated by the hydrophobic method. Unfortunately, this hydrophobic direct bonding is difficult to achieve due to the fact that the Si–H bond is weaker than the Si–OH bond. Similarly, in order to improve the bonding strength and the electrical characteristics of bonded wafer pairs, high-temperature post annealing is inevitable.

In this work, an amorphous Ge intermediate layer is introduced into the Si bonded interface to lower the annealing temperature and achieve good electrical characteristics. The oxide layer is not observed at the Si/Si interface and the ohmic n-Si/n-Si junction is achieved. Due to there being no oxide layer around the interface, the ohmic properties of junctions can be achieved based on this bonding technology. The reduction in electrical resistance across the interface can realize a high-efficiency multi-junction solar cell. Thus, the low-temperature bonding technology based on the a-Ge interlayer can also be widely used to fabricate novel multi-junction solar cells. The carrier transport mechanisms at the p-Si/n-Si bonded interface were also studied using a different thermal annealing process. The crystallization of a-Ge around the Si/Si interface could be responsible for the different carrier transport at the different annealing temperature.

2. Experimental methods

For bonding experiments, the (100)-ordered n-Si (0.018 Ω·cm) and p-Si (0.03 Ω·cm) substrates were used to fabricate n-Si/n-Si and p-Si/n-Si junctions. The diameter and thickness of the wafers were 2 inch and 450 μm, respectively. All the Si substrates were cleaned with the RCA process, and then were dried by a spin dryer. A three-target magnetron sputtering system (TRP-450) was used for the deposition of the a-Ge. After cleaning, the substrates were brought into the vacuum chamber for the deposition of amorphous Ge film. The background pressure was set to 1 × 10^–4 Pa and the sputtering pressure was 0.3 Pa during sputtering. The DC-magnetron sputtering system was used to grow a thin a-Ge layer (3, 5, 10, and 20 nm) on Si substrates with a temperature of 300 K (input power = 20 W). For the first set of samples, the Si substrates with a thin a-Ge layer were immerged into the deionized water for 2 min and then were immediately dried by a spin dryer. For the second set of samples, the fresh a-Ge/Si wafers were instantaneously bonded in the air without immersing them in deionized water. Finally, all samples were attached in air and annealed in the tube furnace to enhance the bonding strength. After bonding, 300 nm thick Pt was grown on both backsides of all bonded wafer pairs. All the bonded wafer pairs were diced into 4 mm² chips for the examination of the electrical properties.

The interfacial bubbles of the bonded wafer pairs were analyzed by a C-mode scanning acoustic microscope (CSAM). The morphology of the bonded interface was examined by a scanning electron microscope (SEM) and cross-sectional transmission electron microscopy (TEM). The I–V characteristic was evaluated to identify the electrical properties.

3. Results and discussion

Fig. 1(a) shows the interfacial bubbles of the bonded wafers (2 inch) with a 20 nm thick a-Ge layer, which were immersed into the deionized water and annealed at 350 °C for 20 h. One can see that a mass of bubbles appears at the Si/Si interface. The appearance of the interfacial bubbles can be explained by the hydrophilic reaction, as shown in Eqs. (1) and (2). For the bonded wafer pairs annealed at 400 °C, as shown in Fig. 1(b), it is surprising that the bubbles almost disappear at the bonded interface, and the two big bubbles can be explained by the absorption of particles on the a-Ge surface before bonding.

Compared to the bonded samples with DI water immersion, the bubble density of the samples without the immersing of DI water clearly decreases at 350 °C, as shown in Fig. 1(c). This indicates that the number of the –OH groups decreases for the absence of DI water immersion and the decrease of –OH groups lead to the reduction of the bubbles. Similarly, for the bonded wafer with annealing at 400 °C, the interfacial bubbles almost disappear. Finally, we obtain a near-void-free bonded interface at a low temperature of 350 °C. Our previous paper^[16] has reported that the high bonding strength can be achieved with the disappearance of bubbles.

$${\rm{Ge}} \!-\! {\rm{OH}}+ {\rm{OH}}\!-\! {\rm{Ge}}\to {\rm{Ge}}\!-\! {\rm{O}}\!-\! {\rm{Ge}}+ {\rm{H}}_2 {\rm{O}} ,$$

(1)

$${\rm{Ge}}+2 {\rm{H}}_2 {\rm{O}}\to {\rm{GeO}}_2+2 {\rm{H}}_2 ,$$

(2)

The SEM photographs of the Si/Si interface annealed at 400 °C for 20 h are shown in Fig. 2. A uniform Ge transition layer with a thickness of ～40 nm was sandwiched between two Si substrates. What is more, no structural defects such as cracks were observed around the Si/Si interface. The uniformity of the bonded interface indicates that the bonded wafers were connected perfectly in crystallography by a thin a-Ge interlayer.

