Silicon Molecular Beam Epitaxy Research Articles

Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy.

Over the past two decades, prototype devices for future classical and quantum computing technologies have been fabricated by using scanning tunneling microscopy and hydrogen resist lithography to position phosphorus atoms in silicon with atomic-scale precision. Despite these successes, phosphine remains the only donor precursor molecule to have been demonstrated as compatible with the hydrogen resist lithography technique. The potential benefits of atomic-scale placement of alternative dopant species have, until now, remained unexplored. In this work, we demonstrate the successful fabrication of atomic-scale structures of arsenic-in-silicon. Using a scanning tunneling microscope tip, we pattern a monolayer hydrogen mask to selectively place arsenic atoms on the Si(001) surface using arsine as the precursor molecule. We fully elucidate the surface chemistry and reaction pathways of arsine on Si(001), revealing significant differences to phosphine. We explain how these differences result in enhanced surface immobilization and in-plane confinement of arsenic compared to phosphorus, and a dose-rate independent arsenic saturation density of 0.24 ± 0.04 monolayers. We demonstrate the successful encapsulation of arsenic delta-layers using silicon molecular beam epitaxy, and find electrical characteristics that are competitive with equivalent structures fabricated with phosphorus. Arsenic delta-layers are also found to offer confinement as good as similarly prepared phosphorus layers, while still retaining >80% carrier activation and sheet resistances of <2 kΩ/square. These excellent characteristics of arsenic represent opportunities to enhance existing capabilities of atomic-scale fabrication of dopant structures in silicon, and may be important for three-dimensional devices, where vertical control of the position of device components is critical.

Studying the Formation of Si (100) Stepped Surface in Molecular-Beam Epitaxy

Experimental studies of the formation of a stepped surface structure during molecular-beam epitaxy of silicon on a Si (100) substrate have been carried out in wide ranges of variation of the substrate temperature and silicon growth rate. The conditions of the transition from a two-domain structure of the Si (100) surface to a single-domain structure associated with the formation of diatomic steps are determined using reflection high-energy electron diffraction. It is shown that the effect of an increase in the substrate temperature on the transition to a single-domain structure is non-monotonic: a single-domain surface forms in the region of relatively low temperatures, whereas a two-domain surface forms at high temperatures. The transition to a single-domain structure during the experiment is possible only, if the silicon growth rate is increased above a certain minimum value.

Study of the crystal structure of silicon nanoislands on sapphire

The results of studies of the crystal structure of silicon nanoislands on sapphire are reported. It is shown that the principal defects in silicon nanoislands on sapphire are twinning defects. As a result of the formation of such defects, different crystallographic orientations are formed in silicon nanoislands on sapphire. In the initial stages of the molecular-beam epitaxy of silicon on sapphire, there are two basic orientations: the (001) orientation parallel to the surface and the (001) orientation at an angle of 70° to the surface.

Thickness uniformity of silicon layers grown from a sublimation source by molecular-beam epitaxy

The thickness distributions of epitaxial layers over the substrate area are experimentally and theoretically studied for deposition from a molecular beam formed by a sublimation source in vacuum. The calculated data agree well with the experimental results for the molecular-beam epitaxy of silicon.

Patterning of silicon nitride for CMOS gate spacer technology. II. Impact of subsilicon surface carbon implantation on epitaxial regrowth

Molecular beam epitaxy of silicon is a key step in the complementary metal–oxide–semiconductor integration process flow as it allows the fabrication of raised source/drains for fully depleted silicon on insulator technology. One of the most important criteria is the surface state of crystalline silicon before epitaxy, which influences the preliminary stages of the epitaxial growth as well as the interface electrical properties. In this work, the authors study the effect of the Si3N4 spacer etching in CH3F/O2/He plasmas on the Si epitaxial regrowth. Using angle resolved x-ray photoelectron spectroscopy, the authors demonstrate that carbon can be implanted into the silicon substrate below the oxygen rich silicon layer that typically forms on the silicon surface during nitride spacer etching. The level of C-Si bonds is modulated by the energy of ions bombarding the silicon surface during the etching process and is correlated to the post-epitaxy silicon thickness. Using N2/H2 plasma post-treatments, the authors show the reduction of the C-Si bonds level leading to a good silicon epitaxial regrowth.

On new two-dimensional structures produced on the Si (111) and Si (100) surface upon molecular-beam epitaxy of cobalt and silicon

Highly perfect epitaxial heterostructure CoSi2 films have been grown on the surface of Si (111) and Si (100) single crystals by the method of molecular-beam epitaxy. The optimal regimes of the film growth with different thicknesses have been determined. It has been shown that short-term annealing of epitaxial films at T = 900–950 K leads to the formation of new CoSi2/Si(111)-2 × 2 and CoSi2/Si(100)−2 × 4 superstructures.

