The term photoconductivity covers all the phenomena by which a change in conductivity—either an increase or decrease—follows absorption of light in the considered materials. Photoconductivity is not an elementary process. It includes several successive or simultaneous mechanisms: optical absorption, hot carrier relaxation, charge carrier transport and recombination. Photoconductivity offers a means of studying many physical properties of materials and, on the other hand, photoconductivity effects have obvious applications for the detection and measurement of light over the whole electromagnetic spectrum. The corresponding literature is very abundant and can be found in many scientific journals and books. Therefore, in this paragraph we don’t intend to review all the published papers dealing with photoconductivity and related effects, but instead to provide a short introduction to the physical effects behind the working principles of recently proposed near-infrared all-silicon photodetectors [18].

The observation of any photoconductivity phenomenon requires the presence of at least one type of mobile charge carrier. Photoconductivity is due to a change in carrier concentration, which results from the generation in the semiconductor of electronic excited states by absorption of light energy. The excited states have a finite lifetime: they lose energy through different processes of relaxation and recombination. Finally, we note that in homogeneous solids, photoconductivity is usually observed by applying an electric field to the material and measuring the resulting photocurrent.

One of the most important aspects is the generation of photocurrent, which is obtained by irradiating a sample with a homogeneous light flux. Inter-band generation, which is due to optical absorption by inter-band electronic transition, is characterized by high absorption coefficients. Intra-band generation refers to free-carrier optical absorption. In this case, the absorption coefficient is usually low and is proportional to the free-carrier concentration. Finally, we have extrinsic generation, which refers to photoionization of impurities and localized centers. There are two main differences with respect to the case of inter-band generation: only one excited carrier is generated, either electron or hole, and the absorption process is characterised by low absorption coefficients [19,20]. Photoconductivity effects related to extrinsic generation have been recently investigated in order to realize NIR silicon photodetectors. Therefore, in the next two subsections, the photoconductivity effect due to mid-bandgap absorption (MBA), and due to surface state absorption (SSA), is reported. Finally, in the last subsection, internal photoemission absorption (IPA) is discussed.

In the process of two-photon absorption, an electron absorbs two photons at approximately the same time, achieving an excited state that corresponds to the sum of the energy of the incident photons. In order to reach the final state, no intermediate state is necessary—only a “virtual state” which doesn’t correspond to any electronic or vibrational energy eigenstate (Figure 4).

The absorption cross section σ describing this process increases linearly with laser intensity according to the following relation: (5) σ = σ ( 2 ) Iwhere σ⁽²⁾ is a coefficient taking into account the probability that the two-photon absorption process occurs. It is clear that in conventional linear optics, the absorption cross section σ is a constant. Atomic transition rate R, due to two-photon absorption scales as the square of the laser intensity, is shown in the following formula: (6) R = σ ( 2 ) I 2 ħ ωIn the absence of significant recombination and diffusion, the differential equation describing the beam propagation in presence of linear and two-photon absorption can be written as [36]: (7) d I / d z = − α I − β I 2where α is the linear absorption coefficient, β is the two-photon absorption coefficient, z is the longitudinal propagation direction, and where the instantaneous irradiance I = I(r, z, t) is a function of time t and transverse position r as well as z. By assuming α as constant, without taking into account the possible influence of lattice heating, band filling, and bandgap renormalization on α, Equation 7 can be solved exactly to give the transmitted irradiance I(r, L, t) in terms of the incident irradiance I(r, 0, t): (8) I ( r ,   L ,   t ) = T 0 I ( r ,   0 ,   t ) 1 + I ( r ,   0 ,   t ) I c 2where: (9) T 0 = ( 1 − R 1 ) ( 1 − R 2 ) e − α Lis the linear transmission of a sample of thickness L and front (rear) surface reflectivity R₁ (R₂).

When the irradiance approaches the critical value I_c2 defined as: (10) I c 2 = ( α / β ) ( 1 − R 1 ) ( 1 − e − α L )the transmission deviates from the linear value and TPA becomes comparable to linear absorption. It is worth noting that when I(r,0,t) = α/β, the two-photon coefficient near the surface equals the linear coefficient. Of course, (1 – R₁) represents the front surface transmission while (1 − e^−αL) is the linear absorption of a sample of thickness L. From a physics point of view, I(r,0,t)/I_c2 compares the linear absorption coefficient α to the two-photon coefficient βI.

