Band Lineup Research Articles

Electrical and physical topography in energy-filtered photoelectron emission microscopy of two-dimensional silicon pn junctions

Photoelectron emission microscopy (PEEM) is a powerful non-destructive tool for spatially resolved, spectroscopic analysis of surfaces with sub-micron chemical heterogeneities. However, in the case of micron scale patterned semiconductors, band line-ups at pn junctions have a built-in lateral electric field which can significantly alter the PEEM image of the structure with respect to its physical dimensions. Furthermore, real surfaces may also have physical topography which can reinforce or counteract the electrically induced distortion at a pn junction. We have measured the experimental PEEM image distortion at such a junction and carried out numerical simulations of the PEEM images. The simulations include energy filtering and the use of a contrast aperture in the back focal plane in order to describe the changes in the PEEM image of the junction with respect to its real physical dimensions. Threshold imaging does not give a reliable measurement of micron sized p and n type patterns. At higher take-off energies, for example using Si 2p electrons, the pattern width is closer to the real physical size. Physical topography must also be quantitatively accounted for. The results can be generalized to PEEM imaging of any structure with a built-in lateral electric field.

XPS Study of Energy Band Alignment between Hf-La Oxides and Si(100)

We have studied the chemical bonding features and electronic states of ultrathin Hf-La oxide using high-resolution x-ray photoelectron spectroscopy (XPS). 6 nm-thick Hf-La oxides with different La/(Hf+La) content were prepared on wet-chemical cleaned p-type Si(100) and Pt layer by thermal decomposition of Hf(DPM: dipivaloymcthanato)4 and La(DPM)3 in O2 ambient at 450℃. From XPS core-line analysis, diffusion and incorporation of Si atoms from the Si substrate into the Hf-La oxide film by 800℃ annealing in O2 ambience were pronounced with increased La content over 35%. The energy band profile of the Hf-La oxide/Si system as a function of La content has been determined by a combination of oxide bandgap (Eg) values and valence band (VB) line-ups. The conduction band (CB) offset has been also estimated using the Eg of the Si substrate, measured Eg, and VB offset. The impact of Si incorporation into Hf-La oxide on CB and VB offsets between the Hf-La(:Si) oxide and Si substrate also discussed in comparison to the energy band diagram of the Hf-La oxide/Pt structure.

Well-thickness dependent electron transport effective mass and mobility in Sb-based quantum wells

We propose a special kind of Sb-based III–V semiconductor quantum wells, which, in contrary to conventional ones, are able to provide novel transport properties: smaller electron transport effective mass and higher mobility with thinner well width. By properly choosing the barrier materials with smaller electron effective mass and proper band lineups, the novel transport property could be achieved because of the penetration effect of the wave-function when interface roughness and alloy disorder scatterings are taken into consideration.

Band bending and determination of band offsets in amorphous/crystalline silicon heterostructures from planar conductance measurements

An analytical model for the calculation of the band bending in amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions is presented and validated by comparison with full numerical simulations. The influence of the various structure properties and parameters, such as the density of states in bulk a-Si:H or at interface defects, the position of the Fermi level in a-Si:H, the temperature dependence of band gaps, is investigated. Significant band offsets imply the presence of a strong inverted layer at the c-Si surface of both (p)a-Si:H/(n)c-Si and (n)a-Si:H/(p)c-Si structures, forming two-dimensional hole and electron gases, respectively. This leads to high sheet carrier densities that have been evidenced from planar conductance measurements. Experimental data obtained on samples coming from various research institutes are analyzed with our model in order to extract the band offsets. We find that the valence band offset ranges between 0.32 and 0.42 eV with an average value at 0.36 eV; the conduction band offset is found between 0.08 and 0.26 eV with a mean value at 0.15 eV. These values are discussed in the frame of the branch point theory for band line-up; they imply that the branch point energy in a-Si:H is almost independent of doping and lies close to mid-gap.

