Spectroscopic second-harmonic generation during ${\mathrm{Ar}}^{+}$-ion bombardment of Si(100)

Spectroscopic and real time optical second-harmonic generation (SHG) has been applied to gain insight into the surface and interface processes during low-energy $(70–1000\mathrm{eV})$ ${\mathrm{Ar}}^{+}$-ion bombardment of $H$ terminated Si(100). The ${\mathrm{Ar}}^{+}$-ion bombardment of the crystalline silicon $\left(c\text{−}\mathrm{Si}\right)$, which creates a layer of amorphous silicon $\left(a\text{−}\mathrm{Si}\right)$, has been studied in the SH photon energy range of $2.7–3.5\mathrm{eV}$. The time-resolved SHG signal has been observed to increase with an order of magnitude upon ion bombardment. Spectroscopic SHG during ion bombardment and after subsequent $\mathrm{Xe}{\mathrm{F}}_2$ dosing indicates that the SHG signal has both a contribution generated at the buried interface between the $a\text{−}\mathrm{Si}$ and the $c\text{−}\mathrm{Si}$ and an additional contribution originating from the $a\text{−}\mathrm{Si}$ surface. By separating these contributions using a critical point model it has been shown that the SHG spectra consist of a sharp resonance at $3.36\mathrm{eV}$ with a linewidth of $0.1\mathrm{eV}$ at the buried $a\text{−}\mathrm{Si}∕c\text{−}\mathrm{Si}$ interface and a much broader resonance at a resonance energy of $3.2\mathrm{eV}$ with a linewidth of $0.5\mathrm{eV}$ at the $a\text{−}\mathrm{Si}$ surface. The former resonance is identified to originate from $E_0^{\prime }∕E_1$ transitions between bulk electronic states in the $c\text{−}\mathrm{Si}$ that are modified due to the vicinity of the interface, while the latter resonance is caused by transitions related to Si-Si bonds in the surface region of the $a\text{−}\mathrm{Si}$. The time-resolved dynamics of the SHG signal can help in understanding the mechanism of ion-beam and plasma etching of silicon.

Figure 1

Horizontal cross section of the high vacuum chamber and the optical setup. The samples are placed in a rotatable sample holder (1) and can be replaced using a load lock. The ${\mathrm{Ar}}^{+}$-ion gun (2) and the $\mathrm{Xe}{\mathrm{F}}_2$ source (3) are placed at 45° and 52° with respect to the sample surface normal. The fundamental laser radiation, provided by a Ti:sapphire laser, and the generated SH radiation propagate through the setup at 74° angle of incidence with respect to the sample surface normal. Detection of the SH radiation takes place by a photomultiplier tube (PMT) connected to photon counting electronics.

Figure 2

Spectroscopic ellipsometry data as a function of photon energy for $c\text{−}\mathrm{Si}$ (dashed lines) and $a\text{−}\mathrm{Si}$ layers (solid lines). (a) Refractive index and extinction coefficient. (b) Linear susceptibility squared. (c) Absorption coefficient. ${\mathrm{Ar}}^{+}$-ion energies of 70, 200, and $1000\mathrm{eV}$ have been used to generate the $a\text{−}\mathrm{Si}$ layers.

Figure 3

Real time SHG intensity generated at $H$ terminated Si(100) subjected to bombardment with $70\mathrm{eV}$ ${\mathrm{Ar}}^{+}$ ions at a flux of $\sim 0.07\mathrm{ML}∕\mathrm{s}$. At $t=0\mathrm{s}$ the ${\mathrm{Ar}}^{+}$-ion bombardment is switched on, at $t=600\mathrm{s}$ the bombardment is switched off, and at $t=1200\mathrm{s}$ the bombardment is switched on again. The SH photon energy is $3.31\mathrm{eV}$ and both the fundamental and the SH radiation are $p$ polarized.

Figure 4

SHG intensity generated at $H$ terminated Si(100) before (open diamonds), during (closed circles) and $24\mathrm{h}$ after (open triangles) bombardment with $70\mathrm{eV}$ ${\mathrm{Ar}}^{+}$ ions as a function of the SH photon energy for $p$ polarized fundamental and SH radiation. In the inset the SHG spectrum before ion bombardment is rescaled to facilitate comparison with the spectrum during ${\mathrm{Ar}}^{+}$-ion bombardment.

