J. H. Huang

Fabrication of antireflective sub-wavelength structures on silicon nitride using nano cluster mask for solar cell application.

We have developed a simple and scalable approach for fabricating sub-wavelength structures (SWS) on silicon nitride by means of self-assembled nickel nanoparticle masks and inductively coupled plasma (ICP) ion etching. Silicon nitride SWS surfaces with diameter of 160–200 nm and a height of 140–150 nm were obtained. A low reflectivity below 1% was observed over wavelength from 590 to 680 nm. Using the measured reflectivity data in PC1D, the solar cell characteristics has been compared for single layer anti-reflection (SLAR) coatings and SWS and a 0.8% improvement in efficiency has been seen.

Silicon Nitride Nanopillars and Nanocones Formed by Nickel Nanoclusters and Inductively Coupled Plasma Etching for Solar Cell Application

The external quantum efficiency of solar cells can be improved by using textured surface with minimum reflection. We have fabricated nanopillars and nanocone structures on silicon nitride surface by means of self-assembled nickel nano particle masks with single step inductively coupled plasma (ICP) ion etching and double step ICP etching, respectively. Thus, sub-wavelength nanopillar and nanoconelike structures displaying low reflectance were obtained readily without the need for any lithography equipment. The formation mechanism of nanopillar and nanocone like structures fabricated on silicon nitride surface has been discussed. The relationship of etching time with structure height and average reflectance spectra has been drawn. # 2009 The Japan Society of Applied Physics

Room-temperature ferromagnetism in self-assembled (In, Mn)As quantum dots

Self-assembled In1−xMnxAs quantum dots (0.19⩽x⩽0.45) have been grown on GaAs (100) substrates by low-temperature molecular beam epitaxy. The microstructure analysis revealed that the uniformly distributed In1−xMnxAs dots have a zinc blende structure as x⩽0.38. Furthermore, all samples exhibit ferromagnetic state at 5K, and their Curie temperatures range from 260to340K varying with x. These (In, Mn)As quantum dots are promising for room-temperature spintronic devices.

Flat band current‐voltage‐temperature method for band‐discontinuity determination and its application to strained InxGa1−xAs/In0.52Al0.48As heterostructures

A method that combines capacitance‐voltage and current‐voltage‐temperature measurements of a n+‐i‐n− (or p+‐i‐p−) heterobarrier structure for reliable determination of the band discontinuity is described. Experimental and analytical procedures for the extraction of the Fermi energies in the doped layers, the location of the flat band condition, and the determination of the barrier height at flat band are given. The effects of nonparabolicity and strain are also considered. Some potential sources of errors encountered in the conventional procedure that is based on the barrier height at zero bias are avoided in this flat band method. The application of this method and other experimental considerations are illustrated by using the strained InxGa1−xAs/In0.52Al0.48As heterointerface as a specific example. The results show that the conduction‐band offset ratio, Qc, is nearly constant at 0.71 for x≤0.54 but appears to change quite abruptly to a fairly constant value of 0.82 for x≥0.58.

Enhancement of exchange coupling between GaMnAs and IrMn with self-organized Mn(Ga)As at the interface

An atomically flat and uniform reaction layer of Mn(Ga)As was found to self-organize at the (Ga,Mn)As∕IrMn interface by postannealing. The Mn(Ga)As layer exhibits strong ferromagnetic characteristics up to the measured 300K. In particular, the manifested horizontal shift of field-cooled hysteresis loops shows a clear signature of exchange bias attributable to the exchange coupling between IrMn and Mn(Ga)As. Implication from composition analyses, exchange-bias effect, and thickness dependence of the Mn(Ga)As layer versus annealing conditions is also discussed.

