Gradient layers of boron-doped diamond on titanium substrates

For the deposition of well-adhesive, low-doped diamond layers on titanium substrates a gradient layer is designed. At first a highly boron-doped diamond layer is deposited, which shows good adhesion to the titanium substrate, followed by a low-boron-doped diamond layer on the surface.The boron-doped diamond layers were deposited on titanium stretch metal substrates by the hot-filament CVD method. It is shown that with increasing boron content during diamond deposition above 6000 ppm B/C the intermediate Ti(C,B) layers becomes very thin and so at high-boron concentrations no problem with layer adhesion occurs. These Ti(C,B)-layers formed during diamond deposition were investigated by standard metallographic preparations. To form a diamond gradient layer on the highly boron-doped diamond the boron content was reduced and a low-doped diamond layer was deposited.Electrochemical cyclic voltammetric measurements show that the lower boron contents at the diamond surface provide better electrochemical properties. These layers show extraordinary electrochemical properties in respect of the gained hydrogen and oxygen overvoltage.     

Analyses of single crystal diamond substrates grown in a pocket substrate holder via MPACVD

High quality single crystal diamond (SCD) substrates are required for several different important current applications. Microwave Plasma Assisted Chemical Vapor Deposition (MPACVD) is a convenient deposition method for high quality substrates. It is hence imperative to synthesize and analyze substrates grown via different CVD techniques. This paper describes the quality of single crystal diamond substrates which have been grown via one such deposition strategy, which is in a “pocket substrate holder” design. The growth process in such a holder helps in depositing substrates which have almost no polycrystalline diamond (PCD) rim growth. The exact pocket holder growth process at high pressures (240 Torr) and high microwave power densities (~ 500 W/cm³) has been discussed in a previous publication [1]. The SCD CVD substrates were analyzed with different characterization techniques. By synthesizing diamond substrates in a pocket holder, the lack of any/almost any PCD rim helped in reducing the amount of stress in the crystals. To study the electronic quality of the substrates, etching experiments were conducted to determine the etch pit density. Nomarski images confirm that the number of etch pits at the edges is higher than at the center of the etched surface thereby implying the feasibility of this simpler method of reducing the etch pit density. The pocket holder process thus not only reduces the PCD rim but also reduces the substrate etch pit density and hence shows good promise of delivering high quality substrates.     

The indentation size effect and the hardness of single crystal diamond on the (001) 110

The hardness of single-crystal diamond is addressed through the analysis of independent Knoop and Vickers indentation microhardness data on the (001) 110. A proportional specimen resistance model is applied to separate the hardnesses into two components, one representative of the indentation size-load effect (ISE) and the other the load-independent hardness. The analysis yields a load-independent Knoop hardness of 4411 kg mm⁻² and a Vickers hardness of 9158 kg mm⁻². These values are compared with similar analyses for sapphire and silicon carbide and are related to the Plendl-Gielisse volumetric lattice energy concept of hardness. The extraordinarily high hardnesses which have been reported for diamond, values in excess of 10 000 kg mm⁻², are attributed to the elastic modulus and friction contributions to the ISE.     

Ion Beam Induced Charge characterization of epitaxial single crystal CVD diamond

IBIC (Ion Beam Induced Charge) technique has been used in order to characterize single crystal epitaxial CVD diamond film with respect to homogeneity and stability of the response (in terms of charge collection efficiency, cce) as a function both of counting rate and of the number of counts per unit surface area. The maximum shift of cce peak, under a 1.2 MeV proton microbeam, is 1.5% for counting rates from 43 to 4330 Hz, while the homogeneity, evaluated as the standard deviation with respect to the average value of cce over strip-like regions 60–100 μm wide and 800–1200 μm long, is 0.5%. Counting rates per unit surface area were between 30 and about 15,000 Hz/mm². A total number of counts per unit area up to 9 10⁶ counts/mm² was reached without noticing any polarization effect due to trapped charge. Moreover, the functionality of a new kind of bulk electrode, realized by a boron doped buffer layer laterally contacted with Ag paste, has been checked by measuring cce at different proton ranges.     