The quantity typically measured to derive β is the transmitted (T) energy, not the irradiance. For a Gaussian spatial and temporal irradiance profile, it can be shown that [36]: (11) T − 1 = T 0 − 1 [ 1 − I 0 2 2 I c 2 ]

It should be noted that this formula has been obtained without taking into account free-carrier absorption (FCA) generated by TPA. When FCA cannot be neglected, it can be included, obtaining a modified and more complex formula for the transmittance [36]. If front and back surface reflectivities are known, it is possible to show that the intercept of an inverse transmission versus irradiance plot corresponds to the linear absorption coefficient, while the initial slope is related to β.

Reintjes and McGroddy [37] isolated TPA in silicon for first time, obtaining a value of β = 1.5 cm/GW at 100 K for picosecond pulses at a wavelength λ = 1,060 nm, and, in 1986, Boggess et al. measured a similar value at 300 K [36]. In 1990, Reitze et al. reported 5 < β < 36 cm/GW by measurements across the direct gap of silicon in the range 550 < λ < 620 nm [38]. In recent years, TPA at telecommunications wavelengths of λ = 1,300 or λ = 1,500 nm has been reported, with values of β ∼ 0.8 cm/GW [39–41]. In 2007, Bristow et al. presented measurements for TPA across the indirect gap (Eg = 1.12 eV) at fundamental wavelengths between 850 and 2,200 nm and compared their results with theoretical predictions [42]. In their work, measurements of β are performed using a z-scan technique [43]. Values of β as a function of λ are illustrated in Figure 5. The data is compared with values measured by others.

MBA has been used to develop integrated optical detectors completely compatible with standard silicon technology and sensitive to C and L optical band wavelengths. Knights et al., in a work for the Conference on Optical Fiber Communication in 2003, proposed a monolithic p-i-n waveguide photodiode based on implantation into a silicon-on-insulator wafer [46]. In particular, the implantation of protons into a rib waveguide with a lateral p-i-n diode (Figure 6) was used to create the deep levels. Thus, defects could be controlled in terms of concentration, depth and location via selective masking. On the other hand, shallow implantation and rapid thermal annealing of the p+ and n+ contact regions allowed for the separation of the contacts to be close to the width of the waveguide without introducing excessive free carrier absorption. The proposed 10-mm-long photodiode exhibited a responsivity of about 8 mA/W at 1,550 nm and a leakage current of about 3 nA, in both cases with 1 V of applied reverse bias.

Bradley, Jessop, and Knights have investigated different factors useful for improving both the performance and repeatability of integrated photodiodes [47,48]. Due to the high deep trap density, carriers are characterized by a lifetime in the nanosecond range and thus by a diffusion length of the order of a few microns. This implies a dependence of device performance on the separation between the waveguide, where the carriers are generated, and the semiconductor p-n junction [38]. Moreover, in 2006, Knights et al. [48] demonstrated the importance of post-implantation annealing on device performance. Figure 7(a,b) reports plots derived by the authors with regards to the dependence of the propagation optical losses and the photocurrent generated on the post-implantation annealing temperature for an integrated photodetector with defects introduced by silicon-ion bombardment. The plots demonstrate that an improvement of device performance can be obtained through low-temperature annealing (around 300 °C). For these temperatures, carrier recombination centers that do not contribute to the carrier generation process are removed. This should imply a reduction of the competing mechanisms of carrier generation and thus a significant improvement both of the photoresponse and optical loss within the waveguide.

Different implanted ions were used by Liu et al. [49]; in particular, helium ions were chosen for their non-reactivity with silicon and for stability of created defects [50]. Liu et al. carried out a detailed analysis of the influences of ion-implantation doses and of annealing temperature, both on the photogenerated current and the optical losses. This analysis led to the fabrication of an optimized integrated photodetector that was 1.7 cm long, and which obtained a maximum value of responsivity of 64 mA/W at 1,440 nm and a leakage current of 0.1 μA, both with a reverse bias of 20 V.

