세종대학교

Properties of Chalcogenides and Their Applications on the Next-Generation of Semiconductors

1.    Introduction

There is a great deal of anticipation that chalcogenides will play a crucial role in the next generation of semi-conductors and memory devices. The development of semi-conductor technology requires miniaturization of devices, high performance, and lower power consumption.  Developing new materials and devices is essential to address these needs. Chalcogenides are expected to play a key role in meeting these requirements.

 For instance, since phase change memory (PCM) uses the reversible phase change property of chalcogenides to implement a non-volatile memory, it has been recognized as a proper alternative that can surmount the limitations of existing memory technologies. Moreover, ovonic threshold switching (OTS) devices are expected to serve as selection devices for resistive switching memory cells and to address the leakage current issues of highly integrated memory arrays by using the threshold voltage (Vth) properties of chalcogenides.      

Two-dimensional transition metal dichalcogenides (TMDCs) and oxychalcogenides have received attention as a channel material for next-generation logic devices due to their high charge mobility.  Accordingly, it is expected that there will be innovative devices with various applications, such as neuromorphic devices and sensors, using these unique electrical properties of chalcogenides. 

The current widespread interest in chalcogenides is attributable to their unique properties. First, they allow for a broad range of adjustment of the electrical properties. Selecting and adjusting materials according to the functions of devices is possible since they exhibit various electrical properties, including metallic, semi-conducting, and insulating. Second, they have the properties of phase change and threshold switching. A few chalcogenides undergo phase changes between amorphous and crystalline states depending on temperature or electrical field, and it allows them to be useful for memory and switching devices. Third, they form two-dimensional structures. Certain chalcogenides can create thin 2D structures up to the atomic level, which are suitable for high-mobility electronic devices. Lastly, they allow the easier engineering adjustment of materials. Through the change of composition and structure, precise adjustments of physical and electrical properties are possible, enabling the development of customized devices.          

Figure 1. Schematic Diagram of Properties of Chalcogenides and Their Applications in Next-Generation Semiconductor Devices

This study examines the unique electrical and structural properties of chalcogenides and makes suggestions for possible research directions for the development of next-generation semi-conductor devices. 

2.    Concepts and Properties of Chalcogenides

Chalcogenides refer to the compounds containing one or more of chalcogens -- including sulfur (S), selenium (Se), and tellurium (Te) -- from group 16 of the periodic table, combined with metals or metalloids.  Although oxygen (O) also belongs to the same group, chalcogens have lower electronegativity than oxygen, leading to stronger covalent bonding than ionic bonding. In addition, their properties are distinct from those of oxides, including the presence of various oxidation states due to the involvement of d orbitals in reactions, and they are therefore classified as different compounds.[1]  

The electronic properties of chalcogenides are mainly determined by their crystal structures and bonding. In the case of crystal structures, the arrangement and symmetry of atoms affect the energy band structure, creating the electrical properties. Furthermore, the ratios and strengths of bonds, such as covalent bonds, ionic bonds, and van der Waals bonds, affect the electrical conductivity and semi-conductor properties of the materials. In particular, the phase changes and OTS properties of chalcogenides, which have recently attracted attention, are determined by the ratios of these bonding types. [2]

Figure 2. Properties of Chalcogenides. (a) Phase Transition and (b) OTS

2.1 Phase Change 

When given thermal or electrical stimuli, chalcogenides exhibit reversible phase transitions between the amorphous and crystalline states. When they are in the amorphous state, they have high residence due to the disordered arrangement of atoms. On the other hand, in the crystalline state, due to the regular and periodic arrangement of atoms, they exhibit lower resistance. [3] The mechanism of phase change involves the movement and rearrangement of atoms, leading to abrupt changes in electrical properties. Since phase changes can be made at high speeds, to the level of hundreds of nanoseconds, it enables the implementation of high-speed memory devices. In addition, thermal stability during the phase change affects data retention and the device's lifespan, which can be optimized by adjusting the composition and structure of materials. A representative phase-change material is Ge2Sb2Te5 (GST), which is widely used in phase-change memory due to its fast transition speed and stable operation. 

2.2 OTS 

OTS refers to the property of maintaining a high level of resistance at or below a threshold voltage (Vth) but rapidly changing to an extremely low level of resistance when the voltage exceeds Vth.  This nonlinear current-voltage behavior is utilized to suppress leakage currents and enhance selectivity in memory arrays. At the threshold voltage or above, the bonds of chalcogen elements shift due to the electric field, resulting in metallic behavior. Additionally, lone pairs of electrons become activated, facilitating charge movement and allowing a large current to flow. [4] Representative threshold switching materials include SiAsTe and GeSe, which are studied for their threshold switching properties in OTS devices. 

