Comparative characteristics of the GaAs/AlGaAs QWIP IDCA by the ISP of the SB RAS (Novosibirsk) and Sofradir (France).
In this article, we present an overview of a focal plane array (FPA) with 640 × 512 pixels based on the AlGaAs quantum well infrared photodetector (QWIP). The physical principles of the QWIP operation and their parameters for the spectral range of 8–10 μm have been discussed. The technology of the manufacturing FPA based on the QWIP structures with the pixels 384 × 288 and 640 × 512 has been demonstrated. The parameters of the manufactured 640 × 512 FPA with a step of 20 μm have been given. At the operating temperature of 72 K, the temperature resolution of QWIP focal plane arrays is less than 35 mK. The number of defective elements in the matrix does not exceed 0.5%. The stability and uniformity of the FPA have been demonstrated.
- GaAs/AlGaAs quantum well infrared photodetector
- focal plane array
- dark current
- noise equivalent temperature difference
The solution to a well-known problem of a finite rectangular quantum well (QW) indicates that the energy position of the quantum subbands is determined by several parameters, namely, the width of the quantum well, the height of the barriers, and the carrier effective masses. The possibility of a rather simple way of monitoring the wavelength of intersubband transitions in a quantum well to create photodetectors has attracted the attention of researchers for a long time . Improving the technology of the molecular beam epitaxy (MBE) has allowed converting these dreams into reality. The quantum well infrared photodetector (QWIP) was first demonstrated on the basis of the AlGaAs/GaAs heterostructure in 1987 . Permanent improvement of the QWIP design, epitaxial growth technologies of the AlGaAs layers, and techniques for device manufacturing allow using the QWIP based on AlGaAs/GaAs heterostructures not only for military purposes but also for various civilian tasks [3, 4, 5, 6, 7].
In this paper, we have described the current state of the manufacturing technology in the Rzhanov Institute of Semiconductor Physics of SB RAS for the focal plane array (FPA) based on the AlGaAs/GaAs QWIP structure.
2. The physical model of the QWIP
A typical heterostructure intended to detect the infrared radiation (IR) is shown in Figure 1. Such heterostructures are commonly formed by a thin (4–6 nm) GaAs QW located between the wide band gap AlGaAs barrier layers. The thickness of these barrier layers (
In such a structure, the electron moves from a ground quantum level
such expressions as ℏ
Conversely, when the electromagnetic wave is polarized along the z axis, the operator is equal to and the expression changes as:
The first integral in expression (Eq.(2)) is nonzero for i and j with different parities, and the last integral is nonzero for
The concentration of the two-dimensional electron gas (2DEG) in the photodetector structures should be large enough to locate the ground quantum level below the Fermi level. The required position of the Fermi level is achieved at the 2DEG concentration (
The doping of the structure outside the GaAs QW (so-called the modulation-doped QWs) leads to significant changes in the QW shape (Figure 2B). Actually, the separation of electrons and ionized donors causes the appearance of an electric field and, therefore, a noticeable bending of the band next to the quantum well region. Therefore, the energy of the quantum levels changes. The calculated energies of the ground and first-excited quantum levels resulted from a self-consistent calculation proved to be equal to 3.2/140.9 meV and −3.1/133.7 meV for the QW doped in middle part and the modulation-doped QW, respectively. These values are given with respect to the Fermi level that lies at the energy ε = 0. As one can easily estimate, the shift in the quantum levels resulted from the changes in the doping location leads to a shift in the intersubband absorption band by 0.5 μm.
The nomograms of the dependence of the intersubband transition line upon the QW width and the barrier height were calculated by the self-consistent solution of the Schrödinger and Poisson equations to compare the rectangular and modulation-doped QWs. Figure 3 shows the calculated nomogram for the QW doped in the middle part. As one can see from Figure 3, λmax is weakly dependent on the QW width in the wavelength region (8–10 μm) worth examining, whereas the required wavelength of the IR absorption is more difficult to obtain in the modulation-doped QWs. One more parameter is added, and the energy of the intersubband transition from the ground to the first excited level depends not only on the well width and the barrier height (or the Al mole fraction in the barrier) but also on the doping level. Therefore, the calculation was made for the modulated-doped QW at a fixed QW width equal to 5.6 nm (Figure 4).
