D. K. Sadana

Ion implantation and low-temperature epitaxial regrowth of GaAs

Channeling and transmission electron microscopy have been used to investigate the parameters that govern the extent of damage in ion‐implanted GaAs and the crystal quality following capless furnace annealing at low temperature (∼400 °C). The implantation‐induced disorder showed a strong dependence on the implanted ion mass and on the substrate temperature during implantation. When the implantation produced a fully amorphous surface layer the main parameter governing the regrowth was the amorphous thickness. Formation of microtwins after annealing was observed when the initial amorphous layer was thicker than 400 A. Also, the number of extended residual defects after annealing increased linearly with the initial amorphous thickness and extrapolation of that curve predicts good regrowth of very thin (<400 A) GaAs amorphous layers produced by ion implantation. A model is presented to explain the observed features of the low‐temperature annealing of GaAs.

Damage induced through megavolt arsenic implantation into silicon

An arsenic beam of 11 MeV was used to achieve a layer of heavily doped silicon centered at 4.4 μ below the surface. Both liquid nitrogen and water‐cooled (100) silicon substrates were exposed to an arsenic dose of 1.9×1015 ions/cm2. Before furnace annealing, cross‐sectional transmission electron microscopy (XTEM) shows buried amorphous regions for both samples with a wider amorphous region for the LN‐cooled sample. The depth distribution of secondary defects on subsequent annealing qualitatively resembles those obtained by lower energy implantation, and the details depend strongly on the substrate temperature during implantation. Secondary ion mass spectrometry (SIMS) was used to verify LSS calculations for the arsenic distribution.

Direct evidence of arsenic clustering in high dose arsenic‐implanted silicon

High dose (5–7.5×1015 cm−2) arsenic implantation was conducted in the energy range 50–120 keV into three types of (100)Si substrates: (i) bare Si, (ii) with a thermally grown screen oxide (775 A), and (iii) preamorphized surface layer produced by self‐implantation. The substrates were subsequently furnace annealed at 600 °C in N2. Cross‐section transmission electron microscopy (XTEM) revealed that in all cases discrete layers of As related clusters and small dislocation loops occurred at depths matching the projected ranges of As into the Si substrates and original amorphous/crystalline interfaces respectively. The atomic profiles of As obtained by secondary ion mass spectrometry (SIMS) from these samples did not show any redistribution of As. The carrier concentration profiles from the spreading resistance measurements indicate that only 30% of As was electrically active. Comparison of XTEM and SIMS suggests that nucleation of the clusters occurred in the regions where As was present above the solid solu...

Recrystallization of buried amorphous layers and associated electrical effects in P+-implanted Si

Abstract The recrystallization behaviour of a buried amorphous layer formed by P+ implantation in (111) Si has been studied using transmission electron microscopy (TEM), secondary-ion mass spectroscopy (SIMS) and Hall measurements. Special TEM specimen preparation techniques were used to study individual sub-surface damage regions in the as-implanted as well as the annealed samples. The TEM results showed that the recrystallization of the amorphous layer began slowly, increased rapidly above 550°C and was complete at 600°C. At temperatures betweeen 750 and 1000°C, two discrete layers of interstitial a/2 ⟨110⟩ and a/3⟨111⟩ dislocation loops formed below the specimen surface, and the surface region concontained stacking-fault tetrahedra. The SIMS results showed that pronounced segregation of phosphorus atoms occurred in the defect-rich regions at 750°C. At higher annealing temperatures P segregation still occurred, but the magnitude of the segregation was reduced because of diffusive effects. The segregatio...

The crystalline to amorphous transformation in silicon

In the present investigation, an attempt was made to understand the fundamental mechanism of crystalline-to-amorphous transformation in arsenic implanted silicon using high resolution electron microscopy. A comparison of the gradual disappearance of simulated lattice fringes with increasing Frenkel pair concentration with the experimental observation of sharp interfaces between crystalline and amorphous regions was carried out leading to the conclusion that when the defect concentration reaches a critical value, the crystal does relax to an amorphous state. Optical diffraction experiments using atomic models also supported this hypothesis. Both crystalline and amorphous zones were found to co-exist with sharp interfaces at the atomic level. Growth of the amorphous fraction depends on the temperature, dose rate and the mass of the implanted ion. Preliminary results of high energy electron irradiation experiments at 1.2 MeV also suggested that clustering of point defects occurs near room temperature. An observation in a high resolution image of a small amorphous zone centered at the core of a dislocation is presented as evidence that the nucleation of an amorphous phase is heterogeneous in nature involving clustering or segregation of point defects near existing defects.

