ICMCTF2012 Session C5-1/F7-1: Polarisation Phenomena in Thin Films and Devices

The classical Electro-Acoustic (EA) technology is a highly developed technology today with applications ranging from telecom, medical, military, scientific, radio and TV, sensors, pharmaceutical industry, etc. Only the telecom industry consumes billions of RF filters annually. It is based on the use of single crystalline piezoelectric materials such as quartz and others. The combination of acoustic waves and low losses in these materials allows the fabrication of compact, low cost devices with extreme performance. Typical EA devices are RF filters, resonators, oscillators, delay lines, various sensors (physical, chemical and biochemical), etc. Amongst the major drawbacks of the EA technology are the limited choice of piezoelectric materials (and hence properties) as well as its incompatibility with the IC technology. Further, the explosive development of mobile communications in recent years has necessitated the use of larger bandwidths thus requiring higher frequencies of operation. This is where the classical EA technology becomes expensive due to increased fabrication costs.

In view of the requirements for high bandwidth, miniaturization and low cost, the so called thin film electro-acoustic (TEA) technology has been recently developed for applications in the microwave region. It makes use of thin piezoelectric films that are grown using the planar technology which makes the IC and the TEA technologies fully compatible with each other. The material of choice so far is AlN while others are currently being developed. The deposition methods employed (PVD, CVD, etc) allow tuning various properties of the thin films (composition, crystallographic structure and texture, roughness, density, stress, etc) which in turn allows to tailor these properties in view of the application in mind. Thus, for instance, the design of filters with a large bandwidth requires the synthesis of highly c-textured AlN films, while in view of resonator operation in liquids (biochemical sensors) excitation of shear waves is needed which in turn requires either films with a c-axis tilted under a certain angle relative to the surface normal or even better, highly a-textured films. In a different perspective, certain applications may require high piezoelectric constants while others may require low losses (both acoustic and electrical) or to exhibit high functional stability with temperature variation, etc. In other words, the race is on for the synthesis of thin piezoelectric films with various functional properties in view of the broad range of potential applications.

The talk will focus on recent advances in the area in terms of both film synthesis and application development.

New types of piezoelectric materials need to be intelligently designed to further improve the performance of modern wireless telecommunication devices, satellites, sensors or optoelectronic devices together with opening new future commercial applications in biomedical engineering, neuroscience and bio-nanotechnology. Our recent physical explanation on the origin of the enhanced piezoelectric effect in wurtzite ScAlN alloys [1] has introduced a simple, free-energy landscape based phenomena in finding new materials with giant piezoelectric response. Here, we present a systematic ab-initio investigation of this strategy on several wurtzite IIIA-IIIB nitride alloys. The special quasirandom structure (SQS) approach provides a successful computational scheme to model substitutional random alloys and predict thermodynamics and, in case of special care, also mechanical and electronic properties. The here presented results will bring a refined understanding of the applicability of the free energy flattening phenomena in the wurtzite IIIA-IIIB nitride alloys and indicate more general rules/strategies in searching for new piezoelectric materials. These new rules and refined strategy will be introduced and discussed by our comparative study.

[1] F. Tasnádi, B. Alling, C. Höglund, G. Wingqvist, J. Birch, L. Hultman, and I. A. Abrikosov, Phys. Rev. Lett. 104, 137601 (2010).

III-nitride materials (InGaAlN) have direct energy gaps that cover the full spectral range from infra-red to UV, but there are significant challenges to their application, primarily due to the presence of strong built-in spontaneous and piezoelectric polarisation fields arising from the underlying wurtzite structure. As a consequence, III-nitrides are well established as blue laser and LED sources, but their efficiency drops rapidly both at longer (green/yellow) and at shorter (UV) wavelengths, due to these polarisation fields.

We consider several approaches being pursued to control the built-in potential, including growth on non-polar and semipolar substrates, use of polarisation-matched alloys and growth of III-N quantum dots (QDs). The design of any optoelectronic device relies on a good knowledge of material parameters. However, there remains controversy regarding several key III-N parameters, including the sign of the shear piezoelectric coefficient, e₁₅, the magnitude of the valence band deformation potentials (which determine light polarisation characteristics), and the influence of potential fluctuations in alloyed heterostructures. We address these issues, and their consequences for polarisation engineering and control.

We argue that e₁₅ < 0, based on an analysis of QD structures grown on non-polar substrates, and on a comparison of wurtzite and (111)-oriented zinc-blende heterostructures. We then show that e₁₅ < 0 allows the built-in potential to be significantly reduced by growth of QD rather than conventional quantum well nanostructures. Using current growth techniques, an InGaN QD in a GaN matrix can reduce the field in a visible emitter by up to 50%, allowing efficient emission to longer wavelengths, of benefit for yellow/green and white light sources.

For UV emitters, we show that careful choice of alloy composition can allow polarisation matching between well and barrier layers. Growth of III-N heterostructures on nonpolar and semipolar substrates can also minimize the potential drop across a quantum well structure over a wide spectral range, with the crystal anisotropy leading to linearly polarized light emission, potentially interesting for applications such as LCD backlight modules. Moreover, using a combination of density functional and valence force field calculations we show that there can be significant local polarisation potential fluctuations within an alloy layer, leading to carrier localization. We discuss the consequences of this localization and of inhomogeneous strain relaxation for recombination both in ideal c-plane and non-polar heterostructures.

