Study on thermal performance of high power LED employing aluminum filled epoxy composite as thermal interface material

This paper elucidates the thermal behavior of an LED employing metal filled polymer matrix as thermal interface material (TIM) for an enhanced heat dissipation characteristic. Highly thermal conductive aluminum (Al) particles were incorporated in bisphenol A diglycidylether (DGEBA) epoxy matrix to study the effect of filler to polymer ratio on the thermal performance of high power LEDs. The curing behavior of DGEBA was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dispersion nature of the Al fillers in polymer matrix was verified with Field Emission Scanning Electron Microscope (FESEM). The thermal performance of synthesized Al filled polymer composite as TIM was tested with an LED employing thermal transient measurement technique. Comparing the filler to polymer ratio, the rise in junction temperature for 60 wt% Al filled composite was higher by 11.1 °C than 50 wt% Al filled composite at cured state. Observed also from the structure function analysis that the total thermal resistance was 10.96 K/W higher for 60 wt% Al filled composite compared to 50 wt% Al filled composite. On the other hand, a significant rise of 9.5 °C in the junction temperature between cured and uncured samples of 50 wt% Al filled polymer TIM was observed and hence the importance of curing process of metal filled polymer composite for effective heat dissipation is discussed extensively in this work.     

Heat sink performances of GaN/InGaN flip-chip light-emitting diodes fabricated on silicon and AlN submounts

Fabricating flip-chip light emitting diodes (FCLEDs) with two good thermal conductivity materials of silicon and aluminum nitride (AlN) as submount are investigated on its output power and heat sink capacity. It is known that many advantages exist in FCLED structures. In addition to the upward emitting light, the downward propagating light is reflected up by a high reflectance contact, increasing the light extraction. The heat generated in the LED flows directly through the interconnect metal of the submount, improving thermal conduction. Except blue shift at the low current injection region (0–0.3 A), the heat induced bang gap narrowing (red shift) at high current injection region (0.3–0.7 A) is observed with a red shift of 8.92 nm for conventional LED, 4.62 nm for silicon submount FCLED, and only 2.87 nm for AlN submount FCLED. The light intensity of FCLEDs with silicon and AlN submounts exhibits 1.6 and 7 times at an injection current of 0.35 and 0.7 A, respectively, larger than that of conventional LED.     

Study on mobility enhancement in MOVPE-grown AlGaN/AlN/GaN HEMT structures using a thin AlN interfacial layer

Al_0.26Ga_0.74N/AlN/GaN high-electron-mobility transistor (HEMT) structures with AlN interfacial layers of various thicknesses were grown on 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy, and their structural and electrical properties were characterized. A sample with an optimum AlN layer thickness of 1.0 nm showed a highly enhanced Hall mobility (μ_Hall) of 1770 cm²/Vs with a low sheet resistance (ρ_s) of 365 Ω/sq. (2DEG density n_s = 1.0 × 10¹³/cm²) at room temperature compared with those of a sample without the AlN interfacial layer (μ_Hall = 1287 cm²/Vs, ρ_s = 539 Ω/sq., and n_s = 0.9 × 10¹³/cm²). Electron transport properties in AlGaN/AlN/GaN structures were theoretically studied, and the calculated results indicated that the insertion of an AlN layer into the AlGaN/GaN heterointerface can significantly enhance the 2DEG mobility due to the reduction of alloy disorder scattering. HEMTs were successfully fabricated and characterized. It was confirmed that AlGaN/AlN/GaN HEMTs with the optimum AlN layer thickness show superior DC properties compared with conventional AlGaN/GaN HEMTs.     

