Chapter 9

PROPERTIES OF DISLOCATIONS INEPITAXIAL ZnO LAYERS ANALYZED BY TRANSMISSION ELECTRON MICROSCOPY

E. Müller1, D. Livinov1, D. Gerthsen1, C. Kirchner2, A. Waag3, N. Oleynik4,A. Dadgar4, and A. Krost4

1Laboratorium für Elektronenmikroskopie, Universität Karlsruhe (TH), 76128 Karlsruhe,Germany; 2Abteilung Halbleiterphysik, Universität Ulm, Albert-Einstein Allee 45, 89081 Ulm,2

Germany; 3Institut für Halbleitertechnik, Universität Braunschweig, Hans-Sommer-Straße 66,38106 Braunschweig, Germany; 4Otto von Guericke Universität Magdeburg, FNW-IEP,IIPostbox 4120, 39016 Magdeburg, Germany

Abstract: The dislocation configuration in epitaxial ZnO layers grown by metal organicvapor phase epitaxy (MOVPE) was analyzed by transmission electronmicroscopy. Misfit dislocations and the majority of threading dislocations arecharacterized by Burgers vectors of the type 1/3 <11-20>. First results on theelectrical activity of dislocations are presented which are obtained by electron holography in a transmission electron microscope. The evaluation of the phasechange of the electron wave in the vicinity of a dislocation yields a negativeline charge of approximately 3 e/nm.

Key words: Metal organic vapor phase epitaxy (MOVPE), dislocations, electrical activityof dislocations, transmission electron microscopy, electron holography

1. INTRODUCTION

ZnO was rediscovered some years ago as a promising material foroptoelectronic devices because ZnO could be grown epitaxially at that time by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE). Although significant progress has been achieved in improving the structural quality of epitaxial ZnO layers, the defect density has to be further reduced before the fabrication of optoelectronic devicesbecomes feasible. Apart from point defects, which determine the electrical properties of the material, high concentrations of extended defects like

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dislocations and stacking faults are present which are known to be detrimental for light-emitting devices. It is therefore of considerable interest to study the configuration of extended defects in ZnO epilayers and correlatethe microstructure with the growth procedure. For this purpose, transmission electron microscopy (TEM) has proven to be a suitable technique because it allows the analysis of extended defects down to the atomic scale. Despite thestrongly increasing activity in epitaxial ZnO growth, only few TEM studieshave been devoted to a detailed study of the defect structure in ZnO. Narayanet al. (1998) have analyzed defects in ZnO deposited by pulsed layer deposition on (0001)Al2O3. The defect structure in MBE-grown ZnO layerswas studied by Hong et al. (2000) and Vigué et al. (2001). For ZnOdeposited directly on Al2O3(0001) substrates, a 30o rotation of the in-planeorientation was observed independent of the growth technique, which reduces the lattice-parameter mismatch from 32 % to 15 %.

The possible influence of dislocations on the overall electrical propertiesof epilayers was recognized recently when electron holography in a transmission electron microscope was used to reveal the electrical activity of edge dislocations in epitaxial GaN layers (Cherns and Jiao, 2001). It is known already for a long time that the electrical activity of dislocations canbe described by electronic states in a partially filled band acting as donors oracceptors depending on the position of the Fermi energy with respect to the occupation limit of the dislocation band (see review of Alexander and Teichler, 1991). If present at a high density, dislocations – in particularcharged dislocations - act as scattering centers which strongly reduces the charge carrier mobility (Gerthsen, 1986). In addition, the charge carrierconcentration can be modified by several orders of magnitude. In a review byOssip’yan et al. (1986), the electronic properties of dislocations in several II-VI semiconductors are summarized and a negative line charge of 1.5 e/nm was assigned to 60o Zn(glide) dislocations on the basal plane of n-ZnO.

The present study is focused on dislocations in ZnO epilayers grown byMOVPE. The distribution of edge, mixed and screw dislocations wasanalyzed for layers grown under different MOVPE conditions using TEM. Electron holography was applied to study the electrical activity of threadingdislocations.

