Two-Dimensional Platinum Diselenide Waveguide-Integrated Infrared Photodetectors.

Low-cost, easily integrable photodetectors (PDs) for silicon (Si) photonics are still a bottleneck for photonic-integrated circuits (PICs), especially for wavelengths above 1.8 μm. Multilayered platinum diselenide (PtSe2) is a semi-metallic two-dimensional (2D) material that can be synthesized below 450 °C. We integrate PtSe2-based PDs directly by conformal growth on Si waveguides. The PDs operate at 1550 nm wavelength with a maximum responsivity of 11 mA/W and response times below 8.4 μs. Fourier-transform IR spectroscopy in the wavelength range from 1.25 to 28 μm indicates the suitability of PtSe2 for PDs far into the IR wavelength range. Our PtSe2 PDs integrated by direct growth outperform PtSe2 PDs manufactured by standard 2D layer transfer. The combination of IR responsivity, chemical stability, selective and conformal growth at low temperatures, and the potential for high carrier mobility makes PtSe2 an attractive 2D material for optoelectronics and PICs.

Introduction

Photonic-integrated circuits (PICs) are maturing as a platform for applications in telecommunications, spectroscopy, diagnostics, biomedical imaging, and gas sensing.[1] PICs tailored for these applications typically rely on infrared (IR) photodetectors (PDs) that are coupled to waveguides and that function as essential components for optoelectronic signal conversion. PDs for the near-IR wavelength range are mainly based on widely used semiconductors such as Si[2] and Ge.[3] Mid-IR PDs are typically made from compound semiconductors such as InGaAs[4] and HgCdTe,[5] with the downside of higher manufacturing cost and the necessity of cryogenic temperature operating conditions. However, a key requirement toward the broad applicability of PICs is the integrability of IR PDs on photonic waveguides. This is not fulfilled by commercial manufacturing methods for the conventional materials because their high deposition and (epitaxial) growth temperatures are not compatible with PIC thermal budgets. This creates a demand for IR PDs with high manufacturability at low-temperature budgets.

Two-dimensional (2D) materials have attracted tremendous attention in the optoelectronic field due to their broadband optical absorption, high carrier mobility, mechanical flexibility, and ease of integration.[6−10] Quantum confinement in 2D materials in the direction perpendicular to their 2D plane leads to novel physical properties that distinguish them from their bulk materials.[11] A wide range of devices such as broadband PDs,[12] modulators,[13,14] lasers,[15] light-emitting diodes,[16] phototransistors,[17−19] and avalanche photodiodes[20] have been demonstrated to emphasize the advantages of 2D materials for optoelectronics.

In particular, the zero-bandgap material graphene has been used to demonstrate high-performance integrated PDs and modulators in the IR region.[21−25] However, graphene requires a layer-transfer process[26,27] because its growth is limited to a few substrate materials and requires high temperatures. 2D black phosphorus is also suitable as an IR PD[28,29] material that can be transferred onto waveguides[30,31] but has the drawback that it is not entirely stable under ambient conditions.[32] Platinum diselenide (PtSe2) is a transition metal dichalcogenide (TMD) with an octahedral lattice structure that is semiconducting with an indirect bandgap of 1.2 eV as a monolayer. As a multilayer material, it becomes semi-metallic.[33] This semi-metallic nature of layered PtSe2 allows its use for broadband IR photodetection in a similar way to graphene.[34−36] In addition, PtSe2 is stable in air and has a high charge carrier mobility compared to other TMDs, with theoretically predicted values of more than 1000 cm2/Vs,[37,38] and has a piezoresistive gauge factor of up to −85.[39−41] A major advantage of PtSe2 is the possibility of direct, large-scale growth on various substrates at temperatures below 450 °C using the thermally assisted conversion (TAC) technique.[34,40,42,43] In combination with the potential for selective and conformal deposition,[44] PtSe2 is suitable for back-end-of-line (BEOL) integration on electronic and photonic wafers.

In this work, we demonstrate the use of layered PtSe2 as an integrated IR PD on Si photonic waveguides. We compare the device performance and material quality of a direct growth integration approach with a wet layer-transfer technique through analytical, electrical, and optical characterization.

