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A comparative study of diode structures based on semi-insulating (SI) GaAs with evaporated Mg vs. Pt contacts is presented. Electric field strength in the vicinity of the contacts is inferred from the detection of α-particles emitted from 241Am radioisotope. It is shown that at zero bias, the structure with Mg-based contacts (MgO/SI GaAs) gives comparatively small signal implying an ohmic behaviour. Contrary, the structure with Pt contacts reveals presence of a non-negligible electric field near the contacts at zero bias voltage, thus indicating presence of a sizeable space charge region near the interface. The results are supplemented by insights from atomistic electronic structure modeling of idealized GaAs/Pt vs. GaAs/Mg and GaAs/MgO interfaces.

The thermal stability of e-Ga 2 O 3 polymorph was studied by complementary methods. Epitaxial films of e-Ga 2 O 3 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) up to 1100 °C was carried out on fragments of pure e-Ga 2 O 3 taken from a very thick layer. The results clearly indicate that e-Ga 2 O 3 initiates modifying its crystallographic structure above 650 °C, as demonstrated by a mild endothermic bent of the DSC curves. However, the effective transition to β-phase occurs quite suddenly at 880–920 °C, depending of the DSC heating rate. XRD and TEM results confirm this evidence. TEM investigation in particular shows that after annealing at 1000 °C and rapid cooling the film is completely made of β-Ga 2 O 3 grains, most of them with orientation (310) respect to the sapphire substrate. However, if the cooling rate is substantially reduced, the converted β-Ga 2 O 3 layer assumes the standard orientation (−201) parallel to (00.1) of the Al 2 O 3 substrate. Based on the results of this study we conclude that e-Ga 2 O 3 may conveniently be used for device fabrication, exploiting its higher crystallographic symmetry and better matching to sapphire. However, all fabrication steps must be carried out at temperatures below 700 °C.

In this paper we report on the single stage deposition of CuInxGa1−xSe2 (CIGS)-based bifacial solar cells on glass coated with Fluorine-doped Tin Oxide (FTO) or Indium Tin Oxide (ITO) by single-stage low-temperature (250 °C) pulsed electron deposition (LTPED). We show that the mechanism of Sodium incorporation during the low-temperature deposition of CIGS on both FTO and ITO leads to the formation of a stable n+/p+ ohmic tunnel junction and photovoltaic efficiencies exceeding 14% can be obtained without any intentional bandgap grading of CIGS. The significant degradation of the cell fill factor with decreasing CIGS thickness is found to be related to the presence of craters left behind by micro-fragments of CIGS target, which are weakly incorporated in the film during the LTPED growth and removed during the subsequent process steps. Evidence is also presented that the low-temperature deposition of CIGS on ITO leads to the formation of a Ga-rich CIGS layer at the interface and to an unintentional compositional grading propagating towards the active region of the solar cells. The defects associated with this grading may be responsible for the loss in FF and Voc with respect to the cells deposited on FTO and Mo back contacts.

We present a comparative study of the symmetric metal-SI GaAs-metal (M-S-M) diodes with full-area contacts on both device sides, in order to demonstrate the effect of contact metal work function in a straightforward way. We compare the conventional high work function Pt contact versus the less explored low work function Mg contact. The Pt-S-Pt, Mg-S-Mg and mixed Mg-S-Pt structures are characterized by the current-voltage measurements, and individual Pt-S and Mg-S contacts are investigated by the X-ray photoelectron spectroscopy (XPS). The transport measurements of Mg-S-Pt structure show a significant current decrease at low bias while the Mg-S-Mg structure shows saturation current at high voltages more than an order of magnitude lower with respect to the Pt-S-Pt reference. The phenomena observed in Mg-containing samples are explained by the presence of insulating MgO layer at the M-S interface, instead of the elementary Mg, as confirmed by the XPS analysis. Alternative explanations of the influence of MgO layer on the effective resistance of the structures are presented. The reported findings have potential applications in M-S-M sensors and radiation detectors based on SI GaAs.

