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We present the design simulation and characterization of a quantum cascade detector operating at 4.3μm wavelength. Array integration and packaging processes were investigated. The device operates in the 4.3μm CO2 absorption region and consists of 64 pixels. The detector is designed fully compatible to standard processing and material growth methods for scalability to large pixel counts. The detector design is optimized for a high device resistance at elevated temperatures. A QCD simulation model was enhanced for resistance and responsivity optimization. The substrate illuminated pixels utilize a two dimensional Au diffraction grating to couple the light to the active region. A single pixel responsivity of 16mA/W at room temperature with a specific detectivity D* of was measured.
© 2016 Optical Society of America
1. Introduction
Mid-infrared detector arrays have been investigated intensively in the past years. Their applicability in a large variety of fields including thermal imaging, remote detection, astronomy and military countermeasure systems promoted the research on so called focal plane arrays (FPA). Microbolometer, HgCdTe [1, 2], InSb [3] and quantum well infrared photodetector (QWIP) F-PAs have already proven their performance and are well established technologies. Despite of their performance, HgCdTe detectors are highly sensitive to their material composition and material distribution. Moreover, batch production relies on the availability of growth and bandgap compatible substrates with respect to absorption in bottom side illuminated designs. QWIP FPAs have been scaled up to provide megapixel and dualcolor imaging [4, 5]. Photoconductive QWIP FPAs require operation temperatures which are not accessible by thermo-electric coolers (TEC) and suffer therefore from relatively heavy space consuming casings. Type II superlattice interband detectors proposed already in 1987 [6] offer focal plane array compatibility [8], detection in the mid-infrared and long wave infrared [9, 10]. Operation at elevated temperatures up to 420K was demonstrated [7]. Thermal imaging up to 180K with a specific detectivity D* of at 150K was shown utilizing a diffusion barrier [11].
Quantum cascade detectors (QCDs) offer the advantages of room temperature operation in combination with the design freedom of quantum cascade based intersubband devices and compatibility to standard InP based growth and fabrication technology. Evolved from photovoltaic QWIPs [12] they utilize a LO-phonon extraction scheme which provides more design freedom. A challenging aspect of QCD design is to maintain a high device resistance while the responsivity remains reasonably high. An advantage of QCDs over well established detector types is their designable operation wavelength, low noise photovoltaic operation mode and therefore room temperature operation capability. With a well established material system detectors for different operation wavelengths can be designed by adjusting the well and barrier dimensions, while the material properties remain the same. For mobile, battery powered applications an uncooled or thermo-electrically cooled mid-infrared imaging FPA is needed. Beside the single pixel performance, scalability to FPA dimensions of several tens of millimeters is necessary for high pixel counts. MBE and MOCVD growth on InP provides these requirements. QCDs have already proven their room temperature performance at a wide wavelength range [13–15] and broadband designs [16–18]. As shown by Hofstetter et al. QCDs around 4.3μm wavelength provide CO2 sensing abilities [19] with high selectivity. Recently QCD performance and robustness at longer wavelengths was increased by a diagonal transition design [20]. The InAs/AlAsSb material system had been introduced for short wavelength high resistance QCDs [21]. Dependent on the desired application the operation wavelength and several QCD parameters have to be chosen. QCD design involves a trade-off between responsivity and device resistance which then influences all other parameters such as the number of periods N, barrier thicknesses, the absorption α and the specific detectivity D*. A description of the design parameters, their influence and optimization considerations is presented in more detail in [22–24].
In this paper we demonstrate a quantum cascade detector in pixel configuration processed as array [Fig. 1]. The detector is substrate bottom side illuminated with an Au diffraction grating processed into the top contact layer. The active region is optimized for a high device resistance. Thereby a resistance increase in the entire temperature range was achieved. Sufficient QCD resistance for operation with the custom read out integrated circuit (ROIC) can be ensured from 80K up to 240K. The presented device is based on the InGaAs/InAlAs material system grown lattice matched by molecular beam epitaxy (MBE) on semi-insulating InP substrate.
Fig. 1 Scanning electron microscope image of a section of the (a) 8×8 pixel array with separate bottom contacts, (b) a detailed SEM image of a single pixel with the corresponding bottom contact (square without grating structure) surrounded by the isolation trench. The inset (c) shows the metal grating in detail.
