Hervé Dumont

Pyrolysis pathways and kinetics of thermal decomposition of diethylzinc and diethyltellurium studied by mass spectrometry

The thermal decomposition of diethylzinc (Et2Zn) and diethyltellurium (Et2Te) has been studied both separately and combined using an isothermal reactor attached to a quadrupole mass spectrometer (QMS). Under helium and hydrogen atmospheres the decomposition of Et2Zn leads to ethene, ethane and n-butane; with D2 as a carrier gas C2H5D is produced. Pyrolysis of Et2Te leads to ethane and ethene only. With helium as the carrier gas, the activation energy for the pyrolysis was E ≈ 52 kcal mol–1 for Et2Zn and E ≈ 50 kcal mol–1 for Et2Te, firstorder kinetics being assumed. Under a hydrogen atmosphere, the determination of the activation energy for the pyrolysis of Et2Zn was inaccurate with a first-order reaction rate, and the activation energy for the pyrolysis of Et2Te was reduced to E ≈ 47 kcal mol–1. In the co-pyrolysis, which is close to the conditions of ZnTe growth, the principal trends are enhancement of Et2Te decomposition and an increase of the thermal stability of Et2Zn.

Photo-assisted growth of ZnTe by metalorganic chemical vapour deposition

Abstract The metalorganic chemical vapour deposition (MOCVD) photo-assisted growth of ZnTe using a xenon lamp has been performed at atmospheric pressure under hydrogen as a carrier gas. Epitaxial growth was achieved on (100) GaAs, (100) GaSb and (100) ZnTe substrates with diethylzinc (DEZn) and diethyltellurium (DETe) as precursors. We have studied the growth rate as a function of the growth temperature, the partial pressure of precursors, the inlet partial pressure ratio R = VI/II, the light intensity and the energy of the irradiating photons. A growth rate enhancement has been observed for illuminated layers grown on GaAs and ZnTe substrates in comparison with those grown without illumination. We have not observed any measurable enhancement for layers grown onto (100) GaSb. The growth rate as a function of the light intensity increases for intensities higher than 20 mW / cm 2 and saturates for P > 120 mW / cm 2 . We relate the growth rate enhancement to irradiating photons with an energy higher than the band gap of ZnTe at the growth temperature.

Mass-spectrometric study of thermal decomposition of diethylzinc and diethyltellurium

A mass-spectrometric study of the thermal decomposition of Group II and VI alkyls involved in ZnTe growth by metal-organic chemical vapour deposition (MOCVD) is reported. The pyrolysis of diethylzinc (Et2Zn) and diethyltellurium (Et2Te) alone and together were followed with the change of the inlet partial pressure ratio (R= Te/Zn = 1 and 5). Under He and H2, the thermal decomposition of Et2Zn organometallic species leads to the formation of n-C4H10, C2H4 and C2H6. The pyrolysis of Et2Te produces ethene in excess of other hydrocarbons. For co-decomposition in an He atmosphere, pyrolysis curves as a function of temperature, with R= 1 show that the temperature corresponding to the half-decomposition ratio (T50) of Et2Te is lowered by 25 °C relative to that of Et2Te alone. For R= 5, the T50 of Et2Zn is increased by 30 °C relative to that of the precursor alone. The kinetics of decomposition of Et2Zn and Et2Te alone showed no thermal hysteresis, whereas a strong hysteresis loop between heating and cooling of the susceptor was recorded during the co-decomposition. The dependence of these effects on R and temperature are discussed in relation to surface reactions.

Analysis of nitrogen incorporation mechanisms in GaAs1-xNx/GaAs epilayers grown by MOVPE

Abstract The incorporation of nitrogen in Ga(As,N)/GaAs grown by metalorganic vapor phase epitaxy has been investigated as a function of growth conditions. The evolution of the solid composition was studied as a function of growth temperature between 500 and 600°C, and gas-phase composition. For instance, at growth temperatures of 510–560°C, the solid composition varies slowly with T , with an apparent activation energy of 0.6 eV. For T >560°C, an exponential decrease is dominantly observed with an apparent much higher activation energy of 3.7 eV. The effect of the growth temperature is analyzed, taking account for the different surface reaction mechanisms. The observed transition between the two regimes is attributed to an increased desorption of nitrogen at high temperatures. In light of experimental results, we propose a surface kinetics model based on the competitive adsorption of group V precursors. We also deduced an energy barrier for adsorption of group V species.

Influence of catalyst droplet diameter on the growth direction of InP nanowires grown on Si(001) substrate

It is demonstrated that the growth direction of InP nanowires grown on (001)-oriented silicon substrate strongly depends on the diameter of the gold catalyst droplets. Small droplets with diameter less than about 15 nm lead to the formation of nanowires leaning on the {111} planes of the zinc blende InP seeds formed in the early stages of growth. Larger droplets lead to the formation of twins in the InP seeds and to the formation of nanowires leaning on the {111} planes of these twinned InP variants, inducing growth directions corresponding to the 〈115〉 directions of the silicon substrate.

