M. Kammler

Femtosecond X-ray measurement of coherent lattice vibrations near the Lindemann stability limit

The study of phase-transition dynamics in solids beyond a time-averaged kinetic description requires direct measurement of the changes in the atomic configuration along the physical pathways leading to the new phase. The timescale of interest is in the range 10-14 to 10-12 s. Until recently, only optical techniques were capable of providing adequate time resolution, albeit with indirect sensitivity to structural arrangement. Ultrafast laser-induced changes of long-range order have recently been directly established for some materials using time-resolved X-ray diffraction. However, the measurement of the atomic displacements within the unit cell, as well as their relationship with the stability limit of a structural phase, has to date remained obscure. Here we report time-resolved X-ray diffraction measurements of the coherent atomic displacement of the lattice atoms in photoexcited bismuth close to a phase transition. Excitation of large-amplitude coherent optical phonons gives rise to a periodic modulation of the X-ray diffraction efficiency. Stronger excitation corresponding to atomic displacements exceeding 10 per cent of the nearest-neighbour distance—near the Lindemann limit—leads to a subsequent loss of long-range order, which is most probably due to melting of the material.

Lateral control of self-assembled island nucleation by focused-ion-beam micropatterning

We demonstrate that the nucleation sites of nanoscale, self-assembled Ge islands on Si(001) can be controlled by patterning the Si surface in situ with a focused ion beam. At low doses of 6000 Ga+ ions per <100 nm spot, the selective growth is achieved without modifying the initial surface topography. At larger doses, topographic effects produced by sputtering and redeposition control the selective nucleation sites. Islands grown on irradiated spots are smaller with higher aspect ratio than islands grown on clean Si(001), suggesting a strong surfactant effect of Ga.

Enhanced Sb segregation in surfactant-mediated-heteroepitaxy: High-mobility, low-doped Ge on Si

Surfactant-mediated epitaxy (SME) allows the growth of smooth, continuous, relaxed, and principally defect free Ge films directly on Si(111); however, the very high surfactant doping level in the range of the solid solubility limit made them unacceptable for most device applications. By using high temperature SME we have reduced the Sb surfactant background doping level by more than three orders of magnitude. This is attributed to an enhanced surfactant segregation without kinetic limitations. The low Sb incorporation has been determined by an electrical characterization: An electron concentration of 1.1×1016 cm−3 and a very high electron Hall mobility of 3100 cm2/V s at 300 K (12 300 cm2/V s at 77 K) suggest an interesting potential of SME grown Ge films for future device applications.

Ultra-fast electron diffraction at surfaces: From nanoscale heat transport to driven phase transitions

Many fundamental processes of structural changes at surfaces occur on a pico- or femtosecond time scale. In order to study such ultra-fast processes, we have combined modern surface science techniques with fs-laser pulses in a pump-probe scheme. Reflection high energy electron diffraction (RHEED) with grazing incident electrons ensures surface sensitivity for the probing electron pulses. Utilizing the Debye-Waller effect, we studied the cooling of vibrational excitations in monolayer adsorbate systems or the nanoscale heat transport from an ultra-thin film through a hetero-interface on the lower ps-time scale. The relaxation dynamics of a driven phase transition far away from thermal equilibrium is demonstrated with the In-induced (8×2) reconstruction on Si(111). This surface exhibits a Peierls-like phase transition at 100K from a (8×2) ground state to (4×1) excited state. Upon excitation by a fs-laser pulse, this structural phase transition is driven into an excited (4×1) state at a sample temperature of 20K. Relaxation into the (8×2) ground state occurs after more than 150 ps.

Surfactant-mediated epitaxy of Ge on Si(111): Beyond the surface

For a characterization of interface and “bulk” properties of Ge films grown on Si(111) by Sb surfactant-mediated epitaxy, grazing incidence x-ray diffraction and transmission electron microscopy have been used. The interface roughness, defect structure, and strain state have been investigated in dependence of film thickness and growth temperature. For all growth parameters, atomically smooth interfaces are observed. For thin Ge layers, about 75% of the strain induced by the lattice mismatch is relaxed by misfit dislocations at the Ge∕Si interface. Only a slight increase of the degree of relaxation is found for thicker films. At growth temperatures below about 600°C, the formation of twins is observed, which can be avoided at higher temperatures.

