Indium phosphide nanowires and their applications in optoelectronic devices

Abstract

Group IIIA phosphide nanocrystalline semiconductors are of great interest among the important inorganic materials because of their large direct band gaps and fundamental physical properties. Their physical properties are exploited for various potential applications in high-speed digital circuits, microwave and optoelectronic devices. Compared to II–VI and I–VII semiconductors, the IIIA phosphides have a high degree of covalent bonding, a less ionic character and larger exciton diameters. In the present review, the work done on synthesis of III–V indium phosphide (InP) nanowires (NWs) using vapour- and solution-phase approaches has been discussed. Doping and core–shell structure formation of InP NWs and their sensitization using higher band gap semiconductor quantum dots is also reported. In the later section of this review, InP NW-polymer hybrid material is highlighted in view of its application as photodiodes. Lastly, a summary and several different perspectives on the use of InP NWs are discussed.

1. Introduction

Indium phosphide (InP) is an important III–V semiconductor, it exists in two crystalline forms wurtzite (WZ) and zinc blende (ZB) with direct band gaps of 1.42 and 1.35 eV at room temperature, respectively, and is a highly promising candidate for construction of viable nano-integrated circuits [1–6]. The nanoscale dimensions of InP in the radial direction exhibit size confinement effects that give novel physical properties to nano-sized InP as compared to bulk materials. One-dimensional geometry on the nanometre (10⁻⁹ m) scale provides an extremely high surface area with a nanoscale radius of curvature and great mechanical flexibility. These properties are advantageous in many chemical and mechanical applications. This review focuses on the synthesis methodologies of InP nanowires (NWs) and their applications in optoelectronic devices, such as solar cells, photodiodes, photodetectors, light-emitting diodes (LEDs) and field effect transistors (FETs). For instance, solar cells or photovoltaics (PVs) have been the subject of prime focus of research for the sake of enhancing the energy conversion efficiency with reduced material and fabrication costs in comparison to the bulk and thin-film PVs [7,8].

A plethora of work has been published recently on III–V NWs [7–9]. However, InP semiconductors among other III–V semiconductors are used in high power and high-frequency electronics because of high electron velocity when compared with other III–V semiconductors, for instance, GaAs. Moreover, InP has the longest lived optical phonons among the compounds with ZB crystal structure [10]. So, in this article we review the various synthesis methods of InP NWs and its use in optoelectronic devices.

In the past few decades, significant progress has been made in the growth of InP NWs in terms of improved uniformity, high yield of products and fine mono-dispersion. Generally, III–V semiconductor NWs have been fabricated using various vapour-phase techniques such as vapour–liquid–solid (VLS) growth [11–14], laser-assisted catalytic growth (LCG) [15–18], oxide-assisted growth [19], template-induced growth [20,21], thermal evaporation growth and the solution-phase approach [22,23].

Owing to a relatively large surface-to-volume ratio, the InP NWs suffer limitations on being defect-free. As a consequence, InP NWs have poor photoluminescence (PL) efficiency. Several attempts have been made to passivate these NWs by growing a shell of higher band gap semiconductor. The shell material plays the role of surface saturation of the dangling bonds resulting in drastic enhancement of PL efficiency [24].

Physical properties exhibited by InP NWs make them unique from their bulk counterparts. Hence the physical properties of InP NWs in combination with their large surface-to-volume ratio make them an interesting class of semiconductor nanomaterial suitable for applications such as single-photon detectors and high-speed electronic devices [6,25].

2. Synthesis of indium phosphide nanowires

InP NWs have been fabricated both by vapour- and solution-phase techniques. Vapour-phase techniques such as molecular beam epitaxy and metal oxide chemical vapour deposition (MOCVD) [26–34] generally produce NWs on the pre-designed substrates [35]. There exists a limit on the physical size of the substrate and the number of substrates that may be processed through a single batch. InP NWs synthesized using solution-phase approaches are relatively cost effective, require a minimum amount of substrate and are operative at low temperatures.

