Elsevier

Applied Surface Science

Volume 481, 1 July 2019, Pages 1120-1126
Applied Surface Science

Full length article
On the presence of Ga2O sub-oxide in high-pressure water vapor annealed AlGaN surface by combined XPS and first-principles methods

https://doi.org/10.1016/j.apsusc.2019.03.196Get rights and content

Highlights

  • XPS and DFT confirm Ga2O's presence in high-pressure water vapor annealed AlGaN/GaN.

  • Broadening and shifts of Ga 3d XPS peak suggest sub-oxide formation.

  • Fitting of deconvoluted XPS profile indicates reasonable inclusion of Ga2O peak.

  • Ga 3d core level shift by initial state approximation falls within that of Ga2O values.

  • DFT–calculated oxidation state of +1 for Ga confirms its presence in Ga2O form.

Abstract

We conducted X-ray photoelectron spectroscopy (XPS) and first-principles calculations based on density functional theory (DFT) to confirm the presence of Ga2O sub-oxide in high-pressure water vapor annealed AlGaN surface. We note that the Ga 3d XPS peak broadens and shifts towards higher binding energies, which suggests surface oxide formation. Deconvoluted Ga 3d XPS profiles between HPWVA-treated and reference samples reveal reasonable inclusion of Ga2O peak, suggesting formation of Ga2O sub-oxide. To theoretically confirm the presence of Ga2O, we calculated the Ga 3d core-level shift using initial state approximation. We obtained a 0.74 eV shift, in reasonable agreement with that of Ga2O. Moreover, based on the calculation of net charge on Ga using DFT, we also obtained a +1 oxidation state for Ga, indicating its existence in Ga2O form. By combining theory and experiment, therefore, we have explored the possibility of the formation of Ga2O sub-oxide, which may provide new avenues for obtaining highly stable operation in GaN-based devices.

Introduction

Gallium nitride's high critical electric field as a direct consequence of its wide bandgap, has permitted AlGaN/GaN high-electron-mobility transistors (HEMTs) to operate in unprecedented output power levels while demanding less input power than the ubiquitous Si-based power devices [[1], [2], [3], [4]]. Higher than 99% efficiencies have been repeatedly demonstrated in power converters employing AlGaN/GaN HEMTs [[5], [6], [7]]. While recent advances have propelled AlGaN/GaN HEMTs to commercial product status [8], the widespread implementation of these devices is still hounded by stability issues particularly by the so-called current collapse. Current collapse is the temporary reduction of drain current following the application of electrical stress at both on- and off-state switching operations [[9], [10], [11]]. According to the widely accepted “virtual gate” model [10], current collapse is predominantly due to electron trapping on the AlGaN surface. Based on the general consensus that AlGaN/GaN HEMT's instability is a surface related issue, surface passivation has become the standard method for addressing current collapse because of its efficacy and simplicity [12,13]. However, it is becoming apparent that passivation itself is insufficient to mitigate current collapse especially in cases where the devices are operated with high voltages. Also, field-plate structures have been an accepted solution for reducing current collapse by relaxing the electric field strength that drives electron injection on the surface [14] and by almost instantaneously recovering the trapped electrons by “field-effect” action [15,16]. State-of-the-art structure-based approaches made possible by electron beam lithography such multi-mesa-channel [[17], [18], [19]] and 3-dimensional field-plate structures [20] have also been demonstrated to be highly effective in suppressing current collapse. Meanwhile, pre-passivation gas plasma treatment of AlGaN surface is another promising alternative against current collapse [[21], [22], [23], [24]]. In a recent work, we have demonstrated highly reduced current collapse in AlGaN/GaN HEMTS treated with high-pressure water vapor annealing (HPWVA) [25]. HPWVA has been known as an effective method of improving properties of dielectric materials [[26], [27], [28], [29]]. Because this HPWVA is a plasma-free process, surface damage usually suffered from bombardment of high energy ionized particles in conventional plasma environment is avoided. Moreover, as it is a low-thermal-budget process using temperature of at most 400 °C, HPWVA evades thermal stress- and metal electrode-related degradation, and thus is a promising alternative avenue for realizing stable and reliable AlGaN/GaN HEMTs.

The reduction in current collapse by HPWVA method is attributed to the elimination of deep level traps because of the incorporation of oxygen species into the AlGaN surface. It is known that native oxide Ga2O3 is usually formed in O2-plasma treated [30,31] and air-exposed [32] AlGaN surfaces. Because Ga2O3 is generally stable, its role in surface passivation has been studied in III-V compound semiconductor devices [[33], [34], [35], [36]]. In HPWVA however, hydrogen atom species are present [28]. These hydrogen atoms can react with Ga2O3 via: Ga2O3 + 4H ➔ Ga2O + 2H2O, to form Ga2O [36,37]. Thus, the presence of Ga2O sub-oxide is highly likely. Hinkle et al. [38] have reported the detection of stable Ga2O interfacial layer and its role in passivation in GaAs. Ga2O interfacial passivation layers have been suggested to give rise to low defect density of III-V surfaces [[39], [40], [41], [42]]. In this work, we report the first confirmation of the presence of Ga2O in HPWVA-treated AlGaN/GaN HEMTS using a combination of X-ray photoelectron spectroscopy (XPS) and first-principles methods.

