Quantum-well Model Research Articles

Analytical Model of InP QWs for Efficiency Improvement in GaInP/Si Dual Junction Solar Cell

Quantum wells (QWs) are proven to enhance the short circuit current and voltage preservation measures help in efficiency enhancement with little degradation of open circuit voltage . Herein, an integrated GaInP/Si dual junction solar cell is proposed by incorporating the QWs in the top cell and carrier‐selective passivated contact at the bottom cell of the design. The QW increases the photon absorption of sub‐bandgap energies, buffer layer reduces the misfits between the lattice mismatched layers, and passivation mechanism improves the overall efficiency. Increasing the number of QWs increases the depletion width of QW region. This reduces the carrier lifetime () and increases the recombinations in the QW region. It results in the reduction in passivation quality and, therefore, reduces the open circuit voltage (). QW strain balancing and measures to reduce the threading dislocations has been considered. Also, the wide bandgap GaInP tunnel junction along with highly doped GaAs tunnel junction has been analyzed to observe the parasitic effect. The GaInP/Si‐integrated model achieves efficiency of 33.39% when the QWs are incorporated in the top cell. The effect of wide bandgap tunnel junction is also evaluated using GaInP tunnel junction.

Dual-peak electroluminescence spectra generated from Al n /12Ga1-n/12N (n = 2, 3, 4) for AlGaN-based LEDs with nonflat quantum wells

The metastability of Al n /12Ga1-n/12N (n = 2, 3, and 4) was investigated by the statistical analysis of electroluminescence (EL) spectra having dual peaks with a peak-to-peak distance (pp) of >10 nm generated from nonflat Al x Ga1- x N (x ∼ 0.2) quantum wells (QWs) fabricated on c(0001) sapphire substrates with a miscut of 1.0° towards the m[100] axis. To explain the origins of the dual-peak EL (DPEL) spectra, which are often observed for AlGaN-QWs with Ga content of greater than 0.7, a nonflat QW model incorporating two metastable compositions, Al(n-1)/12Ga1-(n-1)/12N and Al n /12Ga1-n/12N (n: integer), is proposed. By the statistical analysis of peak wavelengths in DPEL spectra and the verification of EL spectral shapes, two series of featured EL peak wavelengths with intervals of 2–3 nm were obtained from five out of six LED wafers. The two series of featured EL peak wavelengths were assigned by comparison with the calculated EL wavelengths. Then, Al2/12Ga10/12N and Al3/12Ga9/12N were determined to be the origins of peaks with the longer and shorter wavelengths in the DPEL, respectively, in addition to the metastable Al n /12Ga1-n/12N (n= 4–9) compositions observed in our previous studies. When DPEL (pp> 10 nm) appeared, the difference in QW thickness between Al2/12Ga10/12N and Al3/12Ga9/12N tended to be larger than one monolayer (ML), indicating a significant amount of Ga or GaN mass transport. Furthermore, the Al2/12Ga10/12N and Al3/12Ga9/12N QWs are considered to have thicknesses of m ML (m: consecutive integers), suggesting the 1 ML configuration of Al and Ga atoms on the c(0001) plane. In addition, the DPEL obtained from nonflat Al x Ga1- x N (x ∼ 0.25) QWs by another research group was shown to be related to two metastable Al n /12Ga1-n/12N (n = 3, 4), similarly to our one exceptional LED wafer, which also agreed with the model proposed in this work.

The Physics of Interlayer Exciton Delocalization in Ruddlesden-Popper Lead Halide Perovskites.

Two-dimensional (2D) lead halide Ruddlesden-Popper perovskites (RPP) have recently emerged as a prospective material system for optoelectronic applications. Their self-assembled multi quantum-well structure gives rise to the novel interwell energy funnelling phenomenon, which is of broad interests for photovoltaics, light-emission applications, and emerging technologies (e.g., spintronics). Herein, we develop a realistic finite quantum-well superlattice model that corroborates the hypothesis of exciton delocalization across different quantum-wells in RPP. Such delocalization leads to a sub-50 fs coherent energy transfer between adjacent wells, with the efficiency depending on the RPP phase matching and the organic large cation barrier lengths. Our approach provides a coherent and comprehensive account for both steady-state and transient dynamical experimental results in RPPs. Importantly, these findings pave the way for a deeper understanding of these systems, as a cornerstone crucial for establishing material design rules to realize efficient RPP-based devices.

