H.‐J. Gossmann

Implantation and transient B diffusion in Si: The source of the interstitials

Implanted B and P dopants in Si exhibit transient enhanced diffusion (TED) during initial annealing, due to Si interstitials being emitted from the region of the implant damage. The structural source of these interstitials has not previously been identified. Quantitative transmission electron microscopy measurements of extended defects are used to demonstrate that TED is caused by the emission of interstitials from specific defects. The defects are rodlike defects running along 〈110〉 directions, which consist of interstitials precipitating on {311} planes as a single monolayer of hexagonal Si. We correlate the evaporation of {311} defects during annealing at 670 and 815 °C with the length of the diffusion transient, and demonstrate a link between the number of interstitials emitted by the defects, and the flux of interstitials driving TED. Thus not only are {311} defects contributing to the interstitial flux, but the contribution attributable to {311} defect evaporation is sufficient to explain the whole ...

Carbon incorporation in silicon for suppressing interstitial‐enhanced boron diffusion

The effect of substitutional C on interstitial‐enhanced B diffusion in Si has been investigated. Substitutional C was incorporated into B doped Si superlattices using molecular‐beam‐epitaxial growth under a background of acetylene gas. Excess Si self‐interstitials were generated by near‐surface 5×1013/cm2, 40 keV Si implants and diffused at 790 °C. The interstitial‐enhanced diffusion of the B marker layers is fully suppressed for C concentrations of 2×1019/cm3, thus demonstrating that substitutional C acts as a trap for interstitials in crystalline Si. Uniform C incorporation of 5×1018/cm2 significantly reduces the transient enhanced diffusion of a typical B junction implant without perturbing its electrical activity.

Simulation of cluster evaporation and transient enhanced diffusion in silicon

The evaporation of {311} self‐interstitial clusters has recently been linked to the phenomenon of transient enhanced diffusion in silicon. A theory of cluster evaporation is described, based on first‐order kinetic equations. It is shown to give a good account of the data over a range of temperatures. The theory simultaneously explains several of the unexpected features of transient enhanced diffusion, including the apparently steady level of the enhancement during its duration, and the dependence of the duration on implant energy and dose. The binding energy used to match the theory to data is in good agreement with molecular dynamics calculations of cluster stability in silicon.

Trap‐limited interstitial diffusion and enhanced boron clustering in silicon

Boron doped superlattices have been used to detect the diffusion of self‐interstitials in Si. Interstitials were generated in the near‐surface region by 40 keV Si implantation followed by diffusion at 670–790 °C. The interstitial diffusion profile at 670 °C is stationary for t≤1 h, demonstrating that the penetration depth of interstitials is limited by trapping. The concentration of traps is estimated to be ∼1017/cm3. For sufficiently long annealing times, interstitials diffuse beyond the trapping length with an effective trap‐limited diffusivity ranging from ∼6×10−15 cm2/s at 670 °C to ∼1×10−12 cm2/s at 790 °C. The high interstitial supersaturation adjacent to the implant damage drives substitutional B into metastable clusters at concentrations below the B solid solubility limit.

Oxidation enhanced diffusion in Si B‐doping superlattices and Si self‐interstitial diffusivities

A special thin film structure has been grown by low temperature molecular beam epitaxy for an investigation of the properties of self‐interstitials in Si. It consists of a doping superlattice made from B spikes separated from each other by 100 nm of Si. After dry oxidation, the width of each spike is directly proportional to the interstitial concentration at that depth. The superlattice as a whole thus gives a depth profile of the time‐averaged interstitial concentration, allowing the direct determination of the diffusion coefficient of interstitials. The abrupt dopant concentration transitions possible in low‐temperature molecular‐beam‐epitaxy‐grown films allow this investigation in the temperature range 750–900 °C. At 800 °C we find a value of DI=(1.4±0.4)10−13 cm2/s. Performing the experiments as a function of temperature yields DI = D0eEa/kT with D0=102±2 cm2/s and Ea=(3.1±0.4) eV.

