Natural Type IIb Diamond Research Articles

Spatial distribution of defects in natural type IIb diamond after irradiation and annealing

Natural blue diamonds are among the rarest and most valuable of gemstones; however, they are occasionally treated by a variety of methods to improve the color or in attempts to obscure the evidence of treatment. In this study, two sections were cut from a rough naturally sourced type IIb diamond. One was subjected to electron irradiation along one edge. Subsequently, both were isochronally annealed from 300 °C to 1200 °C and the optical defects were documented by changes in infrared (IR) absorption and photoluminescence spectroscopic mapping. The thermal behavior of these centers (TR12, 3H, NV0, GR1, 648.2 and 776.4 nm centers among others) along with their spatial distribution in the irradiated and non-irradiated diamond provided additional insights into their configuration.After irradiation, the uncompensated boron concentration as determined by IR spectroscopy for this sample showed a pronounced decrease particularly at positions close to the irradiation edge. This uncompensated boron decreased further with low temperature annealing (300–600 °C) principally due to interstitial migration. At temperatures >600 °C, the uncompensated boron concentration rebounded due to vacancy migration that likely depletes or annihilates compensating defects. Among the photoluminescence (PL) defects, the 3H center (ZPL = 503.5 nm) showed a much greater thermal stability in the irradiated sample compared to its non-irradiated counterpart.

Characterization of electronic properties of natural type IIb diamonds

Precision admittance spectroscopy measurements were carried out over wide temperature and frequency ranges for a set of natural single crystal type IIb diamond samples. Peaks of conductance spectra vs. temperature and frequency were used to compute the Arrhenius plots, and activation energies were derived from these plots. The capacitance-voltage profiling was used to estimate the majority charge carrier concentration and its distribution into depth of the samples. Apparent activation energies between 315 and 325meV and the capture cross section of about 10−13cm2 were found for samples with uncompensated boron concentrations in the range of 1 to 5×1016cm−3 (0.06–0.3ppm). The obtained boron concentrations are in good coincidence with FTIR results for the samples. Also, a reason for the difference between the observed admittance activation energy and the previously reported ionization energy for the acceptor boron in diamond (0.37eV) is proposed.

Temperature effects on luminescence centers in natural type IIb diamonds

Blue diamonds are among the rarest and most valuable of naturally occurring gemstones. In this study, 12 rough naturally-sourced type IIb diamonds were subjected to HPHT annealing, three different irradiation energies, and then all were stepwise annealed from 200°C to 1100°C and the optical defects were documented by changes in phosphorescence and photoluminescence spectroscopy. Several optical features that are removed from natural type IIb diamonds by HPHT processing, such as 3H, 648.2nm peak, 776.4nm peak, and 660nm band (red) phosphorescence, can be reintroduced into these diamonds with subsequent electron irradiation and annealing at low-to-moderate temperatures. The thermal stability of these centers along with their spatial distribution provided additional insights into their configuration and distinguished them from nitrogen-bearing diamonds.

Phosphorescence in type IIb diamonds

Spectroscopic properties, including phosphorescence spectroscopy, were collected for 354 natural type IIb diamonds, 7 natural type IIa diamonds, along with 24 HPHT-treated diamonds and 18 HPHT synthetic diamonds. The only phosphorescence bands observed in the natural diamonds were centered at 500 and 660 nm. The relative intensity and half-life of these measured bands corresponded well with the boron concentration. None of the treated or synthetic diamonds showed the 660 nm band. However, these samples did have the 500 nm band; the synthetics had much higher intensity than the comparable natural diamonds. The phosphorescence spectra are correlated to data collected from infrared absorption and photoluminescence spectroscopy and these results provide additional evidence of the mechanism responsible for the observed phosphorescence.

Electron emission mechanism of hydrogenated natural type IIb diamond (111)

Clarification of the electron emission mechanism of diamond has been one challenging topic in the field of vacuum nanoelectronics. Electric field of less than 5 V/µm is enough to extract electrons from diamond, which is orders of magnitude lower than the value required for electron emission from metal emitter in general. There have been number of studies on the clarification of electron emission mechanism, however, unified model has not been proposed. The difficulty largely lies in determining the origin of emitted electrons. In this study, we succeeded in clarifying the electron emission mechanism of hydrogenated natural type IIb diamond (111) surface by using combined x-ray photoemission spectroscopy/ultraviolet photoemission spectroscopy/field emission spectroscopy (XPS/UPS/FES) system. Obtained spectra clearly defined the origin of emitted electrons at the valence band maximum (VBM). As applied voltage was increased, the number of emitted electrons increased, however, the origin remained at VBM independent of the applied voltage.

