Perovskite Resonant Tunneling FET with Sequential Negative

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Perovskite Resonant Tunneling FET with Sequential Negative Differential Resistance Peaks Kalpana Agrawal, Vinay Gupta, Ritu Srivastava, and S.S Rajput ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00090 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Electronic Materials

Perovskite Resonant Tunneling FET with Sequential Negative Differential Resistance Peaks Kalpana Agrawal1*, Vinay Gupta1, Ritu Srivastava1 and S.S. Rajput1 1National

Physical Laboratory, Council of Scientific and Industrial Research, Dr. K. S. Krishnan Road, New Delhi, 110012, India. *Email:

[email protected]

Abstract: Vertical resonant tunneling (RT) field effect transistor (VRTFET), fabricated using perovskite (CH3NH3PbI3) has been analyzed for sequential sharp negative differential resistance (NDR) peaks useful in multiple-valued logic devices. NDR peaks are attributed to the sub-bands formation within the parabolic shaped band gap, present at the channel and drain/source interface due to Schottky barriers. Ambipolar CH3NH3PbI3 imparts both p and n mode characteristics with RT NDR peaks. Unprecedentedly high (100 to 1000 V-1) curvature coefficient (ϒ) has been found with two NDR peaks at a short interval, whose positions shift left, with gate bias. Due to ionic nature of CH3NH3PbI3, hysteresis has also been observed in the transfer characteristics. This structure can overcome the limit of 60mV/decade as well as curvature limit of 40V-1, important parameters for analog and digital applications. So, these devices promise for cheaper and easy fabrication at commercial scale operating at ultra low voltage and low power.

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Keywords: perovskite, resonant tunneling FET, negative differential resistance peaks, hysteresis, curvature coefficient (ϒ)

Introduction Increasing demands of high speed, low voltage and low power gadgets, encouraged the researchers to have alternative of silicon metal oxide semiconductor FETs (MOSFETs)1. Tunnel field effect transistor (TFET)2,3 is one of such alternative device, which can impart high speed and low power consumption to the power hungry electronic systems. Their lower sub-threshold slope (SS) results in the low leakage current, main contributor for the power consumption. The minimum SS for silicon MOSFETs is 60mV/decade, which could be decreased using TFETs4. However, obtaining high on current (Ion) and large on/off current ratio (Ion/off) from TFETs is still a challenging job. In 1985, Capasso and Kiehl5,6 proposed RT transistor adopting the heterostructure of AlGaAs/GaAs, which could be used in multiple-valued logic digital circuits etc. The band splitting of the main band in the RTFET 7,8, differentiate it from the TFET9–11. Two or more NDR peaks12,13 occur in RTFET in contrast to only one NDR peak present in TFETs7,14–16. TFETs overcome the (thermal) limit set for SS as 60mV/decade. However, the (thermal) limit set for curvature coefficient (ϒ)11,17–19 as 40V-1 cannot be overcome. ϒ is another useful parameter for future devices used in analog and digital systems, and RTFET is a promising device, having the capability of ϒ, exceeding the limit of 40V-1. The band bending in RTFET results into parabolic (or rectangular) shape energy bands. These bands often splits into discrete energy sub bands 17,20. The parabolic shape band formation is the most probable band shape formed due to band edge tailing21. There is high tunneling probability of charge carriers through the thin parabolic shape barrier as the wavy nature charge particles,