InAs/GaAs quantum dot photodetector photoresponse spectra consist of quantum dot, wetting layer and GaAs components, however, they frequently contain features attributed to defect levels. In this study we focus on the origin of an unwanted negative photovoltage component in order to find a design allowing to eliminate these undesirable effects. Photoelectric properties of vertical InAs/GaAs quantum dot structures grown on semi-insulating (si) and n+-GaAs wafers are analyzed. It is found that the photoresponse drop above 1.37 eV in the photovoltage spectrum, observed with si-GaAs substrate, is originated from shallow defect levels in the unintentional space-charge area created at the interface between the substrate and n+-GaAs buffer. The intrinsic field in that area is opposite to the top pn junction, according to modelling calculations, and it reduces the photoelectric response. Photoelectric characterization of the heterostructure grown on n+-GaAs indicates that the utilization of n+-GaAs substrate helps to avoid this band bending and to eliminate its negative effects, increasing the total photoresponse.

The main goal of this project is to demonstrate the advantages of sensor integration on a remotely controlled robotic platform for increasing operator safety and improving the classification of explosive targets. This is accomplished by combining the imaging provided by radars and an optoelectronic sensor, a time-of-flight (ToF) depth camera. An additional aim is to demonstrate the operability and practicality of the system in a field with landmine simulants having plastic cases.

The project's aim is to prepare portable sensors for explosives based on semiconductor nanowires or carbon nanotubes. Nanostructures permit the preparation of sensitive, very compact and lightweight chemical sensors able to detect the air presence of molecules of explosives emitted by solid explosives, such as Tri-Nitro-Toluene (TNT). Devices based on functionalised nanostructures have the advantage of very low electrical consumption, high chemical sensitivity, and simple technology. This will allow the mounting of the sensor on unmanned aerial vehicles such as drones able to explore hazardous environments without the direct human intervention

Electrical and optical properties of undoped single-phase epsilon-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, fabricated by direct deposition of the epilayers on c-oriented sapphire substrates, exhibited good performance. The physical properties and the photoresponse of epsilon-Ga2O3 make this material very interesting in view of novel applications. (C) 2017 Elsevier B.V. All rights reserved.

Recently some of the authors have proposed a new technique to deposit contacts on CdZnTe detectors that combines the standard electroless procedure and the scanning pipette technique. In this work it is shown that by using a 15 micron inner diameter alumina pipette it is possible to deposit gold dots, pixels and micro-stripes without using the standard photolithographic process. Moreover, it is shown that the performances in terms of charge collection efficiency and energy resolution of detectors prepared with the scanning pipette technique are similar to the ones of detectors prepared with the standard electroless procedure.

Metamorphic InAs/InGaAs quantum dot (QD) structures have been successfully used in optoelectronics as light-emitting and sensing devices. Previous optical and photoelectrical studies showed many benefits such as the shift of the operating range in the infrared and the possibility of engineering their properties via the control of strain. However, photovoltage (PV) kinetic properties of metamorphic QD structures have not been studied yet. PV kinetics in the vertical metamorphic InAs/In0.15Ga0.85As QD and wetting layer (WL) structures compared with conventional pseudomorphic InAs/GaAs QD ones are investigated. A red-shift of the photoresponse from metamorphic QDs in comparison with pseudomorphic QDs was observed, while the PV magnitudes remain similar. Moreover, time-dependent PV measurements reveal faster increase and decay rates in both the metamorphic samples. At the same time, all the structures reveal a non-monotonous kinetics. This kinetics is explained by an unwanted PV component generated in the interface of n+-GaAs buffer to the semi-insulating GaAs substrate, causing a signal opposite to the main PV generated at the top layers of structure. We believe the findings on faster carrier kinetics in metamorphic QDs can be helpful for the development of novel efficient optoelectronic devices. Specifically, the design of metamorphic QD structure is proposed to be improved by: (i) minimizing strain-related defects, (ii) keeping such interface defects far from QDs, (iii) utilizing n+-GaAs substrates.

The low-bias current-voltage characteristics of surface barrier diodes, based on semi-insulating GaAs (SI-GaAs) with different contact metals and area, are reported and analyzed in order to demonstrate the possibility of tuning their low-bias transport characteristics. Novel applications of SI-GaAs emerge for devices with low-current at low-bias requirements, as follows from the possibility of current lowering by as much as two orders of magnitude, achieved by manipulation of the contact metallization and the contact area. The lowest current is observed in the structure with small Mg top and large-area Ti/Pt bottom contact. Since the observed behavior contradicts the conventional ohmic bulk limited and thermionic emission transport models, alternative explanations are discussed. The strong blocking ability of the low-work function Mg contact was attributed to the downwards band-bending, formation of a quasi-degenerate interface region accumulating charge carriers, and the corresponding lowering of the bulk SI-GaAs free carrier concentration. (C) 2013 Elsevier Ltd. All rights reserved.

