Quantum-well Samples Research Articles

Effect of structural defects on InSb/Al xIn 1− x Sb quantum wells grown on GaAs [formula omitted] substrates

We investigate oriented abrupt steps (OASs), a type of surface defect in InSb/Al x In 1− x Sb quantum-well (QW) samples grown on GaAs (0 0 1) substrates. Previous atomic force microscopy studies have reported that the OASs are oriented along the [1 1 0] and [ 1 ̄ 1 0] directions and have an inclination angle of ∼5°–15° with respect to the sample surface. Our plan-view and cross-sectional transmission electron microscopy analyses reveal that the OASs are the terminal edges of threading micro-twins at the sample surface. Hall effect measurements indicate that the density of OASs correlates with the electron mobility in the InSb QWs.

Luminescence energy and carrier lifetime in InGaN/GaN quantum wells as a function of applied biaxial strain

Continuous-wave and time-resolved photoluminescence of InGaN quantum wells are measured as a function of applied biaxial strain, which provides a unique means of altering the built-in polarization field in these structures. The direction and magnitude of the shift of the luminescence-peak energy are quantitatively analyzed within an analytical carrier separation model. It is found that the presently used piezoelectric coefficients of InGaN are not entirely consistent with our experimental results. Instead, consistent interpretation of our data requires the e13 and e33 piezoelectric coefficients of InN to be ∼15% larger than the commonly accepted values. Our analysis allows the assignment of an effective carrier-separation parameter to each investigated quantum-well sample, which quantifies the shift of the luminescence peak energy with the change in the polarization field. The effective carrier separation is found to be zero for narrow quantum wells (<1.5 nm) and asymptotically approaches the full quantum well width for increasing well width. However, heavy doping or increased indium content are found to reduce the effective carrier separation, which is ascribed to screening of the polarization field or localization effects, respectively. A reduction of the carrier lifetime with the application of strain supports the carrier separation model and allows the derivation of a quantity related to the change of the wave function shape with the polarization field.

Mechanisms for photon-emission enhancement with silicon doping in InGaN/GaN quantum-well structures

Material and optical analyses of three InGaN/GaN quantum-well (QW) samples with different silicon-doping conditions were conducted. Quantum-dot (QD) structures were observed in samples of silicon doping either in barriers or wells. The calibrated-radiative lifetimes in both silicon-doped samples showed the consistent trend of the formation of zero-dimensional (0-D) structure upon silicon doping. In optical characterization, the barrier-doped sample showed a blueshift of the photoluminescence (PL) peak, enhancement of integrated PL intensity, reduction of Stokes shift (SS), decrease of carrier-activation energy, and shortening of PL decay time. Except the insignificant PL peak shift, the well-doped sample showed similar trends, although they are not as prominent as the barrier-doped sample. Such results mainly originated from the reduction of the quantum-confined Stark effect (QCSE) within the clusters. Contrary to the interpretation in the past, the major mechanism for QCSE reduction is due to strain relaxation, instead of carrier screening, in conjunction with the formation of QD structures. Such a conclusion is supported by the result of smaller changes of optical behavior in the well-doped sample, in which carrier screening is expected to be more significant. In this sample, besides strain relaxation, enhanced carrier localization (CL) might represent another important mechanism for photon-emission improvement.

Quantum well intermixing enhanced by InP grown by He-plasma assisted GaS source MBE

Studies have been carried out on the enhancement of quantum well intermixing (QWI) using the defects present in an epitaxial layer of InP grown by He-plasma assisted gas source molecular beam epitaxy (He*–InP). The structures investigated were single and double InGaAsP quantum wells (QW) with an InGaAsP barrier. QWI was implemented using rapid thermal annealing (RTA) and measured using low-temperature photoluminescence (PL) spectroscopy. Changes in the PL emission wavelength and intensity were investigated as functions of anneal temperature and time, He*–InP layer thickness, He*–InP layer to QW distance and separation of the QWs in the two QW structure. The PL emission from the single QW structure exhibits a blueshift which increases monotonically with increasing anneal temperature and time, and decreases with the distance between the He*–InP layer and the QW. For the double QW samples, initially only the upper QW emits PL but as the anneal conditions increase in temperature and time the PL emission switches to the lower QW. The defect mechanisms responsible for the interdiffusion processes due to the defects present in the He*–InP layer are discussed, and a tentative explanation of the PL blueshift and intensity behaviour is proposed.

