Quantum Wells In Fields Research Articles

Factors Affecting Terahertz Emission from InGaN Quantum Wells under Ultrafast Excitation

InGaN quantum wells (QWs) grown on c-plane sapphire substrate experience strain due to the lattice mismatch. The strain generates a strong piezoelectric field in QWs that contributes to THz emission under ultrafast excitation. Physical parameters such as QW width, period number, and Indium concentration can affect the strength of the piezoelectric field and result in THz emission. Experimental parameters such as pump fluence, laser energy, excitation power, pump polarization angle, and incident angle can be tuned to further optimize the THz emission. This review summarizes the effects of physical and experimental parameters of THz emission on InGaN QWs. Comparison and relationship between photoluminescence properties and THz emission in QWs are given, which further explains the origin of THz emission in InGaN QWs.

Improved carrier confinement and distribution in InGaN light-emitting diodes with three-layer staggered QWs

Compared with conventional InGaN Quantum Wells (QWs), staggered InGaN QWs offer improved optical and electronic properties. This work studied the carrier concentration, band structure, overlap of hole and electron wave functions, and polarization field of three-layer staggered QWs in the blue spectral region and analyzed them in detail theoretically to explore the source and the dominant mechanism for improvement. Although theoretical studies indicate that the polarization field in QWs of staggered InGaN QWs is larger, the carrier confinement effect is stronger, and the carrier distribution is more uniform. Therefore, three-layer staggered QWs can improve overlapping of the hole and electron wave functions and then enhance the recombination rate so as to increase the optical output power and electroluminescence intensity. Moreover, the performance of the staggered structure C with the lowest indium content at the center of the well is better than that of the step-staggered structure B.

The influence of electromagnetic waves on conductivity tensor in the presence of laser field in quantum wells with parabolic potential for the case of electrons-acoustic phononscattering

In quantum well studies, the conductivity tensor problem is one of the fundamental problems. In this work the topic covered here is the effect of electromagnetic waves on the conductivity tensor with the presence of a laser field in a quantum well with a parabolic potential considering the case ofelectrons-acoustic phonon scattering. By using quantum kinetic equations for electrons in quantum wells with parabolic potential, the author has calculated conductivity tensor with electrons–acoustic phonon scattering in the presence of an electromagnetic wave field and a laser field. The expression tensor for conductivity shows its dependence on the frequency of the electromagnetic wave field, the frequency of the laser field, and other parameters specific to the system. From the conductivity tensor expression plotted the effect of the electromagnetic wave field on the conductivity tensor in the presence of the laser field. The quantum well is discussed and plotted here is the GaAs/GaAsAl quantum well.

Investigation into the Internal Electric-Field Strength in the Active Region of InGaN/GaN-Based LED Structures with Various Numbers of Quantum Wells by Electrotransmission Spectroscopy

The internal electric fields of InGaN/GaN-based green-emission LED heterostructures with various numbers of quantum wells in the active region are investigated by electrotransmission spectroscopy. The frequencies of the observed spectral lines are attributed to possible types of interband transitions. An increase in the number of interband transitions of the “quantum well—quantum barrier” type with an increase in the number of quantum wells is found. This is explained by the nonidentical degree of segregation of In atoms in different GaN barriers layers. The strength of internal electric fields in quantum wells is calculated for various values of the bias of the p–n junction using a series of electrotransmission spectra. It is found that the strength of the internal piezoelectric field decreases from 3.20 to 2.82 MV/cm with an increase in the number of quantum wells.

Исследование напряжeнности внутренних электрических полей в активной области светодиодных структур на основе InGaN/GaN с разным числом квантовых ям методом спектроскопии электропропускания

Abstract The internal electric fields of InGaN/GaN-based green-emission LED heterostructures with various numbers of quantum wells in the active region are investigated by electrotransmission spectroscopy. The frequencies of the observed spectral lines are attributed to possible types of interband transitions. An increase in the number of interband transitions of the “quantum well—quantum barrier” type with an increase in the number of quantum wells is found. This is explained by the nonidentical degree of segregation of In atoms in different GaN barriers layers. The strength of internal electric fields in quantum wells is calculated for various values of the bias of the p – n junction using a series of electrotransmission spectra. It is found that the strength of the internal piezoelectric field decreases from 3.20 to 2.82 MV/cm with an increase in the number of quantum wells.

