2O3 Alloy Research Articles

Assessment of bonding characteristic of β-(AlxGa1−x)2O3 alloys from photoluminescence and x-ray absorption near edge spectroscopy

Bonding characteristics of the β-(AlxGa1−x)2O3 alloy have been assessed from red luminescence and x-ray absorption near edge spectroscopy (XANES). Red luminescence, known as R1 and R2 lines, originates from crystal field split d-levels of unintentionally present Cr3+ ions in the β-(AlxGa1−x)2O3 alloy, is found to blueshift with Al composition. This indicates that the nephelauxetic effect in β-Ga2O3 materials decreases with Al substitution, which results to a decrease in the covalency of the β-(AlxGa1−x)2O3 alloy. The XANES experiments at the oxygen K-absorption edge provide clear evidence of the decrease in the covalency of the alloy, where an absorption peak representing the covalency in the β-Ga2O3 material is found to decrease with an increase in the Al composition. Furthermore, the nature of the observed red luminescence for the β-(AlxGa1−x)2O3 alloy is determined to be excitonic recombination. Our results further confirm the fact that the red luminescence from Cr3+ ions is an excellent local probe to assess the bonding characteristic of the host material.

Observation of mixed-mode behavior of Raman active phonon modes for β-(AlxGa1−x)2O3 alloys

Raman spectroscopy has been used to investigate the phonon mode behavior of a β-(AlxGa1−x)2O3 alloy up to Al compositions of x = 0.474. The alloy displays a mixed-mode behavior, where low frequency (100–200 cm−1) modes corresponding to translation and libration of (Ga1O4) tetrahedral and (Ga2O6) octahedral chains and high frequency (500–800 cm−1) modes related to stretching and bending of (Ga1O4) tetrahedral chains show a monotonic behavior throughout the studied composition range, and display one-mode behavior. On the other hand, medium frequency (300–500 cm−1) phonon modes related to the deformation of (Ga2O6) octahedra and (Ga1O4) tetrahedra chains show sudden change such as the appearance (disappearance) of new (existing) phonon modes corresponding to Al–O (Ga–O) sublattices with Al substitution, and they display a two-mode behavior. The existence of two sublattices is further supported by the observation of preferential occupation of the Al atom at the octahedral atomic site in comparison to the tetrahedral atomic site. The observation of phonon modes related to translation and libration of (Ga1O4) tetrahedral and (Ga2O6) octahedral chains up to the Al composition of x = 0.474 clearly indicates that the long-range periodicity of the monoclinic lattice is maintained throughout the investigated Al composition range. Our results provide an insight into the phonon mode behavior of β-(AlxGa1−x)2O3 alloys, which is necessary to have a better understanding of their physical properties.

NiO/β-(AlxGa1−x)2O3/Ga2O3 heterojunction lateral rectifiers with reverse breakdown voltage >7 kV

NiO/β-(AlxGa1−x)2O3/Ga2O3 heterojunction lateral geometry rectifiers with diameter 50–100 μm exhibited maximum reverse breakdown voltages >7 kV, showing the advantage of increasing the bandgap using the β-(AlxGa1−x)2O3 alloy. This Si-doped alloy layer was grown by metal organic chemical vapor deposition with an Al composition of ∼21%. On-state resistances were in the range of 50–2180 Ω cm2, leading to power figures-of-merit up to 0.72 MW cm−2. The forward turn-on voltage was in the range of 2.3–2.5 V, with maximum on/off ratios >700 when switching from 5 V forward to reverse biases up to −100 V. Transmission line measurements showed the specific contact resistance was 0.12 Ω cm2. The breakdown voltage is among the highest reported for any lateral geometry Ga2O3-based rectifier.

Effect of lattice constraint on structural stability and miscibility of (Al x Ga1−x )2O3 films: a first-principles study

The structural stability and miscibility of (Al x Ga1−x )2O3 alloys are theoretically investigated by means of electronic structure calculations within density functional theory. Effects of lattice constraint due to α-Al2O3(0001) substrate and (Al0.4Ga0.6)2O3 buffer layer on the structural stability and miscibility are evaluated from the energy differences and the excess energies among α, β, and κ phases. For the constrained systems by α-Al2O3(0001) substrate, only (Al x Ga1−x )2O3 alloys with α phase are found to be stabilized. Furthermore, the lattice constraint drastically improves the miscibility of (Al x Ga1−x )2O3 alloys. These results suggest that the lattice constraint is crucial for the stability and miscibility of (Al x Ga1−x )2O3 alloys.