The dark current flowing in the thermal equilibrium through the photodetector under a bias is an important characteristic of a QWIP. Its value is frequently used as a QWIP quality criterion. In QWIPs, the physical reason for the appearance of a dark current is the thermionic electron emission from the ground-filled quantum level to continuum states both above and below the energy barrier
where is the average electron drift velocity in the AlGaAs barrier layers,
The modeling with the help of
Thus, the current density through the heterostructure is finally obtained as:
As one can see from Figure 5, the barrier height decreases at the enhanced doping, which is caused by an increase in the Fermi level accompanied by an increase in the DEG concentration from 1.4 × 1011 to 3.4 × 1011 cm−2.
Figure 6 shows the calculated dependences of the dark current density upon the applied electric field for different donor concentrations.
The optimum 2DEG concentration corresponding to the minimum value of the threshold power of the radiation (NEP) is estimated as follows. The value of NEP is given by:
In its turn, the responsivity is proportional to the absorption coefficient of a single QW α, which, in turn, is proportional to the DEG concentration :
Thus, the maximum sensitivity of the GaAs/AlGaAs QWIP is expected to be around 8.6 μm to ensure the spectral range of the detector (8–10 μm). The calculations have shown that this condition is satisfied by such parameters of the doped QW as the well width
3. Properties of the heteroepitaxial GaAs/AlGaAs MQW structures
Multiple quantum wells GaAs/AlGaAs were obtained by the MBE on the GaAs (100) substrate with a buffer consisting of a 0.4-μm GaAs layer. The doped GaAs layers of 1.5 μm thickness were used as a base contact, whereas the upper ohmic contact was provided by the doped GaAs of 1.2 μm thickness.
It has been found that the best structural perfection of epitaxial GaAs layers and the greatest value of the carrier mobility can be obtained at a growth under conditions ensuring a (3 × 6) surface structure having a stoichiometric composition on the growth surface and being intermediate between the “As-stabilized” (2 × 4) and “Ga-stabilized” (4 × 2) surfaces. At the growth under such conditions, the concentrations of the gallium and arsenic vacancies on the surface are minimal, which provides a minimum concentration of the intrinsic point defects in the crystal. The deviation of the growth conditions toward the gallium or arsenic stabilization leads to the enrichment of the epitaxial layers by the defects of anion or cation sublattices, respectively. Since the (3 × 6) surface is observed in the narrow ranges of the substrate temperature and V/III flow ratio, the growth conditions around the transition from (2 × 4) to (3 × 6) surface can be recommended in order to obtain good process reproducibility.
Such growth conditions being applied, the necessary homogeneity and concentration of the growth defects across the wafer can be achieved. The distribution of the growth defect (“oval” defect) density is shown in Figure 7. One can see that the average density of the defects of 100–200 pcs/cm2 can potentially lead to defectiveness of the photosensitive element matrix by no more than 0.2% (The number of pixels in the array of the photodetectors is 100–300 thousands). A small size of the growth defect in 4–6 microns suggests that even if one pixel is damaged, the neighboring pixels will remain unaffected.
The structure of the samples was examined by the method of studying the cross section of structures by the transmission electron microscopy (TEM). The studies were performed by the JEM-4000EX electron microscope by JEOL (Japan). The samples for the TEM were made in the geometry of the cross section by grinding them with their subsequent thinning during etching by Ar+ ions with an energy of 3–4 keV. The survey was conducted at an accelerating voltage of 400 kV. Figure 8 shows the results of the TEM AlGaAs/GaAs heterostructure. The image represents no structural defects, which indicate a high quality of the heterostructures under study. The measurements carried out by the TEM methods demonstrate a good correspondence of the heterostructure layer thicknesses planned and obtained. Sharp changes in the intensity at the quantum well heterogeneities testify to fairly sharp and smooth transitions from one material to another.
