Lihong Zhu

Study of temperature sensitive optical parameters and junction temperature determination of light-emitting diodes

We investigate the current and temperature dependence of GaN-based high power blue light-emitting diodes and identify a set of temperature sensitive optical parameters (TSOPs) that can provide a real-time solution for determining the junction temperature (Tj). The relationships among Tj, forward current and TSOPs, “center of mass” wavelength, and, in particular, full width at half maximum (FWHM) have been studied, and the relevant mathematic models have been developed. The analysis indicates that using FWHM may yield higher accuracy than using other parameters as TSOPs.

Optimization Studies of Two-Phosphor-Coated White Light-Emitting Diodes

Three-hump InGaN-based white light-emitting diodes (LEDs) precoated with traditional yellow/green phosphors and red-emitting quantum dots (QDs), have been numerically investigated. Under variations of eight correlated color temperatures (CCTs), three wavelengths, two bandwidths, and two peak heights, optimal results of luminous efficacy radiation (LER) and color rendering index (CRI) are identified and retained through filtering off billions of unqualified candidates. These results include LER = 390 lm/W and CRI = 90 [chromaticity difference (Duv)<;0.0054] at CCT = 3000 K. In addition, our photometric and colorimetric sensitivity studies provide the dependence of LER, CRI, CCT, and Duv on LED spectral parameters affected by operating temperatures. Finally, we have discovered that higher instabilities may be induced for cool white LEDs (CCT = 6500 K) than for warm white LEDs (CCT = 3000 K) within the analysis of CCT versus spectral parameters.

Improvements on Remote Diffuser-Phosphor-Packaged Light-Emitting Diode Systems

By modifying traditional remote phosphor-diffuser-packaged light-emitting diode systems, we have managed to increase the luminous efficacy from 145.7 to 162.3 lm/W. One mechanism responsible for this achievement is associated with randomizing the directions of light beams transmitting through an interior diffuser, whose position is optimized based on an overall merit. The other mechanism is identified as the gradual attenuation of the undesirable blue-ring image along the distance from the diffuser toward the phosphor base. The identification of these two mechanisms is verified by luminous efficacy measurements, 3-D image plots, and combined Monte Carlo algorithm ray tracing simulation results. In addition, merits related to correlated color temperatures and issues pertaining to costs are briefly discussed.

Junction-Temperature Determination in InGaN Light-Emitting Diodes Using Reverse Current Method

A method is presented in this study to determine the junction temperature (<i>T</i>_j) of LED in terms of the relationship between the diode reverse current (<i>I</i>_R) and <i>T</i>_j . A theoretical model for the dependence of <i>I</i>_R on <i>T</i>_j is derived on the basis of the Shockley equation and is validated by our experimental results. The method is compared with the conventional forward voltage method, and its advantages have been identified.

Efficiency Droop Improvement in InGaN/GaN Light-Emitting Diodes by Graded-Composition Multiple Quantum Wells

InGaN/GaN light-emitting diodes (LEDs) with a graded-composition multiple quantum well (GQW) were designed not only to investigate the effect of polarization field on the efficiency droop in InGaN/GaN multiple quantum wells (MQWs) but to find out possible solutions to prevent or reduce the efficiency droop in GaN-based LEDs as well. Pulsed electroluminescence measurement, to avoid heating effects partly, revealed that the output intensity peak occurs at ~280A/cm2 current density for the GQW structure, whereas for a conventional structure, it occurs at a much lower current density of ~150A/cm2. The relative quantum efficiency indicates that the efficiency droop in GQW LEDs was reduced effectively due to the reduction in polarization field in the active layer. In the test GQW structure with inverse indium composition variation direction, electrons escape from the active region, which is in company with rapid saturation of output power and decrease in efficiency observed at high current density. It indicates that at high current density, the electron leakage is obvious. These results suggest that the polarization field in the active layer and the consequent electron leakage are probably the main mechanism responsible for the efficiency droop at high current injection levels.

