Ion and Trap Induced Dynamic Phenomena in Salt Semiconductor Devices

State-of-the-art organic light-emitting diodes (OLEDs) used in commercial display technology are complex multilayer structures. In contrast, the light-emitting electrochemical cell (LEC) can be built from a single emissive layer sandwiched between two electrodes. Due to the presence of mobile ions - intermixed with the active material - the single layer can perform all the tasks that take place in an OLED, i.e., electrical charge injection, transport, exciton formation, and radiative recombination. Therefore, LECs have the potential to be fabricated by low-cost solution coating and printing processes. While the structure is simple, the simultaneous occurrence of electronic and mobile ionic charge introduces a complex device functionality. Basic operation principles have been discussed for years in the field and consensus has been reached on the electrochemical doping model. The focus of this thesis is to unravel the interplay of electronic and ionic charges in LECs and to highlight the boundaries and gaps of the firmly established electrochemical doping model. Used methods and conclusions are not restricted to the polymer-based LECs and can be applied to various systems with combined electronic and ionic conduction, as demonstrated for perovskite LEDs.First, effects of ion concentration and active layer thickness variation in the super yellow (SY) polymer LEC (PLEC) are investigated, which play a critical role on the performance. The dependence on the layer thickness reveals the often-overlooked microcavity/Purcell effect, which leads to a strong dependence of the optical outcoupling efficiency on the emitter position (EP) within the active layer. For thicker PLECs, a continuous change of the emission colour during operation provides a direct visual indication of a moving EP. Drift-diffusion simulation of the electrical measurements pinpoints the EP shift to ion dynamics and demonstrates that the only precondition for this event to occur is unequal cation and anion mobilities. Quantitative ion profiles reveal that the movement of ions stops when the intrinsic zone stabilizes, confirming the relation between ion movement and EZ shift. The unresolved origin of device degradation after the ionic steady state has been reached leads to the study of the SY polymer LED (PLED) with the idea to isolate the electronic from ionic effects. Response of PLEDs to electrical driving and breaks covering the timescale from microseconds to (a few) hours reveals a surprisingly slow response of electron traps. Subsequently, the characteristic response of the electron traps to electrical breaks is identified in the PLEC as well. Further investigation by electroabsorption measurement finds that hole trap formation limits the device lifetime, in the exact same manner as for PLEDs. The methodology to unravel dynamic processes due to mobile ions, electronic charges and trap development is applied to perovskite light-emitting diodes (PeLEDs). The application of the optical model based on the microcavity effect to the PeLED is discussed. Experimental thickness variation of the TPBi transport layer of a CsPbBr3 based PeLED indicates the validity of the optical model. A transient voltage pulse reveals an emission feature within the time range of ionic mobility. Transient drift-diffusion simulation is able to describe the observed emission increase as a result of ionic movement.

Spin-valley optoelectronics based on two-dimensional materials

Dmitrii Unuchek

Two-dimensional (2D) materials, in particular graphene and transition metal dichalcogenides (TMDC), have attracted great scientific interest over the last decade, revealing exceptional mechanical, electrical and optical properties. Owing to their layered nature, subnanometer-thick single-layer forms of these crystals can be chemically grown or mechanically isolated, representing an ultimately scaled-down platform for further miniaturization of electronic devices. Indeed, the large family of 2D materials contains hundreds of members, including metals, semiconductors, insulators, and others, covering the whole spectrum of potential applications. Availability of all necessary building blocks for the construction of all-2D solid-state devices makes this platform ideal for fabrication of ultrathin, transparent and flexible devices. Extensively investigated graphene has demonstrated excellent electrical properties. However, the absence of a bandgap is an obstacle for its interaction with light and therefore limits applications in optics. In this aspect, atomically thin TMDC semiconductors appear to be an alternative, more promising platform, as they possess a direct band gap in the visible range in the monolayer form. Despite being only three atoms thick, these semiconductors demonstrate exceptionally large light absorption, efficient light emission, and strong light-matter interaction, attracting justified interest from the optics community. We will focus on their potential applications for optoelectronics in Chapter 4. Even more, broken inversion symmetry and strong spin-orbit coupling in single-layer TMDC crystals reveal a new quantum number, the so-called valley index. Indeed, this unique degree of freedom of charge carriers is known for some materials since the 1970s. However, in the case of 2D semiconductors, spin-valley locking mechanism and valley contrasting optical selection rules open new ways for addressing, manipulation and sensing of this pseudospin, opening the whole new field of valleytronics. Due to the large splitting of the valence band, spin and valley degree of freedom become locked in these atomically thin materials. We employ this unique feature in Chapter 5 for indirect injection of spins into graphene by pumping valley polarized carriers in an adjacent TMDC monolayer. Being gapless, graphene does not allow direct optical injection of spins. Another striking feature of 2D materials is their unique ability to be stacked in vertical heterostructures with strong electrical coupling between layers. The weak van der Waals force, which keeps components together, provides freedom in the assembly process, so that a lattice mismatch or a twist angle becomes unimportant in the first approximation. This provides a novel platform for harvesting complementary properties of the constituent materials, allowing the engineering of new artificial meta-materials as we demonstrate in Chapter 6. Furthermore, synergistic effects in van der Waals heterostructures enable completely new properties and phenomena, which do not exist in the single materials, with twist angle being an important knob for tuning these effects. We observe and exploit such novel phenomena arising in heterostructures of 2D semiconductors in the last chapter of this thesis. On the way to the realization of practical optoelectronic and valleytronic applications, this thesis studies fundamental aspects of rich spin-valley physics of atomically thin semiconductors

