Heteroepitaxy Of Ge Research Articles

Direct REM observation of structural processes on clean silicon surfaces during sublimation, phase transition and epitaxy

Reflection electron microscopy (REM) has been applied to an situ study of silicon sublimation, phase transition and epitaxy. Reversible rearrangement of the stepped Si(111) surface from a regular monatomic-step system to step bunching was observed. During long-time annealing by DC at step bunching temperature the formation of step antibands has been found on the stepped Si(111) surface. The antibands are almost parallel to step bands and contain monatomic steps with opposite sign compared to the initial steps. Rearrangements of the Si(111) surface morphology (regular steps ⇔ step bands system ⇔ step band-antiband system) are reversible. During study of initial stages of homoepitaxial growth on Si(111) the areas free from growth islands near the monatomic step have been observed. These areas are characterized by the (7 × 7) structure on the upper terrace, whereas the structure of these areas on the lower terrace is (1 × 1). The reversible monatomic steps clustering during the superstructural transitions at heteroepitaxy of Ge and Au on surface (111) due to displacement of segments of steps restricting the superstructural domains has been found.

Microstructural evolution during the heteroepitaxy of Ge on vicinal Si(100)

Microstructural evolution during the initial stages of islanding of Ge on vicinal Si(100) has been studied in situ with nanometer resolution in an ultrahigh-vacuum scanning transmission electron microscope. Ge is deposited using molecular-beam-epitaxy (MBE) techniques on vicinal Si(100) misoriented 1° and 5° toward 〈110〉. For MBE-type experiments, there is evidence for metastable growth of the Ge intermediate layer to much greater than the equilibrium critical thickness. The layer may grow up to seven monolayers thick before islanding in the Stranski–Krastanov growth mode. The presence of strong adatom sinks significantly alters the growth and size distribution of the islands when the spacing of these sinks is less than an adatom diffusion distance. Studies of the initial stages of islanding in solid-phase MBE indicate that there is no long-range adatom diffusion. There is an initial fast transformation from a disordered layer growth, followed by a sluggish growth of islands. We have studied the coarsening of these islands at the earliest stages with sensitivity to islands as small as 2 nm in radius. It appears that there is a novel coarsening mechanism influenced by an unstable intermediate layer and dislocation-free islands. In all cases, the dislocation-free islands grow more slowly than those which have relaxed by the introduction of misfit dislocations.

Low-temperature heteroepitaxy of Ge on Si by GeH4 gas low-pressure chemical vapor deposition

The crystalline quality of Ge grown epitaxially on Si can be improved by using GeH4/H2 gas low-pressure chemical vapor deposition at a low temperature of 410 °C and surface-cleaning processing with a purified-H2 flow prior to GeH4 introduction into a CVD reactor. H2 flow processing is carried out at growth temperature. The dislocation density of a 1.3-μm thick Ge film is nearly 2×108 cm−2, which is as low as that of molecular-beam epitaxy (MBE)-grown Ge films. Stacking faults commonly generated in the CVD-grown Ge films at temperatures higher than 700 °C are not observed. The 700 °C heat treatment of the Ge film reduces dislocation density to 2×107 cm−2 and causes no interface reaction between Ge and Si. This technique is superior to conventional chemical vapor deposition (CVD) techniques for Ge epitaxial growth.

Optimization of the heteroepitaxy of Ge on GaAs for minority-carrier lifetime

Growth of Ge on GaAs at reasonably high temperatures, which produces better crystallinity in the Ge, presents serious difficulties due to the dissociation of the GaAs substrate. In this paper, we describe the growth of a low-temperature buffer layer of Ge on GaAs that considerably reduces the effects of decomposition of the GaAs during high-temperature growth of Ge. Using this approach, we present the first report of highly specular, mass-transport-limited, high-temperature growth of Ge on GaAs that is comparable to the homoepitaxy of Ge, although with a reasonably high residual n-type ( ∼10 18 cm -3) doping level. The factors affecting the structural, electrical and optical properties of Ge on GaAs, using such an epitaxial growth technique, were studied. Lifetime variations from very low values to about 0.45 μs were measured by a microwave technique as a function of growth conditions. Significantly, the removal of the surface oxide on the GaAs substrate prior to low-temperature buffer-layer growth, terminating the flow of germane (GeH 4) during the ramp to high growth temperatures, thinner buffer layers, and high-temperature growth of Ge, were found to be important for obtaining long lifetimes.

