Demonstration of Planar Type-II Superlattice-Based Photodetectors Using Silicon Ion-Implantation

: In this letter, we report the demonstration of a pBn planar mid-wavelength infrared photodetectors based on type-II InAs / InAs 1 − x Sb x superlattices, using silicon ion-implantation to isolate the devices. At 77 K the photodetectors exhibited peak responsivity of 0.76 A / W at 3.8 µ m, corresponding to a quantum e ﬃ ciency, without anti-reﬂection coating, of 21.5% under an applied bias of + 40 mV with a 100% cut-o ﬀ wavelength of 4.6 µ m. With a dark current density of 5.21 × 10 − 6 A / cm 2 , under + 40 mV applied bias and at 77 K, the photodetector exhibited a speciﬁc detectivity of 4.95 × 10 11 cm · Hz 1 / 2 / W.

CF 4 :Ar + plasma dry etching in an electron cyclotron resonance-reactive ion etching (ECR-RIE) system ( Figure 1b). This SiO 2 hard-mask is able to completely block the implanted ions from reaching the device active region and top metal contact, as determined using simulation with the stopping range of ions in matter (SRIM) software. The resulting photodetectors ( Figure 1) are circular with diameters ranging from 100 µm to 400 µm. a plasma of SiH4 and N2O gases (Figure 1a). The hard-mask was then lithographically patterned using CF4:Ar + plasma dry etching in an electron cyclotron resonance-reactive ion etching (ECR-RIE) system ( Figure 1b). This SiO2 hard-mask is able to completely block the implanted ions from reaching the device active region and top metal contact, as determined using simulation with the stopping range of ions in matter (SRIM) software. The resulting photodetectors ( Figure 1) are circular with diameters ranging from 100 µm to 400 µm. The wafer was then cut into individual test-pieces that were ion-implanted by Innovion Corporation with typical ion flux, a substrate tilt of 7°, and without substrate cooling. The same SRIM simulation tool was also used to try and estimate the implantation energy and dose required to generate isolation regions. However, due to the complexities of superlattice materials and the lack of empirical data for T2SL in SRIM, it is hard to accurately predict the ion-implantation profile. With silicon as the implanting ion, three different ion-implantation energies (IEng) of 380, 190, and 100 KeV were used and each implantation was performed on different pieces with ion-implantation doses (IDose) of 1.0 × 10 15 , 5.0 × 10 14 , and 1.0 × 10 14 cm −2 , for a total of 9 permutations. A schematic diagram of the device structure and area of ion-implantation are shown in Figure 1e.
After implantation, windows were opened in the hard mask to expose the top metal contact using ECR-RIE plasma dry etching ( Figure 1c) and devices were then wire-bonded ( Figure 1d) to a 68-pin leadless chip carrier. No annealing or other post-implantation thermal treatment was performed. In order to give meaningful comparison, a standard mesa-isolated photodiode was also processed from the same wafer using our standard mesa-isolated photodiode fabrication process, as detailed elsewhere [35]. After the fabrication, the planar and mesa-isolated T2SL devices were loaded into a cryostat and tested optically and electrically.
The dark current densities (JD) of the PT2SL devices with different ion-implantation conditions were measured and compared to the traditional mesa-isolated device for more comprehensive comparison ( Figure 2). The dose and energy must be above some threshold to effectively create isolation regions. The JD values versus temperature under +40 mV applied bias (Vb) are demonstrated in Figure 2 for different IDose (a) and different IEng (b). The wafer was then cut into individual test-pieces that were ion-implanted by Innovion Corporation with typical ion flux, a substrate tilt of 7 • , and without substrate cooling. The same SRIM simulation tool was also used to try and estimate the implantation energy and dose required to generate isolation regions. However, due to the complexities of superlattice materials and the lack of empirical data for T2SL in SRIM, it is hard to accurately predict the ion-implantation profile. With silicon as the implanting ion, three different ion-implantation energies (I Eng ) of 380, 190, and 100 KeV were used and each implantation was performed on different pieces with ion-implantation doses (I Dose ) of 1.0 × 10 15 , 5.0 × 10 14 , and 1.0 × 10 14 cm −2 , for a total of 9 permutations. A schematic diagram of the device structure and area of ion-implantation are shown in Figure 1e.
After implantation, windows were opened in the hard mask to expose the top metal contact using ECR-RIE plasma dry etching ( Figure 1c) and devices were then wire-bonded ( Figure 1d) to a 68-pin leadless chip carrier. No annealing or other post-implantation thermal treatment was performed. In order to give meaningful comparison, a standard mesa-isolated photodiode was also processed from the same wafer using our standard mesa-isolated photodiode fabrication process, as detailed elsewhere [35]. After the fabrication, the planar and mesa-isolated T2SL devices were loaded into a cryostat and tested optically and electrically.
