Transmission electron microscopy study of defects generated during chemical vapor deposition diamond lateral growth
Authors: Lloret Fernando , Araujo Daniel , Eon David , Villar M. Pilar , Bustarret Etienne 
Published: December 2016

Synthetic diamond is one of the most promising materials for high power devices due to its extraordinary physical properties, such as high thermal conductivity (22 W/cm K, 4 times that of Cu), electric breakdown field (>10 MV/cm), and carrier mobility (m
n
=1000 cm
2
/V
s
, m
p
= 2000 cm
2
/ V
s
). Moreover, 3D architectures, i.e. lateral growth, allowed the use of vertical geometries for the design of such devices, in addition to other advantages such as higher miniaturization, distribution of the electric field and reduction of technology steps and costs. In fact, in the quest for power electronic devices sustaining ever higher reverse blocking voltages and forward currents, buried heavily boron doped (p
+
) diamond layers have been shown recently [1] to reduce markedly the on‐state resistance (R
on
) of pseudo‐vertical crystal diamond Schottky diodes. Such advanced designs rely on an improved control of selective 3D overgrowth of dry etched mesa and trenches [2].

Here, MPCVD diamond overgrowth on patterned‐etched diamond substrate is demonstrated to be highly selective depending on the methane concentration. This can be very useful for the design of 3D engineered semiconducting devices such as p‐n junctions. However, some growing conditions are shown to generate defects (dislocations, planar defects…). In addition, boron doping is also shown to induce the generation by a proximity strain related mechanism [3] of another type of defects. Mesa structures were fabricated by reactive‐ion etching (RIE) on masked substrates. Overgrowth was performed by microwave induced plasma chemical vapor deposition (MPCVD). A stratigraphic approach of heavily boron doped layers and undoped ones allows to follow the “history” of the growth, in the vicinity of mesa patterns [4], thanks to further cross section TEM observations. The latter identify and distinguish between extended defects generated by: (i) the boron inclusion, (ii) the strain related to the mesa‐step and (iii) the growth conditions.

Defects are studied using dark field (DF) and weak beam DF (WB) in diffraction contrast modes on focused ion beam lamellas. From the invisibility criterion, and families of Burger vectors have been identified. Based on the position of this defects respect the MESA structure, their origin is identified to be: (i) edge and threading dislocations with type of Burger vector are favorably generated by the boron doping while (ii) planar defects with type of Burger vector are highly influenced by the strain accumulated in the corner of the step generated by the mask before the overgrowth.

In addition, multilayer doping allows identifying the regions of different growth orientation in the mentioned stratigraphic approach. Susceptibility of dislocation generation by boron proximity effects respect to the surface growth orientation is well revealed in such growth geometrical design where several growth orientations have to coexist at the same time. Higher density of type (i) of defects where obtained in closer planes as (111). Finally, the role of the methane concentration in the generation of extended defects will be discussed.