Megaelectronvolt Ion Research Articles

Secondary ion emission from various metals and the semiconductors Si and GaAs induced by mega-electronvolt ion impact

Experiments on secondary ions ejected from six UHV cleaned metals and from Si and GaAs have been performed with Cf-fission fragments both at Darmstadt and using the Tandem accelerator in Orsay. The energy and angular distributions measured for the metal ions by means of a time-of-flight technique are explainable by linear collision sputter theory, when an ionization term is included. Deviations from this theory indicate contributions due to the electronic excitation of the collision-cascade region around the nuclear track. Also, for the semiconductors, collisional sputtering seems to be the dominant process. Doping has no influence on the secondary ion yields.

Megaelectronvolt ion irradiation effects in amorphous carbon: the roles of the CH and CC bonds

The electronic excitation of the target atoms by megaelectronvolt ion bombardment is responsible for the initiation of a number of damage processes. In the work described here we discuss secondary-electron emission, loss of bound hydrogen, and graphite formation in amorphous carbon films under irradiation by heavy ions. A mechanism that appears to underlie these effects is that of CH and CC bond breaking by the secondary electrons that are generated along the track of the incident ion and which allow the transport of the ion's lost energy to regions away from track. Model calculations based on this mechanism are in agreement with the observations.The production by such ion bombardment of small graphite crystals in the poorly conducting amorphous carbon matrix allows the conductivity of the material to be changed continuously over a range of four orders of magnitude. This phenomenon and its possible applications are discussed.

Heavy ion induced desorption (HIID) mass spectrometry of Langmuir‐Blodgett films built from CD‐salts of fatty acids

AbstractLangmuir‐Blodgett films made from cadmium salts of fatty acids have been analyzed by means of the heavy ion induced mass spectrometry in which megaelectronvolt ions are used to desorb sample specific molecules. The method is a useful tool for detecting incomplete salt formation at the subphase and the replacement of the original counter‐ion by ions from the substrate surface. The method exhibits a depth sensitivity which can be varied between 2 to 14 layers depending on the energy loss of the heavy ion.

Secondary ion emission from cesium salts under megaelectronvolt ion bombardment: comparative study and beam secondary effects

A comparative study of the secondary ion yields of various cesium salts (halide, nitrate and sulphate) under bombardment of Ar ions at 9 MeV has been performed using time-of-flight detection techniques. Exceptionally high values (close to 100%) are reported with nitrate and sulphate. This result seems related to the presence of oxygen from which chemically active radicals are formed. However, the use of highly collimated primary ion beams allows observation of a yield variation as a function of primary ion fluence which is similar to what has often been observed with conventional secondary ion mass spectrometry.

Production of milliampere class mass-separated multiply charged ion beam

A new type of microwave ion source for a megaelectron volt ion implanter is designed to produce milliampere class multiply charged ion beams. This ion source has a single stage plasma chamber with both a mirror and an octopole magnetic field to produce milliampere class lower-charge-state ions efficiently. The extracted ion beam is mass separated, and a Kr2+ beam of 2.3 mA and an Ar2+ beam of 3.0 mA are obtained with absorbed microwave powers of 700 and 570 W, respectively. Combination of this new source with an ion accelerator should realize a milliampere class megaelectron volt ion implanter.

Carbon cluster emission from polymers under keV and MeV-ion bombardment

Experiments have been performed to compare secondary ion emission from polymer substrates under kiloelectronvolt ion (secondary ion mass spectrometry, SIMS) and megaelectronvolt ion (plasma desorption mass spectrometry, PDMS) bombardment. The yield of carbon cluster ions has been determined for different polymers. A positive PDMS spectrum of poly(vinylidene fluoride) showing even-numbered carbon clusters to C{sub 200} is presented. The emission of these carbon cluster ions is explained by the strong degradation of the polymer under megaelectronvolt ion bombardment. This is discussed in comparison with the results in SIMS and laser mass spectrometry.

Interaction of megaelectronvolt ion beams with silicon: Amorphization, recrystallization and diffusion

Three areas will be reviewed where the use of megaelectronvolt ion beams has contributed to the understanding of the properties of amorphous silicon: (1) thermodynamic and relaxation effects; (2) thermal and radiation-enhanced diffusion; (3) interfacial segregation.

Lattice damage and silicide formation by deep implantations into silicon

Lattice damage by megaelectronvolt ion implantation has been investigated using transmission electron microscopy (TEM) and depth-resolved measurements of the optical reflectivity change on bevelled samples. Generally, optical reflectivity depends on both the radiation damage and the concentration of implanted atoms. Therefore, extensive investigations of radiation damage due to silicon self-implantation have been performed to avoid any influence of implanted impurities. A distinct interplay of defect generation, interaction and transformation, eventually leading to amorphization of the silicon, has been observed for implantation doses between in the 10 11 – 10 18 cm −2 and at temperatures between 125 and 475 K. A defect generation model describes the dose dependence of damage for a large range of implantation temperatures, using fitting parameters for the relative weights of different microprocesses. Lattice damage due to implantation of nickel, germanium and gold scales well with respect to silicon implantation damage but can show additional features due to superimposed solid state reactions of impurity atoms. For example, amorphization may be suppressed by a high concentration of nickel. Epitaxial NiSi 2 precipitates at type A and type B orientations can be seen by TEM in 450 K samples for doses above 10 17 Ni + cm −2. Damage and nickel accumulation mutually interfere; preferential precipitation can be found at depths of high defect density and transient amorphization occurs at the front flank of the nuclear stopping power profile while, at the profile maximum, amorphization is suppressed. At a dose of 1.3 × 10 18 + cm −2 , a continuous silicide layer forms by coalescence of precipitates. Spreading resistance measurements reveal that, already in the as-implanted state, the local resistivity has dropped at the profile minimum by several orders of magnitude.

