Growth, Boron-distribution And Stress Of Freestanding Boron-doped Diamond Films

Diamond is an important material for many applications due to its unique properties such as high hardness, high thermal conductivity, wide band gap (5.5eV), and so on. Pure diamond is known as an insulator. Generally boron doping can induce an acceptor level (p-type semiconductor). Moreover when the boron doping level increased, the metallic conduction even superconductivity will appear. Recently, heavily B-doped (superconductivity) has been investigated extensively.Freestanding boron-doped CVD diamond films were synthesized by EA-HFCVD process with different B(OCH3)3 flow rates. The growth behavior with different texture, boron concentration, and residual stress of the as grown films were characterized by SEM, XRD and Raman spectra.Based on the XRD and SEM results, the variation in morphology, grain size and crystalline texture are dependent on the incorporation of boron. The undoped diamond film mainly consists of (111) facet with average size of ~50μm. When boron source was introduced (2 - 10 sccm), the appearance of (110) facets is enhanced with respect to (111) facets and the average size decreases to around 8μm, and importantly, the large number of twin appear. However, as the B-flow rate increased up to 20 sccm, the grains show dominated morphologies of both (110) and (111) facets and the number of twin decreases.Fano-type interference can be induced by boron doping and the asymmetry parameter q represents the asymmetric of one-phonon line shape which has been proposed that the smaller the q is, the higher the boron concentration is, so we can obtain the distribution of boron by fitting the Raman spectra. The results show that the values of q for (111) facets, grain boundaries and the center of the whole film are smaller than that for (110) facets, grain surface and the edge of the whole film for all the B-doping levels, respectively. This suggests that, at given boron doping levels, (111) facets take up higher concentrations of boron into the crystal than (110) facets, the boron concentration at grain boundaries is higher than that in grain, the boron concentration is higher at center than that at edge. The q increases from top surface to bottom along the cross section, suggesting that the boron concentration near the surface is higher than that beneath.Generally, doped boron atoms substitute for the carbon atoms and/or occupy neutrally interstitial positions depending on doping level. In these cases, the boron doping leads to stress and consequently influence the structure and properties of those B-doped films. In this paper, the residual and micro stresses were analyzed by XRD. The results show that the residual stress is compressive. With increasing boron flow rate, the stress is gradually decreased. The micro-stress varies as tensile→compressive→tensile in the films fabricated with increasing boron flow rate in the growth processes. It is demonstrated by XRD and SEM that the variations in residual stress and micro-stress as a function of boron doping level are strongly dependent on grain size, growth orientation, and appearance of twins in the boron-doped diamond films. The results are available for designing the high performance diamond-based optoelectronic devices with controlled stress.