Showing papers on "Band offset published in 1997"

Strain effects on the interface properties of nitride semiconductors

Abstract: the range from the deep ultraviolet to the visible. 2 Not surprisingly the potential technological importance of these materials has elicited the interest of a number of theoretical groups. 3‐7 In spite of this, the strained interfaces of these lattice mismatched materials have not been studied. We find that strain effects are significant, inducing changes of 20% to 40% in the value of the band offset and that these changes increase with decreasing in-plane lattice parameter. The AlN/GaN/InN interfaces are all of type I, while the Al 0.5Ga 0.5N/AlN zinc-blende ~001! interface is found to be of type II. Finally, we studied the GaN/AlN wurtzite interface, where qualitatively new features, namely pyroelectric and piezoelectric effects, appear due to the low symmetry of the wurtzite lattice. The standard ab initio plane-wave pseudopotential method 8‐10 was employed in the calculations. The energy cutoff for the plane-wave expansion was 50 Ry to ensure convergence of the nitrogen pseudopotential. We used the equivalent of ten k points for bulk and superlattice calculations in the zinc-blende structure 11 and six k points for calculations of the wurtzite structure. 12 Convergence both in the size of the plane-wave basis and in the number of special points has been carefully checked. The Perdew-Zunger parametrization 13 of the Ceperley-Alder form 14 of the exchange-correlation energy was used. For interface calculations, we employed 414 superlattices ~16 atoms! along the ~001! and ~0001! directions. Nonlocal, norm-conserving pseudopotentials 15‐17 were included using the KleinmanBylander approach. 18 For nitrogen, we used a neutral con

Oxygen vacancy with large lattice distortion as an origin of leakage currents in SiO/sub 2/

Abstract: The origin of electron traps, which induce leakage currents in SiO/sub 2/ films, is predicted to be an oxygen (O) vacancy from first-principles total-energy calculations. Once a neutral O vacancy traps holes, it spontaneously changes to a large distorted structure. The distorted O vacancy can capture electrons, and remains as an electron trap in SiO/sub 2/ films. These results give a microscopic explanation for a trap model that is empirically deduced from experiments. The energy level of the generated electron trap exists near the band offset between the conduction band of SiO/sub 2/ and that of Si. From these results, the electron trap originating from a hole trapped O vacancy can be considered as one origin of the so-called stress-induced leakage currents in SiO/sub 2/ films.

The importance of bandgap narrowing distribution between the conduction and valence bands in abrupt HBTs

Abstract: Abrupt heterojunction bipolar transistors (HBTs) show interfaces where discontinuities in the energy levels appear. Currents through these interfaces are controlled by tunneling and thermionic emission. The values of these currents depend on the form and height of the energy barriers, which are disturbed by the heavy doping effects on semiconductor energy band structure. In this work, the real bandgap narrowing is distributed between the conduction and valence bands according to Jain-Roulston model, and its effect on the base and collector currents of Si/SiGe and InP/InGaAs HBTs is analyzed. This analysis is carried out through a numerical model which combines the drift-diffusion transport in the bulk of transistor with the thermionic emission and tunneling at the base-emitter interface, and an empirically determined surface recombination current.

X-ray photoelectron spectroscopic evaluation of valence band offsets for strained Si1−xGex, Si1−yCy, and Si1−x−yGexCy on Si(001)

Abstract: The crucial parameters which determine the electrical and optical behavior of a heterojunction are the valence and conduction band offsets We demonstrate that carefully performed in situ x-ray photoelectron spectroscopic measurements allow the evaluation of valence band offsets in strained heteroepitaxial systems on Si(001) The result obtained for a Si075Ge025 alloy layer agrees very well with the known value, indicating the reliability of the used method For Si0977C0023 alloy layers tensily strained on Si(001), we could not find any significant valence band offset to Si The observed band gap narrowing is mainly dominated by a conduction band offset Ternary strained Si0727Ge025C0023 alloy layers show the same valence band offset to Si as the appropriate binary Si075Ge025 alloy layers, that is, the valence band offset is dominated by Ge effects

Local conduction band offset of GaSb self-assembled quantum dots on GaAs

Abstract: GaSb self-assembled quantum dots grown by molecular beam epitaxy on GaAs exhibit a staggered (type II) band lineup with a potential barrier in the conduction band. Traditional methods cannot measure this local band offset because of the small (∼50 nm) lateral dot size. The nanometer resolution of ballistic electron emission microscopy is exploited to image individual dots and measure a local band offset of 0.08±0.02 eV.