CdZnTe detectors are commonly exploited for the detection of gamma rays. However, obtaining mechanical stable, low noise contacts on CdZnTe is still an issue. In particular, ohmic contacts would be preferable for high flux applications. In this work, we show that it is possible to obtain mechanical stable gold contacts by electroless deposition in methanol solution. Moreover, we show that electroless deposited nickel contacts are also mechanical stable and are good candidates for the realization of ohmic contacts on high resistivity CdZnTe crystals.

Ge homojunctions were deposited by means of Metal Organic Vapour Phase Epitaxy (MOVPE) on Ge substrates at 550 degrees C and 675 degrees C, using AsH3 as n-type dopant. Ge-n/Ge-p, GaAsn/InGaPn/Ge-n/Ge-p and Ge-n/Ge-p/Ge-p structures were prepared and studied, where n and p identify the layer or substrate doping. Vertical mesa junctions were obtained on the above structures by using conventional photolithographic and evaporation techniques. The junctions were characterized by I-V measurements under dark and illumination conditions and by EBIC technique. It has been observed that the samples grown at lower temperature showed better rectifying I-V characteristics and light conversion efficiency while EBIC results may suggest that a high As diffusion is present in the samples grown at higher temperature. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A bipolar transistor, previously investigated as a possible radiation dosimeter and tested under industrial irradiation conditions in high-activity gamma and high-energy, high-power electron beam facilities has been subjected to stability test in order to understand its behaviour and help to improve its performances. Charge carrier lifetime was measured for several sets of transistors which were then irradiated with various doses (3–60 kGy): seven sets with 60 Co gamma rays and eight with a 10 MeV electron beam. After irradiation all the transistors were measured and each set was divided into three groups: one group was left untreated, the second group was heated at 100 °C for 30 minutes and the third group was heated at 150 °C for 30 minutes, for testing the stability of the lifetime. Our data showed that heat treatment quite successfully eliminates post-irradiation changes in the response. Response measurements of the irradiated transistors, heat-treated and untreated, were carried out at room temperature over several weeks after irradiation to establish post-irradiation stability and assess if these transistors could be used for recording dose history. Calibration curves in the range 3–60 kGy for the thermally treated and untreated devices are presented. Dependence of the response of the transistors on the temperature of the measurements in the range 20–50 °C is reported.

Current–voltage (I–V) characteristics and photocurrent (PC) spectra (600–1000 nm) of the metal–semiconductor–metal (M–S–M) structures based on high-quality undoped semi-insulating (SI) GaAs with AuGeNi backside contact and different semitransparent top contacts (AuGeNi, Pt, Gd and Nd) are reported, and analysed with the help of a simple physical model. It is shown that the dominant peak in the PC spectra and the change of photocurrent sign can be explained by a presence of two Schottky-like barriers at the top and bottom surfaces. In addition, I–V and PC results show dependence on the bias and its polarity, and on the contact metal used. The possible origins of these effects are discussed.

The work reports on a study of the symmetric metal/SI GaAs/metal (M-S-M) diodes in order to demonstrate the effect of contact metal work function. We compare the high work function Pt contact versus the low work function Mg contact. The Pt-S-Pt, Mg-S-Mg and Mg-S-Pt structures are characterized by the current-voltage measurements. The Mg-S-Pt structure show a significant current decrease at low bias while the Mg-S-Mg structure shows saturation current at high voltages more than an order of magnitude lower with respect to the Pt-S-Pt reference. The phenomena observed in Mg-containing samples are explained by the presence of insulating MgO layer at the M-S interface. The reported findings have potential applications in sensors based on SI GaAs.

