β-Ga2O3 Substrates Research Articles

Homoepitaxial growth of Ge doped β-gallium oxide thin films by mist chemical vapor deposition

This study demonstrated homoepitaxial growth of Ge-doped β-Ga2O3 thin films on β-Ga2O3 substrates via mist chemical vapor deposition (CVD) using GeI4, a water-soluble Ge precursor. The carrier concentration of the Ge-doped β-Ga2O3 thin films was controlled by varying the Ge precursor concentration in the solution. A mobility of 66 cm2 V−1 s−1 was obtained at a carrier density of 3.4 × 1018 cm−3 using oxygen carrier gas. X-ray diffraction (XRD) scans 2θ-ω revealed that homoepitaxial Ge-doped β-Ga2O3 thin films were grown on β-Ga2O3 without phase separation. However, the XRD rocking curves revealed that the mist CVD- grown Ge-doped β-Ga2O3 was degraded compared to the substrate as the Ge concentration increased. The surface morphologies of the Ge-doped β-Ga2O3 exhibited atomically flat surfaces with a root mean square roughness of less than 1 nm. These results indicate that the Ge-doped β-Ga2O3 thin films prepared by mist chemical vapor deposition are promising for device applications.

Metalorganic chemical vapor deposition of β-(AlxGa1−x)2O3 thin films on (001) β-Ga2O3 substrates

Phase pure β-(AlxGa1−x)2O3 thin films are grown on (001) oriented β-Ga2O3 substrates via metalorganic chemical vapor deposition. By systematically tuning the precursor molar flow rates, the epitaxial growth of coherently strained β-(AlxGa1−x)2O3 films is demonstrated with up to 25% Al compositions as evaluated by high resolution x-ray diffraction. The asymmetrical reciprocal space mapping confirms the growth of coherent β-(AlxGa1−x)2O3 films (x < 25%) on (001) β-Ga2O3 substrates. However, the alloy inhomogeneity with local segregation of Al along the (2̄01) plane is observed from atomic resolution STEM imaging, resulting in wavy and inhomogeneous interfaces in the β-(AlxGa1−x)2O3/β-Ga2O3 superlattice structure. Room temperature Raman spectra of β-(AlxGa1−x)2O3 films show similar characteristics peaks as the (001) β-Ga2O3 substrate without obvious Raman shifts for films with different Al compositions. Atom probe tomography was used to investigate the atomic level structural chemistry with increasing Al content in the β-(AlxGa1−x)2O3 films. A monotonous increase in chemical heterogeneity is observed from the in-plane Al/Ga distributions, which was further confirmed via statistical frequency distribution analysis. Although the films exhibit alloy fluctuations, n-type doping demonstrates good electrical properties for films with various Al compositions. The determined valence and conduction band offsets at β-(AlxGa1−x)2O3/β-Ga2O3 heterojunctions using x-ray photoelectron spectroscopy reveal the formation of type-II (staggered) band alignment.

Observation of nanopipes in edge-defined film-fed grown β-Ga2O3 substrate and their effect on homoepitaxial surface hillocks

β-Ga2O3 substrates and homoepitaxial films were characterized using the multiphoton excitation photoluminescence (MPPL) method. MPPL emission peaks at 3.2 and 3.36 eV were obtained with broad shoulders on the low energy side. MPPL images showed the presence of nanopipes in the substrate with a unique contrast. Three-dimensional MPPL imaging revealed the distribution and spatial location of the nanopipes. All the nanopipes were elongated along the [010] direction with diameters of approximately 0.4–1.0 μm and lengths of 10–30 μm. The nanopipes were aligned in the [100] and [001] directions with sub-micron spacing in the (00)-oriented substrate. The nanopipes were located under the hillocks at the homoepitaxial surface. The nanopipes are suggested to trigger the hillock growth by transforming into threading dislocations in the homoepitaxial film.