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instead of their climbing over the high energy barrier. The high energy barrier reduces the probability of thermionic emission and creates two or more NDR peaks22,23. For multiple valued logic functions, multiple NDR peaks are required, which need high barrier with deeper parabolic well. Lin et al.24 proposed quasi-hetero junction for RTFET. Some researchers used the vertical hetero-structures25,26 to establish a double barrier tunnel junction for multiple NDR peaks. But, the drawbacks of these hetero-structures are that they operate at very high gate to source voltage (Vgs) and could not be easily reproducible due to the complexity. In today's era, we are moving towards ultra-low voltage and low power devices, which are required to be easily reproducible at commercial scale. In this report, we have proposed a vertical structure for CH3NH3PbI3 based VRTFET with two or more sharp NDR peaks, dependent on Vgs, drain to source voltage (Vds) and Schottky barrier height, present at channel-source/drain interface. Suitable metallic source/drain form rectifying barriers at channel-source/drain interfaces. As source electrode has been made porous for better control of Vgs over the drain current (Ids), the drain-channel interface has been found to be responsible for RT which proves that the presence of double tunnel barriers (hetero-structure) is not essential6,27 for RT phenomena. Results and Discussion In the proposed VRTFET, CH3NH3PbI3 is sandwiched between metallic source and drain to serve as the device channel (Figure 1a). The fabricated device has been shown in Figure 1b. The source terminal has been made porous28,29, to control Ids through Vgs. In all the fabricated devices (>100), the NDR regions have been observed (both in the transfer and output characteristics). The NDR peaks prove the presence of Schottky barrier at the interface of source/drain terminals.



The source being porous, modulate the Ids so only drain-channel interface has been found to be responsible for RT. Aluminum (Al) has been chosen for source and drain contacts to ensure the equal height Schottky barriers at the channel-electrodes interfaces. It has been observed that CH3NH3PbI3 reacts with metal30. But reaction of metal with CH3NH3PbI3 is more pronounced in solutionbased processing rather than vapor deposition. This paper explains the concept of RT occurring due to the Schottky barrier at drain-channel interface, instead of adopting the tedious heterostructures. In the device, first, we have deposited 120nm thick Al layer to form the drain, over which, CH3NH3PbI3 has been vapor deposited. Now, Al (between 10 to12nm) has been deposited for forming the porous source. The gate electrode (Al) which is at the top of device, underneath the combination of insulators, lithium fluoride (LiF) and vendium oxide (V2O5) has been deposited. So, the oxygen and moisture need to cross this insulating layer before reach CH3NH3PbI3. As metal does not react immediately with CH3NH3PbI3, and hence, the device properties remain unaltered at the time of measurement. To minimize the current due to thermionic emission, the barrier at channel-drain interface has been kept high enough, which encourage the charge carriers to tunnel through the thin parabolic shape barrier (Figure 1c) instead of crossing over it. X-ray diffraction pattern (XRD) of CH3NH3PbI3 film (Figure 1d), where the peaks assigned to CH3NH3PbI3 crystal show good crystallinity, due to vacuum evaporation31 with intense diffraction peaks at 14.20 and 28.40 assigned to (110) and (220) diffractions of the tetragonal CH3NH3PbI3 phase32, respectively. In Raman shift for CH3NH3PbI3 film (Figure 1e), the assignment of the band at 60 cm−1 is a clear marker of inorganic component. The 94 cm−1 band is associated with both the Pb−I stretching and to liberation modes of the cations. The 108 cm−1


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and 146 cm−1 are associated with the motion of organic cations. The band at 213 cm−1 can be associated to the torsional mode of the Methylammonium (MA) cations33,34. The surface morphology of CH3NH3PbI3 film35 has been measured by Atomic force microscopy (AFM). The grain size36 and the root mean square (RMS) roughness of the film are found to be 150-250 nm and 7nm respectively (Figure 1f). By using the Gwyddion software, grain size of the perovskite material has been identified. To know the thickness/height of the grain, a line has been drawn through the particle (Figure 1f) to get a height profile graph. The same process has been repeated for many particles to calculate the average size of grain. Operational modes and curvature coefficient (ϒ) Due to the ambipolar37 nature of CH3NH3PbI3, the device has an unique property of being biased either in p mode (holes are the charge carriers) or in n mode (electrons as charge carriers). These modes have been termed as per the nomenclature6,20,38 where the polarity for Vgs and Vds has been chosen opposite39. With no bias applied on the device, electrons and holes are equally distributed in the entire channel. For device, working in p mode, positive Vgs is applied. Since the source is porous, it allows accumulation of negative charges (electrons) at sourcechannel/channel-insulator interface forcing the positive charges (holes) towards channel-drain interface. When negative Vds is applied, holes get attracted towards drain and Ids flows. Similarly, in n mode, negative Vgs causes accumulation of holes at source-channel/channelinsulator interface, forcing the electrons towards channel-drain interface. With positive Vds, electrons move towards drain to current flow. So, the device has two modes: p mode (Vds negative and Vgs positive) and n mode (Vds is positive and Vgs is negative).



