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Recently, nanowires (NWs) with a one-dimensional (1D) columnar shape have gained great attention and have been envisioned as nanoscale materials for next-generation technology with good functionality, superior performance, high integration ability and a potential for low cost. [] Due to those advantages, the limitations, caused by the requirement of lattice and thermal expansion coefficient in the traditional thin film epitaxial technique, can be greatly alleviated, giving more freedom in material combinations and substrate choice. Moreover, the NW geometry can offer more freedom in the band structure engineering and new theories for the device structure design. For example, quantum dots (QDs) can be grown within the NW by stacking the material axially. The formation of these dots does not need the assistance of strain, which is different from the tradition Stranski鈥揔rastanow QDs. Therefore, QDs can be fabricated with a wider range of materials. When made into devices, the NW structure can also have superior performance as compared with the traditional thin film devices.

Traditionally, the NW growth was conducted by using Au catalyst which is a fast-diffusing metal that signi铿乧antly impairs the properties of semiconductors. In our group, self-catalyzed nanowires (NWs) have been developed by using a solid-source molecular beam epitaxy. NWs with a wide range of materials have been demonstrated with high quality, such as GaAs, GaAsP, GaAsSb, InAs, InAsP, InAsSb.

The band gap of the ternary material GaAsP can cover wavelengths ranging from green (550 nm) to near infrared (860 nm) at room temperature, which is one of the most promising III鈭扸 compound semiconductors for photovoltaics and visible light emitters. Thus, the photovoltaics and visible light emitters are two major projects of our GaAsP NWs. 听It has been predicted that a two-junction tandem SC, consisting of a 1.7 eV GaAsP NW junction and a 1.1 eV Si junction, has a theoretical efficiency of 33.8% at 1 sun AM1.5G and 42.3% under 500 suns AM1.5D concentration. If this 1.7/1.1 eV two-junction device is used in water splitting, it is potential to achieve an efficiency of 27.0%.

The growth of GaAsP NWs has been achieved on unpatterned Si substrates with a large Ga flux window. As can be seen in Figure 1, GaAsP NWs are homogeneous in diameter over the whole length. There is a droplet at the very tip of the NW, which clearly indicates the VLS growth mode. For the VLS growth mode, the droplet size decides the NW diameter if there is no VS lateral sidewall growth. Therefore, the homogeneous diameter along the NW length reflects that the Ga droplet size was constant during the growth. This further reveals the well balanced replenishment and consumption of the Ga in the droplets under this V/III ratio. []

Figure 1. (a) 35掳 tilted view SEM image, (b) side view SEM image and (c) length summation histogram of GaAsP NWs. (d) 25掳 tilted view SEM image, (e) side view SEM image and (f) length summation histogram of GaAs NWs. The inset in (c) and (f) are the diameter summation histograms of each sample.

The crystal quality of the NWs was checked by TEM measurement. As can be seen in Figure 2, those NWs are uniform in diameter along the NW length. At the tip of the NW, there is a round droplet, which shows that those NWs were grown by Ga-catalyzed VLS mode. Those NWs have a ZB crystal structure and is almost defect-free at the body part, as confirmed by the electron diffraction pattern shown in the insets. Defects are present at the very tip and very bottom parts. [2,]

Figure 2.TEM image of a GaAsP core NW. The inset is the electron diffraction pattern.

The P/(P+As) in the NW as a function of that in the flux was shown in Figure 3. As can be seen, by adjusting the P/As flux ratio, the GaAsP NWs with a P content ranging from 10% to 75% can be achieved. Moreover, the incorporation coefficient of P was found to be about three times as big as that of the As. This is in stark contrast to the thin film planar epitaxial growth by gas-source or solid-source MBE. []

Figure 3. Red dots are P content in NWs as a function of P/(P + As) beam flux ratio. The blue and pink dots show the planar GaAsP film growth by solid-source and gas-source MBE, respectively.

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By controlling the P composition along the NW length, the growth of defect-free dot-in-wire structure was also achieved (Fig. 4). Without any surface protection layer, an exciton emission line width as narrow as 130 渭eV has been observed. []

Figure 4. (Left) Angular dark-field TEM image of the area containing a GaAs QD in the GaAsP nanowire. Both the GaAs QD and the surrounding GaAsP nanowire are zinc-blende structure without any stacking fault, twinning, or polytype. (Right) the 渭-PL spectra of a GaAs QD.

