Defects in heterostructures

ZnSe-based heterostructures investigated here are grown on GaAs substrates as it is described in section 1.2.2. Therefore, a description of defects that are formed due to the lattice mismatch present at the interface between the substrate and II-VI epilayer (e.g. ZnSe/GAs) or different epilayers (e.g. CdSe/ZnSe) is necessary. This is in principal connected with the degradation behavior of ZnSe-based devices under current injection.

It has been shown that even a small lattice mismatch present at interfaces, gives rise to defect formation inside the epitaxially grown crystal. In the case of ZnSe on GaAs, the latter has the smaller lattice constant. Thus, the ZnSe layer is compressively strained, as indicated in Fig. 1.17. The same situation is valid for CdSe and ZnTe, where the lattice mismatch regarding GaAs is higher (see Table 1.1).

In the opposite case, for instance ZnS, the layer is under tensile strain. Moreover, when the lateral lattice constant of the ZnSe layer equals that of the GaAs substrate and the layer is elongated along the growth direction-then the layer is fully strained

growth direction GaAs ZnSe

pseudomorphic

GaAs ZnSe

relaxed

critical critical

d < d d > d

Figure 1.17: Schematic representation of pseudomorphic and relaxed ZnSe grown on GaAs. Relaxation occurs when the layer thicknessdexceeds the critical thicknessdcritical

.

or pseudomorphic. With increasing layer thickness, the strain energy accumulates, and as soon as the critical thickness (300 nm) [41] is exceeded, the strain is released via the generation of dislocations. Accordingly, the lattice constant of the layer relaxes to its bulk value. Finally, the fully relaxed (layer thicknesses beyond 1000 nm) [41] case is reached and the whole layer has the ZnSe lattice constant, as schematically shown on the right-hand side of Fig. 1.17. The critical thickness does not only depend on the lattice mismatch, but also on the substrate preparation and the growth start procedure.

Figure 1.18: Schematic representation of the nucleation of misfit dislocation networks (D) in the quantum well by a stacking fault (S) and its associated threading dislocations (T). The network is generated under current injection [42].

The different types of crystalline defects that appear in ZnSe-based heterostructures and their influence on the degradation of the processed devices has been studied

inten-1.3 Defects in crystal structure sively (e.g. [43, 42, 44, 26]). Two different types of defects are mainly important in this context:

• stacking faults

• misfit dislocations.

Stacking faults (SF) usually nucleate at the GaAs/ZnSe heterointerface. They are di-rectly related to the substrate surface morphology, which is affected by the substrate preparation, the deoxidation process, and the growth start procedure. Such a SF is bound by partial dislocations, which have a Burgers vector b = 1/3h111iin the case of a Frank-type SF, and a Burgers vectorb = 1/6h121ifor a Shockley-type SF [43]. These SF propagate upwards during growth and form a V-shaped defect in cross section, as shown in Fig. 1.18. In some situations, the partial dislocations of a SF can react and form a perfect 60^◦-dislocation that threads upwards. Such a dissociation process often occurs at a layer interface [43, 42]. The lifetime of a light emitting device depends on the density of stacking faults in the structure, since they are electrically active and act as non-radiative recombination centers [43].

Another negative feature of SFs is that they act as nucleation centers for misfit dis-locations (MD). Misfit disdis-locations are generated, when the layer and the substrate do not have the same lattice constant. For ZnSe-based devices, this is especially relevant in the case of Cd-containing quantum wells. When a SF (or its associated threading dislocation) intersects the highly strained quantum well, the dislocation can dissociate and form a MD in the quantum well. Under current injection, these MD can multiply and finally form a network of dislocations as indicated in Fig. 1.18 [42]. These networks are responsible for the formation of the so-calleddark defects[43]. The defects which are correlated with the Cd-based quantum well degradation will be discussed in chapter 3.

Chapter 2

Experimental techniques

The interaction of the accelerated electrons with the crystal atoms offers the great po-ssibility of obtaining useful information about crystalline materials [45, 46]. Using an assemble of electromagnetic lenses, a real image of the structure can be formed and magnified. With the CM20 UT electron microscope used in this work, point resolution of 0.19 nm can be obtained. This can be used for the determination of crystal properties with an atomic resolution. Therefore, TransmissionElectronMicroscopy (TEM) inves-tigation methods were mainly used in the frame of this work, providing information about crystal quality and chemical composition which can be then correlated with epi-taxial parameters.

This chapter starts with a short description of electron diffraction theory (2.2). Then, for a proper interpretation of TEM micrographs, the process of image formation is shortly explained (2.3) and the different imaging techniques employed for TEM inves-tigations described (2.4). Moreover, due to the fact that the electrons are absorbed in the specimen, thicknesses of the sample less then 200 nm have to be used. There-fore, appropriate sample preparation techniques, pointing out the importance of the specimen preparation for TEM (2.5), are also described. For extensive and descrip-tive presentations of electron microscopy several standard books are available (e.g.:

[47, 48, 49, 50, 51]).

The complexity of the semiconductor structure investigated, requires the compari-son with other investigation methods. Therefore, TEM findings were compared with X-ray diffraction and Grazing incidence X-ray diffraction results (crystal structure in-formation as well as Photoluminescence and Electroluminescence. A brief description of these methods is also given (2.6).

2.1 Basic principle of an electron microscope

During this work, the analyzes of the II-VI semiconductor heterostructures were per-formed using a CM 20 UT (Ultra twin) Philips microscope. The essential components of this electron microscope (and in general) are: the illumination system, the objective lens and the imaging system (Fig. 2.1).