Figure 1. Structure of the device and characterization of CH3NH3PbI3 film. (a) The device structure with Al metallic source, drain and gate where 120nm thick CH3NH3PbI3 film working as an active channel has been sandwiched between source and drain while insulator is deposited at top of this channel along with gate electrode. The active area for the device is 1mm2 where all the electrodes overlap. (b) Actual fabricated device picture where source (S), drain (D) and gate (G). (c) Basic band diagram of the device d, XRD spectra confirming the good crystallinity of the CH3NH3PbI3 film. (e) Raman spectra of CH3NH3PbI3 film f, AFM morphology of CH3NH3PbI3 film (120nm thick) which shows the smoothness of the film with RMS value of 7nm.

Most of the reported papers used first derivative (dI/dV) method 3,12,26 for extracting NDR peaks. Though the position of first NDR peak can be estimated easily, but the position for the second NDR peak is difficult to predict, accurately. For RT, sub bands are formed within a parabolic well due to which two or more NDR peaks are expected. So, dI/dV method is not appropriate for the devices having multiple NDR peaks and some other parameter is required for extraction of information about multiple NDR peaks. We have used the ϒ17 defined as the ratio of the second derivative (d2I/dV2) to dI/dV of the I-V characteristics can be expressed as



d 2 I dI / dV 2 dV

(1)


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p-mode electrical characteristics

The measured device transfer characteristics have been shown in Figure 2a where Vgs has been swept from -2 to 2V and then back to -2V. Vds has been kept fixed at -0.2V, -0.6V and -1V. Here, hysteresis40,41 has been observed which shifts left as Vds varies from -0.2V to -1V. The hysteresis may be due to Vgs induced accumulated ionic migration within the CH3NH3PbI3. The hysteresis loop size increases as Vds increases. The SS, Ion, Ioff and Ion/off have been estimated as 40±5mV/dec, 1.6x10-4A, 2.9x10-9A and 0.6x105, respectively (Figure 2b) at Vds = -1V. Instability of perovskite material is a critical issue which leads to poor device stability. External factors such as oxygen and humidity can be settled by proper encapsulation. For internal factors such as thermal instability can be solved by engineering the grain size and the composition of the material. Ion migration is also a critical issue which could be reduced by lowering the applied external field or adding a suitable interfacial layer between electrodes and channel material for FET.



Figure 2. Electrical characteristics when device behaves like p type. (a) Transfer characteristics of VRTFET where Vgs has been swept from -2 to 2V and then back to -2V at Vds = -0.2V,-0.6V and -1V. Ambipolar property of CH3NH3PbI3 can be clearly seen. (b) SS has been calculated as 40±5mV/dec. at Vds = -1V. Ion , Ioff and Ion/off have been estimated as 1.6x10-4A, 2.9x10-9A and 0.6x105, respectively. (c) Output characteristics where Vds has been swept from 0 to -0.5V while Vgs has been stepped from 0 to 0.5V at the step of 0.1V showing the sharp sequential NDR peaks. (d) ϒ vs Vds curve for Vgs = 0V, 0.1V,0.2V,0.3V,0.4V and 0.5V where peak 1 (red font) and peak 2 (pink font) have been clearly visible and both peaks are shifting towards left.

