This report describes the successful application of a combinatorial chemistry approach to the evaluation of the amorphous transparent oxide semiconductor Al-In-Zn-O, a-AIZO, for use as channel layers in thin film transistors, TFTs. Many technologies, such as computing and electronic displays, rely on the use of the transistor. In particular, for flat panel displays, the development of new TFTs for the control electronics are necessary for thinner displays with better resolution. In addition, transparent materials deposited at low temperatures would enable a new range of applications. To accomplish this, new materials for the TFT channel layer are needed. Transparent oxide semiconductors (TOS) are one alternative the silicon based materials currently in use and the first TOS, amorphous In-Ga-Zn-O, has just gone into production. However, despite its good properties, it suffers from the disadvantage of containing the scarce and expensive metals In and Ga. Several attempts have been made to replace Ga with Al but no systematic study of a-AIZO has been reported. This report describes such a study, using a method known as combinatorial chemistry. Initially, a-AIZO thin films with composition gradients were deposited by DC/RF magnetron sputtering and, following characterization, TFTs with a variety of a-AIZO channel layer composition were manufactured and investigated. Two different compositional areas were found to yield TFTs with good characteristics.

The complex refractive index is reported at room temperature for CdxZn1_xO thin film alloys for Cd composition up to 0.16. The CdxZn1_xO epilayers were grown by molecular-beam epitaxy on smooth ZnO/GaN/sapphire lattice templates. Transmission spectra were recorded by spectrophotometry in the 350-800nm wavelength range. The refractive index and extinction coefficient were derived by an evolutionary algorithm, which optimizes the Sellmeier and Forouhi-Bloomer dispersion models by a least-squares fitting to the experimental data.

CdS is conventionally used as a buffer layer in Cu(In,Ga)Se2, CIGS, solar cells. The aim of this thesis is to substitute CdS with cadmium-free, more transparent and environmentally benign alternative buffer layers and to analyze how the material properties of alternative layers affect the solar cell performance. The alternative buffer layers have been deposited using Atomic Layer Deposition, ALD. A theoretical explanation for the success of CdS is that its conduction band, Ec, forms a small positive offset with that of CIGS. In one of the studies in this thesis the theory is tested experimentally by changing both the Ec position of the CIGS and of Zn(O,S) buffer layers through changing their gallium and sulfur contents respectively. Surprisingly, the top performing solar cells for all gallium contents have Zn(O,S) buffer layers with the same sulfur content and properties in spite of predicted unfavorable Ec offsets. An explanation is proposed based on observed non-homogenous composition in the buffer layer. This thesis also shows that the solar cell performance is strongly related to the resistivity of alternative buffer layers made of (Zn,Mg)O. A tentative explanation is that a high resistivity reduces the influence of shunt paths at the buffer layer/absorber interface. For devices in operation however, it seems beneficial to induce persistent photoconductivity, by light soaking, which can reduce the effective Ec barrier at the interface and thereby improve the fill factor of the solar cells. Zn-Sn-O is introduced as a new buffer layer in this thesis. The initial studies show that solar cells with Zn-Sn-O buffer layers have comparable performance to the CdS reference devices. While an intrinsic ZnO layer is required for a high reproducibility and performance of solar cells with CdS buffer layers it is shown in this thesis that it can be thinned if Zn(O,S) or omitted if (Zn,Mg)O buffer layers are used instead. As a result, a top conversion efficiency of 18.1 % was achieved with an (Zn,Mg)O buffer layer, a record for a cadmium and sulfur free CIGS solar cell. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 717

