Ag/SeO 2 /C Avalanche Type Resonant Tunneling Schottky Barriers

Herein, the design and characterization of Ag/SeO 2 /C avalanche type resonant tunneling devices are reported. Thin pellets of SeO 2 nano-powders pressed under hydraulic pressure of 1.0 MPa which is used as the active material are characterized. They showed tetragonal structure refereeing to space group of 2 4 mbc P and lattice parameters of a b = = 7.866 Å and c = 5.336 Å . The current-voltage characteristic curves have shown that SeO 2 can perform as active media to produce resonant tunneling diodes when forward biased and as avalanche type diode when reverse biased. The peak to valley current ratios of these diodes reached 18.3. In addition, the impedance spectroscopy measurements have shown that the device works in the low impedance mode when operated in the microwave range of frequency near 1.50 GHz. Negative conductance effect is observed in that frequency domain. The features of the Ag/SeO 2 /C nominate them for use as signal amplifiers and microwave oscillators.


Introduction
Selenium dioxide based ceramics has attracted the attention of many researchers due to their wide range of applications in various technology sectors. As for examples, they are used for advanced shielding applications 1 . They also find applications as electrocatalysts 2 . Literature data reported thousands of articles about many technological applications that benefits from selenium dioxide as selenating agent under simple reacting conditions 3 . However, articles which consider SeO 2 as a solid compound suitable for electronic applications are almost absent. For this reason, we are motivated to find new class of applications for this compound. Particularly, here in this work, we are going to produce thin pellets of SeO 2 powders and study their structural, morphological, compositional and electrical properties. The study will allow suggesting practical electronic applications for SeO 2 . We are reporting results that show the selenium dioxide as active material similar to SiO 2 and MoO 3 4,5 . The latter materials capture the focus of thin film transistor technology designers. As for examples, carbon nanotubes (CNT)/SiO 2 /MoO 3 is found ideal composite for charge storage. When used as electrodes in Li batteries it showed high specific capacity up to 0.7 Ah/g over 500 cycles 4 . In addition, stacked layers of Yb/MoO 3 /In 2 Se 3 /Ag thin film transistors are mentioned capable of handling more than one duty at a time. These duties include bandpass/reject filter characteristics in the microwave range of frequency domain and visible light communication signal receivers 5 .

Experimental Details
Powders of high purity of SeO 2 (99.99%,Alpha Aeser) were hydraulically pressed at 1.0 MPa in the shape of pellets of diameter of 1.2 cm and thickness of 0.12 cm. Electrical contacts to the SeO 2 pellets were actualized by using silver at the bottom and carbon point contact on top surface. The top diode area was 3.14 2 10 − × cm 2 . The crystalline nature and morphology of the SeO 2 pellets was tested with Miniflex 600 X-ray diffraction unit and with Coxem 200 scanning electron microscope equipped with EDAX energy dispersive X-ray spectrometer. Electrical measurements were handled with automated Keithley Current-voltage characteristic system and Agilent 4291B 0.01-1.80 GHz impedance analyzer.

