An official website of the United States government
The thermal conductivities of (100) γ-Ga2O3 films deposited on (100) MgAl2O4 substrates with various thicknesses were measured using frequency-domain thermoreflectance. The measured thermal conductivities of γ-Ga2O3 films are lower than the thermal conductivities of (2¯ 01) β-Ga2O3 films of comparable thickness, which suggests that γ-phase inclusions in the doped or alloyed β-phase may affect its thermal conductivity. The thermal conductivity of γ-Ga2O3 increases from 2.3−0.5+0.9 to 3.5±0.7 W/m K for films with thicknesses of 75–404 nm, which demonstrates a prominent size effect on thermal conductivity. The thermal conductivity of γ-Ga2O3 also shows a slight increase as temperature increases from 293 to 400 K. This increase in thermal conductivity occurs when defect and boundary scattering suppress signatures of temperature-dependent Umklapp scattering. γ-Ga2O3 has a cation-defective spinel structure with at least two gallium vacancies in every unit cell, which are the likely source of defect scattering.
more »
« less
Thermal stability and phase transformation of α-, κ(ε)-, and γ-Ga2O3 films under different ambient conditions
Phase transitions in metastable α-, κ(ε)-, and γ-Ga2O3 films to thermodynamically stable β-Ga2O3 during annealing in air, N2, and vacuum have been systematically investigated via in situ high-temperature x-ray diffraction (HT-XRD) and scanning electron microscopy (SEM). These respective polymorphs exhibited thermal stability to ∼471–525 °C, ∼773–825 °C, and ∼490–575 °C before transforming into β-Ga2O3, across all tested ambient conditions. Particular crystallographic orientation relationships were observed before and after the phase transitions, i.e., (0001) α-Ga2O3 → (2¯01) β-Ga2O3, (001) κ(ε)-Ga2O3 → (310) and (2¯01) β-Ga2O3, and (100) γ-Ga2O3 → (100) β-Ga2O3. The phase transition of α-Ga2O3 to β-Ga2O3 resulted in catastrophic damage to the film and upheaval of the surface. The respective primary and possibly secondary causes of this damage are the +8.6% volume expansion and the dual displacive and reconstructive transformations that occur during this transition. The κ(ε)- and γ-Ga2O3 films converted to β-Ga2O3 via singular reconstructive transformations with small changes in volume and unchanged surface microstructures.
more »
« less
- Award ID(s):
- 2324375
- PAR ID:
- 10615125
- Publisher / Repository:
- AIP Publishing
- Date Published:
- Journal Name:
- Applied Physics Letters
- Volume:
- 125
- Issue:
- 9
- ISSN:
- 0003-6951
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Halide vapor phase epitaxial (HVPE) Ga2O3 films were grown on c-plane sapphire and diamond substrates at temperatures up to 550 °C without the use of a barrier dielectric layer to protect the diamond surface. Corundum phase α-Ga2O3 was grown on the sapphire substrates, whereas the growth on diamond resulted in regions of nanocrystalline β-Ga2O3 (nc-β-Ga2O3) when oxygen was present in the HVPE reactor only during film growth. X-ray diffraction confirmed the growth of α-Ga2O3 on sapphire but failed to detect any β-Ga2O3 reflections from the films grown on diamond. These films were further characterized via Raman spectroscopy, which revealed the β-Ga2O3 phase of these films. Transmission electron microscopy demonstrated the nanocrystalline character of these films. From cathodoluminescence spectra, three emission bands, UVL′, UVL, and BL, were observed for both the α-Ga2O3/sapphire and nc-Ga2O3/diamond, and these bands were centered at approximately 3.7, 3.2, and 2.7 eV.more » « less
-
Teherani, Ferechteh H.; Rogers, David J. (Ed.)We demonstrated a metal-organic chemical vapor deposition (MOCVD) of smooth, thick, and monoclinic phase-pure gallium oxide (Ga2O3) on c-plane sapphire using silicon-oxygen bonding (SiOx) as a phase stabilizer. The corundum (α), monoclinic (β), and orthorhombic (ε) phases of Ga2O3 with a bandgap in the 4.4 – 5.1 eV range, are promising materials for power semiconductor devices and deep ultraviolet (UV) solar-blind photodetectors. The MOCVD systems are extensively used for homoepitaxial growth of β-Ga2O3 on (001), (100), (010), and (¯2 01) β-Ga2O3 substrates. These substrates are rare/expensive and have very low thermal conductivity; thus, are not suitable for high-power