Interface Band Diagram Research Articles

Estimating complete band diagrams of non-ideal heterointerfaces by combining ellipsometry and photoemission spectroscopy

In this work, we show that spectroscopic ellipsometry can be combined with photoemission spectroscopy to obtain complete interface band diagrams of non-ideal semiconductor heterointerfaces, such as interfaces between thin-film polycrystalline materials. The non-destructive ellipsometry measurement probes the near-interface bandgap of the two semiconductors (including the buried semiconductor) after the interface has formed. This is important in the non-ideal case where chemical processes during interface growth modify the electronic properties of the two separated surfaces. Knowledge of near-interface bandgaps improves accuracy in conduction band offset measurements of non-ideal interfaces, and it sheds light on their device physics. Both of those positive outcomes are demonstrated in the Cu2ZnSnS4/CdS interface used here as a case study, where the bandgap of both materials decreases by up to 200 meV from the bulk to the near-interface region. This finding reveals a preferential electron-hole recombination channel near the interface, and it yields corrected values for the interfacial conduction band offset.

Large process-dependent variations in band alignment and interface band gaps of Cu2ZnSnS4/CdS solar cells

Electron–hole recombination at the Cu2ZnSnS4/CdS interface is believed to play a major role in limiting the efficiency of Cu2ZnSnS4 solar cells. In this work, we experimentally determine detailed Cu2ZnSnS4/CdS interface band diagrams as a function of process conditions, and correlate them to chemical processes occurring during interface formation and subsequent post-annealing. The newly devised experimental method involves a combination of photoemission spectroscopy and spectroscopic ellipsometry. Our measurements reveal that, under most process conditions, the band gaps of both Cu2ZnSnS4 and CdS decrease by several hundred meV near the interface. Furthermore, interface band bending and conduction band offsets are highly process-dependent and roughly correlated to the amount of chemical interdiffusion. The interface electronic properties are found to be unfavorable under all process conditions studied in this work, either due to a cliff-like conduction band offset, or to substantial band gap narrowing in Cu2ZnSnS4, or to both effects. According to the present study, the least harmful process conditions for the interface electronic properties are a low CdS deposition temperature without post-annealing. Even in such a case, a minimum open circuit voltage loss of 230 mV is expected due to interface- or near-interface recombination.

Color tunable electroluminescence and resistance switching from a ZnO-nanorod–TaOx–p-GaN heterojunction

Well-aligned ZnO nanorods have been prepared on p-GaN–sapphire using a vapor phase transport (VPT) technique. A thin sputtered layer of TaOx is employed as the intermediate layer and an n-ZnO–TaOx–p-GaN heterojunction device has been achieved. The current transport of the heterojunction exhibited a typical resistance switching behavior, which originated from the filament forming and breaking in the TaOx layer. Color controllable electroluminescence (EL) was observed from the biased heterojunction at room temperature. Bluish-white wide band emission is achieved from the forward biased device in both the high resistance and low resistance states, while red emission can only be observed for the reverse biased device in the low resistance state. The correlation between the EL and resistance switching has been analyzed in-depth based on the interface band diagram of the heterojunction.

Universal electronic structure of polar oxide hetero-interfaces.

The electronic properties of NdGaO3/SrTiO3, LaGaO3/SrTiO3, and LaAlO3/SrTiO3 interfaces, all showing an insulator-to-metal transition as a function of the overlayer-thickness, are addressed in a comparative study based on x-ray absorption, x-ray photoemission and resonant photoemission spectroscopy. The nature of the charge carriers, their concentration and spatial distribution as well as the interface band alignments and the overall interface band diagrams are studied and quantitatively evaluated. The behavior of the three analyzed heterostructures is found to be remarkably similar. The valence band edge of all the three overlayers aligns to that of bulk SrTiO3. The near-interface SrTiO3 layer is affected, at increasing overlayer thickness, by the building-up of a confining potential. This potential bends both the valence and the conduction band downwards. The latter one crossing the Fermi energy in the proximity of the interface and determines the formation of an interfacial band offset growing as a function of thickness. Quite remarkably, but in agreement with previous reports for LaAlO3/SrTiO3, no electric field is detected inside any of the polar overlayers. The essential phenomenology emerging from our findings is discussed on the base of different alternative scenarios regarding the origin of interface carriers and their interaction with an intense photon beam.

