Epitaxial Ge Films Research Articles

High-resolution z-contrast imaging of semiconductor interfaces

Using a high-angle annular detector on a high-resolution STEM it is possible to form incoherent images of a crystal lattice characterized by strong atomic number or Z contrast. Figure 1 shows an epitaxial Ge film on Si(100) grown by oxidation of Ge-implanted Si. The image was obtained using a VG Microscopes' HB501 STEM equipped with an ultrahigh resolution polepiece (Cs ∽1.2 mm, demonstrated probe FWHM intensity ∽0.22 nm). In both crystals the lattice is resolved but that of Ge shows much brighter allowing the interface to be located exactly and interface steps to be resolved (arrowed). The interface was indistinguishable in the phase-contrast STEM image from the same region, and even at higher resolution the location of the interface is complex. Figure 2 shows a thin region of an MBE-grown ultrathin super-lattice (Si8Ge2)100. The expected compositional modulation would show as one bright row of dots from the 2 Ge monolayers separated by 4 rows of lighter Si columns. The image shows clearly that strain-induced interdiffusion has occurred on the monolayer scale.

Microstructural studies of epitaxial Ge films grown on [100] GaAs by laser photochemical vapor deposition

Ge films grown on [100] GaAs by laser photochemical vapor deposition (LPVD) in parallel geometry at temperatures (Ts) ranging from ∼240 to 415 °C have been examined by transmission electron microscopy. For 285 ≤Ts≲330 °C, a thin (250–700 Å) epitaxial film is grown initially but a switch to amorphous material is subsequently observed. At higher substrate temperatures (Ts ≳400 °C), thicker (≳800 Å) epitaxial Ge films are grown before the transition to polycrystalline material takes place. In the absence of external 193-nm laser radiation (i.e., growing by conventional low-pressure chemical vapor deposition), the Ge films are completely amorphous (285 ≤Ts≤330 °C) or heavily defected polycrystalline (Ts∼400 °C). The >100 °C temperature reduction for the growth of epi Ge films made possible by LPVD is attributed to the direct production of a species (GeH3) by the laser which is collisionally converted to Ge2H6 en route to the substrate. Upon reaching the [100] GaAs surface, the digermane is pyrolyzed. Experiments with [100] substrates tilted 3° toward [110] yielded thinner (∼120 Å), but smooth, epitaxial films which is attributed to the higher density of available nucleation sites. Films grown at 280–330 °C on [111] oriented GaAs were completely amorphous which appears to arise from reduced adatom mobilities on [111] surfaces. These results clearly demonstrate the feasibility of photochemically generating a species which migrates to the surface and alters the chemistry at a substrate (which is itself not illuminated by the optical source) so as to permit the growth of epitaxial semiconductor films.

Characterization of epitaxial Ge films grown by remote plasma enhanced chemical vapor deposition on Ge(111) and GaAs(111) substrates

Epitaxial Ge films have been grown on Ge(111) and GaAs(111) surfaces using remote plasma enhanced chemical vapor deposition. Prior to growth, all substrates were subjected to an in situ hydrogen plasma treatment to remove surface oxides. The effect of deposition temperature on the epitaxial quality has been studied using reflection high-energy electron diffraction, Rutherford backscattering/channeling spectrometry, and transmission electron microscopy. Reflection high-energy electron diffraction patterns show fundamental streaks on all deposited layers and fractional order streaks on layers grown between 300 and 400 °C. Rutherford backscattering/channeling analysis shows severe degradation in crystalline quality below 300 ° C. Currently, the lowest channeling yields have been for Ge/Ge(111) grown at 350 °C (χmin =4%) and Ge/GaAs(111) grown at 300 °C (χmin =8%). Transmission electron microscopy indicates that the predominant defects in these films are inclusions and microtwins, respectively. The possible origin of these defects is discussed.

