Abstract
1. Introduction
Beta gallium oxide (β-Ga2O3), with a large band gap of about 4.8 eV and a high theoretical breakdown electric field of 8 MV/cm[
The metal oxide semiconductor field-effect transistors (MOSFET) and schottky barrier diodes are two critical power electronics application of β-Ga2O3. Low resistance ohmic contacts are essential for β-Ga2O3 devices to reduce devices’ conduction loss. Several groups[
Up till now, four leading model of current flow in the metal–semiconductor ohmic contact are developed, namely, the thermionic emission theory (TE), field emission (FE), thermal-field emission theory (TFE), and metallic shunts model[
In this letter, Mg/Au is used to prepare ohmic contact with lightly doped β-Ga2O3 because of its low work function, high humidity resistance, oxygen resistance, and corrosion resistance[
2. Experimental
The Sn lightly doped (
The current–voltage characteristics were measured using Keithley 2611A semiconductor characterization system in the range of 300–375 K. The ρc was extracted using the transmission line method (TLM). Hall Effect 5500 PC measurement system was employed to determine the carrier concentration of bulk β-Ga2O3.
3. Results and discussion
Fig. 1 shows the current–voltage characteristics of as-deposited and annealed Mg/Au/β-Ga2O3 contacts samples, as measured from adjacent contact pads with a gap spacing of 0.15 mm. For the as-prepared sample, the I–V curves showed nonlinear behavior, while sample annealed at 400 °C showed the linear behavior of current–voltage curve, which indicates that thermal annealing can lead to the formation of ohmic contacts.
Figure 1.(Color online)
Suzuki et al.[
Fig. 2(a) shows the dependence of the experimentally measured total resistance (RT) on distance (d) between two adjacent contact pads for the annealed sample at different measuring temperatures. The RT can be represented by equation:
Here, RC and RSH are contact resistance and sheet resistance, respectively. The d, W, q, N, μn and S are the distance of adjacent electrodes, width of electrodes, element charge, carrier concentration, carrier mobility and electrode area, respectively. The intercept of line with the ordinate axis corresponds to double RC and its slope is proportional to the RSH. The dependence of RT on d is linear at each measuring temperature, as shown in Fig. 2(a). Fig. 2(b) shows that the RC decreases from 0.96 to 0.8 Ω and RSH increases from 0.3 to 0.56 Ω/□ as measuring temperature rise from 300 to 375 K. As we know, the carrier mobility is affected by temperature and it decrease with increasing measuring temperature at high temperature. Thus, the reason of the increase of RSH with increasing the testing temperature could be attributed to the decrease of electron mobility.
Figure 2.(Color online) (a) Resistance of Mg/Au-
In order to investigate the current transport mechanism of Mg/Au ohmic contact for annealed sample, the ρc was extracted as a function of measuring temperature. The ρc decreased slightly from 4.3 × 10−4 to 1.59 × 10−4 Ω·cm2 in the measuring temperature range from 300 to 375 K, as shown in Fig. 3. According the model of current flow in the metal semiconductor ohmic contact, TFE and FE model may be the basic theories of current transport at the Mg/Au-β-Ga2O3 ohmic contact interface. However, judging from E00, it is calculated to be 0.007 V and qE00/kT (T = 300 K) is 0.27 < 0.5. Where, the m* is 0.28me[
Figure 3.(Color online) Specific contact resistance of the ohmic contact Mg/Au-
For FE theory,
Figure 4.(Color online) The dependence of ln (
4. Conclusion
In summary, ohmic contact has been successfully realized on β-Ga2O3 using low work function Mg/Au stacks after annealing at 400 °C and the current transport mechanism was investigated. The temperature dependence of specific contact resistance of the Mg/Au-β-Ga2O3 ohmic contact decreased with increasing the measuring temperature from 300 to 375 K and qE00/kT is 0.27 < 0.5. Therefore, the current transport mechanism is dominant thermionic emission theory. The effective barrier height extracted is 0.1 eV by fitting experiment data with thermionic emission model. Further investigation will be carried out on the heavily doped β-Ga2O3.
