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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.

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.

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.

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

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.

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 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

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.

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.

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.

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.

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 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.

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 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.

The effects of treatments with hydrogen plasma (hydrogenation) and of subsequent annealing (dehydrogenation) on the photoluminescence efficiency and on the net donor and deep level concentrations in Al x Ga 1−x As grown by molecular beam epitaxy are reported. It is shown that hydrogenation results in the passivation of the shallow donors, whose electrical and optical activities are partially restored after dehydrogenation. It is also shown that deep centres as ME5, ME6 and DX are passivated by hydrogenation. The DX centre is restored to a significant extent by dehydrogenation, whereas the ME5 and ME6 defects are reactivated only to a minor extent. The strong increase in the PL efficiency observed after hydrogenation and its dependence on growth temperature are shown to be consistent with the dominant role of the ME5 and ME6 centres on the non-radiative recombination process in Al x Ga −x As .

We present a method for the evaluation of the Schottky barrier height φ b 0 , Richardson constant A ∗ , characteristic energy E 00 and bias dependence of the barrier height β from the temperature-dependent current-voltage characteristics of Schottky junctions using the thermionic-field emission )TFE) theory. The application of this method to experimental current-voltage characteristics of epitaxial Al/n-Al0.25Ga0.75As ( N = 1.4 × 10 17 cm −3 ) barriers shows that the current flow through these junctions is dominated by TFE with anomalously high E 00 . We conclude that this anomaly may be partly connected with the electric field enhancement at the periphery of the diodes, and with the multistep tunnelling through deep levels.

We report on deep level incorporation in GaAs grown by atomic layer molecular beam epitaxy (ALMBE). In contrast to the case of MBE GaAs, where the main features of the deep level transient spectra strongly depend on the growth temperature T S for 370⩽T S ⩽530 ° C , GaAs grown by ALMBE shows only three peaks, which correspond to the M1, M3 and M4 levels typical of MBE GaAs, together with a fourth trap, that we label M(330). The concentrations of these levels are comparable with those measured in MBE GaAs grown at 600°C and are weakly dependent on T S , in any case being smaller than 1.7×10 13 cm −3 ; these values are up to three orders of magnitude lower than those observed in MBE GaAs grown at comparable temperatures. Thus, as far as deep levels are concerned, ALMBE GaAs grown at 370⩽T S ⩽450° C can be compared with GaAs grown MBE at conventional temperatures (about 600°C).

The coexistence of the DX center with other donor-related bound states has been investigated in Si-doped direct gap AI,Ga I _ xAs, by comparing electron density data obtained by capacitance/voltage and Hall measurements. These experiments were carried out under saturated persistent photoconductivity conditions, as well as during isothermal capture transients. A D ° bound state degenerate in energy with the conduction band is populated through low temperature photo-ionization of the DX center. The D ° level comes very close in energy to the F minimum when the direct-to-indirect gap transition is approached. Under defined conditions, the final stage of isothermal capture transients is dominated by the electron freezing into hydrogenic bound states linked to F.

It is shown that current instabilities, which are frequently observed at low temperatures in forward biased Schottky barriers on n‐type AlGaAs, result from changes in the distribution of negatively charged donors (DX centers) near the metal contact. These changes cannot be ascribed to sample‐heating effects, but they originate from hole injection in the barrier during forward biasing. The dominant mechanisms which are expected to induce the observed distortions in the DX center profile are (i) direct capture of the injected holes by the DX center and (ii) radiative electron‐hole recombination resulting in DX center photoionization. The role of the two mechanisms is discussed.

n H (Hall) and n CV (capacitance-voltage) electron density data are compared in a Si-doped Al 0.35 Ga 0.65 As sample having an N d -N a =2.8×10 18 cm -3 net donor density. Data taken during low-temperature photoionization of DX centers and during temperature variations in the Persistent photoconductivity regime (T

