WO2008011152A2

WO2008011152A2 - Longwave infrared photodetector

Info

Publication number: WO2008011152A2
Application number: PCT/US2007/016466
Authority: WO
Inventors: Xuejun Lu
Original assignee: University Of Massachusetts
Priority date: 2006-07-21
Filing date: 2007-07-20
Publication date: 2008-01-24
Also published as: WO2008011152A9; WO2008011152A3

Abstract

An infrared detector in which a quantum dot (20) heterostructure is located within a resonant cavity (12, 24) that has a two photon absorption characteristic. This provides a long wavelength infrared detector that is thermoelectrically cooled.

Description

TITLE OF THE INVENTION LONGWAVE INFKARED PHOTODETECTOR

CROSS REFERENCE TO RELATED APPLICATIONS The present application claims priority to U.S. Application No. 60/832,303, filed July 21, 2006, the entire contents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Current state-of-the-art high-speed long wavelength infrared

(LWIR) photodetectors are based on narrow-bandgap materials such as HgCdTe and InAsSb-InSb. However, difficulties remain in epitaxial growth of these materials with good quality and high uniformity. Quantum well infrared photo detectors (QWIP) have the advantages of mature epitaxial techniques, high uniformity, as well as engineerable absorption wavelength. The fundamental QWIP limitation, however, is its insensitivity to surface-normal LWIR incidence due to the quantum selection rule. This makes it hard to provide a focal plane array (FPA) for LWIR imaging. Quantum dot infrared photodetectors (QDIP) operate using an intersubband transition in three dimensional (3-D) InAs/GaAs QD heterostructures . They are intrinsically sensitive to normal IR incidence due to the 3-D quantum-confined structure, which eliminates the wave vector selection rules that limit QWIP. They also have high-photoconductive gain (-1000) because of the long (a few hundred ps) excited state carrier lifetime. QDIP' s have surface-normal detection capability and high photoconductive gain, however, because of the strain-induced QD size variation from layer to layer, only a limited number of QD absorption layers have been incorporated in a QDIP. This results in a short absorption length (<0.1μm), and leads to low quantum efficiency and photoresponsivity. In addition, because of the low energy gap (~130meV) needed for LWIR detection, existing LWIR photodetectors are required to operate at liquid nitrogen temperature (77⁰K) to reduce the thermal excited dark current . The requirement for cryogenic systems adds cost and weight, and makes it unsuitable for space/airborne, standalone or portable applications. Thus, further improvements are needed for longwave infrared detectors.

SUMMARY OF THE INVENTION The present invention relates to a quantum dot infrared detector in which a plurality of quantum dot layers are stacked to form a heterostructure that is sensitive to one or more wavelengths or wavelength bands . The array of elements is preferably of a uniform size and distribution and aligned vertically to form an efficient light conversion device. in a preferred embodiment detection is based on a two-photon sequential absorption process with resonant cavity enhanced quantum efficiency. In another embodiment, the heterostructure can be formed to detect infrared wavelength at a plurality wavelengths .

A preferred embodiment of the invention uses a plurality of vertically stacked quantum dot layers. These layers can be positioned between buffer layers with a high energy barrier. This can increase the transition energy to more than 200 meV and thus reduce the thermal noise. The photo-response is based on a two- photon sequential absorption with high resonant cavity enhanced quantum efficiency. The high quantum efficiency and low thermal noise level provide for operation at temperatures above 150° k. Consequently, the detector can be thermoelectrically cooled and consequently used for a variety of applications.

A preferred embodiment use at least five layers of InAs/AlGaAs in a stacked quantum dot structure between a metallic thin film mirror on a first side and a Bragg grating (Fabry Perot) on a second side to define a reflective cavity. The Bragg grating can be a distributed Bragg grating (DBR) made with a plurality of layers of GaAs/AlGaAs, for example, that also acts as a bandpass filter to exclude wavelengths outside the detection range . A preferred embodiment of the invention further includes a method of fabricating an infrared detector using photolithographic techniques . In one example, a starting substrate can be a semi- insulating gallium arsenide (GaAs) substrate, followed by a stack of layers forming a distributed Bragg grating that provides a first reflector. The first electrode is than formed followed by a spacer layer and a plurality of quantum dot layers . A cavity spacer is then followed by a contact layer and a second electrode. The second electrode can be formed with a reflective metal to provide a second reflector that forms a resonant cavity in combination with the first reflector. The resulting monolithically integrated structure can then be mounted to a readout circuit and a thermoelectric cooler to provide an infrared detector.

