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Xray detectors play a pivotal role in development and advancement of humankind, from farreaching impact in medicine to furthering the ability to observe distant objects in outer space. While other electronics show the ability to adapt to flexible and lightweight formats, stateoftheart Xray detectors rely on materials requiring bulky and fragile configurations, severely limiting their applications. Lead halide perovskites is one of the most rapidly advancing novel materials with success in the field of semiconductor devices. Here, an ultraflexible, lightweight, and highly conformable passively operated thin film perovskite Xray detector with a sensitivity as high as 9.3 0.5 C Gy1 cm2 at 0 V and a remarkably low limit of detection of 0.58 0.05 Gy s1 is presented. Various electron and hole transporting layers accessing their individual impact on the detector performance are evaluated. Moreover, it is shown that this ultrathin formfactor allows for fabrication of devices detecting Xrays equivalently from front and back side. (VLID)5744134 Version of record

Semiconductors of group III-V, in particular III-Nitrides and their compounds,have shown an increasing interest from the scientific community in the last years.

Research in the renewable energy field is mandatory to provide alternatives to fossil fuels in energy production and photovoltaics is very promising as solar energy is widely distributed. Moreover, the related technology is nowadays easily available, it can be integrated with buildings and presents limited environmental issues. Silicon Oxi-Nitride (SiOxNy) thin films have been characterized in view of the application in thin-film solar cells as well as in wafer based silicon solar cells, like Silicon HeteroJunction (SHJ) solar cells. Material properties like optical and electronic properties of the film, composition and structure of the nanocrystals have not been studied yet. In addition, the role of deposition conditions and precursor gas concentrations on these properties is also not clear. The results presented in the contribution aim to clarify several aspects concerning nc-SiOxNy film properties.

The aim of this chapter is a throughout description and discussion of surface photovoltage spectroscopy. The basic physical principles, experimental details, and relevant results of the method are described, and the capability of the method to extract material properties like optical band gap and defect-related states is discussed. The method presents several advantages, as it allows for the identification of conduction versus valence band nature of the defect-related transitions and the defect level positions within the band gap. Moreover, it allows for the detection of relatively low densities of surface defects as well as their cross-sections. The application of the method to different materials and structures is discussed, ranging from bulk semiconductors to low-dimensional systems, to nanostructures.

Semiconductors containing nanopores have gained a renewed interest as they are able to adsorb and interact with atoms and molecules and can thus be used in several interesting and emerging applications. Ge film and bulk samples, self-implanted with Ge+ ions, have shown to have a sponge-like nanoporous structure [1], as shown in fig.1. Possible applications of porous semiconductors include various novel sensors, solar cells, optoelectronic devices; they can be used for catalysis, biological molecular isolation and purification or electrodes for micro-fuel cells [2 and reference therein]. Optoelectronic characterization of the layers are carried out by Surface Photovoltage (SPV) Spectroscopy, a powerful technique which allows for obtaining detailed information on material optical properties, such as electronic transitions at gap states, and band to band transitions [2,3]. In addition, structural and microscopic analyses have been carried out in order to identify amorphous or crystalline phases and the average size of the nano- pores. Different nanoporous (np) structures have been investigated: crystalline and amorphous np-Ge obtained by implantation of bulk Ge, as well as crystalline and amorphous np-Ge obtained by ion implantation of Ge film grown on Si substrates by molecular beam epitaxy and sputtering. The effect of Au nanoparticles embedded within the nanoporous structure has been also investigated. Changes in the SPV spectra as a function of ion implantation fluence and annealing treatments have been found and discussed on the basis of the structural properties of the samples [4]. A significant enhancement of the SPV signal in np-Ge samples decorated with Au nanoparticles has been shown, and related to enhanced light trapping effects. SPV spectra of np-Ge thin films show the main peak, which corresponds to band-to-band transitions, significantly blue shifted with respect to the same peak in bulk np-Ge (fig.2). This result has been assigned to a quantum confinement effect occurring at Ge nano- walls separating the nanopores. In addition, a strong enhancement of the SPV signal has been observed due to light confinement effects. Quantum confinement and light trapping effects demonstrated in nanoporous Ge film deposited on Si substrate can be of major interest for future photovoltaic applications of thin film solar cells. [1] G Impellizzeri et al Nanotechnology 23, 395604 (2012) [2] D Cavalcoli et al Microporous and Mesoporous Mat 196, 175–178 (2014) [3] L Kronik and Y. Shapira, Surf Sci Rep. 37,1 (1999) [4] D Cavalcoli et al, in Semiconductors and Semimetals, Elsevier, (2015)

