Ga2O3 heterostructures

Vacuum 141 (2017) 103e108

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Valence and conduction band offsets in AZO/Ga2O3 heterostructures Patrick H. Carey IV a, F. Ren a, David C. Hays b, B.P. Gila b, S.J. Pearton b, *, Soohwan Jang c, Akito Kuramata d, e a

Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA c Department of Chemical Engineering, Dankook University, Yongin 16890, South Korea d Tamura Corporation, Sayama, Saitama 350-1328, Japan e Novel Crystal Technology, Inc., Sayama, Saitama 350-1328, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 March 2017 Received in revised form 21 March 2017 Accepted 24 March 2017 Available online 27 March 2017

We report on the determination of band offsets in rf-sputtered Aluminum Zinc Oxide (AZO)/single crystal b-Ga2O3 (AZO/Ga2O3) heterostructures using X-Ray Photoelectron Spectroscopy. The bandgaps of the materials were determined by Reflection Electron Energy Loss Spectroscopy as 4.6 eV for Ga2O3 and 3.2eV for AZO. The valence band offset was determined to be
0.61eV ± 0.23eV, while the conduction band offset was determined to be
0.79 ± 0.34 eV. The AZO/Ga2O3 system has a nested, or straddling, gap (type I) alignment and provides a convenient method for reducing contact resistance on Ga2O3based device structures. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction There is significant recent interest in the development of the bpolytype of Ga2O3 [1e12]. This has a wide bandgap of ~4.6 eV, can be grown in bulk form from melt sources and has a theoretical breakdown field of ~8 MV cm
1, which is higher than either GaN or SiC [1e5]. Ga2O3 is promising for solar blind UV detectors [13], as well as extreme environment electronics (high temperature, high radiation, and high voltage (low power) switching [1e12,14e16]. High quality bulk Ga2O3 crystals can be grown by edge-defined film-fed (EFG) growth using Ir crucibles, by Czochralski or by float zone with excellent quality and control of conductivity. Excellent results for Ga2O3-based power rectifiers, metalsemiconductor field effect transistors (MESFETs) and metal-oxide field effect transistors (MOSFETs) have been reported [1,3e7,9e12], along with various types of solar blind photodetectors [13]. One common limitation noted in these results is the need for improved Ohmic contacting processes [1,3,4]. High quality Ohmic contacts are a prerequisite for any device and should provide low contact resistance at moderate anneal temperatures. Additional contact resistance leads to slower device switching speeds as well as reliability issues due to local contact heating during current

* Corresponding author. E-mail address: [email protected]fl.edu (S.J. Pearton). http://dx.doi.org/10.1016/j.vacuum.2017.03.031 0042-207X/© 2017 Elsevier Ltd. All rights reserved.

flow during device operation. The usual approaches to lowering contact resistance involve surface etching or cleaning to reduce barrier height or increase of carrier concentration of the surface through preferential loss of oxygen [3,6,7,14e16]. To date, contact schemes involving deposition of Indium Tin Oxide (ITO) or dry etching in BCl3/Ar to enhance the surface n-type conductivity, followed by Ti/Au annealed at 500  C have been common [1,3,7,9]. Specific contact resistances of ~4.6e8  10
6 U cm
2 were reported for Ti/Au contacts on n-Ga2O3 epitaxial layers in which Si was implanted and annealed at 925  C, followed by dry etching, metal deposition and annealing at 470  C [14,15]. Many published current-voltage (I-V) characteristics for contacts on Ga2O3 are only quasi-linear at low current and show the need for improved contact approaches. Cr has a low work function of 4.5 eV and is currently one of the best choices for Ohmic contacts on n-Ga2O3. For an n-type semiconductor, to achieve an Ohmic contact means that the work function of the metal must be close to or smaller than the electron affinity of the semiconductor (affinity of b-Ga2O3 is ~4.00 ± 0.05 eV) [16,17] and this limits the available options. Another approach is to deposit a transparent conducting oxide such as Aluminum Zinc Oxide (AZO). AZO thin films have been widely studied for transparent and flexible device applications such as liquid crystal displays, plasma display panels, electronic paper displays, organic light emitting diode, solar cells, touch panels, gas sensors and other optoelectronics devices [18e24]. The most commonly used transparent

