High quantum efficiency dots-in-a-well quantum dot infrared photodetectors with AlGaAs confinement enhancing layer

(1)

High quantum efficiency dots-in-a-well quantum dot infrared photodetectors with

AlGaAs confinement enhancing layer

H. S. Ling, S. Y. Wang, C. P. Lee, and M. C. Lo

Citation: Applied Physics Letters 92, 193506 (2008); doi: 10.1063/1.2926663

View online: http://dx.doi.org/10.1063/1.2926663

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/19?ver=pdfcov Published by the AIP Publishing

Articles you may be interested in

High-performance, long-wave (10.2m) InGaAs/GaAs quantum dot infrared photodetector with quaternary In0.21Al0.21Ga0.58As capping

Appl. Phys. Lett. 99, 181102 (2011); 10.1063/1.3657142

Very long wavelength quantum dot infrared photodetector using a modified dots-in-a-well structure with AlGaAs insertion layers

Appl. Phys. Lett. 98, 103507 (2011); 10.1063/1.3563709

Effects of well thickness on the spectral properties of In 0.5 Ga 0.5 As Ga As Al 0.2 Ga 0.8 As quantum dots-in-a-well infrared photodetectors

Appl. Phys. Lett. 92, 193507 (2008); 10.1063/1.2927487

Influence of quantum well and barrier composition on the spectral behavior of InGaAs quantum dots-in-a-well infrared photodetectors

Appl. Phys. Lett. 91, 173508 (2007); 10.1063/1.2802559

Resonant cavity enhanced In As In 0.15 Ga 0.85 As dots-in-a-well quantum dot infrared photodetector J. Vac. Sci. Technol. B 25, 1186 (2007); 10.1116/1.2746054

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 140.113.38.11 On: Wed, 30 Apr 2014 23:05:20

(2)

High quantum efficiency dots-in-a-well quantum dot infrared photodetectors

with AlGaAs confinement enhancing layer

H. S. Ling,1S. Y. Wang,2,a兲 C. P. Lee,1and M. C. Lo1 1

Department of Electronic Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan

2_{Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 106, Taiwan} 共Received 1 April 2008; accepted 18 April 2008; published online 13 May 2008兲

We demonstrate the high quantum efficiency InAs/In0.15Ga0.85As dots-in-a-well共DWELL兲 quantum

dot infrared photodetectors 共QDIPs兲. A thin Al0.3Ga0.7As layer was inserted on top of the InAs

quantum dots共QDs兲 to enhance the confinement of QD states in the DWELL structure. The better confinement of the electronic states increases the oscillation strength of the infrared absorption. The higher excited state energy also improves the escape probability of the photoelectrons. Compared with the conventional DWELL QDIPs, the quantum efficiency increases more than 20 times and the detectivity is about an order of magnitude higher at 77 K. © 2008 American Institute of Physics. 关DOI:10.1063/1.2926663兴

In the past decade, quantum dot infrared photodetectors 共QDIPs兲 have been widely investigated with different struc-tures and materials because of their potential to become low cost, high temperature operation infrared detectors.1–8With the self-assembled In共Ga兲As/GaAs quantum dots 共QDs兲, several encouraging results have been demonstrated with op-eration temperatures higher than 150 K.5–9 However, with the simple QD structure, the tuning of the detection wave-length is relatively difficult because of the limitation of the self-assembled growth. To overcome this drawback, many efforts have been focused on the dots-in-a-well 共DWELL兲 structure which provides the flexibility to adjust the elec-tronic states with the quantum well.8–19 The DWELL struc-ture with GaAs wells and AlGaAs barriers has been investi-gated for different spectral ranges.18 The QDIPs made with the DWELL structure have an additional advantage of lower dark current because the ground state energy is usually lower. Recently, high quality 640⫻512 DWELL QDIP im-aging focal plane arrays have been demonstrated.19

However, the QDIPs made from the DWELL structure usually suffer from poor quantum efficiencies. In DWELL QDIPs, the infrared absorption comes from the transition be-tween the ground state and the excited state in which the electronic wave function extends to the quantum well region. With the inserted InGaAs quantum well, the absorption strength associated with the bound-to-bound transition be-comes weaker due to the decreased dipole element. Besides, the photoexcited electrons in the DWELL QDIPs have lower energy relative to the GaAs barrier. So the escape probability for the excited carriers to become free is lower and, as a result, the operation voltage is higher. It has been demon-strated that the quantum efficiency of DWELL QDIPs can be increased by increasing the QD density with Sb surfactants.11 However, an additional Sb source is required and the QD growth process is more complicated than the conventional devices.

