Fully Deep‐UV Transparent Thin Film Transistors Based on SrSnO3

Ultra‐wide bandgap semiconductors are gaining attention for their promising properties for UV optoelectronics and UV transparent electronics as well as high‐power applications. Among them, La‐doped SrSnO3 exhibits excellent properties both for deep‐UV transparent oxide semiconductors and deep‐UV transparent conducting oxide. Here, the demonstration of thin film transistors (TFTs) with full deep‐UV transparency is reported, including electrodes, gate oxide, and substrate. The lightly La‐doped SrSnO3 for the channel layer is grown on MgO (100) substrates with buffer layers by pulsed laser deposition. TFTs with a metal—insulator–semiconductor structure are fabricated using high‐k perovskite dielectric LaScO3 as the gate oxide. A degenerately La‐doped SrSnO3 is used as the gate, the source, and the drain electrodes to obtain good ohmic contact with the channel layer as well as UV transparency. The resultant device shows a field effect mobility value of ≈24 cm2 V−1 s−1 and an on/off ratio >106. The optical transmittance of the entire device (including the substrate) is found to be >75% at 300 nm in wavelength. Furthermore, the electrical characteristics of the device exhibit excellent stability under visible irradiation. This research highlights the potential of SrSnO3 in advancing the field of UV optoelectronics and UV transparent electronics.


Introduction
Recently, deep-UV (DUV,  = 200-300 nm) optoelectronics has attracted considerable attention for their emerging and wide-ranging applications, including water purification, [1] flame detection, [2] and biomolecule sensing. [3]For instance, solarblind UV photodetectors are being explored for their capacity to DOI: 10.1002/aelm.202300547[6][7] As the field of DUV optoelectronics matures with the development of solar-blind UV photodetectors, [4][5][6][7] UV light-emitting diodes, [8] and UV laser diodes, [9] the importance of DUV-transparent electronic devices is becoming increasingly evident.Among them, thin film transistors (TFTs) with DUV transparency could be used to create next-generation optoelectronic devices such as biosensors [10] and enhanced solar-blind photodetectors [7] in addition to increasing the efficiency of UV optoelectronic devices.The realization of a TFT with full transparency in the DUV region would open up new possibilities for this emerging field.A major challenge toward realizing DUVtransparent TFT is obtaining materials with suitable electrical and optical properties.The development of such devices requires a DUV-transparent oxide semiconductor (DUV-TOS) for the active layer, a DUVtransparent conducting oxide (DUV-TCO) for the conductive electrodes, and a dielectric layer for the gate oxide.However, this is more challenging than it may initially appear.As the bandgap of a semiconductor widens, doping becomes increasingly difficult due to a rise in the dopant's ionization energy, self-trapping of carriers, and a higher probability of compensation by impurities. [11]For instance, while -Ga 2 O 3 with its ultra-wide bandgap of 4.8 eV has been investigated as a promising material, numerous attempts to dope -Ga 2 O 3 further for enhanced conductivity have resulted in a significant decrease in DUV transmittance. [12,13]As a result, reports of successful DUV-TCOs with high conductivity are extremely rare.
Recently, SrSnO 3 has received considerable attention as a material suitable for DUV applications.[19][20][21][22] These studies attribute the high mobilities of these materials to their conduction band originating from the highly dispersive Sn 5s orbital. [17,18]Furthermore, La has proven to be an effective n-type dopant due to its shallow donor level-in the case of BaSnO 3 , the level is even within the conduction band. [18,19]Also, by substituting La for the Ba or Sr site, doping occurs away from the SnO 6 octahedral network, which minimizes disruption in the conduction band structure, even in the case of degenerate doping. [20]In other words, by altering the concentration of La-dopant of SrSnO 3, SrSnO 3 can serve as either a semi-insulating semiconductor with high electron mobility or a metallic electrode with high conductivity, all the while exhibiting DUV transparency.This makes SrSnO 3 extremely appealing for DUV applications, especially DUV-transparent TFTs, since they require both DUV-TOS and DUV-TCO.
In recent years, a few reports have been published on TFTs based on SrSnO 3 .The first demonstration of a SrSnO 3based field effect transistor (FET) in a metal-semiconductor FET (MESFET) configuration was reported in 2018. [23]Subsequent improvements using a recessed gate [24] and a demonstration of RF characterization [25] are followed by the same group.In 2020, the metal-insulator-semiconductor (MIS) configuration of SrSnO 3 -based FET with the field effect mobility of ≈14 cm 2 V −1 s −1 was reported, albeit with a low on/off ratio of ≈10 2 . [26]Additionally, there are some reports demonstrating TFTs based on other semiconductors with ultra-wide bandgaps, including amorphous Ga 2 O 3 , [27][28][29] -Ga 2 O 3 , [30] Ga 2 O 3 :CdO, [31] -Ga 2 O 3 , [32] ZnGa 2 O 4 , [33] In 0.5 Ga 0.5 O, [34] and SnGaO. [35]However, all those reported TFTs have employed metal or indium tin oxide electrodes that are opaque in the UV range, so to our best knowledge, no fully DUV-transparent TFT comprising electrodes based on DUV-TCOs has been reported to date.
Here, we report on SrSnO 3 -based thin film transistors (SSO TFTs) with full DUV transparency, utilizing La-doped SrSnO 3 as both active layer and conductive electrodes of the device.The resultant device showed an on/off current ratio >10 6 , a threshold voltage of −3.29 V, and field effect mobility of ≈24 cm 2 V −1 s −1 .To measure the optical transmittance of the device, we constructed a multilayer of thin films that constitute the active area of the TFT.Optical measurement on such a sample allowed us to determine the transmittance of the device as ≈75% at 300 nm in wavelength.Furthermore, we investigated the transport properties of the devices under exposure to various wavelengths of light.Impressively, the device demonstrates excellent stability when irradiated with visible light.

