Quantum-confined Stark Shift Research Articles

Faraday–Stark optoelectronic effect

An optoelectronic effect based on Faraday or Kerr rotation and the quantum-confined Stark shift is demonstrated. It is novel in that the degree of rotation is controlled by an electric field as opposed to a magnetic field. By applying an electric field to a quantum well structure, the Faraday rotation can be tuned into resonance, thereby varying the rotation angle of plane polarized light. We have observed a resonant rotation of 10° in GaAs at a magnetic field of 1 T. By applying a gate voltage of 2 V to a GaAs/AlGaAs superlattice structure, we observed a change in rotation of 1.3° at 2 K. The Faraday–Stark effect could be useful in electrically controlled light switches and high-speed optical modulators.

A voltage-tunable multicolor triple-coupled InGaAs/GaAs/AlGaAs quantum-well infrared photodetector for 8–12 μm detection

A voltage-tunable multicolor triple-coupled quantum-well infrared photodetector (TC-QWIP) has been developed for 8–12 μm detection. The TC-QWIP consists of three coupled quantum wells formed by an enlarged Si-doped InxGa1−xAs quantum well and two undoped GaAs quantum wells separated by two thin AlyGa1−yAs barriers. Two TC-QWIP structures with varying indium and aluminum compositions were designed and demonstrated. Due to the strong coupling effect of the asymmetrical quantum wells, three bound states (E1, E2, E3) are formed inside the quantum wells of the TC-QWIP. The main detection peak wavelength is due to E1→E3 bound states transition for both devices, while two secondary detection peaks due to E1→E2 and E1→Ec continuum states transitions under different biases were also observed. In addition, a strong quantum-confined Stark shift effect for the E1→E3 transition was observed in the wavelength range of 8.2–9.1 and 10.8–11.5 μm for QWIP-A and QWIP-B, respectively; both devices exhibit a linear dependence of detection peak wavelength on the applied bias voltage. A spectral responsivity of Ri=0.05 A/W and background limited performance (BLIP) detectivity DBLIP*=6.1×109 cm√Hz/W were obtained at Vb=5 V, λp=8.6 μm, and TBLIP=66 K for QWIP-A, while Ri=0.33 A/W and DBLIP* =1.63×1010 cm√Hz/W at Vb=4 V, λp=11.2 μm, and TBLIP=50 K were obtained for QWIP-B.

Very large blue and red Stark shifts of the excitonic transition in InGaAs-InP-InAsP antisymmetric coupled quantum wells

A new strained InGaAs-InP-InAsP antisymmetric coupled quantum well (CQW) structure with both very large blue and red quantum-confined Stark shifts for the first heavy-hole-to-electron excitonic transition, Ehh1fEe1, is studied theoretically in this paper. In the antisymmetric coupled quantum well, an antisymmetric-like pair of potential profiles between the shallow-deep conduction band profile and the deepshallow valence band profile are formed. The sub-band eigen-energies, E, and the associated envelope wave functions in the CQW structures with or without an applied electric field are calculated by the transfer-matrix method. The effect of strain on the pseudomorphic layers has been taken into account. Results indicate that the strained InGaAs-InP-InAsP antisymmetric CQW structure exhibits significant enhancement of the blue and red Stark effects in the Ehh1fEe1 transition. The influences of various antisymmetric CQW structural parameters, such as the total well width, the individual well width, the central barrier thickness and the composition of the strained layer on the quantum-confined Stark shift, as well as the envelope wave function overlap, are studied systematically. These strong Stark effects in the antisymmetric CQW structure may have potential applications in sophisticated new electronic devices, such as optical switching devices.

Quantum confined Stark shift in arbitrarily shaped single quantum wells by Fourier series method

A new technique for obtaining energies and wavefunctions in the Fourier series method in the as-grown and annealing induced diffusion modified single quantum wells under applied d.c. electric field perpendicular to the growth direction is presented. The ground state and the first excited energies and wavefunctions are compared with those obtained in the inverse power method. Both methods give energies and wavefunctions in complete agreement with each other.

Electroabsorption modulation at 1.3 μm 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 μm 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 xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , 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.

Heavy- and light-hole band crossing in a variable-strain quantum-well heterostructure

A variable-strain quantum-well semiconductor heterostructure has been designed and studied. Mutually opposite slopes for heavy- and light-hole band edges are obtained by grading the strain within the quantum well from compressive to tensile. A simultaneous red and blue quantum-confined Stark shift for the heavy- and light-hole transitions, respectively, and a unique bias-controlled crossover have been observed by modulation spectroscopy. This indicates that the heavy and light holes experience opposite fields created by the same strain at the same location.

Enhanced Stark Shift in GaAs/AlGaAs Step Quantum Well Under an Applied Electric Field

Theoretical calculations of the quantum confined Stark shift for the first heavy hole exciton (hh1-e1) in step quantum well are given. A significant change in Stark shift is achieved in specially stepped potential shape, which is more than two times larger than the conventional rectangular quantum well. The structure has the advantage of a large change in absorption, which is one of the best candidates for the optical modulators utilizing the quantum confined Stark effect (QCSE) in GaAs/AlGaAs. Relations between QCSE and stepped structure are discussed in the optimization for large Stark shift.