All the described photodetectors have been characterized by a very large traversal section in the order of tens of μm², and by absorption lengths of 1 to 2 cm. Thus, the bandwidth of these devices was rather limited. Reducing the waveguide dimensions and optimizing the implantation in the silicon waveguide, Geis et al. [51] demonstrated the feasibility of robust high-frequency integrated photodiodes. The main concept is related to the decrease of the transversal cross section of the waveguide. In fact, this scaling induces a larger overlap between the optical mode and the implanted region and thus increases optical absorption, reducing the required device length and increasing the frequency response.

After a rigorous process of optimization [51,52], the photodetector proposed by Geis et al. had a cross section of about 0.11 μm² showing an internal responsivity of 0.5–0.8 A/W at 1,550 nm, a reverse bias of about 5 V, a leakage current of 2.5 nA/mm and a bandwidth of 10–20 GHz. Further, to increase the device bandwidth, Geis and his co-authors optimized the external circuit components [53] such as the resistance and capacitance of the silicon wings on the sides of the rib wavelength (Figure 8) and on the contact pads.

This optimization allowed minimizing the effect of the parasitic components and thus increased the detector bandwidth above 50 GHz. Unfortunately, Geis’s devices have low optical absorption, requiring a diode on the order of milimetres in length to absorb more than 50% of the incoming light. Recently, for the OFC/NFOEC 2010 conference in San Diego, Shafiiha et al. proposed a drastic reduction of the device size using a resonance structure [54]; in particular, the light from a straight waveguide coupled into a small-defect Si+ implanted ring resonator (Figure 9).

The ring resonator [55–57] was developed in order to equal the power coupling between the ring and the bus waveguide and the round-trip loss of the ring resonator at the resonance wavelength. In this condition, most of the transmission light is absorbed, increasing electron-hole generation. In Figure 9(a), the transverse section of the ring waveguide is sketched, and the overlap between the region of the optical mode, the defect area obtained and the depletion region of the p-i-n diode is illustrated. In Figure 9(b), a SEM image of the 15-μm-radius-ring integrated photodetector is reported. The photocurrent of the ring detector under a reverse bias of −2 V is illustrated in Figure 10.

In particular, photocurrents for both before and after a burn in process are reported. The burning-in process, that was also used by Geis et al. [52], consists of a forward bias with a current density of 300 mA/cm for 5 minutes and makes possible an increase in the responsivity of the ring photodetector from 0.02 A/W up to about 0.1 A/W at 1,549 nm and −2 V. Reported leakage current is 0.1 nA at −2 V of applied reverse bias. The compactness of the device allows reaching a 3-dB bandwidth of about 7 GHz at −2 V reverse bias. However, all the reported performances of the ring photodetector are relative to the resonance wavelength and thus the C+L bands are not completely covered.

To overcome this limitation, Doylend et al. recently proposed a new ring-resonator-based photodetector [58]. The device was characterized by a responsivity of 0.14 A/W with a reverse bias of −10 V and a leakage current of 0.2 nA; i.e., performance that is comparable with that of Shafiiha’s device. However, the novelty of the proposed structure is the possibility of tuning the photodetector response in a wavelength range from 1,510 nm to 1,600 nm. In particular, resonance wavelength shift has been obtained by thermo-optic tuning of the ring resonance, using a resistive metal strip which overlaps the structure.

Although MBA has been well-known for quite some time, very few papers concerning silicon-based bulk devices have been reported in the literature, and yet several integrated devices are fabricated and characterized. In 2001, Wu et al. [59] observed a remarkable increase of IR absorbance in surface microstructured silicon wafers produced by laser irradiation in the presence of SF₆. They found that in the microstructuring process, a high concentration of impurities and structural defects were incorporated into the silicon lattice, most likely producing bands of impurity states in the band gap that could absorb infrared radiation. The ion channelling spectra obtained from microstructured wafer show a background level higher than that of crystalline silicon but lower than that of amorphous silicon. These measurements indicate that the material is crystalline and is likely to have a high density of defects. The background remains high in ion channelling spectra from samples annealed for 3 hours at 1,200 K, indicating that a significant amount of disorder in the microstructured material is unaffected by annealing under these conditions. Moreover, the resulting subgap absorption is enhanced by the surface texture. These remarkable properties were then exploited for the fabrication of a silicon-based detector for infrared radiation [60]. The diode characteristics and responsivity depend strongly on processing conditions including laser fluence, substrate doping, and thermal annealing temperature. Figure 11 shows a schematic diagram of the device, whose active area is approximately 5 mm².

