GaN Cladding Layers Research Articles

Numerical Investigation of the Impact of ITO, AlInN, Plasmonic GaN and Top Gold Metalization on Semipolar Green EELs

In this paper, we present the results of a computational analysis of continuous-wave (CW) room-temperature (RT) semipolar InGaN/GaN edge-emitting lasers (EELs) operating in the green spectral region. In our calculations, we focused on the most promising materials and design solutions for the cladding layers, in terms of enhancing optical mode confinement. The structural modifications included optimization of top gold metalization, partial replacement of p-type GaN cladding layers with ITO and introducing low refractive index lattice-matched AlInN or plasmonic GaN regions. Based on our numerical findings, we show that by employing new material modifications to green EELs operating at around 540 nm it is possible to decrease their CW RT threshold current densities from over 11 kA/cm2 to less than 7 kA/cm2.

Application of Hexagonal Boron Nitride to a Heat-Transfer Medium of an InGaN/GaN Quantum-Well Green LED.

Group III-nitride light-emitting diodes (LEDs) fabricated on sapphire substrates typically suffer from insufficient heat dissipation, largely due to the low thermal conductivities (TCs) of their epitaxial layers and substrates. In the current work, we significantly improved the heat-dissipation characteristics of an InGaN/GaN quantum-well (QW) green LED by using hexagonal boron nitride (hBN) as a heat-transfer medium. Multiple-layer hBN with an average thickness of 11 nm was attached to the back of an InGaN/GaN-QW LED (hBN-LED). As a reference, an LED without the hBN (Ref-LED) was also prepared. After injecting current, heat-transfer characteristics inside each LED were analyzed by measuring temperature distribution throughout the LED as a function of time. For both LED chips, the maximum temperature was measured on the edge n-type electrode brightly shining fabricated on an n-type GaN cladding layer and the minimum temperature was measured at the relatively dark-contrast top surface between the p-type electrodes. The hBN-LED took 6 s to reach its maximum temperature (136.1 °C), whereas the Ref-LED took considerably longer, specifically 11 s. After being switched off, the hBN-LED took 35 s to cool down to 37.5 °C and the Ref-LED took much longer, specifically 265 s. These results confirmed the considerable contribution of the attached hBN to the transfer and dissipation of heat in the LED. The spatial heat-transfer and distribution characteristics along the vertical direction of each LED were theoretically analyzed by carrying out simulations based on the TCs, thicknesses, and thermal resistances of the materials used in the chips. The results of these simulations agreed well with the experimental results.

Al-free cladding-layer blue laser diodes with a low aspect ratio in far-field beam pattern * * Project supported by the National Key R&D Program (Nos. 2017YFB0403100, 2017YFB0403101), the National Natural Science Foundation of China (Nos. 61534007, 61404156, 61522407, 61604168, 61775230), the Key Frontier Scientific Research Program of the Chinese Academy of Sciences (No. QYZDB-SSW-JSC014), the Science and Technology Service Network

c-plane GaN-based blue laser diodes (LDs) were fabricated with Al-free cladding layers (CLs) and deepened etching depth of mesa structure, so the aspect ratio of the far-field pattern (FFP) of the laser beam can be reduced to as low as 1.7, which is nearly the same as conventional AlGaInP-based red LDs. By using GaN CLs, the radiation angle of the laser beam θ⊥ is only 10.1° in the direction perpendicular to the junction plane. After forming a deeply etched mesa, the beam divergence angle parallel to the junction plane of FFP, θ//, increases from 4.9° to 5.8°. After using the modified structure, the operation voltage of LD is effectively reduced by 2 V at an injection current of 50 mA, but the threshold current value increases. The etching damage may be one of the main reasons responsible for the increase of the threshold current.

High-power blue laser diodes with indium tin oxide cladding on semipolar (202¯1¯) GaN substrates

We demonstrate a high power blue laser diode (LD) using indium tin oxide as a cladding layer on semipolar oriented GaN. These devices show peak output powers and external quantum efficiencies comparable to state-of-the-art commercial c-plane devices. Ridge waveguide LDs were fabricated on (202¯1¯) oriented GaN substrates using InGaN waveguiding layers and GaN cladding layers. At a lasing wavelength of 451 nm at room temperature, an output power of 2.52 W and an external quantum efficiency of 39% were measured from a single facet under a pulsed injection current of 2.34 A. The measured differential quantum efficiency was 50%.