The role of native defect inhomogeneity on the minority-carrier properties in n-type GaAs LEC has been investigated. An array of Au/GaAs Schottky diodes has been evaporated along the wafer diameter; then the net donor concentration and the hole diffusion length were measured by C–V and SPV methods, respectively. The distribution of extended defects was measured by etch pit observations and electron beam induced current (EBIC) imaging. In order to correlate diffusion length and recombination centre radial distribution, a deep level transient spectroscopy (DLTS) investigation was carried out. A W-shaped distribution of etch pit density ranging from 2 × 10 4 to 1 × 10 5 cm -2 was found, while an M-shaped diffusion length within the interval (0.7−2.3) × 10 -4 cm was observed. The lifetime τ of the minority carriers has been directly measured by the photocurrent decay method and a value ranging in the interval (5−10) × 10 -9 s was found. Some hypotheses on the dominant trap controlling the diffusion length and the lifetime and on its interaction with dislocations are discussed. The inhomogeneous distribution of the transport properties has been explained on the basis of different point-extended defects interaction in differently dislocated regions.

In this work we present the results of an electrical and structural characterization of molecular beamepitaxy (MBE) grown InAs/In 0.15 Ga 0.85 As quantum dot (QD) structures having InAs coverages of 2.0 and3.0ML that are, respectively, below and above the expected critical value for QD ripening in the InAs/GaAssystem. The samples have been investigated by atomic force microscopy (AFM), transmission electronmicroscope (TEM), capacitance–voltage ( C – V ), and deep level transient spectroscopy (DLTS) techniques.The results obtained by the above techniques are compared and discussed.© 2008 Elsevier B.V. All rights reserved. 1. Introduction In recent years, optoelectronic devices based on semiconductorquantum dots (QD) structures have been of great interest becauseoftheirpromisingtechnologicalapplications[1,2],suchasdevelop-ment of lasers that emit in the 1.3 and 1.55!m telecommunicationbands.BytheuseofselfassembledInAsQDgrownonmetamorphicInGaAs confining layers the emission over these wavelength rangescould be achieved [3]. One of the approaches to tune the emissionof InAs/InGaAs QD nanostructures towards longer wavelengths isthe increase of InAs coverage to enlarge the QD size. However theincrease of InAs coverage beyond a critical value is expected toresult in QD ripening [4], as it has been reported in previous stud-ies on InAs/GaAs QD structures [5].Neverthelessdetailedstudieson the ripening effects in InAs/InGaAs QD structures have not yetbeen reported. In this work we present the results of an electricalandstructuralcharacterizationofInAs/In

Capacitance and reverse current–voltage measurements have been performed as a function of temperature on Au/n-GaAs Schottky diodes with InAs quantum dots (QDs) embedded between two GaAs spacers. The spacers were either doped or undoped containing a QD layer equivalent to 3 monolayer (ML) QD coverage. In the investigated temperature range of 77–350 K and for all the diodes measured, the apparent free electron concentration close to the QDs layer is lower compared to the concentration at the interface between GaAs doped buffer layer and GaAs n+-substrate, showing that the QDs induce more traps than those created at a conventional interface. Theoretical fitting of the capacitance–voltage characteristics yields the values of the energy level, the sheet concentration and the dispersion of the distribution of the confined states induced by the InAs quantum dots, which is higher for the diode with doped GaAs spacers than for the diode with undoped spacers. The effective activation energies of the traps, obtained from the effective generation lifetime calculated from reverse voltage current, agree well with the results obtained from the theoretical fitting.

We present a study on the effects of quantum dot coverage on the properties of InAs dots embedded in GaAs and in metamorphic In0.15Ga0.85As confining layers grown by molecular beam epitaxy on GaAs substrates. We show that redshifted emission wavelengths exceeding 1.3μm at room temperature were obtained by the combined use of InGaAs confining layers and high quantum dot coverage. The use of high InAs coverage, however, leads to detrimental effects on the optical and electrical properties of the structures. We relate such behaviour to the formation of extended structural defects originating from relaxed large-sized quantum dots that nucleate in accordance to thermodynamic equilibrium theories predicting the quantum dot ripening. The effect of the reduced lattice-mismatch of InGaAs metamorphic layers on quantum dot ripening is discussed in comparison with the InAs/GaAs system.

