Fully transparent field-effect transistor with high drain current and on-off ratio

We report a fully transparent thin-film transistor utilizing a La-doped BaSnO 3 channel layer that provides a drain current of 0.468 mA/ μ m and an on-off ratio of 1.5 × 10 8 . The La-doped BaSnO 3 channel is grown on a 100–150 nm thick unintentionally doped BaSnO 3 buffer layer on a (001) MgO substrate by molecular-beam epitaxy. Unpatterned channel layers show mobilities of 127–184


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scitation.org/journal/apm masks during growth, i.e., without photolithography. Attempts to use photolithography to define smaller devices have been plagued by the creation of oxygen vacancies in the BaSnO 3 film, including the undoped BaSnO 3 buffer layer, during the ion-milling process. These vacancies make the undoped BaSnO 3 buffer layer conductive, shunt the device, and degrade performance. 11 This problem is well known in other oxides as well. 12 Here, we report growth of high mobility La-doped BaSnO 3 thin films on MgO substrates using molecular-beam epitaxy and use reactive ion etching (RIE) to pattern BaSnO 3 -based TFTs. With appropriate conditions, this etching technique enables BaSnO 3 to be etched with a controllable etch rate while preserving surface roughness and electrical properties including conductivity and mobility. The resulting micrometer-scale photolithography defined BaSnO 3 -based transparent TFT has a peak field-effect mobility of 17.2 cm 2 /V −1 s −1 , an on-off ratio over 1.5 × 10 8 , I D over 0.468 mA/μm, and a peak transconductance (gm) of 30.5 mS/mm. As we describe below, this is one of the highest performance fully transparent oxide TFTs ever reported.
Epitaxial La-doped BaSnO 3 films were grown on (001) MgO substrates by adsorption-controlled growth using molecular-beam epitaxy with barium, lanthanum, and SnO 2 sources. The details of the growth process are given elsewhere. 6 Epitaxial growth of single-phase La-doped BaSnO 3 on MgO substrates was confirmed by x-ray diffraction (XRD) measurements; the full width at half maximum of the BaSnO 3 002 peak is 0.31 ○ (see Fig. S1 of the supplementary material). MgO substrates, with a bandgap of about 7.8 eV, 13 are used to avoid the potential complications of persistent photoconductivity from the substrate arising from light exposure during the photolithography process. 14 On the MgO substrate, an undoped BaSnO 3 layer followed by a La-doped BaSnO 3 layer is grown to form the channel of the TFT as is typical for BaSnO 3 -based TFTs. 10,[15][16][17][18][19] The purpose of the undoped BaSnO 3 layer is to reduce the density of threading dislocations that propagate into the overlying doped (and conducting) BaSnO 3 . Even with such a buffer layer, the threading dislocation density of BaSnO 3 films grown on MgO as well as other commonly used substrates is of order 10 11 cm −2 ; 6,15,20 an ideal substrate for BaSnO 3 has yet to be demonstrated, but several lattice matched candidates are being developed. [21][22][23] The heterostructures for TFTs fabrication consist of a 100-150 nm thick unintentionally doped BaSnO 3 buffer layer followed by about 10 nm of La-doped BaSnO 3 . In test structures, the Hall mobility of a 10-nm-thick La-doped BaSnO 3 layer (grown on an MgO substrate with an undoped BaSnO 3 buffer layer) reaches 184 cm 2 V −1 s −1 at a carrier concentration of 6.5 × 10 19 cm −3 . Despite the thin La-doped BaSnO 3 film and the use of a nonperovskite MgO substrate, this value is the highest mobility reported for BaSnO 3 thin films; it is slightly higher than the previous record achieved on a DyScO 3 substrate. 6 The Hall mobility of a 10-nm thick La-doped BaSnO 3 layer (on a 150 nm thick undoped BaSnO 3 buffer layer) with a carrier concentration of 1.3 × 10 19 cm −3 is over 90 cm 2 V −1 s −1 . This decrease in mobility as the carrier concentration in La-doped BaSnO 3 is reduced below about 7 × 10 19 cm −3 is typical of BaSnO 3 films 6,15,20 and is a result of scattering from the huge density of threading dislocations, which become less effectively screened from the mobile charge carriers as the carrier concentration is reduced. 4,20 The mobility, limited by the threading dislocations, is given by 24,25 and e, c, Z, N td , n, ε, and m * are the electron charge, c-axis lattice parameter, charge state of a unit cell in a threading dislocation, density of threading dislocations, carrier concentration, dielectric permittivity, and effective mass, respectively. The fitting parameters are given in the supplementary material. The fitting achieved using this formula is overall in good agreement with the experimental data (see Fig. S6 of the supplementary material). The mobilities of our La-doped BaSnO 3 films on MgO are the highest yet reported at the relatively low carrier concentrations and low thickness of La-doped BaSnO 3 layers, 4,15,26,27 which are needed to be able to fully deplete the TFT channel when a voltage is applied to the gate.