2. QCD design
2.1. Heterostructure
At shorter mid-infrared wavelengths a resonant tunneling extraction scheme is more favorable than a diagonal optical transition. Due to the higher energy of the optical transition in this wavelength range back scattering to the ground level of the active region is reduced. The band structure of one period is illustrated in Fig. 2(a). The active transition takes place in well A, where A′ denotes the active well of the next period. The simulated extraction efficiencies are calculated for all subsequent levels including the LO-phonon staircase (B-J). An electron in the ground level of well A can be excited to one of the three degenerated excited states in well A (delocalized between A, B and C) which have extraction efficiencies of 81.84%, 80.58% and 86.04%, respectively. These efficiencies express the probability of an electron, excited to the corresponding level, to reach and to be captured in the ground level of the next QCD period (well A′). The extraction efficiency is calculated as a weighted average of these levels.
Fig. 2 Band diagram (a) of one QCD period with the corresponding extraction efficiencies for all levels of the extractor and LO-phonon staircase calculated for 300K. The active region consists of 20 periods. The simulated QCD responsivity (b) is based on the simulated absorption efficiency of a 45° mesa device.
The total extraction probability calculated considering the absorption strength for each level allows for extraction optimization from the active well (A/A′) to the first extractor level (B/B′). In order to identify bottlenecks, the extraction efficiency is calculated also for all following LO-phonon staircase levels. The transition between well J and A′ is chosen to exhibit a higher energy difference to prevent thermally excited carriers from back-filling the lower staircase levels of the previous period. This is important to achieve a temperature insensitive responsivity as thermal back-filling would reduce the absorption efficiency at elevated temperatures. The device consists of 20 periods with a total active region thickness of 1.36μm sandwiched between a 800nm InGaAs top contact layer and a 500nm InGaAs bottom contact layer, both Si doped to 1.5e18cm−3. A period of the active region consists of the layer structure 4.3 /6.5/0.85/6.5/0.9/6.5/1.0/5.5/1.2/4.5/1.45/4.5/1.7/4.5/2.05/4.0/2.4/4.0/2.85/3.0nm starting with the active well A, InGaAs in bold, InAlAs normal and the 8e17cm−3 doped well underlined.
The Vienna Schrödinger Poisson (VSP2) frameworks semi-classical Monte-Carlo transport simulator originally developed for quantum cascade laser design was extended for QCD design and simulations [20, 25]. The tools allow for simulation of the intersubband scattering rates for short wavelength detectors, the absorption, differential device resistance, responsivity and the specific detectivity. The model accounts for all possible scattering transitions and is based on three consecutive periods. For extraction efficiency simulation, a scattering event contributes to the current flow, if it scatters from the middle period to the right or to the left adjacent period. From the resulting scattering matrix the outbound scattering rates of the ground levels are deleted and all electrons are initially set to be in the excited state(s) of the active well (A/A′). The scattering matrix is then applied n times to the initial population. After n events the electrons accumulate in the ground states which have no outbound scattering. The ratio between the amount of electrons in the ground state of A to A′ then gives the total extraction efficiency. The absorption efficiency ηeff is modeled for 45° mesa devices considering the facet transmission Tfacet ≈ 70% and all possible optical excited intersubband transitions by the absorption coefficient in the active region.
where Np is the number of QCD periods and Lp the length of a period. The simulated responsivity R(λ) is calculated based on the absorption efficiency. In contrast to QWIPs the capture probability pc for QCDs can be assumed to be close to unity. The peak responsivity is then given by with the total detector current Is, the total incident radiation power Ps, the elementary charge q, Plancks constant h, speed of light c, absorption efficiency ηabs, photodetector gain gp and the extraction efficiency pe. In-plane non-parabolicity is included in the model to correctly predict the red-shift with increasing temperatures [Fig. 2(b)]. The QCD differential resistance model is based on the work of Delga et al. [26]. All scattering transitions between subbands are replaced by a conductance. An equivalent conductance circuit is extracted, which is solved by node analysis to obtain the device resistance. The model accounts for inter period transitions by periodic repetition of the scattering matrix.2.2. Optical and array design
The QCD is designed as a substrate bottom side surface-normal illuminated pixel device. A QCD array was processed and flip-chip bonded to a CMOS ROIC. Since intersubband detectors are only sensitive to light with the electric field component polarized in growth direction a coupling scheme to the active region is required. Light coupling is achieved by a two dimensional 2nd order Au grating with a grating period of p = 1.4μm, depth d = 450nm and duty cycle 0.4. The pixel to pixel pitch is 225μm in both directions, with a pixel size of 109μm × 109μm. The pixel pitch is determined by the size of the prototype RIOC amplifier circuit for each pixel. This size can be reduced in future designs.
