Deep Quantum Wells Research Articles

Photovoltaic Detector Based on Type II Heterostructure with Deep AlSb/InAsSb/AlSb Quantum Well in the Active Region for the Mid-Infrared Spectral Range

Photodetectors for the spectral range 2-4 μm, based on an asymmetric type-II heterostructure p-InAs/AlSb/InAsSb/AlSb/(p, n)-GaSb with a single deep quantum well (QW) or three deep QWs at the heterointerface, have been grown by metal-organic vapor phase epitaxy and analysed. The transport, luminescent, photoelectric, current-voltage, and capacitance-voltage characteristics of these structures have been examined. A high-intensity positive and negative luminescence was observed in the spectral range 3-4 μm at high temperatures (300–400 K). The photosensitivity spectra were in the range 1.2–3.6 μm (T = 77 K). Large values of quantum efficiency (η = 0.6–0.7), responsivity (Sλ = 0.9–1.4 A·W1), and detectivity D*λ 3.5·1011 to 1010 cm·Hz1/2·W−1) were obtained at T = 77–200 K. The small capacitance of the structures (C = 1.5 pF at V = −1 V and T = 300 K) enabled an estimate of the response time of the photodetector at τ = 75 ps, which corresponds to a bandwidth of about 6 GHz. Photodetectors of this kind are promising for heterodyne detection of the emission of quantum-cascade lasers and IR spectroscopy.

Resonance coulomb trapping of electrons in a deep quantum well

The role of the Coulomb interaction in trapping of electrons in a deep quantum well is investigated. By an example of a three-level quantum well, the fundamental mechanisms of trapping of electrons are considered: upon interaction with optical phonons and Coulomb interaction of electrons with one another. The corresponding trapping probabilities and lifetimes of electrons are calculated. With regard to the effect of Auger recombination on the charge-carrier distribution in a quantum well, the system of rate equations for the nonstationary regime is solved and the time dependences of electron concentrations at the ground energy level in the quantum well are determined. The contribution of each recombination process is shown.

High Sensitivity InAs DQW Linear Hybrid Hall ICs with InAs Deep Quantum Well Hall Elements

SUMMARY By electrically connecting and placing an InAs deep quantum well (DQW) Hall element as a magnetic sensor chip and an Si IC linear amplifier in a small plastic package, a very small InAs DQW linear hybrid Hall IC (InAs DQW LHHIC) with a high magnetic field sensitivity has been developed. The output voltage of the hybrid Hall IC showed a very small temperature coefficient of 0.02%/°C and the response time was very low, less than 3 μs. By using this InAs DQW LHHIC, practical current sensors with high sensitivity, high accuracy, and temperature stability have been developed.

Terahertz oscillation of resonant tunneling diodes with deep and thin quantum wells

Terahertz oscillation of resonant tunneling diodes with deep and thin quantum wells

Impact of the interfaces in the charge trap layer on the storage characteristics of ZrO2/Al2O3 nanolaminate-based charge trap flash memory cells

The charge trap flash memory cells incorporating high-k ZrO2/Al2O3 nanolaminate as charge trapping layers and amorphous Al2O3 as tunneling and blocking layers were prepared, investigated and optimized. The interfaces between ZrO2/Al2O3 nanolaminate play an important role in the charge storage characteristics. With increasing number of the interfaces in the charge trapping layer, the memory window increases first and then decreases due to electrostatic repulsion between the trapped electrons. A satisfactory retention performance was observed in the optimized cell structure, which was attributed to the deep quantum wells between the ZrO2/Al2O3 nanolaminate.

Acceleration of carrier recovery in a quantum well semiconductor optical amplifier due to the tunneling effect

In this paper, we theoretically demonstrate that carrier recovery can be accelerated through the tunneling effect in a novel (to our knowledge) quantum well (QW) semiconductor optical amplifier. In the active region, we design the repeated element, including a shallow QW and a following deep QW. Through numerical calculation, we find this novel structure is helpful for improving the dynamic characteristics. In the single element, the shallow QW acts as a perfect carrier reservoir, while the deep QW acts as a “real” active region. Gain recovery time is shortened significantly.

