S.H. Li

Decomposition mechanisms of tertiarybutylarsine

Abstract As a new source compound to replace AsH 3 for organometallic vapor phase epitaxy (OMVPE) of III/V semiconductors, tertiarybutylarsine (TBAs) has the advantages of low decomposition temperatures, lower safety hazards, and low carbon contamination in OMVPE grown GaAs layers. The vapor pressure of TBAs was measured, and is given by log 10 P ( Torr ) = 7.500 − 1562.3/ T ( K ). The decomposition mechanisms of TBAs were studied in a D 2 ambient using a time-of-flight mass spectrometer to analyze the gaseous products. Although a free radical mechanisms would seem the most likely, it is not the dominant route for decomposition. Instead, unimolecular processes are the preferred pathway. Two such reactions are proposed. The major step is intramolecular coupling yielding AsH and isobutane. At higher temperatures β-elimination becomes important, producing AsH 3 and isobutene. The reactions are catalyzed by GaAs surfaces, but not by silica. The temperature dependence of the reaction rates was studied, and Arrhenius parameters for the rate constants are given.

Decomposition mechanisms of trimethylgallium

Abstract The thermal decomposition of trimethylgallium (TMGa) has been studied in a variety of carrier gases, using a time-of-flight mass spectrometer to analyze the products and obtain kinetic information. N 2 and He give almost identical pyrolysis curves. Addition of toluene in He shifts the decomposition to higher temperatures; thus methyl radical attack on the parent TMGa is important in the inert carriers. H 2 and D 2 accelerate the reaction compared to N 2 . Addition of a small amount of CH 3 radicals from trimethylindium pyrolysis lowers the pyrolysis temperature significantly, indicating a chain reaction. The products of a D 2 /toluene mixture show that the active species are the H or D atoms rather than CH 3 radicals. These data show that the major reactions involved in TMGa decomposition in N 2 or He are (CH 3 ) 3 Ga → CH 3 + (CH 3 ) 2 Ga and CH 3 + (CH 3 ) 3 Ga → CH 4 + CH 2 Ga(CH 3 ) 2 . In H 2 a chain reaction takes place: CH 3 + H 2 → CH 4 + H and H + (CH 3 ) 3 Ga → CH 4 + CH 3 + CH 3 Ga. Numerical modeling was used to test these proposals. By this means it was determined that in addition to the reactions above, a key reaction is decomposition of CH 2 Ga(CH 3 ) 2 to give CH 2 GaCH 3 + CH 3 . As with other main group organometallics, the entire decomposition mechanism is complex.

Kinetics of the reaction between trimethylgallium and arsine

The kinetics of the reaction between trimethylgallium (TMGa) and AsH3 were studied in a flow tube reactor with D2 as the carrier gas and using a time-of-flight mass spectrometer to analyze the products. Addition of TMGa accelerates AsH3 decomposition. Like wise, AsH3 lowers the pyrolysis temperature of TMGa. The data from the decomposition at a series of V/III ratios shows that the stoichiometry is exactly 1:1 Experiments using a methyl radical scavenger show that gas phase free radical reactions are not important in the temperature range studied. The variation in rate constant with surface area shows that the rate determining step is predominatly heterogeneous. The only product in D2 is CH4, so there is no independent gas phase decomposition of TMGa. The lack of H2 and C2H6 indicates that independent surface pyrolysis of either TMGa or AsH3 does not occur. Two mechanisms are consistent with our data: (i) heterogeneous decomposition of an adduct formed in the gas phase and (ii) a Langmuir-Hinshelwood mechanism consisting of reaction between undecomposed adsorbed TMGa and AsH3 molecules. Based on measurements of growth rate versus pressure of TMGa and AsH3, the latter is the most likely pathway.

