Abstract
Compared with the monochromatic detector, the two-color detector has higher detection accuracy and is the typical representative of third-generation infrared detectors. Type-II superlattice has outstanding advantages such as wide absorption spectrum and high uniformity, and can be used to prepare two-color detectors. Annealing technology can improve the performance of two-color type-II superlattice devices by improving the quality of sidewall passivation and the quality of contact between materials and electrodes. By conducting a series of annealing experiments at different temperatures on the device, the effect of annealing on the performance of mid-/long-wavelength two-color type-II superlattice infrared focal plane devices is analyzed and studied. By optimizing the annealing process, the long-wavelength corresponding impedance of the pnp structure device can be increased by 5.6 times, and the long-wavelength corresponding impedance of the npn structure device can be increased by 31%. Compared with improving device performance by optimizing material structure and device structure, annealing has outstanding advantages such as high efficiency and simplicity. This study has certain reference significance for the performance improvement of mid-/long-wavelength two-color type-II superlattice infrared focal plane devices.
Keywords
0 Introduction
In the late 1980s, type II superlattices began to be used in the preparation of infrared detectors. Compared with traditional infrared detector materials, it has outstanding advantages such as wide absorption spectrum and high uniformity [1-4], so it is widely used in the preparation of infrared detectors. Related studies have shown that two-color detectors have higher detection accuracy than monochromatic detectors and can reduce false alarm rates [5]. Applying type II superlattices to the field of two-color detection can give full play to its performance advantages and thus prepare high-performance two-color infrared focal plane detectors. The latest research work reported so far has completed a pixel pitch of 12 Preparation of medium/long wave dual-color type II superlattice infrared focal plane detector with an array size of 1280 ×720 [6].
In order to achieve effective pixel isolation and improve the detector signal-to-noise ratio, the type II superlattice detector needs to form a mesa junction in the focal plane array [7-8]. Compared with the single-color device, the etching depth of the two-color device needs to be further deepened, so higher requirements are placed on the etching quality and passivation [9] and electrode preparation quality. This increases the difficulty of device preparation. Annealing technology can be used to optimize the contact between the material and the passivation layer and between the material and the electrode, thereby optimizing the device performance.
This paper studies the effect of annealing on the electrical properties of medium/ long-wavelength dual-color type II superlattice infrared focal plane devices with pnp and npn structures. By analyzing the changes in the electrical properties of the devices under annealing at different temperatures, the relevant annealing process is optimized. The results show that the long-wavelength corresponding impedance of the pnp structure device can be increased by 5.6 times, and the long-wavelength corresponding impedance of the npn structure device can be increased by 31%, achieving an improvement in device performance.
1 Experimental plan
Fig.1Schematic diagram of the position and structure of the two-color type II superlattice device in the circuit : (a) pnp structure; (b) npn structure.
Type II superlattice materials of pnp and npn structures grown on substrates were selected to prepare devices, so as to analyze the influence of annealing on devices with different structures. First, a mesa structure of corresponding depth was prepared by dry etching process, then a passivation layer was grown, electrode contact holes were prepared, electrodes were grown, and finally the device structure was prepared. The pixel spacing, array scale and preparation method of devices with different structures were consistent. The electrical properties of the prepared devices were tested by semiconductor analyzer, mainly I-V and impedance change with voltage test to ensure the normal electrical properties of the device itself . Then the device was placed in an annealing device and annealed at different temperatures in H2 and Ar environments. During the annealing process, the device state was ensured to be stable, and the time of each annealing was controlled and kept consistent.
Figure 1 shows the position and structure of two different types of dual-color type II superlattice devices in the circuit. They use two back-to-back structures, pnp and npn, that is, diode structures with opposite directions. The response to infrared light of different bands is achieved by setting corresponding absorption layers in different layers. In the table structure, the long-wave response area is in the upper part and the medium-wave response area is in the lower part, which can realize medium/long-wave dual-color infrared detection.
2 Experimental Results
Fig.2I-V and impedance curves of the pnp structure two-color type II superlattice device as a function of voltage : (a) before annealing; (b) after annealing.
The prepared pnp structure two-color type II superlattice device was annealed at a certain temperature. The device was tested for I-V and impedance variation with voltage before and after annealing. Figure2 (a) is a graph of the device I-V and impedance variation with voltage before annealing, and Figure2 (b) is a graph of the device I-V and impedance variation with voltage after annealing.
I-V curves before and after annealing and the test curves of impedance versus voltage, it can be seen that the long-wave corresponding impedance in the impedance versus voltage curve after annealing increases significantly, with an increase of up to 5.6 times, from 1.6 MΩ increased to 9 MΩ; after annealing, an obvious flat area appears on the long wave side of the I-V curve, and the impedance corresponding to the medium wave in the impedance change with voltage test curve decreases slightly . The current corresponding to the flat area of the medium wave of the I-V curve increases by about one time, and the flat area corresponding to the medium wave decreases. The reasons for this phenomenon are analyzed below.
