**Applications**

**Chapter 4**

**Provisional chapter**

**Nanowires for Room-Temperature Mid-Infrared**

**Nanowires for Room-Temperature Mid-Infrared** 

DOI: 10.5772/intechopen.79463

InAs-based nanowires hold a promise to offer transformational technologies for infrared photonic applications. Site-controlled InAs nanowire growth on low-cost Si substrates offers the practical integration advantages that silicon photonics benefits from. This includes the realisation of cheap photonic circuitries, light emitters and detectors that are otherwise expensive to realise with III/V material-based substrates. This chapter details the growth development of advanced faceted multi-quantum well structures within InAs nanowires using molecular beam epitaxy. We review the crystal structure for the faceted quantum wells along with an analysis of their optical emission characteristics which shows quantum confinement and localisation of the carriers on the quantum well nanostructure. This enables tuning of the emission wavelength and enhanced emission intensity up to the technologically important room-temperature operation point.

**Keywords:** indium arsenide, nanowires, multi-quantum wells, molecular beam epitaxy,

Developments that took place in the past few decades in the semiconductor industry have allowed the realisation of III–V one-dimensional (1D) structures [1], such as nanowires (NWs), that have attracted increasing attention as promising materials for the fabrication of midinfrared nanoscale devices [2]. III–V semiconductors NWs have many interesting physical and optical properties due to their narrow band gap [3], small electron effective mass [4], very high electron mobility [5], along with a great potential for realising nanoscale devices [1, 6, 7].

photoluminescence, infrared photonics, silicon photonics

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Emission**

**Emission**

Aiyeshah Alhodaib, Yasir J. Noori,

and Andrew R.J. Marshall

**Abstract**

**1. Introduction**

http://dx.doi.org/10.5772/intechopen.79463

Anthony Krier and Andrew R.J. Marshall

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Aiyeshah Alhodaib, Yasir J. Noori, Anthony Krier

#### **Nanowires for Room-Temperature Mid-Infrared Emission Nanowires for Room-Temperature Mid-Infrared Emission**

DOI: 10.5772/intechopen.79463

Aiyeshah Alhodaib, Yasir J. Noori, Anthony Krier and Andrew R.J. Marshall Aiyeshah Alhodaib, Yasir J. Noori, Anthony Krier and Andrew R.J. Marshall

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79463

#### **Abstract**

InAs-based nanowires hold a promise to offer transformational technologies for infrared photonic applications. Site-controlled InAs nanowire growth on low-cost Si substrates offers the practical integration advantages that silicon photonics benefits from. This includes the realisation of cheap photonic circuitries, light emitters and detectors that are otherwise expensive to realise with III/V material-based substrates. This chapter details the growth development of advanced faceted multi-quantum well structures within InAs nanowires using molecular beam epitaxy. We review the crystal structure for the faceted quantum wells along with an analysis of their optical emission characteristics which shows quantum confinement and localisation of the carriers on the quantum well nanostructure. This enables tuning of the emission wavelength and enhanced emission intensity up to the technologically important room-temperature operation point.

**Keywords:** indium arsenide, nanowires, multi-quantum wells, molecular beam epitaxy, photoluminescence, infrared photonics, silicon photonics

## **1. Introduction**

Developments that took place in the past few decades in the semiconductor industry have allowed the realisation of III–V one-dimensional (1D) structures [1], such as nanowires (NWs), that have attracted increasing attention as promising materials for the fabrication of midinfrared nanoscale devices [2]. III–V semiconductors NWs have many interesting physical and optical properties due to their narrow band gap [3], small electron effective mass [4], very high electron mobility [5], along with a great potential for realising nanoscale devices [1, 6, 7].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In recent years, silicon photonics attracted a significant research effort because of the potential benefits of integrating optoelectronics functions within silicon (Si) CMOS electronic devices [8]. The growth of InAs nanowires on low-cost Si substrates paves the way towards lowcost infrared detection technologies [7]. The unique geometry of NWs structures offers new silicon photonics architectures for sensing to be operating in the mid-infrared spectral range [9], hence improving the control over the size [10], higher flexibility in sample processes [11, 12] and more freedom for band-gap engineering [13]. The field of InAs-based NWs growth in particular has attracted many researchers interest and have been extensively investigated in order to fabricate nanoscale devices including field-effect transistors [14], solar cells [15], sensor applications [16], lasers [12] and photodetectors [17].

large-scale product, due to their high surface diffusion, fast growth rates and high throughput [28]. However, in comparison to MOCVD, solid-source MBE offers several advantages in low impurity incorporation due to the ultrahigh vacuum growth environment and the highly pure elemental growth species [26], the very accurate control over the composition of the deposited monolayer and their doping, and finally the ability to grow advanced radial and axial core-shell heterostructures via sophisticated in situ growth-monitoring methods [29].