Quantum-Confined Stark Shift Due to Piezoelectric Effect in InGaAs/GaAs Quantum Wells Grown on (111)A GaAs

In0.2Ga0.8As/GaAs strained quantum wells (SQWs) were grown on GaAs (111)A just-oriented, 1° off and 5° off toward [110]- and [001]-oriented substrates. Dependence of strain relaxation on substrate orientation was studied by photoluminescence (PL) spectroscopy. Samples grown on GaAs (111)A 5° off toward [001]-oriented substrates showed the best optical characteristics and this substrate orientation was chosen for making p-i-n diodes. The PL spectrum shows the influence of a built-in electric field due to the piezoelectric effect. The blueshift of PL peaks with applied bias was demonstrated in a p-i-n structure. The PL peak corresponding to a 10 nm SQW blueshifted as much as 24 meV with only 1.2 V applied reverse bias.

Simultaneous blue- and red-shift of light-hole and heavy-hole band in a novel variable-strain quantum well heterostructure

We have designed and studied a new type of strained semiconductor quantum well structure, variable-strain quantum well. The strain within the quantum well is graded from compressive to tensile in order to obtain mutually opposing slopes for the heavy- and light-hole band edges, causing the heavy and light holes to experience opposite fields created by the same strain. A unique bias controlled crossover with a simultaneous red and blue quantum confined Stark shift for the heavy- and light-hole transitions, respectively, has been observed by electroreflectance spectra. This results in a bias controlled change of the polarization properties and the transition energies suitable for polarization controllable photonic devices.

Low Temperature Grown Thin Inalas Step Graded Buffers for Application on Optical Modulator at 1.3 μM

ABSTRACTPreliminary results are described about the growth, structure and properties of multiple quantum well(MQW) Quantum Confined Stark Effect (QCSE) modulators grown on GaAs substrates for operation at 1.3μm in wavelength. Step graded InAlAs buffer layers grown at low temperature by molecular beam epitaxy (MBE) with a total thickness of 0.3 μm are used to relieve the strain caused by the lattice-mismatch between the GaAs substrate and the In0.35Ga0.65As/In0.35Al0.65As MQW heterostructure. X-ray diffraction spectra show that significant lattice relaxation takes place in the buffer. A quantum confined Stark shift of the exciton absorption peak of 48meV was obtained with an applied electric field of 130KV/cm, measured in PIN diode structures consisting of 30 period 95ÅIn0.35Ga0.65As/100ÅIn0.35Al0.65As MQWs on a 3 stage compositionally step graded InxAl1−xAs buffer doped with Si to 5*1017/cm3 grown on a nominally 1018/cm3 n-type doped GaAs substrate.

Electric field effects on the photoluminescence in modulation-doped pseudomorphic AlGaAs/InGaAs/GaAs single quantum wells

Electric field effects on the photoluminescence spectra of modulation-doped AlGaAs/InGaAs/GaAs single quantum wells grown by metalorganic chemical vapor deposition are studied. The electron density is continuously varied by the use of a Schottky gate. Parity forbidden transition is observed in the samples with a high electron density of more than 1×1012 cm−2. The luminescence line shape has strong dependence on the external bias. External field-induced blue and red shifts of the optical transition at the n=1 conduction subband are found. The mechanism for this can be related to the quantum-confined Stark shift, due to the competition between the built-in field and the external field. No photoluminescence intensity enhancement is observed at the Fermi edge.

Spectroscopic study of piezo-electric field effects in InGaAs/GaAs multi-quantum wells grown on (111)B oriented GaAs substrates

An optical study of a series of high quality piezo-electric (PZ) (111)B GaAs-(InGa)As strained multi-quantum wells is reported. Well defined Δn≠0 transitions (E1HH2, E1HH3 and E2HH1) are observed with comparable strength to the Δn=0 transition, as a result of the asymmetric well profile induced by the piezo-electric field. In PLE the onset of the E1HH1 continuum is clearly seen, allowing the deduction of an exciton binding energy of 9 meV. Appyting a bias to oppose the PZ field reduces the field in the well, which decreases the quantum confined Stark shift and weakens the Δn≠0 transitions. At high bias, corresponding to flat band in the well, strong lifetime broadening is observed. Good agreement between theory and experiment is found only by using a value for the PZ constant ∼30% smaller than the commonly accepted value.

Quantum-confined Stark shift observed by electroluminescence and circularpolarized luminescence excitation spectroscopy in GaAs/AlGaAs coupled quantum wells

The quantum-confined Stark effect in electroluminescence (EL) spectra was observed using Al0.3Ga0.7As/GaAs symmetric coupled quantum-well (CQW) structures. The Stark shift of the transition between the ground electron state and heavy-hole state was found to be symmetrical with regard to the flatband bias for EL and for photoluminescence (PL), in which the electric field was applied in reverse direction. Circularly polarized photoluminescence excitation (CPPLE) spectroscopy was also employed to clarify the origins of optical transitions. Six transitions between electrons and light and heavy holes in the CQWs were clearly identified by PL and by polarization of CPPLE measurement.

