Theoretical investigation on the interaction of nitrogen with dislocations in single crystal CVD diamond

In recent experimental studies, evidence for an atmosphere of nitrogen around the cores of dislocations was found in irradiated single crystal CVD diamond. In this paper, we present a first-principles study on the interaction of nitrogen with dislocations in single crystal CVD diamond, where dislocations are observed as mixed-type 45° and 90° edge-type dislocations lying along <100>. We find a strong binding of nitrogen to the dislocation core for both types of dislocations and show that our results are in consistency with the experiment.     

X-ray micro beam analysis of the photoresponse of an enlarged CVD diamond single crystal

Diamond is one of the most promising materials for developing innovative electronic devices. Chemical vapour deposition (CVD) homoepitaxial growth allows the synthesis of high quality single crystal diamond plates. However, the use of these crystals for electronic applications is hampered by their small area (typically of the order of 10 mm²). Large areas are desired to ensure efficient particle or radiation detection with pixelated devices. By growing a thick CVD layer it is possible to enlarge the initial area of the substrate by a factor of 2 since growth also occurs laterally from the substrate.In this work, by using an X-ray collimated synchrotron radiation beam, the detection and charge collection properties of an enlarged CVD single-crystal diamond are used as a point-to-point probe to study the material quality. It was found that stress and dislocation density are correlated with the detection properties of the enlarged regions. The sensitivity of the device is affected by the vertical-to-lateral growth interface and the enlarged material quality seems to be correlated with the distance from this interface.     

Luminescence centers in proton irradiated single crystal CVD diamond

Diamond displays a large variety of luminescence centers which define its optical properties and can be either created or modified by irradiation. The main purpose of the present work is to study the radiation hardness of several of such centers in homoepitaxial single-crystal CVD diamond by following the evolution of photoluminescence and ionoluminescence upon 2 MeV proton irradiation. Luminescence decays were observed with values of the fluence at half of the starting luminescence (F_1/2) of the order of 10¹⁴ cm^? 2. The 3H center displayed a non-monotonic behavior, with a growing behavior and a subsequent decay with a rather high F_1/2 value (in the order of few a 10¹⁶ cm^? 2), maintaining at the highest fluences an intensity significantly higher than the blue A-band. A simple model based on a double-exponential trend was defined to fit with satisfactory accuracy the evolution of the 3H center. Several PL centers (namely: 3H, TR12, 491 nm and 494 nm) exhibited clear correlations and anti-correlations in their fluence dependences, which were considered in the attempt to acquire some insight into their possible alternative attributions.     

Single crystal CVD diamond as an X-ray beam monitor

Synchrotron X-ray beams may now be focussed to < 1 μm and devices with similar spatial precision are required to monitor the beam position or provide real-time information for position control. Over the X-ray energy range 5–25 keV, diamond is an outstanding material for the fabrication of ‘semitransparent’ beam monitors. Tests were made at the ESRF with ‘electronic grade’, single crystal diamond samples which were patterned with metal contacts and operated as solid-state ionization chambers. A uniform spatial response flat within 0.2% was seen when the devices were mapped with a sub-micron synchrotron X-ray beam, and the photoelectric current generated was linear with the beam intensity and showed no time lag effects. The beam induced currents were measured both at DC and at the RF frequency of the synchrotron. A sensitivity of < 13 nm to beam movement was obtained.     

Elastic properties of single crystal diamond made by CVD

Brillouin light scattering has been used to investigate the elastic properties of high quality homoepitaxial diamond layers about 1 mm thick that have been elaborated by microwave plasma assisted chemical vapour deposition. Taking advantage of the detection of different acoustic modes, a complete elastic characterization of the crystal has been achieved. Three single crystal elastic constants, namely, c₁₁, (c₁₁ − c₁₂) / 2 and c₄₄ have been selectively determined, respectively, from the frequency of the longitudinal and of the shear horizontal bulk modes travelling parallel to the film surface. These determinations are in agreement with the frequency of the observed surface modes and of the bulk waves propagating at different angles from a normal single crystal film plane and consistent with the properties of natural diamond. By adding a low amount of nitrogen ranging from 2 to 50 ppm in the gas phase, the growth rates were increased from 6 to 33 μm/h whereas the mechanical properties of the resulting layers remained close to those of natural diamond.     

