Energy Band Offsets Research Articles

Bonding Constraints at Interfaces Between Crystalline Si and Stacked Gate Dielectrics

ABSTRACTThis paper discusses chemical bonding effects at Si-dielectric interfaces that are important in the implementation of alternative gate dielectrics including: i) the character of interfacial bonds, either isovalent with bond and nuclear charge balanced as in Si-SiO2, or heterovalent, with an inherent mismatch between bond and nuclear charge, ii) mechanical bonding constraints related to the average number of bonds/atom, Nay, and iii) band offset energies that are reduced in transition metal oxides due to the d-state origins of the conduction band states. Applications are made to specific classes of dielectric materials including i) nitrides and oxide/nitride stacks and ii) alternative high-K gate materials.

Electrical characterization of GaN/SiC n-p heterojunction diodes

GaN/SiC heterojunction diodes have been fabricated and characterized. Epitaxial n-type GaN films were grown using metalorganic chemical vapor deposition (MOCVD) and electron cyclotron resonance assisted molecular beam epitaxy (ECR-MBE) on p-type Si-face 6H-SiC wafers. The I–V characteristics have diode ideality factors and saturation currents as low as 1.2 and 10−32 A/cm2, respectively. The built-in potential in the MOCVD- and ECR-MBE-grown n-p heterojunctions was determined from capacitance–voltage measurements at 2.90±0.08 eV and 2.82±0.08 eV, respectively. From the built-in potential the energy band offsets for GaN/SiC heterostructures are determined at ΔEC=0.11±0.10 eV and ΔEV=0.48±0.10 eV.

Measurement of band offsets in Si/Si1−xGex and Si/Si1−x−yGexCy heterojunctions

Realization of group IV heterostructure devices requires the accurate measurement of the energy band offsets in Si/Si1−xGex and Si/Si1−x−yGexCy heterojunctions. Using admittance spectroscopy, we have measured valence-band offsets in Si/Si1−xGex heterostructures and conduction-band and valence-band offsets in Si/Si1−x−yGexCy heterostructures grown by solid-source molecular-beam epitaxy. Measured Si/Si1−xGex valence-band offsets were in excellent agreement with previously reported values. For Si/Si1−x−yGexCy our measurements yielded a conduction-band offset of 100±11 meV for a n-type Si/Si0.82Ge0.169C0.011 heterojunction and valence-band offsets of 118±12 meV for a p-type Si/Si0.79Ge0.206C0.004 heterojunction and 223±20 meV for a p-type Si/Si0.595Ge0.394C0.011 heterojunction. Comparison of our measured band offsets with previously reported measurements of energy band gaps in Si1−x−yGexCy and Si1−yCy alloy layers indicates that the band alignment is type I for the compositions we have studied and that our measured band offsets are in quantitative agreement with these previously reported results.

Energy band offsets of SiGeC heterojunctions

We report on conduction and valence band offsets in thick, relaxed Ge-rich Si 1− x− y Ge x C y alloys grown by solid source molecular beam epitaxy on (100) Si substrates. X-ray photoemission spectroscopy was used to measure the valence band energies with respect to atomic core levels, and showed that C increased the valence band maximum of SiGeC by +48 meV/%C. The bandgap energies were obtained from optical absorption, and were combined with the valence band offsets to yield the conduction band offsets. For SiGeC/Si heterojunctions, the offsets were typically 0.6 eV for the valence band and 0.38 eV for the conduction band, with a staggered type II alignment. These offsets can provide significant electron and hole confinement for device applications.

ZnSe-ZnCdSe single quantum wells: dispersion relations and absorption processes

We have investigated theoretically the optical properties of strained (001) ZnSe-(Zn, Cd)Se single quantum wells with a low cadmium content (~10%). Due to the band offset and strain energy, the electron-to-light-hole transitions are marginally type I. We compute the dispersion relations and joint density of states for a typical structure away from Λ and show that the degeneracy can occur between lh 1 and hh 3 sub-bands in a large area of the Brillouin zone. This can cancel the selection rules for interband processes.

A Statistical Thermodynamic Model for Studying Energy Band Structure of Semiconductor Heterostructures

