Designs and applications of corrugated QWIPs

Infrared Physics & Technology 47 (2005) 76–90 www.elsevier.com/locate/infrared

Designs and applications of corrugated QWIPs K.K. Choi a,*, C. Monroy a, A. Goldberg a, G. Dang a, M. Jhabvala b, A. La b, T. Tamir c, K.M. Leung c, A. Majumdar d, Jinjin Li d, D.C. Tsui d a

U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, USA b NASA, Goddard Space Flight Center, Greenbelt, MD 20771, USA c Polytechnic University, Brooklyn, NY 11201, USA d Princeton University, Princeton, NJ 08544, USA Available online 12 April 2005

Abstract In this paper, we will describe the performance of two long wavelength 1024 · 1024 corrugated quantum well infrared photodetector focal plane arrays (C-QWIP FPAs) with cutoff wavelengths at 8.6 and 9.0 lm, respectively. The FPAs are background limited (BLIP) at around 76 K in an f/1.8 optical system. In addition to the high performance of these C-QWIPs, the corresponding FPAs are also easily producible, making them ideal for large production. We will discuss the optimization of the detectors for different applications. Since corrugated coupling is wavelength insensitive, it is capable of broadband and multi-color detection. We will present a GaAs/AlGaAs broadband detector based on a binary superlattice design. Incorporating the broadband characteristic in a high gain InGaAs/InP material, C-QWIPs with large background photocurrent can be obtained for high speed applications. For multi-color detection, we have investigated two different approaches. One is based on a voltage-tunable, twocolor QWIP material, which can be switched between two detection wavelengths simply by changing the detector bias. Stacking two of these similar QWIPs together and separating them with a middle contact layer, a voltage tunable, four-color detector array can be fabricated. A second approach is to combine a broadband QWIP material with a wavelength-selective light coupling method. Using a light coupling geometry to control the detection wavelength of individual pixels, a large number of wavelengths can be detected based on a single broadband detector material. Published by Elsevier B.V. PACS: 42.25.Fx; 73.63.Hs; 85.35.Be; 85.60.Gz Keywords: Infrared detector; Quantum well; Light-coupling; FPA

*

Corresponding author. Tel.: +1 301 394 0495; fax: +1 301 394 1746/5451. E-mail address: [email protected] (K.K. Choi).

1350-4495/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.infrared.2005.02.013

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1. Introduction Quantum well infrared photodetector (QWIP) technology has reached a high level of maturity, and QWIP cameras are now commercially available. The major advantage of this technology is derived from the starting detector material, which is easily available and highly versatile. The material can be purchased in large quantities at low cost, and its absorption properties can be tailored precisely. It has a wide range of detection wavelengths from 3 to 25 lm and the spectral width can be varied from 0.5 to several microns [1,2]. The material has good mechanical strength and the substrate is transparent to the infrared. The material possesses a depletion layer on its surface. This layer acts as a natural passivation in protecting the detector active material from the surfaces, thus allowing the use of small pixels in high resolution focal plane arrays (FPAs). Meanwhile, the small photoconductive gain, g, of the detector also contributes to the viability of small unit cells. It increases the effective charge capacity of a readout integrated circuit (ROIC) by 1/g times, and thus allows high density ROICs with small charge capacity to achieve high sensitivity detection. With all of these material attributes, QWIPs are well poised to be a competitive infrared technology. However, QWIPs need to be operated at low temperatures, and large format FPAs are still in short supply. These shortcomings are exacerbated by the grating fabrication in the FPAs. Although gratings are currently the standard approach for light coupling [3–6], they have several drawbacks in FPA applications. For example, the coupling is wavelength specific, requires electron-beam lithography for the shorter wavelengths, produces pixel cross-talk, and is less effective in small pixels. These shortcomings need to be mitigated for further development of QWIPs.

2. Figures of merit To further improve the QWIP technology, we proposed another coupling structure, known as corrugated QWIPs (C-QWIPs) [7,8]. The detector does not suffer from the problems associated with

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gratings, and the optimized structure will give a large quantum efficiency, g, and a small photoconductive gain. In general, the sensitivity of an FPA is measured by the noise equivalent temperature difference, NEDT. Under BLIP conditions, pffiffiffiffiffiffiffiffiffiffiffi 2gN p ni NEDT ¼ dN p ¼ ; ð1Þ CN p dT

