A. S. W. Wong

Effects of AlAs interfacial layer on material and optical properties of GaAs∕Ge(100) epitaxy

GaAs∕AlAs∕Ge(100) samples grown at 650°C with AlAs interfacial layer thickness of 0, 10, 20, and 30nm were characterized using transmission electron microscopy, secondary ion mass spectrometry (SIMS), and photoluminescence (PL) techniques. SIMS results indicate that the presence of an ultrathin AlAs interfacial layer at the GaAs∕Ge interface has dramatically blocked the cross diffusion of Ge, Ga, and As atoms, attributed to the higher Al–As bonding energy. The optical quality of the GaAs epitaxy with a thin AlAs interfacial layer is found to be improved with complete elimination of PL originated from Ge-based complexes, in corroboration with SIMS results.

Interfacial characteristics and band alignments for ZrO2 gate dielectric on Si passivated p-GaAs substrate

The interfacial characteristics and band alignments of high-k ZrO2 on p-GaAs have been investigated by using x-ray photoelectron spectroscopy and electrical measurements. It has been demonstrated that the presence of Si interfacial passivation layer (IPL) improves GaAs metal-oxide-semiconductor device characteristics such as interface state density, accumulation capacitance, and hysteresis. It is also found that Si IPL can reduce interfacial GaAs-oxide formation and increases effective valence-band offset at ZrO2∕p-GaAs interface. The effective valence-band offsets of ZrO2∕p-GaAs and ZrO2∕Si∕p-GaAs interfaces are determined to be 2.7 and 2.84eV, while the effective conduction-band offsets are found to be 1.67 and 1.53eV, respectively.

Photovoltaic characteristics of p-β-FeSi2(Al)/n-Si(100) heterojunction solar cells and the effects of interfacial engineering

Heterojunction solar cells with Al-alloyed polycrystalline p-type β-phase iron disilicide [p-β-FeSi2(Al)] on n-Si(100) were investigated. The p-β-FeSi2(Al) was grown by sputter deposition and rapid-thermal annealing. Photocurrent of ∼1.8 mA/cm2 and open-circuit voltage of ∼63 mV were obtained for p-β-FeSi2(Al)/n-Si(100)/Ti/Al control cells with indium-tin-oxide (ITO) top electrode. Open-circuit voltage increased considerably once thin Al layer was deposited before amorphous-FeSi2(Al) deposition. Furthermore, device performances were found to improve significantly (∼5.3 mA/cm2 and ∼450 mV) by introducing germanium-nitride electron-blocking layer between ITO and p-β-FeSi2(Al). The improvement is attributed to the formation of epitaxial Al-containing p+-Si at p-β-FeSi2(Al)/n-Si(100) interface and suppressed back-diffusion of photogenerated electrons into ITO.

Characterization of sputtered TiO2 gate dielectric on aluminum oxynitride passivated p-GaAs

Structural and electrical characteristics of sputtered TiO2 gate dielectric on p-GaAs substrates have been investigated. It has been demonstrated that the introduction of thin aluminum oxynitride (AlON) layer between TiO2 and p-GaAs improves the interface quality. X-ray photoelectron spectroscopy and transmission electron microscopy results show that the AlON layer effectively suppresses the interfacial oxide formation during thermal treatment. The effective dielectric constant value is 1.5 times higher for the TiO2∕AlON gate stack compared to directly deposited TiO2 on p-GaAs substrates, with a comparable interface state density. The capacitance-voltage (C-V), current-voltage (I-V) characteristics, and charge trapping behavior of the TiO2∕AlON gate stack under constant voltage stressing exhibit an excellent interface quality and high dielectric reliability, making the films suitable for GaAs based complementary metal-oxide-semiconductor technology.

