Elsevier

Microelectronics Journal

Volume 39, Issues 3–4, March–April 2008, Pages 582-585
Microelectronics Journal

Study of the recombination around the excitonic region of MBE ZnSe:Cl thin films

https://doi.org/10.1016/j.mejo.2007.07.110Get rights and content

Abstract

The recombination processes around the excitonic region of undoped ZnSe and chlorine doped ZnSe thin films were studied by continuous-wave photoluminescence (cw-PL) and time-resolved photoluminescence (TRPL) spectroscopies. Samples with different chlorine concentration were obtained by varying the temperature of the Cl source. The evolution of the PL signal and its decay time were analyzed as a function of temperature. Activation energy (Ea) values associated to the quenching of the D0X and band-to-band emission were obtained from the temperature dependent cw-PL experiments. The activation energy was lower for the film with higher Cl content. The characteristic exponential decay time (τPL) of the PL signal decreased with increasing Cl concentration.

Introduction

Wide-band-gap II–VI semiconductors such as ZnSe have been extensively studied because they can be used to develop laser diodes and optoelectronic devices operating in the blue-green spectral region. The formation and propagation of non-radiative defects are the major degradation mechanisms that limit the lifetime during room-temperature operation of ZnCdSe based blue-green laser diodes. The fabrication of a laser diode requires high levels of n- and p-type doping in order to obtain the appropriate electrical properties [1]. However, the presence of impurities produces defects, mainly from non-electrically active centers that affect the optical emission. The study of carrier recombination processes will provide a better understanding of the defects and associated levels that degrade the structural and electrical properties of the ZnSe layers employed in the fabrication of the devices. In this work, we present our results on continuous-wave photoluminescence (cw-PL) and time-resolved photoluminescence (TRPL) experiments near the band-edge region of chlorine-doped zinc selenide (ZnSe:Cl) thin films. In particular, we analyze the temperature dependence of the spectral and temporal evolution of the exciton-bound-to-neutral donor emission (D0X) in nominally undoped and several ZnSe:Cl samples. In a previous work, we reported that at high chlorine concentrations, the excitonic and band-to-band transitions are absent and only an impurity band (∼2.07 eV) is observed [2]. In this work, we focus on low chlorine concentrations where the excitonic and/or the band-to-band transitions are present.

Section snippets

Experimental details

ZnSe:Cl thin films were grown by molecular beam epitaxy (MBE) on GaAs (0 0 1) substrates by using Zn, Se, and ZnCl2 sources. The substrate temperature was set to 325 °C for the growth of the ZnSe:Cl samples. In order to obtain different Cl concentrations, the ZnCl2 effusion cell temperature was fixed to 75, 90, 105, and 120 °C. Details on the sample growth were published in Ref. [3]. The thickness of the ZnSe:Cl films was around 0.5 μm. An undoped ZnSe (ZS) film with ∼1.7 μm thickness was grown at a

Results and discussion

Fig. 1 shows the cw-PL spectra measured at 15 K corresponding to the undoped ZnSe sample and of the ZnSe:Cl films doped with different chlorine concentrations, the employed nomenclature is given in Table 1. The near band-edge transition around 2.79 eV is observed only in samples with low Cl concentration. The so-called self-activated (SA) band emission centered at 2.07 eV was observed in all samples, although this emission is much more intense in samples with higher impurity concentration [2]. The

Summary

CW-PL experiments showed that the D0X emission (2.797 eV) is present in undoped ZnSe (ZS) and slightly doped ZnSe:Cl (ZSC75) thin films. The D0X emission is more intense and sharper in ZSC75 than in ZS. This behavior is associated to the larger amount of donors introduced by the chlorine impurities. The sharp emission can be taken as an indication of the improvement of the crystal quality of the film due to strain reduction. The Ea values calculated from temperature dependent cw-PL around the

Acknowledgments

We would like to thank Dr. Juan P. Martínez at the ICM-UV (Spain) who let us use the TRPL setup of his laboratory. AEMC wishes to thank to the CONACYT, the Students Mobility Programme of the UNAM and the University of Valencia, Spain, for financial support. This work was partially supported by CONACYT, Mexico.

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