Optical characteristics of type-II hexagonal-shaped GaSb quantum dots on GaAs synthesized using nanowire self-growth mechanism from Ga metal droplet.

We report the growth mechanism and optical characteristics of type-II band-aligned GaSb quantum dots (QDs) grown on GaAs using a droplet epitaxy-driven nanowire formation mechanism with molecular beam epitaxy. Using transmission electron microscopy and scanning electron microscopy images, we confirmed that the QDs, which comprised zinc-blende crystal structures with hexagonal shapes, were successfully grown through the formation of a nanowire from a Ga droplet, with reduced strain between GaAs and GaSb. Photoluminescence (PL) peaks of GaSb capped by a GaAs layer were observed at 1.11 eV, 1.26 eV, and 1.47 eV, assigned to the QDs, a wetting-like layer (WLL), and bulk GaAs, respectively, at the measurement temperature of 14 K and excitation laser power of 30 mW. The integrated PL intensity of the QDs was significantly stronger than that of the WLL, which indicated well-grown GaSb QDs on GaAs and the generation of an interlayer exciton, as shown in the power- and temperature-dependent PL spectra, respectively. In addition, time-resolved PL data showed that the GaSb QD and GaAs layers formed a self-aligned type-II band alignment; the temperature-dependent PL data exhibited a high equivalent internal quantum efficiency of 15 ± 0.2%.

Direct Transfer of Light's Orbital Angular Momentum onto a Nonresonantly Excited Polariton Superfluid.

Recently, exciton polaritons in a semiconductor microcavity were found to condense into a coherent ground state much like a Bose-Einstein condensate and a superfluid. They have become a unique testbed for generating and manipulating quantum vortices in a driven-dissipative superfluid. Here, we generate an exciton-polariton condensate with a nonresonant Laguerre-Gaussian optical beam and verify the direct transfer of light's orbital angular momentum to an exciton-polariton quantum fluid. Quantized vortices are found in spite of the large energy relaxation involved in nonresonant pumping. We identified phase singularity, density distribution, and energy eigenstates for the vortex states. Our observations confirm that nonresonant optical Laguerre-Gaussian beam can be used to manipulate chirality, topological charge, and stability of the nonequilibrium quantum fluid. These vortices are quite robust, only sensitive to the orbital angular momentum of light and not other parameters such as energy, intensity, size, or shape of the pump beam. Therefore, optical information can be transferred between the photon and exciton-polariton with ease and the technique is potentially useful to form the controllable network of multiple topological charges even in the presence of spectral randomness in a solid state system.

Electrical properties and thermal stability in stack structure of HfO2/Al2O3/InSb by atomic layer deposition.

Changes in the electrical properties and thermal stability of HfO2 grown on Al2O3-passivated InSb by atomic layer deposition (ALD) were investigated. The deposited HfO2 on InSb at a temperature of 200 °C was in an amorphous phase with low interfacial defect states. During post-deposition annealing (PDA) at 400 °C, In-Sb bonding was dissociated and diffusion through HfO2 occurred. The diffusion of indium atoms from the InSb substrate into the HfO2 increased during PDA at 400 °C. Most of the diffused atoms reacted with oxygen in the overall HfO2 layer, which degraded the capacitance equivalent thickness (CET). However, since a 1-nm-thick Al2O3 passivation layer on the InSb substrate effectively reduced the diffusion of indium atoms, we could significantly improve the thermal stability of the capacitor. In addition, we could dramatically reduce the gate leakage current by the Al2O3 passivation layer. Even if the border traps measured by C-V data were slightly larger than those of the as-grown sample without the passivation layer, the interface trap density was reduced by the Al2O3 passivation layer. As a result, the passivation layer effectively improved the thermal stability of the capacitor and reduced the interface trap density, compared with the sample without the passivation layer.

Al2O3 Passivation Effect in HfO2·Al2O3 Laminate Structures Grown on InP Substrates.

The passivation effect of an Al2O3 layer on the electrical properties was investigated in HfO2-Al2O3 laminate structures grown on indium phosphide (InP) substrate by atomic-layer deposition. The chemical state obtained using high-resolution X-ray photoelectron spectroscopy showed that interfacial reactions were dependent on the presence of the Al2O3 passivation layer and its sequence in the HfO2-Al2O3 laminate structures. Because of the interfacial reaction, the Al2O3/HfO2/Al2O3 structure showed the best electrical characteristics. The top Al2O3 layer suppressed the interdiffusion of oxidizing species into the HfO2 films, whereas the bottom Al2O3 layer blocked the outdiffusion of In and P atoms. As a result, the formation of In-O bonds was more effectively suppressed in the Al2O3/HfO2/Al2O3/InP structure than that in the HfO2-on-InP system. Moreover, conductance data revealed that the Al2O3 layer on InP reduces the midgap traps to 2.6 × 1012 eV-1 cm-2 (compared to that of HfO2/InP, that is, 5.4 × 1012 eV-1 cm-2). The suppression of gap states caused by the outdiffusion of In atoms significantly controls the degradation of capacitors caused by leakage current through the stacked oxide layers.

Controlling the defects and transition layer in SiO2 films grown on 4H-SiC via direct plasma-assisted oxidation.

