Quantum-well Samples Research Articles

S-shaped negative differential resistance in 650 nm quantum well laser diodes

The appearance of an S-shaped negative differential resistance (NDR) in GaInP/AlGaInP quantum well (QW) lasers has been observed in both single and multi-QW devices at temperatures between 100 and 200 K. Light–current ( L– I) and current–voltage ( I– V) relationships have been measured in detail at temperatures from 100 to 250 K, using three QW samples with different values of cladding layer thickness (1.0, 0.5 and 0.3 μm). The dependence of the S-shaped NDR characteristics on cladding thickness and laser output intensity shows that the high resistivity p-cladding layer is the cause of NDR. It is because of the impact ionisation of impurities in the p-cladding layer that an S-shaped NDR is produced. An impurity ionisation energy of ∼62 meV in p-type (Al 0.7Ga 0.3) 0.52In 0.48P is obtained from the I–V curve, which is consistent with that obtained from a hydrogenic model and accepted values.

Phonon Replica Dynamics in High Quality GaN Epilayers and AlGaN/GaN Quantum Wells

We present an experimental study of the exciton and phonon replica dynamics in high quality GaN epilayers and AlGaN/GaN quantum wells (QW) by means of picosecond time-resolved photoluminescence (PL) measurements. A non-exponential decay is observed both at the zero phonon line (ZPL) and at the n = 1 LO replica. Time-resolved spectra unambiguously assign the replica to the free exciton A recombination. Optical migration effects are detected both in the epilayer and the QWs samples and disappear as the temperature increases up to 60–90 K. Even though the sample quality is comparable to state-of-the-art samples, localization effects dominate the exciton dynamics at low temperature in the studied GaN based strucures.

Effects of Confinement on the Coupling between Nitrogen and Band States in InGaAs1?xNx/GaAs (x ? 0.025) Structures: Pressure and Temperature Studies

We report photo-modulated reflectance studies under applied pressure and variable temperature, and related calculations, that probe the influence of N-related resonant anti-bonding states on the electronic structure of dilute III–N–V quantum wells (QWs). Three InyGa1—yAs1—xNx/GaAs multiple QW samples with N contents of 0–2.5% are investigated up to 85 kbar at 300 K, and for 300–10 K at 1 atm. While the temperature dependence is only minimally affected by the N content, the pressure shifts of the intersubband transition energies depend significantly on the percentage of N. The linear pressure coefficients are much smaller than those of the InGaAs band gap. A ten-band k · p model is in broad accord with the observed pressure dependence; the predicted non-linear shift is somewhat larger than measured.

Coherent Excitation Spectroscopy of Disordered Quantum-Well Structures

We study semiconductor quantum-well samples with different disorder strengths by coherent excitation spectroscopy with particular emphasis on disorder-mediated couplings of exciton subensembles. Our experiments show that there is no coherent coupling of excitons in different quantum islands and in a strongly inhomogeneously broadened sample. Latest results with polarization-resolved coherent excitation spectroscopy indicate a disorder-induced dephasing of biexcitons in the inhomogeneously broadened sample.

Coherent Excitation Spectroscopy of Disordered Quantum-Well Structures

We study semiconductor quantum-well samples with different disorder strengths by coherent excitation spectroscopy with particular emphasis on disorder-mediated couplings of exciton subensembles. Our experiments show that there is no coherent coupling of excitons in different quantum islands and in a strongly inhomogeneously broadened sample. Latest results with polarization-resolved coherent excitation spectroscopy indicate a disorder-induced dephasing of biexcitons in the inhomogeneously broadened sample.

Disorder-induced dephasing in semiconductors

Microscopic calculations including energetic disorder and Coulomb correlations up to third order in the laser field are performed. The resulting four-wave-mixing signals show polarization-dependent dephasing induced by diagonal disorder. The correct modeling of this disorder-induced dephasing requires the proper inclusion of Coulomb correlations. The theoretical results are in good qualitative agreement with measurements performed on a variety of quantum-well samples.

