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In this letter, a standard deviation based optimization technique has been applied on High Resolution X-ray Diffraction symmetric and asymmetric scan results to accurately determine the Aluminum molar fraction and lattice relaxation of Molecular Beam Epitaxy grown compositionally graded Aluminum Gallium Nitride (AlGaN)/Aluminum Nitride/Gallium Nitride (GaN) heterostructures. Mathews–Blakeslee critical thickness model has been applied in an alternative way to determine the partially relaxed AlGaN epilayer thicknesses. The coupling coefficient determination has been presented in a different perspective involving sample tilt method by offset between the asymmetric planes of GaN and AlGaN. Sample tilt is further increased to determine mosaic tilt ranging between 0.01° and 0.1°.

A comparative assessment of AlGaN/GaN high-electron-mobility transistors (HEMTs) grown by molecular beam epitaxy on silicon and sapphire substrates has been carried out. Large-area power GaN HEMTs with identical device dimensions were fabricated on both substrates. A thicker AlN buffer layer was used for the GaN HEMT on silicon to achieve similar quality and uniformity of GaN epitaxy for rational comparison with that grown on sapphire. Direct-current analysis and physical characterization were carried out to understand the performance of the devices. Mathematical measurement of the instability of the current–voltage (I–V) characteristic at high applied drain bias was carried out to evaluate the performance of both devices. An improved two-dimensional (2D) analysis of the I–V characteristic was performed from a thermal perspective including appropriate scattering effects on the 2D electron gas mobility. The experimental and analytical studies were correlated to reveal the effects of temperature-sensitive scattering phenomena on the mobility as well as on the I–V characteristic at high drain bias in terms of lattice thermal heating. It is observed that the HEMT on Si has improved stability compared with sapphire due to its weaker scattering phenomena at high drain bias, associated with its thermal conductivity. Simulation of 2D thermal mapping was also carried out to distinguish the hot-spot regions of the devices. The comparable electrical performance of these devices illustrates the viability of AlGaN/GaN HEMTs on Si(111) to achieve low-cost stable devices with better thermal power handling for high-voltage applications.

AlGaN/GaN high electron mobility transistors (HEMTs) functionalized with ZnO nanorods were used for sensing CO in the concentration range of 50–500 ppm balanced with air at ambient and temperatures from 25 to 250 °C. An increase of the HEMT drain current was observed for exposure to the CO-containing ambient. The sensing limitation was improved from 100 ppm to 50 ppm by increasing the sensor temperature from room temperature to 250 °C. The sensing response was also enhanced from 0.09% to 0.34% by increasing the sensor temperature. Reliable and repeatable current changes with the introduction of a low CO concentration of 50 ppm and also rapid response times of ∼40 s and recovery times ∼15 s were demonstrated.

In this study, we report on a demonstration of hydrogen sensing at low temperature using SnO2 functionalized AlGaN/GaN high electron mobility transistors (HEMT). The SnO2 dispersion was synthesized via a hydrothermal method and selectively deposited on the gate region of a HEMT device through a photolithography process. The high electron sheet carrier concentration of nitride HEMTs provides an increased sensitivity relative to simple Schottky diodes fabricated on GaN layers. The morphology and crystalline properties of the SnO2-gate, together with the texture of the multilayer films on the device were investigated by SEM, HRTEM, EDS and XRD. The effects of annealing treatment on the crystalline properties of the SnO2-gate, and gas sensing properties of SnO2-gated HEMT sensors were studied. The SnO2-gated HEMT sensor showed fast and reversible hydrogen gas sensing response at low temperature.

Wurtzite CdxZn1−xO epilayers with cadmium concentrations ranging from x = 0.02 to 0.30 were investigated using photoluminescence, transmission/reflection spectroscopy, and atomic force microscopy. The CdxZn1−xO photoluminescence peak was found to shift through the visible region from 421 (2.95 eV) to 619 nm (2.0 eV) as the cadmium concentration was increased from 2% to 30%. An additional broad photoluminescence peak was observed and is attributed to deep levels – the center of the broad peak was found to shift from 675 to 750 nm as the cadmium concentration was increased. RMS roughness of the epilayers increased from 1.5 nm (x = 0.02) to 9.2 nm (x = 0.30), as determined from atomic force microscopy. The demonstrated visible wavelength tunability throughout the visible range verifies the viability of using wurtzite CdxZn1−xO compounds for visible light emission in future optoelectronic devices.

