PacSurf2022 Session TF-WeP: Thin Films Poster Session

The precision achievable in XPS is very good. Accurate quantitation from relative peak intensities is more difficult (1). Normalizations for photoionization cross-sections, σ, and variation of analysis depths, 𝜆, are required, but the procedures are well understood. Separation of the intrinsic photoelectron spectrum from its associated scattered electron background is on less secure ground, as there are several approaches, and implementation requires using the software of the instrument vendor or commercial analysis package vendor. The situation is most difficult when an XPS core level “peak” consists of strong overlapping structure spread over a wide BE range, such as with Fe 2p and O1s in Fe₂O₃ (2). We have examined, for a single crystal sample, the implementation of different background removal procedures: Tougaard (U2 version), T, Shirley, S, Linear, and total signal. The choice of the high BE endpoint, implying there is no further intrinsic signal beyond that BE, is most important. It is possible to adjust endpoints to return the “expected” answer, 40% atomic Fe, but this is somewhat arbitrary, and assumes 100% accuracy of the relative σ's, and relative 𝜆's, in addition to assuming a) that the Transmission Function of the instrument is correct and b) for our particular case here (and much of the prior literature!), that a small signal from OH is properly accounted for in the analysis. The best consistency is obtained by a compromise between a wide enough range (start to end points) of background removal to include all observable substructure associated with Fe 2p and with O1s, while keeping the BE range very similar for Fe2p and O1s. If such a situation is achievable, there is only a small difference between using T, or S, backgrounds. Even a linear background, or no removal at all, will return values within 10% of the T or S derived Fe %age values. The reason for the close agreement of T and S compositions, despite the huge difference in the amounts of backgrounds removed, is that the background is proportional to the signal generating it, so provided the same removal procedure is adopted for both Fe2p and O1s, the functional form of the removal is of secondary importance. It is important to note that the analyst should never pick ranges where substructure is included for one of the elements, but not the other.

References

1. C. R. Brundle and B. V. Crist, J. Vac. Sci. Tech A, 38(4) Jul/Aug 2020; doi:10.1116/1.5143897
2. P. S. Bagus, C. J. Nelin, C. R. Brundle, B. V. Crist, N. Lahiri, and K. M. Rosso, Phys. Chem. Chem. Phys., 2022, 24, 4562-4575.

In this study, we suggest channel array transistor (BCAT) structure with dual insulator(HfO₂ and SiO₂) to solve short channel effect(SCE) in the cells of dynamic random-access memory (DRAM) using 2D TCAD simulations BCAT is widely used to make high performance sub 30nm DRAM cell transistors. However, as the device size becomes smaller, device characteristics deteriorate due to SCE. We changed the single insulator currently used for BCAT to dual insulator to improve SCE characteristics. The electrical properties were analyzed according to the thickness changes of HfO₂ and SiO₂ while the total thickness of the insulator is equal. As a result, the higher the HfO₂ layer ratio, the better the value of Threshold voltage, Drain induced barrier lower (DIBL), body effect and swing, 25.2%, 39.8%, 20.9% and 7.0% respectively. HfO₂ with Highly dielectric permittivity would have increased electronic polarization in the insulator and reduced changes by external stress other than gate voltage. Finally, we also discuss disadvantages such as the insulator breakdown to suggest optimal device design scheme in terms of insulator.

Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) have been extensively studied because of their applicability to next-generation display. Because SiO₂ exhibits excellent insulating properties and uniformity, the doped Si and thermally grown SiO₂ are generally used as a gate electrode and a gate insulator in the conventional AOS TFTs, respectively. However, the thermally grown SiO₂ cannot be used as the gate insulator in AOS TFTs of the next-generation display since they are required to be transparent in the visible light region and compatible to the flexible substrate.

Therefore, in this study, we fabricated indium-zinc oxide (IZO) TFTs with HfO₂/Al₂O₃ gate insulator deposited by low temperature atomic layer deposition (ALD). Because Al₂O₃, a representative high-k dielectric material, has a wide bandgap (~7.5 eV), low leakage current can be obtained when used for TFTs as a gate insulator. However, since Al diffusion into the channel layer occurs easily due to the small ionic radius of Al cations, an HfO­₂ high-k dielectric layer was deposited as a buffer layer to reduce the defects at the channel-insulator interface. Also, the high-k dielectric insulator has the advantage of reducing power consumption by lowering the driving voltage of the fabricated TFTs.

