A new method for growing bulk single-crystal silicon carbide (SiC) can produce high-quality SiC wafers for optoelectronics applications, say engineers at Okmetic AB (Linköping, Sweden), a member of the Okmetic group (Vantaa, Finland). "We have been building on our long-term work with Linköping University," says Asko Vehanen of Okmetic. "This has borne fruit in the form of a radically new method for SiC crystal growth, high-temperature chemical vapor deposition (HTCVD)." Using a continuous flow of pure gases containing silicon and carbon species, the HTCVD method produces SiC crystals with tightly controlled electrical and optical properties (see figure on page 12). According to Vehanen, the group has developed the first high-purity semi-insulating SiC wafers. "The main advantage is a significant increase in ingot quality micropipes, background doping levels, etc.and the degree of process control," says Vehanen. "We have also demonstrated SiC wafers that so far cannot be produced with sublimation, for example p-type wafers."
Okmetic is refining its relationship with Aixtron group (Aachen, Germany) member Epigress AB (Lund, Sweden), which produces the reactors for the HTCVD process. That company also has ties to the university. "In 10 years Linköping University has developed SiC growth technologies together with Okmetic, ABB, and Epigress," says Erik Janzén at Linköping University. "We will also expand our research on nitrides grown on SiC substrates."
Epigress is slated to supply Okmetic with a gas foil rotation (GFR) hot-wall SiC CVD system, says Goran Berg of Epigress. This system can accommodate three 2-in. or single 3- and 4-in. wafers.
SiC is a strong contender for the development of short-wavelength, high-brightness devices, say Reed Electronics Research (Sutton, UK) analysts in Optoelectronics: A Study of the Worldwide Semiconductor Optoelectronic Component Industry 2005. "The total market for these type of devices will reach U.S. $714 million for blue lasers and light-emitting diodes (LEDs) by 2005," the report says. "White LEDs based on these wide bandgap semiconductors could add an additional U.S. $806 million by 2005."
In a related story, Osram Opto Semiconductors GmbH (Regensburg, Germany) has demonstrated a continuous-wave (CW), blue-emitting indium gallium nitride (InGaN) laser. The work is the culmination of a research project funded by the German government and includes collaborators at the Fraunhofer Institut für Angewandte Festkörperphysik (Freiburg, Germany) and the universities of Stuttgart, Braunschweig, and Ulm.
Success did not happen overnight. "Even the first pulsed blue laser we developed in our labs in Regensburg represented a huge success for us," recalls Alfred Lell, project manager for the blue laser project at Osram Opto. The laser emitted microsecond pulses at approximately 410 nm but required a current of 1200 mA and a voltage of 33 V.
The HTCVD process involves exposing a seed crystal to a continuous flow of high-purity gases containing Si and C species to produce multiwafer ingot.
"This corresponds to an electric power of around 40 W, and a lot of heat is generated in the process," says project coordinator Volker Härle of Osram Opto. "In order to achieve a CW blue laser, we had to find ways and means to reduce the current, voltage, and therefore the power required to operate the laser."
To build the 0.5-mm x 0.3-mm x 0.1-mm laser chip, the group deposited various layers onto the SiC substrate using metal-oxide vapor phase epitaxy (MOVPE). Reducing the threshold current of the laser was a key step to reducing power consumption. It became evident that one efficient way of doing this was to optimize the InGaN quantum well (QW) active zone in which the light is generated. Specific coordination and adaptation of the composition and the thickness and the spacing of the QWs, combined with a precise definition of their quantity, ultimately led to success. With the aid of ridge waveguide technology, it was possible to limit the light-emitting range to a width of 3 µm, thus keeping the threshold current within defined limits.