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We report sensitive and selective AlGaN/GaN High Electron Mobility Transistor (HEMT)-based sensor for Lead ion (Pb2+) detection. The gate region of the HEMT was functionalized by 2,5-dimercapto-1,3,4-thiadiazole (DMTD). The response of the sensor is observed by monitoring drain to source current ( $\text{I}_{\textsf {DS}}$ ) for different concentrations of Pb2+ ions at a fixed drain to source voltage ( $\text{V}_{\textsf {DS}}$ ). Our sensor reaches the lower detection limit of 0.018 ppb, which is much lower than the standard detection limit recommended by the World Health Organization (WHO) for drinking water. Furthermore, the sensor exhibited a rapid response time of ~4 seconds and high sensitivity of $0.607~\mu \text{A}$ /ppb. Moreover, the selectivity analysis was performed and found that the sensor was highly selective towards Pb2+ ions. The change in electron concentration at 2-dimensional electron gas (2DEG) upon the capture of Pb2+ ions at gate region by DMTD, causes a change in the $\text{I}_{\textsf {DS}}$ , which showed excellent sensing response towards Pb2+ ions. The highly sensitive, selective, and rapid detection of Pb2+ ions paves the way for stable sensing performance based on DMTD functionalized AlGaN/GaN HEMT sensor.

This paper demonstrates a novel AlGaN/GaN high-electron mobility transistor (HEMT)-based cadmium ion (Cd2+) sensor with mercaptopropionic acid (MPA) and glutathione (GSH) functionalization. The sensing response of the sensor was analyzed by detecting Cd2+ ions at different concentrations. The AlGaN/GaN HEMT sensor exhibits excellent response with the sensitivity of $0.241~\mu \text{A}$ /ppb, a fast response time of ~ 3 s, and a lower detection limit of 0.255 ppb. The observed lower detection limit is significantly lower than the World Health Organization (WHO) standard recommended limit for Cd2+ ions in drinking water. Furthermore, the sensor showed good selectivity of Cd2+ ions toward other heavy metal ions. The results indicate that the binding properties of GSH to Cd and the sensitivity of 2-D electron gas toward the variation of charges at the gate region make the device highly sensitive with rapid detection of Cd2+ ions.

In the present work, studies have been carried out on the structural and morphological properties of kesterite Copper Zinc Tin Sulphide (CZTS) thin films deposited on Molybdenum (Mo) foils. The effect of oxygen content in CZTS processed at various sulphurization temperatures i.e. 550 °C to 700 °C is studied. Structural properties of these samples are evaluated by using X-ray diffraction (XRD), crystalline quality decreases with the increase of oxygen level (∼ 52 % atomic weight percentage) at 550 °C and 700 °C sulphurization temperatures. Phase formation of CZTS and existence of secondary phases are further confirmed by Raman spectroscopy. Formation of nanorods with higher oxygen level at 550 °C and 700 °C are observed. The atomic percentages of Zn/Sn, S/metal, Cu/(Zn+Sn) are also dependent on sulphurization temperatures and the oxygen atomic weight percentages.

Compared with bulk-Si wafer, Al x Ga1– x N/gallium nitride (GaN) heterostructures grown on a 150-mm silicon-on-insulator (SOI) substrate with a 35-nm-thick Si overlayer are shown to have $\sim 50$ % less wafer bowing. As a result, the 2-D electron gas mobility and the sheet-resistivity uniformity on SOI are improved due to a lower defect density. In terms of device performance, high-electron-mobility transistors (HEMTs) fabricated on the Al x Ga1– x N/GaN-on-SOI exhibit $\sim 20.5$ % higher saturation drain current as compared with the bulk-Si counterparts. However, due to the poorer conductivity of the buried oxide layer, the Al x Ga1– x N/GaN-on-SOI HEMT suffers greater self-heating, with $\sim 50$ K higher channel temperature. With mitigation of self-heating, the Al x Ga1– x N/GaN-on-SOI, in view of its more superior structural and thermal stability, should offer an attractive alternative for integration of the GaN technology with the Si CMOS platform.

Structural properties and solar cells performance of sputter grown sub-micrometer thick (~600 nm) Cu 2 ZnSnS 4 (CZTS) films are investigated for low cost and large scale photovoltaic deployment. The CZTS films are deposited using single quaternary stoichiometric target at room temperature. X-ray diffraction, micro-Raman spectroscopy, and scanning electron microscopy measurements were carried out to investigate the structural property and secondary phase formation. The intense phonon peak at 334 cm −1 in the visible Raman spectra confirms the deposition of single phase CZTS films. CZTS-based thin film solar cells show excellent performance with open circuit voltage ~600 mV and over all cell efficiency of ~3.07%, exhibiting commercial potential toward large scale fabrication of photovoltaic devices.

