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

Using temperature as observable of the frequency response of RF CMOS amplifiers

The power dissipated by the devices of an integrated circuit can be considered a signature of the circuit's performance. Without disturbing the circuit operation, this power consumption can be monitored by temperature measurements on the silicon surface. In this paper, the frequency response of a RF LNA is observed by measuring spectral components of the sensed temperature. Results prove that temperature can be used to debug and observe figures of merit of analog blocks in a RFIC. Experimental measurements have been done in a 0.25 mum CMOS process. Laser probing techniques have been used as temperature sensors; specifically, a thermoreflectometer and a Michaelson interferometer.Peer ReviewedPostprint (author's final draft

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

Compositional tuning of ferromagnetism in Ga1-xMnxP

We report the magnetic and transport properties of Ga1-xMnxP synthesized via ion implantation followed by pulsed laser melting over a range of x, namely 0.018 to 0.042. Like Ga1-xMnxAs, Ga1-xMnxP displays a monotonic increase of the ferromagnetic Curie temperature with x associated with the hole-mediated ferromagnetic phase while thermal annealing above 300 C leads to a quenching of ferromagnetism that is accompanied by a reduction of the substitutional fraction of Mn. However, contrary to observations in Ga1-xMnxAs, Ga1-xMnxP is non-metallic over the entire composition range. At the lower temperatures over which the films are ferromagnetic, hole transport occurs via hopping conduction in a Mn-derived band; at higher temperatures it arises from holes in the valence band which are thermally excited across an energy gap that shrinks with x.Comment: To be published in Solid State Communication

Optical Properties of III-Mn-V Ferromagnetic Semiconductors

We review the first decade of extensive optical studies of ferromagnetic, III-Mn-V diluted magnetic semiconductors. Mn introduces holes and local moments to the III-V host, which can result in carrier mediated ferromagnetism in these disordered semiconductors. Spectroscopic experiments provide direct access to the strength and nature of the exchange between holes and local moments; the degree of itineracy of the carriers; and the evolution of the states at the Fermi energy with doping. Taken together, diversity of optical methods reveal that Mn is an unconventional dopant, in that the metal to insulator transition is governed by the strength of the hybridization between Mn and its p-nictogen neighbor. The interplay between the optical, electronic and magnetic properties of III-Mn-V magnetic semiconductors is of fundamental interest and may enable future spin-optoelectronic devices.Comment: Topical Revie