A trend of increased near-surface valence band maximum band bending with increasing O∕Ga relative... more A trend of increased near-surface valence band maximum band bending with increasing O∕Ga relative fraction was observed, extrapolating to 2.7eV±0.1eV for pristine GaN surfaces (0% O 1s peak area). This trend of apparent oxide overlayer coverage affecting the band bending linearly could lead to better understanding and characterization of oxidized GaN surfaces to control band bending for sensors or other devices.
Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attr... more Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attracted increasing interest of researchers. Indeed, the literature on the field reports apparently contradictory results about the effect of a metal on graphene doping. Here, we elucidate the effect of metal nanostructuring and oxidation on the metal work function (WF) and, consequently, on the charge transfer and doping of graphene/metal hybrids. We show that nanostructuring and oxidation of metals provide a valid support to frame WF and doping variation in metal/graphene hybrids. Chemical vapour-deposited monolayer graphene has been transferred onto a variety of metal surfaces, including d-metals, such as Ag, Au, and Cu, and sp-metals, such as Al and Ga, configured as thin films or nanoparticle (NP) ensembles of various average sizes. The metal-induced charge transfer and the doping of graphene have been investigated using Kelvin probe force microscopy (KPFM), and corroborated by Raman s...
Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows th... more Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows the production of large area graphene as required for its application as transparent conductive layer substituting ITO. CVD-graphene presents a polycrystalline structure that strongly influences both mechanical and electrical properties. Nowadays, it is well consolidated, among different users of graphene, that analyzing graphene grains after growth, is important for quality-control. In fact, graphene is a well ordered material and contains internal boundaries, commonly known as "grain boundaries". When graphene is grown, the carbon atoms within each growing grain are lined up in a specific pattern, depending on the crystal structure of sample. With growth, each grain impact others and forms interfaces where the atomic orientations differ. It has been established that the transport properties of the graphene improve as the grain size increases. Therefore, the growth conditions mu...
In this study, we present a two-dimensional dielectric grating which allows achieving high absorp... more In this study, we present a two-dimensional dielectric grating which allows achieving high absorption in a monolayer graphene at visible and near-infrared frequencies. Dielectric gratings create guided-mode resonances that are exploited to effectively couple light with the graphene layer. The proposed structure was numerically analyzed through a rigorous coupled-wave analysis method. Effects of geometrical parameters and response to the oblique incidence of the plane wave were studied. Numerical results reveal that light absorption in the proposed structure is almost insensitive to the angle of the impinging source over a considerable wide angular range of 20°. This may lead to the development of easy to fabricate and experimentally viable graphene-based absorbers in the future.
Graphene is a material with exceptional optical, electrical and physicochemical properties that c... more Graphene is a material with exceptional optical, electrical and physicochemical properties that can be combined with dielectric waveguides. To date, several optical devices based on graphene have been modeled and fabricated operating in the near-infrared range and showing excellent performance and broad application prospects. This paper covers the main aspects of the optical behaviour of graphene and its exploitation as electrodes in several device configurations. The work compares the reported optical devices focusing on the wavelength tuning, showing how it can vary from a few hundred up to a few thousand picometers in the wavelength range of interest. This work could help and lead the design of tunable optical devices with integrated graphene layers that operate in the NIR.
Europium-doped hydroxyapatite Ca10(PO4)6(OH)2 (3% mol) powders were synthesized by an optimized c... more Europium-doped hydroxyapatite Ca10(PO4)6(OH)2 (3% mol) powders were synthesized by an optimized chemical precipitation method at 25 °C, followed by drying at 120 °C and calcination at 450 °C and 900 °C. The obtained nanosized crystallite samples were investigated by means of a combination of inductively coupled plasma (ICP) spectroscopy, powder X-ray diffraction (PXRD), Fourier Transform Infrared (FTIR), Raman and photoluminescence (PL) spectroscopies. The Rietveld refinement in the hexagonal P63/m space group showed europium ordered at the Ca2 site at high temperature (900 °C), and at the Ca1 site for lower temperatures (120 °C and 450 °C). FTIR and Raman spectra showed slight band shifts and minor modifications of the (PO4) bands with increasing annealing temperature. PL spectra and decay curves revealed significant luminescence emission for the phase obtained at 900 °C and highlighted the migration of Eu from the Ca1 to Ca2 site as a result of increasing calcinating temperature.
