Weidan Li

NUCLEATION OF GAAS ON CAF2/SI(111) SUBSTRATES

Two different nucleation modes in epitaxial growth of GaAs on CaF2/Si(111) substrates have been identified. At low temperature (below 500 °C). GaAs nucleates on CaF2(111) surface with a three‐dimensional mode. When Ts is high (higher than 590 °C), two‐dimensional nucleation of GaAs can be achieved on the CaF2/Si(111) surface under a proper As flux. A thorough investigation of this phenomenon has been carried out using Rutherford backscattering spectrometry. A stable As layer has been observed on the CaF2(111) surface when the CaF2/Si(111) substrates are treated at high temperatures and under a high enough As flux (≥2×10−5 mbar). The formation of this As layer is understood as a consequence of a chemical reaction between As adatoms and the CaF2 surface. The existence of the As layer modifies the surface free energy of CaF2/Si(111) substrates and leads to a transition in the GaAs nucleation mode. By controlling the nucleation mode, high quality GaAs has been grown on the top of CaF2/Si(111) substrates witho...

Chemical Mechanical Polishing of Low Dielectric Constant Oxide Films Deposited Using Flowfill Chemical Vapor Deposition Technology

In this work, the properties and chemical mechanical polishing (CMP) characteristics of thin films of a new low dielectric constant (low-κ) oxide deposited using Flowfill chemical vapor deposition (CVD) technology are presented. This oxide film consists of silicon dioxide network with methyl groups incorporated and has a dielectric constant K as low as ∼2.7. The film properties were studied using Fourier transform infrared spectroscopy (FTIR), spectroscopic ellipsometry, Rutherford backscattering, atomic force microscopy, and capacitance-voltage measurements. The refractive index, as low as 1.38, was measured using spectroscopic ellipsometry. The surface was found to be more hydrophobic compared to conventional CVD oxide. The stretching mode of the Si-O bond peak in the FTIR spectrum shifts to lower wavenumber, which corresponds to lower Si-O bonding energy, with increase in the methyl concentration inside the film. The CMP removal rate decreases as the methyl concentration in the film increases. An atomically smooth surface with root mean square surface roughness <1 nm over an area 2 X 2 μm was obtained after CMP. Our results suggest that the incorporation of methyl groups results in a reduction in the CMP removal rate. We speculate that the diffusion of water into the film is probably the CMP removal rate-limiting step.

Characterization of methyl-doped silicon oxide film deposited using Flowfill™ chemical vapor deposition technology

In this article, methyl-doped silicon oxide films deposited using Flowfill™ chemical vapor deposition (CVD) technology have been chracterized for use in inter-layer dielectrics application. Films with different methyl concentrations were deposited and characterized in order to study the effect of methyl concentration on film properties. Material properties including chemical composition and bonding structure, density, dielectric constant (κ), refractive index, thermal stability, resistance to moisture absorption, leakage current, and hardness were investigated. The films have a κ as low as 2.7 and were found to be thermally stable up to 550 °C. They show excellent resistance to moisture absorption. Low-leakage current and breakdown voltage higher than 3 MV/cm were obtained. Their hardness is lower than silicon oxide deposited using plasma-enhanced CVD but is higher than most polymer and nanoporous low-dielectric constant (low-κ) materials. The chemical mechanical polishing (CMP) characteristics of these f...

Optimization of GaAs epitaxy on CaF2/Si(111) substrates

Abstract Strain-free GaAs epitaxial layers on Si can be achieved by introducing a CaF 2 buffer layer. Here, molecular beam epitaxial growth of GaAs on CaF 2 /Si(111) substrates has been thoroughly investigated. Three growth regimes have been distinguished. In the low temperature regime, growth was always in a three-dimensional mode, while, in the high temperature regime, two-dimensional nucleation was achieved. However, continued growth in the high temperature regime resulted in the formation of twins after epitaxial initiation. Between those two regimes, there is a narrow temperature window in which high quality GaAs can be grown on the top of CaF 2 /Si(111). Under optimal growth conditions in this window, 1 μm thick GaAs layers were grown which exhibited good crystal quality (a minimum ion channeling yield of 3.5% was achieved at the GaAs surface) along with specular surface morphology. Such high quality GaAs on the top of CaF 2 /Si(111) substrates was achieved previously only by using electron-beam modification of the CaF 2 surface.

