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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. [ABSTRACT FROM AUTHOR]

The in situ metalorganic chemical vapor deposition (MOCVD) growth of Al2O3 dielectrics on β-Ga2O3 and β-(AlxGa1−x)2O3 films is investigated as a function of crystal orientations and Al compositions of β-(AlxGa1−x)2O3 films. The interface and film qualities of Al2O3 dielectrics are evaluated by high-resolution x-ray diffraction and scanning transmission electron microscopy imaging, which indicate the growth of high-quality amorphous Al2O3 dielectrics with abrupt interfaces on (010), (100), and ( 2 ¯ 01) oriented β-(AlxGa1−x)2O3 films. The surface stoichiometries of Al2O3 deposited on all orientations of β-(AlxGa1−x)2O3 are found to be well maintained with a bandgap energy of 6.91 eV as evaluated by high-resolution x-ray photoelectron spectroscopy, which is consistent with the atomic layer deposited (ALD) Al2O3 dielectrics. The evolution of band offsets at both in situ MOCVD and ex situ ALD deposited Al2O3/β-(AlxGa1−x)2O3 is determined as a function of Al composition, indicating the influence of the deposition method, orientation, and Al composition of β-(AlxGa1−x)2O3 films on resulting band alignments. Type II band alignments are determined at the MOCVD grown Al2O3/β-(AlxGa1−x)2O3 interfaces for the (010) and (100) orientations, whereas type I band alignments with relatively low conduction band offsets are observed along the ( 2 ¯ 01) orientation. The results from this study on MOCVD growth and band offsets of amorphous Al2O3 deposited on differently oriented β-Ga2O3 and β-(AlxGa1−x)2O3 films will potentially contribute to the design and fabrication of future high-performance β-Ga2O3 and β-(AlxGa1−x)2O3 based transistors using MOCVD in situ deposited Al2O3 as a gate dielectric. [ABSTRACT FROM AUTHOR]

Growths of monoclinic (AlxGa1−x)2O3 thin films up to 99% Al contents are demonstrated via metalorganic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa) as the Ga precursor. The utilization of TMGa, rather than triethylgallium, enables a significant improvement of the growth rates (>2.5 μm h−1) of β‐(AlxGa1−x)2O3 thin films on (010), (100), and (2¯01) β‐Ga2O3 substrates. By systematically tuning the precursor molar flow rates, growth of coherently strained phase pure β‐(AlxGa1−x)2O3 films is demonstrated by comprehensive material characterizations via high‐resolution X‐ray diffraction (XRD) and atomic‐resolution scanning transmission electron microscopy (STEM) imaging. Monoclinic (AlxGa1−x)2O3 films with Al contents up to 99, 29, and 16% are achieved on (100), (010), and (2¯01) β‐Ga2O3 substrates, respectively. Beyond 29% of Al incorporation, the (010) (AlxGa1−x)2O3 films exhibit β‐ to γ‐phase segregation. β‐(AlxGa1−x)2O3 films grown on (2¯01) β‐Ga2O3 show local segregation of Al along (100) plane. Record‐high Al incorporations up to 99% in monoclinic (AlxGa1−x)2O3 grown on (100) Ga2O3 are confirmed from XRD, STEM, electron nanodiffraction, and X‐ray photoelectron spectroscopy measurements. These results indicate great promises of MOCVD development of β‐(AlxGa1−x)2O3 films and heterostructures with high Al content and growth rates using TMGa for next‐generation high‐power and high‐frequency electronic devices. [ABSTRACT FROM AUTHOR]

The lack of p-type doping has impeded the development of vertical gallium oxide (Ga2O3) devices. Current blocking layers (CBLs) using implanted deep acceptors have been used to demonstrate vertical devices. This paper presents a pioneering demonstration of in situ Mg-doped β-Ga2O3 CBLs grown using metal–organic chemical vapor deposition. The Mg-doping density during growth was calibrated by quantitative secondary ion mass spectroscopy. Electrical test structures were designed with in situ Mg doped layers with various targeted Mg doping concentrations. The effectiveness of the CBL is characterized by using temperature-dependent current–voltage measurements using n-Mg-doped-n structures, providing crucial insight into the underlying mechanisms. Pulsed measurements show similar blocking characteristics as DC. To further validate the experimental results, a TCAD simulation is performed, and the electrically active effective doping is found to be dependent on the Mg-doping density, offering an alternate perspective on the optimization of CBL performance. Breakdown measurements show a peak 4 MV/cm field strength. [ABSTRACT FROM AUTHOR]

