Epitaxial β-Ga2O3 Research Articles

From Amorphous to β-Gallium Oxide: Practical Implementation of Energetics Considerations in Process Design and Optimization

Gallium oxide (Ga2O3) is a wide bandgap material (bandgap ~4.0 eV – 5.2 eV) with a large breakdown field that has considerably high figures of merit (FOM) in power handling compared to other wide bandgap semiconductor materials in use today (such as GaN and SiC) [1, 2]. Gallium oxide is also expected to expand the operating spectral range of optoelectronic devices to deep UV.Properties of gallium oxide depend on its crystal structure; amorphous [3, 4] as well as different crystalline forms [5] of this material have been used in electronic and optoelectronic devices. Among gallium oxide crystalline polymorphs, β-Ga2O3 has attracted the most attention because it is the most stable gallium oxide polymorph and, therefore, can ultimately be obtained by heating other gallium oxide polymorphs (and even amorphous gallium oxide) at sufficiently high temperatures (ca. 550°C and above); this polymorph can also be obtained from the melt at high temperatures (ca. 1800°C) using bulk crystal growth techniques [1, 6]. In the thin film form, growing high quality β-Ga2O3 is only possible on very limited substrates (e.g., β-Ga2O3 native substrate and sapphire) while having to carefully choose very specific process conditions based on each process and the instrument being used.In this work, we present strategies and guidelines, based on energetics considerations, that make it possible to design epitaxial deposition processes that achieve β-Ga2O3 thin films at low temperatures (< 300°C). We use the atomic layer deposition (ALD) technique to achieve dense and pinhole-free films of amorphous gallium oxide. Then, we revise the deposition process conditions step-by-step so that the energetics of the process can lead us to obtain high quality epitaxial β-Ga2O3 at low temperatures while not being limited to β-Ga2O3 native substrates or very specific (or instrument-dependent) process conditions. The results presented in this work facilitate the implementation of Ga2O3 in next generation wide bandgap electronic devices.

Enhanced solar-blind photoresponse characteristics in β-Ga2O3 epitaxial films on large miscut sapphire substrates

Realizing high performance solar-blind region photodetector are highly desired for the next-generation civil and military surveillance applications. As an attractive wide band gap semiconductor, β-Ga2O3 with a suitable band gap show great potential as the ideal candidate for solar-blind detection without the need of bandgap engineering. Here, we found that the crystallinity and photoresponse properties of epitaxial β-Ga2O3 films can be modified by selecting the substrate with different miscut angles. Especially, enhanced characteristics of responsivity and specific detectivity in β-Ga2O3 thin films grown on sapphire substrate with 6° miscut angle could reach 1.36 A/W and 5.6 × 1014 Jones at 5 V under 5 µW/cm2 254 nm light, respectively, which are superior than those grown on other miscut angle substrates. The results demonstrate an effective strategy to develop future high performance solar-blind photodetectors.

Ta-doped epitaxial β-Ga2O3 films deposited on SrTiO3(100) substrates by MOCVD

In this work, we successfully prepared epitaxial β-Ga2O3 films with different Ta doping concentrations on SrTiO3(100) substrates. XPS measurement indicated that Ta mainly existed in the β-Ga2O3 lattice as +5 valence state, and the actual doping concentration of the Ta element were determined. XRD and TEM results proved the obtained films were high quality β-Ga2O3 single crystal films, and the epitaxial relationship between the film and substrate was β-Ga2O3(100)//SrTiO3(100) with β-Ga2O3 [001]//SrTiO3<011>. The film with 0.2% Ta doping concentration presented the highest Hall mobility of 11.9 cm2 V−1 s−1, whereas the lowest resistivity was 68.8 Ω cm when the Ta concentration is 1.0%. By controlling the Ta concentration, the carrier concentration of the β-Ga2O3 films could be effectively regulated in the range of 4.15 × 1015–1.12 × 1017 cm−3. The average transmittances of the films in the visible light region exceeded 95%.

