AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY FACULTY OF MATERIALS SCIENCE AND CERAMICS Department of Silicate Chemistry and Macromolecular Compounds

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1 AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY FACULTY OF MATERIALS SCIENCE AND CERAMICS Department of Silicate Chemistry and Macromolecular Compounds Prof. dr hab. Dominik Dorosz REVIEW of MSc. Eng. Djalal Benyahia s doctoral dissertation entitled Technology of the buffer layers on GaAs substrate for the molecular beam epitaxial growth of III-V infrared detector material systems The subject of the review is MSc. Eng. Djalal Benyahia s doctoral dissertation entitled "Technology of the buffer layers on GaAs substrate for the molecular beam epitaxial growth of III-V infrared detector material systems", promoted by prof. dr hab. inż. Antoni Rogalski, Ordinary Member of the Polish Academy of Sciences. The review have been done at the request of Prof. dr hab. inż. Stanisław Cudziło, the Dean of the Faculty of Advanced Technologies and Chemistry, The Military University of Technology in Warsaw. The Subject and purpose of the work Photonics in the last century was indicated as one of the main development direction of our civilization. This conviction was resulted from the unique feature of the photon as the fastest way of transmitting information known to us. The best example, honored with the Nobel Prize (Charles Kao, 2009, "groundbreaking achievements relating to light in fibers for optical communication"), is fiber optic telecommunications, which revolutionized the modern world, creating virtually unlimited possibilities in terms of social communication. This achievement were connected with dynamic development of not only low loss silica optical fiber and laser sources, but also efficient detectors in the m spectral range. This last need led to the invention of InGaAs semiconductor which is now wildly used in optoelectronics devices. The next most important material which opened new possibility in Infra-red (IR) detection was HgCdTe (MCT). His excellent physical properties like spectral range to 32 m (independence of lattice constant with composition), long life times (1-10 s), high absorption coefficient (10-4 cm -1 ) and quantum efficiency (~70%) caused that MCT is widely used in thermal imaging and night vision applications. Significant development of the IR detector was possible not only by material invention but also a superlattice idea combined with technology of thin layers deposition mainly MBE and MOCVD. It led to propose a quantum well infrared (QWIPs) and superlattice (SL) photodetectors. The advantage of the letter one is that depends on the SL

2 localisation the spectral range and decrease of avalanche noise may be modified. The other type of SL is a strained layer superlattice photodetector which provides a thin buffer layers to eliminate lattice mismatch between different materials. This approach gives the possibility of producing semiconductors with even a high difference of lattice constants, but also to modify the active area of the detector by locating superlattice structure in it. This area includes the subject of Djalal Benyahia s doctoral dissertation to elaborate superlattice photodetector based on InAs/GaSb T2SL (Type-II Superlattice). He formulated the aim to use MBE method to growth III-V semiconductor on GaAs substrate. The realization of the thesis undoubtedly required a systematic experimental research, undertaken by Author as the proposed substrate is characterised with high lattice mismatch to the InAs/GaSb T2SL structure. It should be noted that the aim of the work is unambiguously directed at solving a specific technological problem, which is a concept of suitable buffer layer, and the use of different techniques to propose an innovative aspect of the work. Proving the thesis required the Author to carry out technological and experimental research and preceded by the analysis of the state of knowledge. I declare that the assumptions adopted by the Author are correct, and the purpose and thesis of the work have been formulated correctly. Layout and work evaluation Numbering 127 pages, the work is divided into two parts: theoretical (chapters 1-2) and experimental (chapter 3-5). Additionally, such parts of the dissertation as: abstract, Author s publications and conferences, list of figures and tables, conclusion and bibliography were exposed. In the theoretical part, which is less than half the volume of work, the subject area was characterized, justifying the purpose and indicating the physical and technological aspects of current photonics structures used in IR region. The first chapter presents general information about IR detectors form the principles to the superlattice idea. At the beginning, Author discusses advantages and disadvantages of currently used materials and compare their properties. In particular he points out HgCdTe and GaAs/AlGaAs quantum well properties showing also their limitations which are connected with i.e. weakness of Hg-Te bonds and performance limitations at higher temperatures respectively. Then the most perspective type-ii InAs/GaSb strained layer superlattice (InAs/GaSb T2SL) structure have been described. Author underlined its most important advantageous (wide energy bandgap to 32 m, low dark current, reduced Auger recombination, high working temperature) and clearly pointed out technological problem of GaAs substrate lattice mismatch with GaSb. In order to overcome this problem some techniques have been proposed. Author based on the literature discusses their advantages 2