Thermal conductivity of AlN thin films deposited by RF magnetron sputtering

Aluminum nitride (AlN) film, which is being investigated as a possible passivation layer in inkjet printheads, was deposited on a Si (1 0 0) substrate at 400 °C by radio frequency (RF) magnetron sputtering using an AlN ceramic target. Dependence on various reactive gas compositions (Ar, Ar:H₂, Ar:N₂) during sputtering was investigated to determine thermal conductivity. The crystallinity, grain size, and Al–N bonding changes by the gas compositions were examined and are discussed in relation to thermal conductivity. Using an Ar and 4% H₂, the deposited AlN films were crystalline with larger grains. Using a higher nitrogen concentration of 10%, a near amorphous phase, finer morphology, and an enhanced Al–N bonding ratio were achieved. A high thermal conductivity of 134 W/mk, which is nine times higher than that of the conventional Si₃N₄ passivation film, was obtained with a 10% N₂ reactive gas mixture. A high Al–N bonding ratio in AlN film is considered the most important factor for higher thermal conductivity.     

Failure and stress analysis of through-aluminum-nitride-via substrates during thermal reliability tests for high power LED applications

The objective of this study is to evaluate the reliability of through-aluminum-nitride-via (TAV) substrate by comparing those experimental results with the finite element simulation associated with measurements of aluminum nitride (AlN) strength and the thermal deformation of Cu/AlN bi-material plate. Two reliability tests for high-power LED (Light emitting diode) applications are used in this study: one is a thermal shock test from − 40 °C to 125 °C, the other is a pressure cook test. Also, the strength of AlN material is measured by using three-point bending test and point load test. The reliability results show that TAV substrates with thicker Cu films have delamination and cracks after the thermal shock test, but there are no failure being found after the pressure cook test. The determined strengths of AlN material are 350 MPa and 650 MPa from three-point bending test and point load test, respectively. The measurement of thermal deformation shows that the bi-material plate has residual-stress change after the solder reflow process, also indicating that a linear finite element model with the stress-free temperature at 80 °C can reasonably represent the stress state of the thermal shock test from − 40 °C to 125 °C without considering Cu nonlinear effect. The further results of the finite element simulation associated with strength data of AlN material have successfully described those of the reliability test.     

Defects annealing in 4H–SiC epitaxial layer probed by low temperature photoluminescence

The thermal evolution of defects induced in 4H–SiC by multiple implantation of C ions was investigated by Low Temperature Photoluminescence in the temperature range 450–1000 K. The photoluminescence spectra show sharp luminescent lines (alphabet lines) in the wavelength range 426–440 nm upon irradiation and thermal treatment at 450 K induces the appearance of a new line at 427 nm (D_I centre). The trend shown by the luminescence lines as a function of the temperature is quite complex. The alphabet lines intensity increases up to 850 K, whereas at higher temperature decreases with an activation energy of 2.0 eV, suggesting that the defect, responsible for these lines, is the Si-vacancy. The luminescence yield of D_I centre is always increasing as a function of the temperature, with a higher slope from 750 K, suggesting a correlation to the reconfiguration and to the annealing of point defects.     

A study on the heat dissipation of high power multi-chip COB LEDs

In this study, the heat dissipation efficiencies of high power multi-chip COB (Chip-on-Board) LEDs with five different chip gaps were compared by assessing their junction temperature (T_j) and thermal resistance (R_th). Junction temperatures were measured using an IR camera and were also simulated by computational fluid dynamics (CFD) software. The effects of heat sinks with different surface areas, heat slugs made of different materials and different injection currents (different wattages) on high power LED junction temperatures are discussed. In addition, the optical characteristics of the LED, such as its lumens and luminous efficiency are evaluated. The experimental results show that a chip with a smaller gap has a higher junction temperature and more thermal resistance, and the junction temperature difference between the LEDs with the smallest and largest chip gaps is 3.12 °C. Optical performance analyses show that the LED with a larger chip gap has higher lumens and higher luminous efficiency. Thus, higher junction temperatures reduce the optical performance of high power LEDs.     