Growth, structure and luminescence properties of multilayer Si/β-FeSi 2NCs/Si/…/Si nanoheterostructures

Multilayer structures (up to 15 layers) with β-FeSi 2 nanocrystallites (NCs) buried in silicon crystalline lattice were grown by successive repetition of reactive deposition epitaxy (RDE) or solid phase epitaxy (SPE) of thin iron film on Si(100) or Si(111) substrates and silicon molecular beam epitaxy (MBE) (100–200 nm). Cross-section high resolution transmission electron microscopy (HR TEM) images and ex situ optical and Raman spectroscopy data prove that NCs formed in silicon matrix have the structure and optical properties of β-FeSi 2. The growth conditions provide no dislocations in silicon lattice were found in the course of TEM analysis. Two types of NCs depth distribution were observed: (i) layered that corresponds to iron RDE and (ii) uniform that occurs in the case of iron SPE. The uniform NCs distribution points out the fact that during a growth process NCs moves up to the surface. In spite of small nanocrystallites size (5–50 nm) and their distribution in silicon cap layers the significant photoluminescence (PL) signal at 0.8 eV was observed for all grown samples.

Optical Properties of Multiple, Delta-doped Si:B/Si Layers

ABSTRACTReliable fabrication of high-speed, delta-doped transistors and better understanding of two-dimensional metal-insulator transitions can be achieved using silicon molecular beam epitaxy (MBE). However, this fabrication technique should be performed with care, avoiding dopant segregation on epitaxial Si surfaces and improving the doping efficiency. Here we report comprehensive structural and optical investigations of MBE-grown Si/delta-doped Si:B multilayer structures. Measurements of Auger electron spectroscopy, Raman scattering, optical reflection and photoluminescence are performed. Our results indicate nearly metallic conductivity at room temperature with metal-insulator phase transition near T ∼100 K. In contrast to recently reported data, no enhancement of photoluminescence at room temperature is found. Occasionally, a few samples in specific areas exhibit strong photoluminescence at 1.4-1.6 micron attributed to structural defects, most likely due to B segregation.

Arsenic surface segregation during in situ doped silicon and Si 1−xGe x molecular beam epitaxy

Surface segregation has been a major factor limiting dopant incorporation in silicon and Si 1− x Ge x epitaxy. This paper discusses arsenic incorporation and surface segregation during mixed gas- and solid-source molecular beam epitaxy of silicon and Si 1− x Ge x . The use of Si 2H 6 gas as the silicon source and careful control of growth conditions enable calculation of the surface arsenic concentration during growth from its effect on the gas-source growth rate. A wide range of surface arsenic concentrations were examined to determine the near-equilibrium relationship between the surface arsenic concentrations and the incorporated arsenic concentrations in the silicon epilayers. A new model based on 2-dimensional islanding of surface arsenic dimers is proposed to explain the observed segregation relationship and the changing surface morphology. The effects of germanium on arsenic surface segregation in Si 1− x Ge x are also discussed qualitatively.

Scanning probe microscopy for silicon device fabrication

We present a review of a detailed fabrication strategy for the realisation of nano and atomic-scale devices in silicon using phosphorus as a dopant and a combination of ultra-high vacuum scanning probe microscopy and silicon molecular beam epitaxy (MBE). In this work we have been able to overcome some of the key fabrication challenges to the realisation of atomic-scale devices including the identification of single P dopants in silicon, the controlled incorporation of P atoms in silicon with atomic precision and the minimisation of P segregation and diffusion during Si encapsulation. Recently, we have combined these results with a novel registration technique to fabricate robust electrical devices in silicon that can be contacted and measured outside the ultra-high vacuum environment. We discuss the importance of our results for the future fabrication of atomic-scale devices in silicon.

Electrical and Schottky contact properties of Pt/n‐Si 1– x Ge x /n‐Si(100) heterostructure

We report on n-type (100) oriented Si 0 . 7 6 Ge 0 . 2 4 samples grown by silicon molecular beam epitaxy (Si-MBE). The grown wafer was, first, cut into small pieces. Some of these pieces were annealed under an inert gas atmosphere at 600 °C, 700 °C and 800 °C for 1h to induce partial relaxation in Si 1 - x Ge x . The formation of Schottky junction was made by Pt deposition on n-Si 0 . 7 6 Ge 0 . 2 4 . The electrical properties of, both, the unannealed and annealed Pt/n-Si 0 . 7 6 Ge 0 . 2 4 /n-Si were studied by current-voltage (I-V) and capacitance-voltage (C-V) measurements. These measurements have been done under different temperatures, and Schottky barrier heights have been determined. Also, these results have been compared with Pt / n-Si.

シリコンの分子線エピタキシー（ＭＢＥ）プロセスにおける基板のサーマルクリーニング温度の低温化

A modified RCA cleaning method was employed for cleaning silicon substrates before silicon molecular beam epitaxy (MBE) growth. Three kinds of wet chemical processes using the conventional SC1 and SC2 solutions were employed in this study. The type of the solution and/or the dipping time at the final step of the wet chemical process was changed. When the cleaning process was ended with SC 1 solution, thermal cleaning temperature (Ttc) of 800°C was required to obtain a smooth surface of the silicon substrate. On the other hand, when it was ended with SC2 solution, a smooth surface was successfully obtained even at Ttc 690°C. SC2 was a preferable solution at the final step of the modified RCA cleaning method for lowering Ttc, and the roughness became smaller with increasing the dipping time in the final SC2 solution.