单晶CVD金刚石作为功能材料的应用

阐述单晶CVD金刚石的发展、特性及应用.分析了单晶CVD金刚石在工业与技术应用方面的优越性.目前其重要应用领域主要包括:半导体器件;微波技术;监测器件与检测系统;光纤通讯以及光信息存储技术等.     

Anisotropic dry etching of boron doped single crystal CVD diamond

Semiconducting boron doped single-crystal CVD diamond has been patterned using aluminum masks and an inductively coupled plasma (ICP) etch system. For comparison insulating HPHT diamond samples were also patterned using the same process. Diamond etch rates above 200 nm/min were obtained with an O₂/Ar discharge for a gas pressure of 2.5 mTorr using 600 W RF power. We have accomplished the fabrication of structures with a minimum feature size of 1 μm with vertical sidewalls in both CVD and HPHT diamond. The ICP etching produced smooth surfaces with a typical root-mean-square surface roughness of 3 nm. The dependence of etch rate on bias voltage was somewhat different for the two types of diamond. However, for all samples both the etch rate and anisotropy were found to improve with increasing bias voltage.     

Photoconductive properties of lightly N-doped single crystal CVD diamond films

In the preparation of high power diamond photoswitches, thick (more than 100 μm) lightly nitrogen-doped single crystals were grown at LIMHP, for which Differential Interference Contrast Microscopy, Raman spectroscopy, photoluminescence, and cathodoluminescence have confirmed good morphology and very low but well-controlled impurity doping level. In order to evaluate the effect of nitrogen incorporation on the electronic properties of these films, photoconductivity measurements have been carried out. In an initial study, I–V and transient photocurrent measurements were conducted on several films with N-doping from 0 to 20 ppm intentionally added to the gas phase during growth, resulting into nitrogen concentrations lower than 100 ppb in the film. The results of these measurements are presented showing typical semiconductor behavior in terms of gain versus settling time, relatively high external quantum efficiency (EQE) and corresponding derived μτ (mobility × lifetime) product. In particular, samples with no nitrogen showed EQEs of several hundreds while their settling time was quite long (tens of seconds). However, samples with small nitrogen addition were observed to have settling times decreasing below a few seconds while EQEs close to 10 showed that a compromise could be found between efficiency and response time.     

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.     

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.     

Reduction of dislocation densities in single crystal CVD diamond by using self-assembled metallic masks

The development of diamond-based electronic devices designed to operate at high power is strongly hampered by the lack of low dislocation single crystal material. Dislocations in Chemically Vapor Deposited (CVD) diamond are indeed generally responsible for leakage current, seriously deteriorating the performance of the devices. They can be due to defects such as polishing damage or contamination found at the substrate's surface, or they can directly originate from existing bulk defects that extend into the homoepitaxial layer. Although significant improvements have been achieved by using adapted surface treatments, dislocations found in CVD diamond grown on standard quality single crystal substrates are still typically in the range 10⁵–10⁶ cm^− 2. In this work, we report on a new growth strategy aiming at preventing threading dislocations from propagating into CVD diamond layers. It is based on the selective masking of existing defects revealed at the surface of the substrates by Pt nanoparticles. The interaction of dislocations with such embedded particles has been assessed and critical remarks are given as to the use of this technique in order to reduce dislocation densities in synthetic diamond.     

