Self-Assembled III-Phospide Quantum Dots Grown by Metalorganic Chemical Vapor Deposition
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temperature.'
Also,
a "blue-shift" of emission spectrum to shorter
wavelengths due to multi-dimensional quantization is expected. Among various fabrication techniques, direct growth of self-assembled quantum dots (SAQD or simply QD) on a planar surface is a most promising one, due to its ability to form virtually defect free 3-D confined structures, without employing etching that could cause degradation of the semiconductor materials. SAQDs are typically grown in a Stranski-Krastanow mode 2, which is a 3-D growth that follows a 2-D growth due to induced strain from lattice mismatch between the substrate and the materials used for SAQD. Coherent SAQDs are known as dislocation-free QDs. Arsenide-based QD structures were first investigated. For example, the growth of GaAs/InAs QD 3 ; AIAs/InAs QD 4 ; InGaAs/InAs QD 5 ; InP/InAs QD 6; GaP/InAs QD 7 ; GaAs/InGaAs QD 8 ; and AIGaAs/InGaAs QD 9 were reported previously. Those QD structures often showed the expected "zero-dimensional" PL characteristics. While arsenide-based structures have been extensively investigated, phosphide-based QD structures, which have a wider bandgap, have been relatively less studied. In this work, Ino s5 Al o0 49P/InP SAQDs on GaAs substrates are grown and characterized to study the optimum condition and growth characteristics for InP SAQDs. GaP/InxAl1 .xP SAQDs are also grown and compared to InP SAQD morphology.
39 Mat. Res. Soc. Symp. Proc. Vol. 583 © 2000 Materials Research Society
Table I: Growth parameter summary in this study. System
Ino. 51A10 .49P/InP QD
Growth parameter Variable Growth temperature Growth time
Growth condition 450C
1.125ML Post purge time 0min V/111 ratio 4366
GaP/ InxAlI.P QQD
D composition
15ML growth I 500C At550C 1.6875ML I 3.75ML At 550C, after 7.5ML growth Imin I 3mi At 550C, 2.25ML growth 1092 2183 At650C 475C
x=-0.7
550C 7.5ML 5min 514
x=-0-.85
EXPERIMENT SAQDs, cladding layers, and buffer layers are grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) in EMCORE GS3200-UTM reactor system, operating at -60 Torr, using H 2 as a carrier gas. Sources used in the growth are trimethylindium (TMIn), triethylgallium (TEGa), and trimethylaluminum (TMA1) for column III elements, and phosphine (PH 3 ) and arsine (AsH3 ) for column V elements. For InP SAQDs embedded in In0 51A10. 49P, GaAs buffer and 0.5[tm thick In 0 51A10.4 9P cladding layers are first grown on a GaAs (100) substrate, followed by InP QDs. The In0 51A10.49P cladding layer composition is analyzed using high-resolution X-ray rocking curves to determine the degree of lattice matching to substrate. The layer is calibrated to have less than -100 arc second angle separation relative to the GaAs substrate in order to minimize the strain effect from the cladding layer. InP SAQDs are grown with various growth temperatures and times to see the effect on QD average sizes and densities. The post-purging effect is studied by varying the purging time and temperature. The effect of V/Ill ratio on SAQD morphology is also studied. For S
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