Overlapping effects of the optical transitions of GaNAs thin films grown by molecular beam epitaxy
Introduction
Through the substitution of arsenic by nitrogen atoms in the gallium arsenide (GaAs) host lattice the gallium arsenide nitride (GaNxAs1-x, where x is the mole fraction of N) ternary compound is formed. This material has been widely researched due to the attractive large bandgap bowing observed even for small atomic percentage of nitrogen (%N) incorporation since it was experimentally demonstrated in the early 1990s [1]. For instance, within the diluted regime (%N < 3) the band gap energy is reduced by ~100 meV per %N [2]. On this way, the development of the GaNAs has been a subject of great interest due to the potential applications in optoelectronic devices, covering emission/absorption processes between red to ultraviolet [3], [4], [5], [6].
The behavior of the band structure of the GaNAs system is explained by the band anti-crossing model (BAC), which describes the role of nitrogen incorporation on a wide variety of III–N–V alloys. The BAC indicates that anti-crossing interaction between the electronic states of the localized nitrogen and energy states of the GaAs host matrix split the conduction band into two sub-bands labeled as E- and E+ [7]. The band gap of the GaNAs system is related to E- while the E+ level is an extra conduction-band edge of the GaNAs. Thus, the GaNxAs1-x bandgap can be tuned in by the %N used in the alloy, according to BAC. As it was mentioned earlier, a small increment of %N decreases the bandgap energy, E-, while at the same time increases the energy separation between the maximum of the valence band and the E+ level.
GaNAs splitting of the conduction band makes this material very prominent for a wide variety of devices, e.g. the intermediate-band solar cell concept, in which energy states are located within the forbidden bandgap of the host solar cell, allowing for the absorption of sub-bandgap energy photons [5]. The GaNAs quantum structures such as wells and dots exhibit interesting properties that can improve the optical fiber communications where the emission in the 1.3 and 1.55 μm wavelengths can be reached [6]. Consequently, the experimental analysis of the GaNAs and GaNAs/GaAs system band structure with non-destructive techniques is mandatory in order to determine and improve their optical and electrical behavior.
Photoreflectance spectroscopy (PR) has been the most employed technique to analyze the band structure in GaNAs layers with thickness above 250 nm where the features associated to E- and E+ have been shown [8], [9], [10], [11], [12], [13], being the presence of E+ the best evidence for the identification of the band anti-crossing interaction of dilute nitrides predicted by the BAC [10]. The PR spectrum is obtained when the dielectric constant of the material is perturbed by a periodic photomodulation of an electric field. The photoexcited electron-hole pairs created by the pump source changes the reflectivity coefficient of the analyzed structure, providing information about their band structure [14].
The band structure of a semiconducting material can be alternatively studied by the analysis of the dielectric function via room temperature spectroscopic ellipsometry (SE). GaNAs thin films have been previously explored with this technique, being the presence of the E1 (~2.9 eV), E1+Δ1 (~3.1 eV), and E2 (~4.8 eV) the critical-point transitions that have been mostly observed and studied [15], [16], [17], [18], [19]. It was demonstrated that these critical points are sensitive to %N, whose energy position blueshifts when the x value of the alloy is raised. Conversely, the range of 1 to 2 eV, where the E- and E+ critical points should be revealed in the SE spectrum, has been scarcely studied to determine the presence of the conduction band splitting in the alloy [20,21].
In this report, the authors used non-destructive PR and SE techniques with the aim to understand the band structure of the GaNAs layers grown on GaAs in the range of 1 to 2 eV, the energy region where the conduction band splitting of the alloy should be displayed for %N < 2. The nitrogen concentration and thickness of the GaNAs layer were chosen to allow mixing between the diverse band gaps transitions that are present in GaNAs alloy and those originated in GaAs layers at room temperature. We demonstrated that low temperature PR facilitates the analysis of the GaNAs electronic structure by the separation of the energy levels. Finally, a comparative study between PR and SE is presented, where the presence of both the E- and E+ critical points are shown in congruence with BAC.
Section snippets
Experimental details
The GaNAs layer was grown on epi‑ready GaAs (100) SI-substrates using a Molecular beam epitaxy (MBE) system equipped with solid source III–V materials (7N5 Gallium and Arsenic purities). Nitrogen radicals were produced using an Oxford Applied Research radio frequency plasma source at 250 W that produces a neutral beam with high N-radical flux and approximately zero ion content [22]. High purity N (5N5) was introduced into the plasma source using a mass flux controller and a leak valve. Before
Results and discussion
A wide variety of experimental techniques have been applied to measure %N and lattice disorder of the dilute III-V nitride compounds while their band structure has been restricted principally to PR analysis together with the evaluation of the E- emission by photoluminescence spectroscopy [23]. The BAC for GaNAs indicates that the energy of the conduction bands formed by the inclusion of nitrogen in the Gallium arsenide lattice is given by Eq. (1)here EM are the
Conclusions
In summary, we have presented a strategy for identification of the E+ gap of the GaNAs/GaAs system, which has usually been claimed as the figure of merit to evaluate the quality of the GaNAs alloy. For achieving a precise experimental determination of the presence/absence of the conduction band splitting by spectroscopies such as PR or SE, the influence of the signal of another critical point that lies in the same energy range predicted by the BAC must be considered. The study of the E- and E+
Author statement
We would like to state that all authors have seen and approved the final version of the manuscript being submitted. The manuscript is the authors' original work and it has not been published and neither has been submitted simultaneously elsewhere.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the financial support from CEMIE-SOL 22, FRC-UASLP, C19-FAI-05-18.18 and CONACYT-Mexico through grants: INFR-2015-01-255489, CB 2015-257358, PNCPN2014-01-248071 and the Cátedras CONACyT (Project No. 44).
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