MeV Ion Implantation Research Articles

Direct fabrication and IV characterization of sub-surface conductive channels in diamond with MeV ion implantation

In the present work we report about the investigation of the conduction mechanism of sp2 carbon micro-channels in single crystal diamond. The structures are fabricated with a technique which employs a MeV focused ion-beam to damage diamond in conjunction with variable thickness masks. This process changes significantly the structural properties of the target material, because the ion nuclear energy loss induces carbon conversion from sp3 to sp2 state mainly at the end of range of the ions (few micrometers). Furthermore, placing a mask with increasing thickness on the sample it is possible to modulate the channels depth at their endpoints, allowing their electrical connection with the surface. A single-crystal HPHT diamond sample was implanted with 1.8 MeV He+ ions at room temperature, the implantation fluence was set in the range 2.1×1016-6.3×1017 ions cm-2, determining the formation of micro-channels with a graded level of damage extending down to a depth of about 3 μm. After deposition of metallic contacts at the channels’ endpoints, the electrical characterization was performed measuring the I-V curves at variable temperatures in the 80-690 K range. The Variable Range Hopping model was used to fit the experimental data in the ohmic regime, allowing the estimation of characteristic parameters such as the density of localized states at the Fermi level. A value of 5.5×1017 states cm-3 eV-1 was obtained, in satisfactory agreement with values previously reported in literature. The power-law dependence between current and voltage is consistent with the space charge limited mechanism at moderate electric fields.

Accelerator based synthesis of hydroxyapatite by MeV ion implantation

Accelerator based MeV ion implantation of Ca 2+ and P 2+ into the titanium substrate to form hydroxyapatite (HA) has been carried out. Calcium hydroxide was formed after heating the calcium implanted titanium in air at 80 °C for 3 h. Upon subsequent annealing for 5 min at 600 °C HA was formed on the surface. Penetration depth of the HA layer in this method is much higher as compared to keV ion implantation. By elemental analysis, Ca/P ratio of the HA was found to be 1.76 which is higher than the ideal 1.67. This higher Ca/P ratio is attributed to the higher penetration depth of the MeV technique used.

Strain evolution in Si substrate due to implantation of MeV ion observed by extremely asymmetric x-ray diffraction

We studied the strain introduced in a Si(111) substrate due to MeV ion implantation using extremely asymmetric x-ray diffraction and measured the rocking curves of asymmetrical 113 diffraction for the Si substrates implanted with a 1.5 MeV Au2+ ion at fluence values of 1×1013, 5×1013, and 1×1014/cm2. The measured curves consisted of a bulk peak and accompanying subpeak with an interference fringe. The positional relationship of the bulk peak to the subpeak and the intensity variation of those peaks with respect to the wavelengths of the x rays indicated that crystal lattices near the surface were strained; the lattice spacing of surface normal (111) planes near the surface was larger than that of the bulk. Detailed strain profiles along the depth direction were successfully estimated using a curve-fitting method based on Darwin’s dynamical diffraction theory. Comparing the shapes of resultant strain profiles, we found that a strain evolution rapidly occurred within a depth of ∼300 nm at fluence values between 1×1013 and 5×1013/cm2. This indicates that formation of the complex defects progressed near the surface when the fluence value went beyond a critical value between 1×1013 and 5×1013/cm2 and the defects brought a large strain to the substrate.

Anti-Site Defects are Found at Large Distances from Localised H and He Ion Implantations

1 MeV ion implantations of 4H SiC have been performed to various doses with ion probes of 5 µm diameter. Defect introduction has been studied by microscopic photoluminescence.

Direct fabrication of three-dimensional buried conductive channels in single crystal diamond with ion microbeam induced graphitization

We report on a novel method for the fabrication of three-dimensional buried graphitic micropaths in single crystal diamond with the employment of focused MeV ions. The use of implantation masks with graded thickness at the sub-micrometer scale allows the formation of conductive channels which are embedded in the insulating matrix at controllable depths. In particular, the modulation of the channels depth at their endpoints allows the surface contacting of the channel terminations with no need of further fabrication stages. In the present work we describe the sample masking, which includes the deposition of semi-spherical gold contacts on the sample surface, followed by MeV ion implantation. Because of the significant difference between the densities of pristine and amorphous or graphitized diamond, the formation of buried channels has a relevant mechanical effect on the diamond structure, causing localized surface swelling, which has been measured both with interferometric profilometry and atomic force microscopy. The electrical properties of the buried channels are then measured with a two point probe station: clear evidence is given that only the terminal points of the channels are electrically connected with the surface, while the rest of the channels extends below the surface. IV measurements are employed also to qualitatively investigate the electrical properties of the channels as a function of implantation fluence and annealing.

