Surface morphology and Bi incorporation in GaSbBi(As)/GaSb films

Adam Duzik n, Joanna M. MillunchickDepartment of Materials Science and Engineering, The University of Michigan, 2300 Hayward St., HH Dow Bldg., Ann Arbor, MI 48109, USA

a r t i c l e i n f o

Article history:Received 5 July 2013Received in revised form16 November 2013Accepted 1 December 2013Communicated by A. BrownAvailable online 14 December 2013

Keywords:A1. Atomic force microscopyA1. SurfacesA1. X-ray diffractionA3. Molecular beam epitaxyB1. Bismuth compoundsB1. Semiconducting III–V materials

a b s t r a c t

Several GaSbBi(As)/GaSb films were grown to investigate the effects of Bi on GaSb surface morphologyand bulk composition as a function of growth conditions. Scanning electron microscopy of the surfaceshows several biphasic droplets consisting of Ga- and Bi-rich phases � 1 μm in diameter form on thesurface. Some of these droplets exhibit more unusual features such as facets, sub-droplets, and dropletetching into the underlying film. Bi droplet coverage shows a direct increase with increasing Bi:Gaand Bi:Sb BEP ratios. Rutherford backscatter and X-ray diffraction analyses of these films show Biconcentration of up to 12% and a concurrently increasing unintentional As concentration of up to 9.3%,suggesting the presence of a strain auto-compensation mechanism during film growth. Once Biconcentration reaches 10–12%, Bi incorporation saturates, with excess Bi atoms instead accumulatingin the droplets.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction and motivation

A number of benefits have been observed when growing III–V-Bisemiconductors with Bi, including surface smoothing [1–4], bandgap reduction [5–11], spin–orbit coupling [12,13], and preservedelectron mobility [14–18]. Such properties are promising for creat-ing long wavelength based optoelectronic devices or novel spin-tronic devices. However, Bi does not readily incorporate into III–Valloys owing to its large size. This has necessitated low growthtemperatures and growth rates to achieve appreciable incorpora-tion [4,19–23].

One potential method of alleviating this difficulty is to use aIII–V substrate with a larger lattice constant, such as GaSb whichhas a lattice constant of 6.096 Å. Such GaSbBi alloys grown byliquid phase epitaxy exhibit a bandgap reduction of 40 meV for aGaSb0.996Bi0.004 alloy [24]. Given that GaSb already has a smallerbandgap than GaAs, these two factors make GaSbBi attractive forlong wavelength devices, but to date, very little work has beendone on increasing Bi incorporation into GaSb. As with GaAs,the tendency of Bi towards surface segregation necessitates lowgrowth temperatures to achieve appreciable incorporation, asdemonstrated by Song et al. [25]. In that work, the presence ofdroplets or lack thereof depended on the BEP ratios of Ga, Sb, andBi. Bi concentrations of 0.2–0.6% were also reported for flat,droplet-free films according to secondary ion mass spectroscopy(SIMS) and Rutherford backscatter spectroscopy (RBS).

In this work we examine Bi incorporation into GaSb as well asthe formation and structure of surface droplets as a function ofgrowth conditions. The surface morphology is explained in light ofthe Ga–Bi phase diagram. Bi concentrations of up to 12% areobserved for Ga and Sb growth rates of 0.6 ML/s under a simulta-neous Bi growth rate of 1.0 ML/s. Significant As incorporation is alsoobserved from background sources and is directly proportional tothe amount of incorporated Bi. We propose that this occursspontaneously to compensate for the strain generated in thesefilms, as seen from the calculated film relaxation. These films arereferred to as GaSbBi(As) films, where the parentheses indicateunintentional As incorporation.

2. Background and methods

All GaSbBi(As) films were grown on GaSb(001) substrates usingmolecular beam epitaxy (MBE). BEP was measured with a retract-able ion gauge. Sample surface oxide desorption was carried out at520 1C as measured by a pyrometer. 200 nm thick GaSb bufferlayers were grown at 485 1C, exhibiting a streaky ð1� 3Þ reflectionhigh-energy electron diffraction (RHEED) pattern. Samples werethen cooled at 20 1C/min to 300 1C, with the Sb BEP shut off at asubstrate temperature of E425 1C to prevent the formation of að1� 5Þ surface reconstruction or polycrystalline Sb on the surface.