Current Gain and Offset Voltage in an InGaP/GaAsSb/GaAs Double Heterojunction Bipolar Transistor

An In0.5Ga0.5P/GaAs0.93Sb0.07/GaAs heterostructure is grown by metal-organic chemical vapor deposition and demonstrated to be an alternative to InGaP/GaAs systems for use in single heterojunction bipolar transistors (SHBTs). The In0.5Ga0.5P/GaAs0.93Sb0.07/GaAs double heterojunction bipolar transistor (DHBT) device structures are identical to the control SHBTs except for the incorporation of antimony to the base layer. The InGaP/GaAsSb/GaAs DHBT has a lower turn-on voltage than the control InGaP/GaAs SHBT. Details of the effect of the base current on the offset voltage in the studied devices are explored. The dc current gain β of the InGaP/GaAsSb/GaAs DHBT is virtually independent of collector current at 300 K. The current gain of the GaAsSb-based DHBT is greater than 40 at a very small collector current of 1 μA at 300 K. The dependence of the current gain on temperature at various collector currents is discussed. The variation of β with temperature is slight at high currents over the range of 300 K-390 K. Furthermore, the current gain of the InGaP/GaAsSb/GaAs DHBT exhibits almost no dependence on the collector-base voltage VCB, demonstrating the nonblocking “type-II” band lineup at the base/collector heterojunction.

High-Quality Growth of GaInNAs for Application to Near-Infrared Laser Diodes

GaInNAs was proposed and created in 1995. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material with a bandgap energy that corresponds to near infrared. By combining GaInNAs with GaAs, an ideal band lineup for laser-diode application is achieved. This paper presents the reproducible growth of high-quality GaInNAs by molecular beam epitaxy. Examining the effect of nitrogen introduction and its correlation with impurity incorporation, we find that Al is unintentionally incorporated into the epitaxial layer even though the Al cell shutter is closed, followed by the concomitant incorporation of O and C. A gas-phase-scattering model can explain this phenomenon, suggesting that a large amount of N2 gas causes the scattering of residual Al atoms with occasional collisions resulting in the atoms being directed toward the substrate. Hence, the reduction of the sublimated Al beam during the growth period can suppress the incorporation of unintentional impurities, resulting in a highly pure epitaxial layer.

First principles calculations of the band offset at SrTiO3−TiO2 interfaces

We report on first principles calculations of the properties of the epitaxial SrTiO3−TiO2 (anatase) heterojunction, with an emphasis on the electronic band profile and lineup at the interface. The valence and conduction band offsets are calculated as a function of the number of anatase layers deposited onto the SrTiO3, as well as of the position of an oxygen vacancy with respect to the interface. It is shown that the presence of oxygen vacancies in the SrTiO3 is a way to effectively lower the barrier heights at the interface. Our results are in agreement with recent experiments reporting nearly zero band offset.

Effects of Transition Metal Dichalcogenide Molybdenum Disulfide Layer Formation in Copper–Zinc–Tin–Sulfur Solar Cells from Numerical Analysis

This study demonstrates the effects of transition metal dichalcogenide, MoS2 layer formation in between the copper–zinc–tin–sulphide (CZTS) absorber layer and Mo back contact from theoretical study and numerical modeling. The objective of this study is to elucidate the effects of n or p type MoS2 on the overall CZTS solar cell performance. Energy band line-up of Mo/MoS2/CZTS interface is analyzed to elucidate the interface properties. It is found out that p-MoS2 layer in CZTS solar cell induces the same adventitious effect as p-MoSe2 in CIGS solar cell. However, n-MoS2 layer has detrimental effect on the CZTS solar cell by creating an additional back contact diode with p-CZTS layer and an ohmic contact with Mo layer. Thickness, bandgap energy and carrier concentration of n-MoS2 all have been varied in the numerical simulation to observe its effects on the cell performance parameters. The results from numerical simulation show that MoS2 layer as thin as 50 nm is sufficient enough to induce adverse effect on the solar cell performance. This could be caused by the increase in series resistance of the solar cell as n-type MoS2 would inhibit hole current into Mo back contact due to the hole barrier between n-type MoS2 and Mo back contact. The increase in MoS2 bandgap and carrier concentration also results in detrimental effect to the performance of the cell mainly due to the possibility of electrons to drift towards the back contact and recombine.

