ISSO Post Doctoral Fellowship Minigrant Project

Title: Miniature Multifunction Fluorescence Sensors on Si Substrates

Existing fluorometry-based sensors are not compatible with applications in rapidly changing and harsh environments because of their large size, high costs, and low reliability. Real time measurement micro-sensors capable of performing multiple functions in extreme conditions are required to solve these problems. Integration of the fluorometer components on a silicon chip would result in miniature, inexpensive, rugged, MEMS-compatible multifunctional devices, ideally suited for environmental analysis and control. Such integration was not possible up till recently due to a lack of semiconductor UV/blue LEDs and tunable photodectectors. The challenge currently is to achieve similar device characteristecs on Si substrates.

Recent develpments in III-V nitride materials allow now for the fabrication of ultraviolet/blue optical sources and tunable photodetectors on silicon. Integration of these devices (high chemical and thermal strength) with well developed silicon micromachining, and signal conditioning technologies would result in system with superior performance and allow applications in extreme conditions (high temperature, pressure, and chemical reativity).

The purpose of this ISSO mini-grant project is to investigate the development of optoelectronic chemical sensors based on group III-nitride materials. The compounds GaN, AlN, InN, and their alloys are optically active from 650nm (InN) to 200nm (AlN) and thus are ideally suited for use in UV-VIS chemical sensors. Emission and detection devices can be separately tailored to specific wavelengths and grown on the same chip. AlInGaN devices could offer many advantages over current optical chemical sensors such as high chemical and thermal stability, smaller size, and higher sensitivity. When grown on Si wafers, such sensors would be compatible with current Micro Electro Mechanical Systems (MEMS) technology and ideally suited for environmental analysis and control.

The objective of this mini-grant project is to extend our effort on III-N growth for the ISSO chemical sensor project on sapphire substrates into the growth of III-N layers on silicon wafers for similar applications. Since we have previously demonstrated n- and p-type GaN on Si, this project focused on the following two areas: (1) growth of In_xGa_1-xN layers with varying values of x, and (2) growth of Al_xGa_1-xN with varying compositions x.

A range of In/Ga flux ratios were explored to study the effect of growth conditions on In_xGa_1-xN layers. Only at growth temperatures of 600°C or less was a substantial amount of indium incorporated into the film. This is due to the higher re-evaporation rate of In as compared to Ga at these temperatures. By adjusting the ratio of In to Ga during growth, mole fractions of up to 42% In were achieved on silicon substrates. However, there were problems with uniform indium incorporation. Specifically, the indium tended to separate out into 2 or more distinct compositions of In_xGa_1-xN. Further work is needed in order to address this problem.

To grow Al_xGa_1-xN layers of various compositions, a range of Ga/Al flux ratios were explored at a growth temperature of 750°C. In the case of Al_xGa_1-xN, it is the higher sticking coefficient of the Al that strongly determines the film composition. Composition of the layers ranged from 7% Al to 42% as determined by CL measurements. Using the CL data, we were able to calibrate the count rate in SIMS measurements with Al mole fraction. Now at the completion of the project, we have demonstrated that we can grow In_xGa_1-xN and Al_xGa_1-xN layers by RFMBE on silicon substrates. Indium mole fractions up to 42% have been demonstrated for In_xGa_1-xN, although there are still some phase separation issues that need to be resolved. Al_xGa_1-xN, films with up to 42% Al have been fabricated. Thus our work indicates that it should be feasible to transfer our techniques for integrated optoelectronic chemical sensors from sapphire onto Si substrates.