Wide Gap Materials Research Articles

Measurements of Charge Collection Efficiency of p<sup>+</sup>/n Junction SiC Detectors

Silicon carbide is a promising wide-gap material because of its excellent electrical and physical properties, which are very relevant to technological applications. In particular, silicon carbide can represent a good alternative to Si in applications like the inner tracking detectors of particle physics experiments [1]. In this work p+/n SiC diodes realized on a medium doped (1×1015 cm -3), 40 µm thick epitaxial layer are exploited as detectors and measurements of their charge collection properties under beta particle radiation from Sr90 source are presented. Preliminary results till 900 V reverse voltage show a good collection efficiency of 1700 e- and a collection length (ratio between collected charges and generated e-h pairs/µm) equal to the estimated width of the depleted region.

Quantum information processing exploiting defects in wide gap materials

Wide-gap materials, and especially defects and dopants in them, have characteristics that could enable quantum information processing. Spectroscopy, in particular, is a powerful resource, and spectroscopic methods might be used to manipulate quantum information. In some cases, the methods might lead to optically-controlled spintronics and, conceivably, to quantum information processing at room temperature. Other classes of device, whilst of more limited value technologically, do illustrate the pervasive role of quantum physics that is sometimes overlooked. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic processes in rare earth activated wide gap materials

This paper is a review of charge exchange processes in rare earth doped insulating materials. After a description of experimental approaches for the location of impurity ions in the host gap, charge transfer and photoionization phenomena are presented. Experimental data are discussed and interpreted using simple models.

High Speed Heterostructure Metal-Semiconductor-Metal Photodetectors

A. Colaa,∗, B. Nabet, Xiying Chen and F. Quaranta CNR-IMM sez. di Lecce, via Arnesano 73100 Lecce, Italy Electrical and Computer Engineering Dept., Drexel University 318 Commonwealth, Philadelphia, PA, 19104, USA In this work we review the properties of a class of metal−semiconductor– metal photodetectors based on heterojunction structures. Particularly, an AlGaAs/GaAs device is detailed in which the absorption region is in the GaAs layer, and a two-dimensional electron gas is formed at the heterointerface due to δ-doping of the widegap material. This heterostructure metal– semiconductor–metal photodetector also contains an AlGaAs distributed Bragg reflector that forms a resonant cavity for detection at 850 nm. The beneficial effect of the two-dimensional electron gas in the GaAs absorption layer in terms of speed and sensitivity is demonstrated by comparing samples with and without doping in the AlGaAs layer. The design and the physical properties of the grown epitaxial structure are presented, together with the static and dynamic characteristics of the device in time domain. In particular, photocurrent spectra exhibit a 30 nm wide peak at 850 nm, and time response measurements give a bandwidth over 30 GHz. A combination of very low dark current and capacitance, fast response, wavelength selectivity, and compatibility with high electron mobility transistors makes this device suitable for a number of application areas, such as Gigabit and 10 Gigabit Ethernet, wavelength division multiplexing, remote sensing, and medical applications.

Electrical and Photonic Functions Originating from Low-Dimensional Structures in Wide-Gap Semiconductors LnCuOCh (Ln=lanthanide, Ch=chalcogen): A Review

This article reviews novel electrical and optical properties found for epitaxial thin films of wide-gap semiconductors, LnCuOCh (Ln = lanthanide, Ch = chalcogen). This material series has a two-dimensional crystal structure composed of alternately stacked (Ln 2 O 2 ) 2 + and (Cu 2 Ch 2 ) 2 - layers. Distinctive properties such as high hole mobility, degenerate p-type conduction, room temperature exciton, and large optical nonlinearity were found and these are attributed to two-dimensional electronic structure arising from the layered structure, i.e., a narrow-gaped and hole-conductive (Cu 2 Ch 2 ) 2 - layer is sandwiched by wide-gaped insulating (Ln 2 O 2 ) 2 + layers. In particular, the wide-gap p-type metallic conduction was the first demonstration among any class of wide-gap materials including GaN: Mg. Realization of epitaxial thin films for these materials by reactive solid-phase epitaxy led to these discoveries which make LnCuOCh promising materials for optoelectronic devices utilizing the high p-type conductivity and/or the room temperature exciton.

Development of semiconductor detectors based on wide-gap materials

The properties of promising materials for uncooled semiconductor x- and γ-ray detectors are examined. Work being performed in this field in our country is briefly described. The spectrometric parameters of detectors based on CdTe, GaAs, HgI2, and TlBr crystals are presented. Small, according to sensitivity, detectors based on CdxZn1-xTe and HgI2 have the best room-temperature resolution. Individual samples and experimental batches of such detectors have been tested and used under industrial and field conditions. The results obtained with TlBr crystals are interesting.