Fabrication Process of SSO TFT
What we fabricated is top-gate thin film transistors composed of epitaxial films (Figure 1).The devices are fabricated on MgO (001) substrate with sequentially strain-relaxing buffer layers comprised of SrHfO 3 and BaHfO 3 . [15]For the active layer of the TFT, a 20 nm-thick layer of Sr 0.996 La 0.004 SnO 3 (0.4% SLSO) was deposited on buffer layers using the pulsed laser deposition (PLD) technique.The deposition process for the La-doped SrSnO 3 films was similar to our previous work, [15] and we used stencil masks for patterning instead of etching.Subsequently, 50 nm-thick Sr 0.96 La 0.04 SnO 3 (4% SLSO) films were deposited for the source and drain electrodes.Its sheet resistance ranged from 130 to 160 Ω, corresponding to the resistivity of 0.65-0.80mΩ cm, consistent with our previous report. [15]For the gate dielectric layer, a 250 nm-thick LaScO 3 (LSO) layer was used.Lastly, a 50 nmthick 4% SLSO film was deposited for the gate electrode.The channel width (W) and the channel length (L) are measured to be 120 and 60 μm, respectively.Detailed information about the fabrication process can be found in the Experimental Section.It is worth noting that the patterned films are clearly distinguishable in the optical microscopic image (Figure 1b), but it is due to the interference effect; the films do not have inherent "colors" in the visible region.All materials used to construct the TFT were chosen for their bandgaps >4.6 eV, ensuring that the entire device, including the substrate, is transparent in the deep-UV range of the spectrum.X-ray reciprocal space mapping (RSM) for the entire heteroepitaxial layers is shown in Figure S1 (Supporting Information), demonstrating that each layer was grown epitaxially.Cross-sectional energy dispersive spectroscopy (EDS) analyses by scanning electron microscope (SEM) are presented in Figure S2 (Supporting Information), also confirming the intended multilayer growth.
Our prior studies on BaSnO 3 thin films and their corresponding devices employed the same cooling condition after the deposition of BaSnO 3 : we filled the chamber with oxygen until the partial pressure of oxygen (P O ) reaches 600 Torr, before proceeding with cooling.This method effectively suppresses the formation of oxygen vacancies in thin films.However, for SrSnO 3 , exposing the films to high temperature and high oxygen pressure tends to degrade their electrical properties, presumably by changing its phase among its many polymorph phases. [36]A number of studies reporting on highly conductive SrSnO 3 thin films have utilized either vacuum annealing or vacuum cooling after deposition. [14,15]However, this approach can lead to complications during device fabrication, especially with regard to the formation of oxygen vacancies in the dielectric layer, which degrades its dielectric strength.In this study, we have opted to cool the samples in the same atmospheric conditions used for deposition.This strategy is designed to maintain the integrity in both the SrSnO 3 channel layer and LSO dielectrics.

