Prof. Dr. Martin Stutzmann
Academic Research Focus
- Semiconductor materials
- Characterization of optical and electronic properties
- Light trapping structures
- Laser processing
- Characterization of optical and electronic properties
- Light trapping structures
- Laser processing
Fields of Application
Semiconductor Materials, Solar Cells / Photovoltaics
Publications
16.
T Höldrich, A Wieland, F Pantle, J Winnerl, M Stutzmann
Selective growth and characterization of GaN nanowires on SiC substrates Journal Article
In: Journal of Crystal Growth, vol. 665, pp. 128194, 2025, ISSN: 0022-0248.
@article{nokey,
title = {Selective growth and characterization of GaN nanowires on SiC substrates},
author = {T H\"{o}ldrich and A Wieland and F Pantle and J Winnerl and M Stutzmann},
url = {https://www.sciencedirect.com/science/article/pii/S0022024825001423},
doi = {https://doi.org/10.1016/j.jcrysgro.2025.128194},
issn = {0022-0248},
year = {2025},
date = {2025-09-01},
journal = {Journal of Crystal Growth},
volume = {665},
pages = {128194},
abstract = {GaN on SiC is a promising material combination for high power devices, where especially nanostructures, such as nanowires or nanofins, are a space and resource saving solution. In this work we demonstrate the selective area growth of GaN nanowires on SiC substrates, using the polytype 6H-SiC. We investigate the influence of the Si- and C-polarity of the substrate on the structural properties of the GaN nanowires by scanning electron microscopy and photoluminescence spectroscopy. On both substrates uniform and hexagonal nanowires are achieved for the respective optimal growth temperature, which is determined to be 20$circ $C higher for Si-polarity. As the polarity combination of the SiC substrate and GaN nanowires strongly influences the electrical properties at the heterointerface due to different charge accumulations, it is necessary to investigate the epitaxial relationship. X-ray diffraction revealed that the GaN nanowires exclusively adopt the orientation of the underlying SiC lattice, leading to an in-plane epitaxial relationship of (11¯00)GaN/(11¯00)6H-SiC. Polarity-selective wet chemical etching and Kelvin probe force microscopy showed that the GaN nanowires preserve the polarity of the substrate, thus, either a predominantly metal-polar (Si-polar/Ga-polar) or non-metal-polar (C-polar/N-polar) orientation is present. The complete epitaxial relationship on both substrate polarities can be identified as (11¯00)[0001]GaN||(11¯00)[0001]6H-SiC for the large majority of NWs at their respective optimum growth temperatures.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
GaN on SiC is a promising material combination for high power devices, where especially nanostructures, such as nanowires or nanofins, are a space and resource saving solution. In this work we demonstrate the selective area growth of GaN nanowires on SiC substrates, using the polytype 6H-SiC. We investigate the influence of the Si- and C-polarity of the substrate on the structural properties of the GaN nanowires by scanning electron microscopy and photoluminescence spectroscopy. On both substrates uniform and hexagonal nanowires are achieved for the respective optimal growth temperature, which is determined to be 20$circ $C higher for Si-polarity. As the polarity combination of the SiC substrate and GaN nanowires strongly influences the electrical properties at the heterointerface due to different charge accumulations, it is necessary to investigate the epitaxial relationship. X-ray diffraction revealed that the GaN nanowires exclusively adopt the orientation of the underlying SiC lattice, leading to an in-plane epitaxial relationship of (11¯00)GaN/(11¯00)6H-SiC. Polarity-selective wet chemical etching and Kelvin probe force microscopy showed that the GaN nanowires preserve the polarity of the substrate, thus, either a predominantly metal-polar (Si-polar/Ga-polar) or non-metal-polar (C-polar/N-polar) orientation is present. The complete epitaxial relationship on both substrate polarities can be identified as (11¯00)[0001]GaN||(11¯00)[0001]6H-SiC for the large majority of NWs at their respective optimum growth temperatures.
15.
E Sirotti, S Böhm, G Grötzner, M Christis, L I Wagner, L Wolz, F Munnik, J Eichhorn, M Stutzmann, V Streibel, I D Sharp
Amorphous nitride semiconductors with highly tunable optical and electronic properties: the benefits of disorder in Ca–Zn–N thin films Journal Article
In: Materials Horizons, 2024, ISSN: 2051-6347.
@article{nokey,
title = {Amorphous nitride semiconductors with highly tunable optical and electronic properties: the benefits of disorder in Ca\textendashZn\textendashN thin films},
author = {E Sirotti and S B\"{o}hm and G Gr\"{o}tzner and M Christis and L I Wagner and L Wolz and F Munnik and J Eichhorn and M Stutzmann and V Streibel and I D Sharp},
url = {http://dx.doi.org/10.1039/D4MH01525H},
doi = {10.1039/D4MH01525H},
issn = {2051-6347},
year = {2024},
date = {2024-12-16},
journal = {Materials Horizons},
abstract = {Semiconducting ternary nitrides are a promising class of materials that have received increasing attention in recent years, but often show high free electron concentrations due to the low defect formation energies of nitrogen vacancies and substitutional oxygen, leading to degenerate n-type doping. To achieve non-degenerate behavior, we now investigate a family of amorphous calcium\textendashzinc nitride (Ca\textendashZn\textendashN) thin films. By adjusting the metal cation ratios, we demonstrate band gap tunability between 1.4 and 2.0 eV and control over the charge carrier concentration across six orders of magnitude, all while maintaining high mobilities between 5 and 70 cm2 V−1 s−1. The combination of favorable electronic properties, low synthesis temperatures, and earth-abundant elements makes amorphous Ca\textendashZn\textendashN highly promising for future sustainable electronics. Moreover, the successful synthesis of such materials, as well as their broad optical and electrical tunability, paves the way for a new class of tailored functional materials: amorphous nitride semiconductors \textendash ANSs.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Semiconducting ternary nitrides are a promising class of materials that have received increasing attention in recent years, but often show high free electron concentrations due to the low defect formation energies of nitrogen vacancies and substitutional oxygen, leading to degenerate n-type doping. To achieve non-degenerate behavior, we now investigate a family of amorphous calcium–zinc nitride (Ca–Zn–N) thin films. By adjusting the metal cation ratios, we demonstrate band gap tunability between 1.4 and 2.0 eV and control over the charge carrier concentration across six orders of magnitude, all while maintaining high mobilities between 5 and 70 cm2 V−1 s−1. The combination of favorable electronic properties, low synthesis temperatures, and earth-abundant elements makes amorphous Ca–Zn–N highly promising for future sustainable electronics. Moreover, the successful synthesis of such materials, as well as their broad optical and electrical tunability, paves the way for a new class of tailored functional materials: amorphous nitride semiconductors – ANSs.
