Unusual strategies for using indium gallium nitride grown on silicon (111) for solid-state lighting

Abstract

Properties that can now be achieved with advanced, blue indium gallium nitride light emitting diodes (LEDs) lead to their potential as replacements for existing infrastructure in general illumination, with important implications for efficient use of energy. Further advances in this technology will benefit from reexamination of the modes for incorporating this materials technology into lighting modules that manage light conversion, extraction, and distribution, in ways that minimize adverse thermal effects associated with operation, with packages that exploit the unique aspects of these light sources. We present here ideas in anisotropic etching, microscale device assembly/integration, and module configuration that address these challenges in unconventional ways. Various device demonstrations provide examples of the capabilities, including thin, flexible lighting "tapes" based on patterned phosphors and large collections of small light emitters on plastic substrates. Quantitative modeling and experimental evaluation of heat flow in such structures illustrates one particular, important aspect of their operation: small, distributed LEDs can be passively cooled simply by direct thermal transport through thin-film metallization used for electrical interconnect, providing an enhanced and scalable means to integrate these devices in modules for white light generation.

Fig. 1.

Schematic illustration of arrays of InGaN μ-ILED arrays (A) before and (B) after anisotropic etching of the near-interfacial region of a supporting Si (111) wafer. The colors correspond to the InGaN (light blue), the contact pads (gold), and a thin current spreading layer (red). SEM images of a dense array of μ-ILEDs on a Si (111) wafer (C) before and (D) after this type of anisotropic etching process. The insets provide magnified views (colorized using a scheme similar to that in A). SEM images of the region of the μ-ILED structure that connects to the underlying silicon wafer (E) before and (F) after etching. Break-away anchors serve as fracture points during retrieval of μ-ILEDs from Si (111) wafer. SEM images of a representative μ-ILED, shown in sequence, (G) after undercut, (H) after removal from the Si wafer, and (I) after assembly onto a receiving substrate (colorized for ease of viewing).

Fig. 2.

SEM images of the interconnection process for a representative InGaN μ-ILED, shown in sequence, (A) after assembly onto a optically transparent substrate (e.g., glass or plastic), (B) after spin-coating a photo-sensitive polymer, (C) after self-aligned via formation using a BSE process, and (D) after deposition and patterning of a metallic interconnect layer. The colorized regions correspond to the contact pads (gold), a thin current spreading layer (red), and Al interconnects (green). Optical images of various lighting modules based on arrays of μ-ILEDs (E) plastic and (F, G) glass substrates.

Fig. 3.

SEM images of arrays of released InGaN μ-ILEDs with dimensions from (A) 25 × 25 μm², (B) 50 × 50 μm², (C) 75 × 75 μm² to (D) 150 × 150 μm². The colorized regions correspond to the contact pads (gold), and thin current spreading layers (red). (E) Corresponding current density-voltage (J-V) characteristics for μ-ILEDs with the dimensions shown in (A). The inset provides a plot of current density as a function of μ-ILED area, measured at 6 V. (F) Current density-voltage (J-V) characteristics and emission spectrum (inset) of a representative device before undercut etching on the Si wafer, and after assembly onto a glass substrate.

Fig. 4.

(A) Schematic illustration of the process for fabricating flexible, white lighting modules, achieved by integrating patterned, encapsulated tiles of YAG:Ce phosphor islands with arrays of InGaN μ-ILEDs. (B) Color chromaticity plotted on a CIE 1931 color space diagram for μ-ILEDs integrated with phosphors with thicknesses of 60 μm, 80 μm, and 105 μm. Optical images of a fully interconnected array of μ-ILEDs (C) without phosphor, (D) with a laminated film of encapsulated YAG:Ce phosphor islands (500 × 500 μm²), and (E) with a laminated diffuser film.

Fig. 5.

(A–D) Temperature distributions for isolated InGaN μ-ILEDs with Al interconnects [300 nm and 1,000 nm-thick for (A–B) and (C–D), respectively] at input powers of (A) 7.8 mW, (B) 16.4 mW, (C) 8.4 mW, and (D) 18.0 mW captured using a QFI Infra-Scope Micro-Thermal Imager (left) and calculated by analytical models (right). (E) Surface temperature for μ-ILEDs with Al interconnect thicknesses of 300 nm (black) and 1,000 nm (red) extracted from experiments (dots) and computed using the analytical model (lines) as a function of input power. (F) Three-dimensional plot of the surface temperature as function of device size and interconnect thickness, at a constant heat flux of 400 W/cm². Temperature distribution for (G) a macrosize LED (i.e., 1 × 1 mm²), and (H) an array of 100 μ-ILEDs (i.e., 100 × 100 μm²) at a spacing of 2 mm. (I) μ-ILEDs surface temperature vs. spacing for an array of 100 μ-ILEDs.