Properties of LaScO 3 Dielectric Layer
To evaluate the key properties of a TFT, such as field-effect mobility (μ FE ), we must first determine the capacitance of gate dielectric (C ox ).Hence, to understand the dielectric properties of the LSO layer, we constructed a capacitor with 4% SLSO electrodes and a 200 nm-thick LSO film (Figure 2a).LSO is chosen for its lattice match with SrSnO 3 in addition to its great dielectric properties.Figure 2b illustrates the capacitance (C p ) and the dissipation factor (tan ) when an AC voltage of 30 mV at a frequency ranging from 10 3 to 10 5 Hz is applied.From the measured C p , the dielectric constant () of LSO is calculated to be 30.This agrees well with the previously reported value of 28, [37] measured from LSO grown on BaSnO 3 electrodes.Finally, a DC voltage was applied in order to measure the leakage current and the breakdown field.As shown in Figure 2c, a sudden increase in current was observed, which indicates the breakdown field (E BD ) of 3.68 MV cm −1 .
Further, we examined the rapidly increasing current density before the breakdown to estimate the barrier height between LSO and SrSnO 3 .Fowler-Nordheim (F-N) tunneling process can be described by the following relation [38] : where J, E, m* LSO , and Φ are the current density, the electric field, the effective mass of conduction electrons in LSO, and the barrier height, respectively.To derive the value of Φ from the above equation, we plotted the ln(J/E 2 ) versus E −1 graph and got a well-fitted line in the high electric field region ≈3 MV cm −1 , which suggests that the F-N tunneling mechanism dominates leakage current (Figure S3, Supporting Information).Based on the fitted slope and assuming the electron effective mass of LSO as 0.4 m 0 , Φ is calculated to be 0.80 eV.As the barrier height is determined by the energy difference between the conduction band minimum (CBM) of LSO and the fermi level of 4% SLSO, it is necessary to establish the energy level of the fermi level of 4% SLSO to calculate the conduction band offset.Assuming a parabolic band, carrier concentration and fermi level follow the relations where n, m*, E F , E CB , and T are the carrier concentration, the effective mass of the conduction electron of SrSnO 3 , the fermi level, the energy level of the conduction band minimum, and the temperature, respectively.Using m* = 0.4 m 0 , n = 4.25 × 10 20 cm −3 for 4% SLSO, [15] we calculated E F − E CB to be 0.51 eV.Finally, by adding E F − E CB to Φ, the conduction band offset is found to be 1.31 eV.
It is worth noting that the offset in the conduction band of LSO and BaSnO 3 was reported to be about the same, ≈1.2 eV. [37]It will require further investigation to understand why the conduction band offset between SrSnO 3 and LSO is not smaller than that of BaSnO 3 /LSO.Also, we note that the deposition of the LSO layer did not result in a notable increase in conductivity of the SrSnO 3 channel layer, contrary to the case of BaSnO 3 that formed a 2D electron gas (2DEG) at the interface with the LSO layer. [37,39]