Experimental Section

The PtSe2 PDs were realized on rib waveguides with a 50 nm step height on silicon-on-insulator (SOI) substrates. Optical access is provided through two grating couplers that are optimized for a wavelength of 1550 nm and transverse electric (TE) polarization. We fabricated two sets of samples: one using direct growth and another using a wet transfer method of the PtSe2 layers. For the first set, a 10.8 nm thick layer of prepatterned sputtered platinum (Pt) was converted into a 27 nm thick layered film of PtSe2 directly on the waveguides using TAC.[34] Pt is a highly reactive material. A thin barrier layer of Al2O3 was therefore deposited with atomic layer deposition (ALD) to protect the waveguides and grating couplers from the reaction of Pt with the top Si photonic layer during the TAC growth. For the second set of samples, PtSe2 films of 7.6, 13.7, and 23.5 nm thickness were grown by TAC on separate silicon substrates with a 90 nm thermal silicon oxide (SiO2) layer and were wet-transferred onto the Si waveguides (see Methods for details). The thicknesses of the PtSe2 films were measured by atomic force microscopy (AFM) (Supporting Information). In both sample sets, the PtSe2 patches came in contact with nickel/aluminum (Ni/Al) electrodes that had a distance of 5 μm to the waveguides. The width of the PtSe2 channels on the waveguide along the light propagation direction for all PDs is W = 50 μm. Schematic cross sections of the integrated PDs and scanning electron microscopy (SEM) images of a transferred and a directly grown PD are shown in Figure a–d.

Figure 1

Schematic cross sections and false color SEM micrographs of transferred (a,c) and directly grown (b,d) PtSe2 PDs on silicon waveguides. The wrinkles visible in the SEM image in (c) are due to wet transfer and consequently absent in (d). The green dashed lines indicate the direction of the cross sections in (a,b).

The topography of the PtSe2 films around the waveguides is different for the two different integration methods. The wet transfer method presumably leads to small air gaps around the sidewalls of the Si waveguides, which is known from graphene devices,[45] as indicated schematically in Figure a. With the wet transfer process, the conformal coverage around the waveguide is not possible because of the limited flexibility of the transfer polymer. In addition, wrinkles in the PtSe2 layer are visible that result from the wet transfer process (Figure c). In the case of directly grown PtSe2, the sputtered Pt covers the sidewalls of the waveguides, which results in a conformal PtSe2 film around the waveguide (Figure b,d). This feature underlines the advantage of direct 2D material growth over layer transfer, which often requires additional planarization processes to avoid tearing of the 2D films at the sharp edges of the waveguides[24,46] and reduces the strain effects arising from the bending of the 2D films over the waveguides.[47,48]

Raman spectroscopy was carried out on all devices to confirm the successful formation and quality of PtSe2 layers after fabrication (Figure a). The spectra of various PtSe2 films on the SOI substrate exhibit two characteristic peaks at approximately 177 and 206 cm–1, which represent the Eg and A1g modes of layered PtSe2, respectively.[49] The Eg peak originates from the in-plane vibration of selenium (Se) atoms and the A1g peak is caused by the out-of-plane vibration of Se atoms. As the number of PtSe2 layers increases, a red shift in the position of both peaks and an increase in the intensity ratio of the two peaks I(A1g)/I(Eg) are observed. This behavior can be explained by an increasing out-of-plane contribution due to an increase of van der Waals interactions between the layers.[49] The full width at half-maximum (FWHM) of the Eg peak indicates the material quality of PtSe2.[50,51] For high-quality TAC-grown PtSe2 films, the FWHM is smaller than 5 cm–1.[41] For our samples with a directly grown PtSe2 film, the FWHM of the Eg peak is 4.8 cm–1. For the samples with transferred PtSe2, this value varies between 5.1 and 5.6 cm–1. In both cases, these values indicate a high material quality which is sufficient for device integration.[41] We note that there is still a quality gap between highly crystalline exfoliated films and thin films grown by various techniques.

Figure 2

(a) Raman spectra of PtSe2 with different thicknesses of transferred PtSe2 (gray, red, and blue lines) and direct growth (green line). The spectra represent the average of area scans. They were recorded with an integration time of 1 s and averaged over 10 accumulations. (b) Drain current (Ids) as a function of drain-source voltage (Vds) for different PtSe2 PDs. (c) Evanescent field absorption of PtSe2 films on the waveguide at 1550 nm wavelength. The absorption per propagation distance for 7.6, 13.7, 23.5, and 27 nm thick (directly grown) PtSe2 is 0.06, 0.38, 0.85, and 1.2 dB/μm, respectively.