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conducting oxide (TCO) material is ITO, because of its high conductivity and optical transparency over visible wavelengths. However, the relative scarcity and high cost of indium has focused attention on alternatives such as AZO [20]. To be used as a contact layer on Ga2O3, we need to establish the band alignment to ensure that there is no barrier to electron transport from the contact layer into the Ga2O3. In this paper, we report on the determination of the band alignment in the AZO/Ga2O3 heterostructure. We employ X-Ray Photoelectron Spectroscopy (XPS) to determine the valence band offsets and by measuring the respective bandgaps of the AZO (3.2 eV) and Ga2O3 (4.6 eV), we were also able to determine the conduction band offset in AZO/Ga2O3 heterostructures.

the problem worse, our experience with oxides on conducting substrates has been that the differential charging is minimized with the use of an electron gun. Calibrations with and without the gun

2. Experimental The AZO was deposited by RF magnetron sputtering on Ga2O3 and quartz substrates at room temperature using a 3-in. diameter target of pure AZO. The RF power was 125 W and the working pressure was 5 mTorr in a pure Ar ambient. The dc bias on the electrode under these conditions is in the range 30e40 V. The bulk b-phase Ga2O3 single crystals with (
201) surface orientation (Tamura Corporation, Japan) were grown by the edge-defined filmfed growth method. Hall effect measurements showed the sample was unintentionally n-type with an electron concentration of ~3  1017 cm
3. To obtain the valence band offsets, X-Ray Photoelectron Spectroscopy (XPS) survey scans were performed to determine the chemical state of the AZO and Ga2O3 and identify peaks for high resolution analysis (31). Note that the Ga2O3 samples were cleaned using solvent rinses and loaded immediately were not exposed to air prior to the subsequent Reflection Electron Energy Loss Spectroscopy (REELS) measurements to avoid complications from surface contamination [26e28]. The latter may lead to less accurate band gap measurements when using REELS [29
33]. The samples for XPS were briefly exposed to ambient during transfer and loading into the XPS analysis chamber. A Physical Electronics PHI 5100 XPS with an aluminum x-ray source (energy 1486.6 eV) with source power 300 W was used, with an analysis area of 2 mm  0.8 mm, a take-off angle of 50 and an acceptance angle of ±7. The electron pass energy was 23.5eV for the high resolution scans and 187.5eV for the survey scans. The approximate escape depth (3l sin q) of the electrons was <80 Å [34]. All of the peaks are well-defined in this system and we didn't need to curve fit the data. Charge compensation was performed using an electron flood gun, due to the dielectric nature of the films. The charge compensation flood gun is often not sufficient at eliminating all surface charge, and additional corrections must be performed. Using the known position of the adventitious carbon (C-C) line in the C 1s spectra at 284.8 eV, charge correction was performed. An optimized peak fit to the carbon 1s spectrum on IGZO using only a simple peak model involving a single C-C peak at 284.8 can lead to errors of 0.5eV or more. Therefore, additional peaks are generally added. In our work, we add one peak constrained to be 1.5eV above the main peak and of equal FWHM. This higher binding energy peak is ascribed to alcohol (C-OH) and/or ester (C-O-C) functionality. A further high binding energy peak, attributed to O-C]O, is added with a position constraint of 3.7eV above the main peak. All peaks are constrained to a peak area ratio of 2:1:1. During the measurements, all the samples and electron analyzers were electrically grounded so they were performed providing a common reference Fermi level. Differential charging is a serious concern for photoemission dielectric/semiconductor band offset measurements [32]. While the use of an electron flood gun does not guarantee that differential charging is not present and in some cases could make

Fig. 1. XPS survey scans of AZO, 1.5 nm AZO on Ga2O3 and Ga2O3 bulk sample.

Fig. 2. XPS spectra of core levels to valence band maximum (VBM) for bulk Ga2O3 (a) and thick film AZO (b).