2. EXPERIMENTAL TECHNIQUES

The TEM investigations were performed in a 200 keV Philips CM 200FEG/ST microscope which is equipped with a field emission gun. Dislocation Burgers vectors b were analyzed on the basis of the b g = 0extinction criterion using different imaging vectors g. The weak-beam

Properties of dislocations in epitaxial ZnO layers 101

technique was applied to improve the resolution compared to conventional bright-field and dark-field images (see e.g. Williams and Carter, 1996). Cross-section samples along the <11-20>- and <1-100>-zone axes were prepared applying the technique described by Strecker et al. (1993) using 3.5keV Xe+ ions for the final ion milling to minimize radiation damage with anion current of 1 mA and an incidence angle of 14 degrees.

For electron holography, the microscope is equipped with a Möllenstedt biprism, which is installed in the selected-area aperture holder. Theelectrostatic potential of the biprism was set at approximately 180 V leadingto an interference fringe distance of 0.2 nm. Holograms were obtained byorienting a straight dislocation line parallel to the biprism whereas the reference wave is transmitted through the undisturbed crystal on the otherside of the biprism. The reconstruction procedure for the amplitude and phase of the electron wave is outlined in detail by Lehmann and Lichte (2002).

All investigated samples were grown by MOVPE on Al2O3(0001)substrates. Details of the growth procedure can be found in context with theanalyzed samples in chapter 3.

3. DISLOCATIONS IN EPITAXIAL ZnO LAYERS

3.1 Structural and morphological properties

Dislocations are characterized by a Burgers vector and line direction,which determine the dislocation type. Perfect dislocations in hexagonalcrystals have Burgers vectors ba=1/3<11-20>, bc=[0001] and ba+c=1/3<11-23> (Hull, 1975). Misfit dislocations are generated at the interface betweenthe substrate and epitaxial layers, if they are grown on a substrate with adifferent lattice parameter. The density of misfit dislocations depends on thelattice-parameter mismatch and - during the early stages of the growth – on the layer thickness. The threading dislocation density is correlated with the density of misfit dislocations but can be minimized by optimizing the epitaxial growth.

Figure 1 shows cross-section TEM images of a MOVPE-grown ZnOlayer on Al2O3(0001) with a 40 nm GaN buffer layer deposited prior to theZnO growth. Isopropanol and Diethylzinc were used as precursors for the ZnO deposition with a VI/II-flux ratio of 36 at total reactor pressure of 400mbar and hydrogen as a carrier gas. The substrate temperature was 380 oC.

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Figure 1. Cross-section weak-beam images of the same area of a MOVPE-grown ZnO layeron Al2O3(0001) with a 40 nm GaN buffer layer taken with (a) g = {0002} under g,3gconditions and (b) g = {11-20} under g,3g conditions.

Figure 2. Bright-field image of the interface between the GaN/Al2O3(0001) interface and theZnO layer taken with g = {11-20}. The black line marks a GaN island.

Threading dislocations extend typically along the [0001] direction in epilayers with a hexagonal crystal structure, which has been also observed for MBE-grown ZnO epilayers (Hong et al., 2000). One possible origin of this preferred line direction was attributed to the fact that the dislocations form small angle grain boundaries between columnar grains (Vigué et al., 2001) slightly twisted around the [0001] direction which is visible in Fig.1(b) by contrast variations in the ZnO layer. Figure 1(a) was taken with g ={0002} where edge dislocations with ba = 1/3<11-20> do not show any contrast. The contrast of screw dislocation is extinct in Fig. 1(b) taken with g= {1-100}. It is obvious from Fig. 1, that the majority of dislocations exhibit edge-type character while the density of dislocations with bc and ba+c isrelatively low.

Imaging the interface region (Fig.2) shows, that the GaN buffer layerconsists of relatively large islands (a GaN island is indicated by the black line in Fig.2), which could contribute to the mosaicity of the ZnO film.

Properties of dislocations in epitaxial ZnO layers 103

The threading dislocation configuration depends significantly on thebuffer layer morphology and growth procedure, which can be inferred from Fig. 3. This ZnO film was grown on a thick GaN buffer layer using a procedure outlined in detail by Dadgar et al. (2004). A high-temperature annealing treatment was applied after the buffer layer growth and again afterthe deposition of the ZnO. Due to the thick GaN buffer layer, the mismatchbetween the Al2O3(0001) and ZnO is reduced from 15 % to 1.8 % which is accompanied by a reduction of the threading dislocation density. It is remarkable that the threading dislocations in the GaN buffer layer do not continue in the ZnO film but annihilate at the ZnO/GaN interface. The dislocation lines in the ZnO deviate frequently from the [0001] direction indicating a low degree of mosaicity, which is supported by the homogeneous contrast of the ZnO layer. The extinction of the dislocation contrast in Fig.2(a,b) shows that the majority of the dislocations is characterized by Burgers vectors of the type ba=1/3<11-20> as in the layershown in Fig.1. However, different dislocation types are the result of the random dislocation line orientations.

