A voltage-controlled tunable two-color infrared photodetector using GaAs/AlAs/GaAlAs and GaAs/GaAlAs stacked multiquantum wells Yaohui Zhang,a) D. S. Jiang, J. B. Xia, L. Q. Cui, C. Y. Song, and Z. Q. Zhou National Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
W. K. Ge Department of Physics, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong
~Received 6 November 1995; accepted for publication 13 February 1996! A voltage-controlled tunable two-color infrared detector with photovoltaic ~PV! and photoconductive ~PC! dual-mode operation at 3–5 mm and 8–14 mm using GaAs/AlAs/AlGaAs double barrier quantum wells ~DBQWs! and bound-to-continuum GaAs/AlGaAs quantum wells is demonstrated. The photoresponse peak of the photovoltaic GaAs/AlAs/GaAlAs DBQWs is at 5.3 mm, and that of the photoconductive GaAs/GaAlAs quantum wells is at 9.0 mm. When the two-color detector is under a zero bias, the spectral response at 5.3 mm is close to saturate and the peak detectivity at 80 K can reach 1.031011 cmHz1/2/W, while the spectral photoresponsivity at 9.0 mm is absolutely zero completely. When the external voltage of the two-color detector is changed to 2.0 V, the spectral photoresponsivity at 5.3 mm becomes zero while the spectral photoresponsivity at 9.0 mm increases comparable to that at 5.3 mm under zero bias, and the peak detectivity ~9.0 mm! at 80 K can reach 1.531010 cmHz1/2/W. Strictly speaking, this is a real bias-controlled tunable two-color infrared photodetector. We have proposed a model based on the PV and PC dual-mode operation of stacked two-color QWIPs and the effects of tunneling resonance with narrow energy width of photoexcited electrons in DBQWs, which can explain qualitatively the voltage-controlled tunable behavior of the photoresponse of the two-color infrared photodetector. © 1996 American Institute of Physics. @S0003-6951~96!03815-X#
Recently quantum well infrared photodetectors ~QWIPs! have attracted much attention because of their practical applications.1–3 Compared with HgCdTe and InSb bulk materials, the quantum wells can provide a flexibility of energyband tailoring for QWIPs to operate at different atmospherical spectral windows by tuning external bias.4–6 In practice, the bias-controlled tunable two-color infrared photodetectors are requested to be sensitive at only one atmospherical window under a specific bias while the photoresponse at other windows should be completely zero. It is quite difficult to realize this request for usual multicolor QWIPs using multistack, asymmetric, and symmetric quantum well structures.5–8 In this letter, we report a bias-controlled tunable twocolor GaAs/AlAs/AlGaAs and GaAs/AlGaAs stacked QWIPs with ~PV! and ~PC! dual-mode operation at 3–5 and 8–14 mm. Under zero bias, this device operates only at the window of 3–5 mm while no photoresponse occurs at other atmospherical windows, and the peak detectivity ~5.3 mm! at 80 K can reach 1.031011 cmHz1/2/W. When the bias is tuned to 2.0 V, the photodetector changes to operate only at 8–14 mm and the peak detectivity ~9.0 mm! at 80 K is found to be 1.531010 cmHz1/2/W. We suggest that the voltagecontrolled tunable operation of the detector originates from the PV and PC dual-mode operation of this stacked multiquantum wells and tunneling resonance with narrow energy a!