This optimized sample exhibits responsivity of 50 mA/W at 1,330 nm and 35 mA/W at 1,550 nm at 0.5 V of applied reverse bias; its capacitance is 64 nF/cm², while the signal rise time is 10 ns and the fall time is 30 ns. Device leakage current is 120 μA/cm² at 0.5 V of applied reverse bias.

In 2007, Geis et al. [51] observed a low photoresponse for an unimplanted diode and attributed it to a surface effect of the waveguide even if its performances were a strong function of the operating environment (such as humidity and surface chemistry). Baehr-Jones et al. [61] enhanced the effect of the surface states optimizing the overlap between the optical mode and the surface of the waveguide. The SEM image and a top view of one of the devices characterized by the authors are shown in Figures 12(a,b), respectively.

Small conductions arms intersects the waveguide defining the active regions of the photodetector. The waveguide was developed with a cross section of 500 nm × 100 nm in order to increase overlap between the optical mode and the waveguide surface. In this way, part of the optical mode is absorbed by the surface states and a change in conductivity of the device is achieved. For the device illustrated in Figure 12, a responsivity of 36 mA/W for a bias of 11 V at a wavelength of 1,575 nm was measured. A device leakage current of 0.12 μA at −11 V is reported as well. Moreover, the authors reported a detailed analysis to demonstrate that the measured photocurrent is due to the energy state of the surface defects and not to other effects such as heating, two-photon absorption or internal photoemission.

In order to enhance the photocurrent generated by SSA, Chen et al. [62] used a p-i-n diode embedded into a silicon micro-ring resonator. Figure 13(a) shows a top view of Chen’s proposed device; whereas in Figure 13(b), the waveguide dimension obtained on a SOI wafer with a 1-μm-thick buried oxide layer is illustrated. A low-temperature deposited oxide layer with a thickness of 1 μm has been used to cover the waveguide. In Figure 13(b), the intensity contour of the simulated TE optical mode field is reported and the spatial overlapping with the Si/SiO₂ interfaces can be noted. This overlapping makes possible achievement of photogenerated current by SSA.

The ring resonator was characterized by a Q factor of about 8,000. With this value, the responsivity detected at the resonance wavelength (1,541.5 nm) for 0 V bias was about 0.12 mA/W and resulted twenty times that of the responsivity measured for off-resonance wavelengths. Increasing the bias voltage to −15 V, the responsivity increased to 0.25 mA/W but the dark current reached 2.5 nA. Due to the presence of a resonator, the device became more sensitive to temperature changes. In particular, the authors measured a redshift of the resonance wavelength that was explained by the thermo-optical effect and the heating due to the carrier recombination. Finally, the authors noted a slight linear absorption-induced photovoltaic effect, with a power generation efficiency of about 0.05 mW/W.

For all the previous described methods, a photon at a wavelength of 1.55 um that travels in the silicon waveguide is efficiently absorbed if its energy allows an inter- or intra-band transition of an electron (or hole). Another way to absorb the photon is to lower the energy gap that the electrons (holes) must overcome. This approach is based on the use of a semiconductor/metal interface. The potentialities of such a method for the realization of an all-silicon photodetector have been demonstrated by the theoretical work of Casalino et al. [29], and recently the effect has been employed for an integrated structure as well [63]. In Figure 14, the integrated photodetector proposed by Casalino et al. [63] is schematically illustrated. A rib waveguide was terminated on a deep trench that reached down to the buried oxide layer of the SOI wafer. A Cu/p-Si Schottky contact was fabricated on the vertical surface of the deep trench.

