Picosecond Carrier Lifetime in InGaAsP Grown by He Plasma-Assisted Molecular Beam Epitaxy

InGaAsP grown by He plasma assisted Molecular Beam Epitaxy has a 15 ps carrier lifetime and a sharp band edge allowing for an ultrafast response and a strong optical nonlinearity at the telecommunications wavelength of 1.55 μm. S.D. Benjamin, Li Qian, J.E. Ehrlich, P.W.E. Smith, B.J. Robinson, D.A. Thompson Picosecond carrier lifetime in InGaAsP... 1 Picosecond Carrier Lifetime in InGaAsP Grown by He Plasma-Assisted Molecular Beam Epitaxy S.D. Benjamin, Li Quan, J.E. Ehrlich and P.W.E. Smith Department of Electrical and Computer Engineering University of Toronto 10 King's College Rd, Toronto, Ontario Canada M5S 1A4 (416) 978-0602 B.J. Robinson, and D.A. Thompson Centre for Electrophotonic Materials and Devices McMaster University 1280 Main Street West, Hamilton, Ontario Canada L8S 4L7 Materials with large optical nonlinearities and ultrafast response times are required for compact ultrafast all optical devices. However there is a trade-off for nonlinear optical materials in terms of sensitivity vs speed. One can make use of the large nonlinearity near a resonance in a material but the response is typically relatively slow. Ultrafast nonlinearities can be obtained far from resonance, but these nonlinearities are quite small. Semiconductor materials can exhibit large optical nonlinearities for light resonant with the band gap. However the carrier lifetime, and thus the response of S.D. Benjamin, Li Qian, J.E. Ehrlich, P.W.E. Smith, B.J. Robinson, D.A. Thompson Picosecond carrier lifetime in InGaAsP... 2 semiconductor layers grown under standard conditions, is too slow for ultrafast all optical devices. In the GaAs-based material system, large optical nonlinearities and ultrafast response times have been achieved by Molecular Beam Epitaxy growth at lower than standard temperatures. Excess As is incorporated into the crystal during growth leading to recombination centers which can reduce the carrier lifetime to picoseconds or less. However this technique does present some difficulties. In the range of temperatures used, it is difficult to ascertain and control the substrate temperature during growth. Subsequent growth at standard temperatures on top of low temperature grown layers will anneal those layers. Significant sub-band gap absorption is also present in low temperature grown layers accompanied by a smearing of the band edge. The excess As incorporated induces strain in the lattice and high quality crystal growth is limited by a critical thickness. In the InGaAsP/InP quaternaries required for the telecommunication wavelengths of 1.3 and 1.55 μm, additional group V incorporation calibration may be required at the low growth temperatures. We present a new growth technique to achieve short carrier lifetimes: He Plasma Assisted Molecular Beam Epitaxy. This technique yields a material with an ultrashort carrier lifetime, while maintaining the sharp bandedge of material grown without a He plasma. A 2 μm thick epilayer of InGaAsP (undoped, bandgap wavelength of 1.55 μm) was grown at 435°C by gas source molecular beam epitaxy with the sample continuously exposed during growth to a He-plasma stream generated by a electron cyclotron resonance source. In recent work on S.D. Benjamin, Li Qian, J.E. Ehrlich, P.W.E. Smith, B.J. Robinson, D.A. Thompson Picosecond carrier lifetime in InGaAsP... 3 InP grown with a He-plasma, it has been shown that the properties of the material are reproducible and thermally stable for rapid thermal anneals up to 800°C. For comparative purposes, two additional InGaAsP samples, each 2μm thick, were also grown. Motivated by a recent report in the literature, we grew one sample without He-plasma at a low substrate temperature estimated to be 200°C and intentionally doped with Be at 7×10cm. The second comparative sample was grown under conditions similar to those for the He-plasma sample, but with no plasma. For the He-plasma grown sample, standard pump probe measurements, as shown in Fig. 1, yielded a single exponential decay from which a 15 ps carrier lifetime was obtained. This response time is sufficiently rapid for many potential applications of all optical devices. Fig. 2 shows the absorption for wavelengths tuning through the bandedge of the InGaAsP samples as obtained via transmission measurements. There is little difference between the steepness of the band edge between the He-plasma grown sample and the standard grown sample. We would thus expect that the large optical nonlinearity of the standard grown material would be preserved in the He-grown sample. Our first attempt at low temperature grown InGaAsP, however, does not display an identifiable band edge and has substantial absorption out to 5 μm. Evidently, the growth of He-plasma assisted InGaAsP is much more straightforward than the growth of good crystal quality low temperature grown InGaAsP. He plasma assisted grown InGaAsP is a convenient means to obtain an ultrafast response and a strong nonlinearity at the important telecommunications wavelength of 1.55 μm. We are planning to demonstrate ultrafast all optical switching devices using this material. S.D. Benjamin, Li Qian, J.E. Ehrlich, P.W.E. Smith, B.J. Robinson, D.A. Thompson Picosecond carrier lifetime in InGaAsP... 4