Spatial carrier distribution in InP/GaAs type II quantum dots and quantum posts

We performed a detailed investigation of the structural and optical properties of multi-layers of InP/GaAs quantum dots, which present a type II interface arrangement. Transmission electronic microscopy analysis has revealed relatively large dots that coalesce forming so-called quantum posts when the GaAs layer between the InP layers is thin. We observed that the structural properties and morphology affect the resulting radiative lifetime of the carriers in our systems. The carrier lifetimes are relatively long, as expected for type II systems, as compared to those observed for single layer InP/GaAs quantum dots. The interface intermixing effect has been pointed out as a limiting factor for obtaining an effective spatial separation of electrons and holes in the case of single layer InP/GaAs quantum-dot samples. In the present case this effect seems to be less critical due to the particular carrier wavefunction distribution along the structures.     

Simple and rapid determination of carrier concentration and mobility profiles in GaAs

We describe a simple and fast method to obtain carrier concentration and Hall mobility profiles of GaAs multilayer structures on semi-insulating substrates from automatic van der Pauw-Hall measurements using a continuous chemical etching technique. In addition, a simplified procedure for determining conductivity profiles is presented. Applications to GaAs multilayer structures prepared by vapour phase epitaxy demonstrate the high depth resolution of these techniques.     

Measurement of lifetime of nonequilibrium charge carriers in single-crystal silicon

An equation of continuity describing decay of nonequilibrium charge carriers after illumination of semiconducting silicon by a long duration pulse of light with wavelength of 1.06 µm is solved by numerical methods in a one-dimensional approximation. It is shown that, when the volume recombination lifetime of minority carriers is 500 µs and higher, the known formula for “thin” samples 1/τ_eff = 1/τ _V + 2s/d cannot be used to evaluate the effective lifetime of thin plates (10 µm or above). Estimations of the range of variation of excess density in which the instantaneous lifetime achieves saturation are carried out; this takes place in the second part of the relaxation curve if the wafer thickness does not exceed three diffusion lengths of minority carriers. It is demonstrated that the formula scaled to an infinite recombination rate is applicable for thicknesses of the sample within the range from one to five diffusion lengths. For higher thicknesses, it is necessary to apply corrections taking into account the dependence of instantaneous lifetime on the excess density of nonequilibrium charge carriers.     

Charge carrier recombination via isoelectronic traps involving excitons in compensated semiconductors

The lifetimes of electrons and holes in semiconductors were studied with respect to their recombination via excitons formed on isoelectronic traps. The curve of carrier lifetime versus trap concentration is nonmonotonic and exhibits a maximum. The effect is related to a sharp decrease in the concentration of major carriers under full compensation conditions and is accompanied by sharp increase in the semiconductor resistivity.     

Electrical characterization and carrier transport mechanisms of GaAs p/i/n devices for photovoltaic applications

GaAs p/i/n diodes made by Metal-Organic Vapour Phase Epitaxy were examined by electrical measurements for evaluating the optimum i-region for use as solar cells. Four series of samples were prepared and studied each one with a different i-region width. The performance of the devices was examined by means of Admittance spectroscopy as well as classical current–voltage and capacitance–voltage characterization, allowing the calculation of the minority carriers lifetime (τ_eff) and the diodes ideality factors. The values of the τ_eff were found to lie between 8.7 ps and 0.14 ns for i-region widths between 0 and 0.8 μm. These results were used to model the multilayer structure with the two-diode representation and explain the conductance mechanisms inside the diodes. This modeling showed that the recombination/generation currents were dominating in forward biased diodes and the ohmic loss current in reverse bias.     

Electrical characterization of InAs/GaAs quantum dot structures

The electrical properties of InAs quantum dots (QD) in InAs/GaAs structures have been investigated by space charge spectroscopy techniques, current–voltage and capacitance–voltage measurements. Au/GaAs/InAs(QD)/GaAs Schottky barriers as well as ohmic/GaAs/InAs(QD)/GaAs/ohmic structures have been prepared in order to analyze the apparent free carrier concentration profiles across the QD plane, the electronic levels around the QD and the electrical properties of the GaAs/InAs(QD)/GaAs heterojunction. Accumulation and/or depletion of free carriers at the QD plane have been observed by Capacitance–Voltage (C–V) measurements depending on the structure parameters and growth procedures. Similarly, quantum dot levels which exhibit distributions in energy have been detected by Deep Level Transient Spectroscopy (DLTS) and Admittance Spectroscopy (AS) measurements only on particular structures. Finally, the rectification properties of the InAs/GaAs heterojunction have been investigated and the influence of the related capacitance on the measured capacitance has been evidenced.     