The focus in earliest studies was on optimising the growth conditions, analyse or control the crystal structure for such wires. However, the crystal phase is particularly relevant to PL studies because the band gap and hence emission wavelength are phase dependent. In general, bulk InAs grown by conventional epitaxial techniques have a zinc-blende phase, hence its band gap (EgInAs (ZB)) is well characterised at 0.415 eV. However, by contrast, a consensus has not yet been reached around a single band gap for Wurtzite InAs (EgInAs (Wz)). While low-temperature studies build initial understanding, only a few optical emission studies have been reported, due to the very poor optical efficiency of these materials, very strong non-radiative surface and difficulties of performing spectroscopy in the IR spectral region [30]. For instance, the first low-temperature PL of InAs NWs was reported by Sun et al. [30] for InAs NWs on Si having both pure WZ and ZB crystal phases, with band energy (0.41–0.425 eV) corresponding to the above band edge surface state-related recombination, with a slight shift at increasing the temperature. They also noticed a blue shift due to quantum confinement depending on the wire diameter and not on the structure changes. Furthermore, Trägårdh et al. [31] predicted a WZ band gap of 0.54 eV from extrapolation-fitted photocurrent measurements at 5 K on

Möller et al. [32] reported temperature-dependent PL studies that enable them to estimate the wurtzite band gap to be 0.458 eV at low temperatures. Koblmuller et al. [11] have gone further to report PL temperature dependence of InAs NWs having a WZ phase structure and a peak position 0.411 eV at 15 K. Despite the strong PL emission at low temperatures, the emission was only reached 130 K before the signal was quenched [31]. The wires show a 25-meV blue shift due to confinement by reducing the wire diameters from 100 to 40 nm with respect to bulk InAs due to quantum confinement and dominant surface effect which limits PL efficiencies. The non-radiative recombination causing this quenching could originate from lattice defects, surface states or Auger recombination [31]. Recently, Rota et al. [33] estimated the energy gap for InAs WZ nanowires to be 0.477 eV, higher than the ZB band gap by 59 meV which does not depend on the nanowires size and carrier confinement. The spread of results that ranges from 0.41 to 0.54 eV may originate from polytypism, with a further complication being atmospheric absorption in the commensurate spectral range. However, while low-temperature PL measurements have supported initial studies confirming the crystal structure, emission at room temperature will be required for most practical applications. A common route to suppressing non-radiative recombination at the surfaces is the in situ growth of a wider band-gap shell which has been employed to InAs wires by Treu et al. [34] using an InAsP shell where they

times enhancement of the PL emission up to room temperature [35]. Also,

GaAs/AlGaAs core-shell NWs showed improved PL intensity compared to bulk GaAs NWs caused by the reduction in the surface states, which was found to be effective in enhancing the PL emission intensity, allowing it to persist up to room temperature. In addition to suppressing

segment of the composition 0.14 < x < 0.48.

Nanowires for Room-Temperature Mid-Infrared Emission

http://dx.doi.org/10.5772/intechopen.79463

57

single InAs NWs with a centrally placed InAs1−xP<sup>x</sup>

demonstrated 10<sup>2</sup>

Semiconductor NWs in nanotechnology can be synthesised through two main approaches, one is called top-down and the other bottom-up approaches [18]. The idea behind the topdown approach is to etch out and remove the crystal planes of the material from larger pieces which is usually already present on the substrate to form the nanowires. This approach mostly dominates in industry for large-scale fabrication. Few researches demonstrated successful production of NWs using this approach, for example, using InP and InGaAsP/InP materials [11, 19]. To the best of our knowledge, until today, there has not been any report of InAsSb fabricated using top-down approach; this approach shows to some extent the ability to be producing NWs for some III–V materials. However, this method suffers from drawbacks such as wire surface contamination or damages after etching treatment which hinders achieving an optimum optical performance. In the bottom-up approach, the nanostructures are built up on the substrate by adding atoms layer by layer in an ordered manner, offering the growth of a very high uniformity to the crystal structures, with a higher relative controllability in the NWs growth rate. There are two methods within the bottom-up fabrication of nanowire growth techniques: the template directed and the free-standing methods. Most of the reported NWs have been fabricated using the second approach by random or site-controlled growth.

