There has been a lack of research for proper understanding of defects relaxing the strained lattice of InGaN/(Al)GaN quantum wells. This motivates us to find the relation among the defects, the piezoelectric field (FPZ), and the bandgap shrinkage under high injection. In this work, five similar-structure near-ultraviolet (NUV) light-emitting diodes (LEDs) are used to find systematically that the increase of point defects in the sample decreases both the peak wavelength and FPZ. This effect clearly indicates that the strain relaxation is induced by defects. We propose a model that consistently explain the observed changes in macroscopic characterizations of NUV LEDs.
Light-emitting diodes (LEDs) are considered as the most promising candidate for the next-generation lighting. They are also widely used as backlight units (BLU) for displays and light sources for various other applications. To extend the range of applications of the LEDs, the improvement of the LED performance is required. Since LEDs are optoelectronics devices that convert the electrical power into the optical power, the understanding of how photons generated in LEDs behave is most important. In this paper, we have investigated the optical processes in InGaN-based LEDs using electroreflectance (ER) and photocurrent (PC) spectroscopies. We have chosen some factors that affects the optical transition in LEDs and performed the ER and PC spectroscopies by changing those factors.
In InGaN/GaN blue light-emitting diodes (LEDs) widely utilized for general lighting, there exist various material issues that lead to the unwanted nonradiative recombination. In this paper, we utilize various characterization techniques to investigate the nonradiative recombination mechanisms in LED devices. With the characterization techniques such as temperature-dependent external quantum efficiency, current-voltage, and electroluminescence spectra, we show that different nonradiative recombination processes such as the Shockley-Read-Hall recombination and the defect-assisted tunneling can play roles in the LED devices. Information on the dominant nonradiative recombination obtained by these analyses can be used for further improving the quantum efficiency of the device.
The efficiency droop in light-emitting diodes (LEDs) represents a gradual decrease of the internal quantum efficiency
(IQE) with increasing current. Experimentally, the IQE droops are strong functions of material, epitaxial and chip
structures, and operating temperature. Recently, we have proposed an IQE droop model as the saturation of the radiative
recombination rate at low current and subsequent increase in the nonradiative recombination rates at high current. Once
the radiative recombination rate begins to saturate at an active region, the carrier density as well as the nonradiative
recombination rate rapidly increase there. Eventually, the IQE droop appears from the increase in the nonradiative
recombination rate being much larger than that in the radiative one. A dominant nonradiative recombination process is
not solely determined for each LED chip, but it could vary with current level and operating temperature. As temperature
decreases, in general, the IQE droop becomes larger with the peak IQE occurring at an extremely small current level. We
test the droop model by investigating the radiative and nonradiative recombination processes separately from the
cryogenic to room temperature. The characterization methods include comparative efficiency study between
photoluminescence (PL) and electroluminescence (EL), open-circuit voltage under resonant PL excitation, interrelations
of current-voltage-light characteristics, and EL spectra of color-coded quantum wells (QWs). Although a sudden increase
of the nonradiative recombination rate is an apparent cause of the IQE droop, the saturation of the radiative
recombination rate is the common trigger behind the IQE droop issue.
We have grown LED structures on top of a robust n-type GaN template on 8-inch diameter silicon
substrates achieving both a low dislocation density and a 7 um-thick template without crack even at a
sufficient Si doping condition. Such high crystalline quality of n-GaN templates on Si were obtained by
optimizing combination of stress compensation layers and dislocation reduction layers. Wafer bowing of LED
structures were well controlled and measured below 20 μm and the warpage of LED on Si substrate was
found to strongly depend on initial bowing of 8-inch Si substrates. The full-width at half-maximum (FWHM)
values of GaN (0002) and (10-12) ω-rocking curves of LED samples grown on 8-inch Si substrates were 220
and 320 arcsec. The difference between minimum and maximum of FWHM GaN (0002) was 40 arcsec. The
dislocation densities were measured about 2~3×108/cm2 by atomic force microscopy (AFM) after in-situ SiH4
and NH3 treatment. The measured quasi internal quantum efficiency of 8-inch InGaN/GaN LED was ~ 90 %
with excitation power and temperature-dependent photoluminescence method. Under the un-encapsulated
measurement condition of vertical InGaN/GaN LED grown on 8-inch Si substrate, the overall output power of
the 1.4×1.4 mm2 chips representing a median performance exceeded 484 mW with the forward voltage of 3.2
V at the driving current of 350 mA.