In order to make micrometer scale TFTs, it is critical to be able to lithographically pattern the high mobility La-doped BaSnO 3 channel layer without degrading its electrical performance. A few methods have been used in the past to pattern a La-doped BaSnO 3 channel layer. Early BaSnO 3 TFTs used shadow masks to define the channel structure during deposition. 10,[15][16][17][18][19] This method can pattern the La-doped BaSnO 3 layer without degrading its surface roughness and electrical properties, but the channel length is limited to about 100 μm. More recently ion-milling has been employed to pattern the La-doped BaSnO 3 layer. Although ion-milling can etch La-doped BaSnO 3 to micrometer scale dimensions with controllable etch rate, the size of the channel length reported was about 100 μm. 11 Importantly, the resulting BaSnO 3 TFT with an undoped BaSnO 3 layer could not be depleted at room temperature and its on-off ratio was only 2. The reason for the inability to deplete the TFT at room temperature was attributed to significant charge trapping, 11 but our own experiments reveal that undoped BaSnO 3 becomes conductive when subjected to ion-milling. The conductivity of an undoped BaSnO 3 layer increased more than 10 8 times following ion-milling even when we use the smallest available acceleration voltage of our equipment, 400 V. Such an enhancement in conductivity of the undoped BaSnO 3 layer is consistent with it is being caused by the creation of oxygen vacancies from the high energy bombardment of Ar ions during the ion-milling process, as has been reported for SrTiO 3 . 12 The undesired conductance enhancement in unintentionally doped BaSnO 3 resulting from ion-milling can be reduced by annealing the sample in flowing oxygen at temperatures of about 800 ○ C, but even after such aggressive annealing, the conductance does not fully return to its original state. The effect of annealing on ionmilled undoped BaSnO 3 in flowing O 2 at temperatures from 300 ○ C to 800 ○ C is given in the supplementary material (see Fig. S7 of the supplementary material). Therefore, we believe that the conductance of the underlying undoped BaSnO 3 buffer layer caused by the ion-milling process is partially responsible for the poor roomtemperature performance of the prior BaSnO 3 -based TFT device subjected to ion-milling. 11 Ion-milling can also alter film stoichiometry and lead to charge trapping from defects along the side walls. For this reason, we have developed an alternate etching process to pattern the BaSnO 3 layers into a TFT.

ARTICLE scitation.org/journal/apm
In order to overcome the limitations of previous techniques used to define BaSnO 3 -based TFTs, we employ reactive-ion etching (RIE) using chlorine and argon gases to etch the BaSnO 3 epitaxial stack. By using a reactive gas such as chlorine, the RIE technique can etch a BaSnO 3 film with a chemical process rather than a purely physical process, in contrast to ion-milling. Under appropriate conditions, the undoped BaSnO 3 film conductivity issue that we ascribe to the creation of oxygen vacancies can be resolved. Furthermore, with RIE, we are able to achieve a controllable etch rate that is not dependent on sample quality. In short, our RIE etching process for BaSnO 3 provides a controllable etch rate while preserving surface roughness and electrical properties including conductivity and mobility-qualities critical to the fabrication of high-performance TFTs. BCl 3 gas can also be used for RIE etching of BaSnO 3 while preserving surface roughness and electrical properties including conductivity and mobility, but the etch rate is lower than when chlorine gas is used. Therefore, using chlorine gas without adding BCl 3 gas is preferable. A higher etch rate is desirable to reduce etch time, but there is a trade-off between the etch rate and damage to the BaSnO 3 film. In making the TFTs, the etch rate has been maximized without damaging the properties of the BaSnO 3 film by adjusting RIE etching parameters. The RF power of the RIE is around 80-120 W with a DC voltage in the 200-250 V range. The total gas pressure is around 1-2 mTorr, and a gas ratio between 1:5 and 5:1 is used for the Cl 2 and Ar gases. 28 The device patterning and fabrication process was completed in the cleanroom environment of Cornell's NanoScale Science and Technology Facility (CNF), except for the atomic layer deposition (ALD) process. For light exposure during photolithography, an Autostep 200 stepper was used. The channel layer was etched using a PT720/740 RIE etching tool. For device isolation, the channel layer was etched down about 18 nm, considerably beyond the 10 nm thickness of the La-doped BaSnO 3 top layer and well into the undoped BaSnO 3 buffer layer. After the etching process, indium tin oxide (ITO) was deposited as source and drain contacts using a PVD75 sputtering tool. The source and drain contacts were patterned by lift-off. A blanket HfO 2 gate dielectric layer was then deposited over the full structure by atomic-layer deposition (ALD), at 300 ○ C with ozone as an oxidant, which leads to have a deposition rate of 0.8 Å/cycle. The thickness of the HfO 2 layer was 20 nm. To complete the TFT, ITO was deposited again on the gate dielectric to form the gate electrode using the same sputtering tool as used earlier in the process. The gate contact was also patterned by lift-off. Electrical measurements were performed using a Cascade probe station with a Keithley 4200 SCS system. All device measurements presented were measured at room temperature in ambient air. Figure 1 shows a schematic of the BaSnO 3 -based TFT measured in Figs. 2 and 3, together with a scanning electron microscope image of the fully patterned device. The mobility and carrier concentration of the as-grown La-doped BaSnO 3 channel layer in this sample are μ = 65.2 cm 2 V −1 s −1 and n = 1.81 × 10 19 cm −3 , respectively, from Hall measurements at room temperature. The channel length and width of the TFT are L = 0.3 μm and W = 0.93 μm, respectively. This is the smallest BaSnO 3 -based TFT yet reported. 10,11,[15][16][17][18][19] Further scaling is limited by the resolution of the light exposing tool.