A preferred embodiment of the invention includes an electrically-controllable multi-spectral quantum dot infrared light detector (QDIP) . The QDIP of vertically-stacked InAs quantum dots layers with two different capping layers for MWIR and L WIR absorption, respectively. The multi-spectral QDOP is capable of simultaneously detecting multi-spectral normal incidence through inter-subband transitions in the three-dimensional (3-D) confined quantum dot nanostructures . The QDIP showed multi-color IR detection bands centered at 5.6/μm, 7.7/μm and 10.0/μm, respectively. By tuning the bias voltage, the detection band can be individually turned on. High photodetectivity of >2.3xl0¹⁰cmHz^1/2/W were obtained for these IR detection bands. The voltage-controllable detection band selection enables real-time system reconfiguration to focus on the band of interest. The vertically-stacked device structure can provide a focal plane array (FPA) . Different bands can have different capping layers in the structure .

The quantum dot elements preferably have an average diameter in a range of 10 to 40μm and preferably between 20 and 30μm. The average quantum dot size preferably varies less than 20% across the structure with the dots aligned vertically. A focal plane array can be formed with an two dimensional array of mesa type detector elements . The array can have at least 10x10 pixel elements, for example, or as many as, hundreds or thousands of elements depending on the desired resolution.

A preferred detector has a plurality of wavelength detection bands such as a first band in a mid-wavelength infrared range (MWIR) between 3 and about 6 microns and a second band in the long wavelength infrared range (LWIR) between 7 and 12 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a quantum dot heterostructure in accordance with the invention.

FIG. 2 is a schematic conduction band diagram illustrating a two photon absorption process for the present invention.

FIG. 3 graphically illustrates the dark current of the heterostructure of the present invention in comparison with a conventional QDIP.

FIGs 4A-4I illustrates a process sequence for fabricating a quantum dot heterostructure within a resonance cavity in accordance with the invention.

FIG. 5 shows the mounting of the monolithically fabricated device to form the detector.

FIGs . 6A and 6B are atomic force microscopy images of top and side views of a quantum dot structure fabricated in accordance with a preferred embodiment of the invention. FIG. 7 shows a cross-sectional view of an XTEM image of a vertically aligned quantum dot heterostructure made in accordance with a preferred embodiment of the invention.

FIG. 8A is a cross-sectional view of a multispectral quantum dot heterostructure in accordance with a preferred embodiment of the invention.

FIG. 8B shows current voltage characteristics of devices with and without a separation layer.

FIGs. 9A and 9B schematically illustrate band diagrams of a multispectral quantum dot infrared photodetector at different bias levels in accordance with a preferred embodiment of the invention.

FIG. 10 is a photoluminescence spectrum of a multispectral quantum dot infrared photodetector compared with a single color mid wavelength quantum dot photodetector. FIG. 11 shows the photocurrent spectrum of a quantum dot infrared photodetector at four different bias voltages.

FIG. 12A and 12B are graphical diagrams of dark current density and noise current density, respectively, as a function of bias voltage at four different temperatures . FIGs. 13A and 13B graphically illustrate photoresponsivity and photodetectivity, respectively, as a function of bias voltage at different temperatures.

FIGs. 14A-14E illustrate a focal plane array system in accordance with a preferred embodiment of the invention. FIG. 15 is a process sequence in accordance with a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A thermal-electrically cooled LWIR photodetector that can specifically overcome existing technical problems. The invention includes using a two-photon sequential absorption (TPSA) process in InAs QD heterostructures for LWIR photodetecting. By increasing the energy barrier from 130meV to 260meV based on the nonlinear TPSA process, this substantially suppresses thermal- excited dark current level. By using a resonant cavity there is an increase in the quantum efficiency and a reduction background noise. The schematic structure and the simplified conduction band diagram of the resonant cavity QDIP 10 are shown in FIG. 1 and FIG. 2, respectively.