Current–voltage (IV) measurements of the PtSe2 detectors with different thicknesses show near-linear behavior for small source-drain voltages, which indicates semi-metallic characteristics of the PtSe2 films and Ohmic contacts to the Ni/Al electrodes (Figure b). The resistance of PtSe2 decreases with increasing layer thickness. The directly grown PtSe2 layer exhibits similar properties to the transferred layers. However, as the drain voltages are increased, the IV curves clearly deviate from the linear behavior, which indicates the existence of back-to-back Schottky barriers at the interfaces (Figure S3 in the Supporting Information). This is confirmed by taking the derivative of the IV curves, which show that a weak nonlinearity is also present for smaller bias voltages and slightly stronger for thinner films (Figure S4 in the Supporting Information). The latter is in line with previous reports that have observed semiconducting behavior for thin TAC films.[43] These films are still multilayer films, and the reason of their semiconducting behavior is under discussion because in theory, the semi-metal to semiconductor transition should happen predominantly for monolayer films.[52] The signatures of Schottky barriers at the contact interfaces of thicker films, which are expected to be semi-metallic, have been explained by the coexistence of semi-metallic and semiconducting domains in the TAC-grown films.[53] Nevertheless, the resistivities of nickel/aluminum contacts to PtSe2 films have been measured with the transfer length method (TLM) to vary between 0.7 and 2 kΩ μm for the same batch of samples in a previous study.[41] Such values allow us to neglect the effect of the metal contacts on the total device resistance for our purposes and to use the two-probe configuration for further analysis. This is an important observation because the direct growth process includes Pt deposition and TAC on the sidewalls and at the edges and corners, which is the key enabler for the direct integration of PtSe2 on photonic structures.

The light–matter interaction of the PtSe2 films was characterized for IR light with a wavelength of λ = 1550 nm that was coupled into one grating coupler through a single-mode fiber. The fiber-to-fiber losses of the grating couplers and the waveguides of 8 dB were measured on separate test structures and subtracted to obtain these values. First, we measured the specific absorption of the evanescent field by the PtSe2 films located at 10 nm distance to the waveguide using structures with different PtSe2 dimensions (Figure c). The resulting absorption was 0.06, 0.38, 0.85, and 1.2 dB/μm for 7.6, 13.7, 23.5, and 27 nm PtSe2, respectively.

The opto-electric response of the PtSe2 PDs was investigated by measuring the photocurrents with a lock-in amplifier while modulating the light intensity at a frequency of 1 kHz (see also the Supporting Information). The photocurrents were measured as a function of laser light power for each PD with a width of 50 μm (along the waveguides) and a length of 13 μm (perpendicular to the waveguides, i.e., distance between the contacts) for an applied bias voltage of Vds = 4.5 V across the device. All detectors exhibit a linear dependence on light power for the measured range. The highest photocurrent is observed for the device with the thickest PtSe2 film which is directly grown on the waveguide (Figure a). The intrinsic responsivity is defined as R = Iph/Popt, where Iph is the photocurrent and Popt is the optical power arriving at the PD after subtracting the losses of the grating couplers and waveguides. Measurements of the responsivities at different bias voltages ranging from Vds = −4.5 to 4.5 V at a fixed laser output power of 9 mW are plotted in Figure b. Increasing Vds results in higher photocurrents and consequently higher responsivities. At 4.5 V applied bias voltage, this intrinsic responsivity yields R = 0.8 mA/W for the thinnest PtSe2 layer (7.6 nm) up to a maximum value of R = 11 mA/W for the PD with a directly grown PtSe2 layer of 27 nm thickness (Table ). Different devices on each chip were measured and statistics of the responsivities are also shown in Figure c, which confirms that the responsivity of the devices increases with the number of PtSe2 layers.

Figure 3

Optical measurements of PtSe2 PDs. All devices are 50 μm wide along the waveguide direction. (a) Photocurrent as a function of light power at 4.5 V bias voltage. All PtSe2 PDs respond linearly to the light power. (b) Responsivity vs bias voltage calculated at 9 mW laser output power. (c) Box plot of responsivities for different thickness PtSe2 PDs measured at 4.5 V applied bias. (d) Time-resolved measurement of the PtSe2 PDs at 8 mW optical power and 4.5 V bias voltage. The measured rise and fall times for the devices are between 8 and 13 μs.

Table 1

Summary of the Different PtSe2 PDs; Maximum Responsivity, NPDR, and Rise/Fall Time of All the PDs

PtSe2	thickness (nm)a	number of layers (estimate)	absorption (dB/μm)	max responsivity (mA/W)b	NPDR (W–1)b	rise/fall time (μs)b
sample A: grown on Si/SiO2, wet-transferred	7.6	11	0.6	0.87	7.5	8.6/13.1
sample B: grown on Si/SiO2, wet-transferred	13.7	20	0.38	2.2	1.3	8.7/9.9
sample C: grown on Si/SiO2, wet-transferred	23.5	34	0.85	7.5	1.6	8.5/9.7
sample D: directly grown on Si waveguide	27	39	1.2	11	1.9	8.4/8.7

A single layer PtSe2 has a thickness of about 0.7 nm.[68] The number of layers for each film can be estimated accordingly.

