xN Quantum Research Articles

Hydrostatic Pressure and Built-In Electric Field Effects on the Donor Impurity States in Cylindrical Wurtzite GaN/AlxGa1−xN Quantum Rings

Within the framework of the effective mass approximation, the ground-state binding energy of a hydrogenic impurity is investigated in cylindrical wurtzite GaN/AlxGa1-xNstrained quantum ring (QR) by means of a variational approach, considering the influence of the applied hydrostatic pressure along the QR growth direction and the strong built-in electric field (BEF) due to the piezoelectricity and spontaneous polarization. Numerical results show that the donor binding energy for a central impurity increases inchmeal firstly as the QR radial thickness(ΔR)decreases gradually and then begins to drop quickly. In addition, the donor binding energy is an increasing (a decreasing) function of the inner radius (height). It is also found that the donor binding energy increases almost linearly with the increment of the applied hydrostatic pressure. Moreover, we also found that impurity positions have an important influence on the donor binding energy. The physical reasons have been analyzed in detail.

Minimizing the impact of surface potentials in axial InxGa1−xN/GaN nanowire heterostructures by reducing their diameter

We study the influence of the diameter of axial InxGa1−xN/GaN nanowire heterostructures on the electron and hole confinement in the InxGa1−xN quantum disk using an eight-band k · p model. Elastic relaxation as well as polarization and surface potentials are fully taken into account. Our calculations indicate that a reduction of the nanowire diameter diminishes the influence of the surface potential and thus leads to a significantly increased spatial overlap of electron and hole wave functions. The results suggest that a reduction of the nanowire diameter below 40 nm can significantly improve the internal quantum efficiency of nanowire-based light emitters.

Effects of interface roughness on photoluminescence full width at half maximum in GaN/AlGaN quantum wells

Low temperature photoluminescence (PL) measurements have been performed for a set of GaN/AlxGa1−xN quantum wells (QWs). The experimental results show that the optical full width at half maximum (FWHM) increases relatively rapidly with increasing Al composition in the AlxGa1−xN barrier, and increases only slightly with increasing GaN well width. A model considering the interface roughness is used to interpret the experimental results. In the model, the FWHM's broadening caused by the interface roughness is calculated based on the triangle potential well approximation. We find that the calculated results accord with the experimental results well.

Electron transport through cubic InGaN/AlGaN resonant tunneling diodes

We theoretically study the electron transport through a resonant tunneling diode (RTD) based on strained AlxGa1−xN/In0.1Ga0.9N/AlxGa1−xN quantum wells embedded in relaxed n- Al0.15Ga0.85N/strained In0.1Ga0.9N emitter and collector. The aluminum composition in both injector and collector contacts is taken relatively weak; this does not preclude achieving a wide band offset at the border of the pre-confinement wells. The epilayers are assumed with a cubic crystal structure to reduce spontaneous and piezoelectric polarization effects. The resonant tunneling and the thermally activated transfer through the barriers are the two mechanisms of transport taken into account in the calculations based on the Schrödinger, Poisson and kinetic equations resolved self-consistently. Using the transfer matrix formalism, we have analyzed the influence of the double barrier height on the resonant current. With an Al composition in the barriers varying between 30% and 50%, we have found that resonant tunneling dominates over the transport mediated by the thermally activated charge transfer for low applied voltages. It is also found that the designed n-type InGaN/AlGaN RTD with 30% of Al composition in the barriers is a potential candidate for achieving a resonant tunneling diode.

Ternary effects on the optical interface phonons in onion-like GaN/AlxGa1−xN quantum dots

The interface optical phonons and its ternary effects in onion-like quantum dots are studied by using dielectric continuum model and the modified random-element isodisplacement model. The dispersion relations, the electron–phonon interactions and ternary effects on the interface optical phonons are calculated in the GaN/Al x Ga 1 − x N onion-like quantum dots. The results show that aluminium concentration has important influence on the interface optical phonons and electron–phonon interactions in GaN/Al x Ga 1 − x N onion-like quantum dots. The frequencies of interface optical phonons and electron–phonon coupling strengths change linearly with increase of aluminium concentration in high frequency range, and do not change linearly with increasing aluminium concentration in low frequency range. • The random-element isodisplacement model and dielectric continuum model are used. • The ternary effects on IO phonons in onion-like GaN/Al x Ga 1 − x N QDs are studied. • The ternary effects on electron–IO phonon coupling in QDs are studied.