We report on the fabrication and characterization of an ultraviolet (UV) light-emitting diode (LED) based on a p-n junction MgZnO/ZnO/AlGaN/GaN semiconductor triple-heterostructure (THS). Radio-frequency (RF) plasma-assisted molecular-beam epitaxy (MBE) has been employed to grow individual epitaxial layers of ZnO, MgxZn1−xO, and the complete heterostructure on c-plane GaN/sapphire templates. Various growth strategies have been used to optimize the quality of the ZnO layers as well as to precisely control the composition of the MgxZn1−xO compound. Cross-sectional transmission electron microscopy (TEM) study shows the excellent crystalline quality of the pseudomorphically grown ZnO active region of the device. A strong electroluminescence (EL) emission associated with ZnO excitonic transition was observed up to 650 K. The results shown in this paper strongly suggest the viability of RF plasma-assisted MBE in the development of next-generation UV emitters using ZnO-based materials.

We investigated different Er3+ defect sites found in Er-doped GaN layers by site-selective combined excitation-emission spectroscopy and studied the role of these sites in different direct and multistep excitation schemes. The layers were grown by molecular beam epitaxy and were 200 nm thick. Two majority sites were found along with several minority sites. The sites strongly differ in excitation and energy transfer efficiencies as well as branching ratios during relaxation. For this reason, relative emission intensities from these sites depend strongly on emission and excitation. The sites were identified for several transitions and a comprehensive list of energy levels has been compiled. One of the minority sites appears strongly under ultraviolet excitation above the GaN band gap suggesting that this site is an excellent trap for excitation energy of electron-hole pairs.

The impact of electron injection on the minority carrier diffusion length was studied at various temperatures in p-type Mg-doped GaN, Al 0.2 Ga 0.8 N, and Al 0.2 Ga 0.8 N/GaN superlattices, which were homogeneously and modulation (barrier only) doped. The activation energies for the electron injection-induced effect were found to be in the range from 190 to 260 meV, which is close to the thermal ionization energy of the Mg-acceptor. The obtained results are in agreement with the previously proposed model of minority carrier transport enhancement due to charging of Mg-centers in p-(Al)GaN. The activation energy of the electron injection effect observed in Al 0.2 Ga 0.8 N is consistent with the deepening of Mg-acceptor level due to the incorporation of Al into GaN lattice. The activation energy in the Al 0.2 Ga 0.8 N/GaN superlattice, homogeneously doped with Mg, indicates that the main contribution to the effect comes from the capture of injected electrons by the wells.

AlGaN/GaN high-electron mobility transistors (HEMTs) show decreases in extrinsic transconductance, drain-source current threshold voltage, and gate current as a result of irradiation with 40 MeV protons at doses equivalent to decades in low-earth orbit, The data are consistent with the protons creating deep electron traps that increase the HEMT channel resistance. Postirradiation annealing at 300°C was able to restore ∼ 70% of the initial gm and IDS values in HEMTs receiving proton doses of 5 × 1010 cm-2.

Thin (100 A) layers of MgO or Sc2O3 grown by molecular beam epitaxy were investigated for surface passivation of AlGaN/GaN high electron mobility transistors. A variety of pre-treatments such as UV/O3 or in situ heating were employed prior to the dielectric deposition. Under optimized conditions, the MgO produced an increase in drain–source current, threshold voltage and extrinsic transconductance, which is consistent with passivation of surface states in the AlGaN. The absence of hydrogen in the dielectrics makes them attractive candidates for long-term stable passivation of the HEMTs.

Real time detection of vitellogenin in largemouth bass serum was demonstrated using AlGaN/GaN HEMTs. Anti-vitellogenin antibodies were chemically anchored to the gold-coated gate area of the HEMT by thioglycolic acid. The potential difference that occurred from the vitellogenin antigen/antibody interaction induced a drain current change. The sensor was able to detect as low as 4 µg/mL Vtg. Successful detection in serum samples shows that HEMTs have the potential to be used as biological sensors in real-world applications (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A 13.56 MHz rf plasma atom source with an alumina discharge tube was characterized for use with oxygen. For a rf input power of 500 W the maximum degree of dissociation was found to be 70% at an indicated source pressure of 0.20 Torr, and a dissociation of approximately 40% was obtained at an indicated source pressure of 0.60 Torr. Corresponding atom fluxes of 6.4×1019 atoms cm−2 s−1 and 1.66×1020 atoms cm−2 s−1, respectively were calculated at the source exit from the measured degree of dissociation and gas throughput. The energy distributions of the charged particle emissions from the source were also measured. It was found that the source produced positive ions with an average energy of 278 eV and electrons with an average energy of 8 eV. The ion and electron spatial distributions were also measured. The electron and positive ion currents were measured as a function of source pressure. Plasma emission spectra in the range 500–1000 nm were monitored at two different source pressures corresponding to max...

Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nitride (GaN) thin films—an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam epitaxy on c-plane sapphire substrates (α-Al2O3). Room temperature isotropic and anisotropic mode VASE measurements were made at angles of incidence between of 20° and 80°. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite crystal structure of GaN. The sizable off-diagonal elements (Aps and Asp) of the Jones matrix indicate that the optical axis 〈c〉 of the c-plane sample are slightly off from the surface normal due to a small miscut of substrates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angl...

We study the electrical characteristics (current vs voltage, I/V) of Co, In, Mg, Mn, Ni, and Zn each with an Au overlayer to determine their usefulness as ohmic contact metals to p-type GaN. For all the metals, none of the I/V relationships are completely linear even after annealing. At a fixed voltage of 3V Co, In, Ni, and Zn have comparable current levels, whereas Mg and Mn are almost an order of magnitude less. Due to the various mechanisms by which the metals may form ohmic contacts, we further examine the metals in multilayer combinations in an attempt to reduce contact resistance. Three p-type GaN wafers with carrier concentrations of 1.2 × 1017, 1.5 × 1017 and 4.7 × 1017 cm−3 are used with Ni/Au metallizations as a common standard for comparison. The lowest average specific contact resistance obtained in this study is with Co/Au at 0.0081 ohm-cm2. In addition to comparing magnitudes of contact resistances, thermal aging studies of the metal contacts are performed from 300 to 700°C for 6 h periods to determine its effect on their electrical stability. In this test, Ni/Au is found to be the most electrically invariant with thermal aging prior to failure. However, the temperature at which it fails occurred sooner than that for many of the other metallizations examined (e.g., Co/Au, In/Au, and Zn/Au). The temperature for failure is arbitrarily defined to be the temperature that the contact resistance degrades to twice its pre-thermal-aging contact resistance.

Gallium nitride wafer epitaxy on large diameter substrates is critical for the future fabrication of large area UV linear or 2D imaging arrays, as well as for the economical production of other GaN-based devices. Typical group III-nitride deposition is now performed on 2-inch diameter or smaller sapphire substrates. Reported here are visible blind, UV GaN p-in photodetectors which have been fabricated on 3-inch diameter (0001) sapphire substrates by RF atomic nitrogen plasma MBE. The uniformity across the wafer of spectral responsivity and shunt resistance (R0) for the p-i-n photodetectors has been characterized. Spectral responsivity and 1/f noise as a function of temperature exceeding 250°C will be presented for the GaN p-i-n photodetectors. Spectral response with >0.17 A/W at peak wavelength and having 4-6 orders of magnitude visible rejection has been achieved. 1/f noise typically less than 10-14 A/Hz1/2 at room temperature also has been achieved with GaN p-i-n photodiodes. The results have been correlated with proposed models for dark current and 1/f noise in GaN diodes.

The effect of bias voltage polarity on the hydrogen sensing characteristics of AlGaN/GaN heterostructure Schottky diodes is reported. Under forward bias, there was a maximum observed in the sensitivity for hydrogen detection. For reverse bias, the hydrogen detection sensitivity increased proportionally to the bias voltage. A detection limit of 10 ppm of H2 in N2 was achieved under reverse bias with a current increase of 14% as compared to a detection limit of 100 ppm of H2 for a similar current change under forward bias.

Direct measurements of both the dissociation fraction and the atom flux have been carried out for the first time to characterize a radio frequency (rf) atomic nitrogen source. The measurements have been made over a range of source pressures and rf powers. Dissociation fractions of up to 0.4 and atomic nitrogen fluxes of up to 0.85×1018 atom s−1 were obtained. In addition these results have been correlated with spectroscopic measurements of the ratios of atomic and molecular optical emissions from the source plasma in order to provide a way of estimating in situ the atomic flux.

A linear motion Ga source has been constructed and tested for smooth flux variation. The beam flux can be monitored by sensors such as atomic absorption, RHEED, and pyrometric interferometry. We demonstrated stable and reproducible operation of the source using sensor feedback. Constant layer composition can be maintained despite Ga thermal desorption at high temperatures. Arbitrary composition profile can be easily achieved for band gap engineering.

