Ellipsometric studies of Be x Zn 1 2x Se between 3 eV and 25 eV K. Wilmers* and T. Wethkamp Institut fu ¨r Festko ¨rperphysik, TU Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany and Max-Planck-Institut fu ¨r Festko ¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany N. Esser, C. Cobet, and W. Richter Institut fu ¨r Festko ¨rperphysik, TU Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany M. Cardona Max-Planck-Institut fu ¨r Festko ¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany V. Wagner, H. Lugauer, F. Fischer, T. Gerhard, and M. Keim Physikalisches Institut, Universita ¨t Wu ¨rzburg, Am Hubland, D-97074 Wu ¨rzburg, Germany ~Received 27 August 1998! We have determined the dielectric function of Be x Zn 12x Se above the fundamental absorption edge for the full composition range (0 <x <1) by spectroscopic ellipsometry in the UV and VUV range. The spectra show pronounced features in the photon energy range between 2.7 and 25 eV. By comparing the ZnSe spectra ( x 50) to literature data, we assign the observed structures to interband transitions at specific points or regions of the Brillouin zone. The corresponding assignment for BeSe is made by tracking the systematic dependence of these structures on the Be content x from x 50 to x 51. E 0 has the steepest slope dE 0 / dx and exhibits a bowing vs x. On samples with a large Zn content ( x ,0.2) two structures show up at high energies ~15.7 and 16.9 eV! which have not been observed before. We assign these structures to transitions from the Zn 3 d bands to a higher conduction band at the L point and at the G point. @S0163-1829~99!01515-5# I. INTRODUCTION Devices based on II-VI semiconductors, such as ZnSe la- ser diodes, have a reduced lifetime compared to III-V based devices due to defect formation favored by the high ionicity and the smaller bond energies of conventional II-VI materi- als. Recently, however, it has been demonstrated that Be-VI compounds have a high degree of covalent bonding and bond energies similar to GaN. 1 This should entail a lattice harden- ing and slower degradation with time. Therefore, II-VI compounds containing beryllium have become interesting materials for optoelectronic devices like green and blue laser diodes. 1–5 For instance, quarternary compounds, such as Be x Mg y Zn 1 2x 2y Se/GaAs(001), may be used to tailor the optical and electronic properties over a wide range by varying the composition ( x , y ) under condi- tions of lattice matching to GaAs. 1,2 Beryllium chalcogenides, however, are a rather new class of materials; their optical and electronic properties have not yet been elucidated. As we show below, BeSe has a funda- mental band gap of 5.5 eV while the band gap of ZnSe is 2.7 eV. Thus, by varying the stoichiometry in Be x Zn 1 2x Se a wide range of band-gap energies is covered and even small differences of the beryllium content in alloys cause large band-gap shifts. The technique of spectroscopic ellipsometry is well estab- lished for investigating the optical properties of bulk materi- als and thin films. 6,7 It facilitates the direct determination of the complex dielectric function e 5e 1 1i e 2 and thus circum- vents the need for a Kramers-Kronig transformation. e is directly related to the electronic band structure of the mate- rial through interband transitions. Wide band-gap materials exhibit their main interband transitions within the spectral range from 3 to 15 eV, i.e., in the UV/VUV spectral region. Since this range is not fully accessible to conventional ellip- someters, which are limited to approximately 6 eV, we used the VUV ellipsometer at BESSY in the range of 2.5 to 25 eV with a synchrotron radiation source. Various Be x Zn 1 2x Se samples with different beryllium contents were investigated at room temperature. The results obtained for the two binary compounds ZnSe and BeSe were compared to band-structure calculations and dielectric func- tions derived therefrom. We compared reflectivity data avail- able in the literature for ZnSe to the reflectivity calculated from our ellipsometry measurements. No experimental data in the VUV spectral range were previously available for the beryllium containing al- loys. Local-density approximation density-functional theory ~LDA-DFT! calculations for BeSe ~Ref. 8! predict that, in contrast to ZnSe, the interband transition at the L point from the top valence band to the lowest conduction band has a higher energy than that at the X point. Our experimental data for BeSe do not allow for an unambiguous interpretation of structures in e because the main structures of the BeSe di- electric function appear very close to each other in energy, and thus are difficult to resolve. In this article, however, we show that an assignment of spectral features becomes pos- sible when tracking the evolution of the dielectric function with varying x. II. EXPERIMENTAL DETAILS The experiments were carried out with the VUV ellipsom- eter at the 2m-Seya-Namioka beamline at BESSY. High in- PHYSICAL REVIEW B 15 APRIL 1999-I VOLUME 59, NUMBER 15 PRB 59 0163-1829/99/59~15!/10071~5!/$15.00 10 071 ©1999 The American Physical Society