Antiferromagnetic phase transition in a single MnTe monolayer

We investigate a quantum-well sample containing a monolayer of MnTe in its center. The exciton Zeeman splitting is measured as a function of temperature by polarization-dependent photoluminescence excitation experiments. The dependence of the inverse Zeeman splitting shows a kink at a critical temperature of 50 K. Simultaneously, a peak in the linewidth of the e1–hh1 transition is observed at the same temperature. The critical temperature agrees very well with the predicted Nèel temperature obtained within the mean-field approximation for a single monolayer MnTe. After annealing the sample, the Zeeman splitting drastically increases and no critical phenomena are observed anymore.

Photoluminescence dynamics of amorphous Si/SiO2 quantum wells

We have studied photoluminescence (PL) spectrum and dynamics of amorphous silicon (a-Si) based quantum wells at low temperatures. In a-Si/SiO2 quantum-well samples with thin a-Si layers, the PL spectra appear in the visible spectral region. With a decrease of the a-Si well thickness, the PL peak energy shifts to higher energy and the PL lifetime becomes shorter. The well-thickness and temperature dependence of the PL lifetime show that the nonradiative recombination of carriers occurs at the a-Si/SiO2 interface and the lifetime is determined by the energy relaxation process in the a-Si well and the carrier diffusion to the interface. The quantum confinement and localization of carriers in a-Si/SiO2 quantum-well structures will be discussed.

Ultrafast Coherent Control of Excitons Using Pulse-Shaping Technique

In this paper, we report on the ultrafast coherent control of excitons in quantum wells using a phase-locked pulse sequence generated by pulse-shaping techniques. The pulse-shaping system with a double liquid-crystal spatial light modulator (SLM), and a Ti3+-sapphire laser is used to generate phase-locked pulses. The ultrafast coherent control of the exciton population and polarization is demonstrated by the observation of the reflectivity change in the pump-probe and the diffracted power in the degenerate-four-wave-mixing (DFWM) measurements. In single quantum wells, good coherent control characteristics with 87% coherent carrier destruction are demonstrated at the low excitation power of 0.15 mW (∼1.1×1010 photons/cm2). In addition, the preliminary manipulation of the exciton population is demonstrated by the phase-locked triple pulses. In the case of coupled quantum wells, the modulation of the SLM is carefully controlled and the coherent control of both the exciton population and the polarization is demonstrated in the two different coupled quantum-well samples.

Effect of ion induced damage on carrier lifetimes in strained CdZnSe/ZnSe quantum wells

Studies of the effects of reactive ion etching on molecular beam epitaxy grown CdxZn1−xSe/ZnSe strained quantum wells (QWs) using photoluminescence (PL) and time-resolved reflectivity measurements are reported. The shallow (50 nm cap layer) QW samples exhibit a blueshift in their PL peak position as a function of etch voltage up to a certain point, after which the blueshift is reduced. The reduction in the blueshift of the PL spectrum is strongly correlated with a reduction in the carrier lifetimes measured by transient reflectivity. From these experiments we suggest that the initial blueshift is a result of ion damage at the surface interacting with strain in the QW. On the other hand, the reduced carrier lifetime at higher voltages is a result of more severe structural damage in the QW.

Effect of quantum well location on single quantum well p-i-n photodiode dark currents

The photocurrent available from a p-i-n solar cell can be increased by the addition of quantum wells (QWs) to the undoped region. At the same time the QWs reduce the open-circuit voltage by introducing areas of lower band gap where recombination is enhanced. This increase in recombination should be as small as possible for the most favorable effect on the photovoltaic efficiency of the device. Theoretical considerations indicate that nonradiative recombination, which is the dominant loss mechanism in AlxGa1−xAs/GaAs QW structures, may be reduced by positioning the QWs away from the point where the electron-hole product is a maximum. For p-i-n diodes, where recombination is greatest at or near the center of the space charge region, this means locating the QWs closer to the doped regions. Spectral response should not be affected so long as the QWs are still located within the field bearing region. Thus, improved photovoltaic performance may be expected through strategic location of the QWs. We report on measurements on a series of Al0.36Ga0.64As p-i-n photodiodes, three of which contained a single 87 Å GaAs QW within the i region, and one which was a control sample with no QW. The three QW samples were grown with the QW located nearer to the p-doped layer, centrally, and nearer to the n-doped layer, respectively. Spectral response measurements confirm that for good quality samples photocurrent is independent of QW location within the depleted region. Contrary to expectations, the dark current is highest for the sample with the QW located closer to the n region. We analyze these results in terms of structure and doping profile, and compare them with the predictions of a self-consistent model. The observed behavior is attributed to a relatively high unintentional background doping in the intrinsic region.

