Article pubs.acs.org/crystal
Hetero-Twinning in Chemical Epitaxy of PbS Thin Films on GaAs Substrates Anna Osherov, Vladimir Ezersky, and Yuval Golan* Department of Materials Engineering and the Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel ABSTRACT: Twinning, i.e., inversion of atomic stacking sequence along a specific crystallographic axis, is a commonly occurring phenomenon in thin film growth. A very large number of reports have shown cases of conventional twinning within a single polycrystalline phase, where the plane of contact between the two twinned parts, called the composition plane, coincided with the twinning plane. Here we show for the first time twinning in heteroepitaxial monocrystalline PbS semiconductor thin films that are deposited from aqueous solutions under ambient conditions. The chemically deposited films show a distinct, well-defined “hetero-twin” relationship with monocrystalline GaAs substrates of various orientations: (100), (111), and (110). Notably, the same twin relationship, with the (111) twin plane and [1̅1̅2] twinning direction was observed in all studied cases regardless of the composition plane.
■
INTRODUCTION Heteroepitaxial growth of monocrystalline thin films from solution is unusual and beneficial for heterojunction-based electronic and optoelectronic devices.1 Lead sulfide (PbS) and gallium arsenide (GaAs) are technologically important semiconductor materials, with applications in sensing and emitting devices in the infrared range.2,3 Multiple carrier generation, recently suggested in chemically deposited (CD) lead sulfide monocrystalline films,4 points out this material as a promising candidate for replacing Ge in tandem solar cells. Understanding the mutual tendencies of materials to develop epitaxial relations, and the nature of the resulting heterojunction interfaces, is essential for the development of electronic and optoelectronic devices with improved performance. However, to date, no universal tools exists for a priori prediction of the orientation relations between different material pairs. In a recent review, we have summarized a fairly large body of evidence for “chemical epitaxy”: growth of CD semiconductor films that show well-defined orientation with respect to the underlying substrate at relatively low temperatures.1 In particular, PbS films have been shown to demonstrate chemical epitaxy with InP,5 Ge,5−7 CdS,7 PbSe,8 and GaAs.8−10 Previously reported CD of PbS on GaAs(100) substrates resulted in nonconventional orientation relationship between the film and the substrate with (011)PbS∥(100)GaAs and [01̅1]PbS∥[011̅]GaAs.8−10 It was proposed that this rather unusual orientation relationship of PbS on GaAs(100) could be favored due to a close lattice match (less than 1%) in the 3:2 superstructure formed between 3d(220)GaAs and 2d(200)PbS, similar to that previously reported for epitaxial CdSe nanocrystals electrodeposited on Au(111).11 Understanding of the complex interplay between structure and properties requires detailed characterization of the interface; we have recently © 2012 American Chemical Society
shown that despite similarity in the orientation and morphology of CD PbS films deposited on GaAs(111)A and GaAs(111)B, the chemical nature of the film surfaces as well as film/substrate interfaces are markedly different.12 Furthermore, control over film orientation allowed control over the PbS surface oxide species that evidently affect the electronic properties of the surface.12 Strain accommodation during growth commonly occurs via dislocation generation, stacking faults, or twinning. The most frequently occurring symmetry rule of twinning is that the crystal structure of one of the parts is the mirror image of the crystal structure of the other part, in a certain crystallographic plane called the twinning plane.13 In fcc-type crystals, twins always occur on (111) planes because such a twin has a relatively low formation energy as the nearest neighbor atomic distances and bond angles remain unchanged.14 Moreover, when the fcc deposit is grown in ⟨111⟩ out-of-plane orientation on a (111) surface, it is frequently found that two or more growth twin orientations can be present, all of them having close-packed directions aligned with the substrate.15 While the crystal structure and the material composition are identical on both sides of the twin boundary, the wave function symmetry mismatch due to opposite orientation gives rise to considerable electron scattering at the periodically distributed interface and consequently affect interfacial properties.14 Furthermore, electronic structure calculations of twin boundaries and twinning superlattices in PbS predict that such films are expected to exhibit unusual and interesting electronic and optical properties.14,16 Received: April 11, 2012 Revised: June 26, 2012 Published: July 2, 2012 4006
dx.doi.org/10.1021/cg3004883 | Cryst. Growth Des. 2012, 12, 4006−4011
Crystal Growth & Design
Article
Figure 1. (a) X-TEM micrograph of PbS/GaAs(111)B, (b) SAED of the PbS/GaAs(111) interface. Inset: Indexing for the diffraction spots showing the PbS[11̅0] and GaAs[11̅0] zone axes (taking into account matrix/twin orientation relations). Mirror plane between film and substrate is indicated on the diffraction pattern.
