The iodate ion IO3–
very effectively injects charge into Si through the five-electron process described in Table
. The induced etching is highly exothermic and rapid. HIO3
concentrations in the range of 5
M have all been found to produce rapid etching.
Etching in fluoride solutions leads to the formation of the hexafluorosilicate ion SiF62–
bubbles by the reaction scheme
For brevity, (aq) is dropped. Thus, as mentioned above, salts containing Na or K should be avoided as a source of IO3– to avoid precipitation. Using NaIO3, we could easily induce precipitation of Na2SiF6 with sufficiently extensive etching. Alkali metal hexafluorosilicates are white powders.
Observing the etching of Si in HIO3
HF, one sees bubble formation on the crystal accompanied by the formation of a rough-looking surface and dark particulates. Rinsing the crystal in ethanol releases the particulates and causes the solution to take on the typical red coloration of a tincture of iodine. Clearly, I2
is being formed during etching. Figure
displays a typical SEM micrograph of a Si substrate etched in an aqueous solution of HIO3
/HF. The surface is rough and pitted with no indication of the formation of a nanoporous layer.
Figure 1 SEM micrograph of Si(100) substrate etched in 0.002M HIO3 in 48% HF for 30min.
The precipitation of I2
can be avoided by the addition of ethanol to the etchant. Ethanol addition significantly reduced the etch rate of most stain etchants
]; however, that was not a problem for the highly reactive iodate system. Bubble formation and roughening were again observed. In this case, the etchant gradually turned red, reached a maximum intensity, and then became clear. At this point, the bubbling also ceased. I2
was being produced by the reduction of IO3–
; however, it was also being consumed by a secondary reaction. The dissolution and etch rates can be enhanced by performing the etching with simultaneous ultrasonic agitation.
As shown in Figure
, significant surface restructuring is possible with HIO3
HF etching. The surfaces are extremely rough and have a gray to black appearance. The lack of visible photoluminescence and the absence of a significant Si-H peak in the infrared absorption spectrum indicate the absence of a nanoporous layer. The surface features are largely uncontrollable. The same sample can exhibit regions covered with pyramids, as shown in Figure
a when the substrate is (100) oriented. These pyramids have smooth faces, but in between, there is significant roughness and a significant number of circular pits 45 to 110
nm in diameter. Other regions will exhibit rectangular features as shown in Figure
b. As the micrograph in panel (d) shows, these roughly 150-nm features are pits rather than pores. If the substrate orientation is switched to (111), triangular rather than rectangular pits are found. This is to be expected for anisotropic etching with etch rates that are dependent on surface orientation as found in the alkaline etching of Si
]. Other regions exhibit corral-like structures (on both (111) and (100) orientations) with nested pits within pits and significant roughness, as shown in panel (c). Regions such as those shown in panels (b) and (c) or the between-pyramid regions might be labeled por-Si; however, because they do not exhibit a depth significantly longer than their width, large surface area (i.e., the lack of significant Si-H IR absorption), nor photoluminescence, we refrain from designating them as por-Si.
We believe that IO3–-induced etching is capable of producing nanoporous silicon. This generates a great deal of surface area, which enhances the reactivity of the Si substrate toward other species. This facilitates such major restructuring of the surface because the chemistry of iodine-containing species in solution is extremely complex, and it is impossible to separate IO3–-induced chemistry from that induced by I–, I2, and I3–. As we will show below, all of these species are capable of etching Si. The first two lead to smooth isotropic etching, and the last leads to anisotropic etching. The combination and balance of these chemistries with IO3–-induced etching leads to the great variety of surface features observed.
