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Integration of n-type and p-type quantum-well infraredphotodetectors for sequential multicolor operation

E. Dupont,a) M. Gao,b) Z. Wasilewski, and H. C. LiuInstitute for Microstructural Sciences, National Research Council, Ottawa, Ontario K1A 0R6, Canada

Received 16 January 2001; accepted for publication 2 February 2001

A multicolor infrared photodetector based on the epitaxial integration of an n-type with a p-type

GaAs/AlGaAs quantum-well stack is experimentally demonstrated. Additionally, a quantum-wellGaAs light-emitting diode is inserted between the stacks to achieve up-conversion of mid-infraredradiation to near-infrared signal. This device shows a remarkable selectivity on wavelength:depending on the bias voltage the peak wavelength detection can be switched on and off between9.1 and 4.85 m. 2001 American Institute of Physics. DOI: 10.1063/1.1359482

The technology of quantum-well infrared photodetector QWIP is presently developed to the stage of commercial-ization of single-color focal plane arrays FPA and demon-stration of two-color arrays.1 3 The recent two-color FPAdemonstrated in Refs. 2 and 3 uses three lead pixels for

independent reading of the two QWIP stacks.4

This is madeat the cost of complexity of the readout circuit and hybrid-ization, of the grating performance, and to a smaller extent,of the fill factor 70% . Interface dual-band FPAs developedat JPL1 solves the first two problems, but it sacrifices the fillfactor to 50% for each wavelength. For some applications, asequential reading of the color may be sufficient. For thispurpose the wavelength sensitivity of the QWIP can be con-trolled by the bias. With such a device, sequential multicolorinfrared imaging is possible without sacrificing the fill factorof the FPA, and standard readout circuits can be used. Dif-ferent approaches have been tested for sequential wavelengthdetection: Grave5 and Tsai6 tested devices where different

quantum well stacks are simply grown on top of each other.Liu7 and Lenchyshyn8 looked in detail at the operation oftwo lead devices composed of different QWIP stacks sepa-rated by conducting layers. Another option is to take advan-tage of the voltage control of the carrier escape probability atdifferent energies in asymmetric QW structures.9,10

Following our research on the integration of QWIP withlight-emitting diode LED for pixelless infrared imaging,we show in this letter that sequential two-color imaging ispossible with the QWIPLED technology. The device dem-onstrated here is a combination of n-type QWIP, LED, andp-type QWIP for sequential detection at 9 or 5 m. Oneattractive quality of this device is that the p-QWIP stack,unlike the n-QWIP, does not need a light coupler such as adiffraction grating. This can be an advantage when making amulticolor FPA with such a structure. The operation ofQWIPLED devices is based on direct injection of carriersphotoexcited in the QWIP by mid or far infrared M/FIRradiation into the LED active region, and subsequent emis-sion of near infrared NIR radiation. Thus, a QWIPLEDoperates as a converter of M/FIR to NIR light. An important

technological advantage of the QWIPLED approach is thatit allows fabrication of large two-dimensional FPAs withNIR output, which can be easily imaged by well-developeddevices such as Si CCDs. This approach could eliminate theneed for hybrid integration of QWIP with a Si readout circuit

and opens up the possibility of a pixelless imaging scheme.11

The sample was grown by molecular beam epitaxy on asemi-insulating 001 GaAs substrate. The layer structure isschematically shown in Fig. 1. The parameters of the p-typeQWIP are derived from earlier optimization studies. The sec-ond light-hole subband LH2 is in resonance with thebarrier12,13 for large in-plane polarization absorption 0.05% peak absorption per well . The 250 barrier iswide enough to prevent interwell tunneling in a 5 mp-QWIP stack.14 The optimum doping concentration of a 5m p-QWIP was found to be about 121012 cm2 forhigh dark current limited detectivity and highest backgroundlimited infrared performance BLIP temperature.14 Increas-

ing the doping to 41012 cm2 would result in slightlyhigher integrated absorption, but at the expense of a largeincrease in thermionic emission. In total, the dark currentdetectivity would be reduced, as well as the BLIP tempera-ture. However, to avoid high field formation in the p-QWIPwe deliberately tried to bring the impedance of the short-wavelength p-QWIP closer to the long-wavelength

a Electronic mail: emmanuel.dupont@nrc.cab Present address: Zenastra Photonics Inc., 2305 St. Laurent Bvd., Ottawa,

Ontario K1G 4J8. FIG. 1. Schematic of the n-QWIP/LED/p-QWIP structure.

APPLIED PHYSICS LETTERS VOLUME 78, NUMBER 14 2 APRIL 2001

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n-QWIP. For this purpose we chose a high carrier concen-tration in the well ( 41012 cm2). A similar way of think-ing was adopted when choosing the doping concentration inthe n-QWIP: The low Si concentration of 11011 cm2 isalso meant to bring the resistivity of the long-wavelengthstack closer to that of the shorter-wavelength stack. The LEDstructure between the two QWIPs should have little influenceon the LED resistance since it is negligible when turned on.

Mesa devices of 290140 m2 in size were processedby standard GaAs processing and a 45 facet was polished tocouple the infrared radiation with the n-type intersubbandtransitions. An open window 200100 m2 on the top ofthe mesa allows us to test the LED.

