Team T
Investigative UH PI: Abdelhak Bensaoula, Ph.D., Research Associate Professor, Space Vacuum Epitaxy Center (SVEC) Department of Physics College of Natural Science and Mathematics Houston, TX 77204-5507 Phone: [phone]; Fax: [phone] E-mail: [e-mail] UH Co-PI: David Starikov, Sr. Research Scientist, SVEC Houston, TX 77204-5507 Phone: [phone]; Fax: [phone] E-mail: [e-mail] NASA-JSC PI: Richard L. Sauer, PE Medical Sciences Division 2101 NASA Road 1, Code SD3 Houston, TX 77058 Phone: [phone] 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: [phone]; Fax: [phone] E-mail: [e-mail] UH PDAF: Chris Boney, Ph.D. Nitride Materials Research Group, SVEC Houston, TX 77204-5507 Phone: [phone]; Fax: [phone] E-mail: [e-mail]
HE PURPOSE OF THIS PROJECT IS TO INVESTIGATE THE development of optoelectric 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--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-Doctoral Fellow 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. 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 experi-
SENSOR--Dr. Abdelhak Bensaoula (above) examines the growth of nitride layers by radio-frequency gas-source molecular beam epitaxy (RFMBE).
UH/UHCL ISSO--Y1999-2000--13
Figure 1. Photoluminescence spectrum of a In0.5Ga0.5N layer grown on sapphire.
Figure 2. Cathodo-luminescence spectra of AlxGa1-xN layers with compositions ranging from 15% to 42% Al.
SAPPHIRE--Dr. Chris Boney examines experiments on sapphire with the direct deposition of GaN followed by successive layers in the vacuum chambers of the Space Vacuum Epitaxy Center (SVEC). SVEC has expertise in thin films. ments were performed on sapphire wafers when growth conditions had been narrowed down. For experiments done on Si substrates, a 200 thick AlN buffer layer was deposited between 750 and 800C 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 InxGa1-xN layers with varying values of x for emission and detection windows and (2) growth of AlxGa1-xN layers for emission wavelengths less than 363 nm. Layers were characterized by photoluminescence (PL), cathodo-luminescence (CL), secondary ion
Figure 3. Transmission spectra from three layers grown on single-side polished sapphire: GaN, AlGaN, and AlN. The adsorption edge of the AlGaN layer is at 239 nm (5.19 eV) which corresponds to a Al mole fraction of about 71 percent. 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 InxGa1-xN layers deposited on Si. Substrate temperature was varied between 600 and and 650C was found to have a profound impact on the indium incorporation of the film. At 650C, no indium was found in the layers for any indium flux as determined from PL and SIMS. Only by lowering the growth temperature to 600C was a substantial amount of indium incorporated into the film. This is a consequence of 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 percent In were achieved. However, there were problems with uniform indium incorpora-
14--Y1999-2000--ISSO UHCL/UH
GROWTH PATTERNS--Agnes Tempez, graduate student in physics, deals with the growth of AlxGa1-xN for high energy applications performed on Si (111) substrates. tion. For growths on Si, the indium tended to separate out into two or more distinct compositions of InxGa1-xN. For experiments performed on sapphire, the growth temperature of 600C was fixed and the In/Ga ratio was adjusted. Layers grown in this manner showed much less InxGa1-xN phase separation as illustrated by the PLdata shown in Fig. 1. By changing the relative fluxes, compositions of up to 50 percent indium mole fraction have been achieved without phase separation. The corresponding optical emission for this particular layer is around 520 nm, which is roughly the lower energy (longer wavelength) limit that we should need for our chemical sensors. The initial investigation on the growth of AlxGa1-xN for higher energy (lower wavelength) applications was performed on Si(111) substrates. Prior to AlGaN growth at 750C, AlN was deposited at 800C followed by GaN at 800C. A range of Ga/Al flux ratios were explored to determine the compositional dependence on the group III fluxes. In the case of AlxGa1-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 4000 in order to prevent cracking during post-growth cooling. Composition of the layers on silicon ranged from 7 percent Al to 42 percent as determined by CLemission peaks, shown in Fig. 2. 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 percent have been achieved (Fig. 3). In conclusion, we are making good progress on the development of nitride-based integrated optoelectronic chemical sensors. We have demonstrated growth of InxGa1-xN and AlxGa1-xN layers by RFMBE on sapphire substrates, with indium mole fractions up to 50 percent for InxGa1-xN, and AlxGa1-xN, films ...