Fig. 1 represents the cross-sectional schematic diagram of IZO TFTs with HfO₂/Al₂O₃ gate insulator. The Al₂O₃ gate insulator and HfO₂ insulator buffer were deposited on a heavily doped p-type Si substrate by low temperature ALD at 120 ℃ using tri-methyl-aluminum (TMA), tetrakis [ethyl-methyl-amino] hafnium (TEMAHf) and H₂Oas an Al precursor, a Hf precursor and an oxidant, respectively. The IZO channel layers and S/D electrodes of the fabricated TFTs were deposited by radio-frequency (RF) magnetron sputtering and thermal evaporator, respectively. The RF magnetron sputtering was conducted with IZO (In: Zn = 90 wt.%: 10wt.%) target in R.T. and 2.0 x 10^-3 Torr. After depositing the channel layers, annealing was performed using a hot plate at 250 ℃ for 1h. The fabricated TFTs were analyzed with a semiconductor parameter analyzer (Elecs Co. EL423).

Fig. 2 shows (a) output curves and (b) transfer curves of IZO TFTs with HfO₂/Al₂O₃ gate insulator. The electrical properties of the fabricated TFTs exhibited 6.02 cm² / Vs of carrier mobility (μ_sat), 1.18 x 10⁶ of on-off current (I_ON/OFF), 0.55 V of threshold voltage (V_th), and 0.31V/dec of subthreshold swing (SS). Afterward, to improve the electrical properties, we are due to optimize the IZO TFTs with HfO₂/Al₂O₃ gate insulator by varying the thickness of insulator, the thickness ratio of Al₂O₃ and HfO₂, and growth temperature, respectively.

Very thin surface coatings in the range of a few nanometers are very challenging to analyze. Identification and knowledge of the distribution of molecular species within these surface coatings are very important for the final performance of most materials. Such surface information can be obtained using advanced surface analysis techniques such as tandem (MS/MS) time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and 3D profilometry. In addition, the gas cluster ion beam (GCIB) sputter source provides new opportunities for the analysis of such thin organic coatings.

In this work, the analysis of very thin organic surface coatings is presented. These organic coatings formed (self-assembled) in a corrosive chloride solution on a brass surface. The molecular conformation was completed using tandem ToF-SIMS capability by describing the molecular fragmentation mechanisms of the various precursor ions. Moreover, organometallic complexes were identified on the surface, which formed between the metal ions released due to corrosion and the organic molecules. Such analyses are still very rare and new to the SIMS database in general. After obtaining the ToF-SIMS signals describing the molecule, 3D distribution analysis was performed using GCIB sputtering associated with 3D ToF–SIMS imaging. The chloride was found to be located below the coating, indicating that the formation of metal chlorides is faster than the adsorption of the organic molecules. The latter was also confirmed by the GCIB-XPS analyses using Ar cluster sputter beams at different acceleration energies and cluster sizes. Finally, surface topography and agglomeration of the molecules in the surface coating were demonstrated by AFM (smaller surfaces) and 3D profilometry (larger surfaces).

References

[1] FINŠGAR, Matjaž. Time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy study of 2-phenylimidazole on brass. Rapid communications in mass spectrometry, 2021, vol. 35.

[2] FINŠGAR, Matjaž. The influence of the amino group in 3-amino-1,2,4-triazole corrosion inhibitor on the interface properties for brass studied by ToF-SIMS. Rapid communications in mass spectrometry, 2021, vol. 35.

[3] FINŠGAR, Matjaž. Advanced surface analysis using GCIB−C₆₀⁺⁺-tandem-ToF-SIMS and GCIB-XPS of 2-mercaptobenzimidazole corrosion inhibitor on brass. Microchemical journal, Volume 159, December 2020, 105495

The transition metal dichalcogenides (TMDC) form a group of layered, highly anisotropic compounds which exhibit interesting and unusual physical properties. Among these TMDC materials, WSe₂ has been a subject of great interest. Besides common characteristics of TMDC family i.e., van der Waals layered structure, indirect-to-direct bandgap transition in the monolayer regime, and spin-valley coupling, WSe₂ also constitutes a high quantum yield in 2D system and can be synthesized on a large area by chemical vapor deposition, opening various potential applications. To meticulously design and understand 2D optoelectrical device’s function correctly, the optical constants of monolayer WSe₂ are needed. Although there are a few studies on the dielectric functions, systematic study on temperature dependence of critical points (CPs) of monolayer WSe₂ has not been reported, yet. In this work, we report the dielectric function of monolayer WSe₂ from 0.74 to 6.42 eV at temperatures from 40 to 350 K using dual rotating compensators ellipsometry. The sample is a large area WSe₂ thin film grown on sapphire substrate by low pressure chemical vapor deposition. The sample's quality was confirmed by AFM, Raman, photoluminescence spectra, and spectroscopic ellipsometry. The CP energies were determined by standard lineshape analysis of numerically calculated second derivatives of ε with respect to energy. Several CPs are distinguished at low temperature where the CPs are blue shifted and sharpened as a result of the reduced lattice constant and electron-phonon interaction. Especially, by carefully examining the region from 1 to 2 eV, the existence of three peaks can be diagnosed, which can be identified by the combination of a neutral exciton A, a negatively charged exciton Aˉ, and a superposition of biexciton emission (AA) with defect-bound exciton emission (L1). The B-exciton structure also shows a significantly asymmetric lineshape, indicating contributions of at least two CP structures. These results will be useful for physical understanding and application for the device based on WSe₂.