EPFL2019

Lattice-matched AlInN alloys for nitride-based optoelectronic devices

Julien Dorsaz

Gallium Nitride (GaN) and its ternary alloys with aluminium and indium have met a growing interest in the last decade. These semiconductors have a large direct bandgap and can be doped with either silicon (Si) for n-type and magnesium (Mg) for p-type layers. Consequently, they are attractive targets for the fabrication of electrically driven light-emitting diodes, laser diodes and photodetectors, operating in the UV and visible range. Nitride semiconductors are also robust ans stable at high temperatures, suggesting their use in high frequency – high power electronic devices. The research on the III-nitride semiconductors is still in its infancy and, although several devices have been demonstrated and commercialized, there are still many issues to be solved. In particular, the III-nitride materials suffer from the lack of an adequate lattice-matched substrate for epitaxial growth, the high density of defects and dislocations, the poor quality of the p-type layers (low hole concentration and mobility), localization effects in indium containing layers, chemical inertness, etc… Also, due to the material quality issues, some electrical and optical parameters are only approximatively known. AlInN alloys have not attracted much attention compared to the InGaN and AlGaN ternary alloys, due to their unstable growth conditions. High-quality AlInN layers were deposited by metalorganic vapor-phase epitaxy (MOVPE). AlInN with an indium content of 18% is lattice-matched to GaN. It has a bandgap energy of about 4.35 eV and a refractive index contrast to GaN of 7% at 450 nm. It is shown that the optical, electrical and structural characteristic of epitaxial AlInN layers start degrading above a critical thickness of 200 nm, due to an ongoing pollution of the MOVPE reactor by products of the reaction. The distributed Bragg reflector (DBR) is an essential building block for advanced optoelectronic devices such as vertical-cavity surface emitting lasers (VCSELs). The introduction of the lattice-matched AlInN quarter-layers allows to grow thick DBR structures, with limited strain issues. It is used with GaN quarter-layers for visible reflectors and AlGaN quarter-layers for UV reflectors Optimizing the growth conditions of AlInN layers leads to the realization of DBRs with reflectivity above 99 % in the blue wavelength range (410-450 nm) as well as in the UV wavelength range (345 nm). The novel AlInN/GaN DBR is introduced in various microcavity LED (MCLED) structures. The basic features of the MCLED devices are presented, as well as optical simulations obtained using the transfer matrix representation (TMR) formalism, coupled with a dipole approximation for source terms. Electrical characterizations on these devices show that an intra-cavity contact (ICC) scheme should be used to achieve a sufficient electron injection in the active region. The external quantum efficiency of MCLED devices with thick AlInN/GaN DBRs (12 pairs) were lower than expected, due to the degradation of the crystalline quality of the active layers caused by the pollution of the reactor. Another requirement for the realization of a nitride VCSEL is the optimization of the electrical injection of carriers in the optical cavity. Assuming that circular metallic contacts are used, the carriers should spread laterally and be injected in the active region under the top DBR. This is generally achieved by the combination of a current spreading cap layer, which helps for the lateral injection of the carriers, and a current confinement layer, which inhibit the injection of the carriers in the active region outside of the vertical cavity. For the III-nitride structures, several methods are available for the lateral injection: n+/p+ tunnel junctions, doped-AlGa(In)N/GaN superlattices, semi-transparent electrodes, etc… A novel technique is developed to oxidize selectively surface or buried AlInN layers and form a stable, semi-insulating compound, with a refractive index of about 1.8. A detailed description of the technique is presented. It can be applied to form current confinement layers in VCSELs, but also layers for high reflectivity distributed Bragg reflectors (DBRs), layers for improved light confinement in planar waveguides or insulating buffer layers for field effect transistors (FETs). Also, oxidized AlInN could be used as a sacrificial layer for the preparation of free-standing GaN substrates. In this work, the technique was applied to form a current confinement aperture in a LED device. A buried AlInN layer was oxidized laterally, leaving a small 5 µm aperture in a 28 µm × 28 µm mesa structure. Optical and electrical confinement are demonstrated. With the availability of a high-reflectivity lattice-matched nitride DBR and a technique to produce small current apertures, both based on the new AlInN material, the realization of an electrically injected VCSEL becomes a realistic goal. An ideal structure is realized and discussed.

