Investigation of a nanostructured GaP/MoS₂ p-n heterojunction photodiode

Transition metal dichalcogenides (TMDCs) attract a much attention because of their versatile properties and promising potential for future electronics. One of them is molybdenum disulfide (MoS₂)—an n-type semiconductor—that exhibits a direct bandgap of 1.8 eV in the monolayer form and an indirect bandgap of 1.29 eV in the multilayer and bulk form.^1,2 Its unique properties encourage intensive research aimed at combining MoS₂ with other p-type semiconductors to form various heterojunctions and obtain more effective electron devices. MoS₂ has so far been used in many diverse applications, including photodetectors,^3,4 light emitting diodes (LEDs),^5,6 and field-effect transistors (FETs).⁷ 

It was demonstrated that substrates with nanostructures induced high-quality epitaxial growth of MoS₂ even if the deposited layered MoS₂ structures were largely mismatched. It was concluded that MoS₂ favored growth at the edges of nanostructures and edged MoS₂ nanostructures exhibited preferential active nucleation sites. Such nanostructures had strong broadband light absorption and optimized charge transport along the 2D vector plane.⁸ 

Previous studies reported also on PN junction diodes based on an n-type MoS₂/p-type WSe₂ heterostructure with vertically aligned atomic layers⁹ and on MoS₂/WS₂ heterostructure transistors that exhibited extraordinary electrical and optoelectronic properties, which far exceeded those of individual MoS₂ and WS₂ materials.¹⁰ Another option was to use a layered graphene structure for the preparation of a heterojunction between MoS₂ and graphene substrate. It was demonstrated that the MoS₂ bandgap increased with the decreasing thickness of the material, and MoS₂/graphene p-n junction diodes had I–V characteristics that were precisely tuned by adjusting the thicknesses of the MoS₂ films.^11,12 MoS₂ was also grown on silicon.¹³ Photovoltaic performance and optical and electrical properties of MoS₂/p-Si solar cells were correlated with the growth of thin MoS₂ films and the influence of MoS₂ microstructure. As the thickness of the films increased, their microstructure evolved from amorphous to microcrystalline. If they were thicker than 10 nm, their bandgap showed a maximum of 1.3 eV as a result of fewer band tail states in the films.¹⁴ The structures were prepared on substrates with a high-density of edges.

Another work focused on the formation of a heterostructure between a PtO₂ monolayer and MoS₂ by means of Van der Waals forces. With its valence-band maximum and conduction-band minimum separated in different layers, the PtO₂/MoS₂ heterostructure was identified as a type-II heterostructure with a strong adsorption of visible light.¹⁵ 

Recent papers kept bringing reports on the fascinating properties of MoS₂ and its great application potential in the field of photodetectors.^3,4,16,17 An excellent overview of different MoS₂ photodetector applications was presented in Ref. 18. However, the lack of MoS₂ with p-type conductivity has remained to be a significant problem. This drawback is usually overcome via heterojunctions, and a functional PN heterojunction between MoS₂ and another material is usually a sign that good quality technology was used.

We previously investigated the deposition of attractive materials for advanced applications, such as ZnO and MoS₂, on planar gallium phosphide (GaP) substrates and on GaP substrates structured with GaP nanocones (NCs).^19,20

As was mentioned above, MoS₂ material shows a tendency to favor edge growth. GaP NCs are very convenient for such type of growth thanks to the enormous number of nanosized edges in their sidewalls.¹⁹ 

This paper reports on the properties of GaP/MoS₂ heterojunctions that were prepared on such nanocone-structured GaP substrates and on reference planar GaP substrates.

The technology of experimental samples involved three stages: (1) growth of GaP NCs, (2) deposition of Mo layers on the nanocone-structured GaP and planar GaP substrates, and (3) turning the Mo layers into MoS₂ layers by sulfuration.

Nanocone growth: Dense GaP NC assemblages were grown at dense gold seeds on planar GaP substrates by metal organic vapor phase epitaxy (MOVPE) at optimized growth temperatures. The NCs were grown close to one another, and the resulting high-density GaP NC assemblage possessed a high-density of nanosized crystallographic facets, ledges, and edges.^19,20 GaP NCs were grown on p-type Zn doped (p = 1 × 10¹⁸ cm⁻³) GaP⟨111⟩B substrate by a combination of standard 2D growth and a vapor–liquid–solid (VLS) growth using 30 nm gold particles as seeds.

