Roberto Fornari

• Professor (Full) at University of Parma

Photoelectronic properties of orthorhombic undoped κ-Ga 2 O 3 epitaxial thin films, grown on sapphire substrates by metal-organic chemical vapour deposition, were evaluated under X-ray irradiation (CuK α line, 8.05 keV) for the first...

Electrical and optical properties of nominally-undoped high-resistivity κ -Ga2O3 epitaxial films are presented, along with the ultraviolet detection performance of solar-blind photodetectors fabricated from these epilayers. The photodetectors exhibited a solar-blind rejection ratio higher than 10⁴, reproducible on–off switching times and a remarkab...

A novel broadband p-n UV photodiode is presented, that integrates an ultrawide band gap n-type κ-Ga2O3 epitaxial film doped with silicon (Si) and a p-type NiO polycrystalline film, to provide a planar NiO/κ-Ga2O3 p-n heterojunction for UV detection applications. The proposed device features a circular design, with a diameter of 240 μm, resulting i...

Ultra-wide bandgap semiconductors like Ga2O3, presenting intrinsic spectral selectivity in the UV-C region of the electromagnetic spectrum, are especially well suited for application as solar-blind photodetectors. In this work, photodiodes based on a planar hybrid heterojunction between Ga2O3 and an organic semiconductor are fabricated. Specificall...

For the first time, single-phase κ-Ga2O3 films were grown on (001) p-type GaAs and BxGa(1-x)As/GaAs templates using Metal-Organic Chemical Vapor Deposition (MOCVD). The films were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). TEM investigations revealed a thin polycrystalline nu...

Antimony selenide (Sb2Se3) is an Earth-abundant and non-toxic material that stands out as a promising absorber for the fabrication of thin film solar cells. Despite significant advancements in recent years, all the devices reported in the literature exhibit open-circuit voltages well below the theoretical value. Identifying the factors contributing...

A systematic investigation of the optical properties of β‐, α‐, and κ‐phase gallium oxide (Ga 2 O 3 ) polymorphs is conducted by UV–vis spectrophotometry through the Swanepoel method and temperature‐dependent photoluminescence. Using the same approach and apparatus allows similarities and differences between these three phases to be directly establ...

The “photo‐gain effect” amplifying the DC photocurrent of κ‐Ga 2 O 3 UV‐C photoresistors is analyzed by means of 2D numerical simulations and linked to the capture of photogenerated holes by deep donor levels, probably associated with oxygen vacancies. The resulting ionization of the deep donors leads to an increase in the electron density, hence t...

Electrical contacts are of the greatest importance as they decisively contribute to the overall performance of photoresistors. Undoped κ-Ga2O3 is an ideal material for photoresistors with high performance in the UV-C spectral region thanks to its intrinsic solar blindness and extremely low dark current. The quality assessment of the contact/κ-Ga2O3...

Gallium oxide is an ultrawide-bandgap semiconductor with proven applications in the fields of solar-blind UVC photodetectors and power electronics. This oxide has one thermodynamically stable polymorph, the monoclinic β phase, and four additional metastable phases, called α, γ, κ, δ. They present bandgaps between 4.6 and 5.3 eV but different crysta...

Antimony selenide is a very promising material for photovoltaic applications, with the potential to become a competitive alternative to more traditional silicon, CdTe and CIGS-based technologies. A notable feature of Sb2Se3 is its strong anisotropy, and this property is reflected in the performance parameters of the solar cell. In this study a nove...

A collection of extensive chapters that cover fundamentals, technology and device applications of modern semiconductors

Atomically thin molybdenum disulfide (MoS2) is a two-dimensional semiconductor with versatile applications. The recent adoption of liquid molybdenum precursors in chemical vapor deposition has contributed significantly to the reproducible wafer-scale synthesis of MoS2 monolayer and few-layer films. In this work, we study the effects of the carrier...

The in situ passivation of a methylammonium lead triiodide (MAPbI3) phase spin-coated via a one-step process was experimentally investigated to elucidate their fundamental properties. Structural analysis revealed that MAPbI3 adopts a tetragonal crystal structure with a small excess of PbI2 (0.03 M) segregating at grain boundaries. Optical character...

In this study, a dual-frequency method for the impedance/admittance measurements on illuminated junctions has been critically analyzed as appropriate methodological approach for the measure of the depletion capacitance, both under steady-state light and during on-off illumination cycles. A planar Pt/κ-Ga2O3 Schottky diode was used as a test structu...

