Thin film field-effect transistor with ZnO:Li ferroelectric channel

1. Introduction

Zinc oxide is a transparent, wide bandgap semiconductor with many interesting properties. Some compounds of A^III–B^V groups, such as GaN or GaAs, exhibit similar properties and find successful applications in optoelectronics and microwave-integrated circuits.^1,2,3 Unlike these compounds, ZnO is a common material with nontoxic and biocompatible properties. In the last two decades, significant progress has been made in improving the technology of fabricating thin films based on metal oxides, and this progress has made it possible to obtain epitaxial ZnO thin films that retain some of their inherent attractive properties.^4,5 One of these properties is associated with the presence of spontaneous polarization: When 10% Li is added, the film acquires ferroelectric properties with spontaneous polarization $P_S$ aligned with the crystallographic C axis.^6,7 Spontaneous polarization, as in traditional ferroelectrics, is compensated.^8,9 The compensation process leads to the bending of the conduction bands and the appearance of localized states.¹⁰ In particular, it has been shown in ZnO:Ga/ZnO:Li two-layer systems that the interboundary charge is due to two factors: The first factor is the difference between the Fermi levels and conductivities of the ZnO:Ga and ZnO:Li films, the second factor is the presence of polarization in the ZnO:Li films and the absence of polarization in the ZnO:Ga film. These factors lead to band bending and limitation of charge carrier mobility. Additional traps due to surface states form a depletion region with a screening of spontaneous polarization, which can be used as a channel in a ZnO:Li/ZnO:Ga or ZiO:Li/Ag field-effect transistor (FET).

The goal of this work is to obtain a field-effect transistor with the ability to switch the static current–voltage characteristics of the transistor. The measurements were carried out on a field-effect transistor with an n-type channel. A distinctive feature of traditional field-effect transistors is the use of a ZnO:Li film as a channel. Manipulation control of the electrical characteristics of the field-effect transistor is carried out by switching the direction of spontaneous polarization of the ferroelectric semiconductor film.

2. Experimental

Zinc oxide thin films doped with lithium or gallium impurities were obtained by electron beam deposition in a vacuum using (001) oriented sapphire or Al₂O₃ ceramic plate as substrates. The targets for sputtering were prepared by solid-phase synthesis; the concentration of lithium or gallium in the target was 10at.% and 2at.%, respectively. All films were fabricated under the same conditions: Electron energy was 6keV, substrate temperature was $250\pm 1^{\circ }$C, and growth rate was 0.14nm/s.

The planar structure of the field effect transistor was obtained using a stencil mask. First, a ZnO:Ga film was deposited onto the substrate; this film has a high conductivity and serves as a drain and source of transistor. Then, a ZnO:Li ferroelectric semiconductor film was deposited to serve as a channel of a field-effect transistor. The method used leads to the production of oxygen-deficient films; therefore, they were further annealed in air to saturate them with oxygen. This leads to a decrease in oxygen vacancies and as a consequence, donor centers too decrease in the ZnO:Li film. It is important to mention that annealing in air has minimal impact on the conductivity of ZnO:Ga films. During annealing, the change in the conductivity was controlled. Next, an MgF₂ film was deposited on the resulting structure as a gate insulator (Fig. 3). MgF₂ films were obtained by thermal vacuum deposition. The conductivity was measured on the structure of a field-effect transistor without a gate electrode (the ratio of the width W to the length L of the channel was 1:1 or 1:10). The magnitude of spontaneous polarization $P_S$ of ferroelectric semiconductor ZnO:Li film was determined using the Sawyer–Tower scheme and the Mertz method.¹¹ Thermally-deposited argentum was used as electrodes. The study of fatigue processes during multiple polarization switching in a ferroelectric semiconductor film was carried out using the same setup. For measurements, a rectangular signal was used. The rise time of the pulse front was no more than 10ns. The change in repolarization currents during multiple cyclic switching of polarization on films of various thicknesses with varying external parameters (the frequency and amplitude of the external field), as well as the change in the current maximum and switching time with an increase in the number of switching cycles N, were studied.

