GaN Technology
Introduction
Wide band gap gallium nitride (GaN) and related III-V nitrides are promising materials for applications in optoelectronics. Despite the fact that there are already many GaN based blue light-emitting diodes (LEDs) and laser diodes (LDs) on the market, there are still many problems related to material quality and device structure optimization. The main problem with the GaN based optoelectronic devises is the poor crystalline quality and stress of the semiconductor layers due to the large lattice mismatch between sapphire substrate and epitaxial GaN layer. Also doping of III-nitrides and processing of hexagonal crystal structures are very challenging tasks.
Figure 1. Cyan GaN based LED working at 20 mA operation current. Advanced p-type contact and current spreading layer enables uniform light output with high efficiency.
Figure 2. Near UV-LED operating at 20 mA current. The peak wavelength is 405 nm, and optical output 7 mW.
Fabrication and characterization methods
All the GaN based LED and laser structures are fabricated with the metal organic vapor phase epitaxy system (MOCVD) in Micronova. The multi-wafer (3x2") GaN MOCVD system suitable for both research and production of gallium nitride structures was installed in 2003. The MOCVD system is optimized for high-purity growth of GaN compounds with excellent wafer uniformity. Micro and Nanosciences Laboratory has an extensive collection of processing and characterization systems available for LED and laser research. Both optical and electrical properties of LEDs and LDs can be studied extensively in the laboratory. Growth rate and surface morphology of the semiconductor layers are examined with in-situ reflectometry during MOCVD growth process. Atomic force microscopy (AFM), scanning electron microscopy (SEM) and optical microscopy can be used for surface analysis. Crystal quality and layer characteristics can be analyzed with X-ray diffraction measurements (XRD). Optical properties of the LED and laser structures are characterized with photoluminescence (PL) and electroluminescence (EL) measurements. Hall measurements are used to get knowledge of the electrical properties of the semiconductor layers.
Figure 4. AFM surface scans of GaN layers grown on sapphire with a) standard "two step" method and b) optimized "multistep" method. Threading dislocations are seen as dark dots.
Further information
Blue LEDs by MOVPE Technology. Presentation from Micronova Research seminar, December 2005.
Fabrication of InGaN Quantum Wells for LED Applications. Sami Suihkonen, PhD thesis 2008.
Fabrication of Heteroepitaxial Templates for GaN-Based Optoelectronic Devices. Teemu Lang, PhD thesis 2007.