Strained Quantum Wells Research Articles

Wavelength Filter and Converter by NDFWM in Strained Quantum Wells

A novel kind of wavelength filter and converter using nondegenerate four-wave mixing (NDFWM) in an optical amplifier with tensile and compressively strained quantum wells (T/C SQW) is reported in this paper. The configuration of the T/C SQW converting filter is designed first, followed by the establishment of the coupling model of the device. The influences of the property parameters of the optical amplifier with T/C SQW on the output efficiency, 3dB bandwidth, and the frequency shift are mainly discussed. The device can serve as both filter and wavelength converter with high conversion efficiency and narrow bandwidth on the order of 0.01 nm. Meanwhile, the frequency conversion range, which is decided by the gain bandwidth of the semiconductor media, is up to 5000 Ghz in the considered case.

Monolithic integration of laser and passive elements using selective QW disordering by RTA with SiO/sub 2/ caps of different thicknesses

Area-selective disordering of an InGaAs strained quantum wells (QWs) was performed by rapid thermal annealing with thick and thin SiO/sub 2/ caps. Fabry-Perot lasers were fabricated on a selectively-disordered wafers, and a lasing wavelength difference as large as 23 nm was obtained between the lasers in 300-nm and 30-nm capped areas. We present fabrication of lasers integrated with disordered passive waveguides and demonstrate significant reduction of the passive waveguide loss from roughly 40 cm/sup -1/ to 3 cm/sup -1/. A distributed Bragg reflector laser with QW disordering in the grating area was demonstrated, and remarkable improvement of laser characteristics was achieved compared to a laser without disordering.

Raman Study of Elastically Strained Bulk and Layered Structures Based on CdTe

Bulk CdTe bars under uniaxial stress, as well as single and multiple CdTe strained quantum wells (QWs), with varying barrier composition (degree of strain), have been studied by resonant Raman spectroscopy and photoluminescence excitation (PLE) at low temperature (T ≤ 90 K). Combining, a) piezo-Raman results from bulk CdTe samples, b) misfit-strain-induced and piezoelectric-field-induced phonon shifts measured on QWs, and c) measured Gr�neisen parameters, we estimate strain- and field- Phonon Deformation Potentials (PDPs) for the TO and LO phonons of CdTe.

AlGaAs/GaAs/InGaAs composite MQW structures for photovoltaic applications

AlGaAs/GaAs/InGaAs composite MQW structures were theoretically studied and simulated. The computer simulation indicated that an appropriate composite MQW, both with symmetrical and non-symmetrical structures, could keep | ψ| 2 of quantized carriers at proper locations in electric-field-tilted quantum wells, so the efficient transition by photon absorption would be possible and applicable for photovoltaic cells which have the composite MQW as the active region. The AlGaAs/GaAs MQW and GaAs/InGaAs (4 x) SQW structures were separately prepared by MBE and were evaluated for their spectral responses. The AlGaAs/GaAs MQW has a high response at a short wavelengths (peak at 685 nm) due to the quantized states in GaAs wells, while the GaAs/InGaAs SQW has a broader spectral response covering longer wavelengths (600–850 nm) because of the strong absorption in the GaAs barrier and substrate. However, (4 x) photoluminescence peaks at 900–1100 nm that werefound from GaAs/InGaAs strained quantum wells at room temperature are promising evidence for the longer wavelength spectral response. The AlGaAs/GaAs/InGaAs composite SQW and MQW samples were experimentally prepared by MBE techniques and tested for their optical properties. The broader photoluminescence peak was observed and reflected the nature of the composite structure. The study on the photospectral response of composite MQW structures has been conducted which provides the basic information for high performance solar cell design.

1.26 μm intersubband transitions in In0.3Ga0.7As/AlAs quantum wells

We observed room-temperature intersubband transitions at 1.26 μm in n-doped type-II In0.3Ga0.7As/AlAs strained quantum wells. An improved tight-binding model was used to optimize the structure parameters in order to obtain the shortest wavelength intersubband transition ever achieved in a semiconductor system. The corresponding transitions occur between the first confined electronic levels of the well following mid-infrared optical pumping of electrons from the barrier X- valley into the well ground state.