In this paper, we numerically and experimentally demonstrate how to modulate the amplitude and ph... more In this paper, we numerically and experimentally demonstrate how to modulate the amplitude and phase of a microwave ring resonator by means of few-layers Chemical Vapour Deposition (CVD) graphene. In particular, both numerical and experimental results show a modulation of about 10 dB and a 90 degrees-shift (quadrature phase shift) when the graphene sheet-resistance is varied. These findings prove once again that graphene could be efficiently exploited for the dynamically tuning and modulation of microwave devices fostering the realization of (i) innovative beam-steering and beam-forming systems and (ii) graphene-based sensors.
The use of dry hydrogen plasma etching is evaluated for determination of GaN polarity and critica... more The use of dry hydrogen plasma etching is evaluated for determination of GaN polarity and critically compared to wet etching in NaOH. It is shown that hydrogen plasma etching is effective in revealing inversion domains (IDs) and some types of dislocations. This is because the surface morphology is unchanged by the hydrogen treatment, and, hence, the surface reactivity is not masked.
In this contribution, we address two critical and interesting aspects from both fundamental and t... more In this contribution, we address two critical and interesting aspects from both fundamental and technological point of views, which are the polarity of ZnO and the interface reactivity and stability to hydrogen and nitrogen. The effects of atomic hydrogen and nitrogen produced by radiofrequency (r.f. ,13.56 MHz) H2 and N2 plasmas and of temperature on the optical, compositional and structural properties of Zn- and O-polar ZnO have been studied. It is found that Zn-polar ZnO is highly reactive with atomic hydrogen while O-polar ZnO is almost inert. Conversely, both polarities react with nitrogen, with the O-polar ZnO showing a larger reactivity toward N-atoms than the Zn-polarity.
Journal of Vacuum Science Technology B Microelectronics and Nanometer Structures, 2002
The effects of sapphire nitridation temperature on the properties of the GaN epitaxial layers gro... more The effects of sapphire nitridation temperature on the properties of the GaN epitaxial layers grown by rf plasma-assisted molecular-beam epitaxy were investigated. It is found that a nitridation temperature as low as 200 °C acts as a singular point for producing a homogeneous AlN ...
GaN surface passivation processes are still under development and among others hydrogen treatment... more GaN surface passivation processes are still under development and among others hydrogen treatments are investigated. In this study, we use non-destructive optical and electrical probes such as spectroscopic ellipsometry (SE) and surface potential Kelvin probe microscopy (SP-KPM) in conjunction with non-contact atomic force microscopy (AFM) for the study of the different reactivity of Ga-polar and N-polar GaN epitaxial layers with atomic hydrogen. The GaN epitaxial layers are grown by molecular beam epitaxy on sapphire (0 0 0 1) substrates, and GaN and AlN buffer layers are used to grow N-polar and Ga-polar films, respectively. The atomic hydrogen is produced by a remote rf (13.56 MHz) H 2 plasma in order to rule out any ion bombardment of the GaN surface and make the interaction chemical. It is found that the interaction of GaN surfaces with atomic hydrogen depends on polarity, with N-polar GaN exhibiting greater reactivity. Furthermore, it is found that atomic hydrogen is effective in the passivation of grain boundaries and surface defects states.
Nitride materials are critical for a range of applications, including UV-visible light emitting d... more Nitride materials are critical for a range of applications, including UV-visible light emitting diodes (LEDs). Advancing the performance, reliability and synthesis of AlGaN/GaN and InGaN/GaN heterojunction devices requires a systematic methodology enabling characterization of key metric like alloy composition, thickness and quality possibly in real time. This contribution reports on the real time characterization of the plasma assisted molecular beam epitaxy of AlGaN/GaN and InGaN/GaN heterostructures. Spectroscopic ellipsometry real time monitoring has revealed a number of key process and material iusses, such as the roughening of the GaN templates depending on plasma exposure during the substrate cleaning step, the composition of the alloy and the growth mode. Parameters like the plasma conditions, the surface temperature and the atomic flow ratio are investigated to understand the interplay process-material composition-structure-optical properties.
