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 Université de Sherbrooke, Sherbrooke, Québec J1K 2R1 Canada

2 Outline 1.Motivation 2.Modeling heat distribution and photoluminescence (PL) in QW wafers 3.Temperature profiles induced in InGaAs/InGaAsP wafers by moving laser beam 4.PL shift profiles 5.Summary

3 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. Motivation > E 0 E 1 E 2 E 3 Quantum well intermixing

4 Current state of the problem [1] A. McKee, et. al., IEEE J. Quantum Electron., vol. 33, pp. 45–55, Jan [2] B.S.Ooi,et. al. IEEE J. Quantum Electron., vol. 40, pp.481–490, May 2004 Simulations [1] predict transition region ~300μm using CW Nd:YAG laser irradiation (photoabsorbtion induced disordering) with a shadow mask [1]. Also, pulsed laser IR disordering (2-step process) has been proposed (~2μm transtion region possible [2]). 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. Motivation >

5 Moving laser beam In previous work [3] 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. [3] 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. Motivation >

6 Finite Element Method simulations Heat transfer PDE: Subdomain equation: Q -  (k  T) =  C p (  T/  t) Boundary equation: k  T=q 0 + h(T inf – T) + εσ(T amb 4 – T 4 ) For correct results temperature dependent thermal conductivity k and optical absorption α should be taken into account. 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. Computation details >

7 1.Take diffusion coefficient as parameter 2.Find concentration profile for given D and time 3.Find energy profiles for electrons and holes (take into account bandgaps, band offsets, bandgap bowing) 4.Solve Schrödinger equation, find energy levels and PL 5.Approximate results as some function D(PL shift) If T(t)=const (like with RTA): L D = – diffusion length. Otherwise one needs to solve numerically D assumed to be the same for different atomic species. Finding PLshift(D) Computation details >

8 Finding D(T) and PLshift(T, t) Computation details > Compare simulations and experimental PLshift(T anneal ) data for the same annealing time, find D(T anneal ) Build Arrhenius plot lnD(1/kT) and find parameters for D=D 0 exp(-E A /kT) Now we can find PL shift for any T and time.

9 Laser power density and surface damage To achieve T needed for intermixing, different power needed for different beam diameters. For small diameters 30W/mm 2 ). Needed power density can be reduced using background heating. Computation details > 270 W/mm W/mm W/mm 2

10 Power density for moving beam 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 T QWI increases a little, but fluence drops down significantly (shorter dwell time). Computation details > T QWI

11 d=12μm Depth, μm Lateral, μm d=100μm Depth, μm Lateral, μm Depth dependence For small beam diameters T drops down with depth very fast. InP is transparent to Nd:YAG wavelength at RT, but E g (InP)=1.165eV at 500°C, and α= cm -1 at higher T. Thus, all the energy is absorbed on the surface and goes inside only by heat conduction. Computation details >

12 Scanning speed and bg heating For small samples slower speed results in raise of background temperature. For big wafers heat dissipates faster and temperature profiles don’t depend on scanning speed (laser power is adjusted to achieve same T surface ). Background heating helps to achieve wanted T. Temperature profiles >

13 Temporal T behavior during scan To calculate PL shift profile for moving beam we need: calculate concentration and energy profiles using given T(t) and D(T) at different distances from line center, solve Schrödinger equation and find PL shift. PL shift profiles >

14 PL shift profile for moving beam PL shift profile shape doesn’t depend on T max. PL shift profiles > Due to varying T(t), PL shift profile for moving beam differs from that of stationary beam, although temperature profiles are the same. Higher temperatures reduce processing time significantly.

15 Processing time for 100nm PL shift along one 2-inch line assuming T max =1073K (which requires 90s to get the same PL shift with RTA). Practical applications will require shifts < 50nm. PL shift resolution and processing time

16 Summary 1.Irradiation with the moving CW Nd:YAG laser been has been investigated for selective area writing of the QWI material. 2.For large size wafers (2 inch) temperature profiles don’t depend on scanning speed (assuming that beam power is adjusted to achieve the same T max ). 3.Processing time to achieve targeted PL (badgap) shifts depends on beam diameter and T max. 4.To achieve reasonable processing time without loss in resolution a) QWs should be very close to surface, b) T max should be as high as allowed by material decomposition temperature 4.Background heating can be used to further decrease processing time (especially for deep QWs) but decreasing also resolution. 5.Lateral PL shift resolution of 5μm is feasible (InGaAs/InGaAsP QW material system) with the 12μm beam Laser-RTA. Support Natural Sciences and Engineering Research Council of Canada (NSERC) Canada Research Chair (CRC) Program