Search Results - (Author, Cooperation:N. Samarth)

2018-01-11

American Physical Society (APS)

1098-0121

1095-3795

Physics

Semiconductors I: bulk

2014-07-25

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

1089-7550

AIP Digital Archive

Physics

We report photoluminescence (PL) results obtained on p-type ZnSe epilayers grown by molecular beam epitaxy. As an acceptor dopant, we used an active nitrogen beam produced by a free radical nitrogen source. On the basis of a detailed analysis of PL data we propose a simple semiquantitative method for a quick and contactless evaluation of the net acceptor concentration in p-type ZnSe. In particular, we show that the intensity ratio of the donor–acceptor pair (DAP) emission to the acceptor-bound exciton (I1) emission strongly depends on both the excitation power and the quality of the sample, and because of that it cannot by itself be regarded as a good measure of the net acceptor concentration. On the other hand, the intensity of the DAP emission under saturation excitation shows a simple direct proportionality to the net acceptor concentration, thus providing a reliable tool for determining the relative doping level in p-type ZnSe films.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The FCC Heisenberg spin lattice with AF NN interactions is a known model of an inherently frustrated system. Due to the continuous ground state degeneracy, the spin order in this lattice is particularly sensitive to weak symmetry-breaking perturbations, such as Dzyaloshinski–Moriya (D-M) interactions or anisotropic strains. These effects may create a rich variety of colinear and noncolinear structures. The limited availability of naturally existing prototypes has resulted in little experimental work in this field. However, the emergence of novel epitaxial structures containing zinc-blende type MnSe and MnTe, which are very close approximations of the NN FCC Heisenberg model, opens new opportunities for such studies. In many of these systems the MnSe and MnTe layers are strongly strained (c/a∼0.95–1.05) due to lattice mismatch effects. The resulting anisotropy in the Mn-Mn exchange may lead to a dramatic change in the spin structure—e.g., studies of MnSe/ZnTe superlattice systems have revealed a transition from a type III AF order to an entirely new incommensurate helical state not yet seen in any FCC aniferromagnet.1 In this paper we report neutron diffraction data from "thick'' (∼1 μm) Co0.25Mn0.75Se films. Detailed mapping of the magnetic diffraction intensity reveals a shift from perfect type III lattice points, showing that detectable incommensurate effects may be produced even by weak residual stresses. Our results point out that noncolinear effects arising from strains may be indistinguishable from canting produced by D-M exchange,2 which may additionally complicate the detection of this weak interaction in Mn chalcogenides.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

New zinc-blende Mn-based magnetic semiconductors offer unique insights into the magnetic order and critical behavior of frustrated antiferromagnets. Molecular beam epitaxy grown samples exist both in the form of strained superlattices, with strain induced anisotropies (MnSe/ZnSe and MnTe/ZnTe), and in the form of near-isotropic bulk MnTe. The tetragonally distorted strained superlattices exhibit a second-order phase transition, consistent with symmetry arguments favoring such behavior. In contrast, the bulk-like epitaxial layers of MnTe have a first-order phase transition, associated with magnetostriction, unknown to an unaccounted for by the previous studies.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We report neutron diffraction studies of zinc-blende strained MnSe/ZnSe superlattices. Such systems are examples of a fcc Heisenberg antiferromagnet of the third kind. The results show clearly long-range order in the MnSe layer plane, and no magnetic coupling across the passive layers. The strain affects the relative domain population. Measurements of the diffracted intensity versus T show that the AF phase transition in the MnSe layers is of second order with TM = 107 ± 5 K.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We have designed a low-temperature femtosecond-resolved near-field scanning optical microscope to study spatiotemporal excitonic spin behavior in magnetic semiconductor heterostructures. Local disorder introduced by focused-ion-beam implantation reduces the otherwise large Zeeman splittings in modest magnetic fields, creating a planar spin-dependent energy landscape for diffusing carriers. Near-field polarization-resolved static and femtosecond measurements map out excitonic spin behavior with ∼125 nm spatial resolution, revealing spin-dependent diffusion. We demonstrate the applicability of two distinct time-resolved techniques in the near field, and discuss limitations on the measurement of polarized luminescence from semiconductors in the near field. © 1996 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We have developed a new femtosecond-resolved optical technique with which one may examine magnetic spin dynamics in near-atomic scale structures by employing a highly sensitive, ultrafast measurement of the Faraday rotation. We apply this spectroscopy to novel band-gap-engineered II–VI diluted magnetic semiconductor heterostructures of two types to demonstrate the ability to simultaneously monitor electronic and magnetic interactions in a quantum geometry. The experiments show that these dynamics evolve on widely different time scales, and reveal the onset and decay of magnetization due to carrier spin scattering with femtosecond temporal resolution (300 fs). Although photoexcited carriers recombine within several hundred picoseconds, they leave behind a magnetic "footprint'' that persists considerably longer and relaxes through an entirely different spin-lattice mechanism. Time-resolved magnetic measurements reveal the unusual dynamical properties of low-dimensional systems incorporating magnetic spins as compared to traditional semiconductor heterostructures.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the growth of a new closely lattice-matched II-VI heterostructure: ZnTe/CdSe (Δa/a∼0.3%). Epilayers of zinc blende CdSe grown on ZnTe buffer layers are shown to have much better quality than those grown earlier (with a 7% mismatch) on GaAs substrates. This permitted the first successful growth of high quality superlattices of ZnTe/CdSe. The superlattices were studied by x-ray diffraction, transmission electron microscopy, and optical techniques. Results of photoluminescence and optical transmission measurements show that ZnTe/CdSe superlattices have a very small valence-band offset.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Excitonic absorption has been investigated in multiple quantum wells of (Zn,Cd)Se/ZnSe in an effort to increase the electron-hole Coulomb interaction. Well-defined absorption peaks for the n=1 heavy hole exciton are observed at room temperature showing evidence for a reduced exciton-optical phonon scattering rate.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Optical characteristics of a new, weakly strained wide-gap II-VI quantum well have been studied with emphasis on the confined exciton states. Strong luminescence efficiency in the blue is observed with spectra showing only weak alloy disorder effects. Magneto-optical studies yield a measurement for the band offsets in a model which properly includes exciton effects.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have obtained optically pumped laser action in the form (Zn,Cd)Se/ZnSe single quantum well structures prepared by molecular beam epitaxy. Near-room-temperature lasing under pulsed excitation has been achieved and high repetition quasi-continuous mode operation at temperatures so far up to 120 K. Rapid gain switching has generated pulses of approximate 20 ps in duration.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report on studies of optically pumped laser action in (Zn,Cd)Se/ZnSe multiple quantum well structures prepared by molecular beam epitaxy on lattice-matched bulk (Ga,In)As substrates. Room-temperature lasing under pulsed excitation with threshold pump intensity at I≈500 kW/cm2 has been achieved, together with high repetition "quasi-continuous'' mode operation at temperatures so far up to 100 K.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the growth of cubic (zinc blende) CdSe epilayers on [100] GaAs substrates by molecular beam epitaxy. The lattice constant of the CdSe epilayers is 6.077 A(ring), and the energy gap is 1.75, 1.74, and 1.67 at 10, 80, and 300 K, respectively.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have employed short-wavelength holographic laser lithography and reactive ion etching to define wire and dot-like patterns in ZnSe thin epitaxial films and heterostructures with spatial feature size to better than 100 nm. Photoluminescence measurements suggest that surface damage from etching may be much less severe than in III-V semiconductors.〈hedend〉

