T. Hsu

Correlation between silicon hydride species and the photoluminescence intensity of porous silicon

The role of silicon hydride species in the photoluminescence intensity behavior of porous Si has been studied. The surfaces of luminescent porous Si samples were converted to a predominate SiH termination using a remote H plasma. The as‐passivated samples were then immersed in various concentrations of hydrofluouric solutions to regulate the recovery of SiH2 termination on the surface. Photoluminescence measurements and transmission Fourier‐transform infrared spectroscopy have shown that predominant silicon monohydride (SiH) termination results in weak photoluminescence. In contrast, it has been observed that the appearance of silicon dihydride (SiH2) coincides with an increase in the photoluminescence intensity.

Homoepitaxial films grown on Si (100) at 150°C by remote plasma-enhanced chemical vapor deposition

Low‐temperature silicon epitaxy is critical for future generation ultralarge scale integrated circuits and silicon‐based heterostructures. Remote plasma‐enhanced chemical vapor deposition has been applied to achieve silicon homoepitaxy at temperatures as low as 150 °C, which is believed to be the lowest temperature reported to date. Critical to the process are an in situ remote plasma hydrogen cleaning of the substrate surface in an ultrahigh vacuum growth chamber prior to epitaxy, and substitution of thermal energy by remote plasma excitation via argon metastables and energetic electrons to dissociate silane and increase adatom mobility on the surface of the silicon substrate. Excellent crystallinity with very few defects such as dislocations and stacking faults is observed.

Very low defect remote hydrogen plasma clean of Si (100) for homoepitaxy

We discuss a remote hydrogen plasma cleaning technique which is effective at removing carbon, nitrogen and oxygen from a silicon surface that has been pretreated with a wet chemical clean and a final dilute HF dip. It has been found that for best results, air exposure of the wafer after the HF dip must be minimized, as the ability of the H plasma clean to remove oxygen seems to be reduced on wafers which have been exposed to air for several hours. The atomic H supplied by the clean also results in different hydrogen terminated surface reconstructions depending on the substrate temperature. We observe a (1 × 1) RHEED pattern at clean temperatures below 190° C which is believed to be due to disordered silicon monohydride and dihydride termination, a (3 × 1) pattern between 200 and 280° C, corresponding to ordered monohydride and dihydride coverage, transitioning to a (2 × 1) pattern around 300° C due to monohydride termination. A dilute HF dip produces a (1 × 1) pattern due to disordered (3 × 1) cells,i.e. disordered monohydride and dihydride coverage. The passivating effects of the dilute HF dip and of the remote hydrogen plasma clean have also been discussed. The two processes have been found to result in similar hydrogen coverage of the wafer surface which in turn results in similar surface passivation. Both the dilute HF dip and thein situ remote H plasma clean have been found to protect the wafer from contamination for up to 15 min in ambient air and for an indefinite period under vacuum.

Cleaning and passivation of the Si(100) surface by low temperature remote hydrogen plasma treatment for Si epitaxy

