Search

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.

session Poster (Q.PII.32); International audience

Scanning Electron Microscopy (SEM) is a powerful characterization method, especially for probing nanoscaled structures. Recently, new amazing applications have been demonstrated [1]. In this frame, this contribution deals with a study on GaAs nanowires (NWs), grown vertically aligned but subsequently driven to self-assembly during SEM observations. The electron beam induced bending has been rarely reported to affect post-growth NW orientation [2, 3] and few hypotheses have been advanced on the nature of the force which drives the NWs to bow [1, 2, 3]. No effort has been dedicated to understand the coupled bundling effect. We will elucidate the rising of NWs clusters into the frame of adhesion, friction, and compliance properties of natural and bio-mimetic contact surfaces [4]. We will report on the NWs bundling in the frame of lateral collapsing of fibrillar architectures [5]. We will demonstrate that the mechanical and surface properties of the NWs together with the geometrical design of the arrays play a key role in avoiding or favoring the NWs self-assembly. Besides, we will evidence that the electron beam-mediated bending coupled to adhesion-induced bundling of GaAs NWs shows thus the way to turn the SEM electron beam into a patterning tool for the selective fabrication and control of NW integration at the nanoscale. The possible applications range from NWs assembly on their growth substrate for in situ devices’ fabrication to the realization of sophisticated anisotropic surface designs leading to the engineering of anisotropic physical properties like wetting, electrical and thermal transport, electromagnetic polarization as well as cell and tissue growth.

Deep Level Transient Spectroscopy was performed on InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) in order to determine the effect of the dislocation density on the deep intragap electronic levels. The LEDs were grown by Metalorganic Vapor Phase Epitaxy (MOVPE) on GaN templates with a high dislocation density (HDD) of 8x109 cm-2 and a low dislocation density (LDD) of 3x108 cm-2. Three trapping levels for electrons were revealed, named A, A1 and B, with energies EA ≈ 0.04 eV, EA1 ≈ 0.13 eV and EB ≈ 0.54 eV, respectively. The trapping level A has a much higher concentration in the LEDs grown on the template with a high density of dislocations. The logarithmic dependence of the peak amplitude on the bias pulse width for traps A and A1 identifies the defects responsible for these traps as associated with linearly arranged defects. We conclude that traps A and A1 are dislocation-related intragap energy levels.

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.

The quaternary nitride-based high electron mobility transistor (HEMT) has been recently a focus of interest because of the possibility to grow lattice-matched barrier to GaN and tune the barrier bandgap at the same time.

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.

La Sindrome di Churg-Strauss è una patologia autoimmune sistemica che può interessare numerosi distretti corporei, non da ultimo il sistema nervoso; si tratta per la maggior parte dei casi di una neuropatia periferica ma è stato osservato anche il coinvolgimento del sistema nervoso centrale. Le Linee Guida correnti non forniscono evidenze sufficienti all’identificazione di esami clinico-strumentali che garantiscano diagnosi di certezza eziolo-gica, al di fuori della biopsia del parenchima cerebrale, dirimente ma chiaramente non praticabile di routine. Proponiamo il percorso clinico di una paziente con diagnosi anamnestica di Sindrome di Churg-Strauss giunta alla nostra attenzione per deficit neurologici ad esordio ictale., Bollettino della Società Medico Chirurgica di Pavia, Vol 124, N° 2 (2011)

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.

GaN-based high electron mobility transistors with excellent high-frequency and high-power performance characteristics obtained from AlInN/ GaN heterostructures already reached market maturity. Nearly lattice-matched AlInN/ GaN heterostructures exhibit typical 2D electron gas (2DEG) density confined at the heterointerface whose density is of the order of 1013 cm-2. Unfortunately, these electrons forming the 2DEG may suffer from poor in-plane transport properties due to alloy disorder induced scattering. AlN interlayer helps keeping the electrons better confined in the GaN channel and prevents 2DEG electrons from alloy scattering. The present contribution deals with AlInN/AlN/GaN heterojunctions grown by MOCVD with different AlN thicknesses studied by Surface Photovoltage Spectroscopy (SPS). The 2DEG at the AlN/GaN interface has been characterized by Hall effect and Current –Voltage characteristics. By Surface Photo-voltage Spectroscopy a band gap shift has been detected and its dependence on the 2DEG electron density was studied. The energy gap was found to increase as a function of the free carrier concentration at the heterointerface, which is related to the 2DEG density, due to the combined effect of Moss-Burnstein and renormalization models. Their application allows us to calculate a parameter which quantitatively accounts for the non parabolic conduction band and to evaluate the effective mass dependence on free carrier concentration. This work was supported by the EU under Project No. PITN-GA-2008 213238-RAINBOW.