ABRUPT BORON PROFILES BY SILICON-MBE

Surface segregation and diffusion are the dominant mechanisms for profile smearing. However in the low temperature regime below 600°C diffusion is negligible. We investigated the dopant profile during silicon molecular beam epitaxy (MBE) in silicon (100). A method for measurement of the adlayer density of segregating dopant atoms is suggested. We utilize the results of this experiment to generate very sharp boron profiles. For the doping we use the pre-build up method with constant boron flux.

Substrate preparation and low-temperature boron doped silicon growth on wafer-scale charge-coupled devices by molecular beam epitaxy

Silicon charge-coupled devices (CCDs) are extensively used for commercial and scientific imaging in the visible to near-infrared wavelength range of 450 to 850 nm. Ground-based astronomers require large-scale high-performance CCDs with high sensitivity at wavelengths from the 320 nm atmospheric cutoff to 900 nm. We report on wafer-scale low-temperature silicon molecular beam epitaxy to enable ultraviolet (UV) detection utilizing silicon-fabrication-facility-compatible surface preparation. Characterization of the UV response-enhanced backside-illuminated CCDs fabricated with this technique show near 100% internal quantum efficiency in the wavelength range of 200 to 900 nm.

A novel measurement method of segregating adlayers in MBE

A novel method for measurement of the adatom density of segregating dopant atoms is suggested. The determination of the surface density is completely based on post growth concentration profile measurements. The dependancies of the surface segregation ratio on different growth parameters can be extracted. The method was tested on the segregation of boron in silicon (100) molecular beam epitaxy. The strong decay of segregation with decreasing temperature was confirmed and quantified for a selected set of parameters. The boron segregation was shown to be also strongly dependant on doping concentration.

Depth Profile of Delta-doped Semiconductors by X-Ray Reflection Technique

Delta-doping is one of the methods to realize high concentration doping in semiconductors, by which the doped atoms are confined in the thin layer in semiconductor crystals. Sb and Er delta doping layers in silicon crystals were grown at different temperatures by silicon molecular beam epitaxy (MBE). An oxygen ambient was also used in the Er delta doping process. Synchrotron radiation X-ray reflection technique was employed to measure the dopant depth distribution. Results showed that the growth temperature should be below 300°C for obtaining narrow Sb or Er delta doping in order to avoid the surface segregation of Sb or Er atoms. Thermal stability of such delta doped structure was also studied by this technique, no significant change was observed in Sb delta doped Si crystal after annealing at 500°C for 30 min. Besides, suppressing effect of surface segregation of dopant Er by oxygen was investigated. With the oxygen ambient, narrow Er delta doping was realized at growth temperature of 400°C, which was 100°C higher than that in the normal MBE epitaxy.

Investigation of Surface Formation Process of Silicon Molecular Beam Epitaxy by Atomic Force Microscopy

Evaporated Silicon molecules are epitaxially grown on (100), (111) and (110) oriented single crystalline Si substrates by MBE, and their molecular level surface formation processes are observed by both AFM and RHEED. In the case of growth on (100) and (110) at 800°C, the surface was covered with many nuclei at first, then, they began to combine together. In the case of (111) at 800°C, giant steps were formed at first and subsequent growth changed the length and edge shape of steps; however, the steps themselves could never be eliminated. It is now proposed that the molecular level surface formation model be adapted to take these experimental results into consideration.

Surface segregation behavior of B, Ga, and Sb during Si MBE: Calculations using a first-principles method

The potential energies of B, Ga, and Sb dopant atoms in the three top layers of Si(100) surfaces were evaluated using density-functional calculations of the model clusters. The different behaviors of the dopants in surface segregation during silicon molecular-beam epitaxy can be understood by considering the dopant-Si bond energies as the driving force for segregation. The energy of the B-Si bond is greater than that of the Si-Si bond, which precludes segregation, while the weaker Ga-Si and Sb-Si bonds favor it.

Bistable diodes grown by silicon molecular beam epitaxy

Si devices with two double δ-doped (one p- δ and one n- δ) junctions were grown by silicon molecular beam epitaxy (MBE). A new I–V bistability phenomenon was observed in those devices. We used a `band switching' mechanism to explain the bistability. Devices with different doping and spacer widths were grown to test the proposed mechanism. Finally, a detailed temperature dependence measurement of the I–V characteristics was shown to confirm our proposed mechanism.

Kinetic roughening of Si surfaces and surfactant effect in low temperature molecular beam epitaxy growth

Thanks to an experimental (in situ reflection high-energy electron diffraction and ex situ high-resolution electron microscopy) and a theoretical probabilistic cellular automaton study of surface kinetic roughening in low temperature silicon molecular beam epitaxy, we achieve a clear correlation between the surface roughness and the microscopic morphology of the growing layer. A transition in the growth mechanisms between a perfect epitaxy regime and another one displaying structural defects is shown. It may explain previous unusually observed deviations of the surface roughness scaling behavior unpreviewed by current theories. The effect of gallium atoms as «surfactants» is also investigated. High-resolution electron microscopy comparison of layers grown with and without gallium shows its role in the surface morphology smoothing.