⇦Chinese Physics Letters, 2020, Vol. 37, No. 7, Article code 077302 Band Alignment at the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ Interface with CHF$_{3}$ Treatment Hao Liu (刘浩), Wen-Jun Liu (刘文军)*, Yi-Fan Xiao (肖懿凡), Chao-Chao Liu (刘超超), Xiao-Han Wu (吴小晗), and Shi-Jin Ding (丁士进) Affiliations State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, Shanghai 200433, China Received 15 April 2020; accepted 25 May 2020; published online 21 June 2020 Supported by the Guangdong Province Key Technologies Research and Development Program (Grant No. 2019B010128001), the National Natural Science Foundation of China (Grant No. 61774041), and the Shanghai Science and Technology Innovation Program (Grant No. 19520711500).
*Corresponding author. Email: wjliu@fudan.edu.cn Citation Text: Liu H, Liu W J, Xiao Y F, Liu C C and Wu X H et al. 2020 Chin. Phys. Lett. 37 077302    Abstract The energy band alignment at the atomic layer deposited Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface with CHF$_{3}$ treatment was characterized by x-ray photoelectron spectroscopy and secondary ion mass spectrometry (SIMS). With additional CHF$_{3}$ plasma treatment, the conduction band offset increases from 1.95${\pm}$0.1 eV to 2.32${\pm}$0.1 eV; and the valence band offset decreases from 0.21${\pm}$0.1 eV to $-$0.16${\pm}$0.1 eV. As a result, the energy band alignment changes from type I to type II. This energy band alignment transition could be attributed to the downshift of the core-level of Ga $3d$, resulting from the Ga–F bond formation in the F-rich interfacial layer, which is confirmed by the SIMS results. DOI:10.1088/0256-307X/37/7/077302 PACS:73.20.At, 73.22.-f, 73.40.Qv, 71.15.-m © 2020 Chinese Physics Society Article Text Recently, $\beta$-Ga$_{2}$O$_{3}$ has emerged as a promising candidate for applications in next-generation high-power electronic devices because of its ultrawide bandgap of 4.6–4.9 eV, which is larger than that of SiC (3.2 eV) or GaN (3.4 eV).[1–4] The ultrawide bandgap allows $\beta$-Ga$_{2}$O$_{3}$ to possess Baliga's figure of merit of 3444 and a theoretical breakdown field of 8 MV/cm.[5–9] Generally, the gate dielectric can be used as a potential barrier to both electrons and holes, further the band offset can act as a potential barrier for carrier transportation through the interface. Therefore, the understanding of the band offset is crucial to understand electrostatic properties of heterojunction devices and the transport properties across the interface.[10] Gate dielectrics with a wide bandgap and a high dielectric constant are needed to fabricate $\beta$-Ga$_{2}$O$_{3}$ MOSFETs and high-power $\beta$-Ga$_{2}$O$_{3}$ devices, because the resulting high capacitance can lower the device operating voltage by reducing the influence of the interface state.[11–14] Furthermore, the desired band offset of larger than 1 eV to conduction band is helpful for a high-power $\beta$-Ga$_{2}$O$_{3}$ device to minimize gate leakage current.[2,14,15] As mentioned previously, the gate dielectric materials employed are limited to a few choices, such as Al$_{2}$O$_{3}$,[1] SiO$_{2}$,[13] and HfO$_{2}$.[16] In this work, we investigate the impact of CHF$_{3}$ treatment on the band offsets of atomic layer deposited Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ heterojunctions. The energy band alignment of the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ heterojunction changes from type I to type II after CHF$_{3}$ plasma treatment, and the conduction band offset (CBO) changes from 1.95 eV to 2.32 eV, which mainly results from the existence of an F-rich interfacial layer causing the downshift of the core-levels of Ga $3d$. The Sn-doping $\beta$-Ga$_{2}$O$_{3}$ substrate was ultrasonically cleaned with acetone, isopropanol, deionized water 10-min each, blowing dry by high-purity nitrogen (99.99%). The doping concentration of the substrates is $4.4 \times 10^{18}$ atoms/cm$^{3}$. Prior to Al$_{2}$O$_{3}$ deposition, $\beta$-Ga$_{2}$O$_{3}$ substrates were treated using CHF$_{3}$ plasma at the pressure of 3 Pa for the processing time of 60 s (Samco RIE-10NR). The RF power was 30 W and CHF$_{3}$ flow rate was 30 sccm. After CHF$_{3}$ plasma treatment, $\beta$-Ga$_{2}$O$_{3}$ substrates were transferred to an atomic layer deposition (ALD) chamber (Picosun, R-200 Advanced, Finland). The Al$_{2}$O$_{3}$ film was deposited onto $\beta$-Ga$_{2}$O$_{3}$ using precursors of trimethylaluminum and O$_{2}$ plasma at temperature of 200℃, the growth rate of Al$_{2}$O$_{3}$ is 0.1 nm per cycle. Four samples were prepared for x-ray photoelectron spectroscopy measurements (XPS), i.e., bulk $\beta$-Ga$_{2}$O$_{3}$ substrate, 3 nm-Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface without treatment, 3 nm-Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface with CHF$_{3}$ treatment, and 40 nm-Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ sample (bulk Al$_{2}$O$_{3}$). The XPS measurement was carried out using K-Alpha$^+$ (Thermo Fisher) equipped with a monochromatic Al $K_{\alpha 1}$ radiation source of 1486.7 eV. All high-resolution spectra were collected in a step of 0.1 eV. To eliminate the charging effect, C $1s$ signal located at 284.8 eV was used to correct the XPS spectra. Secondary ion mass spectrometry (SIMS) (PHI Adept 1010, Japan) with Cs primary ion beam at 3 keV was employed with the samples for profiling Ga, Al, and H concentrations at different depths, and positive ions were collected, and charge compensation was carried out. The VBO at the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface can be evaluated by following mathematical expression based on Kraut's work,[17] $$\begin{align} \Delta E_{\rm V}={}&{(E}_{\rm core}^{\rm {Ga}_{2}O_{3}}-E_{\rm VBM}^{\rm {Ga}_{2}O_{3}})-(E_{\rm core}^{\rm {Al}_{2}O_{3}}-E_{\rm VBM}^{\rm {Al}_{2}O_{3}})\\ &-{(E}_{\rm core}^{\rm {Ga}_{2}O_{3}}-E_{\rm core}^{\rm {Al}_{2}O_{3}}),~~ \tag {1} \end{align} $$ where $E_{\rm core}^{\rm {Ga}_{2}O_{3}}-E_{\rm VBM}^{\rm {Ga}_{2}O_{3}}(E_{\rm core}^{\rm {Al}_{2}O_{3}}-E_{\rm VBM}^{\rm {Al}_{2}O_{3}})$ is the energy difference between core-level Ga $3d$ (Al $2p$) and VBM for bulk $\beta$-Ga$_{2}$O$_{3}$ (bulk Al$_{2}$O$_{3}$), and $E_{\rm core}^{\rm {Ga}_{2}O_{3}}-E_{\rm core}^{\rm {Al}_{2}O_{3}}$ is the energy difference between core-level Ga $3d$ and core-level Al $2p$ for Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface. Then, the CBO of the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ heterojunction was calculated by the formula $$ \Delta E_{\rm C}=E_{\rm bg}^{\rm {Ga}_{2}O_{3}}-E_{\rm bg}^{\rm {Al}_{2}O_{3}}-\Delta E_{\rm V},~~ \tag {2} $$ where $E_{\rm bg}^{\rm {Al}_{2}O_{3}}(E_{\rm bg}^{\rm {Ga}_{2}O_{3}})$ are the energy bandgaps of Al$_{2}$O$_{3 }(\beta$-Ga$_{2}$O$_{3}$). The estimated $E_{\rm bg}^{\rm {Al}_{2}O_{3}}$ is 6.81$\pm$0.1 eV, which is calculated around the O $1s$ peak in the XPS spectrum and is consistent with recent works,[10,11] and for $\beta$-Ga$_{2}$O$_{3}$ the ${E}_{\rm bg}^{\rm {Ga}_{2}O_{3}}$ is 4.65$\pm$0.1 eV according to our previous report.[18]
cpl-37-7-077302-fig1.png
Fig. 1. The valence band spectrum and core-level spectrum for (a) bulk Al$_{2}$O$_{3}$ and (b) bulk $\beta$-Ga$_{2}$O$_{3}$. The core-level spectra of Ga $3d$ and Al $2p$ were measured at the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface (c) for the sample without treatment and (d) for the CHF$_{3}$ plasma-treated sample.
