Joungchel Lee

Rotating-compensator multichannel ellipsometry: Applications for real time Stokes vector spectroscopy of thin film growth

A multichannel spectroscopic ellipsometer based on the rotating-compensator principle was developed and applied to measure the time evolution of spectra (1.5–4.0 eV) in the normalized Stokes vector of the light beam reflected from the surface of a growing film. With this instrument, a time resolution of 32 ms for full spectra is possible. Several advantages of the rotating-compensator multichannel ellipsometer design over the simpler rotating-polarizer design are demonstrated here. These include the ability to: (i) determine the sign of the p-s wave phase-shift difference Δ, (ii) obtain accurate Δ values for low ellipticity polarization states, and (iii) deduce spectra in the degree of polarization of the light beam reflected from the sample. We have demonstrated the use of the latter spectra to characterize instrument errors such as stray light inside the spectrograph attached to the multichannel detector. The degree of polarization of the reflected beam has also been applied to characterize the time evo...

Coherent heteroepitaxy of Bi2Se3 on GaAs (111)B

We report the heteroepitaxy of single crystal thin films of Bi2Se3 on the (111)B surface of GaAs by molecular beam epitaxy. We find that Bi2Se3 grows highly c-axis oriented, with an atomically sharp interface with the GaAs substrate. By optimizing the growth of a very thin GaAs buffer layer before growing the Bi2Se3, we demonstrate the growth of thin films with atomically flat terraces over hundreds of nanometers. Initial time-resolved Kerr rotation measurements herald opportunities for probing coherent spin dynamics at the interface between a candidate topological insulator and a large class of GaAs-based heterostructures.

Advances in multichannel spectroscopic ellipsometry

Abstract Since the development and perfection of automatic ellipsometers, an effort that began nearly three decades ago, spectroscopic ellipsometry (SE) has increased in popularity as a non-destructive tool for characterizing the optical properties and layered structure of bulk solids and thin films. With the more recent development of multichannel ellipsometers for high-speed spectral scanning, in situ real-time SE has emerged as a very powerful tool for characterizing the time evolution of the optical properties and layered structure of materials during preparation and processing. High-speed spectral scanning has also opened up the possibility of real-time monitoring and control for complex material systems that cannot be fully characterized using well-established real-time measurements at one (or a few) wavelength(s). In this review, we will first describe the most recent developments in multichannel, multiparameter SE, in which spectroscopic information is obtained in addition to the ellipsometric parameters (ψ, Δ). Then we will discuss recent applications of multichannel SE to amorphous semiconductor film growth in order to obtain material properties in complex multilayer and graded-layer device structures. In general in this review, the focus will be on recent examples of the characterization of complex optical systems of technological interest. Practical situations will be described in which the dielectric functions of films are deposition-dependent and evolve with thickness due to particle and grain size effects and intentional compositional gradients, and in which film growth occurs on rough surfaces, leading to optical models for film growth that require interface and surface roughness layers.

Effects of processing conditions on the growth of nanocrystalline diamond thin films: real time spectroscopic ellipsometry studies

Abstract Real time spectroscopic ellipsometry has been applied to characterize the preparation of ∼2000-A thick nanocrystalline diamond films on Si substrates by microwave plasma-enhanced chemical vapor deposition. Diamond films prepared under different conditions, including variations in the substrate temperature and CH4+H2+O2 gas mixture, have been studied. In addition to providing accurate Si substrate surface temperatures for diamond film growth via an ellipsometry-based calibration, the real time approach yields information on the following processes: (i) generation of initial substrate damage by the seeding process used to enhance diamond nucleation, (ii) annealing of the seeding damage that occurs upon heating the substrate to the deposition temperature, and (iii) the structural evolution of the diamond film throughout the nucleation and bulk film growth regimes. In the nucleation regime, the following information on the diamond film can be extracted: (i) the diamond mass thickness evolution with time, (ii) the sp2 C and void volume fractions in the nucleating layer, and (iii) the thickness at which nuclei make contact, the latter providing an estimate of the nucleation density. In the bulk layer growth regime, the time evolution of the following information can be extracted: (i) the bulk layer thickness, (ii) the surface roughness layer thickness, (iii) the mass thickness of sp2 C in the bulk layer, and (iv) the void volume fraction in the bulk layer. In the nucleation regime, we find that high quality diamond nanocrystals form under a wide range of gas compositions for substrate temperatures above 700°C. It is proposed that nucleation is controlled by a disordered carbon phase embedded in the substrate surface during the seeding process, in which the Si wafer is abraded with diamond powder. Above 700°C, the disordered C at the substrate surface is believed to recrystallize in the diamond phase upon exposure to an initial H2 plasma prior to diamond film growth. A common feature of diamond growth under all deposition conditions is the incorporation of a large concentration of sp2 C when diamond nuclei coalesce (typically after a thickness of 200–400 A). The maximum concentration occurs under conditions for which poorer quality bulk films are obtained, including low substrate temperatures ( 0.02). The sp2 C is trapped within ∼300 A of the substrate interface, but under optimum conditions of growth (i.e., the diamond growth region of the CHO gas phase diagram), little additional sp2 C forms with continued growth after the first 300 A. The formation of sp2 C in the coalescence stage has been attributed to shadowing effects that lead to a reduction of atomic H relative to C-containing precursors arriving at shadowed surfaces. For the nanocrystalline diamond films studied here, it is found that films having the lowest volume fraction of sp2 C at the end of the 2000 A deposition also exhibit the lowest void volume fraction and the smoothest surfaces.

