F. J. Pacheco

Growth of III-nitrides on Si(1 1 1) by molecular beam epitaxy: Doping, optical, and electrical properties

The growth of high-quality III-nitrides by plasma-assisted molecular beam epitaxy on Si(1 1 1) substrates is addressed. A combination of optimized AIN buffer layers and a two-step growth process leads to GaN layers of high crystal quality (8 arcmin X-ray diffraction full-width at half-maximum) and flat surfaces (57 A rms). Low-temperature luminescence spectra, dominated by excitonic emissions at 3.465 ± 0.002 eV, reveal the presence of a biaxial tensile strain of thermal origin. AlGaN layers, grown within the alloy range 0.10 < x < 0.76, have flat surfaces and exhibit strong excitonic luminescence. Si-doping of GaN and AlGaN produces n-type films reaching electron densities up to 2 × 10 19 and 8 × 10 19 cm -3 , respectively. From photoluminescence and Hall data analysis a Si-donor ionization energy between 50 and 60 meV is derived in GaN. The exciton bound to Si neutral donors at 3.445 eV redshifts while the c-axis lattice parameter decreases as the Si-doping increases, indicating an enhancement of the biaxial tensile strain in the film. This strain increase is a consequence of a strong reduction of the density of dislocations reaching the free surface, due to a particular grain size and orientation governed by the presence of Si donors. Be-doping is also achieved on GaN giving the shallowest acceptor activation energy reported so far, around 90-100 meV. However, there is a severe limitation of the Be incorporation on substitutional sites, leading to the formation of complex, deep defects.

Design of InGaAs linear graded buffer structures

The relaxation of compositionally graded InGaAs buffers, with and without uniform cap layers, has been studied. Simple InGaAs linear‐graded layers on GaAs substrates never reach complete relaxation. The residual strain in these structures produces a dislocation‐free strained top region while the rest of the buffer is nearly completely relaxed through misfit dislocations, as observed by transmission electron microscopy (TEM). This strained top region is analyzed and its thickness compared with theoretical calculations. The effects of different cap layers on the relaxation behavior of the graded buffer has been studied by double crystal x‐ray diffraction, TEM, and low temperature photoluminescence, and results compared with predictions of the models. The optical quality of the cap layer improves when its composition is close to the value that matches the lattice parameter of the strained surface of the grade. The design of linear graded buffers having a strain‐free cap layer with high crystalline quality is...

Influence of the surface morphology on the relaxation of low-strained InxGa1-xAs linear buffer structures

Abstract The relaxation of low-strained InGaAs linear-graded buffer layers up to 30 and 60% In content grown on GaAs substrates is studied. Strain-limited designs allow to preserve bidimensional (2D) growth mode with a relaxation driven by dislocation multiplication for large thickness, as in the case of single layers. However, it is shown that low-strained linear buffer layers are not able to inhibit tridimensional (3D) growth mode. Under standard growth conditions (500°C), the surface morphology evolves from a near-flat one for samples up to x = 0.3, to a rough one at higher In content, characterised by saw-tooth morphology with well-defined facets. The development of the surface roughness changes the strain relaxation mechanism from dislocation multiplication inside the layer to surface elastic strain relaxation plus surface nucleation and a high density of threading dislocations. A constant surface strain in linear grades during all the dislocation-driven relaxation is proposed to allow the observed 2D to 3D transition. The influence of the grading rate, composition steps, and growth temperature on the surface morphology and the relaxation are analysed. Appropriate growth conditions and buffer design up to high Indium content allow to produce structures with improved structural and optical quality.

A mechanism for the multiple atomic configurations of inversion domain boundaries in GaN layers grown on Si(111)

Atomic structure investigation has been carried out on inversion domain boundaries in GaN layer grown on Si(111) by molecular-beam epitaxy. The comparison of the stacking sequences between simulated and experimental high resolution electron microscopy images shows the existence of two different atomic configurations for the inversion domain boundaries: the Holt model coexists with the V model inside the same layers. A mechanism allowing the switch from one model to the other by the interaction with the I1 stacking fault is proposed.