Semiconductor photodetectors can work only in specific material-dependent light wavelength ranges, connected with the bandgaps and absorption capabilities of the utilized semiconductors. This limitation has driven the development of hybrid devices that exceed the capabilities of individual materials. In this study, for the first time, a hybrid hete...

Raman spectroscopy, a versatile and nondestructive technique, was employed to develop a methodology for gallium oxide (Ga 2 O 3 ) phase detection and identification. This methodology combines experimental results with a comprehensive literature survey. The established Raman approach offers a powerful tool for nondestructively assessing phase purity...

Orthorhombic κ-Ga2O3 thin films were grown for the first time on polycrystalline diamond free-standing substrates by metal-organic vapor phase epitaxy at a temperature of 650°C. Structural, morphological, electrical, and photoelectronic properties of the obtained heterostructures were evaluated by optical microscopy, X-ray diffraction, current-volt...

The interfacial properties of a planar SnO/κ-Ga2O3 p–n heterojunction have been investigated by capacitance–voltage (C–V) measurements following a methodological approach that allows consideration of significant combined series resistance and parallel leakage effects. Single-frequency measurements were carried out in both series- and parallel-model...

Sb2Se3 is a typical V2VI3 binary chalcogenide compound characterized by a single crystalline phase and a fixed composition. Sb2Se3 displays a narrow energy gap ranging from 1.1 to 1.3 eV, which are quite optimal values for single-junction solar cells. Earth-abundant and non-toxic components make this material a good candidate for low-cost thin-film...

Among the most promising material for thin-film photovoltaics, Antimony Selenide plays a pivotal role, being constituted of Earth-abundant elements and owing to its optical and electrical properties. Recently, ASe solar cells exhibited relatively good photovoltaic performances, reaching a considerable efficiency close to 9 %. Despite these remarkab...

Unintentionally doped (001)‐oriented orthorhombic κ‐Ga2O3 epitaxial films on c‐plane sapphire substrates are characterized by the presence of ≈ 10 nm wide columnar rotational domains that can severely inhibit in‐plane electronic conduction. Comparing the in‐ and out‐of‐plane resistance on well‐defined sample geometries, it is experimentally proved...

SnO/𝜀-Ga2O3 vertical p–n diodes with planar geometry have been fabricated on c-plane Al2O3 and investigated by current–voltage measurements. The effects of the in-plane conduction through the Si-doped 𝜀-Ga2O3 layer on the diode performance and their relevance have been evaluated. A significant series resistance is observed, which shows typical feat...

In this study, experimental and theoretical investigations have been performed on nominally undoped -Ga2O3 films deposited on (0001)-Al2O3 substrates by Metal-Organic Vapor Phase Epitaxy (MOVPE) using different O and Ga precursor ratios. Hydrogen and Helium were used as carrier gas. Low-temperature Cathodoluminescence (CL) broad emissions extending...

Deep and shallow electronic states in undoped and Si-doped ε-Ga2O3 epilayers grown by MOVPE on c-oriented Al2O3 were investigated by cathodoluminescence, optical absorption, photocurrent spectroscopy, transport measurements, and electron-paramagnetic-resonance. Nominally undoped films were highly resistive, with a room temperature resistivity varyi...

BGaAs/GaAs epilayers and BInGaAs/GaAs quantum well (QW) have been prepared using metal-organic chemical vapor deposition under different growth conditions, and their physical and structural properties have been examined. SEM-EDS investigation showed a dependence of surface properties of the ternary compound on the growth conditions. High-resolution...

Gallium oxide (Ga2O3) is an ultra-wide bandgap material, which has recently attracted widespread attention for holding promising applications in power electronics and as solar blind UV photodetectors, outclassing GaN or SiC in terms of a larger bandgap and higher breakdown voltages. The orthorhombic κ phase (also referred to as ε) has sparked parti...

The good control of the n-type doping is a key issue for the fabrication of efficient devices based on ε-Ga2O3 epilayers. In this work we studied the possibility of doping the ε-Ga2O3 thin films, epitaxially grown on c-oriented sapphire by metal-organic chemical vapor deposition, by means of a post-deposition treatment. For the first time, the n-ty...