Growth and magnetic properties of self-assembled (In, Mn)As quantum dots

Self-assembled In0.79Mn0.21As quantum dots were successfully grown on GaAs (001) substrates by low-temperature molecular beam epitaxy. Atomic force microscopy and high-resolution transmission electron microscopy confirm the formation of quantum dots. High-resolution lattice image suggests that In0.79Mn0.21As dots are single phase with zinc-blend structure. The dots exhibit typical ferromagnetic state at 5K and demonstrate a Curie temperature of ∼290K which is much higher than those of (In, Mn)As diluted magnetic semiconductor alloys ever reported. The significant increase in Curie temperature can be attributed to the much higher Mn content in the dots, and the possible enhancement of the hybridization strength between the quantum-confined holes in the dots and the itinerant holes in the semiconductor valence band.

Distribution of electronic reconstruction at the n-type LaAlO3/SrTiO3 interface revealed by hard x-ray photoemission spectroscopy

We investigated the electronic reconstruction at the n-type LaAlO3/SrTiO3 interface with hard x-ray photoelectron spectroscopy (HAXPES) under grazing incidence. By exploiting the collapse of evanescent x-ray waves and the abrupt increase of x-ray absorption at the critical incidence angle, our HAXPES study reveals a 2% electronic reconstruction from Ti4+ to Ti3+ occurring near the interface. Such an electronic reconstruction also extends from the interface into SrTiO3 with a depth of about 48u2009A (∼12 unit cells) and an estimated total charge transfer of ∼0.24 electrons per two-dimensional unit cell.

Coexistence of exchange bias and magnetization pinning in the MnOx/GaMnAs system

Coexistence of exchange bias (H(E)) and magnetization (M) shift was observed in as-grown and field-annealed MnO(x)/Ga(0.95)Mn(0.05)As bilayers. It was found that H(E) initially decreases with the annealing time t(a) and then increases when t(a) > 30 min, while the M shift remains almost unchanged with t(a). X-ray photoelectron spectroscopy (XPS) analysis reveals that MnO(x) is composed of MnO and Mn(3)O(4), and the volume amount ratio of Mn(3)O(4) to MnO increases with increasing t(a). A simple model based on a uniform MnO-Mn(3)O(4) interface with constant pinned uncompensated interfacial spins is proposed to account for the observed exchange-biased phenomena in the bilayers.

Self-assembly and magnetic properties of MnAs nanowires on GaAs(001) substrate

The in-plane aligned MnAs nanowires have been grown by molecular-beam epitaxy on GaAs(001) substrates at high growth temperature (≥450u2009°C). A discontinuous growth with break intervals (50 s’ interval per 10 s’ growth) was employed. The obtained nanowires were identified to be mainly type-B hexagonal MnAs. The influences of growth temperature and As4/Mn flux ratio on the nanowires’ morphology were investigated. Both high growth temperature and high As4/Mn flux ratio are necessary for the growth of uniaxially aligned MnAs nanowires with high aspect ratio. The magnetic anisotropy of the nanowires and their multimodal size distributions contribute to the large coercivity and special shape of the M-H loops along the magnetic easy axis, which is [11¯02]MnAs∥[110]GaAs. However, the longer growth time would lead to the both azimuthal alignments of the MnAs wires and the weakening of the magnetic anisotropy.

Two-dimensional arsenic precipitation in superlattice structures of alternately undoped and heavily Be-doped GaAs grown by low-temperature molecular beam epitaxy

The precipitation of arsenic in superlattice structures of alternately undoped and [Be]=2.4×1019u2002cm−3 doped GaAs grown at low temperatures has been studied using transmission electron microscopy. Novel precipitate microstructures were observed in annealed samples, including preferential accumulation of precipitates toward each interface of Be-doped GaAs and the following grown undoped GaAs. Specifically, after 800u2009°C annealing, the precipitates are totally confined in Be-doped regions, forming two-dimensional dot arrays near the aforementioned interfaces. Data are also presented to show that the heavily Be-doped GaAs has a smaller lattice constant than the undoped GaAs. A strain-induced mechanism was proposed to account for the segregation of As clusters.