On the reliable analysis of indium mole fraction within InxGa1−xN quantum wells using atom probe tomography

Surface crystallography and polarity are shown to influence the detection probability of In, Ga, and N ions during atom probe tomography analysis of InxGa1−xN m-plane, c-plane, and (202¯1¯) quantum wells. A N deficit is observed in regions of the reconstruction generated from Ga-polar surfaces, and the probability of detecting group-III atoms is lower in InxGa1−xN quantum wells than in GaN barrier layers. Despite these artifacts, the detected In mole fraction is consistent throughout a given quantum well regardless of the crystal orientation of the quantum well or the evaporation surface from which the reconstruction was generated.

Raman gain in a Boron based Group-III nitride quantum well

Electron Raman scattering of a hydrogenic impurity is studied using exact diagonalization method in a BxGa1−xN/BN coupled quantum well. Intersubband scattering rates, in a Boron based wide band gap GaN, are considered. BxGa1−xN semiconductor is taken as inner quantum well and BN material is taken as barrier material. The effect of quantum confinement on the differential cross section of Raman scattering, with and without the impurity, is obtained. The built-in internal electric field is included throughout the calculations. The third order susceptibility with the incident photon energy is calculated with and without doping impurity. The donor hydrogenic binding energy and its low lying excited states are computed taking into account the geometrical confinement. The binding energy is obtained for various impurity position and the Boron alloy content in BxGa1−xN quantum well. It is brought out that the geometrical confinement and built-in internal electric fields have great influence on the optical properties of the semiconductor.

Barrier penetration effects on thermopower in semiconductor quantum wells

Finite confinement effects, due to the penetration of the electron wavefunction into the barriers of a square well potential, on the low–temperature acoustic-phonon-limited thermopower (TP) of 2DEG are investigated. The 2DEG is considered to be scattered by acoustic phonons via screened deformation potential and piezoelectric couplings. Incorporating the barrier penetration effects, the dependences of diffusion TP and phonon drag TP on barrier height are studied. An expression for phonon drag TP is obtained. Numerical calculations of temperature dependences of mobility and TP for a 10 nm InN/In xGa1−xN quantum well for different values of x show that the magnitude and behavior of TP are altered. A decrease in the barrier height from 500 meV by a factor of 5, enhances the mobility by 34% and reduces the TP by 58% at 20 K. Results are compared with those of infinite barrier approximation.

Influence of the barrier composition in GaN/InxAl1−xN quantum wells properties

GaN/InxAl1−xN quantum wells with varying In composition in the InxAl1−xN barrier and varying GaN well thickness were grown by metal–organic vapor phase epitaxy on a c-plane sapphire. The as-grown samples were characterized by high resolution X-ray diffraction to determinate the composition and the relaxation state. The surface was observed by atomic force microscopy and the photoluminescence was measured at low temperature. A competition between the confinement effect and electric field effect has been evidenced. Under a critical well thickness, the luminescence is blue-shifted with the decrease of the In composition while above this critical thickness, the opposite behavior is observed.

Strain Engineering of Nanowire Multi-Quantum Well Demonstrated by Raman Spectroscopy

An analysis of the strain in an axial nanowire superlattice shows that the dominating strain state can be defined arbitrarily between unstrained and maximum mismatch strain by choosing the segment height ratios. We give experimental evidence for a successful strain design in series of GaN nanowire ensembles with axial InxGa1-xN quantum wells. We vary the barrier thickness and determine the strain state of the quantum wells by Raman spectroscopy. A detailed calculation of the strain distribution and LO phonon frequency shift shows that a uniform in-plane lattice constant in the nanowire segments satisfactorily describes the resonant Raman spectra, although in reality the three-dimensional strain profile at the periphery of the quantum wells is complex. Our strain analysis is applicable beyond the InxGa1-xN/GaN system under study, and we derive universal rules for strain engineering in nanowire heterostructures.

