**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.

**Keywords:** germanium telluride, phase change material, switching, reconfigurable circuit, terahertz technology

## **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].

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 ( *ñ*) 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]:

$$E = \frac{V}{d} \tag{1}$$

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

$$R = \frac{\rho l}{A} \tag{2}$$

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

**Figure 2.** *Resistivity variation in GeTe wires because of external electric field [1].*
