The use of ENDOR to identify the atomic structure of defects in diamond

Although nearly 100 paramagnetic defects have been catalogued in diamond by spin Hamiltonian parameters measured by electron paramagnetic resonance (EPR), very few of these have been unambiguously associated with an atomic model. It has been necessary to use electron nuclear double resonance (ENDOR) to obtain enough information to make proper assignment of such models. The reason for the limitation of EPR, and the way in which ENDOR overcomes these limitations are discussed. The interpretation of hyperfine structure in terms of unpaired electrons in molecular orbitals, and of quadrupole interactions in terms of all electrons, paired and unpaired, as a source of information about molecular structure in diamond, is evaluated by reference to some well documented examples. The measurements so far made by ENDOR on defects in diamond are reviewed, and the salient contribution for the assignment of a model for each defect is explained. The details revealed by ENDOR considerably increase knowledge about defects, particularly those involving substitutional nitrogen atoms. This in turn helps in understanding the complex electron and atom, migration processes which go on under appropriate conditions of temperature and pressure, or optical excitation. The possibilities are discussed for using ENDOR to increase the number of well characterized centres.     

硼/氮原子共注入金刚石的原子级研究

利用Tersoff势和分子动力学方法研究了室温下500 eV的能量粒子硼(4个)和氮(8个)共注入金刚石晶体中晶体结构的变化特征和缺陷分布特征.结果表明：粒子注入金刚石后产生的空位比间隙原子更靠近晶体的近表层分布；间隙原子主要以四面体间隙(T形)和哑铃状分裂间隙的形式存在于晶体中，T形间隙结构更容易存在，并且大部分间隙原子富集在空位的周围；注入金刚石中的硼原子和氮原子有78%左右处于替代位置,硼、氮原子之间的键长比完整金刚石结构的键长短13%，硼氮原子成键有利于减少金刚石晶格的畸变程度.     

电子辐照金刚石的光致发光研究

本文利用488 nm及325 nm激光器比较了超纯、氮掺杂及硼掺杂三种金刚石经电子辐照所形成的光学中心. 结果表明, 金刚石中引入施主或受主原子后, 形成了新的光学中心, 如NV, DB1中心.     

First principles cluster investigation of electronic structure and hyperfine interaction for nitrogen in diamond

The self-consistent field Unrestricted Hartree-Fock cluster procedure has been used to study the location, electronic structure and hyperfine properties of nitrogen impurity in diamond. From the analysis of the potential energy curve for nitrogen along the <111> axis, it was found that nitrogen is located at a position 0.3A away from the substitutional site towards the plane formed by the three nearest neighbour carbon atoms. The calculated values of the magnetic hyperfine constants and nuclear quadrupole coupling constants for¹⁴N agree very well with values obtained from electron paramagnetic resonance and electronnuclear double resonance measurements.     

Substitutional-interstitial interactions in niobium-titanium alloys: An internal friction study

The influence of additions of interstitial oxygen and nitrogen on the internal friction spetrum of the niobium-1 at-% titanium alloy was studied. The nature of the various observed relaxation processes introduced by the presence of substitutional titanium is discussed. A thermodynamic analysis was carried out for two pronounced interaction peaks attributed to the stress-induced reorientation of single oxygen or nitrogen atoms around single titanium atoms and the respective binding energies were estimated. It was also found that, over the range of oxygen concentration studied, interstitial oxygen is completely removed from random migration by substitutional titanium atoms acting as trapping centres, whilst nitrogen population is always partitioned between mobile and trapped atoms even at very low relative nitrogen concentrationsC _N/C _Ti. A possible reason for the different behavior of oxygen and nitrogen is suggested.     

EPR studies of a two-nitrogen-atom centre in natural,plastically-deformed diamond

The hyperfine structure of an EPR system in a natural, brown diamond implies that the system is due to a centre containing two nitrogen atoms on almost equivalent sites. X-ray topographic evidence shows that the sample has been plastically deformed and it is suggested that a possible model for the defect is that of an ionized, substitutional N pair with a dislocation nearby.     

Transport studies in boron-ion implanted type II<Emphasis Type="Italic">a</Emphasis> diamond

We have investigated low-temperature electrical transport mechanisms in the surface layer of a type IIa diamond which has been heavily implanted with boron-ions at low temperatures and then annealed at high temperatures. The boron atoms occupy substitutional sites giving rise to a heavily doped wide-bandgap semiconductor. The dc-conductivity results suggest that for the maximum boron doping that has been achieved, the diamond sample is close to the insulator-metal transition. A model to account for the observed increase in activated boron centres with ion dose is presented. On the insulating side of the transition, the data are interpreted in terms of variable-range hopping laws.     