(a). Vapour–liquid–solid synthesis

VLS growth was first proposed by Wagner & Ellis [11], to explain the formation of silicon (Si) NWs which were grown at high temperature by reduction of Si tetrachloride (SiCl₄) in the presence of metal nanoparticles. An alloy is formed from a metal nanoparticle and reactant species (silicon atoms from the reduction reaction) when the VLS growth commences. The alloy fully melts into a liquid droplet at the eutectic composition. The liquid droplet then serves as a preferential site for the absorption of reactant species and upon super-saturation, reactant species nucleate in the liquid droplet. Incoming reactant species instead of forming another nucleus in the droplet condense onto the minute nuclei present in the droplet to minimize the total surface energy. Axial growth commences as more reactant species condense onto the growing nuclei and eventually a solid NW is formed. On cooling, the liquid droplet freezes into a solid and excess reactant species crystallizes out bridging the NW with solid metal nanoparticles.

Hui et al. [36] reported a facile method for the synthesis of high-density InP NWs on the amorphous substrate using gold nanoclusters as catalyst. NWs were grown via VLS–chemical vapour deposition. Hui et al. took a pre-cleaned substrate and thermally evaporated a 0.5 nm thin film of Au. Then a dual zone horizontal tube furnace was used to grow InP NWs. A boron nitride crucible containing solid InP powder was positioned in the up-stream zone, whereas the substrate was placed in the downstream zone. Pressure and temperature of 0.6 torr and 800°C was maintained for pre-annealing of Au-coated substrate (for around 10 min), in order to get gold nanoclusters that acted as a catalyst during the reaction. After that, vaporization of InP powder was done at around 750–770°C. The growth temperature for the substrate was set at 440–480°C. H₂ was used as a carrier gas. The growth was interrupted by turning off heaters thereby lowering the temperature to room temperature. It was inferred that the growth time and source temperature are the crucial parameters that dictate the morphology of InP NWs. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations were carried out to obtain morphology and crystal structure information and the results showed that InP NWs existed in a ZB crystal structure with significant twin defects and intermixed with a small amount of WZ phase but when configured as FETs they still exhibited excellent electrical performance.

In the VLS approach [11], growth of InP NWs using metal catalyst is mostly practised [37–39] but the catalyst may affect the device performance by incorporating into the NWs. Considering this drawback, scientists have tried to develop an alternative method for InP NWs synthesis by using selective-area metal-organic vapour-phase epitaxy (SA-MOVPE) in the absence of catalyst [40–44].

(b). Metal-organic vapour-phase epitaxy for indium phosphide nanowires growth

One of the first published experiments using the MOVPE process (also termed as metal-organic chemical vapour deposition (MOCVD)) was the growth of a thin film in 1969 [45]. At that time, the point of focus was to produce high purity and high-quality single-crystal materials for semiconductor devices. In the ideal case of epitaxial techniques for semiconductor NWs growth, the lattice structure of the underlying crystal, such as a substrate, is replicated in the growing layer. In MOVPE, metal-organic vapour precursor provide the constituent atoms of the layer. Trimethylindium (TMI) and tertiarybutylphosphine (TBP) are typically used as metal-organic precursors for the growth of InP NWs. Figure 1 presents the schematic of InP NWs growth by MOVPE.

Figure 1.

Schematic of InP NWs growth by MOVPE: (a) gold (Au) nanoparticles are deposited on substrate of choice, (b) precursors of In and P in vapour-phase are supplied and the Au nanoparticle alloys with the precursors, (c) nucleation occurs at the nanoparticle–substrate interface and (d) the NWs keep growing at the nanoparticle–substrate interfaceunless the supply of ingredients is stopped [45]. (Online version in colour.)

The main processes involved in MOVPE growth of InP NWs are: (i) the mass transport from the vapour-phase to the substrate surface, (ii) TMI and TBP decomposition, and (iii) indium and phosphorous atoms diffusion and adsorption on the substrate surface followed by desorption. One or more of these processes are the limiting factors during InP NWs growth. Hence different growth temperature regimes can be defined. VLS growth of InP NWs by MOVPE with [34] or without [46–48] metal catalyst particles typically takes place at temperatures well below 500°C. At these temperatures, growth is limited by reaction kinetics. The atoms can contribute to the growth of a single NW only if they are able to reach the liquid droplet at the NW tip by surface diffusion or by direct impingement [49]. Moreover, lower temperatures result in the incomplete decomposition of metal-organic molecules. For example, the decomposition temperatures of TBP and TMI molecules are 475°C and 325°C, respectively.