Results and Discussion
The results of the X-ray diffraction measurements on SeO 2 samples are shown in Figure 1a. It is clear from the figure that the samples exhibit polycrystalline nature. An analysis of the XRD patterns which were carried out with "Crystdiff" software packages have shown that the powder based samples prefers crystallization in tetragonal phase corresponding to the space group lines/cm 2 . Compared to literature data, the crystallite sizes and dislocation density values of the SeO 2 pellets are sufficient to explain their physical properties using band theory of solids. These two structural parameters also allow application of these pellets in optoelectronic technology 9,10 . Figure 2b show the results of the energy dispersive X-ray spectroscopy for the SeO 2 pellets. It is clear from the spectra that the material is composed of Se, O and Pt only. Platinum appears because it was coated onto the pellets to prevent electron contaminations. In general, SeO 2 samples are pure and exhibit homogeneous stoichiometry in all parts of the sample. Energy dispersive X-ray spectroscopy (EDS) revealed compositional ratios of 62.2 at% Se and 37.8 at. % O. The data suggested some oxygen deficiency in the samples. On the other hand, the scanning electron microscopy images (SEM) for the studied samples are illustrated in Figure 1c. In accordance with SEM images which represent an enlargement by 10000 times, the surface is continuous and almost no cracks existed in the samples.
The hot probe technique has the p-type conductivity of SeO 2 . As shown in the energy band diagram presented in Figure 2a, from electronic point of view, as the electron affinity and energy band gap of SeO 2 are 1.89 eV 11 and 3.70 eV 12,13 , respectively, the work function ( of SeO 2 is located below the middle of the energy band gap ( ). Since the work function is larger than (   is created by ionization process under the influence of high electric fields. They are important devices for microwave oscillation. In these diodes applying large reverse voltage across the diode terminals widens the space charge region from the p + side (majority carriers in SeO 2 ) to the minority carriers ( n) region. This is called reach through condition. It states that as the reverse voltage increases, the reversebiased depletions region will eventually reach through to the forward-biased depletion region. As also illustrated in the inset of Figure 2b, when the C/SeO 2 avalanche diodes are forward biased, the current show peaky behavior in which it initially rises reaching a maxima of ( p I = 0.32 µA) at 0.65  14 . PVCR of 6.2 is reported for AlSb/GaInAsSb quantum well RTD's 16 . PVCR of 62 was also observed for Ge/BN interfaces 17 . These types of devices occupy large space in communication technology as they are sources of negative conductance which is a main factor in Gigahertz/ terahertz communication technology 14,18,19 . Particularly, when a system operates in the region of negative differential conductivity (NDC), where current falls rather than rises with increasing electric field, the density of carriers and their drift mobility strongly affects the NDC effect. Under conditions of NDC there is a tendency to enhance nonuniformities that would otherwise be damped. As for examples, in the Gunn effect charge carriers exists in high and low states of mobility. Charge carriers in the high state of mobility move easier through the pellets than charge carriers of low mobility. When the pellets are unbiased most of the charge carriers exist in high mobility state. Applying a voltage to the samples, forces current flows from high mobility states. Gunn effect dominates when a sufficiently large voltage is applied forcing motion of charge carriers into the state of lower mobility. This leads them moving more slowly and decreasing the electrical conductivity of the material is observed 20 . As an avalanche effect, large voltage biasing is expected to be associated with large electrical current values. However, the ability of the nonmetallic solids to carry this huge current is limited by the scarcity of mobile charge carriers (hence current decreases with increasing applied voltage leading to negative differential conductance) . Strong applied electric fields frees large number of charge carriers from the structure of the solid (impact ionization) and large current dominates again 14 .
On the other hand fitting the reverse current as function of applied reverse voltage (inset of Figure 3a) showed a nonlinear quadratic relation. Namely, the reverse current is given by It is well known that the avalanche breakdown happens when a high reverse voltage is applied across the terminal of (Ag/SeO 2 /C) diodes. As the reverse applied voltage is increased, the electric field across the junction increases. This electric field exerts a force on the charge carriers at the junction and frees them from covalent bonds. These freed charge carriers start moving with high velocity across the junction and collide with the other atoms, thus creating freer carriers resulting in a rapid increase in net current. The other possible mechanism for the large reverse current is the Zener breakdown. Namely, when the device is reverse biased, the kinetic energy of the carriers increases. The high-velocity carriers collide with other atoms and give rise to other free carriers. These free carriers, in turn, give rise to a high value of reverse saturation current. The main difference between Zener breakdown and avalanche breakdown is their mechanism of occurrence. Zener breakdown occurs because of the high electric field. The avalanche breakdown occurs because of the collision of free electrons with atoms. Both these breakdowns can occur simultaneously 14 .
As practical application of the C/SeO 2 avalanche/RTDs we have recorded the impedance spectra. The measured impedance (Z) capacitance (C) and conductance (G) spectra are shown in Figure 3a, b and c, respectively. It is clear from Figure 3a that the higher the signal frequency is, the lower the impedance value. Z decreases by two orders of magnitude as the frequency is increased from the radiowave (0.01 GHz) to the microwave frequency domain (~1.80 GHz). Low impedance in the microwave frequency domain is necessary for signal amplification especially in 5G mobile technologies 21 . They are employed to realize shunt capacitance in circuits 22 . As it is also readable from Figure 3b, the capacitance spectra follow the same trend of variation like that of impedance spectra. Namely, in the radiowave frequency domain, C sharply falls with increasing f . When the microwave frequency domain is reached, C slowly vary tending to remain constant. As the increasing f reaches 1.50 GHz, resonance-antiresonance peaks are observed. On the other hand, the conductance spectra which is the harvest of RTD, tends to remain positive and constant in the frequency domain of 0.01-1.50 GHz. When f > 1.56 GHz, the conductance show negative values. As seen from Figure 3c, the narrow peak exhibit minimum at 1.56 GHz. Negative conductance (NG) effects is observed. NG effects and resonance-antiresonance in the capacitance was previously observed in Se/Se stacked layers comprising Ag nanosheets in is structure 23 . It is a sign of resonant tunneling diodes and beneficial for supplying the electronic circuits with energy for maintaining microwave oscillations 24 . It can also be employed for compensating the loss from the inductor-capacitor tank during resonance 25 . NG effect is believed to originate from the existence of large density of deep states within a narrow energy range above the valence band 26 . It could also originate from the avalanche effect which we observed in this work.

Conclusions
In the current work, we have shown that thin SeO 2 pellets which exhibit tetragonal structure can be employed as an electronic device. The electrical characterizations have shown that Ag/SeO 2 /C structure exhibit resonant tunneling diode characteristics with peak to value current ratios of 18.3. The devices also behaved as sources of negative conductance which can be used to compensate losses in microwave resonators (inductor-capacitor tanks). The devices showed wide transient impedance tunability making it suitable for microwave oscillations.