Comparison of p–n junction formed by BF2 and B implantation in silicon microstrip detector with low and high thermal budget: impact of fluorine on electrical characteristics

The influence of crystal damage on the electrical properties and the doping profile of the implanted p⁺–n junction has been studied at different annealing temperatures using process simulator TMA-SUPREM4. This was done by carrying out two different implantations; one with implantation dose of 10¹⁵ BF₂⁺ ions/cm² at an energy of 80 keV and other with 10¹⁵ B⁺ ions/cm² at 17.93 keV. Substrate orientation 1 1 1 of phosphorus-doped n-type Si wafers of resistivity 4 kΩ cm and tilt 7° was used, and isochronally annealing was performed in N₂ ambient for 180 min in temperature range between 400°C and 1350°C. The diode properties were analysed in terms of junction depth, sheet resistance. It has been found that for low thermal budget annealing, boron diffusion depth is insensitive to the variation in annealing temperature for BF₂⁺-implanted devices, whereas, boron diffusion depth increases continuously for B⁺-implanted devices. In BF₂⁺-implanted devices, fluorine diffusion improves the breakdown voltage of the silicon microstrip detector for annealing temperature upto 900°C.For high thermal budget annealing, it has been shown that the electrical characteristics of BF₂⁺-implanted devices is similar to that obtained in B⁺-implanted devices.     

The effect of dose rate and implant temperature on transient enhanced diffusion in boron implanted silicon

The effect of ion implantation dose rate and implant temperature on the transient enhanced diffusion (TED) of low energy boron implants into silicon was investigated. The implant temperature was varied between 5 and 40°C. The beam current was varied from 0.035 to 0.35 mA/cm². Three different defect regimes were investigated. The first regime was below the formation of any extended defects (5 keV B⁺ 2 × 10¹⁴/cm²) visible in the transmission electron microscope. The second regime was above the {311} formation threshold (2×10¹⁴/cm²) but below the subthreshold (type I) dislocation loop formation threshold. The final regime was above both the {311} and dislocation loop formation threshold (10 keV 5×10¹⁴/cm²). TED for these conditions is shown to be over after annealing at 750°C for 15–30 min. Secondary ion mass spectroscopy results for the three different damage regimes indicate that there is no measurable effect of dose rate or implant temperature on TED of boron implanted silicon for any of the damage regimes. It should be emphasized that the dose and energy of the boron implants is such that none of these implants approached the amorphization threshold. Above amorphization dose rate and implant temperature have dramatic effects on TED, but it appears that below the amorphization threshold there is little effect. These results suggest that for a given energy it is the ion dose not the extent of the implant damage that determines the extent of TED in boron implanted silicon.     

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.     

Transient-enhanced diffusion in shallow-junction formation

Shallow junctions are formed in crystalline Si by low-energy ion implantation of B⁺, P⁺, or As⁺ species accompanied by electrical activation of dopants by rapid thermal annealing and the special case of spike annealing. Diffusion depths were determined by secondary ion-mass spectroscopy (SIMS). Electrical activation was characterized by sheet resistance, Hall coefficient, and reverse-bias diode-leakage measurements. The B⁺ and P⁺ species exhibit transient-enhanced diffusion (TED) caused by transient excess populations of Si interstitials. The electrically activated fraction of implanted dopants depends mainly on the temperature for B⁺ species, while for P⁺ species, it depends on both temperature and P⁺ dose. The relatively small amount of diffusion associated with As⁺ implants is favorable for shallow-junction formation with spike annealing.     