Charge transition levels of carbon-, oxygen-, and hydrogen-related defects at the SiC/SiO2interface through hybrid functionals

We calculate charge transition levels of various defects at the SiC/SiO2 interface within a scheme based on hybrid density functionals, which accurately reproduce the involved band gaps and band offsets. The defect levels are first evaluated in bulk models of the interface components and then aligned with respect to the interface band diagram through the use of a model interface showing good structural and electronic properties. Interface-specific polarization effects are evaluated separately through classical electrostatics. We considered carbon-related defects involving single atoms and dimers in both the crystalline SiC substrate and the amorphous SiO2 oxide. Our investigation also comprises oxygen-and hydrogen-related defects, including the Si-Si bond (O vacancy), the Si-2-C-O structure, the peroxy linkage, and the hydrogen bridge (Si-H-Si). Among the defects studied, the Si-2-C-O structure represents the best candidate for the high defect density measured near the conduction band of SiC.

Surface photovoltage spectroscopy ‐ method and applications

AbstractSurface photovoltage spectroscopy (SPS) allows for obtaining a detailed picture of the electronic structure of semiconductors. In SPS, changes in band bending at the free semiconductor surface are monitored as a function of external illumination. Surface photovoltage spectroscopy can provide detailed, quantitative information on bulk properties (e.g. bandgap and conductivity type, defect states energy, and lifetime) and can be used for complete construction of surface and interface band diagrams, including the measurement of energy levels in quantum structures. Measurements using SPS are contactless and non‐destructive. In addition, they can be performed both in situ and ex situ, at any reasonable temperature, on many semiconducting materials, at any ambient and at any lateral resolution down to the atomic scale. In this review an overview of SPS‐related theory is presented, the SPS experimental set‐up is described and applications for the characterization of a wide variety of materials and structures are presented (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron energy band alignment at the NiO/SiO2 interface

Study of the NiO/SiO2 interface band diagram using optical absorption, photoconductivity, and internal photoemission measurements reveals that deposited Ni oxides have a band gap of 3.2±0.1 eV, with the top of the valence band at 5.3±0.1 eV below the conduction band of SiO2 and showing no measurable sensitivity to the oxygen surplus in the layer up to 20%. Annealing in a reducing ambient (forming gas) at 400 °C results in narrowing of the band gap and an up shift in the occupied electron state edge by ≈1 eV, which is tentatively associated with partial reduction in the NiO layer.

Influence of passivating interlayer on Ge/HfO 2 and Ge/Al 2O 3 interface band diagrams

The energy band alignment between Ge, HfO 2 and Al 2O 3 was analyzed as influenced by passivating interlayers (ILs) of different composition (GeO 2, Ge 3N 4, Si/SiO x ). From internal photoemission and photoconductivity experiments we found no IL-sensitive dipoles at the Ge/HfO 2 interfaces, the latter being universally characterized by conduction and valence band offsets of 2.1 and 3.0 eV, respectively. However, in the case of HfO 2 growth using H 2O-based atomic layer deposition, the Ge oxide IL appears to have a narrower bandgap, 4.3 eV, than the 5.4–5.9 eV gap of bulk germania. Accordingly, formation of this IL yields significantly reduced barriers for hole and, particularly, electron injection from Ge into the insulator. Changing to a H-free process for HfO 2 and Al 2O 3 deposition suppresses the formation of the narrow-gap Ge oxide.

Electronic structure at interfaces of cubic Gd 2O 3 with embedded Si nanocrystals

The electronic structure of silicon nanocrystals (∼2–6 nm in size) embedded in cubic Gd 2O 3 epi-layers grown on (1 1 1) Si was analyzed using spectroscopic ellipsometry and photocharging methods. With decreasing nanocrystal size down to the 2 nm range, the optical absorption exhibits a spectacular shift in spectral threshold to 2.9 ± 0.1 eV, as compared to the 1.12 eV absorption edge of the bulk Si crystal. This shift suggests a significant influence of quantum confinement on the Si nanocrystal/oxide interface band diagram, which effect is shown to be predominantly caused by an upshift of the nanocrystal conduction band.