Figure 3. Cross-section weak-beam images of the same area of a MOPVE-grown ZnO layeron Al2O3(0001) with a thick GaN buffer layer taken with (a) g = {0002} and (b) g = (11-20)taken under g,3g conditions.

The large fraction of threading dislocations with ba can be understood by analyzing the misfit dislocations at the interface between the buffer layer and the ZnO film. Figure 4(a) displays a <11-20> zone-axis high-resolution TEM (HRTEM) image of the interface of the sample depicted in Fig.3. The {1-100} Fourier-filtered image Fig.3(b) shows that the misfit dislocations

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are characterized by one {1-100} plane inserted from below as expected dueto the larger a lattice parameter of the ZnO.

Figure 4. (a) HRTEM image of the interface between the GaN buffer layer and ZnO along the<11-20>-zone axis and (b) Fourier-filtered image showing only {1-100} lattice fringes. Thearrows indicate {1-100} fringes inserted from the substrate side.

Figure 5 shows schematically the misfit dislocation orientation and possible Burgers vectors (grey vectors). The electron-beam direction and dislocation lines are aligned parallel to the [11-20] direction. 60o or 120o

misfit dislocations are characterized by ba = 1/3 [-12-10] and 1/3 [-2110].The edge component be = ½ [1-100] is compatible with an inserted (1-100)plane as observed in Fig.4(b). It can assumed that three set of misfit dislocations along the three <11-20> directions are present in the (0001)interface plane.

The generation of misfit dislocations with ba can be understood by considering the efficiency of the different possible Burgers vectors to relax the lattice-parameter mismatch. Dislocations with ba are most efficientbecause they are characterized by the longest Burgers vector edge component be in the (0001) interface plane among all possible dislocation Burgers vectors. In addition, the strain energy of dislocations is b2 whichfavors the formation of dislocations with the shortest overall Burgers vectorba. Since dislocations cannot end inside a perfect crystal and the Burgers vector is conserved along the dislocation line, the threading segments of misfit dislocations are also characterized by ba as observed is Fig.1 and Fig.3. The above analysis suggests that improving the structural quality of the interface and using a thick GaN buffer layer should minimize threadingdislocations with Burgers vectors of the type 1/3 <11-20>.

Properties of dislocations in epitaxial ZnO layers 105

Figure 5. Schematic representation of the orientation of a misfit dislocation (bold black line)and possible Burgers vectors (grey vectors) in the (0001) interface plane

3.2 Electrical activity of dislocations

Due to the strong disturbance of the translational symmetry of the crystal,dislocations with an edge component are expected to generate deep electronic states in the energy gap of a semiconductor. If the Fermi level does not coincide with the occupation limit of the band of electronic statesthe dislocation will be charged. To maintain the overall neutrality of thematerial, a charged dislocation is required to be shielded by a space charge region of opposite sign. The resulting potential V at a distance r from thedislocation line was already calculated by the Read (1954),

0

( )4

QV (4

2 2R rR2

1ll R rRlnln 2 2

R rRR2 22 22 2r R2 22 2222 (1)

where Q=q e is the charge per dislocation length and = 8.4 the dielectricconstant of the material (Viswanatha et al., 2004). The Read radius R =

/q / describes the radius of space charge region where Nsc isdensity of compensating charges available for screening.

Electron holography is a TEM technique that allows the retrieval of thephase of the electron wave. This provides the possibility to study the interaction of the incident electrons with the electrostatic potential of a charged dislocation with high spatial resolution. Fig.6 indicates the

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orientation of the dislocation with respect to the electron-beam direction, which has been used in our study.

Figure 6. Geometry used for electron holography investigations: the electron beam is orientedalong the z direction and the dislocation along y perpendicular to the electron-beam direction.

After passing a sample with a thickness t, the phase of the transmitted beam is shifted with respect to the vacuum. If a charged dislocation is present, the mean inner potential V0 of the material is locally modified by the potential V(r) ( 2 2r x z2xx ) which induces a phase shift given by Eq.(2).

