S. K. Chan

ZnSe nanowires epitaxially grown on GaP(111) substrates by molecular-beam epitaxy

We report molecular-beam epitaxy growth of single crystalline ZnSe nanowires with uniform diameters (∼10 nm) on GaP(111) substrates. The growth process was based on the Au-catalyzed vapor-liquid-solid deposition. As determined by electron microdiffraction and high-resolution transmission electron microscopy, ZnSe nanowires grew generally along the 〈110〉 and 〈112〉 directions with the orientation relationship of (111)ZnSe wire//(111)GaP and 〈110〉ZnSe wire//〈110〉GaP. The dominant defects were found to be twins at the interface between the substrate and the nanowires along the (111) plane.

Efficient second harmonic generation from large band gap II-VI semiconductor photonic crystal

Dramatic enhancement of second harmonic generation (SHG) near the photonic band edge was observed in a one-dimensional ZnSe∕ZnMgS semiconductor photonic crystal (PC) structure. Over two orders of magnitude increase in SHG intensity was observed at the photonic band edge at ∼1400nm compared to the nonphase matching region. The maximum SHG conversion efficiency of 0.8% is observed in only seven micrometers length of crystal. This enhancement came from a combination of large ZnSe second order susceptibility coefficient (χ(2)), high density of optical modes and phase matching of the fundamental and second harmonic waves near the photonic band edge due to modification of the dispersion curve by the PC structure.

Control of growth orientation for epitaxially grown ZnSe nanowires

ZnSe nanowires (NWs) were grown on (111), (100), and (110)-oriented GaAs substrates by molecular-beam epitaxy via the vapor-liquid-solid reaction. The size dependence of NW growth orientation was studied by varying the Au catalyst size. Through detailed transmission electron microscopy studies, it was found that ⟨111⟩ orientation is the growth direction for NWs with size ⩾30nm, while NWs with size around 10nm prefer to grow along the ⟨110⟩ direction, with a small portion along the ⟨112⟩ direction. These observations have led to the realization of vertical ZnSe NWs with size around 10nm grown on a GaAs(110) substrate.

Growth behaviors of ultrathin ZnSe nanowires by Au-catalyzed molecular-beam epitaxy

Ultrathin ZnSe nanowires grown by Au-catalyzed molecular-beam epitaxy show an interesting growth behavior of diameter dependence of growth rates. The smaller the nanowire diameter, the faster is its growth rate. This growth behavior is totally different from that of the nanowires with diameters greater than 60nm and cannot be interpreted by the classical theories of the vapor-liquid-solid mechanism. For the Au-catalyzed nanowire growth at low temperatures, we found that the surface and interface incorporation and diffusion of the source atoms at the nanowire tips controlled the growth of ultrathin ZnSe nanowires.

Simultaneous enhancement of the second- and third-harmonic generations in one-dimensional semiconductor photonic crystals

We report on observation of simultaneous strong enhancements of second-harmonic generation (SHG) and third-harmonic generation (THG) in visible spectra region in a one-dimensional ZnSe-ZnMgS semiconductor photonic crystal. These enhancements come from phase matching and a high density of modes near the photonic band edge. Our result shows that the THG is due to direct /spl chi//sup (3)/ process instead of the cascaded two-step /spl chi//sup (2)/ process. Measured forward SH conversion efficiency close to 1% for only a few microns thick film demonstrates potential device applications. Theoretical calculations are in good agreement with the experimental measurements.

Formation mechanism of nanotrenches induced by mobile catalytic nanoparticles

⟨110⟩ oriented nanotrenches were generated by thermally annealing an ultrathin Au layer deposited on ZnSe surfaces of different orientations. Results from a number of structural and chemical analyses indicate that they were induced by the migration of Au-alloy droplets through a catalytic reaction with ZnSe. Highly aligned nanotrenches can be achieved on (100)-oriented substrates, which can potentially serve as templates for the fabrication of one-dimensional nanostructures of various materials.

The formation of an aligned 1D nanostructure on annealed Fe/ZnSe bilayers.

A highly aligned one-dimensional (1D) nanostructure was realized at the surface of Fe/ZnSe bilayers grown on GaAs(001) substrates through thermal annealing. These 1D nano-grooves were found to align along the [110] direction resulting in bent reflection high energy electron diffraction (RHEED) patterns when the sample was rotated relative to the e-beam. A model based on Ewald construction is presented to explain the unusual RHEED observation. The formation mechanism of this 1D nanostructure is possibly related to the minimization of surface energy, together with an Fe-Se exchange interaction and Fe-induced decomposition of several top ZnSe atomic layers during thermal annealing.