Studies of Scotopic/Photopic Ratios for Color-Tunable White Light-Emitting Diodes

Three- and four-hump InGaN-based white light-emitting diodes (LEDs) have been computationally investigated. The investigation includes three-hump LEDs precoated with green- and red-emitting quantum dots (QDs) and four-hump LEDs precoated with green-, yellow-, and red-emitting QDs. Results show scotopic/photopic (S/P) ratios 3.80 and color rendering indices (CRIs) ≥ 70 for three-hump LEDs and S/P ratios 3.90 and CRIs ≥ 70 for four-hump LEDs under a correlated color temperature (CCT) of 45 000 K. Furthermore, for both three- and four-hump LEDs under the condition of CCT 5000 K, optimal results in this paper exhibit capabilities of exceeding S/P = 2.50 and simultaneously satisfying the criterion of CRI ≥ 70. Our simulation procedure is conducted under variations of nine CCTs, several wavelengths, and peak heights. Optimized results of S/P ratios and CRIs are identified and retained through filtering off a great number of unqualified candidates. Finally, present findings may help improve the quality of eye visions pertaining to LED illumination and help reduce the electrical energy consumption related to LED usage.

Tailoring the hole concentration in superlattices based on nitride alloys

By introducing Mg-doped InGaN/AlGaN strained-layer superlattice (SL) as p-type layer, the performance of p-type Ohmic contact is improved as compared with AlGaN/GaN and InGaN/GaN SLs. InGaN/AlGaN SL yields higher hole concentration due to larger oscillation of the valence band edge and smaller activation energy. The calculated average hole concentration in InGaN/AlGaN SL shows a twofold increase compared to that in AlGaN/GaN SL at the same Mg-doped level. The measured sheet hole density for ten periods of InGaN/AlGaN SL is as high as 4.4×1014 cm−2. Finally, Ni/Au contacts on Mg-doped InGaN/AlGaN SL with specific contact resistance of 7.3×10−5 Ω cm2 are realized.

Determining Junction Temperature in InGaN Light-Emitting Diodes Using Low Forward Currents

We propose an experimental method that determines junction temperatures in light-emitting diodes by measuring currents while holding the low forward voltages constant. In this procedure, we first calibrate current-temperature-rela- tionship parameters under the condition of negligible thermal generation. With one of the two parametric values, we discover the existence of a forward voltage peak that yields most sensitive measurements of the junction temperature. Results show a nearly linear relationship between the algorithmic currents and temperature reciprocals with high testing precision.

Improved Quantum Efficiency in Semipolar

We investigated the comparative structural and optical properties of semipolar InGaN/GaN multiple quantum wells (MQWs) grown on the (1̅101) facet GaN/sapphire substrate by metal-organic chemical vapor deposition using lateral epitaxial overgrowth. The scanning electron microscopy (SEM), photoluminescence (PL), and temperature-varying time-resolved photoluminescence measurement were performed to investigate the structure and optical properties. The cross-sectional SEM image shows that the stripe triangular structure of the QW with semipolar (1̅101) planes is obtained as sidewall facets with the mask stripes aligned along the GaN α-axis. The structural and optical advantages of semipolar orientations were confirmed by a modurate shift of the PL peak energy, higher internal quantum efficiency, and lower radiative recombination lifetime than the MQWs on (0001) GaN grown by conventional methods. The results were obtained because of the reduced polarization fields in semipolar InGaN/GaN MQWs comparing with that in polar (0001) MQWs.

(1\bar{1}01)

The barrier thickness dependences of the optical properties of In-graded InGaN/GaN quantum wells (QWs) are studied by photoluminescence measurement and theoretical calculation. The internal quantum efficiencies (IQEs) of the In-graded QWs with 8 and 16 nm barrier thicknesses and the conventional QWs with 16 nm barrier thickness are 35.0, 31.2, and 22.8% at 300 K, respectively. Besides, the In-graded QWs with 8 nm barrier thickness obtain the shortest radiative lifetime at 10–300 K. Calculation results demonstrate that combining the In-graded QWs with thinner barriers effectively increases the IQE by reducing the polarization field of the InGaN/GaN QWs.