GaAsBi quantum well structures were grown using a 32P RIBER (Bezons, France) MBE system. Substrates were pieces of a semi-insulating GaAs substrate soldered with indium on a silicon wafer mounted on the substrate holder to be loaded in the MBE system. The substrate thermocouple temperature for the molybdenum substrate holder was first calibrated by using a band edge thermometry system (BandIT). The control of the growing material was performed by reflection high-energy electron diffraction (RHEED). Our MBE system is designed to grow in the RIBER ‘optimal cell/sample oven’ geometry which leads to high thickness uniformity on 2-in. samples even though the 32P Riber MBE system is not normally designed to get high uniformity on these large surface areas. In such a geometry, substrate rotation is required to be used continuously during the growth, since the fluxes are not converging towards the substrate holder center.
QWs with different Bi contents and widths were grown, and the results presented here come from the QW emitting at the longer wavelength. They were grown after careful calibration of the growth conditions, the GaAs growth rate, i.e., the V/III ratio, the substrate temperature, and the Bi content, on thick GaAsBi layers. Note that we do not have any flux gauge in our MBE system, so the Bi control was carried out via the cell temperature.
The investigated QW sample consists of a 500-nm-thick buffer GaAs layer, the GaAsBi/GaAs QW, and finally a 100-nm-thick cap layer. After the growth of the buffer layer at 580°C, the temperature is lowered to 365°C, the value selected for the QW growth. The As cell valve opening is reduced in order to yield the As4 flux corresponding to a V/III atomic ratio close to unity, as needed for GaAsBi growth ; the As cell is a RIBER VCAS700 cracker (Bezons, France) one whose nose temperature is set to 650°C, thus mostly ejecting As4 species. At the same time, the Ga cell temperature is decreased to a value which leads to a low growth rate for GaAs, of the order of 0.25 ml/s. After substrate temperature cooling, care is taken to get temperature stabilization since this parameter plays a major role in Bi incorporation . At this step, the Ga and Bi cell shutters are opened simultaneously. For the first period of growth, bismuth plays the role of a surfactant for the low-temperature-grown GaAs , until a (2 × 1) reconstruction of a bismuth-rich GaAs surface  is observed, the required condition for efficient incorporation of this element into GaAs . Then, the bismuth element contributes to the formation of a GaAsBi QW. At the end of the QW growth, the Bi cell shutter is closed first. The Ga shutter is only closed once a 5-nm layer of GaAs has been grown; we have observed that the GaAs RHEED pattern deteriorates for a GaAs layer thickness higher than 5 to 10 nm, after the floating bismuth was incorporated in GaAs or desorbed . The growth is then interrupted to heat the structure temperature to 520°C. Finally, a 30-nm-thick GaAs layer is grown at 0.7 ml/s at this temperature, and the 100-nm-thick barrier growth is completed while the temperature is raised to 580°C.
Once grown, the GaAsBi/GaAs QW structures were analyzed by stationary photoluminescence using the 514-nm line of an argon laser and a GaInAs photodetector. The quantum well emitting at 300 K at the longer wavelength, 1.23 μm, was selected. The sample was cleaved into pieces, which were subjected to ex situ RTA in an AnnealSys AS-One system (West Newbury, MA, USA). RTA was carried out in a nitrogen atmosphere during 30 s at annealing temperatures of 650°C, 700°C, 750°C, and 800°C. Samples were covered with a GaAs substrate during annealing in order to prevent surface degradation by arsenic desorption.
High-resolution X-ray diffraction (HR-XRD) was performed on the as-grown sample using a D8 Discover Bruker (Karlsruhe, Germany) equipment in order to determine its thickness and strain, from which we deduce its bismuth content. It was also used on the annealed samples to get insight into the evolution of their structural properties upon annealing. Secondary ion mass spectrometry (SIMS) with a CAMECA IMS-6f (Gennevilliers, France) was employed to measure the profile of the bismuth element within the structure for the as-grown and annealed samples. Primary Cs+ ions were accelerated at 3 kV, while the positive secondary ions were collected at 2 kV. Transmission electron microscopy (TEM) in conventional and high-resolution (HR) modes was carried out on the as-grown QW sample. A <110 > -oriented cross-sectional sample was thinned by mechanical polishing and ion milling. HR-TEM observations were performed at 200 kV on a TECNAI F-20 (E.A. Fischione Instruments, Inc., Export, PA, USA), equipped with a spherical aberration corrector tuned to avoid the delocalization effect at the interface and to achieve a 0.12-nm resolution.
Time-resolved photoluminescence (PL) spectroscopy was performed at room temperature on the as-grown and annealed samples. Optical excitation was provided by focusing 1.5-ps pulses generated by a mode-locked Ti-sapphire laser with 80-MHz repetition frequency. The laser wavelength was set to λexc = 795 nm with 20-mW incident power, focused to a 50-μm diameter spot at the sample surface. The signal was recorded using a S1 photocathode Hamamatsu streak camera (Hamamatsu, Shizuoka, Japan) with an overall time resolution of 8 ps. The signal was recorded in the high-energy side (1,120 to 1,220 nm) of the PL spectrum.