Photovoltaic and flexible deep ultraviolet wavelength detector based on novel \u03b2-Ga2O3/muscovite heteroepitaxy

Flexible and self-powered deep ultraviolet (UV) photodetectors are pivotal for next-generation electronic skins to enrich human life quality. The fabrication of epitaxial β-Ga2O3 thin films is challenging on flexible substrates due to high-temperature growth requirements. Herein, β-Ga2O3 (stackrel{-}{2} 0 1) films are hetero-epitaxially grown on ultra-thin and environment-friendly muscovite mica which is the first time β-Ga2O3 epitaxy growth on any flexible substrate. Integration of Gallium oxide with muscovite enables high-temperature processing as well as excellent flexibility compared to polymer substrates. Additionally, the metal–semiconductor-metal (MSM) photodetector on β-Ga2O3 layer shows an ultra-low dark current of 800 fA at zero bias. The photovoltaic peak responsivity of 11.6 µA/W is obtained corresponding to very weak illumination of 75 μW/cm2 of 265 nm wavelength. Thermally stimulated current (TSC) measurements are employed to investigate the optically active trap states. Among these traps, trap with an activation energy of 166 meV dominates the persistence photocurrent in the devices. Finally, photovoltaic detectors have shown excellent photocurrent stability under bending induced stress up to 0.32%. Hence, this novel heteroepitaxy opens the new way for flexible deep UV photodetectors.

Investigation of β-Ga2O3 films and β-Ga2O3/GaN heterostructures grown by metal organic chemical vapor deposition

In this work, (−2 0 1) β-Ga2O3 films are grown on GaN substrate by metal organic chemical vapor deposition (MOCVD). It is revealed that the β-Ga2O3 film grown on GaN possesses superior crystal quality, material homogeneity and surface morphology than the results of common heteroepitaxial β-Ga2O3 film based on sapphire substrate. Further, the relevance between the crystal quality of epitaxial β-Ga2O3 film and the β-Ga2O3/GaN interface behavior is investigated. Transmission electron microscopy result indicates that the interface atom refactoring phenomenon is beneficial to relieve the mismatch strain and improve the crystal quality of subsequent β-Ga2O3 film. Moreover, the energy band structure of β-Ga2O3/GaN heterostructure grown by MOCVD is investigated by X-ray photoelectron spectroscopy and a large conduction band offset of 0.89 eV is obtained. The results in this work not only convincingly demonstrate the advantages of β-Ga2O3 films grown on GaN substrate, but also show the great application potential of MOCVD β-Ga2O3/GaN heterostructures in microelectronic applications.

Tailoring the solar-blind photoresponse characteristics of β-Ga2O3 epitaxial films through lattice mismatch and crystal orientation

Realizing manipulation of the photoelectric properties of wide bandgap semiconductors is a main challenge for successful next-generation functional optoelectronics. As an intriguing wide bandgap semiconductor, β-Ga2O3 (e.g. ∼4.9 eV) is emerging as a promising candidate for photodetectors operating in the solar-blind region. Here, we show that by selecting substrates with different symmetries and lattice parameters (i.e. (100) MgO, (100) MgAl2O4 and (0001) α-Al2O3), epitaxial β-Ga2O3 films with (100)-or -orientation could be fabricated. We found that the photoresponse characteristics are strongly correlated with the lattice mismatch and film orientation. In particular, (100)-oriented β-Ga2O3 film grown on MgO substrate with smaller lattice mismatch exhibited a 254 nm responsivity of 0.1 A · W−1 and detectivity of 4.3 × 1012 Jones, which are approximately an order of magnitude higher than that of the -oriented β-Ga2O3 film. Our work may provide a strategy to develop further high performance solar-blind photodetectors.

Low 114 cm−3 free carrier concentration in epitaxial β-Ga2O3 grown by MOCVD

We report on record low free carrier concentration values in metalorganic chemical vapor deposition (MOCVD) grown β-Ga2O3 by using N2O for oxidation. Contrary to the pure oxygen, the N2O oxidant produced β-Ga2O3 thin films co-doped with nitrogen and hydrogen, but the incorporation efficiency of both impurities is strongly dependent on key MOCVD growth parameters. An array of growth conditions resulted in β-Ga2O3 thin films with N and H concentrations ranging as high as ∼2 × 1019 cm−3 and ∼7 × 1018 cm−3, respectively, to films with no SIMS detectable N and H was identified. Films grown without detectable N and H concentrations showed a room temperature electron mobility of 153 cm2/V s with the corresponding free carrier concentration of 2.4 × 1014 cm−3. This is the lowest room temperature carrier concentration reported for MOCVD grown β-Ga2O3 with excellent electron mobility. A thin β-Ga2O3 buffer layer grown using N2O reduced the net background concentration in an oxygen grown film and is attributed to the compensation of Si at the film/substrate interface by N, which acts as a deep acceptor. The results show that the use of the N2O oxidant can lead to low background concentration and high electron mobility, which paves the road for the demonstration of high-performance power electronic devices with high breakdown voltages and low on-resistances.

