Wet chemical etching experiments were first carried out on InAs and GaSb bulk materials, all samples were processed into mesas using standard optical lithography and wet chemical etching with the chemical solution based on citric acid （
C6 H8 O7, 100 %）, orthophosphoric acid （ H3P O4, 85 %） and hydrogen peroxide （ H2 O2, 30 %）. The wet etching rate and roughness of mesa sidewalls were measured by step profiler and atomic force microscope （AFM）, respectively. Then the optimized etching solution was applied to fabricate single pixel InAs-based SL detectors. The InAs-based superlattices were grown by molecular beam epitaxy. The layered structure of the InAs-based T2SLs long wavelength infrared detector was shown in Figure 1, consisted of a 1 μm Si-doped InAs buffer layer, followed by a 50 period Si-doped 22 ML InAs/9 ML Ga（As）Sb n-type superlattice, a 200 period lightly Be-doped 22 ML InAs/9 ML Ga（As）Sb absorber region, a 50 period Be-doped 22 ML InAs/9 ML Ga（As）Sb p-type superlattice, and finally a 50 nm Be-doped GaSb cap layer. The detectors are designed to receive the irradiance from the front sides. The architecture of the single-pixel detectors can be found in our previous paper.
2 Result and discussion
2.1 Etching of InAs and GaSb bulk materials
The chemical reactions of InAs and GaSb etching with citric acid （
C6 H8 O7）, orthophosphoric acid （ H3P O4） and hydrogen peroxide （ H2 O2） are as follows,
2GaSb + 6
H2 O2→ G a2 O3+ S b2 O3+ 6 H2O . （1）
2InAs + 6
H2 O2→ I n2 O3+ A s2 O3+ 6 H2O . （2）
InAs + 4
H2 O2→ InAs O4+ 4 H2O . （3）
M2 O3+ 7 H3P O4→ M（ H2P O4 ）3+ M2（HP O4 ）3+ MP O4+ 6 H2O
（M = Ga or As or Sb or In）
b2 O3+ 2 C6 H8 O7→ 2（Sb（ C6 H4 O7）（ H2O）） + H2O + 2 H+ .
Among the above chemical reactions,
H2 O2is the oxidizing agent. InAs and GaSb oxidized with H2 O2firstly, then the products are dissolved in water or reacted with H3P O4. S b2 O3is poorly soluble in water or H3P O4,while it can react with C6 H8 O7to form a water-soluble complex. Therefore etchants containing C6 H8 O7is necessary for GaSb, while etchants without C6 H8 O7is feasible for InAs.
The etching rate and surface roughness with different etchants for InAs bulk materials were shown in Table 1. When
H3P O4: H2 O2= 1:1 and without C6 H8 O7, the surface is the smoothest and the roughness is only 0.4 nm, which was shown in Figure 2 （a）. While maintaining the ratio of H3P O4: H2 O2= 1:1, the surface roughness is increased with increasing the proportion of C6 H8 O7. The presence of C6 H8 O7does not improve the InAs mesa sidewalls morphology, similar to reports in the literatur e. When the proportion of H2 O2is slightly more than that of H3P O4, it has little effect on the surface roughness, while the surface roughness is increased with increasing H3P O4content. That is because if H3P O4content is increased, the dihydrogen phosphate will further react with H3P O4, which lead to form a poorly soluble salt （monohydrogen phosphate or normal phosphate）. The presence of these complexes will adsorb on the mesa sidewalls to form a dense film and prevent the etching reaction to continue and strongly deteriorate the mesa surface sidewalls morpholog y.
Table 1 Etching rate and surface roughness with different etchants for InAs bulk materials.
C6 H8 O7: H3P O4: H2 O2
Surface roughness （nm） 0:0.1:1 0.26 1.4 0:0.5:1 0.35 0.5 0:1:1 0.45 0.4 0:5:1 0.35 10.9 0:10:1 0.25 15.6 0.2:1:1 0.33 1.1 1:1:1 0.32 2.7
Fig. 2 AFM pictures of the etching surface of （a） InAs bulk material, （b） GaSb bulk material and （c） InAs-based superlattices with the optimized etchants, respectively.