■
RESULTS AND DISCUSSION To further investigate the orientation relations in the PbS/ GaAs system,9,10 monocrystalline PbS films were deposited from solution onto various faces of GaAs: (100), (111)A (Gaterminated), (111)B (As-terminated), and (110). Cross sectional transmission electron microscopy (X-TEM) was applied for morphological and crystallographic characterization in each of the PbS/GaAs systems. The X-TEM image of PbS/ GaAs(111)B shown in Figure 1a shows the characteristic triangular, 3-fold symmetrical surface morphology of the PbS film. Corresponding selected area electron diffraction (SAED) shown in Figure 1b indicated a (111)PbS∥(111)GaAs and [11̅0]PbS∥[11̅0]GaAs orientation relationship, which initially appeared to be a case of conventional heteroepitaxy. However, more careful examination of the diffraction pattern reveals “heterotwin”-like orientation relations between the film and the substrate, with a twinning plane that coincides with the interfacial plane. No apparent differences in PbS film microstructure and morphology were observed when GaAs(111)B substrates were replaced with GaAs(111)A (not shown). However, substantial differences in orientation, surface composition, and physical characteristics of the PbS(111) layers were revealed upon deposition on oppositely terminated GaAs(111) substrates. 12 PbS(111)A was obtained on GaAs(111)A, and PbS(111)B was obtained on GaAs(111)B.12 We note that identical spot electron diffraction patterns (rather than arcs or rings as in polycrystalline films) repeat within different samples and along the entire length of the film and not only in designated regions. To further elucidate the origin of these orientation relations, PbS films were deposited onto GaAs(100) and GaAs(110) substrates. Samples prepared in two perpendicular [011] and [01̅1] in-plane directions of the GaAs(100) substrate (Figure 2a−c), and two perpendicular [11̅0] and [001]in-plane directions of the GaAs(110) (Figure 2d-f) were examined using X-TEM. Schematic illustrations of these PbS/GaAs cross sectional directions are shown in the insets of Figure 2a,d. The PbS/GaAs(100) micrograph shown in Figure 2a revealed large density of threading dislocations within the PbS film that are likely to arise from misfit strain. Corresponding selected area electron diffraction taken along the [01̅1]GaAs zone axis and
In this paper we report heteroepitaxial, twin-like growth of CD PbS films with respect to GaAs in three different substrate orientations. This phenomenon is reported for the first time in semiconductor thin films.
■
METHODS
Basic Materials and Chemicals. Thiourea (ACS analytical >99.0% Aldrich reagent), lead nitrate (Aldrich, analytical 99.99+%), and sodium hydroxide AR were used without further purification. Substrates were single crystal GaAs(100), GaAs(110), GaAs(111)A, and (111)B (AXT, one side polished, undoped, ± 0.1° miscut). Chemical Bath Deposition. PbS films were deposited from solution with final composition of 1 mM Pb(NO3)2, 5.7 mM CS(NH2)2, and 146 mM NaOH at final 12 < pH < 13. Fresh stock solutions of 17.5 mM Pb(NO3)2, 1 M CS(NH2)2, and 57 mM NaOH were prepared. The deposition solution was prepared by stirring 10 mL of 57 mM NaOH solution with 25 mL of distilled water followed by slow addition of 2 mL of 17.5 mM Pb(NO3)2. Finally, 2 mL of 1M CS(NH2)2 was added with additional stirring. Film growth was carried out at 20 °C for various periods of time. Characterization Methods. High Resolution Scanning Electron Microscopy (HR-SEM). The morphology of the films was observed using a JEOL 7400F field emission gun SEM without coating the samples. Secondary electrons were used to obtain the topography images. Acceleration voltages ranged from 2 to 5 kV. Transmission Electron Microscopy (TEM). TEM imaging was carried out using a JEOL 2010 instrument operating at 200 keV. Selected area electron diffraction patterns were obtained from both the PbS films and the GaAs substrates. TEM Sample Preparation. Cross sections were prepared by cutting the sample into slices normal to the interface and gluing them together face-to-face using M-Bond 610 adhesive (Allied HighTech Ltd.). The samples were polished with a precision small-angle tripod holder on a series of diamond polishing papers (Allied HighTech Ltd.) until a thin wedge was formed. The sample was then glued to a Cu slot support (1 × 2 mm2), and final thinning was done by Ar ion milling using a Gatan model 691 precision ion polishing system. Software. Schematic illustrations of the orientation relations models were obtained using CaRine Crystallography software. Simulated stereographic projections were obtained using TwinCrystal software. 4007