This paper presents the results of a study of the hydrogen-passivated Si(100) surface prepared by a remote hydrogen plasma treatment which serves the dual purpose of cleaning and passivating the Si(100) surface prior to low temperature Si epitaxy by Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD). The remote hydrogen plasma treatment was optimized for the purposes of cleaning and passivation, respectively. To achieve a clean, defect-free substrate surface, the remote hydrogen plasma process was first optimized using Transmission Electron Microscopy (TEM) and Auger Electron Spectroscopy (AES). For hydrogen passivation, the substrate temperature was varied from room temperature to 250° C in order to investigate the degree of passivation as a function of substrate temperature by examining the amount of oxygen readsorbed on the substrate surface after air exposure. Low temperature Si expitaxy was subsequently performed on the air-exposed substrates without further cleaning to evaluate the effectiveness of the hydrogen passivation. It was found that better Si surface passivation is achieved at lower substrate temperatures as evidenced by the fact that less oxygen is observed on the surface using AES and Secondary Ion Mass Spectroscopy (SIMS) analyses. The amount of readsorbed oxygen on the H-passivated Si surface after a two hour air exposure was found to be as low as 0.1 monolayer from SIMS analysis. Using Reflection High Energy Electron Diffraction (RHEED) analysis, different surface reconstructions ((3 × 1) and (1 × 1)) were observed for H-passivated Si surfaces passivated at various temperatures, which was correlated to the results of AES and SIMS analyses. Epitaxial growth of Si films at 305° C was achieved on the air-exposed Si substrates, indicating a chemically inert Si surface as a result of hydrogen passivation. A novel electron-beam-induced-oxygen-adsorptiom phenomena was observed on the Hpassivated Si surface. Scanning Auger Microscopy (SAM) analysis was performed to study the reaction kinetics as well as the nature of Si—H bonds on the H-passivated Si surface. Preliminary results show that there is a two-step mechanism involved, and oxygen adsorption on the H-passivated Si surface due to electron beam irradiation may be due to the formation of O-H groups rather than the creation of Si—O bonds.

Remote plasma-enhanced CVD of silicon: reaction kinetics as a function of growth parameters

The reaction kinetics in Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) have been studied for a chamber pressure of 200 mTorr, rf powers between 4 and 8 W, diluted silane flow rates between 5 and 40 sccm, and temperatures between 190 and 480° C. The observed temperature dependence of growth rate reveals a change in activation energy at 300–325° C, suggesting that hydrogen desorption is the rate limiting step in the deposition reaction. A strong dependence of growth rate on rf power has been attributed in part to the extension of the glow discharge region closer to the substrate at higher rf powers. Growth rate has been shown to increase when the sample is positioned closer to the glow, indicating that the reaction precursor is a short-lived species, probably SiH2 or SiH3. Growth rate has been shown to exhibit a sublinear dependence on silane partial pressure. Oxygen incorporation in the deposited films has been studied using Secondary Ion Mass Spectroscopy (SIMS), and it has been found that the main source of oxygen contamination is the process gases. However, it has also been found that “point of use” purification of the process gases reduces water and oxygen contamination significantly, reducing the oxygen incorporation in the films by an order of magnitude.

The use of Langmuir probe measurements to investigate the reaction mechanisms of remote plasma-enhanced chemical vapor deposition

Langmuir probe measurements of plasma density and electron temperature have been used to investigate the reaction kinetics in remote plasma-enhanced chemical vapor deposition (RPCVD) of Si on Si (100) substrates. The increased growth rate for negative substrate bias indicates that positively charged ions are involved in the deposition reaction. A comparison of growth rate and plasma density data indicates that the growth rate is proportional to the ion flux. It is concluded that the rate limiting reaction in RPCVD is H desorption from the hydrogenated Si surface by ion bombardment.

Defect microstructure in low temperature epitaxial silicon grown by RPCVD

Defect characterization of epitaxial silicon films grown by low temperature remote plasmaenhanced chemical vapor deposition (RPCVD) under various conditions is discussed. The film morphology and crystallinity have been examined by defect etching/Nomarski optical microscopy and transmission electron microscopy. Prior to epitaxial growth, anex situ wet chemical clean and anin situ remote hydrogen plasma clean were performed to remove the native oxide as well as other surface contaminants such as carbon. A damage-free (100) Si surface with extremely low concentrations of carbon and oxygen as confirmed byin situ Auger electron spectroscopy can be achieved using this cleaning technique at temperatures as low as 250°. Low temperature Si homoepitaxy was achieved by RPCVD on lightly doped (100) Si substrates. Growth parameters such as silane flow rate (partial pressure), chamber pressure, and substrate temperature were varied during epitaxial growth to investigate the dependence of film quality on these parameters. For comparison,in situ remote hydrogen plasma and epitaxial growth were also performed on heavily dopedp-type (100) Si substrates. Finally, the results of epitaxial growth at temperatures as low as 150° are presented.

