incorporation of sb and as in mbe grown gaasxsb1 x layers

Incorporation of Sb and As in MBE grown GaAsxSb1-x layers Tobias Zederbauer, Aaron Maxwell Andrews, Don MacFarland, Hermann Detz, Werner Schrenk, and Gottfried Strasser Citation: APL Materials 5, 035501 (2017); doi: 10.1063/1.4973216 View online: http://dx.doi.org/10.1063/1.4973216 View Table of Contents: http://aip.scitation.org/toc/apm/5/3 Published by the American Institute of Physics Articles you may be interested in High ionic conductivity in confined bismuth oxide-based heterostructures APL Materials 4, 121101121101 (2016); 10.1063/1.4971801 Magnetic, electronic, and optical properties of double perovskite Bi2FeMnO6 APL Materials 5, 035601035601 (2016); 10.1063/1.4964676 Molecular beam epitaxy of Cd3As2 on a III-V substrate APL Materials 4, 126110126110 (2016); 10.1063/1.4972999 Research Update: Nanoscale surface potential analysis of MoS2 field-effect transistors for biomolecular detection using Kelvin probe force microscopy APL Materials 4, 100701100701 (2016); 10.1063/1.4964488 APL MATERIALS 5, 035501 (2017) Incorporation of Sb and As in MBE grown GaAsx Sb1−x layers Tobias Zederbauer,1,a Aaron Maxwell Andrews,1 Don MacFarland,1 Hermann Detz,2,3 Werner Schrenk,3 and Gottfried Strasser1,3 TU Wien, Institute of Solid State Electronics, Floragasse 7, 1040 Wien, Austria Academy of Sciences, Dr Ignaz Seipel-Platz 2, 1010 Wien, Austria TU Wien, Center for Micro- and Nanostructures, Floragasse 7, 1040 Wien, Austria Austrian (Received 30 September 2016; accepted December 2016; published online 13 January 2017) With the increasing interest in low effective mass materials for intersubband devices, mixed As-Sb compounds, like GaAsx Sb1☞x or Alx In1☞x Asy Sb1☞y , gain more and more attention The growth of these materials, however, still provides significant challenges due to the complex interaction between As and Sb In this work, we provide an indepth study on the incorporation of Sb into the GaAsx Sb1☞x layers and compare our findings to the present literature on this topic It is found that both the composition and the crystal quality of GaAsx Sb1☞x layers are strongly influenced by the growth rate due to the As-for-Sb exchange reaction which takes place at the growing surface, and that high crystal quality can be achieved when the growth is performed under Sb limited conditions © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4973216] Quantum Cascade Lasers (QCLs) are a reliable source of coherent light in the mid-infrared (MIR) and terahertz (THz) regimes While high output power and room temperature operation have been achieved in the MIR, QCLs are still facing significant challenges in the THz regime The search for concepts which allow for higher performance of these devices shifts the focus of the research towards low effective electron mass well materials like Gax In1☞x As1,2 and InAs,3–6 and thus compounds containing Sb, like GaAsx Sb1☞x or Alx In1☞x Asy Sb1☞y ,7 become more and more interesting as barrier materials The growth of high quality layers of these materials, however, is still challenging since the presence of two group V species creates a non-trivial growth environment GaAsx Sb1☞x is an ideal material to study the interaction between As and Sb, since the presence of only one group III material limits the space for interpretations of the experimental results Molecular beam epitaxy (MBE) growth of GaAsx Sb1☞x was first demonstrated by Chang et al in 1977.8 Nakata and co-workers9 have grown high quality layers of GaAsx Sb1☞x using As4 and Sb4 at substrate temperatures between 470 ◦ C and 490 ◦ C and a growth rate of 1.3 µm h☞ Since then, numerous publications were devoted to understanding the incorporation of As and Sb into this alloy The inverse relation between the growth temperature (Tg ) and the incorporation of Sb into GaAsx Sb1☞x was first demonstrated by Klem et al.10 and is generally agreed on in the literature The role of the growth rate, however, is still subject to debate Almuneau et al reported on the incorporation of Sb into GaAsx Sb1☞x , AlAs1☞x Sbx , and Alx Ga1☞x Asy Sb1☞y 11 For all three alloys, a linear relation between the Sb flux, normalized by the total flux of the group III elements, and the Sb mole fraction in the layer was found From these results, it was concluded that the incorporation of Sb has to be close to unity Similarly, Semenov et al found an increasing As mole fraction in GaAsx Sb1☞x and Alx Ga1☞x Asy Sb1☞y alloys for increasing group III fluxes and constant group V fluxes.12 This is attributed to the fact that higher growth rates will lead to a shortage of Sb and unoccupied lattice sites will be filled by excess As a tobias.zederbauer@tuwien.ac.at 2166-532X/2017/5(3)/035501/6 5, 035501-1 © Author(s) 2017 035501-2 Zederbauer et al APL Mater 5, 035501 (2017) On the other hand, the reverse has also been stated by other authors An inverse relation between