Ion/molecule reactions in SiH4/H2S and GeH4/H2S mixtures.

The gas phase ion chemistry of silane/hydrogen sulfide and germane/hydrogen sulfide mixtures was studied by ion trap mass spectrometry (ITMS), in both positive and negative ionization mode. In positive ionization, formation of X/S (X = Si, Ge) mixed ions mainly takes place via reactions of silane or germane ions with H(2)S, through condensation followed by dehydrogenation. This is particularly evident in the system with silane. On the other side, reactions of H(n)S(2)(+) ions with XH(4) (X = Si, Ge) invariably lead to formation of a single X-S bond. In negative ionization, a more limited number of mixed ion species is detected, but their overall abundance reaches appreciable values, especially in the SiH(4)/H(2)S system. Present results clearly indicate that ion processes play an important role in formation and growth of clusters eventually leading to deposition of amorphous solids in chemical vapor deposition (CVD) processes.

Gas-phase ion chemistry of GeH(4)/B(2)H(6) mixtures.

The gas phase ion-molecule reactions in positively and negatively ionized germane/diborane mixtures have been studied by ion trap mass spectrometry. Reaction sequences and rate constants for the most interesting processes have been determined. In positive ionization, formation of Ge-B bonds exclusively occurs through condensation reactions of B(n)H(m)(+) ions with germane, followed by H(2) or BH(3) loss. No reactions of ions from germane with B(2)H(6) were observed under the experimental conditions used here. In negative ionization, the Ge(n)H(m)(-) (n = 1, 2) ion families react with diborane to yield the Ge(n)B(p)H(q)(-) (p = 1, 2) ions, again via dehydrogenation and BH(3) loss, while diborane anions proved to be unreactive. In both positive and negative ionization, Ge-B ions reach appreciable abundances. The present results afford fundamental information about the intrinsic reactivity of gas-phase ions and provide valuable indications about the first nucleation steps ultimately leading to amorphous Ge and B-doped semiconductor materials by chemical vapor deposition methods.

Negative gas-phase ion chemistry of silane: A quadrupole ion trap study.

Silicon clusters are of considerable interest for their importance in astrophysics and chemical vapour deposition processes, as well as from a fundamental point of view. Here, we present a quadrupole ion trap study of the self-condensation ion/molecule reactions of anions of silane. In the high-pressure regime, several ion clusters are formed with increasing size: the largest ions detected are Si5Hn- (n = 0-3). Selective ion isolation and storage allowed detection of the main reaction sequences occurring in the reacting system. The most frequent condensation step is followed by single or multiple dehydrogenation, this latter being particularly observed for the high-mass reactant ions. As a consequence, the most abundant ions in the mass spectra are those with a low content of hydrogen, namely Si2H-, Si3H-, and Si4H-. These results are discussed with reference to literature data on silicon cluster anions and related systems.

Gas-phase ion chemistry in XH4--C3H4--ZH3 (X==Si, Ge; Z==N, P) mixtures.

The gas-phase ion chemistry of silane-allene-ammonia, germane-allene (or propyne)-ammonia (or phosphine) systems was studied by ion trap mass spectrometry. Reaction sequences were determined and rate constants were measured for the main processes observed. The mixture containing silane displays higher reactivity with respect to that with germane. Comparison with analogous systems provides useful information about the reactivity of different hydrocarbon molecules and the different affinities of silicon and germanium towards nitrogen and phosphorus. The most interesting product ions observed are those containing Si (or Ge), C and N (or P) elements together, as these ion species may be considered precursors of doped amorphous carbides, which are widely used in semiconductor devices.

Gas-phase ion chemistry of the allene-phosphine and silane-allene-phosphine systems.

The gas-phase ion chemistry of allene-phosphine and silane-allene-phosphine mixtures was studied by ion trap mass spectrometry. Rate constants of the main processes were measured and compared with the collisional rate constants to determine the reaction efficiencies. For the binary mixture, the highest yield of C- and P-containing ions is obtained with a 1 : 1 partial pressure ratio among the reagents. In the ternary mixture, formation of ion species containing Si, C and P together is mainly achieved in reactions of Si/P ions with allene, with a lower contribution from reactions of Si/C and C/P ions with phosphine and silane, respectively. The formation of ternary ion clusters is related to their possible role as precursors of amorphous silicon carbides doped with phosphorus, obtained by deposition from properly activated silane-allene-phosphine mixtures.

Gas-phase ion chemistry in the ternary silane-propyne-phosphine system.

The gas-phase ion chemistry of propyne-phosphine and silane-propyne-phosphine mixtures was studied by ion trap mass spectrometry. For the binary mixture, the effect of different partial pressures of the reagents on the yield of C and P-containing ions was evaluated. Reaction sequences and rate constants were determined and reaction efficiencies were calculated from comparison of experimental and collisional rate constants. In the ternary silane-propyne-phosphine systems, the reaction pathways leading to formation of Si(m)C(n)P(p)H(q) (+) ions were determined and the rate constants of the most important steps were measured. For some ion species, selected by double isolation procedures (MS/MS), the low ion abundances prevented determination of the reaction rate constants. Si, C and P-containing ions are mainly produced in reactions of Si(m)P(p)H(q) (+) ions with propyne, while the reactivity of the Si(m)C(n)H(q) (+) ions towards PH(3) and of the C(n)P(p)H(q) (+) ions towards SiH(4) is very low. The formation of hydrogenated Si--C--P ions is interesting for their possible role as precursors of amorphous silicon carbides doped with phosphorus, obtained in a single step, by deposition from properly activated silane-propyne-phosphine mixtures.