Reported at 4.5 V bias voltage.

The normalized photocurrent-to-dark current ratio (NPDR), defined as responsivity divided by the dark current, is another important parameter for evaluating the sensitivity of the PDs. Larger NPDR values indicate better suppression of dark currents and lower noise-equivalent power. The NPDRs of all the PDs have been calculated and listed in Table . The NPDR values for the PtSe2 PDs are similar to those reported for graphene PDs but have the strong advantage of enabling direct growth instead of transfer.[22,54]

Time-resolved measurements were performed on the PtSe2 PDs (Figure d). The rise and fall times are defined by the time it takes for the photocurrent to reach 10 and 90% of the maximum value in rising and decaying curves, respectively, or by fitting eqs and 2:[55]where τ is the rise/fall time constant and t1 or t2 is the time it takes for switching the laser on or off, respectively. The measured rise and fall times for the PDs are listed in Table . The values for all the detectors are between 8 and 13 μs. Consequently, our PDs provide a faster response time than most of the reported TMD-based IR PDs so far.[56,57] The performance parameters of merit for some 2D-based IR PDs are summarized in Table . The transient time (τ), that is, the time needed for the photocarriers to reach the metal contacts, can be calculated through the carrier drift model by eq :[58]where L is the length of the channel perpendicular to the propagation of the light, μ is the electron mobility, and Vds is the applied bias. To approximate the carrier mobility, PtSe2 films with the same thicknesses as used for the PDs and grown in the same batch were wet-transferred onto a Si substrate with the 90 nm SiO2 layer. After fabrication of the contacts and patterning of the PSe2 films, the field-effect mobility of all the film thicknesses was calculated using the transconductance method in eq :[59,60]Here, W is the channel width along the waveguide and Vds is the voltage applied to the device channel. gm is defined as , where Ids is the drain-source current and Vg is the back-gate voltage, and is the back-gate capacitance of SiO2. ε and ε0 are the relative and vacuum permittivity and tox is the thickness of SiO2. The extracted field-effect mobilities for all PtSe2 films were between 2.7 and 3.9 cm2/Vs. This relatively low value can be attributed to defects and the polycrystallinity of the films grown by the TAC method.[39,47] Literature data on mobilities of PtSe2 films show broad variability ranging from values below 1 cm2/Vs for TAC-grown films[42,61,62] to 210 cm2/Vs for exfoliated films.[38] Most of the studies report mobilities below 50 cm2/Vs.[63−67] Using the extracted carrier mobility, the transient time for a device with L = 13 μm and 4.5 V bias voltage is 0.6 ns.

Table 2

Characteristics and Performance Metrics of PDs Based on 2D Materials

materials	growth methods	wavelength	responsivity	response time	ref
graphene	CVD	1550 nm	180 mA/W	2.7 psa	(73)
BP	exfoliation	3.8 μm	11.3 A/W	0.3 ms	(74)
MoS2	MOVPE	360–960 nm	920 A/W	0.5/1.15 s	(57)
MoS2	exfoliation	980 nm	2.3 A/W	50 ms	(19)
HfS2	CVD	808	3.8 × 105 A/W	8 ms	(75)
MoTe2	CVD	980	6.4 A/W	31/21 ms	(76)
MoTe2/Gr	exfoliation	1300 nm	0.2 A/W	7 psa	(77)
PtSe2	TAC	635–2.7 μm	7.8 mA/W	6/9 μs	(36)
PtSe2	CVD	1550 nm	0.19 mA/W	17/39 ps	(78)
PtSe2	TAC	1550 nm	11 mA/W	8.4/8.7 μs	this work

Calculated from the reported 3 dB bandwidth.

Potential applications for photonic-integrated IR PDs beyond the telecommunication wavelengths are sensing, diagnostics, thermal imaging, and free space communication.[60,69] Therefore, we have studied the broadband absorption of PtSe2 films with different thicknesses for the wavelength range from 1.2 to 28 μm using Fourier-transform IR (FTIR) spectroscopy. We wet-transferred PtSe2 films with thicknesses of 7.6, 13.7, and 23.5 nm onto separate Si substrates and measured their absorbance (A) by FTIR spectroscopy while subtracting the absorbance of the Si substrate (Figure a). All PtSe2 films exhibit a small peak at 1729 cm–1 (5784 nm). The absorption coefficients (α) of the PtSe2 films were also calculated from their absorbance by eq ,where A is the absorbance of the material and t is the film’s thickness (Figure b). The absorption coefficient of the sample with the thickest PtSe2 (23.5 nm) layer behaves differently from the other two samples: while its absorption decreases at first in a similar way to the 7.6 and 13.7 nm thick films, it starts to increase to a steady value as the wavelength increases further. The coefficients of the thinner films remain low as the wavelength increases. The absorption of the 23.5 nm thick PtSe2 is nonzero even at 28 μm wavelength (0.04 eV) which can be considered metallic behavior.[70] This result suggests that PtSe2 may also be suitable for long-wavelength mid-IR PDs.

