GaAsBi atomic surface order and interfacial roughness observed by STM and TEM

Abstract

ukAbstract. GaAsBi is an interesting ternary material for opto-electronic applications. Bi fractions of 11-13% allow 1.55 µ m emissions from a range of bulk and QW structures. GaAsBi has shown strong room temperature photoluminescence. The temperature insensitive band gap and large spin orbit splitting are attractive optoelectronic features, however the typical full width half maximum is 2.5 times greater than GaAs. In solid source molecular beam epitaxy (MBE), near stoichiometric fluxes and low growth temperatures are necessary to achieve the desired Bi content. In order to explore photoluminescence linewidth broadening and the accommodation of strain a joint scanning tunnelling microscopy (STM) and scanning transmission electron microscopy (STEM) study of quasi-bulk and QW structures has been undertaken. IntroductionThe bismuth alloy GaAs 1-x Bi x has a number of interesting properties, namely band gap reductions of 88 meV/% and large spin orbital splitting [1,2]. The narrow, temperature insensitive band gap offered by such an alloy is particularly well suited to the fabrication of infra red emitters and photodetectors [2], however certain complications in the growth process have limited the impact of this alloy.The strong tendency towards surface segregation and droplet formation of Bi during epitaxy has led to unconventional low temperatures growth parameters. Dilute bismuthides can be grown at around 430 °C up to ~1% Bi [3], however for high compositions lower growth temperatures are necessary. It is common to grow at temperatures as low as 280 °C in order to ensure high quality, smooth epilayers are produced with >10% Bi fractions [4]. X-ray Diffraction (XRD) and photoluminescence (PL) data show high quality crystalline material with strong room temperature PL can be achieved. There are two restrictions to molecular beam epitaxy (MBE) growth of this ternary alloy. Firstly, in the 335 -400 °C growth temperature range the film thickness/composition is restricted via strain related undulations and eventually dislocations. Secondly, below 335 °C undulations and dislocations are reduced due to thermal limitations, however point defects strongly affect material quality.In this paper, scanning tunnelling microscopy (STM) and scanning transmission electron microscopy (STEM) are performed on samples grown at each of these limits in order to investigate observed XRD and PL phenomena.

Effect of post-growth anneal on the photoluminescence properties of GaSbBi

“…The height of the PL peak is greatly enhanced as a result of Bi incorporation and the peak width is decreased. This is in contrast to MBE-grown GaAsBi where the presence of Bi is found to increase and broaden the band gap transition peak [14]. The peak at 0.72 eV seems to be a characteristic of Bi-containing GaSb which we earlier attributed to Bi-related complex defects [8].…”

Scanning transmission electron microscopy measurement of bismuth segregation in thin Ga(As,Bi) layers grown by molecular beam epitaxy

“…∼17 nm GaAs 1‐x Bi x layers were grown at different temperatures (details in Table , where the second sample also contains a separate quantum well), capped by GaAs and the Bi content fitted using X‐ray diffraction (XRD) scans as well as the peaks in photoluminescence spectroscopy observed at wavelengths of ∼1200 nm for sample LE2 and at ∼1450 and ∼1600 nm for STB29. More details on growth and optical characterisation can be found in reference .…”

“…Scanning Transmission Electron Microscopy (STEM) was performed using two different instruments, a JEOL 2010F with ∼0.3 nm probe size operated with a 9.5 mrad semi‐angle of convergence at 197 kV, where some preliminary results were included in a previous conference publication , and a double aberration corrected JEOL Z3100 R005 operated with ∼0.15 nm probe size and 100 pA beam current by using a 21 mrad semi‐angle of convergence at 300 kV acceleration voltage . High‐angle annular dark‐field (HAADF) images were recorded with collection angles of ∼55 mrad (JEOL 2010F) or ∼65 mrad (JEOL R005), respectively.…”