Multiple Quantum Well Samples Research Articles

InGaAs ∕ InP quantum well intermixing studied by high-resolution x-ray diffraction and grazing incidence x-ray analysis

Following a study of implantation enhanced interdiffusion of InGaAs∕InP multiple quantum well (MQW) structures by cross-sectional scanning tunneling microscopy (XSTM), the techniques of low temperature photoluminescence spectroscopy, high-resolution x-ray diffraction (HRXRD), and grazing incidence x-ray analysis (GIXA) are used to independently investigate the suitability of a square well model for the interdiffused MQW profiles, and the observed dependence of strain development as a function of the implanted ion range relative to the MQW stacks. In agreement with previous XSTM findings, when ions are implanted through the MQWs, HRXRD measurements indicate equivalent extents of interdiffusion occurring on both sublattices, while GIXA measurements further indicate the compositional profiles to be non-Fickian and compatible with uniformly broadened square well distributions. Following shallow ion implants (where ions are deposited between the MQWs and the sample surface), (004) HRXRD measurements indicate preferential group V interdiffusion. Dynamical simulations of the superlattice envelope in the (004) HRXRD rocking curves show the compositional profiles to be non-Fickian and compatible with a square well model for the broadened compositional profiles. Additional analysis of the (001) bilayer spacing from previously published XSTM linescan data for this structure is also consistent with this finding. Results of a preliminary photoluminescence and HRXRD investigation of disordering induced by indium implants and the effects of extended annealing on a series of MQW samples (with and without implantation) are presented. Implications for the implantation enhancement of interdiffusion in the InGaAs∕InP material system are discussed. The interpretation of quantum well interdiffusion experiments in this material system in terms of Fickian diffusion models warrants revision in light of the present findings.

Optical pumping of IV-VI semiconductor multiple quantum well materials using a GaSb-based laser with emission at λ=2.5μm

Room-temperature photoluminescence (PL) measurements of IV-VI semiconductor multiple quantum well (MQW) structures using diode laser optical pumping at two different excitation wavelengths, 2.5 and 0.91μm, are described. Active region temperatures during continuous-wave optical pumping were determined from blueshifts in PL emission energies. Temperatures were between 22.7 and 29.5°C lower for 2.5-μm pumping as compared to 0.91-μm pumping at the same power level of 100mW. Heating effects are also shown to be smaller for MQW samples with more narrow PbSe wells.

Influence of the deposition time of barrier layers on optical and structural properties of high-efficiency green-light-emitting InGaN∕GaN multiple quantum wells

We reported the effect of the deposition time of barrier layers on optical and structural properties of high-efficiency green-light-emitting InGaN∕GaN multiple quantum wells (MQWs) by photoluminescence (PL) and high-resolution x-ray diffraction techniques. The MQW samples on (0001)-plane sapphire substrates were prepared with a ramping method by metalorganic chemical deposition. It was found that the structural or interface quality of the MQW system improved as the deposition time of barrier layers increased from 10 to 14min, but lattice relaxation was still observed. The relaxation degree decreases from 33% to 6% as the deposition time increases. Temperature-dependent PL measurements from 12 to 300K indicated that the integrated PL intensities start to decay rapidly as temperature rises above 50K for the sample with the shorter deposition time, and above 100K for the sample with the longer deposition time. The luminescence thermal quenching of the two samples suggests the two nonradiative recombination centers based on a fit to Arrhenius plot of the normalized integrated PL intensity over the entire temperature range. The first centers at higher temperatures show less difference for the two samples. The second centers at lower temperatures can be attributed to the trapping of carriers at the rough interface for the sample with the shorter deposition time and to the thermal quenching of bound excitons for the sample with the longer deposition time, respectively.

Optical properties of GaN/AlN multiple quantum wells

Optical properties of GaN/AlN multiple quantum wells (MQW) have been investigated by Raman scattering, photoluminescence and photoluminescence excitation measurements. A careful examination of the Raman spectrum reveals the fact that the constituent layers of GaN/AlN MQWs are well strained. The experimental results of emission and absorption in MQWs were compared with the calculated solutions of the finite quantum well and the bound states involved in the optical transitions were identified. It is found that the interband transitions up to n=3 bound state can be observed in the strained GaN/AlN MQWs sample. The temperature dependence of the heavy-hole transitions shows an interesting phenomenon, in which the peak energy first increases with increasing temperature and then decreases with the temperature rapidly. The observation can be explained in a consistent way by the strain effects of lattice mismatch due to the interplay between the thermal expansion of GaN and AlN layers. Our results indicate that pseudomorphic GaN/AlN MQWs with good quality can be readily grown, and their applications in optoelectronics can be expected in the near future.