cpl-37-7-077302-fig2.png
Fig. 2. Detailed band diagrams at the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interfaces (a) for the sample without treatment and (b) for the CHF$_{3}$ plasma-treated sample.
To obtain the VBO of Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface, the core-level Ga $3d$ and Al $2p$ spectra were measured for bulk $\beta$-Ga$_{2}$O$_{3}$ and bulk Al$_{2}$O$_{3}$, respectively. Figures 1(a) and 1(b) present the VBM and core-level spectra of the bulk Al$_{2}$O$_{3}$ and $\beta$-Ga$_{2}$O$_{3}$, respectively. The intersection between the leading slope and the baseline indicates the VBM of the sample. The binding energy difference (BED) between the VBM and Al $2p$ is 70.92$\pm$0.1 eV for bulk Al$_{2}$O$_{3}$; and for bulk $\beta$-Ga$_{2}$O$_{3}$, the BED between the VBM and Ga $3d$ is 17.02$\pm$0.1 eV. The core-level Al $2p$ and Ga $3d$ spectra of the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface are shown in Figs. 1(c) and 1(d); and the BED is 54.11$\pm$0.1 eV and 53.74$\pm$0.1 eV for the sample without and with the CHF$_{3}$ treatment, respectively. Thus, the $\Delta E_{\rm C}$ of the untreated Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface calculated by Eq. (2) is 1.95$\pm$0.1 eV, which is comparable to that at the $\gamma$-Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface;[19] while the $\Delta E_{\rm C}$ increases by 0.37$\pm$0.1 eV for the sample with CHF$_{3}$ treatment. The detailed band diagrams of the sample without and with CHF$_{3}$ plasma are depicted in Fig. 2. The CHF$_{3}$ plasma treated sample has a higher $\Delta E_{\rm C}$ than the sample without treatment, implying the superiority in suppressing leakage current for $\beta$-Ga$_{2}$O$_{3}$ MOSFETs because of the increased electron barrier height. In addition, the band alignment transits from type I to type II.
cpl-37-7-077302-fig3.png
Fig. 3. Al $2p$ and Ga $3d$ XPS spectra (a) for the sample without treatment and (b) for the CHF$_{3}$ plasma-treated sample.
cpl-37-7-077302-fig4.png
Fig. 4. SIMS measurement (F, Al and Ga) in the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ heterojunction with CHF$_{3}$ plasma treatment.
