Reaction intermediates and their respective kinetics influence the overall growth rate of a thin film by CVD. The reaction chemistry at the surface is becoming more important as growth temperatures are lowered into regimes where the rate of film growth is limited by the desorption rate of a product. Germane and digermane, two of the most commonly used commercial precursors, were examined and these studies are summarized below. These two precursors were used to provide a foundation on which to build our later investigations. Our attention then turned to the surface reactions of substituted germanes. We are interested in understanding the important chemical features needed for a molecule to be a successful precursor for chemical vapor deposition. Each of the precursor systems that we have examined are summarized below.
Germane: We have studied the adsorption and decomposition of germane on the Si(100) surface. Germane has a relatively low sticking probability, ~10-3 at 110 K and the adsorption step is precursor-mediated. The molecule dissociatively chemisorbs on Si by cleavage of a Ge-H bond to yield an adsorbed germyl group and an adsorbed hydrogen atom. The germyl groups decompose at higher surface temperatures by the sequential loss of hydrogen to the silicon surface, so that by 600 K all of the hydrogen resides on the silicon surface as Si—monohydrides (SiH groups). Complete transfer of hydrogen to the Si surface from decomposing germyl groups is observed for the lowest surface coverages. At higher GeH3 surface coverages (high GeH4 exposures), site blocking becomes important as the germyl groups decompose and the hydrogen thermal desorption exhibits states from a distribution of both Ge and Si surfaces sites.
We also determined that the presence of small amounts of preadsorbed Ge can strongly affect the desorption kinetics of hydrogen from the decomposition of GeH4. The presence of preadsorbed Ge shifts the majority of H2 desorption into the lower temperature state (Ge—influenced sites) and desorption from the Si-influenced sites is greatly reduced.
Digermane: Ge2H6 is the other Ge-hydride system that we examined. The focus of this study was to determine the decomposition mechanism of this molecule on Si(100). This study was originally undertaken so see if a Si surface covered with only GeH3 could be generated by simple cleavage of the Ge-Ge bond. We determined that digermane adsorbs molecularly at a surface temperature of 110 K. Upon warming the surface to between 150 and 200 K, the digermane molecule dissociates by Ge-Ge bond scission. This is easily observed in ultraviolet photoelectron spectroscopy experiments, as evidenced by the removal of the Ge-Ge bonding molecular orbital with heating. The GeH3 covered Si surface decomposes at temperatures above 200 K by the sequential loss of hydrogen from GeH3 to the Si surface, in a similar mechanism to what was observed for the GeH4 system. Desorption of hydrogen from the surface at higher temperatures is dominated by two desorption states that can be related to desorption from either Ge surface sites or Si sites. There is also experimental evidence for desorption of H2 from sites that involve both Ge and Si atoms.
Tetramethylgermane: We have examined the interactions of tetramethylgermane, Ge(CH3)4, with Si(100) at 110 K. Ge(CH3)4 adsorbs molecularly at this low temperature. Heating the Ge(CH3)4 covered surface results in both the desorption and decomposition of this molecule. The products of the surface reaction are adsorbed CH3 and Ge(CH3)3. Ge(CH3)3 desorbs at higher surface temperatures in a broad desorption state centered near 250 K. Thus, all the germanium-containing portions of the molecule desorbs from the surface at low temperature. The remaining surface methyl groups are stable to about 600 K and decompose to adsorbed carbon and hydrogen. The kinetics of methyl decomposition were measured by static secondary ion mass spectrometry (SSIMS) and a pseudo-first order preexponential of 1±5•108 s-1, and an activation energy of 29±1 kcal mol-1 were obtained. These values represented, to our knowledge, the first kinetic measurements for methyl decomposition on Si(100).
Diethylgermane: We have also investigated the mechanism for the surface decomposition of (C2H5)2GeH2 (or GeH2Et2) on Si(100). The purpose of this study was to explore the utility of using GeH2Et2 as an alternative precursor for GeH4 or Ge2H6 in CVD applications. This strategy requires that atomically clean Ge be deposited at the surface without the incorporation of carbon during film growth. The results of our study look encouraging. We are able to deposit submonolayer quantities of Ge from GeH2Et2 without carbon retention at the surface.
We have obtained detailed information about the surface mechanism for the decomposition of GeH2Et2. The adsorption of GeH2Et2 at 110 K on Si(100) is dissociative and the initial decomposition step involves the cleavage of the Ge-C bonds with transfer of the ethyl groups to the Si surface. At higher temperatures, the surface ethyl groups undergo b-hydride elimination to produce gas phase ethylene. It is this process of ethylene production that we believe reduces the possibility of carbon retention at the surface. The GeH2 that is produced by the cleavage of the Ge-C bonds transfers hydrogen atoms to the Si surface in a manner similar to what we observed earlier for GeH4 and Ge2H6. This surface process results in the clean deposition of Ge (within our detection limits of XPS and SIMS) with the evolution of H2 and C2H4 into the gas phase. The adsorption of GeH2Et2 at 110 K and the heating to 1000 K that results in the deposition of Ge is a self-limiting reaction. In other words, we are able to deposit a maximum of 0.25 ML of Ge per reaction cycle in this manner. This self-limiting behavior may have important implications in atomic-level processing of ultrathin Ge structures.
Monoethylgermane: As an extension of the GeH2Et2 studies, the adsorption and decomposition of GeH3Et was explored. GeH3Et contains one less ethyl group compared to GeH2Et2 and this property led to a higher coverage of Ge in a given reaction cycle. This is due to fewer steric interactions at the surface compared to GeH2Et2. A maximum coverage of 0.31 ML of Ge per reaction cycle was observed without carbon incorporation.
Triethylgermane: The site blocking effect
of surface ethyl groups was extended to this system also. The
presence of three ethyl groups within the triethylgermane reduced
the efficiency of germanium deposition per reaction cycle.