Radiation hardness of single crystal CVD diamond detector tested with MeV energy ions

The spectroscopic properties of a commercial high purity single crystal diamond detector (1 mm² area, 500 μm thickness) have been studied using focused ion beams (H, He and C ions) in the MeV energy range. A measured relative energy resolution of 1.3% (FWHM = 25 keV) for the detection of 2 MeV protons demonstrated a good spectroscopic performance of the CVD diamond device, which makes it useful for the detection of light ions or atoms. To test the radiation hardness of the diamond detector, it was selectively irradiated with a 6.5 MeV focused carbon beam up to a fluence of 10¹¹ ions/cm². Reliable measurement of the ion fluence was accomplished by means of the microprobe single ion technique IBIC (ion beam induced charge). After irradiations that produced selectively damaged regions in the diamond detector, low current mode IBIC microscopy has been performed to measure the degradation of the charge collection efficiency (CCE). In order to get a better understanding of the detector performance after irradiation, different ions with the end of a range smaller, equal and larger than the extend of the damaged layer were used as IBIC probes. The same experimental procedure of irradiation and IBIC microscopy has been performed on a detector grade silicon PIN diode in order to directly compare the radiation hardness of diamond and silicon. The presented results show that the single crystal CVD diamond is less radiation hard for the spectroscopy of short range heavy ions compared to the high resistivity silicon, which is contrary to the results obtained for diamond detectors exposed to the high energy particles.     

Calibration of boron concentration in CVD single crystal diamond combining ultralow energy secondary ions mass spectrometry and high resolution X-ray diffraction

In this work, we combine ultralow energy secondary ion mass spectrometry (uleSIMS) and high resolution X-ray diffraction (HRXRD) for the analysis of boron doped single crystal diamond. CVD layers of nominal boron concentrations 1.3 · 10²¹ At/cm³, 2.0 · 10²¹ At/cm³, 6.9 · 10²¹ At/cm³ and 3.8 · 10²² At/cm³ have been characterized by uleSIMS at 1 keV, 500 eV and 300 eV using normal incidence O₂⁺ and by HRXRD.We show that the enhancement of ¹²C⁺ signal as a function of boron concentration observed in uleSIMS analysis of heavily doped layers compared to intrinsic diamond is due to a change in the ionization probability for carbon rather than to a change in erosion rate or post-ionization in the gas phase. For this reason, calibrating the boron concentration using a relative sensitivity factor (RSF) derived from an implanted reference material, which will naturally have a different ionization probability for carbon, is not accurate and a calibration curve needs to be obtained from an independent analysis technique such as nuclear reaction analysis (NRA).Simulation and measurement of HRXRD 113 asymmetric and 004 symmetric reflections have confirmed that the layers grew coherently to the substrate. The peak strain in the samples has been obtained from the HRXRD patterns by comparison between simulation and experiments, being ∼ 0.41 · 10^− 3, ∼ 0.43 · 10^− 3 and ∼ 1.30 · 10^− 3 for the first three samples. The peaks in the experimental profiles are broad compared to the simulations of uniform layers, which is explained by the graded composition of the layers, shown by the uleSIMS profiles.     

Production of single crystal diamond needles by a combination of CVD growth and thermal oxidation

Single crystal diamond needles were produced by combining chemical vapor deposition of a thin polycrystalline diamond film and its subsequent thermal oxidation. The deposition process has been carried out in a direct discharge activated hydrogen–methane gas mixture with parameters providing (100) textured diamond film growth. The oxidation has been performed by heating the deposited film in an air atmosphere with a temperature allowing the selective etching of the smallest fraction of the diamond crystallites in the film. As a result of this procedure, perfectly shaped micrometer sized single crystal diamond pyramids were obtained. The pyramids have a rectangular base plane with an apex tip curvature radius of about 2 to 10 nm. The atomic structure of the pyramids was established using high resolution transmission electron microscopy. We propose a model explaining the mechanism of the pyramids' formation.     

