Chapter 6
Summary and conclusion
The aim of this work was the study of degradation effects and structural properties in the active region of the ZnSe based semiconductor laser diodes employing the TEM technique. In such a device, the light is either produced in a quantum well or in quan-tum dots. Therefore, the TEM experiments were focused on two directions: one regard-ing the quaternary CdZnSSe quantum wells within the active region of as grown MBE laser structures as well as degraded structures and second to CdSe/ZnSSe superlattices containing quantum dots, grown by MEE.
An intense research of more than 15 years has been performed on electrically-pumped ZnSe based laser diodes from the time when reliable p-type doping of these structures was possible. Several experimental techniques, regarding growth, electrical, optical and structural characteristics, were employed to study these devices in order to obtain a laser diode with a maximum lifetime of more than 400 h, produced by Sony in 1998.
However, in the university research groups, the lifetime of these diodes remains in a range of minutes, probably due to the non-advanced methods used in the device pro-cessing .
During the development of the ZnSe laser diodes, TEM was found extremely useful for the improvement of the crystal quality of these heterostructure by decreasing the defect density, leading to better electrical and optical characteristics. However, the lim-ited stability of these devices under current injection is a major problem which is closely connected with the rapid degradation of the quantum well. During operation, one ob-serves a decreasing efficiency of the device caused by dark defects developing in the active region. Furthermore, the degradation is promoted via recombination enhanced defect reaction (REDR) processes where the defects are activated by nonradiative re-combination of carriers at the site of an already existing defect. It was also found that growth related point defects plays a major role in the degradation process.
In the epitaxy group of the Institute of Solid State Physics in Bremen, intensive work was dedicated to the processing of such ZnSe based laser diodes. These investiga-tions led to the development of the first quaternary Cd-rich quantum well and five-fold CdSe/ZnSe quantum dot laser diodes which showed emission with a wavelength close to 560 nm. However, the lifetime of these devices is still limited to minutes in cw mode.
Therefore, in the framework of this PhD thesis, Cd-rich quantum well and quantum dot structures with a wavelength in the 560 nm range were analyzed.
First, the TEM investigations were based on the comparison between the properties of as-grown and degraded quaternary CdZnSSe quantum wells with high Cd content. It
was found that, despite of the high lattice mismatch of the quantum well relative to the waveguides, the entire as-grown laser structure shows a high structural quality. This was seen in conventional TEM micrographs taken from the quantum well area using a (002) three beam imaging condition in both [110]and [010] orientations. Furthermore, this was also confirmed by the low defect density obtained from HRXRD measurements and the sharp quantum well emission peak seen in the low temperature PL measure-ments. Processed to devices, these laser structures showed an emission wavelength of about 560 nm gained from electroluminescence spectra, taken in pulse-mode, LED mode as well as in cw mode. On the other hand, the L-J characteristics showed an in-creased light output power above the threshold of 750 A/cm2. Moreover, the quatum efficiency was 69 %.
To study the degradation mechanism, the structure was electrically degraded, in pulse mode at 20 A/cm2 constant current density. Discontinuous contrast fluctuations around the quantum well area of the degraded specimen became visible in (002) DF TEM micrographs. These contrasts were more pronounced in the p-side of the epitaxial structure. One possible explanation for such contrast fluctuations could be the outd-iffusion of Cd from the well. Furthermore, high resolution two beam (111) images of the same region revealed the presence of extended defects confined to the active region.
These observations indicate that the degradation of the laser diodes is connected with the formation of the mentioned defects in the optical active region leading to a partial relaxation of the quantum well.
Experimental and theoretical calculations revealed that the degradation of high struc-tural quality ZnSe-based LED is mainly due to the instability of the shallow nitrogen ac-ceptor itself [103]. The concentration of “interstitial nitrogen selenium vacancy” defect complexes (Ni-VSe) (∼2.1 eV above the valence band edge), being energetically more favorable then N on Se site at any hole concentration higher than 1014 cm−3, is grad-ually enhanced during the LD operation. As positively charged species, they diffuse under the applied forward bias to the active region which is under compression. At the active region they are trapped and consequently cause the degradation of the laser performance [104]. The observed enhancement of Cd outdiffusion to the p-side can be explained also by the low amount of electrically active nitrogen atoms (only 2% are active donors). Non-active N might play a role for the enhancement of point defect diffusion. The TEM results were also supported by electroluminescence measurements taken at constant current density. The small emission red shift observed at the begin-ning of the degradation experiment, can be attributed to a possible strain relaxation of the QW due to extended defects in the QW area, and the strong blue shift, seen in the next stage, to the outdiffusion of Cd from the well.