Deep-level noise characterization of MOVPE-grown β -Ga2O3

We measure the low-frequency noise in epitaxial β-Ga2O3 grown by MOVPE. Both 1/f and generation-recombination noise components are well resolved. The Hooge parameters characterizing the 1/f noise are 3 × 10–4 at room temperature and 2 × 10–5 at temperatures near 200 K. Mid bandgap trap states result in generation-recombination noise that is analyzed using temperature dependent low-frequency deep-level noise spectroscopy. Trap levels with energies of 165, 127, and 37 meV below the conduction band minimum are characterized in terms of density and activation energy.

Epitaxial growth and interface band alignment studies of all oxide α-Cr2O3/β-Ga2O3 p-n heterojunction

Epitaxial growth of α-Cr2O3(p-type) on c-Al2O3 and β-Ga2O3 (n-type) on α-Cr2O3(p-type) has been carried out to make an all oxide epitaxial n-type β-Ga2O3/p-type α-Cr2O3 heterojunction using RF sputtering. A valence band offset of 3.38 ± 0.2 eV at the heterojunction is determined using Kraut's method. From the bandgap measurements of α-Cr2O3 and β-Ga2O3, the conduction band offset of 1.68 ± 0.2 eV at the heterojunction is obtained. Thus, the band alignment at this heterojunction is found to be staggered (Type-II), which leads to the confinement of electrons and holes in the β-Ga2O3 layer and α-Cr2O3 layer, respectively. Our results provide a pathway to design all oxide optoelectronic devices based on a p-n heterojunction consisting of n-type β-Ga2O3 and p-type α-Cr2O3.

Growth and characterization of β-Ga2O3 thin films on different substrates

β-Ga2O3 thin films were grown on the substrates of sapphire, GaN, and single crystals of β-Ga2O3, using plasma-assisted molecular beam epitaxy. By varying deposition conditions, pure-phase epitaxial β-Ga2O3 thin films were obtained, and the crystal quality of the as-grown films was optimized. A systematic characterization and a detailed analysis were performed on the films, including the nucleation process, surface morphology, crystal quality, thermal stability, as well as electrical and optical properties. Optical absorption was investigated using photothermal deflection spectroscopy, which provides detailed information about sub-gap optical absorption. Photocurrent measurements indicated a pronounced persistent photo-conductivity of β-Ga2O3. A blue-UV emission with an energy of 3–3.5 eV was observed by cathodoluminescence spectroscopy. The Fermi level position of the as-grown film was determined based on temperature-dependent electrical conductivity measurements. It is proposed that oxygen vacancies in the film form a defect band at around Ec-0.8 eV that pins the Fermi level and is related to the observed photocurrent and cathodoluminescence characteristics.

Puzzling robust 2D metallic conductivity in undoped β-Ga2O3 thin films

Here, we report the analogy of an extremely stable topological-like ultra-wide bandgap insulator, a solid that is a pure insulator in its bulk but has a metallic conductive surface, presenting a two-dimensional conductive channel at its surface that challenges our current thinking about semiconductor conductivity engineering. Nominally undoped epitaxial β-Ga2O3 thin films without any detectable defect (after a range of state-of-the-art techniques) showed the unexpectedly low resistivity of 3 × 10−2 Ωcm which was found to be also resistant to high dose proton irradiation (2 MeV, 5 × 1015cm−2 dose) and was largely invariant (metallic) over the phenomenal temperature range of 2 K up to 850 K. The unique resilience and stability of the electrical properties under thermal and highly ionizing radiation stressing, combined with the extended transparency range (thanks to the ultra-wide bandgap) and the already known toughness under high electrical field could open up new perspectives for use as expanded spectral range transparent electrodes (e.g., for UV harvesting solar cells or UV LEDs/lasers) and robust Ohmic contacts for use in extreme environments/applications and for novel optoelectronic and power device concepts.

MOCVD grown epitaxial β-Ga2O3 thin film with an electron mobility of 176 cm2/V s at room temperature

In this work, we report record electron mobility values in unintentionally doped β-Ga2O3 films grown by metal-organic chemical vapor deposition. Using degenerately Sn-doped regrown n+ β-Ga2O3 contact layers, we were able to maintain Ohmic contact to the β-Ga2O3 films down to 40 K, allowing for reliable temperature-dependent Hall measurement. An electron mobility of 176 cm2/V s and 3481 cm2/V s were measured at room temperature and 54 K, respectively. The room and low temperature mobilities are both among the highest reported values in a bulk β-Ga2O3 film. A low net background charge concentration of 7.4 × 1015 cm−3 was confirmed by both temperature dependent Hall measurement and capacitance-voltage measurement. The feasibility of achieving low background impurity concentration and high electron mobility paves the road for the demonstration of high performance power electronics with high breakdown voltages and low on-resistances.

