Excitonic Interband Transitions Research Articles

Electronic characterization of InSb quantum wells

We have fabricated InSb quantum-well structures with high electron mobilities that are limited by scattering due to crystalline defects and remote ionized Si dopants. The integer quantum Hall effect is observed in these structures and has an unusual temperature dependence. Interband exciton transitions in a parabolic quantum well are used to determine an InSb/Al0.09In0.91Sb conduction-band offset ratio of 57%±2%.

Dynamic structure factor of diamond and LiF measured using inelastic X-Ray scattering

The dynamic structure factors S(q-->,omega) of diamond and LiF have been measured using inelastic x-ray scattering. The experimental data are compared to results of ab initio calculations, which take into account the interaction of the excited electron with the remaining hole. In diamond, the vicinity of the indirect band gap and its momentum dependence are studied. In LiF, a larger energy range, which covers the fundamental exciton, the plasmon, and several interband transitions, is investigated. Calculations and measurements agree quite well and emphasize the need to properly include the interaction of the excited electron in the conduction band with the hole in the valence band.

Electrooptical Properties of InGaAs/GaAs Strained Single Quantum Wells

The electrooptical properties of (100) and (111)B InGaAs/GaAs strained single quantum wells have been studied by the photoreflectance spectroscopy at various temperatures and under different biases. There is a Si δ-doping sheet under the GaAs surface to screen the surface built-in electric field from the quantum well. The spectral features consist of the excitonic interband transitions in the quantum well, and the Franz-Keldysh oscillations from the band edge transitions of the GaAs barrier near the surface. The piezoelectric field tilts the (111)B single quantum well toward the δ-doping and causes subband filling by the electrons from the δ-doping centers. With a proper external bias to repel electrons from the tilted quantum well, the excitonic interband transition in the (111)B system is enhanced.

Normal-incidence infrared photoconductivity in Si p-i-n diode with embedded Ge self-assembled quantum dots

Room-temperature infrared photoconductivity in a single layer of Ge self-assembled quantum dots incorporated into silicon p-i-n diode is reported. An in-plane polarized photocurrent response with two bias controlled maxima at 2.9 μm and at 1.7 μm wavelength has been observed. The two kinds of absorption driven by reverse bias in opposite ways are ascribed to the intraband hole bound-to-continuum transition and to the interband excitonic transition.

Photovoltage and photoreflectance spectroscopy of InAs/GaAs self-organized quantum dots

We present a detailed study of the interband excitonic transitions of InAs/GaAs self-organized quantum dots (QDs) based on photovoltage (PV), photoreflectance (PR) and photoluminescence (PL) spectroscopy. At room temperature, the interband absorption transitions of QDs have been observed by using PV spectrum, which clearly exhibits four well-resolved excitonic absorption peaks. The absorption line shape is Gaussian-like. Furthermore, the corresponding excitonic transitions are also observed in PR experiment at 77 K. The first derivative of a Gaussian profile can fit the experimental data well.

Observation of excitonic transitions in InSb quantum wells

We report the observation of interband exciton transitions in InSb/AlxIn1−xSb multi-quantum-well samples. The exciton peaks are identified with the use of a simple quantum well model. The strain present in the InSb wells alters the spectrum significantly from that for unstrained III–V materials and makes it possible to use the exciton spectrum in determining the band offset.