含硼金属包覆膜的物相结构与含硼金刚石的生成

使用以FeB为硼源的含硼粉末冶金铁基触媒,在六面顶压机上高温高压合成含硼金刚石单晶.金相观察发现,金刚石金属包覆膜由粗大的板条状渗碳体和细密的莱氏体共晶组织构成.X射线衍射(XRD)发现,金属包覆膜的物相组成为(Fe,Ni)3C、(Fe,Ni)3(C,B)、石墨(Gr)以及γ-(Fe,Ni)(A).使用透射电镜(TEM)在包覆膜中发现了颗粒状的Fe3(C,B),条状的γ-(Fe,Ni) 和颗粒状的Fe23(C,B)6.电子探针分析(EPMA)结果表明,硼元素在包覆膜中存在浓度梯度,越接近含硼金刚石,硼元素的含量越高.分析认为,高温高压下硼是以铁-碳-硼化合物的形式通过金属包覆膜向金刚石晶体扩散的,Fe3(C,B)或(Fe,Ni)3(C,B)极有可能是含硼金刚石生长的直接碳源和硼源.     

Synthesis of boron-doped homoepitaxial single crystal diamond by microwave plasma chemical vapor deposition

The deposition of boron-doped homoepitaxial single crystal diamond is investigated using a microwave plasma-assisted chemical vapor deposition system. The objective is to deposit high-quality boron-doped single crystal diamond and establish the relationships between the deposition conditions and the diamond growth rate and quality. Experiments are performed using type Ib HPHT diamond seeds as substrates and growing diamond with varying amounts of diborane in a methane–hydrogen gas mixture. The deposition system utilized is a 2.45 GHz microwave plasma-assisted CVD system operating at 135–160 Torr. Experiments are performed with methane concentrations of 4–6% and diborane concentrations of 5–50 ppm in the feedgas. Diamond is deposited with growth rates of 2 to 11 µm/h in this study. The deposited diamond is measured to determine its electrical conductivity and optical absorption versus wavelength in the UV, visible and IR portions of the spectrum. Data is presented that relates the growth rate and diamond properties to the deposition conditions including substrate temperature and feedgas composition.     

Deposition of thick boron-doped homoepitaxial single crystal diamond by microwave plasma chemical vapor deposition

The deposition of high quality single crystal boron-doped diamond is studied. The experimental conditions for the synthesis of 1–2 mm thick boron-doped diamond are investigated using a high power density microwave plasma-assisted chemical vapor deposition reactor. The boron-doped diamond is deposited at a rate of 8–11.5 μm/h using 1 ppm diborane in the feed gas as the boron source, and the capability to overgrow defects is demonstrated. The experimental study also investigates the deposition of diamond with both 10 ppm diborane and 2.5–500 ppm of nitrogen added to the feedgas. Synthesized material properties are measured including the electrical conductivity using a four-point probe and the substitutional boron content using infrared absorption.     

Free-standing diamond films grown on cobalt substrates

Diamond films were grown directly on cobalt substrates, using microwave plasma-assisted chemical vapour deposition. Although cobalt is known to inhibit the nucleation of diamond and enhancing the formation of graphite, we were able to grow relatively thick films (∼190 μm). The films were easily detached from the substrates. The poor adhesion allows the possibility of obtaining free-standing diamond films without chemical etching. Micro-Raman spectroscopy showed the 1332 cm⁻¹ characteristic Raman peak of diamond and the 1580 cm⁻¹, 1360 cm⁻¹ bands of graphite, on the growth surface and backside of the films, respectively. Through scanning electron microscopy and X-ray diffraction we were able to monitor film thickness and morphology with growth evolution. The results showed the (111) preferential growth morphology for the film with higher growth rate. By energy dispersive X-ray spectroscopy it was only possible to detect cobalt in the back of the films, but not in the surface. The role of cobalt in the film growth is discussed.     