Reflection electron energy loss spectroscopy during initial stages of Ge growth on Si by molecular beam epitaxy

Using a conventional reflection high-energy electron diffraction gun together with an electron energy loss spectrometer, we have combined in situ measurements of inelastic scattering intensities from Si L2,3 and Ge L2,3 core losses with reflection electron diffraction data in order to analyze the initial stages of Ge heteroepitaxy on Si(001). Diffraction data indicate an initial layer-by-layer growth mode followed by island formation for Ge thicknesses greater than 0.8–1.1 nm. The electron energy core loss data are consistent with a simple model of grazing incidence electron scattering from the growing Ge film. Reflection electron energy loss spectroscopy is found to be highly surface sensitive, and the energy resolution and data rate are also sufficiently high to suggest that reflection electron energy loss spectroscopy may be a useful real-time, in situ surface chemical probe during growth by molecular beam epitaxy.

Surface Analysis During the Growth of Ge and GexSi1−x Alloys on Si by Reflection Electron Energy Loss Spectrometry

ABSTRACTUsing a conventional 30 keV reflection high energy electron diffraction gun on a custom built molecular beam epitaxy system together with an energy loss analyser, we have combined in situ measurements of inelastically scattered electrons from Si L2,3 (99 eV) and Ge L2,3 (1217 eV) core losses with reflection electron diffraction data in order to study the initial stages of Ge heteroepitaxy on Si (001). Diffraction data indicate Stranski-Krastanov growth mode and indicate islanding for Ge thicknesses greater than 0.3 nm. The normalized core loss intensities from electron energy loss data are consistent with a simple model based on grazing incidence electron scattering from the growing Ge film.Additional experiments designed to determine the surface composition sensitivity from Si K (1840 eV) and Ge L2,3 ionization cross-sections reveal that accurate values of composition for a dilute GexSi1−x alloy (x=0.02) can be obtained from raw data when compared with Rutherford Backscattering and Transmission Electron Microscopy/X-ray microanalysis measurements.

Ge Heteroepitaxy on (100) Si Substrates by Ion Assisted Deposition

ABSTRACTGe films have been grown by ion-assisted deposition on Si(100) substrates. Initial studies were conducted to determine the growth and morphology of Ge using a modified crucible arrangement and ionization chamber. It was observed that a narrow substrate temperature range existed for the deposition of smooth Ge films without ion assistance. Ionization and acceleration appear to induce additional damage to the film without improving crystalline quality.

Initial stage of heteroepitaxy of Ge on Si(111)-7 × 7 and (100)-2 × 1 surfaces studied by low-energy electron-loss spectroscopy (LEELS)

The initial stage of heteroepitaxial growth of Ge on Si surfaces has been investigated by low-energy electron-loss spectroscopy (LEELS) combined with low-energy electron diffraction (LEED) and Auger-electron spectroscopy (AES). Ge was deposited onto Si(111)-7 × 7 and (100)-2 × 1 surfaces maintained at RT, 350°C and 500°C. At RT, the LEELS spectrum of Si(111) and (100) surfaces evolves to that of amorphous Ge (a-Ge) with increase in Ge thickness. Below one monolayer (1 ML) coverage, the surface plasmon of Si splits into two peaks due to a Ge-induced transition at localized GeSi bonds. At high temperatures, the bulk plasmon maintains about the same energy up to ∼ 30 Å deposition of Ge, consistent with a pseudomorphic growth of thin Ge layers (∼ 4 ML) followed by islands formation on the layers. With 30 Å deposition, the surface-related structures in the LEELS Spectrum of Ge on Si(111) become similar to those of a clean Ge(111) surface, while those of Ge on Si(100) are rather weak compared with those of the clean Ge(100) surface. The (7 × 7) structure of the Si(111) surface is replaced by a (5 × 5) one which may be caused by the formation of GeGe bonds with Ge deposition of 1.5–2 ML. However, in spite of the appearance of the (5 × 5) pattern in LEED, the LEELS measurements reveal that the back-bond surface states of Si remain up to some critical thickness depending on the deposition and/or annealing temperature.