Near and mid infrared silicon/germanium based photodetection

Short-period silicon/germanium (Si m Ge n ) superlattice and SiGe quantum well (QW) structures have been grown by molecular beam epitaxy (MBE) on (100)Si substrates for near (1.3 μm) and mid-infrared (3–5 μm; 8–12 μm) detection. For the near IR detection – suitable for fibre optical communication within a Si integrated circuit (IC) chip – short-period Si m Ge n superlattices with period lengths of several atomic monolayers (ML; e.g. m= n=5 ML; 1 ML~1.4 Å) and 2–4 monolayers wide p-doped Ge quantum well layers separated by 20 MLs of Si and embedded in two 10-nm thick Si 1− x Ge x layers have been grown. In the superlattice structure the zone folding effect (U. Gnutzmann, K. Clausecker, Appl. Phys. 3 (1974) 9) has been predicted to produce strong interband transitions near 0.8 eV (≈1.3 μm), in the latter one the sharp Si/Ge hetero-interfaces break the k-selection rules and strong localisation of electron and hole wave function favour a strong interband excitonic transition at 1.3 μm. This results in a rather efficient room temperature photo- and electroluminescence and in sufficient absorption. An integrated waveguide/photodetector deposited on a SIMOX (Si substrate with separation by implantation of oxygen) substrate has been fabricated and an external quantum efficiency of 11% with an impulse response time of 400 ps has been observed. For the mid IR range (3–5 μm) highly p-doped Si/SiGe quantum well detectors have been deposited on an undoped, double-sided polished Si substrate based on hetero-internal photoemission (HIP) over the Si/SiGe barrier. The absorption and photocurrent spectra have been measured from fabricated mesa detectors at 77 K. The photoresponse spectrum of the HIP detectors is shown to be widely tunable in the technological important wavelength band 3–5 μm by choice of Ge-content, well thickness and doping level. Quantum efficiencies of ~1% at 4 μm and 77 K have been achieved from SiGe HIP structures, dark currents as low as 10 −8 A/cm 2 can be obtained by modulation doping. Detectivity values of D* of 10 9 cm √Hz/W have been achieved, the quantum efficiency spectrum is considerably broader and up to a factor of 4 higher than Pt:Si at 4 μm.

Temperature dependent study of the band edge excitons of ReS 2 and ReSe 2

ReS 2 and ReSe 2 are diamagnetic indirect semiconductors and belong to the family of transition-metal dichalcogenides crystallizing in the distorted octahedral layer structure of triclinic symmetry. We have measured the temperature dependence of the spectral features in the vicinity of the direct gaps E g d in the temperature range betwee 25 and 526 K using piczoreflectance (PzR). The polarization dependence of the PzR spectra provides conclusive evidence that the features are associated with the interband excitonic transitions from different origins. From a detailed lineshape fit to the PzR spectra, we have been able to determine accurately the temperature dependence of the energies and broadening parameters of the band-edge excitons. The parameters that describe the temperature variation of the transition energies and broadening function have been evaluated and discussed.

Temperature dependence of the energies and broadening parameters of the interband excitonic transitions in wurtzite GaN

We have performed contactless electroreflectance studies of the interband excitonic transitions of GaN thin films in the temperature range between 15 and 475 K. The parameters that describe the temperature dependence of the interband transition energies and the broadening function, \ensuremath{\Gamma}(T), of the excitonic features are evaluated. The exciton-longitudinal optical phonon coupling constant, which is determined from an analysis of \ensuremath{\Gamma}(T), is considerably larger than that for a number of the cubic III-V semiconductors.

Refractive index of the alkali halides.mI. Constant joint density of states model

The electronic contribution to the imaginary part of the dielectric function of the alkali halides is assumed to be proportional to ${\mathrm{E}}^{\mathrm{\ensuremath{-}}2}$ above the onset of optical absorption, which is equivalent to the assumption of a constant joint density of states (CJDOS) model. In this approach the electronic contribution to the refractive index can be described by two parameters, the long-wavelength limit of the refractive index ${\mathrm{n}}_{\mathrm{\ensuremath{\infty}}}$ and the frequency of the onset of optical absorption ${\mathrm{E}}_{\mathrm{x}0}$ . Various fits of the corresponding model function to experimental dispersion data show that ${\mathrm{E}}_{\mathrm{x}0}$ is closely related to the frequency of the lowest exciton. Exciton absorption and interband transitions from the p valence bands to s and d conduction bands make the main contributions to the imaginary part of the dielectric function. The relative shifts of the s and d bands with respect to the p bands with density have different signs, and make different contributions to the photoelastic behavior. The CJDOS model is extended to account for the different behavior of s and d bands, and it is shown that the available literature data for the density dependence of the refractive index can be related to the energies of the s and d bands, derived from optical spectra and band-structure calculations. Shock data for LiF and NaCl are discussed, as well as low-pressure results for KCl, KBr, and KI. Recent high-pressure results for CsI show a strong nonlinear density dependence of the refractive index, in contrast to the almost linear behavior of the lighter alkali halides. Within the CJDOS model, this nonlinear behavior can be related to the closure of the band gap in CsI.

