An official website of the United States government
β-phase gallium oxide (Ga2O3) is an emerging ultrawide bandgap (UWBG) semiconductor with a bandgap energy of ∼ 4.8 eV and a predicted high critical electric field strength of ∼8 MV/cm, enabling promising applications in next generation high power electronics and deep ultraviolet optoelectronics. The advantages of Ga2O3 also stem from its availability of single crystal bulk native substrates synthesized from melt, and its well-controllable n-type doping from both bulk growth and thin film epitaxy. Among several thin film growth methods, metalorganic chemical vapor deposition (MOCVD) has been demonstrated as an enabling technology for developing high-quality epitaxy of Ga2O3 thin films, (AlxGa1−x)2O3 alloys, and heterostructures along various crystal orientations and with different phases. This tutorial summarizes the recent progresses in the epitaxial growth of β-Ga2O3 thin films via different growth methods, with a focus on the growth of Ga2O3 and its compositional alloys by MOCVD. The challenges for the epitaxial development of β-Ga2O3 are discussed, along with the opportunities of future works to enhance the state-of-the-art device performance based on this emerging UWBG semiconductor material system.
more »
« less
Improving crystal quality of β -phase MgGaO thin films by using low-temperature homo-buffer layer
Ultra-wide bandgap (UWBG) semiconductors are promising for many applications, such as power electronics and deep-ultraviolet photonics. In this research, UWBG β-phase magnesium gallium oxide (MgGaO) thin films with a bandgap of 5.1 eV were grown using low-temperature homo-buffer layers in a plasma-assisted molecular beam epitaxy system. The role of the growth temperature and thickness of low-temperature buffer layer on the quality of the active layer was studied using x-ray diffraction and transmission electron microscopy and by analyzing the properties of metal–semiconductor–metal photodetector devices based on these films. It is found that lower buffer growth temperature at 300 °C leads to higher crystal quality of active layer. For the same low buffer growth temperature, different crystal quality in the active layer is attained with different buffer layer thickness. A buffer layer thickness at 40 nm has the best active layer quality with the highest photo current under 265 nm illumination and long decay time as a result of reduced recombination of photo-generated carriers through fewer defects in the active layer.
more »
« less
- Award ID(s):
- 2105566
- PAR ID:
- 10416109
- Date Published:
- Journal Name:
- Applied Physics Letters
- Volume:
- 122
- Issue:
- 21
- ISSN:
- 0003-6951
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Background carbon incorporation and film cracking issue in (001) β-Ga2O3 films grown by metalorganic chemical vapor deposition (MOCVD) are investigated. Quantitative secondary ion mass spectrometry analysis shows that increasing the O2 flow rate significantly reduces carbon concentration, suggesting the importance of optimizing the VI/III ratio and growth temperature to achieve low compensation and controllable doping in MOCVD of (001) Ga2O3 films. MOCVD growth of (001) β-Ga2O3 films with a film thickness of 25 μm at a growth rate of 10 μm/h is achieved. However, film cracking remains a persistent challenge. Reducing the growth rate by adjusting the trimethylgallium (TMGa) flow rate or increasing chamber pressure effectively suppresses cracking, but it results in slower growth rates. In addition, lower growth temperature and high chamber pressure can help suppressing surface reconstruction and reduce the formation of cracking. Buffer layers grown at 850 °C, 100 Torr, and 58 μmol/min of TMGa significantly improve surface morphology of drift layers. Moreover, the use of AlGaO buffer layers with 8% of Al and a thickness of ∼130 nm leads to a lower crack density. X-ray rocking curve analysis confirms high crystalline quality at a growth rate of 10 μm/h, with no degradation observed from the introduction of an AlGaO buffer layer. These optimized growth conditions effectively improve surface smoothness and minimize defects. Results from this work provide fundamental insights in MOCVD epitaxy of β-Ga2O3 on (001) Ga2O3 substrates, revealing the opportunities and challenges of MOCVD (001) β-Ga2O3 thin films with fast growth rates for high-power electronic device technology.more » « less
-
