AVS2012 Session EM+OX-WeA: Oxides and Dielectrics for Novel Devices and Ultra-dense Memory

The rectenna structure we focus on consists of a micro-antenna and a rectifying metal-insulator-metal (MIM) diode for converting electromagnetic wave induced alternating current on the antenna to a direct current. The antenna couples to ambient or directed electromagnetic (EM) radiation, and the diode rectifies the AC signal for DC current and power generation. The frequency or the wavelength of the EM signal dictates the physical dimensions of the antenna, and the operating frequency of the diode. Here one of the main challenges in achieving DC output upon IR radiation is to fabricate a diode capable of operating in the infrared range of frequencies, and more specifically at 30 Terahertz corresponding to 10 micrometer infrared radiation or approximately 300oK. To meet this challenge, we are fabricating metal-insulator-metal diode structures with oxide thicknesses of on the order a couple of nanometers or less.

An application of significant interest is a high resolution, high speed IR imager that can operate at room temperatures. Expanding from a single pixel to a complete large array is a challenge because the added complexity may give rise to electromagnetic coupling between adjacent elements, which needs to be accounted for. In addition, IR imaging, which corresponds to 30 terahertz region of operation, is special because it represents an ‘in-between’ region between radio and optical frequencies. At 10 micron wavelengths, the photons are typically too small in energy to economically convert their AC power into DC using semiconductor or quantum based photodetectors. At the same time, their frequency is too high to utilize standard PN or Schottky barrier diode based rectification. We thus explore the use of a micro-antenna coupled to a MIM diode for AC to DC conversion. A major difficulty here is to develop a diode that responds quickly enough to be forward biased during one part of the AC cycle and reverse biased to the other half of the cycle, without having the parasitic capacitance of the diode short out the signal. The rectenna we are developing uses a Nickel/Nickel-Oxide/Nickel (Ni-NiO-Ni) MIM structure, fabricated and designed using unique modeling and processing techniques.

In the conventional silicon-on-sapphire (SOS) technology with the epitaxy of Si on r-plane (1-102) sapphire, typical device region do not need reverse bias between the substrate and device area for electrical separation, because SOS wafer is separated by the sapphire itself, a best insulator. The advantage is that the sapphire insulator is very thick, which engenders an ultra-small capacitance and therefore it can reduce parasitic capacitance and leakage current at a high operating frequency. However, SOS wafer has a limitation in carrier mobilities due to the silicon material. The mobilities of SiGe can be a few times higher than those of silicon due to the high carrier mobilities of germanium (p-type Si: 430 cm²/V·s, p-type Ge: 2200 cm²/V·s, n-type Si: 1300 cm²/V·s, n-type Ge: 3000 cm²/V·s at 10¹⁶ per cm³ doping density). Therefore, RF devices which are made with rhombohedral SiGe on c-plane sapphire can potentially run a few times faster than RF devices on SOS wafers.

NASA Langley’s rhombohedral epitaxy uses an atomic alignment of the [111] direction of cubic SiGe on top of the [0001] direction of the sapphire basal plane (c-plane). It shows a sample of rhombohedrally grown SiGe on c-plane sapphire with a single crystalline percentage of 95%. Twin defects exist only at the edge of the wafer. The electron mobilities of the tested samples are between those of single crystal Si and Ge. For instance, the electron mobility of 95% single crystal SiGe is 1538 cm²/V·s which is between 350 cm²/V·s (Si) and 1550 cm²/V·s (Ge) at 6x10¹⁷ /cm³ doping concentration. Typically, a rhombohedral single crystal SiGe has 2 or 3 times higher carrier mobility than monocrystalline silicon. If the defects in SiGe can be removed, transistors with higher operational frequencies can be fabricated for a new generation of ultrafast chipsets.

MIM tunnel diodes have been proposed for high speed applications such as hot electron transistors, IR detectors, and optical rectennas for IR energy harvesting as well as backplanes for LCDs. The majority of these applications require highly asymmetric and non-linear I-V behavior at low applied voltages. The standard approach to achieving asymmetric operation in MIM devices is through the use of metal electrodes with different workfunctions (Φ_M). However, the amount of asymmetry achievable using this method is limited by the Φ_M difference that can be obtained using practical metals. In this work, we use an alternative approach to achieving asymmetric and non-linear operation – engineering of the insulating tunnel barrier using nanolaminate pairs of insulators with different bandgaps and band-offsets to produce asymmetric tunnel barriers. Electrons tunneling from one metal electrode to the other will see a different shape barrier depending on the direction of tunneling and the bias applied.

Recently, we found that atomic scale roughness at the bottom metal-insulator interface can dominate the I-V characteristics of MIM diodes, even overwhelming the influence of Φ_M difference. By using the amorphous metal ZrCuAlNi (ZCAN) as an ultra-smooth bottom electrode in combination with high quality dielectrics deposited via ALD, we were able to fabricate MIM diodes dominated by FN tunneling. In this work, nanolaminate insulator stacks consisting of either HfO₂/Al₂O₃, Ta₂O₅/Al₂O₃,ZrO₂/Al₂O₃, or HfO₂/ZrO₂ are deposited on sputtered ZCAN bottom electrodes via ALD. Al dots form the top gate electrode.

MIIM I-V characteristics were found to be sensitive to the relative thickness as well as the arrangement of the individual dielectric layers. In the ZCAN / Al₂O₃ / HfO₂ / Al orientation, the asymmetric tunnel barrier opposes the effect of the asymmetric Φ_M, a larger current is measured at positive bias, and asymmetry is lower than for a neat Al₂O₃ insulator device. However, in the ZCAN / HfO₂ / Al₂O₃ / Al orientation, the asymmetric tunnel barrier enhances the asymmetric Φ_M, a larger current is measured at negative bias, and asymmetry is greater than for neat Al₂O₃ or HfO₂ insulator devices. High asymmetry is seen when conduction in both dielectric layers is dominated by FN tunneling rather than bulk limited mechanisms. Ta₂O₅/Al₂O₃, e.g., did not show an enhancement in asymmetry.