In order to identify the crystallinity of the amorphous Ge film and the evolution of bubble density at different temperatures, the HRTEM image of the Si/Si interface of the samples (3 nm) was obtained, as shown in Fig. 3. It can be seen in Fig. 3(a) that a clear amorphous transient layer of ~5 nm in thickness is formed and the hydrophilic oxidation layer cannot be observed at the Si/Si interface. Furthermore, we have performed the EDS spectroscopy analysis of the bonded interface in our previous paper^[17]. Further, the signal of the O atoms is absent. This indicates that no oxide layer was formed at the bonded interface. This suggests that the a-Ge/a-Ge layers at the bonded interface integrate well. For the bonded wafer pairs with annealing at 350 °C for 20 h, the recrystallization of the amorphous transient layer was not observed at the Si/Si interface. However, for the samples annealed at 400 °C, as shown in Fig. 3(b), a number of dark areas around the Ge/Si interface can be observed in the white circle. This indicates that the poly-crystalline Ge has formed at these dark areas. In addition, the recrystallization of the amorphous Ge film starts at the a-Ge/Si interface. Due to the large difference of the coefficient of thermal expansion between Si (2.6 × 10^–6 K^–1) and Ge (6 × 10^–6 K^–1), the a-Ge layer suffers from high stress during the post annealing process. Ref. [18] reported that the amorphous Ge film on the polymer substrate introduced a stress in a polymer substrate, which can further reduce the crystallization. Thus, the crystallization of amorphous Ge film triggered by the induction of the Si substrate and the high stress in amorphous Ge (stress-induced crystallization) plays a major role in the crystallization process. In addition, as shown in Fig. 3(b), the oxidation layer also cannot be observed at the Si/Si interface. Thus, the semiconductor to semiconductor bonding is achieved.

This crystallization process can also explain the disappearance of the bubbles after post annealing at 400 °C, as illustrated in the following. The Si substrates with the fresh a-Ge layer show intensely hydrophilic properties (contact angle = ~3° no shown here). With the DI water immersion, the hydrophilic a-Ge film absorbs a mass of –OH groups. The thermal annealing process at 350 °C induces the hydrophilic reaction, leading to the increase of bubbles at the bonded interface. It is significant to note that when 400 °C annealing is conducted, the crystallization of a-Ge takes place and the bubbles disappear; it suggests that the polycrystalline Ge (poly-Ge) can absorb the H₂ at the bonded interface due to the existence of the grain boundary. The H₂ may migrate to the grain boundary, resulting in the disappearance of the bubbles at 400 °C.

The I–V characteristic of the bonded n-Si/n-Si wafers (2 × 2 mm², 20 pieces) with 20 nm thick amorphous Ge interlayer annealed at 400 °C for 20 h was measured at 300 K, as shown in Fig. 4(a). It can be seen that the I–V characteristics of twenty Si/Si pieces were similar to each other and reveal ohmic features. This indicates that the electrical properties of n-Si/n-Si junctions based on the oxide-free and bubble-free a-Ge bonded interfaces were relatively good.

Fig. 4(b) shows the I–V characteristics of n-Si/n-Si junctions annealed at different temperatures and those of the n-Si substrates. One can see that the I–V curves of all the bonded wafer pairs show linear characteristics. By the fitting of the I–V curve (least square method), the resistances of the n-Si/n-Si junctions annealed at 300, 350, 400 °C, and the n-Si substrates were determined to be 0.51, 0.32, 0.24, and 0.22 Ω·cm², respectively. The resistance of n-Si/n-Si junctions decreases with the increase of the annealing temperature. This can be attributed to the recrystallization of a-Ge at the interface with the elevation of the annealing temperature. As we all know, the poly-Ge shows higher electrical conductivity and lower material defects. Thus, this variation trend of the I–V curve with temperature may be due to the increase of the electrical conductivity and the decrease of the defect density at the interface. It has been reported that a potential barrier and depletion layer is formed at the bonded interface due to the defect states trap carriers^[19]. With the reduction of defect state density, the potential barrier height can be lower and the conductive properties of the Si-based junctions can be improved^[10]. Thus, in our work, the height of the potential barrier across the bonded interface decreases with the increase of the annealing temperature.

These results are very different from those found in conventional direct wafer bonding^[9], where the resistances at the bonded interfaces are larger than their bulk resistance. This was demonstrated to be attributed to the residual oxide layer formed around the bonded interface. Therefore, we can conclude that the Si/Si bonded wafers without an oxide layer show excellent electrical properties.

In order to confirm the effect of the amorphous Ge interlayer thickness on the electrical properties of Si-based PN junctions, the I–V characteristics of the samples with different amorphous Ge interlayer thickness are shown in Fig. 5. It indicates that the current density of the Si-based PN junction annealed at 350 °C significantly decreases with the augment of the amorphous Ge thickness, while that of the samples annealed at 400 °C only shows a small decrease with the augment of the amorphous Ge interlayer thickness. We can explain this feature as follows.

As mentioned above, the amorphous Ge interlayer at the Si/Si interface does not crystallize when the samples were annealed at 350 °C. Thus, with the augment of the amorphous Ge interlayer thickness, the resistance at the bonded interface increases due to the introduction of the amorphous material with a low migration rate. However, when the wafer pairs were annealed at 400 °C, a part of the a-Ge transforms to be poly-Ge. The migration rate of poly-Ge is larger than that of the a-Ge. As a result, the current density of the samples shows a little decrease when the thickness of the amorphous Ge interlayer increases.

