Uncooled Infrared Detector Using a Thin InAsSb Layer ... - IEEE Xplore

IEEE SENSORS JOURNAL, VOL. 6, NO. 5, OCTOBER 2006

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Uncooled Infrared Detector Using a Thin InAsSb Layer Acting as a Gate on a GaAs Field-Effect Transistor Yossi Paltiel, Ariel Sher, Arie Raizman, Daniel Majer, A. Arbel, Aviram Feingold, Member, IEEE, J. Levy, and Ron Naaman

Abstract—The demand for high-quality low-cost uncooled infrared (IR) photodetectors have significantly increased in recent years. In this paper, a novel concept of utilizing InAsSb as a midwave IR uncooled detector is introduced. According to the approach used in this paper, the InAsSb detection layer acts as gate over a GaAs field-effect transistor (FET). IR light is absorbed in the detection layer and changes the surface potential of the transistor. The current in the transistor, which is very sensitive to those changes, should yield a sensitive detector. The same concept can be generalized to other adsorbents that absorb light at the various range of the spectrum. The advantage of using the mature technology of GaAs for achieving a low-cost efficient uncooled IR detector is clear. The experimental results presented here, using InAsSb as the absorbing layer, serve as a proof of the general concept. Index Terms—InAsSb, infrared (IR) sensors, nanodevices, uncooled detectors.

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HE TERNARY InAsSb alloy is an attractive candidate for the realization of detectors operating at near room temperature in the 3–5-µm spectral region [1], [2]. The conventional approach, which is based either on p-n or p-i-n structures of these ternaries, was studied in recent years [3], [4]. Promising results indicate that these detectors may compete with the microbolometers or pyroelectric uncooled detectors. Here, we present an innovative concept for utilizing the InAsSb as uncooled infrared (IR) photodetector. A thin layer of InAsSb is grown on top of a gateless GaAs field-effect transistor (FET). Free carriers that are generated as a result of the absorbed IR light in the layer diffuse to the InAsSb–GaAs interface, and as a result, the surface potential of the FET changes. When the FET operates in the subthreshold limit, the current driven through the FET is very sensitive to minute changes of the surface potential (Fig. 1). Thus, a highly responsive detector can be fabricated in which the response to the wavelength is controlled by the deposited layer, whereas the gain is controlled by the FET-like structure. A similar Manuscript received June 2, 2005; revised September 4, 2005. The associate editor coordinating the review of this paper and approving it for publication was Prof. Eugenii Katz. Y. Paltiel, A. Sher, and A. Raizman are with the Solid State Physics Group, Electro-Optics Division, Soreq NRC, Yanve 81800, Israel (e-mail: [email protected]; [email protected]). D. Majer, A. Arbel, A. Feingold, and J. Levy are with Chiaro Networks, Jerusalem 95484, Israel. R. Naaman is with the Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel (e-mail: [email protected]). Digital Object Identifier 10.1109/JSEN.2006.881344

Fig. 1. (a) Device schematics: A thin layer of InAsSb is deposited on the gate area of a GaAs FET. When the device is illuminated by IR radiation, it is absorbed in the InAsSb layer and changes the surface potential of the GaAS FET. This, in turn, changes the current flowing in the FET at a constant voltage. (b) Picture of one of the fabricated devices. Several GaAs FETs with different active areas (300–500 000 µm2 ) and aspect ratios (0.03–20) were processed on the same chip.

concept has been demonstrated in a previous paper in which a molecular controlled semiconductor resistor (MOCSER) was demonstrated as a high-quality gas sensing [5], [6] detector. In the MOCSER, adsorbed molecules are deposited on the gate area. Any chemical change in these molecules is translated to a change in the electrochemical potential of the adsorbed layer and, hence, to a change in the electric potential on the surface of the FET. This change is expressed as a change of the resistivity of the FET. To further improve the response, one can grow an n-type narrow-gap material on top of a low p-type widegap material. Illumination of the narrow-gap material generates electrons, which deplete the channel of the transistor even further, creating a noticeable current change in the detector. Those two mechanisms above operate simultaneously to produce a detectable signal at high temperatures. To prove the validity of the new approach, we grew thin layers of InAsSb on top of GaAs FET devices and measured the currents generated in the FET due to the IR absorbed in the

