Sang M. Han

Selective growth of Ge on Si(100) through vias of SiO2 nanotemplate using solid source molecular beam epitaxy

We demonstrate that Ge can be selectively grown on Si(100) through openings in a SiO2 nanotemplate by solid source molecular beam epitaxy. The selectivity relies on the thermal instability of GeO and SiO near 650 °C. Ge islands grow in the template windows and coalesce on top of the template, forming an epitaxial lateral overgrowth (ELO) layer. Cross-sectional transmission electron microscopy images show that the Ge seeds and the ELO layer are free of threading dislocations. Only stacking faults are generated but terminate within 70 nm of the Ge–Si interface, while twins along {111} planes are observed in the ELO layer. The threading-dislocation-free Ge seeds and ELO layer are attributed to epitaxial necking as well as Ge–Si intermixing at the interface.

Heteroepitaxy of high-quality Ge on Si by nanoscale Ge seeds grown through a thin layer of SiO2

We demonstrate that high-quality Ge can be grown on Si covered with a thin layer of chemical SiO2. When the oxidized Si substrate is exposed to Ge molecular beam, 7-nm-wide seed pads form in the oxide layer and “touchdown” on the underlying Si. Upon continued exposure, Ge selectively grows on the seed pads rather than on SiO2, and the seeds coalesce to form an epitaxial lateral overgrowth (ELO) layer. The Ge ELO layer is characterized by transmission electron microscopy and etch-pit density (EPD). The Ge ELO layer is free of dislocation network, but stacking faults exist near the Ge-SiO2 interface. A fraction of these stacking faults propagate to the surface, resulting in EPD less than 2×106cm−2. The high quality Ge ELO layer is attributed to a high density of nanoscale Ge seed pads interspaced by 2–12-nm-wide SiO2 patches.

Morphological evolution and strain relaxation of Ge islands grown on chemically oxidized Si(100) by molecular-beam epitaxy

We have previously demonstrated that high-quality Ge can be grown on Si by the touchdown process, where chemically oxidized Si is exposed to a Ge molecular beam. The causes of strain relaxation in the Ge epilayer were also proposed and discussed. Herein, we present a detailed analysis on the morphological evolution and strain relaxation of nanoscale Ge islands on SiO2-covered Si in order to identify the mechanisms by which the high-quality epilayer forms. During the touchdown, the Ge seeds are anchored to the underlying Si. This immobility of Ge islands gives rise to a unique bimodal size distribution during coarsening. Three events are observed during coalescence: (1) merging of two small (<10nm) islands largely driven by surface diffusion, (2) merging of a small island and a big island (∼50nm), and (3) merging of two big islands. The coalescence of two small islands is characterized by the formation of twins or stacking faults at the two merging fronts. In contrast, no stacking fault or grain boundary r...

Anhydrous solution synthesis of germanium nanocrystals from the germanium(II) precursor Ge[N(SiMe3)2]2

A convenient, simple, single-source solution synthesis of Ge nanocrystals via thermal reduction of Ge(II) precursor Ge[N(SiMe3)2]2 in a non-coordinating solvent at 300 degrees C and 1 atm Ar is described.

Impact of leakage current and electrolysis on FET flow control and pH changes in nanofluidic channels

We have fabricated multiple-internal-reflection Si infrared waveguides integrated with an array of nanochannels sealed with an optically transparent top cover. The channel walls consist of a thin layer of SiO2 for electrical insulation, and gate electrodes surround the channel sidewalls and bottom to manipulate their surface charge and zeta-potential in a fluidic field effect transistor (FET) configuration. This nanofluidic device is used to probe the transport of charged molecules (Alexa 488) and to measure the pH shift in nanochannels in response to an electrical potential applied to the gate. During gate biasing for FET operation, laser-scanning confocal fluorescence microscopy (LS-CFM) is used to visualize the flow of fluorescent dye molecules (Alexa 488), and multiple internal reflection-Fourier transform infrared spectroscopy (MIR-FTIRS) is used to probe the characteristic vibrational modes of fluorescein pH indicator and measure the pH shift. The electroosmotic flow of Alexa 488 is accelerated in response to a negative gate bias, whereas its flow direction is reversed in response to a positive gate bias. We also measure that the pH of buffered electrolyte solutions shifts by as much as a pH unit upon applying the gate bias. With prolonged application of gate bias, however, we observe that the initial response in flow speed and direction as well as pH shift becomes reversed. We attribute these anomalous flow and pH shift characteristics to a leakage current that flows from the Si gate through the thermally grown SiO2 to the electrolyte solution.

