# Output Properties of Transparent Submount Packaged FlipChip Light-Emitting Diode Modules
## 1 Introduction  
The LED efficiency and manufacturing cost are the key issues to affect the commercialization share of solid-state lighting. There are a lot of methods to enhance the LED's luminous efficiencies, such as improving the LED's epitaxial quality, modifying the LED's epitaxial structure [1], increasing the surface roughness of LEDs [2; 3; 4; 5], and improving the heat dissipation, etc. Recently, due to the high power operation of indoor and outdoor lighting, flip chip (FC) technology has been widely used; it can solve the heat accumulation problem effectively. Intense research is being carried out by different groups to improve the light output efficiency of FCLEDs [6; 7; 8; 9; 10]. However, most of the FCLED submounts used today are not transparent, which decreases the output light extraction. Besides using highly refractive submounts, an alternative method to increase the FCLED efficiency through lowering the submount absorption is proposed here by using high transparency mounting materials.  
In fact, it seems that there is a trade-off between the transparency and the thermal conductivity of the submount materials. Materials such as aluminum and copper have very high thermal conductivity, but they are not transparent [11]. On the other hand, a material such as glass is transparent but its thermal conductivity is very poor when compared to metallic aluminum or copper. It is known that heat dissipation has always been an issue in LED technology. So far, only 18% of the external quantum efficiency is achieved by LEDs [12], with the rest of the power or energy still dissipating out from LEDs as heat. Many groups have proposed different kind of heat-sinking methods with different materials [13; 14], but there is still a lack of research on the transparent materials. Although [PERSON] _et al._ also proposed the architecture of a LED combined with a zinc oxide (ZnO) transparent submount [15], the price of ZnO substrates is expensive, and the LED's operation currents are only up to 80 mA. Therefore, we have to investigate further to make some practical applications in this area.  
In general, to enhance the light extraction implies not only to increase the light output but also to decrease the heat generation. One prospect in the field of improving the light extraction of LEDs is called \"bi-directional LEDs\", where light can be collected in both directions through transparent substrates. Furthermore, FCLEDs with transparent submounts can also be used to emit different mixing colors in opposite directions easily. The proposed model for bi-directional LEDs is demonstrated in the inset of Figure 1. By having a transparent substrate, LEDs can emit light in all directions, just like the traditional tungsten light bulb. Unfortunately, as mentioned above, most transparent substrates are not good thermal conductors, and the accumulated heat will severely affect the output performance of LEDs. Most LED manufacturers today prefer to use a silver mirror between the LED dice and the non-transparent metal submount to reflect the light back [16; 17]. So, if the metal submount can be replaced by transparent substrates, which can also dissipate heat well. For example, by using sapphire or graphene deposited on glass, an efficient bi-directional LED can be achieved in a cost-effective manner, also with the benefit of a wide light output angle.  
Silicon has high stability, low cost and is readily available in large wafer sizes. However, it is very challenging to grow high quality GaN-based LEDs directly on Si due to the large mismatch in lattice constants and the thermal expansion coefficients between GaN and Si [18; 19; 20; 21; 22; 23; 24; 25]. Despite all these disadvantages of silicon, in this work we use it as a traditional non-transparent reference submount material due to its easy processing characteristics.