Development of green, yellow, and amber light emitting diodes using InGaN multiple quantum well structures

InGaN/GaN light emitting diode (LED) structures with different GaN barrier growth temperatures have been grown by metalorganic chemical vapor deposition (MOCVD). Atomic Force Microscopy (AFM)-Conductive AFM (CAFM) analysis has been performed to study the submicron scale electrical properties of the LED structures. The effect of high O impurity concentration, introduced during the low temperature barrier layer growth, on the MQW LED structure has been explored. It is observed that (VGa-ON)2- point defects formed in the MQW layer serve as deep-level traps and lead to defect-assisted tunneling. Increasing barrier growth temperature decreases O impurity incorporation and thus improves the device performance. INTRODUCTION InGaN/GaN MQW structures grown by MOCVD have been widely studied in the solid state lighting field for the past few decades. The role of introduced impurities and point defects formed during the epitaxial growth process and their effects on the optical properties of MQW structures have drawn a great deal of attention. Several previous investigations have been done on the effects of GaN barrier growth temperature on the characteristics of InGaN/GaN MQWs [1-3], in which the InGaN quantum wells were grown at low temperature from 700-760oC, while the growth temperature of the GaN barriers was varied over the range of 700-950oC. It was suggested that GaN barriers grown at temperatures below 800oC leads to the generation defects and In inclusions and higher barrier growth temperature up to 910oC improves the crystal quality of GaN barrier by suppressing the formation of deep level related defects. In the present work, the electrical properties of InGaN/GaN LED structure with same barrier growth temperature as the well temperature is studied. The effects of induced O impurity are discussed.EXPERIMENT The InGaN/GaN MQW and LED samples were grown on (0001) sapphire substrates with low temperature GaN buffer layers using Veeco K465i MOCVD reactor. 2.2µm undoped GaN layer and 2.7µm n-GaN layer were grown at 1085oC. 4 pairs of In0.14Ga0.86N/GaN (2.6 nm/10 nm) quantum wells were grown at 760oC for LED structure Al. 6 pairs of such quantum wells were grown at 760oC and 860oC for the InGaN wells and GaN barriers, respectively, for LED structure B1. 20nm p-AlGaN layer and 95nm p-GaN layer were grown at 980oC on top of the MQW structures to form the LED structures. AFM analysis was done on a Bruker Icon system and by using TESPA probe. CAFM module is mounted to the AFM system for conductivity studies of the sample. Extended-tunneling AFM tip holder and PFTUNA probes are used for AFM-CAFM analysis.EXPERIMENT RESULTS An AFM-CAFM study on LED structure with various reverse biases has been done on A1 and shown in Figure 2. Notice that in Figure (b) and (c), the color bar is from 450nA to 300nA, while 450nA to -400nA in Figure (d). High density of spots of early breakdown has been observed when the reverse bias increases. The high reverse current at those spots may owe to the defect-assisted tunneling by deep-level traps located in the MQW active layer [4]. This is consistent with the high O concentration and high point defects density in the MQW layer reported in Ref. [3]. These point defects cause leakage and deteriorate the device performance [5].CONCLUSION In summary, the InGaN/GaN MQW LED structure with 760oC quantum barrier growths has been investigated by AFM-CAFM system to study the submicron electrical properties of the sample. High density of spots of early breakdown caused by defect-assisted tunneling by (VGa-ON)2- complex point defects located in the MQW layer. These point defects cause leakage and deteriorate the device performance.