The growth was performed in an AIX 200 MOVPE low-pressure reactor at a temperature of 650 °C and pressure of 100 mbar. The NCs were doped with Zn to have p-type conductivity. They grew at a rate of about 120 nm min⁻¹ and reached between 1.5 and 3.5 µm in height depending on the growth duration. Their average diameter was between 600 and 1000 nm at the base and about 30 nm under the Au seed, which gave a ratio of ∼30.^21,22 Figure 1 presents a SEM side view image of high-density GaP NCs.

Molybdenum deposition: thin Mo films were sputtered on the nanocone-structured and reference planar GaP⟨111⟩B substrates by DC magnetron sputtering in an argon atmosphere (10⁻³ mbar) from a molybdenum target at room temperature. The films were 3–10 nm thick, as was determined using a quartz crystal balance within the sputtering equipment.

Molybdenum sulfuration: MoS₂ was prepared by sulfuration of the Mo layers. The process was performed at an optimized temperature and duration of 700 °C and 15 min in a furnace supplied with nitrogen. It was heated at a rate of 25 °C min⁻¹ to the target process temperature. A detailed Raman study was done to determine an appropriate amount of sulfur for the sulfuration of the Mo layers that was necessary to prevent the formation of other sulfur compounds, such as GaS and Ga₂S₃.

Photodiode devices were processed on GaP/MoS₂ heterostructures that were formed on (i) the nanocone-structured GaP substrates and (ii) reference planar GaP substrates. The photodiodes were prepared by providing the heterostructures with ohmic contacts based on indium. A 60 nm thick indium layer was blank evaporated onto the backsides of the GaP substrates and another 60 nm thick indium layer was deposited on top of MoS₂ through a shadow metallic mask with circular openings 200 μm in diameter. The spacing between the ohmic contact areas was 1 mm, and the photodiode area was close to 1 mm². The processing of the ohmic contacts was completed by annealing the In layers at 450 °C in a forming gas atmosphere.

Figure 2 shows Raman spectra excited at 514 nm by an Ar laser from the planar GaP substrate with and without a MoS₂ layer and from the nanocone-structured GaP substrate with and without a MoS₂ layer.

The figure reveals a dominant GaP transverse optic (TO) mode at 366 cm⁻¹ and a longitudinal optic (LO) mode at 404.9 cm⁻¹. Bulk (3D) MoS₂ exhibited E¹_2g and A¹_1g values of 383 and 408 cm⁻¹, respectively, and a 2D monolayer of MoS₂ showed values of 385 and 404 cm⁻¹, respectively.¹² Our samples exhibited MoS₂-related Raman shifts at around 383.5 and 408.3 cm⁻¹. The latter was overlapped by a longitudinal optical mode (LO) of GaP with a maximum of 404.9 cm⁻¹.

For comparison, the nanocone-structured GaP substrate showed a shift and narrowing of the Raman spectra for the longitudinal optic (LO) and transverse optic (TO) modes. The high frequency Raman line from the GaP NCs was downshifted by ∼1 cm⁻¹ relative to the LO phonon from bulk GaP. It was observed that narrow polar nanowires exhibited both size- and shape-related effects in Raman spectra. Such phenomena were observed for GaP nanowires²¹ and in high-density GaP NCs.¹⁹ A new observation is that a thin MoS₂ layer on top of the nanocone-structured GaP substrate did not change the position of the LO phonon but increased its intensity compared with that from the planar heterostructure. In addition, the measurement showed that the NCs played an important role in the quality of the MoS₂ layer. This is described in detail in Fig. 3.

Figure 3 compares Raman spectra, normalized to GaP (TO mode), from the GaP/MoS₂ heterostructures on the nanocone-structured GaP substrate and reference GaP substrates along with deconvoluted signals from the edgy nanocone-structured GaP/MoS₂ heterostructure. Both samples were prepared in the same sulfuration run. The MoS₂-related Raman signal from the GaP/MoS₂ heterostructure was significantly higher than the material that was prepared on the GaP substrate with the NCs. This supports an assumption that the growth of MoS₂ was encouraged at the GaP nanocone-structured surface with a high density of edges. The increase in the Raman signal indicated a better quality of the MoS₂ layer. This was very important for the formation of a GaP/MoS₂ heterojunction. The assumption was supported by electrical and photoelectrical characterization of the heterojunction diodes. Figure 3 shows that the frequency differences between the E¹_2g and A¹_1g Raman peaks of 24.8 cm⁻¹ are consistent with the previously reported values for MoS₂ layers of different thicknesses (>10ML to 10 nm).^17,23