Advances in the science and technology of device quality semiconductor microstructures opened new directions in the making of heterojunction devices which are much faster than the conventional silicon devices. In order to fully appreciate the potantial electronic and optoelectronic applications of heterostructures there are material issues that must be fully investigated. One of the most important issues is the understanding and determination of the discontinuity in the energy band structure of the heterostructure at its interface and has received world wide interest among the device physicists and engineers over the three decades [1]. These energy band offsets affect various device properties such as the injection efficiency of heterojunction bipolar transistors (HBTs) and the carrier confinement in the recently developed modulation doped field effect transistors (MODFETs)[2]. A statistical thermodynamic model is presented to study the band offsets and temperature and strain induced nonlinearities in the band structure of lattice matched and pseudomorphic heterostructures. The model uses an average internal reference energy level (obtained using the universal tight bonding theory [3]), at which the bonding (valence-like) and antibonding (conduction-like) states are equal, to define the thermodynamic equilibrium for a heterostructure at any temperature and strain. Use of such internal reference level allows one to incorporate the effects of third nearest neighboor interactions on the band offset calculations. The temperature and strain induced nonlinear effects on the band structure is calculated by using two universal statistical thermodynamic postulates [4]: (i) The free electrons and holes are electrically charged weakly interacting quasi-chemical particles; (ii) The electron-hole pairs are generated by the charge transfer from the bonding (antibonding) states toantibonding (bonding) states in tetrahedrally coordinated semiconductors and semimetals. It is shown that there is a good agreement between the model predictions and experimental data for the band offsets and temperature and pressure induced nonlinearities in the energy bandgaps of lattice matched AlGaAs/GaAs and HgCdTe/CdTe heterostructures for over wide range of temperatures and pressures. It is also shown that in the HgTe/CdTe system, the applied pressure moves up the energy of \Gamma_{6c} antibonding states above the energy of the \Gamma_{8v} bonding states and HgTe becomes a diamond type semiconductor above certain pressure with finite bandgap. Above the transition pressure of 25 Kbar, the band alignment at the HgTe/CdTe interface transforms from type III to conventional type I.

Measurements of the energy band offsets of [formula omitted] and [formula omitted] heterojunctions

Discontinuities in the energies of the conduction and valence bands at semiconductor heterojunctions are important parameters for device design. We describe experiments using X-ray photoelectron spectroscopy with measurements of valence-band energies with respect to core-levels of metastable, coherently strained Si 1− x Ge x alloy layers and of thick Ge 1− y C y alloy layers. For strained Si 1− x Ge x alloys on Si, we have found that the valence band offset increased with the Ge fraction x with most of the offset in the valence band. We obtained a valence band offset of 0.22 eV for x=0.23, in good agreement with theoretical calculations. For Ge 1− y C y alloys, we found very little shift in the valence band energies with the C fraction y. Since the optical bandgap of GeC increased with the C fraction y, most of the offset for Ge 1-yC y Ge heterojunction was in the conduction band. Based on the measurements of the energy band offsets of Si 1−xGe x Si , we infer that the major portion of bandgrap discontinuity of Ge 1− y C y on Si is in the valence band. Ge 1− y C y alloys are new matastable materials that open up a new region for group IV heterostructures.

Investigation of the role of ZnSe films on GaAs using acoustoelectric voltage spectroscopy

The electrical properties of the ZnSe/GaAs heterostructure have been investigated using the acoustoelectric voltage spectroscopy technique, and in particular, the role of the high-resistivity ZnSe on the surface passivation of the GaAs substrate has been evaluated. From the transverse acoustoelectric voltage (TAV) spectra, the carrier type and concentration, energy band offsets, and the energy levels of various trap states at the heterostructure interface have been found. The effect of heterostructure epitaxial layer on the surface properties of GaAs has been studied by comparing the normalized changes in TAV amplitude for samples of various epitaxial layers and different thicknesses. From all these measurements, surface recombination velocities (S) have been evaluated. For the pseudomorphic ZnSe films (thickness ≤0.15 μm) on GaAs, a reduction in S has been found. As the thickness of the ZnSe film was increased, the presence of a large number of interface states due to the introduction of misfit dislocations was detected using TAV measurements.

Determining energy-band offsets in quantum wells using only spectroscopic data

We have developed an experimental technique for accurately determining energy-band offsets in semiconductor quantum wells (QW) based on the fact that the magnitude of the ground-state light-hole (LH) energy is more sensitive to the depth of the valence-band well than is the ground-state heavy-hole (HH) energy. In a lattice-matched, unstrained QW system, this behavior causes the energy difference between the LH and HH excitons to go through a maximum as the well width, Lz, increases from zero. Calculations show that the position, and more importantly, the magnitude of this maximum is a sensitive function of the valence-band offset, Qv, the parameter which determines the depth of the valence-band well. By using Qv, or alternatively Qc=1−Qv, as an adjustable parameter and fitting experimentally measured LH-HH splittings as a function of Lz, an accurate determination of band offsets can be derived. However, we further reduce the experimental uncertainty by plotting LH−HH as a function of HH energy (which is, itself, a function of Lz) rather than Lz, since then all of the relevant data values can be precisely determined from absorption spectroscopy alone. Using this technique, we have derived the conduction-band offsets for several material systems, including lattice-mismatched systems and, where a consensus has developed, have obtained values in good agreement with other determinations.

Optical Properties of (InAs)1/(GaAs)m Strained-Layer Superlattices

We report the photoluminescence (PL) and absorption (ABS) properties of (InAs)1/(GaAs)m strained-layer superlattices with m=7, 10, 20, and 30 monolayers grown on a (001) GaAs substrate by molecular-beam epitaxy. We have clearly observed the ABS bands of the heavy-hole and light-hole excitons associated with the first subbands and the PL band of the heavy-hole exciton. It is found that the exciton energies increase as the GaAs-layer thickness increases. The GaAs-layer-thickness dependence of the Exciton energy is reasonably explained by the calculated results on the basis of an effective-mass approximation including strain effects. In addition, the band-offset energy of the GaAs/InAs heterojunction is discussed.