The QDIP can include five InAs/Al_xGai_-xAs QD layers 20, for example, embedded in a λ resonant cavity (λ = lOμm) . The aluminum composition x will vary between 0 and 0.4 depending on the application. The bottom mirror of the resonant cavity can comprise 5 pairs of GaAs/AlGaAs distributed Bragg gratings 24 (DBR) formed over a GaAs semi-insulating substrate 26. The top mirror 12 is gold, which has over 98% reflectance at λ = lOμm, and is formed over an n⁺ contact layer. Spacers 16, 18 can be used to separate the quantum dot structure from the overlying and underlying layers, respectively. The LWIR incident light 28 experiences multiple reflections at the two mirrors in the cavity and builds up the optical field at the resonant wavelength. This increases the effective absorption length of the QDIP and thus leads to high quantum efficiency. The cavity also enhances the optical field intensity at the QD absorption layers . The enhanced optical intensity of the QD absorption layers enables QDIPs based on nonlinear absorption process. In addition, the resonant cavity functions as a bandpass filter, which only allows LWIR incidence within the pass-band of the resonant cavity and rejects all other wavelengths. This reduces background noise and thus increases photodetectivity. The quantum dot structure thus comprises a plurality of at least five layers in which the quantum dot elements are positioned in vertical alignment to form a columnar structure 25.

As shown in FIG. 2, of the QDIP is based on the TPSA process. In this process, an electron confined in the QD heterostructure is excited to a first excited state by absorbing a LWIR photon. It then absorbs another LWIR photon and reaches the second excited state, where it is collected by the electrodes. In this example, the TPSA process provides a substantial energy gap increase from 130meV to 260τneV for LWIR detection. This energy gap increase suppress the thermally excited dark current level by a factor of over 10⁴. This dark current reduction has been measured as is shown in Figure 3.

The dark current reduction was measured and compared with a conventional QDIP device. Four orders of magnitude smaller dark current was achieved. FIG. 3 shows the dark current in the inAs/AlGaAs QD heterostructure at different operating temperature as compared with that of a conventional InAs/GaAs QD heterostructure. The dark blue trace is our device at 77⁰K and the light blue trace is a conventional device at 77"K. The yellow trace is our device at 180⁰K. As shown in Figure 3, the dark current of the InAs/AlGaAs QD heterostructure is roughly four orders of magnitude smaller than that of a conventional QDIP. At 180°K, the dark current is roughly on the same level as the dark current of conventional QDIP at 77°K, which enables operation at 180°K. By increasing the number of aligned QD layers, higher operating temperature enables thermal-electrically cooled operation at 220^βK. The reduced dark current provides a system that achieves ultra-low noise QDIPs for thermal-electrically cooled operation. The TPSA process is different from two-photon absorption

(TPA) in that the intermediate state is a real state rather than a virtual state as in the TPA process. Since the lifetime of a real intermediate state is much longer than that of a virtual state, the TPSA absorption coefficient is much larger than that of a TPA process. Due to the long excited state lifetime (a few hundred ps, rather than a few ps in bulk GaAs) and the increased wave-function overlaps in QD nanostructures, the TPSA absorption coefficient is much higher than that of bulk materials . An enhancement factor of >10² can occur. The high TPSA coefficient allows the design of a QDIP based on this nonlinear process with high energy barrier for dark current reduction.

Table 1 summarizes the performance of a QDIP detector compared with the existing approaches .

By increasing the quantum efficiency and enhancing optical intensity in the QD absorption layers through the resonant cavity and decreasing the thermal-excited dark. current level by increasing the energy barrier, the QDIP provides LWIR detection without cryogenic systems .

Based on a two-photon sequential absorption (TPSA) process in InAs QD heterostructures of the present invention provide an increased energy barrier from 130meV to 260meV. A substantial suppression of thermal-excited dark current level occurs by using a resonant cavity to increase the quantum efficiency and reduce background noise .

The invention overcomes the difficulties associated with low temperature operation and offers high-speed, high sensitivity LWIR sensor technology without cryogenic systems for numerous space and airborne based IR imaging and stand-alone environmental monitoring applications, including thermal remote sensing, heat capacity mapping for earth resource locating, environment and atmosphere monitoring, target tracking and discrimination, as well as IR spectroscopy and medical diagnostics.

