Higher Performance Metal-Insulator-Metal Diodes using Multiple Insulator Layers

Research Article

Austin J Nanomed Nanotechnol. 2014;1(1): 1004.

Higher Performance Metal-Insulator-Metal Diodes using Multiple Insulator Layers

Aydinoglu F1,3*, Alhazmi M,1,3, Cui B2,3, Ramahi OM3, Irannejad M1,3, Brzezinski A1,3 and Yavuz M1,3

1Department of Mechanical and Mechatronics Engineering, University of Waterloo, Canada
2Department of Electrical and Computer Engineering, University of Waterloo, Canada
3Waterloo Institute for Nanotechnology, University of Waterloo, Canada

*Corresponding author: Yavuz M, Department of Mechanical and Mechatronics Engineering, University of Waterloo, Canada and Waterloo Institute for Nanotechnology, University of Waterloo, Canada

Received: December 05, 2013; Accepted: December 27, 2013; Published: December 31, 2013


It is found that by repeating two insulator layers with different electron affinities and keeping the total insulator thickness constant, the asymmetry and nonlinearity values can have significant impact on the behavior of Metal- Insulator-Metal diodes. The asymmetry value of a diode with a double insulator layer was recorded as 3, however, for a quadra insulator layer diode; the asymmetry value was recorded as high as 90. The new MIM diode design promises a strong impact on emerging applications such as energy harvesting from fast switching electromagnetic waves.

Keywords: Metal-Insulator-Metal diodes; Schottky diodes


A rectifier with a high response time is critical for energy harvesting, infrared detectors, hot electron transistors [1,2], liquidcrystal display backplanes [3], macro-electronics [4], field-emission cathodes [5], and switching memories [6]. Schottky diodes are commonly used in these applications because of their fast response time. However, Schottky diodes do not operate efficiently at high THz frequencies. A Metal-Insulator-Metal (MIM) diode, which is a thin insulator layer sandwiched between two metals, is superior to Schottky diodes beyond the 12 THz [7]. The cut-off frequency of the diode is inversely proportional to the junction area of the diode. A MIM diode can operate at high Hz frequencies by reducingthe junction area to nano-scale. Bareiss et al. [8] fabricated 93 nm diameter MIM diodes and claimed that these nano-scale diodes can roperly operate up to 219 THz. MIM diodes can operate at the THz range due to their femto-second fast transport mechanism of quantum tunneling [9]. The tunneling phenomenon is one of the most important factors in MIM diodes. The thickness of the insulator changes the tunneling efficiency exponentially [10]. To achieve tunneling, the thickness of the insulator should be less than 10 nm [11]; however, to make the tunneling efficient, the thickness should be further reduced. The insulator layer of the diode can be grown in different ways including sputter oxidation, vapor deposition, thermal oxidation, anodic oxidation, electron-beam deposition and atomic layer deposition. Atomic layer deposition (ALD) is considered as thebest way to provide uniform, pinhole free and ultra-thin oxide layers. This highly controllable deposition process makes this technique highly compatible for the growth of the insulator layer of the MIM diodes.

In addition to the operation frequency and the tunneling efficiency, the current-voltage (I-V) curve is another important parameter in determining the diode's performance. An ideal diode has to have an asymmetrical I-V curve to achieve rectification. The asymmetry is simply defined as forward current divided by reverse current (|IF /IR|). A MIM diode could have an asymmetric I-V curve if a single insulator is sandwiched between two different metals since each side has a different barrier height value,φ, which is a potential difference between the two materials. Another significant characteristic in I-V curve is turn-on voltage.

The diode's non-linearity is given by:

( dI dV ) V I MathType@MTEF@5@5@+=feaaguart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVCI8FfYJH8YrFfeuY=Hhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfeaY=biLkVcLq=JHqpepeea0=as0Fb9pgeaYRXxe9vr0=vr0=vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaKazaaiabiqaaG5ejugObabaaaaaaaaapeGaaiikaKazfaiadaWcaaqcKbaGa8aabaqcLbAapeGaaeizaiaabMeaaKazaaiapaqaaKqzGgWdbiaabsgacaqGwbaaaiaacMcajqwbacWaaSaaaKazaaiapaqaaKqzGgWdbiaabAfaaKazaaiapaqaaKqzGgWdbiaabMeaaaaaaa@4989@

