Department of Engineering Sciences

Our recent PhD theses

Patrice Bras (2017)

Sputtering-based processes for thin film chalcogenide solar cells on steel substrates

Abstract - Thin film chalcogenide solar cells are promising photovoltaic technologies. Cu(In,Ga)Se2 (CIGS)-based devices are already produced at industrial scale and record laboratory efficiency surpasses 22 %. Cu2ZnSn(S,Se)4 (CZTS) is an alternative material that is based on earth-abundant elements. CZTS device efficiency above 12 % has been obtained, indicating a high potential for improvement. In this thesis, in-line vacuum, sputtering-based processes for the fabrication of complete thin film chalcogenide solar cells on stainless steel substrates are studied. CIGS absorbers are deposited in a one-step high-temperature process using compound targets. CZTS precursors are first deposited by room temperature sputtering and absorbers are then formed by high temperature crystallization in a controlled atmosphere. In both cases, strategies for absorber layer improvement are identified and implemented. The impact of CZTS annealing temperature is studied and it is observed that the absorber grain size increases with annealing temperature up to 550 °C. While performance also improves from 420 to 510 °C, a drop in all solar cell parameters is observed for higher temperature. This loss is caused by blisters forming in the absorber during annealing. Blister formation is found to originate from gas entrapment during precursor sputtering. Increase in substrate temperature or sputtering pressure leads to drastic reduction of gas entrapment and hence alleviate blister formation resulting in improved solar cell parameters, including efficiency. [...]

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Da Zhang (2017)

On the Low Frequency Noise in Ion Sensing

Abstract - Ion sensing represents a grand research challenge. It finds a vast variety of applications in, e.g., gas sensing for domestic gases and ion detection in electrolytes for chemical-biological-medical monitoring. Semiconductor genome sequencing exemplifies a revolutionary application of the latter. For such sensing applications, the signal mostly spans in the low frequency regime. Therefore, low-frequency noise (LFN) present in the same frequency domain places a limit on the minimum detectable variation of the sensing signal and constitutes a major research and development objective of ion sensing devices. This thesis focuses on understanding LFN in ion sensing based on both experimental and theoretical studies. The thesis starts with demonstrating a novel device concept, i.e., ion-gated bipolar amplifier (IGBA), aiming at boosting the signal for mitigating the interference by external noise. An IGBA device consists of a modified ion-sensitive field-effect transistors (ISFET) intimately integrated with a bipolar junction transistor as the internal current amplifier with an achieved internal amplification of 70. The efficacy of IGBA in suppressing the external interference is clearly demonstrated by comparing its noise performance to that of the ISFET counterpart. [...]

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Christopher Frisk (2017)

Modeling and electrical characterization of Cu(In,Ga)Se2 and Cu2ZnSnS4 solar cells

Abstract - In this thesis, modeling and electrical characterization have been performed on Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) thin film solar cells, with the aim to investigate potential improvements to power conversion efficiency for respective technology. The modeling was primarily done in SCAPS, and current-voltage (J-V), quantum efficiency (QE) and capacitance-voltage (C-V) were the primary characterization methods. In CIGS, models of a 19.2 % efficient reference device were created by fitting simulations of J-V and QE to corresponding experimental data. Within the models, single and double GGI = Ga/(Ga+In) gradients through the absorber layer were optimized yielding up to 2 % absolute increase in efficiency, compared to the reference models. For CIGS solar cells of this performance level, electron diffusion length (Ln) is comparable to absorber thickness. Thus, increasing GGI towards the back contact acts as passivation and constitutes largest part of the efficiency increase. For further efficiency increase, majority bottlenecks to improve are optical losses and electron lifetime in the CIGS. In a CZTS model of a 6.7 % reference device, bandgap (Eg) fluctuations and interface recombination were shown to be the majority limit to open circuit voltage (Voc), and Shockley-Read-Hall (SRH) recombination limiting Ln and thus being the majority limit to short-circuit current and fill-factor. Combined, Eg fluctuations and interface recombination cause about 10 % absolute loss in efficiency, and SRH recombination about 9 % loss, compared to an ideal system. [...]