Origin and removal of stacking faults in Ge islands nucleated on Si within nanoscale openings in SiO2

We have previously reported that Ge films formed after nucleation of Ge islands within nanometer size openings in SiO2 and their subsequent coalescence over the SiO2 template exhibit threading dislocation densities below 106 cm−2. However, these films contain a density of twin/stacking fault defects on the order of 5 × 1010 cm−2 that emanate primarily from the Ge-SiO2 interface. Most of these faults self-terminate within 200 nm of the interface; however, a total of 5 × 107 cm−2 propagate to the Ge surface. These defects are found to be detrimental to the morphology and minority carrier lifetime in III-V films integrated onto the Ge-on-Si virtual substrates. We have found that annealing the Ge islands during the initial stage of coalescence eliminates stacking faults, but further Ge growth leads to a film containing a threading dislocation density of 5 × 107 cm−2. To explain the origin of the twin/stacking fault defects in the Ge films and their removal after annealing Ge islands, we have studied the Ge is...

Two-photon absorption of matrix-free Ge nanocrystals

The authors demonstrate that solution synthesized Ge nanocrystals (NCs) display a highly nonlinear optical absorption. The Ge NCs with an average diameter of 5±2nm are synthesized from germanium(II) bis(trimethylsilyl)amide with hexadecylamine surfactants at 300°C and 1atm in argon atmosphere. The resulting Ge NCs in a powder form are then dispersed on a silica glass substrate. Femtosecond pulses at 820nm wavelength from a mode-locked Ti:sapphire laser are used to measure a two-photon absorption coefficient of the deposited Ge NCs. The calculated coefficient ranges from 1190to1940cm∕GW.

Effect of threading dislocation density and dielectric layer on temperature-dependent electrical characteristics of high-hole-mobility metal semiconductor field effect transistors fabricated from wafer-scale epitaxially grown p-type germanium on silicon substrates

We report the electrical characteristics of Schottky contacts and high-hole-mobility, enhancement-mode, p-channel metal semiconductor field effect transistors (MESFETs) fabricated on Ge epitaxially grown on Si substrates. The Ge film covers the entire underlying Si substrate at the wafer scale without mesas or limited-area growth. The device performance is characterized primarily as a function of threading dislocation density in the epitaxial Ge film (2 × 107, 5 × 107, 7 × 107, and 2 × 108 cm−2) and dielectric layers (SiO2, Al2O3, and HfO2) inserted between gate metal and Ge. The thin dielectric layers (∼1.3 nm) are used to unpin the Fermi level. The device performance improves with decreasing threading dislocation density and the use of HfO2. The hole mobility in the Ge film with 2 × 107 cm−2 dislocation density, obtained from Hall measurements, is 1020 cm2/V-s. Capacitance-voltage measurements on Schottky contacts provide the energy-dependent interfacial trap density of 6 × 1011 cm−2 eV−1, while current...

Empirical correlation for minority carrier lifetime to defect density profile in germanium on silicon grown by nanoscale interfacial engineering

High-quality Ge-on-Si heterostructures have been explored for many applications, including near infrared photodetectors and integration with III–V films for multijunction photovoltaics. However, the lattice mismatch between Ge and Si often leads to a high density of defects. Introducing annealing steps prior to and after full Ge island coalescence is found to reduce the defect density. The defect density in Ge is also found to decrease with increasing dopant density in Si substrates, likely due to the defect pinning near the Ge-Si interface by dopants. The authors establish an empirical correlation between the minority carrier lifetime (τG) and the defect density in the Ge film (ρD) as a function of distance from the Ge-Si interface: τGe = C/ρD, where C is a proportionality constant and a fitting parameter which is determined to be 0.17 and 0.22 s/cm2 for Ge films grown on low-doped, high-resistivity Si substrates and high-doped, low-resistivity Si substrates, respectively. The effective minority carrier ...

Dislocation reduction in heteroepitaxial Ge on Si using SiO2 lined etch pits and epitaxial lateral overgrowth

We report a technique that significantly reduces threading dislocations in Ge on Si heteroepitaxy. Germanium is first grown on Si and etched to produce pits in the surface where threading dislocations terminate. Further processing leaves a layer of SiO2 only within etch pits. Subsequent selective epitaxial Ge growth results in coalescence above the SiO2. The SiO2 blocks the threading dislocations from propagating into the upper Ge epilayer. With annealed Ge films grown on Si, the said method reduces the defect density from 2.6 × 108 to 1.7 × 106 cm−2, potentially making the layer suitable for electronic and photovoltaic devices.