Optical investigation on the annealing effect and its mechanism of GaInAs/GaNAs quantum wells

Thermal annealing of GaInAs/GaNAs quantum wells (QWs) as well as other nitrogen- and indium-contained QW structures grown by molecular beam epitaxy and its effect on optical properties are investigated. The photoluminescence (PL) and photovoltaic (PV) spectra of annealed GaInAs/GaNAs QWs show that the luminescence properties become degraded due to the N diffusion from the GaNAs barrier layers to the GaInAs well layer. Meantime, the annealing-induced blueshift of the PL peak in this QW system is mainly induced by the change of In distribution, suggesting that the In reorganization is greatly assisted by the N-induced defects. The elucidation of annealing effect in GaInAs/GaNAs QW samples is helpful for a better understanding to the annealing effect in the GaInNAs/GaAs QWs.

Ultrafast carrier relaxation in GaN,In0.05Ga0.95N,and anIn0.07Ga0.93N/In0.12Ga0.88Nmultiple quantum well

Room-temperature, wavelength-nondegenerate ultrafast pump/probe measurements were performed on GaN and InGaN epilayers and an InGaN multiple quantum well (QW) structure. Carrier relaxation dynamics were investigated as a function of excitation wavelength and intensity. Spectrally resolved sub-picosecond relaxation due to carrier redistribution and QW capture was found to depend sensitively on the wavelength of pump excitation. Moreover, for pump intensities above a threshold of $100\ensuremath{\mu}{\mathrm{J}/\mathrm{c}\mathrm{m}}^{2},$ all samples demonstrated an additional emission feature arising from stimulated emission (SE). SE is evidenced as accelerated relaxation $(<10\mathrm{ps})$ in the pump-probe data, fundamentally altering the redistribution of carriers. Once SE and carrier redistribution is completed, a slower relaxation of up to 1 ns for GaN and InGaN epilayers, and 660 ps for the multiple QW sample, indicates carrier recombination through spontaneous emission.

Proton-irradiation-induced intermixing of InGaAs quantum dots

Proton irradiation was used to create interdiffusion in In0.5Ga0.5As quantum dots (QDs), grown by low-pressure metalorganic chemical vapor deposition. After 25-keV proton irradiation, the QD samples were annealed at two temperatures (700 or 750 °C) for 30 s. It was found that much lower annealing temperatures were needed to recover the photoluminescence signals than in the quantum-well case. Large blueshifts (120 meV) and narrowing of the photoluminescence spectra were seen. Various doses (5×1013–1×1015 cm−2) and implant temperatures (20–200 °C) were used to study the interdiffusion processes in these samples. In QD samples, much lower doses were required to achieve similar energy shifts than reported in quantum-well samples.

Mapping of strain and electric fields inGaAs/AlxGa1−xAsquantum-well samples by laser-assisted NMR

The usefulness of semiconductor heterostructures derives from the possibility to engineer their electronic and optical properties to match the requirements of many different applications. Optically detected nuclear magnetic resonance provides the possibility to map microscopic properties of such samples with a high spatial resolution through the splitting of resonance lines. In a multiple quantum-well sample, we measure the distortion of the crystal lattice and find variations of the order of ${10}^{\ensuremath{-}5}$ over distances of a few mm. Internal electric fields also cause resonance line splittings. Comparing the electric field-induced resonance line splittings in different quantum wells, we mapped the vertical variation of the electric field from a Schottky contact with a spatial resolution of some 40 nm.

Structural control of Rashba spin-orbit coupling in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum wells

We investigated the correlation between the Rashba spin–orbit coefficient α and potential shape of the quantum wells (QW), where α values are experimentally deduced from the weak antilocalization analysis. We studied the gate I–V properties of the QW samples and have obtained results consistent with the potential shapes predicted for these QWs.

Anisotropic structural and electronic properties of InSb/Al xIn 1− xSb quantum wells grown on GaAs (0 0 1) substrates

Structural defects in InSb/Al x In 1− x Sb quantum-well samples grown on (0 0 1) GaAs substrates have been investigated using transmission electron microscopy. A high density of dislocations is created at the interface between the InSb nucleation layer and the substrate, while {1 1 1}-planar defects are introduced at various locations. An InSb/Al x In 1− x Sb interface 920 nm above the InSb/GaAs interface acts as a trapping site for many dislocations. The majority of the {1 1 1}-planar defects are micro-twins with {1 1 1}-bilayer thickness fluctuations. The anisotropic distribution of the micro-twins correlates with the measured anisotropic electron mobility in the quantum well.

