Adrián Alfaro-Martínez

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.

ZnCdSe/ZnCdMgSe quantum well infrared photodetector

We present the design, fabrication and characterization of a II-VI ZnCdSe/ZnCdMgSe quantum well infrared photodetector with a bound to quasi-bound transition centered at 8.7 μm. Absorption, photocurrent measurements yield results consistent with conventional III-V QWIPs.

MBE growth of ZnCdSe/ZnCdMgSe quantum-well infrared photodetectors

The authors report the growth of quantum well infrared photodetectors (QWIPs) made from wide band gap II-VI semiconductors. ZnCdSe/ZnCdMgSe QWIPs in both medium-wave infrared and long-wave infrared regions were grown by molecular beam epitaxy on InP substrates. High-resolution x-ray diffraction and photoluminescence measurements showed that the as-grown samples have high structural and optical quality. Spectral responses with peaks at 8.7 μm and 4.0 μm have been obtained.

Influence of ZnSe capping of CdSe layers in the growth mode of ZnCdMgSe/CdSe/ZnCdMgSe heterostructures

The authors present a photoluminescence study of the optical and structural properties of ZnSe/CdSe/ZnSe layers grown by atomic layer epitaxy, embedded within barriers of ZnCdMgSe lattice-matched to the InP (001) substrate. The authors show that a few ZnSe monolayers surrounding the CdSe layer inhibit quantum dot formation and induce the growth of quantum islands of CdSe. For the case of a 2 monolayers coverage of CdSe, quantum islands of 1 and 2 monolayers thickness were observed. These quantum islands (or quantum terraces) present intense luminescence even at room temperature.

Photoluminescence Study of Potential Fluctuations in 2.5 and 3 Monolayers Thick CdSe Ultra‐Thin Quantum Wells

CdSe ultra‐thin quantum wells (UTQWs) can be grown by atomic layer epitaxy (ALE) without thickness fluctuations. However, the evolution of the excitonic peak energy as a function of temperature exhibits an S‐shaped curve typical of potential fluctuations. These are originated from small interface composition fluctuations, as evidenced by the larger amplitude of the S‐curve of a 2.5 monolayer (ML) CdSe UTQW, finished with a fractional ML, compared to the results for a 3 ML UTQW.

Peculiar enhancement of the excitonic emission of CdSe/ZnSe quantum wells at ∼ 90 K when excited with a HeCd laser

The close coincidence at low temperatures of the HeCd blue laser line (442 nm, Elaser = 2.808 eV) with the ZnSe bandgap, Eg = 2.821 eV, and with the excitonic emission at ∼2.80 eV, allows the observation of peculiar effects during photoluminescence studies of CdSe/ZnSe quantum wells with a typical experimental setup. One effect is the enhancement of the excitonic emission at ∼ 90 – 100 K; the second effect is the presence of strong longitudinal optical (LO) phonon lines (in a broad temperature range) due to resonant Raman scattering. Here, we will show that the enhancement of the excitonic emission, that can be misinterpreted as caused by an intrinsic temperature dependent behavior of the quantum wells, is due to the high absorption of the blue laser light by the barriers when the ZnSe bandgap coincides with Elaser at ∼ 90 K, electron and holes produced in the barriers diffuse to the quantum wells enhancing their excitonic emission.

Growth and Excitonic Emission of CdSe Ultra‐Thin Quantum Wells Without Thickness Fluctuations

Due to the cation and anion surface reconstruction properties, one Cd‐Se atomic layer epitaxy (ALE) cycle produces a coverage of 0.5 CdSe monolayers. In this work we demonstrate that even when an odd number of cycles are deposited to produce ultra‐thin quantum wells, under the appropriate growth conditions, the photoluminescence spectrum indicates the absence of thickness fluctuations. A single excitonic peak is detected in the whole sample.

Ultra-thin quantum wells and fractional monolayer quantum dots of II-VI semiconductors for optoelectronic applications

We will present the results of the combined growth by molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE) of CdSe/ZnSe and CdTe/ZnTe ultra-thin quantum wells (UTQWs), characterized by means of photoluminescence spectroscopy (PL). In spite of its reduced thickness, a single QW, few monolayers (ML) thick, is able to produce bright emission even under low power excitation. These nanostructures are of great interest for optoelectronic applications such as LEDs and diode lasers. In-situ, reflection high energy electron diffraction (RHEED) experiments indicated the 2D growth of the UTQWs and the real-time temporal analysis of the intensity of RHEED pattern features demonstrated the absence of degradation of their structural quality during the ALE growth. Fractional monolayer quantum dots (FMQDs) produced by the deposition of /spl sim/0.5 ML of CdSe present very intense and narrow deep blue excitonic emission. The sharp luminescence of the FMQDs is indicative of their highly uniform size and shape. Even though the 1ML QWs are very thin, the calculated transitions obtained by means of the envelope function in the effective mass approximation are in quite good agreement with the observed excitonic transitions.