By means of this technological solution, a very narrow semiconductor/metal barrier transverse to the optical field coming out from the waveguide has been achieved. The integrated photodetector was characterized by a responsivity of 0.08 mA/W at a wavelength of 1,550 nm with a reverse bias of −1 V. Measured dark current at −1 V was about 10 nA. Moreover, the authors assert that the thinness of the Cu/p-Si Schottky barrier could enable a speed operation in the gigahertz range. An indirect evaluation of the bandwidth of the detector was reported to confirm the operation speed potentialities. A bandwidth of about 3 GHz was measured by Zhu et al. [64] on a Schottky-barrier-based integrated photodetector, where the junction was achieved by a nickel silicide layer (NiSi₂) on silicon [65,66] (Figure 15).

In order to achieve both a suitable optical absorption and an efficient photoexcitation of metal electrons or holes across the silicide/silicon interface, the authors proposed the lengthening of a thin silicide layer on the surface of a SOI waveguide. The authors reported a detailed analysis on the influence of the silicide layer dimension on the performance of both NiSi₂/p-Si and NiSi₂/n-Si diodes. The overall performance of the NiSi₂/p-Si structure has resulted better than that relative to the NiSi₂/n-Si interface, due to the lower Schottky barrier height of the NiSi₂/p-Si structure. In particular, a responsivity of about 4.6 mA/W at a wavelength of 1,550 nm and a reverse bias of −1 V was estimated for the NiSi₂/p-Si diode against the value of 2.3 mA/W of the NiSi₂/n-Si junction. Moreover, a measured dark current of 3 nA was reported.

Recently, Akbari et al. [67] have proposed enhancing the interaction between the optical field and the metal/semiconductor barrier by the use of surface plasmon polariton (SPP) [68,69]; i.e., an optical surface wave propagating along the interface between a metal and a dielectric [70]. The performances achieved are interesting and comparable with other Schottky-barrier-based integrated photodetectors. In particular, the authors measured a responsivity of about 0.8 mA/W at a wavelength of 1,550 nm and a bias voltage of −100 mV for a 35-μm-long SPP detector obtained by an Al strip on n-Si; while the dark current was about 6 μA.

Recently, some research groups have published papers on IPA-based NIR silicon bulk photodetectors working at room temperature. Lee et al. [71] investigated the spectral responsivity of Al-porous silicon Schottky barrier photodetectors in the wavelength range 0.4–1.7 μm. The structure of the PS photodetector was Al (finger type)/PS/Si/Al (ohmic), and the active area was 18 mm². The photodetectors show strong photoresponsivity in both the visible and the infrared bands, especially at 1.55 μm. The photocurrent can reach 1.8 mA at a reverse bias of 6 V under illumination by a 1.55-μm, 10-mW laser diode. The corresponding quantum efficiency is 14.4%; this high value comes from a very high surface-area-to-volume ratio, of the order of 200–800 m²/cm³ of porous silicon. The dark current is −5 μA at −10 V. The quantum efficiency versus illumination power under 1.55 μm semiconductor laser diode illumination with the bias voltage as a parameter is shown in Figure 16. The quantum efficiency is almost a linear function of the illumination power. In addition, the quantum efficiency increases and gradually saturates with reverse-bias voltage. The frequency response was 200 MHz and is limited by the low carrier mobility.

More recently Casalino et al. [72,73] presented the realization and the characterization of a resonant cavity enhanced (RCE) photodetector, completely silicon compatible and working at 1.55 μm (see Figure 17). The resonant cavity is a vertical-to-the-surface Fabry-Perot structure. It is formed by a buried reflector, a top mirror interface and, in the middle, a silicon cavity. The buried reflector is a Bragg mirror, realized by alternating layers of amorphous hydrogenated silicon (a–Si:H) and silicon nitride (Si₃N₄). A Schottky metal layer (Cu), working both as active (absorbing) layer and as cavity mirror, is deposited above the silicon layer. The novelty of the device is the idea of enhancing the internal photoemission absorption by the microcavity effect. The room temperature responsivity measurements on the device reveal a peak responsivity of about 2.3 μA/W and 4.3 μA/W, respectively, for 0 V and −10 mV of applied reverse bias for a device having radius of active area of 2 mm [72]. Recently the same authors have improved device performances by reducing the radius of active area down to 40 μm obtaining a responsivity of 8 μA/W at −100 mV. In this device a bandwidth in the GHz range was estimated, too [73].

