Jiang Fengyi

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.

Different properties of GaN-based LED grown on Si(111) and transferred onto new substrate

InGaN MQW LEDs, grown by metal-organic chemical vapor deposition (MOCVD) on Si(111) substrates, were successfully bonded and transferred onto new Si substrate. After chemical etching Si(111) substrate and inductively coupled plasma (ICP) etching the transferred LED film to Si-doped layer, a vertical structure GaN blue LEDs were then fabricated. The characteristics of the lateral structure LED (grown on Si) and the vertical structure LED (bonded on Si) were investigated. It shows the performance of vertical structure LEDs had obviously been improved compared to the lateral structure LEDs and the tensile stress in GaN layer of vertical structure LEDs is smaller than that in lateral structure LEDs.

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.

Vacancy and H Interactions in Nb

The vacancy and H interactions in bcc Nb are important due to their implication in understanding of the H induced damage of Nb metallic membrane used in H2 separation and purification application. Using density functional theory, the vacancy formation energy and vacancy (Vac)-H interaction energies are calculated. The results show that vacancies have a strong trapping effect on H atoms, which lowers the formation energy of Vac-nH clusters substantially. The concentration of Vac-nH clusters is evaluated using a statistical model and the dependence of the concentration on the H-to-M ratio is obtained. It is shown that the concentration of the Vac-nH clusters can be as high as 10−3 at 573K, i.e. one Vac-nH cluster per 1000 atoms, in good agreement with the experimental observations.

Effects of Carrier Gas on Carbon Incorporation in GaN

GaN epitaxial layers were grown on Si (111) substrates by metal organic chemical vapor deposition. Carbon concentrations in the films grown in different ambients were measured by secondary ion mass spectrometry. The results show that the carbon incorporation is strongly dependent on the H2 flow rate when the NH3 flow rate is small, but insensitive to the H2 flow rate when the NH3 flow rate is sufficient large. We conclude that H2 can inhibit the dissociation of NH3 and result in a less active N source; an insufficiently active N source causes more N vacancies, which enhances carbon incorporation.

Structural, Morphology and Optical Properties of Epitaxial ZnO Films Grown on Al2O3 by MOCVD

Epitaxial ZnO films are grown on Al2O3 (0001) by the MOCVD method. These films are high quality wurtzite crystals with (0001) orientation. Big hexagonal crystallites (diameter from several decades to 100 μm) are found on the surface. Inside these crystallites, a stronger luminescence is observed compared with the plain area. Transmission electronic microscopy reveals that the film is thicker inside the hexagonal crystallites than the plain area, and some crystallites are not connected with each other and are slightly rotated with respect to their neighbours.