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Pawel Strak

Publications and source records attributed to Pawel Strak.

15 recordsLinked to original sources
arXiv2026

Role of quasi-Fermi levels in Si- and Mg-related optical absorption in nitride laser diodes (LDs): material context

Optical absorption and reabsorption of light emitted from active regions in nitride laser diodes (LDs) have been shown to reduce the light extraction efficiency of these devices. It was proven that the presence of Si and Mg may considerably increase the optical absorption. This effect is much stronger in the high-energy (short-wavelength) range of the spectrum. The absorption increase is directly related to the ionization of the Si donor and Mg acceptor levels, which are controlled by the electron and hole quasi-Fermi levels. It is shown that the absorption may be increased because of the higher ionization of Mg caused by the compensation in the p-type region and the high ionization of Si in the n-type region. It was explained theoretically why optical efficiency is increased by removal of doping in waveguides. It was also shown that good material quality leads to a low absorption level, especially in the Mg-doped p-type part of the device.

cond-mat.mtrl-sci↗
arXiv2026

Hydrogen at GaN(0001) surface control of Fermi level pinning: Mg activation of p-type conductivity -- Nakamura process deciphered

Ab initio calculations were used to disentangle the mystery of Nakamura activation of p-type in Mg doped MOVPE grown gallium nitride, the key process leading to the 2014 Nobel Prize in Physics. Calculations were used to obtain the equilibrium state of the hydrogen atom deep in the GaN bulk and at the GaN(0001) surface. It was shown that the H position within bulk GaN depends on the Fermi level: in n-type GaN, it is located in the channel, whereas in p-type GaN, it is attached to the N atom, breaking one of the GaN bonds. In contrast, at the GaN(0001) surface, H is attached in the on-top position for any hydrogen coverage; for low and high H-coverage, the Fermi level is pinned at the Ga - broken bond state and at the valence band maximum (VBM), respectively. The diffusion path from the bulk to the surface was obtained when the Fermi level was high and low, the barrier was zero, and $ΔE_{bar} \approx 1.717 eV$, which effectively blocked hydrogen escape into the vapor. Thus, high H coverage, that is, high hydrogen pressure in the vapor, prevents H from escaping from the bulk to the surface, whereas at low coverage (low hydrogen pressure), the process is barrierless. It is therefore proven that the hydrogen escape control step in the Nakamura process is the transition of hydrogen from the bulk to the surface, which is controlled by the position of the Fermi level at the surface. Molecular hydrogen desorption from the surface is easy for high H coverage and difficult for low, thus opposite to observed experimentally thus this process is not the determining step in activation. A full thermodynamic estimate of the maximal partial pressure of hydrogen in the vapor, corresponding to the transition of the Fermi level from the Ga-broken bond state to the VBM, was used to establish the maximal hydrogen pressure limit for the p-type Mg activation process.

cond-mat.mtrl-sci↗
arXiv2025

Strain distribution in GaN/AlN superlattices grown on AlN/sapphire templates: comparison of X-ray diffraction and photoluminescence studies

Series of GaN/AlN superlattices (SLs) with various periods and the same thicknesses of GaN quantum wells and AlN barriers have been investigated. X-ray diffraction, photoluminescence (PL) and transmission electron microscopy (TEM) techniques were used to study the influence of thickness of AlN and GaN sublayers on strain distribution in GaN/AlN SL structures. Detailed X-ray diffraction measurements demonstrate that the strain occurring in SLs generally decreases with an increase of well/barrier thickness. Fitting of X-ray diffraction curves allowed determining the real thicknesses of the GaN wells and AlN barriers. Since blurring of the interfaces causes deviation of calculated data from experimental results the quality of the interfaces has been evaluated as well and compared with results of TEM measurements. For the samples with thinner wells/barriers the presence of pin-holes and threading dislocations has been observed in TEM measurements. The best quality of interfaces has been found for the sample with a well/barrier thickness of 3 nm. Finally, PL spectra showed that due to Quantum-Confined Stark Effect the PL peak energies of the SLs decreased with increasing the width of the GaN quantum wells and AlN barriers. The effect is well modelled by ab initio calculations based on the density functional theory applied for tetragonally strained structures of the same geometry using a full tensorial representation of the strain in the SLs.