Te antisite incorporation in Zn S1-x Tex thin films

We investigate Te-related intrinsic defects in ZnS 1˛xTex thin films. First-principles calculations were performed for ZnS with Te as impurity atoms. The results show that the substitutional incorporation of Te in group VI sites cannot form defect states deep in the energy band gap of ZnS. On the other hand, our calculations on both Te antisite and interstitial defects result in deep-level states. X-ray photoelectron spectroscopy sXPSd and time-of-flight secondary ion mass spectrometry sTOF-SIMSd were used to study the chemical structures of three molecular-beam-epitaxy-grown ZnS1˛xTex samples, xh0.005, 0.019, 0.026j. An asymmetry was detected in the XPS spectra of both Te 3d3/2 and Te 3d5/2 core levels, which indicates that a small portion of the incorporated Te atoms is bonded to a more electronegative element. A peak at m/ z of 158 was detected in the SIMS depth profiles of these samples and its characteristics match that of TeS components of the lattice matrix. This experimental evidence strongly supports the existence of Te antisite defects in the thin films. We believe that the origin of the highly luminescent centers in ZnS 1˛xTex thin films is possibly attributed to the deep-level states generated from Te anti-site incorporation. The optoelectronic properties of III-V or II-VI compound semiconductors can be altered by incorporating an element with the same number of valence electrons as one of the host elements. These isovalent semiconductor systems, AC1˛xDx, show a continuous change in optical properties when x varies from 0t o 1. GaP 1˛xAsx, 1 ZnSe1˛xTex, 2 and ZnS1˛xSex 3 are some of the well-studied systems. In some systems, the impurities form deep-level bound states in the forbidden band gap of the host crystals, which are called isoelectronic centers sIECsd. The IECs can be generally treated as point defects which localize the wave function of the carriers. The typical examples include sid a GaP:N system 4 in which bound states are attributed to the isolated N atoms or pairs of N atoms with different internuclear pair separation, and siid a ZnS:Te system in which bound states were thought to be caused by isolated Te atoms or Tensno 2d clusters. 5‐7 In general, IECs form deep-level states within the forbidden band gap due to the difference in electronegativity and atomic size as compared with the replaced host atoms. These centers act as either electron or hole traps for excited carriers. Effective trapped exciton recombination at IECs leads to enhanced luminescence. We recently reported that highquality ZnS1˛xTex s0 x 1d single-crystal alloy films can be grown on GaAs and Si substrates by the molecular-beamepitaxy sMBEd technique. 8 Strong photoluminescence in the yellow to blue spectral region, with room-temperature external quantum efficiencies of 2‐4 % at an unoptimized excitation wavelength of 365 nm, was observed. In 1972, Baldereschi and Hopfield carried out a tight-binding calculation assuming Te isoelectronic substitution on the ZnS:Te system. 9 By introducing the effect of relaxation of the host crystal into the screened impurity potential of IECs, they predicted the existence of a bound state due to Te IECs based

Competitive antiferromagnetic and ferromagnetic coupling in a CrSe/Fe/GaAs(111)B structure

A novel transition from the negative exchange bias (NEB) to the positive exchange bias (PEB), due to coexistent and competitive ferromagnetic and antiferromagnetic coupling in a molecular-beam-expitaxy-grown CrSe/Fe/GaAs(111)B structure, is described. The source of the unusual PEB effect was found to originate from the interface at the hetero-junction of Fe/GaAs(111)B while the NEB effect resulted from the CrSe/Fe bilayer structure. Phenomenological models are presented to explain the mechanism of the PEB effect and the varying asymmetry of the magnetic hysteresis of this structure as a function of the measured temperature.

Molecular-beam-epitaxy-grown CrSe∕Fe bilayer on GaAs(100) substrate

A novel CrSe∕Fe bilayer structure has been fabricated on a GaAs (100) substrate by the molecular beam epitaxy technique. Microstructural characterizations have revealed that the Fe layer is a single-crystalline bcc structure with the orientation relationship of (100)Fe‖(100)GaAs, while the top CrSe layer shows four preferred hexagonal domains with their c axis each along one of the four upward-pointing ⟨111⟩ directions of the underlying Fe lattice. The magnetic hysteresis loops of this bilayer structure measured by a superconducting quantum interference device magnetometer demonstrate a strong exchange bias effect with a negative exchange bias field as high as −48.4Oe at 5K. The magnetization reversal process shows an abrupt transition nature at temperature from 5to300K. An enhancement of the coercivity not accompanied by the exchange bias field was observed at temperature higher than and well above the blocking temperature. We have interpreted these observations based on the well-established exchange spr...