Band offsets at the GaInP/GaAs heterojunction

Abstract: We have measured current–voltage curves and the temperature dependence of the zero bias conductance for a p-type Be-doped GaInP/GaAs heterojunction grown by the molecular beam epitaxy method. We have determined the valence band offset ΔEν from both measurements and find it to be 310 meV within 5% of accuracy. Similarly, we find for an n-type Si-doped sample that the conduction band offset ΔEC is 95 meV. First-principles calculations have been carried out for the atomic and electronic structures of the interfaces. For the thermodynamically favored interfaces, the valence band offset is found not to be sensitive to atomic relaxations at the interface. The calculated values are in good agreement with the experiments.

Experimental determination of quantum dipoles at semiconductor heterojunctions prepared by van der Waals epitaxy: Linear correction term for the electron affinity rule

Abstract: We determined the band offset of eight layered heterocontacts consisting of combinations between SnS2, SnSe2, WSe2, MoS2, MoTe2, InSe, and GaSe. The comparison of offsets predicted by the electron affinity rule (EAR) revealed a systematic deviation. Due to the absence of structural dipoles in layered materials, this deviation corresponds to the magnitude of the quantum dipoles at the interface which allows the development of a quantum dipole correction term for the EAR. The corrected EAR is still a linear rule. The error margin of the corrected EAR lies well within the experimental error of photoemission spectroscopy experiments, thus proving the general applicability of linear rules for the determination of the band offset.

Determination of the band offset of GaInP-GaAs and AlInP-GaAs quantum wells by optical spectroscopy

Abstract: We report the determination of band offset ratios, using photoluminescence excitation measurements, for GaInP/GaAs and AlInP/GaAs quantum wells grown by gas-source molecular beam epitaxy. To reduce the uncertainty related to the intermixing layer at heterointerfaces, the residual group-V source evacuation time was optimized for abrupt GalnP/GaAs (AlInP/GaAs) interfaces. Based upon thickness and composition values determined by double-crystal x-ray diffraction simulation and cross-sectional transmission electron microscopy, the transition energies of GalnP/GaAs and AlInP/GaAs quantum wells were calculated using a three-band Kane model with varying band-offset ratios. The best fit of measured data to calculated transition energies suggests that the valence-band offset ratio (γ band discontinuity) was 0.63 ± 0.05 for GalnP/GaAs and 0.54 ± 0.05 for AlInP/GaAs heterostructures. This result showed good agreement with photoluminescence data, indicating that the value is independent of temperature.

On the conduction mechanism of hydrogenated nanocrystalline silicon films

Abstract: A hetero-quantum-dots (HQD) model for hydrogenated nanocystalline silicon films (nc-Si∶H) is proposed. The main contents of our model are: (i) the nanocrystalline grains and their amorphous conterparts have very different band gap and band structures. As a result, they form heterojunction like structures in the interface regions, where the band offset effects dramatically reduce the activation energy and the grains act like quantum dots; (ii) in the presence of an external field, the activated electrons in the quantum dots conduct via quantum tunneling through the interface barriers. By means of the HQD model, we have identified the conduction of nc-Si∶H as thermal assisted tunneling process. Our results show that there are two distinctive regimes for the conductivity of nc-Si∶H:(1) low temperature regime,where there is a simple activation energy ΔE; (2) high temperature regime, where ΔE is enhanced by the temperature effect of the electronic tunneling in the nanoscale particales. The theory is in good agreement with the experiments. Finally, we propose a perfect conductivity formula for nc-Si∶H films.

