Long-Wavelength Infrared InAs/InGaSb Type-II ... - SPIE Digital Library

Long-Wavelength Infrared InAs/InGaSb Type-II Superlattice Photovoltaic Detectors K.A. Anselm, H. Ren, M. Vilela, J. Zheng, C.H. Lin Applied Optoelectronics, Inc. Sugar Land, TX 77478 V. Nathan AFRL/VSSS, Kirtland AFB, NM 87117 G. Brown AFRL/MLPO, Wright-Patterson AFB, OH 45433-7707 ABSTRACT The design and characteristics of very long wavelength InAs/InGaSb strained layer superlattice photodiodes are presented. These photodiodes have cutoff wavelengths ranging from 12 to longer than 15 microns, and are among the longest wavelengths reported for photovoltaic detectors made using this material system. Structural, optical and electrical properties are reported. Measured quantum efficiencies are as high as 10% at 10µm for a 0.7µm thick structure at 77K. The absorption coefficients are excellent, however, the electrical properties still need improvement.

INTRODUCTION The detection of infrared signals in the long wavelength range, 8-12 microns and beyond, is important for commercial and military applications. HgCdTe has been the dominant material system for such applications for more than the past two decades. Despite considerable progress, this material system is difficult to work with and has problems with uniformity and stability of the epitaxially grown material as well as large dark currents and short lifetimes[1,2], especially at very long wavelengths. Infrared detectors using III-V semiconductor compounds instead of HgCdTe have been available through band-gap engineering techniques in recent years[3]. However, the inherent ultra-short lifetime due to phonon scattering (< 50 ps) and high thermal generation rates limit the quantum efficiency and operating temperature [3,4]. The intrasubband transitions for n-type quantum wells (QWs) are forbidden except for incident light with a z polarization parallel to the growth direction, which complicates the fabrication techniques. Furthermore, the photoconductor structure strongly increases the dark current. The InAs/InGaSb type-II strained layer superlattice(SLS), was proposed in the late 80’s by Smith and Mailhiot[5] as an alternative for HgCdTe for long wavelength infrared detection. The superlattice consists of alternating thin (nm scale) layers of semiconductors in which the conduction band of one layer is below the valence band of the other (type II band alignment). This band alignment, in conjunction with thin layers that allow the electron and hole wavefunctions to overlap, lead to the formation of energy bands with transition energies that can be designed to correspond to a wide range of cutoff wavelengths including very long wavelength infrared. The epitaxial growth of these materials is not as mature as other III-V compounds, but this structure has shown promise as a long-wavelength IR photodiode material[6]. The absorption coefficients are comparable to HgCdTe and the structures promise smaller dark currents for long wavelength IR detectors due to a larger effective mass and longer minority carrier lifetimes due to suppressed Auger recombination[5]. Recently, significant progress has been made in the epitaxial growth of these superlattices[7] and the use of these superlattices in photodiodes[8,9] . Low background material has been grown by molecular beam epitaxy (MBE). It has also been shown that the reduction of crystalline defects in the material plays a significant role in the dark current. For small area diodes, the sidewall leakage also contributes significantly to the dark current, but can be improved with proper sidewall passivation. Photoconductors detecting at wavelengths longer than 20microns have been demonstrated [10], but most of the progress in photodiodes has been for structures less than 11microns[11]. In this paper, we report on the growth of photodiodes at wavelengths greater than 12 microns.

Photodetectors: Materials and Devices VI, Gail J. Brown, Manijeh Razeghi, Editors, Proceedings of SPIE Vol. 4288 (2001) © 2001 SPIE · 0277-786X/01/$15.00

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DESIGN AND THEORY The type-II strained layer superlattice consists of multiple thin layers of suitable semiconductor materials; in this case InAs and InxGa1-xSb epitaxially grown on a GaSb substrate. The conduction band of bulk InAs lies below the valence band of bulk GaSb. The band structure of the superlattice is illustrated in Fig. 1. When the carriers are confined within a superlattice the energy levels shift, but due to the type-II band alignment, the cutoff wavelength can be designed for very long cutoff wavelengths through the proper choice of layer thicknesses and compositions. Due to the number of design parameters (constituent layer thickness and composition, interface types, strain, etc.) and the number of properties affected by the superlattice design (cutoff wavelength, absorption coefficient, effective mass, Auger recombination, etc.), numerical models provide essential guidance for the design of the superlattice materials. An 8x8 k·p finite element program was developed to model the electronic band structure, wavefunctions, and optical matrix elements of multilayer structures and is discussed in more detail in previous publications[10,12]. Using this model, the absorption coefficient as a function of wavelength of an SLS is calculated and shown in figure 2 for an InAs layer thickness of 42Å and an In 0.23Ga0.77Sb thickness of 20Å. For these narrow gap structures, the calculated energy bands are sensitive to small variations in the calculation parameters. Nevertheless, good agreement has been achieved between modeling and measured cutoff wavelengths for a wide range of structures[9]. For these calculations, the effect of the interface layers was not considered. The interface layers play a significant role in the quality and net strain of the superlattices, but their effect on the cutoff wavelength is difficult to model.

