Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2002-11-18
2004-12-28
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S094000, C117S095000, C117S090000, C117S106000, C428S642000, C428S640000, C428S620000
Reexamination Certificate
active
06835246
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the growth of heteroepitaxial layers on silicon (Si) substrates and, more particularly, to the use of micro- and nanoscale, 1-dimensional and 2-dimensional periodic and random structures generated on silicon and other substrates for forming compliant, thin films suitable for gettering defects and for accommodating lattice and thermal expansion mismatches during heteroepitaxial growth thereon.
BACKGROUND OF THE INVENTION
Heteroepitaxial growth of pseudomorphic compound semiconductor films on silicon (Si) substrates has been a subject of enduring commercial interest due in part to applications in optoelectronics integrated circuits (See, e.g., O. Wada and J. Crow in
Integrated Optoelectronics
, edited by M. Dagenais et al., Academic Press (1995)), and low-cost, low-weight, high-efficiency solar cells having high mechanical strength (See, e.g., S. R. Messenger et al., 26
th
IEEE PVSC, 995 (1997)). High-quality heteroepitaxial growth on Si substrates beyond critical thickness h
c
is difficult to realize because of lattice constant and thermal expansion coefficient mismatches. For example, lattice expansion mismatch leads to misfit and threading dislocations resulting in a well-known crosshatch pattern for a SiGe layer system (See, e.g.,
Germanium-Silicon Strained Layers and Heterostructures
, Suresh C. Jain, Advances in Electronics and Electron Physics, Supplement 24, Academic Press (1994)). Due to a large thermal expansion coefficient mismatch during for cooling after growth; that is, between 8.35×10
−6
and 5.9×10
−6
for germanium (Ge) and between 4.27×10
−6
and 2.57×10
−6
for Si (See, e.g., M. T. Currie et al., Appl. Phys. Lett. 72, 1718 (1998)), a high density of microcracks (See, e.g., B. G. Yacobi et al., Appl. Phys. Lett. 51, 2236 (1987)) and wafer bowing (See, e.g., S. Sakai, Appl. Phys. Lett. 51, 1069 (1987)) has been observed.
For these types of lattice and thermal expansion mismatched systems, the performance of minority carrier devices such as solar cells and laser diodes is degraded due to enhanced recombination at the lattice dislocations (See, e.g., P. M. Sieg et al., Appl. Phys. Lett. 73, 3111 (1998)), although moderately successful majority carrier devices have been reported (See, e.g., R. M. Sieg et al., J. Vac. Sci. Technol. B16, 1471 (1998)). Research directed to growth of high-quality (defect density <10
5
cm
−2
) heteroepitaxial films on Si substrates has been a field of active research for many years.
Several distinct approaches have evolved with varying levels of success:
A. Graded Layer Approach
One manner of eliminating, or reducing lattice and thermal mismatches is to form a “virtual” substrate by growing a graded composition of the desired heteroepitaxial film on a defect-free Si substrate (See, e.g. D. J. Paul, Adv. Mater. 11, 191 (1999)). A layer having constant composition and the desired lattice parameter can then be grown on this buffer layer. By grading the composition, the misfit strain is distributed throughout the buffer layer thickness resulting in a three-dimensional misfit dislocation network. The primary objective of the constant composition layer is to achieve complete relaxation along with spatial separation from the underlying network of dislocations. Graded SiGe films have been prepared with low (between 10
5
and 10
6
cm
−2
) defect densities (See, e.g., J. H. Li et al., J. Appl. Phys. 82, 2881 (1997)). However, several difficulties remain with this approach including surface roughness due to a pronounced <110> crosshatch pattern that creates difficulties in lithographic patterning (See, e.g., M. A. Lutz et al., Appl. Phys. Lett. 66, 724 (1995)). Moreover, as the germanium concentration is increased, the crosshatch surface roughens further leading to an overlap of underlying strain fields, which tends to block threading dislocation glide and enhances dislocation pileups (See, e.g., S. B. Samavedam and E. A. Fitzgerald, J. Appl. Phys. 81, 3108 (1997)).
