R. Kúdela

Aluminum oxide as passivation and gate insulator in GaAs-based field-effect transistors prepared in situ by metal-organic vapor deposition

Application of GaAs-based metal-oxide-semiconductor (MOS) structures, as a “high carrier mobility” alternative to conventional Si MOS transistors, is still hindered due to difficulties in their preparation with low surface/interface defect states. Here, aluminum oxide as a passivation and gate insulator was formed by room temperature oxidation of a thin Al layer prepared in situ by metal-organic chemical vapor deposition. The GaAs-based MOS structures yielded two-times higher sheet charge density and saturation drain current, i.e., up to 4 × 1012 cm−2 and 480 mA/mm, respectively, than the counterparts without an oxide surface layer. The highest electron mobility in transistor channel was found to be 6050 cm2/V s. Capacitance measurements, performed in the range from 1 kHz to 1 MHz, showed their negligible frequency dispersion. All these results indicate an efficient suppression of the defect states by in situ preparation of the semiconductor structure and aluminum oxide used as a passivation and gate insu...

Fabrication of a vector Hall sensor for magnetic microscopy

We have developed a micromachined Hall sensor for scanning the entire magnetic field vector whose active dimensions are an order of magnitude smaller (∼5 μm) than the smallest existing vector field sensor. It is realized by patterning three Hall probes on the tilted faces of epitaxy-overgrown GaAs-based pyramidal-shaped mesa structures. Data from these “tilted” Hall probes are used to reconstruct the full magnetic field vector.

Study of narrow InGaP/(In)GaAs quantum wells

Narrow InGaP/GaAs/InGaP and InGaP/InGaAs/InGaP quantum wells (QWs) prepared by MOVPE were studied by photoluminescence and HRXRD in this work. A detailed analysis of the results has shown the presence of band gap fluctuations at the interfaces in the InGaP/(In)GaAs/InGaP system. A series of experiments with various thickness of QWs from 1 to about 19 monolayers allowed us to distinguish various influences on the interface quality. It is shown that diffusion plays only a negligible role at our growth temperature of 560°C. Immiscibility in a thin In1−xGaxAs1−yPy interlayer that forms at the interface along with surface kinetic processes are suggested to be the driving force for composition variations at the interfaces. The impact of MOVPE growth techniques minimising the inhomogeneities at the interface is discussed.

InGaAs/GaAs metal-oxide-semiconductor heterostructure field-effect transistors with oxygen-plasma oxide and Al2O3 double-layer insulator

Surface condition before an insulator deposition is the key issue for the preparation of reliable GaAs-based metal-oxide-semiconductor (MOS) devices. This study presents the preparation and properties of InGaAs/GaAs MOS structures with a double-layer insulator consisting of an oxygen-plasma oxide covered by Al2O3. The structures were oxidized during 75 s and 150 s. Static measurements yielded a saturation drain current of ∼250 mA/mm at VG = 1 V. Capacitance measurements showed improved performance in the depletion region compared with the structures without the double-layer insulator. Trapping effects were investigated by conductance vs. frequency measurements. The trap state density was in order of 1011 cm−2·eV−1 with a continuous decrease with increased trap energy. The carrier mobility evaluation showed peak values of 3950 cm2/V·s for 75 s and 4570 cm2/V·s for 150 s oxidation times with the sheet charge density ≅2 × 1012 cm−2. The results demonstrate great potential of the procedure that was used to pr...

Anisotropic surface structure in ordered strained InGaP

We have studied the surface structure of ordered In x Ga 1-x P organometallic vapour phase epitaxy (OMVPE)-grown layers using optical microscopy, atomic force microscopy (AFM), and synchrotron topography. The layers were intentionally lattice mismatched (0.388 ≤ x ln < 0.552), and they exhibited a surface structure with three basic features. The first one is a fine island structure with the size of surface features in the range of 10 nm, which is very similar for all layers regardless of their misfit. This fine structure is superposed to surface undulations with lateral dimensions in the micrometer scale. The surface structure of the strained layers (tensile and compressed) follows the dislocation line pattern revealed by synchrotron topography. The change of the dominant misfit dislocation direction from [011] to [0 - 11] is observed for the layer still under tension with Δa/a = - 3.28 x 10 3 . The best surface morphology and no misfit dislocations are observed for the slightly compressed layer with Δa/a = + 9.42 x 10 4. With increased compression in the layers, we observed at first the creation of large (probably metal) precipitates and then the formation of a misfit dislocation net. The third feature observed on the surface of ordered layers is the presence of hillocks. Their density, shape and orientation depend on lattice mismatch.