Residual strains in epitaxial fluorides on Si(111) substrates

Growth conditions for molecular beam epitaxy of SrF2 directly on Si(111) have been optimized for excellent crystal quality. A χmin of 2.6% was demonstrated. Lattice distortion measurements were carried out by ion channeling along off‐normal channeling directions in the strained SrF2 layers. The measured residual tensile strain versus the film thickness demonstrated a special thickness: When films were thinner than this thickness, their strain dropped dramatically as thickness decreased, and when films were grown thicker than it, the strain gradually decreased with increasing film thickness. This unique behavior was successfully modeled as the result of competition between the large lattice mismatch between SrF2 and Si and the large difference in thermal coefficients of expansion. Residual strain remained even in SrF2 films thicker than 600 nm, unlike CaF2 on Si(111) for which CaF2 films thicker than 300 nm were observed to have no strain.

Molecular beam epitaxial growth of GaAs on CaF2/Si(111) substrate

In this work, molecular beam epitaxial (MBE) growth of GaAs on CaF2/Si(111) substrates has been studied with both Rutherford backscattering spectroscopy and transmission electron microscopy. It has been observed that, under certain conditions, a chemical reaction between As adatoms and CaF2 layers can be induced, by which a more stable As layer on the CaF2 surface is formed. The existence of the As layer improves the Ga wettability on the CaF2 surface and, if properly controlled, leads to two‐dimensional (2D) nucleation of GaAs on the CaF2/Si(111) surface as opposed to the more commonly observed three‐dimensional growth. In subsequent growth, two kinds of twins have been observed. All samples were observed to have microtwins near the GaAs/CaF2 interface. These twins can be suppressed during the first 1000 A, if the layer is grown in a narrow optimal growth window. Otherwise, the growth will be in a 3D mode at lower temperature, or, it will suffer from the formation of big rotational twins at higher temper...

Surface morphology of epitaxial CaF2/Si(111) and its influence on subsequent GaAs epitaxy

Atomic force microscopy (AFM) has been intensively used to investigate the surface morphology of CaF2 films. The atomic steps of CaF2 grown on tilted Si substrates are observed to bunch up during growth, which leads to the formation of large, atomically flat plateaus separated by polyatomic steps. Both the shape and the size of those steps strongly depend upon the misorientation direction of the Si substrate. Quantitative AFM analysis indicates that the size of the plateaus on the CaF2 surface grown on Si substrates tilted towards the [112] azimuth is about eight times smaller than the size of those grown on a Si substrate tilted towards the opposite direction (the [112] azimuth). Smaller plateaus on the CaF2 surface are observed to result in smaller twins which are more easily suppressed if proper growth conditions are chosen. High resolution scanning electron microscopy has also been employed to study the surface of GaAs films grown on CaF2/Si(111) substrates with different misorientation directions....

Characterization of methyl-doped silicon oxide film for inter-layer dielectrics application

In this work, methyl-doped silicon oxide films deposited using Flowfill/sup TM/ chemical vapor deposition (CVD) have been characterized for use in inter-layer dielectrics (ILD) application. Films with different methyl contents were studied to understand the effects of incorporated methyl groups on the properties. Chemical composition and bonding structure, dielectric constant (k), refractive index (RI), density, thermal stability, moisture permeability, and hardness were investigated. Chemical mechanical polishing (CMP) of these films and their stability under O/sub 2/, N/sub 2/ and H/sub 2/ plasma treatments were also studied.

Strain Relief in SrF 2 Epitaxial Films on Si(111) Substrates

Molecular beam epitaxial (MBE) growth condition of SrF 2 directly on Si(111) substrates has been optimized in terms of both X min and the surface morphology. Lattice distortion measurements were carried out with ion channeling along off-normal channeling directions in the strained layers grown at the optimal condition. The relationship of residual strain vs. film thickness for SrF 2 on Si(111) was provided by the first time. The experimental data demonstrated a special thickness in this relation, at which the derivative of strain vs. film thickness changes its sign. This unique behavior was understood as the result of competition between the large lattice mismatch and the large thermal mismatch between SrF 2 and Si.

Growth condition dependence of RHEED pattern from GaAs(111)B surface

A 3-dimensional phase diagram is introduced to describe the dependence of the RHEED pattern from GaAS (111)B surface on growth conditions. The 2 {times} 2, transitional (1 {times} 1), and {radical}19 {times} {radical}19 surface reconstructions correspond to different zones in the phase diagram. A equation is given for the planes that separate these zones, which fit experimental data well. Homoepitaxial films on GaAs(111)B grown in the 2 {times} 2 region generally have bad crystal quality as determined by the ion channeling, and growth in the {radical}19 {radical}19 region generally yields rough surface morphology. At higher substrate temperatures ({approximately} 650{degrees}C), featureless films with minimum ion channeling yields of less than 4% are achieved. 13 refs., 6 figs.