Jjh Joost Gielis

On the c-Si surface passivation mechanism by the negative-charge-dielectric Al2O3

Al2O3 is a versatile high-κ dielectric that has excellent surface passivation properties on crystalline Si (c-Si), which are of vital importance for devices such as light emitting diodes and high-efficiency solar cells. We demonstrate both experimentally and by simulations that the surface passivation can be related to a satisfactory low interface defect density in combination with a strong field-effect passivation induced by a negative fixed charge density Qf of up to 1013 cm−2 present in the Al2O3 film at the interface with the underlying Si substrate. The negative polarity of Qf in Al2O3 is especially beneficial for the passivation of p-type c-Si as the bulk minority carriers are shielded from the c-Si surface. As the level of field-effect passivation is shown to scale with Qf2, the high Qf in Al2O3 tolerates a higher interface defect density on c-Si compared to alternative surface passivation schemes.

Negative charge and charging dynamics in Al2O3 films on Si characterized by second-harmonic generation

Thin films of Al2O3 synthesized by atomic layer deposition provide an excellent level of interface passivation of crystalline silicon (c-Si) after a postdeposition anneal. The Al2O3 passivation mechanism has been elucidated by contactless characterization of c-Si/Al2O3 interfaces by optical second-harmonic generation (SHG). SHG has revealed a negative fixed charge density in as-deposited Al2O3 on the order of 1011 cm−2 that increased to 1012–1013 cm−2 upon anneal, causing effective field-effect passivation. In addition, multiple photon induced charge trapping dynamics suggest a reduction in recombination channels after anneal and indicate a c-Si/Al2O3 conduction band offset of 2.02±0.04 eV.

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...

The nucleation behavior of supercooled water vapor in helium

The nucleation behavior of supersaturated water vapor in helium is experimentally investigated in the temperature range of 200–240 K. The experiments are performed using a pulse expansion wave tube. The experimental results show a sharp transition in the nucleation rates at 207 K. We suggest that the transition is due to the transition of vapor/liquid to vapor/solid nucleation (ordered with decreasing temperature). A qualitative theoretical explanation is given based on the classical nucleation theory and the surface energy of ice.

a-Si:H∕c-Si heterointerface formation and epitaxial growth studied by real time optical probes

The deposition of amorphous and epitaxial silicon thin films on H-terminated Si(100) has been studied in real time by the simultaneous application of spectroscopic ellipsometry, attenuated total reflection infrared spectroscopy, and optical second-harmonic generation. The morphology development of the films could be monitored nonintrusively in terms of critical point resonances and H bonding resolving the abruptness of the film-substrate interface and providing a clear distinction between direct heterointerface formation, nanometer-level epitaxial growth, and epitaxial breakdown.

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.

Electric field induced surface passivation of Si by atomic layer deposited Al 2 O 3 studied by optical second-harmonic generation

Recently, we have demonstrated that ultrathin (≪30 nm) films of Al<inf>2</inf>O<inf>3</inf> synthesized by (plasma-assisted) atomic layer deposition (ALD) provide an excellent level of surface passivation of c-Si which may find important applications in (high-efficiency) solar cells. In this contribution, the Al<inf>2</inf>O<inf>3</inf> passivation mechanism has been further elucidated by the contactless characterization of the c-Si/Al<inf>2</inf>O<inf>3</inf> interface by optical second-harmonic generation (SHG). SHG has revealed effective field-effect passivation of the c-Si surface caused by a negative fixed charge density of 5×10¹² cm⁻² in an annealed, 11 nm thick Al<inf>2</inf>O<inf>3</inf> film while it is on the order of 10¹¹ cm^™2 in the as-deposited film which shows negligible passivation. A comparison with SHG measurements on a 84 nm thick a-SiN<inf>x</inf>:H film treated in a conventional firing furnace has revealed the presence of a positive fixed charge density of 2×10¹² cm⁻² which further corroborates the SHG analysis and results.

Amorphization of Si(100) by Ar+-ion bombardment studied with spectroscopic and time-resolved second-harmonic generation

The surface and interface sensitive technique of optical second-harmonic generation (SHG) has been applied spectroscopically and time-resolved before, during, and after low energy (70–1000 eV) Ar+-ion bombardment of H-terminated Si(100). The photon energy range of the fundamental radiation was ℏω=0.76–1.14 eV. Besides physical sputtering of the silicon, ion bombardment of crystalline silicon damages and amorphizes the top layer of the sample and thereby creates a layered structure of amorphous silicon (a-Si) on crystalline silicon. The SHG radiation, which is sensitive to the Ar+-ion flux, ion energy, and the presence of reactive gas species, originates from the top surface of the sample and from the interface between a-Si and c-Si. From a comparison with the SHG results obtained at a fundamental radiation of ℏω=1.3–1.7 eV, it is concluded that the SHG radiation during and after creation of this structure dominantly originates from the tails of electronic transitions in the E0′/E1 energy region rather tha...

A Real-Time Study of the a-Si:H/c-Si Interface Formation using Ellipsometry, Infrared Spectroscopy, and Second Harmonic Generation

The formation of the c-Si/a-Si:H interface has been studied in real time during the deposition of a-Si:H on H-terminated c-Si using hot-wire CVD. Several optical diagnostics were used simultaneously: spectroscopic ellipsometry revealing information about the film morphology during the initial stage of growth; attenuated total reflection infrared spectroscopy yielding the H-bonding configuration and depth profile; and optical second harmonic generation providing insight into c-Si and a-Si:H interface and surface states. In this paper these techniques are introduced and initial results 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.