Contact-free investigation of the EL2-defect in the surface of GaAs wafers

The EL2 as the most important intrinsic defect in semi-insulating GaAs was already deeply studied in the past. All investigation methods applied so far to semi-insulating GaAs explore the entire depth of the sample and are therefore not suitable to analyse e.g. influences of surface treatments. In this work the detection of EL2 in the surface of samples by the application of microwave detected photo induced current transient spectroscopy will be presented. A correlation between height and sign of the signals and the concentration of acceptors, furthermore of the compensation ratio of the EL2, the position of the Fermi level as well as the specific resistivity was found. The results can be simulated by solving rate equations for the measurement process

Oxygen in GaAs and its relation to the EL3 defect investigated by TSC and PICTS

Thermally stimulated current (TSC) and photo induced current transient spectroscopy (PICTS) were used to investigate deep traps in oxygen-rich medium resistivity GaAs. We observed an abnormal behaviour of the TSC signal at a temperature of about 200 K, where the TSC current is lower than the dark current. This results in a negative peak occurring in the temperature range where the signal of the well known EL3 defect in undoped semi-insulating GaAs is present. For verification, comparative PICTS-measurements were performed, which exhibit the same negative peak at similar temperatures. The activation energy for this negative peak was determined to be ~ 0.6 eV (TSC). This is in good agreement with published values for the EL3 ranging from 0.56 to 0.68 eV. Furthermore, we illustrate that the depth of the peak increases with increasing oxygen content. Our results support the assumption, that oxygen on an arsenic site – the so called off centre oxygen oc-OAs – should be the origin of the EL3

Two-dimensional imaging of O2, H2O2, and glucose distributions by an array of 400 individually addressable microelectrodes

A monolithic array of 400 individually addressable microelectrodes was fabricated in a modified CMOS process. The platinum electrodes, each 36 µm x 36 µm in size, were arranged in a square matrix of overall area 1 cm². Individual addressing of each electrode in the array was realized by two on-chip shift registers coupled with array control logic. In the electrochemical scanning mode, the 400 electrode signals were read successively. The array was used in supporting electrolyte solutions for imaging of oxygen distributions detected amperometrically at -0.8 V vs Ag/AgCl. Hydrogen peroxide and glucose distributions were imaged amperometrically at +0.8 V vs Ag/AgCl. Prior to imaging glucose distributions, the enzyme glucose oxidase was entrapped into the conducting polymer polypyrrole siumultaneously on all array electrodes. Two minutes was needed for a successive readout of all 400 microelectrode signals