Tong Yuzhen

Optical design of adjustable light emitting diode for different lighting requirements

Light emitting diode (LED) sources have been widely used for illumination. Optical design, especially freedom compact lens design is necessary to make LED sources applied in lighting industry, such as large-range interior lighting and small-range condensed lighting. For different lighting requirements, the size of target planes should be variable. In our paper we provide a method to design freedom lens according to the energy conservation law and Snell law through establishing energy mapping between the luminous flux emitted by a Lambertian LED source and a certain area of the target plane. The algorithm of our design can easily change the radius of each circular target plane, which makes the size of the target plane adjustable. Ray-tracing software Tracepro is used to validate the illuminance maps and polar-distribution maps. We design lenses for different sizes of target planes to meet specific lighting requirements.

Deflection Reduction of GaN Wafer Bowing by Coating or Cutting Grooves in the Substrates

GaN films on sapphire substrates are obtained using the metal-organic chemical vapor deposition growth technique. We present two methods to reduce the GaN wafer bowing caused by the mismatch of the thermal expansion coefficients (TECs) between the film and the substrate. The first method is to use coating materials on the back side of the substrate whose TECs are smaller than that of the GaN films. The second is to cut grooves on the back side of the sapphire substrate and filling the grooves with appropriate materials (e.g., tungsten, silicon nitride). For each method, we minimize wafer bowing and even reduce it to zero. Moreover, the two methods can reduce stress concentration and suppress the propagation of cracks in the GaN/sapphire structure.

Fabrication of extremely thermal-stable GaN template on Mo substrate using double bonding and step annealing process

A new layer transfer technique which comprised double bonding and a step annealing process was utilized to transfer the GaN epilayer from a sapphire substrate to a Mo substrate. Combined with the application of the thermal-stable bonding medium, the resulting two-inch-diameter GaN template showed extremely good stability under high temperature and low stress state. Moreover, no cracks and winkles were observed. The transferred GaN template was suitable for homogeneous epitaxial, thus could be used for the direct fabrication of vertical LED chips as well as power electron devices. It has been confirmed that the double bonding and step annealing technique together with the thermal-stable bonding layer could significantly improve the bonding strength and stress relief, finally enhancing the thermal stability of the transferred GaN template.

Low temperature photoluminescence properties of InGaN films grown on (0 1 1 2) Al2O3 and (0 0 0 1) Al2O3 substrates by low pressure MOVPE

Abstract Low temperature photoluminescence (PL) properties of InGaN films grown on (0 1 1 2) Al2O3 and (0 0 0 1) Al2O3 substrates by low pressure MOVPE were studied at 11 K. In the PL spectrum of a In 0.11 Ga 0.89 N AlN buffer/(0 1 1 2)Al2O3 film, only one near band edge emission with wavelength 371.4 nm was observed in the range from 350 nm to 630 nm. For InxGa1−xN/GaN/GaN buffer/(0 0 0 1)Al2O3 films with x = 0.02 and 0.035, similar PL spectra of GaN films, the recombinations of an exciton bound to a neutral donor (I2 lines) were observed and seem to be dominant. The peak energy positions were 3.467 eV, 3.454 eV and 3.449 eV for x = 0, 0.02 and 0.03, respectively. It was found that FWHM roughly increase as x increases. Donor-Acceptor (D-A) pair combination near 3.27 eV and their LO-phonon replica (near 3.18 eV) also appeared in the PL spectra. The LO-phonon energy of InxGa1−xN shift to high energy compared to GaN. It is suggested that the D-A pair recombination contributes to the transitions from a shallow donor to a carbon-related acceptor level.

Numerical study on jet effect in the vertical showerhead MOCVD reactor

Numerical simulations are conducted about jet effects in the vertical showerhead MOCVD reactor. By varying the reactor height, nozzle spacing, jet speed and susceptor rotating speed, the flow, temperature and concentration fields in the reactor are simulated, so as to gain insights into the jet effect in vertical MOCVD reactor. It is concluded that (1) in the reactor chamber below the showerhead, velocity, temperature and concentration are all in periodic fluctuation, and the fluctuations decay gradually from the susceptor center to the edge; (2) the vertical velocity at the susceptor center is larger than that in the edge, the precursor concentration in the center is higher than that in the edge, and the temperature in the center is lower than that in the edge; (3) increasing the reactor height and the susceptor rotating speed, decreasing the nozzle spacing and the jet outlet velocity, the radial fluctuations of both axial jet velocity and precursor concentration will be depressed.

Influence of Growth Temperature and Trimethylindium Flow of InGaN Wells on Optical Properties of InGaN Multiple Quantum-Well Violet Light-Emitting Diodes

An InGaN multiple-quantum-well (MQW) violet-light-emitting diode (LED) is grown by low-pressure metalorganic chemical vapour deposition. It is found that photoluminescence wavelength of the InGaN MQW violet LED is lengthened with increasing growth temperature and with the increasing trimethylindium flow of the InGaN wells. The electroluminescence peak wavelength of the violet LED are about 401 nm with full width at half maximum of 14 nm, and the output power in injection current of 20 mA at room temperature is 4.1 mW.

P-Type GaN Directly Grown by Low Pressure Metalorganic Vapor Phase Epitaxy

P-type GaN was grown on Al3O2 substrate by low pressure metalorganic vapor phase epitaxy without any post treatment. The p-type conduction of as-grown GaN has the typical characteristics of a light-doped or high-compensation semiconductor, that is a hole concentration of 2.2 × 1017 cm-3 at 77 K, which changes to n-type with an electron concentration of 2.7 × 1017 cm-3 at room temperature. After thermal annealing under a N2 ambient, it showed the characteristics of a heavy-doped semiconductor and the hole concentration was enhanced reaching a hole concentrations of 4.2 × 1017 cm-3 at 77 K and 5.7 × 1017 cm-3 at room temperature.