00( ( ))0

t

EC V V r dz( ( ))(0E ( 0CE (2)

The constant CE = 7.29.106 is determined by the electron energy which was measured precisely for our electron microscope by convergent beam electron diffraction (Kruse et. al., 2003). Equation (2) only applies if kinematicaldiffraction conditions are chosen. To eliminate the influence of dynamicalelectron diffraction, the sample was tilted out of the zone axis by minimizingthe excitation of the Bragg reflections in the diffraction pattern.

In previous work (Cherns and Jiao, 2001) dislocations with [0001]-line directions were aligned parallel to the electron beam, which is adequate tomaximize the contribution of the line charge to the phase shift of the electronwave. However, the formation of pits at the intersection line of thedislocation with the surface due to ion etching of the sample is difficult toavoid and to detect. By analyzing embedded dislocations, thicknessmodifications, which also shift the phase with respect to the surroundingmaterial according to Eq.(2), can be eliminated definitely. In addition, dynamical contributions to the phase shift are more difficult to exclude if

Properties of dislocations in epitaxial ZnO layers 107

dislocations and electron beam are parallel. The dislocation strain field can locally tilt the lattice such that dynamical diffraction conditions apply in the vicinity the dislocation, i.e. Bragg reflections are strongly excited, even if kinematical conditions are chosen for the undisturbed crystal. Thecontribution of dynamical phase shifts can be minimized in our approach as will be shown later in this section.

On the basis of Eq.(2), a comparison of the phase shift measured by electron holography and the calculated yields the line charge q. Figure 7shows the evaluated amplitude and phase of an electron hologram for the transmitted beam. The dislocation line direction is indicated by the dashed line in the amplitude image. Due to the noise, the position of the dislocation is visible only by a weak dark contrast in the phase image.

Figure 7. Reconstructed amplitude and phase of the electron wave in an area containing adislocation indicated by the dashed line in the reconstructed amplitude.

To reduce the noise, the phase profile perpendicular to the dislocation line along the x direction was obtained by averaging the phase along the ydirection inside the dark frame indicated in the phase image. The resulting phase profile is plotted in Fig.8. The line represents the best fit of theexpected phase shift from Eq. (1) and (2) using the line-charge density as a fit paramter. The best fit was obtained for a line charge q = 3 e/nm-1. Thecalculation was carried out iteratively by optimising both, the line charge q and R, because the Read radius R also depends on the line charge. Since thedensity of positive screening charges NSC is not exactly known we have varied Nsc in the range from 1017cm-3 to 1018cm-3, which yields Read radii between 98 nm and 30 nm. The error due to the variation of Nsc correspondsto q = 0.024 e/nm-1. The retardation of the electron wave at thedislocation indicates that the dislocation is negatively charged. As only the electrical field induced by the charged dislocation contributes to the

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measured phase change with respect to the undisturbed crystal, the thicknessintegration interval in Eq.(2) was chosen to be twice the Read radius.Therefore, an accurate value for the sample thickness is not required.

Figure 8. Measured phase change perpendicular to the dislocation line and fit according to Eq.(1) and Eq.(2).

The value for the line charge is reliable only if the effects of dynamicaldiffraction can be neglected. To assess the influence of dynamical diffractionon the phase shift, the ’worst’ case was considered by assuming that the dislocation locally tilts the crystal from kinematical into {0002} or {11-20}two-beam conditions. The resulting dynamical phase shift was calculated using the Bloch wave formalism of the EMS program package (Stadelmann,1987). Figure 9 shows the phase difference for the transmitted beam in theundisturbed crystal as a function of the thickness comparing the phase underkinematical conditions and for {0002} or {11-20} two-beam conditions. For the tilt angle the experimental values (1.5° perpendicular and 5° along thedislocation line with respect to the <1-100> zone axis) were taken into theaccount. It follows from Fig. 9, that dynamical diffraction is negligible if the distorted region in Bragg condition is smaller than 7 nm. With the experimental geometry with a dislocation orientation perpendicular to the electron-beam direction the assumption of a laterally limited distortion field is more likely to be satisfied compared to a parallel orientation.

Properties of dislocations in epitaxial ZnO layers 109

Figure 9. Phase difference for the transmitted beam in the undisturbed crystal comparing thephase under kinematical conditions and for (lower curve) {0002} or (upper curve) {11-20}two-beam conditions as a function of the thickness.

