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This paper describes the tribochemical reactions of silicon against a polymer pad as studied by attenuated total internal reflection Fourier transform infrared spectroscopy. The evolution of the silicon surface and chemical species formed on the surface during rubbing have been studied in solutions with values of pH 4, 7, and 10. It is shown that the process of material removal (that can be tailored to function as a form of tribochemical polishing) proceeds most intensively in the solution with pH 10. The tribochemical reaction of silicon hydrides, e.g., SiH x and SiOSiH x ⇄ Si-(OH) x , (x = 1...3) is observed directly and believed to be the limiting step in the material removal process during rubbing in an alkaline solution. Then, hydrolysis of Si-H bonds leads to the formation of silanol groups on the surface (Si-OH), that in turn weaken the Si-Si backbonds and allow material to be removed from the surface in the form of silicon hydroxide or, most commonly, silicic acid (Si(OH) 4 ). The simultaneous action of friction and chemical reactions enhances the dissolution of the material at the asperity contact zones, which results in surface smoothing. No evidence of the formation and abrasion of surface silica layer has been found during rubbing.

We show that it is possible to extend the lower limit of the spectral range of Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy on Si surfaces from 1500 to ∼850 cm−1 using a Ge ATR prism coated with a thin epitaxial layer of Si, making accessible the especially important fingerprint region of organic molecules. The utility of the ATR prism is demonstrated by collecting FTIR spectra of several surface terminations of Si (H, oxide, 3-aminopropyltrimethoxysilane, and DNA) that contain peaks in the newly accessible spectral region.

Germanium films were deposited on GaAs (100) substrates with or without an epiready surface oxide at temperatures between room temperature (RT) and 500 °C using an ultrahigh‐vacuum e‐beam deposition system. The film at 100 °C on a substrate with a surface oxide had a flat absorption curve over the wave‐number range investigated, 500–4000 cm−1, with an absorption of less than 10/cm at 1000 cm−1 (10 μm wavelength). Films deposited at RT and 50 °C on substrates with a surface oxide had comparable low absorption, but they contained an absorption peak at 830 cm−1 associated with the Ge—O bonds. Although all three films were amorphous, the films deposited at the lower temperatures were more porous. This enabled oxygen to percolate in from the atmosphere to form the Ge—O bonds. The films deposited at 150 °C and above on substrates with a surface oxide and at 100 °C with the surface oxide removed thermally in situ prior to deposition the Ge films, and the single crystal films deposited at 400 and 500 °C on oxide‐...

Understanding and controlling localised strain in semiconductor devices is one of the critical challenges confronting the designer of new technologies. Due to continuous scaling down in very large and ultralarge scale integration there is a growing requirement for techniques with sujjicientcapacity to characterise localised strain quantitatively with a resolution in the micrometre to nanometre range. The present paper reports such a technique, namely, electron diffraction contrast imaging (EDCI) using transmission electron microscopy (TEM). As a case study the characterisation of localised strain in a semiconductor laser structure is presented. The strain in the structure is modelled using finite element calculations, and the EDCI results are simulated using the ‘Simeon’ software. The three-dimensional effects of TEM specimen preparation on the interpretation of EDCI results are also reported.MST/3250

By in-situ annealing of metastably strained (Si)/GexSi1−x/Si heterostructures in a transmission electron microscope, it is possible to directly observe and measure misfit dislocation propagation velocities. These measurements correspond to different regimes than conventional measurements of dislocation velocities in bulk Ge and Si, as in the present experiments motion at extremely high resolved applied (lattice-mismatch) shear stresses, of the order 1 GPa, and very low propagating dislocation lengths, 10 to 100 nm, is studied. This typically ensures that dislocation motion in the kink non-collision regime is observed, as opposed to the kink collision regime generally assumed in bulk measurements. By variation of the GexSi1−x/Si interface orientation, it is also possible to study the motion of total versus partial dislocations.

X-ray interferometry has been used to study the profile structures of chemisorbed alkylsiloxane (hexadecyl-, octadecyl-, and eicosyl-) monolayer films on Ge/Si multilayer substrates of the type 2(Ge 2 Si 30 ), i.e., two superlattice unit cells, each consisting of 2 Ge monolayers and 30 Si monolayers, as fabricated by molecular beam epitaxy (MBE). Analysis was performed in three steps. First, based on the structural specifications of the cleaned, bare substrates acquired form the MBE fabrication, the actual profiles structures of selected multilayer substrates were determined via a model refinement analysis of their meridional X-ray diffraction

Molecular‐beam epitaxial growth of GexSi1−x on 〈100〉 porous Si substrate was studied using Rutherford backscattering spectrometry, transmission electron microscopy, and defect etching. This study was stimulated by the theoretically predicted possibility of stress field reduction in a lattice‐mismatched film grown on a patterned substrate. The experimental results show predominantly 60° dislocations with long misfit segments, and no reduction of dislocation density is observed by the use of porous Si substrates. The nature of interconnected growth area of the porous Si substrate is believed to be the reason for the ineffectiveness of the stress reduction.