Effect of AlN buffer layer thickness on the properties of GaN films grown by pulsed laser deposition

The GaN films are grown by pulsed laser deposition (PLD) on sapphire, AlN(30 nm)/Al₂O₃ and AlN(150 nm)/Al₂O₃, respectively. The effect of AlN buffer layer thickness on the properties of GaN films grown by PLD is investigated systematically. The characterizations reveal that as AlN buffer layer thickness increases, the surface root-mean-square (RMS) roughness of GaN film decreases from 11.5 nm to 2.3 nm, while the FWHM value of GaN film rises up from 20.28 arcmin to 84.6 arcmin and then drops to 31.8 arcmin. These results are different from the GaN films deposited by metal organic chemical vapor deposition (MOCVD) with AlN buffer layers, which shows the improvement of crystalline qualities and surface morphologies with the thickening of AlN buffer layer. The mechanism of the effect of AlN buffer layer on the growth of GaN films by PLD is hence proposed.     

Effect of ethyl cellulose on thermal resistivity of thixotropic ZnO nano-particle paste for thermal interface material in light emitting diode application

A large amount of heat trapped inside Light Emitting Diode (LED) is the consequence of large thermal resistance between the heat source and the heat sink. Zinc oxide (ZnO) thick film was screen-printed from thixotropic paste that consisted of binder, filler and solvent to act as thermal interface material. Structural, surface morphology, vibrational and thermal properties of the samples were studied by means of Field Emission Scanning Electron Microscopy (FESEM), Atomic Force Measurement (AFM), Fourier Transform Infrared Spectroscopy (FTIR) and Thermal Transient Tester (T3ster). XRD analysis revealed that the formation of hexagonal wurtzite ZnO powder, which is free of hydroxide. FESEM results indicated that 50 wt% of filler loading in the thick film had created longer thermal transportation chain. The surface roughness of thick film displays variation in the range of from 64.8 to 218 nm. The presence of ZnO and binder were confirmed by FTIR spectrum at 518 cm⁻¹ and 668 cm⁻¹ to 2974 cm⁻¹, respectively. Thermal characterization reveals a drop in film’s resistivity with the higher content of filler loading of 50 wt% and 55 wt%. The lowest rise in junction temperature of tested LED is reported to be 14.7 °C of 50 wt% of filler loading.     

Low-voltage organic thin film transistors with hydrophobic aluminum nitride film as gate insulator

This paper reports on the low-voltage (<5 V) pentacene-based organic thin film transistors (OTFTs) with a hydrophobic aluminum nitride (AlN) gate-dielectric. In this work, a thin (about 50 nm), smooth (roughness about 0.18 nm) and low-leakage AlN gate dielectric is obtained and characterized. The AlN film is hydrophobic and the surface free energy is similar to the organic or the polymer films. The demonstrated AlN–OTFTs were operated at a low-voltage (3–5 V). A low-threshold voltage (−2 V) and an extremely low-subthreshold swing (∼170 mV/dec) were also obtained. Under low-voltage operating conditions, the on/off current ratio exceeded 10⁶, and the field effect mobility was mobility was 1.67 cm²/V s.     

Thermal stability of lead-free CH3NH3SnxI3 systems (0.9 ≤ x ≤ 1.1) for photovoltaics

Methylammonium-tin-iodide (MASn_xI₃, 0.9 ≤ x ≤ 1.1) systems were prepared by precipitation process in aqueous solutions. The “as prepared” MASn_xI₃ systems exhibited a tetragonal crystalline phase (space group I4cm) with polyhedral crystallites (length 50–400 µm). The as prepared samples were annealed at T = 150 °C for t = 8 h under nitrogen and synthetic air. Under nitrogen, the CH₃NH₃Sn_xI₃ systems adopt nearly-cubic tetragonal structure (space group P4mm) with crystallites of 2–4 µm length whereas a degradation process with formation of non-crystalline phases occurred in air. The differential thermal analysis (DTA) profile in nitrogen revealed events at T = 247 °C, T = 297 °C (decomposition of CH₃NH₃Sn_xI₃ systems into methylamine (CH₃NH₂), hydroiodic acid (HI) and SnI₂), and in the range T = 342–373 °C (melting of SnI₂) respectively. The thermal profile in air showed endothermic events at T = 139 °C and T = 259 °C with additional events at onset temperatures of T = 337 °C and T = 423 °C respectively which correspond to the formation of Sn(IV)-O binds and to the decomposition of methylamine. Static thermogravimetry analysis (TG), performed at T = 85 °C and T = 150 °C for t = 2 h, revealed a linear weight loss as a function of the time. The optical absorption spectra displayed absorbance edges in near infrared range, at 1107.0 nm (x = 0.9), 1098.6 nm (x = 1.0) and 1073.2 nm (x = 1.1) respectively.     

