Tunable photodetectors based on strain compensated GaInAsSb/AlAsSbmultiple quantum wells grown by molecular beam epitaxy

GaInAsSb/AlGaAsSb strain compensated multiple quantum well (MQW) p-i-n structures grown by molecular beam epitaxy on GaSb substrates have been successfully fabricated into tunable photodiodes, which showed a large photoresponse peak shift up to 30 meV (80 nm) at 77 K under a reverse bias of 10 V due to quantum confined Stark effect (QCSE). The QCSE persists up to room temperature and the values of the excitonic absorption peak shifts agree well with the calculated results. Based on the observed QCSE, an electrically tunable resonant cavity enhanced photodetector with strain compensated MQW structure is proposed and modeled     

Large Stark shift of the interband transition in two-step quantumwells

Large and near-linear Stark shifts of the electron-heavy-hole ground state excitonic transition were observed in photoluminescence (PL) measurements for a two-step quantum-well (TSQW) structure. The Stark shift was 40 meV while a corresponding square well shifted only 20 meV at a field of 70 kV/cm. The observed Stark shifts agreed well with calculations. The large Stark shift of the TSQW was achieved on a global-to-local state transition realized via tailor-made quantum well (QW) parameters. This structure is an ideal candidate for optoelectronic devices based on the quantum confined Stark effect (QCSE)     

Asymmetric quantum wells with enhanced QCSE: modulation behaviour and application for integrated laser/modulator

We have optimized asymmetric InGaAsP QWs with respect to the quantum confined Stark effect (QCSE). We have found structures with a red shift of 40 meV at a field of 70 kV/cm. Moreover, it was our aim to find structures with a strong blue shift. A new principle for the integration of a laser and a modulator is presented, which is based on the application of blue shift asymmetric QW's in the active layer for laser and modulator simultaneously. A calculation of the laser and modulation properties for a QW-structure with 27 meV blue shift at a field variation of 70 kV/cm is presented.     

Carrier screening of electric field and electroabsorption saturation in InGaAs-GaAs quantum well structure

Saturation of the electroabsorption is obtained in the quantum confined Stark effect (QCSE) at low optical intensities and low electric fields, in a strained InGaAs-GaAs quantum well modulator structure operating at the exciton energy. This effect is attributed to carrier-induced screening of the internal electric field, and its implications for devices using the QCSE are discussed.<>     

Strain effect on intersubband transitions in rolled-up quantum well infraredphotodetectors

Pre-strained nanomembranes with four embedded quantum wells (QWs) are rolled up into threedimensional (3D) tubular QW infrared photodetectors (QWIPs),which are based on the QW intersubband transition (ISBT).A redshift of~0.42 meV in photocurrent response spectra is observed and attributed to two strain contributions due to the rolling of the pre-strained nanomembranes.One is the overall strain that mainly leads to a redshift of~0.5 meV,and the other is the strain gradient which results in a very tiny variation.The blue shift of the photocurrent response spectra with the external bias are also observed as quantum-confined Stark effect (QCSE) in the ISBT.     

Optoelectronic properties of n-type CdZnTe 2-DEGsingle-quantum-well heterostructures

Optoelectronic properties of asymmetrically strained II-VI CdZnTe single-quantum-well structures grown by molecular-beam epitaxy are reported. Indium doping CdZnTe n-type using a two-dimensional electron gas heterostructure achieved a carrier mobility of 5000 cm²·V^-1 s^-1 at 40 K. A shallow donor ionization energy of 14.5 meV was determined from Hall effect measurements. Fabrication of a large-area-mesa heterostructure device allowed us to investigate exciton absorption of the mixed type-I and type-II single quantum well. Control of the electron concentration in the quantum well allows optical absorption modulation using both the quantum-confined Stark effect (QCSE) and phase-space absorption quenching. Separation of electron and hole photocurrents in different layers is demonstrated and results in photogain. A heavy-hole red-shift of 9.9 meV/V due to the reverse QCSE is reported     

