Heavy Cr doping of ZnSe by molecular beam epitaxy

Abstract

INTRODUCTIONThe incorporation of transition-metal ions into a semiconductor host is an effective technique for producing material useful for fabricating lasers. In particular, the small crystal-field splitting found for transition-metal ions in II-VI materials allows for laser operation in the 2-5-m spectral range. 1 Lasers based on these materials are of interest for commercial and military applications for optical identification of chemicals. Many molecules have strong and unique absorption spectra in the infrared. As examples, molecules of H 2 O, CO 2, CO, N 2 O, and HCN can be accurately detected in the mid-infrared. 2,3 The incorporation of chromium into ZnSe produces an infrared-laser material that operates at room temperature and is tunable from about 2-3 m. 4 Of technological interest, the absorption band that produces the laser emission occurs at about 1,800 nm. This wavelength is currently accessible using commercial-diode lasers, and in principle, a compact and efficient laser-diode-pumped tunable solid-state laser can be built. Chromium substituting on the Zn site of the tetrahedrally coordinated II-VI lattice results in a 3d 4 electronic configuration, often referred to as Cr 2ϩ by the community, 5 and is the desired electronic configuration to allow laser fabrication. Previous incorporation of chromium into zinc selenide has focused on bulk crystals through either the addition of chromium to the melt 1 or postgrowth-isothermal diffusion. 6 The use of epitaxial-growth techniques would allow fabrication of complex heterostructures that may have advantages for certain applications. As an example, growth of ZnSe:Cr on GaAs should allow for production of integrated structures, opening the possibility of integrated pump and waveguide lasers in the same package. We present results that indicate molecular beam epitaxy (MBE) is a viable approach for incorporating optically active Cr in the ZnSe-host matrix at levels up to ϳ10 19 atoms cm Ϫ3 in epitaxial layers, comparable to that achieved for bulk techniques. EXPERIMENTAL DETAILSAll epitaxial ZnSe layers were grown in a custombuilt MBE system at West Virginia University (Morgantown, WV) on undoped, semi-insulating GaAs (100) substrates. The substrates were initially degreased, etched in H 2 SO 4 :H 2 O 2 :H 2 O (8:1:1) for 5 min at room temperature, and rinsed in flowing deionized water for 5 min. The substrates were further cleaned in situ at a substrate temperature of 400°C Epitaxial ZnSe layers were grown by molecular beam epitaxy (MBE) to study Cr incorporation with the long-term goal of demonstrating an alternate route for achieving transition-metal-doped lasers. Concentrations between 10 15 atoms cm Ϫ3 and 4 ϫ 10 20 atoms cm Ϫ3 were achieved. Secondary ion-mass spectroscopy (SIMS) concentration profiles strongly suggest that surface segregation and accumulation of Cr occurs during growth. Photoluminescence (PL) measurements indicate Cr is incorporated in the optically active Cr 2ϩ state up to levels of ϳ10 19 cm Ϫ3 . Electron paramagnetic resonance (EP...

Progress in Cr2+and Fe2+doped mid‐IR laser materials

“…A series of Cr-doped ZnSe and ZnTe epilayers for mid-IR optical studies were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs (001) substrates [34][35][36][37]. Chromium concentrations in the doped epilayers ranged from 10 14 to 10 20 cm −3 .…”

Structure characterization of MBE-grown (Zn,Cr)Se layers

“…To our knowledge, the only experimental data relating an increase of the lattice parameter value for MBE-grown (Zn,Cr)Se layer to an increase of Cr content are presented in Ref. [18]. According to the authors, the X-ray diffraction and the reflection high-energy electron diffraction (RHEED) indicated that high structural layer quality was maintained for Cr incorporation up to ∼10 19 atoms cm −3 (corresponding to the Zn 1−x Cr x Se mixed crystal composition x ∼ 0.001).…”

“…The precision of XRD measurements is not as good as that for HRXRD so we will not use the numerical values given in Ref. [18] for our estimations.…”

“…Meanwhile, a progress in the technology also made it possible to grow selected crystals containing up to a few percent of Cr. The bulk Hg 1−x Cr x Se mixed crystals [14,15] were the first DMS of this type, (Zn,Cr)Te and (Zn,Cr)Se thin layers were the next examples ( [16,17,18], respectively). In some cases, the ferromagnetic character of investigated samples was clearly established up to the room temperature, as was reported for (Zn,Cr)Te layers, for which the Cr content corresponds to the Zn 1−x Cr x Te nominal mixed crystal composition x = 0.2 [19,20].…”

Luminescence properties of Cr2+:ZnSe: Comparison between a single crystal and a film deposited by RF magnetron co-sputtering

“…1b the total Cr concentration in the studied film (without distinguishing the valence state of the chromium) is 3 times larger than the one of the single crystal. Contrarily to the Cr 2 + :ZnSe films deposited by MBE [22], the Cr concentration in both studied samples is quite uniform along the thickness.…”