Thin-film solar cells based on Cu(In,Ga)Se2 contain a thin buffer layer of CdS in their standard configuration. In order to avoid cadmium in the device for environmental reasons, Cd-free alternatives are investigated. In this thesis, ZnO-based films, containing Mg or S, grown by atomic layer deposition (ALD), are shown to be viable alternatives to CdS. The CdS is an n-type semiconductor, which together with the n-type ZnO top-contact layers form the pn-junction with the p-type Cu(In,Ga)Se2. From device modeling it is known that a buffer layer conduction band (CB) position of 0-0.4 eV above that of the Cu(In,Ga)Se2 layer is consistent with high photovoltaic performance. For the Cu(In,Ga)Se2/ZnO interface this position is measured by photoelectron spectroscopy and optical methods to –0.2 eV, resulting in increased interface recombination. By including sulfur into ZnO, a favorable CB position to Cu(In,Ga)Se2 can be obtained for appropriate sulfur contents, and device efficiencies of up to 16.4% are demonstrated in this work. From theoretical calculations and photoelectron spectroscopy measurements, the shift in the valence and conduction bands of Zn(O,S) are shown to be non-linear with respect to the sulfur content, resulting in a large band gap bowing. ALD is a suitable technique for buffer layer deposition since conformal coverage can be obtained even for very thin films and at low deposition temperatures. However, deposition of Zn(O,S) is shown to deviate from an ideal ALD process with much larger sulfur content in the films than expected from the precursor pulsing ratios and with a clear increase of sulfur towards the Cu(In,Ga)Se2 layer. For (Zn,Mg)O, single-phase ZnO-type films are obtained for Mg/(Zn+Mg) < 0.2. In this region, the band gap increases almost linearly with the Mg content resulting in an improved CB alignment at the heterojunction interface with Cu(In,Ga)Se2 and high device efficiencies of up to 14.1%.

La thèse présentée ici a pour objectif de développer des matériaux innovants et performants pour la fabrication de la couche tampon des cellules photovoltaïques en couches minces à base de Cu(In,Ga)Se2 (CIGS). Pour la première fois, des couches minces d'In2(S,O)3 et de Zn(O,S) ont été réalisées par dépôt chimique en phase vapeur par flux alternés assisté par plasma afin de remplacer la couche tampon traditionnelle en sulfure de cadmium. En apportant des espèces plus réactives, cette méthode permet d'effectuer des réactions qui ne pourraient pas avoir lieu par procédé thermique. La comparaison des deux procédés a permis l'évaluation de leurs atouts et de leurs contraintes. Par exemple, l'In2(S,O)3 n'a pu être synthétisé que par cette méthode, via des mécanismes surfaciques d'échange entre des radicaux d'oxygène et le soufre de l'In2S3. Pour augmenter les performances des cellules CIGS/In2(S,O)3 jusqu'à 11,9%, le procédé de synthèse initial a été amélioré en corrélant les études de Spectroscopie Photoélectronique X et celles de spectrométrie de masse. En parallèle, il a été montré que la température de croissance avait un effet notable sur les propriétés opto-électroniques des cellules CIGS/Zn(O,S) et qu'il existait des optimums de performance à basse (Tdep < 160°C) et haute (Tdep > 200°C) températures. L'optimum situé à basse température s'explique par les propriétés favorables des couches minces de Zn(O,S) synthétisées par procédé thermique, tandis que celui situé à haute température est dû à l'existence de mécanismes d'interdiffusion à l'interface Zn(O,S)/CIGS. Un rendement de 15,6% a pu ainsi être obtenu. / This thesis focuses on the development of innovative and efficient materials for the fabrication of the buffer layer of Cu(In,Ga)Se2 (CIGS) thin film solar cells. For the first time, In2(S,O)3 and Zn(O,S) thin films were synthesized by Plasma Enhanced Atomic Layer Deposition (PEALD) in order to substitute the conventional cadmium sulfide buffer layer. By creating reactive species, this deposition technique allows reactions which could not be possible using thermal ALD. The comparison of both methods allows the evaluation of their respective assets and constraints. For instance, In2(S,O)3 thin films could only be achieved using PEALD through exchange reaction mechanisms between oxygen radicals from the plasma and sulfur atoms of In2S3 growing film. In order to obtain CIGS/In2(S,O)3 solar cells with efficiencies of 11.9%, the initial deposition process was improved by correlating X-Ray Photoelectron Spectroscopy and Quadrupole Mass Spectrometry analyses. At the same time, the deposition temperature proved to have a crucial effect on CIGS/Zn(O,S)-ALD device opto-electronic properties and we evidenced the existence of two deposition temperature ranges, at Tdep < 160°C and Tdep > 200°C, where the performances are enhanced. In the low temperature range, the high performances were explained by specific Zn(O,S) properties, while at high temperature they are enhanced by favorable interdiffusion mechanisms at the CIGS/Zn(O,S) interface. Increasing the deposition temperature allowed the fabrication of CIGS/Zn(O,S) solar cells with efficiencies up to 15.6%.

