Microscopie de biréfringence et caractérisation de matériaux à grand gap

THÈSE Pour obtenir le grade de DOCTEUR DE L’UNIVERSITÉ DE GRENOBLE Spécialité : « Matériaux, Mécanique, Génie Civil et Électrochimie » Arrêté ministériel : août 2006 Présentée par Thi Mai Hoa LE Thèse dirigée par «Thierry OUISSE» «Laboratoire des Matériaux et du Génie Physique (LMGP) » préparée au sein du dans l'École Doctorale « Ingénierie – Matériaux, Mécanique, Énergétique, Environnement, Procédés (IMEP-2) » Microscopie de biréfringence et caractérisation de matériaux grand gap Thèse soutenue publiquement le « Mars 2014 », devant le jury composé de : Monsieur, Thanh Vinh LE Professeur, Université d’Aix-Marseille, (Rapporteur) Monsieur, Efstathios POLYCHRONIADIS Professeur, Université Thessaloniki, (Rapporteur) Monsieur, Jocelyn ACHARD Professeur, Université Paris 13, (Président) Monsieur, Didier CHAUSSENDE Chargé de recherche CNRS, (Membre) Monsieur, Thierry OUISSE Professeur, Grenoble INP, (Directeur de thèse) Table of contents TABLE OF CONTENTS Abstract i Acknowledgments iii Chapter General introduction Foreword………………………………………… ……………………………… A brief review of Silicon Carbide (SiC)……… … ………………………………5 1.1 Structure and properties of Silicon Carbide…… …………… ……… 1.1.1 Crystal structure… ………………………………… ……… 1.1.2 Properties of Silicon Carbide… ………… ………… …… 1.2 Crystallographic defects in SiC material and their characterization… ….9 A brief review of Diamond……………………………….… …… ……………14 2.1 Structure and properties of Diamond……………………… …… … 14 2.1.1 Crystallographic structure………………………… ……… 14 2.1.2 Properties of diamond……………………………… …… …15 2.2 Crystallographic defects in Diamond and their characterization… 17 About the necessity of studying crystallographic defects .20 About the necessity of developing non-destructive characterization techniques 22 Layout of the thesis 23 References……… …… …………………………………………………………… 24 Chapter Birefringence and rotating polariser method Introduction .32 Birefringence or Double refraction 32 Table of contents 2.1 The dielectric tensor and the refractive index 32 2.2 Birefringence 36 Elasticity 39 3.1 Stress and strain 39 3.1.1 Stress 39 3.1.2 Strain 41 3.2 Hooke’s law 42 3.3 Relation between compliance and stiffness 43 3.4 The matrix and tensor notations 43 3.5 Transformation rule of the stress tensor 44 Stress- induced birefringence 47 Birefringence microscopy .49 5.1 Introduction 49 5.2 Conventional set-up for birefringence measurements 51 5.2.1 The nature of light 51 5.2.2 Linear and circular polarized light .52 5.2.3 Quarter-wave plate .53 5.2.4 Plane polariscope .54 5.2.5 Circular polariscope 55 5.3 Experimental setup of the rotating polarised method .56 Birefringence modelling 59 6.1 Birefringence images of a dislocation in 6H-SiC crystals .59 6.1.1 Introduction 59 6.1.2 Birefringence modelling 60 6.1.3 The calculation of the residual stress field and birefringence image 62 6.2 Birefringence images of a dislocation in diamond 65 6.2.1 Introduction 65 6.2.2 The calculation of the birefringence image of a dislocation in Diamond .65 References 70 Table of contents Chapter Birefringence microscopy study of dislocations in SiC substrates Introduction .75 Experimental birefringence image of dislocations in Silicon carbide 76 2.1 Introduction 76 2.2 Sample and experimental method 76 2.3 Results and discussion 77 Comparison between experiment and theory 80 3.1 Introduction 80 3.2 Sample and experimental method 81 3.2.1 Sample .81 3.2.2 Experimental method 82 3.3 Results and discussion 82 The influence of the residual stress upon the birefringence pattern .87 4.1 Introduction 87 4.2 Results and discussion 87 Observing dislocations with a small component of the Burgers vector in the basal plane 90 5.1 Introduction 90 5.2 Results and discussion 91 Experimental and simulated birefringence patterns induced by dislocations with different Burgers vectors 92 6.1 Introduction 92 6.2 Results and discussion 93 Dependence of the birefringence pattern on the dislocation length along the z axis 96 7.1 Introduction 96 7.2 Results and discussion 97 Comparison between birefringence and KOH etching 100 8.1 Introduction 100 8.2 Experimental method .101 Table of contents 8.3 Results and discussion 101 The correlation between the birefringence and the substrate thickness .111 9.1 Introduction 111 9.2 Sample and experimental method 114 9.3 Results and discussion 115 10 Conclusion 122 References 123 Chapter Birefringence microscopy study of dislocations in Diamond Introduction 131 Birefringence patterns due to dislocations and inclusions in diamond 133 2.1 Samples and