Spontaneous nanostructure formation in GaAsBi alloys C. Ryan Tait ⇑,1 , Lifan Yan 1 , Joanna M. Millunchick Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA article info Article history: Received 20 March 2018 Received in revised form 19 April 2018 Accepted 24 April 2018 Available online 25 April 2018 Communicated by H. Asahi Keywords: A3. Molecular beam epitaxy B2. Semiconducting III-V materials B2. Semiconducting ternary compounds B1. Bismuth compounds B1. Alloys abstract Bismuth containing semiconductor alloys are of increasing interest for their potential applications as infrared devices, though material quality in these materials has often proven problematic. This work shows a series of GaAsBi films grown by molecular beam epitaxy over a range of temperatures, 340 °C–315 °C, with all other parameters equal. Atomic force microscopy shows that there is an increase in the surface roughness of the films from 0.8 nm to 2.7 nm RMS. Transmission electron microscopy and atom probe tomography reveals the onset of spontaneous nanostructure formation in the films including nanopores, lateral composition modulation, and Bi rich GaAsBi clusters. The phenomena are shown to correlate and linked to reaching a growth instability due to the differences in surface diffusivity of Bi and As adatoms. Ó 2018 Published by Elsevier B.V. III-V semiconductor alloys containing Bi, often referred to as bismides, have recently drawn attention because of their predicted properties related to the large bandgap reduction with increasing Bi incorporation and large spin orbit bowing [1–3]. Growth of bis- mides is quite challenging because Bi desorbs from the surface at typical growth temperatures, thus films need to be grown at a low temperature to incorporate any amount of Bi. Growth of these materials is further complicated by the formation of group III or Bi droplets [4–7]. It has been predicted that the highest Bi incorpora- tion in GaAsBi occurs when Ga droplets formed on the surface [8], but growth under these conditions has been shown to result in Bi compositional inhomogeneity [9] and Ga-enriched embedded nanowires [10]. In addition, lateral composition modulation of Bi [11] and clustering of both Bi and As has been shown [12,13].A complete understanding of what conditions are conducive to these deleterious bulk defects is crucial for further development of bis- mide based devices. In this paper, a series of GaAsBi bulk layers were grown on of GaAs (0 0 1) substrates at varying temperatures. It is shown that as the temperature decreases the composition of Bi in the film increases as expected. However many other interesting phenomena also arise with decreasing growth temperature, including lateral composition modulation, surface roughness, Bi clustering, and nanopores formation. These features appear to be correlated and are proposed to be related to a growth instability arising due to in differences in surface diffusivity between the Bi and As adatoms. The GaAsBi films analyzed by this work were grown on GaAs(0 0 1) substrates by molecular beam epitaxy (MBE). All sources were solid source effusion cells, and the As 2 flux was pro- vided by a valved cracking source. Growth temperatures were measured by optical pyrometer having an operational temperature range of 250–1400 °C. All growths began with the deposition of a 250 nm-thick GaAs buffer layer deposited at 600 °C, a Ga growth rate of 0.97 ML/s, and an As to Ga BEP ratio of 3.5, which is expected to result in a ratio of 1.1 As atoms for every 1 Ga atom at the surface. Next, a series of 500 nm-thick GaAsBi films were deposited at a Bi beam equivalent pressure (BEP) of 1.0  10 7 torr, and substrate temperatures varying between 315 and 340 °C. These sets of growth conditions result in droplet-free growth over each of the temperatures studied. The surfaces of the films were further characterized using Atomic Force Microscopy (AFM) in tap- ping mode. The results of multiple 25  25 lm 2 AFM scans show that the surfaces are generally flat with the Root Mean Squared (RMS) roughness ranging from 0.8 to 2.7 nm with decreasing sub- strate temperature (Table 1). This finding is not surprising because as the growth temperature decreases the diffusivity of adatoms also decreases, resulting in rougher films. The average Bi composi- tion was inferred from (0 0 4) X-ray diffraction scans assuming the lattice parameter obeys of Vegard’s law and the lattice constant for GaBi is 6.33 Å [14]. These measurements indicate that the Bi com- position decreases slightly from 2% to 1.5% with increasing tem- perature (Table 1), consistent with increased Bi desorption with increasing temperature [4,15]. https://doi.org/10.1016/j.jcrysgro.2018.04.026 0022-0248/Ó 2018 Published by Elsevier B.V. ⇑ Corresponding author. E-mail address: rtait@umich.edu (C.R. Tait). 1 Equal contribution. Journal of Crystal Growth 493 (2018) 20–24 Contents lists available at ScienceDirect Journal of Crystal Growth journal homepage: www.elsevier.com/locate/crys