Advances in remote plasma-enhanced chemical vapor deposition for low temperature in situ hydrogen plasma clean and Si and Si 1- x Ge x epitaxy

Remote plasma-enhanced chemical vapor deposition (RPCVD) is a low temperature growth technique which has been successfully employed inin situ remote hydrogen plasma clean of Si(100) surfaces, silicon homoepitaxy and Si1- xGex heteroepitaxy in the temperature range of 150–450° C. The epitaxial process employs anex situ wet chemical clean, anin situ remote hydrogen plasma clean, followed by a remote argon plasma dissociation of silane and germane to generate the precursors for epitaxial growth. Boron doping concentrations as high as 1021 cm−3 have been achieved in the low temperature epitaxial films by introducing B2H6/He during the growth. The growth rate of epitaxial Si can be varied from 0.4Å/min to 50Å/min by controlling therf power. The wide range of controllable growth rates makes RPCVD an excellent tool for applications ranging from superlattice structures to more conventional Si epitaxy. Auger electron spectroscopy analysis has been employed to confirm the efficacy of this remote hydrogen plasma clean in terms of removing surface contaminants. Reflection high energy electron diffraction and transmission electron microscopy have been utilized to investigate the surface structure in terms of crystallinity and defect generation. Epitaxial Si and Si1-xGex films have been grown by RPCVD with defect densities below the detection limits of TEM (~105 cm-2 or less). The RPCVD process also exploits the hydrogen passivation effect at temperatures below 500° C to minimize the adsorption of C and 0 during growth. Epitaxial Si and Si1-xGex films with low oxygen content (~3 × 1018 cm-3) have been achieved by RPCVD. Silicon and Si/Si1-xGex mesa diodes with boron concentrations ranging from 1017 to 1019 cm-3 in the epitaxial films grown by RPCVD show reasonably good current-voltage characteristics with ideality factors of 1.2-1.3. A Si/Si1-xGex superlattice structure with sharp Ge transitions has been demonstrated by exploiting the low temperature capability of RPCVD.In situ plasma diagnostics using single and double Langmuir probes has been performed to reveal the nature of the RPCVD process.

The photoluminescence spectra of porous silicon boiled in water

The photoluminescence (PL) of porous Si that has been hydrogenated in boiling water has been investigated. The PL intensity is observed to increase with a concurrent shift of the spectral peak to shorter wavelengths. These effects can be explained in terms of size effects in the microstructures of porous Si. The spectral changes after annealing and after rehydrogenation are similar to the behavior of a‐Si:H. We conclude that hydrogen plays an important role in the luminescence of porous Si.

Defect microstructure in single crystal silicon thin films grown at 150°C–350°C by the remote plasma-enhanced chemical vapor deposition

Defect microstructure in terms of defect density and impurity concentration of epitaxial Si films grown by low temperature Remote Plasma-enchanced Chemical Vapor Deposition (RPCVD) in the temperature range of 150–305° C has been investigated using Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectroscopy (SIMS), Reflection High Energy Electron Diffraction (RHEED) and defect etching/Nomarski microscopy. Defect density in the epitaxial Si films is found to be a strong function of growth temperature in the temperature range under study, indicating that thermal excitation is an important source of energy, in addition to plasma excitation, for driving surface reactions in the RPCVD expitaxial process. Impurity concentrations of H, O and C in the epitaxial films have been determined by SIMS analysis. The trace amounts (∼1 ppm) of oxygen and water vapor in the reactant gas (2%SiH4/He) was identified to be an important source of oxygen in the epitaxial Si films. An oxygen concentration as low as 3 × 1018 cm-3 in the epitaxial Si film grown at 150° C has been achieved through the use of a gas purifier. The higher hydrogen concentration in the films grown at lower temperatures is believed to be due to insufficiently rapid hydrogen desorption from the surface during growth. The results of characterization using TEM and SIMS are discussed to elucidate the atomistic mechanisms of Si epitaxial growth by RPCVD.