Elucidating the role of the InGaN UL in the efficiency of InGaN based light-emitting diodes

Blue III-nitride light-emitting diodes (LEDs) are widely used nowadays in solid-state lighting, as white light can be produced by combining yellow phosphors and blue LEDs. This technology has the advantage to have a higher luminous efficiency than incandescent light bulbs and fluorescent tubes. This high efficiency is attributed to InGaN quantum wells (QWs), which exhibit an internal quantum efficiency (IQE) higher than 90 % at 300 K. However, even if blue LEDs are industrially produced, the high IQE of InGaN QWs is still barely understood.

In InGaN/GaN-based LEDs, an InGaN layer grown before the InGaN/GaN QW is often found. This InGaN layer named an underlayer (UL) is used in all commercial devices and is known to increase drastically the efficiency of LEDs. However, the physical mechanism behind the efficiency improvement is debated in the literature.

In the first part of this work, a study of the different existing hypotheses proposed is conducted. The conclusion is that the role of the InGaN UL is to capture point defects before they reach the InGaN QWs. Indeed, points defects react with InGaN and are transformed into non-radiative centers when they are buried in the InGaN alloy. We call such point defects reacting with indium "surface defects" (SDs).

To explore the SD capture by the InGaN UL, we studied different kinds of UL: InGaN UL, InAlN UL, and low-temperature GaN UL. We found that indium atoms play a key role in the capture of SDs. Indeed, the number of SDs buried in the UL depends on its thickness and indium composition. As a consequence, GaN layers are not able to bury SDs as efficiently. A model based on segregation of SDs was proposed based on our observations. As InGaN UL are present in all LEDs, we concluded that SDs are problematic for all LEDs independently of the type of reactor. The growth temperature of GaN was identified as the parameter which generates SDs. Indeed, for temperatures higher than 820 \degres C in MOVPE, SDs are created. Different impurities were searched by secondary ion mass spectrometry measurements, but no correlation was found between the sample efficiency and the impurity concentration. Our hypothesis is that the stability of the GaN surface is affected by the temperature and therefore leads to the formation of SDs, which could be intrinsic defects such as nitrogen vacancies. However, we do not have any experimental proof about the actual nature of SDs yet.

In the last part of this work, near UV LEDs with InAlN UL were designed and characterized. Single QW (SQW) LEDs emitting around 405 nm with an InAlN/GaN superlattice UL exhibit an IQE around $\sim$ 70 %, which is higher than for a SQW LED without UL ( $\sim$ 10 %). The $I$ - $V$ characteristic is similar for both samples thanks to a highly doped region placed at the interface between the InAlN/GaN superlattice UL and the GaN spacer.