Er 2 O 3 thin films have been deposited by low-pressure metalorganic chemical vapor deposition (M... more Er 2 O 3 thin films have been deposited by low-pressure metalorganic chemical vapor deposition (MOCVD) also plasma assisted (RP-MOCVD), using tris(isopropylcyclopentadienyl)erbium and O 2 on Si(1 0 0), Si(1 1 1) and corning glass substrates. The RP-MOCVD approach produced highly (1 0 0)-oriented, dense and mechanically stable Er 2 O 3 films with columnar structure, while films with (1 1 1) texture are deposited by MOCVD. A high refractive index of 2.1 at 589.3 nm comparable to that of bulk single crystalline Er 2 O 3 , a high transparency in the vis-near UV range and an optical band-gap of 6.5 eV have been found, which make Er 2 O 3 interesting as antireflective and protective coating. A static dielectric constant k $ 12, a density of interface traps as low as 4.2 Â 1010 cm 2 eV À1 , for 5-10 nm thick Er 2 O 3 layers grown on Si(1 0 0), render the present Er 2 O 3 films interesting also as high-k dielectric in CMOS devices.
The effect of sapphire nitridation temperature on the chemistry and microstructure of the sapphir... more The effect of sapphire nitridation temperature on the chemistry and microstructure of the sapphire substrate/GaN interface, nucleation layer, and of the GaN epilayers grown by rf plasma assisted molecular beam epitaxy is investigated. It is found that a sapphire nitridation temperature ...
Au nanoclusters have been deposited on Si (001) surfaces by sputtering of a metallic Au target us... more Au nanoclusters have been deposited on Si (001) surfaces by sputtering of a metallic Au target using an Ar plasma. Different wet and dry treatments of the Si (001) surface, including dipping in HF solution and exposure to H2 and N2 plasmas, have been applied and the ...
Fully microcrystalline silicon, μc-Si, thin films have been deposited on corning glass by plasma ... more Fully microcrystalline silicon, μc-Si, thin films have been deposited on corning glass by plasma enhanced chemical vapor deposition (PECVD) using SiH4-H2. The effects of the surface treatment and of the deposition temperature on microstructure of μc-Si films are investigated by “in situ” laser reflectance interferometry (LRI), “ex situ” spectroscopic ellipsometry (SE) and Raman spectroscopy. LRI indicated the existence of a
A trend of increased near-surface valence band maximum band bending with increasing O∕Ga relative... more A trend of increased near-surface valence band maximum band bending with increasing O∕Ga relative fraction was observed, extrapolating to 2.7eV±0.1eV for pristine GaN surfaces (0% O 1s peak area). This trend of apparent oxide overlayer coverage affecting the band bending linearly could lead to better understanding and characterization of oxidized GaN surfaces to control band bending for sensors or other devices.
Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attr... more Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attracted increasing interest of researchers. Indeed, the literature on the field reports apparently contradictory results about the effect of a metal on graphene doping. Here, we elucidate the effect of metal nanostructuring and oxidation on the metal work function (WF) and, consequently, on the charge transfer and doping of graphene/metal hybrids. We show that nanostructuring and oxidation of metals provide a valid support to frame WF and doping variation in metal/graphene hybrids. Chemical vapour-deposited monolayer graphene has been transferred onto a variety of metal surfaces, including d-metals, such as Ag, Au, and Cu, and sp-metals, such as Al and Ga, configured as thin films or nanoparticle (NP) ensembles of various average sizes. The metal-induced charge transfer and the doping of graphene have been investigated using Kelvin probe force microscopy (KPFM), and corroborated by Raman s...
Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows th... more Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows the production of large area graphene as required for its application as transparent conductive layer substituting ITO. CVD-graphene presents a polycrystalline structure that strongly influences both mechanical and electrical properties. Nowadays, it is well consolidated, among different users of graphene, that analyzing graphene grains after growth, is important for quality-control. In fact, graphene is a well ordered material and contains internal boundaries, commonly known as "grain boundaries". When graphene is grown, the carbon atoms within each growing grain are lined up in a specific pattern, depending on the crystal structure of sample. With growth, each grain impact others and forms interfaces where the atomic orientations differ. It has been established that the transport properties of the graphene improve as the grain size increases. Therefore, the growth conditions mu...