dx.doi.org/10.1021/cg3004883 | Cryst. Growth Des. 2012, 12, 4006−4011
Crystal Growth & Design
Article
Figure 2. (a) X-TEM micrograph of PbS/GaAs(100) structure. Inset: Schematic illustration indicating the direction of cross section for the diffraction patterns shown in (b and c). (b) SAED of the PbS/GaAs(100) interface. Inset: Indexing for the diffraction spots showing the [101]̅ PbS and [011̅]GaAs zone axes. Mirror plane is indicated on the diffraction pattern. (c) SAED of the PbS/GaAs(100) interface. Inset: Indexing for the diffraction spots showing the PbS[010] and GaAs[011] zone axes. (d) X-TEM micrograph of PbS/GaAs(110) system. Inset: Schematic illustration indicating direction of cross section for the diffraction patterns shown in (b and c). (e) SAED of the PbS/GaAs(100) interface. Inset: Indexing for the diffraction spots showing the PbS[110̅ ] and GaAs[110̅ ] zone axes. Mirror plane is indicated on the diffraction pattern. (f) SAED of the PbS/ GaAs(100) interface. Inset: Indexing for the diffraction spots showing the PbS[111̅] and GaAs[001] zone axes.
relations between film and substrate (see mirror plane indicated on the diffraction pattern), however in this case, the twinning plane is distinct from the interfacial (contact) plane. Furthermore, a 19° misorientation between [111]GaAs and
indexing for the diffraction spots shown in Figure 2b indicates the (101)PbS∥(100)GaAs and [101̅]PbS∥[01̅1]GaAs orientation relationship. Also here, careful examination of the diffraction pattern (Figure 2b) revealed heterotwin-like orientation 4008
dx.doi.org/10.1021/cg3004883 | Cryst. Growth Des. 2012, 12, 4006−4011
Crystal Growth & Design
Article
Figure 3. Superimposed stereographic projection and real space illustration for (a,b) PbS(111)/GaAs(111), (c,d) PbS(110)/GaAs(100), and (e,f) PbS(112)/GaAs(110). Solid red and yellow circles in the stereographic projections represent parent and twin pole, respectively. Overlapping positions of twin and matrix planes are indicated by green circles. The large circle on the projection corresponds to the zone axes of reflections in the diffraction patterns shown in Figures 1 and 2, accordingly.
GaAs interface along [11̅0]GaAs zone axis shown in Figure 2e indicated (112)PbS∥(110)GaAs and [11̅0]PbS∥[11̅0]GaAs orientation relations. Also, in this case, the twinning plane was distinct from the interfacial (contact) plane. A misorientation of 13° between [111]GaAs and [111]PbS was observed (Figure 2e). Similarly, SAED from the PbS/GaAs(100) interface along [001]GaAs zone axis shown in Figure 2f, indicated the (112)PbS∥(110)GaAs and [111̅]PbS∥[001]GaAs orientation relations and again confirmed the monocrystalline nature of the
[111]PbS directions is observed (Figure 2b), indicating mutual tendency to align the {110}PbS∥{100}GaAs low index planes. In a similar way, SAED from the PbS/GaAs(100) interface along [011]GaAs zone axis shown in Figure 2c verified the existence of the (101)PbS∥(100)GaAs and [010]PbS∥[011]GaAs orientation relations and furthermore confirmed the monocrystalline nature of the layer. Similarly, the X-TEM micrograph of PbS films deposited on GaAs(110) shown in Figure 2d revealed a network of dislocations. Corresponding SAED from the PbS/ 4009
dx.doi.org/10.1021/cg3004883 | Cryst. Growth Des. 2012, 12, 4006−4011
Crystal Growth & Design
Article
Figure 4. 3D real space schematic illustrations of the atomic arrangement on the GaAs substrate wafers. (a) GaAs(111), emphasizing three identical ⟨110⟩GaAs. (b) HRSEM images showing the 3-fold symmetrical topography of PbS films chemically deposited on GaAs(111). (c) GaAs(100), emphasizing two dissimilar, perpendicular [011]GaAs and [01̅1]GaAs. (d) Corresponding HRSEM image, confirming preferential alignment of PbS along one preferential ⟨110⟩ direction. (e) GaAs(110), showing the perpendicular [11̅0]GaAs and [001]GaAs directions. (f) Corresponding SEM image supporting preferential alignment along these directions as confirmed by cross sectional TEM.
