Reflectance Difference Signal Research Articles

Normal-incidence reflectance difference spectroscopy based on a liquid crystal variable retarder.

We propose liquid crystal variable retarder-based reflectance difference spectroscopy for normal-incidence measurements. Principles, instrumentation, data collection and reduction, and calibration procedures are provided. The signal noise is better than 10-3, and the spectral range is from 1.6 to 2.4 eV with 346 photon energy channels. As a demonstration, reflectance difference signals of a multilayer pentacene film on poly (ethylene terephthalate) (PET) film are presented with different polarization azimuths. The characteristic peaks at 1.8 and 1.97 eV, corresponding to the Davydov splitting of pentacene crystal, are observed, which indicate well-ordered in-plane anisotropic structure of pentacene crystal film on PET. Thanks to normal incidence, this design is immune to adjusting the optical structure for the measurements with different working distances, and the objective lens is easily integrated to realize microarea measurements.

A liquid crystal variable retarder-based reflectance difference spectrometer for fast, high precision spectroscopic measurements

We present a liquid crystal variable retarder (LCVR)-based design for reflectance difference (RD) spectrometry, which offers one high quality RD spectrum in the visible range within several seconds. The measurement principle and the instrument development of this design are provided. As the LCVR is a key component, investigations are focused on its wavelength-dependent optical characteristics, the systematic errors induced by its imperfections, and the temperature effect on the measurement results. With careful calibration and data correction, the qualities of corrected RD spectra, defined as standard deviations of the RD signals as a function of wavelength and time, are better than 3×10−4 and 1.3×10−3, respectively.

Well-width dependence of in-plane optical anisotropy in (001) GaAs/AlGaAs quantum wells induced by in-plane uniaxial strain and interface asymmetry

The well-width dependence of in-plane optical anisotropy (IPOA) in (001) GaAs/AlxGa1−xAs quantum wells induced by in-plane uniaxial strain and interface asymmetry has been studied comprehensively. Theoretical calculations show that the IPOA induced by in-plane uniaxial strain and interface asymmetry exhibits much different well-width dependence. The strain-induced IPOA is inversely proportional to the energy spacing between heavy- and light-hole subbands, so it increases with the well width. However, the interface-related IPOA is mainly determined by the probability that the heavy- and light-holes appear at the interfaces, so it decreases with the well width. Reflectance difference spectroscopy has been carried out to measure the IPOA of (001) GaAs/AlxGa1−xAs quantum wells with different well widths. Strain- and interface-induced IPOA have been distinguished by using a stress apparatus, and good agreement with the theoretical prediction is obtained. The anisotropic interface potential parameters are also determined. In addition, the energy shift between the interface- and strain-induced 1H1E reflectance difference (RD) structures, and the deviation of the 1L1E RD signal away from the prediction of the calculation model have been discussed.

In-diffusion of oxygen vacancies near step edges dominates the oxidation of perovskitefilms

By a Monte Carlo simulation of the deposition process and a mean-field theorymodel of the surface structure, we have successfully reproduced experimentaloblique-incidence optical reflectance difference signals from laser molecular beam epitaxy ofNb:SrTiO3 ona SrTiO3 substrate in ambient oxygen. We find that during deposition the concentration of oxygenvacancies close to step edges is larger than that on flat terraces and remains stable. Wesuggest that oxygen vacancies diffusing on the surface tend to accumulate near step edgesdue to their slow in-diffusion rate there, and that this in-diffusion dominates the oxidationof as-deposited film.

Understanding the optical anisotropy of oxidized Si(001) surfaces

First-principles optical-response calculations of oxidized and strained Si structures are presented. Local Si lattice deformations accompanying the oxidation of Si bulk bonds cause pronounced optical anisotropies that account very well for the reflectance difference signal measured during oxide growth [Yasuda et al., Phys. Rev. Lett. 87, 037403 (2001)]. In contrast, calculations for various energetically favored $(4\ifmmode\times\else\texttimes\fi{}2)$ and $(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructed overlayer structures fail to reproduce the experiment.