Figure 2c shows the output characteristics with Vds being swept from 0 to -0.5V while, Vgs steps from 0 to 0.5V with 0.1V interval. Two distinct NDR peaks are visible so formation of two sub bands (sub band 1 and sub band 2) is expected. At fixed Vgs = 0.1V, Ids increases with Vds, and then tries to saturate. However, perfect saturation has not been observed. When Vds reaches to 0.36V, first NDR peak has been observed due to alignment of sub band 1 with drain Fermi level (EFD). Further increase in Vds decreases Ids due to the misalignment of sub band 1 with EFD.


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When sub band 2 aligns with EFD, Ids again increases and a second NDR peak results at Vds = 0.4V. If Vds increases, sub band 2 misaligns and Ids decreases. Beyond this Vds, Ids increases abruptly. As the total applied voltage (Vgs +Vds) reaches to 0.5V, it lowers the Schottky barrier at channel- drain interference so thermionic emission starts, resulting in abrupt rise of Ids. Similar results are obtained at Vgs = 0.2V, 0.3V and 0.4V with no NDR peaks beyond Vgs = 0.5V (implying Vds = 0V to maintain Vgs+Vds=0.5V). This is due to the Vgs attaining the pinning voltage6,15. In these devices, leakage current has been observed in the nano-meter range (Figure S1) when operating at low voltage (2>3. Thus, the sub-band moves up (Figure 3c) and the tunneling occurs at lower Vds. This phenomenon of sub bands shifting present due to both, Vgs and Vds has been described (Figure 3d to 3i), by showing only drain side band bending. In Figure 3d, at Vgs = 0V, sub band 1 prominently appears with a trace of sub band 2. When Vgs = 0.1V (Figure 3e), parabolic well becomes deeper so that the sub bands 1 and 2 both, shift little upwards to cause two NDR peaks. At Vgs = 0.2V, these sub bands move up forcing NDR peaks to occur at lower Vds (Figure 3f). When Vgs = 0.3V or 0.4V (Figure 3g and 3h respectively), similar trends continue with shifting of the sub bands upward and approaching the drain Fermi level (EFD) and causing NDR regions at lower Vds. At, Vgs = 0.5 (Figure 3i), sub band 1 totally disappears, while some portion of sub band 2 still visible. Now, HOMO of channel and EFD align having negligible interface barrier so thermionic emission overtakes RT. Sub bands alignments with EFD have been shown (Figure 3j to 3n), for fixed Vgs and sweeping Vds. EFD moves towards sub band 1 with the sweeping of Vds (0 to -0.5V), increasing the Ids (Figure 3j). As sub band 1 aligns with EFD, NDR peak 1 occurs. EFD moves further downward, misaligns with sub band 1 so Ids decreases (Figure 3k). Further increase in Vds, EFD aligns with sub band 2 and Ids increases again, getting NDR peak 2 (Figure 3l). Sub band 2 misaligns as Vds increases further to decrease Ids (Figure 3m). At Vds higher than this voltage, forces Ids to increase



abruptly due to thermionic emission as there is negligible drain-channel interface barrier (Figure 3n). The HOMO of the channel also moves downward as Vds decreases for fixed Vgs.

Figure 3. Study of band diagram having sub bands in the parabolic well for p type. (a) Band diagram without biasing where EF of channel aligns with Fermi level of source and drain metal. (b) Band diagram with biasing where Vgs is varied from 0 to 0.5V. Accordingly, HOMO/LUMO of channel moves upward forming deeper parabolic well. (c) Sub band movement as Vgs increases. Here, w is the width of sub band which is constant at all values of Vgs (d to i) When Vgs is applied from 0 to 0.5V at 0.1V interval, sub bands shift more upwards. (j to n) At fixed Vgs, when Vds is swept from 0 to -0.5V, sub band alignment and misalignment with EFD results in NDR peaks.