experimental method 133 2.1.1 Samples 133 2.1.2 Experimental method .133 2.2 Results and discussion 134 Identification of the dislocations in single crystal CVD diamond by combining experiment and simulation 143 3.1 Experimental method .143 3.2 Results and discussion 144 Dependence of the birefringence pattern on the dislocation length along the zaxis 152 4.1 Introduction 152 4.2 Results and discussion 152 Investigation of the characteristic of the HPHT substrate before and after chemical vapour deposition (CVD) growth .158 5.1 Introduction 158 5.2 Results and discussion 158 Investigation of the dislocation propagation from the HPHT substrate into the CVD layer 161 6.1 Introduction 161 Table of contents 6.2 Samples and experimental method 161 6.3 Results and discussion 162 Conclusion 167 References 167 General conclusion 174 Abstract Wide band gap (WBG) semiconductor materials such as Silicon Carbide (SiC) and Diamond have outstanding material properties Many applications can benefit from WBG semiconductors In order to improve material quality as well as to increase the range of technological applications of the WBG semiconductor materials, it is necessary to decrease or minimize the number of extended defects This research work deals with the assessment, modelling and development of analytical techniques based upon the use of optical microscopy The thesis dedicated to the identification of structural defects in Silicon Carbide (SiC) and diamond materials Extended defects in 6H-SiC wafer and diamond materials were characterized by birefringence microscopy The measured birefringence patterns of individual dislocations modelled In the case of SiC, a good agreement is obtained between theory and experiment, which led to the proper determination of the Burgers vector values and background residual stress All observed dislocations were almost vertical dislocations with a mixed character Sometimes, their orientation changes resulting in the observation of a faint birefringence pattern We compared birefringence data with etch pits formed after KOH etching Combining both techniques is a method to discriminate between pure screw dislocations and mixed or pure edge dislocations Typical dislocations in single crystal CVD diamond were determined by birefringence measurement and quantitatively modelled Although the simulated images only approximate the experimental ones, the individual dislocations are determined to be threading edge or mixed dislocations with a possible Burgers vector a/2(011) or a/2(110) Sometimes, the vertical dislocations can convert to a horizontal or slightly tilted line and then turn vertical again resulting in the observation of two separated birefringence patterns The dislocation propagation from the HPHT substrate into the CVD layer has been investigated by simultaneously analysing the HPHT substrate and the CVD layer in the same sample region Keywords: Birefringence microscopy, crystallographic defects, Diamond, Silicon Carbide, Chemical Vapour Deposition i Résumé Les matériaux semi-conducteurs large bande interdite (WBG) tels que le carbure de silicium (SiC) et le diamant ont des propriétés matérielles exceptionnelles De nombreuses applications peuvent bénéficier des semi-conducteurs WBG Afin d'améliorer la qualité de ces matériaux, ainsi que pour augmenter leur gamme d'applications technologiques, il est nécessaire de diminuer ou réduire au minimum le nombre de défauts étendus Ce travail de recherche a porté sur l'évaluation, la modélisation et le développement de techniques d'analyse s’appuyant sur l'utilisation de la microscopie optique La thèse s’est focalisée sur l'identification des défauts structurels dans le carbure de silicium (SiC) et le diamant Les défauts étendus dans des substrats 6H-SiC et du diamant ont été caractérisés par microscopie de biréfringence et les figures de biréfringence des différentes dislocations mesurées ont été modélisées Dans le cas du SiC, un bon accord est obtenu entre théorie et expérience ce qui a amené la détermination valide des vecteurs de Burgers et contraintes résiduelles d’arrière-plan Presque toutes les dislocations observées étaient des dislocations verticales caractère mixte Parfois, leur orientation change résultant dans l'observation d'un motif de faible biréfringence Nous avons comparé les données de biréfringence avec les gravures formées après attaque KOH La combinaison des deux techniques est une faỗon de discriminer entre les dislocations vis pures et les dislocations mixtes ou coins Les dislocations typiques dans du diamant monocristallin CVD ont été déterminées par