1. Introduction
In recent years, Ⅲ-V group nanowires (NWs) have attracted increasing attention because of their special geometrical characteristics, and unique electronic and optical properties for applications in nanometer-scale devices[1-5]. Among them, GaAs NWs have been extensively studied due to their direct bandgap and high optical performance, which in combination make them suitable for nano-optoelectronic devices, such as solar cells, photodetectors, and light-emitting diodes[6-10]. To date, a great deal of work has been done to grow GaAs NWs via a vapor-liquid-solid (VLS) method using Au-catalyzed[11-17] and Ga self-catalyzed[18-24] growth modes. The realization of the morphology and structure control is one of the most important issues in these studies. Careful manipulation of the morphology and structure of GaAs NWs will enhance the performance of future GaAs-based devices, including heterostructure devices and may also give access to interesting physics. Many factors affect on the morphology and structure of GaAs NWs and among them some key parameters have been tuned to control GaAs NW growth. For example, carefully selected Ga and As fluxes were employed to control the diameter and growth rate of GaAs NWs[18, 24, 25]. In the meantime, finely tuned growth temperature was shown to achieve high uniformity and desirable density of GaAs NWs[26, 27]. Different substrate orientations were also adopted for different growth directions of GaAs NWs[28-31]. In the previous work, by choosing appropriate As flux and opening and closing the Ga cell shutter during NW growth, we have obtained various combinations of wurtzite (WZ), zinc blende (ZB), and defect-section (DS) in a single GaAs NW, whose length is determined by the duration of opening or closing the Ga shutter[24]. All these aforementioned tunings for growth were made during or before GaAs NW growth. However, significantly, the cell shutter closing sequence that happens at the end of the growth could influence the morphology and structure of GaAs NWs at their tips, which is the NWs growth front. To the best of our knowledge, this important factor for GaAs NW growth has not been systematically investigated yet.
In this paper, self-catalyzed GaAs NWs are grown on Si (111) substrates by molecular-beam epitaxy (MBE). At the end of the GaAs NW growth, the Ga and As cell shutters are closed with different sequences and the effect of the cell shutter closing sequence on the morphology and structural phase of self-catalyzed GaAs NWs is investigated. When the growth of GaAs NWs is terminated by closing the Ga and As cell shutters simultaneously and by closing the Ga cell shutter first and then closing the As cell shutter 1 minute later, we find that the respective obtained GaAs NWs with and without Ga droplets can both keep vertical growth, and their structural phase transitions at the top of NWs follow the triple-phase-line (TPL) shift mode. GaAs NWs are also grown on Si (111) substrates with different Ga flux existing times after closing the As cell shutter. We find that these cell shutter closing sequences result in the growth direction of GaAs NW changing from [111] to non-[111]. Illustrations of the morphology evolution of GaAs NWs are provided and the structural phase transition at the end part of these GaAs NWs is also investigated, which confirms that the TPL shift mode is available even for the growth terminated with different Ga flux existing times. Our work will provide useful information for better understanding of the growth mechanism and realizing the morphology and structure control of GaAs NWs.
2. Experiments
GaAs NWs were grown in a solid source MBE system (VG 80). Commercial p-type Si (111) wafers were used as the substrates. Before being loaded into the MBE chamber, the Si substrates were pretreated by chemical etching. At first, we removed the native oxidized layer completely using a HF solution (5%). Then, the substrates were coated with a new oxidized layer by dipping the Si substrate in a solution of H
The morphology and crystal structure of the GaAs NWs were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL2100 operated at 200 kV). The chemical compositions of the GaAs NWs were investigated by X-ray energy-dispersive spectroscopy (EDX). For TEM analysis, GaAs NWs were removed from the growth substrate via sonication in ethanol and then drop-cast onto lacey carbon grids.
3. Results and discussion
To investigate the effect of the cell shutter closing sequence on the morphology and structure of GaAs NWs, we began our work by growing GaAs NWs on Si (111) substrates and the NW growth was terminated by closing the Ga and As cell shutters simultaneously. Figs. 1(a) and 1(b) show 25

Next, we grew GaAs NWs on Si (111) substrates and the growth was stopped by closing the Ga cell shutter 1 minute before closing the As cell shutter. This cell shutter close sequence is expected to consume the Ga droplet completely. Fig. 2(a) shows a 25
GaAs NWs were also grown on Si (111) substrates with different Ga flux existing times after closing the As cell shutter. That is to say, at the end of the GaAs NW growth, we closed the As cell shutter first and then closed the Ga shutter after several seconds. Figs. 3(a)-3(c) show the 25