Enhancement-mode vertical (100) β-Ga2O3 FinFETs with an average breakdown strength of 2.7 MV cm−1

In this work, we report on the realization of vertical (100) β-Ga2O3 FinFET devices for the use in power electronics applications. The experiments are carried out on structures consisting of highly conducting (100) β-Ga2O3 substrates with a doping concentration N D of 3 × 1018 cm−3, and epitaxially grown layers with N D of 5 × 1016 cm−3 for the drift and channel region. The fabricated FinFET devices feature enhancement-mode properties with a threshold voltage of +4.2 V and on/off-current ratio of 105. Moreover, breakdown measurements of these devices reveal an average breakdown strength of 2.7 MV cm−1. Additional device simulation indicates the presence of electric field peaks near the gate edge outside the active device as high as 7 and 5 MV cm−1 in the Al2O3 gate oxide and β-Ga2O3 semiconductor, respectively.

Suboxide vapor phase epitaxy for growth of high-purity gallium oxide

We propose using gallium suboxide, Ga2O, as a Ga source for the growth of high-purity Ga2O3 by vapor phase epitaxy. It is shown in a thermochemical analysis that the suboxide can be generated effectively in the reaction between Ga2O3 and Ga and subsequently be utilized for the epitaxial growth of Ga2O3. A demonstration of Ga2O3 crystal growth was carried out on β-Ga2O3 (001) substrates with Ga2O and O2 used as the gaseous precursors, resulting in high-purity epitaxial layers. No possible donor impurities from the sources or growth environment, such as Si or Sn, were detected in the grown layers.

Band alignment and electrical properties of NiO/β-Ga2O3 heterojunctions with different β-Ga2O3 orientations

Due to the difficulty of p-type doping in β-Ga2O3, NiO/β-Ga2O3 heterojunction becomes a promising candidate for fabricating bipolar devices. In this work, we performed a comparative study on the band alignment and electrical properties of the NiO/β-Ga2O3 heterojunctions with different β-Ga2O3 orientations. NiO thin films were sputtered on three β-Ga2O3 substrates with (−201), (001) and (010) orientations and were subsequently fabricated into NiO/β-Ga2O3 heterojunction diodes. A type-Ⅱ band alignment between the NiO and β-Ga2O3 was identified by the transmittance spectra and X-ray photoelectron spectroscopy (XPS) measurements. The valence band offsets (VBOs) of the NiO/β-Ga2O3 heterojunctions on (−201), (001) and (010) substrates were determined to be (2.12 ± 0.06) eV, (2.44 ± 0.07) eV and (2.66 ± 0.07) eV, respectively, and their corresponding conduction band offsets (CBOs) were (1.22 ± 0.06) eV, (1.49 ± 0.07) eV and (1.86 ± 0.07) eV. In addition, the fabricated heterojunction diodes showed good rectification properties with different turn-on voltages, which was in good agreement with the XPS results. The revealed influence of substrate orientations on the properties in NiO/β-Ga2O3 heterojunctions were of great importance for the design and optimization of β-Ga2O3-based heterojunction devices.

Si doping of β -Ga2O3 by disilane via hybrid plasma-assisted molecular beam epitaxy

Obtaining uniform silicon concentration, especially with low concentrations (ranging from 1 × 1016 to 1 × 1018 cm−3) by molecular beam epitaxy, has been challenging due to oxidation of a silicon solid source in the oxide environment. In this work, Si doping of β-Ga2O3 (010) films by diluted disilane as the Si source is investigated using hybrid plasma-assisted molecular beam epitaxy. The impact of growth temperature, disilane source concentration, and disilane flow rate on Si incorporation was studied by secondary ion mass spectrometry. Uniform Si concentrations ranging from 3 × 1016 to 2 × 1019 cm−3 are demonstrated. Si-doped β-Ga2O3 films with different silicon concentrations were grown on Fe-doped β-Ga2O3 (010) substrates. The electron concentration and mobility were determined using van de Pauw Hall measurements. A high mobility of 135 cm2/V s was measured for an electron concentration of 3.4 × 1017 cm−3 at room temperature.