14.
M Rostami, B Yang, F Haag, F Allegretti, L Chi, M Stutzmann, J V Barth
In: Applied Surface Science, vol. 674, pp. 160880, 2024, ISSN: 0169-4332.
@article{nokey,
title = {Influencing the surface quality of free-standing wurtzite gallium nitride in ultra-high vacuum: Stoichiometry control by ammonia and bromine adsorption},
author = {M Rostami and B Yang and F Haag and F Allegretti and L Chi and M Stutzmann and J V Barth},
url = {https://www.sciencedirect.com/science/article/pii/S0169433224015939},
doi = {https://doi.org/10.1016/j.apsusc.2024.160880},
issn = {0169-4332},
year = {2024},
date = {2024-11-15},
journal = {Applied Surface Science},
volume = {674},
pages = {160880},
abstract = {Gallium nitride (GaN), a wide bandgap semiconductor with absorption and emission in the ultraviolet/visible range, is proposed as an alternative to metallic surfaces for assembling organic molecular structures aiming at optoelectronic applications. However, the formation of a persistent surface oxide layer in air considerably limits the use of GaN for well-defined interfaces. In this work, we have investigated, characterized and processed n-type free-standing c-plane hexagonal wurtzite GaN crystals grown by hydride vapor phase epitaxy and ammonothermal growth methods. Surface cleaning and full removal of the oxide layer on GaN surfaces could be reproducibly achieved via sputtering and annealing cycles, as evidenced by X-ray photoelectron spectroscopy and low-energy electron diffraction. Scanning tunneling microscopy, however, indicated substantial roughening of the GaN surface and the formation of unwanted Ga-rich islands and clusters. Although ammonia (NH3) and bromine (Br) treatments compensated the N/Ga atoms ratio reduced by sputtering, the surface morphology remained rough, exhibiting randomly shaped and distributed hillocks. In addition, we studied the effect of electron bombardment on the surface quality of GaN during NH3 annealing, on-surface debromination and polymerization of 1,3,5-tris(4-bromophenyl) benzene on GaN, and the removal of Ga atoms by Br atoms during the desorption.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Gallium nitride (GaN), a wide bandgap semiconductor with absorption and emission in the ultraviolet/visible range, is proposed as an alternative to metallic surfaces for assembling organic molecular structures aiming at optoelectronic applications. However, the formation of a persistent surface oxide layer in air considerably limits the use of GaN for well-defined interfaces. In this work, we have investigated, characterized and processed n-type free-standing c-plane hexagonal wurtzite GaN crystals grown by hydride vapor phase epitaxy and ammonothermal growth methods. Surface cleaning and full removal of the oxide layer on GaN surfaces could be reproducibly achieved via sputtering and annealing cycles, as evidenced by X-ray photoelectron spectroscopy and low-energy electron diffraction. Scanning tunneling microscopy, however, indicated substantial roughening of the GaN surface and the formation of unwanted Ga-rich islands and clusters. Although ammonia (NH3) and bromine (Br) treatments compensated the N/Ga atoms ratio reduced by sputtering, the surface morphology remained rough, exhibiting randomly shaped and distributed hillocks. In addition, we studied the effect of electron bombardment on the surface quality of GaN during NH3 annealing, on-surface debromination and polymerization of 1,3,5-tris(4-bromophenyl) benzene on GaN, and the removal of Ga atoms by Br atoms during the desorption.
13.
F Rauh, J Dittloff, M Thun, M Stutzmann, I D Sharp
In: ACS Applied Materials & Interfaces, vol. 16, no. 5, pp. 6653-6664, 2024, ISSN: 1944-8244.
@article{nokey,
title = {Nanostructured Black Silicon as a Stable and Surface-Sensitive Platform for Time-Resolved In Situ Electrochemical Infrared Absorption Spectroscopy},
author = {F Rauh and J Dittloff and M Thun and M Stutzmann and I D Sharp},
url = {https://doi.org/10.1021/acsami.3c17294},
doi = {10.1021/acsami.3c17294},
issn = {1944-8244},
year = {2024},
date = {2024-01-24},
urldate = {2024-01-24},
journal = {ACS Applied Materials \& Interfaces},
volume = {16},
number = {5},
pages = {6653-6664},
abstract = {Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is a powerful method for probing interfacial chemical processes. However, SEIRAS-active nanostructured metallic thin films for the in situ analysis of electrochemical phenomena are often unstable under biased aqueous conditions. In this work, we present a surface-enhancing structure based on etched black Si internal reflection elements with Au-coatings for in situ electrochemical ATR-SEIRAS. Using electrochemical potential-dependent adsorption and desorption of 4-methoxypyridine on Au, we demonstrate that black Si-based substrates offer advantages over commonly used structures, such as electroless-deposited Au on Si and electrodeposited Au on ITO-coated Si, due to the combination of high stability, sensitivity, and conductivity. These characteristics are especially valuable for time-resolved measurements where stable substrates are required over extended times. Furthermore, the low sheet resistance of Au layers on black Si reduces the RC time constant of the electrochemical cell, enabling a significantly higher time resolution compared to that of traditional substrates. Thus, we employ black Si-based substrates in conjunction with rapid- and step-scan Fourier transform infrared (FTIR) spectroscopy to investigate the adsorption and desorption kinetics of 4-methoxypyridine during in situ electrochemical potential steps. Adsorption is shown to be diffusion-limited, which allows for the determination of the mean molecular area in a fully established monolayer. Moreover, no significant changes in the peak ratios of vibrational modes with different orientations relative to the molecular axis are observed, suggesting a single adsorption mode and no alteration of the average molecular orientation during the adsorption process. Overall, this study highlights the enhanced performance of black Si-based substrates for both steady-state and time-resolved in situ electrochemical ATR-SEIRAS, providing a powerful platform for kinetic and mechanistic investigations of electrochemical interfaces.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is a powerful method for probing interfacial chemical processes. However, SEIRAS-active nanostructured metallic thin films for the in situ analysis of electrochemical phenomena are often unstable under biased aqueous conditions. In this work, we present a surface-enhancing structure based on etched black Si internal reflection elements with Au-coatings for in situ electrochemical ATR-SEIRAS. Using electrochemical potential-dependent adsorption and desorption of 4-methoxypyridine on Au, we demonstrate that black Si-based substrates offer advantages over commonly used structures, such as electroless-deposited Au on Si and electrodeposited Au on ITO-coated Si, due to the combination of high stability, sensitivity, and conductivity. These characteristics are especially valuable for time-resolved measurements where stable substrates are required over extended times. Furthermore, the low sheet resistance of Au layers on black Si reduces the RC time constant of the electrochemical cell, enabling a significantly higher time resolution compared to that of traditional substrates. Thus, we employ black Si-based substrates in conjunction with rapid- and step-scan Fourier transform infrared (FTIR) spectroscopy to investigate the adsorption and desorption kinetics of 4-methoxypyridine during in situ electrochemical potential steps. Adsorption is shown to be diffusion-limited, which allows for the determination of the mean molecular area in a fully established monolayer. Moreover, no significant changes in the peak ratios of vibrational modes with different orientations relative to the molecular axis are observed, suggesting a single adsorption mode and no alteration of the average molecular orientation during the adsorption process. Overall, this study highlights the enhanced performance of black Si-based substrates for both steady-state and time-resolved in situ electrochemical ATR-SEIRAS, providing a powerful platform for kinetic and mechanistic investigations of electrochemical interfaces.