At a typical growth temperature like 350°C for catalyst-free InP NWs, the decomposition efficiency of TMI is highly dependent on temperature and most of the TBP molecules do not decompose at all [50]. However, if the real chemical catalytic action of metal nanoparticle catalysts is expected, the decomposition temperature of the metal-organic molecules can be considerably lower [51].

Bhunia et al. [31] carried out research work on vertically aligned and surface mounted InP NWs growth using gold nanoparticle-assisted MOVPE-VLS technique. Semiconducting InP wafers were used as a substrate over which the solution was spread. These InP wafers were then dried under vacuum and transferred to the MOVPE reactor containing TMI and TBP precursors (for indium and phosphorous precursors) with hydrogen as a carrier gas.

Pre-growth annealing was done at around 540°C. This annealing helped in the removal of initial native oxide on the surface and also in forming an initial gold-indium eutectic liquid, necessary for the VLS growth. This occurred when molten gold particles consumed indium from the substrate, at elevated temperate. In order to study the morphology of InP NWs grown through this method, SEM and TEM measurements were carried out.

Similar experiment was carried out by Mattila et al. [34] for the fabrication of InP NWs using MOVPE in the presence of colloidal gold nanoparticles as catalyst. The same precursors and conditions were followed as reported earlier [31,32,52]. For characterization, SEM was performed in order to find the NWs dependence on the growth temperature in terms of NWs orientation and dimensions. The SEM images showed that the size and shape of InP NWs was highly temperature dependent (figure 2) and TEM images suggested that the NWs grown at higher temperature exhibited smooth morphology and constant diameter. PL studies were also carried out which showed a blue shift of 80 meV in comparison to the substrate. X-ray diffraction (XRD) and TEM revealed the WZ crystal structure of InP NWs instead of ZB which resulted in the blue shift of the PL spectra. Mattila et al., followed this study with another investigation on InP NWs growth using catalyst-free MOVPE approach. The PL of the NWs peaked at 1.38 eV exhibiting a blue shift of approximately 30 meV from the bulk InP band gap (1.35 eV). Sulfur (S)-doped InP NWs were also fabricated by Iqbal et al. [53] using MOVPE. S-doping was obtained by using H₂S as S precursor and HCl was used in order to avoid radial growth. These NWs had an average diameter of approximately 100 nm and length of 1.6 μm. The authors cleverly designed the experiment to differentiate the PL of InP NWs from the InP substrate. The as-grown InP NWs on InP substrate were excited at 488 nm (2.54 eV) and the PL of the NWs PL peaked at photon energies of 1.35 and 1.49 eV suggesting that the InP substrate existed in ZB crystal structure and NWs existed in WZ crystal structure, respectively.

Figure 2.

SEM images of InP NWs taken at a tilt of 10° for samples grown at temperatures between450°C and 420°C. Reproduced with permission from [34].

Mohan et al. [43] reported the growth of core–multishell array of InP/InAs/InP NWs using a relatively new approach of VLS, i.e. SA-MOVPE, where no catalyst was used [40,54]. The core and shell consisted of InP NWs with strained InAs quantum well layer in between. TBP and TMI were used in conjunction with 5% arsine (AsH₃) and hydrogen. They consulted the already reported work for the conditions necessary for the InP core and InAs shell growth [42,43]. The experimental conditions were altered for the growth of the InP outer shell. SEM and TEM images showed that the epitaxially grown InP NWs were highly uniform, with vertical orientation and periodic alignment. The core–multishell NWs of Si/Ge [55] and GaN/InGaN [56,57] reported before the work of Mohan et al., were fabricated using two different approaches, i.e. catalyst-assisted VLS and vapour-phase deposition, whereas Mohan and co-workers made use of pure epitaxial catalyst-free method. Their work also revealed that the key factors affecting the radial or axial direction of InP NWs growth are: growth temperature and TBP partial pressure. Optimum conditions like growth temperature of 600°C and TBP partial pressure of 5.5×10⁻³ atm were found to favour the lateral growth of InP over InAs sidewall facets. The measurements were carried out confirming the formation of strained InAs quantum wells on InP sidewalls. The SEM measurements suggested that the grown nanostructures were highly uniform and well defined.