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105

and verified that was the case. This procedure has been described in detail previously [29,30]. The valence band maximum (VBM) was determined by using a linear extrapolation method, ie. it is determined by linear fitting the leading edge of the valence band and linearly fitting the flat energy distribution and finding the intersection of these two lines. The core-level peaks were referenced to the top of valence band for the thick IGZO and the thick film of dielectrics. To determine the valence band offset, the binding energy differences between the valence band and the selected core peaks for the single thick layers were combined with the core-level binding energy differences for the heterojunction sample. Spectra from insulating samples can be charge corrected by shifting all peaks to the adventitious C 1s spectral component (C-C, C-H) binding energy set to 284.8 eV. The C1s spectrum typically has C-C, C-O-C and O-C]O components and optimization involves constraining these additional peaks. This charge correction is used for chemical analysis, but not band offset measurements. The method of Kraut et al.(31) using x-ray photoemission spectroscopy has been established as a reliable way to determine band offsets at the hetero-junction interface. This method has also been successfully used to provide insights into interfacial properties between different materials (31). It is based on using an appropriate shallow core-level position as a reference. Generally, this approach is based on the assumption that the energy

difference between the core-level positions and valence-band maximum (VBM) are both fixed in the bulk. Reflection electron energy loss spectroscopy (REELS) was employed to measure the bandgaps of the AZO and Ga2O3. REELS is a surface sensitive technique capable of analyzing electronic and optical properties of ultrathin gate oxide materials because the low-energy-loss region reflects the valence and conduction band structures (33). REELS spectra were obtained using a 1 kV electron beam and the hemispherical electron analyzer.

Fig. 3. Reflection electron energy loss spectra to determine the bandgap of bulk Ga2O3 (a) and for thick AZO (b).

Fig. 4. High resolution XPS spectra for the vacuum-core delta regions of the bulk Ga2O3 (a) and AZO (b).

3. Results and discussion Fig. 1 shows the XPS survey scans of thick (200 nm) AZO, 1.5 nm AZO on Ga2O3 and Ga2O3 bulk crystals. The spectra show the films themselves free from contaminants (although there is adventititious carbon on the surface) and consistent with past published XPS data on these materials [17]. The valence band maximum (VBM) was determined by linearly fitting the leading edge of the valence band and the flat energy distribution from the XPS measurements, and finding the intersection of these two lines, as shown Fig. 2 for the bulk Ga2O3 (a) and thick AZO (b). The VBM was measured to be 3.2 ± 0.2 eV for Ga2O3,

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which is consistent with previous reports [17] and 2.53 ± 0.2 eV for the AZO. The bandgap of the Ga2O3 was determined to be 4.6 eV, as shown in the REELS spectra in Fig. 3 (a). The band gap was determined from the onset of the energy loss spectrum [33]. The measured band gap for the sputtered AZO was 3.2 ± 0.3 eV

(Fig. 3(b)), which is consistent with literature values. The difference in bandgaps between AZO and Ga2O3 is therefore 1.4 eV and thus has the potential to provide a significant conduction band offset to enhance electron transport for improved Ohmic contacts. To determine the actual band alignment and the respective valence and conduction band offsets, we examined the core level spectra for the samples. High resolution XPS spectra of the VBM-core delta region are shown in Fig. 4 for the Ga2O3 (a) and thick AZO (b) samples. These were used to determine the selected core level peak positions of the AZO

Ga2 O3 AZO Þ Ga2O3 and the AZO ðECore
ECore Ga

2 O3

. Fig. 5 shows the XPS

spectra for the respective Ga2O3 to AZO-core delta regions ððECore
EVBM ÞRef :Ga2 O3
ðECore
EVBM ÞRef :AZO Þ of the heterostructure samples. These values are summarized in Table 1 for the three samples examined and these were then inserted into the following equation to calculate the valence band offset DEv:

DEV ¼ ðECore
EVBM ÞRef :Ga2 O3
ðECore
EVBM ÞRef :AZO AZO  Ga2 O3 AZO
ECore
ECore

Ga2 O3

Fig. 6 shows both a simplified and detailed band diagram of the AZO/Ga2O3 heterostructure. Our data shows this is a nested, type I alignment, with a valence band offset of 0.61 ± 0.23 eV and the conduction band offset DEC is then 0.79 ± 0.34 eV using the

Fig. 5. High resolution XPS spectra for the Ga2O3 to AZO-core delta regions.