Inhomogeneous broadening of optical transitions observed in photoluminescence and modulated reflectance of polar and non-polar InGaN quantum wells

In this work, the broadening of interband transitions in InGaN/GaN quantum wells (QWs) resulting from structural inhomogeneities is analyzed. The role of a polarization-induced electric field in the mechanism behind the inhomogeneous broadening observed in photoluminescence (PL) and electromodulated reflectance (ER) spectra of InGaN QWs dedicated to green/blue lasers is explained. Spectra of both polar and nonpolar QWs are simulated within the random QW model distinguishing contributions of individual transitions taking into account QW inhomogeneities (QW width and indium content fluctuations). On this basis, we interpret the ER and PL spectra measured for a polar multiple QW InGaN/GaN structure. The built-in electric field shifts the emission wavelength to red and enhances the broadening of optical transitions. It is clearly shown that for polar QWs the Stokes shift can be easily overestimated if PL spectra are compared with ER spectra since the intensity of the fundamental transition observed in ER spectra significantly decreases with the increase in QW width. In this way, an ER signal related to excited states starts to dominate. This effect is strongly enhanced by QW inhomogeneities.

Accessing externally induced spatially-resolved strain in GaAs thin-film solar cells by electroluminescence imaging

Externally induced strain effect was investigated in GaAs thin-film solar cells by electroluminescence (EL) imaging method. The strained regions demonstrated not only enhancement of local EL intensity but also redshift of the EL emission peaks. The observations were explained well by a gradient quantum-well (GQW) model correlated with tensile strain in the GaAs thin films. External biaxial strain was considered in this model which was accordingly obtained from the energy shift of the EL spectra. Finally, the spatially resolved strain in GaAs thin-film solar cells was obtained from the EL images by relating the EL emission intensity to the shift of the EL spectra. This work provides a potential standard method for diagnosis of externally induced strain in thin-film solar cells by EL measurements.

Optical efficiency and gain dynamics of modelocked semiconductor disk lasers.

Compact optically pumped passively modelocked semiconductor disk lasers (SDLs) based on active quantum wells (QWs) such as vertical external-cavity surface-emitting lasers (VECSELs) or modelocked integrated external-cavity surface-emitting lasers (MIXSELs) are wavelength-versatile sources that offer a unique combination of gigahertz pulse repetition rates and short pulse durations. In this paper, we present record-short pulses of 184 fs from a gigahertz MIXSEL emitting at a center wavelength of 1048 nm. This result comes at the expense of low optical-to-optical pump efficiency (<1%) and average output power limited to 115 mW. We experimentally observe that shorter pulses significantly reduce the macroscopic gain saturation fluence and develop a QW model based on rate equations to reproduce the gain saturation behavior and quantitatively explain the VECSEL and MIXSEL modelocking performances. We identify spectral hole burning as the main cause of the reduced gain at shorter pulse durations, which in combination with the short lifetime of the excited carriers strongly reduces the optical pump efficiency. Our better understanding will help to address these limitations in future ultrafast SDL designs.

Deeper insight into phase relations in ultrathin Pb films

Films of $\mathrm{Pb}(111)$ with thickness ranging from 1 to 16 monolayers (ML) were grown on $\mathrm{Ge}(111)$ and examined by angle-resolved photoemission spectroscopy. The measured thickness dependences of the work function and surface energy from the same set of samples were both well described by damped sinusoidal oscillatory functions with the same period, but the oscillation phases differ by $1/4$ period between the two quantities, as judged by the node positions. This difference agrees with the prediction of a generic quantum-well model. The varied node positions of damped oscillations of the work function and surface energy were examined in terms of the details of Pb subband dispersions, indicating the different roles of the density of states at the Fermi level to both thin-film properties. The surface dipoles of Pb films were found to have no relevant effects on the phase shift of thickness-dependent work functions except for the amplitudes of oscillations. The total boundary phase shifts of the quantum well states, extracted from the energy positions of the quantum-well states for both $\mathrm{Pb}/\mathrm{Ge}(111)$ and freestanding Pb films, are compared and related to the thickness-dependent work function and surface energy.

