Y. J. Park

Application of atomic-force-microscope direct patterning to selective positioning of InAs quantum dots on GaAs

The application of atomic-force-microscope (AFM) direct patterning to the selective positioning of InAs quantum dots (QDs) on a (100) GaAs substrate has been proposed and experimentally implemented. The AFM direct patterning was used to generate various patterns of several tens of nanometers in size, and InAs QDs were subsequently grown by a metalorganic chemical vapor deposition technique. A nonuniform distribution of the QDs was observed near the patterns. The detailed shape of the QD distribution and the size of the QDs depended on the geometrical properties such as the sidewall angle, the spacing, and the width of the patterns. We have been able to ascertain, through our work, what growth conditions are necessary for QDs’ alignment along the patterns.

Mn-implanted dilute magnetic semiconductor InP:Mn

Unintentionally doped bulk InP was prepared by the liquid encapsulated Czochralski method and subsequently implanted with various doses of Mn+. The properties of Mn+-implanted InP:Mn were investigated by various measurements. The results of energy dispersive x-ray peaks displayed injected concentrations of Mn of 0.8% and 8.8%, respectively. The results of photoluminescence (PL) measurement showed that optical broad transitions related to Mn appeared near 1.089, 1.144, and 1.185 eV in samples with various doses of Mn+. It was confirmed that the photoluminescence peaks near 1.089, 1.144, and 1.185 eV were Mn-correlated PL bands by the implantation of Mn. Ferromagnetic hysteresis loops measured at 10 K were observed and the temperature-dependent magnetization showed ferromagnetic behavior around 90 K, which almost agreed with the theoretical prediction (Tc∼70 K).

Ferromagnetic behavior of p-type GaN epilayer implanted with Fe+ ions

p-type GaN epilayers were prepared by metalorganic chemical vapor deposition and subsequently implanted with Fe+ ions. The properties of Fe+ implanted GaN epilayers were investigated by various measurements. The results of photoluminescence measurement show that optical transitions related to Fe appear at 2.5 eV and around 3.1 eV. It was confirmed that the photoluminescence peak at 2.5 eV is a donor-Fe acceptor transition and the photoluminescence peak around 3.1 eV is a conduction band-Fe acceptor transition. Apparent ferromagnetic hysteresis loops measured at 10 and 300 K were observed, and the temperature-dependent magnetization displayed a ferromagnetic behavior persisting above 350 K.

A study on the temperature dependence of the threshold switching characteristics of Ge2Sb2Te5

We investigated the temperature dependence of the threshold switching characteristics of a memory-type chalcogenide material, Ge2Sb2Te5. We found that the threshold voltage (Vth) decreased linearly with temperature, implying the existence of a critical conductivity of Ge2Sb2Te5 for its threshold switching. In addition, we investigated the effect of bias voltage and temperature on the delay time (tdel) of the threshold switching of Ge2Sb2Te5 and described the measured relationship by an analytic expression which we derived based on a physical model where thermally activated hopping is a dominant transport mechanism in the material.

Influence of arsenic during indium deposition on the formation of the wetting layers of InAs quantum dots grown by migration enhanced epitaxy

We compared the structural and optical properties of InAs∕GaAs quantum dots grown by migration enhanced epitaxy, with and without arsenic, during indium deposition. The uniformity and size of the quantum dots are enhanced in a sample without arsenic. As a result, narrower and longer wavelength photoluminescence is observed in this sample. Furthermore, the thickness of the wetting layers is reduced by ∼20% in the sample without arsenic, and this result agrees well with the speculation that metallic indium has a smaller driving force for corrugating the InAs wetting layers before they are transformed from two-dimensional to three-dimensional layers. Additionally, the photoluminescence linewidth of the sample without arsenic is insensitive to the cryostat temperature due to two major factors: the reduced thickness of the wetting layers and the enhanced uniformity. In the sample with arsenic, however, the photoluminescence linewidth shows typical anomalies.