Electron microscopy study of SiC obtained by the carbonization of Si(111)

A SiC thin film grown by propane carbonization of a Si(111) substrate has been characterized by transmission electron microscopy and scanning electron microscopy techniques. This study reveals the presence of planar defects in the SiC layer and voids in the Si(111) substrate as well as misfit dislocations at the SiC/Si interface. The resulting SiC layer consists of a mosaic structure and is shown to have low stress.

AlN buffer layer thickness influence on inversion domains in GaN/AlN/Si(111)

Abstract The AlN buffer layer thickness influence on the inversion domains (IDs) in GaN/AlN/Si(111) grown by plasma assisted molecular beam epitaxy (MBE) is studied by transmission electron microscopy (TEM). The GaN layer polarity is determined by convergent beam electron diffraction (CBED). The AlN buffer layer thickness notably affects to the IDs density and the GaN epilayer polarity. Moreover the threading dislocation distribution existence also depends on such thickness.

Origin of Inversion Domains in GaN/AlN/Si(111) Heterostructures Grown by Molecular Beam Epitaxy

Using high-resolution electron microscopy, the formation of inversion domains inside GaN layers grown by molecular beam epitaxy (MBE) on Si(111) was investigated. It was shown that when the buffer layer thickness is small (15 nm), the inversion domains form inside a nitrogen polar layer at the steps on the Si(111) surface. As the thickness of the AlN buffer layer is increased to 35 nm, the MBE GaN layer polarity switches to gallium polarity, but inversion domains still form. In this situation they originate at the GaN/AlN interface.

Properties of Homoepitaxial and Heteroepitaxial GaN Layers Grown by Plasma-Assisted MBE

This work presents a comparative study of the growth by plasma-assisted molecular beam epitaxy (MBE) of GaN layers on four different substrates: Si(111), Al2O3(0001), GaN/Al2O3 and ELOG GaN/Al2O3 templates. Optimization of the growth parameters for the case of growth of GaN layers on silicon substrates leads to smooth films with surface roughness below 5 nm, intense low temperature photoluminescence (PL) (15 meV FWHM) and X-ray diffraction (XRD) values of 8.5 arcmin. (FWHM). The quality of the material clearly improves when growing on sapphire substrates obtaining intense low- and-room temperature PL (10 and 54 meV FWHM, respectively) and XRD values of 6.5 arcmin (FWHM). The best GaN epilayers (intense low-temperature PL emissions with FWHM of 4 meV) are obtained when growing homoepitaxially on high quality GaN/Al2O3 templates, reproducing the optical and structural properties of the template underneath. Finally, the dislocation density decreases drastically from (6 to 10) × 109 cm2 for the case of silicon substrate to 〈106 cm–2 for the GaN layers grown on the ELOG templates.

Structural characterization of high temperature AlN intermediate layer in GaN grown by molecular beam epitaxy

Transmission electron microscopy has been used to study the structural quality of GaN grown on sapphire by plasma assisted molecular beam epitaxy using high temperature AlN intermediate layers with different thicknesses. The introduction of an AlN intermediate layer with an optimum thickness is observed to minimize the density of dislocations reaching the overgrown GaN surface. In this sample, the measured threading dislocation density reaching the surface of 1 x 10 10 cm -2 resulted to be seven times lower than that of a reference sample, without any AlN interlayer. The bending at the GaN/AlN interface and following interactions between dislocations have been observed in cross-sectional transmission electron micrographs. This fact explains the decrease of dislocation density reaching the GaN surface.

A study of the defect structure in GaAs layers grown at low and high temperatures on Si(001) substrates

Abstract In this work we report the defect structure of GaAs layers grown on Si (001) substrates by atomic layer molecular beam epitaxy at 350 °C and conventional molecular beam epitaxy at 580 °C, investigated by transmission electron microscopy. It is found that the GaAs layers grown at 580 °C show high threading dislocation densities, whilst the majority defect types in the GaAs layers grown at 350 °C are stacking faults and microtwins. These results can be explained by taking into account that the mobility of partial dislocations depends on both the stress in the epitaxial layer and the growth temperature. According to this dependence, a transition from dissociated threading dislocations at a low temperature (350 °C) to perfect threading dislocations at a high temperature (580 °C) can be expected.