Modulated Beam Mass Spectrometry Research Articles

Probing the disilane adsorption kinetics: An alternative approach

The adsorption kinetics and subsequent dissociation of disilane during gas source molecular beam epitaxy on Si(001) surface is studied in situ using modulated beam mass spectrometry, thermal desorption spectroscopy, reflection high energy electron diffraction and growth of epitaxial layers involving repeated cycles of disilane adsorption and hydrogen desorption. The dissociation of disilane molecules is found to occur sequentially and the major intermediate reaction products are ${\mathrm{SiH}}_{2}$ and SiH. At temperatures above $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ disilane dissociates readily to give two silicon atoms and all six hydrogen atoms and forms the monohydride $(2\ifmmode\times\else\texttimes\fi{}1)+(1\ifmmode\times\else\texttimes\fi{}2)$ surface. The formation of a Si-monohydride surface also passivates against further adsorption and dissociation of disilane. The main reaction pathway for the decomposition of ${\mathrm{SiH}}_{2}$ to SiH is identified and studied as a function of incident flux and growth temperature. This process is found to be controlled by the number of unsaturated dangling orbitals.

Modulated-beam studies of the layer-by-layer etching of GaAs(0 0 1) using AsBr3: identification of the reaction mechanism

Modulated-beam mass spectroscopy (MBMS) has been used to study the reaction mechanism of the layer-by-layer etching of GaAs(0 0 1) using AsBr3 under molecular beam epitaxy conditions. It is shown that GaBr is the main etching product and its “delay” time with respect to the incident AsBr3 flux exhibits a strong dependence on substrate temperature, changing from 12 ms in the reaction limited regime at 387°C to less than 0.5 ms at 560°C. Desorbing As-containing species appear to have very short surface lifetimes throughout the temperature range investigated and the additional As2 flux supplied from a solid source has no effect on the etching rate. The results suggest a reaction pathway where the rate limiting step is the formation or desorption of GaBr and not the decomposition of AsBr3.

Thermal decomposition mechanisms of bis(2‐fluoro‐2,2‐dinitroethyl) formal (FEFO) and bis(2‐fluoro‐2,2‐dinitroethyl) difluoroformal (DFF) from simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) measurements

AbstractThe simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) technique has been applied to measure the vapor pressures and evaluate the thermal decomposition chemistry of two energetic liquids, bis(2‐fluoro‐2,2‐dinitroethyl)formal (FEFO) and bis(2‐fluoro‐2,2‐dinitroethyl)difluoroformal (DFF). The resulting heat of vaporization (ΔHvap) and vapor pressure at 25°C are 20.3 ± 0.2 kcal/mol and 0.4 ± 0.1 millitorr for FEFO, and 17.3 ± 0.2 kcal/mol and 5.1 ± 1.1 millitorr for DFF. The thermal decomposition of FEFO indicates there are six major pyrolysis pathways. The results suggest that FEFO initially decomposes at 150°C by rearrangement of the nitro group (NO2) to the nitrite group (ONO), followed by loss of NO. Some NO2 is also formed at 170°C. Between 200°C–300°C, further pyrolysis occurs. In one pathway, the FEFO backbone remains intact and a high molecular‐weight product is formed. The other three pathways involve scission of the FEFO backbone; one yielding CO2 (possibly N2O), one yielding CH2O and CO, and one yielding C3H2NOF. Differences in the thermal decomposition behaviors in the liquid and gas phases are observed. In the thermal decomposition of DFF, the formal fluorine atoms stabilize the backbone structure. Numerous minor thermal decomposition products are also reported.