In recent years photodetector structures have become more complex in response to more stringent performance demands such as higher bandwidth and lower noise. There are, however, fundamental performance limitations such as the responsivity/bandwidth tradeoff that have not been adequately addressed. At present, it appears that significance increases in performance will be the result of major technological breakthroughs rather than incremental improvements in conventional structures. Our productive approach is the implementation of resonant-cavity structures.

X-ray interferometry/holography was applied to meridional x-ray diffraction data to determine uniquely the profile structures of a single monolayer of an integral membrane protein and a peripheral membrane protein, each tethered to the surface of a solid inorganic substrate. Bifunctional, organic self-assembled monolayers (SAMs) were utilized to tether the proteins to the surface of Ge/Si multilayer substrates, fabricated by molecular beam epitaxy, to facilitate the interferometric/holographic x-ray structure determination. The peripheral membrane protein yeast cytochrome c was covalently tethered to the surface of a sulfhydryl-terminated 11-siloxyundecanethiol SAM via a disulfide linkage with residue 102. The detergent-solubilized, photosynthetic reaction center integral membrane protein was electrostatically tethered to the surface of an analogous amine-terminated SAM. Optical absorption measurements performed on these two tethered protein monolayer systems were consistent with the x-ray diffraction results indicating the reversible formation of densely packed single monolayers of each fully functional membrane protein on the surface of the respective SAM. The importance of utilizing the organic self-assembled monolayers (as opposed to Langmuir-Blodgett) lies in their ability to tether specifically both soluble peripheral membrane proteins and detergent-solubilized integral membrane proteins. The vectorial orientations of the cytochrome c and the reaction center molecules were readily distinguishable in the profile structure of each monolayer at a spatial resolution of 7 A.

Photoemission spectroscopy is used to demonstrate that Ge segregates to the first atomic layer of Ge0.5Si0.5(100)2×1 and that the second layer is predominantly Si. Comparison of the resolved signals from the dimer atoms of the reconstructed (100)2×1 surfaces of Ge, Si, and equiatomic Ge‐Si alloy shows that the surface layer of the alloy is extremely Ge rich and the second layer is occupied mainly by Si atoms. This result is in good agreement with theoretical predictions.

The preceding chapter laid out a physical basis for molecular beam deposition. In this chapter that conceptual framework will be translated into the reality of common laboratory apparatus and practices. As a starting point, classic semiconductor molecular beam epitaxy (MBE) [1,2] will be briefly described. Then, the focus will turn to leading edge equipment and technique issues that should be of interest to the broader thin film deposition audience for whom this volume is intended. Those issues include: alternate thermal deposition sources, ion beam doping, direct sensing of deposition fluxes and true deposition rate, and production considerations of uniformity, multiple wafer deposition, automation and particulate control.

The use of patterned Si substrates (specifically porous Si) for the reduction of heteroepitaxialfilm strain energy [S. Luryi and E. Suhir, Appl. Phys. Lett. 4 9, 140 (1986)] has been studied recently by several research groups. We report experimental studies and discuss the validity of the original concept. We studied molecular beam epitaxial growth of continuous Ge x Si1−x film on 〈100〉 porous Si substrates using Rutherford backscattering spectrometry, transmission electron microscopy, and x‐ray rocking curves. The results show predominantly 60° dislocations with long misfit segments. There is no reduction in either strain in the films or dislocation density compared to the samples grown on regular Si substrates. In order to reduce defect densities, the stress fields from different growth areas of patterned substrate must not interfere. Arguments based on crystallographic considerations are presented which demonstrates that, first, the porous Si substrate is not suitable for such applications because of the nature of interconnected growth areas, and second, any kind of decaying stress field requires a certain degree of wafer warpage which does not always present in a realistic MBE‐grown heterostructure. It is the conclusion of this discussion that the strain energy in a continuous heteroepitaxialfilmgrown on patterned substrates does not reach a limiting value, but continues to increase with the film thickness.