Optimization of RTA parameters to produce ultra-shallow, highly activated B+, BF 2 + , and As+ ion implanted junctions

The effects of time, temperature, ramp-up, and ramp-down rates with rapid thermal annealing employing a STEAG AST SHS3000 were investigated on 1.0 and 2.0 keV ¹¹B⁺, 2.2, 5.0, and 8.9 keV ⁴⁹BF ₂ ⁺ , and 2 KeV ⁷⁵As⁺, 1E15/cm² samples implanted in a Varian VIISion-80 PLUS ion implanter at 0^o tilt angles. These annealed samples were analyzed by four-point probe, secondary ion mass spectrometry (SIMS), and in select cases by spreading resistance profiling (SRP) and transmission electron microscopy (TEM). To ensure reproducibility and to minimize oxidation enhanced diffusion as an uncontrolled variable, the O₂ background concentration in N₂ was maintained at a controlled low level. Under these conditions, ramp-rates alone were found not to be significant. Spike anneals (1050°C, ～ 0 s) with fast ramp-rates (240°C/s) and fast cool down rates (86°C/s) provided the shallowest junctions, while still yielding good sheet resistance values. Post annealed samples were examined for extended defect levels (by TEM) and trapped interstitial concentrations. Fluorine concentration measurements were employed to qualitatively explain differences in the B diffusion from ¹¹B⁺ and ⁴⁹BF ₂ ⁺ ion implants at various energies. The 2.2 keV ⁴⁹BF ₂ ⁺ “fast” spike annealed sample at 1050°C exhibited limited, if any, enhanced diffusion, yielding a SIMS junction depth of 490Å, an electrical junction of 386Å (by SRP) and a sheet resistance of 406 ohm/sq.     

Influence of boron clustering on the emitter quality of implanted silicon solar cells: an atom probe tomography study

The use of ion implantation doping instead of the standard gaseous diffusion is a promising way to simplify the fabrication process of silicon solar cells. However, difficulties to form high‐quality boron (B) implanted emitters are encountered when implantation doses suitable for the emitter formation are used. This is due to a more or less complete activation of Boron after thermal annealing. To have a better insight into the actual state of the B distributions, we analyze three different B emitters prepared on textured Si wafers: (1) a BCl₃ diffused emitter and two B implanted emitters (fixed dose) annealed at (2) 950°C and at (3) 1050°C (less than an hour). Our investigations are in particular based on atom probe tomography, a technique able to explore 3D atomic distribution inside a material at nanometer scale. Atom probe tomography is employed here to characterize B atomic distribution inside textured Si solar cell emitters and to quantify clustering of B atoms. Here, we show that implanted emitters annealed at 950 °C present maximum clusters due to poor solubility at lower temperature and also highest emitter saturation current density (J_0e = 1000 fA/cm²). Increasing the annealing temperature results in greatly improved J_0e (131 fA/cm²) due to higher solubility and a consequently lower number of clusters. BCl₃ diffused emitters do not contain any B clusters and presented the best emitter quality. From our results, we conclude that clustering of B atoms is the main reason behind higher J_0e in the implanted boron emitters and hence degraded emitter quality. Copyright © 2015 John Wiley & Sons, Ltd.     

Silicon preamorphization and shallow junction formation for ULSI circuits

The effects of preamorphization of silicon by¹⁹F,²⁸Si and⁷⁴Ge implants preceding¹¹B implants have been investigated in this work and compared with BF₂ molecular implants. For shallow boron implanted layers less than 0.2 μm deep, the beam purity of BF₂ implants is crucial as well as proper preamorphization of the silicon to eliminate any inadvertent channeling. Preamorphization of silicon can be achieved with either⁷⁴Ge,²⁸Si or¹⁹F implants. Data from SIMS, TEM, RBS and diode leakage current measurements have all consistently shown that the best results are obtained with⁷⁴Ge preamorphization, followed by²⁸Si- and¹⁹F-preamorphization. RTA of preamorphized silicon at 1000° C for 10s in a dry argon ambient is preferred for shallow junctions. Furnace anneals at 950° C for 45 min of⁷⁴Ge preamorphized samples have resulted in practically perfect PN junctions.     

