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We present a near-infrared (NIR) spectrum measurement method using a Schottky photodetector enhanced by surface plasmon resonance (SPR). An Au grating was fabricated on an n-type silicon wafer to form a Schottky barrier and act as an SPR coupler. The resulting photodetector provides wavelength-selective photodetection depending on the SPR coupling angle. A matrix was pre-calculated to describe this characteristic. The spectrum was obtained from this matrix and the measured photocurrents at various SPR coupling angles. Light with single and multiple wavelengths was tested. Comparative measurements showed that our method is able to detect spectra with a wavelength resolution comparable to that of a commercial spectrometer.
© 2016 Optical Society of America
1. Introduction
Compact spectrometers in the near-infrared (NIR) range are useful for portable and handheld spectrum measurements for purposes such as the non-destructive evaluation of fruit quality (e.g., measurements of soluble solid content and firmness during storage) [1] and the self-monitoring of blood glucose, which has an absorption band at a wavelength of approximately 1.7 μm [2]. Several detection principles enable spectral observations using portable devices, such as Fabry-Pérot interferometers [3] and gratings combined with photodetector arrays, which are widely used in commercial compact NIR spectrometers in particular. These gratings diffract the incident light into different angles depending on its wavelength, thereby allowing the photodetector array to detect different wavelengths separately. Spectrometers can provide abundant information on tested samples, and reduction of the device size is beneficial for various NIR spectroscopy applications, such as spectral measurements using endoscopes for disease diagnosis [4]. Micro-electro-mechanical system (MEMS) techniques can be used to fabricate grating-based microspectrometers [5–7]. However, one major difficulty with grating-based microspectrometers is that an optical path of a certain length is necessary to achieve sufficient spectral resolution [8], which hinders further downscaling.
Here, we propose a method of measuring NIR spectra based on a Schottky photodetector with an Au grating to achieve high spectral resolution without a lengthy optical path. The Au grating is fabricated directly on the surface of the photodetector. It serves to couple the incident light to the surface plasmon resonance (SPR) on the Au surface. A Schottky barrier beneath the Au grating is used to transform the absorbed energy into a photocurrent such that the photodetector generates a photocurrent under SPR conditions. The SPR on the surface of a metal grating occurs at different incidence angles depending on the wavelength; therefore, the photocurrents corresponding to different incidence angles contain information on the spectrum of the incident light. Because the SPR phenomenon exhibits a sharp absorption line width with respect to the wavelength, the SPR-based method is well suited for achieving a high spectral resolution.
A Schottky photodetector with SPR-based photocurrent enhancement has previously been proposed for application as a wavelength-selective photodetector [9–11]. One of these previous works reported the feasibility of using such a Schottky photodetector with gratings for spectroscopic purposes [9]. In their method, 4 pairs of detectors are arrayed in a line behind a concave lens. The concave lens causes the incident light beam to diverge such that the angle of incidence on each pair of detectors is different; thus, each pair of detectors is assigned to measure photocurrent of a certain wavelength, however, the photocurrent contained the response of a nearby component of the resonant wavelengths and a continuous spectrum of the light was not constructed due to the limitation of the length of the optical path and scale of each photodetector. In our proposed method, the photodetector is rotated to change the angle of incidence. We constructed a matrix to describe the photocurrent responses of the detector at various incidence angles to the spectrum in the NIR wavelength range with a wavelength interval of a few nanometers. Using this matrix, the photocurrents measured at these incidence angles can be converted into the light power spectrum. In this way, a continuous spectrum can be obtained using the proposed Schottky photodetector. Because the proposed method does not require an optical length for wavelength dispersion, the device can be further downscaled.
2. Theory
The structure of the proposed photodetector is shown in Fig. 1(a). An Au grating is fabricated on top of an n-type silicon substrate. The grating has a pitch of a and is perpendicular to the plane of incidence. An Al film on the backside of the silicon substrate makes an ohmic contact. The spectrum of the transverse magnetic (TM)-polarized light that is incident on the Au grating is measured.
Fig. 1 Schematic diagram of the (a) structure, (b) band diagram and (c) SPR enhancement of the proposed Schottky photodetector.
