Germanium Telluride: A Chalcogenide Phase Change Material with Many Possibilities

Abstract

Germanium telluride (GeTe) is a chalcogenide phase change material which is nonvolatile and changes its phase from amorphous state to a highly conductive crystalline state at approximately 180–230°C temperature, dropping the material’s resistivity by six orders of magnitude. These temperature-induced states lead to different physical and chemical properties, making it a suitable candidate for optical storage, reconfigurable circuit, high-speed switching, terahertz (THz), and satellite applications. Besides, GeTe-based devices offer complementary metal oxide-semiconductor (CMOS) compatibility and simplified, low-cost fabrication processes. In this chapter, three applications of GeTe will be discussed. They are as follows: (1) how GeTe can be utilized as DC and RF switching material with their high OFF/ON resistivity ratio, (2) how GeTe can contribute to current THz technology as split-ring resonators and modulators, and (3) effect of threshold voltage on GeTe for reconfigurable circuits.

1. Introduction

Switching plays the most significant role in circuit technology. Phase change material (PCM) such as germanium telluride (GeTe)-based switching can provide reliable switching performance with fewer mechanical components and higher device lifetime. When an external stimulus is provided to GeTe, it can behave like both conductors and insulators which are dependent on that external stimulus. This phenomenon can be used as DC switching mechanism [1]. It is crucial to study the properties of both amorphous and crystalline states of selected materials, different component geometries, and perfect amount and type of external stimulus, etc. GeTe is a promising candidate for RF switching applications because of its suitable and small resistivity in crystalline phase. Uniform heating-based phase transition is needed to attain RF switching which provides massive resistance change. Phase transition done by voltage pulses can provide the above advantage. When current pulse is provided for long time, induced heat goes above the crystallization temperature and recrystallization starts in GeTe films. On the other hand, amorphous phase returns in GeTe films when an increased current pulse is provided for shorter time [2, 3].

Even though terahertz (THz) technology can play vital role in several research fields, i.e. communication, biomedical, security, etc., it is often overlooked as there is little to no simple methods available for production, detection, and modulation of THz waves. Nevertheless, there are some technologies available which can be exploited to utilize THz waves. Among those, technologies like photonic crystals, quantum cascade lasers, and metamaterials are considered to be most promising. GeTe-based resonators and modulators can be applied in these applications [4, 5]. Besides switching and THz applications, GeTe is applicable in reconfigurable devices. These devices have the ability to be used for multiple applications which are very useful and cost-effective for circuit and other applications. For a basic RC circuit, by tuning the resistivity and permittivity of phase change materials, cutoff frequency can be changed because cut-off frequency is directly related to resistance and capacitance. Resistance can be changed by controlling the geometries and chemical structures of phase change materials-based resistors fabricated within the circuits. This change can alter the state of reconfigurable devices [6]. This chapter will discuss the applicability of GeTe in these three applications.

2. DC and RF switching applications

2.1 GeTe strips for direct current (DC) switching

Here, a new DC switch design is presented where no mechanical component is required to control the current flow in GeTe wire. Micromachined parallel wire designs were fabricated to apply external electric fields across the GeTe wire. First, oxide layer was created on Si substrate. GeTe wires/strips were patterned and deposited using RF sputtering method. Traditional lift-off method provided perfect GeTe wire shape. After that, gold contact pads (using Ti adhesion layer underneath) were patterned and deposited on top of GeTe wire using evaporation method. These GeTe wires were around 200 nm thick. Using the similar method, parallel gold wires/plates were patterned and deposited around the GeTe wire. These parallel plates were used to provide electric fields. Gap between the wires were 80 and 100 μm. Figure 1 shows the design of GeTe test structure [1].

Figure 1.

Design of GeTe test structure for DC switching testing [1].