High thermal stability of vacancy clusters formed in MeV Si-self-ion-implanted Si

We have shown that considerable vacancy defects, introduced by MeV Si self-ion implantation, can survive a 900°C∕5min annealing for gate formation. By analyzing the trap-limited Si interstitial diffusion, we have characterized these vacancy clusters. Furthermore, we show that the remaining vacancies are sufficient to reduce B diffusion. The study suggests that MeV ion implantation, a promising approach for ultrashallow junction formation in metal-oxide-semiconductor device fabrication, can be inserted before gate formation (involving high temperature annealing) to avoid irradiation damage on gate structures.

Diffusion of antimony in silicon in the presence of point defects

We have investigated the diffusion of Sb in Si in the presence of defects injected by high-energy implantation of Si ions at room temperature. MeV ion implantation increases the concentrations of vacancies, which induce transient-enhanced diffusion of Sb deposited in Si. We observed a significant enhancement of Sb diffusion. Secondary ions mass spectroscopy has been performed on the implanted samples before and after annealing. Rutherford-backscattering spectrometry has been used to characterize the high-energy implantation damage. By fitting diffusion profiles to a linear diffusive model, information about atomic scale diffusion of Sb, i.e. the generation rate of mobile state Sb and its mean migration length were extracted.

Demonstration of an erbium-doped microdisk laser on a silicon chip

An erbium doped micro-laser is demonstrated utilizing $\mathrm{SiO_{2}}$ microdisk resonators on a silicon chip. Passive microdisk resonators exhibit whispering gallery type (WGM) modes with intrinsic optical quality factors of up to $6\times{10^{7}}$ and were doped with trivalent erbium ions (peak concentration $\mathrm{\sim3.8\times{10^{20}cm^{-3})}}$ using MeV ion implantation. Coupling to the fundamental WGM of the microdisk resonator was achieved by using a tapered optical fiber. Upon pumping of the $^{4}% I_{15/2}\longrightarrow$ $^{4}I_{13/2}$ erbium transition at 1450 nm, a gradual transition from spontaneous to stimulated emission was observed in the 1550 nm band. Analysis of the pump-output power relation yielded a pump threshold of 43 $\mathrm{\mu}$W and allowed measuring the spontaneous emission coupling factor: $\beta\approx1\times10^{-3}$.

Characterization of three-dimensional microstructures in single-crystal diamond

We report on the Raman and photoluminescence characterization of three-dimensional microstructures fabricated in single crystal diamond with a Focused Ion Beam (FIB) assisted lift-off technique. The fabrication method is based on MeV ion implantation, followed by FIB micropatterning and selective chemical etching. In a previous publication we reported on the fabrication of a micro-bridge structure exhibiting waveguiding behavior [ P. Olivero, S. Rubanov, P. Reichart, B. Gibson, S. Huntington, J. Rabeau, Andrew D. Greentree, J. Salzman, D. Moore, D. N. Jamieson, S. Prawer, Adv. Mater., 17 (20) (2005) 2427]. In the present work, Raman and photoluminescence spectroscopies are employed to characterize the structural quality of such microstructures, particularly as regards the removal of residual damage created during the machining process. Three-dimensional microstructures in high quality single crystal diamond have many applications, ranging from integrated quantum-optical devices to micro-electromechanical assemblies.

SIMS investigation of gettering centres produced by phosphorus MeV ion implantation

The ion implantation is a well-known standard procedure in electronic device technology for precise and controlled introduction of dopants into silicon. Damages caused by implantation act as effective gettering zones, collecting unwanted metal impurities. In this work, the consequences of high-energy ion implantation into silicon and of subsequently annealing were analysed by means of secondary ion mass spectrometry (SIMS). The differences in impurities gettering behaviour were studied in dependence of the implantation dose and annealing time at T = 900 °C.

