Galia Pozina

Mechanism for low-temperature photoluminescence in GaNAs/GaAs structures grown by molecular-beam epitaxy

The mechanism for low-temperature photoluminescence (PL) emissions in GaNAs epilayers and GaAs/GaNxAs1 - x quantum well (QW) structures grown by molecular-beam epitaxy is studied in detail, employ ...

Growth and excitonic properties of single fractional monolayer CdSe/ZnSe structures

Single fractional monolayer (FM) CdSe/ZnSe structures have been grown by molecular beam epitaxy (MBE), employing both conventional MBE and migration-enhanced epitaxy (MEE). A precise calibration of the FM mean thickness in the range of 0.15–3.0 ML has been performed for both techniques, revealing more than a 3.5 times lower Cd incorporation ability for the MEE mode at the same Cd and Se incident fluxes. Steady-state and time-resolved photoluminescence spectroscopy is used to characterize the intrinsic morphology of the CdSe FMs, with a special emphasis on the submonolayer thickness range. Both MBE and MEE grown samples exhibit inhomogeneity of the excitonic system, which can be explained by coexistence of a homogeneous alloylike layer and relatively large CdSe 2D clusters. The MEE samples display smaller fluctuations of the layer thickness and island sizes.

Group III-nitride based hetero and quantum structures

The present paper attempts an overview of a presently very active research field: the III-nitrides and their interesting possibilities for a range of device applications employing heterostructures and low-dimensional quantum structures. The family of materials containing AlN, GaN, InN and the alloys between them span a range of direct bandgaps between 6.2 and 1.9 eV, with very large band offsets in type I heterojunctions, which is very favourable for a number of interesting device concepts. A very important feature of these materials is the dominant influence of strong polarisation fields (spontaneous as well as piezo-electric) on the physical properties of multilayer structures, as well as on devices. Exciton binding energies are large, and excitonic effects are therefore important at room temperature. Many alloy systems, in particular InGaN, have a high miscibility gap, leading to a strong tendency for phase separation and consequently to many novel physical properties which yet have to be explored in detail. Localization effects for carriers and excitons are very important in quantum structures based on these alloys. Devices based on III-N heterostructures cover a wide range, from optical devices (violet lasers, LEDs covering a range from UV to red, white LEDs, photodetectors, UV cameras) to high-frequency power devices, both unipolar transistors (AlGaN/GaN HEMTs) and bipolar HBTs.

Mechanism for rapid thermal annealing improvements in undoped GaNxAs1−x/GaAs structures grown by molecular beam epitaxy

A systematic investigation of the effect of rapid thermal annealing (RTA) on optical properties of undoped GaNAs/GaAs structures is reported. Two effects are suggested to account for the observed dramatic improvement in the quality of the GaNxAs1−x/GaAs quantum structures after RTA: (i) improved composition uniformity of the GaNxAs1−x alloy, deduced from the photoluminescence (PL), PL excitation and time-resolved measurements; and (ii) significant reduction in the concentration of competing nonradiative defects, revealed by the optically detected magnetic resonance studies.

Properties of molecular-beam epitaxy-grown GaNAs from optical spectroscopy

GaNxAs1−x layers with different nitrogen concentrations x grown on (001)GaAs substrates by molecular-beam epitaxy have been studied by photoluminescence, optical absorption, and Raman spectroscopy. The content of nitrogen in the layers was determined by x-ray diffraction and secondary-ion-mass spectrometry. The samples can be classified in three categories with respect to the concentration of N: with doping nitrogen concentration, with average content of N less than 0.3, and with x close to 1. From optical measurements and from analysis of x-ray diffraction spectra, different phases are observed in the GaNxAs1−x layers: GaAs, GaN, and the solid ternary solution GaNxAs1−x. In Raman spectra both GaAs-like and GaN-like optical phonons are observed. We have estimated the fundamental band-gap energy in the GaNxAs1−x alloy with low nitrogen concentration up to x=0.04 from absorption measurements, and in GaNxAs1−x with high nitrogen concentration x>0.96 from photoluminescence spectra. Fitting of the experimental...

Time-resolved studies of photoluminescence in GaNxP1−x alloys: Evidence for indirect-direct band gap crossover

Time resolved photoluminescence spectroscopy is employed to monitor the effect of N incorporation on the band structure of GaNP alloys. Abrupt shortening in radiative lifetime of near-band gap emissions, arising from excitonic radiative recombination within N-related centers, is found to occur at very low N compositions of around 0.5%, i.e., within the same range as the appearance of the direct-band gap-like transitions in the photomodulated transmission spectra of GaNP reported previously. The effect has been attributed to an enhancement in oscillator strength of optical transitions due to band crossover from indirect to direct-band gap of the alloy.

Reducing Thermal Resistance of AlGaN/GaN Electronic Devices Using Novel Nucleation Layers

Currently, up to 50% of the channel temperature in AlGaN/GaN electronic devices is due to the thermal-boundary resistance (TBR) associated with the nucleation layer (NL) needed between GaN and SiC substrates for high-quality heteroepitaxy. Using 3-D time-resolved Raman thermography, it is shown that modifying the NL used for GaN on SiC epitaxy from the metal-organic chemical vapor deposition (MOCVD)-grown standard AlN-NL to a hot-wall MOCVD-grown AlN-NL reduces NL TBR by 25%, resulting in ~10% reduction of the operating temperature of AlGaN/GaN HEMTs. Considering the exponential relationship between device lifetime and temperature, lower TBR NLs open new opportunities for improving the reliability of AlGaN/GaN devices.

Bound exciton dynamics in GaN grown by hydride vapor-phase epitaxy

Temperature-dependent time-resolved photoluminescence measurements were performed on thick GaN layers grown by hydride vapor-phase epitaxy on Al2O3 substrates. Radiative lifetimes were determined for the neutral-donor-bound exciton with position at 3.478 eV and for two neutral-acceptor-bound excitons at 3.473 and 3.461 eV. We report a value of 3600 ps for the radiative lifetime of the acceptor-bound exciton transition at 3.461 eV. The dominant mechanism responsible for the nonradiative recombination of the bound excitons is shown to be connected with dissociation of the bound excitons into free excitons

Origin of multiple peak photoluminescence in InGaN/GaN multiple quantum wells

Optical spectroscopy has been performed for a set of In0.12Ga0.88N/GaN multiple quantum wells (MQW) grown by metalorganic vapor phase epitaxy at 820 degrees C. Time-resolved, temperature- and power ...

Er/O and Er/F doping during molecular beam epitaxial growth of Si layers for efficient 1.54 μm light emission

Er, together with oxygen or fluorine as co-dopants, has been incorporated into Si during molecular beam epitaxial growth using co-evaporation of Si and Er containing compounds. The Er doping concentration using both Er2O3 and ErF3 can reach a level of ∼5×1019 cm−3 without precipitation, which is at least one order of magnitude higher than a previously reported solid solubility limit for Er in Si. Growth, structural, and luminescence characterization of these Er/O and Er/F doped Si samples are reported. In particular, 1.54 μm electroluminescence has been observed from Er/O doped Si layers at room temperature through hot electron impact excitation.