Pro-cessed to devices these structures show stimulated emission around 560 nm. The laser diodes discussed here are planar gain guided structures with a 10µm Pd/Au stripe on the p-side as can be seen in Fig. 3.1. Further details about the growth of the laser diodes can be found in the work of M. Klude [20].
The quaternary Cd rich QWs are under compressive strain, which, as was seen in Sec.3.1, is one of the driving forces for the degradation of the ZnSe laser diodes. There-fore, in order to compensate the strain, in addition to the conventional structures, a series of laser structures with non-doped nominal 5 nm thin ZnSSe films positioned neighboring to the active layer was grown. The sulphur content in this thin layers is about 25% in comparison with 6% present in the conventional ones. The tensile strain induced by higher S concentrations, in the combination with the Cd induced compres-sive strain leads to strain reduction in the QW area. The difference between the LD structures is the modified arrangement of the ZnSSe barrier layers: on n-side, p-side and respectively both sides of the QW. At room temperature, all these LDs show the same emission wavelength around 520 nm. Since the process of Cd diffusion is enhanced in the p- doped material, due to the higher density of in-grown point defects, the p-side ZnSSe barrier structure is the most important one for the purpose of this work.
3.3 Analysis of as-grown CdZnSSe quantum well
For TEM investigations, the as-grown structure was prepared in cross-section geom-etry in two different orientations: h110i and, respectively, h010i zone axis. However, the sample preparation of the laser structures is experimentally challenging. Since the whole epitaxial structure is about 2µm, the active region is located at about 1µm below the surface. In plan view geometry, using the conventional preparation method, the
Figure 3.2: Cross section TEM of the active region of the laser structure. The image was taken using{002}three beam imaging condition inh010izone axis.
direct investigations of the QW are rather difficult due to the transparency limits of the TEM specimen in the electron beam direction.
In principle, the interest is focused on two different informations: structural qual-ity and chemical distribution of the QW. The structural defects present in zincblende
Figure 3.3:Line scan intensity profiles taken across (left) and along (right) the CdZnSSe QW, p- and respectively n- ZnSSe waveguides seen in the three beam cross-section im-age taken inh010izone axis (Fig. 3.2)
structures can be observed in TEM using (220) and (111) reflections, where the strain component is predominant in the process of the contrast image formation. For compo-sition investigations, the (002) chemically sensitive reflection is chosen.
Figure 3.4: Cross section TEM of the active region of the laser structure. The image was taken using{002}three beam imaging condition inh011izone axis.
Figure 3.2 shows a cross-section TEM image of the CdZnSSe quantum well region.
The micrograph is obtained in a three beam imaging condition. Experimentally this is achieved by tilting the beam to approximately 5 around the axis parallel to the growth direction in a way that the systematic row corresponding to the {002} reflection is strongly excited. The primary beam is kept onto the optical axis and the objective aper-ture is inserted in a way that only the primary beam, {002} and{002}beams are con-tributing to the image formation. The mentioned imaging conditions correspond to the situation in which the chemical information is preponderant and the strain component is minimized. The CdZnSSe quantum well and the p-type ZnSSe and n-type ZnSSe waveguides are clearly distinguished. An approximately uniform quantum well and
3.3 Analysis of as-grown CdZnSSe quantum well
Figure 3.5: Line scan intensity profile taken across (left) and along (right) the CdZnSSe QW, p- and respectively n- ZnSSe waveguides seen in three beam cross-section image taken inh011izone axis (Fig. 3.4)
respectively small roughness of the interfaces with the waveguides and cladding lay-ers can be seen. The contrast variations present over the whole image are connected to surface contamination due to ion milling and possibly growth related point defects.
Further information about the CdZnSSe QW is obtained by performing line scans across
Figure 3.6:Low temperature PL spectra of the laser structure with quaternary CdZnSSe quantum well dominated by the QW signal (The measurement was performed by C.
Kruse)
the whole active region seen in the image. Line scans are obtained using the DALI soft-ware package. The procedure consists of choosing a symmetrical area with a few pixel width across the QW. The intensity is averaged on this width and the value obtained is then normalized to an arbitrary value from the ZnSSe waveguide far away from the QW. For an easy interpretation of the scan, the origin of the measured distance is
cho-sen in the middle of the QW and the direction of the scan is from the p- to the n- ZnSSe waveguide. One of these line scans is shown in the left side of Fig. 3.3. The aspect of the scan is symmetrical and, from the measurement of the FWHM of about 4 nm, no broadening of the QW area could be detected. Apart from this, a line scan performed along the QW area (right side of Fig. 3.3) revealed a homogeneous Cd distribution in the quantum well inside relatively small fluctuations observed also in line scans made along ZnSSe waveguides.
Figure 3.7: Reciprocal space map of the (224) Bragg reflection obtained by HRXRD for as-grown specimen. (The measurement was performed by G. Alexe)
The corresponding image taken in h110i zone axis can be seen in Fig. 3.4. The QW is found to have a relatively small rough interface with respect to the lower n-type and upper p-type ZnSSe waveguide layers as already observed in Fig. 3.2. The contrast in-homogeneities present all over the image are also related to the presence of as grown point defects and the surface contamination due to TEM preparation. A line scan inten-sity profile taken from the quantum well area is depicted in Fig. 3.5 resulting in a 4.3 nm thick smooth QW. This is confirmed by several linescans made over the whole QW area. The corresponding line scan along the QW revealed also a homogeneous QW in the limits of the background noise unavoidable in the image.
The TEM observations are in good agreement with photoluminescence measure-ments. The low-temperature PL spectrum of the laser structure can be seen in Fig. 3.6.
The spectrum is dominated by the sharp quantum well emission at 2.334 eV where the FWHM of the luminescence signal was only 14 meV. The value of FWHM is close to the theoretical values expected for a perfectly mixed CdZnSSe alloy with 43% Cd and 4% S
3.4 Operational characteristics and degradation