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Figure 2. A calculated absorption spectrum for an InAs (42Å)/In 0.23Ga0.77Sb(20Å) superlattice. The calculated energy gap is 98meV correpsonding to a cutoff wavelength of 12.6µm. 184

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Figure 3 shows the calculated cutoff wavelength changes as a function of the layer thicknesses and composition. There is a wide range of layer combinations that can be used to achieve a particular cutoff wavelength. Other constraints such as strain, absorption coefficient, and epitaxial growth considerations are used to determine which combination of layer thickness is used. The data show that for the range of thicknesses depicted in Fig. 3, one monolayer (~3Å) change in the thickness of one of the layers results in more than 1 µm shift in the cutoff wavelength. Likewise, a change in the composition of 1% (absolute) results in an anticipated shift of more than a half micron. Molecular beam epitaxy has a demonstrated capability to achieve thickness and composition uniformity of 1% relative, implying that cutoff wavelength uniformity on the order of 0.1µm should be achievable. The contribution of the interfaces in not considered in the calculations. The range of thicknesses used in Fig. 3 are also nearly strain compensated, however the contribution of the interfaces cannot be neglected in the device optimization. InSb-like interfaces tend to drive the net strain to be more compressive whereas InGaAs like interfaces tend to drive the net strain tensile.

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c) Figure 3. The calculated cutoff wavelength of InAs/InGaSb superlattice designed for very long wavelength IR detection. a) shows the cutoff wavelength for an InAs thickness of 42Å as the In0.23G0.77aSb thickness varies. b) shows the cutoff wavelength as the InAs layer thickness varies for an In 0.23Ga0.77Sb layer thickness of 20Å. c) shows the cutoff wavelength as the In composition varies for an InAs layer thickness of 20Å and an InGaSb layer thickness of 42Å.


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EXPERIMENTAL The strained layer superlattice photodiodes were grown by molecular beam epitaxy (MBE) on GaSb (100) substrates. Two different designs were used. The first design used an InAs(43.6Å)/In 0.23Ga0.77Sb(17.2Å) SLS. The substrate and GaSb buffer layer were p-type. On top of this layer, a 0.7µm thick SLS was grown unintentionally doped. Subsequent to the active region, a graded n-type buffer layer was grown and was capped by and n-type InAs layer. The graded layer is thin and was designed to affect the electrical properties, but should not impact the absorption. The x-ray diffraction data are shown in figure 4. The substrate peak and the superlattice peaks are all clearly discernible and have narrow FWHM. There is a broad background that is observed and is believed to be due to the graded layers. The period is determined to be 61.8Å, which is slightly longer than the designed period. The net strain in the superlattice is 0.15% compressive. The second design used an InAs(45.4Å)/In0.23Ga0.77Sb (19.0Å) strained layer superlattice. The substrate was p-type GaSb and had a p-type GaSb buffer layer. On top of this layer the unintentionally doped 2µm thick SLS was grown. The structure was capped with an n-doped SLS layer and a heavily doped (10 18cm-3) InAs contact layer. The x-ray diffraction data is shown in Fig. 4b. The background signal is absent due to the lack of a graded buffer layer. The superlattice period is 62.0Å, which is slightly shorter than the designed period. The linewidths of the superlattice peaks are much broader than in sample a, possibly due to changes in period during the growth or poor interface quality. The net strain is 0.13% compressive. From this information, it is not possible to determine both the thickness and composition of the layers within the structure. Even if one value were known, the significant contribution of the interfaces to the net strain prevents more detailed determination of the actual structure from the x-ray data. The measured external quantum efficiency and relative response per photon for photodiode a) are shown in Fig. 5. The 50% cutoff wavelength is approximately 12µm. The quantum efficiency was measured using radiometric methods with optical bandpass filters. The relative spectral response was measured using a grating monochromator and comparing the signal to a reference detector. Accounting for reflection from the surface, the quantum efficiency is slightly less than would be expected based on the thickness of the device (0.7µm) and the calculated absorption coefficients. At short wavelengths, the quantum efficiency increases dramatically and has an internal quantum efficiency of nearly 50% if a top surface reflectivity of 30% is assumed. Thus the collection of photogenerated carriers must be quite good for this structure. Although the onset of this effect occurs at a different wavelength, it is consistent with the increase in absorption at short wavelengths shown in the calculation results of Fig. 2. This strong absorption is due to transitions between the valence band and a higher energy conduction band. The spectral response of photodiode b) is shown in Fig. 6. The 50% cutoff wavelength is seen to be near 15.5µm at 77K and is quite broad, extending to more than 18µm. A broad wavelength cutoff is often related to a short carrier diffusion length since the longer wavelength photons are absorbed deeper in the device and are collected less efficiently than carriers generated closer to the diode junction. The quantum efficiency was not measured radiometrically but is estimated to be several times less than sample a), despite having a substantially thicker absorption layer. The lower quantum efficiency of sample b) results from poorer electrical properties and carrier collection. The main reason is possibly lower material or interface quality. The superlattice period and cutoff wavelength are summarized in Table 1. It should be noted that the measured period and strain are nearly identical even though the cutoff wavelengths are significantly different. Based on the estimated sensitivities illustrated in Fig. 3, such a difference in wavelength could not be explained by the x-ray data. Due to the number of degrees of freedom in the layer design and the contributions of the interfaces, the period and strain cannot uniquely define the SLS energy gap, as is illustrated in Table 1.

