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We report on the physical and electrical properties of AlN as grown by reactive-atomic-beam deposition under various growth and anneal conditions. The physical characterization shows that AlN grown on hydrogen terminated (HF-last) Si has a thin layer of Si3N4 at the interface while growth of AlN on SiO2 or Al2O3 suffers from nucleation problems. The AlN on HF-last Si, grown and annealed at 650 °C, shows high interfacial quality and four to five orders of magnitude lower leakage current as compared to SiO2. In contrast, poor interfacial properties and leakage current are reported for the AlN grown on SiO2 or Al2O3. The relative permittivity of the AlN is found to be close to 18 and the equivalent thickness of the interfacial layer is approximately 6 A. The effective electron mobility in Al-gated N-channel metal-oxide-silicon field-effect transistors with a 22 A thick AlN layer peaks at ∼130 cm2/V s. The relatively low mobility is attributed to a combination of high concentration of fixed charge (>5×1012 cm...

We have studied the effect of growth temperature on the growth mode and defect density of (LaxY1−x)2O3/Si(111) and Si(111)/(LaxY1−x)2O3/Si(111) heterostructures by transmission electron microscopy. We have also investigated the epitaxial relationships between fluorite and bixbyite oxides on different Si surfaces in the framework of a model which suggests that the mismatch between the oxygen sublattice and the Si substrate is the most important criterion for establishing the epitaxial relation. In addition, during attempts to achieve perfect lattice matching between (LaxY1−x)2O3 and Si by incorporating lanthanum beyond the near-lattice matched composition, we observe the formation of a phase-separated microstructure. A strain energy argument is developed to explain this observation.

A series of GaN:Mg structures were grown in molecular beam epitaxy, using either one or two rf nitrogen sources, and in metalorganic chemical vapor deposition systems with varying Mg flux. Acceptor energies were measured using the Hall effect and admittance spectroscopy techniques. The acceptor energies were found to be different for the two methods, i.e., 135–155 meV for the Hall effect measurement and 80–115 meV for the admittance spectroscopy measurement. The apparently small acceptor energies from the admittance spectroscopy measurement were explained, through a simulation process, by the combined effects of (1) high Mg acceptor concentration with no other free carrier sources, and (2) the Mg emission kinetics assisted by the Frenkel–Poole field effect in the GaN:Mg structures.

We report the effects of ultrahigh vacuum and nitrogen annealing on self-aligned W-gated HfO2 n-field-effect transistors. While rapid thermal annealing in a nitrogen ambient significantly improved the channel mobility, there is a corresponding increase in the gate dielectric thickness of up to 20A. Here we show that this increase is due to the formation of interfacial oxides between the HfO2 and the silicon, caused by the diffusion of oxygen from the W gates. The results point to the importance of oxygen solubility in metals, which may be significant in many candidate gate electrodes.

Future microelectronics will be based upon silicon or germanium-on-insulator technologies and will require an ultrathin (

We report on controlled interfacial oxide formation within epitaxial (LaxY1−x)2O3/Si(111) heterostructures under UHV environments. Results indicate that exposure of these epitaxial films to molecular oxygen right after deposition results in the formation of an amorphous interfacial layer thicker than that expected when a bare silicon surface is exposed to molecular oxygen under the same conditions. The results imply significant oxygen diffusion through the epitaxial dielectric and reaction at the silicon–oxide interface. Arguments have been developed to explain these observations.

Many of the proposed high permittivity gate dielectrics for silicon-based microelectronics rely on a stack configuration, with an SiO2 buffer layer to provide an interface. We describe a means for creating gate dielectrics with a direct yttrium silicate–silicon interface through the solid-state reaction of yttria and silicon oxynitride, avoiding the preparation of an oxide-free silicon surface. Characterization by medium-energy ion scattering indicates complete consumption of the underlying oxide through silicate formation during high-temperature annealing. Furthermore, the silicate dielectric exhibits small flat-band voltage shifts, indicating low quantities of charge, without passivation steps. Creation of a silicate–silicon interfaces by a simple route may enable the study of an alternate class of dielectrics.

