Maria C. Tamargo

ZnCdSe/ZnCdMgSe quantum cascade electroluminescence

This letter reports electroluminescence emission from a ZnCdSe∕ZnCdMgSe quantum cascade (QC) structure. With a two-well QC active region design, the II-VI heterostructure was grown lattice matched on an InP substrate by molecular beam epitaxy. Deep etched mesas were electrically pumped at current densities up to 10kA∕cm2, producing optical emission centered near 4.8μm, in good agreement with the structure design. The light is predominantly TM polarized, confirming its intersubband origin. Electroluminescence was observed from 78to300K.

Direct-to-indirect band gap crossover for the BexZn1−xTe alloy

We have investigated the growth and optical properties of a set of BexZn1−xTe epitaxial layers having different composition, with x ranging from 0–0.7. Comparison of the reflectivity and the photoluminescence spectra allowed us to locate the direct-to-indirect band gap crossover for this alloy at x≈0.28. The Γ→Γ direct band gap exhibits a linear dependence on composition over the entire compositional range and can be fitted to the equation EgΓ(x)=2.26*(1−x)+4.1*x. It increases linearly with BeTe content at a rate of 18 meV for a change of 1% in BeTe content. The Γ→X indirect band gap for BexZn1−xTe can be fitted to the equation EgX(x)=3.05*(1−x)+2.8*x−0.5*x*(1−x), suggesting that the energy of the indirect Γ→X transition for ZnTe is about 3.05 eV.

Dielectric functions and critical points of BexZn1−xTe alloys measured by spectroscopic ellipsometry

Using a rotating analyzer spectroscopic ellipsometer, we have investigated the complex dielectric function of a series of ternary BexZn1−xTe thin films in the energy range between 0.7 and 6.5 eV for alloy concentrations between x=0.0 and x=0.52. After determining the alloy concentrations using x-ray diffraction and photoluminescence techniques, a standard inversion technique was used to obtain the optical constants from the measured ellipsometric spectra. Analyzing the second derivative of both the real and the imaginary parts of the dielectric constant, we have deduced the critical point parameters corresponding to the electronic transitions in the Brillouin zone. We find that the energy of the critical points with respect to Be concentration does not show any bowing effects unlike many other II–VI semiconductor ternary alloys.

Room temperature and narrow intersubband electroluminescence from ZnCdSe/ZnCdMgSe quantum cascade laser structures

We report ZnCdSe/ZnCdMgSe Quantum Cascade structures with “two-phonon” and “bound-to-continuum” active region designs. The electroluminescence shows more than 3 times higher luminescence efficiency and 40% narrower linewidth (<30 meV) than previous reports. The measured turn-on voltage matches closely the calculated value, indicating the improved electron transport characteristics in these structures. A waveguide design suitable for mode confinement in this material system is also presented, which resulted in a structure with a single narrow electroluminescence peak for all temperatures from 80 to 300 K.

Percolation behavior in the Raman spectra of ZnBeTe alloy

Zone-center longitudinal (LO) and transverse (TO) optical phonons of Zn1−xBexTe epilayers are identified by Raman spectroscopy. On top of the expected BeTe- and ZnTe-like modes, which obey the modified-random-element-isodisplacement model, we observe an extra BeTe-like (TO, LO) doublet at intermediate x values. It has the same atypical characteristics as its BeSe-like counterpart in ZnBeSe alloy. Its activation validates our percolative picture for multimode description in Be–chalcogenide alloys that open the attractive class of mixed crystals with a sharp contrast in the bond stiffness. Also, the local modes of Be in ZnTe and of Zn in BeTe are identified at 411 cm−1 and 195 cm−1, respectively.