Highly effective p-type doping of diamond by MeV-ion implantation of boron

Boron ions (2 MeV) were implanted into type IIa natural diamond samples to create deep buried conducting layers. The implantation was carried out at room temperature, followed by furnace annealing at temperatures up to 1650 °C. Data of Hall effect and conductivity measurements are presented in a wide temperature range, which clearly show p-type conduction with an activation of up to 30% of the implanted boron atoms, which is the highest value yet reported for ion implanted diamond.

Highly stripped ion sources for MeV ion implantation

A joint research and development effort whose ultimate goal is to develop an intense, high charge state, ion source for mega-electron-volt ion implanters has been initiated. Present day high-energy ion implanters utilize low charge state (usually single charge) ion sources in combination with radio frequency (rf) accelerators. Usually, a MeV Linear Accelerator (MV LINAC) is used for acceleration of a few milliamperes. It is desirable to have instead an intense, high charge state ion source on a relatively low energy platform [direct current (dc) acceleration] to generate high-energy ion beams for implantation. This endeavor is a continuation of earlier research, which resulted in generating ions like Pb+7 and Bi+8 and ion currents exceeding 200 mA. The natural next step is to convert and optimize ion charge state enhancement techniques to generate B, P, As, and Sb ions, and adapt them to a dc implanter. A number of schemes are to be pursued simultaneously. The most promising approach is to be developed into a commercial ion source.

Selective excitation of erbium in silicon-infiltrated silica colloidal photonic crystals

Optically active erbium ions in the silica and silicon sections of a Si-infiltrated silica colloidal photonic crystal can be separately addressed. A face-centered cubic colloidal crystal composed of 860 nm silica colloids was made by self-assembly under controlled drying conditions. It was then infiltrated with Si using chemical vapor deposition at 550 °C. Next, the photonic crystal was doped with erbium ions by 2 MeV ion implantation. The erbium ions were activated by thermal anneals at 400 and 750 °C, and showed clear photoluminescence at 1.5 μm in both the Si and silica parts of the photonic crystal. By varying measurement temperature and excitation wavelength the erbium ions were selectively excited in Si and/or silica. In this way the local optical density of states in these photonic crystals can be selectively probed. The emission linewidth for Er3+ in crystalline Si is relatively narrow and fits well within the calculated photonic band gap. The long luminescence lifetime of Er in Si makes these photonic crystals an ideal geometry to measure effects of the optical density of states on spontaneous emission.

Laser damage probability studies of fused silica modified by MeV ion implantation

Energetic ions in the MeV regime have pronounced effects on the stress-state and geometry of fused silica. In particular, Polman and co-workers have shown that 4 MeV xenon ions cause substantial changes in thin films and microspheres of fused silica. For example, 2 μm wide trenches in thin films can be partially closed and microspheres substantially distorted. In our study, we investigate implantation into bulk silica and the subsequent response to high intensity ultra violet light. Specifically, we compare the damage threshold of fused silica to intense ultra violet light at 355 nm before and after room temperature ion bombardment and find little change despite clear alteration of the stress-state in the glass. We have also performed molecular dynamics simulations in order to understand the underlying effects that lead to obscuration of optics under laser and ion irradiation.

Abnormal distribution of defects introduced into MgO single crystals by MeV ion implantation

The distribution of defects introduced into MgO single crystals formed by MeV ion implantation was studied and then compared to the distribution calculated by using TRIM code. First, MgO single crystals were implanted with 3.1 MeV Au ions at a dose of 2 × 10 16 cm −2. Cross-sections of the crystals examined by using transmission electron microscopy revealed an abnormal distribution of the defects, namely, that the peak defect density occurred at a deeper depth (1.7–1.9 μm) than implanted-Au distribution (0.5–1.6 μm), although the peak defect density normally occurs at a shallower depth than the peak implanted-Au concentration. The ion implanted MgO crystals were irradiated with 20 keV X-ray to examine the effect of electronic excitation on the defect distribution. The X-ray irradiation considerably reduced the defect density in the implanted layer, indicating that the abnormal defect distribution is due to the effect of electronic excitation around the energetic ion tracks.