In this study, we would like to tune the color temperature of the high power phosphor Light-Emitting Diodes (LEDs) with the single LED of red, green, yellow and blue, respectively. The starting color temperature of the white phosphor LED will be set at 7500K (D75 white light), then changing the voltage of the single LED of the red, green, yellow and blue, respectively, to find the best tuning function for the color temperature and luminous efficiency. These results exhibited that changing the voltage of red LED had the broader color temperature from 7500 K to 1500 K and low luminous variation from 54 to 41 lm /W than green and blue LEDs. Though the green and yellow LEDs had the low luminous variation from 47 to 51 lm /W and from 40 to 47 lm /W, respectively, but the color temperature of green and yellow LEDs could only fine turn from 7500K to 8200K and 4700K to 7500K, respectively. The luminous efficiency had rapid variation from 27 to 42 lm /W and out of the color temperature of 20000K for the voltage tuning of blue LED. Therefore, the voltage tuning of red LED could show the warm color temperature for the human life. The variation of color shifts (Δuv) of red and green LEDs changed from small to large with the increasing of the enhanced voltage. But the variation of Δuv of the yellow and blue LED changed nothing with the increasing of the enhanced voltage.

Structural and optical characteristics of InN/GaN multiple quantum wells grown by metalorganic chemical vapor deposition

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 InxGa1−xN/InyGa1−yN QW and three-layer staggered InyGa1−yN/InxGa1−xN/InyGa1−yN QW structures are investigated as active region to enhance the spontaneous emission radiative recombination rate (Rsp) for LEDs emitting at 520–525 nm. Numerical analysis shows significant enhancement of Rsp for both two-layer and three-layer staggered InGaN QWs as compared to that of the conventional InzGa1−zN 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.

Three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520–525 nm were grown by metal-organic chemical vapor deposition by employing graded growth-temperature profile. The use of staggered InGaN QW, with improved electron-hole wave functions overlap design, leads to an enhancement of its radiative recombination rate. Both cathodoluminescence and electroluminescence measurements of three-layer staggered InGaN QW LED exhibited enhancements by 1.8–2.8 and 2.0–3.5 times, respectively, over those of conventional InGaN QW LED.

GaN-based light emitting diodes (LEDs) with a total thickness of 4.8 µm have been grown by metal–organic chemical vapor deposition on 4-in. Si(111) substrate. The structural property has been revealed by the measurements of high-resolution X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It can be clarified that the LEDs in sample have the good interfaces and layer periodicities for the strained-layer superlattices and multiple quantum wells. In addition, the optical property in the light output power has been evaluated. As a result, LEDs with a maximum output power of 3.3 mW and a high saturation operating current of 400 mA have exhibited the good device performance.

Here, we report on the design tradeoffs between traditional hexagonal and emerging cubic InXGa(1−X)N/GaN-based green (520nm≤λ≤550nm) light-emitting diodes with special emphasis on the electron blocking layer, number of quantum wells, and thicknesses of quantum wells and barriers. We identified three crucial design rules for cubic green light-emitting diodes: (1) no need for an electron blocking layer; (2) use of a wide quantum well; and (3) choice of thin quantum barriers in multi-quantum well light-emitting diode designs. These design rules increase the internal quantum efficiency of cubic green light-emitting diodes by ∼30.5% under 100A/cm2 injection with respect to traditional designs. Overall, the design rules of cubic light-emitting diodes and their differentiating nature from the traditional, hexagonal ones are crucial for the advent of next-generation cubic light-emitting diodes.

The epitaxial growth of novel GaN-based light-emitting diode (LED) on Si (100) substrate has proved challenging. Here in this work, we investigate a monolithic phosphor-free semi-polar InGaN/GaN near white light-emitting diode, which is formed on a micro-striped Si (100) substrate by metal organic chemical vapor deposition. By controlling the size of micro-stripe, InGaN/GaN multiple quantum wells (MQWs) with different well widths are grown on semi-polar () planes. Besides, indium-rich quantum dots are observed in InGaN wells by transmission electron microscopy, which is caused by indium phase separation. Due to the different widths of MQWs and indium phase separation, the indium content changes from the center to the side of the micro-stripe. Various indium content provides the wideband emission. This unique property allows the semipolar InGaN/GaN MQWs to emit wideband light, leading to the near white light emission.

