Mobility and threshold voltages comparison of zinc nitride-based thin-film transistor fabricated on Si and glass

The present work reports the fabrication and characterization of high mobility thin-film transistors, where zinc nitride is used as the active layer (∼100 nm thick). For the TFT, the active layer was deposited at room temperature on different substrates (Si-p type and glass) by RF magnetron sputtering method and annealed at 350 °C post-fabrication and HfO2 was used as the gate insulation layer (∼50 nm thick). The obtained value of field-effect mobility was greater than 5 cm2 Vs−1, with optical bandgap ∼3.07 eV. The two MIS (metal insulator semiconductor) structures were analyzed using I–V and C–V measurements. It is demonstrated that Zinc Nitride is a potential candidate to be used as an active layer in TFT fabrication. The threshold voltages of the device built on Si and glass substrates were obtained as 0.8 volts and 2.6 volts respectively.


Introduction
Currently, many semiconductors materials such as silicon, germanium, gallium arsenide, etc are being used as an active layer in a thin-film transistor (TFT), with SiO 2 or HfO 2 as a dielectric material for gate insulation.
TFT is a field-effect transistor (FET) with its basic characteristics and physical structure same as that of metal-oxide field-effect transistor (MOSFET) with some important differences. TFTs show important applications viz. integrated displays, photovoltaic, sensing devices, and integrated logic devices [1][2][3][4]. The optical band gap of Zn3N2 was identified as a direct band-gap and its value varied from 2.2 eV to 3.5 eV [5][6][7][8][9].
Zinc nitride (Zn 3 N 2 ) is a group of II-V compound semiconductors used in electronics and optoelectronics applications owing to its many attractive features such as wide direct bandgap, high refractive index, low cost, and ecological friendliness. The electrical properties of Zn 3 N 2 have been extensively studied. Its resistivity is reported to be in the range of 10 −3 to 10 −1 Ωcm depending on the method of fabrication and substrate, carrier mobility ∼10 −1 -100 cm 2 V −1 s −1 and Hall measurements report it to be n-type [10][11][12][13][14]. Metallic contacts with Al, Au, and Ag show good ohmic behavior.
This work is focused on the deposition of zinc nitride (Zn 3 N 2 ) as an active layer through nitridation of Zn using RF plasma sputtering technique on glass and Si substrate and its structural, microstructure, and electrical characterization.

Experiment
TFT was deposited on the p-type Si (111, ρ<0.01 Ω cm) wafer and glass substrate simultaneously, which was cut into a correct dimension. RCA (Radio Corporation of America) and piranha method were used for cleaning the Si and glass substrate respectively and Al metal contacts were deposited using thermal vapor deposition thereafter to be used as gate contact for the bottom gate TFT. The next layer was of HfO 2 of approximately 50 nm thickness as a gate insulator, deposited by RF sputtering [15,16], The next layer was HfO 2 of 50 nm thickness as a gate insulator, which was deposited by RF sputtering, annealed at 600°C for 1 h to improve its crystalline properties, for which the sputtering machine present substrate heating setup [17]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. And then Zn 3 N 2 was used as an active layer semiconductor with a thickness of about 100 nm. The active layer is characterized by reactive conditions with zinc (99.9% purity) 2-inch diameter target with RF sputtering and N 2 as reactive gas (flow rate 10 sccm) while Ar as carrier gas (flow rate in 10 sccm). The radio frequency power of sputtering used for GK was 100 watt where the base and deposition pressures were 5×10 -6 mbar and 3×10 -3 mbar, respectively.
Similarly, after the deposition of Zn 3 N 2 , it was annealed at 350°C for 30 min to improve crystallinity. The drain and source contacts were fabricated with gold (Au) by dc sputtering through masking, While Al metal was used to make the gate contact, as shown in figure 1. While back surface contact fabrication was done in the case of the Si substrate (p-type) while the upper surface contact fabrication was used for the glass substrate, the electron beam vapor deposition technique was used for the deposition of the Al layer contact.
Crystallinity and thickness of the films were studied using grazing angle (0.25°) XRD with x-ray reflectivity (XRR) using X'Pert Pro of PAN-alytical (Cu Kα1) system. AFM Bruker multi-mode 8 was used to investigate the roughness and surface morphology.
Field emission scanning electron microscopy (Nova Nano-FESEM 450) was used to study thin-films surface morphology with EDS giving the chemical analysis. UV-vis spectroscopy was done by Perkin Elmer's LAMBDA-750 UV-Vis-NIR spectrophotometer. The electrical characteristics were measured by the Semiconductor Devices Analyzer of Agilent B1500A Technologies The deposition parameters as shown in the above table (1) were kept the same for both thin films (Zn 3 N 2 and HfO 2 ) so that their physical properties could be compared on the same basis. The XRD diffraction spectrum shows a polycrystalline structure of HfO 2 and Zn 3 N 2 [18,19]. In one of the XRD spectrums of both multi-layer spectrums which deposit on the Si substrate, the crystalline peak of Si can be  Deposition rate 2 nm min −1 2 nm min −1 8