A preliminary study of technology and performance of radiation detectors based on bulk semi-insulating (SI) GaAs with P+ blocking electrodes is presented. Detectors with blocking electrode system formed either by P+(GaAs)/N–(SI GaAs) homojunction or P+(Al0.3Ga0.7As)/N–(SI GaAs) heterojunction are prepared. Room-temperature I–V characteristics, measurement and evaluation of pulse–height spectra using 241Am and 57Co radionuclide sources were performed for the characterization of the fabricated detectors. The aim of the study is to verify the possibility to improve the performances of SI GaAs radiation detectors based on the Schottky contact by using the more sophisticated P+–N junction technology. The obtained results and the possibility of future improvements of SI GaAs radiation detectors to be used in digital X-ray imaging are discussed.

Localized states in Au/n-GaAs Schottky diodes, with InAs quantum dots (QDs) embedded in GaAs confining layers, were studied by means of low frequency noise measurements at temperatures ranging from 160 K to 299 K. Diodes containing a single array or three arrays of QDs were used; they all exhibited generation–recombination noise at low forward currents, which we attribute to local traps located in the GaAs layer. In the diode with a single array of QDs, a shallow trap level was detected with the activation energy about 0.037 eV, located above the Fermi level. In the diodes with three arrays of QDs we observed, in addition to the shallow level, a deep level located 0.1 eV below the midgap.

We report on GaAs samples containing an InAs quantum dot matrix grown from 3 monolayers of InAs. In this case, most of the mechanical stress relaxes by misfit dislocations. Capacitance measurements have been performed on macroscopic devices and, also, from a scanning microscope in which the capacitance is measured between the probe and sample surface. It was found that the electrical characteristics dramatically change during the capacitance measurements. This is explained by a degradation of the quantum dot layer which is attributed to the generation of point defects and/or dislocations. These results draw attention to the fact that, at the microscopic scale measurement, a small current may result in a large local current density which, in turn, degrades the device. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work, basic tasks related to the spectrometric performance of X- and γ-ray radiation-imaging detectors based on semi-insulating (SI) GaAs are studied. The state of the art in this field is reported. The importance of a low-noise front-end read-out electronic chain is demonstrated. Four different charge-sensitive preamplifiers are tested and compared. The investigation of SI GaAs detectors considered for imaging applications is focused on the study of different electrode technologies. A blocking electrode is obtained by a surface Schottky barrier or an MBE-grown P + layer. An ohmic N + contact is fabricated using alloyed AuGeNi metallization or an advanced, quasi-ohmic non-injecting system created by a non-stoichiometric buffer grown by low-temperature MBE at the metal-semiconductor interface. This latter approach is considered as an improvement of the “non-alloyed” ohmic contact. The pulse height spectra obtained with 241 Am and 57 Co sources and corresponding detector energy resolution are evaluated. The role of detector geometry and temperature on the spectrometric performance is studied.

In AlxGa1-xSb, the Te impurity originates a simple donor state (D level), whose occupancy equilibrates rapidly with the conduction band; it coexists with a DX state which is responsible for the persistent photoconductivity effect. The D level is linked to the L conduction band minima and becomes resonant with the conduction band for x

The photoluminescent and photoelectric properties of a metamorphic InAs/In0.15Ga0.85As structure with InAs quantum dots (QDs) grown on an InGaAs metamorphic buffer (MB) layer are studied. The structure shows the an efficient photoluminescence (PL) signal at 0.93 eV and a week weak band by the InAs wetting layer near 1.2 eV at room temperature (RT), the bands shift to 1.02 and 1.26 eV respectively being cooled to 10 K. Spectral position of the QD emission band corresponds to the telecom window at 1.3 µm (0.95 eV). The structure with vertical contacts is found to be photosensitive in the spectral range above 0.9 eV at RT. All the components of the structure from QDs at 0.9 eV to the MB above 1.22 eV are observed in the photovoltage and photoconductivity spectra. The indirect effect of GaAs manifests itself as only with the drop of the signal at 1.36 eV. No effect of defect centers on the PL and photoelectrical properties is found, although they are observed in the absorption spectrum. The band structure provides a good collection of charge carriers to the InAs QDs which leads to high PL signal. These results confirm that the proposed metamorphic QD structure is a viable option for the development of optoelectronic devices, as the presence of defects due to strain relaxation of the MB does not forbid an efficient collection of carriers into QDs.