Understanding the thermal stability and degradation mechanism of β-Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) is crucial for their high-power electronics applications. This work examines the high temperature performance of the junctionless lateral β-Ga2O3 FinFET grown on a native β-Ga2O3 substrate, fabricated by metal-assisted chemical etching with Al2O3 gate oxide and Ti/Au gate metal. The thermal exposure effect on threshold voltage (Vth), subthreshold swing (SS), hysteresis, and specific on-resistance (Ron,sp), as a function of temperature up to 298 °C, is measured and analyzed. SS and Ron,sp increased with increasing temperatures, similar to the planar MOSFETs, while a more severe negative shift of Vth was observed for the high aspect-ratio FinFETs here. Despite employing a much thicker epilayer (∼2 μm) for the channel, the high temperature performance of Ion/Ioff ratios and SS of the FinFET in this work remains comparable to that of the planar β-Ga2O3 MOSFETs reported using epilayers ∼10–30× thinner. This work paves the way for further investigation into the stability and promise of β-Ga2O3 FinFETs compared to their planar counterparts. [ABSTRACT FROM AUTHOR]

In this Letter, the role of background carbon in metalorganic chemical vapor deposition (MOCVD) β-Ga2O3 growth using trimethylgallium (TMGa) as the Ga precursor was investigated. The quantitative C and H incorporations in MOCVD β-Ga2O3 thin films grown at different growth rates and temperatures were measured via quantitative secondary ion mass spectroscopy (SIMS). The SIMS results revealed both [C] and [H] increase as the TMGa molar flow rate/growth rate increases or growth temperature decreases. The intentional Si incorporation in MOCVD β-Ga2O3 thin films decreases as the growth rate increases or the growth temperature decreases. For films grown at relatively fast growth rates (GRs) (TMGa > 58 μmol/min, GR > 2.8 μm/h) or relatively low temperature (<950 °C), the [C] increases faster than that of the [H]. The experimental results from this study demonstrate the previously predicted theory—H can effectively passivate the compensation effect of C in n-type β-Ga2O3. The extracted net doping concentration from quantitative SIMS {[Si]-([C]-[H])} agrees well with the free carrier concentration measured from Hall measurement. The revealing of the role of C compensation in MOCVD β-Ga2O3 and the effect of H incorporation will provide guidance on designing material synthesis for targeted device applications. [ABSTRACT FROM AUTHOR]

This Letter reports a high performance β -Ga2O3 thin channel MOSFET with T gate and degenerately doped (n++) source/drain contacts regrown by metal organic chemical vapor deposition. Highly scaled T-gate with a gate length of 160–200 nm was fabricated to achieve enhanced RF performance and passivated with 200 nm silicon nitride. Peak drain current (ID,MAX) of 285 mA/mm and peak transconductance (gm) of 52 mS/mm were measured at 10 V drain bias with 23.5 Ω mm on resistance (RON). Metal/n++ contact resistance of 0.078 Ω mm was extracted from transfer length measurements. RON is possibly dominated by interface resistance between channel and highly doped n++ regrown layer. A gate-to-drain breakdown voltage of 192 V is measured for LGD = 355 nm resulting in average breakdown field (EAVG) of 5.4 MV/cm. This EAVG is the highest reported among all sub-micron gate length lateral FETs. Current gain cut off frequency (fT) of 11 GHz and record power gain cut off frequency (fMAX) of approximately 48 GHz were extracted from small signal measurements. fT is limited by DC-RF dispersion due to interface traps which needs further investigation. The fT·VBR product is 2.112 THz V for 192 V breakdown voltage. Device surpasses the switching figure of merit of Silicon and competitive with mature wide bandgap devices. [ABSTRACT FROM AUTHOR]