12.
M Christis, A Henning, J D Bartl, A Zeidler, B Rieger, M Stutzmann, I D Sharp
Annealing-Free Ohmic Contacts to n-Type GaN via Hydrogen Plasma-Assisted Atomic Layer Deposition of Sub-Nanometer AlOx Journal Article
In: Advanced Materials Interfaces, vol. n/a, no. n/a, pp. 2300758, 2023, ISSN: 2196-7350.
@article{nokey,
title = {Annealing-Free Ohmic Contacts to n-Type GaN via Hydrogen Plasma-Assisted Atomic Layer Deposition of Sub-Nanometer AlOx},
author = {M Christis and A Henning and J D Bartl and A Zeidler and B Rieger and M Stutzmann and I D Sharp},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/admi.202300758},
doi = {https://doi.org/10.1002/admi.202300758},
issn = {2196-7350},
year = {2023},
date = {2023-12-01},
journal = {Advanced Materials Interfaces},
volume = {n/a},
number = {n/a},
pages = {2300758},
abstract = {Abstract A plasma-assisted atomic layer deposition (PE-ALD) process is reported for creating ohmic contacts to n-type GaN that combines native oxide reduction, near-surface doping, and encapsulation of GaN in a single processing step, thereby eliminating the need for both wet chemical etching of the native oxide before metallization and thermal annealing after contact formation. Repeated ALD cycling of trimethyl aluminum (TMA) and high-intensity hydrogen (H2) plasma results in the deposition of a sub-nanometer-thin (≈8 r{A}) AlOx layer via the partial transformation of the GaN surface oxide into AlOx. Hydrogen plasma-induced nitrogen vacancies in the near-surface region of GaN serve as shallow donors, promoting efficient out-of-plane electrical transport. Subsequent metallization with a Ti/Al/Ti/Au stack results in low contact resistance, ohmic behavior, and smooth morphology without requiring annealing. This electrical contracting approach thus meets the thermal budget requirements for Si-based complementary metal\textendashoxide\textendashsemiconductor structures and can facilitate the design and fabrication of advanced GaN-on-Si heterodevices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract A plasma-assisted atomic layer deposition (PE-ALD) process is reported for creating ohmic contacts to n-type GaN that combines native oxide reduction, near-surface doping, and encapsulation of GaN in a single processing step, thereby eliminating the need for both wet chemical etching of the native oxide before metallization and thermal annealing after contact formation. Repeated ALD cycling of trimethyl aluminum (TMA) and high-intensity hydrogen (H2) plasma results in the deposition of a sub-nanometer-thin (≈8 Å) AlOx layer via the partial transformation of the GaN surface oxide into AlOx. Hydrogen plasma-induced nitrogen vacancies in the near-surface region of GaN serve as shallow donors, promoting efficient out-of-plane electrical transport. Subsequent metallization with a Ti/Al/Ti/Au stack results in low contact resistance, ohmic behavior, and smooth morphology without requiring annealing. This electrical contracting approach thus meets the thermal budget requirements for Si-based complementary metal–oxide–semiconductor structures and can facilitate the design and fabrication of advanced GaN-on-Si heterodevices.
11.
F Pantle, S Wörle, M Karlinger, F Rauh, M Kraut, M Stutzmann
Environmental sensitivity of GaN nanofins grown by selective area molecular beam epitaxy Journal Article
In: Nanotechnology, vol. 34, no. 17, pp. 175501, 2023, ISSN: 0957-4484.
@article{nokey,
title = {Environmental sensitivity of GaN nanofins grown by selective area molecular beam epitaxy},
author = {F Pantle and S W\"{o}rle and M Karlinger and F Rauh and M Kraut and M Stutzmann},
url = {https://dx.doi.org/10.1088/1361-6528/acb4f6},
doi = {10.1088/1361-6528/acb4f6},
issn = {0957-4484},
year = {2023},
date = {2023-02-13},
journal = {Nanotechnology},
volume = {34},
number = {17},
pages = {175501},
abstract = {Nanostructures exhibit a large surface-to-volume ratio, which makes them sensitive to their ambient conditions. In particular, GaN nanowires and nanofins react to their environment as adsorbates influence their (opto-) electronic properties. Charge transfer between the semiconductor surface and adsorbed species changes the surface band bending of the nanostructures, and the adsorbates can alter the rate of non-radiative recombination in GaN. Despite the importance of these interactions with the ambient environment, the detailed adsorption mechanisms are still not fully understood. In this article, we present a systematic study concerning the environmental sensitivity of the electrical conductivity of GaN nanofins. We identify oxygen- and water-based adsorbates to be responsible for a quenching of the electrical current through GaN nanofins due to an increased surface band bending. Complementary contact potential difference measurements in controlled atmospheres on bulk m- and c-plane GaN reveal additional complexity with regard to water adsorption, for which surface dipoles might play an important role besides an increased surface depletion width. The sensitive reaction of the electrical parameters to the environment and surface condition underlines the necessity of a reproducible pre-treatment and/or surface passivation. The presented results help to further understand the complex adsorption mechanisms at GaN surfaces. Due to the sensitivity of the nanofin conductivity on the environment, such structures could perform well as sensing devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Nanostructures exhibit a large surface-to-volume ratio, which makes them sensitive to their ambient conditions. In particular, GaN nanowires and nanofins react to their environment as adsorbates influence their (opto-) electronic properties. Charge transfer between the semiconductor surface and adsorbed species changes the surface band bending of the nanostructures, and the adsorbates can alter the rate of non-radiative recombination in GaN. Despite the importance of these interactions with the ambient environment, the detailed adsorption mechanisms are still not fully understood. In this article, we present a systematic study concerning the environmental sensitivity of the electrical conductivity of GaN nanofins. We identify oxygen- and water-based adsorbates to be responsible for a quenching of the electrical current through GaN nanofins due to an increased surface band bending. Complementary contact potential difference measurements in controlled atmospheres on bulk m- and c-plane GaN reveal additional complexity with regard to water adsorption, for which surface dipoles might play an important role besides an increased surface depletion width. The sensitive reaction of the electrical parameters to the environment and surface condition underlines the necessity of a reproducible pre-treatment and/or surface passivation. The presented results help to further understand the complex adsorption mechanisms at GaN surfaces. Due to the sensitivity of the nanofin conductivity on the environment, such structures could perform well as sensing devices.