The dark current densities (J D ) of the PT2SL devices with different ion-implantation conditions were measured and compared to the traditional mesa-isolated device for more comprehensive comparison ( Figure 2). The dose and energy must be above some threshold to effectively create isolation regions.   (Figure 2b) leads to a reduced dark current density. At higher implantation energy each impinging ion can generate more damage to the structure and can penetrate deeper causing more effective device isolation. At a higher IDose the dark current reduction is due to more ions being available to counteract the epitaxial doping and more ions causing more damage to the T2SL [26]. The best electrical performance was achieved with IEng = 380 KeV and IDose = 1.0 × 10 15 cm −2 . However, even with these conditions the dark current is still slightly higher than that of the reference mesa-isolated device at 77 K. For a higher temperature of 150 K the difference is higher, which is not desirable. The performance degradation at a high temperature operation of the P2TSL device is probably related to the nature of the defects created by the ionimplantation process [26]. While not a superior approach, this could still be a promising result for the development of the ion-implantation process for planar FPAs.
Based on the SRIM simulations for the optimized condition (IEng = 380 KeV and IDose = 1.0 × 10 15 cm −2 ), the depth of the ion concentration peak inside the T2SL material was estimated to be 1 µm with a straggle of 115 nm. This corresponds to ions affecting the entire top p-contact, barrier, and reaching 150 nm into the n-type MWIR absorption region. Increasing the implantation dose and energy further may yield a further reduction of the dark current however, the energy/dose of bombardment ions cannot be very low or isolation will not be attained, nor can they be unlimitedly high, or damagerelated conduction effects or hopping conduction effects could increase the dark current [26,27]. This risk is borne out by the comparatively larger dark current at a high temperature, which may be related to the nature of the defects created by the ion-implantation process [26], which must be addressed via further optimization. It is worth noting that further study in simulation and experimental procedure of different aspects of ion-implantation on T2SL material is an appealing subject of study for future direction of this research but it is out of the scope of the present work. The intention of presenting different aspects of ion-implantation in this work is solely to draw a guideline for future work.
At Vb = +40 mV and T = 77 K, a 200 µm diameter optimized PT2SL device (IEng = 380KeV and IDose = 1.0 × 10 15 cm −2 ) shows the JD of 5.21 × 10 −6 A/cm2. If the temperature is increased to 100 K the JD goes to 4.10 × 10 −5 A/cm 2 . At 150 K, it increases to 2.68 × 10 −3 A/cm 2 (Figure 3a). An Arrhenius plot of the differential resistance area product at zero bias (R0 × A) indicates that the dark current is dominated by different mechanisms in different temperature regimes (Figure 3b). Above 100 K, the detector is diffusion limited with an activation energy (Eactivation) of 230 meV. This Eactivation is very close to the expected bandgap of the MWIR InAs/InAs1−xSbx superlattices in this temperature regime. From 100 to 77 K, the detector becomes generation-recombination limited with an Eactivation of 108 meV.  (Figure 2b) leads to a reduced dark current density. At higher implantation energy each impinging ion can generate more damage to the structure and can penetrate deeper causing more effective device isolation. At a higher I Dose the dark current reduction is due to more ions being available to counteract the epitaxial doping and more ions causing more damage to the T2SL [26]. The best electrical performance was achieved with I Eng = 380 KeV and I Dose = 1.0 × 10 15 cm −2 . However, even with these conditions the dark current is still slightly higher than that of the reference mesa-isolated device at 77 K. For a higher temperature of 150 K the difference is higher, which is not desirable. The performance degradation at a high temperature operation of the P2TSL device is probably related to the nature of the defects created by the ion-implantation process [26]. While not a superior approach, this could still be a promising result for the development of the ion-implantation process for planar FPAs.
Based on the SRIM simulations for the optimized condition (I Eng = 380 KeV and I Dose = 1.0 × 10 15 cm −2 ), the depth of the ion concentration peak inside the T2SL material was estimated to be 1 µm with a straggle of 115 nm. This corresponds to ions affecting the entire top p-contact, barrier, and reaching 150 nm into the n-type MWIR absorption region. Increasing the implantation dose and energy further may yield a further reduction of the dark current however, the energy/dose of bombardment ions cannot be very low or isolation will not be attained, nor can they be unlimitedly high, or damage-related conduction effects or hopping conduction effects could increase the dark current [26,27]. This risk is borne out by the comparatively larger dark current at a high temperature, which may be related to the nature of the defects created by the ion-implantation process [26], which must be addressed via further optimization. It is worth noting that further study in simulation and experimental procedure of different aspects of ion-implantation on T2SL material is an appealing subject of study for future direction of this research but it is out of the scope of the present work. The intention of presenting different aspects of ion-implantation in this work is solely to draw a guideline for future work.