GaInN light-emitting diodes using separate epitaxial growth for the p-type region to attain polarization-inverted electron-blocking layer, reduced electron leakage, and improved hole injection

A GaInN light-emitting diode (LED) structure is analyzed that employs a separate epitaxial growth for the p-type region, i.e., the AlGaN electron-blocking layer (EBL) and p-type GaN cladding layer, followed by wafer or chip bonding. Such LED structure has a polarization-inverted EBL and allows for uncompromised epitaxial-growth optimization of the p-type region, i.e., without the need to consider degradation of the quantum-well active region during p-type region growth. Simulations show that such an LED structure reduces electron leakage, reduces the efficiency droop, improves hole injection, and has the potential to extend high efficiencies into the green spectral region.

Pulsed high-power AlGaN-cladding-free blue laser diodes on semipolar (202¯1¯) GaN substrates

We demonstrate high-power AlGaN-cladding-free blue laser diodes (LDs) on semipolar (202¯1¯) GaN substrates with peak output powers and external quantum efficiencies (EQEs) that are comparable to state-of-the-art commercial c-plane devices. Ridge waveguide LDs were fabricated on (202¯1¯) GaN substrates using InGaN waveguiding layers and GaN cladding layers. The devices lased at 454 nm at room temperature. We measured an output power of 2.15 W, an EQE of 39%, and a differential quantum efficiency of 49% from a single facet with a pulsed drive current (current density) of 2.02 A (28.1 kA/cm2).

Waveguide Optimization for Semipolar (In,Al,Ga)N Lasers

In this work the optical waveguiding in semipolar InGaN-based laser diodes is analyzed. Different designs of the separate confinement heterostructure with AlGaN or GaN cladding layers and GaN or InGaN waveguide layers are studied. The influence of waveguide material, thickness and composition on the optical confinement factor Γ, the accumulated strain energy E and the refractive index contrast is calculated. Measurements of the threshold and the far field intensity distributions of lasers with differing waveguide design confirm the predictions from model calculations. The optimum waveguide for a 410 nm single quantum well laser is found to consist of a symmetric In0.04Ga0.96N waveguide of 2×85 nm thickness with GaN cladding layers.

GaN based LEDs with semipolar QWs employing embedded sub-micrometer sized selectively grown 3D structures

We present LED structures with embedded semipolar {101¯1} quantum wells based on 2-inch c-plane GaN templates grown on c-plane sapphire substrates. Using selective area epitaxy, we achieved periodic GaN stripe structures with triangular cross-section with dimensions of a few 100nm on continuous areas of several cm2. These structures exhibit semipolar side facets on which GaInN quantum wells with reduced piezoelectric fields have been deposited. The small dimensions of these structures allow complete embedding by GaN cladding layers eventually resulting in a flat c-plane surface. Consequently, our approach allows conventional device processing to be applied. Structural, optical, and electrical characterization is presented and the influence of mask material and pattern on the performances of LED structures is investigated.

Internal Quantum Efficiency and Nonradiative Recombination Rate in InGaN-Based Near-Ultraviolet Light-Emitting Diodes

The internal quantum efficiency (IQE) of three types of InGaN-based near-UV light-emitting diodes (LEDs) grown on different substrates was studied by excitation-power-density-dependent and temperature-dependent photoluminescence (PL) spectroscopy. IQE was evaluated under both the band-to-band excitation of GaN cladding layers and the selective excitation of InGaN active layers. A correlation between IQE and threading dislocation density was clearly observed under both excitation conditions. In addition, the dynamics of radiative and nonradiative recombinations was studied by time-resolved PL spectroscopy. The rate of nonradiative recombination was evaluated by measuring temperature-dependent nonradiative recombination lifetime. The validity of our method for estimating IQE is discussed on the basis of the correlation between IQE and nonradiative recombination rate for three LEDs with different threading dislocation densities.