We have examined the stability of Al2O3/Si heterostructures and show that significant Al diffusion occurs into the silicon for temperatures of 1000 °C and more. This may be caused by dissociation of small quantities of Al2O3 and subsequent dissolution of the Al into the silicon. Such diffusion may be reduced, though not eliminated via an interfacial silicon oxynitride diffusion barrier. Using long channel metal gate Al2O3/Si n field effect transistor data, we show that anneals at 1000 °C result in a degradation of the electron mobility by a factor of 2.

We demonstrate a ternary (LaxY1−x)2O3 thin-film oxide that can be grown epitaxially on Si(111) substrates with a lattice constant that can be matched to twice the lattice constant of silicon. We further show that silicon can then be deposited epitaxially (though with a high defect density) on this oxide such that epitaxial silicon/oxide/silicon structures may be grown. We discuss the microstructural relationships and the growth modes for the oxide on silicon and silicon on oxide growths.

We demonstrate that depositing Ta diffusion barriers under ultra‐high vacuum conditions without in situ oxygen dosing allows for variations both in microstructure and in the concentration of chemical impurities that severely degrade barrier performance. The effects of deposition pressure, in situ oxygen dosing at interfaces, hydrogen and oxygen contamination, and microstructure on diffusion barrier performance to Cu diffusion for electron‐beam deposited Ta are presented. 20 nm of Ta diffusion barrier followed by a 150 nm Cu conductor were deposited under ultra‐high vacuum (UHV, deposition pressure of 1×10−9 to 5 ×10−8 Torr) and high vacuum (HV, deposition pressure of 1×10−7 to 5×10−6 Torr) conditions onto 〈100〉 Si. In situ resistance furnace measurements, Auger compositional depth profiling, secondary ion mass spectrometry, and forward recoil detection along with scanning and transmission electron microscopy were used to determine the electrical, chemical, and structural changes that occurred in thin‐film Ta diffusion barriers upon annealing. Undosed HV deposited Ta barriers failed from 560 to 630 °C, while undosed UHV barriers failed from 310 to 630 °C. For UHV Ta barriers, in situ oxygen dosing during deposition at the Cu/Ta interface increased the failure temperatures by 30–250 °C and decreased the range of failure temperatures to 570–630 °C. Undosed UHV Ta barriers have no systematic relationship between failure temperature and deposition pressure, although correlations between breakdown temperature, oxygen and hydrogen concentrations, and microstructural variations were measured.

We demonstrate the potential for ultrathin aluminum-oxide films as alternate gate dielectrics for Si complementary metal–oxide–semiconductor technology. Films are deposited in ultrahigh vacuum utilizing atomic beams of aluminum and oxygen on Si(100) surfaces. We show device-quality Si(100)/Al2O3 interfaces with interfacial trap densities in the 1010 cm−2 eV−1 range, and with leakage current densities five orders of magnitude lower than what is observed in SiO2 insulators at the same equivalent electrical thickness. As-grown films possess an amorphous-to-microcrystalline structure, depending upon the deposition temperature, and any interfacial layers between the Si(100) and Al2O3 layer are

We report on high effective mobilities in yttrium-oxide-based n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) with aluminum gates. The yttrium oxide was grown in ultrahigh vacuum using a reactive atomic-beam-deposition system. Medium-energy ion-scattering studies indicate an oxide with an approximate composition of Y2O3 on top of a thin layer of interfacial SiO2. The thickness of this interfacial oxide as well as the effective mobility are found to be dependent on the postgrowth anneal conditions. Optimum conditions result in mobilities approaching that of SiO2-based MOSFETs at higher fields with peak mobilities at approximately 210 cm2/V s.

The stability of Al2O3 films during thermal processing will help determine their usefulness as an alternative gate dielectric for advanced complementary metal-oxide-semiconductor devices. We used medium energy ion scattering and atomic force microscopy to examine the degradation of ultrathin Al2O3 layers under ultrahigh vacuum annealing and the effects of low-temperature oxidation. No degradation is observed at 900 °C, but voids appear at higher temperatures. Growth of interfacial SiO2 takes place during low-pressure oxidation at 600 °C, which may limit the capacitance of extremely thin structures.