The optical simulations for light coupling were conducted with the active regions absorption αisb(λ) retrieved from our QCD model. Absorption was taken into account only in growth direction to account for the intersubband selection rule. For all materials the mid-infrared complex refractive indices ṉ(λ) were taken into account. The grating depth, duty cycle and grating period were optimized for maximum coupling efficiency. The Au layer of the grating is used as the top contact of the pixels and serves as the bonding pad for Au thermo-compression flip-chip bonding.
3. Results
The QCD material was first characterized as a 100μm × 100μm mesa device. As intersubband devices are subjected to the intersubband selection rule, a 45° facet was polished onto the sample for light coupling. The photocurrent spectra were recorded by FTIR (Fourier transform infrared) spectroscopy illuminated by a broadband globar source. In order to determine the detectors wavelength dependent responsivity R(λ) the power density spectrum and total power of the broadband globar source were recorded. With respect to the measured parameters the responsivity from Eq. (2) can be expressed as
The photocurrent spectrum Is(λ) is obtained by the FTIR recorded photocurrent spectrum normalized by the measured total photocurrent Itot. The total incident power Pi is determined by the spatial power distribution of the focused globar source and its fraction impinging on the 45° facet. The factor M accounts for the polarization selection and the cryostat window transmission. The cryostat ZnSn window transmission was determined to be 75%. For mesa devices the polarization was accounted with a factor of 0.5 since 45° facet devices are only sensitive to light polarized in growth direction (M = 0.75 × 0.5). Due to the 2D metal array the pixel detectors are sensitive to unpolarized light (M = 0.75).
Figure 3 shows the temperature dependent responsivity and differential resistance of a 45° mesa device. The two pronounced valleys are due to the strong CO2 absorption bands. In contrast to the simulation a responsivity increase between 80K and 160K can be observed. This may be accounted to carrier freeze-out or the increasing dominance of coherent tunneling and electron-electron scattering which are not included in the simulation model. The characteristic red-shift with increasing temperatures is due to strong in-plane non-parabolicity. Due to the optimization for higher operation temperatures, the CO2 absorption is mostly aligned with the photocurrent peak at temperatures between 240K and 300K. The measured responsivity is slightly below the simulated performance illustrated in Fig. 2(b). This mismatch can be accounted to the Ti /Au metal-semiconductor contact resistances and the contact layer to first QCD period resistances, which are not included in the simulator model. From previous experiments we noticed that the design of the contact layer to the first QCD period is relevant, especially if the contact layer would be low doped [27]. The pixels are processed as sparse pixels which do not cover the maximum available area. This design decision is due to the resistance limitations of the prototype read out circuit. With the pixel mesa size of 109 × 109μm a resistance of 13.9kΩ at 240K operation temperature could be realized which is well within the ROIC specifications [Fig. 4(b)]. From the finished array single pixels were directly wire bonded. The single pixel measurements were conducted prior to flip-chip bonding. Figure 4(a) shows the single pixel responsivity of a typical processed array with a SiNx anti-reflection coating (ARC). As for the 45° mesa devices strong CO2 absorption is observed. The responsivity increases between 80K and 160K and decreases towards 18mA/W at the design temperature. Due to the high coupling and absorption efficiency of the pixel design the responsivity decrease between liquid N2 temperature and room temperature is less pronounced in comparison to 45° mesa devices. The simulated peak absorption efficiency of a single pixel based on the real sample geometry is illustrated in Fig. 4(d).
Fig. 3 Photocurrent spectrum and temperature performance of the 4.3μm 20 period QCD design measured with a 100μm × 100μm mesa device. The illumination is through a 45° polished facet.
Fig. 4 Photocurrent spectrum and temperature performance (a) of the 4.3μm 20 period QCD design measured with a 109μm × 109μm pixel device. The illumination is through the ARC coated substrate side. The two visible valleys in the spectrum are due to the strong CO2 absorption in the beam path. The simulated and measured differential resistance R0 (b) show good agreement in the entire temperature range. The measured peak responsivity RPeak and the specific detectivity D* (c) were measured by a calibrated globar source. The field of view Θ for is π. The grating coupling simulation of the absorption efficiency (d) of a single pixel with αabs(λ) provided by the QCD simulator. The inset depicts the normalized electric field component Ey due to the metal grating coupling.