This letter reports vacuum annealing of lateral field-plated $\beta $ -Ga2O3 MOSFETs with significant current recovery and improvement in the on-state resistance, $\text{R}_{\text {on}}$ , after Reactive Ion Etching (RIE) induced damage. We fabricate and characterize MOSFETs based on Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) grown $\beta $ -Ga2O3 wafers to better understand the effects of vacuum annealing. We see a clear trend of vacuum annealed devices showing no reduction in breakdown voltages, $\text{V}_{\text {br}}$ , as compared to polymer passivated MOSFETs. This trend holds for identical gate-drain separation, $\text{L}_{\text {gd}}$ , varying from $20 ~\mu \text{m}$ to $60 ~\mu \text{m}$. Devices show up to 10 times reduction in $\text{R}_{\text {on}}$ as compared to previously reported $\text{R}_{\text {on}}$ for SU-8 passivated devices. For MBE sample, $\text{V}_{\text {br}}$ of 7.16 kV for a $\text{L}_{\text {gd}}$ = $40 ~\mu \text{m}$ device, with an average field strength of 1.79 MVcm $^{{-{1}}}$ and peak drain current density of 40 mA/mm, is reported. $\text{R}_{\text {on}}$ is $897 \Omega $. mm, giving Baliga’s Figure of Merit (BFOM) as 5.71 MWcm $^{{-{2}}}$. For a $\text{L}_{\text {gd}}$ = $60 ~\mu \text{m}$ device, we report record high breakdown of 8.56 kV with BFoM of 4.9 MWcm $^{{-{2}}}$. For MOCVD grown sample, a $\text{L}_{\text {gd}}$ = $60 ~\mu \text{m}$ device has $\text{V}_{\text {br}}$ of 6.11 kV, $\text{R}_{\text {on}}$ of 1.98 $\text{k}\Omega $. mm and corresponding BFoM of 1.88 MWcm $^{{-{2}}}$. Transfer Length Method (TLM) analysis indicates incomplete post etch current recovery after vacuum annealing. [ABSTRACT FROM AUTHOR]

Metalorganic chemical vapor deposition (MOCVD) growths of β-Ga2O3 on on-axis (100) Ga2O3 substrates are comprehensively investigated. Key MOCVD growth parameters including growth temperature, pressure, group VI/III molar flow rate ratio, and carrier gas flow rate are mapped. The dependence of the growth conditions is correlated with surface morphology, growth rate, and electron transport properties of the MOCVD grown (100) β-Ga2O3 thin films. Lower shroud gas (argon) flow is found to enhance the surface smoothness with higher room temperature (RT) electron Hall mobility. The growth rate of the films decreases but with an increase of electron mobility as the VI/III molar flow rate ratio increases. Although no significant variation on the surface morphologies is observed at different growth temperatures, the general trend of electron Hall mobilities are found to increase with increasing growth temperature. The growth rates reduce significantly with uniform surface morphologies as the chamber pressure increases. By tuning the silane flow rate, the controllable carrier concentration of (100) β-Ga2O3 thin films between low-1017 cm−3 and low-1018 cm−3 was achieved. Under optimized growth condition, an (100) β-Ga2O3 thin film with RMS roughness value of 1.64 nm and a RT mobility of 24 cm2/Vs at a carrier concentration of 7.0 × 1017 cm−3 are demonstrated. The mobilities are primarily limited by the twin lamellae and stacking faults defects generated from the growth interface. Atomic resolution scanning transmission electron microscopy reveals the formation of twin boundary defects in the films, resulting in the degradation of crystalline quality. Results from this work provide fundamental understanding of the MOCVD epitaxy of (100) β-Ga2O3 on on-axis Ga2O3 substrates and the dependence of the material properties on growth conditions. The limitation of electron transport properties of the (100) β-Ga2O3 thin films below 25 cm2/Vs is attributed to the formation of incoherent boundaries (twin lamellae) and stacking faults grown along the on-axis (100) crystal orientation. [ABSTRACT FROM AUTHOR]

