C.-W. Hu

Ge-Sn semiconductors for band-gap and lattice engineering

We describe a class of Si-based semiconductors in the Ge1−xSnx system. Deuterium-stabilized Sn hydrides provide a low-temperature route to a broad range of highly metastable compositions and structures. Perfectly epitaxial diamond-cubic Ge1−xSnx alloys are grown directly on Si(100) and exhibit high thermal stability, superior crystallinity, and crystallographic and optical properties, such as adjustable band gaps and lattice constants. These properties are completely characterized by Rutherford backscattering, low-energy secondary ion mass spectrometry, high-resolution transmission electron microscopy, x-ray diffraction (rocking curves), as well as infrared and Raman spectroscopies and spectroscopic ellipsometry. Ab initio density functional theory simulations are also used to elucidate the structural and spectroscopic behavior.

Low temperature chemical vapor deposition of Si-based compounds via SiH3SiH2SiH3 : Metastable SiSn/GeSn/Si(100) heteroepitaxial structures

Growth of Si1−xSnx alloys on Ge1−ySny-buffered Si(100) was achieved via reactions of SnD4 and SiH3SiH2SiH3 at 275°C. Kinetic studies indicate that unprecedented low growth temperatures are made possible by the highly reactive SiH2 groups. The authors obtain supersaturated metastable compositions (y∼25%) near the indirect to direct band gap crossover predicted by first principles simulations. Extensive characterizations of composition, structure, and morphology show that the SiSn∕GeSn films grow lattice matched via a “compositional pinning” mechanism. The initial Raman observations of Si–Sn bond vibrations in a condensed phase are discussed in the context of simulated bond distributions in the alloys.

Growth of self-assembled GaN quantum dots via the vapor–liquid–solid mechanism

Self-assembled nanometer-scale GaN quantum dots were fabricated on 6H–SiC(0001) substrates via the formation of Ga liquid droplets and their subsequent nitridation with a supersonic gas source seeded with NH3 molecules. The entire process was observed and controlled in situ and in real time in a low-energy electron microscope. The microstructure of the quantum dots was studied by high-resolution cross-sectional transmission electron microscopy illustrating the perfectly coherent wurtzite structure of GaN quantum dots with 5 nm base width. Spatially resolved cathodoluminescence spectra yield the characteristic band edge emission near 3.48 eV for larger size GaN dots.

Low-temperature pathways to Ge-rich Si1−xGex alloys via single-source hydride chemistry

We report rapid low-temperature (300–470°C) growth of Si0.50Ge0.50, Si0.33Ge0.67, Si0.25Ge0.75, and Si0.20Ge0.80 alloys on Si(100) using heavy single-source hydride molecular compounds (H3Ge)nSiH4−n (n=1–4). Incorporation of the entire SiGe, SiGe2, SiGe3, and SiGe4 framework of these precursors into the film provides precise control of morphology, composition, and strain. Low-energy electron microscopy analysis indicates that the (H3Ge)xSiH4−x (x=2–4) species are highly reactive, with H2 desorption characteristics comparable to those of Ge2H6, despite the presence of strong Si–H bonds in their molecular structure.

Structural and optical properties of coherent GaN islands grown on 6H-SiC(0001)-(√3×√3)

Coherent islands of GaN with base widths in the range of 100 to 500 nm were grown on 6H-SiC(0001)-(√3×√3) surfaces via the vapor–liquid–solid (VLS) mechanism. The microstructure of GaN islands was studied by high-resolution cross-sectional transmission electron microscopy. The morphological details of the islands were imaged by atomic force microscopy and the same assembly of islands was identified in a scanning electron microscope in which site-specific cathodoluminescence (CL) spectroscopy was conducted on individual islands. The broadening of the CL linewidths together with the shift to lower wave numbers in the E2 Raman mode detected by micro-Raman spectroscopy suggest the existence of tensile strain in the GaN islands. The strain is due to the heavy Si doping of the GaN islands by Si adatoms on the (√3×√3) substrate surface during the VLS growth process.