Present address: Paul-Drude Institut fu¨r Festko¨rperelektronik, Hausvogteiplatz-5-7, 10117 Berlin, Germany. Electronic mail:
[email protected]
width of photoexcited electrons in GaAs/AlAs/AlGaAs ~DBQWs!. The two-color QWIP samples were grown by a VG MK II molecular beam epitaxy ~MBE! system on ~100! semiinsulating GaAs substrates. The quantum wells of two-color QWIP, sandwiched between two n-type (n5231018 cm23 ) 1.0 mm contact layers, consisted of two stacks with 25 quantum wells. The stack near the substrate was designed as GaAs/Ga0.7Al0.3As multiple quantum wells ~MQWs! sensitive at the long-wave infrared ~LWIR! window ~8–14 mm!, with each unit in the stack consisted of 40 A GaAs well ~doped to n5231018 cm23 ) and 250-A Ga0.7Al0.3As barriers. The second stack was designed to be sensitive at the middle-wave infrared ~MWIR! window ~3–5 mm!, with a 25-period of GaAs/AlAs/Ga0.7Al0.3As DBQW with a well width of 50 A ~doped to n52.031018 cm23 ) and two undoped barriers each of which has a 15-A AlAs inner barrier and a 200-A Al0.3Ga0.7As outer barrier. Figure 1 shows the energy band diagram of this kind of photodetector. The intersubband transition of the GaAs/AlGaAs quantum well is from bound state to continuum state ~B-C!, and that of the GaAs/AlAs/AlGaAs DBQWs is from bound state to quasicontinuum state ~B-QC!. The use of the thin AlAs inner barriers in DBQWs provides the following advantages:9 ~1! enhancement of the energy and intensity of the intersubband absorption; ~2! the thin AlAs layers act as tunning barriers of photoexcited electrons; ~3! the strong asymmetry between GaAs/AlAs and AlAs/ GaAs heterojunctions due to MBE growth is responsible
2114 Appl. Phys. Lett. 68 (15), 8 April 1996 0003-6951/96/68(15)/2114/3/$10.00 © 1996 American Institute of Physics Downloaded¬11¬Nov¬2010¬to¬159.226.228.7.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright;¬see¬http://apl.aip.org/about/rights_and_permissions
FIG. 1. The schematic energy-band diagrams for two-color QWIP structure.
for the photovoltaic behavior of GaAs/AlAs/GaAlAs DBQWs with MWIR photoresponse.
FIG. 3. The peak photocurrent at 5.3 and 9 mm vs applied voltage of twocolor QWIP.
Devices were processed out of the grown wafers and a circular mesa with 200 mm in diameter was etched down to the n 1 bottom contact layer. Au/GeNi was evaporated onto the top of each mesa and n 1 bottom contact layer for ohmic contacts. The detectors were back illuminated through a 45° polished facet and their photoresponsivity spectra were measured at 80 K using a Bruker Fourier transform infrared ~FTIR! spectrometer. Figure 2 displayed the spectral response of the two-color QWIP at 80 K under different voltage. The polarity is defined as positive when the higher potential is applied on the top of a mesa. The two-color QWIP under zero bias can operate only at MWIR window and no photoresponse occurs at LWIR window. When the positive voltage increases, the photoresponsivity at 5.3 mm of the two-color QWIP decreases till zero at a bias of 2.0 V while the photoresponsivity at 9.0 mm increases from zero to that comparable to the peak photoresponsivity at 5.3 mm under zero bias. In addition, we have measured the peaked detectivity of the twocolor QWIP at 80 K under different bias. The peak detectivity of the two-color QWIP at zero bias achieved at 1.0 31011 cmHz1/2/cm, and the peak detectivity at a bias of 2.0 V can reach 1.531010 cmHz1/2/W. Therefore, this kind of QWIP can meet the request of two-color infrared detection in practice. Figure 3 shows the bias dependence of the peak photoresponsivity at 5.3 and 9.0 mm at a temperature of 80 K. From this figure, we know that the MWIR-QWIP ~GaAs/
AlAs/GaAlAs DBQWs! is a typical photovoltaic detector while LWIR-QWIP ~GaAs/GaAlAs MQWs! is photoconductive. It should be noted that the peak photoresponsivity at 5.3 mm has the maximum value at a bias of 20.8 V, and the transport of photoexcited electrons seems to be resonant under this external bias. In order to discuss the bias-controlled tunable mechanism of the two-color QWIP, we have measured the absorption spectra of the two-color QWIP under zero bias at 77 and 300 K as shown in Fig. 4. The measurement was performed with a Bruker Fourier transform infrared spectrometer using a waveguide geometry. From this figure, one can see that, at 77 and 300 K, the two-color QWIP has two infrared absorption peaks near 1850 and 880 cm21 , which result from the intersubband transition of GaAs/AlAs/GaAlAs DBQWs and GaAs/GaAlAs MQWs, respectively. Although there is no photoresponse at 8–14 mm under zero bias as shown in Fig. 2, we can clearly observe the infrared absorption due to the intersubband transition. Moreover, the energy position of photoresponse peaks of the two-color QWIP has little change with applied bias. Therefore, the external voltage has little influence on infrared absorption, and the bias-controlled tunable operation of the two-color QWIPs photoresponsivity is mainly attributed to the transport process of photoexcited electrons instead of infrared absorption. In addition, the blueshift of the two peaks with decreasing temperature is induced by the many-body effects of the two-dimensional electron gas in quantum wells.10 We believe that the photovoltaic effects in QWIP originates from the strong asymmetry between the GaAs/
FIG. 2. The spectral response of two-color QWIP at 80 K under different bias.