cond-mat.mtrl-sci↗
arXiv2024

Polarization spontaneous and piezo: fundamentals and their implementation in ab initio calculations

Fundamental properties of spontaneous and piezo polarization are reformulated and critically reviewed. It was demonstrated that Landau definition of polarization as a dipole density could be used to the infinite systems. The difference between the bulk polarization and surface polarity are distinguished thus creating clear identification of both components. The local model of spontaneous polarization was created and used to calculate spontaneous polarization as the electric dipole density. It was shown that the proposed local model correctly predicts c-axis spontaneous polarization values of the nitride wurtzite semiconductors. It was also shown that the proposed model predicts zero polarization in the plane perpendicular to the c-axis, in accordance with symmetry requirements. In addition, the model results are in accordance with polarization equal to zero for zinc blende lattice. These data confirm the basic correctness of the proposed model. The spontaneous polarization values obtained for all wurtzite III nitrides (BN, AlN, GaN and InN) are in basic agreement with the earlier calculations using Berry phase and slab models of Bernardini et al. {Bernardini et al. Phys Rev B 56 (2001) R10024 & 63 (2001) 193201} but not with Dreyer et al. {Dreyer et al. Phys. Rev X 6 (2016) 021038}. Wurtzite nitride superlattices ab initio calculations were performed to derive polarization-induced fields in the coherently strained lattices showing good agreement with the polarization values. The strained superlattice data were used to determine the piezoelectric parameters of wurtzite nitrides obtaining the values that were in basic agreement with the earlier data. Zinc blende superlattices were also modeled using ab initio calculations showing results that are in agreement with the absence of polarization of all nitrides in zinc blende symmetry.

cond-mat.mtrl-sci↗
arXiv2024

Pressure-dependent bandgap study of MBE grown {CdO/MgO} short period SLs using diamond anvil cell

Semiconductor superlattices have found widespread applications in electronic industries. In this work, short-period superlattice structure (SLs) composed of CdO and MgO layers was grown using the plasma-assisted molecular beam epitaxy (PA-MBE) technique. The optical property of the SLs was investigated by absorption measurement at room temperature. The ambient-pressure direct bandgap was found to be 2.76 eV. The pressure dependence of fundamental bandgap has been studied using a diamond anvil cell (DAC) technique. It has been found that the band-to-band transition shifts toward higher energy with applied pressure. The bandgap of SLs was varied from 2.76 eV to 2.87 eV with applied pressure varied from 0 to 5.9 GPa. The pressure coefficient for the direct bandgap of SLs was found to be 26 meV/GPa. The obtained experimental result was supported by theoretical results obtained using DFT calculations. The volume deformation potential was estimated using the empirical rule. We believe our findings may provide valuable insight for a better understanding of {CdO/MgO} SLs towards their future applications in optoelectronics.

cond-mat.mtrl-sci↗
arXiv2024

Coulomb contribution to Shockley-Read-Hall (SRH) recombination

Defect-mediated nonradiative recombination, known as Shockley-Read-Hall (SRH) recombination is reformulated. The introduced model considers Coulomb attraction between charged deep defect and the approaching free carrier, showing that this effect may cause considerable increase of the carrier velocity approaching the recombination center. The effect considerably increases the carrier capture rates. It is demonstrated that in the typical semiconductor device or semiconductor medium, the SRH recombination cannot be neglected at low temperatures. The SRH is more effective in the case of low doped semiconductors. Effective screening by mobile carrier density could reduce the effect, leading to SRH rate increase.

cond-mat.mtrl-sci↗
arXiv2022

Polarization doping ab initio verification of the concept charge conservation and nonlocality