We report on the electrical and microstructural characteristics of La- and Y-based oxides grown on silicon substrates by ultrahigh vacuum atomic beam deposition, in order to examine their potential as alternate gate dielectrics for Si complementary metal oxide semiconductor technology. We have examined the issues of polycrystallinity and interfacial silicon oxide formation in these films and their effect on the leakage currents and the ability to deposit films with low electrical thickness. We observe that polycrystallinity in the films does not result in unacceptably high leakage currents. We show significant Si penetration in both types of films. We find that the interfacial SiO2 is much thicker at ∼1.5 nm for the Y-based oxide compared to the La-based oxide where the thickness is

We demonstrate that Si layers overgrown on GaN/sapphire substrate structures can be transferred onto host substrates by using the technique of laser-induced metallization of GaN at the GaN/sapphire interface via the reaction GaN=Ga+1/2N2. This may allow the grafting of films of Si onto substrates such as glass or plastic for microelectronic applications. We examine the debonding mechanism and propose that it occurs due to pressures of the order of a few thousand atmospheres generated with the release of nitrogen from the GaN metallization reaction.

We demonstrate the selective area growth of gallium nitride on patterned Si(111)/GaN/SiO2 wafers by metalorganic molecular beam epitaxy using triethyl gallium as a Ga source. We show that such selective area deposition may be used to grow isolated microcolumns of GaN with lateral dimensions of tens of nanometers on Si/SiO2 wafers. Via high resolution cathodoluminescence imaging we show that such microcolumn structures are highly luminescent inspite of a large surface to volume ratio, indicating that nonradiative recombination at free surfaces is not a significant issue in this system.

We demonstrate the operation of multicolored light emitting diodes (LEDs) on a silicon wafer using ultraviolet/violet GaN LEDs grown by molecular beam epitaxy on Si(111) in conjunction with organic dye based color converters. These organic converters, applied on top of the nitride diodes, absorb the nitride LED electroluminescence at ∼360 nm, and fluoresce in the green (∼530 nm), and orange (∼600 nm). In this fashion, multicolored light emitters may be made to operate side by side and monolithically on the same Si wafer in an approach that may be attractive for miniature GaN based full color displays.

We report the fabrication and characterization of GaN-based double heterostructure light emitting diodes grown by molecular beam epitaxy on Si(111) substrates. Light emitting diode operation is achieved by using the conducting Si(111) substrate as a backside n contact and a standard transparent Ni/Au p contact. We observe electroluminescence peaked in the ultraviolet ∼360 nm, with a full width at half maximum of ∼17 nm and in the violet at ∼420 nm. Electron microscopy studies indicate a high density of threading and planar defects. Consequences of these are discussed.

This invention provides a novel hybrid organic-inorganic semiconductor light emitting diode. The device consists of an electroluminescent layer and a photoluminescent layer. The electroluminescent layer is an inorganic GaN light emitting diode structure that is electroluminescent in the blue or ultraviolet (uv) region of the electromagnetic spectrum when the device is operated. The photoluminescent layer is a photoluminescent organic thin film such as tris-(8-hydroxyquinoline) Al, Alq3, deposited onto the GaN LED and which has a high photoluminescence efficiency. The uv emission from the electroluminescent region excites the Alq3 which photoluminesces in the green. Such a photoconversion results in a light emitting diode that operates in the green (in the visible range). Other colors such as blue or red may be obtained by appropriately doping the Alq3. Furthermore, other luminescent organics in addition to Alq3 may be used to directly convert the uv or blue to other wavelengths of interest. The invention provides the benefits of simplicity and ease of fabrication, since a complete redesign of the structure is not necessary to change emission wavelength, and the possibility for making displays by spatially varying the deposition of the emissive layer.

We examine the epitaxial incorporation behavior of the volatile p-type dopant Mg at high growth temperatures during the molecular beam epitaxy of GaN on the [0001] surface, and report interesting doping behavior. The net Mg incorporation is independent of the arriving Mg flux over a flux variation of two decades, and dependent strongly on the growth temperature of the epilayer, limiting the net hole concentrations obtainable to the low 1017 cm−3 range. These results are explained in terms of incorporation either through a surface Mg phase, or via the availability of specific sites on the surface for Mg incorporation.

We report a study of the desorption behavior of Ga on the GaN(0001) surface and the growth behavior of GaN during molecular beam epitaxy. A desorption activation energy of 2.2±0.2 eV is measured for Ga adatoms. Porous columnar features in the GaN microstructure are observed that are enhanced by higher growth temperatures and eliminated by growing Mg or Si doped GaN. We propose a model for this observation.

