APPLIED PHYSICS LETTERS 93, 261104 共2008兲

Terahertz Si:B blocked-impurity-band detectors defined by nonepitaxial methods P. Rauter,1,a兲 T. Fromherz,1 S. Winnerl,2 M. Zier,2 A. Kolitsch,2 M. Helm,2 and G. Bauer1 1

Institute of Semiconductor and Solid State Physics, University of Linz, Altenberger Str. 69-Linz, Upper Austria 4040, Austria 2 Institute of Ion Beam Physics and Materials Research, Forschunsgzentrum Dresden-Rossendorf, P.O. Box 510119, 01314 Dresden, Germany

共Received 30 October 2008; accepted 9 December 2008; published online 29 December 2008兲 The molecular beam epitaxial 共MBE兲 fabrication of blocked-impurity-band detectors 共BIB兲 has been a technologically complex and delicate matter ever since its demonstration in silicon, and has not been adapted for other material systems offering detection onsets at lower terahertz frequencies. We report the fabrication and characterization of a vertical Si:B BIB, circumventing the intrinsically troublesome MBE growth of an ultrapure blocking layer by employing ion implantation. We present a thorough characterization of our device, which exhibits highly competitive figures of merits. Our results not only increase the accessibility of BIB fabrication tools for ultrasensitive terahertz detection but also open a road to other material systems. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3059559兴 Blocked-impurity-band 共BIB兲 detectors are state-of-theart devices for highly sensitive detection of midinfrared to far infrared radiation. They are employed for astronomical imaging, e.g., in the orbital Spitzer Space Telescope. The BIB concept, as introduced in Ref. 1, is based on the infrared absorption of charge carriers frozen out in the impurity band formed in a relatively highly doped semiconductor layer 关impurity layer 共IL兲兴. The generation of dark current via hopping transport within the impurity band is prevented by growing an ultrapure intrinsic layer 关blocking layer 共BL兲兴 on top of the IL. Counterdoping of the IL introduces a charged depletion region under an externally applied bias and results in a strong field increase near the BL, enabling impact ionization of impurity-band carriers and the operation of BIB structures as solid state photomultipliers.2 Up to now the BIB concept has been applied in Si-based structures grown by molecular beam epitaxy 共MBE兲, where the growth of a sufficiently pure BL on top of a highly doped IL is a technologically extremely delicate process. Si BIBs are sensitive to infrared radiation down to 200 cm−1,3 where the goal to push the sensitivity of BIBs further into the far infrared has induced considerable endeavor to implement the concept in the Ge 共Ref. 4兲 and GaAs 共Ref. 5兲 material systems. However, realization was prevented by technological difficulties in growing a BL of sufficient quality, even though theoretical considerations aimed at finding alternate operation modes for circumventing these difficulties.6 The proposal of abandoning the conventional, but troublesome and Si-confined, BIB fabrication technology of MBE growth and use ion implantation as a means of introducing the required doping concentrations into an ultrapure bulk substrate is highly promising. It dangles the possibility of finally extending the BIB concept to other material systems as well as a technologically simpler means of fabricating Si-based BIBs.7 However, the device presented in Ref. 7 is a planar one, holding intrinsic disadvantages concerning array fabrication and exhibiting noncompetitive performance. a兲

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0003-6951/2008/93共26兲/261104/3/$23.00

We report the fabrication of ion-implanted vertical Si:B BIB structures using a backside etching process through the Si substrate in order to establish an electrical contact to the buried BL. The devices where characterized by Fourier infrared spectroscopy and by measuring the photocurrent 共PC兲 response to blackbody radiation, revealing highly competitive figures of merit. The samples were fabricated by implanting the required high-concentration boron doping as well as the lowconcentration phosphorus counterdoping using the highenergy ion-implantation facilities at the Forschungszentrum Dresden-Rossendorf. The high-purity device layer 共phosphorus concentration ⬍1013 cm−3兲 of a silicon-on-insulator 共SOI兲 wafer served as a substrate, enabling the use of intrinsic pure pulled bulk silicon as a favorable substitute for the conventional BL grown by MBE. As stated in Ref. 3, an impurity concentration below 5 ⫻ 1013 cm−3 is required in order to sufficiently suppress dark current up to high applied voltages and ensure a low-noise/high-gain operation of the BIB device. The 10 ␮m thick device layer is separated from the 300 ␮m thick handling wafer by 1 ␮m of thermally grown oxide. The implantation parameters were chosen in a way to establish homogeneous boron and phosphorus concentration profiles down to a substrate depth of 6 ␮m, leaving a BL of 4 ␮m thickness. As suggested by Crystal-TRIM simulations, the dopants were implanted in four steps, where the boron implantation energies were 150 keV, 1 MeV, 2 MeV, and 3 MeV, and those for phosphorus were 900 keV, 2.7 MeV, 5.5 MeV, and 8 MeV. Samples S1 and S2 differ in both boron and phosphorus doping, where the target concentrations for S1 were 5 ⫻ 1017 cm−3 共implantation dose of 5 ⫻ 1013 cm−2兲 and 5 ⫻ 1014 cm−3 共dose of 5 ⫻ 1010 cm−2兲, respectively. The implantation process of S2 aimed at concentrations of 1 ⫻ 1018 cm−3 共boron dose of 1 ⫻ 1014 cm−2兲 and 2 ⫻ 1015 cm−3 共phosphorus dose of 2 ⫻ 1011 cm−2兲. Note that the used implantation energies result in a very thick IL of 6 ␮m, which is employed in our work following conventional BIB dimensions3 for a study of the principle, but can be scaled down and optimized for further applications in