Highly efficient InGaN/GaN LEDs grown on 4- and 8-inch silicon substrates comparable to those on sapphire
substrates have been successfully demonstrated. High crystalline quality of n-GaN templates on Si were obtained by
optimizing combination of stress compensation layers and dislocation reduction layers. The full-width at half-maximum
(FWHM) values of GaN (0002) and (10-12) ω-rocking curves of n-GaN templates on 4-inch Si substrates were 205 and
290 arcsec and those on 8-inch Si substrate were 220 and 320 arcsec, respectively. The dislocation densities were
measured about 2~3×108/cm2 by atomic force microscopy (AFM) after in-situ SiH4 and NH3 treatment. Under the unencapsulated
measurement condition of vertical InGaN/GaN LED grown on 4-inch Si substrate, the overall output power
of the 1.4×1.4 mm2 chips representing a median performance exceeded 504 mW with the forward voltage of 3.2 V at the
driving current of 350 mA. These are the best values among the reported values of blue LEDs grown on Si substrates.
The measured internal quantum efficiency was 90 % at injection current of 350 mA. The efficiency droops of vertical
LED chips on Si between the maximum efficiency and the efficiency measured at 1A (56.69 A/cm2) input current was
5%.
We investigate the effect of carrier distribution characteristics in InGaN multiple-quantum-well (MQW) structures on
the efficiency droop of light-emitting diodes (LEDs). Here, three kinds of inhomogeneous carrier distributions are
studied; inhomogeneous carrier distribution in the vertical direction between QWs, that in the horizontal direction of a
QW plane due to the current crowding in the LED chip, and that inside QW materials by carrier localization in the Inrich
areas. It is found, by numerical simulation, that the built-in polarization field in InGaN MQWs makes the hole
distribution between QWs more inhomogeneous, which enhances the efficiency droop. In addition, nonuniform current
spreading is also found to have a significant influence on the efficiency droop by the inhomogenous carrier distribution
in the plane of a QW. When the carrier distribution characteristic is investigated in a microscopic scale, the localization
of carriers in the In-rich areas is expected to reduce the effective active volume where carriers are able to recombine,
and enhances the efficiency droop due to the large increase in the carrier density at inhomogeneously distributed In-rich
regions.
We present our approach to measure the profile of nonuniformly bent GaN epi-wafers grown on sapphire substrates. By
using a laser displacement sensor, the position of the epi-wafer is accurately measured and mapped. From the measured
profile data, analysis of stress distributions over the nonuniformly bent wafer is performed by using a theoretical model.
We show the result of theoretical analysis of how the stress tensors distribute over a wafer. The estimated stress tensors
are related with optical properties such as photoluminescence of the wafer.
We have proposed an efficiency droop model which can comprehensively explain experimental IQE droop phenomena
occurring at different temperatures, materials, and active structures. In our model, carriers are located and recombined
both radiatively and nonradiatively inside randomly distributed In-rich areas of InGaN-based QWs and the IQE droop
originates from the saturated radiative recombination rate and the monotonically increasing nonradiative recombination
rate there. Due to small effective active volume and small density of states of In-rich areas, carrier density is rapidly
increased even at low current density and the radiative recombination rate is easily saturated by different distributions of
electrons and holes in the momentum k-space. A measurement method that can separately estimate the radiative and
nonradiative carrier lifetimes just at room temperature is theoretically developed by analyzing the time-resolved
photoluminescence (TRPL) response. The method is applied to a blue InGaN/GaN QW LED. The experimental results
show that the radiative carrier lifetime increases and the nonradiative carrier lifetime saturates with increasing TRPL
laser power, which is one of direct evidences validating our IQE droop model.
High quality epitaxial growth and uniform current spreading are essential to III-nitride light emitting diodes (LEDs) for
superior wall-plug efficiency and reliability. An analysis method of current spreading based on 3-dimensional circuit
modeling is introduced. We have investigated influences of the current spreading in the lateral-electrode type blue LEDs
of 320 × 320 μm2 size theoretically and experimentally. It is known that the current spreading can be greatly reduced by careful design of electrode pattern. Uniform current spreading is very important to improve electrical and optical
characteristics such as series resistance, efficiency droop, leakage current with operational time leakage current, and
electrostatic discharge (ESD) voltage. A method improving ESD voltage is presented by inserting floating metal near the n-electrode. About 4 times larger ESD voltages are experimentally measured at LEDs with floating metal compared to conventional LEDs without one. The internal quantum efficiency (IQE) is the most important factor affecting overall LED performances. A measurement method of the IQE measurable just at room temperature is proposed and demonstrated. The method utilizes both the time responses of the time-resolved photoluminescence (TRPL) as a function of the excitation femtosecond laser pumping power and their theoretical analysis based on the carrier rate equation.
