A. Mahajan

Surface chemistry of diethylsilane and diethylgermane on Si(100): An atomic layer epitaxy approach

The surface chemistry of diethylsilane (DES) and diethylgermane (DEG) on the Si(100) surface was studied using high‐resolution electron‐energy‐loss spectroscopy and temperature programmed desorption. High‐resolution electron‐energy‐loss spectra indicate that the precursors chemisorbed dissociatively as (C2H5)2MHx(ad) and (2−x)H(ad) [x=0,1; M=Si or Ge] groups at room temperature and that the ethyl groups remain bonded to the precursor Si or Ge. Thermal annealing of DEG covered surfaces indicated that the ethyl groups remain attached to Ge at least up to the desorption temperature. Ethyl ligands react to form ethylene via a β‐hydride elimination pathway. The ethylene desorption peak temperature from DEG dosed surfaces was approximately 40 K lower than that from DES dosed surfaces (733 K). We propose that ethylene desorbs from Ge atoms rather than from Si surface atoms. The hydrogen remaining on the surface after ethylene desorption desorbs from the β1 state, with peak maxima at 810 and 780 K for DES/Si(100)...

Adsorption and decomposition of diethylsilane and diethylgermane on Si(100): Surface reactions for an atomic layer epitaxial approach to column IV epitaxy

The room‐temperature adsorption and desorption kinetics of diethylsilane (DES) and diethylgermane (DEG) on the Si(100)‐(2×1) surface were investigated under ultrahigh vacuum using temperature programmed desorption, high resolution electron energy loss spectroscopy, and Auger electron spectroscopy. DES and DEG adsorb at room temperature in a self‐limiting fashion, reaching saturation (0.4 and 0.3 monolayers, respectively), at exposures above 30 and 350 L, respectively. Temperature programmed desorption of the DES‐saturated and DEG‐saturated surfaces revealed only two species, hydrogen and ethylene, desorbing from either surface. In both systems, the hydrogen atoms desorbed primarily through the recombinative desorption of monohydride species, while the ethyl groups decomposed via β‐hydride elimination and subsequently desorbed as ethylene. The hydrogen desorption peak temperature was 794 K for the DES‐saturated surface and 788 K for the DEG‐saturated surface. The desorption peak temperature for ethylene was significantly lower in the DEG/Si(100) system (700 K) than in the DES/Si(100) system (730 K) because of a lower activation energy and higher pre‐exponential factor for β‐hydride elimination from DEG‐dosed Si(100). High resolution electron energy loss spectra of the DEG‐saturated surface support an adsorption mechanism in which the ethyl groups remain bonded to the incoming germanium atom throughout the adsorption process.

Hydrogen desorption on various H-terminated Si(100) surfaces due to electron beam irradiation: Experiments and modeling

Hydrogen desorption from (2×1) and (3×1) H‐terminated Si(100) surfaces due to irradiation by electron beams with 2–5 keV beam energies has been investigated both experimentally and theoretically. Auger electron spectroscopy (AES) has been employed to monitor Si, O, and C signals periodically with continuous irradiation of an electron beam on H‐terminated Si(100) surfaces. An incubation phenomenon is observed in the time evolution profiles of the Si, O, and C AES signals for all H‐terminated Si(100) surfaces. The incubation period is believed to be associated with the time required for desorption of hydrogen from the H‐terminated Si surface as a result of electron beam irradiation. Among (2×1) and (3×1) H‐terminated Si(100) surfaces, the (3×1) surface is found to have greater hydrogen coverage than (2×1) surface. The hydrogen desorption cross section is found to range from 4×10−19 to 8×10−18 cm2 and decrease with increasing beam energy in the 2–5 keV range.

