Reducing optical and electrical losses in germanium via nanostructures and surface passivation

Joonas Isometsä

Reducing optical and electrical losses in germanium via nanostructures and surface passivation

Julkaisun otsikon käännös: Optisten ja sähköisten häviöiden minimointi germaniumissa käyttäen pinnan nanokuviointia ja pintapassivointia

Joonas Isometsä

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Germaniumilla (Ge) on useita merkittäviä etuja piihin verrattuna, kuten varauksenkuljettajien suurempi liikkuvuus ja kapeampi kielletty energiaväli, mikä tekee siitä houkuttelevan substraattimateriaalin erilaisiin optoelektronisiin sovelluksiin. Niiden hyödyntäminen on kuitenkin jäänyt vähäiseksi johtuen germaniumin pinnan haasteellisista optisista ja elektronisista ominaisuuksista, joista merkittävimmät ovat heijastava peilipinta ja huomattava pintarekombinaatio. Aluksi tässä väitöskirjassa tutkitaan pintaheijastuksien vähentämistä kehittämällä nanorakenteita. Nanorakenteet mahdollistavat minimaalisen heijastuvuuden laajalla spektrialueella, jolloin tuloksena on täysin musta Ge. Työssä kehitetään kaksi erilaista valmistusprosessia mustalle Ge:lle, toinen induktiivisesti kytketyn plasman reaktiivisen ionisyövytyksen (ICP-RIE) avulla ja toinen metallikatalysoidun syövytyksen (MACE) avulla. ICP-RIE:n osoitetaan kykenevän tuottamaan mustaa Ge:tä, jolla heijastuvuus on alle 1% 400-1600 nm aallonpituusalueella. Vastaavasti MACE-käsitellyn pinnan keskimääräinen heijastuvuus pysyy jonkin verran korkeampana noin 9% keskimääräisellä heijastuksella. Siitä huolimatta, MACE-prosessi tarjoaa joitakin selkeitä etuja ICP-RIE:hen verrattuna, kuten alhaisemmat prosessointikustannukset, mikä tekee siitä houkuttelevan. Työn toinen pääteema on vähentää pinnan rekombinaatiota kehittämällä atomikerroskasvatettu (ALD) alumiinioksidipohjainen (Al2O3) passivointiprosessi. ALD Al2O3 tarjoaa tehokkaan passi-voinnin kiillotetuille Ge-pinnoille, tuloksena parhaimmillaan vain 6,55 cm/s suuruinen pintarekombinaationopeus. Tämä saavutetaan optimoiduilla prosessiparametreilla, kuten suolahappo (HCl) esikäsittelyllä ja pinnoitetun kalvon jälkilämmittämisellä 400 °C lämpötilassa 30 minuuttia. Tarkemman karakterisoinnin jälkeen havaitaan, että alhainen SRV perustuu suureen pintavarauksen määrään, eikä niinkään pinnan tehokkaaseen kemialliseen passivointiin. Kemiallisen passivoinnin parantamiseksi kehitetään ALD-esikäsittelyksi tyhjiössä suoritettava lämpökäsittely, jonka seurauksena kemiallinen passivointi paranee mutta samalla käsittely vähentää pintavarauksen määrää ja lopulta passivoinnin tehokkuus pysyy ennallaan. Työn lopullinen tavoite on yhdistää kehitetyt nanokuviointi- ja pintapassivointiprosessit ja siten samanaikaisesti pienentää sekä heijastus- että rekombinaatiohäviöitä. ICP-RIE:llä valmistetussa mustassa Ge:ssä havaitaan kemikaalijäämiä, jotka heikentävät pinnan passivointia. Syklisellä suolahappo ja vetyperoksidipohjaisella puhdistusprosessilla jäämät saadaan poistettua mutta samalla pinnan morfologia muuttuu ja heijastuvuus kasvaa. Täten heijastuvuuden ja pinnan passivoinnin välillä on tehtävä kompromissi. Esimerkiksi ilman heijastuvuuden liialllista heikkenemistä (< 2%), saadaan jo merkittävä parannus passivointiin (SRV ~ 30 cm/s). MACE:n tapauksessa pinnan tehokas passivointi vaikuttaa olevan monimutkaisempaa. Tässä työssä saavutetut tulokset antavat hyvän pohjan korkean hyötysuhteen optoelektronisten laitteiden suunnitteluun.

