High-sensitivity modulation-doped quantum dot infrared photodetectors

Microelectronic Engineering 63 (2002) 185–192 www.elsevier.com / locate / mee

High-sensitivity modulation-doped quantum dot infrared photodetectors K. Hirakawa a,b , *, S.-W. Lee a,b , Ph. Lelong a , S. Fujimoto a , K. Hirotani a , H. Sakaki a a

b

Institute of Industrial Science, University of Tokyo, 4 -6 -1 Komaba, Meguro-ku, Tokyo 153 -8505, Japan CREST, Japan Science and Technology Corporation, 1 -4 -25 Mejiro, Toshima-ku, Tokyo 171 -0031, Japan

Abstract We have designed and fabricated quantum dot (QD) infrared photodetectors which utilize photoionization of self-assembled InAs QDs and lateral transport of photoexcited carriers in the modulation-doped AlGaAs / GaAs two-dimensional channels (modulation-doped quantum-dot infrared photodetectors; MD-QDIPs). A broad photocurrent signal has been observed in the mid-infrared range. A very large photoconductive gain of the order of 10 5 – 6 is achieved by long lifetimes as well as high mobilities of photoexcited carriers in the modulationdoped conduction channels.  2002 Elsevier Science B.V. All rights reserved. Keywords: Quantum dots; Infrared photodetectors; Heterostructures; Quantum wells

The detection of mid-infrared (MIR) radiation has increasing importance in applications such as thermography, infrared cameras, remote sensing, biology, chemistry, pollution monitoring, etc. Quantum well infrared photodetectors (QWIPs) which utilize intersubband transition in quantum wells have brought a great technological impact by making it possible to detect MIR radiation using wide bandgap materials such as GaAs and Si [1]. However, QWIPs have drawbacks such as intrinsic insensitivity to the normal incidence radiation and a relatively large dark current due to scatteringassisted tunneling processes. It has been expected that such drawbacks can be overcome using quantum dots (QDs) as active elements of the photodetectors. Recently, quantum dot infrared photodetectors (QDIPs) using InAs self-assembled QDs have been fabricated and successful operation in the MIR range has been demonstrated [2–10]. Most of the structures reported so far utilize a vertical transport through stacked self-assembled QDs, simulating the operation of QWIPs. However, because of unavoidable inhomogeneity in the size and spatial alignment of the QDs, the tunneling transport across multiple self-assembled QD layers is strongly affected by disorder. Consequently, it seems difficult to achieve high sensitivities in the vertical transport structures. Recently, we have proposed and fabricated a new type of QD infrared photoconductive detector, * Corresponding author. Tel.: 181-3-5452-6260; fax: 181-3-5452-6262. E-mail address: [email protected] (K. Hirakawa). 0167-9317 / 02 / $ – see front matter PII: S0167-9317( 02 )00606-8

 2002 Elsevier Science B.V. All rights reserved.

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modulation-doped quantum dot infrared photodetectors (MD-QDIPs), which combines photoionization of self-assembled InAs QDs and subsequent high-mobility transport at modulation-doped heterointerfaces [11–13]. The device structure is intended to improve the detector performance by maximizing mobilities and lifetimes of photoexcited carriers. In designing MD-QDIPs, knowledge of electronic structures of the QDs is essential; in particular, the energy positions of the bound states in the QDs with respect to the conduction / valence band edges of GaAs are indispensable. Optimization of device structures is also important for realizing high-sensitivity photodetection. In this work, we have investigated the electronic structures in InAs QDs by measuring MIR photocurrent spectra of n- and p-type MD-QDIPs. Together with photoluminescence data, the band diagram for the QDs embedded in GaAs matrix has been obtained. Furthermore, we have systematically investigated the lifetime of photoexcited carriers to clarify the relationship between the responsivity and the device structures. It is found that the lifetime depends exponentially on the distance between the heterointerface channel and the QD layer. A lifetime of the order of 0.1–1 ms and a photoconductive gain as large as | 10 5 – 6 were observed even at T 5 77 K. The obtained extremely large photoconductive gains enable a high sensitivity operation of MD-QDIPs. The samples were grown on (001) semi-insulating GaAs substrates by molecular beam epitaxy. The details of the growth procedure have been described elsewhere [11]. Self-assembled InAs QDs were grown at 470 8C under As beam pressure of 1.2 3 10 25 mbar. The atomic force microscope (AFM) observation revealed that the average diameter and height of the grown QDs were 20 and 7 nm, respectively, and that the density was 0.8 3 10 11 cm 22 (see the inset of Fig. 1). Each QD layer was embedded in the middle of a wide GaAs quantum well (QW) with a thickness of 2d. d was varied from 20 to 100 nm. The 30-nm-thick Al 0.2 Ga 0.8 As barriers were d-doped with Si (Be) to supply 11 22 electrons (holes) to the QDs. Doping densities in the MD-QDIP (1 3 10 cm ) samples were set in a way such that only s-bound states in the QDs were occupied. Fig. 1 shows the photoluminescence