Room-temperature current–voltage (I–V) characteristics of the diodes were measured using an Agilent 4155C semiconductor parameter analyzer in dark and under focused halogen white light. Figure 4 compares I–V curves of the GaP/MoS₂ heterojunctions on the planar and nanocone-structured GaP substrates. As the forward voltage was increased, the forward current seemed to increase exponentially. It started to increase markedly at turn-on voltages (V_on) of 1.3 and 0.7 V and a forward current of 1 µA for diodes on the planar and nanocone-structured GaP substrates, respectively. The rectifying ratio (I₊/I₋) of the diodes measured at ±1 V was about 11.4 for those on the planar substrate and 33 for those on the nanocone-structured GaP substrate.

According to the measured data, the diodes had an open-circuit voltage V_OC of ∼0.25 V with different values of short-circuit current for the GaP/MoS₂ heterostructures on the planar and nanocone-structured GaP substrates. The results show that the GaP/MoS₂ diodes prepared on the nanocone-structured GaP substrates had better properties compared with those on the planar structures.

Figure 5 shows the photocurrent of the diodes vs light intensity. It is evident that the GaP/MoS₂ diode prepared on the nanocone-structured GaP substrate exhibited a linear dependence of the photocurrent vs illumination intensity, while the planar diode showed a tendency for saturation at higher intensities. The diodes with NCs showed an ∼2.5 times higher photocurrent compared with the planar diodes. This feature was also confirmed with the spectral response measurement. It was done with a standard spectroscopy measurement set-up using a 100 W halogen lamp, high resolution monochromator, and lock-in technique in a short circuit arrangement to assure the linearity of the devices under study. The responsivity was calibrated at 650 nm and an illumination power of 12.5 µW at the diode area. Figure 6 presents the responsivity of the GaP/MoS₂ heterostructure diodes on the nanocone-structured GaP and planar substrates measured between 400 and 1100 nm. The spectrum exhibited two evidently separated regions between 400 and 550 nm, related to the generation of carriers in GaP, and between 550 and 1100 nm, linked with the generation of carriers in the MoS₂ layer. The significant peaks at 450 and 510 nm represent the generation of carriers because of direct and indirect transitions of the GaP bandgap (E_g = 2.75 and 2.21 eV).

GaP NCs that were grown at 650 °C had a large surface area and were confined to complex crystallographic facets and edges. We demonstrated that nanostructured surfaces composed of such NCs are appropriate for the deposition of active chemical materials, such as thin Mo layers that were sulfurated at 700 °C and transformed into MoS₂. The quality of the interface between GaP and MoS₂ formed by sulfuration led to the formation of an active PN heterojunction.

A p-GaP/n-MoS₂ heterostructure was prepared on planar and nanocone-structured GaP substrates for the first time. The heterostructure was characterized by electrical and optical measurement. The Raman scattering characterization confirmed that a MoS₂ layer was formed on both types of GaP substrate, and the current–voltage characterization showed that a PN junction was formed between the p-GaP and n-MoS₂ materials.

The GaP/MoS₂ heterostructure on the planar and nanocone-structured substrates exhibited turn-on voltages of 1.3 and 0.7 V at a forward current of 1 µA, respectively, and rectifying ratios measured at ±1 V of about 11.4 and 33, respectively. The planar heterostructure had a weaker photocurrent generation than the nanocone-structured one. The GaP/MoS₂ heterostructures on the planar and nanocone-structured substrates exhibited short circuit currents of 0.14 and 0.36 µA at an open circuit voltage of ∼0.25 V, respectively. This improvement of the diode properties can be attributed to a better quality of the heterojunction formation on the nanocone-structured substrate, which can be related to the action of the highly facetted and edged surface of the GaP NC assemblages. It was reported that edges played a similarly active role in the growth of MoS₂ on graphene, WSe₂, and silicon.

This research was supported by the Science and Technology Assistance Agency under Grant Nos. APVV-20-0264 and APVV-20-0437 and by VEGA Project No. 2/0104/17.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.