Nucleation and bulk film growth kinetics of nanocrystalline diamond prepared by microwave plasma‐enhanced chemical vapor deposition on silicon substrates

Real time spectroscopic ellipsometry has been applied to characterize the substrate temperature (T) dependence of the deposition rates for nanocrystalline diamond thin films prepared by microwave plasma‐enhanced chemical vapor deposition on seeded Si substrates. With the real time capability, it is possible to determine the rates at which the diamond mass thickness (i.e., volume per area) increases during the early nucleation and bulk film growth regimes. The increases in the nucleating and bulk diamond growth rates with T for 400<T<800 °C are consistent with activation energies of ∼17 and 8 kcal/mol, respectively. The results reported here provide insights into the nature of the low‐T growth rate limitations for diamond films on nondiamond substrates.

Low temperature plasma process based on CO-rich CO/H2 mixtures for high rate diamond film deposition

A low temperature process (350 °C<T<500 °C) for nanocrystalline diamond film growth by microwave plasma-enhanced chemical vapor deposition has been developed that yields high rates (up to 2.5 μm/h) at relatively low plasma powers (0.5 kW). In contrast to the widely used H2-rich mixtures of CH4 or CO and H2 that exhibit monotonic decreases in the diamond growth rate as T is reduced from 800 to 400 °C, CO-rich mixtures of CO and H2 exhibit an increase and sharp maximum as T is reduced. The results suggest a different diamond growth mechanism from the CO-rich mixtures.

Analysis of the ellipsometric spectra of amorphous carbon thin films for evaluation of the sp3-bonded carbon content

Abstract Using spectroscopic ellipsometry (SE), we have measured the optical properties and optical gaps of a series of amorphous carbon (a-C) films ∼ 100–300 A thick, prepared using a filtered beam of C + ions from a cathodic arc. Such films exhibit a wide range of sp 3 -bonded carbon contents from 20 to 76 at.%, as measured by electron energy loss spectroscopy (EELS). The Tauc optical gaps of the a-C films increase monotonically from 0.65 eV for 20 at.% sp 3 C to 2.25 eV for 76 at.% sp 3 C. Spectra in the ellipsometric angles (1.5–5 eV) have been analyzed using different effective medium theories (EMTs) applying a simplified optical model for the dielectric function of a-C, assuming a composite material with sp 2 C and sp 3 C components. The most widely used EMT, namely that of Bruggeman (with three-dimensionally isotropic screening), yields atomic fractions of sp 3 C that correlate monotonically with those obtained from EELS. The results of the SE analysis, however, range from 10 to 25 at.% higher than those from EELS. In fact, we have found that the volume percent sp 3 C from SE using the Bruggeman EMT shows good numerical agreement with the atomic percent sp 3 C from EELS. The SE-EELS discrepancy has been reduced by using an optical model in which the dielectric function of the a-C is determined as a volume-fraction-weighted average of the dielectric functions of the sp 2 C and sp 3 C components.

Alignment and calibration of the MgF 2 biplate compensator for applications in rotating-compensator multichannel ellipsometry

Biplate compensators made from MgF2 are being used increasingly in rotating-element single-channel and multichannel ellipsometers. For the measurement of accurate ellipsometric spectra, the compensator must be carefully (i) aligned internally to ensure that the fast axes of the two plates are perpendicular and (ii) calibrated to determine the phase retardance delta versus photon energy E. We present alignment and calibration procedures for multichannel ellipsometer configurations with special attention directed to the precision, accuracy, and reproducibility in the determination of delta (E). Run-to-run variations in external compensator alignment, i.e., alignment with respect to the incident beam, can lead to irreproducibilities in delta of approximately 0.2 degrees . Errors in the ellipsometric measurement of a sample can be minimized by calibrating with an external compensator alignment that matches as closely as possible that used in the measurement.

Dual rotating-compensator multichannel ellipsometer: Instrument development for high-speed Mueller matrix spectroscopy of surfaces and thin films

A multichannel ellipsometer in the dual rotating-compensator configuration has been developed for applications in real time Mueller matrix spectroscopy of anisotropic surfaces and thin films. The sequence of optical elements for this instrument configuration can be denoted PC1r(5ω)SC2r(3ω)A, where P, S, and A represent the polarizer, sample, and analyzer. In this sequence, C1r and C2r represent the first and second compensators, rotating with angular frequencies of 5ω and 3ω, respectively, where ω=π/TC is the base angular frequency (corresponding to 2 Hz) and TC=0.25 s is the fundamental optical period. The instrument can provide 170 point spectra over the wavelength range from 235 nm (5.3 eV) to 735 nm (1.7 eV) in all 16 elements of the unnormalized Mueller matrix with minimum acquisition and repetition times of TC=0.25 s. In this study, instrumentation calibration procedures are demonstrated in the transmission geometry without a sample, including (i) alignment of the two MgF2 zero-order biplate compens...

Rotating-compensator multichannel ellipsometry for characterization of the evolution of nonuniformities in diamond thin-film growth

A multichannel spectroscopic ellipsometer based on the rotating-compensator principle has been applied to obtain the evolution of spectra (1.5–4.0 eV) in the normalized Stokes vector of the light beam reflected from the surface of a nanocrystalline diamond film during growth. Spectra in the ellipsometry angles (ψ, Δ) provide the time evolution of the microstructure and optical properties of the film in thin layers, whereas the spectra in the degree of polarization provide the time evolution of nonuniformities in the growth process attributed to light scattering by diamond nuclei in the initial stage of growth and to thickness gradients over the probed area in thicker layers.