There is a recent interest to tailor the properties of group IIIA nitrides by alloying with group IIIB transition metal nitrides, both for optoelectronic and electroacoustic applications. For example, it has been shown for AlN alloyed with ScN that there is an increase in the piezoelectric response d₃₃ [1] as well as electromechanical coupling [2]. The increase in d₃₃ was theoretically predicted to be an intrinsic alloying effect [3] related to a metastable layered hexagonal phase of ScN. This brings a strong mismatch with the wurtzite group IIIA nitrides, so the challenge is to extend the obtainable composition range of such alloys. The solubility and stability of the alloys has been investigated for w-Sc_xAl_1-xN, results show elemental fluctuations on the nanoscale for x=0.2 [4]. YN possesses the same metastable phase as ScN [5], but has not yet been explored for alloying with group III nitrides. Preliminary theoretical predictions suggest increased softening of the material with addition of Y, but less pronounced increase in piezoelectric constant e₃₃ as compared to ScAlN.

Here, we present experimental and theoretical results from growth and structural, electrical, as well as optical characterization of wurtzite Y_xAl_1-xN thin films with 0≤x≤0.2. Dual reactive magnetron sputter deposition from elemental Y and Al targets was used to grow thin films onto Al₂O₃(0001) and Si(001) substrates in N₂/Ar discharge in an UHV chamber. ERDA showed that the films are stoichiometric with respect to the metal/N ratio and that they contain less than 2 at.% of impurities. XPS measurements show that the N1s peak is shifting towards lower energies with addition of Y. According to I-V and C-V measurements performed on Y_0.13Al_0.87N/TiN/Al₂O₃ structures, films show no leakage current, low dielectric dissipation and relative dielectric constant increases up to 11, as compared to 9 in a pure AlN sample. Results from optical measurements showing changes in the bandgap for different Y concentrations will be presented as well. Initial HRTEM studies show that films are polycrystalline and have a columnar microstructure. For films with x=0.13 there is no detectable elemental segregation according to STEM. XRD confirms that both lattice parameters c and a are affected when increasing Y concentration. The results follow preliminary theoretical predictions, thus suggesting that compound forming is a solid solution.

[1] M. Akiyama, et al., Adv. Mater. 21, 5 (2009).

[2] G. Wingqvist, et al., Appl. Phys. Lett. 97, 11 (2010).

[3] F. Tasnádi, et al., Phys. Rev. Lett. 104, 13 (2010).

[4] C. Höglund, et al., Physical Review B, 81, 22 (2010).

[5] Y. Cherchab, et al., Physica E, 40, 3 (2008).

Recent theoretical calculations have revealed the origin of the anomalous, 400% increase of the piezoelectric coefficient in scandium aluminum nitride alloys [1]. In this work, the piezoelectric response during nanoindentation of Sc_xAl_1-xN (0 ≤ x ≤0.2) thin films has been investigated. Films of 250-500 nm thick were deposited by reactive magnetron sputtering from elemental Al and Sc targets onto Al₂O₃(0001) substrates with a conductive TiN(111) seed layer, at substrate temperatures in the range 400-800 °C. Microstructure and composition were analyzed by x-ray diffraction and transmission electron microscopy. Structural analysis confirms epitaxial growth; all films are c-axis oriented and have a columnar structure, but the alloying of AlN with ScN results in a deterioration of the crystalline quality due to phase instabilities.

Nanoindentation with simultaneous measurement of load and electrical voltage was used to characterize the nanoscale electromechanical properties of the piezoelectric films. Testing was done by a TI-950 Hysitron Triboindenter configured to perform electrical measurements with a conductive Berkovich boron-doped diamond tip. For all compositions, the films show a linear relationship of the applied force to the generated voltage. For loads ranging from 0.1 to 11 mN, output maximum voltages from 2 to 60 mV were obtained, depending on applied forces and composition. Consistent values of generated voltages were measured after multiple force cycles with no hysteresis observed in the results. No influence of the nanoindentation loading rate on peak voltage generation was detected. The results were also correlated to data obtained by piezoresponse force microscopy (PFM).

[1] F. Tasnádi, B. Alling, C. Höglund, G. Wingqvist, J. Birch, L. Hultman, and I. A. Abrikosov, Phys. Rev. Lett. 104, 137601 (2010).

This letter studies the hot-carrier effect in indium–gallium–zinc oxide (IGZO) thin film transistors with symmetric and asymmetric source/drain structures. The different degradation behaviors after hot carrier stress in symmetric and asymmetric source/drain device indicate that different mechanisms dominate the degradation. Since the C-V measurement is highly sensitive to the trap state compared with the I-V characteristics, thus, the C-V curves are utilized to analyze the hot carrier stress induced trap state generation. Furthermore, the asymmetric C-V measurements (gate-to-drain capacitance, gate-to-source capacitance) are useful to analyze the trap state location. For asymmetric device structure, different source/drain structure under hot carrier stress will induce asymmetric electrical field and cause different degradation behaviors. In this work, the on-current and subthreshold swing (S.S) degrade under low electrical field, whereas the apparent V_t shift occurs under large electrical field. The different degradation behavior indicates that the trap state generates under low electrical field and channel-hot-electron (CHE) effect occurs under large electrical field.