Calculation of linear and nonlinear intersubband optical absorptions in a quantum well model with an applied electric field

Analytic forms of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained for general asymmetric quantum well systems using the density matrix formalism, taking into account the intrasubband relaxation. Based on this model, we calculate the electric field dependence of the linear and the third-order nonlinear intersubband optical absorption coefficients of a semiconductor quantum well. The energy of the peak optical intersubband absorption is around 100 meV (wavelength is 12.4 μm). Thus, electrooptical modulators and photodetectors in the infrared regime can be built based on the physical mechanisms discussed here. The contributors to the nonlinear absorption coefficient due to the electric field include 1) the matrix element variation and 2) the energy shifts. Numerical results are illustrated.     

Electroabsorption modulation at 1.3 /spl mu/m on GaAs substrates using a step-graded low temperature grown InAlAs buffer

We have achieved quantum confined Stark effects (QCSE) on In/sub 0.38/Ga/sub 0.62/As-In/sub 0.38/Al/sub 0.62/As multiple-quantum-well (MQW) structures, operating at 1.3 /spl mu/m grown on GaAs substrates. A quantum confined Stark shift of the exciton absorption peak of 47 meV was obtained with an applied electric field of 190 KV/cm, measured on surface normal PIN diodes. The structure is grown by MBE on a novel three-stage, compositionally step graded, In/sub x/Al/sub 1-x/As buffer, doped with Si to 5/spl middot/10/sup 17//cm/sup 3/, on an n-type GaAs substrate. The total thickness of the buffer is 0.3-0.6 mm, which is considerably smaller than that of linearly graded buffer layers. This structure can be used in both waveguide modulators and surface normal F-P type modulators on GaAs substrates.     

Quantum-confined Stark effects in semiconductor quantum disks

Quantum-confined Stark effects (QCSE's) on excitons in semiconductor quantum disks with finite-potential barriers have been calculated as a function of disk size parameters by a variational calculation in an effort to examine possible application to optical devices. The calculations agree with experimental data reported so far. Although the exciton binding energy, E_b, for smaller diameters is large at zero bias, it decreases more with increasing electric field, which is contrary to the E_b behavior in a spherical quantum dot and quantum well. This larger decrease results in a smaller red Stark shift. Both the red Stark shift and the oscillator strength can be controlled by changing disk diameter and height. The analysis shows that favorable QCSE characteristics, i.e., a large red Stark shift at a small electric field with large oscillator strength, can be obtained     

Field screening in (111)B InAsP/lnP strained quantum wells

Low-temperature photoluminescence measurements were performed on InAsP/InP strained quantum wells grown on InP (lll)B substrates by gas-source molecular beam epitaxy. The emission energy was observed to increase as the pump-power density increased. This was attributed to the screening of the internal piezoelectric field by photo-generated carriers. The energy shift was as large as 35 meV for an InAs_0.28P_0.72/InP quantum well with a lattice mismatch of ~0.9%. A similar structure with a smaller strain showed saturation of the energy shift with increasing pump-power density. We performed a model calculation which includes the quantum confined Stark effect, and this saturation was correlated with a flat-band structure of the quantum well due to the nearly complete screening of the built-in electric field.     

垒层Si掺杂对多量子阱InGaN绿光LED性能的影响

InGaN系绿光LED的量子阱结构具有较高的In含量,InN与GaN之间较大的晶格失配度使得绿光器件的量子限制Stark效应更显著。对内建电场的屏蔽可以有效提高载流子的辐射复合效率。论文探讨了绿光多量子阱中垒层的Si掺杂对绿光器件性能的影响。研究发现,多量子阱中垒层适度Si掺杂(3.4×1016 cm-3)可以改善多量子阱结构界面质量和In组分波动,在外加正向电流的作用下更大程度地屏蔽极化电场;同时,还能够增强电流的横向扩展性,提高活化区的有效发光面积。然而,多量子阱中垒层的过度Si掺杂对于绿光LED器件的性能带来诸多的负面影响,比如加剧阱垒晶格失配、漏电途径明显增加等,致使器件光效大幅度降低。     

