Scott A. Ecelberger

A microelectromechanical‐based magnetostrictive magnetometer

The principles of operation of a microelectromechanical (MEMS)‐based magnetometer designed on the magnetoelastic effect are described. The active transduction element is a commercial (001) silicon microcantilever coated with an amorphous thin film of the giant magnetostrictive alloy Terfenol‐D [(Dy0.7Te0.3)Fe2]. In addition to the magnetostrictive transducer, basic components of the magnetometer include: (a) mechanical resonance of the coated‐microcantilever through coupling to an ac magnetic field; and (b) detection by optical beam deflection of the microcantilever motion utilizing a laser diode source and a position‐sensitive detector. Currently, the sensitivity of this MEMS‐based magnetostrictive magnetometer is ∼1μT.

Structural and electrical properties of reactively sputtered InN thin films on AlN‐buffered (00.1) sapphire substrates: Dependence on buffer and film growth temperatures and thicknesses

An extensive investigation of InN overlayers on AlN‐buffered (00.1) sapphire by reactive magnetron sputtering has been undertaken and the dependencies of several basic materials properties (film thickness, development and quality of heteroepitaxy, film morphology, and electrical transport) on such key deposition parameters such as the growth temperatures of the insulating AlN buffer layer and the InN overlayer and their thicknesses have been determined. Three prominent effects of the AlN buffer layer are (1) the stabilization of heteroepitaxial growth over a broad range of film and buffer layer growth temperatures; (2) the attainment of a higher Hall mobility (up to 60 cm2/V s) over much of the same range; and, (3) the retention of heteroepitaxial growth, higher Hall mobility, and pseudo‐two‐dimensional growth even in the limit of an InN layer of ∼40 A. In the context of a structure‐zone model, the AlN buffer layer is projected to effectively raise the growth temperature of the InN thin film. The increase...

Compact two‐step laser time‐of‐flight mass spectrometer for in situ analyses of aromatic organics on planetary missions

RATIONALE A miniature time-of-flight mass spectrometer measuring 20 cm in length has been adapted to demonstrate two-step laser desorption/ionization (LDI) in a compact instrument package for enhanced organics detection. Two-step LDI decouples the desorption and ionization processes, relative to traditional LDI, in order to produce low-fragmentation mass spectra of organic analytes. Tuning the UV ionization laser energy would allow control of the degree of fragmentation, which might enable better identification of constituent species. METHODS A reflectron time-of-flight mass spectrometer prototype was modified to allow a two-laser configuration, with IR (1064 nm) desorption followed by UV (266 nm) postionization. A relatively low ion extraction voltage of 5 kV was applied at the sample inlet. RESULTS The instrument capabilities and performance were demonstrated with analysis of a model polycyclic aromatic hydrocarbon, representing a class of compounds important to the fields of Earth and planetary science. Two-step laser mass spectrometry (L2MS) analysis of a model PAH, pyrene, was demonstrated, including molecular ion identification and the onset of tunable fragmentation as a function of ionizing laser energy. Mass resolution m/Δm = 380 at full width at half-maximum was achieved for gas-phase postionization of desorbed neutrals in this highly compact mass analyzer. CONCLUSIONS Achieving L2MS in a highly miniaturized instrument enables a powerful approach to the detection and characterization of aromatic organics in remote terrestrial and planetary applications. Tunable detection of molecular and fragment ions with high mass resolution, diagnostic of molecular structure, is possible on such a compact L2MS instrument. The selectivity of L2MS against low-mass inorganic salt interferences is a key advantage when working with unprocessed, natural samples, and a mechanism for the observed selectivity is proposed.

Heteroepitaxial growth of InN on AlN‐nucleated (00.1) sapphire by ultrahigh vacuum electron cyclotron resonance‐assisted reactive magnetron sputtering

A novel deposition technique, ultrahigh vacuum electron cyclotron resonance (ECR)‐assisted reactive magnetron sputtering, has been developed for the preparation of group IIIA nitride thin films. In initial experiments, thin films of the semiconductor InN have been deposited on AlN‐seeded (00.1) sapphire substrates, and the properties of these films studied as a function of growth temperature. Comparison to InN thin films grown by conventional reactive magnetron sputtering shows enhanced Hall mobilities (from about 50 to over 80 cm2/V s), a decreased carrier concentration (by about a factor of 2), an increased optical band gap, and an apparent reduction in homogeneous strain that is in part due to film relaxation induced by the ECR beam and in part to enhanced nitrogen content and more nearly stoichiometric films.

