10.1117/2.1201105.003750 Growing thin films that contain embedded voids Pavel Frajtag, Salah Bedair, Aadhithya Hosalli, Geoffrey Bradshaw, and Nadia El-Masry Uniform voiding defects in thin films offer unique designs for brighter and more efficient LEDs. Over the last decade gallium nitride (GaN) has become the semiconductor material of choice in several optical and elec- tronic devices. In particular, GaN can help realize the poten- tial of solid-state lighting (SSL), a multi-billion dollar emerging technology using LEDs that promises to fundamentally alter lighting and contribute to energy savings. Unfortunately, GaN and other III-nitride materials—such as aluminum and indium nitrides or their alloys—suffer from a high density of disloca- tions and other defects (10 8 –10 10 cm 2 ) because of the lack of lattice-matched substrates. In general, these defects act as non- radiative recombination and scattering centers that impact the diffusion length 1 and minority carrier lifetime, reduce thermal conductivity, 2 and form easy pathways for impurity diffusion. Thus, they limit the performance, reliability, breakdown voltage, and lifetime 3 of both optoelectronic and power devices. Here, we discuss our approach to reduce GaN defects by controlling the void density. Dislocations generated at the GaN/sapphire interface run through the crystalline structure of the films and terminate at free surfaces, affecting the function of active, multi-quantum well (MQW) layers. To combat this, our embedded voids approach (EVA) 4 intentionally introduces a high density of micro-voids—a few micrometers in length and less than a micrometer in diameter—into the GaN layer near its interface with the substrate. By doing so, we create an efficient ‘trapping zone’ near the GaN/sapphire interface, where the voids act as both sinks for defects as well as expansion joints for lattice mismatches. The active layers of III-nitride epitaxial films are then grown on the void-embedded layer, free from dislocation disturbance. EVA is a three-step process (see Figure 1). First, we grow bulk GaN by metal organic chemical vapor deposition on the sap- phire substrate. Then, we form GaN nanowires (NWs) from the Figure 1. 3D schematics of (a) bulk-grown gallium nitride (GaN) film on a sapphire substrate, (b) GaN nanowires (NWs) formed using the maskless inductively coupled plasma-reactive ion etching (ICP-RIE), and (c) NW overgrowth and void formation. 5 Figure 2. (a) Schematic of maskless ICP-RIE for GaN NW formation, where the etching gas is composed of boron trichloride (BCl 3 ) and chlorine (Cl 2 ) in a 1:5 ratio and (b) high resolution scanning electron micrograph of GaN NWs produced. 5 bulk-grown film using maskless inductively coupled plasma- reactive ion etching (ICP-RIE, see Figure 2). Maskless ICP-RIE is based on the assumption that dislocations and other defects represent sites of high elastic energy where localized elevated etching rates proceed. Thus, we can form good quality GaN NWs that exhibit superior optical, electrical, and x-ray quality compared to bulk material. Additionally, because GaN NWs are a strain-free, low defect semiconductor material with high sur- face ratio, they can be used in a variety of applications. 5–7 In the final step of EVA, we overgrow epitaxial GaN on the GaN NW template. Continued on next page