E. Gombia

15% efficient Cu(In,Ga)Se2 solar cells obtained by low-temperature pulsed electron deposition

An approach to low-cost production of Cu(In,Ga)Se2 (CIGS) solar cells based on pulsed electron deposition (PED) has achieved a crucial milestone. Lab-scale solar cells with efficiencies exceeding 15% were obtained by depositing CIGS from a stoichiometric quaternary target at 270 °C and without any post-growth treatment. An effective control of the p-doping level in CIGS was achieved by starting the PED deposition with a layer of NaF tailored to generate the optimum Na diffusion. These results show that PED is a promising technology for the development of a competitive low-cost production process for CIGS solar cells.

Low frequency noise of GaAs Schottky diodes with embedded InAs quantum layer and self-assembled quantum dots

The electrical properties of InAs quantum layer (QL) and self-assembled quantum-dots (QDs), embedded in GaAs, are investigated by low-frequency noise measurements using Au/n-GaAs Schottky diodes as test devices. The measurements are carried out in the forward conduction regime with forward current IF as a parameter. Current–voltage and capacitance–voltage measurements indicate that GaAs and GaAs/InAs-QL Schottky diodes are nearly ideal, even though defects are present in the space–charge region of GaAs/InAs-QD Schottky diodes. In GaAs and GaAs/InAs-QL Schottky diodes, the power spectral density of the current fluctuations, S1, shows 1/f behavior and is proportional to IF2, which is explained by modulation of the barrier height due to trapping and detrapping phenomena. In GaAs/InAs-QD Schottky diodes, S1 shows 1/fγ (with γ≈0.6) behavior and is proportional to IF2 in the low current region and proportional to IF2.5 in the high current region. These noise data are explained by the generation of band tail sta...

Investigation of single electron traps induced by InAs quantum dots embedded in GaAs layer using the low-frequency noise technique

The properties of the traps induced by InAs quantum dots (QDs), embedded in a GaAs layer grown by molecular beam epitaxy, are investigated by the low-frequency noise measurements using the Au∕n-GaAs Schottky diode as a test device. The forward current noise spectra are composed of two noise components: a 1∕f-like noise at low frequencies and a generation-recombination (g-r) noise at higher frequencies. The 1∕f noise is ascribed to the mobility fluctuations within the space-charge region. The obtained Hooge parameter (αH=6×10−5) is larger than the expected value considering the phonon or impurity scattering mechanism, indicating the presence of the defects associated with QDs. The analysis of the g-r noise gives a single trap of density of about 1.6×1014cm−3 in the part of the GaAs layer located above the QDs.

Electrical characterization of self-assembled InAs/GaAs quantum dots by capacitance techniques

Abstract The electrical characteristics of GaAs/InAs/GaAs structures containing self-assembled quantum dots (QD) or pseudomorphic layers (PSL) of InAs have been investigated by capacitance–voltage ( C – V ) measurements and deep-level transient spectroscopy (DLTS). The depth profiles of the apparent electron concentration obtained by C – V measurements show significant carrier depletion centered around the position of the InAs layer on both QD and PSL samples. In contrast, an accumulation peak, whose position depends on the temperature and the test signal frequency, is detected at low temperature only on QD samples. In addition to the M1, M3, and M4 traps, which are commonly detected in GaAs grown by molecular beam epitaxy (MBE), DLTS investigations show two InAs-related levels located at 60 and 480 meV below the GaAs conduction band edge. The shallower level, which is observed only on QD samples, is associated with an energy level induced by the dots. The deeper level, detected on both QD and PSL samples, is due to defects related to the InAs insertion. The influence of the above levels on the C – V characteristics is discussed.

The effects of quantum dot coverage in InAs/(In)GaAs nanostructures for long wavelength emission

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.

Low-frequency noise spectroscopy in Au∕n-GaAs Schottky diodes with InAs quantum dots

The temperature dependence of low-frequency noise in Au∕n-GaAs Schottky diodes, with InAs quantum dots (QDs) embedded in the GaAs confining layers, is investigated in the temperature range of 77–298 K and at frequencies from 1 Hz to 5 kHz. Diodes prepared on samples with similar structure but without QDs exhibit 1∕f behavior. In diodes containing QDs, in addition to the 1∕f noise at low frequencies, generation-recombination (g-r) noise at higher frequencies was observed, related to single energy traps in the GaAs layer. Analysis of the experimental data has shown that the g-r noise is related to three traps with activation energies 0.234, 0.09 and 0.075 eV, corresponding to the ground and two excited confined states in the QDs.

Photoelectric properties of the metamorphic InAs/InGaAs quantum dot structure at room temperature

We present the study of optical and photoelectric properties of InAs quantum dots (QDs) grown on a metamorphic In0.15Ga0.85As buffer layer: such nanostructures show efficient light emission in the telecom window at 1.3 μm (0.95 eV) at room temperature. We prepared a sample with vertical geometry of contacts isolated from the GaAs substrate. The structure is found to be photosensitive in the spectral range above 0.9 eV at room temperature, showing distinctive features in the photovoltage and photocurrent spectra attributed to QDs, InAs wetting layer, and In0.15Ga0.85As metamorphic buffer, while a drop in the photoelectric signal above 1.36 eV is related to the GaAs layer. No effect of defect centers on the photoelectrical properties is found, although they are observed in the absorption spectrum. We conclude that metamorphic QDs have a low amount of interface-related defects close to the optically active region and charge carriers can be effectively collected into InAs QDs.

Preparation and characterisation of Au/InGaP/GaAs Schottky barriers for radiation damage investigation

High quality Au/InGaP Schottky diodes have been prepared as efficient test structures for a study of the radiation hardness of InGaP as space solar cell material. A detailed characterisation of the metal–semiconductor barriers obtained on both n (free carrier concentration ranging from 3×l015 to 1.2×l018 cm−3) and p-type (3.5×1017 cm−3) InGaP epitaxial layers lattice matched to GaAs substrate has been performed using current–voltage, capacitance–voltage and internal photoemission techniques. Excellent electrical properties were found for low doped (ideality factor of 1.05–1.06, rectification ratio of about 1010 at 0.7 V, reverse current lower than 1×10−12 A at −2 V) as well as heavily doped samples (rectification ratios of about 105 at 0.6 V). The barrier height values calculated by the different techniques were compared and discussed. Deep level transient spectroscopy (DLTS) spectra obtained on unirradiated samples did not show detectable deep levels with the exception of the heaviest doped sample showing a weak peak associated to the DX centre. After electron irradiation at 9 MeV with doses ranging from 5×l013 to 1.5×1015 e cm−2 the samples exhibited a broad dominant peak (activation energy in the 0.90–0.93 eV range) whose intensity increased linearly with the absorbed dose. The broadening of the peak and the observed increase of the corresponding trap concentration with the doping level suggest that this peak could be associated to complexes due to the interaction of primary defects, created by high irradiation energy, with each others and with the shallow impurities.

Influence of preparation procedure on the characteristics of Schottky barriers fabricated in situ on MBE GaSb

Abstract 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.

Evaluation of the diffusion length of minority carriers in bulk GaAs

Abstract 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.