2.1 SPR coupling on the grating
Surface plasmons (SPs) are collective oscillations of electrons that propagate along the interface between a dielectric and a metal. SPs exist only for TM-polarized light. The propagation constant for SPs traveling along such an interface is derived from Maxwell’s equations as follows [12,13]:
where εm is the permittivity of the metal, εd is the permittivity of the dielectric medium that is in contact with the metal, ω is the angular frequency of the incident light, and c is the speed of light. Gratings are typically used to increase the propagation constant of incident light by means of diffraction to allow light that is propagating in free space to be coupled to SPs. The horizontal component of the propagation constant of the diffracted light at an incidence angle θ is [13]:where a is the pitch of the grating and n is the diffraction order. SPs occur when the horizontal component of the propagation constant of the diffracted light, k0, matches that for SPs, ksp. According to Eqs. (1) and (2), under the assumption that the grating pitch, dielectric permittivity and diffraction order remain constant during a measurement, SPs arise at a different SPR angle for each wavelength.2.2 Detection of SPR-enhanced photocurrent
In the proposed device, the Schottky barrier at the interface between the Au and the n-type Si is used to electrically detect the SPR. When a metal is in contact with a semiconductor, the electrons in the semiconductor diffuse into the metal, causing the Fermi levels of the two materials to be equal at thermal equilibrium. As a result, a potential energy barrier, namely, the Schottky barrier ΦB, is formed (Fig. 1(b)). When SPR occurs, a large number of hot electrons are generated because of the decay of SPs. These hot electrons have sufficient energy to cross over the Schottky barrier and produce a large photocurrent, Iph. The intense absorption of light by SPs gives rise to an enhanced photocurrent that is much larger than that in the absence of SPR. Therefore, the curve of the photocurrent versus the incidence angle exhibits a peak at an SPR angle that depends on the wavelength (Fig. 1(c)). Therefore, SPR photocurrent measurements at various angles contain information on the spectrum of the incident light.
2.3 Spectrum calculation
The power of the light that is incident on a photodetector, Pin, is related to the photocurrent Iph by a sensitivity coefficient R, which is called the responsivity, as shown in the following equation [14]:
Under the assumption that the polychromatic incident light is discretized with respect to the wavelength such that it possesses n wavelength components, , each wavelength component has a power denoted by , respectively. Then, the photocurrent is the sum of the photocurrents arising from each wavelength component. Using Eq. (3), the photocurrent at an angle can be expressed asThis equation can be written in matrix form as I = RP:where I is the vector of the photocurrents at each angle; P is a vector of the powers of each wavelength component, representing the spectrum of the light; and R is a transformation matrix, which we will henceforth call the responsivity matrix. The responsivity matrix is composed of elements (i, j = 1, 2, …, n), each of which represents the responsivity for wavelength λi at incidence angle θj. The responsivity matrix R represents the characteristics of the photodetector and can be predetermined using a monochromatic NIR laser prior to spectral measurements. The diagonal elements (i = j) of the matrix are equal to the responsivity for each wavelength at its SPR angle θj. Each diagonal element is larger than all other elements in the same row and column because of the SPR enhancement, and the matrix is regular. Therefore, once the responsivity matrix R has been obtained, the spectrum P of the incident light can be calculated using the equation P = R−1I.3. Fabrication
The fabrication process for the proposed photodetector is illustrated in Fig. 2(a). First, an n-type silicon wafer with a resistivity of 1~20 Ω‧cm was placed in a 1% hydrofluoric acid solution to remove native silicon dioxide. Photoresist (OFPR 23cp, Tokyo Ohka, Japan) was then spin coated onto the wafer with the rotation speed of 6000 rpm for 30 s. The thickness of the photoresist was estimated to be 1 μm. The linear grating pattern was exposed to the photoresist, and the photoresist was developed (NMD-3, Tokyo Ohka, Japan) to form a grating pattern. Then the 100-nm-thick Au was deposited on the photoresist grating patterns using EB evaporator (EX-400, Ulvac, Japan). After the deposition of Au on the grating, the photoresist was removed using a stripper solution (Hakuri 104, Tokyo Ohka, Japan). In this way, the Au evaporated on the photoresist pattern was lifted off so that the Au directly evaporated on the n-Si formed an Au grating. Then, 100-nm-thick Au was deposited again to completely cover the n-Si surface with Au. Finally, a 100 nm thick aluminum film was deposited on the bottom of the wafer as a cathode electrode. A fabricated Schottky photodetector is depicted in Fig. 2(b). To confirm the grating groove height h and the Au layer thickness t, a fabricated Schottky photodetector was split at the center of the sensing area. The h and t values measured from the SEM image of the split sample were 105 nm and 100 nm, respectively. The pitch and fill factor of the Au grating were 3.2 μm and 0.5, respectively. The Schottky barrier height of the fabricated photodetector was calculated to be 0.69 eV following procedures presented in ref [15] where device area was 561 mm2 in the calculation. Because the obtained barrier height was smaller than the energy bandgap of silicon (1.12 eV) and corresponded to the cut-off wavelength of 1800 nm, it was confirmed that NIR photodetection was possible with the fabricated device.