Growth kinetics during metal-organic chemical vapour deposition of ZnTe

Abstract ZnTe films are grown by metal-organic chemical vapour deposition starting from pyrolysis of a mixture of diethylzinc (DEZn) and diethyltellurium (DETe) diluted with H2. The decomposition process of these two organometallic precursors is monitored with a mass spectrometer; DEZn leads to n− C 4 H 10 with other hydrocarbons (C2H4 and C2H6) as reaction products and DETe leads to C2H4. When both organometallic precursors are present, a strong hysteresis loop of the signal intensity when heating and cooling the susceptor has been recorded, as a result of a catalytic reaction between the gas phase and the surface. The experimental growth rate V at 400 °C vs. organometallic concentration is described in terms of a kinetic model involving pyrolysis processes and a surface-catalysed reaction. The model explains the asymmetric behaviour of V; for a given DETe pressure (10−4 atm), the ZnTe growth rate as a function of DEZn partial pressure passes through a maximum ( p max DEZn ≈ 10 −4 atm ; V max ≈ 2.4 μ m h −1 ), whereas, at the same constant DEZn partial pressure, V follows a sublinear law with the increase in DETe partial pressure and saturates to V = 4.3 μ m h −1 .

Carbon and hydrogen incorporation in ZnTe layers grown by metalorganic chemical vapor deposition

The metalorganic chemical vapor deposition growth of ZnTe has been performed at atmospheric pressure under helium and hydrogen carrier gases. Epitaxial growth was achieved on GaAs (100) substrates with the combination of diethylzinc and diethyltellurium as precursors. We have studied the incorporation of carbon and hydrogen in as-grown layers of ZnTe by secondary ion mass spectroscopy analysis and out-diffusion experiments with different carrier gases and growth temperatures. The amount of carbon and hydrogen incorporated in the ZnTe layers greatly depends on the nature of the gas considered. Under helium atmosphere, the amount of carbon and hydrogen incorporated are greater than under H2 with an origin from organometallic precursors.

Recombination Dynamics in ZnxCd1-xS Single Quantum Well Grown by Photoassisted Metalorganic Vapour Phase Epitaxy by Time-Resolved Photoluminescence Spectroscopy.

A higher zinc composition of illuminated epilayers (Zn0.38Cd0.62S) was employed to form the barrier whereas unilluminated growth sequences were adopted to grow the well (Zn0.30Cd0.70S) in the photoassisted metalorganic vapour phase epitaxy (MOVPE). Time-integrated photoluminescence spectra clearly show two peaks at 437 nm (2.83 eV) and 443 nm (2.80 eV) attributed to the barrier and to the well respectively. The time evolution of these spectra shows an increasing intensity of emission from the well with a time delay due to the confinement and recombination of carriers in the well. Time-resolved measurements have shown a time delay of 60 ps between the barrier and the well. These results indicate the formation of type-I band structure.

Photoassisted metal organic vapour-phase epitaxy of ZnTe on GaAs

Abstract Photoassisted epitaxy of ZnTe on GaAs by a metal organic vapour-phase process has been performed using sources of diethylzinc and diethyltellurium or diisopropyltellurium. Illuminating the layer during growth with a xenon lamp increases the growth rate. Growth rate enhancement was found to be a function of light wavelength and intensity. Only photons having energy higher than the bandgap of ZnTe increase the growth rate. The characterization of the layers by X-ray diffraction and of the surface morphology by Nomarski microscopy is reported. We observed the degradation of the surface morphology of irradiated layers in comparison with unilluminated layers. We tentatively explain the growth rate enhancement by a photocatalytic surface process involving electron - hole pairs created in the ZnTe layers.

Growth mode and effect of carrier gas on In0.53Ga0.47As/InP surface morphology grown with trimethylarsine and arsine

Abstract Different growth mode have been observed for InGaAs/InP grown with trimethylarsine and arsine by Metalorganic Vapor Phase Epitaxy (MOVPE) when changing the carrier gas. The surface has been investigated by Atomic Force Microscope (AFM) for epilayers grown at 600°C under pure hydrogen or a mixture of hydrogen and nitrogen as carrier gas. The step/terrace surface morphology was observed for InP/InP and InGaAs/InP (001) using 0.2° off substrates. InP epilayers grown under nitrogen flow show step-bunched terraces as large as 170 nm. The effect of the group V source for InGaAs/InP has been studied. It is shown that the step edge characteristic of step flow growth appears for lattice-matched InGaAs/InP grown with arsine. When using TMAs and hydrogen as a carrier gas, the growth mode and surface roughness depends greatly on V/III ratio and growth temperature. Under nitrogen flow with the combination of TMI+TMG+TMAs, pit-like defects (5–8 nm deep) are visible at high surface concentration (109–1010/cm2). When increasing V/III ratio, 3D growth occurs simultaneously with pit-like defects, recovering the whole surface of the sample. Various surface morphology characteristics of InGaAs epilayers assessed by AFM characterisation will be presented and discussed.