Transient annealing of defects in irradiated silicon devices

The annealing of radiation-produced defects in semiconductor devices is discussed briefly for⁶⁰Co gamma-ray and 1-MeV electron damage, and in detail for fast-neutron damage. The effects on the reordering processes of varying the material parameters and the irradiation conditions are considered. Transient annealing of neutron damage near room temperature has been investigated for a wide variety of devices, and the data are presented in generalized form to increase their usefulness to device and circuit designers. Based on the experimental results, physical models are suggested for the reordering processes which occur during the annealing of neutron damage. Electron density is shown to be the most important factor governing the rate of transient annealing. Annealing factors are estimated for very early times (1 µs) following neutron exposure. Suggestions are made to minimize the effects of transient annealing on devices.     

DLTS Study of plastically deformed copper-doped n-type germanium

Classical deep level transient spectroscopy (DLTS) and its modification are used to study the time constants of electron capture by substitutional Cu _s ^2? atoms and thermal electron emission from Cu _s ^3? atoms in plastically deformed Cu-doped n-type germanium. The activation energy E _σ, the electron capture cross-section, the energy E ₃ of the third acceptor level of Cu _s/3? atoms, and the ionization entropy are determined. The lack of E ₃-level broadening, the exponential capture kinetics for a filling-pulse duration of t _p ? 1 ms, the fact that the Cu _s/2?/3?-atom recombination parameters are independent of the dislocation density, and the low concentration of Cu _s/2?/3? atoms in the deformed samples suggest that the DLTS spectra are due to Cu _s/2?/3? atoms located outside the Read cylinders.     

Theoretical estimates of the sheet resistance of Gaussian n-type ion-implanted layers in semiconductors: Phosphorus in silicon

The variation of sheet resistance of Gaussian if n-type implanted layers in semiconductors with high ion dose has been estimated in terms of theoretical electron mobilities. Ionized impurity scattering and lattice scattering are considered to be the dominant mechanisms determining the total mobility. The former is obtained from the classical Brooks-Herring theory and its extension to heavy doping due to Moore and Ehrenreich, both incorporating a screened Coulomb potential having a screening length corresponding to the local donor concentration, while the latter is obtained from the calculations of Norton et al. Sheet resistance against dosage curves are presented for phosphorus implants in silicon and compared with the universal curves of Smith and Stephen, and with some experimental data.     

Paramagnetic defects in silicon carbide crystals irradiated with gamma-ray quanta

The results of the first observations of paramagnetic defects in SiC crystals irradiated with gamma-ray quanta are reported. Three types of defects, designated as γ1, γ2, and γ3, were detected in irradiated 4H-SiC:Al and 6H-SiC:Al crystals using electron spin resonance (ESR) measurements. All these centers have almost the same parameters of the spin-related Hamiltonian with S=1/2 and feature an appreciable anisotropy of the g-factors. The γ1 centers are almost coaxial with the local z-axis oriented approximately along one of the directions of the Si-C bond that does not coincide with the c-axis. The γ2 and γ3 centers have a lower symmetry, although the orientation along the above bonds is clearly pronounced. The values of the largest g-factor (g _z) decreases in the sequence from γ1 to γ3. The γ1 signal can be detected at temperatures of 3.5–15 K; the γ2 and γ3 signals are detectable at temperatures of 10–35 and 18–50 K, respectively. The hyperfine interaction of an unpaired electron in the γ1 center with a nucleus of the ²⁹Si isotope is detected for certain orientations of the crystal. The γ1, γ2, and γ3 centers cease to exist at a temperature of 160°C; it is concluded that the ESR signals of these centers are related to defects in the C sublattice. It is assumed that the γ1, γ2, and γ3 centers have a common origin and are related to the low-temperature (γ1) and high-temperature (γ2 and γ3) modifications of the same center. The models of a defect in the form of either a carbon vacancy or a complex incorporating an Al impurity atom and a C atom that occupies the silicon site or interstice are discussed.     

