Influence of Molar Concentration: Sol-Gel Synthesized Magnesium Oxide Thin Films for High Power Light Emitting Diode Thermal Management

Sol-gel technique was used to synthesize 0.6 M and 0.8 M molar concentrations of magnesium oxide thin on aluminium substrate and enforced into heat spreader material in thermal management of lighting devices. X-ray diffraction analysis confirm the films to be polycrystalline cubic in nature with (200), (220) and (222) orientations. 20 layers from 0.8 M showed higher intensity from the X-ray analysis. Relative thermal conductivity measurement of 20 layers from 0.6 M showed a value of (26.3 W/mK) which appears to improve over other layers. At 700 mA operating current, 20 layers from 0.6 M showed higher junction temperature difference of 37.16 °C as well as higher thermal resistance difference of 5.9 K/W when compared to bare aluminium substrate. All coated films are within the accepted color correlated temperature safe operating range (< 3300 K) of light emitting diodes, addition to this, 20 layers MgO thin films from 0.6 M displayed a recommendable illuminance as well as color correlate temperature performance from optical characterization. Atomic force microscope surface analysis for both molar concentrations showed an improvement in surface smoothness over bare substrate. Overall, magnesium oxide thin films can be perfectly enforced as heat spreader for high power light emitting diodes.


Introduction
Light emitting diode (LED) have taking the place of conventional light sources in street lighting, traffic signals, residential, automobile, outdoor and advertisement board lighting [1][2][3]. Numerous advantages of LEDs, such as, miniaturization of their package, low-voltage power supply demand for their operation, no ultra -violet (UV) or infrared light emission, environmentally friendly, output energy efficient, long lifespan, flexible display design, resistant to shock, resistance to vibration as well as impact, all these features makes them beneficial over conventional lighting [1,4]. Despite all those advantages of LEDs, miniaturization of LEDs resulted in generation of excessive heat within the package, thereby resulting in rise of junction temperature, thermal runaway, shorten of the LEDs lifetime, poor efficiency and overall failure of the LEDs [2]. Therefore, need arises to properly address the heat management of the LEDs, so that, the rapid heat generated within them be remove via a suitable thermally conductive path. Doing so will keep the device components in a safe operating temperature condition.  [1,3], due to some of its excellent properties, among which are; good chemical and thermal stability, electrical insulation, high thermal conductivity (40 W/mK), large band gap of 7.8 eV. The properties include, dielectric constant (9.8), as well as 12 MV/cm breakdown field and good technology applicability [5,6]. Findings from other researchers in the same group reported a lower junction temperature (T j ) of 66.30 °C using Al 2 O 3 filled epoxy TIM [1], and lower thermal resistance of 13.81 K/W and lower T j of 47.32 °C from silver doped ZnO thin film TIM [7]. This study is aimed in using MgO thin film as a heat spreader between LED package and heat sink. Doing so will provide efficient high thermally conductive path, complete removal of excessive heat generated within the LED package, reduction in both LED's junction temperature and thermal resistance, solution to LEDs thermal management difficulties, improvement of LED lifespan, brightness, and performance. Various deposition methods were used in synthesis of magnesium oxide thin films, these includes, chemical vapor deposition, spin coating, spray pyrolysis, sputtering technique and vacuum evaporation [5,8,9]. Spin coating is used in this work due to its advantages of cost effectiveness, large and difficult area coating, stoichiometry control and uniformity of the films over the substrates during coating [5].
This paper reports the effects of molar concentration, variations in number of layers, film thickness, surface roughness level, thermal conductivity measurements, thermal transient analysis, structural and optical analyses as parameters that will brings about production of impressive MgO thin film heat spreader.

