Eliezer Dovid Richmond

Surface treatment of (11̄02) sapphire and (100) silicon for molecular beam epitaxial growth

A low‐temperature surface preparation technique for molecular beam epitaxial growth of silicon on sapphire and silicon is described. Thermal desorption of regrown oxide has been accomplished at 850 °C and epitaxial growth at 650 °C. A comparison of two surface treatment techniques for silicon (100) and sapphire (1102) substrates is reported.

Unique properties of molecular beam epitaxy silicon on sapphire using in situ high-temperature substrate annealing compared with chemically vapor deposited silicon on sapphire

Abstract The dramatically different and superior properties of molecular beam epitaxy (MBE) of silicon on sapphire (SOS) have been investigated and compared with standard commercially available chemically vapor deposition (CVD) SOS. X-ray diffraction reveals a 24% reduction in the strain of the MBE SOS relative to the CVD SOS. The MBE SOS has a 40% higher electron Hall mobility at room temperature. At liquid N 2 temperatures, the electron Hall mobility of the MBE SOS increases instead of decreasing as in CVD SOS. The microtwin differential volume fraction profile for MBE SOS is lower by more than an order of magnitude compared with that of CVD SOS; it decreases faster with distance from the Si-sapphire interface; and it effectively goes to zero at 0.3 μm from the interface. The average Si-sapphire interface charge for MBE SOS is −8.0×10 10 cm −2 , which is negative and more than an order of magnitude lower than the interface charge of 2×10 12 cm −2 for CVD SOS. Some of the unique features of the Naval Research Laboratory VG80 Si MBE/Surface-Analytical System are discussed.

Elimination of microtwins in silicon grown on sapphire by molecular beam epitaxy

Using transmission electron microscopy, we have examined a number of (001) silicon thin films grown on (1012) sapphire substrates by molecular beam epitaxy (MBE). We have found that for silicon films less than 0.55 μm thick, microtwins are very much in evidence. For silicon films greater than 700 nm thick, however, dislocations, rather than microtwins, are the predominant defect. It is our conjecture that dislocation extension, and the associated disappearance of microtwins in thicker MBE‐grown SOS films, is analogous to the generation of misfit dislocations in silicon‐germanium films grown on silicon or germanium substrates by MBE; furthermore, these observations can be understood via the concept of excess stress that has been recently formulated by Tsao, Dodson, and others. The persistence of microtwins in SOS grown by chemical vapor deposition, as opposed to MBE‐grown SOS, can be understood in terms of Dodson and Tsao’s formulation of plastic deformation kinetics in thin films.

X-ray photoelectron spectroscopy analysis of the growth kinetics of Ge on Si(001)

Abstract The growth kinetics of Ge on Si(001) is determined from the X-ray photoelectron measurements of the Si 2p core level intensity and the Ge 2p 3 2 core level intensity as a function of the Ge overlayer thickness. The growth kinetics of the Ge overlayer follow a Stranski-Krastanov growth mode. The critical thickness is ≈2.2 ML (where ML means monolayers). The change in the growth kinetics, from a layer-by-layer mode to an island nucleation-and-growth mode is spontaneous. The distribution and the partial relaxation of the misfit strain of this pseudomorphic system are measured by the change in the full width at half-maximum of the Si 2p and Ge 2 p 3 2 core levels. The strain is found to be confined to the Ge overlayer. Partial relaxation of the misfit strain precedes the transition from the layer-by-layer mode to the island nucleation-and-growth mode. Scanning electron micrographs confirm that the critical thickness occurs in the interval 1.8 ML of Ge to 2.8 Ml of Ge. A second transition occurs at ≈6.9 ML of Ge and corresponds to the nucleation and growth of large three-dimensional islands.

