Numerical Determination of Shallow Electronic States Bound by Dislocations in Semiconductors

Farvacque, J. L.; François, P.

doi:10.1002/1521-3951(200102)223:3<635::aid-pssb635>3.0.co;2-k

Cited by 18 publications

(8 citation statements)

References 24 publications

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“…According to theoretical work based on effective mass theory and deformation potentials, the shear stresses will not affect the s-type conduction band minimum but only the p-type valence band maximum. Thus, such screw dislocations would bind exclusively holes [6,9]. Excitons then are formed by binding electrons to these bound holes by Coulomb interaction.…”

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confidence: 99%

“…However, they may, due to the limited size of presently computable supercells, fail in correctly describing bound states due to the long-range strain field appropriately. Continuum approaches based on the effective mass approximation and deformation potentials (e.g., k · p calculations, or numerical solutions of the Schrödinger equation [6,9]) are appropriate to provide a reliable and accurate description of the bound states due to the long-range strain field. A major drawback of treating shallow states due to the long-range strain field by a continuum approach is the treatment of the high strain close to the dislocation core.…”

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confidence: 99%

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Origin of the unusually strong luminescence ofa-type screw dislocations in GaN

2014

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Based on luminescence studies and density functional theory calculations we identify the origin of the unusually strong luminescence of a-type screw dislocations in GaN. In contrast to previous models where only a localization of the holes was considered, density functional theory calculations show a localization of both electrons and holes in the dislocation strain field. This strain field causes a mixing of the s-type state at the conduction band minimum with the next highest state that has p character and is thus susceptible to the shear strain induced by the dislocation.Dislocations are linear defects that induce electronic states into the forbidden gap of semiconductors. Commonly deep states are assigned to the presence of dangling bonds along the dislocation core [1,2] while shallow states are allotted to the long-range strain field of the dislocation causing a shift of the band edges [3][4][5][6]. According to this generally accepted picture, charge carriers either recombine nonradiatively at the dislocation core or they may be bound in the form of excitons in one-dimensional bands split off from the valence and conduction bands due to the long-range strain field. These bound excitons can be measured by photoluminescence and appear there in the form of optical transitions redshifted from the band edges [7,8]. A particularly interesting case is the perfect screw dislocation in a direct band gap semiconductor. The core of a perfect screw dislocation has no dangling bonds and exhibits in first order a pure shear strain field. According to theoretical work based on effective mass theory and deformation potentials, the shear stresses will not affect the s-type conduction band minimum but only the p-type valence band maximum. Thus, such screw dislocations would bind exclusively holes [6,9]. Excitons then are formed by binding electrons to these bound holes by Coulomb interaction.GaN is considered a model system for group III nitride semiconductors. Screw dislocations with a-type Burgers vector (Burgers and line vectors b = 1 3 1120 and l = 1 3 1120 , respectively) are the most important threading dislocation in III-nitride heterostructures grown on nonpolar and/or semipolar substrates. Strained heterostructures with these orientations have recently attracted considerable interest in optoelectronic applications, since they allow us to reduce the quantum confined Stark effect and thereby to improve the efficiency of light-emitting devices [10,11]. In the present paper we will present a combined experimental and theoretical work on dislocation-bound excitons at individual a-type screw dislocations.Our experimental analysis of a-type screw dislocations, freshly induced into semi-insulating GaN bulk crystals by scratching, show strong polarized luminescence at 3.34 eV. We further investigated the electronic structure of these defects * Corresponding author: martin.albrecht@ikz-berlin.de by developing and applying a quasicontinuum approach that combines first-principles calculations with elasticity theory. Our analysi...

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Origin of the unusually strong luminescence ofa-type screw dislocations in GaN

2014

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“…Therefore, the lattice distortion, introduced by the dislocation strain field and self-consistently taken into account in the ab initio calculations, may be supposed to be at the origin of the shallow states in the gap. This hypothesis was proposed and confirmed in the works presented in [29,30], that solved both numerically and analytically the Schrödinger equation of the envelope function in the presence of the dislocation strain field binding potential and found that these circumstances lead to the formation of shallow one-dimensional bands for both electrons and holes in the energy gap. According to [29,30], in GaN, the electronic bound states due to the dislocation deformation potential should lie at ∼100 meV below the conduction band.…”

Section: Introductionmentioning

confidence: 58%

“…This hypothesis was proposed and confirmed in the works presented in [29,30], that solved both numerically and analytically the Schrödinger equation of the envelope function in the presence of the dislocation strain field binding potential and found that these circumstances lead to the formation of shallow one-dimensional bands for both electrons and holes in the energy gap. According to [29,30], in GaN, the electronic bound states due to the dislocation deformation potential should lie at ∼100 meV below the conduction band. Also, this last result was confirmed through ab initio calculations performed on big clusters (∼60 000 atoms) [31], which showed that the core orbitals are fully reconstructed leading to core structures in agreement with HR-TEM pictures and eliminating the possible existence of any deep states.…”

Section: Introductionmentioning

confidence: 58%

Effect of threading dislocations on carrier mobility in AlGaN/GaN quantum wells

Carosella¹,

Farvacque²

2008

J. Phys.: Condens. Matter

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The various scattering mechanisms induced by dislocations have been reviewed and adapted to the case of threading dislocations in AlGaN/GaN quantum wells. These scattering mechanisms can be classified into two categories, the first one issuing straightforwardly from the dislocation strain field, the other one being due to the Coulomb potential created by electrons trapped on the energy states that dislocations may create in the GaN band gap. For the first category of mechanisms (strain field effects), we indicate that edge dislocations can only be connected with the so-called deformation potential, the piezoelectric coupling being ruled out because of the particular geometry of the threading dislocation strain fields. We show that the dislocation deformation potential can only be responsible for a very weak, even negligible, effect on the carrier mobility. Then, after a survey of the various results found in the literature concerning the possible existence of dislocation energy states we conclude that dislocations are responsible for the existence of shallow acceptor states below the conduction band and propose a model for describing the potential associated with such states when filled by electrons. More particularly, we show that the linear dislocation charge density resulting from the trapped carriers at dislocation states can not be uniform, as it is systematically assumed in the literature, and we propose a description of this linear charge density as a function of the dislocation energy state position and of the various features characterizing the quantum well. Using the scattering potential induced by such a spatially-dependent dislocation charge density together with the usual scattering mechanisms allows us to give an estimation of their effect on the free carriers’ mobility. We particularly show that at low carrier density (∼1012 cm−2) the mobility is mainly determined by the combination of dislocation scattering mechanisms and intrinsic scattering mechanisms. Finally we suggest that our model could be employed for determining the position of the dislocation energy level in the gap.

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“…The Kohn-Sham matrix eigenelements are then numerically solved using an iterative procedure and their eigenvalues allow us, finally, to calculate the full quantum well energy E T (L) which turns out to be a function of the parameter L. This last value L is chosen so as to minimize E T . This variational procedure, already used in [14,15], allows one to get precise numerical results with a relatively low number of plane waves (∼50 in the present 1D localized case). Figure 3 illustrates the first two wavefunctions found in the case of the triangular potential shown in figure 1.…”

Section: Numerical Determination Of the Quantum Well Energy Statesmentioning

confidence: 99%