An approach to delay the degradation is the introduction of strain-compensating ZnSSe barrier layers near to the active region. Using this approach, conventional LEDs and LDs with differently arranged barriers near to the highly strained QW were grown.
A high structural quality of these structures was proved by TEM and the achieved en-hancement of the lifetime was more than one order of magnitude in case of the struc-tures with p-side ZnSSe barrier. Further investigations revealed that a layer with high sulfur concentration of about 25 % does not only act as strain compensating layer but also as a blocking layer for diffusion of vacancies and consequently also blocks the dif-fusion of Cd into the p-layers. As further investigation, it was planned to combine elec-trical degradation and annealing experiments to clarify more the degradation process.
However, these experiments could not be finished within this work.
Another recent approach is the use of CdSe dots as active region of ZnSe based laser diodes. Despite of the fact that the growth of CdSe dots is experimentally challenging, a higher stability of the devices is expected, when CdSe/ZnSSe stack structures are used to form the active region. Furthermore, for optical applications, a homogeneous morphology of the dots is beneficial, because this leads to a more uniform emission spectrum.
Therefore, the study of chemical composition and ordering phenomena in CdSe-Zn(S)Se QD supperlattices is extremely important in order to obtain better performance in comparison with the quantum well structures. Employing cross-section and plan view geometry, a series of five-fold CdSe/Zn(S)Se structures with different spacer layer thickness (2, 4.5 and 8 nm) and a ten-fold structure (4.2 nm ZnSSe spacer) were investi-gated and compared. For the five-fold series, the nominal values of S-concentration in the ZnSSe spacers were 40, 25 and 12 % and an average S concentration around 35, 29 and 15 % was determined from DF(002)images in combination with the simulations of the image intensity, performed using the EMS software. It follows that the nominal and the experimental values are in good agreement. However, in the case of Cd concentra-tion (nominally around 40%) in the CdSe dot layers, the values measured (around 20%) are underestimated due to the fact that the accuracy of the measurement is hampered by the limitations of the experimental method used. Two main factors are responsi-ble for the errors: the loss of the image intensity around 40 %Cd and the influence of the background intensity due to inelastic scattering and thermal diffuse scattering.
Moreover, taking into account that an intermixing occurs at the interface between CdSe and ZnSSe spacer layers, it is expected that the dot layer also contains S and Zn. As a consequence, for obtaining the composition of such quaternary quantum dot layer, an additional source of information is needed, namely the lattice parameter, which is also varying with composition. The lattice parameters of the quaternary CdZnSSe dot layer for the five-fold series were derived from HRXRD measurements. Combining them with TEM results acquired from{002}DF images, Cd and S concentrations in the quaternary CdZnSse dot layer were estimated. The measured Cd concentrations in the CdZnSSe dot layers were 18, 20 and 22%, and the S concentrations were 6, 1.5 and 0
%. As expected, the S concentration decreases when the ZnSSe spacer layer thickness increases.
Better accuracy of the concentration values can be obtained in general from (002) high resolution images employing the CELFA technique. This method could be suc-cessfully applied in the case of ternary compounds. For quaternary compounds the situation is complicated by the fact that there is only one quantity that can be measured.
Therefore, a recent idea is to use an interference pattern of the primary,{002}and{220}
which can provide information about the image intensity and lattice parameters, both depending on composition. In this case, CELFA and DALI techniques have to be used.
However, to obtain such an image a proper objective lens aperture is needed, which will be processed with the focused ion beam machine. However, this approach is still in the project stage.
On the other hand, the experimental results regarding the alignment of the CdSe dots revealed a strain driven ordering process, in which the strain fields from buried QDs leads to heterogeneous nucleation conditions for the QDs in the subsequently de-posited layers. If the spacer thickness becomes too large, the superposition of the strain
fields originating from individual QDs leads to a diminished strain modulation at the surface. From theory this effect is expected to become important, when the spacer layer thickness becomes comparable to the average QD distance. In the present case of a five-fold CdSe/ZnSSe stack series, the lateral and vertical ordering of the dots was detected for samples with 4.5 nm ZnSSe spacer layer thickness or smaller and no ordering effects in the case of an 8 nm ZnSSe spacer thickness. These alignment effects were observed in cross-section{220}DF images taken in[110]and respectively[010]zone axis. This is in agreement with the theoretical predictions and the experimental results obtained for other material systems such as conventional III-VI, GaN/AlN(0001) or PbSe/PbTe(111) QD stacks. With increasing stacking number (here the comparison was made between the five and ten-fold structures with approximately the same spacer thickness) the or-dering is enhanced, supporting the fact that the strain field around the dots from lower CdSe layer forms the nucleation point for the formation of dots in the uppers layers.