Comparison of GaN and AlN nucleation layers for the oriented growth of GaN on diamond substrates

In this study, {0001} oriented GaN crystals have been grown on freestanding, polycrystalline diamond substrates using AlN and GaN nucleation layers (NLs). XRD measurements and SEM analysis showed that the application of a thin AlN NL gives the best structural results, because AlN has a thermal expansion coefficient in between GaN and diamond and thus delocalizes the stress to two interfaces. The optical quality of the layers, investigated with Raman microscopy and photoluminescence spectroscopy, is similar. Although no lateral epitaxy is obtained, new insight is gained on the nucleation of GaN on diamond substrates facilitating the growth of GaN epilayers on polycrystalline diamond substrates.     

Nucleation and growth of diamond films on single crystal and polycrystalline tungsten substrates

Silicon has been the most widely studied substrate for the nucleation and growth of CVD diamond films. However, other substrates are of interest, and in this paper, we present the results of a study of the biased nucleation and growth of diamond films on bulk single and polycrystalline tungsten. Diamond films were nucleated and grown, using a range of bias and reactor conditions, and characterized by Raman spectroscopy and scanning electron microscopy (SEM). High-quality (100) textured films (Raman FWHM<4 cm⁻¹) could be grown on both single and polycrystalline forms of the tungsten substrate. On carefully prepared substrates, by varying the bias treatment, it was possible to determine the nucleation density over a 4–5 order range, up to ∼10⁹ cm⁻². Raman measurements indicated that the diamond films grown on bulk tungsten exhibited considerable thermal stress (∼1.1 GPa), which, together with a thin carbide layer, resulted in film delamination on cooling. The results of the study show that nucleation and growth conditions can be used to control the grain size, nucleation density, morphology and quality of CVD diamond films grown on tungsten.     

High quality thick CVD diamond films homoepitaxially grown on (111)-oriented substrates

The development of diamond power electronic devices based on p–n junctions strongly relies on the ability to achieve efficient n-type doping which has so far been the limiting step. (111)-oriented diamond films offer the advantage of a higher activity and incorporation of dopants. In this respect, growing high-quality films by Plasma Assisted Chemical Vapour Deposition (PACVD) on this orientation is critical. Other applications such as those based on nitrogen-vacancy (NV) centres could also benefit from the availability of high-quality (111)-oriented substrates. Due to the preferential orientation of the NV bond along the < 111 > direction, higher emission intensity and easier alignment of the magnetic field are expected. However (111) CVD films are plagued by twinning and defects that are easily formed on this orientation. Good quality (111) CVD films have been obtained but only for low thicknesses (< 1 μm) and at extremely low growth rates.In this paper, diamond growth was carried out by high power PACVD on (111)-oriented high pressure high temperature substrates prepared from octahedral-shape crystals. It was found that under conditions of high temperature and low methane concentration, the growth rate in the < 100 > direction is almost completely inhibited which ensures that penetration twins cannot develop. In this case smooth films with a thickness over 100 μm were successfully obtained at 6 μm/h. Although the crystalline quality is still below that of conventional (100) CVD films, the growth of such thick (111) CVD films opens the way to their integration into electronics applications.     

Temperature dependent creation of nitrogen-vacancy centers in single crystal CVD diamond layers

In this work, we explore the ability of plasma assisted chemical vapor deposition (PACVD) operating under high power densities to produce thin high-quality diamond layers with a controlled doping with negatively-charged nitrogen-vacancy (NV⁻) centers. This luminescent defect possesses specific physical characteristics that make it suitable as an addressable solid-state electron spin for measuring magnetic fields with unprecedented sensitivity. To this aim, a relatively large number of NV⁻ centers (> 10¹² cm^− 3) should ideally be located in a thin diamond layer (a few tens of nm) close to the surface which is particularly challenging to achieve with the PACVD technique. Here we show that intentional temperature variations can be exploited to tune NV⁻ creation efficiency during growth, allowing engineering complex stacking structures with a variable doping. Because such a temperature variation can be performed quickly and without any change of the gas phase composition, thin layers can be grown. Measurements show that despite the temperature variations, the luminescent centers incorporated using this technique exhibit spin coherence properties similar to those reached in ultra-pure bulk crystals, which suggests that they could be successfully employed in magnetometry applications.     

