Physico-Chemical nanomaterials science

Intersectoral boron-impurity-related fluctuations of local electrical properties in semiconductor HPHT diamond plates of different orientations
Nikolenko A.S.1, Strelchuk V.V.1, Lytvyn P.M. 1, Malyuta S.V. 1, Danylenko I.M.1, Gontar O.G.2, Starik S.P.2, Kovalenko T.V.2, Ivakhnenko S.O.2
1 V.E. Lashkaryov Institute of Semiconductor Physics NASU, pr. Nauky 41, 03028 Kyiv, Ukraine 
E-mail: nikolenko@isp.kiev.ua
2 V. Bakul Institute for Superhard Materials NASU, Avtozavodska str. 2, 04074 Kyiv, Ukraine

Diamond due to its unique physical properties - a record-high thermal conductivity value (22 Wt/(cm·K) at T = 300K), breakdown voltage (10 MV/cm), holes and electrons mobility (1800 and 2200 cm2·V-1·c-1) - is a promising semiconductor for high-power and high-frequency electronics. There are two main methods of diamond growth - crystallization from carbon solutions in molten metals at high pressure and high temperature (HPHT) and chemical vapor deposition (CVD). Both methods enable the growth of the samples with high structural perfection and purity, almost identical in the cost of production of sizes up to 5 - 10 mm. The problem of semiconducting diamond is associated with a limited choice of dopants that form acceptor (B: 0.37 eV) or donor (N: 1.7 eV, P: 0.57 eV) energy levels. Boron is the most appropriate impurity to obtain semiconducting p-type diamond. Nitrogen, in terms of atomic size, is the main compensating donor impurity. Incorporation of donor and acceptor impurities into the diamond lattice under HPHT conditions depends on their activity, which is also the function of the solvent composition. To obtain semiconducting diamonds with decreased nitrogen and increased boron content, new carbon solvents based on Fe-Al-Mg, Fe-Co-Mg and Fe-Co-Al-Mg are being developed. This enables the synthesis of HPHT diamonds with composition and electrical properties meeting the needs of solid-state electronics. However, HPHT diamonds usually reveal the non-uniform zonal-sectoral distribution of dopants related to the selective absorption of impurities by the crystal faces. Usually, octahedral {111} sectors have the maximum nitrogen and boron content. In order to control the spatial distribution of defects and impurities in diamond crystals, further development of local characterization techniques is important. Kelvin probe force microscopy (KPFM) is a fast and informative way of such nanoscale characterization.

Fig.1. HPHT diamond plate grown in the Ni-Mn-C system with the addition boron and cut perpendicular to the growth axis. Visualization of impurity distribution (a) and deformation fields (b) by transmission and polarization optical microscopy. Surface potential maps at intersectoral boundaries (c, d). The insets show the diagram of the growth sectors in correspondence with the potential maps.
In this work, the KPFM method is applied to quantitatively characterize the variation of surface potential in boron-doped HPHT diamond plates cut perpendicular (fig.1) and parallel to the growth axis. The intersectoral jumps of the surface potential and the distribution of surface potential in the axial and radial directions of the growth sectors are studied, and the influence of the built-in strain fields on the intrasectoral distribution of impurities is revealed. The obtained results are complete with the µ-Raman and µ-FTIR mapping data concerning the distribution of structural quality, strains, and boron content through the interfaces of different growth sectors. A high level of structural perfection of intersectoral boundaries is demonstrated (absence of twins, massive dislocation walls, impurity gettering) and, accordingly, the absence of electrically active defects in these boundaries. The features of the band structure of local sections of multisector diamond plates are estimated. Thus, the proposed comprehensive approach allows for reliable estimation of local structural and electro-physical parameters of HPHT diamond plates, which is useful for solving the actual problems of novel diamond electronics.