Doping of Si thin films by low-temperature molecular beam epitaxy

Two‐dimensional doping sheets (‘‘δ doping’’) are integral parts of many novel semiconductor device concepts. Deep submicron design rules require junction depths significantly below 100 nm. This level of control is difficult to achieve with ion implantation. We discuss the application of thermal, coevaporative doping with Sb and elemental B during Si molecular beam epitaxy at growth temperatures below ≊300 °C to this problem. We show that it is possible to create structures with very high doping levels, yet with very sharp doping transitions. Delta‐doping spikes with a full width at half maximum of <2.7 nm and <4.0 nm have been obtained by secondary‐ion mass spectrometry for Sb and B, respectively, with corresponding up‐slopes of 2.5 and 0.94 nm/decade. Homogeneously doped films show full activation up to NSb≊6×1020 cm−3 and NB≳1×1021 cm−3. Mobilities agree with bulk values at corresponding concentrations. Mesa‐isolated pn junctions exhibit ideality factors of 1.05.

Implantation and transient boron diffusion: the role of the silicon self-interstitial

Abstract The phenomena associated with the transient diffusion of implanted B in Si are reviewed. The Si self-interstitial flux, which causes the B interstitialcy diffusion, has been directly observed by the use of B diffusion marker layers fabricated by low-temperature crystal growth. Low-energy and low-dose Si implants in surface layers permit the experimental separation of the source and flux of interstitials. The clustering of B is directly correlated with the supersaturation of injected Si interstitials. The diffusivity of the Si interstitial is dominated by the presence of trapping sites. Implantation produces rod-like defects which consist of interstitial precipitates on {311} planes as a single monolayer of hexagonal Si. The number of Si interstitials evaporating from the {311} defect is sufficient to explain the whole of the transient diffusion.

Germanium partitioning in silicon during rapid solidification

Pulsed laser melting experiments were performed on GexSi1−x alloys (x≤0.10) with regrowth velocities ranging from 0.25 to 3.9 m/s. Analysis of post‐solidification Ge concentration profiles, along with time‐resolved melt depth measurements, allowed determination of the liquid‐phase diffusivity Dl for Ge in Si and the dependence of the Ge partition coefficient k on interface velocity v. A Dl of 2.5×10−4 cm2/s was measured. The k vs v data were analyzed using various models for partitioning, including both the dilute and nondilute Continuous Growth Models (CGM). Extrapolating to zero velocity using the partitioning models, an equilibrium partition coefficient of approximately 0.45 was obtained. Best fitting of partitioning data to the nondilute CGM yields a diffusive speed of 2.5 m/s. These measurements quantify previous indications of partitioning observed in other studies of pulsed laser processed GexSi1−x alloys.

LOW-TEMPERATURE SI MOLECULAR BEAM EPITAXY : SOLUTION TO THE DOPING PROBLEM

A major problem in group IV molecular beam epitaxy (MBE) is the difficulty to incorporate and control dopants due to the low incorporation probability and strong segregation in Si at typical growth temperatures. It is demonstrated here that growth at low temperatures yields a solution to this doping problem making thermal, coevaporative doping with excellent control possible in Si MBE without the need for any post‐growth annealing. Unity incorporation and activation of Sb with concentrations reaching 5×1019 cm−3 are achieved for epitaxial growth of Si on Si(100) at temperatures of 325 °C. Hall electron mobilities in the films are close to bulk values indicating the high quality of the films. Capacitance‐voltage measurements on Sb δ‐doped films have full widths at half maximum of ≲50 A, the narrowest Sb‐doping profiles in Si determined with an electrical technique.

Evolution of terrace size distributions during thin‐film growth by step‐mediated epitaxy

Under certain conditions the process of thin‐film epitaxy can be envisioned as atom deposition onto a crystalline substrate with subsequent surface diffusion along the surface terraces and eventual atomic bonding at a surface step. This process of step‐mediated growth is currently being explored as a mechanism to form two‐dimensional periodic structures. Such schemes require a periodic step distribution, i.e., uniform terrace lengths, to succeed. In this paper we use a model based on step‐mediated growth to present an analytical derivation of the approach to uniform terrace lengths on a stepped surface, given a terrace length distribution of finite width at the outset. The results show that growth interruption is of no advantage and that in general the approach to uniform terrace lengths is quite slow. The width of the terrace length distribution varies approximately as the inverse 4th root of the deposited coverage. This will only occur if the atoms attach themselves predominately at the up‐step of each ...