Resistivity and switching property of GeTe are highly dependent on the deposition condition. Upon application of an external stimuli, an amorphous insulating GeTe film transitions into conducting polycrystalline. As the GeTe strip is placed in between two parallel plates, electric field (E) across the device, that is generated by adding bias to the plates, can be found from the applied voltage (V) across the parallel wires and the distance (d) between them (shown in Eq. (1)). GeTe strip resistance can be determined according to the Ohm’s law. To prove the phase transition, GeTe resistivity ($\rho$) is calculated using the measured strip resistance (R) and the length (l) and cross-sectional area (A) of GeTe strip (shown in Eq. (2)). Any drastic change of resistivity due to electric field implies a phase change in the GeTe wire [1]:

$$E=\frac{V}{d}$$E1

$$R=\frac{\rho l}{A}$$E2

Electric field was created across the GeTe wires by applying voltage across the parallel plates and resistivity of GeTe wires changed. So, transition from conductor phase to insulator phase occurred. This applied electric field becomes higher and lower, respectively, when voltage is gradually increased and decreased. DC probes were connected with the gold contact pads (test pads) which gave the resistance values. Increase in resistivity was found about three to five orders of magnitude, depending on the different lengths and widths of GeTe wires. It was also found that GeTe wires, while in the volatile regime, regained their initial material phase when the external voltage was eliminated. Figure 2 shows the resistivity variation according to different applied voltages and different sizes of GeTe wires. The transition happened when the applied voltage was in between 37 to 45 V [1].

Figure 2.

Resistivity variation in GeTe wires because of external electric field [1].

2.2 GeTe-based coplanar waveguide (CPW) radio frequency (RF) switches

2.2.1 Fabrication process for GeTe RF switches

For this fabrication process, oxide and nitride layers were used as isolation layers. Some substrates had oxide layer, and some have nitride layers. Silicon substrate was thermally oxidized to make silicon dioxide (300 nm thickness) film. On the other hand, plasma-enhanced chemical vapor deposition (PECVD) was used to make silicon nitride (100 nm thickness) film. On top of these isolation layers, GeTe strips for signal line were deposited. GeTe layers were fabricated using RF sputtering method (parameters were 300 W RF power, 10 mTorr pressure, 20.1 sccm Ar gas flow for ignition, and 2 min time). The prepared GeTe films were in amorphous phase because of room temperature deposition. GeTe strips had length of 30 μm, width of 10 μm, and thickness of 100 nm. After that, bimetallic Ti/Au ground lines and bimetallic Ti/Au pads on both sides of GeTe strips were deposited using E-beam evaporation method and lift-off method. Ten-nanometer-thick Ti works as an adhesion layer for 100 nm gold layer, and it is essential to get better electrical contact. Figure 3 shows the fabricated GeTe CPW RF switching devices [2].

Figure 3.

(a) Microscopic image of a device with GeTe strip; (b) a smaller device on sapphire substrate for I-V measurement; and (c) a CPW RF switch having signal and ground lines [2].

2.2.2 Performance analysis of GeTe RF switches

Before doing RF switching testing, thermal characterization was done. Thermal characterizations were done using thermal chuck where devices were directly placed on the chuck. Figure 4 shows the thermal characterization results, and it is found that all devices show resistance change of ~six orders of magnitude. Figure 4a and b shows nonvolatile phase transitions. These devices were fabricated using oxide and nitride isolation layer, and GeTe film resistance never came back to its original values. This is an indication of permanent crystallization. Figure 4c and d shows volatile phase transitions. These devices were directly fabricated on substrates (c-Si and Al₂O₃ substrates, respectively) without any isolation layer, and they came back to their original values. Also, Figure 4c does not show ideal I-V behavior because of higher conductivity of c-Si substrate [2].

Figure 4.

Semi-logarithmic resistance versus temperature curves of GeTe: (a) on SiO₂/c-Si substrate; (b) on Si₃N₄/c-Si substrate; (c) on c-Si substrate; and (d) on Al₂O₃ substrate (LINEAR scale curves are in the insets) [2].

This heat-induced phase transition can be explained by the phenomena of heat transfer via conduction through multiple layers of materials. For GeTe on SiO₂/c-Si and GeTe on Si₃N₄/c-Si, there were two layers of materials in between GeTe layer and thermal chuck. For GeTe on c-Si and GeTe on Al₂O₃, there was only one layer of material in between GeTe layer and thermal chuck. Heat vertically moves upward direction from thermal chuck. Small crystallites are formed due to this upward flow from substrate to GeTe via dielectric layer, and these crystallites are responsible for the GeTe resistance change. Additionally, GeTe on Al₂O₃ shows volatile transition following a different path. This is because of the difference in thermal expansion coefficients between Al₂O₃ and GeTe (5.3 and 0.56, respectively), which may result breaking of the conductive crystallites throughout the cooling process [2].

