Reflectance Difference Spectroscopy of II-VI Compounds

Metal Organic Chemical Vapour Deposition (MOCVD) is a key manufacturing process for microelectronic and opto-electronic devices. The advanced functional materials deposited include compound semiconductors, but also metals, high dielectric materials, ferroelectrics, insulators etc. MOCVD of epitaxially grown binary, ternary and quaternary semiconductor structures is an essential process for electronic and opto-electronic devices. These range from optoelectronic aluminium, gallium, indium based arsenide and phosphide semiconductors used in communications industry to the wide band gap nitrides. In this epitaxy process the net added value of a 2-6 inch substrate wafer can amount from a few thousand Euro to hundred thousand Euro. However, in MOCVD environments the deposition/ epitaxy process is hardly monitorable, nor due to the lack of in-process monitoring techniques closed loop controllable. Within this project we focussed on in situ monitoring and control of the MOCVD deposition process for the two material systems, which are the most difficult to control: a) ternary and quaternary {Al, Ga, In}-{As, P, Sb} semiconductors, which allow to modify the energy gap from the middle infrared to the visible yellow, provided, that the parameters and device specifications (stoichiometry, strain, doping, homogeneity) are met and reproducible b) The other material system investigated in this project is gallium nitride (GaN), a wide band gap semiconductor and its related compounds aluminium gallium nitride (AlGaN) and indium gallium nitride (InGaN). Work in this material system has been triggered by the search for blue laser diodes, high power (microwave) semiconductor devices and high storage capacity. This project used, improved and combined real time monitoring techniques and closed loop feedback control of epitaxial crystal growth, where the sensors in situ used are purely optical ones, compatible with different industrially used (e.g. planetary) MOCVD growth reactors and systems. The embedded sensors are compatible to the MOCVD set-up and have been developed with the final aim of proving their industrial applicability and adaptability to MOCVD systems. These techniques are 1) (in situ) Spectroscopic Ellipsometry- SE, 2) (in situ) infrared Spectroscopic Ellipsometry- IRSE, 3 (in situ) Reflectance Anisotropy Spectroscopy- RAS and reflectometry, 4 (in situ) Raman spectroscopy -RS, and 5) (in situ) X- ray diffraction- XRD. It was shown, that on-line monitoring (and closed loop control) tools 1) cut the cost and time required for the determination of process parameters for new MOCVD epitaxy/deposition processes by at least 50%. This is especially true for the currently required calibration runs. 2) allowed the design of new device structures (e.g. VCSEL structures, custom designed composition variation within a few hundred Angstroms, reproducible overgrowth of structures with cap layers, etc.) MOCVD is specifically appropriate for these device designs, because mass flows can be controlled without thermal dead times, 3) by building an industrially used MOCVD system with attached embedded sensors allowing closed loop control. This research will pave the way to novel (opto-) electronic device structures as e.g. quantum dot devices and single electron transistors, because the monitoring and closed loop control techniques are strongly supporting nanotechnology.