F. M. Morales

Model for the thickness dependence of electron concentration in InN films

A model for the influence of different contributions to the high electron concentration in dependence on the film thickness of state-of-the-art InN layers grown by molecular-beam epitaxy is proposed. Surface accumulation has a crucial influence for InN layers 10μm.

The role of Si as surfactant and donor in molecular-beam epitaxy of AlN

The growth of Si-doped AlN(0001) thin films on Al2O3(0001) substrates by plasma-induced molecular-beam epitaxy is reported. We have found that Si positively affects the epitaxy being an effective surfactant for AlN growth with a remarkable impact on the crystal quality. It was proven that the characteristic surface reconstruction sequences frequently related to the Al adatoms are obviously Si induced on AlN(0001) surfaces. It was also observed that heavy doping conditions result in volume segregation of Si on the threading dislocation network and in the formation of an amorphous (AlO)(SiO)N cap layer caused by surface oxidation of the accumulated Al and segregated Si. The electron affinity was measured to be smaller than 0.5eV on the clean AlN surface after removing of the cap layer using Ar+ sputtering.

Raman studies of Ge-promoted stress modulation in 3C–SiC grown on Si(111)

We present a study of the stress state in cubic silicon carbide (3C–SiC) thin films (120 and 300 nm) grown by solid-source molecular-beam epitaxy (SSMBE) on Si(111) substrates modified by the deposition of germanium prior to the carbonization of Si. μ-Raman measurements were used to determine the residual stress existing in the 3C–SiC layers. The stress is found to decrease linearly with increasing Ge quantity but with different strength depending on the 3C–SiC thickness deposited after the introduction of Ge. Based on secondary ions mass spectroscopy (SIMS) and transmission electron microscopy (TEM) analyses it is suggested that the Ge introduced prior to the carbonization step remains in the near-interface region and reduces the Si outdiffusion, which further reduces the stress state of the 3C–SiC layers.

Ge-modified Si(100) substrates for the growth of 3C-SiC(100)

An alternative route to improve the epitaxial growth of 3C-SiC(100) on Si(100) was developed. It consists in covering the silicon wafers with germanium prior to the carbonization step of the silicon substrate. Transmission electron microscopy and μ-Raman investigations revealed an improvement in the residual strain and crystalline quality of the grown 3C-SiC layers comparable to or better than in the case of 3C-SiC grown on silicon on insulator substrates. These beneficial effects were reached by using a Ge coverage in the range of 0.5–1 monolayer.

SiC voids, mosaic microstructure and dislocations distribution in Si carbonized layers

Abstract The defect structure of SiC/Si layers obtained by carbonization of Si is reported by means of transmission electron microscopy in high-resolution (HREM) and conventional (CTEM) modes. 3C–SiC was obtained after a rapid thermal annealing treatment and good interfacial quality is reported in terms of small void dimensions and densities. Moreover, high misfit dislocation densities are observed close to the Si/SiC interface and inside the SiC layer without observable generation of threading dislocations. The mosaic grain structure is also evidenced, with low misorientation with respect to the substrate. These results are encouraging for further growth of III–N alloy heterostructures.

Evaluation of interpolations of InN, AlN and GaN lattice and elastic constants for their ternary and quaternary alloys

Structural and compositional data were collected for several high-quality strained InGaN, InAlN, AlGaN and InAlGaN layers with various compositions. Based on these results, the InN/AlN/GaN molar fractions of every film were indirectly estimated by the application of Vegards law (Vegard 1921 Z. Phys. 5 17) to lattice and elastic constants of the binaries and by an alternative approach proposed by (Williams et al 1978 J. Electron. Mater. 7 639) (even considering possible mistakes in its application). True compositions were independently assessed by x-ray spectroscopy or Rutherford backscattering spectrometry. The results of both interpolation models are highlighted in utilizable contour and surface ternary plots of lattice constants and biaxial strain relaxation coefficients calculated for the whole compositional range. Calculated compositions from Vegards law best fit the measured values. Finally, it is geometrically demonstrated that there are wrong assumptions in the Williams et al model, so that Vegards law should only be used to successfully determine the compositions of ternary and quaternary III-N nitrides.