The quantum dot infrared detector with resonant cavity is grown by molecular beam epitaxy (MBE) as shown in the example of FIGs 4A-4I showing a fabrication sequence 40. The heterostructure is grown on a semi-insulating GaAs substrate 42 (FIG. 4A) . The MBE growth sequence is listed as the following first a GaAs buffer layer 44 is formed over the substrate (FIG. 4B) , a DBR bottom mirror 46 is formed (FIG. 4C) followed by, GaAs bottom contact (n+) layer 48 (FIG. 4D) , a GaAs cavity spacing layer 50 (FIG. 4E) , five periods of InAs/AlGaAs QD layers 52 (FIG. 4F) , GaAs cavity spacing layer 54 (FIG. 4G) , and a GaAs top contact (n+) layer 56 (FIG. 4H) . After the MBE growth the sample is e-beam evaporated with Ni/Ge/Au metal layer 58 (FIG. 41) , which functions as top mirror for the resonant cavity and top electrode for the QDIP. The cross section diagram of the growth layers is shown in Fig 1.

After MBE growth, the GaAs wafer contains the QD heterostructures . Standard photo-lithography and wet etching (RIE) were performed on the wafer to form individual QDIPs. The electrode is formed by standard thermal deposition and lift-off metalization procedures followed by rapid thermal annealing (RTA) to ensure Ohmic contacts were formed on both of the top and bottom electrodes. FIG. 5 shows the mounting of the fabricated structure 100 onto a readout circuit 118 and thermoelectric cooler device 120 using a flip chip mounting process 130 to provide the detector 140.

The size and density of quantum dot structures used in accordance with the invention were measured using atomic force microscopy. Fig. 6A shows a top view of a quantum dot layer in which the lateral size of the dots are about 25nm in diameter. The density of the dots is about 2.9xl0¹⁰/cm². Fig. 6B shows a side perspective view of the structure shown in Fig. 6A. Shown in Fig. 7 is a cross-sectional of an XTEM image of a five layer vertically stacked columnar quantum dot structure . The columnar structure is formed in which dots are formed along parallel aces 210, 212 (FIG. 8A) . MuIti-spectral infrared photodetectors , including Mid-IR and long wave IR photodetectors, are of great importance in numerous applications, including temperature registration, objects detection and discrimination, target tracking, remote sensing, environment monitoring, material analysis and medical diagnostics. Quantum-well based multicolor infrared photo detectors (QWIPs) have certain limitations. The QDIP detectors are based on three dimensional (3-D) confinement using InAs/GaAs QD nanostructures . Compared with QWIPs, QDIPs offer numerous advantages for infrared photodetectors, including intrinsic sensitivity to normally incident infrared light, low-leakage current due to three- dimensional (3-D) electron confinement, low dark current because of the significantly suppressed electron-phonon scattering, and high-photoconductive gain due to the increased excited state carrier lifetime. For conventional MBE growth, the size of the QDs depends on several different growth parameters such as substrate temperature, growth rate, and material flow rate, which makes the QD size tuning quite complicated and usually nonrepeatable .

A preferred embodiment of the present invention utilizes a two-color QDIP incorporating vertically-stacked InAs quantum dot layers with two different capping layers for MWIR and LWIR absorption. The detector showed IR detection bands centered at 5.6μm, 7.7μm and 10. Oμm, respectively. By tuning the bias voltage, the detection band can be individually turned on. High photodetectivity of >2.3XlO¹⁰CTnHz³⁷Vw were obtained for these IR detection bands. The voltage-controllable detection band selection enables real-time system reconfiguration to focus on the band of interest. Such vertically-stacked device structure can be utilized for a focal plane array (FPA) . Structure of the multi-spectral QDIP photodetector 200 is shown in Fig. 8A. The multi-spectral photodetector consists of InAs quantum dot layers in two different cap layers (206, 208), InGaAs, and GaAs, for MWIR 202 and LWIR 204 absorption, respectively. Each of the QDIP absorption bands can include 10- periods of InAs/InGaAs QD layers sandwiched between the top and bottom electrodes 216, 220 separated from the electrodes by n⁺ GaAs contacting layers 214, 218.