Experimental Details
0.6 and 0.8 molarity were used to prepare MgO solution separately in this study. Magnesium acetate tetrahydrate, calculated nitric acid and diethanolamine were dissolved in ethanol. The solution was sonicated at 50 °C. The solution was heated as well as stirred simultaneously for the period of 3 hrs using 60 °C. The solution was then kept for 24 hrs to undergo ageing.
Al5052 substrate was used in this study. Detergent, acetone, methanol along with dionised water were used to clean the substrate. The solutions were then deposited on the cleaned substrates, spun using 3000 rpm and 30 s parameters, followed by drying for 5 min at a temperature of 200 °C. The circle is then repeated for several times (5,10,5,20) in order to have different film thickness. After that, the coated MgO were annealed at a temperature of 500 °C for 2 hrs.
Thermal performance of the MgO thin films as heat spreader for CREE (3.6 W) white LED CXA 1304 was tested by thermal transient tester (T3ster), X-ray diffractor (Bruker D8) was used to study the crystallinity of the films. Atomic force microscope (AFM) (ULTRA objectives surface imaging system, GmbH) was applied to study the morphology of the samples. Thermal conductivity and optical analysis were carried out respectively using HOT DISC method and spectrophotometer.

Thin Film Thickness and AFM Analysis
Thin film thickness is one of the key features of TIMs and heat spreaders. Thin film thickness of various layers of the MgO from 0.6 M and 0.8 M concentration were measured and presented in table 1. The thickness increases as the number of layers increases from 5 to 20 layers, similarly, the thickness increases as the molar concentration increases from 0.6 M to 0.8 M. Films with thickness in the range of 300 nm to 800 nm are expected to give recommendable results [10]. Therefore, 20 layers are expected to be best for the study purpose.
Spin coated MgO thin films surface roughness from 0.6 M and 0.8 M were carried out using AFM machine. Figure 2   The measured values were summarized and presented in table 1. Table 1 showed that, 20 layers appears to have higher values of surface roughness from both concentrations, these was due to higher crystallite size and agglomeration of particles [5,11]. Research had proven that, thin films with smooth surface do possess higher thermal conductivity and perform more efficiently [10], however, research carried out here showed that, 20 layers from 0.6 M with surface roughness (121 nm) performs better, this was due to influence of the film thickness and thermal conductivity of MgO (~40 W/mK).

Thermal Transient Analysis
A perfect heat spreader is expected to have high thermal conductivity, low thermal resistance (R th ) as well as low junction temperature (T j ) values [1]. MgO thin film where spin coated and used as a heat spreader between CREE (3.6 W) LED package and Al5052 substrate. Thermal resistance together with junction temperature measurements of LED mounted on MgO heat spreader were conduct using (350, 500 and 700) mA input currents. Figure 4 represent the analyzed thermal resistance ranges of LED fixed on bare as well as different layers of MgO thin films and powered at 700mA. The figure showed an improvement in performance of the LED by reduction in the R th of the coated films compared to that of the bare substrate.

Optical Analysis
Optical properties for LEDs like Color-Correlated Temperature (CCT) in connection with luminance (LUX) were carried out using hand hold spectrometer. A good TIM or heat spreader are expected to maintain low CCT values to enable them stay within the safe operating range [12].    Considering all the results displayed by 20 layers of 0.6 M from all the analysis carried out, it proved its practical capability of been used as heat spreader toward improving thermal management of LEDs packaging system.

Conclusion
500 °C and 2 hours were annealing temperature and annealing period used to synthesized 0.6 M and 0.8 M magnesium oxide thin films using spin coating technique. They were used as heat spreader between CREE 3.6 W light emitting diode and aluminum substrate. XRD analysis revealed all the films were polycrystalline cubic with (200), (220) and (222) orientations. The intensities of the films increase as the number of layers increases from 5 to 20 layers, similarly it increases from 0.6 M to 0.8 M. 20 layers MgO film from 0.6 M was suggested suitable to be used as heat spreader between light emitting diodes package and metal substrate for thermal management of LEDs due to its thermal conductivity (26.3 W/mK), higher thermal resistance difference (5.9 K/W) and higher junction temperature difference (37.16 °C) . Beside these, it showed a lower surface roughness (121 nm) compared to that of 20 layers from 0.8 M and optical performance of LED fixed on it was within the safe operating range (<3300 K), were it showed appreciable performance in both color correlated temperature and illuminance.