Surface segregation of bulk oxygen on oxidation of epitaxially grown Nb-doped SrTiO3 on SrTiO3(001)

We studied the epitaxy of 10 mol % Nb:SrTiO3 on a SrTiO3(100) substrate under an interrupted pulsed-laser-deposition condition. By monitoring the recovery behaviors of reflection high-energy electron diffraction intensity and an optical reflectance difference signal from the growth surface, we observed that, at temperatures above 630 °C, the oxidation of an as-deposited Nb:SrTiO3 monolayer was achieved by the diffusion of oxygen vacancies in the monolayer into the substrate.

An In-Situ Oblique-Incidence Optical Reflectance Difference Study of Co Electrodeposition on a Polycrystalline Au(111) Surface

AbstractWe studied submonolayer and multilayer deposition of Co on Au(111) using in-situ oblique-incidence optical reflectance difference (OI-RD). We show that the optical technique is highly sensitive and accurate in determining the electrodeposited film thickness and growth mode. We found that the optically determined thickness of the ultrathin Co film is in very good agreement with that deduced from the integration of the anodic current during cyclic voltammetry (CV). From a weak oscillatory behavior of the optical reflectance difference signal, it seems that the growth of electrodeposited Co on Au(111) under pulsed deposition condition proceeds by a combination of three dimensional island and quasi layer-by-layer growth modes.

Self-assembled growth of InAs-quantum dots and postgrowth behavior studied by reflectance-difference spectroscopy

Initial stages of self-assembled growth of InAs dots on GaAs(001) and a postgrowth evolution of the deposited surface have been investigated by reflectance-difference (RD) spectroscopy and reflection high-energy electron diffraction (RHEED). A significant change in the RD spectrum occurs even at 0.1-monolayer (ML) deposition of InAs. With increasing InAs deposition, the As-dimer-elated RD sign is inverted, which indicates that the original As dimer along the [110] direction of the c(4×4) As-rich GaAs(001) surface becomes dimerized along the [−110] direction. Postgrowth behavior has been studied at 480°C after 1.9-ML deposition of InAs. During postgrowth, the As-dimer-related RD signal returns to signals observed at lower InAs deposition. This behavior gives evidence that the InAs coverage is reduced by postgrowth. Furthermore, positive offsets that increase in amplitude with photon energy appear in the RD spectra of the postgrown samples.

Theoretical analysis of thin film epitaxial growth monitored by differential reflectance

The thin film growth at the level of a single molecular layer can be achieved via laser molecular beam epitaxy approach. Such a growth can be monitored not only via the reflection high-energy electron diffraction technique, but also with the use of relative reflectivity change between s- and p-polarized light. The epitaxy growth of thin film is modeled as a microscopic two-dimensional (2D) gratings on the surface of the substrate with a period over two orders of magnitude less than the wavelength of probe light. The solution of Maxwell’s equations for such a 2D gratings leads to an effective multilayer medium model, from which the reflectance-difference (RD) signal in the process of epitaxy growth can be calculated in the framework of general optics. The simulation results demonstrate that the growth of thin film at the atomic level can be detected by such an optical RD technique. The theoretical calculations of RD signal are in agreement with the experimental measurements both in the order of magnitude and variation character of RD signal curve.

Step-Induced Optical Anisotropy of Vicinal Si(001)

It is demonstrated, using reflectance difference spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction, combined with deliberate straining of the surface, that the presence of atomic steps dramatically changes the optical anisotropy of the Si(001) surface. The step-induced reflectance difference signal originates predominately from rebonded steps and is comparable in magnitude to that of the terrace signal.

