G. Patrick Watson

The measurement of deep level states caused by misfit dislocations in InGaAs/GaAs grown on patterned GaAs substrates

Rectangular Schottky diodes were fabricated on In0.06Ga0.94As grown by organometallic vapor phase epitaxy on GaAs substrates patterned with mesas. The density of α and β misfit dislocations at the strained‐layer interface changed with the size of the rectangular mesas. Since all mesas (four sizes and two orientations) are processed simultaneously, all other defect concentrations are expected to remain constant in each diode. Scanning cathodoluminescence showed that the misfit dislocation density varied linearly with rectangle size. Deep‐level transient spectroscopy showed that an n‐type majority‐carrier trap is present at 0.58 eV below the conduction band with a concentration that increases with increasing α‐type misfit dislocation density. The β misfit dislocation density had no influence the deep level spectra, indicating that this trap is related to the cores of only α‐type misfit dislocations. The capture rate trend corroborates the view that the trap is associated with the dislocation cores and not w...

From Relaxed GeSi Buffers to Field Effect Transistors: Current Status and Future Prospects

We review the current status of relaxed GeSi buffer layers, high-mobility two-dimensional electron and hole gases fabricated on top of these layers, and GeSi-based field effect transistors (FETs). Recent progress in these fields is emphasized. A brief summary is then given for a variety of GeSi-based FET fabrications to date. Finally, the future prospects of GeSi-based FETs to be integrated into Si VLSI production is discussed.

The SCattering with Angular Limitation in Projection Electron-Beam Lithography (SCALPEL) System

A SCALPEL\circledR (SCattering with Angular Limitation in Projection Electron-beam Lithography) proof-of-concept lithography system, comprising a tool, a reticle and a resist, has been designed to address the critical issues that must be investigated to determine if this approach is viable as a practical lithographic technology.

Dose modification proximity effect correction scheme with inherent forward scattering corrections

A new approach to dose modification proximity effect correction (PEC) has been proposed that accounts for both short range and long range scatter in advanced direct-write electron beam lithography systems. This scheme can be applied to device writing and to optical mask making where critical dimension (CD) control is of increasing importance. This technique is unique because detailed knowledge of the short range scatter dose distribution is unnecessary; by correcting only for long range scatter, both short and long range compensation is obtained. The new approach is based on the fact that if the characteristic distance of short range scatter is about one third or less of the smallest feature, and if each feature is at least five addressable pixels in the beam writer, then the feature can be printed at its coded dimension if the resist clearing or threshold dose is at the midpoint of the dose profile. Si wafers coated with a positive tone e-beam resist were exposed to a test pattern using no PEC, conventio...

The thermal stability of lattice mismatched InGaAs grown on patterned GaAs

Patterning and etching substrates into mesas separated by trenches before the growth of mismatched (by about 1% or less) epitaxial layers considerably reduces the interface misfit dislocation density when the layer thickness exceeds the critical thickness. Such films are in a metastable state, since misfit dislocations allow the epitaxial layers to relax to an in-plane lattice parameter closer to its strain-free value. Thermal annealing (from 600 to 850° C) has been used to study the stability of these structures to explore the properties of the misfit dislocations and their formation. The misfit dislocation density was determined by counting the dark line defects at the InGaAs/GaAs interface, imaged by scanning cathodoluminescence. InGaAs epitaxial layers grown on patterned GaAs substrates by organometallic chemical vapor deposition possess a very small as-grown misfit dislocation density, and even after severe annealing for up to 300 sec at 800° C the defect density is less than 1500 cm−1 for a In0.04Ga0.96As, 300 nm thick layer (about 25% of the dislocation density found in unpatterned material that has not been annealed). The misfit dislocation nucleation properties of the material are found to depend on the trench depth; samples made with deeper (greater than 0.5 μm) trenches are more stable. Molecular beam epitaxially grown layers are much less stable than the above material; misfit dislocations nucleate in much greater numbers than in comparable organo-metallic chemical vapor deposited material at all of the temperatures studied.