Selectively doped GexSi1−x/Si strained layer heterostructures have been grown in a single quantum well configuration on 〈001〉‐Si substrate using molecular beam epitaxy. The modulation doping effect has been observed in p‐type structures only; although both n‐ and p‐type double heterostructures were grown. We have investigated the effects of: (i) alloy layer thickness (well width), (ii) doping setback, and (iii) cladding layer dopant concentration, on the hole mobilities. At present, optimum structures show peak hole mobilities in excess of 3300 cm2V−1s−1 at 4.2 K, sheet charge densities of 3.0–14×1011 cm−2, and hole effective mass m*h =0.32±0.03 m0. It is found that the low temperature (T∼10 K) hole mobility, in structure having x=0.2 is relatively insensitive to alloy layer (well) thickness in the range 100 A≲Lw≲Lc; where Lc is the critical thickness marking the transition from strained layer to relaxed, misfit accommodated, alloy growth. For Lw>Lc, misfit dislocation scattering dramatically decreases μh...

Patterned epitaxial silicon films are grown on silicon substrates by molecular beam deposition (MBE) through an oxide masking layer. Epitaxial patterns faithfully reproduce the oxide mask, which is fabricated by conventional photolithographic processes. Resolution is determined solely by the mask, and linewidths below 1 μ are demonstrated. Sample morphology and crystallinity are examined by scanning and transmission electron microscopy, and Nomarski interference contrast microscopy in conjunction with chemical defect etching. These techniques confirm that patterned films are epitaxial and free of observable defects. A chemical liftoff step dissolves the oxide mask and leaves an elevated silicon pattern. Patterned film growth requires no modification to the MBE apparatus.

Epitaxial silicon films have been grown on single‐crystal Si (100) substrates by evaporation from an e‐gun source in ultrahigh vacuum and have been doped with gallium and with aluminum from separate oven sources. Gallium doping profiles have been controlled accurately for substrate temperatures in the range 600–800 °C and for carrier densities in the range 1014–5×1017 cm−3. Examples are given of abrupt changes in doping level. Measured drift mobilities in the films are within 15% of values for bulk silicon. Crystallographic properties of the films are comparable to those of the substrates and are suitable for device applications. Films doped with aluminum exhibit comparable electrical and crystallographic properties, but good control of the doping profile has not been achieved for the range of parameters studied.

The observation of Bragg diffraction from supernanometre regions of 'amorphous' silicon reveals the existence of crystalline clusters too small to be observed by X-ray diffraction in bulk. The clusters are imaged by high-resolution transmission electron microscopy. These observations support a submicrocrystalline model for the structure of non-crystalline solids.

We report the results of studies which have been made on heteroepitaxial layers of GaAs and AlGaAs grown by metalorganic chemical vapor deposition on composite substrates that consist of four different types of heteroepitaxial layered structures of Ge and Ge-Si grown by molecular beam epitaxy on (100)-oriented Si substrates. It is found that of the four structures studied, the preferred composite substrate is a single layer of Ge ∼1 µm thick grown directly on a Si buffer layer. The double-crystal X-ray rocking curves of 2 µm thick GaAs films grown on such substrates have FWHM values as small as 168 arc sec. Transmission electron micrographs of these Ge/Si composite substrates has shown that the number of dislocations in the Ge heteroepitaxial layer can be greatly reduced by an anneal at about 750° C for 30 min which is simultaneously carried out during the growth of the GaAs layer. The quality of the GaAs layers grown on these composite substrates can be greatly improved by the use of a five-period GaAs-GaAsP strained-layer superlattice (SLS). Using the results of these studies, low-threshold optically pumped AlGaAs-GaAs DH laser structures have been grown by MOCVD on MBE Ge/Si composite substrates.

A method for tethering proteins to solid surfaces has been utilized to form vectorially oriented monolayers of the detergent-solubilized integral membrane protein Ca(2+) -ATPase from the sarcoplasmic reticulum (SR). Bifunctional, organic self-assembled monolayers (SAMs) possessing "headgroup" binding specificity for the substrate and "endgroup" binding specificity for the enzyme were utilized to tether the enzyme to the substrate. Specifically, an amine-terminated 11-siloxyundecaneamine SAM was found to bind the Ca(2+)-ATPase primarily electrostatically. The Ca(2+)-ATPase was labeled with the fluorescent probe 5-(2-[(iodoacetyl)amino]ethyl)aminonaphthalene-1-sulfonic acid before monolayer formation. Consequently, fluorescence measurements performed on amine-terminated SAM/enzyme monolayers formed on quartz substrates served to establish the nature of protein binding. Formation of the monolayers on inorganic multilayer substrates fabricated by molecular beam epitaxy made it possible to use x-ray interferometry to determine the profile structure for the system, which was proved correct by x-ray holography. The profile structures established the vectorial orientation of the Ca(2+)-ATPase within these monolayers, to a spatial resolution of approximately 12 A. Such vectorially oriented monolayers of detergent-solubilized Ca(2+)-ATPase from SR make possible a wide variety of correlative structure/function studies, which would serve to elucidate the mechanism of Ca(2+) transport by this enzyme.

