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Abstract
Semiconductor ion sensors that respond to the surface electric charge in a solution are used for chemical and biological sensing. Photonic sensors exploiting such a response in the photoluminescence intensity enable a simple system consisting only of a photopump source and a photodiode; however, their sensitivity is usually lower than that of electric sensors, such as ion-sensitive field-effect transistors. This study employed a GaInAsP semiconductor honeycomb photonic crystal slab as a photonic sensor structure and obtained a high ion sensitivity. The surface recombination, which is the origin of the ion sensitivity, was enhanced by increasing the surface-to-volume ratio and moderately suppressing the photopump level. Nevertheless, a sufficient signal-to-noise ratio was maintained by improving the light extraction efficiency. Moreover, a high pH sensitivity of 0.27 dB/pH, which is six times that without photonic crystals, was obtained and resulted in a pH resolution of 0.011 at pH ∼7 comparable with that of electric sensors.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Semiconductors interacting with ions in a solution via charged functional groups at the surface are capable of acquiring electrical information on chemicals and biomolecules directly attached to the surface [1,2]. The ion-sensitive field-effect transistor (ISFET) [3,4], light addressable potentiometric sensor (LAPS) [5,6], and electrolyte-insulator-semiconductor (EIS) capacitive sensor [7,8] are the devices exploiting such a property. The ISFET has been commercialized as a pH sensor with a 0.01 resolution. However, such electric sensors need a reference electrode and external circuit, making the system complicated and unstable [1]. In addition to these electric ones, photonic ion sensors have also been studied. Semiconductors that change their photoluminescence (PL) intensity depending on the surface charge [9–18] enable non-contact, simple, and low-cost sensing. However, their sensitivity is overall lower than that of electric sensors [1].
The principle of the PL intensity change was first explained by the dead layer model (DLM) [12,13,19,20]. The Schottky barrier formed near the semiconductor surface in a solution acts as a dead layer (depletion layer) for the excited electrons and holes, and the radiative recombination is suppressed. The Schottky barrier is modified by the surface charge associated with ions and then reflected in the PL intensity. The nonradiative surface recombination is later shown to be also involved in the principle. Excited minority carriers are accumulated at the surface by the Schottky barrier, and the surface recombination is accelerated. However, some literatures highlight one or the other principle and sometimes cannot decompose these effects [13]. We have studied GaInAsP semiconductor photonic crystal (PC) nanolaser biosensors operated by photopumping [21,22] and independently found ion sensitivity [23,24]. Here the laser emission intensity was modified by the pH of the solution and the adsorption of the charged media. Moreover, the surface recombination principle was suggested from the measurement of the PL intensity and the lifetime [23,25]. This is a reasonable result because the hole array of the PC structure has a large surface-to-volume ratio (SVR). The surface recombination is further enhanced by operating the device at a low carrier density (= low pump power condition), in which the radiative recombination is relatively low.
This study demonstrates photonic ion sensors showing a high pH sensitivity. A honeycomb PC without nanocavities is formed into GaInAsP slab, and the PL is observed in the pH solutions, as schematically shown in Fig. 1. The PL intensity is particularly increased or decreased depending on the pH because the surface recombination is intentionally enhanced by a large SVR and a low pump power. The surface recombination degrades the internal quantum efficiency of the PL; nevertheless, a sufficient signal-to-noise ratio (S/N) is maintained by optimizing the PC to enhance the light extraction efficiency. As a result, it achieves an excellent pH sensitivity and resolution. We comprehensively report herein on the device design and fabrication and the theory and evaluation of the pH sensitivity.
