Double – Gold/Nitrogen – Nanosecond-Laser Doping of Gold-Coated Silicon Wafer Surface in Liquid Nitrogen

1. Introduction

Nowadays silicon (Si) is reviving as a basic nano- and optoelectronic material, being a key component material in CMOS-compatible planar photonic integrated circuits [1], extended in their functionality by heterogeneous integration with III-V heterostructures [2]. Photonic integrated circuits are demanded in ultrafast telecommunication and other signal-processing modules, utilizing ultimate speed of light, ultrafast switching etc. [3].

Hyperdoping of semiconductors, including Si, through non-equilibrium condensed – solid- or liquid- − phase mechanisms, such as ion implantation [4,5], pulsed laser melting of surface dopant layers along with the semiconductor substrate [6,7] or doping in pulsed laser-activated chemically-active fluids [8,9,10], was broadly utilized to enhance p-n-junction functionality (dimensions, response time, etc.) [11] or specific near-IR or mid-IR light absorption for their optoelectronic response [12,13]. In the latter case, the dopant could form a rather deep intermediate donor (acceptor) impurity band in the semiconductor bandgap [14], still merging with the corresponding conduction (valence) band at high impurity concentrations ~10²⁰-10²¹ cm^-3 [15], resulting in an “insulator-to-conductor” transition [16]. Additionally, the related spontaneous amorphization of the material due to internal impurity-induced stresses [17], considerable reduction of carrier mobility due their scattering on impurity centers or thermally-ionized free carriers [18], and their trapping at the deep impurity-related traps [19] set a natural doping limit at high doping concentrations, requiring elaborate management of impurity intragap energetic states via control of dopant concentrations, chemical and charge states [20,21]. Double or even multiple doping of semiconductors opens a way to overcome the concentration-related “insulator-to-conductor” transition obstacle by formation of a few intermediate donor (acceptor) bands across the semiconductor bandgap [22]. So far, double doping was performed, minimum, in two steps – one for each dopant [23], with the interference (synergy or competition) of the different dopants during the doping process being still a “black-box” issue, potentially depending on chemical, concentration and other factors.

Recently, Si wafer was laser-doped by nitrogen (N) in a laser plasma-activated liquid-nitrogen (LN) environment [24], similarly to boron carbide in previous studies [10]. Nitrogen impurity is known as a key dopant for improving Si crystalline structure by enhancing oxygen precipitation to improve the intrinsic gettering ability of silicon wafers [25], locking dislocations to increase mechanical strength [26] and annihilating microdefects [27] (voids and interstitial-type dislocation aggregates via accumulation of vacancies in N-V complexes [28]), both in floating-zone-grown and Czochralski-grown silicon crystals. Bulk doping with nitrogen atoms in substitutional (N_s) positions as deep donor centers could occur (not only in dislocations, grain boundaries and cellular structure), when the critical content of nitrogen ( ~10¹⁵ atom/cm³ [29]) is locally exceeded in the Si crystal [30]. By itself, N-doped Si exhibits strong sub-bandgap infrared (IR) absorption [31,32], suitable for empowering optoelectronic, photovoltaic and detector performance [33,34] and offering a wide range of potential applications.

However, the challenging introduction of N into Si via non-equilibrium ion implantation meets difficulties in producing high N concentrations due to the very low solid and liquid solubility limits of only 4.5×10¹⁵ and 6.0×10¹⁸ atoms/cm³ [35], respectively, crystal damage and amorphization [36], activation of interstitial N into substitutional sites [37], with the dominant and most stable N defect in Si being the di-interstitial pair (N_i-N_i) [38]. Then, additional laser annealing converts nitrogen into active N_sform, with the highest achieved total N concentration in Si to date, of 0.5 ± 0.2 at. % (2.5×10²⁰ at./cm³) [39]. For the maximal implanted nitrogen concentrations of below 10²⁰ atoms/cm^-3 Si remains crystalline, but amorphous Si and SiN appear for nitrogen concentrations above 3×10²⁰ atoms/cm^-3, transforming into precipitated Si₃N₄ upon annealing at 1000⁰C or in continuous layer of crystalline Si₃N₄ for concentrations above 4×10²² atoms/cm³ [40,41], while the excess nitrogen could be trapped and make blistering during high-temperature annealing. Alternatively, reactive laser or plasma-enhanced chemical vapor deposition (PECVD) in nitrogen atmosphere, requiring high-energy plasma species to activate nitrogen and subsequent thermal annealing of as-deposited films to improve their crystalline quality, was reported to result in a-Si₃N₄ films [42,43], promising for fabrication of low-loss waveguides and integrated-photonics elements [44]. Direct laser treatment in low-pressure ammonia [42], high-pressure ammonia or nitrogen gases [45] provided chemical (a-Si₃N₄ [42]) or doping (up to 6 at. % [45], comparing to 12 at. % by PECVD [31]) nitrifying of Si surface, respectively.