New approach to local anodic oxidation of semiconductor heterostructures

We have experimentally explored a new approach to local anodic oxidation (LAO) of a semiconductor heterostructures by means of atomic force microscopy (AFM). We have applied LAO to an InGaP/AlGaAs/GaAs heterostructure. Although LAO is usually applied to oxidize GaAs/AlGaAs/GaAs-based heterostructures, the use of the InGaP/AlGaAs/GaAs system is more advantageous. The difference lies in the use of different cap layer materials: Unlike GaAs, InGaP acts like a barrier material with respect to the underlying AlGaAs layer and has almost one order of magnitude lower density of surface states than GaAs. Consequently, the InGaP/AlGaAs/GaAs heterostructure had the remote Si-delta doping layer only 6.5 nm beneath the surface and the two-dimensional electron gas (2DEG) was confined only 23.5 nm beneath the surface. Moreover, InGaP unaffected by LAO is a very durable material in various etchants and allows us to repeatedly remove thin portions of the underlying AlGaAs layer via wet etching. This approach influences LAO technology fundamentally: LAO was used only to oxidize InGaP cap layer to define very narrow (approximately 50 nm) patterns. Subsequent wet etching was used to form very narrow and high-energy barriers in the 2DEG patterns. This new approach is promising for the development of future nano-devices operated both at low and high temperatures.

Elimination of surface band bending on N-polar InN with thin GaN capping

0.5–1 μm thick InN {0001} films grown by molecular-beam epitaxy with N- or In-polarity are investigated for the presence of native oxide, surface energy band bending, and effects introduced by 2 to 4 monolayers of GaN capping. Ex situ angle-resolved x-ray photo-electron spectroscopy is used to construct near-surface (GaN)/InN energy profiles, which is combined with deconvolution of In3d signal to trace the presence of InN native oxide for different types of polarity and capping. Downwards surface energy band bending was observed on bare samples with native oxide, regardless of the polarity. It was found that the In-polar InN surface is most readily oxidized, however, with only slightly less band bending if compared with the N-polar sample. On the other hand, InN surface oxidation was effectively mitigated by GaN capping. Still, as confirmed by ultra-violet photo-electron spectroscopy and by energy band diagram calculations, thin GaN cap layer may provide negative piezoelectric polarization charge at the G...

Local anodic oxidation by AFM tip developed for novel semiconductor nanodevices.

The local anodic oxidation (LAO) by the tip of atomic force microscope (AFM) is used for fabrication of nanometer-scaled structures and devices. We study the technology of LAO applied to semiconductor heterostructures, theoretically and experimentally as well. The goal is to improve the LAO process itself, i.e., to create narrow LAO lines that form high-energy barriers in the plane with the 2D electron gas. In the first part we show the electric field distribution in the system tip-sample during LAO. For samples with low-conductive cap layer the maximum electric field is shifted apart the tip apex, which leads to wide oxide lines. Our Monte Carlo (MC) calculations show how the height of the energy barrier in the system depends on the geometry of the created lines (trenches), and on voltage applied to the structure. Based on the calculations, we have proposed a novel LAO technology and applied it to InGaP/AlGaAs/GaAs heterostructure with doping layer only 6 nm beneath the surface. The doping layer can be oxidized easily by the AFM tip in this case, and the oxide objects can be removed by several etchants. This approach to the LAO technology leads to narrow LAO trenches (approximately 60 nm) and to energy barriers high enough for room- and low-temperature applications.

On-tip sub-micrometer Hall probes for magnetic microscopy prepared by AFM lithography

We developed a technology of sub-micrometer Hall probes for future application in scanning hall probe microscopy (SHPM) and magnetic force microscopy (MFM). First, the Hall probes of approximately 9-mum dimensions are prepared on the top of high-aspect-ratio GaAs pyramids with an InGaP/AlGaAs/GaAs active layer using wet-chemical etching and non-planar lithography. Then we show that the active area of planar Hall probes can be downsized to sub-micrometer dimensions by local anodic oxidation technique using an atomic force microscope. Such planar probes are tested and their noise and magnetic field sensitivity are evaluated. Finally, the two technologies are combined to fabricate sub-micrometer Hall probes on the top of high-aspect ratio mesa for future SHPM and MFM techniques.

Growth of In1−xGaxAs by vapour phase epitaxy and analysis of growth conditions

Ternary epitaxial layers In1-xGaxAs have been prepared on GaAs and InP substrates by hydride vapour phase epitaxy. An analysis of growth conditions reported in this article demonstrates that the incomplete reaction of HCl with In and Ga together with the deposition of In1-yGayAs on the walls of the reactor plays an important role. It is shown that different compositions of gas mixtures flowing into the reactor can give the same equilibrium partial pressures of reactants over the substrate. We have studied some special cases where epitaxial growth is possible using the analytical equations derived in this paper. Efficiency of the reaction of HCl with In and Ga sources was estimated by fitting the experimental data.