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InAsSb. This sensing strategy has several advantages. Unlike previous papers [3], [4], the detectors are realized on a GaAs substrate and are therefore processed using the mature GaAs technology. The signal-to-noise ratio is improved due to the transistor gain, and detection is enabled at higher temperatures. For low-cost room-temperature detectors, the simple production and high signal-to-noise ratio are important advantages. The regrowth process of the InAsSb thin layers on top of a molecular beam epitaxy (MBE) GaAs FET structure was carried out in a Thomas Swan metal-organic vapor phase epitaxy (MOVPE) machine with a vertical reactor. Trimethylindium (TMI), tertiarybutylarsine (TBAs), and trimethylantimony (TMSb) were used as precursors. The growth temperature was 440 ◦ C, and the pressure was 400 torr. Three types of samples were used: a control sample without regrowth of InAsSb on top of the gate area and two regrown samples of thin InAsSb with thicknesses of 10 and 20 nm. A crucial step before the regrowth process was the cleaning of the GaAs oxidation with HCl. The presence of the thin InAsSb layer was confirmed by a Philips PW 1380 X-ray horizontal powder diffractometer, using the Kβ line of the GaAs substrate as a calibration reference. The antimony concentration was deduced from thicker layers grown in similar growth conditions and measured by a double crystal diffractometer. The antimony concentration was found to be 6%–8%, which corresponds to an energy band gap of ∼0.3 eV at room temperature. The layer was n-doped to 5 × 1016 1/cm3 . The MBE transistor structure was supplied by ProComm. The FET structure was grown with the following layer structure: spacer—1000 nm of undoped GaAs; super lattice—ten layers of super lattice combining 35 nm of undoped 35% AlGaAs and 5 nm of undoped GaAs, channel of 50 nm n-doped 5 × 1017 1/cm3 , and capping of 8 nm undoped GaAs. The gateless GaAs FETs were manufactured with areas ranging from 300 up to 500 000 µm2 and aspect ratios in the range of 0.03–20 using the following steps. n-type ohmic contacts were defined using the resist liftoff process with Ni : Ge : Au : Ni : Au metallization. Two steps of etching were used in order to avoid contact between the conducting InAsSb layer and the ohmic contacts, namely 1) selective InAsSb wet etching using citric acid (C6 H8 O7 ) and 30% diluted (H2 O2 ) with a (C6 H8 O7 )/(H2 O2 ) ratio of 1 : 5 and 2) mesa GaAs etch using H2 PO3 /H2 O2 /H2 O in a 1 : 1 : 40 ratio. The GaAs etch process was followed by wafer back side metallization of Ni : Ge : Au : Ni : Au and RTP alloying. For passivation, a silicon nitride layer was deposited and etched using a photolithography mask and dry inductively coupled plasma (ICP) etching/reactive ion etching (RIE). Lastly, metal pads were defined using the resist liftoff process with Ti : Pt : Au metallization. A picture of a packaged device is shown in Fig. 1(b). Devices with different sizes and aspect ratios were fabricated from all the sample types. Dark current I(V ) was measured, and a FET-like dark current behavior was found. For measuring the spectral response of the detector, a 1000-K blackbody source was used as the IR source. An optical setup was utilized for collecting the light from the blackbody source through a chopper and filters and onto the device. Using a lock-in amplifier, the response at the different wavelength


Fig. 2. (a) Room-temperature response using a blackbody source and a Ge 2.5-µm high-pass filter. The control device shows no response to the IR light, whereas the device with a 20-nm InAsSb layer on the gate of the FET shows a 17% change is the current when illuminated. The device active area was 330 000 µm2 , and the aspect ratio was 0.03. (b) Change in the response current of the detector using different high-pass filters. A cutoff is expected at 4 µm based on the InAsSb energy gap, but a slow decay is visible at lower wavelengths.

regimes was measured. Fig. 2(a) shows the response of a control device that contains only the gateless FET structure without the InAsSb layer and the response of an active device with InAsSb layer deposited on its gate area. The device active area is 330 000 µm2 , and its aspect ratio is 0.03. The measurement was performed at room temperature and the blackbody radiation was passed through a Ge 2.5-µm high-pass filter. The current through the control device does not change when illuminated with the IR light. However, the device with the InAsSb absorbing layer does change its current when exposed to the IR radiation. At room temperature, the current decreases in the active device when irradiated with IR radiation. The device with the 20-nm-thick InAsSb layer shows a 17% decrease of the current when illuminated with the same light for a 2-V bias. For the 10-nm-thick gate, a change of 9% in the current was obtained using the same conditions. The comparison with the 20-nm results indicates that the InAsSb absorption layer plays the dominant role in the effect of the current change and that the optimization of the thickness of the absorption layer is crucial. The response gives a clear proof that the device can operate as a room-temperature midwavelength IR (MWIR) detector. To estimate the change in the surface potential, the results at room