High quality epitaxial crystalline Cu(In,Ga)Se2 (CIGS) films were grown on n-type (1 0 0)--Germanium (Ge) substrates using pulsed electron deposition (PED) technique at a remarkably low substrate temperature of 300 °C, thanks to the high-energy of adatoms arriving to the substrate. The crystalline quality was confirmed by X-ray diffraction techniques and from Transmission Electron Microscopy and the only defects found were twin boundaries along the (1 1 2) direction in these CIGS films; surprisingly neither misfit dislocations nor Kinkerdall voids were observed. A 100 meV optical band located below the band edge was observed by Photoluminescence technique. Current-voltage and capacitance-voltage measurements confirm an intrinsic p-type conductivity of CIGS films, with a free carrier concentration of ?3.5×1016 cm-3. These characteristics of crystalline CIGS films are crucial for a variety of potential applications, such as more efficient absorber layers in single-junction and as an integral component of multi-junction thin-film solar cells.

In this work, bulk semi-insulating (SI) InP wafers of four various producers have been used for the fabrication of radiation detectors. The tested detectors were prepared starting from the different materials in just one run in order to be sure that their performances were not influenced by technological processes. On one type of material various electrode technologies were used with the aim to analyze their role on the detector performances. The fabricated detectors were tested for detection performance by the 241 Am and 57 Co gamma-ray sources at below room temperature. The best detector was calibrated and tested also using 133 Ba and 137 Cs gamma sources. The best detector gives an energy resolution of 7 keV FWHM and a charge collection efficiency (CCE) of 82% (59.5 keV photopeak) at a temperature of 216 K. According to our knowledge, these results are the best which have been obtained with InP radiation detectors till now. The operation of SI InP detector at room temperature is demonstrated and discussed. In general, the CCE and the energy resolution of radiation detectors fabricated on bulk SI InP materials showed a strong dependence on the electrode technology as well as the base substrate quality.

High resistivity CdTe crystals were grown by Bridgman and by physical vapour transport techniques without intentional doping. Novel synthesis and pre-growth treatment procedures were developed for controlling background impurities concentration and stoichiometry of the source material. A detailed characterisation of the deep levels responsible for the high resistivity behaviour is reported.

We report on the epitaxial growth of Cu(In,Ga)Se 2 (CIGS) thin films on n-type Ge substrates using the Pulsed Electron Deposition (PED) technique. Single-crystal films with smooth interface and no dislocations were obtained at a substrate temperature of 300°C with thickness above 1.5μm. CIGS/Ge epilayers were characterized in terms of Transmission Electron Microscopy, X-Ray Diffraction and Photoluminescence. Preliminary results are very promising for a variety of potential applications as absorber layer in single and multi-junction thin-film solar cells.

Unexpected excess current was obtained in GaAs/InAs quantum dot structures at low temperatures and low current levels. This excess current exhibited instabilities with changing the bias, and over the time. It has been concluded that the excess current is a minority injection current connected with recombination through defects originated from the formation of QDs. The instabilities are connected with unstable occupation of energy levels induced by the above defects, which depend on temperature and on the current level. Excess currents have also been obtained in annealed AlGaAs/GaAs structures. These excess currents exhibited memory effect, which was probably connected with formatin of defects during annealing.

Beryllium (Be) diffusion after rapid thermal annealing experiments is studied in heavily doped GaAs structures grown by MBE. SIMS measurements show that in p/p + structures, Be diffusion is reduced by increasing the As 4 /Ga flux ratios. In contrast, no effect is observed in p/p + /p structures. Furthermore, Be concentration profiles measured after annealing experiments performed at 770 and 850°C for 30 s indicate that Be redistribution is almost independent of the annealing temperature. These results are discussed in terms of a substitutional interstitial diffusion mechanism.

The Schottky barrier height in Al/n-In0.35Ga0.65As was engineered using thin p-type near-interface In0.35Ga0.65As layers grown by molecular beam epitaxy. The effect of the thickness and doping level of the p-type layer on the barrier height was also studied by computer simulation. A good agreement was obtained between the calculated and experimental barrier height values. An experimental Schottky barrier height of 0.67 eV with an ideality factor of 1.15 has been achieved.

