Electroluminescence Analysis and Simulation of the Effects of Injection and Temperature on Carrier Distribution in InGaN-Based Light-Emitting Diodes with Color-Coded Quantum Wells (original) (raw)
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IET Optoelectronics, 2009
Staggered InGaN quantum wells (QWs) are investigated both numerically and experimentally as improved active region for light-emitting diodes (LEDs) emitting at 520-525 nm. Based on a self-consistent six-band k. p method, band structures of both two-layer staggered In x Ga 12x N/In y Ga 12y N QW and three-layer staggered In y Ga 12y N/In x Ga 12x N/In y Ga 12y N QW structures are investigated as active region to enhance the spontaneous emission radiative recombination rate (R sp) for LEDs emitting at 520-525 nm. Numerical analysis shows significant enhancement of R sp for both two-layer and three-layer staggered InGaN QWs as compared to that of the conventional In z Ga 12z N QW. Significant reduction of the radiative carrier lifetime contributes to the enhancement of the radiative efficiency for both two-layer and three-layer staggered InGaN QW LEDs emitting at 520-525 nm. Three-layer staggered InGaN QW LEDs emitting at 520-525 nm was grown by metal-organic chemical vapour deposition (MOCVD) by employing graded-temperature profile. Power density-dependent cathodoluminescence (CL) measurements show the enhancement of peak luminescence by up to 3 times and integrated luminescence by 1.8-2.8 times for the three-layer staggered InGaN QW LED. Electroluminescence (EL) output power of the staggered InGaN QW LED exhibits 2.0-3.5 times enhancement as compared to that of the conventional InGaN QW LED. The experimental results show the good agreement with theory.
Carrier transport and emission efficiency in InGaN quantum-dot based light-emitting diodes
Nanotechnology, 2017
We present a study of blue III-nitride light-emitting diodes (LEDs) with multiple quantum well (MQW) and quantum dot (QD) active regions (ARs), comparing experimental and theoretical results. The LED samples were grown by metalorganic vapor phase epitaxy, utilizing growth interruption in the hydrogen/nitrogen atmosphere and variable reactor pressure to control the AR microstructure. Realistic configuration of the QD AR implied in simulations was directly extracted from HRTEM characterization of the grown QD-based structures. Multi-scale 2D simulations of the carrier transport inside the multiple QD AR have revealed a non-trivial pathway for carrier injection into the dots. Electrons and holes are found to penetrate deep into the multi-layer AR through the gaps between individual QDs and get into the dots via their side edges rather than via top and bottom interfaces. This enables a more homogeneous carrier distribution among the dots situated in different layers than among the later...
Spontaneous emission and characteristics of staggered InGaN quantum-well light-emitting diodes
Quantum Electronics, IEEE …, 2008
A novel gain media based on staggered InGaN quantum wells (QWs) grown by metal-organic chemical vapor deposition was demonstrated as improved active region for visible light emitters. Fermi's golden rule indicates that InGaN QW with step-function like In content in the well leads to significantly improved radiative recombination rate and optical gain due to increased electron-hole wavefunction overlap, in comparison to that of conventional InGaN QW. Spontaneous emission spectra of both conventional and staggered InGaN QW were calculated based on energy dispersion and transition matrix element obtained by 6-band formalism for wurtzite semiconductor, taking into account valence-band-states mixing, strain effects, and polarization-induced electric fields. The calculated spectra for the staggered InGaN QW showed enhancement of radiative recombination rate, which is in good agreement with photoluminescence and cathodoluminescence measurements at emission wavelength regime of 425 and 500 nm. Experimental results of light-emitting diode (LED) structures utilizing staggered InGaN QW also show significant improvement in output power. Staggered InGaN QW allows polarization engineering leading to improved luminescence intensity and LED output power as a result of enhanced radiative recombination rate.
Journal of Physics D: Applied Physics, 2008
This paper discusses radiative recombination efficiency in electroluminescence of InGaN-based light-emitting diodes prepared on the (1 01 0) plane. Radiative efficiency was studied over a wide range of temperatures and drive currents on four types of LED samples with different InGaN active-layer thicknesses. Efficiency was minimally affected by active-layer thickness, yet was a strong function of temperature and current. Efficiency reduction at high current was observed on these LEDs, which confirms strain-induced electric polarization fields are not a dominant mechanism. Luminescence intensity was found to be proportional to the square root of current at low temperature. Acceptor freeze-out was suggested to induce hole depletion at increased current; shortage of holes resulted in reduced efficiency and triggered off electron injection into the p-type layer to sustain total current. Injected electrons were shown to lead to the square-root relationship by solving rate equations.
Journal of Applied Physics, 2006
The temperature dependence of the electroluminescence ͑EL͒ spectral intensity has been investigated in detail between T = 20 and 300 K at various injection current levels for a set of two blue InGaN / GaN multiple-quantum-well ͑MQW͒ light-emitting diodes ͑LEDs͒ with and without an additional n-doped In 0.18 Ga 0.82 N electron reservoir layer ͑ERL͒. The radiative recombination efficiency of the main blue emission band ͑ϳ480 nm͒ is found to be significantly improved at all temperature regions and current levels when the additional ERL is introduced. For high injection currents I f , i.e., large forward bias voltages V f , a quenching of the EL intensity is observed for T Ͻ 100 K for both LED structures, accompanying appearance of short-wavelength satellite emissions around 380-430 nm. Furthermore, the low-temperature intensity reduction of the main EL band is stronger for the LED without the ERL than with the ERL. For low I f , i.e., small V f , however, no quenching of the EL intensity is observed for both LEDs even below 100 K and the short-wavelength satellite emissions are significantly reduced. These results of the main blue emission and the short-wavelength satellite bands imply that the unusual evolution of the EL intensity with temperature and current is caused by variations of the actual potential field distribution due to both internal and external fields. They significantly influence the carrier capture efficiency by radiative recombination centers within the active MQW layer and the carrier escape out of the active regions into high-energy recombination centers responsible for the short-wavelength satellite emissions.