At V b = +40 mV and T = 77 K, a 200 µm diameter optimized PT2SL device (I Eng = 380 KeV and I Dose = 1.0 × 10 15 cm −2 ) shows the JD of 5.21 × 10 −6 A/cm2. If the temperature is increased to 100 K the J D goes to 4.10 × 10 −5 A/cm 2 . At 150 K, it increases to 2.68 × 10 −3 A/cm 2 (Figure 3a). An Arrhenius plot of the differential resistance area product at zero bias (R 0 × A) indicates that the dark current is dominated by different mechanisms in different temperature regimes (Figure 3b). Above 100 K, the detector is diffusion limited with an activation energy (E activation ) of 230 meV. This E activation is very close to the expected bandgap of the MWIR InAs/InAs 1−x Sb x superlattices in this temperature regime. From 100 to 77 K, the detector becomes generation-recombination limited with an E activation of 108 meV. The devices were optically characterized under front-side-illumination ( Figure 4) using a calibrated 1000 °C blackbody source along with a Fourier transform infrared (FTIR) spectrometer (Bruker IFS 66 v/S). No anti-refection (AR) coatings were applied to the device. The devices can operate unbiased, but it can still operate up to +40 mV before it gets saturated (Figure 4b). At 77 K and +40 mV bias, the 100% cut-off wavelength of the MWIR PT2SL device was 4.6 µm with a peak responsivity of 0.76 A/W at 3.8 µm, corresponding to a quantum efficiency (QE) of 21.5% (Figure 4a). At 150 K, the peak responsivity increases to 1.09 A/W, corresponding to a QE of 32.6%. These values are only slightly lower (17% lower at 150 k and 12% lower at 77 K) than those of the reference mesaisolated device (Table 1). We speculate that this trend could be related to the optical contribution of the sloped mesa sidewalls in acting like mirrors and slightly enhancing the QE of mesa-isolated devices. It is worth noting that we tested the uniformity of the performances of the planar devices and the level of electrical isolation was not fully tested.  The devices were optically characterized under front-side-illumination ( Figure 4) using a calibrated 1000 • C blackbody source along with a Fourier transform infrared (FTIR) spectrometer (Bruker IFS 66 v/S). No anti-refection (AR) coatings were applied to the device. The devices can operate unbiased, but it can still operate up to +40 mV before it gets saturated (Figure 4b). At 77 K and +40 mV bias, the 100% cut-off wavelength of the MWIR PT2SL device was 4.6 µm with a peak responsivity of 0.76 A/W at 3.8 µm, corresponding to a quantum efficiency (QE) of 21.5% (Figure 4a). At 150 K, the peak responsivity increases to 1.09 A/W, corresponding to a QE of 32.6%. These values are only slightly lower (17% lower at 150 k and 12% lower at 77 K) than those of the reference mesa-isolated device ( Table 1). We speculate that this trend could be related to the optical contribution of the sloped mesa sidewalls in acting like mirrors and slightly enhancing the QE of mesa-isolated devices. It is worth noting that we tested the uniformity of the performances of the planar devices and the level of electrical isolation was not fully tested. The devices were optically characterized under front-side-illumination ( Figure 4) using a calibrated 1000 °C blackbody source along with a Fourier transform infrared (FTIR) spectrometer (Bruker IFS 66 v/S). No anti-refection (AR) coatings were applied to the device. The devices can operate unbiased, but it can still operate up to +40 mV before it gets saturated (Figure 4b). At 77 K and +40 mV bias, the 100% cut-off wavelength of the MWIR PT2SL device was 4.6 µm with a peak responsivity of 0.76 A/W at 3.8 µm, corresponding to a quantum efficiency (QE) of 21.5% (Figure 4a). At 150 K, the peak responsivity increases to 1.09 A/W, corresponding to a QE of 32.6%. These values are only slightly lower (17% lower at 150 k and 12% lower at 77 K) than those of the reference mesaisolated device (Table 1). We speculate that this trend could be related to the optical contribution of the sloped mesa sidewalls in acting like mirrors and slightly enhancing the QE of mesa-isolated devices. It is worth noting that we tested the uniformity of the performances of the planar devices and the level of electrical isolation was not fully tested.   In order to ensure the ion-implantation is fully isolating, the diodes optical characterization of different size diodes was performed (Figure 4c). At all operating temperatures, the QE values are similar across a broad range of diode sizes (Figure 4c) and this is indicative that the ion-implantation is effective at defining the device active region and suggests this approach will scale down to pixel-size diodes and be a promising approach for FPA applications.