The spectra of the piezomodulated reflection of the MQW structure in a visible spectral range at a liquid nitrogen temperature were measured to control the aluminum mole fraction in the AlGaAs barrier layers. Mechanical vibrations of the ceramic plate caused the modulation of the mechanical stresses in the structure, the modulations of the real Δεr and the imaginary Δεi parts of the dielectric constant, and as a consequence, the modulation of the reflection coefficient. The reflection variances ΔR are related to the variation of the dielectric constant by the Seraphin ratio :
where α and β are Seraphin coefficients. The energy position of the peaks in the piezoreflection spectrum corresponds to the electronic transitions in the structure under study. Typical piezoreflection spectra of the MQW structure are shown in Figure 9A. The aluminum mole fraction in the barrier layer is calculated from the peak energy. The peak at the 1.484 eV corresponds to the transitions from the levels of residual acceptors (neutral carbon atoms) to the conduction band in the GaAs substrate. The band gap of the GaAs at the 77 K is
The measurement of the QW width was carried out by means of the photoluminescence (PL) spectroscopy at a liquid nitrogen temperature along a line corresponding to the transition from the ground electron level to the ground level of the heavy holes
To control the electron concentration in the grown structures with MQWs, the C-V characteristics were measured. Those measurements were carried out with the specially fabricated Schottky barriers of the TiAu with a diameter of 200 μm and a C-V bridge operating at frequencies of 1 to 100 kHz. The upper contact layer had been previously etched and then measured. According to the measured C-V dependences, the concentration dependences upon the depletion region depth
To calculate parameters of the multiplexer (capacitance and integration time), it is necessary to know the field dependences of the dark current. The measured dependencies of the dark current upon the bias for the QWIP structure made up of 30 periods of QW and barriers (
4. Selecting the parameters of the FPA on the basis of the GaAs/AlGaAs QWIP
While choosing a number of QW periods in the QWIP structure, the following must be taken into account: (A) an increase in a number of structure periods leads to an increase in the absorption coefficient, but, in turn, (B) the overall probability of capturing photoexcited electrons back into the QW, (C) the growth time of the heterostructure, (D) the mesa depth, and (E) the magnitude of the voltage applied to the structure increase as well. As a result, the sensitivity of the QWIP increases very weakly with an increasing number of the GaAs/AlGaAs layer periods . Therefore, a heterostructure with 30 periods of the GaAs QW has been chosen.
A 2D diffraction grating with the parameters determined by the spectral range and properties of the dielectric applied is required to provide the absorption of a normal-incident light and increase the quantum efficiency of the QWIP . The required etching depth of the lattice (
The thickness of the upper contact layer of n+ doped GaAs was increased up to 1200 nm to adapt the fabrication technology for ohmic contacts. At a lower thickness of the contact layer, the metal may penetrate into the QW region as a result of diffusion during a long (5 minutes) annealing.
The multiplexer capacity and integration time of the signal were estimated by the dependency dark current upon the bias at the 77 K (Figure 11). The current at the bias voltage equal to 0.5 V is about 10−10 A, which, in the presence of the capacitance in the multiplexer equal to 6 × 106 electrons, allows integrating the signal during 10 ms.
5. The fabrication technology for the FPA on the basis of the GaAs/AlGaAs QWIP
The focal plane arrays on the basis of GaAs/AlGaAs QWIP structures were fabricated by the complex of coordinated technological operations based on photolithography processes using functional dielectric and metallic layers, etching processes, and chemical treatments of the heterostructures in the regimes determined by the physicochemical properties of the MQW layers. The technology development was carried out at the FPA with 384 × 288 elements with the 25 microns pitch. The structure control was carried out by the optical and scanning electron (LEO-1430, SU8220) microscopes.
5.1. Formation of the diffraction grating and mesa structure
A diffraction grating and mesa structures were formed by dry anisotropic etching of the GaAs layer in a remote gas-discharged plasma (ICP RF) of the BCl3, argon, and nitrogen. The obtained diffraction grating holes are square shaped with rounded corners and vertical walls (Figure 12A). The view of the mesa structure walls formed by etching the GaAs at a given depth (2.4 μm) at the optimum ratios of reagent gases, the power of the RF and ICP generators, the reactor pressure, substrate temperature, heating, and etching time are shown in Figure 12B.