Temperature effect on exchange coupling and magnetization reversal in antiferromagnetically coupled (Co/Pd) multilayers

Magnetization reversal of antiferromagnetically coupled (AFC) soft and hard (Co/Pd) multilayers was studied as a function of temperature. While the hard [Co(0.3 nm)/Pd(0.8 nm)]×10 was kept unchanged, the softness of the [Co(t)/Pd(0.8 nm)]×3 was controlled by varying the thickness t of the Co sublayer. Clear two-step hysteresis loops were observed for all the investigated multilayers with t ranging between 0.4 and 1 nm. The spin reorientation of the soft layer magnetization from in-plane direction to out-of-plane direction was investigated from 50 to 300 K. The antiferromagnetic field HAFC measured from the shift of the minor hysteresis loop reveals a good agreement to the quantum-well model. From the out-of-plane hysteresis loop of the uncoupled soft layer, its magnetization shows an in-plane orientation for t ≥ 0.6 nm. The strong HAFC helps to induce an out-of plane orientation of the soft layer with a linear decrease of its coercivity with temperature. These investigated structures show the possibility to reduce the unwanted stray field and improving the out-of-plane anisotropy even for relatively thicker soft layer.

Quantum confinement and band offsets in amorphous silicon quantum wells

Quantum wells (QWs) are nanostructures consisting of alternating layers of a low and high band-gap semiconductor. The band gap of QWs can be tuned by changing the thickness of the low band-gap layer, due to quantum confinement effects. Although this principle is well established for crystalline materials, there is still controversy for QWs fabricated from amorphous materials: How strong are the confinement effects in amorphous QWs, where, because of the disorder, the carriers are localized to start with? We prepare an atomistic model of QWs based on a-Si:H to gain insight into this problem. The electronic structure of our atomistic QWs model is described with first-principles density functional theory, allowing us to study the confinement effects directly. We find that the quantum confinement effect is rather weak, compared to experimental results on a similar system.

Quasiparticle interference scattering of spin-polarized Shockley-like surface state electrons: Ni(111)

We report on a detailed quasiparticle interference (QPI) scattering study of the Ni(111) surface by low-temperature scanning tunneling spectroscopy (LT-STS). While conventional constant-separation STS shows two broad features, which are interpreted as the ${\ensuremath{\Lambda}}_{3}$ bulk band and $sp$-like Shockley-type surface state ($sp$-SS), energy-dependent Fourier-transformed QPI maps reveal the band dispersion of the underlying surface electronic features. We find two electronlike branches in the $sp$-SS dispersion, which are interpreted as the exchange-split minority and majority spin part. The exchange splitting is determined to $\ensuremath{\Delta}{E}_{\mathrm{ex}}=100\ifmmode\pm\else\textpm\fi{}8$ meV. In addition, a holelike $d$-derived surface resonance is found. Band onsets and effective electron masses are determined by fitting the band dispersion with a parabola at small $k$ values. Hybridization effects with bulk electronic states are observed towards larger $k$ values. Prominent quantum confinement phenomena of the $sp$-SS are observed in STS data obtained within vacancy islands. The results can be interpreted within a one-dimensional quantum-well model.

Different views on the electronic structure of nanoscale graphene: aromatic molecule versus quantum dot

Graphene's peculiar electronic band structure makes it of interest for new electronic and spintronic approaches. However, potential applications suffer from quantization effects when the spatial extension reaches the nanoscale. We show by photoelectron spectroscopy on nanoscaled model systems (disc-shaped, planar polyacenes) that the two-dimensional band structure is transformed into discrete states which follow the momentum dependence of the graphene Bloch states. Based on a simple model of quantum wells, we show how the band structure of graphene emerges from localized states, and we compare this result with ab initio calculations which describe the orbital structure.

