Unexpected bismuth concentration profiles in metal-organic vapor phase epitaxy-grown Ga(As1−xBix)/GaAs superlattices revealed by Z-contrast scanning transmission electron microscopy imaging

Abstract

A set of GaAs1−xBix/GaAs multilayer quantum-well structures was deposited by metal-organic vapor phase epitaxy at 390 °C and 420 °C. The precursor fluxes were introduced with the intent of growing discrete and compositionally uniform GaAs1−xBix well and GaAs barrier layers in the epitaxial films. High-resolution high-angle annular-dark-field (or “Z-contrast”) scanning transmission electron microscopy imaging revealed concentration profiles that were periodic in the growth direction, but far more complicated in shape than the intended square wave. The observed composition profiles could explain various reports of physical properties measurements that suggest compositional inhomogeneity in GaAs1−xBix alloys as they currently are grown.

The Effect of the Bi Precursors, (CH3)3Bi and (C2H5)3Bi, on the Metal‐Organic Vapor Phase Epitaxy of GaAs1‐yBiy Films

“…b). Considering our previous TEM studies, the first Bi spike in each period is associated with the Bi source flux into the growth system, resulting in the growth of the GaAs 1‐ y Bi y layer. Based on the observation with the growth of GaAs 1‐ y Bi y single layers using the TEBi source (Sample E1 in Table ), it is expected that there will be some residual Bi on the GaAs 1‐ y Bi y film surface.…”

Interfacial Mixing Analysis for Strained Layer Superlattices by Atom Probe Tomography

“…The sample designs included a thick (43.5 nm) motif SL composed of five repetitions of In 0.37 Al 0.63 As (20.1 nm)/In 0.67 Ga 0.33 As (23.4 nm) with 1% tensile/compressive strain relative to the InP substrate, nominally and respectively, and a thin (4.1 nm)-motif SL composed of 124 repetitions of In 0.44 Al 0.56 As (1.9 nm)/In 0.6 Ga 0.4 As (2.2 nm) nominally strained +0.56%/−0.47% relative to the substrate, respectively. However, they provide rather little insight into the composition profile within the motif, which is critical to device performance, as the same high-resolution X-ray diffraction (HRXRD) data typically can be fit with the same accuracy with several composition profiles that have the same periodicity and average composition as the sample [3]. Transmission electron microscopy (TEM) provides details of the atomic structures of the layers and interfaces, most readily in 2D projections [4].…”

“…These simulations can provide an accurate measurement of the periodicity of the SL and the composition of the motif averaged over its volume. However, they provide rather little insight into the composition profile within the motif, which is critical to device performance, as the same high-resolution X-ray diffraction (HRXRD) data typically can be fit with the same accuracy with several composition profiles that have the same periodicity and average composition as the sample [3].…”

Atom probe tomography evidence for uniform incorporation of Bi across the growth front in GaAs1−Bi /GaAs superlattice

“…In order to grow high-quality epitaxial GaAs 1-x Bi x with uniform Bi concentration, we need to understand how Bi atoms are incorporated in the growth front and subsequently distributed in the grown films. X-ray diffraction (XRD) and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) have been used previously [15] to investigate the Bi concentration profile in the GaAs 1-x Bi x /GaAs superlattice investigated in this study. The XRD method allows deduction of the Bi concentration in the layers that comprise the superlattice, but it is based on fitting the diffraction pattern to a simulation for which the generic shape of the concentration profile is an input.…”

“…Often the square wave form of a perfect superlattice is assumed. In the previous study [15], the shape of the concentration profile was deduced from HAADF-STEM ("Z-contrast") images, rather than assumed, to refine the concentration determination. In APT studies of GaAs and GaAs-based materials have been reported as well [22][23][24][25].…”