Fig. 1. Photoluminescence spectrum measured on a n-type MD-QDIP sample at 18 K. The inset shows the atomic force microscope image of the quantum dots grown on the surface of the sample.

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Fig. 2. Schematic band diagram of the n-type MD-QDIP structure.

(PL) spectrum measured on the n-type sample. From the PL spectra, the interband energy distance between electron and hole 1s-bound states in the QDs, E1s (e-h), was determined to be approximately 1.2360.03 eV. The p-type samples showed almost identical PL spectra (not shown here). Fig. 2 illustrates a schematic band diagram of the n-type MD-QDIP. This basic unit was repeated for 10 times in the actual devices to increase the infrared absorption. Ohmic contacts were fabricated by depositing and annealing AuNiGe (InZn) alloy on the surfaces of n(p)-type samples. The interelectrode separation, L, was set to be 100 mm. The photocurrent spectra of the MD-QDIPs were measured in a single-pass normal incidence geometry by a Fourier transform spectrometer. The sample was mounted in a variable temperature cryostat with ZnSe windows and unpolarized light from a globar was used for excitation. Fig. 3(a) shows the photocurrent spectra of the n-type MD-QDIP with d 5 60 nm measured at various temperatures, T. In the figure, the responsivity spectrum of a calibrated InGaAs photodiode operated at 300 K is also plotted for reference. The photocurrent shows a broad peak in a photon energy range of 140–450 meV and is attributed to the photoionization of the localized 1s-bound states in the QDs to the quasi-three-dimensional (3D) continuum in the conduction band of GaAs. From the spectra, the photoionization threshold of the 1s-electron states in the QDs, E1s (e), is determined to be 140610 meV (indicated by an arrow). Due to a parasitic series resistance effect at ohmic contacts, this particular device exhibited the highest responsivity at T 5 80 K. The observed responsivity is approximately one order of magnitude larger than that of typical QWIPs [1] and is due to high mobility and long lifetime of photoexcited carriers at heterointerfaces. It is also noted that a weak photosensitivity is observed even at T 5 200 K. A similar measurement was done for a p-type MD-QDIP and its MIR photocurrent spectrum measured at T 5 30 K is plotted in Fig. 3(b). The fine structures around 200 meV are due to absorption by organic glue and are not relevant to the discussions here. From this spectrum, the photoionization threshold for the 1s-hole states in the QDs, E1s (h), is determined to be 110610 meV. By summarizing the PL and MIR photocurrent data for the n- and p-type devices, we can draw a band diagram for a QD embedded in GaAs matrix, as shown in Fig. 4. It is noted that E1s (e) 1 E1s (e-h) 1 E1s (h) amounts

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Fig. 3. (a) Infrared photocurrent spectra measured at various temperatures on a n-type MD-QDIP with d560 nm. The arrow in the figure indicates the position of photoionization threshold for the 1s-electrons. The figure also shows the reference photocurrent spectrum of a calibrated InGaAs photodiode operated at 300 K. (b) Infrared photocurrent spectrum measured on a p-type MD-QDIP with d560 nm measured at 30 K. The arrow in the figure indicates the position of photoionization threshold for the 1s-heavy holes.

to 1.4860.04 eV and is close to the bandgap of GaAs, suggesting reliability of the obtained band diagram. On the basis of the above information, we have calculated the electronic states and the photoionization probability of the QDs in the n-type MD-QDIPs [14,15]. In the calculation, we assumed that a lens-shaped QD with a diameter of 20 nm and a height of 2 nm sits on a 2 monolayer

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Fig. 4. The determined band diagram for the self-assembled InAs quantum dots embedded in GaAs matrix.

