Multiple Quantum Well Samples Research Articles

Characteristics of emission polarization in a-plane nanorods embedded with InGaN/GaN multiple quantum wells

Nonpolar (a-plane) GaN nanorod arrays with embedded InxGa1−xN/GaN (x=0.09, 0.14, 0.24, and 0.30) multiple quantum wells (MQWs) grown on r-plane sapphire have been fabricated successfully by self-assembled Ni nanomasks and subsequent inductively coupled plasma reactive-ion etching. After nanorod fabrications, the polarization ratio of the emission from MQWs with lower indium composition (x=0.09 and 0.14) slightly decreases but apparently increases by at most 79% for the samples with higher indium composition (x=0.24 and 0.30). Competition between the effect of multiple scattering, strain relaxation and reduction in localized centers, expected in a-plane MQW samples, are attributed to the variations in the polarization ratios after the nanorod formation.

Optical characterization of quaternary AlInGaN epilayer and multiple quantum wells grown by a pulsed metalorganic chemical vapor deposition

The optical properties of quaternary AlInGaN epilayers and AlInGaN/AlInGaN multiple quantum wells (MQWs) grown by a pulsed metalorganic chemical vapor deposition have been investigated by means of photoluminescence (PL) and time-resolved PL measurements. The PL emission peaks in both AlInGaN epilayers and MQWs show a blueshift with increasing excitation power density. The PL intensities of MQWs are much stronger (∼3–4 times) than that of the epilayer. The PL emission intensities ( I emi) of both AlInGaN epilayers and MQW samples increase superlinearly with increasing excitation power density ( I exc), following a power-law form, I emi ⧜ I exc β . The PL decay times of MQWs are longer than that of epilayer. The longer PL decay times may be due to a stronger localization effect of carriers/excitons at band tail states and wave function separation caused by the quantum confined Stark effect. These results indicate that AlInGaN/AlInGaN MQWs grown by a PMOCVD are promising materials for ultraviolet light emitting diode (LED) applications similar to the InGaN/InGaN system for blue LED applications.

MBE growth and transport properties of silicon δ-doped GaAs/AlAs quantum well structures for terahertz frequency detection

We present the electrical characterization of n-type GaAs/AlAs multiple quantum well (MQW) structures designed for terahertz (THz) radiation sensing at cryogenic temperatures. The samples were grown by solid source molecular beam epitaxy (MBE) and were δ-doped with silicon atoms at each potential well center. Temperature dependent Hall effect data show that (i) the conduction in these planar doped structures is thermally activated below 180 K, (ii) the free carriers sheet densities are near the metal–insulator transition, and (iii) the low temperature mobility is controlled by ionized impurities scattering. The study of the magneto-transport properties at 1.3 K further indicates that only the fundamental electronic sub-band of the two-dimensional electronic gas is populated. The MQW samples were then processed into lateral mesa-shaped photodetectors to investigate their spectral response in the THz frequency range. The preliminary experimental results for the proposed detection scheme, which involves transitions in the confined shallow donor impurity states, are described.

Optical transition dynamics in ZnO/ZnMgO multiple quantum well structures with different well widths grown on ZnO substrates

We report the optical properties of ZnO/ZnMgO multiple quantum well (MQW) structures with different well widths grown on ZnO substrates by molecular beam epitaxy. Photoluminescence (PL) spectra show MQW emissions at 3.387 and 3.369 eV for the ZnO/ZnMgO MQW samples with well widths of 2 and 5 nm, respectively, due to the quantum confinement effect. Time-resolved PL results show an efficient photogenerated carrier transfer from the barrier to the MQWs, which leads to an increased intensity ratio of the well to barrier emissions for the ZnO/ZnMgO MQW sample with the wider well width.