Passivation of threading dislocations in an incommensurate Ge0.36Si0.64/Si structure is studied using hydrogen plasma anneal. The reverse current of pn junction diodes made of the above structure is reduced by more than 30 times after hydrogenation. Associated improvements in the current‐voltage (I‐V) characteristics is also observed. Capacitance‐voltage (C‐V) measurements reveal that the shallow dopants neutralized by hydrogenation reactivate at lower temperatures than the passivated deep level defects. Secondary‐ion mass spectroscopy (SIMS) analysis established the depth of diffusion of hydrogen under the experimental conditions. Work in this direction could eventually lead to the integration of infrared detectors with Si very large scale integration (VLSI).

Epitaxial silicon films have been grown by molecular beam epitaxy (MBE) on (1012) sapphire and (100) spinel substrates. Film morphology and electrical quality are strong functions of substrate preparation and vacuum purity. Under optimized conditions, single‐crystal growth occurs at 650–800 °C and 700–900 °C on sapphire and spinel substrates, respectively. Films initially grow in islands which coalesce and gradually form smooth continuous layers. On sapphire, coalescence occurs at ∼500 A and films grow smoothly above ∼2000 A. On spinel coalescence occurs at ∼0.5 μm and smooth growth above ∼1 μm. Thick films on either substrate have mobilities close to bulk. As thickness decreases, mobilities fall gradually and values cannot be measured in films less than twice the coalescence thickness. At thicknesses of 0.5 and 1.0 μm, MBE SOS films have mobilities in excess of those reported for comparably doped CVD layers.

We report a new photoluminescence defect spectrum with a no‐phonon transition at 1.0192 eV which emerges in many silicon layers grown on silicon substrates by molecular beam epitaxy. Comparison of the no‐phonon transition and the chief local mode (ℏω=7.5 meV) to the well‐established Cu‐related spectrum at 1.0145 eV suggests that the new defect incorporates Cu as well. Deep level transient measurements support the presence of Cu in the epilayers. We suggest that this defect spectrum has recently been observed by others but was not identified and associated with Cu.

The self‐consistent ab initio pseudopotential results of C. G. Van de Walle and R. M. Martin [J. Vac. Sci. Technol. B 3, 1256 (1985)] have been combined with a phenomenological deformation potential theory to estimate the band gap and band offsets for coherently strained multilayers of GexSi1−x/Si for growth on 〈001〉 GeySi1−y substrates. It is found that ΔEc is negligible and the narrower GexSi1−x gap falls within the wider Si gap (type I band alignment) if the Si in the multilayers is cubic, whereas ΔEc can be appreciable and the GexSi1−x conduction‐band edge tends to be higher in energy than the Si conduction‐band edge(type II band alignment) if both the Si and the GexSi1−x are strained. In particular, the present results resolve the seeming paradox which arose from interpretations of modulation doping experiments using heterojunctions grown either on Si〈001〉 substrates or on an unstrained alloy buffer layer.

GexSi1−x alloy layers grown on (100) silicon substrates by molecular beam epitaxy and amorphized by ion irradiation at −196 °C are shown to recrystallize epitaxially during subsequent ion irradiation at 275 °C. This ion beam annealing process has been examined for two different sample configurations: the first consisting of a thin amorphous layer extending from the surface to about half the thickness of the alloy layer, and the second consisting of a thick amorphous layer extending beyond the alloy layer into the underlying silicon. In both cases, ion beam annealing results in epitaxial crystallization of the alloy layer. Results are reported for alloy composition in the range from Ge0.1Si0.9 to Ge0.8Si0.2.