InAs/GaAs lasers with very thin active layer

InAs/GaAs laser structures based on atomically thin InAs strained quantum wells were prepared by metal–organic vapour phase epitaxy. The dependence of electroluminescence spectra on the thickness, as well as on the number of InAs quantum wells, was studied. The temperature dependence of the mode structure and optical output power were studied in the range from 25 to 100°C. The position of laser emission can be shifted by changing the thickness and the number of InAs active layers from 1.15 to 1.4 eV. Wavelength switching with increasing operating temperature and excitation current was observed.

Temperature dependence of the photoluminescence properties and band gap energy of InxGa1−xAs/GaAs quantum wells

The effective band gap energy of InxGa1−xAs/GaAs strained quantum wells (QWs) is investigated by photoluminescence spectroscopy (PL) in the range 12–295 K. The temperature dependence of the band gap energy of strained QWs correlates well with that of bulk InxGa1−xAs of similar composition. Deviations from the band gap variation of bulk material at low temperatures (12–90 K) are interpreted in terms of exciton localization. The differences ΔE(12 K) between the measured PL peak energies and the expected transition energies at 12 K (obtained by simulating the measured temperature dependence of the PL peak positions by the well-known Varshni relation) are suggested to be closely related to the Stokes shifts that often exist between PL and PL excitation spectra of QWs. A linear relation is found between the PL full-width at half-maximum measured at 12 K and ΔE for a range of QWs prepared under different growth conditions. Excitonic recombination is inferred to be dominant in the PL transitions at the highest temperatures investigated—even at room temperature.

Theoretical Analysis of Polarization Insensitive InGaAsP Multiple-Quantum-Wells Eletroabsorption Modulators with Negative Chirp

Polarization sensitivity of InGaAsP electroabsorption (EA) modulators is analyzed by the fractional-dimensional approach. Chirping parameter α is then calculated using Kramers-Krönig relations. It is found that polarization-insensitive and negative chirp operation can be realized simultaneously for InGaAsP multiple-quantum-wells (MQW) EA modulators with optimized well width and amount of strain. We propose a polarization-insensitive InGaAsP EA modulator with 9-nm-wide 0.38% tensile strained quantum wells. The chirping parameter α is estimated to be around -2 for the first 3 dB of transmission, while the modulation characteristics remain polarization-independent up to 80 kV/cm, corresponding to an extinction ratio of over 15 dB.

Thermally detected optical absorption, reflectance, and photoreflectance of In(As,P)/InP quantum wells grown by gas source molecular beam epitaxy

This work reports an extensive optical study of a series of In(As,P)/InP strained quantum wells grown by gas source molecular beam epitaxy with various thicknesses and compositions. Thermally detected optical absorption, reflectance, and photoreflectance measurements have been performed in order to determine all the exciton energies. An envelope function model including band nonparabolicity, intervalence band coupling, and also possible group V element exchange at the interfaces, is used to interpret the experimental data. The fit of the optical transition energies leads to an accurate determination of the crucial structural and optical parameters. The arsenic concentration inside the wells is evaluated and the conduction band offset ratio (Qc=0.70) as well as the bowing parameter of In(As,P) (C=0.14 eV) are determined. These studies also provide useful information about the nanometer-scale P–As interface mixing.

Selective Disordering of InGaAs Strained Quantum Well by Rapid Thermal Annealing with SiO2 Caps of Different Thicknesses for Photonic Integration

Area-selective disordering of an InGaAs strained quantum well was performed by rapid thermal annealing with thick and thin SiO2 caps. The lasing wavelength difference as large as 23 nm was obtained between Fabry-Perot lasers in 300 nm and 30 nm capped areas. We present fabrication of lasers integrated with disordered passive waveguides and demonstrate significant reduction of the passive waveguide loss from roughly 40 cm-1 to 3 cm-1.