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© 2008 American Institute of Physics

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FIG. 1. 共Color online兲 Spatial variation in energy bands and impurity levels in a biased BIB structure. The thick lines show the calculated bending of the VB and conduction band of S1 at an externally applied voltage of 1.88 V. The thin lines indicate the impurity-band edges, where the ⫺ and + symbols represent the ionized acceptor and donor atoms, respectively. The inset sketches a vertical cross section of the device together with the fabrication steps as described in the text.

order to utilize implantation at low energies of standard facilities. The ion implantation was followed by a 90 min long annealing step at 1100 ° C. The device layer was structured into mesas of 400⫻ 400 ␮m2 by reactive ion etching. A contact to the BL was established by etching holes aligned to the mesas through the handle wafer from the sample backside. The Bosch process employed for this purpose stopped at the SOI wafer’s thermal oxide, which was afterward removed by hydrofluoric acid. Aluminum contacts were deposited on both sides of the structure and alloyed at 380 ° C. The samples’ performance was measured by applying voltage to the mesa top contact 共IL兲 in respect to the grounded back contact 共BL兲. Thus, operation in the gain mode is expected for positive applied bias. Figure 1 sketches the devices’ fabrication and band structure. The samples’ dark current characteristics were measured in a liquid helium vessel, assuring low radiation background. The broken lines in Fig. 2, showing the dark current curves, exhibit a strong asymmetry due to the devices’ structure. At a negative bias of as low as ⫺1 V, electric fields are sufficiently high for a breakthrough and a strong increase in dark current results. This is due to the fact that for negative bias −1

10

Blackbody S1 Dark S1 Blackbody S2 Dark S2

−3

Current (A)

10

−5

10

10−7 10−9 10−11

T=4.2K

−13

10

−2

−1

0

1 2 Voltage (V)

3

4

FIG. 2. 共Color online兲 I-V characteristics. The broken lines show the dark current curves of S1 and S2 exhibiting a strong asymmetry and featuring breakthrough for high negative voltages. Note S1’s thresholdlike behavior for positive voltages, which is due to the onset of dark current gain. The solid 共S1兲 and dotted 共S2兲 lines present both samples’ responses to 300 ° C blackbody radiation. Similar to the dark current, the PC response of S1 shows threshold characteristics and features a sharp rise over two orders of magnitude, indicating the onset of high PC gain by impact ionization.

all of the applied voltage drops at the BL.8 In contrast, for both samples the dark current at positive bias remains below 10−10 A up to higher applied voltages, as expected from a BIB featuring a BL of sufficient purity. At a low temperature of 4.2 K, charge carriers freeze out in the impurity band, and even though holes can be efficiently transported within the impurity band by hopping, at low bias the BL prevents the carriers from reaching the sample’s back contact. Next to the BL-IL interface a depletion region is established. The phosphorus ions lead to a significant voltage drop over the depletion region, where the depletion width decreases with increasing counterdoping concentration.1 In an optimized BIB structure the counterdoping concentration is therefore chosen to be as low as possible while maintaining homogeneity, which is required to prevent local high-field regions forming dark current leaks. As expected from both the higher boron 共stronger impurity-band broadening兲 and phosphorus 共higher electric fields兲 concentrations, the dark current of S2 starts to increase at lower voltages of 1.5 V than for S1 at 1.9 V. For S2 it originates from impurity-band carriers tunneling into the valence band 共VB兲 at the high-field interface between IL and BL and rises slowly, in contrast to the breakthrough at negative voltages. On the contrary, for S1 the dark current remains below 10−10 A up to 1.9 V and then rises abruptly, indicating that the tunneling current experiences gain by impact ionization of impurity-band carriers. The reproducible dip in the dark I-V characteristic at 2.1 V is due to a change in field distribution and/or a self-stabilization of the gain process.9,10 The detector performance of the structures was studied by illuminating the helium-cooled samples with infrared radiation through a polyethylene outer and a diamond inner cryostat window. The solid lines in Fig. 2 present the samples’ current response to the chopped radiation of a blackbody radiator at 300 ° C, measured in lock-in technique. For S2, the PC response increases slowly, where only at a high bias of 3.3 V a current jump of a factor of 2 can be identified as the onset of gain. However, at this voltage the dark current is far too high for proper BIB operation, indicating that the counterdoping for S2 is too high to establish a sufficiently wide gain region prior to an increase in dark current. The PC response for S1 exhibits a sharp increase in PC between 1.8 and 1.9 V by two orders of magnitude, which can only by attributed to the onset of gain. Above 1.8 V a charge carrier excited by the blackbody radiation from the impurity band into the VB gains sufficient energy, while drifting along the field of the depletion region to cause a charge avalanche by impact ionization. Concluding from the height of the jump, PC gain by impact ionization reaches values of about 100. In order to quantify the structure’s responsivity, its photospectral characteristic was measured using Fourier transform spectroscopy in the far infrared and midinfrared. Figure 3 presents S1’s spectrally resolved responsivity at a bias of 1.7 V, as calculated from the measured PC spectra, the I-V characteristics, the known spectral radiation density of the blackbody, and the transmission of the cryostat windows. The strong noise above 100 meV is due to the polyethylene window’s weak transmission at these energies. The onset of the infrared response lies at about 30 meV, which is consistent with Ref. 3 and significantly below the isolated boron ionization energy 共45 meV兲 due to the impurity-band broadening. The fringes seen in the spectrum are due to interference oscillations in the 10 ␮m thin device. The PC spectrum was used to calculate the dependence of

