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The crystalline quality of the transition region between buffer layer and epilayer in MOMBE (metalorganic molecular beam epitaxy) and plasma-MOVPE (metalorganic vapour phase epitaxy) grown GaAs layers on Si(100) is analysed by Raman scattering with variation of the penetration depth and besides bevel polishing or step etching. The region with considerable lattice imperfections is essentially confined to the original buffer layer.
We have investigated the influence of the deposition of a passivating silicon nitride layer on the electron transport in InAlAs/InGaAs/InAlAs quantum wells capped with a thin InGaAs layer. This structure is grown by Molecure Beam Epitaxy lattice-matched on InP substrates for the fabrication of High Electron Mobility Transistors. The InGaAs cap layer is added to reduce the ohmic contact resistance. In a first step to illuminate the complicated structural changes involved when passivating fully processed devices, we studied the influence of silicon nitride layer deposition on the virgin epilayers. The electrical transport properties were characterised by resistance, mobility, carrier concentration and magnetoresistance measurements, while Raman spectroscopy was used to probe the vibrational properties which are influenced by the free carriers. Because of the small penetration depth of the laser light mainly the lattice vibrations in the InGaAs ohmic contact layer were observed in the Raman spectra. Before passivation this thin highly doped cap layer is depleted, as confirmed by magnetoresistance measurements, that show a single conducting path in the two-dimensional electron gas formed in the InAlAs/InGaAs/InAlAs QW, and only bulk-like InGaAs phonons are observed in the cap layer. After passivation a shift to lower frequencies of the GaAs-like mode indicates a high electron concentration in the InGaAs cap. The results of the Raman analysis are in good agreement with the electrical transport data that show a considerably lower resistance after passivation. Magnetoresistance measurements on passivated layers confirmed the creation of an additional conducting path in the InGaAs cap layer introduced by the silicon nitride deposition process.
Using molecular beam epitaxy, thin films perovskite-type oxide SrxBa1−xTiO3 (0≤×≤1) has been grown epitaxially on Si(001) substrates. Reflection high energy electron diffraction measurements and X-ray diffraction analysis indicate that high quality heteroepitaxy on Si takes place with SrxBa1−xTiO3(001)//Si(001) and SrxBa1−xTiO3//Si. Extensive atomic simulations have been carried out to understand the initial growth mechanism of the oxide layers on silicon. SrTiO3 layers grown directly on Si have been used as the gate dielectric for the fabrication of MOSFET devices. By varying the growth conditions the thickness of the amorphous interfacial silicon oxide layer formed during the growth of the oxide layers has been engineered to minimize the device short channel effects. An effective oxide thickness <10 Å has been obtained for a 110 Å thick SrTiO3 dielectric film with interface state density around 6.4 × 1010 cm−2 eV-1, and the inversion layer carrier mobilities of 220 cm2 V−1 s−1 and 62 cm 2 V−ls−1 for NMOS and PMOS devices, respectively. The gate leakage in these devices is 2 orders of magnitude smaller than a comparable SiO2 gate dielectric MOSFET.
Metalorganic chemical vapor deposition (MOCVD) has been used to grow (Ba,Sr)TiO3 thin films on Ir/SiO2/Si substrates. β-diketonates of Ba, Sr, and Ti were used as the precursors, and delivered to the reactor via direct-liquid injection. Growth rate and film thickness were monitored by in-situ spectroscopic ellipsometry, and determined after growth. Film growth was studied as a function of film thickness, composition, substrate temperature, and mixture of O2 and N2O with and without microwave plasma enhancement. Dense, mirror-like films were obtained under all conditions except when pure oxygen plasma enhancement was used. Surface roughness of the films appears strongly dependent on film thickness and composition. Film composition and growth temperature determine growth texture of the films. This paper describes these results as well as the correlation between these results and dielectric properties.
Microstructure in the SrTiO3/Si system has been studied using high-resolution transmission electron microscopy and image simulations. SrTiO3 grows heteroepitaxially on Si with the orientation relationship given by (001)STO//(001)Si and STO//Si. The lattice misfit between the SrTiO3 thin films and the Si substrate is accommodated by the presence of interfacial dislocations at the Si substrate side. The interface most likely consists of Si bonded to O in SrTiO3. The alternative presentation of Sr and Si atoms along the interface leads to the formation of 2× and 3× Sr configurations. Structural defects in the SrTiO3 thin film mainly consist of tilted domains and dislocations.
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