Synchrotron X-ray based characterization of technologically relevant III-V surfaces and nanostructures

Abstract

Innovative design and materials are needed to satisfy the demand for efficient and scalable devices for electronic and opto-electronic applications, such as transistors, LEDs, and solar cells. Nanostructured III-V semiconductors are an appealing solution, combining the excellent functional properties of III-V materials with the flexibility typical of nanostructures, such as the nanowires (NWs) studied here. However, there are a number of open challenges, that currently hinder the performance of III-V nanostructure devices: first, the surface quality of III-V materials is still one of their main limiting factors. Other problems specific of III-V NWs are the control of dopant incorporation - crucial for their functionalization -, and of their structural inhomogeneity (e.g. lattice strain and tilt), that can affect opto-electronic performance. These problematics require a set of non-trivial cutting-edge characterization tools: here an approach based on a combination of X-ray synchrotron techniques is demonstrated.
Synchrotron based X-ray photoelectron spectroscopy (XPS) has been used to study the surface chemistry of III-V model systems and to monitor industrially relevant processing on them. A new passivation process improving the surface quality of InAs substrates used for electronics has been investigated: the surface structure and composition resulting from thermal oxidation followed by ex situ deposition of a high-k material via atomic layer deposition (ALD) has been assessed with XPS. The implementation of this passivation approach in gate stacks showed improvements in performance, that were attributed to the specific stoichiometry of the thermal oxide. The dynamics of the ALD process on InAs was also studied in situ with ambient pressure XPS: it was observed that the chemisorption of the precursor is an important step to ensure a good quality of the high-k oxide deposition.
Dopant evaluation in NWs is challenging due to their small dimensions. Here, a first approach to this problem was to perform XPS to study the effects of Zn dopant incorporation on the surface of GaAs NWs, used for solar cells. High doping conditions during growth were found to form a Zn layer on the outside of the NW that suppresses the native oxides, which are generally a cause of poor passivation of III-V surfaces. In another experiment, XPS scanning microscopy was used to study surface Zn doping in an InP NW with an axial pn junction, also used for solar cells. The surface potential drop along the junction was monitored in operando, while applying a bias to the NW device, and it was found smaller than what expected for the bulk. Finally, a quantitative evaluation of Zn dopants incorporation in III-V NWs was studied for the first time with nano-focused X-ray fluorescence, due to the excellent combination of low detection limits and spatial resolution. Dopant gradients and memory effects were noted along InP and InGaP NWs, showing complex dopant incorporation mechanisms during the growth.
The structural inhomogeneity in InGaN nano-pyramids for next generation LEDs was also investigated. The influence of different processing parameters on lattice and strain were studied with full field X-ray diffraction microscopy. This imaging technique uses Bragg diffraction intensity as contrast mechanism and has a large field of view, useful for imaging at once large areas patterned with pyramids, giving valuable statistical consistency. The growth parameters providing the best lattice quality and homogeneity were assessed.
This thesis shows how cutting edge synchrotron characterization methods can provide useful information for improving III-V surfaces and nanostructures for next generation devices. Moreover, in most cases advances in the characterization methods are achieved, that can be relevant also in other and broader scientific fields.

Abstract (Swedish)