The structural stability and electrical performance of SiO2 grown on SiC via direct plasma-assisted oxidation were investigated. To investigate the changes in the electronic structure and electrical characteristics caused by the interfacial reaction between the SiO2 film (thickness ~5 nm) and SiC, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), density functional theory (DFT) calculations, and electrical measurements were performed. The SiO2 films grown via direct plasma-assisted oxidation at room temperature for 300s exhibited significantly decreased concentrations of silicon oxycarbides (SiOxCy) in the transition layer compared to that of conventionally grown (i.e., thermally grown) SiO2 films. Moreover, the plasma-assisted SiO2 films exhibited enhanced electrical characteristics, such as reduced frequency dispersion, hysteresis, and interface trap density (Dit ≈ 1011 cm-2 · eV-1). In particular, stress induced leakage current (SILC) characteristics showed that the generation of defect states can be dramatically suppressed in metal oxide semiconductor (MOS) structures with plasma-assisted oxide layer due to the formation of stable Si-O bonds and the reduced concentrations of SiOxCy species defect states in the transition layer. That is, energetically stable interfacial states of high quality SiO2 on SiC can be obtained by the controlling the formation of SiOxCy through the highly reactive direct plasma-assisted oxidation process.

Structural and Electrical Properties of EOT HfO2 (<1 nm) Grown on InAs by Atomic Layer Deposition and Its Thermal Stability.

We report on changes in the structural, interfacial, and electrical characteristics of sub-1 nm equivalent oxide thickness (EOT) HfO2 grown on InAs by atomic layer deposition. When the HfO2 film was deposited on an InAs substrate at a temperature of 300 °C, the HfO2 was in an amorphous phase with an sharp interface, an EOT of 0.9 nm, and low preexisting interfacial defect states. During post deposition annealing (PDA) at 600 °C, the HfO2 was transformed from an amorphous to a single crystalline orthorhombic phase, which minimizes the interfacial lattice mismatch below 0.8%. Accordingly, the HfO2 dielectric after the PDA had a dielectric constant of ∼24 because of the permittivity of the well-ordered orthorhombic HfO2 structure. Moreover, border traps were reduced by half than the as-grown sample due to a reduction in bulk defects in HfO2 dielectric during the PDA. However, in terms of other electrical properties, the characteristics of the PDA-treated sample were degraded compared to the as-grown sample, with EOT values of 1.0 nm and larger interfacial defect states (Dit) above 1 × 10(14) cm(-2) eV(-1). X-ray photoelectron spectroscopy data indicated that the diffusion of In atoms from the InAs substrate into the HfO2 dielectric during the PDA at 600 °C resulted in the development of substantial midgap states.

Hot Carrier Trapping Induced Negative Photoconductance in InAs Nanowires toward Novel Nonvolatile Memory.

We report a novel negative photoconductivity (NPC) mechanism in n-type indium arsenide nanowires (NWs). Photoexcitation significantly suppresses the conductivity with a gain up to 10(5). The origin of NPC is attributed to the depletion of conduction channels by light assisted hot electron trapping, supported by gate voltage threshold shift and wavelength-dependent photoconductance measurements. Scanning photocurrent microscopy excludes the possibility that NPC originates from the NW/metal contacts and reveals a competing positive photoconductivity. The conductivity recovery after illumination substantially slows down at low temperature, indicating a thermally activated detrapping mechanism. At 78 K, the spontaneous recovery of the conductance is completely quenched, resulting in a reversible memory device, which can be switched by light and gate voltage pulses. The novel NPC based optoelectronics may find exciting applications in photodetection and nonvolatile memory with low power consumption.

Effects of nitrogen incorporation in HfO(2) grown on InP by atomic layer deposition: an evolution in structural, chemical, and electrical characteristics.

We investigated the effects of postnitridation on the structural characteristics and interfacial reactions of HfO2 thin films grown on InP by atomic layer deposition (ALD) as a function of film thickness. By postdeposition annealing under NH3 vapor (PDN) at 600 °C, an InN layer formed at the HfO2/InP interface, and ionized NHx was incorporated in the HfO2 film. We demonstrate that structural changes resulting from nitridation of HfO2/InP depend on the film thickness (i.e., a single-crystal interfacial layer of h-InN formed at thin (2 nm) HfO2/InP interfaces, whereas an amorphous InN layer formed at thick (>6 nm) HfO2/InP interfaces). Consequently, the tetragonal structure of HfO2 transformed into a mixture structure of tetragonal and monoclinic because the interfacial InN layer relieved interfacial strain between HfO2 and InP. During postdeposition annealing (PDA) in HfO2/InP at 600 °C, large numbers of oxidation states were generated as a result of interfacial reactions between interdiffused oxygen impurities and out-diffused InP substrate elements. However, in the case of the PDN of HfO2/InP structures at 600 °C, nitrogen incorporation in the HfO2 film effectively blocked the out-diffusion of atomic In and P, thus suppressing the formation of oxidation states. Accordingly, the number of interfacial defect states (Dit) within the band gap of InP was significantly reduced, which was also supported by DFT calculations. Interfacial InN in HfO2/InP increased the electron-barrier height to ∼0.6 eV, which led to low-leakage-current density in the gate voltage region over 2 V.