PALTIEL et al.: UNCOOLED IR DETECTOR USING A THIN InAsSb LAYER ACTING AS A GATE ON A GaAs FET

temperature were compared with those obtained on the same device using a negative-bias metal gate [5], [6]. Applying a bias between 20 and 40 meV reduced the current at the same amount as obtained with the InAsSb gate. These results demonstrate that a very small change in the surface potential can be detected and that, with an optimized device, even better results can be expected. The emissivity of one at 1000 K is 5.7 W/cm2 , which is based on the Planck formula for blackbody. The Ge filter transmits only above 2.5 µm. The InAsSb cutoff is at 3.5 µm. Therefore, only 25% of the energy of the blackbody is available for detection by the device. Our optical system collects 2 cm2 of blackbody light and concentrates it on the sample with a spot size that is approximately four times larger than our device active area. These approximations indicate that roughly 0.7 W of radiation have reached the active device area. The quantum efficiency is given by η = αL, where α ∼ 104 W/cm is the absorption coefficient, and L is the layer thickness. Hence, in the case of the 10-nm-thick layer, only about 7 mW of radiation was absorbed, which translates to 1017 photons at 3 µm. The change in the current through the transistor is about 10−4 A. Hence, the device response is of the order of ∼14 mA/W. Compared with the most common devices of p-i-n InAsSb, which typically shows 1 A/W, the response in the current is small. However, the thickness of the absorbing layer in common devices is 3 µm, which is 1500 times thicker than in this paper. The response is expected to be improved by optimizing the transistor structure and the absorbing layer. Two directions can be chosen. The first is using a thicker IR absorbing layer. Our estimations show that better absorption will create band bending and a large depletion of the channel. Using this approach, we expect that the optimized absorbing layer should be about 1 µm thick. Such a layer should be grown on a lattice-matched GaSb transistor. Alternative approach is to grow many layers of nanodots on the gate. Using this method, we will be able to have both higher absorption and also better control of the spectral response. The energy gap in the grown InAsSb, at room temperature, corresponds to the cutoff in the absorption at about 4 µm. Fig. 2(b) shows the response of the detector using a blackbody emitter and different high-pass filters. A slow decay of the response is apparent before the 4-µm cutoff. At high temperature, an indistinct cutoff can only be observed, however, at low temperatures, a clear cutoff is seen. Although this phenomenon was seen before [7], a clear explanation for it is not available. At 77 K, the effect of the IR radiation on the current increases by more than an order of magnitude [Fig. 3(a)]. However, the effect of the illumination on the current reverses direction. The current is now enhanced when the device is illuminated as opposed to the room-temperature case where the current is decreased. This phenomenon is not well understood since it is not likely that the transistor carriers change. It is more probable that the surface potential has switched direction at low temperature. According to our interpretation, there are two reasons that could create such an effect. Either a change in surface potential or surface impurities or the two-dimensional (2-D) Hall gas at the interface is accepting, at low temperature, excited holes from the absorbing layer. The type I band alignment between the

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Fig. 3. (a) Response of the detector to a 1000 K blackbody IR source filtered at different regimes. Measurement was done at 77 K. An increase by more than an order of magnitude is seen in the detectors current with the MWIR illumination. (b) Photoresponse of the thin InAsSb layer (line) compared with the detector response (dash) using the same layer as a gate, illustrating a gain of 100 in our device.

wide-gap GaAs (low p) and the narrow-gap InAsSb (low n) is creating 2-D Hall gas at the interface. This Hall gas should be a good acceptor for holes and thereby affect the current in a way opposite of the effect caused by the depletion and by electrons located at surface states. At low temperature, the effect of holes accepting is expected to be stronger than the contribution from the other mechanisms. If this interpretation of the observation is correct, it means that by using type II band alignment like that in GaSb/InAsSb, it will be possible to form 2-D electron gas that will increase the response of the device to light. The ratio between the changes in the current in the FET to the photoresponse of the InAsSb layer is the gain of the device. By growing the same layer of InAsSb both on the gate area of the transistor and between two ohmic contacts, the photoresponse of the layer could be measured and directly compared to the change in the current through the transistor. The results are shown in Fig. 3(b). The change in the current through the FET with the InAsSb gate is by two orders of magnitude larger than the photoresponse of the InAsSb itself.

