High nsitivity silicon-bad VIS/NIR photodetectors
James E. Carey1, Catherine H. Crouch1,2 †, Michael A. Sheehy3, Mengyan Shen2, Cynthia M. Friend3, and Eric Mazur1,2*
1Division of Engineering and Applied Sciences, 2Department of Physics, and 3Department of Chemistry and Chemical Biology, Harvard University, 9 Oxford St., Cambridge, MA 02138
We report fabrication of a silicon-bad photodiode that is ten times more nsitive than commercial silicon PIN photodiodes at visible wavelengths and that can be ud at wavelengths up to 1650 nm. At room temperature and –0.5 V the responsivity of the device is 54.4 A/W at 1000 nm, 38 mA/W at 1300 nm, and 25 mA/W at 1550 nm. We fabricate the devices by irradiating a crystalline silicon wafer with femtocond lar puls in the prence of sulfur hexaflouride. The irradiation creates a diode junction between the undisturbed substrate wafer and a highly disordered sulfur-doped surface layer.
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* Electronic mail: mazur@physics.harvard.edu; Fax: +1 617/495-9837
经典残局† Prent address: Department of Physics, Swarthmore College, Swarthmore, PA 19081
Silicon (Si) is the most commonly ud miconductor in microelectronics and optoelectronic devices. Becau the band gap of ordinary silicon is 1.07 eV, the absorption and the photorespon of silicon decrea precipitously for wavelengths longer than 1100 nm. Conquently, crystalline Si photodetectors are innsitive to the two primary telecommunications wavelengths, 1300 nm and 1550 nm. Therefore other miconductor materials, such as germanium (Ge), are typically ud in detectors for the wavelengths, but the materials are more expensive and difficult to integrate into silicon-bad microelectronics. Devices using SiGe alloys on Si achieve responsivities (photocurrent per unit incident power) as high as 150 mA/W at 1300 nm but require complex fabrication techniques [1-3]. Extending the nsitivity of Si-bad photodetectors to longer infrared wavelengths is therefore an active area of rearch [1-4].
We previously reported that silicon surfaces microstructured in the prence of sulfur containing gas with high-intensity femtocond [5, 6] or nanocond [7] lar puls have near-unity absorption from the near-ultraviolet (250 nm) to the near-infrared (2500 nm), at photon energies well below the band gap of ordinary silicon [8, 9]. Here we report that under rever bias, the microstructured surfaces can be ud to fabricate photodiodes that are nsitive to infrared wavelengths up to 1650 nm and that have greatly enhanced photorespon to visible wavelengths.
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To microstructure our samples, we irradiate n-doped, Si (111) wafers (r = 8–12 W•m) with a 1-kHz train of 100-fs lar puls at normal incidence in an 0.67-bar atmosphere of
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. Each pul has an average fluence of 4 kJ/m2 and a 1/e spot size of 150 x 150 m m2.
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数学课In order to structure an area larger than this spot size, the Si wafer is translated relative to the lar beam at a speed such that any given spot on the surface is expod to an average of 500 puls. The resulting surface is covered with conical microstructures that are 2–3 m m tall and spaced by 2–3 m m; the uppermost few hundred nanometers of the microstructures are modified, with crystalline silicon beneath [6].
Following lar irradiation, samples are annealed in flowing argon gas for 30 minutes at 800 K. Electrical contacts are then fabricated by thermal evaporation of a 4-nm layer of Cr followed by a 100-nm layer of Au on both the structured and unstructured sides. Metal is deposited on the entire unstructured side; on the microstructured side, metal is deposited in two circular areas 1 mm in diam
eter and 3 mm apart center-to-center. The resulting devices have a 2 ¥ 4 mm2 active area. Figure 1 shows a schematic diagram of a vertical cross-ction through the device and a bright-field transmission electron micrograph of a cross-ction of the interface between the lar-affected surface layer. We measure the spectral responsivity of the devices using light from a 300-W xenon arc lamp that is pasd through a monochromator with a spectral resolution of 0.1 nm. The monochromator is scanned in 25-nm increments over the range 400 – 1650 nm. Using a calibrated photodiode and a variable neutral density filter, we adjust the power at each wavelength to 1 m W and then measure the photocurrent through the device. Figure 2 shows the spectral responsivity of the microstructured silicon at a bias of –0.1 V and –0.5 V as well as the responsivity of the standard Si PIN photodiode ud for calibration. At a
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bias of –0.1 V, the responsivity of the device is greater than 1 A/W for wavelengths from 400 nm to 1175 nm, with a peak responsivity of 12.1 A/W at 975 nm. The responsivities are nearly ten times greater than tho of commercially available Si PIN photodiodes. In addition, the responsivity does not drop steeply until 1200 nm, 100 nm further into the near infrared than commercially available Si PIN photodiodes. At longer wavelengths (1200-1650 nm), the responsivity is weaker, but still remarkably high for Si-bad devices. For a bias of –0.1 V we measure a responsivity of 14 mA/W at
1300 nm and 11 mA/W at 1550 nm. For a bias of –0.5 V bias the responsivity ris to 38 mA/W at 1300 nm and 25 mA/W at 1550 nm—four orders of magnitude higher than responsivities measured in Si avalanche photodiodes before amplification—and is comparable to levels reported for devices bad on SiGe alloys [1-3, 10]. Relative to the photocurrent measured with zero bias, we measure responsivities approximately 100 and 400 times higher for –0.1 V and –0.5 V, respectively. Both gain and photocurrent increa with bias voltage, although the dark current and noi also increa. When the incident power is incread the responsivity decreas but still remains substantially higher than that of a standard Si photodiode.
祖逖闻鸡起舞Figure 3 shows the current-voltage characteristics of the device without illumination. The diodic curve shows low dark current values for rever bias up to –3 V. The dark current density at –1.0 V bias is 3.5 A/m2; as the rever bias is incread to –20 V, the dark current increas to 115 A/m2. At rever bias voltages exceeding –100 V, the dark current increas rapidly with time due to heating. If the bias voltage is removed and the devices are allowed to cool, the dark current returns to its original value.
Changing the lar processing conditions allows optimization of devices for particular applications. For instance, increasing the lar fluence to 8 kJ/m2 produces a device with less dark current and gr
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eater open circuit voltage, but with slightly less photocurrent. Such devices are better suited for photovoltaic applications.
To understand the remarkable current-voltage characteristics and spectral responsivity of the devices, we examined the structure and composition of the lar-affected surface layer. Bright-field transmission electron microscopy indicates that the surface layer consists of silicon nanocrystallites and nanopores embedded in a disordered matrix; lected area electron diffraction from the damaged region indicates that the region has crystalline order [6]. In addition, we measured Raman spectra of microstructured samples (not shown). The show a red-shift and broadening of the main Si peak, which is consistent with the prence of micro- or nanocrystallites [11]. Secondary ion mass spectrometry and Rutherford backscattering measurements show that the atomic concentration of sulfur in the surface layer is about 1% [6-8]. Given that sulfur is more soluble in amorphous Si than in crystalline Si [12], the majority of the sulfur is mostly likely contained within the disordered part of the lar-affected layer rather than the nanocrystallites. Becau the lected area diffraction pattern is dominated by the signal from the nanocrystallites, it is difficult to determine the detailed structure of the disordered matrix surrounding the nanocrystallites. However, the structure of the matrix is likely to be similar to amorphous silicon; the rapid cooling that follows short-pul irradiation has long been known to form amorphous material [13], and the plume material
