Unstrained Quantum Research Articles

Optical absorption coefficient of quantum wires in strain and electric fields with intermixing interfaces

We have theoretically studied the optical absorption coefficient of quantum wires in electric and strain fields with intermixing interfaces. The potential profiles are governed by the intermixing heterojunctions, internal strain due to the lattice mismatch, the external electric field and stress. The second Fick's law describes the wires' intermixing heterojunctions due to the alloy interdiffusion. We adopt the Green's function method to solve the Poisson equation for the displacement to determine the internal strain due to the lattice mismatch. The single-band Schr\"odinger equation in the effective mass approximation is used to describe the conduction subband structures while the four-band Kohn-Luttinger Hamiltonian is used to describe the valence subband structures. In solving the Schr\"odinger equation and the Kohn-Luttinger Hamiltonian, we expand the wave functions for the electrons and holes by using linear combinations of the two-dimensional harmonic oscillator wave functions to yield two matrix equations. These matrix equations are numerically solved for their eigen-energies and their corresponding eigenfunctions. We investigate some physical properties of the unstrained quantum wire $(\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{As})$ and the strained wire $(\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se})$ as examples. When the interdiffusion becomes stronger, on one hand, the internal strain is relaxed leading to the band gap shrinkage (if the initial strain ${\ensuremath{\epsilon}}_{0}<0$) and, on the other hand, the potential profiles are deformed and the effective band gap becomes wider. Variations of the transition energies with external stress show the anticrossing effect and variations of the transition energies with external electric field show the field emission effect. The hole effective masses can be enhanced or become electronlike by applying stress to the wire. The oscillator strengths of the dipole-allowed intersubband transitions for the $y$ and $z$ polarizations are calculated to infer possible optical transitions. The optical absorption coefficients together with the joint density of states are calculated.

Resonance Raman scattering by optical phonons in unstrained germanium quantum dots

The position and shape of the ${E}_{1},\phantom{\rule{0.3em}{0ex}}{E}_{1}+{\ensuremath{\Delta}}_{1}$ Raman resonance depending on the size of unstrained Ge quantum dots were studied. The dots were grown in $\mathrm{Ga}\mathrm{As}∕\mathrm{Zn}\mathrm{Se}∕\mathrm{Ge}∕\mathrm{Zn}\mathrm{Se}$ structures on (111)-oriented GaAs substrates using molecular-beam epitaxy. A shift of the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ resonance energies by up to $0.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ was observed. The dependence of the shift on quantum dot size was shown to be well described by a cylindrical model using quantization of the bulk Ge electron-hole states that form excitons at the two-dimensional critical point of the interband density of states. A separate display of the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ resonances in the quantum dots related to transformation of the interband density of states into the \ensuremath{\delta} function due to quantization of the energy spectrum was observed.

Hierarchical Self-Assembly ofGaAs/AlGaAsQuantum Dots

A novel structure containing self-assembled, unstrained GaAs quantum dots is obtained by combining solid-source molecular beam epitaxy and atomic-layer precise in situ etching. Photo-luminescence (PL) spectroscopy reveals light emission with very narrow inhomogeneous broadening and clearly resolved excited states at high excitation intensity. The dot morphology is determined by scanning probe microscopy and, combined with single band and eight-band k.p theory calculations, is used to interpret PL and single-dot spectra with no adjustable structural parameter.

Ultrafast polarization modulation induced by the “virtual excitation” of spin-polarized excitons in quantum wells: Application to all-optical switching

The operation of an all-optical coherent polarization switch that makes use of spin-polarized virtual excitons in unstrained quantum wells is thoroughly investigated experimentally over a wide range of excitation intensities. The device is shown to exhibit a 415 fs switching time and a contrast ratio of >300:1 at ∼100 K in a thin (40 well) sample. The switching mechanisms are discussed in terms of the circular optical selection rules and the virtual excitation is studied by performing differential measurements for various input polarizations. The polarization state changes induced by the spin-polarized virtual population are measured and their contributions to the switch signal separated using a combination of time-averaged, time-resolved, and spectrally resolved ellipsometric techniques.