Critical behaviour of super-heated (1900 - 2000 K) vapours

We report on the experimental observation of a critical phenomenon (phase-transition-like) in super-heated vapours inside an alumina effusive oven in the temperature range of 1900 - 2000 K. This phenomenon is studied by modulated molecular beam mass spectrometry and the main observations are dramatic discontinuous decay of the flux by a factor of within and a simultaneous dramatic rise of a signal. Nearly complete thermal reversibility is demonstrated. Under flow conditions with 2 - 10 Torr helium, and are also observed. Both exhibit the same critical behaviour as . Tentative interpretations are in terms of heterogeneous gas - surface critical kinetic phenomena or vapour condensation leading to formation of liquid-like films followed by complete disintegration and oxidation.

Increasing the range of growth temperatures available for GaAs selective area growth using triisopropylgallium and arsine

This paper reports the first assessment of triisopropylgallium (TIPGa) for the selective area growth of GaAs using chemical beam epitaxy. Modulated beam mass spectrometry has been used to study the decomposition of TIPGa on oxides of silicon and GaAs, with both pre-cracked arsine and tris(dimethylamino)arsenic (TDMAAs) as the group V source. Experiments have been performed as a systematic function of growth temperature, both with and without the group V flux, and a comparison made with the use of triethylgallium (TEGa) in place of TIPGa. Using pre-cracked arsine as the group V source, we have demonstrated that TIPGa does not decompose on a silicon oxide surface for all growth temperatures above 450°C, whereas temperatures over 560°C are required to achieve total selectivity when using TEGa as the group III source. Studies using TIPGa and TDMAAs also demonstrate selectivity across a wide temperature range.

A detailed time of flight study of the cracking pattern of trimethylgallium; implications for MOMBE growth

Time of flight measurements of trimethylgallium (TMGa) have been used to obtain detailed information regarding the effect of mass spectroscopic detection on organometallic molecules, under ultra high vacuum. The technique adopted allows the determination of “cracking patterns” for all species, trimethylgallium, dimethylgallium (DMGa) and monomethylgallium (MMGa), derived from the parent molecule. This information has important significance for in situ modulated beam mass spectroscopy (MBMS) growth studies of GaAs from this precursor.

Atomic force microscopy studies of substrate cleaning using tris(dimethylamino)arsenic and tris(dimethylamino) antimony and investigations of surface decomposition mechanisms

Atomic force microscopy studies have been performed on substrates cleaned in a chemical beam epitaxy reactor using tris(dimethylamino)arsenic (TDMAAs) or tris(dimethylamino)antimony (TDMASb) at substrate temperatures well below those required for thermal desorption of the native oxide. For the first time, we report surface oxide removal from InAs(001) at 370°C using TDMAAs, and from InSb(001) and GaSb(001) at 405°C and 495°C respectively, using TDMASb. Atomically flat surfaces are achieved which are suitable for epitaxial growth without a buffer layer. Preliminary experiments indicate InSb growth may be achieved using uncracked TDMASb with trimethylindium. Decomposition studies of TDMAAs on GaAs using modulated beam mass spectrometry show an important change in the decomposition efficiency of TDMAAs above 480°C.

Thermal Decomposition Studies of a New, gem-Dinitroalkyl Nitramine 1,3,3-Trinitroazetidine ( TNAZ ).

The initial results from a study of the thermal decomposition of TNAZ and TNAZ-\.'sN02 using the simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) technique are presented. The major products formed in the decomposition of TNAZ are N02 and NO with slightly lesser amounts of H20, HCN, COIN2, C01!N20 and l-nitroso-Ld-dinitroazetidine (NDNAZ). The last mentioned product (NDNAZ) is shown to be an important intermediate in the decomposition of TNAZ and its decomposition has also been examined by synthesizing it independently. Major products formed in this decomposition of NDNAZare NO with smaller amounts of H20, HCN, COIN2 and C01!N20. The temporal behaviours of the ion signals associated with the various thermal decomposition products from TNAZ, TNAZ-I-15N02 and NDNAZ are also presented. They illustrate the evolution sequence of the various products that are associated with the different reaction pathways that control the decomposition of these materials. In particular, the study of the ISN-Iabeled sample revealed that N02 originates from both the likely sites in the TNAZ molecule and that the cleavage of the nitramine-N02 group precedes that of the C-N02 cleavage, resulting in similar sequences in the formation of NO and NDNAZ also.