where ni is the number of noise electrons accumulated in a frame time, Np is the number of photoelectrons collected, and C = hm/kT2 is the thermal contrast. For a given integration time s, Np = ggUs, where U is the in-band photon flux falling on the pixel. Substituting this Np into Eq. (1) yields pffiffiffi 2 1 pffiffiffiffiffiffiffiffi : NEDT ¼ ð2Þ C gUs Hence, a small NEDT requires a large g. The situations in which Eq. (2) is applicable are when the detection is conducted at high frame rates, using slow optics, or under low backgrounds, in which Np is limited by the duration of signal collection and not by the readout circuit. Under such conditions, besides a large g, it is also useful to expand the detector absorption width and preserve the pixel fill factor so that optical flux received by the detector can be maximized. On the other hand, when the application permits a long integration time, in which Np approaches the maximum capacity Nc of the ROIC, Eq. (1) then becomes pffiffiffi rffiffiffiffiffiffi 2 g NEDT ¼ : ð3Þ Nc C In this case, NEDT depends only on g. In fact, the readout can be viewed as having an effective charge capacity of Nc/g, and a small g will lead to a small NEDT. From Eqs. (2) and (3), a detector should have a large g and a small g. The value of their product gg, which is referred to as the conversion efficiency (CE), does not affect the sensitivity in both cases and therefore, in principle, can assume any value. Nevertheless, CE determines the integration time s = Nc/ggU in the second case. If an application has certain speed requirement, it will impose a

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minimum value on CE. In this case, a small g is feasible only if it is supported by a large g.

3. Corrugated QWIPs A C-QWIP consists of a collection of trapezoidal corrugations of active material. The light coupling is achieved by reflection at the corrugated sidewalls. A typical C-QWIP detector geometry is shown in Fig. 1. The angle of the sidewall indicated in Fig. 1(b) is fixed by the crystallographic plane exposed by chemical etching and was experimentally determined to be 50 ± 2. The period p of the corrugations is thus related to the etching depth t and the width d at the top of the corrugations by p = 2t cot(50) + d. Using ray-tracing techniques within the geometric-optical (GO) model, one can calculate the external quantum efficiency for unpolarized light g in terms of the absorption coefficient a of vertically polarized, parallel propagating light. Assuming that the angle of the corrugation sidewalls is 45 instead of 50, the value of g calculated from the decay of light along the ray path is given by [8]     4n 1 eap  2at gða; p; tÞ ¼ t þ 1  e ; þ K 0 2 2a ð1 þ nÞ p ð4Þ where K0 is the internal unpolarized quantum efficiency due to light scattering at the ends of the corrugations. The geometry of a C-QWIP is fixed by the ratio 2t/p. Within the 45 sidewall approximation, 2t/p = 1 yields a triangle while a smaller value

Fig. 1. (a) 3-D view of a typical C-QWIP detector pixel. The central unetched area is for indium bump bonding. (b) Ray diagram in a side view showing a typical light path. The solid path is for thick substrates and the dashed lines show an additional path due to substrate reflection.

Fig. 2. Theoretical g of C-QWIPs vs. p for a = 0.22 lm1 for different S = 2t/p, under the geometric-optical (GO) model with K0 = 0 and thick substrate. The approximate value of g for thinned substrate can be read off from the same curve at twice the period.

provides a shallower trapezoid. Detectors with a fixed 2t/p have a fixed projection area fill factor S (=2t/p) available for light reflection. C-QWIPs with different S thus fall into different classes. In Fig. 2, the calculated g for thick substrates and a = 0.22 lm1 (typical for a narrow band GaAs QWIP) is plotted as a function of p under different classes. The value of g increases initially with p because of the larger active volume. However, when p  1/a, most of the light incident under the sidewall is absorbed, in which case g is limited purely by its class geometry. For triangular corrugations (S = 1), the maximum g is 0.5 · 0.71 · S = 0.36, which is half of the unpolarized unity incident power times the transmission coefficient of the substrate. In principle, g can be increased using an anti-reflection coating and its value can reach 0.5 for S = 1. In Fig. 2, a larger triangular corrugation is seen to have a larger g and at the same time, it also contains more quantum well layers, so that g is smaller. Therefore, a large triangular corrugation will exhibit a large g and a small g simultaneously. When the substrate is thin relative to the lateral dimension of the detector pixel, Fig. 1(b) indicates that light reflected from the substrate will retrace a similar path, so that the total path length doubles. Thus, g for a thinned detector can be read off from the same curve at twice the value of p. For exam-

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ple, the efficiency g in a detector with p = 10 lm and S = 1 will increase after thinning from 21 to 27.5% (read from p = 20 lm), i.e. a 13% increase. The quantity g in Fig. 2 determines the level of optical absorption, and thus the magnitude of the photocurrent Ip. When the dark current Id is considered, NEDT in both Eqs. (2) and (3) is larger by a factor of (1 + Id/Ip) [9]. Consequently, the detection sensitivity depends on g/Id. Furthermore, BLIP is achieved only when Ip > Id. Hence the detector performance is enhanced by either increasing Ip or reducing Id. Due to the decreased active volume in the corrugated coupling geometry, Id is reduced by a factor (p  t)/p. Therefore, when comparing with other couplings schemes (e.g., grating coupling and edge coupling that do not offer dark current reduction), a new quantity Q can be defined by Qg