In this study, we designed a modified DWELL structure to enhance both the absorption strength and the escape prob-ability. In this structure, a thin Al0.3Ga0.7As layer was

in-serted on top of the InAs QD layer. This added layer pro-vides better confinement for the excited state wave function in the QD region and also elevates the excited state energy. The overall quantum efficiency is therefore improved.

Two samples were prepared in this study by a Veeco GEN-II molecular beam epitaxy system on 共001兲 semi-insulating GaAs substrates. One is the confinement enhanced DWELL 共CE-DWELL兲 QDIP while the other is a conven-tional DWELL QDIP. In each sample, the active region con-tains ten layers of QDs separated by 53 nm GaAs barrier layers and is sandwiched between two 500 nm n+_GaAs

con-tact layers. For the CE-DWELL structure, 2.2 ML of InAs QDs were deposited on a 2 nm In_0.15Ga_0.85As layer and then followed by a 2.5 nm Al0.3Ga0.7As confinement layer and a

4.5 nm In0.15Ga0.85As.␦-doped Si layers with a concentration

of 1⫻1010 _cm−2_{were inserted 2 nm before each QD layer to}

provide electrons to the QDs. The schematic of the CE-DWELL sample structure is shown in Fig.1共a兲. For the con-ventional DWELL sample, the Al_0.3Ga_0.7As layer was re-placed by an In0.15Ga0.85As layer while other growth

parameters were kept to be the same. A surface QD layer with the same growth condition as the embedded QD layers was also deposited for atomic force microscopy共AFM兲 mea-surement. Similar QD densities were determined in both samples to be about 2.1⫻1010_cm−2_{. Given the doping}

den-sity used, we estimated an average number of carriers in each QD around 0.5.

Due to the strain distribution, the inserted thin Al0.3Ga0.7As layer was expected to aggregate in areas

with-out QDs leaving the tips of the QDs uncovered. Such a prop-erty is favored since the structure could provide the barrier that enhances the confinement in the lateral direction but not block the transport of electrons in the vertical direction. In order to confirm this, the structure of the active layer was examined by the cross-sectional transmission electron mi-croscopy共TEM兲 image shown in Fig.1共b兲. It is clearly seen that the AlGaAs layer is flat instead of conforming to the shape of the InAs QDs. The TEM image shows the height of the QD is about 5 nm and the base width is about 26 nm.

The confinement effect of the AlGaAs layer was then verified with the photoluminescence共PL兲 measurement. The a兲_{Electronic mail: [email protected].}

APPLIED PHYSICS LETTERS 92, 193506共2008兲

(3)

PL spectra were taken with a liquid nitrogen cooled micro-PL system with high excitation density to reveal the higher levels of the QDs. Figure2shows the measured spec-tra of the two samples mentioned above. Four spec-transition peaks which arise from the DWELL states were revealed in the spectra of both samples. Clear leverage of the transition energies was identified in the CE-DWELL sample. The en-ergy difference of the ground state in the two samples is about 62 meV, while the energy difference is 82 meV for the third excited state. The insertion of the AlGaAs layer pro-vides more confinement effect to the higher excited states. The possibility that the observed blueshift might come from the change of dot size and composition due to different capped materials is ruled out because it has been reported that the insertion of an Al contained layer should cause a redshift for the ground state energy due to the suppression of the In segregation.20Furthermore, our AFM and TEM results show that the QD shape and size for the two samples are quite similar. So the blueshift of the energy states in our sample should be the result of the better confinement pro-vided by the inserted AlGaAs barrier layer.

Standard processing techniques were then applied for the device fabrication. 260⫻370␮m2_{mesas with AuGe contact}

rings were formed to allow normal incidence measurement from the mesa top. The device characteristics at different temperatures were measured using a close cycled helium cry-ostat. In all measurements, the bottom contact is referred as ground. The photocurrent responsivity spectra were mea-sured by Fourier transform infrared spectroscopy and cali-brated by a 1000 C blackbody radiation source.

