P-polarized Reflectance Research Articles

P-polarized reflectance spectroscopy: A highly sensitive real-time monitoring technique to study surface kinetics under steady state epitaxial deposition conditions

This paper describes the results of real-time optical monitoring of epitaxial growth processes by p-polarized reflectance spectroscopy (PRS) using a single wavelength application under pulsed chemical beam epitaxy (PCBE) condition. The high surface sensitivity of PRS allows the monitoring of submonolayer precursor coverage on the surface as shown for GaP homoepitaxy and GaP on Si heteroepitaxy as examples. In the case of heteroepitaxy, the growth rate and optical properties are revealed by PRS using interference oscillations as they occur during growth. Superimposed on these interference oscillations, the PRS signal exhibits a fine structure caused by the periodic alteration of the surface chemistry by the pulsed supply of chemical precursors. This fine structure is modeled under conditions where the surface chemistry cycles between phosphorus supersaturated and phosphorus depleted surfaces. The mathematical model describes the fine structure using a surface layer that increases during the tertiarybutyl phosphine (TBP) supply and decreases during and after the triethylgallium (TEG) pulse, which increases the growing GaP film thickness. The imaginary part of the dielectric function of the surface layer is revealed from the turning points in the fine structure, where the optical response to the first precursor pulse in the cycle sequence changes sign. The amplitude of the fine structure is determinated by the surface layer thickness and the complex dielectric functions for the surface layer with the underlying bulk film. Surface kinetic data can be obtained by analyzing the rise and decay transients of the fine structure.

GaxIn1-xP/GaP Heterostructures on Si(001) Substrate

AbstractIn this contribution we report on the real-time monitoring of low temperature growth of epitaxial GaxIn1-xP/GaP heterostructures on Si(100) by pulse chemical beam epitaxy, using tertiary butylphosphine (TBP), triethylgallium (TEG), and trimethylindium (TMI) as source materials. Both step-graded and continuously graded heterostructures have been investigated. The composition of the GaxIn1-xP epilayers has been analyzed by various techniques including X-ray diffraction, Rutherford backscattering, Auger, and Raman spectroscopy. Good correlation has been found between X-ray diffraction, RBS, and Vegard's law compositional analysis. We used Ppolarized Reflectance Spectroscopy (PRS) and Laser Light Scattering (LLS) to monitor the growth rate and surface morphology during growth. The information gained by these techniques has been utilized in the improvement of the surface preconditioning as well as to optimize the initial heteroepitaxial nucleation and overgrowth process. We studied the optical response to the compositional changes in the surface reaction layer (SRL) during the exposure of the surface to either sequential or synchronous pulses of TEG and TMI. The cross sectional TEM analysis indicates that the synchronous exposure results in an abrupt GaxIn1-xP/GaP interface while the sequential exposure does not which may suggest a compositionally graded interlayer formation. For heteroepitaxial GaxIn1-xP films on Si, a buffer layer of GaP is found to be necessary for optimum uniformity of the GaxIn1-xP layer. The selective growth of GaxIn1-xP on Si(001) is accessed.

Real-time Characterization of the Optical Properties of an ultra-thin Surface Reaction Layer during Growth

AbstractThe average optical properties of an ultra-thin surface reaction layer (SRL) during growth by pulsed chemical beam epitaxy (PCBE) can be quantitatively accessed by p-polarized reflectance spectroscopy (PRS), as demonstrated on the example of heteroepitaxial GaP growth. Under PCBE growth conditions, the surface of the substrate is exposed to pulsed ballistic beams of tertiarybutyl phosphine [TBP, (C4H9)PH2] and triethylgallium [TEG, Ga(C2H5)3]. The pulsed precursor supply causes a period in composition and thickness modulated SRL, monitored as a fine structure that is superimposed on interference oscillations, resulting from back reflection at the substratelayer interface with increasing layer thickness. The amplitude of this fine structure undergoes a period amplitude modulation and exhibits turning points at which the response to the first precursor pulse changes sign. The turning points can be characterized by the expression R4(Φmax)=R4(Φmin), which describes the maximal and minimal values of the temporally modulated phase factor in the SRL, using a four layer stack description. The positions of these turning points are not affected by the thickness of the SRL, which allows the computation of the average complex dielectric function of the SRL independent of its thickness. In the next step, the average thickness of the SRL can be extracted from the amplitude of the observed fine structure.

