Far Infrared Spectroscopy of In 0.53 Ga 0.47 As Quantum Wells on InP(100)
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Far Infrared Spectroscopy of In0.53Ga0.47As Quantum Wells on InP(100) N. L. Rowell1), D. J. Lockwood2), P. J. Poole2), and G. Yu1) 1) Institute for National Measurement Standards and 2)Institute for Microstructural Sciences National Research Council, Ottawa, Canada K1A 0R6 ABSTRACT Polarized far infrared reflectance was measured at oblique incidence for In0.53Ga0.47As / InP multiple quantum wells grown by chemical beam epitaxy on InP(100) wafers. Both the well thickness (0.25 - 20 nm) and number of periods (10 - 40) were varied. The reflectance spectra contained sharp Berreman modes at the frequencies of the transverse (TO) and longitudinal (LO) optical phonons. The contributions of the individual phonons were resolved with the model fits. Interface layer phonon modes were observed with intensity increasing with number of wells. The interface layers were 0.6 nm thick and of different composition to adjoining wells consistent with cross-sectional scanning tunneling microscope results on the same samples. The variation due to phonon confinement of the InAs- and GaAs-like LO and TO phonon frequencies was obtained. INTRODUCTION Phonon frequencies are strongly influenced by the reduced dimensionality in semiconductor quantum wells. In this paper we use an infrared optical approach for the study of the confinement effect on vibrational modes in alloy quantum wells. The method provides additional or complementary information to that available from other optical or scanning methods like Raman scattering [1] or high-resolution microscopy [2] . Although there have been infrared (IR) [3] (and Raman [4]) studies for bulk InGaAs alloys, InGaAs strained layers [5], and extensively for GaAs/AlAs superlattices [6] , no systematic study has been reported for InGaAs/InP quantum wells (QWs) on InP(100). Also, while recent Raman work [1] has shown a need for such IR spectroscopy to aid the identification of phonon modes, standard IR methods such as attenuated internal reflection are too qualitative to be helpful for thin layers [7]. The method here, a quantitative IR technique based on measuring Berreman modes [8] at oblique angles of incidence, has no adjustable parameters and thus can provide accurate information about thin film phonons. The technique, which is the basis for IR ellipsometry [9], takes advantage of the difference between the dielectric constants of different layers. As a far-IR technique, the method avoids the carrier-induced line shifting possible in QW Raman scattering [10]. This method permits the calculation of phonon frequencies for thin layers and their thicknesses. The InGaAs/InP quantum well samples were grown at 480 oC on heavily-doped (100) InP substrates (Si ~ 5 x 1018 cm-3) using a Riber 32P chemical beam epitaxy system. Sequence A of Reference [11] was employed for gas switching during growth of the QWs. The multiple quantum well (MQW) samples had 130 nm InP buffer layers followed by 10 to 40 In0.532Ga0.468As QWs separated by undoped InP spacers 20 nm thick. At this Ga fraction, the QWs were unstr
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