The characteristic curves of the BaSnO 3 TFT are shown in Fig. 2. Figure 2 vs V GS . As shown in Fig. 2(a), the TFT can be depleted completely at room temperature-a first for a photolithographically defined BaSnO 3 -based TFT. 11 The field-effect mobility of the device is calculated from the relation μFE = ( L C ox WV DS ) ∂I DS ∂V GS . 30 I DS , L, Cox, and W are the drain-to-source current, the channel length, the capacitance of the gate dielectric per unit area, and the channel width, respectively. The capacitance of the HfO 2 gate dielectric was measured as shown in Fig. S9 of the supplementary material with a metal-oxide semiconductor (MOS) structure with dimensions 50 μm × 50 μm. This MOS capacitor has the same La-doped BaSnO 3 channel layer and the same ITO gate electrode as the TFT, i.e., the layers measured in the MOS capacitor were deposited on the MgO wafer and patterned into the MOS capacitor test structure and into the TFT. For calculating μFE, the maximum Cox at the given voltage range has been used so as to not overestimate μFE. The calculated μFE is 17.2 cm 2 V −1 s −1 . The large difference between the field-effect mobility and the Hall mobility can be attributed to the contact resistance. The contact resistance includes the sheet resistance of the ITO contact layer and interface resistance between the ITO contact layer and the La-doped BaSnO 3 channel layer. In the supplementary material (see Fig. S10), we utilize the measured value of the contact resistance to calculate the field-effect mobility; the result is 72 cm 2 V −1 s −1 , which is comparable to the Hall mobility of the as-grown channel layer. From the details of this calculation, it is clear that the high contact resistance is significantly limiting the TFT performance; a lower resistance contact will increase the field-effect mobility and thus the drain current. There is dispersion in the capacitance at measurement frequencies higher than 100 kHz, and it is believed to be related to the trapping of charge by defects or impurities within the HfO 2 film and at the interface between the HfO 2 and La-doped BaSnO 3 channel layer since the cutoff frequency is 51.6 MHz. The cutoff frequency is calculated from the relation f = 1 2πRC and is much higher than 100 kHz. The on-off ratio is over 1.5 × 10 8 . This is the highest on-off ratio among BaSnO 3 TFTs when an undoped BaSnO 3 buffer layer is used. 10,11,[15][16][17][18] The subthreshold swing has been calculated from the equation S = ( , and S is 0.15 V dec −1 . Figure 2 at V DS = 1 V. This is much higher than the previous record transconductance in a BaSnO 3 based TFT, which is only 2 mS/mm. 11 Figure 2(c) shows the output characteristic of the device, which is I D vs V DS (drain-to-source voltage) by varying V GS from 2 to −7 V. As shown in Fig. 2(c), the drain current reaches over 0.468 mA/μm. The slow increase in the drain current with respect to V DS is attributed to the high contact resistance. The resistance of the ITO/La-doped BaSnO 3 /ITO channel structure vs channel length is plotted in Fig. S10 of the supplementary material. The source/drain contact resistance between the ITO contact layer and the Ladoped BaSnO 3 channel layer, which is 5.1 kΩ μm, is calculated by The drain current vs channel length at V GS = 2 V and V DS = 5 V is plotted in Fig. 3. The drain current is inversely proportional to the overall length of the channel except at the shortest channel length of 0.3 μm, showing little degradation with respect to device scaling. The close agreement between the calculated field-effect mobility (with the effect of contact resistance included) and Hall mobility indicates that the deviation from linear behavior at the short channel length of 0.3 μm in the inset of Fig. 3 is likely due to the contact resistance and not short channel effects such as velocity saturation. At a channel length of 0.3 μm, the channel resistance is comparable to the contact resistance and the effective voltage applied to the channel is greatly diminished, lowering drain current. A lower resistance contact is expected to reduce the deviation evident in Fig. S11 (a) between the ideal square-law transistor model and actual TFT performance.