Furthermore, as a direct consequence of the lateral ordering of the dots, the mean lat-eral dot distance from the above mentioned images, was measured as well. The values were around 11 nm and 15 nm for 2 nm and 4.5 nm ZnSSe spacer thickness specimens and 16 nm for the ten-fold stack specimen with 4.2 nm ZnSSe spacer thickness. In plan-view geometry, the alignment of the dots is also indicated by the network of regularly distributed contrasts seen in DF {022}images which do not follow a crystallographic direction. However, the mean lateral distance between the features was calculated from these images being in good agreement with results obtained from cross-section speci-mens.
An anisotropy of the lateral alignment of the CdSe dots could be detected in cross-section geometry, by comparison of the {220} dark field images in two differenth110i zone axis. The¡
220¢
DF images taken in [110] zone axis revealed the presence of con-trasts from strain fields surrounding the CdSe dots. In (220) DF images from £
110¤ zone axis, this contrast vanishes. Furthermore, this is supported by the plan-view im-ages. The actual shape and orientation of the quantum dots are not known, and also the process leading to such preferential alignment is not clear at present. The TEM results regarding the ordering and anisotropy of the dots were also supported by GISAXS mea-surements, where the measured mean lateral distances for the sample under investiga-tion were found to be close to the TEM values. However, due to stronger fluctuainvestiga-tions of the measurements, this can be attributed to the anisotropic elastic strain distribution combined with surface diffusion. A possible explanation could be obtained with the support of final element calculations where a proper model for composition, shape and distribution of the dots is needed.
Finally, there is a lot of work that can be done in the future, for the improvement of the quaternary Cd based laser structure as well as CdSe/ZnSSe dot structure. Neverthe-less, during this thesis it was possible for the first time to investigate the microstructure of degraded CdZnSSe quantum wells and intensive work on CdSe/ZnSSe quantum dot structures clarified important structural properties of the CdSe dots.
List of Tables
1.1 Some physical parameters of the II-VI compounds used in this thesis in comparison with GaAs. The values are taken from the latest edition of theLandolt-B¨onstein[11] . . . 4 1.2 Different types of dislocations and corresponding Burgers vectors inf cc
crystal structures, whereaindicates the lattice constant of the material. . . 19 2.1 Important parameters of CM20 UT electron microscope used during this
work . . . 25 4.1 Sulphur content in the ZnSSe spacers determined from{002}DF images . 82 4.2 Cd and S content in the CdZnSSe spacer determined from XRD and TEM 89 5.1 Mean lateral distancehΓi between the CdSe stacked dots as determined
from cross-section¡ 220¢
DF images taken in[110]and[010]zone axis . . . 98 5.2 Comparison of thehΓi values obtained from{220}images of samples B
and D . . . 104 5.3 Mean lateral distance hΓi measured from cross-section hΓiCS and plan
viewhΓiP V images . . . 109 5.4 Mean lateral distancehΓimeasured from cross-sectionhΓiCS images and
GISAXShΓiGISAXS experiments . . . 111
List of Figures
1.1 a)ABC stacking sequence for zincblende structures b) Zincblende
polar-ity and c) Non-primitive unit cell . . . 2
1.2 Linear variation of lattice constant with composition for CdxZn1−xSe and ZnSeyS1−y ternary compounds varying the Cd and S content, respectively. 4 1.3 The components of the stress tensor for a body crystal. . . 5
1.4 Variation of bandgap energy of ZnSSe depending on S content . . . 6
1.5 Lattice constant versus bandgap for binary compounds investigated in this thesis [20]. The range of wavelength for visible spectrum covered by this compounds is also depicted. . . 7
1.6 Simplified picture of an island of a deposited film . . . 9
1.7 growthmode1 . . . 9
1.8 growthmode2 . . . 10
1.9 growthmode3 . . . 10
1.10 MBE . . . 11
1.11 Analogy of MBE (a) with MEE (b) growth mode for ZnSe. The dash lines delimit the beginning and the end of the MEE cycle. . . 12
1.12 a) Interstitial impurity atom, b) Edge dislocation, c) Self interstitial atom, d) Vacancy, e) Precipitate of impurity atoms, f) Vacancy type dislocation loop, g) Interstitial type dislocation loop, h) Substitional impurity atom (taken from www.tf.uni-kiel.de) . . . 13
1.13 Burger circuit in a crystal which contains an:a) edge dislocation, b) screw dislocation (taken from www.tf.uni-kiel.de) . . . 14
1.14 a) Dislocation glide b) Dislocation climb . . . 16
1.15 a) Intrinsic stacking fault where the fault sequence is ABCABABCA b) Ex-trinsic stacking fault where the fault sequence is ABCABACABCA (taken from www.tf.uni-kiel.de) . . . 17
