Mo Chunlan

Status of GaN-based green light-emitting diodes*

GaN-based blue light emitting diodes (LEDs) have undergone great development in recent years, but the improvement of green LEDs is still in progress. Currently, the external quantum efficiency (EQE) of GaN-based green LEDs is typically 30%, which is much lower than that of top-level blue LEDs. The current challenge with regard to GaN-based green LEDs is to grow a high quality InGaN quantum well (QW) with low strain. Many techniques of improving efficiency are discussed, such as inserting AlGaN between the QW and the barrier, employing prestrained layers beneath the QW and growing semipolar QW. The recent progress of GaN-based green LEDs on Si substrate is also reported: high efficiency, high power green LEDs on Si substrate with 45.2% IQE at 35 A/cm2, and the relevant techniques are detailed.

Comparative study of ZnO and GaN films grown by MOCVD

ZnO and GaN films were grown by MOCVD. AFM, DCXRD and photoluminescence were used to study the surface morphologies, structural and optical properties of the films. By a comparison of the measurement results, it was shown that the structural and optical properties of the ZnO films are superior to the GaN films. The (102) FWHM and the free/bound exciton intensity ratio of the ZnO films are the best results ever reported for ZnO films. To evaluate the overall quality of the GaN films, an InGaN/GaN MQW LED was fabricated and the LED showed good I-V characteristics and its light output power was 6 mW at 20 mA, which indicated the good quality of the GaN layers and then indirectly suggested the high quality of the ZnO films.

High optical efficiency GaN based blue LED on silicon substrate

After nearly ten years of exploration, our team firstly had a breakthrough in the technologies of materials growth and thin film chip manufacturing for high efficiency GaN based blue LED on silicon substrate in the world. The internal quantum efficiency and extraction efficiency of the vertical structure blue LED chips reached 80%. These LED chips have been realized mass production, and been successfully used in street lamps, miners lamp, down light, bulb light, flashlight and display imaging etc. In this paper, the related key technologies are comprehensive and systematically introduced. Selective-Area-Growth and Maskless-Micro-Epitaxial-Lateral-Overgrowth technologies were invented, by which a high crystalline quality GaN film was achieved with an only 100 nm AlN buffer layer. A set of systemic technologies were invented for manufacturing vertical thin film structure LED on silicon substrate, included high reflectance low resistance P type ohmic contact electrode, high stability low resistance N type electrode ohmic contact, surface roughening, complementary electrode, releasing residual tensile stress of GaN film technologies etc. At 350 mA (35 A/cm2), the light output power of blue LED (450 nm) on silicon is 657 mW, and external quantum efficiency of it is 68.1%.

Stress Distribution in GaN Films grown on Patterned Si (111) Substrates and Its Effect on LED Performance

Crack free GaN films were grown on 1200×1200μm2 patterned Si (111) substrates and 36 light emitting diodes (LEDs) were fabricated in each pattern unit. Spatial distribution of the tensile stress in the pattern units and its influence on the LED performance are studied by micro-Raman and electroluminescence (EL). The Raman shift of the GaN E2 mode shows that the tensile stress is the maximum at the center, partially relaxed at the edge, and further relaxed at the corner. With the stress relaxation, the EL wavelength has a significant blue shift and the luminous intensity shows a great enhancement.