Revisiting the “In-clustering” question in InGaN through the use of aberration-corrected electron microscopy below the knock-on threshold

The high intensity of light emitted in InxGa1−xN/GaN heterostructures has been generally attributed to the formation of indium-rich clusters in InxGa1−xN quantum wells (QWs). However, there is significant disagreement about the existence of such clusters in as-grown InxGa1−xN QWs. We employ atomically resolved CS-corrected scanning transmission electron microscopy and electron energy loss spectroscopy at 120 kV—which we demonstrate to be below the knock-on displacement threshold—and show that indium clustering is not present in as-grown In0.22Ga0.78N QWs. This artifact-free, atomically resolved method can be employed for investigating compositional variations in other InxGa1−xN/GaN heterostructures.

Ternary mixed crystal effects on interface optical phonon and electron-interface optical phonon coupling in wurtzite GaN/AlxGa1−xN quantum wells

Within the framework of the modified random-element isodisplacement model and dielectric continuum model, the interface optical phonon and electron-interface optical coupling in GaN/AlxGa1−xN quantum wells with different aluminum concentration are studied in a fully numerical manner. The results show that aluminum concentration has important influence in the interface optical phonon and electron–phonon coupling in GaN/AlxGa1−xN quantum wells. When the aluminum concentration is lower (0.03<x<0.13), there are two branches interface optical phonon in high frequency range. When the aluminum concentration is higher (0.13<x<1.00), there are four branches interface optical phonons in high and low frequency range. Comparing the electron–phonon coupling in lower and higher aluminum concentration range, we found that the electron–phonon coupling in higher aluminum concentration range is stronger.

Optical gain analysis of c-InGaN quantum wells on unstrained c-In0.31Ga0.69N templates

We investigate the optical properties of c-InxGa1−xN (x = 0.31–0.44) quantum wells (QWs) on unstrained c-In0.31Ga0.69N templates in the green-to-red spectral range using self-consistent multiband k·p theory. The transverse-electric- and transverse-magnetic-polarized optical gains are much higher for QWs on unstrained c-In0.31Ga0.69N templates compared with conventional templates because of a smaller internal electric field and strong valence band mixing. Using c-InxGa1−xN QWs on c-In0.31Ga0.69N templates is expected to reduce the threshold carrier density in the green range and extend the operable wavelength into the red range.

Hartree-Fock Problem of Electron-Hole Pair in Quantum Well GаN

We present microscopic calculations of the absorption spectra for GaN=AlxGa1 xN quantum well systems. Whereas the quantum well structures with the parabolic law of dispersion exhibit the usual bleaching of an exciton resonance without shifting a spectral position, the significant red-shift of an exciton peak is found with increasing the electron-hole gas density for a wurtzite quantum well. The energy of the exciton resonance for a wurtzite quantum well is found. The obtained results can be explained by the influence of the valence band structure on quantum confinement effects. The optical gain spectrum in the Hartree–Fock approximation and the Sommerfeld enhancement are calculated. A red shift of the gain spectrum in the Hartree–Fock approximation with respect to the Hartree gain spectrum is found.

Phonon-assisted intersubband transitions in wurtzite GaN/InxGa1−xN quantum wells

A detailed numerical calculation on the phonon-assisted intersubband transition rates of electrons in wurtzite GaN/InxGa1−xN quantum wells is presented. The quantum-confined Stark effect, induced by the built-in electric field, and the ternary mixed crystal effect are considered. The electron states are obtained by iteratively solving the coupled Schrödinger and Poisson equations. The dispersion properties of each type of phonon modes are considered in the derivation of Fermi's golden rule to evaluate the transition rates. It is indicated that the interface and half-space phonon scattering play an important role in the process of 1–2 radiative transition. The transition rate is also greatly reduced by the built-in electric field. This work can be helpful for the structural design and simulation of new semiconductor lasers.