10.
A Henning, J D Bartl, L Wolz, M Christis, F Rauh, M Bissolo, T Grünleitner, J Eichhorn, P Zeller, M Amati, L Gregoratti, J J Finley, B Rieger, M Stutzmann, I D Sharp
Spatially-Modulated Silicon Interface Energetics Via Hydrogen Plasma-Assisted Atomic Layer Deposition of Ultrathin Alumina Journal Article
In: Advanced Materials Interfaces, vol. 10, iss. 6, pp. 2202166, 2022, ISSN: 2196-7350.
@article{nokey,
title = {Spatially-Modulated Silicon Interface Energetics Via Hydrogen Plasma-Assisted Atomic Layer Deposition of Ultrathin Alumina},
author = {A Henning and J D Bartl and L Wolz and M Christis and F Rauh and M Bissolo and T Gr\"{u}nleitner and J Eichhorn and P Zeller and M Amati and L Gregoratti and J J Finley and B Rieger and M Stutzmann and I D Sharp},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/admi.202202166},
doi = {https://doi.org/10.1002/admi.202202166},
issn = {2196-7350},
year = {2022},
date = {2022-12-16},
urldate = {2022-12-16},
journal = {Advanced Materials Interfaces},
volume = {10},
issue = {6},
pages = {2202166},
abstract = {Abstract Atomic layer deposition (ALD) is a key technique for the continued scaling of semiconductor devices, which increasingly relies on scalable processes for interface manipulation of structured surfaces on the atomic level. While ALD allows the synthesis of conformal films with utmost control over the thickness, atomically-defined closed coatings and surface modifications are challenging to achieve because of 3D growth during nucleation. Here, a route is presented toward the sub-nanometer thin and continuous aluminum oxide (AlOx) coatings on silicon substrates for the spatial control of the surface charge density and interface energetics. Trimethylaluminum in combination with remote hydrogen plasma is used instead of a gas-phase oxidant for the transformation of silicon dioxide (SiO2) into alumina. Depending on the number of ALD cycles, the SiO2 can be partially or fully transformed, which is exploited to deposit ultrathin AlOx layers in selected regions defined by lithographic patterning. The resulting patterned surfaces are characterized by lateral AlOx/SiO2 interfaces possessing 0.3 nm step heights and surface potential steps exceeding 0.4 V. In addition, the introduction of fixed negative charges of 9 × 1012 cm−2 enables modulation of the surface band bending, which is relevant to the field-effect passivation of silicon and low-impedance charge transfer across contact interfaces.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Atomic layer deposition (ALD) is a key technique for the continued scaling of semiconductor devices, which increasingly relies on scalable processes for interface manipulation of structured surfaces on the atomic level. While ALD allows the synthesis of conformal films with utmost control over the thickness, atomically-defined closed coatings and surface modifications are challenging to achieve because of 3D growth during nucleation. Here, a route is presented toward the sub-nanometer thin and continuous aluminum oxide (AlOx) coatings on silicon substrates for the spatial control of the surface charge density and interface energetics. Trimethylaluminum in combination with remote hydrogen plasma is used instead of a gas-phase oxidant for the transformation of silicon dioxide (SiO2) into alumina. Depending on the number of ALD cycles, the SiO2 can be partially or fully transformed, which is exploited to deposit ultrathin AlOx layers in selected regions defined by lithographic patterning. The resulting patterned surfaces are characterized by lateral AlOx/SiO2 interfaces possessing 0.3 nm step heights and surface potential steps exceeding 0.4 V. In addition, the introduction of fixed negative charges of 9 × 1012 cm−2 enables modulation of the surface band bending, which is relevant to the field-effect passivation of silicon and low-impedance charge transfer across contact interfaces.
9.
F Pantle, M Karlinger, S Wörle, F Becker, T Höldrich, E Sirotti, M Kraut, M Stutzmann
Crystal side facet-tuning of GaN nanowires and nanofins grown by molecular beam epitaxy Journal Article
In: Journal of Applied Physics, vol. 132, no. 18, pp. 184304, 2022.
@article{nokey,
title = {Crystal side facet-tuning of GaN nanowires and nanofins grown by molecular beam epitaxy},
author = {F Pantle and M Karlinger and S W\"{o}rle and F Becker and T H\"{o}ldrich and E Sirotti and M Kraut and M Stutzmann},
url = {https://aip.scitation.org/doi/abs/10.1063/5.0098016},
doi = {10.1063/5.0098016},
year = {2022},
date = {2022-10-22},
journal = {Journal of Applied Physics},
volume = {132},
number = {18},
pages = {184304},
abstract = {GaN nanostructures are promising for a broad range of applications due to their 3D structure, thereby exposing non-polar crystal surfaces. The nature of the exposed crystal facets, i.e., whether they are a-, m-plane, or of mixed orientation, impacts the stability and performance of GaN nanostructure-based devices. In this context, it is of great interest to control the formation of well-defined side facets. Here, we show that we can control the crystal facet formation at the nanowire sidewalls by tuning the III\textendashV ratio during selective area growth by molecular beam epitaxy. Especially, the N flux serves as a tool for controlling the growth kinetics. In addition, we demonstrate the growth of GaN nanofins with either a- or m-plane side facets. Based on our observations, we present the underlying nanostructure growth mechanisms. Low temperature photoluminescence measurements show a correlation of the formation of structural defects like stacking faults with the growth kinetics. This article demonstrates the controlled selective epitaxy of GaN nanostructures with defined crystal side facets on large-scale available AlN substrates.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
GaN nanostructures are promising for a broad range of applications due to their 3D structure, thereby exposing non-polar crystal surfaces. The nature of the exposed crystal facets, i.e., whether they are a-, m-plane, or of mixed orientation, impacts the stability and performance of GaN nanostructure-based devices. In this context, it is of great interest to control the formation of well-defined side facets. Here, we show that we can control the crystal facet formation at the nanowire sidewalls by tuning the III–V ratio during selective area growth by molecular beam epitaxy. Especially, the N flux serves as a tool for controlling the growth kinetics. In addition, we demonstrate the growth of GaN nanofins with either a- or m-plane side facets. Based on our observations, we present the underlying nanostructure growth mechanisms. Low temperature photoluminescence measurements show a correlation of the formation of structural defects like stacking faults with the growth kinetics. This article demonstrates the controlled selective epitaxy of GaN nanostructures with defined crystal side facets on large-scale available AlN substrates.
