E. Kaufmann

Size control and midinfrared emission of epitaxial PbTe∕CdTe quantum dot precipitates grown by molecular beam epitaxy

Epitaxial quantum dots with symmetric and highly facetted shapes are fabricated by thermal annealing of two-dimensional (2D) PbTe epilayers embedded in a CdTe matrix. By varying the thickness of the initial 2D layers, the dot size can be effectively controlled between 5 and 25nm, and areal densities as high as 3×1011cm−2 can be achieved. The size control allows the tuning of the quantum dot luminescence over a wide spectral range between 2.2 and 3.7μm. As a result, ultrabroadband emission from a multilayered quantum dot stack is demonstrated, which is a precondition for the development of superluminescent diodes operating in the near infrared and midinfrared.

Emission properties of 6.7μm continuous-wave PbSe-based vertical-emitting microcavity lasers operating up to 100K

A detailed analysis of midinfrared cw lasing of IV–VI vertical-cavity surface-emitting devices is presented. The structures, based on high-finesse microcavities containing PbSe as active medium, show optically pumped cw laser emission up to temperatures of 100K at a long wavelength of 6.7μm. Stimulated emission with a very narrow beam divergence below 1° and a large temperature tuning range of 70nm is found. The measured linewidth of the laser emission is only 0.6nm, limited by the spectrometer resolution with a strong narrowing with respect to the linewidth of the subthreshold signal. The observed cw output power amounts up to 1.2mW at 85K.

Quantum dots with coherent interfaces between rocksalt-PbTe and zincblende-CdTea)

The formation of PbTe quantum dots (QDs) in a crystalline CdTe host matrix is demonstrated by the annealing of a coherent, heteroepitaxial PbTe layer clad between CdTe layers. The resulting QDs have a centrosymmetric shape and they exhibit intense room-temperature mid-infrared photoluminescence due to an electron-hole pair recombination in the narrow-gap PbTe. The intense luminescence approves the high quality of the QD interfaces, between the sixfold coordinated rocksalt structure of PbTe and the fourfold coordinated zincblende structure of CdTe. To gain further insight into the structural interface properties, we compare quantitatively multislice simulations of HRTEM images with first-principles total-energy calculations in the repeated-slab approximation. The most drastic effect occurs at the electrostatically neutral (110) interface, where we find a lateral spatial offset between the two crystal halves due to rebonding across the interface. For the two polar (001) interfaces, significantly different l...

Mid-infrared high finesse microcavities and vertical-cavity lasers based on IV–VI semiconductor/BaF2 broadband Bragg mirrors

We report on molecular beam epitaxially grown high-reflectivity broadband Bragg mirrors for mid-infrared devices using IV–VI semiconductors and BaF2. This material combination exhibits a high ratio between the refractive indices of up to 3.5, leading to a broad mirror stop band with a relative width of 75%. To verify the high quality of the PbEuTe/BaF2 Bragg mirrors, we study a half-wavelength microcavity formed by mirrors with only three periods. The resonance of the microcavity has a narrow linewidth of 5.2 nm corresponding to a very high finesse of 750. From this, a mirror reflectivity higher than 99.7% is deduced, in good agreement to transfer matrix simulations. Furthermore, we demonstrate mid-infrared continuous-wave vertical-cavity surface-emitting lasers based on these mirrors. Optical excitation of laser structures with a PbSe active region results in stimulated emission at various cavity modes between 7.3 and 5.9 μm at temperatures between 54 and 135 K. Laser emission is evidenced by a strong li...

PbTe and SnTe quantum dot precipitates in a CdTe matrix fabricated by ion implantation

We present rock-salt IV-VI semiconductor quantum dots fabricated by implantation of Pb+, Te+, or Sn+ ions into epitaxial zinc-blende CdTe layers. PbTe and SnTe nanoprecipitates of high structural quality are formed after implantation by thermal annealing due to the immiscibility of dot and matrix materials. For samples implanted only with Pb+, intense continuous-wave photoluminescence peaked at 1.6 μm at 300 K is found. In contrast, for PbTe quantum dots fabricated by coimplantation of Pb+ and Te+, the 300 K emission peak is observed at 2.9 μm, indicating luminescence from much larger dots.

Size-controlled quantum dots fabricated by precipitation of epitaxially grown, immiscible semiconductor heterosystems

Epitaxial quantum dots with symmetric and highly faceted shapes are fabricated by thermal annealing of two-dimensional (2D) PbTe epilayers embedded in a CdTe matrix. This novel self-organization scheme is based on the immiscibility of the involved semiconductor materials, which originates from the different bulk bonding configurations and the concomitant lattice-type mismatch. By varying the thickness of the initial 2D layers, the dot size can be controlled in a range between 5 and 25 nm, with areal densities as high as 3 × 1011 cm−2. Control of the quantum dot size allows for photoluminescence tuning over a spectral range between 2.2 and 3.7 µm. Multilayer quantum dot stacks with systematically varying sizes yield ultra-broadband emission, and thus a precondition for the development of superluminescence diodes operating in the near- and mid-infrared.

Molecular Beam Epitaxial Growth and Photoluminescence Characterization of PbTe/CdTe Quantum Wells for Mid-Infrared Optical Devices

This paper describes molecular beam epitaxial growth of PbTe/CdTe quantum wells on (100)-oriented GaAs substrates and characterization of their photoluminescence spectra. Despite of the differences in crystal structure and thermal expansion coefficient between PbTe and CdTe, an intense mid-infrared emission was observed even at higher temperatures than 300K. The energy of the emission peak showed blue shift with decreasing well width and had a positive dependence on temperature in agreement with that of bulk PbTe, indicating that the emission resulted from electron-hole recombination in a type I quantum well. Multiple peaks, however, were found in the PL spectra, and analysis of the temperature dependence of PL peak energy revealed that the thermal mismatch between CdTe and PbTe promoted the peak separation. A PbTe/CdTe double quantum well showed a higher efficiency of the PL emission. These results indicate a promising application of this heterosystem to cleaved-edge cavity laser diodes operating at room temperature.