Spectroscopic and electrical calculation of band alignment between atomic layer deposited SiO₂ and β-Ga₂O₃ (⁠$2¯01$⁠)

Recently, wide bandgap β-Ga₂O₃ (Ga₂O₃) has received much attention as an attractive semiconductor for power electronics and UV detector applications due to its large bandgap of 4.6–4.9 eV.^1–3 In addition, high bulk electron mobilities in Ga₂O₃ lead to a Baliga Figure of Merit (BFoM), which exceeds that of SiC and GaN,^4,5 which makes it as an attractive choice for next generation of power semiconductor devices. Moreover, large area bulk crystals of Ga₂O₃ can be grown using scalable crystal growth technologies.^6–10 Both doped and semi-insulating bulk crystals are available commercially.^4,5,11,12 Epitaxial growth of Ga₂O₃ by molecular beam epitaxy (MBE)^4,13,14 and ion implantation doping technology¹⁵ has also been reported. All these factors make Ga₂O₃ a strong candidate for next generation power electronics. Ga₂O₃ metal-oxide semiconductor field effect devices (MOSFETs) with high breakdown voltages, large ON/OFF ratios, and high temperature operation have been recently demonstrated.^5,15–17 These devices use atomic layer deposited (ALD) Al₂O₃ as a gate barrier. The conduction band offset between Al₂O₃ and Ga₂O₃ has been determined to be 1.5–1.7 eV.^18,19 A higher conduction band offset is preferred to reduce thermal leakages during high temperature operation of power devices. However, the large bandgap of Ga₂O₃ limits the choice of gate dielectrics. In addition to be used as gate barrier, dielectrics are also used for passivation and electric field profiling by field plates in power semiconductor devices.

Silicon dioxide (SiO₂) is an attractive gate barrier material for Ga₂O₃ due to its large bandgap of ∼9 eV.²⁰ However, there is no report of the band parameters between SiO₂ and Ga₂O₃. In this letter, we report the band alignment between ALD SiO₂ and β-Ga₂O₃ (⁠$2¯01$⁠). Silicon dioxide deposited by ALD has great potential to serve as a gate dielectric in Ga₂O₃ based power device because of the expected large conduction band offset at the interface of SiO₂/Ga₂O₃ and also because of its large critical breakdown electric field (∼10 MV/cm (Ref. 21)). In this work, the conduction band offset of ALD-SiO₂/Ga₂O₃ hetero-junction was characterized using X-ray photoelectron spectroscopy (XPS) as well as the tunneling current through metal-oxide-semiconductor capacitors (MOSCAPs).

β-Ga₂O₃ (⁠$2¯01$⁠) crystals studied here was grown at Tamura Corporation with an n-type doping (Sn doped) density of ∼$9×1018/cm3$⁠. A surface root mean square (rms) roughness of 0.13 nm was measured by atomic force microscopy (AFM) on these wafers. For XPS characterization, a thin layer (∼3 nm) of SiO₂ was deposited on Ga₂O₃ by plasma-enhanced ALD in an Oxford FLEXAL system at 300 °C with trisdimethylaminosilane (3DMAS) and O₂ plasma at 250 W. Standard solvent degreasing procedure was used to clean the wafers before SiO₂ deposition. For band offset calculation by XPS, core level spectra of Si in bulk SiO₂ are necessary. We use 40 nm thick SiO₂ on Si as the bulk standard. For electrical studies, a 40 nm layer of SiO₂ was deposited on Ga₂O₃. The growth rate was calibrated on silicon wafers to be 0.71 Å/cycle. For electrical characterization, the MOSCAP structure, shown in Figure 1, was fabricated. First, the top Ti/Au electrodes were defined by standard photolithography and lift-off technique. Next, the silicon oxide and Ga₂O₃ were etched by CF₄/Ar based reactive ion etching. And finally, the bottom Ti/Au electrodes are defined. The sample was then annealed at 300 °C for 1 h to reduce the contact resistance.¹⁷ 

XPS measurements were performed using a Physical Electronic PHI VersaProbe 5000 equipped with a hemispherical energy analyzer. A monochromic Al Kα X-ray source (1486.6 eV) was operated at 25.3 W and 15 kV. The energy of the analyzer was operated at a pass energy of 117.5 eV for survey acquisitions and 23.50 or 11.75 eV for high-resolution acquisitions. The energy resolution was 0.025 eV for high resolution spectra or 1.0 eV for survey spectra. The operating pressure of XPS was $<4×10−6$ Pa (⁠$3.0×10−8$ Torr) and the background pressure was $<1×10−6$ Pa (⁠$7.5×10−9$ Torr). Dual charge neutralization was utilized to reduce the effects of charging on the acquired signal. Binding energies were calibrated by setting the CH_x peak in the C 1s envelope at 284.8 eV to correct for charging effects.^18,22 However, the valence band offset (VBO) measurement is not sensitive to charging effects.

