The ferroelectric and antiferroelectric properties of ZrO2 ultrathin films (~12 nm in thickness) prepared by atomic layer deposition (ALD) were tailored by introducing sub-nanometer interfacial layers between the ZrO2 ultrathin film and top and bottom Pt electrodes. In terms of polarization switching ability, the ferroelectricity of ZrO2 ultrathin films was significantly enhanced by an HfO2 interfacial layer (i.e., a Pt/HfO2/ZrO2/HfO2/Pt layered arrangement). While, a TiO2 interfacial layer (i.e., a Pt/TiO2/ZrO2/TiO2/Pt layered arrangement) led to a transition from ferroelectricity to antiferroelectricity. The modulation of ferroelectricity and antiferroelectricity of ZrO2 ultrathin films by the interfacial layers can be achieved without post-annealing.
Wurtzite aluminum nitride (AlN) of space group P63mc has long been recognized as a non-ferroelectric material, lacking the polarization switching ability. This paper reports the induction of ferroelectricity in a single crystalline epitaxial AlN ultrathin film with a thickness of 8−10 nm. The ferroelectric AlN epilayer was grown on a single crystalline GaN layer, forming a [0001]-oriented AlN/GaN epitaxial heterostructure with two reversible polar variants: [000-1] and [0001]. The AlN epilayer exhibited soft ferroelectricity with large switching currents and a polarization value of ~3.0 μCcm-2 during a 180° polarization switch. The AlN epilayer was prepared by the atomic layer deposition technique at 300°C in conjunction with in-situ atomic layer annealing. The two-dimensional electron gas (2DEG) at the AlN/GaN interface could be manipulated by the ferroelectric switching in the AlN epilayer. Strain engineering via lattice mismatch at the AlN/GaN interface was the key to creating a ferroelectric AlN/GaN heterojunction. Based on the reciprocal space mapping analysis, the AlN ferroelectricity is believed to be stemming from the out-of-plane compressive strain and inplane tensile strain present in the [0001]-oriented AlN epilayer. The discovery of low-temperature prepared, CMOScompatible AlN ultrathin films with soft ferroelectric characteristics will undoubtedly spur new fundamental and applied research in low-dimensional ferroelectric systems based on the AlN/GaN heterojunction.
In this study, we have successfully grown blue LED structure on ZnO template (deposited on sapphire substrate by
atomic layer deposition, ALD) by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD).
Although GaN semiconductor material is very similar to ZnO in many ways, i.e. relatively small lattice mismatch ~1.8 %
compared with traditional sapphire substrate~16 %, it still has a big challenge when GaN-based LEDs grow on ZnO
template at usually growth temperature near 1100°C. With too high a temperature and a long deposited time, it would
cause reaction at GaN/ZnO interface which is a vital reason that degrades the GaN crystalline quality. In view of this, we
introduced an optimized thin AlN cover layer on ZnO template protecting the underneath ZnO layer and then obtained a
real work LED structure. Meanwhile, the TEM measurement characterized the epilayer crystalline structure. The optical
properties also were carried out by photoluminescence and electroluminescence analysis. Finally, with a suitable
fabrication of LED processing, the ZnO template may has the potential as a sacrificial layer by chemical etching
technical instead of conventional laser lifted-off.
The morphologic changes of living cells under drug interactions were studied by using 80-nm gold nanoparticles and dark-field optical section microscopy. The gold nanoparticles were coated with poly (L-lysine), which attached to the membranes of various cells by way of electrostatic attractive force. A three-dimensional (3-D) morphological image was obtained by measuring the peak scattering intensities of gold nanoparticles at different focal planes. An algorithm for the reconstruction of 3-D cell morphology was presented. With the measured nanoparticle images and calculations, we show morphologic changes of lung cancer cells under the interaction of cytochalasin D drug at different times.
The radiative and nonradiative recombinations involved in efficient light-emitting metal-oxide-silicon tunneling diodes have been studied. The radiative recombination coefficient in the silicon light-emitting diode was previously found by us to be one order of magnitude greater than that of the bulk silicon. However, the nonradiative Shockley-Read-Hall recombination still dominates the carrier recombination processes near the Si/SiO2 interface. In the present work, we show by using the voltage-dependent photoluminescence that the position of the Fermi level near the Si/SiO2 interface significantly influences the nonradiative recombination rates. The nonradiative recombination states are shown to capture electrons much more effectively. This study suggests that significant reduction in nonradiative recombination is essential for efficient light emission from silicon.
