Sheet resistance variations of phosphorus implanted silicon at elevated temperatures

Silicon has been implanted to high doses (2 × 10¹⁵–2 × 10¹⁶ ions cm^?2) with 40 keVP⁺ at constant temperatures in the range 20–300°C. The sheet resistance following implantation is shown to have a break point at 167°C in its temperature variation characteristic. It is suggested that sheet resistance variations result from changes in the depth of the amorphous layer formed during implantation. A dose rate effect has also been observed. The behaviour correlates with the variations which have been observed on colour-banded wafers. It is also demonstrated that small temperature rises during implantation can result in significant variations in the sheet resistance, even after high temperature annealing.     

Shallow phosphorus diffusion profiles in silicon

Concentration profiles have been determined for phosphorus diffusion into silicon. These have been fitted to the solution of Fick's diffusion equations using a model where a moving boundary separates two distinct phases. Thus, it is concluded that the silicon surface region is part of a different phase from the rest of the diffused layer. For the short diffusion times studied, the phase boundary reaction is the rate limiting process and the phase boundary moves at a nearly constant rate. In the region beyond the phase boundary the transport of phosphorus is controlled by two diffusion species, characterized by two substantially different diffusion coefficients. The slow diffusant is present mainly in a transition region between the phase boundary and the fast diffusant dominated region. The fast diffusant has a maximum concentration at the phase boundary. The linear rates of movement of the phase boundary, the concentrations of the fast diffusant at the phase boundary, and the diffusion constants for the slow and the fast diffusants were obtained over the temperature range from 820 to 1100°C. As the diffusion temperature increases the diffusion constants for the slow and the fast diffusants approach each other. At 1100°C, the diffusion profile can be represented by a single diffusion constant. For long diffusion times the phase boundary became less distinct.     

Concentration-dependent diffusion of boron and phosphorus in silicon

A calculation is made of the tail of the impurity concentration profile resulting from concentration-dependent diffusion from a constant surface concentration into a semi-infinite medium. The calculation predicts that if the concentration dependence at low impurity concentrations is negligible, the low concentration portion of the doping profile should still take the familiar form,C = C'_{s} erfc (x/2 D_{i^{frac{1}{2}}}t^{frac{1}{2}}). Diis the commonly known diffusion coefficient at low impurity concentrations, whileC'_{s}is the "apparent" surface concentration.C'_{s}depends on the actual surface concentration and also depends on how the diffusion coefficient varies with impurity concentration at high concentrations. It is a constant for a given diffusion system but could be orders of magnitude higher than the actual surface concentration. Empirical data have been obtained for boron and phosphorus diffusions in silicon and found to be in good agreement with this prediction.     

Unpinning the GaAs Fermi level with thin heavily doped siliconoverlayers

A theoretical investigation is made into the unpinning of the GaAs Fermi level (E_f) at Schottky contacts by thin interfacial layers of heavily doped Si. The pinning mechanism is assumed to be a plane of interface states in the GaAs. The method used is to solve Poisson's equation numerically as a two-point boundary problem across the semiconductor. The results show that E_f can be moved from its pinned position to near the edge of the silicon valence band maximum or conduction band minimum at the Si/GaAs heterojunction with heavily doped p-Si or n-Si overlayers, respectively. This unpinning is observed with and without thick metallization on the Si. Exactly analogous results are obtained for Ge interfacial layers. The results are in good agreement with the experimental results obtained by J.R. Waldrop and R.W. Grant (1988). Although the unpinning is seen to occur for interface-state densities sufficient to pin E_f at the free GaAs surface, interface-state densities high enough to result in significant E_f pinning at metal/GaAs contacts are seen to prevent such unpinning by Si interfacial layers. It is therefore suggested that Si (or Ge) deposition gives rise to fewer interface states in the GaAs than does metal deposition     

Silicon dioxide masking of phosphorus diffusion in silicon

Doped oxide diffusion condition was used to diffuse P-32 radioisotope into 1100°C thermal SiO₂. Two phosphorus containing diffusing species were identified. Their diffusion characteristics were used with the literature data on the kinetics of phosphosilicate glass formation to construct a multiple-zone model of P₂O₅ diffusion into thermal SiO₂. This physical model was quantitatively verified by several types of masking experiments. In addition, the effects of dry-wet and dry-wet-dry oxidations (at 1100 and 1200°C) on the masking capability of the thermal oxides were examined.     