Passively mode-locked diode-pumped surface-emitting semiconductor laser

A surface-emitting semiconductor laser has been passively mode locked in an external cavity incorporating a semiconductor saturable absorber mirror. The gain medium consists of a stack of 12 InGaAs-GaAs strained quantum wells, grown above a Bragg mirror structure, and pumped optically by a high-brightness diode laser. The mode-locked laser emits pulses of 22-ps full-width at half maximum duration at 1030 nm, with a repetition rate variable around 4.4 GHz.

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.

Modulation-doped ZnSe/(Zn,Cd,Mn)Se quantum wells and superlattices

The fabrication of “magnetic” two-dimensional electron gases in modulation-doped ZnSe/Zn1−x−yCdxMnySe quantum wells and superlattices is described, with a focus on correlating growth conditions with low temperature transport, atomic force microscopy, and photoluminescence. Optimization of the growth parameters results in a two-fold increase in 0.3 K mobility (∼23 000 cm2/V s) for the host ZnCdSe two-dimensional electron gasses (2DEGs) and a similar relative increase for magnetic 2DEGs. The improved sample growth is applied to the fabrication of modulation doped, strained-layer magnetic superlattices containing up to 25 periods of moderately strained quantum wells, without significant reduction in mobility.

Effect of ion induced damage on carrier lifetimes in strained CdZnSe/ZnSe quantum wells

Studies of the effects of reactive ion etching on molecular beam epitaxy grown CdxZn1−xSe/ZnSe strained quantum wells (QWs) using photoluminescence (PL) and time-resolved reflectivity measurements are reported. The shallow (50 nm cap layer) QW samples exhibit a blueshift in their PL peak position as a function of etch voltage up to a certain point, after which the blueshift is reduced. The reduction in the blueshift of the PL spectrum is strongly correlated with a reduction in the carrier lifetimes measured by transient reflectivity. From these experiments we suggest that the initial blueshift is a result of ion damage at the surface interacting with strain in the QW. On the other hand, the reduced carrier lifetime at higher voltages is a result of more severe structural damage in the QW.

High-speed wavelength-division multiplexing and demultiplexing using monolithic quasi-planar VCSEL and resonant photodetector arrays with strained InGaAs quantum wells

We demonstrate the high-speed (gigabit-per-second) operation of a wavelength-division-multiplexed optical interconnect, which is implemented by multiplexing the optical data from a multiple-wavelength vertical-cavity surface-emitting laser (VCSEL) array into a single optical fiber, and demultiplexing the composite data stream using an array of resonance-enhanced photodetectors (REPDs) with matching resonance wavelengths. By using VCSELs and REPDs with a new quasi-planar oxide-confinement design for improved high-speed performance, and using strained InGaAs-GaAs quantum wells to achieve a better tradeoff between optical responsivity and wavelength selectivity, wavelength-division-multiplexing (WDM) operation has been demonstrated under 1-Gb/s data modulation, with an optical crosstalk rejection ratio of better than -10 dB for wavelength channels that are spaced 4 nm apart.

Temperature dependence of photoluminescence from InAsP/InP strained quantum well structures grown by metalorganic chemical vapor deposition

In this article, we report the growth and characterization of InAsP/InP strained single quantum well (SSQW), strained single quantum well (SSQW) stack, and strained multiple quantum well (SMQW) structures on (1 0 0)-oriented InP substrates grown by metalorganic chemical vapor deposition. Double-crystal X-ray diffraction, photoluminescence (PL) and transmission electron microscope (TEM) are used to characterize the strained quantum wells. The high-quality crystalline InAs y P 1 −y (72Å)/InP SSQW structure with y⩽0.36 exhibits a 9.9 meV full-width at half-maximum (FWHM) of 10 K PL spectra. The peaks in the PL spectra for SSQW stack structure with a well thickness of 8, 14, and 35 Å vanish above 100, 150, and 296 K, respectively, presumably due to the decrease of photons yielded by electron–hole recombination in thinner quantum well regions on increasing the temperature. The PL peak emission energy dependence of well thickness in the InAsP/InP SSQW stack structure is in good agreement with the calculated results. In addition, the variations of the PL peak energy and FWHM in all the InAsP/InP SSQW, SSQW stack, and SMQW structures are described in detail.