The illumination system comprises the electron gun and the system of condenser lenses. Its task is the formation of an electron beam and focusing it onto the speci-men. The entire electron gun is a triode, where theLaB₆ source is used as the cathode.

Figure 2.1: Optical column of the electron microscope

In addition to this cathode, there is a Wehnelt cylinder, and an anode at earth potential with a hole in its center. The cathode is attached to the high tension cable, which it is connected to the high-voltage power supply. When the LaB₆ source is heated to high enough temperature, the electrons have enough energy to overcome the natural barrier (“work function“ (Φ)). The cathode has a negative potential (-200 kV) with respect to the anode. The electrons are accelerated towards the anode and pass it through the anode hole, with a velocity greater than half the speed of light.

To get a controllable beam of electrons through the hole in the anode and into the microscope, a small negative bias is applied to the Wehnelt cylinder. The electrons com-ing from the cathode are converged to a point calledgun crossoverlocated between the Wehnelt cylinder and the anode. Therefore, the Wehnelt acts as a simple electrostatic lens: the first lens in the microscope. The gun crossover is considered as the object for the condenser system.

The condenser system consists of two magnetic lenses: the first condenser lens C₁ and the second condenser lensC2, and an aperture. This can operate in two principal modes: parallel beam and convergent beam. The first mode is used for TEM imaging and diffraction, while the second is used for scanning (STEM) imaging, microanalysis

2.1 Basic principle of an electron microscope and microdiffraction. The C₁ lens is located close to the anode and forms a demagni-fied image of the gun crossover, with a diameter of 10-100 µm, depending on the lens excitation. This is calledspot size(smallest illuminated area).

In order to focus the beam onto the specimen asecond condenser lens(C₂) is used. The C2uses the C1crossover as the object. This lens has a variable current control and hence variable focal length. As a result, C₂ affects the convergence of the beam and also the diameter of the illuminated area on the specimen. When a parallel beam is required (high-resolution observations), C2 is adjusted in a way to produce an underfocused image of the C₁ crossover. The C₂ condenser aperture (situated below) controls the fraction of the beam which is allowed to hit the specimen. It therefore helps to control the intensity of illumination and the width of the illuminated area. In conclusion, the condensor system prepares the electron beam for the interaction with the specimen.

Parameter Value

High-tension 200 keV

de Broglie wavelength 2.51 pm Spherical abberationCs 0.5 mm Chromatic abberationC_c 1 mm

Point resolution 0.19 nm Beam convergenceθ_c 1.3-2.5 mrad

Table 2.1: Important parameters of CM20 UT electron microscope used during this work

Objective lens The most important lens in an electron microscope is the objective lens (OL). The CM20 is equipped with an ultra twin (UT) objective lens, which is constructed as a symmetric lens. In the symmetric lens design, the electron experiences nearly half of the objective lens magnetic field before reaching the specimen, and this upper half of the field is known asobjective condensor lens. Responsible for this is the upper polepiece of the objective lens. It can be used to demagnify further the incident probe, down to the nanometer size range.

The first function of the objective lens is to bring the various diffracted electron beams, produced after the interaction of the electron beam with the specimen, to a crossover while introducing minimal aberrations and second function is to form the first intermediate image of the specimen in the image plane. The influence of the lens aberrations will be discussed in the section devoted to theory of imaging formation.

Then, this image is subsequently magnified by the intermediate lens and the projector lens system.

The objective lens is characterized by two different planes: theback focalplane (BFP) andimageplane. In the back focal plane of the objective lens thediffraction pattern (DP) is formed. In this plane the objective aperture (OA) can be inserted. Its function is to select electrons which are diffracted by specific lattice planes to contribute to the image, and thereby affects the appearance of the image, as the contrast of the final image. By inserting the aperture and tilting the beam, different types of images can be formed (see Sec. 2.4). To form diffraction patterns from small areas of the specimen, a “selected area“

aperture is used. This operation is calledselected-areadiffraction and consists of inserting

the selected aperture in the image plane of the objective lens, in a way that a virtual aperture is created in the specimen plane. Therefore, any electron that hits the specimen outside this area defined by the virtual aperture will be absorbed by the diaphragm in the image plane. Thus, it will be excluded from contributing to the diffraction pattern that is projected onto the viewing screen in diffraction mode.

Imaging system The first intermediate lens magnifies the initial image that is formed by the objective lens. When the object plane of the intermediate lens is the BFP of the objective lens, the diffraction pattern is shown on the viewing screen. When the object plane of the intermediate lens is located on the image plane of the objective lens, a mag-nified image of the specimen is formed. Magnification in the electron microscope can be varied from several hundred to several hundred thousand times depending on the setting of intermediate lens and projector lens. Moreover, at very low magnifications, the objective lens is switched off.

Recording system The image (DP) seen on the viewing screen can be recorded us-ing a TV camera, negatives, imagus-ing plates or a CCD camera. The images obtained in this work were recorded on negatives or imaging plates. Negatives have the disad-vantage of the non-linear relation between grey-level and intensity. A better quality of the recorded image can be obtained using imaging plates. Animaging plateis a flexible electron detector, where an active layer of tiny crystals locally stores high energetic ra-diation. The storage crystals are made from doted barium fluoro-bromide embedded in a blue colore resin. The electron irradiation excites the crystals in their luminescence center to a semi-stable state. By an illumination with red laser light (Ditabis scanner), the crystals are excited again and stimulated to release the stored information as blue luminescence signal. The amount of blue light released depends on the electron dose during the first excitation. Using the imaging plates, the quality of the recorded im-age is highly improved, because they show a high dynamic range and linear transfer of contrast.