Investigation of indium-rich InGaN alloys and kinetic growth regime of GaN

Nils Asmus Kristian Kaufmann

Nowadays, light emitting diodes (LEDs) and laser diodes (LDs) are part of our daily life. More and more devices incorporate InGaN-based optoelectronic devices. In fact, since the first demonstration of a candela-class InGaN-based LED in the beginning of the nineties, those LEDs have quickly become popular. The efficiency of blue InGaN LEDs is nowadays very high and when coated with a yellow phosphor, they can efficiently emit white light. Such white LEDs are more and more used for different lighting applications, from home lighting to car lighting. Another application of blue InGaN emitters that is found in many households is the Blu-ray disc, based on a 405 nm LD. The great success of the III-nitrides in the blue range created a strong interest to extend the wavelength range into the green, as efficient green emitters are missing. However, their performance is still low compared to their blue counterparts. The goal of this dissertation is to investigate issues that limit the performance of green InGaN LEDs and LDs. To achieve efficient green emission, the growth temperature needs to be reduced to incorporate more indium into the active region. In this context, the present study is divided into two parts. In the first one, the growth regime of GaN as a function of the temperature is investigated and the physical origin of the surface features is discussed. When lowering the temperature, the so called Ehrlich-Schwöbel barrier, a barrier at the step-edges, causes a different attachment probability for the adatoms arriving from the different sides. This asymmetry can lead to the appearance of three different kinetic surface morphologies: hillocks, fingers, or step-bunching. After presenting the theory, the three different regimes are demonstrated for GaN, and the growth parameters that allow to control the morphology are investigated. Indeed, the different morphologies usually reported for the different growth techniques can be attributed to the different growth parameters commonly used. In the end of the first part, the influence of the underlaying morphology on the properties of the active region of an LED is discussed. In the second part, the impact of the thermal budget on the properties of InGaN quantum wells (QWs) is discussed. Active regions with a high In content can degrade easily when the p-type layer is grown on top, due to being exposed to a too high thermal budget. This is especially critical for green LDs which require thick cladding layers, i.e. a long growth time. The origin of the degradation and the parameters influencing it are discussed first: when an In-rich QW is annealed at a too high temperature, its emission intensity is reduced and dark spots appear on PL maps. The annealing temperature causing this degradation depends on the indium content, the QW thickness and the number of QWs. It is shown that these dark spots exhibit a peculiar emission spectra, with a characteristic emission at 2.75 eV. This emission is tentatively attributed to nitrogen vacancies that are created by the formation of metallic indium clusters in the active region. Then the focus is laid on the beneficial effects that can take place when an In-rich active region is exposed to a moderate thermal budget. Apart from reducing the linewidth of the emission, a strong improvement in photoluminescence intensity can be achieved. This improvement increases for high indium content QWs, i.e. when the growth temperature is low. In fact, this improvement is more pronounced for molecular beam epitaxy grown structures whose active regions are grown at a much lower temperatures than their metal organic vapor phase epitaxy counterparts. This is tentatively ascribed to the reduction of point defects by thermal annealing. As a consequence, a moderate annealing can be beneficial for both to reduce the non-radiative recombinations and the linewidth. It is finally proposed to perform a short annealing step of high indium content QWs before growing the p-type (cladding) layer at low temperature.

EPFL2013

Defect passivation in zinc tin oxide: improving the transparency-conductivity trade-off and comparing with indium-based materials

Oscar Esteban Rucavado Leandro

Transparent conductive oxides (TCOs) are essential in technologies coupling light and electricity. Due to their good optoelectronic properties and the production scalability, Sn-doped indium oxide (In2O3:Sn) is the preferred TCO in industrial applications. Nonetheless indium is scarce in the Earth's crust and its availability might be compromised over the next decades. To address this issue, this doctoral project aims to (i) improve the optoelectronic properties of In-free TCOs seeking to match those of In2O3:Sn, and (ii) refine the optoelectronic properties of In2O3-based films to decrease the In consumption in applications. To accomplish this, we investigated the links between the defects, the microstructure and the optoelectronic properties of two families of TCOs, indium- and tin-based oxides. First, we studied the evolution in the optoelectronic properties and microstructure of amorphous zinc tin oxide (a-ZTO) when annealed up to 500C in oxidizing, neutral, and reducing atmospheres. We show that annealing in atmospheric pressure at temperatures > 300C decreases the detrimental subgap absorptance while increasing the electron mobility (mu). Thermal treatments in reducing atmospheres increase the free-carrier density (Ne) and the subgap absorptance. None of the thermal treatments resulted in important changes in the amorphous microstructure. Combining these results and density functional theory (DFT) calculations, oxygen deficiencies (VO) were identified as the source of detrimental subgap absorption. VO can act as donors but also as electron scattering centres. Based on these results, a-ZTO with ÎŒ up to 35 cm2/Vs, is demonstrated by high-temperature defect passivation. Profiting from the microstructure stability of a-ZTO, this material was used as a recombination junction in a tandem solar cell which require a high-temperature step in its processing. The passivation scheme might be problematic for temperature-sensitive technologies. Therefore, we demonstrated an alternative low-temperature passivation method, which relies on co-sputtering a-ZTO with SiO2 . Using two Sn-based oxides with different composition and microstructure: a-ZTO and SnO2 , and results of DFT calculations, we demonstrate that SiO2 contribution is twofold. (i) The oxygen from SiO2 passivate the VO in SnO2 and a-ZTO. (ii) The formation energy of the ionized VO is lowered by the silicon-atoms, enabling defects that do not contribute to the subgap absorptance. This passivation scheme improves the optical properties without affecting the electrical conductivity, overcoming the optoelectronic trade-off in Sn-based TCOs. Finally, we study an In-based high-ÎŒ TCO: Zr-doped In2O3. Films of In2O3:Zr with thickness from 15 nm-100 nm were sputtered in the amorphous state and annealed in different atmospheres. Annealing in air yields fully crystalline films with high transparency and a high-mu limited by phonon and ionized impurity scattering. 15 nm-thick films exhibit an average absorptance of < 0.5 % (between 390 nm-2000 nm) and an mu of 50 cm2/Vs increasing to 105 cm2/Vs for 100 nm-thick films. Alternatively, annealing in a neutral or reducing atmospheres result in higher mu for films thinner than 50 nm as a high Ne is maintained. The demonstration of thickness reduction while keeping high lateral mu makes In2O3:Zr is an alternative to reduce In in applications such as flexible displays, solar cells and light emitting diodes.

