P. de Mierry

Stress control in GaN grown on silicon (111) by metalorganic vapor phase epitaxy

The strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy has been investigated. The insertion of AlN/GaN superlattices was found to decrease the stress sufficiently for avoiding crack formation in an overgrown thick (2.5 μm) GaN layer. X-ray diffraction and photoluminescence measurements are used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layers. A reduction of threading dislocation density is also observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed with a full width at half maximum of the bound exciton line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) x-ray rocking curve also attests the high crystalline quality of GaN on Si (111), when using these AlN/GaN superlattices.

The effect of the Si/N treatment of a nitridated sapphire surface on the growth mode of GaN in low-pressure metalorganic vapor phase epitaxy

In this letter, we studied the effect of the high-temperature Si/N treatment of the nitridated sapphire surface followed by the deposition of a low-temperature GaN nucleation layer on the growth mode of GaN in low-pressure metalorganic vapor phase epitaxy. It was shown that the nucleation layer, initially flat and continuous, converts to wide isolated truncated hexagonal islands having {1–101} facet planes and a top (0001) plane, after heating up to 1150 °C. The coalescence of these GaN islands yields a reduction of the total number of extended defects from the 1010–1011 cm−2 range usually obtained down to the low 109 cm−2 range for the best samples.

Pyramidal defects in metalorganic vapor phase epitaxial Mg doped GaN

A transmission electron microscopy study of structural defects induced by the introduction of Mg during the growth of metalorganic vapor phase epitaxy GaN is presented. These defects are assumed to be pyramidal inversion domains with an hexagonal base and {1123} inclined facets. The tip of the pyramids is always pointing toward the [0001] direction, i.e., in a Ga-terminated film, toward the substrate and in a N-terminated film, toward the surface. A chemical quantitative analysis shows that these pyramidal defects are Mg rich. They are present in all the studied films, independent of the doping level.

Growth of high quality crack-free AlGaN films on GaN templates using plastic relaxation through buried cracks

A method is presented to achieve thick high quality crack-free AlGaN layers on GaN. This method uses jointly plastic relaxation and lateral growth. In a first step, plastic relaxation by cracking and misfit dislocation introduction is realized. Then the cracks are overgrown to obtain a smooth surface. By this reproducible technique, we grew smooth metal-organic chemical vapor deposition Al0.2Ga0.8N films with a threading dislocation density as low as 5×108 cm−2. This result is the best ever reported for crack-free AlGaN growth over a large area. The control of the complete plastic relaxation opens up perspectives for the realization of high performance devices. In order to explain the mechanisms involved in the full relaxation of the AlGaN/GaN heterostructure, we propose a relaxation scheme and discuss its different steps.

Luminescence and absorption in InGaN epitaxial layers and the van Roosbroeck–Shockley relation

Wurtzite InxGa1−xN (0.01≲x≲0.14) films have been grown by metalorganic vapor phase epitaxy on sapphire substrates. Integrated photoluminescence intensity and line shapes have been studied as functions of temperature and alloy composition x. We compare the “effective” InGaN band gap energy assessed by photothermal deflection spectroscopy with a “mean” band gap energy calculated from room temperature photoluminescence spectra utilizing the van Roosbroeck–Shockley relation and assuming a Gaussian energy dependence of the subband gap absorption coefficient. The Stokes’ shift between band gap energy and 300 K photoluminescence peak is explained by this model.Wurtzite InxGa1−xN (0.01≲x≲0.14) films have been grown by metalorganic vapor phase epitaxy on sapphire substrates. Integrated photoluminescence intensity and line shapes have been studied as functions of temperature and alloy composition x. We compare the “effective” InGaN band gap energy assessed by photothermal deflection spectroscopy with a “mean” band gap energy calculated from room temperature photoluminescence spectra utilizing the van Roosbroeck–Shockley relation and assuming a Gaussian energy dependence of the subband gap absorption coefficient. The Stokes’ shift between band gap energy and 300 K photoluminescence peak is explained by this model.

Semipolar GaN films on patterned r-plane sapphire obtained by wet chemical etching

It is shown that (112¯2)-oriented GaN films can be achieved from r-sapphire patterned by chemical etching. Growth first occurs selectively from the inclined c-facet of sapphire, leading finally to a fully coalesced layer with (112¯2) orientation. The structural and optical quality of these layers was assessed by x-ray diffraction, cathodoluminescence and photoluminescence measurements. The results clearly show that the quality of (112¯2) GaN on patterned r-sapphire is markedly improved in comparison with (112¯2) GaN on m-sapphire.

Crack-Free Thick GaN Layers on Silicon (111) by Metalorganic Vapor Phase Epitaxy

The tensile strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy was investigated. Thick (0.9-2.5 μm) GaN layers without any crack were deposited on Si(111) using AIN/GaN superlattices as templates. X-ray diffraction and photoluminescence measurements were used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layer. Evolution of strain as a function of the GaN layer thickness is also determined. Dislocation reduction (from 10 10 to 2.5 x 10 9 cm -2 ) is observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed, with full width at half maximum of the I 2 line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) X-ray rocking curve attests the high crystalline quality of GaN on Si(111 ), when AlN/GaN superlattices are used.

Indium incorporation above 800 °C during metalorganic vapor phase epitaxy of InGaN

We have studied the indium incorporation into InGaN ternary alloys during low-pressure metalorganic vapor-phase epitaxy as a function of the trimethylindium flow and the growth temperature in the 800–860 °C range. Partially relaxed InxGa1−xN bulk films with indium compositions 0.02≲x≲0.14 have been grown. In relation to the band-gap energy at room temperature, determined by photothermal deflection spectroscopy, we find a downward band-gap bowing of 2.65±0.15 eV. The required change of the trimethylindium flow as a function of the growth temperature, necessary to obtain isocomposition InGaN films, can be described by an Arrhenius law. We find an indium desorption energy of 0.8±0.3 eV.

Improved semipolar (112¯2) GaN quality using asymmetric lateral epitaxy

Semipolar (112¯2) GaN films were obtained by epitaxial lateral overgrowth from (112¯2) GaN templates patterned with SiO2 stripes 7 μm wide with 3 μm spacing, oriented along the [11¯00] GaN in-plane direction. The growth conditions were optimized in order to promote a fast growth rate along the +c [0001] direction. The crystal expands both laterally and vertically until a situation where it overgrows the adjacent crystal, thus stopping the propagation of stacking faults and threading dislocations. The growth anisotropy and filtering of defects is observed by cross-sectional scanning electron microscopy and cathodoluminescence. The lowering of defect density is confirmed by x-ray diffraction measurements. The photoluminescence spectrum of the coalesced epitaxial lateral overgrowth of the (112¯2) epilayers exhibits a strong band edge emission and a low emission band at 3.41 eV, assigned to the remaining stacking faults.

Influence of high Mg doping on the microstructural and optoelectronic properties of GaN

A transmission electron microscopy study of a wide range of p-type GaN samples reveals that high Mg doping has a strong influence on the polarity of GaN. The main characteristic of Mg-doped metal organic vapour phase epitaxy (MOVPE) and bulk GaN is the presence of pyramidal inversion domains (PIDs). It is shown that the appearance of PIDs is correlated with a decrease of the free hole concentration and with the appearance of the blue photoluminescence band which is characteristic of MOVPE-grown Mg-doped GaN. A tentative model based on electrostatic considerations is proposed for this blue luminescence band.