Analysis of TM mode light extraction efficiency enhancement for deep ultraviolet AlGaN quantum wells light-emitting diodes with III-nitride micro-domes

Analysis of transverse magnetic (TM) mode light extraction efficiency enhancement for AlGaN quantum wells (QWs) based deep ultraviolet (UV) light-emitting diodes (LEDs) with III-nitride micro-hemisphere and micro-dome structures on the p-type layer are studied and compared to that of the conventional deep-UV LEDs with flat surface. The transverse electric (TE) and TM components of the spontaneous emission of AlGaN QWs with AlN barriers were calculated by using a self-consistent 6-band k∙p method, which shows the TM component overtakes the TE component and becomes the dominant contribution of the spontaneous emission when the Al-content of the AlGaN QWs is larger than 0.66. The TM mode light extraction efficiency of the deep-UV LEDs emitting at 250 nm with AlGaN micro-domes as compared to the conventional LEDs with flat surface is calculated based on three dimensional finite difference time domain (3D-FDTD) method. The effects of the III-nitride micro-dome diameter and height as well as the p-type layer thickness on the light extraction efficiency were comprehensively studied. The results indicate optimized light extraction efficiency enhancement (>7.3 times) of the dominant TM polarized spontaneous emission for deep-UV LEDs with III-nitride micro-domes.

Resonant thermotunneling design for high-performance single-junction quantum-well solar cells

In a material system displaying a negligible valence band offset, which enables the smooth transport of holes, we show that the conduction band (CB) confinement energies and barrier thicknesses can be designed to favor a sequential thermionic promotion and resonant tunneling of electrons to the CB continuum resulting in an overall faster carrier collection. Using 1 eV dilute nitride semiconductor quantum wells that are embedded in conventional GaAs solar cells, we present practical energy-level engineering designs that significantly facilitate the collection of all photogenerated carriers within several picoseconds (10 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{-12}$</tex></formula> s) from deep quantum wells rather than several nanoseconds, as it is the case for conventional designs. A preliminary evaluation of a GaAs/GaAsN multiquantum well device that incorporates such thermotunneling design indicates potential for significant efficiency improvement over a conventional GaAs solar cell, thus surpassing the Shockley–Queisser efficiency limit for a single-junction device.

Shallow–Deep InGaN Multiple-Quantum-Well System for Dual-Wavelength Emission Grown on Semipolar ( $$ 11\bar{2}2 $$ ) Facet GaN

We report growth and characterization of a shallow–deep InGaN/GaN multiple-quantum-well (MQW) system for dual-wavelength emission grown on semipolar ( $$ 11\bar{2}2 $$ ) facet GaN. Structural and optical properties of the InGaN multiple-quantum-well system were investigated by scanning electron microscopy (SEM), cross-sectional scanning transmission electron microscopy (XSTEM), photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved photoluminescence (TRPL) measurements. Cross-sectional transmission electron microscopy (XTEM) revealed that the growth rate of the InGaN well layers on the (0001) flat top microfacet (~500 nm) was about six times as fast as on the ( $$ 11\bar{2}2 $$ ) inclined facet, whereas the growth rate of GaN barrier layers on the (0001) flat top facet was roughly 4.5 times as large as that on the ( $$ 11\bar{2}2 $$ ) facet. A room-temperature PL spectrum showed dual-wavelength light emission of the shallow–deep InGaN multiple-quantum-well system situated at 2.720 eV (455 nm) and 2.967 eV (418 nm). The Stokes shifts between the two PL peaks and the two “effective bandgaps” were ~260 meV in energy for the deep quantum wells and ~233 meV for the shallow quantum wells. The TRPL decay demonstrated the short radiative recombination lifetime on the order of several nanoseconds in the InGaN MQW system. Realization of the shallow–deep InGaN multiple-quantum-well system with emission wavelength controllability would be useful to achieve III-nitride-based multicolor light-emitting devices for displays.

Photovoltaic detector based on type II heterostructure with deep AlSb/InAsSb/AlSb quantum well in the active region for the midinfrared spectral range

Photodetectors for the spectral range 2-4 μm, based on an asymmetric typeII heterostructure pInAs/AlSb/InAsSb/AlSb/( p, n)GaSb with a single deep quantum well (QW) or three deep QWs at the het� erointerface, have been grown by metal-organic vapor phase epitaxy and analyzed. The transport, lumines� cent, photoelectric, current-voltage, and capacitance-voltage characteristics of these structures have been examined. A highintensity positive and negative luminescence was observed in the spectral range 3-4 μm at high temperatures (300-400 K). The photosensitivity spectra were in the range 1.2-3.6 μm (T = 77 K). Large values of the quantum yield (η = 0.6-0.7), responsivity (Sλ = 0.9-1.4 A W -1 ), and detectivity ( = 3.5 × 10 11 to 10 10 cm Hz 1/2 W -1 ) were obtained at T = 77-200 K. The small capacitance of the structures (C = 7.5 pF at V = -1 V and T = 300 K) enabled an estimate of the response time of the photodetector at τ = 75 ps, which corresponds to a bandwidth of about 6 GHz. Photodetectors of this kind are promising for heterodyne detec� tion of the emission of quantumcascade lasers and IR spectroscopy.

