Ruben Lieten

Tensile strained GeSn on Si by solid phase epitaxy

We demonstrate single crystalline GeSn with tensile strain on silicon substrates. Amorphous GeSn layers are obtained by limiting the adatom surface mobility during deposition. Subsequent annealing transforms the amorphous layer into single crystalline GeSn by solid phase epitaxy. Excellent structural quality is demonstrated for layers with up to 6.1% of Sn. The GeSn layers show tensile strain (up to +0.34%), which lowers the difference between direct and indirect band transition and makes this method promising for obtaining direct band gap group IV layers. GeSn with 4.5% Sn shows increased optical absorption compared to Ge and an optical band gap of 0.52 eV.

Tensile-Strained GeSn Metal–Oxide–Semiconductor Field-Effect Transistor Devices on Si(111) Using Solid Phase Epitaxy

We demonstrate tensile-strained GeSn metal–oxide–semiconductor field-effect transistor (MOSFET) devices on Si(111) substrates using solid phase epitaxy of amorphous GeSn layers. Amorphous GeSn layers are obtained by limiting the adatom surface mobility during deposition. Subsequent annealing transforms the amorphous layer into single-crystalline GeSn by solid phase epitaxy. Single-crystalline GeSn layers with 4.5% Sn and 0.33% tensile strain are fabricated on Si(111) substrates. To verify the structural quality of thin-film GeSn as a channel material, we fabricate ultrathin GeSn p-channel MOSFETs (pMOSFETs) on Si(111). We demonstrate junctionless depletion-mode operation of tensile-strained GeSn(111) pMOSFETs on Si substrates.

Solid phase epitaxy of amorphous Ge on Si in N2 atmosphere

We demonstrate a straightforward and economical way to obtain smooth germanium layers of high quality on silicon. Thin amorphous germanium layers deposited by plasma enhanced chemical vapor deposition on Si(111) substrates are transformed into single crystalline and smooth layers by solid phase epitaxy in N2 atmosphere. The crystal orientation of the substrate has a clear influence on the crystal quality. This is most likely due to a different growth mode, namely, layer-by-layer for Si(111) and three-dimensional growth for Si(001). The amorphous germanium layer can roughen during annealing due to mobile atoms on the surface. This can be effectively suppressed by annealing in N2 ambient. Electrical measurements show high charge mobility.

Photoluminescence Studies of Polarization Effects in InGaN/(In)GaN Multiple Quantum Well Structures

The impact of polarization fields in multiple quantum well (MQW) structures is revealed by photoluminescence measurements and band diagram calculations. We observe a blue shift of luminous energy of 33 meV and an increased light emission of 19% for InxGa1-xN/InyGa1-yN MQWs with respect to InxGa1-xN/GaN MQWs. Band diagram calculations show a lowering of the polarization fields and an increase in wave function overlap of 22% by adding indium into the barriers. We therefore attribute the observed blue shift and increased emission to an improved electron and hole wave function overlap due to lower electric fields in InxGa1-xN/InyGa1-yN structures.

Strain relaxation in GaN nanopillars

In this work, we demonstrate the direct measurement of the strain state at the surface of nanostructures by in-plane X-ray diffraction. GaN tapered nanopillars have been fabricated by dry etching of a highly strained epilayer. The strain of the surface as function of pillar height shows an exponential relaxation which can be described by a single relaxation parameter. Additionally, we have simulated the strain relaxation and distribution of nanopillars. The impact of the pillar geometry on the strain relaxation has been discussed. In agreement with the measurements, an exponential relaxation of the strain is observed.

Mg doping of GaN by molecular beam epitaxy

We present a systematic study on the influence of growth conditions on the incorporation and activation of Mg in GaN layers grown by plasma-assisted molecular beam epitaxy. We show that high quality p-type GaN layers can be obtained on GaN-on-silicon templates. The Mg incorporation and the electrical properties have been investigated as a function of growth temperature, Ga?:?N flux ratio and Mg?:?Ga flux ratio. It was found that the incorporation of Mg and the electrical properties are highly sensitive to the Ga?:?N flux ratio. The highest hole mobility and lowest resistivity were achieved for slightly Ga-rich conditions. In addition to an optimal Ga?:?N ratio, an optimum Mg?:?Ga flux ratio was also observed at around 1%. We observed a clear Mg flux window for p-type doping of GaN?: 0.31% < Mg?:?Ga < 5.0%. A lowest resistivity of 0.98???cm was obtained for optimized growth conditions. The p-type GaN layer then showed a hole concentration of 4.3 ? 1017?cm?3 and a mobility of 15?cm2?V?1?s?1. Temperature-dependent Hall effect measurements indicate an acceptor depth in these samples of 100?meV for a hole concentration of 5.5 ? 1017?cm?3. The corresponding Mg concentration is 5 ? 1019?cm?3, indicating approximately 1% activation at room temperature. In addition to continuous growth of Mg-doped GaN layers we also investigated different modulated growth procedures. We show that a modulated growth procedure has only limited influence on Mg doping at a growth temperature of 800??C or higher. This result is thus in contrast to previously reported GaN?:?Mg doping at much lower growth temperatures of 500??C.

Evidence of the metal-insulator transition in ultrathin unstrained V2O3 thin films

We report the strain state and transport properties of V2O3 layers and V2O3/Cr2O3 bilayers deposited by molecular beam epitaxy on (0001)-Al2O3. By changing the layer on top of which V2O3 is grown, we change the lattice parameters of ultrathin V2O3 films significantly. We find that the metal-insulator transition is strongly attenuated in ultrathin V2O3 layers grown coherently on Al2O3. This is in contrast with ultrathin V2O3 layers grown on Cr2O3 buffer layers, where the metal-insulator transition is preserved. Our results provide evidence that the existence of the transition in ultrathin films is closely linked with the lattice deformation.

Reversible phase changes in Ge–Au nanoparticles

We demonstrate a reversible phase transition in nanoparticles composed of a binary eutectic alloy, Ge–Au. The structure, 9 nm diameter nanoparticles embedded in silica, can be switched from bilobe to mixed using a 30 ns ultraviolet laser pulse. The structure can be switched back to bilobe by heating at 80 °C. The bilobe/mixed switching can be performed on the same sample at least ten times. Synchrotron X-ray diffraction studies reveal that the bilobe structure contains crystalline Ge and Au while the mixed structure consists of crystalline Ge and β Ge–Au.

Hydrogen and inert species in solid phase epitaxy

The incorporation of hydrogen during deposition of amorphous germanium can influence solid phase epitaxy in many ways. We show that Ge–H bonds are not important during the crystallization process. However, atomic hydrogen is important during deposition to obtain a highly disordered layer. We have found that highly disordered layers can also be obtained when using a beam of inert gas species during ultrahigh vacuum deposition. These inert species effectively increase the disorder of the layer by limiting the surface mobility of adsorbed germanium atoms. In this way subsequent solid phase epitaxy can be improved significantly.

Misoriented domains in (0001)-GaN/(111)-Ge grown by molecular beam epitaxy

Structural characterization has been performed on (0001)-GaN epilayers grown on (111)-Ge substrates using plasma assisted molecular beam epitaxy. By combining high-resolution x-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy, it has been shown that the GaN epilayer consists of misoriented domains. The domains are rotated about the GaN-[0001] (Ge-[111]) zone axis by 8° with respect to each other and by ±4° with respect to the Ge substrate. These domains need to be eliminated to reduce grain boundary defects and improve GaN crystal quality.