Atomic and electronic structures of Si/Ge(100) interfaces studied by high-resolution photoelectron spectroscopy and scanning tunneling microscopy

The close similarity of silicon and germanium, isoelectronic group-IV elements, makes the integration of Ge layers on Si substrates suitable for technology development, but the atomic and electronic structures of Si1-xGex surfaces are still an open issue, in particular, for the alloy systems where Si is deposited on the Ge substrate. In this study, utilizing low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy using synchrotron radiation, we demonstrate that the formation mechanisms of the Si-on-Ge structures are controlled by two interface phenomena, namely Si indiffusion and Ge segregation on top of this surface. Employing these phenomena and controlling the Si quantity, one can synthesize the well-defined crystalline Ge-(2 x 1)/Si1-xGex/Ge(100) stacks where the number of Si atoms at the host Ge lattice sites can be tuned. Using the obtained data on the atomic and electronic structures of such systems, we also propose a method for interface engineering of Ge/Si/Ge stacks with tailored properties as promising templates for growing the device junctions

Stabilization of unstable and metastable InP native oxide thin films by interface effects

III-V semiconductor - oxide interfaces have attracted huge interest due to their substantial potential in electronic applications. However, due to the extreme complexity of the modeling of the interfaces, there are only few ab initio studies of these interfaces.Several model interfaces of native InPO4 oxides are designed in this study. It is shown that energies of the (quasi-)coherent interfaces are much smaller than energies of the incoherent interfaces. Furthermore, it is pointed out that the interface energy can stabilize oxide structures not found in bulk form. Relatively small strain energy and configurational match imply a small interface energy.It is estimated that the gap state density of the In-terminated quasi-coherent interfaces is small or zero. However, partial oxidation of the substrate P atoms, which can be induced, e.g., by non-stoichiometry of the oxide, causes distinct gap states. This is a mechanism to explain Fermi level pinning of the III-V - oxide interfaces. Non-stoichiometric compositions are also investigated. Experimental results on InP native oxide growth are discussed. The models can be used to study various properties of the interfaces and more complex models including, e.g., dislocations or non-planar surfaces can be based on the models.</p

Effects of thermal vacuum nitridation of Si(100) surface via NH3 exposure

Low temperature treatments to control the Si-interface properties become more and more relevant to the broad Si-based electronics and photonics technology when the back-end-of-line processing is developed and the integration of hybrid materials on the Si platform increases. In this work we have investigated effects of NH3 nitridation of three different Si surfaces in ultrahigh-vacuum (UHV) chamber at 400 °C: (i) nitridation of well-defined Si(100) (2 × 1)+(1 × 2) cleaned by the high-temperature flash heating, (ii) nitridation of the Radio Corporation of America (RCA)-cleaned H-terminated Si(100) with the final HF dip, and (iii) nitridation of the RCA-treated (without the final HF dip) Si(100) which includes so-called wet-chemical oxide of SiO2. X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy/spectroscopy measurements show that nitrogen incorporates into subsurface layers of clean Si and into the SiO2 chemical-oxide layer, when the materials are exposed to NH3 background in UHV chamber without a plasma source at 400 °C or even at room temperature. XPS results indicate that the nitridation does not remove oxygen from the SiO2 chemical oxide. The nitridation of SiO2 is also found to increase the density of electron levels at 3 to 4 eV above the Fermi level. Electrical measurements of atomic-layer deposited HfO2/Si(100) capacitors with and without the nitridation support that the method has potential to decrease amount of interface defects and to control interface properties.​​​​​​​</ul

Atomic-Scale Modification of Oxidation Phenomena on the Ge(100) Surface by Si Alloying

Properties of Ge oxides are significantly different from those of widely used Si oxides. For example, the instability of GeOx at device junctions causes electronic defect levels that degrade the performance of Ge-containing devices (e.g., transistors and infrared detectors). Therefore, the passivating Si layers have been commonly used at Ge interfaces to reduce the effects of Ge oxide instability and mimic the successful strategy of Si oxidation. To contribute to the atomic-scale knowledge and control of oxidation of such Si-alloyed Ge interfaces (O/Si/Ge), we present a synchrotron radiation core-level study of O/Si/Ge, which is combined with scanning probe microscopy measurements. The oxidation processes and electronic properties of O/Si/Ge(100) are examined as functions of Si amount and oxidation doses. In particular, the incorporation of Si into Ge is shown to cause the strengthening of Ge−O bonds and the increase of incorporated oxygen amount in oxide/Ge junctions, supporting that the method is useful to decrease the defect-level densities.</p

Observation of Si 2p Core‐Level Shift in Si/High‐κ Dielectric Interfaces Containing a Negative Charge

Negative static charge and induced internal electric field have often been observed in the interfaces between silicon and high‐κ dielectrics, for example Al2O3 and HfO2. The electric field provides either beneficial (e.g., field‐effect passivation) or harmful (e.g., voltage instability) effect depending on the application. Different intrinsic and extrinsic defects in the dielectric film and interface have been suggested to cause the static charge but this issue is still unresolved. Here spectroscopic evidence is presented for a structural change in the interfaces where static charge is present. The observed correlation between the Si core‐level shift and static negative charge reveals the role of Si bonding environment modification in the SiO2 phase. The result is in good agreement with recent theoretical models, which relate the static charge formation to interfacial atomic transformations together with the resulting acceptor doping of SiO2