Enhancement of the Stark effect in coupled quantum wells foroptical switching devices

Significant enhancement of the Stark effect on the electronic state and the optical dipole moments of coupled quantum wells is shown theoretically. The multiband effective mass theory (k&oarr;-p&oarr;), which takes into account coupling between heavy- and light-hole states of the coupled quantum wells is used. Mixing of states in the coupled quantum wells leads to the splitting of subband energy levels. An applied electric field causes repulsion between the split levels as well as the spin-splitting of the valence-subband structure. Comparison with the single quantum well shows that the optical dipole moment is substantially more reduced for the coupled quantum wells at the same electric field because of enhanced charge separation in this structure. A variational method is used to solve the exciton problem in coupled quantum wells. Calculated exciton peak positions versus electric field show very good agreement with recent experiments. Calculated exciton absorption spectra for the ground state show the quenching of the exciton peak at F=30 kV/cm at 5 K. These results may have interesting applications to low-voltage optoelectronic switching devices based on the quantum-confined Stark effect     

GaAs量子阱中退极化效应对光学整流的影响

用弛豫时间近似和紧致密度矩阵方法,在施加偏向电场的GaAs方量子阱中,研究了退极化场对光学整流的影响。结果表明,退极化场使共振峰的位置向高能方向发生移动。对应三个不同的偏向电场 , ,和 的共振峰的位置分别为 , 和 .共振峰位置的偏移量分别为5.96 、5.99 、6.28 ,其偏移量随偏向电场的增大而略有增大。     

Growth and photoluminescence study of ZnSe quantum dots

We report detailed photoluminescence (PL) studies of ZnSe quantum dots grown by controlling the flow duration of the precursors in a metal-organic chemical vapor deposition system. The growth time of the quantum dots determines the amount of blue shift observed in the PL measurements. Blue shift as large as 320 meV was observed, and the emission was found to persist up to room temperature. It is found that changing the flow rate and the total number of quantum dot layers also affect the peak PL energy. The temperature dependence of the peak PL energy follows the Varshni relation. From analyzing the temperature-dependent integrated intensity of the photoluminescence spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of the exciton in ZnSe quantum dot.     

Transport and optics in quantum wires fabricated by MBE overgrowth on the (110) cleaved edge

The MBE double-growth technique that we call cleaved-edge overgrowth has, over the past several years, proved itself to be especially suitable for making quantum wires of the very highest quality. We will review our recent progress in measuring the transport and quantum optics characteristics of these wires, and the MBE growth issues that arise with cleaved-edge overgrowth fabrication. Our transport experiments have resulted in 250 Å wide quantum wires with ballistic mean free paths exceeding 10 μm. We verify the prediction that in the ballistic regime the electron conductivity in a quantum wire is independent of the wire length and shows quantized steps proportional to e²/h. The deviation of our observed step heights from exactly e²/h is taken as evidence for correlated electron behaviour. The electrons are tightly confined on three sides by atomically smooth GaAs/AlGaAs heterojunctions and in the fourth direction by an electric field. This results in a quantum wire of nominal square cross-section 250 × 250 Å. Magneto-transport measurements reveal quantum wire sub-band separations in excess of 20 meV as well as the symmetries of the wave functions of the one-dimensional modes. For optics studies our quantum wires are made using cleaved-edge overgrowth to form a line junction as two quantum wells are made to intersect with the cross-section forming a letter ‘T’. This line intersection separately forms a quantum wire bound-state for holes, for electrons, and even for excitons. We have characterized our optical wires by PL, by PLE, and by scanning near-field optics. An important application of this work is our demonstration of the first quantum laser using this T-geometry.     