The etching rate and surface roughness with different etchants for GaSb bulk materials were shown in Table 2. For the wet etching of GaSb,
C6 H8 O7have to be contained as a complexing agent to react with S b2 O3to form a soluble product. When C6 H8 O7: H3P O4: H2 O2= 10:1:1, the smoothest surface was obtained and the roughness is only 0.7 nm, which was shown in Figure 2 （b）. The surface roughness is gradually increased with reducing the proportion of C6 H8 O7. And the surface roughness is gradually increased with increasing the proportion of H3P O4, while when the proportion of H2 O2is more than that of H3P O4, the surface roughness change slightly. This is similar to the results of InAs bulk materials.
2.2 Etching of InAs-based superlattices
Through the above experiments, it was found that for InAs-based SL materials,
H2 O2was used as an oxidant, H3P O4was used to react with the oxide products and C6 H8 O7was used as a complexing agent. The optimized proportion of H2 O2and H3P O4is around 1:1 and the proportion of H2 O2can be slightly more than that of H3P O4. The C6 H8 O7content in the etching etchants is related to the Ga（As）Sb thickness ratio in InAs-based superlattice. Keeping H2 O2: H3P O4= 1:1 and adding C6 H8 O7, the etching rate and surface roughness with different etchants for InAs-based superlattices were investigated, as shown in Table 3. When C6 H8 O7: H3P O4: H2 O2= 3:1:1, the surface roughness is the smallest, only 1 nm. The AFM picture was shown in Figure 2 （c）.
Table 3 Etching rate and surface roughness with different etchants for InAs-based superlattices
C6 H8 O7: H3P O4: H2 O2
Surface roughness （nm） 10:1:1 0.32 8.3 3:1:1 0.45 1.0 1:1:1 1.2 3.5
The InAs-based superlattice LWIR detector was fabricated by the optimized etchants of
C6 H8 O7: H3P O4: H2 O2= 3:1:1 （Sample 311）. At the same time, another sample was used for comparison that etched by the etchants of C6 H8 O7: H3P O4: H2 O2= 10:1:1 （Sample 1011）. The SEM pictures of the InAs-based superlattice mesa sidewalls of （a） sample 1011 and （b） sample 311 were shown in Fig. 3. The etching surface of sample 311 is smoother than that of sample 1011.
Fig. 3 SEM pictures of the InAs-based SL photodetectors etched with （a）
C6 H8 O7: H3P O4: H2 O2= 10:1:1 and （b） C6 H8 O7: H3P O4: H2 O2= 3:1:1 at room temperature.
图3 在室温下，分别使用腐蚀液（a）C6H8O7:H3PO4:H2O2 = 10:1:1 和（b）C6H8O7:H3PO4:H2O2 = 3:1:1制备InAs基超晶格单元器件时，获得的器件腐蚀表面和侧壁的SEM图
Figure 4 （a） shows the current responsivity spectrum of the InAs-based SL detector measured at 81 K. The 50 % cut-off wavelength of the detectors reaches 12 μm. The fabricated photodiodes have a similar peak responsivity of 1.6 A/W at 81 K, corresponding to quantum efficiency （QE） of 38 %. Figure 4 （b） shows the dark current density and dynamic differential resistance-area product values （RA） of sample 311 （red dots） and sample 1011 （black dots） with mesa area of 200 × 200 μ
m2. The dark current density of sample 311 and sample 1011 are 5.7 × 1 0-3A/c m2and 9.2 × 1 0-3A/c m2, respectively, under a bias of -20 mV at 81 K. The surface resistivity ρSurfaceof two samples were calculated by a linear least squares fitting （see Figure 4 c） between the R0 A-1（ R0A denotes the differential-resistance-area-product at zero bias） of diodes and P/A ratio based on the following equation:
Fig. 4 （a） Current responsivity spectrum of detectors etched with
C6 H8 O7: H3P O4: H2 O2= 3:1:1 at 81 K （b） I-V characteristic for devices etched with C6 H8 O7: H3P O4: H2 O2= 10:1:1 （black dots） and C6 H8 O7: H3P O4: H2 O2= 3:1:1 （red dots） （c） The dependence of R0 A-1at zero bias on P/A ratio for the two detectors at 81 K.
