U. Rossow

Depth inhomogeneity of porous silicon layers

The two possible causes of depth inhomogeneities of the microstructure of porous silicon are changes in the HF concentration with depth and a varying chemical etching rate of the porous silicon layer. During anodization chemical etching will become important for microporous silicon — e.g. p‐porous silicon — due to the large internal surface area, especially at long etching times. On the other hand, a considerable decrease of the HF concentration will occur during etching with high current densities to produce p+‐porous silicon with high porosities. We have investigated the depth inhomogeneity of porous silicon layers by spectroscopic ellipsometry, Raman spectroscopy and photoluminescence measurements. From a line shape analysis of the Raman signal a size distribution of nanocrystals is deduced. For p‐porous silicon smaller nanocrystals are found near the surface of the layer; for p+‐porous silicon etched with high current densities smaller nanocrystals are found near the porous silicon/substrate interface.

Thermal desorption of amorphous arsenic caps from GaAs(100) monitored by reflection anisotropy spectroscopy

Abstract Arsenic layers were deposited on top of molecular beam epitaxy (MBE) grown GaAs(100) layers by cooling the samples after growth in an As4 flux down to temperatures of about - 5°C. Raman scattering has shown that these arsenic layers are adsorbed in on amorphous structure (Surf. Sci. 269/270 (1992) 797). Consequently, the As overlayers, the so called As-“caps”, show no anisotropy in reflectivity, which means that no reflection anisotropy spectroscopy (RAS) signal is detected from the caps. After storage in air for several weeks desorption experiments were performed in ultra-high vacuum (UHV) in order to recover the GaAs(100) surface. The samples were heated stepwise to temperatures of at least 545°C. After desorption of most of the As layer a first RAS signal can be detected. From this stage on the RAS spectra undergo strong variations with increasing annealing temperature. This is correlated with a decreasing As content of the GaAs surfaces, as revealed by means of Auger electron spectroscopy (AES) and also with LEED patterns of different symmetries. The RAS spectra, however, turn out to be much more sensitive than the observable changes in the symmetry of the LEED pattern. RAS is therefore suitable to monitor not only the decapping procedure but, once calibrated, also allows for a fine tuning of the As/Ga ratio at the surface.

Heteroepitaxy of GaP on Si(100)

In this article, we analyze the kinetics of heteroepitaxial growth of GaP on Si(100) by pulsed chemical beam epitaxy on the basis of results obtained by real‐time optical process monitoring. In view of the large barrier to epitaxial growth on oxygen or carbon contaminated silicon surface elements and the low stacking fault energy for GaP, residual contamination of the silicon surface contributes to defect formation in the initial phase of GaP heteroepitaxy on Si, and requires special measures, such as surface structuring, to limit the propagation of defects into the epitaxial film. The control of the supersaturation during the first 10–20 s of film formation is essential for the quality of subsequent epitaxial growth and is limited to a narrow process window between three‐dimensional nucleation and overgrowth at low Ga supersaturation and gallium‐cluster formation at high Ga supersaturation. Steady state heteroepitaxial growth is described by a four‐layer stack substrate/epilayer/surface reaction layer (S...

Investigation of different oxidation processes for porous silicon studied by spectroscopic ellipsometry

Abstract In this paper we investigate the oxidation of porous silicon by O3, H2O2, and, for comparison, in normal air. Such an oxidation may serve as passivation for porous silicon in applications in order to prevent devices from degradation. The changes in the dielectric function caused by this oxidation was monitored by spectroscopic ellipsometry. Application of both H2O2 and O3 resulted in a significant lowering of the values of the imaginary part of the dielectric function as expected when oxidizing the inner surfaces of these layers. For a multilayer structure we show that ozone treatment of this structure indeed passivates that sample against further oxidation in air as studied over an extended period of time (3 months).

Surface and interface effects on ellipsometric spectra of crystalline Si

We present the first systematic investigation of the differences among reference-quality ellipsometrically measured pseudodielectric function 〈e〉 spectra of crystalline Si, which are nominally used to approximate the bulk dielectric function of this material. In addition to the expected influence of residual overlayers, we identify surface-local-field and energy-derivative effects, the latter representing shifts between bulk and measured critical point energies, as well as changes in excited-carrier lifetimes due to the surface. Model calculations indicate that these four effects account for nearly all differences among spectra studied, although a second-energy-derivative component appears at the E1 transition in some cases. The isotropic contribution to the surface-local-field effect is observed for the first time.