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Piotr Szaniawski (2017)

From Light to Dark: Electrical Phenomena in Cu(In,Ga)Se2 Solar Cells

Abstract - In Cu(In,Ga)Se2 (CIGS) solar cells the CIGS layer serves as the light absorber, growing naturally p-type. Together with an n-type buffer layer they form a p-n heterojunction. Typically, CdS is used as a buffer, although other, less toxic materials are investigated as alternatives. The intrinsic p-type doping of CIGS layers is the result of complex defect physics. Defect formation energies in CIGS are very low or even negative, which results in extremely high defect concentrations. This leads to many unusual electrical phenomena that can be observed in CIGS devices. This thesis mostly focuses on three of these phenomena: light-soaking, light-on-bias, and light-enhanced reverse breakdown. Light-soaking is a treatment that involves illuminating the investigated device for an extended period of time. In most CIGS solar cells it results in an improvement of open-circuit voltage, fill factor, and efficiency that can persist for hours, if not days. The interplay between light-soaking and the remaining two phenomena was studied. It was found that light-soaking has a strong effect on light-on-bias behavior, while the results for light-enhanced breakdown were inconclusive, suggesting little to no impact [...]

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Malkolm Hinnemo (2017)

On the Road to Graphene Biosensors

Abstract - Biosensors are devices that detect biological elements and then transmit a readable signal. Biosensors can automatize diagnostics that would otherwise have to be performed by a physician or perhaps not be possible to perform at all. Current biosensors are however either limited to particular diseases or prohibitively expensive. In order to further the field, sensors capable of many parallel measurements at a lower cost need to be developed. Field effect transistor (FET) based sensors are possible candidates for delivering this, mainly by allowing miniaturization. Smaller sensors could be cheaper, and enable parallel measurements.

Graphene is an interesting material to use as the channel of FET-sensors. The low electrochemical reactivity of its plane makes it possible to have graphene in direct contact with the sample liquid, which enhances the signal from impedance changes. Graphene-FET based impedance sensors should be able to sense almost all possible analytes and allow for scaling without losing sensitivity. [...]

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Yi Ren (2017)

Annealing of Cu2ZnSn(S,Se)4 Thin Films: A Study of Secondary Compounds and Their Effects on Solar Cells

Abstract  - Kesterite Cu2ZnSnS4 (CZTS) is interesting as a sustainable photovoltaic technology due to its earth-abundant elements and suitable semiconducting properties. To date, a record efficiency of 12.6% has been achieved but further improvements are required to reach high efficiency for industrial implementation. Among the limiting issues is the understanding of the annealing process, which is crucial in promoting high material quality. In particular, the knowledge of the effects of segregated secondary compounds on solar cell performance is lacking.

In contrast to formation of ZnS particles throughout CZTS film, it is notable that SnS forms and usually segregates on the CZTS top and rear surfaces. The influence of SnS on CZTS solar cells was studied by electron beam induced current measurements. It is found that SnS presence on the CZTS surfacecan introduce “dead area”, whereas it seems beneficial for solar cell current when accumulates on the CZTS rear. For SnS passivation and from investigation of the passivation effect from an Al2O3 thin layer at the CZTS rear, improvement in overall device performance could not be demonstrated, due to either poor CZTS bulk or non-optimal device structure. The limitation in CZTS bulk quality was shown from a thickness study where carrier collection saturated already about 700-1000 nm CZTS thickness. [...]

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Shabnam Mardani (2016)

Copper and Silver Metallization for High Temperature Applications

Abstract - High-temperature electrical- and morphological-stability of interconnect is critical for electronic systems based on wide band gap (WBG) semiconductors. In this context, the thermal stability of both Ag and Cu films with Ta and TaN films as diffusion barriers and/or surface-capping layers at high temperatures up to 800 oC is investigated in this thesis.

The investigation of un-capped Ag films with either Ta or TaN diffusion barrier layers shows electrical stability upon annealing up to 600 °C. Degradation occurs above 600 °C mainly as a result of void formation and Ag agglomeration. Sandwiching Ag films between Ta and/or TaN layers is found to electrically and morphologically stabilize the Ag metallization up to 800 °C. The barrier layer plays a key role; the β-to-α phase transition in the underlying Ta barrier layer is identified as the major cause for the morphological instability of the film above 600 °C. This phase transition can be avoided using a stacked Ta/TaN barrier. Furthermore, no observable Ta diffusion in Ag films is found. [...]