layer. The TEM image in Figure 2a depicts the high defect density of the films, with characteristic X-ray rocking curve broadening >1500 arcsec. Crystallographic analysis of the experimental diffraction patterns shown in Figures 1 and 2 confirmed that film/ substrate orientation relations in all studied cases have a parent/twin relationship. This is further confirmed in the superimposed film/substrate (twin/matrix) stereographic projections for the cases PbS/GaAs(111), PbS/GaAs(100), and PbS/GaAs(110) shown in parts a, c, and e of Figure 3, respectively. Small red and yellow circles represent parent and twin poles, correspondingly, while green circles indicate overlapping positions of twin and matrix planes. Pole indices for the matrix are indicated above the circle and for the twin
beneath the circles. The large circles on the projections correspond to zones of reflections appearing in the diffraction patterns. The zone axes are indicated on the stereographic projections by dashed line circles. Furthermore, real space illustrations of the twin−matrix interface visualizing the complex interplay between composition plane, twin plane, and twin direction are shown in Figures 3b,d,f. The superimposed stereographic projections underline the existence of the (111) twin plane with [11̅ 2̅ ] twinning direction in all studied cases. It can be easily seen that for the case of PbS(111)/GaAs(111) shown in Figure 3a, the mutual position of the parent and twin poles on the large circle corresponds to the [11̅0]PbS∥[101̅]GaAs zone axes and perfectly fits the positions of the corresponding reflections in the diffraction 4010
dx.doi.org/10.1021/cg3004883 | Cryst. Growth Des. 2012, 12, 4006−4011
Crystal Growth & Design
Article
orientation, similar to the case of Al twinning in Al-TiN multilayers.18
pattern (Figure 1b). For the cases of PbS/GaAs(100) and PbS/ GaAs(110), conventional twinning was accompanied by additional rotation about the PbS zone axis as indicated in Figure 3c,d and Figure 3e,f, respectively. Examination of the PbS(110)/GaAs(100) sample from the [101]PbS∥[011̅]GaAs zone axes suggests that the mutual positions of the parent and twin poles on the corresponding large circle perfectly fit the positions of the corresponding reflections in the diffraction pattern (Figure 2b). However, further inspection of the PbS(110)/GaAs(100) structure from the [111̅]PbS∥[001]GaAs zone axes reveal that 19.47° rotation about the PbS zone axes is required in order to sustain twin relations. In agreement with this prediction, a misorientation of 19° between (111)PbS and (111)GaAs was observed in Figure 2c. This is depicted in the real space schematic illustration of the PbS/GaAs interface in Figure 3d. Similarly, crystallographic analysis of the PbS(112)/ GaAs(110) pair along the [11̅0]PbS∥[11̅0]GaAs zone axes indicates perfect match in the positions of corresponding reflections in the diffraction pattern (Figure 2e), while examination along the PbS[111̅]∥ GaAs[001] zone axes shows that additional rotation of 15.8° about the PbS zone axis is required in order to match the experimental diffraction pattern (Figures 2e and 3f). Figure 4a shows 3D real space schematic illustrations of the GaAs substrate depicting the hypothetical atomic arrangement on the GaAs(111) surface. As can be seen, the (111) planes are unique in the zinc blende (F43̅ m) structure. Furthermore, it should be noticed that atomic and subatomic arrangement in all three ⟨110⟩ in-plane directions in the GaAs(111) surface are identical. Therefore, no preferential alignment along a singular in-plane direction is expected, as can be seen in the corresponding SEM image in Figure 4b. This is not the case for the GaAs(100). The 3D real space schematic illustration shown in Figure 4c indicates different atomic arrangement of the surface (striped red spheres) when viewed along the two perpendicular ⟨110⟩ directions due to the directional subsurface