In$_x$Ga$_{1-x}$N–GaN-Based Solar Cells With a Multiple-Quantum-Well Structure on SiCN–Si(111) Substrates

In this letter, the author describes an effective method of obtaining a high photovoltaic efficiency of In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> N-GaN-based solar cells with a multiple-quantum-well (MQW) structure using a SiCN-Si (111) substrate. The MQW region had a higher solar cell absorption than the non-MQW In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> N-GaN sample. Under an air mass 1.5 global solar spectrum, a maximum photovoltaic efficiency of 5.43% in the MQW sample over the control sample was observed. It was found that the device and fabrication technology developed in this study are applicable to the realization of solar cells with high open-circuit voltages ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> ) of 2.86-2.96 V, high short-circuit current densities ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> ) of 2.40-2.45 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and high fill factors of 74.8%-76.5%.

High Photovoltaic Efficiency of InxGa1-xN/GaN-Based Solar Cells with a Multiple-Quantum-Well Structure on SiCN/Si(111) Substrates

In this paper, I describe an effective method of obtaining a high photovoltaic efficiency of InxGa1-xN/GaN-based solar cells with a multiple-quantum-well (MQW) structure using a SiCN/Si (111) substrate. The MQW region had a higher solar cell absorption than the non-MQW InxGa1-xN/GaN sample. Under an air mass 1.5 global solar spectrum, an average increase of 38.5% in the photovoltaic efficiency of the MQW sample over the control sample was observed. It was found that the device and fabrication technology developed in this study are applicable to the realization of solar cells with high open-circuit voltages (Voc) of 2.92 to 3.16 V, high short-circuit current densities (Jsc) of 55.1 to 56.5 mA/cm2, and high fill factors of 89.5 to 91.8%.

Ambipolar spin diffusion and D'yakonov-Perel' spin relaxation in GaAs quantum wells

We report theoretical and experimental studies of ambipolar spin diffusion in a semiconductor. A circularly polarized laser pulse is used to excite spin-polarized carriers in a GaAs multiple quantum-well sample at 80 K. Diffusion of electron and spin densities is simultaneously measured using a spatially and temporally resolved pump-probe technique. Two regimes of diffusion for spin-polarized electrons are observed. Initially, the rate of spin diffusion is similar to that of density diffusion and is controlled by the ambipolar diffusion coefficient. At later times, the spin diffusion slows down considerably relative to the density diffusion and appears to be controlled by a nonconstant (decreasing) spin-diffusion coefficient. We suggest that the long-time behavior of the spin density can be understood in terms of an inhomogeneous spin-relaxation rate, which grows with decreasing density. The behavior of the spin-relaxation rate is consistent with a model of D'yakonov-Perel' relaxation limited by the Coulomb scattering between carriers.

Mid-infrared photoluminescence of PbSe/PbSrSe multiple quantum wells grown by molecular beam epitaxy

Mid-Infrared photoluminescence of PbSe/PbSrSe multiple quantum wells （MQWs） grown by molecular beam epitaxy is studied. High-order satellite peaks are observed by high resolution X-ray diffraction, which indicates the quantum well structure has sharp interfaces. Temperature dependence of photoluminescence spectra shows that the MQWs make good confinement of electrons and holes. At the same measurement temperature, the photoluminescence peaks of the MQW sample show a clear blue shift compared with that of the bulk PbSe material. We find that the intensity of photoluminescence is dependent on the measurement temperature. When temperature increases from 150 K to 230 K, the PL intensity reaches its maximum at 230 K, and with further increasing the intensity decreases slowly. We can still observe strong PL intensity above room temperature, which indicates MQWs have the potential to be used in room-temperature_operating mid-infrared optoelectronic devices.

Effect of Interface Roughness and Dislocation Density on Electroluminescence Intensity of InGaN Multiple Quantum Wells

Effects of interface roughness and dislocation density on the electroluminescence (EL) intensity of InGaN multiple quantum wells (MQWs) are investigated. It is found that the EL intensity increases with the number of satellite peaks in the x-ray diffraction experiments of InGaN MQW samples. It is indicated that the rough interface will lead the reduction of EL intensity of InGaN MQW samples. It is also found that the EL intensity increases with the decrease of dislocation density which is characterized by the x-ray diffraction measurements. It is suggested that the EL intensity of InGaN MQWs can be improved by decreasing the interface roughness and dislocation density.

