Energy-band alignment of (HfO2)x(Al2O3)1-x gate dielectrics deposited by atomic layer deposition on β-Ga2O3 (-201)

Accepted Manuscript Title: Energy-band alignment of (HfO2 )x (Al2 O3 )1−x gate dielectrics deposited by atomic layer deposition on ␤-Ga2 O3 (-201) Authors: Lei Yuan, Hongpeng Zhang, Renxu Jia, Lixin Guo, Yimen Zhang, Yuming Zhang PII: DOI: Reference:

S0169-4332(17)33008-8 https://doi.org/10.1016/j.apsusc.2017.10.075 APSUSC 37418

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APSUSC

Received date: Revised date: Accepted date:

1-9-2017 9-10-2017 10-10-2017

Please cite this article as: Lei Yuan, Hongpeng Zhang, Renxu Jia, Lixin Guo, Yimen Zhang, Yuming Zhang, Energy-band alignment of (HfO2)x(Al2O3)1-x gate dielectrics deposited by atomic layer deposition on ␤-Ga2O3 (-201), Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.075 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Energy-band alignment of (HfO2)x(Al2O3)1-x gate dielectrics deposited by atomic layer deposition on β-Ga2O3 (-201) Lei Yuana, Hongpeng Zhanga, Renxu Jiaa,*, Lixin Guob, Yimen Zhanga, Yuminga Zhang a

School of Microelectronics, Xidian University, Xi'an 710071, People's Republic of China

b

School of Physics and Optoelectronic Engineering, Xidian University, Xi'an 710071, People's Republic of China

Graphical Abstract

Highlights   

The conduction band offset between (HfO2)x(Al2O3)1-x and β-Ga2O3 are determined to be 1.42-1.53 eV. the value of Eg, △Ec, and △Ev for (HfO2)x(Al2O3)1-x/β-Ga2O3 change linearly with x, which can be expressed by 6.98-1.27x, 1.65-0.56x, and 0.48-0.70x, respectively. β-Ga2O3 MOS capacitor with (HfO2)x(Al2O3)1-x dielectric shows lower leakage current and higher critical electric field compared with which with Al2O3 dielectric.

Abstract

Energy band alignments between series band of Al-rich high-k materials (HfO2)x(Al2O3)1-x and β-Ga2O3 are investigated using X-Ray Photoelectron Spectroscopy (XPS). The results exhibit sufficient conduction band offsets (1.42-1.53 eV) in (HfO2)x(Al2O3)1-x/β-Ga2O3. In addition, it is also obtained that the value of Eg, △Ec, and △Ev for (HfO2)x(Al2O3)1-x/β-Ga2O3 change linearly with x, which can be expressed by 6.98-1.27x, 1.65-0.56x, and 0.48-0.70x, respectively. The higher dielectric constant and higher effective breakdown electric field of (HfO2)x(Al2O3)1-x compared with Al2O3, coupled with sufficient barrier height and lower gate leakage makes it a potential dielectric for high voltage β-Ga2O3 power MOSFET, and also provokes interest in further investigation of HfAlO/β-Ga2O3 interface properties. 1. Introduction Recently, monoclinic gallium oxide (β-Ga2O3) has been considered as a promising material for high-power transistors due to its ultra-wide bandgap (Eg) of 4.7-4.9eV compared with SiC and GaN, which enables β-Ga2O3 to have a theoretical breakdown field (Ec) of 8 MV/cm [1-3]. Additionally, large-area crystal of Ga2O3 could be fabricated with simple and low-cost melt-growth methods, offering Ga2O3 an economic advantage over other ultra-wide bandgap semiconductors such as AlN and diamond [4-5]. In case of device fabrication, metal-oxide-semiconductor field-effect transistors based on β-Ga2O3 develop rapidly in the last two years. Green A J et al. reported a record gate to drain electric field strength >3.8 MV/cm based on Sn-doped epitaxial layer, which exceeds the theoretical limits of bulk GaN and SiC [6] ; Wong M H et al. successfully fabricated Ga2O3 MOSFET with field plated to relive the electric field crowding at the gate electrode periphery, while the breakdown voltage was 750V [7]. Ke Zeng et al. recently achieved β-Ga2O3 MOSFETs using spin-on-glass doping technology with an ON/OFF ratio of 108 and Vbr of 382 V [8]. However, most of reported β-Ga2O3 MOSFETs focused on the depletion-mode since the preparation of p-type Ga2O3 is certainly difficult due to the presence of oxygen vacancy. As to the device performance improvement, high-k dielectric, high conduction band offset (△Ec), and high oxide/β-Ga2O3 interface quality are major concerns.