HfOxNy gate dielectric on p-GaAs

Plasma nitridation method is used for nitrogen incorporation in HfO2 based gate dielectrics for future GaAs-based devices. The nitrided HfO2 (HfOxNy) films on p-GaAs improve metal-oxide-semiconductor device characteristics such as interface state density, accumulation capacitance, hysteresis, and leakage current. An equivalent oxide thickness of 3.6 nm and a leakage current density of 10−6 A cm−2 have been achieved at VFB−1 V for nitrided HfO2 films. A nitride interfacial layer (GaAsO:N) was observed at HfO2–GaAs interface, which can reduce the outdiffusion of elemental Ga and As during post-thermal annealing process. Such suppression of outdiffusion led to a substantial enhancement in the overall dielectric properties of the HfO2 film.

Energy-band alignments of HfO2 on p-GaAs substrates

Interfacial reaction and the energy-band alignments of HfO2 films on p-GaAs substrate were investigated by using x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. It has been demonstrated that the alloying of HfO2 with Al2O3 (HfAlO) can significantly reduce native oxides formation and increases the valence-band offsets (VBOs) at HfO2∕p-GaAs interface. In addition, the effects of Si interfacial passivation layer on band alignments have also been studied. VBO at HfO2∕p-GaAs, HfAlO∕p-GaAs, and HfO2∕Si∕p-GaAs interfaces were 2.85, 2.98, and 3.07eV, respectively.

Role of AlxGa1−xAs buffer layer in heterogeneous integration of GaAs/Ge

The material and optical properties of the GaAs/AlxGa1−xAs/Ge structures grown by metalorganic chemical vapor deposition were examined and found to be dependent of the Al content x. SIMS and PL measurements show that the 10 nm AlxGa1−xAs buffer layer with x = 0.3 and 0.6 are equally effective in suppressing the outdiffusion of Ge, whereas x = 1.0 gives the most abrupt interface. The best morphology with surface rms of 0.3 nm is obtained in the structure with x = 0.3 buffer layer. Analysis on change of strain in the AlxGa1−xAs buffer layer suggests that the compressive strain at the AlxGa1−xAs-GaAs interface is compensated by the tensile strain at the Ge-AlxGa1−xAs interface when x ∼ 0.3. AlxGa1−xAs lattice matched to Ge is crucial for better result in surface morphology, but higher Al content is preferred for eliminating the interdiffusion of atoms at the heterointerface.

Understanding the Growth of β-FeSi2 Films for Photovoltaic Applications: A Study Using Transmission Electron Microscopy

The microstructure of β-FeSi 2 films grown on Si using magnetron sputtering has been examined using various electron microscopy techniques. After annealing, the differences in interfacial roughness and grain size with different target materials are investigated using secondary electron and transmission electron microscopy techniques. Here, we study the variation in microstructures with sputtered materials. We observed, for Fe sputtered onto Si followed by rapid thermal anneal, the formation of nanosized FeSi 2 grains (∼120 nm) with a rough surface and film/Si interface. These morphologies and microstructure are very different when FeSi 2 is sputtered onto Si and annealed; instead, the formation of micrometer FeSi 2 grains (∼1 to 5 μm) with sharp surfaces and interfaces is observed. In addition, the effect of oxygen on the growth of FeSi 2 has also been studied. Our results show that oxygen impurities in the films result in the formation of Si x O y nanoparticles in the FeSi 2 matrix upon anneal.

Enhanced ferromagnetic Fe-rich germanide film grown using magnetron sputtering employing a post-deposition anneal

We describe the formation of a ferromagnetic Fe-rich germanide layer by a two-step process involving magnetron sputtering and annealling. A thin epitaxial iron (epi-Fe) film is deposited on the Ge (0 0 1) substrate and then annealed at 275 °C in nitrogen inducing germanide formation. Surprisingly, we observe an enhancement in saturation magnetization in germanide films. Secondary ion mass spectroscopy and x-ray diffraction confirm the formation of a thin Fe-rich germanide layer whilst high resolution transmission electron imaging suggests it to be Fe3Ge.