In-situ advantages for InP based materials

XRD referenced nk database for InP and related materials

InP based materials exhibit higher electron mobility and higher frequency response compared to GaAs. This makes InP HBTs a good candidate for next generation trans-impedance amplifiers in optical fiber communications and for 5G applications. Moreover, since InP HBT’s base bandgap energy is much lower than that of GaAs HBTs, the InP based device’s turn-on voltage and related power consumption are significantly lower. However, the high-yield MOCVD growth of device grade quaternary InGaAsP and InGaAlAs structures precisely lattice matched to InP is rather challenging, especially on larger wafers. The solution is in-situ process control based on accurate high temperature quaternary nk data.
Together with Dr. Tony SpringThorpe’s team at National Research Council of Canada and Christoph Hums and his co-workers at Fraunhofer HHI Berlin (Germany), LayTec has further improved the accuracy level of its nk database for these two quaternary material systems. At ICMOVPE XVIII, we presented the results of this work in the talk "MOCVD of InGaAsP/InP based device structures: full replacement of ex-situ process calibration by advanced in-situ metrology". During lattice matched growth of InGaAs on InP in an AIXTRON Crius reactor, the high-resolution wafer bow sensing (EpiCurve®TT Gen3 with ARS module) reached a resolution of 0.2km-1!

Two effects that contribute to the wafer bow were carefully separated: the lattice mismatch between layer and substrate as well as the vertical temperature gradient across the wafer resulting from temperature difference between wafer pocket and shower-head. In result, the lattice match of InGaAs to InP could be tuned in-situ with a ±50ppm resolution an accuracy that formerly could be achieved only by ex-situ X-ray diffraction (XRD). Download the PDF of the talk here 

Selected examples:

Fig. 1a shows the 633 nm refractive index of InGaAsP and InGaAlAs in the full composition range at three relevant growth temperatures T1<T2<T3. With n(x,T) and k(x,T) available for the full range of lattice matched quaternary compositions, precise and quantitative process control becomes straight forward.

Fig. 1: Control of device related InGaAsP and InGaAlAs film growth on InP: When lattice matched growth is validated by in-situ wafer bow sensing, both quaternary material systems can be treated as an effective quasi-ternary mixture: (InGaAs)x(InP)1-x and (InGaAs)x(AlGaAs)1-x , respectively.
Fig. 1a) The respective composition range (x=0…1) covers the reference (PL) wavelength range 0.92–1.65 µm for InGaAsP and 0.85–1.65 µm for InGaAlAs. The three lines for each material system give the 633 nm refractive index at three wafer temperatures.
Control of device related InGaAsP and InGaAlP film growth on InP
Fig. 1b) The quaternary/ternary layers (steps 17–29) are lattice matched to InP (step 15) as can be seen from the unchanged wafer curvature (green line). Hence, the measured 633 nm reflectance data (blue line) of an InP/InGaAsP device tructure can be exactly fitted (red line) yielding all compositions x and all growth rates. The –3 K reduction in wafer temperature due to the changed As/P ratio is a real effect.
Control of device related InGaAsP and InGaAlP film growth on InP
For this highly precise nk database, growth rates and accurate lattice match were carefully gauged to ex-situ XRD. Wafer temperatures were measured by  EpiTT that had been calibrated by AbsoluT.
For more information please feel free to contact or call +49(0)30 89 00 55-0. 

Analysis of nanometer scaled quaternary films with EpiNet 2016

LayTec's new release of the control and analysis software EpiNet2016 offers completely new analysis features for our customers interested in high-accuracy statistical process control (SPC) of related device growth processes. Fig.1 gives an example:

Fig. 1: Screenshot of the EpiNet 2016: data analysis of an InGaAsP/InP device structure on InP(001): the thickness of the three very thin InGaAsP layers in steps 2, 6, 10 is: 28.5nm, 48.7nm and 100.3nm respectively. The table in the lower part of the figure gives the sequence of analysis functions for routine and automated SPC of this device growth process.

The thickness of very thin InGaAsP layers in a device stack grown in an AIXTRON Planetary Reactor® on InP(001) is determined by a well selected set of automated analysis operations. First, several InP layers are utilized for permanent in-situ high-accuracy re-calibration of all reflectance channels (yellow lines) in long lasting epi runs. Second, the lattice matching of the quaternary layers is verified by wafer bow analysis (not shown). Third, the composition of the quaternary material is determined at the thick InGaAsP layer in step #14. And finally, based on this information, the thickness of the thin InGaAsP layers in steps #2, #6 and #10 is accurately measured by double-wavelength thickness analysis.

For better understanding of growth processes, LayTec offers related training courses for process engineers and quality managers. To learn more, please contact or call our sales engineer +49 30 89 00 55-0.