GaN based heterostructures have recently gained increased interest due to their applications for High Electron Mobility (HEMT) transistors with excellent high-frequency (GHz) and high-power performance characteristics. Nearly lattice-matched (LM) AlInN/ GaN heterostructures exhibit typical 2D electron gas (2DEG) density confined at the heterointerface whose density is of the order of 1.0 × 1013 cm-2, but these electrons forming the 2DEG show poor mobilities due to alloy disorder induced scattering. The insertion of AlN interlayer helps keeping the electrons better confined in the GaN channel and prevents 2DEG electrons from alloy scattering within the AlInN barrier [1]. In this work AlInN/AlN/GaN heterojunctions grown by MOCVD with different AlN (interlayer) thicknesses have been investigated by Surface Photovoltage Spectroscopy (SPS). The density of the two dimensional electron gas forming at the interface has been measured by Hall effect. A band gap shift has been detected by SPS 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. The application of the model allowed us to calculate a parameter which quantitatively account for the non parabolic conduction band (E*) and to evaluate the effective mass dependence on free carrier concentration.

In0.14 Al0.86N/AlN/GaN heterostructures with different AlN interlayer thicknesses (0-7.5nm) and different barrier layer thicknesses have been investigated by Current-Voltage (I-V) characteristics with Schottky contacts in a planar back-to-back configuration at 300K. Changes in the slope of I-V characteristic have been observed for increasing bias and attributed to the onset of electrical conduction at the AlN/GaN interface, where the two-dimensional electrons gas (2DEG) is located. A new model has been proposed to extract the 2DEG properties from the room temperature I-V measurements. By accurate modelling of the heterostructure and by using the 1-D Schrödinger Poisson solver, the characteristics of the two-dimensional electrons gas (2DEG) forming at the AlN/GaN interface have been calculated. By comparing I-V and Hall Effect results, we assessed the reliability of common I-V analyses in the determination of the major transport properties of the 2DEG. Capacitance-Voltage measurements are under study for further analysis and also to compare the extracted results from proposed model and other methods.

MOCVD grown Al1-xInxN/AlN/GaN and InxGa1-xN/GaN heterostructures have been characterized by Atomic Force Microscopy in semi-contact and conductive mode. In-related alloys consist of grain-like structures indicating step flow and 3D-growth with threading dislocations (TDs) of density equal to ~108/cm2, the origin of which is mostly attributed to lattice mismatch between GaN and sapphire. These TDs with screw or mixed components terminate at the surface of overgrown layers as V-defects, which are six-facetted inverted pyramidal structures. Strain relaxation mechanism, formation of cracks and its propagation to the surface of the samples were also investigated. With phase-imaging (in semi-contact AFM), we have traced sites of indium segregation in the V-defects, surface-relaxation and crack propagation in In-related alloys. These sites in V-defects and cracks were found to be highly conductive by current-AFM either due to the presence of In-segregation or due to lowering of potential barrier as a consequence of strain-relaxation. They may be the possible dominant cause of leakage current in Schottky diodes. Further, current-voltage characteristics were obtained using AFM tip and barrier lowering due to image charges or trapped charges at the interface could be the conduction mechanism present in these structures. This has allowed us to calculate the local 2DEG density. This work was supported by the EU under Project No. PITN-GA-2008 213238_RAINBOW

InAlN/AlN/GaN heterostructures have a high potential to be used in HEMTs because of their ability to provide high electron mobility and high 2D electron gas density. Here we report the characterization of MOCVD grown Al0.84In0.16N/AlN/GaN heterostructures using Four Probe Current-Voltage (I-V) measurements. InAlN/AlN/GaN heterostructures with for different AlN interlayer thicknesses (0-7.5nm) and also different barrier layer thicknesses have been explored. The four probe Current-Voltage measurements have been performed at room temperature. A change of the conduction mechanism has been observed and a model has been developed to extract the 2DEG properties by the current-voltage characteristics. By comparing I-V and Hall effect results we could assess the reliability of common I-V analyses in the determination of the major transport properties of the 2DEG.. This work was supported by the EU under Project No. PITN-GA-2008 213238_RAINBOW

