M. S. Goorsky

Synthesis of Si1−yCy alloys by molecular beam epitaxy

We have synthesized pseudomorphic Si1−yCy (y≤0.05) alloys and strained layer superlattices on silicon by molecular beam epitaxy using solid sources for carbon and silicon. The introduction of C into substitutional sites in the silicon lattice is kinetically stabilized by low‐temperature growth conditions (500–600 °C) and relatively high Si fluxes, against extremely low C solubility (10−6 at 1420 °C) and the thermodynamically favored silicon carbide phases. Higher temperature growth leads to an islanded morphology. At lower temperatures, disruption of epitaxy occurs via the formation of highly twinned layers or even amorphous growth. The temperature window for alloy growth is reduced as the C concentration is increased. X‐ray diffraction, transmission electron microscopy, secondary ion mass spectroscopy, and Raman spectroscopy confirm the growth of pseudomorphic, tetragonally strained alloy layers with no detectable silicon carbide precipitation. These alloy layers allow for the engineering of Si‐based lat...

Lattice contraction due to carbon doping of GaAs grown by metalorganic molecular beam epitaxy

Epitaxial layers of GaAs have been grown by metalorganic molecular beam epitaxy (MOMBE) with atomic carbon concentrations ranging from 4×1017 to 3.5×1020 cm−3. The dependences of GaAs lattice parameter and hole concentration on atomic carbon concentration have been determined from x‐ray diffraction, Hall effect, and secondary‐ion mass spectrometry measurements. For atomic carbon concentrations in excess of 1×1019 cm−3, the hole concentrations are less than the corresponding atomic carbon concentrations. Lattice parameter shifts as large as 0.2% are observed for carbon concentrations in excess of 1×1020 cm−3, which results in misfit dislocation generation in some cases due to the lattice mismatch between the C‐doped epilayer and undoped substrate. Over the entire range of carbon concentrations investigated, Vegard’s law accurately predicts the observed lattice contraction.

Thermal stability of Si1−xCx/Si strained layer superlattices

The thermal stability of epitaxial silicon‐carbon alloys grown by molecular beam epitaxy on (001) silicon was investigated using high resolution x‐ray diffraction, transmission electron microscopy, and secondary ion mass spectroscopy measurements. Different superlattices, with alloy compositions of Si0.997C0.003, Si0.992C0.008, and Si0.985C0.015, all nominally 6 nm thick, with 24 nm Si spacer layers were employed. Annealing studies determined that there are different pathways to strain relaxation in this material system. At annealing temperatures of 900 °C and below, the structures relax only by interdiffusion, indicating that these layers are stable during typical device processing steps. At temperatures of 1000 °C and above, SiC precipitation dominates with enhanced precipitation in the Si1−xCx layers with the highest initial carbon content.

Characterization of epitaxial GaAs and AlxGa1−xAs layers doped with oxygen

Intentional oxygen doping (≳1017 cm−3) of GaAs and Al0.30Ga0.70As epitaxial layers was achieved during metalorganic vapor phase epitaxy through use of an oxygen‐bearing metalorganic precursor, dimethylaluminum methoxide (CH3)2AlOCH3. The incorporation of oxygen and very low levels of Al (AlAs mole fraction <0.005) in the GaAs layers leads to the compensation of intentionally introduced Si donors. Additionally, deep levels in GaAs associated with oxygen were detected. The introduction of dimethyl aluminum methoxide during AlxGa1−xAs growth did not alter Al mode fraction or degrade the crystallinity of the ternary layers, but did incorporate high levels of oxygen which compensated Si donors. The compensation in both GaAs and Al0.30Ga0.70As indicates that high resistivity buffer layers can be grown by oxygen doping during metalorganic vapor phase epitaxy.

Determination of epitaxial AlxGa1−xAs composition from x‐ray diffraction measurements

The correlation between the aluminum composition in epitaxial AlxGa1−xAs and double crystal x‐ray diffraction measurements was quantitatively determined. The angular separation ΔΘ, between the diffraction peaks from the AlxGa1−xAs layer grown by metalorganic vapor phase epitaxy and the GaAs substrate increased nonlinearly with the Al content, which was independently determined using photoluminescence and electron microprobe measurements. The calibration curve was used to determine AlAs materials parameters. The AlAs lattice constant and Poisson ratio were determined to be 5.6622 A and 0.275, respectively, assuming that the GaAs parameters are 5.65325 A and 0.311.

