Growth and Characterisation of Mg-Based Low Dimensional Systems for Blue-Green Optoelectronics

Abstract

In this paper a brief overview of the state-of-the-art of the growth and characterisation of Mg-based II-VI heterostructures is reported. Recent studies on MgSe binary compound, ZnMgSe ternary alloy and ZnSe/ZnMgSe multiple quantum wells (MQWs) grown by metalorganic vapour phase epitaxy MOVPE are presented. The low pressure MOVPE growth of MgSe, ZnMgSe and ZnSe/ZnMgSe MQWs on (100)GaAs by using ditertiarybuthylselenide, dimethylzinc:triethylammine (Me 2 Zn:Et 3 N), and bismethylcyclopentadienylmagnesium [(MeCp) 2 Mg] as Se, Zn and Mg precursors, respectively is reported. MgSe layers turned out to be mosaics made of (111)- and (100)-oriented MgSe crystals in their rocksalt phase; no signatures of the zincblend (ZB) phase was observed. ZnMgSe ternary epilayers were however grown in the ZB phase throughout the compositional range studied (0.10<x c <0.46). The ZnMgSe solid-vapour distribution curve deviates from linearity due to the different Mg and Zn alkyl vapour diffusion coefficients. A detailed structural analysis of MOVPE-grown ZnSe/ZnMgSe MQWs on (100)GaAs is also reported for the first time. Several ZnSe/Zn 0.83 Mg 0.17 Se MQWs having 16 monolayers (ML)(4.4 nm) thick wells, an alloy barrier thickness ranging between 32 ML (10.5 nm) and 16 ML (4.5 nm) and a number of periods varying between 6 and 12 were deposited on GaAs after a 4.2 nm thin pseudomorphic ZnSe buffer layers. The MQW modulation of as-grown samples was shown by secondary ion mass spectrometry (SIMS) elemental depth profiling measurements. The crystalline properties of ZnSe/Zn 0.83 Mg 0.17 Se MQWs were investigated by high resolution X-ray diffraction (HRXRD), whose spectra were simulated by means of a XRD model within the kinematical approximation, allowing us to determine the sample built-in strains and alloy composition. Whilst the latter value confirmed what expected on the basis of the MOVPE solid-vapour distribution curve, the determination of residual elastic strain in the MQWs structures allowed a systematic investigation as function of MQWs parameters. In the case of 6 periods ZnSe/Zn 0 83 Mg 0.17 Se MQWs having 32 ML thick barriers (total thickness 105 nm) diffraction patterns recorded in asymmetrical configuration indicate almost no relaxation of the structure, within the HRXRD sensitivity. HRXRD of ZnSe/ Zn 0.83 Mg 0.17 Se MQWs structures grown under the same conditions (well and barrier width of 16 ML and Mg content equal to 0.17) but with a different number of periods (6, 9, 12) and so with different total thickness (68, 98, 127 nm) show that zeroth and first order satellite peaks broaden with increasing the number of periods even for nominally pseudomorphic MQWs as a result of strain fluctuations and/or defect nucleation in the MQW structure. Between 9 and 12 periods MQWs structures begin to plastically relax. 10K PL spectra of ZnSe/ZnMgSe MQWs with the same well width show a sharp peak that red-shifts with the Mg content in the barriers. This can be ascribed to an increased tensile strain in the ZnSe well dominating over the confinement effect. Fixing the Mg amount in the barrier, the PL peak red-shifts with increasing the well width.