Decreasing Interface Defect Densities via Silicon Oxide Passivation at Temperatures Below 450 degrees C

Low-temperature (LT) passivation methods (700 degrees C). Therefore, the LT passivation of SiOx/Si has long been a research topic to improve application performance. Here, we demonstrate that an LT (<450 degrees C) ultrahigh-vacuum (UHV) treatment is a potential method that can be combined with current state-of-the-art processes in a scalable way, to decrease the defect densities at the SiOx/Si interfaces. The studied LT-UHV approach includes a combination of wet chemistry followed by UHV-based heating and preoxidation of silicon surfaces. The controlled oxidation during the LT-UHV treatment is found to provide an until now unreported crystalline Si oxide phase. This crystalline SiOx phase can explain the observed decrease in the defect density by half. Furthermore, the LT-UHV treatment can be applied in a complementary, post-treatment way to ready components to decrease electrical losses. The LT-UHV treatment has been found to decrease the detector leakage current by a factor of 2

Effects of post oxidation of SiO2/Si interfaces in ultrahigh vacuum below 450 °C

Growing SiO2 layer by wet-chemical oxidation of Si surfaces before growth of another insulating film(s) is a used method to passivate Si interfaces in applications (e.g., solar cell, photodiode) at low temperatures (LT) below 450 °C. We report on potential of LT ultrahigh-vacuum (UHV) treatments combined with the wet-chemical oxidation, by investigating effects of LT-UHV oxidation after the wet-chemical growth of SiO2 and before growing Al2O3 film on top of SiO2/Si. This method modifies the SiO2/Si and is found to (i) decrease defect-level density, (ii) increase negative fixed charge density, and (iii) increase carrier lifetime for Al2O3/SiO2/p-Si, as compared to state-of-the-art SiO2/p-Si reference interfaces without LT-UHV. X-ray photoelectron spectroscopy shows that the LT-UHV treatment decreases amount of Si+3 oxidized atoms in chemically grown SiO2 and also amount of carbon contamination. In order to pave the way for further tests of LT-UHV in silicon technology, we present a design of simple UHV instrument. The above-described benefits are reproduced for 4-inch silicon wafers by means of the instrument, which is further utilized to make LT-UHV treatments for complementary SiO2/Si interfaces of the native oxide at silicon diode sidewalls to decrease the reverse bias leakage current of the diodes.​​​​​​​</ul

Quantifying the Impact of Al Deposition Method on Underlying Al2O3/Si Interface Quality

Oxide-semiconductor interface quality has often a direct impact on the electrical properties of devices and on their performance. Traditionally, the properties are characterised through metal-oxide-semiconductor (MOS) structures by depositing a metal layer and measuring the capacitance-voltage (C-V) characteristics. However, metal deposition process itself may have an impact on the oxide and the oxide-semiconductor interface. The impact of magnetron sputtering, e-beam evaporation, and thermal evaporation on an Al2O3/Si interface was studied, where atomic layer deposited (ALD) Al2O3 was used, by MOS C-V and Corona Oxide Characterization of Semiconductors (COCOS) measurements. The latter allows characterisation of the interface also in its original state before metallisation. The results show that sputtering induces significant damage at the underlaying Al2O3/Si interface as the measured interface defect density Dit increases from 1011 cm−2eV to 1013 cm−2eV. Interestingly, sputtering also generates a high density of positive charges Qtot at the interface as the charge changes from –2 · 1012 cm−2 to +7 · 1012 cm−2. Thermal evaporation is found to be a softer method, with modest impact on Dit and Qtot. Finally, we show that Alnealing heals the damage but has also a significant impact on the charge of the film recovering the characteristic negative charge of Al2O3 (∼ –4 · 1012 cm−2).Peer reviewe

Surface passivation of Germanium with ALD Al2O3: Impact of Composition and Crystallinity of GeOx Interlayer

Germanium is an excellent material candidate for various applications, such as field-effect transistors and radiation detectors / multi-junction solar cells, due to its high carrier mobilities and narrow bandgap, respectively. However, efficient passivation of germanium surfaces has 10 remained challenging. Recently the most promising results have been achieved with atomic layer deposited (ALD) Al2O3, but the obtainable surface recombination velocity (SRV) has been very sensitive to the surface state prior to deposition. Based on X-ray photoelectron spectroscopy (XPS) and Low-energy electron diffraction (LEED), we show here that the poor SRV obtained with the combination of HF and DIW surface cleaning and ALD Al2O3 results from a Ge suboxide interlayer (GeOx, x < 2) with compromised quality. Nevertheless, our results also demonstrate that both the composition and crystallinity of this oxide layer can be improved by a combination of low-temperature heating and a 300-Langmuir controlled oxidation in ultrahigh-vacuum (LT-UHV treatment). This results in the reduction of the interface defect density (Dit) allowing us to reach SRV values as low as 10 cm/s. Being compatible with most device processes due to the low thermal budget, the LT-UHV treatment could be easily integrated into many future devices and applications.Peer reviewe