Oxidation of strained Si in a microwave electron cyclotron resonance plasma

Abstract: Electron cyclotron resonance plasma oxidation of strained Si on relaxed Si1−xGex buffer layers in O2 ambient at room temperature is reported. The electrical properties of grown oxide have been characterized and compared with thermally grown oxides using a metal-oxide semiconductor structure. At a low field, the accumulation of holes in the buried Si1−xGex layer, due to the type-II band offset, has been observed. The experimental results from thermally grown oxides have been compared with the simulation results obtained using a heterostructure Poisson solver.

Investigation of the GaAs/Si heterojunction band lineup with capacitance and current versus voltage measurements

Abstract: The GaAs/Si heterojunction band lineup has been investigated using capacitance versus voltage ( C-V ) and current versus voltage ( I-V ) measurements on n-GaAs/p-Si (n ≈ 10 16 cm −3 ) heterojunction diodes. Such an approach was possible because of the fabrication of diodes with near-ideal characteristics; ideality factors of less than 1.10 were deduced from forward bias I-V measurements. The heterojunction built-in voltage was determined by extrapolation from C −2 - V plots, and band offsets of Δ E C = 0.03 eV and Δ E V = 0.27 eV were calculated at 300 K for the conduction and valence bands, respectively. The heterojunction energy band diagram was constructed, I-V measurements at various temperatures in the vicinity of 300 K were used to extract the potential barrier to electron transport across the junction, and the results were consistent with those from C-V . The results also agree well with theoretical models.

Effect of Crystal Symmetry on Electronic Structures of CuInSe 2 and Related Compounds

Abstract: The first-principles band calculations by using the full-potential linearized augmented plane wave method have been used to understand the electronic structures of the chalcopyrite CuInSe2 and the defect stannite CuIn3Se5. It is found that the symmetrized wave function and the d-d mixing as well as the p-d hybridization play important roles in causing the anomalous large band gap of CuIn3Se5. The valence band offset between CuInSe2 and CuIn3Se5 is evaluated to be 0.52 eV. This value is in good agreement with the observed value.

Electrical and optical properties of p-SiC/n-GaN heterostructures

Abstract: We report on the optical, electrical and structural properties of GaN films heteroepitaxially grown by low pressure chemical vapor deposition on 6H-SiC substrates. We employed photoluminescence (PL), Hall effect measurements, scanning tunneling microscopy (STM) and X-ray analysis to determine the quality of our films. Heterojunction diodes were fabricated on p-type SiC and characterized by temperature dependent current–voltage and capacitance–voltage techniques. The results are interpreted within the thermionic emission model and the barrier found is attributed to the conduction band offset between 6H-SiC and wurtzite GaN. The diodes show electroluminescence of the donor-acceptor pair recombination type of 6H-SiC at room temperature. By analysis of the injection behavior we can interpret our data, determining the high valence band offset between 6H-SiC and α -GaN to 0.67 eV. This high valence band offset favors applications for hetero-bipolar transistors (HBT).

The electronic structure of alloys and related heterojunctions

Abstract: The dependences on composition of the bond lengths and energy bands of quaternary alloys are calculated on the basis of tight-binding theory under the virtual-crystal approximation. For a quaternary alloy lattice matched to InP, a type II staggered band line-up is observed when the Al composition is larger than 24%. At this point the conduction band discontinuity of heterostructures disappears. The effective electron mass associated with the conduction band minimum is estimated on the basis of the theory, and is in good agreement with the cyclotron resonance experimental results. The relative conduction band discontinuity is determined to be 72.2% for . The band offsets of the related heterojunctions are calculated, and compared with experimental and previous theoretical results.

Band Discontinuities of Perfectly Lattice‐Matched GaSb(n)/GaAlAsSb(p)/GaSb(p) Double Heterojunction

Abstract: Band discontinuities in the perfectly lattice-matched GaSb(n)/Ga 0.53 AL 0.47 As 0.04 Sb 0.96 (P)/GaSb(p) double heterostructure have been determined by capacitance-voltage studies. The measured conduction and valence band-offsets are ΔE c = 0.35 eV and ΔE v , = 0.24 eV, furnishing a fractional offset Q = ΔE c /(ΔE c + ΔE v ) equal to 0.59.

