Strained Quantum Wells Research Articles

应变量子阱能带偏置的分析与计算

应变量子阱能带偏置的分析与计算

InGaN/GaN multiple‐quantum well heterostructures for solar cells grown by MOVPE: case studies

AbstractWe investigate the influence of growth temperature, p ‐doping with bis‐cyclopentadienyl magnesium (Cp2Mg) and number N of multi‐quantum wells on the surface morphology, the electrical and optical properties of InGaN‐based solar cells grown by metal‐organic vapour phase epitaxy. Atomic force microscopy measurements show no influence of multiple‐quantum well number on the surface morphology, but a smoothing with the increase of the Cp2Mg flow. Electrochemical capacitance‐voltage profiling exhibits an increase of the Na‐Nd concentration when increasing the Cp2Mg flow from 250 to 700 sccm. X‐ray diffraction analysis and transmission electron microscopy measurements confirm completely strained quantum wells with similar superlattice period for N=5 to 30. Finally, first solar cells have been demonstrated with a maximum external quantum efficiency of 38% at 380 nm wavelength for N=30. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Absence of nonlocal resistance in microstructures of PbTe quantum wells

AbstractWe report on experiments allowing to set an upper limit on the magnitude of the spin Hall effect and the conductance by edge channels in quantum wells of PbTe embedded between PbEuTe barriers. We reexamine previous data obtained for epitaxial microstructures of n‐type PbSe and PbTe, in which pronounced nonlocal effects and reproducible magnetoresistance oscillations were found. Here we show that these effects are brought about by a quasi‐periodic network of threading dislocations adjacent to the BaF2 substrate, which give rise to a p‐type interfacial layer and an associated parasitic parallel conductance. We then present results of transport measurements on microstructures of modulation doped PbTe/(Pb,Eu)Te:Bi heterostructures for which the influence of parasitic parallel conductance is minimized, and for which quantum Hall transport had been observed, on similar samples, previously. These structures are of H‐shaped geometry and they are patterned of 12 nm thick strained PbTe quantum wells embedded between Pb0.92Eu0.08Te barriers. The structures have different lateral sizes corresponding to both diffusive and ballistic electron transport in non‐equivalent L valleys. For these structures no nonlocal resistance is detected confirming that PbTe is a trivial insulator. The magnitude of spin Hall angle γ is estimated to be smaller than 0.02 for PbTe/PbEuTe microstructures in the diffusive regime.

Strain compensation techniques for red AlGaInP-VECSELs: Performance comparison of epitaxial designs

We present a strain-compensation design for non-resonantly pumped vertical external cavity surface-emitting lasers for emission in the red spectral range around 665nm. Here, the VECSEL chip is based on a metal-organic vapor-phase epitaxy grown (GaxIn1−x)0.5P0.5/[(AlxGa1−x)yIn 1−y]0.5P0.5 multi-quantum-well structure with 20 compressively strained quantum wells. By introducing tensile strained quaternary barriers and cladding layers in a 5×4 QW design, we could compensate for the compressive strain introduced by the quantum wells. Photoluminenscence measurements of structures with different numbers of quantum well packages reveal a more homogenous quantum well growth due to the strain-compensation technique. Furthermore, with the strain compensation technique, the output power could be increased over 30% compared to our conventional structures.

The design of electroabsorption modulators with negative chirp and very low insertion loss

Electroabsorption modulators (EAMs) with negative chirp and very low insertion loss are numerically designed with asymmetric intra-step-barrier coupled double strained quantum wells (AICD-SQWs) based on InGaAlAs material. For this purpose, the electroabsorption coefficient is calculated over a range of wells layer strain from compressive (CS) to tensile (TS). The chirp parameter and insertion loss for TE input light polarization are evaluated from the calculated electroabsorption spectra, and their Kramers-Krönig transformed refractive index changes. The results of the numerical simulation show that the best range of left and right wells strain for EAMs based on AICD-SQWs with negative chirp and very low insertion loss are from 0.032% to 0.05% (TS) and −0.52% to −0.50% (CS), respectively.

Size dependent effective band gap, optical absorption coefficients and refractive index changes in a wide ZnS/Zn1−xMgxS strained quantum well

Exciton binding energy of a confined heavy hole exciton is investigated in a Zn1−xMgxS/ZnS/Zn1−xMgxS single strained quantum well with the inclusion of size dependent dielectric function for various Mg content. The effects of interaction between the exciton and the longitudinal optical phonon are brought out. The effect of exciton is described by the effective potential between the electron and hole. The interband emission energy as a function of well width is calculated for various Mg concentration with and without the inclusion of dielectric confinement. Non-linear optical properties are carried out using the compact density matrix approach. The dependence of nonlinear optical processes on the well width is investigated for different Mg concentration. The linear, third order non-linear optical absorption coefficients values and the refractive index changes of the exciton are calculated for different concentration of magnesium content. The results show that the exciton binding energy is found to exceed LO phonon energy of ZnS for x>0.2 and the incorporation of magnesium ions and the effect of phonon have great influence on the optical properties of ZnS/Zn1−xMgxS quantum wells.