Despite the small value of the TPA coefficient, ranging from 0.44 cm/GW to 0.9 cm/GW at a wavelength of 1,550 nm [74], silicon optical waveguides have potential for use in TPA because of their long interaction lengths and low optical dispersion in silicon. The photocurrent generated by the TPA effect can efficiently collect embedding a p-i-n diode in the silicon waveguide. In Figure 18, the TPA-based integrated detector proposed by Liang et al. in 2002 is reported [75].

In Figure 18, the measured photocurrent for a bias voltage of −3 V is reported and the typical non-linear behaviour of the TPA effect can be noted. Liang’s device was used to perform an optical autocorrelation for measuring ultra-short laser pulses, but it was not suitable as a photodetector due to the high required optical input power value. The key to obtaining micrometer devices sensitive to low optical input is to strengthen the interaction between light and matter by tightly confining the light, i.e., employing high-Q microresonators. Bravo-Abad et al. [76] theoretically studied the feasibility of using p-i-n diode embedded silicon micro-ring or photonic crystal resonators for ultrafast photodetection. Starting from these premises, Tanabe et al. [77] proposed an infra-red photodetector based on a p-i-n integrated into a high-Q width-modulated line-defect photonic crystal (PhC) nanocavity [78,79]. The sketch of the device proposed by the Japanese authors is reported in Figure 19. The nanocavity is characterized by a lattice constant a = 420 nm, hole radius r = 108 nm, slab thickness t = 204 nm, Wi = 8.72 μm and Ww = 8.4 μm; whereas the input and output waveguides are 1.05a wide and are in-line connected with the cavity through barrier line defects 0.98a wide and 11a long.

The PhC nanocavity is characterized by an intrinsic Q factor of 8.4 × 10⁵. This high Q value permits the obtention of a high generation of photocurrent into the cavity region, enabling the possibility of detecting even low input power. In particular, an external quantum responsivity of 16 mA/W at a resonance wavelength of aproximately 1,500 nm, has been measured. Furthermore, on the one hand, the very small dimensions of the device allow achievement of a low dark current value (about 15 pA at room temperature for 3 V of reverse bias), and on the other hand, a small capacitance (a value of 9.5 × 10⁻¹⁸ F was estimated). The resistance for the p and n regions limits the speed of the proposed device at 0.1 GHz.

A silicon microdisk resonator with a laterally embedded p-i-n diode was chosen by Chen et al. [80] to enhance the TPA photocurrent. TPA is a very weak effect which is difficult to exploit for the realization of bulk all-silicon photodetectors with appreciable efficiency. For this reason, as shown in the literature, TPA exploitation in bulk silicon devices at 1,550 nm is aimed at improving the sensitivity of two-photon absorption autocorrelators, utilizing, as the two-photon absorber, a silicon avalanche photodiode (APD) [81–83]. The avalanche process in APDs amplifies the weak IR two-photon absorption photocurrent, thereby greatly improving the sensitivity of the autocorrelation measurement. More recently, TPA in all-silicon bulk detectors has been exploited for a novel profilometry technique at λ = 1.55 μm [84]; compared with conventional profilometry, TPA has a much wider dynamic range without the need for high-speed devices or complicated computation. Shi et al. [85] fabricated a hemispherical nearly-intrinsic silicon TPA photodetector operating at a wavelength of 1.3 μm and emitted from a CW laser. The detector was made of nearly-intrinsic silicon crystal with a resistivity of 6,000 Ω/cm and made into a hemisphere with a radius of 3 mm. The silicon hemisphere was used as both a detector and a solid immersion lens (SIL) in the experiments. The bottom of the detector is (1̅10) plane, on which aluminum electrodes were evaporated. The electrodes consisted of concentric circular and annular metal contacts with a spacing of 0.15 mm and a radius of 0.5 mm for the central electrode. The contacts between the aluminum and the silicon hemisphere are considered to be ohmic contacts. Because TPA substantially occurs in the vicinity of the focused spot (the center of the hemisphere), the concentric electrodes collect the photo-excited carriers efficiently.

The responsivity of the detector was about 2 μA/W at 1,300 nm and 1 V of applied bias. This silicon photodetector is easy to fabricate and is useful in autocorrelation for measuring ultra-short laser pulses with wavelengths in the region of 1.2–2.1 μm.