Martensitic transformation and the stress induced by 3 MeV ion implantation in an austenite stainless steel sheet

Investigations were carried out on the relation between the martensite transformation and the stress induced by 3 MeV ion implantation in an austenite stainless steel (SS) sheet. Gold ions were implanted into the austenite SS sheet and the implantation dose was varied between 1 × 10 16/cm 2 and 3 × 10 16/cm 2. The microstructures in the ion-implanted region were examined by using transmission electron microscopy equipped with an energy dispersive X-ray spectrometer. The depth profile of the martensite volume fraction in the ion-implanted region was obtained by using a modified version of the direction comparison method, and the residual stress in the ion-implanted layer was measured by a low incident beam angle X-ray diffraction method.

Minimisation of spanning dislocations in the crystallisation of deep amorphous silicon wells through the alignment of volume edges with fast growth directions

Deep amorphous silicon wells were created in a crystalline substrate by MeV ion-implantation. The sides of the rectangular amorphous volume were aligned normal to fast growth directions. Upon heating at elevated temperatures crystallisation proceeded inwards at both lateral and vertical interfaces. The progress of the epitaxy and the number and nature of the secondary structures formed were investigated through both plan-view and cross-sectional transmission electron microscopy analyses. The number of spanning dislocations was significantly reduced in this aligned configuration compared with the situation where the sides of the well were not coincident with crystallographic directions. The reduction in the number of spanning dislocations allowed a more complete characterisation of other defects which arise uniquely from the crystallisation of volumes in this geometry.

Simple expression for vacancy concentrations at half ion range following MeV ion implantation of silicon

Mean concentrations CD of aggregated vacancy-type point-defect structures in float-zone Si implanted with H+, B+, Si+, O+, and Ge2+ ions at energies between 0.45 and 4.0 MeV have been measured as a function of ion dose φ at depths ∼RP/2 (half projected ion range) by beam-based positron spectroscopy. By adjusting φ to φA using factors given by the code TRIM, one arrives at the universal expression CD=(2.79×1010) φA0.63; CD (cm−3) can be estimated to ±50% for MeV ions implanted for φA from 109×1013 cm−2, which corresponds to an upper limit dose approaching 1014 cm−2 for 2 MeV Si+ implantation.

Investigation of the environment of Cr ions implanted into sapphire

MeV ion implantation of transition metals into sapphire (Al 2O 3), followed by thermal annealing, can produce optically active layers for use in solid state laser technology. We have investigated ion implantation of Cr into sapphire wafers, with co-implantation of O, to encourage formation of optically active Cr 3+. The Cr concentration was 0.113 at.%. Annealing temperatures ranged from 600 to 1450 °C. Damage production and recovery were observed with Rutherford backscattering spectrometry and ion channeling (RBS-C). The photoluminescence between 695 and 710 nm of Cr implanted sapphire measured at low temperatures (∼77 K) exhibits fine structure related to Cr–Cr exchange interactions. The variation of the fine structure with annealing temperature was investigated to determine the nature of the local environment of the Cr 3+ ions. Cr 3+ exists in the stoichiometry of Cr 2O 3, which implies the Cr ions should substitute for an Al lattice ion to be optically active. RBS-C was used to study the evolution of the Cr profile with increasing annealing temperature.

Development of a continuously variable energy radio frequency quadrupole accelerator for SiC power semiconductor device fabrication

High energy ion implantation is an important process for controlling deep junction conductivity of SiC semiconductor devices. An radio frequency quadraupole (RFQ) accelerator can accelerate high current ions, but the conventional RFQ accelerator is unable to accelerate various ions with different energies. In order to form the box profile with a low number of defects, a high current MeV ion implanter was constructed using a newly designed continuously variable energy RFQ accelerator and high temperature ion implantation chamber. Results of the experiment showed that the resonance frequency varied from 11.7 to 29.3 MHz, so the output beam energy could be changed over a range of about 6.3 times. The results also showed a higher Q-value of over 5000 compared with that of a conventional machine of 1500. Beam acceleration tests were performed using Al ions, and beams were accelerated from 20 to 750 keV using an radio frequency power of 15 kW. The ions were implanted into 4H-SiC, and the projected ranged was 0.76 μm measured by secondary ion mass spectroscopy.