Self-ion implantation on bulk Ge induces the formation of nanopores, and their growth, structure and transport properties have been recently reported [1]. Semiconductors containing nanopores have gained a renewed interest as they are able to adsorb and interact with atoms and molecules and can thus be used in several interesting and emerging applications. In this contribution we report on the influence of ion implantation and subsequent nanopore formation on optoelectronic properties of Ge samples. The analyses are carried out by Surface Photovoltage (SPV) Spectroscopy, a powerful technique to get detailed information on material optical properties [2]. Different nanoporous (np) structures have been investigated: crystalline and amorphous np-Ge obtained by implantation of bulk Ge, as well as crystalline and amorphous np-Ge obtained by ion implantation of Ge film grown on Si substrates by molecular beam epitaxy and sputtering. Changes in the SPV spectra as a function of ion implantation fluence and annealing treatments are discussed on the basis of the structural properties of the samples. Quantum confinement effects in nanoporous Ge film have been demonstrated, while a significant SPV enhancement in np-Ge samples decorated with Au nanoparticles has been shown. These results can be of major interest for future photovoltaic applications of thin film solar cells. G Impellizzeri et al Nanotechn 23, 395604 (2012) L Kronik and Y. Shapira, Surf Sci Rep. 37,1 (1999)

The Scanning Probe Microscopy is a very useful technique that provides a variety of information about the surface of materials. Atomic Force Microscopy (AFM) is used in this work in order to investigate the topography and the local conductivity of a system composed of silicon nano-crystals (Si NCs) embedded in a dielectric matrix (SiC). This system is studied in view of its application as a tunable band-gap absorber in all-silicon multi-junction photovoltaic cells. SiC/Silicon Rich Carbide (SRC) multilayers are produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) and are subsequently annealed in order to obtain NCs formation. The properties of the multilayers have been studied as a function of the NCs size. Macroscopical as well as microscopical characterization are carried out in order to understand the properties of the multilayers. AFM maps are obtained in tapping mode in order to investigate the topography of the layers and the energy dissipated between the tip and the sample. Conductivity measurements as a function of temperature show that the conductivity increases with SRC layer thickness. Local IV measurement are performed with the AFM in contact mode with a conducting probe and the measurements show a similar trend with respect to SRC layer thickness. Conductive-AFM is performed to identify conductive paths at the nano-scale. Regions with different conductivity have been observed, which could be related to compositional variations (Si content) and unintentional doping by contamination with nitrogen and oxygen. The results are correlated with crystalline fraction and structural properties of the layers as obtained by optical analyses and transmission electron microscopy measurements, respectively.

Silicon heterojunction solar cells (SHJ) attracted a lot of attention in recent years due to their high efficiency potential and low temperature production process. In the present study nano-crystalline Silicon Oxy-Nitrides layers (nc-SiOxNy) are studied in view of their application in SHJ cells since they suffer less parasitic absorption. SiOxNy films deposited by Plasma Enhanced Chemical Vapour Deposition with different parameters were subsequently annealed in order to obtain the formation of nanocrystals. Structural and compositional analyses were performed by Raman and Fourier-Transform Infra-Red (FTIR) spectroscopy and optical characterization. IV measurements were carried out in order to investigate the electrical properties of the layers. Microscopical analyses were performed with Scanning Electron and Atomic Force Microscope (AFM). Samples show high conductivity ranging from 1-40 S/cm demonstrating that the dopants become electrically active and that the layers become nanocrystalline. Through AFM phase contrast analysis we found differences in tip-sample energy dissipation that can be due to variations in the composition and/or crystalline/amorphous phase. Raman spectroscopy has allowed us to detect the evolution of the crystallinity with deposition conditions and annealing time. FTIR shows peak shifts with deposition and annealing parameters. The combination of macroscopical and microscopical techniques gives a fundamental insight on the properties of the material.