Julkaisun otsikon käännös	Optisten ja sähköisten häviöiden minimointi germaniumissa käyttäen pinnan nanokuviointia ja pintapassivointia
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	Savin, Hele, Vastuuprofessori Vähänissi, Ville, Ohjaaja
Kustantaja	Aalto University
Painoksen ISBN	978-952-64-1677-9
Sähköinen ISBN	978-952-64-1678-6
Tila	Julkaistu - 2024
OKM-julkaisutyyppi	G5 Artikkeliväitöskirja

Tutkimusalat

germanium
musta germanium
pintapassivointi
atomikerroskasvatus

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-64-1678-6

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Repo, P. (Manager) & Rissanen, A. (Other)
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@phdthesis{5e1bf1fa6e184a109182550b7cfcea60,

title = "Reducing optical and electrical losses in germanium via nanostructures and surface passivation",

abstract = "Germanium (Ge) offers several distinct advantages over silicon, including higher carrier mobility and narrower bandgap, making it an attractive substrate material for various optoelectronic ap-plications, such as near-infrared detectors, multijunction solar cells, and thermophotovoltaics. However, the full potential of Ge has not been realized yet due to challenges related to optical and electrical losses, namely high reflectivity and high recombination at the surfaces. First, this dissertation addresses the high reflectivity of Ge surfaces by developing nanostructures, which could enable minimal reflectance over a wide spectral range resulting in visibly pitch-black Ge (b-Ge). b-Ge processes for two different fabrication methods, inductively-coupled plasma reactive-ion etching (ICP-RIE) and metal-assisted chemical etching (MACE), are developed. ICP-RIE is shown to be capable of producing b-Ge with a reflectance below 1% across 400–1600 nm wavelength range, whereas the MACE b-Ge reflectance remains somewhat higher at an average of 9%. Nevertheless, the MACE process provides some advantages over ICP-RIE, such as being considerably lower cost, making it equally competitive. In order to address the second main challenge, i.e. the high surface recombination, an ALD Al2O3-based surface passivation process is developed. ALD Al2O3 is demonstrated to provide efficient passivation for polished Ge surfaces, achieving a surface recombination velocity (SRV) of 6.55 cm/s. This is obtained with optimized process parameters such as an HCl pre-treatment and a post-anneal in 400 °C for 30 mins. After more detailed characterization, it is identified that the low SRV is based on a strong field-effect rather than good chemical passivation. In order to improve the chemical passivation, an ultra-high vacuum anneal as an ALD pre-treatment is employed. Consequently, the chemical passivation is improved but as a trade-off, the field-effect is reduced leading to a similar level of overall passivation. Finally, the above results are combined targeting a simultaneous reduction of reflective and recombination losses. In the case of ICP-RIE fabricated b-Ge, some chemical residues are observed to deteriorate the surface passivation. Cyclical HCl and H2O2-based cleaning process is seen to remove the residues but simultaneously leads to changes in surface morphology and reflectance. Hence a trade-off between reflectance and surface passivation is necessary. As an example, without deteriorating the reflectance too much (< 2%), an SRV of 30 cm/s is obtained. In the case of MACE, achieving efficient surface passivation seems to be more complicated. The obtained results in this thesis give a good basis for designing high-efficiency optoelectronic devices. ",

keywords = "germanium, black germanium, surface passivation, atomic layer deposition, germanium, musta germanium, pintapassivointi, atomikerroskasvatus, germanium, black germanium, surface passivation, atomic layer deposition",

author = "Joonas Isomets{\"a}",

year = "2024",

language = "English",

isbn = "978-952-64-1677-9",

series = "Aalto University publication series DOCTORAL THESES",

publisher = "Aalto University",

number = "35",

address = "Finland",

school = "Aalto University",

}

TY - BOOK

T1 - Reducing optical and electrical losses in germanium via nanostructures and surface passivation