A low-temperature fabrication of Al $x$ Ga 1–x N/GaN MISHEMTs on a 200-mm GaN-on-Si substrate using a common metal stack (Ti/Al/NiV) for concurrent source, drain, and gate formation is demonstrated. An ohmic contact resistance of 0.8 $\Omega $ -mm and a specific contact resistivity of $6\times 10^{\mathrm {-6}} \Omega $ -cm2 are achieved at an annealing temperature of 500 °C. The fabricated MISHEMTs have excellent electrical characteristics: an $I_{\mathrm{\scriptscriptstyle ON}}/I_{\mathrm{\scriptscriptstyle OFF}}$ ratio of nine decades (due to subnanoampere off -state leakage current up to a −15 V gate bias), and a subthreshold swing of 80 mV/decade (by virtue of a relatively low Al0.23Ga0.77N/GaN interface-trap density on the order of $10^{11}$ cm $^{\mathrm {-2}}$ eV $^{\mathrm {-1}}$ ). The demonstrated approach is thus a promising alternative toward the gold-free GaN-on-Si integration.

A sensor for highly sensitive, selective, and rapid determination of the trace amount of toxic Hg2+ ions is developed for the first-time using molybdenum disulfide (MoS2) functionalized AlGaN/GaN high electron mobility transistor (HEMT). The vertically aligned, flower-like MoS2 structures are synthesized through a simple hydrothermal route and applied on the gate region of AlGaN/GaN HEMT. The scanning electron microscopy, Raman spectroscopy, and X-ray diffraction are performed for structural characterization of MoS2. Further, the sensing of Hg2+ ions is performed by electrical characterizations of MoS2 functionalized AlGaN/GaN HEMT. The sensor showed an excellent sensitivity of 0.64 μA/ppb and detection limit of 0.01152 ppb with the rapid response time of 1.8 s. The sensor exhibits the linear range of detection from 0.1 ppb to 100 ppb and highly selective behavior towards Hg2+ ions. The results demonstrated that the MoS2 possess excellent Hg2+ ions capture property, that could be attributed to the complexation of Hg2+ ions with sulfur and the electrostatic interaction between MoS2 and Hg2+ ions alters the drain to source current (IDS) of the HEMT at a constant drain to source voltage (VDS). Therefore, the MoS2 based AlGaN/ GaN HEMT devices have a huge potential for next-generation ion sensing applications.

We have carried out a systematic study on the vertical breakdown behavior of GaN-based epitaxial layers on 200-mm-diameter Si(111) substrates with different carbon concentrations. The GaN layers were grown by metalorganic chemical vapor deposition by tuning growth parameters such as temperature and pressure. This study shows that, when the carbon concentration of the wafers is below 1 × 1018 cm−3, the vertical leakage currents are high with breakdown voltages below 260 V. The wafers that reveal lower vertical leakage currents are those with moderate carbon doping concentrations (between ∼1 × 1018 cm−3 and 2.5 × 1018 cm−3), whereas wafers with higher carbon concentrations (≥4 × 1018 cm−3) give rise to poor breakdown behavior caused by lower crystal quality and rougher surface morphology. The uniformity of vertical breakdown behavior across a 200-mm wafer shows that wafers with low crystal quality also exhibit poor uniformity. Moreover, when the carbon doping level is relatively high (between ∼2 × 1018 cm−3 and 2.5 × 1018 cm−3), good uniformity with variation of ∼5% can be achieved.

Nonpolar a-plane InN films were grown on r-plane sapphire substrate by plasma assisted molecular beam epitaxy with GaN underlayer. Effect of growth temperature on structural, morphological, and optical properties has been studied. The growth of nonpolar a-plane (1 1 -2 0) orientation was confirmed by high resolution X-ray diffraction study. The film grown at 500 °C shows better crystallinity with the rocking curve FWHM 0.67° and 0.85° along [0 0 0 1] and [1 -1 0 0] directions, respectively. Scanning electron micrograph shows formation of Indium droplets at higher growth temperature. Room tem-perature absorption spectra show growth temperature dependent band gap variation from 0.74–0.81 eV, consistent with the expected Burstein–Moss effect. The rectifying behaviour of the I–V curve indicates the existence of Schottky barrier at the InN and GaN interface. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InxAl1-xN/AlN/GaN-based high electron mobility transistor (HEMT) structures are epitaxially grown on 200 mm diameter Si(111) substrates by a metal organic chemical vapor deposition technique. The structural and electrical properties of the nitride layers are addressed by high-resolution X-ray diffraction and Hall effect measurements. The fabricated HEMTs with Au-free contact metallization schemes have resulted in an ON-OFF current ratio ∼10 5 . Ti/Al/Ni/W-based Ohmic source/drain contacts, with 1.5 μm RuOx Schottky gate on In0.15Al0.85N/AlN/GaN HEMTs at VDS = 10 V, show a maximum transconductance (gm) and saturation drain current (IDSAT )o f 0.19± 0.01 S/mm and 0.82 ± 0.02 A/mm, respectively, with a reasonable uniformity from the center to

