Energy band alignment of MgO (111)/ZnO (0002) heterojunction determined by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to measure the energy discontinuity in the MgO (111)/ZnO (0002) heterostructure. The valence band offset (VBO) was determined to be 1.22±0.23 eV and a type-I heterojunction with a conduction band offset (CBO) of 3.24±0.23 eV was obtained. The discrepancy of VBO values between MgO/ZnO and ZnO/MgO heterojunctions was mainly attributed to the internal electric field induced by spontaneous polarization effect in ZnO layer.     

Valence band offset of ZnO/BaTiO3 heterojunction measured by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset of the ZnO/BaTiO₃ heterojunction grown by metal-organic chemical vapor deposition. The valence band offset (VBO) is determined to be 0.48±0.09 eV, and the conduction band offset (CBO) is deduced to be about 0.75 eV using the band gap of 3.1 eV for bulk BaTiO₃. It indicates that a type-II band alignment forms at the interface, in which the valence and conduction bands of ZnO are concomitantly higher than those of BaTiO₃. The accurate determination of VBO and CBO is important for use of semiconductor/ferroelectric heterojunction multifunctional devices.     

Band offsets and properties ofSi1 − xGex/Si material systems

Following the upsurge in the study ofSi_1 − xGe_x /Si material systems for high-speed applications, we calculate the band offsets using reformulated tight-binding methods. The calculated value of 0.78 eV for the valence band offset (VBO) of pure substances is in excellent agreement with recent experimental measurements. The VBO for alloy interfaces is 0.78 x. We apply the VBO and conduction band offsets (CBO) to shift in photoluminescence and electron confinement in microstructures respectively. The calculated VBO is strongly dependent on the precursor flux ratio.     

Band offsets at ZnO/SiC heterojunction: Heterointerface in band alignment

Pulsed laser deposited ZnO layers on 6H-SiC substrates showed the six-fold symmetry, indicating a two-dimensional epitaxial growth mode. X-ray photoelectron spectroscopy was employed to study the valence band discontinuity and interface formation in the ZnO/6H-SiC heterojunction. The valence band offset was measured to be 1.38 ± 0.28 eV, leading to a conduction band offset value of 1.01 ± 0.28 eV. The resulting band lineup in epitaxial ZnO/6H-SiC heterojunction is determined to be of staggered-type alignment.     

Valence band offset of MgO/TiO2 (rutile) heterojunction measured by X-ray photoelectron spectroscopy

The valence band offset (VBO) of MgO/TiO₂ (rutile) heterojunction has been directly measured by X-ray photoelectron spectroscopy. The VBO of the heterojunction is determined to be 1.6 ± 0.3 eV and the conduction band offset (CBO) is deduced to be 3.2 ± 0.3 eV, indicating that the heterojunction exhibits a type-I band alignment. These large values are sufficient for MgO to act as tunneling barriers in TiO₂ based devices. The accurate determination of the valence and conduction band offsets is important for use of MgO as a buffer layer in TiO₂ based field-effect transistors and dye-sensitized solar cells.     

Valence band offset of GaN/diamond heterojunction measured by X-ray photoelectron spectroscopy

XPS was used to measure the energy discontinuity in the GaN/diamond heterostructure. The valence band offset (VBO) was determined to be 0.38 ± 0.15 eV and a type-II heterojunction with a conduction band offset (CBO) of 2.43 ± 0.15 eV was obtained.     

Calculation of band offset in chalcopyrite system

A recently reformulated tight binding method is used to calculate valence band offset (VBO) at the CuInSe₂/CuGaSe₂ heterojunction. The hybrid energy is calculated in the s²p² configuration and a new model for the average hybrid energy is used. The theoretical VBO value of 0.05 eV is in good agreement with recent experimental value of 0.04 eV. The value of conduction band offset is 0.60 eV giving a type I alignment. The VBO varies linearly with bond length difference (l), as VBO=(0.24)l.     

Band offsets of epitaxial Tm2O3 high-k dielectric films on Si substrates by X-ray photoelectron spectroscopy

Tm₂O₃ crystalline films have been deposited on Si (0 0 1) by molecular beam epitaxy (MBE). Band alignments of Tm₂O₃/Si gate stacks were studied by X-ray photoelectron spectroscopy (XPS). According to XPS measurements, it can be noted that a valence-band offset of ?3.1 ± 0.1 eV and a conduction-band offset of 2.3 ± 0.3 eV for the Tm₂O₃/Si heterojunction have been obtained. Based on analysis from O 1s energy-loss spectrum, the energy gap of Tm₂O₃ is determined to be 6.5 ± 0.3 eV. A relatively thicker interfacial SiO_x layer was observed for the as-annealed samples. However, no apparent change in band alignment has been observed for Tm₂O₃/Si heterojunction with the formation of interface layer, which has been discussed in detail.     