Shockley-Frank stacking faults in 6H-SiC

We report on Shockley-Frank stacking faults (SFs) identified in 6H-SiC by a combination of low temperature photoluminescence (LTPL) and high resolution transmission electron microscopy (TEM). In the faulted area, stacking faults manifested as large photoluminescence emissions bands located in between the 6H-SiC signal (at ∼2.99 eV) and the 3C-SiC bulk-like one (at ∼2.39 eV). Each of the stacking fault related emission band had a four-fold structure coming from the TA, LA, TO, and LO phonon modes of 3C-SiC. Up to four different faults, with four different thickness of the 3C-SiC lamella, could be observed simultaneously within the extent of the laser excitation spot. From the energy of the momentum-conservative phonons, they were associated with excitonic energy gaps at Egx1 = 2.837 eV, Egx2 = 2.689 eV, Egx3 = 2.600 eV and Egx4 = 2.525 eV. In the same part where low temperature photoluminescence was performed, high resolution transmission electron microscopy measurements revealed stacking faults which, in terms of the Zhdanov notation, could be recognized as SFs (3, 4), (3, 5), (3, 6), (3, 7), (3, 9), (3, 11), (3, 16) and (3, 22), respectively. Among them stacking fault (3, 4) was the most common one, but a faulted region with a (4, 4) 8H-SiC like sequence was also found. Using a type II 6H/3C/6H quantum-well model and comparing with experimental results, we find that the photoluminescence emissions with excitonic band gaps at 2.837 eV (Egx1), 2.689 eV (Egx2), 2.600 eV (Egx3) and 2.525 eV (Egx4) come from SFs (3, 4), (3, 5), (3, 6) and (3, 7), respectively. A possible formation mechanism of these SFs is suggested, which involves a combination of Frank faults with Shockley ones. This provides a basic understanding of stacking faults in 6H-SiC and gives a rapid and non-destructive approach to identify SFs by low temperature photoluminescence.

Theoretical studies of the influence of structural inhomogeneities on the radiative recombination time in polar InGaN quantum wells

AbstractThe influence of two structural inhomogeneities (i.e., the quantum well width and indium concentration fluctuations) on the radiative recombination time (τr) in polar InGaN quantum wells (QWs) was studied within the “random” QW model [M. Gladysiewicz and R. Kudrawiec, J. Phys.: Condens. Matter 22, 485801 (2010)]. In this analysis it is assumed that the τr is inversely proportional to the electron–hole overlap integral which can be calculated within the effective mass approximation. In order to simulate QW inhomogeneities in this model, it was assumed that the QW width and indium concentration vary with a Gaussian distribution where the nominal QW width and indium concentration correspond to the mean value in this distribution and their fluctuations correspond to the deviation from the main value. Obtained results clearly show that the variation of QW width in this system (the first type of QW inhomogeneities) leads to a non‐linear dispersion of the τr whereas the change in indium concentration (the second type of QW inhomogeneities) leads to a linear dispersion of the τr. The two types of QW inhomogenities (QW width and indium concentration fluctuations) lead to a situation where the same emission wavelength can be realized by QWs with significantly different τr constants. Such a behavior of the τr leads to a non‐exponential decay of photoluminescence.

Positron annihilation in Cr, Cu, and Au layers embedded in Al and quantum confinement of positrons in Au clusters

Defect-sensitive and element-selective measurements on ultrathin chrome, copper, and gold layers embedded in aluminium are presented using coincident Doppler broadening spectroscopy (CDBS) with a monoenergetic positron beam. The amounts of positrons implanted in the layers of different thicknesses are calculated and compared with the experimentally gained fractions of positrons annihilating in the buried layers. A high sensitivity was already reached at an Au layer of only 2 nm thickness embedded below 200 nm Al, which was attributed to the highly efficient positron trapping in the Au layer and Au clusters. An implantation and diffusion model describes this high sensitivity for positron trapping layers. A quantum-well model of the positron wave function limits the trapping to gold clusters of a radius larger than 0.23 nm. This result was confirmed experimentally and validated with complementary TEM measurements.

Quantum well states inFe∕Nb(001)multilayers: First principles study

We present a first-principles study to understand the phenomena of interlayer exchange coupling in $\mathrm{Fe}∕\mathrm{Nb}$ multilayers using the linearized-muffin-tin-orbitals method within the generalized gradient approximation. We find that the exchange coupling oscillates with both short and long periodicities, which have been examined in terms of the Ruderman-Kittel-Kasuya-Yosida (RKKY) model as well as the quantum well (QW) model. We have investigated the behavior of the exchange coupling by artificially varying moments of Fe atoms in ferromagnetic layers. For a small moment of Fe, the coupling shows bilinearity in the magnetic moments, implying its RKKY character. However, at higher moments close to that of bulk Fe, the saturation of long-period oscillations is in accordance with the QW model. Quantum well dispersions around the Fermi level demonstrate that the majority-spin bands contribute largely to the formation of quantum well states, which we analyze quantitatively by making use of the phase accumulation model. Our analysis indicates that the quantum well model gives a better description of the oscillatory behavior of the exchange coupling in $\mathrm{Fe}∕\mathrm{Nb}$ multilayers.

