wavelength range are needed for space based applications such as pollution ... Figure 1 shows the typical x-ray diffraction pattern of the photodiode structures.
High performance type-I! InAs/GaSb superlattice photodiodes Hooman Mohseni, Yajun Wei, and Manijeh Razeghi Center for Quantum Devices, Electrical and Computer Engineering Department Northwestern University, Evanston, IL 60208
ABSTRACT We report on the demonstration of high performance p-i-n photodiodes based on type-I! InAs/GaSb superlattices operating in the very long wavelength infrared (VLWIR) range at 80 K. Material is grown by molecular beam epitaxy on GaSb substrates with excellent crystal quality as evidenced by xray diffraction and atomic force microscopy. The processed devices with a 50% cutoff wavelength of
222 tm
show a peak current responsivity about 5.5 A/W at 80 K. The use of binary layers in the
superlattice has significantly enhanced the uniformity and reproducibility of the energy gap. The 90 % to 10 % cut-off energy width of these devices is on the order of 2kT which is about four times smaller compared to the devices based on InAs/Ga1InSb superlattices. Similar photovoltaic devices with cut-off
wavelengths up to 25 im have been measured at 80 K. Our experimental results shows excellent uniformity over a three inch wafer area, indicating the possibility of VLWIR focal plane arrays based on type-TI superlattices.
Keywords: Infrared Detector, Long Wavelength, Superlattice, Type-Il, HgCdTe, Quantum Well.
1.INTRODUCTION Type-IT InAs/Ga1InSb superlattices and heterostructures are emerging as a new material for IR devices due to their flexibility for bandgap engineering and many successful lasers' and detectors2 based on this material system have been demonstrated in recent years. Such flexibility enables one to design the
bandgap over a wide IR range (from 3tm to above 3Oim) while the material is lattice matched to the GaSb substrate. Theoretical calculations3 predict that high performance type-Il infrared detectors in the very long wavelength infrared (VLWIR) range (X>14 rim) can be realized. Infrared detectors in this wavelength range are needed for space based applications such as pollution monitoring and space based astronomy. Currently, the only available detectors in the VLWIR range with high uniformity, high quantum efficiency, and high detectivity are extrinsic silicon detectors which operate below 10K. Consequently, a three-stage cryo-cooler is required which is heavy, bulky, and has a short lifetime. These drawbacks are especially important for space applications since they significantly increase the launch cost.
We have previously demonstrated the first type-Il photoconductive devices grown on GaAs substrate in the X=12 im to X=22 m range operating at 80K4. In this paper, we report on a series of high performance photovoltaic type-Il superlattice detectors which show the same excellent uniformity in the VLWIR range. The main advantage of photovoltaic detectors is their suitability for staring, twodimensional focal plane array (FPA) applications, where low current bias circuitry significantly reduces the array power and heat dissipation requirements.
2. THEORY AND MODELING A four-band k.p superlattice model is use for the calculation of the electrons and holes miniband energy profiles5. The cutoff wavelength of the superlattices were calculated from the energy gap of the rninibands at k0. The full energy band structures of the superlattices were also calculated with an eightband simulation program. The results of this simulation were used for the bandgap engineering of the superlattice for lower Auger recombination rates6'7 and higher dipole matrix elements.
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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The active layer of these p-i-n photodiodes is a short period superlattice and hence, the precise modeling of the device requires the modeling of the carrier transport in such superlattices. However, such modeling proved to be very complex and requires massive numerical calculations8. Therefore, we decided
to adopt the general models available for bulk semiconductors, for these superlattice based devices. Fortunately, the simulated results based on such modeling have shown good agreement with our
.
experimental Similarly, for the calculation of the quantum efficiency we used the calculation developed for bulk
devices9. The overall efficiency of the device is the summation of the efficiency of the n-type layer, depleted layer, and the p-type layer:
(1) 11=lln+11w+11p For a device illuminated from the n-side, the quantum efficiency for each of these layers can be calculated from:
aL —e 1_lfl=
(1-r)aL1
aL—1
-
.