This report focuses on the structural and optical properties of the GaN films grown on p-Si (100) substrates along with photovoltaic characteristics of GaN/p-Si heterojunctions fabricated with substrate nitridation and in absence of substrate nitridation. The high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), Raman and photoluminescence (PL) spectroscopic studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN epifilms grown with silicon nitride buffer layer when compared with the sample grown without silicon nitride buffer layer. The low temperature PL shows a free excitonic (FX) emission peak at 3.51 eV at the temperature of 5 K with a very narrow line width of 35 meV. Temperature dependent PL spectra follow the Varshni equation well and peak energy blue shifts by similar to 63 meV from 300 to 5 K. Raman data confirms the strain free nature and reasonably good crystallinity of the films. The GaN/p-Si heterojunctions fabricated without substrate nitridation show a superior photovoltaic performance compared to the devices fabricated in presence of substrate nitridation. The discussions have been carried out on the junction properties. Such single junction devices exhibit a promising fill factor and conversion efficiency of 23.36 and 0.12 %, respectively, under concentrated AM1.5 illumination.

The structural and optical properties of semipolar (1 1 -2 2) GaN grown on m-plane (1 0 -1 0) sapphire substrates by molecular beam epitaxy were investigated. An inplane orientation relationship was found to be [1 -1 0 0] GaN || [1 2-1 0] sapphire and [-1 -1 2 3] GaN || [0 0 0 1] sapphire for semipolar GaN on m-plane sapphire substrates. The near band emission (NBE) was found at 3.432 eV, which is slightly blue shifted compared to the bulk GaN. The Raman E2 (high) peak position observed at 569.1 cm–1, which indicates that film is compressively strained. Schottky barrier height (φb) and the ideality factor (η) for the Au/ semipolar GaN Schottky diode found to be 0.55 eV and 2.11, respectively obtained from the TE model. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The thermal oxidation process of the indium nitride (InN) nanorods (NRs) was studied. The SEM studies reveal that the cracked and burst mechanism for the formation of indium oxide (In2O3) nanostructures by oxidizing the InN NRs at higher temperatures. XRD results confirm the bcc crystal structure of the as prepared In2O3 nanostructures. Strong and broad photoluminescence spectrum located at the green to red region with maximum intensity at 566 nm along with a weak ultraviolet emission at 338 nm were observed due to oxygen vacancy levels and free excitonic transitions, respectively. The valence band onset energy of 2.1 eV was observed from the XPS valence band spectrum, clearly justifies the alignment of Fermi level to the donor level created due to the presence of oxygen vacancies which were observed in the PL spectrum. The elemental ratio In:O in as prepared In2O3 was found to be 42:58 which is in close agreement with the stoichiometric value of 40:60. A downward shift was observed in the Raman peak positions due to a possible phonon confinement effect in the nanoparticles formed in bursting mechanism. Such single junction devices exhibit promising photovoltaic performance with fill factor and conversion efficiency of 21% and 0.2%, respectively, under concentrated AM1.5 illumination.

The temperature dependent electrical transport behavior of n-n InGaN/Si heterostructures grown by plasma-assisted MBE was studied. Structural characteristics of the as-grown InGaN epilayers were evaluated high resolution X-ray diffraction and composition of InGaN was estimated from photoluminescence spectra using standard Vegard's law. Current density-voltage plots (J-V-T) revealed that the ideality factor (eta) and Schottky barrier height (SBH) (Phi(b)) are temperature dependent and the incorrect values of the Richardson's constant (A**) produced, suggests an inhomogeneous barrier at the heterostructure interface. The higher value of the ideality factor compared to the ideal value and its temperature dependence suggest that the current transport is mainly dominated by thermionic field emission. (C) 2012 Elsevier B.V. All rights reserved.