1.16 The crystal orientations in the opened unfold Thompson tetrahedron. The thicker and thinner arrows can be interpreted as Burgers vectors of perfect or partial dislocations, respectively. . . 18
1.17 Schematic representation of pseudomorphic and relaxed ZnSe grown on GaAs. Relaxation occurs when the layer thickness d exceeds the critical thicknessdcritical . . . 20
1.18 Schematic representation of the nucleation of misfit dislocation networks (D) in the quantum well by a stacking fault (S) and its associated thread-ing dislocations (T). The network is generated under current injection [42]. 20 2.1 Optical column of the electron microscope . . . 24
2.2 Signals generated when a high-energy beam of electrons interacts with a thin specimen. Most of the signals can be detected in different types of TEM. . . 27 2.3 electron scattering . . . 29 2.4 Atomic scattering amplitudes for Cd, Zn,S and Se plotted versus
scatter-ing parameterqin the case of an accelerating voltage of 200 kV . . . 30 2.5 Ewald sphere of radius λ1 in a reciprocal lattice. A Bragg reflection is
excited if the sphere intersects a reciprocal lattice point, e.g. N. . . 32 2.6 Structure factors of the (002) (lower curve) and (004) (upper curve) beams
in CdxZn1−xSe and ZnSxSe1−x, plotted versus the Cd and respectively S concentration x. The curves were computed assuming an acceleration voltage of 200 kV. . . 33 2.7 The normalised structure factors ∂x∂ (FFs(ghkl,x)
s(ghkl,0))which determine the chem-ical sensitivity as indicated in Eq. (2.24) for (002) and (004) reflections. . . 34 2.8 a) Wave function not continuouskk 6=k0k b) Continuity conditionkk =k0k 37 2.9 Ewald sphere showing the positive and negative excitation errors for a
reciprocal lattice point located inside the sphere. . . 37 2.10 The two basic operations of the TEM imaging system involve A)
project-ing the diffraction pattern on the viewproject-ing screen and B) projectproject-ing the image onto the screen. In each case the intermediate lens selects either the back focal plane or the image plane of the objective lens as its object [50]. . . 40 2.11 Transfer function plotted for: a) 50 nm , b)100 and c)150 nm defocus . . . . 43 2.12 First and second passband in CdSe zincblende crystal according to Eq. 2.70 43 2.13 Indexed diffraction patterns of the [110] and [001] zone axes obtain using
the EMS software for a CdSe fcc crystal. . . 45 2.14 Bright-field condition . . . 46 2.15 Dark-field condition. This can be obtain in two configurations:
non-centred (left) and non-centred DF. . . 47 2.16 Bending of the lattice planes of a crystal due to the presence of a dislocation 48 2.17 Principle steps of TEM preparation technique used in the conventional
cross-section preparation method . . . 50 2.18 Principle steps of TEM preparation technique used in the tripod method . 51 2.19 Principal steps of the plan view TEM specimen preparation methods [56] 52 2.20 Experimental setup for PL measurements. The schematic drawing made
by A. Gust. . . 53 2.21 Schematic view of the electro-optical setup. The sample is placed in front
of the optical multimeter for integrated measurement (position A) or in front of the spectrometer for spectrally resolved measurements (position B) [20]. . . 54 2.22 Schematic view of the possible scans in the X-ray diffraction [20]. . . 56 2.23 Schematic setup of the SAXS beamline at ELETTRA synchrotron facility
(Italy). Schematic drawing made by T. Schmidt. . . 57 2.24 Scattering geometry for the case of a position sensitive detector (top) and
a CCD camera (bottom). Schematic drawing made by T. Schmidt. . . 57
List of Figures 3.1 Sketch showing the layers sequence of a laser structure with quaternary
CdZnSSe quantum well [20] . . . 62 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. . 63 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 image taken inh010izone axis (Fig. 3.2) . . . 64 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. . 64 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) . . . 65 3.6 Low temperature PL spectra of the laser structure with quaternary CdZnSSe
quantum well dominated by the QW signal (The measurement was per-formed by C. Kruse) . . . 65 3.7 Reciprocal space map of the (224) Bragg reflection obtained by HRXRD
for as-grown specimen. (The measurement was performed by G. Alexe) . 66 3.8 Electroluminescence spectra taken from the laser structure described in
the previous section (The measurement was performed by M. Klude) . . . 67 3.9 L/j characteristics. (The measurement was performed by M. Klude) . . . . 68 3.10 Change in normalized light output power (left) and wavelength emission
(right) versus operating time at constant current. (The measurement was performed by M. Klude) . . . 69 3.11 Electroluminescence spectra before (black) and after degradation (red)
for harder driving conditions (higher current densities) than in Fig. 3.10.
