**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].

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

#### *Germanium Telluride: A Chalcogenide Phase Change Material with Many Possibilities DOI: http://dx.doi.org/10.5772/intechopen.108461*

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**).

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].

**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].*

#### **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 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

*Germanium Telluride: A Chalcogenide Phase Change Material with Many Possibilities DOI: http://dx.doi.org/10.5772/intechopen.108461*

#### **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 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].*

are important for THz applications as their modulation does not fall below 90% up to 77 cm−1 (2.3 THz) [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].

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].

#### **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

#### **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].*

*Germanium Telluride: A Chalcogenide Phase Change Material with Many Possibilities DOI: http://dx.doi.org/10.5772/intechopen.108461*

**Figure 13.**

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

and smaller cross-sectional area. It was also found that resistors having same device volume and different device geometry showed different resistance values [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 crosssectional 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.

#### **Acknowledgements**

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

*Germanium Telluride: A Chalcogenide Phase Change Material with Many Possibilities DOI: http://dx.doi.org/10.5772/intechopen.108461*
