Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2001-12-13
2004-10-19
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S095000, C117S101000, C117S089000, C117S955000
Reexamination Certificate
active
06805744
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the field of producing device quality (Al)InGaP alloys on lattice-mismatched substrates.
Epitaxial graded composition buffers of In
x
Ga
1−x
P on GaP substrates (In
x
Ga
1−x
P/GaP) are promising substrates for high performance optoelectronic devices. In
x
Ga
1−x
P alloys with large bandgaps that are difficult or impossible to achieve lattice-matched to GaAs substrates can be grown on graded buffers, providing direct bandgap emission of the critical green to orange wavelengths that lie between the capabilities of GaN-based and GaAs-based light emitting diode (LED) and laser diode technologies. In
x
Ga
1−x
P/GaP substrates are also inherently transparent to devices grown on them, which roughly doubles light extraction efficiency in LEDs compared to absorbing substrates such as GaAs. The transparency of In
x
Ga
1−x
P/GaP has also been used to produce negative electron affinity GaAs and InGaAs photocathodes that operate in transmission mode, and a variety of other optoelectronic detectors and modulators can be envisioned to take advantage of a transparent semiconductor substrate. Furthermore, GaP is nearly lattice-matched to Si, so In
x
Ga
1−x
P/GaP is one natural choice for integrating compound semiconductor devices on Si substrates.
Graded buffers are grown to efficiently relieve lattice-mismatch strain between substrates and films of differing lattice constants. For most optoelectronic device applications, direct bandgap compositions of In
x
Ga
1−x
P are desired. The >2% lattice-mismatch between GaP and direct bandgap compositions of In
x
Ga
1−x
P results in heavily defective single heterostructures, due to the large and abrupt introduction of strain at one interface. A graded buffer of In
x
Ga
1−x
P on GaP slowly introduces strain over many interfaces, which minimizes dislocation interactions, maintains a low state of strain, and minimizes dislocation nucleation during growth. Consequently, graded buffers typically have orders of magnitude lower threading dislocation densities than single heterostructures.
The growth of In
x
Ga
1−x
P/GaP has been studied for decades using a variety of growth techniques, including hydride vapor phase epitaxy (HVPE), gas-source molecular beam epitaxy (GSMBE), and metal-organic vapor phase epitaxy (MOVPE). Early HVPE experiments with In
x
Ga
1−x
P/GaP and GaAs
x
P
1−x
/GaAs established some of the basic principles of dislocation dynamics in graded buffers. Since then, visible LEDs have been demonstrated on In
x
Ga
1−x
P/GaP. HVPE has been used to produce LEDs operating at wavelengths from 565 nm to 650 nm, however, device efficiency decreases dramatically when In
x
Ga
1−x
P/GaP is graded beyond x~0.35. GSMBE has been used to grow In
x
Ga
1−
P/GaP with photoluminescence (PL) ranging from 560 nm to 600 nm, with decreasing PL intensity in buffers graded beyond x~0.32.
The agreement of results showing degradation beyond x~0.3 with two very different growth techniques is striking. Both techniques result in the conclusion that material degradation is a natural consequence of increasing lattice-mismatch, presumably through increasing defect density. This intuitive picture is inconsistent with earlier work, which concluded from experimental and theoretical considerations that strain relaxation in graded buffers is a steady-state process, hence defect density should be constant.
Developments in the Ge
x
Si
1−x
/Si system have provided new insights into dislocation dynamics in graded buffers that can aid in understanding In
x
Ga
1−x
P/GaP. It has been demonstrated that the formation of dislocation pileups is the primary cause of material degradation with continued grading in Ge
x
Si
1−x
/Si. Since dislocations immobilized in pileups can no longer glide to relieve strain, pileups force the nucleation of additional dislocations to continue the relaxation process. It has been proposed that pileups were caused by an interaction between dislocations and surface roughness. Misfit dislocation strain fields produce surface undulations and gliding dislocations can be pinned in between, which leads to pileups. Surface roughness increases as more dislocations are pinned, resulting in a recursive and escalating cycle of dislocation pinning and surface roughening. It was then showed that controlling surface roughness by periodic planarization can suppress pileup formation in Ge
x
Si
1−x
/Si and recover a steady-state dislocation density between x=0.3 to x=1. The recovery of steady-state dislocation dynamics is compelling evidence that pileup formation due to the interaction of dislocations and surface roughness is responsible for material degradation with continued grading.
Recent work with In
x
Ga
1−x
P/GaP grown by MOVPE also showed a strong correlation between surface roughness and the density of dislocations and pileups. Pileup formation was tentatively attributed to the proposed mechanisms, but comparison with Ge
x
Si
1−x
/Si results suggests that the sensitivity of defect density to surface roughness is much greater than expected in In
x
Ga
1−x
P/GaP. Related work with In
x
Ga
1−x
As/GaAs noted the presence of “high-energy boundaries” that appeared to pin dislocations, although their overall impact on relaxation was unclear. Defects similar to the “high-energy boundaries” have not been observed in Ge
x
Si
1−x
/Si, so defects of this type may account for the difference in pileup behavior noticed between In
x
Ga
1−x
P/GaP and Ge
x
Si
1−x
/Si.
SUMMARY OF THE INVENTION
In accordance with the invention, the evolution of dislocation dynamics in In
x
Ga
1−x
P/GaP grown by MOVPE is explored. Starting with the question of escalating defect density a previously unrecognized defect microstructure that causes pileups and dominates dislocation dynamics in In
x
Ga
1−x
P/GaP is shown by microscopic characterization and macroscopic modeling. The evolution of microstructure in graded buffers is mapped, and its interaction with dislocation dynamics is modeled. By controlling the new defect microstructure, nearly ideal relaxation behavior dominated by dislocation kinetics is also observed. Through analysis and modeling, a proposed kinetic model for relaxation in graded buffers is confirmed. The evolution of branch defects is used to explain the microstructure of both indium-bearing phosphides and arsenides over a wide range of conditions. The new understanding and control of dislocation dynamics and microstructure are used to derive a set of design rules and an optimization strategy for high quality graded buffer growth. A simple process optimization results in material with a dislocation density of 4.7×10
6
cm
−2
at x=0.39.
Accordingly, the invention provides a method of forming a semiconductor structure including providing a single crystal semiconductor substrate of GaP, and fabricating a graded composition buffer including a plurality of epitaxial semiconductor In
x
(Al
y
Ga
1−y
)
1−x
P alloy layers. The buffer includes a first alloy layer immediately contacting the substrate having a lattice constant that is nearly identical to that of the substrate, subsequent alloy layers having lattice constants that differ from adjacent layers by less than 1%, and a final alloy layer having a lattice constant that is substantially different from the substrate. The growth temperature of the final alloy layer is at least 20° C. less than the growth temperature of the first alloy layer.
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Chin, T.P., J.C.P. Chang, K.L. Kavanagh and C.W. Tu, “Gas-source molecular beam epitaxial growth, characterization, and light-emitting diode app
Fitzgerald Eugene A.
Kim Andrew Y.
Gauthier & Connors LLP
Kunemund Robert
Massachusetts Institute of Technology
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