RHEED monitoring of elastic stresses during MBE growth of group III nitride heterostructures

Abstract

443Heterostructures based on wide bandgap group III nitrides (III N) are widely used in large scale com mercial production of various optoelectronic devices and microwave transistors [1]. However, these device heterostructures are almost always subject to the development of elastic stresses, which is related to a considerable lattice mismatch (up to and above 10%) between layers with different compositions and the absence of commercially available homoepitaxial sub strates. The presence of elastic stresses of different signs significantly influences both the kinetics of epi taxial growth and the energy band diagrams of both materials and device heterostructures [2,3]. In addi tion, relaxation of these stresses can proceed with the formation of various extended defects (e.g., threading dislocations with a density of up to 10 10 cm -2 ) and/or with a change in the morphology of growing layers, which can significantly influence the device charac teristics.Elastic stresses in the III N heterostructures are most frequently studied by means of X ray diffraction (XRD) analysis and laser reflectometry (LR). In the former case, the degree of stress relaxation in hetero structures is determined using the dynamic theory of X ray diffraction on deformed crystals [4] or by simply measuring the radius of curvature of the substrate, which is related via the Stoney formula to the product of elastic stress and layer thickness [5,6]. These mea surements are usually performed upon growth that complicated control and interpretation of results. For this reason, the development of stresses on growing layers is monitored in situ by measuring the substrate curvature using the LR technique [7,8].In the molecular beam epitaxy (MBE) technology, the level of elastic stresses in grown structures is fre quently studied by the method of reflection high energy electron diffraction (RHEED). Using the RHEED data and Ewald's construction, which deter mines the directions of additional reflexes (±0i, i = 1, 2, …) relative to the mirror (00) reflex in the 〈 〉 direction, it is possible to quantitatively estimate and control the relaxation of elastic stresses in heterostruc tures. Although digital processing of RHEED data was originally used as long ago as in 1991 [9], there are rather few research groups (see, e.g., [10]) at present using computer aided RHEED for the analysis of strain relaxation in III N heterostructures. This situa tion is apparently related to methodological problems encountered in this analysis.The present Letter describes a new method devel oped for the statistical analysis of RHEED patterns and demonstrates the results of its application to quantitative monitoring of the crystal lattice parame ter in heterostructures based on various binary and ter nary III N compounds in the course of their MBE growth.The samples of III N heterostructures were grown on c Al 2 O 3 substrates by plasma assisted MBE (PAMBE) using the nitrogen plasma activa tor [2,11]. We have studied the growth of (i) het erostructures based on binary InN/GaN, Ga...

Plasma‐assisted molecular beam epitaxy of AlGaN heterostructures for deep‐ultraviolet optically pumped lasers

“…The inclination mechanism of the strain relaxation during the SL growth by PA MBE has been confirmed by the gradual variation of the distance between reflections in the RHEED pattern by using the technique described in Ref. 49. In contrast, the strain relaxation in the SL with a lower average Al content ( x av = 0.6) occurs almost instantly and proceeds through the transition to 3D growth.…”