Recent results on the electrical properties and detection performances of the semi-insulating InP-based detectors and future programmes are presented. Schottky barrier detectors with a nitride barrier-enhanced interfacial layer and the Schottky back contacts, using material from MASPEC, exhibited a charge collection efficiency (CCE) over 75% and an energy resolution of 4.5% full-width at half-maximum (FWHM) for 5.48 MeV α -particles at RT, CCE over 90% and FWHM less than 3% at 230 K. Furthermore, they are able to detect γ -rays (122 and 60 keV photons) when cooled down to 240 K. New detectors, fabricated with liquid-encapsulated Czochralski material (LEC), produced with a special high-temperature annealing treatment from Japan Energy, coated with an original patented buffer layer and with a symmetrical circular contact configuration using both-sided photolithography and a final electrode metallisation Ti/Pt/Au for both sides, give the best results with a CCE over 88% and a FWHM less than 9% for detection of 122 keV γ -rays.

The electrical characteristics of InAs quantum dot and quantum well structures embedded in GaAs confining layers have been compared and interpreted with the effect of the potential barrier and recharging processes of quantum dots and quantum well.

InAs monolayer (QL) and self-aggregated quantum dots (QD) were grown by atomic layer MBE at 460°C on an n-type GaAs-buffer layer and were capped with a 2×10 16 /cm 3 n-type GaAs layer. QDs significantly reduce the capacitance measured at 1 MHz compared to samples with QL, and the capacitance and conductance of QD samples exhibit strong frequency and temperature dependence. The I – V measurements show that single QDs are laterally coupled depending on the temperature. The fast defect transients measured in the 10 ns to 1 μs range show that the charge and discharge of QDs is similar to extended defects indicating that the broad peak in capacitance DLTS spectra cannot be interpreted as isolated point defects.

The present work describes current–voltage ( I – V ) characteristics of radiation detector structures based on semi-insulating (SI) GaAs obtained with new types of metallizations used to form quasi-ohmic contacts. Three different metals (In, Gd and Mg) having lower work function in comparison with the standard AuGeNi eutectic alloy, were used. These metals are expected to form a band bending at the M–S interface which should give rise to a blocking barrier for holes (“minority” carriers in SI GaAs). Such contacts, which could effectively replace standard alloyed N + “ohmic” contact, show an unusual electrical charge transport as deduced from the measured I – V characteristics. Pulse-height spectra of 241 Am radionuclide source detected by the structures are also reported.

Hole capture at DX centers is evidenced in Schottky diodes made on Te-doped AlGaSb under forward bias conditions. Capacitance versus voltage measurements performed at low temperatures show that the occupancy of the DX centers is affected by sufficiently large forward biases. The current densities required to achieve such modifications are at least one order of magnitude smaller than those needed in Si-doped AlGaAs. Positive deep level transient spectroscopy signals are systematically detected in all the samples studied when forward-bias filling pulses are used. These observations are discussed in terms of minority carrier (hole) injection and subsequent capture by the DX centers.

A preliminary study of electrical properties and detection performance of semi-insulating InP-based particle detectors operated at room temperature is presented. Schottky barrier detectors with the Schottky or P + back contacts exhibit a charge collection efficiency up to about 60% and energy resolution of 7.5% (FWHM) for 5.48 MeV α-particles. The influence of basic material parameters and detector electrodes is discussed with respect to further improvement of SI InP-based particle detectors.

Current–voltage and capacitance–voltage measurements on Al/n–Si/p–Si Schottky junctions prepared by plasma immersion implantation of H + and P + ions have been performed in the temperature range of 80–360 K. Schottky barrier heights up to 0.79 V have been obtained which value is among the highest ones reported for p-Si so far. The unusual electrical properties are discussed in terms of full depletion approximation, lateral inhomogeneity of barrier height, Fermi-level pinning and band gap broadening due to hydrogenation. An overview of electrical behaviour of metal/n/p Schottky junctions is presented.

We have investigated the deep electronic levels in n-In0.35Ga0.65As epitaxial layers grown by molecular beam epitaxy (MBE) on graded InxGa1−xAs buffer/GaAs structures. InxGa1−xAs buffer layers with linear, parabolic and power composition grading law, respectively have been considered. The dependence of the deep levels distribution on the buffer grading law as well as on growth parameters such as the growth temperature and use of As2 or As4 beams is reported.