mesure de biréfringence et quantitativement modélisées Bien que les images simulées soient des approximations des observations expérimentales, les dislocations individuelles apparaissent comme étant des dislocations coins ou mixtes avec un possible vecteur de Burgers a / (011) ou a/ (110) Parfois, les dislocations verticales peuvent se transformer en ligne horizontale ou inclinée, puis revenir nouveau la verticale résultant dans l'observation de deux figures de biréfringence séparées La propagation de dislocations partir du substrat HPHT dans la couche CVD a aussi été étudiée en analysant simultanément le substrat HPHT et la couche CVD dans la même région de l'échantillon Mots-clés: Microscopie de biréfringence, défauts cristallographiques, diamant, carbure de silicium, dépôt vapeur en phase chimique CVD ii Acknowledgements This research work has been achieved at the Laboratory of Materials and Engineering Physics (LMGP) and financially supported by both the Vietnamese and French Governments I wish to express my deep gratitude to my advisor Prof Thierry OUISSE, of Grenoble INP, for giving me the opportunity to complete this thesis, for his excellent support in all situations, for his helpful discussions concerning both experiment and theory, and for kindly spending a lot of time in discussing and correcting my dissertation His wide knowledge, mentorships and encouragements throughout my entire study make this thesis possible I am proud of my mentor and exemplary teacher I would like to thank Bernard CHENEVIER, Director of LMGP, Dr Didier CHAUSSENDE, the SPCS group members as well as all members of LMGP for their great help during my research Particularly, I would like to acknowledge Prof Jocelyn ACHARD, Dr Alexandre TALLAIRE and the LSPM group members for kindly supplying the Diamond samples I also would like to thank all the dissertation committee members and their valuable advice Specially, I am thankful to my parents Vũ Thị Yến and Lê Đức Mậu, my sister and my brother for their love and endless encouragement Many thanks to all my friends from Vietnam, France, Thailand, Korea, Germany and many other countries in the world for their friendship and provided when the support writing this thesis France, December 2013 iii Chapter General introduction Chapter General introduction Chapter Birefringence microscopy study of dislocations in Diamond Figure 4.19 Experimental birefringence images of the sample consisting of the CVD layer on its HPHT type Ib (001) substrate: (a, b) the HPHT substrate before and (c, d) the HPHT substrate after CVD growth, (e, f) the birefringence images of a homoepitaxial diamond film All the images are obtained from the same sample region 163 Chapter Birefringence microscopy study of dislocations in Diamond Figure 4.20 shows experimental birefringence images of the sample consisting of the CVD layer on its HPHT type Ib (001) substrate: (a, b) the HPHT substrate and (c, d) the CVD layer A comparison of the substrate and deposited layer is made in almost the same sample region Figures 4.20 (b) and (d) are the high magnification images of the black-framed area in the Figures 4.20(a) and (c), respectively The dislocations labelled D1 and D2 in the Figure 4.20(b) are observed at almost the same position in Figure 4.20(d) Figure 4.20 Experimental birefringence images of (a, b) the HPHT substrate, (c, d) the CVD layer All the images are obtained from the same sample region Since, by principle, a dislocation cannot terminate inside the sample, it seems rather normal to verify that dislocations pre-existing in the substrate are found to propagate 164 Chapter Birefringence microscopy study of dislocations in Diamond in the CVD layer, when evidenced by birefringence Careful examination of the dislocations in the CVD layer indicates that several dislocations in the substrate tend to propagate into the CVD layer Figure 4.21 shows the birefringence images of the sample consisting of the CVD layer on its HPHT substrate: (a) the HPHT substrate and (b) the CVD layer The two images are obtained from the same sample region by adjusting the focus inside the substrate and CVD layer, respectively The colour scale for the birefringence is in range from 0.02 to 0.17 The image size is 191µm x 254µm From a comparison of Figure 4.21(a) and (b), it can be seen that the images differ, although they are taken from the same sample region The birefringence patterns in Figure 4.21(a) are not the same as in Figure 4.21(b) The birefringence images exhibit a large bright area due to a high residual strain The dislocations labelled from D1 to D5 are