Mass spectra of 2,4-dinitroimidazole and its isotopomers using simultaneous thermogravimetric modulated beam mass spectrometry

The EI mass spectrum of 2,4-dinitroimidazole (2,4-DNI) was studied at 20 and 70 eV ionizing energy using simultaneous thermogravimetric modulated beam mass spectrometry. Product ion formulas and their origin from within the 2,4-DNI molecule were identified from the evaluation of perdeuterated and various 15 N-labeled 2,4-DNI mass spectra. The molecular ion (M +. ; m/z 158) is present in all the spectra and the NO + ion (m/z 30) is the major primary product ion. The base peak is formed by the M .+ ion at 20 eV and by the NO + ion at 70 eV. Other primary product ions are NO 2 + (m/z 46), CH 2 N + (m/z 28) and CHN 2 O 2 + (m/z 73). Fragmentation pathways are postulated based on the identity and relative abundance of the product ions, and on the change in relative abundance of the ions as a function of ionizing energy. Comparisons of the 70 eV mass spectra of 2,4-DNI, 4,5-dinitroimidazole (4,5-DNI) and 1,4-dinitroimidazole (1,4-DNI) show that the fragment product ions have the same m/z values in the three spectra but differ in their relative abundance and in the ion that forms the base peak. The base peak for 2,4-DNI and 4,5-DNI is formed by the ion at m/z 30 (NO + ) and for 1,4-DNI, the ion at m/z 46 (NO 2 + ). This difference is probably due to the nitramine functionality of 1,4-DNI

Molecular Beam Studies on the Laser‐Enhanced Surface Reaction of GaAs(100) with Chlorine

AbstractA cw supersonic Cl2 molecular beam coupled with an angle‐resolved time‐of‐flight (TOF) technique has been used to investigate the laser‐enhanced surface reaction of GaAs(100) with chlorine. The mass and velocity distributions of the major reaction products under 1064 nm laser irradiation have been measured as a function of laser fluence, detection angle, surface temperature and normal component of the translational energy of the incident chlorine molecules. It has been found that increasing both laser fluence and the translational energy of incident chlorine molecules markedly enhance mis surface reaction. The measured flux angular distributions of major reaction products can be fit satisfactorily with a bi‐cosine function. Measurements of the mass and angular distributions of reaction products by a modulated molecular beam mass spectrometry show that the surface temperature effect is obvious for the Cl2/GaAs(100) “dark” thermal reaction. A direct activated dissociative chemisorption is proposed for the mechanism of Cl2 chemisorption on the GaAs(100) surface.

Solid-Phase Thermal Decomposition of 2,4-Dinitroimidazole (2,4-DNI)

AbstractThe solid-phase thermal decomposition of the insensitive energetic aromatic heterocycle 2,4- dinitroimidazole (2,4-DNI: mp 265–274°C) is studied utilizing simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) between 200° and 247°C. The pyrolysis products have been identified using perdeuterated and N-labeled isotopomers. The products consist of low molecular-weight gases and a thermally stable solid residue. The major gaseous products are NO, CO2, CO, N2, HNCO and H2O. Minor gaseous products are HCN, C2N2, NO2, C3H4N2, C3H3N3O and NH3. The elemental formula of the residue is C2HN2O and FTIR analysis suggests that it is polyurea- and polycarbamate-like in nature. The rates of formation of the gaseous products and their respective quantities have been determined for a typical isothermal decomposition experiment at 235°C. The temporal behaviors of the gas formation rates indicate that the overall decomposition is characterized by a sequence of four events; 1) an early decomposition period induced by impurities and H2O, 2) an induction period where CO2 and NO are the primary products formed at relatively constant rates, 3) an autoacceleratory period that peaks when the sample is depleted and 4) a final period in which the residue decomposes. Arrhenius parameters for the induction period are Ea = 46.9 ± 0.7 kcal/mol and Log(A) = 16.3 ± 0.3. Decomposition pathways that are consistent with the data are presented.