Ultrashort Pulse Generation at Quasi-40-GHz by Using a Two-Section Passively Mode-Locked InGaAsP-InP Tensile Strained Quantum-Well Laser

A 1.56 μm passively mode-locked laser diode with a two-section tensile strained multi-quantum-well structure is fabricated. Without any external pulse compression, a Lorentz pulse train with a pulse width of 1.03 ps and a repetition rate of 35.6 GHz is obtained, which is one of the best results that have been reported on similar devices. The optical pulse has a 300 kHz line width and a 50 dB peak over the noise floor in the photodetected radio-frequency electrical spectrum.

Improvement in performance of traveling wave electroabsorption modulator with asymmetric intra-step-barrier coupled double strained quantum wells the active region, segmented transmission-line and mushroom-type waveguide

In this paper, asymmetric intra-step-barrier coupled double strained quantum wells structure (AICD-SQW), segmented transmission-line and mushroom-type waveguide that improve the performance of traveling wave electroabsorption modulators (TWEAM) are presented. The AICD-SQW structure shows that electroabsorption modulator properties such as large change in absorption, high extinction ratio, large Stark shift, very low insertion loss, zero chirp, and higher figures of merit are possible to be achieved simultaneously as compared with intra-step quantum well (IQW). The segmented transmission-line method allows us to design a TWEAM with above 25 Ω impedance and very high bandwidth. Furthermore, increasing the width of p-cladding layer with the same active layer to reduce the resistance in p-i-n mushroom-type waveguide of TWEAM based on AICD-SQW improve the microwave propagation loss and thus the high-speed electro-optical response. For this purpose, the transmission-line microwave properties of TWEAM such as microwave index, microwave loss, and characteristic impedance are obtained for active and passive segments of transmission-line. Then equivalent circuit model elements are extracted. The modulation response of segmented transmission-line mushroom-type TWEAM is obtained by HSPICE simulation and compared to circuit model simulation results of conventional mushroom-type TWEAM counterpart. The analysis results indicate that the segmented transmission-line mushroom-type TWEAM can achieve much wider bandwidth than the conventional mushroom-type TWEAM counterpart, with a small penalty in electro-optic conversion gain due to AICD-SQW structure.

Highly Reliable 1060nm Vertical Cavity Surface Emitting Lasers (VCSELs) For Optical Interconnect

ABSTRACTHigh reliability, low power consumption and high speed laser diodes are required for optical interconnect. We developed 1060nm VCSELs with InGaAs/GaAs strained quantum wells, oxide-confined and double intra-cavity structures for that purpose. As for the power consumption, low power dissipation of 0.14 mW/Gbps at 10 Gbps operation has been achieved. Clear eye openings up to 20 Gbps were confirmed at a low bias current of 5 mA. In the reliability test, accelerated aging tests were performed up to 5,000 hours at 6 mA in three different temperatures, 70 oC, 90 oC and 120 oC. The total number of the VCSELs was 4,898 pcs (approximately 5,000). No failure was observed. Under the normal operating condition of 40 oC and 6 mA, the total device-hours was 7.75×107 hours assuming Ea = 0.35 eV according to Telcordia GR-468-CORE. The random failure rate of 30 FIT with the confidence level (C.L.) of 90 % and 12 FIT with the C.L. of 60 % were estimated. To estimate the wear-out lifetime and the number of FITs, high stressed aging tests with 170 oC and 6 mA were performed. With the acceleration factor of Ea = 0.7 eV in the wear-out failure, the median lifetime was 3,000 hours which was equivalent to 300 years in 40 oC ambient. The FIT numbers due to the wear-out were estimated as 0.3 FIT for 10 years. Compared with the random failure rate of 30 FIT, the wear-out failure rates are considered to be negligible. In the extremely long term aging test with 90 oC and 6 mA, no wear-out trend has been observed in both threshold current and optical power up to 20,000 hours operation. These results indicate that 1060 nm VCSEL is promising light source used in optical interconnect for high performance computers and data centers.