(ML)-thick wetting layer (WL) and that the QD/ WL system is placed at the center of a large cylinder of GaAs (diameter 5 160 nm, thickness 5 60 nm) (see the inset of Fig. 5). We also assumed that the potential depth of the InAs QD in GaAs matrix is 400 meV and the effective mass for electrons is 0.07m 0 [16]. The horizontal axis of Fig. 5 is the energy measured from the conduction band edge of GaAs. It is found that two bound states (1s and 2p) are formed in the QD potential, whose energy positions are indicated by arrows, and that 20 meV-wide quasi-two-dimensional (2D) continuum states due to the WL are formed just below the conduction band edge. Since electrons occupy only the 1s-states in the QDs, we calculate all the possible 1s–p transition probabilities in the system. Fig. 5 shows the result of such calculations. Spiky features in the transition probability are artifacts due to the choice of the boundary condition in our calculation. In the actual situation, the envelope of the calculated transition probability is expected to be observed. The calculated probability for the 1s–2p bound-to-bound transition is very large and cannot be accommodated in this figure. The dominant feature in the spectrum is the peak due to the transition from the 1s-state to the p-like 2D continuum states in the WL. A very broad peak is also seen for the transition to the quasi-3D continuum in the wide QW. It should be noted that the transition probability to the p-states near the exit of the QD potential is suppressed. This suppression is due to the fact that the overlap between the initial 1s-state in the QD and the final p-states in the quasi-3D continuum becomes very small because the wavefunction of the quasi-3D continuum at the exit of the QD potential must be orthogonal to the wavefunctions in the QDs and the WL. From a comparison between the photocurrent spectra shown in Fig. 3(a) and the calculated photoionization spectrum of the 1s-state shown in Fig. 5, one big difference is noticed; that is, the peak due to the 1s–WL transition is missing in the experiment. This fact suggests that in the actual system QDs may not sit on the WL but may be isolated from it, as suggested previously [17]. Finally, we would like to discuss the optimization of the device structures. In photoconductive detectors, the responsivity is proportional to t. In MD-QDIPs, when electrons in the QDs are photoexcited into the conduction band of GaAs, they are transferred to the heterointerfaces and

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Fig. 5. The calculated probabilities for the 1s-to-p dipole transitions. Arrows indicate the positions of the 1s and 2p bound states. Hatched regions denote the quasi-2D continuum in the wetting layer and the quasi-3D continuum in the wide quantum well. The inset shows the geometry which was used for the calculation.

spatially separated from the QDs. Owing to this spatial separation, the probability of recapture of electrons by QDs is significantly reduced and consequently, very long lifetimes are expected. Since the probability of recapture by QDs via phonon scattering is determined by the overlap of the wavefunctions of the bound states in the QDs and the states at the heterointerface, it is expected that t is approximately dependent on d as;

S D

1 d ]~exp 2 ] t d0

(1)

where d 0 is the characteristic length determined by the penetration depth of the two wavefunctions. t in the n-type MD-QDIP was investigated by measuring the waveforms of photocurrent induced by light pulses from a high modulation-speed InGaP light emitting diode (LED) operated at l 5630 nm [18]. When the LED is on, electrons and holes are generated in the conduction and valence bands of QWs, respectively. Because of the depletion field, the photoexcited electrons relax to the heterointerface and holes are captured by the QDs. Then, photoexcited holes recombine with electrons in the QDs either radiatively or non-radiatively. Under LED illumination, these processes reach a steady state. When the LED is suddenly turned off, however, the electrons at the heterointerfaces are slowly captured by the QDs via phonon scattering. This process is expected to be much slower than other relaxation processes (relaxation to the heterointerfaces / QDs, radiative recombinations, etc.) and dominates the decaying process of the photocurrent. Therefore, we can determine the lifetimes of the photoexcited carriers even using visible light pulses. Fig. 6 shows the temporal waveforms of the photocurrent of n-type MD-QDIP samples with various

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Fig. 6. The photocurrent waveforms in n-type MD-QDIPs with various d induced at 77 K by visible light pulses (dotted line) from an InGaP light emitting diode. The inset plots the determined photocarrier lifetimes as a function of d.