Majority of these reported devices have been achieved with Al2O3 or SiO2 gate dielectric[1,6-9] to reduce leakage current at high-temperature and limit tunneling through gate insulator, benefit from sufficient conduction band offset of Al2O3/Ga2O3 (1.5-1.7 eV)[10-11] and SiO2/Ga2O3 (3.6 eV)[12]. Compared with Al2O3 and SiO2, Hf-based materials, which has been widely studied for CMOS devices[13-17], could also be used as gate dielectric of β-Ga2O3 MOSFETs for achieving lower effective oxide thickness (EOT) and high critical electric field. HfO2 [18-20] was already employed as gate dielectrics in β-Ga2O3 MOSFETs. Particularly, M. J. Tadjer et al achieved a record positive threshold voltage which is attractive [19]. However, the large bandgap of Ga2O3 limits the choice of high-k materials as insulating barrier. Most high-k materials have smaller bandgaps than Al2O3 (Eg between 5.5-6.0 eV for both [21-23], thus, HfO2 has relative small △Ec of HfO2/Ga2O3 (1.3 eV)[24]). Intermixing Al2O3 and HfO2 as HfAlO has shown several potential advantages as an insulator for SiC device[25-26]. Firstly, addition of HfO2 boosts the permittivity of Al2O3 and lead to sufficient △Ec on Si or SiC[20]. Secondly, HfAlO has a higher crystallization temperature than that of HfO 2, which could minimize the leakage path in dielectric [26]. It is suggested that HfAlO could emerge a thermally stable and reliable interface on β-Ga2O3 with an appropriate k and △Ec. In this study, we determine the bandgap of Al-rich (HfO2)x(Al2O3)1-x, the valence band offset and the conduction band offset of (HfO2)x(Al2O3)1-x/β-Ga2O3 interface by x-ray photoelectron spectroscopy (XPS) measurement, which is of vital important to certify HfAlO as promising high-k gate dielectric for Ga2O3 power device. 2. Experimental details The experiments consists of two parts. The first part includes the preparation of the samples 1#-4# for the analyze of band alignment of (HfO2)x(Al2O3)1-x/β-Ga2O3 interface, and the fabrication of two

3

MOS oxide samples for electrical measurement. Second part explains the corresponding measurement used in this study. 2.1 Preparation of the samples 1#-4# Single-crystal β-Ga2O3 (-201) substrates (~ 2.1×1017 cm3) were undergone solvent clean (acetone/isopropanol/deionized water) followed by HF die for 10min. The samples were transferred to a Beneq TFS200 ALD System where Al2O3 and HfAlO dielectrics were deposited at 250 ℃. The precursors used were Hf(NCH3(C2H5))4 (TEMAH) and [(CH3)3Al]2 (TMA)for HfO2 and Al2O3, respectively

. The thicknesses of HfAlO films are around 30 nm obtained by

spectroscopic

ellipsometry. The HfAlO/Ga2O3 samples 1#-4# were prepared for XPS measurements and the chemical compositions of different (HfO2)x(Al2O3)1-x in samples 1#-4# are shown in Table. I. To analyze the (HfO2)x(Al2O3)1-x/Ga2O3 interface with electrical measurements, the sample 1# and 3# were used for the fabrication of MOS capacitor after XPS analysis. Before ALD process, a Ti (40 nm)/Au (200 nm) metal stack deposited on the backside with rapid thermal annealing at 500℃ for 3 min to decrease the ohmic contact resistance. After ALD process the Ni (20 nm)/Au (230 nm)/Ni (20 nm) circular anodes are formed on the insulator film using the lift-off technique. 2.2 XPS measurement and electrical measurements The XPS measurements

were performed in Thermo Fisher ESCALAB 250Xi system with

monochromated Al source (hⱱ=1486.6eV) for the excitation of photoelectrons. The C 1s peak (284.8 eV) from surface contamination was used as standard reference. All valence band spectra were recorded with 0.05 eV step, 30 eV pass energy and a 90 take-off angle (normal to surface). The high-frequency (100 KHz) capacitor-voltage (C-V) measurements and current-voltage (I-V) measurements of the asfabricated MOS capacitor structures were performed using the Agilent B1500A analyzer. 3. Result and discussion