the As mole fraction in GaAsx Sb1☞x layers and the arrival rate of Ga atoms was first reported by Klem et al in 1987,13 i.e., higher PGa results in higher Sb incorporation The same effect has been observed by Bosacchi and co-workers14 for GaAsx Sb1☞x layers grown on GaAs substrates The authors concluded that the incorporation of Sb into GaAsx Sb1☞x is more effective at higher growth rates since the availability of Ga sites on the growing surface promotes the dissociation of the Sb species Sun and co-workers15 grew layers of GaAsx Sb1☞x on GaAs at growth rates between 0.2 ML s☞ and 0.9 ML s☞ At growth rates above 0.8 ML s☞ , the authors find a saturation of the Sb mole fraction and hence conclude that the growth rate dependence of the composition can be eliminated if the growth is performed at the higher growth rates The authors assume that high growth rates, and hence, availability of Ga sites, prevent the desorption of Sb which leads to an increase in the incorporation efficiency Selvig et al analyzed GaAsx Sb1☞x and Alx Ga1☞x Asy Sb1☞y layers with Sb mole fractions up to 0.2, grown on GaSb substrates.16 It was found that the incorporation of Sb is decreased at the lower growth rate The authors conclude that a lower Sb composition which is found if the GaAsx Sb1☞x layer is grown at a lower growth rate is due to an As-for-Sb exchange reaction which happens at the growing surface We can see that the reports on the growth of GaAsx Sb1☞x by MBE strongly disagree on the incorporation behavior of Sb While some authors find an increase in the Sb mole fraction with the growth rate, others find the opposite behavior Moreover, the authors reporting to find an increase in the Sb mole fraction with the growth rate present very different explanations for this effect Hence, an in-depth study on the effect of the growth rate on the composition of GaAsx Sb1☞x is necessary in order to understand the mechanisms that define the final layer composition In order to investigate the incorporation of Sb into GaAsx Sb1☞x , a set of samples was grown in a Riber Compact 21 MBE system on free-standing n + InP (001) wafers Within this set, the growth rate (RGaAsSb ) was varied between 0.195 ML s☞ and 1.56 ML s☞ The range of PSb2 was between × 10−7 Torr and 1.0 × 10−6 Torr, a PAs2 between 2.8 × 10−6 Torr and 1.1 × 10−5 Torr was used, and the Tg was varied from 400 ◦ C to 460 ◦ C All pressures in this work are given as absolute values without correction for the specific molecular species For the group V materials, valved cracking cells were used with the cracking zone temperatures set to 850 ◦ C and 1000 ◦ C to produce As2 and Sb2 , respectively For all samples, first, the oxide was thermally desorbed under PAs2 of 1.1 × 10−5 Torr at 520 ◦ C The temperature was then reduced to the growth temperature where a 50 nm thick Ga0.47 In0.53 As buffer layer and a 400 nm thick GaAsx Sb1☞x layer were grown All samples were measured with high resolution X-ray diffraction (HRXRD) to determine their crystal quality and composition To understand the influence of the growth rate on the composition of GaAsx Sb1☞x , four sets of samples have been grown at a Tg of 400 ◦ C and 460 ◦ C For each set, Tg , PAs2 , and PSb2 were kept constant, while the RGaAsSb was varied The results are given in Figure All samples show a decrease in the As mole fraction with increasing RGaAsSb on the low growth rate side (RGaAsSb between ML s☞ and 0.5 × 10−6 ML s☞ ), which is in agreement with Refs 13–16 On the high growth rate side, however, the As mole fraction is increasing with rising RGaAsSb as reported in Refs 11 and 12 Depending on the actual set of parameters chosen in each of these publications, the authors only saw either the low or the high growth rate side which resulted in the discrepancy in their reports In order to gain further insight, samples were grown at different values for Tg , PAs2 , and PSb2 We find that a decrease in PSb2 , under otherwise identical conditions, leads to a shift of the measured compositions in the GaAsx Sb1☞x layers towards higher As mole fractions (Figure 1(b)) While this seems to be an obvious result, it should be noted that the effect is much more dramatic on the high growth rate side of the experiment than on the low growth rate side If we assume a temperature dependent but otherwise constant incorporation coefficient for Sb, as described in Refs 11 and 12, PSb the arsenic mole fraction should be given by x = − C PGa2 , with C being constant for a given Tg On the high growth rate side, the compositions of the GaAsx Sb1☞x layers approach this trend, as indicated by the dashed lines in Figure 1(a) We can conclude that the composition at high growth rates is strongly dependent on the arrival rate of Sb2 molecules, while a different effect limits the incorporation of Sb into the GaAsx Sb1☞x layer at the low growth rates In order to understand the 035501-3 Zederbauer et al APL Mater 5, 