Direct Observation of Hole Carrier-Density Profiles and Their Light-Induced Manipulation at the Surface ofGe

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 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 λ, this change can be persistent (λ = 405, 457 nm) or nonpersistent (λ = 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.

Muon implantation experiments in films: Obtaining depth-resolved information

“…This latter approach has the important advantage of being model independent, but still requires a previous knowledge of the functional relation between the measured µSR parameter and the underlying physical parameter that causes the effect. Another example is the recent determination of the hole carrier concentration profile at the surface of Ge, where a linear relation between the muon depolarization rate and the hole concentration was assumed 8 . A functional relation between a µSR parameter and the underlying physical parameter is not known in advance if the observed effect is a change of the amplitude of the µSR signal with the muon implantation energy, as often observed in semiconductors and insulators.…”

Muon Interaction with Negative- U and High-Spin-State Defects: Differentiating Between C and Si

“…Although probing depths in the submicron range are technically possible, most studies on SiC focus on defects deep in the bulk [17][18][19]. Low-energy μSR (LE-μSR) has been successfully used for the investigation of defect profiles near the surface [20,21] and has also been deployed to study band bending and charge-carrier profiles as well as their manipulation in semiconductors [22,23]. Recently, LE-μSR has also been demonstrated to detect carbon vacancies in the first 120 nm of irradiated n-type 4H -SiC [24].…”

Depth-Resolved Study of the SiO₂- SiC Interface Using Low-Energy Muon Spin Rotation Spectroscopy

“…A sharp reduction in phase was recorded, see Fig. 4 d), indicating that the muon transformed to the diamagnetic state from a neutral precursor state [11]. The changes in muon response as a function of annealing conditions have two potential causes: (i) differences in defect distribution before and after POA, and (ii) differences in carrier concentration.…”

“…The initial phases φ D and φ Mu of the diamagnetic and paramagnetic µSR signals are determined by the position of a specific positron detector with respect to the initial muon spin direction. If the diamagnetic state forms from a neutral precursor state -where the muon spin moves in the opposite direction to the Larmor precession -a negative phase shift of φ D with respect to the muon Larmor precession can be observed for the diamagnetic signal, if the transition rate to the diamagnetic state is on the order of hundreds of MHz [3,11]. The longer the muon stays in the precursor Mu 0 state, the more negative the phase becomes.…”

“…µSR also finds special applications in the field of semiconductors, where muons can replicate the behaviour of hydrogen related defects which otherwise are challenging to probe [2][3][4]. A recent study with low energy (LE)-µSR on germanium * kumar@aps.ee.ethz.ch showed that charge carrier profiles can be determined in the accumulation and depletion regions near the surface [11], whereas in silicon carbide (SiC), the sensitivity of the µSR signal to defects created by proton irradiation has been demonstrated [12,13]. The sensitivity of µSR to important semiconductor parameters combined with the nano-scale resolution of the technique mark its low energy mode as an intriguing choice for investigating device related interface and surface regions.…”