Ion implantation and diamond: some recent results on growth and doping

: We are studying contact formation, regrowth and implantation doping of natural diamond, for future applications to the fabrication of devices from thin film CVD diamond. We are characterizing thin film diamonds of other ONR contractors, using ion beam methods. In the last year we concentrated on the regrowth of ion damaged layers of diamond and the doping of diamond by implantation with Na, Li and F. We observed that complete regrowth of C ion damage diamond occurred below a critical ion dose. Above that dose, a 'green phase' occurred, which was characterized by a golden green color and by the absence of recovery of the lattice, as measured by ion channeling. Implantation doping studies of Li, Na, and F were initiated, and show some promise for n-type doping of diamond. The diffusivity of Li in diamond was studied by neutron depth profiling. In the important area of regrowth of ion damaged diamond, we observed that a completely randomized lattice can be regrown when the damaged level is below a certain threshold value, characterized by a deposited energy equivalent to 12 Frenkel defects per cu nm, or about 7% defects. The kinetic mechanisms for regrowth under these conditions, and the influence of impurities such as F and H on the regrowth, are relevant to CVD growth. (EDC)

Electron microscopy profiling of ion implantation damage in diamond: Dependence on fluence and annealing

ABSTRACTDiamond has an electric-field breakdown 20 times that of Si and GaAs, and a saturated velocity twice that of Si. This results in a predicted cut off frequency for high-power diamond transistors 40 times that of similar devices made of Si or GaAs. Boron is the only known impurity that can be used to lightly dope diamond. This p-type dopant has an activation energy of 0.3 to 0.4 eV, which results in high-resistivity material that is undesirable for devices. However, heavily boron doped diamond has a very small activation energy and a low resistivity and is of device quality. Transistors can be designed that use only undoped and heavily doped diamond. One of the steps in a device fabrication sequence is homoepitaxial diamond growth. Lightly and heavily doped homoepitaxial diamond films were characterized by scanning and transmission electron microscopy, x-ray diffraction, measurements of resistivity as a function of temperature, and secondary ion mass spectroscopy. It was found that under appropriate growth conditions these films are of device quality.

Ion-implanted n-type diamond: electrical evidence

Type IIa diamond crystals were implanted with boron ions with or without prior carbon ion implantation. The samples were kept at liquid nitrogen temperature during both implantation steps. A strong near-edge optical absorption band appeared after implantation, and partially recovered during annealing at 800 °C. For the highest B implantation fluence, optical absorption peaks at 2800 to 3000 cm-1 were observed that were in the same vicinity as the absorption peaks attributed to substitutional boron atoms in natural p-type diamond. Electrical measurements for three of the samples demonstrated well-defined activation energies that could be associated with hopping conduction and/or activation of B dopant atoms. This work shows that p-type doping in diamond by boron ion implantation is feasible, using a suitable combination of low temperature implantation and subsequent annealing.

The doping of the wide-band gap semiconductor diamond has led to the invention of many electronic and optoelectronic devices. Impurities can be introduced into diamond during chemical vapor deposition or high pressure-high temperature growth, resulting in materials with unusual physical and chemical properties. For electronic applications one of the main objectives in the doping of diamond is the production of p-type and n-type semiconductors materials; however, the study of dopants in diamond nanoparticles is considered important for use in nanodevices, or as qubits for quantum computing. Such devices require that bonding of dopants in nanodiamond must be positioned substitutionally at a lattice site, and must exhibit minimal or no possibility of diffusion to the nanocrystallite surface. In light of these requirements, a number of computational studies have been undertaken to examine the stability of various dopants in various forms of nanocrystalline diamond. Presented here is a review of some such studies, undertaken using quantum mechanical based simulation methods, to provide an overview of the crystal stability of doped nanodiamond for use in diamondoid nanodevices.

Materials modification: doping of diamond by ion implantation

Diffusion of hydrogen in undoped, p-type and n-type doped diamonds

We have implanted boron ions into insulating natural diamonds which were predamaged by carbon ion implantation in order to enhance the doping efficiency. All implantations were performed at liquid-nitrogen temperature. Subsequent rapid thermal annealing at 1100 °C produced strong new optical absorption bands near 1060 cm−1, and a sharp absorption at 2962 cm−1 (0.37 eV) which is close to that attributed to substitutional boron in type IIB diamond. We obtained resistivity of the order of 100 Ω cm and carrier activation energy of 0.1 eV for a sample implanted with 2×1015 C and 3×1014 B per cm2, indicating a high substitutional fraction of boron atoms.

Ion beam studies of the static and dynamic properties of dopants in diamond

Heavy phosphorus doping by epitaxial growth on the (111) diamond surface

ABSTRACTWe have investigated the challenging problem of doping diamonds, by co-implanting boron, nitrogen or phosphorus together with carbon into natural insulating type -a1 diamonds. All the implantations were done at liquid nitrogen temperature and then the samples were rapidly heated to 1100 °C. Unlike the previous attempts to dope diamond by room temperature or high temperature ion implantations, this method is expected to yield a higher doping efficiency for the implanted atoms. We have characterized the implanted diamonds with electrical and electron spin resonance (EPR) measurements. Boron doped samples showed low electrical resistivities and the EPR signal showed a strong dependence on the boron fluence, indicating a high substitutional fraction of boron atoms. The samples in which nitrogen and phosphorus were co-implanted with carbon showed lower resistivities compared with samples implanted with carbon only. Preliminary thermo-emf measurements indicated n-type conduction in these samples.

Ion implantation in diamond has long been explored, but many challenges remain before it can be fully realized for practical applications. This difficulty originates from a fundamental problem: lattice damage induced by ion irradiation reduces the effective concentration of impurity atoms. Nevertheless, the successful realization of ion implantation doping would offer major advantages, including localized doping, process simplification, and self-alignment capability using pre-formed electrodes. This topical review provides a comprehensive overview of boron ion implantation in diamond, tracing its development from early research to the latest advancements. We first examine the foundational work and the persistent challenges that have hindered progress for decades, such as low doping efficiency. The latter part of this review focuses on our recent breakthroughs, which have been designed to systematically overcome these long-standing issues. Our findings in the heavy doping regime (10 19 cm −3 ) have achieved a record-breaking doping efficiency of up to 78% with high Hall mobility. Furthermore, in the light doping regime (10 16 cm −3 ), our detailed analysis has identified a deep donor-like defect, which is likely a boron-vacancy complex specific to ion implantation doping. These findings demonstrate that a multi-faceted approach—combining optimal implantation and annealing conditions, a protective layer, and high-quality substrates—can make ion implantation a practical method for localized diamond doping, paving the way for next-generation diamond power devices.

Ion implantation of diamond and diamond films

Highly efficient impurity doping in diamond by ion implantation has been a crucial issue in the field of semiconductor fabrication for several decades. We investigated the electrical properties of heavily B-doped type IIa diamond introduced by ion implantation at room temperature with a shallow and flat impurity concentration of 3.6 × 1019 cm−3 (∼200 ppm) from the surface to ∼130 nm depth, followed by thermal annealing at 1150 and 1300 °C. The activation of the implanted acceptor B was a maximum of 80% for the sample into which B ions were implanted at room temperature followed by 1150 °C annealing. The hole concentration and Hall mobility at room temperature were realized to be higher than 1 × 1014 cm−3 and 110 cm2 V−1 s−1, respectively. We confirmed p-type conductivity and typical activation energy of acceptor B at wide temperatures from −100 to 800 °C for the prepared samples. It was consequently revealed from this study that at least room temperature B-implantation followed by above 1150 °C annealing is sufficiently effective for the electrical activation of B doped in high quality diamond.