Conference-ISSS-4-Formation of ordered Ga-droplets by using atomic-hydrogen assisted Ga self-gathering effect on nano-oxide patterned GaAs ( 111 ) A surface

We demonstrate a novel way for the fabrication of the site-controlled Ga-droplets on GaAs(111)A surface based on oxide patterning process. The process is combined with the atomic force microscope tip-induced local area nanooxidation and atomic hydrogen assisted oxide decomposition (AHAOD). The formation mechanism of Ga-droplets can be explained by the self-gathering effect of the Ga on the modified GaAs(111)A. Furthermore, we suggest the possibility of fabrication of ordered GaAs nanostructures by subsequent supplying of As flux on the site-controlled Ga-droplets. [DOI: 10.1380/ejssnt.2006.161]


I. INTRODUCTION
The site-control processes are key elements for the fabrication of site-controlled nanostructures (SCNS) for the application of next generation quantum nanodevices such as nano circuit elements, quantum logic devices and single-photon emitters with well-ordered nanostructures [1][2][3].
In the present work, a novel fabrication method of the SCNS is demonstrated on the GaAs(111)A surface based on AHAOD process.The site-control mechanism of Gadroplets is also discussed.

II. EXPERIMENTAL
AHAOD was carried out in a Riber 32P molecular beam epitaxy (MBE) chamber equipped with a valved arsenic cracker for the precise control of As 4 flux and a hot tungsten cracker for atomic hydrogen generation [16].The AFM-oxidation was performed in a humidity controllable chamber [16].
To prepare the atomically flat GaAs(111)A surface for the AFM-oxidation process, the undoped-GaAs buffer layer was grown on Si-doped GaAs(111)A±0.3• substrates.After native oxide desorption by conventional thermal cleaning, the substrate temperature was set to 550 • C for the growth of a 300 nm thick GaAs buffer layer.For the atomically flat GaAs surface, the GaAs buffer layer was grown by using a growth rate of 0.2 ML/s with an As 4 /Ga flux ratio of 35.The surface flatness of the GaAs buffer layer was confirmed by AFM.The AFM image showed atomically flat surface with monolayer (ML) steps.
The GaAs substrate with buffer layer was removed from the MBE chamber and mounted on the positioncontrollable AFM to fabricate nano oxide dots.AFMoxidation on the GaAs surface was achieved by applying a bias voltage between the AFM tip (n-Si cantilever) and the GaAs surface under the humidity of 60% at room temperature as shown Fig. 1(a).
In order to decompose the nano oxide dots on the GaAs(111)A surface, the substrate was transferred into the MBE chamber, where the thin native oxide layer and the nano oxide dots were decomposed by atomic hydrogen.Atomic hydrogen was generated by a hot tungsten cracker with temperature of 1400 • C and fed onto the sample surface [16].AHAOD was performed at a substrate temperature of 420 • C and at a H 2 pressure of 3 × 10 −6 Torr.The decomposition of the surface oxide structures was monitored by reflection high-energy electron diffraction (RHEED) pattern during the AHAOD process.The perfect oxide decomposition and cleaned GaAs(111)A surface were confirmed by the observation of clear (2 × 2) reconstruction.The surface quality of the GaAs(111)A after the AHAOD treatments was also confirmed by photoluminescence (PL) measurements.The PL intensity from the quantum well involved a regrown interface was not deteriorated so much when the interface was treated by AHAOD.
Through the all process, a GaAs(001) surface was also used as a reference sample.The surface structures such as oxide dots, Ga-droplets and GaAs nanostructures were confirmed by using the non-contact mode AFM measurements with carbon nanotube cantilevers [16].