Growth, nucleation, and electrical properties of molecular beam epitaxially grown, As-doped Ge on Si substrates

Epitaxial Ge is grown on (100)Si substrates by molecular beam epitaxy (MBE). The effect of various MBE growth conditions on both the nucleation and morphology of Ge grown on Si is studied by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and reflection high-energy electron diffraction (RHEED). These studies indicate that, at the substrate temperatures examined (300–600 °C), heteroepitaxy of Ge on Si favors three-dimensional growth, which is enhanced by both higher growth temperatures and substrate preparation techniques that leave residual surface contamination. Heavily doped n+ Ge layers are obtained using an elemental As source. The electrical properties of these films are evaluated by Hall–van der Pauw measurements. Growth temperatures of 250 °C and optimum As:Ge flux ratios yield electron concentrations as high as 2.5×1020 cm−3. Secondary ion mass spectroscopy (SIMS) and Hall effect data show that for As concentrations which exceed this optimum level, a decrease in both the electron concentration and drift mobility is observed, indicating the presence of electrically inactive As.

Heteroepitaxial growth of Ge films on the Si(100)-2×1 surface

The initial stage of Ge heteroepitaxy on a Si(100)-2×1 surface has been investigated by low-energy electron diffraction (LEED) and Auger-electron spectroscopy (AES). The growth mode of the Ge films was studied by measuring the decrease in the Si(LVV) AES line at 92 eV with an increase in the Ge overlayer thickness. The Ge films deposited at room temperature exhibit layer-by-layer growth up to at least six monolayers. When the substrate is heated up to 350 °C, the growth mode is characterized by the Stranski–Krastanov type; i.e., the first three monolayers of growth is followed by island formation. Although these characteristics of the growth mechanism are similar to the case of Ge on Si(111)-7×7 surfaces, annealing behavior of the Ge films suggests that the bond strength between Ge and Si is stronger on Si(100) than on Si(111) surfaces. In contrast to the case of Ge on Si(111) surfaces, where the original 7×7 superstructure of the Si surface is replaced by a new 5×5 pattern at about two-monolayer coverage of Ge, the original 2×1 LEED pattern is not strongly disturbed up to about 1–2 monolayers of Ge. In addition to the detailed study on the initial stage of heteroepitaxial growth, we observed that thick Ge films deposited onto Si(100) surfaces held at 350 and 470 °C display a sharp 2×1 LEED pattern and demonstrate a single-crystal growth of a Ge(100) face on the Si(100) surface. This is further supported by a measurement of the x-ray diffraction pattern of the Ge films.

The heteroepitaxy of Ge on Si(100) by vacuum evaporation

High quality Ge epitaxial growth has been attained on Si(100) substrates at relatively low temperatures, 350 and 440 °C, by a vacuum evaporation technique. Electrical property and structural quality for the Ge layer were evaluated by Hall measurement, x-ray diffraction, and electron beam diffraction. It is found that the growth layers are p type and that net acceptor concentration and mobility strongly depend on the layer thickness. Of particular significance is the fact that the acceptor concentration decreases most rapidly and hole mobility remarkably increases with the Ge thickness. The acceptor concentration and the mobility at a point larger than 0.7 μm from the lattice mismatched interface are 1.6×1016 cm−3 and 1040 cm2/V sec, respectively. Hole mobility varies as a function of T3/2 and T−3/2 below and above ∼100 °K.

The heteroepitaxy of Ge on Si: A comparison of chemical vapor and vacuum deposited layers

Epitaxial growth of Ge on Si has been investigated by two techniques: vacuum deposition and chemical vapor deposition (CVD). Vacuum-deposited Ge layers (physical vapor deposition, PVD) on heated Si substrates (≲500 °C) have smooth surface morphologies with a surface crystalline quality which improves with Ge layer thickness. Layers prepared by the CVD technique at 500–600 °C are comparable with the PVD prepared layers. Main defects in both PVD and CVD layers are dislocations initiating at the Ge/Si interface. Chemical vapor-deposited Ge layers grown at a substrate temperature of 700–800 °C exhibit poor crystalline quality and often are polycrystalline. Chemical vapor-deposited layers grown at a substrate temperature of 900 °C, again are good quality epitaxial layers. In this case, in addition to dislocations, stacking faults are present. All the studied layers are highly conductive and p-type. The conduction and valence band discontinuities determined from electrical measurements are 0.05±0.04 eV and 0.39±0.04 eV, respectively.