Band diagrams of a multi-spectral quantum dot infrared photodetector at different bias levels are shown in Fig. 9A and Fig. 9B, respectively. The electrically controllable selection of detection bands originates from the asymmetric band structure of the multi-spectral photodetector as indicated in Figs. 9A and 9B. At low bias voltage, the high energy barrier of the GaAs cap layers blocks the photocurrent generated by long wavelength infrared radiation incident and only responds to the mid wavelength infrared radiation that is incident on the detector. As the bias voltage increases, the energy barrier decreases, allowing longer wavelength signals to be detected at different bias voltage levels.

The heterostructure can be grown, as shown in FIG. 15, by molecular beam expitaxy 502 (MBE) using a Veeco GEN II MBE system. In a preferred embodiment, a 0.5μm Si-doped (n+) GaAs contact layer (n = 1 x 10¹⁸cm^"3) was first grown on a semi-insulting GaAs (100) wafer, followed by the growth of 0.3μm lightly Si-doped (n=IxIO¹⁷Cm^"3) layer. A 60nm undoped. GaAs was then grown as a buffer layer. The growth temperature for the GaAs contact and buffer layers was set at 610⁰C. The long wavelength absorption layers 504 were then grown, which includes a stack of ten periods of QD heterostructures . Each period of the QD heterostructures includes three mono-layers (ML) of InAs, 40nm monolayer (ML) In₀.₁₅Ga₀.β₅As cap layer and 45nm GaAs buffer layer. After the growth of each QD layer, the growth was paused for three seconds to allow the migration of the QDs. The QD layers and the In₀.1₅Ga_0-SsAs cap layers were grown at 510⁰C. The growth procedure for the mid wavelength absorption layers was same as that of the long wavelength layers, except that the cap layer was 50nm GaAs. After the growth of the ten-period of quantum dot layers, the wafer was annealed at 610⁰C for 20 seconds for strain relief and defect removal. Lastly, a 0.3μm lightly Si-doped (n=1x10¹⁷cm^"3) GaAs and 0. lμm highly Si-doped (n=lxlO¹⁸Cm^"3) GaAs contact layers were grown, in the final epitaxy steps. After mesa formation 506, metalized electrodes are formed to provide pixel array 508 interconnects 510 are then formed followed by contacts 512. The circuit is then flipped 514 and connected to readout 520 circuit 516 to form a focal plane array.

The photoluminescence (PL) of the sample was measured at room temperature (RT) using a continuous wave argon (Ar³⁺) laser with excitation wavelength of 514nm and laser output power 30OmW. The laser spot size was measured to be ~0.5mm². The photoluminescence spectrum of the multi-spectral QDIP and that of sample with only the MWIR absorption layer at 4K are shown in Fig. 10.

The blue curve is the PL of the multi-spectral QDIP and the red curve is the PL of the MWIR only QDIP. From Fig. 10, two PL peaks can be clearly resolved on the multi-spectral QDIP, corresponding to the two absorption regions. The vertically stacked structure offers great flexibility in absorption region selection and tuning, and increases the absorption layers for high quantum efficiency.

After growth, the wafers were processed into lOOμm-diameter circular mesas using standard photo-lithography and wet etching procedures. The top and bottom electrodes were conventional N-type (Ni (15θA) /Ge (3OθA) /Au(4000A) ) alloys. These electrodes were formed simultaneously on top of and surrounding the mesas by standard E- beam metal evaporation deposition and lift-off processes. The electrodes were subsequently annealed at 460⁰C for 20s at nitrogen flow rate of 20cm³/s to form good Ohtnic contacts. The QDIP chip was then wire-bonded and mounted in a Janis ST-100 dewar with an ZnSe infrared (IR) window. The spectral response of the QDIP at normal incidence was measured using a Bruker Optics Tensor27 FTIR spectrometer. Fig. 11 shows the photo current spectrum of the QDIP at 77K. At a bias voltage of -0.8V, only the LWIR peak at lO.Oμm occurs where the spectrum peaked. When bias increase to -0.5V, the short wavelength peak at 7.7μm start to appear. At positive bias of +0.5V, only the short wavelength peak presented. When bias voltage increased to +0.8V, the photo-excited electrons from the LWIR absorption region can overcome the barriers and the LWIR peak showed up as expected. The absorption at 5 - 6um also showed up at this bias voltage. The dark current of the QDIP was measured at 77K using an HP 4145A semiconductor parameter analyzer. The noise current density was measured using a Stanford Research SR760, FFT Spectrum Analyzer. Figs . 12A and 12B show the dark current density and the noise current density as a function of bias voltages at different temperatures, respectively.

