Ge Nanoclusters Research Articles

Electron beam induced defects in Ge‐implanted SiO 2 layers

Scanning electron microscopy (SEM) and cathodoluminescence (CL) have been used to investigate the irradiation-sensitive defect structure of pure and Ge+-implanted amorphous silicon dioxide layers. CL emission spectra at specimen temperatures between liquid nitrogen (LNT) and room temperature (RT) are identified with particular defect centers including the nonbridging oxygen-hole center (NBOHC: ≡Si−O•) associated with the red luminescence at 650 nm (1.9 eV), the self trapped exciton (STE) with the yellow-green luminescence at 580 nm (2.1 eV) and the Si related oxygen deficient center (SiODC) with the blue (460 nm; 2.7 eV) and ultraviolet UV band (295 nm; 4.2 eV). In Ge doped SiO2 an additional emission band is identified at (410 nm; 3.1 eV). This band corresponds to the Ge related oxygen deficient center (GeODC). The annealing process of Ge+-implanted layer leads first to a strong increase of the violet luminescence due to formation of Ge dimers, trimers and higher aggregates, finally to destruction of the luminescence centers by further growing to Ge nanoclusters. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum dots microstructure and energy spectrum peculiarities

The pyramid-like Ge islands deposited on Si(001) substrate using molecular beam epitaxy at 300 °C reveal quantum dots (QDs) properties. Measurements of x-ray absorption fine structure at the germanium K edge (GeK XAFS measurements) have been performed using total electron yield and fluorescent detection mode. The influence of effective thickness of the germanium film, Ge nanocluster sizes and Si deposition temperature on the QDs microstructure parameters is revealed. Some information concerning the energy structure of the free states of the quantum dot was extract from x-ray absorption near edge structure (XANES) spectra for the first time.

Brillouin scattering and x-ray photoelectron studies of germanium nanoclusters synthesized in SiO2 by ion implantation

Brillouin scattering and x-ray photoelectron spectroscopy (XPS) have been utilized to characterize Ge+-implanted thermal SiO2 layers on a Si substrate with subsequent annealing at 500°C and 1100°C. Sputtering depth profiling in conjunction with XPS studies have been applied to identify the chemical state of elemental Ge and GeO2 precipitations in the SiO2 matrices. The presence of a subsurface GeOx zone as predicted in kinetic 3-dimensional lattice simulations has been confirmed. It is concluded that the intermediate step of Ge oxide formation seems necessary for the creation of Ge nanoclusters. The Ge atomic concentrations obtained from XPS were used to compute the bulk and shear moduli, and consequently the surface acoustic wave (SAW) velocities, for the Ge∕GeO2∕SiO2 systems. These calculations confirm the character of SAW velocity softening as determined from the Brillouin scattering investigations.

Investigation of Nanostructured Germanium/Silicon Dioxide Interfaces

The precipitation of Ge nanoclusters in SiO2 after implantation arises from their immiscibility and is technologically interesting due to their luminescence and potential use for non-volatile memory devices. Exploiting the same immiscibility, we have recently found that oxidation of germaniumimplanted Si surfaces can produce a pileup of Ge in front of the oxidation front and create an atomically sharp interface. In this paper, we examine the band structure and processing of such an interface. Based on ab initio band structure calculations, the band structure of the sharp interface seems to be more favorable for use in electronic devices than the usually diffuse interface in Si/SiO2. Furthermore, we propose an ab initio based Monte Carlo model to simulate such an oxidation of SiGe alloys. The model explains the formation of the sharp interface due to the repulsive interaction between O and Ge. In such cases, Ge is expelled from the oxide and O atoms do not enter the Ge region under the oxidation front, resulting in strong segregation. Furthermore, inclusion of Ge nanoparticles in the oxide is predicted for higher Ge concentrations, in agreement with experimental results.

Photoluminescence and local structure of Ge nanoclusters on Si without a wetting layer

Photoluminescence (PL) originating from single layers of Ge nanoclusters grown on Si(100) via a buffer layer-assisted growth method is investigated. The nanoclusters are characterized by the absence of a wetting layer. They are quasi-zero-dimensional with tunable sizes and exhibit a high cluster density compared to Ge nanoclusters that are formed from the standard Stranski-Krastanov growth. Samples with different cluster sizes show strong and sharp PL in the near infrared. The excitation power and temperature dependencies of PL spectra indicate that the optical transitions are bound-to-bound in nature, suggesting that defects rather than the band structures are associated with the luminescence centers. High-resolution transmission electron microscopy (TEM) reveals that these nanoclusters are amorphous, which could explain the strong defect-related emission.

Formation of Ge nanoclusters on Si(111)-7×7 surface at high temperature

Abstract We report on Ge nanocluster formation on Si(1 1 1)-7 × 7 surface at elevated substrate temperatures during deposition. The shape and size of the Ge clusters are more uniform than those obtained at room temperature due to an increase in the average mobility of the additional atoms. The Ge clusters have a preferential adsorption site in the faulted halves. Some clusters in the faulted and the unfaulted halves exhibit two different features, which indicate the different adsorption energy and chemical activity of the two half-cells. We also observe some clusters forming in a characteristic star shape at a particular positive bias voltage. The formation mechanism and possible structures are discussed.