Cathodoluminescence (CL) investigations are performed on nominally undoped and Si-doped ε-Ga 2 O 3 samples grown by metal-organic vapor phase epitaxy on (0001)-Al 2 O 3 substrates, using different carrier gases. All films exhibit a broad low-temperature CL emission extending over the photon energy range 2-3.4 eV. Emission deconvolution suggests tha...

Gallium oxide is a wide bandgap n-type semiconductor highly interesting for optoelectronic applications (e.g., power electronics, solar blind UV photodetectors). Besides its most thermodynamically stable monoclinic β phase, Ga2O3 can crystallize in different polymorphs; among them the corundum α and the orthorhombic ε phases are the most promising...

In this work, the conduction mechanisms across novel contacts to epitaxial films of pure phase epsilon-Ga2O3 were investigated. Different structures made by sputtered metal and oxide thin films were tested as electrical contacts. I-V characteristics show heterogeneous behaviors, revealing different conduction mechanisms according to the applied bia...

Heterostructures made of GaN and ε-Ga2O3 epitaxial layers may be very interesting because they could exploit the high electron mobility of GaN combined with the ferroelectric character of ε-Ga2O3. We have explored the possibility of using ε-Ga2O3 templates, deposited by metalorganic chemical vapor deposition on sapphire substrates, in order to redu...

A “new” semiconductor can gather the interest of the scientific community only when it adds unprecedented properties to the ones already established, with silicon universally considered as the most widespread and the primary benchmark. In the last decades, this happened for example with III-V and III-N semiconducting compounds, capable of emitting...

The temperature-driven phase transformation of metastable κ-Ga2O3 layers deposited on sapphire was studied by high resolution TEM. Annealing experiments up to 1000 °C were performed either in situ in vacuum within the TEM or ex situ in ambient air. This allowed for the detection of the atomistic mechanisms at the basis of κ to β phase transition. I...

The defects in Si-doped ε-Ga 2 O 3 epitaxial layers have been investigated by electron paramagnetic resonance (EPR) spectroscopy. The results show that Si doping introduces a single, paramagnetic defect, attributed to Si incorporation on the tetrahedral gallium lattice site. It is a spin S = 1/2 center with an axial g tensor with principal values o...

Low resistivity n-type "-Ga2O3 epilayers were obtained for the first time either by adding silane to the gas phase during the metal organic vapour phase epitaxy deposition or by diffusing Sn in nominally undoped layers after the growth. The highest doping concentrations were few 1018 cm−3 and about 1017 cm−3 for Si and Sn doping, with corresponding...

The electronic structure of ε-Ga2O3 thin films has been investigated by ab initio calculations and photoemission spectroscopy with UV, soft, and hard X-rays to probe the surface and bulk properties. The latter measurements reveal a peculiar satellite structure in the Ga 2p core level spectrum, absent at the surface, and a core-level broadening that...

The growth of single crystals remains one of the most challenging technical tasks of materials science. Beside issues related to thermodynamics and kinetics, crystal growth of functional materials, such as semiconductors, includes a variety of physical phenomena, such as momentum, heat and mass transport, heat radiation, phase transitions, capillar...

The momentum scattering time for electron–phonon interaction in β-Ga2O3 was derived within the relaxation time approximation considering all infra-red active optical modes. A first principle calculation was applied to separately obtain the scattering rates due to polar and non-polar phonon–electron interactions, and then spherically averaged coupli...

Single Crystals of Electronic Materials: Growth and Properties is a complete overview of the state-of-the-art growth of bulk semiconductors. It is not only a valuable update on the body of information on crystal growth of well-established electronic materials, such as silicon, III-V, II-VI and IV-VI semiconductors, but also includes chapters on nov...

Indium oxide as thin film and powder was prepared by the sol-gel method. The structural characterization of the film and powder showed that similar crystallite sizes are obtained in samples heat-treated at the same temperature. Thermo-gravimetric analysis and differential scanning calorimetry were performed in order to determine the activation ener...

Microelectronic and optoelectrical applications implicating gallium nitride have turned out a challenge however, their advance is restricted as result of a lack of appropriate substrates. This article investigates in the crystal development of gallium nitride, the procedures directing to GaN bulk crystal being considerably increased in the previous...

Electrical and optical properties of undoped single-phase ε-Ga2O3 epitaxial films prepared by MOCVD are reported. It is shown that this still unexplored polymorph of gallium oxide possesses wide bandgap and very high dark resistivity, thus allowing the design and fabrication of solar-blind UV photodetectors. Simple cost-effective photoresistors, fa...

