Triangular Quantum Wells Research Articles

Effect of tilted magnetic field on the binding energy of a hydrogenic donor in a near triangular quantum well

ABSTRACT Low-dimensional semiconductor systems have attracted considerable attention of the researchers because of their applications such as speed circuits, optical devices etc. The application of external perturbations like magnetic field can change the properties of these systems. Study on tilted magnetic fields is of interest theoretically as it illustrates special confinement effects. In the present study, we calculate the binding energy of a hydrogenic donor in a quantum well with a potential profile proportional to |z|2/3, called Near Triangular Quantum Well (NTQW), in the presence of a magnetic field tilted by an angle θ with the growth axis, using variational method. Calculations of hydrogenic donor binding energies are made by varying the well width (L), Aluminium composition (x), strength of the magnetic field (γ) and the tilt angle (θ). Integrated probability density (P) of finding a hydrogenic donor in NTQW is also analysed as a function of well width (L).

A theoretical study of the effects of electric field, hydrostatic pressure, and temperature on photoionization cross-section of a donor impurity in (Al, Ga)N/AlN double triangular quantum wells

The aim of this research is to analyze the influence of various factors on the photo-ionization cross-section in (Al, Ga)N/AlN double triangular quantum wells. Using the finite difference method, the effects of the electric field, hydrostatic pressure, temperature, and Ga concentration were investigated within the effective mass and parabolic approximations. Our findings show that the photo-ionization cross-section (PICS) is highly dependent on all the variables under consideration. The optical spectra were blue-shifted with increasing electric field and pressure and red-shifted with increasing temperature and impurity displacement far from the center of the structure. Furthermore, it was found that changes in gallium content and impurity position can increase the PICS amplitude. A comparison of the obtained results with the existing literature as a limiting case of the reported problem is also provided, and excellent agreement is found.

Photo-ionisation cross-section of a hydrogenic donor in a near triangular quantum well

ABSTRACT Ground state and first excited state binding energies of a hydrogenic donor in a near triangular quantum Well (NTQW) composed of GaAs/Ga1-xAlxAs are calculated variationally as a function of well width and Al concentration. Photo-ionisation cross-section of the donor is also investigated as a function of photon energy, well width and Al concentration. Our results reveal that the peak value of photo-ionisation cross-section shows a turnover at the well width at which the binding energy is a maximum and at the Al concentration of 0.3. Photo-ionisation shows a strong dependence on well width, Al concentration and photon energy in NTQW than in other nanostructures.

Asymmetrical Gaussian Potential Effects on Strongly Coupled Magnetopolaron Properties in Triangular Confinement Potential Quantum Wells

In this research, the existence of an asymmetrical Gaussian confinement potential (AGCP) along the quantum well (QW) growth direction and of a parabolic potential perpendicular to the polar coordinate direction were considered. The magnetic field and temperature properties of the longitudinal optical (LO)-phonon mean number, ground-state energy (GSE), ground-state binding energy (GSBE) and vibrational frequency (VF) of strongly coupled magnetopolarons in triangular confinement potential QWs (TCPQWs) were investigated according to the quantum statistical theory as well as the linear combination operator and unitary transformation methods. We obtained analytical expressions for the GSE, GSBE, VF and LO-phonon mean number as functions of the applied magnetic field, temperature, AGCP barrier height, AGCP range, polar coordinate system’s polar angle and polar coordinate system’s confinement strength. It was demonstrated by the calculated numerical results that the GSE, GSBE, VF and LO-phonon mean number varied with the related physical quantities. The obtained theoretical results are expected to provide a reference for future research on polarons.

