Dates of performance: 01/01/2000-01/01/2002

Investigative team:
UH PI:Abdelhak Bensaoula, Research Professor, Space Vacuum Epitaxy Center, University of Houston, 4800 Calhoun Rd, SR-1 Room 724, Houston, TX 77004-5507, ph: (713)-743-3621, fx: (713)-747-7724.
UH-Co-PI: David Starikov, Sr. Research Scientist, Space Vacuum Epitaxy Center, University of Houston, 4800 Calhoun Rd, SR-1 Room 724, Houston, TX 77004-5507, ph: (713)-743-3621, fx: (713)-747-7724.
NASA-JSC PI: Richard L. Sauer, PE, Medical Science Division, 2101 NASA Road 1, Phone: (281)-483-2866
NASA-JSC Co-PI: Susan E. Torney, Ph.D., Project Manager, MOD Advanced Projects and Analysis Office, 2101 NASA Road 1, Code DV, Houston, TX 77058, Phone: (281)-483-2866, e-mail: susan.e.torney@jsc.nasa.gov
UH PDAF: Chris Boney, Ph.D., Nitride Materials and Devices Group, SVEC, U of H, Houston, TX, 77204-5507, Phone: (713)-743-3621, Fax: (713)-747-7724, email: cboney@uh.edu

PROJECT SUMMARY

Optical chemical sensors used to measure the concentration of optically active species, usually employ matched pairs of light emitting diodes (LEDs) and photodiodes (PDs) placed in an optically insulated chamber. The necessity for optical insulation requires implementation of special membranes to allow the gases or fluids under test into the chamber yet reject all ambient light. These membranes are sensitive to corrosive environments, They affect the measurement accuracy and require long measurement times. As a consequence, the commercially available sensing systems can handle only very slow sampling rates, are chemically unstable, and lack thermal stability and long lifetimes. Also and due to the large size of such sensors' components (LEDs, PDs, and fiber optics), It is impossible to integrate them into a system compatible with Micro Electro Mechanical Systems (MEMS).

Proposed here, is an optoelectronic chemical sensor based on a combination of III-V nitride layers grown by high vacuum methods on sapphire, that can be simply integrated with MEMS for detection of water and air contaminants. The LED and solar-blind photodiodes, will be fabricated on the same chip, using the same technological process, and have an emission and sensitivity spectrum matched in the ultraviolet (UV) region. The optoelectronic structure can be combined with micromachined silicon wafers attached to sapphire substrates by advanced methods used in the MEMS production. Over the current technology, this new sensor will have high thermal and chemical stability and longer lifetime, lower production costs, miniature size, higher performance, sensitivity and measurement accuracy. Such enhanced capabilities are due to better absorption of UV light by numerous substances and the high mechanical, chemical, and radiation strength of materials used for sensor fabrication. This novel device can be successfully employed as a rugged, compact, and reliable sensor for several military, space, environmental, biomedical, and industrial applications.

The purpose of our ISSO-funded 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. Such integrated devices of AlInGaN materials could offer many advantages over current optical chemical sensors such as high chemical and thermal stability, smaller size, and higher sensitivity.

The objective of this two year project is to develop and fabricate a working prototype of a nitride-based optoelectronic chemical sensor. The sensor will be tested with various concentrations of a known contaminant in water. Towards this goal, the first part of the project has focused on growth of the various materials that will be necessary to fabricate the sensor. Earlier this year a new ISSO Post-doc was hired and he has been working on materials growth issues involved in sensor fabrication. Specifically, the growth of high-quality GaN, AlGaN, and InGaN layers on sapphire is being studied. The basic materials research is nearly complete and we will soon be moving forward to the second part of the project, which will be the growth of multi-layer structures and the processing of these samples into devices for testing. In order to characterize finished sensors, we have recently assembled a test set-up that includes "macro" sensors made from discrete optical components (LEDs, filters, and photodiodes) for comparison purposes.