Im Rahmen dieser Arbeit wurden die optischen Eigenschaften von auf ZnO-basierenden Heterostrukturen untersucht. Besonderes Augenmerk lag hierbei auf ihrer Eignung als aktives Material in Laserdioden für den ultravioletten und sichtbaren Spektralbereich. Es wurde gezeigt, dass ZnO und seine ternären Mischkristalle ZnCdO und ZnMgO erstaunlich vielfältige Anwendungen ermöglichen. Mit diesem Materialsystem lässt sich sowohl ein sehr großer Spektralbereich für Lasertätigkeit abdecken als auch eine Vielzahl von Laseranordnungen realisieren. Im Detail wurde demonstriert, dass sich die Lasertätigkeit von ZnCdO/ZnO Quantengraben-Strukturen vom violetten bis in den grünen Spektralbereich verschieben lässt. Obwohl diese Strukturen starke interne elektrische Felder aufweisen, konnte optisch gepumpte Lasertätigkeit bei Zimmertemperatur bis zu einer Wellenlänge von 510 nm gezeigt werden. Die für die Lasertätigkeit nötige optische Rückkopplung wird durch makroskopische Defekte der Probe verursacht und die Proben fungieren somit als Zufallslaser. Die Herstellung von Mikroresonatoren ermöglichte die Untersuchung des Zusammenspiels von Fabry-Perot- und Zufalls-Rückkopplung. Die experimentellen und theoretischen Ergebnisse zeigen, dass der Schwellengewinn eines Zufallslasers in der Regel größer ist als der des Fabry-Perot-Lasers. Des Weiteren wurde gezeigt, dass hoch reflektierende Braggreflektoren für den ultravioletten und blau/grünen Spektralbereich aus ZnO- und ZnMgO-Schichten hergestellt werden können. Ferner wurden die teils unbekannten Brechungsindexverläufe der verwendeten ternären Materialen erarbeitet und Mikrokavitäten mit ZnO/ZnMgO Quantengraben Strukturen als aktive Schichten realisiert. An diesen Kavitäten konnte bei Temperaturen bis zu 150 K starke Kopplung zwischen Exzitonen und Photonen nachgewiesen werden. Bei Zimmertemperatur konnte vertikal-emittierende Lasertätigkeit im nahen ultravioletten Spektralbereich demonstriert werden. / In the framework of this thesis, the optical properties of ZnO-based heterostructures fabricated by molecular beam epitaxy have been investigated, particularly with regard to their suitability for semiconductor laser devices operating in the ultraviolet and visible spectral range. It turned out that ZnO and its ternary alloys ZnMgO and ZnCdO are extremely versatile. They allow to tune the laser emission in a wide spectral range as well as to realize various laser geometries. In detail, it was shown that the laser emission of ZnCdO/ZnO multiple quantum wells can cover a spectral range from violet to green wavelengths. Although these structures suffer from large built-in electric fields, room temperature laser action under optical pumping was demonstrated up to a wavelength of 510. The optical feedback for lasing is provided by growth imperfections on a macroscopic length scale turning these structures into random lasers. The fabrication of micro-resonators allowed to study the interplay between random and Fabry-Perot feedback. The experimental and theoretical analysis shows that random feedback generally requires a larger gain than under Fabry-Perot feedback. Further, this work demonstrates that ZnO- and ZnMgO-layers can be used to fabricate highly reflective distributed Bragg reflectors for applications in the ultraviolet and blue/green spectral range. The partly unknown dispersion curves of the index of refraction of the employed ternary alloys have been elaborated. This enabled the realization of all monolithic microcavities with ZnO/ZnMgO quantum wells as active zone. For temperatures below 150 K strong exciton-photon coupling is observed in such microcavities. At room temperature, vertical cavity surface emitting laser action in the near UV spectral range is demonstrated for appropriately designed microcavities.