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Silicon and GaPN LEDs united on a single chip

Japanese researchers have designed an optoelectronic integrated circuit for use in one-bit digital counters.

Silicon-based semiconductor devices dominate the microelectronics industry and are used for the fabrication of high density integrated circuits comprising of memory and processing devices. However, silicon has an indirect band gap, which severely limits its use for fabricating photonic devices such as LEDs and lasers. To address this problem, researchers from Toyohashi University of Technology have developed an innovative solution by integrating silicon devices with LEDs. These LEDs are produced using direct band gap nitride compound semiconductors and these novel devices are called optoelectronic integrated circuits (OEICs). Figure 1. One-bit counter consisting of a p-MOSFET and GaPN LED Figure 2. Transmission electron microscope image showing the n-Si/p-GaPN/n-GaPN/GaP/Si heterostructure. Akihiro Wakahara and colleagues at Toyohashi Tech have demonstrated what they say is the first realisation of a one-bit counter circuit OEIC with an optical output consisting of silicon field effect transistors integrated with GaPN LEDs on a single chip. The monolithic integrated circuits were fabricated using lattice matched silicon/GaPN/silicon heterostructures grown on silicon substrates in a dual chamber MBE system. Notably, growth of the silicon capping layer at a high temperature of 850℃led to a dramatic reduction of the threshold voltage to -2.1 V and an increase of the channel mobility of the p-MOSFET to 82 cm2Vs. This improvement is attributed to a decrease in phosphorus incorporation during the growth of the capping layer.


Figure 3. (a) Circuit diagram of the one-bit counter; (b) Optical image of an actual circuit; (c) Synchronisation of the LED emission with input and output circuit voltages

The one-bit counter circuit fabricated using the n-Si/p-GaPN/n-GaPN/GaP/n-Si heterostructure exhibited normal operation, where red light emission from the input and output indicators was in synchronisation with the input and output logical voltages.  

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