The improvements in efficiency and reliability are essential to realize high performance light emitting diodes (LEDs). Uniform current spreading is very important to enhance both efficiency and reliability simultaneously. 3-dimensional circuit modeling and analysis method was developed in order to figure out influences of current crowding on performances. The method was applied to the top-surface emitting-type LEDs of 320 320 μm2 size. It was found that the current crowding was closely related to the improvements in series resistance, saturation of light output power, leakage current with operational time, and endurance of electrostatic discharge (ESD). Defects due to high ESD stress were observed at current crowding area. In order to suppress the peak electric field and enhance the ESD voltage, the floating metal was inserted near the n-electrode. About 4 times larger ESD voltages were experimentally measured at LEDs with floating metal structure compared to without one. A LED with the lozenge shape is fabricated to enhance light extraction efficiency over conventional design, i.e., rectangular parallel piped chip. A lozenge shaped LED has an additional advantage of easy chip making since it has natural crystallographic cleavage planes. About 11% larger light extraction efficiency was experimentally obtained in the lozenge shaped LEDs than conventional rectangular chips.
In this study, effects of n-electrode patterns to the current spreading in the active region were analyzed on the blue
vertical light emitting diode (VLED) with GaN/InGaN multi quantum well (MQW). Several n-electrode patterns of the
VLED are designed, analyzed qualitatively, and investigated its effect to current spreading in the active region. A 3-dimensinal circuit model whose parameters are experimentally extracted from an actual VLED chip is adopted for the
quantitative analysis of current spreading. The n-electrode patterns are modeled and simulated by simple electrical
circuits in order to find the current distribution and current-voltage characteristics of devices. Based on theoretical
analysis results, blue VLEDs with different n-electrode patterns were fabricated and a series of measurements were
carried out. Analytic and experimental results for different n-electrode pattern showed quite similar tendencies. Finally,
we proposed some design methodologies for improved current spreading.
A strain analysis model in the pseudomorphically grown epitaxial multilayer system is investigated. Analytical formulas
of strain parameters in each epitaxial layer are derived as the following assumptions: (1) the substrate thickness is finite,
(2) the in-plane lattice constant is the same for all epitaxial layers for dislocation free growth, (3) the stress along the
crystal growth direction is constant, but not necessary zero, (4) the in-plane lattice constant is determined such that the
total strain energy. We find the residual stress affect the electronic properties of epitaxially grown multilayer system even
though in-plane lattice constant is unchangeable compare with no stress along the crystal growth direction.
We investigated the dependency of waveguide structures on ripples of far-field patterns in 405nm GaN-based laser diodes theoretically and experimentally. As the n-type cladding layer thickness decreases, the passive waveguide modes strongly interact with an active layer mode. This suggests that the thicknesses of n-AlGaN/GaN superlattice clad and n-GaN waveguide layers have significant influences on FFP ripples. We successfully obtained very smooth far-field patterns perpendicular to the junction plane by optimizing both n-AlGaN/GaN clad layer thickness and n-GaN waveguide layer thickness.
A highly integrated 10 Gb/s transmitter optical sub-assembly was fabricated and characterized for XFP transceiver. As a
light source, uncooled 1.3 μm high-speed distributed feedback laser diode (DFB-LD) was fabricated and assembled on
AlN sub-mount with a monitoring PD, a matching-resistor, and a bias-Tee with spiral-inductor. A glass sealed metallic
low-loss TO-stem with in-line leads was newly presented. We developed a small-signal equivalent circuit model based
on measured S-parameters in order to verify RF characteristics of LD and passive components. The eye-diagram of 10
Gb/s NRZ patterns with a PRBS 231 -1 was opened clearly without mask violation. At 85°C, -3-dB bandwidth was
measured as high as 11 GHz and 75-km transmission was successfully achieved with very low penalty.
We reported the nonlinear properties of transfer curves of multiple quantum well (MQW) traveling-wave electroabsorption modulators (TW-EAMs) in analog optical link applications. A new method to extract the optical absorption coefficient of TW-EAMs was developed. By using the method, we investigated the dependence of the transfer curvs on both the input optical power and the bias voltage. The relationships between the RF output power and bias voltage as well as RF input power were studied experimentally and theoretically. A SFDR as high as 128 dB-Hz4/5 was successfully obtained by adjusting the bias level as well as optical input power at 10.0 GHz.
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