Result and discussion
Deposition power 100 watt 100 watt 9 Source and Drain ohmic contacts Au Au 10 Gate ohmic contacts Al Al seen which shows the difference in the multi-layer structure of deposit TFT on both substrates. The film shows a lower number of peaks on the glass substrate, indicating that the crystalline structure of the deposit thin-film on the Si substrate develops more sharply than the glass substrate, mainly due to the Si surface of the glass surface. And the other major reason, where on one hand the glass is amorphous while Si crystalline on the other, the crystalline nature of Si causes the thin-film deposited on it to be more crystalline.
The thickness of the thin-films was determined using the XRR technique shown in figure (3), which is capable of studying the effect of density on the optical properties of thin-films as well as changes in the refractive index of the film.
With the XRR technique, the thickness of the Zn 3 N 2 and HfO 2 layers were measured as different signal layer thin films that were obtained at ∼108 nm and ∼61 nm, respectively [20,21]. The roughness and density of the films were also calculated from the XRR shown in table (2).
The surface morphology and topology of Zn 3 N 2 and HfO 2 films were characterized by AFM and FESEM. The experiment was done in contact mode AFM. The average surface roughness using AFM was obtained as ∼23 nm and ∼12 nm for Zn 3 N 2 and HfO 2 thin films respectively.
Here the roughness of Zn 3 N 2 is higher than the roughness of HfO 2 because the thickness of Zn 3 N 2 is twice that of HfO 2 and we know that as the thickness of thin-film increases, the roughness of thin-film also increases shown in figure (4) [22].
FESEM with the EDX spectrum in figures (5) and (6) show the surface morphology with the EDAX spectrum of the two films (Zn 3 N 2 and HfO 2 ) separately.  EDAX confirms the presence of the constituent elements. The micrographs reveal uniform films and corroborated by the low roughness values known through AFM.
The optical band gap was calculated using the Tauc-plot shown in the inset of figures 7(a) and (b). The obtained direct band gap is 3.07 eV, falling in the range reported in the literature [23].
Hall effect measurement of Zn 3 N 2 thin-film confirmed that It is an n-type semiconductor for which Hall mobility value μ H =2.34 (cm 2 Vs −1 ) and charge carrier concentration value n e =2.3×10 20 (cm −3 ) were obtained, confirming that our TFT is n-channel Works simultaneously in enhancement mode. Figures 8(a) and (b) give the C-V curves of the two substrates, the AC signal had a frequency 1 MHz with an amplitude 100 mV. For the Si substrate, the typical behavior of an MIS (metal insulator semiconductor) is seen. Considering that the AC signal had a relatively high frequency, the electrons did not get time to respond and the device did not enter the strong inversion region of operation. The value of capacitance per unit area was calculated from the graph.      Since the diameter of the drain and source gold ohmic contact was 1 mm, the area of MIS capacitor was about 0.007 85 cm 2 , the values of MIS capacitance obtained from the CV graph were about 5.28 nF and 4.49 nF for Si and glass-based MIS structure, respectively. After this, the capacitance per unit area values was obtained 0.672 μF cm −2 and 0.571 μF cm −2 respectively. The slight difference in gate capacitance is due to this as you can see in figure 1. In the case of Si substrate the gate is made as a back contact while in the glass substrate case the gate is made as a surface contact, here silicon the case of the substrate, the voltage provided to the gate crosses the two different surfaces Al metal, Si substrate and reaches the gate whereas, in contrast to the voltage provided to the gate in the case of the glass substrate, it is not so, so the gate A slight increase in capacitance may be due to the Schottky-diode effect (between Al metal andp-type silicon). Figures 9(a) and (b) shows the comparison of the transfer characteristics Zn 3 N 2 (thickness ∼100 nm) based TFT with HfO 2 (thickness ∼50 nm) gate dielectric for different substrates Si and Glass. Higher on current, higher on/off current ratio, and better sub-threshold region are seen. And figures 9(a) and (b) shows the output characteristics (V d v/s I d ) of the Zn 3 N 2 TFT with 500 μm channel length and width is 2000 μm, where drain current (I d ) is plotted as a function of the drain to source voltage (V d ) for gate voltage (V g ) ranging from 0 to 5 voltages, and the output characteristics clearly show a saturation region, the current crowding effect in both TFT at a low value of V ds is appreciated shown in figures 10(a) and (b), In the above output characteristics TFT is working in enhancement-mode where drain voltage Vd=0 is the limit at which TFT can switch from enhancement-mode to depletion-mode, due to this thin switching boundary some microampere current in TFT Flow may occur. The transverse characteristics at a constant drain voltage (V d =5 V) are shown in figure (9).