I d ð45Þ p g ¼ : ¼g Id p  t 1  S=2

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S  t/p = 0.884. In Fig. 4, we plot the absorption, reflection and transmission as a function of p for the same a. The values of g calculated from the two models follow the same trend and are quite close to each other. However, the MTL model reveals a finite transmission in the entire calculated p range, which is due to the presence of sharp corners in the corrugations. The transmission reduces the predicted g in the GO model. In order to prevent transmission of light, we assume a nonabsorbing MgF2 (eMg = 1.64) film and a Cr/Au layer on the inclined sidewalls. The results are shown in Fig. 5. When the light is

ð5Þ

The value of Q is the quantum efficiency normalized to the same (full) dark current. Fig. 3 shows the values of Q for the same detector parameters as in Fig. 2. Q can exceed 50% even without AR coating. The GO predictions were verified by the rigorous EM modal transmission-line (MTL) theory [8,10–12]. In this theory, the C-QWIP structure is assumed to have a sidewall angle of 48 and

Fig. 3. Theoretical Q of C-QWIPs vs. p for a = 0.22 lm1 and different S = 2t/p, under the GO model with K0 = 0 and thick substrate. The value of Q for thinned substrate can be similarly read off from the same curve at twice the period.

Fig. 4. Fractions of absorbed, reflected and transmitted powers vs. period, as determined by the MTL model for S = 0.884. The absorbed power predicted by the GO model is also shown.

Fig. 5. External g vs. period for different values of tMg, as determined by the MTL model. The external g predicted by the GO model is also shown.

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Fig. 6. External g and b vs. wavelength and different values of ei for the C-QWIP with thin substrate.

totally reflected at the metal interface, the values of g under the two models agree quite well for small tMg. We next examine C-QWIPÕs performance when the GaAs substrate is completely removed. With such a thin substrate, and with fixed p = 5.5 lm and S = 0.884, Fig. 6 shows that there is a large variation of g as a function of k. Nevertheless, for most k between 6 and 10 lm, g is above 20% for a = 0.22 lm1 (i.e. ei = 1). Therefore, the broadband nature of the coupling is maintained. Due to interference effects, the ratio of the vertical optical intensity b relative to that of the edge coupling can be very different from GO modeling. For example, b reaches a value of 20 at k = 8.4 lm, with a corresponding g of 50% for ei = 1 even without AR coating.

˚ )/Cr/Au cover on the corrugated sidewalls. (1000 A In addition, an 8 · 8 lm2 ohmic metal contact ˚ Ge/385 A ˚ Au/1000 A ˚ Ag/1500 A ˚ Au) layer (215 A is deposited on top of each pixel. Fig. 7 illustrates the actual pixel cross-sections after etching. The pixel consists of both regular corrugated (type-C) sidewalls as in Fig. 7(a) and hour-glass (type-H) sidewalls as in Fig. 7(b). Due to the presence of the hour-glass sidewalls, the present geometry can have a large Q even with a small S. Fig. 8 shows the calculated g for different sidewall profiles when the substrate is removed and the pixel is covered with epoxy. Both gC and gH are wavelength dependent. However, their variations largely cancel each other, and the combined g of 15% in Fig. 9(c) is rather wavelength insensitive. Near the peak wavelength of 8.4 lm, an average g of 20% is obtained. Accounting for 64% of Id reduction, we find that Q is 54%, similar to that of a structure with triangular cross-section. Although the present design has traded detector performance for processing simplicity, the theoretical values g = 20% and Q = 54% are significantly higher than those obtainable by conventional grating coupling. Large format 1 K · 1 K C-QWIP FPAs with the above pixel geometry have been fabricated on two similar material wafers. The FPA fabricated on one of the wafers has a 9.0 lm cutoff [13,14]. Due to a defect in the ROIC integration

4. Large format C-QWIP FPAs One advantage of corrugated coupling is its effectiveness in small pixels. When the pixel size decreases, one can reduce the number of corrugations in each pixel without reducing its coupling efficiency. For example, we adopted one corrugation per pixel geometry in an 18 · 18 lm2 pixel FPA, with which p = 18 lm. We further selected a layer thickness t of 3.5 lm, so that we could delineate the pixel mesas and create the corrugated geometry in one single chemical etching step. The 3.5 lm-thick QWIP structure contains 62 periods of quantum wells, and there is an MgF2

Fig. 7. Cross-sections of a C-QWIP pixel with p = 18 lm in both x and y directions. All dimensions are in lm.

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thinned to 40 lm thick. Measured from a test structure, it has an 8.6 lm cutoff as shown in Fig. 10. The value of g is measured to be 0.12 at 3 V. Meanwhile, the CE of the FPA is measured to be 3.5% at the same bias. Consequently, g at the detection peak is deduced to be 29%, which is consistent with the 25% in Fig. 8. Since the charge capacity Nc of the present readout is 3 Me, the value of NEDT calculated from Eq. (3) is 16 mK, which represents the temporal noise component. A larger NEDT is expected when other noise sources are taken into account. Fig. 11 shows the pixel total current with f/1.8 optics and 25 C background, which indicates BLIP at 75.5 K where Ip = Id. Fig. 12 shows infrared

Fig. 8. The value g vs. wavelength for unpolarized incidence upon the C-QWIP shown in Fig. 7 for the case with thin substrate and epoxy cover: (a) lamellar xz structure (with typeC sidewalls); (b) lamellar yz structure (with type-H sidewalls; (c) actual 3D structure derived from (a) and (b). The sidewalls of the xz section can be with or without metal layers, as plotted in (a) and (c) by solid and dashed lines, respectively.

