The origin of hysteresis in the drain–source current (IDS)–gate‐source voltage (VGS) characteristics of atomic‐layer‐deposited (ALD) p‐type SnO thin‐film transistors (TFTs) is examined by adding ALD Al2O3 interfacial layers (IL) between the SnO channel layer and the SiO2 gate insulator (GI) layer. SnO TFTs with SiO2 GI exhibit a large hysteresis voltage (Vhy) due to the trap state density near the interface between the SnO active layer and the SiO2 GI (known as the border trap). Both experimental results and theoretical calculations show that the origin of border traps is the SnSi+0 gap states in SiO2, which is induced by the Sn diffusion into the SiO2 layer. The use of Al2O3 films as ILs suppresses this diffusion. The effectiveness, however, is dependent on the thickness, crystallinity, and density of the Al2O3 films. The Vhy of the SnO TFTs can be decreased when the thickness and density of the ILs is increased if the amorphous structure of the Al2O3 IL is maintained after the rapid thermal annealing process. p‐Type ALD SnO TFTs with optimum ILs exhibit a high on‐off ratio of IDS (1.2 × 105), high field‐effect mobility (1.6 cm2 V−1 s−1), and a small Vhy (0.2 V).

We theoretically investigate the mechanism of ferroelectric switching via interlayer shear in 3R MoS2 using first principles and lattice dynamics calculations. First principle calculations show the prominent anharmonic coupling of the infrared inactive interlayer shear and the infrared active phonons. The nonlinear coupling terms generates an effective anharmonic force which drives the interlayer shear mode and lowers the ferroelectric switching barrier depending on the amplitude and polarization of infrared mode. Lattice dynamics simulations show that the interlayer shear mode can be coherently excited to the switching threshold by a train of infrared pulses polarized along the zigzag axis of MoS2. The results of this study indicate the possibility of ultrafast ferroelectricity in stacked two-dimensional materials from the control of stacking sequence.

Increasing capacitance density could be achieved by mainly two methods: increasing the capacitor node area and adopting higher dielectric constant (κ) material. The former relates mostly to integration issues, such as deep capacitor hole etching with an aspect ratio over 50 and filling the hole with the conformal electrode material. The latter is mainly a material issue, requiring appropriate material selection and an extremely conformal film deposition process. It also must be compatible with the electrode material, which means no adverse interfacial reaction and phase-pure high-κ film growth.Given the industry-compatible and matured electrode fabrication process of TiN grown by the atomic layer deposition (ALD) process, ZrO 2 has been the leading high-κ material in DRAM fabrication. This is because the crystalline structure of the thin-film ZrO 2 transforms from the medium-κ (≈20) monoclinic phase to the high-κ (>≈40) tetragonal phase due to the surface-energy effect. [6,7] Also, the ALD process of ZrO 2 is well matured to secure mass production. However, undoped ZrO 2 has suffered from a high leakage current problem. The problem could be ascribed to the local current flow through the grain boundaries of the polycrystalline ZrO 2 [8] and the n-type nature (the Fermi level is close to the conduction band edge) of the material by the presence of oxygen vacancies. [9,10] This problem has been overcome by adopting a thin Al 2 O 3 insertion layer (IL) for the relatively thicker ZrO 2 or doping the ZrO 2 with Al for thinner films. [8][9][10][11][12] However, such a strategy has sacrificed capacitance density owing to the inclusion of the lowκ amorphous Al 2 O 3 layer (κ ∼ 6-9) or the degraded crystallinity of the Al-doped ZrO 2 . [10,13,14] Therefore, the DRAM industry has spent the enormous effort to optimize the dielectric stack structure, but it has become evident that next-generation dielectric material is necessary for further scaling.Among the diverse candidates with even higher κ-values, SrTiO 3 showed a severe incompatibility with the TiN electrode. [15,16] Hf-doping into the ZrO 2 could be a viable and immediate option, but it involves a risk of loss of discharging density and slow operation speed. This problem is owing to the possible involvement of the (anti-) ferroelectric effect of the Hf-doped This study examines the influences of the Al 2 O 3 and Y 2 O 3 insertion layers (ILs) on the structural and electrical features of ZrO 2 thin films for their application to dynamic random access memory capacitors. The ultra-thin Al 2 O 3 IL (0.1-0.2 nm) dissolves into the ZrO 2 layers, which causes the top and bottom portions of the ZrO 2 film to merge and have smaller lattice parameters. However, the thicker Al 2 O 3 IL (>≈0.4 nm) forms a continuous layer and separates the top and bottom portions of the ZrO 2 film. Interestingly, the diffusion of Al does not occur in this case. Overall, the dielectric constant (κ) of the ZrO 2 /Al 2 O 3 /ZrO 2 film is lower than that of the undoped ZrO 2 film due to the involveme...

