This is the accepted version of the journal article: Chi, Zeyu; Tchelidze, Tamar; Sartel, Corinne; [et al.]. «Assessment of large critical electric field in ultra-wide bandgap p-type spinel ZnGa2O4». Journal of Physics D: Applied Physics, Vol. 56, núm. 10 (March 2023), art. 105102. DOI 10.1088/1361-6463/acbb14 This version is available at https://ddd.uab.cat/record/282564 under the terms of the license Assessment of Large Critical Electric Field in Ultra-wide Bandgap p-type Spinel ZnGa2O4 Zeyu Chi1, Tamar Tchelidze2, Corinne Sartel1, Tsotne Gamsakhurdashvili2, Ismail Madaci1, Hayate Yamano3, Vincent Sallet1, Yves Dumont1, Amador Pérez-Tomás5, Farid Medjdoub4, Ekaterine Chikoidze*1 1 Groupe d’Etude de la Matière Condensée (GEMaC), Université Paris-Saclay, UVSQ – CNRS, 45 Av. des Etats-Unis, 78035 Versailles Cedex, France 2 Faculty of Exact and Natural Science, Department of Physics, Ivane Javakhishvili Tbilisi State University, 3 Av. Tchavtchavadze, 0179 Tbilisi, Georgia 3 Department for Integrated Sensor Systems, Danube University Krems, 3500 Krems, Austria 4 IEMN, CNRS, UMR8520, Av. Poincare, 59650, Villeneuve D'Ascq, France 5 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Barcelona, Spain Correspondence author: ekaterine.chikoidze@uvsq.fr Abstract: The spinel Zinc gallate ZnGa2O4 stands out among the emerging ultra-wide bandgap (~5 eV) semiconductors as the ternary complex oxide with the widest gap where bipolar conductivity has been demonstrated. For power and energy electronics, a fundamental property of the material is its critical electric field (𝐸𝐶𝑅 ) although, for ZnGa2O4, is yet unknown. In this work, highly resistive p-type ZnGa2O4 thin films on sapphire and Si substrates were grown by metal organic chemical vapor deposition (MOCVD) to determine both, the remote acceptor concentration and vertical breakdown voltage. Hall Effect measurements confirmed a low carrier concentration at room temperature of ~1011 cm-3. From vertical metal-semiconductor-metal structures the average ECR has been estimated to be of at least 5.3 MV/cm, which already is significantly larger than the one of SiC and GaN. Keywords: ultra-wide band gap, ZnGa2O4, electrical properties, critical electric field 1 1. Introduction Ultra-wide bandgap (UWBG) semiconductor oxides have recently attracted significant attention as a novel platform for ultra-high power electronics and deep ultraviolet opto-electronics [1]. The spinel zinc gallate (ZnGa2O4) stands out within the family of oxide UWBG (Eg ~5 eV) as an advanced semiconductor platform for applications such as photonics (i.e. photodetector [2], phototransistors [3–5]), gas sensors [6], thin-film-transistors [7–9], or optical memory application [10], thus, heralding novel spinel electronics. Spinel oxides have, indeed, a peculiar chemical formula AB2O4, where A and B cations occupy octahedral (Oh) and tetrahedral (Td) sites formed by oxygen atoms. Thus, spinel oxides obey different doping rules from traditional binary semiconductor oxides because of their multivalent elements and different sites, such as the antisite defects due to the inverted distribution of cations [11]. The disordered distribution of ions and vacancies in the material, (caused by