High resistivity halide vapor phase homoepitaxial β-Ga2O3 films co-doped by silicon and nitrogen

Semi-insulating halide vapor phase epitaxial β-Ga2O3 films without intentional dopants introduced during growth are demonstrated. The sheet resistance measured in the 340–480 K range yielded 268–134 kΩ/◻ and an activation energy of 0.81 eV. Room temperature capacitance-voltage measurements at 1 MHz showed evidence of an ultra-low free carrier concentration n-type film with a free carrier concentration near flatband (VFB ∼ 4.4 V) estimated to be &amp;lt;1014 cm−3, resulting in a high breakdown voltage of 2380 V (3.18 MV/cm) measured on a lateral diode without field termination. Secondary ion mass spectroscopy did not reveal Fe compensating species; however, an average Si concentration of about 5 × 1015 cm−3 and an N concentration of about 2 × 1017 cm−3 were detected, suggesting that N acceptors compensated Si donors to result in a nearly intrinsic β-Ga2O3 film. Photoionization spectroscopy suggested the presence of a deep acceptor-like level located at Ec −0.23 eV.

Demonstration of high-responsivity epitaxial β-Ga2O3/GaN metal–heterojunction-metal broadband UV-A/UV-C detector

We demonstrate epitaxial β-Ga2O3/GaN-based vertical metal–heterojunction-metal (MHM) broadband UV-A/UV-C photodetectors with high responsivity (3.7 A/W) at 256 and 365 nm, UV-to-visible rejection >103, and a photo-to-dark current ratio of ∼100. A small (large) conduction (valence) band offset at the heterojunction of pulsed laser deposition (PLD)-grown β-Ga2O3 on metal organic chemical vapor deposition (MOCVD)-grown GaN-on-silicon with epitaxial registry, as confirmed by X-ray diffraction (XRD) azimuthal scanning, is exploited to realize detectors with an asymmetric photoresponse and is explained with one-dimensional (1D) band diagram simulations. The demonstrated novel vertical MHM detectors on silicon are fully scalable and promising for enabling focal plane arrays for broadband ultraviolet sensing.

(Invited) Thick, Low-Doped Homoepitaxial Ga2O3 for Power Electronics Applications

Monoclinic β-Ga2O3 has attracted significant attention as a high power electronics material based on its ultra-wide energy gap (4.9 eV), high theoretical critical field (8 MV/cm), as well as the commercially availability of inexpensive substrates grown from the melt. In order to achieve breakdown field consistent with the theoretical expectations, however, very high quality epitaxial films with very low doping and defect density will be required. Wong et al. have reported a β-Ga2O3 MOSFET with carrier concentration below 4x1014 cm-3, which is the lowest reported value for epitaxial β-Ga2O3 to date [1]. This talk will discuss the structural and electronic properties for thick homoepitaxial β-Ga2O3 with doping as low as 8x1012 cm-3, corresponding to breakdown voltage in excess of 2.38 kV (3.18 MV/cm), measured without field termination for vertical measurements or edge effects correction in the case of lateral devices. Our results show that current β-Ga2O3 halide vapor phase epitaxial technology can produce epilayers with breakdown field approaching the theoretical value of GaN. In addition, we will discuss the annealing of bulk and epitaxial β-Ga2O3 films in O2 atmosphere as a potential pathway to reduce carrier concentration and possibly improve the breakdown voltage of Ga2O3 Schottky diodes. We show that annealing Ga2O3 in O2 will reduce ND-NA by about an order of magnitude, similar to earlier observations by Kuramata et al. [2]. Furthermore, we will present our approach for overcoming the polaron-induced suppression of room-temperature p-type conductivity in Ga2O3, causing holes to preferentially self-trap instead of being available as free carriers [3]. We show that natural p-type semiconducting oxides such as NiO can be integrated with β-Ga2O3 to form a rectifying heterojunction as a fundamental building block for any viable Ga2O3 power device. Finally, we will discuss the low thermal conductivity of Ga2O3 and present our preliminary thermal management simulations and experimental results. [1] A. Kuramata, et al., Jpn. Jour. Appl. Phys. 55, 1202A2 (2016). [2] M.H. Wong, et al., Appl. Phys. Express 10, 041101 (2017). [3] J.B. Varley, et al., Phys. Rev. B 85, 081109(R) (2012).