3. Results and Discussions

ZnO:Li films produced by electron beam deposition in vacuum with an oxygen deficiency were further annealed in air to decrease oxygen vacancies and, consequently, donor centers. In this case, the conductivity of the films decreased to $0.5\times 10^{-3}$ ($\mathrm{\text{Ohm}}\cdot \mathrm{\text{cm}}$)${}^{-1}$. As a result of annealing, compensated semiconductors were obtained containing both donor and acceptor centers with the Hall mobility of charge carriers ${\mu }_{\mathrm{\text{Hall}}}=80$cm²/($\mathrm{\text{V}}\cdot \mathrm{\text{s}}$) and the effective field mobility ${\mu }_{\mathrm{\text{FE}}}=10$–15cm²/($\mathrm{\text{V}}\cdot \mathrm{\text{s}}$).¹² Ferroelectric measurements showed that ZnO:Li films prepared on a sapphire substrate with (001) orientation have a spontaneous polarization: At 8at.% of Li impurity $P_S=2.8\mu \mathrm{\text{C}}∕{\mathrm{\text{cm}}}^2$ and coercive field $E_C\approx 6$kV/cm and at 10at.% — $P_S=5.1\mu$C/cm² and $E_C\approx 10$kV/cm.

Figure 1 shows the results of X-ray diffraction analysis of ZnO:Li films deposited on the sapphire (001) substrate. It can be seen that textured ZnO:Li films with orientation along the (002) axis were obtained.

Figure 2 shows the dependence of the parameter “c” of the zinc oxide crystal lattice on the concentration of lithium ions, as well as the dependences of the full width at half-maximum (FWHM) and film conductivity on the Li concentration. It should be noted that the adding of lithium impurity does not change the quality of the film, as evidenced by minor changes in FWHM.

Figure 3 shows a scheme of a field-effect transistor with a ferroelectric semiconductor channel. On a single sapphire substrate, four identical planar field-effect transistors with dimensions of $2\times 2{\mathrm{\text{mm}}}^2$ were fabricated. Initially, ZnO:Ga film was deposited on a (001) cut sapphire substrate (1). This film is used as source (2) and drain (3) electrodes and has a high conductivity of 20 ($\mathrm{\Omega }$ cm)${}^{-1}$. Next, a ZnO:Li film (4) was deposited on the pre-sputtered source and drain electrodes. After deposition, the resulting structure was annealed in air, as a result the conductivity of the ZnO:Li film decreased, while the conductivity of the ZnO:Ga films changed insignificantly. Next, an MgF₂ film ($\epsilon =4.8$, $\sigma =2\times 10^{-15}$ ($\mathrm{\text{Ohm}}\cdot \mathrm{\text{cm}}$)${}^{-1}$) with 300nm thickness (5), acting as a gate insulator and silver film (6) as the gate electrode was deposited. The thickness of the area of spontaneous polarization screening (7) was assessed using the method described in Refs. 13 and 14. The thickness of this layer was 150–200nm, while the total thickness of the ZnO:Li film in the transistor structure is 300–400nm. Therefore, it can be assumed that the entire thickness of the FET channel represents the $P_S$ screening area (at the $\pm P_S$/Al₂O₃ and $\pm P_S$/MgF₂ transitions). The ZnO:Li film initially had no polarization; therefore, we polarized it using a pulsed voltage. The effective capacitance of the resulting gate-drain junction was $C_i\approx 50$nF/cm².

The study of the ferroelectric parameters was carried out using metal–ferroelectric–metal capacitor (Ag–ZnO:Li–Ag on a sapphire substrate or Ag–MgF₂–ZnO:Li–ZnO:Ga on a sapphire substrate); ZnO:Li film thickness — 950nm, Ag layer — 100nm. Figure 4 shows the ferroelectric hysteresis loop, measured by Sawyer–Tower scheme at 170Hz frequency and $20\pm 1^{\circ }$C. Curve 1 — film after deposition and annealing, 2 — film after $7\ast 10^5$ switching cycles. The measurements allowed to define the spontaneous polarization $P_S\approx 5.14\mu$C/cm² and the coercive field $E_C=7.3$kV/cm. As experiments show, repeated switching of $P_S$ direction in ZnO:Li films leads to a decrease in $P_S$ value, i.e., a fatigue effect is observed.