The role of quasistatic C‐V measurements in investigating DX‐center‐related features in AlGaAs (x=0.25, 0.30, and 0.35) Schottky barriers has been reconsidered under different experimental conditions. The vanishing of the electron‐capture rate by the DX center, at low temperature, is responsible for a frozen‐step‐like density profile of positively charged DX centers near the metal‐semiconductor interface. This causes a knee‐shaped 1/C2‐vs‐V plot and gives rise to an apparent built‐in potential. The low‐temperature freezing in of the free‐electron density in the flatband region has been demonstrated through specific experiments of thermally stimulated capacitance and low‐temperature C‐V measurements performed on the sample cooled at different cooling rates.

The diffusion length of minority charge carriers has been investigated in LEC GaAs, silicon-doped with doping density N D - N A ranging from 10 16 to 10 18 cm -3 , by surface photovoltage (SPV) and electron-beam-induced current (EBIC) of scanning electron microscopy (SEM) measurements. Au Schottky diodes have been evaporated along the diameter of wafers cut from different doping density ingots to determine the variation of minority carrier diffusion length with both the radial position on the slice and the carrier concentration. The diffusion length values obtained by optical and electron excitation enhance systematic differences, which can be explained by the different surface recombination weight in the carrier generation volume and by the injection level, too. In all the examined samples an M-shaped radial variation of the diffusion length has been observed; on the other hand, the mean value of L p increases from 0.5 to 7 μm when the doping concentration increases. The authors correlate this distribution to the electrical inhomogeneity induced by native defects and associated recombination centres. The role of the dislocations, which induce two competitive effects, i.e. an enhanced recombination probability and a precipitate condensation, is here discussed.

The significance of both the density N, and the apparent built-in voltage Va, as usually obtained from C-V measurements on Schottky barriers containing DX centers, is clarified. It is also shown that, owing to the non-equilibrium occupancy of the DX center, at low temperature the electron density in the flat-band region depends on the cooling rate of the sample.

Due to the strong temperature dependence of the capture rate, the DX centre in AlGaAs is expected to show non-equilibrium occupancy at low temperatures. Results of Thermally Stimulated Capacitance and Capacitance-Voltage measurements, carried out at 77 K on Au-Al x Ga 1- x As ( x = 0.25) Schottky barriers, are strongly influenced by the cooling rate. This demonstrate that in practical experiments the free electron density commonly observed at low temperature is far from thermodynamical equilibrium.

An accurate determination of the valence band discontinuity of the lattice matched GaAs/InGaP heterostructure is performed by admittance spectroscopy measurements in p(+)/MQW/n(+) structures grown by metal-organic vapour phase epitaxy (MOVPE) using tertiarybutylarsine and tertiarybutylphosphine as alternative precursors for the V group elements. Through the analysis of the temperature dependence of the resonance frequency (omega) over bar at which the isothermal curves of the conductance over frequency G(omega)/omega have the maximum, the energy separation of 336 +/- 5 meV between the lowest heavy hole quantum level in the GaAs wells and the InGaP valence band top is obtained. A valence band discontinuity of Delta E-V = 346 +/- 5 meV is then derived by accounting for the calculated confinement energy of heavy holes (E-1(hh) = 10 meV). Experimental values of Delta E-V previously reported in the literature spread over the wide range of 300-400 meV. (C) 2007 Elsevier B.V. All rights reserved.