Fig. 10. The spectral response of a test C-QWIP. The peak is at 8.2 lm while the cutoff is at 8.6 lm.

Fig. 9. The upper two of the four quadrants of a 1 K · 1 K C-QWIP FPA operated at 77 K. The cutoff wavelength is at 9.0 lm.

time control, the applied voltage of this particular FPA can only be set to 0.1 V to avoid readout saturation. Despite this readout defect, good infrared images can be obtained at temperatures as high as 77 K, as shown in Fig. 9. The FPA fabricated on the second wafer was backfilled with epoxy and the substrate was

Fig. 11. The temperature dependence of the pixel current with f/1.8 optics and 25 C background. The detection is BLIP at 75.5 K at which the total current (Ip + Id) is twice the current at a low T.

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Fig. 12. Infrared images of a 1 K · 1 K 8.6 lm cutoff C-QWIP FPA operated at 78 K.

images taken at 78 K without two-point gain correction. To determine the pixel cross-talk, the images of a blackbody were taken with aperture diameters ranging from 0.012500 to 0.400 . The pixel size projected on the object plane is 0.030200 . Fig. 13 shows very little cross-talk when the hot target is less than about 6 pixel across. For small targets, the crosstalk is produced by diffraction from individual pixels. With the corrugated geometry, this diffraction effect can be avoided. On the other hand, when the target image is larger than 6 pixels, the pixels collectively begin to act as a single diffrac-

tion grating, and the cross-talk becomes significant. Since this cross-talk is caused by the small pixel size in long wavelength detection, it can be reduced using a larger pixel size. Another solution is to completely remove the substrate.

5. Broadband and high gain C-QWIPs Due to the oscillator strength f sum rule, when the optical transitions are distributed over a wider spectral width B, the value of f and hence a of the QWIP material decrease proportionally. The aB

Fig. 13. The images of circular apertures showing little pixel cross-talk for small targets and substantial cross-talk for large targets. Part of the cross-talk in the horizontal direction is artifact from the readout circuit. The substrate thickness of the FPA is 40 lm.

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product remains approximately constant. Normally, the integrated photocurrent would then remain the same even if the light coupling is not wavelength dependent. However, the large optical path in large corrugation C-QWIPs enables g to be less sensitive to the decrease of a. For example, we consider a detector material with a constant aB product of 0.22 lm1 · 1.4 lm = 0.31 and a C-QWIP with p = 20 lm and t = 10 lm. In Fig. 14, when B increases, a of the material decreases as a function of 1/B. By substituting the corresponding values of a into Eq. (4), g for different spectral widths can be obtained, and its value also decreases. However, g decreases much more slowly than 1/B. As a result, the gB product increases. One 1.0 0.8

η•B

0.6

α

period = 20 µm

0.4

η

0.2 0.0

0

2

4

6

8

10

BANDWIDTH B (µm)

Fig. 14. The values of a (lm1), g and gB as a function of B for a constant aB = 0.31.

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can thus increase the photocurrent by expanding B without increasing either g or g. One example of a broadband QWIP is made from a binary superlattice (SL) structure. Using binary QWs instead of single QWs in the SL allows a more uniform absorption across the spectrum. The present structure consists of 8 repeated binary superlattices for 8–14 lm broadband absorption ˚ thick Al0.19[15], each separated by a 600 A Ga0.81As barrier. Each superlattice consists of ˚ and 75 A ˚. alternate GaAs QWs of widths 70 A ˚ Al0.27Ga0.73As The QWs are separated by 25 A barriers. There is a total of 8 QWs in each superlattice, thus making the total SL period length equal to 0.14 lm. The QWs are doped with Nd = 4 · 1017 cm3. On top of the active material, ˚ doped contact layer. Fig. 15 there is a 1000 A shows the calculated absorption spectrum. The experimental spectral responsivity shown in Fig. 16 agrees quite well with the absorption spectrum except for the shorter long wavelength cutoff. The difference can be attributed to two reasons. One reason is carrier freezeout at low temperatures, which leaves the upper two ground states unoccupied. This will eliminate the two longest wavelength transitions from the spectrum and leads to a shorter cutoff. The second reason is the lower tunneling probability through the blocking barriers for the lowest energy photoelectrons. With this material, an 8–14 lm broadband detector is nevertheless demonstrated.

ABSORPTION COEFF. (A.U.)