Photoluminescence study of InGaN/GaN multiple-quantum-well with Si-doped InGaN electron-emitting Layer

InGaN/GaN multiple-quantum-well (MQW) structure with Si-doped InGaN electron-emitting layer (EEL) was grown by metal–organic chemical vapor deposition and their characteristics were evaluated by photoluminescence (PL) measurements. In a typical structure, a low indium composition and wide potential well was used to be an EEL, and a six-fold MQW was used to be an active layer where the injected carriers recombine. By comparing the PL spectral characteristics of the MQW samples, the PL intensity of MQW with EEL is about 10 times higher than that of typical MQW. Experimental results indicate that the high electron capture rate of the MQW active region can be achieved by employing EEL.

Interband and Intersubband Optical Properties of Doped n-$\hbox{Zn}_{\bf 0.46}\hbox{Cd}_{\bf 0.54}\hbox{Se/Zn}_{\bf 0.24}\hbox{Cd}_{\bf 0.25}\hbox{Mg}_{\bf 0.51}\hbox{Se}$ Multiple Quantum Wells for Intersubband Device Applications

Two heavily doped n-type Zn0.46Cd0.54Se/Zn0.24 Cd0.25Mg0.51Se multiple quantum well (MQW) structures have been grown on InP (0 0 1) substrates by molecular beam epitaxy. Photoluminescence (PL), time-resolved PL, and Fourier transform infrared (FTIR) spectroscopy were performed to characterize their interband and intersubband (ISB) properties. These two MQW samples have similar structures except for different well widths and a different number of periods. Excitation-intensity-dependent PL shows no electronic coupling between the multiquantum wells. The integrated PL intensities and the PL decay times of the MQWs were measured as functions of temperature in the range from 77 to 290 K. Theoretical fittings of temperature dependences of integrated PL intensities and PL decay times indicate that the nonradiative recombination processes observed in our samples can be well described by hole capture by acceptor-like defect centers through multiphonon emissions. ISB absorption spectra of the samples were measured by FTIR and show peak absorption at wavelengths of 3.99 and 5.35 mum for the MQWs with well widths of 28 and 42 A, respectively. Theoretical calculations based on the envelope function approximation confirm that these peaks are due to the transitions from the ground state E1 to the first excited state E2.

AlGaN/GaN multiple quantum wells grown by using atomic layer deposition technique

ABSTRACTIn this work, we report on the growth of ultraviolet (UV) AlGaN/GaN multiple quantum wells (MQWs) structure using atomic layer deposition (ALD) technique. The AlGaN/GaN MQW sample grown on the sapphire substrate consisted of three GaN QWs and four AlGaN barriers comprised AlN/GaN superlattices (SLs). The root-mean-square value of the surface morphology was only 0.35 nm observed from the atomic force microscope image and no crack was found on the surface. Both of the high resolution X-ray diffraction curves and transmission electron microscope images showed sharp interfaces between SLs layers and QWs with good periodicity. These results demonstrate that the ALD could be a very useful technique for controlling the crystalline quality and thickness of the III-nitride epilayer.

Cathodoluminescence Study on Spatial Luminescence Properties of InN/GaN Multiple Quantum Wells Consisting of 1-Monolayer-Thick InN Wells/GaN Matrix

Spatially resolved luminescence properties of InN/GaN multiple quantum wells (MQWs) consisting of nominally one monolayer (1-ML)-thick InN QWs embedded in a GaN matrix are studied by cross-sectional and plan-view cathodoluminescence measurements. First it is confirmed that the dominant emission peaks observed at around 390 nm to 430 nm in the MQWs samples are attributed to the effects of inserting ∼1-ML-thick InN wells in the GaN matrix, resulting in efficient localization of GaN excitons at InN QWs. Furthermore, it is revealed that the detailed structure of the MQWs, such as the thickness distribution and interface sharpness, is very sensitive to the presence of surface defects such as hillocks around screw-component threading dislocations, resulting in different emission wavelengths/energies. This is because the epitaxy process for depositing such thin InN wells is seriously affected by the atomic-level surface structures/properties of the growth front. It will be concluded that it is necessary to use lower dislocation density GaN bulk templates to obtain much higher structural quality InN/GaN MQWs good enough for characterizing their optical properties.

