Bismuth-induced effects on optical, lattice vibrational, and structural properties of bulk GaAsBi alloys
© Sarcan et al.; licensee Springer. 2014
Received: 26 November 2013
Accepted: 19 February 2014
Published: 14 March 2014
Bulk GaAs1 - xBi x /GaAs alloys with various bismuth compositions are studied using power- and temperature-dependent photoluminescence (PL), Raman scattering, and atomic force microscopy (AFM). PL measurements exhibit that the bandgap of the alloy decreases with increasing bismuth composition. Moreover, PL peak energy and PL characteristic are found to be excitation intensity dependent. The PL signal is detectable below 150 K at low excitation intensities, but quenches at higher temperatures. As excitation intensity is increased, PL can be observable at room temperature and PL peak energy blueshifts. The quenching temperature of the PL signal tends to shift to higher temperatures with increasing bismuth composition, giving rise to an increase in Bi-related localization energy of disorders. The composition dependence of the PL is also found to be power dependent, changing from about 63 to 87 meV/Bi% as excitation intensity is increased. In addition, S-shaped temperature dependence at low excitation intensities is observed, a well-known signature of localized levels above valence band. Applying Varshni’s law to the temperature dependence of the PL peak energy, the concentration dependence of Debye temperature (β) and thermal expansion coefficient (α) are determined. AFM observations show that bismuth islands are randomly distributed on the surface and the diameter of the islands tends to increase with increasing bismuth composition. Raman scattering spectra show that incorporation of Bi into GaAs causes a new feature at around 185 cm-1 with slightly increasing Raman intensity as the Bi concentration increases. A broad feature located between 210 and 250 cm-1 is also observed and its intensity increases with increasing Bi content. Furthermore, the forbidden transverse optical (TO) mode becomes more pronounced for the samples with higher bismuth composition, which can be attributed to the effect of Bi-induced disorders on crystal symmetry.
78.55Cr 78.55-m 78.20-e 78.30-j
KeywordsGaAsBi Dilute bismide S-shape Varshni’s law
Recently, it has been realized that the incorporation of a small percentage of bismuth (Bi) into GaAs results in a drastic decrease of bandgap energy, thus making GaAsBi a promising alloy for device applications operating in the near-infrared region [1, 2]. Additionally, spin-orbit splitting energy has been found to increase with increasing Bi content, thereby also establishing GaAsBi as a promising alloy for spintronic applications . GaAsBi can also be considered as an alternative to dilute nitrides whose electron mobility is drastically affected by the influence of nitrogen on the conduction band and nitrogen-induced defects . Since the localized level of bismuth in GaAs only restructures the valence band, it is predictable that the electron mobility is not affected by bismuth content . As a result, incorporation of Bi into GaAs leads to a desirable red shift of bandgap energy, while electron mobility remains unaffected . However, the incorporation of bismuth into the III-V lattice requires low-temperature growth conditions, thus causing formation of the defects as previously experienced in all the members of the highly mismatched alloys . Hence, the optimization of GaAsBi growth conditions to enhance the optical and electrical quality of the alloy is still a challenge. An in-depth study of the fundamental properties of GaAsBi is strongly needed in order to explore its potential for commercial usage .
In the present work, we have studied molecular beam epitaxy (MBE)-grown bulk GaAsBi/GaAs samples with various bismuth compositions using temperature- and intensity-dependent photoluminescence (PL), atomic force microscopy (AFM), and Raman spectroscopy.
GaAsBi epilayers were grown on semi-insulating GaAs substrates using MBE at 360°C to 390°C, while keeping the As/Bi atomic species ratio close to unity. The thickness of the epilayers was in the range of 200 to 250 nm. The temperature was first calibrated, thanks to a bandgap thermometry (BandIT; k-Space Associates, Inc., Dexter, MI USA). The error on growth temperature was estimated to be ±5°. Since bismuth incorporation is known to be highly dependent on substrate temperature, X-ray diffraction was used to accurately determine the bismuth content in the epilayers. All layers were found to be elastically strained. PL measurements were carried out between 40 and 300 K using the 514.5-nm line of an Ar+ laser as an excitation source. The PL signal was dispersed with a 0.5-m high-resolution monochromator and detected using nitrogen-cooled InGaAs photomultiplier. The surface morphology of the samples was monitored and analyzed using AFM in tapping mode. A Jasco NRS 3100 Raman spectrometer (Jasco Corporation, Tokyo, Japan) equipped with a CCD detector was used for recording the micro-Raman spectrum with a diode laser operating at 532 nm as an excitation source. All measurements were performed at room temperature in a back-scattering geometry. The power of laser source was kept as low as a few milliwatts.
Results and discussion
The temperature dependence of PL peak energy at low excitation intensities has a pronounced S-shape characteristic (see Figure 2), which is also typical of the highly mismatched alloys . The S-shaped temperature dependence also originates from the PL of localized states in GaAsBi at low temperatures. When the temperature exceeds 150 K, the PL spectrum is dominated by free exciton emission. With increasing excitation intensity due to the filling of the localized levels, the S-shaped region moves towards lower temperatures (see Figure 2b) and then disappears at higher intensities. This observation can be explained by filling all localized states with photo-generated carriers under high excitation power, thus causing a band-to-band transition of free carriers. Consequently, the PL peak energy blueshifts at low temperatures (≤150 K) under higher excitation intensities and sheds light on the observed discrepancy of the red shift values per Bi% at different excitation intensities (Figure 1b).
Elastic constants of GaAs and GaBi for zinc-blende phase
The bismuth atom is larger in size and has lower vapor pressure in comparison to gallium and arsenic. For this reason, it has a strong tendency to segregate at the surface during GaAsBi growth . It was claimed that bismuth islands could also easily nucleate on the surface . Here, since near-stoichiometric conditions have been used for MBE growth of these layers, the same origin can be proposed as As/Ga ratio was kept slightly higher than the unity during growth. This concludes that the observed islands should be bismuth-related. Further work is clearly to ascertain the origin of these defects.
The effect of Bi composition on optical, lattice vibrational, and structural properties of bulk GaAs1 - xBi x /GaAs alloys was investigated. The compositional dependence of the bandgap was found to be excitation-intensity dependent, which is attributed to the presence of localized states. PL emission at low excitation intensities was observable at low temperatures, indicating that PL originates from localized excitons. On the other hand, at higher excitation intensities, the reason for the observation of the room temperature PL was due to the contribution of free excitons. The temperature-induced shift of the bandgap energy was found to be lower than those of classical III-V alloys and dilute nitrides. The use of Varshni’s law, bandgap at 0 K, thermal expansion coefficient, and Debye temperature was determined as a function of Bi composition. From AFM observations, bismuth islands on the surface were monitored. As bismuth composition increased, the size of the islands also increased. The intensity of Bi-induced mode at approximately 185 cm-1 and the broad feature in the range of 210 to 250 cm-1 were observed to increase with the increase of the Bi composition. Moreover, the observation of the forbidden TO mode in GaAsBi was attributed to Bi-related disorder that degraded the crystal symmetry of the structure.
atomic force microscopy
molecular beam epitaxy
We acknowledge to the COST Action MP0805 for enabling the collaboration possibilities. This work was partially supported by the Scientific Research Projects Coordination Unit of Istanbul University (project number 31160) and The Ministry of Development, Turkey (project number 2010 K121050).
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