Design and Characterization of Au/CdSe/GeO 2 /C MOSFET Devices

2021 Herein, metal-oxide-semiconductor fields effect transistors (MOSFET) are fabricated and characterized. p − type germanium dioxide coated onto Au/ n -CdSe substrates and top contacted with carbon point contacts is used to form the MOSFET devices. The structural investigations which were carried out with the help of X-ray diffraction technique revealed large lattice mismatched polycrystalline layers of CdSe and GeO 2 . The design of the energy band diagram has shown the formation of two Schottky arms (Au/ n − CdSe, C/GeO 2 ) at the interfaces of the n − CdSe/ p − GeO 2 layers. The capacitance-voltage characteristics which are recorded in the frequency domain of 1.0-50.0 MHz revealed the ability of formation of NMOS and PMOS layers. The signal frequency controlled built in potential is tunable in the range of 2.34 and 5.18 eV. In addition, the conductance and capacitance spectral analyses in the frequency domain of 10-1800 MHz revealed the domination of current conduction by tunneling and correlated barriers hoping below and above 760 MHz, respectively. In addition to its features as MOSFET devices, the Au/CdSe/GeO 2 /C hybrid devices are found to be appropriate for use as microwave glasses prior for the fabrication of CdSe base layers. The Cadmium selenide thin films were evaporated onto Au substrates using NORM VCM-600 vacuum evaporator at vacuum pressure of 10 -5 mbar. The source material was CdSe crystal lumps (99.995% Alpha Aesar). Using a high purity GeO 2 powders (Alpha Aesar 99.99%), GeO 2 films of thicknesses of 500 nm were then deposited onto the Au/CdSe films. The films thicknesses were measured with the help of Inficon STM-2 thickness monitor. The structure of the films was investigated with the help of MiniFlex 600 X-ray diffraction unit. The X-ray diffraction patterns were recorded as scanning speed of o 0.5 / min. The produced Au/CdSe/GeO 2 films were masked to locate circular


Introduction
Germanium dioxide thin films have occupied the interest of researchers since years. It is regarded as promising material which can be employed in more than one technology sector.
As for examples, GeO 2 is used in lithium batteries owing to their capability of charge storage 1 . Porous GeO 2 (s)/Ge(c) nanostructures which were used as lithium-ion battery anode revealed capacity of 1.33 Ah/g at a current density of 0.1 A/g 1 . Germanium dioxide crystals are also regarded as smart materials for the production of piezoelectricity. Temperature dependent studies on this crystal indicated the ability to retain large piezoelectric properties in the temperature range of 20-600 o C 2 . In addition, because of the ultra-wide band gap of GeO 2 and high electron and hole mobility values it is nominated for future power electronics 3 . The electron and hole mobility's of the germanium oxide reached 377 and 29 cm 2 /Vs, respectively. Moreover, deposition of ultrathin GeO 2 layers onto MoTe 2 /Ge heterojunctions successfully reduced the dark current density of the MoTe 2 /GeO 2 /Ge photodetectors from 0.44 µA/µm 2 to 0.03 nA/µm 24 . As a result, the photosensitivity is enhanced and the responsivity increased to 15.6 A/W. Furthermore, Germanium oxide nanoparticles which were prepared from the bulk GeO 2 powders using the hydrothermal technique are mentioned exhibiting characteristics of electrically erasable memory devices 5 .
Because of the above mentioned smart features of germanium dioxide, here in this work, we are motivated to find another type of applications for the GeO 2 thin films.
For this reason, germanium dioxide thin films are coated onto Au/CdSe substrates and top contacted with carbon point contacts to form hybrid device structure. The Au/CdSe substrates are selected because they reveal Schottky barriers and able to behave as band stop filters in the microwave range of frequency 6 . The constructed Au/CdSe/GeO 2 /C hybrid devices which are formed of two Schottky arms connected pn junctions are structurally and electrically investigated. The capacitance-voltage characteristics in the frequency domain of 1.0-50 MHz are recorded and analyzed. In addition, the capacitance and conductance spectra in the frequency domain of 10-1800 MHz are considered in detail.