2.3 2D Chalcogenides 

As for a special type of chalcogenides, there are 2D chalcogenides. They have a layered structure with single or multiple atomic layers, and TMDC is a representative of this. TMDC is a compound containing a transition metal element (M) and a chalcogen element (X). The general chemical formula is MX2, and each layer has a structure in which M atoms are sandwiched between X atoms. Weak van der Waals forces act between the layers, forming a 2D structure.

In the band structure of TMDC, there is a direct band gap in a single layer, and it plays an important role for its optical application. When the number of layers increases, it transitions to an indirect band gap, affecting its electrical and optical properties. From the perspective of electrical properties, MoS2 and WS2 are semiconductors with a wide band gap, and they have stability and high electron mobility at high temperatures. On the other hand, TiSe2, VSe2, etc. exhibit metallic properties and can be used as electrode materials. NbSe₂, etc. show superconductivity at low temperatures and can consequently be applied to quantum devices.

In terms of charge mobility, the scattering of charge carriers gets reduced due to the 2D structure, resulting in high mobility. It enables fast switching and low power consumption. In addition, the band gap and electrical properties can be controlled through external electric fields, mechanical strain, and chemical doping, allowing for the development of customized devices in various application fields. 

3.    Semiconductor Devices Using Chalcogenides

Chalcogenides are widely used in electronics, optics, thermos-electrics, and energy devices due to their unique properties.  They are highly anticipated to play a crucial role in next-generation technologies for memory and logic ICs in the field of semiconductors.

3.1 Memory Device 

3.1.2 Phase Change Memory (PCM)

PCM, considered to be a next-generation memory device, is a nonvolatile memory that stores data using the phase change property of chalcogenide. [5] The mechanism for operations of PCM is has two movements to store information: SET applies low-current pulses onto chalcogenides to change them into a crystalline state (low resistance), and RESET applies strong-current pulses for a short period to change them into an amorphous state (high resistance). To be more specific, the SET applies low-current pulses to heat the phase-change material to a temperature of crystalline or higher but below its melting point to promote cystallization. The RESET applies strong-current pulses to heat the phase-change material to a temperature of the melting point or higher and then rapidly cools it to an amorphous state. At the time, a high current is required for the RESET, which might cause issues regarding the power consumption and thermal interference.   

From the perspective of materials and properties, GST is the most widely used due to its fast speed of phase change and stable movement. As for its strengths, they include high switching speed and stable cycle endurance, but it still could have a limitation regarding data storage due to its relatively high power consumption and low phase-change temperature. Doped GST improves thermal stability and data retention through the doping of N, C, and others. GeSb alloy is being studied as a next-generation PCM material due to its fast switching speed and low power consumption. 

The advantages of PCM include fast speed, high endurance, and multi-level storage.  

Figure 3 Phase Transition Memory Device (a) Device Structure; (b) Cross-sectional TEM Image of the Device; and (c) Electrical Properties

3.1.2 Ovonic Threshold Switching (OTS)

OTS devices have a property of threshold switching, so they could be used as selector elements to selectively access the memory cells in a resistance-variable memory array. Their operation principle is that they maintain high resistance at a threshold voltage or lower and, in turn, suppress leakage currents. When a voltage exceeds the threshold voltage, they rapidly switch to a low resistance state, allowing the current to flow. When the voltage drops to the hold voltage or lower, it returns to OFF again.  

From the perspective of materials and properties, Se-based chalcogenides reduce leakage currents with high thermal stability and a wide band gap. Te-based chalcogenides enable low-power operation due to their low threshold voltages, but they may have low thermal stability. Multi-element chalcogenides optimize their properties by combining Si, Ge, As, Se, and Te.

The strengths of OTS devices include high selectivity and fast operation speed, enabling the implementation of simple-structure memory arrays. 

Figure 4 OTS Device (a) Unit Element Stack Structure; (b) Crossbar Array Structure; and (c) Electrical Properties

3.1.3 Selector Only Memory (SOM)

A SOM device integrates selector and memory functions onto a single layer of chalcogenides, in turn simplifying the device’s structure and improving its performance. Its operation principle is to control either the voltage polarity or the current that allows for the adjustment of the threshold voltage to store data.  Then it maintains its state to act as a non-volatile memory. [6, 7]

This device allows a single layer of chalcogenides to perform both its selector and memory functions at once, and it can implement a memory device through an electrode-chalcogenide-electrode single-layer sandwich crossbar array structure. The development of this device is based on the use of the threshold voltage shift phenomenon that occurs in OTS devices as a memory property.  It is similar in many ways to OTS technology.

