M. Vaille

Reduction mechanisms for defect densities in GaN using one- or two-step epitaxial lateral overgrowth methods

A transmission electron microscopy study of the reduction mechanisms for defect densities in epitaxial lateral overgrown (ELO) GaN films is presented. In the standard one step ELO, the propagation of defects under the mask is blocked, whereas the defects in the window regions thread up to the surface. We propose an alternative two step ELO method. In a first step, dislocations close to the edge of the (0001) top facet bend at 90°, thereby producing a drastic reduction in the density of defects above the window. After the coalescence, induced by lateral growth in a second step, dislocations are mainly observed in the coalescence boundaries. The density of defects is decreased to 2×10−7 cm−2 over the entire surface and areas nearly 5 μm wide with 5×106 cm−2 dislocations between the center of the windows and the coalescence boundaries are obtained.

High quality GaN grown by MOVPE

Abstract Several nitrogen precursors have been used for the growth of GaN in MOVPE, but so far the best results were obtained using NH 3 , even though NH 3 does not produce a significant amount of active species at the growing interface. To produce active species from N 2 or NH 3 , a remote plasma-enhanced chemical vapour deposition (RPECVD) process has been implemented. In addition, nitrogen metalorganic precursors, triethylamine and t-butylamine, were also used. To accurately control the critical parameters of the MOVPE of GaN, we have implemented a laser reflectometry equipment, which allows a real-time in situ monitoring of the different steps of the growth, i.e. nitridation of the substrate, nucleation, heat treatment, and deposition. Using an appropriate buffer layer, GaN grown on sapphire using NH 3 as nitrogen precursor, shows sharp low temperature photoluminescence lines (4 meV at 9 K), whereas other nitrogen precursors did not lead to comparable electronic quality.

Study of open-core dislocations in GaN films on (0001) sapphire

This article deals with a plan view and cross section transmission electron microscopy study of columnar defects in GaN films epitaxially grown on sapphire (0001). They are identified as open-core (0001) Burgers vector dislocations. Their behavior along the film thickness is described: it alternates from open core sections (nanopipes) to closed core sections. This alternating behavior is observed in the first 0.5 μm close to the interface with sapphire.

Microstructure of GaN epitaxial films at different stages of the growth process on sapphire (0 0 0 1)

Abstract The microstructure of GaN films at different stages of a classical two steps growth process is studied using TEM. The buffer layer grown at low temperature (600°C) exhibits a mixed cubic-hexagonal columnar microstructure. Numerous defects are present to accomodate the misorientations between micrograins. During the following annealing step up to 1050°C, the microstructure drastically changes: cubic islands remain on the top of a film with hexagonal structure. The buffer layer at this stage is still highly polycrystalline. The microstructure of micrometer thick films grown at 1050°C could be separated in two zones. Close to the interface with sapphire, misfit dislocations, basal stacking faults and nanocavities are observed. We propose a mechanism of relaxation of the strain due to the difference of thermal expansion coefficients which could explain the presence of stacking faults. The existence of nanocavities is supposed to be related to a contamination by oxygen. After a thickness of 0.5 μm, two types of threading defects remain: edge dislocations with 1/3〈1 12¯0〉 Burgers vector which accommodate slight misorientations between grains, and nanopipes. These nanopipes are identified as open core dislocations with (0 0 0 1) Burgers vector. They have an alternating behaviour: close core, open core. The microstructure of this bulk zone duplicates the microstructure of the buffer layer at a higher scale, pointing out the crucial importance of the first steps of the growth.

