Shallow Quantum Wells Research Articles

Spectroscopy studies by reflectance and photoluminescence on shallow quantum wells Al xGa 1− xAs/GaAs types

We report an experimental study by photoluminescence (PL) and reflectivity on shallow quantum wells of Ga 1− x Al x As/GaAs type performed at 10 K and varying aluminium concentration from 1.5% to 4.5%. We present an approach to the investigation of the reflectivity line shape. The calculations of the reflectance line shapes using the classical model of linear dispersion show good agreement with the experimental spectra, in particular, the two types of the line shapes: “dip” for the quantum well and “peak” for the barrier exciton.

Formation ofcharged excitonic complexes in shallow quantum wells of undoped GaAs/AlGaAs structures under below-barrier and above-barrierphotoexcitation

Low-temperature (T≈2 K) photoluminescence (PL) and photoluminescence excitation (PLE) spectra of GaAs/AlGaAs(x = 0.05) structures with shallow quantum wells (QWs) wereinvestigated. It was found that the PLE spectra exhibit anumber of broad bands in the above-barrier energy region; thesebands alternate `in opposite phases' in the spectra of freeexcitons and excitonic complexes (trions) (i.e. an increase inthe exciton luminescence intensity is accompanied by a decreasein the luminescence intensity of the complexes). Effectsoriginating from simultaneous irradiation of the sample by twolaser beams of different wavelengths were studied. In the casewhere the photon energy of the Ti-sapphire laser is tuned toexcite only the QW states, additional pumping by a He-Ne or Ar-ionlaser results in the shift of the equilibrium in theexciton-trion system towards an increase in the concentration of the latter species. On the other hand, upon excitation into certainbarrier states with energies both below and above the barrierbandgap, additional pump shifts the equilibrium in theopposite direction.

The role of Coulombic coupling inelectroabsorption of square quantum wells

The impact of Coulombic coupling between differentsubband pairs, on the quantum confined Stark effect in squarequantum wells, has been studied. It is shown, via detailedcomparison between electroabsorption spectra obtained from afull excitonic Green function calculation, a decoupledexcitonic Green function calculation and measurement, thatinclusion of the Coulombic coupling between different subbandpairs is generally required for correct prediction ofelectroabsorption in square wells, especially for shallow ornarrow square quantum wells. These results are of particular significance for the accurate calculation of electrorefraction,by Kramers-Kronig transformation, of quantum wellelectroabsorption spectra.

Enhanced diffusion as a mechanism for ion-induced damage propagation in GaN

Although GaN is a chemically inert, thermally stable material, it has demonstrated sensitivity to ion damage generated by dry etch processes such as reacting ion etching and inductively coupled plasma etching. Recombination-enhanced diffusion is an important mechanism which has been observed in other III–V semiconductor systems. In this study we examine the possibility of enhanced diffusion in GaN using quantum well (QW) probe structures. The deeper QWs (750 and 1000 Å deep) showed a steady decrease in relative photoluminescence (PL) intensity with time, providing evidence of the cooperative effects of channeling and defect diffusion in deep etch damage propagation in GaN. In contrast, shallow QWs (150 and 250 Å from the surface) showed a slight decrease followed by a gradual increase in relative PL intensity with time which was explained by defect annihilation. Exposure to above band gap illumination, used to simulate and enhance carrier generation during etch, appears to speed defect annihilation in high defect concentration regions resulting in an increase in QW luminescence, where as in lower defect concentration areas, above band gap illumination does not appear to significantly alter QW luminescence. We attribute this difference in behavior to a difference in diffusion constant. The diffusion constant in less damaged regions may be much lower than that of the highly damaged material.

Consequences of thermalization on the photoluminescence dynamics of shallow quantum well heterostructures

We calculate the time-resolved photoluminescence signal of a thermalized population of excitons in quantum wells. The excitons are assumed to be in thermal equilibrium with free carriers either in the well or in the barrier. The results of our calculations based on rate equations for particle and energy losses are compared to experimental data on GaAs/Ga 1− x Al x As shallow quantum wells with Al concentration varying between 1.5 and 17.8%. These structures are shown to be particularly sensitive to thermalization effects over different subbands, to such an extent that the time decay of their excitonic photoluminescence has very little to do with the true exciton radiative lifetime.