In-situ cathodoluminescence (CL) is presented as a technique for determining film composition, optical quality, doping levels, and temperature of MBE grown group III nitride films. Excitation of the films is done with either the Auger or RHEED electron gun operating between 1 and 10 keV. The CL emission is monitored using a 3 nm resolution monochromator. Optimization of the GaN growth process using a RF atomic nitrogen plasma source is discussed using in-situ cathodoluminescence to reduce the yellow defect level present in GaN. Composition and quality of Al x Ga 1-x N films are shown to be quickly determined from the peak position and width. This is extremely useful in the nitride system where reflection high-energy electron diffraction (RHEED) oscillations are not routinely observed. Measurement of the substrate temperature during GaN growth is demonstrated by monitoring the shift in band edge position with temperature. p-type doping and MQW levels observed by CL are shown to allow quick optimization of device and material properties.

IR photo detectors are in high demand for various military and civilian applications, such as airborne surveillance, remote sensing, environmental monitoring, and spectrometry. Recently InAs/GaSb type II superlattice (T2SL) has attracted numerous R and D interest since SLS is the only IR material that has a theoretical prediction of higher performance than HgCdTe. Here we report the improvement of SL photo diodes through a new design with highly-strained type-II superlattice (HS-T2SL). The HS-T2SL consists of a highly compressively strained thick InSb layer at InAs/GaSb interfaces. The presence of coherent strain shifts the band edges such that the SL energy gap is reduced. This reduced band gap is advantageous to photodetectors because longer cut-off wavelengths can be obtained with reduced layer thickness in the strained SL. The highly compressive strain in HS-T2SL also leads to an even higher optical absorption coefficient and lower dark current. Applying this new design resistance-area product (R 0 A) is measured as high as 2.1 Ohm-cm 2 at 85K for 14.8-μm-cutoff photo diodes without any dark current suppression barriers. The fabricated 14.5μm-cutoff photo diode shows Johnson-noiselimited peak detectivity of 8.4×10 10 cmHz 1/2 /W at zero bias at 85K.

We report on the synthesis of Er-doped III–N double heterostructure light-emitting diodes (LEDs) and their electroluminescence (EL) properties. The device structures were grown through a combination of metalorganic chemical vapor deposition (MOCVD) and molecular-beam epitaxy (MBE) on c-plane sapphire substrates. The AlGaN layers, with an Al concentration of ∼12%, were prepared by MOCVD and doped with Si or Mg to achieve n- and p-type conductivity, respectively. The Er+O-doped GaN active region was grown by MBE and had a thickness of 50 nm. The Er concentration was estimated to be ∼1018 cm−3. The multilayer n-AlGaN/GaN:Er/p-AlGaN structures were processed into LEDs using standard etching and contacting methods. Several different LEDs were produced and EL spectra were recorded with both forward and reverse bias conditions. Typically, the EL under reverse bias was five to ten times more intense than that under forward bias. The LEDs displayed a number of narrow emission lines representative of the GaN:Er sys...

The group III nitrides have attracted increasing interest because of their significant potential in optoelectronics such as blue emitter and high temperature electronics. In order to take advantage of bandgap engineering of heterostructures such as GaN/AlN, a controllable growth technique is highly desired. The growth of group III nitrides by MBE (molecular beam epitaxy) requires using energetic nitrogen species which can be generated by techniques such as ion, RF (radio-frequency) and ECR (electron cyclotron resonance) sources. We report nitride results using an RF source to achieve epitaxy on sapphire. The nitrogen flux has been carefully characterized and related to the GaN quality. Schottky diode and ultra-violet (UV) photo-response are also reported.

A promising sensing technology utilizing AlGaN/GaN high electron mobility transistors (HEMTs) has been developed to analyze a wide variety of environmental and biological gases and liquids. The conducting 2DEG channel of GaN/AlGaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. Examples of detecting mercury ions, perkinsus, lactic acid, carbon dioxide, and vitellogenin are discussed in this paper.

Photoelectrochemical (PEC) etching of p-type GaN has been realized in room temperature, 0.1 M KOH solutions. PEC etching of GaN was achieved by applying a positive bias to the surface of the p-GaN layer through a deposited titanium mask. The applied bias reduces the field at the semiconductor surface, which induced the dissolution of the GaN. The effect of bias on etch rate and morphology was examined. It was found that insulating the Ti mask from the KOH solution with Si3N4 significantly increases the etch rate. The rms roughness of the etched region decreased as the bias voltage increased. Etch rates as high as 4.4 nm/min were recorded for films etched at 2 V.

