V. Kirilyuk

High Quality GaN Layers on Si(111) Substrates: AlN Buffer Layer Optimisation and Insertion of a SiN Intermediate Layer

We present a study on the material properties of GaN films grown on (111) silicon substrates by low-pressure metalorganic chemical vapour deposition using AlN buffer layers. This buffer layer is optimised with respect to growth temperature and time for the optical and structural properties of the GaN epilayers. The insertion of a Si x N y intermediate layer significantly increases the optical and structural properties. It results in a reduction of the D 0 X FWHM to 10 meV and in a 2.5-fold increase of its luminescence intensity. The FWHM of symmetric and asymmetric ω-scans are reduced from 832 to 669 arcsec and from 702 to 547 arcsec, respectively.

A combined nonlinear and linear magneto-optical microscopy

New possibilities for magnetic domain studies are demonstrated using a combination of nonlinear magneto-optical microscopy and a conventional linear polarizing microscope. The use of an optical response that is governed by a higher rank tensor offers sensitivity to additional combinations of magnetization directions and optical wave vector and polarization, which is demonstrated in magnetic garnet films of different crystallographic orientations. We observed a nontrivial modulated domain structure in a (210) film and a clear domain contrast for a (111) film, where the linear image only indicated simple up–down domains and no domain contrast for these two situations, respectively.

Exciton-related photoluminescence in homoepitaxial GaN of Ga and N polarities

A photoluminescence (PL) study of GaN homoepitaxial layers grown by metal–organic chemical-vapor deposition demonstrates the high optical quality of N-face layers deposited on vicinal (0001) GaN substrates. In contrast to broad PL emission in exact (0001) layers, narrow-bound (0.9 meV) and free- (A and B) excitonic transitions are observed. By following the PL spectra as a function of temperature and excitation power, the main optical transitions in the Ga- and the misoriented N-face layers are found to be the same. Observed differences are related to the distinct creation of donor and acceptor states in the samples of different polarities.

Thick GaN layers grown by hydride vapor-phase epitaxy: hetero- versus homo-epitaxy

Abstract In this paper, an overview will be given of the growth of thick GaN layers by hydride vapor-phase epitaxy. Two different kinds of substrates were used, that is MOCVD-grown GaN templates on sapphire and GaN single crystals. The layers grown on sapphire-based substrates suffer from the problem of cracking and pit formation. Although the morphology is not mirror-like, the optical and electrical quality of the material is excellent as demonstrated by photoluminescence and Hall–Van der Pauw measurements. The layers grown on Ga-polar GaN single crystals have almost perfect morphologies with only a very low density of pits. For the N-polar substrates the morphology is very rough, exhibiting the same features as are observed for the N-face MOCVD-grown GaN layers, both on sapphire and on N-face GaN single crystals.

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...

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.

Optical investigation of shallow acceptor states in GaN grown by hydride vapor-phase epitaxy

The evolution of low-temperature photoluminescence (PL) spectra with the thickness of the layer (3–400 μm) is investigated on high-quality GaN grown by hydride vapor-phase epitaxy. With increasing layer thickness, three acceptor bound exciton peaks are found to reduce in intensity, although the impurity concentrations, measured by secondary ion mass spectrometry, do not depend on the sample thickness. The observed acceptor transitions are attributed to intrinsic defects, originating from the substrate/layer interface and decreasing in density with the thickness of the layer. The optical properties, studied by reflectance, temperature and excitation power dependent PL, are compared to those of homoepitaxial GaN films grown by metalorganic chemical vapor deposition.

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 .

Growth of GaN epilayers on Si(111) substrates using multiple buffer layers

We present a study of the growth of high quality GaN films on Si(111) substrates by MetalOrganic Chemical Vapor Deposition technique. In order to improve the quality of the epitaxial films we introduced different nucleation or buffer layers and combinations of them. Our results obtained on an optimized AlN nucleation layer will serve as reference point. In order to improve the quality of the epitaxial films we introduced different combinations of nucleation and intermediate layers. The first combination consists of an optimized AlN nucleation layer followed by a 1 µm-thick GaN film, on which we deposited SixNy/GaN intermediate layers. Based on the optimized AlN nucleation layer, we introduced AlGaN/GaN superlattices or AlN intermediate buffer layers. Additionally, we present results on the modification the Si(111) surface with NH3 to promote nucleation from selective GaN islands. In all experiments the total thickness of the GaN epilayers was 3 µm. X-ray diffraction, photoluminescence, Hall measurements and atomic force microscopy were used in order to elucidate the effectiveness of these growth processes. For the most successful deposition scheme, the one with the SixNy/GaN intermediate layers, the resulting GaN layers are of high quality as compared to the other methods. The donor bound exciton, which dominates the photoluminescence spectrum, showed a full width at half maximum (FWHM) of about 50 meV at room temperature and 10 meV at 4K. The FWHM of the symmetric (0002) rocking curves in ?-scan is about 640 arcsec. The root-mean-square roughness, as measured by atomic force microscopy, does not exceed 10 A.

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.