Silicon nano-crystals (Si NCs) embedded in a dielectric matrix are studied in view of their application as tunable band-gap absorbers in all-silicon multi-junction photovoltaic cells. Although high conductivity and high mobility are requirements for the matrix in order to guarantee an efficient carrier collection, the ideal properties of the nc-Si are more related to the ability of absorbing photons beyond a given energy, and emitting photogenerated carriers into the surrounding matrix. The carrier collection efficiency in this composite system is supposed to depend on the barrier height of the surrounding matrix. In the present case we are dealing with Si NCs embedded in a SiC matrix, which guarantees a lower barrier to the Si NCs with respect to SiO2 matrix and better transport properties. SiC/SiC:Si multilayers produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) have been annealed to obtain Si NCs formation. Various experimental conditions (Si NCs size, doping level, crystallinity) have been considered. The structural parameters were determined by Reflectance and Transmittance spectroscopy analyses. From this the calculated thickness of multilayers is in the range of 131-178 nm. Macroscopical as well as microscopical electrical characterizations were carried out in order to understand the conduction mechanisms. Conductivity measurements as a function of temperature showed that the conductivity ranged from 10-6-10-3S/cm and the activation energy range is 0.19-0.4 eV. Atomic Force Microscopy (AFM) was used to characterize the topography of the sample surface. Roughness values are of the order of 0.6 nm while phase contrast dynamical mode AFM mapping allowed us to obtain an estimated average value of the nano-crystal diameter around 10 nm. Conductive-AFM was performed to identify the conductive path at the nano-crystals level. The Si NCs showed enhanced conductivity with respect to the surrounding matrix. The measurements were correlated with optically determined crystallized fraction and with thermally activated transport properties. The combination of electrical with optical measurements, associated to AFM observation at the nanoscale, gives a fundamental insight in the silicon nano-dots conduction properties.

Si NCs embedded in a wide band gap material are promising candidates for all-Si tandem solar cells thanks to the possibility of tuning the NC band gap by varying their size, offered by the NC quantum confinement. The high temperature treatment required for Si crystallization represents a concern for the integration of this material in a tandem solar cell production route. The limiting step for solid phase crystallization is grain nucleation. A high temperature nucleation seeding followed by grain growth at a lower temperature is therefore a promising method to reduce the thermal budget. To this aim, we report on different annealing routes pursued to decrease the thermal budget: in all cases the germs of the c-Si nucleation are created by a high temperature (1150 °C) spike, then the crystallization follows at different temperatures, above and below the 3C-SiC solid phase epitaxial growth temperature (800 °C). The results are discussed in terms of the structural features produced by each process: the time required for complete dehydrogenation, mandatory to avoid disruptive hydrogen evolution during the high temperature spike; Si and SiC crystallization; defect nucleation in c-SiC, correlated to the rate of temperature rise during the annealing; role of the c-Si/SiC interface energy in affecting the growth process. The photovoltaic properties of the resulting structures are estimated based on the measured optical absorption, electronic transitions, and electrical transport.

