Vapor Liquid Solid Growth of Si Wires from SiH 4
3.4 Results from VLS Growth with SiH 4
Taking all of the above considerations into account, Al, Au, Ga, In, and Sn were explored as VLS catalysts with a SiH4 precursor. As noted above, our catalyst search was limited in this case by the inability to reach temperatures higher than ∼600 ◦C in our SiH4 LPCVD system.
A thermal Low Pressure Chemical Vapor Deposition (LPCVD) system was used, with silane (SiH4) as the vapor-phase reactant, and, in some experiments, hydrogen (H2) as a carrier gas. Metal seed particles were created either lithographically using a lift-off process, or simply by thermal evaporation of a thin (5 nm) layer of metal that dewetted from the substrate during deposition to form an array of nanoparticles.
As expected, Au was the best VLS catalyst of the group Al, Au, Ga, In, and Sn. Growth such as depicted in Fig. 3.3 could be achieved with little trouble.
Sn is apparently not thermodynamically favorable as a VLS catalyst for Si, [66] and we saw no wire growth with this catalyst (not shown).
Preliminary experiments with Al catalysts typically yielded thin shells at the edges of the seed particles rather than wires (see Fig. 3.4). Also, the growth rate was much slower than typically seen in the VLS process. This is potentially attributable to the aluminum
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Figure 3.3. Typical example of Si wires grown from Au catalyst with SiH4 precursor. In this case the growth occurred at a total pressure P = 1 Torr, flow rate of 100 sccm, using 5% SiH4 in Ar (i.e., SiH4 partial pressure of 50 mTorr), and temperature T≈ 500 ◦C, for 180 mins. The catalyst was a 5 nm thick evaporated Au film, on a Si(111) substrate. Fig.
(b) is a more detailed image of the same sample as shown in (a).
oxide that forms very rapidly on the Al surface and acts as a barrier to VLS growth. [79]
Evaporation of a thick layer of In on top of the evaporated Al, without breaking vacuum, in an attempt to prevent the Al from oxidizing, did not produce better results (not shown).
For these reasons Al was soon abandoned.
As noted above, Ga is extremely soluble in Si and wires grown with this catalyst were found to be extremely conical, presumably due to the catalyst particle dissolving as the wires grew (Fig. 3.5).
Of the catalysts tried with SiH4, aside from Au we have achieved suitable wire mor- phologies only with In. Representative images of our fabrication results from In catalysts are shown in Fig. 3.6. The wire density as well as the range of wire diameters (∼100 nm to >1 μm) and lengths of many tens of μm to over 100 μm were in the size range desired for wire array solar cell applications - positive signs for the use of In as a catalyst for this application. However, as indicated in the figure, the uniformity of growth across a wafer from unpatterned In was extremely poor, with the In appearing to migrate on the Si(111) surface prior to initiation of wire growth. This typically results in clusters of dense wire growth with large areas of no wire growth in between.
We explored variations in pressure P, temperature T, SiH4 flow rate, and catalyst thickness (deposited by thermal evaporation) in order to optimize growth conditions with
Figure 3.4. Example of Si wires grown from Al seed particles using a pure SiH4 precursor.
Growth was conducted at a temperature T ≈ 600 ◦C, at pressure P = 100 mTorr, and (undiluted) SiH4 flow of 40 sccm, for 60 mins. The catalyst was a 5 nm thick Al film, photolithographically defined to discs of various diameters. Shown are two typical growth morphologies, (a) mostly growth occurred in shells at the very edges of the Al discs, (b) in the few places where wire growth was observed, it was very patchy and the wires tended to be very short and kinked.
Figure 3.5. Typical example of Si wires grown from Ga catalyst with SiH4 precursor. In this case the growth occurred at a total pressure of P = 30 Torr, at a flow rate≈300 sccm with 5% SiH4 in Ar (i.e. SiH4 partial pressure of 1.5 Torr) and temperature T ≈ 500 ◦C, for 180 mins. The catalyst was a 5 nm thick evaporated Ga film, on a Si(111) substrate.
Fig. (b) is a more detailed image of the same sample as shown in (a).
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Figure 3.6. Typical example of Si wires grown from In catalyst with SiH4 precursor. In this case the growth occurred at pressure P = 1 Torr, with a pure SiH4 flow rate of 20 sccm, H2 flow rate of 20 sccm, and temperature T = 500 ◦C, for 60 mins. The catalyst was a 5 nm thick evaporated In film, on a Si(111) substrate. Fig. (b) is a more detailed image of the same sample as shown in (a).
both Au and In as catalyst, using 5% SiH4 in Ar as the growth precursor.
For the Au catalyst, wire morphology improved with increasing substrate temperature and decreasing SiH4 partial pressure in the range T = 300 - 600 ◦C and SiH4 partial pressure P = 0.05 - 1 Torr (in agreement with [39]). Best results were achieved with substrate temperatures of 600 ◦C and SiH4 partial pressure of 0.05 Torr. Flow rate was varied between 40 and 200 sccm but this had relatively little effect on wire morphology.
The thickness of the evaporated Au film also had relatively little effect.
Optimal growth was found to occur at lower T and higherP for In than for Au. For In, if we exceeded substrate temperatures of∼ 500 ◦C we saw non-selective growth forming a very rough film, rather than wire growth. We attribute this to surface diffusion of the In particles upon the Si surface, as well as the high vapor pressure of In relative to Au, [81] [82]
leading to substantial loss of In at high temperatures. When we grew at partial pressures significantly below 1 Torr we saw the growth rate drop and we saw negligible growth after three hours at partial pressures of 0.1 Torr and less. Best results were achieved with substrate temperatures of 500◦C and SiH4 partial pressure of 1 Torr.
A phase diagram for In-catalyzed VLS growth of Si wires under these conditions is presented in Fig. 3.7. As already noted, the uniformity of growth across a wafer from unpatterned In was extremely poor. Despite this, the images show the most typical mor-
Figure 3.7. Partial phase diagram of growth of Si nanowires from In catalyst particles as a function of pressure and temperature. In all cases the growth was achieved with SiH4 diluted to 5% concentration in Ar, with a flow rate of 100 sccm, for three hours. In most cases, catalyst particles were formed by the dewetting of a nominally 5 nm thick evaporated film of In, but the results of growth were found to be fairly relatively insensitive to catalyst layer thickness.
phology.
3.4.1 TEM of SiH4-grown Wires
We have also performed transmission electron microscopy (TEM) on Au-catalyzed, SiH4- grown wires. Firstly samples were grown by taking a p-Si(111) wafer (International Wafer Service), cleaning in RCA 1 solution (5:1:1 H2O:H2O2:NH4OH at 70 ◦C) [83] for 10 mins, etching in buffered HF (Transene), followed by thermal evaporation of 5 nm of Au. Growth was then performed in our LPCVD chamber, at 550◦C and a total pressure of 1 Torr, with a 100 sccm flow rate of 5% SiH4diluted in Ar, for 6 hours. A fragment of the grown sample was then sonicated in 100 μL of IPA for 1 min, and 7 μL of the resulting suspension was dropcast on a lacey carbon grid (SPI Supplies). TEM was then performed by Dr. Carol Garland using a Tecnai F30 transmission electron microscope.
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We observed the expected VLS catalyst droplet at the tip of the wire, and smooth-walled wire with near-circular cross-section from the tip for severalμm towards the base. However, beyond that we see the cross-section become hexagonal, and sawtooth “sub-facets” become increasingly prominent (Fig. 3.8). Also, there is a large amount of Au on the wire surfaces, as seen in Fig. 3.9. The wire cores do appear to be, however, single crystalline (Fig. 3.10).