JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 31, 959–963 (2000) Resonance Raman spectroscopy on strain relaxed CdZnSe/ZnSe quantum wires B. Schreder, 1 A. Materny, 1 W. Kiefer, 1 * G. Bacher, 2 A. Forchel 2 and G. Landwehr 2 1 Institut f¨ ur Physikalische Chemie der Universit¨ at W¨ urzburg, Am Hubland, D-97074 W¨ urzburg, Germany 2 Physikalisches Institut der Universit¨ at W¨ urzburg, Am Hubland, D-97074 W¨ urzburg, Germany Cd 0.35 Zn 0.65 Se/ZnSe and Cd 0.2 Zn 0.8 Se/ZnSe quantum wells grown by molecular beam epitaxy were patterned by electron beam lithography and wet chemical etching to give quantum wires of widths down to the sub- 100 nm range. The splitting between the ZnSe LO phonon of the barrier layers and the ZnSe-like LO phonon of the quantum well layer was investigated by applying different excitation wavelengths. For two wire fields with different wire sizes excitation profiles were recorded, which show features belonging to the incoming and outgoing resonance of the LO phonon fundamental vibration and its first overtone. As a result, the LO phonon wavenumber of the wire shows a small shift on changing the excitation wavelength, which is due to changes in the relative intensities of the contributions of the wire center and the wire edges. The wavenumber positions of these two peak contributions are independent of the different resonance conditions at the edge and in the middle of the wire. Copyright  2000 John Wiley & Sons, Ltd. INTRODUCTION Recently, quantum wires of different II–VI semiconduc- tors have attracted considerable attention. 1–9 This is due to the restriction of the vibrational and electronic (vibronic) system to only one dimension. New physical phenom- ena are expected, which depend strongly on the width of the wires, e.g. a blue-shift of the band gap due to lateral confinement of electrons and holes. Wire sizes down to 13 nm have been achieved with a maximum blue shift of about 20 meV of the smallest wires. 10 Based on our results obtained by resonance Raman spectroscopy and photo- luminescence measurements as a function of the wire width, and theoretical considerations, a dependence of the band gap energy on the coordinates of the quantum well layer cross-section is expected. 5,11,12 The dependence of the band gap energy on the wire coordinates also implies a dependence of the ZnSe-like LO phonon position of the CdZnSe wires on the excitation wavelengths, caused by the different resonance conditions for the peak contribu- tions of the wire edges and the wire center. EXPERIMENTAL The investigated quantum wire samples consist of GaAs substrate on which a GaAs buffer layer, a ZnS 0.06 Se 0.94 buffer layer and subsequently ZnSe/CdZnSe quantum wells were grown with a Riber molecular beam epitaxy (MBE) system. The bottom barrier layer is ZnSe with a * Correspondence to: W. Kiefer, Institut f¨ ur Physikalische Chemie der Universit¨ at W¨ urzburg, Am Hubland, D-97074 W¨ urzburg, Germany; e-mail: wolfgang.kiefer@mail.uni-wuerzburg.de Contract/grant sponsor: Deutsche Forschungsgemeinschaft; Contract/ grant number: 410. Contract/grant sponsor: Fonds der Chemischen Industrie. thickness of 70 nm. The quantum well layers of the two samples under investigation have a thickness of 5 nm for the wells with 20% Cd content (samples A) and 3 nm for the well with 35% Cd content (sample B). As a cap layer, ZnSe of about 15 nm thickness is used. The cad- mium concentration was verified by x-ray diffraction. To obtain wires of widths W down to 13 nm, these quantum wells were patterned by electron beam lithography and wet chemical etching, as described in detail elsewhere. 10,13 The samples used in our experiments 14 consisted of sev- eral areas of 100 ð 100 μm 2 each containing wires of well defined width (see also Fig. 1). In order to obtain laterally resolved Raman data from the samples, a micro Raman set-up was used. The Raman microscope has been described in detail elsewhere. 15 The micro Raman spectra were recorded using different exci- tation wavelengths of an argon ion laser (Spectra Physics Model 166) and of a dye laser (Spectra Physics Model 375B). The sample was placed in a closed-cycle cryostat (CTI Cryogenics), which enabled us to vary the tempera- ture of the sample from room temperature down to about 9 K. In order to avoid sample heating, the measurements were performed using a laser power of only about 5 mW. Micro Raman spectra were recorded applying the scanning multichannel technique. 16 It should be mentioned that the etching depths of the two samples were different. Sample A was deep etched, hence the scattering volume of the ZnSe barrier decreases by the same amount as that of the CdZnSe wire on decreasing the wire size. This leads to a strong decrease in the ZnSe LO phonon signal in the spectra on decreasing the wire size or increasing the exci- tation wavelength (out of resonance excitation). For the ZnSe-like LO phonon of the wire the decrease in the scat- tering volume was compensated by increasing resonance enhancement due to the confinement shift of the absorp- tion gap. In contrast to sample A, sample B was not deep etched, hence the scattering volume of the barrier does not decrease by the same amount as that of the wires and a Copyright  2000 John Wiley & Sons, Ltd. Received 22 February 2000 Accepted 14 April 2000