In order to estimate the detective performance of these novel planar devices in system applications, the specific detectivity (D*) was estimated assuming the device is shot noise-limited ( Figure 5). At 150 K the planar device has a D* of~3.37 × 10 10 cm·Hz 1/2 /W at 3.8 µm, compared to~1.0 × 10 11 cm·Hz 1/2 /W for the mesa-isolated reference device. When the temperature is decreased to 77 K these D* values become even closer, with~4.95 × 10 11 cm·Hz 1/2 /W for planar device and~1.10 × 10 12 cm·Hz 1/2 /W for the mesa-isolated device. These detectivities are comparable to the mesa-isolated reference device and with further optimization of the ion-implantation process as well as refinement to the design of the structure, it should be possible to achieve parity in these 200 µm diameter devices and possibly superior performance in pixel-size devices that are currently dominated by mesa-sidewall leakage.
Photonics 2020, 7, x FOR PEER REVIEW 6 of 8 In order to ensure the ion-implantation is fully isolating, the diodes optical characterization of different size diodes was performed (Figure 4c). At all operating temperatures, the QE values are similar across a broad range of diode sizes (Figure 4c) and this is indicative that the ion-implantation is effective at defining the device active region and suggests this approach will scale down to pixelsize diodes and be a promising approach for FPA applications.
In order to estimate the detective performance of these novel planar devices in system applications, the specific detectivity (D*) was estimated assuming the device is shot noise-limited ( Figure 5). At 150 K the planar device has a D* of ~3.37 × 10 10 ⸱ cm Hz 1/2 /W at 3.8 µm, compared to ~1.0 × 10 11 ⸱ cm Hz 1/2 /W for the mesa-isolated reference device. When the temperature is decreased to 77 K these D* values become even closer, with ~4.95 × 10 11 ⸱ cm Hz 1/2 /W for planar device and ~1.10 × 10 12 ⸱ cm Hz 1/2 /W for the mesa-isolated device. These detectivities are comparable to the mesa-isolated reference device and with further optimization of the ion-implantation process as well as refinement to the design of the structure, it should be possible to achieve parity in these 200 µm diameter devices and possibly superior performance in pixel-size devices that are currently dominated by mesasidewall leakage. Calculation is based on the shot noise limited detectivity equation in the inset where q is the charge of the electron, kb is Boltzmann's constant, J is the dark current density at the applied bias (Vb) of +40 mV, R × A is the differential resistance area product at Vb = +40 mV, Ri is the spectral responsivity, and T is the temperature.
In summary, we have reported silicon ion-implanted planar pBn MWIR photodetectors based on type-II InAs/InAs1−xSbx superlattices. Several ion-implantation energies and doses were studied and the best performance was obtained with IEng = 380 KeV and IDose = 1.0 × 10 15 cm −2 . At 77 K, this optimized planar photodetector exhibits a peak responsivity of 0.76 A/W at 3.8 µm, corresponding to a quantum efficiency of 21.5% under Vb = +40 mV. With a 77 K dark current density of 5.21 × 10 −6 A/cm 2 at the same +40 mV of applied bias, the photodetector exhibits a specific detectivity of 4.95 × 10 11 ⸱ cm Hz 1/2 /W. At 150 K, the optimized planar device's dark current density increases to 2.68 × 10 −3 A/cm 2 and a detectivity drops to 3.37 × 10 10 ⸱ cm Hz 1/2 /W.  Calculation is based on the shot noise limited detectivity equation in the inset where q is the charge of the electron, k b is Boltzmann's constant, J is the dark current density at the applied bias (V b ) of +40 mV, R × A is the differential resistance area product at V b = +40 mV, R i is the spectral responsivity, and T is the temperature.
In summary, we have reported silicon ion-implanted planar pBn MWIR photodetectors based on type-II InAs/InAs 1−x Sb x superlattices. Several ion-implantation energies and doses were studied and the best performance was obtained with I Eng = 380 KeV and I Dose = 1.0 × 10 15 cm −2 . At 77 K, this optimized planar photodetector exhibits a peak responsivity of 0.76 A/W at 3.8 µm, corresponding to a quantum efficiency of 21.5% under V b = +40 mV. With a 77 K dark current density of 5.21 × 10 −6 A/cm 2 at the same +40 mV of applied bias, the photodetector exhibits a specific detectivity of 4.95 × 10 11 cm·Hz 1/2 /W. At 150 K, the optimized planar device's dark current density increases to 2.68 × 10 −3 A/cm 2 and a detectivity drops to 3.37 × 10 10 cm·Hz 1/2 /W.