1.2 1.0

σ = 11 meV

0.8 0.6 σ = 0.3 meV

0.4 0.2 0.0 6

7

8

9

10 11 12 13 WAVELENGTH (µm)

14

15

16

Fig. 15. The absorption spectrum of a binary superlattice QWIP for 8–14 lm broadband detection with two values of line broadening.

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K.K. Choi et al. / Infrared Physics & Technology 47 (2005) 76–90 0.4 V = 1.4 V

RESPONSIVITY (A/W)

DES1

0.3

45 ° EDGE COUPLING

0.2

0.1

0.0

6

7

8

9

10

11

12

13

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WAVELENGTH (µm)

Fig. 16. The experimental spectral responsivity of the binary superlattice QWIP.

A large corrugation produces a large g and a small g. If a large CE (=gg) is desired, one need to increase the gain of the basic material. In literature, InGaAs/InP is known to have a very large photoconductive gain [16], which is about 20 times larger than that of GaAs/AlGaAs for the same number of QW periods N. A narrow band InP QWIP has a typical g of 10 for N = 50. Adopting the same QW structure in a t = 10 lm C-QWIP, N will be 180 and g will be 2.8. With an estimated g = 28% for this detector geometry, CE will be equal to 0.78, comparable to that of HgCdTe detectors. The larger g nevertheless degrades the FPA sensitivity in the Nc limited case. If InP QWIPs can be made broadband, a smaller gain can be selected by adjusting the bias. We have designed and tested a superlattice InP detector. The detector is grown on a semi-insulating (1 0 0) InP substrate. The SL consists of four peri˚ In0.53Ga0.47As QWs, doped at ods of 94 A 17 ˚ undoped InP barriers. n = 5 · 10 cm3, and 27 A There are 19 SLs, each separated by another SL of ˚ In0.53Ga0.47As/27 A ˚ InP barriers. 20 undoped 20 A The entire structure is sandwiched between 0.1 and 1.5 lm thick n-InGaAs (5 · 1017 cm3) top and bottom contact layers. From band structure calculation, the absorption of this structure should span from 7.8 lm to 14.5 m, thus covering the 8–14 lm range. Fig. 17 shows the calculated and the measured spectral responsivity at T = 10 K.

Fig. 17. The measured spectral responsivity of a broadband InGaAs/InGaAsP QWIP under 45 edge coupling (circles) and the calculated spectrum with different line broadening (solid curves).

In order to compare the background photocurrent of the detector with a standard AlGaAs/GaAs narrow band QWIP, we show both window photocurrents in Fig. 18 with 45 edge coupling. Fig. 18 shows that the broadband InP detector has about 40 times higher photocurrent than the corresponding 8.5 lm peak GaAs detector under the same bias. The noise gain is measured to be 15 at 0.04 V/period bias, which confirms the larger gain of the InP material. By increasing N from 19 to 190, a desirable gain of 1.5 can be obtained.

Fig. 18. The background window photocurrents measured from a narrow band GaAs QWIP and a broadband InP QWIP under 45 edge coupling.

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6. Voltage tunable C-QWIPs C-QWIP structures also provide multi-color light coupling. There are a number of multi-color material designs in literature. The present examples are based on a modified voltage tunable superlattice infrared photodetector (SLIP) design [17]. The material structure of the voltage tunable SLIP is illustrated in Fig. 19(a). A single unit cell of the SLIP consists of two different SLs (SL1 and SL2) separated by a thick blocking barrier. In the present example, SL1 is designed for MW detection, and SL2 is designed for LW detection. SL1 is uni˚ ) of the formly doped and the middle part (55 A quantum wells in SL2 is doped with n = 1 · 1018 cm3. When the structure is illuminated, optical transitions between minibands take place in the respective superlattices. Depending on the polarity of the bias, only the photoelectrons from one of the SLs will be passing through the blocking barrier between them and change the conductivity of that barrier. The photoelectrons from the other SL will be passing into the contact layer and will not

Fig. 19. (a) Layer structure of the two-color SLIP. (b) Conduction band diagram and energy levels of one period of the SLIP at zero bias.