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Patrik Ahlberg (2016)

Graphene Implementation Study in Semiconductor Processing

Abstract - Graphene, with its two-dimensional nature and unique properties, has for over a decade captured enormous interests in both industry and academia. This work tries to answer the question of what would happen to graphene when it is subjected to various processing conditions and how this would affect the graphene functionality. The focus is placed on its ability to withstand different thin-film deposition environments with regard to the implementation of graphene in two application areas: as a diffusion barrier and in electronic devices.

With single-layer graphene films grown in-house by means of chemical vapor deposition (CVD), four techniques among the well-established thin-film deposition methods are studied in detail: atomic layer deposition (ALD), evaporation, sputter-deposition and spray-deposition.[...]

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Johan Lindahl (2015)

Atomic layer deposition of zinc tin oxide buffer layers for Cu(In,Ga)Se2 solar cells

Abstract - The aim of this thesis is to provide an in-depth investigation of zinc tin oxide, Zn1-xSnxOy or ZTO, grown by atomic layer deposition (ALD) as a buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells. The thesis analyzes how changes in the ALD process influence the material properties of ZTO, and how these in turn affect the performance of CIGS solar cells.

It is shown that ZTO grows uniformly and conformably on CIGS and that the interface between ZTO and CIGS is sharp with little or no interdiffusion between the layers. The band gap and conduction band energy level of ZTO are dependent both on the [Sn]/([Zn]+[Sn]) composition and on the deposition temperature. The influence by changes in composition is non-trivial, and the highest band gap and conduction band energy level are obtained at a [Sn]/([Zn]+[Sn]) composition of 0.2 at 120  °C [...]

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Shakila Bint Reyaz (2015)

Reconfigurable and Wideband Receiver Components for System-on-Chip Millimetre-Wave Radiometer Front-Ends

Abstract - This thesis presents solutions and studies related to the design of reconfigurable and wideband receiver circuits for system-on-chip (SoC) radiometer front-ends within the millimetre-wave (mm-wave) range. Whereas many of today’s mm-wave front-ends are bulky and costly due to having discrete RF components, single-chip receiver modules could potentially result in a wider use for emerging applications such as wireless communication, short range radar and passive imaging security sensors if realised with adequate performances and at a lower cost. Three main topics are considered in this thesis, monolithic integration of low-loss RF-MEMS (Dicke) switch networks and switched LNAs in MMIC/RFIC foundry processes, designs of SiGe wideband (IF) amplifier and broadband power detectors up to W-band (75-110 GHz) [...]

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Magnus Jobs (2015)

Wireless Interface Technologies for Sensor Networks

Abstract - The main focus of the work presented in this thesis concerns the development and improvement of Wireless Sensor Networks (WSNs) as well as Wireless Body Area Networks (WBANs). WSN consist of interlinked, wireless devices (nodes) capable of relaying data wirelessly between the nodes. The applications of WSNs are very broad and cover both wireless fitness monitoring systems such as pulse watches or wireless temperature monitoring of buildings, among others. The topics investigated in the work presented within this thesis covers antenna design, wireless propagation environment evaluation and modeling, adaptive antenna control and wireless nodes system design and evaluation. In order to provide an end-user suitable solution for wireless nodes the devices require both small form factor and good performance in order to be competitive on the marked and thus the main part of this thesis focuses on techniques developed and data collected to help achieve these goals [...] 

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Erik Särhammar (2014)

Sputtering and Characterization of Complex Multi-element Coatings

Abstract - The thin film technology is of great importance in modern society and is a key technology in wide spread applications from electronics and solar cells to hard protective coatings on cutting tools and diffusion barriers in food packaging. This thesis deals with various aspects of thin film processing and the aim of the work is twofold; firstly, to obtain a fundamental understanding of the sputter deposition and the reactive sputter deposition processes, and secondly, to evaluate sputter deposition of specific material systems with low friction properties and to improve their performance. From studies of the reactive sputtering process, two new methods of eliminating the problematic and undesirable hysteresis effect were found. In the first method it was demonstrated that an increased process pressure caused a reduction and, in some cases, even elimination of the hysteresis. In the second method it was shown that sufficiently high oxide content in the target will eliminate the hysteresis [...]

