L. Macht

Selective photoetching and transmission electron microscopy studies of defects in heteroepitaxial GaN

Photoelectrochemical (PEC) etching has been used to study defects in heteroepitaxial GaN layers. In Ga-polar layers PEC etching reveals only dislocations in the form of filamentary etch features (whiskers). Transmission electron microscopy (TEM) confirmed a one-to-one correspondence between the whiskers and straight threading dislocations, which are mainly of edge and mixed type. In N-polar layers, apart from dislocations, inversion domains (IDs) also give rise to the formation of more complex etch features that also have been confirmed by TEM. IDs of nanometer diameter result in formation of whiskers similar to the dislocation-related ones. However, when the diameter of IDs exceeds a critical size (about 100 nm), crater-like deep etch features are formed during PEC etching. Based on the mechanism of PEC etching of GaN in aqueous KOH solutions, it is argued that inversion domain boundaries are electrically active defects.

Improvement of the optical properties of metalorganic chemical vapor deposition grown GaN on sapphire by an in situ SiN treatment

We have studied the effect of an in situ SiN treatment of sapphire substrates on the optical properties of GaN films grown by metalorganic chemical vapor deposition. The SiN deposition, partially covering the substrate, forms a mask for the formation of nanoscale auto-organized GaN islands. These islands formed upon an increase of the temperature after deposition of a GaN buffer layer on this mask. A photoluminescence study of the GaN epilayers obtained by lateral overgrowth of these islands shows significant enhancement of the luminescence emission intensity of the near band edge peaks and a reduction of the full width at half maximum of the donor bound exciton (D0X) peak by 32% down to 4 meV compared to in our standard process. The GaN films grown using SiN treatment are highly stressed as evidenced by a blueshift of 10 meV in the D0X peak energies. Photoelectrochemical etching in aqueous solution of KOH was applied to reveal the dislocation density. The density of “whisker-like” etch features, which fo...

Complementary study of defects in GaN by photo-etching and TEM

Abstract Photoelectrochemical (PEC) etching in KOH aqueous solutions has been used to study defects in N-polar hetero-epitaxial GaN layers. Detailed cross-sectional TEM study of PEC-etched material revealed that apart from dislocations, also inversion domains (IDs) give rise to the formation of etch features. When the diameter of IDs remains in the tens of nanometer range, the defects are entirely resistant to the etching medium. However, IDs with diameters above the critical value (∼80 nm) are preferentially etched in their centers, resulting in the formation of pronounced crater-like etch features. This indicates that photo-etching of GaN takes place via the ‘electroless’ mechanism, with the critical role of holes for surface reactions and points at the recombinative property of inversion domain boundaries (IDBs).

The direct influence of polarity on structural and electro-optical properties of heteroepitaxial GaN

Different polarities of heteroepitaxial GaN layers are believed to stem from different growth conditions. It follows then that the difference in structural and electro-optical properties between Ga- or N-polar samples can be assumed to have the same cause. An unintentionally n-type doped GaN layer has been grown exhibiting both polarities on the same, single two-inch sapphire substrate, which allows for a thorough study of the differences between those two polarities. In such a case the growth conditions are the same and thus the variation of all the properties must come from the samples polarity or its influence on the growth process. Transmission electron microscopy and etching techniques confirm the large variation in morphology across the surface of the sample to be the result of the polarity difference. Wet chemical etching in a mixture of phosphoric and sulfuric acids, as well as photo-electrochemical etching in aqueous KOH solution, reveal very different defect structures for the two polarities. Hall and PL measurements show a large discrepancy in electro-optical properties suggesting preferential impurity incorporation in the N-polar GaN, which is also confirmed by results of SIMS measurements.