Interband-resonant light modulation by intersubband-resonant light in undoped quantum wells

Interband-resonant light modulation by intersubband-resonant light in undoped quantum wells is investigated. Theoretical calculation for the modulation is carried out by considering not only the excitonic interband-transition but also the continuous level transition between the conduction and valence bands. The modulation characteristics are compared with those of the modulation using n-doped quantum wells. The possibility of the modulation using undoped quantum well is successfully shown by real-time single-shot experiment using Ti-Al/sub 2/O/sub 3/ and CO/sub 2/ lasers for interband- and intersubband-resonant lights at room temperature.

Excitonic Recombination Processes in Undoped and Doped Wurtzite GaN Films Deposited on Sapphire Substrates

ABSTRACTExcitonic recombination processes in GaN films grown by low pressure metalorganic chemical vapor deposition technique have been studied in the temperature range between 6K to 320K by photoluminescence spectroscopy. Low temperature photoluminescence spectra of high resistivity films show well-resolved spectral features associated with the excitonic interband transitions. A detailed spectral analysis allowed us to estimate the exciton binding energy and the energy gap. Spectral studies of Si-doped GaN films demonstrate that the high energy recombination processes in these films are dominated by excitons bound to neutral Si donors. Comparison between the recombination channels in high resistivity and in Si-doped films indicated that Si has a larger exciton binding energy than the unknown pervasive donor in undoped materials. These results confirm the excellent electronic properties of the undoped and doped films.

Optical property of InAsP/InP strained quantum wells grown on InP (111)B and (100) substrates

InAsP/InP strained multiple quantum wells (MQWs) were grown on InP (111)B and (100) substrates by gas-source molecular-beam epitaxy. Specular surfaces were obtained under optimized growth conditions on InP (111)B substrates miscut 1° to the 〈110〉 direction. Photoluminescence, absorption and photoluminescence excitation spectra were taken for InAsP/InP strained MQWs at low temperature. Distinct optical transitions were resolved. Energy-level calculations were carried out for both (111)B and (100) MQWs by taking into account the differences in elastic deformation, strain-induced band-edge shift, valence-band anisotropy, and the piezoelectric effect. The peaks were successfully assigned as different interband excitonic transitions. The best fit of the energy-level calculation to the experimental spectra suggests that the valence-band offset ratios (Qv=ΔEv/ΔEg) for (111)B and (100) InAsP/InP heterostructures are 0.35 and 0.30, respectively. This dependence on the substrate orientation was accounted for in terms of the strain-induced band-edge shift.

Structural and optical characterizations of InAsP/InP strained multiple quantum wells grown on InP (111)B substrates

InAsP/InP strained multiple quantum wells (MQWs) with specular surfaces were grown on InP (111)B substrates by gas-source molecular beam epitaxy. Cross-sectional transmission electron microscopy and high-resolution x-ray rocking curves show that the InAsP/InP (111)B MQW structures possess sharp and uniform interfaces, excellent periodicity, and crystalline quality. Structural parameters were extracted accurately from a dynamical-theory simulation by considering the anisotropy of the (111) strain tensor. Low-temperature photoluminescence and photoluminescence excitation measurements revealed interband excitonic transitions, and the energies agree with a calculation with an internal piezoelectric field taken into account.

Optical Studies of The Uniformity of Thermally Induced In-Plane Strain in Thin Gallium Arsenide Films

ABSTRACTIn-plane anisotropic strain can be employed in the design of a new class of optoelectronic devices, such as high contrast, polarization sensitive spatial light modulators. One of the key issues involved in realizing these devices is obtaining a controllable and uniform in-plane strain. We have studied the uniformity of thermally induced in-plane strain in MOCVD grown GaAs lift-off thin films mounted on LiTaO3 or CaCO3 substrates. The experiment exploits the straininduced splitting of the excitonic interband transition at low temperature through absorption measurements using a Ti-Sapphire laser focused to a spot size less than 100 μm. The polarization vector of the incident light was oriented along an axis which enhances both features. From the energy positions of these transitions, the magnitude as well as the type of the in-plane strain was determined. Topographic scans performed over a 1.4mm X 1.4mm area for the sample bonded to CaCO3, and along a 2 mm line for that bonded to LiTaO3 revealed variations in strain of less than 5%.

