B‐Y. Tsaur

Structural properties of As-rich GaAs grown by molecular beam epitaxy at low temperatures

GaAs layers grown by molecular beam epitaxy (MBE) at substrate temperatures between 200 and 300 °C were studied using transmission electron microscopy (TEM), x‐ray diffraction, and electron paramagnetic resonance (EPR) techniques. High‐resolution TEM cross‐sectional images showed a high degree of crystalline perfection of these layers. For a layer grown at 200 °C and unannealed, x‐ray diffraction revealed a 0.1% increase in the lattice parameter in comparison with bulk GaAs. For the same layer, EPR detected arsenic antisite defects with a concentration as high as 5×1018 cm−3. This is the first observation of antisite defects in MBE‐grown GaAs. These results are related to off‐stoichiometric, strongly As‐rich growth, possible only at such low temperatures. These findings are of relevance to the specific electrical properties of low‐temperature MBE‐grown GaAs layers.

Metal‐semiconductor field‐effect transistors fabricated in GaAs layers grown directly on Si substrates by molecular beam epitaxy

Device‐quality GaAs layers have been grown directly on Si(100) substrates by molecular beam epitaxy. Metal‐semiconductor field‐effect transistors have been fabricated in these layers with transconductance as high as 85 mS/mm and leakage current as low as 1 μA at Vgs =−3 V for gate dimensions of 2.0 μm×200 μm.

AlGaAs double‐heterostructure diode lasers fabricated on a monolithic GaAs/Si substrate

AlGaAs double‐heterostructure diode lasers have been fabricated for the first time on a monolithic GaAs/Si substrate. The heterostructure was prepared by growth of a series of GaAs and AlGaAs layers on a Ge‐coated Si wafer. For pulsed operation at 77 K the lowest threshold current was 260 mA, the highest power output was 1.8 mW per facet, and the highest differential quantum efficiency was 1.2%.

Preparation of superconducting YBa2Cu3Ox thin films by oxygen annealing of multilayer metal films

Superconducting thin films consisting predominantly of YBa2Cu3Ox have been prepared by oxygen annealing of metal films formed by using electron beam evaporation to deposit a three‐layer sequence of Cu, Ba, and Y in nominally stoichiometric proportions, then repeating the sequence five times. For the best superconducting films, which were prepared on yttria‐stabilized cubic zirconia substrates, the onset of superconductivity occurred at 94 K and zero resistivity was observed at 72 K. For the best films prepared on sapphire substrates, the corresponding temperatures were 95 and 40 K.

Zone‐melting recrystallization of encapsulated silicon films on SiO2—morphology and crystallography

Large‐area films of recrystallized Si with (100) texture, preferred in‐plane orientation, and grain sizes ≳1× 10 mm have been produced by using a strip‐heater oven to move a narrow molten zone across Si films on SiO2. A composite encapsulation layer of SiO2 and Si3N4 over the Si leads to smooth surfaces and (100) texture. The 〈100〉 axes of grains tend to lie along the direction of motion of the molten zone. A grid of faceted pits etched anisotropically into the Si permits determination of local orientation. As described elsewhere, the recrystallized films have been used for the fabrication of metal‐oxide semiconductor field‐effect transistors which exhibit electron surface mobilities comparable to devices fabricated on single‐crystal Si.

Low‐dislocation‐density GaAs epilayers grown on Ge‐coated Si substrates by means of lateral epitaxial overgrowth

Single‐crystal GaAs layers have been obtained by means of lateral epitaxial overgrowth seeded within stripe openings in a SiO2 mask over GaAs layers grown on Ge‐coated Si substrates. Transmission electron microscope and scanning cathodoluminescence studies indicate that the laterally overgrown GaAs layers have a dislocation density of less than 104 cm−2, compared to 107–108 cm−2 for the GaAs layers grown directly on the Ge/Si substrates. Initial experiments indicate that the electrical properties of the laterally overgrown layers are comparable to those of conventional GaAs epilayers grown on single‐crystal GaAs substrates.

Synthesis of metastable, semiconducting Ge‐Sn alloys by pulsed UV laser crystallization

Thin microcrystalline films of the metastable semiconducting alloy Ge1−xSnx (x≊0.22) have been formed using excimer laser radiation to crystallize amorphous sputtered films on glass and semiconducting crystalline substrates. X‐ray diffraction, electroreflectance, and Raman spectroscopy have been used to characterize the semiconducting material which is stable to at least 200 °C. The study demonstrates the possibility of extending earlier studies of amorphous Ge1−xSnx alloys into a crystalline regime with a direct band‐gap variable with x from 0 up to about 0.5 eV. The crystallization technique is potentially applicable to the formation of other metastable semiconducting compounds of device potential.

Stress‐enhanced carrier mobility in zone melting recrystallized polycrystalline Si films on SiO2‐coated substrates

Thermal stress is found to have a significant influence on the carrier mobilities in Si films prepared by zone melting recrystallization of polycrystalline Si on SiO2‐coated substrates. Films recrystallized on SiO2‐coated fused quartz substrates exhibit a large tensile stress, which enhances the electron mobility by ∼75% compared to the stress‐free Si films recrystallized on SiO2‐coated Si substrates. In contrast, Si films recrystallized on SiO2‐coated sapphire substrates are under a large compressive stress, which yields an increase of ∼10% in hole mobility compared to the stress‐free films.

Molecular beam epitaxy of GaAs and AlGaAs on Si

Epitaxial layers of GaAs and AlxGa1−x As (0.2≤x≤0.5) have been grown directly on single‐crystal Si (100) substrates, without an intermediate Ge layer, by molecular beam epitaxy (MBE). To improve nucleation, after being chemically cleaned the Si substrates were preheated in the MBE system to reduce surface contamination and achieve ordered surface reconstruction prior to growth. Reflection electron diffraction, Rutherford backscattering channeling, and photoluminescence measurement were used to characterize the epitaxial layers. The AlGaAs layers were found to be superior to the GaAs layers.

Improved techniques for growth of large‐area single‐crystal Si sheets over SiO2 using lateral epitaxy by seeded solidification

Continuous single‐crystal Si sheets over SiO2 with areas of several square centimeters have been produced from poly‐Si films by the LESS technique (lateral epitaxy by seeded solidification). Seeding is achieved either with a narrow stripe opening in a recessed SiO2 layer on a single‐crystal Si substrate or with an external single‐crystal Si seed. N‐channel metal‐oxide‐semiconductor field‐effect transistors (MOSFET’s) fabricated in these films exhibit surface electron mobilities as high as 700 cm2/V s.