M. Ciorga

Temperature dependence of energy gap of amorphous thin films of Zn3P2

Abstract The amorphous thin films of Zn 3 P 2 were studied in transmission and reflection measurements within the temperature range from 10 to 300 K in the vicinity of the energy gap. The absorption coefficient above fundamental absorption edge was calculated and found to follow the Tauc power law. The value of energy gap as a function of temperature was also calculated. The well known formula of this dependence in semiconductors was applied and appropriate parameters were determined. The experimental and calculated data were compared with those for crystalline materials.

Polycrystallisation of amorphous thin films of Zn3P2

Abstract The amorphous thin films of Zn 3 P 2 were elaborated by evaporation in vacuum on cold substrates. The polycrystalline samples were obtained by annealing in vacuum for several hours in temperatures T = 100−250 °C. The evolution of physical properties of layers vs. annealing temperature were studied by means of transmission and reflectivity measurements in wide energy range E = 1.38 – 5.5 eV. The rate of short and long range ordering of ions in crystal lattice after annealing were considered.

The influence of MOCVD process scheme on the optical properties of GaN layers

The GaN layers were grown on sapphire substrates using atmospheric pressure MOCVD system. The new process scheme was developed to improve the quality of epitaxial layers. Additional buffer layers, grown with increasing temperature and increasing V/III ratio, were inserted between the low temperature buffer layer and the high temperature GaN overlayer grown on it. The influence of growth conditions on luminescence properties of GaN layers, especially the yellow emission, was investigated.

Magnetoluminescence measurements of two-dimensional hole gas

Abstract The radiative recombination in the p-type Al 0.5 Ga 0.5 As/GaAs heterostructure interface, so called H-band, was investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. The enhancement of the H-band intensity with excitation at the free exciton energy was observed. Magnetoluminescence measurements, both in Faraday and Voigt configurations, were also performed. Non-excitonic behaviour of investigated line in fields B >3 T and excitonic in lower fields was observed.

Photoluminescence measurements of GaAs grown by liquid phase epitaxy from Ga–Bi solution

Thick, intentionally undoped GaAs epitaxial layers grown by LPE from Ga–Bi solution with different contents of Bi in liquid solvent (from 0 to 82 at.%Bi) were studied by photoluminescence (PL) at temperature hTh=2 K. The dependence of the photoluminescence spectrum on the content of Bi in solution was analysed.

Optical Investigation of Coupled GaAs/Al0.3Ga0.7As Double Quantum Wells Separated by AIAs Barriers

Electronically symmetric coupled double quantum wells are structures where two quantum wells are separated by a thin barrier layer. Both experiment and theory show that when the barrier is so narrow that there is considerable overlap of wave functions in the two wells, the single quantum well electronic one-particle states split into symmetric and antisymmetric states with different energy levels. The splitting of energy levels is very sensitive to barrier width and barrier height and increases with decreasing barrier width and with decreasing barrier height.

Observation of excitons formed by the holes confined at the Al 0.5 Ga 0.5 As/GaAs interface

Abstract The radiative recombination in the Be-doped single heterojunction Al 0.5 Ga 0.5 As/GaAs, so-called H-band, has been studied by photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. Magnetophotoluminescence measurements were also performed. The H-band was interpreted as the recombination of holes confined on the ground level in the quantum well at the interface and electrons from GaAs conduction band. The carriers involved in the recombination originate from 3D excitons excited in the flat band region of GaAs, which diffuse towards the interface. In the PLE spectra two new lines were observed. We interpret them in means of short life-time excitons composed of a confined 2D hole and a free electron, which are excited near the interface.

Magneto-optical studies of quaternary diluted magnetic semiconductors

The reflectivity and photoluminescence measurements of quaternary Cd 1-x-y Zn y Mn x Te were performed in magnetic fields B = 0-5 T in Faraday configuration. The magnetisation measurements were also done on the same samples. The exchange integrals were determined and found to be very close to those calculated in linear approximation taking into account the values of exchange integrals for ternary compounds. The bound magnetic polaron effect on the acceptor bound exciton A 0 X was also studied and the destabilisation magnetic field was found.

Two dimensional hole gas in AIGaAs/GaAs structures in magnetic field

Abstract The emission line related to interface of p-type AIGaAs/GaAs, so called H-band, was investigated by low temperature photoluminescence in magnetic field perpendicular to the interface up to 6 T. From the energetic shift and temperature dependence of intensity of emission line we conclude that electron and hole which recombine are not bound by Coulomb interaction. However, in lower magnetic fields the energetic shift of H-band has exciton-like behaviour.

Effect of above band-gap illumination on structure of free exciton reflection spectra of ZnSe

Abstract The fine structure of free exciton reflection spectra is often observed in bulk and heteroepitaxial ZnSe epilayers of thickness higher than the strain relaxation thickness. This modification of excitonic lines gives no way of direct correlation between optical spectra and energetic structure of semiconductors. We present results on well-resolved measurements and theoretical treatment of reflectance spectra in the free-exciton region of MOVPE-grown ZnSe/GaAs epilayers under additional above band-gap illumination. The measurements were performed at T =77 K, in order to eliminate the polariton phenomena from consideration. A radical change was observed, even the total reversion of the spectral line contour after external illumination of the power of P =1 mW. A multilayer model of the near-surface region was used to analyze the mechanism of lineshape formation, the resonance energy and the decay parameter of free excitons. They were estimated by fitting procedure. It was shown that the spike structure of the ZnSe reflection spectra at high values of damping parameter of excitons is mainly due to the surface electric field and excitonic Stark effect. The characteristic of the electric field distribution in the space-charge region was evaluated for various illumination conditions. The nature of illumination induced surface charged states is discussed.