Loss and Gain Measurements of Tensile-Strained Quantum Well Diode Lasers for Plasmonic Devices at Telecom Wavelengths

We performed an experimental study of tensile strain quantum wells for diode lasers operating at telecom wavelengths and in transverse magnetic polarization. The optical losses of ridge waveguide resonators are measured with passive and active techniques. The results are consistent with the theory and provide a useful material parameter database for this kind of devices. With the aim of implementing active plasmonic devices we studied the effect of the proximity of the metal top contact to the laser active region.

Green/yellow luminescence from highly strained BeZnCdSe quantum wells grown by molecular beam epitaxy

AbstractWe have investigated the luminescent properties of highly strained BeZnCdSe quantum wells (QWs). Single‐QW structures were grown on (001) n‐GaAs substrates by molecular beam epitaxy with various Cd beam pressures. High‐resolution X‐ray diffraction measurements revealed that the lattice mismatch between the BeZnCdSe QW and the GaAs substrate changed from 2.1% to 3.0% as the Cd beam pressure was increased. The photoluminescence (PL) spectra showed that the peak wavelength was nearly proportional to the lattice mismatch for each sample. Fluorescence microscopy measurements revealed that the PL images of all the samples showed a similar luminescence distribution, but the images of samples having the lattice mismatch of 2.8% and 3.0% showed some dark lines. These results suggest that flat QW layers were formed in all the samples, but the lattice relaxation of the QW layer occurred in highly strained samples with lattice mismatches greater than 2.8%. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Size effect on efficiency droop of blue light emitting diode

AbstractIn this paper, the size effects on the efficiency droop (ED) in blue InGaN/GaN quantum well light emitting diode are investigated. The smaller size LEDs can work well under much higher power density, especially when the size is reduced to under 40 micro‐meters. It shows a weaker ED in these small LEDs. Time correlated single photon counting (TCSPC) measurements show a longer electroluminescence lifetime for smaller size LEDs, which implicates the nonradiative recombination is reduced. It is likely due to Aguer recombination reduction by quantum well (QW) band flatened with the device size decreasing. Cathodoluminescence results indicates that the strain in QWs is relaxed both in the whole pillar and along radial direction of the pillar. The better performance of the smaller size LED is likely attributed to strain relaxation (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular beam epitaxy growth of InGaSb/AlGaAsSb strained quantum well diode lasers

2 μm InGaSb/AlGaAsSb strained quantum wells and a tellurium-doped GaSb buffer layer were grown by molecular beam epitaxy (MBE). The growth parameters of strained quantum wells were optimized by AFM, XRD and PL at 77 K. The optimal growth temperature of quantum wells is 440 °C. The PL peak wavelength of quantum wells at 300 K is 1.98 μm, and the FWHM is 115 nm. Tellurium-doped GaSb buffer layers were optimized by Hall measurement. The optimal doping concentration is 1.127 × 1018 cm−3 and the resistivity is 5.295 × 10−3 Ω·cm.

Improvement of saturation optical intensity in electroabsorption modulators with asymmetric intra-step-barrier coupled double strained quantum wells

In this article, the saturation optical intensity in electroabsorption modulators (EAMs) at 1.55 μ m with asymmetric intra-step-barrier coupled double strained quantum well (AICD-SQWs) active region is theoretically investigated and compared with intra-step quantum well (IQW) structure. For this purpose, the two basic escape processes, thermionic emission and tunneling, are considered and the escape times of photogenerated carriers both for the AICD-SQW and the IQW based on the InGaAlAs material system are calculated and compared. From the reduction in escape times, improvement in the saturation optical intensity due to the introduction of the AICD-SQW structure is estimated to be greater than 6 dB at high fields.

On the optical polarization properties of semipolar InGaN quantum wells

Polarized photoluminescence of strained quantum wells grown on c–plane, semipolar (101−2), (112−2), (101−1), (202−1) planes, and nonpolar GaN substrates was studied experimentally and in theory. The observed optical polarization switching between the substrate orientations (101−2) and (112−2) is in accordance with our general model of polarization switching, based on a k→·p→ model of arbitrary substrate orientation. Spectrally resolved measurements of the polarization degree stemming from (101−2) samples show that the maximum of the polarization degree is red–shifted with respect to the maximum of the photoluminescence intensity. We ascribe this effect to an increased polarization of the transitions for higher indium content.

Perforated (In)GaSb quantum wells on GaSb substrates through the use of As2 based in situ etches

The authors investigate the etch modes of GaSb (100) through use of Arsenic (As2) based in situ etching. Three distinct etch modes result from temperature dependent in situ etching. The authors have used this in situ etch process on highly strained (In)GaSb quantum wells (QWs) and have studied the dependence of the in situ etching on substrate temperature and indium composition in the quantum well. The etched quantum wells are capped with an Al0.5Ga0.5Sb barrier and the photoluminescence properties are studied. The authors observe inhomogeneous broadening indicating the possible presence of quantum-sized features with different shapes and sizes.