Figure 3 shows the responsivity spectrum and the re-sponsivity curves at different biases of the two samples at 77 K. The responsivity peak for the CE-DWELL sample is around 8␮m which is shorter than the peak for the DEWLL sample共9.2␮m兲 as expected from the PL measurement. As-suming 70% of the bandgap difference is in the conduction band, the infrared response is identified to be from the tran-sition between the ground state to the third excited state in both samples. This bound to bound transition is consistent with the relatively narrow bandwidth 共⌬␭/␭p兲 of the

spec-trum. Comparing the responsivity curve of the two samples, a clear increase of the responsivity in the CE-DWELL sample is shown over the whole bias region. At −1.2 V and 77 K, the responsivity for the CE-DWELL sample is 0.536 A/W while it is only 0.023 A/W for the conventional sample.

To further probe the origin of the increase of responsiv-ity, the device gain was calculated through the noise mea-surement assuming the G-R noise dominates.21 Due to the limit of the measurement system, noise current smaller than 1⫻10−13_A_/Hz0.5_{could not be correctly measured in lower}

bias range. Both the current gain and the calculated quantum efficiency are shown in Fig.4for comparison. The difference in responsivity is primarily attributed to the difference of quantum efficiency. Compared with the quantum efficiency of the DWELL sample, the quantum efficiency of the CE-DWELL sample is much higher over the measured bias range and increases faster with the applied voltage. The high-est quantum efficiency in CE-DWELL is around 2% at −1 V, which is clearly superior to that in DWELL 共0.08% at −1.3 V兲. It is noticed that the response spectral width is wider in the DWELL sample and the comparison of the peak quantum efficiency might not be fair. Nevertheless, the

inte-FIG. 1.共Color online兲共a兲 The schematic diagram of the CE-DWELL QDIP. 共b兲 The cross-sectional TEM image of the CE-DWELL structure.

FIG. 2. The high excitation energy PL spectra of the two samples. The solid line is the result for the CE-DWELL sample and the dash line is the spec-trum of the DWELL sample. The arrows indicate the ground state and the third excited state transition energies of the two samples.

FIG. 3. The voltage dependence of the peak responsivity of the two samples at 77 K. The insert shows the responsivity spectra of the two samples at −1 V and 77 K.

193506-2 Ling et al. Appl. Phys. Lett. 92, 193506共2008兲

(4)

grated quantum efficiency of the CE-DWELL sample is still superior to the DWELL sample by a factor of 16. The in-serted AlGaAs layer confines the QD’s excited-state wave function to be more localized, so the absorption strength of the ground state electrons is enhanced due to the better wave function coupling. In addition, the excited-state energy in the CE-DWELL sample is about 60 meV higher than that in the DWELL sample共see Fig.2兲. The escape probability is thus

higher and as a result the operation voltage is lower for the CE-DWELL sample. On the other hand, the two samples have essentially similar current gain in the bias region of −1.3– 1.25 V. So the carrier transport property is preserved in the CE-DWELL structure even though the wide bandgap AlGaAs layer was added. This is consistent with our expec-tation since the AlGaAs layer mainly covers the QD edges and leaves the upper portion of the QDs uncovered, thereby serving as the path for the flow of the photocarriers.

The higher ground state energy of the CE-DWELL sample could generate higher dark current and it has been observed in the measurement. The dark current density was 3.82⫻10−4_A_/cm−2 _{at −1 V at 77 K for the CE-DWELL}

sample but it was only 3.3⫻10−5_A/cm−2 _{for the DWELL}

sample under the same condition. However, the increase of the quantum efficiency overcomes the increased dark current. As a result, the overall performance of CE-DWELL QDIPs is far more superior to that of the conventional ones. At 77 K, the highest detectivity measured for the CE-DWELL detector is 1⫻1010_{cm Hz}0.5_{/W 共at −0.9 V兲, which is ten}

times higher than that in the DWELL detector 共1 ⫻109_{cm Hz}0.5_{/W at −1.2 V兲.}

In summary, we designed a modified DWELL structure for QDIPs. A thin AlGaAs layer was inserted on top of InAs QDs as a confinement enhancing layer. This enhanced con-finement effect greatly enhanced both the absorption quan-tum efficiency and the escape probability. At the same time, the transport property of carriers was almost not affected. At 77 K, the maximal quantum efficiency was increased by about 25 times and the peak detectivity was increased by about ten times with a lower bias voltage. Our results dem-onstrate that the confinement enhanced structure is a very promising approach toward the realization of high perfor-mance QDIPs.