Efficient solution processed D1-A-D2-A-D1 small molecules bulk heterojunction solar cells based on alkoxy triphenylamine and benzo[1,2-b:4,5-b′]thiophene units

Two molecules denoted as VC96 and VC97 have been synthesized for efficient (η = 6.13% @ 100 mW/cm² sun-simulated light) small molecule solution processed organic solar cells. These molecules have been designed with the D₁-A-D₂-A-D₁ structure bearing different central donor unit, same benzothiadiazole (BT) as π-acceptor and end capping triphenylamine. Moreover, the optical and electrochemical properties (both experimental and theoretical) of these molecules have been systematically investigated. The solar cells prepared from VC96:PC₇₁BM and VC97:PC₇₁BM (1:2) processed from CF (chloroform) exhibit a PCE (power conversion efficiency) of η = 4.06% (J_sc = 8.36 mA/cm², V_oc = 0.90 V and FF = 0.54) and η = 3.12% (J_sc = 6.78 mA/cm², V_oc = 0.92 V and FF = 0.50), respectively. The higher PCE of the device with VC96 as compared to VC97 is demonstrated to be due to the higher hole mobility and broader IPCE spectra. The devices based on VC96:PC₇₁BM and VC97:PC₇₁BM processed with solvent additive (3 v% DIO, 1,8-diiodooctane) showed PCE of η = 5.44% and η = 4.72%, respectively. The PCE device of optimized VC96:PC₇₁BM processed with DIO/CF (thermal annealed) has been improved up to 6.13% (J_sc = 10.72 mA/cm², V_oc = 0.88 V and FF = 0.61). The device optimization results from the improvement of the balanced charge transport and better nanoscale morphology induced by the solvent additive plus the thermal annealing.     

Optimizing n-type contact design and chip size for high-performance indium gallium nitride/gallium nitride-based thin-film vertical light-emitting diode

The effects of the n-contact design and chip size on the electrical, optical and thermal characteristics of thin-film vertical light-emitting diodes (VLEDs) were investigated to optimize GaN-based LED performance for solid-state lighting applications. For the small (chip size: 1000×1000 µm²) and large (1450×1450 µm²) VLEDs, the forward bias voltages are decreased from 3.22 to 3.12 V at 350 mA and from 3.44 to 3.16 V at 700  mA, respectively, as the number of n-contact via holes is increased. The small LEDs give maximum output powers of 651.0–675.4 mW at a drive current of 350 mA, while the large VLEDs show the light output powers in the range 1356.7–1380.2 mW, 700 mA, With increasing drive current, the small chips go through more severe degradation in the wall-plug efficiency than the large chips. The small chips give the junction temperatures in the range 51.1–57.2 °C at 350  mA, while the large chips show the junction temperatures of 83.1–93.0 °C at 700  mA, The small LED chips exhibit lower junction temperatures when equipped with more n-contact via holes.     

2-Mercaptobenzothiazolate complexes of rare earth metals and their electroluminescent properties

