The band alignment of nonpolar m -plane ZnO 1− x S x /Mg 0.4 Zn 0.6 O heterojunctions

Heterostructures such as heterojunctions, quantum wells, and superlattices are core components of advanced optoelectronic devices. Herein, we attempted the first investigations on the band alignment of nonpolar m -plane oriented ZnO 1 − x S x /Mg 0.4 Zn 0.6 O heterojunctions by X-ray photoelectron spectroscopy. All the heterojunctions were revealed to show a type-I band alignment, and the valence band offset (VBO; Δ E V ) increased significantly, while the conduction band offset ( Δ E C ) decreased insignificantly with increasing S content in the ZnO 1 − x S x layer. Specifically, for the ZnO 1 − x S x /Mg 0.4 Zn 0.6 O heterojunctions with x = 0, 0.13, and 0.22, Δ E V ( Δ E C ) was determined to be 0.24 (0.22), 0.61 (0.17), and 0.79 (0.11) eV, respectively. The VBOs of ZnOS/MgZnO heterojunctions are significantly larger than those of heterojunctions involving only cation-substituted alloys (ZnO/MgZnO or ZnO/CdZnO) due to the opposite shift in the VB maximum of ZnOS and MgZnO with respect to ZnO. Knowing band alignment parameters of the ZnOS/MgZnO interface can provide a better understanding of the carrier transport mechanism and rational design of ZnO-based optoelectronic devices.


I. INTRODUCTION
ZnO is a typical wide bandgap semiconductor suitable for a large variety of device applications. 1 The advantageous characters of ZnO that distinguish it from other semiconductors include direct and wide bandgap (Eg ∼ 3.3 eV at room temperature), large exciton binding energy (60 meV), large piezoelectric constants, strong luminescence, large nonlinear optical coefficients, high melting temperature (2248 K), good biocompatibility, low toxicity, and low cost. [2][3][4][5] In order to optimize the performance of ZnO-based optoelectronic devices, most designs rely on different kinds of heterojunctions for providing carrier and/or optical confinement. A heterojunction is composed of layers of different compositions, whose key parameters are the bandgap of each layer and the valence/conduction band offset between the individual layers. The dynamics of charge carriers in the heterojunction depends on the potential barrier heights, which are quantified by the values of the valence/conduction band offset at the interface between two contacting layers. Following extensive investigations on the bandgap engineering of ZnO by isovalent cation substitution (e.g., MgZnO, CdZnO, and BeZnO alloys), [6][7][8] construction and fabrication of ZnO-based heterojunctions such as ZnO/MeZnO (Me = Mg, Cd, Be, and Ni) 9-12 on various substrates have been demonstrated with different growth techniques. In particular, several experimental studies on the determination of band offsets of the ZnO/MgZnO and ZnO/CdZnO heterojunctions have been reported. 13,14 On the other hand, replacing O 2− ions with isovalent anions is another important way for bandgap engineering of ZnO. 15 For example, introducing S into ZnO can form ZnOS alloys that show a nonlinear variation in bandgaps with the S concentration and show different electrical and optical properties with respect to ZnO due to large electronegativity and size differences between S and O. [16][17][18] At present, high-quality ZnOS thin films with different compositions can be epitaxially grown on sapphire substrates by pulsed laser deposition (PLD). In our previous work, the S solubility limit in single-phase ZnOS epitaxial films was determined, and variations of both the bandgap and lattice ARTICLE scitation.org/journal/adv constants were quantitatively correlated with the S content in the ZnOS alloys. [18][19][20] Because MgZnO and ZnOS alloys show opposite variations in the bandgap energy with the substituent (Mg or S) concentration, we proposed the design of ZnOS/ZnMgO heterostructures, such as superlattices (SLs) and quantum wells (QWs), that have a larger barrier height than ZnO/ZnMgO with appropriate S and Mg concentrations. 19 Although such cation-and anion-substituted ZnO alloy composed heterostructures hold great potential, in reality, ZnO-based heterojunctions involving anionsubstituted alloys such as ZnOS remain unexplored to date. Returning to the ZnO/MeZnO heterojunctions documented so far in the literature, most heterojunctions were grown on a polar surface, such as c-plane orientated sapphire substrates. When a single QW is grown along the c-axis of a hexagonal crystal structure, a built-in electric field arising from the spontaneous polarization occurs, leading to band bending in the QW along the growth direction, 21 which confines the electrons and holes in the conduction band minimum (CBM) and valence band maximum (VBM) of the QW, respectively. 22 The overlap of the electron and hole wave-function is reduced by increasing the well width, leading to an increment in the radiative recombination lifetime (i.e., quantum confined STARK effect, QCSE). 23 These effects encumber the performance and efficiency of light emitting devices. In order to avoid the QCSE, ZnO-based alloy thin films and heterojunctions with nonpolar surface orientation should be developed.
In the present study, based on our previous work on the PLD growth of high-quality ZnOS epitaxial films with a nonpolar surface, 20,24 we attempted and achieved the growth of nonpolar m-plane orientated ZnOS/MgZnO heterostructures on an m-plane sapphire. Then, we used X-ray photoelectron spectroscopy (XPS) to investigate the band alignment of the grown heterojunctions. XPS has been demonstrated to be a direct and powerful tool for characterizing the band alignment of a large variety of heterojunctions, [25][26][27] and herein offers first-hand information on the offsets of both valence and conduction bands between the two layers in the ZnOS/MgZnO heterojunctions.