Surfactant-grown low-doped germanium layers on silicon with high electron mobilities

Abstract We present the first investigation of the electrical properties (electron Hall mobilities and concentrations) of 1-μm thick relaxed Ge layers grown by surfactant-mediated epitaxy (SME) with the surfactant Sb on Si(111) substrates at growth temperatures between 640°C and 720°C. We found that with rising growth temperatures the Ge layer quality improves as is demonstrated by the mobility increases at 300 K and 77 K. A record high electron mobility of 3100 cm2/Vs at 300 K (12 300 cm2/Vs at 77 K) which is close to the Ge lattice mobility was measured in the 720°C layer together with an electron concentration of 1.1×1016 cm−3. This was derived from a differential Hall analysis and is identical to an interpolation value obtained from a comparison with high-quality bulk Ge mobility data. A secondary ion mass spectroscopy (SIMS) analysis yielded a concentration below the detection limit of 2×1017 cm−3 Sb. Our results demonstrate an Sb background doping reduction of three orders of magnitude in SME-grown Ge/Si layers compared to earlier reports. We interpret this as being due to an enhanced surfactant segregation without kinetic limitations at high growth temperatures. The high quality, low doping and high mobility of these Ge layers suggests an exciting potential for future device applications.

Controlled nucleation of dislocations by a spatially localized stress field

We analyze the nucleation of dislocations in silicon at spatially localized stress fields generated by silicon nitride pads having a high intrinsic stress. The nucleation and final configuration of the dislocations were studied using hot-stage transmission electron microscopy and were compared with dislocation simulations based on calculations of the stress fields around the pads. We find that the simulated configurations match well with the experimental data, and we show that the dislocation configuration can be controlled by the pad size.

In-situ observations of self-assembled island nucleation on patterned substrates

We compare in-situ electron microscopy observations of Ge island growth on Si surfaces modulated by different techniques. Si(001) substrates were patterned either using a focused Ga ion beam or lithographically to produce similar feature sizes. In the case of the focused-ion-beam patterned substrates, chemical or strain effects caused by low Ga doses control the positions at which islands nucleate. Nucleation on topographically modulated substrates, with the pattern produced either by high-dose sputtering or lithographically, is controlled by the nature of the side walls of topographic features. The two patterning approaches have different benefits in creating arrays of islands for device applications.

Bi surfactant mediated epitaxy of Ge on Si(111)

Abstract We have tested Bi for the surfactant mediated epitaxy of Ge on Si(111). Islanding of Ge is prevented and a 2D layer growth of smooth and continuous Ge films is observed. The lattice mismatch is accommodated by a periodic array of dislocations confined to the Si/Ge interface. The large covalent radius of Bi reduces the binding energy, allowing very efficient segregation and low doping levels even at low growth temperatures. Unfortunately, this results also in a high Bi desorption flux limiting the possible growth temperatures below 600 °C. Consequently the Ge films show a high defect density in the order of 10 8 cm −2 for stacking faults and 10 9 cm −2 for dislocations which limit electron Hall mobility to values below 700 cm 2 /V s at room temperature.

Surfactant-mediated epitaxy of Ge on Si: progress in growth and electrical characterization

Abstract Smooth, continuous, relaxed, and high quality Ge films have been grown on Si(111) using surfactant-mediated epitaxy (SME). Using high temperature SME we have reduced the Sb surfactant doping level by more than three orders of magnitude below the solid solubility. This enhanced surfactant segregation is attributed to the formation of an ordered (2×1)-Sb reconstruction on the Ge(111) growthfront. With increasing growth temperature the Sb incorporation decreases to 1×10 16 cm −3 at 700°C. This low Sb doping concentration has been determined by electrical characterization. The electron Hall mobility varies strongly with the doping concentration. Record values of 3159 cm 2 /Vs at 300 K suggest interesting potential of SME grown Ge films for future device applications. Capacitance voltage measurements on vertical p + n diodes show a uniform doping profile and are in good agreement with Hall measurements.