Band Gap Discontinuity Research Articles

Simulations of Ge+ and C+ Implantations to form SiGe/Si HBT and Characterization of SiGe and SiGeC Diodes

ABSTRACTSimulations of Ge+ and C+ implantations in Si were performed to study bandgap grading in the SiGeC/Si heterojunction bipolar transistor (HBT). Although no bandgap discontinuity was observed at the base-emitter junction, it was found that a wide-bandgap emitter and a narrow-bandgap base with proper bandgap grading were obtainable with implantation. Electrical characterization of SiGe and SiGeC diodes formed by Ge+ and C+ implantations in Si was carried out. Current-voltage (I-V) measurement results confirm that carbon doping improves the crystalline quality of the germanium-implanted layer. On the other hand, capacitance-voltage (C-V) measurements indicate that both germanium and carbon implantations result in considerable dopant deactivation.

Optical Investigation of CdTe/CdZnTe Heterostructures

We review in this paper some of the optical studies performed on CdTe/CdZnTe heterostructures which demonstrate interesting specific band structure effects in this strain-layer system. Due to the order of magnitude of the three physical parameters which determine the optical properties of such structure (bandgap discontinuity, lattice mismatch induced strain, electron-hole interaction), we observe (i) two types of quantization in CdTe/CdZnTe single quantum well: the first one, for well thickness smaller than 200 Å, is the usual description of a 2D-exciton, whereas the second one, for wider wells, corresponds to the quantization of the motion of the exciton centre-of-mass. (ii) The mixed type nature of the CdTe/CdZnTe superlattice: type I for heavy-hole excitons and type II for light-hole excitons with the possibility of changing the relative energy positions of the type I and type II transitions with the built-in strains. (iii) A dominant piezoelectric effect in the (111) oriented strain-layer superlattices which generates an internal electric field of the order of 104 V/cm and modifies the optical properties of these Te-based heterostructures.

Magnetic tuning of the exciton binding energy and band-offsets determination in a CdTe/Cd1-xMnxTe superlattice

Magnetic tuning of the electronic properties of heterostructures incorporating diluted magnetic semiconductors offers various opportunities which are well illustrated in this optical study of a Cd1-xMnxTe superlattice (x = 6.6%). Due to the exceptional quality of this structure grown on a InSb substrate both the 1S and 2S states of the heavy hole exciton are observed, which allows the determination of the valence band discontinuity at the interface and offers the unique opportunity to study the variation of the exciton binding energy with the quantum well depth. The partition ratio of the band gap discontinuity at the interface between the conduction and valence band is found to be close to 60/40 ie smaller than earlier values obtained from quantum wells with larger values of x but in good agreement with a theoretical estimation.

Identification of the Fe acceptor level in Ga0.47In0.53As

The authors give an unambiguous identification of the Fe acceptor level in Ga0.47In0.53As and a coherent description of its electrical and optical properties by photoluminescence, thermal and photocapacitance spectroscopies. Deep-level transient spectroscopy measurements show a deep trap at Ec-0.34 eV which they identify as due to the single acceptor level Fe3+/Fe2+. This is confirmed by the observation of the internal luminescence transition of Fe2+(5T2 to 5E) detected at 0.35 eV and by a well pronounced resonance peak at 0.34 eV due to the transition to the 5T2 excited state in the sigma n0(h nu ) spectrum of the photoionisation cross section of the Ec-0.34 eV level. Finally they discuss the validity of the empirical rule connecting transition metal levels to predict the band-gap discontinuity in the InP/InGaAs systems using the known positions of the Fe acceptor level in InP and in Ga0.47In0.53As.

Formula omitted] single quantum well structure analysed by reflectance and photoluminescence

A molecular-beam epitaxial GaAs Ga 0.73 Al 0.27 As quantum well structure has been observed and analysed at room temperature using normal reflectance and photoluminescence measurements. The reflectance spectrum, in the energy range 1.3 to 5.5 eV, displays sharp well-defined features due to near-band-gap excitonic effects, which are superimposed on the classical thin film multilayer structure. Transition energies around the Γ point in the Brillouin zone are consistent with photoluminescence measurements and a square well calculation using a conduction band energy gap discontinuity equal to 72% of the total band gap discontinuity.