Rapid assessment of high value samples: An AOTF-LDTOF spectrometer suite for planetary surfaces

We discuss the development of a miniature near-infrared point spectrometer, operating between 1.7-3.45 μm, based on acousto-optic tunable filter (AOTF) technology. This instrument may be used to screen and corroborate analyses of samples containing organic biomarkers or mineralogical signatures suggestive of extant or extinct organic material collected in situ from planetary surfaces. The AOTF point spectrometer will be paired with a laser desorption time-of-flight (LDTOF) mass spectrometer and will prescreen samples for evidence of volatile or refractory organics before the laser desorption step and subsequent mass spectrometer measurement. We describe the AOTF point spectrometer instrument and present laboratory analysis of geological samples of known astrobiological importance. We also present LDTOF spectra of the same samples analyzed with the AOTF, which highlights the value of a comparative data set with the two instruments. We discuss plans for the integration of the two instruments, which is scheduled to take place in the first half of 2012. The AOTF-LDTOF instrument pairing offers the powerful advantage of cross-checked chemical analyses of individual samples, which can reduce chemical and biological interpretation ambiguities.

A compact tandem two-step laser time-of-flight mass spectrometer for in situ analysis of non-volatile organics on planetary surfaces

Two-step laser desorption mass spectrometry is a well suited technique to the analysis of high priority classes of organics, such as polycyclic aromatic hydrocarbons, present in complex samples. The use of decoupled desorption and ionization laser pulses allows for sensitive and selective detection of structurally intact organic species. We have recently demonstrated the implementation of this advancement in laser mass spectrometry in a compact, flight-compatible instrument that could feasibly be the centerpiece of an analytical science payload as part of a future spaceflight mission to a small body or icy moon.

Tiny-TOF-MALDI mass spectrometry for particulate drug and explosives detection

The MALDI (matrix assisted laser desorption/ionization) technique, widely used to desorb and ionize large biomolecules, is applied here to small molecules having low vapor pressure, such as drugs and explosives. Furthermore, we report the coupling of the MALDI technique with a small, highly portable tiny-TOF (fime-of-flight) mass spectrometer developed in our laboratories. This mass spectrometer is a low voltage coaxial reflectron design with a short flight tube that is specifically designed for low molecular weight substances. The reflectron is designed to operate in two different modes that provide an expremely powerful pseudo-tandem mass spectrometry capability that is crucial for field applications. Using this system we have measured mass spectral signatures for cocaine, heroine, and the explosive RDX in the sub- nanogram range. Also reported here are continued developements on advanced MALDI sampling technologies, sensitivity, and mass resolution enhancements of the tiny-TOF, further decreases in system size and weight, and concepts for field operational systems.

Growth Dependence of Thickness, Morphology and Electrical Transport of InN Over Layers on Ain-Nucleated (00.1) Sapphire

The seeded-heteroepitaxial growth, morphology and electrical transport properties of InN overlayers deposited by reactive magnetron sputtering on AIN-nucleated (00.1) sapphire have been investigated. For comparison, InN films were grown directly onto (00.1) sapphire under identical experimental conditions. These unseeded films showed a unimodal growth and were a mixture of textured and broadly heteroepitaxial grains. Low Hall mobility and carrier concentration and high resistivity were typical. In contrast, the AIN-nucleated InN overlayers exhibited a bimodal growth, strongly heteroepitaxial grains, and high Hall mobility. A particularly interesting aspect of the films grown on seeded (00.1) sapphire is the preservation of electrical continuity and high Hall mobility even in the limit of InN overlayers with thicknesses only on the order of 20–40A.

Sputter Deposition of Indium Nitride on The (111) Face of Elemental and Compound Semiconductors

The structure, morphology, and transport properties of thin films of InN grown on several cubic semiconductors has been studied as a function of substrate temperature. Films were deposited using rf-magnetron sputtering onto the (111) face of GaAs, Ge, Si and ZrO 2 . In general, the film structure is such that (00.1) InN parallels the (111) plane of the cubic substrate above some deposition temperature. The in-plane structural coherence duplicates the magnitude of the calculated lattice mismatch between InN and the substrate. Electrical transport properties for growth onto (111) ZrO 2 were characterized by n-type carrier concentration and mobilities ranging up to 44 cm 2 /Vsec. A morphology-induced decrease in electrical mobility is observed for deposition temperatures above 350°C, as shown by SEM.

ECR-Assisted Reactive Magnetron Sputtering of InN

The growth of high-quality thin films of the Group IIIA nitrides is exceedingly difficult given their propensity for nonstoichiometry and the lack of suitable substrates for either homoepitaxial or heteroepitaxial growth. A novel deposition technique, ultrahigh vacuum electron cyclotron resonance-assisted reactive magnetron sputtering, has been developed for the preparation of Group IIIA nitride thin films. Thus far, thin films of the semiconductor InN have been deposited on AlN-seeded (00.1) sapphire substrates, and the properties (structural, morphology, and electrical transport) of these films studied as a function of growth temperature. Comparison to InN thin films grown by conventional reactive magnetron sputtering shows enhanced Hall mobilities (from about 50 to over 100 cm 2 /V-sec), a decreased carrier concentration (by about a factor of 2–3), an increased optical bandgap, and an apparent reduction in homogeneous strain that is in part to be due to film relaxation induced by the ECR beam and in part to enhanced nitrogen content and more nearly stoichiometric films.