the so- called self-doping effect), can lead to intrinsic electron or hole conductivity [12]. Consequently, a wide range of intrinsic conductivities have been reported for ZnGa2O4. The resistivity of native n-ZnGa2O4 can be as low as 2.7×10-2 Ω·cm at room temperature (e.g., [7,13]), while intrinsic p-ZnGa2O4 is, in general, rather insulating at room temperature. Indeed, Chikoidze et al. [14] reported native p-type ZnGa2O4 thin films grown by metal organic chemical vapor deposition (MOCVD), where the intrinsic hole conductivity at high temperature was attributed to the antisite defects ZnGa. Therefore, the intrinsic conductivity of ZnGa2O4 thin films can be controlled well over 10 orders of magnitude by tuning the growth conditions (cation ratio, oxygen pressure, etc.) thanks to this self-doping effect [15]. In agreement with these empirical observations, Sabino et al. [16] suggested by ab-initio simulations that both, n- and p-type conductivities could be achieved in ZnGa2O4, and the hole conductivity is likely achieved by a polaronic band due to excess of Zn. It should be noted, however, that the electronic and optoelectronic devices based on ZnGa2O4, are mostly unipolar (i.e., n-type) (e.g. [17]) and studies on p-type ZnGa2O4 devices are still very limited. However, to exploit the full potential of ZnGa2O4 spinel electronics, bipolar devices (such as p-n junction (photo) diodes, PiN rectifiers, lateral and vertical power MOSFETs) and p-type conductivity should be achieved. The critical electric field (𝐸𝐶𝑅 ) of a material is a crucial parameter for assessing a semiconductor material’s potential as a platform for power and energy electronics applications. In particular, the Baliga’s figure of merit [18] is informative of the on-state/blocking device performance for power switching applications which primarily depends on carrier mobility and the cube of the critical electric field. Representative values of the critical electric field for the most common power electronics semiconductors, such as silicon (0.3 MV/cm), 4H-SiC (3.5 MV/cm) [19], GaN (3.9 MV/cm) [20], or AlGaN (5.9 MV/cm [21]) are well known. For other UWBG such as AlN, the 𝐸𝐶𝑅 values have also been reported (12 MV/cm [22]) while a generally agreed theoretical value for diamond is 10 MV/cm [23]. For β-Ga2O3, a common estimation for the upper limit given in the literature is ~8 MV/cm [24], while this value may be well underestimated as it has recently been experimentally observed to be at least 8.32 MV/cm and 13.2 MV/cm for p-NiO/β-Ga2O3 junctions [25] and Zn-doped β-Ga2O3 [26], respectively). However, to the best of our knowledge, critical electric field values for ZnGa2O4 are not yet known. 2 Therefore, the objective of the present work was to define ZnGa2O4 thin films onto insulating and conducting substrates for assessing the remote concentration and to stablish a preliminary value of the ZnGa2O4 critical electric field , respectively. According to the kinetic impact ionization theory and previous studies on Ga2O3 [26], to achieve large critical electric fields, it is mandatory to obtain a sample with low remote carrier concentration to reduce the onset of the impact ionization process. In the present work, we studied