G. V. Hansson

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.

n‐Si/p‐Si1−xGex/n‐Si double‐heterojunction bipolar transistors

Two different structures of n‐Si/p‐Si1−xGex/n‐Si double‐heterojunction bipolar transistors have been fabricated by molecular beam epitaxy. A common emitter current gain β of about 15 was demonstrated in one kind of structure and the β‐IC curve has been investigated. In the other structure, a novel multistep collector current IC vs collector‐emitter voltage VCE characteristic together with a strong negative resistance behavior was observed at room temperature. In this letter the basic experiments are described; a comparison and a discussion of the two kinds of devices are presented.

Epitaxial growth of Al on Si by thermal evaporation in ultra-high vacuum: growth on Si(100)2 × 1 single and double domain surfaces at room temperature

Abstract Growth of Al on Si(100) in ultra-high vacuum was investigated using in situ RHEED, LEED and AES and ex situ TEM and SEM. The substrates were kept at room temperature during growth. Al was found to grow epitaxially on Si(100)2 × 1 with the orientation relationship Al(110)//Si(100). TEM and RHEED showed that the Al layer had two types of (110)-oriented domains that are 90° rotated with respect to each other in accordance with the following relations Al[001]//Si[011] or Si[011]. RHEED observations during growth demonstrated a continuous change from the original Si(100)2 × 1 pattern to a 1 × 1 pattern after deposition of 2 monolayers (ML). At an Al coverage θ Al = 3 ML , a faint and broad Al-bulk diffraction related intensity was observed in RHEED while a clear Al-bulk diffraction pattern became visible after ~ 4 ML, indicating three-dimensional growth of Al islands. The decay in the Si AES peak-to-peak intensity versus θ A1 could only be modelled by a layer-by-layer growth mode up to θ A 1 = 4 ML . On an off-oriented, essentially single-domain Si(100)2 × 1 surface, an almost single-domain, monocrystalline Al(110) layer was obtained with only a few 90° rotated crystals. Thus the observed Al crystal domains are related to the Si(100)2 × 1 surface domains. The results are explained in terms of Al dimerization on the Si surface and epitaxial growth on the dimerized layer. A growth sequence leading to the growth of Al(110) crystals is also suggested.

Growth of metastable Ge1−xSnx/Ge strained layer superlattices on Ge(001)2×1 by temperature‐modulated molecular beam epitaxy

Single‐crystal metastable diamond‐structure Ge1−xSnx/Ge strained‐layer superlattices (SLS) with x up to 0.24 (the equilibrium solid solubility of Sn in Ge is <0.01) have been grown on Ge(001)2×1 substrates using temperature‐modulated molecular‐beam epitaxy with maximum growth temperatures Ts≤150 °C. In situ reflection high energy electron diffraction combined with postdeposition high‐resolution x‐ray diffraction (HR‐XRD) and cross‐sectional transmission electron microscopy results show that the Ge1−xSnx(001)2×1 alloy and Ge(001)2×1 spacer layers are commensurate. In fact, the alloy layers are essentially fully strained with an average in‐plane lattice constant mismatch of (1±2)×10−5 and an average tetragonal strain in the growth direction of (1.39±0.03)×10−2 as determined from HR‐XRD reciprocal‐space lattice maps obtained using asymmetric (113) reflections. ω broadening of the zero‐order SLS peak was only 30.1 arc sec FWHM, indicating that the degree of mosaicity in these structures is negligible. The int...

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.

Metal–semiconductor junctions on p‐type strained Si1−xGex layers

The electrical properties of Schottky junctions on high‐quality p‐type strained Si1−xGex layers have been studied for Ge fractions 0≤x≤0.24. A multicrystal high‐resolution x‐ray diffractometer was used to investigate the sample’s quality and the strain state and to accurately determine the Ge fraction in the fabricated devices. Several different metals, having a wide range of barrier heights, were used to reach conclusions on the variation of barrier height with the Ge content in the grown strained Si1−xGex layers. It has been found that the Schottky barrier height decreases with increasing Ge fraction in the Si1−xGex layer for the different metals investigated. The change in the barrier height with x has been found to be directly correlated to the valence band discontinuity in the Si/Si1−xGex heterojunction.

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.