Near-infrared emitting In-rich InGaN layers grown directly on Si: Towards the whole composition range

The authors report compact and chemically homogeneous In-rich InGaN layers directly grown on Si (111) by plasma-assisted molecular beam epitaxy. High structural and optical quality is evidenced by transmission electron microscopy, near-field scanning optical microscopy, and X-ray diffraction. Photoluminescence emission in the near-infrared is observed up to room temperature covering the important 1.3 and 1.55 μm telecom wavelength bands. The n-InGaN/p-Si interface is ohmic due to the absence of any insulating buffer layers. This qualitatively extends the application fields of III-nitrides and allows their integration with established Si technology.

Impact of silicon incorporation on the formation of structural defects in AlN

The impact of Si impurities on the structural properties of AlN, grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire is studied. Under nitrogen-rich growth conditions silicon can be homogeneously incorporated up to Si concentrations of [Si]=5.2×1021 cm−3. The presence of silicon on the surface during the growth process is demonstrated to be beneficial for the surface morphology and the structural properties of the AlN films. For [Si] up to (5±3)×1020 cm−3, this surfactant behavior results in a decrease of the surface roughness from 8 nm for undoped layers grown in a nitrogen-rich regime to less than 1 nm. In addition, high resolution x-ray diffraction studies reveal an increase of the average lateral crystal size from 300 nm to more than 1 μm and a simultaneous decrease of the screw dislocation density from 3.8×108 cm−2 for (comparably) weakly doped samples to 2×107 cm−2. At the same [Si] the heterogeneous stress shows a minimum of less than 50 MPa and drastically increases for higher [Si]...

Selective ion-induced intermixing and damage in low-dimensional GaN/AlN quantum structures

Ion-induced intermixing and damage is evaluated in GaN/AlN superlattices of quantum dots (QDs) and quantum wells (QWs) using 100 keV Ar(+) implantation at low temperature (15 K). Despite the similar damage build up at low fluences, a significant increase of the damage accumulation takes place for QDs at high fluences. Elemental depth profiles were fitted with a diffusion model, revealing the higher intermixing efficiency in QD superlattices, significantly higher than for QWs. The scaling of diffusion length with the local fluence and defect concentration is understood on the basis of cascade mixing and migration of defects in the cation sublattice. The selective intermixing/damage of QDs is explained by the promotion of lateral diffusion mechanisms that result in smooth interfaces, as well as by an enhanced diffusivity due to the characteristic strain distribution in QD superlattices.

The role of edge dislocations on the red luminescence of ZnO films deposited by RF-sputtering

The existence of extended defects (i.e., dislocations) in inorganic semiconductors, such as GaN or ZnO, responsible for broad emission peaks in photoluminescence analysis remains unresolved. The possible assignments of these luminescence bands are still matter of discussion. In this study, two different zinc oxide samples, grown under different oxygen partial pressures and substrate temperatures, are presented. Epitaxial and structural properties were analysed by means of X-ray diffraction and transmission electron microscopy techniques. They confirm that the layers are single-phase with a good crystalline quality. Nevertheless, a different density of threading dislocations, with a higher contribution of edge dislocations, was found. Photoluminescence spectroscopy has been used to investigate the optical properties. The steady state luminescence spectra performed at 14K evidenced the donor bound exciton recombination and deep green and red emission bands. The red band with a maximum at 1.78 eV was found to be stronger in the sample grown at lower oxygen pressure which also shows higher density of threading dislocations. From the temperature and excitation density dependence of the red band, a donor acceptor pair recombination model was proposed, where hydrogen and zinc vacancies are strong candidates for the donor and acceptor species, respectively.