Relief of compressive biaxial strains in thin films via microtwins

For heteroepitaxial systems, such as silicon on sapphire, microtwins can usually be observed in the epitaxial layer. It has also been suggested that microtwins play a significant role in strain relief in these systems. From a knowledge of the differential volume fraction of microtwins occurring in a heteroepitaxial systems, it is possible to estimate the greatest possible strain relief due to microtwins. Measurements of the differential volume fraction of microtwins in silicon‐on‐sapphire, however, indicate that the strain relief due to microtwins cannot be greater than 0.7%, even though the lattice mismatch between silicon and sapphire is an order of magnitude larger. Therefore, if the silicon/sapphire interface is coherent, the misfit strain must be relieved by another mechanism.

Molecular beam epitaxy versus chemical vapor deposition of silicon on sapphire

Molecular beam epitaxy (MBE) of Si on sapphire (SOS) has dramatically different and superior properties compared to chemical vapor deposited (CVD) SOS. The strain in the Si epilayer decreases by 21%. A 40% higher electron Hall mobility occurs at room temperature. At LN2 temperatures the electron mobility increases to a level which is more indicative of bulk Si than of CVD SOS. The microtwin differential volume fraction profile is lower by more than an order of magnitude, and decreases below the detectable limit at 300 nm from the interface. The average Si/sapphire interface charge for MBE SOS is −8.0×1010 cm−2, while the interface charge of CVD SOS is 2×1012 cm−2.

Photoreflectance and X-ray studies of silicon films on sapphire

Abstract Photoreflectance was used for the first time to study silicon films on sapphire (SOS). The film thicknesses ranged from 150 to 40 000 A. The 3.4 eV structure was monitored with bulk silicon as a standard. A shift of this structure toward lower energies was observed for the thinner films. As the film thickness increased from 1000 to 40000 A, the energy shift appeared to oscillate about the energy value associated with the bulk silicon sample. The energy shift with thickness of the photoreflectance structure is consistent with that calculated from the X-ray measured strain for the thicker films. The short optical penetration depth at 3.4 eV (≈ 100 A) is essential to allowing photoreflectance investigation of films of this order of thickness.

Single‐crystal germanium grown on (11̄02) sapphire by molecular beam epitaxy

Crystalline germanium films have been successfully grown on the (1102) sapphire surface using molecular beam epitaxy. Growth at temperatures above 700 °C and after preannealing the sapphire substrates above 1100 °C resulted in germanium films with a (110) orientation. A 500 nm germanium film grown at 800 °C after preannealing the sapphire substrate at 1400 °C gave an x‐ray rocking curve width that measured 317 arcsec at half maximum for the (220) reflection.

Xps Analysis of the Sapphire Surface as a Function of High Temperature Vacuum Annealing

For the first time the (1102) surface of sapphire has been investigated by X-ray photoelectron spectroscopy to ascertain chemical changes resulting from annealing in vacuum at 1300° C and 1450° C. As received substrates had a substantial surface C contaminant. For substrates that were chemically cleaned before inserting them into the MBE system no trace of carbon is detected. A residual flourine contaminant results from the cleaning procedure and is desorbed by the vacuum annealing. Spectra of annealed substrates are compared to the unannealed chemically cleaned substrates. The annealed substrates exhibit 0.4 to 0.5 eV shift to higher binding energy of the Al peak and a 0.3 eV shift to higher binding energy of the O peak. In addition, a 2% depletion of oxygen from the surface occurs.

Characteristics of the material improvement process for silicon on sapphire by solid phase epitaxial regrowth

A comprehensive matrix study of the material modification of chemical vapor deposited silicon on sapphire has been made. The modification is effected by applying twice the process of ion implantation with 28 Si+ ions followed by solid phase epitaxial regrowth (SPEG) induced by furnace annealing. The matrix consisted of varying the fluences for the first and second implantation from just above the critical fluence for forming an amorphous phase to 7×1015cm−2. The defect reduction was studied for each sample of the matrix. This study has shown that the density of defects at the silicon/sapphire interface and silicon surface depends on both implantations. The deposited damage energy peak position is estimated to occur at a depth ∼Rp for a substrate temperature of 330 K during implantation. The position of the damage peak affects the estimated damage at the silicon sapphire interface which controls the aluminum outdiffusion. The major reduction in the defect density has already occurred for fluences of the tw...