Molecular beam epitaxy and properties of GaAsBi/GaAs quantum wells grown by molecular beam epitaxy: effect of thermal annealing
© Makhloufi et al.; licensee Springer. 2014
Received: 18 November 2013
Accepted: 31 January 2014
Published: 17 March 2014
We have grown GaAsBi quantum wells by molecular beam epitaxy. We have studied the properties of a 7% Bi GaAsBi quantum well and their variation with thermal annealing. High-resolution X-ray diffraction, secondary ion mass spectrometry, and transmission electron microscopy have been employed to get some insight into its structural properties. Stationary and time-resolved photoluminescence shows that the quantum well emission, peaking at 1.23 μm at room temperature, can be improved by a rapid annealing at 650°C, while the use of a higher annealing temperature leads to emission degradation and blue-shifting due to the activation of non-radiative centers and bismuth diffusion from the quantum well.
Dilute bismuth alloys grown on GaAs attract more and more attention because of their peculiar electronic properties. Adding bismuth to GaAs efficiently decreases the gap energy of this semiconductor  through a change in its valence band properties and increases the spin-orbit interaction . GaAsBi/GaAs quantum wells (QWs) are of interest with a view to fabricate laser diodes which could benefit from these properties, in particular from the higher spin-orbit splitting expected to lower the non-radiative carrier recombination due to Auger mechanisms . Moreover, their emitting wavelength range could meet the requirements for infrared GaAs-based laser diodes as an alternative to low-temperature GaInAs/GaAs  and GaInAsN/GaAs  QWs. Besides, the properties of these III-V alloys are also very promising for photovoltaics . Up to now, literature on GaAsBi has mainly been devoted to thick layers (see  and ref. herein), and only a few papers on the growth of quantum well structures have been published . Here, we present the structural and optical properties of a GaAsBi/GaAs QW grown by molecular beam epitaxy (MBE) and discuss their change after rapid thermal annealing (RTA).
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.
Results and discussion
As-grown quantum well structure
In order to determine the Bi content and QW thickness, HR-XRD analysis is used. As it can be observed in Figure 1b, the presence of thickness fringes and a narrow diffraction peak in the resulting 2θ transverse scan indicates that the grown material is of high quality. It allows us to accurately evaluate the thickness, content, and strain of the QW. The X-ray diffractogram was simulated. This calculation was done using the software provided by Bruker, with GaAs and GaBi lattice parameters taken equal to 0.56353 and 0.633 nm, respectively , and with the elastic constants of GaAs (C11 = 118.81 GPa and C12 = 53.8 GPa) for a structure under complete elastic tetragonal strain. For a thickness of 7.5 nm and a Bi content of 7%, we observe perfect agreement of the experimental rocking curve with the simulated one, further highlighting that the as-grown QW structure exhibits good quality structural properties.
The average strain measured from HR-TEM is 0.016, but this value has to be corrected for surface relaxation effects: the strain before thinning is numerically estimated to be 10% to 20% larger .
Thus, after this correction, the estimated misfit is in the range of 0.0095 to 0.0105. Applying the Vegard’s law, this corresponds to a Bi amount of 7.8% to 8.8%, close to the content provided by HR-XRD. Different analyzed zones have given comparable values of strain. In addition, the measured QW thickness in Figure 2b is measured to be 7 nm, again close to the value estimated through HR-XRD.
Taking into account the values measured by HR-XRD and TEM, the assumption of late incorporation of the bismuth is valid. The thickness of the QW is thinner than the one which would have been obtained had the Bi atoms been directly incorporated as soon as the Bi cell was opened.
Annealed QW samples
The increase of the carrier decay time in the time-resolved luminescence analysis measured for the 650°C annealed sample, in Figure 4b, also supports its better crystallinity, in agreement with the improved emission recorded under CW photoluminescence analysis. On the contrary, for annealing treatment at temperatures higher than 700°C, the room temperature carrier decay time like the other properties of this annealed QW degrades with respect to the as-grown QW, showing again that a too high annealing temperature leads to the activation of additional non-radiative centers.
We have shown that a 7% Bi GaAsBi quantum well grown by molecular beam epitaxy, emitting at 1.23 μm at room temperature is completely elastically strained and exhibits good structural properties, with a uniform thickness, sharp interfaces, and the absence of extended defects. Annealing this QW leads to bismuth out-diffusion as soon as the applied annealing temperature is higher than 650°C. At the latter annealing temperature, the photoluminescence emission and decay time at room temperature are all improved.
Hajer Makhloufi is a PhD student at LAAS. Poonyasiri Boonpeng is a TECHNO Tempus Erasmus Post-doctorate at LAAS. Simone Mazzucato is a visitor researcher at LPCNO. Hélène Carrère holds an assistant professor position at INSA and is a researcher at LPCNO. Julien Nicolai is a CNRS post-doctorant at CEMES. Alexandre Arnoult and Guy Lacoste are CNRS engineers at LAAS. Teresa Hungria is an engineer at INSA. Christophe Gatel (assistant professor at Université Paul Sabatier) and Anne Ponchet (Research Director at CNRS) are researchers at CEMES. Xavier Marie is a professor at INSA and is a researcher at LPCNO. Chantal Fontaine (Research Director at CNRS) is a CNRS researcher at LAAS.
This work was supported by the Paul Sabatier University (AO1 programme), the LAAS-CNRS technology platform (RENATECH), and the LPCNO laboratory. We acknowledge the TECHNO Erasmus Mundus Programme for its support through the post-doctoral grant of Poonyasiri Boonpeng. Part of this work has been supported by the NAIADE project (ANR-11-BS10-017). We would also like to thank the cooperation of COST Action MP0805 and Stephane Calvez for the fruitful discussions.
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