Transistor Properties of SSO TFTs
Figure 3 displays the transistor characteristics of the resultant device.Our device clearly demonstrates the transfer characteristics of an n-type depletion-mode FET (Figure 3a), exhibiting a high on/off ratio (I ON /I OFF ) that exceeds 10 6 .The field effect mobility (μ FE ) is calculated using the relation where g m represents the transconductance and C ox is the capacitance per unit area.This yields a value of ≈24 cm 2 V −1 s −1 .We estimate the threshold voltage (V th ) to be −3.29 V, based on the I D 0.5 -V GS relationship (Figure S4a, Supporting Information).Lastly, the subthreshold swing (S) is calculated from the relation , resulting in a value of 0.87 V dec −1 (Figure S4b, Supporting Information).
We also fabricated an accumulation-mode TFT using the same scheme but with an active layer replaced by slightly less doped La 0.003 Sr 0.997 SnO 3 (0.3% SLSO).The transistor properties of this device are presented in Figure 4.This device exhibited a normally off behavior with a threshold voltage of ≈2.4 V (Figure S4c, Supporting Information) and an impressively low off current (I OFF ), <10 −12 A. This feature is advantageous for applications requiring low energy consumption.While the field effect mobility was relatively modest, at ≈9 cm 2 V −1 s −1 , the accumulation-mode TFT demonstrates a smaller subthreshold swing of 0.23 V dec −1 (Figure S4d, Supporting Information).Compared with the SrSnO 3 -based TFT that Mian Wei previously reported, [26] our device exhibited a higher μ FE value and a higher on-off ratio (I ON /I OFF ).Considering that Mian Wei previously reported thin films of La-doped SrSnO 3 with high Hall mobility exceeding 40 cm 2 V −1 s −1 , [14] this enhanced performance of our device should be attributed to the fabrication process of the device rather than the quality of the active layer.By employing cooling under oxygen pressure used in the deposition process, we circumvent two undesired scenarios -exposing the SrSnO 3 film to high temperature oxygen pressure, which the electrical properties, exposing the LSO film to high temperature and vacuum, potentially reducing the quality of the dielectric layer.Also, our work adopted slightly doped 0.4% SLSO film as an active layer, while Wei used heavily doped 5% La-doped SrSnO 3 .This may be the reason why our device is fully depleted under the field effect and exhibits a low I OFF , even though it is limited by leakage current.In addition, our TFT shows significantly higher mobility compared to other reported TFTs based on various ultra-wide bandgap materials.Furthermore, our TFT incorporates 4% SLSO, which functions as a DUV-TCO, serving as the source, drain, and gate electrodes.8][29][30][31][32][33][34][35]

Optical Transmittance of SSO TFT
To evaluate the optical transmittance of SSO TFTs, we fabricated a multilayer of thin films that constitutes the active area of the resultant TFT (Figure 5a).Because this area contains the greatest number of layers and interfaces, the area is expected to be the least transparent part of the entire device.Next, we measured the optical transmission spectrum of the multilayer, as a function of wavelength, as displayed in Figure 5b.Clear interference fringes are visible in the multilayer spectrum.Analyzing the interference fringes of such a complex multilayer is challenging, but we believe that the interference fringes are primarily due to the reflections at the air/4% SLSO interface and the BaHfO 3 /MgO interface, where the discontinuities in the indices of refraction are the largest.The transmittance of the multilayer sample at 300 nm was ≈75%, which is similar to the transmittance of 73-77.5% observed with 112-120 nm-thick La-doped SrSnO 3 films. [14,15]This result is consistent with the fact that all materials composing SSO TFT exhibit larger bandgaps than those of SrSnO 3 .