.
(x
s,nh
( \ (x
L
(2) —aL1e
cosh—-
(3)
ip =
aLe}
11 = (1 — r){e_ _
}
(4)
where r is the surface reflectivity, a is the optical absorption coefficient, and L and Lh are the electron and
hole diffusion lengths. x,, x, and w are the thickness of the n-type, p-type, and the depleted layer respectively.
3. EXPERIMENTS
3.1 Growth The material is grown by an Intevac Modular Gen II molecular beam epitaxy (MBE) equipped with As and Sb valved cracker sources on p-type GaSh substrates. The photodiode structures were grown at 395°C according to a calibrated pyrometer. The detector structure contains a 1 m GaSb buffer/contact layer doped with Be (p-1x1O'8 cm 3) followed by a 0.5 tm thick InAs/GaSb:Be (p'—ixlO'8 cm3 to 3x10'7 cm3) superlattice. Then, a 2 m thick nominally undoped superlattice was grown and finally, a 0.5 m thick InAs:Si/GaSb (n-1x1O'8 cm3) superlattice was grown and capped with a 100 A thick InAs:Si (n2x10'8 cm3) top contact layer. The structures were annealed in situ at 450°C for 15 minutes under a high As flux.
3.2 Characterization Structural quality of the epitaxial layers was assessed using high resolution x-ray diffraction. Figure 1 shows the typical x-ray diffraction pattern of the photodiode structures. The mismatch between the average lattice constant of the superlattice and the GaSb substrate is about 0.017 %, while the full width at half maximum (FWHM) of the satellite peaks is about 30 arcsec.
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Omega (Degrees) Figure 1 High resolution x-ray diffraction pattern of a sample. The mismatch between the average lattice constant of the superlattice and the GaSb substrate is only about 0.0 17%, while the FWHM of the satellites is about 30 arcsec. The surface morphology of the samples was also studied with a Digital Instrument NanoScope lila atomic force microscope (AFM). Theoretical calculations show that surface and interface roughness lead to
defect-like energy states inside the superlattice energy gap and broadening of the band edges'°. Also, experimental results show a strong correlation between the surface roughness and electrical performance of InAs/Ga1InSb superlattice photodiodes". Figure 2 shows the gray-scale surface morphology of a sample. Wide atomic steps are visible which is an indication of excellent surface smoothness. We could routinely
grow samples with a root mean square (rms) surface roughness below 3 A over a 20 im x 20 im area which is among the best reported values for this material system.
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*5
50 15 ni 25 O8riM
1J1*
Figure 2. AFM
00 nti
gray-scale image of the surface of a sample shows wide atomic steps, indicating an
atomically flat surface over the 10 im xlO im area.
3.3 Processing and Measurement The samples were processed into 400 im x 400 tm mesas using standard lithography and wet etching. Ti/Au contacts were defined for top (150 m x 1 50 jim) and bottom (1 50 m x 300 pm) contacts with metal evaporation and liftoff techniques. No passivation or anti-reflection coating was used on the surface of the devices. Figure 3 shows the schematic diagram of the devices as well as the scanning electron microscope (SEM) image ofa processed device.
/
InAs/GaSb SL Top Contact InAs-n Contact Layer
Bottom Contact
(b) Figure 3 . The schematic diagram (a) and SEM image of a processed device (b).
The samples were then mounted to a cooper heatsink and attached to the cold finger of a helium refrigerator equipped with a temperature controller. The current-voltage (I-V) characterization of the devices was measured with a HP 4155A parameter analyzer at different temperatures as shown in Figure 4.
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10
a)
1 00K —A—-- 80K
a)
U I
1 02
-0.7
.
I
.
I
.
.
I
.
I
.
I
I
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1
0.2
Bias (Volts)
Figure 4. The measured dark current density of a photodiode with a a 50% cutoff wavelength at X=22tm at different temperatures.