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change the resistance of the unit cell. As a result, photoconductivity is observed only in one of the two wavelengths under each bias polarity. Voltage tunability is thus achieved. To increase the infrared absorption, we increase the number of SLIP unit cells [18]. Each unit cell is ˚ thick n-GaAs layer (1 · 1018 separated by a 2000 A 3 cm ). This stacking layer blocks the high energy photoelectrons passing from one unit cell into the next by damping their kinetic energy. If they are allowed to pass into the blocking barrier of the next cell, both wavelengths will be detected simultaneously, and the detection wavelength will no longer be tunable. Spectral responsivity R of the detector was measured at T = 60 K with 45 edge coupling. As shown in Fig. 20(a), LW photoresponse was observed with a large positive bias and MW response was observed with a negative bias. The bias dependence of the MWIR and LWIR peaks is plotted in Fig. 20(b), which clearly demonstrates the voltage tunability. In this detector design, the spectral cross-talk is negligible above a threshold bias, which is important for certain applications. We measured the detector dark current Id vs. Vb from T = 10 to 170 K as shown in Fig. 21, in which the background photocurrent Ibg with f/1.2 optics is also plotted. From Fig. 21, we obtain TBLIP = 70 K for 0 < Vb 6 1.5 V for LW detection and 110 K for 3 6 Vb 6 0 V for MW detection. These values of TBLIP are comparable to those of single-color detectors with similar cut-off wavelengths. We also extracted electron activation energy Ea from the temperature dependence of dark current. The deduced value of Ea = 285 meV as Vb ! 0 V is in good agreement with the calculated value of Ea = 276 meV and the observed cutoff wavelength of 4.5 lm at Vb = 0 V. We tested the corrugated coupling on a similar voltage tunable detector material [16]. The MW SL is made of GaAs wells instead of InGaAs wells, so that its peak wavelength is longer at 5.8 lm. Fig. 22 shows R of a C-QWIP with S = 2t/p = 0.09. Despite the small S adopted, the value of R is half of that with edge coupling for the same material structure. This results shows that C-QWIPs are capable of MW/LW two-color coupling.

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Fig. 21. Bias dependence of dark current Id (solid lines) and background photocurrent Ibg (dashed lines) density of the twocolor SLIP. The detector temperature T is in the 50–170 K range for the dark current curves while T = 10 K for the background photocurrent curve.

Fig. 20. (a) Responsivity R of an edge-coupled SLIP at temperature T = 60 K and bias Vb in the 1.4 to 4 V range. The upper and lower halves of the R-axis are drawn with different scales for clarity. Inset: Scheme for applying bias Vb to and measuring photocurrent Ip of an edge-coupled detector. (b) Bias dependence of peak responsivity Rpk of the MW peaks at kp = 3.9 and 5.2 lm, and the LWIR peak at kp = 9.2 lm at T = 60 K.

7. Quantum grid infrared photodetectors for multi-color detection The voltage tunable corrugated QWIP approach can be extended to four-color detection. This can be achieved by growing two separate stacks of two-color QWIPs and separating them with a middle contact layer. The individual stacks are contacted in the alternate FPA columns. By changing the bias polarity on the pixels, four-color detection is achieved.

Fig. 22. The measured (a) and theoretical (b) spectral responsivity of a 5.8/9.5 lm two-color SLIP under corrugated coupling.

For more than four colors, we adopted another approach, in which the detection wavelength of a pixel is controlled by the light coupling structure. Using a resonant coupling geometry, we enhance the vertical optical electric field in a narrow wavelength range and thus increase infrared absorption at that particular wavelength. When this narrow band coupling geometry is applied to a broadband

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detector material, infrared detection can be performed at selective wavelengths. This approach offers more color detection than other approaches, requires only one indium contact per pixel, and only needs a standard single color readout circuit [19,20]. The coupling structure in our example is known as the quantum grid infrared photodetector (QGIP) [21]. In this scheme, a laminar grid is fabricated into the QWIP material. Metal strips having the same width as the grid lines are placed on top of the grid. Incoming radiation is incident on the bottom of the detector substrate. The geometry of a detector element in the present design is shown in Fig. 23. The present light coupling scheme utilizes multi-pole scattering from the individual metal strips [22], as opposite to the usual collective gridline diffraction. The maximum coupling efficiency occurs when the width w of the metal strips is w¼

Nk ; 2n

ð6Þ

where N is an odd integer, k is the wavelength in free space, n is the refractive index. N = 1 corresponds to the strongest dipole scattering. The coupling is insensitive to the spacing between the grid

Fig. 23. (a) The top view of a QGIP detector element. The numbers are dimensions in microns. (b) The side view of a QGIP. In the present spectrometer, tc = 0.1 lm, tb = 1.83 lm, tm = 0.2 lm, ta = 1.18 lm, s = 4.65 lm, and w is specified in the text.

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lines. The relationship between w and k in Eq. (6) thus provides a simple design rule for multi-color detection. Furthermore, the large wavelength separation between different values of N ensures a one-to-one correspondence between w and k in a wide spectral range, thus eliminating the usual subsidiary peaks. Further analysis using MTL theory shows that the absorption peak of a QGIP can be strengthened and sharpened by extending the grid lines into the bottom contact layer. In this case, each grid line acts as a dielectric waveguide and resonator for the scattered light. While it prohibits the propagation of the reflected wave when k exceeds 2 nw, causing a sharper long wavelength cutoff, a standing wave can also be established inside each grid line, which increases the absorption. Due to these two grid line effects, the peak wavelength is modified from Eq. (6). The efficiency of a coupling scheme is indicated by the quantum efficiency of a detector for a given intrinsic absorption coefficient of the material. Fig. 24 shows the predicted external quantum efficiency g for unpolarized light assuming a typical dielectric constant e = er + iei = 9.722 + 1i across the spectrum in the active layers. The grid parameters are specified in Fig. 23. Fig. 24 shows that the maximum g remains relatively constant across the spectrum for a common grid height. Since the dipole scattering field is concentrated immediately below the metal antennas, the QWIP

Fig. 24. The theoretical g of QGIPs shown in Fig. 23. The numbers show w in microns. The chosen widths are for detection at integral values of wavelengths.