Ion-beam-assisted etching of Si with fluorine at low temperatures

The ion-assisted etching of Si with F atoms has been studied over the temperature range from 77 K to room temperature. Separately controllable beams of F atoms and 1 keV Ar+ ions are used in an ultrahigh-vacuum environment. Neutral etch products are measured with modulated beam mass spectrometry. The ion-assisted etch rate is seen to increase as the temperature is decreased whereas the spontaneous etch rate goes to zero at low temperatures. The nature of the etch products is essentially independent of temperature over this temperature range. Evidence is presented indicating that the spontaneous etching of Si by F atoms at 77 K is blocked by the formation and condensation of Si2F6.

Reaction of β-SiC with thermal atomic hydrogen by modulated molecular beam mass spectrometry

The reaction of polycrystalline β-SiC with thermal atomic hydrogen is investigated by modulated molecular beam mass spectrometry. The temperature and equivalent pressure on the surface are 300–1100 K and 6 × 10 −6 –2 × 10 −5 Torr, respectively. The reaction probability of atomic hydrogen is 9 × 10 −5–5 × 10 −4. Grain boundary effects are negligible and no bulk or surface diffusion is observed. The major reaction products are SiH 4, CH 4, and C 2H 2. SiH 4 and CH 4 formation reactions are linear with respect to the H-atom pressure while C 2H 2 formation is nonlinear. A precursor model is proposed for SiH 4 and CH 4. C 2H 2 appears to be produced in the defect site where SiH 4 forms and desorbs.

Mechanistic studies of the CBE growth of (100) GaAs using the new precursor tri-isopropylgallium

Modulated-beam mass spectrometry and temperature programmed desorption have for the first time been used to investigate the decomposition of tri-isopropyl gallium (TIPGa) on the (100) GaAs surface. Significant differences in the substrate temperature dependence of the GaAs growth rate have been observed between growth using TIPGa and the standard CBE precursor triethylgallium (TEGa). These differences are explained in terms of the balance between desorption and decomposition of adsorbed di-isopropyl gallium radicals shifting in favour of desorption.

Theoretical consideration of the growth kinetics for GaAs and GaSb

An extended MOMBE growth kinetics model is proposed, based on the Robertson model, to explain both the GaAs growth rate variation and modulated beam mass spectroscopy data reported by Martin and Whitehouse. In this model, we assume that (1) MEGa molecules react with ethyl-radicals to form DEGa, (2) excessive group-V molecules on the surface suppress the decomposition of DEGa and enhance the desorption of DEGa, (3) reaction of DEGa with ethyl-radicals to form TEGa is negligible, and (4) effective surface coverage of excessive group-V atoms during growth is determined by the double layer adsorption model including desorption parameters for group-V molecules. The first assumption (1) is found to be a dominant process to explain the behaviour of DEGa desorption at high temperatures. This model can reproduce the dependences of both growth rate and desorbing rate of Ga alkyls on substrate temperature during GaAs MOMBE growth. The use of Sb instead of As produces a significant change in the growth rate variation with substrate temperature and group-V flux for the growth of GaSb, in spite of the use of the same TEGa flow rate. This can be rationalized by the difference in the desorption parameters for Sb and As.

Growth and MBMS studies of reaction mechanisms for In xGa 1− xAs CBE

The reaction mechanism involved in the growth of In x Ga 1− x As lattice matched to InP by chemical beam epitaxy (CBE) was investigated using growth and modulated beam mass spectrometry studies. Emphasis was placed on elucidating how variations in substrate temperature, indium composition and arsenic overpressure influence growth kinetics and how sensitive changes in experimental conditions bring about deviations in the ideal stoichiometry (In 0.53Ga 0.47As) required for lattice matching to InP. Our observations indicate that the compositional variations in the InGaAs stoichiometry at high temperatures (> 485°C) arise because of the changes in the DEG decomposition: desorption branching ratio which is controlled by a temperature- and arsenic pressure-dependent surface population of indium atoms. The low temperature behaviour is governed by the availability of metal surface sites for triethylgallium decomposition which is increased by the presence of surface indium atoms.

