R. Clos

Evolution of stress In GaN heteroepitaxy on AlN/Si(111): From hydrostatic compressive to biaxial tensile

The initial steps of GaN growth on an AlN buffer layer on Si(111) substrates by metalorganic vapor phase epitaxy were investigated using field emission scanning electron microscopy, micro-photoluminescence, as well as by conventional and grazing incidence x-ray diffraction. A series of GaN layers was grown for various times ranging from 7.5 s to several minutes, doubling the growth time for each step. The AlN buffer layer is noncontinuous and consists of (0001)-oriented AlN islands with a mean diameter of about 50 nm. On top of these nucleation centers three-dimensional growth of GaN was observed. With increasing growth times up to 30 s these islands further expanded and their distribution became more homogeneous. At 60 s coalescence started with homogeneously distributed islands, and after 120 s the layer was fully coalesced. The layers grown for 7.5 and 15 s are under a high compressive hydrostatic pressure, which might be enhanced by the lattice mismatch between AlN and GaN. For longer growth times a b...

Optical and structural microanalysis of GaN grown on SiN submonolayers

Lateral overgrowth techniques have demonstrated their ability to strongly reduce the dislocation density in GaN grown on a variety of foreign substrates. The in situ deposition of SiN during metal-organic chemical-vapor phase epitaxy (MOVPE) leads to the formation of a randomly distributed mask layer and induces lateral overgrowth similar to conventional epitaxial lateral overgrowth of GaN. Specifically for GaN on silicon substrate, the insertion of SiN submonolayers is a promising method to reduce not only the dislocation density but also the tensile stress upon Si doping. Besides the advantage of uncomplicated in situ mask formation, it allows complete coalescence and planarization of the overgrown GaN within a layer thickness of about 500 nm depending on the mask thickness, thus reducing the liability to cracking. However, the insertion of ultrathin SiN interlayers and, for thicker GaN stacks, additional stress-compensating low-temperature AIN (LT-AIN) leads to a complicated interplay of stress and dis...

Strains and Stresses in GaN Heteroepitaxy - Sources and Control

We present a study of the sources of strain in GaN heteroepitaxy by in- and ex-situ measurement techniques. With an in-situ curvature measurement technique the strain development can be directly correlated to the different layers and doping in simple and device structures. We show several solutions for strain reduction and control. High-quality devices grown on Si are demonstrated.

Heteroepitaxy of GaN on Silicon: In Situ Measurements

Due to the lack of GaN wafers, so far, group-III nitrides are mostly grown on sapphire or SiC substrates. Silicon offers an attractive alternative because of its low cost, large wafer area, and physical benefits such as the possibility of chemical etching, lower hardness, good thermal conductivity, and electrical conducting or isolating for light emitting devices or transistor structures, respectively. However, for a long time, a technological breakthrough of GaN-on-silicon has been thought to be impossible because of the cracking problem originating in the huge difference of the thermal expansion coefficients between GaN and silicon which leads to tensile strain and cracking of the layers when cooling down. However, in recent years, several approaches to prevent cracking and wafer bowing have been successfully applied. Nowadays, device-relevant thicknesses of crackfree group-III-nitrides can be grown on silicon. To reach this goal the most important issues were the identification of the physical origin of strains and its engineering by means of in situ monitoring during metalorganic vapor phase epitaxy.

Local strain and potential distribution induced by single dislocations in GaN

The presence of a threading edge dislocation terminated at the surface of GaN bulk substrates causes a dipole-like strain state ranging over a several micrometer square area. The local strain state is derived from microphotoluminescence mappings of the near-band-edge spectrum and is quantitatively reproduced by a three-dimensional elastic deformation model approach. These results are compared with the local electrical potential distortion due to the core charge and attracted defects as analyzed by scanning surface-potential microscopy. In contrast to the local strain, the potential profile does not show a dipole-like behavior and decreases laterally faster.

Systematic growth studies of narrow constrictions formed by molecular beam epitaxy on prepatterned substrates

Abstract The overgrowth of prepatterned GaAs substrates by molecular beam epitaxy (MBE) of GaAs/AlGaAs heterostructures is investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). First, mesas with vertical side walls which have a constriction of 1–5 μm, are patterned into a GaAs substrate. During the MBE deposition of GaAs/AlGaAs layers onto this substrate, specific facets form at the boundaries of the patterned area. These facets depend on the shape and orientation of the prepatterning. A detailed study of an overgrown constriction which consists mainly of {110} and intermediate { N 11}A ( N =3 or 4) facets shows that the constriction narrows and that a rectangular shape of the prepatterning is maintained in the MBE process. Inter facet diffusion of adatoms modifies the thickness of the MBE layers in the patterned area. The enhanced thickness of layers in the center of the constriction may be utilized to fabricate dot and wire like structures at well-defined positions. AFM measurements in combination with simulations of the surface diffusion clearly demonstrate that the {110} facets yield the main contribution to the enhanced thickness. { N 11}A facets can only be employed to obtain the above enhancement, if the prepatterned constriction is round-shaped. This sets a limit for down-scaling to the submicron range.