8.
M Kraut, E Sirotti, F Pantle, T Hoffmann, M Stutzmann
Band Gap Control and Properties of Indium–Zinc Oxynitride Thin Films Grown by Molecular Beam Epitaxy Journal Article
In: The Journal of Physical Chemistry C, vol. 126, no. 4, pp. 2070-2077, 2022, ISSN: 1932-7447.
@article{nokey,
title = {Band Gap Control and Properties of Indium\textendashZinc Oxynitride Thin Films Grown by Molecular Beam Epitaxy},
author = {M Kraut and E Sirotti and F Pantle and T Hoffmann and M Stutzmann},
url = {https://doi.org/10.1021/acs.jpcc.1c08630},
doi = {10.1021/acs.jpcc.1c08630},
issn = {1932-7447},
year = {2022},
date = {2022-01-19},
urldate = {2022-01-19},
journal = {The Journal of Physical Chemistry C},
volume = {126},
number = {4},
pages = {2070-2077},
abstract = {The material system of II\textendashIII oxynitride semiconductors has opened new prospects in solar energy harvesting and photocatalysis in recent years due to a tunable band gap and favorable band edge positions with respect to important redox levels. A promising member of this family is In\textendashZn\textendashO\textendashN (IZNO), as its band gap can be tailored to lower values compared to its better studied cousin Ga\textendashZn\textendashO\textendashN. We study IZNO thin films grown on sapphire substrates by molecular beam epitaxy (MBE) and investigate their structural, surface morphology, optical absorption, and photoemission characteristics. We investigate the influence of the constituting elements in the alloy on the position of valence band maxima, conduction band minima, and the structural properties. Through precise variation of the composition, samples with a band gap range between 1.0 and 2.6 eV have been deposited and analyzed. Based on our results, Zn and N have been identified to lower the energy of valence and conduction band edges with respect to the vacuum level, while In and O have the opposite effect. Structural characterization reveals that the samples are polycrystalline with grain sizes of about 30 nm, comprising a mixture of cubic and hexagonal crystal phases with distinct short-range disorder. While in the ternary compounds In\textendashO\textendashN and Zn\textendashO\textendashN metal\textendashoxide bonds are dominant, we elucidate the formation of metal\textendashoxynitride bonds in IZNO. Electrical and optical measurements reveal charge carrier concentrations of 1018\textendash1020 cm\textendash3 and absorption coefficients of 105 cm\textendash1 above about 2 eV excitation energy, accompanied by pronounced free carrier absorption found in samples in the upper carrier concentration range in the infrared energy region. Typical Urbach energies are 80\textendash220 meV, with no clear correlation with the elemental composition. By introducing MBE growth for IZNO, we overcome the limitations typically inflicted by the fabrication methods on stoichiometric InN:ZnO solid solutions and provide unprecedented access to new compounds in this material class.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The material system of II–III oxynitride semiconductors has opened new prospects in solar energy harvesting and photocatalysis in recent years due to a tunable band gap and favorable band edge positions with respect to important redox levels. A promising member of this family is In–Zn–O–N (IZNO), as its band gap can be tailored to lower values compared to its better studied cousin Ga–Zn–O–N. We study IZNO thin films grown on sapphire substrates by molecular beam epitaxy (MBE) and investigate their structural, surface morphology, optical absorption, and photoemission characteristics. We investigate the influence of the constituting elements in the alloy on the position of valence band maxima, conduction band minima, and the structural properties. Through precise variation of the composition, samples with a band gap range between 1.0 and 2.6 eV have been deposited and analyzed. Based on our results, Zn and N have been identified to lower the energy of valence and conduction band edges with respect to the vacuum level, while In and O have the opposite effect. Structural characterization reveals that the samples are polycrystalline with grain sizes of about 30 nm, comprising a mixture of cubic and hexagonal crystal phases with distinct short-range disorder. While in the ternary compounds In–O–N and Zn–O–N metal–oxide bonds are dominant, we elucidate the formation of metal–oxynitride bonds in IZNO. Electrical and optical measurements reveal charge carrier concentrations of 1018–1020 cm–3 and absorption coefficients of 105 cm–1 above about 2 eV excitation energy, accompanied by pronounced free carrier absorption found in samples in the upper carrier concentration range in the infrared energy region. Typical Urbach energies are 80–220 meV, with no clear correlation with the elemental composition. By introducing MBE growth for IZNO, we overcome the limitations typically inflicted by the fabrication methods on stoichiometric InN:ZnO solid solutions and provide unprecedented access to new compounds in this material class.
7.
J D Bartl, C Thomas, A Henning, M F Ober, G Savasci, B Yazdanshenas, P S Deimel, E Magnano, F Bondino, P Zeller, L Gregoratti, M Amati, C Paulus, F Allegretti, A Cattani-Scholz, J V Barth, C Ochsenfeld, B Nickel, I D Sharp, M Stutzmann, B Rieger
Modular Assembly of Vibrationally and Electronically Coupled Rhenium Bipyridine Carbonyl Complexes on Silicon Journal Article
In: Journal of the American Chemical Society, vol. 143, pp. 19505, 2021, ISSN: 0002-7863.
