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

Abstract

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...

On the luminescence of freshly introduced a-screw dislocations in low-resistance GaN

“…Recent calcu lations [15] considering the contributions of higher bands predicted also the existence of an attractive potential for electrons in the conduction band near perfect screw dislocations; this was used in [15] to explain a shift of the DL spectral band by 0.14 eV with respect to the exciton line, but does not allow explana tion of a similar shift by 0.3 eV in our case.…”

“…Based on the latter, it is reasonable to assume that the additional DL band shift by 0.15 eV in our samples in comparison with the DL band in [15] is caused by a difference in the structure of the screw dislocation core, which, in turn, is caused by different doping lev els of materials. In [15], high resistance iron doped GaN samples with perfect screw dislocations were studied.…”

“…In [15], high resistance iron doped GaN samples with perfect screw dislocations were studied. In the experiments presented here, the sam ples were low resistance and not intentionally doped; therefore, it is reasonable to assume that their screw dislocations were dissociated.…”

“…Quite recently, DL obser vation from individual screw dislocations introduced under the same conditions, but into semi insulating GaN, whose emission energy appeared higher, i.e., 3.35 eV at 4 K, was reported [15]. In this case, accord ing to transmission electron microscopy (TEM) data [15], the dislocations in the studied samples were per fect, i.e., they did not undergo the dissociation char acteristic of all glide dislocations in tetrahedrally coor dinated semiconductors.…”

Exciton dynamics at a single dislocation in GaN probed by picosecond time-resolved cathodoluminescence

“…CL is a powerful technique and widely used to study the optical properties of GaN at the nanoscale. [7][8][9][10] Since 1997, it has been confirmed, through a comparison between atomic force microscopy (AFM) and CL images, that dislocations act as non-radiative centers. 2 The non-radiative dislocations have been further identified as pure edge or mixed (edge and screw) type using a comparison between plan-view transmission electron microscopy (TEM) and CL images.…”