We report on the fabrication and characterization of Cu(In,Ga)Se2 (CIGS)-based thin film bifacial solar cells using Al-doped ZnO (AZO) as cost-effective and non-toxic transparent back contact. We show that, by depositing both CIGS and AZO by Low Temperature Pulsed Electron Deposition at a maximum temperature of 250 °C, a good ohmic contact is forme...

The thermal stability of \epsilon-Ga2O3 polymorph was studied by complementary methods. Epitaxial films of \epsilon-Ga2O3 grown on c-oriented sapphire were annealed at temperatures in the range 700-1000 °C and then investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). In addition, Differential Scanning Calorimetry (DSC...

A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga2O3 epilayers grown at low temperature (650 °C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called ε phase; i.e. they e...

The crystal structure and ferroelectric properties of ϵ-Ga2O3 deposited by low-temperature MOCVD on (0001)-sapphire were investigated by single-crystal X-ray diffraction and the dynamic hysteresis measurement technique. A thorough investigation of this relatively unknown polymorph of Ga2O3 showed that it is composed of layers of both octahedrally a...

Growth of gallium oxide thin films was carried out by Metalorganic Chemical Vapor Deposition (MOCVD) at different temperatures. Pure ε-phase epilayers of Ga2O3, with good morphology and structural properties, were obtained, for the first time with this technique, on sapphire at the temperature of 650 °C. XRD analysis performed by high-resolution di...

The question of optical bandgap anisotropy in the monoclinic semiconductor $\beta$-Ga2O3 was revisited by combining accurate optical absorption measurements with theoretical analysis, performed using different advanced computation methods. As expected, the bandgap edge of bulk $\beta$-Ga2O3 was found to be a function of light polarization and cryst...

Electron density and Hall mobility data were simultaneously analyzed in the frame of the relaxation time approximation in order to identify the main scattering mechanisms that limit the carrier mobility in β-Ga2O3 single crystals. The Hall factor correction was self-consistently included in the fitting procedure. The analysis indicates that low-ene...

This chapter describes the most advanced epitaxial technologies applied to the fabrication of devices for energy conversion. Three main categories of devices are considered: high-efficiency multijunction solar cells, thermo-photovoltaic devices, and silicon and germanium thin films on dissimilar materials for solar cell applications. It is shown th...

The lattice parameters of monoclinic beta-Ga2O3 were determined by powder diffraction in a wide temperature range. The experiments provide a quantitative evaluation of the thermal expansion of the unit cell. It was observed that the expansion of the monoclinic cell is not isotropic. These data are of great technological interest for the deposition...

Ga2(1-x)In2xO3 epitaxial layers have been grown on (0001) Al2O3 substrates by metal organic vapour phase epitaxy (MOVPE). The process parameters were optimized and the effects related to the use of two alternative oxygenation sources like O2 and H2O were studied. Different In content x [x = In/(In + Ga)] were investigated in order to determine the...

In this paper, we show by optical and electron microscopy based investigations that vacancies in oxides may cluster and form metallic nanoparticles that induce coloration by extinction of visible light. Optical extinction in this case is caused by generation of localized surface plasmon resonances at metallic particles embedded in the dielectric ma...

SnO2 is a semiconductor with a wide optical bandgap (3.5eV), which makes it an attractive transparent semiconducting oxide (TSO) for electronic and opto-electronic applications. At elevated temperatures it is, however, much more unstable than other TSOs (such as ZnO, Ga2O3, or In2O3). This leads to a rapid decomposition even under very high oxygen...

Epitaxial β-Ga2O3 layers have been grown on β-Ga2O3 (100) substrates using metal-organic vapor phase epitaxy. Trimethylgallium and pure oxygen or water were used as precursors for gallium and oxygen, respectively. By using pure oxygen as oxidant, we obtained nano-crystals in form of wires or agglomerates although the growth parameters were varied i...

The onset of optical absorption in In2O3 at about 2.7 eV is investigated by transmission spectroscopy of single crystals grown from the melt. This absorption is not defect related but is due to the fundamental band gap of In2O3. The corresponding spectral dependence of the absorption coefficient is determined up to α = 2500 cm−1 at a photon energy...

The influence of intrinsic point defects on the electronic structure of n‐type In2O3 single crystals grown by two methods, namely chemical vapor transport and from the melt, was examined by scanning tunneling spectroscopy. This method is a very surface sensitive technique for measuring the local density of states. So far not resolved states within...