Low temperature photoluminescence study for identification of intersubband energy levels inside triangular quantum well of AlGaN/GaN heterostructure

A non-destructive method for extracting the true signatures of intersubband energy level states inside a single triangular quantum well for AlGaN/GaN heterostructure is presented. Herein, we report experiments showing light interaction with high-mobility two-dimensional electron gas (2DEG) in Al0.3Ga0.7N/AlN/GaN based HEMT structure using low-temperature photoluminescence spectroscopy. An interband transition from two-dimensional electron gas (2DEG) subbands to the valence band is identified. These observed transitions were confirmed by comparing the PL spectra of the as grown and top barrier layer etched samples. The luminance peak related to 2DEG disappeared when the top AlGaN barrier layer was removed using reactive ion etching (RIE) system. Furthermore, the emission peak data are also supported by a calculation based on a self-consistent solution of one-dimensional Poisson and Schrodinger equations. The three PL spectra peaks corresponding to the interband transitions from 2DEG subbands to the valence band were reported at 3.269, 3.356 and 3.438 eV respectively. The corresponding intersubband energy states inside the quantum well were extracted (simulated) with 87 (91) and 178 (186) meV energy spacing between E0 & E1 and E0 & E2 respectively. The temperature and excitation power-dependent PL measurement makes it easy to identify the transitions from the 2DEG subbands to valence bands.

Electron accumulation and distribution at interfaces of hexagonal ScxAl1−xN/GaN- and ScxAl1−xN/InN-heterostructures

Electron charges and distribution profiles induced by polarization gradients at the interfaces of pseudomorphic, hexagonal ScxAl1−xN/GaN- and ScxAl1−xN/InN-heterostructures are simulated by using a Schrödinger–Poisson solver across the entire range of random and metal-face ScxAl1−xN-alloys, considering the transition from wurtzite to hexagonal layered crystal structure. In contrast to previous calculations of polarization-induced sheet charges, we use Dryer’s modern theory of polarization, which allows for consideration of the spontaneous polarization measured on ferroelectric ScxAl1−xN-layers. Because the sheet density of the electrons accumulating at the heterostructure interfaces can strongly depend both on the data set of the piezoelectric and structural coefficients and on the alloying region of the ScxAl1−xN-layers in which the transition from the wurtzite to the hexagonal layered crystal structure occurs, we have calculated the charge carrier sheet densities and profiles for three representative data sets and evaluated their relevance for devices. We predict electron sheet densities of (2.26±0.20)×1014cm−2 and (6.25±0.20)×1014cm−2 for all three sets of data for Ni/AlN/InN- and Ni/ScN/InN-heterostructures, respectively. We demonstrate that the polarization-induced interface charges of Ni/ScxAl1−xN/InN-heterostructures are always positive, tend to increase with increasing Sc-content, and can cause electron accumulations that lead to flooding of the triangular quantum wells at the semiconductor interface. We identify Ni/ScxAl1−xN/GaN-heterostructures with 0.13≤x≤0.19 as particularly promising candidates for the processing of energy-efficient high electron mobility transistors due to their missing or low mechanical strain and their large electron sheet densities between (4.11±0.20)×1013cm−2 and (6.37±0.20)×1013cm−2. Furthermore, we present simulation results of highly strained Ni/ScxAl1−xN/GaN-heterostructures for 0.81≤x≤1.0, which point to electron accumulations of up to (8.02±0.40)×1014cm−2. These heterostructures are not suitable for transistor devices, but they may be of great interest for the implementation of low impedance contacts.

Generation of microwave harmonics by non-ohmic surface layers in quantum wells of compound semiconductors at low lattice temperatures

It is well known that the free electron ensemble in the material of a semiconductor structure, at times, may exhibit electrical nonlinearity, when an effectively large electric field is applied. Under the condition, when a microwave field is also superimposed at the input, the efficiency of generation of second harmonic has been estimated here, using the electrical nonlinearity of the surface layers in infinite triangular quantum wells of compound semiconductors at low lattice temperatures. The nonlinear electrical characteristics are obtainable in the widely used framework of the field-dependent effective electron temperature approximation. Some of the explicit low-temperature features, like the degeneracy of the ensemble, and their interaction with the piezoelectric phonons, are taken into account. The efficiency characteristics which are thus obtained for some practically used modulation doped heterostructures of compounds, like InSb, GaAs, and GaN, seem to be interesting. Moreover, the specific low-temperature features are found to effect significant changes in the efficiency characteristics of the layers of such compounds. How the present analysis leaves much scope for further refinement is also discussed.

