Effect of Types of Ga / Si ( 111 ) Reconstructed Structure on Growth Morphology of GaSb Island

Ryuto Machida*, Ryusuke Toda, Keisuke Yoshiki, Shinsuke Hara, Katsumi Irokawa, Hirofumi Miki, Akira Kawazu, and Hiroki I. Fujishiro Faculty of Industrial Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan Faculty of Science and Engineering, Tokyo Denki University, Hatoyama, Saitama 350-0394, Japan


Introduction
III-V compound semiconductors integrated on Silicon (Si) have been attracted attention for the integration of electronic and photonic circuits on a single wafer [1][2][3][4].In these techniques, direct epitaxial growth technique on substrate has the advantage of low cost.Gallium antimonide (GaSb) of III-V compound semiconductors nanostructures, including thin film and quantum dots (QDs), on Si have been intensely studied due to the application as a light source, such as near-infrared lasers for optical communication, in optoelectronic integrated circuits [3][4][5][6][7][8].However, it is difficult to control the density and size of GaSb nanostructures on clean Si surface because of Volmer-Weber (VW) growth mode resulting from a large lattice mismatch between them.It is important to understand the details of the formation process and the growth morphology at the atomic level.In previous study, as the result of using the Ga/Si(111)-√3×√3 reconstructed surface as a template for epitaxial growth, the high-density and uniformly-sized GaSb islands were formed [9].It suggests that the Ga-induced Si(111) reconstructed structures affect the growth morphology of GaSb island.
In this study, we have studied the effect of several types of the Ga/Si(111) reconstructed structure, which is used as a template for epitaxial growth, on the growth morphology of GaSb islands by ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and non-contact atomic force microscopy (NC-AFM).

Experimental
STM observations and sample treatments were performed in a UHV-STM system (JEOL JSTM-4500VT) at a base pressure of 2.0 × 10 −8 Pa.The samples (1.0 × 6.9 × 0.4 mm 3 ) were cut from a phosphorus-doped n-type Si(111) wafer with a resistivity of 2.0 Ω•cm.The substrate temperature was measured above 600 °C using a pyrometer.A clean Si(111) surface was obtained by flashing at 1200 °C for approximately 1 min after outgassing at 600 °C for 12 h.Ga and Sb atoms were evaporated from Knudsen cells.The deposition rate of each atom was estimated by STM observations of the Ga/Si(111) and the Sb/Si(111) reconstructed surfaces [10,11].The deposition rates of Ga and Sb were kept at 0.3 and 2.8 monolayer (ML)/min, respectively (1.0 ML = 7.85 × 10 14 atom/cm 2 ).The Ga/Si(111)-√3×√3, -6.3×6.3,-11×11, and -6.3√3×6.3√3,which were used as the template for epitaxial growth of GaSb, were formed by the Ga deposition at 0.30, 0.45, 0.60, 0.90 ML on a clean Si(111) surface at 550 °C, respectively[Fig.1] [12,13].GaSb was grown on each reconstructed surfaces by the simultaneous deposition of Ga and Sb atoms at 350 °C which is a suitable temperature for the formation of the high-density and uniformly-sized GaSb islands on the Ga/Si(111)-√3×√3 reconstructed surface [9].The surfaces were kept under Sb flux for 60 seconds after the growth of GaSb.STM observations in UHV were carried out at room temperature using chemically etched W-tips.NC-AFM observations of the samples removed from the UHV system were carried out in air by a scanning probe microscopy (SPM) system (SII SPI3800N).