Electroabsorption studies on quasi-three-dimensional and quasi-two-dimensional excitons in (Zn,Cd)Se/Zn(S,Se) structures

The electro-optical properties of (Zn,Cd)Se/Zn(S,Se) quantum well structures grown on GaAs substrates have been studied with differential electroabsorption spectroscopy at room temperature and compared to model calculations. (Zn,Cd)Se wells of 20, 10, and 4×5 nm are investigated, corresponding to well widths of four, two, and one times the exciton Bohr radius in this material system, respectively. We observe the quantum confined Stark effect for the 4×5 nm sample and find a Stark shift of 18 meV in the heavy-hole exciton peak for an electric field change from 82 to 175 kV/cm. In contrast, the 10 nm sample shows a rather weak and more Franz–Keldysh-like signal. We show that the 20-nm-thick quantum-well sample behaves like bulk material, i.e., the electro-absorption signal is well described by Franz–Keldysh oscillations.

Experimental observation of the de Haas–van Alphen effect in a multiband quantum-well sample

We report measurements of magnetic quantum oscillations (the de Haas–van Alphen effect) in a quantum well containing more than one subband. The Fourier transform of the magnetization oscillations shows the expected frequencies proportional to the occupancies of each subband, but additional frequency components, in accordance with theoretical predictions of Alexandrov and Bratkovsky, are also detected. These components relate to the sums and differences of the individual subband frequencies, and are a consequence of coupling of the individual subband occupancies due to oscillation of the electrochemical potential with magnetic field in a canonical Fermi system. This is an experimental verification of the Alexandrov and Bratkovsky theory in a multisubband system.

Coherent Excitation Spectroscopy on Inhomogeneous Exciton Ensembles

A new spectroscopic technique is demonstrated on three characteristically different semiconductor quantum-well samples. Coherent excitation spectroscopy reveals coherent coupling and effectively suppresses the signatures of inhomogeneous broadening and incoherent transfer between various excitonic states in quantum wells. Coherently coupled homogeneous subsystems are analyzed in an inhomogeneously broadened excitonic ensemble.

Growth of device quality InGaP/GaAs heterostructures by gas source molecular beam epitaxy using tertiarybutylphosphine

The feasibility of preparing device quality InGaP/GaAs heterostructures by the gas source molecular beam epitaxial (GSMBE) growth using tertiarybutylphosphine (TBP) was investigated. Undoped and Si doped InGaP layers with GaAs buffer layers as well as InGaP/GaAs quantum well (QW) samples were grown and characterized by atomic force microscopy (AFM), photoluminescence (PL) and Hall measurements. All of the undoped and doped InGaP layers showed narrow PL peaks and electron mobilities comparable to those reported for InGaP layers grown by other methods. Undoped samples showed a residual carrier concentration of 1×10 15 cm −3 and a mobility of 3300 cm 2/V s at room temperature. In the Si-doped samples, the highest electron density of 2×10 19 cm −3 was achieved without carrier saturation. On the other hand, InGaP/GaAs QW samples showed intense and narrow PL emission lines, indicating formation of sharp heterointerfaces.

Photoluminescence characteristics and pit formation of InGaN/GaN quantum-well structures grown on sapphire substrates by low-pressure metalorganic vapor phase epitaxy

We have examined how a growth interruption, caused by closing group-III sources, affects the crystalline quality of InGaN/GaN quantum-well (QW) structures grown by metalorganic vapor phase epitaxy. The QW samples were characterized by their photoluminescence (PL), and by atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy dispersive x-ray (EDX) microanalysis. The PL peak wavelength was strongly dependent on the duration of the growth interruption and on the number of QW layers. AFM measurements revealed that the size of the open hexagonally shaped pits in the QW structures increased dramatically as the interruption duration was lengthened. Through TEM and EDX microanalysis, we found that the formation of these hexahedronal pits, formed due to the growth interruption, causes a large fluctuation in the In composition, especially around the pits, and the presence of such pits in an underlying QW layer strongly affects the In incorporation into the upper QW layers, leading to significant growth-rate variation in an InGaN QW layer and red-shifting of the PL spectra when a multiple-QW structure is grown.