4

3.1 Chemical bonding states analysis The chemical compositions of different (HfO2)x(Al2O3)1-x samples

are determined by the

intensities of XPS lines from full spectra, as shown in Fig. 1. The corresponding elemental compositions of the four samples as well as the value of x are provided in Table I. According to the different x values, the four samples are denoted as 1# Al2O3, 2# (HfO2)0.215(Al2O3)0.785, 3# (HfO2)0.282(Al2O3)0.718, and 4# (HfO2)0.408(Al2O3)0.592. Fig. 1 shows the XPS full spectra for various samples, and the element contents can be observed visually. It is important to list the elemental fractions since the energy bandgap and band offsets of HfAlO are expected to vary with the Al and Hf content [25]. All samples show good stoichiometry and trace carbon element is detected from the surface. Fig. 2 show the XPS spectra of Hf 4f, Al 2p, and O 1s core levels to compare the O and Al chemical bonding states with different compositions. Table II lists the core level (CL) peak positions of Hf 4f, Al 2p, and O 1s. It is observed that all the peak of Hf 4f, Al 2p, and O 1s undergo a shift to lower binding energy with the increase of Hf concentration in (HfO2)x(Al2O3)1-x samples, since Hf is a more ionic cation that Al in (HfO2)x(Al2O3)1-x, and the formation of Hf-O-Al bonding with the increase of Hf concentration. A negative chemical shift of the Al 2p core-level from 75.1 eV for sample 1# to ~74.5 for other samples indicates the formation of Hf-O-Al bonding, which is physically due to the different electronegativeites between Al atom (1.61) and Hf atom (1.3). For O 1s spectra (solid lines in Fig. 2(c)), the Gaussian fitting method is applied as dashed lines. The peak, occured at 531.2 eV, is corresponds to Al-O bonding[25-26]. The peak located at 530.5 eV is attributed to Hf-O bonds, and another peak located at ~532.3 eV is due to the C-O bonding from surface absorbed hydrocarbons[25-26]. From the fitting curve as well as the O 1s spectra, it reveals that Hf-O bonds increase with increasing Hf in HfAlO. 3.2 Energy gap of (HfO2)x(Al2O3)1-x

5

The energy gap of different compositions of (HfO2)x(Al2O3)1-x and Ga2O3 were analyzed by XPS. It is well known that the Eg of the dielectric materials can be obtained by the onsets of single particle excitation or band-to-band transition[10,25-28]. As shown in Fig. 3(a), the Eg of (HfO2)x(Al2O3)1-x and Ga2O3 are extracted from the O 1s energy loss spectra of (HfO2)x(Al2O3)1-x samples and Ga2O3 substrate. The Eg of Ga2O3 is measured as 4.85±0.05 eV, and the Eg of Al2O3 is measured as 6.98±0.05 eV, which is general agree with the previously reported Eg of Ga2O3 [1-2](4.7-4.9 eV) and Al2O3[10-11,2729](7.0 eV). In addition, the Eg of sample 2#-4# are extracted to be 6.71±0.05 eV, 6.62±0.05 eV, and 6.46±0.05 eV, respectively, and a linear change of energy gap value with x in the (HfO2)x(Al2 O3)1-x system is also observed from the above results. Table I. Chemical compositions of (HfO2)x(Al2O3)1-x samples with the extracted HfO2 mole fraction value x. Element compositions 1# 2# 3# 4# Hf% 0 4.16 5.70 8.68 Al% 38.02 30.35 29.05 25.24 O% 61.97 65.50 65.25 66.08 X 0 0.215 0.282 0.408

6

Table II. Core level peak positions of Hf 4f, Al 2p, and O 1s of (HfO2)x(Al2O3)1-x samples. CL peak/eV 1# 2# 3# Hf 4f 5/2 19.21 19.12 Hf 4f 7/2 17.81 17.67 Al 2p 75.12 74.78 74.65 O 1s 531.45 531.16 531.07

4# 19.05 17.49 74.46 530.98

3.3 Band offset of (HfO2)x(Al2O3)1-x/Ga2O3 interface The determination of valence band alignment of HfAlO on β-Ga2O3 substrate was measured by the difference between the valence band maximum (VBM) for (HfO 2)x(Al2O3)1-x samples and Ga2O3 substrate, as shown in Fig. 3(b).The valance band offset (△Ev) can be determined from the following equation:[25,30] Ga2O3 Dielectric Ev  EVBM  EVBM

(1),

O where EDielectric is VBM of gate dielectric, and EGa VBM VBM is VBM of β-Ga2O3. The VBM of samples 1#-4# and 2