III-nitride nanopyramid light emitting diodes grown by organometallic vapor phase epitaxy

Nanopyramid light emitting diodes (LEDs) have been synthesized by selective area organometallic vapor phase epitaxy. Self-organized porous anodic alumina is used to pattern the dielectric growth templates via reactive ion etching, eliminating the need for lithographic processes. (In,Ga)N quantum well growth occurs primarily on the six {11¯01} semipolar facets of each of the nanopyramids, while coherent (In,Ga)N quantum dots with heights of up to ∼20 nm are incorporated at the apex by controlling growth conditions. Transmission electron microscopy (TEM) indicates that the (In,Ga)N active regions of the nanopyramid heterostructures are completely dislocation-free. Temperature-dependent continuous-wave photoluminescence of nanopyramid heterostructures yields a peak emission wavelength of 617 nm and 605 nm at 300 K and 4 K, respectively. The peak emission energy varies with increasing temperature with a double S-shaped profile, which is attributed to either the presence of two types of InN-rich features within the nanopyramids or a contribution from the commonly observed yellow defect luminescence close to 300 K. TEM cross-sections reveal continuous planar defects in the (In,Ga)N quantum wells and GaN cladding layers grown at 650–780 °C, present in 38% of the nanopyramid heterostructures. Plan-view TEM of the planar defects confirms that these defects do not terminate within the nanopyramids. During the growth of p-GaN, the structure of the nanopyramid LEDs changed from pyramidal to a partially coalesced film as the thickness requirements for an undepleted p-GaN layer result in nanopyramid impingement. Continuous-wave electroluminescence of nanopyramid LEDs reveals a 45 nm redshift in comparison to a thin-film LED, suggesting higher InN incorporation in the nanopyramid LEDs. These results strongly encourage future investigations of III-nitride nanoheteroepitaxy as an approach for creating efficient long wavelength LEDs.

Very-High Temperature (200 $^{\circ}$C) and High-Speed Operation of Cascade GaN-Based Green Light- Emitting Diodes With an InGaN Insertion Layer

We demonstrate a novel type of linear cascade green light-emitting diode (LED) arrays as a light source for in-car or harsh environment plastic optical fiber (POF) communications. To further enhance its dynamic and static performance, an InGaN layer is inserted between an n-type GaN cladding layer and InGaN-GaN multiple quantum wells as an efficient current spreading layer. Compared with the control device without that layer, our three-LED cascade array demonstrates a smaller turn-on voltage (9.3 versus 11 V at 20 mA) and a larger output power (25.5 versus 22.5 mW at 180 mA), corresponding to an enhancement of around 31% in wall-plug efficiency. Furthermore, under the constant voltage bias of an in-car battery (12 V), our three-LED array exhibits an electrical-to-optical 3-dB bandwidth (100 versus 40 MHz) performance superior to that of the control device. Even under high-temperature dynamic operation, we observe that the InGaN insertion layer gives strong enhancement of modulation speed with negligible degradation of the output power, unlike the red resonant-cavity LEDs conventionally used for POF. We achieve 200-Mb/s error-free transmission at 200°C which is the highest operation temperature among all the reported high-speed LEDs.

Low-threshold-current-density AlGaN-cladding-free m-plane InGaN/GaN laser diodes

We demonstrate AlGaN-cladding-free m-plane InGaN/GaN laser diodes with threshold current densities that are comparable to state-of-the-art c-plane InGaN/GaN laser diodes. Thick InGaN waveguiding layers and a relatively wide active region with three 8 nm quantum wells were used to provide adequate refractive index contrast with the GaN cladding layers, thus eliminating the need for AlGaN cladding layers. Despite the large active region volume, lasing was achieved at a threshold current density of 1.54 kA/cm2, suggesting that the realization of even lower threshold current densities should be possible by reducing the number of quantum wells in the active region.