Nanoscale semiconductor materials have been extensively investigated as the channel materials of transistors for energy-efficient low-power logic switches to enable scaling to smaller dimensions. On the opposite end of transistor applications is power electronics for which transistors capable of switching very high voltages are necessary. Miniaturi...

There is considerable excitement recently in the field of transparent conducting-oxide-semiconductors due to the successful realization of large-area single crystals of the wide-bandgap semiconductor β-Ga2O3 by bulk growth methods [1]. The availability of bulk β-Ga2O3 crystals led to the rapid demonstration of high-voltage metal-semiconductor field...

We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 Â 10 4 cm À2 , which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN sa...

We present an alternative approach to grow thick GaN single crystalline layers from Ga vapour, transported by an N_2 carrier gas flow, and from nitrogen, excited very near the substrate by 2.45 GHz microwaves at pressure in the 200–800 mbar range. Crystal growth requires high stability of process parameters, especially of plasma operation. A major...

Melt-grown bulk In2O3 single crystals were utilized for systematic heat-treatment experiments, which we performed at temperatures of 200-1400 [degree]C, for 2-300 h and in eight different atmospheres, from oxidizing through vacuum to reducing atmospheres. We found that melt-grown In2O3 single crystals are chemically stable up to 1200-1300 [degree]C...

For the first time a heater-magnet module (HMM), simultaneously generating heat and a traveling magnetic field (TMF), was constructed for an industrial scale G5 multi-crystalline Si crystallizer and extensively tested. Effective melt mixing and precise control of the interface shape have been demonstrated using TMF, which resulted in ingots exhibit...

Kinetic models in the system Al–N–O–C–H are useful to predict the time- and temperature-dependent influences of oxygen related species on aluminum nitride (AlN) bulk crystal growth by the sublimation–recondensation (also known as physical vapor transport, PVT) method. The transient oxygen incorporation is modeled based on non-linear regression anal...

Hexagonal GaN (0001) has been grown by a pseudo-HVPE method on β-Ga2O3 (100) using an intermediate low-temperature buffer layer formed by in situ NH3 treatment of β-Ga2O3 substrate. A simple method for self-separation of bulk GaN from the β-Ga2O3 substrate is reported. The structural properties of the GaN and GaN–β-Ga2O3 interface were investigated...

A single phase InN epitaxial film is grown on a bulk In2O3(111) wafer by plasma-assisted molecular beam epitaxy. The InN/In2O3 orientation relationship is found to be (0001) ∥ (111) and [1100] ∥ [112]. High quality of the layer is confirmed by the small widths of the x-ray rocking curves, the sharp interfaces revealed by transmission electron micro...

Epitaxially strained NaNbO3 films were grown by liquid-delivery spin metal– organic chemical vapour deposition on several oxide substrates, inducing tensile and compressive lattice strain. High-resolution X-ray diffraction measurements reveal that coherently grown compressively strained NaNbO3 films on NdGaO3 exhibit the orthorhombic c phase. With...

The Schottky barrier height of Au deposited on (100) surfaces of n-type β-Ga2O3 single crystals was determined by current-voltage characteristics and high-resolution photoemission spectroscopy resulting in a common effective value of 1.04 ± 0.08 eV. Furthermore, the electron affinity of β-Ga2O3 and the work function of Au were determined to be 4.00...

Germanium nanowires (NWs) were grown onto Ge(111) substrates by the vapor-liquid-solid process using gold droplets. The growth was carried out in a molecular beam epitaxy chamber at substrate temperatures between 370 Degree-Sign C and 510 Degree-Sign C. The resulting nanowire growth rate turns out to be highly dependent on the substrate temperature...

Large-grained Si layers are attractive for cost-effective thin film solar cells. Our experiments using a low-temperature steady-state solution growth procedure for growing Si crystallites on glass show the dominance of Si{111} facets. Recesses observed in the centers of these facets suggest that growth starts at {100} vertices and/or {110} edges an...

The structural properties of the heteroepitaxially grown AlN on off-axis 4H-/6H-SiC by PVT are presented in dependence of the seed polarity. In addition to long-term growth runs (48–72 h) short-term experiments were carried out in order to investigate the initial growth stage. SEM, EDX, HR-TEM and HR-XRD studies show different growth modes for C-po...