Direct Observation Of Hole Carrier-Density Profiles And Their Light-Induced Manipulation At The Surface Of Ge

We demonstrate that, by using low-energy positive muon (mu(+)) 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 mu(+) probe spin on the observable muon spin polarization. By tuning the energy of the low energy mu(+) between 1 and 20 keV, the near-surface region between 10 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 this change can be persistent (lambda = 405, 457 nm) or nonpersistent (lambda = 635 nm) at temperatures less than 270 K. This difference is attributed to the different kinetic energies of the photoelectrons. 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 contactless local probe technique offers the possibility of measuring carrier depth profiles and manipulation directly. Our approach may provide important microscopic information on a nanometer scale in semiconductor device studies.