In this study, we present a two-dimensional dielectric grating which allows achieving high absorp... more In this study, we present a two-dimensional dielectric grating which allows achieving high absorption in a monolayer graphene at visible and near-infrared frequencies. Dielectric gratings create guided-mode resonances that are exploited to effectively couple light with the graphene layer. The proposed structure was numerically analyzed through a rigorous coupled-wave analysis method. Effects of geometrical parameters and response to the oblique incidence of the plane wave were studied. Numerical results reveal that light absorption in the proposed structure is almost insensitive to the angle of the impinging source over a considerable wide angular range of 20°. This may lead to the development of easy to fabricate and experimentally viable graphene-based absorbers in the future.
Graphene is a material with exceptional optical, electrical and physicochemical properties that c... more Graphene is a material with exceptional optical, electrical and physicochemical properties that can be combined with dielectric waveguides. To date, several optical devices based on graphene have been modeled and fabricated operating in the near-infrared range and showing excellent performance and broad application prospects. This paper covers the main aspects of the optical behaviour of graphene and its exploitation as electrodes in several device configurations. The work compares the reported optical devices focusing on the wavelength tuning, showing how it can vary from a few hundred up to a few thousand picometers in the wavelength range of interest. This work could help and lead the design of tunable optical devices with integrated graphene layers that operate in the NIR.
Europium-doped hydroxyapatite Ca10(PO4)6(OH)2 (3% mol) powders were synthesized by an optimized c... more Europium-doped hydroxyapatite Ca10(PO4)6(OH)2 (3% mol) powders were synthesized by an optimized chemical precipitation method at 25 °C, followed by drying at 120 °C and calcination at 450 °C and 900 °C. The obtained nanosized crystallite samples were investigated by means of a combination of inductively coupled plasma (ICP) spectroscopy, powder X-ray diffraction (PXRD), Fourier Transform Infrared (FTIR), Raman and photoluminescence (PL) spectroscopies. The Rietveld refinement in the hexagonal P63/m space group showed europium ordered at the Ca2 site at high temperature (900 °C), and at the Ca1 site for lower temperatures (120 °C and 450 °C). FTIR and Raman spectra showed slight band shifts and minor modifications of the (PO4) bands with increasing annealing temperature. PL spectra and decay curves revealed significant luminescence emission for the phase obtained at 900 °C and highlighted the migration of Eu from the Ca1 to Ca2 site as a result of increasing calcinating temperature.
In this paper, we numerically and experimentally demonstrate how to modulate the amplitude and ph... more In this paper, we numerically and experimentally demonstrate how to modulate the amplitude and phase of a microwave ring resonator by means of few-layers Chemical Vapour Deposition (CVD) graphene. In particular, both numerical and experimental results show a modulation of about 10 dB and a 90 degrees-shift (quadrature phase shift) when the graphene sheet-resistance is varied. These findings prove once again that graphene could be efficiently exploited for the dynamically tuning and modulation of microwave devices fostering the realization of (i) innovative beam-steering and beam-forming systems and (ii) graphene-based sensors.
The use of dry hydrogen plasma etching is evaluated for determination of GaN polarity and critica... more The use of dry hydrogen plasma etching is evaluated for determination of GaN polarity and critically compared to wet etching in NaOH. It is shown that hydrogen plasma etching is effective in revealing inversion domains (IDs) and some types of dislocations. This is because the surface morphology is unchanged by the hydrogen treatment, and, hence, the surface reactivity is not masked.
In this contribution, we address two critical and interesting aspects from both fundamental and t... more In this contribution, we address two critical and interesting aspects from both fundamental and technological point of views, which are the polarity of ZnO and the interface reactivity and stability to hydrogen and nitrogen. The effects of atomic hydrogen and nitrogen produced by radiofrequency (r.f. ,13.56 MHz) H2 and N2 plasmas and of temperature on the optical, compositional and structural properties of Zn- and O-polar ZnO have been studied. It is found that Zn-polar ZnO is highly reactive with atomic hydrogen while O-polar ZnO is almost inert. Conversely, both polarities react with nitrogen, with the O-polar ZnO showing a larger reactivity toward N-atoms than the Zn-polarity.
Journal of Vacuum Science Technology B Microelectronics and Nanometer Structures, 2002
The effects of sapphire nitridation temperature on the properties of the GaN epitaxial layers gro... more The effects of sapphire nitridation temperature on the properties of the GaN epitaxial layers grown by rf plasma-assisted molecular-beam epitaxy were investigated. It is found that a nitridation temperature as low as 200 °C acts as a singular point for producing a homogeneous AlN ...