Interface recombination in GaAs at the GaAs/AlAs interface has been investigated before and after selective ‘‘wet oxidation’’ of the AlAs layer. Time‐resolved photoluminescence of the band‐edge GaAs emission has been used to characterize the interface recombination. Prior to oxidation, the interface recombination is low. After oxidation, the interface recombination has greatly increased, and is comparable to a free GaAs surface in air. However, isolating the GaAs layer from the oxide by a 30 nm layer of Al0.3Ga0.7As allows the interface recombination to remain low after the oxidation. These results help explain the low threshold currents which have been observed in vertical cavity lasers which use wet oxidation of AlAs for current confinement.

Measurements of the polar Kerr rotation and optical coercivity as a function of photon energy have been made on ≊2 μm thick ThMn12‐type crystalline sputtered films of Sm(Fe,Ti)12. The optical coercivity for a Ti containing (222) textured film was 3.10 kOe at a photon energy of 1.25 eV, 2.96 kOe at 1.84 eV, and 2.92 kOe at 2.45 eV. The optical coercivity for a similar composition (002) textured Sm(Fe,Ti)12 film was relatively constant at 1.24 kOe. These samples were measured in air at 300 K without any protective overcoating. The polar Kerr rotation observed for these crystalline films versus the photon energy paralleled that observed for pure Fe. The magnetometer and optical coercivities for the (222) textured Sm(Fe,Ti)12 films were 1.5 and 2.5 times as large, respectively, as for similar composition (002) textured films. Differences in grain sizes incorporated during the direct synthesis of the films onto heated substrates are needed to account for the large differences observed in the coercivities for the (222) versus (002) textured films.

Amorphous thin films of UxAs100−x are ferromagnetic for U‐rich compositions, 49≤x≤75, with a magnetic moment of 0.2 to 0.4 μB per U atom and Curie temperatures, Tc, up to 120 K. Large resistivities are found at 5 K for compositions x

We examined the 4 K photoluminescence spectra of over a dozen Cu2ZnSnS4 films and eight devices. We show that samples deficient in zinc show on average a higher quasi donor-acceptor pair (QDAP) density. However, the QDAP density in samples with the same metal composition also varies widely. Devices prepared with similar metal compositions show different open-circuit voltages and fill factors. These metrics are correlated with the concentration of QDAPs in the absorbers. One additional device with insufficient zinc showed the empirically observed low-efficiency expected for this composition. This sample also showed the highest quasi donor-acceptor pair density of all the devices measured.

We have studied the temperature dependent kinetics of MoSe2 formation between molybdenum and Cu2ZnSnSe4 (CZTSe) films during annealing in the presence of Se. CZTSe is an emerging light-absorbing material for thin film solar cell applications, and thermal treatment of this layer constitutes a critical part of the device processing. The formation of MoSe2 in this system is modeled using a three step mechanism—diffusion of Se through CZTSe, diffusion of Se through MoSe2, and reaction between Se and Mo. Applying the results of the model to experimental results reveals that the MoSe2 formation is limited by the diffusion of Se through the CZTSe layer.

While a few singular reports have demonstrated carbon nanotube (CNT) transistors with subthreshold swings (SS) close to the theoretical limit (60 mV/decade), the majority of devices have more than double the target swing. Here, we show that a low temperature lanthanum oxide dielectric is able to yield a consistently and reproducibly low SS, with an average of 73 mV/decade and a low of 63 mV/decade. This LaOx film is characterized using medium energy ion scattering and shown to be scalable down to 3.5 nm with minimal leakage and a variation in swing of only ±13%. With interface traps playing a dominant role in the switching behavior of CNT transistors, these results reveal the existence of dielectrics with more favorable interfacial characteristics for nanotubes that yield low SS devices.

We have examined Cu2ZnSnSe4 (CZTSe) solar cells prepared by thermal co-evaporation on Mo-coated glass substrates followed by post-deposition annealing under Se ambient. We show that the control of an interfacial MoSe2 layer thickness and the introduction of an adequate Se partial pressure (PSe) during annealing are essential to achieve high efficiency CZTSe solar cells—a reverse correlation between device performance and MoSe2 thickness is observed, and insufficient PSe leads to the formation of defects within the bandgap as revealed by photoluminescence measurements. Using a TiN diffusion barrier, we demonstrate 8.9% efficiency CZTSe devices with a long lifetime of photo-generated carriers.