Epitaxial growth of p-type GaN-based UV photocathode by RF plasma assisted molecular beam epitaxy (MBE) on sapphire, fused silica, and alumina substrates was investigated. The electrical measurements indicted the growth of highly p-type GaN films as thin as 0.1 μm on c-plane sapphire with a thin AlN nucleation layer. Polycrystalline p-type GaN was obtained for growth on fused silica and alumina. Negative electron affinity (NEA) photocathodes were fabricated by cesium activation of the p-type GaN films in vacuum. Quantum efficiency for UV detection on different substrates was then characterized. To study the integration of UV photocathodes with MCPs, direct deposition of p-type GaN films on glass MCPs were done at low growth temperatures by MBE. The detection efficiency of polycrystalline p- GaN photocathodes in reflection mode was much less than the high quality p-type GaN films on sapphire, however, it was comparable to the detection efficiency of the latter measured in the semitransparent mode. This indicates the potential for fabrication of improved photocathodes with higher gain and better spatial and temporal resolutions.

The dissociation fraction and atomic nitrogen flux from an SVTA radio frequency atomic nitrogen source designed for nitrogen doping by MBE have been measured absolutely. Their dependence on source power and pressure conditions has been investigated. Dissociation fractions of up to 40% and atomic fluxes of up to 8×10 17 s −1 were obtained.

GaN / Al 1-x Ga x N-based hetero-structures have demonstrated a versatility in RF electronic applications which is practically unmatched by any other material system. There are many device structures under consideration for use in RF and Power amplifiers, suitable for both commercial and military applications. In this paper, we will discuss HEMT device design and growth of GaN/AlGaN layers on semi-insulating SiC substrates by MBE and MOCVD. Both of the growth techniques have shown high quality GaN /AlGaN epitaxial layers and have demonstrated very uniform epitaxial layers with high mobility. The MBE growth was carried out using RF Plasma Assisted MBE. The MOCVD growth was performed in a close-coupled showerhead reactor operating at low pressure. All HEMT structures were grown on 2-inch semi-insulating SiC substrates. Several of the HEMT wafers grown by these two growth techniques were characterized in detail using AFM measurements of the surface roughness, and non-destructive characterization via contact-less sheet resistance mapping, optical reflectance, and high-resolution X-ray diffraction. Several of the wafers were fabricated into HEMT devices, and the results on these devices are also presented.

Fabrication of Er doped materials deposited on silicon into LED is demonstrated. A strong infrared luminescence is observed at 1540 nm wavelength region. They also have potential applications for planar optical communication.

GaN /AlGaN transistors are being developed for a variety of RF electronic and high temperature elctronics applications that will replace GaAs and Silicon devices and circuits for commercial and military applications. AlGaN/ GaN based HEMT device structure shows significant potential to meet these needs. In this paper, we present a GaN/AlGaN based HEMT design with modeling results, that includes AlN buffer layer followed by AlGaN layers on lattice matched semi-insulating SiC substrates. These devices were grown using RF Plasma Assisted MBE Technique. This approach has demonstrated very uniform epitaxial layers. Key to high quality HEMT structures is the ability to grow high quality AlN Buffer layers. Details of the electrical and optical characteristics of the HEMT layers and devices are presented and a short overview of semi-insulating SiC crystal growth is given.

Writing of nanostructures on thin metal films using atomic force microscopy (AFM) was demonstrated. The writing experiments were done in a nitrogen ambient having variable humidity. Using a p‐type heavily doped silicon AFM tip, a bias voltage was independently applied between the tip and the surface of a thin chromium layer deposited on a Si(100) substrate. Protruded patterns of various shapes were formed only on the water‐adsorbed chromium surface when applying a negative bias on the tip. Their sizes were found to be dependent on the writing time, the bias voltage, and the humidity. The smallest feature size obtained is about 20 nm. From Auger electron spectroscopy (AES) analysis, the products are shown to be Cr oxides. The surface modification mechanism appears to be tip‐induced local oxidation, i.e., anodization.

This report discusses the Phase I results in achieving light emission from Erbium doped Gallium Nitride materials. Erbium is a rare earth material which can emit at specific wavelengths of light, both in the visible and infrared regions of the spectrum. Erbium-doped layers were grown in GaN-based junctions by the molecular beam processing technique. The effects of Erbium in GaN were characterized using photoluminescence (PL) and catholuminescence (CL). Prototype devices were fabricated and electrically pumped light emission from the Er LEDs was observed.