Experimental Details
Gold substrates were coated onto ultrasonically cleaned glasses prior for the fabrication of CdSe base layers. The Cadmium selenide thin films were evaporated onto Au substrates using NORM VCM-600 vacuum evaporator at vacuum pressure of 10 -5 mbar. The source material was CdSe crystal lumps (99.995% Alpha Aesar). Using a high purity GeO 2 powders (Alpha Aesar 99.99%), GeO 2 films of thicknesses of 500 nm were then deposited onto the Au/CdSe films. The films thicknesses were measured with the help of Inficon STM-2 thickness monitor. The structure of the films was investigated with the help of MiniFlex 600 X-ray diffraction unit. The X-ray diffraction patterns were recorded as scanning speed of o 0.5 / min. The produced Au/CdSe/GeO 2 films were masked to locate circular carbon point contacts. The conductivity type of the CdSe and GeO 2 was determined by the hot probe technique. The capacitancevoltage characteristic, capacitance spectra and conductance spectra were measured with the help of Agilent 4291B 1.0 M-1.8 GHz impedance analyzer.

Results and Discussion
The metal-oxide-semiconductor fields effect transistors (MOSFET) which are illustrated in the inset of Figure 1 are fabricated by depositing n − CdSe onto Au substrates and coating p − GeO 2 onto the Au/n −CdSe layers. The resulting Au/n −CdSe/ p − GeO 2 interfaces are painted with carbon point contacts of areas of 7. . The lattice mismatches between CdSe and GeO 2 are 10.06% and 22.92% along the a − and c − axes respectively. Large lattice mismatches causes interfacial stresses and forms three dimensional quantum confinement 8,9 .
From electrical point of view, the work function ( Au qφ ) of Au is 5.34 eV 6 . It is larger than the work function of n −CdSe (4.80 eV). The difference between the two work function forms a Schottky contact between at the Au/n-CdSe interfaces. The work function of p −GeO 2 is not well defined. The electron affinity of GeO 2 is 2.24 eV and the energy band gap is 5. 35 eV 10 are known. Hence the work function of GeO2 can be calculated. Particularly, extrapolation of the published conductivity -reciprocal temperature variations for the Ag/GeO 2 /Ag reveals conductivity activation energy of E σ =17.4 meV above the top of the valance band. This means that the Fermi level is located at The value of the work function of GeO 2 is then 7.58 eV. The work function of p − GeO 2 is much larger than that of carbon (5.10 eV) leading to the formation of another Schottky arm at the C/GeO 2 side. The interface between n −CdSe and p −GeO 2 establishes a pn junction device. The overall established MOSFET device is formed from two Schottky arms attached by a pn junction. The energy band diagram for the device is shown in Figure 2 Figure 3a, b and c. The C V − characteristics display typical MOSFET characteristics. It seems that the device is composed of two MOS devices. Namely, the device displays an inversion mode of PMOS transistor when reverse biased and an inverted mode of operation of NMOS characteristics when forward biased 11 . At the PMOS arm, when activated by lowering the voltage below ~ 0.40 V, the device allows the conduction of holes reaching a minimum capacitance value at . While on the other hand, when the NMOS is operated an inversion layer in the GeO 2 ( player) is created forming n −channel. Conduction in this channel is dominated by electrons. For this channel, the larger the signal frequency, the larger the applied voltage needed to reach the strong inversion condition. It is also noticeable that the change in the capacitance values as the devices switches from strong inversion to weak accumulation states (illustrated in Figure 3a) become less pronounced as the signal frequency increases. As for examples, when the device is in the PMOS mode, the capacitance decreases from 625 pF to 600 pF, from 252 pF to 242 pF and from 172 pf to 171 pf as the signal frequency increases from 5.0 MHz to 10 MHz and reaches 50 MHz, respectively. This behavior is assigned to the charge dynamics. Namely, charges in the depletion layer of PMOS capacitors increase as ~ φ (φ: surface barrier height)  so depletion capacitance decreases as the inverse. Slowly varying signals gives the sufficient time for minority carriers to be generated, drift across depleting regions, or recombine. In contrast to this fact, when signal frequency is high inversion layer carriers can't respond and do not contribute 7 . Analyses of the recorded capacitance-voltage characteristics in accordance with the well know equation, 2 The sum of these two potential reveals built in potential of with s being ~1.99 and 1.80 for PMOS and NMOS channels, respectively. As also appears in Figure 4c, the depletion width increases with increasing signal frequency. The decrease in the free carrier density with increasing signal frequency is ascribed to the inability of the free charge carriers to orient with oscillatory incident electric signals 11 . It is worth noting that calculations that targeted estimation of the , bi V N and W at higher frequency values ( 5.0 F > MHz) revealed built in potential values larger than 10 eV. It indicates the invalidity of the depletion approximation method in the high frequency region. The capacitance response to biasing voltage at high frequencies arises from the diffusion capacitance which usually appears under forward biasing conditions for pn junction devices. However, as our constructed device is hybrid structure formed of two Schottky arms connected to pn junctions, forward biasing of one of the Schottky arms will necessary reverse the biasing of the other. For this reason, the diffusion capacitance appears for both of the NMOS and PMOS devices 7 .