Goto et al. [58] reported the same method of catalyst-free SA-MOVPE for the formation of core–shell p-n junction InP NWs, keeping in view their application in photovoltaic devices, which will be discussed in the latter paragraphs of this review. The method involved electron beam lithography and wet chemical etching to form a 20 nm thin silicon dioxide (SiO₂) film with holes (having around 130 nm diameter) over the p-type InP substrate surface. Growth temperatures of 660°C and 600°C were used for p-type InP NW core and n-type InP NW shell, respectively. TMI, TBP and diethylzinc (DEZ) were used as precursors at specific partial pressures. For the fabrication of an n-type InP shell around the core, slightly higher partial pressure for TBP was used in order to promote lateral growth. For the thin-film growth outside the NWs array, secondary ion mass spectrometry analysis was performed, which showed the concentrations of Zn to be around 6×10¹⁸ atoms cm⁻³ in the p-type layer and Si to be 1×10¹⁸ atoms cm⁻³ in the n-type layer. Zn concentration of up to 2×10¹⁸ atoms cm⁻³ was also detected in the intrinsic layer, which was assumed to be due to the diffusion of Zn atoms from the p-type region. By varying the growth conditions without removing the substrate from the MOVPE chamber the crystal growth sequence was performed continuously.

Surface passivated core–shell InP NWs array solar cells fabrication using catalyst-free SA-MOVPE was reported by Yoshimura et al. [59]. They grew InP core–multishell NWs on SiO₂ masked InP substrate placed in the horizontal MOVPE system. TBP and TMI were as used P and In precursors and DEZ and monosilane (SiH₄) were taken as the p- and n-type dopants, respectively. Growth temperatures of 630°C and 470°C were maintained for p-type core and n-type InP shell, respectively. For the core InP NWs, the partial pressures of the TBP, TMI and DEZn were 6.5×10⁻⁵ atm, 2.7×10⁻⁶ atm and 8.6×10⁻⁸ atm and for the shell layers, the partial pressures of the TBP, TMI and SiH₄ were 4.9×10⁻⁴ atm, 2.7×10⁻⁶ atm and 1.3×10⁻⁷ atm, respectively. One of the advantages of using this approach is easy handling of the NW position, diameter and growth in both lateral and axial directions by controlling growth conditions and adjusting the mask design. SEM analysis revealed highly uniform growth of hexagonal InP core–multishell NWs, having diameter approximately 190 nm, on the masked substrate. Selective-area electron diffraction patterns indicated that the NWs grown by the aforementioned process has a single-crystalline WZ crystal structure without stacking faults.

(c). Solution–liquid–solid synthesis

In comparison to the aforementioned vapour-phase growth methods, solution-phase synthetic routes show various advantages of being low cost, convenient in handling, easy in composition control and operative at relatively low temperatures. These advantages make them a suitable candidate for large-scale synthesis of one-dimensional nanostructures [60]. Various synthetic routes based on controlled precipitation from homogeneous solutions come under the heading of ‘solution-based synthesis of NWs’. This includes hydrothermal or solvothermal approaches, solution-phase methods based on capping reagents, solution–liquid–solid (SLS) processes and low-temperature aqueous-solution methods.

Trentler et al. [60] for the first time introduced an SLS method for the growth of InP polycrystalline fibres as an analogy to the VLS mechanism. These InP fibres had the diameter of 20–50 nm and length up to several micrometres. A yield of 50–100% of InP fibres were obtained using either gallane or tri-tert-butylindane as a precursor and catalytic amounts (10 mol %) of the protic reagents like methanol, thiophenol or diethylamine. Powder XRD analysis, energy-dispersive X-ray (EDX) spectroscopy and TEM was carried out for characterization that confirmed the formation of InP fibres. By using monodisperse nanoparticles (NPs) of low-melting-point metals such as In or Ga as the catalytic seeds enabled them to synthesize highly crystalline InP, InAs and GaAs semiconductor NWs with controlled diameters [60]. The SLS method was further improved to grow the III–V (InP, InAs, GaAs) NWs in a controlled manner [61–67].