The leakage current with and without the f/1.75 room

temperature background radiation was measured at tempera-tures from 90 to 40 K with a close-cycle refrigerator. Theresults are plotted in Fig. 2. One can see that between 9 and14 V the BLIP temperature is 70 K, a value that is typicalfor a 9 m n-QWIP. This shows that above 9 V the long-wavelength stack mostly determines the dark current in thisstructure. The subsequent tests are performed at 80 K in aliquid nitrogen Dewar.

The spectral response of the device was tested in twodifferent ways, as represented in Fig. 3. With a Fourier trans-form spectrometer FTIR the standard way Fig. 3 a is tomeasure the photocurrent probed at a load resistance 1 M

placed in series with the device. With a QWIPLED device

another possibility is to detect the up-converted signal Fig.

3 b i.e., the signal of a Si photodiode that measures theLED emission. Both should be equivalent. The spectral re-sponse of the device is shown in Fig. 4. Looking at thephotocurrent signals at 5 and 15 V, one can see the remark-able wavelength switching capability of this device. At 5 V,the ratio of photoresponse between 4.85 and 9.1 m is 38.When the n-QWIP is turned on 15 V , the ratio between 9.1and 4.85 m is 30. At 5 V the broad resonance peaked at4.85 m, typical of a p-QWIP detector /39% . On theother hand, / for the long-wavelength sharp resonance is

14%. That is typical for an n-QWIP at such low dopingdensity. Due to fast scanning speed of the Michelson infer-ometer of the spectrometer, the slow amplification of thephotodiode signal, and the small photoresponse of thep-QWIP, the up-conversion experiment was successful onlywhen the n-QWIP is turned on. The circledotted curve inFig. 4 showing the up-conversion spectra at 15 V overlays onits equivalent photocurrent spectra solid line in Fig. 4 . Theresponsivity of the device was measured with a calibratedblackbody 1000 K . The result is displayed in the inset ofFig. 4 for two detection wavelengths, 9.2 and 5.1 m. Onecan see an interplay between the p-QWIP and n-QWIP re-sponse: A decrease of responsivity for one wavelength seemsto be correlated with an increase for the other wavelength.

According to an equivalent circuit model described inRefs. 7 and 8, it was demonstrated that this interplay is re-lated to the relative weight between the dynamic resistancesof the two stacks. Therefore, the short-wavelength responseoccurs at low voltage. At higher voltage, when the dynamicresistance of the long-wavelength n-QWIP becomes compa-rable to that of the p-QWIP, the long-wavelength detectionwill start to dominate over the short wavelength. In this typeof QWIPLED device one cannot insert an intermediatecontacting layer to measure the dynamic resistance of eachstack. However, the same circuit model should be valid for

this n-QWIP/LED/p-QWIP device because of the small re-sistance of a turned-on LED. Due to the small leakage cur-

FIG. 2. Voltagecurrent characteristics of a 290140 m2 device for dif-ferent lattice temperatures with solid lines and without dashed lines af/1.75 room temperature background illumination.

FIG. 3. Schematic of two experimental setups for measurement of spectral

response with a FTIR. a standard photocurrent spectra; b the up-conversion scheme with a QWIPLED device.

FIG. 4. Spectral response of a n-QWIP/LED/p-QWIP structure at a tem-perature of 80 K. The thick solid, long-dashed and short-dashed lines rep-resent the photocurrent spectra respectively at 15, 10, and 5 V. The up-converted spectra at 15 V is the circle-dotted curve. The inset represents theresponsivity of the device at 9.2 and 5.1 m as measured with a calibrated

blackbody source.

2068 Appl. Phys. Lett., Vol. 78, No. 14, 2 April 2001 Dupont et al.

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rent below 8.5 V, the numerical aperture of 4 in our experi-mental setup, and the limited detectivity of Si photodiode,

we could measure the LED emission only above 8.5 V, 10nA. Above this bias the external efficiency is constant at1.8%. This value is typical for planar LED with an unityinternal efficiency and a semiconductor/air interface. There-fore, one cannot be certain that between 1.5 and 8.5 V theLED is well turned on, but above 8.5 V the equivalent circuitmodel of two photocurrent sources with their dynamic resis-tance is valid.

Finally, we checked the polarization sensitivity of thedevice when both QWIP response coexist. The result is plot-ted in Fig. 5. The 9 m peak is sensitive to the polarizationof the radiation, as expected from conduction intersubbandtransitions in n-QWIP. The 5 m peak is almost unchangedby the rotation of polarization, proving that it originates fromvalence intersubband transitions in the p-QWIP stack.

We have investigated the possibility of wavelength tun-

ability with the QWIPLED technology. By the combinationof n-type and p-type stacks we have demonstrated that thedetection wavelength can be switched between the two infra-red atmospheric windows with an excellent selectivity. Theup-conversion from 9 m to 825 nm was also demonstrated.Provided progress can be made in the growth of such a thickstructure, the QWIPLED technology is capable of achiev-ing sequential multicolor infrared imaging.

The authors would like to thank D. Wang for the epitax-ial growth of the structure and P. Marshall, P. Chow-Chong,and M. Byloos for the microfabrication of the devices. Thiswork was supported in part by DND.

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FIG. 5. Polarization sensitivity of the bicolor detector at 10 V, 80 K.

2069Appl. Phys. Lett., Vol. 78, No. 14, 2 April 2001 Dupont et al.

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