David L. Dreifus

Diamond devices and electrical properties

Abstract Diamond offers tremendous potential for electronic applications such as field effect transistors. An investigation of the electrical properties of boron-doped homoepitaxial diamond films and the metal-oxide-diamond gate structure was performed. Additionally, field effect transistors were fabricated and characterized. Improvements in the diamond deposition process produced boron-doped homoepitaxial diamond films where the room temperature Hall mobility exceeded 1000 cm 2 V −1 s −1 . Analysis of the temperature-dependent carrier concentration indicated that the compensation was 15 cm −3 . The gate structure for metal-silicon dioxide-boron-doped diamond field effect transistors was evaluated by current-voltage and capacitance voltage measurements. Good correlation of the uncompensated acceptor concentration, determined by capacitance-voltage measurements, and the boron concentration, determined by secondary ion mass spectroscopy, was attained. Preliminary measurements suggested that the density of interface states for this structure was ≈ 10 12 cm −2 eV −1 . Field effect transistors exhibited saturation and pinch-off at temperatures as high as 773 K. The highest normalized transconductance measured was 1.3 mS mm −1 . The field effect transistors were combined into analogue and digital circuits that operated at 523 K and 673 K, respectively.

A review of the electrical characteristics of metal contacts on diamond

Abstract The formation of closely spaced selected area ohmic and rectifying contacts on a given semiconductor surface is of fundamental interest for the fabrication of transistor type devices. However, it has been observed that the formation of good ohmic or rectifying contacts is not always easily accomplished on diamond films grown by chemical vapor deposition (CVD). Contacts established with aluminum or gold on CVD films exhibit highly resistive ohmic or nominally asymmetric behavior, whereas these metals can be used almost routinely to form rectifying contacts on synthetic and natural semiconducting diamond crystals. Considerable effort has been directed towards producing low resistivity ohmic contacts on natural semiconducting crystals. A commonly employed technique is the deposition of metals such as titanium, tantalum and molybdenum with a protective film of gold and a post-deposition anneal. Specific contact resistances of the order of 10 −5 ohmcm 2 have been achieved by this technique. Low-resistance contacts have also been obtained by placing metal contacts on highly doped films. A combination of boron ion-implantation, to achieve a high surface concentration, and a subsequent Ti/Au metallization has been employed for the fabrication of ohmic contacts on polycrystalline diamond films with a specific contact resistance of the order of 10 −6 ohmcm 2 .

Polycrystalline diamond field-effect transistors

Abstract The first demonstration of field-effect transistors fabricated from polycrystalline diamond thin films is reported. Diamond thin films were deposited by a microwave plasma chemical vapor deposition technique. The surface roughness was removed by polishing using a SiO x chemical machining technique. Ion implantation was employed to form a B-doped conducting surface channel with an approximate carrier concentration of5 × 10 18 cm −3 . A low temperature deposition of SiO 2 was used to form the gate dielectric structure. Gate leakage currents were below 10 nA at 25 V. Although the channel did not reach the pinch-off condition, modulation of the channel conductance was observed.

Electron emission measurements from CVD diamond surfaces

Abstract Electron emission measurements on diamond films synthesized by chemical vapor deposition are reported. UV photoemission spectroscopy indicates that the samples exhibit a negative electron affinity after exposure to hydrogen plasma. Secondary electron emission yields vary from 2.2 to 9.2. Field emission current-voltage measurements indicate threshold voltages ranging from 28 to 84 V μm −1 . The film with the highest secondary yield also exhibits the lowest emission threshold.

