C.A. Larsen

Reaction mechanisms in the organometallic vapor phase epitaxial growth of GaAs

The decomposition mechanisms of AsH3, trimethylgallium (TMGa), and mixtures of the two have been studied in an atmospheric‐pressure flow system with the use of D2 to label the reaction products which are analyzed in a time‐of‐flight mass spectrometer. AsH3 decomposes entirely heterogeneously to give H2. TMGa decomposes by a series of gas‐phase steps, involving methyl radicals and D atoms to produce CH3D, CH4, C2H6, and HD. TMGa decomposition is accelerated by the presence of AsH3. When the two are mixed, as in the organometallic vapor phase epitaxial growth of GaAs, both compounds decompose in concert to produce only CH4. A likely model is that of a Lewis acid‐base adduct that forms and subsequently eliminates CH4.

MOVPE growth of GaInAsSb

Ga1−xInxAs1−ySby alloys have been grown by metalorganic vapor phase epitaxy (MOVPE) using trimethyl compounds of Ga In, As, and Sb (TMGa, TMIn, TMAs, and TMSb) plus AsH3 in an atmospheric pressure, horizontal, infrared heated reactor.For the first time, alloys near the center of the region of solid immiscibility have been grown. Alloys with room temperature band gaps of between 0.32 and 0.74 eV (1.7 to 3.9μm) have been grown on InP, InAs and GaSb substrates. The larger band gap alloys, grown on either InP or Gasb substrates, could be grown at temperatures between 580 and 600°C. For the smaller band gap alloys grown on InAs substrates lower temperatures of 480 to 500°C were necessary to avois melting of the layer. These low temperatures caused a reduction of the Ga distribution coefficient from unity value at temperatures of 550°C, to 0.05 at 500°C. The presence of TMSb in the system caused a further reduction in Ga incorporation. The Sb distribution coefficient at 500°C was 0.17. The low temperature photoluminescence of the quaternary alloys routinely consisted of a single peak characteristic of the band gap. The spectral half-width was found to increase for the more metastable alloys. This may indicate the occurence of a significant amount of compositional clustering during growth.

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.

MOVPE growth of InP using isobutylphosphine and tert-butylphosphine

Abstract Two organophosphorus compounds, isobutylphosphine and tertiarybutylphosphine, have been investigated for their possible use as precursors in the metalorganic vapor phase epitaxy (MOVPE) process. They are the first organometallic compounds to be used as phosphorus sources. Pyrolysis studies show that the first decomposition products are phosphine and various organic compounds. The phosphine then pyrolyzes to give phosphorus. The materials are less pyrophoric and less toxic than phosphine. Since they are used as liquids in atmospheric pressure bubblers, they are much safer to use. The compounds have been used to grow epitaxial layers of InP on InP and GaAs substrates using trimethylindium in a flowing hydrogen ambient. No evidence of deleterious gas phase reaction is observed. The optimum temperature for growth using IBP is approximately 630°C. This yields excellent morphology layers with the strongest photoluminescence intensity and narrowest half-width, both of which are comparable to InP grown using PH 3 . Carbon incorporation is nearly identical using IBP and PH 3 . The IBP and TBP are not quite as pure as the best PH 3 . The lowest carrier concentration measured was in the low 10 16 cm −3 range with room temperature mobilities as high as 2800 cm 2 /V·s for TBP. Using TBP slightly higher mobilities of 3100 cm 2 /V· were obtained.

Decomposition kinetics of OMVPE precursors

Abstract The thermal decomposition of trimethylindium and phosphine was studied under flow conditions. Trimethylindium decomposes homogeneously in hydrogen with an activation energy of 35.9 kcal/mol. The pyrolysis is surface dependent when nitrogen is the carrier gas. Phosphine pyrolysis exhibits two regimes, with heterogeneous decomposition occuring below 800°C and homogeneous decomposition above this temperature. The activation energy in the homogeneous range is 69.2 kcal/mol. Indium phosphide has a large catalytic effect on phosphine pyrolysis. the adduct (CH 3 ) 3 In: PH 3 does not form in the gas phase at room temperature, but is stable at 77 K.

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.

OMVPE growth of InP and Ga 0.47 In 0.53 AS as using ethyldimethylindium

We report the organometallic vapor phase epitaxial (OMVPE) growth of InP and Ga0.47In0.53As using a new organometallic indium source, ethyldimethylindium (EDMIn), rather than the traditional sources triethylindium (TEIn) or trimethylindium (TMIn). EDMIn is a liquid at room temperature and its vapor pressure at 17° C was found to be 0.85 Torr using thermal decomposition experiments. The growth results using EDMIn were compared to those using TMIn in the same atmospheric pressure reactor. For InP, use of EDMIn resulted in a high growth efficiency of 1.3 × 104 μm/ mole, which was independent of the growth temperature and comparable to the growth efficiency obtained with TMIn. The high growth efficiency is consistent with the observation of no visible parasitic gas phase reactions upstream of the substrate. The 4K photoluminescence (PL) spectra consist of a peak due to bound excitons and an impurity related peak 38 meV lower in energy. This impurity peak is ascribed to conduction band to acceptor transitions from carbon, due to the decreasing relative intensity of this peak with increasing V/III ratio. The relative intensity of the C impurity peak decreases by five times when the growth temperature is increased from 575 to 675° C, with a corresponding increase in the room temperature electron mobility from 725 to 3875 cm2/ Vs. For GalnAs lattice-matched to InP, use of EDMIn also resulted in a temperatureindependent high growth efficiency of 1.0 x 104 μm/mole, indicating negligible parasitic reactions with AsH3. The In distribution coefficient was nearly constant at a value of 0.9, however the run to run composition variation was slightly higher for EDMIn than for TMIn. The 4K PL showed donor-acceptor pair transitions due to C and Zn. The C impurity peak intensity decreased dramatically with increasing growth temperature, accompanied by an increase in the room temperature electron mobility to 5200 cm2/Vs. Overall, the growth of both InP and GalnAs using EDMIn was qualitatively similar to that using TMIn, although the room temperature electron mobilities were lower for the new source than for our highest purity bottle of TMIn.

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.