FIG. 4. The absorption spectra of two-color QWIP at 77 and 300 K.
Appl. Phys. Lett., Vol. 68, No. 15, 8 April 1996 Zhang et al. 2115 Downloaded¬11¬Nov¬2010¬to¬159.226.228.7.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright;¬see¬http://apl.aip.org/about/rights_and_permissions
Alx Ga12x As heterojunction and Alx Ga12x As/GaAs heterojunction occurred in MBE growth. In fact, the concentration of oxygen in GaAs/Alx Ga12x As heterojunction is always much higher than that in Alx Ga12x As/GaAs heterojunction, and the impurity like oxygen atoms become trap-centers in the interface.11 A very thin negative space-charge layer would be formed at the GaAs/Alx Ga12x As interface since oxygen impurities can trap electrons. Therefore for GaAs/ Alx Ga12x As quantum wells with heavily doped GaAs layers, there must be a built-in electric field in Alx Ga12x As barriers. GaAs/Alx Ga12x As interface with high composition of Al has much large oxygen density. It is thus reasonable to suggest that the photoresponse of GaAs/AlAs/GaAlAs DBQWs with GaAs/AlAs interface is photovoltaic while the biasdependence of photoresponse of GaAs/GaAlAs MQWs is asymmetrically photoconductive under negative and positive external bias as shown in Fig. 3. The MWIR photoconductivity of GaAs/AlAs/GaAlAs DBQWs arises from tunneling of the photoexcited electrons out of the second subband of GaAs quantum wells into the AlGaAs layers. The photoresponsivity strongly depends on the tunneling rate of photoexcited electrons. Although the thickness of AlAs layers in DBQWs in only 15 A, the energy width of the tunneling resonance of photoexcited electrons is 5 meV, which is much smaller than the energy band of the transmission resonance of photoexcited electrons in continuum states of GaAs/GaAlAs MQWs. Therefore, the transport of photoexcited electrons in DBQWs is similar to narrow miniband conduction in superlattices. When the electric field in DBQWs is too large, the tunneling resonance will be destroyed and the photoexcited electrons should be localized within the GaAs quantum wells ~Wannier–Stark localization!. Photoexcited electrons will then return to the ground state through a relaxation process, which results in the dramatic decrease of the photocurrent efficiency. This mechanism can be applied to qualitatively explain the reduction of the peak photoresponsivity at 5.3 mm with increasing negative bias as shown in Fig. 3. The decreasing of peak photoresponsivity at 5.3 mm with increasing positive bias is induced by the drift velocity reduction of photoexcited electrons resulted from the reduction of electric field. However, for GaAs/GaAlAs MQWs, the transport process of photoexcited electrons is quite different, since the continuum band above AlGaAs barriers is broad enough that photoexcited electrons would not be localized even at much higher electric field.12 At a low bias ~below 0.5 V!, most of the voltage applied to the two-color QWIP is dropped across GaAs/AlAs/ GaAlAs DBQWs due to its large resistance at 80 K,6,7 and the electric field in GaAs/GaAlAs MWQs is much smaller. The photoresponse of GaAs/GaAlAs MQWs is very weak though we can observe the obvious infrared absorption. So we can only observe a single photoresponse peak at 5.3 mm as shown in Fig. 2. When positive or negative bias gets further increased, the increase of electric field in GaAs/GaAlAs