We report the finding of photoluminescence (PL) and electroluminescence (EL) studies at silicon bandgap energy for the indium-tin-oxide (ITO)/SiO2/Si metal-oxide-semiconductor (MOS) tunneling diodes. The characteristics of temporal EL response, temperature dependence of EL and PL intensities, and voltage-dependent PL intensity, were used to investigate the radiative recombination and nonradiative Shockley-Read-Hall (SRH) recombination near the Si-SiO2 interface. The temporal EL response indicates that the radiative recombination coefficient in the light-emitting MOS tunneling diode is about ten times larger than that of the bulk silicon. However, the nonradiative SRH recombination is still the dominant carrier recombination process. The intensity of EL was found to be lesser sensitive with temperature than that of PL, which indicates that the nonradiadiative recombination is less thermally active and less efficient for EL. The voltage-dependent PL study shows that the PL intensity increases with the bias voltage. This observation is attributed to the variations of nonradiative SRH recombination rates due to the change of Fermi level with the bias voltage. This study shows that the nonradiative recombination near the Si-SiO2 interface strongly influences the luminescent efficiency.
We report that electroluminescence (EL) at Si bandgap energy is significantly enhanced from the nano-structured metal- oxide-semiconductor (MOS) devices on silicon. The nano- structure is constructed by inserting SiO2 nanoparticles with the size of 12 nm in the oxide layer. The measured EL efficiency of the nano-structured MOS devices is enhanced to be near 10-4, which exceeds the limitation imposed by the indirect bandgap nature of silicon. We also observed the nearly lasing behaviors such as the threshold and resonance modes in the EL characteristics. The enhanced EL efficiency is attributed to simultaneous localization of electrons and holes to form exciton by nano-structure. This causes the process of the phonon-assisted radiative recombination of electron-hole pair more like two-particle (exciton-phonon) collision than three-particle (electron- hole-phonon) collision.
We report room-temperature electroluminescence at Si bandgap energy from Metal-Oxide-Semiconductor (MOS) tunneling diodes. The ultrathin gate oxide with thickness 1 to approximately 3 nm was grown by rapid thermal oxidation (RTO) to allow significant current to tunnel through. The measured EL efficiency of the MOS tunneling diodes increases with the injection current and could be in the order of 10-5, which exceeds the limitation imposed by indirect bandgap nature of Si. We also study the temperature dependence of the electroluminescence and photoluminescence. The electroluminescence is much less dependent on temperature than photoluminescence from Si. The applied external field that results in the accumulation of majority carriers at Si/SiO2 interface in the case of electroluminescence could be the reason for such difference. The involved physics such as optical phonon, interface roughness, localized carriers, and exciton radiative recombination are used to explain the electroluminescence from silicon MOS tunneling diodes.
Silicon is the most important semiconductor material for electronics industry. However, its indirect bandgap makes it hardly emit light, so its applications in optoelectronics are limited. Many efforts had been devoted to converting silicon to light-emitting materials, including porous silicon-based devices, nanocrystalline Si, and so on. In this work, we report electroluminescence on silicon with simple metal-oxide-semiconductor (MOS) structure. The thin oxide is grown by well-controlled rapid thermal oxidation. With extremely thin oxide, significant tunneling current flows through the MOS structure as the metal is properly biased. The tunneled electrons could then occupy the upper energy levels more than the thermal-equilibrium situation. Then luminescence occurs when they have radiative transition to lower energy states. For low biased voltages, the emission occurs around 1150 nm, approximately corresponding to the Si bandgap energy. For large applied voltages, the emission shifts to longer wavelengths and becomes voltage- dependent. MOS structures fabricated on both p-type and n- type silicon exhibit electroluminescence. This is significant because the fabrication of those MOS structures is compatible with CMOS electronics. Therefore, the MOS EL devices provide a particular advantage over other types of luminescence on silicon. The details of the electroluminescence and its physical reason are reported and discussed.
In this paper, we present wide range tunable semiconductor lasers using multiple quantum wells of different widths. Conventional semiconductor-laser gain medium using the quantum-well structure for the wavelength near 0.8 micrometers has only 50 meV bandwidth. Using quantum wells of different widths could significantly broaden the gain bandwidth, so broadband tuning of the laser could be easily achieved. Two types of samples designed for broad bandwidth operation have been used in this experiment. In addition, unlike the past tuning experiments that suffer from serious ASE noise, this experiment demonstrates very high ASE suppression ratio (> 40 dB) over the entire tuning range by using a triangular ring cavity.
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