Superconductivity in heavily B-doped diamond layers deposited on highly oriented diamond films

We deposited a [100]-oriented B-doped diamond layer by three methods to clarify the effects of film morphology on the transition from metallic to superconducting diamond. Heavily B-doped [100]-oriented diamond layers were deposited on [first method] undoped polycrystalline diamond films with [111] faces, [second method] highly oriented undoped diamond (HOD) thin films with a pyramidal surface, and [third method] thick undoped HOD films with a pyramidal surface. We confirmed that the B-doped layer in the third method was oriented in the [100] direction by scanning electron microscopy (SEM). The highest transition temperatures were T_c(onset) = 5.0 K and T_c(zero) = 3.1 K for the B-doped layer deposited on a thick HOD film with a pyramidal surface under a zero magnetic field. By contrast, T_c(onset) was 4.1 K for a heavily B-doped diamond layer deposited on a thin HOD film with a pyramidal surface, and was 3.9 K for a heavily B-doped diamond layer deposited on an undoped polycrystalline diamond film. These differences in T_c for our samples are affected by disorder and effective hole-carrier doping in each sample. Using the third method, we successfully deposited a high-quality B-doped [100] layer in three steps: (first step) depositing a [100] HOD film on a Si [100] substrate, (second step) depositing an HOD film with a pyramidal surface, and (third step) depositing a [100]-oriented B-doped layer. The change in the electronic states due to the B-doping of diamond films and the film morphology were investigated by x-ray photoelectron spectroscopy (XPS) measurements and band calculations.     

Single crystal CVD diamond growth strategy by the use of a 3D geometrical model: Growth on (113) oriented substrates

The quality of single crystal diamond obtained by microwave CVD processes has been drastically improved in the last 5 years thanks to surface pre-treatment of the substrates [A. Tallaire, J. Achard, F. Silva, R.S. Sussmann, A. Gicquel, E. Rzepka, Physica Status Solidi (A) 201, 2419–2424 (2004); G. Bogdan, M. Nesládek, J. D'Haen, J. Maes, V.V. Moshchalkov, K. Haenen, M. D'Olieslaeger, Physica Status Solidi (A) 202, 2066–2072 (2005); M. Yamamoto, T. Teraji, T. Ito, Journal of Crystal Growth 285, 130–136 (2005)]. Additionally, recent results have unambiguously shown the occurrence of (110) faces on crystal edges and (113) faces on crystal corners [F. Silva, J. Achard, X. Bonnin, A. Michau, A. Tallaire, O. Brinza, A. Gicquel, Physica Status Solidi (A) 203, 3049–3055 (2006)]. We have developed a 3D geometrical growth model to account for the final crystal morphology. The basic parameters of this growth model are the relative displacement speeds of (111), (110) and (113) faces normalized to that of the (100) faces, respectively α, β, and γ. This model predicts both the final equilibrium shape of the crystal (i.e. after infinite growth time) and the crystal morphology as a function of α, β, γ, and deposition time.An optimized operating point, deduced from the model, has been validated experimentally by measuring the growth rate in (100), (111), (110), and (113) orientations. Furthermore, the evolution of α, β, γ as a function of methane concentration in the gas discharge has been established. From these results, crystal growth strategies can be proposed in order, for example, to enlarge the deposition area. In particular, we will show, using the growth model, that the only possibility to significantly increase the deposition area is, for our growth conditions, to use a (113) oriented substrate. A comparison between the grown crystal and the model results will be discussed and characterizations of the grown film (Photoluminescence spectroscopy, EPR, SEM) will be presented.     

关于人造含硼金刚石晶体结构及抗氧化性的研究

人造金刚石的抗氧化性是国际上评价其性能优劣的重要指标.文章以粉末铁基触媒中掺入不同含量的供硼剂为硼源,用静压法得到含硼量不同的人造金刚石.利用光学显微镜、X射线衍射仪和差热分析仪对含硼金刚石的晶形、晶体结构和抗氧化性能进行了研究,并与同样工艺条件下得到的普通金刚石进行对比,结果证明此种合成方法能够得到抗氧化性能优异的金刚石.     