III. RESULTS AND DISCUSSION
Figure 1 shows the schematic illustration of the all experimental procedures for GaAs(111)A.Fig. 1(a) shows the experimental set up for the AFM tip-induced oxidation (left) and the formation mechanism for oxide dots on GaAs surface under a bias voltage (right).Under a humid atmosphere, a thin water layer is formed on the top of the GaAs surface.By approaching an AFM tip to a substrate, a very narrow water column is formed between the tip and the substrate as shown in left hand side of Fig. 1(a).The neutral water layer includes OH − and H + ions as shown in right hand side of Fig. 1(a).When an electric field is applied between the tip and the GaAs surface, the electrical field pushes the OH − into the GaAs crystal resulting in the formation of As-and Ga-oxides.If the strength of electric fields increases, we expect that the inter-diffusion depth of the OH − ion will increase.The oxide dots are composed of Ga 2 O 3 and As 2 O x , where x = 1, 3, or 5 stands for the various oxides of arsenic [17].During the oxidation, the GaAs area under the AFM tip is changed to Ga 2 O 3 and As 2 O x accompanied with volume expansion resulting in formation of positive structure like dots.The oval feature directly under the tip in left side of Fig. 1(a) stands for the oxide structure.The sizes of oxide dots can be controlled by changing the number of reacting OH − ions due to the control of the relative humidity, bias voltage (V ), pulse width (P W ), pulse periods (P P ) and pulse duration (P D ) [16]. Figure 1(b) show subsequent decomposition of oxide dots by atomic hydrogen irradiation on GaAs (111)A surface.Surprisingly, we found the big difference in the surface morphology between (001) and ( 111)A surface after the AFM-oxidation and subsequent AHAOD process.That is, the oxide dot area of the GaAs(111)A surface changed to ordered dot-structures while those on (001) surface changed to ordered nanoholes [16].
To investigate the different results between GaAs(001) and (111)A after the AHAOD, we consider the oxide decomposition process and their productions.During AHAOD process, Ga 2 O 3 and As 2 O x can be changed to various species such as water molecules as well as As 2 , Ga 2 O, and Ga by following chemical reactions; where, the upper direction arrow (↑) stand for volatile at a certain temperature.In our experimental results, the surface reconstruction of GaAs(001 Figure 2 shows AFM images and their cross-sectional profiles of the oxide dots grown by the AFM-oxidation on (a) the GaAs(001) and (b) (111)A surfaces.To achieve uniform oxide dots on each surface, the oxidation was carried out under the conditions of the humidity (60%), bias voltage (10 V), pulse period (P P = 10 ms), pulse width (P W = 2 ms) and duration (P D = 1 s).Each surface has similar shape of oxide dots.The height and diameter of oxide dots on GaAs(001) is 3.75 nm and 87 nm, respectively.In case of the GaAs(111)A surface, the height (2.25 nm) and diameter (53 nm) of oxide dots is slightly smaller than those of the GaAs(001) surface.It can be explained by the different surface conductivity of each surface.These results show the site controlled nano oxide structures can be easily achieved by using AFM oxidation on the GaAs(001) and (111)A surfaces.
Figure 3 shows AFM images for the nanostructures (a) on GaAs(001) and (b, c and d) on GaAs(111)A surface during or after AHAOD. Figure 3(a) shows nanoholes which were formed by AHAOD process of nano oxide dots on GaAs(001) at the substrate temperature of 420 • C for 30 minutes.During the oxide decomposition, the reconstruction of the GaAs(001) surface changed from halo to β(2 × 4).After that, the GaAs(001) surface showed the atomically flat and structure free terraces with monolayer steps between nanoholes as shown in Fig. 3 (a).The mean diameter and depth for the nanoholes in GaAs(001) are 80 nm and ∼ 4.5 nm, respectively.Figures 3(b), (c) and (d) show the surface morphologies of GaAs(111)A during or after the AHAOD process.In contrast, the GaAs(111)A surface shows the nanostructures (droplet like) while the GaAs(001) surface shows the nanoholes as shown in Fig 3(d).To confirm the AHAOD process on GaAs(111)A surface, we formed two different sizes of the oxide structures on the same (111)A surface.The lateral sizes of two oxide dots are ∼ 70 nm and ∼ 50 nm (the dotted circles in Fig. 3 oxide dots).During the AHAOD process, the surface reconstruction of GaAs(111)A changed from halo to (2 × 2) structure.Just after the appearance of the (2 × 2), we stopped the AHAOD process.After that, we measured the surface morphologies by AFM as shown in Fig. 3(b) and Fig. 3(c).In case of the large oxide dot (∼ 70 nm), the oxide structure has still remain around the boundary (dotted circle) of GaAs host-crystal and initial oxide dots area, and the center of prior oxide dots area as shown in Fig. 3(b).In case of the small oxide dot (∼ 50 nm), however, the shape of nanostructure was changed from initial shape of oxide dot to a droplet image as shown in Fig. 3(c).Additionally, near the boundary (dotted circle in Fig. 3(c)) of GaAs substrate and prior oxide dot edge showed clear sunken image.For the perfect AHAOD process on GaAs(111)A, we continuously supplied the atomic hydrogen over 10 minutes with same oxide decomposition condition of the GaAs(001) surface after observation of (2 × 2) reconstruction.After that, we found the droplets on the prior oxide dots site as shown in Fig. 3(d).From these results, we assumed that the generated-Ga around oxide structure on the GaAs(001) surface should be migrated toward outside of prior oxide dots area while the Ga gathered around prior oxide dots area (self-gathering effect) on the GaAs(111)A surface due to the difference of Ga migration behavior.In our experimental result, the mean migration length of Ga on the (001) surface is 10 time longer than that of the (111)A surface, which were confirmed by droplet deposition method on Ga-rich GaAs surface.
Figure 4 shows the AFM image of GaAs nanostructure array on GaAs(111)A surface.The inter-dot distance of GaAs nanostructures is 200 nm.We clearly found that well ordered GaAs nanostructures.The nano oxide dots on the GaAs (111)A surface in Fig. 2(b) were decomposed by atomic hydrogen at the substrate temperature of 420 • C for 30 minutes without supplying As flux.After the oxide decomposition process on the GaAs(111) surface, the prior oxide dot position is replace by Ga droplet as shown in Fig. 3(d).And subsequently supplying of As 4 flux with 2 × 10 −5 Torr at the substrate temperature of 420 • C, the nanostructures (dot like) were observed on the GaAs(111)A surface as shown in Fig. 4. The prior oxide dot position was occupied by the GaAs nanostructures and the surface flatness between the GaAs nanostructures was slightly degraded compared to those without the decomposition process as shown in Fig. 2(b).It can be explained by over-cleaning effects due to the atomic-hydrogen kicked away an As or a Ga still remained near the prior position.The AFM image shows that the GaAs nanostructures on GaAs(111)A has slightly different shape compared to that of the prior oxide dots on GaAs(111)A.The mean diameter and height of the GaAs nanostructures are 60 nm and ∼ 1.75 nm, respectively.http://www.sssj.org/ejssnt(J-Stage: http://ejssnt.jstage.jst.go.jp)These results show that the GaAs nanostructure keeps dot like shape and slightly reduces the initial height of droplets accompany with 2D growth occurring at high temperature crystallization.That effect also confirmed by droplet epitaxy on flat GaAs(111)A surface at the substrate temperature of 420 • C. Finally, we suggest that the GaAs SCQDs also can be achieved by using the process.