The substrate effect on InN nanostructures grown by droplet epitaxy has been studied. InN nanostructures were fabricated on Si(111), silicon nitride/Si(111), AlN/Si(111) and Ge(100) substrates by droplet epitaxy using an RF plasma nitrogen source. The morphologies of InN nanostructures were investigated by field emission scanning electron microscopy (FESEM). The chemical bonding configurations of InN nanostructures were examined by x-ray photoelectron spectroscopy (XPS). Photoluminescence spectrum slightly blue shifted compared to the bulk InN, indicating a strong Burstein–Moss effect due to the presence of high electron concentration in the InN dots (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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We present an extensive study on the structural, electrical and optical properties of InN thin films grown on c-Al2O3, GaN(130 nm)/Al2O3, GaN(200 nm)/Al2O3 and GaN(4 mu m)/Al2O3 by using plasma-assisted molecular beam epitaxy. The high resolution X-ray diffraction study reveals better crystalline quality for the film grown on GaN(4 mu m)/Al2O3 as compared to others. The electronic and optical properties seem to be greatly influenced by the structural quality of the films, as can be evidenced from Hall measurement and optical absorption spectroscopy. Kane's k.p model was used to describe the dependence of optical absorption edge of InN films on carrier concentration by considering the non-parabolic dispersion relation for carrier in the conduction band. Room temperature Raman spectra for the InN films grown on GaN show the signature of residual tensile stress in contrast to the compressive stress observed for the films grown directly on c-Al2O3. (C) 2012 Elsevier B.V. All rights reserved.

We present the study involving the dependence of carrier concentration of InN films, grown on GaN templates using the plasma assisted molecular beam epitaxy system, on growth temperature. The influence of InN carrier concentration on the electrical transport behavior of InN/GaN heterostructure based Schottky junctions is also discussed. The optical absorption edge of InN film was found to be strongly dependent on carrier concentration, and was described by Kane's k.p model, with non-parabolic dispersion relation for carrier in the conduction band. The position of the Fermi-level in InN films was modulated by the carrier concentration in the InN films. The barrier height of the heterojunctions as estimated from I – V characteristic was also found to be dependent on the carrier concentration of InN.

The n-type GaN layers were grown by plasma-assisted MBE and either intentionally doped with Si or unintentionally doped. The optical characteristics of a donor level in Si-doped, GaN were studied in terms of photoluminescence (PL) spectroscopy as a function of electron concentration. Temperature dependent PL measurements allowed us to estimate the activation energy of a Si-related donor from temperature-induced decay of PL intensity. PL peak positions, full width at half maximum of PL and activation energies are found to be proportional to the cube root of carrier density. The involvement of donor levels is supported by the temperature-dependent electron concentration measurements. (C) 2012 Elsevier Ltd. All rights reserved.

In this report, the currentvoltage (IV) characteristics of Au/GaN Schottky diodes have been carried out in the temperature range of 300510?K. The estimated values of the Schottky-barrier height (SBH) and the ideality factor of the diodes based on the thermionic emission (TE) mechanism were found to be temperature dependent. The barrier height was found to increase and the ideality factor to decrease with increasing temperature. The conventional Richardson plot of ln(Is/T2) versus 1/kT gives the SBH of 0.51?eV and Richardson constant value of 3.23?X?10-5?A?cm-2?K-2 which is much lower than the known value of 26.4?A?cm-2?K-2 for GaN. Such discrepancies of the SBH and Richardson constant value were attributed to the existence of barrier-height inhomogeneities at the Au/GaN interface. The modified Richardson plot of ln(Is/T2)q2 sigma 2/2k2T2 versus q/kT, by assuming a Gaussian distribution of barrier heights at the Au/GaN interface, provided the SBH of 1.47?eV and Richardson constant value of 38.8?A?cm-2?K-2. The temperature dependence of the barrier height is interpreted on the basis of existence of the Gaussian distribution of the barrier heights due to the barrier-height inhomogeneities at the Au/GaN interface.

Ultra thin films of pure beta-Si3N4 (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using beta-Si3N4 layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/beta-Si3N4/ Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the beta-Si3N4 /Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 +/- 0.05 eV below that of beta-Si3N4 and a type-II heterojunction. The conduction band offset was deduced to be similar to 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/ beta-Si3N4 interface formation. (c) 2011 Elsevier B.V. All rights reserved.

GaN/Si3N4/n-Si and InN/Si3N4/n-Si heterojunctions (HJs) were fabricated using plasma-assisted molecular beam epitaxy for a comparison study. Single-crystalline wurtzite structures of GaN and InN epilayers were confirmed by high-resolution X-ray diffraction and thickness of ultrathin Si3N4 layer was measured by transmission electron microscopy. n-GaN/Si3N4/n-Si HJs show diode-like rectifying current-voltage (I-V) characteristic, while n-InN/Si3N4/n-Si HJs show symmetric nonlinear I-V behavior. The I-V characteristics of both HJs were discussed in terms of the band diagram of HJs and the carrier transport mechanism. The activation energies of carrier conduction were estimated to be similar to 29 meV for GaN/Si3N4/Si and similar to 95 meV for InN/Si3N4/Si HJs. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In the present work, we report the growth of wurtzite InN epilayers on GaN/Si (1 1 1) substrate by plasma-assisted molecular beam epitaxy (PAMBE). The growth parameters such as indium flux, substrate temperature and RF power affect the crystallographic and morphological properties of InN layers, which were evaluated using high resolution X-ray diffraction (HRXRD) analysis and atomic force microscopy (AFM). It is found that excess indium (In) concentrations and surface roughness were increased with increase in In flux and growth temperature. The intensity of HRXRD (0 0 0 2) peak, corresponding to c-axis orientation has been increased and full width at half maxima (FWHM) has decreased with increase in RF power. It was found that highly c-axis oriented InN epilayers can be grown at 450 degrees C growth temperature, 450 W RF power and 1.30 x 10(-7) mbar In beam equivalent pressure (BEP). The energy gap of InN layers grown by optimizing growth conditions was determined by photoluminescence and optical absorption measurement. (C) 2011 Elsevier B.V. All rights reserved.

