Chemical vapour deposition of sp2-hybridised B-C-N materials from organoborons

Sammanfattning: Thin films of sp2-BN are promising materials for graphene and deep-UV optoelectronics. They are typically deposited by thermally activated chemical vapour deposition (CVD) from triethylboron (TEB) and ammonia (NH3) at 1500 °C, albeit in a narrow process window. The aim of this thesis is to establish a better understanding for and to develop CVD of sp2-BN, BxC and BCxNy further. This has been done by fundamental studies of the gas phase and surface chemistries of the organoboron precursor trimethylboron (TMB), studying new substrate materials and by studying plasma CVD.From previous experience with TEB, TMB has been investigated as an alternative precursor. From a study on the gas phase chemistry of TMB in argon and hydrogen ambient, BxC films can be deposited from 600 °C at 5000 Pa and the B/C ratio reaches 3 at susceptor temperatures of 1000 °C. Supporting calculations show that TMB dissociates mainly by α-elimination of CH4 in both ambient, although H2-assisted elimination also occurs in hydrogen ambient. Furthermore, we have demonstrated deposition of BxC films in features with high aspect ratios (up to 2000:1) at 700°C and 5000 Pa, which are much higher temperature and pressure conditions compared to most surface-controlled CVD processes. This was enabled from competitive adsorption of radicals from TMB and H2 on the growing surface. Deposition of sp2-BN from TMB and NH3 was performed between 1200 °C – 1485 °C. The use of TMB instead of TEB allowed for the deposition of epitaxial rhombohedral-BN (r-BN) on nitridated sapphire from 1300 °C and in a wider process window (3000 to 9000 Pa, NH3/TEB from 321 to 1286) and three times higher deposition rate, but at a cost of a higher carbon contamination. The epitaxial relationships are ? − ??(0001) ∥ ? − ???(0001) ∥ ? − ??2?3(0001) out-of-plane and in-plane ? − ??(1120) ∥ ? − ???(1120) ∥ ? − ??2?3(1000) and ? − ??(1120) ∥ ? − ???(1120) ∥ ? − ??2?3(1̅ 000), as determined by φ-scan measurements.For growth on silicon, we studied the feasibility of depositing sp2-BN at 1300 °C, 7000 Pa, and NH3/TEB = 321. Pre-treatments from TEB and NH3 were applied in order to stabilise the silicon surface. It resulted in the growth of amorphous boron nitride (a-BN), regardless of the pretreatment. We brought into light a memory effect involving boron carbide and silane (SiH4) that permitted the growth of orientated crystalline or turbostratic BN grains on the silicon surface, as determined by X-ray diffraction and scanning electron microscopy images. In contrast to the temperature sensitive Si substrate, epitaxial zirconium diboride (ZrB2) templates were studied as a conductive alternative high- substrate to the sapphire (insulator) and silicon carbide (wide bandgap semiconductor). φ-scan measurements showed that r-BN grows with the epitaxial relationship: ? − ??(0001) ∥ ??????−1(111) ∥ ???2(0001) ∥ ???(0001) and ? − ??(1120) ∥ ??????−1(220) ∥ ???2(1120) ∥ ???(1120). The coverage of the surface by epitaxial r-BN grains is found to increase with upon silane exposure prior to growth.In addition, microwave-plasma-activated CVD was studied as an alternative deposition technique. sp2-BCxNy films were deposited from TEB and an Ar-N2 plasma in an approach similar to a 23- factorial design. We observed the effects of the absorbed microwave power, the total gas flow and the N/Ar ratio on the growth rate, composition and morphology. Two deposition regimes were found whether nitrogen or argon is the main gas. The films showed high boron and nitrogen (up to 46 and 41 at. %, respectively) contents and the composition was found not to vary significantly with the deposition parameters. The morphology of the film evolves from granular films to nanosheets. The use of plasma enabled using optical emission spectroscopy to get insight into the deposition chemistry. The relative permittivity κ of the sp2-BCxNy films could be varied between 3 and 35. A strong correlation was found between carbon content and increase of κ.