Impact of process anneals on high-k/β-Ga2O3 interfaces and capacitance

Gallium oxide (β-Ga2O3) is becoming a popular material for high power electronic devices due to its wide bandgap and ease of processing. In this work, β-Ga2O3 substrates received various annealing treatments before atomic layer deposition of HfO2 and subsequent fabrication of metal–oxide–semiconductor (MOS) capacitors. Annealing of β-Ga2O3 with forming gas or nitrogen produced degraded capacitance–voltage (C–V) behavior compared to a β-Ga2O3 control sample with no annealing. A sample annealed with pure oxygen had improved C–V characteristics relative to the control sample, with a higher maximum capacitance and smaller flat-band voltage shift, indicating that oxygen annealing improved the C–V behavior. X-ray photoelectron spectroscopy also suggested a reduction in the oxygen vacancy concentration after O2 annealing at 450 °C, which supports the improved C–V characteristics and indicates that O2 annealing of β-Ga2O3 may lead to better MOS device performance.

Large-area vertical-geometry Pt/(010) β-Ga2O3 Schottky barrier diodes and their temperature-dependent electrical properties

Herein, vertical-geometry Pt/(010) β-Ga2O3 Schottky barrier diodes (SBDs) with various large areas (2.25 × 10−2, 4 × 10−2 to 6.25 × 10−2 cm2) were fabricated on (010) orientation β-Ga2O3 substrates. The electrical properties of different area diodes and the uniformity of diodes with the same area were analyzed. The rectifying ratio of all diodes was determined to be larger than 108 at the bias of ±2 V. The current-voltage (I–V) characteristics of SBD with an area of 2.25 × 10−2 cm2 were investigated in detail from 300 to 450 K. The parameters of the diodes show a strong dependence on temperature, including ideality factor (n), on-resistance (Ron), turn-on voltage (Von), and the Schottky barrier (Φb). The results indicate that a dominant inhomogeneous Schottky barrier exists at the Pt/β-Ga2O3 interface. Such large-area devices exhibit that the value of n is close to 1 and the value of Φb was 1.09 eV at zero bias. In addition, the Von remains low and the stability remains good even after the tests at 450 K. These results indicate that we have successfully fabricated large-area SBDs with excellent performances. This work may contribute to further understanding of the performance of β-Ga2O3 diodes in large areas and harsh environments.

Two-inch Fe-doped β-Ga2O3 (010) substrates prepared using vertical Bridgman method

The growth of large-diameter high-resistivity β-Ga2O3 (010) substrates is important for the low-cost production of lateral Ga2O3 devices. We grew a 2 inch diameter Fe-doped high-resistivity β-Ga2O3 (010) single crystal by using the vertical Bridgman (VB) method, which is expected to grow large-diameter β-Ga2O3 crystals with various crystal orientations. Two-inch substrates were prepared from the obtained crystals, and their crystallinity, concentration of Fe dopants, and electrical properties were investigated. Consequently, a 2 inch β-Ga2O3 (010) substrate, which is comparable to the largest size of (010) substrate prepared using the Czochralski method, was successfully fabricated with the VB method. The in-plane distribution of the X-ray rocking curve from 020 diffraction of the fabricated 2 inch substrate showed that the full widths at half maximums were less than 35 arcsec at almost all measurement points, indicating high crystallinity and high in-plane uniformity. In addition, the crystals contain Fe concentrations in the range of 3.5 × 1018–1.9 × 1019 cm−3, indicating that impurity Si donors are sufficiently compensated by the Fe dopants. Therefore, substrates prepared using the VB method exhibited high resistivities of 6 × 1011–9 × 1012 Ω·cm at room temperature.