Two series of films were grown. The first was a study of filmsgrown at different absolute growth rates (AGR series), with filmsgrown at 0.2, 0.8, and 1.0 (labeled AGR-1, AGR-2, and AGR-3)monolayers per second (ML/s) while maintaining a constantestimated Ga:Sb:Bi growth rate ratio of 1:1:1. Ga and Sb growthrates weremeasured via RHEED intensity oscillations, while Bi growth

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Journal of Crystal Growth

0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jcrysgro.2013.12.001

n Corresponding author. Tel.: 734 936 8250.E-mail addresses: aduzik@umich.edu (A. Duzik),

joannamm@umich.edu (J.M. Millunchick).

Journal of Crystal Growth 390 (2014) 5–11

rates were estimated from Bi desorption rates on GaAsN [1].In this growth regime, the growth rates are limited by Ga BEPalone as determined via GaAs growth rates. The other series ofsamples (BGR series) were grown at a constant Ga and Sb rate of0.6 ML/s, with the Bi growth rate set to 0.2, 0.6, and 1.0 ML/s.All sample growth conditions are given in Table 1, along withmeasured Ga and Bi droplet coverages (θGa and θBi, respectively),Bi and As concentrations, and percentage film strain relaxation.Measured beam equivalent pressures (BEPs) are included forcomparison. From Table 1, the Sb BEP was always less than theGa BEP even though the growth rates are equal. This is due todifferences in the sensitivity of the ion gauge to the two elements.All films were nominally 300 nm thick, after which the samplewas quenched under no overpressure and removed from thevacuum system for characterization. Rutherford backscatter (RBS)measurements using 2 MeV Heþ þ ions normally incident on the

surface with the detector at an angle of 201. The simulationprogram SimNRA was used to simulate the RBS spectra [26,27],in order to reconstruct the individual contributions from surfacedroplets and the film to the overall spectrum. High resolutionX-ray diffraction (HRXRD) rocking curves were taken for the (004)and (224) reflection of each sample at various azimuthal angles fordetermining As concentration and film relaxation.

3. Results

3.1. GaSb growths

Two GaSb buffers, one at 485 1C (B-HT) and another at 300 1C(B-LT), were grown for comparison to the GaSbBi(As) films. TheB-HT sample is simply the GaSb buffer layer grown prior to the low

Table 1GaSbBi growth conditions. All samples consist of a 300 nm GaSbBiðAsÞ film grown at a substrate temperature of 300 1C on a 200 nm GaSb buffer (except for B-HT, which doesnot have the 300 nm film). θGa and θBi correspond to Ga and Bi droplet coverages, respectively.

Sample name Rate (ML/s) Ga:Sb:Bi ratio BEP (10�7 Torr) Coverage % Bi (RBS) % As (XRD) % Relax

Ga Sb Bi Ga Sb Bi θGa θBi

B-HT 0.6 0.8 0 1:1.48:0 3.7 3.6 0 0 0 0 9.0 19B-LT 0.6 0.6 0 1:1:0 4.1 3.6 0 0.34 0 0 4.3 58AGR-1 0.2 0.2 0.2 1:1:1 1.3 0.4 1.5 0.42 0.26 6.0 4.2 58AGR-2 0.8 0.8 0.8 1:1:1 5.4 3.6 4.0 0.14 0.15 2.0 2.3 89AGR-3 1.0 1.0 1.0 1:1:1 6.4 4.3 5.6 0.03 0.22 8.0 7.6 62BGR-1 0.6 0.6 0.2 1:1:0.33 4.4 2.3 1.5 0.03 0.02 10.0 8.9 62BGR-2 0.6 0.6 0.6 1:1:1 4.1 2.3 2.3 0 0.11 10.0 7.7 44BGR-3 0.6 0.6 1.0 1:1:1.67 4.4 2.3 5.3 0.02 0.38 12.0 9.3 54

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Fig. 1. (a) AFM of the B-HT surface, showing the 200 nm GaSb buffer layer before low temperature film growth. (b) AFM of the B-LT surface after growing 300 nm of GaSb at300 1C. (c) XRD of both GaSb samples, where high angle peaks correspond to residual As incorporation in the GaSb buffer (148.5 arcsec) and the low temperature layer(524.0 arcsec). (d) RBS spectra of the B-HT and B-LT films. Inset is a model of the Heþ þ interactions with these samples.

A. Duzik, J.M. Millunchick / Journal of Crystal Growth 390 (2014) 5–116

temperature GaSbBi(As) film growth. Atomic force microscopy(AFM) of the surface (Fig. 1(a)) reveals a typical GaSb surface withterraces, with some surface cracks that suggest a tensile filmstrain. The second sample (B-LT) consists of a high temperaturebuffer followed by a low temperature film grown without Bi,where the Ga and Sb rates were 0.6 ML/s. AFM of the surface(Fig. 1(b)) reveals that the surface is covered by droplets consistentwith the observed disappearance of the (1�3) RHEED patternduring growth. Energy dispersive spectroscopy (EDS) confirmsthat these droplets are completely composed of Ga.