In this study, double doping of Si wafers by nanosecond-laser plasma-mediated nitrifying and nitrogen doping of an Au-film coated, un-doped crystalline Si wafer immersed into liquid nitrogen was explored in different laser treatment regimes and characterized by means of scanning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, IR and Raman/photoluminescence spectroscopy.

2. Materials and Methods

200-μm thick, double-side polished (100) undoped silicon wafer (resistivity >1 kΩ cm) was covered in argon atmosphere by a 11-nm gold (Au) film though magnetron sputtering (SC7620, Quorum Technologies) and then immersed into liquid nitrogen (LN, thickness of 20 mm) in a thermally-isolating polystyrene foam container and linearly laser-scanned over 3mm×2mm area for its gold and nitrogen doping. Laser scanning was performed using a two-axis micro-positioning stage (scanning speed: 10, 25, 40 and 60 µm/s) and a Nd:YAG laser (SOLAR, second harmonic wavelength − 532 nm, full-width at a half-maximum pulse duration − 10 ns, pulse repetition rate − 20 Hz). The radiation was focused onto the immersed sample surface by means of a cylindrical lens with a focal length of 20 cm into a spot 2800 x 140 µm through a 2-cm thick LN layer, producing textured Si regions at the laser exposures N = 280, 110, 70 and 50 pulses/spot (Figure 1). The pulse energy was 10, 14 and 18 mJ, providing the peak laser fluence F = 2.5, 3.5 and 4.5 J/cm², respectively.

Topographies of the nanopatterned/hyperdoped Si surface were visualized by a scanning electron microscope VEGA (TESCAN) equipped by an energy-dispersive X-ray spectrometer Explorer 15 (EDX) for chemical microanalysis at the electron beam energies of 3, 5 and 10 keV.

IR transmission spectra of the nanopatterned/doped Si surface regions and the initial Si wafer were acquired in ambient air by room-temperature Fourier-transform infrared (650–2500 cm⁻¹) spectroscopy, using a spectrometer FT-805 (SIMEX) with a spectral resolution of 4 cm⁻¹. Top view structural characterization of the nanopatterned/doped Si surface layers was performed at room temperature by Raman/photo-luminescence (PL) microspectroscopy at the spectral resolution of 2 cm⁻¹, using a Confotec 520 (SOL Instruments) microscope-spectrometer at a 405 and 532 nm wavelengths.

Chemical bonding of silicon, nitrogen, oxygen, carbon atoms was studied across the nanopatterned/doped Si surface layer by means of x-ray photoelectron spectrometer K-Alpha+ (XPS, ThermoFisher Scientific, Al Kα, hω = 1486 eV). The photoelectron spectra were acquired using a Al K_α-line at an energy accuracy of 0.1 eV and composition accuracy of 0.1 at. %. The spectrometer was calibrated to the binding energies of Au4f_7/2 (83.9 eV), Ag3d_5/2 (368.2 eV), and Cu2p_3/2 (932.6 eV). The XPS spectra of full energy range and region of spectral lines of Au4f, Si2p, N1s, O1s, and C1s were collected from the sample area of 200-µm in diameter, using 30-eV pass energy and 0.1-eV energy step size. The build-in ion (Ar⁺) etching system was used for layer-by-layer analysis and reconstruction of composition distribution in the depth with 20-s (10-nm) etching cycles.

3. Results and Discussions

3.1. Surface Topography and Chemical Composition

Ns-laser interaction of the Au/Si sample with the LN environment was found to proceed via formation of the ablative plasma, resulting in the pronounced microscale ablative surface topography, and its activation (dissociation, excitation by electron impact) of nitrogen molecules in LN, when the threshold fluence ≈1 J/cm² (peak intensity ~0.1 GW/cm²) was exceeded at the peak fluences F = 2.5, 3.5 and 4.5 J/cm² for all used exposures N =50-280 pulses/spot (Figure 2).