@article{nokey,
title = {Modular Assembly of Vibrationally and Electronically Coupled Rhenium Bipyridine Carbonyl Complexes on Silicon},
author = {J D Bartl and C Thomas and A Henning and M F Ober and G Savasci and B Yazdanshenas and P S Deimel and E Magnano and F Bondino and P Zeller and L Gregoratti and M Amati and C Paulus and F Allegretti and A Cattani-Scholz and J V Barth and C Ochsenfeld and B Nickel and I D Sharp and M Stutzmann and B Rieger},
url = {https://doi.org/10.1021/jacs.1c09061},
doi = {10.1021/jacs.1c09061},
issn = {0002-7863},
year = {2021},
date = {2021-11-12},
urldate = {2021-11-12},
journal = {Journal of the American Chemical Society},
volume = {143},
pages = {19505},
abstract = {Hybrid inorganic/organic heterointerfaces are promising systems for next-generation photocatalytic, photovoltaic, and chemical-sensing applications. Their performance relies strongly on the development of robust and reliable surface passivation and functionalization protocols with (sub)molecular control. The structure, stability, and chemistry of the semiconductor surface determine the functionality of the hybrid assembly. Generally, these modification schemes have to be laboriously developed to satisfy the specific chemical demands of the semiconductor surface. The implementation of a chemically independent, yet highly selective, standardized surface functionalization scheme, compatible with nanoelectronic device fabrication, is of utmost technological relevance. Here, we introduce a modular surface assembly (MSA) approach that allows the covalent anchoring of molecular transition-metal complexes with sub-nanometer precision on any solid material by combining atomic layer deposition (ALD) and selectively self-assembled monolayers of phosphonic acids. ALD, as an essential tool in semiconductor device fabrication, is used to grow conformal aluminum oxide activation coatings, down to sub-nanometer thicknesses, on silicon surfaces to enable a selective step-by-step layer assembly of rhenium(I) bipyridine tricarbonyl molecular complexes. The modular surface assembly of molecular complexes generates precisely structured spatial ensembles with strong intermolecular vibrational and electronic coupling, as demonstrated by infrared spectroscopy, photoluminescence, and X-ray photoelectron spectroscopy analysis. The structure of the MSA can be chosen to avoid electronic interactions with the semiconductor substrate to exclusively investigate the electronic interactions between the surface-immobilized molecular complexes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Hybrid inorganic/organic heterointerfaces are promising systems for next-generation photocatalytic, photovoltaic, and chemical-sensing applications. Their performance relies strongly on the development of robust and reliable surface passivation and functionalization protocols with (sub)molecular control. The structure, stability, and chemistry of the semiconductor surface determine the functionality of the hybrid assembly. Generally, these modification schemes have to be laboriously developed to satisfy the specific chemical demands of the semiconductor surface. The implementation of a chemically independent, yet highly selective, standardized surface functionalization scheme, compatible with nanoelectronic device fabrication, is of utmost technological relevance. Here, we introduce a modular surface assembly (MSA) approach that allows the covalent anchoring of molecular transition-metal complexes with sub-nanometer precision on any solid material by combining atomic layer deposition (ALD) and selectively self-assembled monolayers of phosphonic acids. ALD, as an essential tool in semiconductor device fabrication, is used to grow conformal aluminum oxide activation coatings, down to sub-nanometer thicknesses, on silicon surfaces to enable a selective step-by-step layer assembly of rhenium(I) bipyridine tricarbonyl molecular complexes. The modular surface assembly of molecular complexes generates precisely structured spatial ensembles with strong intermolecular vibrational and electronic coupling, as demonstrated by infrared spectroscopy, photoluminescence, and X-ray photoelectron spectroscopy analysis. The structure of the MSA can be chosen to avoid electronic interactions with the semiconductor substrate to exclusively investigate the electronic interactions between the surface-immobilized molecular complexes.
6.
M Kraut, F Pantle, S Wörle, E Sirotti, A Zeidler, F Eckmann, M Stutzmann
Influence of environmental conditions and surface treatments on the photoluminescence properties of GaN nanowires and nanofins Journal Article
In: Nanotechnology, 2021, ISSN: 0957-4484.
@article{nokey,
title = {Influence of environmental conditions and surface treatments on the photoluminescence properties of GaN nanowires and nanofins},
author = {M Kraut and F Pantle and S W\"{o}rle and E Sirotti and A Zeidler and F Eckmann and M Stutzmann},
issn = {0957-4484},
year = {2021},
date = {2021-08-16},
journal = {Nanotechnology},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
5.
A Henning, J D Bartl, A Zeidler, S Qian, O Bienek, C-M Jiang, C Paulus, B Rieger, M Stutzmann, I D Sharp
Aluminum Oxide at the Monolayer Limit via Oxidant-Free Plasma-Assisted Atomic Layer Deposition on GaN Journal Article
In: Advanced Functional Materials, vol. 31, no. 33, pp. 2101441, 2021, ISSN: 1616-301X.
@article{nokey,
title = {Aluminum Oxide at the Monolayer Limit via Oxidant-Free Plasma-Assisted Atomic Layer Deposition on GaN},
author = {A Henning and J D Bartl and A Zeidler and S Qian and O Bienek and C-M Jiang and C Paulus and B Rieger and M Stutzmann and I D Sharp},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202101441},
doi = {https://doi.org/10.1002/adfm.202101441},
issn = {1616-301X},
year = {2021},
date = {2021-06-12},
journal = {Advanced Functional Materials},
volume = {31},
number = {33},
pages = {2101441},
abstract = {Abstract Atomic layer deposition (ALD) is an essential tool in semiconductor device fabrication that allows the growth of ultrathin and conformal films to precisely form heterostructures and tune interface properties. The self-limiting nature of the chemical reactions during ALD provides excellent control over the layer thickness. However, in contrast to idealized growth models, it is challenging to create continuous monolayers by ALD because surface inhomogeneities and precursor steric interactions result in island growth. Thus, the ability to create closed monolayers by ALD would offer new opportunities for controlling interfacial charge and mass transport in semiconductor devices, as well as for tailoring surface chemistry. Here, encapsulation of c-plane gallium nitride (GaN) with ultimately thin (≈3 r{A}) aluminum oxide (AlOx) is reported, which is enabled by the partial conversion of the GaN surface oxide into AlOx using sequential exposure to trimethylaluminum (TMA) and hydrogen plasma. Introduction of monolayer AlOx decreases the work function and enhances reactivity with phosphonic acids under standard conditions, which results in self-assembled monolayers with densities approaching the theoretical limit. Given the high reactivity of TMA with surface oxides, the presented approach likely can be extended to other dielectrics and III\textendashV-based semiconductors, with relevance for applications in optoelectronics, chemical sensing, and (photo)electrocatalysis.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Atomic layer deposition (ALD) is an essential tool in semiconductor device fabrication that allows the growth of ultrathin and conformal films to precisely form heterostructures and tune interface properties. The self-limiting nature of the chemical reactions during ALD provides excellent control over the layer thickness. However, in contrast to idealized growth models, it is challenging to create continuous monolayers by ALD because surface inhomogeneities and precursor steric interactions result in island growth. Thus, the ability to create closed monolayers by ALD would offer new opportunities for controlling interfacial charge and mass transport in semiconductor devices, as well as for tailoring surface chemistry. Here, encapsulation of c-plane gallium nitride (GaN) with ultimately thin (≈3 Å) aluminum oxide (AlOx) is reported, which is enabled by the partial conversion of the GaN surface oxide into AlOx using sequential exposure to trimethylaluminum (TMA) and hydrogen plasma. Introduction of monolayer AlOx decreases the work function and enhances reactivity with phosphonic acids under standard conditions, which results in self-assembled monolayers with densities approaching the theoretical limit. Given the high reactivity of TMA with surface oxides, the presented approach likely can be extended to other dielectrics and III–V-based semiconductors, with relevance for applications in optoelectronics, chemical sensing, and (photo)electrocatalysis.
4.
F Pantle, F Becker, M Kraut, S Wörle, T Hoffmann, S Artmeier, M Stutzmann
Selective area growth of GaN nanowires and nanofins by molecular beam epitaxy on heteroepitaxial diamond (001) substrates Journal Article
In: Nanoscale Advances, vol. 3, no. 13, pp. 3835-3845, 2021.