Influence of polarization fields and depletion fields on photoluminescence of AlGaN/GaN multiple quantum well structures

AbstractWe report on a detailed study of low temperature photoluminescence (PL) in Al0.07Ga0.93N/GaN multiple quantum wells (MQWs). The structures were grown on sapphire with the conventional low temperature AlN nucleation layer and thick GaN buffer layer. Several sets of 5 QW MQW samples were studied, one set with Si doping in the barriers up to or above the metallic limit. Nominally undoped MQW samples were also studied. The spectral behaviour of the doped samples was strongly affected by the near surface depletion field, causing overlap of different spectra from non‐equivalent QWs. The QWs closest to the surface are presumably inactive in some samples, due to a very high depletion field. For the case of undoped samples, on the other hand, the near surface QWs are active and most prominent in the PL spectra. The structure from discrete well width variations is here resolved in the PL spectra. The results demonstrate that for structures with no additional capping layer both the depletion field and the polarisation fields need to be considered in the interpretation of experimental data. The theoretically estimated fields in this work are consistent with the experimental spectra. The presence of localisation even in the case of metallic samples, as observed by a constant PL decay time independent of doping, is discussed in terms of penetration of the hole wave functions into the AlGaN barriers. This localisation is also manifested in a sizeable LO phonon coupling strength in all samples studied.

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.

Photoluminescence of InGaN/GaN multiple quantum wells originating from complete phase separation

The photoluminescence (PL) spectra of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition were investigated by varying the PL temperature and excitation power density. Two different sapphire substrates, an exact c-plane and a c-plane with a 2° offset towards the a-plane, were used. The MQW samples were grown under the same conditions. A shift of the PL peak position to a longer wavelength was observed on the off-axis substrate. At all temperatures, the PL spectra from InGaN/GaN MQWs grown on the on-axis substrate showed a double-peak structure, while those from the MQWs grown on the off-axis substrate showed a single-peak structure. With increasing excitation power density, a larger blueshift was found on the off-axis substrate. These experimental results were discussed based on the segregation effect in InGaN MQWs. The high In composition regions on the off-axis substrate play an important role in the radiative recombination process.

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.

Fabrication of wavelength-shifted In0.2Ga0.8As/GaAs multiple quantum well laser diodes by impurity-free vacancy disordering at different thermal annealing temperatures

We fabricated ridge-waveguide laser diodes with In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) that have undergone impurity-free vacancy disordering (IFVD) using the SiO2 capping layer. The SiO2-capped MQW samples were annealed by rapid thermal annealing (RTA) at different temperatures. The magnitude of the blueshift estimated by the shift of photoluminescence peak energy increases with the increase of RTA temperature due to the enhanced interdiffusion of the MQWs. The electrical characteristics and lasing performance of the annealed laser diodes were compared with those of the as-grown laser diode. The operation wavelengths of 967, 946 and 927 nm with device performance comparable to that of the as-grown device were obtained in the laser diodes fabricated by the MQWs that have undergone IFVD at 850, 900 and 950 °C for 50 s, respectively. Thus, the RTA for the IFVD employed to fabricate wavelength-shifted laser diodes is believed not to seriously degrade the electrical and optical properties of the devices.

Well-width dependence of optical properties of rare-earth ion-dopedZnS0.8Se0.2/undopedZnS multiple quantum wells

We compare optical properties of Sm 3 + -doped ZnS 0 . 8 Se 0 . 2 /undoped ZnS multiple-quantum wells (MQWs) with different well widths (2, 5, 10 monolayers) and bulk ZnS 0 . 8 Se 0 . 2 :Sm 3 + crystal. The excitonic peak in the photoluminescence excitation spectrum of the Sm 3 + luminescence shifts to the shorter-wavelength side with reducing well width, which shows that the excitation of Sm 3 + occurs through the energy transfer from the spatially confined excitons. The activation energy of the thermal quenching of the Sm 3 + luminescence is found to increase with reducing well width. This result is interpreted as due to the difference in the spatial confinement effect on the binding energy of the free exciton. In addition, the quantum efficiency of the Sm 3 + luminescence at 4 K is found to increase remarkably with reducing well width. At this temperature, the quantum efficiency of the two monolayers MQW sample is more than 16 times as high as that of the bulk sample. Probable causes of this fact are discussed.