All-optical injection and control of spin and electrical currents in quantum wells

We show that quantum interference between one- and two-photon absorption can be used to inject spin currents, with or without an accompanying electrical current, in unbiased semiconductor quantum well structures. The directions in which the electrical and spin currents are injected can be coherently controlled, with a relative phase parameter of the optical fields as the control parameter. We characterize the currents for an unstrained quantum well and a quantum well under biaxial compressive strain using the Luttinger-Kohn model; we work out particular examples. If compressive strain is used to appropriately rearrange the subbands, then a degree of spin polarization of the spin currents higher than possible in bulk GaAs can be achieved and maintained even for photon energies well above the band gap.

Strain-induced piezoelectric field effects on light emission energy and intensity from AlInGaN/InGaN quantum wells

We report on the effects of the piezoelectric field and well width on the transition energy and intensity for InGaN quantum well structures with GaN or AlInGaN quaternary barriers. It was found that the emission energy of compressively strained GaN/In0.08Ga0.92N quantum wells exhibits a strong well width dependence not accounted for by quantum confinement subband energy shifting alone. However, for unstrained quantum well layers with quaternary barriers, no emission energy dependence on width was observed due to the elimination of the piezoelectric field, which was measured to be at least 0.6 MV/cm for the strained quantum wells. Furthermore, the unstrained quantum wells demonstrated a higher intensity than their strained counterparts for all quantum well widths investigated. The current data will help clarify the origin of emission in InGaN quantum wells.

INTERSUBBAND TRANSITIONS IN DOPED AND UNDOPED QUANTUM WELL STRUCTURES OF In0.53Ga0.47As/In0.52Al0.48As

We have successfully observed the intersubband absorption in doped as well as in undoped, unstrained quantum well structures of In0.53Ga0.47As/In0.52Al0.48As . The absorption is well resolved near the short-wavelength limit in this structure. Within the accuracy of our measurements, the intersubband transition is observed only in TM mode polarizations in both doped and undoped structures.

Raman E 1, E 1+Δ1 resonance in unstrained germanium quantum dots

Raman scattering by optical phonons in unstrained Ge quantum dots obtained in GaAs/ZnSe/Ge/ZnSe structures was studied using molecular beam epitaxy. A shift in the E 1, E 1+Δ1 resonance energy due to the quantization of the spectrum of electron and hole states in quantum dots was observed. The properties observed were explained with the use of a simplest model of localization with allowance for the spectrum of Ge electron states.

Ge–Si intermixing in Ge quantum dots on Si

We have provided direct evidence for the presence of considerable Si–Ge intermixing in strained and unstrained Ge quantum dots deposited on Si(001) and Si(111). The local structure around Ge was probed by using Ge K-edge X-ray absorption spectroscopy; complementary evidence for intermixing was provided by AFM and STM studies. These results implied that the strain energy in the dots was reduced by Si atoms diffusing into the dots, resulting in a modified form of Stranski–Krastanov growth.

Comparison of hole and electron intersubband absorption strengths for quantum well infrared photodetectors

It is well known that the hole intersubband absorption of normally incident (TE polarized) radiation is nonzero for p-doped quantum well infrared photodetectors (p-QWIP's) which have been fabricated without an optical grating. This present paper shows from k/spl I.oarr//spl middot/p/spl I.oarr/ theory that, for typical p-QWIP designs, this hole intersubband absorption of TE polarized radiation (without the help of an optical grating) is significantly smaller than the electron intersubband absorption of TE polarized radiation in those n-doped QWIP's (n-QWIP's) fabricated with an optical grating. A second result of this present work is that, even when there is significant mixing of the light and heavy hole states, the p-QWIP absorption of TE polarized radiation (without the help of an optical grating) is still much smaller than the n-QWIP absorption of TE polarized radiation (with the help of an optical grating). The reason is that the mixing of light and heavy hole states never increases the amount of |S>-symmetry in the mixed hole wave function beyond the amount of |S>-symmetry which was present in the unmixed, purely light hole state. Finally, this present paper shows from k/spl I.oarr//spl middot/p/spl I.oarr/ theory that strained layer growth on an [001] substrate does not significantly affect the strength of the hole intersubband absorption. The reason is that the Hamiltonian describing uniaxially strained quantum wells has precisely the same (tetragonal) symmetry as the Hamiltonian describing carrier confinement in unstrained quantum wells. All of these results are important in choosing a QWIP device design.

