J. Knall

Indium overlayers on clean Si(100)2×1: Surface structure, nucleation, and growth

The structure, nucleation, growth mechanisms, and morphology of In overlayers on clean Si(100)2×1 surfaces have been investigated as a function of In coverage θ and deposition temperature Ts using reflection high-energy eletron (RHEED), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM). The surface phase diagram for In on Si(100) was determined. Over the entire temperature range investigated, 30 to 600°C, In was found to interact with clean Si(100) surfaces through a Stranski-Krastanov mechanism in which the initially deposited In nucleated and grew two-dimensionally up to a coverage of between 2 and 3 monolayers (ML), depending upon Ts, after which three-dimensional islands were formed. The structure of the two-dimensional layer, as well as the morphology of the islands, was dependent upon θ and Ts. At deposition temperatures between 30 and 150°C, In formed a two-dimensional gas at coverages below 0.1 ML and an In(2×2) structure with chains of In dimers along 〈011〉 directions at coverages between ≈0.1 and 0.5 ML. Further increases in θ resulted in a transition to an In(2×1) structure while single-crystal elongated polyhedral-shaped islands exhibiting preferential growth along [011] and [011] directions were obtained at coverages above ≈ 3 ML. The introduction of even minute amounts of contaminants (≈ 0.01 ML) prevented the formation of the Si(100)2×2-In phase and resulted in the growth of epitaxial hemispherical fcc In islands on Si(100)2×1-In at θ ≳ 2 ML. For Ts between 150 and 420°C, the initial In overlayer on clean Si(100) surfaces exhibited an In(3×4) structure and emispherical In islands were observed at coverages ≳ 2 ML. Above 420°C, a disordered In layer formed on top of the In(3×4) phase at θ≳0.5 ML while (310) Si facets were observed at θ > 0.5 ML and Ts > 550°C. Indium deposition on Si(100)3×4-In surfaces (formed at Ts > 150°C and then cooled to Ts < 150°C before further deposition) resulted in the formation of hemispherical-shaped fcc In islands with a much higher number density than was observed for the elongated polyhedral-shaped In islands grown on Si(100)2×1-In surfaces under the same deposition conditions.

Incorporation of accelerated low-energy (50-500 eV) In+ ions in Si(100) films during growth by molecular-beam epitaxy

A single‐grid broad‐beam ion source was used for low‐energy (50–500 eV) accelerated In+ ion beam doping during growth of Si(100) layers by molecular‐beam epitaxy. Indium incorporation behavior was studied as a function of ion energy (E+In=50–500 eV), substrate temperature (Ts=500–1050 °C), ion flux (J+In=1×109–5×1012 cm−2 s−1), and Si growth rate (R=0.1–1.3 nm s−1). Dopant concentration profiles obtained using secondary ion mass spectroscopy showed that abrupt doping profiles were obtained at Ts<900 °C and R=0.7 μm h−1, the incorporation probability σ+In was close to unity for E+In≥200 eV, while σ+In was less than unity and decreased gradually with increasing Ts for E+In≤100 eV. At Ts≥900 °C, σ+In decreased rapidly with increasing Ts for all ion energies. The incorporation results are interpreted using a qualitative model based on different types of binding sites for In with incident energies ranging from thermal to 500 eV. A procedure, utilizing accelerated ions, for the growth of ultrathin doped layers ...

Indium incorporation during the growth of (100)Si by molecular beam epitaxy: Surface segregation and reconstruction

The In incorporation probability σIn in (100) Si grown by molecular beam epitaxy was found, using secondary ion mass spectrometry (SIMS), to decrease from essentially unity at film growth temperatures Ts of ∼500 °C to <10−4 at 840 °C. SIMS depth profiles of both uniformly doped and modulation‐doped samples showed evidence of strong surface segregation with the amount of profile broadening directly related to σIn(Ts). A combination of in situ electron diffraction and Auger electron spectroscopy was used to show that the surface segregation rate was sufficient over a wide range in Ts and In to Si flux ratios to cause the initial (2×1)‐(100) Si surface reconstruction to transform to (3×4) due to the formation of an ordered In surface layer. The In surface coverage in the (3×4) state was ∼0.05–0.1 monolayer even though the bulk In concentration was ≤2×1017 cm3. The (2×1) to (3×4) surface phase transition was reversible by either terminating film growth and reevaporating the excess surface In or terminating th...

Electrical properties of Si films doped with 200‐eV In+ ions during growth by molecular‐beam epitaxy

A single‐grid ultra‐high‐vacuum‐compatible ion source was used to provide accelerated In+‐dopant beams during Si(100) growth by molecular‐beam epitaxy. Indium incorporation probabilities σ, determined by secondary ion mass spectrometry, in films grown at Ts=800 °C were too low to be measured for thermal In (σIn was 550 °C) . However, for accelerated In+ doping, σIn+ at 800 °C ranged from 0.03 to ∼1 for In+ acceleration energies EIn+ between 50 and 400 eV. Temperature‐dependent Hall‐effect and resistivity measurements were carried out on In+‐doped Si films grown at Ts =800 °C with EIn+=200 eV . Indium was incorporated substitutionally into electrically active sites over a concentration ranging from 2×1015−2×1018 cm−3, which extends well above reported equilibrium solid‐solubility limits. The acceptor‐level ionization energy was 156 meV, consistent with previously published results for In‐doped bulk Si. Room‐temperature hole mobilities μ were in good agreement with the best reported data for B...