Formation and properties of stacking faults in nitrogen-doped 4H-SiC

A high density of double stacking faults consisting of six Si-C bilayers in cubic stacking sequence could be observed in 4H-SiC crystals with nitrogen doping concentrations exceeding 2×10 19 cm −3 after annealing at temperatures above 1100 °C. The double stacking faults form by glide of two partial dislocations in neighboring basal planes. Annealing experiments indicate that the partial dislocations originate from mechanically damaged crystal surfaces. The observed electrical and optical properties of 4H-SiC crystals containing double stacking faults can be well described by a quantum-well model. The conductivity asymmetry is caused by the build-up of potential barriers in c-direction. For the luminescence an indirect radiative recombination of excitons accompanied by momentum-conserving phonons and additionally replicated by the LO( Γ) phonon is suggested.

Increasing Laser Excitation Power Induces Large Blue Shift of the Photoluminescence Peak of Quantum Wells in Gallium Nitride

It is experimentally demonstrated that, as the output power density of a nitrogen laser is increased from 10 to 1000 kW/cm2, a peak in the photoluminescence spectrum of quantum wells (QWs) in GaN shifts by approximately 150 meV. This behavior cannot be interpreted within the framework of the ideal QW model. The observed phenomenon is theoretically explained by the presence of a “tail” in the localized density of states in the QW bandgap and by the filling of bands in the QW by nonequilibrium photogenerated charge carriers. A phenomenological expression for the density of states is proposed, which takes into account the tail in the localized density of states and provides qualitative agreement between theoretical and experimental spectra.

The Energy Levels in Quantum Wells Formed at the Contacts between Cubic and Hexagonal Polytypes of Silicon Carbide

The energies of the ground (e0) and the first excited state (e1) in a quantum well (QW) are evaluated within the framework of the triangular QW model. It is shown that the e0 level in QWs at the contacts between cubic (3C) and hexagonal (NH, N = 2, 4, 6, 8) polytypes of silicon carbide (SiC) can be effectively controlled only by means of doping a wide-bandgap n-NH-SiC polytype with shallow donors.

Interfaces in AlGaSb∕GaSb multiquantum well structures

The Al0.4Ga0.6Sb∕GaSb∕Al0.4Ga0.6Sb quantum well (QW) structures were grown by a molecular beam epitaxy on the GaSb substrates at different growth temperatures (Tg) and with different Sb4∕Ga beam equivalent pressure ratios (BEPR) and were studied using high-resolution x-ray diffraction, photoluminescence (PL), and transmission electron microscopy. The x-ray diffraction analysis showed that the AlGaSb∕GaSb interfaces have smooth Al composition profiles with graded region thicknesses in the range of 0.83nm–2.17nm, depending upon the growth conditions; the normal interfaces are generally sharper than the inverse ones. The low-temperature PL spectra from QWs show relatively broad peaks and energies of the peak emission that can be related to the growth parameters. Good agreement was obtained between the PL peak energies and the transition energies calculated using a QWs model that takes into account the one-dimensionally graded confining potential for describing the experimentally determined Al composition profiles. The interface broadening can be due to Al and Ga segregation, Al diffusion, or Al and Ga intermixing. The broadening increases with the growth conditions that enhance the surface mobility of group-III species, such as high Tg and low V∕III BEPRs. The relatively large full widths at half maxima of the PL features were attributed to the Al composition fluctuations in the barriers, instead of the well thickness ones.

Quantum size effect in two-photon excited luminescence from silver nanoparticles

A direct manifestation of electron energy quantization in metal nanoparticles is observed in two-photon excited luminescence. Experiments reveal the discrete spectra in broadband anti-Stokes photoluminescence from aggregates of silver colloid particles. A theory based on a spherical quantum-well model for metal nanoparticles is in good agreement with experimental observations.