To perform S-parameters analysis of GeTe-based RF switches, S₁₁ and S₂₁ were investigated in frequency range of 10 MHz-20 GHz. It was found that when frequency increased, S₁₁ went down from −11 dB to −16 dB in crystalline phase. In crystalline phase, reflectivity increases because of incorporation of small crystallites due to frequency increase. In amorphous phase, full return loss was found as S₁₁ remained constant according to frequency change. These are shown in Figure 5. S₂₁ was found constant in crystalline phase, and the value was −3 dB. In amorphous phase, S₂₁ had different values in different frequency range. In 10–100 MHz range, it was −55 dB and remained constant. But when frequency increases from 100 MHz to 20 GHz, S₂₁ gradually increased, and the value went up to −28 dB. This might be because of frequency induced crystallization in amorphous phase. The signal line resistance was found as 75 Ω and 3.5 MΩ in ON and OFF states, respectively. For making high-performance RF switch, it was suggested that S₂₁ losses can be reduced by reducing the width of GeTe section which can eventually reduce ON state resistance from 75 Ω to 50 Ω [2].

Figure 5.

Analysis of S parameters for GeTe-based RF switches. In this analysis, 31.6% and 10% of full signal strength are equivalent to −10 dB and −20 dB [2].

3. Applications in terahertz (THz) technologies

3.1 GeTe-based THz split ring resonators

This section will focus on metamaterials compatible with CMOS technology and MEMS fabrication processes. A terahertz (THz) split-ring resonator (SRR) made from germanium telluride (GeTe) has been developed. This metamaterial was chosen for creating tunable response. Approximately at 180–230°C, amorphous GeTe becomes crystalline. The precise phase changing temperature depends on GeTe film thickness, its stoichiometry, and the substrate material. The SRRs here have a 5 μm line width, 20 μm square side length, 3 μm gap width, and a 39 μm periodicity between elements. 300-nm-thick GeTe films were sputter deposited on these devices using 99.99% pure, 50/50 GeTe sputter target (Figure 6a). The sputter chamber was at 10 mT with 20.1 sccm Ar gas flow. 300 nm Au deposition was done for another batch of devices using electron beam evaporator system (Figure 6b). The deposited metals were patterned in bilayer lift-off method [4].

Figure 6.

GeTe gold split-ring resonators (SRRs): (a) GeTe SRRs; (b) GeTe-in-gap SRRs.

At 180–190°C, both types of devices showed abrupt change in transmission line and behaved as metal, indicating GeTe’s crystalline phase. Overall, GeTe SRRs showed transition amplitude drop (Figure 7), whereas GeTe-in-gap SRRs showed altered transmission shape and that did not change even after cooling because of their nonvolatile nature (Figure 8). At temperatures above crystallization temperature, transmission kept falling. Even though it is known that GeTe gets less conductive at higher temperatures, it is observed that GeTe got more conductive along with increasing temperatures. This is because of the presence of thermally induced free carriers at the silicon substrate [4] (Figure 9).

Figure 7.

GeTe SRRs transmission responses for (a) increased and (b) decreased temperature [4].

Figure 8.

GeTe-in-gap SRRs transmission responses for (a) increased and (b) decreased temperature [4].

Figure 9.

Comparison of simulated and measured data: (a) a-GeTe SRRs; (b) c-GeTe SRRs; (c) a-GeTe-in-gap SRR, and (d) c-GeTe-in-gap SRR [4].

Although it was expected to get sharp resonances in the THz transmission response of the GeTe SRRs beyond crystallization temperature, it was not observed. This absence of resonances can be explained by considering ohmic losses in the crystallized GeTe. Therefore, it was found that at THz speeds, c-GeTe’s conductivity nature is not as ideal as gold. In case of in GeTe-in-gap SRRs, it was noticed significant change in transmission response after crystallization. Unlike the GeTe SRRs, impact of ohmic losses was not so daunting. This is for the fact that the devices had Au in mostly with a comparatively small portion of GeTe in the middle. Based on the outcome of GeTe-in-gap SRRs, it can be concluded that GeTe can be successfully utilized along with metals for improving tunability of such devices. GeTe-incorporated metamaterials can be applied in modulation by adding electrical GeTe switching circuits or by optimizing the GeTe film parameters for optical switching [4].