Acknowledgements
This work was supported by the National Key R&D Plan (Nos. 2016YFB0400600, 2016YFB0400601),the National Science Foundation of China (Nos. 11675198, 61376046, 11405017, 61574026), the Fundamental Research Funds for the Central Universities (Nos. DUT15LK15, DUT15RC(3)016, No. DUT16LK29), the Liaoning Provincial Natural Science Foundation of China (Nos. 2014020004, 201602453, 201602176), the China Postdoctoral Science Foundation Funded Project (No. 2016M591434), the Open Fund of the State Key Laboratory on Integrated Optoelectronics (Nos. IOSKL2015KF18, IOSKL2015KF22).
References
[1]
[2] M Higashiwaki, K Sasaki, A Kuramata et al. Gallium oxide (Ga2O3) metal–semiconductor field-effect transistors on single-crystal
[3] M Higashiwaki, K Sasaki, H Murakami et al. Recent progress in Ga2O3 power devices. Semicond Sci Technol, 31, 034001(2016).
[4] K Irmscher, Z Galazka, M Pietsch et al. Electrical properties of
[5] Z Galazka, R Uecker, K Irmscher et al. Czochralski growth and characterization of beta-Ga2O3 single crystals. Cryst Res Technol, 45, 1229(2010).
[6] E G Víllora, K Shimamura, Y Yoshikawa et al. Large-size
[7] N Ueda, H Hosono, R Waseda et al. Synthesis and control of conductivity of ultraviolet transmitting
[8] A Kuramata, K Koshi, S Watanabe et al. High-quality beta-Ga2O3 single crystals grown by edge-defined film-fed growth. Jpn J Appl Phys, 55, 1202A(2016).
[9] H. Aida, K. Nishiguchi, H. Takeda et al. Growth of beta-Ga2O3 single crystals by the edge-defined, film fed growth method. Jpn J Appl Phys, 47, 8506(2008).
[10] W Mu, Z Jia, Y Yin et al. High quality crystal growth and anisotropic physical characterization of beta-Ga2O3 single crystals grown by EFG method. J Alloys Compnds, 714, 453(2017).
[11] K Sasaki, A Kuramata, T Masui et al. Device-quality beta-Ga2O3 epitaxial films fabricated by ozone molecular beam epitaxy. Appl Phys Express, 5, 035502(2012).
[12] E G Víllora, K Shimamura, Y Yoshikawa et al. Electrical conductivity and carrier concentration control in
[13] K Sasaki, M Higashiwaki, A Kuramata et al. Si-ion implantation doping in beta-Ga2O3 and its application to fabrication of low-resistance ohmic contacts. Appl Phys Express, 6, 086502(2013).
[14] P H Carey, J Yang, F Ren et al. Ohmic contacts on n-type beta- Ga2O3 using AZO/Ti/Au. AIP Adv, 7, 095313(2017).
[15] P H Carey, Y Jiancheng, R Fan et al. Improvement of ohmic contacts on Ga2O3 through use of ITO-interlayers. J Vac Sci Technol B, 35, 061201(2017).
[16] T V Blank, Y A Goldberg, E A Posse. Flow of the current along metallic shunts in ohmic contacts to wide-gap III–V semiconductors. Semiconductors, 43, 1164(2009).
[17] T Oyamada, H Sasabe, C Adachi. Formation of MgAu alloy cathode by photolithography and its application to organic light-emitting diodes and organic field effect transistors. Electr Eng Jpn, 152, 37-42(2005).
[18] H Arai, H Nakanotani, K Morimoto et al. Magnesium-gold binary alloy for organic light-emitting diodes with high corrosion resistance. J Vac Sci Technol B, 34, 040607(2016).
[19] R Suzuki, S Nakagomi, Y Kokubun et al. Enhancement of responsivity in solar-blind
[20] S Knight, A Mock, R Korlacki et al. Electron effective mass in Sn-doped monoclinic single crystal beta-gallium oxide determined by mid-infrared optical Hall effect. Appl Phys Lett, 112, 012103(2018).
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