present in both the Figure 4.21(a) and (b) The apparition of new dislocations labelled D6 and D7 is clearly evident in Figure 4.21(b) Apparently, this difference arose due to the fact that new dislocations are formed in the CVD layer, these defects not being initially present in the substrate (see Figure 4.21(a)) Figure 4.21 Experimental birefringence images of the sample consisting of the CVD layer on its HPHT substrate: (a) the HPHT substrate, (b) the CVD layer The two images are obtained from the same sample region 165 Chapter Birefringence microscopy study of dislocations in Diamond The interpretation for the structure of the dislocations D6 and D7 in the CVD layer has been discussed by several researcher groups [11, 15, 16, 34, 40] In a publication [40], by a direct comparison of birefringence images and X-ray topography, Bauer et al reported that the defect which exhibited such a birefringence pattern is the aggregation of dislocations These defects are independently created in the deposited layer In other publication [16, 17], Y Kato et al has reported the stress around a dislocation in CVD epitaxial diamond using a cathodo-luminescence (CL) image By comparison of the birefringence image and CL image, they assumed that there is actually a bundle of dislocations present due to a fine structure at the local stressed area In principle, single dislocations can be detected and identified, but it is difficult to discuss the existence of bundles of dislocations using only birefringence image As a conclusion, we have showed that birefringence microscopy can be used to distinguish defects newly formed in the CVD layer from those already present in the substrate Conclusion  In Chapter 4, we have investigated the characteristics of dislocations in synthetic high-pressure high-temperature (HPHT) type Ib (001) substrates and single crystal CVD diamond In our study, birefringence microscopy is used as a useful tool for analyzing dislocation properties It can easily reveal and locate the various defects in crystals, but the method also has some limitations Substrates with a varying residual stress make it difficult to detect a dislocation Also, if the substrates contain too high dislocation densities, this prohibits the analysis of individual dislocations Besides, the dislocation detection can be achieved only for linear and threading dislocations  We have provided both experimental data and simulated results of the birefringence patterns induced by the dislocations in diamond We presented experimental data and simulated results of typical examples of various dislocation 166 Chapter Birefringence microscopy study of dislocations in Diamond types in single crystal CVD diamond Although the simulated images only approximate the experimental ones, some characteristics of the dislocations can be determined In the most usual case, the individual dislocations are determined to be threading edge dislocations with Burgers vectors a/2(011) or a/2(110) Sometimes, the dislocations can change their orientation due to a number of specific physical processes In particular, dislocations can convert to a horizontal or slightly tilted line 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6HSiC and diamond materials From the results and discussions in chapter and chapter 4, general conclusions are given as follows: The experimental data and simulations demonstrated that the birefringence technique can be a useful tool for investigating dislocation properties Compared to Transmission Electron Microscopy (TEM) or X-ray topography with a synchrotron light beam, it is faster and requires a low-cost equipment The technique has the capability for detecting unit edge dislocations, but the method has intrinsic limitations If the residual stress varies too much across the substrate, it makes difficult to detect all dislocations The substrate containing too high a dislocation density will also prohibit analysis Beside, detection is achievable only for linear, threading dislocations In the case of SiC, it only gives access to the in-plane component of the Burgers vector The characterization of the dislocations in 6H-SiC wafers has been investigated All observed dislocations present a substantial birefringence The experimental birefringence images are in good agreement with the simulated ones From the results of the simulation we can assess the Burgers vector values and background residual stress We note that a vast majority of the observed dislocations presumably lie in a prismatic glide plane They are almost vertical dislocations with probably a mixed character We have