Low‐energy zinc ions have been incorporated in growing MBE GaAs layers producing heavily doped p‐type material. As‐grown layers retain a substantial amount of radiation damage as indicated by the absence of photoluminescence and low hole mobilities. A postgrowth anneal yields layers with low‐temperature luminescence and mobilities comparable to zinc doped LPE epitaxial layers. Annealed layer quality does not depend on substrate growth temperature or ion energy over the ranges 580–650 °C and 100–3000 eV, respectively. The measured zinc ion sticking coefficient has a value of ∼50% and depends only weakly on ion energy. The data suggest that the sticking coefficients are enhanced by a simple ion burial mechanism and not by an electrostatic attraction between the ion and surface atoms.

This paper summarizes a review of recent developments in silicon molecular beam epitaxy. These include ionized doping, heteroepitaxial growth on insulators and silicides, and MBE devices.

The epitaxial crystallization rate of UHV‐deposited amorphous silicon is shown to be 1–2 orders of magnitude lower than the rate reported for ion implanted amorphous layers. The rate of growth is a function of depth, decreasing towards the layer surface. Crystallization is inhibited in samples exposed to air at room temperature. Results indicate that gases migrate at least 1000 A into deposited films and thus suggest that voids are interconnected by microscopic pores.

The effects of arsenic and krypton implantations have been investigated in semi-insulating cadmium telluride crystals. Nearly 100 percent electrical activity was observed when As implants were performed at 300°C in samples subjected to a 24-hour 500°C cadmium vapor anneal prior to implantation. A jet thinning technique was utilized to prepare samples for transmission electron microscopy analysis and a number of micrographs obtained. In2Te3 and CdCl2 precipitates were identified in indium and chlorine compensated materials. Implantation damage was also observed in the form of small vacancy loops.

Silicon molecular beam epitaxy is reviewed emphasizing opportunities for application in VLSI. These include: 1) exploitation of MBE control and uniformity in device scaling and/or process refinement; 2) use of mathematically arbitrary dopant profiles in optimized device structures; 3) MBE heteroepitaxy of crystalline insulators, metals and semiconductors; 4) combination of doping and heteroepitaxial capabilities in heterojunction devices. In each of these areas examples are provided from the recent literature.

Solid phase epitaxy was studied in the GexSi1-x/[100] Si system in the composition range of 4% to 10% Ge. For strained samples regrowth rates decreased with increasing Ge concentrations, with an activation energy of 2.8 and 3.0 eV for the Ge4Si96 and Ge8Si92 alloys respectively. In contrast, the rates of the strain-relaxed samples increased compared with that of pure Si. The minimum yield of fully regrown samples with Ge concentrations lower than 8% recovered to that of the as-deposited samples. However, the minimum yield of a 3500 Å-thick Ge10Si90 alloy increased to 12 % after recrystallization.

The nucleation and propagation of misfit dislocations in Ge-Si strained epilayers on (100) Si have been investigated using transmission electron microscopy and X-ray diffraction topography at low lattice parameter mismatch (˜ 0.8%). Misfit dislocations nucleate as half loops which are predominantly unfaulted (> 90%) at the advancing growth interface. Under the driving force of the epilayer strain, unfaulted half loops glide and expand on inclined { 111 }planes toward the heterointerface (i.e. substrate/epilayer interface). These unfaulted half loops consist of a 60°-dislocation segment which lies along < 011> in a plane parallel to the heterointerface (i.e. (100)) and this segment is connected to the growth interface by two screw dislocation segments which both lie on the same inclined {111} glide plane. As 60° dislocations reach the heterointerface on each of the four inclined {111} variants, they form an orthogonal array of misfit dislocations which lie along [011] and [011]. At higher lattice parameter mismatch (˜ 2%), there appear to be some important changes in the dislocation behavior and these changes result in orthogonal arrays of heterointerface dislocations which are predominantly edge type (i.e. 90°dislocations).

Avalanche photodetectors are important components in the implementation of high bit-rate optical fiber communications. Ga0.47In0.53 As and Ge avalanche photodiodes considered for optical communications systems operating near 1.3μm suffer from an excess noise factor near the theoretical maximum. At the other extreme, Si avalanche photodetectors (APDs) have an excess factor near the absolute minimum. Unfortunately, the optical response of Si APDs is limited-to shorter wavelength, far below 1.3-1.5μm optimum spectral region for optical fiber transmission.

The cause of enhanced diffusion when boron is implanted into crystalline silicon is examined. A report is presented on an experimental non-local diffusion model. Samples are annealed with and without the heavily damaged surface region and profiled by SIMS. Preliminary results show similar diffusion in both sample lots in contradiction to the Fair proposal, implying that the enhanced tail diffusion is a local phenomenon.