A chromium-free etchant for delineation of defects in heavily doped n-type silicon wafers

Delineation of defects in the heavily doped n-type Czochralski silicon wafers by preferential etching is an issue not having been essentially solved. Herein, a chromium-free etchant based on HNO₃–HF–H₂O system, with an optimum volume ratio of V_HNO3%:V_HF%:V_H2O%=20%:45%:35%, has been developed. It can reveal well the defects such as dislocation and oxygen precipitation-induced bulk microdefects (BMDs) in the heavily doped n-type silicon wafers with resistivities even lower than 1 mΩ cm. Moreover, this etchant is appropriate to delineate the defects on (1 1 1), (1 1 0) or (1 0 0) surface of silicon crystal. Furthermore, the density of oxygen precipitation-induced BMDs in the heavily doped n-type silicon wafers derived from the preferential etching using this newly developed etchant correlates well with that derived from scanning infrared microscopy (SIRM) within its detection limit.     

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.     

Micro-and nano-structures in silicon studied by DLTS and scanning probe methods

Presently, there is a high interest in silicon-based optical devices that would render possible the development of fully silicon-based optoelectronics. Being an indirect-gap semiconductor, silicon is poorly efficient as a light emitter since radiative emission is limited by carrier recombination at non-radiative centers. One of the possible approaches to enhance the radiative emission from Si is the controlled introduction of micro-(dislocations) or nano-(nanocrystals) structures, which, providing quantum confinement of free carriers, prevent their diffusion towards non-radiative channels. Dislocations introduced in silicon by plastic deformation and Si nanocrystals embedded in the amorphous silicon matrix have been investigated by junction spectroscopy and scanning probe microscopy methods. The text was submitted by the authors in English.     

A modeling technique for characterizing ion-implanted material using C–V and DLTS data

A new modeling technique for determining free-carrier, shallow-level donor, trap and mobility profiles of ion-implanted GaAs MESFETs is presented. This technique uses the results of C–V and conductance DLTS measurements to calibrate a theoretical model. The model eliminates assumptions commonly made in other simple characterization techniques. A sample case is presented which indicates the importance of this more rigorous treatment.     

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.     

Influence of radiation defects on electrical losses in silicon diodes irradiated with electrons

Silicon diodes with a p ⁺-n junction irradiated with 3.5-MeV electrons (the fluence ranged from 10¹⁵ to 4 × 10¹⁶ cm⁻²) have been studied. It is established that the dependence of the tangent of the angle of electrical losses tanδ on the frequency f of alternating current in the range f = 10²−10⁶ Hz is a nonmonotonic function with two extrema: a minimum and a maximum. Transformation of the dependences tanδ(f) as the electron fluence and annealing temperature are increased is caused by a variation in the resistance of n-Si (the base region of the diodes) as a result of accumulation (as the fluence is increased) or disappearance and reconfiguration (in the course of annealing) of radiation defects. The role of time lag of the defect recharging in the formation of tanδ(f) is insignificant.     

Electrical characterization and DLTS analysis of a gold/n-type gallium nitride Schottky diode

This work describes a comparison of current density–voltage (J–V) and capacitance–voltage (C–V) properties measured as a function of temperature; deep trap properties are measured by deep level transient spectroscopy (DLTS) of Schottky diodes fabricated on n-type gallium nitride (GaN grown by metal organic vapor phase epitaxy (MOVPE). Unexpected behavior in the standard Richardson plot was observed in the temperature range 165–480 K, reflecting a range of Schottky barrier heights and a variation of ideality factor. This was explained by applying a Gaussian spatial distribution of barrier heights across the Schottky diode. C–V measurements were carried out in the temperature range 165–480 K to compare the temperature dependence of the barrier height with those obtained by the Gaussian distribution method. DLTS and high-resolution Laplace DLTS (LDLTS) show a majority carrier peak centered at 450 K.     

Minority-carrier transport parameters in n-type silicon

Minority-carrier diffusion length L, lifetime τ, and diffusion coefficient D in n-type Si are measured at 296 K in the doping range from 10¹⁸ cm^-3 to 7×10¹⁹ cm^-3. The measurement is based on a lateral collection of carriers generated by a spatially uniform light. The distance between the illumination edge and the collection junction is defined by photolithography. This allows simultaneous and independent determination of all transport parameters in the same material. A self-consistency and accuracy check is provided by the relation L ²=Dτ. Details of experimental procedures are described. Empirical best-fit relations for the three parameters are given. The extraction of lifetime and diffusion coefficient was done in the frequency domain, which allows for straightforward elimination of parasitic effects in the nanosecond and subnanosecond range     

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.     