ZnGa2O4 thin films, which are highly resistive p-type in nature due to their deep acceptor native defects as we will show further on. 2. Experimental details ZnGa2O4 thin film were grown in a radio-frequency (RF) heated horizontal MOCVD reactor with separate inlets to avoid premature reactions in the manifold between oxygen and organometallics precursors. Trimethylgallium (TMGa), diethylzinc (DEZn) and, 5.5 N pure oxygen were used as gallium, zinc, and oxygen sources, respectively. The TMGa and DEZn bubbler temperatures were fixed at -10°C and 0°C, respectively. During the growth, the flow rates of TMGa, DEZn, and oxygen were kept at 11 µmol/min, 7 µmol/min, and 1200 sccm, respectively. The growth temperature and the reactor pressure were set at 775°C, and 38 torr, respectively. The growth time was 90 minutes. Argon was used as carrier gas. To analyse the ZnGa2O4 crystal phases, X-Ray diffraction profiles were recorded in θ/2θ configuration between 15° and 115°, using a Rigaku SmartLab apparatus equipped with a Cu-Kα source (λ = 0.1541 nm). The thicknesses of the thin films were determined by means of scanning electron microscopy (SEM) cross-sectional images from a JEOL JSM 7001F scanning electron microscope. For Hall effect measurements, Ohmic contacts were prepared with silver paint at the four corners of the sample. The resistivity and Hall effect measurements were performed, in a Van der Pauw configuration with perpendicularly to current applied magnetic fields varying from 0 to 1.6 T, in 550 to 850 K temperature range. The majority charge carrier sign, concentration, and mobility were then determined. A Keysight B1505A Power Device Analyzer has been used to carry out the vertical breakdown voltage measurements at room temperature. Negative bias has been applied on an isolated Ohmic contact with the grounded substrate to access the breakdown voltage. 3. Results and discussion 3.1. Thermodynamic Analysis of Point Defects and Free Carriers We conducted a thermodynamic analysis of point defects and free carriers for equilibrium condition to establish the optimal growth parameters of the ZnGa2O4 close to the stoichiometry composition. This thermodynamic analysis is carried out for the ZnGa2O4 (crystal) - O2 (gas) system. It is a three-component two-phase system. Thus, according to Gibbs’ phase rule, its degree of freedom is 3, so three thermodynamic parameters can vary freely. All material 3 properties, including defects, free carriers’ concentrations, and conductivity, depend on them. We chose the following independent variables: temperature 𝑇, oxygen partial pressure 𝑃𝑂2 , and 𝑃 the ratio of gallium and zinc partial pressures 𝑥 = 𝑃𝐺𝑎 . 𝑍𝑛 To find the dependence of concentrations of defects and free carriers on these parameters we used the Kroger method of quasi-chemical reactions [27]. Special attention is paid to discern if the concentration of antisite defects GaZn and ZnGa can be controlled in such a way to impact n- and p- type conductivities. The processes we considered with the corresponding mass action law are the following: The creation of a Schottky defect pairs: ø ⇆ 2𝑉𝐺𝑎 + 3𝑉𝑂 , 𝐾𝑆1 = [𝑉𝐺𝑎 ]2 [𝑉𝑂 ]3 (1) ø ⇆ 𝑉𝑍𝑛 + 𝑉𝑂 , 𝐾𝑆2 = [𝑉𝑍𝑛 ][𝑉𝑂 ] (2) Where 𝑉𝑍𝑛 , 𝑉𝐺𝑎 , and 𝑉𝑂 are