Tang et al. [68] developed an experimental procedure for the synthesis of InP NWs where InP reacted with In₂O₃ at high temperature. As a result of the reaction, vapours of indium and phosphorous were formed which were then transported to a low-temperature deposition area followed by recrystallization in the presence of oxide vapours. This whole procedure resulted in the formation of one-dimensional InP nanostructures. The diameter and morphology of InP NWs were observed to be highly dependent on two factors, i.e. deposition temperature and concentration of the oxide vapours. When excess oxide was used, InP nano-tubes were synthesized. Structural analysis of the synthesized NWs was carried out using XRD that furnished the information about crystal structure and phase purity of the products. Morphology was examined by SEM. Similarly, high-resolution field emission transmission electron microscopy (HRTEM) and EDX spectroscopy were used for the elemental composition analysis of the one-dimensional InP nanostructures. The SEM images of InP NWs and EDX are presented in figure 3 and HRTEM images are displayed in figure 4.

Figure 3.

SEM images from a low-temperature furnacearea deposit (a); from the high-temperature furnace area deposit with different InP/In₂O₃ starting weight ratios of (b) 10 : 1, (c) 10 : 2, (d) 10 : 4, (e) elemental analysis of InP through EDX. Reproduced with permission from [68].

Figure 4.

(a) Low-magnification TEM image of InP NWs; the insets are the magnified images ofthe marked regions, (b) high-resolution TEM image of a nearly perfect InP NW and the inset shows the selected area electron diffraction pattern, which is indexed as [1 1 0] zone axis pattern, (c) high-resolution TEM image from the curved InP NWs exhibiting some planar defect. Reproduced with permission from [68].

A more facile route for the fabrication of InP NWs has been presented by Liu et al. [69]. In this method, commercially available indium precursor, i.e. trimethylindium In(CH₃)₃ and more common solvents/surfactants such as 1-octadecene (ODE) and myristic acid (MA) were used. Reaction of mixed solution of tris-trimethylsilylphosphine [P(TMS)₃] and In(CH₃)₃ into a hot ODE containing myristic acid resulted in the formation of InP NWs as flocculent precipitates. These flocculent precipitates were then re-dispersed in toluene forming a light brown suspension that was stable enough not to settle down even after weeks. This characteristic indicated the NWs being surface capped by the surfactant due to high stability. Their solubility in organic solvents helped in NW characterizations and related applications. TEM studies revealed that the NWs yield and purity is highly dependent on the molar ratio of In(CH₃)₃, MA and P(TMS)₃. An optimum yield was obtained when the MA/In/P ratio taken was 3 : 1.5 : 1. Electronic transport properties of these NWs were also investigated in this work. The current–voltage (I–V) measurements and room temperature resistance investigations indicated the presence of very low native defects in InP NWs. Hence, these were found to be potentially promising building blocks for the optoelectronic devices.

Up until 2009, the SLS growth of InP NWs was primarily focused on the use of phosphines or organo-indium complexes containing one or more legating phosphines as precursors [70–74]. The most commonly used phosphine precursor was tris-trimethylsilylphosphine that is intrinsically highly toxic and pyrolytic that is detrimental to the process of scaling up preparation and subsequent mass production of InP NWs essential for economic device fabrication. Therefore, a non-phosphine-based phosphorus precursor is an attractive choice and less hazardous to handle. For example, yellow phosphorus has been used with success, to produce InP NWs via PH₃ as an intermediate in an alkaline and pressurized environment, in a surfactant-templated system [75].

Lim et al. [76] proposed an SLS mechanism using a non-pyrolytic non-phosphine-based phosphorus source with lower toxicity. They treated the solution of PBr₅ in hexadecane with lithium borohydride (LiBH₄) resulting in the formation of yellow precipitates. These yellow precipitates were similar to a solid hydrogen phosphide (PH)_x isolated by Wiberg & Müller-Schiedmayer [77] from the reaction of PBr₃ with LiH in ether as a solvent and more suitable for scaled-up synthesis. Advantageously PBr₅ is less hazardous and is also cost effective in comparison to tristrimethylsilyl phosphine {P(TMS)₃} that makes it even more attractive as a phosphorus precursor. Lim and his co-workers were successful in preparing highly crystalline InP NWs by treating InCl₃ and PBr₅ with LiBH₄ in the presence of pre-formed indium or bismuth metal nanoparticles as seeds to facilitate SLS growth. The crystallinity of InP NWs was confirmed by TEM and electron diffraction patterns.