Our qualitative EDX mapping indicates the homogeneity of the gold and nitrogen introduction into Si, exhibiting the same patterns of the chemical maps (Figure 3a). Quantitative EDX analysis of the Au and N content in the laser texture was performed at 3 keV (probe depth in Si δ₃≈0.02 μm [46]), 5 keV (δ₅≈0.15 μm [46]) and 10 keV (δ₁₀≈0.65 μm [46]). The initial Au/Si sample with the 11-nm thick Au film exhibits the relative Au content C_Au,3≈1.2 at. % (3 keV), C_Au,5≈0.14 at. % (5 keV) and C_Au,10≈0.03±0.02 at. % (10 keV) under these acquisition conditions, respectively, decreasing upon the laser treatment to C_Au/Si,3≈0-0.27 at. %, C_Au/Si,5≈0-0.08 at. % and C_Au/Si,10≈(0-0.04)±0.03 at. %, depending on laser fluence and exposure (Figure 3b). Besides laser ablation, the Au dopant is supposed to migrate into Si for the thickness of the laser-melted/doped Si layer, d ≈ 2√χτ ≈ 0.25 μm for the laser pulsewidth τ≈10 ns and high-temperature (T≤1689 K) thermal diffusivity of Si, χ≈ 0.16 cm²/s [47]. The corresponding integrated contents C_Au,3δ₃ ≈ C_Au,5δ₅ ≈ C_Au,10δ₁₀ ≈ 0.02 at.%μm could be compared to their values in the laser-produced texture – specifically, C_Au/Si,3d ≤ 0.08 at.%μm, C_Au/Si,5d ≤ 0.02 at.%μm and C_Au/Si,10δ₁₀≤0.01±0.01 at.%μm. Obviously, the intermediate EDX analysis regime at the 5 kV-voltage provides the most reliable Au content value (C_Au,5δ₅ ≈ C_Au/Si,5d ≤ 0.02 at.%μm) for δ₅ ≤ d, while the minimal voltage of 3 kV probes the too thin, 20-nm surface layer δ₃ « d, which remains gold-enriched even after the laser treatment C_Au/Si,3d ≤ 0.08 at.%μm > C_Au,3δ₃ ≈ 0.02 at.%μm. In contrast, the 0.6-μm thick layer probed by EDX analysis at the 10-kV voltage is related to the hardly measurable Au concentrations C_Au,10, C_Au/Si,10δ₁₀≤ 0.04±0.03 at. %. Meanwhile, even at the 5-kV voltage there is a large scattering of EDX data for the relative Au content versus laser fluence and exposure around the average level of ≈ 0.03-0.04 at. % without any distinct trends (Figure 3b). Thus, overall one could conclude on minor laser-ablative removal of gold during the laser treatment.

In contrast, nitrogen (N) enters into Si more readily − at much higher contents (up to 2.5 at. % in the 20-nm thick top layer) and much deeper (the difference in the N contents at the 5-kV and 10-kV voltages, i.e., 0.15-μm and 0.6-μm depths is not big − 2.0±0.5 at. % versus 1.5±0.5 at. %, Figure 3c). However, in the case of nitrogen dopant there is a distinct trend in this figure to reduced N content at higher laser fluences, despite apparently stronger laser plasma activation of LN, while apparently due to stronger ablative removal of n-doped Si and ablative screening of the Si surface from the activated gaseous nitrogen. This trend is consistent with the fluence-dependent reduction of the Raman intensity for the 150-cm^-1 peak related to β-Si₃N₄ [48] (see the next section). By the way, these doping results reasonably correlate with N content of 1-3 at. % in 0.3-μm thick Si surface layers laser-treated in LN at similar conditions [24] and of 1-6 at % for ns-laser nitridation till the depths ~0.6 μm in high-pressure N₂, NH₃ gases at laser fluences above 2 J/cm² [45] and 1-5 at. % in the ~1-μm thick layer during ns-laser induced LN-mediated nitridation of SiC at laser fluences above 2 J/cm² [10].