d measured at T577 K with pulsed LED illumination [18]. From the trailing part of the photocurrent waveforms we can determine t. As seen in the figure, the decay time dramatically increases with increasing d. The t determined at T577 K was about 85 ms, 0.36 ms, and 3 ms for d560, 80, and 100 nm samples, respectively, and are plotted for various d in the inset of Fig. 6. As seen in the figure, t depends almost exponentially on d for d .60 nm. The deviation from the exponential dependence for d520 and 40 nm is due to the CR time constant in the measurements. Now, it is possible to estimate the photoconductive gains of the MD-QDIPs, vt /L, where v is the drift velocity of the photoexcited carriers. By substituting v|10 7 cm / s and t |0.1–1 ms, the photoconductive gain is estimated to be as large as |1310 5 – 6 at T577 K for L5100 mm. This extremely large photoconductive gain is |10 5 – 6 times larger than that of typical QWIPs [1] and enables high sensitivity operation of MD-QDIPs. In summary, we have designed and fabricated quantum dot (QD) infrared photodetectors which utilize photoionization of self-assembled InAs QDs and lateral transport of photoexcited carriers in the modulation-doped AlGaAs / GaAs two-dimensional channels (MD-QDIPs). A broad photocurrent signal has been observed in the mid-infrared range. A very large photoconductive gain of the order of |10 5 – 6 is realized by long lifetimes as well as high mobilities of photoexcited carriers in the modulation-doped conduction channels.

Acknowledgements We thank J.-H. Liang of Stanley Electric for providing us with high modulation-speed InGaP LEDs, and G. Bastard, R. Ferreira, and Y. Nakano for fruitful discussions. This work was supported by a

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Grant-in-Aid from JSPS ([13555104), a Grant-in-Aid for COE research from MEXT ([12CE2004), CREST of Japan Science and Technology Corporation, and NEDO international collaboration program.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

For example, B.F. Levine, J. Appl. Phys. 74 (1993) R1. J. Phillips, K. Kamath, P. Bhattacharya, Appl. Phys. Lett. 72 (1998) 2020. D. Pan, E. Towe, S. Kennerly, Appl. Phys. Lett. 73 (1998) 1937. S. Maimon, E. Finkman, G. Bahir, S.E. Schacham, J.M. Garcia, P.M. Petroff, Appl. Phys. Lett. 73 (1998) 2003. S.J. Xu, S.J. Chua, T. Mei, X.C. Wang, X.H. Zhang, G. Karunasiri, W.J. Fan, C.H. Wang, J. Jiang, S. Wang, X.G. Xie, Appl. Phys. Lett. 73 (1998) 3153. N. Horiguchi, T. Fuatasugi, Y. Nakata, N. Yokomaya, T. Mankad, P.M. Petroff, Jpn. J. Appl. Phys. 38 (1999) 2559. T. Cho, J.-W. Kim, J.-E. Oh, S. Hong, Jpn. J. Appl. Phys. 38 (1999) 2442. H.C. Liu, M. Gao, J. McCaffrey, Z.R. Wasilewski, S. Fafard, Appl. Phys. Lett. 78 (2001) 79. S. Sauvage, P. Boucaud, T. Brunhes, V. Immer, E. Finkman, J.-M. Gerard, Appl. Phys. Lett. 78 (2001) 2327. L. Chu, A. Zrenner, G. Bohm, G. Abstreiter, Appl. Phys. Lett. 76 (2000) 1944. S.-W. Lee, K. Hirakawa, Y. Shimada, Appl. Phys. Lett. 75 (1999) 1428. S.-W. Lee, K. Hirakawa, Y. Shimada, Physica E 7 (2000) 499. S.-W. Lee, K. Hirakawa, Y. Shimada, MRS Symposia Proceedings Vol. 607, Infrared Applications of Semiconductors III (2000) p. 147. Ph. Lelong, S.-W. Lee, K. Hirakawa, H. Sakaki, Physica E 7 (2000) 174. Ph. Lelong, K. Hirakawa, S.-W. Lee, K. Hirotani, H. Sakaki, in: N. Miura, T. Ando (Eds.), Proc. 25th Int. Conf. Phys. Semicond., Osaka 2000, Springer, 2001. H. Drexler, D. Leonard, W. Hansen, J.P. Kotthaus, P.M. Petroff, Phys. Rev. Lett. 73 (1994) 2252. C. Kammerer, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, J.-M. Gerard, Phys. Rev. Lett. 87 (2001) 207401. S.-W. Lee, K. Hirakawa, Physica E, in press.