3

Ga2O3 are estimated by linear extrapolation of the valence band states [10,12,26] shown in Fig.3(b). The O EGa is found to be 3.49 eV, which is consistence with reported value of 3.5[17], and the VBMs of VBM 2

3

samples 1#-4# are 3.97 eV, 3.82 eV, 3.77 eV, and 3.68eV, respectively. Therefore, the △Ev of samples 1#-4# are calculated to be 0.48 ± 0.05 eV, 0.33 ± 0.05 eV, 0.28 ± 0.05 eV, and 0.19 ± 0.05 eV, respectively. The △Ev of Al2O3/Ga2O3 are determined to be 0.48±0.05 eV, which is close to reported value of 0.7±0.2 eV.[10] With the above extracted Ga2O3 of Eg (4.863±0.05 eV), the conduction band offset (△Ec) can be simply obtained by: [30] Ec  Eg( dielectric)  Eg( Ga2O3 )  Ev

(2),

where Eg(dielectric) is the Eg of the gate dielectric and Eg(Ga2O3)is the Eg of Ga2O3 (4.86 eV). Hence, △Ec for Al2O3 is calculated as 1.65±0.05 eV, which is consistence with the reported value of 1.5-1.7 eV. For other samples (2#-4#), △Ec of (HfO2)x(Al2O3)1-x/Ga2O3 are 1.53±0.05 eV, 1.49±0.05 eV, and 1.42 7

±0.05 eV, respectively, which show a linear relationship between △Ec and x in the (HfO2)x(Al2 O3)1-x system. The Eg, △Ev, △Ec values extracted by XPS measurements are plotted in Fig. 3(c). It is observed that the linear curves by linear least square fit, which could deduce the following equations:

Eg  6.98  1.27 x

(3a),

Ec  1.65  0.56 x

(3b),

Ev  0.48  0.70 x

(3c),

where x means the mole fraction of HfO2 in (HfO2 )x(Al2O3)1-x. As a result, the electrical characteristic of (HfO2)x(Al2O3)1-x gate dielectrics can

be transformed by changing x easily while keeping the

stoichiometry of the materials. The △Ec for HfAlO of 1.42-1.53 eV is larger than the value (1.3 eV) reported for HfO2[24]. 3.4 Electrical characteristics of HfAlO and Al 2O3 MOS capacitors The C-V and I-V characteristics of a Al2O3 and (HfO2)0.282(Al2O3)0.718/Ga2O3 MOS capacitors are provided and analyzed . Fig. 4 shows the capacitance-voltage (C-V) plots of MOS capacitors. The EOT values are extracted as 8.23 nm for (HfO2)0.282(Al2O3)0.718 and 15.58 nm for Al2O3 from the C-V curve at 100KHz. Based on the EOT and the physical thickness, k was determined to be ~ 7.51 for Al2O3 and ~ 14.21 for (HfO2)0.28 (Al2O3)0.72. In addition, as shown in Fig. 4, both capacitors possess the positive flatband voltage (Vfb) shift, which reveals a negative fixed charge density at the high-k/Ga2O3 interface. Despite HfAlO owns a lower conduction band offset, HfAlO MOS sample results in lower gate leakage than Al2 O3 MOS sample (~ 3.57% at an effective electric field of 4.8 MV/cm) and a higher effective breakdown field, as shown in Fig. 4(c).The effective electric field (Eeff) is calculated using Eeff=(VgVfb)/EOT, taking into account the physical thickness, k and Vfb extracted from C-V curve. The leakage reduction is attributed in part to a higher k and to a decreased density of traps in HfAlO compared to 8

Al2O3[21]. Hence, Al-rich HfAlO exhibits superior properties than Al2O3 with a higher k, lower gate leakage and a higher effective electric field. 4. Conclusions In conclusion, the band alignment of (HfO2)x(Al2O3)1-x films on β-Ga2O3 are experimentally determined using XPS, and the energy gap of (HfO2)x(Al2O3)1-x films are investigated. Al 2p, Hf 4f, O 1s core levels spectra, valence band spectra, and O 1s spectra all show peaks changes with the HfO2 mole fraction value x in (HfO2)x(Al2O3)1-x. The Eg, △Ec, △Ev values of (HfO2)x(Al2O3)1-x on β-Ga2O3 are determined and can be expressed by 6.98-1.27x, 1.65-0.56x, 0.48-0.70x, respectively. The sufficient conduction band offset, the higher dielectric constant and effective breakdown electric field of Al-rich HfAlO compared to Al2O3 make it potentially more suitable to exploit the high breakdown field of βGa2O3 for MOS device. This provokes interest in further analyze of HfAlO/β-Ga2O3 gate stack properties. ACKNOWLEDGMENTS The authors acknowledge the supports of the National Natural Science Foundation of China (Grant No.51472196 and No.61704125), the National Key Research and Development Program of China (Grant No.2016YFB0400500), and Shanxi New-star Plan of Science and Technology (Grant No.2016KJXX-63). REFERENCES 1

