H. C. Mogul

Shallow Si p+-n junctions fabricated by focused ion beam Ga+ implantation through thin Ti and TiSi2 layers

Focused ion beam Ga+ implantation through Ti metal (ITM) and TiSi2 (ITS) layers, followed by rapid thermal annealing (RTA), has been investigated for application in self‐aligned silicide technology. The Ga+ energy was varied from 25 to 50 keV at doses of 1×1013 and 1×1015 cm−2 followed by RTA at 600 °C for 30 s. Depth profiles of the Ga implants were obtained by performing secondary‐ion mass spectrometry. It was observed that higher‐energy and higher‐dose implants yielded good quality p+‐n junction characteristics. Diodes were fabricated to obtain the electrical properties of these silicided junctions. At higher implant energies (≥40 keV) and all doses, I‐V characteristics of ITS diodes showed 100 times lower leakage currents (Ir) than ITM diodes. For low‐energy (<40 keV)/high‐dose implantation the ITS diodes showed a slight improvement in Ir over the ITM diodes, whereas for low‐energy/low‐dose implantation the same Ir was observed.

Localized fabrication of Si nanostructures by focused ion beam implantation

Si nanostructures have been fabricated by focused ion beam implantation (FIB) followed by etching in KOH/IPA. The FIB implantation into Si at a sufficiently high dose (≥1015/cm2) renders the local Si region much less susceptible to chemical etching. This effect has been observed for FIB implantation with Ga, Au, and Si ions. After etching, the implanted layer forms a cantilever structure whose thickness is a function of the implantation energy. At low energies (<30 keV) nanometer‐scale Si structures can be formed using this technique.

Doping-induced selective area photoluminescence in porous silicon

The incubation time (ti) for the onset of porous Si formation by stain etching in HF:HNO3:H2O was observed to be a strong function of dopant type and concentration. For B‐doped p‐Si, ti increased significantly with substrate resistivity (ρ), from ∼0.5 min for 0.004 Ω cm to ∼9 min for 50 Ω cm. P‐doped n‐Si substrates exhibited a ti which decreased with increasing ρ, from ∼10 min for 0.15 Ω cm to ∼8 min for 20 Ω cm. We have utilized the difference in ti between n‐ and p‐type Si to produce selective area photoluminescence (PL) by Ga+ focused ion beam (FIB) implantation doping and B+ broad beam implantation doping of n‐type Si. Using 30 kV FIB Ga+ implantation, PL patterns with submicrometer resolution have been obtained for the first time.

Photoluminescence from stain-etched polycrystalline Si thin films

Visible room‐temperature photoluminescence (PL) has been observed from stain‐etched polycrystalline Si thin films. Poly‐Si thin films deposited on oxidized Si and quartz substrates became porous (PoSi) after stain‐etching in a 1:3:5 solution of HF:HNO3:H2O. Under UV excitation, the stain‐etched doped and undoped poly‐Si films produce uniform orange‐red (∼650 nm) luminescence very similar to that obtained from stain‐etched crystalline Si substrates. Stained amorphous thin films did not exhibit photoluminescence. Luminescent patterns with sub‐micrometer (∼0.6 μm) dimensions have been obtained for the first time from PoSi produced from poly‐Si films.

Crystallinity and Photoluminescence in Stain‐Etched Porous Si

Si thin films were deposited by low pressure chemical vapor deposition on quartz and oxidized Si substrates at temperatures ranging from 540 to 640 o C. X-ray diffraction (XRD) indicates that films deposited ≥590 o C are increasingly polycrystalline with a orientation, while films deposited ≤580 o C are amorphous. After deposition, the Si films were rendered porous (PoSi) by stain-etching in HF:HNO 3 :H 2 O. XRD and Raman spectroscopy indicate that initially polycrystalline films retain their crystallinity after becoming porous. PoSi films deposited ≥590 o C exhibit visible (∼650-670 nm) photoluminescence (PL) at room temperature under UV excitation

Electrochemical capacitance‐voltage depth profiling of nanometer‐scale layers fabricated by Ga+ focused ion beam implantation into silicon

The electrochemical capacitance‐voltage (ECV) profiling technique is employed to measure the active carrier concentration in nanoscale layers fabricated by focused ion beam (FIB) implantation of 3 to 10 keV Ga+ ions into crystalline Si. The carrier concentration profiles obtained by ECV indicate the ability of this technique to probe depths as shallow as 2–3 nm and with a nanometer‐scale depth resolution. The carrier concentration obtained by ECV matches well with the Ga atomic concentration profile detected by secondary‐ion mass spectroscopy, but is almost an order of magnitude higher than that provided by the spreading resistance profile technique.

