Vincent Woods

Real-time optical monitoring of ammonia flow and decomposition kinetics under high-pressure chemical vapor deposition conditions

Understanding the gas phase decomposition kinetics of the chemical precursors involved in the nucleation and thin-film growth processes is crucial for controlling the surface kinetics and the growth process. The growth of emerging materials such as InN and related alloys requires deposition methods operating at elevated vapor densities due to the high thermal decomposition pressure of these materials. High nitrogen overpressure has been demonstrated to suppress the thermal decomposition of InN, but has so far not been explored in chemical vapor deposition experiments. In this contribution we present research results on the decomposition kinetics of ammonia in the laminar flow regime of a high-pressure flow channel reactor. Ultraviolet absorption spectroscopy is applied to analyze absorption features of ammonia with respect to the ammonia flow rate during continuous flow and pulsed ammonia injection. Pulsed ammonia injection has been used to analyze the average gas flow velocity in the high-pressure chemic...

Real-time optical control of Ga1−xInxP film growth by p-polarized reflectance

The engineering of advanced optoelectronic integrated circuits implies the stringent control of thickness and composition. These demands led to the development of surface-sensitive real-time optical sensors that are able to move the control point close to the point where the growth occurs, which in a chemical beam epitaxy process is the surface reaction layer, built up of physisorbed and chemisorbed precursor fragments between the ambient and film interface. In this context, we explored the application of p-polarized reflectance spectroscopy (PRS) for real-time monitoring and control of pulsed chemical beam epitaxy during low-temperature growth of epitaxial Ga1−xInxP heterostructures on Si(001) substrates. A reduced order surface kinetics model has been developed to describe the decomposition and growth kinetics of the involved organometallic precursors and their incorporation in the film deposition. We demonstrate the linkage of the PRS response towards the surface reaction chemistry, composition, film g...

Real time optical characterization of gas flow dynamics in high-pressure chemical vapor deposition

While low-pressure chemical vapor deposition (CVD) methods offer excellent pathways for many compound semiconductors, these growth techniques possess limitations in the growth of high quality compounds with large thermal decomposition pressure such as InN and related materials. To study and extend the growth towards elevated pressures a high-pressure CVD system with integrated real time optical characterization techniques has been established. The built-in real time monitoring techniques allow the characterization of gas flow kinetics, precursor decomposition kinetics, as well as the crucial steps of nucleation and film formation. In this contribution, we report the characterization of process parameter under which the thin film growth process can be maintained under laminar flow condition. Laser light scattering has been proven as the most robust optical tool to characterize the onset of turbulence. Hence, it allows the mapping the pressure and flowing regime under which laminar flow can be maintained.

Surface structure, composition, and polarity of indium nitride grown by high-pressure chemical vapor deposition

The structure and surface bonding configuration of InN layers grown by high-pressure chemical vapor deposition have been studied. Atomic hydrogen cleaning produced a contamination free surface. Low-energy electron diffraction yielded a 1×1 hexagonal pattern demonstrating a well-ordered c-plane surface. High-resolution electron energy loss spectra exhibited a Fuchs–Kliewer surface phonon and modes assigned to a surface N–H species. Assignments were confirmed by observation of isotopic shifts following atomic deuterium cleaning. No In–H species were observed, and since an N–H termination of the surface was observed, N-polarity indium nitride is indicated.

Nucleation and growth of InN by high-pressure chemical vapor deposition: Optical monitoring

The growth of high quality, stoichiometric InN presents a challenge because of the volatility of atomic nitrogen. To overcome the associated difficulties, a high-pressure chemical vapor deposition (HPCVD) system has been developed, which has opened the avenue for achieving stoichiometric single-phase surface compositions for materials such as InN for which thermal decomposition pressures are large at optimum processing temperatures. We report results obtained during InN growth in the pressure range of 2–15bar to achieve the earlier objectives and to obtain insights into the InN nucleation and growth process. Using real-time optical ultraviolet absorption spectroscopy, we characterized the chemistry of the gas-phase precursors as functions of flow, pressure, and temperature. Highly surface sensitive probing on InN nucleation and steady state growth is achieved by principal-angle-reflectance spectroscopy, allowing the characterization of surface chemistry at a submonolayer level. The InN layers grown at low...

Real-time optical monitoring of gas phase dynamics for the growth of InN at elevated pressures

The request for increased performance in high-power / high-frequency optoelectronic devices requires new methods for the fabrication of high quality III nitride alloys that exhibit large thermal decomposition pressure such as InN and related materials. To extend the process and growth window towards elevated pressures, a high-pressure CVD system with integrated real time optical characterization techniques has been constructed. The built-in real-time monitoring techniques allow the characterization of gas flow dynamics, precursor decomposition kinetics, as well as the monitoring of the crucial steps of nucleation and film formation. The gas flow dynamics has been characterized and the process parameter are obtained under which the thin film growth process can be maintained under laminar flow condition. Laser light scattering (LLS) has been proven as the most robust optical tool to characterize the onset of turbulence.

Real-time thickness and compositional control of Ga1−xInxP growth using p-polarized reflectance

Advances in the engineering and design of advanced electro-optical materials require sensors and control strategies that allow tight control over thickness and composition of multilayered structures. In response to this demand, we developed and applied p-polarized reflectance (PR) for real-time optical characterization and control of heteroepitaxial GaP/GaInP growth under pulsed chemical beam epitaxy conditions. For closed-loop control, we applied nonlinear control algorithms (based on nonlinear Kalman filtering) that utilizes the PR signals to adjust the source flows involved in the heteroepitaxial growth of Ga1−xInxP on Si(001). A reduced order surface kinetics model has been formulated to establish the linkage between the surface reaction kinetic and its optical response. These data are linked to compute the compositional and thickness change per time unit, utilizing the monitored PR signals for validation. This allows the establishment of feedback control algorithms, able to control both the growth ra...

The growth of InN and related alloys by high-pressure CVD

The growth of high-quality InN and indium rich group III-nitride alloys are of crucial importance for the development of high-efficient energy conversion systems, THz emitters and detectors structures, as well as for high-speed linear/nonlinear optoelectronic elements. However, the fabrication of such device structures requires the development of growth systems with overlapping processing windows in order to construct high-quality monolithic integrated device structures. While gallium and aluminum rich group III-nitrides are being successfully grown by organometallic chemical vapor deposition (OMCVD), the growth of indium rich group III-nitrides presents a challenge due to the high volatility of atomic nitrogen compared to indium. In order to suppress the thermal decomposition at optimum processing temperatures, a new, unique high-pressure chemical vapor deposition (HPCVD) system has been developed, allowing the growth of InN at temperatures close to those used for gallium/aluminum-nitride alloys. The properties of InN layers grown in the laminar flow regime with reactor pressures up to 15 bar, are reported. Real-time optical characterization techniques have been applied to analyze gas phase species and are highly sensitive the InN nucleation and steady state growth, allowing the characterization of surface chemistry at a sub-monolayer level. The ex-situ analysis of the InN layers shows that the absorption edge in the InN shifts below 0.7 eV as the ammonia to TMI precursor flow ratio is lowered below 200. The results indicate that the absorption edge shift in InN is closely related to the In:N stoichiometry.