Interfacial properties of 2D WS2 on SiO2 substrate from X-ray photoelectron spectroscopy and first-principles calculations

Two-dimensional (2D) WS₂ films were deposited on SiO₂ wafers, and the related interfacial properties were investigated by high-resolution X-ray photoelectron spectroscopy (XPS) and first-principles calculations. Using the direct (indirect) method, the valence band offset (VBO) at monolayer WS₂/SiO₂ interface was found to be 3.97 eV (3.86 eV), and the conduction band offset (CBO) was 2.70 eV (2.81 eV). Furthermore, the VBO (CBO) at bulk WS₂/SiO₂ interface is found to be about 0.48 eV (0.33 eV) larger due to the interlayer orbital coupling and splitting of valence and conduction band edges. Therefore, the WS₂/SiO₂ heterostructure has a Type I energy-band alignment. The band offsets obtained experimentally and theoretically are consistent except the narrower theoretical bandgap of SiO₂. The theoretical calculations further reveal a binding energy of 75 meV per S atom and the totally separated partial density of states, indicating a weak interaction and negligible Fermi level pinning effect between WS₂ monolayer and SiO₂ surface. Our combined experimental and theoretical results provide proof of the sufficient VBOs and CBOs and weak interaction in 2D WS₂/SiO₂ heterostructures.     

Band offsets at the epitaxial anatase TiO2/n-SrTiO3(0 0 1) interface

We have used high-energy resolution X-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO₂(0 0 1)/n-SrTiO₃(0 0 1) heterojunction prepared by molecular beam epitaxy. The valence band offsets range between −0.06 ± 0.05 and +0.16 ± 0.05 eV for anatase thicknesses between 1 and 8 monolayers and three different methods of substrate surface preparation, with no systematic dependence on film thickness. The conduction band offset (CBO) varies over a comparable range by virtue of the fact that anatase and SrTiO₃ exhibit the same bandgap (∼3.2 eV). In contrast, density functional theory predicts the VBO to be +0.55 eV. The lack of agreement between theory and experiment suggests that either some unknown factor in the interface structure or composition excluded from the modeling is influencing the band offset, or that density functional theory cannot accurately calculate band offsets in these oxide materials. The small experimental band offsets have important implications for the use of this interface for fundamental investigations of surface photocatalysis. Neither electrons nor holes are likely to become trapped in the substrate and thus be unable to participate in surface photocatalytic processes.     

Experimental determination of valence band offset at PbTe/Ge(1 0 0) interface by synchrotron radiation photoelectron spectroscopy

The band offset at the interface of PbTe/Ge (1 0 0) heterojunction was studied by the synchrotron radiation photoelectron spectroscopy. A valence band offset of ΔE_V = 0.07 ± 0.05 eV, and a conduction band offset of ΔE_C = 0.27 ± 0.05 eV are concluded. The experimental determination of the band offset for the PbTe/Ge interface should be beneficial for the heterojunction to be applied in new optoelectronic and electronic devices.     

Critical point transitions of wurtzite indium nitride

The optical transmission, photoluminescence, and reflection spectra have been measured on a high-quality wurtzite indium nitride (InN) single crystal in the range of 0.5–20.0 eV. The fundamental bandgap of intrinsic InN has been extracted by taking into account the Burstein–Moss shift, bandgap renormalization and Urbach band tail effects, and found to be very close to the recent strongly re-established value of ∼1.2 eV. With the aid of Adachi's dielectric function model for the vacuum ultraviolet reflection spectra and the empirical pseudopotential method approach for the electron band-structure, we are able to identify up to nine electronic transitions, showing clear picture for the critical point transitions in InN. The temperature dependence of these interband transitions has also been revealed.     

Ab initio study of Yb on the Ge(111)–(3 × 2) and Si(111)–(3 × 2) surfaces

The surface reconstruction, 3 × 2, induced by Yb adsorption on a Ge (Si)(111) surface has been studied using first principles density-functional calculation within the generalized gradient approximation. The two different possible adsorption sites have been considered: (i) H₃ (this site is directly above a fourth-layer Ge (Si) atom) and (ii) T₄ (directly above a second-layer Ge (Si) atom). We have found that the total energies corresponding to these binding sites are nearly the same, indeed for the Yb/Ge (Si)(111)–(3 × 2) structure the T₄ model is slightly energetic by about 0.01 (0.08) eV/unitcell compared with the H₃ model. In particular for the Ge sublayer, the energy difference is small, and therefore it is possible that the T₄, H₃, or T₄H₃ (half of the adatoms occupy the T₄ adsorption site and the rest of the adatoms are located at the H₃ site) binding sites can coexist with REM/Ge(111)–(3 × 2). In contrast to the proposed model, we have not determined any buckling in the Ge = Ge double bond. The electronic band structures of the surfaces and the corresponding natures of their orbitals have also been calculated. Our results for both substrates are seen to be in agreement with the recent experimental data, especially that of the Yb/Si(111)–(3 × 2) surface.     