5.2. The fabrication of ohmic contacts
The ohmic contacts to the base and upper n+ doped GaAs layers (on the mesa surface) were fabricated by the Ge/Au/Ni/Au (20/20/20/100 nm) deposition  after the removal of the native oxide layer from the semiconductor surface by HCl:Н2O (1:8) and annealing during 5 minutes at the 385°C in a hydrogen (Figure 13).
5.3. The mesa structure surface passivation
The SiO2 dielectric layer was deposited by a low-pressure chemical vapor deposition (LP CVD) method at temperatures of 195–250°C to passivate and protect the mesa structure surface. The low temperature of this process excludes the disorder of the surface stoichiometric composition due to the evaporation at high temperatures of the fifth group element. Depending on the synthesis conditions, the layers of LP CVD SiO2 (refractive index 1.46) have a dielectric constant of 5.9–6.5 and leakage currents of 6 ×·10−8–9·× 10−7A/cm2 (
5.4. Production of indium bumps on the FPA and silicon multiplexers
To assemble a FPA by cold welding, indium bumps were fabricated both on the GaAs/AlGaAs QWIP structure and silicon multiplexers. The Tl xLift photoresist was used to produce indium bumps (height of ∼5 μm) by inverse photolithography. The view and cleavage of a separate mesa structure with an indium bump of the FPA of 384 ×·288 elements produced by the developed technology on the GaAs/AlGaAs heterostructures are shown in Figure 14.
5.5. Hybrid assembly of the FPA
The FPA modules were assembled by cold welding of the indium bumps under pressure . The FPA and multiplexer crystals were docked on the M9 setup of Laurier company. The fusion of the indium contacts was performed by heating the module up to the indium melting temperature with the succeeding cooling. The surface autoplanarization is provided during the crystal compression process, which is achieved by installing polyamide stoppers along the perimeter of one of the module elements—an array or multiplexer. The maximum limit mechanical load is determined experimentally from the measurement of the curves of the plastic flow of the indium bumps, their height, and area. The pressure required for the plastic flow of the contacts ranges from 0.3 to 0.9 kg/mm2. The total height of the indium bumps after the FPA assembly is 6–8 μm, which satisfies the requirements of the durability of the FPA hybrid assemblies .
5.6. The GaAs substrate removal from the FPA assembly
The technology of the substrate removal after the assembly of the FPA consisted of the successive processes of the mechanical grinding aimed at removing the main thickness of the GaAs substrate, chemical mechanical polishing and chemical dynamic polishing, in order to obtain a mirror-smooth surface of the array crystal. The processes of the chemical selective etching of the GaAs and heterostructure layers were applied to remove the GaAs substrate from the FPA surface completely (Figure 15) [20, 21, 22].
5.7. Technical characteristics of the multiplexer
The silicon multiplexers by Integral Joint Stock Company (IZ640FD format 640 × 512) made by the CMOS technology and meeting the QWIP requirements were used as a part of the FPA assembly .
The storage capacitance in every cell of the IZ640FD multiplexer was approximately 8 × 106 electrons with a reading noise of 1000 electrons. The adjustable integration time could vary from 100 microseconds to the entire duration of the frame scanning. The electric power consumed at a frame rate of 100 Hz did not exceed 120 mW. The electric power consumed at a frame rate of 100 Hz did not exceed 140 mW in the four output mode and 90 mW at a switch into one output mode, respectively.
The schematic diagram of the input block of the multiplexer is shown in Figure 16, where D is the photoresistor (detector), VD is the detector supply voltage, VB is the voltage specifying the detector bias voltage, VA is the voltage specifying the level of anti-burglary, VS is the skimming voltage, C1 is the integration capacitance in pixel, C2 is the storage capacity in pixel, C3 is the storage capacity in column, K1, K2, and K3 are the keys, A is the amplifier with a controlled gain, and B is the buffer. The signal integration occurs simultaneously on all the array elements, and then, the voltage from the capacitances C2 is line-by-line read out by connecting the key K2 to the column capacitance C3 and the column amplifier A.
