The dynamical process of surface band bending induced by ion bombardment as a function of ion fluence and energy has been studied with a special low energy ion beam system and an x-ray photoelectron spectrometer. It was found that 10 and 100 eV Ar+ bombardment of n- and p-InP (110) samples, which were prepared in ultrahigh vacuum by in situ cleavage of InP (100) bars, moved their surface Fermi levels to a common pinning position 0.95 eV above the valence-band maximum. The observed surface band bending was attributed to the displacement damage in the near-surface region induced by the ion bombardment. A quantitative analysis of the band bending data as a function of ion fluence showed that for the 10 eV Ar+ bombardment the formation of ionized surface states at a density of about 1×1012/cm2 in the band gap of InP was induced by an ion fluence of 5×1016/cm2. As expected, an increase of bombardment energy from 10 to 100 eV greatly raised the formation probability of surface defects. The results imply that very brief plasma exposure is sufficient to cause surface damage to a semiconductor that is severe enough to affect the device performance of the semiconductor.
The capacitively coupled Ar plasma containing oxygen, driven by a radio frequency of 27.12 MHz, is investigated by laser-induced photo-detachment technique assisted with a Langmuir probe. The plasmas with different amounts of oxygen are obtained by changing the flow of Ar and oxygen, each of which is controlled by a mass flow controller. The axial distribution of plasma characteristic can be measured by changing the relative axial position of the Langmuir probe between the parallel electrodes. The electron density and electron temperature are calculated from the current-voltage curve measured by the scanning Langmuir probe, and the electronegativity is obtained from the current curves of the probe with the laser-induced photo-detachment technique. The negative ion density can be calculated from the electron density and the electronegativity. It is shown that with oxygen flow rate increasing, the dissociative attachment of oxygen molecules with electrons will consume the electrons with the middle energy in the electron energy probability function (EEPF) measured by Langmuir probe. The EEPF evolves from Druyvesteyn to Maxwellian distribution due to the thermalization by the e-e interaction with applied power increasing. It is worth mentioning that a depression in the EEPF curve will appear when discharging high-pressure Ar gas containing oxygen. This depression can also be caused by the dissociative attachment of oxygen molecules with electrons where the threshold energy is around 4.5 eV. The axial profile of the electron density is calculated from the EEPF changing from a linear rise in pure Ar plasma to a flater phase of the distribution due to the negative ions such as oxygen introduced into the plasma. The electron temperature changes a little at different axial positions. The rise of negative ion density nearby the sheath of powered electrode is due to the dissociative attachment caused by the collision of oxygen molecules with energetic electrons. In addition, the axial distribution of electronegativity takes on a shape of spoon, which results from the consequence of generation and loss of negative ions in the process of the ambipolar-electric-field-driven diffusion to the plasma center.
Capacitively coupled O2-containing Ar plasma driven by a radio frequency (RF) of 27.12 MHz has been investigated. The electron energy probability function (EEPF) was measured with a Langmuir probe. The electronegativity was measured with a laser-induced photodetachment (LIPD) technique in combination with a Langmuir probe. The probe measurement results show a transition of the EEPF from bi-Maxwellian to single-Maxwellian and finally to a Druyvesteyn distribution as RF input power or discharge pressure was increased. This transition indicates the evolution of the heating mode in the Ar plasma by changing the discharge conditions. Adding electronegative O2 gas into Ar plasma leads to the deviation of the EEPF from the pure Ar plasma case. This deviation becomes more serious at high pressure due to the inelastic collisions of electrons with oxygen molecules. Additionally, the addition of O2 not only lowers the electron density in the axial direction but also smoothens the electron density distribution close to the powered electrode in comparison to the linear electron density with the axis in the Ar plasma case. LIPD measurement results show that electronegativity in 5% O2-containing Ar plasma tends to be high as close to the powered electrode and to be a V-shaped distribution along the axis direction with the increase in the pressure. This behavior of the negative ion distribution may be caused by the combined effects of recombination of negative and positive ions and the pseudo-γ mode of negative ions with oxygen neutrals.
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