zinc, gallium, and oxygen vacancies, respectively; [𝑉𝑍𝑛 ], [𝑉𝐺𝑎 ], and [𝑉𝑂 ] are their concentrations, respectively. The incorporation of oxygen atoms in a crystal from the surrounding atmosphere, when zinc and gallium vacancies are created: 1 [𝑉𝑍𝑛 ] 𝑂 ⇆ 𝑂𝑂 + 𝑉𝑍𝑛 , 𝐾𝑉𝑍𝑛 = (3) 2 2 1/2 𝑃𝑂2 3 [𝑉𝐺𝑎 ]2 𝑂 ⇆ 3𝑂𝑂 + 2𝑉𝐺𝑎 , 𝐾𝑉𝐺𝑎 = (4) 2 2 𝑃𝑂2 3/2 The lattice thermal ionization: ø ⇆ 𝑒 + ℎ, 𝐾𝑖 = 𝑛𝑝 (5) Where 𝑒 and ℎ are electrons and holes, respectively; n and p are their concentrations, respectively. The ionization of zinc, gallium and oxygen vacancies: ′ ]𝑝 [𝑉𝑍𝑛 ′ (6) 𝑉𝑍𝑛 ⇆ 𝑉𝑍𝑛 + ℎ, 𝐾𝑖𝑍𝑛 = [𝑉𝑍𝑛 ] ′ ]𝑝 [𝑉𝐺𝑎 ′ (7) 𝑉𝐺𝑎 ⇆ 𝑉𝐺𝑎 + ℎ, 𝐾𝑖𝐺𝑎 = [𝑉𝐺𝑎 ] ′ ′ Where 𝑉𝑍𝑛 and 𝑉𝐺𝑎 denote positively single-charged zinc and gallium vacancies, respectively; ′ ′ [𝑉𝑍𝑛 ]and [𝑉𝐺𝑎 ] are their concentrations, respectively. The ionization of oxygen vacancy: [𝑉𝑂• ]𝑛 𝑉𝑂• ⇆ 𝑉𝑂 + 𝑒, 𝐾𝑖𝑂 = (8) [𝑉𝑂 ] Where 𝑉𝑂• denotes negatively single-charged oxygen vacancy; [𝑉𝑂• ] is its concentration. Creation of antisite defects via incorporation of metal atoms from the gas phase into vacancies: [𝑍𝑛𝐺𝑎 ] 𝑍𝑛(𝑔) + 𝑉𝐺𝑎 ⇆ 𝑍𝑛𝐺𝑎 , 𝐾𝑍𝑛 = (9) [𝑉𝐺𝑎 ]𝑃𝑍𝑛 4 [𝐺𝑎𝑍𝑛 ] 𝐺𝑎(𝑔) + 𝑉𝑍𝑛 ⇆ 𝑍𝑛𝐺𝑎 , 𝐾𝐺𝑎 = (10) [𝑉𝑍𝑛 ]𝑃𝐺𝑎 Where 𝑍𝑛𝐺𝑎 and 𝐺𝑎𝑍𝑛 are zinc in gallium site and gallium in zinc site anti-site defects, respectively; [𝑍𝑛𝐺𝑎 ] and [𝐺𝑎𝑍𝑛 ] are their concentrations, respectively. And their ionizations: ′ ]𝑝 [𝑍𝑛𝐺𝑎 ′ (11) 𝑍𝑛𝐺𝑎 ⇆ 𝑍𝑛𝐺𝑎 + ℎ, 𝐾𝑖𝑍𝑛𝐺𝑎 = [𝑍𝑛𝐺𝑎 ] • ]𝑛 [𝐺𝑎𝑍𝑛 • (12) 𝐺𝑎𝑍𝑛 ⇆ 𝐺𝑎𝑍𝑛 + 𝑒, 𝐾𝑖𝐺𝑎𝑍𝑛 = [𝐺𝑎𝑍𝑛 ] ′ • Where 𝑍𝑛𝐺𝑎 and 𝐺𝑎𝑍𝑛 are positively and negatively single-charged anti-site defects, ′ • respectively; [𝑍𝑛𝐺𝑎 ] and [𝐺𝑎𝑍𝑛 ] their concentrations, respectively. To find the concentration of defects and free carriers by the system of equations (1) - (12), the charge neutrality condition must be added: ′ ] ′ ] ′ ] 𝑛 + [𝑉𝑍𝑛 + [𝑉𝐺𝑎 + [𝑍𝑛𝐺𝑎 ⇆ 𝑝 + [𝑉𝑂• ] + [𝐺𝑎𝑍𝑛 • ] (13) The connection between zinc, gallium, and oxygen partial pressures are defined by means of the constant of the zinc gallate creation reaction: 2 2 (14) 𝑍𝑛(𝑔) + 2𝐺𝑎(𝑔) + 2𝑂2 (𝑔) ⇆ 𝑍𝑛𝐺𝑎2 𝑂4 (𝑠), 𝐾 = 𝑃𝑍𝑛 𝑃𝐺𝑎 𝑃𝑂2 Having solved the system of equations (1) - (14), one can find the concentrations of defects and free carriers versus temperature, partial pressure of one component and ratio of gallium and zinc pressures. The calculated concentrations of charged defects and free carriers in dependence of oxygen partial pressure are shown in Figure 1. The temperature was chosen to be 775 °C, the ratio 𝑥 = 𝑃𝐺𝑎 equals to 3. These parameters correspond to our actual growth conditions. The 𝑥 = 𝑃𝐺𝑎 ⁄𝑃𝑍𝑛 ratio 𝑃𝑍𝑛 is roughly estimated on the base of TMGa and DEZn flow rates and their formation enthalpies: 𝑃𝐺𝑎 𝐽𝐺𝑎 √2𝑚𝐺𝑎 𝑘𝑇 exp(− ∆𝐻𝐺𝑎 ⁄𝑘𝑇) 𝑥= = 𝑃𝑍𝑛 𝐽𝑍𝑛 √2𝑚𝑍𝑛 𝑘𝑇 exp(− ∆𝐻𝑍𝑛 ⁄𝑘𝑇) Here 𝐽𝐺𝑎 and 𝐽𝑍𝑛 are flow rates of TMGa and DEZn, respectively, 𝑚𝐺𝑎 and 𝑚𝑍𝑛 masses of Ga and Zn atoms, ∆𝐻𝐺𝑎 and ∆𝐻𝑍𝑛 formation enthalpies of TMGa and DEZn. One can distinguish the following regions in the figure that: I. The region of intrinsic n-conductivity, origin of which are oxygen vacancies 𝑉𝑂 . In this region [𝑉𝑂• ] = 𝑛, and they are dominant; II. The first region of self-compensation, here oxygen vacancies [𝑉𝑂• ] and zinc vacancies [𝑉𝑍𝑛 ′ ] • ′ are dominant spices and [𝑉𝑂 ] = [𝑉𝑍𝑛 ]; ′ III. The second region of self-compensation where zinc vacancies [𝑉𝑍𝑛 ] and gallium in zinc sites • ] ′ • [𝐺𝑎𝑍𝑛 are dominant, and [𝑉𝑍𝑛 ] = [𝐺𝑎𝑍𝑛 ]. In this region, the concentration of holes exceeds the concentration of electrons significantly. Around 𝑃𝑂2 = 1 𝑎𝑡𝑚, the concentration of holes achieves 1015 cm-3. It is remarkable that the concentration of holes at a whole range of pressure ′ ′ tails negatively charged gallium vacancies [𝑉𝐺𝑎 ] - the concentrations of 𝑉𝐺𝑎 and holes change 5 the same way when oxygen partial pressure changes and difference between these concentrations is quite small along the whole pressure range. ′ ] IV. The region of hole conductivity, here [𝑉𝐺𝑎 and holes are dominant spices. However, the oxygen pressure is unrealistically high here. Notably, there is a very weak dependence of concentrations of free carriers on 𝑥. For 𝑥 = 10−3 , that is, when the partial pressure of Zn exceeds the partial pressure of Ga three orders of magnitude, the concentration of holes increases only ten times, compared to 𝑥 = 3. Figure 1. The dependence of concentration of point defects and free carriers on oxygen partial 𝑃 pressure, for T = 775 °C, and 𝑥 = 𝑃𝐺𝑎 = 3. 𝑍𝑛 Therefore, the growth of both n- and p-type ZnGa2O4 thin films are feasible by tuning the growth parameters such as, mainly oxygen partial pressure and temperature. The cations concentration ratio weakly affects the conductivity (as experimentally observed [14]). In this work, we concentrate on the regions predicted to correspond to p-type conductivity. Then, the highly-resistive p-type ZnGa2O4 thin layer was grown in the conditions which are input from thermodynamic analysis. 3.2. ZnGa2O4 Thin Film Growth and Structural Properties ZnGa2O4 thin films were grown on two kinds of substrates simultaneously: on conducting p-type Si(111) and c-oriented sapphire Al2O3(0001) (insulating) substrates. ZnGa2O4/Si sample was used to fabricate vertical parallel plane capacitors, and characterized vertically for breakdown voltage capability. While the ZnGa2O4/sapphire thin film was used for the investigation of the electrical transport properties (resistivity, carrier, mobility) at elevated temperatures in a Van Der Pauw configuration. Figure 2 shows the Cu-Kα X-ray diffraction (XRD) spectra of ZnGa2O4 thin films grown on Si(111) and Al2O3(0001). According to the PDF 28-1240 file, it is pure ZnGa2O4 spinel without a secondary phase. Film on Si(111) is polycrystalline without any preferential texturization, similar to 6 XRD pattern reported by Galazka et al. [28] for their melt-grown bulk ZnGa2O4 spinel, milled in a randomly oriented powder. For films on Al2O3(0001) substrates, Bragg peaks appearing at 18.6°, 37.6°, 57.6°, 79.7°, and 106.6° correspond to the (111), (222), (333), (444), and (555) planes of the cubic spinel ZnGa2O4, respectively, as reported by previous literature [14,29]. Figure 2. Cu-Kα XRD spectra of ZnGa2O4 thin film grown on Si(111) (black), and Al2O3(0001) (red). * and # correspond respectively to Al2O3(0001) and Si(111) Bragg reflections. ZnGa2O4 thin films have already been grown polycrystalline on sapphire substrates by MOCVD [30], radio-frequency (RF) magnetron sputtering [31,32], pulsed laser deposition (PLD) [13], with a preferential orientation along [111] of cubic