The Effect of Dose of Nitrogen-Ion Implantation on the Concentration of Point Defects Introduced into GaAs Layers

Secondary-ion mass spectrometry and Rutherford proton backscattering have been used to measure the concentration profiles of nitrogen atoms and examine the defect structure of epitaxial GaAs layers implanted with 250-keV N⁺ ions at doses of 5 × 10¹⁴–5 × 10¹⁶ cm^–2. It was found that no amorphization of the layers being implanted occurs at doses exceeding the calculated amorphization threshold, a concentration of point defects that is formed is substantially lower than the calculated value, and a characteristic specific feature of the defect concentration profiles is the high defect concentration in the surface layer.     

Optical absorption of radio frequency sputtered GaAs(Ti) films

Composition and optical absorption of thin films of GaAs(Ti) and GaAs, deposited by sputtering on glass substrates under different process conditions, have been investigated. The thin films obtained are typically 200 nm thick. ToF–SIMS measurements show a quite constant concentration and good uniformity of Ti profiles along the GaAs(Ti) layers in all cases and EPMA results indicate that Ti content increases with the substrate temperature in the sputtering process. Measurements of the transmittance and reflectance spectra of the GaAs and GaAs(Ti) thin films have been carried out. In the optical characterization of the films it is found that optical absorption is enhanced in all samples containing Ti. The determination of the optical gap from the optical absorption, shows optical gap variations from 1.15 to 1.29 eV in the GaAs thin films, and from 0.83 to 1.13 eV in the GaAs(Ti) thin films. The differences in absorption and EgTAUC observed between samples of GaAs and GaAs(Ti) are consistent with the presence of an intermediate band.     

Transport mechanism analysis of non-equilibrium charge carrier in heterojunctions with GaS-CdTe:Mn thin films

The current-voltage characteristics in the temperature range 170 ÷ 305 K at the direct polarization of GaS-CdTe:Mn heterojunction were studied. The tunneling process of carriers determines the electrical current through the junction. The ratio of the diffusion coefficient and surface recombination velocity in the CdTe:Mn film from the interface of junction was determined from the spectral characteristics of short circuit current and absorption. The non-equilibrium carriers generated in the CdTe:Mn film at the interface of heterojunction at the temperature of 293 K recombine through two recombination levels. At low temperatures (T < 170 K) the concentration of carriers decreases and only one recombination level appears with the short recombination lifetime.     

Electronic characterization of several 100 μm thick epitaxial GaAs layers

Non intentionally doped thick epitaxial GaAs layers, grown by chemical vapour phase epitaxy using a high growth rate, are characterized by different electrical techniques applied on a junction (forward and reverse Current–Voltage characteristics, Current versus time and temperature, Deep Level Transient Spectroscopy, Capacitance–Voltage measurements) in order to detect the defects they contain. Experimental data are coupled with theoretical modelling to determine the electrical characteristics of the layers: type and concentration of the carriers, energy levels and concentrations of the defects, associated minority carrier lifetime. The results obtained indicate that i) the concentration of the residual impurities is in the order of 10¹³ to 10¹⁴ cm⁻³, ii) the layers contain two shallow defects (at 0.12 and 0.21 eV below the conduction band Ec) slightly compensated by a deep defect (at Ec-0.5 eV) and iii) the deep defect is in low concentration (10¹² cm⁻³), resulting in a minority carrier lifetime still limited by the radiative recombination process. Therefore, epilayers obtained using high growth rates exhibit electronic properties very similar to the ones obtained using conventional epitaxial techniques. The good and uniform electronic properties of these layers coupled with the low yield of the fluorescence of GaAs is interesting for high resolution X-ray detection.     