Non-destructive measurement of electron concentration in n-ZnSe by means of reflectance difference spectroscopy

Reflectance difference (RD) spectroscopy was employed for contactless determination of the free-carrier concentrations in n-ZnSe grown by molecular beam epitaxy (MBE). In materials systems that show the linear electro-optic (LEO) effect, the LEO-induced RD signal from the region of the E 1 and E 1 + Δ 1 transitions can be used to estimate the net doping level. In this study we have investigated n-type ZnSe for a wide range of carrier concentrations (9 × 10 16 to 5 × 10 18 cm −3). Our results show that the carrier concentration can be determined from the gauged RD spectra with an accuracy of approx. ± 20% for net donor concentrations of the order of 10 18 cm −3.

Real time monitoring of growth and p-type doping processes in MOVPE of ZnSe

Growth and p-type doping processes in MOVPE of ZnSe have been studied by optical probing methods called RD (reflectance difference) and SPI (surface photo-interference). As for the dopant source, tertiarybutyl-amine (tBNH2) was extensively used in this work, because it is one of the most useful dopant sources in photo-assisted MOVPE of widegap II–VI compounds. Oscillations with monolayer periodicity in RD signal traces have been successfully detected for the first time using a HeNe laser beam in MOVPE of ZnSe. One of the most important findings is that tBNH2 molecules are selectively adsorbed on the “Zn-terminated” surface and not on the “Se-terminated” surface; this suggests that selective doping during Zn-source (DMZn) supply and/or on Zn-terminated surfaces will be effective for improving the doping efficiency.

Investigation of the relationship between reflectance difference spectroscopy and surface structure using grazing incidence X-ray scattering

In this work, in-situ grazing incidence X-ray scattering is used to correlate surface structures observed during organometallic vapor phase growth of GaAs with in-situ optical measurements using reflectance difference spectroscopy (RDS). Our observations of several reconstructions confirm that RDS signals vary among the different surfaces present under vapor phase epitaxy conditions, as also occurs under ultra-high vacuum growth conditions. In addition, the simultaneous observation of intensity oscillations in the X-ray scattering signal and the reflectance difference signal indicates that both of these techniques can be used to monitor layer-by-layer growth processes under some conditions.

Distinguishing the As- or Ga-rich initial reconstruction in short-pulse supersonic nozzle beam epitaxy of GaAs in real time by millisecond time-resolved reflectance difference

For short-pulse supersonic nozzle beam epitaxy growth of GaAs on an As-rich initial surface, we measured the fast evolutions of As dimer density and Ga dimer density in real time by using a millisecond time-resolved reflectance difference (RD) system. Under the impingement of a trimethylgallium short pulse, the (2×4)β, (2×4)γ, and c(4×4) initial reconstructed surfaces show very different transient RD signals which can be used to distinguish the initial reconstruction of the GaAs surface in real time during the epitaxy.

Atomic layer epitaxy of ZnSe using reflectance difference spectroscopy

We applied reflectance difference (RD) spectroscopy to the atomic layer epitaxy (ALE) of ZnSe and proved the self-limiting process of Se atoms by means of in-situ monitoring of the RD intensity. The ALE of ZnSe was performed on a ZnSe buffer layer grown on a GaAs substrate at 275°C. During the ALE growth with the alternate molecular beam supply of Zn and Se atoms, the intensity of the RD signal at 4.2 eV showed a cyclic change between the Zn level and the Se level, respectively. Furthermore, the Se level was found to stay constant both after the pulsed supply of Se atoms and during the continuous supply of Se atoms, while the intensity recovered from the Zn level to the Se level exponentially after the pulsed supply of Zn atoms. These facts show that the self-limiting mechanism prevails only in the atomic layer formation of Se on the ZnSe outermost surface.

Millisecond time-resolved reflectance difference measurements of GaAs grown by short-pulse supersonic nozzle beam epitaxy

We report the millisecond time-resolved reflectance difference (RD) measurements during short-pulse supersonic nozzle beam epitaxy of GaAs using trimethylgallium (TMG) and arsine (AsH3). A rapid rise of RD signal upon the injection of TMG short pulse was observed, and was assigned due to the density change of As dimers on the growing surface. With changes of the TMG pulse width and the substrate temperature, the corresponding changes of the rise time of the RD signal were observed. It is concluded that the rise in the RD signal is related to the As dimer annihilation caused by TMG decomposition, and As dimer formation. Our results also show that the growth amount under monolayer is proportional to the TMG molecular population in the TMG pulse.