EPFL2006

Ferroelectric gate on AlGaN/GaN heterostructures

Lisa Malin

The capability of switching the spontaneous polarisation under an applied electric field in ferroelectric materials can be exploited for the use in low power, non-volatile, re-writable memory devices. Currently available commercially is ferroelectric random access memory, FeRAM, which allows for high speed, low voltage and greater write-erase endurance compared to its two main competitors, Flash and EEPROM, when using the one transistor – one capacitor configuration. However, it is desired to further optimise the configuration in order to obtain better densification, faster access time and better reliability. One way to do such is to pass from the ferroelectric capacitors and develop ferroelectric field effect transistors. Exploiting the phenomenon of ferroelectricity and integrating ferroelectrics with the semiconductor technology has not been simple. The FeFET has been demonstrated using a silicon-based transistor, however commercial devices are not available. Challenges arise mainly due to the high temperature deposition of perovskite ferroelectrics causing the degradation of the ferroelectric/semiconductor interface due to inter-diffusion. Acquiring long term retention of the transistor behavior has also been problematic due to phenomenons such as charge injection and depolarisation. In this thesis a new approach to the problem of semiconductor devices with a ferroelectric gate is explored. Instead of using a silicon-based device, semiconductor heterostructures are investigated. Combining the high mobility channel existing in semiconductor heterostructures, with the non-volatile switching of the polarisation in the ferroelectric gate can pave the way to novel future devices. The AlGaN/GaN semiconductor heterostructure was chosen for two main reasons. The first is a two dimensional electron gas, 2DEG, located at the AlGaN/GaN interface which possesses better transport properties than a single layered semiconductor. Secondly, GaN and its alloys are known to have large chemical and temperature stability making them ideal to withstand the high temperature deposition process of perovskite ferroelectrics. The deposition of two ferroelectric layers onto the AlGaN heterostructure were investigated. Lead zirconium titanate, PZT, a traditional perovskite ferroelectric deposited at high temperature, was chosen for its high remanent polarisation and low coercive field. An alternative ferroelectric gate, the co-polymer poly(vinylidene fluoride/trifluoroethylene), P(VDF/TrFE)(70:30) was deposited and of interest due its low crystallisation temperature and low dielectric constant. Its remanent polarisation is smaller and coercive field larger than that of PZT, but were determined sufficient to observe the depletion effect in the two dimensional electron gas. The goals accomplished in this research were: Development of Ferroelectric Gate Processing: Deposition processes of the ferroelectric layers were developed and optimised in order to obtain a high quality ferroelectric, while maintaining the original transport properties of AlGaNs 2DEG. The processing of HfO2 and MgO buffer layers were developed and investigated for their effects on limiting unwanted inter-diffusion and charge injection. PZT Gate on Al0.3Ga0.7N: The first successful development and observation of a PZT gate on a Al0.3Ga0.7N/GaN heterostrucutre was accomplished. A (111) oriented PZT(40:60) ferroelectric gate on the Al0.3Ga0.7N heterostructure depleted the sheet resistance of the 2DEG by a factor of three when poling the PZT layer with –40V directly to the conductive cantilever used in a piezoresponse force microscope. This decrease in sheet resistance was stable for more than three days. P(VDF/TrFE)(70:30) Gate on Al0.3Ga0.7N: The ferroelectric co-polymer P(VDF/TrFE) (70:30) was investigated as a gate on the Al0.3Ga0.7N heterostructure. When the P(VDF/TrFE) was poled with –30V to a top electrode the sheet resistance was modulated by a factor three, however there was no retention of this modulation. Ferroelectric Spontaneous Polarisation on a Semiconductor Heterostructure: It was theoretically derived that when depositing a ferroelectric layer onto a semiconductor heterostructure its spontaneous polarisation is dramatically reduced due to the depolarisation field and may be suppressed completely. This decay in spontaneous polarisation can be minimised by using a ferroelectric layer with a small dielectric constant, which justifies the use of ferroelectric polymers as the gate material. Domain Writing:The technique of domain writing using piezoresponse force microscopy showed that it was possible to create domain patterning on the ferroelectric layers that were deposited onto the Al0.3Ga0.7N heterostructure with sub-micron line resolution, on the order of 300nm or better. Using ferroelectrics on semiconductors is not only limited to ferroelectric memory devices. Other applications could be ferroelectric nano-lithography, where a ferroelectric pattern is written with a piezoresponse force microscope, PFM, and this pattern is projected onto the transistors channel, potentially creating quantum/ballistic devices. The challenge of integrating ferroelectrics with semiconductors has yet to be fully understood. One of the main advancements that was reached with this research is the understanding of the decrease of the spontaneous polarisation when a ferrroelectric layer is deposited onto a semiconductor heterostructure of finite thickness. The maximisation of the ferroelectrics polarisation on 20nm of Al0.3Ga0.7N can be obtained when minimising the dielectric constant of the ferroelectric layer. The current challenge is to get a better physical understanding of the limitation of the retention of the spontaneous polarisation, while discovering methods for its improvement.

EPFL2007