Possible evidence of a copper-related electron paramagnetic resonance centrein diamond

The EPR centre W36, found in natural type IIb diamond, has beenpreviously attributed to a boron-related point defect, largely on account ofits four line hyperfine structure attributed to 11B. The attribution has beenre-examined, and no simple boron-related site has been found capable ofaccounting for the measured parameters. It is concluded that W36 is much morelikely to be related to copper, probably at a semi-vacancy site (C3VCuVC3)+.An unusual feature of this centre is the very small value of the finestructure parameter D. Two other centres, W10 and W32, with similarly smallvalues of D, which may also be associated with 3d ions, are briefly discussed.

Capacitance–voltage profiling of deuterium passivation and diffusion in diamond Schottky diodes

Abstract Capacitance–voltage measurements have been performed on deuterated boron-doped natural type IIb diamond. They demonstrate the electrical passivation of the boron acceptors, which was manifested by a persistent decrease in capacitance after deuteration. Bias annealing experiments have been carried out to determine the charge state of the diffusing deuterium. The differences between annealing with or without bias lead to the conclusion that boron–deuterium complexes dissociate at about 750 K and that the released deuterium is positively charged.

MeV Ion Implantation Doping of Diamond.

AbstractDiamond-based semiconductor devices offer the promise of operation at high temperatures and under extreme radiation conditions. An essential step in the drive towards operational diamond-based electronic devices is the ability to controllably and reproducibly dope the diamond. Ion implantation is the method of choice for such doping because it offers precise control of the dopant concentration and spatially selective doping is achievable using standard masking techniques. However, compared to silicon, the doping of diamond is complicated by the tendency of the diamond to relax to graphite upon thermal annealing. Furthermore, even if graphitization can be avoided, the compensation of dopants by residual defects has proved in the past to be a limiting factor in obtaining very high mobility material. In this paper, we present a scheme for the effective doping of diamond using MeV ion-implantation. For MeV ion- implantation the doped layer is deeply buried under a cap of undamaged diamond, and so the scheme includes a method using pulsed laser irradiation for making electrical contact to the buried layer. We show that a boron doped layer fabricated by the MeV implantation scheme has, after suitable annealing and removal of these compensating/trapping defects, very high mobility and low compensation ratio. In fact, its electrical properties are quite similar to those of natural boron-doped type IIb diamond.

13C spin–lattice relaxation in natural type-IIb semiconducting diamond

13C spin–lattice relaxation times in the laboratory frame have been measured at 295 K in a field of 4.7 T for two natural type-IIb diamonds using nuclear magnetic resonance spectroscopy (NMR). 13C spin–lattice relaxation is mainly due to mobile hole carriers. The contribution of fixed paramagnetic impurities is at least an order of magnitude smaller. At 295 K, the carrier concentration is estimated to be 1.2×1014 cm−3, resulting in an η, the electron density at the nucleus, of about 320 for natural type-IIb diamonds.

A slow positron beam study of natural and synthetic diamonds

We have depth profiled single crystalline natural type IIA, type IIB and synthetic type IB diamonds with a slow positron beam using Doppler broadening spectroscopy. It was found that the data for all three samples, which differ with respect to defect type and content, could be well described using only a surface and a homogenous bulk component. The bulk positron diffusion lengths measured for these samples were found to be sensitive to the differing defect composition of the samples.

Fine structure in the secondary electron emission peak for diamond crystal with (100) negative electron affinity surface

A fine structure was discovered in the low-energy peak of the secondary electron emission spectra of the diamond surface with negative electron affinity. We studied this structure for the (100) surface of the natural type-IIb diamond crystal. We have found that the low-energy peak consists of a total of four maxima. The relative energy positions of three of them could be related to the electron energy minima near the bottom of the conduction band. The fourth peak, having the lowest energy, was attributed to the breakup of the bulk exciton at the surface during the process of secondary electron emission.

Formation of delta-doped, buried conducting layers in diamond, by high-energy, B-ion implantation

Abstract The electrical properties of a deeply buried, boron-doped layer in type IIa diamond are reported. The layer is fabricated using mega-electron-volt (MeV) B-ion implantation, followed by furnace annealing at 1450 °C. Electrical contact to the buried layer is made using laser-induced graphitization. The room temperature Hall mobility of the acceptors in the buried layer is 585 cm2/Vs, which is the highest value yet reported for ion-implanted diamond, with an acceptor concentration of (3.9±0.3)×1018 cm−3, a compensation ratio of approximately 5%, and an activation energy of 0.354±0.006 eV. The temperature dependence of the mobility is very similar to that observed in natural B-doped type IIb diamond. The cap overlying the implanted region, is, following annealing, a high quality, highly insulating diamond, hence opening the possibility of realizing Field Effect Transistor (FET)-type devices in diamond.

