InAs/GaSb - Wet Etching

InAs/GaSb - Wet Etching

Material Name: InAs/GaSb
Recipe No.: 10347
Primary Chemical Element in Material: In, Ga
Sample Type: Layer
Uses: Etching
Etchant Name: None
Etching Method: Wet etching
Etchant (Electrolyte) Composition: This study focuses on the sidewall roughness and crystallographic orientation selectivity of mid-wave and long-wave InAs/GaSb SL photodiodes while wet etching with etchants based on H3PO4/H2O2/H2O/C6H8O7. The etching of InAs and GaSb bulk materials is also examined in this work.
After photolithography of the MWIR and LWIR materials, a chemical solution based on H3PO4/H2O2/H2O/C6H8O7 was used for wet etching, where the phosphoric acid (85%), hydrogen peroxide (30%) and citric acid (100.0%) solutions were mixed in different proportions. Prior to wet etching, the samples were cleaned by acetone, absolute ethyl alcohol and deionized water to remove organic impurities, with a subsequent cleaning in hydrochloric acid (3.5%) to remove the surface oxides. The samples were then rinsed in deionized water and dried with N2 gas. The etching rates varied for the different solutions, and the sidewall angles and roughness also varied.

To understand the mechanisms of InAs/GaSb SLs wet mesa etching, the InAs and GaSb materials were etched separately by using H3PO4/H2O2/H2O and H3PO4/H2O2/H2O/C6H8O7 solutions in varying proportions for InAs and GaSb substrates, respectively. To analyze the process of chemically etching InAs/GaSb SL photodiodes under various conditions, we vary the proportions of the four wet chemical etchant components including H3PO4, H2O2, C6H8O7 and H2O, and observe the roughness of the mesa sidewalls and the etch depth (i.e., etching rates) via scanning electron microscopy (SEM). The sidewall roughness can be observed via the SEM images, while they cannot be used to measure the roughness. However, the sidewall roughness can be inferred from the surface roughness after etching, and the results of data from atomic force microscopy (AFM) and the SEM images also verify this relationship.

Firstly, we focus on the MWIR and study the role of hydrogen peroxide in the etchant, where the etching rates and the surface roughness are given in Table 2 and in Fig. 1(a). It can be seen that, when the hydrogen peroxide reaches a threshold value, it cannot influence the etching rate. Instead, after the hydrogen peroxide reaches its threshold value, the sidewall roughness increases with the proportions of hydrogen peroxide. Figure 1(b) shows an AFM image of the surface exhibiting the greatest and least amount of roughness.

Next, the roles of citric acid and phosphoric acid in etching the MWIR are studied, and the etching rates and sidewall images are given in Tables 3 and 4. It can be seen that the final etching rate is more correlated with the phosphoric acid and that increased smoothing of the sidewalls and surface occurs with increasing the proportions of citric acid. The corresponding experiments examining the variation of the etchant components on the LWIR InAs/GaSb SLs exhibit the same variational tendency in the etching rates and roughness values as those seen in the MWIR InAs/GaSb SLs.
The presence of complex MPO4 (i.e., InPO4, GaPO4, AsPO4 and SbPO4) and oxides on the surface, however, can strongly deteriorate the mesa surface sidewalls, thus citric acid is used to increase the smoothness of the sidewalls. After the experiment, we obtain an optimized chemical solution (C6H8O7:H3PO4:H2O2:H2O=0.75 g:1 ml:0.6 ml:2 ml) for MWIR InAs/GaSb SLs with an etching rate of 200 nm/min, which produces much smoother sidewalls than the previous etchant (citric acid solution :H3PO4 :H2O2 :H2O=1ml : 1ml : 2 ml : 20 ml, citric acid solution=C6H8O7 :H2O=1g : 1 ml).

In addition, the results of the InAs and GaSb materials show that InAs etchants without citric acid also produce sidewalls with excellent performances, and the GaSb etching requires a more highly concentrated solution than that previously used for InAs/GaSb SLs. Therefore, if we increase the amount of InAs in the SLs, we can reduce the amount of citric acid and, likewise, if the amount of GaSb is increased, an etchant with a higher concentration is needed. There is an obvious anisotropy seen in the etching profile of InAs materials, and in an attempt to reduce this anisotropy we tried different (100) substrates and etchants based upon H3PO4/H2O2/H2O, shown in the SEM image in Fig. 3(a). Although the angles and turning positions vary, the etching profiles are all in the same situation as that shown in Fig. 3(b). We now compare the four different kinds of materials including InAs, GaSb, MWIR SLs and LWIR SLs, whose profiles are shown in Fig. 4, where it can be seen that, even in the same crystallographic orientation, wet chemical etching of InAs and GaSb has markedly different profiles. From the SEM profiles of the MWIR and LWIR SLs, we can see that the profile of each InAs/GaSb SLs corresponds to that of its most prevalent component. For example, LWIR SLs with more InAs have the profile most similar to that of InAs bulk materials.
Procedure (Condition): No data
Note: The roughness and the crystallographic orientation selectivity of etched antimonide-based infrared materials are examined and are used to optimize the chemical mesa etching process of the InAs/GaSb superlattice photodiode with the goal of reducing the dark current. The etchant used is based on phosphoric acid (H3PO4), citric acid (C6H8O7) and hydrogen peroxide (H2O2). The roughness of the mesa sidewalls and etching rates are compared and used to find an optimized etchant.
Reference: HAO Hong-Yue, et al., Wet Chemical Etching of Antimonide-Based Infrared Materials, CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) pp. 107302-1 - 107302-4.

Table 1: The structures of MWIR and LWIR InAs/GaSb photodiodes.

Figure 1: (a) Surface roughness materials in different etchants. (b) AFM image of the surface.

Table 2: Etching rate with different volumes of hydrogen peroxide.

Table 3: Etching rate and surface roughness with increasing the citric acid.

Table 4: Etching rate in etchants with different phosphoric acids.

Figure 3: Nonisotropic of InAs wet etching shown by (a) SEM images and (b) plots.

Figure 4: Profiles of (a) InAs, (b) GaSb, (c) MWIR SLs and (d) LWIR SLs in the (011) crystallographic orientation.