Preferential orientation of NV defects in CVD diamond films grown on (113)-oriented substrates

Thick CVD diamond layers were successfully grown on (113)-oriented substrates. They exhibited smooth surface morphologies and a crystalline quality comparable to (100) electronic grade material, and much better than (111)-grown layers. High growth rates (15–50 μm/h) were obtained while nitrogen doping could be achieved in a fairly wide range without seriously imparting crystalline quality. Electron spin resonance measurements were carried out to determine NV centers orientation and concluded that one specific orientation has an occurrence probability of 73% when (100)-grown layers show an equal distribution in the 4 possible directions. A spin coherence time of around 270 μs was measured which is equivalent to that reported for material with similar isotopic purity. Although a higher degree of preferential orientation was achieved with (111)-grown layers (almost 100%), the ease of growth and post-processing of the (113) orientation make it a potentially useful material for magnetometry or other quantum mechanical applications.     

Pulse thermometers based on synthetic single crystal boron-doped diamonds

A time delay of electrical resistance of boron-doped semiconductor diamond sensor at heating pulses was investigated. We used a symmetric scheme with 2 equivalent 2 × 2 × 1.5 mm³ single crystal elements connected via thermal conductive electrical insulating glue. One of elements served as pulse heater and the other one as a sensor. About 40 ms characteristic time delay of diamond sensor was achieved. It is supposed, that using smaller crystals for sensors and by optimization of their electrical and heat conductivity properties, a regular value of time delay less than 1 ms will be attained. The potential application of diamond thermometers for temperature control in combustion engines is shown.     

Growth strategy for controlling dislocation densities and crystal morphologies of single crystal diamond by using pyramidal-shape substrates

The growth of millimetre-thick diamond single crystals by plasma assisted CVD is complicated by the formation of unepitaxial defects, particularly at the edges of the crystal. These defects tend to encroach on the top surface hence limiting the maximum thickness to typically a few hundreds of micrometres. Dislocations are another type of defects that are also particularly formed at the edges of the crystal. They thread through the diamond film, strongly affecting its characteristics. The growth on pyramidal-shape substrates having different angles and orientations was carried out in an attempt to solve those issues. It was found that the pyramidal-shape tends to disappear after a certain thickness is grown. The inclined faces of the pyramid not only helped in preserving the crystal morphology over a large thickness but also deviated dislocations towards the edges of the crystal, hence limiting their occurrence at the surface. Using this strategy, millimetre-thick diamond single crystals presenting a reduced dislocation density were successfully grown.     

The p-to-n-type conversion of boron-doped diamond layers by deuteration: New findings

The p-to-n-type conversion of particular B-doped homoepitaxially grown diamond layers upon deuterium plasma treatment was discovered three years ago. However, many questions regarding the reproducibility of the effect for samples of different origins remain unanswered up to now, in particular the role of the electrical contacts and the possibility of a surface inversion layer being responsible for the n-type conductivity, the thermal stability and origin of the donor.Here we address the above questions. We show that the p-to-n conversion is closely related to the presence of defects, which can vary, on a macroscopic scale, in different regions of the same sample. The p-type regions containing a small concentration of defects are found to be converted to n-type upon deuteration. In contrast, the regions counting a high defect density exhibit very low p-type conductivity related to passivation of the boron acceptors by deuterium accompanied by a large uptake of D on defects. We show that the n-type conversion can be equally observed with three different kinds of contacts (silver paint, implanted and evaporated). We prove that the n-type conversion is a bulk effect and not a surface effect. We find that the thermal stability of the n-type complex is limited to temperatures lower than 200 °C. The temperature dependence of the carrier concentration measured by the Hall effect at different stages of the conversion process is fitted following the formalism describing the conduction mechanisms in a partially compensated semiconductor. The fact that the donor concentration in the as-deuterated state is very close to the boron concentration strongly suggests that the new-formed donor complexes contain both D and B.