High-quality optical and epitaxial Ge films formed by laser evaporation

High-quality thin films of Ge were deposited by pulsed laser evaporation of molten Ge. Films deposited on 300 °C substrates showed very smooth morphologies and single-crystal grain structures. Energetic ions in the vapor stream, generated by the laser-induced plasma, were observed to affect nucleation and bulk-film growth. Films deposited on ambient temperature substrates by laser evaporation were denser, harder, and exhibited higher values of refractive index and lower levels of intrinsic stress than the films deposited by thermal evaporation.

Electrical Properties of Heteroepitaxial Ge Films on Si(100)–2×1 Surfaces

Electrical properties of epitaxial Ge films grown on Si(100)–2×1 surfaces have been studied by Hall measurement, and their structural properties have been characterized by analyses of LEED patterns, X-ray diffraction, Raman scattering spectra and cross-sectional TEM images. It has been found that the growth layers exhibit p-type conduction, possibly due to the Ge vacancies and the temperature dependences of carrier concentration n(T) and hole mobility µ(T) depend on growth conditions, such as substrate temperature, film thickness and annealing treatment. For thick Ge films grown on coalesced Ge islands, µ(T) is governed by long-range optical phonon scattering, indicating the growth of high structural quality in the Ge films. For relatively thin films composed of Ge islands, on the other hand, µ(T) obeys short-range acoustic phonon scattering due to crystalline imperfections, such as dislocations or defects. Similar experiments have also been performed for thin Ge films grown on predeposited Ge buffer layers on Si substrates.

Atomic Structure and Chemistry of Si/Ge Interfaces Determined by Z-Contrast Stem

ABSTRACTThe technique of Z-contrast STEM provides a fundamentally new and powerful approach to determining the atomic scale structure and chemistry of interfaces. The images produced do not show contrast reversals with defocus or sample thickness, there are no Fresnel fringe effects at interfaces, and no contrast from within an amorphous phase. Such images are unambiguous and intuitively interpretable. In this paper, the technique has been used to directly image subnanometer interdiffusion in ultrathin (SimGen)p superlattices. The Z-contrast image of a (Si8Ge2)p superlattice grown by MBE at 400°C clearly shows significant broadening of the Gerich layer. Also, film formation and misfit accommodation in epitaxial Ge films on (001)Si produced by implantation and oxidation of Si wafers was studied. It was found that the Ge films, which are constrained to grow layer-by-layer, remain completely coherent with the Si substrate to a thickness of 5–6 nm. This is 3 to 6 times thicker than the observed critical thickness for Ge films grown on Si by MBE. It is observed that misfit accommodating dislocations nucleate at the film surface as Shockley partials. The Z-contrast images show these partials can combine to form perfect dislocations whose cores are found to lie entirely in the elastically softer Ge film.

On the origin of superstructures observed on surfaces of Ge and Ge xSi 1−x films grown on Si(111)(7 × 7)

A phase diagram of the structure of the surface grown during molecular beam epitaxy (MBE) of Ge on an Si substrate has been made by using reflection high energy electron diffraction (RHEED). The (111)(5 × 5) superstructure observed on the surface of Ge and Ge x Si 1− x films on Si(111) has been found to appear at the pseudomorphic growth stage and to be induced by compressive epitaxial stresses. The (5 × 5) reconstruction is caused by elastic deformations within the range of 0.6%–4%. Si diffusion to the Ge epitaxial film structure has been found to affect the formation of the Ge(111)(7 × 7) superstructure.

Heteroepitaxy of Ge films on Si(100) surface

Structural and electrical properties of epitaxial Ge films on a Si(100)-2x1 surface have been studied by means of low-energy-electron diffraction (LEED), Auger-electron spectroscopy (AES), X-ray diffraction, Raman scattering and Hall measurements. We found that a thickness of more than 1000 Å is required to grow high quality epitaxial Ge films on the Si substrate at elevated temperature. This is due to island formation at the initial stage of Ge deposition. Thus, special efforts were devoted to elucidate the role of a thin Ge buffer layer fabricated at substrate temperatures below 200 °C used to grow thinner epitaxial Ge films by suppressing island formation. It was found that the thin amorphous buffer layers remained poorly crystallized during the rapid increase in substrate temperature up to 400–500 °C, prior to subsequent Ge deposition on the buffer layer. Consequently, relatively thin single crystal Ge films grew in such a way that the underlying buffer layers accommodate the strain or misfit dislocations near the Si-Ge interface.