(d). Indium phosphide nanowires synthesis by surfactant-free solution–liquid–solid approach

Banerjee et al. [78] grew one-dimensional InP NWs using a hot injection SLS technique without surfactant and protic additives. A single-molecule precursor was used [(PhCH₂)₂InP(SiMe₃)₂]₂, which was prepared by the reaction of indium trichloride (InCl₃), benzyl Grignard and P(SiMe₃)₃. The thermolysis of the precursor on its injection into the hot myristate led to the reduction of In(III) to In(0) which gave nanosized metal droplets and formed InP nanofibres (having diameter of 85–95 nm) from the surface of molten In metal droplets (100 nm diameter). It was deduced that the precursor injection temperature is the key factor involved in the NW length rather than the growth temperature. The minimum injection temperature used in this experiment was 210°C and no NWs formation was observed at temperature lower than 210°C, whereas with increasing temperature, shorter length NWs were formed. Furthermore, the work by Banerjee et al. was in stark contrast to the observations made by Liu et al. [69] according to which myristic acid was the key factor in NWs growth and its absence caused formation of NPs instead. Whereas according to Banerjee et al. the difference in morphology might be due to the usage of single-molecule precursor which provided different paths to InP formation. XRD, HRTEM and electron diffraction studies were carried out to study the InP NW morphology.

(e). Indium phosphide nanowires synthesis via solvothermal process

Zhao et al. [79] were successful in synthesizing branched InP NWs with single-phase crystalline and twinning structures by a solvothermal process using white phosphorus and indium powder as reactants. The reaction was carried out at 180°C in benzene and cetyltrimethylammonium bromide (CTAB) was used as a cationic surfactant. In a typical solvothermal process, yellow P (0.003 mol), indium powder (0.002 mol) and CTAB (0.002 mol) were put into a stainless steel autoclave having a capacity of 40 ml. The autoclave was further filled with benzene up to 62% of the total volume and after sealing it was maintained at 180°C for 12 h. Then it was further heated to 380°C for 6 h in a furnace and afterwards allowed to cool down to room temperature. The samples were collected, washed with dimethylbenzene, absolute ethanol, dilute HCl and distilled H₂O various times to remove the impurities. Finally, the samples were dried in vacuum at 60°C for 8 h. They studied the effect of temperature and surfactant on the morphology of NWs. XRD studies were conducted which revealed the formation of a cubic ZB crystal structure of InP NWs with lattice constant of 5.85  ˚A. On the basis of their findings, they proposed a possible mechanism of NWs growth that is presented in figure 5.

Figure 5.

Pictorial illustration of CTAB-assisted growth of InP NWs [79]. (Online version in colour.)

(f). Synthesis of indium phosphide nanowires via laser-assisted catalytic growth

InP NWs were also grown using the LCG process [18] reported previously [15,16,19,80,81]. A poly-l-lysine (0.1%) functionalized silicon substrate was used for the growth of InP NWs. Dilute solution (up to 10¹⁰–10¹¹ particles ml⁻¹) of gold nanoclusters were dispersed onto the substrate which was then rinsed quickly with water. Atomic force microscopy was performed before placing the substrate in the furnace. Images showed a smooth substrate surface with no aggregation of nanoclusters. The substrates were placed in a quartz tube at the downstream end of the furnace, whereas InP solid was placed at the upstream end, approximately 3–4 cm outside the furnace, where room temperature was maintained in order to avoid thermal evaporation. The furnace heating temperature was around 650–700°C and InP target was ablated for several minutes with an ArF excimer laser (100 mJ pulse⁻¹, 10 Hz). Once the furnace was cooled down, the composition of the individual NWs were also assessed using energy-dispersive X-ray analysis (EDAX). Field emission scanning electron microscopy (FE-SEM) and TEM of the samples were carried out, which helped to estimate the synthesized NWs diameter and length. The LCG process can also help to control the diameter of the NWs. By decreasing the diameter of InP NWs their PL spectra showed a blue shift as a consequence of radial quantum confinement effect for the NWs having diameters less than 15 nm [82].