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Responsivity (A/W)

0.5 0.4 0.3 0.2

S1 @1.7 V

0.1 0 0

50 100 Photon Energy (meV)

150

FIG. 3. 共Color online兲 PC spectrum of S1 measured at 1.7 V. Note that the onset of response at low energies of 30 meV as compared to that of isolated boron at 45 meV results from impurity-band broadening. The fringes are due to resonances resulting from the device thickness of 10 ␮m.

S1’s responsivity on the applied voltage for a photon energy of 50 meV, where the solid line in Fig. 4 shows the result. For comparison, the dark current curve is shown on an arbitrary ordinate axis. At 1.9 V, the PC response of S1 reaches an impressive maximum value of 65 A/W. However, such high responsivity values are only reached at voltages at which the dark current has already risen dramatically to 10−8 A. Thus, even though the structure features gain values of 100, which would even qualify for single-photon detection,1 the dark current at these voltages experiences the same amplification, indicating that it is not generated at the BL-IL interface but within the depletion region and disqualifying the operation of our BIB in this regime. The saturation of the responsivity at voltages above 1.9 V can be attributed 3

This work was supported by the FWF 共Grant No. F2512N08兲 and GME 共both Vienna兲.

1

10

S1 @ 50 meV

101

−1

10 10−1 −3

10

−5

10

−7

10

1

Response to Blackbody Dark Current (a.u.)

−3

10

−5

10 Gain

10−7

Low Dark Current

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Voltage (V)

2

Electrons Per Photon

Responsivity (A/W)

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to a self-stabilization of the charge avalanches.10 Nevertheless, there is a bias window between 1.82 and 1.89 V, within which the dark current still remains below 10−10 A, and the responsivity already reaches 10 A/W, which is equivalent to a number of charge carriers per incident photon 共external quantum efficiency兲 larger than 0.5 and qualifies the structure to compete with conventionally fabricated BIBs. Further optimized BIB fabrication could extend the useful bias window up to external quantum efficiencies ⬎3, corresponding to the responsivity of 65 A/W observed at 1.9 V. In conclusion, we report the fabrication of Si:B BIBs by ion implantation into a high-Ohmic SOI wafer and backside contacting using Bosch etching. S1 exhibits highly competitive responsivities of 10 A/W for photon energies of 50 meV, while dark current remains lower than 10−10 A. By optimizing ion-implantation processes, the regime of high external quantum efficiencies up to 3, equivalent to an avalanche gain of about 100, could be exploited for BIB operation. The demonstration of an ion-implanted vertical Si:B BIB opens a highly promising road to technologically simple and cheap BIB array fabrication with implantation substituting the delicate MBE growth of ultraclean BLs. Ion implantation also constitutes a possible means of applying the BIB concept to other material systems, where growth issues were found to pose impregnable difficulties and to stretch the wavelength detection range of BIBs up to 200 ␮m. Further, implanted Si BIBs open a field of applications by employing the structure as gain region of optically active material grown on top, which is made possible by the buried nature of the backside contacted BL.

2.5

FIG. 4. 共Color online兲 Responsivity and number of charge carriers per incident photon 共external quantum efficiency兲 of S1 at a photon energy of 50 meV. S1 exhibits high peak responsivity values of 65 A/W and external quantum efficiencies of up to 3, although in this regime dark currents 共shown on an arbitrary ordinate axis兲 are high due to dark current gain. However, the onset of high dark current is shifted from the onset of PC gain by 70 mV. Within this work-point window, external quantum efficiency exceeds 0.5, while dark current is still insignificant, qualifying S1 for competitive BIB operation.

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