Analysis of ion implanted diamond

Ion implantation allows controlled introduction of impurities into diamond. A basic problem is to determine if the implanted layers are dominated by substitutional doping or radiation damage effects. Optical and electrical measurements on the implanted diamonds revealed (1) a degradation of the band-gap and sample coloration, (2) no optical absorption levels which would be characteristic of hydrogenic ionization levels, (3) resistivity activation energies of 0.2 to 0.3 eV independent of the ion specie, and (4) no measurable Hall Effect.

Phosphorous implanted layers in diamond were analyzed by means of the channeling technique. It was shown that (1) the diamond retained the implanted phosphorous atoms during a vacuum anneal at 950°C which restored crystalline order, (2) the implanted phosphorous atoms did not assume either substitutional or tetrahedral interstitial sites, (3) the measured projected range for 70 keV phosphorous in diamond of 450 ± 115 Å was consistent with theoretical range calculations and (4) a stable monolayer of oxygen atoms (5.5 × 10¹⁵/cm²) exists on the {111} surface of implanted and annealed diamond. The results of the crystal analysis show that these electrical and optical properties are dominated by radiation damage and not substitutional doping mechanisms.     

锌添加对大尺寸金刚石生长的影响

利用温度梯度法,在6.2—6.4 GPa,1270—1400℃条件下,通过在NiMnCo-C体系中添加不同比例的锌粉成功合成出3 mm左右的大尺寸金刚石单晶.研究了锌添加对金刚石颜色、形貌、内部氮杂质以及晶体结晶度的影响.结果表明:随着锌添加量逐渐增加,晶体的颜色逐渐变浅,晶体的透光性增强;当锌添加比例达到3 wt.%时,晶体表面出现大量不规则的凹坑;晶体内氮杂质主要以C心形式存在,随着锌添加量的增多晶体内氮含量逐渐降低,基于锌的除氮能力总结出两种可能的除氮机制;拉曼光谱测试结果表明,在锌添加量小于3.0 wt.%的研究范围内,锌的添加有利于提高晶体的结晶度.本研究不仅有助于天然金刚石形成机制的探究,而且对丰富金刚石的种类以及扩展人工合成金刚石的应用领域都有着重要意义.     

Effect of excess hydrogen on the electronic properties of passivated diamond

Recently, Yan et al. reported a theoretical approach to overcome the n-type doping problem of diamond by passivating the B acceptor and the reduction of ionization energy of dopants. Using the proposed approach, we have systemically investigated the effect of excess H atoms on the electronic properties of a passivated system [diamond doped with (B+H) complex] based on the variation of Fermi level from first-principle calculations. The results show that the excess H atom is responsible for n-type behavior when all valence electrons of substitutional B atom participate in the hybridization between substitutional B and H atoms. On the contrary, when part of valence electrons of the substitutional B atom participates in the hybridization, H atoms make the passivated system show p-type or insulator behavior. Further study indicates that the excess H atoms are more apt to make the passivated system show p-type rather than n-type behavior under the same conditions. It leads to a much more comprehensive understanding of the interaction between the excess H atoms and the passivated system compared with that of Yan et al. The results may be useful to provide guidance for experimental work on obtaining n-type diamond in the future.     

Structural model of substitutional sulfur in diamond

Based on ab initio calculations, it is found that the donor center of substitutional sulfur(S) in diamond with C_(2v) symmetry is more stable than that with C_(3v)symmetry, which is different from previous reports in literature. The energy difference of C_(2v)and C_(3v)structures is qualitatively affected by the supercell size, and the 216-atom supercell could be proposed as the minimum to obtain stable configuration of substitutional S in diamond. Using supercells of up to 512 atoms, the donor level of substitutional S with C_(2v)symmetry is deep.     

A novel use of hyperfine structure in the electron paramagnetic resonance of interacting pairs of paramagnetic defects in diamond

It is the hyperfine structure of ¹⁴N and ¹³C in the electron paramagnetic resonance (EPR) spectrum which indicates that the unpaired electron of a single substitutional nitrogen atom in diamond is in one of the four anti-bonding N-C orbitals. We show that, for diamonds containing a very high concentration of nitrogen, the hyperfine structure of interacting pairs of nitrogen atoms indicates that for close neighbours there are unique orientations of the constituent N-C bonds, while at larger distances the orientations are random. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electron spin resonance detection and identification of nitrogen centers in nanodiamonds

Individual nitrogen centers N⁰ and nitrogen pairs N ₂ ⁺ have been detected and identified in natural diamond nanocrystals by means of the high-frequency electron spin resonance method. The N⁰ nitrogen centers have been observed in synthetic diamond nanocrystallites with a size of less than 10 nm produced by high-temperature high-pressure sintering of detonation nanodiamonds. Thus, the possibility of the stable state of impurity nitrogen atoms in diamond nanoparticles has been demonstrated.     

