Investigation of melt-grown dilute GaAsN and GaInAsN nanostructures for photovoltaics

AbstractThe present work demonstrates the possibility to use liquid phase epitaxy to incorporate nitrogen in epitaxial GaAsN/GaAs and GaInAsN/GaAs heterostructures, including nanoscaled ones. The structures are grown from Ga - and GaIn - melts containing polycrystalline GaN as a nitrogen source. The red shift of the absorption spectra corresponds to nitrogen content in the epitaxial layers near or less than 0.2 at %. Photoluminescence spectra of dilute nitride GaAsN and GaInAsN show emission from localized nitrogen states - N-nanoclusters of more than two N atoms. These studies show that the melt grown dilute GaAsN and GaInAsN nanostructures can be used for solar cells with extended long wavelength edge

Infrared activity of hydrogen molecules trapped in Si

The rovibrational-translational states of a hydrogen molecule moving in a cage site in Si, when subjected to an electrical field arising from its surroundings, are investigated. The wave functions are expressed in terms of basis functions consisting of the eigenfunctions of the molecule confined to move in the cavity and rovibrational states of the free molecule. The energy levels, intensities of infrared and Raman transitions, effects of uniaxial stress, and a neighboring oxygen defect are found and compared with existing experimental data

Ferromagnetism in Mn doped GaAs due to substitutional-interstitial complexes

While most calculations on the properties of the ferromagnetic semiconductor GaAs:Mn have focussed on isolated Mn substituting the Ga site (Mn$_{Ga}$), we investigate here whether alternate lattice sites are favored and what the magnetic consequences of this might be. Under As-rich (Ga-poor) conditions prevalent at growth, we find that the formation energies are lower for Mn$_{Ga}$ over interstitial Mn (Mn$_i$).As the Fermi energy is shifted towards the valence band maximum via external $p$-doping, the formation energy of Mn$_i$ is reduced relative to Mn$_{Ga}$. Furthermore, under epitaxial growth conditions, the solubility of both substitutional and interstitial Mn are strongly enhanced over what is possible under bulk growth conditions. The high concentration of Mn attained under epitaxial growth of p-type material opens the possibility of Mn atoms forming small clusters. We consider various types of clusters, including the Coulomb-stabilized clusters involving two Mn$_{Ga}$ and one Mn$_i$. While isolated Mn$_i$ are hole killers (donors), and therefore destroy ferromagnetism,complexes such as Mn$_{Ga}$-Mn$_i$-Mn$_{Ga}$) are found to be more stable than complexes involving Mn$_{Ga}$-Mn$_{Ga}$-Mn$_{Ga}$. The former complexes exhibit partial or total quenching of holes, yet Mn$_i$ in these complexes provide a channel for a ferromagnetic arrangement of the spins on the two Mn$_{Ga}$ within the complex. This suggests that ferromagnetism in Mn doped GaAs arises both from holes due to isolated Mn$_{Ga}$ as well as from strongly Coulomb stabilized Mn$_{Ga}$-Mn$_i$-Mn$_{Ga}$ clusters.Comment: 7 figure

Magnetic Interactions and Transport in (Ga,Cr)As

The magnetic, transport, and structural properties of (Ga,Cr)As are reported. Zincblende Ga$_{1-x}$Cr$_{x}$As was grown by low-temperature molecular beam epitaxy (MBE). At low concentrations, x$\sim$0.1, the materials exhibit unusual magnetic properties associated with the random magnetism of the alloy. At low temperatures the magnetization M(B) increases rapidly with increasing field due to the alignment of ferromagnetic units (polarons or clusters) having large dipole moments of order 10-10$^2$$\mu_B$. A standard model of superparamagnetism is inadequate for describing both the field and temperature dependence of the magnetization M(B,T). In order to explain M(B) at low temperatures we employ a distributed magnetic moment (DMM) model in which polarons or clusters of ions have a distribution of moments. It is also found that the magnetic susceptibility increases for decreasing temperature but saturates below T=4 K. The inverse susceptibility follows a linear-T Curie-Weiss law and extrapolates to a magnetic transition temperature $\theta$=10 K. In magnetotransport measurements, a room temperature resistivity of $\rho$=0.1 $\Omega$cm and a hole concentration of $\sim10^{20}$ cm$^{-3}$ are found, indicating that Cr can also act as a acceptor similar to Mn. The resistivity increases rapidly for decreasing temperature below room temperature, and becomes strongly insulating at low temperatures. The conductivity follows exp[-(T$_1$/T)$^{1/2}$] over a large range of conductivity, possible evidence of tunneling between polarons or clusters.Comment: To appear in PRB 15 Mar 200

A carbon-nitrogen complex in gallium phosphide

International audienceIn gallium phosphide containing nitrogen ([N-P] > 10(17) cm(-3)) and carbon ([C-P] > 10(16) cm(-3)) a sharp local vibrational mode at 2087.12 cm(-1) at T=7 K is observed. It is shown that the complex responsible for this local vibrational mode is a carbon-nitrogen complex; the carbon-nitrogen bond is a triple bond aligned along the axes of the lattice. A microscopic structure of the complex is proposed. It is suggested that the equivalent complex exists in GaAs and is characterized by a local vibrational mode at 2088.50 cm(-1) at T=7 K. The carbon atoms involved in these complexes do not contribute to the carbon p-type doping of the crystals

Polarisation dans la résonance paramagnétique électronique de ZnTe/Fe +++

Le champ cristallin introduit une absorption de la composante circulaire anormale du champ hyperfréquence, ce qui se traduit dans un spectre de résonance paramagnétique électronique par une variation angulaire de l'intensité des raies. Cet effet est particulièrement marqué dans le cas du ZnTe/Fe+++ où le paramètre de champ cristallin cubique est exceptionnellement important (a = - 0,261 3 cm-1 ). Des effets analogues existent pour les transitions « interdites » ΔM = 2, 3, 4, 5

Nitrogen-related defects in GaP and GaAs

23rd International Conference on Defects in Semiconductors, Awaji Isl, JAPAN, JUL 24-29, 2005International audienceThere is presently a great interest in dilute III-V nitrides. The investigation of defects in these materials is rather scarce and of importance, as the practical use of these materials relies on the control of the defects present. For this purpose, nitrogen-doped GaP and GaAs crystals have been investigated by Fourier transform spectroscopy. The local vibrational modes of carbon-nitrogen and boron-nitrogen complexes are identified. The carbon-nitrogen complexes sit in T-P or T-A, interstitial sites and are oriented along four-fold axes of the lattice. The local vibrational modes of the boron-nitrogen complex show unusual features: they are accompanied by sidebands and they show a strong stress-induced dichroism at low temperatures. A tentative model of the boron-nitrogen complex is proposed. (c) 2005 Elsevier B.V. All rights reserved