Surface cleaning, electronic states and electron affinity of diamond (100), (111) and (110) surfaces

The effects of cleaning natural type IIb diamond (100), (111) and (110) samples by annealing and hydrogen – or deuterium plasma exposure were investigated by means of ultraviolet photoemission spectroscopy (UPS). Different wet chemical cleaning processes (a conventional chromic acid clean and an electrochemical etch) and a H plasma exposure have been employed to clean natural type IIb semiconducting diamond C(100) wafers. The effects of these processes on the diamond surface have been assessed and compared. As evidenced by Auger electron spectroscopy (AES), an oxygen free surface could be obtained following vacuum annealing to 900°C for the electrochemical process compared to 1050°C for the chromic acid etch. In addition, the technique of atomic force microscopy demonstrated the presence of oriented pits on the surface of samples that were electrochemically etched for long times at high currents. After a H plasma exposure the negative electron affinity (NEA) peak in the UPS spectra doubled in intensity. An anneal to 1100°C resulted in the removal of the sharp NEA feature. A second H plasma treatment resulted in the reappearance of the NEA peak similar to that after the first H plasma exposure. A (2×1) reconstructed low energy electron diffraction pattern was observed subsequent to the anneals as well as the H plasma treatments. The fact that a NEA can be induced or removed repeatedly by means of a H plasma exposure or annealing at 1100°C, respectively, provides evidence to correlate the appearance of a NEA with the presence of a monohydride terminated surface. Corresponding effects were found for (111) and (110) surfaces. A NEA could be induced by a H plasma and removed by annealing at 900 or 800°C for diamond (111) or (110) surfaces, respectively. Following a deuterium plasma exposure the diamond surfaces exhibited a NEA like the ones treated by a hydrogen plasma. Higher annealing temperatures were necessary to remove the NEA for deuterium due to the isotope effect. Values of 79 and 81 V μm −1 were measured for the field emission threshold of the oxygen terminated C(100) and C(110) surfaces, respectively. A value of 25 V μm −1 was determined for the hydrogen terminated C(110) surface.

Electronic transitions of holes bound to boron acceptors in isotopically controlled diamonds

The excitation spectrum of holes bound to substitutional boron acceptors introduced into a 13C diamond is compared with that of boron acceptors in natural Type IIb diamond. While the two spectra are remarkably alike in their general features, the spectrum of the acceptors in 13C diamond is shifted to higher energies by 0.4–1.5 meV. In addition, the presence of substitutional boron relaxes the selection rule associated with translational symmetry, thus allowing the observation of the one phonon spectrum reflecting the density of states of the optical branch as well as the zone center optical phonon.

Hole traps in natural type IIb diamond

Hole traps in natural type IIb diamond have been characterized with Schottky diodes using thermal and optical techniques. The Schottky diode capacitance variation with time was used to determine the detrapping rate of holes. For reverse-biased Schottky diodes the rate was found to be independent of both electric field from 0 to 3.6/spl times/10/sup 5/ V cm/sup -1/, and temperature from -25 to 102/spl deg/C. Optical radiation below 1.4 eV (>890 nm) has no measurable effect on the detrapping rate, while the rate increases with decreasing wavelength below 870 nm, We speculate that the traps are caused by impurities, possibly nitrogen aggregates, distributed over an energy range 1.4-4 eV above the valence band.

Negative electron affinity effects on H plasma exposed diamond (100) surfaces

The effects of annealing and a H plasma exposure on natural type IIb diamond (100) were investigated by means of ultraviolet photoemission spectroscopy (UPS). The diamond (100) surface was found to exhibit a negative electron affinity (NEA) following a 900 °C anneal in ultrahigh vacuum. After a H plasma exposure the NEA peak in the UPS spectra had doubled in height. An anneal to 1100 °C resulted in the removal of the sharp NEA feature. A second H plasma treatment resulted in the reappearance of the NEA peak like after the first H plasma exposure. A 2 × 1 reconstructed low-energy electron diffraction (LEED) pattern was observed subsequent to the anneals as well as the H plasma treatments. The fact that a NEA can be induced or removed repeatedly by means of a H plasma exposure or a 1100 °C anneal respectively provides evidence to correlate the appearance of a NEA with the presence of a monohydride terminated surface.