Calculation of Near-Infrared Intersubband Absorption Spectra in GaN/AlN Quantum Wells

Intersubband transitions in GaN multiple quantum wells (MQW) are expected to be applicable to 1-Tb/s all-optical switches. The near-infrared (1.33–2.15 µm) intersubband absorption spectra of GaN/AlN MQW samples have been theoretically investigated. The measured spectra have been well explained by a model in which zero-voltage drop across one quantum well period and 0.5- to 0.75-nm-thick interface graded layers were assumed. The effective electric field in the well was estimated to be greater than 5 MV/cm. For thick wells, absorption spectra are sensitive to the electric field, and hence to the strain, barrier thickness, and doping level. In thin wells, the effect of the electric field is negligible, but the quality of the interface affects the spectra. The calculation model has been applied to the design of coupled quantum wells (CQW), where faster optical response is expected. Higher-order intersubband absorption at wavelengths of less than 1 µm is also expected to be enhanced in CQWs.

Influence of annealing condition on photoluminescence characteristics of AlGaAs/GaAs multiple quantum well

The energy levels in AlGaAs/GaAs multiple quantum well (MQW) were calculated with transfer matrix method (TMM) and compared with experimental results from photoluminescence (PL) measurements. The comparison shows good agreement. A thin layer of sputtered SiO 2 film was deposited on AlGaAs/GaAs MQW samples and a subsequent rapid thermal annealing (RTA) was carried out. Photoluminescence (PL) measurements showed that the PL characteristics of the wells at different depths beneath the surface of the AlGaAs/GaAs MQW behave differently with the change of RTA condition. The PL intensities of the wells close to the surface and the bottom of the AlGaAs/GaAs MQW are sensitive to the RTA condition, while those of the wells in the middle of the MQW show little variations with the change of RTA condition.

Intersubband transitions in proton irradiated In0.52Ga0.48As/In0.52Al0.48As multiple quantum wells grown on semi-insulating InP substrate

Intersubband transitions in In0.52Ga0.48As/In0.52Al0.48As multiple quantum wells (MQWs) were investigated as a function of 1 MeV proton irradiation dose and thermal annealing temperature. It is observed that proton doses as high as 1×1014 cm−2 do not have a measurable effect on the intensity or the peak position energy of the intersubband transitions. While a dose of 1×1014 cm−2 has shown a detrimental effect on the intersubband transitions in the GaAs/AlGaAs MQWs, the intersubband transitions in InGaAs/InAlAs MQWs withstood proton doses as high as 1×1015 cm−2 and completely depleted after irradiation with a dose of 3×1015 cm−2. Furnace thermal annealing of the heavily irradiated samples shows that the depleted intersubband transitions in InGaAs/InAlAs MQW samples were almost completely recovered.

Phonon emission by photoexcited carriersin InGaN/GaN multiple quantum wells

The effect of well width on both the photoluminescence (PL) andphonon emission in optically excited InGaN multiple quantum well(MQW) samples has been investigated. For narrow MQW samples(w<2.5 nm), the low-temperature PL quantum efficiency is closeto unity with the phonon emission being due mainly to carrierrelaxation in the QWs. For wider MQWs samples the PL quantumefficiency is reduced and the intensity of the phonon emissionincreases. We explain this in terms of the non-radiativerecombination processes in the QWs which result in phononemission and compete with the radiative process.

Electron Energy Relaxation via Acoustic Phonon Emission in GaAs/Ga1?xAlxAs Multiple Quantum Wells: Effects of Base Lattice Temperature

The two-dimensional (2D) electron energy relaxation associated with acoustic phonon emission in GaAs/Ga1—xAlxAs multiple quantum wells (MQW) has been investigated using hot-electron Shubnikov-de Haas (SdH) effect measurements performed at three different base lattice temperatures TL0 ≅ 1.7, 3.5 and 5.9 K. The modulation-doped MQW samples studied have quantum well widths LZ = 51, 75 and 78 Å, and only the lowest subband in each sample is populated with a 2D electron density of about 1.10 × 1016 m—2. The electron temperature (Te) has been determined from the lattice-temperature and electric-field dependencies of the amplitude of the SdH oscillations. The energy-loss rates show a power-law dependence on Te with an exponent γ, which depends on TL0. The experimental results are compared with the current theoretical models for power loss in 2D and 3D semiconductors, which include both piezoelectric and deformation-potential scattering. The electron-temperature dependence of power loss, determined experimentally at liquid-helium temperatures with TL0 ≅ 1.7 K, fits well to both the 2D and 3D theoretical models in the low-temperature regime, while the results at TL0 ≅ 3.5 and 5.9 K fit best to those in the intermediate-temperature regime. The results provide useful information about the relative magnitude of the deformation-potential and piezoelectric contributions to power loss.