The photoresponsivity of the QDIP array was measured using a (1000 K) calibrated blackbody source. The effective received incident power P_in was calculated to 1.8nW from 7.4μm to 10.3μm. The photoresponsivity 91 can be written as:

where, I_ph is the photo current. Fig. 13A shows the photoresponsivity SR as function of bias voltages . The photoresponsivity of 0.8A/W was obtained at the bias voltage of 04V.

The high photoresponsivity is attributable to the high QD density and the reduced defects in the QD active region, which decreases the nonradiative relaxation and thus increases the lifetime of the photo- excited electrons .

The specific photodet4ectivity D* can be written as:

u. ¥^U7^)T (2)

Where, ±_noise is the noise current density in A/Hz^1/2. G is the noise gain, A is the detector area, and I_d and I_ph are the dark current and photocurrent, respectively. Fig. 13B shows the calculated photodetectivity (D*) at various bias voltages at different temperatures. A high photodetectivity of 2.2xl0¹⁰cmHz^1/2W at the biases of -0.8V was achieved.

Thus, a preferred embodiment of the present invention provices an electronically tunable multi-spectral QDIP (with a peak wavelengths of 5.6μm, 7.7um and lO.Oμm, for example) has a high photodetectivity of 2.2xl0¹⁰cmHz^1/2W at a bias of -0.8V. By tuning the bias voltage, the detection band can be individually turned on or off. High photodetectivity of > 2.3xl0¹⁰cmHz^1/2W were obtained for these infrared detection bands. The voltage-controllable detection band selection enables real-time system reconfiguration to focus on the band of interest. The vertically-stacked device structure provides a focal plane array (FPA) detector device.

A focal plane array system 400 can be assembled using the integrated circuit fabrication process as shown in FIGs. 14A-14E. An array of mesa structures 402 is formed is shown in FIG. 14A having a three tiered structure with a first QD periodic structure 404 (e.g. 10 period) between a first contact layer 406 and a second contact layer 408. A second QD periodic structure 440 (e.g. 5 period) is positioned between the second contact layer 408 and a third contact layer 412. A 12x12 array 420 is shown in FIG. 14B and a detailed top view of an individual QDIP mesa 422 having a diameter of lOOμm and top electrode 424 and a bottom electrode 426. A third electrode 409 between the periodic bands can also be used. An individual InAs dot, shown in FIG. 14D, can have a diameter of 25nm and a height of S nm.

As seen in FlG. E, a focal plane area system 400 can be used to detect a two dimensional image 440. A controller 452 with flash 456 and battery 454, is used to control an actuator or motor 442 to position an IR filter 444 and IR lens 446 relative to array 420. A processor 462 is connected to a driver circuit 450 to control the array 420. A readout circuit controls data readout to a video output 458 or to A/D converter 460. Processor 462 converses with memories 474, 475 and arithmetic processor 472 as well as flash memory 464 and SDRAM memory 466. image data can be displayed on display/user interface 468 or transferred to a network through USB port 470.

Many changes in the details, materials and arrangements of parts, herein described and illustrated, can be made by those skilled in the art. Accordingly, it will be understood that the following claims are not limited to the embodiments disclosed herein and can include practices other than those specifically described, and are to be interpreted as broadly as allowed under the law .

Claims

CIAIMS What is claimed is:

1. An infrared light detector comprising: a quantum dot heterostructure having a plurality of layers, each layer having a two dimensional array of quantum dot elements, the elements of the plurality of layers being vertically aligned to form an infrared light detecting structure.

2. The detector of claim 1 wherein the plurality of layers comprises at least five layers .

3. The detector of claim 1 wherein the plurality of layers comprises at least 10 layers .

4. The detector of claim 1 having a first plurality of layers to detect in a first wavelength range and a second plurality of layers to detect in a second wavelength range .

5. The detector of claim 4 wherein the first range is 3-6 microns and the second range is 7-12 microns.

6. The detector of claim 4 further comprising separation layer between layers .

7. The detector of claim 1 further comprising a contacting layer.

8. The detector of claim 1 further comprising a first capping layer over a first plurality of QD layers detecting a shorter wavelength and a second capping layer over a second plurality of QD layers detecting a longer wavelength.