Reflection high energy electron diffraction and reflectance difference studies of surface anisotropy in InGaAs chemical beam epitaxy on flat and vicinal (001) GaAs

InGaAs quantum wells (QWs) were grown in a chemical beam epitaxy (CBE) machine with trimethylindium (TMI), triethylgallium (TEG) and tertiarybutylarsine (TBA) as precursors. Growth was monitored in-situ by reflectance difference (RD) and reflection high energy electron diffraction (RHEED), on both flat and vicinal (2° off in the 〈111〉 A direction) (001)GaAs substrates. The RD was monitored at 632.8 nm. At this wavelength the RD signal from a GaAs surface is primarily related to the absorption by Ga dimers. When InGaAs had been grown, both the average RD signal and the amplitude of the RD oscillations for the subsequent growth of GaAs increased significantly, compared to GaAs growth on GaAs. This In influence was found to persist even after the growth of 20–30 ML of pure GaAs. As a result we were able to monitor growth oscillations with RD and RHEED simultaneously during growth of quantum wells of InGaAs in GaAs. As a conclusion to these observations we suggest that the group III dimer bond concentration, detected in the RD signal, increases.

Effects of pressure and temperature on RD detected growth oscillations

We report on the vacuum chemical epitaxy (VCE) growth of GaAs from triethylgallium and arsine at varying partial pressures of arsine and hydrogen. In situ, monolayer growth oscillations were, for the first time, detected in a hydrogen environment using reflectance difference (RD). These results offer the possibility to link surface mechanisms occuring during chemical beam epitaxy (CBE) with those taking place in metalorganic vapour phase epitaxy (MOVPE) and may lead to the observation of growth oscillations also during MOVPE. Finally, the behaviour of the RD signal as a function of substrate temperature is studied over a wider temperature interval than has previously been reported, giving further information about surface processes.

CBE Growth Characterization of GaAs and InGaAs by Rheed and RD

ABSTRACTTriethylgallium, trimethylindium and tertiarybutylarsine have been used to grow GaAs and InGaAs on (OO1)GaAs. No pre-cracking was used so that the thermal decomposition of tertiarybutylarsine on GaAs could be studied. The growth has been monitored in-situ using reflection high-energy electron diffraction and reflectance-difference. A comparison ismade between the reflectance difference signal and the intensity of the reflection high-energy electron diffraction specular spot as the surface is transformed from As rich to Garich. Using the former technique similar growth transients have also been studied as a function of the temperature. It is found that the incorporation of small amounts of In (less than 5 per cent) in the buffer layer significantly improves the oscillatory behavior of the RD-signal when growth of GaAs is commenced. Finally, the growth dynamics of InGaAs isstudied using reflection high-energy electron diffraction.

Reflectance difference for in situ control of surface V/III ratio during epitaxial growth of GaAs

Optical, in situ, real time control of surface processes during epitaxy is becoming increasingly important for the understanding and control of crystal growth. Here results are presented of in situ studies of the surface V/III balance using the reflectance-difference (RD) method during vacuum chemical epitaxy of (001) GaAs from arsine and triethylgallium. We have found a relationship between the RD signal and the V/III ratio, and we demonstrate the possibility for the use of this relationship for the optimization of growth. We have, by RD, detected a sharply defined, critical V/III ratio, below which the morphology and the photoluminescence intensity deteriorate dramatically, and which can be used to control the conversion from n-type to p-type conductivity. We believe that these observations will be of great importance for the in situ optimization of epitaxial growth, and in eliminating much of the uncertainties involved in reproducing surface V/III ratios.