Proton, deuteron, and helium implantation into GaAs and LiNbO3 for waveguide fabrication

Hydrogen and helium implantation into GaAs, LiNbO3, and other crystals has been shown to produce planar or channel waveguides via defects/damage and/or carrier removal or compensation. We have measured implanted depth distributions of 1H, 2H, and 4He in GaAs and LiNbO3 as functions of ion energy, ion fluence, substrate temperature, and annealing temperature using secondary ion mass spectrometry. We have studied the defect depth distribution for hydrogen‐implanted GaAs by transmission electron microscopy. We have determined the lateral spread of 1H and 2H ions implanted into GaAs by varying the angle of implantation; this relates to the increase in width of closely spaced parallel channel waveguides made using patterned masks.

High resolution structural characterization of the amorphous‐crystalline interface in Se+‐implanted GaAs

High‐resolution transmission electron microscopy is applied to the characterization of the amorphous‐crystalline interface in (100) GaAs implanted at room temperature with Se+. The critical density of energy deposited by nuclear stopping is determined to be 11+2 eV per GaAs molecule. This critical energy density is shown to be defined most naturally by a 50% amorphous‐50% crystalline criterion. The amorphous‐crystalline transition region was found to have a distinct two‐phase nature. Both stacking fault nuclei and channeled damage cascades were detected in the as‐implanted material.

Steady-state thermally annealed GaAs with room-temperature-implanted Si

Semi-insulating Cr-doped single-crystal GaAs samples were implanted at room temperature with 300-keV Si ions in the dose range of (0.17–2.0)×1015 cm–2 and were subsequently steady-state annealed at 900 and 950°C for 30 min in a H2 ambient with a Si3N4 coating. Differential Hall measurements showed that an upper threshold of about 2×1018/cm3 exists for the free-electron concentration. The as-implanted atomic-Si profile measured by SIMS follows the theoretical prediction, but is altered during annealing. The Cr distribution also changes, and a band of dislocation loops ~2–3 kA wide is revealed by cross-sectional TEM at a mean depth of Rp~3 kA. Incomplete electrical activation of the Si is shown to be the primary cause for the effect.

Substitutional placement of phosphorus in ion implanted silicon by recrystallizing amorphous/crystalline interface

Ion implantation of P into (111) Si has been investigated by cross‐sectional transmission electron microscopy and secondary ion mass spectrometry. In the dose range 1×1015–2×1016 cm−2 at 120 keV the formation and width of the amorphous (a) layers were sensitively dependent on wafer temperature. The effect of dynamic annealing on the crystalline (c) to amorphous transformation, and the effect of amorphicity and roughness of the a/c interface on the formation of microtwins have been studied. It has been observed that the P atoms in the amorphous (a) region do not redistribute on subsequent annealing while the P atoms below the a/crystalline (c) interface diffuse rapidly into the Si substrate. The carrier concentration profiles from the annealed samples were found to follow the atomic profiles almost exactly over the depth range extending from the surface to where the original a/c interface existed. However, in the deeper regions, the electrical activity was found to be dependent on the annealing temperature...

Epitaxial reordering of ion-irradiated NiSi2 layers

Abstract Thermally grown (at about 800°C) epitaxial NiSi2 layers on silicon single-crystal substrates were ion irradiated at liquid nitrogen temperature so that the surface region was nearly amorphous but with a crystalline NiSi2 seed near the NiSi2Si interface. The epitaxial reordering of the highly defective region occurs in a layer-by-layer manner and at relatively low temperatures (about 80°C). The activation energy for regrowth was found to be 1.2 ± 0.2 eV. These results indicate that the growth characteristics of NiSi2 without mass transport are drastically different from those where mass transport is required such as in the case of thermally formed NiSi2 from Ni/c-Si couples (where c-Si denotes crystalline silicon).