Enhanced damage due to light in low-damage reactive-ion etching processes

Enhanced dry-etch damage in GaAs/GaAlAs and InGaAs/InAlAs quantum well structures due to additional illumination in low-power reactive-ion etching environments has been studied. We observed that damage in shallow quantum wells is increased significantly when above band-gap laser illumination is present during etching, although there is no measurable effect for illumination subsequent to etching. Contributing factors include the influence on ion channeling and also to a form of radiation-enhanced diffusion. The study shows that the effective defect diffusion coefficient is constrained to lie within the range 10−15 to 10−14 cm2/s.

Thermalization and Photoluminescence Dynamics in Quantum Well Heterostructures

The time dependence of the photoluminescence signal is calculated for a thermalized population of excitons in quantum well heterostructures. A thermal equilibrium between excitons and free carriers either in the well or in the barrier is assumed. The result of our calculations is compared to experimental data obtained on shallow quantum wells. We show that the decay of their excitonic photoluminescence has very little to do with the true exciton decay time.

Spin-polarized quantum transport and magnetic field-dependent carrier density in magnetic two-dimensional electron gases

Low-temperature magneto-transport and magneto-photoluminescence measurements are reported in magnetic two-dimensional electron gas samples formed by modulation doping shallow (Zn,Cd,Mn)Se single quantum wells. The small Fermi energy (∼1–2 meV) and large exchange-induced spin splitting (∼10 meV) results in a completely spin-polarized electron gas at filling factors as high as ν=12. Further, the spin splitting significantly perturbs the confinement potential, resulting in a magnetic field-dependent carrier density.

Lattice mismatch and atomic structure studies on In xGa 1− xAs/In yAl 1− yAs coupled double-step quantum wells

Lattice mismatch and atomic structure studies of In x Ga 1− x As/In y Al 1− y As coupled double-step quantum wells have been performed by transmission electron microscopy (TEM) and electron-diffraction pattern measurements. The high-resolution TEM image of the In x Ga 1− x As/In y Al 1− y As coupled double-step quantum well showed that two sets of a 60-Å In 0.65Ga 0.35As deep quantum well and a 60-Å In 0.53Ga 0.47As shallow step quantum wells bounded by two thick In 0.52Al 0.48As barriers are separated by a 38-Å In 0.52Al 0.48As embedded potential barrier. The selective-area electron-diffraction pattern obtained from TEM measurements on the In x Ga 1− x As/In y Al 1− y As double-step quantum well showed that In x Ga 1− x As active layers were grown pseudomorphologically on the InP buffer layer. The value of the lattice mismatch between the In 0.53Ga 0.47As and the In 0.65Ga 0.35As layers obtained from the high-resolution TEM measurements is 1.6%. A possible crystal structure for the In x Ga 1− x As/In y Al 1− y As coupled double-step quantum well is presented based on the TEM results. These results can help improve understanding of the structural properties for the applications of strained In x Ga 1− x As/In y Al 1− y As coupled double-step quantum wells in new kind of the optoelectronic devices.

Magnetoexciton escape from shallow quantum wells induced by in-plane electric fields

Magnetoexciton escape from shallow quantum wells induced by in-plane electric fields

Electroabsorption in extremely shallow quantum wells: Comparison between theory and experiment

It is shown, via detailed comparison between measured electroabsorption spectra and electroabsorption spectra calculated by a full excitonic Green’s function method, a decoupled excitonic Green’s function method and a variational method, that accurate description of the Coulombic coupling between different subband pairs, including unbound quantum well states above the quantum well edge, is crucial for a correct prediction of electroabsorption in extremely shallow quantum well structures.