Real-time monitoring of heteroepitaxial growth processes on the silicon(001) surface by p-polarized reflectance spectroscopy

In this paper we describe the real-time monitoring by p-polarized reflectance spectroscopy (PRS) using pulsed chemical beam epitaxy (PCBE) of Ga x In 1− x P on Si(001) as an example. For constant source vapor pulse height, width and repetition rate, the formation of the heteroepitaxial film on the Si(001) surface proceeds via a three-dimensional nucleation and overgrowth mechanism. Provision of a high initial supersaturation of the surface drives the nucleation kinetics toward a two-dimensional mechanism. Provision of a high initial supersaturation of the surface drives the nucleation kinetics toward a two-dimensional mechanism. The analysis of the fine structure in the PRS intensity reveals that under the conditions of quasi-steady state growth the surface chemistry cycles between enhanced and diminished P-activity and diminished and enhanced Ga-activity, with the dealkylation of the group III alkyl source molecules constituting a rate limiting step in the growth kinetics. Since the deposition rate per precursor pulse cycle can be determined experimentally, molecular layer epitaxy conditions can be imposed and verified in real-time without reliance on a self-limiting growth mechanism.

Real-time optical monitoring of epitaxial growth: Pulsed chemical beam epitaxy of GaP and InP homoepitaxy and heteroepitaxy on Si

We present a study of the real-time monitoring of the homoepitaxial growth of GaP, InP, and the growth of InP/GaP and GaP/Si(001) heterostructures, combining single wavelength p-polarized reflectance (PRS), reflectance-difference spectroscopy (RDS), and laser light scattering (LLS) during pulsed chemical beam epitaxy with tertiarybutylphosphine, triethylgallium, and trimethylindium sources. The growth rate and the bulk optical properties are revealed by PRS with submonolayer resolution over 1000A of film growth. The surface topography is monitored by LLS providing additional information on the evolution of the surface roughness as well as the nucleation/growth mechanism. The optical surface anisotropy, which is related to surface reconstruction and/or surface morphology, is monitored by RDS and compared with the results of PRS and LLS. The results are discussed with respect to the deposition kinetics, in particular as a function of the V:III flux ratio. The pulsed supply of chemical precursors causes a periodic alteration of the surface composition, which is observed as correlated periodic changes in the RD and PR signals, confirming the high sensitivity of both methods to surface chemistry.

Heteroepitaxy of lattice-matched compound semiconductors on silicon

The heteroepitaxial overgrowth of silicon by nearly lattice-matched compound semiconductors is reviewed in the context of the separation of the chemical problems associated with the initial sealing of the silicon surface by a contiguous epitaxial compound film from the problems associated with the generation of strain during heteroepitaxial growth. Of the mixed compound systems available dilute solid solutions of composition AlxGa1−xNyP1−y and ZnSySe1−y as well as ZnSixGe1−xP are suitable candidates for the exactly lattice-matched epitaxial overgrowth of silicon. Real-time process monitoring by nonintrusive methods is important for gaining an understanding of the epitaxial overgrowth mechanism and for controlling the film properties. A new method, p-polarized reflectance spectroscopy is introduced that provides detailed information about the growth rate per cycle, the bulk optical properties of the film and its topography. Submonolayer resolution is accomplished for thousands of Å of film growth by pulsed chemical beam epitaxy. While the cubic materials considered here generally afford easier control of the electrical and optical properties, the noncubic materials have advantages in the sealing of the silicon surface because of their anisotropic growth and the formation of metastable solid solutions that may permit the graded growth of compound films under exactly lattice-matched conditions. Therefore, no clear-cut preference in the materials selection for nearly lattice-matched overgrowth of silicon by compound semiconductors can be identified at this time.

Migration-enhanced pulsed chemical beam epitaxy of GaP on Si(001)

In this paper, we present results on the heteroepitaxial growth of GaP on Si by migration enhanced pulsed chemical beam epitaxy (PCBE) using additional hydrogen and filament assisted techniques. For the real-time monitoring we applied a new technique, p-polarized reflectance spectroscopy (PRS) which provides information on the bulk, interface and surface properties of the grown GaP/Si heterostructures. The monitoring is initiated close to the Brewster angle for the substrate material, achieving high sensitivity in the initial phase of heteroepitaxial overgrowth. The bulk properties and growth rate are monitored using the quarter wave periodic structure in the reflectance based on interference in the film. The surface vicinity properties are monitored by the periodic fine structures in the intensity of the reflected light, which are correlated with the switching cycles of the metalorganic precursor pulses and are amplitude-modulated in accord with the repeat cycle of the heteroepitaxial growth process. Additional information regarding the surface roughness was revealed by scattered light measurements during the growth of the film. The results of real-time monitoring of nucleation and growth are combined with results of the ex-situ characterization of the GaP/Si heterostructure by cross-sectional transmission electron microscopy (TEM) and atomic force microscopy (AFM) studies to gain an understanding of the critical phase, in which the silicon surface becomes sealed by a contiguous epitaxial film of GaP. The defects formed in the GaP film during this phase determine the quality of the epilayers obtained upon prolonged growth.

