18 Mar 2004
Researchers in Taiwan fabricate high-power red LEDs based on copper substrates.
Red LEDs based on a copper substrate can produce a luminous intensity which is three times higher than that of traditional gallium-arsenide-substrate LEDs, according to two researchers from Taiwan’s National Chiao Tung University.
The duo also claims that their Cu-substrate LEDs can operate at an injection current which is eight times higher than that of traditional GaAs-substrate devices. (Applied Physics Letters 84 1841)
LEDs emitting in the yellow to red region of the spectrum are typically made from AlGaInP and based on a GaAs substrate. But due to the low thermal conductivity of the substrate, these LEDs are limited to low-power applications.
To overcome this drawback, Wei Chih Peng and Yew Chung Sermon Wu fabricated high-power AlGaInP LEDs with Cu substrates using wafer bonding. This involves placing an indium-tin-oxide (ITO) film between the AlGaInP LED structure and the Cu substrate and then applying heat.
“It was found that the sample did not bond at temperature below 400°C. When bonding temperature reached 600°C, the Cu element diffused through the ITO layer and destroyed the LED structure,” say Peng and Wu. “Fortunately Cu did not penetrate the ITO layer when samples were bonded at 500°C for 30 mins.”
According to the authors, copper’s higher thermal conductivity and lower thermal resistance compared with GaAs means that less heat is generated in the LED. This allows the Cu-substrate LEDs to operate a current of up to 800 mA and reach a peak luminous intensity of 1230 mcd.
Peng and Wu also report that the degradation of the intensity was less than 5% after 500 hours running at 20 mA at room temperature.
They add that their Cu-substrate LEDs exhibit a 2 nm redshift at 170 mA compared with a 4 nm redshift for GaAs-substrate devices.