Abstract
Xray photoelectron spectroscopy has been used to measure the valence band offset (VBO) of the wInN/hBN heterojunction. We find that it is a typeII heterojunction with the VBO being −0.30 ± 0.09 eV and the corresponding conduction band offset (CBO) being 4.99 ± 0.09 eV. The accurate determination of VBO and CBO is important for designing the wInN/hBNbased electronic devices.
Keywords:
Valence band offset; wInN/hBN heterojunction; Xray photoelectron spectroscopy; Conduction band offset; Valence band offsetIntroduction
Among the groupIII nitrides, wurtzite InN (wInN) is a very promising semiconductor material of application in highfrequency/highspeed/highpower heterojunction fieldeffect transistors (HFETs) [14], due to its superior electron transport properties, small effective mass, and high mobility. WInN also has been used in field emitter because of its negative affinity (NET) [57]. Hexagonal boron nitride (hBN) is a sp^{2}bonded layered compound isostructural to graphite. Due to its wide band gap, it has been used in microelectronic devices. There were several reports on field emission characteristics of hBN films [810]. Chinaru and his coworkers [11] designed an hBN/GaN field emission device, which appeared lower turning on voltage compared with the conventional devices. The band structures of wInN and hBN are very similar to that of hBN and GaN, so wInN/hBN is promising for field device. It is important to accurately determine the valence band offset (VBO) and conduction band offset (CBO) for using wInN/hBNbased electronic devices. Theoretical and experimental values of band structure have been extensively investigated for the wInN and hBN, respectively, but it is still scarce for the wInN/hBN heterojunction. This letter reports the valence band discontinuity at the wInN/hBN heterojunction interface.
Experimental
Three samples were used in our Xray photoelectron spectroscopy (XPS) experiments. A 40nmthick hBN layer was deposited by ion beam assisted deposition on Si substrate. wInN films were deposited by metal organic chemical vapor deposition (MOCVD): a 5nmthick wInN grown on a prepared hBN layer on Si substrate and a 250nmthick wInN grown on Si(111) substrate. Details of the growth conditions have been presented elsewhere [12]. The crystal qualities of wInN were characterized using the highresolution Xray diffraction apparatus at Beijing Synchrotron Radiation Facility. The full width at half maximum of the Xray diffraction rocking curve (XRC) is 0.75°. The wInN is unintentionally ntype doped, the carrier concentration is 1 × 10^{19} cm^{−3} determined by Hall performed in single field the Van der Pauw geometry at room temperature. However, the hBN is high resistance. XPS measurement was performed on PHI Quantera SXM instrument with Al Kα (hν = 1486.6 eV) as the Xray radiation source and the angle between the Xray source and the detector is 45°. The work function and the Fermi energy level (E_{F}) of the instrument had been carefully calibrated. The surfaces of all the samples were exposed in the air, so existence of impurities (such as oxygen and carbon) on the surface may prevent the precise determination of the valence band maximum (VBM). In order to reduce the contamination effect, all the samples were subjected to surface clean procedure by Ar^{+} bombardment with a voltage of 1 kV at a low sputtering rate of 0.5 nm/min. The total energy resolution of XPS system is about 0.5 eV, and the precision of the binding energy is within 0.03 eV after careful calibration. A low energy electron flood gun was utilized to achieve charge compensation. All XPS spectra were calibrated by the C 1s peak at 284.8 eV from contamination to compensate the charge effect.
Results and Discussion
The VBO can be calculated according to
where is the energy difference between In 3d_{5/2} and B 1s CLs at the wInN/hBN interface. In the terms of and , and are the CLs of wInN and hBN bulk constants of thick films, respectively, and , means the bulk position of the valence band maximum with respect to the E_{f}. The CLs spectra were fitted using Voigt (mixed Lorentz–Gaussian) lineshape and Shirley background. The positions of valence band maximum (VBM) in valence band (VB) spectra are determined by linear extrapolation of leading edges of VB spectra to the base lines in order to account for the finite instrument resolution [13]. Since considerable fitted line to the original measured data has been obtained, the uncertainty of the CL positions should be lower than 0.03 eV, as evaluated by numerous fitting with different parameters. The In 3d5/2 spectra for wInN and wInN/hBN samples, the B 1s spectra for hBN and wInN/hBN samples, and the valence band photoemission for both wInN and hBN samples are shown in Fig. 1. The parameters deduced from Fig. 1 are summarized in Table 1 for clarity. As illustrated in Fig. 1a and 1e, the CL lineshapes of wInN appeared a weak asymmetry. This phenomenon had been investigated by several authors [1417]. Stefan [18] proposed that this associated with plasmon side band. There are two types of plasmon; intrinsic plasmon and extrinsic plasmon. The intrinsic plasmon is that a strong local potential produced when an electron of CL was removed from the matrix, leaving a positive charge of photohole and then the deexcitation of the photohole by quantized excitations the conductionelectron system. However, the extrinsic plasmons are excited by the outgoing photoelectron traveling from the place of the photoexcitation process to the surface [1417]. Energy of extrinsic plasmon is always as smooth background. Considering the couple of the surface plasmons and photoelectrons, the finalstate is projected onto screened component and unscreened component [20]. We attribute the lower energy component to the “screened” finalstate and the higherenergy component to the “unscreened” finalstate. The latter is corresponding to a plasmon satellite at higherbinding energy. Wertheim and his coworker [19,20] used this model to explain screening response in narrow band metal. The energy of the surface plasmon is [17]