For GaAs on Si, similar approaches have been developed. A stress-balance approach based on GaAs
1−x
P
x
(See, e.g., A. Freundlich et al., Appl. Phys. Lett. 59, 3568 (1991)) and AlAs (See, e.g., J. D. Boeck et al., Appl. Phys. Lett. 59, 1179 (1991)) buffer layers has been investigated. An alternative approach is the application of strained layer super lattices of GaP/GaAsP and GaAsP/GaAs to relax lattice mismatch between GaP and GaAs (See, e.g., T. Soga et al., J. Cryst. Growth 77, 498 (1986)). However, there remain problems with a high-density of defects in thick GaAs films and in achieving single domain structure over the entire substrate due to thermal expansion coefficient mismatch between GaAs and Si. This results in stress and changes in lattice constant and band structure for GaAs grown on Si from those grown on GaAs substrates.
B. Finite Area Growth
Mathews, et al., first proposed that limiting the lateral dimensions of the sample prior to growth could reduce the density of threading dislocations (See, e.g., J. W. Mathews et al., J. Appl. Phys. 41, 3800 (1970)). Subsequently, this approach has been extensively investigated for a wide range of material systems. Fitzgerald, et al. investigated misfit dislocations in growth of In
0.05
Ga
0.95
films on 2-&mgr;m-high mesas having various lateral dimensions and geometries on (001) GaAs substrates (See, e.g., E. A. Fitzgerald et al., Appl. Phys. Lett. 52, 1496 (1988)). A reduction of linear interface dislocation density from about 5000/cm to approximately 800/cm for mesas as large as 100 &mgr;m was demonstrated. Yamaguchi et al. in Appl. Phys. Lett. 56, 27 (1989) and E. A. Fitzgerald and N. Chand in J. Electron. Mat., 20, 839 (1991) later extended this approach to GaAs growth on patterned Si substrates. Yamaguchi, et al., teaches that the dislocation density of GaAs on Si is due to thermal stress, and that some stress relief is provided by the finite edges resulting in the reduction in dislocation density. Defect densities were reduced to approximately 1×10
−6
cm
−2
by a combination of thermal cycle annealing and lateral dimensions of about 10 &mgr;m.
Defect densities have the potential of being reduced to <10
5
cm
−2
by growth on substrates with finer lateral dimensions. The finite growth region can either be defined by vertical etching (See, e.g., E. A. Fitzgerald, J. Vac. Sci. Technol. B7, 782 (1989)), or use of an oxide mask (See, e.g., D. B. Noble et al., Appl. Phys. Lett. 56, 51 (1990)).
C. Growth on Nanoscale Structures
In modeling critical layer thickness, h
c
, of strained hetero layers on lattice mismatched nanostructured substrates, Luryi and Suhir determined that critical layer thickness increases sharply as finite seed areas are reduced (See, e.g., S. Luryi and E. Suhir, Appl. Phys. Lett. 49, 140 (1986)). According to this model, for Ge on Si, seed dimensions required are about 10 nm with a separation of approximately 3 nm, which eliminates most low-cost lithographic systems. Porous Si films support somewhat similar structures, and several growth studies were undertaken to evaluate model predictions. GaAs films grown on porous Si were found to contain a high density of microtwins and stacking faults originating from the roughness of the porous Si interface (See, e.g., Y. J. Mii et al., J. Vac. Sci. Technol. B6, 695 (1988)). Ge
x
Si
1−x
films grown on porous Si showed a predominance of 60° dislocations with long misfit segments (See, e.g., Y. H. Xie and J. C. Bean, J. Vac. Sci. Technol. B8, 227 (1990)). In both cases, no reduction in either strain or dislocation density was observed when compared to growth on planar areas. This lack of agreement with the model may be attributed to the interconnected nature of porous Si structure as opposed to the isolated trenches assumed in the model.
D. Growth on Compliant Substrates
An alternative model was later proposed by Lo (See, e.g., Y. H. Lo, Appl. Phys. Lett. 59, 3211 (1991)) based on the prem
Cochran Freund & Young LLC
Freund Samuel M.
Kunemund Robert
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