3 and drawbacks pointing the interfacial misfit arrays technique (IMF) as one of the most perspective. Indeed it is a powerful technique which allow to control atomic arrangement on the condition that we are able to precise control the thickness of the single layers. For that reason Author decided to use Molecular Beam Epitaxy (MBE) which fulfil this condition and additionally allow in-situ monitoring growing of the layer. Detail characterisation of the MBE have been presented in the chapter 2. It includes also all experimental and optimisation methods which have been used in the thesis. What distinguishes here are the MBE growth parameters (temperature, V/III ions ratio, growth speed measurement) and characterisation methods i.e. (DIC, Hall effect, HRXRD, TEM). Against this background, the motivations of the research undertaken were indicated in a systematic way. Apart from the description of the doctoral thesis, Author has pointed important aspects and indirect work tasks that were carried out in order to develop a buffer layers on the GaAs substrate. Author, what should be particularly emphasized, clearly presented the theoretical background, which is strictly related to the subject of the conducted research. The analysis of the state of knowledge was based on properly selected, up-to-date literature covering 233 positions, which allows to conclude that Djalal Benyahia has a deep knowledge of the subject. On this basis, Author planned the experimental part which is concerned with MBE growth technology on GaAs substrate. He started with growth of GaSb epilayers (chapter 3) then InAs and InAsSb (chapter 4) and finally InAs/GaSb T2SL presented in the chapter 5. His obtained results are discussed in every chapter and concluded at the end of thesis. Djalal Benyahia's work layout is transparent and makes it possible to evaluate the Author's achievements. When assessing the results of the work, the transparency of the methodology adopted by the Author should be emphasized at the outset, based on the analysis of literature and experience of research team at the Military University of Technology. Djalal Benyahia proposed important technological work concerning the growth and characterisation of high quality III-V semiconductors (GaSb, InAs, InAsSb, InAs/GaSb T2SL) on the lattice mismatched GaAs substrate. Particular attention has to be paid to the Author s results and discussion which led him to the optimisation of defect free GaSb buffer layer and growth of superlattice finally. In my opinion in Djalal Benyahia's dissertation the following essential elements should be noted: 1. The aim of the investigation is focused into well-defined the need to elaborate new materials for IR applications. Proposed Type-2 superlattice structure based on III-V materials due to its decreased band-to-band tunnelling and reduced Auger recombination may overcome 3

4 known drawbacks of HgCdTe detectors like high dark current value and inability to HOT operation. 2. Substantial problems with GaSb substrates connected with poor structure quality (causing defect density) and high manufacturing cost makes the Author s thesis as a well-founded from the application side. This striving is clearly visible through consecutively planned technological tasks. 3. A systematic research of the MBE growth of GaSb on GaAs substrate by low-temperature and interfacial misfit array techniques have been done. Author used RIBER COMPACT 21- DZ MBE system equipped with Kundsen cells which allow to control fluxes of the substrates in the growth chamber at the partial pressure of Torr. During the growth of epilayers realised on GaAs (001) substrate with 2 offcut, gas control and in-situ characterisation (surface quality - RHEED - reflection high energy electron diffraction, substrate temperature, film residual stress) were performed. 4. The quality of the samples have been tested by Nomarski microscopy, Hall effect system, High-resolution of X-ray difractometer and transmission electron microscopy. Methods used in the dissertation allowed for detail layer characterisation including type of conduction, carrier concentration and mobility which are necessary for the evaluation of semiconductor quality. 5. The first GaSb epilayers which was grown by low temperature technique allowed to investigate different technological conditions like: growth temperature (230 C 520 C), Sb/Ga flux ratio (2.5, 5, 12.5), layer thickness (2 m and 5 m) on their optical and electric properties. Author determined the optimal temperature growth at 420 C (Hall mobility value 956 cm 2 /V s) and showed the possibility to obtain shiny mirror-like GaSb epilayers. 6. In the next approach interfacial misfit array technique was used to obtain higher quality of GaSb epilayers. It should be emphasized that this technique is based on the continuous reliving of the strain at the GaAs/GaSb heterointerface so requires precise control of the process parameters. Author analysed last literature attempts and propose own parameters changing shutter sequence of As and Ga (fig. 3.11). All samples possessed a shiny mirrorlike surface and low dislocations density. Performed analysis of the misfit dislocations allowed to conclude that in the sample (third approach) 90 pure-edge dislocations are present. Also for that sample high carrier mobility 2900 cm 2 /V s (80K) has been measured. 7. The next direction was to grow InAsSb layer on the GaAs substrate. Author in the first step proposed to obtain InAs layers in order to optimise temperature growth (400 C-530 C) and 4