Ge dots on Si (1 1 1) and (1 0 0) surfaces with SiO 2 coverage: Raman study

Germanium quantum dots formed on Si (1 1 1) and (1 0 0)-oriented surfaces coated with ultra-thin oxide layers are studied using Raman spectroscopy technique. Some structural properties (height, stoichiometry and mechanical stresses) of the dots were estimated from Raman data. For analysis of the experimental data, the Raman spectra of Ge nanoclusters containing some hundreds of Ge atoms were calculated numerically. The effects of the resonance enhancement of the intensity of Raman scattering in the Ge-nanoclusters–SiO 2–Si system were discussed. The influence of the lateral sizes of Ge nano-clusters on the frequencies of phonons localized in them was studied using numerical simulation. The influence of multi-layer growth on the structure of the Ge quantum dots was investigated.

Ge dots on Si (111) and (100) surfaces with SiO2 coverage: Raman study

Germanium quantum dots formed on Si (1 1 1) and (1 0 0)-oriented surfaces coated with ultra-thin oxide layers are studied using Raman spectroscopy technique. Some structural properties (height, stoichiometry and mechanical stresses) of the dots were estimated from Raman data. For analysis of the experimental data, the Raman spectra of Ge nanoclusters containing some hundreds of Ge atoms were calculated numerically. The effects of the resonance enhancement of the intensity of Raman scattering in the Ge-nanoclusters–SiO 2 –Si system were discussed. The influence of the lateral sizes of Ge nano-clusters on the frequencies of phonons localized in them was studied using numerical simulation. The influence of multi-layer growth on the structure of the Ge quantum dots was investigated.

Structure and electric property comparison between Ge nanoclusters embedded in Al2O3 and Al2O3/ZrO2

Metal-insulator-semiconductor (MIS) structures containing Ge nanocrystals embedded in both Al2O3 and ZrO2/Al2O3 are fabricated by an ultra-high vacuum electron-beam evaporation method. Secondary ion mass spectroscopy (SIMS) results indicate that Ge embedded in Al2O3 diffuses towards the surface of the Al2O3 layer after annealing at 800°C in N2 ambient for 30 min. Ge embedded in ZrO2/Al2O3 is stable, thus inducing less leakage current. Capacitance voltage studies indicate that annealing can effectively passivate the negatively charged trapping centers. Memory effect of the Ge nanoclusters is verified by hysteresis in the C-V curves in the Al2O3/Ge+Al2O3/Al2O3 and ZrO2/Ge+Al2O3/Al2O3 samples.

Elemental composition of nanoclusters formed by pulsed irradiation with low-energy ions during Ge/Si epitaxy

Multilayer silicon structures with built-in layers of Ge nanoclusters were studied experimentally by Raman light scattering. The built-in layers were formed by the pulsed action of a low-energy beam of intrinsic ions during molecular-beam epitaxy. It is found that the ion-stimulated nucleation and the subsequent growth make it possible to obtain Ge nanoclusters almost free of Si.

Scanning capacitance microscopy and -spectroscopy on SiO 2 films with embedded Ge and Si nanoclusters

Scanning capacitance microscopy (SCM) and -spectroscopy (SCS) were applied to study the charge trapping and the charge retention in silicondioxide layers with embedded Ge- or Si-nanoclusters. The nanoclusters were formed by ion beam synthesis. Applying a dc bias to the conductive nano-tip charge injection into the dielectric was achieved. While SCM images visualize the localized trapped charge in the oxide, the locally recorded d C/d V versus V curves allow to quantify the trap density. The evaluation of d C/d V shifts was used to compare the electronic properties of different oxide layers in dependence on the implantation parameters. In addition it is demonstrated, that combined SCM/SCS measurements are a suitable tool for the examination of the charge decay as well as of the reliability and degradation of oxide layers on a nanoscale.

Preparation of alkyl-surface functionalized germanium quantum dots via thermally initiated hydrogermylation.

A new, thermally initiated hydrogermylation-based method for the synthesis and surface functionalization of air- and moisture-stable germanium quantum dots is reported.

STARK SPECTROSCOPY OF Ge/Si(001) SELF-ASSEMBLED QUANTUM DOTS

Stark spectroscopy was employed to study interband optical transitions in an array of Ge / Si self-assembled quantum dots. The mean diameter and height of the Ge nanoclusters are about 6 nm and 4 nm, respectively. Under an applied electric field splitting of the exciton ground state is observed, implying that the dots possess two permanent dipole moments of opposite sign. We argue that the two possible orientations of the electron-hole dipole in each Ge dot are the result of the spatial separation of electrons which can be excited in Si as well as on top and below the Ge nanocluster. The separation of electron and hole is determined to be (5.1±0.2) nm for the top (apex) electron, and (0.8±0.3) nm for the bottom (base) electron, yielding a distance between the electrons of (5.9±0.5) nm, which is consistent with the staggered band line-up inherent to type-II quantum dots.