035501 (2017) FIG (a) Composition of GaAsx Sb1☞x over the growth rate RGaAsSb at different PAs2 , PSb2 , and Tg On the high growth rate side, the As mole fraction rises with the growth rate since for each curve the PSb2 is constant and hence the ratio PSb decreasing The dashed lines indicate a fit to − C P Ga the falling growth rate Supplying less Sb2 (b) leads to PSb PGa is via C On the low growth rate side, the As mole fraction is rising with a higher As mole fraction Supplying less As2 (c) leads to a lower As mole fraction in the GaAsx Sb1☞x layer On the high growth rate side, samples grown at a Tg of 400 ◦ C and 460 ◦ C show an almost identical behavior (d) On the low growth rate side, the incorporation of Sb into GaAsx Sb1☞x is strongly decreased at a higher temperature influence of the Tg , samples were grown at a Tg of 460 ◦ C under otherwise identical conditions (Figure 1(d)) We can see that there is an almost perfect overlap with the samples grown at Tg of 400 ◦ C on the high growth rate side This indicates that the sticking of Sb does not depend on Tg in PSb this temperature range, and that this slope is solely dependent on the PGa2 fraction On the low growth rate side, however, we see a strong increase in the As mole fraction at a higher Tg In addition to the composition of the GaAsx Sb1☞x layers, their crystal quality might give a hint on the incorporation of Sb In order to understand the influence of the growth rate on the crystal quality, samples grown at different growth rates but similar compositions were examined by HRXRD A comparison of the crystal quality of GaAsx Sb1☞x layers grown at 0.227 ML s☞ and at 0.870 ML s☞ is shown in Figure Subfigure (a) shows the rocking curve scans of the two layers The sample grown at a lower growth rate shows a full width at half maximum (FWHM) of 258 arc sec while the peak corresponding to the layer grown at a higher growth rate shows a FWHM of only 36 arc sec HRXRD reciprocal space maps (RSMs) around the InP (224) diffraction peaks of the two layers are shown in subfigures (b) and (c) The in-plane lattice constant measured for the layer grown at the low growth rate deviates from that of the InP substrate, indicating partial relaxation Using the measured in-plane and out-of-plane lattice constants and using the stiffness components given in Ref 17, a relaxation of 25% was calculated The layer grown at a higher growth rate (subfigure (c)) shows no deviation in the in-plane lattice constant and hence can be assumed to be fully strained The effect of early relaxation can also be seen by comparing the crystal quality of highly lattice mismatched layers The two samples shown in Figure 1(b) which have been grown at a lower PAs2 and at a RGaAsSb of ∼0.45 ML s☞ and 0.88 ML s☞ have almost identical compositions However, the layer grown at a higher growth rate shows a rocking curve peak FWHM of 1140 arc sec while the layer grown at a lower growth rate shows a peak width of 1710 arc sec which is an increase of about 50% This observation indicates that the mechanism, controlling the Sb incorporation at low growth rates, facilitates the strain relaxation in the GaAsx Sb1☞x layers From these results we can conclude that the crystal quality of GaAsx Sb1☞x layers is not solely defined by the relaxation due to lattice mismatch Layers grown at low growth rates already show partial relaxation at compositions which yield a fully strained layer when a high growth rate is used Different mechanisms for the incorporation of Sb into GaAsx Sb1☞x layers were proposed in the literature Semenov and co-workers found an increase in the As mole fraction with increasing growth rates and attributed this to a shortage of Sb These observations can be confirmed for the high growth rate side of the experiment Three independent studies found a decrease in the As mole fraction with an increasing growth rate and provided different theories to explain their observations 035501-4 Zederbauer et al APL Mater 5, 035501 (2017) FIG Triple axis rocking curve (a) and RSM ((b) and (c)) scans of GaAsx Sb1☞x layers grown at different growth rates The sample grown at a lower growth rate of 0.227 ML s☞ shows FWHM of 258 arc sec, while the layer grown at 0.0870 ML s☞ shows a FWHM of only 36 arc sec The RSMs around the InP (224) diffraction peak show a relaxation of 25% for the GaAsx Sb1☞x layer grown at the low growth rate and a fully strained layer for the sample grown at the high growth rate Sun et al suggested that this increase in the Sb mole fraction is related to the availability of Ga sites which prevent the desorption of Sb species and hence increase the incorporation of Sb into the GaAsx Sb1☞x layers Under this assumption, the sticking coefficient of Sb would have to increase Sb super linearly with the availability of Ga sites since the PPGa fraction is decreasing when the RGaAsSb is raised Furthermore the Sb sticking coefficient should be independent of