Electrical contacts to ultrananocrystalline diamond

Abstract

The contact behavior of various metals on n-type nitrogen-doped ultrananocrystalline diamond ͑UNCD͒ thin films has been investigated. The influences of the following parameters on the current-voltage characteristics of the contacts are presented: ͑1͒ electronegativity and work function of various metals, ͑2͒ an oxidizing acid surface cleaning step, and ͑3͒ oxide formation at the film/contact interface. Near-ideal ohmic contacts are formed in every case, while Schottky barrier contacts prove more elusive. These results counter most work discussed to date on thin diamond films, and are discussed in the context of the unique grain-boundary conductivity mechanism of the nitrogen-doped UNCD.Diamond thin films are a promising platform material for a large number of electronic applications, including chemical sensors, 1 bioimplantable electronics, and high-power radiation hard field effect transistors. However, many limitations exist which hinder the commercial application of diamond films, for example: High cost, lack of reliable n-type doping, high surface roughness, and high film stress. In addition, a crucial limitation for the materials integration of diamond is the lack of reliable low contact resistance ohmic contacts which can be fabricated at low temperatures.Ultrananocrystalline diamond ͑UNCD͒ films have a set of unique structural and tribological characteristics which obviate many of the limitations just mentioned. 2-6 Deposited via microwave plasma-enhanced chemical vapor deposition ͑MPECVD͒ with unique Ar-rich plasma chemistries, UNCD is an extremely fine-grained ͑2-5 nm͒, smooth ͑24 nm rootmean-square roughness͒, 98% sp 3 diamond film with atomically abrupt grain boundaries. Strikingly, UNCD films can be doped with nitrogen to produce very highly conductive n-type diamond, 3 which is fully active at room temperature and demonstrates reasonable mobilities. Substitutional donor nitrogen in bulk diamond does not result in conduction due to its 1.7 eV activation energy, but the mechanism of nitrogen doping in UNCD is quite different. Theoretical calculations 7 indicate that nitrogen is preferentially incorporated into grain boundaries, which consist of a combination of sp 2 , sp 3 , and other bonded carbon. This nitrogen is energetically favored to form bonding states which result in a lone electron pair and a carbon dangling bond, promoting grain boundary conductivity. The conductivity of nitrogendoped UNCD is correlated with the amount of nitrogen in the films as measured by high mass resolution secondary ion mass spectroscopy. Initial measurements indicate a roomtemperature conductivity of 140 ⍀ Ϫ1 cm Ϫ1 at a nitrogen content of 0.2 at. %. Temperature dependent conductivity measurements indicate a range of thermally activated conduction mechanisms. 3 The conductive UNCD/contact system is thus a complicated one, as the grain boundaries-not diamond grainsform the conductive pathways. The same electrically active states which promote conduction may exist in significant numbers at the UNCD/contact interface, st...

Substrate-diamond interface considerations for enhanced thermionic electron emission from nitrogen doped diamond films

“…Utilization of an n-type nitrogen incorporated UNCD interstitial layer in the device structure suggests the absence of a Schottky barrier at the substrate-diamond interface due to the high nitrogen incorporation and due to the graphitic grain boundaries. 32 In fact, it has been reported that metal contacts to ntype UNCD films are typically ohmic in nature; independent of the metal work function. 33 As the thermionic emission current is affected by device resistivity, the interface electrical resistance of the emitter layer structure may limit the emission.…”

Are Diamonds a MEMS' Best Friend?

“…Metallic ohmic contacts were achieved on UNCD surfaces with sputter-deposited Al, Au, Cr, Cu, Pt, and Ti layers, all at a thickness of 200 nm [28], [29]. Currentvoltage contact characteristics were used to calculate layer resistance from the slopes of these curves, including the resistance of the UNCD plus the resistance of the metal contacts themselves.…”

Fundamentals of ultrananocrystalline diamond (UNCD) thin films as biomaterials for developmental biology: Embryonic fibroblasts growth on the surface of (UNCD) films

“…The growth conditions were as follows: (a) substrate temperature: 800°C; (b) pressure: 200 mbar; (c) microwave power: 1000 W; (d) growth time: 60 min; and (e) gas flow: Ar (98.4 sccm) and CH 4 (1.6 sccm). UNCD films were produced, using the well established Ar-rich/CH4 plasma chemistry developed and extensively used at Argonne National Laboratory [1][2][3][4][5][6][7].…”

“…UNCD films are grown using Ar-rich [(Ar (99%)/ CH 4 (1%)] plasmas, using Microwave Plasma-Enhanced Chemical Vapor Deposition (MPCVD). UNCD films exhibit a unique set of complementary properties, such as chemical inertness, low friction and high wear resistance, high hardness, and good electrical transport properties when appropriately doped [5][6][7]. Carbon films including diamond and diamond-like carbon (DLC) thin films are also being explored as coatings for artificial joints and artificial heart valves.…”