GaAs growth using tertiarybutylarsine and trimethylgallium

Abstract Tertiarybutylarsine (TBAs) is an ideal alternative to highly toxic AsH 3 for organometallic vapor phase epitaxy (OMVPE). Its decomposition was studied in a flow tube reactor using D 2 as the carrier gas. The products were analyzed in a time-of-flight mass spectrometer. The major products are isobutane (C 4 H 10 ), isobutene (C 4 H 8 ) and AsH 3 . The decomposition proceeds via two routes: intramolecular coupling, to produce C 4 H 10 and AsH, and β-elimination at higher temperatures which yields C 4 H 8 and AsH 3 . Addition or trimethylgallium (TMGa) has little effect on the rate and product distribution of TBAs decomposition; however, the pyrolysis of TMGa is altered greatly by the presence of TBAs. Numerical modeling shows that the main pathway to remove TMGa is via reaction with the AsH from the coupling step. In OMVPE growth the substrate temperatures are high enough that some TMGa decomposes independently, in which case the AsH serves to prevent adsorption of the CH 3 radicals and thus results in low carbon contamination.

Pyrolysis of tertiarybutylphosphine

AbstractThe reaction mechanism for the pyrolysis of tertiarybutylphosphine (TBP) has been studied in an atmospheric pressure flow tube reactor using a time-of-flight mass spectrometer to analyze the gaseous products. D2 was used as the carrier gas in order to label the reaction products. The temperature and time dependence of TBP pyrolysis were investigated above a silica surface, which was found to have no effect on TBP decomposition. However, the pyrolysis rate and products are strongly dependent on the input TBP concentration, suggesting the TBP pyrolysis involves second order reactions. A simple free radical mechanism model is proposed which includes 4 major reactions:C4H9PH2 = C4H9 + PH2 C4H9 + C4H9PH2 = C4H10 + C4H10 + C4H9PHC4H9PH = C4H9 + PHC4H9 = C4H8 + H.Arrhenius parameters for these reactions are reported.

The effect of supplemental t-butyl radicals on the pyrolysis of tertiarybutylarsine, tertiarybutylphosphine, and ditertiarybutylarsine

Azo-t-butane (ATB), a source of t-butyl radicals, has been used for the first time to study the mechanism for the decomposition of the organometallic vapor phase epitaxy (OMVPE) precursors TBAs (t-C4H9AsH2), DTBAs ((t-C4H9)2AsH) and TBP (t-C4H9PH2) in an inert He ambient. At a precursor to ATB ration of 2, 100% decomposition of TBAs and 45% decomposition of TBP can be induced at approximately 300°C, and at a ratio of 1, 50% decomposition of DTBAs can be induced at the same temperature. These temperatures are far below those at which the precursors normally decompose in the absence of ATB. The results strongly suggest that a t-butyl radical attack reaction occurs in the decomposition. Besides the hydrocarbon reaction products C4H10, C4H8, and C8H18, DTBAs was produced in the TBAs + ATB system, (C4H9)2PH and (C4H9PH)2 were found for the TBP + ATB system, and (C4H9)3As was observed for the DTBAs + ATB system. The study of these reaction products leads to an understanding of the bond strengths in the precursors and the reaction mechanisms during OMVPE growth.

Study of tertiarybutylphosphine pyrolysis using a deuterated source

The pyrolysis of tertiarybutylphosphine (TBP) and the reaction mechanism for the organometallic vapor‐phase epitaxial growth of GaP using TBP and trimethylgallium (TMGa) in a He ambient have been studied. A deuterated TBP source (C4H9PD2) was used to label the reaction products and to distinguish possible mechanisms. The reaction was found to be independent of the ambient, and C4H9D was found to be a major product species. However, its relative concentration was determined to increase as the input concentration of the reactant increased. The concentration of C4H9D is even higher when the pyrolysis was catalyzed by GaP surfaces. However, the addition of TMGa retarded both the production of C4H9D and the pyrolysis of TBP. Another species, CH3D, becomes the major deuterated product observed under these conditions. The results lead to the following conclusions: (1) an important reaction for TBP pyrolysis is of second order: the tert‐butyl radical (C4H9) attacks TBP; (2) the PH and PH2 species on GaP surfaces,...