M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, and S. Yamakoshi, Appl. Phys. Lett., 100 (2012)

013504-1-013504-3. 2

M. Orita, H. Ohta, M. Hirano, and H. Hosono, Appl. Phys. Lett., 77 (2000) 4166-4168.

3

H. H. Tippins, Phys. Rev.140 (1965) A316–A319.

4

H. Aida, K. Nishigchi, H. Takeda, N. Aota, K. Sunakawa, and Y. Yaguchi. J. Appl. Phys., 47 (2008)

8506-8509. 9

5

Z. Galazka, R. Uecker, K. Irmscher, M. Albrecht, D. Klimm, M. Pietsch, M. Brutzam, R. Bretram, S.

Ganschow, and R. Fornari, Crys. Res. Technol., 45 (2010) 1229-1236. 6

A. J. Green, K. D. Chabak, E. R. Heller, R. C. Fitch, M. Baldini, A. Fiedler, K. Irmscher, G. Wagner,

Z. Galazka, S. E. Tetlak, A. Crespo, K. Leedy, and G. H. Jessen, IEEE Electron Device Lett.37 (2016) 902-905. 7

M. H. Wong, K. Sasaki, A. Kuramata, S. Yamakoshi, and M. Higashiwaki, IEEE Electron Device Lett,

37 (2016) 212-215. 8

K Zeng, J. S. Wallace, C. Heimburger, K. Sasaki, A. Kuramata, T. Masui, J. A. GardellaJr, and U.

Singisetti, IEEE Electron Device Lett. 38 (2017) 513-516. 9

H. Zhou, M. Si, S. Alghamdi, G. Qiu, L. Yang, and P. D. Ye, IEEE Electron Device Lett.38 (2017)

103-106. 10

T. Kamimura, K. Sasaki, M.H. Wong, D. Krishnamurthy, A. Kuramata, T.Masui, S. Yamakoshi, and

M. Higashiwaki, Appl. Phys. Lett. 104 (2014) 192104-1-192104-5. 11

T-H. Hung, K. Sasaki, A. Kuramata, D. N. Nath, P. S. Park, C. Polchinski, and S. Rajan, Appl. Phys.

Lett., 104 (2014) 162106-1-162106-3 (2014). 12

Y. Jia, K. Zeng, J. S. Wallace, J. A. Gardella, and U. Singisetti, Appl. Phys. Lett., 106 (2015) 102107-

1-102107-4. 13

G. He, J. Liu, H. Chen, Y. Liu, Z. Sun, X, Chen, M. Liu, and L. Zhang, J. Mater. Chem. C, 2(2014)

5299-5308. 14

G. He, B. Deng, H. Chen, X. Chen, J. Lv , Y. Ma , and Z. Sun, APL Mater., 1.1 (2013) 012104.

15

J. Zhang, G. He, L. Zhou, H. Chen, X. Chen, X. Chen, B. Deng, J. Lv, and Z. Sun, J. Alloy. Compd.,

611(2014) 253-259. 16

G. He, J. Gao, H. Chen, J. Cui, Z. Sun, and X. Chen, ACS Appl. Mater. Inter., 6(2014) 22013-22025.

10

17

G. He, X. Chen, and Z. Sun, Surf. Sci. Rep., 68(2013) 68-107.

18

N. A. Moser, A. Crespo, S. E. Tetlak, A. J. Green, K. D. Chabak, and G. H. Jessen, Device Research

Conference (DRC), 2016 74th Annual. IEEE.(2016) 1-2. 19

M. J. Tadjer, N. A. Mahadik, V. D. Wheeler, E. R. Glaser, L. Ruppalt, A. D. Koehler,K. D. Hobart, C.

R. Eddy, Jr., and F. J. Kub, J. Sol. Stat. Sci. and Technol. 5 (2016) P468-P470. 20

N. A. Moser, J. P. Mccandless, A. Crespo, K. D. Leedy, A. J. Green, E. R. Heller, K. D. Chabak, N.

Peixoto, and G. H. Jessen, Appl. Phys. Lett.110 (2017) 143505-1-143505-5. 21

S. Sayan, N. V. Nguyen, J. Ehrstein, T. Emge, E. Garfunkel, M. Croft, X. Zhao,D. Vanderbilt, I.

Levin, E. P. Gusev, H. Kim, and P. J. McIntyre, Appl. Phys. Lett.86 (2005) 152902-1-152902-3. 22