Stability and electronic properties of two-dimensional Ga2O3 and (MxGa1-x)2O3 (M = Al, Ga) alloys

The alloying of Ga2O3 monolayer with oxides of the same main group element (Al and In) is simulated by the special quasirandom structures method. The stability and electronic properties of the Ga2O3 monolayer and (MxGa1-x)2O3 monolayer alloys are systematically investigated by first-principles calculations. The (AlxGa1-x)2O3 monolayer alloy becomes more stable with the increase of Al2O3 content, but the stability of (InxGa1-x)2O3 monolayer alloy worsens with the increase of In2O3 content. The (AlxGa1-x)2O3 monolayer alloy has relatively better thermodynamic stability, which would be easily synthesized at room temperature. The orbital-projected band structure and charge density reflect that the effect of alloying Ga2O3 with Al2O3 and In2O3 on the electronic properties of materials is mainly on the conduction band. Interestingly, we find that the charge density of the VBM and CBM states for Ga2O3 monolayer and its monolayer alloys exhibit the characteristic of complete separation in real space, i.e., an abnormal carrier separation. Through the calculation of the band-gap bowings parameter, it is predicted that the band gap of the monolayer alloy can continuously be tuned in the range of 3.38 ∼ 7.02 eV. These results would shed some light on the application of the Ga2O3 monolayer and its monolayer alloys in optoelectronic devices.

Growth of (SmxGa1−x)2O3 by molecular beam epitaxy

The (SmxGa1−x)2O3 alloy system is a potential new dielectric for compound semiconductors such as GaAs. Using molecular beam epitaxy under metal-modulated growth conditions, we grew the binary oxide, Sm2O3, at two substrate temperatures (100 and 500 °C) and optimized the structural, morphological, and electrical properties of the films. Decreasing the Sm cell temperature suppressed the formation of the monoclinic phase and promoted the growth of the cubic phase. Next, the ternary oxide, (SmxGa1−x)2O3, was deposited to investigate the effects of Ga incorporation. Optimization experiments were used to determine the effects of substrate temperature and samarium cell temperature (i.e., growth rate) on film stoichiometry, phase distribution, and microstructure in these films. Films grown at 500 °C showed significant surface roughness and the presence of multiple crystalline phases. Since all of the Sm-based oxides (i.e., samarium oxide with and without gallium) were found to have unbonded Sm metal, annealing experiments were carried out in oxygen and forming gas to determine the effects of annealing on film stoichiometry. The motivation behind annealing in forming gas was to see whether this commonly used technique for reducing interface densities could improve the film quality. GaAs metal-oxide-semiconductor diodes with (SmxGa1−x)2O3 showed breakdown fields at 1 mA/cm2 of 4.35 MV/cm, which decreased with increasing Sm unbonded metal content in the films.

(Digital Presentation) MOCVD Growth of Ga2O3, Algao and Heterostructures

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability. It has a band gap of 4.5-4.9 eV, much wider than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). The monoclinic β-phase Ga2O3 represents the thermodynamically stable crystal among the known five known phases (α, β, γ, d, ε/κ). The breakdown field of β-Ga2O3 is predicted to be 6-8 MV/cm, which is much larger than that of the 4H-SiC and GaN. These unique properties make β-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. Single crystal β-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques. Metalorganic chemical vapor deposition (MOCVD) growth technique was demonstrated to produce high quality β-Ga2O3 thin films and its ternary (AlxGa1-x)2O3 alloys. Control of background and n-type doping in β-Ga2O3 will be discussed. Record carrier mobilities of 194 cm2/V·s at room temperature and ~10,000 cm2/V·s at low temperature were measured for MOCVD β-Ga2O3 thin films. Growth and fundamental understanding of β-(AlxGa1-x)2O3 are still limited. The incorporation of Al in beta-phase Ga2O3 has not been well understood, although it was predicted up to 60% of Al composition could be incorporated into β-Ga2O3. MOCVD growth of β-AlGaO targeting for Al composition of > 40% will be discussed. N-type doping capability as a function of Al composition in (AlxGa1-x)2O3 is another important fundamental question. Charge carrier transport properties in (AlxGa1-x)2O3 will be discussed. In addition, MOCVD growth of different phases of Ga2O3 and AlGaO will be presented. Acknowledgement: The authors acknowledge the funding support from the Air Force Office of Scientific Research FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir) and the National Science Foundation (1810041, 2019753, 1755479).