Hot electron energy relaxation via acoustic-phonon emission in GaAs/Ga1−xAlxAs multiple quantum wells: well-width dependence

The well-width dependence of the two-dimensional (2D) electron energy relaxation associated with acoustic-phonon emission in GaAs/Ga1−xAlxAs multiple quantum wells (MQWs) has been investigated using Shubnikov-de Haas (SdH) effect measurements in the temperature range of 3.3–15 K and at applied electric fields up to 300 V m−1. The modulation-doped MQW samples studied have quantum-well widths (Lz) in the range between 40 and 145 Å; only the lowest subband in each sample is populated with a 2D carrier density in the range from 1.04 × 1016 m−2 to 1.38 × 1016 m−2. The power loss–electron temperature characteristics of the samples have been obtained from the lattice temperature and electric field dependences of the amplitude of the SdH oscillations. It is found that the power loss decreases markedly when Lz increases from 40 Å to about 120 Å and it increases for Lz > 120 Å. The experimental results are compared with the current 2D and three-dimensional (3D) theoretical models for power loss, which include both piezoelectric and deformation-potential scattering. The electron-temperature dependence of power loss determined experimentally fits well to both the 2D and 3D theoretical models in the intermediate-temperature regime but with different values for the acoustic deformation potential. It is shown that for samples with Lz in the range of 40–120 Å, the 2D model predicts a dependence of power loss on the well width, which is similar to that observed experimentally. The 3D model, however, predicts a power loss which increases with increasing well width and describes well the well-width dependence of the experimental power loss for wide wells (Lz ≥ 120 Å). The results provide useful information about the relative magnitude of the deformation-potential and piezoelectric contributions to power loss.

Phonon and Photon Emission from Optically Excited InGaN/GaN Multiple Quantum Wells

The effect of the well width on both the photoluminescence (PL) and phonon emission in InGaN multiple quantum well (MQW) samples has been investigated. For narrow MQW samples (w = 1.25, 2.5 nm) the low temperature PL quantum efficiency is close to unity with the phonon emission being due mainly to carrier relaxation in the QWs. For wider MQWs the PL quantum efficiency is reduced and the intensity of the phonon emission increases. We explain this in terms of non-radiative recombination processes in the QWs which result in phonon emission.

Photoluminescence Excitation Spectroscopy of InxGa1xN/GaN Multiple Quantum Wells with Various In Compositions

Luminescence properties of InxGa1—xN/GaN multiple quantum wells (MQWs) with various In compositions have been studied by means of photoluminescence excitation (PLE) spectroscopy. The clear peak due to the absorption of InxGa1—xN quantum wells was observed in the PLE spectrum of the MQW sample with x < 0.01 at 4 K, and the Stokes shift was estimated to be 63 meV. It was found from temperature-dependent PLE measurements that the Stokes shift was independent of temperature up to 300 K. This result suggests that the large Stokes shift cannot be explained only by the effect of carrier localization due to compositional fluctuation.

Photoluminescence Excitation Spectroscopy of InxGa1-xN/GaN Multiple Quantum Wells with Various In Compositions

Luminescence properties of In x Ga 1-x N/GaN multiple quantum wells (MQWs) with various In compositions have been studied by means of photoluminescence excitation (PLE) spectroscopy. The clear peak due to the absorption of In x Ga 1-x N quantum wells was observed in the PLE spectrum of the MQW sample with x < 0.01 at 4 K, and the Stokes shift was estimated to be 63 meV. It was found from temperature-dependent PLE measurements that the Stokes shift was independent of temperature up to 300 K. This result suggests that the large Stokes shift cannot be explained only by the effect of carrier localization due to compositional fluctuation.

Phonon and Photon Emission from Optically Excited InGaN/GaN Multiple Quantum Wells

The effect of the well width on both the photoluminescence (PL) and phonon emission in InGaN multiple quantum well (MQW) samples has been investigated. For narrow MQW samples (w = 1.25, 2.5 nm) the low temperature PL quantum efficiency is close to unity with the phonon emission being due mainly to carrier relaxation in the QWs. For wider MQWs the PL quantum efficiency is reduced and the intensity of the phonon emission increases. We explain this in terms of non-radiative recombination processes in the QWs which result in phonon emission.