S. Roorda

Hyperuniformity in amorphous silicon based on the measurement of the infinite-wavelength limit of the structure factor

We report the results of highly sensitive transmission X-ray scattering measurements performed at the Advanced Photon Source, Argonne National Laboratory, on nearly fully dense high-purity amorphous-silicon (a-Si) samples for the purpose of determining their degree of hyperuniformity. A perfectly hyperuniform structure has complete suppression of infinite-wavelength density fluctuations, or, equivalently, the structure factor S(q→0) = 0; the smaller the value of S(0), the higher the degree of hyperuniformity. Annealing was observed to increase the degree of hyperuniformity in a-Si where we found S(0) = 0.0075 (±0.0005), which is significantly below the computationally determined lower bound recently suggested by de Graff and Thorpe [de Graff AMR, Thorpe MF (2010) Acta Crystallogr A 66(Pt 1):22–31] based on studies of continuous random network models, but consistent with the recently proposed nearly hyperuniform network picture of a-Si. Increasing hyperuniformity is correlated with narrowing of the first diffraction peak and extension of the range of oscillations in the pair distribution function.

Comment on "The local structure of amorphous silicon".

Treacy and Borisenko (Reports, 24 February 2012, p. 950) argue from reverse Monte Carlo modeling of electron diffraction and fluctuation electron microscopy data that amorphous silicon is paracrystalline and not described by a continuous random network. However, their models disagree with high-resolution x-ray measurements and other evidence, whereas the agreement with fluctuation electron microscopy is at best qualitative.

Segregation and formation of MnP particles during rapid thermal annealing of Mn-implanted InP and GaP

We have studied the structural and magnetic properties of Mn implanted (1–5×1016 cm−2, 200 keV) into InP and GaP substrates, before and after rapid thermal annealing. As revealed by Rutherford backscattering spectrometry, secondary ion mass spectrometry, and transmission electron microscopy measurements, implantation results in an amorphous surface layer 300 nm deep, and subsequent annealing gives rise to epitaxial recrystallization of this layer accompanied by a segregation of most of the Mn into clusters about 60 nm in diameter at the surface. Magnetic measurements indicate ferromagnetic behavior only for the annealed samples with TC close to 290 K, characteristic of bulk MnP, whose presence is confirmed by diffraction data. In addition, there is no evident dependence of the magnetic and structural properties on the doping type or level of the substrates.

Wilson ratio in Yb-substituted CeCoIn5

We have investigated the effect of Yb substitution on the Pauli-limited, heavy-fermion superconductor, CeCoIn5. Yb acts as a non-magnetic divalent substituent for Ce throughout the entire doping range, equivalent to hole doping on the rare-earth site. We found that the upper critical field in (Ce,Yb)CoIn5 is Pauli limited, yet the reduced (H,T) phase diagram is insensitive to disorder, as expected in the purely orbitally limited case. We use the Pauli-limiting field, the superconducting condensation energy and the electronic specific-heat coefficient to determine the Wilson ratio (RW), the ratio of the specific-heat coefficient to the Pauli susceptibility in CeCoIn5. The method is applicable to any Pauli-limited superconductor in the clean limit.

Surfactant-mediated growth of ferromagnetic Mn [delta]-doped Si

We present an investigation of Mn {\delta}-doped layers in Si(001) grown by molecular beam epitaxy. We discovered that a Pb surfactant has significant effect on the structural and magnetic properties of the submonolayer of Mn, which depends on the Si capping layer growth temperature, T_Si, and the Mn coverage, {\theta}_Mn. The results presented in this paper identify three regions in the growth-phase-diagram characterized by distinct magnetic behaviors and crystal structures. In one region, X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM) experiments indicate that MnSi nanocrystallites form with B2-like crystal type structures. At the optimal growth conditions, T_Si = 200 C and {\theta}_Mn = 0.26 monolayer, a ferromagnetic phase develops with a Curie temperature T_C > 400 K and a saturation moment m_sat = 1.56 {\mu}_B/Mn, whereas T_C drops to zero for a control sample prepared without Pb. For T_Si > 200 C, MnSi-B20 type precipitates form with a T_C ~ 170 K. Rutherford backscattering spectroscopy shows that the increase in the remanent magnetization in these two phases is possibly correlated with an increase in the Mn substitutional fraction, which suggests that a Si_{1-x}Mn_x dilute magnetic semiconductor may be forming in the matrix between the precipitates. Density functional calculations show that Pb changes the pathway by which the Mn atoms access the Si substitutional sites, Mn_Si. While the Pb increases the formation energy of Mn_Si at the Si surface, it enables substitutional incorporation by lowering the formation energy of Si vacancies by 0.92 eV.