In terms of materials and manufacturing technologies, GeSe-based chalcogenides are suitable for SOMs due to their low threshold voltage and high durability, and currently, Te-group chalcogenides, such as SiGeAsTe, are also being explored. 

The advantages of SOMs include structural simplicity, fast operation speed, energy efficiency, and long device life. Especially, due to its low operating voltage compared to PCM, there is less thermal interference with surrounding memory cells, which is advantageous for achieving high integration. Since high-temperature operation and material movement are not involved, there is less deterioration of the device caused by operation. 

Figure 5. SOM Device (a) Structure; (b) Cross-Sectional TEM Image; and (c) Electrical Properties

3.2 High Mobility TFT for Logic IC 

High-mobility thin-film transistors (TFTs) play a key role in displays, sensors, and logic devices, and their performance can be improved by using chalcogenides as channel materials. 

The most representative TFT is MoS2. It has a honeycomb structure in a single layer and is of a tri-layer of S-Mo-S. In terms of its electrical properties, it has a direct band gap in a single layer and exhibits high electron mobility (up to 200 cm2/Vs or more). Such high charge mobility enables fast switching and low power consumption, and due to its thinness, mechanical flexibility, and high optical transmittance, it can be used for flexible displays and wearable sensors. 

4. Challenges and Recent Studies 

4.1 Challenges and Current Studies on Memory Devices  

PCM still has several challenges to overcome, including power consumption, thermal stability, and integration, despite its outstanding properties. In particular, a high current is required during the RESET, which causes heat, leading to heat interference and deterioration of stored data and device lifespan.  However, as the integration of devices increases, the thermal interference problem is becoming more severe.

OTS devices have challenges such as threshold voltage control and durability improvement. Depending on the operation of the device, Vth movement occurs, so there must be new materials to prevent it, Also, durability must be enhanced so that the properties are maintained even during repeated switching. 

SOM’s operation principle is relatively free of the heat generation problem of PCM and the Vth movement issue of OTS. However, there should be academic explorations to enhance the stability of threshold voltage and durability in order to optimize the properties of materials. 

In recent studies, there have been attempts to explore the optimization of material composition, develop deposition technologies, and enhance reliability. In terms of optimization of material composition, GeSe-based materials have been studied to optimize threshold voltage and durability by adjusting the ratio of Ge and Se, and studies have also been conducted to improve thermal stability and switching characteristics by combining elements such as Si, Ge, As, and Se as multi-element alloys. [8]. 

In terms of device structure, research is being conducted to develop high-density memory arrays and improve storage capacity by implementing 3D stacking through vertical structures (VSOM). In terms of deposition technology, selective ALD is being utilized to selectively form thin films and facilitate the implementation of 3D structures. For this, it is important to maintain an amorphous state through low-temperature deposition.

In this regard, ALD of chalcogenides has been studied for the process of producing amorphous GeTe, GeSe, SbTe, and GST thin films through ligand-driven exchange reactions of chalcogen silicon compound precursors, and accordingly, its applicability to VSOM devices is being studied. [9-12]

Figure 6. Future Assignments for SOM Devices

4.2 Challenges and Current Studies on High Mobility TFT

In recent studies, there have been several attempts to enhance the surface conditions, develop the heterojunction structures, and improve device stability, among others. 2D semiconductors do not bond in the vertical direction, making it difficult to connect metal electrodes. To solve this problem, chemical treatment of the surface and insertion of interface layers are under examination. 

In terms of developing the heterojunction structures, when a 3D dielectric film is connected to a 2D semiconductor material such as TMDC, an electronic state is formed at the interface, which can deteriorate the properties of the 2D semiconductor material, so researchers are exploring how to integrate 2D dielectric films such as h-BN.

In terms of improving the device stability, it is necessary to develop protective layers and others to complement the easily oxidized characteristics and secure stable operating performance.

In this context, Bi2O2Se and Bi2SeO5 have been examined. They are 2D chalcogen compounds with a layered structure, in which metal oxide layers and chalcogen layers exist repeatedly, and the layers interact with each other through van der Waals forces. Bi2O2Se is a semiconductor material with high electron mobility, and Bi2SeO5 can be utilized as a high-k thin film without degradation of channel characteristics. [13] These materials had only been possible to grow by CVD on STO or Mica single crystal substrates that are not used in exfoliation or semiconductors, but recently, ALD deposition on SiO2 substrates have been reported. [14] 

5. Conclusion

Chalcogenides shed light on the innovations in semiconductor devices due to their unique and distinctive electrical properties and diverse structure. With consistent efforts on the academic research and development of technologies, chalcogenides have soared to a core material for next-generation semi-conductors and are highly expected to play a key role in the information society of the future. These academic efforts would promote the development of semiconductor devices of high-performance and low power consumption, the key elements of up-coming technologies, such as artificial intelligence, IoT, and autonomous driving, and these lead to innovative changes to the entire industry, as well as our daily lives.  