Electron leakage and its suppression via deep-well structures in 4.5- to 5.0-μm-emitting quantum cascade lasers

The equations for threshold-current density Jth and external differential quantum efficiency d of quantum cascade lasers (QCLs) are modified to include electron leakage and the electron-backfilling term corrected to take into account hot electrons in the injector. We show that by introducing both deep quantum wells and tall barriers in the active regions of 4.8-µm-emitting QCLs, and by tapering the conduction-band edge of both injector and extractor regions, one can significantly reduce electron leakage. The characteristic temperatures for Jth and d, denoted by T0 and T1, respectively, are found to reach values as high as 278 and 285 K over the 20 to 90°C temperature range, which means that Jth and d display 2.3 slower variation than conventional 4.5- to 5.0-µm-emitting, high-performance QCLs over the same temperature range. A model for the thermal excitation of hot injected electrons from the upper laser level to the upper active-region energy states, wherefrom some relax to the lower active-region states and some are scattered to the upper miniband, is used to estimate the leakage current. Estimated T0 values are in good agreement with experiment for both conventional QCLs and deep-well QCLs. The T1 values are justified by increases in both electron leakage and waveguide loss with temperature

Features of dual-wavelength generation in a vertical-external-cavity surface-emitting laser

The steady-state stability of a dual-wavelength vertical-external-cavity surface-emitting laser has been analyzed. It is shown that, in the laser with independent active regions containing quantum wells (QWs) of different depths, this state is always stable. The increment of growth and the frequency of oscillations of small deviations from the steady state are determined in the presence of coupling between the active regions, which is caused by the absorption of short-wavelength radiation in deep QWs. In the region of violation of the steady-state stability, the generation proceeds in the form of short pulses, with the almost simultaneous excitation of the optical fields at both frequencies.

Electroluminescence in p-InAs/AlSb/InAsSb/AlSb/p(n)-GaSb type II heterostructures with deep quantum wells at the interface

Luminescent characteristics of asymmetric p-InAs/AlSb/InAsSb/AlSb/p-GaSb type II heterostructures with deep quantum wells at the heterointerface are studied. The heterostructures were grown by metalorganic vapor phase epitaxy. Intense positive and negative luminescence was observed in the range of photon energies of 0.3–0.4 eV with a forward and reverse bias, respectively. Dependences of the spectra and intensities for positive and negative luminescence on the pumping current and on the temperature are studied in the range of 77–380 K. It is established that, at a temperature higher than 75°C, intensity of negative luminescence surpasses that of positive luminescence by 60%. The suggested heterostructures can be used as lightemitting diodes (photodiodes) with switched positive and negative luminescence in the mid-IR spectral range of 3–4 μm.

Charge carrier recombination mechanisms in Sb-containing quantum well laser structures

The dynamics of interband luminescence in structures with InGaAsSb-based quantum wells and barriers of different types was studied at different temperatures and excitation levels. The lifetime of optically injected charge carriers in quantum wells at different temperatures and optical excitation levels was determined. An increased recombination rate in structures with deep electron quantum wells was discovered; it is associated with the occurrence of resonance Auger recombination. It was concluded that the application of quinary solid solutions as barriers in laser structures for a 3—4 fum wavelength range is to be preferred.

Effects of carrier escape and capture processes on quantum well solar cells: a theoretical investigation

A theoretical model is proposed to study the effects of carrier escape and capture processes on the photocurrent of quantum well solar cells (QWSCs). The results show that solar cells with very deep quantum wells (QWs) will suffer from extremely slow escape processes and their photocurrent can be inferior to their bulk counterparts. The results suggest that only when the escape time is at least two-order of magnitude smaller than the carrier lifetime of QWs, solar cells will benefit from QW structures. The optimal band gap energies of QW materials for achieving the maximum photocurrent are also calculated and discussed.

Highly temperature insensitive, deep-well 4.8 μm emitting quantum cascade semiconductor lasers

4.8 μ m emitting, quantum cascade (QC) lasers that suppress carrier leakage out of their active regions to the continuum have been realized by using deep (in energy) quantum wells in the active regions, tall barriers in and around the active regions, and tapered conduction-band-edge relaxation regions. The characteristic temperature coefficients T0 and T1 for the threshold current density Jth and slope efficiency, respectively, reach values of 238 K over the 20–60 °C temperature range, which means that Jth and the slope efficiency vary with temperature half as fast as those of conventional QC lasers. In turn, significantly improved continuous wave performance is expected.

Optical excitation of nonidentical quantum wells in the active region of a vertical-external-cavity laser

Optical excitation of a semiconductor structure comprising two nonidentical quantum wells (QWs) separated by barriers absorbing the pump radiation has been numerically simulated. It is shown that the equalization of populations in the QWs can be achieved in two ways: (i) shift of the deeper QW to a region of weak carrier generation and (ii) introduction of a blocking layer preventing the transport of carriers.