Determination of the resonant-state energy of an In 0.53Ga 0.47As/AlAs pseudomorphic resonant tunneling barrier grown by MBE

The lowest resonant-state energy, E 0, of an In 0.53Ga 0.47As/In 1− x Al x As (0.48≤ x≤1) pseudomorphic resonant tunneling barrier structure was determined by the temperature dependence of thermionic emission current through this resonant state. Using this value of E 0, the conduction band discontinuity, Δ E c, of the heterointerface was determined by a calculation accounting for both the band nonparabolicity and the strain. Δ E c increased linearly with the A1As mole fraction, x. For an In 0.53Ga 0.47As/A1As resonant tunneling barrier (RTB), we have determined the value of Δ E c to be about 60% of the energy band gap discontinuity.

Structural and optical studies on molecular beam epitaxially grown (InAs) m(AlAs) m and (InAs) m(GaAs) m short-period strained layer superlattices

Binary consisting (InAs) m (AlAs) m and (InAs) m (GaAs) m short-period strained layer superlattice InP substrates were grown and their structural and optical properties were examined. Mirror-like surface morphologies were obtained as long as their total thicknesses were less than a critical value. X-ray diffraction measurements showed the formation of atomically ordered periodic superstructure. Photoluminescence spectra measured at 77 K revealed good cry stalline quality in spite of about 7% lattice mismatch between the binary layers. The peak energy changes greatly on (InAs) m (IlAs) m according to the layer thickness, while it varies by a small amount on (InAs) m (GaAs) m structures, which reflects to the layer band gap discontinuities between InAs and AlAs. These structures can be substituted for InAlAs and InGaAs alloys.

Quantum-confined Stark effects on a GaAs cluster embedded in AlxGa

Quantum-confined Stark effects on the electronic structure of a GaAs microcluster with 63.5-A\r{} (120-a.u.) radius embedded in the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As bulk crystal were investigated, within the effective-mass approximation, based on the Weyl-Titchmarsh theory. This approach provides both point and continuous spectral functions for the problem. Numerical calculations were performed on a continuous energy spectrum under an electric field for single-particle states of electron and heavy hole, and for an exciton state, using a 57:43 split of the band-gap discontinuity between the conduction and valence bands. The spherical-heterojunction potential barrier effects on the exciton state are discussed with regard to both exciton binding energy and exciton stability. The exciton Coulomb binding energy for the confined exciton in a cluster was found to be much larger than that in a one-dimensional superlattice system. Also, the Coulomb interaction brings about still longer exciton lifetimes compared with the external electrostatic field. Field ionization of the exciton is inhibited by the strong confined effects caused by the existence of the spherical quantum well.

Band-gap discontinuities for Ge/ZnSe(100) and Si/ZnSe(100): A photoemission study.

Ultraviolet- and x-ray-photoemission and low-energy electron-diffraction studies during room-temperature growth of Ge/ZnSe(100) and Si/ZnSe(100) heterojunctions show that these interfaces are abrupt but disordered. Heterojunction valence-band discontinuities were found to be 1.65\ifmmode\pm\else\textpm\fi{}0.1 and 1.55\ifmmode\pm\else\textpm\fi{}0.1 eV for Ge/ZnSe and Si/ZnSe, respectively, as determined from the valence-band maxima for ZnSe(100) and the growing overlayers of Ge or Si, with corrections made for band bending. Complementary calculations of the difference in core-level binding-energy positions for the substrate and the overlayer confirmed these results. The final Fermi-level-pinning positions were 1.75\ifmmode\pm\else\textpm\fi{}0.1 and 1.85\ifmmode\pm\else\textpm\fi{}0.1 eV above the valence-band maximum of ZnSe.

Thin superlattices and band-gap discontinuities: The (110) diamond-boron nitride interface.

The recent progress in chemical-vapor deposition of diamond on a variety of substrates suggests that diamond may soon be incorporated into electronic devices. As a first step in the investigation of interfaces involving diamond, the linear augmented-plane-wave method is applied to calculate the electronic structure and band-edge discontinuities at the (110) interface of diamond and BN. The use of core levels in identifying the valence-band discontinuity is investigated. By studying N-layer/N-layer C/BN (110) superlattices with N=1, 3, and 5 it is found that three layers of each material are sufficient to give a central layer which has a core-level position nearly representative of the bulk. The calculated valence-band discontinuity is 1.42\ifmmode\pm\else\textpm\fi{}0.04 eV, which (using the experimental band gaps of 5.5 eV for diamond and \ensuremath{\sim}6.2 for BN) leaves a conduction-band discontinuity of 0.7 eV. Due to the heteropolar potential in BN, the lowest conduction band at the interface is the X-point band in BN in spite of the smaller band gap of diamond. This study also shows that the average interstitial potentials in diamond and BN are nearly identical, which suggests another possibility for estimating band discontinuities from bulk quantities alone. The effects of strain at a diamond-on-BN heterojunction are discussed briefly.