Real-time Optical Monitoring of Epitaxial Growth Processes by p-Polarized Reflectance Spectroscopy

AbstractIn this paper we introduce a real-time optical probe technique, p-polarized reflectance spectroscopy (PRS), for the monitoring of epitaxial growth processes. GaP heteroepitaxy by pulsed chemical beam epitaxy (PCBE) is used as an example. PRS allows to follow the deposition process with submonolayer resolution, utilizing a fine structure that is superimposed to the interference oscillations in the reflected intensity. This fine structure is explained by the periodic alteration of the surface reaction chemistry under pulsed chemical precursor supply. In the case of epitaxial GaP growth, it is modeled for a four layer stack, including an ultra-thin surface reaction layer of periodically changing thickness do(t) and dielectric function εo(t) tied to the periodic surface exposure to tertiarybutyl phosphine (TBP) and triethylgallium (TEG) pulses, respectively. The imaginary part of the dielectric function, εO2, of this surface reaction layer can be determined directly from the distance of the inflection points in the fine structure, where the optical response to the first precursor pulse in the cycle sequence changes sign, from the closests interference minimum. The surface reaction kinetics can be studied by analyzing the decay time characteristic in the transients of the fine structure.

Real-time Optical Monitoring of GaxIn1−xP/GaP Heterostructures on Silicon

AbstractIn this paper we report the combined application of p-polarized reflectance spectroscopy (PRS), reflectance difference spectroscopy (RDS), and laser light scattering (LLS) to investigate the heteroepitaxy of GaxIn 1−xP/GaP on Si by pulsed chemical beam epitaxy (PCBE) with tertiarybutylphosphine (TBP), triethylgallium (TEG), and trimethylindium (TMI) precursors. Both, PRS and RDS follow the growth process with submonolayer resolution utilizing a periodic fine structure signal, which is caused by a periodic alteration of thickness and composition of an ultra-thin surface reaction layer during the periodic TEG and TBP exposure of the surface. After the transition from GaP growth to GaxIn 1−xP growth, the RDS oscillations are reoriented after about five precursor cycles in a new oscillation periodicity, where the response to the TBP pulse has the opposite direction. The ratio of the changes in the amplitudes of RDS signals as a response to TEG and TMI surface exposure is used to estimate the composition fo GaxIn 1−xP. The PRS fine structure is maintained after switching to GaxIn 1−xP growth with a separate feature for each TEG and TMI surface exposure. The amplitude ratio of these features changes during growth.

Birefringent beam splitter for intracavity mode selection in high-power multimirror lasers

The design and application of an uncoated sapphire plate that acts as both the beam splitter and the output coupler of an interferometric laser resonator are described. Output coupling is provided at one of the surfaces by p-polarized (TM) reflection near the Brewster angle, and axial-mode selection is enforced at the other surface by s-polarized (TE) reflection at the same angle of incidence. The design is discussed in the context of the phase-locked, rf linac free-electron laser, in which the coupling of adjacent optical pulses at the beam splitter induces temporal phase coherence among all the pulses in the output beam; this coherence is manifested in the frequency domain as a reduction in the number of axial modes per rf frequency interval. The Michelson and Fox-Smith resonator designs are compared, and applications to cavity dumping are discussed.

Characterization of the linear optical properties of a multiple quantum well structure in the sheet-model approximation

The possibility of determining the linear optical response tensor of a multiple quantum well (MQW) structure from optical reflection spectra is discussed. Taking into account local-field effects, the optical parameters of the MQW system are derived directly from the sheet conductivity tensor of the individual quantum well. For an amorphous Si/SiO 2 MQW structure deposited on a crystalline Si substrate, both s- and p-polarized reflection spectra were measured at different angles of incidence in the near-infrared and visible frequency regions. It was shown that the s-polarized spectra can be characterized by means of only one (complex) parameter, whereas in general it is impossible to account for the p-polarized spectra parametrically in a full range of angles of incidence even with four angular independent parameters. Finally, it was demonstrated that Feibelman's d-parameters can only be used to describe the experimental spectra at near-normal and grazing angles of incidence.

Surface and Defect Structure of Epitaxial Gallium Phosphide on Si(001)

Epitaxial GaP films grown on silicon by chemical beam epitaxy contain planar defects that are related to three dimensional island formation of GaP nucleating on the silicon substrate. Attention in the surface preconditioning and to the surface chemistry during the initial stages of nucleation and growth allow the control from pronounced three-dimensional growth behavior toward almost two-dimensional growth. New results on defect structure and surface morphology obtained by a combination of atomic force microscope (AFM) and a new method p-polarized reflectance spectroscopy (PRS) are presented and related to growth conditions during the initial stages of heteroepitaxy.