Development of semiconductor detectors based on wide-gap materials

Development of semiconductor detectors based on wide-gap materials

The carrier contribution to the elastic constants in superlattices of non-parabolic semiconductors with graded interfaces under magnetic quantization: simplified theory and suggestion for experimental determination

An attempt is made to study the carrier contribution to the elastic constants under magnetic quantization in III–V, II–VI, PbTe/ PbSnTe, strained layer and HgTe/CdTe superlattices (SLs) with graded interfaces and compare the same with that of the constituent materials by formulating the respective dispersion laws. It is found, taking GaAs/Ga 1− x Al x As, CdS/CdTe, PbTe/ PbSnTe, InAs/GaSb and HgTe/CdTe with graded interfaces as examples, that the second and third order elastic constants exhibit oscillatory dependence with the inverse quantizing magnetic field due to the Shubnikov-de-Haas (SdH) and allied SL effects and increases with increasing electron concentration in an oscillatory manner in all the cases. The nature of oscillation is totally band structure dependent and the width of the finite interface enhances the numerical values of the second and third order elastic constants for all the aforementioned SLs. The numerical values of the second and third order elastic constants in graded superlattices are greater than that of the constituent materials. We have suggested an experimental method of determining the electronic contribution to the elastic constants in materials having arbitrary dispersion laws and the theoretical results are in quantitative agreement with the suggested relationship. In addition, the well-known expressions for the bulk specimens of wide gap materials have also been obtained as special cases of our generalized analysis under certain limiting conditions.

Luminescence of molecules adsorbed on surfaces of wide gap materials

We report on the photoluminescence spectra of thin films of chain-like π-conjugated molecules on well-defined surfaces of wide gap inorganic materials. The aim is to study the energy transfer processes across the organic/substrate interface which limit the luminescence of molecules in close contact to substrate surfaces. We discuss quaterthiophene (4 T) adsorbed on the c(2×2)-ZnSe(0 0 1) surface and tetracene (Tc) on the surface of a thin epitaxial Al 2O 3 film on Ni 3Al(1 1 1). For thin films vapor-deposited at low substrate temperatures, we observe no luminescence signal for both systems, which indicates the presence of fast luminescence quenching processes at the organic/inorganic interface. After annealing, luminescence spectra corresponding to those of the bulk crystals are obtained. This can be explained by the formation of 3D-crystallites, which effectively separate most molecules from the organic/inorganic interface.

Preface: phys. stat. sol. (c) 1/S2

The papers in this special issue of physica status solidi (c) are selected manuscripts including diverse research lines presently in development in the ambit of the NanoSemiMat network in Brazil. The 3rd Workshop on Semiconductor Nanodevices and Nanostructured Materials (NanoSemiMat-3) took place in Salvador, Bahia, Brazil, at the Catussaba Resort Hotel, during the period of 24–27 March 2004. The NanoSemiMat network is part of the Brazilian Initiative on Nanoscience and Nanotechnology (N&N), with strategic cooperative research support in this area. The initiative started in 2001, through the formation of four research networks nationwide in different scientific fields associated to NanoScience and Nanotechnology (N&N). The 3rd Workshop on Nanodevices and Nanostructured Materials (NanoSemiMat-3) is an evolution of the two previous meetings which were held in Recife, PE, Brazil and Natal, RN, Brazil in 2002 and 2003, respectively. The meeting comprised 16 invited plenary talks, each 30 minutes long, given by eminent researchers from Brazil, Canada, France, Germany and the United States of America. These invited talks extend through different topics of N&N associated to Nanodevices and Nanostructured Materials: Photodetectors, Lasers and LEDs, Porous Materials, New Materials, and New Technologies, among others. There were short talks presented by representatives of the other N&N networks in Brazil dealing with Molecular Technology and Interfaces, Nanostructured Materials and Nanobiotechnology. Also a poster session, with about 60 presentations, highlighted the main research activities presently being developed by the network members at the different sites which constitute the NanoSemiMat network. The presentations reflected theoretical and experimental research lines which lead to the development of basic and applied research in nanostructured semiconductor materials such as III–V and II–VI, Si and SiC based nanodevices, wide gap materials, ceramics, polymers, porous materials, optical and transport properties of low dimensional structures, magnetic nanostructures and structures under the influence of high fields, spintronics and sensor applications. The participants of the workshop came from 20 research institutions within Brazil and from 7 research laboratories and universities in Europe and North America. In total about 120 researchers, members of the network, invited researchers, representatives of supporting and funding agencies in Brazil, undergraduate and graduate students, technical staff and supporting personal as well as researchers from complementary fields were present. The realization of the NanoSemiMat-3 was possible due to the financial support of the Brazilian Ministry of Science and Technology (MCT) and the Brazilian National Research Council (CNPq) and the logistic support of Federal University of Bahia. All activities during the NanoSemiMat-3 were open to the general public with interest in nanoscience and nanotechnology. In this third workshop of the series, we highlight the expansion of its format, with plenary and invited talks, poster sessions, as well as the presence of seven invited speakers from abroad. We expect that the continuation of the NanoSemiMat series will be a forum for discussions of state-of-the-art research developed in Brazil on N&N and the multidisciplinary field of semiconductor nanodevices and nanostructured materials as well as its superposition to other branches of science. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