@article{nokey,
title = {Selective area growth of GaN nanowires and nanofins by molecular beam epitaxy on heteroepitaxial diamond (001) substrates},
author = {F Pantle and F Becker and M Kraut and S W\"{o}rle and T Hoffmann and S Artmeier and M Stutzmann},
url = {http://dx.doi.org/10.1039/D1NA00221J},
doi = {10.1039/D1NA00221J},
year = {2021},
date = {2021-05-05},
journal = {Nanoscale Advances},
volume = {3},
number = {13},
pages = {3835-3845},
abstract = {GaN-on-diamond is a promising route towards reliable high-power transistor devices with outstanding performances due to better heat management, replacing common GaN-on-SiC technologies. Nevertheless, the implementation of GaN-on-diamond remains challenging. In this work, the selective area growth of GaN nanostructures on cost-efficient, large-scale available heteroepitaxial diamond (001) substrates by means of plasma-assisted molecular beam epitaxy is investigated. Additionally, we discuss the influence of an AlN buffer on the morphology of the GaN nanostructures. The nanowires and nanofins are characterized by a very high selectivity and controllable dimensions. Low temperature photoluminescence measurements are used to evaluate their structural quality. The growth of two GaN crystal domains, which are in-plane rotated against each other by 30°, is observed. The favoring of a certain domain is determined by the off-cut direction of the diamond substrates. By X-ray diffraction we show that the GaN nanostructures grow perpendicular to the diamond surface on off-cut diamond (001) substrates, which is in contrast to the growth on diamond (111), where the nanostructures are aligned with the substrate lattice. Polarity-selective wet chemical etching and Kelvin probe force microscopy reveal that the GaN nanostructures grow solely in the Ga-polar direction. This is a major advantage compared to the growth on diamond (111) and enables the application of GaN nanostructures on cost-efficient diamond for high-power/high-frequency applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
GaN-on-diamond is a promising route towards reliable high-power transistor devices with outstanding performances due to better heat management, replacing common GaN-on-SiC technologies. Nevertheless, the implementation of GaN-on-diamond remains challenging. In this work, the selective area growth of GaN nanostructures on cost-efficient, large-scale available heteroepitaxial diamond (001) substrates by means of plasma-assisted molecular beam epitaxy is investigated. Additionally, we discuss the influence of an AlN buffer on the morphology of the GaN nanostructures. The nanowires and nanofins are characterized by a very high selectivity and controllable dimensions. Low temperature photoluminescence measurements are used to evaluate their structural quality. The growth of two GaN crystal domains, which are in-plane rotated against each other by 30°, is observed. The favoring of a certain domain is determined by the off-cut direction of the diamond substrates. By X-ray diffraction we show that the GaN nanostructures grow perpendicular to the diamond surface on off-cut diamond (001) substrates, which is in contrast to the growth on diamond (111), where the nanostructures are aligned with the substrate lattice. Polarity-selective wet chemical etching and Kelvin probe force microscopy reveal that the GaN nanostructures grow solely in the Ga-polar direction. This is a major advantage compared to the growth on diamond (111) and enables the application of GaN nanostructures on cost-efficient diamond for high-power/high-frequency applications.
3.
F Pantle, F Becker, M Kraut, S Wörle, T Hoffmann, S Artmeier, M Stutzmann
Selective area growth of GaN nanowires and nanofins by molecular beam epitaxy on heteroepitaxial diamond (001) substrates Journal Article
In: Nanoscale Advances, vol. 3, no. 13, pp. 3835-3845, 2021.
@article{nokey,
title = {Selective area growth of GaN nanowires and nanofins by molecular beam epitaxy on heteroepitaxial diamond (001) substrates},
author = {F Pantle and F Becker and M Kraut and S W\"{o}rle and T Hoffmann and S Artmeier and M Stutzmann},
url = {http://dx.doi.org/10.1039/D1NA00221J},
doi = {10.1039/D1NA00221J},
year = {2021},
date = {2021-05-05},
journal = {Nanoscale Advances},
volume = {3},
number = {13},
pages = {3835-3845},
abstract = {GaN-on-diamond is a promising route towards reliable high-power transistor devices with outstanding performances due to better heat management, replacing common GaN-on-SiC technologies. Nevertheless, the implementation of GaN-on-diamond remains challenging. In this work, the selective area growth of GaN nanostructures on cost-efficient, large-scale available heteroepitaxial diamond (001) substrates by means of plasma-assisted molecular beam epitaxy is investigated. Additionally, we discuss the influence of an AlN buffer on the morphology of the GaN nanostructures. The nanowires and nanofins are characterized by a very high selectivity and controllable dimensions. Low temperature photoluminescence measurements are used to evaluate their structural quality. The growth of two GaN crystal domains, which are in-plane rotated against each other by 30°, is observed. The favoring of a certain domain is determined by the off-cut direction of the diamond substrates. By X-ray diffraction we show that the GaN nanostructures grow perpendicular to the diamond surface on off-cut diamond (001) substrates, which is in contrast to the growth on diamond (111), where the nanostructures are aligned with the substrate lattice. Polarity-selective wet chemical etching and Kelvin probe force microscopy reveal that the GaN nanostructures grow solely in the Ga-polar direction. This is a major advantage compared to the growth on diamond (111) and enables the application of GaN nanostructures on cost-efficient diamond for high-power/high-frequency applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
GaN-on-diamond is a promising route towards reliable high-power transistor devices with outstanding performances due to better heat management, replacing common GaN-on-SiC technologies. Nevertheless, the implementation of GaN-on-diamond remains challenging. In this work, the selective area growth of GaN nanostructures on cost-efficient, large-scale available heteroepitaxial diamond (001) substrates by means of plasma-assisted molecular beam epitaxy is investigated. Additionally, we discuss the influence of an AlN buffer on the morphology of the GaN nanostructures. The nanowires and nanofins are characterized by a very high selectivity and controllable dimensions. Low temperature photoluminescence measurements are used to evaluate their structural quality. The growth of two GaN crystal domains, which are in-plane rotated against each other by 30°, is observed. The favoring of a certain domain is determined by the off-cut direction of the diamond substrates. By X-ray diffraction we show that the GaN nanostructures grow perpendicular to the diamond surface on off-cut diamond (001) substrates, which is in contrast to the growth on diamond (111), where the nanostructures are aligned with the substrate lattice. Polarity-selective wet chemical etching and Kelvin probe force microscopy reveal that the GaN nanostructures grow solely in the Ga-polar direction. This is a major advantage compared to the growth on diamond (111) and enables the application of GaN nanostructures on cost-efficient diamond for high-power/high-frequency applications.
