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Femtosecond laser-drilling-induced HgCdTe photodiodes F.-X. Zha,1,2,* M. S. Li,1 J. Shao,2 W. T. Yin,3 S. M. Zhou,1 X. Lu,2 Q. T. Guo,1 Z. H. Ye,3 T. X. Li,2 H. L. Ma,1 B. Zhang,2 and X. C. Shen2,1 1
Physics Department, Shanghai University, Shanghai 200444, China National Laboratory for Infrared Physics and Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China 3 Research Center for Advanced Materials and Devices, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China *Corresponding author:
[email protected]
2
Received December 3, 2009; revised February 24, 2010; accepted February 24, 2010; posted March 2, 2010 (Doc. ID 120675); published March 24, 2010 Femtosecond-laser drilling may induce holes in HgCdTe with morphology similar to that induced by ionmilling in loophole technique. So-formed hole structures are proven to be pn junction diodes by the laser beam induced current characterization as well as the conductivity measurement. Transmission and photoluminescence spectral measurements on a n-type dominated hole-array structure give rise to different results from those of an ion-milled sample. © 2010 Optical Society of America OCIS codes: 040.3060, 220.4000, 300.6470.
Laser drilling is used to remove the material of the irradiated region with high beam powers. The technique has in recent years been broadly used in micromachining various micro-/nanostructures or devices [1–5]. Our recent experiments have also shown that laser drilling may induce local electrical typeinversion in p-type HgCdTe, forming a localized pn junction associated with the hole [6]. As a potential fabrication tool for photodiode array, laser drilling is of unique advantages in comparison with the conventional hybrid IR focal plane array techniques [7,8]. For instance, the diode formation with laser drilling is a “clean” technique free of lithography and allows directly beam-writing of a diode array. Moreover, the array-fabrication techniques such as the beaminterference in laser drilling [4,5] also allow the method to be a time-efficient fabrication tool for large-scale array devices. Most attractively, the noncontact character of the laser beam-writing opens an opportunity to fabricate the diodes allowing the materials to be maintained in an isolated growth environment. In this Letter, we report the electrical and optical properties of laser-drilled holes. The material used is a vacancy-doped p-type HgxCd1−xTe 共x = 0.24兲 layer grown on a CdZnTe substrate by liquid phase epitaxy. The thickness of the epilayer is approximately 20 m. The materials were first passivated with a 0.25-m-thick ZnS layer on top before the diode fabrication process. The experimental setup on laser drilling was described elsewhere [6,9]. The pn junctions were characterized with a laser-beam-induced current (LBIC) microscopy at the temperature of 77 K [6]. The conductivity experiment was conducted with Keithley 4200 source-measure units using a home-built probe station at 130 K without cold shield and with the diode effectively viewing a 2 sr room temperature background. Optical characterization was performed with a Fourier-transformation infrared spectrometer including room-temperature 0146-9592/10/070971-3/$15.00
transmission (TR) and 77 K photoluminescence (PL) measurements [10]. The inset in Fig. 1 shows the atomic force microscopy image of a laser drilled hole generated with a beam power of 30 mW and with a 20⫻ objective lens for beam focusing. The corresponding line profile across the hole is indicated by the solid curve in Fig. 1. The dotted curve is the line profile of the other hole, formed with the same beam power (30 mW) but using a 100⫻ objective lens for beam focusing. It is surprising that the depth profiles with the two lenses are quite different. Regardless of the interesting dynamics on the phenomenon, we simply point out the resemblance between the morphologies induced with the 20⫻ objective lens and the loophole structure [7,8]. The pn junction formation of the hole was corroborated by both the LBIC and conductivity experiments as shown below. The LBIC result is illustrated in Fig. 2(a). According to the principle of the LBIC [6,11], the curve fea-
Fig. 1. (Color online) Atomic force microscopy of a laser drilled hole generated with 30 mW beam power and the objective lens of 20⫻. The inset presents a three-dimensional plot of the image. The solid curve is the topographic line profile across the hole as designated by the dashed line in the inset. The triangle topographic line profile (dotted curve) results from a laser drilled hole formed with the use of a 100⫻ objective lens and the same laser power. © 2010 Optical Society of America
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Fig. 2. (Color online) Electrical properties of holes formed with 60 mW beam power and a 20⫻ objective lens in laser drilling. (a) A LBIC curve across the hole along with the illustration of the pn junction model. The marked distance of 24 m measures the lateral dimension of the typeconversed area which is associated with a hole of 12 m diameter. (b) The reverse bias I/V characteristics and the dynamic differential resistances of a laser drilled hole (solid dotted curve) and an ion-milled diode (open circle curve). The inset is the micrograph of a measured hole (The white bar represents 20 m), around which the golden square is the evaporated gold electrode.