Evidence of near-surface localization of excited electronic states in crystalline Si

Surface- and interface-related spectra, obtained either directly by techniques such as reflectance-difference (-anisotropy) spectroscopy or indirectly by subtracting pseudodielectric function spectra obtained ellipsometrically on surfaces with different chemical termination, exhibit features related to energy derivatives of the bulk dielectric function. We argue that these spectra provide direct evidence that the excitations involved are localized both in space and time. These data unequivocally indicate that critical point energies obtained from above-band-gap ellipsometric or reflectrometric optical spectra are not necessarily equal to bulk values, and that surface chemical and structural termination is at least one contributing factor. Present surface-optical calculations do not include these effects, which may explain, in part, remaining discrepancies between theory and experiment.

Molecular layer epitaxy by real-time optical process monitoring

In this paper we consider modern methods of optical process monitoring and control in the context of atomic layer epitaxy. One specific method, p-polarized reflectance spectroscopy (PRS), is chosen to assess details of layer-by-layer growth. We show that PRS monitoring under conditions of steady-state growth by pulsed chemical beam epitaxy (PCBE) can achieve the deposition of molecular layers of GaP on silicon (100) deposited with a precision of 5%, which can be improved by reducing the growth rate and increasing the period of time averaging of the reflectance data. Since in the nucleation period prior to formation of a contiguous heteroepitaxial film inhomogeneous surface chemistry and roughening complicates the modeling of the overgrowth process, advances in both experimental methods and theory are required for extending the control to non-steady-state growth conditions. Results of simultaneous single-wavelength PR monitoring and laser light scattering measurements in conjunction with atomic force microscopy studies of short period heteroepitaxial overgrowth processes are presented. The extension of PRS to the monitoring of organometallic chemical vapor deposition at higher pressures is also discussed.

Interpretation of surface‐induced optical anisotropy of clean, hydrogenated, and oxidized vicinal silicon surfaces investigated by reflectance‐difference spectroscopy

Using reflectance‐difference spectroscopy, we determine surface‐induced optical anisotropy (SIOA) spectra of clean, hydrogenated, and oxidized (113) and vicinal (001) Si surfaces to obtain a better understanding of the origin of the optical response of surfaces and interfaces. Hydrogenation was performed either by etching in dilute HF or by exposing clean surfaces to atomic hydrogen. Hydrogenated and oxidized vicinal surfaces show energy‐derivativelike spectra that roughly scale with offcut angle, indicating step‐induced behavior, and exhibiting features near 3.4 and 4.2 eV, the threshold energies of the (E1, E0′) and E2 interband critical points of bulk Si, respectively. The appearance of derivativelike line shapes indicates that bulk threshold energies become dichroic near the surface due to the surface‐induced modification of the potential, as supported by model calculations. However, direct integration yields dielectric functions somewhat different from bulk values, indicating that the surface affects...

Influence of the formation conditions on the microstructure of porous silicon layers studied by spectroscopic ellipsometry

Abstract The influence of various parameters of the electrochemical process on the resulting microstructure of porous silicon layers was studied by spectroscopic ellipsometry. The first parameter, the etching time, is known to determine the layer thickness. In addition, although the current density is kept constant, the microstructure of the layers changes, at least in the top part probed by the light, with increasing etching time. A change in the microstructure was also found upon leaving the samples in the electrolyte for a certain time interval after the end of the electrochemical process. The main effect in both situations seems to be an increase in porosity of the layers. For long etching times or applied illumination with short wavelength light during the etching process the spectra indicate that the diameters of the crystals comprising the silicon skeleton are reduced.

The influence of nanocrystals on the dielectric function of porous silicon

Abstract Porous silicon films consist of a complicated network of crystals with diameters down to the nanometer range. In the smallest crystals phonons and electrons are localized. From the Raman spectra the distributions of the nanocrystal diameters are estimated. The influence of the microscopic structure on the dielectric function is studied by spectroscopic ellipsometry. The observed reduction in the height of the imaginary part of the dielectric function is related to the porosity of the layers and to the state of oxidation. A broadening and slightly changed energies of the Van Hove singularities in the joint density of states (optical gaps) are found for the porous Si layers. By performing simulations of the dielectric function for porous Si, more information about the topology of the layers is obtained.