2. Structure and optical characteristics
2.1 Structure and photonic band
A uniform PC slab without nanocavities has a large SVR caused by the hole arrangement and enhances the light extraction under the second-order diffraction condition, called the second Γ point (Γ2) in the phonic band [26–31]. We first examined herein the three following PC structures (upper panel, Fig. 2): the square lattice PC (called Square PC) with SVR$= {{2\pi r} \mathord{\left/ {\vphantom {{2\pi r} {({a^2} - \pi {r^2})}}} \right.} {({a^2} - \pi {r^2})}};$ the triangular lattice close-packed PC (Close-packed PC) with SVR$= {{2\pi r} \mathord{\left/ {\vphantom {{2\pi r} {[({{\sqrt 3 } \mathord{\left/ {\vphantom {{\sqrt 3 } 2}} \right. } 2}){a^2} - \pi {r^2}]}}} \right. } {[({{\sqrt 3 } \mathord{\left/ {\vphantom {{\sqrt 3 } 2}} \right. } 2}){a^2} - \pi {r^2}]}};$ and the triangular lattice honeycomb PC (Honeycomb PC #1) with SVR$= {{2\pi r} \mathord{\left/ {\vphantom {{2\pi r} {[({{3\sqrt 3 } \mathord{\left/ {\vphantom {{3\sqrt 3 } 4}} \right. } 4}){{a^{\prime}}^2} - \pi {r^2}]}}} \right. } {[({{3\sqrt 3 } \mathord{\left/ {\vphantom {{3\sqrt 3 } 4}} \right. } 4}){{a^{\prime}}^2} - \pi {r^2}]}},$ where a or a′ is the hole pitch and 2r is the hole diameter. Their photonic bands are shown in the middle panel of Fig. 2, where a commercial software (Lumerical, FDTD Solutions) was used for the calculation based on the assumption that the average refractive index of GaInAsP slab was 3.4; the refractive index of the surrounding solution was 1.33; the normalized parameter 2r/a or 2r/a′ was 0.55; and the slab thickness was 180 nm. Dipoles in the same orientation were placed at the slab center as the excited sources to simulate the in-plane polarized emission from a compressively strained single quantum well (SQW) that we used in the experiment. As seen in the band diagrams, Γ2 is located at high frequencies above the light line of the solution, and its in-plane wave vector is zero, indicating that light can be extracted in the vertical direction. At Γ2, four and six band edges are crowded in the square and triangular lattices, respectively, although some bands are outside the figures [32]. In the following, we will focus on the band edges on the low-frequency side (i.e., a/λ < 0.6), as indicated by circles.
Fig. 2. Scanning electron micrograph (SEM) of the PCs formed in the GaInAsP epilayers (upper panel), photonic bands (middle panel, 2r/a or 2r/a′ = 0.55; the blue lines show the light lines of the solution, and the inset shows the Brillouin zone), and PL spectrum with different 2r/a or 2r/a′ (lower panel; thick lines represent the envelope of all the spectra for different a or a′). (a) Square PC (2r/a = 0.55). (b) Close-packed PC (2r/a = 0.55). (c) Honeycomb PC #1 (2r/a′ = 0.55).
2.2 Fabrication
We used GaInAsP epilayers of ∼180 nm total thickness as the slab, which were grown on InP substrate by metal organic chemical vapor deposition (MOCVD) and included ∼4 nm SQW and separate confinement heterostructure layers [33]. Although the epilayers are undoped, they usually show a weak n-type conductivity because indium is replaced with carbon during the MOCVD. A 30 µm square two-dimensional PC pattern was formed into the epilayers using electron-beam lithography (Elionix, ELS-G100) and HI inductively coupled plasma etching (Samco, ICP-200ip). An air-bridge PC slab was formed by HCl partial etching of the InP substrate (upper panel, Fig. 2). Finally, ∼3 nm ZrO2 was deposited by atomic layer deposition (Cambridge NanoTech, Savannah) to suppress the epilayer etching by acidic or basic ions under photopumping [23]. ZrO2 is transparent at near-infrared and chemically stable even against strong acid/base [34]. It also has point of zero charge around 7, thereby being electrically neutral, and shows an ideal Nernst response [35,36] and sufficient sensitivity to the surface charge like Ta2O5, which is widely used in ISFETs [4].
2.3 PL measurement
In the first measurement of the PL spectrum, the devices were immersed in ultrapure water, and the pulsed pump light at a wavelength λ of 980 nm was focused to a 20 µm diameter spot through 50× objective lens. The emitted light was detected by the same objective lens and analyzed by an optical spectrum analyzer after the pump light elimination by a notch filter and a long-pass filter (lower panel, Fig. 2). All the spectra for different a were superimposed, and their envelopes were indicated by thick lines. The PL intensity increased around the condition of Γ, and several peaks originated from the multiple band edges of Γ2. The PL peaks shifted to a higher or lower frequency as 2r/a increased or decreased, respectively, in accordance with the shift of Γ2. These results can confirm that the light extraction efficiency ηext was enhanced by the Γ2 condition.
