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Resistive switching memory has been studied actively for next-generation computing. Several binary oxides, such as HfO2 and Ta2O5, are widely adapted as resistive switching layers; however, there are several limitations to obtaining an energy- and area-efficient operation with sufficient reliability. Rare-earth oxide materials exhibit interesting resistive switching properties because of their reactivities with active anion species. In this study, atomic-layer-deposited GdOx films were investigated as an active switching layer for resistive switching memory. Post-deposition annealing of as-grown GdOx films enhanced the tunability of the resistance states, which can be useful for the multilevel operation of nonvolatile memory and neuromorphic computing applications. The effects of the crystallization and hygroscopic nature of the GdOx film are investigated for elucidating the change in the resistive switching characteristics and its underlying mechanism.

AlN thin films were deposited with different thicknesses (5 and 20 nm) by atomic layer deposition and we explored the electron transport mechanisms in AlN/GaN Schottky junctions. Higher ideality factor and higher m values were observed in the current–voltage plots when the thickness of the AlN was 5 nm. The reverse bias leakage current for 5-nm-thick AlN was explained based on the Poole–Frenkel emission. Capacitance–voltage analysis revealed a higher interface state density for 5 nm thick AlN. These results might be associated with the defective interfacial layer present near the AlN/GaN interface. Because of the strain relaxation, the upper region of the AlN layer was degraded in the case of 20-nm-thick AlN, which may have strongly affected the interfacial properties of the Pt/AlN junction strongly.

Resistive switching memory or memristors have been extensively studied worldwide, driven by perspective applications in nonvolatile memory, processing-in-memory, and neuromorphic computing hardware. Binary oxides are the most prevalent memristive materials; however, non-oxide materials can also exhibit resistive switching. In this study, ternary nitride alloy memristors were fabricated using AlGaN films and were investigated. AlGaN memristors exhibit highly linear and repeatable resistive switching characteristics. Structural and chemical analyses indicate a plausible mechanism of the conduction channel formation in the ternary nitride memristors.

Resistive random access memory (RRAM) has been developed as a next-generation nonvolatile memory because of its fast operation speed, low power consumption, high density, and simple structure. Non-oxide materials such as AlN-based RRAM also exhibit low operation energy and large on/off ratios. However, AlN-based RRAM may deteriorate upon oxidation when exposed to air/moisture. In addition, chemical reactivity between the electrode and the switching layer material affects device stability. In this study, four kinds of top electrode materials (Al, Ti, TiN, and Pt) were used in an AlN/TiN stack and a water-resistant encapsulation layer was used to prevent the degradation of AlN-based RRAM. The electrical properties of the device were measured at weekly intervals for 7 weeks. The devices containing Al and Ti top electrodes showed degradation of resistance states despite being encapsulated in a thin Al2O3 layer. In contrast, the devices with TiN and Pt electrodes maintained their resistance states and switching properties regardless of the encapsulation layer. These trends in degradation can be explained by the electrode and AlN reactivity with moisture based on fundamental thermodynamics.

The electrical and interfacial properties of HfO2/Al2O3 and Al2O3/HfO2 dielectrics on AlN/p-Ge interface prepared by thermal atomic layer deposition are investigated by capacitance–voltage(C–V) and current–voltage(I–V) measurements. In the C–V measurements, humps related to mid-gap states are observed when the ac frequency is below 100 kHz, revealing lower mid-gap states for the HfO2/Al2O3 sample. Higher frequency dispersion in the inversion region is observed for the Al2O3/HfO2 sample, indicating the presence of slow interface states A higher interface trap density calculated from the high-low frequency method is observed for the Al2O3/HfO2 sample. The parallel conductance method, applied to the accumulation region, shows border traps at 0.3~0.32 eV for the Al2O3/HfO2 sample, which are not observed for the Al2O3/HfO2 sample. I–V measurements show a reduction of leakage current of about three orders of magnitude for the HfO2/Al2O3 sample. Using the Fowler-Nordheim emission, the barrier height is calculated and found to be about 1.08 eV for the HfO2/Al2O3 sample. Based on these results, it is suggested that HfO2/Al2O3 is a better dielectric stack than Al2O3/HfO2 on AlN/p-Ge interface.

We investigated the electrical and interfacial properties of AlN/4H-SiC interfaces prepared by atomic layer deposition (ALD). The conductance–voltage (G/ω–V) characteristics were identified based on two types of traps: fast and slow. Fast traps located in the semiconductor energy gap close to the AlN/SiC interface showed both narrow and wide energy distributions. Slow traps with defects in the AlN layer exhibited a narrow energy distribution. According to X-ray photoelectron spectroscopy analysis, the formation of AlN during the initial ALD deposition was unclear and some of the oxygen atoms present on the SiC surface were converted into Al2O3. Both O 1s and Al 2p core level spectra shifted to lower energies as the AlN thickness increased. This was associated with the strain relaxation originating from the lattice mismatch between AlN and SiC.