Improved stability of a phase change memory device using Ge-doped SbTe at varying ambient temperature

Ge-doped SbTe (Ge–ST) was compared with Ge2Sb2Te5 (GST) for its potential use in the phase-change memory with improved stability at varying ambient temperature (TA). Device characteristics such as RESET current, RESET resistance, and SET resistance of Ge–ST devices were found to vary significantly less with TA than those of GST devices. From measured carrier density, mobility, and optical band gaps, these findings are interpreted to derive from a metallic nature of the crystalline Ge–ST in contrast with a semiconducting nature of the crystalline GST as well as a relatively weaker covalent bonding in amorphous Ge–ST.

Optical and magnetic properties of Mn+-implanted GaAs

Neutron-transmutation-doped GaAs samples were prepared by irradiating the middle-level neutrons into the semi-insulating GaAs grown by a liquid encapsulated Czochralski method and subsequently implanted with Mn+. The characteristics of the Mn+-implanted neutron-transmutation-doped GaAs (namely, the implantation of Mn+ subsequent to neutron-transmutation-doping) were investigated by various measurements. The result of the energy dispersive x-ray peak displayed an injected Mn concentration of 9.65%. The photoluminescence peaks related to carbon and germanium acceptors were resolved, and the peaks related to Mn due to a neutral Mn acceptor were evidently observed. It is found that the proper activation for the neutral Mn acceptor starts from a relatively low annealing temperature of 600°C for 15min. The atomic force microscopy and magnetic forcemicroscopy images showed that magnetic clusters were well formed. The ferromagnetic hysteresis loop measured at 10K was observed, and the temperature-dependent magnet...

Controlled recrystallization for low-current RESET programming characteristics of phase-change memory with Ge-doped SbTe

An investigation was conducted to examine the high RESET-current (IRESET) problem of phase-change memory (PCM) using a fast growth-dominated Ge-doped SbTe (GeST). By examining material and device characteristics of GeST with varying Sb-to-Te ratio from 1.80 to 3.82, the growth rate of crystallization was found to play an important role in determining IRESET and SET speed of the device. Lower IRESET obtained with decreasing Sb-to-Te ratio was ascribed to lower growth rate leading to smaller degree of recrystallization during melt-quenching. With shrinkage of device dimensions, GeST-PCM of a lower Sb-to-Te ratio may become increasingly promising due to its lower IRESET and scaled SET speed.

Origin of clear ferromagnetism for p-type GaN implanted with Fe+ (5 and 10at.%)

The systematic enhancement of ferromagnetic hysteresis loops for GaN implanted with high doses of Fe (5→10at.%) takes place with an increase in the annealing temperature from 700to850°C. The trends of magnetic properties coincide with the results of the increased full width at half maximum of triple axis diffraction for GaN (0002), including the appearance of GaFeN, the enhanced Fe-related photoluminescence transitions, and the systematic increase in sizes of symmetric spin ferromagnetic domains GaFeN in atomic force microscopy and magnetic force microscopy.

Enhanced Curie temperature of InMnP:Zn—TC∼300K

P-type bulk InP was prepared by the liquid encapsulated Czochralski method and subsequently diffused with Mn by heat treatment after the evaporation of Mn on top of InP:Zn using a molecular beam epitaxy system. The characteristics of Mn-diffused InMnP:Zn were investigated by an energy dispersive x-ray spectroscopy, photoluminescence, and a superconducting quantum interference device magnetometer measurements. The samples were characterized by transmission electron microscopy and no evidence of secondary phase formation of InMnP:Zn was found. The results of energy dispersive x-ray peak displayed injected concentration of Mn near 3%. The results of photoluminescence measurement showed that optical broad transitions related to Mn appeared around 1.2eV and it was confirmed that the transitions around 1.2eV were Mn-related band by the diffusion of Mn into InP:Zn. Clear ferromagnetic hysteresis loops were observed at 10 and 300K and the temperature-dependent magnetization showed ferromagnetic behavior around 30...