Growth morphology of GaSb island on Ga/Si(111) structures
Figure 2 shows the NC-AFM images of GaSb islands grown on the template surfaces where (a) the √3×√3-, (b) 6.3×6.3-,(c) 11×11-, and (d) 6.3√3×6.3√3-Gawere formed.On the √3×√3-Ga, the polygonal islands were spread over a wide surface area [black arrow in Fig. 2(a)], and the large dome-shaped island was rarely formed [white arrow in Fig. 2(a)].As the result of GaSb growth on the 6.3×6.3-Ga of which the coverage was slightly higher than that of the √3×√3-Ga, the small dome-shaped islands were formed [white arrow in Fig. 2(b)] along with the polygonal islands over a wide surface area [black arrow in Fig. 2(b)].Further increasing of a Ga coverage of the template Ga/Si(111) surface, large dome-shaped islands were formed with scattering [white arrows in Figs.2(c) and (d)].
It was reported that the hexagonal-and pyramid-shaped GaSb islands were formed on the Ga/Si(111)-√3×√3 at the growth temperature of 350 °C [9].Taking the same growth condition into account, it is indicated that the polygonal islands shown in Fig. 2(a) correspond to the hexagonaland pyramid-shaped GaSb islands.However, as increasing the Ga coverage of the template Ga/Si(111) surface, the number of the polygonal islands decreases.Instead, the dome-shaped islands were formed and the size of the islands increases.These results indicate that the growth morphology is changed by the types of the template Ga/Si(111) surface.

Growth process of GaSb island on Ga/Si(111) structures
Figures 3 and 4 show the STM images of GaSb on the Ga/Si(111)-11×11 and -6.3√3×6.3√3depending on the coverage, respectively.On the 11×11-Ga, the two-dimensional (2D) layer composed of a disordered pattern with inverted triangular protrusions was widely formed at 1.5 ML of GaSb [Fig.3(a)].Although the 2D layer was also observed as the result of slightly increase of the coverage [Fig.3(b)], the high 2D islands with ordered protrusions were locally formed [inset in  The 2D layer composed of a disordered pattern with the inverted triangular protrusions corresponds to the initial GaSb layer [14,15].On the Si(111) surface, GaSb grows up to second layer with same pattern.On the other hand, the high 2D island shown in Fig. 3(b) corresponds to the third GaSb layer where the strained-GaSb(111)-2×2 structure is formed with the dislocation network [15].The third GaSb layer also has the residual compressive strain by the lattice mismatch, and plays a role of a strong nucleation site of the dome-shaped island to relieve the strain [15].These STM results indicate that (1) the flat 1 st and 2 nd GaSb layers are widely formed, (2) the 3 rd GaSb layer is formed on the local surface area, and (3) the dome-shaped island abruptly grows on the local 3 rd layer, on the 11×11-and 6.3√3×6.3√3-Ga.By contrast, GaSb on √3×√3-Ga begins to grow from the small domain of the initial GaSb layer and the faceted island is formed as the result of the continuation of the layer-by-layer (multilayer) growth [9].The abrupt nucleation, which results in the dome-shaped island, is rarely observed on the above 3 rd GaSb layer.Therefore, it is  suggested that the difference of the template Ga/Si(111) structures affects the morphology of the initial GaSb layer and the difference of the initial growth state changes the strain relaxation mechanism of the island growth process.As mentioned above, it is important to clarify the initial growth state in order to understand the difference of growth process of GaSb between on the √3×√3-Ga and other Ga-induced reconstructed surfaces.Figure 5 shows the STM images of the initial growth state of GaSb on the Ga/Si(111), which was the surface formed by exposing only Sb atoms on the √3×√3-[Fig.5(a)] and 6.3√3×6.3√3-Ga[Fig.5(b)] at 350 °C.On the √3×√3-Ga, two types of the pattern were observed [Fig.5(a)].One was the zigzag line pattern, and the other was the cross-hatched pattern.The cross-hatched pattern was separated by the zigzag line pattern, and formed the local domain.On the other hand, the only cross-hatched pattern was observed on the 6.3√3×6.3√3-Ga[Fig.5(b)].The cross-hatched pattern was also observed at the lower terrace.The zigzag line pattern and cross-hatched pattern correspond to the Sb/Si(111)-2×1 reconstructed structure and initial GaSb layer, respectively [14,16].These results indicate that the initial GaSb layer is formed by the reaction between Ga atoms deposited on the surface and adsorbed Sb atoms.However, Ga/Si(111)-√3×√3 is formed by a coverage of about 0.3 ML [12].This coverage is expected to be poor to cover the surface by the initial GaSb layer.Therefore, the 2×1-Sb is formed by the adsorption of Sb atoms on the bare Si(111) surface which is the outside area of the local initial GaSb layer domain formed by the thermal diffusion.On the other hand, the Ga/Si(111)-6.3√3×6.3√3 is formed by at a coverage of about 1.0 ML [17].Considering that the cross-hatched pattern was also observed at the lower terrace, the 2 nd GaSb layer was also formed on 6.3√3×6.3√3-Ga.Therefore, the coverage of 0.9 ML Ga is plenty to cover the surface by the initial GaSb layer.It is inferred that these differences of the initial growth state would affect the growth process and morphology of GaSb island.
Based on these results, we discuss the growth process of GaSb on the Ga/Si(111) surface in Fig. 6.On the Ga/Si(111) surface induced by a low Ga coverage (i.e.,√3×√3-Ga), the first GaSb layer forms many small domains with the separation by the 2×1-Sb structure [Fig.6(a)].Because the GaSb layer is not formed on the 2×1-Sb [18], the GaSb layer continues to grow on the small domain of the first GaSb layer.When GaSb grows until third layer, the surface structure is changed to that of the intrinsic GaSb(111) [15].On this layer, the internal compressive strain by the lattice mismatch should be induced.However, the size of the 2D island (3 rd GaSb layer) is small because the growth of the island is restricted by the initial layer.On the small 2D island, it is expected that the strain would be slightly released to the lateral direction by the stretch of the atom arrangement on the third layer [Fig.6(b)].Therefore, GaSb on the Ga/Si(111) surface induced by a low Ga coverage continues to grow in a multilayer growth without the abrupt nucleation (i.e., the dome-shaped island) due to weak internal strain [Fig.6(c)].As the result of this growth process, the small faceted islands are formed with high density.On the other hand, on the Ga/Si(111) surface induced by a high Ga coverage (i.e., 11×11-and 6.3√3×6.3√3-Ga), the 1 st and 2 nd GaSb layers were widely formed without the domain by the separation with the 2×1-Sb structure [Fig.6(d)].The 3 rd GaSb layer, which has the structure of the intrinsic GaSb(111), can be formed with the comparatively large size because the size restriction by the initial layer does not exist [Fig.6(e)].The more large compressive strain could be accumulated because the stretch of the atomic arrangement is difficult.The abrupt nucleation would occur on the 3 rd GaSb layer, and the dome-shaped island is formed to release the internal strain [Fig.6(f)].The density of the dome-shaped island does not increase because of the adsorption atoms converging to the dome-shaped island.
It is suggested that the template Ga/Si(111) structures affect the growth morphology of GaSb island by the above growth processes.In addition, the previous study showed that the substrate temperature also affected the growth morphology of GaSb islands on Ga/Si(111) and only dome-shaped islands were formed by substituted Sb atoms on Si(111) above 400 °C [9].Therefore, it is important (1) to prepare the template Ga/Si(111) formed by a low Ga coverage (about 0.3 ML) and ( 2) to heat the substrate at the appropriate temperature (about 350 °C) in order to form the high-density and uniformly-sized GaSb islands.

