Carbon Nanotube Schottky DiodesUsing Ti−Schottky and Pt−OhmicContacts for High FrequencyApplicationsHarish M. Manohara,* Eric W. Wong, Erich Schlecht, Brian D. Hunt, andPeter H. SiegelJet Propulsion Laboratory, California Institute of Technology, 4800 Oak GroVe DriVe,Pasadena, California 91109Received May 4, 2005; Revised Manuscript Received May 13, 2005ABSTRACTWe have demonstrated Schottky diodes using semiconducting single-walled nanotubes (s-SWNTs) with titanium Schottky and platinum Ohmiccontacts for high-frequency applications. The diodes are fabricated using angled evaporation of dissimilar metal contacts over an s-SWNT.The devices demonstrate rectifying behavior with large reverse bias breakdown voltages of greater than−15 V. To decrease the series resistance,multiple SWNTs are grown in parallel in a single device, and the metallic tubes are burnt-out selectively. At low biases these diodes showedideality factors in the range of 1.5 to 1.9. Modeling of these diodes as direct detectors at room temperature at 2.5 terahertz (THz) frequencyindicates noise equivalent powers (NEP) potentially comparable to that of the state-of-the-art gallium arsenide solid-state Schottky diodes, inthe range of 10-13W/xHz.For high-frequency applications in the range of 30 GHz to3 THz (microwave to submillimeter wave regime), diodesare of particular interest as detectors, mixers, and frequencymultipliers.1In particular, solid-state Schottky diodes (rec-tifying metal-semiconductor junctions) are employed becauseof their higher switching speeds, and their inherent suitabilityfor low-voltage, high-current applications. The state-of-the-art utilizes solid-state Schottky diode detectors for roomtemperature sensor systems and Schottky diode multipliersfor submillimeter wave power generation. However, abovea few hundred GHz the inherent parasitic capacitance(proportional to semiconductor junction area) and resistance(inversely proportional to electron mobility) of these devices,because of the limitations of the fabrication process and thematerial properties, severely limits the achievable sensitivityfor detection (direct detector noise equivalent power or NEP∼10-12W/xHz, heterodyne NEP ∼10-17W/xHz for cooledoperation at 4 K),1and generated power at THz frequencies(only microwatts of power up to 1.5 THz).1From the materialpoint of view, carbon nanotubes2offer an excellent alternativeto their solid-state counterparts because of their small junctionareas due to their physical dimensions (<1 to 2 nm diameter),high electron mobilities (up to 200 000 cm2/V-s as reportedby Durkop et al.),3and low estimated capacitances (tens ofaF/µm),4,5leading to predicted cutoff frequencies in the THzrange.5The electronic properties of single-walled carbon nano-tubes (SWNTs) have been studied in detail.6-10The synthesisof SWNTs results in tubes that are either metallic (m-SWNTs) or semiconducting (s-SWNTs) depending on theirchirality. Semiconducting SWNTs typically exhibit p-typeconductivity for measurements done in air, for reasons stillunder discussion.11-15Earlier studies have employed s-SWNTs to develop Schottky-barrier-contact field effecttransistors (FETs),4,16and rectifying junctions based on CNTdefects,4double gates,17or crossed m- and s-SWNTs.18Burkeet al. have studied in detail the AC response of s-SWNT-FETs using phenomenological models5and through mea-surements at 2.6 GHz.19Interestingly, the latter workdemonstrates a significantly decreased AC impedance (whencompared to DC impedance) of the device (at 4 K) becauseof a possible capacitive coupling between the nanotube andthe contact pads. In fact, in m-SWNT circuits they measureAC impedances (∼1.7 kΩ) much lower than the quantumlimited resistance for a 1-D system (h/4e2∼ 6.25 kΩ).4While, this is encouraging, a further reduction in parasiticsthat hinder the AC performance of an electronic device canbe achieved by employing a Schottky diode design in whicha substrate-less membrane architecture can be employedsimilar to an earlier reported monolithic membrane diode(MoMED) design for a 2.5 THz receiver system.20Atheoretical study conducted by Leonard et al.21concludedNANOLETTERS2005Vol. 5, No. 71469-147410.1021/nl050829h CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 05/27/2005that, unlike in planar junction Schottky diodes, the Fermilevel pinning in carbon nanotube Schottky diodes does notcontrol the device properties, and as a result the thresholdmay be tuned for optimal device performance. They showedthat for these devices the Schottky barrier height is controlledby the metal work function, unaffected by the Fermi levelpinning, which offers the possibility of controlling the barrierheight by the choice of the metal.In this letter, we demonstrate Schottky diodes created bydepositing two dissimilar metals at the two ends of p-types-SWNTs, one metal with lower work function (Φ) than thatof the SWNT (ΦNT∼ 4.9 eV) to make a Schottky contactand the other with higher Φ than that of the SWNT to makean Ohmic contact. We also show the predicted performanceof these diodes as detectors at high frequencies by calculatingtheir voltage responsivity and NEP using analytical models.The metal deposition was conducted using a self-alignedangled evaporation technique to deposit both metals with asingle photoresist mask. The choice of metals used aretitanium (Ti) for the Schottky contact (ΦTi) 4.33 eV <ΦNT; ΦNT∼ 4.9 eV) and platinum (Pt) for the Ohmic contact(ΦPt) 5.65 eV > ΦNT). Both single s-SWNT devices andmultiple s-SWNT devices have been tested. For highfrequency operation, it is absolutely essential to develop thesediodes with high yield of s-SWNTs per device grown inparallel, as explained later. SWNTs were grown using ironcatalysts on a silicon (Si) substrate with ∼400 nm thickthermal oxide layer. The iron nanoparticles (FeNPs) used togrow SWNTs were synthesized similarly to a previouslypublished procedure.22The distribution of nanoparticlediameters was 5.8 ( 2.0 nm as determined by transmissionelectron microscopy (TEM) using an Akisha EM-002b at100 kV. As described previously,23monolayers of FeNPswere patterned onto oxidized silicon substrates using 350nm thick PMMA (MicroChem, 950K, 4% in chlorobenzene).All the growths were done at 850 °C using methane (CH4-1500 sccm) and hydrogen (H2-50 sccm) at a pressure of780 Torr. The resulting SWNTs were characterized by atomicforce microscopy (AFM) using a DI Nanoscope III withsilicon