Material structure and device processing issues related to the performance of high-frequency and high-power AlGaN-GaN HEMTs are discussed. Results on selected issues like transport properties of highly resistive buffer layers, persistent photoconductivity in 2DEG structures, preparation and properties of ohmic and Schottky-barrier contacts and HEMT devices, as well as analysis of deep traps in HEMTs are addressed in this paper in detail.

High performance ultraviolet (UV) detectors have been fabricated using plasma-enhanced molecular beam epitaxy. The realized AlGaN Schottky detectors exhibit high responsivity, sharp spectral cutoff and high shunt resistance of several giga-ohms for 0.5 mm2 active area devices. Quantitative measurements have been carried out on these detectors in the photon energy range from 10 eV (from approximately 1200 to 120 nm in wavelength). Very short UV spectral measurement of these AlGaN detectors is reported for the first time using high intensity sources. The detectors exhibited almost eight orders of magnitude in response dynamic range in that spectral span. Reliability of the devices is evaluated after exposure to repeated DUV irradiation.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High performance UV detectors have been fabricated using group III-nitride materials grown by molecular beam epitaxy. GaN PIN detectors exhibit near quantum efficiency limited responsivity, sharp spectral cutoff, and high shunt resistance of several hundred mega-ohms for 0.5 mm 2 active area devices. Comparison of PIN and Schottky devices is presented. The capabilities of group III-nitride based UV detectors is discussed in relation to suitability in UV sensing applications such as high temperature flame sensing, UV-B solar radiation monitoring, and high intensity UV dosimetry.

Generalized variable angle spectroscopic ellipsometry (VASE) and Raman scattering have been employed to study the optical anisotropy of GaN/Sapphire structures. The GaN films were grown hydride vapor phase epitaxy and molecular beam epitaxy on both m-plane and c-plane sapphire ((alpha) -Al2O3) substrates, respectively. Anisotropic optical phonon structure of sapphire have been measured, based on which the optical axis of sapphire substrate has been determined. A 541 cm-1 TO phonon of GaN grown on m-plane sapphire substrate has been discovered experimentally which is due the coupling of A1 and E1 TOs. Optical axis orientation of GaN film on m-sapphire has been fully determined by the anisotropic angular dependence of the coupled TO phonon. Off-diagonal elements Apst and Aspt of transmission VASE (TVASE) are very sensitive parameters related to the optical anisotropy. The optical axis orientation of GaN on m-sapphire has also been accurately determined by TVASE at two special sample positions. The optical anisotropy due to GaN film and sapphire substrate has been successfully separated at 90 degree(s) samples position allowing to study the optical anisotropy of GaN film only.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Multiple AlGaN/GaN heterostructure has been grown on Si(111) substrate by molecular beam epitaxy with different buffer growth conditions. Its influence on physical and electrical properties of two-dimensional electron gas (2DEG) has been investigated. Correlation between growth temperature variation in AlN intermediate layer and thick GaN buffer layer on 2DEG transport property has been observed. Besides the variation in growth temperatures, dissimilar partial doping in the thick GaN buffer has also been studied. Impact of different silicon substrate doping has been examined to inspect the electrical properties of high electron mobility transistors (HEMT). DC characteristics of large area fabricated HEMT have been compared in terms of current, transconductance, and linearity for power amplifier applications.

The study assessed the effect of low doses of fatty acids from fish or flaxseed oil on plasma lipid concentrations in normal humans consuming diets with either high (0.87, n = 11) or low (0.48, n = 15) dietary polyunsaturated/saturated fatty acid (P/S) ratios. The dose of (n-3) fatty acids reflected an (n-3) intake that could easily be attained by selection of foods in a normal diet. The individuals were initially supplemented with olive oil [35 mg 18:1/(kg body weight.d)], and then were randomly assigned to either flaxseed or fish oil [35 mg 18:3(n-3) or 35 mg 20:5(n-3) + 22:6(n-3)/(kg body weight.d), respectively] treatments. Participants consumed each oil supplement for 3 mo. Blood samples were drawn for analysis at the end of each 3-mo period. Plasma triacylglycerol, total, LDL and HDL cholesterol concentrations, and lipoprotein fatty acid concentrations are shown. Fish oil reduced plasma triacylglycerol and increased lipoprotein levels of 20:5(n-3) and 22:6(n-3). The flaxseed oil did not alter plasma triacylglycerol level and produced small changes in 20:5(n-3) and 22:6(n-3) concentrations. Total, LDL and HDL cholesterol levels were not affected by either (n-3) fatty acid. Significant differences in plasma triacylglycerol concentrations and total and LDL cholesterol levels were found between the two dietary P/S groups after all oil treatment periods. Levels of 18:3(n-3), 20:4(n-6), 20:5(n-3), and 22:6(n-3) in LDL were also different in high vs. low dietary P/S groups for all oil treatments and in the VLDL for the olive oil and fish oil supplementation. This study indicates that low intake of purified fish oil induces changes in plasma triacylglycerol, 20:5(n-3) levels in VLDL, LDL, and HDL, and 22:6(n-3) levels in LDL and HDL that are apparent after 3 mo and which might influence atherogenicity of lipoprotein particles in normal free-living individuals.

