GOX 2022 Session HM-MoP: Heterogeneous Material Integration Poster Session

The impact of post-bond annealing on the structural and thermal characteristics of 140-nm thick exfoliated (201) β-Ga₂O₃ (via H⁺ ion implantation [1]) direct wafer bonded to (0001) 4H-SiC was studied. For these studies, 30 nm amorphous alumina was grown on the β-Ga₂O₃ substrates prior to bonding as an interlayer between the β-Ga₂O₃ and 4H-SiC. The surface activated bonding technique was utilized for bonding, which induces a thin ~nm amorphous interfacial region at the bonded interface (alumina | 4H-SiC) [2]. We demonstrate annealing the bonded structure at 800 °C in ambient air up to 1 hour is beneficial: (1) removal of residual strain in the exfoliated β-Ga₂O₃ layer that was due to the exfoliation implant, (2) reduction of lattice mosaicity in the β-Ga₂O₃ layer, and (3) recrystallization of the amorphous bonded interfacial region. The thermal characteristics correspondingly improve with the improvement in structural characteristics. The thermal conductivity of the as-bonded β-Ga₂O₃ layer was 2.9 W/m∙K and the thermal boundary conductance (TBC) of the bonded interface was 66 MW/m²∙K [2]. After annealing at 800 °C for 1 hour, triple-axis X-ray diffraction ω:2θ measurements showed a reduction in strain for the β-Ga₂O₃ layer and the symmetric (201) rocking curve widths. Transmission electron microscopy images of the bonded interface show that the amorphous bonded interfacial region recrystallized. We simultaneously observe a doubling of the β-Ga₂O₃ thermal conductivity to 6.0 W/m∙K and a twenty percent increase in the TBC. While the previous results showed the promise of exfoliation of b-Ga₂O₃ on 4H-SiC, here we demonstrate that annealing further improves both structural and thermal properties.

References:

1. M. Burel, et al., Jpn. J. Appl. Phys., 36, 1636 (1997).

2. Z. Cheng, et al., ACS Appl. Mater. Interfaces, 12, 40 (2020).

The authors M.E.L., K.H., Y.W., Z.C., J.S., S.G., and M.S.G. would like to acknowledge the support from the Office of Naval Research through a MURI program, grant No. N00014-18-1-2429.

Ga₂O₃ is a promising material for next generation electronics. Recently, we demonstrated that plasma-enhanced atomic layer deposition (PEALD) can be used to attain heteroepitaxial metastable phases [1], specifically α and ε/κ, and the full stoichiometric range of (Al_xGa_1-x)₂O₃ films. PEALD is a conformal, energy-enhanced synthesis method for thin films with many advantages, including: deposition at reduced growth temperatures, access to metastable phases, elimination of miscibility gaps, and improved crystallinity and growth rates over conventional ALD. In this work, we present new insights into the microstructure and uniformity of metastable ε-Ga₂O₃ films integrated on GaN. Additionally, we explore the use of PEALD for fabricating metastable nucleation layers that can be expanded by more traditional, faster deposition techniques.

Transmission electron microscopy (TEM) was used to correlate PEALD parameters with the resulting Ga₂O₃ microstructure on GaN substrates. A low temperature, low pressure PEALD regime produced ~95% epsilon phase films with a strained interfacial layer and 10-50nm grains. The remaining portion was a combination of alpha and beta phases that were primarily segregated towards the surface of the film. In comparison, a higher temperature, high-pressure growth regime produced uniform, pure epsilon phase films with 2-5 nm grains. The abrupt interface was fully relaxed with misfit dislocations at a spacing of ~2nm, explaining the resulting ε-Ga₂O₃grain size. There was no evidence of kappa or gamma phase as often seen with low temperature heterogeneous integration of Ga₂O₃ films.

While PEALD is beneficial for depositing thin films of metastable phases, practical devices often require much thicker barrier and active layers. For this reason, we investigated integrating PEALD metastable Ga₂O₃ films with traditional semiconductor growth techniques, such as molecular beam epitaxy (MBE), capable of extending these layers beyond 100 nm in thickness. The same MBE conditions were used to deposit Ga₂O₃ films on GaN substrates with and without PEALD ε-Ga₂O₃ nucleation layers.Those deposited without the PEALD nucleation layer produced β-phase films, while those with nucleation layers resulted in pure ε-phase films. We also show a similar capability using metal-organic chemical vapor deposition (MOCVD).This shows importance of PEALD for realizing practical device structures using metastable phases.This concept could also be expanded to α-Ga₂O₃ film, though this is more easily achieved directly on sapphire substrates.

[1] Wheeler et al., Chem. Mater. 2020, 32, 1140-1152

Gallium oxide (Ga₂O₃) is a promising semiconductor for the next-generation power devices, due to its ultra-wide bandgap (4.9 eV), high electron saturation velocity (1.1 × 10⁷ cm/s), and high breakdown electric field (8 MV/cm) and the mature growth technique for large diameter native substrates. While many high-performance unipolar devices, e.g., field-effect transistors (FET) and Schottky diodes, have been reported recently, the lack of p-type Ga₂O₃ limits the development of Ga₂O₃ based bipolar devices. Wafer bonding of p-type diamond with Ga₂O₃ and growth of p-type polycrystalline on Ga₂O₃ have been attempted to realize bipolar (pn) devices with limited performance. The recent semiconductor grafting approach shows the potential of forming semiconductor heterostructures without concerning about lattice mismatch. The grafting approach could be exploited to form p-n heterojunctions by combining n-type Ga₂O₃ with monocrystalline p-type semiconductors. In this talk, preliminary results of p-Si/n-Ga₂O₃ pn diode fabricated by semiconductor grafting will be presented. The Ga₂O₃ epi layer consists of Sn-doped 150 nm n- (2e17 cm^-3) on a Sn doped 500-600µm n⁺-Ga₂O₃ (5e18 cm^-3) substrate. A ~185 nm thick boron doped (5e19 cm^-3) single crystalline Si (native oxide free) was released from SOI and transferred to the top of the n-Ga₂O₃ epi substrate. A thermal anneal of 350 °C for 5 mins was performed to form chemical bonding while the top layer of Ga₂O₃ is expected to serve as the interface passivation layer. Ni/Au/Cu/Au metal stack was deposited on the p-Si as anode contact and a Ti/Au/Cu/Au was deposited on the backside (n+) Ga₂O₃ as cathode contact. Without any further thermal annealing, we have achieved an I-V characteristic with I_on/I_off = 1.5e8 at ±2 V and ideality factor n = 1.4. The device performances were compared with a Schottky diodes fabricated using the same epi substrate. The study has shown the feasibility of grafting in developing Ga₂O₃-based bipolar devices, despite that further improvement of device performance is expected.