A quadrupole ion trap and Fourier transform ion cyclotron resonance concerted study of the kinetics of an ion/molecule association reaction: a chemometric approach.

The effects of different experimental parameters on rate constant measurements performed by mass spectrometry were investigated with a two-level fractional factorial design. This chemometric technique allows a study of the effects of selected factors and of their interactions on the response of an experiment by performing a limited number of analyses. The selected factors were: sample pressure, energy of the ionising electrons, reaction time and ionisation time. In this work, two mass spectrometric techniques were compared: Fourier transform ion cyclotron resonance (FT-ICR) and quadrupole ion trap (QIT) mass spectrometries. Experimental results were obtained from a study of a reaction system consisting of the condensation between triethylphosphite and its fragment ion (CH(3)CH(2)O)(2)P(+). Apparent bimolecular rate constants are clearly larger when determined by QIT than by FT-ICR, because of collisional stabilisation of the adduct ion by helium buffer gas introduced in the QIT spectrometer. However, the QIT rate constant extrapolated to zero helium pressure is almost identical to the FT-ICR value; this supports the conclusion regarding the buffer gas effect. Minor effects evidenced by the chemometric method were attributed to the sample pressure and to the reaction time.

Gas-phase ion chemistry in the ternary SiH4-C3H6-NH3 system.

The gas-phase ion chemistry of propene-ammonia and silane-propene-ammonia mixtures was studied by ion trap mass spectrometry. As far as the binary mixture is concerned, the effect of different molar ratios of the reactants on the trend of ion species formed was evaluated, the ion-molecule reaction processes were identified and the rate constants for the main processes were measured. The results were compared with the collisional rate constants to determine the reaction efficiencies. In the ternary silane-propene-ammonia mixture the mechanisms of formation of Si(m)C(n)N(p)H(q)(+) clusters were elucidated and the rate constants of the most important steps were measured. For some species, selected by double isolation (MS/MS), the low abundance of the ions allowed us to determine the reaction paths but not the rate constants. Ternary ions are mainly formed by reactions of Si(m)C(n)H(q)(+) ions with ammonia, whereas a minor contribution comes from reactions of Si(m)N(p)H(q)(+) ions with propene. On the other hand, the C(n)N(p)H(q)(+) ions showed a very low reactivity and no step leading to ternary ion species was identified. The formation of hydrogenated ternary ions with Si, C and N has a basic importance in relation to their possible role as precursors of amorphous silicon carbides doped with nitrogen obtained by deposition from silane-propene-ammonia mixtures properly activated.

Gas-phase ion chemistry in germane-propane and germane-propene mixtures.

Germane-propane and germane-propene gaseous mixtures were studied by ion trap mass spectrometry. Variations of ion abundances observed under different partial pressure ratios and mechanisms of ion-molecule reactions elucidated by multiple isolation steps are reported. In addition, the rate constants for the main reactions were experimentally determined and compared with the collisional rate constants to obtain the reaction efficiencies. The yield of ions containing both Ge and C atoms is higher in the germane-propene than in the germane-propane system. In the former mixture, chain propagation takes place starting from germane ions reacting with propene and proceeds with the formation of clusters such as Ge(2)C(4)H(n) (+) and Ge(3)CH(n) (+).

Gas-phase ion chemistry in the ternary SiH(4)-C(3)H(6)-PH(3) system.

Propene-phosphine and the silane-propene-phosphine gaseous mixtures were studied by ion trap mass spectrometry. For the binary mixture the variation of ion abundances under different partial pressures and the mechanisms of ion-molecule reactions are reported. Moreover, the rate constants of the main processes were measured and compared with the collisional rate constants to determine the reaction efficiencies. In the ternary silane-propene-phosphine mixture the mechanisms of formation of Si(m)C(n)P(p)H(+)(s) ion clusters were elucidated, but the complexity of the system and the low abundances of the ions usually isolated by successive steps prevented the determination of rate constants. The hydrogenated ternary ions are mainly formed by reactions of Si(r)P(s)H(+)(t) ions with propene, whereas a minor contribution comes from reactions of Si(m)C(n)H(+)(p) ions with phosphine. The C(v)P(w)H(+)(z) ions show very low reactivity with silane. The formation processes of these species are discussed in relation to their possible role as precursors of amorphous silicon carbides doped with phosphorus obtained by deposition from properly activated silane-propene-phosphine mixtures.

Negative ion clusters in self-condensation of GeH4.

Formation of negative ion clusters from GeH4 has been studied as a function of germane pressure, in the 25-450 mTorr range, by chemical ionisation mass spectrometry. At the lowest pressures, only the GeHn- (n = 0-3) ion family is formed, whilst at increasing pressures GemHn- (m = 1-9) ion clusters of increasing size are observed in the mass spectra. A variable contribution of the ions with different hydrogen content is observed as a function of the pressure of germane in all the GemHn- (m = 1-9) clusters. Increasing pressures induce a general increase of ion species with a low content of hydrogen atoms. In fact, at 450 mTorr, 38% of the ion current is due to the bare Gem- (m = 2-5) clusters and 83% to the sum of abundances of the GemHn- ions without hydrogen (n = 0) and with a number of hydrogen atoms not higher than the number of germanium atoms (n = 0-m). This trend suggests that a contribution of negative clusters to the deposition of the amorphous solid a-Ge:H from gaseous systems containing GeH4, activated by radiolytic methods, can enhance the formation of solids with a low hydrogen content, which show better photoelectrical properties.