The passivation layer grown in the long-wave response detection area is located in the sidewall area of the table. Due to the inclination of the sidewall, its smoothness is somewhat lower than that of the bottom, resulting in a certain degree of decrease in the passivation coverage quality of the sidewall compared to the bottom. Annealing will make the passivation layer have a closer contact with the sidewall of the table, thereby improving the passivation coverage quality of the long-wave response area. Compared with the performance degradation caused by annealing excited surface states [10], the improvement of passivation quality at this time is dominant, which improves the overall long-wave detection performance.
Compared with the long-wave response area, the passivation quality of the medium-wave response area is less affected by annealing. In addition, since the deep mesa structure is a positive trapezoidal structure, the medium-wave detection area below has a higher duty cycle, so the annealing excited surface state has a greater impact on the medium-wave detection area, resulting in a slight decrease in the medium-wave corresponding impedance and a certain degree of increase in the current. However, for the optimization of two-color detection performance, it is tolerable to greatly improve the long-wave performance and slightly reduce the medium-wave performance.
At the same time, annealing will reduce the Schottky barrier between the material and the electrode, thereby weakening the impact of the Schottky barrier on device performance, improving the contact quality between the material and the electrode, reducing contact resistance, and improving device performance.
Fig.3I-V and impedance curves of the pnp structure two-color type II superlattice device after the annealing temperature is further increased.
By controlling a single variable and increasing the annealing temperature of the prepared pnp structure two-color type II superlattice device, it was annealed. After annealing, the device was tested for I-V and impedance variation with voltage (results shown in Figure3) . As can be seen from Figure3, the impedance corresponding to the long wave did not increase further ( but decreased together with the impedance corresponding to the medium wave) , from 9 MΩ down to 6 MΩ; after annealing, the flat area on the long-wave side of the I-V curve becomes significantly smaller and the current corresponding to the medium-wave of the I-V curve further increases, and the flat area on the medium-wave side also begins to decrease. This shows that with the increase of annealing temperature, after the long-wave performance of the device is improved to a certain extent through annealing, the passivation quality improvement has reached its limit and there is no room for further improvement. At this time, the surface state plays a leading role, causing the device performance to deteriorate at a specific temperature, which also verifies that the device performance will only be improved by annealing at a specific temperature.
Fig.4I-V and impedance curves of the npn structure two-color type II superlattice device as a function of voltage: (a) before annealing; (b) after annealing.
The above results show that the medium and long wave detection performance of the two-color detection device can be effectively improved by applying the annealing process to the pnp structure two-color type II superlattice device. The same annealing process is used to anneal the npn structure two-color type II superlattice device, and the results are analyzed to verify whether the process is applicable to the npn structure two-color type II superlattice device.
Figure4 (a) is a graph showing the I-V and impedance variation with voltage test of the npn structure two-color type II superlattice device before annealing, and Figure4 (b) is a graph showing the I-V and impedance variation with voltage test of the npn structure two-color type II superlattice device after annealing.
By comparing I-V curves before and after annealing and the test curves of impedance variation with voltage, it can be seen that the long wave in the impedance variation curve corresponds to the impedance from 1.6 before annealing to MΩ increased to 2.1 after annealing MΩ, there is no obvious flat area in the long-wave corresponding area of the I-V curve, the medium-wave corresponding current increases by about one-fold, and the flat area corresponding to the medium-wave slightly decreases. This shows that the annealing effect of npn structure devices and pnp structure devices is not the same, and the long-wave corresponding impedance increases by about 31%, which is a small increase.
npn structure devices is the same as that of pnp structure devices, but the extent of performance improvement is different. This is related to the effect of annealing on device performance: on the one hand, annealing will affect the passivation contact effect, and on the other hand, annealing will affect the surface state of the device. In addition, the potential barriers between npn structure type II superlattice and pnp structure type II superlattice as different types of materials and electrode metals are also different. The surface states of npn structure type II superlattice materials and pnp structure type II superlattice materials are also different, which will lead to different annealing effects of different types of materials.
3 Conclusion
By analyzing the I-V and impedance-voltage curves of pnp and npn structure two-color type II superlattice devices before and after annealing, it can be seen that annealing can improve long-wave performance without significantly sacrificing medium-wave performance. In particular, the pnp structure device can increase the long-wave corresponding impedance by 5.6 times after annealing, and the npn structure device can increase the long-wave corresponding impedance by 31%, thereby improving the overall performance of the two-color type II superlattice device. Compared with improving device performance by optimizing material structure and device structure, annealing has outstanding advantages such as high efficiency and simplicity, and provides a way to optimize the performance of other two-color devices. In the future, annealing experiments in other atmospheres will be systematically carried out to analyze the mechanism of the annealing process in more depth, so as to further optimize device performance.