Piezoelectric-field effects on transition energies, oscillator strengths, and level widths in (111)B-grown (In,Ga)As/GaAs multiple quantum wells

We report a spectroscopic study of high-quality piezoelectric (PZ) (111)B GaAs/(In,Ga)As strained multiple quantum wells. Normally forbidden \ensuremath{\Delta}n\ensuremath{\ne}0 transitions (E1HH2, E1HH3, E2HH1) are observed strongly due to the asymmetric well profile induced by the PZ field. Applying a bias to oppose the PZ field reduces the quantum-confined Stark shifts and weakens the \ensuremath{\Delta}n\ensuremath{\ne}0 transitions. At high bias, corresponding to flat band in the well, strong lifetime broadening is observed. Good agreement between theory and experiment is found, but only by using a value for the PZ constant \ensuremath{\sim}30% smaller than the commonly accepted value.

Large observed exciton shifts with electric field in InGaAs/InGaAsP stepped quantum wells

We report the experimental realization of asymmetric stepped InGaAs/InGaAsP quantum wells. The structure was designed to optimize the quantum-confined Stark shift. We have observed a shift of 30 meV in the heavy hole exciton absorption peak over an electric field change of 50 kV/cm. This shift is double that observed for the same structure without the stepped wells.

Optimization of molecular beam epitaxy grown pin multiple quantum well electroabsorption modulators for the 1.5-μm wavelength region

Comparisons of several molecular beam epitaxy grown GaInAs/AlInAs and GaInAs/Al0.34Ga0.14In0.52As 12-period multiple quantum well structures show that the largest quantum-confined Stark shift (97 nm at −6 V) can be obtained by using the latter structure with the growth temperature set at 500 °C. Deleterious effects of growth interruptions and system contamination are identified in the photocurrent spectra and their physical interpretations are given. The potential benefit of modulating the substrate temperature during growth was found to be negated by the need to introduce growth interruptions.

Enhancement of the Quantum-Confined Stark Shift in Disordered, Strained InGaAs/GaAs Single Quantum Wells

Theoretical results are presented showing how quantum well disordering modifies the quantum-confined Stark effect in strained InGaAs/GaAs single quantum wells. An error function distribution is used to model the constituent atom composition after interdiffusion. It is shown that for a sufficiently long interdiffusion the exciton Stark shift in the disordered quantum well is greater than in the as-grown quantum well and that the change in electroabsorption near the fundamental absorption edge is larger in the disordered well than in the as-grown well for the same applied electric field. These results demonstrate the potential of using disordering to achieve improved performance in strained InGaAs/GaAs quantum well electroabsorption modulators.

Numerical analysis of a step-coupled well: A novel quantum-confined Stark shift structure

A new quantum-confined Stark shift structure, the step-coupled well, is introduced. The feature of the new structure is using a square well of only one quantum state to enhance the Stark shift of the step well. The design rule of 10-μm modulator is suggested. The transfer matrix technique is used for numerical calculation. Numerical results show that the new structure produces a larger Stark shift (13 meV) than the step well (7 meV) under the same bias (20 kV/cm). A lower electric field is thus needed for the step-coupled well to produce the same Stark shift, so that the leakage problem can be minimized and a high switching speed can be achieved.

Electromodulation spectroscopy of CdS microcrystallites embedded in polymer films

CdS microcrystallites embedded in acrylonitrile-styrene copolymer films are studied by the electromodulation spectroscopy of absorption and photoluminescence(PL) to clarify the electronic structure near the absorption edge for the first time. It is elucidated that three states exist near the absorption edge. One is the lowest exciton state, another is a surface state, and the other is a D-A state. The electroabsorption spectra are explained by the quantum confined Stark shift of the lowest exciton. The electromodulation spectra are explained by assuming a surface state and a D-A state.

Quantum-confined Stark shift for differently shaped quantum wells

The quantum-confined Stark shift was calculated, using a numerical method, for eight differently shaped simple Al0.4Ga0.6As/AlxGa1-xAs quantum wells. The dependence of the electron and heavy-hole ground-state interband transition energy on external electric field, quantum well profile and its thickness was investigated. Calculations also include the excitation binding energy, the overlap of the electron and hole wavefunctions and their average spatial separation. A wider well has a larger Stark shift, independent of its shape. An extensive comparison was made of the field response to differently shaped wells having the same zero-field electron ground-state energy (78 meV). The thinnest was a 51 AA wide square well and the thickest a 261 AA asymmetric triangular well. The symmetric and asymmetric triangular wells were found to exhibit the largest Stark shifts but also had a larger reduction of the overlap and exciton binding energy. The square well, on the contrary, had the smallest Stark shift but also smaller variation of the overlap and exciton binding energy. Other wells exhibited characteristics between these two extreme cases.