High thermal stability of AlGaAs/GaAs V-grooved quantum wire

In this article, we make a systematic comparison of the photoluminescence (PL) properties of AlGaAs/GaAs V-grooved quantum wire (QWR) structures with their counterpart quantum well (QWL) structures grown in the same system. The QWR structures exhibit constant photoluminescence (PL) intensity up to a temperature Tc of 90, 140, and 170 K for a 2, 5, and 9 nm QWR, respectively. The 9 nm QWR sample showed the highest relative thermal stability with respect to its counterpart 9 nm QWL sample, its PL intensity being stronger than that of the 9 nm QWL up to 270 K. This temperature is very close to room temperature, at which most opto-electronic devices operate. The time-resolved PL measurements provide further confirmation of the PL results. These data show that the PL thermal stability of the V-grooved QWR structures, despite the increased interface to volume ratio, is now comparable to that of the counterpart QWL structures, which is a basic condition for the superior properties of QWR structures to be brought into practical play.

Sub-picosecond electron relaxation of near-infrared intersubband transitions in n-doped (CdS/ZnSe)/BeTe quantum wells

We report on intersubband (ISB) absorption and ultrafast ISB energy relaxation of carriers in n-type doped (CdS/ZnSe)/BeTe quantum wells (QWs), grown by molecular-beam epitaxy. The highly n-type doped QW samples were obtained by introducing a few monolayer CdS into a ZnSe/BeTe QW, and ISB absorption with a peak wavelength as short as 1.62 μm, covering 1.55 μm within its absorption bandwidth, was achieved. The ISB carrier relaxation was investigated by means of femtosecond (∼150 fs) one-color pump and probe technique at the ISB absorption peak. The ISB carrier relaxation time of 270 fs was observed in the sample with the absorption peak at 1.82 μm. The slow decay component with a time constant of a few ps, which has been observed in ZnSe/BeTe QWs, was not observed in the (CdS/ZnSe)/BeTe QWs, indicating that the Γ(ZnSe)–X(BeTe) electron transfer is suppressed, as expected from the band alignment.

Comparative study of ultrafast intersubband electron scattering times at ∼1.55 μm wavelength in GaN/AlGaN heterostructures

We report on a comparative study of room temperature intersubband electron scattering lifetimes in GaN/AlGaN single and coupled double multiple quantum well (QW) samples with peak absorption wavelengths ranging from 1.4 to 1.7 μm. Using time-resolved pump-probe spectroscopy electron scattering times as short as ∼160 fs have been measured for a coupled QW sample and ≲300 fs for single QW samples. While no significant dependence on the excitation power has been observed, a decrease of the scattering times with increasing probe wavelength has been measured and may be attributed to monolayer fluctuations in the samples.

Quasiregular quantum-dot-like structure formation with postgrowth thermal annealing of InGaN/GaN quantum wells

Postgrowth thermal annealing of an InGaN/GaN quantum-well sample with a medium level of nominal indium content (19%) was conducted. From the analyses of high-resolution transmission electron microscopy and energy filter transmission electron microscopy, it was found that thermal annealing at 900 °C led to a quasiregular quantum-dot-like structure. However, such a structure was destroyed when the annealing temperature was raised to 950 °C. Temperature-dependent photoluminescence (PL) measurements showed quite consistent results. Blueshift of the PL peak position and narrowing of the PL spectral width after thermal annealing were observed.

Lattice Strain Induced by Ion Implantation inAlGaAs/AlGaInAs Quantum Well Samples

Si+ ions of 350 keV have been implanted into AlGaAs/AlGaInAs quantumwell samples in the dose range from 5×1013cm-2to5×1014cm-2. The Raman spectra and high resolution x-raydiffraction (HRXRD) were measured from these implanted samples as well asthe non-implanted sample. In the implanted layers the average strain which wasevaluated by HRXRD increases with the implantation doses and varies from0.0011 to 0.0029. The quantum well interface intermixing effect andcompositional modification are also observed from HRXRD. At the higher doses, anabnormal annealing procedure takes place and it partly removes damage, butthe strain remains almost unchanged in the epilayers.