Correlated electron—hole transitions in wurtzite GaN quantum dots: the effects of strain and hydrostatic pressure

Within the effective-mass and finite-height potential barrier approximation, a theoretical study of the effects of strain and hydrostatic pressure on the exciton emission wavelength and electron—hole recombination rate in wurtzite cylindrical GaN/AlxGa1−xN quantum dots (QDs) is performed using a variational approach. Numerical results show that the emission wavelength with strain effect is higher than that without strain effect when the QD height is large (> 3.8 nm), but the status is opposite when the QD height is small (< 3.8 nm). The height of GaN QDs must be less than 5.5 nm for an efficient electron—hole recombination process due to the strain effect. The emission wavelength decreases linearly and the electron—hole recombination rate increases almost linearly with applied hydrostatic pressure. The hydrostatic pressure has a remarkable influence on the emission wavelength for large QDs, and has a significant influence on the electron—hole recombination rate for small QDs. Furthermore, the present numerical outcomes are in qualitative agreement with previous experimental findings under zero pressure.

Phonon and electron-hole plasma effects on binding energies of excitons in wurtzite GaN/InxGa1−xN quantum wells

The binding energy of an exciton screened by the electron-hole plasma in a wurtzite GaN/InxGa1−xN quantum well (in the case of 0.1 < x < 1 within which the interface phonon modes play a dominant role) is calculated including the exciton-phonon interaction by a variational method combined with a self-consistent procedure. The coupling between the exciton and various longitudinal-like optical phonon modes is considered to demonstrate the polaronic effect which strongly depends on the exciton wave function. All of the built-in electric field, the exciton-phonon interaction and the electron-hole plasma weaken the Coulomb coupling between an electron and a hole to reduce the binding energy since the former separates the wave functions of the electron and hole in the z direction and the later two enlarge the exciton Bohr radius. The electron-hole plasma not only restrains the built-in electric field, but also reduces the polaronic effect to the binding energy.

Non-ideal energy selective contacts and their effect on the performance of a hot carrier solar cell with an indium nitride absorber

The hot carrier solar cell is a third generation photovoltaic device that extracts photo-generated carriers before they thermalise. In this work, the efficiency of a hot carrier solar cell with a 50 nm indium nitride (InN) absorber layer has been calculated, taking into account the realistic transport properties of energy selective contacts. The cell performance has been modeled considering the carrier extraction through contacts as ballistic. A potential practical implementation of a hot carrier solar cell, with contacts based on an InXGa1−XN/InN/InXGa1−XN quantum well structure, has been proposed, with calculated maximum efficiency of 37.15% under 1000 suns.

Ionized Acceptor Bound Exciton States in Wurtzite GaN/AlxGa1−xN Cylindrical Quantum Dot

Based on the framework of the effective-mass approximation, the ionized acceptor bound exciton (A−, X) binding energy and the emission wavelength are investigated for a cylindrical wurtzite (WZ) GaN/AlxGa1−xN quantum dot (QD) with finite potential barriers by means of a variational method. Numerical results show that the binding energy and the emission wavelength highly depend on the QD size, the position of the ionized acceptor and the Al composition x of the barrier material AlxGa1−xN. The binding energy and the emission wavelength are larger when the acceptor is located in the vicinity of the left interface of the QD. In particular, the binding energy of (A−, X) complex is insensitive to the dot height when the acceptor is located at the left boundary of the QD. The ionized acceptor bound exciton binding energy and the emission wavelength are both increased if Al composition x is increased.

Optical studies of GaN nanocolumns containing InGaN quantum disks and the effect of strain relaxation on the carrier distribution

AbstractThe optical properties of GaN nanocolumns containing pairs of InxGa1–xN (x ∼ 0.1) quantum disks (QDisks) have been experimentally measured. Strain simulations reveal an inhomogeneous distribution of strain, and furthermore, a relaxation between successive QDisks along the growth direction of the column. Although this inhomogeneous distribution of strain may indicate a lateral separation of carriers in the QDisk, the decay lifetime of the emission suggests far less carrier separation. The quantum confined stark effect is found to create an inhomogeneous distribution of charge within the QDisks along the growth direction of the rod. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)