Imp Igor Aarts

Direct and highly sensitive measurement of defect-related absorption in amorphous silicon thin films by cavity ringdown spectroscopy

Cavity ringdown spectroscopy has been applied to hydrogenated amorphous silicon (a-Si:H) showing that this fully optical method is suited for the detection of defect-related absorption in thin films with a minimal detectable absorption of 1×10−6 per laser pulse and without the need for a calibration procedure. Absolute absorption coefficient spectra for photon energies between 0.7 and 1.7 eV have been obtained for thin a-Si:H films (4–98 nm) revealing a different spectral dependence for defects located in the bulk and in the surface/interface region of a-Si:H.

Optical second-harmonic generation in thin film systems

The surface and interface sensitive nonlinear optical technique of second-harmonic generation (SHG) is a very useful diagnostic in studying surface and interface properties in thin film systems and can provide relevant information during thin film processing. An important aspect when applying SHG is the interpretation of the SHG response. In order to utilize the full potential of SHG during materials processing it is necessary to have a good understanding of both the macroscopic and the microscopic origin of the SHG response, particularly in thin film or multilayer systems where the propagation of radiation is another important aspect that should be considered carefully. A brief theoretical overview on the origin of the SHG response and a description of the propagation of radiation will be given. Furthermore, several methods will be discussed that might reveal the possible macroscopic and microscopic origins of the SHG response in thin film systems. The different approaches will be illustrated by examples...

Spectroscopic second harmonic generation measured on plasma-deposited hydrogenated amorphous silicon thin films

Optical second harmonic generation (SHG) has been measured for plasma-deposited thin films of hydrogenated amorphous silicon (a‐Si:H) at different polarization states for pump photon energies between 1.0 and 1.7eV. Distinct resonance peaks are observed in this energy range and it is shown that the SH signal originates from an isotropic contribution at both the film-surface and substrate-interface region. The possibility that the SH signal originates from surface and interface dangling bond states of a‐Si:H is discussed.

Absence of the enhanced intra-4f transition cross section at 1.5μm of Er3+ in Si-rich SiO2

We present measurements of the optical absorption cross section of the I15∕24→I13∕24 transition at 1.5μm of Er3+ ions embedded in SiO2 and Si-rich oxide, using cavity ringdown spectroscopy on thin films. The peak absorption cross section for Er3+ embedded in Si-rich oxide (10at.% excess Si) was found to be (8±2)×10−21cm2 at 1536nm, similar to typical values for Er embedded in SiO2. The data imply that the silicon nanoclusters incorporated in Si-rich oxide do not enhance the peak cross section of the Er3+ I15∕24−I13∕24 transition by 1-2 orders of magnitude, contrary to what has been reported in earlier work.

Absolute in situ measurement of surface dangling bonds during a-Si:H growth

Hydrogenated amorphous silicon (a-Si:H) is regarded as a model system in mechanistic studies of amorphous semiconductor film growth, where the key reactive site is generally considered to be a surface “dangling bond.” Employing an ultrahigh-Q (≈1010) monolithic optical resonator, the authors probe the creation of dangling-bond defects during growth of a-Si:H from a predominantly SiHx (x=0–3) radical flux by detecting the associated near-IR subgap absorption with evanescent wave cavity ringdown spectroscopy. They find the apparent dangling-bond creation rate [(5±3)×1012cm−2s−1] and steady-state surface density [(5±2)×1011cm−2] to be considerably lower than expected from dangling-bond-based growth mechanisms.

Circular dichroism in second harmonic generation from oxidized Si (001)

Circular dichroism in second harmonic generation (SHG) is often related to molecules and materials with chiral structures. In this letter, we report circular dichroism in SHG from anisotropic achiral oxidized Si (001) at room temperature. The dichroism value depends on the azimuthal angles of the crystal axes. Due to the simple nonlinear susceptibility elements involved, we were able to attribute the dichroism in SHG to interference between particular terms of the bulk electric quadrupole and surface dipole contributions. The presence of a phase shift between the bulk and surface SHG is required to observe circular dichroism.

In Situ Defect Spectroscopy: Probing Dangling Bonds During a-Si: H Film Growth by Subgap Absorption

This paper describes a novel optical diagnostic that was recently introduced to the field of Si-based thin films for probing defect states such as dangling bonds in a-Si:H. This diagnostic is based on the cavity ringdown spectroscopy (CRDS) technique. When applied in situ or real time during film growth it can provide new insights into the a-Si:H film properties as well as into the fundamental surface processes during growth. In this paper the CRDS technique is described and both the ex situ application on as-deposited a-Si:H films as well as the in situ application during a-Si:H film growth are presented

Novel in situ and real-time optical probes to detect (surface) defect states of a-Si:H

This paper describes two novel optical diagnostics that were recently introduced to the field of Si-based thin films, in particular for probing defect states present in the bulk and at the surface of a-Si:H films. It is expected that these diagnostics, when applied in situ or real time during film growth, can provide new insights into the a-Si:H film properties as well as into the fundamental surface processes during growth. The first method is cavity ringdown spectroscopy (CRDS). From ex situ measurements on a-Si:H thin films, it is shown that this method is very powerful for measuring absolute defect-related absorptions at subgap energies without the need for a calibration procedure, even for films as thin as 4 nm. It is also shown that the method can be used for measuring rare-earth dopants – here Er 3+ in silicon-rich oxide – to the extent that issues about absorption cross-sections can be resolved by using thin samples instead of waveguides. Furthermore, the in situ application of the method for thin films is discussed by presenting the evanescent-wave cavity ringdown (EW-CRDS) technique. The second method is spectroscopic second harmonic generation (SHG). It has been found that this non-linear optical technique yields a photon energy dependent signal for as-deposited a-Si:H films and that this signal has a contribution from a-Si:H surface states. From a comparison with c-Si surface science studies, the possible origin of the signal from surface Si dangling bonds and strained Si-Si bonds is discussed. The application of SHG during real-time film growth is also presented.

Photonic Crystal Properties Of Sion (N=1.56)

We report for the first time the use of the relatively low index material SiON (n=1.56) for photonic crystal applications. With this system it is possible to enlarge the complete TE-band gap of a 2D-photonic crystal by properly designing the unit cell filling. This may be practically realised because of the larger dimensions of photonic crystals in low index materials, which allows less stringent etch conditions when compared to traditional InP based materials. The optimal distribution of the refractive index is found by placing the displacement field in high or in low dielectric material. The structure is defined by two figures of merit, the width of the band gap and the smallest feature size, which is the key factor in etching the structure. In addition, we suggest the use of a graded-index profile to reduce out of plane scattering. This graded index profile can be realised by controlling the nitrogen flow during deposition of the SiON layer.