HRXRD (Fig. 1(c)) of the B-HT sample shows a broad peak148.5 arcsec higher than the substrate peak, indicating that thelattice parameter of the film is in tensionwith respect to the substrate,accounting for the cracks observed in Fig. 1(a). We attribute this toAs incorporation due to a residual As overpressure. Using the rule of

mixtures and Bragg's law, the As composition of this film is 0.9%.HRXRD of the B-LT film (Fig. 1(c)) shows two high angle peaks, oneof which is at the same position as the B-HT peak, and a higherangle peak indicating an As concentration of 4.3%. The firstappears from residual As incorporation into the 200 nm bufferlayer, while the second originates from the subsequent 300 nmlow temperature GaSb layer. We attribute the increased As con-centration to the lower Sb BEP and lower temperature conditionsused in growing the low temperature film, which result in reducedsite competition with Sb and reduced As desorption. RBS (Fig. 1(d))of the B-HT sample is typical of GaSb with sharp Ga and Sb leadingedges and flat energy plateaus. The Sb leading edge in B-LT is morerounded due to the Ga droplets on the surface. As the incomingHeþ þ ions reach the surface, some will encounter bare GaSb (seethe inset of Fig. 1(d)), resulting in the dashed spectrum in Fig. 1(d).

0.5-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-0.2 -0.1 0 0.1 0.2 0.3 0.4

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Fig. 2. SEM (a–c) and EDS (d–f) of the AGR sample surfaces. In the EDS, Ga (red) and Bi (green) are concentrated in the droplets, while Sb (blue) is only in the film. XRD (g–i)and RBS (j–l) of these films. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

A. Duzik, J.M. Millunchick / Journal of Crystal Growth 390 (2014) 5–11 7

The remainder will be incident on the Ga droplets, some of whichwill backscatter from the droplets while the others penetrate intothe underlying film. The droplets are round and thus of non-uniform thickness, resulting in a distribution of Heþ þ energylosses. The total spectrum is thus the weighted average, based onthe coverage of the droplets, of the ions backscattering from theGaSb buffer layer (dashed line in Fig. 1(d)) and the droplet coveredfilm (dash-dotted line in Fig. 1(d)). The residual As does not appearin either spectra as the mass resolution of RBS is insufficient todistinguish the As and Ga signals.

3.2. Absolute growth rate (AGR) series

The growth of GaSbBi(As) often leads to the formation of dropletson the surface. SEM of the surface grown at 0.2 ML/s (AGR-1)(Fig. 2(a)) shows a surface with a wide size distribution of irregularlyshaped biphasic droplets. Moreover, many small sub-droplets on thelarger droplets are visible, which appear as brighter regions in thesecondary electron image but are not resolved in the EDS image.Increasing the absolute growth rate to 0.8 ML/s (AGR-2) results inlarge (up to 1 μm in diameter), biphasic, hemispherical droplets withclear delineation of phase boundaries as seen in Fig. 2(b). A growthrate of 1.0 ML/s (AGR-3, Fig. 2(c)) sees a return of the irregularlyshaped droplets, but with a smaller droplet density and sizedistribution and less distinct phase separation.

Plan-view EDS measurements of all three surfaces (Fig. 2(d–f))show the droplets consist of Ga (red) and Bi (green), but not Sb(blue), in agreement with observations in the GaAsBi materials system[28,29]. In AGR-1, droplets cover 68% of the surface, with Bi segregat-ing to opposite ends of the droplets. In AGR-2 and AGR-3, surfacedroplets cover 29% and 25% of the surface, respectively. The biphasicnature of the droplets can be explained by the Ga–Bi phase diagram[30]. Ga and Bi are miscible at the growth temperatures used here. Asthe sample cools below the melting point of Bi, 271 1C, Bi begins tosolidify leaving behind a Ga-rich liquid. As cooling drops below 193 1C,any remaining Ga will segregate to the surface, forming the sub-droplets observed in Fig. 2(c).

These data show that while the total absolute BEP increases toachieve the faster growth rate, the droplet coverage on the surfacedecreases, primarily in the reduction of the amount of Ga inthe droplets. As the absolute growth rate increases, the Sb BEPincreases more than the Ga BEP in order to maintain the relativegrowth rates. It also appears that the droplet morphology canchange depending on the relative amounts of Bi and Ga present inthem. When one is much more prevalent than the other, theirregular shaped droplets form; but when the ratio is 1, roundeddrops form.