Populärvetenskaplig sammanfattning:
En studie publicerad av NASA:s Goddard Institute for Space Studies år 2019 visade att de genomsnittliga globala temperaturerna under de senaste fyra åren (2015-2018) var de högsta som någonsin registrerats i människans historia. Den nyheten passar väl in i de alarmerande prognoserna om global uppvärmning, och kräver snabba och radikala handlingar. Att studera “tekniskt relevanta III-V halvledares nanostrukturer och ytor med hjälp av synkrotronljus”, som är en omformulering av denna avhandlings titel, syftar till att ge ett litet bidrag till denna fråga. Det är dock svårt att se ett enkelt samband mellan titeln och den globala uppvärmningen. Att klargöra avhandlingens beståndsdelar kan kanske hjälpa till att belysa kopplingen.
Halvledare, som t.ex. kisel (Si), är material som finns närvarande i hela vårat vardagsliv. De har två typer av laddningar som rör sig inuti: elektroner (negativa) och "hål" (positiva). Genom att lägga till orenheter som kallas "dopants" i halvledare, kan man få "n-dopning" när de flesta av laddningar är elektroner, eller "p-dopning" när de är hål, beroende på dopant. p och n dopade halvledare kan kombineras för att skapa dioder och transistorer, som ofta används i datorer. Dessutom kan, i lysdioder (mer kända som LEDs), hål och elektroner återförenas och generera fotoner, som producerar ljus. Halvledare är också avgörande för att fånga upp solenergi: fotoner kan generera ström och denna effekt utnyttjas i solceller.
Framsteg inom halvledarteknik kan ha stor inverka på miljön: genom att öka prestandan hos apparaterna kan energiförbrukningen minskas markant, samtidigt som effektivare solceller kan ge renare energi.
I den här avhandlingen studeras prover av klass III-V halvledare som är nära att bli tillämpningsbara. III-V:er är föreningar av två (eller flera) element, en som tillhör III-gruppen (t.ex. In, Ga, och Al) och en av V-gruppen (t.ex. N, As, och P) av den periodiska tabellen. III-V halvledare har enastående ledningsförmåga och effektiva egenskaper för att generera ljus. Detta gör dem till perfekta kandidater för framtida enheter: till exempel är indiumarsenid (InAs) optimalt för transistorer, indiumfosfat (InP) för solceller och indiumgalliumnitrid (InGaN) för LEDs.
III-V:er har faktiskt varit kända under lång tid och undersökts redan före Si. Anledningen till att de inte har ersatt Si beror främst på den relativt höga kostnaden och den dåliga kvaliteten på deras ytor, vilket är avgörande för elektronik och solceller. Under de senaste decennierna, har forskning om dessa material upplevt en renässans, på grund av nya metoder för att förbättra ytorna och också för möjligheten att tillverka III-V halvledare som nanostrukturer, som t.ex. nanotrådar (NW) som studeras i denna avhandling. NWs är nålformade objekt som är cirka en mikrometer (1 μm = 0,000001 m) långa och med diametrar på cirka 30-200 nanometer (1 nm = 0,001 μm). De erbjuder en mycket flexibel grund för att kombinera olika III-V material utan att producera stora deformationsfält (som kan hämma prestandan) i samma nanostruktur. Många fysiska processer, som laddningstransport, är i sig effektivare i NWs än i materialets normala storlek.
För att utveckla banbrytande effektiva nanostrukturenheter behövs dock en fullständig förståelse av fysiken bakom dem. Framsteg måste uppnås i deras behandling genom att 1) förbättra ytkvaliteten, 2) kontrollera NW-dopningen, 3) övervaka den strukturella deformationen.
Karaktärisering av III-V ytor och NWs är därför nödvändigt, men detta är extremt utmanande på grund av deras ringa storlekar. Avhandlingen föreslår ett steg framåt i denna riktning genom att använda en kombination av avancerade tekniker baserade på röntgenljus från en synkrotronkälla. Röntgenljus som produceras av en synkrotron - en cyklisk partikelaccelerator - är optimala på grund av dess mycket höga intensitet, som är utmärkt för detaljerade kemiska och strukturella analyser och även mikroskopi, när ljusstrålen är ordentligt fokuserad.
En ny behandling för att förbättra kvaliteten på InAs-ytor undersöktes med en teknik som kallas fotoelektronspektroskopi (XPS). Denna metod är baserad på insamling av provelektroner som matas ut av röntgenljuset, som fungerar som kemiska fingeravtryck. Genom att utveckla denna teknik, studerades den ytkemiska utvecklingen under depositionen av ett högkvalitativt oxidskikt i realtid. Resultaten visade de viktiga aspekterna i denna behandling. XPS användes också för att studera dopants på NWs ytor, som visade sig ha en stark effekt på ytan egenskaper.
Resultatet av dopning i NWs är generellt svårt att förutsäga och mäta. För första gången har en röntgenstråle med ett fokus på bara några tiotals nanometer använts för att kartlägga dopants-distribution i III-V NWs. Dopants räknades tack vare deras emission av karakteristiskt röntgenljus, en effekt som kallas fluorescens. Resultaten är användbara för att förstå hur man optimerar tillverkningen för att få den önskade dopants-distributionen för effektiva NW-solceller.
Slutligen studerades de deformationsseffekter som orsakas av tillverkning av NWs-mönster som används för LEDs med en teknik som kallas full field röntgens-diffraktionsmikroskopi. Denna metod är baserad på diffraktion, en reflektion av röntgenstrålar som endast förekommer i specifika riktningar, beroende på atomavståndet, som självt beror på deformation. För första gången erhölls bilder på InGaN NW-mönster med kontrasten som ges av deras olika deformationer. Denna studie hjälper till att hitta de bästa parametrarna för att tillverka högkvalitativa NW LEDs.
Denna avhandling utforskar ett brett spektrum av viktiga aspekter i specifika III-V nanostrukturer. Användningen av moderna röntgenkarakteriseringsverktyg syftar till att ge konkreta svar för att förbättra dessa material och enheter, med förhoppningen att göra dem mer effektiva.

