Optimization of InGaAs Metamorphic Buffer For Laser Diodes Using EpiCurve TT
(Nanowerk News) Temperature characteristics of laser diodes (LDs) on GaAs substrate depend on the quality and the thermal resistance of InGaAs metamorphic buffer layer. At the Electronic Materials Symposium (July 2013, Japan), NTT Photonics Laboratories (NTT) presented a new method for optimization of the metamorphic buffer layer for freestanding quasi-InGaAs substrates.
Mr. Ryo Nakao from NTT uses LayTec‘s in-situ metrology system EpiCurve® TT to improve layer thickness and indium content in order to fabricate a thin InGaAs metamorphic buffer with low thermal resistance. The in-situ tool helps understand the MOVPE growth conditions and shows the changes in wafer curvature caused by residual strain during MOVPE growth.
In-situ curvature data of InGaAs buffer and quasi-InGaAs substrate on GaAs substrate.
Fig. 1 shows in-situ curvature data for a 1250 nm metamorphic InGaAs buffer layer and a quasi-InGaAs substrate layer. The growth can be separated into 3 parts: I - Coherent (pseudomorphic) growth of the buffer, II - Growth with relaxation, III - Free-standing (unstrained) growth. During relaxation, the in-plane lattice constant of the In0.12Ga0.88As layer increases. After the buffer has reached 1250 nm, its lattice constant exactly matches the lattice constant of In0.10Ga0.90As. As a result, the wafer curvature does not change during the subsequent growth of In0.10Ga0.90As layer.
The authors call this final growth phase “free-standing” because the In0.10Ga0.90As layer grows with its natural lattice constant and creates a quasi-substrate similar to an In0.10Ga0.90As wafer for the later device growth.
Further experiments with the same indium content and different buffer thicknesses showed that if the relaxation is not sufficient (buffer thickness=1000 nm), the quasi-In0.10Ga0.90As substrate growth is compressively strained. However, over-relaxation (1600 nm) results in a tensile strain.
The researchers plotted these changes to obtain a map of the correlation between thickness and curvature. For a free-standing quasi-InGaAs substrate, they choose a buffer thickness with no further variation in curvature over time during In0.10Ga0.90As growth.
This work shows that in-situ curvature measurements are a decisive part of an innovative technology that is developed to improve characteristics of LDs.