In an attempt to explore the role of growth rates and thickness on the performance of the MOSFET devices, we have re-prepared the devices by coating GeO 2 layer of thickness of 250 nm at high and slow deposition rates of 11.7 /s Å and 5.2 /s Å , respectively. The measured C V − characteristics are shown in Figure 5a and b, respectively. It is clear from the figure, that the slower the deposition rate, the more accurate the collected data and the more stable the capacitance response to voltage excitations. Compared to the 500 nm thick GeO 2 films which were prepared at slow deposition rates (Figure 3a), the C V − curve of the 250 nm thick device indicates the formation of PMOS device and the inverted NMOS channel is absent. In addition, the value of the capacitance at particular voltage is much larger for the CdSe coated with 250 nm GeO 2 layers at slower rates. The effect of thickness and deposition rate is also evident from the calculated free carrier density. The free carrier density for films prepared at slow rates decreased from 2.18 20 10 × cm -3 to 7.24 17 10 × cm -3 as the film thickness is increased from 250 to 500 nm. The large number of free carriers (~2 0 hand, increasing the thickness is mentioned enhancing the mobility of charge carriers in thin film transistors. This is just because the thickness of the active channel layer is increased 12 . Similar conditions apply for the films prepared at faster rates. For GeO 2 layers of thicknesses of 250 nm prepared at fast rates the free carrier density is 2.4 18 10 × cm -3 . It is mentioned that high deposition rates causes poor crystallinity. In this case, some oxygen atoms might not be bonded leading to the high number of free charge carriers 13 . Figure 6a illustrates the capacitance spectra for the Au/CdSe/GeO 2 /C MOSFET devices being recorded at low biasing voltage (V=0.10 V) and in a wide range of frequency (10-1800 MHz). As seen from the figure, the capacitance sharply decreases with increasing signal frequency. The larger the frequency, the more pronounced  the decrease. To explain the origin of the capacitance spectra we employ the previously reported Qasrawi-Ershov method. In that approach, the capacitance is assumed to be composed of geometrical and dynamical parts known as o C and 1 C , respectively. The dynamical part of the capacitance is due to holes and due to electrons oscillatory motion. The total capacitance take the form 14 , ( )  The related fitting parameters which revealed good correlation between the theoretically estimated and experimentally measured conductivities are shown in Table 1. In accordance with the table, the scattering time needed for hoping of charged particles through correlated barriers is the same as that we found for scattered holes and scattered electrons (estimated from capacitance spectra modeling). The estimated phonon frequency value being ~333 cm -1 is consistent with that reported as 333 cm -1 for transverse optical phonons in the u E mode of oscillation of GeO 2 10 . The good consistency between the experimentally determined and theoretically estimated conductivity and capacitance spectral data assures the domination of the quantum mechanical tunneling at low frequencies below 760 MHz and the domination of correlated barrier hopping at high frequencies [15][16][17][18] . The poor fitting of the total conductivity at high frequencies (1000-1600 MHz) can ascribed to the existence of more than one kind of correlated and tunneling barriers which needs additional fitting to explore its origin 19 .
It is interesting to mention the existence of the wide band gap GeO2 layer has remarkable roles on achieving band filter characteristics. Earlier studies on Au/CdSe indicated that optimizing a microwave band filter characteristics is not possible unless the CdSe layers are sandwiched with Yb nanosheets of thicknesses of 40 nm 6 . For Au/CdSe/Yb/CdSe/C devices, band stop filter characteristics with notch frequency of ~1500 MHz 6 . Yb/CdSe/C 20 tunneling barriers also did not display microwave band filter characteristics. For CdSe substrates achieving band filter characteristics always need to be stacked with materials having wider energy band gaps.
As for examples, coating CdSe (E g =1.78 eV) onto CdS

Conclusions
In the current study, we have shown that it is possible to fabricate a metal-oxide-semiconductor field's effect transistor (MOSFET) from the CdSe/GeO 2 heterojunctions coated onto Au substrate and top contacted with carbon point contacts. The fabricated MOSFET devices are found to be beneficial for use as passive mode devices. The formed energy bands diagrams which is verified by the capacitance-voltage characteristics indicated the workability for both of the PMOS and NMOS channels appropriately. The capacitance and conductance spectra which are studied in the frequency domain that extends to microwave regions assure the usability of this device as microwave cavities.