On the core NW, a layer of shell with the same composition was grown. The nominal structure is a GaAs_0.8P_0.2听core with a shell of the same composition. As can be seen in Figure 5a, the NWs have very smooth on side facets and highly regular tip. The same as the core NW, the defects are mainly concentrated at the very tip and bottom part, leaving the body part almost defect free. As confirmed by the dark field TEM and annular dark field scanning TEM (ADF-STEM) measurement shown in Figure 5b and c, the segment presented is completely defect-free. The shell has the zinc blend crystal structure which is copied from the core NW. [4,]

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Figure 5. (a) 25掳 tilted SEM image, (b) Conventional ADF-TEM image and (c) High-magnification annular dark field image of core-shell GaAs_0.8P_0.2听NWs. Inset in (b) is electron diffraction pattern.

To achieve better position control of the NWs, patterned Si substrates were used.听 However, the growth of NWs on patterned Si substrates has a long term issue of achieving good yield and repeatability. We found for the first time that this issue is caused by the native oxide and/or SiO₂听residue prior to NW growth. By introducing an in-situ thermal cleaning and a Ga pre-deposition technique, the VLS growth of NWs with uniform morphology has been achieved for the first time. Figure 6a and b shows a SEM image of the core-shell GaAsP NWs. The core-shell NW array has uniform morphology with smooth sidewalls. Figure 6c shows the room-temperature emission from the NWs with peak intensity at ~740 nm. This reveals their good crystal quality and demonstrates their potential application in photovoltaics, visible emitters, and photonic crystals. []

Figure 6. SEM images (tilt angle = 25掳) of positioned (a) core and (b) core-shell NW arrays. (c) Room temperature PL spectrum of the core-shell NW arrays.听听听听听听听听听听

The single nanowire solar cells (SNWSC) has been fabricated with those GaAsP NWs and the results are shown in Fig. 7 and table I. The SNWSC achieved an efficiency of 10.2%, which set a world record for the single NW SC. This demonstrates the great potential of GaAsP NWs in making high efficiency SC. []

Figure 7. (a) I-V characteristics of the SNWSC under dark (blue dots) and AM1.5G illuminated conditions (red dots). The black line is a fit to the data. Inset is the scanning electron microscope image of the SNWSC device (b) P鈥揤 result of the SNWSC.

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TABLE I.听 Summary of the results from the SNWSC

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The application of the GaAsP NWs and the effect of InAsP passivation in the water splitting have been investigated and shown in Fig. 8. A wafer-scale solar-to-hydrogen conversion efficiency of 0.5% has been achieved for the GaAsP NW photocathode with a low surface coverage of ~1.8 脳 10⁸听cm^鈭�2. []

Figure 8. (a) The steady-state current density of the photocathodes measured at 0.1 V versus RHE under AM 1.5G illumination. (b) Current density potential characteristics of GaAsP NW photocathode.

The growth of high quality GaAsSb NWs has also been demonstrated, which can be seen in Fig. 7. These NWs are defect free and have zinc blend crystal structure.

Figure 9 SEM image of GaAsSb NWs.

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In-based NWs have significant advantages due to the high mobility and small & direct bandgap, which make them promising in areas such as high-mobility transistors, infrared photodetectors, and Thermophotovoltaic (TPV). We have demonstrated the growth of high-quality InAs (Fig. 8a), InAsP (Fig. 8b) [], and InAsSb (Fig. 8c) [] NWs on Si substrates.

Figure 10. SEM image of (a) InAs, (b) InAsP, and (c) InAsSb NWs.

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Upon incorporation of antimony, we observed a drastic change in the crystal structure polytype from wurtzite dominant in the InAs nanowires (17% ZB) to almost pure zincblende in the InAs0.85Sb0.15 (99% ZB) (Fig. 9). We observed a significant increase of the domain length and field-effect mobility as the stacking fault density in the InAs1鈭抶Sbx nanowires decreased. [10]

Figure 11. (a鈥揹) High-resolution TEM characterization of catalyst-free InAs and InAs_1鈥搙Sb_x听nanowires with increasing antimony content. Twins (T), stacking faults (intrinsic I and extrinsic E) and grain boundaries (G) are indicated in the figures. White arrows show the growth direction. The scale bar is 5 nm for all images. (e) Percentage of zinc-blende structure in the InAs and InAs_1鈥搙Sb_x听nanowires as a function of antimony content. (f) Defect density in the InAs and InAs_1鈥搙Sb_x听nanowires as a function of antimony content. The black diamonds represent the total number of defects including twins, stacking faults, and grain boundaries. Error bars represent plus or minus one standard deviation.

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