Section snippets

Methodology

In order to examine the effect of HPWVA on the AlGaN surface, an Al0.20Ga0.80N/GaN heterostructure grown on a 4H-SiC substrate by metal organic chemical vapor deposition (MOCVD) is subjected to HPWVA as shown in Fig. 1. Together with the sample, a pre-determined amount of water corresponding to the desired pressure of 0.5 MPa at the annealing temperature of 400 °C, is placed inside the chamber. The chamber is then sealed and heated up to 400 °C for a duration of 30 min, and then allowed to cool

Methodology

To theoretically confirm the presence of Ga2O in HPWVA-treated AlGaN surface as shown by the XPS above, we used first-principles method based on density functional theory [49,50] to determine the core-level shift and oxidation states of relevant Ga and oxygen atoms in a suitable model surface. The calculations are implemented in Vienna Ab-initio Simulation Package (VASP) [51,52], where we used the projector augmented wave (PAW) method [53] to treat the ion-electron interaction and the local

Conclusions

AlGaN/GaN is treated with high-pressure water vapor annealing (HPWVA) and is characterized using X-ray photoelectron spectroscopy (XPS). At photoelectron escape angles of 15°, the Ga 3d XPS peak broadens and shifts towards higher binding energies, suggesting surface oxide formation. Deconvoluted XPS profile of the reference sample shows Ga2O3 and GaN peaks, however, that of the HPWVA-treated sample can only be fitted if Ga2O peak component is also included. Escape angle dependence of the Ga 3d

Acknowledgments

MCS Escaño would like to extend gratitude to Research Center for Development of Far-Infrared Region, University of Fukui for research funds. JT Asubar would like to acknowledge the support from JSPS Grant-in-Aid for Scientific Research C No. 15K06013. The calculations are done using the ACCMS supercomputer of Kyoto University as supported by CII, University of Fukui and the High-Performance Computing Cluster Fukui (HPCCF).

References (60)

  • Y.-F. Wu et al.

    kV-class GaN-on-Si HEMTs enabling 99% efficiency converter at 800 V and 100 kHz

    IEEE Trans. Power Electron.

    (2014)
  • T. Kikkawa et al.

    Commercialization and reliability of 600 V GaN power switches

    Proc. IEEE IRPS

    (2015)
  • M.A. Khan et al.

    Current/voltage characteristic collapse in AlGaN/GaN heterostructure insulated gate field effect transistors at high drain bias

    Electron. Lett.

    (1994)
  • R. Vetury et al.

    The impact of surface states on the DC and RF characteristics of AlGaN/GaN HFETs

    IEEE Trans. Electron Devices

    (2001)
  • K. Nishiguchi et al.

    Evaluation of off-bias-stress induced surface charging at AlGaN/GaN surface using a dual-gate transistor structure

    Jpn. J. Appl. Phys.

    (2014)
  • B.M. Green et al.

    The effect of surface passivation on the microwave characteristics of undoped AlGaN/GaN HEMTs

    IEEE Trans. Electron Devices

    (2000)
  • T. Hashizume et al.

    Surface passivation of GaN and GaN/AlGaN heterostructures by dielectric films and its application to insulated-gate heterostructure transistors

    J. Vac. Sci. Technol. B

    (2003)
  • W. Saito et al.

    Suppression of dynamic on-resistance increase and gate charge measurements in high-voltage GaN-HEMTs with optimized field-plate structure

    IEEE Trans. Electron Devices

    (2007)
  • M.T. Hasan et al.

    Current collapse suppression by gate field-plate in AlGaN/GaN HEMTs

    Electron Device Lett.

    (2013)
  • J.T. Asubar et al.

    Highly reduced current collapse in AlGaN/GaN high-electron-mobility transistors by combined application of oxygen plasma treatment and field plate structures

    Jpn. J. Appl. Phys.

    (2016)
  • K. Ohi et al.

    Current stability in multi-mesa-channel AlGaN/GaN HEMTs

    IEEE Trans. Electron Devices

    (2013)
  • J.T. Asubar et al.

    Improved current stability in multi-mesa-channel AlGaN/GaN transistors

    Phys. Stat. Sol. C

    (2014)
  • J.T. Asubar et al.

    Reduced thermal resistance in AlGaN/GaN multi-mesa-channel high electron mobility transistors

    Appl. Phys. Lett.

    (2014)
  • A. Suzuki et al.

    Improved current collapse in AlGaN/GaN HEMTs with 3-dimensional field plate structure

    IEEE IMFEDK Tech. Dig.

    (2015)
  • D.J. Meyer et al.

    SF6/O2 plasma effects on silicon nitride passivation of AlGaN/GaN high electron mobility transistors

    Appl. Phys. Lett.

    (2006)
  • M. Tajima et al.

    Effects of surface oxidation of AlGaN on DC characteristics of AlGaN/GaN high-electron-mobility transistors

    Jpn. J. Appl. Phys.

    (2009)
  • Y. Hori et al.

    Characterization of interface states in Al2O3/AlGaN/GaN structures for improved performance of high-electron-mobility transistors

    J. Appl. Phys.

    (2013)
  • J.T. Asubar et al.

    Impact of oxygen plasma treatment on the dynamic on-resistance of AlGaN/GaN high-electron-mobility transistors

    Appl. Phys. Express

    (2015)
  • J.T. Asubar et al.

    Current collapse reduction in AlGaN/GaN HEMTs by high-pressure water vapor annealing

    IEEE Trans. Electron Devices

    (2015)
  • T. Sameshima et al.

    Improvement of SiO2 properties by heating treatment in high pressure H2O vapor

    Jpn. J. Appl. Phys.

    (1997)
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