图4 （a）腐蚀液为C6H8O7:H3PO4:H2O2 = 3:1:1时，在81 K下测得的器件的电流响应光谱；（b）在81 K下，腐蚀液分别为C6H8O7:H3PO4:H2O2 = 10:1:1（黑点）和C6H8O7:H3PO4:H2O2 = 3:1:1（红点）时，测得的暗电流密度和动态差分电阻面积乘积值（RA）随偏压的变化；（c）在81 K下，两个样品的R0A-1和P/A比之间的线性关系
R0 Abulkis the bulk differential-resistance-area-product, P is the perimeter of the diode mesa, and A is the cross-sectional area of the detector. ρSurfaceof sample 311 is 4.4 × 1 03Ωcm, which is almost eight times larger than that （5.1 × 1 02Ωcm） of sample 1011, indicating a good surface quality obtained by the optimized etchants and an InAs-based SL LWIR detector with enough low surface leakage currents has been fabricated.
Wet chemical etching of InAs-based InAs/Ga（As）Sb superlattice long wavelength infrared photodiodes was studied in this paper. The etching experiments using citric acid, orthophosphoric acid and hydrogen peroxide were carried out on InAs, GaSb bulk materials and InAs-based superlattices with different solution ratios.
H2 O2was used as an oxidant, H3P O4was used to react with the oxide products and C6 H8 O7was used as a complexing agent. The optimized proportion of H2 O2and H3P O4is around 1:1 and the proportion of H2 O2can be slightly more than that of H3P O4. The C6 H8 O7content in the etching etchants is related to the Ga（As）Sb thickness ratio in InAs-based superlattice. An optimized etching solution for the InAs-based superlattices has been obtained. The etched surface roughness is only 1 nm. The InAs-based LWIR detectors with 50 % cut-off wavelength of 12 μm were fabricated. The photodetectors etched with optimized solution ratio show low surface leakage characteristic. At 81 K, the surface resistivity ρSurfaceof the detector is 4.4 × 1 03Ωcm.
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Wet chemical etching of InAs-based InAs/Ga（As）Sb superlattice long wavelength infrared photodiodes was studied in this paper. The etching experiments using citric acid, orthophosphoric acid and hydrogen peroxide were carried out on InAs, GaSb bulk materials and InAs/Ga（As）Sb superlattices with different solution ratios. An optimized etching solution for the InAs-based superlattices has been obtained. The etched surface roughness is only 1 nm. InAs-based superlattice LWIR detectors with 50 % cut-off wavelength of 12 μm were fabricated. The photodetectors etched with optimized solution ratio show low surface leakage characteristic. At 81 K temperature, the surface resistivity
开展了InAs基InAs/Ga（As）Sb II类超晶格长波红外探测器的湿法腐蚀工艺研究.选择的腐蚀液由柠檬酸、磷酸和过氧化氢组成，先后在InAs、GaSb体材料和InAs/Ga（As）Sb II类超晶格上进行了湿法腐蚀实验，分别获得了其最佳的腐蚀液组分及配比.使用优化的磷酸系腐蚀液对InAs/Ga（As）Sb II类超晶格进行腐蚀，获得的腐蚀表面粗糙度仅为1 nm.然后使用改进的工艺制备了50 ％截止波长为12 μm的超晶格长波单元器件，实验结果表明磷酸系腐蚀液可以获得低暗电流密度的InAs基InAs/Ga（As）Sb II类超晶格长波红外探测器.另外，在81 K下，该探测器的表面电阻率（
Long wavelength infrared （LWIR） photo-detectors have important applications in the fields of geoexploration, marine and environmental monitoring, meteorological forecast, etc. InAs/GaSb Type-II superlattices （SLs） have showed excellent opto-electrical properties for infrared detection and high performance focal plane arrays based on this novel material have been demonstrated
Numerous wet chemical etchants have been investigated on GaSb-based InAs/GaSb type-II superlattice materials
This work was supported by the National Natural Science Foundation of China （NSFC） with Grant No. 61534006, 61505237, 61505235, 61404148, the National Key Research and Development Program of China with Grant No.2016YFB0402403 and the Natural Science Foundation of Shanghai with Grant No. 15ZR1445600 and No. 16ZR1447900.