II. EXPERIMENTAL
In this work, seven samples including three single-layer Composition analysis of the single-layer films and band alignment determination of the bilayer heterojunctions were accomplished by XPS core-level (CL) and valence band (VB) spectra measurements using an Escalab 250Xi system (Thermo Fisher Scientific, USA) with a photon energy of 1486.6 eV (Al Kα radiation from a monochromatic x-ray source) under ultrahigh vacuum conditions. Ar-ion sputtering was used to clean surfaces of the films and perform step-wise etching of the bilayer heterojunctions. The sputtering parameters for etching were fixed at an Ar pressure of 5 × 10 −6 mbar, Ar + ion energy of 1 keV, and sputtering period of 2 min. Multiple cycles of the sputtering-XPS test were performed until the CL signal of Mg 2p from MgZnO was detected by XPS. This eventually allowed simultaneous detection of CL signals from both layers of the ZnOS/MgZnO heterojunctions. The final thickness of the ZnOS layer was estimated to be ∼5 nm. All spectra presented in this paper were calibrated by two steps: first, the charging effect induced by Ar + bombardments was removed by considering the binding energy shifts of Zn 2p 3/2 before and after sputtering. Then, recalibration referring to the C 1s hydro-carbon peak at 284.5 eV was performed to remove the charging effect induced by x-ray irradiation. [29][30][31] The S contents in the ZnOS films deposited under oxygen pressures of 5 Pa and 3.5 Pa were determined to be approx. 13% and 22%, respectively, while the Mg content in the MgZnO films was measured to be and maintained at 40%. X-ray diffraction (XRD) was performed using a four-circle single-crystal diffractometer (D8 Discover, Bruker GmbH) with a Cu Kα 1 monochromatic radiation source (λ = 0.154 06 nm) for the structural characterization of the films and heterojunctions. Optical transmittance of the films was measured by using a UV-Vis-NIR spectrophotometer (Shimadzu UV 3600) to evaluate optical bandgaps of the films.