3. Ion sensitivity
3.1 Expression for the PL intensity
The carrier density N in a semiconductor active layer and the measured PL power PPL are expressed in the following rate equations [37,38]:
3.2 pH sensitivity
We now focus on H+ as an iontronic element, which changes vs and B and, hence, A, ηr, ηs, and PPL. In the n-type GaInAsP, the Schottky barrier with a depletion layer is formed particularly on the high pH side. As aforementioned, minority carriers (holes in this case) are accumulated at the surface under this condition, and the surface recombination is accelerated, resulting in a large vs and A. We previously measured the PL lifetime τ for the close-packed PC of the same semiconductor coated with ZrO2 by time-resolved single photon counting. The pH dependence of the nonradiative recombination lifetime τnr = 1/A was then evaluated [23,25]. Considering τnr to be equal to the surface recombination lifetime in the PC, vs was estimated from vs = (τnrS/Va)−1 to be 1.4–1.7 × 104 cm/s at pH = 7. The experimental results that will be shown later are well explained by the calculation results with vs = 1.7 × 104 cm/s. This value is slightly larger than 1.3 × 104 cm/s reported in the past by our group [27] and 1.0 × 104 cm/s, which is a typical value for GaInAsP in the literature [39]. This increase of vs might be affected by the ZrO2 coating which increases surface states. We also evaluated vs = 1.0 × 104 cm/s at pH = 2 and vs = 1.9 × 104 cm/s at pH = 11. Although the intermediate change was slightly nonlinear, we simply approximated the change by the following linear equation:
We also predicted that B was changed by the depletion layer. The following linear equation explained the later experimental results: Let us consider the situation where the ion concentration in the solution changes from c to c′ and the PL intensity changes from PPL to PPL′ via the change of vs and B. The ion sensitivity SIon can then be defined as3.3 Structural dependence
Figure 3(a) shows the dependence of ηr and ηs in the close-packed PC (2r/a = 0.60) at pH = 2 and 11, which was calculated with the carrier density N. Here, Eqs. (5) and (6) were used for vs and B, respectively, with a known Auger recombination coefficient of C = 5.0 × 10−29 cm6/s. With the increase of N, ηs decreases from unity, and ηr increases from zero because of the N dependence of the surface recombination and the N2 dependence of the radiative recombination. Accordingly, ηr is saturated and gradually decreased when N is too large because of the Auger recombination. The pH sensitivity |SpH| was calculated from Eqs. (1)–(6) and (9) (Fig. 3(b)) assuming Va = 1.26 µm3 for the 20 µm diameter pump spot and an SQW thickness of 4 nm, ηabs = 0.26 for the absorption of the pump light in all the GaInAsP layers [41], and τ0 = 10 ns. Compared with that of the unpatterned wafer (SVR = 0), |SpH| with the PC is greatly increased at a low Ppump. Figure 3(c) shows the |SpH| calculated with the hole pitch in different PC structures. A smaller hole pitch increases the SVR, ηs, and pH sensitivity. The pH sensitivities are almost the same (|SpH| ∼ 0.15 dB/pH) among the square, close-packed, and honeycomb #1 PCs under the condition of Γ2 at λ = 1550 nm.
Fig. 3. Calculation results of the pH sensitivity |SpH|. (a) Radiative recombination efficiency ηr and nonradiative fraction ηs as a function of the carrier density N for close-packed PC (a = 800 nm, 2r/a = 0.60). (b) pH sensitivity as a function of N under the same condition of (a). (c) pH sensitivity in various PCs as a function of the hole pitch at Ppump = 10 mW. 2r/a = 0.60 for square and close-packed PCs, 2r/a′ = 0.60 for honeycomb PCs #1, and 2rs/a′ = 0.24 for honeycomb #2 are assumed. Γ2 represents that of the lowest frequency band edge of Γ2 at λ = 1550 nm.
This was experimentally confirmed by fixing the fabricated devices in a dimethylpoly-siloxane microfluidic channel. The PL intensity was then measured by successively injecting different pH solutions by an autosampler. The pH solution was prepared by adding HCl or KOH to a 100 mM KCl solution. The measurement was started at pH = 11 with the Ppump small enough such that PPL was not buried in the noise level of the used optical power meter, and it was set to be constant for the other pH. Therefore, Ppump was changed for different structures. The structure with a high light extraction efficiency allowed a lower Ppump. The time-averaged PL intensities at pH = 2, 7, and 11 were then used to obtain the linear regression curves for the experimental |SpH| in [dB/pH]. Figure 4 shows the designed value of the SVR, the relative power PPL/Ppump exhibiting the change in the light extraction efficiency ηext, and the so-obtained |SpH| for the three structures, where 2r/a or 2r/a′ was fixed at approximately 0.55, and a or a′ was changed. In any PC structure, |SpH| was maximized at an a slightly smaller than the condition giving a maximum ηext (indicated by a black dot) because a smaller a increases the SVR. The SVR at the maximum ηext condition was almost the same between the structures. The |SpH| values were also similar (i.e., 0.15–0.20 dB/pH), which roughly agreed with the calculation values in Fig. 3(c).