Donor doping of corundum (AlxGa1−x)2O3

Corundum (AlxGa1−x)2O3 alloys have been proposed as a candidate ultrawide-bandgap oxide for a number of applications, but doping is unexplored. We examine the prospects for n-type doping with H, Si, Ge, Sn, Hf, Zr, and Ta in corundum (AlxGa1−x)2O3 alloys using first-principles calculations. All of the dopants are shallow donors in corundum Ga2O3. In the (AlxGa1−x)2O3 alloy, they transition from shallow to deep donors at Al compositions that are unique to each donor. Si and Hf remain shallow donors up to the highest Al contents in corundum (AlxGa1−x)2O3 alloys and are still shallow even as the (AlxGa1−x)2O3 bandgap exceeds 6.5 eV. Finally, we address the detrimental role of cation vacancies as compensating deep acceptors and suggest that doping in a hydrogen-rich environment under cation-rich conditions can be used to overcome this problem.

Electron mobility in ordered β -(AlxGa1−x)2O3 alloys from first-principles

Alloying Ga2O3 with Al2O3 yields diverse structural phases with distinctive optoelectronic properties, making them promising candidates for ultrawide bandgap semiconductors in next-generation power electronics. Yet, there is a lack of sound knowledge of the carrier dynamics in the (AlxGa1−x)2O3 alloys due to their structural complexity. Herein, we focus on the ordered β-(AlxGa1−x)2O3 alloys, predict their carrier mobility, and determine the intrinsic electron mobility limit based on solving linearized Boltzmann transport equations from first principles. The predicted electron mobility for ordered β-(Al0.25Ga0.75)2O3 and β-(Al0.5Ga0.5)2O3 alloys at 300 K, respectively, is 103.6 and 80.60 cm2/V s, demonstrating excellent agreement with literature experiments. Such low electron mobility is limited by the intrinsically strong polar optical phonon (POP) scattering process. As the Al content further increases, the alloy's electron mobility further reduces mainly due to the enlarged Pauling ionicity, Fröhlich coupling constant, and POP scattering. This work provides physical insight into the carrier dynamics in ordered β-(AlxGa1−x)2O3 alloys and seeks to improve the electron mobility for potential applications in high-power electronics.

Determination of Al occupancy and local structure for β-(AlxGa1−x)2O3 alloys across nearly full composition range from Rietveld analysis

Al occupancy and local structure (bond lengths and bond angles) for monoclinic β-(AlxGa1−x)2O3 alloys, with Al compositions (x) up to 90%, have been determined from Rietveld refinement of x-ray diffraction data. An Al atom preferentially occupies an octahedron (Oh) atomic site in comparison with a tetrahedron (Td) atomic site. However, a sizable number of Td atomic sites, i.e., 20% for Al composition of 5%, remain occupied by Al atoms, which is found to increase sharply with Al composition. The Oh atomic sites are not fully occupied by Al atoms even for the Al composition of 90%. The lattice parameters (bandgap) of the β-(AlxGa1−x)2O3 alloy decrease (increase) linearly with Al composition, but a change in slope of the variation of both lattice parameters and bandgap is observed at around Al composition of 50%. The lattice is found to be distorted for Al compositions more than 50% as indicated by a large change in the bond angles. The lattice distortion is determined to be the origin for the observed change in slope of the variation of both lattice parameters and bandgap for a monoclinic β-(AlxGa1−x)2O3 alloy system. Our results provide an insight into the local structure of β-(AlxGa1−x)2O3 alloys, which are required to have better understanding of their physical properties.