Figure 5

Real time SHG intensity generated at $H$ terminated Si(100) subjected to bombardment with $70\mathrm{eV}$ (closed circles), $200\mathrm{eV}$ (open squares), and $1000\mathrm{eV}$ (open circles) ${\mathrm{Ar}}^{+}$ ions at fluxes of $\sim 0.07\mathrm{ML}∕\mathrm{s}$. At $t=0\mathrm{s}$ the ${\mathrm{Ar}}^{+}$-ion bombardment is switched on, at $t=600\mathrm{s}$ the bombardment is switched off, and at $t=1200\mathrm{s}$ the bombardment is switched on again. The SH photon energy is $3.31\mathrm{eV}$ and both the fundamental and the SH radiation are $p$ polarized.

Figure 6

SHG intensity as a function of the SH photon energy generated at $H$ terminated Si(100) during bombardment with ${\mathrm{Ar}}^{+}$ ions at energies of $70\mathrm{eV}$ (closed circles), $200\mathrm{eV}$ (open squares), and $1000\mathrm{eV}$ (open circles). Both the fundamental and SH radiation are $p$ polarized.

Figure 7

(a) Real time SHG response of $H$ terminated Si(100) (closed circles) bombarded with $70\mathrm{eV}$ ${\mathrm{Ar}}^{+}$ ions at a flux of $\sim 0.07\mathrm{ML}∕\mathrm{s}$ followed by two small doses of $\mathrm{Xe}{\mathrm{F}}_2$ (in total $\sim 30$ ML). Between $t=0\mathrm{s}$ and $t=600\mathrm{s}$ the sample is exposed to ion bombardment. Between $t=930\mathrm{s}$ and $t=960\mathrm{s}$ and between $t=1060\mathrm{s}$ and $1120\mathrm{s}$ the $\mathrm{Xe}{\mathrm{F}}_2$ beam is switched on. The SH photon energy is $3.31\mathrm{eV}$ and both the fundamental and the SH radiation are $p$ polarized. (b) The count rate due to $\mathrm{Xe}{\mathrm{F}}^{+}$ as measured with the quadrupole mass spectrometer (QMS) during dosing.

Figure 8

SHG intensity as a function of the SH photon energy generated at $H$ terminated Si(100) during bombardment with $1000\mathrm{eV}$ ${\mathrm{Ar}}^{+}$ ions (open circles) and after subsequent $\sim 400$ ML $\mathrm{Xe}{\mathrm{F}}_2$ dosing (closed squares) for $p$ polarized fundamental and SH radiation.

Figure 9

Two-layer optical model to describe the SHG response of ion bombarded $c\text{−}\mathrm{Si}$, consisting of vacuum, a layer of $a\text{−}\mathrm{Si}$ with thickness $d$ and a semi-infinite slab of $c\text{−}\mathrm{Si}$, with refractive indices $n_1$, $n_2$, and $n_3$, respectively. SHG is generated in polarized sheets placed in vacuum gaps at the surface $(z=0^{+})$ and the buried interface $(z=-d)$. Multiple reflections of (a) the fundamental radiation and (b) SH radiation generated at the $a\text{−}\mathrm{Si}$ surface and (c) SH radiation generated at the buried $a\text{−}\mathrm{Si}∕c\text{−}\mathrm{Si}$ interface are shown schematically.

Figure 10

Experimental (symbols) and simulated (solid lines) SHG spectra measured at $H$ terminated Si(100) during bombardment with ${\mathrm{Ar}}^{+}$ ions at energies of (a) $70\mathrm{eV}$, (b) $200\mathrm{eV}$, and (c) $1000\mathrm{eV}$ and (d) after $\mathrm{Xe}{\mathrm{F}}_2$ dosing directly after $1000\mathrm{eV}$ ion bombardment. The solid lines in (a)–(d) are fits to the data using a superposition of two CP-like resonances. The dashed and the dotted lines represent the individual resonances ${\chi }_{I,zzz}^{\left(2\right)}$ at the buried interface between $a\text{−}\mathrm{Si}$ and $c\text{−}\mathrm{Si}$ and ${\chi }_{S,zzz}^{\left(2\right)}$ at the $a\text{−}\mathrm{Si}$ surface, respectively, without the propagation functions $A_{L,zzz}$ taken into account. In (e) the linear susceptibility squared as in Fig. 2b is given in the same photon energy range as the SHG spectra.