spinel and equivalent direction for Al2O3(0001). Analogously, ZnGa2O4 thin films have also been grown on Si substrates by atomic layer deposition (ALD) [29], PLD [33], and RF magnetron sputtering [32]. A larger lattice mismatch between ZnGa2O4 and Si(100) (which may also contain a native ultra-thin SiO2 interlayer) results in a relatively weaker orientation of crystallite in the ZnGa2O4 grown on Si substrate than that on sapphire (0001) [32]. The full width at half maximum (FWHM) values were measured as 0.29° and 0.49° for ZnGa2O4 (111) on sapphire and Si(100) substrates, respectively, indicating also a smaller size of crystallites in the case of Si(111) substrates with respect to Al2O3(0001) ones. 3.3. Electrical Transport Properties Hall and electrical transport properties were studied for ZnGa2O4 grown on Al2O3 crystals, as the sapphire substrate is insulating up to high temperatures thus preventing parasitic leakage through the substrate. Figure 3-a shows a typical Ohmic current−voltage (I−V) behavior at different temperatures. The ohmic contact was made with silver paste painted at four corners of the sample in a square shape. The resistivity versus temperature for the spinel ZnGa2O4 sample is shown in Figure 3-b. At 850 K, the resistivity was measured as 1.9×102 Ω·cm, and increased to 5.3×104 Ω·cm at 550 K. Hall voltage (𝑉𝐻 ) versus magnetic field (0 – 1.6 T) at 700 – 850 K for a ZnGa2O4 sample is shown in Figure 3-c, the positive 𝑉𝐻 increases with a perpendicularly applied magnetic field in a Van der Pauw configuration, which evidenced that our spinel ZnGa2O4 thin film has p-type conductivity at elevated temperature. The temperature dependence for Hall hole concentration is shown in Figure 3- d for p-type ZnGa2O4 film, the hole concentration was measured between 8.8×1011 and 3.7×1015 cm- 7 3 at 550 – 850 K. Although the hopping behavior starts to pronounce below 600 K, extrapolating the band-activated regime of the hole concentration versus temperature at the higher temperatures, the hole density is below 1011 cm-3 at 300 K, confirming the low level of p-type doping with the applied growth conditions. The mobility of the p-type ZnGa2O4 thin film varies between 9 and 15 cm2/(V·s). Figure 3. (a) Ohmic I−V characteristics at 700K and 850 K (b) Resistivity versus temperature. (c) Hall voltage (𝑉𝐻 ) versus magnetic field at different temperatures (700 – 850 K). The Hall voltage is positive and increase with the increasing applied magnetic field, indicating the p-type conductivity in ZnGa2O4 layer. (d) Temperature dependence for Hall free hole concentration for ZnGa2O4 thin film. 3.4. ZnGa2O4 Critical Electric Field To determine the vertical critical electric field experimentally, vertical parallel plane capacitors has been fabricated by depositing Ti/Au contacts (with a size of 170 µm × 300 µm) onto p-type ZnGa2O4 grown onto the conducting Si thin films (Figure 4-a). Figure 4-b shows the scanning electron microscopy (SEM) cross-section image revealing that the thickness of the ZnGa2O4/Si is 600 nm. The vertical hard breakdown voltage 𝑉𝐵𝑅 was in the range of 270 to 350 V (Figure 4-c). It can be pointed out that a low leakage current below 1 mA/mm2 is observed all the way to the breakdown voltage. A thin SiO2 layer (2 nm) at the interface ZnGa2O4/Si can often be seen [32] after the growth. To exclude the influence of the SiO2 layer, a bare Si substrate with a native SiO2 (2 – 3 nm thick layer) has been characterized as well. The vertical breakdown voltage of around 10 V [26] is considered to produce a negligible contribution to the experimentally measured 𝑉𝐵𝑅 of p-type ZnGa2O4/Si structure. 