The operating mode of a planar Cd(Zn)Te device in its most basic form consists of two electrodes, one of which is biased under negative potential while the other which is typically connected to a charge sensitive pre-amplifier for signal amplification. It is also usually required that the detectors have sufficiently high resistivity such that the leakage current induced by the operating voltage is sufficiently low. However, it is not enough to have material with high bulk resistivity. Surface preparation has a strong effect on the electronic properties of the device and is of paramount importance to achieve functioning and reproducible detectors. The effect of anode and cathode surface morphology on detector performance has been investigated by A. Hossain et al. (1). Furthermore the effects of using different chemical etchants for electrode surface preparation has been investigated by (2) Cd(Zn)Te. Finally, the effect of lateral edge morphology on device leakage current has been demonstrated by Crocco et al. (3). Despite these recent advancements in preparing the surface of Cd(Zn)Te devices for detector applications, large asymmetries in the electronic properties of Cd(Zn)Te detectors are quite common. Indeed, we have observed differences in leakage current several orders in magnitude depending on which electrode of the planar configuration is used as the cathode. In fact, this phenomenon has been observed for both oriented as well as non-oriented crystals. Therefore an understanding of the material properties which contribute to this asymmetry is of paramount importance from a detector fabrication perspective. For the development of patterned electrode geometries, correct selection of this surface is crucial. In the case of oriented crystals, it has been observed that the (111)A and (111)B surface terminations may have some influence on the electrical properties of the device. Shown below in Fig. 1 is the structure of (111) oriented Cd(Zn)Te, showing the alternating crystallographic planes of Cd(Zn) and Te atoms. Taking into account the unit cell of the crystal, the surface which will terminate in Te is referred to as the (111)B surface, and the surface which will terminate in Cd(Zn) is the (111)A surface.

Silicon nano-crystals (Si NCs) embedded in a dielectric matrix are presently studied in view of their application as tunable band-gap absorbers in all-silicon multi-junction photovoltaic cells. Although high conductivity and high mobility are requirements for the matrix in order to guarantee an efficient carrier collection, the ideal properties of the nc-Si are more related to the ability of absorbing photons beyond a given energy, and emitting photogenerated carriers into the surrounding matrix. The carrier collection efficiency in this composite system is supposed to depend on the barrier height of the surrounding matrix. It is therefore very important to clarify the electrical behavior of the Si NCs when inserted in the specific matrix that will be used for the final device. In the present case we are dealing with Si NCs embedded in a silicon carbide matrix, which guarantees a lower barrier to the Si NCs with respect to SiOx matrix and better transport properties. This work deals with the study of the conduction mechanisms in the system composed by silicon nano-crystals within the SiC matrix. SiC/SiC:Si multilayers produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) have been subsequently annealed to obtain Si nano-crystal formation. Various experimental conditions (silicon nano-crystal size, doping level, crystallinity) were considered. The structural parameters were determined by Reflectance and Transmittance spectroscopy analyses. From this the calculated thickness of multilayers is in the range of 131-178 nm. Macroscopical as well as microscopical electrical characterizations were carried out in order to understand the conduction mechanisms. Conductivity measurements as a function of temperature showed that the conductivity ranged from 10-6-10-3S/cm and the activation energy range is 0.19-0.4 eV. Atomic Force Microscopy (AFM) was used to characterize the topography of the sample surface. Roughness values are of the order of 0.6 nm while phase contrast dynamical mode AFM mapping allowed us to obtain an average value of the nano-crystal diameter around 10 nm. Conductive-AFM was performed to identify the conductive path at the nano-crystals level. The Si NCs showed to have higher conductivity with respect to the surrounding matrix. The measurements were correlated with optically determined crystallized fraction and with thermally activated transport properties. The combination of electrical with optical measurements, associated to AFM observation at the nanoscale, gives a fundamental insight in the silicon nano-dots conduction properties.