In order to understand the growth process of the top sections of GaAs NWs grown with different Ga flux existing times after closing the As cell shutter, the morphology evolution diagrams of GaAs NWs are shown in Figs. 4(a)-4(d). Fig. 4(a) shows an evolution process of GaAs NWs with the Ga and As cell shutters closed at the same time. As GaAs NWs are still in an As-rich atmosphere after closing the Ga and As cell shutters, Ga droplets are consumed a bit, which makes the TPL pass through the edges between the top and side facets of the NWs, and DS and WZ crystal structure appear as discussed in Fig. 1(f). As shown in Fig. 4(b), if we close the As cell shutter first and keep the Ga cell shutter open for 20 s, the Ga droplet will become too large to be stable and slide down to the sidewalls of vertical GaAs NWs gradually, and nucleation happens along one of the non-vertical directions. Further extending Ga flux existing time to 60 s, the length of the non-vertical GaAs NW on the new direction becomes longer (Fig. 4(c)). The V/Ⅲ BEP ratio of GaAs NWs was changed to 9 with the same procedure as Fig. 4(b), the Ga droplet also became too large to be stable and slide down to the NW sidewalls gradually as shown in Fig. 4(d). However, a stable Ga droplet on the top of the GaAs NW will form soon and then crystallize quickly due to the high V/Ⅲ BEP ratio, which keeps a vertical tip for the NWs. After closing the Ga cell shutter, the Ga droplet would be exhausted gradually under the As-rich atmosphere, as shown in Figs. 3(d) and 4(d).

As discussed above, GaAs NWs' growth direction and morphology can be tuned by varying the Ga flux existing time after closing the As cell shutter. To investigate the structure information of these NWs and confirm the growth direction of kinked GaAs NWs, TEM observations were carried out. Fig. 5 shows the TEM images for GaAs NWs grown with a V/Ⅲ BEP ratio of 7.6 and Ga flux existing time for 60 s after closing the As cell shutter (corresponding SEM image is shown in Fig. 3(c)). The rectangles in Fig. 5(a) highlight the regions where the HRTEM images were recorded. Figs. 5(b)-5(g) show the HRTEM images taken from the regions A, B, C, D, E and F in Fig. 5(a), respectively. As shown in Fig. 5(b), the body part of GaAs NW is grown along the [111] direction (vertical to Si (111) substrate). Figs. 5(c) and 5(e) are the HRTEM images of the corner part (B and D sections) of the GaAs NW, and we can see that the growth direction of the GaAs NW changes from [111] to non-[111]. The angle between the [111] and non-[111] directions is 70.5

Figs. 6(a)-6(c) show the TEM images of a typical GaAs NW grown with the V/Ⅲ BEP ratio of 9 and Ga flux existing time for 20 s after closing the As cell shutter (its SEM images are shown in Fig. 3(d) and Fig. 4(d)). As can be seen from Fig. 6(a), the diameter changes obviously at the top part of NW, which is consistent with the SEM results above. The Ga droplet has been consumed completely due to the high V/Ⅲ BEP ratio and As-rich atmosphere. Fig. 6(c) depicts a HRTEM image corresponding to the section of NW marked with the red rectangle as shown in Fig. 6(b). It is found that almost the whole GaAs NW presents a ZB structure except the top part, where there is a structural phase transition from ZB to WZ and finally to ZB again. The structural phase transition near the end of NW can be explained by the TPL shift mode[24]. For a better understanding of the TPL shift mode in this case, illustrations of the effect of the Ga droplet size on the crystal structure of the GaAs NW are shown in Fig. 6(d). The nucleation mechanism is based on the shift of the TPL position and nucleation sites along the NW during the whole growth process. The shift of the TPL position is related to the evolution of the shape and volume of the Ga droplet at the end of NW growth[24]. It is well known that when the droplet only wets the top facets of the NWs, TPL nucleation favors the WZ phase growth in the standard VLS mechanism[24, 35], and a pure ZB structure is favored when the side-walls of the NWs are wetted by Ga droplets due to the low liquid-vapor surface energy of Ga droplets[19, 24, 36, 37]. In our case, GaAs NWs present the ZB structure due to the TPL being on the side-wall instead of on the top of the NW (Fig. 6(d)). When the As cell shutter is closed and Ga flux still exists, the volume of Ga droplet will become larger gradually, which makes the Ga droplet unstable. TPL nucleation is not suitable at this moment and thus the ZB structure forms (Fig. 6(e)). The Ga droplet will become stable again for the large diameter of the NW (Fig. 6(f)). After 20 s, the Ga flux is also stopped, and the Ga droplet begins to be consumed under the As-rich atmosphere. The TPL retreats along the side-wall of the NW, generating a transition layer consisting of defects, until it reaches the top of the NW (Fig. 6(g)). When the TPL shifts to the top facet and the Ga droplet wets only the top facet, the WZ structure is formed due to the TPL nucleation (Fig. 6(h)). When the droplet becomes small to some degree it will move along the NWs' top facet leading to a ZB phase again which is because the unconstrained nucleation on the flat surface as well as the possible breakdown of the condition for TPL nucleation (Fig. 6(i))[24, 33 35]. Thus the structural transition at the top part of the NW can be explained by the TPL shift mode.

4. Conclusion
In conclusion, self-catalyzed GaAs NWs are grown on Si (111) substrates by MBE. At the end of GaAs NW growth, the effect of different closing sequences of the Ga and As cell shutters on the morphology and structural phase of the GaAs NWs is investigated. After closing the As cell shutter, several Ga flux existing times have been tried and it is shown to be critical for GaAs NWs growth by changing the growth direction from [111] to