Field and photo-field electron emission from self-assembled Ge?Si nanostructures with quantum dots

Monolayer and multilayer Ge nanocluster structures were prepared on Si(1 0 0) using molecular beam epitaxy. The cluster size was ⩽10 nm and cluster density was ∼1010 cm−2. A stable field electron emission was obtained from these structures, showing current peaks in the current–voltage characteristics, which may be attributed to the resonant electron tunneling via the energy levels of the nanocluster potential well. For cluster multilayers, the current–voltage curves also showed current peaks with a complex shape. The cluster multilayer structures had a considerable temperature sensitivity, as well as photosensitivity, in the wavelength range from 0.4 to 10 μm.

Pulsed-laser modification of germanium nanoclusters in silicon

The effect of pulsed ruby laser radiation on Ge nanoclusters grown on a (100)-oriented Si substrate is studied. The energy density of radiation corresponds to the melting threshold of the Si surface. Changes in the structure of nanoclusters are analyzed by comparing the experimental Raman spectra to those calculated in terms of Born-von Karman and Vol’kenstein models. It is established that the action of one pulse changes the cluster size and partly relieves the compression. Still greater changes take place in a sample subjected to ten pulses. The Ge nanoclusters transform into clusters of GexSi1−x solid solution, presumably due to the stress-and vacancy-aided diffusion. Laser-induced thermal processes in germanium nanoclusters in silicon are numerically simulated.

Cluster coarsening and luminescence emission intensity of Ge nanoclusters in SiO2 layers

SiO 2 layers 180 nm thick are implanted with 120 keV Ge+ ions at a fluence of 1.2×1016 cm−2. The distribution and coarsening evolution of Ge nanoclusters are characterized by Rutherford backscattering spectrometry and transmission electron microscopy and the results are correlated with photoluminescence measurements as a function of the annealing temperatures in the 400 °C⩽T⩽900 °C range. At 400 °C we observe a monomodal array of clusters characterized by a mean diameter 〈φ〉=2.2 nm which increases to 〈φ〉=5.6 nm at 900 °C. This coarsening evolution occurs concomitantly with a small change of the total cluster–matrix interface area and an increase of the Ge content trapped in observable nanoclusters. However, at 900 °C a significant fraction of up to about 20% of the Ge content still remains distributed in the matrix around the nanoparticles. The results are discussed in terms of possible atomic mechanisms involved in the coarsening behavior that lead to the formation of the oxygen deficiency luminescence centers.

Fabrication of Ge nanoclusters on Si with a buffer layer-assisted growth method

Size selectable Ge nanoclusters are formed on Si using a buffer layer-assisted growth method. A condensed inert gas layer of xenon, with low surface free energy, was used as a buffer to prevent direct interactions of deposited Ge atoms with Si substrates during Ge nanocluster growth. The scanning tunneling microscope studies indicate absence of a strained wetting layer between Ge nanoclusters. These nanoclusters are substantially smaller and denser than the Ge hut clusters that are formed with the normal Stranski–Krastanov growth mode. The morphology of the nanoclusters can be tuned over a wide range, which is very desirable for studying the three-dimensional confinement effect.

Field and photo-field electron emission from self-assembled Ge–Si nanostructures with quantum dots

Monolayer and multilayer Ge nanocluster structures were prepared on Si(1 0 0) using molecular beam epitaxy. The cluster size was ⩽10 nm and cluster density was ∼10 10 cm −2. A stable field electron emission was obtained from these structures, showing current peaks in the current–voltage characteristics, which may be attributed to the resonant electron tunneling via the energy levels of the nanocluster potential well. For cluster multilayers, the current–voltage curves also showed current peaks with a complex shape. The cluster multilayer structures had a considerable temperature sensitivity, as well as photosensitivity, in the wavelength range from 0.4 to 10 μm.

Structural and electrical characteristics of Ge nanoclusters embedded in Al2O3 gate dielectric

Structural and electrical characteristics of the metal–insulator–semiconductor (MIS) structures of Al/Al2O3/Si containing Ge nanoclusters are experimentally demonstrated. Secondary ion mass spectroscopy results indicate the out-diffusion of Ge after annealing at 800 °C in N2 ambient for 30 min. An increment of leakage current is observed due to the out-diffusion of Ge. Capacitance–voltage studies indicate that annealing can effectively passivate the negatively charged trapping centers. Memory effect of the Ge nanoclusters is verified by the hysteresis in the C–V curves in the annealed sample.

Linear and third-order nonlinear optical absorption of amorphous Ge nanoclusters embedded in Al2O3 matrix synthesized by electron-beam coevaporation

Amorphous germanium (α-Ge) nanoclusters embedded in Al2O3 matrix are synthesized on fused-quartz glass substrate at room temperature by vacuum electron-beam coevaporation. Linear optical transmittance measurements reveal an indirect optical absorption edge with a blueshift as large as 1.6 eV due to the quantum confinement effects. The Z-scan method is used to study the third-order nonlinear absorption process of the synthesized samples, which clearly show a two-photon nonlinear process. Results also suggest that Ge nanocluster density or size is of great influence to the value of nonlinear absorption coefficient β.