semiconductor devices. The c-plane sapphire is typically used as a substrate for high-power devices. The β-Ga2O3 grows in the (¯2 01) direction on sapphire. In this direction, the presence of high-density oxygen dangling bonds, frequent stacking faults, twinning, and other phases and planes impede the heteroepitaxy of thick β-Ga2O3. Previously phase stabilizations with SiOx have been reported for tetragonal and monoclinic hafnia. We were able to grow ~580nm thick β-Ga2O3 on sapphire by MOCVD at 750 oC through phase stabilization using silane. The samples grown with silane have a reduction in the surface roughness and resistivity from 10.7 nm to 4.4 nm and from 371.75 Ω.cm to 135.64 Ω.cm, respectively. These samples show a pure-monoclinic phase determined by x-ray diffraction (XRD); have tensile strain determined by Raman strain mapping. These results show that a thick, phase-pure -Ga2O3 can be grown on c-plane sapphire which can be suitable for creating power devices with better thermal management.more » « less
-
β-Ga2O3 is an emerging ultra-wide bandgap semiconductor, holding a tremendous potential for power-switching devices for next-generation high power electronics. The performance of such devices strongly relies on the precise control of electrical properties of β-Ga2O3, which can be achieved by implantation of dopant ions. However, a detailed understanding of the impact of ion implantation on the structure of β-Ga2O3 remains elusive. Here, using aberration-corrected scanning transmission electron microscopy, we investigate the nature of structural damage in ion-implanted β-Ga2O3 and its recovery upon heat treatment with the atomic-scale spatial resolution. We reveal that upon Sn ion implantation, Ga2O3 films undergo a phase transformation from the monoclinic β-phase to the defective cubic spinel γ-phase, which contains high-density antiphase boundaries. Using the planar defect models proposed for the γ-Al2O3, which has the same space group as β-Ga2O3, and atomic-resolution microscopy images, we identify that the observed antiphase boundaries are the {100}1/4 ⟨110⟩ type in cubic structure. We show that post-implantation annealing at 1100 °C under the N2 atmosphere effectively recovers the β-phase; however, nano-sized voids retained within the β-phase structure and a γ-phase surface layer are identified as remanent damage. Our results offer an atomic-scale insight into the structural evolution of β-Ga2O3 under ion implantation and high-temperature annealing, which is key to the optimization of semiconductor processing conditions for relevant device design and the theoretical understanding of defect formation and phase stability.more » « less
-
β-Ga2O3 is a promising ultra-wide bandgap semiconductor whose properties can be further enhanced by alloying with Al. Here, using atomic-resolution scanning transmission electron microscopy, we find the thermodynamically unstable γ-phase is a ubiquitous structural defect in both β-(AlxGa1−x)2O3 films and doped β-Ga2O3 films grown by molecular beam epitaxy. For undoped β-(AlxGa1−x)2O3 films, we observe γ-phase inclusions between nucleating islands of the β-phase at lower growth temperatures (∼500–600 °C). In doped β-Ga2O3, a thin layer of the γ-phase is observed on the surfaces of films grown with a wide range of n-type dopants and dopant concentrations. The thickness of the γ-phase layer was most strongly correlated with the growth temperature, peaking at about 600 °C. Ga interstitials are observed in the β-phase, especially near the interface with the γ-phase. By imaging the same region of the surface of a Sn-doped β-(AlxGa1−x)2O3 after ex situ heating up to 400 °C, a γ-phase region is observed to grow above the initial surface, accompanied by a decrease in Ga interstitials in the β-phase. This suggests that the diffusion of Ga interstitials toward the surface is likely the mechanism for growth of the surface γ-phase and more generally that the more-open γ-phase may offer diffusion pathways to be a kinetically favored and early forming phase in the growth of Ga2O3. However, more modeling and simulation of the γ-phase and the interstitials are needed to understand the energetics and kinetics, the impact on electronic properties, and how to control them.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0214500
