M.L. Wise

Atomic layer controlled deposition of SiO2 and Al2O3 using ABAB… binary reaction sequence chemistry

Abstract Sequential ABAB… surface chemical reactions can be employed for atomic layer controlled deposition. We have examined the binary reactions SiCl 4 +2H 2 O⋌SiO 2 +4HCl for SiO 2 deposition and 2Al(CH 3 ) 3 +3H 2 O ⋌ Al 2 O 3 + 6CH 4 for Al 2 O 3 deposition. Each binary reaction (A + B ⋌ products) was performed sequentially by individual exposures to the A reactant and then the B reactant. If each surface reaction is self-limiting, repetitive ABAB… cycling may produce layer-by-layer controlled growth. For example, the individual “A” and “B” surface reactions for SiO 2 deposition can be described by (A) Si–OH + SiCl 4 ⋌ Si–O–SiCl 3 + HCl, (B) Si–Cl + H 2 O ⋌ Si-OH + HCl. We have studied ABAB… binary reaction sequences for SiO 2 and Al 2 O 3 deposition using laser-induced thermal desorption, temperature-programmed desorption and Auger electron spectroscopy techniques on single-crystal Si(100) surfaces. Fourier transform infrared spectroscopy techniques were also employed to examine these binary reaction schemes on high surface area SiO 2 and Al 2 O 3 samples. Controlled deposition of SiO 2 and Al 2 O 3 was demonstrated and optimized utilizing the above techniques. Under the appropriate conditions, each surface reaction was self-terminating and atomic layer controlled growth was a direct consequence of the binary reaction sequence chemistry.

Atomic layer growth of SiO2 on Si(100) using SiCl4 and H2O in a binary reaction sequence

Abstract The atomic layer control of SiO2 growth can be accomplished using binary reaction sequence chemistry. To achieve this atomic layer growth, the binary reaction SiCl4 + 2H2O → SiO2 + 4 HCl can be divided into separate half-reactions: (A) SiOH ∗ +SiCl 4 →SiOSiCl ∗ 3 +HCl , SiCl ∗ +H 2 O→SiOH ∗ +HCl , where the asterisks designate the surface species. Under the appropriate conditions, each half-reaction is complete and self-limiting and repetitive ABAB... cycles should produce layer-by-layer-controlled SiO2 deposition. The atomic layer growth of SiO2 thin films on Si(100) was achieved at temperatures from 600–680 K with reactant pressures from 1–50 Torr. These experiments were performed in a small high pressure chamber situated in an ultrahigh vacuum (UHV) apparatus. This design couples high pressure conditions for film growth with an UHV environment for surface analysis using laser-induced thermal desorption (LITD), temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). The controlled growth of a stoichiometric and chlorine-free SiO2 film on Si(100) was demonstrated using these techniques. SiO2 growth rates of approximately 0.73 ML of oxygen (1.1 A of SiO2) per AB cycle were obtained at 600–680 K. Additional vibrational spectroscopic studies performed in a second vacuum chamber utilized transmission Fourier transform infrared (FTIR) experiments on high surface area, oxidized porous silicon to monitor the surface species during the binary reaction sequence chemistry. These FTIR measurements observed the SiCl stretching vibration at 625 cm−1 and the SioH vibration at 3740 cm−1 and confirmed that each half-reaction was complete and self-limiting. These studies illustrate the feasibility of atomic-layer-controlled SiO2 growth and have determined the reactant pressures and substrate temperatures required for the SiO2 binary reaction sequence chemistry.

Diethylsilane on silicon surfaces: Adsorption and decomposition kinetics

The adsorption and decomposition kinetics of diethylsilane (DES), (CH3CH2)2SiH2, on silicon surfaces were studied using laser‐induced thermal desorption (LITD), temperature programmed desorption, and Fourier transform infrared (FTIR) spectroscopic techniques. LITD measurements determined that the initial reactive sticking coefficient of DES on Si(111) 7×7 decreased versus surface temperature from S0≊1.7×10−3 at 200 K to S0≊4×10−5 at 440 K. The temperature‐dependent sticking coefficients suggested a precursor‐mediated adsorption mechanism. FTIR studies on high surface area porous silicon surfaces indicated that DES adsorbs dissociatively at 300 K and produces SiH and SiC2H5 surface species. Annealing studies also revealed that the hydrogen coverage on porous silicon increased as the SiC2H5 surface species decomposed. CH2=CH2 and H2 were the observed desorption products at 700 and 810 K, respectively, following DES adsorption on Si(111) 7×7. The ethylene desorption and growth of hydrogen coverage during eth...

