Direct observation of hole carrier density profiles and their light induced manipulation at the surface of Ge

Abstract

We demonstrate that, by using low-energy positive muon (µ + ) spin spectroscopy as a local probe technique, the profiles of free charge carriers can be directly determined in the accumulation/depletion surface regions of p-or n-type Ge wafers. The detection of free holes is accomplished by measuring the effect of the interaction of the free carriers with the µ + probe spin on the observable muon spin polarization. By tuning the energy of the low-energy µ + between 1 keV and 20 keV the near-surface region between 10 nm and 160 nm is probed. We find hole carrier depletion and electron accumulation in all samples with doping concentrations up to the 10 17 /cm 3 range, which is opposite to the properties of cleaved Ge surfaces. By illumination with light the hole carrier density in the depletion zone can be manipulated in a controlled way. Depending on the used light wavelength λ this change can be persistent (λ = 405, 457 nm) or non-persistent (λ = 635 nm) at temperatures < 270 K. This difference is attributed to the different kinetic energies of the photo-electrons. Photoelectrons generated by red light do not have sufficient energy to overcome a potential barrier at the surface to be trapped in empty surface acceptor states. Compared to standard macroscopic transport measurements our contact-less local probe technique offers the possibility of measuring carrier depth profiles and manipulation directly. This new approach may provide important microscopic information on a nanometer scale in semiconductor device studies.