The NWs growth mechanisms are usually governed by the famous vapour-liquid-solid (VLS) [20] or vapour solid (VS) mechanisms [21]. In the VLS approach, metal droplets are deposited on the growth substrate either through self-induced (e.g. In for InAs) or foreign metal catalyst (such as Au droplets) followed by subsequent nanowire synthesis. In the VS growth mechanism, the material starts in a vapour form inside the growth chamber and then the layers are deposited on top of the substrate epitaxially layer by layer in the solid phase, and such processes can be lithographically patterned or self-assembled. In the case of self-assembled, the NWs are grown randomly on the surface and have a variation of diameter and length. However, this kind of growth may lead to an unintentional kinking in the grown NWs [14]. Thus, controlling over position and size (diameter and length) of the NWs are required to fabricate efficient nanowire-based devices, hence allowing their applications to be realised at a large scale.

As more progress is made towards realising efficient optoelectronic devices, many methods are adapted for NWs growth, such as pulsed laser deposition [22], chemical beam epitaxy [23], metal organic chemical vapour deposition MOCVD [24] and molecular beam epitaxy (MBE) [25]. The most popular methods to grow NWs are (MOCVD) [26] and (MBE) [27]. MOCVD system and related gas phase techniques are used in the production of commercial large-scale product, due to their high surface diffusion, fast growth rates and high throughput [28]. However, in comparison to MOCVD, solid-source MBE offers several advantages in low impurity incorporation due to the ultrahigh vacuum growth environment and the highly pure elemental growth species [26], the very accurate control over the composition of the deposited monolayer and their doping, and finally the ability to grow advanced radial and axial core-shell heterostructures via sophisticated in situ growth-monitoring methods [29].

In recent years, silicon photonics attracted a significant research effort because of the potential benefits of integrating optoelectronics functions within silicon (Si) CMOS electronic devices [8]. The growth of InAs nanowires on low-cost Si substrates paves the way towards lowcost infrared detection technologies [7]. The unique geometry of NWs structures offers new silicon photonics architectures for sensing to be operating in the mid-infrared spectral range [9], hence improving the control over the size [10], higher flexibility in sample processes [11, 12] and more freedom for band-gap engineering [13]. The field of InAs-based NWs growth in particular has attracted many researchers interest and have been extensively investigated in order to fabricate nanoscale devices including field-effect transistors [14], solar cells [15],

Semiconductor NWs in nanotechnology can be synthesised through two main approaches, one is called top-down and the other bottom-up approaches [18]. The idea behind the topdown approach is to etch out and remove the crystal planes of the material from larger pieces which is usually already present on the substrate to form the nanowires. This approach mostly dominates in industry for large-scale fabrication. Few researches demonstrated successful production of NWs using this approach, for example, using InP and InGaAsP/InP materials [11, 19]. To the best of our knowledge, until today, there has not been any report of InAsSb fabricated using top-down approach; this approach shows to some extent the ability to be producing NWs for some III–V materials. However, this method suffers from drawbacks such as wire surface contamination or damages after etching treatment which hinders achieving an optimum optical performance. In the bottom-up approach, the nanostructures are built up on the substrate by adding atoms layer by layer in an ordered manner, offering the growth of a very high uniformity to the crystal structures, with a higher relative controllability in the NWs growth rate. There are two methods within the bottom-up fabrication of nanowire growth techniques: the template directed and the free-standing methods. Most of the reported NWs

have been fabricated using the second approach by random or site-controlled growth.

The NWs growth mechanisms are usually governed by the famous vapour-liquid-solid (VLS) [20] or vapour solid (VS) mechanisms [21]. In the VLS approach, metal droplets are deposited on the growth substrate either through self-induced (e.g. In for InAs) or foreign metal catalyst (such as Au droplets) followed by subsequent nanowire synthesis. In the VS growth mechanism, the material starts in a vapour form inside the growth chamber and then the layers are deposited on top of the substrate epitaxially layer by layer in the solid phase, and such processes can be lithographically patterned or self-assembled. In the case of self-assembled, the NWs are grown randomly on the surface and have a variation of diameter and length. However, this kind of growth may lead to an unintentional kinking in the grown NWs [14]. Thus, controlling over position and size (diameter and length) of the NWs are required to fabricate efficient nanowire-based devices, hence allowing their applications to be realised at a large scale.