5 As/In ratio ( ). Because of the lattice mismatch (GaAs-InAs,7.2%) he investigated 2 m and 5 m thick layers expecting that dislocations density decreases with the thickness. In results high quality of InAs layers was elaborated. Their Hall mobility value was better than reported in the literature. Hence the right conclusion of the Author to use it as a buffer for superlattice development. 8. Comparative study of the InAsSb growth on the GaAs substrate with and without InAs buffer layer (optimised before) was performed. Author proposed wide range of InAs1-xSbx compositions (x= ) which was a correct approach as we consider the lowest energy bandgap (0.1eV) which we can obtain for the InAs0.35Sb0.65 semiconductor. It was clearly seen that InAsSb structures with InAs buffer layer have higher Hall mobility and lower carrier concentration which suggests reducing the influence of the GaAs/InAsSb lattice mismatch. Moreover, the change of conduction form n-type (InAsSb) to p-type after doping with Be has been shown. 9. Finally Author based on the previous results decided to use GaAs substrate to grow type-ii InAs/GaSb superlattice. After detail literature analysis he proposed own deposition procedure (fig. 5.2) and performed an experiment to grow 30 periods of 10ML InAs/ 10ML GaSb T2SL. In conclusion the shutter sequence using InSb-like and GaAs-like interfaces were chosen for the next experiments. In the next step he investigated temperature growth of GaAs/InAsSb T2SL on GaAs substrates with 2 offcut (<110>) and without offcut. In result he pointed out significant difference between optimal temperature which was 330 C and 400 C respectively. 10. Different GaAs/InAsSb T2SL have been grown i.e.: 10ML InAs/10ML GaSb, 13ML InAs/7ML GaSb, 24ML InAs/7ML GaSb for MWIR and LWIR spectral range. Their properties have been characterised and discussed comparatively with last literature. Assessing Djalal Benyahia's doctoral dissertation, I conclude that it does not have factual errors (only single mistakes in references to drawings and citations) and was implemented correctly. The most important aspects and achievements include: - Up-to-date literature review with conducted a thorough discussion of the obtained results, explaining the observed properties at the every stage of his dissertation. - Optimisation of the interfacial misfit array technique to obtain high quality of GaSb epilayers. Detail characterisation of density and type of dislocations at the GaAs/GaSb interface using the angle defined by the Kaganger (table 3.6) and analysis reciprocal space map (RSM). Further analysis of GaSb layers and GaAs/GaSb interface using XTEM, SAED 5

6 and HAADF-STEM methods which proofed not only very good GaSb layer quality, but also excellent developed technology. - Elaboration of high quality InAs layers on GaAs substrate with carrier mobility of cm 2 /Vs. Performed a differential Hall measurement and proved that defects are located only at the GaAs/InAs interface. - Investigations on GaAs/InAsSb growth with InAs buffer layer in the wide range of Sb concentration which has huge potential application up to 32 m. - Optimisation technology of type II InAs/GaSb superlattice growing, including shutters sequence with InSb-like and GaAs-like interfaces. - Analysis the influence of the GaAs substrate (0 - offcut and 2 - offcut) on the growth temperature of 10ML InAs/10ML GaSb T2SL. Comparative research between the growth on GaAs and GaSb substrates the same SL structures, showing technological readiness of applying the GaAs substrate to obtain IR detectors. - Detail analysis of electrical properties (Hall concentration, mobility and resistivity) of 10ML InAs/10ML GaSb and 24ML InAs/7ML GaSb T2SL and discussion about their conduction features. - Finally proved that mismatched GaSb buffer substrate is capable to grow type II InAs/GaSb superlattice characterised by low carrier concentration and high mobility. - Clear conclusions and future aims. Author conducts a discussion for and against of proposed sensing structures and formulates future plans. It proofs his scientific maturity. - Doctoral thesis verified by 15 journals (indexed WoS, h-index=3, 17 times citied) and 4 international conferences. Conclusion Summing up, I find that Djalal Benyahia solved the scientific and research problem posed at work, requiring both a wide range of theoretical knowledge and technology in the field of optoelectronics. The achievements presented in the work of Djalal Benyahia allow me to state that, in accordance with the "Act on Scientific Degrees and Titles as well as Degrees and Titles in Art", dated March 14, 2003, as amended, it meets all the requirements doctoral dissertations and I apply for admission to public defence. In addition, on the basis of the scientific achievements indicated in the review and also WoS citation report, I apply for the distinction of the doctoral dissertation. Krakow, r. 6