Homoepitaxial Si-doped Gallium Oxide films by MOCVD with tunable electron concentrations and electrical properties

Doping technology with stable carrier concentration regulation is a guarantee for optimizing β-Ga2O3-based high-power electronic devices and optoelectronic devices. In this study, β-Ga2O3 homoepitaxial epitaxial films with stable and tunable electron concentrations were grown on Fe-doped (010)-oriented β-Ga2O3 substrates by using metal-organic chemical vapor deposition (MOCVD). XRD shows that the basic bulk properties of β-Ga2O3 film, i. e. the structure and crystal orientation, are stable with low Si-doping, however, its electrical properties vary remarkably depending on concentrations of Si ions incorporated by Si source flow rate during the film growth process. Hall measurement is utilized to confirm high-quality β-Ga2O3 films with tunable electron concentration of 1017–1019 cm−3. As a consequence, the electron concentration-dependent behavior of β-Ga2O3 Schottky barrier diodes (SBDs) was studied. The β-Ga2O3 SBDs exhibited better forward current output electrical performance with reduction in built-in potential difference (Vbi) from 2.4 V to 0.3 V and on-resistance (Ron) from 257 Ω cm to 26 Ω cm. Our results suggest that stable and tunable electron concentration provides high-quality material assurance for constructing power electronic devices of Ga2O3.

Interface engineering in epitaxial growth of sputtered β-Ga2O3 films on Si substrates via TiN (111) buffer layer for Schottky barrier diodes

The native amorphous silicon oxide (SiO2) layer formed on the Si substrate leads to the (100) preferred orientation for β-phase gallium oxide (β-Ga2O3), which encounters various defects in β-Ga2O3, such as twin boundaries and stacking faults etc. The (111) preferred orientation of titanium nitride (TiN) hetero-buffer layer can be used in the interface between β-Ga2O3 and Si substrates to reduce the lattice mismatch between them and increase the (˗201) preferred orientation for β-Ga2O3. The lattice mismatch between TiN (111) and β-Ga2O3 (˗201) is 0.76%, which is less than the β-Ga2O3 (˗201)/Si (111) about ˗6.3%. The β-Ga2O3 and TiN films were grown using radio-frequency magnetron sputtering on Si substrates, which possess polycrystalline nature as revealed using X-ray diffraction patterns and high-resolution transmission electron micrographs. The optimal parameters are found as, process atmosphere: Ar = 10 sccm, RTA: 800 °C for β-Ga2O3/TiN (300 nm)/Si. This work highlights the effect of the TiN hetero-buffer layer, the process atmosphere, and annealing temperatures on the microstructural and surface morphology of β-Ga2O3 films. The lateral Schottky barrier diode (SBD) were fabricated using the optimized β-Ga2O3/TiN (300 nm)/Si film. The Baliga's Figure of Merit (BFOM) of the lateral SBD is 7.60 × 10−2 kW/cm2, which exhibits 4 order of BFOM higher than that of β-Ga2O3/Si SBD due to interface engineering. The β-Ga2O3/TiN (300 nm)/Si hetero-structure demonstrates a promising material to be employed in the next generation β-Ga2O3 based power devices.

Coherent growth of β-(Al x Ga1−x )2O3 alloy thin films on (010) β-Ga2O3 substrates using mist CVD

Herein, we demonstrate β-(Al x Ga1−x )2O3 thin films that were coherently grown on a (010) β-Ga2O3 substrate using mist chemical vapor deposition (CVD). X-ray diffraction and reciprocal space mapping results revealed that the β-(Al x Ga1−x )2O3 thin films were of high-crystalline quality and were grown coherently to attain an Al content of 18.3% as measured by Rutherford backscattering spectroscopy. Importantly, based on their surface morphologies, the coherently grown β-(Al x Ga1−x )2O3 thin films have atomically flat surfaces. These results indicate that mist CVD is a promising technique for β-(Al x Ga1−x )2O3/β-Ga2O3 heterojunction devices.

Ultrawide bandgap vertical β-(AlxGa1−x)2O3 Schottky barrier diodes on free-standing β-Ga2O3 substrates