Growth and Etching of Germanium Films by Chemical Vapor Deposition in a GeCl4 ‐ H 2 Gas System

The etching and growth of germanium films are investigated using a gas system in the temperature range of 490°–565°C. At relatively low partial pressures less than , epitaxial growth of Ge is observed on Ge (100) surfaces, whereas at partial pressures higher than , etching of the Ge film is found to occur. In the experiments utilizing patterned substrates, where the surface consists of defined areas of Ge and , Ge is found to deposit selectively only on the exposed Ge regions. The growth reactions of Ge epitaxial films proceed through the Langmuir‐Hinshelwood mechanism: the surface reaction takes place between two hydrogen atoms dissociatively adsorbed and a surface‐adsorbed molecule. molecules adsorbed on the surface are formed directly from molecules, not through gas‐phase reduction by hydrogen. On the other hand, the etching reaction of Ge films is proved to be a reverse disproportionation reaction: . Based on the analyses of growth and etching reactions, the equation of the Ge epitaxial film's growth rate is derived as a function of partial pressure, hydrogen partial pressure, and growth temperature.

Epitaxial growth of Ge films on GaAs (285–415 °C) by laser photochemical vapor deposition

Epitaxial Ge films have been grown on [001]GaAs by laser photochemical vapor deposition (LPVD) for substrate temperatures (Ts) in the 285≤Ts≤415 °C interval. Films 400–700 Å thick were grown at rates up to ∼1 Å s−1 (0.6–5 nm/min) by photodissociating GeH4 with an ArF (193 nm) laser in parallel geometry (i.e., laser beam passes above and parallel to substrate). The quality of the Ge/GaAs heterointerfaces has been examined by plan-view and cross-sectional transmission electron microscopy. Convergent beam electron diffraction, selected area diffraction patterns, and lattice images show the LPVD films to be epitaxial, whereas films grown in the absence of ultraviolet laser radiation (i.e., by conventional chemical vapor deposition) are amorphous (Ts=305 °C) or heavily defected polycrystalline (Ts=415 °C). Epitaxy apparently hinges on the laser production of a species which migrates more than 10–60 mean free paths to reach the substrate (or is collisionally transformed en route into a more stable species) and dramatically alters the surface chemistry. The large number of collisions involved points to the transformation of photochemically generated GeH2 or GeH3 into Ge2H6 as the key gas phase reaction.

Deposition, post-deposition annealing, and characterization of epitaxial Ge films grown on Si(100) by pyrolysis of GeH4

As a first step towards developing heterostructures such as GaAs/Ge/Si entirely by chemical vapor deposition, Ge films have been deposited on (100) Si by the pyrolysis of GeH4. The best films are grown at 700° C and are planar and specular, with RBS minimum channeling yields of ≈4.0% (near the theoretical value) and defect densities of 1.3 x 108 cm−2. Variations of in-situ cleaning conditions, which affect the nature of the Si substrate surface, greatly affect the ability to get good epitaxial growth at 700° C. The majority of the defects found in the Ge films are extrinsic stacking faults, formed by dissociation of misfit and thermal expansion accommodation dislocations. The stacking fault density is not significantly reduced by post-deposition annealing, as is the case for the dislocations observed in MBE Ge films. It is suggested that lowering the CVD growth temperature through the use of high vacuum deposition equipment would result in dislocation defects like those of MBE films which could then be annealed more effectively than stacking faults. Films with defect densities equivalent to MBE Ge films (~2 x 107 cm−2) could then probably be produced.