GaN surface passivation processes are still under development and among others hydrogen treatment... more GaN surface passivation processes are still under development and among others hydrogen treatments are investigated. In this study, we use non-destructive optical and electrical probes such as spectroscopic ellipsometry (SE) and surface potential Kelvin probe microscopy (SP-KPM) in conjunction with non-contact atomic force microscopy (AFM) for the study of the different reactivity of Ga-polar and N-polar GaN epitaxial layers with atomic hydrogen. The GaN epitaxial layers are grown by molecular beam epitaxy on sapphire (0 0 0 1) substrates, and GaN and AlN buffer layers are used to grow N-polar and Ga-polar films, respectively. The atomic hydrogen is produced by a remote rf (13.56 MHz) H 2 plasma in order to rule out any ion bombardment of the GaN surface and make the interaction chemical. It is found that the interaction of GaN surfaces with atomic hydrogen depends on polarity, with N-polar GaN exhibiting greater reactivity. Furthermore, it is found that atomic hydrogen is effective in the passivation of grain boundaries and surface defects states.
Nitride materials are critical for a range of applications, including UV-visible light emitting d... more Nitride materials are critical for a range of applications, including UV-visible light emitting diodes (LEDs). Advancing the performance, reliability and synthesis of AlGaN/GaN and InGaN/GaN heterojunction devices requires a systematic methodology enabling characterization of key metric like alloy composition, thickness and quality possibly in real time. This contribution reports on the real time characterization of the plasma assisted molecular beam epitaxy of AlGaN/GaN and InGaN/GaN heterostructures. Spectroscopic ellipsometry real time monitoring has revealed a number of key process and material iusses, such as the roughening of the GaN templates depending on plasma exposure during the substrate cleaning step, the composition of the alloy and the growth mode. Parameters like the plasma conditions, the surface temperature and the atomic flow ratio are investigated to understand the interplay process-material composition-structure-optical properties.
Er 2 O 3 thin films have been deposited by low-pressure metalorganic chemical vapor deposition (M... more Er 2 O 3 thin films have been deposited by low-pressure metalorganic chemical vapor deposition (MOCVD) also plasma assisted (RP-MOCVD), using tris(isopropylcyclopentadienyl)erbium and O 2 on Si(1 0 0), Si(1 1 1) and corning glass substrates. The RP-MOCVD approach produced highly (1 0 0)-oriented, dense and mechanically stable Er 2 O 3 films with columnar structure, while films with (1 1 1) texture are deposited by MOCVD. A high refractive index of 2.1 at 589.3 nm comparable to that of bulk single crystalline Er 2 O 3 , a high transparency in the vis-near UV range and an optical band-gap of 6.5 eV have been found, which make Er 2 O 3 interesting as antireflective and protective coating. A static dielectric constant k $ 12, a density of interface traps as low as 4.2 Â 1010 cm 2 eV À1 , for 5-10 nm thick Er 2 O 3 layers grown on Si(1 0 0), render the present Er 2 O 3 films interesting also as high-k dielectric in CMOS devices.
The effect of sapphire nitridation temperature on the chemistry and microstructure of the sapphir... more The effect of sapphire nitridation temperature on the chemistry and microstructure of the sapphire substrate/GaN interface, nucleation layer, and of the GaN epilayers grown by rf plasma assisted molecular beam epitaxy is investigated. It is found that a sapphire nitridation temperature ...
Au nanoclusters have been deposited on Si (001) surfaces by sputtering of a metallic Au target us... more Au nanoclusters have been deposited on Si (001) surfaces by sputtering of a metallic Au target using an Ar plasma. Different wet and dry treatments of the Si (001) surface, including dipping in HF solution and exposure to H2 and N2 plasmas, have been applied and the ...
Fully microcrystalline silicon, μc-Si, thin films have been deposited on corning glass by plasma ... more Fully microcrystalline silicon, μc-Si, thin films have been deposited on corning glass by plasma enhanced chemical vapor deposition (PECVD) using SiH4-H2. The effects of the surface treatment and of the deposition temperature on microstructure of μc-Si films are investigated by “in situ” laser reflectance interferometry (LRI), “ex situ” spectroscopic ellipsometry (SE) and Raman spectroscopy. LRI indicated the existence of a
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Papers by Giovanni Bruno