Selective epitaxy of GaAs, AlxGa1−xAs, and InxGa1−xAs

Abstract Many device structures benefit from the ability to selectively deposit epitaxial materials. Through the use of a masking material, such as Si3N4 or SiO2, on the substrate surface, patterns generated through standard lithographic procedures can be used to define regions for selective deposition. Highly selective growth can be achieved through the use of growth precursors which contain halogens, such as (C2H5)2GaCl and (C2H5)2AlCl. These compounds decompose, most probably, to the volatile mono-halogen species, e.g. GaCl, and also generate HCl in the gas phase as a reaction by-product. We present experimental results on the morphology and growth behavior of GaAs, AlxGa1−xAs, and InxGa1−xAs using this selective epitaxy technique. Electri cal and optical characterization has been carried out on these materials and selectively grown structures produced by this technique. The interface between the selectively grown material and the underlying substrate was investigated and the conditions for achieving high quality electrical interfaces were determined. A thermodynamic model of this growth chemistry predicts the trends in composition and growth rate. The thermodynamic model, based on the quasi-equilibrium of the halogen-based compounds with the substrate surface, indicates that the growth behavior is very similar to the inorganic-based growth of these compounds. Experimental applications of this technique to high speed digital device structures and sub-micron dimensioned optical structures are presented.

Characterization of Si/SiGe strained‐layer superlattices grown by ultrahigh vacuum/chemical vapor deposition technique

We employed high‐resolution double‐crystal x‐ray diffraction and transmission electron microscopy to characterize Si/Si1−xGex strained‐layer superlattices grown by ultrahigh vacuum/chemical vapor deposition technique. Rocking curve analyses showed uniform layer thickness and alloy composition across superlattices of 10 periods. Extensive dynamical x‐ray simulation indicated that heterointerfaces were abrupt and the Si layer was found to be 206±5 A thick and SiGe layer was 8.25% Ge and 185±5 A thick. The thickness values were confirmed by the cross‐sectional transmission electron microscopy. A tilt angle of 26 arcsec was observed between the (001) planes in the superlattice and the substrate, resulting from steps on the surface of 〈100〉 2° off oriented Si substrates.

Selective growth of silicon‐germanium alloys by atmospheric‐pressure chemical vapor deposition at low temperatures

Dichlorosilane and germane were used to grow silicon‐germanium alloys at temperatures as low as 550 °C at atmospheric pressure. The silicon‐germanium alloy composition was varied over the range 15%–44%. Films containing high Ge mole fractions were grown at a temperature of 625 °C and below and exhibit smooth surface morphology. Silicon‐germanium/silicon multilayers with abrupt heterointerfaces have been achieved. Selective growth of silicon‐germanium on oxide patterned silicon wafers was also demonstrated. A significant feature of the selective deposition is the lack of faceting at the oxide sidewall, which has been commonly observed in high‐temperature silicon growth.

Acceptor doping of (Al,Ga)As using carbon by metalorganic vapor phase epitaxy

Abstract Carbon doping of Al x Ga 1− x As with x = 0 to 0.3 has been investigated using trimethylarsine (TMAs) as the carbon precursor. Carbon concentrations from 5×10 17 to ⋍ 10 20 cm -3 have been achieved using AsH 3 /TMAs mixtures or TMAs alone. The carbon concentration increases with aluminum composition and decreasing AsH 3 /TMAs ratio. A contraction in the lattice constant is observed for carbon concentrations larger than ⋍ 10 18 cm -3 which is attributed to substitutional carbon. The carbon incorporation is non-uniform at higher carbon concentrations. The electrical activation of carbon in (Al,Ga)As is however, very low, of the order of 5%.

The role of dislocation‐dislocation interactions in the relaxation of pseudomorphically strained semiconductors. II. Experiment−The high‐temperature relaxation of ultrahigh‐vacuum chemical‐vapor‐deposited SiGe thin films

The thermal relaxation of SiGe films deposited by ultrahigh‐vacuum chemical vapor deposition was studied by annealing the films for times up to 21/2 h at a temperature of 950 °C. Strain relaxation was determined by misfit dislocation density obtained by planar‐view transmission electron microscopy and by double‐crystal x‐ray diffractometry. When the relaxation process requires relatively few dislocations (≲2 μm), the films relax to a remnant strain which is in agreement with previous experimental measurements; however, when higher densities of misfit dislocations were generated, substantially larger remnant strains were observed. This is interpreted as resulting from energetic interactions among the dislocations and analyzed in terms of the theory developed previously. It is found that the cutoff distance for dislocation interactions is substantially greater than the film thickness and a value of 1.4±0.5 μm is determined for 75–150‐nm‐thick films. Limited data from the literature also indicate a cutoff di...