The nitride layers in our investigation were grown by radio-frequency gas source molecular beam epitaxy (RFMBE). This method used an EPI Uni-Bulb plasma source to generate active nitrogen species while standard effusion cells supplied the group III metals. As part of our preliminary work, we grew several layers on commercial grade Si(111) substrates which are significantly less costly than sapphire wafers of the same size. Later experiments were performed on sapphire wafers when growth conditions had been narrowed down.

For experiments done on Si substrates, a 200Å thick AlN buffer layer was deposited between 750 and 800°C prior to growth of GaN, InGaN, or AlGaN films. Experiments on sapphire were begun with either direct deposition of GaN followed by subsequent layers or by the same AlN buffer layer used for silicon substrates. Since we have previously demonstrated n- and p-type GaN, this work focused on two main objectives: (1) growth of In_xGa_1-xN layers with varying values of x for emission and detection windows, and (2) growth of Al_xGa_1-xN layers for emission wavelengths less than 363nm. Layers were characterized by photoluminescence (PL), cathodo-luminescence (CL), secondary ion mass spectroscopy (SIMS), and x-ray diffraction (XRD).

A range of substrate temperatures and In/Ga flux ratios were explored to study the effect of growth conditions on In_xGa_1-xN layers deposited on Si. Substrate temperature was varied between 600 and 650°C and was found to have a profound impact on the indium incorporation of the film. At 650°C no indium was found in the layers for any indium flux as determined from PL and SIMS. Only by lowering the growth temperature to 600°C 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. However, there were problems with uniform indium incorporation. For growths on Si, the indium tended to separate out into 2 or more distinct compositions of In_xGa_1-xN.

For experiments performed on sapphire, a growth temperature of 600°C was fixed and the In/Ga ratio was adjusted. Layers grown in this manner showed much less In_xGa_1-xN phase separation as confirmed by PL measurements. By changing the relative fluxes, compositions of up to 50% indium mole fraction have been achieved without phase separation. The corresponding optical emission for this particular layer is around 520nm, which is roughly the lower energy (longer wavelength) limit that we should need for our chemical sensors.

The initial investigation on the growth of Al_xGa_1-xN for higher energy (lower wavelength) applications was performed on Si(111) substrates. Prior to AlGaN growth at 750°C, AlN was deposited at 800°C followed by GaN at 800°C. A range of Ga/Al flux ratios were explored to determine the compositional dependence on the group III fluxes. In the case of Al_xGa_1-xN, it is the higher sticking coefficient of the Al that strongly determines the film composition. Due to the lattice and thermal expansion mismatches between the layers and the Si substrate, cracking of the films was a problem. Therefore layer thickness had to be kept below about 4000Å in order to prevent cracking during post-growth cooling. Composition of the layers on silicon ranged from 7% Al to 42% as determined by CL measurements.

Work has recently begun on the growth of AlGaN on sapphire substrates. Because the substrate is transparent, measurement of the transmission of the film as a function of wavelength can be used to determine the bandgap, and hence the Al mole fraction, of the layer. Only a few growths have been performed to date, but Al compositions of up to 71% have been achieved.

In conclusion, we are making good progress on the development of nitride-based integrated optoelectronic chemical sensors. We have demonstrated growth of In_xGa_1-xN and Al_xGa_1-xN layers by RFMBE on sapphire substrates, with indium mole fractions up to 50% for In_xGa_1-xN, and Al_xGa_1-xN, films with up to 71% Al. Currently, we can fabricate layers that are optically active from 200nm (AlN) up to 520nm (InGaN). The first part of the project, the individual growth of the necessary materials, is almost complete. In the near future we will integrate these layers into device structures and fabricate the samples into integrated optoelectronic chemical sensors. We will begin to work closely with JSC in order to test and optimize sensor performance with the goal of being able to accurately detect the concentration of a known contaminant in water.