The effect of hole confinement on photoluminescence from Er in SiGe/Si quantum wells

We have investigated the photoluminescence (PL) decay from Er implanted, or grown, into strained SiGe/Si multiple quantum well (MQW) structures grown by molecular beam epitaxy, and compared it with that of Er implanted into crystalline Si. The Er concentration in all MQW structures was of the order of 4 × 10 18 cm −3. Prior to the erbium implant, PL originating from confined states in the strained MQWs confirmed that this was an optically active MQW sample. The implanted MQW sample was re-grown at 550 °C for 5 h and X-ray diffraction measurements after re-growth showed that a high degree of strain had been retained. The Er emission at 1.54 μm was more intense from all MQW structures than from the implanted Si, but the PL decay time was not significantly different, suggesting that the excitation efficiency is improved when Er is in a quantum well structure and close to a long lived hole population, but the recombination efficiency is largely unaffected. When the excitation laser power density was increased by 10 times, the PL decay from the Er in the MQW structures was largely unaffected, but the PL decay time from Er in Si reduced; this is discussed in terms of non-radiative competition at end-of-range damage. It is proposed that the presence of a long-lived hole population increases the potential for efficient excitation of Er in the MQW structures.

Improvements of quantum efficiency and thermal stability by using Si delta doping in blue InGaN/GaN multiple quantum well light‐emitting diodes

AbstractThe improvements of quantum efficiency and thermal stability by using Si delta doping in blue InGaN/GaN multiple quantum well (MQW) structures have been investigated. The high‐resolution X‐ray diffraction (HR‐XRD) indicated better interface properties in the MQWs for Si delta doping structures. The Arrhenius plots of the temperature‐dependent photoluminescence (PL) spectra were used to reveal the enhancements of the radiative recombination. The activation energy was increased from 115 to 140 meV for the InGaN/GaN MQW sample with Si delta doping. The improvements of the thermal stability and activation energy were attributed to the supply of electrons from the Si delta doping barrier region into the InGaN well layer and the increase in hole capturing by the higher energy barrier in the valence band. Si delta doping process plays a crucial role in carrier recombination to significantly suppress carrier leakage, which not only increases the exciton confinement, but also improves the quantum efficiency. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structural and Optical Properties of Nonpolar InGaN/GaN Multiple Quantum Wells Grown on Planar and Lateral Epitaxially Overgrown m-Plane GaN Films

The structural and optical properties of nonpolar (1100) m-plane InGaN/GaN multiple quantum wells (MQWs), grown simultaneously on reduced-defect lateral epitaxially overgrown (LEO) m-plane GaN and high-defect-density planar m-plane GaN, were investigated. Atomic force microscopy images of the LEO m-plane MQW sample revealed the presence of smooth steps on the Ga-face wings. The presence of stacking-fault-related defects was observed in the window and the N-face wing regions. Transmission electron microscopy on the sample revealed abrupt quantum well interfaces. High-resolution X-ray diffraction analysis showed that the In incorporation was the same in the LEO sample and in the planar sample. Spot-excitation cathodoluminescence measurement revealed a similar emission wavelength for the nearly defect-free laterally overgrown regions and the defective window regions. However, the emission from the reduced-defect-density wings of the LEO m-plane GaN was more than six times stronger than the emission from the neighboring defective windows. Wide-area photoluminescence measurement showed a similar emission wavelength for the LEO sample and the planar MQW sample.

Large g factors of higher-lying excitons detected with reflectance difference spectroscopy in GaAs-based quantum wells

Exciton g factors in GaAs-based quantum wells (QWs) were evaluated by reflectance difference spectroscopy (RDS) under a weak magnetic field. The well-width dependence of the n=1 heavy-hole exciton (1H1E) g factor agrees well with the reported results, demonstrating RDS as a sensitive tool for detection of g factor. By comparison, the n=1 light-hole exciton g factor increases with the well width, and shows a larger value than that of 1H1E. In a 20-nm-wide Al0.02Ga0.98As∕AlAs multiple QW sample, the g factors of up to ten excitons are obtained, and the higher-lying exciton g factors are found to be one order larger than that of the 1H1E exciton.