Where dI, dV, I and V are variations in current, variation in voltage, current and voltage respectively. High non-linearity and high asymmetry cannot be achieved by a single insulator layer in MIM diodes [10]. MIM diodes with double insulator layers can overcome this challenge [10]. The barrier height value at each interface plays the key role regarding the tunneling efficiency, asymmetry and non-linearity in MIM diodes. The barrier height value is defined as the difference between the work function Φ of the metal and the electron affinity χ of the insulator at metal-insulator interfaces and is the difference between two electron affinity values of the insulators at insulator-insulator interfaces. If there is only one insulator used, there will be two different potential barrier values which are between metal one and the insulator, and between the metal two and the insulator. These two interfaces determine the turn on and breakdown voltages depending on the work functions of the metals. If there are two insulators, there will be three different barrier height values. If the order of the insulators is changed, the I-V characteristics will also be changed due to the change in barrier height at each interface.

In this work, a series of MIM diodes with double and four insulator layers, metal-insulator-insulator-metal (MI2M), and metal-insulator-insulator-insulator-insulator-metal (MI4M) were fabricated and characterized, in order to observe the impact of the number of insulators in a MIM diode performance.


The MI2M's bottom metal, M1, was chosen as Chromium (Cr), and the top metal, M2, were considered Titanium (Ti), based on their work functions properties. The insulator layers TiO2 and Al2O3 were considered due to their difference in the electron affinity. Conventional photolithography followed by electron beam evaporation and atomic layer deposition (ALD) techniques, were used to fabricate the MIM diodes on the SiO2 substrate.

A 60 nm thick Cr was deposited on a SiO2 substrate by e-beam evaporation. After a lift-off process, insulator layers were deposited by ALD. The 34 cycles of the TiO2 and 14 cycles of the Al2O3 were deposited by one atom layer by one atom layer (see supplementary information), at deposition rate of 0.044 nm/cycle and 0.105nm/cycle, respectively for TiO2 and Al2O3. The total thickness of the insulator layers in the MI2M diode was 3 nm, with each single insulator layer thickness of 1.5 nm. After a second photolithography process, 100 nm Ti was deposited by e-beam evaporation technique. A second lift-off process was performed to remove the photoresist and un-patterned areas of the second metal. The oxide layers onto the M1 were removed by using the reactive ion etching (RIE) process with Ar. The MI4M diode with, the same bottom and top electrodes as MI2M structure were fabricated with the procedure as aforementioned except that 17 cycles of the TiO2, 7 cycles of the Al2O3, 17 cycles of the TiO2, 7 cycles of the Al2O3, respectively, were deposited by ALD as insulator layers. Therefore, in the MI4M diode, the thickness of each insulator layer was 0.75 nm, and the total thickness of the insulator layer in the MI4M diode was 3 nm which was the same value used in the MI2M diode. A schematic diagram of the fabricated Cr/TiO2/Al2O3/Ti MI2M diode and Cr /TiO2/Al2O3/TiO2/Al2O3/Ti MI4M diode are shown in Figure 1.

Results and Discussion

The MI2M diode and MI4M diodes were fabricated by keeping the total thickness of the insulator layer as 3 nm to investigate the impact of the number of insulator layers on the electrical properties of the diodes. There are three different interfaces between two metals in the MI2M diode. Each interface value. Figure 1b shows the energy band diagram of the MI2M diode without applying any voltage. The shape of the energy band diagram was determined by the work functions and the electron affinities. The electron affinity of the insulators changes with the thickness [12] and film morphology. The electron affinities of the ALD deposited TiO2 and Al2O3 of 15 nm thickness are 3.9 eV (χ1) and 2.8 eV (χ2), respectively. The work function of the Cr was 4.5 eV (φ1), and that of Ti was 4.33 eV (φ2). Therefore, the barrier heights was obtained as Φ1 = Φ1 - χ1 = 0.6 eV, Φ2 = χ1 - χ2 = 1.1 eV, and Φ3 = φ2 - χ2 = 1.53 eV. The shape of the energy band diagram also changes by applying a bias to one of the metals. When a negative bias was applied to the Ti, the probability of an electron tunneling from Ti to Cr becomes higher than from Cr to Ti (Figure 1c). However, the tunneling probability of an electron tunneling from Cr to Ti becomes greater than from Ti to Cr when a positive bias is applied to the Ti as shown in Figure 1d.

Citation: Yavuz M, Alhazmi M, Cui B, Ramahi OM, Irannejad M, et al. Higher Performance Metal-Insulator-Metal Diodes using Multiple Insulator Layers. Austin J Nanomed Nanotechnol. 2013;1(1): 1004. ISSN:2381-8956