Growth of α-(AlGa)2O3 alloy thin films on c-sapphire substrates by mist chemical vapor deposition using acetylacetonated Al and Ga solutions

α-Ga2O3 is a semi-stable phase of Ga2O3 and is known as an ultra-wide-bandgap semiconductor material. α-(AlGa)2O3 alloys are important for their applications in electronic and optoelectronic devices. We investigated the growth mechanism and process of α-(AlGa)2O3 alloys by mist chemical vapor deposition using acetylacetonated Al and Ga aqueous solutions. The contribution of acetylacetonated Al ions to the epitaxial growth was investigated. The effects of an anchoring mechanism on the mosaic spread were experimentally evaluated. Investigating X-ray diffraction profiles, strain relaxation processes in film formations are discussed.

Trace Al component in ε-(AlxGa1-x)2O3 alloy films and film-based solar-blind photodetectors

ε-(AlxGa1-x)2O3 alloy thin films containing Al fractions of 0, 2.9 at%, and 4.7 at% with ε-Ga2O3 templates were fabricated on (0001) c-sapphire by tin-assisted PLD technique. Sn is a surface-mediated surfactant to stabilize the formation of epsilon phase. XRD reveals that the peak position of ε-(AlxGa1-x)2O3 shifts to a higher diffraction angle and that ε-(AlxGa1-x)2O3 is accompanied by a deterioration of crystal quality when the Al composition increases from 0 to 4.7 at%. With Al content increasing, XPS demonstrates the appearance of more aluminum atoms bound to oxygen atoms and higher oxygen vacancy density in ε-(AlxGa1-x)2O3 films. The optical bandgap for ε-(AlxGa1−x)2O3 thin films with various Al components are 4.97 eV, 5.09 eV and 5.16 eV, respectively. The solar-blind photodetectors were fabricated on ε-(AlxGa1-x)2O3 alloy thin films with various Al fractions. The photo-to-dark current ratio, responsiveness, detectivity and response time (rise time/decay time) for ε-(AlxGa1-x)2O3 (x = 2.9 at%) PDs were 2.7 × 103, 4.2 A/W, 1.23 × 1013 Jones and 2.09 s/0.49 s.

Infrared-active phonon modes and static dielectric constants in α-(AlxGa1−x)2O3 (0.18 ≤ x ≤ 0.54) alloys

We determine the composition dependence of the transverse and longitudinal optical infrared-active phonon modes in rhombohedral α-(AlxGa1−x)2O3 alloys by far-infrared and infrared generalized spectroscopic ellipsometry. Single-crystalline high quality undoped thin-films grown on m-plane oriented α-Al2O3 substrates with x = 0.18, 0.37, and 0.54 were investigated. A single mode behavior is observed for all phonon modes, i.e., their frequencies shift gradually between the equivalent phonon modes of the isostructural binary parent compounds. We also provide physical model line shape functions for the anisotropic dielectric functions. We use the anisotropic high-frequency dielectric constants for polarizations parallel and perpendicular to the lattice c axis measured recently by Hilfiker et al. [Appl. Phys. Lett. 119, 092103 (2021)], and we determine the anisotropic static dielectric constants using the Lyddane–Sachs–Teller relation. The static dielectric constants can be approximated by linear relationships between those of α-Ga2O3 and α-Al2O3. The optical phonon modes and static dielectric constants will become useful for device design and free charge carrier characterization using optical techniques.

Electronic properties and stability of M2O3 (M = Al, Ga, In) and alloy (MxGa1‐x)2O3 in α and β phases: A theoretical study

AbstractGa2O3 is clearly emerging as an important wide band‐gap semiconductor. Band‐gap engineering is now highly demanded for expanding its applications. Alloying with the same group of metal oxides is a straightforward and effective way. In this work, by using hybrid density functional theory calculations, the structural, electronic properties, and phase stability of group IIIA (Al, Ga, In) metal oxides and their ternary alloys (MxGa1‐x)2O3 (M = Al, In) in the corundum and monoclinic phases are systematically investigated. The lattice constants, elastic constants, modulus, formation energies, band‐gaps, band‐gap deformation potentials, band‐edge alignments, band‐gap bowing, and ternary alloy formation energies are obtained. The basic relations between the geometric structure and electronic properties are discussed. It is found that the cation ordered structure is the most stable alloy structure in the monoclinic phase, rather than the random alloy structure as is commonly thought. A phase stability diagram of the (MxGa1‐x)2O3 alloys is established, showing that the stable phase of the alloy changes from the monoclinic phase to the corundum phase when the incorporation of Al2O3 (In2O3) is greater than 69% (76%). These results can be used to understand the relative experimental data and shed some light on the synthesis and device design efforts of Ga2O3.

