Photoluminescence at Dislocations in GaAs

Abstract

Photoluminescence measurements with high spatial and spectral resolution on ^-type GaAs at helium temperatures show the reduction of radiative quantum efficiency immediately at individual deformation-induced dislocations to be orders of magnitude stronger than near grown-in dislocations.Dislocations in semiconductors can lead to formation of distinct electronic levels and cause inhomogeneities of charge and dopant distributions. 1 Radiative recombination of electron-hole pairs is known to be affected by the presence of dislocations. 2 *" 4 However, the details of the fundamental processes of recombination at and around dislocations are still insufficiently understood. For GaAs, this question has recently become of exceptional importance because the quantum efficiency of injection lasers is degraded by dislocation effects. 5 " 8 In this paper we present the first definitive correlations between dislocations and recombination by measuring luminescence spectra directly at and adjacent to individual dislocations of various origin. High spatial resolution on the specimen surface is combined with high spectral resolution at helium temperatures for identification of the specific electronic transitions in w-type GaAs.A special helium cryostat 9 enabled us to position the samples as close as 12 mm to an external microscope illuminator system, which is used to focus light from a He-Ne laser onto the sample for photoluminescence excitation and to collect the recombination radiation for analysis in a grating spectrometer. The cryostat was mounted on a micrometer stage with motion feasible in both coordinates within the plane of the sample surface. A spatial definition of better than ± 3 pm was achieved.The excitation intensity was kept low to avoid disturbances. Less than 0.8 mW of the 1.96-eV laser radiation impinged on the sample. We convinced ourselves of the independence of our results upon excitation level by inserting a series of neutral-density filters. Sample temperature during excitation was approximately 10 K.The samples to be discussed here were w-type GaAs, Se-doped, with room-temperature electron concentrations between (3 and 4) xlO 17 cm" 3 . Electron mobility at room temperature was typically 3300 cm 2 V" 1 sec" 1 . We estimate the compensation to be of the order of 20%, mostly due to silicon acceptors. The surfaces, of (100) orientation, could be etched after the luminescence was taken in order to correlate the details of the spectra with dislocation etch-pit distributions or dopant striations. 10

Benefit of Microscopic Diffusion Measurement for the Characterization of Nanoporous Materials

“…Under the measuring conditions of tracer-(i.e., self-) diffusion and tracer permeation (or self-permeation, if one chooses an analogous terminology), diffusivity and permeability both decrease with increasing loading. This behavior reflects the inhibition of site exchange between neighboring guest molecules, which increases with increasing loading and, under ideal single-file conditions, would result in a reduction of the diffusivity down to zero [133,142]). The coincidence in the trends of the diffusivities and surface permeabilities for the Zn(tbip) crystals under study is taken as an indication that the finite rate of surface permeation is caused by the total blockage of a large number of channel openings, with only a few of them essentially being freely accessible.…”

“…The fact that diffusion is not ideally one-dimensional may be a result of the microporosity of SBA-15, which ensures matter exchange between adjacent channels. Moreover, transport perpendicular to the main channel direction may be a result of lattice defects which, though often essentially invisible to conventional methods of structural analysis, from diffusion measurements [47,65,142] may be expected to occur quite commonly in nanoporous materials.…”

“…Zn(tbip) (H 2 tbip = 5-tert-butyl isophthalic acid) [141,142] is a highly stable representative of the family of metal organic frameworks. As a result of this stability, it was possible to perform dozens of subsequent adsorption and desorption cycles with the same crystal, without any perceptible loss in reproducibility.…”

Room Temperature Photo-Luminescence Imaging of Dislocations in GaAs Wafers

“…The gettering process accumulates impurities and native defects [13,14) into each dislocation plane of misplaced atoms that forms a "wall". The disordered region (or "core") associated with a dislocation line is generally less than one micron in diameter [15) but getters impurities from a much larger surrounding volume of material, as much as 50-100 ~m away. Thus, individual dislocation cores are not usually visible within "cell walls", which appear as regions tens to hundreds of microns thick and have a much higher PL emission efficiency than do areas of relatively lower dislocation density in the interior of the "cell" (see Figs.…”

Period-doubling reconstructions of semiconductor partial dislocations

“…5 However, many of these device architectures rely on latticemismatched epitaxy, for which threading dislocations are inevitable. Because these dislocations act as recombination centers for charge carriers [6][7][8] and reduce the device efficiency, the density of dislocations in the active region of the device must be minimized, generally by carefully varying the lattice constant in a thick graded layer. [9][10][11] It may also be possible to mitigate the impact of these dislocations via passivation with impurities, [12][13][14][15] or by controlling the type 16 or direction of the remaining dislocations.…”