1_{D. Pan, E. Towe, and S. Kennerly,}_{Appl. Phys. Lett.} _{75, 2719}_共1999兲. 2_{S. Y. Wang, S. D. Lin, H. W. Wu, and C. P. Lee,}_{Appl. Phys. Lett.} _78,

1023共2001兲.

3_{S. Y. Wang, S. C. Chen, S. D. Lin, C. J. Lin, and C. P. Lee, Infrared Phys.}

Technol. 44, 527共2003兲.

4_{H. Lim, W. Zhang, S. Tsao, T. Sills, J. Szafraniec, K. Mi, B. Movaghar,}

and M. Razeghi,Phys. Rev. B 72, 085332共2005兲.

5_{P. Bhattacharya, X. H. Su, S. Chakrabarti, G. Ariyawansa, and A. G. U.}

Perera,Appl. Phys. Lett. 86, 191106共2005兲.

6_{L. Jiang, S. S. Li, N. T. Yeh, J. I. Chyi, C. E. Ross, and K. S. Jones,}_Appl. Phys. Lett. 82, 1986共2003兲.

7_{S. Chakrabarti, A. D. Stiff-Roberts, P. Bhattacharya, S. Gunapala, S.}

Ban-dara, S. B. Rafol, and S. W. Kennerly,IEEE Photon. Technol. Lett. 16,

1361共2004兲.

8_{X. Lu, I. Vaillancourt, and M. J. Meisner,}_{Appl. Phys. Lett.} _{91, 051115}

共2007兲.

9_{H. Lim, S. Tsao, W. Zhang, and M. Razeghia,}_{Appl. Phys. Lett.} _90,

131112共2007兲.

10_{R. S. Attaluri, J. Shao, K. T. Posani, S. J. Lee, J. S. Brown, A. Stintz, and}

S. Krishna,J. Vac. Sci. Technol. B 25, 1186共2007兲.

11_{P. Aivaliotis, L. R. Wilson, E. A. Zibik, J. W. Cockburn, M. J. Steer, and}

H. Y. Liu,Appl. Phys. Lett. 91, 013503共2007兲.

12_{E. T. Kim, Z. Chen, and A. Madhukar,}_{Appl. Phys. Lett.} _{79, 3341}_共2001兲. 13_{Z. Ye, J. C. Campbell, Z. Chen, E. T. Kim, and A. Madhukar,}_{J. Appl.}

Phys. 92, 7462共2002兲.

14_{S. Raghavan, P. Rotella, A. Stintz, B. Fuchs, S. Krishna, C. Morath, D. A.}

Cardimona, and S. W. Kennerly,Appl. Phys. Lett. 81, 1369共2002兲. 15_{S. Raghavan, D. Forman, P. Hill, N. R. Weisse-Bernstein, G. von Winckel,}

P. Rotella, S. Krishna, S. W. Kennerly, and J. W. Little,J. Appl. Phys. 96,

1036共2004兲.

16_{S. Krishna, Infrared Phys. Technol. 47, 153}_共2005兲.

17_{R. S. Attaluri, S. Annamalai, K. T. Posani, A. Stintz, and S. Krishna,}_J. Vac. Sci. Technol. B 24, 1553共2006兲.

18_{G. Jolley, L. Fu, H. H. Tan, and C. Jagadish,}_{Appl. Phys. Lett.} _{91, 173508}

共2007兲.

19_{S. D. Gunapala, S. V. Bandara, C. J. Hill, D. Z. Ting, J. K. Liu, S. B.}

Rafol, E. R. Blazejewski, J. M. Mumolo, S. A. Keo, S. Krishna, Y. C. Chang, and C. A. Shott,Infrared Phys. Technol. 50, 149共2007兲. 20_{Z. Y. Zhang, B. Xu, P. Jin, X. Q. Meng, Ch. M. Li, X. L. Ye, and Z. G.}

Wang,J. Appl. Phys. 92, 511共2002兲.

21_{S. Y. Wang, M. C. Lo, H. Y. Hsiao, H. S. Ling, and C. P. Lee, Infrared}

Phys. Technol. 50, 166共2007兲. FIG. 4. 共Color online兲 The current gain 共triangles兲 and quantum efficiency

共squares兲 of the two samples at different voltages at 77 K.

193506-3 Ling et al. Appl. Phys. Lett. 92, 193506共2008兲