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have investigated the epitaxial growth of Zn1−x CdxSe epilayers and ZnSe/Zn1−x CdxSe superlattices (0≤x≤1) on (100)GaAs. Although thick epilayers of Zn1−x CdxSe are prone to defect formation with increasing Cd content, the structural and optical characteristics improve remarkably when Zn1−x CdxSe is in the form of thin layers within ZnSe/Zn1−x CdxSe superlattices. High quality superlattices can be grown for x≤0.35. The characterization of these systems using transmission electron microscopy, x-ray diffraction, reflectivity, and photoluminescence is reported.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Diluted magnetic semiconductors (DMS) are semiconducting alloys containing a random distribution of substitutional magnetic ions (e.g., Mn++ in Cd1−xMnxSe). Magnetic properties of DMS—such as spin glass behavior due to lattice frustration, magnon excitations in a random system of spins, superexchange, and short range antiferromagnetic order—are of considerable interest in their own right. In addition, understanding these properties is important because they strongly influence the electronic phenomena in DMS via the sp-d exchange interaction between the band electrons and the localized magnetic moments. We review the most recent results concerning magnetic susceptibility, Mn++-Mn++ nearest-neighbor exchange, and magnetic short-range order in these materials. On this basis, we can present a fairly complete and unified picture of magnetic properties of DMS. In addition, we point out some of the issues and challenges that lie ahead.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Laser action in the blue-green at room temperature has been achieved in pulsed optically pumped (Zn,Cd)Se/ZnSe multiple quantum well structures at threshold intensities Ith≈30 kW/cm2. Continuous-wave operation in a II-VI semiconductor laser has also been demonstrated for the first time, here above 100 K. The role of excitons is found to be of importance in defining the lasing mechanism up to room temperature in these quasi-two-dimensional wide-gap heterostructures.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

P-type nitrogen-doped ZnSe grown on n+-GaAs by molecular beam epitaxy has been studied by deep-level transient spectroscopy (DLTS) and double correlation DLTS. To achieve p-type doping of ZnSe, we employed an active nitrogen beam produced by a free-radical plasma source. Four hole traps—with activation energies of 0.22, 0.51, 0.63, and 0.70 eV—were detected by DLTS. Two of these—those at 0.51 and 0.63 eV—have never been observed before in ZnSe. They are probably introduced to the material by nitrogen doping. The properties of the other two traps—at 0.22 and 0.70 eV—support the hypothesis that both of them are associated with native defects, in agreement with earlier reports. To our knowledge this is the first report about direct experimental investigation of deep states in p-type ZnSe.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Distributed feedback operation of optically pumped (Zn,Cd)Se/ZnSe quantum-well lasers has been realized at room temperature. The periodic grating structure was fabricated by short wavelength holographic lithography and dry etching techniques. These results are relevant also in the context of the new blue-green diode lasers, based on this same quantum-well system.

Electronic Resource