Fig. 4. Designed SVR and measured PPL/Ppump and |SpH| for three PCs: (a) Square PC, (b) Close-packed PC, and (c) Honeycomb PC #1. PPL/Ppump is proportional to the light extraction efficiency ηext. The black dots indicate the maximum values of PPL/Ppump and |SpH|.
3.4 Structural optimization
The honeycomb-structure PC has a large filling factor of the semiconductor and a large flexibility in tuning the structural parameters. We investigated a honeycomb-structure PC with six small holes of diameter of 2rs arranged at each lattice point (Honeycomb PC #2) with SVR$= {{2\pi {r_\textrm{s}}} \mathord{\left/ {\vphantom {{2\pi {r_\textrm{s}}} {[({{\sqrt 3 } \mathord{\left/ {\vphantom {{\sqrt 3 } 8}} \right. } 8}){{a^{\prime}}^2} - \pi {r_\textrm{s}}^2]}}} \right. } {[({{\sqrt 3 } \mathord{\left/ {\vphantom {{\sqrt 3 } 8}} \right. } 8}){{a^{\prime}}^2} - \pi {r_\textrm{s}}^2]}},$ as shown in Fig. 5(a) to further increase the SVR with maintaining ηext at Γ2. In addition to the lattice constant a, the inter-apex distance a′ of the unit cell and the small airhole pitch a′′ are defined as parameters. Figure 5(b) shows the photonic band of honeycomb PC #2. Although some bands degenerate at Γ2, the overall look of the bands is similar to that of honeycomb PC #1 in Fig. 2(c). Figures 5(c) and (d) show a comparison of the square of the modal electrical field |E(r)|2 and that of the in-plane wave vector component |F(k)|2 at the lowest frequency band edge of Γ2 between honeycomb PCs #1 and #2. They are almost the same between the two structures even though the SVR increases 2.1 times higher for honeycomb PC #2 than that of honeycomb PC #1. In the figure showing |F(k)|2, the inner and outer white circles show the detectable angle by a 50× objective lens and the light line of the solution, respectively. From the ratio of the |F(k)|2 components in the former circle to those in the latter circle, the coupling efficiency of PL to the objective lens is estimated as 71% for #1 and 63% for #2. The smaller value for #2 was reflected in the PL intensity, as shown in Fig. 5(e). The PL intensity increased with the increase of 2rs/a′. This behavior was similar to that for #1 in Fig. 2(c). The PL peak wavelength blueshifted when 2rs/a′ increased because of the decrease in the effective modal index and the shift of Γ2 to higher frequencies. The PL peak wavelength at 2rs/a′ = 0.22 roughly agreed with that at 2r/a′ = 0.60 for #1. Although the integrated PL intensity slightly decreased, a high ηext was confirmed for #2 from this figure.
Fig. 5. Honeycomb PC #2 consisting of six small holes at each lattice point. (a) SEM image of the fabricated structure. (b) Photonic band for a′′/a′ = 0.3 and 2rs/a′ = 0.24. The blue lines indicate the light line of the solution, and the inset shows the Brillouin zone. (c, d) Calculation results of the normalized modal electrical field |E(r)|2 = Ex2 + Ey2 (upper panel) and the in-plane wave vector component |F(k)|2 (lower panel) of the lowest frequency band at Γ2 in honeycomb PC #1 (2r/a′ = 0.60) and honeycomb PC #2 (2rs/a′ = 0.22, a′′/a′ = 0.3), respectively. F(k) is obtained by spatial Fourier transforming E(r). The outer and inner white circles indicate the light line of the solution and the detectable angle of 50 × objective lens with a numerical aperture of 0.55, respectively. (e) PL spectra for different 2rs/a′ for a′ = 480 nm and a′′/a′ = 0.3 (gray filled lines). The PL spectrum without gray is that of honeycomb PC #1 (2r/a′ = 0.60) for comparison.
Figure 6(a) shows the designed SVR and the measured PPL/Ppump and |SpH| for #2 with a′ = 480 nm. All of them increased with the increase of 2rs. Figure 6(b) presents a comparison of |SpH| for different a′. The smaller a′ was, the higher |SpH| became because of the enhanced SVR. Adjacent holes were combined in this fabrication when a′ < 420 nm; therefore, the structure with a′ = 420 nm and 2rs/a′ = 0.23 gave the maximum |SpH| of 0.27 dB/pH. This value almost agreed with the theoretical value shown in Fig. 3(c) and six times higher than 0.046 dB/pH measured for the unpatterned wafer.