ARTICLE scitation.org/journal/adv
Bandgaps of the ZnOS and MgZnO films were estimated based on UV-Vis spectroscopy. The absorption coefficients α of the films were calculated from the transmittance measurements and plots of (αhv) 2 as a function of photon energy hν, which are shown in Fig. 1(b). Due to an allowed direct transition of ZnOS and MgZnO, the relationship (αhv) 2 ∝ (hν − Eg) holds good, from which the optical bandgap can be estimated by extrapolating the linear portion of the (αhv) 2 curve to the axis of hν. 32 The bandgaps are found to be 3.32, 3.00, and 2.88 eV for ZnO 1−x Sx (x = 0, 0.13, and 0.22), and 3.78 eV for Mg 0.4 Zn 0.6 O, respectively. The decrease in the bandgap of ZnOS with increasing S concentration is consistent with our previous report. 20 Band offset measurements for the ZnOS/MgZnO heterojunctions were performed following the widely spread method suggested by Kraut et al. 26 The method makes use of XPS for high-accuracy measurements of heterojunction band discontinuities by determining binding energy differences between the CL and VBM. In the specific case of ZnOS/MgZnO, the value of the valence band offset (VBO) (ΔE V ) can be determined based on the following equation: Here, ΔE Then, the conduction band offset (ΔE C ) can be calculated using the following equation: Here, ΔEg = ΔE MgZnO g − ΔE ZnOS g is the bandgap energy difference between the MgZnO and ZnOS single-layer films.
According to this method, for deducing ΔE V between ZnOS and MgZnO, we need to determine the binding energies of Zn 2p 3/2 in the ZnOS, MgZnO single-layer films, and the bilayer ZnOS/MgZnO heterojunction, as well as the VBM of both singlelayer films. Figure 2 of the films can be estimated by extrapolating the linear portion to the extended baseline of the VB spectra, following the widely used procedure. 27 The VBM values of the ZnO 1−x Sx films with x = 0, 0.13, and 0.22 are deduced to be 2.94 ± 0.02, 2.90 ± 0.04, ± 0.04 eV. It is noteworthy that there exists another portion with a smaller slope at the end of the VB edge for all the samples, which is known as the band tail originating from defects. 33 A similar phenomenon has been frequently found in the VB region of a variety of semiconductor materials. 34 Table I summarizes the experimental results of Eg, E Zn2p3/2 , E VBM , and E Zn2p3/2 − E VBM for the single-layer ZnOS and MgZnO films. According to these data, a diagram of band alignments among the ZnOS/MgZnO heterojunctions can be schemed, as shown in  Fig. 5). Based on the experimental data in Table I  heterojunctions (up to 0.79 eV) are significantly larger than those of the heterojunctions such as ZnO/MgZnO 13,25 and ZnO/CdZnO 35 (less than 0.17 eV). Generally, for ZnO and related alloys, the top of the valence band is a bonding state with a significant contribution from the coupling between the anion-p and cation-d orbitals, while the bottom of the conduction band, being an antibonding state, stems primarily from both the anion-s and cation-s orbitals. 36 Since Zn, but not Mg, contains 3d electrons, the anion-p and cation-d coupling in MgZnO would be weakened as a result of the absence of Mg 3d electrons, resulting in the lowered VBM of MgZnO with respect to ZnO, which becomes more pronounced in MgZnO with a higher Mg content. 37 This effect, however, leads only to small valence band offsets between ZnO and MgZnO alloys according to the general "common anion rule". For the anion-substituted ZnOS alloys, due to the much enhanced p-d coupling between S and Zn as evidenced by a higher VBM of ZnS than ZnO, 38 their VBMs lie significantly higher than those of ZnO or MgZnO, as illustrated in Fig. 4. The VBM of ZnOS shifts upwards with an increasing S content, which is in good agreement with theoretical calculations of Persson et al. 38 It is the opposite shifts of VBM in ZnOS and MgZnO relative to ZnO that lead to larger valence band offsets in the ZnOS/MgZnO heterojunctions than in ZnO/MgZnO. This is an advantageous character for device applications where holes play a major role. It is noteworthy that our present work further indicates a type-II (staggered) band alignment for the ZnO/ZnOS heterojunctions, as seen from the

IV. CONCLUSIONS
In summary, the band alignment of ZnO 1−x Sx/Mg 0.4 Zn 0.6 O heterojunctions was determined by XPS. It was found that type-I heterojunctions were formed between ZnOS and MgZnO. Valence band offsets ΔE V were determined to be 0.24, 0.61, and 0.79 eV, while conduction band offsets ΔE C were calculated to be 0.22, 0.17, and 0.11 eV for x = 0, 0.13, and 0.22, respectively. Extending the most studied ZnO/MgZnO heterojunctions to both anion-and cationsubstituted ZnO-based heterojunctions enabled us to first find the type-II band alignment in the ZnO/ZnOS heterojunctions. More importantly, it implies a great freedom for the rational design of various ZnOS/MgZnO heterostructures with a desired band alignment for high-performance optoelectronic devices as both the cation-and anion-substitutes can tune the ZnO band structure in different ways, leading to various VBMs and CBMs of corresponding ZnO alloys.