The electrical transport properties of InN/GaN heterostructure based Schottky junctions were studied over a wide temperature range of 200-500 K. The barrier height and the ideality factor were calculated from current-voltage (I-V) characteristics based on thermionic emission (TE), and found to be temperature dependent. The barrier height was found to increase and the ideality factor to decrease with increasing temperature. The observed temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. Such inhomogeneous behavior was modeled by assuming the existence of a Gaussian distribution of barrier heights at the heterostructure interface. (C) 2011 Elsevier Ltd. All rights reserved.

Investigations were carried out on the ambient condition oxidation of self-assembled, fairly uniform indium nitride (InN) quantum dots (QDs) fabricated on p-Si substrates. Incorporation of oxygen in to the outer shell of the QDs was confirmed by the results of transmission electron microscopy (TEM), X-ray photoemission spectroscopy (XPS). As a consequence, a weak emission at high energy (similar to 1.03?eV) along with a free excitonic emission (0.8?eV) was observed in the photoluminescence spectrum. The present results confirm the incorporation of oxygen into the lattice of the outer shell of InN QDs, affecting their emission properties. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Hexagonal Ge3N4 layer was prepared on Ge surface by in situ direct atomic source nitridation and it is promising buffer layer to grow GaN on Ge (111). The valence band offset (VBO) of GaN/Ge3N4/Ge heterojunctions is determined by X-ray photoemission spectroscopy. The valence band (VB) of Ge3N4 is found to be 0.38?+/-?0.04?eV above the GaN valance band and 1.14?+/-?0.04?eV below the Ge. The GaN/Ge3N4 and Ge3N4/Ge are found type-II and type-I heterojunctions, respectively. The exact measurements of the VBO and conduction band offset (CBO) are important for use of GaN/Ge3N4/Ge (111) heterosystems.

InN layers were directly grown on Ge substrate by plasma-assisted molecular beam epitaxy (PAMBE). The valence band offset (VBO) of wurtzite InN/Ge heterojunction is determined by X-ray photoemission spectroscopy (XPS). The valence band of Ge is found to be 0.18 ± 0.04 eV above that of InN and a type-II heterojunction with a conduction band offset (CBO) of ~ 0.16 eV is found. The accurate determination of the VBO and CBO is important for the design of InN/Ge based electronic devices.

The removal of native oxide from Si (1 1 1) surfaces was investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectra (SIMS) depth profiles. Two different oxide removal methods, performed under ultrahigh-vacuum (UHV) conditions, were carried out and compared. The first cleaning method is thermal desorption of oxide at 900 degrees C. The second method is the deposition of metallic gallium followed by redesorption. A significant decrease in oxygen was achieved by thermal desorption at 900 degrees C under UHV conditions. By applying a subsequent Ga deposition/redesorption, a further reduction in oxygen could be achieved. We examine the merits of an alternative oxide desorption method via conversion of the stable SiO(2) surface oxide into a volatile Ca(2)O oxide by a supply of Ga metals. Furthermore, ultra thin films of pure silicon nitride buffer layer were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma followed by GaN growth. The SIMS depth profile shows that the oxygen impurity can be reduced at GaN/beta-Si(3)N(4)/Si interfaces by applying a subsequent Ga deposition/redesorption. (C) 2011 Elsevier B.V. All rights reserved.

We report the structural and optical properties of a-plane GaN film grown on r-plane sapphire substrate by plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction was used to determine the out-of-plane and in-plane epitaxial relation of a-plane GaN to r-plane sapphire. Low-temperature photoluminescence emission was found to be dominated by basal stacking faults along with near-band emission. Raman spectroscopy shows that the a-GaN film is of reasonably good quality and compressively strained. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Indium nitride (InN) epilayers have been successfully grown by nitrogen-plasma-assisted molecular beam epitaxy (NPA-MBE) on Si (111) substrates using different buffer layers. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by high resolution X-ray diffraction (HRXRD). The Raman studies show the high crystalline quality and the wurtzite lattice structure of InN films on the Si substrate using different buffer layers and the InN/β–Si3N4 double buffer layer achieves minimum FWHM of E2 (high) mode. The energy gap of InN films was determined by optical absorption measurement and found to be in the range of ~ 0.73–0.78 eV with a direct band nature. It is found that a double-buffer technique (InN/β–Si3N4) insures improved crystallinity, smooth surface and good optical properties.