3rd Workshop on Semiconductor Nanodevices and Nanostructured Materials (NanoSemiMat-3)

The 3rd Workshop on Semiconductor Nanodevices and Nanostructured Materials (NanoSemiMat-3) took place in Salvador, Bahia, Brazil, 24–27 March 2004. The NanoSemiMat network is part of the Brazilian Initiative on Nanoscience and Nanotechnology (N&N). The papers include the following topics: Photodetectors, Lasers and LEDs, Porous Materials, New Materials, New Technologies, Molecular Technology and Interfaces, Nanostructured Materials and Nanobiotechnology. The presentations reflect theoretical and experimental research on nanostructured semiconductor materials such as III–V and II–VI, Si and SiC based nanodevices, wide gap materials, ceramics, polymers, porous materials, optical and transport properties of low-dimensional structures, magnetic nanostructures and structures under the influence of high fields, spintronics and sensor applications. This issue is devoted to Prof. J. R. Leite, Sao Paulo, former Regional Editor of physica status solidi and Guest Editor in memoriam of the present Proceedings.

Theoretical and Experimental Study on Anisotropy of XANES/ELNES of Wide-Gap Materials

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Proceedings of the Conference on Photo‐responsive Materials

The Conference on Photo-responsive Materials took place 25–29 February 2004 at Kariega Game Reserve, South Africa. It was focused primarily on III-nitrides (GaN, AlGaN, InGaN), SiC, ZnO and other wide gap materials, chalcopyrites such as CuInSe2 for photovoltaic applications, and novel photoresponsive materials including polymers. Material-related issues within the scope of the conference were crystal growth and epitaxy, characterisation (photoluminescence, DLTS, IR spectroscopy), defect-related studies, processing, and opto-electronic devices (modelling and novel structures).

First-principles calculations of ELNES and XANES of selected wide-gap materials: Dependence on crystal structure and orientation

Theoretical calculations of electron energy-loss near-edge structure (ELNES) and x-ray absorption near-edge structure (XANES) of selected wide-gap materials including ${\text{TiO}}_{2}$, AlN, GaN, InN, ZnO, and their polymorphs are performed using a first-principles method. Calculations of 39 $K$ and ${L}_{3}({L}_{2,3})$ edges are made using large supercells containing 72 to 128 atoms. A core hole is included in the final state, and the matrix elements of the electric dipole transition between the ground state and the final state are computed. Structures of some metastable crystals are optimized by a plane-wave basis pseudopotential method. Spectral differences in ELNES and XANES among polymorphs are quantitatively reproduced in this way. The origin of the spectral differences is pursued from the viewpoint of chemical bondings. Crystallographic orientation dependence of ELNES and XANES is also examined both by experiment and theory. The dependence is found to be much larger in $K$ edges than that in ${L}_{3}({L}_{2,3})$ edges.

MIP-type organic solar cells incorporating phthalocyanine/fullerene mixed layers and doped wide-gap transport layers

We describe a simple organic solar cell structure that allows to study in detail loss mechanisms due to non-ohmic contacts, structural drawbacks and material selection: the MIP-type (metal-intrinsic-p-doped) structure discussed here represents the p-side and the active layer within a PIN type solar cell architecture. We here adapt the bulk hetero-junction concept using blends of zinc-phthalocyanine (ZnPc) and buckminsterfullerene C 60 . Furthermore, we use doped wide-gap materials as hole transport layers that enable a more sophisticated solar cell development. The samples are characterized by combination of current voltage characteristics, impedance spectroscopy and capacitance–voltage measurements. We present an evolution of MIP-type structures which improves all solar cell parameters; in particular, excessive series resistance and contact problems reducing the fill factor could be almost completely eliminated.