Valence-band offset of p-NiO/n-ZnO heterojunction measured by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy was used to measure the valence-band offset (VBO) of the NiO/ZnO heterojunction grown on quartz substrate by radio frequency (RF) magnetron sputtering. Core levels of Ni 2p and Zn 2p were used to align the VBO of p-NiO/n-ZnO heterojunction. The valence-band offset (ΔEV) is determined to be 1.47 eV. According to the band gap of 3.7 eV for NiO and 3.37 eV for ZnO, the conduction-band offset (ΔEC) in the structure was calculated to be 1.8 eV, and it has a type-II band alignment.     

Photoemission spectroscopy at MOVPE-prepared InGaAs(100) surface reconstructions

Angular resolved ultraviolet photoemission spectroscopy at BESSY was employed to study the electronic structure of the three different, (4 × 3)-, (2 × 4)-, and (4 × 2)-surface reconstructions of In_0.53 Ga_0.47As, which was grown lattice-matched to InP(100). The surfaces have been prepared using metal organic vapor phase epitaxy (MOVPE). For spectroscopy, a dedicated transfer system was employed and samples were transferred contamination-free from the MOVPE reactor to UHV-based analysis tools. For the different surface reconstructions, the Γ ? Δ ? X direction was scanned while varying the photon energy between 10 eV and 28 eV. We observed two surface states in the photoelectron spectra on all of these surface reconstructions in addition to the bulk derived valence band emissions. Different binding energies of the surface states originating from different surface band bending were detected and described.     

Ge/ZnSe(100)异质结能带偏移的同步辐射光电子能谱研究

利用同步辐射光电子能谱研究了Ge/ZnSe（100）极性界面的能带连接问题．表面灵敏的芯能级谱显示出Ge原子与Se原子在界面处存在较弱的化学反应．利用芯能级技术，测量了该异质结的价带偏移，为1.76±0.1eV．用界面键极性模型对ZnSe（100）极性表面对价带偏移的影响进行了讨论，理论与实验符合较好 关键词：     

Performance of mid-wave T2SL detectors with heterojunction barriers

A heterojunction T2SL barrier detector which effectively blocks majority carrier leakage over the pn-junction was designed and fabricated for the mid-wave infrared (MWIR) atmospheric transmission window. The layers in the barrier region comprised AlSb, GaSb and InAs, and the thicknesses were selected by using k · P-based energy band modeling to achieve maximum valence band offset, while maintaining close to zero conduction band discontinuity in a way similar to the work of Abdollahi Pour et al. [1] The barrier-structure has a 50% cutoff at 4.75 μm and 40% quantum efficiency and shows a dark current density of 6 × 10⁻⁶ A/cm² at −0.05 V bias and 120 K. This is one order of magnitude lower than for comparable T2SL-structures without the barrier. Further improvement of the (non-surface related) bulk dark current can be expected with optimized doping of the absorber and barrier, and by fine tuning of the barrier layer design. We discuss the effect of barrier doping on dark current based on simulations. A T2SL focal plane array with 320 × 256 pixels, 30 μm pitch and 90% fill factor was processed in house using a conventional homojunction p–i–n photodiode architecture and the ISC9705 readout circuit. High-quality imaging up to 110 K was demonstrated with the substrate fully removed.     

Transparent CdO/n-GaN(0001) heterojunction for optoelectronic applications

Undoped CdO films were prepared by sol–gel method. Transparent heterojunction diodes were fabricated by depositing n-type CdO films on the n-type GaN (0001) substrate. Current–voltage (I–V) measurements of the device were evaluated, and the results indicated a non-ideal rectifying characteristic with I_F/I_R value as high as 1.17×10³ at 2 V, low leakage current of 4.88×10⁻⁶ A and a turn-on voltage of about 0.7 V. From the optical data, the optical band gaps for the CdO film and GaN were calculated to be 2.30 eV and 3.309 eV, respectively. It is evaluated that interband transition in the film is provided by the direct allowed transition. The n-GaN (0001)/CdO heterojunction device has an optical transmission of 50–70% from 500 nm to 800 nm wavelength range.     

Influence of growth temperature and deposition duration on the structure,surface morphology and optical properties of InN/YSZ (1 0 0)

InN films with the wurtzite structure have been grown directly on YSZ (1 0 0) substrate by the RF-magnetron sputtering technique. Strongly (0 0 2) oriented films with smooth surfaces (0.7–2.9 nm surface roughness depending on substrate temperature), were grown within 30 min. Films deposited for 60 min developed three-dimensional (3D) pyramidal islands on top of their surfaces, which diminished the residual elastic strain. The optical absorption edge and PL peak energy around 1.7 eV were found to redshift with increasing film thickness and substrate temperature.