Surface chemistry of diethylsilane and diethylgermane on Ge(100)

The adsorption and desorption kinetics of diethylsilane (DES) and diethylgermane (DEG) on a Ge(100) (2×1) surface have been studied using temperature‐programmed desorption (TPD), Auger electron spectroscopy (AES), and high‐resolution electron energy‐loss spectroscopy. DES and DEG adsorb at room temperature in a self‐limiting fashion. Data indicate that both precursors dissociatively chemisorb, producing a hydrogen‐ and ethyl‐terminated surface. TPD of DES‐saturated and DEG‐saturated surfaces revealed only two desorbing species, ethylene and hydrogen. The ethylene signal results from the decomposition of the ethyl groups via β‐hydride elimination, with a desorption peak temperature of ∼616 K for both precursors. The hydrogen TPD spectra were also similar for both precursors and consisted of two peaks: a low‐temperature peak corresponding to hydrogen desorption from the monohydride state and a high‐temperature peak corresponding to desorption of hydrogen produced by β‐hydride elimination of the ethyl groups...

In situ P-doped Si and Si 1–x Ge x epitaxial films grown by remote plasma enhanced chemical vapor deposition

Remote plasma-enhanced chemical vapor deposition has been applied to growin- situ doped n-type epitaxial Si and Si1−xGex with the introduction of phosphine. Growth rates and dopant incorporation have been studied as a function of process parameters (temperature, rf power, and dopant gas flow). Growth rates remain unaltered with the introduction of PH3 during deposition, unlike in many other low temperature growth techniques. Phosphorus incorporation shows a linear dependence on PH3 flow rate, but has little if any dependence on the other growth parameters, such as radio frequency power and substrate temperature, for the ranges of parameters that were examined. Phosphorus concentrations as high as 4 × 1019 cm−3 at 14 W have been obtained.

Adaptive temperature program ALE of Si1 − xGex/Si heterostructures from Si2H6/Ge2H6

Abstract We describe a thermally-driven atomic layer epitaxy technique that allows submonolayer thickness control over the growth of planar heterostructures. Our temperature programmed desorption measurements show that the temperature required to desorb essentially all passivating hydrogen species decreases sharply with increasing Ge coverage (99.9% removed after 5 min at 730 K on Si and 5 min at 580 K on Ge). In adaptive temperature program-atomic layer epitaxy, this coverage dependence of the hydrogen desorption rate is exploited to supply sufficient heat to reactivate the surface, while minimizing the impact of other thermally activated processes, such as surface segregation and island formation that are also driven by high concentrations of germanium. Temperature programmed desorption, Auger electron spectroscopy and cross-section transmission electron microscopy indicate that Ge interface segregation is rapid for such growth conditions, but limited in spatial extent to several monolayers. The onset of three-dimensional island growth was observed only for concentrated (> 20%-Ge) alloy films.

Hydrogen plasma cleaning of the Si(100) surface: Removal of oxygen and carbon and the etching of Si

Silicon etch rates and the in situ remote H plasma cleaning effectiveness in a Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) system have been measured under conditions of varying substrate temperature, exposure time, and hydrogen pressure. An incubation phenomenon is observed in the Si etch rate as a function of exposure time to the plasma species at a substrate temperature of 250°C. The etch rate is observed to increase from 20 A/hr. for 45 min. exposure to 70 A/hr. for a 4 hour exposure. The dependence of the etch rate on the plasma discharge pressure shows the etch rate to decrease from 72 A/hr. at 50-55 mTorr to 4 A/hr. for pressures greater than 125 mTorr. The substrate temperature is found to have the greatest effect on both the cleaning of the surface and the Si etch rate, with 250AC resulting in a surface with oxygen and carbon concentrations less than the detection limit for Auger Electron Spectroscopy. On wafers exposed to atomic H at both 150 and 400AC, oxygen, carbon and nitrogen were detected after 4 hr. exposures. The etch rate is inversely related to temperature, consistent with earlier results. At a substrate temperature of 150AC, Reflection High Energy Electron Diffraction (RHEED) shows the diffraction pattern to change from a streak pattern observed at higher temperatures to a spot pattern, indicating a roughened surface after 4 hrs. of etching. Wafers cleaned at temperatures above 150AC yielded streak patterns even after 4 hr. exposure. Increasing the H pressure during the clean had no effect on the final RHEED pattern.