Optical spectroscopic study of electric field sharing effects in piezoelectric GaN/Al0.65Ga0.35N multi-quantum well structures

We have studied a series of GaN/Al0.65Ga0.35N samples of varying well and barrier widths by cathodoluminescence and photoluminescence spectroscopy. These samples consist of unstrained GaN quantum wells and strained Al0.65Ga0.35N barriers. Some of the samples exhibit micro-cracks on their surface around which the barriers are partially relaxed and the quantum wells strained. The observation of the quantum well emission under two different strain conditions allows the relative effects of spontaneous polarization and piezoelectric polarization on the emission energies to be compared. We are able to accurately model the emission energies for both strain configurations if we consider the large electric fields present within the quantum wells to be due almost entirely to piezoelectric and field sharing effects.

Ge–Si intermixing in Ge quantum dots on Si(001) and Si(111)

Exploiting Ge K-edge x-ray absorption spectroscopy we provide direct evidence of Si–Ge intermixing in self-organized strained and unstrained Ge quantum dots on Si, and provide a quantitative measurement of the average composition. For Ge/Si(001) dots with equivalent thickness in the range 5.8–38 nm and morphology ranging from that typical of coherently strained to that associated with relaxed dots we find that the average Si composition is approximately 30%. For Ge/Si(111), we find that the wetting layer has a Si composition near 50%. We discuss these results in terms of the energetics of dot formation and argue that strain-enhanced diffusion of Si into Ge should be considered as an important factor in minimizing the strain energy of the system.

Giant electric fields in unstrained GaN single quantum wells

We demonstrate that, even in unstrained GaN quantum wells with AlGaN barriers, there exist giant electric fields as high as 1.5 MV/cm. These fields, resulting from the interplay of the piezoelectric and spontaneous polarizations in the well and barrier layers due to Fermi level alignment, induce large redshifts of the photoluminescence energy position and dramatically increase the carrier lifetime as the quantum well thickness increases.

Spin–orbit coupling effects on the optical gain of strained quantum wires

The effects of spin–orbit split-off band on InGaAs/InP strained quantum wire structures are theoretically investigated using 6×6 Luttinger–Kohn Hamiltonian. The strong coupling of spin–orbit split-off band with other states has considerable effects on optical properties. It shows that the peak gains are underestimated for the unstrained and the tensile strained quantum wires in 4×4 Luttinger–Kohn model. The change of the density of states and the dipole moments are the main factors for the inconsistent results between the 4×4 and the 6×6 models in the linear optical gain. In the tensile and the unstrained quantum wire structures, the threshold carrier densities are reduced by spin–orbit coupling. The compressive strained structure has the lowest threshold carrier density and the highest differential gain. As a whole, the differential gain is improved by the spin–orbit coupling effects.

Monte Carlo simulations of capture into quantum well structures

The purpose of this paper is to simulate capture events into quantum well structures of the kinds studied in luminescence experiments in order to understand the carrier dynamics, to extract local electron and hole capture times and to relate microscopic carrier processes to the luminescence signal. To do this, a quantum mechanical model for charge capture by a quantum well has been incorporated into a self-consistent Monte Carlo simulation of carrier transport in quantum well laser diodes. The capture model makes use of the established technique of modifying Fermi's golden rule to calculate a dimensionless capture probability instead of a capture rate. This model has been used to simulate time-resolved photoluminescence experiments on a variety of InGaAsP-based structures which have unstrained quantum wells. Local electron and hole capture times have been extracted from the transport data for simulations performed at 300 K, and these times oscillate as a function of well width. Wells 85-90 Å wide permit fast electron capture into a second electronic subband (0.56 ps capture time) and hole capture into a second light hole band (0.44 ps). This may be contrasted with a 75 Å well for which electron capture into the single conduction subband exceeds 1.7 ps, and the peak hole capture time is 1.1 ps.

