_xH_(2x+2). Byway of examples, the silicon precursor can be or include one or more of silicon tetrachloride (SiCl₄), trichloro-silane (SiCl₃H), dichlorosilane (SiCl₂H₂), monochlorosilane (SiClH₃), hexachlorodisilane (HCDS), octachlorotrisilane (OCTS), a silicon iodide, or a silicon bromide.

In accordance with other examples, the silicon precursor can include an amino-based precursor, such as hexakis(ethylamino)disilane (AHEAD), SiH[N(CH₃)_2]3(3DMASi), BDEAS (bis(diethylamino)silane), di-isopropylaminosilane; or an oxysilane-based precursor, such as tetraethoxysilane Si(OC₂H₅)₄. In some cases, the silicon precursor can include two or more precursors, such as, for example, a halogenated precursor (e.g., a silicon halide compound noted above) and a silane precursor.

Exemplary dopant precursors include typical n-type and p-type dopants, such as arsine, phosphine, and the like.

During step 104, first silicon-containing nanoparticles are provided to the substrate surface. As the nanoparticles contact the substrate surface, islands of first silicon-containing material may form on the substrate surface and eventually coalesce into a continuous first silicon-containing epitaxial layer. The first silicon-containing epitaxial layer may be monocrystalline and serve as a template for further epitaxial layers (e.g., formed during step 106 and as repeated during loop 108). In some cases, the entire first layer may be monocrystalline. In some cases, the first layer can form as non-monocrystalline material in some areas.

A thickness of the first silicon-containing epitaxial can be between greater than zero or about 2 nm and about 10 nm, between about 2 nm and about 20 nm, or between about 2 nm and about 100 nm.

Step 106 of forming a second silicon-containing epitaxial layer overlying the first silicon-containing epitaxial layer by providing second silicon-containing nanoparticles can be similar to step 104, except a composition of the first silicon-containing epitaxial layer differs from a composition of the second silicon-containing epitaxial layer. The differing compositions of the first silicon-containing epitaxial layer and the second silicon-containing epitaxial layer can allow one of the layers to be selectively etched relative to the other of the first silicon-containing epitaxial layer and the second silicon-containing epitaxial layer. Step 106 can include, for example, forming a silicon germanium layer.

As with step 104, step 106 can include forming the second silicon-containing nanoparticles in situ (e.g., using a second plasma-enhanced chemical vapor deposition process) or providing the second silicon-containing nanoparticles to the reaction chamber. Further, step 106 can include initially forming an island of second silicon-containing material, which can coalesce to form a monocrystalline second silicon-containing epitaxial layer.

Step 106 can be performed in the same reaction chamber used during step 104. Alternatively, step 106 can be performed in another reaction chamber, such as another reaction chamber in the same cluster tool as the reaction chamber used during step 104 or of another reactor system. A temperature and/or pressure for step 106 can be the same or similar to the temperature and/or pressure described above in connection with step 102.

When forming the second silicon-containing nanoparticles, the second silicon-containing nanoparticles can be formed in a manner similar to the manner described above. For example, a silicon precursor, a dopant precursor (if any), a germanium precursor, and optionally an inert gas and/or a reactant can be provided to the reaction chamber, and a plasma can be formed within the reaction chamber to thereby form nanoparticles comprising second silicon-containing material, such as silicon germanium. In accordance with other examples of the disclosure, other silicon compositions or alloys may be formed or used as the first or second silicon-containing nanoparticles or the first or second silicon-containing epitaxial layer.

The silicon precursor, reactant (if any), and noble gas can be the same or similar to those described above in connection with step 104. Exemplary germanium precursors include germanes, such as germane (GeH₄), digermane (Ge₂H₆), trigermane (Ge₃H₈), or germylsilane (GeH₆Si), and halogen compounds, such as GeBr₄or other suitable germanium-containing precursor. By way of particular examples, the germanium precursor can include one or more of germane and germanium tetrachloride (GeCl₄) or GeCl_xH_4-x.

The plasma power and partial pressure of the silicon precursor can be as noted above. Further, during step 106, one or more of the silicon precursor, any reactant, any noble gas, and/or the plasma power can be pulsed as described above. Further, the substrate or substrate support can be biased—or not—as noted above.

When forming second silicon-containing nanoparticles comprising silicon germanium, volumetric flow rates and flow ratios of the silicon precursor and the germanium precursor can vary in accordance with desired composition of the second layer.

Exemplary second silicon-containing epitaxial layers can include about 70 to about 80 atomic percent, or about 70 to about 90 atomic percent, or about 70 to about 99 atomic percent silicon and/or about 1 to about 30 atomic percent, or about 5 to about 20 atomic percent, or about 15 to about 30 atomic percent or about 20 to about 30 atomic percent germanium. In some embodiments, the germanium content within the second silicon-containing epitaxial layer may not be constant, but rather may be varied, such that the germanium content (and/or other component) may have a graded composition within the first layer. The grading can be accomplished by, for example, manipulating a flowrate of one or more of the silicon precursor and the germanium precursor during in situ formation of the second silicon-containing nanoparticles.

A thickness of the second silicon-containing epitaxial layer formed during step 106 can be between greater than zero or about 2 nm and about 4 nm, between about 2 nm and about 10 nm, between about 2 nm and about 20 nm, or between about 2 nm and about 100 nm.

In some cases, a dilution gas or an inert gas, or a reactant gas, such as hydrogen, can be provided to the reaction chamber during one or more of steps 104 and 106. In some cases, the dilution or inert or reactant gas can be continuously flowed to the reaction chamber during steps 104, 106 and loop 108.

As illustrated in FIG. 1, method 100 can include purge 110 after step 104 and/or purge 112 after step 106. Purge steps 110, 112 can be affected either in time or in space, or both. For example, in the case of temporal purges, a purge step can be used, e.g., in the temporal sequence of providing a purge gas to the reaction chamber, wherein the substrate does not move. In the case of spatial purges, a purge step can include moving a substrate from a first location, through a purge gas curtain, to a second location.

Method 100 can also include step 116 of etching the superlattice to form a superlattice feature. By way of examples, the superlattice can be etched using a reactive ion etch, such as a reactive ion etch employing a SF