Measurement of the backscatter coefficient using resist response curves for 20–100 keV electron beam lithography on Si

The effective backscatter coefficient η is a quantity that must be known with precision so that the proximity effect can be adequately compensated to minimize feature size variations in electron beam lithography. A unique technique to measure η that does not require the precise form of the backscatter dose distribution was employed. This method simply compares the resist response near the center of printed features that are much smaller and much larger than the characteristic range of the long range scatter. This technique was already employed to estimate the backscatter coefficient on Si at 100 keV beam energies. We have extended this measurement to determine η at lower beam energies. Results show that η on Si is 0.38, 0.50, 0.55, and 0.46 for 100, 50, 40, and 20 keV beam energies, respectively. Monte Carlo simulations indicate a trend of decreasing η with increasing beam energy, consistent with the experimental results except at 20 keV.

Influence of trench depth on the misfit dislocation density at strained epitaxial layer interfaces grown on patterned GaAs substrates

The number of misfit dislocations in strained, epitaxial InGaAs layers can be significantly reduced by growing on patterned substrates. Etched trenches on these substrates block misfit dislocation propagation from mesa to mesa. To determine the minimum trench depth needed to block misfit dislocations, 200 μm×200 μm mesas separated by 10‐μm‐wide trenches of varying depths were etched into a GaAs substrate before organometallic chemical vapor deposition of 300 or 600 nm In0.04Ga0.96As strained layers (several times the critical thickness). Three isolation regimes are seen in the 300‐nm‐thick InGaAs samples. The shallowest trenches, regime I, below 300 nm, do not completely block all dislocations. Misfit dislocations are blocked by trenches 300–500 nm deep, regime II, after they glide down the mesa walls and stop at the far side of the trenches. Trenches greater than 500 nm, regime III, stop dislocations at the mesa edges. The 600‐nm‐thick InGaAs layers with trench depths greater than 450 nm completely stopp...

The Fabrication of Self-Aligned Ohmic Cobalt Contacts to Relaxed, N-Type Si 0.7 Ge 0.3

Co(SiGe) x contacts have been formed on low defect density, relaxed Si 0.7 Ge 0.3 layers by conventional self-aligned contact processing techniques. Test structures measuring the contact metal sheet resistance indicate that the resistivity is high for anneal temperatures from 450°C to 750°C. The lowest sheet resistivity was 100 Ω/□, about 10 times the resistivity of a comparable amount of CoSi 2 . Contact resistivities, measured by the transmission line method, were as low as 2 × 10 −5 Ω cm 2 . There is a large discrepancy between contact resistivities measured by transmission line and 4 point Kelvin test structures that may be due to the fabricated contact sizes.

Variation of lattice parameter with silicon concentration in n‐doped, liquid‐encapsulated Czochralski GaAs single crystals

The lattice parameters of several Si‐doped and undoped GaAs wafers have been measured, using the Bond x‐ray diffraction technique. The relative lattice parameters of wafers from the same boule were found to decrease monotonically from seed to tail by as much as 6×10−5(δa/a), following the same trend as the increase in Si content ranging from 1.3 to 9.4×1018 cm−3 (measured by secondary‐ion mass spectroscopy). A plot of the change in lattice parameter versus the Si concentration shows a linear trend with a slope which is three times larger than that predicted by Vegard’s law. Possible explanations for this discrepancy are discussed.

Characterizing GHOST proximity effect correction effectiveness by determining the worst-case error

Background proximity effect corrections such as GHOST and the proposed SCALPEL correction technique have been analyzed to determine how well they can perform in situations where the correction dose distribution is not ideal. A technique has been developed that evaluates the largest possible correction dose error by determining the worst-case exposure pattern. This pattern is found to be unlikely to occur in integrated circuit patterns, so that the dose errors found in realistic situations are expected to be much smaller. For Gaussian shaped GHOST correction energy distributions, the worst-case pattern resembles a disk or a bull’s eye. The optimal correction dose distribution has a width parameter of about 90% of the backscatter width. The worst-case background dose error is less than +/−2% for a backscatter coefficient of 0.40. For a correction dose that resembles a disk of constant intensity, the worst-case error is typically larger than the case of a Gaussian shape. The smallest worst-case error occurs ...