Conclusion
The effect of several types of the Ga/Si(111) reconstructed structure, which is used as a template for epitaxial growth, on the growth morphology and process of GaSb islands have been studied.On the √3×√3-Ga, the polygonal islands were spread over a wide surface area.On the other Ga-induced reconstructed surfaces, the dome-shaped islands were formed.As increasing Ga coverage of the template Ga/Si(111), the size of the dome-shaped island increases.On the 11×11-and 6.3√3×6.3√3-Ga, the dome-shaped island was abruptly grown on the GaSb 2D island (i.e., the 3 rd GaSb layer) locally formed on the flat initial GaSb layers (i.e., the 1 st and 2 nd layer).These growth processes of GaSb island was different from that on the √3×√3-Ga.At the initial growth state, the first GaSb layer forms many small domains with the separation by the 2×1-Sb structure on the √3×√3-Ga, while the 1 st and 2 nd GaSb layers were widely formed on the 6.3√3×6.3√3-Ga.It is suggested that these initial growth states affect the growth process and growth morphology of GaSb island.It is important to prepare a template Ga/Si(111) surface at about 0.3 ML Ga and to heat the substrate at about 350 °C during GaSb deposition in order to form the high-density and uniformly-sized GaSb islands.

Fig. 6 .
Fig. 6.Schematic illustrations of the growth process of GaSb on the Ga/Si(111) surface induced by (a)-(c) a low and (d)-(f) a high Ga coverages.