Excitation intensity dependence of photoluminescence from narrow 〈100〉- and 〈111〉A-grownInxGa1−xAs/GaAssingle quantum wells

We have used excitation intensity-dependent photoluminescence (PL) spectroscopy to study strained ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ $(x\ensuremath{\approx}0.23)$ single quantum wells grown along the 〈001〉 and $〈111〉A$ directions. PL intensities from these two quantum-well samples exhibit a linear dependence on incident power density over the excitation intensity range examined. The linewidth of the emission from the 〈100〉 well is well described by band-filling effects as ${I}_{\mathrm{exc}}$ is increased. By careful analysis of the excitation intensity dependence of the 〈111〉 linewidth and the corresponding blueshift of the PL peak energy, the strain-induced piezoelectric field is estimated to be 70 kV/cm.

GaNAs/GaInAs short-period superlattice quantum well structures grown by MOCVD using TBAs and DMHy

The MOCVD growth of GaInNAs/GaAs quantum wells (QWs) using TBAs and DMHy was studied for realizing long wavelength lasers. In this study, we propose a short-period superlattice using GaNAs and GaInAs to achieve high nitrogen (N) incorporation. This may solve one of the problems on the limitation of N incorporation in GaInNAs compared with a GaNAs. By comparing the peak wavelength of a photoluminescence with a sample without the DMHy flow, we observed a 20 nm red shift of the emission wavelength from a GaNAs/GaInAs QW sample. The N composition in the GaNAs layer was 0.004 and the required DMHy flow was less than 1/10 compared with that of a GaInNAs QW. An intermediate layer inserted QW structure was also grown using the technique enabling a large N incorporation in GaNAs.

High-field cyclotron resonance and electron–phonon interaction in modulation-doped multiple quantum well structures

A systematic study of electron cyclotron resonance (CR) in two sets of GaAs/Al0.3Ga0.7As modulation-doped quantum-well samples (well widths between 12 and 24 nm) has been carried out in magnetic fields up to 30 T. Polaron CR is the dominant transition in the region of GaAs optical phonons for the set of lightly doped samples, and the results are in good agreement with calculations that include the interaction with interface optical phonons. The results from the heavily doped set are markedly different. At low magnetic fields (below the GaAs reststrahlen region), all three samples exhibit almost identical CR which shows little effect of the polaron interaction due to screening and Pauli-principle effects. Above the GaAs LO-phonon region (B>∼23 T), the three samples behave very differently. For the most lightly doped sample (3×1011cm−2) only one transition minimum is observed, which can be explained as screened polaron CR. A sample of intermediate density (6×1011cm−2) shows two lines above 23 T; the higher frequency branch is indistinguishable from the positions of the single line of the low density sample. For the most heavily, doped sample (1.2×1012cm−2) there is no evidence of high frequency resonance, and the strong, single line observed is indistinguishable from the lower branch observed from sample with intermediate doping density. We suggest that the low frequency branch in our experiment is a magnetoplasmon resonance red-shifted by disorder, and the upper branch is single-particle-like screened polaron CR.

Carrier relaxation and recombination in an InGaN/GaN quantum well probed with time-resolved cathodoluminescence

Spatially, spectrally, and temporally resolved cathodoluminescence (CL) techniques have been employed to examine the optical properties and kinetics of carrier relaxation for metalorganic chemical vapor deposition grown InGaN/GaN single quantum wells (QWs). Cathodoluminescence wavelength imaging of the QW sample revealed local band gap variations, indicating the presence of local In composition fluctuations and segregation during growth. A detailed time-resolved CL study shows that carriers generated in the boundary regions will diffuse toward and recombine at InN-rich centers, resulting in a strong lateral excitonic localization prior to radiative recombination.