Spin lifetimes in diluted magnetic semiconductor spin superlattices

The growth of spin superlattice structures in which spin-up and spin-down carriers occupy alternating layers in the structure has recently been reported. We report here measurements of electron and hole spin lifetimes and spin relaxation processes. In dc photoluminescence spectra, both the higher-energy (+1/2,+3/2) and lower-energy (−1/2,−3/2) heavy-hole exciton interband transitions are observed, even though it would be energetically favorable for the spin-up carriers to first relax to the spin-down state before radiative recombination. From the field dependence of the intensity ratios of these components and a rate equation model, we determine the heavy hole τhs and electron τes spin lifetimes relative to the radiative lifetime τr, with τhs/τr≊4.5 and τes/τr≊0.08, so that the heavy-hole spin lifetime is ≊50 times longer than that of the electron. This is attributed to the strain-induced splitting of the heavy- and light-hole bands, which prevents fast spin relaxation of the holes.

Piezoelectric-field-induced localization of barrier states in {211}-oriented InAs/GaAs superlattices.

We observe localized states whose energy is resonant with the continuum in GaAs with periodically inserted {211}-InAs sheets. Evidence for these states comprises excitonic interband transitions above the gap of GaAs, observed by piezoreflectance and photoluminescence-excitation spectroscopies. This localization has its origin in the strain-induced electric field existing in the InAs layers, which we show by simulations of the dielectric function based on envelope-function-type calculations.

Characterization of GaAs/GaAsP strained multiple quantum wells grown by gas-source molecular beam epitaxy

GaAs/GaAs1−xPx strained multiple quantum well (SMQW) structures with x ranging from 0 to 0.5 were prepared by gas-source molecular beam epitaxy. High-resolution x-ray rocking curves and transmission electron microscopy show the multilayered GaAs/GaAsP structures to possess good periodicity and crystalline quality. Photoluminescence and photoluminescence excitation spectra show sharp and distinct peaks that are attributed to the various interband excitonic transitions in quantum wells. An energy-level calculation based on an envelope function model suggests Qv=ΔEv/ΔEg=0.42 for strained GaAs/GaAs0.61P0.39 quantum well structure. Moreover, a 90-meV valence-band offset for a GaAs(76.5 Å)/GaAs0.68P0.32(76 Å) SMQW structure is determined from photoluminescence measurements under hydrostatic pressures.

Long hole spin relaxation times in diluted magnetic semiconductor heterostructures

We report the observation of long spin lifetimes for heavy holes in (Zn,Mn)Se and (Zn,Fe)Se based heterostructures. These long spin relaxation times are observed in both simple strained epilayers, as well as in complex spin superlattice structures in which the spin up and spin down carriers occupy alternating layers of the superlattice. In photoluminescence spectra, both the higher energy (+1/2, +3/2) and lower energy (−1/2, −3/2)heavy hole exciton interband transitions are observed, even though it would be energetically favorable for the spin up carriers to first relax to the spin down state before radiative recombination. From the magnetic field dependence of the intensity ratio of these components and a rate equation model, we find that the heavy hole spin lifetime τhs is substantially longer than the electron spin lifetime τes, with τhs /τr ≈ 4 and τes /τr ≈ 0.06, where τr is the radiative recombination time. This is attributed to the strain induced splitting of the heavy and light hole bands, which inhibits mixing of the hole spin states and subsequent dipole-allowed transitions producing fast spin relaxation.

Interband transitions in InxGa1-xAs/In0.52Al0.48As single quantum wells studied by room-temperature modulation spectroscopy.

Room-temperature phototransmittance and electrotransmittance of single quantum wells of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As with ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As barriers have been used to study the excitonic interband transitions between the confined conduction- and valence-band states. Peak assignment has been confirmed by photocurrent spectroscopy. The lattice-matched (x=0.53) and strained (x=0.6) structures were considered for two different well widths of 50 and 250 \AA{}. Transition energies and broadening parameters were measured from the spectra of the wide well samples and studied as a function of the principal quantum number. Reasonably good agreement between theory and experiment has been achieved by using published values of the electronic band-structure parameters. An observed monotonic increase of the linewidth with the quantum number has been associated with the presence of well-width fluctuations due to rough interfaces.