P. Vennéguès

Epitaxial Lateral Overgrowth of GaN

Since there is no GaN bulk single crystal available, the whole technological development of GaN based devices relies on heteroepitaxy. Numerous defects are generated in the heteroepitaxy of GaN on sapphire or 6H-SiC, mainly threading dislocations (TDs). Three types of TDs are currently observed, a type (with Burgers vector 1/3〈〉); c type (with 〈0001〉) and mixed a+c (1/3〈〉). The Epitaxial Lateral Overgrowth (ELO) technology produces high quality GaN with TD densities in the mid 106 cm—2, linewidth of the low-temperature photoluminescence (PL) near-bandgap recombination peaks <1 meV and deep electron traps reduced below 1014 cm—3 (compared to mid 1015 cm—3 in standard GaN). Numerous modifications of the ELO process have been proposed in order either to avoid technological steps (mask-less ELO) or to improve it (pendeo-epitaxy). Basically developed on either sapphire or 6H-SiC, the ELO technology is also achievable on (111)Si or (111)3C-SiC/Si provided that an appropriate buffer layer is grown to avoid cracks. More sophisticated technologies have been implemented to further increase the useable part of the ELO GaN surface (two technological steps, three-step ELO). Unfortunately, in-depth understanding of the basic ELO process is still missing, i.e. of the growth anisotropy and bending of dislocations.

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.

Anisotropic morphology of nonpolar a-plane GaN quantum dots and quantum wells

The growth of (11–20) or a-plane quantum dots and quantum wells by plasma-assisted molecular-beam epitaxy has been studied. It is shown that Ga-rich conditions lead to the formation of quantum dots, whereas quantum wells are obtained in N-rich conditions. Combining various experimental techniques, it is furthermore demonstrated that quantum dot nucleation along [1–100] and quantum well morphology in the (1–100) plane are influenced by anisotropic growth of AlN buffer layer. Moreover, it is established that peculiar morphological features of quantum dots and quantum wells, in particular the asymmetric shape of quantum dots, are related to the polar character of the [0001] direction in wurtzite nitride material.

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.

Luminescence and reflectivity studies of undoped, n- and p-doped GaN on (0001) sapphire

GaN grown by three different methods (MOVPE, GSMBE and HVPE) have been studied using temperature (T) dependent reflectivity and photoluminescence (PL). Both non intentionally doped (MOVPE, GSMBE and HVPE), n- and p-doped samples (MOVPE and GSMBE) have been investigated. Reflectivity is used to obtain intrinsic transition energies. These energies vary with the amount of strain in the crystal. Growth parameters influencing this strain state are discussed. Using MOVPE (T-g approximate to 1050 degrees C) and GSMBE (T-g approximate to 800 degrees C), it is possible to grow samples whose low temperature PL spectra are dominated by free and bound excitons and their phonon replica. Intentional n-type doping up to 10(20) cm(-3) is easily achieved with Si. For n much greater than 10(18) cm(-3), the spectra broaden and exhibit a blue shift, attributed to band filling. p-Type doping has been attempted using Mg, C and Ca. Ca doping led to compensated samples. C doping using CCl4 resulted in n-type samples, due to simultaneous oxygen incorporation in the layers; a strong enhancement of the 3.27 eV donor acceptor pair PL is also observed in this case. p-Type doping up to 10(18) cm(-3) has been achieved with Mg. With increasing densities, a deepening of the donor acceptor pair PL energy is observed. For high Mg doping, the spectra are dominated by a blue band in the 2.8 eV range, involving deep electron states

Molecular-beam epitaxy of gallium nitride on (0001) sapphire substrates using ammonia

Ammonia is used for growing undoped GaN layers by gas source molecular-beam epitaxy on c-plane sapphire substrates. The growth mode is layer by layer as shown by the observation of reflection high-energy electron diffraction intensity oscillations. The structural quality is studied by x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Low-temperature photoluminescence (PL) and reflectivity demonstrate intrinsic excitonic emission. Room-temperature PL exhibits a strong band-edge intensity and a weak deep-level emission, the so-called yellow band. Finally, secondary ion mass spectroscopy is carried out to check the residual impurity levels of Si, C, and O.

Interface structure and anisotropic strain relaxation of nonpolar wurtzite (1120) and (1010) orientations: ZnO epilayers grown on sapphire

The interface properties between nonpolar ZnO and sapphire have been studied using high resolution transmission electron microscopy. Two nonpolar orientations are investigated: a- and m-orientations corresponding to [112¯0] and [101¯0] crystallographic directions. After the definition of the epitaxial relationships and the resulting initial lattice mismatch, we show that nonpolar ZnO can be grown on sapphire with perfectly flat interfaces. Geometrical misfit dislocations are observed at the interface ZnO/sapphire and their density gives the residual strain in the layer. A strong anisotropy in the strain relaxation is found along the two perpendicular in-plane directions. This anisotropy may be explained in terms of initial anisotropic mismatch yielding different relaxation processes. A domain matching epitaxy is observed in m- and a-oriented layers for mismatches larger than 9% while a lattice matching epitaxy, in which the relaxation is driven by nucleation and glide of dislocations, is observed in a-ori...

Epitaxial relationships between GaN and Al2O3(0001) substrates

GaN thin layers (200 Angstrom) were grown by gas-source molecular beam epitaxy on c-plane Al2O3 substrates, Transmission electron microscopy reveals that two different epitaxial relationships may occur, The well-known GaN orientation with the c axis perpendicular to the Al2O3 surface and [ ](GaN)parallel to[ ](Al2O3) is observed when the substrate is nitridated prior to the growth. On the other hand, GaN layers deposited on bare Al2O3 surfaces exhibit a different crystallographic orientation: [ ](GaN)parallel to[ ](Al2O3) and [ ](GaN)parallel to[ ](Al2O3). This corresponds to a tilt of about 19 degrees of the c axis with respect to the substrate surface

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.