Growth and characterization of ultra‐thin quantum wells of II–VI semiconductors for optoelectronic applications

Abstract

Atomic layer epitaxy (ALE) was used to grow CdTe and CdSe quantum wells (QWs) with 1 to 4 monolayer thickness. With this ultra-thin quantum wells (UTQWs) it is possible to cover the red-blue spectral range. Low temperature photoluminescence spectroscopy reveals that most of these UTQWs present very intense and narrow emission and, depending on growth conditions, no thickness fluctuations are observed over the whole sample area. The heterostructures were grown at different temperatures between 260 and 290 °C. Both CdTe and CdSe UTQWs grown at the higher temperature presented blue-shifted emission compared to those grown at lower temperatures. This effect is attributed to Cd losses by reevaporation and Cd substitution by Zn. This effect of the temperature can be employed for fine tuning of the UTQW emission. The critical thickness of CdSe/ZnSe grown by ALE is estimated to be 5 ML. 1 Introduction After the first blue-green laser diode, produced in 1991 from wide-band-gap II-VI semiconductors [1], many efforts have been addressed to improve their lifetime and spectral range; it is desirable to produce full color emission devices based on II-VI semiconductors. Zn 1-x Cd x Se/ZnSe quantum wells (QWs) with x < 0.25 and thickness of a few nm have been employed as the active region of blue-green LEDs and lasers grown on GaAs substrates. To extend the emission range towards lower energies, ternary alloys with a higher Cd content are required. However, the increase of Cd in the QW increases the lattice mismatch with the ZnSe barrier, the consequence is that strain and alloy disorder produce the degradation of the optical and structural properties of the QWs. In order to overcome these difficulties a broader range of materials have been employed. For example, full color LED emission has been demonstrated with ZnCdSe/ZnCdMgSe pseudomorphic QWs grown on InP [2]; CdZnSSe quaternary alloys have been used to produce laser emission in the 2.48 to 2.21 eV range (bluish green to green-

Factorization: little or great algorithm?

“…The remaining (unphysical) eigenfunctions u = c + u + + c − u − (c ± ∈ C) have no nodal points in R but diverge for both → ±∞. By choosing one such untrivial u with c ± = 0 as a Darboux generator, one ends up with a nonlocal deformation shifting V (x) to the right as x → −∞ and to the left as x → +∞ (or vice versa); thus producing a non-periodic V (x) in which two contradictory displacements either collide or diverge (see figure 1), giving the contact effect or a quantum well in the middle [154]. The new results for the second-order SUSY displacements were recently obtained by Samsonov et al [155].…”

Infrared reflectance and transmission spectra in II-VI alloys and superlattices

“…[8][9][10][11][12][13][14] By varying the composition and controlling the lattice constants in ternary or quaternary alloys, we can achieve greater flexibility of tuning emission and absorption wavelengths for high-efficiency solid-state light emission sources. [15][16][17][18][19][20][21][22][23][24][25][26][27] Earlier, the applications of II-VI materials for photonic devices were hampered primarily by the availability of poor-quality crystals and the difficulty of managing doping. [28][29][30][31][32][33] Progress in the modern crystal growth techniques such as metalorganic vapor phase epitaxy (MOVPE), 34 metalorganic chemical vapor deposition, 35 molecular beam epitaxy (MBE), 36,37 chemical beam epitaxy, 38 and hot wall epitaxy 39 has offered higher quality and greater versatility in the preparation of thin films with controlled doping on many convenient substrates.…”

Red to Blue Excitonic Emission with Ultra-Thin Quantum Wells and Fractional Monolayer Quantum Dots of II–VI Semiconductors

“…More details about substrate preparation and buffer layer growth can be found in ref. UTQWs of CdTe grown at T s =270 °C presented narrower single peaks for thickness above 1ML [3]. Considering the need of a high sample reproducibility and precise control of the QW thickness, we employed the atomic layer epitaxy (ALE) growth method for the production of the UTQWs.…”