A Purely Spectroscopic Technique for Determining Energy Band Offsets in Quantum Wells

ABSTRACTWe have developed a novel experimental technique for accurately determining band offsets in semiconductor quantum wells (QW). It is based on the fact that the ground state heavy- hole (HH) band energy is more sensitive to the depth of the valence band well than the light-hole (LH) band energy. Further, it is well known that as a function of the well width, Lz, the energy difference between the LH and HH excitons in a lattice matched, unstrained QW system experiences a maximum. Calculations show that the position, and more importantly, the magnitude of this maximum is a sensitive function of the valence band offset, Qy, which determines the depth of the valence band well. By fitting experimentally measured LH-HH splittings as a function of Lz, an accurate determination of band offsets can be derived. We further reduce the experimental uncertainty by plotting LH-HH as a function of HH energy (which is a function of Lz ) rather than Lz itself, since then all of the relevant parameters can be precisely determined from absorption spectroscopy alone. Using this technique, we have derived the conduction band offsets for several material systems and, where a consensus has developed, have obtained values in good agreement with other determinations.

Determination of Γ electron and light hole effective masses in AlxGa1−xAs on the basis of energy gaps, band-gap offsets, and energy levels in AlxGa1−xAs/GaAs quantum wells

Experimental data of energy gaps, band offsets, and energy levels in AlxGa1−xAs/GaAs quantum wells are utilized for the determination of Γ electron and light hole effective masses in AlxGa1−xAs compounds on the basis of the author’s relations between these quantities. The temperature dependences of electron and light hole effective masses in GaAs are also obtained.

Electrical characteristics and energy-band offsets in n- InAs0.89Sb0.11/ n- GaSb heterojunctions grown by the liquid phase epitaxy technique

Liquid-Phase-Epitaxy (L.P.E.) grown heterojunctions of n-type InAs 0.89Sb 0.11 lattice-matched to n-type Te-doped GaSb(100) substrates, were studied by capacitance-voltage measurements at T = 77 K. By using the electric displacement continuity, it is shown that the band-lineup of this material is of the broken type with conduction-band and valence-band offsets ΔE c = 0.82 eV and ΔE v = 0.36. The electron affinity of InAs 0.89Sb 0.11 alloy is also determined to be X = 4.87 eV.

Determination of band offsets using the photovoltaic effect: Application to the PbS/PbSe heterostructure

The energy-band offsets at a semiconductor heterointerface can be determined by measuring the temperature and wavelength dependence of the photovoltaic response. Results for the PbS/PbSe structure show a type I band alignment with a conduction-band offset ΔEc of ∼48 meV. The method is sensitive to small band offsets and can be applied to the study of other semiconductor systems, especially narrow band-gap materials.

Light scattering determination of band offsets in [formula omitted] and [formula omitted] quantum wells: A comparative study

We determine the energy band offsets in GaAs Al xGa 1−xAs and GaSb Al xGa 1−xSb quantum wells with a new light-scattering method. We measure electronic intersubband transitions in the conduction band of the quantum wells, and use this information to deduce the conduction band discontinuity ΔE c. The light scattering technique is very accurate because ΔE c can be determined regardless of the valence band structure or exciton binding energies. Our results indicate that the valence band offset of the two systems are very similar, suggesting the validity of a weaker form of the Common Anion Rule.

Unified model of ideal metal–semiconductor contacts and semiconductor heterojunctions

A many-electron model is proposed to describe the electronic structure of metal–semiconductor interfaces and semiconductor heterojunctions. The model is utilized to calculate the self-consistent one-electron potential in the vicinity of the interface. The boundary conditions ensuring thermal, mechanical, and electron-transfer equilibrium are imposed explicitly. Numerical calculations predict the validity of Schottky’s phenomenological boundary condition for metal–semiconductor contacts, the applicability of the electron affinity rule for semiconductor heterojunctions, and systematic correlations between Schottky barrier heights and heterojunction energy band offsets. The latter three results emerge as consequences of the rigorous and complete imposition of thermodynamic equilibrium boundary conditions and of the use of a model of the ion-core charge density in which atomic reconstructions at the interface relative to the vacuum surfaces are not considered explicitly.

On the determination of the energy band offsets in Hg1−xCdxTe heterojunctions

The energy band offsets play a critical role in the electrical properties of heterojunctions. We raise here serious questions regarding the precision and accuracy of the measured and calculated energy band offsets in HgCdTe heterostructures. We argue that the Anderson model prescription for the offsets is not useful and is not correct for abrupt heterojunctions when narrow band gap semiconductors are the constituents of the heterostructure. New values for the photoionization energy of Hg1−xCdxTe are given and the reliability of these values for determining the offset is discussed.