Structural properties of heteroepitaxial Ge films on a Si(100)-2×1 surface

Structural and electrical properties of epitaxial Ge films grown on a Si(100)-2×1 surface have been studied by means of low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), x-ray diffraction, and Raman scattering. The attempt has been made to find the growth conditions, such as the substrate temperature TSi and Ge thickness dGe, for the single-crystalline Ge (100) on the Si surface. The sharp (2×1) LEED pattern of Ge (100) accompanied by the absence of any AES signals from the Si substrate is unambiguous evidence for the growth of single-crystalline Ge films. The structural quality of the Ge films was examined by the peak position and width of the Ge(400) Bragg line in x-ray diffraction. From these results, we found that more than 1000-Å Ge thickness is required to grow the high-quality epitaxial Ge films on the Si substrate at elevated temperatures. This is due to island formation at the initial stage of Ge deposition. Therefore, we also studied the role of thin Ge buffer layers fabricated at low temperatures below TSi=200 °C to grow thinner epitaxial Ge films without island formation. It was found that the amorphous buffer layers are poorly crystallized during the rapid increase in TSi up to 400–500 °C, prior to subsequent Ge deposition on the buffer layer. Consequently, single-crystalline Ge films are grown at relatively thin dGe in such a way that the underlying buffer layers accommodate the strains or misfit dislocations near the Si-Ge interface. The single-crystalline Ge films with and without the buffer layers thus grown on the Si substrates were also characterized by the line shape of the phonon Raman scattering spectra. Depending on the details of sample preparation, the spectra exhibited a characteristic change in shape including peak position and width. Having estimated the probing depth of the Raman scattering, we were able to gain deep insight into not only the Ge–Ge bond strength, but also the inhomogeneity of the Ge films and the extent of the strains into the Ge films, which were not accessible by LEED or x-ray diffraction.

Bevelled plan-view TEM specimen preparation and an artifact due to dimpling

The microstructure and metallic impurity gettering efficiency of misfit dislocation (MD) networks generated in Si/Si-2%Ge epitaxial films has been examined using TEM. Metallic elements were evaporated onto the top surface where alternating open windows and oxide masks were defined. The subsequent annealing permitted metal (i.e., gold in the present discussion) to diffuse through open windows into the wafer. The details of the sample structure and results can be found elsewhere in this proceedings. It is desirable to make microscopic observations in both cross-section and in plan-view. This contribution briefly outlines our procedure for making bevelled plan-view TEM specimens and describes a novel specimen preparation artifact.The top surface was treated with aqua regia (75% HCI, 25% HNO3) for a few seconds in order to remove the gold after the diffusion anneal. 2 mm square sections were cut from a gold diffused wafer and half of the top surface was polished at 2° angle with 6 μm and 1 μm diamond paste followed by final polishing with cyton using a vibrating lapping machine

Laser Photochemical Vapour Deposition of Epitaxial Ge Films on GaAs from GeH4 Using NH3 as a Sensitiser

ABSTRACTThe viability of ammonia as a sensitiser for the epitaxial growth of Ge on GaAs by laser photochemical vapour deposition (LPVD) has been investigated. Specifically NH3/GeH4/He (0.8/5/95 sccm, 5.5 Torr total pressure) mixtures have been irradiated by a 193nm ArF excimer laser in parallel geometry for substrate temperatures, Ts<400°C. As evidenced by a dramatic acceleration in Ge film growth rate, the NH3 efficiently couples the laser radiation to the GeH4 precursor molecule. The microstructures of LPVD Ge films grown with and without NH3 have been examined by TEM, and the epitaxial nature of both types of films has been verified, although some subtle differences are noted. Chemical analysis of the deposited films has been carried out using Auger spectroscopy, X-ray photoelectron spectroscopy and secondary ion mass spectroscopy. Our results show that there is little or no nitrogen incorporation into the Ge films grown in the presence of NH3, and that hydrogen contamination in our films is minimal. The beneficial effect of NH3 on the growth rate of LPVD Ge films is attributed to the photolytic production of hydrogen atoms which efficiently decompose GeH4 by hydrogen abstraction collisions.