A cross-sectional SEM view (Fig. 3) of an AGR-2 droplet revealsthat the droplets etch the substrate, similar to that of Ga into GaAs[31,32]. We propose that the droplet has a relatively high amountof Ga, as pure Bi droplets have not been observed to etch GaAs atthese temperatures [33]. The etched region has a smooth interfacewith the underlying GaSbBi(As) film, indicating that the droplet wasuniform in composition during etching, then separated into distinctphases during cooling. The two phases form a vertical boundarywithin the droplet, minimizing the interfacial area between thetwo phases. This suggests a high interfacial energy between the solidphases of Ga and Bi, likely from the dissimilar crystal structures of thetwo solids. Moreover, the difference in convexity of the droplet phasesat the solid–vacuum interface is easily distinguished, with theGa-rich phase having a convex surface and the Bi-rich phasehaving a concave surface. This behavior can be explained fromthe difference in density between Ga, Bi, and the underlying GaSb(5.91, 9.81, and 5.61 g/cm3, respectively). Since the solid Ga andGaSb have similar densities, there is little change in the shape of

the droplet upon solidification. The denser Bi phase contracts uponsolidification, resulting in the buckling of the surface.

HRXRD of these samples (Fig. 2(g–i)) shows peaks higher inangle than the substrate peak, meaning that the film latticeparameter is less than that of the substrate. This unexpected resulthas been reported in work by Song et al. and is attributed in thatwork to Bi-induced vacancy formation in GaSb [25,34]. However,we show that there is unintentional As incorporation in thesefilms as a result of a residual background As pressure. Given thatneither the Bi nor the As compositions are known, RBS character-ization of these films is required, shown in Fig. 2(j–l). The shape ofthe RBS spectra are altered due to the presence of droplet on thesurface. We consider the film spectrum to be a weighted averageof three components: bare GaSbBi(As), Ga droplets on GaSbBi(As)(labeled Ga/GaSbBi(As)), and Bi droplets on GaSbBi(As) (Bi/GaSbBi(As)). We assume that the nominal film thickness of 300 nm, whilethe droplet height data is taken from AFM and the average etchingdepth. The coverage of the Ga and Bi droplets, which determinesthe weighting factor, was determined from the EDS maps. Oncethe contribution of each component is considered, the agreementbetween the simulated spectra and the experimental data is verygood. Any observed slope of the Sb and Bi plateaus occurs becauseof the Bi droplets, which form a peak at the RBS Bi leading edgeand then decrease with decreasing channel. Depending on the sizeand thickness of the droplets, the Bi droplet RBS signal can reachwell into the Sb plateau, producing the observed spectra. Account-ing for these effects and simulating the spectra of all three samples(Fig. 2(j–l)) allow us to calculate the Bi concentration in theunderlying GaSbBi(As) film (see Table 1). Despite the steadilyincreasing absolute growth rate, the Bi concentration starts at 6%for AGR-1, falls to 2% for AGR-2, then rises back to 8% for AGR-3.The other properties that follow this trend are the amount of Bi inthe surface droplets and the Bi:Sb BEP ratio (see Table 1). Thissuggests an absolute growth rate of Bi is less important than theratio of the Bi to Sb BEP. Taking the Bi composition into account,the As concentration is calculated from the HRXRD spectra (seeTable 1) to be 4.2%, 2.3%, and 7.6% for AGR-1, AGR-2, and AGR-3,respectively.

3.3. Relative Bi growth rate (BGR) series

The trends in the AGR series suggest that the relative growthrates are a stronger determinant of the droplet morphology andfilm concentrations than absolute growth rate. Three GaSbBi(As)

Fig. 3. Cross-sectional view of AGR-2 droplet etching.

A. Duzik, J.M. Millunchick / Journal of Crystal Growth 390 (2014) 5–118

films were grown at fixed Ga and Sb rates of 0.6 ML/s as a functionof relative Bi growth rates of 0.2 ML/s, 0.6 ML/s, and 1.0 ML/s,labeled BGR-1, BGR-2, and BGR-3 respectively, in Table 1. SEM ofthese surfaces is shown in Fig. 4(a–c). In all three cases, severaldroplets cover the surface, whose sizes vary solely according to therelative Bi:Ga BEP. In BGR-1, several small droplets�1 μm arepresent (Fig. 4(a)). Upon increasing the Bi rate in BGR-2, fewer,larger droplets form (Fig. 4(b)). As the Bi rate is increased furtherto 1.0 ML/s (BGR-3), large irregularly shaped droplets 2–3 μm insize cover the majority of the surface area, and sub-droplets areobserved (Fig. 4(c)). Fig. 4(d–f) shows EDS maps of the same threesurfaces, and as with the AGR-1 and AGR-3, the droplets areprimarily composed of Bi with smaller amounts of Ga present.