3. Applications of indium phosphide nanowires

In almost every single step of photo-conversion in optoelectronic devices, the NWs geometry offers various potential advantages like reduced reflection, extreme light trapping, facile strain relaxation, improved band gap tuning and increased defect tolerance [83] over the wafer-based or thin-film-based solar cells. Maximum efficiency of the solar cell is not enhanced above standard limits by exploiting the NWs, rather they result in reduction of the quantity of material necessary to approach those limits, hence making them cost effective. Additionally, in the current photovoltaic technology, complex single-crystalline semiconductor devices can be fabricated directly on low-cost substrates and electrodes such as aluminium foil, stainless steel and conductive glass, using these NWs [83].

(a). Indium phosphide nanowire-based solar cells

Photovoltaic device fabrication using core–shell p-n junction InP NWs array was reported by Goto et al. [58]. The method for the synthesis of NWs has been discussed in §2b. After the NWs were grown using SA-MOVPE, cyclotene resin was used as a transparent electrical resin to fill the spaces between NWs via spin coating. Tips of the NWs were exposed by removing overlaid excess resin through reactive ion etching which was followed by sputtering of transparent indium tin oxide (ITO) film electrode, at room temperature, on the NWs array and then heated at 400°C to reduce ITO resistance. A silver electrode having a comb-shaped structure was also formed on the transparent ITO electrode. Au–Zn alloy was used to form the backside electrode of the substrate. Various measurements of open circuit voltage, short-circuit current and fill factor indicated a solar power conversion efficiency of 3.37% under AM1.5G illumination.

Yoshimura et al. [59] put forward an idea of higher energy efficiency NW-based PVs using a passivation technique, by using core–multishell InP NWs (grown using the catalyst-free SA-MOVPE method) for solar cell fabrication. Reflectance measurements carried out for these core–multishell NWs, which confirmed enhanced light absorption thereby increasing the short-circuit current density and elevating the energy conversion efficiency by 6.35% under AM1.5G illumination. Recently, Wallentin et al. [84] fabricated millimetre-sized p-i-n doped InP NWs array solar cells. The diameter and the length of the NWs were 180 nm and 1.5 μm, respectively. The p-i-n doped InP NW-based solar cells exhibited the record efficiency of 13.8% with short-circuit current (J_sc), open circuit voltage (V _oc) and fill factor (FF) 24.6 mA cm⁻², 0.779 V and 72.4%, respectively. The schematic of millimetre-sized InP NWs solar cell is presented in figure 6.

Figure 6.

Pictorial representation of InP NWs array solar cell [84]. (Online version in colour.)

(b). Indium phosphide nanowires-polymer hybrid as photodiode

In order to increase the carrier collection efficiency and to eliminate the need for expensive substrate, Novotny et al. [85] designed a InP NW-polymer photodiode in which they grew n-type InP NWs directly on the ITO electrode, as displayed in figure 7. Experimental analysis revealed the ohmic contact between InP NWs and ITO electrode. These NWs were then coated with a high hole mobility polymer [poly(3-hexylthiophene)]. The inclusion of InP NWs enhanced the forward-biased current by six to seven orders of magnitude in comparison to the polymer-only device. Further studies showed the high rectification of these photodiodes with a low ideality factor of 1.31. Photo-response of this hybrid device is a promising alternative to polymer solar cells and photodetectors technology [7–9].

Figure 7.

Schematic of an NW-polymer hybrid device [85]. (Online version in colour.)

(c). Indium phosphide nanowires as photodetector

Wang et al. [6] carried out the optical investigation of individual free-standing InP NWs synthesized using the LCG technique [15,16,19,79,80]. PL measurements of these InP NWs revealed polarization anisotropy, which formed the basis of photoconductivity (PC)-based photodetectors, where individual InP NWs act as the device element. Furthermore, the nano-sized photodetector device element fabricated by Wang et al. is smaller than the previously reported [86,87] polarization-sensitive quantum well-based detectors, which are often sensitive to only monochromatic light. By contrast, the NWs photodetectors reported in Wang’s work can be exploited to create high-density optical interconnects, as optically gated switches and photonic-based devices where bandwidth can be widely increased via polarization detection.