Photo-and Bias-Stability of SSO TFT
The photo-stability of devices is crucial for advanced applications in optoelectronics and transparent electronics.For instance, transparent TFTs utilize transparent oxide semiconductors such as amorphous indium gallium zinc oxide (a-IGZO), which offer high transparency in the visible region.However, their transport properties often suffer significant instability under visible light exposure, hindering their applications like transparent displays and necessitating the additional cost for light shielding. [40]o evaluate the light stability of SSO TFTs, we analyzed the transfer characteristics (I D -V GS ) under irradiation with various wavelengths (Figure 6a).The intensity of the illuminated light was measured to be within the range of 0.5-1.5 mW cm −2 (Figure S5, Supporting Information).Figure 6b presents the transfer characteristics of the depletion-mode SSO TFT both in the dark and under irradiation of different wavelengths.
Remarkably, up to a wavelength of 400 nm, the device displays negligible difference under illumination.However, when exposed to of 380 nm or shorter wavelengths, the device starts to exhibit a slight shift in V th .This shift becomes more pronounced with wavelengths shorter than 320 nm.Given that the bandgap of SrSnO 3 corresponds to a cutoff wavelength of ≈270 nm, we attribute the shift to the subgap states of SrSnO 3 . [41,42]These results indicate that SSO TFTs exhibit excellent stability under visible light, an attribute not common among traditional transparent TFTs.Nonetheless, it also highlights the need for further investigation into SSO TFTs to mitigate the instability under the near-UV light.Several methods have been suggested by a number of works such as channel thickness reduction, treatments, and utilizing double-gate structure. [43,44]n addition, the stability of an SSO TFT under negative and positive bias stress, along with negative bias illumination stress has been investigated (Figure S6, Supporting Information).Very little bias stress dependence has been observed in the I d -V GS curve including its threshold voltage, although some changes in the off-state current were observed.

Conclusion
In summary, we successfully fabricated and investigated deep-UV transparent TFTs based on SrSnO 3 .The resultant device, utilizing La-doped SrSnO 3 and LSO layers, exhibited clear characteristics of an n-type FET, with a high on/off ratio and impressive field effect mobility of ≈24 cm 2 V −1 s −1 .An evaluation of the optical properties of our devices confirmed high transparency in the deep-UV range, with a transmittance of ≈75% at 300 nm.Also, our TFTs exhibited excellent stability under visible light, a trait not shared by traditional transparent TFTs, though some instability under near-UV light was noted.We present a summary table for the recent TFT data using semiconductors with ultrawide bandgap in Table S1 (Supporting Informations)and compare their performances.Our work provides a promising foundation for the development of advanced optoelectronics and transparent electronic applications that requires fully deep-UV transparency or solar-blindness.
For the fabrication of the TFT, a 55 nm-thick BaHfO 3 film was grown first on MgO (001) substrate (5 mm × 5 mm × 0.5 mm, MTI Korea) while oxygen partial pressure and target-to-substrate distance were kept at 100 mTorr and 51 mm, respectively.Second, a 33 nm-thick SrHfO 3 buffer layer was grown on the BaHfO 3 layer while oxygen partial pressure and target-to-substrate distance were kept at 100 mTorr and 45 mm, respectively.Following that, a 20 nm-thick 0.3-0.4% SLSO film was grown by alternatively depositing from SrSnO 3 and 1% SLSO ceramic targets, with a stencil mask.It was ensured that the growth rate of each cycle was lower than the height of one unit cell of SrSnO 3 .Oxygen partial pressure and target-to-substrate distance were kept at 150 mTorr and 63 mm, respectively.After deposition of 0.4% SLSO film, a 250 nm-thick LaScO 3 film was grown with a stencil mask, while oxygen partial pressure and target-tosubstrate distance were kept at 100 mTorr and 55 mm, respectively.Finally, a 4% SLSO gate electrode layer was grown with a stencil mask while oxygen partial pressure and target-to-substrate distance were kept at 150 mTorr and 63 mm, respectively.After each deposition, the substrate heater was turned off and the sample was cooled to room temperature in oxygen pressure during the deposition.
For the fabrication of the LaScO 3 capacitor, 50 nm-thick 4% SLSO film was grown first, with BaHfO 3 and SrHfO 3 buffer layers, by following the same process of the channel layer of the TFT.Then, a 200 nm-thick LaScO 3 layer was grown, also sharing the same process as the dielectric layer of the TFT.Finally, 50 nm-thick 4% SLSO film was grown again as the top electrode, with the same process as the gate electrode of the TFT.As in the case of the TFT, after each deposition, the substrate heater was turned off and the sample was cooled to room temperature in oxygen pressure during the deposition.
For the fabrication of the multilayer for optical measurement, each layer was grown without a stencil mask, and all films were grown at once, without cooling in the middle of the process.The deposition of each layer followed the same process as its counterpart in the TFT fabrication.After deposition of all layers, the substrate heater was turned off and the sample was cooled to room temperature in oxygen pressure during the last deposition.
Measurement of Electric Properties: The transfer characteristics (I D -V GS ) and the output characteristics (I D -V DS ) of fabricated TFTs were measured using a semiconductor characterization system (SCS-4200, Keithley) at room temperature in air.Indium was used to easily make contact to probe tips.The capacitance of LaScO 3 dielectric was measured by the same system, too.
Measurement of Optical Properties: To measure the optical transmittance of the samples, a grating spectrometer (Cary 5000, Bruker) was utilized.It employs a quartz iodide lamp light source for wavelengths ranging from 2000 to 350 nm and a deuterium UV light source for wavelengths below 350 nm.Samples were mounted on a holder with a 3 mm diameter hole.
To measure the photo-stability of transport properties of the TFTs, a Xe lamp (450 W) with a UV monochromator system was used while the characterization was performed with the aforementioned semiconductor characterization system.The intensity of the incident light was measured with a photodiode power sensor (S120VC, Thorlabs).