The theory of resonant Raman scattering: finite quantum wells in an electric field

The authors present a theoretical analysis of the effects of an electric field on the subbands in a quantum well with the use of a finite-potential-barrier model. Through the wavefunctions, they have determined the enhancement in the Raman cross section for photon energies near the electronic resonance in GaAs-Ga1-xAlxAs superlattices. Both the energy positions and the general shape of the resonant curves agree with those derived theoretically based on the two-dimensionality of the quantum states in such superlattices. They find distinct physical effects for fields parallel and perpendicular to the quantum-well layers. The calculations are performed using the 57:43 split of band-gap discontinuity between conduction and valence bands. They find good agreement with available experiments with no fitted parameters.

Electronic structure of Fibonacci copolymers and superlattices

The electronic spectrum of quasi-one-dimensional Fibonacci copolymers has been calculated numerically for a realistic valence band-conduction band situation. It is shown that the density-of-states of a n-th generation Fibonacci quantum well structure can be interpreted as a superposition of the spectra of two periodic superlattices of F n−2 ( A 2 B) and F n−3 ( AB) segments. The ratio of the peak intensity in the band gap discontinuity regions is given by the golden mean τ. Absorption spectrum and negative conductance measurements are proposed.

Photocollection efficiency of GaAs/AlAs/GaAs p +-i-n and n +-i-p photodiodes

Measurements of the photocollection efficiency of GaAs/AlAs/GaAs diodes are reported. The quantum efficiency measurements demonstrate that the conduction band discontinuity between AlAs and GaAs is small — most of the band gap discontinuity is accommodated by the valence band. These results suggest that series resistance may be a problem in p-type heteroface cells (owing to the large majority carrier barrier); n-type heterofaces should display low series resistance due to the isotype heterojunction, even when the junction is not intentionally graded.

Single and coupled double-well GaAs/AlGaAs and energy-dependent light-hole mass

Photoluminescence spectra of single and coupled double wells were reanalyzed assuming a 62% partition of the band-gap discontinuity to the conduction band and an energy-dependent light-hole effective mass. The photoluminescence peak positions of the single well are little affected when 62% instead of 85% is adopted for the partition value. The photoluminescence spectra of the coupled wells consist of a main peak and its satellites. Energy differences between the peak and its satellites are in good agreement with the calculated values when they are ascribed to transitions between doublet states in the conduction-band well and doublet states in the valence-band well, assuming a 62% partition and reasonable energy dependence of the light-hole mass. This coupled double well offers a first evidence for the energy-dependent light-hole mass.

Electrical and optical properties of parabolic semiconducting quantum wells

We investigate theoretically the optical and electrical properties of parabolic semiconducting quantum well structures. In our calculations, we assume that the confinement of the carriers is in an infinite parabolic well. We show that the carrier mobility in the plane perpendicular to the direction of confinements is directly proportional to the harmonic oscillator length λ whose value depends upon the partitioning of the band gap discontinuity between the conduction and valence bands. We have also calculated the linewidth for intra-subband resonances which should occur for electromagnetic radiation polarized in the direction of carrier confinement and show that the linewidth is inversely proportional to λ and directly proportional to the temperature when the linewidth is dominated by acoustic phonon scattering. The absorption coefficient for interband optical transitions shows equally spaced steps as a function of photon energy where the value of the spacing between adjacent steps depends upon the partitioning of the band gap discontinuity. Carrier freeze-out in the intrinsic conduction occurs due to the presence of zero point energies in the conduction and valence bands arising from the carrier confinement. These zero point energies also are found to depend upon the partitioning of the band gap discontinuities. Therefore, information about the partitioning of the energy band gap discontinuity between the conduction and valence bands can be obtained by measuring these various optical and electrical transport properties of a parabolic quantum well semiconducting structure under those conditions when the model of an infinite parabolic well approximates the real system.