This shows that long channel devices are also affected by high contact resistance. The overall results show the Zn 3 N 2 films deposited at room temperature as a potential candidate as high mobility semiconductor for flexible TFTs.  The improvement of transfer characteristics is associated with a higher transconductance g m which is proportional to the gate oxide capacitance per unit area C ox of the device provided by insulating layer HfO 2 as shown from equation (1).
Where μ FE is the electron field-effect mobility and C ox is the gate capacitance per unit area of gate insulator from C-V characteristics figures 7(a) and (b), the channel width (W) and length (L) are 2000 μm and 500 μm respectively and V T is the threshold voltage. Since the structure of this TFT is the bottom gate structure, it is less likely to have a frigs effect due to the lower sheet resistance affect in it compared to the top gate structure. This is why fabrication of nano-thin MOSFET type, bottom gate structure type is done. The electron field-effect mobility and threshold voltage were extracted from the square root of I ds versus V gs using equation (2) of the saturation regime. The transverse characteristics shown in figure 8 were used to calculate field-effect mobility, for which taking the dots at 10 different fixed points on the graph and deriving its corresponding coordinate values (Vg, Id), using equation (2), the average The field-effect mobility was calculated, shown in table 3.
The values expected for threshold voltages V T have been extracted by the fitting tangent to the (V g versus (I d ) 1/2 ) curves shown in figure (9) and calculated 0.8 volts and 2.6 volts respectively for Si and Glass substrate TFT.
TFTs that have been formed into devices on various substrates (Si and glass) also influence the field-effect electric dynamics of the MOSFET. This mobility difference arises due to different contact resistances which are the result of high contact resistance between different layers and collective tuning of different Fermi levels between different interfaces which are also responsible for generating many electric parameters. Such as current crowding, degradation of the transconductance, or impact ionization, among others, etc.
The transverse characteristics at a constant drain voltage (V d =5 V) are shown in figure (9). The subthreshold swing is characterized by the transverse characteristics in the current-voltage curves of a MOSFET, which is calculated by the inverse of the slope of the curve between Log I d versus V gs . This is mainly in the sub-threshold region, the drain current behavior is controlled by the gate terminal which is similar to the exponentially decreasing current of a forward-biased diode.
The drain current ON-OFF ratio of the device is calculated from equation (4).
Where σ and d is the conductivity and thickness of the semiconductor layer of the TFTs, respectively. Both devices have different contact resistance, mainly due to different substrates. The aluminum metal coating is used for making gate ohmic contact where the back contact coating method is used for other Si substrates while the direct coating method is used for another and glass substrate as shown in figure (9), this is also a major reason for the difference in threshold voltage of both devices.
The high contact resistance masks the real value of field-effect mobility and reduces the on-current. Also published reports indicate that contact resistance may affect the sub-threshold region [24][25][26].
On the other hand, the value extracted for field-effect mobility greater than 5 cm 2 Vs −1 is better than those reported 0.2 cm 2 Vs −1 [27][28][29] although different mobility values reported in the literature are a function of different annealing temperatures.

Conclusion
TFTs based on Zn 3 N 2 film (∼100 nm) as an active layer and HfO 2 (∼50 nm) as insulator were fabricated using two different substrates (Si and glass) and characterized. The better electrical characteristics for the TFT on Si correspond to a higher transconductance in the device. Also, TFT on Si shows better threshold voltage (V T ) and mobility as compared to that of TFT on the glass substrate, due to better metallic contact provided by Si substrate. The output characteristics indicate that the long channel device is also affected by the high contact resistance, therefore the real field-effect mobility maybe even higher.