Compensation and persistent photocapacitance in homoepitaxial Sn-doped β-Ga2O3

The electrical properties of epitaxial β-Ga2O3 doped with Sn (1016–9 × 1018 cm−3) and grown by metalorganic chemical vapor deposition on semi-insulating β-Ga2O3 substrates are reported. Shallow donors attributable to Sn were observed only in a narrow region near the film/substrate interface and with a much lower concentration than the total Sn density. For heavily Sn doped films (Sn concentration, 9 × 1018 cm−3), the electrical properties in the top portion of the layer were determined by deep centers with a level at Ec-0.21 eV not described previously. In more lightly doped layers, the Ec-0.21 eV centers and deeper traps at Ec-0.8 eV were present, with the latter pinning the Fermi level. Low temperature photocapacitance and capacitance voltage measurements of illuminated samples indicated the presence of high densities (1017–1018 cm−3) of deep acceptors with an optical ionization threshold of 2.3 eV. Optical deep level transient spectroscopy (ODLTS) and photoinduced current transient spectroscopy (PICTS) detected electron traps at Ec-0.8 eV and Ec-1.1 eV. For lightly doped layers, the compensation of film conductivity was mostly provided by the Ec-2.3 eV acceptors. For heavily Sn doped films, deep acceptor centers possibly related to Ga vacancies were significant. The photocapacitance and the photocurrent caused by illumination at low temperatures were persistent, with an optical threshold of 1.9 eV and vanished only at temperatures of ∼400 K. The capture barrier for electrons causing the persistent photocapacitance effect was estimated from ODLTS and PICTS to be 0.25–0.35 eV.

High reverse breakdown voltage Schottky rectifiers without edge termination on Ga2O3

Vertical geometry Ni/Au-β-Ga2O3 Schottky rectifiers were fabricated on Hydride Vapor Phase Epitaxy layers on conducting bulk substrates, and the rectifying forward and reverse current-voltage characteristics were measured at temperatures in the range of 25–100 °C. The reverse breakdown voltage (VBR) of these β-Ga2O3 rectifiers without edge termination was a function of the diode diameter, being in the range of 920–1016 V (average value from 25 diodes was 975 ± 40 V, with 10 of the diodes over 1 kV) for diameters of 105 μm and consistently 810 V (810 ± 3 V for 22 diodes) for a diameter of 210 μm. The Schottky barrier height decreased from 1.1 at 25 °C to 0.94 at 100 °C, while the ideality factor increased from 1.08 to 1.28 over the same range. The figure-of-merit (VBR2/Ron), where Ron is the on-state resistance (∼6.7 mΩ cm2), was approximately 154.07 MW·cm−2 for the 105 μm diameter diodes. The reverse recovery time was 26 ns for switching from +5 V to −5 V. These results represent another impressive advance in the quality of bulk and epitaxial β-Ga2O3.

Temperature-Dependent Characteristics of Ni/Au and Pt/Au Schottky Diodes on β-Ga2O3

Schottky diodes were formed on bulk or epitaxial β-Ga2O3 using Ni/Au or Pt/Au and the electrical characteristics measured as a function of temperature in the range 25–200°C. The barrier heights were 1.07 eV (Ni/Au) and 1.04 eV (Pt/Au) at 25°C. The barrier heights increased with temperature, while the on-state resistances (RON) decreased over the same range. The temperature coefficient of reverse breakdown voltage (VB), β, was −4 mV/K for Ni/Au and −0.1 mV/K for Pt/Au. The figure-of-merit (VB2/RON) was above 3 MW.cm−2 at 25°C for Ni/Au diodes and was still ∼1 MW.cm−2 at 200°C. The reverse recovery times were also measured as a function of temperature and were of the order of 21–28 ns over the range 25–150°C for both epi and bulk diodes. The results show the already high quality of bulk and epitaxial β-Ga2O3 and the potential of this material for high temperature power electronics.

A self-powered deep-ultraviolet photodetector based on an epitaxial Ga2O3/Ga:ZnO heterojunction

An epitaxial β-Ga2O3/Ga:ZnO heterojunction based self-powered DUV photodetector with an excellent wavelength selectivity and a high DUV/visible rejection ratio.

Room-temperature laser annealing for solid-phase epitaxial crystallization of β-Ga2O3 thin films

The epitaxial crystallization of β-Ga2O3 thin films on NiO-buffered α-Al2O3(0001) substrates via the solid-phase crystallization of amorphous Ga2O3 thin films by KrF excimer laser annealing at room temperature (RT) was examined. The results of X-ray and reflection high-energy electron diffraction measurements indicated that the epitaxial β-Ga2O3 thin films were fabricated by RT laser annealing. The optical bandgap of the thin films was estimated to be 4.9 eV from the results of UV/vis transmittance measurements. In the cathodoluminescence spectrum, UV–green luminescence was observed for the thin films. These optical properties are similar to those of bulk β-Ga2O3.