Control of deposition rate in remote plasma enhanced chemical vapor deposition of GexSi1−x/Si heteroepitaxial films

The addition of germane and dopant gases significantly alter the growth kinetics of low temperature Si epitaxy. Germane, phosphine, and diborane have been reported to both enhance and retard film growth rate in various processes. The growth kinetics of remote plasma enhanced chemical vapor deposition are largely unaffected by the addition of GeH4 or by in situ doping. Adsorption sites are created by low energy ion bombardment and are only minimally dependent on temperature for activation. Ion‐induced gas phase reactions also play an important role in the deposition via formation of precursors which have greater sticking probabilities and insertion rates into the hydrogenated surface than for the direct reactions of SiH4 and GeH4 with the Si surface in thermal chemical vapor deposition.

In Situ Low Temperature Cleaning and Passivation of Silicon by Remote Hydrogen Plasma for Silicon-Based Epitaxy

Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD), which involves nonthermal, remote plasma excitation of precursors, has been demonstrated to be a novel and attractive technique for low temperature (150-450C) Si and Si l-x Ge x epitaxy for applications in Si ULSI and novel Si heterostructure devices which require compact doping profiles and/or heterointerfaces. An in situ low temperature remote hydrogen plasma clean in the Ultra-High Vacuum (UHV) deposition chamber in order to achieve a chemically passive, hydrogenated Si surface with minimal O, C and N contamination, is a critical component of the process. The ex situ wet chemical cleaning consists of ultrasonic degreasing and a modified RCA clean, followed by a final dilute HF dip. The in situ clean is achieved by remote plasma excited H, where H introduced through the plasma column is r-f excited such that the plasma glow does not engulf the wafer. In situ AES analysis shows that the remote H plasma clean results in very substantial reduction of the C, O and N contamination on the Si surface. We believe that the H plasma produces atomic H which, in turn, produces a reducing environment and has a slight etching effect on Si and SiO 2 by converting them to volatile byproducts. TEM analysis of the wafers subjected to this clean indicate that defect-free surfaces with dislocation loop densities below TEM detection limits of 10 5 /cm 2 are achievable. Corroborating evidence of achieving an atomically clean, smooth Si surface by remote H plasma clean as obtained from in situ RHEED analysis will also be presented. After in situ H cleaning at low pressures (45 mTorr), typically for 30 min. at a substrate temperature of 310 C, we observe both stronger integral order streaks compared to the as-loaded sample and the appearance of less intense half-order lines indicative of a (2 × 1) reconstruction pattern, indicating a monohydride termination. A (3 × 1) reconstruction pattern is observed upon H plasma clean at lower temperatures (250 C), which can be attributed to an alternating monohydride and dihydride termination. Results of air exposure of hydrogenated Si surfaces by AES analysis indicate that the (3 × l) termination is chemically more inert towards readsorption of C and 0. Successful Si homoepitaxy and Si/Si l-x Ge x heteroepitaxy under a variety of surface cleaning conditions prove that by a combination of these cleaning techniques, and by exploiting the inertness of the H-passivated Si surface, very low defect density films with 0 and C levels as low as 1X10 18 cm −3 and 5×10 17 cm −3 , respectively, can be achieved.

Growth of GexSi1-x/Si heteroepitaxial films by remote plasma chemical vapor deposition

GexSi1−x/Si heteroepitaxial thin films have been grown using the low‐temperature remote plasma‐enhanced chemical vapor deposition (RPCVD) approach, in which the substrate is kept remote from the glow discharge, and an Ar plasma is employed to indirectly activate the reactant gases (SiH4 and GeH4) and drive the chemical deposition reactions. Secondary ion mass spectroscopy (SIMS), plan‐view and cross‐sectional transmission electron microscopy (TEM), and in situ reflection high‐energy electron diffraction (RHEED) have been employed to analyze the films with different Ge mole fractions and thicknesses. Abrupt Si/GexSi1−x heterointerfaces with the Ge concentration changing by 10× in about 30 A (SIMS resolution limit) have been achieved. Commensurate growth has been observed for layers whose thicknesses are below the critical layer thicknesses (CLTs). Crystalline GexSi1−x/Si films with high mole fractions of Ge (up to 60%), which are thicker than the CLTs, show relaxation of misfit strain. This results in more...