Minority-carrier transport parameters in degenerate n-type silicon

Direct measurements of the minority-hole transport parameters in degenerate n-type silicon were done by analyzing transient photocurrent in the frequency domain. Minority-hole mobility is found to increase with doping for dopings larger than 4×10¹⁹ cm^-3 . The ratio of minority-hole to majority-hole mobility is found to be about 2.8 at N_D=7.2×10¹⁹ cm^-3. The measured lifetime shows a strongly Auger-dependent mechanism. The extracted Auger coefficient at 296 K is C_n =2.22×10^-31 cm⁶-s^-1, and is in agreement with that reported on other works. Self-consistent checking is used to validate the accuracy of the measured results     

Popcorn noise and generation-recombination noise observed in ion-implanted silicon resistors

Experimental results show that, for low currents, the generation-recombination noise component increases with current, and, at higher currents, it decreases inversely with current. For currents greater than a certain value, a generation-recombination noise component is scarcely observed.     

DLTS characterization of defects in GaN induced by electron beam exposure

The deep level transient spectroscopy (DLTS) technique was used to investigate the effects of electron beam exposure (EBE) on n-GaN. A defect with activation energy of 0.12 eV and capture cross section of 8.0×10^–16 cm² was induced by the exposure. The defect was similar to defects induced by other irradiation techniques such as proton, electron, and gamma irradiation. In comparison to GaN, the EBE induced defects in other materials such as Si and SiC are similar to those induced by other irradiation methods.     

Diffusion of ion-implanted boron impurities into pre-amorphized silicon

Boron ion implantation into pre-amorphized silicon is studied. Pre-amorphization is performed either by F⁺ or Si⁺ implantation prior to B⁺ implantation at 10 keV with 3×10¹⁵ ions/cm². Broadening of the boron profile can be suppressed markedly in the pre-amorphized layers. For instance, the as-implanted depth at a B concentration of 1×10¹⁸ atoms/cm³ decreases from 0.19 to 0.1 μm for implantation into a pre-amorphized layer compared to B implantation into crystalline silicon. After annealing at 950°C, B atoms diffuse much more rapidly in the pre-amorphized layers than in the crystalline silicon case. Nevertheless, shallower junctions are obtained with the use of pre-amorphization. For dual F⁺ and B⁺ implantation at F⁺ doses above 1×10¹⁵ F⁺/cm², fluorine is found to segregate to the peak of the boron profile during annealing. Fluorine is also trapped at the peak of the as-implanted fluorine profile peak and near the amorphous–crystalline interface. The effects of fluorine dose and anneal temperature on the F precipitation are described and compared to results for BF⁺₂ implants.     

Thermal acceptors in irradiated silicon

Comparative analysis of the conditions for the formation of shallow acceptor centers upon high-temperature annealing in silicon irradiated with electrons, neutrons, and energetic ions is performed. The introduction of a sufficiently large (in comparison with the initial concentration of impurities and defects) concentration of radiation-induced distortions of the silicon lattice is shown to lead to the formation of thermal acceptors stable up to annealing temperature of ～650°C. The acceptor formation is supposed to be due to the interaction of background acceptor impurities (supposedly boron) with vacancies “stored” in multivacancy clusters and released upon their breakup.     

Defect analysis of n-type silicon strained layers

The structural and electrical properties of n-type silicon strained layers, sandwiched between Si_1−xGe_x layers, with x=0.15, 0.20 and 0.30 have been investigated using a combination of analytical techniques. Here, the focus is on the application of deep level transient spectroscopy (DLTS) on p–n junction structures, to assess non-radiative generation-recombination centres. It will be demonstrated that successful analysis can only be applied if the edges of the devices are chemically passivated. Finally, it is shown that for low-leakage diodes, the quantum-well properties can, in principle, be extracted from the combined DLTS and capacitance–voltage/capacitance–temperature characteristics.     

Structural and electrical profiles for double damage layers in ion-implanted silicon

(111) Si specimens were implanted at room temperature with 5 × 1014 cm?2, 120 keVP+ ions, and annealed at 950°C. Transmission electron-microscope cross-section specimens revealed two discrete damage layers at depths of 95 and 190 nm. Electrical profiles showed decreases in carrier concentration and/or mobility at these two depths.