Figure 5 shows the dependences of the conductivity change $\mathrm{\Delta }{\sigma }_s$ and field mobility ${\mu }_{\mathrm{\text{FE}}}$ on the surface charge density Q in the FET channel. The measurements were carried out on the structure of the FET (Fig. 3), but as the gate insulator, we used air with 0.5mm thickness. Curve 1 — the spontaneous polarization vector is directed from the substrate to the gate of the transistor, curve 2 — the spontaneous polarization vector is directed from the gate to the substrate. Based on these data, the effective field mobility ${\mu }_{\mathrm{\text{FE}}}$ was estimated from the ratio ${\mu }_{\mathrm{\text{FE}}}=\mathrm{\Delta }{\sigma }_s∕\mathrm{\Delta }Q$. It is seen that the effective field mobility does not depend on the surface charge density in films with spontaneous polarization direction (2). In films with spontaneous polarization direction (1), the effective field mobility increases linearly with the surface charge density.

Quantitative experiments were also carried out to determine the time and field dependences of polarization reversal; measurements were carried out according to the Merz scheme.¹¹ In these experiments, voltage was applied to the film in the form of a meander — alternately two pulses of positive polarity and two pulses of negative polarity (see inset in Fig. 6). The first pulse leads to repolarization; the second pulse probes the state. Typical results for bias currents with an antiparallel $P_S$ first pulse and a parallel $P_S$ second pulse are shown in Fig. 6. The recorded current is due to two mechanisms: Charging a capacitor with a ferroelectric semiconductor dielectric and polarization switching. At full polarization switching, the area under the curve in Fig. 6(a) is equal to total charge $Q=\epsilon {\epsilon }_{0}SE+2S{P}_S$, where S is the area of the electrodes and E is the applied field. The area under curve b is equal to Q = $\epsilon {\epsilon }_0$SE. Based on these two measurements, the magnitude of the spontaneous polarization $P_S$ can be determined. This method is still the most direct method for studying switching in crystals with very low conductivity.

For ZnO:Li film, the switching time $t_S$ depends on the applied field E and obeys an exponential law for the fields from 1 to 1.5kV/cm; as the applied voltage increases, the switching time decreases. It should be noted that with an increase in the applied voltage, the number of switching cycles decreases due to a decrease in the magnitude of spontaneous polarization, i.e., there is a fatigue effect. In particular, at 18kV/cm already after 10³ switching cycles, the spontaneous polarization decreases by a factor of two.

Figure 7 shows the drain–source current $I_{\mathrm{\text{DS}}}$ as a function of the drain–source voltage $U_{\mathrm{\text{DS}}}$ for different gate-to-source voltages $U_{\mathrm{\text{GS}}}$. It should be noted that the ratio of the drain current in the closed state to the open one is more than 10⁷, and the channel resistance is $\sim 1.5\times 10^8$Ohm. Dependences of drain–source current $I_{\mathrm{\text{DS}}}$ on drain–source voltage $U_{\mathrm{\text{DS}}}$ have been approximated by an expression using a quadratic model :

where $\beta ={\mu }_{\mathrm{\text{FE}}}\cdot \epsilon \cdot W∕(d\cdot L)$, ${\mu }_{\mathrm{\text{FE}}}$ — electron mobility, $\epsilon$ — the dielectric constant. From Eq. (1), the following values were obtained: ${\mu }_{\mathrm{\text{FE}}}=1.5{\mathrm{\text{cm}}}^2$/Vs, $U_{\mathrm{\text{th}}}=30$V.

The ferroelectric channel thickness was of the order of the spontaneous polarization screening depth ($t\approx 400$nm), which guarantees the presence of spontaneous polarization in the channel. Thus, the main contribution to the $I_{\mathrm{\text{DS}}}$-$V_{\mathrm{\text{DS}}}$ output characteristic is made by the polarization screening region. On the other hand, the threshold voltage $U_{\mathrm{\text{th}}}$ can be estimated using the following expression :

where $\varphi$ is the difference of energies between the MgF₂ gate insulator (4.8eV) and ZnO channel (3.3eV), i.e., $\varphi =1.5$eV, $C_{\mathrm{\text{OX}}}$ is the capacitance of the field-effect transistor and ${\psi }_b$ is the half-width of the charge $Q_f$ accumulated in the dielectric.