The existence of high threading dislocation density (TDD) in GaN-based epilayers is a long unsolved problem, which hinders further applications of defect-sensitive GaN-based devices. Multiple-modulation of epitaxial lateral overgrowth (ELOG) is used to achieve high-quality GaN template on a novel serpentine channel patterned sapphire substrate (SCPSS). The dislocation blocking brought by the serpentine channel patterned mask, coupled with repeated dislocation bending, can reduce the dislocation density to a yet-to-be-optimized level of ∼2 × 10(5) to 2 × 10(6) cm(-2). About 80% area utilization rate of GaN with low TDD and stress relaxation is obtained. The periodical variations of dislocation density, optical properties and residual stress in GaN-based epilayers on SCPSS are analyzed. The quantum efficiency of InGaN/GaN multiple quantum wells (MQWs) on it can be increased by 52% compared with the conventional sapphire substrate. The reduced nonradiative recombination centers, the enhanced carrier localization, and the suppressed quantum confined Stark effect, are the main determinants of improved luminous performance in MQWs on SCPSS. This developed ELOG on serpentine shaped mask needs no interruption and regrowth, which can be a promising candidate for the heteroepitaxy of semipolar/nonpolar GaN and GaAs with high quality.

Semi-polar InGaN/GaN multiple quantum well solar cells with spectral response at up to 560 nm

We have demonstrated the growth of quaternary AlInGaN compounds at different growth temperatures and pressures with metalorganic chemical vapor deposition (MOCVD). The optical properties of the samples have been investigated by photoluminescence (PL) at different temperatures. The results show that the sample grown at higher temperature (850 °C) exhibits the best optical quality for its sharp band edge luminescence and weak yellow luminescence. The AlInGaN exhibited three-dimensional (3D) growth mode at higher pressure. The band edge emission almost disappeared. With the optimization of AlInGaN growth parameters, we replaced the traditional barrier in InGaN/GaN multiple quantum wells (MQWs) with AlInGaN barriers. The peak wavelength for the InGaN/AlInGaN-MQW based light emitting diodes (LEDs) was very stable at various injection current levels because of the polarization-matched InGaN/AlInGaN MQWs.

This paper reports a new model of current versus voltage for light emitting devices with a quantum well. These devices include light emitting diodes (LEDs) and laser diodes (LDs). In LED or LD operation, nearly all electrons and holes are injected into the quantum well and recombine there. As a result, the forward current consists of mainly the recombination current. This is in contrast to the popular Sah-Noyce-Shockley (SNS) diode equation where the dominating current is the diffusion current. The SNS model assumes that recombination in the depletion region is negligible under typical forward bias condition. This is opposite to what actually happens in LEDs or LDs with quantum wells. However, SNS equation has been used directly to describe LEDs and LDs with quantum wells for several decades probably because of its simplicity. Another probably reason is that there is no other I-V model available. In this study, I-V curves calculated using the new model agree well with results measured on GaN/sapphire LEDs with InGaN quantum wells. In the calculation, junction temperature T_j rather than case temperature T_c is used to achieve better agreement.