the PSb2 Hence, increasing PSb2 would have a similar effect on the high and on the low growth rate side of the experiment In Figure 1, however, we see that increasing the PSb2 has a much stronger effect on the high growth rate side Moreover, the incorporation of As would have to be independent of the RGaAsSb , since it would counteract the increase in the Sb mole fraction Bosacchi et al suggested that the dissociation of Sb4 into Sb2 is enhanced when a higher density of Ga sites is available The arguments which speak against this mechanism proposed, by Sun et al., can also be applied to this theory Moreover, an enhanced incorporation of Sb at higher growth rates was also found when Sb4 is cracked into Sb2 by using a cracker cell This indicates that the dissociation of Sb4 is not the predominant mechanism in this experiment In Figure we also see that the growth temperature has a stronger influence on the composition of the GaAsx Sb1☞x layer at low growth rates The fact that the high growth rate side for both curves overlaps shows that the desorption of Sb is only slightly dependent on the Tg in this temperature range This agrees with the line-of-sight QMS measurements by Kaspi and co-workers.18,19 Since the desorption of Ga can be neglected at these temperatures20 and, hence, the availability of Ga sites is temperature independent, the difference between the GaAsx Sb1☞x layers grown at different temperatures has to be explained by a different effect Both mechanisms cannot provide an explanation for (1) the lower Sb incorporation at elevated Tg and (2) an enhanced tendency for the relaxation of GaAsx Sb1☞x layers grown at low growth rates Selvig and co-workers conclude that the increase in the Sb mole fraction with higher growth rates is related to an As-for-Sb exchange reaction which takes place at the growth surface.16 According to this mechanism, the Sb mole fraction increases with the growth rate since the time an Sb site is exposed to As flux decreases, and hence, the rate at which this event takes place is reduced The As-for-Sb exchange reaction was previously studied by Losurdo and co-workers21 by exposing steady GaAs and GaSb surfaces to Sb and As fluxes, respectively The authors found a transformation of the GaSb into GaAs while the reverse reaction is not found and conclude that the As for Sb exchange is favored due to the difference in the enthalpy of formation of the two reactions, 2GaSb + As2 → 2GaAs + Sb2 , ∆H ◦ = −47.6 kJ mol−1 (1) 035501-5 Zederbauer et al APL Mater 5, 035501 (2017) and GaSb + As2 → GaAs + AsSb2 , ∆H ◦ = −33.9 kJ mol−1 (2) with respect to the reverse reactions 2GaAs + Sb2 → 2GaSb + As2 , ∆H ◦ = 47.6 kJ mol−1 (3) GaAs + Sb2 → GaSb + AsSb2 , ∆H ◦ = 13.7 kJ mol−1 (4) and TEM analysis of GaAs layers exposed to Sb2 flux shows relatively smooth interfaces while GaSb layers exposed to As4 flux show As-Sb clusters This mechanism provides a good explanation for all the phenomena which have been found regarding the incorporation of Sb into GaAsx Sb1☞x layers Since the exchange reaction takes place at the growing surface, the chance for an Sb atom to be replaced by an As atom is larger at a lower growth rate At higher Tg the reaction happens more frequently since the energy barrier for this reaction is more easily overcome Since the imperfections such as particulates and impurities strongly influence the relaxation of strain in epitaxial layers,22,23 the early relaxation of GaAsx Sb1☞x layers grown at low growth rates can be linked to the presence of AsSb clusters originating from the As-for-Sb exchange reaction Of the three mechanisms proposed in the literature to describe the incorporation of Sb into GaAsx Sb1☞x layers, the As-for-Sb exchange reaction is the only one which can describe all aspects found in the experiments In conclusion, we have studied the incorporation of Sb into GaAsx Sb1☞x layers grown on InP by MBE and compared our results to the studies on this topic found in the literature 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J Appl Phys 65, 2220–2237 (1989) 23 E A Fitzgerald, “The effect of substrate growth area on misfit and threading dislocation densities in mismatched heterostructures,” J Vac Sci Technol., B: Microelectron Nanometer Struct.–Process., Meas., Phenom 7, 782–788 (1989)  ... effective electron mass well materials like Gax In1 ? ?x As1 ,2 and InAs,3–6 and thus compounds containing Sb, like GaAsx Sb1 ? ?x or Alx In1 ? ?x Asy Sb1 ☞y ,7 become more and more interesting as barrier materials... reported on the incorporation of Sb into GaAsx Sb1 ? ?x , AlAs1? ?x Sbx , and Alx Ga1? ?x Asy Sb1 ☞y 11 For all three alloys, a linear relation between the Sb flux, normalized by the total flux of the group... desorption of Sb which leads to an increase in the incorporation efficiency Selvig et al analyzed GaAsx Sb1 ? ?x and Alx Ga1? ?x Asy Sb1 ☞y layers with Sb mole fractions up to 0.2, grown on GaSb substrates.16