Low-temperature photoluminescence study of exciton recombination in bulk GaAsBi
© Mazzucato et al.; licensee Springer. 2014
Received: 13 November 2013
Accepted: 25 December 2013
Published: 13 January 2014
The exciton recombination processes in a series of elastically strained GaAsBi epilayers are investigated by means of time-integrated and time-resolved photoluminescence at T = 10 K. The bismuth content in the samples was adjusted from 1.16% to 3.83%, as confirmed by high-resolution X-ray diffraction (HR-XRD). The results are well interpreted by carrier trapping and recombination mechanisms involving the Bi-related localized levels. Clear distinction between the localized and delocalized regime was observed in the spectral and temporal photoluminescence emission.
Over the last few years, much attention has been paid to the growth and investigation of dilute bismides, with potential applications for high-efficiency solar cells and for optoelectronic devices in the 1- to 1.55-μm wavelength range [1–3]. Adding even a small amount of Bi to arsenides strongly affects the valence band structure and induces a significant lowering of their bandgap energy, up to approximately 88 meV% of Bi , and a significant increase of the spin-orbit (SO) split-off energy, resulting from a valence band anticrossing behavior [5, 6]. On the contrary, the conduction band is barely affected by the Bi atoms, but the electron spin properties, which depend critically on the SO interaction, can be tuned in dilute bismides, making them suitable candidates for spintronics applications . In addition, the incorporation of Bi yields a significant carrier localization in the valence band, affecting the band-to-band recombination energy and visible as a deviation from the Varshni curve at low temperature (S-shape),  in a similar way as observed in dilute nitrides [9, 10]. The origin of this S-shape behavior is attributed to localized states due to alloy disorder, cluster formation, and potential fluctuations in GaAsBi induced by Bi incorporation [11, 12].
A study on the shallow localized states associated with Bi clusters near the top of the GaAsBi valence bandgap was performed by Lu et al. . This study was done at room temperature, where the thermal energy already masks most of the contribution of the shallowest levels. Here instead, we investigate the exciton dynamics in different GaAsBi epilayers at T = 10 K, as function of incident power, being able to distinguish between the localized and free carrier regime.
Bi fraction of the investigated GaAsBi samples
The samples were mounted in a closed cycle He-cooled cryostat, where the temperature varied from 10 to 300 K. 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 fixed at λexc = 795 nm to allow both the GaAs and GaAsBi layer to be excited, and the beam was focused on a 50-μm diameter spot at the sample surface. The incident power was varied by means of neutral density filters from 0.01 to 150 mW, which corresponds to a typical photon flux at the sample surface from 2.5 × 1010 to 3.8 × 1014 cm−2, respectively. Assuming that GaAsBi has the same absorption coefficient as GaAs, we estimate an average photon number absorbed in the GaAsBi layer from 109 to 1014 cm−3. Time-integrated and time-resolved photoluminescence (PL), measured along the sample growth direction, were collected using a S1 photocathode Hamamatsu streak camera (Hamamatsu Photonics K.K., Naka-ku, Japan) with an overall time resolution of 8 ps, as a function of incident power and sample temperature.
Results and discussion
The energy red shift of the PL peak with increasing Bi% is clearly evidenced, in agreement with the literature results . In our case, the amplitude of this shift is equal to about 75 meV/Bi%.
With increasing incident power, the localized levels saturate, giving rise to delocalized excitons and to an increase in the FWHM. This is probably due to inhomogeneous broadening caused by fluctuations in the local Bi composition, valence band potential, and strain distribution, and eventually band filling.
The change in the FWHM with Pin is illustrated in Figure 4 for three samples, where the two different processes depending on the Pin clearly appear. All five samples follow the same u-shaped trend, with a minimum FWHM in the Pin region between 0.5 and 20 mW, as already observed by Mazur et al.  in GaAsBi QW samples under CW excitation power. The excitation power corresponding to this minimum for each sample will be referred as PMIN.
At low intensity, excitons tend to be highly localized and cannot be separated, so they recombine radiatively. By increasing Pin, filling of the localized states occurs, and delocalized excitons start recombining, with the PL emission energy approaching the theoretical Varshni curve.
The spectral and temporal dependence of the PL emission of GaAsBi bulk epilayers with different Bi contents from 1.16% to 3.83% was used to characterize the localized levels dominating at low lattice temperature and low incident power. Although the localized excitons exist even at our highest Pin, we managed to distinguish the delocalized and localized exciton contributions by fitting the PL spectra with two separate Gaussians and therefore investigate their mutual relation as function of Pin. The results show the band filling effect occurring at higher excitation intensity and the increase of the density of localized exciton states at higher Bi content.
SM is a post-doc researcher at LPCNO. HL is an undergraduate student at INSA. HC is an associate professor at LPCNO. HM is a PhD student at LAAS. AA is a CNRS engineer at LAAS. CF is a CNRS researcher at LAAS. TA and XM are professors at LPCNO.
This work was supported by the Université Paul Sabatier AO1 program, the LAAS-CNRS technology platform (RENATECH), and the LPCNO laboratory. We would also like to thank the cooperation with COST Action MP0805.
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