OMVPE growth mechanism for GaP using tertiarybutylphosphine and trimethylgallium

Abstract The pyrolysis mechanism of tertiarybutylphosphine (TBP) on a GaP surface and also of mixtures of TBP and trimethylgallium (TMGa) for the growth of GaP were investigated. The reaction products for the decomposition of TBP alone on a GaP surface are similar to the products on an SiO 2 surface, i.e. C 4 H 10 , C 4 H 8 , PH 3 and H 2 ; however, the products are more strongly dominate by the C 4 H 10 species. The addition of TMGa to the gas mixture formed additional products CH 4 ,CH 3 D and (CH 3 ) x PH 3− x ( x = 1–2). The increased decomposition of TMGa at high TBP/TMGa ratios indicates that TBP enhances TMGa decomposition, but the decomposition of TBP is hindered by the presence of TMGa. The decomposition of TBP on a GaP surface occurs via a chain reaction. Surface adsorbed PH x species ( x =1–2) react with adsorbed TBP to form the volatile reaction product C 4 H 10 , and also propagate the PH x radicals. The terminating step in this chain reaction is the recombination of the adsorbed PH x species to form P x H 2 , and/or PH 3 molecules which then desorb. Additional chain terminating steps occur when TMGa is introduced. The PH x species react with adsorbed TMGa molecules to grow GaP, thus depleting the surface radical population, resulting in reduced TBP decomposition. The reactions between the surface adsorbed radicals resulting from TBP decomposition and the group III alkyls may be responsible for the low carbon incorporation observed in the organometallic vapor phase epitaxial (OMVPE) growth using TBP.

Decomposition studies on triisopropylantimony and triallylantimony

The pyrolysis of triisopropylantimony ((C3H7)3Sb) and triallylantimony ((C3H5)3Sb) has been investigated mass-spectrometrically in He and D2 using a SiO2 flow tube reactor at atmospheric pressure. Both temperature and time dependencies of percent decomposition were studied and the reaction products were analyzed. The overall decomposition processes for both compounds were found to be homogeneous and first order. (C3H7)3Sb pyrolyzes at 250-350° C with no effect of the ambient gas. However, C3H6, C3H8, and C6H14 (2,3-dimethylbutane) were produced in He whereas C3H3D appeared in D2. The pyrolysis is believed to begin via bond cleavage to generate the free C3H7 radicals that, in turn, recombine and disproportionate. Isopropyl radicals react slowly with D2, producing the C3H7D detected. For (C3H5)3Sb, the pyrolysis takes place at 100-160° C. The only major product is C6H10 (1,5-hexadiene). Both the pyrolysis rate and products were independent of the ambient. Two possible mechanisms, homolysis and reductive coupling, are discussed. Assuming that homolysis is the rate-limiting step for the pyrolysis of both (C3H7)3Sb and (C3H5)3Sb, bond strengths of 30.8 and 21.6 kcal/mole for C3H7—Sb and C3H6—Sb were determined from the experimental data. When either (C3H7)3Sb or (C3H5)3Sb was mixed with trimethylindium, a nonvolatile, liquid material, probably an adduct, was formed.

Decomposition studies of tertiarybutyldimethylantimony

The vapor pressure, decomposition temperature, decomposition products, and decomposition reaction order are reported for a novel organometallic vapor-phase epitaxy (OMVPE) Sb precursor, tertiarybutyldimethylantimony (TBDMSb, C4H9(CH3)2Sb). The TBDMSb vapor pressure is 7.3 Torr at 23° C. The 50% decomposition temperature is 300° C for both He andD2 ambients in a flow tube reactor with a residence time of approximately 3.2 sec at 300° C. The decomposition products are primarily C4H10, C4H8, and TMSb in both ambients. The overall decomposition reaction is first order. The decomposition mechanism is believed to be homolysis followed by recombination and disproportionation reactions for C4H9 and (CH3)2Sb groups. Added trimethylgallium (TMGa) has no measurable effect on either the pyrolysis rate or the products. Apparently, TMGa and TBDMSb do not interact during pyrolysis nor do they form a room temperature adduct. No room temperature adduct between TMGa and TBDMSb was formed. It is believed that TBDMSb is a promising Sb precursor for low temperature OMVPE growth.