G. Lüpke

Characterization of semiconductor interfaces by second-harmonic generation

Abstract In recent years, optical second-harmonic generation has matured into a versatile and powerful technique for probing the electronic and structural properties of buried solid–solid interfaces. In particular Si/SiO2 interfaces have been studied extensively and significant progress has been made in understanding the sensitivity of second-harmonic generation to interface defects, steps, strain, roughness, electric fields, carrier dynamics, and chemical modifications. The macroscopic and microscopic aspects of the second-order nonlinear optical effects at buried semiconductor interfaces have been treated theoretically by first-principles calculations and semi-empirical models. It is the aim of this review article to present a comprehensive overview of recent accomplishments, current understandings and future directions in this field.

Vibrational lifetimes of hydrogen in silicon

Characterization of defect and impurity reactions, dissociation, and migration in semiconductors requires a detailed understanding of the rates and pathways of vibrational energy flow, of the energy transfer channels, and of the coupling mechanisms between local modes and the phonon bath of the host material. Significant progress in reaching this goal has been accomplished in recent landmark studies exploring the excitation and dynamics of vibrational states associated with hydrogen in silicon. The lifetime of the Si–H stretch mode is found to be extremely dependent on the local solid-state structure, ranging from picoseconds for interstitial-like hydrogen, hundreds of picoseconds for hydrogen–vacancy complexes, to several nanoseconds for hydrogen bonded to Si surfaces—over three orders of magnitude variation. Such large variations in lifetime (transition probability) are extraordinarily rare in solid-state science. The level of theoretical investigation into the vibrational lifetime of the Si–H oscillato...

Semiconductor point defect concentration profiles measured using coherent acoustic phonon waves

Coherent acoustic phonon interferometry is used to quantitatively measure depth-dependent point defect concentrations in semiconductor systems with a depth range of the order of tens of microns. Using time-resolved pump-probe techniques, the optical response of ion-beam irradiated GaAs crystals is analyzed as a function of defect concentration ranging over four orders of magnitude. Varying the ion dose quantitatively relates changes in the optical response to local defect concentrations. Thermal annealing is shown to reduce the effect on the optical response, indicating recovery of the crystal lattice through self-interstitial-vacancy recombination.

Ultrafast magnetization dynamics of epitaxial Fe films on AlGaAs (001)

Uniform magnetization precessions are generated by ultrafast optical excitation along the in-plane easy axis [100], as well as along the hard axis [1-10], in epitaxial Fe films grown on AlGaAs (001) over a wide range of applied magnetic fields. From the temporal evolution of the coherent magnetization precession, we determine the magnetic anisotropy constants and damping parameters which are crucial in designing fast magnetic switching devices and spintronics devices.

Charge control of antiferromagnetism at PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 interface

The interfacial spin state of the multiferroic heterostructure PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 and its dependence on ferroelectric polarization is investigated with magnetic second-harmonic generation at 78 K. The spin alignment of Mn ions in the first unit cell layer at the heterointerface can be tuned from ferromagnetic to antiferromagnetic exchange coupled, while the bulk magnetization remains unchanged. Multiple domains of both phases coexist as the ferroelectric polarization is switched. The results will help promote the development of new interface-based functionalities and device concepts.

Characterization of boron charge traps at the interface of Si/SiO2 using second harmonic generation

We report results from optical second harmonic generation studies of boron charge traps near the interface of Si/SiO2. Our data suggest that a static electric field at the interface is formed during the oxide growth process due to the presence of negative boron ions (B−) in the silicon substrate and positive boron ions (B+) in the oxide. We demonstrated that the B+ state traps could be filled through the creation of neutral boron states created by internal photoelectron emission. By fitting our data, we found that the effective interface susceptibility |χ(2)| depends on doping concentration.

Control of magnetic contrast with nonlinear magneto-plasmonics

The interaction between surface plasmons (SP) and magnetic behavior has generated great research interest due to its potential for future magneto-optical devices with ultra-high sensitivity and ultra-fast switching. Here we combine two surface sensitive effects: magnetic second-harmonic generation (MSHG) and SP to enhance the detection sensitivity of the surface magnetization in a single-crystal iron film. We show that the MSHG signal can be significantly enhanced by SP in an attenuated total reflection (ATR) condition, and that the magnetic contrast can be varied over a wide range by the angle-of-incidence. Furthermore, the magnetic contrast of transverse and longitudinal MSHG display opposite trends, which originates from the change of relative phase between MSHG components. This new effect enhances the sensing of magnetic switching, which has potential usage in quaternary magnetic storage systems and bio-chemical sensors due to its very high surface sensitivity and simple structure.

Time-resolved magneto-optical imaging of Y1Ba2Cu3O7−δ thin films in high-frequency AC current regime

We present a time-resolved magneto-optical (MO) imaging study of high-temperature superconductor (HTS) in a high-frequency alternating current (AC) regime. The evolution of the magnetic flux density distribution in YBa2Cu3O7−δ (YBCO) thin film samples is studied as a function of the phase of the applied AC current. A quantitative analysis of the data shows that the maxima of the AC current density is shifted from the edges further inside the sample, which may be caused by the higher self-induced field in that region. This technique can be used to study magnetic flux evolution in HTS films and coated conductors in the high-frequency current regime.

Spin-orbit interaction tuning of perpendicular magnetic anisotropy in L10 FePdPt films

The dependence of perpendicular magnetic anisotropy Ku on spin-orbit coupling strength ξ is investigated in L10 ordered FePd1−xPtx films by time-resolved magneto-optical Kerr effect measurements and ab initio density functional calculations. Continuous tuning of Ku over a wide range of magnitude is realized by changing the Pt/Pd concentration ratio, which strongly modifies ξ but keeps other leading parameters affecting Ku nearly unchanged. Ab initio calculations predict a nearly quadratic dependence of Ku on ξ, consistent with experimental data. Ku increases with increasing chemical order and decreasing thermal spin fluctuations, which becomes more significant for samples with higher Pt concentration. The results demonstrate an effective method to tune Ku utilizing its sensitivity on ξ, which will help fabricate magnetic systems with desirable magnetic anisotropy.

Surface plasmon-enhanced transverse magnetic second-harmonic generation

We present experimental studies on surface plasmon (SP) enhanced transverse magnetic second-harmonic generation (T-MSHG) in single-crystal iron films grown by molecular beam epitaxy at room temperature on MgO (001) substrates. We show that it is possible to achieve both strongly enhanced T-MSHG intensity and high magnetic contrast ratio under attenuated total reflection configuration without using complex heterostructures because MSHG is generated directly at the iron surface where SPs are present. The T-MSHG has a much larger contrast ratio than transverse magneto-optical Kerr effect (T-MOKE) and shows great potential for a new generation of bio-chemical sensors due to its very high surface sensitivity. In addition, by analyzing the experimental results and the simulations based on SP field-enhancement theory, we demonstrate that the second-order susceptibility of MSHG shows great anisotropy and the tensor χ(xzz)(odd) is dominant in our sample.