Multibandgap quantum well wafers by IR laser quantum well intermixing: simulation of the lateral resolution of the process. O. Voznyy , R. Stanowski, J.J. Dubowski Department of Electrical and Computer Engineering Research Center for Nanofabrication and Nanocharacterization
O. Voznyy, R. Stanowski, J.J. Dubowski
Department of Electrical and Computer Engineering
Research Center for Nanofabrication and Nanocharacterization
Université de Sherbrooke, Sherbrooke, Québec J1K 2R1
E0 E1 E2 E3Quantum well intermixing
Multibandgap materials are needed for creation of photonic integrated circuits (lasers, modulators, waveguides, multi-color detectors etc. fabricated on same wafer)
Quantum well intermixing (QWI) – interdiffusion of wells and barriers resulting in the change of the well width, potential barrier height and energy of confined states.
Simulations  predict transition region ~300μmusing CW Nd:YAG laser irradiation (photoabsorbtion induced disordering) with a shadow mask. Also, pulsed laser IR disordering(2-step process) has been proposed (~2μm transtion region possible).
Our aim is to investigate Laser-RTA (annealing with a moving CW laser beam) as a flexible (1-step process) and potentially cost-effective technique.
 A. McKee, et. al., IEEE J. Quantum Electron., vol. 33, pp. 45–55, Jan. 1997.
 B.S.Ooi,et. al. IEEE J. Quantum Electron., vol. 40, pp.481–490, May 2004
In previous work  array of 12 lines of intermixed GaAs/AlGaAs QW material was successfully written with 5cm/s, 0.7mm CW Nd:YAG laser beam in a
14 mm x6 mm sample.
This approach has the potential to write complex patterns of intermixed material.
 J.J. Dubowski, et. al., Proc. SPIE, 5339, (2004).
Quantum well PL peak position
measured across the sample
irradiated with a fast scanning laser beam
that was used to generate a 12-line pattern.
To find heat distribution in a wafer we used FEMLAB commercial software.
Geometry is divided into small mesh elements with their own PDE parameters. Then the resulting system of PDEs is solved.
Heat transfer PDE:
Subdomain equation: Q - (kT)= Cp(T/t)
Boundary equation:kT=q0 + h(Tinf – T) + εσ(Tamb4 – T4)
For correct results temperature dependent thermal conductivity k and optical absorption α should be taken into account.
If T(t)=const (like with RTA):
LD = – diffusion length.
Otherwise one needs to solve numerically
D assumed to be the same for different atomic species.
Compare simulations and experimental PLshift(Tanneal) data for the same annealing time, find D(Tanneal)
Build Arrhenius plot lnD(1/kT) and find parameters for
Now we can find PL shift
for any T and time.
To achieve T needed for intermixing, different power needed for different beam diameters.
For small diameters <0.5mm power densities become higher than surface damage threshold (>30W/mm2).
Needed power density can be reduced using background heating.
With laser fast scanning (Laser-RTA) we can heat samples to same temperatures, with smaller beam diameters and avoid surface damage.
Power needed to heat the wafer to TQWI increases a little,but fluence drops down significantly (shorter dwell time).
0 50 100 Lateral, μm
0 50 100 Lateral, μmDepth dependence
For small beam diameters T drops down with depth very fast.
InP is transparent to Nd:YAG wavelength at RT, but
Eg(InP)=1.165eV at 500°C, andα=104-106cm-1 at higherT.
Thus, all the energy is absorbed on the surface and goes inside only by heat conduction.
Due to varying T(t), PL shift profile for moving beam differs from that of stationary beam, although temperature profiles are the same.
PL shift profile shape doesn’t depend on Tmax.
Higher temperatures reduce processing time significantly.
a) QWs should be very close to surface,
b) Tmax should be as high as allowed by material decomposition temperature
Natural Sciences and Engineering Research Council of Canada (NSERC)
Canada Research Chair (CRC) Program