3.2 Improved THz modulations using GeTe

Here, a study has been done on exploiting a single GeTe layer as a thin film THz modulator. A 100-nm-thick GeTe layer was deposited in same manner as it has been discussed in the previous subsection. Apart from that, a sapphire wafer was used as the control sample for the reference measurements. The reason behind choosing sapphire is because its rhombohedral elementary cell is comparable to c-GeTe. Following Figure 10 shows time-dependent transmitted THz signals. It is evident from the figure that signal attenuation is highest around crystallization temperature. Here, rising of signal peak attenuation and time delay between signal generation and detection have been seen. These changes are directly correlated with increased absorbance and refractive index, respectively [5].

Figure 10.

THz-TDS response of GeTe film from room temperature to 250 °C: (a) complete response of positive and negative time domain peaks; (b) close-up view of positive time domain peak [5].

Figure 11a shows that the absorbance increases as the temperature is increased. Highest absorbance was observed at the transition/crystallization temperature. Also, high absorbance region starts at 100 °C from 70 to 100 cm⁻¹. This region corresponds to incident radiation above 2 THz. Below transition temperature film stays in polycrystalline phase and acts as insulator. Region for below transition temperature phonon vibrations do not last long as the films are thin with pinholes and other film anomalies compared to substrate beneath. As a result, the absorbance was low. Figure 11b shows that without sapphire substrate’s influence, for the GeTe only samples, around 88.5% to 91.5% modulation depths from crystalline GeTe were found. However, modulation of 99% at approximately 77 cm⁻¹ (2.3 THz) is observed when c-GeTe/sapphire heated to 250°C. For our case, GeTe only samples are important for THz applications as their modulation does not fall below 90% up to 77 cm⁻¹ (2.3 THz) [5].

Figure 11.

(a) GeTe film absorbance spectra with reference to sapphire substrate at room temperature to 250°C and again cooled to room temperature, across the full spectrum 10–110 cm⁻¹; (b) THz transmittance modulation of GeTe/sapphire and GeTe at 250°C and GeTe at room temperature [5].

4. Reconfigurable circuit applications

4.1 Fabrication of horizontal and vertical GeTe resistors

Two different geometries were chosen here. One is vertical GeTe resistor and other is horizontal GeTe resistor. In both horizontal and vertical resistors, there are bottom and top metal layers, GeTe layer and an isolation layer. To fabricate the horizontal resistor (Figure 12), firstly, a bottom metal layer (100 nm Au on top of 10 nm Ti) was patterned and deposited on silicon wafer using photolithography, e-beam evaporation, and traditional lift-off methods. Heat dissipation was happened via this bottom thermal conduction layer which is very important to maintain uniform crystallization. Next, an isolation layer consisting of 100 nm silicon nitride (Si₃N₄) layer was prepared using plasma-enhanced chemical vapor deposition (PECVD) method. This layer created an isolation in between bottom metal layer and top contact pads. On top of the isolation layer, 100–200 nm thin GeTe line resistor (shape of narrow bridge) was deposited using high vacuum RF sputtering method in room temperature. Then, top metal contact pads (250 nm Au on top of 10 nm Ti) were patterned and deposited on both sides of GeTe resistor. In these horizontal resistors, inter-electrode distances were ranged from 5 μm to 10 μm. In these horizontal resistors, resistor length was represented by separation between two metal pads and resistor area represented by the product of GeTe thickness with distance between metal pads [6].

Figure 12.

(a) Top view; (b) cross-sectional view of GeTe horizontal resistor; (c) a GeTe horizontal resistor with 5 μm inter-electrode distance [6].

