Si photodiodes have been deployed in numerousapplications owing to their low cost, low noise, andcompatibility with complementary metal-oxide semi-conductor (CMOS) technology [1,2]. There has beensignificant research on techniques to maximize thespectral range over which high quantum efficiencycan be achieved [3–6]. UV responsivity can beenhanced by hydrogenated amorphous silicon [3] orhybrid material structures like GaN/Si [4] anda-Si:H/a-SiC:H [5]. A SiGe/Si structure [6] has beenused to enhance the photon detection in the IR range.Surface texturing provides a broad-spectral-band,antireflection coating and enables light trappinginside the semiconductor, which improves opticalabsorption [7]. The Si photovoltaic industry hasemployed several surface-texturing techniques toenhance solar cell efficiency [8–10]. Previously, ahigh responsivity of 119A=W at 960nm has beenachieved in a laser-textured Si photodetector withsulfur doping [11]; however, the 3dB bandwidth isonly 1:2kHz as a result of a high density of deep-leveltraps. A backside-illuminated, laser-textured com-mercial Si p-i-n photodiode has demonstratedimproved quantum efficiency in the NIR spectral
range due to better long-wavelength light trapping[12]. In this paper, a top-illuminated, laser-texturedSi photodiode with a 300nm shallow p-n junctiondepth is reported. A wide spectral range from theUV (220nm) to NIR (1100nm) is detected. An exter-nal quantum efficiency of is 62% achieved at the245nm wavelength, which is comparable to commer-cial UV-enhanced Si photodetectors. An optical tran-sient response measurement yielded a bandwidth of60MHz for 1064nm incident light.
2. Experimental
A. Texture Formation
p-type float-zone monocrystalline Si (100) (resistivity2Ω cm) wafers were diced into 2 cm × 2 cm chips. Thechips were mounted on an X-Y stage inside avacuum chamber (base pressure ∼1mbar). Thechamber was then backfilled with 400mbar of sulfurhexafluoride gas. The Si chips were textured byraster scanning with laser pulses (pulse energy0:6mJ, wavelength 800nm, and pulse duration130 fs) at a repetition rate of 1KHz from a regenera-tively amplified Spectra Physics Ti-sapphire lasersystem. The laser spot diameter was 450 μm andthe scanning speed was 6mm=s. Scanning wasadjusted such that laser lines overlapped suitablyto generate a uniform surface texture over the siliconsurface. The laser beam was focused onto the sample
surface with a 200mm focal length lens and the laserfluence was adjusted to be 0:9 J=cm2 using a Glanlaser calcite polarizer. The mean height of the peaksof the as-textured surface was estimated to be 4 μm.
B. Device Fabrication
To remove laser-induced damage, as-texturedsurfaces were etched in a solution of 25% NaOHfor 40 s followed by isotropic etching (HNO3:CH3COOH:HF, 30∶10∶4) for 4 s. Finally, the sampleswere cleaned using the Interuniversity Microelectro-nics Center process [13] to remove organic and metalcontamination. An nþ-p junction was formed byapplying a commercially available phosphorus-dopedoxide (P509 from Filmtronics) spin-on-dopant solu-tion on the textured surface and diffusing in a furnacefor 15 min at 900 °C. Note that this results in a shal-low (∼300nm) p-n junction that follows the contour ofthe textured surface. After the diffusion, the spin-onfilmwas removed by dipping in dilute HF solution. Toachieve surface passivation the wafers were dryoxidized at 800 °C for 30 min, followed by depositionof 70nm of SiNx by plasma-enhanced chemicalvapor deposition. Front-illuminated mesa structureswith a diameter of 250 μm were fabricated on thesepassivated, textured Si wafers by inductive coupledplasma reactive ion etching. Buffered HF etchantwas used to remove part of the oxide thin film onthe textured surface to open the contact area formetaldeposition. Ti/Au (200nm=5000nm)wasdeposited onthe front surface as the p- and n-type contacts. An Alfilm was deposited on the back side as a reflector forlong-wavelength light. Additional details about thesurface texturing and cleaning process are presentedin [14]. Figure 1(a) and 1(b) shows the as-texturedsurface and the surface after chemical etching andpassivation. Figure 1(c) is a completed photodiode.
3. Results and Discussion
Figure 2 shows the dark current versus bias of thetextured Si photodetector. The dark current reaches100nA at 10V reverse bias; this relatively high darkcurrent is probably due to lack of passivation of themesa side walls with a thermal oxide after the mesaetch. The slow increase of current under forward biasindicates a relatively high contact resistance, whichis attributable to the Schottky-like contact betweenthe lightly-doped p-type substrate (1016=cm−3) andthe p-metal contact.