Special features of the periodic-pulse-train laser radiation confinement in doped gallium arsenide and zinc selenide

Experimental data on the pulsed laser radiation confinement in compensated GaAs and ZnSe with deep impurity levels are reported for the laser wavelength λ=1.55 μm and a pulse repetition frequency of up to 100 kHz. It is demonstrated that an increase in the pulse repetition rate is accompanied by a decrease in the energy confinement threshold and by an increase in the radiation attenuation coefficient. These effects are explained by the accumulation of nonequilibrium charge carriers related to a dependence of the recombination time constant on the concentration of free impurity centers.     

The damage-dependent atom and carrier profiles in phosphorus-implanted silicon

The substantial redistribution of implanted phosphorus after annealing at 550 °C in silicon was investigated by measuring the atom concentration profile with a radioactive tracer technique and by determining the carrier concentration profile from Hall effect measurements on the same sample. The profiles were analysed by the method of moments and examined as a function of the pre-bombardment fluence of ³¹P and/or ⁴⁰Ar ions, and the results were correlated with theoretically calculated implantation-induced damage profiles. A Pearson I density function constructed with four moments interpolated from theoretical values and a Pearson IV distribution with empirically derived moments were compared with the experimental profiles.     

Non-equilibrium carrier relaxation in sprayed CdSe films

The rise and decay of the photocurrent in CdSe films prepared by spray pyrolysis were investigated. The decay time at an illumination of 10 mW cm^?2 was 1.6 ms. The dependence of the decay time on the light intensity, the background illumination and the chopping frequency was examined. The lifetime of the majority carriers, as deduced from the decay curve, was found to vary with the illumination, which gives a clue to the superlinearity shown in the illumination-current characteristics of these films. At 5 mW cm^?2 the lifetime was 0.8 ms and at 10 mW cm^?2 it was 1.6 ms. The amplitude of photoconductivity decrased by 33% when the frequency of the chopper was increased from 50 to 270 Hz.     

Theory of the field-assisted photomagnetoelectric effect in thin films: Experiment on n-type GaAs and applications

The field-assisted photomagnetoelectric (FAPME) effect is the generalization of the classical PME effect obtained by the application of a (strong) longitudinal electric field: photogenerated carriers are magnetoconcentrated by the Lorentz force and consequently the current-voltage (I(V)) law becomes non-linear. The theory of the FAPME effect is proposed for thin semiconductor layers, where the recombination rate at the film-substrate interface is a key parameter. The roles of other important factors (electric and magnetic fields, lifetime, dimensions) are also systematically investigated. Experimental data for the open-circuit voltage, the photomagnetoresistance and the I(V) characteristics were obtained for n-type GaAs films 1.5 μm thick in magnetic and electric fields of up to 13 T and 1.5 x 10⁴ V m^-1 respectively. A satisfactory agreement with the theory is found, in spite of some inherent limitations (film non-uniformity, influence of the semi-insulating GaAs substrate). Complementary magnetotransport measurements were performed without photoexcitation in the temperature range 77–300 K. These numerous and independent experiments are simultaneously exploited for the assessment of several basic properties of GaAs: the lifetime, the surface recombination velocities, the Hall and drift mobilities and the scattering mechanisms.     

On the determination of carrier mobility in compensated GaAs

The applicability of the Brooks-Herring and Conwell-Weisskopf formulas in calculations of the carrier mobility associated with scattering by ionized impurities is examined usingn-type GaAs as an example. The Brooks-Herring approximation is shown to be inapplicable at large compensation ratios, where the Conwell-Weisskopf formula is more accurate. The applicability limits of the two formulas are established for calculations of carrier mobility from dopant concentration and compensation ratio and for calculations of the concentration of ionized centers (compensation ratio) from carrier concentration and mobility. The predicted applicability limits are consistent with experimental data.     