ture of a pair of positive/negative peaks associated with the hole demonstrates a loophole-type pn junction, which consists of a central n region and the surrounding outer part of the p region. Regarding Fig. 2(a), the overall diameter of the photosensitive area is approximately 50 m. The distance between the positive and negative peaks measures the lateral dimension of the conversed area (n region), which is 24 m, larger than the hole’s diameter of 12 m. For the conductivity experiment, gold electrodes were evaporated on both the holes and a remote p region. The electrodes covering the holes are prevented from short-circuit to the p region due to the existence of ZnS passivation layer. The micrograph of such a hole is shown in the bottom inset in Fig. 2(b). The current/voltage I/V characteristics display an obvious rectification effect. In order to have insight into the junction mechanism, the results shown in Fig. 2(b) are presented in logarithmic plots of the currents along with the plots of the dynamic resistances 共RD兲. The solid dotted lines result from the laser-drilled hole. For comparison, the results of an ion-milled loophole diode are shown together by the open circle lines. In contrast to the linear slope of the ion-milled diode, the I / V characteristic of the laser-drilled hole manifests a turn at the location designated by the dashed circle. Similar feature was ever observed with a n+–n–p junction [12]. On the other hand, the hump in the RD ⬃ voltage curve is also similar to that shown by Elliott et al. [13], who took into account the impact ionization along with the band to band tunneling to
interpret the reverse bias feature of a p–n–-type diode. Regarding those models, the laser drilled pn junctions measured are likely a type of asymmetry junction. The maximum RD is reasonably high 共⬃1 M⍀兲, optimistic for application consideration. Regarding the dynamic process of junction formation, laser drilling is different from ion milling. In ion milling, a local type-inversion in a p-type HgCdTe monolith is due to the diffusion of excess Hg atoms created by the ion bombardment in the surface and the depth of the type-inversion is proportional to the etching time, typically tens of minutes [7,8]. In laser drilling, the material is removed with the intense surface vaporization in extremely short time (microseconds or less) [14]. In order to have insights into the type-inversion mechanism, TR and PL experiments were applied. Because a hole is much smaller than the minimum light spot size available in the optical experiments, the measurable signal intensity should be collected from a set of holes instead of an individual. In order to focus on the property of the type-inversion region, a hole array was designed by suitable choice of the pitch of the array. Referring to Fig. 2(a), the peak distance of 24 m in fact defines the dimension of the hole’s n region. Thus, the optical property of a hole array with the pitch of ⱕ24 m should be dominated by the type-inversion region. The pitch of the hole array in the experiment is 18 m. The micrograph of a part of the sample is shown in the inset in Fig. 3(a). The overall dimensions of the array are 0.9 mm⫻ 1 mm, adequate for the optical experiments which involve light spots less than 0.5 mm in diameter generally. Figure 3(a) displays the results from both the hole array (solid line) and a p region (dashed curve). For comparison, the same experiments were also conducted on an ion-milled sample fabricated with the same material as shown in Fig. 3(b) (solid curve), which also includes the result of its p region for comparison (dashed curve). Generally, the spectral feature of the laser drilled holes is conformable to the others, indicating that the laser drilling has not altered the band structure of the material. Comparing with the unprocessed p regions, both the laser-drilled and ion-milled samples display gradual variation tails (as marked by line A in Fig. 3) at the lower energy aspect of the absorption edge (as marked by line B), which should be attributed to impurities induced by laser drilling or ion milling. Note in Fig. 3(b) that both the absorption edge and PL peak are obviously blueshifted from the counterparts of the p regions, with the amounts of 5 and 9 meV, respectively, whose difference arises from the measurement temperatures (room temperature and 77 K, respectively). In terms of an ion-milled sample, the blueshift is a generally observed phenomenon [15,16]. The missing of blueshift with the laser drilled sample might imply a different donor mechanism from that for ion milling. Nonetheless, the data are not yet sufficient to exclude the contribution of mercury, supposing that fast diffusion of Hg atoms was still possible in the ultrashort period concurrently with the evaporation of
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vantages, which enable one to fabricate the devices directly with materials being maintained in a growth environment so as to minimize the generation of blind elements of the diode array. F.-X. Zha acknowledges the Shanghai Leading Academic Discipline Project (number S30105) and the innovation project of Shanghai University. J. Shao acknowledges the Program of Shanghai Subject Chief Scientist (08XD14047). The authors thank Ning Li and Fei Yin for their help in this work. References
Fig. 3. (Color online) Room temperature TR spectra and 77 K PL for both a laser drilled hole-array and an ion milled sample. The inset in (a) is the micrograph of a laser drilled array with a white bar representing 50 m. TR and PL spectra of both the hole-array and the ion-milled sample region are shown by the solid lines in (a) and (b), respectively. The data of the p regions along with each sample are shown correspondingly by the dotted lines in each frame. The TRs of both laser drilled and ion-milled samples display gradually varied tails at the lower energy aspect (indicated by line A) of the absorption edge (indicated by line B). In contrast to the data of the laser drilled sample in (a), the PL and TR of the ion milled sample in (b) display obvious blueshifts with the amounts as indicated.
most atoms in laser ablation. The donor mechanism and the pn junction properties are the important topics that deserve further attention in the next step research. In conclusion, this Letter shows the pn junction formation of laser drilled holes with the LBIC and conductivity experiments. Optical characterization seems to imply a different type-conversion mechanism for laser drilling from that for ion milling. The most attraction of laser drilling for device fabrication may be due to its noncontact and lithography-free ad-
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