Integration of low dimensional crystalline Si into functional epitaxial oxides

In this work we show that by efficiently exploiting the growth kinetics during molecular beam epitaxy (MBE) one could create Si nanostructures of different dimensions. Examples are Si quantum dots (QD) or quantum wells (QW), which are buried into an epitaxial rare-earth oxide, e.g. Gd2O3. Electrical measurements carried out on Pt/Gd2O3/Si MOS capacitors comprised with Si-QD demonstrate that such well embedded Si-QD with average size of 5nm and density of 2×1012cm−2 exhibit very good charge storage capacity with suitable retention (∼105s) and endurance (∼105 write/erase cycles) characteristics. The Pt/Gd2O3/Si (metal–oxide–semiconductor (MOS)) basic memory cells with embedded Si-QD display large programming window (∼1.5–2V) and fast writing speed and hence could be a potential candidate for future non-volatile memory application. The optical absorption of such Si-QD embedded into epitaxial Gd2O3 was found to exhibit a spectral threshold maximum up to 2.9±0.1eV depending on their sizes, inferring a significant influence of quantum confinement on the QD/oxide interface band diagram.Ultra-thin single-crystalline Si-QW with epitaxial insulator (Gd2O3) as the barrier layers were grown by a novel approach based on cooperative vapor phase MBE on Si wafer with sharp interfaces between well and barriers. The current–voltage characteristics obtained for such structure exhibits negative differential resistance at lower temperature, making them a good candidate for resonant tunneling devices.

Transport mechanisms in manganite-titanate heterojunctions

We have found that the current-voltage characteristics of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3(\ensuremath{-}\ensuremath{\delta})}∕\mathrm{Nb}:\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ rectifying junctions are quantitatively well described by (thermally assisted) tunneling with an effectively temperature-independent Schottky barrier under no magnetic field, while those of the oxygen deficient junction remarkably deviate from such a simple behavior as magnetic field is applied. These results indicate the junction magnetoresistance arising from magnetic-field changes of the interface band diagram via the strong electron-spin coupling in manganites.

Energy band alignment at the (100)Ge/HfO2 interface

The Ge/HfO2 interface band diagram was directly determined using internal photoemission of electrons and holes from Ge into the Hf oxide. The inferred offsets of the conduction and valence band at the interface, 2.0±0.1 and 3.0±0.1 eV, respectively, suggest the possibility to apply the deposited HfO2 as an insulator on Ge. The post-deposition annealing of the Ge/HfO2 structures in oxygen results in ∼1 eV reduction of the valence band offset attributed to the growth of GeO2 interlayer.

Surface photovoltage spectroscopy of semiconductor structures: at the crossroads of physics, chemistry and electrical engineering

AbstractThe possibility of obtaining a detailed picture of the electronic structure makes surface photovoltage spectroscopy (SPS) eminently suitable for bridging the gap between the chemical, physical, optical and electrical properties of semiconductors. In SPS, changes in band bending (both at the free semiconductor surface and at buried interfaces) are monitored as a function of external illumination. Surface photovoltage spectroscopy can provide detailed, quantitative information on bulk properties (e.g. bandgap and type, carrier diffusion length and lifetime) and can be used for complete construction of surface and interface band diagrams, including the measurement of energy levels in quantum structures. A particular strength is that a comprehensive analysis of surface and bulk defect state distributions and properties is made possible. Measurements using SPS are contactless and non‐destructive. In addition, they can be performed both in situ and ex situ, at any reasonable temperature, on any semiconducting material, at any ambient and at any lateral resolution down to the atomic scale. This review starts with an overview of SPS‐related surface and interface theory, describes the SPS experimental set‐up and presents applications for surface and interface characterization of a wide variety of materials and structures, cross‐correlating them with other methodologies. Copyright © 2001 John Wiley & Sons, Ltd.

Some efforts beyond the local density approximation

AbstractThe exchange‐correlation density functional can be expressed as a many‐body perturbation series in terms of the Coulomb interaction using the exact Kohn‐Sham orbitals as the basis. A self‐consistent equation is derived for the exact exchangecorrelation potential. This perturbation approach forms a basis for going beyond the local density approximation (LDA). The discontinuity in the exchange‐correlation potential for semiconductors calculated by the perturbative approach gives a good account of the discrepancy of the band gap calculated in LDA. The discontinuity also plays an important role in the interface band diagrams. A theory to account for the interaction effects of localized d or f orbitals is reviewed and the physics of the applications to a model test, to some 3d transition metals, and to heavy fermions is discussed. The perturbative approach to improvement beyond LDA tends to be computation‐intensive and to be system‐specific. © 1995 John Wiley & Sons, Inc.