We have demonstrated the electroluminescent (EL) properties of 2-mercaptobenzothiazolate complexes of rare earth metals [Ln(mbt)₃, Ln = Y, Sm, Eu, Gd, Tb, Dy, Tm] using simple non-doped two-layer organic light emitting diode with the configuration of indium tin oxide/N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine/Ln(mbt)₃/Yb. It was found that 2-mercaptobenzothiazolate complexes have highly efficient intra-energy transfer from the singlet to the triplet state of the ligand, and then to the excited state of the central lanthanide ions. Thus Y(mbt)₃ and Gd(mbt)₃ exhibit the broad ligand-centered emission with maximum near 600 nm and Dy(mbt)₃, Tb(mbt)₃ and Tm(mbt)₃ complexes exhibit pure sharp emission bands from the intra f–f transitions of lanthanide ions Tb³⁺: ⁵D₄ → ⁷F₆ (492 nm), ⁵D₄ → ⁷F₅ (547 nm), ⁵D₄ → ⁷F₄ (589 nm), ⁵D₄ → ⁷F₃ (624 nm); Dy³⁺: ⁴F_9/2 → ⁶H_13/2 (575 nm) and Tm³⁺: ³H₄–³H₆ (795 нм).     

The influence of thickness and ammonia flow rate on the properties of AlN layers

Undoped AlN layers have been grown on c-plane sapphire substrates by metal-organic chemical vapor deposition in order to study the effects of ammonia (NH₃) flow rate and layer thickness on the structural quality and surface morphology of AlN layers by high-resolution X-ray diffraction, scanning electron microscopy, and atomic force microscopy. Lower NH₃ flow rate improves crystallinity of the symmetric (0 0 0 2) plane in AlN layers. Ammonia flow rate is also correlated with surface quality; pit-free and smooth AlN surfaces have been obtained at a flow rate of 70 standard cm³ per minute. Thicker AlN films improve the crystallinity of the asymmetric (1 0 1̄ 2) plane.     

Fabrication of NbN/AlN/NbN junctions with Al embedding circuits on Si membrane for 1.5 THz SIS mixers

A process has been developed to fabricate NbN tunnel junctions and 1.5 THz SIS mixers with Al electrodes and Al/SiO₂/Al microstrip tuning circuits on thin Si membranes patterned on silicon on insulator wafers (SIMOX). High Josephson current density (J_c up to 2×10⁴ A/cm²) NbN/AlN/NbN and NbN/MgO/NbN SIS junctions have been fabricated with a reasonably good V_m quality factor and energy gap values close to 5 meV at 4.2 K on (100) oriented 3 inches SIMOX wafers covered by a thin (∼8 nm) MgO buffer layer. The sputtering conditions critically influence the dielectric quality of both AlN and MgO tunnel barriers as well as the surface losses of NbN electrodes. 0.6-μm Si/SiO₂ membranes are obtained after processing of a whole wafer and etching the individual chips in EDP. Such a technology is applied to the development of a waveguide/membrane SIS mixer for use around 1.5 THz.     

Integrity of copper–hafnium,hafnium nitride and multilayered amorphous-like hafnium nitride metallization under various thickness

The barrier properties and failure mechanism of sputtered Hf, HfN and multilayered HfN/HfN thin films were studied for the application as a Cu diffusion barrier in metallization schemes. The barrier capability and thermal stability of Hf, HfN and HfN/HfN films were determined using X-ray diffraction (XRD), leakage current density, sheet resistance (R_s) and cross-sectional transmission electron microscopy (XTEM). The thin multi-amorphous-like HfN thin film (10 nm) possesses the best barrier capability than Hf (50 nm) and amorphous-like HfN (50 nm). Nitrogen incorporated Hf films possess better barrier performance than sputtered Hf films. The Cu/Hf/n⁺–p junction diodes with the Hf barrier of 50 nm thickness were able to sustain a 30-min thermal annealing at temperature up to 500 °C. Copper silicide forms after annealing. The Hf barrier fails due to the reaction of Cu and the Hf barrier, in which Cu atoms penetrate into the Si substrate after annealing at high temperature. The thermal stabilities of Cu/Hf/n⁺–p junction diodes are enhanced by nitrogen incorporation. Nitrogen incorporated Hf (HfN, 50 nm) diffusion barriers retained the integrity of junction diodes up to 550 °C with lower leakage current densities. Multilayered amorphous-like HfN (10 nm) barriers also retained the integrity of junction diodes up to 550 °C even if the thickness is thin. No copper–hafnium and copper silicide compounds are found. Nitrogen incorporated hafnium diffusion barrier can suppress the formation of copper–hafnium compounds and copper penetration, and thus improve the thermal stability of barrier layer. Diffusion resistance of nitrogen-incorporated Hf barrier is more effective. In all characterization techniques, nitrogen in the film, inducing the microstructure variation appears to play an important role in thermal stability and resistance against Cu diffusion. Amorphousization effects of nitrogen variation are believed to be capable of lengthening grain structures to alleviate Cu diffusion effectively. In addition, a thin multilayered amorphous-like HfN film not only has lengthening grain structures to alleviate Cu diffusion, but block and discontinue fast diffusion paths as well. Hence, a thin multilayered amorphous-like HfN/HfN barrier shows the excellent barrier property to suppress the formation of high resistance η′-(Cu,Si) compound phase to 700 °C.     