9. The detector of claim 8 wherein the first capping layer comprises GaAs.

10. The detector of claim 8 wherein the second capping layer comprises InGaAs .

11. The detector of claim 1 wherein the detector is coupled to a thermoelectric cooler that operates at 100°k or lighter.

12. The detector of claim 1 further comprising a readout circuit and a thermoelectric cooler.

13. The detector of claim 1 further comprising a focal plane array system.

14. The detector of claim 13 wherein the detector array comprises a lens optically coupled to a light receiving surface of the detector.

15. The detector of claim 13 further comprising a driver circuit.

16. The detector of claim 13 further comprising a processor.

17. The detector of claim 16 wherein the processor is connected to a user interface and a memory.

18. The detector of claim 13 further comprising a controller coupled to lens actuator.

19. The detector of claim 13 wherein the detector has an array of at least 10x10 pixel elements.

20. The detector of claim 1 wherein the device has at least three contacting layers .

21. The detector of claim 19 wherein each pixel element has a mesa structure.

22. The detector of claim 1 wherein each quantum dot element has a diameter between 10 and 40 microns.

23. The detector of claim 1 wherein each quantum dot element has a diameter between 20 and 30 microns.

24. The detector of claim 1 wherein the quantum dot size varies less than 20% across the structure.

25. The detector of claim 1 wherein the quantum dot elements are aligned vertically along parallel axes .

26. The detector of claim 1 wherein the detector is formed on a GaAs semi-insulating substrate.

27. The detector of claim 1 wherein the quantum dot elements comprise InAs .

28. The detector of claim 27 further comprising a cap layer of InGaAs .

29. A method of forming a quantum dot infrared detector comprising: forming a plurality of quantum dot layers, each layer having an array of quantum dot elements, the elements from each layer being aligned vertically; and forming an electrode contact .

30. The method of claim 29 further comprising forming a first plurality of layers to detect a first wavelength and a second plurality of layers to detect a second wavelength.

31. The method of claim 29 wherein the elements have less then a 20% variation in diameter in each layer.

32. the method of claim 29 wherein the elements are aligned vertically along a plurality of parallel axes.

33. The method of claim 29 further comprising forming a focal plane array including at least 10x10 pixel elements .

34. The method of claim 30 further comprising a separation layer .

35. An infrared detector comprising: a quantum dot heterostructure having an energy barrier with a first confined excited state and a second excited state, the heterostucture having a two photon absorption characteristic in an infrared range; and a first electrode and a second electrode that collects a photocurrent .

36. The infrared detector of claim 35 wherein the quantum dot heterostucture comprises InAs and GaAs.

37. The infrared detector of claim 35 further comprising a readout circuit.

38. The infrared detector of claim 35 further comprising a thermoelectric cooler.

39. The infrared detector of claim 35 wherein the quantum dot heterostructure comprises InGaAs and GaAs .

40. The infrared detector of claim 35 wherein the quantum dot heterostructure comprises InAs and AlGaAs .

41. The infrared detector of claim 35 wherein the quantum dot heterostructure comprises InAs and InP.

42. The infrared detector of claim 35 wherein the quantum dot heterostructure comprises InAs and InGaAsP.

43. The infrared detector of claim 35 wherein the first reflective layer comprises the first electrode.

44. A method of fabricating an infrared detector comprising: forming a quantum dot heterostructure the heterostructure having a first confined excited state and a second excited state such that the heterostructure has a two photon absorption characteristic in an infrared range; and forming an electrode contact with the heterostructure to provide an infrared detector.

45. The method of claim 44 further comprising forming an electrode structure to collect photocurrent .

46. The method of claim 44 further comprising forming the heterostructure from a pair of materials selected from the group comprising InAs and GaAs, InGaAs and GaAs, InAs and AlGaAs, InAs and InP, or InAs and InGaAsP.

47. The method of claim 44 further comprising forming a first reflective layer and a second reflective layer to forma resonant cavity.

48. The method of claim 44 further comprising forming a focal plane array detector.

49. The method of claim 44 further comprising coupling the detector to a thermoelectric cooler.

50. The method of claim 44 forming a focal plane array having at least 10x10 pixel elements.