Impact of phosphorus gettering parameters and initial iron level on silicon solar cell properties

We have studied experimentally the effect of different initial iron contamination levels on the electrical device properties of p‐type Czochralski‐silicon solar cells. By systematically varying phosphorus diffusion gettering (PDG) parameters, we demonstrate a strong correlation between the open‐circuit voltage (V_oc) and the gettering efficiency. Similar correlation is also obtained for the short‐circuit current (J_sc), but phosphorus dependency somewhat complicates the interpretation: the higher the phosphorus content not only the better the gettering efficiency but also the stronger the emitter recombination. With initial bulk iron concentration as high as 2 × 10¹⁴ cm⁻³, conversion efficiencies comparable with non‐contaminated cells were obtained, which demonstrates the enormous potential of PDG. The results also clearly reveal the importance of well‐designed PDG: to achieve best results, the gettering parameters used for high purity silicon should be chosen differently as compared with for a material with high impurity content. Finally we discuss the possibility of achieving efficient gettering without deteriorating the emitter performance by combining a selective emitter with a PDG treatment. Copyright © 2012 John Wiley & Sons, Ltd.     

On phosphorus diffusion in silicon under oxidizing atmospheres

Phosphorus diffusion in silicon has been carried out in both inert (nitrogen) and oxidizing (90% nitrogen plus 10% oxygen, dry oxygen, steam) atmospheres, over a wide temperature range (1000–1200°C) and for doping concentrations usually encountered in the silicon planar technology. The experimental data, interpreted on the basis of the Kato and Nishi theoretical model taking into account the redistribution phenomena at the moving oxide-silicon interface, show that the phosphorous diffusion coefficient is strongly influenced by the nature of the ambient atmosphere in which the diffusion is carried out. Two different values for the activation energy of the diffusion process, E_i = 3·5 eV for the inert and E₀ = 2·5 eV for the oxidizing conditions, have been found. These values seem to confirm the phosphorous diffusion mechanism based on E-centers for the inert case, while for the oxidizing case a different diffusion mechanism should be considered.     

Measurement of the minority-carrier transport parameters in heavily doped silicon

A new method for simultaneous measurement of bandgap narrowing and diffusion length in a heavily doped n⁺substrate is proposed. The method uses planar test pattern at the front side of the substrate to determine the hole minority-carrier current injected from a p-type emitter and diodes at the rear side to measure diffusion lengths. The method can be generalized such that the minority-carrier diffusion constant can be estimated and the use of extrapolated literature data can be avoided. Results of measured values of bandgap narrowing and diffusion length versus impurity concentration are given for n-type material.     

The kinetics of open tube phosphorus diffusion in silicon

Using a novel liquid-phase dilution technique, open-tube diffusion of radiophosphorus into silicon was carried out for the first time under intrinsic oxide non-accumulation conditions. The existence of the surface-barrier rate control of the deposition process in the open-tube technique was established from the concentration profiles. An in-depth analysis of the vapor-solid partition coefficient of the phosphorus-silicon system showed the dramatic contribution of oxygen in the incorporation of phosphorus into silicon in the open-tube process. The fitted deposition and redistribution profiles of phosphorus in silicon supported the concept of equivalent diffusion kinetics in both the deposition and redistribution steps. The present study also supported the Kennedy-Murley redistribution kinetics model quantitatively.     

Tracer investigations on the diffusion of phosphorus from doped oxide sources

Tracer investigations on the diffusion of phosphorus from phosphorus silicate glass (PSG) are described in this paper. Concentration profiles in the silicon and the PSG are given. Investigations were made also on the interaction between PSG and silicon dioxide and between PSG and silicon nitride. The shape of the concentration profiles obtained in the system PSG/silicon dioxide and the loss of phosphorus to the ambient depend on the phosphorus concentration in the PSG. From annealing experiments with PSG/silicon nitride layers it could be derived that phosphorus escapes from the PSG with high phosphorus concentration in the form of the pentoxide.     

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.     

Relation between the optoelectronic parameters of amorphous hydrogenated silicon films deposited at high temperatures and their microstructure

The optical modulation spectra and photoconductivity of a number of a-Si:H films containing 5–6 at.% hydrogen with different microstructure parameters (R=0.2–0.8) have been investigated. Information about the defect density in the films, spreading of band edges (tailing), the gap width, and the product of the mobility and the lifetime of the electrons has been obtained. The microstructure is shown to have a substantial effect on the optoelectronic parameters of films with low hydrogen content. Fiz. Tekh. Poluprovodn. 32, 121–123 (January 1998)     

Measurement of steady-state minority-carrier transport parameters in heavily doped n-type silicon

The relevant hole transport and recombination parameters in heavily doped n-type silicon under steady state are the hole diffusion length and the product of the hole diffusion coefficient times the hole equilibrium concentration. These parameters have measured in phosphorus-doped silicon grown by epitaxy throughout nearly two orders of magnitude of doping level. Both parameters are found to be strong functions of donor concentration. The equilibrium hole concentration can be deduced from the measurements. A rigid shrinkage of the forbidden gap appears as the dominant heavy doping mechanism in phosphorus-doped silicon.     