In this work, we study the emergence of polarization doping in AlxGa1-xN layers with graded composition from a theoretical viewpoint. We demonstrate that the charge conservation law applies for fixed and mobile charges separately, leading to nonlocal compensation phenomena involving bulk fixed and mobile charge and polarization sheet charge at the heterointerfaces. The magnitude of the effect allows obtaining technically viable mobile charge density for optoelectronic devices without impurity doping (donors or acceptors). Therefore, it provides an additional tool for the device designer, with the potential to attain high conductivities: high carrier concentrations can be obtained even in materials with high dopant ionization energies, and the mobility is not limited by scattering at ionized impurities.

cond-mat.mtrl-sci↗
arXiv2019

Phase-amplitude functional theory -- new ab initio calculation method for large size systems

New method for ab initio calculations of the properties of large size system based on phase-amplitude functional is presented. It is shown that Schrodinger equation for many electrons complex system including large size molecules, or clusters and also periodic systems could be translated into functional of two variables, attributed to many electron wavefunctions: phase and the amplitude (i.e. square root of total electron density). The equations for the phase and the amplitude are derived. The kinetic and Coulomb interaction energy are expressed in function of these variables. The equations for one-electron wavefunctions, necessary for the energy spectrum are derived using these two variables.

physics.comp-ph↗
arXiv2019

Exact method of determination of the recombination mode from time resolved photoluminescence data

A method of data analysis is proposed for the determination of the carrier recombination processes in optically excited matter measured by time-resolved photoluminescence, differentiating monomolecular, bi-molecular, tri-molecular, and higher order molecular processes. Our method allows to determine whether the so-called ABC model describes time evolution of optical relaxation of the excited system. The procedure is applicable to the time evolution of any optically excited medium. As an illustration, the method is successfully applied to III-nitride polar and non-polar multi-quantum wells. We show that in this case the mono- and bi-molecular processes determine the carrier relaxation, and the tri-molecular Auger recombination contribution is negligible.

physics.optics↗
arXiv2018

On Composite Discontinuous Galerkin Method for simulations of electric properties of semiconductor devices

In this paper, a variant of discretization of the van Roosbroeck equations in the equilibrium state with the Composite Discontinuous Galerkin Method for the rectangular domain is discussed. It is based on Symmetric Interior Penalty Galerkin (SIPG) method. The proposed method accounts for lower regularity of the solution on the interfaces of devices' layers. It is shown that the discrete problem is well-defined and that discrete solution is unique. Error estimates are derived. Finally, numerical simulations are presented.

math.NA↗
arXiv2013

General aspects of the vapor growth of semiconductor crystals - a study based on DFT simulations of the NH3/NH2 covered GaN(0001) surface in hydrogen ambient

Vapor growth of semiconductors is analyzed using recently obtained dependence of the adsorption energy on the electron charge transfer between the surface adsorbed species and the bulk [Krukowski et al. J. Appl. Phys. 114 (2013) 063507, Kempisty et al. ArXiv 1307.5778 (2013)]. Ab initio calculations were performed to study the physical properties of GaN(0001) surface in ammonia-rich conditions, i.e. covered by mixture of NH3 molecules and NH2 radicals. The Fermi level is pinned at valence band maximum (VBM) and conduction band minimum (CBM) for full coverage by NH3 molecules and NH2 radicals, respectively. For the crossover content of ammonia of about 25% monolayer (ML), the Fermi level is unpinned. It was shown that hydrogen adsorption energy depends on the doping in the bulk for the unpinned Fermi level, i.e. for this coverage. Surface structure thermodynamic and mechanical stability criteria are defined and compared. Mechanical stability of the coverage of such surfaces was checked by determination of the desorption energy of hydrogen molecules. Thermodynamic stability analysis indicates that initally equilibrium hydrogen vapor partial pressure steeply increases with NH3 content to attain the crossover NH3/NH2 coverage, i.e. the unpinned Fermi level condition. For such condition the entire range of experimentally accessible pressures belongs showing that vapor growth of semiconductor crystals occurs predominantly for unpinned Fermi level at the surface, i.e. for flat bands. Accordingly, adsorption energy of most species depends on the doping in the bulk that is basis of the possible molecular scenario explaining dependence of the growth and the doping of semiconductor crystals on the doping in the bulk

cond-mat.mtrl-sci↗
arXiv2013

Fermi level influence on the adsorption at semiconductor surfaces - ab initio simulations

