Etching Kinetics of IIIâV Semiconductors Coupled with Surface

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Etching Kinetics of III-V Semiconductors Coupled with Surface Passivation Investigated by Scanning Electrochemical Microscopy Jie Zhang, Junhui Lai, Wei Wang, Pei Huang, JingChun Jia, Lianhuan Han, Zhao-Wu Tian, Zhong-Qun Tian, and Dongping Zhan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01868 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 15, 2017

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The Journal of Physical Chemistry

Etching Kinetics of III-V Semiconductors Coupled with Surface Passivation Investigated by Scanning Electrochemical Microscopy Jie Zhang,† Junhui Lai, Wei Wang, Pei Huang, Jingchun Jia, Lianhuan Han, Zhao-Wu tian, ZhongQun Tian, Dongping Zhan* State Key Laboratory of Physical Chemistry of Solid Surfaces (PCOSS), Collaborative Innovation Centre of Chemistry for Energy Materials (iChEM), and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

ABSTRACT Because of their unique physical and electric properties and chemical stability, the etching kinetics of III-V semiconductors is of great significance in the fabrication of functional microdevices. However, the produced oxides results in a surface passivation and hinders the etching progress. Taking into account the passivation and depassivation of the insoluble oxides, we investigated the etching kinetics of III-V semiconductors (i.e., n-GaAs, n-InP and n-GaP) by scanning electrochemical microscopy (SECM) as well as finite element method (FEM). By considering the coupling effect of mass transport process and surface reactions, a dynamic deformed geometry module was adopted to figure out both the etching rate and passivation rate individually and futher correlate the kinetic parameters to the topography of etching pits. The results show that the etching process will be slowed down or even be ceased with the increasing

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kinetic rate of surface passivation. Based on the component analysis of the passivation layer, ligands are suggested as additives to increse the solubility of the cations and to avoid the formation of metal oxides. By tuning the relative rates of the etching and passivation processes, the etching pit will be deepened with little increase of diameter. SECM is proved powerful in the kinetic investigation of complex etching system coupled with surface processes. This model allows the prediction of etching resolution and removal rate with consideration of the passivation effect, which is valuable for the electrochemical microfabrication (ECM) on III-V semiconductor wafers.

1. INTRODUCTION Because of the high saturated electron velocity, high electron mobility and appropriate band gap, III-V semiconductors including gallium arsenide (GaAs), gallium phosphide (GaP) and indium phosphide (InP), are widely used in microdevices such as light-emitting diodes,1 laser diodes,2 solar cells,3 and photocatalysis.4-6 To improve the performance of the microdevices, various three-dimentional micro/nano-structures (3D-MNS) are fabricated on the surface of the III-V semiconductors.7 For example, in the solar cell system, porous structures or nanowire arrays were fabricated on GaAs surface to increase the light-harvesting efficiency and, thus, enhance the photovoltaic conversion efficiency. The ever increasing demands of miniaturized and integrate microdevices requires the high efficeint, low cost and green microfabrication methods applicable to III-V semiconductors. Among numerous microfabrication techniques, ECM takes the advantages of high efficiency, no tool wear, no thermal stress and no mechanical stress.8-15 Moreover, ECM has the capability to fabricate complex 3D-MNS which can not be completed by other techniques. However, the workpieces are usually conductive because the fundamentals of ECM are the cathodic deposition


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and anodic dissolution. For the workpiece with poor conductivity, localized chemical etching induced by electrogenerated etchant were developed.16-24 However, the uncontrollable diffusion of etchant ruins the machining accuracy, and the piont-by-piont removal mode is of low efficiency. In 1992 Tian proposed confined etchant layer technique (CELT) in which scavengers were employed to control the diffusion distance in electrolyte solution of the electrogenerated etchant.25 The confined etching layer (CEL) can be compacted to the nanoscale and, thus, the machining accuracy is ensured. Most importantly, the mold electrode can be of large area to realize the mass replication of complex 3D-MNS on various workpieces regardless of their conductivity.26-36 Because the etching process occurs in a ultrathin-layer electrolyte system, to make it controllable, it is essential to figure out the kinetic parameters and ascertain the coupling effect of the complex CELT reaction system. In turn, the kinetic investigation can provide a guideline on how to opitimize the reaction system and improve the machining accuracy. SECM has been proved a powerful method to investigate the kinetics of chemical etching reactions.17, 21, 37 Based on the kinetic parameters obtained by SECM, Arbitrary Lagrange-Eulerian (ALE) method was adopted to analyze the topography of etching pits.38-40 The etching rate constant was obtained from the linear relationship between etching depth and etching time under different etching kinetics. We have studied the kinetics of confined chemical etching system of n‑GaAs by SECM and FEM simulation, and corelated the reaction kinetics to the topography of etching pits.4143

However, due to the ultrathin electrolyte between the SECM tip and the semiconductor substrate,

the decomposition or precipitation of the etching products, e.g., the metallic oxides, would make the surface passivated and hinder the etching processes. Indeed, the surface passivation involves the subsequent reactions of the etching products and depends on the kinetic rates of both the etching reaction and their own. If the surface passivation were serious, the kinetic rates obtained