n mode electrical characteristics


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Transfer characteristics in n mode have been shown in Figure 4a where Vgs sweeps from -3 to 2V and then back to -3V to observe the hysteresis. Vds, has been kept fixed at 0.2, 0.6V and at 1V. We have observed that at low Vds (0.2V or 0.6V), hysteresis loop is larger than in p mode (Figure 2a). It may be due to faster migration of ions in this mode. The SS, Ion, Ioff and Ion/off have been found to be 40±5mV/decade, 6.5x10-5A, 4.4x10-9A and 1.5x104, respectively at Vds = 1V (Figure 4b). The output characteristics with Vds being swept from 0 to 1V, and Vgs varying from 0 to -1V at an interval 100mV have been divided into two regions for clarity (Figure S3). For Vgs, stepping from 0 to -0.5V (Figure 4c), only one NDR peak can be clearly observed but second NDR peak is not visible. When Vgs has been stepped from -0.6 to -1V (Figure 4d), more NDR peaks at an interval of 20 to 30mV can be observed, confirming the sequential RT. As NDR peaks are not clearly visible with dIds/dVds (Figure S4), again ϒ method has been used for analyzing peaks (Figure 4e), where left shifting of the peaks is prominent. At Vgs= 0V (Figure 4e), two peaks (peak 1 and peak 2) at a short interval of 60mV on Vds axis have been observed (peak 1 at Vds = 0.52V and peak 2 at Vds = 0.58V). However, in output characteristics (Figure 4c), only one NDR peak is visible, which implies that the thermionic emission starts due to lowering of effective barrier (drain-channel interface) near to the voltage when peak 2 occurs at Vds=0.58V. As Vgs varies (-0.1V to -0.5V), gap between peaks increases from 60mV to 240mV, which can be attributed to increase in the gap between sub bands. At Vgs = -0.5V, peak 1 almost disappears while peak 2 appears. At Vgs = -0.6V, along with peak 2, two more peaks (peak 3 at -0.90V Vds and peak 4 at -0.92V Vds) appear at an interval of ~20mV. At Vgs = -0.7V, peak 1 vanishes while



peak 2 is at Vds= 0.12V. Peak 3 and peak 4 have been observed at Vds ranging between -0.79V and -0.82V at a gap of 30mV. At Vgs = -0.8V, peak 1 and peak 2 vanish completely while peak 3 and peak 4 have been observed. Similarly, at Vgs = -0.9V and -1V, peak 3 and peak 4 have been visible. The interval between peak 3 and peak 4 ranges between 20 to 30mV. As Vgs increases, these peaks shift left at lower Vds. So, we can say, when EFD aligns with any sub band within parabolic well of channel-drain interface, Ids increases. Ids decreases when EFD misaligns with sub band. The higher values of ϒ ranging between 103V-1 and 1074V-1 have been observed in this mode as well (Table SII).


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Figure 4. Electrical characteristics when device behaves like n type. (a) Transfer characteristics where Vgs has been swept from -3 to 2V and then back to -3V to see the full hysteresis. Vds, has been kept fixed at 0.2, 0.6V and at 1V. (b) The SS, Ion, Ioff and Ion/off have been found to be as 40±5mV/decade, 6.5x10-5A, 4.4x10-9A and 1.5x104, respectively at Vds = 1V. (c) Output characteristics where Vds has been swept from 0 to 1V and Vgs has been stepped from 0 to -0.5V. (d) Output characteristics where Vds has been swept from 0 to 1V and Vgs has been stepped from 0.6 to -1V. (e) The ϒ vs Vds curve where two peaks, peak1(red font) and peak 2 (pink font) have been clearly visible when Vgs has been stepped from 0V to -0.5V and both peaks are shifting towards left. At Vgs = -0.6V, two more peaks (peak 3 and peak 4) have been observed. At Vgs = -0.7V, peak 1 vanishes while rest of three peaks are visible. At Vgs = -0.8V, -0.9V and -1V, peak 1 and peak 2 are not visible, only peak 3(sky blue font) and peak 4(orange font) are visible.

Hysteresis behavior has been observed due to ion migration. Ion migration will be more if applied electric field is high. In our devices, as operating voltage is very less (

Perovskite Resonant Tunneling FET with Sequential Negative

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