Transparent conducting oxides, such as Ga-doped ZnO (GZO) and Al-doped ZnO (AZO) are attractive materials for high-performance plasmonic devices operating at telecommunication wavelengths. In this contribution, we compare the growth of epsilon-near-zero GZO and AZO films on sapphire by two different deposition techniques: molecular beam epitaxy (MBE) and atomic layer deposition (ALD). For MBE of GZO, a multiple buffer consisted of a high-temperature MgO layer, a low-temperature ZnO, followed by a high-temperature ZnO layer is employed to assure the crystalline quality of the GZO film. By controlling the growth parameters, including Ga doping level, VI/II ratio, substrate temperature, we are able to produce GZO films at 350 °C with electron mobility between 30 and 50 cm2/V.s, electron concentration up to 7×1020 cm-3, and resistivity down to 2.5×10-4 Ω.cm. For ALD of AZO, without using any buffer, by reducing the Al pulse duration, we are able to grow the AZO films under a large ratio of Al to Zn pulses of 1:6, which improves the activation of Al as an effective dopant. Hence AZO films with electron concentration above 7×1020 cm-3, electron mobility between 10 and 20 cm2/V.s, and resistivity below 6×10-4 Ω.cm have been obtained at 250 °C. The corresponding epsilon-near-zero point in the ALD-grown material was tuned down to 1470 nm. Our data indicate that the ALD method provides a low-temperature route to plasmonic TCOs for telecommunication wavelength range. Effect of electron mobility on optical loss and, therefore, plasmonic figure of merit is discussed.more » « less
-
The EuO/SrTiO3 heterojunction is a promising combination of a ferromagnetic material and a two-dimensional electron system. We explore the deposition of Eu metal on SrTiO3/Si pseudo-substrates, with varying SrTiO3 (STO) thickness, under ultrahigh vacuum conditions. By varying the thickness of the STO layer (2-10 nm) and the deposition temperature (20-300 °C), we investigate the process by which oxygen is scavenged from STO by Eu. In situ x-ray photoelectron spectroscopy is used to investigate the electronic structure of the nominal Eu/STO/Si stack. We find that as a result of Eu deposition, epitaxial EuO is formed on thick STO (6-10 nm), leaving behind a highly oxygen-deficient SrTiO3-δ layer of ∼4 nm in thickness. However, if the thickness of the STO layer is comparable to or less than the scavenging depth, the crystal structure of STO is disrupted and a solid state reaction between Eu, Si, and STO occurs when the deposition is done at a high temperature (300 °C). On the other hand, at a low temperature (20 °C), only a 1-2 nm-thick EuO interlayer is grown, on top of which the Eu metal appears to be stable. This study elucidates the growth process under different conditions and provides a better understanding and control of this system.more » « less
-
We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow α-(AlxGa1−x)2O3 films on (110) sapphire substrates over the 0 < x < 0.95 range of aluminum content. In S-MBE, 99.98% of the gallium-containing molecular beam arrives at the substrate in a preoxidized form as gallium suboxide (Ga2O). This bypasses the rate-limiting step of conventional MBE for the growth of gallium oxide (Ga2O3) from a gallium molecular beam and allows us to grow fully epitaxial α-(AlxGa1−x)2O3 films at growth rates exceeding 1 µm/h and relatively low substrate temperature (Tsub = 605 ± 15 °C). The ability to grow α-(AlxGa1−x)2O3 over the nominally full composition range is confirmed by Vegard’s law applied to the x-ray diffraction data and by optical bandgap measurements with ultraviolet–visible spectroscopy. We show that S-MBE allows straightforward composition control and bandgap selection for α-(AlxGa1−x)2O3 films as the aluminum incorporation x in the film is linear with the relative flux ratio of aluminum to Ga2O. The films are characterized by atomic-force microscopy, x-ray diffraction, and scanning transmission electron microscopy (STEM). These α-(AlxGa1−x)2O3 films grown by S-MBE at record growth rates exhibit a rocking curve full width at half maximum of ≊ 12 arc secs, rms roughness <1 nm, and are fully commensurate for x ≥ 0.5 for 20–50 nm thick films. STEM imaging of the x = 0.78 sample reveals high structural quality and uniform composition. Despite the high structural quality of the films, our attempts at doping with silicon result in highly insulating films.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0147948