MQWs enhances the photoresponsivity at 9.0 mm. However, for GaAs/AlAs/GaAlAs DBQWs, the increasing negative bias induces the localization of photoexcited electrons while positive bias results in reduction of the drift velocity of photoexcited electrons, the photoresponsivity at 5.3 mm thus gets a dramatical decrease under relatively high positive or negative voltage. Therefore, the voltage-tunable two-color detection can be observed at this kind of two-color QWIP. It should be noted that it is very important for the photovoltaic effects of GaAs/AlAs/GaAlAs DBQWs to be adjusted to achieve the maximal peak photoresponsivity at 5.3 mm under a bias close to zero ~20.8 V!. Therefore, the voltage-tunable two-color detection can be realized for this kind of QWIP structure. In conclusion, we have demonstrated a bias-tunable twocolor ~5.3 and 9 mm! infrared detector using stacked GaAs/ AlAs/AlGaAs DBQWs and GaAs/AlGaAs QWs. At zero bias, the two-color QWIP is sensitive only at the MWIR atmospherical window and its peaked detectivity ~5.3 mm! at 80 K can reach 1.031011 cmHz1/2/W. When the external bias increases to 2.0 V, the photoresponse at the MWIR window tends to disappear, and the two-color QWIP becomes sensitive at the LWIR atmospherical window and the peaked detectivity ~9.0 mm! at 80 K is 1.531010 cmHz1/2/W. This is a well-defined real bias-tunable two-color infrared detector. The bias-controlled tunable performance originates from the photovoltaic and photoconductive dual-mode operation of these QWIP structures and the effects of tunneling resonance with narrow energy width of photoexcited electrons in GaAs/AlAs/AlGaAs DBQWs. The authors are grateful to Professor Kun Huang and Houzhi Zheng for their support and encouragement of this work. Thanks are due to Fangjie Zhuang for device fabricating, to Yinghai Chen for photocurrent measurements. This work was supported by the presidential foundation of Chinese Academy of Sciences and the National Science Foundation of China. B. F. Levine, J. Appl. Phys. 74, R1 ~1993!. M. A. Kinch and A. Yariv, Appl. Phys. Lett. 55, 2093 ~1989!. 3 K. K. Choi, M. Z. Tidrow, M. Taysing-Lara, W. H. Chang, C. H. Kuan, C. W. Karley, and F. Chang, Appl. Phys. Lett. 63, 908 ~1993!. 4 I. Grave, A. Shakouri, N. Kuze, and A. Yariv, Appl. Phys. Lett. 60, 2362 ~1992!. 5 K. Kheng, M. Ramsteiner, H. Schneider, J. D. Ralston, F. Fuchs, and P. Koidl, Appl. Phys. Lett. 61, 666 ~1992!. 6 K. L. Tsai, K. H. Chang, C. P. Lee, K. F. Huang, J. S. Tsang, and H. R. Chen, Appl. Phys. Lett. 62, 3504 ~1993!. 7 M. Z. Tidrow, K. K. Choi, A. J. DeAnni, and W. H. Chang, Appl. Phys. Lett. 67, 1800 ~1995!. 8 Y. H. Wang, J. C. Chiang, S. S. Li, and P. Ho, J. Phys. Phys. 76, 2538 ~1994!. 9 H. Schneider, F. Fuchs, B. Dischler, J. D. Ralston, and P. Koidl, Appl. Phys. Lett. 58, 2234 ~1991!. 10 I. P. Loehr and M. O. Manasreh, Semiconductor Quantum Wells and Superlattices for Long-Wavelength Infrared Detectors, edited by M. O. Manasreh ~Artech House, Boston, London, 1993!, Chap. 2. 11 T. Achtnich, G. Burri, M. A. Py, and M. Ilegems, Appl. Phys. Lett. 50, 1730 ~1987!. 12 H. C. Liu, Appl. Phys. Lett. 60, 1597 ~1992!. 1 2
2116 Appl. Phys. Lett., Vol. 68, No. 15, 8 April 1996 Zhang et al. Downloaded¬11¬Nov¬2010¬to¬159.226.228.7.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright;¬see¬http://apl.aip.org/about/rights_and_permissions