(The measurement was performed by A. Ueta) . . . 69 3.12 Bright-field cross section TEM image of the active region of the laser
structure. The image was taken along theh011izone axis. . . 70 3.13 Comparison between line scans taken from degraded and non-degraded
QW areas of the image seen in Fig. 3.12 (left) and line scan made along the degraded QW (right) . . . 71 3.14 Reciprocal space map of the (224) Bragg reflection obtained by HRXRD
for the degraded specimen. (The measurement was performed by G. Alexe) 71 3.15 Two-beam{111}high resolution image of the degraded specimen
corre-lated with the areas where the contrast variation was observed. . . 72 3.16 Comparison between the lifetime measurements performed for the
refer-ence, p-side and n-and p-side barrier structures. (The measurement was performed by A. Gust) . . . 73 3.17 TEM micrograph of the laser structure with ZnSSe barrier positioned on
the p-side . . . 74 4.1 Schematic design of the five-fold stack CdSe/ZnSSe QD stack structure . . 78 4.2 Cross-section {002} dark field image of sample A with d = 8nm ZnSSe
spacer thickness . . . 80 4.3 Cross-section{002}dark field image of sample B withd= 4.5nm ZnSSe
spacer thickness . . . 80
4.4 Cross-section {002} dark field image of sample C with d = 2nm ZnSSe spacer thickness . . . 81 4.5 Line scan performedLeft: across the CdSe/ZnSSe superlattice andRight:
along the CdSe and ZnSSe layers on a{002}dark-field image of the sam-ple A (d= 8nmZnSSe spacer thickness) . . . 81 4.6 Line scan performedLeft: across the Cd Se/ZnSSe superlattice andRight:
along the CdSe and ZnSSe layers on{002}dark-field image of the sample B (d= 4.5nmZnSSe spacer thickness) . . . 82 4.7 Line scan performed across the Cd Se/ZnSSe superlattice on{002}
dark-field image of the sample C (d= 2nmZnSSe spacer thickness) . . . 83 4.8 Intensity profiles obtained using Bloch wave computations for a ternary
ZnSSe alloy for different specimen thickness . . . 83 4.9 Intensity profiles obtained using Bloch wave computations for a ternary
CdZnSe alloy for different specimen thickness . . . 85 4.10 Intensity profile of CdZnSSe alloy for S concentration of 0, 6 and 12% . . . 86 4.11 Experimental ω/2θ scan of the (004) reflection and the corresponding
simulated profile for sample B (4.5 nm). The measurement was per-formed by G. Alexe. . . 86 4.12 3D intensity profiles (50, 60 and 70 nm sample thickness) obtained a for
quaternary CdZnSSe compound when both Cd and S are varied. . . 87 4.13 3D plot of the CdZnSSe lattice constant as a function of Cd and S
concen-tration . . . 88 4.14 Intersection between the isolines corresponding to the normalised
inten-sity value and the experimental lattice parameter, respectively, of CdZnSSe layer (sample C). . . 88 4.15 Intersection between the isolines corresponding to the normalised
inten-sity value and the experimental lattice parameter, respectively, of CdZnSSe layer (sample B). . . 89 4.16 Intersection between the isoline corresponding to the normalised
inten-sity value and the experimental lattice parameter, respectively, of CdZnSSe layer (sample A). . . 90 5.1 Schematic drawing showing the local lattice plane rotation due to shear
strain . . . 96 5.2 ¡
220¢
dark-field image of the structure with nominal 4.5 nm ZnSSe spacer taken in[110]zone axis (sample B) . . . 97 5.3 ¡
220¢
dark-field image of the structure with nominal 2 nm ZnSSe spacer taken in[110]zone axis (sample C) . . . 97 5.4 ¡
220¢
dark-field image of the structure with 8 nm ZnSSe spacer taken in [110]zone axis (sample A) . . . 98 5.5 (004)dark-field of the specimen with 4.5 nm ZnSSe spacer taken in[110]
zone axis (sample B) . . . 98 5.6 (004) dark-field of the specimen with 2 nm ZnSSe spacer taken in [110]
zone axis (sample C) . . . 99 5.7 (004)dark-field image of the specimen with 8 nm ZnSSe spacer taken in
[110]zone axis (sample A) . . . 99