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to grow nanodots on the GaAs surface, or to use organic molecules to attach nanocrystals, provides a very flexible way to achieve highly sensitive multispectral device with narrow absorption lines. ACKNOWLEDGMENT The authors would like to thank M. Mizrahy and B. Bejerano from Soreq NRC for outstanding technical assistance and Dr. A. Zussman from Soreq for helpful discussions. R EFERENCES

Fig. 4. Room-temperature zero-voltage ac response of the FET with an InAsSb gate compared with a control device. A 1.3-µm laser modulated by a chopper illuminated both devices.

The gating effect of the 20-nm InAsSb induces a change in current that is 100 times larger compared to a single-channel 20-nm InAsSb. Those results should be taken cautiously since the photoresponse current depends on the mobility, density, and lifetime in the thin InAsSb layer. These properties are bound to be far from ideal in a layer grown on the GaAs substrate. We believe that a much higher response is possible with the optimization of the absorption layer and the transistor. The voltage that is needed to fully deplete the FET channel is lower than 1 V, which indicates that it is possible to design a very sensitive absorption gate. When the light is chopped, the device could be tested in an ac mode. The signal was obtained from the detectors at zero drain–source bias. Fig. 4 shows the response of a control and active devices to a 1.3-µm laser modulated by a chopper. In this mode, the laser light changes the capacitance of the FET inducing an ac response. Fig. 4 shows the very small response of the control device and a much larger effect in the active FET. The small response of the control sample is probably due to the chopper being very close to the detector (5 cm apart) and therefore inducing an oscillating blackbody radiation. The large response of the active detector is an indication that in this working mode, the gate-transistor capacitance is changed due to light absorption. This mode of work, although more complicated, can be important when measuring very small signals. In summary, we presented a novel uncooled IR detector concept. A thin InAsSb layer is acting as a gate on top of a GaAs FET. We measured the response of the detector to IR radiation, at room temperature, and compared it to the photoresponse of an InAsSb thin layer. The change in the full device current was two orders of magnitude higher compared to the change of the photocurrent of the absorbing 20-nm InAsSb layer. The results presented here show the validity of the concept. More work is required in order to improve the detector. Clearly varying the thickness of the adsorbed layer is one of the parameters to be optimized [8]. In addition, the adsorbed layer properties can be modified to allow multispectral response. The ability

[1] A. Rogalski, “InAs1−x Sbx infrared detectors,” Prog. Quantum Electron., vol. 13, no. 3, pp. 191–231, 1989. [2] R. Triboulet, “Alternative small gap materials for IR detection,” Semicond. Sci. Technol., vol. 5, no. 11, pp. 1073–1079, Nov. 1990. [3] A. Rakovska, V. Berger, X. Marcadet, B. Vinter, K. Bouzehouane, and D. Kaplan, “Optical characterization and room temperature lifetime measurements of high quality MBE-grown InAsSb on GaSb,” Semicond. Sci. Technol., vol. 15, no. 1, pp. 34–39, Jan. 2000. [4] G. Marre, B. Vinter, and V. Berger, “Strategy for the design of a noncryogenic quantum infrared detector,” Semicond. Sci. Technol., vol. 18, no. 4, pp. 284–291, Apr. 2003. [5] K. Gartsman, D. Cahan, A. Kadyshevitch, J. Libman, T. Moav, R. Naaman, A. Shanzer, V. Umansky, and A. Vilan, “Molecular control of a GaAs transistor,” Chem. Phys. Lett., vol. 283, pp. 301–306, 1998. [6] D. Cahen, K. Gartsman, A. Kadyshevitch, R. Naaman, and A. Shanzer, “Hybrid organic–inorganic semiconductor structures and sensors based thereon,” U.S. patent 6,433,356, Aug. 13, 2002. European patent 0935748. [7] I. Kimukin, N. Biyikli, and E. Ozbay, “InSb high-speed photodetectors grown on GaAs substrate,” J. Appl. Phys., vol. 94, no. 8, pp. 5414–5416, Oct. 2003. [8] M. Leibovitch, L. Kronik, B. Mishori, Y. Shapira, C. M. Hanson, A. R. Clawson, and P. Ram, “Determining band offsets using surface photovoltage spectroscopy: The InP/In0.53 Ga0.47 As heterojunction,” Appl. Phys. Lett., vol. 69, no. 17, pp. 2587–2589, Oct. 1996.

