Germanosilicate Films Research Articles

Nonstoichiometric Germanosilicate Films on Silicon for Microelectronics: Memristors and Other Applications

Nonstoichiometric Germanosilicate Films on Silicon for Microelectronics: Memristors and Other Applications

Проявление квантоворазмерных эффектов в нанокристаллах и аморфных нанокластерах германия в плeнках GeSixOy

The non-stoichiometric germanosilicate films of two types GeOx[SiO](1-x) and GeOx [SiO2](1-x) were obtained by high-vacuum evaporation of GeO2 and either SiO or SiO2 powders and sputtering on a cold Si (001) substrate. The as-deposited and annealed (550 and 650 oC, 1 hour) samples were studied by IR spectrocopy, electron microscopy, Raman spectroscopy and photoluminescence (PL). From an analysis of Raman spectra, it was found that the as-deposited GeO[SiO2] film did not contain germanium clusters, and the as-deposited GeO[SiO] film contained amorphous germanium clusters; according to electron microscopy, it’s size was ~3 nm. According to IR spectroscopy, the films contained Si-O, Ge-O, and Si-O-Ge bonds. After annealing at 550 °C, amorphous germanium clusters were detected in both films, and after annealing at 650 °C, germanium nanocrystals were observed. A wide PL band with a maximum of 1050 nm was found in the as-deposited films at low temperatures. PL is probably due to defects, presumably oxygen vacancies and excess germanium atoms. Annealing causes a transformation of both film structure and PL spectra. In films containing germanium nanoclusters, PL is observed with a maximum of 1500–1600 nm. In this case, the PL signal from defects decreased. The temperature dependence of the intensity of the PL peaks was studied; it decreased with increasing temperature but remained at temperatures up to 200 K. The contribution to the PL from germanium nanocrystals formed during annealing is discussed.

Luminescence of unfused 95%SiO2–5%GeO2 amorphous films with fluorine additive: No evidence for presence of GeODC(I) defects found

Photoluminescence (PL) of unfused amorphous germanosilicate films with fluorine additive is studied in 2–8.5eV spectral range. Experiments are based on films deposited on silica substrates by means of the surface-plasma chemical vapor deposition (SPCVD). Films of about 100μm in thickness with “high F” (~4.2wt.%) and “low F” (~0.5wt.%) fluorine content have been fabricated for the experiments. KrF (248nm), ArF (193nm) and F2 (157nm) excimer lasers are used to pump PL. It is found that absorption and luminescence associated with germanium oxygen deficient centers (GeODCs) in “high F” and “low F” films differ. In the “high F” unfused film absorption coefficient of the band at 5eV as well as intensity of the blue PL band at 3.1eV are significantly greater. This film proves features of the so called GeODCs(II), which symbolize twofold coordinated germanium defects in silica network. In the “low F” unfused film absorption band at 5eV is feebly marked. Poorly resolved PL intrinsic to GeODCs(II) can be detected in this film under the KrF laser pump. The most significant PL features are revealed under deeper UV pump by ArF and F2 lasers. Spectral positions of PL bands excited by these lasers correspond to GeODCs(II). However PL decay kinetics dramatically differs from that one intrinsic to GeODCs(II). Noticeable growth of PL intensity caused by permanent (half an hour and more) exposure to ArF and/or F2 lasers takes place, indicating GeODC(II) formation. It is found that considerable body of fluorine additive has the same effect as profusion for “low F” and/or fluorine free germanosilicate amorphous material synthesized by SPCVD. In the “high F” film yield of GeODC(II) PL pumped by the F2 laser remains high. This speaks for the suppression of the competitive 7.6eV absorption band associated with SiODCs(I) by fluorine additive indicating a decrease in the content of this type of defects in the material. High yield of GeODC(II) luminescence pumped by deep UV photons as well as hypothetical similarity of SiODC(I) and GeODC(I) permit one to conclude that GeODCs(II) are the only defects dominating in the materials under study.

Optical and physical properties of solgel-derived GeO_2:SiO_2 films in photonic applications

The functionality of optical components relies heavily on the composition-dependent properties of germanosilicate materials, which include the refractive index, photosensitivity, and microstructural properties. Recent studies and parallel developments are presented of germanosilicate films with composition x of Ge content (i.e., xGeO(2):(1-x)SiO(2)) that were synthesized by the solgel process for various integrated photonic applications undertaken. The following novel aspects are discussed with respect to the effect of composition of the glassy films (0.05</=x</=0.40): determination of spectral optical properties, UV imprinting of optical waveguides with relatively large index change (Dn), and quantum-well intermixing enhancement observed in InGaAs(P)/InP quantum-well optical devices. The implications of the results are discussed.