Pyrolytic and Laser Photolytic Growth of Crystalline and Amorphous Germanium Films from Digermane (Ge2H6)

ABSTRACTHigh-purity digermane (Ge2H6, 5% in He) has been used to grow epitaxially oriented crystalline Ge films by pyrolysis. Amorphous Ge:H films also have been deposited by pyrolysis and ArF (193 nm) laser-induced photolysis. The amorphous-to-crystalline transition and the film's morphology was studied as a function of deposition conditions. The film's microstructure, strain and epitaxial quality were assessed using x-ray diffraction curves and scanning and transmission electron microscopy. It was found that commensurate, coherently strained epitaxial Ge films could be grown pyrolytically on (100) GaAs at low (0.05–40 m Torr) Ge2H6 partial pressures, for substrate temperatures above 380°C.

Strain in ultrathin epitaxial films of Ge/Si(100) measured by ion scattering and channeling.

We describe a channeling analysis of the strain associated with ultrathin (2--6 monolayers) epitaxial films of Ge embedded in Si. Measured strains are at the level of 3--4%, exceeding that accessible in bulk materials. These values, in both single-layer films and Ge-Si superlattices, are in approximate agreement with bulk elastic properties suggesting that Poisson's ratio is applicable at the few-monolayer level.

Low-temperature epitaxy of Si and Ge by direct ion beam deposition

Amorphous, polycrystalline, and epitaxial thin films of Si and Ge have been grown by ion beam deposition (IBD) under ultrahigh-vacuum conditions. IBD involves the direct deposition of ions onto single-crystal substrates from mass- and energy-analyzed beams with energies of 10 to 200 eV. The IBD films were characterized by Rutherford backscattering, ion channeling, cross-section transmission electron microscopy, and x-ray diffraction. The effects of substrate temperature, ion energy, and substrate cleaning were studied. Differences in the formation of epitaxial thin films on p- and n-type Si substrates were observed with n− Si showing better epitaxy at low temperatures. Epitaxial overlayers which showed good minimum yields by ion channeling (3%–4%) have been produced at temperatures as low as 375 °C for Ge on Ge(100) and Si on Si(100).

Statistical equilibrium in particle channeling

We have used a unique sample configuration consisting of thin, epitaxial films of Ge sandwiched within a Si(100) lattice to explore the conditions for statistical equilibrium in particle channeling. Indications of nonequilibrium came about from a strong asymmetry in the off-normal angular scans. We find the depth necessary to achieve statistical equilibrium to be ∼2200 Å for 1.8 MeV He particle channeling along the Si〈110〉 direction. This measurement provides a useful guide for the interpretation of channeling experiments in a variety of near-surface applications.

Strained Layer Semiconductor Films: Structure and Stability

ABSTRACTStrained layer epitaxy is a process for the formation of new materials with a strain and composition modulation in the one to one hundred monolayer range. Two aspects of strained epitaxial growth are discussed in this paper. We first consider the clustering process, a basic limitation in strained layer epitaxy. A second experiment examines the strain in few monolayer epitaxial films of Ge embedded in Si(100). We show that the strain in these monolayer films is comparable to that expected from bulk elastic constants and discuss the new properties of these films.

Remote plasma-enhanced chemical-vapor deposition of epitaxial Ge films

Epitaxial Ge films have been deposited at 300 °C using a remote plasma-enhanced chemical-vapor deposition technique where metastable He atoms flow downstream from the plasma region to dissociate GeH4 molecules into deposition precursor species. Ge epitaxy is demonstrated on Ge(111), Si(100), and GaAs(111)Ga face substrates. An in situ cleaning process that involves a moderate thermal bake at 300 °C and a hydrogen plasma etch of the native oxides is integral to the process. Reflection high-energy electron diffraction is used to examine surface quality just prior to and after deposition. Uniform integral order diffraction streaks and fractional order reconstruction features observed from the Ge epilayers indicate that high quality Ge epitaxial layers can be grown using remote plasma-enhanced chemical-vapor deposition.