Quantum-confinement effects in the conduction band of deposited germanium nanocrystals are measured to be greater than in similar-sized silicon nanocrystals. The germanium particles are condensed out of the gas phase and their electronic properties are determined with x-ray absorption spectroscopy. The conduction band edge shifts range from 0.2 eV for 2.7 nm particles up to 1.1 eV for 1.2 nm particles.

Strong quantum-confinement effects in the conduction band of germanium nanocrystals

High-yield synthesis of germanium nanowires (NWs) and core−shell structures is achieved by the chemical vapor deposition (CVD) of dicyclopentadienyl germanium ([Ge(C5H5)2]). The one-dimensional (1D) nanostructures are formed on an iron substrate following a base-growth model in which an Fe−Ge epilayer functions as a catalytic bed. The wire growth is selective and no catalyst particles are observed at the tip of the NWs, which is contrary to the characteristic feature of a 1D growth based on the vapor−liquid−solid (VLS) mechanism. The diameter and length of the NWs were in the ranges 15−20 nm and 25−40 μm, respectively, as found by high-resolution electron microscopy. Both axial and radial dimensions of the NWs can be controlled by adjusting the precursor feedstock, deposition temperature, and size of alloy nuclei in the Fe−Ge epilayer. High precursor flux produced coaxial heterostructures where single-crystalline Ge cores are covered with an overlayer of nanocrystalline Ge. Single-crystal Ge nanowires exh...

Germanium Nanowires and Core−Shell Nanostructures by Chemical Vapor Deposition of [Ge(C5H5)2]

Nanocrystals (NCs), representing a zero-dimensional system, are an ideal platform for exploring quantum phenomena on the nanoscale, and are expected to play a major role in future electronic and photonic devices. Here we review recent progress in the growth, characterization and utilization of some group-IV semiconductors (Si and Ge), metal and high-k NCs for silicon planar technology compatible light-emitting and floating gate memory devices. We first introduce the size-dependent electrical and optical properties of Si and Ge NCs. We outline some of the schemes to achieve light emission from indirect band gap Si and Ge NCs embedded in different high band gap oxide matrices. In particular, special emphasis is given on the review of the advances in Ge NCs because of some of their intriguing electronic and optical properties. We then describe the use of semiconductor and metal NCs as floating gates for non-volatile memory devices to achieve high data retention and faster program/erase speeds. The exploitation of high-k oxides with tunable and variable injection barriers for improved charge storage devices is discussed. Finally, the integration of single and multilayer metallic NCs and multilayer high-k oxides as floating gates is explored by the fabrication and testing of memory transistors.

https://iopscience.iop.org/article/10.1088/0022-3727/46/15/153001/pdf

Nanocrystals for silicon-based light-emitting and memory devices

We have examined the Raman spectra of violet and infrared emitting Ge+-implanted SiO2 films with special emphasis upon annealing temperature (AT) dependence of Raman scattering. We found that the complete spectrum mainly consists of three bands at 220–280, 300, and 430 cm−1, corresponding to scattering of Ge-related components, Ge nanocrystallites, and localized Si–Si optical phonons in the Ge neighborhoods, respectively. The Ge crystalline band shows an obvious AT dependence. The theoretical result from the phonon confinement model can predict its linewidth change with AT, but cannot explain its constant peak frequency. Based on the experimental result from x-ray diffraction, we attributed the discrepancy mainly to the compressive stress exerted on Ge nanocrystallites, which leads to the upshift of Ge crystallite peak thereby basically compensating the downshift caused by the confinement on phonon frequency.

Annealing temperature dependence of Raman scattering in Ge+-implanted SiO2 films

The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.

The one phonon raman spectrum in microcrystalline silicon

Germanium (Ge) films have been fabricated by the cluster‐beam technique. In transmission electron microscope (TEM) micrographs, the film deposited on a substrate at room temperature appears smooth and uniform, while that deposited at liquid nitrogen temperature consists of nanostructures with diameters from 4 to 40 nm. The x‐ray diffraction (XRD) patterns show that the crystalline structure for either deposition temperature is not the ordinary diamond structure but is tetragonal. The absorption spectra are distinct from that of bulk Ge. The photo‐oxidized Ge nanostructures exhibit blue light emission, which is strong enough to be recognized with the naked eye even under a room light.

Tetragonal germanium flims deposited by the cluster-beam evaporation technique

The valence-band x-ray photoelectron spectroscopy (XPS) spectra of the Ge nanocrystals deposited by the cluster-beam evaporation technique were studied, and suggest the presence of the tetragonal crystal structure (ST-12) of Ge in the Ge nanocrystals. Although the ST-12 structure of Ge is known to be metastable and is transformed to the diamond structure, which is the crystal structure of bulk Ge, at relatively low temperatures, the XPS and absorption spectra of the Ge nanocrystals annealed at various temperatures show that the obtained ST-12 structure is thermally stable. The critical temperature for the phase transformation of the Ge nanocrystals is found to be higher than 700 °C. These results are consistent with our earlier study using Raman spectroscopy.

Density of states of the tetragonal-phase germanium nanocrystals using x-ray photoelectron spectroscopy

The photo-oxidation of germanium (Ge) nanostructures deposited by the cluster-beam evaporation technique is systematically studied. The significant photo-oxidation is found in the Ge-liquid nitrogen temperature (LNT) film, which was deposited on substrates whose temperature was kept at liquid nitrogen temperature during the deposition but not in the Ge-RT film, whose substrate temperature was room temperature. The difference in photo-oxidation of both types of Ge films is explained by denseness of Ge nanostructures. The threshold photon energy for photo-oxidation is close to the optical band gap estimated from the absorption spectrum of the as-deposited Ge-LNT film. The modified electron-active oxidation model is proposed to explain the photo-oxidation of Ge nanostructures. It is also hypothesized that the final size of Ge cores is determined by the incident photon energy and that the size of Ge cores becomes more uniform for prolonged photo-oxidation.

Photo-oxidation of germanium nanostructures deposited by the cluster-beam evaporation technique

Photoluminescence study on oxygen-containing silicon nanostructures

The structure and the vibrational properties of germanium films deposited by the cluster-beam technique were investigated mainly using Raman spectroscopy. Germanium films were deposited on Si substrates at room temperature (Ge-RT) and liquid N 2 temperature (Ge-LNT). Raman as well as other techniques (XRD, TEM, optical absorption and PL) were used to characterize these films. The nanostructures were observed in Ge-LNT by TEM micrograph, while flat continuous images were found in Ge-RT. Raman spectra of both Ge films showed a broad peak around 270 cm −1 different from that of cubic Ge (300 cm −1 ). Ge-RT films showed a line at 300 cm −1 by annealing the films above 700°C, while Ge-LNT did not give such line up to the 800°C annealing. The relation between these spectra and the observed nanostructure is discussed.

S. Sato

Papers

Tetragonal germanium flims deposited by the cluster-beam evaporation technique

Density of states of the tetragonal-phase germanium nanocrystals using x-ray photoelectron spectroscopy

Photo-oxidation of germanium nanostructures deposited by the cluster-beam evaporation technique

Photoluminescence study on oxygen-containing silicon nanostructures

Raman spectroscopy of germanium films deposited with cluster-beam technique