Figure 4

(a) Absorbance of PtSe2 using FTIR spectroscopy. Inset: magnified view of the peak at 1729 cm–1 (5784 nm). (b) Absorption coefficients of PtSe2 films with different thicknesses, calculated from their absorbance.

We have demonstrated the integration of PtSe2 PDs into silicon photonic waveguides and demonstrated their functionality at a wavelength of 1550 nm. We have shown that layered PtSe2 can be utilized for IR photodetection with high responsivity. Our PtSe2 PDs can be synthesized directly on the photonic waveguide structures, on wafer scale, and at CMOS-compatible temperatures using TAC. The directly grown films show higher performance than reference devices transferred with a typical 2D layer-transfer method. The highest responsivity of 11 mA/W was achieved for a directly grown PtSe2 PD, which reached a fast response time of 8.4 μs. FTIR data indicate that PtSe2 is suitable for photodetection well into the mid-IR range, which opens opportunities for applications in food safety, agriculture, gas detection, on-chip spectroscopy, or imaging. The direct growth of PtSe2 on waveguides creates a new perspective for the integration of 2D materials with PICs. Our results show that multilayered PtSe2 is a promising candidate for high-responsivity optoelectronic applications in the near- and mid-IR regime, including the direct integration on commercial semiconductor technology platforms. This includes silicon nitride (SiN) photonics as the growth methods do not require any crystalline surfaces, in contrast to epitaxial processes.[40,66,71,72]

Methods

Device fabrication: Silicon rib waveguides with a 50 nm step height were fabricated on a 150 mm SOI wafer with 220 nm top Si and 3 μm buried oxide layers using i-line (365 nm) photolithography and reactive ion etching (RIE). Grating couplers designed for the 1550 nm wavelength were realized at the end of the waveguides by electron beam lithography and subsequent RIE. 10 nm of Al2O3 was deposited on the wafer by ALD to protect the waveguides and grating couplers from the diffusion of Pt into the top Si layer during the growth process and also to avoid the formation of a Schottky junction between PtSe2 and the bottom Si. The thin Al2O3 works as a cladding for grating couplers and increases their coupling efficiency.

The SOI wafer was diced after fabricating the photonic base components, and PtSe2 PDs were fabricated on different dies of the wafer.

For the directly grown sample, a 10.8 nm Pt layer was sputtered onto the waveguides using a predefined lithography pattern and a lift-off process. The Pt layer was converted into PtSe2 of 27 nm thickness. PtSe2 films with thicknesses of 7.6, 13.7, and 23.5 nm were grown on separate Si/SiO2 (90 nm) substrates and then wet-transferred onto the separate samples using potassium hydroxide (KOH) solution. All PtSe2 films were grown using this TAC process, as described in detail in previous publications.[34,49] For wet transfer, a support layer of poly(methyl methacrylate) (PMMA) was spin-coated on top of the PtSe2 films. Then, the PMMA film was scratched, and a few droplets of KOH solution were dropped on the scratched areas. KOH causes delamination of PtSe2 films from underlying SiO2. After delamination and release of PtSe2/PMMA films from the substrates, they remained floating on the deionized water for few days and then were transferred onto the final substrates using the fishing technique. The samples were dried in air, and then the PMMA layer was removed from their surfaces using acetone and isopropanol. The transferred PtSe2 films were patterned using contact lithography and RIE. Afterward, all PtSe2 films came in contact with 15 nm Ni and 50 nm Al using contact lithography and a subsequent lift-off process.

Electrical characterization: Electrical measurements were performed in a Lakeshore chamber connected to a Keithley SCS4200 source meter unit under ambient conditions.

FTIR measurements: The absorption spectra were calculated from transmittance measurements. The data were recorded from 0.05 eV (400 cm–1) up to 0.99 eV (8000 cm–1) using a Bruker Vertex 80v Fourier-transform spectrometer with a spectral resolution of 0.5 meV (4 cm–1) and averaged over 64 scans.