Properties of Quantum Well Excitons in GaN/AlGaN and InGaN/GaN/AlGaN UV, Blue, Green, and Amber Light Emitting Diode Structures

Optical spectra of GaN, In-doped GaN, and InGaN single quantum well (SQW) structures were compared to explore the role of In for the emission mechanisms in the SQW light emitting diodes (LEDs). The internal electric field, F, due to spontaneous and piezoelectric polarization in strained quantum wells (QWs) naturally induces the quantum confined Stark effect (QCSE) that reduces the electron–hole (e–h) wavefunction overlap due to rapid ionization of excitons causing a serious decrease of the emission efficiency especially for the GaN SQW, although a clear QW excitonic absorption peak was observed. The low energy tail of the emission spectrum of InGaN amber SQW LEDs extends below the bulk InN bandgap, indicating that F is not completely screened by injected carriers. However, doping and alloying of In in GaN improve the emission efficiency despite the fact that the effective bandgap inhomogeneity is increased with increasing InN mole fraction. In is considered to support effective localization of QW excitons, and the Coulomb interaction between the e–h pair is maintained provided that the QW thickness is smaller than the free exciton Bohr radius.

Influence of growth temperature of InAsP/InP strained multiple quantum wells grown by metalorganic chemical vapor deposition

In this article, we report the influence of growth temperature on the luminescent and structural properties of InAsyP1−y/InP strained multiple quantum wells (SMQWs) and strained single quantum wells (SSQWs) grown by metalorganic chemical vapor deposition (MOCVD). The strained quantum wells are characterized by high-resolution transmission electron microscope (TEM), photoluminescence (PL), and double-crystal x-ray diffraction (DC-XRD). An AsH3/(AsH3+PH3) gas flow ratio of 0.50% and 1.48% at 580 and 650 °C growth temperatures, respectively, will result in an InAsP layer with y=0.3 solid composition. The experimental PL emission energies at 10 K at different well thicknesses for the InAsyP1−y/InP SSQWs grown at 580 and 650 °C are in well agreement with the trend of the calculated curves. The TEM lattice image of an InAsP/InP SSQW grown at 580 °C on the order of two monolayers has been demonstrated. The InAsP/InP SSQW structure grown at 580 °C appears to be extremely abrupt, uniform, free of misfit dislocations, and narrow PL linewidth. Besides, the growth of InAsP/InP SMQWs at 580 °C maintains its structural integrity throughout the deposition sequence with smooth interface and well-defined periodicity. However, the InAsP/InP SSQWs or SMQWs exhibit an adverse property at 650 °C growth temperature. From the above results, the lower growth temperature is necessary for the InAsP/InP SMQW growth by MOCVD.

InGaAs/InP heterostructures with strained quantum wells and quantum dots (λ=1.5–1.9 µm)

Strongly strained InxGa1−x As/In0.53Ga0.47As/InP heterostructures with indium content x=0.69−1.0 in the active region were investigated experimentally and theoretically. Two types of structures were obtained by vapor-phase epitaxy from metalorganic compounds: 1) with isolated compression-strained quantum wells and 2) with self-organized nanosize InAs clusters (quantum dots). The temperature dependence of the quantum radiation efficiency of samples with quantum wells in the temperature range 77–265 K is characterized by T 0=43 K. One reason for the low value of T 0 is electron delocalization in the active region. The maximum radiation wavelength obtained in structures with quantum dots is 1.9 µm at 77 K.

AlGaInAs/InP strained-layer quantum well lasers at 1.3 µm grown by solid source molecular beam epitaxy

AlxGayIn1−x−yAs/InP strained-layer multiple-quantum-well lasers emitting at 1.3 µm have been grown by solid source molecular beam epitaxy, and the performance characteristics have been studied. The lasers contain 4, 5, or 6 compressively strained quantum wells in the active region. They exhibit low transparency current densities, high gain coefficients, and high characteristic temperatures compared to conventional GaInAsP/InP quantum well lasers. The results show that desired lasing features can be achieved with relatively simple layer structures if the doping profiles and waveguide structures are properly designed and the material is grown to high structural perfection.