Ultrawide bandgap β-(AlxGa1−x)2O3 vertical Schottky barrier diodes on (010) β-Ga2O3 substrates are demonstrated. The β-(AlxGa1−x)2O3 epilayer has an Al composition of 21% and a nominal Si doping of 2 × 1017 cm−3 grown by molecular beam epitaxy. Pt/Ti/Au has been employed as the top Schottky contact, whereas Ti/Au has been utilized as the bottom Ohmic contact. The fabricated devices show excellent rectification with a high on/off ratio of ∼109, a turn-on voltage of 1.5 V, and an on-resistance of 3.4 mΩ cm2. Temperature-dependent forward current-voltage characteristics show effective Schottky barrier height varied from 0.91 to 1.18 eV while the ideality factor from 1.8 to 1.1 with increasing temperatures, which is ascribed to the inhomogeneity of the metal/semiconductor interface. The Schottky barrier height was considered a Gaussian distribution of potential, where the extracted mean barrier height and a standard deviation at zero bias were 1.81 and 0.18 eV, respectively. A comprehensive analysis of the device leakage was performed to identify possible leakage mechanisms by studying temperature-dependent reverse current-voltage characteristics. At reverse bias, due to the large Schottky barrier height, the contributions from thermionic emission and thermionic field emission are negligible. By fitting reverse leakage currents at different temperatures, it was identified that Poole–Frenkel emission and trap-assisted tunneling are the main leakage mechanisms at high- and low-temperature regimes, respectively. Electrons can tunnel through the Schottky barrier assisted by traps at low temperatures, while they can escape these traps at high temperatures and be transported under high electric fields. This work can serve as an important reference for the future development of ultrawide bandgap β-(AlxGa1−x)2O3 power electronics, RF electronics, and ultraviolet photonics.

Process exploration of β-Ga2O3 blind hole processing by water-assisted femtosecond laser technology

β-Ga2O3 is a promising high-performance material with broad application prospects in the micro-electro-mechanical-systems (MEMSs). It can be used in extreme environments such as ultralow temperatures and intense radiation, but its hardness, brittleness, and easy cleavage are the primary defect in processing. As a substrate material for the next generation of large-scale integrated circuits, blind hole processing of β-Ga2O3 has attracted intense attention. In this work, we investigated the blind hole on (010) β-Ga2O3 surfaces treated with a femtosecond-pulse laser at the wavelength of 1030 nm and the pulse width of 285 fs. Water-assisted fs laser micromachining is proposed to solve the deposition of pore debris within the hole in the air environment. Furthermore, the influence of laser pulse energy, laser pulse numbers, liquid layer thickness, and laser repetition rate on the surface morphology of the blind hole was studied by using the control variable method. The results show that precise processing of β-Ga2O3 blind holes can be achieved by controlling fs laser processing parameters. In this work, the proposed liquid-assisted fs laser machining was proved to be a reliable tool for performing high-quality β-Ga2O3 blind hole processing, which has important implications for device applications of β-Ga2O3 substrates.

Self-aligned patterning on β-Ga2O3 substrates via backside-exposure photolithography

β-Ga2O3 has not been patterned using backside-exposure lithography, despite its high potential applications in future power electronics and its transparency to conventional lithography light sources. Here, a patterned metal on a β-Ga2O3 substrate surface was used as an embedded photomask to demonstrate the applicability of backside-exposure lithography to β-Ga2O3 substrates. Self-aligned photoresist patterning, lift-off, and etching processes were demonstrated for an AZ5214E photoresist in both the positive and negative processing modes. The findings suggest that backside-exposure photolithography is a promising fabrication approach for future β-Ga2O3-based devices.

Optimization of chemical mechanical polishing of (010) β-Ga2O3

Smooth (< 0.5 nm rms) and subsurface damage-free (010) β-Ga2O3 was achieved with low-pressure chemical mechanical polishing. An applied pressure of 1 kPa along with colloidal silica and poromeric polyurethane polishing pads rotating at 30 rpm was found to be the optimal polishing parameters for (010) β-Ga2O3. Using higher pressures typically employed in the current literature induced subsurface damage in the substrates. Diffuse scatter intensity of triple-axis x-ray rocking curves was used to determine the presence of subsurface lattice damage, which was quantified by measuring peak widths below the half maximum (i.e., FWXM where X < 0.5). The initially rough surfaces of (010) β-Ga2O3 substrates due to wafer slicing and grinding were lapped and polished. A 5 μm Al2O3 slurry followed by a 0.3 μm Al2O3 slurry was used as the primary lap material removal step. The material removal rates were ∼20 and ∼9 μm/h, respectively. Then, chemical mechanical polishing was performed using colloidal alumina followed by colloidal silica. The removal rates were ∼1.3 and ∼0.4 μm/h, respectively. Only colloidal silica showed the complete removal of subsurface damage. The final (020) β-Ga2O3 rocking curve FWHM was ∼13″ and FW(0.001)M was ∼120″, which matches the widths of commercially available pristine (010) β-Ga2O3. A final cleaning step using dilute bleach and dilute citric acid to remove residual silica slurry particles from the surface was demonstrated.