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Milena De Albuquerque Moreira (2014)

Synthesis of Thin Piezoelectric AlN Films in View of Sensors and Telecom Applications

Abstract - The requirements of the consumer market on high frequency devices have been more and more demanding over the last decades. Thus, a continuing enhancement of the devices’ performance is required in order to meet these demands. In a macro view, changing the design of the device can result in an improvement of its performance. In a micro view, the physical properties of the device materials have a strong influence on its final performance. In the case of high frequency devices based on piezoelectric materials, a natural way to improve their performance is through the improvement of the properties of the piezoelectric layer. The piezoelectric material studied in this work is AlN, which is an outstanding material among other piezoelectric materials due to its unique combination of material properties [...]

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Mathias Grudén (2014)

Wireless Sensor Network Systems in Harsh Environments and Antenna Measurement Techniques

Abstract - Wireless sensor network (WSN) has become a hot topic lately. By using WSN things that previously were difficult or impossible to measure has now become available. One of the main reasons using WSN for monitoring is to save money by cost optimization and/or increase safety by letting the user knowing the physical status of the monitored structure. This thesis considers four main topics, empirical testing of WSN in harsh environments, antenna designs, antenna measurements and radio environment emulation. The WSN has been tested in train environment for monitoring of ball bearings and inside jet engines to monitor strain of blades and temperatures. In total, two investigations have been performed aboard the train wagon and one in the jet engine. The trials have been successful and provide knowledge of the difficulties with practical WSN applications. The key issues for WSN are robust communication, energy management (including scavenging) and physical robustness [...]

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Sara Lotfi (2014)

Design and Characterization of RF-LDMOS Transistors and Si-on-SiC Hybrid Substrates


Abstract - With increasing amount of user data and applications in wireless communication technology, demands are growing on performance and fabrication costs. One way to decrease cost is to integrate the building blocks in an RF system where digital blocks and high power amplifiers then are combined on one chip. This thesis presents LDMOS transistors integrated in a 65 nm CMOS process without adding extra process steps or masks. High power performance of the LDMOS is demonstrated for an integrated WLAN-PA design at 2.45 GHz with 32.8 dBm output power and measurements also showed that high output power is achievable at 5.8 GHz. For the first time, this kind of device is moreover demonstrated at X-band with over 300 mW/mm output power, targeting communication and radar systems at 8 GHz [...]

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Ling-Guang Li (2013)

Fabrication and Characterization of Si-on-SiC Hybrid Substrates

Abstract - In this thesis, we are making a new approach to fabricate silicon on insulator (SOI). By replacing the buried silicon dioxide and the silicon handling wafer with silicon carbide through hydrophilic wafer bonding, we have achieved silicon on crystalline silicon carbide for the first time and silicon on polycrystalline silicon carbide substrates at 150 mm wafer size. The conditions for the wafer bonding are studied and the surface and bond interface are characterized. Stress free and interfacial defect free hybrid wafer bonding has been achieved. The thermally unfavourable interfacial oxide that originates from the hydrophilic treatment has been removed through high temperature annealing, denoted as Ox-away. Based on the experimental observations, a model to explain the dynamics of this process has been proposed. Ox-away together with spheroidization are found to be the responsible theories for the behaviour. The activation energy for this process is estimated as 6.4 eV. Wafer bonding of Si and polycrystalline SiC has been realised by an intermediate layer of amorphous Si [...]

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Jörn Timo Wätjen (2013)

Microscopic Characterisation of Solar Cells : An Electron Microscopy Study of Cu(In,Ga)Se2and Cu2ZnSn(S,Se)4 Solar Cells

Abstract - The sun provides us with a surplus of energy convertible to electricity using solar cells. This thesis focuses on solar cells based on chalcopyrite (CIGSe) as well as kesterite (CZTS(e)) absorber layers. These materials yield record efficiencies of 20.4 % and 11.1 %, respectively. Especially for CZTS(e), the absorber layers often do not consist of one single desired phase but can exhibit areas with deviating material properties, referred to as secondary phases. Furthermore, several material layers are required for a working solar cell, each exhibiting interfaces. Even though secondary phases and interfaces represent a very small fraction of the solar cell they can have a profound influence on the over-all electrical solar cell characteristics. As such, it is crucial to understand how secondary phases and interfaces influence the local electrical characteristics [...]

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