1 Experiment
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 （H3 PO4 , 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 （H3 PO4 ） and hydrogen peroxide （H2 O2 ） are as follows,2GaSb + 6
H2 O2 → Ga2 O3 + Sb2 O3 + 6H2 O . （1）2InAs + 6
H2 O2 → In2 O3 + As2 O3 + 6H2 O . （2）InAs + 4
H2 O2 → InAsO4 + 4H2 O . （3）2
M2 O3 + 7H3 PO4 → M（H2 PO4 ）3 +M2 （HPO4 ）3 + MPO4 + 6H2 O. （4）
（M = Ga or As or Sb or In）
S
b2 O3 + 2C6 H8 O7 → 2（Sb（C6 H4 O7 ）（H2 O）） +H2 O + 2H+ . （5）
Among the above chemical reactions,
H2 O2 is the oxidizing agent. InAs and GaSb oxidized withH2 O2 firstly, then the products are dissolved in water or reacted withH3 PO4 . Sb2 O3 is poorly soluble in water orH3 PO4, while it can react withC6 H8 O7 to form a water-soluble complex. Therefore etchants containingC6 H8 O7 is necessary for GaSb, while etchants withoutC6 H8 O7 is feasible for InAs.The etching rate and surface roughness with different etchants for InAs bulk materials were shown in Table 1. When
H3 PO4 :H2 O2 = 1:1 and withoutC6 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 ofH3 PO4 :H2 O2 = 1:1, the surface roughness is increased with increasing the proportion ofC6 H8 O7 . The presence ofC6 H8 O7 does not improve the InAs mesa sidewalls morphology, similar to reports in the literature[18] . When the proportion ofH2 O2 is slightly more than that ofH3 PO4 , it has little effect on the surface roughness, while the surface roughness is increased with increasingH3 PO4 content. That is because ifH3 PO4 content is increased, the dihydrogen phosphate will further react withH3 PO4 , 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 morphology[15] .Table 1 Etching rate and surface roughness with different etchants for InAs bulk materials.
表1 InAs体材料表面腐蚀速率和粗糙度随腐蚀液组分和配比的变化
C6 H8 O7 :H3 PO4 :H2 O2 Etching rate
（μm/min）
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.
NOTE: 图2.（a）InAs体材料（b）GaSb体材料和（c）InAs基超晶格材料分别在使用优化的腐蚀工艺后，测得的腐蚀表面的AFM形貌图
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 O7 have to be contained as a complexing agent to react with Sb2 O3 to form a soluble product. WhenC6 H8 O7 :H3 PO4 :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 ofC6 H8 O7 . And the surface roughness is gradually increased with increasing the proportion ofH3 PO4 , while when the proportion ofH2 O2 is more than that ofH3 PO4 , 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 O2 was used as an oxidant,H3 PO4 was used to react with the oxide products andC6 H8 O7 was used as a complexing agent. The optimized proportion ofH2 O2 andH3 PO4 is around 1:1 and the proportion ofH2 O2 can be slightly more than that ofH3 PO4 . TheC6 H8 O7 content in the etching etchants is related to the Ga（As）Sb thickness ratio in InAs-based superlattice. KeepingH2 O2 :H3 PO4 = 1:1 and addingC6 H8 O7 , the etching rate and surface roughness with different etchants for InAs-based superlattices were investigated, as shown in Table 3. WhenC6 H8 O7 :H3 PO4 :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
表3 InAs基超晶格材料表面腐蚀速率和粗糙度随腐蚀液组分和配比的变化
C6 H8 O7 :H3 PO4 :H2 O2 Etching rate
（μm/min）
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 :H3 PO4 :H2 O2 = 3:1:1 （Sample 311）. At the same time, another sample was used for comparison that etched by the etchants ofC6 H8 O7 :H3 PO4 :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 :H3 PO4 :H2 O2 = 10:1:1 and （b）C6 H8 O7 :H3 PO4 :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 × 10-3 A/cm2 and 9.2 × 10-3 A/cm2 , respectively, under a bias of -20 mV at 81 K. The surface resistivityρSurface of two samples were calculated by a linear least squares fitting （see Figure 4 c） between theR0 A-1 （R0 A 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 :H3 PO4 :H2 O2 = 3:1:1 at 81 K （b） I-V characteristic for devices etched withC6 H8 O7 :H3 PO4 :H2 O2 = 10:1:1 （black dots） andC6 H8 O7 :H3 PO4 :H2 O2 = 3:1:1 （red dots） （c） The dependence ofR0 A-1 at 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
Where
R0 Abulk is the bulk differential-resistance-area-product, P is the perimeter of the diode mesa, and A is the cross-sectional area of the detector.ρSurface of sample 311 is 4.4 × 103 Ωcm, which is almost eight times larger than that （5.1 × 102 Ωcm） 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.3 Conclusion
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 O2 was used as an oxidant,H3 PO4 was used to react with the oxide products andC6 H8 O7 was used as a complexing agent. The optimized proportion ofH2 O2 andH3 PO4 is around 1:1 and the proportion ofH2 O2 can be slightly more than that ofH3 PO4 . TheC6 H8 O7 content 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 resistivityρSurface of the detector is 4.4 × 103 Ωcm.References
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Abstract
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
摘要
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（
Introduction
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
Contributions Statement
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.