Grating coupling is designed around the peak at 2350cm−1 with the electric field component in growth direction shown in the inset of Fig. 4(d). A slightly higher doping was used in the model to match the simulations with the experiment. The optimization for increased QCD resistance results in a high detectivity even at room temperature [Fig. 4(c)].
The arrays substrate side was polished and reduced in thickness to ≈ 250μm. For the presented 8 × 8 array complete substrate removal is not necessary due to the small total size, large pixel pitch and comparable high operation temperature which results in a low ΔT ≈ 70K. The finished 8 × 8 pixel device is packaged into a 16 pin TO-8 housing [Fig. 5(b)] with a spot welded sealed cap purged with N2. The assembly consists of the two stage TEC, the thermo-compression bonded array and ROIC, a Ge window soldered to the cap and a Cu cold shield [Fig. 5(a)]. The cold shield is thermally connected with the TEC. The Au thermo-compression bonded prototype array with the ROIC is wire bonded to the TEC element and to the pins of the TO-8 housing [Fig. 5(c)].
Fig. 5 The housing of the prototype consists of (a) the cap, the base with the two stage thermo-electric cooler, the bonded ROIC with the QCD array, a cold shield and the anti-reflection coated Ge window. The array wire bonded viewed in illumination direction is depicted in (b). The packaged array (c) is purged with N2. Flip-chip bonding and packaging as depicted was developed and conducted by Faunhofer IZM Berlin.
4. Conclusion and outlook
Based on our customization of the VSP2 frameworks single particle Monte-Carlo transport simulator we have shown a high performance QCD designed at the CO2 absorption bands around 4.3μm. These detector specific models enabled for a resistance increase and an over all performance increase in comparison to previously shown III/V material based mesa devices at this wavelength [14, 19, 20, 22, 28]. The calculation of the extraction efficiencies for every extractor level allows to identify bottlenecks and to increase the total extraction efficiency pe. Processing improvements for the grating structure could further improve pixel performance.
With the performance of our QCD design we show the potential of this detector type and envision its further improvement to be used for arrays in imaging applications. Remote sensing would benefit from spectrally narrow detection as provided by QCDs and provide additional scene information based on the design wavelength and the thereby detectable substances such as CO2.
5. Materials and methods
5.1. Array processing
The MBE grown material is cleaned with acetone in an ultrasonic bath and isopropanol prior to all processing steps. A 420nm SiNx hardmask is deposited for the 2nd order diffraction grating etch process and structured by electron beam lithography. After the e-beam lithography the grating dimensions were evaluated by atomic force microscopy. The SiNx hardmask is etched with an CHF / O2 dry etch process optimized for highly anisotropic etching. The grating is structured into the top contact layer by reactive ion etching. In the next step the pixels are structured by reactive ion etching with a SiNx nitride hardmask. The pixels are isolated from each other by a wet chemically etched 6μm wide isolation trench which cuts through the bottom contact layer. The next step involves the deposition of an insulation layer of 350nm SiNx and the opening of the contact windows. The Ti/Au (10nm/440nm) contacts are sputter deposited using a negative resist lift-off process. The relatively high Au layer thickness is a requirement for the Au thermo-compression flip-chip bonding process. After contact deposition the sample is cleaned with an ultrasonic acetone bath followed by isopropanol and cleaved to the final array size. The arrays have to be cleaved with minimal remaining edge width surrounding the array to make the ROIC’s digital and analog I/O bond-pads accessible by the bond wires. The samples InP substrate bottom side is then polished by wet mechanical polishing. The 580nm SiNx anti-reflection coating is then deposited by plasma enhanced chemical vapor deposition directly onto the polished substrate bottom side. Au thermo-compression flip-chip bonding is done directly onto the grating surface of the pixels and the corresponding bottom contacts. Due to the relatively low mesa height no planarization step is necessary.
Acknowledgments
This work was supported by the FP7 EU-project ICARUS, the Austrian Science Funds (FWF) within SFB project F49-09 NextLite and the framework of the Doctoral School “Building Solids for Function” (project W1243).
References and links
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