Band offsets at heavily strained III - V interfaces

Abstract: In consequence of the progress made in epitaxial techniques, strained layers are important parts of a few of today's optoelectronic devices. Much attention has thus been paid to the question of how the band line-up at a heterojunction is affected when one or both constituents are strained. This article reports experimental results on band offsets at heavily strained heterojunctions of arsenide compounds. The InAs/GaAs and InAs/AlAs heterojunction valence-band offsets are markedly strain dependent. Emphasis is placed on photoemission experiments and on the modifications of the classical core-level method due to the strain. The results are compared with theoretical data from both `model' theories and self consistent calculations.

Type II recombination and band offset determination in a tensile strained InGaAs quantum well

Abstract: Photoluminescence measurements under different excitation powers and time-resolved photoluminescence experiments were carried out at low temperature on tensile strained In0.3Ga0.7As quantum wells with InGaAs barriers lattice matched to InP. Evidence of a type II recombination is found between carriers confined in the tensile strained layer and in the lattice matched one. This study allows us to propose a precise determination of the light holes band offset in the In0.3Ga0.7As/In0.53Ga0.47As system.

Isotype heterojunctions with flat valence or conduction band

Abstract: We show that isotype heterojunctions with a perfectly flat majority or minority carrier band edge can be realized by modulation doping of arbitrary continuous alloy grading. The required impurity distribution is obtained analytically from the knowledge of the compositional grading and band structure parameters in the grading. This analytic relationship is exact for heterojunctions in which the grading fields are negligible in comparison with the atomic fields. We illustrate the design of flat valence-band heterojunctions for application in high-reflectivity low-resistance distributed Bragg reflectors for vertical-cavity lasers. The presented formalism enables the design of isotype heterojunctions with arbitrary band-edge profiles.

Lateral superlattice solar cells

Abstract: A novel structure which comprises of a lateral superlattice as the active layer of a solar cell is proposed If the alternating regions A and B of a lateral superlattice ABABAB… are chosen to have a Type-II band offset, it is shown that the performance of the active absorbing region of the solar cell is optimized In essence, the Type-II lateral superlattice region can satisfy the material requirements for an ideal solar cells active absorbing region, ie simultaneously having a very high transition probability for photogeneration and a very long minority carrier recombination lifetime

First-principles calculations of band offsets of heterostructures

Abstract: A first-principles pseudopotential method combined with the virtual-crystal approximation (VCA) is used to calculate band offsets of heterostructures. It was found that both the valence and conduction band offsets vary linearly with the alloy composition. Our results are in good agreement with the experimental data.

Compositionally Dependent Band Offsets In Aln/AlxGa1-xN Heterojunctions Measured By Using X-Ray Photoelectron Spectroscopy

Abstract: Although GaN has been extensively studied for applications in both light emitting and high power devices, the AlN/GaN valence band offset remains an area of contention. Values quoted in the literature range from 0.8eV (Martin)[1] to 1.36eV (Waldrop)[2]. This paper details an investigation of the AIN/AlxGa1-xN band offset as a function of alloy composition. We find an AlN/AlxGa1-xN valence band offset that is nearly linear with Al content and an end point offset for AlN/GaN of 1.36 ± 0.1 eV. Samples were grown using radio frequency plasma assisted molecular beam epitaxy and characterized with x-ray photoelectron spectroscopy(XPS). Core-level and valence-band XPS data for AIN (0001) and AlxGa1-xN (0001) samples were analyzed to determine core-level to valence band maximum (VBM) energy differences. In addition, oxygen contamination effects were tracked in an effort to improve accuracy. Energy separations of core levels were obtained from AlN/AlxGa1-xN(0001) heterojunctions. From this and the core-level to valence band maximum separations of the bulk materials, valence band offsets were calculated.