Band Alignment of Al2O3 on α-(AlxGa1-x)2O3

X Ray Photoelectron Spectroscopy was used to measure valence band offsets for Al2O3 deposited by Atomic Layer Deposition on α-(AlxGa1-x)2O3 alloys over a wide range of Al contents, x, from 0.26–0.74, corresponding to a bandgap range from 5.8–7 eV. These alloys were grown by Pulsed Laser Deposition. The band alignments were type I (nested) at x <0.5, with valence band offsets 0.13 eV for x = 0.26 and x = 0.46. At higher Al contents, the band alignment was a staggered alignment, with valence band offsets of − 0.07 eV for x = 0.58 and −0.17 for x = 0.74, ie. negative valence band offsets in both cases. The conduction band offsets are also small at these high Al contents, being only 0.07 eV at x = 0.74. The wide bandgap of the α-(AlxGa1-x)2O3 alloys makes it difficult to find dielectrics with nested band alignments over the entire composition range.

Structural characters and band offset of Ga2O3–Sc2O3 alloys

Alloy engineering is a promising approach to optimize the electronic properties and application of the ultrawide bandgap semiconductor Ga2O3. Here, the structural and electronic properties of (ScxGa1−x)2O3 alloys are studied using density functional theory with the Heyd–Scuseria–Ernzerhof hybrid functional. Hexagonal (ScxGa1−x)2O3 alloys show more negative formation enthalpies than (AlxGa1−x)2O3 alloys, and the increments in the positive formation enthalpies for monoclinic (ScxGa1−x)2O3 alloys are different from the (AlxGa1−x)2O3 alloys. (ScxGa1−x)2O3 alloys will undergo the compressive strain if grown on the Ga2O3 substrate. The bandgaps range from 4.78 to 5.44 eV for monoclinic (ScxGa1−x)2O3 and from 5.17 to 6.10 eV for hexagonal (ScxGa1−x)2O3. It is noted that Ga2O3/(ScxGa1−x)2O3 heterojunctions keep the type-II band alignments and whose conduction and valence band offsets can be significantly and negligibly enlarged by increasing Sc concentration, respectively. The large conduction band offsets for Ga2O3/(ScxGa1−x)2O3 heterojunctions allow (ScxGa1−x)2O3 alloys to be an electron blocking layer for the Ga2O3 device, and ease the problems of parasitic conduction in the field effect transistor.

Al Composition Dependence of Band Offsets for SiO2 on α-(AlxGa1−x)2O3

Valence band offsets for SiO2 deposited by Atomic Layer Deposition on α-(AlxGa1-x)2O3 alloys with x = 0.26–0.74 were measured by X-ray Photoelectron Spectroscopy. The samples were grown with a continuous composition spread to enable investigations of the band alignment as a function of the alloy composition. From measurement of the core levels in the alloys, the bandgaps were determined to range from 5.8 eV (x = 0.26) to 7 eV (x = 0.74). These are consistent with previous measurements by transmission spectroscopy. The valence band offsets of SiO2 with these alloys of different composition were, respectively, were −1.2 eV for x = 0.26, −0.2 eV for x = 0.42, 0.2 eV for x = 0.58 and 0.4 eV for x = 0.74. All of these band offsets are too low for most device applications. Given the bandgap of the SiO2 was 8.7 eV, this led to conduction band offsets of 4.1 eV (x = 0.26) to 1.3 eV (x = 0.74). The band alignments were of the desired nested configuration for x > 0.5, but at lower Al contents the conduction band offsets were negative, with a staggered band alignment. This shows the challenge of finding appropriate dielectrics for this ultra-wide bandgap semiconductor system.

