Full length articleOn the presence of Ga2O sub-oxide in high-pressure water vapor annealed AlGaN surface by combined XPS and first-principles methods
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).
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2022, Applied Surface ScienceCitation Excerpt :It is observed that the FWHM of In2O3 is a bit large (2.6 eV), but it is not much different from the FWHM of In2O3 reported in the previous literature [49], within a reasonable range. Although related studies proposed the existence of gallium low-valence oxide (Ga2O) [50–53], there are no direct characterizations to prove its existence. Moreover, Ga2O is far less stable than Ga2O3, and therefore there is no Ga2O peak in the fitting of Fig. 5a and d.
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1st and 2nd authors contributed equally.