On intrinsic Stokes shift in wide GaN/AlGaN polar quantum wells

The interpretation of electromodulated reflectance (ER) spectra of polar quantum wells (QWs) is difficult even for homogeneous structures because of the built-in electric field. In this work we compare the room temperature contactless ER and photoluminescence (PL) spectra of polar GaN/AlGaN QWs with the effective-mass band structure calculations. We show that the emission from the ground state transition is observed in PL but the ER is dominated by transitions between excited states. This effect results from the polarization-induced built-in electric field in QW that breaks the selection rules that apply to square-like QWs, allowing many optical transitions which cannot be separately distinguished in the ER spectrum. We develop the guidelines for the identification of optical transitions observed in PL and ER spectra. We conclude that an intrinsic Stokes shift, i.e., a shift between emission and absorption, is present even for homogeneous GaN/AlGaN QWs with large width, where the electron–hole wavefunction overlap for the fundamental transition is weak.

Micro-photoluminescence mapping of surface plasmon-coupled emission from InGaN/GaN quantum wells

Micro-photoluminescence (PL) mapping was investigated to elucidate the detailed mechanism of surface plasmon (SP)-enhanced emissions from InGaN/GaN quantum wells (QWs) with Ag or Al coating. The PL wavelengths were obviously red-shifted with Ag films, while the PL peak wavelengths were not changed with Al coating. The relationship between the PL peak intensity and the PL wavelength at each pixel showed a positive or negative correlation for the uncoated part of the InGaN/GaN QWs with blue or green emission, respectively. We found that these correlations disappeared in the Ag-coated regions. These results suggest that the energy transfer from the excitons to the SPs should be much faster than that in the exciton localization and charge screening processes of the piezoelectric field in QWs. These effects were not observed for the Al-coated regions because the mechanism of PL enhancement should be quite different as we have suggested previously.

Micro-photoluminescence mapping of surface plasmon enhanced light emissions from InGaN/GaN quantum wells

We conducted photoluminescence (PL) mapping to elucidate the detailed mechanism of PL enhancement based on the energy transfer from excitons to surface plasmon polaritons (SPPs) in silver-coated InGaN quantum wells (QWs). The PL mapping for bare InGaN QWs showed positive or negative correlations between the PL peak intensity and wavelength. These correlations are normally caused by exciton localization and the quantum confined Stark effect, respectively; however, they did not appear in the silver-coated region of the InGaN QWs, and the wavelength distribution shifted into shorter wavelengths due to the SP-induced PL enhancement. These results suggest that the energy transfer from the excitons to the SPPs should be much faster than that in the exciton localization and charge screening processes of the piezoelectric field in QWs.

Theoretical investigation of magnetoresistivity oscillations modulated by a terahertz field in quantum wells with parabolic potential

The magnetoresistivity (MR) in a parabolic quantum well (PQW), subjected to a crossed dc electric field and magnetic field, modulated by a terahertz field (TF), is theoretically calculated. The electron - acoustic phonon interaction is taken into account at low temperatures. In the case of absence of the TF, the Shubnikov - de Haas oscillations are observed. The temperature dependence of the relative amplitude of these oscillations is in good agreement with previous theories and experiments in some two-dimensional electron systems. In the presence of the TF, there exist the oscillations in the MR which are similar to those observed experimentally in some two-dimensional electron systems. The amplitude of these oscillations increases with increasing the TF amplitude (intensity).

Bandgap behavior of InGaN/GaN short period superlattices grown by metal‐organic vapor phase epitaxy

To obtain short period superlattices (SPLS) of InxGa1−xN/GaN in most cases molecular beam epitaxy has been applied. In this work the metal‐organic vapor phase epitaxy was used for obtaining similar structures and their quality as well as light emission features are studied. Thanks to control of growth parameters it was possible to fabricate InxGa1−xN/GaN SPSLs with structural quality of structures grown by MBE. They contain around 30% of indium in quantum wells (QWs) and different number m of atomic monolayers in QWs, and n. in the barriers. X‐ray diffraction and transmission electron microscope measurements have confirmed that the designed SPSLs structures were obtained. An agreement between the experimental results of photoluminescence measurements and theoretically predicted band gap behavior of SPSLs composed of mInxGa1−xN/nGaN was achieved. Built‐in electric field in quantum wells and barriers of the selected structures were determined using a simplified method of calculation.