Results of boron implantation into silicon diodes and metaloxide-semiconductor gate-controlled turn-off thyristors

50 MeV boron ions have been implanted into silicon wafers homogeneously with doses between 5×10 11 and 2×10 14 cm −2 . Prior to the implantation the wafers have been processed in an integrated-circuit line to generate the early structures of the desired device. After the irradiation, processing of the devices is continued without any specific annealing step. The forward-voltage drop, the breakdown voltage and the switching behaviour of metaloxide-semiconductor gate-controlled turn-off thyristors and diodes have been investigated. It is shown that megaelectronvolt ion implantation into silicon is a promising way of creating structures buried within the bulk material of power devices.

Millimeter-wave planar InP Schottky diodes and their small-signal equivalent circuit

Two planar indium phosphide Schottky diode designs have been fabricated and analyzed for millimeter-wave detector applications up to 150 GHz. Device structure and fabrication are discussed, and small-signal equivalent circuit models are presented. The following topics are included: the planar InP diode structure fabricated by megaelectron volt ion implantation. DC and RF measurements, circuit model values, and 94-GHz small-signal detector performance. The zero-bias detector sensitivity at 94 GHz was measured to be as high as 400 mV/mW, and the calculated tangential signal sensitivity was -56 dBm. >

Experimental microwave results on an ion-implanted GaAs slow-wave monolithic structure

The use of mega-electronvolt ion implantation for the realization of a GaAs monolithically compatible device is demonstrated. Ion implants up to 6 MeV in energy are used employing Si and S atoms. The fabricated device is an electromagnetic slow-wave microstrip-like structure designed for performance into the millimeter-wave regime. Phase shift θ and insertion loss L measurements are performed for frequencies of 2-18 GHz at room temperature. Comparison of the experimental ion-implanted device results to epitaxial device results indicates comparable performance, with no more than a 30-percent reduction in θ but with an improvement in loss behavior, namely an L reduction upto 40 percent.

Intermixing between germanium and silicon thin films in the presence of a fast interstitial diffuser

The interdiffusion between germanium and silicon is relatively slow. Generally speaking, an annealing temperature of not less than 800°C is needed to cause any significant diffusion in Ge/Si thin film couples in a reasonable time. In this investigation we study the effect of fast interstitial diffusers on the intermixing between germanium and silicon thin films. The samples generally consist of the following structure SiO 2/Ni(≈100–340 Å)/Ge(≈3000 Å)/Si(≈3000 Å) Nickel is known to be a fast interstitial diffuser in germanium and silicon. Megaelectronvolt ion backscattering and X-ray diffraction were used to examine the intermixing and the structure of the samples before and after annealing. It was found that germanium and silicon mixed significantly faster at relatively low temperatures in the presence of a fast interstitial diffuser whereas little or no intermixing was observed in diffusion couples without any fast diffuser at temperatures of about 750°C.

Enhancement of adhesion by megaelectronvolt ion bombardment

It has been demonstrated that the adhesion of a wide variety of film-substrate combinations can be dramatically improved by megaelectronvolt ion bombardment. Although the results are similar in most cases, it is becoming clear that several mechanisms may be involved in the process. For example, in the 35Cl 4+ ion bombardment (6–21 MeV) of gold films on vitreous SiO 2 substrates, there are two distinct dose regimes where enhanced adhesion occurs. One is centered at 10 13 35Cl 4+ ions cm −2, coincident with the formation of bombardment-induced surface cracks in the SiO 2. The other occurs above an ion-energy-dependent threshold (about 10 14 35Cl 4+ ions cm −2). We have also observed that the adhesion enhancement for gold on GaAs depends strongly on the type and level of doping. For example, both silicon- and tellurium-doped n-type GaAs have a sharp threshold in enhanced adhesion near 10 14 35Cl 4+ ions cm −2 for 18 MeV 35Cl + ion irradiation. However, although chromium-compensation-doped p-type GaAs reaches the same final adhesion levels, there is a smooth increase versus dose starting below 10 12 35Cl 4+ ions cm −2. Several possible mechanisms will be discussed and compared with the data.

Cross Sections for Dielectronic Recombination ofB2+andC3+via2s→2pExcitation

Dielectronic recombination cross sections for the Li-like ions B/sup 2 +/ and C/sup 3 +/ via 1s /sup 2/2s..-->..1s /sup 2/2p excitation are reported. The amount of electron capture attending the passage of megaelectronvolt (atomic mass unit) ion beams through a collinear, magnetically confined, space-charge--limited electron beam is observed as a function of relative energy. The results agree well with distorted-wave calculations.

Pseudomorphic Structure at the Interface of Ge on Si(111) Studied by High-Energy-Ion Scattering

Megaelectronvolt ion scattering is applied to a study of the initial stage of heteroepitaxy of the Ge/Si(111) system and it is shown that the interface is uniform and pseudomorphic at first, after which island formation takes place and strains are introduced at the interface. This change occurs after deposition of a threshold film thickness of \ensuremath{\sim} 3 monolayers. More than 10 monolayers are distorted at the interface after deposition of about 30 monolayers of germanium.