3.2. XPS Depth Profiling of Chemical States for Si and Its Dopants

XPS analysis for the sample surfaces indicates the considerable oxidation (Si-O, 103.0 eV) and nitridation (Si-N, 101.9 eV) of pure elemental Si (doublet of Si2p_3/2, 99.4 eV and Si2p_1/2, 100.0 eV) (Figure 4a). Nitrogen N1s appears in Figure 4b in the form of SiN_x/Si phase (single peak at 397.3 eV) [49,50], which somewhat increased versus exposure N and decreases versus F (Figure 4 f). The dopant gold is presented by the doublet of Au4f_5/2 and Au4f_7/2, where each line is composed by two components – metallic gold Au_M and cluster gold Au_CL [51] (Figure 4c), with the overall gold content raising at the maximal fluence of 4.5 J/cm² (Figure 4f), apparently, as a result of ablative plasma screening of the sample surface. The typical surface concentrations are ≈50 at. % (Si), 7-9 at. % (N), ≈40 at. % (O), ≤ 0.1 at. % (Au), i.e., much higher owing to the nanometer-thick XPS probing depth, as compared to the typical energy-dependent (sub)micrometer EDX probing depths.

During the etching of the samples with the 10-nm step in the range of 0-50 nm one could observed the fast purification of Si in the 10-nm surface layer (Figure 5a and Figure 6a), while the other dopants are reduced in their concentrations across the total 50-nm thick etched layer eventually (Figure 5 and Figure 6). At the minimal fluence F= 2.5 J/cm² (also at 3.5 J/cm²) gold is almost absent in the sample (Figure 5d), but O- and N-components rather slowly decrease into the bulk Si (Figure 5b,c). In contrast, at the maximal fluence F= 2.5 J/cm² the emerging gold eventually diminishes in its concentration into the bulk Si, with its minor Au_M component decreasing faster, as compared to Au_CL (Figure 6d). Moreover, importantly, inside the 50-nm Si layer the emerging gold replaces nitrogen and oxygen, which are remaining only on the surface. Surprisingly, even the marginal (~0.1 at. %) content of gold such strongly affects the multi-percent (up to 10 at. %) content of nitrogen and oxygen, indicating its fast distribution in the laser melt and prevention of the following nitrogen/oxygen dissolution. The depth-dependent nitrogen and gold distributions are summarized below in Figure 7.

3.3. Raman and FT-IR Spectroscopic Characterization

Raman characterization of the laser-modified/doped Si surface layer was performed at 532-nm pumping (Figure 8), where the penetration depth of the 532-nm radiation in crystalline c-Si (~1 μm [52]) enables to probe the entire modified layer, using the initial c-Si sample as a reference. Specifically, in the panoramic Raman spectra (Figure 8a) one could observe a number of characteristic spectral features: the main LO/TO optical-phonon peak at 521 cm^-1 with a weak amorphous Si (a-Si) shoulder at 480 cm^-1 (if any), zone-edge acoustic-phonon LA peak at 300 cm^-1, and 144-cm^-1 peak of β-Si₃N₄ or 153-cm^-1 peak of α-Si₃N₄ [48] in the first-order (one-phonon) Raman region (Raman I), accompanied by the second-order (two-phonon) Raman band at 960-970 cm^-1 (Raman II). Surprisingly, comparing to the reference c-Si spectra, one can’t observe any Raman peaks of doping nitrogen centers [32,53], starting at the Raman wavenumbers above 500 cm^-1.

The main 521-cm^-1 LO/TO Raman peak appears slightly red-shifted in the laser-modified Si and the initial Au/Si samples (Figure 8b), comparing to the reference c-Si sample. The observed minor fluence- and exposure-dependent shifts Δω≈2-4 cm^-1 could be related to nanocrystalline state of the recrystallized material [54,55] with its nanocrystallite size D [54]

$$D=a{\left({\displaystyle \frac{A}{\Delta \omega }}\right)}^{1/\gamma },$$

(2)

where the calibration constant A=47.41cm^-1, the Si lattice parameter a =0.543 nm, and the exponent γ=1.44 [54]. The calculated Si nanocrystallites occasionally vary in size in the range D = 4-6 nm almost independently on the laser fluence and exposure (Figure 8b, inset). Likewise, second-order (Raman II) band at 960-970 cm^-1, which usually represents the crystalline quality of the Si lattice, varies in its peak intensity insignificantly and almost independently on the laser fluence and exposure (Figure 8c). In contrast, the 144-cm^-1 peak of β-Si₃N₄ or 153-cm^-1 peak of α-Si₃N₄ [48], sensitively acquired by the Raman spectroscopy as a nitrified surface Si layer, demonstrates rather clear trend to the reduced intensity versus laser fluence (Figure 3c), discussed above in the previous section.