Strong piezoelectric effects in unstrained GaN quantum wells

Optical properties of GaN/AlGaN single quantum well (SQW) and multi quantum well (MQW) structures grown by nitrogen plasma assisted MBE on MOCVD-grown GaN/sapphire have been characterised by low temperature photoluminescence. Photoluminescence (PL) peaks corresponding to emission from very narrow (10 Å wide) SQWs are blue shifted with respect to the bulk GaN emissions but reveal strong red shift for wider SQWs (40 and 60 Å wide). Structural properties of SQW and MQW structures have been characterised by X-ray reciprocal lattice mapping in order to determine the strain conditions and composition of ternary AlGaN alloys. The results clearly demonstrate that in all structures under investigation the ternary (barrier) alloys are fully strained to underlying MOCVD-grown GaN/sapphire substrates. Despite of the fact that GaN quantum wells (QW) are unstrained, photoluminescence experiments clearly reveal the presence of extremely strong electric fields in the QW regions, which can be attributed to the interplay of the piezoelectric-type polarisation in the well and barrier layers due to Fermi level alignment.

Artificial ferroelectricity in coupled strained quantum dots

A multiple quantum dot structure consisting of alternating strained and unstrained quantum dots and having nonzero net dipole moment, i.e., possessing ferroelectric properties, is proposed. Curie temperature and spontaneous polarization are estimated to be of the order of 20 K and 10−5 C/m2, respectively, for the case of InGaAs quantum dots grown on InP substrate and are shown to be insensitive to the lateral arrangement of the dots. Practical applications in information processing and storage are considered.

Bias independent heavy- and light-hole degeneracy in InGaAs/InGaAsP quantum wells

We present a type of strained semiconductor quantum-well structure that exhibits bias-independent heavy- and light-hole degeneracy. This effect is achieved by inserting thin layers of highly strained material in an unstrained quantum well. By adjusting the thickness and the position of the highly tensile strained layers, the quantum confined Stark effect for the heavy and light holes can be engineered separately to control the bias dependent polarization properties. Experimental results on such a structure agree well with the theory. These unique bias-dependent polarization properties have important applications in optoelectronic devices when specific polarization properties are required.

Strong Piezoelectric Effects in Unstrained GaN Quantum Wells

AbstractOptical properties of GaN/AIGaN single quantum well (SQW) and multi quantum well (MQW) structures grown by nitrogen plasma assisted MBE on MOCVD-grown GaN/sapphire have been characterised by low temperature photoluminescence. Photoluminescence (PL) peaks corresponding to emission from very narrow (10 Å wide) SQWs are blue shifted with respect to the bulk GaN emissions but reveal strong red shift for wider SQWs (40 and 60 Å wide). Structural properties of SQW and MQW structures have been characterised by X-ray reciprocal lattice mapping in order to determine the strain conditions and composition of ternary A1GaN alloys. The results clearly demonstrate that in all structures under investigation the ternary (barrier) alloys are fully strained to underlying MOCVD-grown GaN/sapphire substrates. Despite of the fact that GaN quantum wells (QW) are unstrained, photoluminescence experiments clearly reveal the presence of extremely strong electric fields in the QW regions, which can be attributed to the interplay of the piezoelectric-type polarisation in the well and barrier layers due to Fermi level alignment.

Efficient single-mode oxide-confined GaAs VCSEL's emitting in the 850-nm wavelength regime

Single- and multimode vertical-cavity surface-emitting lasers (VCSELs) with three unstrained GaAs quantum wells (QWs) and emission wavelengths around 850 nm have been fabricated using molecular beam epitaxy (MBE) for crystal growth. Wet chemical etching and subsequent selective oxidation are applied for current confinement. The influence of oxide layer position on lateral index guiding is studied in detail in order to increase maximum single-mode output power. A device of 3-μm active diameter and reduced index guiding shows maximum single-mode output power of 2.25 mW with a side-mode suppression ratio (SMSR) of more than 30 dB for high-efficiency oxidized VCSELs.