A detailed understanding of the atomic configuration of the compound semiconductor surface, especially after reconstruction, is very important for the device fabrication and performance. While there have been numerous experimental studies using the scanning probe techniques, further theoretical studies on surface reconstruction are necessary to promote the clear understanding of the origins and development of such subtle surface structures. In this work, therefore, a pressure-temperature surface reconstruction diagram was constructed for the model case of the InAs (001) surface considering both the vibrational entropy and configurational entropy based on the density functional theory. Notably, the equilibrium fraction of various reconstructions was determined as a function of the pressure and temperature, not as a function of the chemical potential, which largely facilitated the direct comparison with the experiments. By taking into account the entropy effects, the coexistence of the multiple reconstructions and the fractional change of each reconstruction by the thermodynamic condition were predicted and were in agreement with the previous experimental observations. This work provides the community with a useful framework for such type of theoretical studies.

This work reports on the theoretical equilibrium crystal shapes of GaAs and InAs as a function of temperature and pressure, taking into account the contribution of the surface vibration, using ab-initio thermodynamic calculations. For this purpose, new (111)B reconstructions, which are energetically stable at a high temperature, are suggested. It was found that there was a feasible correspondence between the calculated equilibrium shapes and the experimental shapes, which implied that the previous experimental growth was performed under conditions that were close to equilibrium. In this study, GaAs and InAs were selected as prototype compound semiconductors, but the developed calculation methodology can also be applied to other III–V compound semiconductor materials.

The wurtzite (WZ) structure AlN is a well-known piezoelectric material with a spontaneous polarization (P s ) as high as %130 μC cm À2 . [1] However, such a high P s cannot be switched reversibly by the electric field due to its presumably higher coercive field (E c ) than its typical breakdown field (E bd ). [2] This problem has severely limited the usefulness of AlN in ferroelectric devices. Such a high E c could be positively related to the high switching barrier (E a ) for the P s switching. Several other oxide materials with the WZ structure had also been examined as potential candidates for ferroelectricity. [3,4] Among them, doped ZnO has shown ferroelectricity, [5][6][7] which suggests that its E c is lower than its E bd . The doping of Sc into AlN was experimentally reported recently, allowing the (Al,Sc)N solid solution (SS) to significantly decrease the E c and making ferroelectric switching feasible. The E c and the remanent polarization (P r ) decreased with the increasing fraction of ScN and ranged from 1.5 to 4.5 MV cm À1 and 85-110 μC cm À2 , respectively. [8] The reason for the ferroelectricity manifestation was presumed to be the reduction of E a through ScN alloying in the theoretical study using a disordered (Al,Sc)N SS. [9] To avoid the configuration-dependent property variation, the AlN/ScN superlattice (SL), the ordered structure, was also recently studied. [10,11] However, both studies focused on the effect of strain in a fixed composition of an (AlN) 1 /(ScN) 1 SL. The problem of the concomitant reduction in the P s (and P r ) by the incorporated Sc in the (Al,Sc)N might be less severe in SL than in SS. Despite the importance of this aspect, it has not been studied yet.Therefore, in this study, we expanded the previous theoretical studies by encompassing various compositions of the (AlN) n / (ScN) m SL and examining the configuration effects by comparing them with those of the (Al,Sc)N SS, based on ab initio calculations. The composition and configuration effects on the ferroelectric behavior were explored by considering the atomic structure, phase stability, and key properties (E a and P s ) required for ferroelectric application at the same time. The calculated E a is a measure of the experimental E c , [12] and the calculated P s corresponds to the experimental P r , considering the almost ideal P-E hysteresis curve observed in (Al,Sc)N. [8,13] Both E a and P s were found to have decreased as the ScN fraction increased in the SL and SS structures. However, the degree of reduction in SL was less significant than in SS, as seen in the analysis of the discrepancies in the atomic structure and energy landscape during the ferroelectric switching. The insights from this study were used to propose a clear strategy for tailoring the ferroelectric properties by controlling the composition and structure. Results and DiscussionThe most stable phases of AlN and ScN are the WZ and rock salt (RS), respectively. Both materials have a layered hexagonal phase (H-phase) as a metastable state. The detail...

Identification of microstructural evolution of nanoscale conducting phase, such as conducting filament (CF), in many resistance switching (RS) devices is a crucial factor to unambiguously understand the electrical behaviours of the RS-based electronic devices. Among the diverse RS material systems, oxide-based redox system comprises the major category of these intriguing electronic devices, where the local, along both lateral and vertical directions of thin films, changes in oxygen chemistry has been suggested to be the main RS mechanism. However, there are systems which involve distinctive crystallographic phases as CF; the Magnéli phase in TiO2 is one of the very well-known examples. The current research reports the possible presence of distinctive local conducting phase in atomic layer deposited SrTiO3 RS thin film. The conducting phase was identified through extensive transmission electron microscopy studies, which indicated that oxygen-deficient Sr2Ti6O13 or Sr1Ti11O20 phase was presumably present mainly along the grain boundaries of SrTiO3 after the unipolar set switching in Pt/TiN/SrTiO3/Pt structure. A detailed electrical characterization revealed that the samples showed typical bipolar and complementary RS after the memory cell was unipolar reset.

Through DFT calculations, a Be0.25Mg0.75O superlattice having long apical Be–O bond length is proposed to have a high bandgap (>7.3 eV) and high dielectric constant (∼18) at room temperature and above.