8 Taking the experimentally determined 600 nm thickness for the ZnGa2O4 layer and extracting bare Si substrate related 𝑉𝐵𝑅 into account, the average vertical critical electric field across the film can be as high as 𝐸𝐶𝑅 = 5.3 MV/cm, which is already significantly larger than the one from SiC and (single crystal) GaN. Therefore, the blank of breakdown field for ZnGa2O4 in the literature for assessing WBG semiconductors [34] can be filled as shown in Table I. Figure 4. (a) Sketch of the vertical heterojunction structure with Ti/Au metal contacts. (b) SEM cross- section image of ZnGa2O4 polycrystalline film deposited on Si(111). The estimated thickness of the film is 600 nm. (c) Experimental I-V curves of vertical breakdown voltage measurements through the ZnGa2O4/Si structure, indicating a hard breakdown voltage up to 350 V at room temperature. Table I. Comparison of various wide-bandgap semiconductor materials properties [26,34,35]. *: experimental 𝐸𝐶𝑅 values measured by authors. aCommon value form literature. bp-NiO/n-Ga2O3 [25] C p-Ga2O3 [22] 4H-SiC GaN β-Ga2O3 ZnGa2O4 Hexagonal Hexagonal Cubic Crystal Structure Monoclinic (Wurtzite) (Wurtzite) (Spinel) Space Group P63mc P63mc C2/m Fd-3m Direct bandgap, 𝑬𝒈 (eV) 3.2 3.4 4.8 4.8 Dielectric constant, 𝜺 9.7 9 10 [36] 10.4 [37] 8 [24], 8.32b, a Critical electric field, 𝑬𝑪𝑹 (MV/cm) 3.5 3.9 > 5.3* 5.7c, 13.2c [26] Electron mobility, µ (cm2/(V·s)) 1000 1250 300 100 106 (300 K) Hole mobility, µ (cm2/(V·s)) 34 (300 K) [39] ~10 (high T) [26] ~10 (high T) [38] Saturation velocity, 𝒗𝒔 (107 cm/s) 2 2.5 1.8 – 2 – Thermal conductivity, λ (W/cm·K) 4.9 2.3 0.1 – 0.3 0.22 4. Conclusion In this work, following a thermodynamic analysis, highly resistive undoped polycrystalline ZnGa2O4 thin film of 600 nm were grown on p-type silicon substrate by MOCVD. The conductivity and hole concentration of the film at 550 K were determined to be as low as 5 ×104 Ω·cm and 9 ×1011 cm-3, respectively. The average breakdown voltage of ZnGa2O4/Si vertical structures was 𝑉𝐵𝑅 = 350 9 V, resulting in a critical electric field 𝐸𝐶𝑅 of 5.3 MV/cm. This value is already near twice the breakdown electrical fields of SiC and GaN and is in line with previous demonstration of its binary counterpart, Ga2O3. We believe that this first experimental estimation of 𝐸𝐶𝑅 for polycrystalline ZnGa2O4 will trigger further studies related to nanocrystal and high quality and homo-epitaxial films growth for future possible integration of spinel ZnGa2O4 and doped relatives for electronic and optoelectronic devices. Acknowledgement The present work is a part of “GALLIA” International Research Project, CNRS, France. GEMaC colleagues acknowledge financial support of French National Agency of Research (ANR), project “GOPOWER”, CE-50 N0015-01. 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