Structural properties and band offset determination of p-channel mixed As/Sb type-II staggered gap tunnel field-effect transistor structure

The structural properties and band offset determination of p-channel staggered gap In0.7Ga0.3As/GaAs0.35Sb0.65 heterostructure tunnel field-effect transistor (TFET) grown by molecular beam epitaxy (MBE) were investigated. High resolution x-ray diffraction revealed that the active layers are strained with respect to “virtual substrate.” Dynamic secondary ion mass spectrometry confirmed an abrupt junction profile at the In0.7Ga0.3As/GaAs0.35Sb0.65 heterointerface and minimal level of intermixing between As and Sb atoms. The valence band offset of 0.37 ± 0.05 eV was extracted from x-ray photoelectron spectroscopy. A staggered band lineup was confirmed at the heterointerface with an effective tunneling barrier height of 0.13 eV. Thus, MBE-grown staggered gap In0.7Ga0.3As/GaAs0.35Sb0.65 TFET structures are a promising p-channel option to provide critical guidance for the future design of mixed As/Sb type-II based complementary logic and low power devices.

Role of InAs and GaAs terminated heterointerfaces at source/channel on the mixed As-Sb staggered gap tunnel field effect transistor structures grown by molecular beam epitaxy

The structural, morphological, defect properties, and OFF state leakage current mechanism of mixed As-Sb type-II staggered gap GaAs-like and InAs-like interface heterostructure tunnel field effect transistors (TFETs) grown on InP substrates using linearly graded InxAl1-xAs buffer by molecular beam epitaxy are investigated and compared. Symmetric relaxation of >90% and >75% in the two orthogonal 〈110〉 directions with minimal lattice tilt was observed for the terminal GaAs0.35Sb0.65 and In0.7Ga0.3As active layers of GaAs-like and InAs-like interface TFET structures, respectively, indicating that nearly equal numbers of α and β dislocations were formed during the relaxation process. Atomic force microscopy reveals extremely ordered crosshatch morphology and low root mean square roughness of ∼3.17 nm for the InAs-like interface TFET structure compared to the GaAs-like interface TFET structure of ∼4.46 nm at the same degree of lattice mismatch with respect to the InP substrates. The GaAs-like interface exhibited higher dislocation density, as observed by cross-sectional transmission electron microscopy, resulting in the elongation of reciprocal lattice point of In0.7Ga0.3As channel and drain layers in the reciprocal space maps, while the InAs-like interface creates a defect-free interface for the pseudomorphic growth of the In0.7Ga0.3As channel and drain layers with minimal elongation along the Δω direction. The impact of the structural differences between the two interface types on metamorphic TFET devices was demonstrated by comparing p+-i-n+ leakage current of identical TFET devices that were fabricated using GaAs-like and InAs-like interface TFET structures. Higher OFF state leakage current dominated by band-to-band tunneling process due to higher degree of defects and dislocations was observed in GaAs-like interface compared to InAs-like interface where type-II staggered band alignment was well maintained. Significantly lower OFF state leakage current dominated by the field enhanced Shockley-Read-Hall generation-recombination process at different temperatures was observed in InAs-like TFET structure. The fixed positive charge at the source/channel heterointerface influences the band lineup substantially with charge density greater than 1 × 1012/cm2 and the band alignment is converted from staggered gap to broken gap at ∼6 × 1012/cm2. Clearly, InAs-like interface TFET structure exhibited 4× lower OFF state leakage current, which is attributed primarily to the impact of the layer roughness, defect properties on the carrier recombination rate, suggesting great promise for metamorphic TFET devices for high-performance, and ultra-low power applications.

Band Lineup Issues Related with High-k/SiO2/Si Stack

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Mid-wave infrared HgCdTe nBn photodetector

A unipolar, barrier-integrated HgCdTe nBn photodetector with all n-type doping and a type-I band lineup is experimentally demonstrated. Planar mid-wave infrared (MWIR) nBn devices exhibit current-voltage (I-V) characteristics that are consistent with band inversion in reverse bias, indicating a barrier-influenced behavior. Dark current saturation is observed beyond a reverse bias of approximately −0.8 V. Bias-dependent photoresponse is observed in the mid-wave infrared with a cut-off wavelength around 5.7 μm. Numerical modeling based on experimental results predicts an internal peak quantum efficiency of approximately 66%.

