Fig 5.23 shows the results of sputter-depositing CdS and both CdS and ITO layers onto CdTe NW arrays grown on CdTe/Mo substrates. Fig 5.23a shows an array of NWs and Fig 5.23b shows the tip of an individual NW following sputter deposition of CdS. Images of FIB milled cross sections, of core-double shell ITO/CdS/CdTe NWs are shown in Fig 5.23c and d.
Following sputter coating of CdS the NW arrays remained intact (Fig 5.23a). Moreover, the NWs were completely coated with CdS, both radially around the NW and longitudinally
105 along the NW axis – no visibly bare regions were observed. However, there was some
evidence of shadowing occurring, as towards the base of some tilted NWs, the thickness of the shell on the underside of the NW appeared to be thinner than on the top side. Sputter
deposition of ITO also coated the NWs completely. Elemental contrast in the cross-sectional images of the core-double shell structures show that the core and shell regions are well
defined. Fig 5.23c shows an example of a NW coated uniformly; the CdTe core is ~ 200 nm in diameter, and the CdS and ITO shells are ~ 200 nm and ~ 100 nm thick respectively.
However, the uniformity of the shells was inconsistent, as shown in Fig 5.23d. Here, the CdS and ITO shells are much thicker on one side of the NWs than the other.
Fig 5.23: SEM images of core-shell structures generated by sputtering of CdS and ITO/CdS onto CdTe NWs. a) and b) CdS coated CdTe NWs. The CdS layer is rough and covers both the NW and catalyst tip (b). c) and d) Sputtered double-shells of ITO and CdS on CdTe NWs. The shell structures were frequently non-uniform. The images are of FIB-milled cross-
sections, with the NWs having been held in place on a Si substrate by Pt.
5.5.5 Discussion
The suitability of each of the deposition methods for the generation of shell coatings is now discussed, as are the details of some of the structures observed.
106 CBD is considered to be unsuitable for shell growth due to the damage the process
inflicted on the NW arrays, despite its ability to generate highly uniform coatings.
MOCVD is also considered to be unsuitable due to the highly non-uniform growth of both CdZnS and CdS. The dendritic morphology of the CdZnS growth is undesirable within the context of the NW solar cell (described in Chapter 1), and whilst CdS deposition did not yield dendritic growth, it failed to completely coat the CdTe NWs, leaving bare regions – this would result in shunts in a solar cell.
Nevertheless, the nature of the secondary NWs, observed for CdZnS growth, is now discussed. Most notably, the removal of the Zn from the growth procedure quenched
secondary NW growth, which implies that Zn may be responsible for their nucleation. Indeed, the hexagonal lattice constants of these structures, obtained from TEM analysis, agreed with reported values for ZnS. Although ZnS typically adopts the zinc-blende phase at low
temperature, wurtzite phase ZnS NWs have indeed previously been grown at just 180°C by hydrothermal synthesis - growing preferentially along the c-axis, catalyst free 51. They have also been VLS-grown by pulsed laser vaporization at 950°C using Au catalysts 52. Here, the absence of catalysts at the secondary NW tips suggests the VLS mechanism was not
responsible.
It is likely that the secondary NW growth was crystallographically driven, as they are arranged in a number of distinct rows around the NW circumference – either 3, 4 or 6 rows. As the primary CdTe NWs adopt different growth directions, then they will have different cross-sectional shapes; this being investigated in greater detail in Chapter 6. A <112> NW for example could have a rectangular cross-section, defined by two {110} facets and two {111} facets. Accordingly, this would provide four separate growth directions, which are
perpendicular to the primary NW growth axis, for secondary NWs to adopt. Indeed, in Fig 5.22b there appears to be 4 definite rows of secondary NWs, each rotated around the primary NW by 90° with respect to its neighbor, and so we assume the primary wires’ axis is <112>. NWs growing in a <111> direction on the other hand may have a hexagonal cross-section, with 6 sidewall facets from which secondary NWs could grow. The more pillar-like
morphology of these structures, obtained after a longer growth time (Fig 5.22b) is attributed to lateral growth.
Sputter deposition is considered to be the most suitable method for the generation of shell layers, as it did not disrupt the NWs themselves, and resulted in the complete coating of the CdTe NWs with CdS, although in some cases the coatings were thicker on one side of the NWs than the other. The non-uniformity is attributed to shadowing effects, this being
107 supported by the observation that the side of the NWs on which the layers were thickest is the same for both CdS and ITO layers. A further benefit to sputter deposition is that the CdS and ITO layers can be deposited sequentially without breaking vacuum in the system used here.