Potential role of hole accumulation via photoionization and electron tunneling during laser-assisted field evaporation of insulators with high energy photons

The laser-assisted field evaporation of oxides by deep UV (257.5 nm wavelength, 4.8 eV photon energy) and UV (355 nm, 3.5 eV) laser light was investigated by measuring the detection rate as a function of laser pulse energy. The experimental approach, where the voltage and hence the electrostatic field are held constant, was performed on the metallic alloy Ni-20Cr as well as chromia and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>SrTiO</a:mi><a:mn>3</a:mn></a:msub></a:mrow></a:math>. The detection rate was recorded as a function of laser pulse energy as well as field estimates. The experiments confirm evaporation being facilitated by a thermal pulse via the laser for the metallic alloy, thereby validating the strategy. For oxides, the relation did not follow an Arrhenius law expected for a temperature pulse. Instead, the measurements closely followed an expression of the form <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>A</b:mi><b:msub><b:mi>x</b:mi><b:mn>1</b:mn></b:msub><b:mo>+</b:mo><b:mi>B</b:mi><b:msup><b:mrow><b:msub><b:mi>x</b:mi><b:mn>2</b:mn></b:msub></b:mrow><b:mn>2</b:mn></b:msup><b:mo form="prefix">exp</b:mo><b:mrow><b:mo>{</b:mo><b:mrow><b:mo>−</b:mo><b:mfrac><b:mi>C</b:mi><b:msub><b:mi>x</b:mi><b:mn>2</b:mn></b:msub></b:mfrac></b:mrow><b:mo>}</b:mo></b:mrow></b:mrow></b:math>, where <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:msub><d:mi>x</d:mi><d:mn>1</d:mn></d:msub><d:mo>=</d:mo><d:msub><d:mi>E</d:mi><d:mi>Laser</d:mi></d:msub></d:mrow></d:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mi>x</e:mi><e:mn>2</e:mn></e:msub><e:mo>=</e:mo><e:mrow><e:mo>(</e:mo><e:mrow><e:msub><e:mi>T</e:mi><e:mn>0</e:mn></e:msub><e:mo>+</e:mo><e:mi>k</e:mi><e:mspace width="0.16em"/><e:msub><e:mi>E</e:mi><e:mi>Laser</e:mi></e:msub></e:mrow><e:mo>)</e:mo></e:mrow></e:mrow></e:math> with <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mi>A</g:mi></g:math>, <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mi>B</h:mi></h:math>, <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mi>C</i:mi></i:math>, and <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mi>k</j:mi></j:math> being constants, <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:msub><k:mi>T</k:mi><k:mn>0</k:mn></k:msub></k:math> the base temperature, and <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:msub><l:mi>E</l:mi><l:mi>Laser</l:mi></l:msub></l:math> the laser pulse energy. Hence, we propose that the evaporation mechanism involves a reduction of the energy barrier through accumulation of charge carrier holes at the apex. Hole generation occurs via photoionization and photoinduced tunneling of electrons. Investigations with a variation of the electrostatic field during evaporation of chromia underline the proposed mechanisms. Previously unexplained laser-assisted field evaporation of MgO, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:msub><m:mi mathvariant="normal">Fe</m:mi><m:mn>3</m:mn></m:msub><m:msub><m:mi mathvariant="normal">O</m:mi><m:mn>4</m:mn></m:msub></m:mrow></m:math>, and GaN in atom probe experiments with UV laser light fits with the new observations. Further theoretical and experimental research is needed to confirm or refute the proposed mechanism.