Fig. 6 illustrates the temperature dependence of I–V characteristics for the samples annealed 350 and 400 °C. Note that the slope of the current at reverse-bias voltages between 0.3 and 0.75 V is almost invariant to temperature. It indicates that the trap-assisted tunneling process is the dominant mechanism of the carrier transport across the interfaces. It was reported that the characterization of the current for reverse-bias voltages can be explained by a numerical analysis of trap-assisted tunneling through 0.2 eV traps^[20]. As shown in Fig. 6, the ideality factor of both samples increases with the decrease of the detected temperature. This indicates that with the decrease of temperature, tunneling current is dominant at low temperatures, and this phenomenon is strengthened with the decrease of temperature. For the sample annealed at 350 °C, the measured ideality factor values, which are larger than 2, suggest that the trap-assisted tunneling process dominates the conduction mechanism. For the sample annealed at 400 °C, the increasing annealed temperature has increased the ideality factor of the junctions from 2.11 to 1.59 (at 295 K). It suggests that the increasing annealed temperature improves the performance of the device.

As we all know, the activation energy (E_a) of PN junction diodes is a significant factor to characterize the carrier transport mechanism of PN junction diodes^[21]. When E_a ≈ E_g/2, the dark current is dominated by the carrier recombination mechanism, when E_a < E_g/2, trap-assisted tunneling plays a major role in the dark current, and when E_a ≈ E_g, the diffusion current can be responsible for the dark current. E_a is given by Eq. (3) at reverse bias voltage –1 V.

$${E_{\rm a}} = \frac{{\partial (\ln J)}}{{\partial (\frac{1}{{kT}})}},$$

(3)

where J is the current density across the junctions, T is the test temperature, and k_B is the Boltzmann constant. As shown in Fig. 7, the E_a of p-Si/n-Si junctions annealed at 350 °C is approximately 0.34 eV; this is a little smaller than E_g/2 of a-Ge (E_g = 0.88 eV)^[22]. Thus, the carrier transport mechanism for the p-Si/n-Si junctions annealed at 350 °C can be explained by the trap-assisted tunneling model^[20]. It was reported that the a-Ge has a high density of states in the band-gap^[23], especially for the a-Ge without the passivation of hydrogen. Mott et al.^[24] suggested that the electron transferring channel is formed by these localized states above the Fermi level. Thus, the carriers tunneling through the a-Ge layer by the capture and emission process of traps play a major role at this temperature. The E_a of p-Si/n-Si junctions annealed at 400 °C is approximately 0.51 eV, which is close to E_g/2 of a-Ge. This indicates that generation recombination current dominates the reverse current. When the temperature increases to 400 °C, the amorphous Ge starts to crystallize, leading to the decrease of the residual a-Ge layer thickness. This also results in the reduction of the localized state density in the Ge transition layer. The effect of trap-assisted tunneling becomes weak due to the thin a-Ge, instead, the carrier is easier to recombine at the traps in poly-Ge.

Figs. 8(a) and 8(b) show schematic band diagrams of the Si-based PN junctions at reverse-bias voltage. These p-Si/n-Si band diagrams are based on the carrier concentrations of Si, which are 6.25 × 10¹⁸ cm^–3 (n-Si) and 5.70 × 10¹⁷ cm^–3 (p-Si). The diffusion potentials are found to be 1.0 V and the reverse-bias voltage is –1 V. As shown in Fig. 8(a), for the junctions annealed at 350 °C，the a-Ge film has a high density of localized states in the gap. The high density of interface states above the fermi level would be sufficiently great to form an electron transferring channel. Then electrons can tunnel through the bonded interface directly. For the samples annealed at 400 °C, as the a-Ge layer was recrystallized gradually, the misfit dislocations were introduced into the Ge/Si interface. Meanwhile, the density of localized states decreased at the bonded interface. The recombination of electrons and holes occurs at the Ge/Si interface, as shown in Fig. 8(b). The carrier transport mechanism for the reverse-biased Si-based PN junctions is consistent with the generation-recombination-current model.

4. Conclusion

The effect of annealing temperature and a-Ge layer thickness on the electrical properties of p-Si/n-Si and n-Si/n-Si junctions manufactured by introducing a thin a-Ge interlayer is studied. The bubbles at the bonded interface can be totally eliminated when 400 °C-annealing is conducted and the bubble density drastically decreases at 350 °C when the DI water immersion of a-Ge is absent. The n-Si/n-Si junction shows excellent ohmic features. The resistance of the n-Si/n-Si junction decreases with the increase of annealing temperature and is finally close to the bulk resistance of n-Si at 400 °C. This is due to the crystallization of amorphous Ge with annealing at 400 °C. The resistance of the bonded junction increases as the thickness of a-Ge increases from 3 to 20 nm. The E_a of p-Si/n-Si annealed at 350 and 400 °C are extracted to be 0.34 and 0.51 eV, respectively. Thus, trap-assisted tunneling and generation-recombination models can be responsible for the carrier transport at the p-Si/n-Si bonded interface annealed at 350 and 400 °C, respectively.