用EPR方法研究人造金刚石中杂质氮

 本文用电子顺磁共振（EPR）方法，研究人造金刚石中氮的电子结构。提出用自旋浓度来表征氮含量的方法，并对不同方法制得的金刚石进行测定。     

The role of 14N and 13C hyperfine structure in characterizing point defects in diamond

Hyperfine structure (hfs), of either ¹³C or ¹⁴N, observed in the electron paramagnetic resonance (EPR) spectrum, has been a remarkably powerful indicator of the models of paramagnetic defects in diamond. It has often been valuable or necessary to use the much higher resolution of electron nuclear double resonance (ENDOR). Measurements of hfs have recently allowed considerable progress in understanding the nature of defects which have been uncharacterized for many years. A rich variety of defects involving up to at least 4 N atoms, which readily substitute for C atoms, has been found in diamond. The role of ¹⁴N hfs is reviewed in solving problems where different aspects are relevant. The use of synthetic diamond, enriched by up to 10% in ¹³C, has greatly facilitated the construction of models for centres produced by radiation damage, where the only information is from ¹³C hfs. Both ¹⁴N and ¹³C hfs have confirmed models of Ni related centres. This revised version was published online in August 2006 with corrections to the Cover Date.     

EPR study of the transformations in nickel containing centres at heated synthetic diamonds

Electron paramagnetic resonance (EPR) was used to investigate the transformation of as grown nickel and nitrogen defects at the annealing of synthetic diamonds, obtained by the temperature gradient method. Structural models and formation mechanisms of the seven nickel containing paramagnetic centres (NE1-NE7) in synthetic diamonds are discussed. A common structural fragment of NE1-NE4 centres is a double semivacancy, in centre of which Ni⁺ ion is located. The NE6, NE7 centres are proved to operate as shallow electronic traps. The effects observable for NE5 centres are supposed due to an internal electronic transformation in them. The problem of charge compensators for nickel ions is also discussed. The features observable in the charge transfer processes under X-ray irradiation for nickel and nitrogen containing centres suggest that donor nitrogen serves as a bulk charge compensator for substitutional nickel and NE1 centres.     

Fabrication of single optical centres in diamond—a review

Colour centres in diamond are rapidly becoming one of the leading platforms for solid-state quantum information processing applications. This is due in large part to the remarkable properties of the nitrogen-vacancy colour centre. From initial demonstrations of room-temperature single photon generation and spin single spin readout and quantum control, diamond nanocrystals are also finding application in magnetometry and biosensing. This review discusses the state of the art in the creation of isolated and small ensembles of optically active diamond defect centres, including nitrogen and nickel-related centres.     

Further study of the difference in nitrogen aggregation among (111) and (100) growth sectors of synthetic diamonds

Abstract

Single nitrogen atoms in synthetic diamond crystals aggregate in pairs by heat treatment under high pressure after electron irradiations. We find that the rate constants of nitrogen aggregation in the (111) growth sectors grown by the various solvents are about ten times greater than those of the (100) growth sectors. Furthermore the rate constant of nitrogen aggregation of the same sector is almost the same value. (shown Table-1). We have considered a few reasons for this fact. It may be accurate that a form of nitrogen atoms contained in (111) growth sectors of an as-grown diamond differs from that in (100) grown sectors and that the former accelerates nitrogen aggregation. Consequently we propose one nitrogen migration and aggregation model.     

Mössbauer study of Fe in 3C-SiC following 57Mn implantation

Our investigations on substitutional and interstitial Fe in the group IV semiconductors, from ⁵⁷Fe Mössbauer measurements following ⁵⁷Mn implantation, have been continued with investigations in 3C-SiC. Mössbauer spectra were collected after implantation and measurement at temperatures from 300 to 905 K. Following comparison with Mössbauer parameters for Fe in Si, diamond and Ge, four Fe species are identified: two due to Fe in tetrahedral interstitial sites surrounded, respectively, by four C atoms (Fe_i.C) or four Si atoms (Fe_i,Si) and two to Fe in (or close to) defect free or implantation damaged substitutional sites. An annealing stage at 300–500 K is evident. Above 600 K the Fe_i,Si fraction decreases markedly, reaching close to zero intensity at 905 K. This is accompanied by a corresponding increase in the Fe_i,C fraction.     

Comparative Study of Substitutional N and Substitutional P in Diamond

Based on density functional theory calculations, it is found that for substitutional N in diamond the C_(3v) symmetry structure is more stable, while C_(3v) and D_(2d) symmetry patterns for the substitutional P in diamond have comparable energies. Moreover, the substitutional N is a deep donor for diamond, while P is a shallow substitutional n-type dopant. This is attributed to the different doping positions of dopant(the N atom is seriously deviated from the substitutional position, while the P atom nearly locates in the substitutional site), which are determined by the atomic radius.