investigated the influence of the background stress upon the birefringence patterns in a systematic way by comparing experimental and simulated birefringence patterns with different levels of background stress This is the first time that such an investigation was successfully achieved We have discussed the dependence of the birefringence pattern on the dislocation length along the c-axis We can conclude that in the most usual case the dislocations 175 General conclusion line was threading throughout the whole sample thickness, but sometimes they can change their orientation We have presented a comparison between KOH etch pits and birefringence images We note that a basal plane dislocation can turn into a vertical dislocation In such cases, it is not detectable if the length of the vertical dislocation line is too small It is thus totally excluded to count all emerged dislocation by birefringence measurements Sometimes, we even found micropipes devoid of any birefringence, so that some micropipes can be missed as well KOH etching allows us to estimate the emerging basal plane and pure edge dislocations, but does not allow us to discriminate between pure screw and mixed dislocations It is clear that both techniques are complementary We have investigated the correlation between the birefringence and the substrate thickness The results allow us to conclude that in order to quantitatively model the birefringence pattern induced by the dislocations, the substrate thickness is required to be smaller than approximately 300 µm, and larger than approximately 60µm We have investigated the characteristics of synthetic high-pressure hightemperature (HPHT) type Ib (001) substrates and single crystal CVD diamond by birefringence microscopy Birefringence images due to dislocations and inclusions in diamond have been presented We have also identified the dislocations in single crystal CVD diamond by comparing experiments and simulations The birefringence images clearly reveal a contrast between different regions The dislocation density of single crystal CVD diamond as well as the HPHT substrate is estimated to be ∼104cm-2, which is very close to the dislocation density reported in some publications We have combined experimental data and simulations to identify various dislocation types in single crystal CVD diamond Although the simulated images only approximate the experimental ones, some characteristics of individual dislocations can be determined In the most usual case, the observed dislocations are determined to be threading edge or mixed dislocations with a possible Burgers vector a/2(011) or 176 General conclusion a/2(110) Sometimes, the dislocations can convert to a horizontal or tilted line and then turn vertical again The characteristics of the HPHT substrate before and after CVD growth were studied The results indicate that it is possible to detect defects in the HPHT substrate before and after CVD growth In most cases, the position of the dislocations does not change, but the properties of the residual stress in the HPHT substrate are modified by the CVD growth The dislocation propagation from the HPHT substrate into the CVD layer is discussed In our studies, we cannot give an unambiguous conclusion about defect propagation From the results, we can only conclude that birefringence microscopy can be used to distinguish new defects formed in the CVD layer from those already present in the HPHT substrate We have identified the type of individual dislocations in single crystal CVD diamond by comparing the experiments and simulations, but on a small number of substrates To confirm and extend our results, it would be necessary to compare birefringence images and e.g., optical microscope images of etch pits or X-ray topographs Birefringence microscopy is a non-destructive technique It can represent a useful method for investigating dislocation structure of transparent materials Therefore, it should to be extended to other materials (e.g., LiF) 177 ... recorded from a 6H-SiC wafer [21] 11 Chapter General introduction Defect etching of SiC in molten potassium hydroxide (KOH) is a very common method to detect dislocations S A Sakwe et al have developed... de la microscopie optique La thèse s’est focalisée sur l'identification des défauts structurels dans le carbure de silicium (SiC) et le diamant Les défauts étendus dans des substrats 6H-SiC et. .. caractérisés par microscopie de biréfringence et les figures de biréfringence des différentes dislocations mesurées ont été modélisées Dans le cas du SiC, un bon accord est obtenu entre théorie et expérience