Dynamics of nonlinear optical rectification, second, and third harmonic generation in asymmetric triangular double quantum wells due to static electric and magnetic fields

Dynamics of nonlinear optical rectification, second, and third harmonic generation in asymmetric triangular double quantum wells due to static electric and magnetic fields

The Confinement Profile Effect on the Optical Properties in Different Inverse-shaped Single InGaN/GaN Quantum Wells

In this work, the effects of size, and temperature on the linear and nonlinear optical properties in InGaN/GaN inverse parabolic and triangular quantum wells (IPQW and ITQW) for different concentrations at the well center were theoretically investigated. The indium concentrations at the barriers were fixed to be always xmax = 0.2. The energy levels and their associated wave functions are computed within the effective mass approximation. The expressions of optical properties are obtained analytically by using the compact density-matrix approach. The linear, nonlinear, and total absorption coefficients depending on the In concentrations at the well center are investigated as a function of the incident photon energy for different values of temperature and quantum wells size. The results show that the In concentrations, size and temperature have a significant effect on these optical properties. The positions of the resonance peaks of the absorption coefficients were blue-shifted under increasing indium compositions in the quantum wells (InGaN) and temperature while they were red-shifted with the increase in the thickness of the wells. Moreover, the amplitudes of the resonance peaks were enhanced under the increase of the In composition, the temperature, and the thickness of the quantum wells. The optical absorption in ITQW structure is slightly greater than that in IPQW one.

Effect of Ridge Width on the Lasing Characteristics of Triangular and Rectangular InAs/In0.53Ga0.47As Quantum Well Lasers

The lasing characteristics of InP-based InAs/In0.53Ga0.47As quantum well (QW) lasers with different ridge widths are investigated. Two groups of lasers are grown for comparison, one with active triangular QW regions and the other with rectangular QW regions. Their output powers, characteristic temperatures (T0), external differential quantum efficiencies (ηd) and junction temperatures (Tj) are analyzed and compared. The parameter of ridge width is found to play an important role in the performance of the lasers. In triangular QW lasers, by broadening the ridge width from 8 to 12 μm, output power and ηd of the lasers are decreased for the temperature range of 100–320 K due to heating effect. But by broadening the ridge width from 8 to 100 μm in rectangular QW lasers, output power has about 3.5 time increase at 100 K and ηd also has a little increase for temperatures from 100 to 180 K due to much larger emission area and much faster heat dissipation. Tj, the real temperature of the active region, is also found to have accelerated increase at high injection current and heat sink temperature. Besides, compared to the rectangular QW laser of the same ridge width, the improved thermal performance of triangular QW laser is also demonstrated.

InAs triangular quantum wells grown on InP/SiO2/Si heterogeneous substrate for mid-infrared emission

The properties of InAs/In0.53Ga0.37As triangular quantum wells (QWs) grown on an InP/SiO2/Si integrated substrate by ion-slicing technology are investigated. The material structure and growth quality are characterized by the X-ray diffraction (XRD) and transmission electron microscope measurements. The photoluminescence (PL) spectra at various temperatures are also analyzed. The PL peak wavelengths red-shift from 1.94 to 2.13 μm with the increase of temperature from 12.4 to 300 K. The experimental results of the QWs on InP/SiO2/Si substrate are found to be comparable with the performance of the same QWs grown on an InP substrate. The results are promising for future integration of Si with InP-based optical devices for the applications of light emission in mid-infrared wavelength range.

