Songcheol Hong

Coupled GaAs/AlGaAs quantum‐well electroabsorption modulators for low‐electric‐field optical modulation

Experimental and theoretical studies are presented for exciton transitions in p‐i‐n GaAs/AlGaAs multiple coupled‐quantum‐well structures where each quantum well consists of two identical wells with a thin barrier. Electroabsorption and photocurrent studies are carried out to identify how the excitonic peaks respond to transverse electric fields. With a careful choice of the dimensions of the coupled quantum well, it is seen that the lowest heavy‐hole exciton peak moves at a rate ∼2.5 faster than in a square well. Thus strong modulation is obtained at much lower electric fields. The nature of the higher‐energy transitions is also studied.

Intersubband absorption in strained InxGa1−xAs/Al0.4Ga0.6As (0≤x≤0.15) multiquantum wells

We report, for the first time, temperature‐dependent intersubband absorption data in doped pseudomorphic InxGa1−xAs/Al0.4Ga0.6As (0≤x≤0.15) multiquantum wells. In this composition range the absorption resonance varies in the range 6–7 μm for 50 A wells, which agrees extremely well with theoretical calculations.

Interdiffusion and wavelength modification in In0.53Ga0.47As/ In0.52Al0.48As quantum wells by lamp annealing

In0.53Ga0.47As/ In0.52Al0.48As single quantum well structures grown by molecular beam epitaxy were pulse annealed by a halogen lamp to determine the stability of their optical properties after such thermal treatment. The annealing time and temperature were 5 s and 650–850 °C, respectively. The shift in energy of the main peak in the low‐temperature photoluminescence spectra was modeled by considering Al‐Ga interdiffusion at the heterointerface and solving the appropriate Schrodinger equation for this region. The estimated interdiffusion constants D are ∼10−16–10−15 cm2/s in this temperature range, which are almost three orders higher than the corresponding values reported for GaAs/ AlxGa1−xAs. For longer annealing times, up to 30 min, the linewidth (full width at half‐maximum) of the excitonic transition in the 11 K photoluminescence spectrum continuously decreased from 12.5 to 7.7 meV, while the intensity maintained a high value.

Enhancement in excitonic absorption due to overlap in heavy‐hole and light‐hole excitons in GaAs/InAlGaAs quantum well structures

In this letter we present experimental and theoretical results for excitonic transitions in coherently strained GaAs/InGaAlAs multiquantum well structures grown on a GaAs substrate. Absorption spectra of the structure with the substrate removed show an extremely sharp exciton peak with an absorption constant corresponding to nearly twice the value of that in the lattice‐matched GaAs/AlGaAs system. Theoretical calculations suggest that the biaxial tensile strain in the well region, occurring after the substrate is removed, causes the heavy‐hole and light‐hole exciton states to coincide for a specific composition of the quaternary alloy. A comparison between the experiments and theory is made and the potential for devices based on this phenomenon is discussed.

Excitonic energies and inhomogeneous line broadening effects in InAlAs/InGaAs modulator structures

The exciton problem in the In0.52Al0.48As/In0.53Ga0.47As quantum well in the presence of static transverse electric field is studied. We report results on variation of exciton size, binding energies, and emission energies as a function of electric field and well size. Inhomogeneous line broadening of exciton lines due to interface roughness and alloy disorder is also calculated. Alloy disorder in the well and barrier regions is found to be very important and its control may be critical in the performance of this system as a high efficiency modulator.

Study of excitons in an arbitrarily shaped GaAs/Al0.3Ga0.7As single quantum well in the presence of static transverse electric field

A method is developed to calculate exciton parameters in an arbitrarily shaped quantum well by using numerical methods in conjunction with the variational method. We solve the Schrodinger equation of the relevant system both by the Monte Carlo method and by the ordinary differential equation solving method, and find that the two approaches yield similar results. Our technique is readily applied to arbitrarily shaped GaAs/AlGaAs quantum wells, and allows us to study the variation of exciton parameters with respect to transverse electric field. We report the results for exciton binding energies, electron and hole tunneling rates, and exciton volumes, which are essential to predicting electro‐optical properties of quantum well. Results are presented for the square, graded, and asymmetric quantum wells, and the effects of the shape of the well on exciton properties are discussed. We find that there is only small dependence of the exciton binding and emission energy on the well shape, but a strong dependence o...

System requirements and feasibility studies for optical modulators based on GaAs/AlGaAs multiquantum well structures for optical processing

Explores the potential of the optoelectronics programmable spatial light modulators (PSLM) based on GaAs/AlGaAs multiquantum well structure for optical processing systems. The authors focus on examining the system requirements for the rapid unbiased bipolar incoherent calculator (RUBIC) architecture from the point of view of dynamic range and signal to noise ratio. The physics of the modulator and the potential transmittance errors expected in state-of-art devices are discussed and correlated to system requirements. They find a dynamic range of 10/sup 3/ for multiplication in the RUBIC architecture should be possible for systems based on PSLM technology only if temperature control of +or-1 degrees K is possible. >

Study of the low‐temperature line broadening mechanisms for excitonic transitions in GaAs/AlGaAs modulator structures

We examine the dominant line broadening mechanisms for excitonic lines in a quantum well in the presence of strong transverse electric fields. We have calculated the line broadening due to interface roughness effects and lifetime broadening effects. We find that the line broadening effects due to interface roughness increase rapidly as the transverse electric field is increased and for a 100‐A Al0.3Ga0.7As/GaAs well, increase by a factor of ∼2.0 as the field is increased from 0 to 100 kV/cm. The broadening due to lifetime effects is very sensitive to the band lineup assumed and is much smaller than interface roughness effects even for one monolayer interface roughness.

Theory of electric field-induced optical modulation in single and multiquantum well structures using a Monte Carlo approach

A Monte Carlo approach is used in conjunction with the variational method to calculate quasi-bound levels in arbitrary quantum wells in the presence of transverse electric field. This approach is expected to be superior to the conventional variational approach since the results do not depend upon the choice of any starting wave solution. The technique is applied to Al 0.3 Ga 0.7 AS/GaAs quantum wells. The shift in the ground state level energies is calculated, along with the tunneling rates for the electrons and holes. We find that the tunneling rates are very sensitive to the band edge discontinuity distribution and to the barrier width in the case of multiquantum wells. The exciton binding energy is also calculated. Due to high tunneling rates at high electric fields, the photoluminescence is expected to be quenched. Consequences of these results are examined.

Theoretical investigation of an integrated all-optical controller-modulator device using QCSE in a multiquantum well phototransistor

Theoretical investigation of an all-optical controller-modulator device based on excitonic transitions for the purpose of a general logic implementation are discussed. The device is based on the quantum confined Stark effect of the heavy-hole excitonic transition in a multiquantum well. The device consists of three basic elements: modulator, controller, and load. The controller is a heterojunction phototransistor with multiquantum wells in the base-collector depletion region. This allows an amplified photocurrent-controlled voltage feedback with low light levels, allowing one to change the state of the modulator. A detailed analysis of the sensitivity of this device in various modes of operation (i.e. floating base and contacted base) is presented. Studies on the cascadability of the device as well as its integrating-threshold properties are also presented. Switching of the resistive load and optically active load with less than 10 mu W total power is demonstrated to be feasible. >