4. Experiment and results
4.1 Responsivity matrix
The experimental setup for measuring the responsivity matrix is depicted in Fig. 3. A wavelength-tunable NIR laser source (8163B, Agilent, USA) was used to generate light with single wavelengths from 1470 to 1570 nm in steps of 5 nm. The NIR light from the laser source was passed through a polarizer to produce TM-polarized light. The photodetector was placed on a rotation stage with the grating grooves perpendicular to the plane of incidence. The rotation stage was controlled by a stepper motor. A DC voltage current monitor (6242, ADCMT, Japan) measured the photocurrent of the photodetector. The laser power at each wavelength was measured by a power meter (PM320E-SC122C, Thorlabs, USA) to calculate the responsivity. The experiments were performed in a dark environment at room temperature.
First, the photocurrent versus incidence angle was measured for each wavelength. The incidence angle was varied from 20 to 28° in steps of 0.1°. The results are shown in Fig. 4(a). Each curve showed a peak due to SPR enhancement. With increasing wavelength, this peak shifted toward larger angles. The incidence angle at the peak position was taken to be the measured SPR angle for each wavelength. Theoretical calculations using Eqs. (1) and (2) indicate that these SPR angles correspond to a diffraction order of n = −3. Both the measured and calculated values are shown in Fig. 4(b). The measured SPR angles at 1470 to 1570 nm range from 20.9 to 27°; these values are shifted by approximately 0.9° compared with the calculated values. This shift can be primarily attributed to the difference in the permittivity of the Au film between the calculated and experimental values. The difference in the grating profiles is also a factor. However, the experimental results are consistent with the calculations because in both cases, the SPR angles exhibit approximately linear variations with wavelength with nearly identical slopes.
Then, the responsivity R at each incidence angle was obtained by dividing the photocurrent by the laser power at the corresponding wavelength that was measured using the power meter. Finally, we used the responsivities at the various SPR angles to construct a regular R matrix; for example, the SPR angle is θ1 = 20.9° for λ1 = 1470 nm, θ2 = 21.2° for λ2 = 1475 nm, θ3 = 21.5° for λ3 = 1480 nm, and so on. The measured 21 × 21 responsivity matrix is shown in Fig. 5. The elements in each column are the responsivities for incident light with the same wavelength at different SPR angles. The diagonal elements are the responsivities at each wavelength in the presence of SPR. They are larger than all other elements in the same row and column because of the SPR enhancement. Therefore, the measured responsivity matrix (R) is invertible. This measured responsivity matrix was used for the spectrum calculations presented in the following section.
4.2 Spectrum measurements
A light source (SC450-2, FIANIUM, UK) and an integrated acousto-optic tunable filter (AOTF) (NIR2, FIANIUM, UK) were used as the NIR light source for the spectrum measurement tests. White laser light from the source was directed via an optical fiber into the AOTF, which controlled the wavelength of the output light. The AOTF had 8 channels. Each channel could produce laser light with a given single wavelength. Test light with multiple wavelengths was obtained by using several channels simultaneously. Except for the light source, the experimental setup for the spectrum measurements was identical to that for the responsivity matrix calculation. The spectra of the light from the AOTF were measured using the proposed photodetector. A commercial NIR spectrometer (SOL2.2A, BWTEK, USA) was also used to measure the spectrum for comparison. The wavelength resolution of the SOL2.2A spectrometer is approximately 4.5 nm; this is close to the resolution of 5 nm of the proposed photodetector, which corresponds to the wavelength increment used to construct the responsivity matrix.