6. References

[1]    F. Jellinek, "Transition metal chalcogenides. relationship between chemical composition, crystal structure and physical properties," Reactivity of Solids, vol. 5, no. 4, pp. 323-339, 1988, doi: 10.1016/0168-7336(88)80031-7.

[2]    D. Lencer, M. Salinga, B. Grabowski, T. Hickel, J. Neugebauer, and M. Wuttig, "A map for phase-change materials," Nat. Mater., vol. 7, no. 12, pp. 972-977, 2008, doi: 10.1038/nmat2330.

[3]    A. V. Kolobov, P. Fons, A. I. Frenkel, A. L. Ankudinov, J. Tominaga, and T. Uruga, "Understanding the phase-change mechanism of rewritable optical media," Nat. Mater., vol. 3, no. 10, pp. 703-708, 2004, doi: 10.1038/nmat1215 http://www.nature.com/nmat/journal/v3/n10/suppinfo/nmat1215_S1.html.

[4]    M. Zhu, K. Ren, and Z. Song, "Ovonic threshold switching selectors for three-dimensional stackable phase-change memory," MRS Bull., vol. 44, no. 9, pp. 715-720, 2019, doi: 10.1557/mrs.2019.206.

[5]    S. R. Ovshinsky, "Reversible Electrical Switching Phenomena in Disordered Structures," Phys. Rev. Lett., vol. 21, no. 20, p. 1450, 1968. doi: 10.1103/PhysRevLett.21.1450.

[6]    S. Hong et al., "Extremely high performance, high density 20nm self-selecting cross-point memory for Compute Express Link," in 2022 International Electron Devices Meeting (IEDM), 3-7 Dec. 2022 2022, pp. 18.6.1-18.6.4, doi: 10.1109/IEDM45625.2022.10019415. 

[7]    I. M. Park et al., "Enhanced Endurance Characteristics in High Performance 16nm Selector Only Memory (SOM)," in 2023 International Electron Devices Meeting (IEDM), 9-13 Dec. 2023 2023, pp. 1-4, doi: 10.1109/IEDM45741.2023.10413748. 

[8]    T. Ravsher et al., "Polarity-Induced Threshold Voltage Shift in Ovonic Threshold Switching Chalcogenides and the Impact of Material Composition," phys. status solidi (RRL) – Rapid Research Letters, vol. 17, no. 8, p. 2200417, 2023, doi: https://doi.org/10.1002/pssr.202200417.

[9]    V. Pore, T. Hatanpää, M. Ritala, and M. Leskelä, "Atomic Layer Deposition of Metal Tellurides and Selenides Using Alkylsilyl Compounds of Tellurium and Selenium," J. Am. Chem. Soc., vol. 131, no. 10, pp. 3478-3480, 2009, doi: 10.1021/ja8090388.

[10]    T. Eom et al., "Conformal Formation of (GeTe2)(1–x)(Sb2Te3)x Layers by Atomic Layer Deposition for Nanoscale Phase Change Memories," Chem. Mater., vol. 24, no. 11, pp. 2099-2110, 2012, doi: 10.1021/cm300539a.

[11]    T. Eom et al., "Combined Ligand Exchange and Substitution Reactions in Atomic Layer Deposition of Conformal Ge2Sb2Te5 Film for Phase Change Memory Application," Chem. Mater., vol. 27, no. 10, pp. 3707-3713, 2015, doi: 10.1021/acs.chemmater.5b00805.

[12]    S. Yoo, C. Yoo, E.-S. Park, W. Kim, Y. K. Lee, and C. S. Hwang, "Chemical interactions in the atomic layer deposition of Ge–Sb–Se–Te films and their ovonic threshold switching behavior," J. Mater. Chem. C, vol. 6, no. 18, pp. 5025-5032, 2018, doi: 10.1039/C8TC01041B.

[13]    T. Li and H. Peng, "2D Bi2O2Se: An Emerging Material Platform for the Next-Generation Electronic Industry," Accounts of Materials Research, vol. 2, no. 9, pp. 842-853, 2021, doi: 10.1021/accountsmr.1c00130.

[14]    H. Park et al., "Direct Growth of Bi2SeO5 Thin Films for High-k Dielectrics via Atomic Layer Deposition," ACS Nano, vol. 18, no. 33, pp. 22071–22079, 2024, doi: 10.1021/acsnano.4c05273.