Characterization of ZnSe nanocrystals grown by vapor phase epitaxy

Abstract

This paper reports the application of scanning electron microscopy, x-ray diffraction, and photoluminescence techniques for characterization of ZnSe nanocrystals grown on GaAs (100) substrate from the vapor phase. The applied characterization techniques show the evidence for coexistence of two sets of nanocrystals with rather different characteristic sizes. In addition, the lowest energy levels of spherically shaped nanocrystals are calculated in the framework of the effective-mass approximation and compared with photoluminescence data. In the last ten years the field of semiconductor nanostructures (SNS) has known a strong development owing to demands in advanced optoelectronic devices, chemical sensors, or even biological labels. A comprehensive up-to-date review on SNS applications can be found in Ref. 1.Semiconductors that attract the most attention are elemental (Ge, Si) ones as well as III-V and II-IV compounds. In the pursuit of higher and higher quality and lower fabrication costs, a variety of physical and chemical techniques have been tested in regard to their capability to produce the above materials in nanoscale size and with properties requested from industries. The full list of these diverse techniques existing in a variety of modifications would span a volume. Nevertheless, two major groups can be distinguished among all efforts to produce the SNS: synthesis in colloidal solutions [1,2] and gas-phase synthesis [3][4][5]. Very challenging strategies in the SNS field are nanoscale lithography [6,7] and biogenic approach [2,8]. Each of the techniques used has its own advantages and drawbacks and obviously produce different results.This work reports characterization of ZnSe nanocrystals produced by the technique that utilizes the gas-phase synthesis route involving evaporation, nucleation, and growth. The growth technique used is a variant of vapor phase epitaxy (VPE) that uses no alien substances except extremely pure hydrogen, which plays the role of a carrier for transfer of the vapor of the source material to the deposition zone. This technique facilitates the growth of really uncontaminated ZnSe as opposed to the previously reported VPE route utilizing reaction between vapors of (CH 3 )Zn:N(C 2 H 5 ) and H 2 Se in a counterflow jet reactor [9]. For many years the ZnSe-based nanostructures have been the focus of much attention for applications in all solid-state blue and green lightemitting devices. For such applications the purity of working media is of great importance.For this study a series of samples with ZnSe nanocrystals atop GaAs(100) substrates were prepared by VPE and assessed by scanning electron microscopy (SEM), x-ray diffraction, and low-temperature photoluminescence (PL). Only substrates with the prevalent grain size around 0.2-0.3 mm were used. This was assessed by x-ray diffraction topography.A well-polished, 1´1 cm GaAs substrate was mounted in a horizontal cylindrical quartz reactor with three axial channels for the vaporization of source ma-