The photodetector was exposed to beams of NIR light with 3 spectral shapes: a single wavelength of 1500 nm; triple wavelengths of 1480, 1510, and 1540 nm; and quadruple wavelengths of 1490, 1510, 1530, and 1550 nm. The photocurrents generated under these light sources were measured while varying the incidence angle and were converted into power spectra using the responsivity matrix. The photocurrents versus the incidence angles are plotted for the three tested light profiles in the top row of graphs in Fig. 6, whereas the calculated spectra are shown in the bottom row. The spectra measured by the SOL2.2A are also plotted in Fig. 6 for comparison. The photocurrent generated under exposure to the light with a single wavelength of 1500 nm exhibited an SPR angle of 22.7° (Fig. 6(a)), consistent with the previous experiment. Both spectra, from the photodetector and the spectrometer, indicate an incident wavelength of 1500 nm. The spectral resolution achieved by the photodetector is comparable to that of the commercial spectrometer. The full width at half maximum (FWHM) of the spectra is 10 nm.
Fig. 6 Photocurrents versus incidence angles and spectra measured using the proposed photodetector and a commercial spectrometer under exposure to light with (a) single and (b), (c) multiple wavelengths.
The test results for the light beams with multiple wavelengths of 1480, 1510, and 1540 nm and of 1490, 1510, 1530, and 1550 nm are shown in Figs. 6(b) and 6(c), respectively. These two measured photocurrent curves contain three and four peaks, respectively, corresponding to the wavelength components. The two spectra calculated using the responsivity matrix reveal the precise shapes of the light spectra, which are consistent with those measured by the commercial spectrometer. Both show distinctive peaks that correspond to the input wavelength components of the tested light. The differences between the maximal wavelength values measured by the two detectors are within 5 nm. Therefore, the SPR-enhanced Schottky photodetector has been proven to be able to detect light spectra with a high wavelength resolution comparable to that of a commercial spectrometer.
The wavelength resolution of the proposed spectrum measurement method could be improved in several ways. In this study, the responsivity matrix was constructed with a wavelength increment of 5 nm; decreasing the wavelength increment would result in a larger responsivity matrix with a higher wavelength resolution. Similarly, the angular rotation increment, which was 0.1° in this study, should be reduced. The limit on the resolution improvement is determined by the FWHM of the SPR curves. Previous studies have reported that sharper SPR curves can be obtained by optimizing the configuration of the metal grating or selecting an appropriate diffraction order [16,17]. Therefore, using these techniques, it is possible to achieve high-resolution SPR-based spectrum measurements while overcoming the limitation imposed by the required length of the optical path. In addition, because the fabrication process for the proposed photodetector is compatible with the MEMS process, the size of this photodetector can be further reduced via integration with micro-actuators, such as a comb-drive tilting mirror, for varying the incidence angle.
5. Conclusion
In this paper, we propose a method of measuring light spectra using an SPR-enhanced Schottky photodetector that does not require an optical path for the dispersion of different wavelengths. The photodetector is fabricated by patterning an Au grating on a silicon substrate. The incident light is wavelength-selectively coupled to SPs and detected as photocurrents at the SPR angles. This process occurs on the surface of the Au grating and at its interface with the silicon substrate; therefore, the optical path that is needed in conventional spectrometers based on diffraction gratings and photodetectors is not required here. Because the position of the SPR angle varies with the wavelength of the incident light, we propose the use of a responsivity matrix to convert the angular distribution of the measured photocurrents into the wavelength spectrum of the incident light. For the experimental demonstration of the proposed photodetector, a 21 × 21 matrix consisting of the responsivities for NIR light with wavelengths from 1470 to 1570 nm in steps of 5 nm at the various corresponding SPR angles was constructed. Incident light beams with single and multiple wavelengths were produced and measured using both the proposed photodetector and a commercial spectrometer for comparison. The spectral resolution of the proposed photodetector was nearly identical to that of the commercial spectrometer. The measured FWHM for the single-wavelength spectrum was 10 nm. For light with multiple wavelengths, the differences between the wavelength components measured using the two methods were within 5 nm. The wavelength resolution of the proposed method is determined by the sharpness of the SPR, which can be improved by optimizing the grating design. The fabrication process for the proposed photodetector is compatible with the semiconductor fabrication process; therefore, the photodetector can be integrated with MEMS actuators. Because it overcomes the optical-path limitation, the proposed method offers the potential of realizing a compact spectrometer with a high spectral resolution.
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