Electrical properties of nanometer‐scale Si p+‐n junctions fabricated by low energy Ga+ focused ion beam implantation

Diodes have been fabricated by on‐axis Ga+ focused ion beam (FIB) implantation at 4–25 keV into n‐Si 〈100〉 wafers doped to 2×1015/cm3. Post‐implantation anneal was performed at 600 °C for 30 s to electrically activate the Ga and to regrow the implanted layer. SIMS measurements performed to obtain the Ga concentration depth profile indicate good agreement with trim simulation even at low energies. At 4 keV an electrical junction depth of 15 nm is obtained from spreading resistance profiling (SRP). The junction depth was found to vary linearly with energy over the range explored. The electrical properties of the diodes were obtained from I‐V characteristics. The leakage current density of the 5 keV diode was measured to be 1 and 20 nA/cm2 at a reverse bias of 1 and 5 V, respectively. The corresponding leakage current density values for the 10 and 15 keV diodes were between 25% and 50% lower than those reported for 5 keV. The reverse bias breakdown voltage was between 105 and 110 V for all diodes. The combin...

Selective-area room temperature visible photoluminescence from SiC/Si heterostructures

SiC/Si heterostructures have been patterned by reactive ion etching with CHF3/O2 to produce SiC‐covered and Si‐exposed regions with lateral dimensions of 2.5 to ∼500 μm. The patterned samples were then anodized in HF/ethanol solutions. Short anodization times (<3 min) result in selective‐area UV‐induced visible photoluminescence (PL), with a peak located at 650 nm, being observed at 25 °C from only the SiC‐covered regions. The emission is generated by porous Si (PoSi) selectively formed under the SiC cap and transmitted through the wide band‐gap SiC layer. Longer etching times result in nonselective PL.

Ultrashallow Si p+–n junction fabrication by low energy Ga+ focused ion beam implantation

The fabrication of ultrashallow Si p+–n junctions by low energy Ga+ focused ion beam implantation has been investigated at energies ranging from 5 to 15 keV. Post‐implantation rapid thermal annealing was performed at 600 °C for 30 s to activate the implanted Ga and to regrow the implanted layer. Secondary ion mass spectroscopy (SIMS), spreading resistance profile (SRP), and cross‐sectional transmission electron microscopy (TEM) have been employed to characterize the resulting Ga atomic concentration depth profile and the structure of the implanted layer. For 5 keV Ga+ implantation, the cross‐sectional TEM (xTEM) measurement yielded an amorphous layer thickness of 9 nm and a line of end‐of‐range defects 16 nm below the surface [after rapid thermal annealing (RTA)]. The SIMS profiles indicate that only minor Ga channeling occurred during implantation. The SRP measurements give a junction depth of only 20 nm for the 5‐keV Ga implants. Leakage current density of 20 nA/cm2 has been measured at 5 V reverse bias.

Si Oxyhydrides on Stain‐Etched Porous Si Thin Films and Correlation with Crystallinity and Photoluminescence

Porous Si has been fabricated from amorphous and polycrystalline Si films by stain-etching in HF:HNO 3 :H 2 O. Infrared transmission measurements have revealed an absorption peak at 880-890 cm -1 only in crystalline porous Si samples. This peak is probably due to an SiH 2 bending mode in the presence of oxygen. Similarly, only crystalline PoSi films exhibit visible (∼650-670 nm) photoluminescence under UV excitation. Amorphous PoSi samples do not luminesce even after very long etch times, in spite of greatly increased porosity. Therefore, it appears that there exists a unique correlation between the presence of crystallinity in the starting Si film and the presence of surface oxyhydrides and photoluminescence after stain-etching