Yossi Paltiel received the B.Sc. degree in mathematics and physics from the Hebrew University of Jerusalem, Jerusalem, Israel, and the M.Sc. degree in physics, in the field of semiconductors, and the Ph.D. degree in physics, in the field of superconductivity, from the Department of Physics, Weizmann Institute of Science, Rehovot, Israel. He worked for two years in Chiaro Networks, Jerusalem, which is an electrooptics startup company, and then joined the Solid State Physics Group, Electro-Optics Division, Soreq NRC, Yanve, Israel, in 2003. His current fields of interest are III–V semiconductors, mainly Sb-based semiconductors for infrared photodetectors and devices, metalorganic vapor phase epitaxy and molecular beam epitaxy growth, optoelectronics, and nanotechnology. His papers last year include works done using nanodots, quantum well infrared photodetectors, diodes, noise, THz, and uncooled detectors.

Ariel Sher received the B.Sc., M.Sc., and Ph.D. degrees from the Faculty of Science, Hebrew University, Jerusalem, Israel, in 1997, 1982, and 1988, respectively. Since 2001, he has lead the Solid State Physics Group of Soreq NRC, Yanve, Israel. His research interests include infrared materials and devices, epitaxial growth by LPE and MOCVD of HgCdTe, and narrow-bandgap antimonide compounds.

PALTIEL et al.: UNCOOLED IR DETECTOR USING A THIN InAsSb LAYER ACTING AS A GATE ON A GaAs FET

Arie Raizman received the B.Sc., M.Sc., and Ph.D. degrees from the Department of Physics, Tel-Aviv University, Tel-Aviv, Israel. His Ph.D. thesis was in the field of magnetic resonance. He is a Research Physicist with the Solid State Physics Group, Electro-Optics Division, Soreq NRC, Yanve, Israel. His main research area is characterization of thin layer structures by high-resolution X-ray diffraction and metal–organic chemical vapor deposition growth of III–V structures for infrared applications. Dr. Raizman was a member of the Israel Physical Society Committee.

Daniel Majer received the B.Sc. degree in physics from the Tel-Aviv University, Tel-Aviv, Israel, and the M.Sc. and Ph.D. degrees in physics from the Weizmann Institute of Science, Rehovot, Israel. The subject of his Ph.D. dissertation was the phase diagram of high-temperature superconductors. He co-founded Chiaro Networks, Jerusalem, Israel, in 1997 and served as its Chief Scientist. His main focus there was to develop and then bring to full manufacturing a fast optical switch based on a GaAs electrooptical switch. He has published more than 40 scientific papers and is the holder of 29 U.S. patents.

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J. Levy received the the B.Sc. degree in physics from the Technion—Haifa, Israel, the M.Sc. degree in experimental plasma physics from the University of California at Los Angeles, and the D.Sc. degree in experimental physics from the Technion. He has 25 years of technical and operation experience in the semiconductor and optical industries. He is currently the Vice President (VP) R&D of Novatrans, working in the field of nanotechnology transistors. Prior to this, he served as VP ElectroOptics of Chiaro Networks, Jerusalem, Israel, which is an optical communication company, where he was responsible for the development of a high-speed optical switch. He was part of the founding team and served as VP Operations at Tower Semiconductor, where he was in charge of a 500-member team devoted to R&D, engineering, and operations. He served at National Semiconductor in various managerial positions in R&D and operations, including director of process and device engineering and operations management. At MERET Optical Communications (a Californiabased startup that was later acquired by AMOCO Corporation), he designed and analyzed high-speed electro-optic systems. He has been issued (or is in the process of being issued) several patents in the areas of nanotechnology, electrooptic communications, advanced microelectronic processing, and CMOS image sensors.

A. Arbel, photograph and biography not available at time of publication.

Aviram Feingold (M’03) is a Process Engineering Manager with over 15 years of experience in semiconductor wafer processing, both in R&D and production industries, such as AT&T Bell Laboratories, Murry Hill, NJ, Tower Semiconductors, Migdal Haemek, Israel, Chiaro Networks, Jerusalem, Israel, and Novatrans, Herzlia, Israel.

Ron Naaman received the Ph.D. degree from the Weizmann Institute of Science, Rehovot, Israel. After spending three years at the Department of Chemistry of Stanford University and Harvard University, he joined the Weizmann Institute of Science, where he is currently a Professor with its Department of Chemical Physics. His current research interests involve the investigation of electronic properties of hybrid organic–inorganic devices and of self-assembled organic monolayers. Dr. Naaman is a Fellow of the American Physical Society.