here n is the carrier concentration, ε(∞) and m^{*} are the highfrequency dielectric constant and the effective mass of the conduction electron, respectively. The ratio between surface and bulk frequencies is with ε(∞) = 8.4. The calculated result is 0.945. For the unintentionally doped ntype wInN, the carrier concentration is much lower than that of metal. The energy of plasmon is always less than 1 eV, which is comparable with the intrinsic linewidth. The screened and unscreened peaks overlapped, resulting in an asymmetric core lineshape [19,21]. The In 3d_{5/2} consists of three peaks by Voigt fit using Shirley background, the positions of peak locked at 443.71, 444.54, and 445.45 eV, as shown in Fig. 1a. The binding energy of 443.71 and 444.54 eV associated with screened final state and the unscreened final states, respectively. The binding energy of 445.45 eV belongs to InO bond. Similarly, in the wInN/hBN system, the binding energy of 443.78 and 444.67 eV belong to screened and unscreen final states, respectively. According to previous reports [15], it is suggested that the energy 446.15 eV is InO bonding in Fig. 1e.
Figure 1. CL spectra of In 3d_{5/2} recorded on wInN bulk (a) and wInN/hBN (e) Samples, B 1s spectra on hBN bulk (c) and wInN/hBN heterojunction (f). Samples, wInN VB spectra (b) and hBN VB spectra. All peaks have been fitted to Voigt line shapes using Shirley background, and the VBM values are determined by linear extrapolation of the leading edge to the base line
The energy differences between the unscreened and the screened are 0.83 and 0.89 eV for the bulk and the heterojunction. It should be noted that the plasmon frequency varies with the surface electron concentration. BN 1s peak in the bulk and heterojunction is 190.1 and 190.37 eV, respectively. The VBM lineshape and the CL positions of hBN are very similar to the latest reports [18,22].
Due to the peak and linewidth of higherbinding energy (unscreened finalstate) depend on the excitement of bulk, surface treatment [14,17], we choose the lowerbinding energy components related to screened finalstate for VBO calculation, in this letter. At room temperature, the wInN band gap is 0.64 eV [23]. Taniguchi and his coworker have calculated the band gap of hBN about 5.97 eV [24]. According to obtained data, the VBO is calculated to be −0.30 eV. The CBO () is given by the formula . and are the band gap of hBN and wInN, respectively. is calculated to be 4.99 eV. According to these results, a typeII band alignment forms at the wInN/hBN heterojunction, as shown in Fig. 2.
Figure 2. Room temperature VBM and CBM lineup of the wInN/hBN heterojunction, showing a typeII band alignment
We noted that lattice mismatch between wInN (0001) and hBN (0001) is 20%, so the critical thickness is estimated to be about 1 monolayer (ML). The residual stress in the film is very small. Because of the small linear pressure coefficient of InN (~0.06 meV/Gpa) [25], the change of band gap induced by the stress can be neglected. In addition, it is well known that the nitrides are piezoelectric materials. Martin measured the piezoelectric effects of nitrides [23,26]. According to his results, we estimate the magnitude of the field is in the order of 10^{8} V/m. Due to the critical thickness is about 1 ML, the band bending caused by the piezoelectric effect is about 0.06 eV. The effect of interface states is to shift the potential within the sampled region on both sides of the interface by the same constant value. And then, any potential shift which due to band bending induced by interface states can be canceled. Considering above condition, accumulative total error is about 0.09 eV.
Conclusions
In summary, the VBO of wInN/hBN heterojunction has been measured by XPS to be 0.30 ± 0.09 eV, and the corresponding CBO is 4.99 ± 0.09 eV, so it belongs to a typeII band lineup. Based on the calculation, the effect of piezoelectric caused by the lattice mismatch and band bending by the surface state can be neglected. The accurate determination of the band alignment of wInN/hBN is important for designing the devices.
Acknowledgments
The authors are grateful to Professor Huanhua Wang and Dr. Tieying Yang of the Institute of High Energy Physics, Chinese Academy of Science. This work was supported by National Science Foundation of China (No.60776015, 60976008), the Special Funds for Major State Basic Research Project (973 program) of China (No.2006 CB604907), and the 863 High Technology R&D Program of China (No.2007AA03Z402,2007AA03Z451).
Open Access
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