Stark effect in vertically coupled quantum dots in InAs-GaAs heterostructures

The results of studies of hole energy states in vertically coupled quantum dots in InAs-GaAs p-n heterostructures by deep-level transient spectroscopy are reported. Spectra were recorded at different reverse-bias voltages. Levels related to bonding and antibonding s and p states of vertically coupled quantum dots were revealed. The energies of these states significantly depend on an external electric field applied to a heterostructure. This dependence was attributed to the quantum-dimensional Stark effect for the hole states of vertically coupled quantum dots. In addition to this, it was found that the energy of thermal activation of carriers from vertically coupled quantum dots depends on the conditions of isochronous annealing that was carried out both with the reverse bias switched-on and switched-off and both in the presence and absence of illumination. These changes, as in the case of isolated quantum dots, are typical of a bistable electrostatic dipole formed by carriers, localized in a coupled quantum dot, and ionized lattice point defects. The built-in electric field of this dipole reduces the energy barrier for the carriers in the coupled quantum dot. The investigated structures with vertically coupled quantum dots were grown using molecular-beam epitaxy taking account of self-assembling effects.     

Multiple quantum well optical modulator structures using surfaceacoustic wave induced Stark effect

Multiple-quantum-well (MQW) optical modulator structures using the quantum-confined Stark effect (QCSE) manifested by the application of an electric field induced by a propagating surface acoustic wave (SAW) are proposed. The magnitudes of the parallel E_∥ and perpendicular E_⊥ electric-field components are computed to determine the electroabsorption and change of index of refraction for the AlGaAs-GaAs system. The proposed structures are shown to be less cumbersome than conventional p-i-n configurations used to impress E. In addition, an enhanced performance Bragg modulator/switch which utilizes the QCSE as well as the acoustic phase grating produced by the SAW beam is described     

Charge accumulation effects in InGaAs/GaAs [111]-oriented piezoelectric multiple quantum wells

We show that charge accumulation in piezoelectric [111]-oriented multiple quantum wells (MQWs), with average electric fields opposing the field in the barriers, inhibits the shift of optical transitions by externally applied electric fields. This effect is due to the screening of the average electric field as photogenerated electrons and holes drift towards the opposite edges in the MQW region due to this average field. The resulting dipole flattens the envelope potential and hence precludes the change of energy levels with variations of external voltage. This behavior has been observed in different device configurations employing InGaAs/GaAs MQW embedded in a p-i-n diode by low temperature photoluminescence (PL) and photocapacitance spectroscopies under different bias conditions. In addition to these ‘self-locked’ transitions we also observed other peaks in the PL spectra related to the charge accumulation effect and that are qualitatively explained using Hartree calculations.     

Modulation of intersubband absorption in tunnel-coupled quantum wells in electric fields

Modulation of absorption of middle-infrared radiation in double tunneling-coupled quantum wells in longitudinal electric fields is studied. A specific feature of the quantum wells is the small separation in energy between the two lower levels. As a consequence, the levels may exhibit “anticrossing” even in low transverse electric fields. An interpretation of the change in intersubband absorption is suggested. The interpretation is based on the assumption that a transverse electric-field component may appear in the structure. The change in the absorption coefficient is calculated taking into account the redistribution of electrons between size-quantization subbands and the changes in the temperature of electrons in the subbands in the longitudinal electric field, as well as the changes in the optical matrix elements, the energies of transitions, and the concentrations of electrons in the subbands in the transverse electric field. The possibility of using the structure for the efficient modulation of middle-infrared light with the photon energy 136 meV is shown.     

Electroabsorption and electrooptic effect in SiGe-Si quantum wells:realization of low-voltage optical modulators

We have calculated the behavior of the band-to-band absorption coefficient in square, coupled, and graded bandgap Si_0.6Ge _0.4-Si quantum wells as a function of the transverse electric field. It is seen that due to the weak confinement of the electrons (ΔE_c⩽20 meV) the absorption of photons with energy equal to the interband transition energy can be reduced at very small values of the transverse electric field. This phenomenon lends itself to the design of efficient amplitude modulators. In addition, the resulting change in the refractive index is also large and the corresponding linear electrooptic coefficient is calculated to be as large as 1.9×10^-10 m/V in square wells. This effect could prove to be the basis for the realization of efficient Si-based electrooptic modulators. Device designs are discussed