Photoluminescence (PL) of high quality GaN epitaxial layer grown on β-Si 3 N 4 /Si (1 1 1) substrate using nitridation–annealing–nitridation method by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated in the range of 5–300 K. Crystallinity of GaN epilayers was evaluated by high resolution X-ray diffraction (HRXRD) and surface morphology by Atomic Force Microscopy (AFM) and high resolution scanning electron microscopy (HRSEM). The temperature-dependent photoluminescence spectra showed an anomalous behaviour with an ‘S-like’ shape of free exciton (FX) emission peaks. Distant shallow donor–acceptor pair (DAP) line peak at approximately 3.285 eV was also observed at 5 K, followed by LO replica sidebands separated by 91 meV. The activation energy of the free exciton for GaN epilayers was also evaluated to be ∼27.8±0.7 meV from the temperature-dependent PL studies. Low carrier concentrations were observed ∼4.5±2×10 17 cm −3 by measurements and it indicates the silicon nitride layer, which not only acts as a growth buffer layer, but also effectively prevents Si diffusion from the substrate to GaN epilayers. The absence of yellow band emission at around 2.2 eV signifies the high quality of film. The tensile stress in GaN film calculated by the thermal stress model agrees very well with that derived from Raman spectroscopy.

Negative differential capacitance (NDC) has been observed in n-GaN/p-Si heterojunctions grown by plasma assisted molecular beam epitaxy (PAMBE). The NDC is observed at low frequencies 1 and 10 kilohertz (kHz) and disappeared at a higher testing frequency of 100 kHz. The NDC is also studied with temperature and found that it has disappeared above 323 ∘C. Current–Voltage ( I – V ) characteristics of n-GaN /p-Si heterojunction were measured at different temperatures and are attributed to the space-charge-limited current (SCLC). A simple model involving two quantum states is proposed to explain the observed NDC behavior.

The electrical transport behavior ofn-nindium nitride nanodot-silicon (InN ND-Si) heterostructure Schottky diodes is reported here, which have been fabricated by plasma-assisted molecular beam epitaxy. InN ND structures were grown on a 20 nm InN buffer layer on Si substrates. These dots were found to be single crystalline and grown along [0 0 0 1] direction. Temperature-dependent current density-voltage plots (J-V-T) reveal that the ideality factor (η) and Schottky barrier height (SBH) (ΦB) are temperature dependent. The incorrect values of the Richardson constant (A**) produced suggest an inhomogeneous barrier. Descriptions of the experimental results were explained by using two models. First one is barrier height inhomogeneities (BHIs) model, in which considering an effective area of the inhomogeneous contact provided a procedure for a correct determination ofA**. The Richardson constant is extracted ~110 A cm-2K-2using the BHI model and that is in very good agreement with the theoretical value of 112 A cm-2K-2. The second model uses Gaussian statistics and by this, mean barrier heightΦ0andA**were found to be 0.69 eV and 113 A cm-2K-2, respectively.

Ultra thin films of pure silicon nitride were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2 p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si 3 N 4 (Si 4+ ) but also in the intermediate nitridation states with one (Si 1+ ) or three (Si 3+ ) nitrogen nearest neighbors. The Si 2 p core-level shifts for the Si 1+ , Si 3+ , and Si 4+ components are determined to be 0.64, 2.20, and 3.05 eV, respectively. In annealed sample it has been observed that the Si 4+ component in the Si 2 p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si 3 N 4 by molecular beam epitaxy (MBE).

Non-polar a-plane GaN films were grown on an r-plane sapphire substrate by plasma assisted molecular beam epitaxy (PAMBE). The effect of growth temperature on structural, morphological and optical properties has been studied. The growth of non-polar a-plane (1 1 −2 0) orientation of the GaN epilayers were confirmed by high resolution X-ray diffraction (HRXRD) study. The X-ray rocking curve (XRC) full width at half maximum of the (1 1 −2 0) reflection shows in-plane anisotropic behavior and found to decrease with increase in growth temperature. The atomic force micrograph (AFM) shows island-like growth for the film grown at a lower temperature. Surface roughness has been decreased with increase in growth temperature. Room temperature photoluminescence shows near band edge emission at 3.434–3.442 eV. The film grown at 800 °C shows emission at 2.2 eV, which is attributed to yellow luminescence along with near band edge emission.