The external quantum efficiency was measuredfrom 220nm to 1100nm. A xenon lamp filtered by agrating monochromator was used to generate amonochromatic in-signal from 220nm to 600nm.The incident light spot was focused inside the n-typecontact ring, which is about 150 μm in diameter. Thephotocurrent was then measured with a lock-inamplifier. By comparing with the response of aUV-enhanced Si photodiode, which is continuouslycalibrated and NIST traceable, the quantum effi-ciency of the textured photodiode in the spectralrange between 220nm and 600nm was calculated.
Fig. 1. SEM images of (a) as-laser-textured surface; (b) surfaceafter chemical etching and passivation; (c) photodetector fabri-cated on laser-textured surface.
Fig. 2. Dark current versus voltage characteristic for the 250-μm-diameter textured Si photodetector.
In the spectral range 220nm to 600nm the quantumefficiency was calculated by comparing the responseof the textured photodiode to that of a calibratedUV-enhanced Si photodiode. Similarly, a tungsten-halogen lamp and a Si photodiode calibrated inthe range from 350nm to 1100nm were used todetermine the quantum efficiency from the visibleto the NIR. To remove the higher-order short wave-lengths generated in themonochromator, a high-passoptical filter with a cutoff wavelength of 500nm wasused for wavelengths >500nm. Figure 3 shows themeasured external quantum efficiency from 220nmto 1100nm. The measurement was carried out at−1V bias. Further increasing the reverse bias didnot significantly enhance the response. The photoresponse is relatively flat with a quantum efficiency>80% from 490nm to 780nm. The broad spectralresponse can be attributed to the antireflection char-acteristics of the laser-textured surface. The quan-tum efficiency remains above 70% from 430nm to890nm. Compared with the improved NIR responseof laser-textured Si in reference [12], the quantumefficiency drops off very quickly from 900nm to1100nm. This is due to the fact that these texturedphotodiodes have a much smaller active area thatresults in decreased light trapping for longer-wavelength light. Better optical confinement canbe achieved by using a larger detector area or byreducing the wafer thickness. Both the textured sur-face and the shallow p-n junction (300nm) enhancethe collection efficiency of minority carriers gener-ated near the surface by UV light. As a result, thequantum efficiency at 245nm is 62%, which iscomparable to that of commercial UV-enhanced Siphotodetectors. The high UV efficiency indicatesthat surface recombination does not overwhelm theresponse. We conclude that texturing followed bysurface passivation results in a relatively defect-freesurface. There is a local minimum at 280nm wherethe efficiency drops to 42%, which is the result of theoptical absorption coefficient peak of Si at 4:4 eVphoton energy [15]. Similarly, the quantum efficiency
peak at 245nm wavelength can also be attributed toa local minimum in the absorption coefficient at5:1 eV photon energy [15]. Compared with the com-mercial UV Si photodetector, the quantum efficiencyof the textured Si detector between 250nm and350nm is lower. This may be due to the fact thatthe light penetration depth in this range is very shal-low and therefore surface recombination plays agreater role in the photocurrent response. Surfacedefects caused by texturing become more important,as does the fact that the textured surface area ismuch larger.
The bandwidth was extracted from measurementsof the temporal response to 1064nm optical pulseswith peak power of 2mW. The optical pulse widthwas less than 1ns and the detector was biased at−1V. Figure 4(a) shows the transient response.The full width at half-maximum (FWHM) and falltime were 7:3ns and 10ns, respectively. The −3dBbandwidth was estimated as 60MHz as determined
Fig. 3. (Color online) External quantum efficiency of textured Siphotodetector and two commercial (UDT) calibrated Si photodetec-tors under −1V bias condition.
Fig. 4. (Color online) (a) 1064nm optical transient response oftextured Si photodetector under 0V bias condition. (b) Fouriertransform of the pulsed transient response of the textured Siphotodetector.
from the Fourier transform of the transient response[Fig. 4(b)]. Since the absorption at 1064nm is weak,many of the carriers are generated far below thesurface, which results in long, diffusion-limitedlifetimes.
4. Conclusion
A top-illuminated, surface-textured Si photodetectorhas been characterized under −1V bias condition.The 250-μm-diameter detector exhibited FWHM of7:3ns and bandwidth of 60MHz for 1064nm light.The external quantum efficiency achieved broadbandresponse with quantum efficiency>70% from 430nmto 890nm and high UV quantum efficiency of 62%at 245nm.
This work has been supported as part of the Centerfor Energy Nanoscience, an Energy FrontierResearch Center funded by the U.S. Department ofEnergy (DOE), Office of Science, Office of BasicEnergy Sciences under award no. DE-SC0001013.
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