The effect of ion-implantation damage on dopant diffusion in silicon during shallow-junction formation

Low-thermal-budget annealing of ion-implanted BF ₂ ⁺ , P, and As in Si was studied for shallow-junction formation. Implant doses were sufficient to amorphize the silicon surface region. Low-temperature furnace annealing and rapid-thermal annealing of ionimplanted boron, phosphorus and arsenic in silicon exhibit a transient enhanced diffusion regime resulting injunction depths considerably deeper than expected. The origin of this transient enhanced diffusion is the annealing of ion-implantation damage in the silicon substrate. We have found that point-defect generation during the annealing of either shallow end-of-range damage or small clusters of point defects dominates the transient enhanced diffusion process depending upon the annealing temperature and time. The net effect of damage annealing is to reduce the activation energy for dopant diffusion by an amount equal to the activation energy of the supersaturation of point defects in silicon. Models which can describe the transient enhancement characteristics in dopant diffusion during both furnace and rapid-thermal annealing of these implants are discussed.     

Crystal damage and the properties of implanted p-n junctions in silicon

The influence of crystal damage on the properties of implanted p-n junctions has been studied by variation of the amount of initial damage, variation of the recovery process, and variation of the residual damage. This was done by carrying out implantations at - 196, 25 and 700°C with 10¹⁵ B⁺/cm² at an energy of 50 keV, and at 25°C with 10¹⁵ BF₂⁺ at an energy of 250 keV and 10¹⁵ Ga⁺/cm² at an energy of 70 keV. Substrate orientations of both 〈111〉 and 〈100〉 were used, and annealing was done in a temperature range between 400 and 1100°C. Gettered as well as non-gettered slices were used for 〈111〉 oriented substrates. The diode properties were analyzed with the aid of Shockley-Read-Hall recombination statistics. Depending upon crystal history and processing, different traps are found to dominate the reverse current. Traps caused by the gettering of contamination as well as those caused by the damage itself play a role. The number of traps is found to be smaller than 10¹²/cm³ for well annealed diodes, resulting in a reverse current density of 0.2 nA/cm² at 1 V reverse bias.     

Solar‐grade boron emitters by BF3 plasma doping and role of the co‐implanted fluorine

We investigate the electrical properties and dopant profiles of boron emitters performed by plasma immersion ion implantation from boron trifluoride (BF₃) gas precursor, thermally annealed and passivated by silicon oxide/silicon nitride stacks. High thermal budgets are required for doses compatible with screen‐printed metal pastes, to reach very good activation rates. However, if good sheet resistances and saturation current densities may be obtained, we met strong limitations of the implied open‐circuit voltage of the n‐type Czochralski silicon substrates, which is incompatible with high‐efficiency solar cells. Such limitations are not encountered with beamline where pure B⁺ ions are implanted. Efforts on the passivation quality may improve the implied open‐circuit voltage but are not sufficient. We provide experimental comparison between beamline and plasma immersion allowing us to discriminate the causes explaining this observation (implantation technique or ion specie used) and to infer our interpretation: The co‐implantation of fluorine seems to indirectly impact the lifetime of the core substrate after thermal annealing. Copyright © 2015 John Wiley & Sons, Ltd.     

Photothermal Activation of Shallow Dopants Implanted in Silicon

Dopant impurities were implanted at high dose and low energy (10¹⁵ cm⁻², 0.5–2.2 keV) into double-side polished 200 mm diameter silicon wafers and electrically activated to form p–n junctions by 10 s anneals at temperatures of 1,025, 1,050, and 1,075°C by optical heating with tungsten incandescent lamps. Activation was studied for P, As, B, and BF₂ species implanted on one wafer side and for P and BF₂ implanted on both sides of the wafer. Measurements included electrical sheet resistance (Rs) and oxide film thickness. A heavily boron-doped wafer, which is optically opaque, was used as a hot shield to prevent direct exposure to lamp radiation on the adjacent side of the test wafer. Two wafers with opposing orientations with respect to the shield wafer were annealed for comparison of exposure to, or shielding from, direct lamp illumination. Differences in sheet resistance for the two wafer orientations ranged from 4% to 60%. n-Type dopants implanted in p-type wafers yielded higher Rs when the implanted surface was exposed to the lamps, as though the effective temperature had been reduced. p-Type dopants implanted in n-type wafers yielded lower Rs when the implanted surface was exposed to the lamps, as though the effective temperature had been increased. Effective temperature differences larger than 5°C, which were observed for the P, B, and BF₂ implants, exceeded experimental uncertainty in temperature control.     