InN quantum dots (QDs) were fabricated on Si(111) substrate by droplet epitaxy using an RF plasma-assisted MBE system. Variation of the growth parameters, such as growth temperature and deposition time, allowed us to control the characteristic size and density of the QDs. As the growth temperature was increased from 100 C to 300 degrees C, an enlargement of QD size and a drop in dot density were observed, which was led by the limitation of surface diffusion of adatoms with the limited thermal energy. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to assess the QDs size and density. The chemical bonding configurations of InN QDs were examined by X-ray photo-electron spectroscopy (XPS). Fourier transform infrared (FTIR) spectrum of the deposited InN QDs shows the presence of In-N bond. Temperature-dependent photoluminescence (PL) measurements showed that the emission peak energies of the InN QDs are sensitive to temperature and show a strong peak emission at 0.79 eV.

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth.

One of the scientific challenges of growing InN quantum dots (QDs), using Molecular beam epitaxy (MBE), is to understand the fundamental processes that control the morphology and distribution of QDs. A systematic manipulation of the morphology, optical emission, and structural properties of InN/Si (111) QDs is demonstrated by changing the growth kinetics parameters such as flux rate and growth time. Due to the large lattice mismatch, between InN and Si (~8%), the dots formed from the Strannski–Krastanow (S–K) growth mode are dislocated. Despite the variations in strain (residual) and the shape, both the dot size and pair separation distribution show the scaling behavior. We observed that the distribution of dot sizes, for samples grown under varying conditions, follow the scaling function.

The indium nitride (InN)-based nanometric-objects were grown directly on a c-sapphire substrate by using plasma-assisted molecular beam epitaxy (PAMBE) at different substrate temperatures. High resolution X-ray diffraction (HRXRD) reveals the InN (0002) reflection and full width at half maximum (FWHM) found to be decreased with increasing the growth temperature. The size, height and density of the grown nanometric-objects studied by scanning electron microscopy (SEM) has remarkable differences, evidencing the decisive role of substrate temperature. Photoluminescence (PL) studies revealed that the emission energy is shifted towards the higher side from the bulk value, i.e., a blue shift in the PL spectra was observed. The temperature dependence of the PL peak position shows an ``S-shaped'' emission energy shift, which can be attributed to the localization of carriers in the nanometric-objects.

Nanosized hexagonal InN flower-like structures were fabricated by droplet epitaxy on GaN/Si(111) and GaN flower-like nanostructure fabricated directly on Si(111) substrate using radio frequency plasma-assisted molecular beam epitaxy. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the crystallinity and morphology of the nanostructures. Moreover, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to investigate the chemical compositions and optical properties of nano-flowers, respectively. Activation energy of free exciton transitions in GaN nano-flowers was derived to be ∼28.5 meV from the temperature dependent PL studies. The formation process of nano-flowers is investigated and a qualitative mechanism is proposed.

Ferroelectric/semiconductor heterostructures have attracted great interest for future optoelectronic and electronic devices due to ferroelectric polarization induced interfacial charge coupling at the junction. Here, we report on the fabrication of GaN/BaTiO3 (GaN/BTO) heterostructure based Schottky junction on c-sapphire using pulsed laser deposition and its performance in UV detection. The Schottky-type BTO/GaN heterojunction for high temperature selective ultraviolet photo detection is demonstrated ranging from 313 to 423 K. The temperature-dependent parameters such as responsivity (R λ ), sensitivity (S), specific detectivity (D *), and barrier height ( b) and ideality factor (η) of the device have been quantified. Calculated parameters are systematically compared with the effect of dark and UV light. The responsivity increases with respect to temperature up to 413 K and then decrease. The highest UV (λ = 360 nm) responsivity is found to be 45.54 and 45.21 A W−1, at temperature 393 and 413 K respectively under +5 V bias. The Schottky barrier height of BTO/GaN heterojunction increased with increasing temperature in presence of dark as well as UV light while ideality factor decrease in both the cases. Overall, the efficiency of BTO/GaN device is optimized at 413 K. We have observed high sensitivity of BTO/GaN device as compared to GaN device with respect to applied bias.

The room temperature ferromagnetic behavior of InN nanostructures grown by molecular beam epitaxy (MBE) is explored by means of magnetization measurements. The saturation magnetization and remanent magnetization are found to be strongly dependent on the size of the nanostructures. This suggests that the ferromagnetism is essentially confined to the surface of the nanostructures due to the possible defects. Raman spectroscopy shows the existence of indium vacancies which could be the source of ferromagnetic ordering in InN nanostructures.