English popular science summary:
A study published by the Goddard Institute for Space Studies of NASA in 2019 put in evidence that the average global temperatures of the last four years (2015-2018) were the highest ever recorded in human history. This recent news fits well into the alarming forecasts on global warming, demanding prompt and radical interventions. Studying “technologically relevant III-V semiconductor nanostructures and surfaces with techniques based on synchrotron radiation”, which is a rephrasing of the title of this thesis, aims to give a little contribution to this issue. Apparently, there is not a straightforward connection between the title and the global warming: a clarification on the elements of this thesis can help in highlighting the link.
Semiconductors, like for instance silicon (Si), are materials present in almost all aspects of our everyday life. They have two different types of charges that can move in the solid: electrons (negative) and “holes” (positive). By adding impurities called “dopants” into semiconductors, one can obtain “n type doping” when most of the charge carriers are electrons, or “p type doping” when they are holes, depending on the dopant. p and n doped semiconductors can be combined together to create diodes and transistors, widely used in computers. Moreover, in the class of the light emitting diodes, better known as LEDs, holes and electrons can recombine together generating photons, that is producing light. Semiconductors are also crucial in harvesting solar energy: the photons can generate a current and this effect is exploited in solar cells.
Advances in semiconductor technology can have a big impact for the environment: by boosting the performance of devices, the energy consumption can be sensibly decreased, while more efficient solar cells can provide more clean energy.
Here, samples of the class of the III-V semiconductors close to realistic applications are studied. III-Vs are compounds of two (or more) elements, one belonging to the III group (for example In, Ga, and Al) and one of the V group (for example N, As, and P) of the periodic table. III-V semiconductors have outstanding charge transport and charge-photon conversion properties, making them perfect candidates for future devices: for example, indium arsenide (InAs) is optimal for transistors, indium phosphate (InP) for solar cells and indium gallium nitride (InGaN) for LEDs.
III-Vs have actually been known for a long time and investigated even before Si. The reason why they have not supplanted Si is mainly due to the relatively high cost and to the poor quality of their surfaces, which is crucial for electronics and solar cells. In the last decades, the research on these materials experienced a renaissance, due to new methods for improving the surfaces and especially for the possibility of implementing III-V semiconductors in nanostructures, like for instance the nanowires (NW) studied in this thesis. NWs are needle shaped objects ca. 1 micron (1 µm = 0.000001 m) long and with diameters of ca. 30-200 nanometers (1 nm = 0.001 µm), and they offer a very flexible platform to combine different III-V materials avoiding large strain fields (that can hamper performances) in the same nanostructure. Many physical processes, like charge transport, are intrinsically more efficient in NWs than in their bulk counterpart.
However, for developing cutting-edge efficient nanostructure devices, a complete understanding of the physics behind them is still needed. Advances need to be achieved in their processing by 1) improving the surface quality, 2) controlling the NW doping, 3) monitoring the structural strain.
A characterization of III-V surfaces and NWs is therefore needed, but this is very challenging due to their small size. This thesis proposes a step forward in this direction by using a combination of advanced techniques based on X-rays from a synchrotron source. The X-rays produced by a synchrotron - a cyclic particle accelerator - are ideal because of their very high intensity, excellent for detailed chemical and structural analyses and even microscopy, when focused properly.
A new processing to improve the quality of InAs surfaces was investigated with a technique called synchrotron based X-ray photoelectron spectroscopy (XPS). This method is based on the collection of the sample electrons ejected by the X-rays, acting as chemical fingerprint. By pushing this technique to its limits, the surface chemistry evolution during the deposition of a high quality oxide layer was studied in real time. The results showed the critical aspects in this industrially relevant processing. XPS was also used for studying the dopants on the surface of NWs, that were found to have a strong effect on the surface properties.
The doping incorporation in NWs is in general difficult to predict and measure. For the first time, an X-ray beam with a focal spot of only few tens of nanometers was used to map the dopant distribution in III-V NWs. The dopants were identified and counted thanks to their emission of characteristic X-rays, an effect called fluorescence. The results are useful to understand how to tailor the processing to have the desired dopant distribution for very efficient NW solar cells.
Finally, the strain effects caused by patterning arrays of NWs used for LEDs were studied with a technique called full field X-ray diffraction microscopy. This method is based on diffraction, a selective reflection of X-rays that occurs only in specific directions, depending on the atomic spacing: if there is strain, the atomic spacing (and the diffraction angle) is different. For the first time, images of InGaN NW arrays were obtained with the contrast given by their different strain. This study helps to find the optimal parameters for fabricating high quality NW LEDs.
This thesis explores a wide range of criticalities in specific III-V nanostructures. The use of modern X-ray based characterization tools is aimed to give concrete answers to improve these materials and devices, with the hope of making them more energetically efficient.

• author: Troian, Andrea
• supervisor: Rainer Timm; Anders Mikkelsen
• opponent: Professor Kummel, Andrew, University of California, San Diego, USA
• organization: Synchrotron Radiation Research; NanoLund: Centre for Nanoscience
• publishing date: 2019-03
• type: Thesis
• publication status: published
• subject: Condensed Matter Physics
• keywords: synchrotron radiation, III-V semiconductors, high-k oxides, passivation, doping, XPS, AP-XPS, SPEM, XRF, full field X-ray diffraction microscopy, Fysicumarkivet A:2019:Troian
• defense location: Lundmarksalen, Astronomihuset, Sölvegatan 27, Lund
• defense date: 2019-04-12 09:15:00
• ISBN: 978-91-7753-978-0
• ISBN: 978-91-7753-979-7
• language: English
• LU publication?: yes
• id: 7f64b980-f0e2-476b-bd53-08592ed23577
• date added to LUP: 2019-03-18 12:13:26
• date last changed: 2020-03-10 17:21:09