V. Donchev

A vector model for analysing the surface photovoltage amplitude and phase spectra applied to complicated nanostructures

An approach is presented for comprehensive and reliable analysis of the surface photovoltage (SPV) amplitude and phase spectral behaviour in various semiconductor materials and structures. In this approach the SPV signal is represented as a radial vector with magnitude equal to the SPV amplitude and angle with respect to the x-axis equal to the SPV phase. This model is especially helpful in complicated nanostructures, where more than one SPV formation processes arises during the spectrum run. The value of the proposed model has been demonstrated by the successful explanation of seemingly contradictory SPV amplitude and phase spectra of AlAs/GaAs superlattices with embedded GaAs quantum wells, grown on different GaAs substrates. This has provided useful information about the investigated nanostructures. The need for simultaneous examination of both SPV amplitude and SPV phase spectra in order to obtain a correct understanding of the experimental data is emphasized.

Characterization of thick epitaxial GaAs layers for X-ray detection

Abstract We have studied the current–voltage and capacitance–voltage characteristics of p/i/n structures made on non-intentionally doped epitaxial GaAs layers grown by the chemical reaction method. Deep level transient spectroscopy demonstrates that these layers contain a low defect concentration. X-ray photoconductivity shows that the diffusion length is large. The homogeneity of the properties of these layers, which has been evaluated over large area (cm 2 ), is confirmed by photoluminescence mapping.

Band offset of GaAs-GaInP heterojunctions

N+ GaAs-n GaInP lattice-matched heterostructures, grown by metalorganic vapour phase epitaxy, have been studied by capacitance-voltage, current-voltage and current-temperature techniques. This allowed the determination of the conduction band offset in three different and independent ways. The value obtained (0.24-0.25 eV) has been verified by photoluminescence and photoluminescence excitation on a 90 angstrom thick GaAs well in GaInP grown under the same conditions.

Electronic transport through semiconductor barriers

Electron transport across rectangular barriers, made of 200 AA thick GaAlAs layers embedded in GaAs, and triangular barriers at the (n+)GaAs-(n)GaInP interface has been studied. Current versus voltage and temperature characteristics have been analysed in order to extract the different mechanisms that induce this transport, and to determine the temperature and electric field range in which they apply. At low temperature and high field the current is driven by the Fowler-Nordheim regime. At higher temperatures the current is dominated by a defect-induced mechanism. This mechanism consists of the thermal emission of electrons into the barrier conduction band from defects lying in the barrier that can be refilled by tunnelling. The defect involved appears to be the deep state associated with the donor impurity, i.e. the DX centre. This study demonstrates that the apparent band offset depends strongly on the experimental conditions under which it is measured.

Modular method for calculation of transmission and reflection in multilayered structures.

We describe a new modular method for the calculation of wave propagation in stratified media based on the direct use of reflection and transmission coefficients. Within this reflection from the left, transmission, and reflection from the right (LTR) method we define addition and multiplication operators that enable the theoretical construction of any multilayered structures from substructures. This modular concept allows for the design and analysis of complex multilayer structures for optical devices.

Optical and acoustic phonon modes in strained InGaAs/GaAs rolled up tubes

Rolled-up semiconductor tubes of various diameters made of alternating In0.215Ga0.785As/GaAs layers have been investigated by means of Raman scattering. The optical and acoustic phonon modes of individual tubes have been studied and compared with the characteristics of the surrounding material. After tube formation, the frequency of the phonon modes shifts with respect to the as-grown material and disorder activated modes are observed. The frequency shifts are related to the residual strain in the tubes through the deformation potential approximation. Good agreement with atomistic valence force field simulations and x-ray micro-diffraction measurements is found. By comparison with x-ray data, a Raman strain constant K = 0.65 is proposed for In0.215Ga0.785As. In the low frequency range, acoustic mode doublets are observed on the tubes that are absent in the surrounding material. They show clear evidence of the formation of periodic superlattices after the rolling-up process, and give insight into the quali...

Optical properties of multi-layer type II InP/GaAs quantum dots studied by surface photovoltage spectroscopy

We present a low-temperature (73 K) study of the optical properties of multi-layer type II InP/GaAs self-assembled quantum dots by means of surface photovoltage (SPV) spectroscopy, taking advantage of its high sensitivity and contactless nature. The samples contain 10 periods of InP quantum dot planes separated by 5 nm GaAs spacers. The SPV amplitude spectra reveal two major broad peaks, situated at low and high energies, respectively. These features are analyzed taking into account the type II character of the structure, the quantum coupling effects, the spectral behavior of the SPV phase, and the photoluminescence spectra. As a result they have been attributed to optical transitions in the quantum dots and the wetting layers, respectively. The main mechanism for carrier separation in the SPV generation process is clarified via the analysis of the SPV phase spectra. The influence of the substrate absorption on the SPV spectra is discussed in details.

Interdiffused InAs/InGaAlAs quantum dashes-in-well structures studied by surface photovoltage spectroscopy

We report the study of interband optical transitions in the interdiffused InAs quantum dash (QD) in InAlGaAs quantum well (QW) structures using room temperature surface photovoltage (SPV) spectroscopy. SPV signals have been detected from all relevant portions of both the as-grown and interdiffused structures including the QD, QW, and cladding layer. The effect of group-III intermixing on the interband optical transition energies in the interdiffused structures has also been revealed by the SPV spectroscopy, and the results have been confirmed by photoluminescence measurements. The SPV investigation shows that the compositional intermixing occurs not only between the dash and the surrounding well but also between the well and the surrounding barrier. The results demonstrate the potential of the SPV spectroscopy as a nondestructive, contactless method to characterize optical transitions in complex semiconductor nanostructures at room temperature.

High-temperature excitons in GaAs quantum wells embedded in AlAs/GaAs superlattices

Photoluminescence (PL) spectra of GaAs quantum wells embedded in short-period AlAs/GaAs superlattices have been measured at 2 K and at room temperature. Two approaches have been applied in order to investigate the mechanisms of radiative recombination in these structures. In the first one, we studied the excitation density dependence of the PL intensity. In the second approach a line-shape analysis of the PL spectra is performed by means of a statistical model, which includes both free exciton, and free carrier recombinations. The fit based on this model reproduces with high accuracy the experimental spectra and allows to assess the relative contributions of excitons and free carriers to the radiative recombination process. The results of both approaches indicate the predominance of free excitons in the radiative recombination at room temperature.

The effect of an additional infrared laser on the carrier collection efficiency of InAs quantum dots

We report a micro-photoluminescence study on the influence of single and multi-quantum dots (QDs) on the exposure by a low-energy laser, in addition to the principal exciting laser. At low temperatures, the presence of the low-energy laser effectively quenches the single QD luminescence. This can be explained in terms of an induced screening of a built-in electric field, which plays an important role as a carrier capture mechanism. The influence of the low-energy laser is successively decreasing when the capture efficiency is increased either by elevated crystal temperature or by increased QD densities, full consistent with the proposed model.