For comparison, FT-IR spectroscopy is related to multi-micrometer penetration depths in Si [52], being rather insensitive to its sub-micrometer thick surface modifications, e.g., optically-thin surface layers of amorphous a-, α- or β-Si₃N₄ phases (absorption range of 700-1100 cm^-1 [24,48,56]). Specifically, in our study the microtextured/doped Si surfaces exhibit considerable broadband (2-10 μm) light scattering/trapping, becoming evident both in the strongly reduced IR reflectance and transmittance acquired in air (Figure 9). The high-wavenumber increase of extinction coefficient indicates that long-wavelength absorption of free carriers, which could emerge due to thermal ionization of the predominating donor nitrogen centers and are present inside gold nanoparticles, appears negligible (see also our electro-physical characterization data below). Basically, Au- and, mostly, N-dopant IR absorption occurs in nitrogen pair (N_i-N_i) centers at 770 and 960 cm^-1 [53], off-center substitutional nitrogen N_s (550, 770 and 890 cm^-1 [53], aggregates with vacancies (V) and Si self-interstitials (I) – (N_s-N_s), (N_i-N_i)V, (N)2I etc. [32], being enhanced by scattering/trapping. Importantly, the spectral variations of reflectance, transmittance and extinction coefficient are lower at the maximal fluence F = 4.5 J/cm², supporting the observation of stronger ablative plasma screening on the Si surface under this fluence and corresponding exposures.

Finally, we performed room-temperature van der Paw and Hall measurements of electrical characteristics of the laser-modified Si surface layer. These measurements exhibit the high carrier mobility of 1050 cm²/Vs and carrier sheet concentration ~2×10¹⁰ cm^-2 (bulk concentration ~10¹⁴-10¹⁵ cm^-3), which are reasonably consistent with the corresponding room-temperature parameters in nitrogen-implanted/annealed Si samples ((3-4)×10² cm²/Vs and ~10¹⁷ cm^-3) [40]. The high carrier mobility implies the good crystalline quality of the layer, while the rather low carrier concentration indicates negligible dopant segregation effects and low thermal ionization of the deep donor states of nitrogen (0.2-0.3 eV [40]) and donor/acceptor states of gold (Au donor/acceptor centers) in Si.

4. Conclusions

Novel double-impurity doping process for Si surfaces was explored, proceeding via nanosecond-laser co-melting of a top Au film and a Si wafer substrate in a laser plasma-activated LN environment. The fluence- and exposure-independent surface micro-spike topography contains minor Au (~0.1 at. %) and major N (~10 at. %) dopants, localized in a sub-100-nm thick surface layer. X-ray photoelectron spectroscopy indicated bound surface (SiN_x) and bulk doping nitrogen states, with the nitrogen and oxygen dopant bulk distributions strongly reduced by the competing gold dopant with its two orders of magnitude lower concentrations. The slight reduction of the crystalline order was revealed by in the second-order Si phonon band, along with nanoscale dimensions of the resolidified Si nano-crystallites, negligible a-Si abundance and Si₃N₄ surface coating layer. IR 2D-mapping exhibited the only broad structureless absorption bands in the range of 1000-5000 cm^-1 related to dopant absorption and light-trapping in the surface micro-relief. The room-temperature electrical characteristics of the laser-nitrified Si layer – carrier mobility of ~1×10³ cm²/Vs and background carrier sheet concentration ~10¹⁴-10¹⁵ cm^-3 – indicated almost defect-free structure of the doped layer without abundant thermally-ionized shallow impurity centers and Au-segregated nanoparticles, being superior regarding previously reported characteristics of nitrogen-implanted/annealed Si samples. This novel facile double-element laser-doping procedure paves the way toward local maskless on-demand introduction of multiple intra-gap intermediate donor and acceptor bands in Si, providing its related broadband IR photoconductivity for optoelectronic applications.