500 °C operation of β-Ga2O3 field-effect transistors

We demonstrated 500 °C operation of field-effect transistors made using ultra-wide bandgap semiconductor β-Ga2O3. Metal–semiconductor field-effect transistors were fabricated using epitaxial conductive films grown on an insulating β-Ga2O3 substrate, TiW refractory metal gates, and Si-implanted source/drain contacts. Devices were characterized in DC mode at different temperatures up to 500 °C in vacuum. These variable-temperature measurements showed a reduction in gate modulation of the drain current due to an increase in gate leakage across the gate/semiconductor Schottky barrier. Devices exhibited a reduction in transconductance with increasing temperature; despite this, drain current increased with temperature due to a reduction in threshold voltage caused by the de-trapping of electrons from deep-level traps. Devices also showed negligible change in semiconductor epitaxy and source/drain contacts, hence demonstrated recovery to their room-temperature electrical properties after the devices were tested intermittently at different high temperatures in vacuum. The mechanism of gate leakage was also explored, which implicated the presence of different conduction mechanisms at different temperatures and gate electric fields.

Metalorganic vapor-phase epitaxy of β-(AlxGa1-x)2O3 on ([formula omitted] 0 1) Ga2O3 substrates

Thin β-(AlxGa1-x)2O3 films were grown on (2¯ 0 1) β-Ga2O3 substrates via metal organic chemical vapor deposition (MOCVD). The Al content and band gap energy of β-(AlxGa1-x)2O3 was quantified using high-resolution X-ray photoelectron spectroscopy, through which a band gap energy of 5.26 eV was determined for a 24.1 % Al content. A smooth surface with a bar-like particle morphology was observed via atomic force microscopy and scanning electron microscopy. It was proposed that the film growth exhibited a step bunching model and the roughness of β-(AlxGa1-x)2O3 was decreased at higher Al contents. Finally, β-(AlxGa1-x)2O3/Ga2O3 multi-quantum well structures with a high crystal quality and sharp interface were grown and characterized.

Quantitative x-ray diffraction analysis of strain and interdiffusion in β-Ga2O3 superlattices of μ-Fe2O3 and β-(AlxGa1−x)2O3

Superlattices composed of either monoclinic μ-Fe2O3 or β-(AlxGa1−x)2O3 with β-Ga2O3 spacers are grown on (010) β-Ga2O3 substrates using plasma-assisted molecular beam epitaxy. High-resolution x-ray diffraction data are quantitatively fit using commercial dynamical x-ray diffraction software (LEPTOS) to obtain layer thicknesses, strain, and compositions. The strain state of β-(AlxGa1−x)2O3 and μ-Fe2O3 superlattices as characterized using reciprocal space maps in the symmetric (020) and asymmetric (420) diffraction conditions indicates coherent growths that are strained to the (010) β-Ga2O3 lattice. β-(AlxGa1−x)2O3 and μ-Fe2O3 superlattices grown at hotter substrate temperatures result in crystal structures with better coherency and reduced defects compared to colder growths. The growth rate of μ-Fe2O3 is ∼2.6 nm/min at Tsub = 700 °C and drops to ∼1.6 nm/min at Tsub = 800 °C due to increased Fe interdiffusion at hotter substrate temperatures. Scanning transmission electron microscopy data of a μ-Fe2O3 superlattice grown at Tsub = 700 °C confirm that there is significant diffusion of Fe atoms into β-Ga2O3 layers.