M/Semi-insulating (SI) GaAs/M diodes with novel Mg-based metallization are investigated using current-voltage measurement, ?-particle and photon detection. An anomalous decrease of the reverse current and peculiar photon spectra has been observed with such contact explainable by a strong electron accumulation due to the creation of a quasi-degenerate region at the Mg/SI-GaAs interface.

Deep Level Transient Spectroscopy (DLTS) investigations of plastically deformed, n-type silicon have been performed. DLTS spectra revealed four lines usually found in deformed silicon but they were unusually dominated by a broadened level located at 0.40 eV from the conduction band edge. This trap resulted to be the most localized at the dislocations, while the other traps are probably related to point defects. The measured DLTS line widths have been simulated by the introduction of a broadening parameter δ, whose dependence on the dislocation density has been studied. Some hypotheses on the physical mechanisms responsible for the line broadening have been advanced. A tentative identification of the defects responsible for the deep levels observed has been performed.

Schottky barriers have been prepared on molecular beam epitaxy (MBE)-grown n-type GaSb by depositing Al in situ. In order to prevent both Sb evaporation at high temperatures and Sb condensation during cooling of the epilayers before the metal deposition, the Sb shutter has been closed at a properly chosen temperature. It is shown that the time, Ar, spent at this temperature with the Sb shutter closed strongly affects both the I-V and C-V characteristics of the barriers. The barrier height values determined by I-V measurements are unreliable due to the non-ideal features of the diodes and to hole injection which is pointed out by deep level transient spectroscopy (DLTS) measurements. Conventional C-V measurements give surprisingly large values of the diffusion potentials and of the donor concentration. These values can be corrected by combining a modified dual-frequency technique with an excess capacitance correction. It is shown that, consistent with the I-V investigation, increased Δ t values result in a significant worsening of the barrier quality.

The realization of low resistivity and stable contacts on p-type CdTe is still challenging, especially if high annealing temperatures must be avoided. In this paper, the electrical characteristics of Sb/sub 2/Te/sub 3/ contacts on p-type CdTe single crystals are studied. It is found that the resistivity of the contacts is low and constant with time.

The coexistence of the DX center with nonmetastable bound states of the ordinary substitutional configuration of the donor impurity is extensively investigated in Si-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As samples having different x AlAs molar fractions in the direct gap region. The occupation of nonmetastable states is evidenced by comparing the ${\mathit{n}}_{\mathit{CV}}$ ``electron density,'' as derived from capacitance-voltage measurements in Schottky diodes, with the ${\mathit{n}}_{\mathit{H}}$ Hall density. In samples of compositions not far from the direct-to-indirect gap transition and with doping levels in the ${10}^{18}$-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ range, a nonmetastable state SX, degenerate in energy with the conduction band, can reach a significant occupancy when the saturated persistent photoconductivity condition is approached during low-temperature photoionization of DX centers. On the other hand, when the free-electron density is smaller than a critical density of a few ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, electrons freeze out into a localized S\ensuremath{\Gamma} state, or into a low mobility impurity band, linked to the \ensuremath{\Gamma} conduction-band edge. A finite occupancy of the SX or S\ensuremath{\Gamma} state gives rise to a significant ${\mathit{N}}_{{\mathit{D}}^{0}}$ density of substitutional donors in the neutral ${\mathit{D}}^{0}$ charge state having a strongly correlated spatial distribution. Electron capture into donor states, DX or not, has a spatially selective character, as can be evidenced by low-temperature mobility data under conditions where impurity scattering dominates. For DX centers, this is demonstrated by the hysteretic behavior of low temperature (${\mathit{T}}_{0}$) ${\mathrm{\ensuremath{\mu}}}_{\mathit{H}}$ vs ${\mathit{n}}_{\mathit{H}}$ data, where any given ${\mathit{n}}_{\mathit{H}}$ value is reached through DX center photoionization steps or through electron capture via a proper thermal cycling. When ${\mathit{N}}_{{\mathit{D}}^{0}}$ is negligible the hysteresis amplitude is maximum. However, whenever ${\mathit{N}}_{{\mathit{D}}^{0}}$ reaches significant values, either at the ${\mathit{T}}_{0}$ temperature or during the thermal cycle, the hysteresis amplitude vanishes. This is systematically observed in all the cases where the occupation of SX or S\ensuremath{\Gamma} states is independently demonstrated through the analysis of ${\mathit{n}}_{\mathit{CV}}$ and ${\mathit{n}}_{\mathit{H}}$ data. Two diverse complementary effects are proposed to explain the observed vanishing of the hysteresis amplitude. \textcopyright{} 1996 The American Physical Society.

