in the p-side, n-side and both sides were investigated [106]. The influence of the bar-rier/barriers upon the degradation was detected by performing lifetime measurements, with a constant current density of 50 A/cm−2, in DC mode as well as under pulsing con-ditions, for all samples from the series. These measurements were then compared with the one obtained for the strained QW structure. As can be seen in Fig. 3.16 the lifetimes for p-side and n-p side barrier structures were one order of magnitude higher then the reference QW strained structure. If in the case of strained QW the lifetime was 11±2 min, the p-side barrier structure was operating for 133±34 min [4].
Therefore, the TEM micrograph of the as-grown structure with p-side ZnSSe barrier taken inh011izone axis is shown in Fig. 3.17. Near to the homogeneous CdZnSSe QW, the p-side ZnSSe barrier layer (5 nm) is clearly distinguished by its higher brightness in comparison with the ZnSSe waveguide. Moreover, the interfaces QW/barrier and barrier/waveguide are very smooth indicating a good structural quality. One also can see a sharp interface between the waveguides and cladding layers pointing out to an excellent crystal quality of the entire laser structure.
These lifetime measurements emphasise that the ZnSSe barrier layer located in the p-side acts not only as strain compensating layer, but as a layer which protects against the formation and propagation of vacancies near the active region or, in other words, against Cd outdiffusion. The additional S in the barrier means that more Zn-S bonds are present, which are stronger than the Zn-Se bonds due to a higher melting point of ZnS compared with ZnSe.
Systematic experiments of electrical degradation and annealing on quaternary QW structures with ZnSSe barriers were initiated. The first TEM investigations were not relevant, however they promise to give a clarifying information on vacancy migration and Cd diffusion processes. These measurements are in progress and could not be com-pleted in the framework of this thesis.
It has to be noted that another alternative for improving the lifetime of the II-VI LDs is the use of CdSe/ZnSSe QD superlattice as active region of the laser structure instead of conventional QW structures. Using this approach a resulting emission wavelength of 560 nm was possible [99, 107]. Furthermore, this structure was processed as ridge structure where the top layers were etched down beneath the 10µm injection stripe by ion beam etching about 1µm into the middle of p-waveguide layer. The QD structure showed no essential degradation under DC mode within an operating time of 600 h, which is more then three order of magnitude higher then the QW structures. Studies of the CdSe/ZnSSe QD superlattice are presented in the next chapters.
3.7 Summary
Structural investigations comprising TEM and HRXRD combined with electro-optical characterisation have been performed on the active region of the ZnSe based laser struc-tures with Cd rich quaternary CdZnSSe quantum well grown by MBE. The research was focused mainly on the degradation mechanism of these diodes. The as-grown laser structure showed a very good structural quality. The {002} three beam micrographs pointed out a high uniformity of the QW and homogeneous distribution of Cd whereas in the{111}micrographs the defect density it was found to be very low. These findings
are in agreement with HRXRD results as well as low-temperature PL measurements.
However, despite of the high structural quality of the grown material, after short oper-ation, the devices dies. The{002}BF TEM images of the degraded specimens revealed the presence of localised areas where Cd diffusion took place, and a closer look using HR{111}microscopy showed the presence of extended defects at the QW area. These extended defects are located where the Cd diffusion was observed. The degradation is connected with the existence of as-grown point defects which can migrate and give rise to extended defects in the QW region (partial relaxation of the QW). These defects can be a starting point for the process of a Cd diffusion from the QW.
Chapter 4
Chemical distribution in CdSe/ZnSSe superlattices
4.1 Research status and motivation
ZnSe based semiconductor devices using Cd based quantum well structures, in which electrons are confined on a nanometer scale in one direction are well known and in-tensively studied. In the last decade, a lot of effort was dedicated for creating confine-ment of carriers in three dimensions, quantum dots (QDs). In particular CdSe/ZnSe QDs have been the focus of intense research [108, 109, 110] due to their potential appli-cations for fabrication of light emitting devices such as laser diodes. Using CdSe as quantum dots, lasing in the blue-green spectral range has been already demonstrated [111, 112]. Furthermore, for QD lasers a lower current densities are expected, allowing laser operation with reduced heat load. This is extremely important for II-VI semicon-ductor devices, because the degradation effects induced during operation (see chapter 3) are still a major challenge. Recently, experimental evidence for an improved degra-dation behavior using CdSe/ZnSSe quantum dot stack structures as an active region was reported [4], yielding lifetimes up to three orders of magnitude longer than those of CdSe quantum well structures. In addition, the gain of the CdSe dot based lasers has been demonstrated to be significantly larger than for corresponding quantum well lasers [113].
From the optimized epitaxial growth of these structures to device operation, there are still several steps that have to be clearly understood and improved. It is well known that due to the high lattice mismatch between CdSe/ZnSe the formation of self assem-bled CdSe QDs is favoured. However, it is not yet clear if the CdSe QD formation is a result of the Stranski Krastanov growth mode. In comparison with other material sys-tems the CdSe dots are rather small and strong intermixing occurs [114]. It was also reported [115] that Cd segregation occurs under certain growth conditions. Therefore, it may be supposed that the growth conditions during overgrowth of CdSe by ZnSe influence the strong intermixing, as well as the structural and electronic properties of the CdSe QD structures. The importance of the method used for cap layer deposition as well as the substrate temperature were also exploited. The usage of MEE technique (see Sec.1.2.3) for the deposition of the ZnSe cap layer pointed out QD formation already at 1,9 ML CdSe deposition and could be explained as a segregation enhanced reorganiza-tion process [116].
By high-resolution XRD it was found that the stacking fault densities are higher for capping by MBE under stoichiometric conditions. A detail defect structure characteri-zation performed by Litvinov et. al. [55], using TEM, revealed the existence of pairs of stacking faults lying on one of the two pairs of lattice planes given by{(111)−(111)}
and {(111)−(111)} bounded by Shockley partial dislocations. Furthermore, they are developing at the CdSe/ZnSe interface inside of Cd rich regions (islands).
In the case of the influence of the substrate temperature during growth of the ca-pping ZnSe layer a decrease of stacking fault densities with increasing temperature was observed [114]. Moreover, the amount of CdSe deposit and the Cd concentration in the (Cd,Zn)Se layers decrease with increasing growth temperature [117] caused by an in-creased desorption of segregating Cd atoms during cap layer growth. Several others TEM studies regarding the chemical composition and strain state of CdSe/ZnSe quan-tum dot heterostructures are also reported [54, 118, 119, 120].
The chemical analysis for CdSe/ZnSSe stack structures is rather difficult in the con-ditions of the diminished chemical contrast due to the fact that CdSe QD are relatively small in comparison with other material systems and strong intermixing during over-growth occurs. However, despite of the experimental limits imposed by such weak compositional fluctuations, using {002} DF TEM technique in combination with the HRXRD measurements of the lattice parameters, a semiquantitative evaluation of aver-age Cd content in the dots and S concentration in the spacers is possible.