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material uses a relatively thin active layer for a larger photoconductive gain. It uses the same binary superlattice shown in Fig. 15 for broadband absorption. The total QWIP thickness below the metal layer is 1.28 lm, approximately the same as the penetration depth of the scattering field. To accommodate the tall grid line requirement, a thick (2.5 lm) bottom contact layer is grown. In this experiment, 1.83 lm is etched into the bottom contact layer to achieve a total grid height of 3.11 lm. A set of six QGIPs with w = 1.22, 1.43, 1.80, 1.90, 2.06 and 2.50 lm were tested. The spacing among the grid lines is about 4.65 lm. The detector dimensions are measured using a scanning electron microscope. The detectors are labeled from A to F, respectively. Fig. 25 shows their spectral responsivity R with a substrate bias of 1 V and at temperature of 15 K. In Fig. 25, we also show R of a plain detector (S) without grid structures under normal incidence. The finite responsivity of S is due to stray light scattered from the wafer edges, and the spectrum of S reflects the intrinsic absorption of the detector material. With QGIP coupling, detectors with different w detect at different wavelengths, ranging from 8.5 to 12 lm. Since each detector has only one detection peak, we have demonstrated the multi-color capability of QGIPs. Note that the peak R of each QGIP is much larger than that of S measured at the same k in spite of the much less active material in the QGIPs. This observation indicates the effectiveness of QGIP coupling.

Fig. 25. The spectral responsivity of QGIP A to E and a plain detector S at Vsub of 1 V.

Since the responsivity of a QGIP is affected by both the grid coupling efficiency, v, and the wavelength dependence of the intrinsic absorption, v can be extracted by taking the ratio R(QGIP)/ R(S), in which case, all the material related properties will be factored out. The normalized v, which is assumed to be the normalized responsivity ratio, is shown in Fig. 26(a). Free from the material absorption roll-offs, the grid shows a wider range of wavelength selectivity, ranging from 8 to 13 lm. Apparently, the QGIPs are also sensitive to stray light, which causes a finite baseline for the detectors. In Fig. 26(b), we calculated the theoretical v based on the grid parameters. To model after the experiment, a finite baseline has been added to the theoretical predictions. In general, the observed peak wavelength agrees with the prediction as shown in Fig. 27, which validates the present device concept. The discrepancy for F could be due to the smaller wavelength selectivity for wider grid lines, which makes it more susceptible to the presence of stray light. The measured spectral widths are somewhat wider than the theoretical predictions. Note that while w is on the order of 1.5 lm, the length of each grid line is 400 lm and there are

Fig. 26. (a) The experimental normalized v for the six QGIPs. The presence of a constant baseline for all QGIPs signifies the presence of stray light. (b) The calculated normalized v with a finite baseline added.

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Fig. 27. The measured v peak position (circles) and the calculated curves.

about 180 grid lines in each detector. Given the large number of long and thin grid lines, spectral widening due to w fluctuation is expected. By using shorter grid lines, one should be able to obtain narrower spectral lines. The shoulder around k = 14 lm in the v curve of F in Fig. 26(a) can be due to the QGIP coupling. To assess the effects of stray light, the substrate of two other detectors were thinned from the original 620 lm to about 200 lm. One of the detectors has the dimensions very close to C and the other (G) has a grid line width of 2.33 lm. Their normalized

Fig. 28. The experimental normalized v (curves with symbols) for QGIPs with partially thinned substrate. The dashed curves show the calculated v with the same grid parameters of the corresponding detectors.

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v spectra are shown in Fig. 28. Indeed, with smaller edge surfaces for random scattering, the constant baseline is lowered, and the spectral response is more consistent with the theoretical predictions. The present detector elements are too big to be integrated into staring imaging FPAs, and they are suitable only for non-imaging spectroscopic applications. The detector offers a theoretical wavelength selectivity of Dk/k = 13%. By lowering the doping density of the material, the optical damping at resonance can be reduced, whereby one obtains a smaller linewidth. For example, when ei is reduced to 0.2, Dk/k decreases to 6.7%. Therefore, one can also control the spectral resolution of a QGIP through the doping level of the detector material. For multi-color imaging FPAs, we designed a new pixel pattern, in which the grid lines are distributed radially from a central circular bonding area as shown in Fig. 29. Using this ‘‘daisy’’ QWIP structure, a pixel size as small as 18 · 18 lm2 can be made. Since wavelength selection is based on the width and not on the spacing of the grid lines, this geometry should retain its wavelength selectivity. If one groups 4 · 4 of such pixels with different widths into a unit cell, 16 colors can be detected simultaneously. A 1024 · 1024 single color FPA can thus be converted into a 16-color 256 · 256 FPA. The signal from a single color or a combination of colors can be readout for adaptive multicolor imaging.