Lateral conductivity effects have been investigated in self-organised InAs quantum dot (QD) structures grown in a GaAs matrix with different cap layers. Current-voltage (I-V), capacitance-voltage (C-V), DLTS, capacitance and conductance frequency dependence, fast defect transient (FDT), and electron beam induced conductivity (EBIC) measurements were applied. The conductivity in the QD plane decays within a distance of 10 microns. The capacitance transients are dominated by the local QD-plane transversal conductivity and by the free carrier transport, in the cap layer. The nonequilibrium free carrier created by electron beam excitation develop a potential barrier at macroscopic distances from the electrical contacts.

Admittance spectroscopy was employed to investigate the electrical properties of buried GaSb homojunctions. A strong p-type surface layer was obtained in n-type GaSb bulk crystals through diffusion of Zn atoms. The acceptor impurities were introduced into Te-doped substrates by epitaxially growing a heavily p(Zn)-doped GaAs layer. Current-voltage investigation, after the removal of the GaAs cap layer, demonstrated the formation of the GaSb p-n homojunction. However, the p-n interface of the junctions resulted to be not due to the presence of Zn. In fact, secondary ion mass spectrometry pointed out only a small penetration depth of Zn atoms in the GaSb substrate (similar to 100 nm), whereas electron beam induced current investigation demonstrated that the p-n junction interface was located more deeply into the substrate (similar to 1 mu m). Admittance spectroscopy and capacitance-voltage investigations led to attribute the change of conduction type from n to p of GaSb beyond the Zn penetration depth to the formation of lattice acceptor defects, influenced by the thermal annealing processes during and after the GaAs growth. An attempt to explain the formation of the buried junction in terms of atomic inter-diffusion is provided, in order to justify, from a microscopic point of view, the low diffusivity of Zn in GaSb, and the depth of the p-n junction interface in the substrate. (C) 2013 AIP Publishing LLC.

We correlate the Si concentration measured by secondary ion mass spectrometry (SIMS) and the net donor concentration in GaAs:Si grown by atomic layer molecular beam epitaxy (ALMBE); Si was supplied during: (a) both the As and the Ga subcycles, (b) the As subcycle, and (c) the Ga subcycle; the layers were grown at temperatures in the 300–530°C range. The results show that Si incorporation and its compensation depend on the Si-supply scheme and that the extent of compensation decreases with the growth temperature. We also study the deep levels in the ALMBE GaAs grown under the above conditions. Our results show the occurrence of M1, M3 and M4 levels with concentrations that are: (i) essentially independent of both the Si supply scheme and the ALMBE growth temperature, (ii) close to those of MBE GaAs grown at 600°C, and (iii) up to 2 orders of magnitude lower than that of GaAs prepared by molecular beam epitaxy (MBE) at similar temperatures.

The work reports on the characterization of high purity epitaxial 4H-SiC grown by liquid phase epitaxy and the performances of Au-Ni/4H-SiC detectors fabricated on the same material. X-ray diffraction and topography as well as I-V, C-V and DLTS measurements are used for the evaluation of the material properties and device characteristics. The X-ray detection abilities are evaluated by pulse-height spectra measurements of the 241Am. Preliminary results of the detector hardness to fast neutron and gamma ray radiations are also reported.

The work reports on the study of metal/semi-insulating GaAs/metal systems with different contacts: Mg, Gd, In and AuGeNi eutectic. Charge carrier transport through the interface is characterized by the current-voltage measurements. Observed results: i) opposite generally accepted 'ohmic, bulk limited' charge transport at low bias, ii) are in contradiction with the thermionic emission model of charge transport in M-SI GaAs, and iii) demonstrate the unpinning of the Fermi level at the M-SI GaAs interface. © 2012 IEEE.