Valence band offset of β-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy
© Wei et al.; licensee Springer. 2012
Received: 28 August 2012
Accepted: 24 September 2012
Published: 10 October 2012
A sample of the β-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the β-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the β-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the β-Ga2O3/GaN structure is 1.40 ± 0.08 eV.
GaN has been used in many applications including field-effect transistors and high-electron mobility transistors [1, 2]. However, the power-handling capability of these devices is limited by the leakage current through the Schottky gate. To solve this problem, GaN-based metal-oxide-semiconductor (MOS) or metal-insulator-semiconductor structures are being widely investigated. Ga2O3 can be used as the gate dielectric medium for GaN-based MOS devices to suppress the gate leakage current . Because gallium oxide can crystallize in monoclinic crystalline form (β-Ga2O3) in the process of fabricating a β-Ga2O3/GaN-based MOS structure, the β-Ga2O3 layer can be formed on GaN epitaxially. In this case, the β-Ga2O3 layer can be formed as an oxide (insulator) layer with a certain crystalline structure within the MOS structure. Apart from the crystalline quality of the Ga2O3 layer, the band parameters, such as band offsets, also play an important role in the current transport mechanism. These parameters determine the barrier for hole or electron transport across the interface. There is a similar influence to that on the current transport mechanism in a β-Ga2O3/GaN dual-color photodetector .
The growth and structural characterization of Ga2O3/GaN heterostructures by dry thermal oxidation on GaN have been reported extensively [5–7]. However, to date, the band alignment of the Ga2O3/GaN heterostructure has not yet been determined experimentally. In this paper, the Ga2O3/GaN heterostructures were fabricated by a thermal process method. Because the Ga2O3 can be grown epitaxially on GaN, the Ga2O3/GaN heterostructure with the epitaxial relationship between the Ga2O3 and GaN layers allows us to evaluate the band offset of the heterostructure. X-ray photoelectron spectroscopy (XPS) is a powerful tool for measurement of the valence band offsets (VBOs) of heterostructures. Experimental measurements of the VBO for the Ga2O3/GaN heterostructure by XPS were conducted, and the conduction band offset (CBO) was also calculated. These measurements are important for understanding the current transport mechanism of Ga2O3/GaN-based electronic devices.
To measure the VBO values, three samples were used: a 6-μm-thick GaN layer grown on a c-plane sapphire substrate as sample I, a 200-nm-thick Ga2O3 layer on a GaN template as sample II, and an approximately 5-nm-thick Ga2O3 layer on a GaN template as sample III. The GaN samples were grown on c-plane (0001) sapphire substrates by metal organic chemical vapor deposition (MOCVD). Trimethylgallium and blue ammonia were used as the Ga and N sources, respectively, for MOCVD growth. In our experiments, the GaN sample was thermally oxidized in a 600 ml/min oxygen ambient for 10 min at 900°C, and an approximately 5-nm-thick Ga2O3 layer was obtained on the GaN surface. The GaN sample was thermally oxidized in the same condition but for 8 h, and a 200-nm-thick Ga2O3 layer was obtained on the GaN surface. The GaN thin-film surface has a root-mean-square (RMS) roughness of 0.3 nm as revealed by AFM. The RMS roughness of the approximately 5-nm-thick Ga2O3 layer surface is 2.7 nm. The Ga2O3 thickness was measured by XPS, and the Ga2O3 crystal structures were characterized using an X-ray diffraction (XRD) apparatus. The XRD measurements were carried out using an X'Pert Pro MPD diffractometer (CuKα radiation; PANalytical B.V., Almelo, The Netherlands) with an X'Celerator detector. The XRD patterns were then refined using the HighScore Plus (PANalytical B.V.) and FullProf software packages. The XPS measurements were performed at room temperature using a PHI Quantera SXM instrument (Physical Electronics GmbH, Ismaning, Germany) with AlKα (hv = 1486.6 eV) as the X-ray radiation source, which had been carefully calibrated based on the work function and the Fermi level (EF). The total energy resolution of this XPS system is approximately 0.5 eV, and the accuracy of the observed binding energy is within 0.03 eV after careful calibration . Before taking the measurements, the XPS apparatus is calibrated by fitting to the Fermi edge of an Ar+-bombarded silver sample. The accuracy of the observed binding energy (368.26 ± 0.03 eV for Ag 3d5/2) is within 0.03 eV. When the sample is measured, a large number of electrons are excited and emitted from the sample, so the sample is always positively charged and the resulting electric field can affect the measured kinetic energy of the photoelectrons. A low-energy electron flood gun was used to achieve charge compensation, and all of the XPS spectra were calibrated using the C1s peak at 284.8 eV from contamination to compensate for the charge effect. In order to avoid the pernicious effect of surface contamination on the XPS measurement of the Ga2O3/GaN heterojunction, an Ar+ bombardment with a voltage of 1 kV at a low sputtering rate of 0.5 nm/min was carried out.
Results and discussion
XPS core-level spectra curve-fitting results and VBM positions used to calculate VBO of the Ga 2 O 3 /GaN heterostructure
Binding energy (eV)
In Figure 2a,e, the N1s peaks in both the GaN and Ga2O3/GaN samples have quite asymmetrical shapes and consist of three components. The two lower binding energy components are associated with the Ga Auger peaks [11, 12], and the higher binding energy component is considered to be from Ga-N bonding.
As shown in Table 1, the energy difference between N1s and the VBM of the GaN film (EN 1sGaN − E VBM GaN) is 394.96 eV, the energy difference between O1s and the VBM of the Ga2O3 film is 528.05 eV, and the energy difference between the N1s and O1s core levels in GaN and Ga2O3, , is 134.34 eV. The Ga2O3/GaN VBO is therefore 1.25 ± 0.08 eV for the O1s-N1s combination.
VBOs calculated for the Ga 2 O 3 /GaN heterostructure using different combinations of the XPS core levels
In summary, β-Ga2O3 films have been grown on a wurtzite GaN underlayer with an epitaxial relationship of β-Ga2O3 (−201)//wurtzite GaN (002). The VBO of the β-Ga2O3 (−201)/wurtzite GaN (002) heterostructure has been measured by XPS to be 1.40 ± 0.08 eV, with a corresponding CBO of 0.10 ± 0.08 eV from the calculation. Accurate determination of the VBO of GaN/Ga2O3 is critical for the design and application of Ga2O3/GaN-based electronic and optoelectronic devices .
This work was supported by the National Key Basic R&D Plan (973 Project) of China (Grant nos. 2012CB619301 and 2012CB619306).
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