Diffusion of zinc into ion-implanted gallium arsenide

A study has been made of the diffusion of zinc, from a vapour source, into GaAs slices which had been previously implanted with various ion species. A radiotracer sectioning technique was used to measure the zinc diffusion profiles. It was found that the various implanted ion species (H⁺, He⁺, N⁺, Zn⁺, As⁺) had different effects on the zinc diffusion. The results could not be attributed solely to native defects produced by radiation damage. The heavier ion species increased the zinc concentration in the implanted region, but not beyond. The lighter species substantially increased the zinc diffusion rate and altered the resultant concentration profiles. Uphill diffusion was seen in slices which had been given a single high energy H⁺ implant. The results obtained are compared to those of Radiation-Enhanced-Diffusion experiments. It is suggested that the rate of incorporation of dopant species into the host semiconductor lattice is an important influence on the diffusion mechanism and the shape of the concentration profile.     

Redistribution of ion-implanted impurities in silicon during diffusion in oxidizing ambients

A closed form expression for the redistribution of ion-implanted impurities in silicon during diffusion in an oxidizing ambient is derived, based on a theoretical model proposed by Huang and Welliver. It is shown that the computed results for boron profiles agree very well with experimental data. The closed form solution requires considerably less computer execution time than the usual numerical solution.     

The gettering of boron by an ion-implanted antimony layer in silicon

Secondary ion mass spectrometry has been employed to reveal the gettering of implanted B by an annealed, implanted Sb layer. It is shown that the gettering of B is significant, and may be caused by electric-field-enhanced diffusion of the B as well as by solubility enhancement of the electrically-active Sb. These results emphasize the first-order importance of cooperative effects between donors and acceptors in diffusion profile calculations.     

Diffusion of delta doped boron in silicon following oxidation

Samples from a silicon wafer containing a boron doped delta layer were exposed to plasma oxidation and rapid thermal processing (RTP). It is demonstrated experimentally for the first time that there is no enhanced diffusion resulting from oxidation by plasma anodisation, making it well suited to complement low temperature processing.<>     

Electron-beam annealing of ion-implanted silicon

A scanning-electron-beam zapping system for the annealing of ion-implanted semiconductors is described. Experiments have been conducted on silicon implanted with various doses of arsenic and boron. Results show that for a given beam power full annealing is achieved above a threshold exposure.     

Diffusion of boron and phosphorus in silicon during high-temperature ion implantation

A theoretical model is proposed for describing the accelerated diffusion of boron and phosphorus impurity in silicon during high-temperature implantation. The model takes into account the production and diffusion of impurities and point defects and the formation and decomposition of defect-impurity pairs. The formation of dislocations and an increase (decrease) of their size during bombardment is taken into account. Dislocations are sinks for defects and impurities. The computed profiles are compared with existing data. It is shown that dislocations play an important role as trapping centers. Oscillatory behavior of the impurity profile was observed near the surface; this behavior is due to impurity implantation and diffusion processes occurring simultaneously. Fiz. Tekh. Poluprovodn. 31, 385–389 (April 1997)     

High-efficiency ion-implanted silicon solar cells

The development of solar cells with AM1 coversion efficiency of 18 percent is reported. The cells comprise an n⁺-p-p⁺structure fabricated from float zone silicon having resistivity of 0.3 Ω . cm. The n⁺and p⁺regions are formed by low energy ion implantation and thermal annealing. An important feature of cell fabrication is the growth of SiO2passivation for reduction of surface recombination velocity. Details of both cell fabrication and testing are reported.     

Electrical characterization and modelling of high energy pre-amorphized PN silicon junctions

Shallow P⁺N junctions were obtained using germanium pre-amorphization step to reduce the high diffusivity of boron implanted in silicon. The germanium implantation step was performed under different conditions of temperature: ambient temperature and nitrogen temperature. P-type doping was obtained by boron implantation at relatively low energy. To characterize and simulate the electrical behaviour of such samples, steady state current-voltage measurements have been performed at different temperatures varying between 172 and 294 K. The results show a close dependence between the current-voltage characteristics of the samples and their technological parameters of manufacturing. The pre-amorphization step at ambient temperature seems to improve the electrical behaviour of the junction. To simulate the electrical characteristics of the studied samples, a reliable model has been developed based on the classical Spice formulas and taking into account additional phenomena. The simulated curves satisfactorily fit the experimental results for all the samples.     

Comparison of boron and neon damage effects in boron ion-implanted resistors

Boron and neon damage implants were used in fabricating integrated-circuit resistors in silicon. Resistor properties were studied as functions of damaging ion species and dose. Sheet resistances in the 10000 ?/? range were obtained with low temperature and voltage sensitivities and good d.c. isolation.     

The behaviour of boron molecular ion implants into silicon

Implants of boron molecular ions into silicon have been studied using a variety of experimental techniques, but with emphasis on sheet resistance annealing characteristics and transmission electron microscopy. Boron halide compound molecules have been implanted and equivalent dose sequential implants of atomic species used as control conditions. The implants studied were B⁺, BCl₂⁺, BCl⁺, Cl⁺ + B⁺, BF₂⁺, BF⁺ and B⁺ + F⁺ at 25 keV/B atom and B⁺, BBr₂⁺ and Br₂⁺ + B⁺ at 12 keV/B atom.The implantation of molecular ions enables conditions of varying damage to be studied with constant dose, dose rate and energy of the dopant species. In addition to damage effects the halogen atoms produce species effects in the implanted zone. The escape of the halogen atoms has been measured as a function of the annealing temperature.The significant differences which exist between the behaviour of silicon implanted with these various conditions are considered with reference to the damage structures observed by transmission electron microscopy. The boron-fluorine molecular implants are shown to offer some advantages as a means of implanting boron.     

