Temperature dependence of Bi behavior in MBE growth of InGaAs/InP
Introduction
Bismuth (Bi) containing III–V alloys have attracted much attention because they are thought to have temperature-insensitive bandgaps, which can be used as active layers in semiconductor lasers to achieve temperature-insensitive lasing wavelengths [1], [2]. For conventional semiconductor lasers made of InGaAsP, the lasing wavelength is very sensitive to the ambient temperature and fluctuates with the variation of the temperature, because the bandgap (and refractive index) of InGaAsP is sensitive to the temperature. In this situation, a massive Peltier device has to be equipped in a wavelength-division-multiplexing (WDM) system to stabilize the temperature by controlling the current. Replacing the InGaAsP alloy with Bi-containing III–V alloys can eliminate the need for a Peltier device and make laser devices less expensive. GaAsBi alloys have been successfully fabricated by molecular beam epitaxy (MBE) [3] and metalorganic chemical vapor deposition (MOCVD) [4]. Their temperature-insensitive bandgaps were confirmed by photoreflectance spectra. It was found that only a few percent Bi in GaAs is necessary to obtain the temperature-insensitive bandgap desired for the optoelectronic devices for applications in communication networks. Furthermore, the addition of N into GaAsBi to form the GaNAsBi alloy can make this quaternary alloy lattice match to a GaAs substrate with a 1.3 μm emission wavelength [5]. Recently, new quaternary InGaAsBi alloy films have been created on InP substrates by MBE [6]. Bi incorporation in the InGaAs layer was confirmed by X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS).
A primary concern in obtaining good crystal growth by MBE or other related vapor-phase techniques is the growth mode of the film. It was shown that the addition of a surfactant enhances the two-dimensional layer-by-layer growth mode. This modified growth process is now called the surfactant-mediated growth method. It has had a strong impact on the development of technologically important materials in devices, such as heterostructures used for laser applications. Bi can be also considered to be a surfactant in strained-layer growth. The effectiveness of this has been demonstrated in many systems, such as Ge/Si [7], GaN [8], and GaNAs [9]. A surface layer of Bi could, in principle, have a profound effect on the processes that occur at the surface during the growth of these films, because the interface energy between the Bi surfactant and the semiconductor surface is likely to be significantly lower than the energy of the semiconductor–vacuum interface.
Most studies have shown that Bi cannot be incorporated in III–V alloys, such as InGaAs, and can act only as a surfactant. However, on the basis of our results, Bi can be successfully incorporated into InGaAs to form the quaternary InGaAsBi alloy. To clarify this contradiction, systematic examinations were carried out to investigate the effect of growth temperature on Bi behavior in the MBE growth of InGaAs/InP.
Section snippets
Experimental procedure
InGaAs(Bi) was grown on a (0 0 1)-oriented InP substrate by MBE using solid sources of Ga, In, and Bi, and a valved arsenic cracker supplying As4. The substrates were degreased and etched by Br2/CH3OH etchants to remove residual impurities on the surfaces, and they were mounted on a 2-in Mo block of a substrate holder using In solder. Before the initiation of growth, native oxides of the InP substrates were desorbed by heating the substrates with an As flux impinging on the substrate surface. A
Results and discussion
Bi can be successfully incorporated into an InGaAs film to form the quaternary InGaAsBi alloy reported by the authors [6]. The peak position of the epilayer in XRD shifts toward a low value with Bi doping, which indicates that Bi is incorporated and located in the InGaAs lattice sites. The Bi incorporation was also confirmed by RBS measurements. Bi signals shadowed along both [1 1 0] and [1 0 0] directions in RBS angular scans give clear evidence that Bi atoms were substitutionally incorporated in
Conclusions
We have investigated the temperature dependence of Bi behavior in the MBE growth of InGaAs/InP. Bi can be successfully incorporated into InGaAs films at growth temperatures below 350 °C. The amount of incorporated Bi in InGaAs films decreases rapidly with increasing growth temperature. At normal growth temperatures for the InGaAs films, there is almost no detectable Bi content in the film. However, the addition of Bi can act as a surfactant in the MBE growth of InGaAs/InP at normal temperatures.
Acknowledgments
This study is partially supported by a grant for Regional Science and Technology Promotion from the Ministry of Education, Culture, Sports, Science, and Technology.
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