Figure 8(a) shows the time dependence of the drain–source current $I_{\mathrm{\text{DS}}}$ at a stable drain–source $U_{\mathrm{\text{DS}}}$ and gate–source $U_{\mathrm{\text{GS}}}$ voltages. The current is controlled by switching the direction of spontaneous polarization in the ZnO:Li ferroelectric semiconductor channel. Measurements have shown that this structure withstands 10⁵-10⁶ polarization switching. Figure 8(b) shows the electrical circuit for switching a transistor from state a to state b. The sequence of steps for switching the direction of spontaneous polarization: (i) Switch 4 is closed, the source and drain have the same potential; (ii) generator 2 supplies a switching pulse, (iii) the switching process is controlled by the voltmeter 6 by current through resistor R2, (iv) for reading the recorded information, switch 4 is opened, and voltage sources 2 and 3 supply voltage to the source–drain and gate–source, respectively, (v) the magnitude of the voltage drop across resistance R2 determines state a or b.

Figure 9(b) shows the time dependence of $U_{\mathrm{\text{DS}}}$ voltage and binary signal levels “0” and “1” when the $U_{\mathrm{\text{GS}}}$ voltages are applied to the gate of the transistor (Fig. 9(a)). The pulse A leads to the switching of the spontaneous polarization direction, i.e., information is recorded, and the pulse B probes the $P_S$ direction, i.e., information is read without its destruction. A change in the $P_S$ direction leads to a significant change in the $I_{\mathrm{\text{DS}}}$ current (see Fig. 8(a)) and, as a result, to a change in the value of the $U_{\mathrm{\text{DS}}}$ voltage. This state remains for 10⁶s, while the change of $U_{\mathrm{\text{DS}}}$ does not exceed 5%. The $U_{\mathrm{\text{DS}}}$ voltage at negative and positive polarization is 5.8V and 4V, respectively. Thus, the difference between the levels of binary signals “0” and “1” is equal to 1.8V.

Studies have been carried out on FET parameters for use as a memory element. We controlled changes in the $I_{\mathrm{\text{DS}}}$ current during a single recording of information and multiple readings (reading without destroying the recorded information). For an FET in the “0” state, the current decreases by 5% in 100 days, and by 10% in 150 days. For FET in state “1”, the $I_{\mathrm{\text{DS}}}$ decreases by 2.5% in 100 days, and by 8% in 150 days. At multiple write–read processes, the $I_{\mathrm{\text{DS}}}$ value decreases by 10% in 10⁷ cycles.

Figure 10 shows a schematic diagram of a ferroelectric field-effect transistor. The bending of the conduction band and valence band of a ferroelectric semiconductor, caused by the polarization screening charges, is shown.^8,15 The mechanism of the conductivity change in the field-effect transistor channel is due to a change in $P_S$ direction. In fact, we can consider the developed structure as a dual-gate transistor.^14,16 Top gate (6 on Fig. 3) is the traditional metallic gate, while the electric field of the virtual bottom gate is due to the $P_S$ screening charge (7 on Fig. 3). The channel is formed at the ZnO:Li-Al₂O₃ transition (in the area of spontaneous polarization screening). The simultaneous presence of ferroelectric and semiconductor properties in ZnO:Li films is crucial for analyzing the mechanism of charge carrier transport. Due to the semiconductor nature of the ZnO:Li film, the channel can have both conduction electrons and bound charges due to the screening of spontaneous polarization. These bound charges are equivalent to the charges created by the gate in the FET channel. The surface charge has two specific values: $+P_S$ and $-P_S$. The behavior of a transistor with a ferroelectric channel is akin to that of a transistor with a channel made of a conventional semiconductor¹⁷ (with the distinction that its properties can be controlled by altering the direction of polarization).

4. Conclusion

The field-effect transistor with an n-type channel has been developed based on a zinc oxide wide-gap semiconductor. Specifically, a ZnO:Li film exhibiting the properties of a ferroelectric semiconductor with low conductivity was used as the channel for the transistor. Measurements of the static characteristics of the transistors and the processes of writing and reading of information were carried out. The channel of the field-effect transistor is formed in the region of screening of spontaneous polarization, because ZnO:Li is a ferroelectric semiconductor. The fundamental possibility of switching the static characteristics of the field-effect transistor is demonstrated when the direction of spontaneous polarization of the ferroelectric semiconductor film in the FET channel is changed. These results have potential applications in creating a manipulation field-effect transistor.

Acknowledgment

The work was supported by the Science Committee of RA in the frames of the research project N 21T-1C150.

ORCID

Armen Poghosyan  https://orcid.org/0000-0001-6657-1701