Boron-implanted silicon resistors

The sheet resistance of silicon resistors implanted with boron at room temperature has been experimentally determined for doses from 5 × 10¹² to 2 × 10¹⁶ cm^?2. The results have been compared with the V calculated values. Two methods for minimizing the temperature coefficient, TCR, are described, and their merits and disadvantages are discussed. For a 1-kΩ/□ resistor, TCR can be reduced to 1000 ppm/°C by implanting ¹¹B⁺ at low energy, 5–10 keV, and to less than 100 ppm/°C by implanting a suitable dose of Ar⁺ damage. In a two-terminal resistor, the end effect of the total sheet resistance on TCR and on voltage coefficient VCR was also investigated.     

Ultra-shallow raised p+−n junctions formed by diffusion from selectively depositedIn-situ doped Si0.7Ge0.3

In this paper, a novel raised p⁺−n junction formation technique is presented. The technique makes use ofin- situ doped, selectively deposited Si_0.7Ge_0.3 as a solid diffusion source. In this study, the films were deposited in a tungsten halogen lamp heated cold-walled rapid thermal processor using SiCl₂H₂, GeH₄, and B₂H₆. The microstructure of the Si_0.7Ge_0.3 layer resembles that of a heavily defected epitaxial layer with a high density of misfit dislocations, micro-twins, and stacking faults. Conventional furnace annealing or rapid thermal annealing were used to drive the boron from thein- situ doped Si_0.7Ge_0.3 source into silicon to form ultra-shallow p⁺−n junctions. Segregation at the Si_0.7Ge_0.3/Si interface was observed resulting in an approximately 3:1 boron concentration discontinuity at the interface. Junction profiles as shallow as a few hundred angstroms were formed at a background concentration of 10¹⁷ cm⁻³.     

Effect of ion doping on the dislocation-related photoluminescence in Si+-implanted silicon

The study is concerned with the effect of the additional implantation of Si samples with C⁺, O⁺, B⁺, P⁺, and Ge⁺ impurity ions followed by annealing at 800°C on the behavior of the dislocation photoluminescence line D1, induced in the samples by implantation with Si⁺ ions at a stabilized temperature followed by annealing in an oxidizing Cl-containing atmosphere. It is established that the intensity of the D1 line strongly depends on the type of incorporated atoms and the dose of additional implantation. An increase in the D1 line intensity is observed upon implantation with oxygen and boron; at the same time, in other cases, the D1 luminescence line is found to be quenched. The mechanisms of such behavior, specifically, the role of oxygen and its interaction with implanted impurities are discussed.     

An EPR study of defects induced in 6H-SiC by ion implantation

Crystalline (0001) plane wafers of n-type 6H-SiC have been implanted at room temperature with 200 keV Ge⁺ ions in the dose range 10¹² to 10¹⁵ cm⁻². Electron paramagnetic resonance (EPR) measurements have been made on these samples both before and after annealing them at temperatures in the range room temperature to 1500°C. The as-implanted samples have a single isotropic and asymmetric line EPR spectrum whose width, ΔB_pp, increases with ion dose before falling when a buried continuous amorphous layer is produced. This increase is interpreted in terms of the change in the relative intensity of a line with g = 2.0028 ± 0.0002, ΔB_pp = 0.4 mT associated primarily with carbon dangling bonds in a-SiC and a line with g in the range 2.0033 to 2.0039 of uncertain origin. The variation with anneal temperature of the populations of these defects is reported.     

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.     