The electrical and interfacial characteristics of AlN/GaN and AlGaN/GaN heterostructures prepared by thermal atomic layer deposition (ALD) were investigated in this work. According to the parallel conductance method, the AlGaN/GaN sample showed two border traps with energies of ~ 0.38 and ~ 0.46 eV, which were not observed for the AlN/GaN sample. The AlN/GaN sample also showed a lower interface trap density and a lower reverse leakage current. X-ray photoelectron spectroscopy analysis indicated that during the initial ALD deposition process, most of the Al and N atoms tended to combine with oxygen atoms as opposed to Ga atoms. As a result, AlGaN layer formation was delayed, resulting in the creation of a defective interfacial layer at the AlGaN/GaN interface. Thus, our results indicate that the initial ALD process should focus on removing the defective interfacial layer during AlGaN deposition for device optimization.

Resistive random-access memories (RRAM) have been extensively studied because of their advantages such as low operating voltage, high reliability, and simple structure. Among the various types of materials for RRAMs, such as oxides, nitrides, sulfides, and chalcogenides, AlN shows resistive switching phenomena with low energy and high speed via the formation of Al-rich conducting channels owing to the generation of nitrogen vacancies. Moreover, AlN has a large band gap (~ 6.2 eV), high thermal conductivity, and dielectric constant. Therefore, AlN can find application as a gate dielectric material, functional layer, and resistive switching layer for RRAM applications. In this study, AlN thin film is deposited by thermal atomic layer deposition (ALD), which is a self-limiting technique through ligand exchange between the precursor molecules and surface functional groups. We use trimethylaluminum (TMA) and NH3 as the metal precursor and reaction gas, respectively. We obtain growth rates of 0.05–0.16 nm/cycle at a wafer temperature of 274–335 °C. Structural and chemical properties of the AlN films grown at various temperatures are investigated by X-ray diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The electrical properties of these AlN films are studied by fabricating the devices having an Al/AlN/Pt stack.

The interfacial and electrical properties of atomic layer deposited Gd2O3 with an AlN layer on n-GaN were investigated. According to X-ray photoelectron spectroscopy spectra, the formation of Ga–O bonds that is significant near the Gd2O3/GaN interface was suppressed near the AlN/Gd2O3/GaN and Gd2O3/AlN/GaN interfaces. Larger amounts of oxygen atoms across the dielectric layers were observed for AlN/Gd2O3/GaN and Gd2O3/AlN/GaN junctions, which in turn produced the dominant peak corresponding to O–Al bonds. The flatband voltage shift in capacitance–voltage hysteresis characteristics was highest for the Gd2O3/AlN/GaN junction, indicating the highest interface and oxide trap densities. In addition, AlN/Gd2O3/GaN and Gd2O3/AlN/GaN junctions showed the highest interface state densities in the energy ranges of 0.1–0.2 eV and 0.4–0.6 eV, respectively. The reverse leakage currents were explained by Fowler–Nordheim (FN) for Gd2O3/GaN and AlN/Gd2O3/GaN junctions and by trap assisted tunneling (TAT) for the Gd2O3/AlN/GaN junction.

AlGaN films with high Al content (Al/Ga ∼5.5) were successfully grown via thermal atomic layer deposition at low temperature (342 °C) using trimethylaluminum and triethylgallium as Al and Ga precursors, respectively, and ammonia as reactant gas. Incorporation of GaN into AlN is evidenced by the dependence of the growth rate on the pulse ratio of AlN and GaN subcycles. Chemical analysis reveals the composition of the AlGaN film and the existence of GaN chemical bonding state irrespective of the pulse ratio. Although the chemical composition of AlGaN film was little affected by the pulse ratio between AlN and GaN cycles, the electrical properties of the films could be modulated. Layer-by-layer growth with close to theoretical dielectric constant could be achieved by the introduction of sufficient number of AlN subcycles. Density functional theory calculations were utilized to assess the surface reaction mechanism of the Al and Ga precursors and ammonia reactant during deposition, which show smaller reactivity of the Ga precursor compared to that of Al would affect the doping ratio of the ALD AlGaN films.