Ion beam induced luminescence in natural diamond

We used ion beam induced luminescence (IBIL) to study the evolution of damage during ion implantation of natural type-IIa diamond with 30 keV carbon ions, for different sample temperatures in the range 35–300 K. Blue band-A luminescence decays with irradiation dose from an initial value of ∼10−3 photons/e-h pair. This decay was modeled to obtain an effective damage cross section of ∼40 Å2, which is roughly independent of temperature. We explored the use of IBIL as a tool to characterize the processing of p-n junctions in natural type-IIb diamond and apply it to find conditions that determine the ultraviolet response of light-emitting diodes.

Characterization of rectifying contacts on natural type IIb diamond

Metal-semiconducting diamond (MS) and metal-insulating diamond-semiconducting diamond (MIS) contacts were fabricated on the same type IIb single crystal diamond. Direct comparisons of MS and MIS structure characteristics were made by analysis of current-voltage and differential capacitance-voltage (C-V) data. Both the MS and MIS contacts exhibited good rectifying characteristics, with a 5 V rectification ratio ≳106. The depletion layer uncompensated acceptor concentration measured in both structures by C-V analysis was ∼1.8×1016 cm−3.

Angle-resolved photoemission of diamond (111) and (100) surfaces; negative electron affinity and band structure measurements

Angle-resolved photoemission from both (100) and (111) oriented, natural type IIb diamond was used to study the electronic structure of diamond. A negative electron affinity (NEA) was found on the 2×1 reconstructed diamond (100) surface. Photoemission spectra of NEA surfaces exhibit a distinctive peak due to emission from the conduction band edge. This peak was used as a reference point to correlate spectra of (100) and (111) surfaces with a NEA. A feature at ∼7.5 eV above the conduction band edge appears in the spectra of both the (111) and (100) surfaces. Angle-resolved photoemission measurements were used to determine the energy dispersion of this feature for various off-normal angles. The results for the surfaces were correlated, and compared to a calculated band structure. Based on this comparison we attribute this feature to emission from secondary electrons which are collected in a conduction band level at ∼7.5 eV above the conduction band edge.

Influence of interfacial hydrogen and oxygen on the Schottky barrier height of nickel on (111) and (100) diamond surfaces.

In this paper we report on Schottky barrier height measurements of nickel on both diamond (111) and (100) surfaces, as a function of surface preparation. The Schottky barriers of thin (5 \AA{}) nickel films on natural type-IIb (p-type semiconducting) diamond (111) and (100) surfaces were determined with ultraviolet photoemission spectroscopy. Exposing the diamond (111) surfaces to an argon plasma while heated to 350 \ifmmode^\circ\else\textdegree\fi{}C resulted in a change from a negative-electron-affinity surface to a positive-electron-affinity surface. This effect was used as an indication that a hydrogen-free surface had been obtained. Deposition of a monolayer of nickel on the hydrogen-free diamond (111) surface resulted in a Schottky barrier height of 0.5 eV. The nickel caused the surface to exhibit a negative-electron-affinity surface. Nickel deposited on a diamond (111) surface with a negative electron affinity, indicative of a monohydride-terminated surface, resulted in a \ensuremath{\sim}1.0-eV Schottky barrier height. Diamond (100) surfaces were prepared by vacuum annealing to temperatures ranging from 500 \ifmmode^\circ\else\textdegree\fi{}C to 1070 \ifmmode^\circ\else\textdegree\fi{}C. The various anneals resulted in a lowering of the electron affinity by up to \ensuremath{\sim}1 eV, which resulted in a negative electron affinity after the surface had been annealed to \ensuremath{\sim}1000 \ifmmode^\circ\else\textdegree\fi{}C. Oxygen was initially present on the surface but could not be observed after the 1000 \ifmmode^\circ\else\textdegree\fi{}C anneal. The removal of oxygen and the appearance of a negative electron affinity coincided with the appearance of a 2\ifmmode\times\else\texttimes\fi{}1 surface reconstruction. Nickel was deposited after the various anneals, and Schottky barrier heights were found, ranging from 1.5 eV for the 545 \ifmmode^\circ\else\textdegree\fi{}C-annealed surface to 0.7 eV for the 1070 \ifmmode^\circ\else\textdegree\fi{}C-annealed surface. These measurements suggest that for both the (111) and the (100) diamond surfaces the presence of chemisorbed species, such as hydrogen and oxygen, results in an increase in the Schottky barrier height.