EPFL2019

Characterisation of Silicon Heterojunction Solar Cells Beyond Standard Test Conditions

Jean Cattin

The global photovoltaic market is mostly dominated by solar cells based on crystalline silicon (c-Si), which are covering 95% of the market. This thesis concerns silicon heterojunction (SHJ), a high-efficiency technology with a 2% market share in 2018, but showing a steady global production capacity growth and potential for industry-compatible modules with a conversion efficiency above 21%. This thesis focuses on improving the device efficiency in real operating conditions. More specifically, different effects affecting a cell used in non-standard conditions are assessed, including different temperatures and illuminations, during prolonged exposure to light and the effect of surface inhomogeneities. The main SHJ technology specificity consists of the use of thin hydrogenated amorphous silicon (a-Si) layers that provide excellent c-Si surface passivation. This enables very high operating voltages compared to other c-Si-based technologies, which directly reduces the loss of efficiency induced by higher operating temperatures. The cell's properties are measured at very low temperatures are studied, which amplifies charges transport phenomena. The voltage shows a deviation from its theoretical curve, which we explain to be related to a decrease of the positive contact holes selectivity due to the influence of the transparent conductive oxide layer. An increase in the resistive losses at low temperature is measured. These losses are caused by a resistivity increase in the different a-Si layers and interfaces because of a thermal energy decrease at low temperatures. These effects may also affect cell performances at room temperature and under operating conditions. This implies that optimisation at standard test conditions (STC), generally used to assess the performance of PV modules, does not necessarily result in the same optimum as in real climatic conditions, which generally correspond to a higher temperature and/or a lower illumination. We show that by adding carbon in the front doped layer, a transparency increase, and thus a current gain is observed at the expense of larger ohmic losses, which overall reduces the efficiency at STC. Using simulations and real climate data, we show that using such a layer leads to a relative energy production gain of 0.5 to 0.8%, both in a moderate and arid climate. Another specificity of SHJ cells is a slight efficiency increase when exposed to light for a few days. We show that in case of a too-small positive doped layer charges reserve, cell performance degradation is observed instead of an improvement. In order to avoid such degradation, the positively doped layer should be sufficiently thick and doped to screen the influence of the subsequent transparent conductive oxide layer. A treatment in which a forward bias voltage is applied to the cell allows triggering the gain in efficiency without causing the detrimental effects of light exposure. Finally, the influence of a non-homogeneous surface is studied. Parasitic electric currents are induced between the well-passivated area and poor passivation area through the metallisation. Using dedicated samples, correspondence between these currents, and an often measured drop of the low injection lifetime is described and linked to the detrimental effect of either the sample edges or by surface defects. Concretely, these effects can affect the performances of the cell at low illumination, reducing the performances in real outdoor conditions.

EPFL2020