Figure 3 shows the XPS spectra of Al $2p$ and Ga $3d$ for the sample without treatment and for the CHF$_{3}$ plasma-treated sample. The peak of Al $2p$ shifts slightly from 74.36$\pm$0.1 eV to 74.46$\pm$0.1 eV, and the peak of Ga $3d$ increases from 20.25$\pm$0.1 eV to 20.72$\pm$0.1 eV. As indicated in Fig. 4, seen from the SIMS results, there is an F peak of $1.2\times 10^{22}$ cm$^{-3}$ located at the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface. Additionally, some of the F ions diffuse into Al$_{2}$O$_{3}$ and $\beta$-Ga$_{2}$O$_{3}$ because of small size of F ions, and/or in situ annealing effect. On the other hand, the tails of Ga in Al$_{2}$O$_{3}$ and Al in $\beta$-Ga$_{2}$O$_{3}$ could be due to the diffusion caused by primary beam bombardment in the process of SIMS measurement, Ga and Al have larger atomic radius than F so that their diffusion are less obvious.[20] During CHF$_{3}$ plasma treatment, F was diffused into the $\beta$-Ga$_{2}$O$_{3}$ surface, because of its higher electronegativity (4.0), F has a propensity to attract electrons and may further form dipoles with other element atoms (e.g., Al and Ga). Therefore, the Ga–O bond could be replaced by Ga–F, and thus the binding energy of the Ga $3d$ peak shifts upward. In Fig. 3, the increment is 0.47$\pm$0.1 eV for the Ga $3d$ peak, and 0.1$\pm$0.1 eV for the Al $2p$ peak, leading to a 0.37$\pm$0.1 eV CBO enlargement. The peak of Ga $3d$ may shift to a higher binding energy, in agreement with the reports by Seaward et al.[21] and Vakulka et al.,[22] suggesting that the band offsets of Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ heterojunction could be effectively modified by Ga–F formation in F-rich interfacial layer. The change in band offsets measured by XPS resulted from the interface charge and interface dipole effect, which however cannot be differentiated by the current characterization. More investigation is needed, but it is beyond the scope of this work. In conclusion, using XPS, we have investigated the band offsets of the atomic layer deposited Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ heterojunction with CHF$_{3}$ plasma treatment. It is found that the core-level Ga $3d$ shifts remarkably downward by 0.47$\pm$0.1 eV and the core-level of Al $2p$ increases only 0.1$\pm$0.1 eV, which causes a CBO change of 0.37$\pm$0.1 eV at the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface. The band alignment changes could be due to the higher binding energy of Ga–F bond formation in the F-rich interfacial layer. These observations may help in the design and optimization of $\beta$-Ga$_{2}$O$_{3}$-based devices via surface plasma treatment in the future. References Al2O3/ $\beta $ -Ga2O3(-201) Interface Improvement Through Piranha Pretreatment and Postdeposition AnnealingDepletion-mode Ga 2 O 3 metal-oxide-semiconductor field-effect transistors on β-Ga 2 O 3 (010) substrates and temperature dependence of their device characteristicsLow resistance ohmic contacts on wide band‐gap GaN3.8-MV/cm Breakdown Strength of MOVPE-Grown Sn-Doped $\beta $ -Ga 2 O 3 MOSFETsHigh-voltage field effect transistors with wide-bandgap β -Ga 2 O 3 nanomembranesField-Plated Ga 2 O 3 MOSFETs With a Breakdown Voltage of Over 750 VElectron mobility in single‐ and polycrystalline Ga 2 O 3Some electrical properties of the semiconductor βGa2O3Band alignment of Al2O3 with (−201) β-Ga2O3Band alignment and electrical properties of Al 2 O 3 / β -Ga 2 O 3 heterojunctionsDevelopment of gallium oxide power devicesLarge conduction band offset at SiO 2 /β-Ga 2 O 3 heterojunction determined by X-ray photoelectron spectroscopyRecent progress in Ga 2 O 3 power devicesEditors' Choice Communication—A (001) β-Ga 2 O 3 MOSFET with +2.9 V Threshold Voltage and HfO 2 Gate DielectricLeakage current conduction mechanisms and electrical properties of atomic-layer-deposited HfO 2 /Ga 2 O 3 MOS capacitorsPrecise Determination of the Valence-Band Edge in X-Ray Photoemission Spectra: Application to Measurement of Semiconductor Interface PotentialsBand alignment of In 2 O 3 /β-Ga 2 O 3 interface determined by X-ray photoelectron spectroscopyEpitaxial growth and electric properties of γ-Al 2 O 3 (110) films on β-Ga 2 O 3 (010) substratesBand alignment of HfO 2 /multilayer MoS 2 interface determined by x -ray photoelectron spectroscopy: Effect of CHF 3 treatmentSurface contamination and damage from CF4 and SF6 reactive ion etching of silicon oxide on gallium arsenide
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