Optical detection and dynamics of surface transient processes in plasma CVD of hydrogenated amorphous silicon

By in situ diagnosis using p-polarized light reflection from an a-Si:H growing surface, we have verified the so-called growth zone, i.e. the transient topmost atomic layers, and analyzed its relaxation process quantitatively. A stabilized He-Ne laser was linearly p-polarized and impinged on an rf plasma depositing a-Si:H surface and the reflected light intensity was measured by a photodiode. A slight and gradual change was detected in the reflected light intensity when the film deposition was halted by turning off the plasma. The observed reflection pattern including this slight and clear change can be well simulated by taking into account the presence and relaxation of the growth zone with its refractive index and the absorption coefficient lower than those of the bulk network. The relaxation rate increased with an increase of substrate temperature. During this relaxation stage, the growing surface layer is considered to be densified by liberating hydrogen into the rigid bulk amorphous network.

Optical properties of semiconductor superlattices in the far infrared

In the far infrared, superlattice dielectric functions can be well represented by an effective-medium model. The overall symmetry of a superlattice is uniaxial, and each of the principal components of the resulting dielectric tensor typically contains a different series of resonances. Here we examine some consequences that are of importance to many of the spectroscopy experiments that are undertaken. We describe sharp Brewster modes near longitudinal frequencies in p-polarized oblique incidence reflectivity and give details of guided wave and surface modes, including interface polaritons at the superlattice–vacuum interface. After discussing the basic optical principles we examine their application to the measurement of phonon confinement parameters.

Reflectance of nonlocal conducting superlattices.

We calculate the p-polarized reflectance spectra of a semi-infinite superlattice made up of alternating spatially dispersive conducting layers, accounting exactly within a hydrodynamic model, for the propagation of plasma waves. Besides the expected structure originated from the infinite-superlattice bulk-plasma resonances, we find an extra series of peaks which arise from the semi-infinite-superlattice surface resonances. The latter originate from standing plasma waves at the surface layer, whose resonant frequencies are larger than those in the interior layers due to confinement effects.

Optical diamagnetic response of a niobium quantum well: p-polarized reflection coefficient

On the basis of a self-consistent local-field calculation, the p-polarized reflection coefficient of a niobium quantum well deposited on crystalline quartz is determined as a function of frequency and well width. It is demonstrated that the local field induced along the quantum well drives the infrared response.

Polarization Conversion Using Prism-coupled Surface Plasmon-polaritons

Abstract Evanescent prism coupling through a thin silver film provides a convenient mechanism for excitation of surface plasmon-polaritons (SPPs). However, the SPP is observed as a dip in p-polarized reflectivity, which may be inconvenient in sensor application. Here we observe the effect of rotation of the plane of incident polarization. This leads to the observation of polarization mixing at SPP excitation, and a peak reflectivity signal is obtained between crossed polarizers. This provides a powerful mechanism for SPP observation as a peak, which may be more easily monitored.

Anisotropic off-normal incidence optical reflection from GaP (110) surfaces

This article contains a theoretical study for off-normal incidence surface induced optical anisotropy (SIOA). The discrete dipole approximation was used to calculate the off-normal incidence optical response of slabs. By means of the two slab approach those results were converted to semi-infinite reflectivities. The calculated ellipsometric parameter δΔ shows large variations near the Brewster angle, but only the p-polarized reflection has a clearly increased SIOA sensitivity. So experimentally a straightforward determination of ΔRp should be preferred. Advantages have to be sought in the optical observation of surface state related phenomena at subbandgap conditions.

Influence on reflectivity spectra of an exciton-free surface layer of a spatially dispersive thin film

We have calculated the p-polarized reflectivity (Rp) spectra of a spatially dispersive semiconductor thin film with an exciton-free layer (dead layer) on both surfaces, deposited on a metal substrate, in the vicinity an excitonic transition, using the surface impedance method. Two different well known additional boundary conditions (ABC) are invoked at the interfaces between the two exciton-free surfaces and the spatially dispersive film. The results presented for the case of a CdS film on an Al substrate show qualitative as well as quantitative differences between the two ABC's spectra. The dead layer presence makes additional structure on Rp at ω ω T=0.99935 and 1.00255 for the Ting et. al. and the Pekar ABC's spectrum respectively.

Liquid polymer conformation on solid surfaces

Infrared spectra of thin films of polyperfluoroethers on metal and silicon dioxide surfaces were obtained for films with thicknesses between 1 and 1000 nm by p-polarized reflection and modulated polarization techniques. Changes in frequencies and relative intensities of vibrational peaks with film thickness suggest that the conformation of liquid polymers on surfaces is unlike bulk conformation, with polymer chains extended preferentially along the surface.