Electrical and optical properties of parabolic semiconducting quantum wells

Abstract We investigate theoretically the optical and electrical properties of parabolic semiconducting quantum well structures. In our calculations, we assume that the confinement of the carriers is in an infinite parabolic well. We show that the carrier mobility in the plane perpendicular to the direction of confinements is directly proportional to the harmonic oscillator length λ whose value depends upon the partitioning of the band gap discontinuity between the conduction and valence bands. We have also calculated the linewidth for intra-subband resonances which should occur for electromagnetic radiation polarized in the direction of carrier confinement and show that the linewidth is inversely proportional to λ and directly proportional to the temperature when the linewidth is dominated by acoustic phonon scattering. The absorption coefficient for interband optical transitions shows equally spaced steps as a function of photon energy where the value of the spacing between adjacent steps depends upon the partitioning of the band gap discontinuity. Carrier freeze-out in the intrinsic conduction occurs due to the presence of zero point energies in the conduction and valence bands arising from the carrier confinement. These zero point energies also are found to depend upon the partitioning of the band gap discontinuities. Therefore, information about the partitioning of the energy band gap discontinuity between the conduction and valence bands can be obtained by measuring these various optical and electrical transport properties of a parabolic quantum well semiconducting structure under those conditions when the model of an infinite parabolic well approximates the real system.

Determination of the conduction-band discontinuity between In0.53Ga0.47As/In0.52Al0.48As using n+-InGaAs/InAlAs/n−-InGaAs capacitors

The conduction-band discontinuity (ΔEc) between In0.53Ga0.47As and In0.52Al0.48As is determined by current-voltage measurements on n+-InGaAs/InAlAs/n−-InGaAs capacitors. Current is found to be dominated by thermionic emission conduction down to 180 K. Barrier heights are determined from the slope of ln(J/T2) vs 1/T where good straight-line fits are obtained in the thermionic emission range. After correcting for the Fermi level a conduction-band discontinuity of 0.51±0.04 eV is obtained representing 70% of the total band-gap discontinuity. Furthermore, capacitance-voltage measurements are fit to classical capacitance-voltage theory and show that no charge is present in the InAlAs insulating layer.

Determination of band-gap discontinuity in AlGaAs/GaAs system by quantum oscillations of photoluminescence intensity

An anomalous excitation intensity dependence of photoluminescence (PL) in high-quality Al 0.3Ga 0.7As/GaAs quantum wells was observed at low temperatures. The PL intensity I PL was seen to increase as I PL ∝ I EX m with increasing excitation intensity I EX and the power m varied periodically between 1 and 2, depending on the well width. This phenomenon can be understood in terms of the carrier trapping efficiency under resonant and off-resonant conditions. From the theoretical analysis of the effect, the conduction band off-set can be estimated to be 59–70%, considerably smaller than the value of Dingle et al.

Determination of band-gap discontinuity in AlGaAs/GaAs system by quantum oscillations of photoluminescence intensity

Abstract An anomalous excitation intensity dependence of photoluminescence (PL) in high-quality Al0.3Ga0.7As/GaAs quantum wells was observed at low temperatures. The PL intensity IPL was seen to increase as IPL ∝ IEXm with increasing excitation intensity IEX and the power m varied periodically between 1 and 2, depending on the well width. This phenomenon can be understood in terms of the carrier trapping efficiency under resonant and off-resonant conditions. From the theoretical analysis of the effect, the conduction band off-set can be estimated to be 59–70%, considerably smaller than the value of Dingle et al.

Optical properties of very thin GaInAs(P)/InP quantum wells grown by gas source molecular beam epitaxy

Single quantum wells of GaInAs and GaInAsP isolated by 150-Å-thick InP barriers have been grown by gas source molecular beam epitaxy. The quantum wells ranged in thickness from 5 to 30 Å. Photoluminescence and transmission electron microscopy were used to characterize them. Intense luminescence was observed at low temperature even from the thinnest of the wells. The spectral shifts due to the quantum size effect were 450 meV for the quaternary 5-Å well and 534 meV for the ternary 5-Å well. The latter constitutes approximately 85% of the total band-gap discontinuity. The emission linewidths suggest that on a scale larger than the exciton diameter the interface fluctuates less than one monolayer.