In the vertical resistor (Figure 13), Ti/Au bottom contact pads were patterned and deposited on Si wafer. This process was similar to the process of bottom metal thermal conduction layer of horizontal resistor, but this bottom contact pad was the bottom electrode. 100 nm Si₃N₄ isolation layer was prepared on top of it. Next, hole was patterned and created in the Si₃N₄ isolation layer using photolithography and reactive ion etching (RIE) methods. Via this hole, GeTe with desired thickness was deposited using RF sputtering method, and this GeTe film connected the top and bottom electrodes. Then, Ti/Au top metal contact pads were patterned and deposited on top of GeTe which fully covered the GeTe film. In these vertical resistors, resistor length was represented by GeTe thickness and resistor area represented by the size of via hole [6].

Figure 13.

(a) Top view; (b) cross-sectional view of GeTe vertical resistor; (c) a GeTe vertical resistor with 3 μm hole diameter [6].

4.2 Thermal and electrical characterization of GeTe resistors

Impact of direct heating (Joule) and indirect heating (thermal) and the relation between geometrical parameters and threshold voltage have been revealed. First, amorphous phase resistance was investigated for both horizontal and vertical GeTe resistors according to their thickness (Figure 14a). It was found that the resistivity of vertical resistors was hugely dependent on layer thickness of GeTe. 100 nm and 400 nm thick vertical resistors gave resistance values of 1 × 10⁴ Ω and 8 × 10⁴ Ω, respectively. Again, horizontal resistors showed greater resistance values than vertical resistors having same thickness. It was because of their larger inter-electrode distance and smaller cross-sectional area. It was also found that resistors having same device volume and different device geometry showed different resistance values [6].

Figure 14.

(a) Resistance of different GeTe resistors in amorphous phase, and (b) phase transition of GeTe horizontal resistor using thermal conduction method [6].

To understand the amorphous to crystalline transitional behavior of GeTe resistors via thermal conduction method, temperature of thermal chuck was increased up to 300°C (Figure 14b). Transition in horizontal resistors was happened at 220–230°C where resistance change was found at MΩ range (a sudden drop from MΩ to Ω). On the other hand, in Joule heating-based phase transition, vertical resistors with different thickness and same hole area were used. As thickness increased from 90 nm to 400 nm, voltage for transition increased from 0.5 V to 3.25 V (Figure 15a) and threshold field also increased from 3.3 V/μm to 8.13 V/μm. For horizontal resistors, threshold voltage and threshold field are much higher because of larger inter-electrode distance (with same cross-sectional area). It was also established that the effect of cross-sectional area on threshold voltage is very negligible (Figure 15b). Finally, it can be concluded that despite having relatively complex fabrication process than horizontal resistors, vertical resistors can be fit for low-power reconfigurable circuits applications because of their smaller inter-electrode distance [6].

Figure 15.

Joule heating-based transition of GeTe vertical resistors: (a) effect of inter-electrode distance with same cross-sectional area and (b) effect of cross-sectional area with same inter-electrode distance [6].

5. Conclusion

This book chapter discusses about how it is possible to exploit a chalcogenide phase change material like GeTe for DC and RF switching, THz technology, and reconfigurable circuits applications. A GeTe-based test structure was fabricated for DC switching testing. It was found that phase transition occurs when at 37 to 45 V applied voltages. More importantly, GeTe wire was able to go back to its original phase when the voltage was removed. A PCM device was fabricated for analyzing GeTe’s performance for RF switching applications. It was evident from the results that a thinner GeTe section in the device can reduce ON state resistance and hence improve RF switching performance. Following that, different types of GeTe-based SRRs were fabricated for applications related to terahertz (THz) technologies. It is found that, for certain device designs and GeTe placements, GeTe can provide significant tunability of metamaterial devices. For a GeTe/sapphire system, 92%–99% modulation depths were achieved. By observing the modulation depth results, it can be assumed that incorporating GeTe into modulators, filters, and other THz technology devices will be beneficial for current THz technology. Moreover, from the experimental results found from GeTe resistors, it can be concluded that even though vertical resistors are harder to fabricate, they are suitable for low-power reconfigurable circuits applications as they have smaller inter-electrode distance. Based on all the experiments discussed in the book chapter, it is justified to say that GeTe is a perfect candidate for representing the positive impact of chalcogenide phase change material on current THz technology field.

Acknowledgments

The authors want to thank all the authors and co-authors whose works have been discussed in this book chapter.