High-temperature operation of polycrystalline diamond field-effect transistors

Operation of polycrystalline diamond field-effect transistors (FETs) at temperatures up to 285 degrees C and drain-to-source voltages of up to 100 V has been demonstrated. The devices were fabricated from B-doped polycrystalline diamond grown by a microwave plasma-enhanced chemical vapor deposition (CVD) technique. At 150 degrees C, the devices exhibited saturation of drain current and a peak transconductance of 65 nS/mm. These are the first polycrystalline diamond devices to demonstrate saturation. Device characteristics at 250 degrees C also show saturation and increased transconductance of 300 nS/mm. Characterization was not performed at temperatures exceeding 285 degrees C due to gate leakage current above 10 nA.<<ETX>>

Bias assisted etching of diamond in a conventional chemical vapor deposition reactor

A novel technique for selectively etching diamond films is presented. This letter describes a technique by which diamond may be etched in a conventional plasma assisted chemical vapor deposition (CVD) reactor at rates comparable to those reported for both electron cyclotron resonance and reactive ion etching techniques. This technique involves negatively biasing the diamond film, while it is immersed in a mostly hydrogen containing plasma. Negative dc bias assisted etching of CVD diamond films is performed in both microwave and dc plasma reactors over a wide range of temperatures and pressures. Speculation on the etching mechanisms is also included.

Metal‐intrinsic semiconductor‐semiconductor structures using polycrystalline diamond films

The electrical characteristics of a metal‐intrinsic semiconductor‐semiconductor structure formed by Al‐undoped polycrystalline diamond‐B‐doped polycrystalline diamond were investigated. Boron‐doped diamond films containing B‐to‐C ratios of 400 and 4000 ppm in gas phase were deposited on (111)‐oriented B‐doped Si substrates. Subsequently, undoped diamond layers were deposited on the B‐doped diamond films for 60 min. The existence of a bilayer structure in terms of the atomic B concentration was confirmed by a secondary‐ion mass spectroscopy. Significant improvements in the rectifying characteristics could be obtained with the introduction of an undoped diamond layer.

Effect of Annealing in Air on Electrical Resistances of B-Doped Polycrystalline Diamond Films

The temperature dependent electrical behavior of as-deposited B-doped polycrystalline diamond films was investigated. Diamond films with various atomic B concentrations were deposited using a microwave plasma chemical vapor deposition technique. The atomic B concentrations were determined using secondary ion mass spectroscopy. Resistances were measured between 25 and 525° C in air. Similar electrical behavior was observed for films with atomic B concentrations less than 3×1017 cm-3 in which the resistance increased over 6 orders of magnitude after heating to 525° C. The variation in resistance between the heating and cooling cycles decreased with increasing B concentration. Little or no variation in the resistance was observed for films with atomic B concentrations higher than 4×1018 cm-3. The variable temperature dependent behavior can be explained by a surface conduction mechanism.

Passive Diamond Electronic Devices

Diamond electronic device exploration began over 70 years ago with evaluations of natural stones for potential use as radiation detectors.1 Often thousands of natural stones were painstakingly evaluated for their “counting” properties in the presence of ionizing radiation.2, 3 Over the years, the electrical, mechanical, optical, and magnetic properties have been studied in natural stones.1 These properties were often found to be superior to those of any other material. The development of synthetic high temperature, high pressure and explosion-based processes for producing synthetic diamonds provided large quantities of similar material beginning in the 1950’s.1 Industrial use of diamonds was primarily limited to cutting and abrasive applications with a few niche applications involving optical windows or heat spreaders.4 Unfortunately, high quality diamond, either naturally occurring or produced synthetically, was still limited to a few square millimeters. Thus, diamond has not found wide acceptance as a material for electronic device fabrication owing to the availability of reproducible high quality large-area, inexpensive material.

Rectifying diodes with a metal/intrinsic semiconductor/semiconductor structure using polycrystalline diamond films

Abstract Rectifying diodes with a metal/intrinsic semiconductor/semiconductor structure formed by Al/undoped polycrystalline diamond/B-doped p-type polycrystalline diamond film were investigated. The undoped diamond/B-doped diamond bilayer films were grown by microwave plasma chemical vapor deposition. Good rectification ratios of 10 4 -10 5 at 5 V were obtained for atomic B concentrations between 1 × 10 18 and 1 × 10 19 cm −3 . A reverse-bias breakdown voltage in excess of 20 V was observed for an undoped diamond layer 0.4 μm thick. Furthermore, a rectification ratio of 10 3 at 5 V was obtained, even at 300°C. For comparison, metal/semiconductor diodes formed by Al/B-doped polyerystalline diamond film were fabricated, but only a non-linear non-rectifying behavior was observed.