Adsorption and decomposition of diethylgermane on Si(111) 7×7

Germanium was deposited on Si(111) 7×7 by the adsorption and thermal decomposition of diethylgermane [(CH3CH2)2GeH2] (DEG). The DEG reaction products were CH2■CH2 and H2, which desorbed at 700 and 800 K, respectively, as observed by laser‐induced thermal desorption and temperature programmed desorption techniques. The desorption of atomic Ge was also monitored at approximately 1200 K. The production of ethylene was consistent with a β‐hydride elimination mechanism for the surface ethyl groups, i.e., Ge—CH2CH3→GeH+CH2■CH2. The initial sticking coefficient of DEG decreased with increasing surface temperature and a saturation coverage was obtained after exposures of E≳700 L at 200 K. This saturation behavior indicates that DEG may be useful for the controlled growth of Ge atomic layers on silicon surfaces.

Modeling silicon epitaxial growth with SiH2Cl2

Abstract Silicon epitaxial growth with SiH 2 Cl 2 was modeled using measured SiH 2 Cl 2 adsorption kinetics and H 2 , HCl, and SiCl 2 desorption kinetics from studies on Si(111) 7×7 surfaces. The predicted growth rates were compared with growth rates measured recently by Regolini et al. as a function of surface temperature between 923 and 1523 K. The agreement between the predicted and measured growth rates was very good. At lower temperatures between 923 and 1173 K, the silicon growth rates varied exponentially with temperature. The calculations revealed that HCl desorption is rate-limiting in this growth regime controlled by the availability of free surface sites. At temperature above 1173 K, the silicon growth rates were proportional to the SiH 2 Cl 2 pressure. This higher temperature region is controlled by the incident reactant flux and the reactive sticking coefficient of SiH 2 Cl 2 . The agreement between the calculations and measurements indicates that silicon epitaxial growth rates during low pressure chemical vapor deposition can be interpreted in terms of gas kinetic theory and adsorption and desorption surface kinetics.

Ethyl group decomposition kinetics following adsorption of diethylsilane, diethylgermane, and ethylsilane on Si(111) 7 × 7

Abstract The kinetics of ethyl group decomposition on Si(111)7 × 7 were measured using laser-induced thermal desorption techniques following the adsorption of diethylsilane (DES), diethylgermane (DEG), and ethylsilane (ES). Earlier infrared spectroscopic studies have shown that ethyl species are present after the dissociative chemisorption of DES, DEG, and ES on silicon surfaces. As the ethyl groups decompose via a β-hydride elimination mechanism, the hydrogen coverage increases and ethylene desorbs into the gas phase, i.e. Si-CH2CH3(ad) → Si-H(ad) + CH2 = CH2(g). LITD measurements determined that the ethyl group decomposition kinetics consisted of a fast initial step, followed by a slower second step. Decomposition kinetics for DES, DEG, and ES could be described by a model consisting of two concurrent first-order decomposition processes. The similar ethyl group decomposition rate constants for DES, DEG, and ES suggested that both hydrogen and ethyl groups transfer upon adsorption to free dangling bonds on Si(111)7 × 7. Measured activation barriers for the β-hydride elimination reaction were slightly larger than the thermodynamic barrier, but much smaller than the calculated gas-phase barrier. The dependence of the β-hydride elimination mechanism on the availability of free dangling bonds was studied by adsorption of additional atomic hydrogen following ES and DES exposures. The presence of additional hydrogen adsorption did not significantly affect the ethyl group decomposition kinetics. These results suggested that β-hydride transfer occurs through a four-center transition state and is not dependent on adjacent free dangling bond sites.

Adsorption kinetics for ethylsilane, diethylsilane, and diethylgermane on Si(111) 7×7

The adsorption kinetics for ethylsilane (ES), diethylsilane (DES), and diethylgermane (DEG) on Si(111) 7×7 were studied using laser‐induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The initial reactive sticking coefficients were determined as a function of surface temperature using LITD measurements. In these experiments, the ethyl coverage vs adsorption time was monitored using CH2=CH2 (ethylene) LITD signals that were produced by the β‐hydride elimination of the surface ethyl groups, e.g. Si–CH2CH3(ad)→Si–H(ad)+CH2=CH2(g). The initial reactive sticking coefficients were S0≊2×10−3, 4×10−3, and 5×10−2 for DES, ES, and DEG, respectively, at 200 K. As expected from a precursor‐mediated adsorption model, the initial reactive sticking coefficients were observed to decrease with increasing surface temperature. Experiments with preadsorbed hydrogen also demonstrated that the initial reactive sticking coefficients of DES and DEG were reduced as a function of hydrogen cove...