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Table 1. A summary of the SLS parameters including the measured cutoff wavelength and the x-ray data.

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b) Figure 4. The x-ray rocking curves for the superlattices a)with and b)without the graded layers.


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Figure 6. The measured relative photoresponse of sample b as determined by FTIR spectroscopy. The 50% cutoff wavelength is 15.5µm and is measured near zero bias. The same curve is plotted as a function of

Sample a) was processed into a 150µm square sample for electrical measurements. The current-voltage curves are shown in Fig. 7 for temperatures ranging from 40K to 170K. Clear rectification of the signal is seen and there is an initial strong decrease in the reverse bias current with temperature. However, the leakage current becomes temperature insensitive below about 110K and is still significant even at 40K. The same trend can be seen by examining the area-normalized impedance at zero bias (R0A) as a function of temperature. These results are shown in Fig. 8 as a function of inverse temperature. At higher temperatures, R0A changes with temperature as would be expected for a diffusion limited diode with a bandgap energy of 80meV. The fitted line is shown in the figure. The bandgap energy is estimated to be closer to 95meV based on calculations and the measured absorption edge. As the temperature is decreased below approximately 100K, the slope changes and the impedance becomes temperature independent. This behavior is typical for trap assisted tunneling. The tunneling process dominates any generation-recombination mechanisms. Only a few devices were processed from this

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sample, but evidence from devices operating at shorter wavelengths indicates that sidewall leakage is likely the dominant mechanism for the leakage. For a comparable bandgap, RoA of these devices is an order of magnitude below that for passivated HgCdTe photodiodes at 77K. Extrapolating the diffusion limited regime to lower temperatures would result in an R0A comparable to HgCdTe.

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CONCLUSIONS Two different device structures were investigated with 50% cutoff wavelengths. One had a 50% cutoff wavelength of 12µm and very good absorption and carrier collection. The measured quantum efficiency agreed reasonably well with the calculated absorption coefficients. Although the quantum efficiency was excellent for such a thin device, the electrical properties are limited by trap assisted tunneling. Extrapolation of the diffusion limited case to lower temperatures suggests that the performance could be comparable to HgCdTe if the leakage mechanism can be identified and eliminated. The other structure had a 50% cutoff wavelength of 15.5µm with an absorption tail extending beyond 18µm; among the longest wavelengths reported for a photodiode in this material.

ACKNOWLEDGEMENTS The authors would like to acknowledge the AFRL/VSSS under contract#F29601-97-D-01444 for their support of this work. We would also like to thank John Hubbs and Ball Aerospace for providing radiometric quantum efficiency measurements.

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