Disordering of the ZnCdSe single quantum well structure by Cd diffusion
Applied Physics Letters
The effects of annealing on a ZnCdSe single quantum well ͑SQW͒ structure with ZnCdSSe/ZnSSe superlattice optical guiding layers are investigated. X-ray diffraction and photoluminescence ͑PL͒ measurements showed disordering of a ZnCdSSe/ZnSSe superlattice after annealing at about 500°C. The PL peak energy of the SQW shifted to the higher energy side, and the linewidth narrowed in the sample annealed at 300°C. Cadmium diffusion was confirmed by secondary ion mass spectrometry. We found that the
... sordering of the ZnCdSSe/ZnSSe superlattice and the changes in the emissions from the SQW were due to the Cd diffusion. © 1996 American Institute of Physics. ͓S0003-6951͑96͒05149-2͔ In recent years, considerable effort has been devoted to the investigation of ZnSe-based II-VI compound semiconductors for blue-green laser diodes. 1-3 The Zn 1Ϫx Cd x Se (0рxр0.3) quantum well is frequently used as an active layer, and ZnS y Se 1Ϫy (yХ0.08) or a superlattice structure is used as the optical guiding layer. We previously reported that use of a superlattice as the optical guiding layer improves the quality of a ZnCdSe active layer, makes the strain easier to control, and creates a large difference in the refractive index between the active layer and guiding layer. 4 Superlattice structures can be disordered by ion implantation or thermal annealing of GaAs/AlGaAs devices, and understanding the characteristics of disordering in II-VI superlattices is important for improving the performance of II-VI devices. Several researchers have investigated the thermal annealing effects of II-VI compounds grown by molecular beam epitaxy, especially, p/n-type ZnSe, ZnMgSSe, and ZnCdSe single quantum well ͑SQW͒ structures. 5,6 It is clear that the carrier concentration of doped II-VI compounds decreases as a consequence of annealing above about 300°C, and the photoluminescence ͑PL͒ linewidth from a ZnCdSe SQW becomes narrower. However, the effects of thermal annealing on a quantum well with a superlattice structure have not been investigated. These thermal annealing effects are important in the characterization of II-VI compounds. In this letter, we discuss thermal annealing effects on the characteristics of a ZnCdSe SQW structure with a ZnCdSSe/ ZnSSe superlattice optical guiding layer. The samples were grown by molecular beam epitaxy on ͑001͒-oriented semiinsulating GaAs substrates. The source materials were solidsource Zn, Se, ZnS, and Cd. The growth temperature was 300°C. The samples consisted of a ZnSe buffer layer ͑500 nm͒, a guiding layer, a 10-nm Zn 0.7 Cd 0.3 Se SQW, and another guiding layer. The guiding layer in sample A consisted of ZnSe; in sample B, it was a three-period superlattice of alternating 10-nm Zn 0.78 Cd 0.22 S 0.2 Se 0.8 and 10-nm ZnS 0.16 Se 0.84 ; and in sample C, it was a six-period superlattice of the same materials. These samples were annealed at temperatures between 180 and 600°C for 15 min in a nitrogen atmosphere. The samples used for secondary ion mass spectrometry ͑SIMS͒ measurements were annealed at 500°C for 5 h. Photoluminescence ͑PL͒ measurements were carried out at 77 K to study changes in the emission due to thermal annealing. The excitation source was a 325-nm He-Cd laser with an intensity of 0.5 mW. Figure 1 shows the PL spectra of sample A at several annealing temperatures. There is a very intense sharp peak at 497 nm from the ZnCdSe SQW in Fig. 1͑a͒ . The intensity of this emission was slightly higher in the sample annealed at 250°C, apparently because the crystallinity was improved by the low-temperature annealing. The intensity decreased in the sample annealed at 450°C, and this peak disappeared in the sample annealed at 510°C. Instead of the peak that disappeared, another peak appeared at 450 nm, and deep level emissions were observed around 550 and 650 nm as shown in Fig. 1͑d͒ . The emission from the SQW in samples B and C are eliminated by annealing at 510°C. In the sample annealed at 600°C, only deep level emissions were observed. These emissions were the same as in the undoped ZnSe film annealed at 510°C and 600°C. Therefore, the observed emissions in the samples annealed above 550°C are from the ZnSe buffer layer. This indicates that the quantum well structures are disordered by annealing at about 500°C. Figure 2 shows the dependence of emission properties from samples A and B as functions of the annealing temperature. Both samples had the same tendencies: that is, peak energy shifts to the higher energy side and the linewidth narrows as annealing temperature rises. Ichimura et al. have FIG. 1 . PL spectra of sample A: ͑a͒ as-grown; ͑b͒ annealed at 250°C; ͑c͒ annealed at 450°C; ͑d͒ annealed at 510°C; and ͑e͒ annealed at 600°C.