Research on III-nitride semiconductors is achieving new heights due their high potential applications in photonics and electronics. The wide range of variation of band gap of III-N alloys allows for interesting optoelectronic applications. Low dimensional structures with high band-offsets (AlN/GaN or InN/GaN) are the basis for high efficient Light Emitting devices. Moreover, the polarization –induced high electric field, strongly localized at the interface of these heterojunctions, induces a very good confinement of 2 dimensional electron gas, which has been applied to the realization of high mobility field effect transistors [1]. However, GaN-based semiconductors are mostly grown epitaxially on sapphire, and due to the large lattice mismatch and the differences in the thermal expansion coefficients, the structures usually contain a high density of threading dislocations (TDs). While growth procedures and structures of TDs are well known [2], their electronic properties are still debated. Dislocations in GaN are known to be negatively charged and to affect mobility and electrical conduction and leakage current of GaN based devices [3,4]. Alloying of GaN with In or Al led to even a more complex scenario, as In atoms easily segregate at dislocations due to its high surface diffusivity, changing their electronic properties. In the present contribution we will show conductive AFM and phase contrast AFM studies of TDs in GaN andAl/In GaN ternary alloys to evidence the role of the strain and the composition on the electrical properties of dislocations in III-nitrides. Local I-V curves were measured at the dislocations. Metallorganic Chemical Vapor Deposition (MOCVD) grown GaN layers and heterostructures made of AlInN/AlN/GaN with different AlN thickness, InGaN/GaN with varying percentage of In, and AlGaN/GaN with varying percentage of Al were examined. The samples were obtained by different growers (AIXTRON, III-V Lab). Surface morphology, phase separation, defect structures and their effect on the electrical properties of TDs will be reported. The comparison between the results obtained in the different alloys allowed us to understand the role of In and Al on the TDs electrical properties. [1] M. Gonschorek, J.-F. Carlin, E. Feltin, M. A. Py, N. Grandjean, V. Darakchieva, B. Monemar, M. Lorenz, and G. Ramm, J. Appl. Phys. 103, 093714 (2008). [2] A. Mouti, J.-L. Rouvière, M. Cantoni, J.-F. Carlin, E. Feltin, N. Grandjean, and P. Stadelmann, Phys. Rev. B 83, 195309 (2011). [3] D. C. Look and J. R. Sizelove, Phys. Rev. Lett. 82, 1237 (1999). [4] P. J. Hansen,Y. E. Strausser, A. N. Erickson, E. J. Tarsa, P.Kozodoy, E. G. Brazel, J. P. Ibbetson, U. Mishra, V. Narayanamurti, S. P., DenBaars, and J. S. Speck, Appl. Phys. Lett. 72, 2247 (1998). Acknowledgments This work was supported by the EU under Project No. PITN-GA-2008-213238 (RAINBOW). The Project partners (AIXTRON and III-V Lab) are gratefully acknowledged for providing samples.

Silicon nanocrystals, ncSi, embedded in a dielectric matrix are presently studied in view of their application as tunable band-gap absorbers in all-silicon multijunction photovoltaicsa. Althoug high conductivity and high mobility are requirements for the matrix in order to guarantee an efficient carrier collection, the ideal properties of the ncSi are more related to the ability of absorbing photons beyond a given energy, and then emit the photogenerated carrier into the extended states of the surrounding matrix. At present, the role of doping in this context has not fully established, also because doping of nanocrystals suffers from limitations due to segregation at the surface and formation of electrically inactive complexes with dangling bondb, as well as size dependent activation efficiencyd. At the nanoscale, higher conductivity has also been observed either on the ncSi surface or within the ncSi itseld, depending on the doped of intrinsic nature of the ncSi.4. Finally, overlapping of the wavefunction, and therefore carrier collection efficiency, is also predicted to depend on the barirer height of the surrounding matrix. It is therefore very important to clarify the electrical behaviour of the ncSi when inserted in the specific matrix that will be used for the final device. In our case we focus on silicon carbide, which quarantees lower barrier to the ncSi, and better transport properties.

GaN based heterostructures have recently gained increased interest due to their applications for high electron mobility transistors. In this letter AlInN/AlN/GaN heterojunctions grown by metal-organic chemical-vapor deposition with different AlN thicknesses have been investigated by surface photovoltage spectroscopy. The density of the two-dimensional electron gas (2DEG) forming at the interface has been measured by Hall effect. A band gap shift has been detected and its dependence on the 2DEG electron density at the AlN/GaN interface has been analyzed on the basis of the Moss–Burstein and renormalization effects.