The interface trap charge density D it can be calculated using 31 where k, T, and e are the Boltzmann constant, temperature, and electron charge, respectively. The calculated D it is 5.03×10 12 eV −1 cm −2 . A comparison of the drain current vs on-off ratio among transparent oxide channel TFTs is shown in Fig. 4. 3,5,10,[15][16][17][18][19] The high I D of the highest performance TFT with a transparent oxide channel was achieved on cleaved and transferred flakes of β-Ga 2 O 3 onto an opaque substrate; 48 this is not a scalable technology. These data are indicated as gray marks. Our BaSnO 3 -based TFT exhibits one of the best performances of all transparent oxide TFTs and is comparable to the best TFTs made with transparent oxide channel materials (in device structures in which other elements are not transparent). Its drain current is second best among all fully transparent scalable oxide channel TFTs. 57 In summary, a fully transparent submicron TFT based on BaSnO 3 has been fabricated with a high drain current and on/off current ratio. This breakthrough is made possible by (1) high mobility bare films in combination with (2) the development of a micrometer-scale etching method that preserves the surface roughness, conductivity, and mobility of BaSnO 3 films. These results demonstrate the tremendous potential of BaSnO 3 for the future of transparent electronics.

SUPPLEMENTARY MATERIAL
See the supplementary material for further data and discussion of the structural and electrical properties. This includes Xray diffraction θ-2θ and ϕ-scans, a reflection high-energy electron diffraction (RHEED) intensity oscillation during the growth of a BaSnO 3 thin film, a reciprocal space map (RSM) of a BaSnO 3 thin film, atomic force microscope (AFM) images of the surface of a La-BaSnO 3 film before and after etching in dilute nitric acid, a plot of mobility vs carrier concentration for the La-BaSnO 3 /undoped BaSnO 3 /MgO heterostructures, a comparison of the 2-probe resistance of RIE-etched BaSnO 3 and ion-milled BaSnO 3 films following post annealing in flowing O 2 , the band diagram of the BaSnO 3based TFT device fabricated with a HfO 2 gate dielectric and an ITO gate contact, the capacitance vs V GS characteristics of an MOS ITO/ALD-grown HfO 2 /La-BaSnO 3 capacitor, extraction of the contact resistance from the ITO/La-doped BaSnO 3 /ITO channel structure, analysis of the field-effect mobility that includes the high contact resistance, and a simulation of the ideal output characteristics of the BaSnO 3 -based FET. We annealed an ion-milled undoped BaSnO3 sample in flowing O2 at temperatures from 300 ℃ to 800 ℃. At each temperature the sample was annealed for 2 hours. The resistivity of the sample was measured by a 2-probe measurement after each anneal. The result is plotted below. As can be seen from the plot, the resistivity of the undoped BaSnO3 cannot be recovered by annealing at any temperature in the 300 ℃ to 600 ℃ range.
Annealing at 700 ℃ and 800 ℃ improves the film resistivity, but it is still lower than that of Supplementary Fig. S10. Extraction of contact resistance of an ITO/BaSnO3/ITO channel structure with respect to the distance between the contacts. The contact resistance is estimated by extrapolating a linear fit to the data of total channel resistance (RtotalW) vs. channel length (Lch). The estimated contact resistance of the ITO layer is 5.1 kΩ• m. The high contact resistance of the ITO source-drain contacts slows down the increase of the drain current as VGS increases.
Our calculation of the field-effect mobility is 72 cm 2 V -1 s -1 , which is comparable to the Hall mobility of the as-grown channel layer. The details of this calculations are as follows.
From Supplementary Fig. S10, the contact resistance of ITO is 0.51 Ω•cm. The normalized resistance of the La-BaSnO3 channel with respect to the channel width is calculated by Rs×L, where Rs and L are sheet resistance and channel length, respectively. A sheet resistance of , < , 1 , and are the channel width, effective mobility, capacitance per unit area of the gate dielectric, and channel length, respectively. The slower increase of IDS vs. VDS in the measured output characteristics than that of the simulated output characteristics is attributed to the high contact resistance of ITO as depicted with the dashed line in supplementary Fig. 8.