Influence of selective GaN islands on optical properties of GaN films grown on sapphire substrate

Abstract We report on the growth of GaN films on sapphire substrates by low-pressure metalorganic vapor phase epitaxy using selective GaN islands as buffer layer. These islands are produced by a silane (SiH4) treatment of the sapphire surface at high temperature, followed by a low temperature GaN buffer deposition. A photoluminescence (PL) study demonstrates that this growth process significantly enhances the luminescence emission of the donor bound exciton (D°X) recombination and leads to a very narrow peak with a full width at half maximum (FWHM) of 4 meV. This width is about 30% smaller as compared with a standard process using a low temperature buffer layer. Changes in PL peak energies due to the residual strain were linked to the growth mode of the GaN epilayers. Photo-electrochemical (PEC) etching in aqueous solutions of KOH was applied to the GaN epilayers. ‘Whisker-like’ structures were observed on the surface of etched GaN samples by scanning electron microscope (SEM). These structures result from distribution of dislocations in GaN films. Their density was reduced from 6×109 cm−2 in standard GaN films to 8×108 cm−2 in the GaN layers grown on the selective islands.

Microphotoluminescence mapping of laterally overgrown GaN layers on patterned Si (111) substrates

Spatial distribution of optical properties of GaN layers grown on patterned Si (111) substrates by maskless metalorganic chemical vapor deposition has been investigated. The Si substrates were prepared with a pattern of 1.5μm diameter holes at a 3.5μm distance from each other. The holes were overgrown by GaN until coalescence, creating GaN areas with no substrate underneath. Microphotoluminescence mapping measurements with 0.8μm lateral resolution show a five-fold increase in luminescence intensity coming from the overhang areas as compared to the layer directly over the substrate. This is accompanied by a slight redshift of the luminescence peak wavelength. Photoelectrochemical etching shows that the dislocation density is much lower in those areas while the photoluminescence redshift is attributed to lesser strain relaxation resulting from a lower dislocation density.Spatial distribution of optical properties of GaN layers grown on patterned Si (111) substrates by maskless metalorganic chemical vapor deposition has been investigated. The Si substrates were prepared with a pattern of 1.5μm diameter holes at a 3.5μm distance from each other. The holes were overgrown by GaN until coalescence, creating GaN areas with no substrate underneath. Microphotoluminescence mapping measurements with 0.8μm lateral resolution show a five-fold increase in luminescence intensity coming from the overhang areas as compared to the layer directly over the substrate. This is accompanied by a slight redshift of the luminescence peak wavelength. Photoelectrochemical etching shows that the dislocation density is much lower in those areas while the photoluminescence redshift is attributed to lesser strain relaxation resulting from a lower dislocation density.

Improvement of the Optical and Structural Properties of MOCVD Grown GaN on Sapphire by an in-situ SiN Treatment

In this paper, a study on the material properties of GaN films on sapphire substrates grown by low-pressure metalorganic vapor phase epitaxy using selective GaN islands as buffer layer is presented. The optical quality of the GaN layer grown with the optimized island buffer layer is significantly increased compared to the standard two-step method, i.e. a reduction of the D 0 X FWHM to 4 meV and an increase of the luminescence intensity. A blue shift of the excitonic transitions revealed an increase in residual stress. Rocking curve measurements and photo-enhanced etch experiments (PEC) demonstrated that the dislocation density decreases from 6 x 10 9 to 8 × 10 8 cm -2 .

Investigation of optical and structural properties of GaN grown by hydride vapor-phase epitaxy

Photoluminescence and reflectance experiments of high quality GaN layers grown by hydride vapor-phase epitaxy reveal narrow (2-3 meV) free and bound exciton transitions. Quenching of three acceptor-bound exciton peaks is observed with increasing film thickness (3.5-400 μm). Changes in the PL features appear to be independent of the impurity concentrations, as measured by secondary ion mass spectrometry, and the dislocation densities, obtained by photo-enhanced chemical etching. Therefore, the observed acceptor levels are assigned to intrinsic defects originating from the substrate/layer interface and decreasing in density with the thickness of the film.