Table 1 Values of band offsets determined in these experiments (eV). Reference Ga2O3

Reference AZO

Thin AZO on Ga2O3

Ga2O3 metal core

Ga2O3 VBM

Metal Core level

Metal core - Ga2O3 VBM

AZO VBM

Zn 2p3/2 core level

Zn 2p3/2 - VBM

DCL Ga 2p3/2 - Zn 2p3/2

Valence band offset

Ga2p3/2

3.20

1118.10

1114.90

2.53

1021.92

1019.39

96.12


0.61

Fig. 6. Summary (a) and detailed (b) band diagrams for AZO/Ga2O3 heterostructure.

P.H. Carey IV et al. / Vacuum 141 (2017) 103e108

following equation:

DEC ¼ EgGa2 O3
EgAZO
DEV

[5]

ie. DEC ¼ 4:6eV
3:2eV
0:61eV ¼ 0:79eV. In these equations EgGa2 O3 is the bandgap of the Ga2O3 and EgAZO is the bandgap of the AZO. The fact that the AZO has a smaller bandgap and the correct alignment for electron transport from the contact into the Ga2O3 means that it is therefore a good choice as an enhancement layer for reducing Ohmic contact resistance on n-type Ga2O3. The lack of high quality Ohmic contacts is still a liming issue on current Ga2O3 devices and thus any method that enhances electron transport across the contact/Ga2O3 heterointerface is valuable. This is a technique that is used in compound semiconductors, where smaller gap materials like InAs or InGaAs are used on GaAs to lower the Omic contact resistance. We should also point out the possibility of the band offsets shifting when annealing such contacts, as it is usually found that post-deposition annealing further reduces contact resistance. Since AZO also has good thermodynamic stability on Ga2O3, in its undoped form it could also be a good choice as a surface passivation layer to prevent surface conductivity changes that are common in electronic oxides upon exposure to hydrogencontaining ambients [25,26].

[13]

4. Summary and conclusions

[14]

The AZO/Ga2O3 heterojunction has a nested gap alignment of band offsets with a valence band offset of
0.61eV ± 0.06eV and a conduction band offset of
0.79 eV ± 0.80 eV determined from XPS measurements. This is useful information for Ohmic contact schemes on n-type Ga2O3 devices.

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The project or effort depicted was also sponsored by the Department of the Defense, Defense Threat Reduction Agency, HDTRA1-17-1-011, monitored by Jacob Calkins. The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred. The research at Dankook was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01058663), and Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2015M3A7B7045185). Part of the work at Tamura was supported by “The research and development project for innovation technique of energy conservation” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. We also thank Dr. Kohei Sasaki from Tamura Corporation for fruitful discussions. References [1] Masataka Higashiwaki, Kohei Sasaki, Hisashi Murakami, Yoshinao Kumagai, Akinori Koukitu, Akito Kuramata, Takekazu Masui, Shigenobu Yamakosh, Recent progress in Ga2O3 power devices, Semicond. Sci. Technol. 31 (2016) 034001. [2] M. Mohamed, C. Janowitz, R. Manzke, Z. Galazka, R. Uecker, R. Fornari, J.R. Weber, J.B. Varley, C.G. Van de Walle, The electronic structure of Ga2O3, Appl. Phys. Lett. 97 (2010) 211903. [3] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Development of gallium oxide power devices, Phys. Stat. Solidi A 211 (2014) 21. [4] Kelson D. Chabak, Neil Moser, Andrew J. Green, Dennis E. Walker Jr., Stephen E. Tetlak, Eric Heller, Antonio Crespo, Robert Fitch, Jonathan P. McCandless, Kevin Leedy, Michele Baldini, Gunter Wagner, Zbigniew Galazka, Xiuling Li, Gregg Jessen, Enhancement-mode Ga2O3 wrap-gate fin field-effect transistors

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