P. W. Peacock and J. Roberston, Appl. Phys. Lett. 92(2002) 4712-4720.

23

J. Robertson, J. Vac. Sci. and Technol. B, 18 (2000) 1785.

24

V. D. Wheeler, D. I. Shahin, M. J. Tadjer, and C. R. E. Jr, ECS J. Solid. State. Sc., 6 (2017) Q3052-

Q3055. 25

H. Y. Yu, M. F. Li, B. J. Cho, C. C. Yeo, M. S. Joo, D. L. Kwong, J. S. Pan, C. H. Ang, J. Z. Zheng,

and S. Ramanathan , Appl. Phys. Lett. 81 (2002) 376-378. 26

R. Suri, C. J. Kirkpatrick, D. J. Lichtenwalner and V. Misra, Appl. Phys. Lett. 96 (2010) 042903-1-

042903-3. 27

M. L. Huang, Y. C. Chang, C. H. Chang, T. D. Lin, J. Kwo, T. B. Wu, and M. Hong, Appl. Phys. Lett.

89 (2006) 012903-1-012903-3. 28

C. M. Tanner, Y. C. Perng, C. Frewin, S. E. Saddow, and J. P. Chang, Appl. Phys. Lett. 91 (2007)

203510-1-203510-3. 29

N. V. Nguyen, O. A. Kirillov, W. Jiang, W. Wang, J. S. Suehle, P. D. Ye, Y. Xuan, N. Goel, K. W.

Choi, W. Tsai, and S. Sayan, Appl. Phys. Lett.93 (2008) 082105-1-082105-3.

11

30

R. X. Jia, L. P. Dong, Y. X. Niu, C. Z. Li, Q. W. Song, X. Y. Tang, F. Yang, and Y. M. Zhang,

Chinese Phys. B. 24(2015) 038103-1-038103-4.

Figure captions: FIG. 1. The XPS full spectra for various (HfO2)x(Al2O3)1-x samples FIG. 2. XPS spectra for (a) Hf 4f, (b) Al 2p and (c) O 1s CLs taken from sample 1#-4#. The CL peak positions of Hf4f, Al 2p and O 1s shift towards higher binding energy with increasing Al components. For O 1s spectra, solid lines are experimental data and dashed lines stand the fitting curve. FIG. 3. The O 1s energy loss spectra (a) and XPS valence band spectra (b) for (HfO2)x(Al2O3)1-x samples and bulk Ga2O3 substrate. (c) Dependence of Eg, △Ev, and △Ec for (HfO2)x(Al2O3)1-x on HfO2 mole fraction x. The solid lines are linear least square fits of the data points. FIG. 4. C-V plots of MOS capacitors on β-Ga2O3with (a) Al2O3 and (b) (HfO2)0.282(Al2O3)0.718. Fig.3(c) is the gate leakage current density (Jg) vs. the effective dielectric field (Eeff) plot of MOS capacitors. The insets of (a) and (b) are plots of Jg vs. Eeff.

12

FIG. 1

13

FIG. 2 14

FIG. 3 15

0.8

0.3

Eeff (MV/cm)

0.2

0.1

(b) 0.7

Jg (A/cm2)

0.4

1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10 1E-11 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Capacitance (F/cm2)

(a) Jg (A/cm2)

Capacitance (F/cm2)

0.5

0.6 0.5 0.4

1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10 1E-11 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Eeff (MV/cm)

0.3 0.2 0.1

0.0 -5

-4

-3

-2

-1

0

1

2

3

4

5

0.0 -5

-4

-3

Gate Voltage (V)

-1

Al2O3

1E-4

HfAlO

1E-5 1E-6 1E-7 1E-8 1E-9 1E-10 1E-11 1.5

2.0

0

1

2

Gate Voltage (V)

(c)

1E-3

Jg (A/cm2)

-2

2.5

3.0

3.5

4.0

4.5

Eeff (MV/cm)

FIG. 4

16

5.0

5.5

3

4

5