Structural morphology and Mn distribution in annealed InP coimplanted with P and Mn

We have studied the structural, chemical, and magnetic properties of InP following coimplantation with high energy P and Mn ions to fluences in excess of 1016 cm−2 and subsequent annealing above 600 °C. Rutherford backscattering spectrometry, x-ray diffraction, secondary ion mass spectrometer, and transmission electron microscopy measurements reveal that rapid thermal annealing causes only a partial epitaxial recrystallization of the layer amorphized by the implantations; the remaining region up to the surface crystallizes incoherently but retains most of the implanted Mn. Magnetic measurements demonstrate that fully three quarters of the Mn has been incorporated into MnP nanoparticles embedded into this polycrystalline InP region. The remaining Mn is most probably incorporated into the InP lattice but gives only a paramagnetic response that can be described by a simple Brillouin function at temperatures down to 5 K.

Understanding subtle changes in medium-range order in amorphous silicon

Based on a detailed study of the radial distribution function (RDF) of a model for amorphous silicon (a-Si), we address the relation between short-range rearrangements and an increase in medium-range order induced by thermal relaxation. Recent experimental measurements have shown that a small peak appears in the RDF around 4.7 Å upon annealing, along with other subtle changes, and this is attributed to ordering among the dihedral angles. We show that, although this is a possible explanation, an increase in short-range order (up to second neighbors) is not only necessary for these changes to occur, but could also be their sole cause. To clarify the nature of disorder in the amorphous system, correlations among dihedral and tetrahedral angles are examined. The bivariate probability distribution of these two variables reveals small correlations between dihedral and tetrahedral angles, associated with the staggered and eclipsed conformations. In the first case, bond angles around 112.5[Formula: see text] are favored versus 120[Formula: see text] in the second case. Bond angles between 95[Formula: see text] and 100[Formula: see text] are less probable in both conformations.

Terahertz Generation Using Implanted InGaAs Photomixers and Multi-wavelength Quantum Dot Lasers

We report on a study of terahertz (THz) generation using implanted InGaAs photomixers and multi-wavelength quantum dot lasers. We carry out InGaAs materials growth, optical characterization, device design and fabrication, and photomixing experiments. This approach is capable of generating a comb of electromagnetic radiation from microwave to terahertz. For shortening photomixer carrier lifetime, we employ proton implantation into an epitaxial layer of lattice matched InGaAs grown on InP. Under a 1.55 μm multi-mode InGaAs/InGaAsP quantum dot laser excitation, a frequency comb with a constant frequency spacing of 50 GHz generated on the photomixer is measured, which corresponds to the beats of the laser longitudinal modes. The measurement is performed with a Fourier transform infrared spectrometer. This approach affords a convenient method to achieve a broadband multi-peak coherent THz source.

Photoluminescence from single nitrogen isoelectronic centers in gallium phosphide produced by ion implantation

Single emitters formed from two nitrogen isoelectronic traps in GaP are created by low energy implantation. Several dyad configurations are individually resolved, establishing that ion implantation can produce multi-impurity single emitters with high luminescence yield. Measured dyad concentrations significantly exceed those predicted from simulations, suggesting that their formation is strongly enhanced by implantation defects. Annealing at 600 °C optimizes the luminescence yield and higher temperatures lead to the physical dissociation of dyads. The dissociation activation energy increases with interatomic separation, indicating that nearest neighbor dyads are energetically unfavorable and that their concentration can be adjusted with a simple temperature treatment.

Dominant structural defects in amorphous silicon

The nature of disorder in amorphous silicon (a-Si) is explored by investigating the spatial arrangement and energies of coordination defects in a numerical model. Spatial correlations between structural defects are examined on the basis of a parameter that quantifies the probability for two sites to share a bond. Pentacoordinated atoms are found to be the dominant coordination defects. They show a tendency to cluster, and about 17% of them are linked through three-membered rings. As for tricoordinated sites, they are less numerous, and tend to be distant by at least two bond lengths. Typical local geometries associated to under and overcoordinated atoms are extracted from the model and described using partial bond angle distributions. An estimate of the formation energies of structural defects is provided. Using molecular-dynamics calculations, we simulate the implantation of high-energy atoms in the initial structure in order to study the effect of relaxation on the coordination defects and their environments.