Microscopic indium distribution and electron localization in zinc blende InGaN alloys and InGaN/GaN strained quantum wells

In order to give an atomic level understanding of the light emission mechanism and seek In distribution patterns closely related to the elusive electron localization centers, we optimize the crystal structure of zinc blende In x Ga1−x N (0≤x≤1) alloys with different In distributions and investigate their electronic structures using first-principles calculations. Our results show that In x Ga1−x N forms a random alloy, in which several-atom In–N clusters and In–N chains can exist stably with a high concentration due to their small formation energy. These In–N clusters and chains form more easily in zinc blende structure than in wurtzite structure. The band gap of zinc blende In x Ga1−x N alloys insensitively depends on the In distribution. Moreover, we find that both small In–N clusters and straight In–N chains with three or more In atoms, acting as radiative recombination centers, highly localize the electrons of the valence band maximum state and dominate the light emission of Ga-rich In x Ga1−x N alloys. The strains of In x Ga1−x N layers can enhance the electron localization in In x Ga1−x N/GaN strained quantum wells. Our results are in good agreement with experiments and other calculations.

Light emission from several-atom In–N clusters in wurtzite Ga-rich InGaN alloys and InGaN/GaN strained quantum wells

In order to understand the mechanism of light emission and to seek the special In-related crystal microstructures associated with the elusive electron localization centers, we consider four representative In configurations (uniform, small In–N clusters, short In–N chains, and a combination of clusters and chains) in wurtzite Ga-rich In x Ga 1− x N alloys and In x Ga 1− x N/GaN strained quantum wells (QWs), respectively, and investigate their electronic structures using powerful first-principles calculations. We find that the several-atom In–N clusters can exist stably with a high concentration due to their small formation energy and play an important role in Ga-rich In x Ga 1− x N alloys and QWs. Unlike previous In–N-chain or In-rich quantum dot-like viewpoints, as radiative recombination centers, the several-atom In–N clusters, especially the c-plane clusters, highly localize electrons at the valence band maximum (VBM) and dominate the light emission if clusters and chains coexist in Ga-rich In x Ga 1− x N alloys. The microscopic arrangement of In atoms in the alloy strongly influences its band gap and bowing parameter. Moreover, the strains of the In x Ga 1− x N layer can enhance the electron localization of the VBM state around the clusters. The physical reasons have been analyzed in-depth. Our results are in good agreement with experiments and other calculations.

Molecular beam epitaxy growth of GaAs on an offcut Ge substrate

Molecular beam epitaxy growth of GaAs on an offcut Ge (100) substrate has been systemically investigated. A high quality GaAs/Ge interface and GaAs film on Ge have been achieved. High temperature annealing before GaAs deposition is found to be indispensable to avoid anti-phase domains. The quality of the GaAs film is found to strongly depend on the GaAs/Ge interface and the beginning of GaAs deposition. The reason why both high temperature annealing and GaAs growth temperature can affect epitaxial GaAs film quality is discussed. High quality In0.17Ga0.83As/GaAs strained quantum wells have also been achieved on a Ge substrate. Samples show flat surface morphology and narrow photoluminescence line width compared with the same structure sample grown on a GaAs substrate. These results indicate a large application potential for III—V compound semiconductor optoelectronic devices on Ge substrates.

Hysteretic magnetoresistance and thermal bistability in a magnetic two-dimensional hole system

Colossal negative magnetoresistance and the associated field-induced insulator-to-metal transition, the most characteristic features of magnetic semiconductors, are observed in n-type rare earth oxides and chalcogenides, p-type manganites, n-type and p-type diluted magnetic semiconductors (DMS) as well as in quantum wells of n-type DMS. Here, we report on magnetostransport studies of Mn modulation-doped InAs quantum wells, which reveal a magnetic field driven and bias voltage dependent insulator-to-metal transition with abrupt and hysteretic changes of resistance over several orders of magnitude. These phenomena coexist with the quantised Hall effect in high magnetic fields. We show that the exchange coupling between a hole and the parent Mn acceptor produces a magnetic anisotropy barrier that shifts the spin relaxation time of the bound hole to a 100 s range in compressively strained quantum wells. This bistability of the individual Mn acceptors explains the hysteretic behaviour while opening prospects for information storing and processing. At high bias voltage another bistability, caused by the overheating of electrons10, gives rise to abrupt resistance jumps.