As more progress is made towards realising efficient optoelectronic devices, many methods are adapted for NWs growth, such as pulsed laser deposition [22], chemical beam epitaxy [23], metal organic chemical vapour deposition MOCVD [24] and molecular beam epitaxy (MBE) [25]. The most popular methods to grow NWs are (MOCVD) [26] and (MBE) [27]. MOCVD system and related gas phase techniques are used in the production of commercial

sensor applications [16], lasers [12] and photodetectors [17].

56 Nanowires - Synthesis, Properties and Applications

The focus in earliest studies was on optimising the growth conditions, analyse or control the crystal structure for such wires. However, the crystal phase is particularly relevant to PL studies because the band gap and hence emission wavelength are phase dependent. In general, bulk InAs grown by conventional epitaxial techniques have a zinc-blende phase, hence its band gap (EgInAs (ZB)) is well characterised at 0.415 eV. However, by contrast, a consensus has not yet been reached around a single band gap for Wurtzite InAs (EgInAs (Wz)). While low-temperature studies build initial understanding, only a few optical emission studies have been reported, due to the very poor optical efficiency of these materials, very strong non-radiative surface and difficulties of performing spectroscopy in the IR spectral region [30]. For instance, the first low-temperature PL of InAs NWs was reported by Sun et al. [30] for InAs NWs on Si having both pure WZ and ZB crystal phases, with band energy (0.41–0.425 eV) corresponding to the above band edge surface state-related recombination, with a slight shift at increasing the temperature. They also noticed a blue shift due to quantum confinement depending on the wire diameter and not on the structure changes. Furthermore, Trägårdh et al. [31] predicted a WZ band gap of 0.54 eV from extrapolation-fitted photocurrent measurements at 5 K on single InAs NWs with a centrally placed InAs1−xP<sup>x</sup> segment of the composition 0.14 < x < 0.48. Möller et al. [32] reported temperature-dependent PL studies that enable them to estimate the wurtzite band gap to be 0.458 eV at low temperatures. Koblmuller et al. [11] have gone further to report PL temperature dependence of InAs NWs having a WZ phase structure and a peak position 0.411 eV at 15 K. Despite the strong PL emission at low temperatures, the emission was only reached 130 K before the signal was quenched [31]. The wires show a 25-meV blue shift due to confinement by reducing the wire diameters from 100 to 40 nm with respect to bulk InAs due to quantum confinement and dominant surface effect which limits PL efficiencies. The non-radiative recombination causing this quenching could originate from lattice defects, surface states or Auger recombination [31]. Recently, Rota et al. [33] estimated the energy gap for InAs WZ nanowires to be 0.477 eV, higher than the ZB band gap by 59 meV which does not depend on the nanowires size and carrier confinement. The spread of results that ranges from 0.41 to 0.54 eV may originate from polytypism, with a further complication being atmospheric absorption in the commensurate spectral range. However, while low-temperature PL measurements have supported initial studies confirming the crystal structure, emission at room temperature will be required for most practical applications. A common route to suppressing non-radiative recombination at the surfaces is the in situ growth of a wider band-gap shell which has been employed to InAs wires by Treu et al. [34] using an InAsP shell where they demonstrated 10<sup>2</sup> times enhancement of the PL emission up to room temperature [35]. Also, GaAs/AlGaAs core-shell NWs showed improved PL intensity compared to bulk GaAs NWs caused by the reduction in the surface states, which was found to be effective in enhancing the PL emission intensity, allowing it to persist up to room temperature. In addition to suppressing loss through non-radiative recombination, PL intensities and quenching temperatures can be increased by acting to raise the radiative recombination rate. In a very recent study, Jurczak et al. demonstrated a 10-fold enhancement of InAs NWs PL emission using an InP core-shell layer that passivates the surface states to reduce the rate of non-radiative recombination [35]. This research direction is attracting many researchers today in order to develop advanced optoelectronic devices and nanoscale photonic applications [13]. Progress in this direction will provide further insight into the optical emission and energy band-gap properties, hence improving the use of these materials, especially for infrared detectors and emitters. This chapter discusses the concept of developing novel InAsSb/InAs multi-quantum wells (MQWs) NWs on Si (111) substrate structures within InAs nanowires, as significant step towards viable nano- and quantum emitters in the extended IR wavelength range. We review the growth process for these structures, the crystal structural characterisation. Finally, we discuss their optical properties along with developing a band structure for the NWs of this material.