Anomalous Rashba spin–orbit interaction in electrically controlled topological insulator based on InN/GaN quantum wells

We study theoretically the topological phase transition and the Rashba spin–orbit interaction in electrically biased InN/GaN quantum wells. We show that that for properly chosen widths of quantum wells and barriers, one can effectively tune the system through the topological phase transition applying an external electric field perpendicular to the QW plane. We find that in InN/GaN quantum wells with the inverted band structure, when the conduction band s-type level is below the heavy hole and light hole p-type levels, the spin splitting of the subbands decreases with increasing the amplitude of the electric field in the quantum wells, which reveals the anomalous Rashba effect. Derived effective Rashba Hamiltonians can describe the subband spin splitting only for very small wave vectors due to strong coupling between the subbands. Furthermore, we demonstrate that for InN/GaN quantum wells in a Hall bar geometry, the critical voltage for the topological phase transition depends distinctly on the width of the structure and a significant spin splitting of the edge states lying in the 2D band gap can be almost switched off by increasing the electric field in quantum wells only by a few percent. We show that the dependence of the spin splitting of the upper branch of the edge state dispersion curve on the wave vector has a threshold-like behavior with the on/off spin splitting ratio reaching two orders of magnitude for narrow Hall bars. The threshold wave vector depends weakly on the Hall bar width, whereas it increases significantly with the bias voltage due to an increase of the energetic distance between the s-type and p-type quantum well energy levels and a reduction of the coupling between the subbands.

Performance improvement of VLC system by using GaN-based LEDs with strain relief layers

The GaN-based blue light-emitting diodes (LEDs) with superlatticelike strain relief layer (SRL) and without SRL were fabricated by metal organic vapor phase epitaxy (MOVPE). The luminescence characteristics of the different LEDs were measured experimentally and calculated theoretically. It is demonstrated that the efficiency droop of GaN-based LEDs is suppressed using the SRL due to the reduction of polarization electric field in quantum wells. This is beneficial to the mitigation of LED light output power-injection current (L-I) nonlinearity. Furthermore, using the different LEDs as light source, the performance of visible-light communication (VLC) system was also investigated. The results reveal that the VLC system performance is improved when the LEDs with SRL are applied. The reason is that the transmitted signal error decreases as the LED L-I nonlinearity mitigates. As a result, the GaN-based LEDs with SRL can be applied in the VLC to improve the system performance.

Electron density magnification of the collective spin-orbit field in quantum wells

The spin-orbit field acting on the spin waves of a spin-polarized electron gas is studied by inelastic light scattering on a CdMnTe quantum well. Above-barrier illumination allows us to vary the electronic density and control the collective Rashba and Dresselhaus coupling constants. We demonstrate that the enhancement between the single-particle and the collective spin-orbit field increases with increasing electronic density. This result is reproduced by a first-principles calculation. This behavior, which is opposite to usual Coulombic spin enhancements, reveals a novel aspect of the interplay of spin-orbit and Coulomb interactions in collective spin modes.

Internal Quantum Efficiency Enhancement by Relieving Compressive Stress of GaN-Based LED

By die-attaching GaN-based light-emitting diodes (LEDs) on Si substrates with eutectic AuSn solder, the external quantum efficiency of the LEDs was enhanced by 3.5% at a current input of 120 A/cm 2 . The enhancement of the external quantum efficiency of the die-attached LED chips is attributed to a reduction in compressive stress and piezoelectric fields in quantum wells after eutectic AuSn die-attachment. Raman and photoluminescence analyses were used to estimate the reduction in compressive stress and piezoelectric field in quantum wells, which is 277.8 MPa and 0.056 MV/cm, respectively, after AuSn die-attachment.

Performance improvement of AlGaN-based deep ultraviolet light-emitting diodes by using staggered quantum wells

The optical and physical properties of AlGaN-based deep ultraviolet light-emitting diodes (UV LEDs) with various specific designs of staggered quantum wells (QWs) are numerically investigated. Detailed analysis has been carried out on the light output power, energy band, overlap of electron and hole wavefunctions, carrier concentration, radiative recombination rate, spontaneous radiative spectrum and internal quantum efficiency. The simulated results reveal that the deep UV LEDs with the staggered quantum wells exhibit better performance than their conventional counterpart due to the diminished piezoelectric polarization fields in QWs which can increase the density of electron and hole and the overlap of the electron and hole wavefunctions, and thus enhance the radiative recombination rate.

