Formation of nonuniformity in ZnSe/ZnMgSSe quantum well structures during MOVPE on GaAs(0 0 1) misoriented by 10° to (1 1 1)A plane

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Abstract

ZnSe/ZnMgSSe quantum well structures grown by metal-organic vapor-phase epitaxy on GaAs substrate misoriented from (0 0 1) to (1 1 1)A by 10° were studied. Low-temperature cathodoluminescence spectra of these structures showed doubling QW emission lines. The doubling was observed also at addition of Cd, Mg and S in the QWs with concentration of several percents. The reason of this effect was investigated by X-ray reflectometry and diffraction methods. Increasing of scattering with increasing penetration depth was found. It evidences changing QW parameters during the growth. Decomposition of ZnMgSSe solid solution was also observed. The main reason of the doubling was supposed to be due to mutual diffusion of Zn and Mg atoms at QW interfaces. The diffusion is stimulated by relaxation of internal stains or nonuniform distribution of point defects due to the ZnMgSSe decomposition.

Introduction

Using of ZnMgSSe quaternary compounds to achieve both high band gap (3–3.1 eV) and lattice period matching with the GaAs substrate is important condition to obtain low defect periodic structures with resonant-periodic gain for effective lasers with longitudinal optical and electron-beam pumping, radiating in blue spectral range (∼2.7 eV) [1], [2]. Emission line of ZnSe quantum wells (QWs) can undergo short-wave shift due to unintentional doping by Mg atoms with concentration up to 1% when the structures are grown by metal-organic vapor-phase epitaxy (MOVPE) at temperature 450–460 °C. In order to tune the emission line to required wavelength of 460–465 nm and increase the luminescence intensity one adds Cd with concentration of 1–2% in the QWs. This leads to increasing crystal lattice misfit between the ZnMgSSe and the QW layers that can be the factor of degradation, especially for the structures containing 20–30 QWs required for such lasers. For reduction of this misfit it is possible to add S in the QW also. Low temperature cathodoluminescence (CL) of these structures reveals doubling of QW emission line that finally worsens characteristics of the laser [3]. The reason of this doubling, apparently, is connected with peculiarities of QW formation, as is an object of the given research.

Section snippets

Experimental

ZnSe:Cd,Mg,S/ZnMgSSe structures with two and three identical QWs were grown by MOVPE on GaAs substrates misoriented by 10° from a (0 0 1) plane to a (1 1 1)A plane. Growth was carried out with Veeco reactor at low pressure (85 torr) hydrogen and Tgr=450–460 °C [3]. Thickness of the QW layers for all structures studied was about 8 nm while thickness of top, barrier and bottom ZnMgSSe layers was 73 nm that was monitored by time and rate of growth. Only for the ♯437 structure, the thickness of the first

Cathodoluminescence

CL spectra of five structures with different QW composition are presented in Fig. 1.

Typical CL spectrum of the structure with 2 pure ZnSe QWs contains a weak emission line of ZnMgSSe with maximum at 3.07 eV and full width at half maximum (FWHM) of 20 meV and two lines at 2.81 and 2.84 eV due to QW emission. The FWHM of these last lines is about 18 meV but each line is a superposition of two components. At surface excitation by e-beam with Ee=3 keV (average penetration depth is about 50 nm), only the

X-ray analysis

Reflectrometric curves for six different structures ♯♯396, 431, 435, 437, 418 and 419 are presented in Fig. 2. First five structures contain 2 QWs while the last structure has 3 QWs. For clearness the curves are shifted along the vertical axis with logarithmical scale.

The reflectometric curves contain fringes. The fringes of the structures with 2 QWs are doublets whereas those of 3 QWs are triplets. Angular distance between the same peaks in the nearest doublets or triplets corresponds to the

Discussion

Experimental results show that some QW parameters change during the growth. The most real reason of this change is mutual diffusion of Zn and Mg through the QW interfaces. The reflectometric curvers presented in Fig. 2 confirm this assumption. They show an increase of X-ray diffuse scattering with depth. The deeper layer, the longer diffusion time and the stronger lattice disordering due to Zn–Mg diffusion occur. Meanwhile the curves demonstrate a good flatness of the growth surfaces except the

Conclusions

It was found that QW parameters change during MOVPE of the ZnSe/ZnMgSSe structures on GaAs(1 0 0) substrates with the 10° misorientation to (1 1 1)A plane at T=450–460 °C. It leads to doubling the QW emission line. The doubling increases with addition of Cd and S while decreases with addition of Cd and Mg. The reason of such a doubling is likely to be due to mutual diffusion of Zn and Mg through the QW interfaces. The Mg diffusion is stimulated by formation of structure defects as result of a

Acknowledgments

Authors thank Michael D. Tiberi for giving structures. The work was supported by Russian Foundation for Basic Research (Gr. 07-02-01139) and Program “Scientific schools of Russia” (Gr. NSh-3168.2.2008).

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