(d). Indium phosphide nanowires as light-emitting diodes

Gudiksen et al. [5] investigated the use of single InP NW p±n junctions as nanoscale LEDs. A schematic of InP NW LED is shown in figure 8. Individual InP NW devices, in forward bias, exhibited highly polarized light emission from p±n junctions due to the one-dimensional structure of NWs that is blue-shifted due to radial quantum confinement effect. These NW LEDs had an efficiency of 0.1% that can substantially be increased further. They further suggested that it can be possible to engineer an electronically driven single-photon source with well-defined polarization by the introduction of quantum dot hetero-structure during NW synthesis within a p±n diode. Such an NW-based device could be extremely useful in quantum cryptography and information processing [88].

Figure 8.

Schematic illustration of InP NW LED [5]. (Online version in colour.)

(e). Indium phosphide nanowires as resonant tunnelling diodes

Bjork et al. [39] carried out I–V characterization of one-dimensional InAs–InP hetero-structures (figure 9) and observed an interesting nonlinear behaviour which resulted due to double barrier resonant tunnelling [1]. This suggested the transport phenomena in super lattice NWs and the possibility of controlling the electronic band structure by carefully selecting the constituent materials. This kind of new architecture is attractive especially for thermoelectric applications [1], as the lattice thermal conductivity can be reduced by the interfaces present between NWs via blocking the phonon conduction along the wire axis, whereas the electrical conduction may be sustained from the unusual electronic band structures due to the periodic potential perturbation.

Figure 9.

Composition profile of InAs nanowhisker containing InP hetero-structures [38]. (Online version in colour.)

(f). Field effect transistors

Leiber and co-workers [25] used InP NWs to assemble some functional nanoscale devices in which they exploited NWs electrical properties which were controlled through selective doping. These doped NWs function as nanoscale FETs, and can be assembled into crossed-wire p±n junctions that exhibit rectifying behaviour. Later, Jiang et al. [89] reported the use of radial core–shell InAs–InP NW in FETs. The InP shell segment (figure 10) provided the band off-set of approximately 0.52 eV that provided a good confinement potential for electrons also for thermally generated holes. They grew InP shells of thickness 2–3 nm around an InAs core and electrical measurements were carried out at room temperature. The core–shell NWs FETs showed improved trans-conductance and on-current in comparison to the simple InAs NWs FETs.

Figure 10.

Cross-sectional representation of an InAs–InP core–shell NW and band diagram [89].

Shen et al. [90] reported a single-step thermochemical method for the synthesis of pearl-like ZnS-decorated InP NWs. These ZnS-sensitized InP NWs were then used in the fabrication of FETs, which exhibited p-type transistor performance and a decent response to UV-light exposure. Thermal activation behaviour was also observed for these NWs in response to their electronic transport properties, which were investigated at different temperatures.

4. Conclusion

In this review article, we have presented the various synthesis routes of InP NWs. InP NWs have been synthesized by both vapour- and solution-phase routes, over the years and researchers are trying to bring innovations in order to use InP NWs in devices more effectively. We have also presented a number of characterization techniques, for instance, SEM, TEM, XRD and PL. The SEM furnished the information about the morphology of the NWs, while TEM and XRD provided information about the crystal structure and defects in the NWs. The PL gave information regarding the band gap and carriers (electron and hole) transport in InP NWs. Owing to their good optoelectronic properties, InP NWs has resulted in a new paradigm for the fabrication of many advanced devices like photodiodes, photodetectors, thermo-electrical devices, FETs, LEDs and solar cells.

Authors' contributions

All the authors contributed equally to this work.

Competing interests

The authors declare no competing interest.

Funding

This study was supported by the Higher Education Commission (HEC) of Pakistan, equipment/ research grants (PD-IPFP/HRD/HEC/2013/1983) and (20-3071/NRPU/R&D/HEC/13).