Enhancement of Intersubband Absorption in GaInN/AlInN Quantum Wells

Recently, there has been increasing interest in III-nitride semiconductors for the fabrication of ultrahigh-speed intersubband (ISB) devices. First of all it is due to the large conduction band discontinuity available in these materials. The very deep quantum wells (QW) which can be fabricated, are used as an active region for ISB devices for telecommunication applications (1.55 and 1.3 μm). Moreover the ISB absorption recovery times in nitride semiconductors are extremely short, typically of the order of 150-400 fs due to enhanced electron-phonon interaction in these highly ionic materials [1]. One can expect that ISB devices could provide ultrafast data transfer in a single channel of optical fiber. The predicted speed should be in the range of 0.1-1 Tbit/s bit-rate regime, which is significantly larger than the speed achieved with current bipolar technology. Our work has focused on the growth of GaInN/AlInN multi QWs on GaN substrates. Properly choosing growth parameters we were able to fabricate high quality ISB structures exhibiting intense ISB absorptions at 1.55 μm [2]. Due to a partially strain compensation even quite thick structures were crack free. However, an application of such structures in highspeed ISB devices requires extremely good optical and electrical parameters, which should be appropriate for μ-size devices. In the present work we show a significant enhancement of ISB absorption for 15.5 A thick GaInN QWs due to increase of the AlInN barrier width. The enhancement of ISB absorption was more then 3 times for enlarged barrier width from 3 to 6 nm. Also, we observe that the barrier width does not influence the energy of interband and intersubband transitions within experimental accuracy. The experimental results have been compared with theoretical calculations which were performed within the electron effective mass approximation solving the Schrodinger and Poisson equations self-consistently. Strain effects as well as the effect of spontaneous and piezoelectric polarization have been included in these calculations. A good agreement between experimental data and theoretical calculations has been observed for the investigated samples. By analyzing the electron wave functions, it has been concluded that the electrons significantly penetrate the quantum well barriers and that the QWs are coupled despite the fact that the energy coupling between QW levels is very weak. The observed enhancement of ISB transitions can be associated with the effect of better localization of the second electron level in QW. This finding seems to be very important and useful to optimize ISB devices. In this paper this issue will be discussed in details. This work was partially supported by the Polish Ministry of Science and Higher Education as scientific projects Nos. N507 011 32/0500 and N202 41090 33.

Theoretical study of auger recombination processes in deep quantum wells

The basic processes and mechanisms of Auger recombination of nonequilibrium carriers in a semiconductor heterostructure with deep InAs0.84Sb0.16/AlSb quantum wells (QWs) are analyzed. It is shown that a zero-threshold Auger recombination process involving two heavy holes predominates in sufficiently narrow QWs, and a resonant process involving two electrons is dominant in wide QWs. The range of QW widths at which the Auger recombination is suppressed in a given structure to the greatest extent (suppression region) is determined. In this case, the threshold process involving two electrons remains the basic nonradiative recombination process, with its probability being several orders of magnitude lower than those for the zero-threshold and resonant mechanisms. In turn, the zero-threshold mechanism involving two electrons is totally impossible in the heterostructure under study because of the large conduction-band offset (which markedly exceeds the energy gap). Also, the range of emission wavelengths that corresponds to the suppression region is estimated. It is shown that the interval calculated belongs to the mid-IR range.

Growth and surface passivation of near-surface InGaAs quantum wells on GaAs (1 1 0)

The growth and surface passivation of near-surface InGaAs quantum wells (QWs) on GaAs (1 1 0) substrate have been investigated. Triangular shaped small islands, approximate areal density of 10 7 cm - 2 , are observed on metal organic vapor phase epitaxially grown GaAs single layer and InGaAs multi-quantum wells (MQWs) surfaces in a wide range of growth temperatures. By optimizing the growth conditions, high quality In 0.22 Ga 0.78 As QWs with optical properties comparable to the same structure grown on GaAs (1 0 0) are obtained. Near-surface single QWs are used to study the surface passivation. Epitaxially in situ grown mono-layer thick GaP and InP layers as well as surface phosphorization with tertiarybutylphosphine (TBP) are utilized as passivation methods. Passivation significantly increases photoluminescence (PL) intensity and carrier lifetime of near-surface QWs. The best passivation efficiency is obtained by surface phosphorization with TBP on (1 1 0)-oriented near-surface QW while the ultra-thin InP layer is the best on (1 0 0)-oriented near-surface QW. After 7 months of air exposure, all passivated near-surface QWs still show high PL intensity comparable to deep QW while the PL intensity of unpassivated samples degraded severely. Also, the differences between the optical properties of QWs on GaAs (1 1 0) and (1 0 0) substrates are observed and discussed.