Organic solar cells with sensitized phosphorescent absorbing layers

We demonstrate an increase of the exciton diffusion length (L_D) of a fluorescent donor layer via the process of sensitized phosphorescence, whereby initially generated singlet excitons are transferred to triplet excitons via a properly chosen dopant molecule. Using a poly(p-phenylene vinylene) host doped with the phosphorescent molecule platinum octaethylporphyrin, L_D increases from 4 ± 1 to 9 ± 1 nm for an optimal doping of 5 wt%. As a result of the increased L_D, the photocurrent from the doped layer increases by 40%. By doping with the fluorescent, non-metalated analogue octaethylporphyrin, a reduction in photocurrent is shown, providing further evidence of the sensitized phosphorescence mechanism.     

Phase-change immersion cooling high power light emitting diodes and heat transfer improvement

A new cooling method of ethanol direct-contact phase-change immersion cooling was proposed in the thermal management of high power light emitting diodes (LED) and the feasibility of this cooling method was investigated. The heat generated by LED was measured firstly using two types of power systems: DC power and LED driver. Then the heat dissipation performance was evaluated under different experimental conditions. The results indicate that startup process of the cooling system is quick and only 450 s is needed to reach steady-state under heat load of 42.78 W. The minimum thermal resistance of 1.233 °C/W is obtained when liquid filling ratio is 33.14%. The junction temperature of LED under different absolute pressures is much lower than the limited value of 120 °C. Baffle with total height of 140 mm, bottom space height of 20 mm and distance away from substrate surface of LED of 8 mm improves heat transfer performance best due to ethanol self-circulating in the cooling receiver. Overall, the ethanol phase-change immersion cooling is an effective way to make sure high power LED work reliably and high efficiently.     

A highly efficient OLED based on terbium complexes

A neutral ligand 9-(4-tert-butylphenyl)-3,6-bis(diphenylphosphineoxide)-carbazole (DPPOC) and its complex Tb(PMIP)₃DPPOC (A, where PMIP stands for 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) were synthesized. DPPOC has a suitable lowest triplet energy level (24,691 cm^?1) for the sensitization of Tb(III) (⁵D₄: 20,400 cm^?1) and a significantly higher thermal stability (glass transition temperature 137 °C) compared with the familiar ligand triphenylphosphine oxide (TPPO). Experiments revealed that the emission layer of the Tb(PMIP)₃DPPOC film could be prepared by vacuum co-deposition of the complex Tb(PMIP)₃(H₂O)₂ (B) and DPPOC (molar ratio = 1:1). The electroluminescent (EL) device ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB; 10 nm)/Tb(PMIP)₃ (20 nm)/co-deposited Tb(PMIP)₃DPPOC (30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP; 10 nm)/tris(8-hydroxyquinoline) (AlQ; 20 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) exhibited pure emission from terbium ions, even at the highest current density. The highest efficiency obtained was 16.1 lm W^?1, 36.0 cd A^?1 at 6 V. At a practical brightness of 119 cd m^?2 (11 V) the efficiency remained above 4.5 lm W^?1, 15.7 cd A^?1. These values are a significant improvement over the previously reported Tb(PMIP)₃(TPPO)₂ (C).