An analytical model for electron tunneling in triangular quantum wells

An analytical expression for the decay rate of quasi-bound states (QBS) in a triangular quantum well is derived by considering the resonance scattering of particles from the triangular potential profile. Asymptotic properties of the Airy functions that are solutions to the Schrödinger equation for a linear or triangular potential and a perturbative expansion for a small broadening of the QBS are utilized to derive the expression for its location E 0 as well as the decay rate Γ that further gives the net tunneling current from the well. The triangular well model is commonly used to represent the band-bending near a heterojunction due to the electrostatic field. This compact expression shows excellent agreement with a full numerical solver and improvement over Wentzel–Kramers–Brillouin based calculations is demonstrated.

Imaging confined and bulk p-type/n-type carriers in (Al,Ga)N heterostructures with multiple quantum wells

The current state of dopant assessment for the optimization of the III-nitride-based heterostructures for high frequency, high power, and light emission applications relies heavily on quantitative chemical analysis techniques. In such complex heterostructures, determination of p-type carrier density of the cap layer, control of background concentration, and assessment of polarization induced confined carriers are necessary for the realization of optimal devices. None of these can be completely inferred from chemical analysis owing to several material and growth issues including poor activation of Mg, presence of O impurities, and amphoteric nature of carbon impurities. Here, as regions of interest require nanometer resolution, especially near the interfaces featuring triangular quantum wells and exhibiting electron/hole confinement, exploitation of the behavior of the nanosize metal–semiconductor junction formed between the metallic scanning probe microscopy probe and the III-nitride surface is promising for carrier determination. By combining two techniques sensitive to local change in capacitance and rectifying characteristic of conduction at the nanoscale, the nature of free carriers originating from extrinsic n-type and p-type dopants and polarization induced confined carriers, two-dimensional electron gas and hole gas, were eventually revealed across III-nitride heterostructures.

Room temperature photon induced electrical tuning of intersubband transition in GaN HEMT for terahertz applications

A new and versatile mechanism for electrical tuning of intersubband transitions (ISBT) in GaN High Electron Mobility Transistor (HEMT) device at room temperature is presented. In present study, experimental demonstration is provided which clearly discriminate ISBT from any other transitions induced by deep level traps, defects, etc. in 100 nm GaN HEMT device at room temperature. The strong interaction of light with two-dimensional electron gas (2DEG) inside asymmetrical triangular quantum well of GaN HEMT is investigated. The resultant ISBT of the carriers can be explained through pinning of the fermi level inside the quantum well by applying an electrical field along growth direction through gate. Presently intersubband (ISB) based devices are operated at cryogenic temperature to minimize the thermal lattice vibration. The inherent advantage of conduction band tuning through external bias in HEMT structure as demonstrated in this works can leads to room temperature device operation feasibility.

Influence of magnetic field and Coulomb field on the Rashba effect in a triangular quantum well

The influence of magnetic field and Coulomb field on the Rashba spin–orbit interaction in a triangular quantum well was studied using Pekar variational method. We theoretically derived the expression of the bound magnetopolaron ground-state energy. The energy of the bound magnetopolaron splits under the influence of the Rashba effect. From this phenomenon, it is concluded that the effects of orbital and spin interactions on the polaron energy in different directions must be considered. Because of the contribution of the magnetic field cyclotron resonance frequency to the Rashba spin–orbit splitting, the energy spacing becomes larger as the magnetic field cyclotron resonance frequency increases. Compared to the bare electron, the bound polaron is more stable, and the energy of bound polaron split is more stable.

Influence of position-dependent effective mass on the nonlinear optical properties in Al Ga1−As/GaAs single and double triangular quantum wells

Abstract In the present work, the linear and nonlinear optical absorption coefficients (OACs) and third harmonic generation (THG) in AlGaAs/GaAs single and double triangular quantum wells (TQWs) have been studied, emphasizing on the influence of position dependent effective mass (PDEM) and the shapes of confined potential. We calculate the OACs and THG within the compact density-matrix approach. Simultaneously, we obtain the reliable energy eigenvalues and their corresponding eigenfunctions of double and single TQWs system with PDEM and constant effective mass (CEM) via the finite difference method. The theoretical finding show that (1) the energy levels with PDEM have a significant decrease with respect to the energy levels with CEM. (2) The correct inclusion of PDEM is the cause of a noticeable difference in linear and nonlinear OACs and THG with respect to the use of CEM. According to the results, it is deduced that the PDEM plays a vital and considerable role in the electronic and optical properties of single and double TQWs.