Multi-step rapid thermal annealing of boron and indium implanted Hg1−xCdxTe

In this work, we are reporting the use of a two-step rapid thermal annealing (RTA) process (250°C, 100s+340°C, 30s) for the annealing of Hg_1−xCd_xTe (MCT) implanted layers over p-type (x=0.22) substrates. We report a high value of electrical activation (70%) of the indium implants after this short RTA treatment in inert Ar atmosphere. The need of two RTA steps in the annealing recipe is shown, and so the role played by each of them: the first step annihilates the implantation damage, while the second one produces the impurity electrical activation. However, for the boron case, no electrical activity was found after several annealing processes, behaving as an inert species for the case of this bulk MCT material. We also point out the change on the substrate electrical characteristics induced by the thermal treatments, and report the convenience of a subsequent low temperature furnace annealing (200°C, 72 h) to reduce back the bulk carrier concentration to values low enough to achieve an n⁺-p IR detector structure.     

Electrically active defects in shallow pre-amorphisedp + n junctions in silicon

An examination of shallow pre-amorphisedp ⁺ n junctions in silicon has revealed three distinct defect related phenomena determined largely by the annealing temperature and relative location of the junction and the amorphous-crystalline (α-c) boundary. For temperatures below 800‡ C all samples displayed leakage currents of ∼10⁻³ A/cm² irrespective of the amorphising atom (Si⁺, Ge⁺ or Sn⁺). The generation centres responsible were identified to be near mid-gap deep level donors lying beyond the α-c interface. For samples annealed above 800‡ C, the leakage current was determined by the interstitial dislocation loops at the α-c boundary. If these were deeper than the junction, a leakage current density of ∼10⁻⁵ A/cm² resulted. From the growth of these loops during furnace annealing it was concluded that the growth was supported by the influx of recoil implanted silicon interstitials initially positioned beyond the α-c boundary. In the case where the as-implanted junction was deeper than the α-c boundary, annealing above 800° C resulted in a transient enhancement in the boron diffusion coefficient. As with the dislocation loop growth, this was attributed to the presence of the recoil implanted silicon interstitials.     

Exfoliation and blistering of Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te substrates by ion implantation

As part of a series of wafer bonding experiments, the exfoliation/blistering of ion-implanted Cd_0.96Zn_0.04Te substrates was investigated as a function of postimplantation annealing conditions. (211) Cd_0.96Zn_0.04Te samples were implanted either with hydrogen (5×10¹⁶ cm⁻²; 40–200 keV) or co-implanted with boron (1×10¹⁵ cm⁻²; 147 keV) and hydrogen (1–5×10¹⁶ cm⁻²; 40 keV) at intended implant temperatures of 253 K or 77 K. Silicon reference samples were simultaneously co-implanted. The change in the implant profile after annealing at low temperatures (<300°C) was monitored using high-resolution x-ray diffraction, atomic force microscopy (AFM), and optical microscopy. The samples implanted at the higher temperature did not show any evidence of blistering after annealing, although there was evidence of sample heating above 253 K during the implant. The samples implanted at 77 K blistered at temperatures ranging from 150°C to 300°C, depending on the hydrogen implant dose and the presence of the boron co-implant. The production of blisters under different implant and annealing conditions is consistent with nucleation of subsurface defects at lower temperature, followed by blistering/exfoliation at higher temperature. The surface roughness remained comparable to that of the as-implanted sample after the lower temperature anneal sequence, so this defect nucleation step is consistent with a wafer bond annealing step prior to exfoliation. Higher temperature anneals lead to exfoliation of all samples implanted at 77 K, although the blistering temperature (150–300°C) was a strong function of the implant conditions. The exfoliated layer thickness was 330 nm, in good agreement with the projected range. The “optimum” conditions based on our experimental data showed that implanting CdZnTe with H⁺ at 77 K and a dose of 5×10¹⁶/cm² is compatible with developing high interfacial energy at the bonded interface during a low-temperature (150°C) anneal followed by layer exfoliation at higher (300°C) temperature.