Measurement of Structural Properties:
To measure the structural properties of SSO TFTs, RSM was conducted by using SmartLab equipped with a Cu K1 source ( = 1.5406Å, Rigaku) at room temperature.An X-ray cross-beam optics system, a Ge (220) 2-bounce monochromator, and a 1D semiconductor detector (Hypix-3000) were used.SEM and EDS analysis was performed by using a field emission scanning electron microscope (JSM-7800F Prime, JEOL)

Figure 1 .
Figure 1.Schematics of the fabricated TFT.a) An illustration of the scheme structure of the device.b) The top view of the device pictured by an optical microscope.c) A cross-sectional diagram of the device.

Figure 2 .
Figure 2. Dielectric properties of LaScO 3 on 4% SLSO.a) The schematic of the capacitor.A 200-nm-thick LSO dielectric layer was inserted between 4% SLSO contacts.b) The capacitance and the dissipation factor of the LSO.The dielectric constant () was calculated from the measured capacitance (C p ) and the dimensions of the capacitor.c) The J-E characteristic of LSO capacitor.The breakdown field (E BD ) is determined by the rapidly increasing current density (J).

Figure 3 .
Figure 3. Transistor characteristics of a depletion-mode TFT.a) Transfer (I D -V GS ) characteristics of the depletion-mode TFT (V DS = 1 V).The absolute values of the gate leakage current (|I G |) and field effect mobility (μ FE ), which is calculated from transconductance (g m ), are also plotted.The on/off current ratio I ON /I OFF exceeds 10 6 .b) Output characteristics of the depletion-mode SSO TFT.Gate voltage bias (V GS ) was varied from 0 to 15 V with the interval of 3 V.The device exhibits excellent pinch-off behavior.

Figure 4 .
Figure 4. Transistor characteristics of an accumulation-mode TFT.a) Transfer characteristics and b) output characteristics of the accumulation-mode SSO TFT.Gate voltage bias was varied from 12 to 32 V with the interval of 4 V.

Figure 5 .
Figure 5. Optical transmittance of the device.a) The schematic of the prepared sample.This multilayer of thin films constitutes the active area of the fabricated TFT.b) The optical transmittance spectrum of the prepared sample.The spectrum of the MgO substrate is also plotted (in red).The inset shows a magnification of the spectra in the UV region.A dotted vertical line represents the wavelength of 300 nm.

Figure 6 .
Figure 6.The photo-stability of the device.a) The schematic of the measurement.Transfer (I D vs V GS ) characteristics were measured with the irradiation of monochromatic light, which was generated by a Xe lamp and a monochromator.b) Transfer characteristics in the dark and under various wavelengths of illumination.