Analysis of the RBS for each sample (Fig. 4(j–l)) shows thatthese films have a higher Bi concentration. BGR-1 and BGR-2 havethe same nominal Bi concentration (10%), but the amount of Bi inthe droplets is larger for BGR-2, indicating that the higher incidentBEP was absorbed into the droplets rather than incorporated intothe film. Furthermore, the Bi BEP for BGR-3 is twice that of BGR-2,but the amount of incorporated Bi is only 12%, while the amount ofBi in the droplets has increased from 11% to 38%. Thus, themajority of incident Bi atoms incorporate into droplets once thefilm concentration reaches 10–12%. In contrast, the increased Bi:SbBEP ratio between AGR-1/AGR-2 and AGR-3/AGR-2 corresponds toa 4–6% increase in Bi concentration but only a 7–11% increase in Bidroplet coverage. With the Bi concentrations established with RBS,

0.5-0.2 -0.1 0 0.1 0.2 0.3 0.4

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Fig. 4. SEM (a–c), EDS (d–f), XRD (g–i), and RBS (j–l) of the BGR sample surfaces.

A. Duzik, J.M. Millunchick / Journal of Crystal Growth 390 (2014) 5–11 9

the As concentration in the GaSbBi(As) films may be determinedas before from the HRXRD data shown in Fig. 4(g–i).

3.4. Discussion

The coverage of the Bi droplets θBi increases with the Bi:Sb andBi:Ga BEP ratios as plotted in Fig. 5 for all the GaSbBi(As) films inthis study. θBi as a function of Sb:Ga ratio was not plotted as thisratio was not varied in this study. Ga droplet coverage θGa is ratherlow in most of the GaSbBi(As) samples studied here.

Fig. 6 shows a plot of the Bi concentration as a function of Asconcentration. These data show that there is a strong correlationbetween the Bi and As compositions independent of the growthconditions. EDS spectra (Fig. 7) of the films confirm a small butsignificant As Kα signal at E10.5 keV for all samples with an Asconcentration greater than 4%. Given that the incorporation of Aswas unintentional, these data suggest that As incorporates inresponse to the strain induced in the growing film due tothe incorporation of Bi. Because the lattice parameter of GaAs(5.653 Å) is less than that of GaSb (6.096 Å) and the calculatedlattice parameter of GaBi (6.324 Å) [35], incorporation of As canoffset the lattice mismatch in the growing film. It takes about oneAs atom to compensate for two Bi atoms, but the trend in Fig. 6shows the films have three As atoms for every four Bi atoms, lowerthan the ideal amount. As a result, there is a net residual filmlattice contraction as the As concentration is higher than thisrequired amount. This technique for strain compensation has been

widely used in other systems, such as InGaAs/InGaAsP multiplequantum wells [36], GaAsNBi, [8,35,37], and InGaAs/InP [38–40].The unusual feature in these data is that the amount of incorpo-rated As does not depend on the supplied As BEP, that is, the Asoverpressure was residual as opposed to intentional. Thus, it islikely that more Bi may be incorporated into GaSb in the presenceof a higher As overpressure.

4. Conclusion

In this work, several GaSbBi(As) films were grown on GaSbsubstrates to study Bi incorporation. We find that the surfaces ofthese films are covered by biphasic Bi–Ga droplets, sometimeswith small sub-droplets and facets forming due to Ga segregationfrom the Bi-rich regions. Droplet etching into the film is alsoobserved. Bi concentrations of up to 12% were achieved, withexcess Bi incorporating into the droplets instead of the film.Relative Bi growth rate was found to be a stronger determinantthan the absolute growth rate of morphology and composition. AllBi-containing films also contained a proportional amount of As,thus explaining the observed reduction in the lattice parameterof this film. The Bi and As concentrations were directly related,

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

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Fig. 5. Plot of the Bi droplet coverage vs. flux ratios. Lines are only guides forthe eye.

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Fig. 6. Plot of As concentration vs. Bi concentration in all the GaSbBi(As) filmsgrown in this study. Line through the data is only a guide for the eye.

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A. Duzik, J.M. Millunchick / Journal of Crystal Growth 390 (2014) 5–1110

indicating a strain auto-compensation effect occurred from incor-porating both species.

Acknowledgments

This research was supported by a Materials World Networkgrant from the National Science Foundation DMR 0908745. Theauthors would also like to acknowledge the University of MichiganElectron Microbeam Analysis Laboratory in generating the SEMimages in this publication.

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