Aim
Characterization of bulk polarization phenomena such as the bulk photo-voltaic effect (BPVE) in semiconductors requires the preparation of two electrical contacts on the sample, which inevitably introduces additional defects in the semiconductor-metal interface what is especially challenging for ultrawide bandgap semiconductors with high resistivity.
Solution
Surface photovoltage (SPV) spectroscopy using a Kelvin probe is used as a nondestructive, contactless method for characterizing the change of the contact potential difference (DCPD) in carbon doped GaN single crystals. By designing a unique setup capable of measuring DCPD up to ± 200 V, large photovoltages could be detected, without the need to deposit electrical contacts.
Application example
The band gap of GaN is 3.4 eV. As an example, figure 1 shows a spectrum of DCPD for a carbon doped GaN crystal (GaN:C). The corresponding SPV signal reached more than 13 V at 3 eV, i.e., under excitation of certain defect states, SPV signals were much larger than expected from the band gap (for more details see [1]). A change of the direction of DCPD and a signature in the derivative were found near the band gap. Under excitation with a laser diode (445 nm) at higher intensity, the maximum signal was obtained for GaN:C with a carbon concentration of 9×1018 cm-3 and amounted to about 23 V, far exceeding the band gap (figure 2).