The demand of highly efficient and deformable flexible organic light-emitting diodes (FOLEDs) has recently increased drastically with the rapid advance of wearable devices. To develop a high-performance FOLEDs, the development of flexible transparent conductive electrodes (FTCEs) and emitting materials is essential. Mesh-structured electrodes are one of the promising FTCEs as substitute of indium tin oxide [1,2], owing to their excellent optoelectrical properties with highly deformable mechanical properties. Despite such remarkable properties, a few characteristics other than transmittance or deformability must be considered to practically use mesh electrodes for demonstration of high-performance FOLEDs [3]. One of the intrinsic problems is the bumpy surface of mesh pattern, which induces leakage current or charge accumulation. Yet, PEDOT:PSS or SU-8 are used for planarize the rough surface however, these layers are lack of conductivity, which increases the turn-on voltage of FOLEDs [4]. In addition to the rough surface issue, the work function mismatch between the FTCE and organic transport or injection layers deteriorate the charge injection efficiency.

In addition to the development of FTCEs, it is hard to get high-performance of FOLED without the development of emission layer. To date, thermally activated delayed fluorescence (TADF) emitters have shown improved device performance however, few research have been focused on verifying the stability of the materials.

Herein, high-performance and flexible blue TADF OLEDs are proposed using nickel (Ni)-doped zinc oxide (ZnO)/silver (Ag)/ZnO mesh (Ni:ZAZ) electrode and a boron-based TADF emitter. The Ni doping is conducted through co-sputtering process of ZnO and Ni dopants. As shown in Fig. 1, the Ni:ZAZ mesh electrode shows remarkably high optical transparency (>90%), and low sheet resistance (<10 Ω sq^–1), together with superior flexibility. Addtionally, the increase of work function (4.8 eV -> 5.1 eV) is observed via surface doping. Notably, the similar (or enhanced) optical and electrical property is observed after Ni doping, which indicates the effective penetration of Ni dopant to the surface of ZnO. The Ni:ZAZ mesh electrode is then applied as an anode of flexible OLEDs with no additional layers for planarization.

We also synthesized a boron-based deep blue TADF dopant material in powder for thermal evaporation system, and co-evaporated with the host material. As a result, the proposed FOLEDs exhibited superior device efficieny, which is attributed to the enhanced opto-electrical properties of Ni:ZAZ mesh electrode and highly efficient boron-based TADF emitter material.

As computing technologies such as big data, artificial intelligence and machine learning develops, the development of memory device that stores and processes vast amounts of data is also required. From various performance of memory device, phase change memory (PCM) is the powerful candidate for next-generation memory. Ge₂Sb₂Te₅ (GST-225) based PCM has outstanding performance such as non-volatility, long write endurance and high inherent scalability with existing complementary metal-oxide-semiconductor process. Despite its advantages, the GST-225 based PCM is hindered by reliability such as low switching speed, low crystalline temperature, and resistance drift.

In this study, we propose GeSb-based phase change memory device that can obtain a fast switching speed, owing to the fragile Sb-Sb bond while excluding Te atoms that degrade repeatability of memory operations due to high vapor pressure and low melt point. In addition, we insert the NiOx layers on the GeSb layer to generate the thermal boundary resistance (TBR) at the interface of the GeSb layer and the NiOx layer. The superlattice-like NiOx/GeSb multilayer film has the higher crystallization temperature than the single layer of GeSb and suppresses heat dissipation by TBR to efficiently utilize the heat required for phase change in GeSb layer. The adding of NiOx can improve the stability of GeSb material and lower power consumption phase change memory devices. The results of the I-V sweep from PCM devices with different structures.The threshold voltage of the NiOx/GeSb superlattice PCM device is 1.6 V, which shows significantly lower power than the GeSb-based PCM device (~ 3.4 V). Consequently, NiOx/GeSb multilayer might show low power consumption behavior on pulse operations and thermal stability by inserting NiOx layers.