RF atomic nitrogen plasma molecular beam epitaxy (MBE) was used to deposit gallium nitride (GaN) p-i-n junction photovoltaic detectors on (0001) sapphire. The detectors consisted of a bottom contact layer n-type silicon doped to 5 × 1018 cm−3. The intrinsic layer was undoped and possessed an n-type background carrier concentration of 1 × 1016 cm−3. The top /p-GaN layer was doped with magnesium to give a Hall concentration of 5 × 1017 cm−3. The p-type GaN cathodoluminescence (CL) spectra showed a strong 372 nm emission level in contrast to the 430 nm level observed in MOCVD samples. These layers were fabricated into 1 × 10 element detector arrays using a chlorine-based reactive ion etch (RIE) and refractory metal ohmic contacts. Peak responsivity of 0.11 AAV on detectors without anti-reflection coating were obtained at the GaN bandedge of 360 nm. The ultraviolet (UV) to visible rejection ratio was greater than 103 − 104 and was accredited to the reduction of the yellow defect levels in MBE material. Preliminary results on AlxGa1−xN detectors with responsivity peaks at 313 and 343 nm are presented as well.

The effect of ambient temperature on the detection sensitivity of carbon monoxide (CO) using ZnO nanorod-gated AlGaN/GaN high electron mobility transistor (HEMT) sensors was studied over a range of temperatures from 25 to 400 °C. An increase of the HEMT drain current was observed for exposure to the CO-containing ambients, due to chemisorbed oxygen on the ZnO surface reacting with CO to form CO2 and releasing electrons to the oxide surface, increasing the counter charges in the two-dimensional electron gas channel of the HEMT. By increasing the detection temperature from 25 °C to 150 °C, the CO detection sensitivity, ΔI/I, and detection limit were significantly improved from 0.23% to 7.5% and from 100 ppm to ∼30 ppm, respectively. However, the sensitivity of the CO detection was degraded by the decrease of mobility and saturation drain current of HEMT at temperatures higher than 200 °C.

InAs/InGaSb strained-layer superlattices have been deposited for long wavelength infrared (IR) detectors. The superlattice structure consists of alternating periods of InAs and InGaSb deposited upon GaSb or GaAs substrates. The lattice mismatch between the two layers is accommodated by strain which modifies the electronic and optical properties of the material. These properties can be controlled by varying the layer thickness, composition, and heterointerface properties. Molecular beam epitaxy was used to deposit the InAs/InGaSb superlattice and reflection high energy electron diffraction (RHEED) patterns were observed during growth. It was found that RHEED oscillations and x-ray diffraction patterns were heavily influenced by the incident As2 flux used during the InAs layers. Precise control of the As flux with a valve cracker resulted in RHEED intensity oscillations continuing throughout the superlattice and excellent x-ray diffraction patterns. Fast Fourier transform IR spectroscopy of the superlattices indicate absorption around 13 micrometers .© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The carbon monoxide (CO) detection sensitivities of ZnO nanorod-gated AlGaN/GaN high electron mobility transistors were measured over a range of temperatures from 25–150 °C. Once the sensor was exposed to the CO-containing ambient, the drain current, I, of the high electron mobility transistors increased due to chemisorbed oxygen on the ZnO surface reacting with CO, forming CO2 and releasing electrons to the oxide surface. Although the sensor could detect CO as low as 100 ppm at room temperature, the detection sensitivity, ΔI/I, was only around 0.23%. By increasing the sensor temperature to 150 °C, the detection sensitivity was improved by a factor of over 30% to 7.5%.