Nearly lattice matched Al0.84In0.16N/AlN/GaN heterostructures have a high potential to be used in HEMTs because of their ability to provide high electron mobility [1] and high 2D electron gas density [2]. However, microscopical electrical properties and morphology have not been deeply investigated up to now. Here we report the characterization of MOCVD grown Al0.84In0.16N/AlN/GaN heterostructures using Atomic force microscopy and conductive-Atomic force microscopy. We have used Atomic Force Microscopy for topography maps and conductive atomic force microscopy to obtain the current-maps at nanoscale. A bias is applied to the Atomic Force Microscopy conductive tip and the resulting current is mapped over the selected area. The analyses of these current maps allow for understanding the current flow in these structures. Figure 1 shows an example of a topography map over one of the described structure. By varying the applied bias to the tip a current-voltage characteristic is obtained which allows for the study of the electronic transport mechanism in these heterostructures. By using Fowler-Nordheim tunnelling model for the conduction mechanism in the metal (tip) oxide semiconductor (MOS) system the electronic transport mechanism has been explained. References: [1] Jinqiao Xie, Xianfeng Ni, Mo Wu, Jacob H. Leach, Ümit Özgür, and Hadis Morkoç, APPLIED PHYSICS LETTERS 91 (2007) 132116 [2] M. Gonschorek, J.F. Carlin, E. Feltin, M. A. Py, N. Grandjean, V. Darakchieva, B. Monemar, M. Lorenz, and G. Ramm, JOURNAL OF APPLIED PHYSICS 103, (2008) 093714

Hydrogenated nanocrystalline silicon (nc-Si:H) is an attractive material for photovoltaic applications, nevertheless some of its physical properties have been investigated only in recent times. In particular, the investigation of the transport mechanisms has up to now led to controversial results. This is mainly due to the complexity of nc-Si:H, as several phases and many defects and impurities coexist. The doping process further increases the complexity of the system as dopant atoms can segregate at nanocrystals (ncs) or at the boundaries between different phases. An extended study of the conduction mechanisms at microscopic level of nc-Si:H thin films is here reported. The films have been deposited by Low Energy Plasma Enhanced Chemical Vapor Deposition, at deposition temperatures from 200 to 400°C and SiH4 dilution ratios from 1% to 50%, which resulted in crystal fractions ranging from 25 to 75%. p-type and n-type doped layers were obtained by using B2H6 and PH3 gases, respectively. Sub-micron resolution current maps have been obtained by conductive atomic force microscopy. In the undoped samples all the maps presented a clear evidence of enhanced conduction in the ncs, while the disordered tissue surrounding them was mostly non-conductive. The conduction, furthermore, occurs mainly at the ncs independently of the crystalline fraction of the films. Doped films show a quite different behavior: nanocrystals are still more conductive than the surrounding tissue, but their localization in the map is different from that of intrinsic films. These results have been compared with macroscopic conductivity measurements. A unified model to interpret both microscopic and macroscopic results is advanced.

Electronic properties of Hydrogenated nanocrystalline Silicon for Photovoltaic Applications Authors : Daniela Cavalcoli and Anna Cavallini Department of Physics, University of Bologna, viale C. Berti-Pichat 6/II, 40127 Bologna, Italy. Resume : Hydrogenated nanocrystalline silicon (nc-Si:H) is a promising material for photovoltaic applications, mainly due to its higher stability with respect to amorphous silicon, nevertheless its physical characterization has been carried out only recently, and several material properties are still not clearly understood. The present contribution deals with the study of electrical conductivity and below band gap spectroscopy in intrinsic and doped nc-Si:H films grown by LEPECVD (Low Energy Plasma Enhanced Chemical Vapor Deposition). The films have been deposited using SiH4 and H2 precursor gases, at temperatures ranging from 200 to 400°C, SiH4 dilution ratios from 1% to 50%, on several substrates: crystalline Si, glass, glass covered with ITO or ZnO. The crystal fraction ranged from 25 to 75%, p-type and n-type doped layers were obtained by adding to the precursor gases B2H6 and PH3, respectively. Sub-micron resolution current maps have been obtained by conductive atomic force microscopy (C-AFM) applying a fixed voltage to a conductive tip. The maps of the intrinsic films present a clear evidence of enhanced conduction in the nanocrystals, while the disordered tissue surrounding the nano-grains is mostly nonconductive. In doped films the maps show a quite similar behavior, but the spatial distribution of conductive nanocrystals differs from the one of the intrinsic films. Surface photovoltage spectroscopy (SPS) has been applied to intrinsic nc-Si:H films. The spectra have allowed for the determination of defect states, energy gap and Urbach tails in the films. Several characteristics of the films were obtained: slight n-type conductivity, optical gap around 1.5 eV, Urbach tails around 50meV and the presence of intra-gap transitions relevant to defective states.

