在陶瓷基板上制造的高压薄膜 GaN LED

High-voltage thin-film GaN LEDs with the emission wavelength  of 455 nm were fabricated on ceramic substrates (230 W/m·K). The highvoltage operation was achieved by three cascaded sub-LEDs with dielectric  passivation and metal bridges conformally deposited on the side walls.  Under the driving power of 670 W/cm² , the high-voltage LEDs exhibit  much alleviated efficiency droop and the operative temperature below 80  °C. The excellent performances were attributed to the improved current  spreading within each sub-LED and the superior heat sinking of the ceramic  substrate.

An intuitive way to defer the occurrence of carrier overcrowding, and thus efficiency  droop, is by improving the uniformity of current spreading over the entire emitting area of  LEDs. In this regard, the high-voltage (HV) LED serves as a promising solution. A HV  LED comprises several sub-LEDs (cells) connected in series, leading to high operation  voltage and low driving current in comparison with the conventional large-area LED at the  same input power. Since the emitting area of each constituent cell is decreased, HV LEDs  exhibit improved current spreading owing to the reduced series resistance. Moreover, HV  LEDs require less power transformers from regular voltage sources, which not only cuts the  power loss but also improves device reliability because of the simplified package  components.

In addition to the growth and the fabrication methods, cooling the operation temperature is  also an effective approach to mitigate the droop effect. The LEDs with superior thermal  management should exhibit improved quantum efficiencies at high driving currents, which can be explained by the suppressed nonradiative Shockley-Read-Hall recombination and the  decreased electron leakage from the active region. Heat sinking is particularly important  for the devices grown on sapphire considering its poor thermal conductivity (~36 W/m·K). To  address the thermal issue of sapphire substrates, ceramics are among the most favorable in  light of their very high thermal conductivities (> 200 W/m·K). The substitution of ceramic  substrate for sapphire can be accomplished through the thin-film technique, where the  sapphire-based LED structure is firstly wafer-bonded to a foreign substrate and then the LED  structure is detached from sapphire by laser lift-off .

Compared with Si, which is currently the most used substrate for thin-film structures, ceramic substrates are more suitable for high-power LEDs because of their superior  cooling capabilities. Moreover, ceramics hold two other advantages over Si: i) the ceramic  substrate is an insulator, which favors the fabrication of HV LEDs since the isolation of each  constituent cell must be properly processed in order to ensure the serial connection through  the metal bridges. The conductive nature of Si substrates makes it very difficult for the  isolation process as the potential leakage currents cannot only flow through the side walls, but  also through the substrate. ii) Ceramics exhibit the robustness that is not attainable with Si.  The fragility of Si substrates often induces wafer cracks during the processes of wafer  bonding and laser lift-off, which greatly sacrifices the production yield. The crack problem  can be effectively eased with ceramic substrates. Despite these advantages, the research of  thin-film LEDs on ceramics is scarcely found to date, not to mention the ceramics for HV  devices.

In this study, HV thin-film LEDs are fabricated on ceramic substrates to demonstrate their  excellent capabilities of current spreading and thermal dissipation, both of which contribute to  the reduced efficiency droop at the current densities beyond 200 A/cm² . The devices are built  via novel processing techniques, including the high-reflection and low-contact-resistance  electrode to p-type GaN (p-GaN), and conformal deposition of the side-wall passivation layer  and the metal bridges. Compared to the sapphire-based devices, the HV thin-film LEDs on  ceramic substrates not only exhibit much improved wall-plug efficiencies (WPE), but also  operate at a significantly cooled junction temperature. Systematic characterizations of the  proposed device structure will be presented.

Fig1

The sapphire-based LED is built with standard process, including the dry etching to form  the mesa area, the deposition of indium tin oxide (ITO) layer to assist current spreading on pGaN, and the coating of reflective metal on the backside of sapphire to improve light  extraction. The process of Ceramic-LV starts from a sequential deposition of the reflective  metal (1-nm Ni/200-nm Ag) and the bonding metal (50-nm Ti/1-μm Au/2-μm In/1-μm  Au/50-nm Ti) on p-GaN, followed by the wafer bonding with the ceramic substrate. The  sapphire substrate is then detached by laser lift-off. On the ceramic substrate, the undoped  GaN is firstly removed by dry etching, and the mesa area defined by photolithography is revealed by the second dry etching down to the reflective metal. To further enhance light  extraction, surface roughening of n-GaN is attained with KOH wet etching. The side walls of  each die are passivated by a dielectric layer (700-nm Al₂O₃), and finally Cr/Au is deposited  for the P- and N-electrodes. The Ceramic-HV LED is fabricated with the process flow similar  to that of Ceramic-LV. An important difference is the removal of the reflective and the  bonding metal layers down to the ceramic substrate, which is performed in order to isolate the  three constituent cells. After passivating the side walls with Al₂O₃ using plasma-enhanced  chemical vapor deposition, Cr/Au metal bridges are conformally deposited on each cell to  realize the serial connection.

Figure 3(a) presents the optical output power of the LEDs at increasing current densities.  The output power was collected by an integrating sphere in which the LEDs were packaged  using transistor outline (TO) can without any encapsulation. The measurement with the  sapphire-based and the Ceramic-LV samples stopped when the output power was saturated. It  is clear that the devices on ceramic substrates deliver the optical power much higher than that  on sapphire. At the current density of 100 A/cm² , the optical power measured on ceramic  substrates is around twice that on sapphire, i.e. ~0.38 W vs. ~0.19 W. The enhanced output  power of the ceramic LEDs are mainly due to the improved current spreading discussed in  Fig. 2(a), and the superior heat dissipation of the ceramic substrate . In addition, the  increased light extraction by the roughened n-GaN surface should be the third factor leading  to the boosted optical power of the ceramic LEDs .