Study of interfaces between phase‐change material Ge2Sb2Te5 and prevalent complementary metal‐oxide semiconductor materials by XPS

The energy band alignment between Ge2Sb2Te5 (GST) and common microelectronic materials such as dielectrics and metals was investigated using XPS. Energy band lineups of GST with various complementary metal‐oxide semiconductor dielectrics, that is, SiO2, HfO2, Ta2O5 and Si3N4, were thus determined, which can be used as for phase change memory device engineering and integration with complementary metal‐oxide semiconductor technology. Hole barrier height at metal/GST interface is reduced slightly by increasing the work function of the metal. Doping GST with nitrogen leads to an increase in hole barrier height at the interface between metals and nitrogen‐doped GST. Significant pinning of metal Fermi level towards the valence band energy of undoped and nitrogen‐doped GST was discovered. This leads to low hole barrier height and good ohmic contact formed between metals and GST. The hole barrier height between crystalline GST and various metals appears to increase as compared with the amorphous state. Copyright © 2012 John Wiley & Sons, Ltd.

Optical Dielectric Function Modeling and Electronic Band Lineup Estimation of Amorphous High-k LaGdO3 Films

Optical constants of pulsed laser ablated amorphous high-k LaGdO3 (a-LGO) thin films on quartz (0001) substrates were measured and analyzed. Refractive index (n ∼ 2.05–2.29) and extinction coefficient (k ∼ 0.004–0.017) were parameterized by analysis of the UV/Visible transmission spectra employing a Cauchy-Urbach dispersion model in the 300–750 nm spectral window and confirmed the applicability of this model in this wavelength region. The composition and chemistry of these dielectric films were studied by utilizing ATR-FTIR spectroscopy. The deduced pinning factor (S), 0.5 suggests that LGO/Si is an interacting interface. The parameter S was applied in the Cowley-Sze relation to estimate the interface trap density (Dit) to be 1.08 × 1013 states cm−2 eV−1. As grown LGO/p-Si heterojunction showed a downward Fermi-level (FL) pinning of 0.39 ± 0.02 eV. A complete Type I straddled band lineup of this gate dielectric/semiconductor heterostructure was determined by applying the charge neutrality level (CNL) model. The CNL of LGO was deduced to be 2.11 ± 0.05 eV above valence band maximum.

Electrical properties of atomic layer deposited aluminum oxide on gallium nitride

We report on our investigation of the electrical properties of metal/Al2O3/GaN metal-insulator-semiconductor capacitors. We determined the conduction band offset and interface charge density of the alumina/GaN interface by analyzing the capacitance-voltage characteristics of atomic layer deposited Al2O3 films on GaN substrates. The conduction band offset at the Al2O3/GaN interface was calculated to be 2.13 eV, in agreement with theoretical predications. A non-zero field of 0.93 MV/cm in the oxide under flat-band conditions in the GaN was inferred, which we attribute to a fixed net positive charge density of magnitude 4.60 × 1012 cm−2 at the Al2O3/GaN interface. We provide hypotheses to explain the origin of this charge by analyzing the energy band line-up.

Self-assembled growth of GaSb type-II nanorods aligned along quasiperiodic multiatomic steps on vicinal (111) B GaAs

Self-assembled GaSb nanorods are formed on a vicinal (111) B substrate by molecular beam epitaxy. An atomic force microscope study shows that GaSb nanorods are elongated and aligned along quasiperiodic GaAs multiatomic steps, where the average period of the GaSb nanorod array is nearly twice as large as that of the underlying GaAs multisteps. On average, GaSb nanorods are about 30 nm in width, 84 nm in length, and 2.5 nm in height. In photoluminescence (PL) measurements, the peaks of the GaSb nanorods and wetting layer are observed at 1.07 and 1.29 eV. The PL peaks shift toward higher energies with increasing excitation power density, suggesting that the band lineup exhibit a type-ІІ staggered alignment. In addition, we investigate the temperature dependence of the integral PL intensity of the GaSb nanorods.