Regarding the accuracy of the determined line charge, it has to be taken into account that the fit of the phase profile strongly depends on the dielectric constant of ZnO with significantly differing values published in the literature (Landolt Börnstein, 1987). Another source of error is thesample thickness. If the sample thickness is smaller than the diameter of the Read cylinder (2 R) the electrical field of the line charge leaks into thevacuum region. Thus the effective dielectric constant decreases, leading to asmaller value of q. For an accurate treatment of this situation the field distribution outside the sample has to be calculated requiring an additionalparameter for the fit of the measured phase curve.

The determined value for q is large, but it agrees with the trend towards high line charges in ZnO according to Ossip’yan et al. (1986). The discrepancy between our measured value and the value published for 60o

(Zn)gilde dislocation can be explained by the different core structure of threading dislocations which do not lie on the (0001) plane. In our case, wedid not analyse the dislocation Burgers vector because the dislocation could not be identified anymore after taking electron holograms. It has to be noted that the electrical activity of dislocations is not necessarily induced byintrinsic electronic dislocation states but can be also due to a high

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concentration of point defect accumulated in the vicinity of the dislocation core.

The high negative dislocation line charge in n-ZnO indicates that dislocations can provide acceptor states with a high concentration if the dislocation density is high. This will lower the free electron concentration. Due to the amphoteric nature of the dislocation states they act as donors, if the Fermi energy is located below the occupation limit of the dislocationstates. Pinning of the Fermi energy at the occupation limit of the band isexpected if the concentration of shallow donors is reduced and the shallow acceptor density is increased. Therefore, dislocation states could contribute to the difficulty of obtaining p-type ZnO with low resistivity.

4. SUMMARY

The morphology and Burgers vectors of threading and misfit dislocations in MOVPE-grown ZnO heterostructures grown on Al2O3(0001) substrates were analyzed by transmission electron microscopy. Most threadingdislocation are characterized by 1/3 <11-20> Burgers vectors which can be considered as threading segments of misfit dislocations with the same Burgers vectors. The line directions of threading dislocations extend typically along the [0001] growth direction yielding a high fraction of edge dislocations. High-temperature annealing treatments during the ZnO growthlead to different line directions and consequently different threading dislocations types. The threading dislocation density can be reduced by thegrowth of a thick GaN buffer layer and the optimization of the interfacequality between the GaN buffer and the ZnO film. Analyzing the phase of the transmitted beam by electron holography, indications for a high negativeline charge at threading dislocations are found. This observation emphasizes that dislocations can have a strong influence on the mobility and concentration of charge carriers in ZnO if they are present at a high density.

REFERENCES

Alexander, H., and Teichler, H., Dislocations, 1991, in: Materials Science and Technology,Vol. 4, chap.6, W. Schröter, ed., North-Holland Pub. Company, pp. 249.

Cherns, D., and Jiao, C.G., 2001, Phys. Rev. Lett. 87, 205504.Dadgar, A., Oleynik, N., Forster, D., Deiter, S., Witek, H., Bläsing, J., Bertram, F., Krtschil,

A., Diez, A., Christen, J., and Krost, A., 2004, J. Cryst. Growth 267, 140 .Gerthsen, D., 1986, phys. stat. sol. (a) 97, 527.Hong, Soon-Ku, Ko, Hang-Ju, Chen, Y., Yao, T, 2000, J. Cryst. Growth 209, 537.Hull, D., Introduction to Dislocations, 1975, 2nd edition, Pergamon Press, pp.122.d

Properties of dislocations in epitaxial ZnO layers 111

Kruse, P., Rosenauer, A., and Gerthsen, D., 2003, Ultramicroscopy 96, 11. Landolt Börnstein, 1987, Springer-Verlag, Heidelberg, 165Lehmann, M., and Lichte, H., 2002, Microscopy and Microanalysis 8, 447.Narayan, J., Dovidenko, K., Sharma, A.K., and Oktyabrsky, 1998, S., J. Appl. Phys. 84, 2597.Osip’yan, Yu. A., Petrenko, V.F., Zaretski, A.V., and Whitworth R.W., 1986, Adv. Phys. 35

115.Read, W.T., 1954, Phil. Mag. 45, 775.Stadelmann, P.A., 1987, Ultramicroscopy 21, 131.Strecker, A., Salzgeber, U., and Mayer, J., 1993, Prakt. Metallographie 30, 482.Vigué, F., Vennéguès, P., Vézian, S., Laügt, M., and Faurie, J.-P., 2001, Appl. Phys. Lett. 79,

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PART III: ROLE OF HYDROGEN