Epitaxial growth of κ-phase Ga2O3 thin films is investigated on c-plane sapphire, GaN- and AlN-on-sapphire, and (100) oriented yttria stabilized zirconia (YSZ) substrates via metalorganic chemical vapor deposition. The structural and surface morphological properties are investigated by comprehensive material characterization. Phase pure κ-Ga2O3 films are successfully grown on GaN-, AlN-on-sapphire, and YSZ substrates through a systematical tuning of growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, whereas the growth on c-sapphire substrates leads to a mixture of β- and κ-polymorphs of Ga2O3 under the investigated growth conditions. The influence of the crystalline structure, surface morphology, and roughness of κ-Ga2O3 films grown on different substrates are investigated as a function of precursor flow rate. High-resolution scanning transmission electron microscopy imaging of κ-Ga2O3 films reveals abrupt interfaces between the epitaxial film and the sapphire, GaN, and YSZ substrates. The growth of single crystal orthorhombic κ-Ga2O3 films is confirmed by analyzing the scanning transmission electron microscopy nanodiffraction pattern. The chemical composition, surface stoichiometry, and bandgap energies of κ-Ga2O3 thin films grown on different substrates are studied by high-resolution x-ray photoelectron spectroscopy (XPS) measurements. The type-II (staggered) band alignments at three interfaces between κ-Ga2O3 and c-sapphire, AlN, and YSZ substrates are determined by XPS, with an exception of κ-Ga2O3/GaN interface, which shows type-I (straddling) band alignment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In situ etching using Ga flux in an ultra-high vacuum environment like molecular beam epitaxy is introduced as a method to make high aspect ratio three-dimensional (3D) structures in β -Ga2O3. Etching of β -Ga2O3 due to excess Ga adatoms on the epilayer surface had been viewed as non-ideal for epitaxial growth especially since it results in plateauing and lowering of the growth rate. In this study, we use this well-known reaction from epitaxial growth to intentionally etch β -Ga2O3. We demonstrate etch rate ranging from 2.9 to 30 nm/min with the highest reported etch rate being only limited by the highest Ga flux used. Patterned in situ etching is also demonstrated and used to study the effect of fin orientation on the sidewall profiles and dopant (Si) segregation on the etched surface. Using in situ Ga etching, we also demonstrate 150 nm wide fins and 200 nm wide nanopillars with high aspect ratio. This etching method could enable future development of highly scaled vertical and lateral 3D devices in β -Ga2O3. [ABSTRACT FROM AUTHOR]

Single α-phase (AlxGa1−x)2O3 thin films are grown on m-plane sapphire (α-Al2O3) substrates via metalorganic chemical vapor deposition. By systematically tuning the growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, the epitaxial growth of high-quality phase pure α-(AlxGa1−x)2O3 films (0 ≤ x ≤ 1) is demonstrated with smooth surface morphologies and alloy homogeneities by comprehensive material characterization. The asymmetrical reciprocal space mapping reveals fully relaxed films for α-(AlxGa1−x)2O3 films with x ≤ 0.5. The coherent growth of α-(AlxGa1−x)2O3/α-Al2O3 superlattice structures is demonstrated with abrupt interfaces and uniform Al distribution for higher Al compositions at x = 0.78 in the α-(AlxGa1−x)2O3 layer. The influence of growth parameters, such as growth temperature and chamber pressure, on the phase stabilization and Al incorporation in the α-(AlxGa1−x)2O3 films is investigated. While lower growth temperatures facilitate the phase stabilization of α-Ga2O3 thin films, lower chamber pressure leads to higher Al incorporation in α-(AlxGa1−x)2O3 films. High resolution x-ray photoelectron spectroscopy was utilized for determining the Al compositions and bandgaps of α-(AlxGa1−x)2O3. Furthermore, the evolution of the valance and conduction band offsets at α-Al2O3/α-(AlxGa1−x)2O3 heterojunctions is evaluated with the variation of Al compositions, which reveals the formation of type-I (straddling) band alignment between α-Al2O3 and α-(AlxGa1−x)2O3. [ABSTRACT FROM AUTHOR]