Maximum bit-rate analysis of various symmetric extremely shallow quantum well self-electro-optic effect devices under optimized input power

We investigate the effects of high input power on the performance of optical bistable symmetric self-electro-optic effect devices (SSEEDs) using extremely shallow quantum wells (ESQWs). In this study we consider four ESQW SEEDs: antireflection (AR)-coated ESQW S-SEEDs (E-SEEDs), back-to-back AR-coated ESQW S-SEEDs (BBESEEDs), asymmetric Fabry-Pe´ rot (AFP)-ESQW S-SEEDs (AE-SEEDs), and back-to-back AFP-ESQW S-SEEDs (BBAE-SEEDs). As the input power increases, device performances such as on/off contrast ratio and on/off reflectivity difference are seriously degraded due to the ohmic heating and exciton saturation. But switching speed of the device increases up to a certain value and then begins to decrease. With reasonable optimization of the input power, we simulate and analyze the system bit rate of a cascading optical interconnection system using the various ESQW S-SEEDs changing the number of quantum wells for the external bias of 0 V and 25 V. The BBE-SEED with 48 quantum wells shows the maximum bit rate of 100 Mbytes/s at 0-V bias, while the E-SEED with 60 QWs shows the maximum bit rate of 109 Mbytes/s at 25-V bias. In AFP cavity structures, the AE-SEED with 18 QWs and the BBAE-SEED with 28 QWs show maximum bit rate of 311 and 156 Mbytes/s, respectively, at 0-V bias.

Excitonic recombination dynamics in shallow quantum wells

We report a comprehensive study of carrier-recombination dynamics in shallow ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum wells. At low crystal temperature (2 K), the excitonic radiative recombination time is shown to be strongly enhanced in shallow quantum wells with $x>0.01,$ consistently with a model that takes into account the thermal equilibrium between the three-dimensional exciton gas of the barrier and the two-dimensional exciton gas, which are closer in energy as x decreases. Furthermore, we demonstrate the existence of a thermally activated escape mechanism due to the low effective barrier height in these structures. The nonradiative recombination is shown to dominate the carrier dynamics for temperatures as low as 10 K for $x\ensuremath{\approx}0.01.$ Our experimental observations are analyzed using three different variational exciton calculations. In particular, we study the crossover from the two-dimensional to the three-dimensional behavior of the exciton, which occurs for x as low as 0.01 and affects mainly the oscillator strength, whereas the transition energies in shallow quantum wells can be calculated, to a large extent, using the same approximations as for conventional quantum wells. The peculiar behavior of the oscillator strength at the crossover to the weak confinement regime is obtained by expansion in a large basis.

Effects of design parameters on non-biased optical bistable devices using multiple quantum well nipi-diode structure

We have studied the effect of design parameters on the performance of non-biased optical bistable (NOB) devices fabricated with non-resonant multiple quantum wells nipi-diode structures. This device made of extremely shallow quantum wells as intrinsic regions allows both a large low-field electroabsorption change and exciton ionization, and consequently, ensures both large electric field swing and strong light absorption without external bias voltage. By optimizing stack pairs of non-resonant nipi-structures, the performance of the NOB device was significantly enhanced with a large signal contrast ratio, low heat power dissipation, and low drive voltage, while maintaining low switching energy, high signal difference, and layout simplification.

Microprobe spectroscopy of localized exciton states in II–VI quantum wells

We have investigated exciton localization in Zn1−xCdxSe/ZnSe quantum wells by microprobe, spatially resolved luminescence. We found that compositional fluctuations in Cd-rich (x=0.23) quantum wells induce the formation of a tail of states in the gap, where a continuum of localized exciton states forms. On the other hand, shallow quantum wells (x=0.11) exhibit sharp lines due to excitons localized at monolayer well-width fluctuations extending over a few hundred nanometers. The observed results are consistent with the occurrence of localized exciton lasing in deep ZnCdSe quantum wells.