AU - Isometsä, Joonas

PY - 2024

Y1 - 2024

N2 - Germanium (Ge) offers several distinct advantages over silicon, including higher carrier mobility and narrower bandgap, making it an attractive substrate material for various optoelectronic ap-plications, such as near-infrared detectors, multijunction solar cells, and thermophotovoltaics. However, the full potential of Ge has not been realized yet due to challenges related to optical and electrical losses, namely high reflectivity and high recombination at the surfaces. First, this dissertation addresses the high reflectivity of Ge surfaces by developing nanostructures, which could enable minimal reflectance over a wide spectral range resulting in visibly pitch-black Ge (b-Ge). b-Ge processes for two different fabrication methods, inductively-coupled plasma reactive-ion etching (ICP-RIE) and metal-assisted chemical etching (MACE), are developed. ICP-RIE is shown to be capable of producing b-Ge with a reflectance below 1% across 400–1600 nm wavelength range, whereas the MACE b-Ge reflectance remains somewhat higher at an average of 9%. Nevertheless, the MACE process provides some advantages over ICP-RIE, such as being considerably lower cost, making it equally competitive. In order to address the second main challenge, i.e. the high surface recombination, an ALD Al2O3-based surface passivation process is developed. ALD Al2O3 is demonstrated to provide efficient passivation for polished Ge surfaces, achieving a surface recombination velocity (SRV) of 6.55 cm/s. This is obtained with optimized process parameters such as an HCl pre-treatment and a post-anneal in 400 °C for 30 mins. After more detailed characterization, it is identified that the low SRV is based on a strong field-effect rather than good chemical passivation. In order to improve the chemical passivation, an ultra-high vacuum anneal as an ALD pre-treatment is employed. Consequently, the chemical passivation is improved but as a trade-off, the field-effect is reduced leading to a similar level of overall passivation. Finally, the above results are combined targeting a simultaneous reduction of reflective and recombination losses. In the case of ICP-RIE fabricated b-Ge, some chemical residues are observed to deteriorate the surface passivation. Cyclical HCl and H2O2-based cleaning process is seen to remove the residues but simultaneously leads to changes in surface morphology and reflectance. Hence a trade-off between reflectance and surface passivation is necessary. As an example, without deteriorating the reflectance too much (< 2%), an SRV of 30 cm/s is obtained. In the case of MACE, achieving efficient surface passivation seems to be more complicated. The obtained results in this thesis give a good basis for designing high-efficiency optoelectronic devices.

AB - Germanium (Ge) offers several distinct advantages over silicon, including higher carrier mobility and narrower bandgap, making it an attractive substrate material for various optoelectronic ap-plications, such as near-infrared detectors, multijunction solar cells, and thermophotovoltaics. However, the full potential of Ge has not been realized yet due to challenges related to optical and electrical losses, namely high reflectivity and high recombination at the surfaces. First, this dissertation addresses the high reflectivity of Ge surfaces by developing nanostructures, which could enable minimal reflectance over a wide spectral range resulting in visibly pitch-black Ge (b-Ge). b-Ge processes for two different fabrication methods, inductively-coupled plasma reactive-ion etching (ICP-RIE) and metal-assisted chemical etching (MACE), are developed. ICP-RIE is shown to be capable of producing b-Ge with a reflectance below 1% across 400–1600 nm wavelength range, whereas the MACE b-Ge reflectance remains somewhat higher at an average of 9%. Nevertheless, the MACE process provides some advantages over ICP-RIE, such as being considerably lower cost, making it equally competitive. In order to address the second main challenge, i.e. the high surface recombination, an ALD Al2O3-based surface passivation process is developed. ALD Al2O3 is demonstrated to provide efficient passivation for polished Ge surfaces, achieving a surface recombination velocity (SRV) of 6.55 cm/s. This is obtained with optimized process parameters such as an HCl pre-treatment and a post-anneal in 400 °C for 30 mins. After more detailed characterization, it is identified that the low SRV is based on a strong field-effect rather than good chemical passivation. In order to improve the chemical passivation, an ultra-high vacuum anneal as an ALD pre-treatment is employed. Consequently, the chemical passivation is improved but as a trade-off, the field-effect is reduced leading to a similar level of overall passivation. Finally, the above results are combined targeting a simultaneous reduction of reflective and recombination losses. In the case of ICP-RIE fabricated b-Ge, some chemical residues are observed to deteriorate the surface passivation. Cyclical HCl and H2O2-based cleaning process is seen to remove the residues but simultaneously leads to changes in surface morphology and reflectance. Hence a trade-off between reflectance and surface passivation is necessary. As an example, without deteriorating the reflectance too much (< 2%), an SRV of 30 cm/s is obtained. In the case of MACE, achieving efficient surface passivation seems to be more complicated. The obtained results in this thesis give a good basis for designing high-efficiency optoelectronic devices.

KW - germanium

KW - black germanium

KW - surface passivation

KW - atomic layer deposition

KW - germanium

KW - musta germanium

KW - pintapassivointi

KW - atomikerroskasvatus

KW - germanium

KW - black germanium

KW - surface passivation

KW - atomic layer deposition

M3 - Doctoral Thesis

SN - 978-952-64-1677-9

T3 - Aalto University publication series DOCTORAL THESES

PB - Aalto University

ER -

Reducing optical and electrical losses in germanium via nanostructures and surface passivation

Abstrakti

Tutkimusalat

Pääsy asiakirjaan

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