Y. Xi

Junction–temperature measurement in GaN ultraviolet light-emitting diodes using diode forward voltage method

A theoretical model for the dependence of the diode forward voltage (Vf) on junction temperature (Tj) is developed. An expression for dVf∕dT is derived that takes into account all relevant contributions to the temperature dependence of the forward voltage including the intrinsic carrier concentration, the band-gap energy, and the effective density of states. Experimental results on the junction temperature of GaN ultraviolet light-emitting diodes are presented. Excellent agreement between the theoretical and experimental temperature coefficient of the forward voltage (dVf∕dT) is found. A linear relation between the junction temperature and the forward voltage is found.

Influence of junction temperature on chromaticity and color-rendering properties of trichromatic white-light sources based on light-emitting diodes

Trichromatic white-light sources based on light-emitting diodes (LEDs) offer a high luminous efficacy of radiation, a broad range of color temperatures and excellent color-rendering properties with color-rendering indices (CRIs) exceeding 85. An analysis of the luminous efficacy and CRI of a trichromatic light source is performed for a very broad range of wavelength combinations. The peak emission wavelength, spectral width, and the output power of LEDs strongly depend on temperature and the dependencies for red, green, and blue LEDs are established. A detailed analysis of the temperature dependence of trichromatic white LED sources reveals that the luminous efficacy decreases, the color temperature increases, the CRI decreases and the chromaticity point shifts towards the blue as the junction temperature increases. A high CRI>80 can be maintained, by adjusting the LED power ratio so that the chromaticity point is conserved.

Junction and carrier temperature measurements in deep-ultraviolet light-emitting diodes using three different methods

The junction temperature of AlGaN ultraviolet light-emitting diodes emitting at 295nm is measured by using the temperature coefficients of the diode forward voltage and emission peak energy. The high-energy slope of the spectrum is explored to measure the carrier temperature. A linear relation between junction temperature and current is found. Analysis of the experimental methods reveals that the diode-forward voltage is the most accurate (±3°C). A theoretical model for the dependence of the diode forward voltage (Vf) on junction temperature (Tj) is developed that takes into account the temperature dependence of the energy gap. A thermal resistance of 87.6K∕W is obtained with the device mounted with thermal paste on a heat sink.

Junction temperature in light-emitting diodes assessed by different methods

The junction temperature of red (AlGaInP), green (GaInN), blue (GaInN), and ultraviolet (GaInN) light-emitting diodes (LEDs) is measured using the temperature coefficients of the diode forward voltage and of the emission-peak energy. The junction temperature increases linearly with DC current as the current is increased from 10 mA to 100 mA. For comparison, the emission-peak-shift method is also used to measure the junction temperature. The emission-peak-shift method is in good agreement with the forward-voltage method. The carrier temperature is measured by the high-energy-slope method, which is found to be much higher than the lattice temperature at the junction. Analysis of the experimental methods reveals that the forward-voltage method is the most sensitive and its accuracy is estimated to be ± 3°C. The peak position of the spectra is influenced by alloy broadening, polarization, and quantum confined Stark effect thereby limiting the accuracy of the emission-peak-shift method to ±15°C. A detailed analysis of the temperature dependence of a tri-chromatic white LED source (consisting of three types of LEDs) is performed. The analysis reveals that the chromaticity point shifts towards the blue, the color-rendering index (CRI) decreases, the color temperature increases, and the luminous efficacy decreases as the junction temperature increases. A high CRI > 80 can be maintained, by adjusting the LED power so that the chromaticity point is conserved.

Junction Temperature in Ultraviolet Light-Emitting Diodes

The junction temperature and thermal resistance of AlGaN and GaInN ultraviolet (UV) light-emitting diodes (LEDs) emitting at 295 and 375 nm, respectively, are measured using the temperature coefficient of diode-forward voltage. An analysis of the experimental method reveals that the diode-forward voltage has a high accuracy of ±3°C. A comprehensive theoretical model for the dependence of diode-forward voltage (Vf) on junction temperature (Tj) is developed taking into account the temperature dependence of the energy gap and the temperature coefficient of diode resistance. The difference between the junction voltage temperature coefficient (dVj/dT) and the forward voltage temperature coefficient (dVf/dT) is shown to be caused by diode series resistance. The data indicate that the n-type neutral regions are the dominant resistive element in deep-UV devices. A linear relationship between junction temperature and current is found. Junction temperature is also measured by the emission-peak-shift method. The high-energy slope of the spectrum is explored in the measurement of carrier temperature.