Redox-couple interactions with undoped single crystalline CVD diamond

Intrinsic CVD diamond is well known as insulator, however if the surface is hydrogen terminated, a surface conduction can be detected if exposed to air and covered by an adsorbate layer. In this paper we show that hydrogen terminated intrinsic single crystalline CVD diamond undergoes an insulator metal transition if immersed into redox electrolyte solutions with chemical potentials below the valence-band maximum. We have applied cyclic voltammetry experiments using different redox couples to characterize this phenomenon. The experiments reveal a large chemical window (> 4.6 V) of undoped diamond. We detect the same formal potentials as reported in the literature for H-terminated boron doped polycrystalline diamond. The peaks show comparable symmetries but are shifted towards higher and lower potentials, and are significantly broadened. We attribute this to the rate limited electron transfer at the interface. The results are discussed based on the transfer doping model.     

CVD diamond dosimetric response evaluated by X-ray absorbers method

The DC electronic response of a small area polycrystalline diamond film irradiated by a low energy X-ray beam has been analysed. The relationship between the induced photocurrent and the X-ray flux has been investigated making use of an X-ray absorber method. Aluminium foils have been used to attenuate the direct beam and the metal–diamond–metal device response tested at several electric field values. The X-ray photocurrent shows a linear response of the device that has been observed for a large range of X-ray flux values.     

Extreme UV photodetectors based on CVD single crystal diamond in a p-type/intrinsic/metal configuration

We report on extreme UV (EUV) photodetectors based on CVD single crystal diamond in a p-type/intrinsic/metal configuration fabricated and tested at Roma “Tor Vergata” University laboratory, operating in a sandwich geometry. Particular care has been devoted to the design of the device geometry in order to take advantage of the internal junction electric field and to minimize the signal contribution arising from secondary electron emission, which is known to strongly affect the detection properties in the UV and EUV regions. The device has been characterized in the EUV spectral region by using both He and He–Ne DC gas discharge radiation sources and a toroidal grating vacuum monochromator, with 5 Å wavelength resolution. The reproducibility test has been performed on several photodetectors showing a high uniformity of the device performances. The devices showed negligible undesired effects such as persistent photocurrent and memory effects, resulting in extremely promising stability features of the p-type/intrinsic/metal structured device. The devices have been tested at different bias voltages between 0 and 15 V, showing the best performances at 0 bias voltage. Finally, the external quantum efficiency (EQE), as well as the responsivity have been measured in the range 20 to 100 nm.     

Characterization of phosphorus doped CVD diamond films by cathodoluminescence spectroscopy and topography

A phosphorus doped homoepitaxial diamond film made by a CVD method was investigated by cathodoluminescence spectroscopy and topography. An exciton peak bound to phosphorus at 239 nm was observed from {001} as well as {111}, but the wavelength and width of the peak varied depending on location of the sample. This may be due to the difference of crystal perfection. It was found in the spectra that the slope with an array of weak peaks has a maximum of approximately 270 nm. This broad band may be an indicator of incorporation of phosphorus, although all the phosphorus doped diamonds do not exhibit the band. No other peak expected to be related to phosphorus was found, but several peaks commonly observed from CVD diamond were seen instead. These were also dependent on growth surface. Peaks at 415, 482, 500, 514 and 532 nm were strong on {111} surfaces of the non-epitaxial crystallites, whereas the 575-nm peak and another system of the 532-nm peaks were stronger in {001} epitaxial layers, as reported earlier.     

Preparation of low index single crystal diamond surfaces for surface science studies

In this review I summarise the procedures that have been developed to prepare well ordered, low index single crystalline diamond surfaces for surface science studies. Particular emphasis is placed on methods to smooth as polished surfaces with the aim to obtain well developed terraces with atomic order by different atomic hydrogen etching techniques.     

Neutron irradiation studies with detector grade CVD diamond

Detector grade CVD diamond samples have been irradiated with neutrons from a spallation source with an average energy of 5 MeV up to fluences of 6.5×10¹⁵ n cm⁻². After each irradiation stage the samples were characterised in terms of their charge collection efficiency, priming ratio and priming dose. It was found that the charge collection efficiency degrades with increasing neutron fluence for all investigated samples. The sensitivity to neutron radiation is found to be strongly sample-dependent. The experimental data can be well described by a simple damage model.