Isothermal H2 desorption kinetics from Si(100) 2 × 1: dependence on disilane and atomic hydrogen precursors

Abstract Isothermal H2 desorption kinetics from the Si(100)2 × 1 surface were studied using laser-induced thermal desorption (LITD) techniques. Disilane (Si2H6) and atomic hydrogen were used as the hydrogen precursors. Atomic hydrogen deposits only hydrogen adatoms and H2 subsequently desorbs from a nearly atomically flat Si(100)2 × 1 surface. Disilane deposits both hydrogen and silicon adatoms that may produce an atomically rough Si(100) surface. This surface roughening with silicon adatoms simulates silicon chemical vapor deposition and may affect the H2 desorption kinetics. The isothermal LITD studies revealed first-order H2 desorption kinetics for both precursors. An activation barrier of Ed = 57.2 ± 2.6 kcal/mol and a pre-exponential factor of vd = 2.21 × 1015±1 s-1 were measured for the atomic hydrogen precursor. An activation barrier of Ed = 54.3 ± 2.3 kcal/mol and a pre-exponential factor of vd = 2.32 × 1014±1 s-1 were determined for the disilane precursor. Within the experimental error, the isothermal H2 desorption kinetics were not significantly affected by the hydrogen source. The similar desorption kinetics are attributed to the surface mobility of the silicon adatoms deposited with disilane. The silicon adatoms can either diffuse to nearby step edges or form Si(100)2 × 1 islands on the underlying Si(100)2 × 1 surface. The first-order H2 desorption kinetics are explained by the concerted desorption of H2 from two hydrogen atoms prepaired on the same silicon dimer on the Si(100)2 × 1 surface.

Reaction kinetics of H2O with chlorinated Si(111)-(7 × 7) and porous silicon surfaces

Abstract Atomic layer control of SiO2 film growth can be achieved on silicon surfaces using the SiCl4 +H2O reaction applied in an ABAB … binary reaction sequence (A) SiCl ∗ + H 2 O→ SiOH ∗ + HCl , (B) SiOH ∗ + SiCl 4 → SiOSiCl 3 ∗ + HCl , where the asterisks indicate the surface species. The reaction of H2O with SiCl∗ species on the silicon surface is an important initial step for this controlled SiO2 deposition. In this study, the reaction of H2O on Si(111)-(7 × 7) surfaces chlorinated by SiCl4 exposures was studied using laser-induced thermal desorption (LITD), temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) techniques. Complementary transmission FTIR experiments monitored the SiCl, SiH and SiOSi vibrations of the surface species during the reaction of H2O on chlorinated porous silicon surfaces. At temperatures T≤700 K, the oxygen uptake resulting from H2O adsorption was small, and chlorine loss was negligible on chlorinated Si(111)-(7 × 7) and porous silicon surfaces. In contrast, both oxygen uptake and chlorine removal were measurable and thermally activated at T>700 K. The kinetics of oxygen uptake and chlorine loss were also studied on the Si(111)-(7 × 7) surface versus chlorine coverage. The oxygen uptake rates in the temperature range from 700–820 K were independent of the initial surface chlorine coverage. A simple kinetics model employing H2O adsorption kinetics and H2, HCl and SiCl2 desorption kinetics was used to explain the temperature threshold at ∼ 700 K and to determine the reaction mechanism. These model calculations were consistent with chlorine loss that was rate-limited by HCl desorption that occurs at T>700 K. The independence of oxygen uptake on the initial chlorine coverage was attributed to the similarity between the HCl and H2 desorption kinetics. In contrast to recent observations on the chlorinated SiO2 surface, these results indicate that the first H2O reaction in the initial AB sequence on chlorinated silicon surfaces does not involve a direct substitution reaction with an SiCl∗ surface species. Rather, the reaction is consistent with a Langmuir-Hinshelwood mechanism involving H2O adsorption followed by HCl desorption.

Adsorption and decomposition of diethyldiethoxysilane on silicon surfaces: New possibilities for SiO2 deposition

Diethyldiethoxysilane is an organosilicate that may offer new possibilities for SiO2 deposition. In this study, the adsorption and decomposition of diethyldiethoxysilane (DEDEOS) was examined on Si(100)2×1 and porous silicon surfaces using laser‐induced thermal desorption (LITD), temperature‐programmed desorption (TPD), Fourier‐transform infrared (FTIR), and Auger electron spectroscopy techniques. The FTIR studies revealed that DEDEOS dissociatively adsorbs on porous silicon and deposits ethyl and ethoxy species. These species are observed to decompose via a β‐hydride elimination mechanism at ∼700 K. In agreement with this mechanism, TPD studies on Si(100)2×1 observed ethylene (C2H4) at ∼700 K and H2 desorption at ∼800 K. Additionally, the controlled deposition of SiO2 was achieved on Si(100)2×1 using repetitive cycles of DEDEOS adsorption at 300 K followed by thermal annealing at 820 K for 300 s. After the rapid deposition of an oxygen coverage of θO∼2.4 ML, the oxygen deposition rate decreased and reach...