Performance of many III-V compound heterostructure devices relies on exact control of dopant placement and concentration. This requires the dopant source to be responsive and reproducible. We report a novel electron beam evaporation source for both carbon and silicon dopant incorporation in MBE growth of III-V compounds. Carbon in particular has attracted much interest as a replacement of beryllium for p-type doping in III-V compounds. Not only high doping levels are achievable, its low diffusion coefficient makes abrupt interface possible even after high temperature processing and operation. This source provides a convenient means for generating high carbon flux. It is equipped with a compact flux monitor such that the dopant levels can be adjusted in real time. The GaAs samples show excellent photoluminescence peaks and mobility results. Good carbon doping for GaSb was reported for the first time.

Pyrometry Interferometry (PI) is a powerful technique for in-situ sensing of the wafer temperature and growth rate. Evaluation of the two parameters would allow exact process control required for sophisticated device fabrication and material processing. The PI technique analyzes the interference patterns of the thermal radiation from the growing layer with a changing thickness d at growth temperature T. Since it is non-contact, applicable to all semiconductor materials and insensitive to wafer motion, the method is an ideal candidate for real time process control. We use a reflection assisted method to aid real time computation of these parameters. One could select the wavlength of interest to optimize the temperature and layer thickness resolution. We present data on MBE grown quarter wavelength stacks of GaAs and AlAs, and silicon oxidation to show P1 is extremely useful for growth of surface emitting laser and for silicon processing.

The effects of relative humidity on sensing characteristics of Pt-gated AlGaN/GaN high electron mobility transistor diode based hydrogen sensors were investigated. The absorbed water and oxygen molecules blocked available Pt surface adsorption sites for H2 absorption and reduced the hydrogen sensing sensitivity compared to low humidity conditions. The hydrogen sensing sensitivity decreased proportional to the relative humidity. However, the presence of humidity improved the sensor recovery characteristics after exposure to the hydrogen ambient.

Endocrine disrupters are known to have negative effects on the environment and human health. Real time detection of vitellogenin, an endocrine disrupter biomarker, was demonstrated using AlGaN/GaN high electron mobility transistors (HEMTs). Anti-vitellogenin antibodies were chemically anchored to the gold-coated gate area of the HEMT and immobilized with thioglycolic acid. The potential difference that occurs from the vitellogenin antigen-antibody interaction-induced caused a drain current change in the HEMT. The HEMT sensor was tested for vitellogenin detection both in phosphate buffer saline and largemouth bass serum.

Enhancement mode AlN/GaN high electron mobility transistors (HEMTs) were fabricated from originally depletion-mode structures using oxygen plasma treatment on the gate area prior to the gate metallization. Starting with a depletion mode AlN/GaN HEMT, the threshold voltage of the HEMT could be shifted from −3.2 to 1 V depending on the oxygen plasma treatment time to partially convert the AlN barrier layer into Al oxide. The gate current was reduced and the current-voltage curves show metal-oxide semiconductor diodelike characteristics after oxygen plasma treatment.

Antibody-functionalized, Au-gated AlGaN∕GaN high electron mobility transistors (HEMTs) were used to detect botulinum toxin. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN∕GaN HEMT drain-source current showed a rapid response of less than 5s when the target toxin in a buffer was added to the antibody-immobilized surface. We could detect a range of concentrations from 1to10ng∕ml. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN∕GaN HEMTs for botulinum toxin detection.

Electrical and microstructural characterizations of the Ti/Al/Mo/Au Ohmic contacts to ultrathin AlN/GaN heterostructures were carried out. It was found that as-deposited contacts had linear I-V behavior due to high tunneling current across the thin AlN barrier. A contact resistance of 0.455 Ω mm was obtained for samples annealed at 800 °C without any premetallization plasma treatment. Transmission electron microscopy studies showed that despite the use of Ti, the AlN layer remained intact. Mushroom-shaped TiN protrusions were formed only along threading dislocations, which terminated in the AlN layer. The TiN protrusions acted as metal plugs/spikes thereby aiding carrier transport.

The origin of hysteresis in capacitance-voltage (C-V) characteristics was studied for Schottky diodes prepared on AlGaN∕GaN transistor structures with GaN (Fe) buffers. The application of reverse bias leads to a shift of C-V curves toward higher positive voltages. The magnitude of the effect is shown to increase for lower temperatures. The phenomenon is attributed to tunneling of electrons from the Schottky gate to localized states in the structure. A technique labeled “reverse” deep level transient spectroscopy was used to show that the deep traps responsible for the hysteresis have activation energies of 0.25, 0.6, and 0.9eV. Comparison with deep trap spectra of GaN buffers and Si doped n-GaN films prepared on GaN buffers suggests that the traps in question are located in the buffer layer.