Redistribution of phosphorus implanted into silicon doped heavily with boron

The special features of redistribution of phosphorus implanted into silicon wafers with a high concentration of boron (N _B=2.5×10²⁰ cm^?3) were studied. It is shown that, in silicon initially doped heavily with boron, the broadening of concentration profiles of phosphorus as a result of postimplantation annealing for 1 h in the temperature range of 900–1150°C is significantly less than in the case of lightly doped silicon. The results are interpreted in terms of the impurity-impurity interaction with the formation of stationary boron-phosphorus pairs. The binding energy of boron-phosphorus complexes in silicon was estimated at 0.6–0.8 eV.     

Two-step rapid thermal diffusion of boron into silicon using a boron nitride solid diffusion source

A two-step rapid thermal diffusion process of boron into silicon using a boron nitride solid diffusion source is described. During the first step, HBO² glass is transferred onto the silicon wafer from the diffusion source by keeping the temperature of the silicon wafer at 750° C while the diffusion source is at about 900° C. Boron is, then, diffused into the silicon wafer from HBO² glass at 1000° C or 1100° C in N₂ during the second step. Extremely shallow junctions with junction depths of about 20 nanometers and sheet resistances of about 350 ohms/sq can be achieved with this method as well as relatively deep junctions with junction depths of about 175 nanometers and sheet re-sistances of about 55 ohms/sq. When the diffusion is performed at 1100° C, both the junction depth and electrically active boron concentration at the surface increase as the ambient gas is changed from N₂ to O₂ while the sheet resistance decreases. A boron rich layer which has high resistivity is not formed at the surface when the diffusion is performed at 1100° C in O₂ ambient. This work was supported by Ministry of Science and Technology, Korea     

Diffusion of impurities during epitaxy

The diffusion of impurities in both epitaxial layers and the substrate is considered. The differential equations and boundary conditions which describe the problems are derived and solutions are presented for both the idealizations of a semi-infinite substrate and the true thickness of the substrate. Several types of boundary conditions are considered. For the cases of diffusion of impurities with the substrate considered as semi-infinite, the results from digital computation are given in tabular and graphical form. The method of application of the information to the design and production of epitaxial structures is indicated. A comparison is made between the solutions accounting for diffusion during epitaxy and solutions for a rationalized situation in which diffusion is computed by simpler means, and the simpler method is shown to be unsatisfactory for epitaxial calculations.     

Excess noise measurements in ion-implanted silicon resistors

The low-frequency noise spectra of partially annealed boron-implanted silicon resistors with various geometries are measured. The implantation energies are 50, 80 and 110 keV and the doses are 2·5 × 10¹² cm^?2, 1·0 × 10¹³ cm^?2 and 1·0 × 10¹⁴ cm^?2. The spectra exhibit thermal noise and $\text{?}{}^{\mathrm{?n}}$ (excess) noise exclusively. Investigations indicate that the contracts from the implanted layer to the electrode generally contribute small amounts to the total excess noise observed. The excess noise exhibits a strong dependence on the sheet resistance of the layers, while the dependence on substrate bias, implantation energy, and on temperature is relatively weak. A discussion of the results is given in terms of a volume effect. Noise measurements on implanted layers, produced under carefully controlled conditions, show promise as a tool to investigate excess noise.     

Ionization of impurities in silicon

A model for calculation of the percentage of ionized dopant atoms as a function of the doping concentration and temperature is proposed. The results are compared with experiment. Analytical approximations that facilitate practical applications of the model are given.     

Optically controlled characteristics in an ion-implanted silicon MESFET

OPFETs (optical FETs) are useful as compatible IC transducers in optical communication systems, although APDs (avalanche photo diodes) have higher multiplication gain and speed of response. Studies have been made on the optically controlled characteristics of an ion-implanted Si MESFET which show that drain-source current can be enhanced with increasing radiation flux intensity and lower wavelength of operation. Furthermore, the threshold voltage is found to be reduced under normally OFF conditions and increased under normally ON conditions for higher flux density and lower wavelength. The effect of radiation becomes predominant over the impurity concentration at flux densities greater than or equal to 10¹⁸/m² and wavelengths less than or equal to 0.78 μm.     

Electrical measurements on ion-implanted LPCVD polycrystalline silicon films

The electrical conduction properties of ion implanted polycrystalline silicon films have been studied. The polysilicon films were deposited by pyrolysis of silane at 647°C in LPCVD system onto oxide-coated silicon wafers to a thickness of 0.6 μm. Dopants were itroducd by implanting with boron or phosphorus ions, accelerated to 145 keV; doses ranged from 1 × 10¹² cm^?2 to 1 × 10¹⁵ cm^?2. Film resistivities spanning 8 orders of magnitude were obtained using this doping range. Current-voltage characteristics of polysilicon resistors were measured at temperatures ranging from 24 to 140°C. The associated barrier heights and activation energies were derived. The grain-boundary trapping states density was estimated to be 5 × 10¹² cm^?2. We found that both dopant atom segregation and carrier trapping at the grain boundaries play important roles in polysilicon electrical conduction properties. However, within the dose range studies, the dopant atom segragation is most detrimental to the film conductivity for doses < 1 × 10¹³ cm^?2; as the dose is increased, carrier trapping effects become more pronounced for doses up to 5 × 10¹⁴ cm^?2. For doses ? 5 × 10¹⁴ cm^?2, conduction due to carriers tunneling through the potential barriers at grain boundaries has to be considered.