Simulation of low-temperature arsenic diffusion from a heavily doped silicon layer

Low-temperature (500–800°C) diffusion of As from a heavily doped Si layer was simulated on the basis of a dual pair mechanism. The anomalously high diffusion rate is attributed to excess self-interstitials accumulated in the layer during the preceding high-temperature diffusion stage. The shift of the concentration profile in the region of intermediate values of concentrations is caused by the presence of a maximum in the concentration dependence of the diffusivity. This shift is attributed to the considerable role that negatively charged self-interstitials play in the arsenic diffusion process (f _I ^? ≈0.4).     

Transport properties of microcrystalline silicon at low temperatures

The dark conductivity and photoconductivity along with pulsed electron spin resonance have been measured over a wide temperature range with a high crystallinity hydrogenated microcrystalline silicon (μc-Si: H) sample. The transport mechanism in μc-Si: H is discussed on the basis of these measurements. Striking similarities in the temperature dependences of the dark conductivity and photoconductivity between μc-Si: H and some well-studied materials, such as hydrogenated amorphous silicon, suggest that at low temperatures hopping of carriers between localized states dominates the transport properties of μc-Si: H. Fiz. Tekh. Poluprovodn. 32, 905–909 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Effect of extended phosphorus diffusion gettering on chromium impurity in HEM multicrystalline silicon

We have investigated the extended phosphorus diffusion gettering (PDG) effect on chromium impurities (Cr) in p-type multicrystalline silicon (mc-Si) grown by Heat Exchanger Method (HEM). The study was made after phosphorous diffusion and according to different extended annealing temperatures. The secondary ion mass spectrometry (SIMS) analysis revealed a significant accumulation of ⁵²Cr in heavily phosphorus doped (HPD) region. Using quasi-steady state photoconductance (QSSPC) technique, the apparent lifetime dependent minority carrier density curves have been obtained. The results showed an increment of the bulk minority carrier lifetime for specific annealing temperatures. Appropriate calculations based on QSSPC results allowed us to determine the lifetime curves associated to gettered impurities. Their fitting by Shockley-Read-Hall (SRH) model reveal that the origin of the lifetime increment is the reduction of interstitial chromium (Cr_i) density in the bulk. Furthermore, the estimation of electron to hole capture cross-section ratio (k=σ_n/σ_p) through the modelling of apparent lifetime curves using Hornbeck–Haynes model, confirmed the effectiveness of Cr_i gettering and identified the nature of dominant recombination centres after gettering process.     

Determination of diffusion,partition and sticking coefficients for boron,phosphorus and antimony in silicon

Our model (1) for impurity transfer into epitaxial films grown by sublimation at U.H.V. is improved by the use of a more complete set of boundary conditions to be met by the solution of the diffusion equation. Detailed measurements are given of impurity profiles for layers grown from boron, phosphorus or antimony doped sources. These are used to obtain their diffusion and partition coefficients in the source as well as their sticking coefficient on the substrate. The dependence of these parameters on temperature is also reported, giving a basis for prediction of profiles under various experimental conditions.     

A model of high-and low-temperature phosphorus diffusion in silicon by a dual pair mechanism

A model of phosphorus diffusion in silicon was developed on the basis of a dual pair mechanism; according to this model, the contribution of the impurity-vacancy (PV) and impurity-self-interstitial (PI) pairs to diffusion is accounted for directly in terms of the phosphorus diffusion coefficient. A violation of thermodynamic equilibrium in relation to native point defects occurs as a result of diffusion of the PI neutral pairs. At the high-temperature diffusion stage, the phosphorus diffusion is described by a single diffusion equation with the diffusion coefficient dependent on both the local and surface phosphorus concentrations; whereas at the next (occurring at lower temperatures) stage, the phosphorus diffusion is described by two diffusion equations for the total concentrations of the components containing phosphorus and self-interstitials. An anomalously high rate of the low-temperature diffusion is ensured by excess self-interstitials accumulated in the doped layer during the preceding high-temperature diffusion. The model makes it possible to quantitatively account for the special features of the phosphorus diffusion in a wide range of the surface concentrations at both the high (900–1100°C) and lower (500–700°C) temperatures.     

Specific features of the segregation-related redistribution of phosphorus during thermal oxidation of heavily doped silicon layers

A model of the diffusion-segregation redistribution of phosphorus in an SiO₂/Si system during thermal oxidation of highly doped silicon layers is developed taking into account the formation of a peak of surface impurity concentration at the interface. The formation of this surface concentration peak is attributed to a change in the free energy of the impurity atoms near the silicon surface. This process is simulated by a diffusion-segregation equation. It is shown that the developed diffusion-segregation model is quite adequate for describing the phosphorus redistribution occurring during the oxidation of uniformly doped silicon layers. For the oxidation of implanted silicon layers, it was found that the segregation coefficient of the phosphorus at the SiO₂/Si interface is not constant but depends on time in the same way as the efficiency of transient enhanced diffusion in silicon. This phenomenon is explained by the reactivity of the impurity segregation during the thermal oxidation of silicon, when excess point defects in the implanted silicon layer affect both the oxidation process and the capture of impurity atoms by the growing silicon dioxide.