2.
M Kraut, E Sirotti, F Pantle, C M Jiang, G Grotzner, M Koch, L I Wagner, I D Sharp, M Stutzmann
Control of Band Gap and Band Edge Positions in Gallium-Zinc Oxynitride Grown by Molecular Beam Epitaxy Journal Article
In: Journal of Physical Chemistry C, vol. 124, no. 14, pp. 7668-7676, 2020, ISSN: 1932-7447.
@article{,
title = {Control of Band Gap and Band Edge Positions in Gallium-Zinc Oxynitride Grown by Molecular Beam Epitaxy},
author = {M Kraut and E Sirotti and F Pantle and C M Jiang and G Grotzner and M Koch and L I Wagner and I D Sharp and M Stutzmann},
url = {\<Go to ISI\>://WOS:000526331500009},
doi = {10.1021/acs.jpcc.0c00254},
issn = {1932-7447},
year = {2020},
date = {2020-04-09},
journal = {Journal of Physical Chemistry C},
volume = {124},
number = {14},
pages = {7668-7676},
abstract = {Gallium-zinc oxynitride (GZNO) is a promising material system for solar-driven overall water splitting, as it exhibits a tunable band gap in the visible range, beneficial positions of valence and conduction band edges, and promising long-term stability. Fabrication of GZNO is traditionally accomplished via a solid state reaction pathway. This limits the growth of thin films or large single crystals and the precise control of the composition, which complicates investigations about fundamental properties of the material, including, for example, the influence of the single constituent ratios on the band gap. In this work, we present the growth of GZNO thin films on sapphire by plasma-assisted molecular beam epitaxy (MBE). The thin films exhibit a crystallite size of up to 50 nm and a wurtzite crystal structure with distinct short-range disorder. Variations of Ga/Zn and N/O flux ratios are found to influence the optical absorption edge of the alloy without major impact on the Urbach energy. Controlled change of the composition of the alloy reveals that the band gap reduction is caused by both an increased valence band energy, which is correlated with the N content, and a decrease of the conduction band energy which is induced by increasing Zn content. Based on these findings, GZNO thin films with band gaps of down to 2.0 eV were fabricated and their photoelectrical properties assessed. Using MBE, we overcome compositional restrictions typically associated with stoichiometric GaN:ZnO solid solutions and provide unprecedented access to new compounds within this materials class. In doing so, we elucidate the specific role of individual elements on band edge energetics and demonstrate new routes to band gap engineering for future photocatalytic and photoelectrochemical applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Gallium-zinc oxynitride (GZNO) is a promising material system for solar-driven overall water splitting, as it exhibits a tunable band gap in the visible range, beneficial positions of valence and conduction band edges, and promising long-term stability. Fabrication of GZNO is traditionally accomplished via a solid state reaction pathway. This limits the growth of thin films or large single crystals and the precise control of the composition, which complicates investigations about fundamental properties of the material, including, for example, the influence of the single constituent ratios on the band gap. In this work, we present the growth of GZNO thin films on sapphire by plasma-assisted molecular beam epitaxy (MBE). The thin films exhibit a crystallite size of up to 50 nm and a wurtzite crystal structure with distinct short-range disorder. Variations of Ga/Zn and N/O flux ratios are found to influence the optical absorption edge of the alloy without major impact on the Urbach energy. Controlled change of the composition of the alloy reveals that the band gap reduction is caused by both an increased valence band energy, which is correlated with the N content, and a decrease of the conduction band energy which is induced by increasing Zn content. Based on these findings, GZNO thin films with band gaps of down to 2.0 eV were fabricated and their photoelectrical properties assessed. Using MBE, we overcome compositional restrictions typically associated with stoichiometric GaN:ZnO solid solutions and provide unprecedented access to new compounds within this materials class. In doing so, we elucidate the specific role of individual elements on band edge energetics and demonstrate new routes to band gap engineering for future photocatalytic and photoelectrochemical applications.
1.
S J Hao, M Hetzl, V F Kunzelmann, S Matich, Q L Sai, C T Xia, I D Sharp, M Stutzmann
Sub-bandgap optical spectroscopy of epitaxial beta-Ga2O3 thin films Journal Article
In: Applied Physics Letters, vol. 116, no. 9, 2020, ISSN: 0003-6951.
@article{,
title = {Sub-bandgap optical spectroscopy of epitaxial beta-Ga2O3 thin films},
author = {S J Hao and M Hetzl and V F Kunzelmann and S Matich and Q L Sai and C T Xia and I D Sharp and M Stutzmann},
url = {\<Go to ISI\>://WOS:000519225900001},
doi = {10.1063/1.5143393},
issn = {0003-6951},
year = {2020},
date = {2020-03-02},
journal = {Applied Physics Letters},
volume = {116},
number = {9},
abstract = {Room temperature sub-gap optical absorption spectra measured by photothermal deflection spectroscopy were investigated for hetero- and homo-epitaxial beta-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy as well as for a bulk crystal. The absorption spectra show a pronounced exponential Urbach tail with slope parameters of 120-150 meV in the spectral region between 4.5 and 5 eV, indicating an unusually large self-trapping energy of excitons. In addition, an absorption band related to deep defects is observed in the spectral region from 2.5 to 4.5 eV. The steepness of the Urbach tail as well as the strength of the defect-related absorption can be influenced and optimized by annealing at 900-1000 degrees C in an oxygen atmosphere. Similar features were also observed for bulk beta-Ga2O3 crystals and for homoepitaxial beta-Ga2O3 layers. The present results for beta-Ga2O3 are compared and discussed in the context of similar measurements for other wide-bandgap semiconductors of current interest in electronics and photocatalysis: GaN, ZnO, TiO2, and BiVO4.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Room temperature sub-gap optical absorption spectra measured by photothermal deflection spectroscopy were investigated for hetero- and homo-epitaxial beta-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy as well as for a bulk crystal. The absorption spectra show a pronounced exponential Urbach tail with slope parameters of 120-150 meV in the spectral region between 4.5 and 5 eV, indicating an unusually large self-trapping energy of excitons. In addition, an absorption band related to deep defects is observed in the spectral region from 2.5 to 4.5 eV. The steepness of the Urbach tail as well as the strength of the defect-related absorption can be influenced and optimized by annealing at 900-1000 degrees C in an oxygen atmosphere. Similar features were also observed for bulk beta-Ga2O3 crystals and for homoepitaxial beta-Ga2O3 layers. The present results for beta-Ga2O3 are compared and discussed in the context of similar measurements for other wide-bandgap semiconductors of current interest in electronics and photocatalysis: GaN, ZnO, TiO2, and BiVO4.