Chemical adsorption of the species at semiconductor surfaces is analyzed showing the existence of the two contributions to adsorption energy: bond creation and charge transfer. It is shown that the energy of quantum surface states is affected by the electric field at the surface, nevertheless the potential contribution of electron and nuclei cancels out. The charge transfer contribution is Fermi level independent for pinned surfaces. Thus for Fermi level pinned at the surface, the adsorption energy is independent on the Fermi energy i.e. the doping in the bulk. The DFT simulations of adsorption of hydrogen at clean GaN(0001) and silicon at SiC(0001) surfaces confirmed independence of adsorption energy on the doping in the bulk. For the Fermi level nonpinned surfaces the charge contribution depends on the position of Fermi level in the bulk. Thus adsorption energy is sensitive to change of the Fermi energy in the bulk, i.e. the doping. The DFT simulations of adsorption of atomic hydrogen at 0.75 ML hydrogen covered GaN(0001) surface confirmed that the adsorption energy may be changed by about 2 eV by the doping change from n- to p-type.

cond-mat.mtrl-sci↗
arXiv2012

High efficiency UV emitters - theoretical investigation of GaN/AlN heterostructures

Density functional theory simulations were used to obtain physical properties of GaN/AlN system. Combination of these two compounds into multiquantum well (MQW) structure will induce strong electrostatic effect leading to emergence of high magnitude dipole layers at the AlN/GaN interfaces, which were first postulated by {Tersoff Phys. Rev. B 30(8) pp.4874 (1984)} and already identified in GaN/InN by {Romanowski et al. J. Phys. Chem C 114. 14410 (2010)}. When combining GaN and AlN in to a heterostructure a spatial projection of wavefunctions indicate that valence band offset between states becomes of order of 0.85 V. Systematic analysis of influence of number of Ga atomic layers on the properties of wells have shown that for thickness up to 4 Ga layers, GaN behave as carriers locating potential minimum rather, while for larger thickness it is a standard quantum well. In all cases wells has strongly localized quantum states close to valance band maxima (VBM) and conduction band minimum (CBM). The calculated oscillator strength values rapidly decreases for the well thickness in excess of 8 Ga layers (~21 Å) which indicates that wells for UV emitters should be much thinner than these based on InGaN/GaN systems. The Quantum Confined Stark Effect (QCSE) related changes of the transition energy in function of the geometric arrangement were also obtained.

cond-mat.mtrl-sci↗
arXiv2011

Polarization and polarization induced electric field in nitrides - critical evaluation based on DFT studies

Density Functional Theory (DFT) calculations were used to evaluate polarity of group III nitrides, such as aluminum nitride (AlN), gallium nitride (GaN) and indium nitride (InN) providing physically sound quantitative measure of polarity of these materials. Two different approaches to polarization of nitride semiconductors were assessed and the conclusions have been used to develop models. It was shown that Berry phase formulation of the electron related polarization component provides a number of various solutions, different for various selection of the simulated volume. The electronic part gives saw-like pattern for polarization. A total number of these solutions, related to well known scaling of the geometric phase, is equal to the number of valence electrons in the system. Summation with similar pattern for ionic part provides several polarization values. Standard dipole density formulation depends on the selection of the simulation volume in periodic continuous way. Using a condition of continuous embedding into the infinite medium, and simultaneously, the zero surface charge representation at crystal boundary provides to physically sound solution. This solution is corresponding to maximal and minimal polarization values and also corresponds to different physical termination of the crystal surfaces, either bare or covered by complementary atoms. This change leads to polarization and electric field reversal. The polarization and related fields in finite size systems were obtained.

cond-mat.mes-hall↗