Band offsets at metalorganic chemical vapor deposited β-(AlxGa1−x)2O3/β-Ga2O3 interfaces—Crystalline orientation dependence

The β-(AlxGa1−x)2O3 alloy represents an emerging ultrawide bandgap semiconductor material for applications in high-power electronics and deep ultraviolet optoelectronics. The recent demonstrations of orientation-dependent epitaxial growth of high quality β-(AlxGa1−x)2O3 films have unlocked prospects for the development of ultrahigh frequency β-(AlxGa1−x)2O3/Ga2O3 based transistors with high-power tolerance. To control the electronic and optical properties of β-(AlxGa1−x)2O3/Ga2O3 heterostructure-based devices, an understanding of the band offsets between β-(AlxGa1−x)2O3 and β-Ga2O3 is crucial. However, there have been no systematic experimental studies on the evolution of the band offsets between differently oriented β-(AlxGa1−x)2O3/Ga2O3 heterointerfaces as a function of Al compositions. This work presents the valence and conduction band offsets at (010) and (2¯01) oriented β-(AlxGa1−x)2O3/Ga2O3 interfaces using x-ray photoelectron spectroscopy. β-(AlxGa1−x)2O3 films with x ≤ 0.35 and x ≤ 0.48 were grown by metalorganic chemical vapor deposition on (010) and (2¯01) oriented β-Ga2O3 substrates, respectively. The determined band offsets reveal the formation of a type-II (staggered) band alignment at (010) oriented β-(AlxGa1−x)2O3/Ga2O3 and a type-I (straddling) heterojunction between (2¯01) β-(AlxGa1−x)2O3 and β-Ga2O3. For both crystalline orientations, the valence and conduction band offsets are found to increase with increasing Al content with a much weaker variation in the valence band offsets as compared to the conduction band offsets. Among different orientations investigated, such as (010), (2¯01), and (100), the largest conduction band offset occurs at a β-(AlxGa1−x)2O3/Ga2O3 interface with (100) orientation, providing opportunities for excellent electron confinement with a high-density two-dimensional electron gas. Results from this study on the tunable and orientation-dependent band offsets with the variation of the Al alloy fraction will provide guidance for the design of heterostructures in future β-(AlxGa1−x)2O3/Ga2O3 based devices.

Bandgap widening and behavior of Raman-active phonon modes of cubic single-crystalline (In,Ga)2O3 alloy films

The influence of Ga incorporation into cubic In2O3 on the electronic and vibrational properties is discussed for (In1−x,Gax)2O3 alloy films grown by molecular beam epitaxy. Using spectroscopic ellipsometry, a linear dependence of the absorption onset on the Ga content x is found with a blueshift of up to 150 meV for x = 0.1. Consistently, the fundamental bandgap exhibits a blueshift as determined by hard x-ray photoelectron spectroscopy. The dependence of the absorption onset and the effective electron mass on the electron concentration is derived from the infrared dielectric functions for a Sn doped alloy film. The influence of alloying on phonon modes is analyzed on the basis of Raman spectroscopic measurements. The frequencies of several phonon modes are identified as sensitive measures for the spectroscopic determination of the Ga content.

Bandgap bowing parameter and alloy fluctuations for β-(AlxGa1−x)2O3 alloys for x ≤ 0.35 determined from low temperature optical reflectivity

A bandgap bowing parameter of 0.4 ± 0.2 eV for β-(AlxGa1−x)2O3 alloys, with Al compositions (x) up to 0.35, has been determined from the bandgap obtained from low temperature optical reflectivity, which suppresses the effect of electron–phonon interaction on the bandgap. A length scale of inhomogeneity of 0.21 ± 0.03 times of the electron–hole mean free path length has been estimated for β-(AlxGa1−x)2O3 alloys. The unit cell of β-(AlxGa1−x)2O3 alloys compresses, and the lattice parameters vary linearly with Al substitution. Our results provide insight into bandgap engineering and alloy disorder for β-(AlxGa1−x)2O3 alloys, which are an important material system for applications in deep ultraviolet opto-electronic devices.