Very high quality AlN grown on (0001) sapphire by metal-organic vapor phase epitaxy

Very high quality AlN epitaxially grown on (0001) sapphire by metal-organic vapor phase epitaxy is investigated by atomic force microscopy, x-ray diffraction, and photospectrometry. A clear and continuously linear step-flow pattern with sawtooth shaped terrace edges is observed in atomic force microscopic images. Triple-axis x-ray rocking curves show a full width at half maximum of 11.5 and 14.5arcsec for the (002) and (004) reflections, respectively. KOH etching reveals an etch-pit density of 2×107cm−2, as deduced from atomic force microscopy measurements. The optical transmission spectrum shows a sharp absorption edge with a band gap energy of 6.10eV.

Light Extraction in GaInN Light-Emitting Diodes using Diffuse Omnidirectional Reflectors

A theoretical and experimental analysis of light extraction in GaInN light-emitting diodes LEDs employing diffuse omnidirectional reflectors is presented. The diffuse omnidirectional reflector consists of GaN, a Ni/Au current spreading layer, a SiO2 layer roughened by Ar ion etching, and a Ag layer. Randomly distributed polystyrene spheres are used as an etch mask. The diffusely reflected power is enhanced by two orders of magnitude for a roughened reflector surface compared with a planar surface. The GaInN LEDs with diffuse omnidirectional reflectors show a higher light output 3.3% and a lower angular dependence of emission than LEDs with specular reflectors. The enhancement is attributed to reduced trapping of light within the high-index GaN semiconductor.

Quantitative assessment of diffusivity and specularity of surface-textured reflectors for light extraction in light-emitting diodes

Surface-textured reflectors fabricated by natural lithography and ion beam etching have a specular and a diffusive component of the reflectivity. The diffusely and specularly reflected powers of surface-textured reflectors are measured and analyzed quantitatively in terms of a theoretical model. The diffusive-power-to-total-power ratio is determined and shown to strongly depend on the surface texture. The light extraction efficiency from a waveguide clad by a partially diffuse reflector is analyzed and shown to be enhanced.

Second-order optical nonlinearity in bulk PbO/B2O3 glass

Abstract By investigating the second-harmonic generation (SHG) of the bulk PbO/B 2 O 3 glass samples with different compositions after thermal poling, it was found that there was an optimal poling temperature for each sample with different compositions and there was also a relation between optimal poling temperature and glass transition temperature. At their own optimal poling temperatures, the samples had different frequency doubling efficiencies with the same applied voltage. We also found that the frequency doubling efficiency of PbO/B 2 O 3 glass increased with the increase of poling voltage. An induced dipole model was proposed to explain the super-quadratic relation between the SHG intensity and the poling voltage.

Recombination dynamics in ultraviolet light-emitting diodes with Si-doped AlxGa1−xN∕AlyGa1−yN multiple quantum well active regions

Ultraviolet (UV) light-emitting diodes with AlxGa1−xN∕AlyGa1−yN multiple quantum well active regions, doped in the barriers with different Si doping levels, show a sharp near-band edge emission line (UV luminescence). Some samples have a broad subband gap emission band centered at about 500 nm (green luminescence) in addition to the near-band edge emission. The electroluminescence intensities of the UV and green emission line are studied as a function of the injection current. For the sample grown on the AlN substrate under optimized growth conditions, the UV luminescence intensity increases linearly with the injection current, following a power law with an exponent of 1.0, while the green luminescence intensity increases sublinearly with the injection current. On the contrary, the samples grown on the sapphire substrate show a superlinear (to the power of 2.0) and linear (to the power of 1.0) dependence on the injection current for the UV and green luminescence, respectively. A theoretical model is propo...