High performance, room temperature, broadband II-VI quantum cascade detector

We report on the experimental demonstration of a room temperature, II-VI, ZnCdSe/ZnCdMgSe, broadband Quantum Cascade detector. The detector consists of 30 periods of 2 interleaved active-absorption regions centered at wavelengths 4.8 μm and 5.8 μm, respectively. A broad and smooth photocurrent spectrum between 3.3 μm and 6 μm spanning a width of 1030 cm–1 measured at 10% above baseline was obtained up to 280 K, corresponding to a ΔE/E of 47%. Calibrated blackbody responsivity measurements show a measured peak responsivity of 40 mA/W at 80 K, corresponding to a detectivity of about 3.1×1010 cmHz/W. Bias dependent photocurrent measurements revealed no significant change in the spectral shape, suggesting an impedance matched structure between the different active regions.

High detectivity short-wavelength II-VI quantum cascade detector

We demonstrate the first II-VI based short-wave (λ ≤ 4 μm) Quantum Cascade Detector. Peak responsivity and background limited detectivity of 0.1 mA/W and 2.5×10¹⁰ cm√Hz/W, respectively, were measured at 80 K.

Size controlled synthesis of monodisperse PbTe quantum dots: using oleylamine as the capping ligand

We report a simple method to synthesize monodisperse hydrophobic PbTe quantum dots followed by a stability study of the as-synthesized quantum dots in air. We provide evidence that air-stable PbTe quantum dots can be synthesized using this method. PbCl2 complexed with oleylamine was used as the Pb precursor. Tellurium powder dissolved in tri-n-octylphosphine was used as the Te precursor. Oleylamine was used as the only capping ligand. The size and shape of the PbTe quantum dots were controlled by changing variables such as injection temperature, growth temperature, and growth time. Both Pb to oleylamine and Pb to Te feed mole ratios have been examined to obtain the optimal synthetic conditions. The PbTe quantum dots can be changed from hydrophobic to hydrophilic through ligand exchange using 4-mercaptopyridine to replace oleylamine as the capping ligand. The colloidal PbTe quantum dots were characterized by transmission electron microscopy, high resolution transmission electron microscopy, selected area X-ray diffraction, energy-dispersive X-ray spectroscopy, FT-IR spectrometer, 31PNMR spectrometer, and powder X-ray diffraction. The sizes of the PbTe quantum dots synthesized ranged from 2.6 nm to 14.0 nm with a relative standard deviation of ∼5.6 to 9.1%. The shape of the PbTe quantum dots was either spherical ( ∼9 to 10 nm).

Percolation-based vibrational picture to estimate nonrandom N substitution in GaAsN alloys

The number of N atoms in N-rich regions mostly due to nonrandom N incorporation in GaAsN (N∼4%), referred to as the Nr rate, is studied using a nonstandard Raman setup that addresses transverse symmetry. The Ga–N optical range shows a two-mode signal which discriminates between the N-poor (Np) and N-rich (Nr) regions. This is discussed via a percolation-based picture for Be-chalcogenide alloys, which exhibit mechanical contrast with regard to the shear modulus. This applies to GaAs–GaN even though the contrast is in the bulk modulus. The balance of Nr/Np strength provides a Nr rate of ∼30%, i.e., much larger than the corresponding Be rate of ∼4% in random Be-based alloys.

Enhancement of p-type doping of ZnSe using a modified (N+Te)δ -doping technique

Delta doping techniques have been investigated to enhance the p-type doping of ZnSe. Tellurium was used as a codopant for improving the nitrogen doping efficiency. The net acceptor concentration (NA−ND) increased to 1.5×1018 cm−3 using single δ doping of N and Te (N+Te), while it was limited to 8×1017 cm−3 by δ doping of N alone. A promising approach was developed in which three consecutive δ-doped layers of N+Te were deposited for each δ-doping cycle. An enhancement in the (NA−ND) level to 6×1018 cm−3 has been achieved in ZnSe using this technique. The resultant layer has an average ZnTe content of only about 3%. This doping method shows potential for obtaining highly p-type doped ohmic contact layers without introducing significant lattice mismatch to ZnSe. Low-temperature photoluminescence spectra reveal some Te-related emissions.