Subnanometer-scale size effects in electronic spectra ofSi∕SiO2multiple quantum wells: Interferometric second-harmonic generation spectroscopy

Resonant electronic structure of amorphous-$\mathrm{Si}∕\mathrm{Si}{\mathrm{O}}_{2}$ multiple quantum wells (MQWs) is studied by means of second-harmonic generation (SHG) spectroscopy and SHG interferometric spectroscopy of the MQW samples with the Si quantum well thickness $d$ ranging from $1.00\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}0.25\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The observed modification of the SHG spectra upon decreasing $d$ is interpreted using a combination of the resonant two-subband approximation for the nonlocal optical response of each quantum well with the generalized transfer-matrix formalism for the description of light propagation across the whole MQW structure. Agreement with the experiment shows that the description of the quadratic optical response of the MQW structure within the model of a nonlocal piecewise-continuous medium remains valid on the subnanometer scale.

Growth of GaNAs/GaAs Multiple Quantum Well by Molecular Beam Epitaxy Using Modulated N Radical Beam Source

GaNAs/GaAs multiple quantum well (MQW) structures have been grown on GaAs(001) substrates by molecular beam epitaxy (MBE) using modulated N radical beam source under optimized conditions, wherein the amount of N2 gas flow, RF-power and shutter sequence are systematically controlled. Clear and flat GaNAs/GaAs interfaces were observed in the cross-sectional transmission electron microscopy (TEM) measurements. Fine MQW structures originating from the precise control of the modulated N radical beam have been demonstrated as clear satellite peaks from the X-ray diffraction (XRD) measurements and sharp photoluminescence (PL) peaks. The step-like behaviors in the absorption spectra which reflect the density of state in two-dimensional systems, were clearly observed for all MQW samples.

Optical Properties of Zn0.46Cd0.54Se/Zn0.24Cd0.25Mg0.51Se Multiple Quantum Wells for Infrared Photodetector Applications

ABSTRACTRecently, lattice-matched Zn0.46Cd0.54Se/ZnCdMgSe multiple-quantum-wells (MQWs) have been recognized as very promising materials to fabricate intersubband (ISB) devices such as quantum cascade lasers and mid-infrared photoconductors. These structures have important applications in biological and chemical detections. The ISB transition covers a wide mid-infrared wavelength range from 1.3 μm to a few tens of μm.In this work, two heavily doped n-Zn0.46Cd0.54Se/Zn0.24Cd0.25Mg0.51Se MQW structures have been grown on InP (001) substrate by molecular beam epitaxy. Temperature dependent steady-state photoluminescence (SSPL), temperature dependent time- resolved photoluminescence (TRPL), and Fourier transform infrared spectroscopy (FTIR) were performed to characterize their interband and ISB properties. These two MQW samples have similar structures except different well widths and different number of periods. The integrated SSPL intensities and the PL decay times of the MQWs were measured as functions of temperature in the range from 77 K to 290 K. The luminescence efficiency of the sample with 28 Å well width is larger than that of the sample with 42 Å well width although both samples exhibit similar temperature dependence of PL intensity. Time-resolved PL measurements show that the PL decay times of both samples decrease with increasing temperature. From 77 K to 290 K, the decay time of the sample with 28 Å well width is in the range of 440 ps ∼ 120 ps and is much longer than that of the sample with 42 Å well width, which is in the range of 65 ps ∼ 25 ps. Strong non-radiative recombinations dominate the luminescence behavior of the wider MQWs. Intersubband absorption spectra of the samples were measured by FTIR and show peak absorption at wavelengths of 3.99 μm and 5.35 μm for the MQWs with well widths of 28 Å and 42 Å, respectively, falling within the 3-5 Åm range, which is of great interest for the infrared photodetector applications.