Methane Influence on Diamond Schottky Barrier Diode Performance

The use of synthetic p-type diamond for high power electronic applications has been of significant interest, and the success in the performance of such devices highly depends on the control of the growth process of diamond films. Despite recent advances in the synthesis of chemical vapor deposition (CVD) single crystal diamond (SCD), the device performance still remains far below the theoretical capacities.¹ Therefore, the optimization of the deposition conditions of B-doped layers has a crucial role in the improvement of electronic devices such as Schottky barrier diodes.<br/>In a previous study² it was shown that by increasing the methane concentration from 0.5% to 3% in the CVD plasma, the boron incorporation increased from 10¹⁷ cm^-3 to well above the Mott transition 10²¹ cm^-3, leading to heavily B-doped SCD films (<i>p</i>⁺). This fact was corroborated by first-principle density functional theory calculations. It was revealed that the presence of CH₂ sites has no significant impact on the binding energy of a boron atom. However, the CH₂ sites enhance the chance of a so-called H-defect site formation, <i>i.e. </i>a desorption of an H atom on the diamond surface thereby providing additional boron binding sites.<br/>In this work, the previously studied <i>p</i>⁺ layers² were used to fabricate pseudo-vertical Schottky barrier diodes to investigate the effect of methane concentration during the growth of B-doped layers on the eventual device performances. Within this framework, the optimized growth conditions were used to deposit lightly B-doped diamond layers (<i>p</i>^-) with an acceptor concentration of (1.0 0.5) 10¹⁶ cm^-3, as measured by the Hall effect, on the said <i>p</i>⁺ layers. These <i>p</i>⁺ layers were grown by microwave plasma-enhanced CVD on 3 3 mm²Ib (100)-oriented high pressure, high temperature substrates. Ti (40 nm) / Au (60 nm) and Zr (30nm) / Au (50 nm) contacts were deposited on the <i>p</i>⁺ and <i>p</i>^- layers as the Ohmic and Schottky contacts, respectively.<br/>The electrical characteristics of the rectifiers exhibited good reproducibility regardless of the methane concentration used during the deposition of the <i>p</i>⁺ layers and the size of diodes, indicating the uniformity of the Zr / diamond interface over large areas. In addition, a very low reverse current of 10^-12 A was measured, pointing to the high crystalline quality of the grown layers. The forward operation current was boosted proportionally from the rectifiers with the <i>p</i>⁺ layer grown at the lowest [CH₄]/[H₂] of 0. 5%, to the diodes with the <i>p</i>⁺ layer grown at the highest [CH₄]/[H₂] of 3 %. This is associated with the increase of boron incorporation in the diamond layers by increasing the methane concentration during the CVD growth. The highest current density (10³ A/cm²) was found for the diodes with the <i>p</i>⁺ layers grown at 2% and 3% methane concentrations. The capacitance-voltage measurements of the diodes showed a punch-through behavior with a very low doping level (10¹⁴cm^-3).<br/><b>References:</b><br/>H. Umezawa <i>et al.</i>, Diam. Relat. Mater., <b>24</b>, 201-205, 2012.<br/>R. Rouzbahani <i>et al.</i>, Carbon, <b>172</b>, 463-473, 2021.