6. Opto-electronic characteristics of the FPA assembly
At the final stage, the opto-electronic characteristics of the fabricated FPA assembly were determined. For this purpose, the assembly was placed in a nitrogen cryostat with an entrance window made of the ZnSe. The operating temperature of 65 K was achieved due to the pressure pumped down by a vacuum pump. A cooled diaphragm provided a relative aperture of 1:2. To measure the sensitivity of the FPA, the module illumination was made by an extended-type absolutely black body. It should be noted that the high parameters of the FPA assembly both the absolute values of the signals and their homogeneity with respect to the array elements are supposed to be essential.
The distribution histogram of the total current (dark + photo signal from the 300 K background) for the FPA assembly thinned up to 170 μm is shown in Figure 17A. The integration time of the signal was chosen to be 9 ms. Figure 17B shows the distribution histogram of the temperature sensitivity of the ST at the 300 K background upon the pixels of the FPA module BM20. Its average value is rather high and equal to 23.2 mV/K. Figure 17C shows the distribution of the noise voltage Vn at the output of the photoreceptor module BM20 at a background of 300 K. All the histograms are rather narrow, which demonstrates the high uniformity of the array parameters. The noise equivalent temperature difference NETD = Vn/ST of the FPA module BM20 is shown in Figure 17D. The average value of a NETD for nondefective pixels at the FPA temperature of 67 K is 22.2 mK. A number of defective elements with a NETD over 70 mK is 0.15%. A typical spectrum of photosensitivity of a 640 × 512 FPA is shown in Figure 18. An example of the IR image detected by a 640 × 512 FPA assembly module equipped by the germanium lens with D/F = 1:2 aperture is shown in Figure 19.
The developed technology for the FPA assembly is reproducible and has a rather high yield percentage of the suitable products well seen from Figure 20 showing the scatter of the noise equivalent temperature differences and a number of defective elements for the 640 × 512 FPA assembly manufactured on the 5 MBE grown heterostructures.
6.1. Integrated detector cooler assembly
The fabricated and tested 640 × 512 FPA were installed in the body of a vacuum cryostat integrally coupled with a microcryogenic system. Resulted integrated detector cooler assembly (IDCA) is showed in Figure 21. A vacuum cryostat performs thermal isolation of the array from the environment to guarantee effective cooling of the photodetector up to the operating temperature (T = 68–72 K). The radiation from the detected objects is fed to the FPA through an input window made up of the germanium with the antireflection in the range of 8–10 μm and a cooled diaphragm with a relative aperture F/2 designed to reduce the background illumination. A low pressure in the vacuum cryostat is provided by a getter, whose reactivation is carried out by passing an electric current through it. Typical operating temperatures for the FPA based on the QWIP with the wavelength range 8–10 μm are 68–72 K. Thus, the powerful microcryogenic systems ensuring a cooling capacity at an operating temperature of 70 K not less than 0.4 W and the power consumption not more than 20 W are needed to provide the required temperature in a full range of climatic conditions.
As one can see from Table 1, the parameters of the developed 640 × 512 QWIP IDCA are comparable with those of the Sofradir products.
|ISP SB RAS (Russia)|
|Array format||640 × 512||640 × 512|
|Pixel pitch, μm||20||20|
|Peak sensitivity, μm||8.5–8.6||8.5|
|Integration time, ms||6||7|
|Operating temperature, K||72||70–73|
The technology of manufacturing the AlGaAs/GaAs QWIP FPA has been discussed. The parameters of a FPA of 640 × 512 format with a 20-μm pitch for a spectral range of 8–10 μm have been described. At an operating temperature of 72 K, the temperature resolution of the QWIP FPA is less than 35 mK. The frame rate is 100 Hz. A number of defect elements in the array do not exceed 0.5%. It is shown that the parameters of the QWIP FPA fabricated by Rzhanov Institute of Semiconductor Physics of SB RAS meet the world class standards.