The use of tertiarybutylphosphine and tertiarybutylarsine for the metalorganic molecular beam epitaxy of the In 0.53Ga 0.47As/InP and In 0.48Ga 0.52P/GaAs materials systems

This paper describes a study on the use of thermally cracked tertiarybutylphosphine (TBP) and tertiarybutylarsine (TBA) with elemental Ga and In sources for the metalorganic molecular beam epitaxy (MOMBE) growth of the In 0.53 Ga 0.47 As/InP and In 0.48 Ga 0.52 P/GaAs materials systems. Modulated beam mass spectroscopy was used to characterize the thermal decomposition of these Group-V metal alkyls. Results indicate that As 2 and P 2 are the dominant growth species produced when cracker temperatures greater than 700°C are used. These conditions result in high quality epitaxial layers with essentially zero oval defects (less than l/cm 2 ) despite the use of elemental Group-Ill sources. Results of doping and heterointerface studies indicate that these Group-V precursors are suitable replacements for hydride sources. Application of these precursors for device structure growth including heterojunction bipolar transistors (HBTs) and resonant tunneling diodes (RTDs) is also described.

Sticking and Desorption Coefficients of As4 and As2 During Group V and Group III Controlled MBE Growth

ABSTRACTReflection High Energy Electron Diffraction (RHEED) oscillations under arsenic and gallium-controlled Molecular Beam Epitaxy (MBE) growth conditions have been used to measure the sticking and desorption coefficients of As2 and As4. The coefficients are obtained from measurements of the arsenic incorporation rates. Comparisons are made with measurements obtained from desorption rates using modulated beam mass spectroscopy. The transition from gallium to arsenic-controlled growth is observed to occur after excess gallium atoms accumulate on the surface. The maximum intrinsic arsenic sticking coefficients occur when the maximum number of gallium atoms can be incorporated for a given arsenic flux. The intrinsic maximum arsenic sticking coefficients are found to be 0.75 and 0.50 for As2 and As4, respectively. During galliumcontrolled growth, the arsenic sticking coefficients are independent of substrate temperature as long as the sticking coefficient of gallium is equal to one. However, a temperature dependent maximum gallium-controlled arsenic sticking coefficient exists. It can be measured by the maximum Ga to As4 flux ratio that produces specular film surfaces. During gallium-controlled growth, the Ga to As flux ratios are shown to be equal to the gallium-controlled arsenic sticking coefficients. The activation energy for arsenic desorption during arsenic-controlled growth conditions was measured as -0.50 eV for independent As4 and As2 incident fluxes. During gallium-controlled growth with incident As4 fluxes, an activation energy for arsenic desorption of -0.70 eV was measured for the maximum gallium-controlled arsenic sticking coefficients.

Dual atom beam studies of etching and related surface chemistries

Abstract

Atomic and Molecular Beam Studies of Etching and Related Surface Chemistries

ABSTRACTResults will be presented using a new ultrahigh vacuum system designed to investigate the complex surface chemistries prevailing in plasma-assisted etching processes which use more than one halogen or halogen-hydrogen gas mixtures. In this new apparatus, two separate controllable beams of atoms can be directed onto a surface and energetic ion bombardment of the surface can be introduced. Neutral species evolved or reflected from the surface are monitored with modulated beam mass spectrometry. Etch rates as small as 10−3 Å/sec can be detected. The chemical systems which have been used are Si-F-Cl, Si-F-H and Si-CI-H and some unexpected phenomena observed with these systems will be described. Some catalytic reactions have also been observed with these gases in the presence of polycrystalline Ni and Pd surfaces and another example of an oscillatory surface reaction has been found.