21 February 2017

MIT optimizes QCL epitaxy using LayTec's EpiCurve TT

In-situ metrology system maker LayTec AG of Berlin, Germany says that new in-situ results by Dr Christine Wang and colleagues at the MIT Lincoln Laboratory in the USA have been presented in a recent paper (Wang C.A. et al., Journal of Crystal Growth (2016); http://dx.doi.org/10.1016/j.jcrysgro.2016.11.029).

The team worked on improving metal-organic chemical vapor deposition (MOCVD) growth of indium phosphide (InP)-based quantum cascade lasers (QCLs) in a Veeco D-125 multi-wafer (3x2") reactor. Wang reported two major findings:

(1) The growth of high-performance QCL structures requires the deposition of a complex sequence of coupled quantum wells (AlInAs, GaInAs). During this process, the cumulative indium surface segregation must be carefully compensated in the growth recipe to keep the targeted lattice matching throughout the full QCL structure.

Figure 1: In-situ measurements of AlInAs/GaInAs MQWs: reflectance at 450nm (blue) and wafer curvature (black): (a) varying GaInAs well layer thickness dw with constant 10nm AlInAs barrier layer thickness dB; (b) varying AlInAs barrier layer thickness dB with constant GaInAs 10nm well layer thickness dw.

(2) Even under optimized growth conditions, a certain interface grading is unavoidable, especially at the GaInAs-to-AlInAs interfaces. However, optimum QCL performance can be achieved by taking into account these interface grading effects in the calculations of the QCL target structure.

EpiCurve TT was the key to these findings. Figure 1(a) shows in-situ reflectance at a wavelength of 450nm and the curvature of the two decisive MQW runs. Reflectance oscillations correspond to each barrier and well layer, and each layer is easily resolved. In figure 1(b), the high-resolution wafer bow sensing verified the compressive strain accumulation at the AlInAs-to-GaInAs interface. After recipe optimization, the 450nm reflectance could be used as a characteristic finger-print of the formation of every single graded interface among the hundreds constituting a single QCL layers.

Tags: LayTec Metrology MOCVD

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