Ab initio calculation of effective work functions for a TiN/HfO2/SiO2/Si transistor stack

Ab initio techniques are used to calculate the effective work function (Weff) of a TiN/HfO2/SiO2/Si stack representing a metal-oxide-semiconductor (MOS) transistor gate taking into account first order many body effects. The required band offsets were calculated at each interface varying its composition. Finally, the transitivity of local density approximation (LDA) calculated bulk band lineups were used and completed by many body perturbation theory (MBPT) bulk corrections for the terminating materials (Si and TiN) of the MOS stack. With these corrections the ab initio calculations predict a Weff of a TiN metal gate on HfO2 to be close to 5.0 eV.

Electronic structure of indium tin oxide/nanocrystalline TiO2 interfaces as used in dye-sensitized solar cell devices

Dye-sensitized solar cells are typically prepared under ambient conditions and contamination is inevitably introduced during the fabrication process. Hence, the electronic structure and charge injection properties of the indium tin oxide (ITO)/nanocrystalline titanium dioxide (TiO2) interface was studied by photoemission spectroscopy (PES) in the presence of environmental contaminants. The interface was formed by in situ multi-step electrospray thin film deposition of TiO2 nanoparticles onto ITO substrates cleaned prior in solvent under ambient conditions. In between deposition steps, the samples were characterized with PES yielding the band line-up at the ITO/TiO2 interface. In addition, the band line-up before and after annealing of the TiO2 layer was determined. The results of these measurements have in common that there are only small charge injection barriers between the valence bands of the oxides (∼0–0.2 eV), but more significant barriers for electron injection from TiO2 to ITO (∼0.3–0.5 eV), which has the potential to affect the performance of solar cell device structures. Another focus of the experiments was to investigate whether an earlier reported work function lowering measurement artifact occurring during ultraviolet photoemission spectroscopy (UPS) measurements on environmentally contaminated metal oxide surfaces would affect the characterization of ITO/TiO2 interfaces. For this purpose additional band line-up measurements using low intensity x-ray photoemission spectroscopy (LIXPS) were performed. LIXPS has been demonstrated to not cause the artifact, i.e., can be used to detect its occurrence. The results of these experiments show that the UV-induced work function reduction is not observed on the annealed interface, but that at contaminated interfaces care must be taken during data evaluation to take the artifact into account.

Ambiguity in the magnitude and direction of the derived interface dipole in lanthanum aluminate heterostructures: Implications and proposed solution

The electric field strength of interface dipoles cannot be measured directly but can be derived from measurable electronic properties such as the valence band offset (VBO) and electron affinity using photoemission techniques. In this study, we found that the measurements of these two values are affected by differential charging and surface contaminants, respectively. This can affect both the polarity and the strength of the derived interface dipole and therefore might have implications regarding the understanding of oxide-semiconductor band alignment. Our overall band lineup and derived interface dipole in lanthanum aluminate (LAO) heterostructures agree excellently with a popular charge-neutrality level model. This would not be possible without the accurate measurement of VBO and electron affinity in LAO heterostructures.

Band lineup in amorphous/crystalline silicon heterojunctions and the impact of hydrogen microstructure and topological disorder

We determine the band offsets in amorphous/crystalline silicon [a-Si:H/c-Si{111}] heterojunctions using combined data from photoelectron spectroscopy and surface photovoltage measurements on structures comprising a-Si:H layers with device-relevant thickness ($10 \mathrm{nm}$). Altering the a-Si:H hydrogen (H) content ${C}_{\mathrm{H}}$ by the choice of deposition conditions, we observe a systematic retreat of the a-Si:H valence band edge leading to an increase of the band gap and the valence band offset $\ensuremath{\Delta}{E}_{V}$ with ${C}_{\mathrm{H}}$ by about $13 \mathrm{meV}/\mathrm{at}.\mathrm{ }\mathrm{%}\mathrm{ }\mathrm{H}$. The discrepancy with the $30\ensuremath{-}40 \mathrm{meV}/\mathrm{at}.\mathrm{ }\mathrm{%}\mathrm{ }\mathrm{H}$ predicted by theory can be consistently explained by the compensating effect of enhanced topological disorder imposed by the increasing density of microvoids as revealed by an analysis of the H microstructure. Thus we highlight the necessity of explicitly including the details of the H configuration in a theoretical treatment of the a-Si:H/c-Si heterojunction.