We have examined the stability of TbScO3 on Si(100) using medium energy ion scattering. At high temperatures the dielectric decomposes into a Tb-rich silicate layer near the substrate, and a Sc-rich layer near the surface. Interfacial SiO2 is consumed in the reaction. We find that Sc2O3 by itself does not react with SiO2 while Tb2O3 readily forms a silicate. This difference in reactivity drives the vertical separation of metal ions. Consideration of the fundamental chemistry of rare-earth scandates suggests that rare-earth scandates are unstable in the presence of SiO2

High efficiency Cu2ZnSnS4 solar cells have been fabricated on glass substrates by thermal evaporation of Cu, Zn, Sn, and S. Solar cells with up to 6.8% efficiency were obtained with absorber layer thicknesses less than 1 μm and annealing times in the minutes. Detailed electrical analysis of the devices indicate that the performance of the devices is limited by high series resistance, a “double diode” behavior of the current voltage characteristics, and an open circuit voltage that is limited by a carrier recombination process with an activation energy below the band gap of the material.

The energy band gap, the band alignment with Si, and the chemical bonding of 4–5 nm thick (TbxSc1−x)2O3 dielectric films were investigated as a function of composition. Films with x=0, 0.5, and 1 were prepared by a molecular beam deposition technique on silicon substrates. The structure of the dielectric films was characterized by high resolution transmission electron microscopy. We found that upon deposition, a silicate and a silicon oxide layer were formed at the dielectric/silicon interface for all compositions. X-ray photoelectron spectroscopy was used to determine the band gap, as well as the energy band alignment with Si and the chemical structure of the films. Energy gap values of 6.0±0.2 and 7.5±0.2 eV were obtained for pure Sc2O3 and Tb2O3, respectively, while for the mixed layer (x=0.5) a value of 6.8±0.2 eV was extracted. It was found that the valence band offset does not change with Sc addition to Tb2O3, while the conduction band offset increases with x, from a value of 2.9±0.2 eV for the x=0 ...

We report the effect of La2O3 capping layers on HfO2/SiO2/Si dielectrics, proposed for use in threshold voltage tuning of field effect transistors. Depth profiling with medium energy ion scattering shows that an initial surface layer of La2O3 diffuses through the HfO2 at elevated temperatures, ultimately converting some of the thin interfacial SiO2 into a silicate. Core-level photoemission measurements indicate that the additional band-bending induced by the La2O3 only appears after diffusion, and the added charge resides between the HfO2 and the substrate.

Photocatalysis has widespread applications from solar cells to photolithography. We studied the photocatalytic properties of TiO2 films of thicknesses down to 2 nm, grown on n-type and p-type silicon wafers, using the oxidation of isopropanol as a model system. Direct in vacuo mass spectrometry measurements were performed under irradiation above the TiO2 bandgap. We present a model consistent with our experimental results, which indicate that only near-surface electron-hole pair generation is relevant and that the reaction rate can be controlled by varying the substrate Fermi level in going from n-type to p-type silicon, by approximately a factor of 2.

We report on molecular beam synthesis and high temperature electrical characterization of crystalline ceria thin films. It is found that CeO2 forms a mismatched interface with Al2O3 (0001) with the orientation relation of (111) CeO2‖(0001)Al2O3 and [110] CeO2‖[12¯10]Al2O3. The electrical conductivity was found to be 0.08S∕cm in air and independent of PO2 down to 10−16atm at 1058K. It is likely that the highly oriented nature of the film leads to altered electrical conductivity phenomena at lower oxygen partial pressures.

The authors have examined the role of sub nanometer La2O3 and LaN cap layers interposed in Si∕HfO2∕TiN high-k gate dielectric stacks in tuning the flatband and threshold voltages of capacitors and transistors. High performance, band edge n metal oxide field effect transistors with channel lengths down to 60nm may be fabricated without significant compromise in mobility, electrical thickness, and threshold voltage. They have carried out a microstructural evaluation of these stacks and correlated these results with the electrical behavior of the devices.