Efficiency and efficiency retention in InGaN LEDs has recently received considerable attention. In this realm, we investigated internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) of c-plane InGaN LEDs designed for emission at $420 nm from the active region which contains multiple quantum wells (MQWs) of different barrier height (either In 0.01 Ga 0.99 N or In 0.06 Ga 0.94 N barriers) and thickness (3 and 12 nm) as well as a 9-nm double heterostructure (DH). Pulsed electroluminescence (EL) and laser excitation powerdependent measurements indicated that both the relative EQE and the IQE were enhanced due to the incorporated two-layer InGaN stair-case electron injector (SEI) with indium mole fraction steps of 4 and 8% as compared to the conventional AlGaN electron blocking layer (EBL). Furthermore, the lowered In 0.06 Ga 0.94 N interwell barriers (LB) instead of the traditional In 0.01 Ga 0.99 N barriers improved the EQE and the IQE of MQW LEDs. Specifically, the MQW LEDs with the 6period 2-nm In 0.2 Ga 0.8 N quantum well and 3-nm In 0.06 Ga 0.94 N barrier structure showed 6% higher IQE at an injected carrier density of 6 Â 10 18 cm À3 and 35% higher EQE as compared to that of the same structure with a higher In 0.01 Ga 0.99 N barrier. The DH LEDs showed 30% higher EQEs compared to MQW LEDs, albeit at a relatively higher injection current density of 150 A/cm 2 . The relatively low EQE in the DH LEDs at low injection levels is attributed to spatial separation of electrons and holes due to confinement in the interfacial triangular well and thus the associated decrease in radiative efficiency and possible increase in nonradiative recombination due to degradation of material quality with increasing InGaN layer thickness.
Influence of temperature on different optoelectronic characteristics of InGaN light emitting diodes
Optical and Quantum Electronics, 2017
We have measured the electroluminescence (EL) and carrier lifetime characteristics in InGaN/Sapphire purple light emitting diode (LED), namely, UV3TZ-405-30 in a temperature range from 350 to 120 K and have compared them with those of a similar LED (UV3TZ-395-15) but with different Indium concentration, measured earlier. While it is found that for the present device the EL intensity decreases drastically with lowering of temperature after reaching a maximum (99%) at 228 K, this is markedly different from the previous device where intensity continues to increase monotonically till lowest temperature. This qualitatively distinct temperature dependence indicates difference in nature of localisation of carriers in the multiple quantum wells for varying Indium content in the two devices. The light-current-temperature data have been analysed in terms of the semiconductor rate equations to determine different optoelectronic properties. Next, estimating the ideality factor from the current-voltage (I-V) measurements, the effective carrier lifetime has been evaluated from the open circuit voltage decay process. Using the above measurements, the temperature dependence of the internal quantum efficiency of the device has been calculated and it is found to attain a maximum value of 99.88% at 228 K. Unlike all previous calculations, a unique feature of the present approach has been to include the effect of temperature dependence of the radiative recombination coefficient (B) in the rate equation analysis. Finally, a comparative study of the temperature dependence of the different optoelectronic properties of both devices is presented with and without this effect.
Journal of the Electrochemical Society, 2012
The aim of this study is to determine the emission and carrier transport characteristics of Trimethylindium (TMIn)-treated InGaN green light emitting diodes (LEDs) by using quantum efficiency and time-resolved electro-luminescence measurements. As TMIn treatment time increased, a more homogeneous indium composition and low V-shaped defect density lead to slightly blue-shifted peak position, narrower spectrum width, and better luminescence efficiency. In addition, the ns-scale response time shows efficient carrier injection and carrier transport. The shorter response times of the longer-TMIn-treated LED suggest that a lower V-shaped density is beneficial to carrier injection into the quantum wells and that a slight carrier localization helps carrier recombination. Furthermore, a μs-scale decay time represents inefficient carrier recombination in the active region. The longer the TMIn treatment time, the shorter the response time, the faster the radiative decay rate, and the slower the nonradiative decay rate. With a forward applied voltage, lower V-shaped defect density, un-reduced polarization field, carrier delocalization, and weaker Auger recombination in the TMIn-treated samples lead to the inevitable efficiency droop. The resulting recombination dynamics are correlated with the device characteristics and performance of the TMIn-treated LEDs. The research results provide important information to solve the efficiency droop of LEDs.
Japanese Journal of Applied Physics, 2008
The phenomena of electroluminescence in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with an n-AlGaN layer and a superlattice of 10 periods of InGaN (10 Å)/GaN (15 Å) serving as the electron tunneling layer (ETL) have been investigated in detail over a broad temperature range from 20 to 300 K at various injection currents. Compared with conventional LEDs with a well-designed ETL, quantum efficiency and temperature insensitivity are found to be improved when an n-AlGaN layer is inserted. This is attributed to the localization effect of the n-AlGaN layer being stronger than that of the ETL layer, as analyzed using the Varshini formula and band-tail model. Nevertheless, the inserted ETL layer with the purpose of improving the carrier injection into the active layer not only increases the carrier recombination quantity, which leads to a marked increase in output light emission intensity, but also reduces the light emission intensity compared with sample with the n-AlGaN layer. Consequently, inserting a blocking layer between an active layer and a p-GaN layer may increase the output light emission intensity of the sample with an ETL.