There are two possible production paths for I2
. The first is the reduction of IO3–
listed in Table
. The second is reaction (2)
which occurs spontaneously when IO3–
are mixed. A relatively small amount of I2
can be lost to adsorption on the surface
]. Two possible sinks for I2
, apart from simple precipitation of I2
(s), are a thermally initiated etch reaction involving I2
as well as a fluoride species (because I2
in solution by itself does not etch silicon in solution at room temperature, see below),
1 or 2 and y
4 or 2) and the reaction with iodide to form triiodide,
The stoichiometry of reaction (3) is notional. By a thermal reaction, we intend to imply that it is initiated by the chemical reaction of I2
dissociative adsorption on the surface of silicon as opposed to a hole injection step. Subsequent steps involve some unidentified combination of fluoride species. Because the first step does not involve hole injection, this reaction would not be constrained by quantum confinement effects to be self-limiting
] and would, therefore, be capable of destroying por-Si. It should be noted that all silicon tetrahalides are unstable in water and subject to reactions of the type
Iodide production can occur from the reduction of I2
listed in Table
(though the rate of this should be low based on the E°
value), the reduction of triiodide,
or an alternative iodate reduction reaction,
The chemical reactions of Cl2
, and I2
, when exposed as gases to Si surfaces, are known to preferentially attack step sites
]. In semiconductor processing, this reaction is carried out at high temperatures; nonetheless, an analogous reaction in solutions would be expected to destroy por-Si. The high surface area and defect-laden surfaces of por-Si would be much more susceptible to such reactions in comparison to a polished silicon surface. Therefore, it is not surprising that the production of halogens in solution should lead to the destruction of por-Si. That Br2
destroys por-Si was previously reported by Kelly and co-workers
Now, we turn to the etch chemistry of I–
, and I3–
with polished Si and por-Si surfaces. To do so, we made up a series of solutions from HI, I2
, or NaI
. These were dissolved either in water or in water/ethanol solutions and exposed to mirror finish Si(100) and Si(111) surfaces. The Si surfaces were hydrogen terminated as their oxide layers were stripped by etching first in HF. Alternatively, a layer of por-Si was produced with either a FeCl3·
HF or a V2
HF solution. The layer exhibited a uniform blue or green color indicative of a homogeneous por-Si film. Immediately after being made and rinsed, these layers were exposed to this set of solutions. No etching of either the polished Si surfaces or the por-Si films was noted. Iodide, iodine, and triiodine solutions do not react with either flat, defect-free surfaces or the high-surface-area, defect-laden surfaces of por-Si when HF is not also added to the solution. This result is consistent with the work of Haber et al.
] who showed that I2
/methanol solutions can lead to the formation of Si-I and Si-OCH3
bonds; however, they gave no indication for an increase in surface area or other signs of etching. That iodide-containing but HF-free solutions can affect the surface of silicon or por-Si but do not etch the surface is also consistent with other reports of solar cell characteristics and flatband potentials
] or photoluminescence properties
] that shift upon exposure to iodide- or triiodide-containing solutions.
We investigated the etch behavior of solutions containing I–
, and I3–
to which HF has also been added. Neither HI
HF nor I2
HF solutions exhibit appreciable reactivity with polished Si surfaces. We note no bubbling on the faces of the crystals, and the surfaces retain their mirror finishes after rinsing. Occasionally, a few small bubbles may appear after many minutes on scratches or the edges of the crystals. This indicates that there is some reactivity with defects but very low reactivity with well-ordered terraces. An I3–
-containing solution can roughen a polished Si surface. This is a slow process requiring about half an hour or more. As shown in Figure
, this etch process also exhibits a degree of crystallographic anisotropy. We exposed a p-type Si(100) crystal to, for example, a 0.005
M NaI in 3:1 ethanol/HF solution or an 0.004
M in I2
M in NaI in 25% HF. Most of the etch pits are circular. However, as shown in Figure
, some of the pits gradually convert to inverted pyramids, often with approximately 100-nm circular pits at their apex. The pyramids are 750
nm to 2
μm on a side. There is no evidence of nanoporous silicon on such samples. No bubbles formed during etching. This may be related to the slow etch rate.
Now, we address the etch chemistry of por-Si films with solutions containing I–
, and I3–
to which HF has also been added. HI
ethanol, and HI
HF solutions all remove por-Si layers accompanied by the formation of bubbles. The rate at which the por-Si is destroyed is dependent on the concentration of the iodine species. Interestingly, when I–
destroy por-Si, they leave behind a nearly mirror-like Si substrate. These two species, therefore, facilitate step flow etching in HF solutions. This is consistent with their lack of reactivity with the terraces of polished surfaces. I3–
-containing solutions, on the other hand, remove por-Si to reveal a rough and pitted surface much like the one shown in Figure
. Again, this is consistent with the more aggressive and anisotropic nature of I3–
-induced etching. All three of these species destroy nanoscale Si structures; therefore, the initiation step and etch rate of each reaction must not be constrained by quantum confinement.
These results are general for the other halogens and halogenates. A 0.04-M Br2
HF solution led to etching of flat Si surfaces, which remained flat after etching. If the solution was exposed to a por-Si layer, the por-Si was removed. As mentioned above, this is consistent with the work of Bressers et al.
]. A solution of 0.07
HF also etched Si. By the solution color change observed, it was clear that the reaction of BrO3–
in solution. As expected, the etching of a polished Si surface led to a flat surface after etching. Similarly, a 0.2-M KClO3
HF solution led to the etching of polished silicon, which resulted in the formation of Cl2
in solution and a flat final surface. Neither one of these oxidants etched Si with the same degree of anisotropy as observed with iodate. This may be due to the more reactive nature of Br2
as compared to I2
and the lack of a species analogous to I3–