Polarization modulation effect of BeO on AlGaN/GaN high-electron-mobility transistors

We investigate the polarization modulation effect of a single-crystalline BeO layer on AlGaN/GaN high-electron-mobility transistors (HEMTs). The BeO layer with macroscopic polarization on top of the AlGaN barrier layer increases the 2-dimensional electron gas density in the triangular quantum well (QW) at the interface of the AlGaN/GaN heterostructure. Electronic band bending of BeO and a deeper triangular QW observed from the simulated conduction band profile indicate that the BeO layer can modify the polarization field at the AlGaN/GaN interface. A ∼20-nm-thick single-crystalline BeO thin film is grown on AlGaN/GaN HEMTs by atomic-layer deposition. Room-temperature and variable-temperature Hall-effect measurements confirm that the HEMT with BeO forms a channel with a 14% increase of the sheet carrier concentration as compared with a conventional HEMT. An improved output performance is also observed in the I-V characteristics which confirms the polarization modulation effect of the BeO layer.

Magnetic field driven bound magnetic polaron with composition effect in semimagnetic triangular quantum well

Theoretical investigation has been carried out on the bound magnetic polaron (BMP) due to the strong exchange interaction between the charge carriers bound to an acceptor/donor impurity and the localized spins of Mn2+ ion in a semimagnetic triangular quantum well as a function of externally applied magnetic field. The effect of various combinations of the composition of Mn2+ ion in the well (xin) and in the barrier (xout), in such a way that xout – xin = x is always the same on the donor BMP, has been studied and the effect of triangular confining potential on the BMP associated with the heavy and light holes bound to an acceptor impurity has also been addressed. The binding energy of the acceptor/donor impurity has been calculated using variational technique in the effective mass approximation and the interaction between the magnetic moment of the Mn2+ ions and the spin of the carrier has been treated using mean field theory.

Thermionic emission of carriers in InGaN/(In)GaN multiple quantum wells

The room temperature photoluminescence (PL) intensity of InxGa1-xN/InyGa1-yN multiple quantum wells were correlated with the structural properties such as polarization strength and ground-state quantum levels for the triangular quantum wells. For x – y ≤ 0.2, the PL intensity increased with increasing the In-content of the well. In this region, we found that the PL intensity can be well described by the thermionic emission process with an activation energy corresponding to the energy difference between the valence band of barrier and the quantum level of holes in the well at the well/barrier interface. The pre-factor of the exponential component of effective barrier, reflecting the effective density of nonradiative recombination centers in the barriers, was 3.7 times larger for the samples with GaN barriers than that with InGaN barriers, indicating that the PL intensity is also influenced by the optical quality of barriers when the effective barrier height is relatively low.

Improved Performance of Near UV GaN-Based Light Emitting Diodes With Asymmetric Triangular Multiple Quantum Wells

Near-ultraviolet (NUV) light-emitting diodes (LEDs) have been used in several potential applications such as UV curing and biochemical sensors. However, the internal quantum efficiency (IQE) of NUV-LEDs is still a crucial issue. To improve the IQE, in this paper, an asymmetric triangular multiple quantum well (MQW) structure was used, which exhibited a higher emission efficiency and lower efficiency droop with nitrogen face-oriented inclination. This is in contrast to the trend observed in blue LEDs. Furthermore, we demonstrated that holes are more confined in MQWs with nitrogen face-oriented inclination than in MQWs with gallium face-oriented inclination. Moreover, simulations revealed that the IQE improved by approximately 32% compared with that of symmetric square MQW NUV-LEDs; this trend was also confirmed through experimental results. The external quantum efficiency of thin-film flip-chip LEDs with nitrogen face-oriented inclination MQWs was 52%.