atomic layer in tetrahedral sites of the fcc-based zinc blende GaAs substrate (green spheres). Again, examination of the surface topography in the SEM images of PbS films confirms preferential alignment of the ⟨110⟩PbS direction with only one of the two possible ⟨110⟩GaAs directions in the (100)GaAs surface (Figure 4d).9 In the case of GaAs(110), there is only one ⟨110⟩GaAs in-plane direction as indicated in the 3D real space schematic illustration in Figure 4e, therefore preferential alignment along this crystallographic direction is observed in the SEM image shown in Figure 4f. It has been previously established that preferential lead absorption was obtained on the more reactive, As-terminated, GaAs(111)B face, indicating stronger affinity of Pb to As compared to Ga in GaAs.12 It is reasonable to assume that during nucleation, transformations with smallest atomic displacements and lattice strains are normally favored with respect transformations which involve greater changes. Furthermore, comparison of the radii of Ga in GaAs and Pb in PbS, point out very close proximity (ca. 1.35 Å for Ga and ca. 1.32 Å for Pb),17 therefore causing relatively small distortion of the lattice at the interface. The above-mentioned suggests that first deposition layer is comprised of Pb atoms in natural continuation of the Ga sublattice in GaAs but with the chemical species changed from Ga to Pb. However for the consecutive PbS layer, the twinned orientation seems to be preferred over the untwinned
■
CONCLUSIONS We can conclude that twin-like epitaxy is a general phenomenon in the PbS/GaAs system and not an accidental occurrence. Superimposed stereographic projections confirmed the presence of conventional twinning elements, with (111)twin plane and [1̅1̅2] twinning direction observed in all cases studied. Preferential alignment of a ⟨110⟩PbS in-plane growth direction with a ⟨110⟩ in-plane direction of the GaAs substrate was observed on all GaAs faces studied. In the cases of PbS/GaAs(100) and PbS/GaAs(110), conventional twinning was found to be accompanied by additional rotation about the PbS zone axis.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: 972-7-6461474. Fax: 972-7-6472944. E-mail: ygolan@ bgu.ac.il. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Osherov, A.; Golan, Y. MRS Bull. 2010, 35, 790−796. (2) Bode, D. E. Physics of Thin Films, Vol. 3; Academic Press: New York, 1996. (3) Herman, M. A., Richter, W. & Sitter, H. Epitaxy; Springer-Verlag: Berlin Heidelberg, 2004. (4) Pijpers, J. J. H; Ulbricht, R.; Tielrooij, K. J.; Osherov, A.; Golan, Y.; Delerue, C.; Allan, G.; Bonn, M. Nature Phys. 2009, 5, 811−814. (5) Isshiki, M.; Endo, T.; Masumoto, K.; Usui, Y. J. Electrochem. Soc. 1990, 137, 2697−2700. (6) Davis, J. L.; Norr, M. K. J. Appl. Phys. 1966, 37, 1670−1674. (7) Watanabe, S.; Mita, Y. J. Electrochem. Soc. 1969, 116, 989−993. (8) Osherov, A.; Shandalov, M.; Ezersky, V.; Golan, Y. J. Cryst. Growth 2007, 304, 169. (9) Osherov, A.; Ezersky, V.; Golan, Y. Eur. Phys. J. Appl. Phys. 2007, 37, 39−47. (10) Osherov, A.; Ezersky, V.; Golan, Y. J. Cryst. Growth 2007, 308, 334−339. (11) Golan, Y.; Margulis, L.; Rubinstein, I.; Hodes, G. Langmuir 1992, 8, 749−752. (12) Osherov, A.; Matmor, M.; Froumin, N.; Ashkenasy, N.; Golan, Y. J. Phys. Chem. C 2011, 115, 16501. (13) Hall, S.; Mcmahon, B. International Tables for Crystallography, 2nd ed.; Springer: Berlin Heidelberg, 2001. (14) Ikonic, Z.; Srivastava, G. P.; Inkson, J. C. Phys. Rev. B 1997, 55, 9286−9289. (15) Kelly, A.; Groves, G. W.; Kidd, P. Crystallography and Crystal Defects; John Wiley & Sons Inc.: New York, 2000. (16) Ikonic, Z.; Srivastava, G. P.; Inkson, J. C. Superlattices Microstruct. 1995, 17, 393−396. (17) Kraus, W.; Nolze, G. J. Appl. Crystallogr. 1996, 29, 301−303. (18) Bhattacharyya, D.; et al. Appl. Phys. Lett. 2010, 96, 093113− 093116.
4011
dx.doi.org/10.1021/cg3004883 | Cryst. Growth Des. 2012, 12, 4006−4011