Surface Photovoltage Spectroscopy (SPS) [1] is a valuable method for the non-contact and non-destructive investigation of several semiconductor bulk properties, such as optical band-gap, defect-related optical transitions, minority carrier diffusion length and lifetime, Van Hove singularities or phase inhomogeneities. Moreover, SPS allows for obtaining a detailed picture of the electronic structure of surfaces and interfaces, of quantum structures such as quantum wells and superlattices. In SPS, changes in band bending (both at the free semiconductor surface and at buried interfaces) are monitored as a function of external illumination. In the present contribution the physical principles of the method and the experimental details will be discussed, together with several applications concerning a wide variety of materials, alloys and low dimensional semiconductor structures. SPS has been applied to study hydrogenated nanocrystalline silicon films (nc-Si:H) [2]. nc-Si:H is a very complex material made by Si nano-crystals dispersed in hydrogenated amorphous Si (a-Si:H) matrix that is very promising for photovoltaic applications. Notwithstanding its interesting applications, many issues regarding its electronic and optical properties and the influence of structural defects on them are not completely understood yet. The SPS analyses of nc-Si:H films have allowed for the determination of defect states, energy gap and Urbach tails in the films. Several characteristics of the films were obtained: slight n-type conductivity, optical gap around 1.5 eV, Urbach tails around 50meV and the presence of intra-gap transitions relevant to defective states. Optical transitions (around 1.86 eV and 1.14 eV) were also detected and attributed to the optical gap of the amorphous phase and crystalline phases, respectively. Another application will be shown, relevant to Cd1–xZnxTe radiation detectors studied by SPS [3]. SPV spectra have been obtained on semi-insulating Cd1-xZnxTe samples with varying Zn concentration. A reliable and very accurate determination of the energy gap in Cd1-xZnxTe (with x ranging from 0 to 6.76%) has been achieved, needed to assess the exact Zn concentration which significantly affects the transport and lattice parameters of the alloys. It was also found that the electron-hole recombination at the surface has a strong effect on the determination of band gap energy through SPV spectra. Moreover a band structure feature relevant the Γ8 splitting of the valence band in CdTe has been found. A final application will concern Surface photovoltage spectroscopy of AlInN based semiconductor heterostructures. The SPS has allowed for the determination of energy gap in buried semiconductor layers and of intra-gap defect states. References: [1] L. Kronik Y. Shapira Surf. Interface Anal. 31, 954 (2001) [3] A. Cavallini, D. Cavalcoli, M. Rossi, A. Tomasi, S. Pizzini, D. Chrastina, G. Isella. PHYSICA. B, Cond. Matt 401-402, 519 - 522. (2007). [2] D. Cavalcoli, B. Fraboni, and A. Cavallini J Appl Phys 103, 043713 (2008).

International audience; The system described in this paper is aimed at improving the clinical workflow of post-stroke patients under oral anticoagulant therapy (OAT). The system helps both physicians and patients during the periodic control visits necessary to assess the anticoagulation status and the next therapeutic plan. Controls represent a burden for both patients, which after blood drawing must wait for the result, and for physicians, that, after assessing the therapy plan, must communicate it to patients, face-to-face or by telephone. A system is proposed, which embeds an algorithm for the patient-tailored calculation of the drug dosage and scheduling, and an automatic telephone dialogue for the communication of the therapy plan, once it has been validated or adjusted by the physician.