IV. CONCLUSIONS
In conclusions, we demonstrate the experimental results on the site-control process of Ga-droplets on the GaAs(111)A surface by the combination of AFMoxidation and the AHAOD process.The formation mechanism of Ga-droplets can be explained by the generation of Ga from AHAOD and the self-gathering effect of the Ga on (111)A.In addition, we suggested the possibility for the fabrication of the highly ordered the GaAs nanostructures by combination of AFM-oxidation, AHAOD process on GaAs(111)A surface.
FIG. 1: The schematic illustration of the experimental procedures for GaAs surface.(a) The schematic figure of the oxide formation on the GaAs surface under a humidity condition (lift) and the concept of oxidation process under a bias voltage (right).(b) The schematic figure of a Ga-droplet formation due to the excess Ga gathering effect during the AHAOD process on GaAs(111)A.(c) The GaAs nanostructure formation by supplying of As flux into the Ga-droplet on GaAs(111)A.

FIG. 2 :
FIG. 2: The AFM images and their cross-sectional profiles of the oxide dots grown by the AFM-oxidation on (a) the GaAs(001) and (b) (111)A surfaces (scale bar 200 nm).
Figure3shows AFM images for the nanostructures (a) on GaAs(001) and (b, c and d) on GaAs(111)A surface during or after AHAOD.Figure3(a)shows nanoholes which were formed by AHAOD process of nano oxide dots on GaAs(001) at the substrate temperature of 420 • C for 30 minutes.During the oxide decomposition, the reconstruction of the GaAs(001) surface changed from halo to β(2 × 4).After that, the GaAs(001) surface showed the atomically flat and structure free terraces with monolayer steps between nanoholes as shown in Fig.3 (a).The mean diameter and depth for the nanoholes in GaAs(001) are 80 nm and ∼ 4.5 nm, respectively.Figures3(b), (c) and (d)show the surface morphologies of GaAs(111)A during or after the AHAOD process.In contrast, the GaAs(111)A surface shows the nanostructures (droplet like) while the GaAs(001) surface shows the nanoholes as shown inFig  3(d).To confirm the AHAOD process on GaAs(111)A surface, we formed two different sizes of the oxide structures on the same (111)A surface.The lateral sizes of two oxide dots are ∼ 70 nm and ∼ 50 nm (the dotted circles in Fig.3(b) and Fig. 3(c) represent the initial diameter of FIG. 4: The AFM images for GaAs nanostructure array on GaAs (111)A surface.The inter-dot distance of GaAs nanostructures is 200 nm.