Fabrication of sol–gel 70GeO2–30SiO2 thick films from TEOG and DEOS and investigation of the 5 eV band

Crack-free germanosilicate films, 3.6 µm-thick and containing up to 70 mol% germanium dioxide, which are inaccessible through the conventional sol–gel process, were fabricated using tetraethyl orthogermanate (TEOG) and diethylorthosilicate (DEOS) as precursors for germania and silica, respectively. The studies using viscosity, TEM and SEM revealed that DEOS contributed largely to stabilizing the GeO2–SiO2 sol and suppressing crack formation in thick films. XRD study showed that the films remained amorphous after being sintered at 600 °C in air for 60 min and annealed at 550 °C under a flowing H2/N2 atmosphere for 120 min. An intense absorption band at around 241 nm was distinctly observed in the films. The intensity of this absorption band was found to be effectively bleached by UV illumination. Weak photoluminescence emission bands which originated from the neutral oxygen di-vacancy were detected near 375 and 275 nm. Therefore, the 5 eV absorption band observed in this work was mainly caused by the neutral oxygen monovacancy. A saturated absorptivity change of the UV-bleachable band after prolonged illumination was found to be 389 cm−1 for 70GeO2–30SiO2 films.

Enhanced photosensitivity in sol–gel derived 20GeO2:80SiO2 thin films

We investigate the photosensitivity of binary 20GeO2:80SiO2 (germanosilicate) inorganic films. The samples have been fabricated by the sol–gel spin-coating method and the densification has been performed by rapid thermal annealing at various temperatures ranging from 500 °C to 1000 °C. The –OH absorption bands in the Fourier-transform infrared (FTIR) spectra and the refractive-index data show that the films annealed below 900 °C are porous and the films annealed at 900 °C and above are dense. An ultraviolet (UV) KrF laser at 248 nm has been used to induce the change in the refractive index of the samples. We have achieved a large refractive-index change (Δn) of 0.0098 after UV illumination in excess of 1 min for our dense germanosilicate films. This UV-induced refractive-index change is attributed to the formation of GeE’/SiE’ centers from Ge–Ge/Si–Si (neutral oxygen monovacancy) and Ge2+ centers and to the creation of oxygen deficiency related defects. From our experiments, the oxygen deficiency related defects correspond to the absorption band at 620–740 cm-1 in the FTIR spectra and these are the defects which make a large contribution to Δn. The attenuation coefficient of the as-deposited and UV-illuminated dense samples is about 0.42 dB/cm at 1550 nm. For porous samples, UV exposure has densified the samples to some extent.

Characterization of thermally poled germanosilicate thin films

We report measurements of the nonlinearity profile of thermally poled low-loss germanosilicate films deposited on fused-silica substrates by PECVD, of interest as potential electro-optic devices. The profiles of films grown and poled under various conditions all exhibit a sharp peak ~0.5 microm beneath the anode surface, followed by a weaker pedestal of approximately constant amplitude down to a depth of 13-16 microm, without the sign reversal typical of poled undoped fused silica. These features suggest that during poling, the films significantly slow down the injection of positive ions into the structure. After local optimization, we demonstrate a record peak nonlinear coefficient of ~1.6 pm/V, approximately twice as strong as the highest reliable value reported in thermally poled fused silica glass, a significant improvement that was qualitatively expected from the presence of Ge.

Piezoelectricity in Poled Ge-Doped Silica Thin Films

This paper presents piezoelectricity in germanium-doped silica (germanosilicate) films produced by poling treatment. The germanosilicate films were prepared on Si substrates by RF magnetron sputtering. The films were poled by electric fields of 2–4 ×107 V/m at a temperature above 300°C. Before the poling, no piezoelectric response was observed. After the poling, a 0piezoelectric response caused by normal stress T33 on the film surface appeared. The maximum value of the piezoelectric constant d33 of the poled film was larger than d11 of quartz by 20–30%. Various applications of the piezoelectric Ge:SiO2 film are expected to emerge.

Channel waveguides formed by ion implantation of20% Ge-doped silica

The implantation of 20% Ge-doped silica with MeV Si or Ge ions has been used to produce singlemode channel waveguides. The germanosilicate film was grown by plasma enhanced chemical vapour deposition. For implantation with either Si or Ge ions, the attenuation loss was measured as 0.15–0.25 dB/cm at 1300 nm and 1.5–1.8 dB/cm at 1550 nm.

Direct UV writing of buried singlemode channelwaveguides in Ge-doped silica films

Germanosilicate film waveguides fabricated by plasma enhanced chemical vapour deposition have been shown to possess high sensitivity towards UV induced index changes. As an illustration of possible future applications, direct point-to-point writing of buried singlemode channel waveguides using a focused, continuous wave, UV laser beam is demonstrated.