The multiple quantum well (MQW) structure is a good candidate to monolithically integrate lasers and waveguides for realizing photonic integrated circuits. For such an application, selective disordering techniques are required, because these can make it possible to define a waveguide section laterally by inducing local changes in absorption and refractive indices of a MQW structure, which is needed to fabricate waveguide devices including laser. There are several techniques to selectively disorder the I I I -V MQW structures. These are impurity induced disordering [1,2], ion implantation disordering [3, 4], and impurity-free disordering [5-12] followed by thermal treatment. Impurity induced disordering and ion implantation disordering can disorder MQW structure perfectly, but these techniques introduce many defects and high doping concentrations which may deteriorate the performances of waveguide devices because of losses from scatterings by defects and free carrier absorption in the waveguide. Impurity-free techniques employ dielectrics, such as SiO2 or SiN, a capped annealing technique, which employs rapid thermal annealing (RTA) [5-10] or sealed ampoule annealing with an As overpressure [11, 12]. Impurity-free techniques do not disorder MQW structure completely, but disorder it enough to fabricate waveguides and lasers [9-12]. For impurity-free disordering, there are various film growth techniques, such as chemical vapour deposition (CVD) [11], e-beam evaporation [6, 8, 9], sputtering [5], and plasma enhanced chemical vapour deposition (PECVD) [7, 10, 12], which can deposit SiN and/or Si te films on a MQW structure. The behaviour of disordering is affected by the film quality used in the impurity-free disordering technique. Therefore, if an optimum film growth condition can be found by varying process conditions, then selective disordering can be achieved with the same material without surface degradation which should be accompanied in the disordering process without an encapsulation. The characteristics of film deposited by the PECVD technique can be varied by varying process conditions, such as substrate temperature, ratio of reactant gas, and RF power [13]. As a first step to finding a process-dependent impurity-free disordering, we varied the RF power. We carried out impurity-free disordering using a PECVD SiN cap layer with RTA. We found that this technique could disorder the GaAs/A1GaAs MQW structure and the disordering was enhanced by using SiN film grown at high RF power. We used a GaAs/A1GaAs MQW laser structure which was grown by a metal organic chemical vapour deposition (MOCVD) technique on Si-doped n ÷ GaAs substrate. The structure has the following layers from the top of the substrate: 0.5/xm of n (1018 cm -3) GaAs buffer, 1/zm of n (1017 cm -3) A10.4~Ga0.s3As, 0.1/xm of undoped A10.24Ga0.76As, four 7 nm undoped GaAs quantum wells with 10 nm A10.24Ga0.76As barrier, 0.1/xm of undoped A10.24Ga0.76As, 1/zm of p (1017 cm -3) A10.47Ga0.53As, and 0.2/xm of p+ (10 ~8 cm -3) GaAs. In the substrate design, separate confinement design is chosen to enhance the optical confinement in the quantum wells. SiN films were deposited by a PECVD technique for cap layers. We used dilute silane (5% Sill4 in N:) and high purity NH 3 (99.999%) and N2 (99.9999%) as a reactant gas. During plasma deposition, the ratio of partial pressure (PNH3/Psi~,) was kept at 1, total pressure was kept as 120 Pa by adding N2 gas and the substrate temperature was 300 °C. SiN films were grown with various RF powers (0 W, 60 W and 90 W). The thicknesses and refractive indices, for each RF power condition, were determined by an ellipsometer (Gaertner, Ll17) and a surface profiler (Tencor, alpha step 200). Disordering of MQW samples was accomplished by RTA at 850 °C for 35 s in Ar atmosphere with a heating rate of 65 °C/s. Disordering of MQW

In this work, we present a method to synthesize arrays of hexagonal InGaN submicrometer platelets with a top c-plane area having an extension of a few hundred nanometers by selective area metal-organic vapor-phase epitaxy. The InGaN platelets were made by in situ annealing of InGaN pyramids, whereby InGaN from the pyramid apex was thermally etched away, leaving a c-plane surface, while the inclined {101̅1} planes of the pyramids were intact. The as-formed c-planes, which are rough with islands of a few tens of nanometers, can be flattened with InGaN regrowth, showing single bilayer steps and high-quality optical properties (full width at half-maximum of photoluminescence at room temperature: 107 meV for In0.09Ga0.91N and 151 meV for In0.18Ga0.82N). Such platelets offer surfaces having relaxed lattice constants, thus enabling shifting the quantum well emission from blue (as when grown on GaN) to green and red. For single InGaN quantum wells grown on the c-plane of such InGaN platelets, a sharp interface between the quantum well and the barriers was observed. The emission energy from the quantum well, grown under the same conditions, was shifted from 2.17 eV on In0.09Ga0.91N platelets to 1.95 eV on In0.18Ga0.82N platelets as a result of a thicker quantum well and a reduced indium pulling effect on In0.18Ga0.82N platelets. On the basis of this method, prototype light-emitting diodes were demonstrated with green emission on In0.09Ga0.91N platelets and red emission on In0.18Ga0.82N platelets.

Internal quantum efficiency improvement of InGaN/GaN multiple quantum well green light-emitting diodes