Figures 2(a)–2(c) show the XPS spectra for 40 nm SiO₂/Si, β-Ga₂O₃, and SiO₂ (∼3 nm)/β-Ga₂O₃ structure. XPS results were curve fitted with a Gauss-Lorentzian band type with Shirley background subtraction^23–25 with curve fitting limits as follows: binding energy ±0.4 eV, FWHM ±0.2 eV, and % Gauss = 92%. The core level (Ga 2p_3/2) spectra on bare Ga₂O₃ shows a single peak (1118.4 eV) corresponding to Ga-O bond.¹⁸ The valence band maxima (VBM) were found by the linear extrapolation of the valence band states,^18,24,25 the VBM of Ga₂O₃ was found to be 3.69 eV above the Fermi level (E_F), as shown in Figure 2(b). For the SiO₂ (3 nm)/Ga₂O₃ sample, in addition to the Si-O bonds, the XPS spectrum shows the Ga 3p peaks from underneath the ALD-SiO₂ layer. Next, the valence band offset was calculated by the following equation:^18,26

where the subscripts indicate the XPS peak and the superscripts indicate the sample. From the measured XPS profiles, $(ESi2pSiO2/Ga2O3−EGa2p3/2SiO2/Ga2O3)$⁠, $(EGa2p3/2 Ga2O3−EVBMGa2O3)$⁠, and $(ESi2pSiO2−EVBMSiO2)$ are −1015.51, 1114.72, and 98.78 eV, respectively, which gives a valence band offset (⁠$ΔEV$⁠) of 0.43 eV. Figures 3(a) and 3(b) show core level and the loss structure of O 1s on SiO₂/Si and bare Ga₂O₃ samples. From the loss peak, the band gap of Ga₂O₃ and SiO₂ was found to be 4.54 eV and 8.6 eV,^18,27 respectively. The conduction band offset is calculated by

where $ΔEg$ is the band gap difference between Ga₂O₃ and SiO₂ and $ΔEV$ is the calculated valence band offset. Taking the band gap difference to be 4.06 eV, a conduction band offset of 3.63 eV is calculated.

In addition to XPS measurement, electrical characterization of the MOSCAPs was also carried out to calculate the conduction band offset between SiO₂ and Ga₂O₃. C-V characteristic of MOS diode is shown in Figure 4. Both first derivative of C-V and flatband capacitance method indicate a flatband voltage about 9.7 V, suggesting the existence of negative surface charge between SiO₂ and n-Ga₂O₃. An electron density of 9.7 × 10¹⁸cm⁻³ for n-Ga₂O₃ was estimated using differential capacitance-voltage profile technique,²⁸ which is given by

where $n(W)$ is the carrier density, W is the depth from surface of metal and oxide, $Ks$ is relative permittivity of the channel, and A is the area of contact. The current-voltage characteristics of the MOSCAP are shown in Figure 5. The current in the reverse bias is negligibly small (not shown), while in the forward direction current remains low till 50 V then rises rapidly due to Fowler-Nordheim (F-N) tunneling. Destructive breakdown of the MOSCAPs was observed at ∼60 V both in the forward and the reverse bias conditions. We extract the conduction band offset from the F-N tunneling current in forward bias.¹⁸ When sufficient forward bias is applied F-N tunneling takes place²⁹ as indicated in the inset of Figure 5. The F-N tunneling current which depends on ΔE_c is given by:^18,29

where J is the current density, q is the electron charge, h is the Plank's constant, m₀ is the free electron mass, and m_ox is the electron effective mass in oxide. E_ox is the electric field strength in the oxide, which can be calculated easily if the diode is in the strong accumulation region at large forward bias voltages. $ΔEC$ was extracted from the slope (S) of $ln(J/Eox2)$ vs. $(1/Eox)$⁠, as shown in Figure 6. The slope of this curve in the F-N tunneling regime is measured which is given by¹⁸ 

Using the calculated slope (S) and assuming an electron effective mass of SiO₂ is 0.4m₀,^29,30 the $ΔEC$ was calculated to be 3.76 eV, which is close to the result obtained from XPS. Taking bandgap of SiO₂ and Ga₂O₃, band offset obtained from XPS, doping density as $9×1018/cm3$⁠, barrier height between Ti and SiO₂ as 3.34 eV, a calculated band diagram at zero bias is shown in Figure 7.

In summary, we evaluated the band alignment between ALD SiO₂ and n-doped β-Ga₂O₃ (⁠$2¯01$⁠) by XPS and electrical measurements. The conduction band offset is determined to be 3.63 and 3.76 eV by XPS and electrical measurements, respectively. The large conduction band offset is useful for power devices, especially for high temperature operation. However, the dielectric constant of SiO₂ is lower than Al₂O₃, which reduces the equivalent oxide thickness (EOT). A composite gate dielectric stack with thin interfacial SiO₂ layer and thicker Al₂O₃ layer can be used to obtain both large conduction band offset and lower EOT.

This work was partly supported by an ONR Grant (No. N000141310214) monitored by Dr. Paul A. Maki. A portion of this work was performed in the University at Buffalo Electrical Engineering Cleanroom and the Materials Characterization Laboratory, part of the university's Shared Instrumentation Laboratories. The authors would like to thank Dr. Brian Thibeault for the deposition of the SiO₂ films.