InAs/GaAs quantum dot (QD) structures were grown by molecular beam epitaxy (MBE) with InAs coverages θ continuously graded from 1.5 ML to 2.9 ML. A critical coverage of 2.23 ML is found, above which the islands undergo ripening, which causes a fraction of quantum dots to increase in size and to eventually relax through the formation of pure, edge-type misfit dislocations which propagate towards the surface in the form of V-shaped defects. Concomitant with ripening, extended-defect related traps with activation energies of 0.52 and 0.84 eV were observed, and regarded as the cause of the significant worsening of the optical and electrical properties in high coverage structures. Their relationship with the observed dislocations is discussed.

An EBIC study of bulk micro defects (BMDs) in not-intentionally contaminated epi Si submitted to both relaxation and segregation gettering and with an unknown initial contaminant concentration below the sensitivity of DLTS has been carried out. It allowed to establish which was the contaminant (Fe) and the possible range of its initial concentration. An evaluation of the behaviour of the BMDs during relaxation gettering and of the contribution of relaxation gettering to the overall gettering process was performed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

By studying the competition between internal and p(+) gettering, it was possible to identify the unwanted contaminant and estimate its concentration in unintentionally contaminated Si epi-structures simultaneously submitted to both gettering options. The contaminant concentration in the thermally untreated samples was below the DLTS detectability limit. Stratigraphic DLTS on thermally treated samples and EBIC study of the internal gettering allowed to establish that the contaminant was Fe. The occurrence of internal gettering even in presence of p(+) gettering in p/p(+) epi-structures suggested that the initial Fe concentration could have been in the range similar to 1.5 x 10(9)-5.2 x 10(10) cm(-3). Internal gettering is effective in p(+) substrate because of the low onset temperature of p(+) gettering.

This paper shows a comprehensive experimental and numerical investigation of proton-irradiated diodes for high-power snubberless applications. By means of DC and transient current–voltage measurements, OCVD extraction of lifetimes, C–V profiling, and DLTS trap characterization, a wide set of parameters was experimentally extracted and fed into a physically accurate mixed-mode simulation model. The numerical results are shown to be consistent with the available measured data, for example in showing the much better turn-off softness of proton-irradiated samples versus electron-irradiated ones. The complete physical/electrical model set-up in this work can now be used as an aid in the design and development of new proton-irradiated diodes.

A GaAs-based p-i-n junction grown by MOVPE and using the high purity and low dissociation temperature tertiarybutylarsine (TBAs) precursor, was studied. Such a structure is suitable to play the role of a sub-cell in a multi-junction photovoltaic device. Electrical measurements showed excellent properties of the junction; in particular, the forward current-voltage characteristic was detected over ten orders of magnitude, showing: a rectification ratio of about 1010 at 1.2 V, low reverse current and low series resistance. Capacitance measurements confirmed the low contaminant concentration of the unintentionally doped intrinsic region (of the order of 1014 cm–3). A fill factor equal to 0.81, confirming low background doping and indicating low series resistance, has been measured for the illuminated junction. The long wavelength tail of the spectral response, controlled by the diffusion length of the minority carriers (holes) in the n-type base region of the junction, has been fitted, giving a hole diffusion length equal to 2 µm. The comparison with an identical structure, grown by MOVPE with the use of the traditional arsine precursor, showed that the p-i-n junction here studied has some superior electrical and photoelectrical properties, in particular a lower dark-current. Although the primary use for devices of this kind is as a part of a more complex heterostructure, the p-i-n junction studied in this work also achieves a good projected conversion efficiency as a single junction cell (21.4 ± 1)% at AM1.5g. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A method is presented for the evaluation of the Schottky barrier height φ b0 , the Richardson constant A*, the characteristic energy E 00 , and the bias dependence of the barrier height β from the temperature dependent current-voltage characteristics of Schottky junctions by using the thermionic-field emission (TFE) theory. This method is applied to experimental results measured in epitaxial Al/undoped Al x Ga 1-x As/n-Al 0.25 Ga 0.75 As junctions, and it is found that the current flow through these junctions is dominated by the TFE with anomalously high E 00 . It is concluded that this anomaly may be partly connected with the electric field enhancement at the periphery of the diodes, and with the multistep tunnelling through deep levels and/or DX centres.