We report on the ferroelectric polarization-induced switchable interfacial coupling modulations in BaTiO3/GaN heterojunction transport behaviour. The ferroelectric barium titanate, BaTiO3 (BTO) was integrated with polar semiconductor gallium nitride (GaN). BTO with a tetragonal structure was deposited on a wurtzite (0 0 0 1) epitaxial GaN/c-Al2O3 substrate by pulsed laser deposition, which was further confirmed by x-ray diffraction and Raman spectroscopy. BTO/GaN heterojunctions with resistive switching behaviour exhibited modulations in transport characteristics due to the interfacial coupling. The ferroelectric nature and interfacial coupling effect of this heterojunction was confirmed with the help of piezo-response force microscopy. A valence band offset of 0.82 eV and conduction band offset of 0.62 eV were obtained for BTO/GaN heterojunctions by x-ray photo-electron spectroscopy. This interfacial coupling phenomenon was analysed and its effect on the carrier conduction in the heterojunction was investigated by band alignment studies.

In this letter, the effect of Ge diffusion at the gate area of AlxGa1-xN/GaN high electron mobility transistors (HEMTs) on a thin silicon-on-insulator (SOI) substrate has been investigated. The pinch-off voltage shifted toward enhancement mode type operation behavior due to the Ge diffusion through the surface of the thin GaN cap layer. An anomalous hump observed in high frequency C-V plot is due to the electron confinement at the Ge-GaN/AlGaN interface. The threshold voltage reduces by 0.8 to 1.0 V after Ge diffusion. The drive current and the transconductance further reduced as compared to the control sample due to the parallel channel formation at the top Ge-GaN/AlxGa1-xN interfaces. Published version

Multiple energies of heavy ion implantation with inert-gas ion (84Kr+) were carried out on AlGaN/GaN high-electron-mobility transistors (HEMTs) for planar device isolation. Thermal stability of the implantated samples were also investigated by isochronal annealing at 500, 600, 700, and 800 °C (each temperature for 1 h.). Due to the damages created by heavy ions (84Kr+) in the GaN lattice, the implant-isolated Al0.27Ga0.73N/GaN HEMT samples exhibited better thermal stability than 40Ar+-implant-isolation. This was also confirmed by Rutherford backscattering spectrometry in channeling condition and ultraviolet micro-Raman spectroscopy measurements. With reference to mesa-isolated AlGaN/GaN HEMTs, the buffer breakdown voltage is also stable in the implant-isolated AlGaN/GaN HEMTs. An enhanced OFF-state breakdown voltage was also realized in the implant-isolated AlGaN/GaN HEMTs. The inert gas heavy ion implantation (84Kr+) is a viable solution for the fabrication of thermally stable planar AlGaN/GaN HEMTs even...

The authors report on the fabrication and characterization of low-temperature processed gold-free Ohmic contacts for AlxGa1−xN/GaN high electron mobility transistors (HEMTs). The HEMT structure grown on a 200-mm diameter Si(111) substrate is used in this study. Using the Ti/Al/NiV metal stack scheme, the source/drain Ohmic contact optimization is accomplished through the variation of Ti/Al thickness ratio and thermal annealing conditions. For an optimized Ti/Al stack thickness (20/200 nm) annealed at 500 °C for 30 s with smooth contact surface morphology, a specific contact resistivity of ∼6.3 × 10−6 Ω cm2 is achieved. Furthermore, with gold-free Ni/Al gates, the fabricated HEMTs exhibit ION/IOFF ratio of ∼109 and a subthreshold swing of ∼71 mV/dec. The demonstrated gold-free contact schemes thus provide a solution toward the implementation of GaN-based HEMT process on a Si foundry platform. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

This paper describes a method to optimally combine wide band gap Al2O3 with high dielectric constant (high-κ) Ta2O5 for gate dielectric applications. (Ta2O5)x(Al2O3)1−x thin films deposited by thermal atomic layer deposition (ALD) on GaN-capped AlxGa1−xN/GaN high electron mobility transistor (HEMT) structures have been studied as a function of the Ta2O5 molar fraction. X-ray photoelectron spectroscopy shows that the bandgap of the oxide films linearly decreases from 6.5 eV for pure Al2O3 to 4.6 eV for pure Ta2O5. The dielectric constant calculated from capacitance-voltage measurements also increases linearly from 7.8 for Al2O3 up to 25.6 for Ta2O5. The effect of post-deposition annealing in N2 at 600 °C on the interfacial properties of undoped Al2O3 and Ta-doped (Ta2O5)0.12(Al2O3)0.88 films grown on GaN-HEMTs has been investigated. These conditions are analogous to the conditions used for source/drain contact formation in gate-first HEMT technology. A reduction of the Ga-O to Ga-N bond ratios at the oxide...