The influence of substrate characteristics on the contact angles between liquid gallium and gallium arsenide crystals

Parameters influencing the interfacial interaction between liquid gallium and single crystalline gallium arsenide were investigated under vacuum by means of the sessile-drop technique over the temperature range ∼ 30 to 200° C. In addition to their dependence on the anisotropy of the {111} planes, contact angles in the Ga(I)/GaAs(s) system were found to be sensitive to the degree of misorientation and the direction of tilt of these planes. Furthermore, contact angles were found to be dependent on the size of the liquid drop and on the surface roughness of the substrate. In agreement with theoretical expectations the measured angles increased with increasing roughness of the GaAs surfaceS. However, these angles were found to be unaffected by the presence of N₂, Ar, and He atmospheres, and by the nature and concentration of charge carriers in the substrate.     

Defect induced shortening of excess carrier lifetime in single period nipi structures

Abstract

High sensitivity for detecting optically generated excess carriers in the mid infrared spectrum is offered by nipi structures of PbTe. The built in nipi potential separates the electron–hole pairs in real space which leads to a high excess carrier lifetime τ. The temperature dependence of τ in PbTe single period nipi structures grown on BaF₂ substrates under repeated thermal cycling ranging from 300 to 12 K was studied in the present paper. The typical behaviour of the lifetime τ deviates from theory. This was attributed to the fact that the lattice mismatch and the different thermal expansion of BaF₂ and PbTe generate a temperature dependent strain which causes strain induced lattice defects. Besides misfit dislocations at the interface, threading dislocations are generated, which form channels for enhanced recombination of the excess carriers. A special temperature cycling technique is presented which can change the number and the arrangement of the strain induced defects and leads to higher excess carrier lifetimes τ at low temperatures.     

Impact of photoluminescence temperature and growth parameter on the exciton localized in BxGa1-xAs/GaAs epilayers grown by MOCVD

In this work, B_xGa_1-xAs/GaAs epilayers with three different boron compositions were elaborated by metal organic chemical vapor deposition (MOCVD) on GaAs (001) substrate. Structural study using High resolution X-ray diffraction (HRXRD) spectroscopy and Atomic Force Microscopy (AFM) have been used to estimate the boron fraction. The luminescence keys were carried out as functions of temperature in the range 10–300 K, by the techniques of photoluminescence (PL). The low PL temperature has shown an abnormal emission appeared at low energy side witch attributed to the recombination through the deep levels. In all samples, the PL peak energy and the full width at half maximum (FWHM), present an anomalous behavior as a result of the competition process between localized and delocalized carriers. We propose the Localized-state Ensemble model to explain the unusual photoluminescence behaviors. Electrical carriers generation, thermal escape, recapture, radiative and non-radiative lifetime are taken into account. The temperature-dependent photoluminescence measurements were found to be in reasonable agreement with the model of localized states. We controlled the evolution of such parameters versus composition by varying the V/III ratio to have a quantitative and qualitative understanding of the recombination mechanisms. At high temperature, the model can be approximated to the band-tail-state emission.     

Time-resolved photoinduced thermoelectric and transport currents in GaAs nanowires

In order to clarify the temporal interplay of the different photocurrent mechanisms occurring in single GaAs nanowire based circuits, we introduce an on-chip photocurrent pump-probe spectroscopy with a picosecond time resolution. We identify photoinduced thermoelectric, displacement, and carrier lifetime limited currents as well as the transport of photogenerated holes to the electrodes. Moreover, we show that the time-resolved photocurrent spectroscopy can be used to investigate the drift velocity of photogenerated carriers in semiconducting nanowires. Hereby, our results are relevant for nanowire-based optoelectronic and photovoltaic applications.     

Study of charge collection efficiency in 4H–SiC Schottky diodes with C ions

The Charge Collection Efficiency (CCE) in 4H–SiC epitaxial Schottky diodes is studied as a function of the applied reverse bias. Three SiC types, with different doping concentrations, were used to detect ¹²C ions at 14.2, 28.1 and 37.6 MeV. In two SiC types we observe minority charge carriers, generated by the incoming ion in the neutral region, which diffuse to the depleted layer and are finally collected. The minority carriers contribution to the CCE is well reproduced by an analytical equation which represents the contribution from the charge carriers diffusing to the depletion region from the semiconductor substrate. From a best fit procedure, the minority carriers diffusion length is extracted and the minority carriers lifetime is calculated. Finally, the dependence of the lifetime on the doping concentration is investigated.