Improved Performance of GaN-Based Blue LEDs With the InGaN Insertion Layer Between the MQW Active Layer and the n-GaN Cladding Layer

In this study, we demonstrate the effect of GaN-based blue light-emitting diodes (LEDs), using an InGaN layer inserted between the n-type GaN cladding layer and the active layer (InGaN/GaN multiple quantum well), on improving device performances. With a 20-mA current injection, the results indicate that the typical output power (or forward voltage) of light-emitting diodes grown with, and without, the InGaN insertion layer are approximately 18.1 (3.1) and 15.3(3.5) mW (V), respectively. This corresponds to an enhancement in output power (wall-plug efficiency) of around 18% (33%), with the use of the InGaN insertion layer. In addition, the electrostatic discharge (ESD) endurance voltages increased from 1000 V to 6000 V when the InGaN insertion layer was applied to the GaN/sapphire-based LEDs. The improvement of output power and ESD endurance voltage could be mainly due to the fact that the Si-doped InGaN insertion layer played the role of a current-spreading layer, which led to a lower possibility of junctions suffering a large current density in specific local sites.

A well protection layer as a novel pathway to increase indium composition: a route towardsgreen emission from a blue InGaN/GaN multiple quantum well

We have investigated the effects of a well protection layer (WPL) onthe optical and crystal properties of an InGaN/GaN multiple quantumwell (MQW). The five-pair MQW, consisting of an InGaN well grown at750 °C and a GaNbarrier grown at 850 °C, was simply embedded between GaN cladding layers on a sapphire (0001) substrate. Whilethis dual-temperature MQW growth scheme seemed better suited to the GaN barrierquality, it exposed the volatile InGaN well to a higher temperature ambient during theramping-up process to grow the barrier. In order to prevent damage to the fragile well, athin GaN WPL was subsequently coated on each well layer at the same temperature beforeramping-up the temperature. Consequently, it was found that the WPL directly influencedthe indium composition and optical properties of the MQW. The indium composition wasin fact increased, as was evident from x-ray diffraction experiments. In addition,photoluminescence measurements showed that the emission peak wavelength was increasedfrom 464 to 520 nm. These results provide evidence that the WPL effectively suppressesindium re-evaporation during the ramping-up time. The present study proposesthat the WPL leads to a new way to increase the wavelength of InGaN/GaNMQWs.

Pressure dependence of optical transitions inIn0.15Ga0.85N/GaNmultiple quantum wells

The effects of hydrostatic pressure on optical transitions in ${\mathrm{In}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ multiple quantum wells (MQW's) have been studied. The optical transition associated with confined electron and hole states in the MQW's was found to shift linearly to higher energy with pressure but exhibit a significantly weaker pressure dependence compared to bulklike thick epitaxial-layer samples. Similar pressure coefficients obtained by both photomodulation and photoluminescence measurements rule out the possibility of the transition involving localized states deep in the band gap. We found that the difference in the compressibility of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ and GaN induces a tensile strain in the compressively strained ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ well layers, partially compensating the externally applied hydrostatic pressure. This mechanical effect is primarily responsible for the smaller pressure dependence of the optical transitions in the ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ MQW's. In addition, the pressure-dependent measurements allow us to identify a spectral feature observed at an energy below the GaN band gap. We conclude that this feature is due to transitions from ionized Mg acceptor states to the conduction band in the p-type GaN cladding layer rather than a confined transition in the MQW's.

Growth of GaN, InGaN, and AlGaN films and quantum well structures by molecular beam epitaxy

GaN, AIGaN and InGaN films have been grown by molecular beam epitaxy (MBE) using RF plasma sources for the generation of active nitrogen. These films have been deposited homoepitaxially onto GaN/SiC substrates and hetero-epitaxially onto LiGaO 2 substrates. LiGaO 2 is an ordered and closely-lattice-matched orthorhombic variant of the wurtzite crystal structure of GaN. A low-temperature AIN buffer layer is necessary in order to nucleate GaN on LiGaO2. Thick GaN and AIGaN layers may then be grown once deposition is initiated. InGaN has been grown by MBE at mole fractions of up to 20% as a quantum well between GaN cladding layers. The indium containing structures were deposited onto GaN/SiC substrates to focus the development effort on the InGaN growth process rather than on heteroepitaxial nucleation. A modulated beam technique, with alternating short periods of (In, Ga)N and (Ga)N, was used to grow high-quality InGaN. The modulated beam limits the nucleation of metal droplets on the growth surface, which form due to thermodynamic limitations. A narrow PL dominated by band edge luminescence at 421 nm results from this growth technique. Growth of GaN at high temperatures is also reported.