D.J. Robbins

Evolution of surface morphology and strain during SiGe epitaxy

Abstract We have studied the morphology of x = 0.20−0.26 Si1−xGex alloy layers during vapour phase epitaxial (VPE) growth at 610–750 °C using in situ laser light scattering (LLS) and ex situ atomic force microscopy (AFM). A rippled surface morphology is a clearly defined intermediate form in the roughness evolution, preceding misfit dislocation generation. Transmission electron microscopy (TEM) shows that microscopic strain fluctuations are associated with the thickness variations. Using a simple one-dimensional sinusoidal model for the form of the strain variations, we demonstrate that a flat uniformly strained layer can be energetically unstable with respect to this rippled topography when the ripple period exceeds a critical value. Data from the literature concerning roughness on pseudomorphically strained InGaAs/GaAs, SiGe/Si and Ge/Si epitaxial layers shows an inverse trend between the lateral roughness scale and the mismatch strain, which is consistent with the model. An undulating topography therefore provides a means of partial elastic strain relaxation, which is general for strained-layer epitaxy. Finally, we discuss the role of surface diffusion in enabling the changes in Si1−xGex surface morphology observed by AFM.

Optical properties of porous silicon films

Abstract The optical constants of porous silicon layers formed in wafers with resistivities of 0.01–25 Omega cm and densities of 20%–75% were determined by ellipsometry in the wavelength range 0.28–0.7 microm and by reflectance and transmission from 0.6 to 2.5 microm. The IR refractive index was found to decrease as the density was reduced and an effective medium model was used to obtain density values from the optical data. Good agreement was obtained with gravimetric densities for 0.01 Omega cm samples on the assumption of a mixture of crystalline silicon and air, but material of higher resistivity showed apparently reduced optical densities because of the presence of some oxidized silicon. The optical measurements indicate that the material is a chemical mixture (SiO x ) at high oxygen concentrations. e 1 and e 2 spectra showed that 0.01 Omega cm samples had better crystallinity than higher resistivity samples and this was confirmed by the observation of amorphous-like bands in photoluminescence spectra of the latter material.

Spectroscopic ellipsometry characterization of strained and relaxed Si1-xGex epitaxial layers

Spectroscopic ellipsometry has been used to study thick, relaxed and thin, strained epilayers of Si1−xGex on Si in the range 0.1<x<0.25. Dielectric functions of relaxed Si0.87Ge0.13 and Si0.8Ge0.2 have been obtained and long‐wavelength absorption coefficient values, required for interference fringe fitting, shown to be higher than measured previously. The dielectric function of strained Si0.78Ge0.22 has been measured for the first time and the effects of strain on the critical points shown to be consistent with deformation potential theory. An interpolation procedure has been developed for the fitting of layer composition and thickness, and excellent agreement with conventional techniques obtained for a series of uncapped single epilayers. The surface roughness of Si1−xGex epilayers has been studied as a function of time and deposition temperature and shown to play an important role in the modeling. The application of the technique to the characterization of buried strained layers is discussed.

Spectroscopic ellipsometry of Si1−xGex epilayers of arbitrary composition 0≤x≤0.255

Critical point (CP) transition energies have been calculated for strained Si1−xGex (0≤x≤0.255) between 2.5 and 3.5 eV from Lorentzian fits to the second differential of reference dielectric function spectra. E1 and E’0 transition energies are similar to those of the relaxed alloy. Comparison with deformation potential theory shows E1+Δ1 to be coincident with E’0 due to a strain‐induced up shift in the former’s transition energy. The reference spectra and CP transition energies are used in an interpolation procedure to analyze spectroscopic ellipsometry spectra of both uncapped and buried layers of strained Si1−xGex. Compositions and thicknesses are obtained in good agreement with alternative techniques.

In-situ dual-wavelength and ex-situ spectroscopic ellipsometry studies of strained SiGe epitaxial layers and multi-quantum well structures

In-situ dual-wavelength ellipsometry and ex-situ spectroscopic ellipsometry have been used to study strained Si 1-x Ge x /Si multilayers. Reference dielectric function spectra of strained Si 1-x Ge x with 0.06<x<0.29 have been obtained for the first time and an interpolation procedure based on the strain dependence of the dominant critical point energies developed. Good agreement with composition and thickness values from corroborative techniques was obtained when the effects of strain were taken into account

Spectroscopic ellipsometry of epitaxial Si {100} surfaces

The dielectric spectra of Si {111} and Si {100} orientations are shown to be equivalent using ex situ spectroscopic ellipsometry on clean epitaxial surfaces. The peak values for the real and imaginary parts (er, i) of the dielectric function exceed those previously reported, values of ei (at 4.25 eV)≥47 being obtained. Surface features with lateral scales of ≊0.5–2 μm, do not affect the dielectric spectra significantly. The high dielectric function peaks indicate that the nanometer lateral‐scale roughness on these epitaxial surfaces is very small.

Absorption in p‐Si1−xGex quantum well detectors

The normal incidence absorption between 2 and 14 μm in a pseudomorphic p‐Si0.81Ge0.19/Si multiple quantum well sample with doping 5×1012 cm−2 per well is described by a Drude conductivity characteristic of free carriers, with an in‐plane mobility of 32 cm2/V s and a relaxation time of 5.5 fs at 77 K. When the absorption is scaled with dopant concentration these parameters predict quantum efficiencies for quantum well infrared photodetectors in reasonable agreement with experiment.

The surface roughness and optical properties of high quality Si epitaxial layers

Abstract New recently published Si dielectric function data collected from specially prepared epitaxial Si are compared with literature values. Atomic force and optical microscopy studies of the epitaxial surfaces revealed them to be smooth on the fine scales to which ellipsometry is sensitive. The use of these new dielectric function data is demonstrated to lead to significantly improved fitting of ellipsometric data collected from a thermal oxide on Si structure.

Si1−XGeX/Si quantum well infrared photodetectors

P-type Si1−XGeX/Si quantum well infrared photodetectors for thermal imaging applications have been grown by low pressure vapour phase epitaxy for the first time. Good control of periodicity in these pseudomorphic multiple quantum well structures on Si substrates was demonstrated by real-time ellipsometry during growth and subsequently confirmed by transmission electron microscopy. The photoresponse spectrum was tuned in the 8–13 Μm waveband by varying the Ge fraction in the Si1−XGeX quantum wells. Dark currents were reduced by doping only the centre regions of the quantum wells.

Real Time Observation of Periodic Step Arrays During Silicon Vapour Phase Homoepitaxy

Light scattering has been used to characterise the periodic surface topography which develops on (001) Si epitaxial layers growing by a step flow mechanism. The periodicity provides a measure of the average kink density along the misorientation steps which develop a sawtooth form during growth. The diffracting properties of the layers have been determined after growth by angle-resolved scattering measurements. In-situ light scattering at fixed angle has been used to fol low the build-up of the periodic step arrays. The periodic structure is a metastable state of the surface maintained by the supersaturation during growth, and it decays on switching off the silane source gas or when the temperature is reduced below the point at which surface diffusion will support the sawtooth step shape.