Synthesis of Si-Ge nanoscale structures via deposition of single-source (GeH3)4−nSiHn hydrides

Growth of nanoscale islands with distinct Si0.33Ge0.67, Si0.25Ge0.75, and Si0.20Ge0.80 compositions and uniform sizes is conducted on Si(100) via dehydrogenation of the single-source hydrides (H3Ge)2SiH2, (H3Ge)3SiH, and (H3Ge)4Si, respectively. High-spatial-resolution electron energy loss spectroscopy and Raman spectroscopy indicate homogeneous elemental concentrations within and among islands and confirm that their Si-Ge content is predetermined by the stoichiometry of the corresponding precursors. Z-contrast electron microscopy reveals distinct and perfectly epitaxial islands with atomically sharp interfaces grown via a smooth and continuous wetting layer ∼10A thick. Cross-sectional electron microscopy shows monomodal distributions of islands with defect-free microstructures. Low-energy electron microscopy studies of the film formation reveal that the growth proceeds via the Stranski-Krastanov mode. Assemblies of coherent quantum dots with highly controlled Ge-rich concentrations produced by this metho...

Optical characterization of Si1−xGex nanodots grown on Si substrates via ultrathin SiO2 buffer layers

Growth of Si1−xGex nanodots with x=0, 0.33, 0.67, and 1.0 was accomplished on ultrathin SiO2 buffer layers of 1–2 ML on Si(001) and Si(111) substrates using single-source gaseous precursors at 550°C. The Si1−xGex dots have diameters of ∼10nm and an areal density of ∼1011cm−2. Raman spectroscopy conducted on the nanodots shows that they are relaxed and their compositions correlate closely with the molecular ratios in the precursors used in their fabrication. Photoluminescence (PL) spectra were taken with reduced laser power density which enhanced the PL contribution from the nanodots while suppressing the PL contribution from the Si substrate. Two groups of PL peaks were observed, in the ranges of 0.8–1.0 and 1.0–1.1eV. The first group in the 0.8–1.0eV range shows peaks similar to those observed in Si with dislocations. The second group of peaks at the 1.0–1.1eV range shows an increase in intensity with increasing Ge concentration in the dots. However, both groups of peaks appear to be reproducible in Si s...

MORPHOLOGICAL CONTROL OF GaN BUFFER LAYERS GROWN BY MOLECULAR BEAM EPITAXY ON 6H–SiC(0001)

The growth of GaN buffer layers of thickness 10–25 nm directly on 6H–SiC(0001) substrates was studied using low energy electron microscopy, atomic force microscopy and cross-sectional transmission electron microscopy. The Ga flux was supplied by an evaporative source, while the NH3 flux came from a seeded beam supersonic jet source. By monitoring the growth in situ and by suitably adjusting the Ga/NH3 flux ratio, smooth basal-plane-oriented GaN layers were grown on hydrogen-etched SiC substrates at temperatures in the range of 600–700°C. The growth proceeds via nucleation of small flat islands at the step edges of the 6H–SiC(0001) substrate surface. The islands increase in size with a lateral-to-vertical growth ratio of ~10 and eventually coalesce into a quasicontinuous layer. A highly defective substrate surface was found to be detrimental to the growth of flat buffer layers.

Controlled striped phase formation on ultraflat Si(001) surfaces during diborane exposure

We have used low-energy electron microscopy to study spontaneous step formation in “striped” domains on ultraflat Si(001)-(2×1) surfaces during B2H6 exposure at elevated temperatures. We show that the size and arrangement of striped domains are kinetically limited, and propose that the limiting factor is the supply of diffusing Si surface adatoms. By adding controlled amounts of extra Si to ultraflat terraces, it is possible to foster the formation of very large (>5 μm) single-domain striped regions with adjustable stripe widths.