Control of higher spectral components by spatially inhomogeneous fields in quantum wells

The propagation of a spatially inhomogeneous few-cycle laser field, linked to surface plasmon polaritons, through an ensemble of quantum wells is investigated under two conditions. It is found that the transmitted spectra sensitively depend on the spatial inhomogeneity due to the nonlinear propagation effects. Under the small-frequency mismatch condition, the transmitted spectral distribution is continuous for the spatially homogeneous case. However, when spatial inhomogeneity is introduced, the distribution is discrete instead, with clearly even- and odd-order harmonic peaks coexisting, which is related to the strong-field reshaping effect and the inversion symmetry breaking due to the introduction of the spatially inhomogeneous field. As for the large-frequency mismatch condition, only odd-order harmonics exist, no matter how strong the spatial inhomogeneity is. Under both conditions, the propagation effect can obviously enhance the intensity of higher spectral components.

Importance of non linear piezoelectric effect in Wurtzite III-N semiconductors

The piezoelectric coefficients of all the binary III-N wurtzite semiconductors for both first and second order has been used to evaluate the impact of non linear piezoelectricity on quantum wells and quantum dots of III-N materials and its alloys. The often neglected second order coefficients have instead significant effects on the strain induced piezoelectric polarization. The comparison of our predictions with the experimentally obtained values reveals a much better match with the internal piezoelectric fields in quantum wells and superlattices. The impact of our semi empirical approach is quite pronounced in III-N quantum dots as the excitonic properties remain highly influenced.

Electron mobility and drift velocity in selectively doped InAlAs/InGaAs/InAlAs heterostructures

An increase in the electron mobility and drift velocity in high electric fields in quantum wells of selectively doped InAlAs/InGaAs/InAsAs heterostructures is obtained experimentally via controlling the composition of semiconductors forming the interface. The electron mobility at the interface in the In0.8Ga0.2As/In0.7Al0.3As metamorphic structure with a high molar fraction of In (0.7–0.8) is as high as 12.3 × 103 cm2 V−1 s−1 at room temperature. An increase in the electron mobility by a factor of 1.1–1.4 is attained upon the introduction of thin (1–3 nm) InAs layers into a quantum well of selectively doped In0.53Ga0.47As/In0.52Al0.48As heterostructures. A maximal drift velocity attains 2.5 × 107 cm/s in electric fields of 2–5 kV/cm. The threshold field Fth for the intervalley Γ-L electron transfer (the Gunn effect) in the InGaAs quantum well is higher than in the bulk material by a factor of 2.5–3. The effect of two- to threefold decrease in the threshold field Fth in the InGaAs quantum well is established upon increasing the molar fraction of In in the InAlAs barrier, as well as upon the introduction of thin InAs inserts into the InGaAs quantum well.

Direct current driven by ac electric field in quantum wells

It is shown that the excitation of charge carriers by ac electric field with zero average driving leads to a direct electric current in quantum well structures. The current emerges for both linear and circular polarization of the ac electric field and depends on the field polarization and frequency. We present a micoscopic model and an analytical theory of such a nonlinear electron transport in quantum wells with structure inversion asymmetry. In such systems, dc current is induced by ac electric field which has both the in-plane and out-of-plane components. The ac field polarized in the interface plane gives rise to a direct current if the quantum well is subjected to an in-plane static magnetic field.

Electroreflectance investigations of quantum confined Stark effect in GaN quantum wells

In this paper we present room temperature electroreflectance studies of GaN quantum wells (QWs) with different well width. The electroreflectance measurements were performed with external voltage applied to the structure therefore it was possible to tune the electric field inside QW up to its completely screening and furthermore even reversing it. The analysis of QW spectral lines showed the Stark shift dependence on applied voltage and well width reaching about 35 meV for highest voltage and widest well width. It was possible to obtain the condition of zero electric field in QW. Both broadening and amplitude of QW lines are minimal for zero electric field and increases for increasing electric field in QW. The energy transition is maximum for zero electric field and for increasing electric field it decreases due to Stark effect. Neither amplitude and broadening parameter nor energy transition does not depend on the direction of electric field. Only parameter that depends on the direction of electric field in QW is phase of the signal. The analysis of Franz-Keldysh oscillations (FKOs) from AlGaN barriers allowed to calculate the real electric field dependence on applied voltage and therefore to obtain the Stark shift dependence on electric field. The Stark shift reached from −12 meV to −35 meV for 450 kV/cm depending on the well width. This conditions were established for highest forward voltages therefore this is the value of electric field and Stark shift caused only by the intrinsic polarization of nitrides.