Phase change memory (PCM) is a next-generation memory storing data with a large difference in resistance between crystalline and amorphous of phase change materials. PCM has outstanding properties such as scalability, high operation speed and non-volatility. However, conventional PCM has some issues to overcome, such as high reset current and set/reset asymmetry. Among them, poor adhesion between the phase change materials and SiO2, interfacial layer dielectric, which can cause problems during memory operation. Also, reset behavior takes higher current consumption due to its melt-quenching process, so improving the thermal efficiency is necessary. The thermal conductivity and interfacial properties between the heater and the phase change materials affect nucleation, so that electrical properties may change by inserting the interfacial layer. In this study, we examine the effects of the interfacial layer between GeTe and heater on crystallization and electrical properties. 1.5nm-thick HfO2 layer was deposited with atomic layer deposition (ALD) on the bottom electrode contact surrounded by SiO2, and then 20nm-thick GeTe was deposited thereon with ALD. The diameter of bottom electrode contact is varied from 120 to 2000 nm. TiN as top electrode was grown by sputtering. The electrical properties were characterized with varying pulse width and bias polarity. GeTe-based PCM exhibits the abrupt set and reset switching, while more gradual transition is observed in the device with HfO2 insertion layer. Additionally, GeTe film was not grown without insertion layer, whereas, ALD growth of GeTe film on HfO2 layer was facile with improved adhesion. As a result, HfO2 insertion layer affects both adhesion and the electrical properties of PCM.

Phase change memory (PCM) has been developed as one of the next generation memory candidates due to its high read/write speeds, high densities, and non-volatile characteristics. This memory uses the phase change of the chalcogenide materials by Joule heating. However, there is a problem of requesting high reset current, thereby increasing power consumption during melt-quench process for its amorphization. As a way to solve the high reset current, interfacial PCM (iPCM) was suggested. It is based on the metastable states formed in superlattice-like GeTe/Sb2Te3 stack of high quality. This PCM operates with a reversible structural change between high and low resistance states without melt-quench process. However, if the heating from electrode is too excessive during the iPCM operation process, unwanted melting can occur and iPCM characteristics can be lost. Therefore, it is necessary to study the thermal effects of superlattice-like phase change material and electrode in order to ensure stability in conventional PCM or iPCM operation. In this study, we conduct time-domain thermoreflectance study on the molecular-beam-epitaxially grown phase change materials; GeTe and Sb2Te3. Phase change layer was consisted of single or superlattice-like structure. TiN metal layer was used to see the thermal boundary resistance between the metal and phase change materials. Au metal layer was deposited as a photothermal transducer layer due to its stability and thermoreflectance coefficient value. Thermal conductivities of signle and superlattice-like thin films are compared to analyze the thermal effects. In addtion, heat conduction between the metallic TiN and phase change material of high quality is considered.

The interfacial and electrical properties of atomic layer deposited Gd

In this work, we investigated the interfacial properties of AlN/GaN heterostructure with HfO2/Al2O3 (single bilayer) and HfO2/Al2O3/HfO2/Al2O3 (double bilayer) dielectrics prepared by atomic layer deposition (ALD). From capacitance–voltage measurements, significant frequency dispersion was observed for the single bilayer. The interface traps for the double bilayer showed the exponential dependence of trap time constant to the applied voltage, indicating the high quality and uniform interface, whereas the single bilayer deviated from this dependence, revealing the non-uniformity of oxide charges. According to current–voltage measurements, the double bilayer showed much lower leakage current than the single bilayer. X-ray photoelectron spectroscopy measurements showed that Hf–Al–O bonding formed wider region for the double bilayer. For the single bilayer, the outdiffusion of Ga atoms into the AlN layer and the formation of Al–OH were observed more significantly. This work indicates that the double bilayer is a promising dielectric on the AlN/GaN heterostructure.

The interfacial properties of AlN/GaN heterostructures with different dielectric layers such as Al2O3, HfO2, and HfO2/Al2O3 prepared by atomic layer deposition were investigated. Interface state density versus energy level plots obtained from the Terman method revealed the peculiar peaks at ∼0.25 eV for the samples with Al2O3 and HfO2/Al2O3 and at ∼0.52 eV for the sample with HfO2, associated with nitrogen vacancy-related defects. According to the parallel conductance method, both the interface and border traps were observed for the sample with Al2O3. However, the border traps were not observed with including an HfO2 layer. The lowest interface trap density and the reverse leakage current were obtained for the sample with an HfO2/Al2O3 bilayer. Analysis of x-ray photoelectron spectroscopy spectra obtained from the HfO2 layers showed the formation of Hf–Al–O bonding for the sample with HfO2 while such formation was not observed for the sample with HfO2/Al2O3. These results indicate the superior interfacial quality of AlN/GaN heterostructures with an HfO2/Al2O3 bilayer.