Simple theory of the optical absorption coefficient in nonparabolic semiconductors

A simple theory is developed of the optical absorption coefficient (OAC) in nonparabolic semiconductors on the basis of the three-band model of Kane, by considering the wave-vector (k⃗) dependence of the optical matrix element. It has been found that the OAC is proportional to [(ℏω)2−Eg2]1/2 (ℏω and Eg are the energy of the incident radiation and the bandgap, respectively) instead of [(ℏω)−Eg]1/2, the conventional form. It has been demonstrated, by taking n-InAs, n-InSb, Hg1−xCdxTe, and In1−xGaxAs1−yP1−y lattice matched to InP as examples, that the OAC increases with increasing photon energy and the value of the same coefficient in the three band model of Kane is greater than that in parabolic energy bands in all the cases. The well-known result for wide gap materials having parabolic energy bands has also been obtained from our generalized formulation under certain limiting condition.

Tunneling photoconductivity computations of multi-quantum well p-i (nano)-n photovoltaic nanostructures by means of the causal Green's function

We propose a first principles method of calculating the photo-component of tunneling conductivity of a specific superlattice (SL) photovoltaic structure: an InP/InGaAs p-i(SL)-n solar cell. The method we follow is based on the generalized Greenwood–Kubo formulation and on the causal form of the Green's function. The vertical conductivity (along the growth direction of the device), under illumination, involves a group of parameters directly connected to the geometry of the device (hence it is on one's disposal) and the quantum size effects in the quantum wells of the superlattice. The latter is incorporated in the intrinsic region of p-i-n solar cells and bring along the advantage of carrier tunneling, which is possible if thin layers of wide-gap material (InP at 25 A/layer) are grown successively on narrow band-gap material (lattice-matched In0.53Ga0.47As at 7 nm/layer) to form a sequence of quantum wells.

Tunneling photoconductivity computations of multi-quantum well p-i (nano)-n photovoltaic nanostructures by means of the causal Green's function

We propose a first principles method of calculating the photo-component of tunneling conductivity of a specific superlattice (SL) photovoltaic structure: an InP/InGaAs p-i(SL)-n solar cell. The method we follow is based on the generalized Greenwood–Kubo formulation and on the causal form of the Green's function. The vertical conductivity (along the growth direction of the device), under illumination, involves a group of parameters directly connected to the geometry of the device (hence it is on one's disposal) and the quantum size effects in the quantum wells of the superlattice. The latter is incorporated in the intrinsic region of p-i-n solar cells and bring along the advantage of carrier tunneling, which is possible if thin layers of wide-gap material (InP at 25 A/layer) are grown successively on narrow band-gap material (lattice-matched In 0.53Ga 0.47As at 7 nm/layer) to form a sequence of quantum wells.

Characterization of p–n junctions of diamond and c-BN by cathodoluminescence andelectron-beam-induced current

p–n junctions were fabricated in diamond and cubic BN for ultraviolet light emittingdiodes. These junctions were characterized by means of cathodoluminescence (CL) andelectron-beam-induced current (EBIC). The line profile of CL spectra clearly revealed thedifferent doping regions. EBIC profiles showed the band profile of these structures. Thecomparison of CL and EBIC results confirmed that, in both cases, the electrical fields existat the interfaces between p and n regions. The light emitted from these structuresunder the current injection was compared with CL spectra from the p and nregions. It is noted that EBIC and CL are a suitable tool for the characterization ofp–n junction devices of wide-gap materials with small minority carrier diffusionlength.

MIP-type organic solar cells incorporating phthalocyanine/fullerene mixed layers and doped wide-gap transport layers

We describe a simple organic solar cell structure that allows to study in detail loss mechanisms due to non-ohmic contacts, structural drawbacks and material selection: the MIP-type (metal-intrinsic-p-doped) structure discussed here represents the p-side and the active layer within a PIN type solar cell architecture. We here adapt the bulk hetero-junction concept using blends of zinc-phthalocyanine (ZnPc) and buckminsterfullerene C60. Furthermore, we use doped wide-gap materials as hole transport layers that enable a more sophisticated solar cell development. The samples are characterized by combination of current voltage characteristics, impedance spectroscopy and capacitance–voltage measurements. We present an evolution of MIP-type structures which improves all solar cell parameters; in particular, excessive series resistance and contact problems reducing the fill factor could be almost completely eliminated.