Correlation between exciton decay time and Stokes shift in digital magnetic heterostructures

The exciton recombination time and the exciton photoluminescence Stokes shift are systematically investigated in a set of CdTe/Cd 1 - x Mg x Te quantum-well samples containing MnTe layers with the thickness below or equal to 1 ML. Similar to diluted magnetic semiconductors, these digital magnetic heterostructures exhibit strongly magnetic-field-dependent photoluminescence lines which are inhomogeneously broadened. Both quantities under investigation depend on the Mn content, on the details of the Mn distribution within the quantum wells, and on the energy of detection. Three sets of samples are investigated: digital magnetic heterostructures containing four MnTe layers with equidistant spacing and various thickness of the MnTe layers, quantum wells containing 1-ML MnTe at various positions in the quantum well, and quantum wells containing in total 1-ML MnTe but distributed in the quantum well in different ways. For all samples, the exciton recombination time is strongly correlated to the Stokes shift. This correlation is caused by exciton localization, leading to a mixing of radiatively decaying states with small momentum with nonradiative states with large momentum. In our experiment, however, a linear dependence between the exciton decay time and the Stokes shift is observed, which is not yet explained by theory, to our knowledge. Although, the luminescence properties in semimagnetic semiconductors are affected by exchange interactions leading to the formation of magnetic polarons, this process can be ruled out as the origin of the observed correlation.

Carrier Dynamics in Group-III Nitride Low-Dimensional Systems: Localization versus Quantum-Confined Stark Effect

Continuous-wave and time-resolved optical spectroscopy is used to examine a variety of InGaN/GaN quantum-well and quantum-box samples, grown by molecular beam epitaxy. The results are analyzed in order to clarify the respective influences of electric fields and of carrier localizations on radiative recombinations. The coupling of electron–hole pairs with LO-phonons is also studied in detail, from careful analysis of the size-dependent intensities of LO-phonon replica. From our attempt of modelling the Huang-Rhys factor, S, for excitons in these systems, we conclude that the observed optical recombinations are rather those of electrons and holes separately localized on different potential fluctuations.

Carrier Dynamics in Group-III Nitride Low-Dimensional Systems: Localization versus Quantum-Confined Stark Effect

Continuous-wave and time-resolved optical spectroscopy is used to examine a variety of InGaN/GaN quantum-well and quantum-box samples. grown by molecular beam epitaxy. The results are analyzed in order to clarify the respective influences of electric fields and of carrier localizations on radiative recombinations. The coupling of electron-hole pairs with LO-phonons is also studied in detail. from careful analysis of the size-dependent intensities of LO-phonon replica. From our attempt of modelling the Huang-Rhys factor, S. for excitons in these systems, we conclude that the observed optical recombinations are rather those of electrons and holes separately localized on different potential fluctuations.

MBE growth of BeZnCdSe quaternaries, MgSe/BeZnCdSe superlattice and quantum well structures on InP substrates

BeZnCdSe quaternaries, MgSe/BeZnCdSe superlattice (SL) and BeZnCdSe quantum well (QW) structures were grown on InP substrates by molecular beam epitaxy for the first time. As for a lattice-matched Be 0.08(Zn 0.30Cd 0.70) 0.92Se sample, a single-peak photoluminescence (PL) spectrum was observed at the temperature range from 15 K to room temperature. The PL peak wavelength and the full-width at half-maximum (FWHM) value at 15 K were 572 nm and 8.8 meV, respectively. PL measurements of MgSe/BeZnCdSe SL samples were observed single-peak emissions in the green-to-blue color range from 525 to 470 nm at 15 K with changing MgSe layer-thickness ratio from 0.20 to 0.60. As for the QW sample consisting of four BeZnCdSe QWs with a different well width, PL peaks corresponding to each QW were observed in the wavelength range from 459 to 561 nm at 15 K. By fitting calculated transition energies in the QWs to the PL peak energies, the band offset ratio between the BeZnCdSe well and the MgSe/BeZnCdSe SL barrier was estimated to be ΔE c : Δ E v =9 : 1 .

Semimagnetic self-organized Cd1−xMnxTe quantum dots generated by postgrowth thermal annealing

We demonstrate the possibility to fabricate self-organized semimagnetic quantum dots by the growth of two-dimensional layers and subsequent thermal annealing. In particular, CdTe/CdMgTe quantum-well samples containing four narrow MnTe barriers are characterized by optical methods. After annealing well above the growth temperature, from time-resolved photoluminescence (PL), PL experiments under selective excitation, as well as micro-PL measurements, we find clear evidence for the presence of quantum dots. For a sample with a average Mn content of 5%, the dot luminescence shows a redshift of 18 meV under an applied magnetic field of 4 T, due to the giant Zeeman effect.