Mg-enhanced lateral overgrowth of GaN on patterned GaN/sapphire substrate by selective Metal Organic Vapor Phase Epitaxy

Selective and lateral overgrowth by Metal Organics Vapour Phase Epitaxy (MOVPE) was carried out until coalescence to produce smooth and optically flat thick GaN layers. A GaN epitaxial layer is first grown using atmospheric pressure Metalorganic Vapour Phase Epitaxy on a {0001} Al 2O3. substrate. Then a 30Å silicon nitride dielectric film is deposited in-situ by reaction of silane and ammonia to form a selective mask. Afterwards, the openings and the figures in the dielectric films are achieved using standard photolithographic technology. Stripes openings in the mask, revealing free GaN surface, are aligned in the 〈1010〉 direction. Typical stripes spacing and width are 10 μm and 5 μm respectively. These patterned layers are further on used for epitaxial regrowth of GaN by MOVPE. The growth anisotropy and therefore the coalescence process is achieved by introducing (MeCp)2Mg in the vapour phase. A two-step process is reported which allows a dramatic reduction of threading dislocations density not only above the masked areas but also above the windows opened in the mask. With this process, very sharp bound exciton luminescence peaks are measured at low temperature in the overgrown GaN.

A Two‐Step Method for Epitaxial Lateral Overgrowth of GaN

We report on a two-step process for the epitaxial lateral overgrowth of GaN. In the first step, the selective area epitaxy proceeds at low temperature and GaN stripes with a triangular cross section are rapidly obtained. Then, in a second step, these stripes are used as seeds for epitaxial lateral overgrowth, the growth conditions being changed either by increasing the growth temperature or by introducing a Mg precursor. Fully coalesced and planar GaN samples are thereby achieved. A comparison between samples grown without and with this two-step process evidences an additional reduction in the threading defects density with the two-step process. The dislocations emerging from the seeds are drastically reduced to 5 × 106 to 107 cm—2 between the coalescence boundaries.

Alternative N precursors and Mg doped GaN grown by MOVPE

In this paper, we address two different aspects relevant to the growth of GaN. The first part concerns alternative nitrogen source whereas in the second part, we report experimental results on Mg doping. Several nitrogen precursors have been used for the growth of GaN in MOVPE. To produce active species from N2 or NH3, a remote Plasma Enhanced Chemical Vapour Deposition (RPECVD) process has been implemented. In addition, nitrogen organic precursors, triethylamine and t-butylamine were also used. To accurately control the critical parameters of the MOVPE of GaN, we have implemented a laser reflectometry, which allows a real time in situ monitoring of the different steps of the growth. MeCp2Mg was used as Mg precursor for the p doping study. The dependence on the partial pressure of Mg precursor of dopant incorporation, electrical activity and growth rate are reported.

Study of (Al,Ga)N Bragg Mirrors Grown on Al2O3(0001) and Si(111) by Metalorganic Vapor Phase Epitaxy

A series of selective (Al,Ga)N quarter-wave reflectors has been grown on sapphire and silicon substrates by metalorganic vapor phase epitaxy. The microstructure of the mirror structures, consisting of GaN quarter-wave layers alternating with AlN. AIGaN or AlN/GaN short-period superlattices, has been assessed in terms of a joint X-ray diffraction (XRD) and transmission electron microscopy (TEM) study.

Microstructural analysis of III-V nitrides grown on 6H-SiC by metal-organic vapour phase epitaxy (MOVPE )

Abstract We have studied the growth mechanisms of GaN and AlGaN layers with low aluminium incorporation (

High quality ELO-GaN layers on GaN/Al2O3 patterned substrates by halide vapour phase epitaxy

Abstract The growth of thick GaN layers by halide vapour phase epitaxy (HVPE) on patterned metal organics vapour phase epitaxy (MOVPE)-GaN/Al 2 O 3 substrates and their characterisation is reported. The growth on small area features has shown that lateral overgrowth was enhanced following preferential crystallographic directions. Double crystal X-ray diffraction (DCXRD) assessment in ω scan showed 50 arcsec full width at half maximum (FWHM) for layers grown on a small area hexagonal holes field. On the way towards the realisation of self-supported GaN substrates, the present study was extended to epitaxial lateral overgrowth (ELO) on large surface GaN/Al 2 O 3 patterned substrates to achieve full coalescence. Hexagonal holes and parallel stripes features were used. Structural, electrical and optical characterisation of such layers was performed, underlining the promising quality of these materials.