Fig. 6. pH sensitivity of honeycomb PC #2. (a) Designed SVR and measured PPL/Ppump and |SpH| as a function of 2rs. (b) |SpH| measured for different a′.
3.5 pH resolution
The pH resolution around pH = 7 was evaluated for honeycomb PC #2 that yielded the maximum sensitivity. Here, 1 M 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), which has a buffer capability around a neutral pH, was used to accurately control the pH. Figure 7 shows the PL intensity when the pH was changed from 6.8 to 7.4 with a 0.2 step. An intensity change of approximately 0.1 dB was obtained for the step. The average intensity for each pH was then calculated, and |SpH| was determined from the slope to be 0.47 dB/pH. This value was 1.7 times higher than that in Fig. 6 because the 1 M concentration of HEPES was much higher than 100 mM of KCl. The higher electrolyte concentration, which shortened the Debye length, increased the change in surface charge density associated with the pH [24] and enhanced the sensitivity. From the intensity fluctuation in the three standard deviations 3σ, S/N was estimated to be 88.7/pH. Its inverse value provided pH resolutions of 0.011, which was comparable with that achieved by ISFETs.
Fig. 7. Real-time observation of the relative PL intensity for different pH around 7 in honeycomb PC #2 (a′ = 420 nm, 2rs/a′ = 0.23). Each plot was obtained by 30 times averaging.
3.6 Discussion
Now, let us compare pros and cons of electric and photonic sensors. The ISFET is a compact pH sensor that has been commercialized, but it needs a reference electrode, external circuit and encapsulation, which affect their long-term stability [1]. The EIS capacitive sensor detects electrochemical reaction, but it cannot be as compact as ISFETs because of a larger sensing area [42]. The LAPS can visualize the charge distribution, which is applicable to cell imaging [6], but it needs a scanner of pump light, making the system complicated [43]. A CMOS ion image sensor is another device that acquires the charge distribution with a simpler system [44,45], while the spatial resolution is limited by the pixel size. As compared with them, photonic sensors using a pump source and photodetector allow the sensor unit to be separated, much smaller, and disposable. Our sensor is attractive because of its simple system, a high sensitivity, and resolution comparable to electric sensors’.
GaInAsP semiconductors have a relatively narrow electronic band gap; hence, the interaction between the carriers in the semiconductor and the ions in the solution is suppressed, and a stable sensitivity is obtained in the wide pH range from acid to base. A disadvantage is that the Si photodetector cannot be used because the emission wavelength is 1.2–1.6 µm. In contrast, an advantage is that various optical components cultivated in optical communications can be used, including GaInAs photodetectors. Some studies attempted to apply a bias voltage to increase the pH sensitivity in such semiconductors [16,46]. A pH resolution of 0.05 was reported in GaN [16], but a measurable pH range was limited because of its wide band gap. GaAs is a more standard semiconductor, but its surface is much more unstable, and the photocorrosion under photopumping is a problem [47]. The GaInAsP PC achieves a stable sensitivity with a pH resolution of 0.011 without a bias voltage. The calculated sensitivity based on the change of the surface recombination determined by vs and SVR and that of the radiative recombination coefficient B based on the DLM well explained the experimental results.
There are rooms for optimizing the PC structure for a high SVR as well as the light extraction efficiency to obtain higher sensitivity and resolution. For instance, acquiring sufficient light output by expanding the area of the PC structure and pump spot will enable photopump density to be reduced and the surface recombination to be enhanced further. The improvement of the light extraction to upper direction by tilting the sidewall of the holes may also be effective [48].
4. Conclusions
We proposed herein an ion sensor based on the change of PL in the GaInAsP semiconductor and demonstrated an enhanced sensitivity by introducing a PC structure. The change of the surface recombination was enhanced by the increase of the SVR and the low pump power, for which the low radiative efficiency was compensated for by a high light extraction efficiency of the PC, resulting in an enhanced pH sensitivity. The pH sensitivity of the honeycomb-structure PC composed of six small holes at each lattice point was particularly six times higher than that of the unpatterned structure. The pH resolution was evaluated to be 0.011 at pH ∼7. This device achieves such a high resolution even without bias voltage and has the potential to be a platform for iontronic sensing in a variety of applications, such as chemical analysis, biosensing, cell activity monitoring, and in vivo live cell imaging.
Funding
Japan Society for the Promotion of Science (#16H06334).
Acknowledgments
The author would like to thank Dr. Yosuke Terada and Dr. Hiroyuki Ito for their meaningful discussion on the FDTD calculation and Mr. Akihiro Sakata for the discussion on the optimal condition of atomic layer deposition.
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