Fig. 29. A ‘‘daisy’’-QWIP pixel for adaptive multi-color imaging.

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8. Conclusion In this paper, we have described the design and optimization of C-QWIPs with much improved quantum efficiency and photoconductive gain. C-QWIPs with large corrugations will give a large g and a small g simultaneously, both of which are needed for high sensitivity infrared imaging. For high speed applications where a large conversion efficiency is more desirable, InP based C-QWIPs can be utilized. We have also shown that C-QWIP coupling is effective in small pixels as well as in broadband and voltage tunable multi-color detectors. Combined with its simple processing steps, large format, high sensitivity, high resolution, high speed and multi-color FPAs can be produced in large quantities, in short production time, and at low cost. The introduction of corrugated coupling thus represents a significant step in QWIP technology development. In this paper, we also discussed using quantum grid infrared photodetectors for narrow band coupling. When this coupling scheme is applied to a broadband detector material, a specific wavelength can be singled out for detection. One can therefore design a spectrum of detection wavelengths in a single focal plane array for adaptive multi-color imaging.

References [1] K.K. Choi, J. Appl. Phys. 73 (1993) 5230. [2] K.K. Choi, S.V. Bandara, S.D. Gunapala, W.K. Liu, J.M. Fastenau, J. Appl. Phys. 91 (2002) 551. [3] J.Y. Andersson, L. Lundqvist, J. Appl. Phys. 71 (1992) 3600.

[4] H. Schneider, P. Koidl, M. Walther, J. Fleissner, R. Rehm, E. Diwo, K. Schwarz, G. Weimann, Inf. Phys. Tech. 42 (2001) 283. [5] M. Sundaram, S.C. Wang, M.F. Taylor, A. Reisinger, G.L. Milne, K.B. Reiff, R.E. Rose, R.R. Marin, Inf. Phys. Tech. 42 (2001) 301. [6] S.D. Gunapala, S.V. Bandara, A. Singh, J.K. Liu, S.B. Rafol, E.M. Luong, J.M. Mumolo, N.Q. Tran, D.Z.-Y. Ting, J.D. Vincent, C.A. Shott, J. Long, P.D. Levan, IEEE Trans. Electron. Dev. 47 (2000) 963. [7] C.J. Chen, K.K. Choi, M.Z. Tidrow, D.C. Tsui, Appl. Phys. Lett. 68 (1996) 1446. [8] K.K. Choi, K.M. Leung, T. Tamir, C. Monroy, IEEE J. Quant. Electron. 40 (2004) 130. [9] K.K. Choi, C.J. Chen, A.C. Goldberg, W.H. Chang, D.C. Tsui, Proc. SPIE 3379 (1998) 441. [10] L. Yan, M. Jiang, T. Tamir, K.K. Choi, IEEE J. Quant. Electron. 35 (1999) 1870. [11] M. Jiang, T. Tamir, S. Zhang, Opt. Soc. Am. A 18 (2001) 807. [12] C.-H. Lin, K.M. Leung, T. Tamir, Opt. Soc. Am. A 19 (2002) 2005. [13] A. Goldberg, K.K. Choi, M. Jhabvala, A. La, P. Uppal, N. Winn, 2003 SPIE AeroSense, Orlando FL, 21–25 April 2003. [14] M. Jhabvala, K.K. Choi, A. Goldberg, A. La, S. Gunapala, 2003 SPIEÕs 48th Annual Meeting, 3–8 August 2003. [15] R. Ellis, A. Majumdar, K.K. Choi, J. Reno, D.C. Tsui, Appl. Phys. Lett. 84 (2004) 5127. [16] M. Razeghi, M. Erdtmann, C. Jelen, F. Guastavinos, G.J. Brown, Y.S. Park, Inf. Phys. Tech. 42 (2001) 135. [17] C.C. Chen, H.C. Chen, C.H. Kuan, S.D. Lin, C.P. Lee, Appl. Phys. Lett. 80 (2002) 2251. [18] A. Majumdar, K.K. Choi, J.L. Reno, D.C. Tsui, Appl. Phys. Lett. 83 (2003) 5130. [19] P. Mitra, F.C. Case, J.H. McCurdy, S.A. Zaidel, L.T. Claiborne, Appl. Phys. Lett. 82 (2003) 3185. [20] K.K. Choi, G. Dang, J.W. Little, K.M. Leung, T. Tamir, Appl. Phys. Lett. 84 (2004) 4439. [21] L.P. Rokhinson, C.J. Chen, D.C. Tsui, A.G. Vawter, K.K. Choi, Appl. Phys. Lett. 74 (1999) 759. [22] J. Mao, A. Majumder, K.K. Choi, D.C. Tsui, K.M. Leung, C.H. Lin, T. Tamir, G.A. Vawter, Appl. Phys. Lett. 80 (2002) 868.