Carrier dynamics in shallow GaAs/AlGaAs quantum wells

We report a comprehensive study of carrier relaxation, recombination and escape mechanisms in a set of nine high-quality GaAs/Al x Ga 1− x As shallow quantum wells (SQW), under various conditions of applied electric field, temperature and excitation energy, by means of time-integrated and time-resolved photoluminescence. Our experimental findings are analyzed theoretically and bring a better understanding of SQW properties as well as conventional QWs. In a biased SQW at low temperature, it is shown that photo-carrier escape via direct tunneling results in a strong quenching of the luminescence at fields one order of magnitude smaller than what prevails in conventional QWs. Apart from the field-activated escape process, we demonstrate the existence of thermally activated escape dynamics due to the low effective barrier height in SQWs. Time-resolved photoluminescence at low temperature reveals both a major increase in the relaxation times and radiative recombination times in SQWs, in good agreement with our theoretical model.

UHV-ECR etching and in-situ analysis of wide bandgap II–VI nanostructures

A low damage in situ compatible etching technology for the patterning of wide bandgap II–VI materials has been developed. Thermally assisted ECR-etching in a UHV chamber is used for the pattern transfer into the semiconductor heterostructure while in situ and ex situ photoluminescence (PL) spectroscopy is performed to analyze the etched structures. As one main result, the quantum efficiency in shallow etched near surface quantum wells is reduced after sample exposure to the atmosphere, indicating the creation of non-radiative loss channels at the surface after exposure to air. Moreover, the potential of the UHV etching technology for the fabrication of nanostructures is demonstrated by realizing optically active CdZnSe/ZnSe quantum wires with lateral widths down to 20 nm showing clear lateral confinement effects in the PL spectra.

Interdiffused AlGaAs-GaAs quantum well for improved electroabsorptive modulation

This is a theoretical study of the effects of two as-grown structural parameters on the modulation properties of Al/sub x/Ga/sub 1-x/As-GaAs quantum wells (QWs), which are the Al concentration in barrier and the thickness of the well layer serving as initial conditions before interdiffusion. The results show that, with a larger Al concentration and a wider well width, the range of interdiffusion for an enhanced electroabsorption (EA) change increases with both of these parameters, while insertion loss increases with the former and decreases with the latter. However, the increase in loss is lower than that of the rectangular QW for the same magnitude of absorption change. The range of a tunable absorption-peak wavelength produced by interdiffusion increases with increasing Al concentration and decreases with increasing well width. Moreover, in a moderately interdiffused QW, the required bias reduces for the same level of EA modulation. For the best device operation, interdiffused QWs with the Al concentration between 0.3 and 0.4 and well width between 10 and 12 nm are most suitable for developing a general-purpose electroabsorptive modulator. When applied in high-speed modulators, the EA of a wide and shallow QW active-region structure can be further enhanced by the use of corresponding interdiffusion.

Abnormal photocurrent–voltage behavior of GaAs/AlGaAs multiple shallow quantum well p-i-n diodes

We have observed the abnormal photocurrent–voltage (I–V) behavior in GaAs/AlGaAs multiple shallow quantum wells p-i-n diodes. Under the illumination of a laser, two current plateaus were developed at the negative conductance region of the I–V curve, along with some hystereses depending on the scan direction. At the first plateau, two major oscillations of ∼120 kHz and ∼37 MHz were observed with several minor oscillations of frequencies below the latter, while this latter component was uniquely at the other plateau. Analyzing the electrical and the optical oscillations, we explain that one hysteresis at the first plateau was due to the low frequency bias-circuit oscillations, whereas the other at the next plateau was attributed to the intrinsic behavior of the p-i-n diode.

High Power Performance of Nonbiased Optical Bistable Devices Using Multiple Shallow Quantum Well p-i-n-i-p Diodes

We studied the laser power dependence of the performance of nonbiased optical bistable devices (NOBDs) which are composed of two identical multiple shallow quantum wells p-i-n-i-p diodes connected in series. Under the illumination of the laser of a wavelength corresponding to that of the exciton absorption, the diode revealed both the maximum photocurrent and the large negative differential resistance in the forward bias region, fulfilling the conditions of the nonbiased optical bistable operation without any external bias voltage. With the laser power up to 2 mW in a circle of 10 µm diameter, the reflectivity change of ∼20% and the contrast ratio of 2:1 were maintained between the on- and off-states of the NOBD. These results ensure that the proposed device is a good candidate for high-speed optical bistable operation using a high power laser.