W.-X. Ni

Er/O and Er/F doping during molecular beam epitaxial growth of Si layers for efficient 1.54 μm light emission

Er, together with oxygen or fluorine as co-dopants, has been incorporated into Si during molecular beam epitaxial growth using co-evaporation of Si and Er containing compounds. The Er doping concentration using both Er2O3 and ErF3 can reach a level of ∼5×1019 cm−3 without precipitation, which is at least one order of magnitude higher than a previously reported solid solubility limit for Er in Si. Growth, structural, and luminescence characterization of these Er/O and Er/F doped Si samples are reported. In particular, 1.54 μm electroluminescence has been observed from Er/O doped Si layers at room temperature through hot electron impact excitation.

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.

A silicon molecular beam epitaxy system dedicated to device-oriented material research

Abstract Design, performance test, doping capability and grown material quality of a Balzers UMS 630 Si MBE system are reported, particularly concerning measures to obtain good quality of grown films. Good stability, reproducibility and uniformity of deposition rates (Si and Ge) and doping concentrations (Sb and B) have been obtained for growth on a 4 inch Si wafer with sample rotation using a mass-spectrometry controlled e-beam evaporation system, and home-made doping sources, respectively. The quality of grown undoped and modulation doped Si and SiGe layered structures were evaluated using high-resolution XRD, XTEM, SIMS, Hall, and PL measurements. Intense and sharp excitonic PL transitions and high carrier mobility obtained from the grown Si SiGe heterostructures and quantum wells grown at a wide substrate temperature range (320–650°C) indicate high crystalline quality of grown films. Finally, test HBT structures with a thin SiGe base have been made. Good dc characteristics and frequency performance were obtained.

Electron mobility enhancement in Si using doubly δ‐doped layers

Large enhancements in the electron mobility are reported for structures containing a pair of closely spaced Sb δ‐doped layers in Si. The room‐temperature mobility is enhanced by a factor of 2 compared to corresponding uniformly doped layers of singly δ‐doped structures. Even higher mobilities were obtained by using a Schottky gate on top and applying a voltage to adjust the potential well. With an effective gate voltage of ∼−0.3 V the mobility was 1200 cm2 V−1 s−1 at room temperature, which is an enhancement by a factor of 10 relative to the layer with equivalent bulk doping concentration. The high mobility is attributed to wave functions with nodes at the δ‐doped layers.

Mechanism for thermal quenching of luminescence in SiGe/Si structures grown by molecular beam epitaxy: Role of nonradiative defects

Thermal quenching of photoluminescence from SiGe/Si quantum wells (QWs) grown by low-temperature molecular beam epitaxy is shown to be significantly improved by postgrowth thermal annealing. The dominant mechanism responsible for this improvement is shown to be a reduction of grown-in nonradiative defects, such as vacancy-related complexes. Postgrowth hydrogenation is demonstrated to be less effective as compared to thermal annealing in removing the nonradiative defects. Selective optical excitation has been used to determine the relative contributions of nonradiative recombination channels present in the SiGe QWs and the Si barriers.

Characterization of highly boron-doped Si, Si1 − xGex and Ge layers by high-resolution transmission electron microscopy

Abstract Cross-sectional transmission electron microscopy (XTEM) has been used to characterize the defect structure of as-grown and annealed highly boron-doped Si, Si 1 − x Ge x ( x ≤ 0.18) and Ge layers grown by molecular beam epitaxy. The structures have also been analyzed with two-dimensional (2D) reciprocal space mapping using high-resolution X-ray diffraction (HRXRD). The boron concentration ( C B ) was in the range from 3 × 10 19 to 8 × 10 20 cm −3 . Si and Si 1 − x Ge x layers were grown at 400°C and Ge layers at 325°C. XTEM micrographs show no crystalline defects in Si and Si 1 − x Ge x samples for C B ≤ 3 × 10 20 cm −3 . However, for C B = 8 × 10 20 cm −3 , B precipitation in the form of epitaxial layer (2D) precipitates on (001) planes in Si and Si 1 − x Ge x and both (001) and (113) planes in Ge was observed. After annealing the B-doped Si and SiGe samples with C B = 8 × 10 20 cm −3 at 1000°C for 15 min, a large number of discrete, 3D, B-related precipitates were observed. For B-doped Ge samples, the thermal stability was poor and B precipitation and severe roughening were observed after annealing at 650°C for 15 min.

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.

HIGH QUALITY SI/SI1-XGEX LAYERED STRUCTURES GROWN USING A MASS-SPECTROMETRY CONTROLLED ELECTRON-BEAM EVAPORATION SYSTEM

High quality, strained Si/Si1−xGex layered structures have been grown at temperatures in the range 400–625 °C, using a solid‐source molecular‐beam epitaxy (MBE) system with a mass‐spectrometry‐based loop‐control to improve the accuracy and stability of the evaporation rates. Good control of the growth parameters has been achieved as verified by, e.g., high‐resolution x‐ray diffraction. Very high intensities and extremely small peak widths, down to 2.7 meV for the XNP transition at low excitation levels of photoluminescence spectra, indicate high crystalline quality of the layers. It is shown that some previously reported defect‐related luminescence from MBE‐grown SiGe layers is not intrinsic to the MBE process.

Influence of ion bombardment on Si and SiGe films during molecular beam epitaxy growth

Growth of Si and SiGe layers using molecular beam epitaxy was carried out with the substrate at a floating, positive or negative bias, in order to investigate effects of ion bombardment on the crystalline quality of grown materials. Although ion energies (100–1500 eV) and ion/atom flux ratios (∼0.005) used in the experiments were quite low, significant lattice distortion along the growth direction (Δa⊥/as up to ∼300 ppm) was observed by high resolution x‐ray diffraction from the Si layers grown at 420 °C. At the same time, a broadband transition was observed in photoluminescence measurements from both Si and SiGe layers. Based on results of the annihilation behavior during postgrowth treatments using thermal annealing and hydrogenation, we attribute these effects to the ion bombardment induced formation and injection of different types of pointlike defects and defect clusters, which degrade the optical and electrical properties of grown layers.

Concentration transient analysis of antimony surface segregation during Si(001) molecular beam epitaxy

Antimony surface segregation during Si(001) molecular beam epitaxy (MBE) was investigated at temperatures Ts = 515−800 °C using concentration transient analysis (CTA). The dopant surface coverage θ, bulk fraction γ, and incorporation probability σ during MBE were determined from secondary-ion mass spectrometry depth profiles of modulation-doped films. Programmed Ts changes during growth were used to trap the surface-segregated dopant overlayer, producing concentration spikes whose integrated area corresponds to θ. Thermal antimony doping by coevaporation was found to result in segregation strongly dependent on Ts with θSb values up to 0.9 monolayers (ML): in films doped with Sb+ ions accelerated by 100 V, θSb was less than or equal to 4 × 10−3 ML. Surface segregation of coevaporated antimony was kinetically limited for the film growth conditions in these experiments.