Adsorption and desorption kinetics of In on Si(100)

Abstract The structure and surface morphology of In overlayers on Si(100) surfaces were investigated as a function of substrate temperature and surface coverage using low-energy and reflection high-energy electron diffraction as well as Auger electron spectroscopy. Desorption kinetics of adsorbed In was studied with modulated-beam desorption and temperature-programmed desorption spectroscopies. Indium was found to grow on Si(100) according to a Stranski-Krastanov mechanism with the initial formation of several two-dimensional phases preceding the nucleation and growth of three-dimensional In islands. Binding energies and frequency factors were extracted from the desorption measurements using a model based on first-order desorption from several interdependent surface phases. First-order and zeroth-order kinetics were observed for the total desorbing flux from coexisting surface phases.

Electrical properties of Si(100) films doped with low‐energy (≤150 eV) Sb ions during growth by molecular beam epitaxy

A low‐energy ultrahigh‐vacuum compatible ion gun with single‐grid optics was used to provide accelerated Sb ion doping during the growth of Si(100) by molecular beam epitaxy (MBE). The incorporation probability of accelerated Sb in MBE Si films grown at 800 °C with an ion acceleration potential of 150 eV was near unity, more than four orders of magnitude higher than for thermal Sb. The films exhibited complete dopant substitutionality and temperature‐dependent electron mobilities were equal to the best reported bulk Si values for Sb concentrations up to 2×1019 cm−3, more than an order of magnitude higher than obtainable by thermal Sb doping during Si MBE. Transmission electron microscopy examination of all films showed no evidence of dislocations or other extended defects.

DOPANT INCORPORATION KINETICS AND ABRUPT PROFILES DURING SILICON MOLECULAR BEAM EPITAXY

Abstract Controlled dopant incorporation behavior during the growth of single crystal silicon films by molecular beam epitaxy (MBE) is crucial for most device applications. However, since all dopants except boron exhibit low incorporation probabilities and/or a high degree of surface segregation, achievement of sharp doping layers with well defined concentration levels is not straightforward. In order to overcome these problems, techniques involving either the use of accelerated low-energy ions (secondary and direct implantation) or solid-phase epitaxy regrowth have been developed. Recent results on dopant incorporation using low-energy secondary and direct implantation are presented in this paper. Using indium as a model dopant, it is shown that the incorporation probability during secondary implantation, as measured by secondary-ion mass spectroscopy, exhibits a complicated dependence on the film growth temperature and the indium flux. From a detailed investigation of the adsorption/desorption behavior of indium on Si(100) surfaces, it was found that the incorporation behavior was directly correlated to corresponding changes in the segregated indium surface overlayer. Physical models describing dopant incorporation during silicon MBE are discussed and particular emphasis is placed on a newly developed multi-site model. This model is based on an exchange process for dopant atoms moving between potential wells corresponding to different lattice sites in the near-surface region. Five different dopant sites, including surface, bulk and three intermediate sites were used and surface segregation, incorporation, and bulk diffusion were accounted for by solving simultaneous rate equations. The model is demonstrated in the case of both thermal and accelerated antimony doping, to fit experimental incorporation data both as a function of growth temperature and growth rate very well. Finally, results on δ-doped structures are also presented.

Strain-affected band offsets at Si/Si1−xGex(100) heterojunction interfaces studies with x-ray photoemission

The band offsets at strained Si/Si 1− x Ge x (100) heterojunction interfaces have been studied by X-ray photoemission in combination with ion beam doping during MBE growth. Both Δ E c and Δ E v derived from the core level spectroscopy measurements show large differences in band lineup for heterostructures grown with different strain configurations. The results are in good agreement with published calculations of band offsets.

Low-Energy Accelerated-Ion Doping of Si during Molecular Beam Epitaxy: Incorporation Probabilities, Depth Distributions, and Electrical Properties

Molecular-beam epitaxy (MBE) has become a well established technique for single-crystal Si film growth. However, almost all of the dopants commonly used in bulk Si present problems when co-evaporated with Si during MBE. For example, the incorporation probabilities σ of thermal Sb(1), Ga(2), In(3), and As(4) dopant beams range from ~ 10–4 to < 10–8at typical MBE Si growth temperatures Ts. Moreover, σ decreases exponentially with increasing Ts making control of dopant concentrations difficult. These problems arise due to strong segregation of dopant atoms to the growth front and/or rapid desorption of dopant atoms.(5) Dopant accumulation on the surface leads to highly distorted doping profiles with segregation-induced broadening as large as ~ 300 nm.(6) High dopant surface coverages during growth also limit the maximum concentrations Cmax obtainable (e.g. Cmax is ≤ 1018 cm-3 for thermal Sb) without degrading electronic properties due to introduction of structural defects.