Photonic studies of defects and amorphization in ion beam damaged GaAs surfaces
In the present investigation, a comprehensive photonic characterization and analysis of low energy Ar⁺ ion beam processed GaAs surfaces is presented. The purpose of this investigation was to evaluate the damage and amorphization introduced at the surface and sub-surface regions by ion bombardment. Ar⁺ ion beam etching was selected in order to rule out the possibility of producing any additional effects at the interface due to chemical reactions in the case of reactive ion etching.
After a brief review of the concepts and underlying physics, several photonic structures are introduced. The basic theory governing the photovoltaic devices and photoconductive samples is discussed. The preparation and characterization techniques of ion beam processed GaAs samples are described. An automated photovoltaic materials and devices (PVMD) system was developed. Asyst, a Forth based scientific software was selected to write the source codes for data acquisition and reduction. The inherent fast execution times of the software allows data acquisition in real time, ensuring the quasi-steady state condition. The electrical and optical evaluation procedures developed and employed for the present investigation are discussed.
One of the striking features of the ion beam bombardment on semi-insulating (SI) GaAs samples was the observation of persistent photoconductivity. A phenomenological model for optically generated ion beam induced metastable defect state formation was proposed to explain the persistent photoconductivity. Presence of two or more exponential curves in the relaxation mode indicates the distributed nature of the traps within the band gap. A conjectural flat-band energy diagram was introduced to elucidate the proposed model. The observed dark and photoconductivity response model was based on the distributed lumped electrical components analysis. Fundamental transport equations were employed in the analysis of the lumped electrical components model.
Metal-Insulator-Semiconductor (MIS) type Schottky barrier diodes and photodiodes were fabricated employing both thermal and anodic oxides. Diode parameters were evaluated as a function of ion-beam energy. An increase in reverse saturation current density accompanied by an increase in the ideality factor was observed, indicating the presence of trap-assisted tunneling and a region of high recombination. The effective barrier height was generally lowered; however, no monotonic correlation with the ion energy was observed. It is proposed that the mechanisms described in previous studies (e.g. tunneling, stoichiometry effects, ion penetration depth) were dominated by the effect of Fermi level pinning at the electronic states of process-induced defects. Deep level transient spectroscopy (DLTS) indicated the presence of at least two distinct deep trap levels, at 0.32 eV and at 0.52 eV below the conduction band edge, as a consequence of ion beam etching. The EL2 peak was evident in the virgin sample and vanished in the ion beam etched samples and such observation is in agreement with our proposed model. The photovoltaic response was characterized using illuminated current-voltage (I-V) and spectral response measurements. The ratio of external quantum efficiencies of IBE devices to unetched device indicates the regions and relative extent of the damage. Since the damage has a impact on the band-bending due to excess carrier generation, the sub-bandgap photon absorption response reveals the degree of disorder. XPS results indicated an increased surface sensitivity and change in Ga/As ratio as a function of ion beam energy.
The modelling of ion-beam-processed samples was considered and several computer programs which simulate their operation are described. The depth of amorphization was calculated using the Lindhard-Scharff-SchiΦtt (LSS) theory and the standard projected range and straggle parameters, and experimental parameters. A large difference was observed in the values calculated using LSS theory and experimentally measured values, using optical probes. The difference was explained in light of the Collision-Cascade model.