Spectral photoresponse was measured using a Galaxy 3000 FTIR spectrometer system. The
samples were illuminated through the front side at normal incidence. Absolute response of the photodetectors was calculated using a blackbody test set, which is composed of a blackbody source (Mikron 305), preamplifier (EG&G PA-6), lock-in amplifier (EG&G 5209), and chopper system (Stanford Research System SR540). The responsivity of a device with 222.tm operating under a small reverse bias at T80K is shown in Figure 5.
7 6 5
4 rJD
C
3
2
0
4
6 8 10 12 14 16 18 20 22 24 26 Wavelength (nm)
Figure 5. The spectral responsivity of a device under a small reverse bias of VB-l2OmV at 80K. Dotted lines indicate quantum efficiencies of 30%, 50%, and 70%.
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4. RESULTS AND DISCUSSION The detectivity of the device D* can be calculated from the value of the RA product and the current responsivity R1:
D*=RI \
4kT
(5)
where k is the Boltzmann constant and T is the absolute temperature. Detectors with X=22tm show a peak current responsivity ofabout 5.5A/W and R0AO.l at T=50K leading to a Johnson noise limited detectivity ofabout 3.5xlO10cmHz"2/W.
In order to study the major components of the dark current at T=80K, the current-voltage characteristic of the devices was modeled. We used a similar formalism as reported in reference 1 3 . Figure
6 shows the measured and modeled current densities versus the applied bias for devices with 2=l6 m. The calculated current density shows a good agreement to the measured values for forward and reverse biases. We assumed an effective mass of me=O.O3 m0 for electrons and mh=O.4 m0 for holes based on previous theoretical calculations1° and experimental results2. Based on the experimental measurements on similar devices'2, we also assumed an electron mobility parallel to the growth direction of ie= 1000 cm2/Vs (the mobility of the holes is not significant in the diffusion current, since the device has a junction). Using C-V measurement, we extracted the unintentional background carrier concentration to be about p=2. lxlO'5 cm3. The fitting parameters for the model were carrier lifetime ;='c1=22O ns and generationrecombination lifetime in the depleted layer GR=°6 ns. In contrast to HgCdTe, tunneling is not significant even at high values of the reverse bias due to the higher effective mass of the electrons in type-TI superlattices. However, generation-recombination current is the dominant source of dark current for these devices at T=80K, and hence further improvement ofthe growth should increase RAand detectivity.
I 000 c'
E
0
;:io C,)
C
01 0.1 0.01 -0.40
-0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 Bias
Figure
(Volts)
6. The measured and calculated current density of a device with 2=l6m at 1=80K. Generation-
recombination is the main current component ofthe dark current at this temperature.
Our previous results from devices in the long wavelength JR range have shown that the four-band k.p model can accurately predict the transition energies between the conduction and different valance bands at k11=013. Figure 7 shows a good agreement between the transition energies calculated from the fourband model at critical values ofk1, and the optical response ofa photodiode at T=9.5K. The photoresponse increases at the onset ofa possible transition at k10 and decreases at the edge ofthe Brillouin zone.
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Energy (meV) 100
50
ci.)
C,)
00 U) ci)
4 6 810121416182022242628 Wavelength (pm)
Figure 7. The optical spectral response of a photodiode at T9.5K. The arrows indicate transition energies at the k1=0 and the edge of the Brillouin zone in the growth direction kinax calculated with a four-band model. The uniformity of the bandgap energy of these detectors is an important issue for infrared imaging applications. Such uniformity over the area of a focal plane array (FPA) leads to a lower value of NEAT especially when D*>10IcmHzl'2/W. Moreover, the uniformity of the bandgap over a pixel area leads to a sharp cutoff wavelength. We have previously shown that the uniformity of the bandgap energy over a three-inch wafer area is extremely good and is only about 5meV for a superlattice with X=22im4. Figure 8 shows the photoresponse of different photodiodes with cutoff wavelength from 1Otm to above 20im. The 90% to 10% cutoff energy of the responses are below 2kT for all of the devices, indicating an excellent bandgap uniformity over the detector area (400 im x 400 nm). This value is at least four times smaller compared to the similar devices based on InAs/Ga1InSb (0. 15
