Highly efficient doping of boron into high-quality homoepitaxial diamond films
Introduction
Impurity doping into diamond is an important technique for fabricating diamond-based electronic devices. The activation energy of the dopant must be large in diamond because of its low dielectric constant. Therefore, the impurity must be doped highly to realize comparable resistivity to those of conventional semiconductors at room temperature (RT). In the case of boron-doped diamond, an impurity concentration greater than 1019 cm− 3 is necessary to achieve RT resistivity of the order of 1 Ω cm. Such high doping growth considerably degrades the crystalline perfection because rigid diamond crystals are unable to accept high impurity contents; consequently, they form electronic defects. This feature renders it difficult to conduct deposition of diamond with reasonable RT resistivity.
Several researchers have optimized diamond growth conditions for boron-doping using microwave-plasma chemical vapor deposition (MPCVD) and high RT Hall mobilities greater than 1000 cm2 V s− 1 [1], [2], [3], [4]. These excellent values, illustrating the high potentiality of diamond as a material for electronic devices, were achieved only for moderately doped specimens whose acceptor densities were ca. 1017 cm− 3. On the other hand, electrical and optical properties of highly boron-doped samples with reasonably low resistivity have not been characterized systematically. In addition, the effect of hopping conduction might be superimposed on the carrier transport mechanism of the previously reported highly doped samples because of insufficient care in crystalline quality and relative lack of information on hole concentration and mobility [5], [6]. Detailed investigations using high-quality highly doped diamond are therefore indispensable for elucidating the advantages of diamond from an electronic material point of view.
Another subject that should be investigated is the incorporation mechanism of boron from gas phase into substitutional sites of diamond crystal. Some previous reports have suggested that an increased methane concentration ratio in the gas phase enhances the efficiency of incorporation into diamond crystal [4], [7], [8]. This phenomenon suggests that the diamond growth mode, such as growth rate, affects the incorporation quantity of substitutional boron, in addition to the B / C ratio in the gas phase (Herein, we designate the boron / carbon concentration ratio as a B / C ratio and define the incorporation efficiency as the substitutional B / C ratio in diamond crystal divided by the B / C ratio in the gas phase.). That enhancement effect is, however, not used as a method to obtain highly doped samples because the crystalline quality of diamond film is known to be inferior when the methane concentration increases substantially under the conventional low-power MPCVD condition.
Recently, we reported high-quality undoped diamond films that were grown at a high growth rate using high-power MPCVD with higher methane concentration [9], [10], [11], [12]. Considering the incorporation enhancement effect mentioned above, this advantageous feature of the high-power MPCVD is considered appropriate to obtain high-quality highly doped diamond films. This study investigates the boron incorporation efficiency under high-power MPCVD and characterizes the resultant highly conductive high-quality diamond films.
Section snippets
Experimental
Homoepitaxial diamond thin films were deposited using a high-power MPCVD system (ASTeX AX-5400; Seki Technotron) with a 5.0 kW microwave generator. Substrates were high-temperature/high-pressure type-Ib (100) diamond single crystals with a mirror-polished surface that was 3 × 3 mm2. The source gas was a mixture of H2 that had been purified in palladium (purity > 9 N) and high purity CH4 (purity 7N). Trimethylboron, B(CH3)3, diluted with hydrogen into 100 ppm (purity 6N5) was used as a doping gas. The
Results and discussion
Fig. 1(a) and (b), respectively, show OM and SEM images of the 2 ppm B-doped sample. The growth rate in the present conditions was 3.5 μm h− 1, which is ca. 30 times faster than previously reported for the highest mobility sample grown by MPCVD [1]. The total film thickness examined was 25 μm; it comprised a B-doped layer and an undoped layer beneath the B-doped one. The resultant film surface was macroscopically flat; it had no non-epitaxial crystallites in the entire sample surface area. Some
Conclusion
High-quality conductive homoepitaxial diamond (100) films were grown using high-power MPCVD. Macroscopically flat boron-doped diamond films showing strong free and bound exciton recombination emissions were deposited at the high growth rate of ca. 3.5 μm. The boron incorporation efficiency under the high-power MPCVD condition is higher by more than one order of magnitude than that of the conventional low-power MPCVD conditions. The boron-doped diamond epitaxial layers exhibited higher carrier
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