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on the fresh substrate with short etching time would not work in the practical ECM processes with long etching time. Experimentally the depth~t curve will deviate from the linear relationship with the etching process going on. In this case, the coupling effect of the surface passivation reactions should be taken in to account, and strategies should be proposed to avoid surface passivation and to make the etching process controllable. In this paper, we investigated the etching kinetics of III-V semiconductors by SECM and FEM simulations by considering the surface passivation. A deformed geometry module coupled with mass transport and surface processes were adopted to obtain the kinetics of both the etching reaction and the passivation reaction of n-GaAs, n-GaP and n-InP, and to correlate these kinetic parameters to the topographies of etching pits. The results showed that surface passivation was caused by the limited solubility of the dissolved ions in the ultrathin electrolyte. The increase of kinetic passivation rate led to a serious deviation of depth~t curves from the linear relationship. The surface passivation can be avoid by adding ionic ligands as the additives in the electrolyte solution. 2. THEORY AND SIMULATIONS Bromide (Brˉ) is adopted in experiments as the precursor to generate the etchant bromine (Br2):

6 Br − → 3 Br2 + 6e−

(1)

The etching processes involved in the confined chemical etching of n-GaAs are expressed as followed: 3 Br2 + GaAs + 6 H2O → H3GaO3 ⋅ H3 AsO3 + 6 HBr

k1 (2)

H3GaO3 ⋅ H3 AsO3 + α GaAs → α GaAs ⋅1 2 (Ga 2O3 ⋅ As 2O3 ) + 3 H2O

k2 (3)


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In the case of n-GaP: 3 Br2 + GaP + 6 H2O → H3GaO3 ⋅ H3PO3 + 6 HBr

k3 (4)

H3GaO3 ⋅ H 3 PO 3 + β GaP → β GaP ⋅ 1 3 [ Ga 2O 3 ⋅ Ga(PO 3 )3 ] + 2 H 2O + 2 H +

k4 (5)

And in the case of n-InP: 3 Br2 + InP + 6 H2O → H3InO3 ⋅ H3PO3 + 6 HBr

k5 (6)

k6 (7) H3InO3 ⋅ H 3 PO3 + γ InP → γ InP ⋅ 1 3 [In 2O3 ⋅ In(PO3 )3 ] + 2 H 2O + 2 H + where, equation 1 is the electrochemical reaction for the generation of etchant Br2, and equation 2-7 are the etching processes for n-GaAs, n-GaP and n-InP. The etching rate constant of GaAs, GaP and InP by Br2 expressed by equation 2, 4 and 6 are defined as k1, k3 and k5, respectively. The passivation rate constant expressed by equation 3, 5 and 7 are defined as k2, k4 and k6, respectively. Although the oxides generation and dissolution steps were considered, these reaction formulas are still simplified and the side reactions and intermediates such as Br•, PH3, P2O5, InPO4 and GaPO4 are neglected. The mass transport in an axisymmetric cylindrical coordinates abides by Fick’s second law: ∂Ci 2 = Di ∇ Ci ∂t -3

(8) where i represents redox species in solution, C (mol cm ) and D (cm s ) is the concentration and 2

-1

diffusion coefficient of the species, respectively. Herein, we consider the diffusion coefficient of Br- and Br2 are the same with a value of 1.77 × 10-5 cm2 s-1. The diffusion coefficient of H3GaO3, H3AsO3, H3InO3, and H3PO3 are defined with a value of 1.0 × 10-5 cm2 s-1. The geometry of FEM model for SECM experiments is shown in Figure 1. The geometry parameters are defined as follows: a is the radius of tip, rsub is the radius of GaAs substrate, rglass is the radius of sealing glass around the Pt wire, and d is the distance between tip and substrate. The simulations and


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experiments are carried out using a 25-μm-diameter Pt ultrmicroelectrode with a RG of 2. The boundaries represented by the numbers in Figure 1 are described as followed. On the symmetry/insulation boundaries 1, 4 and 5, no electrochemical reaction occurs. Thus, there is no flux normal to these boundaries for redox species i: ∇Ci ⋅ n = 0

(9)

where n is the inward unit vector normal to the surface. On the boundary 2, the diffusion-limited oxidation of Br- to Br2 is defined as:

where

CBr* -

1 * CBr- = 0, CBr2 = CBr 2 -

(10)

is the bulk concentration of Br .

On the boundary 3, the etching reactions in equation 2-8 are considered as quasi-first order and irreversible processes. The flux of species normal to semiconductor surface is related to the etching rate constant of semiconductor. Thus, the flux boundary for GaAs is described as:

DBr2 ∇CBr2 ⋅ n = −3k1CBr2 (1-θ )

(11)

DBr- ∇CBr- ⋅ n = 6k1CBr2 (1-θ )

(12)

DH3GaO3 ⋅H3AsO3 ∇CH3GaO3 ⋅H3AsO3 ⋅ n = k1CBr2 (1-θ ) - k2CH3GaO3 ⋅H3AsO3

(13)

The flux boundary for GaP is described as:

DBr2 ∇CBr2 ⋅ n = −3k3CBr2 (1-θ )

(14)

DBr- ∇CBr- ⋅ n = 6k3CBr2 (1-θ )

(15)

DH3GaO3 ⋅H3PO3 ∇CH3GaO3 ⋅H3PO3 ⋅ n = k3CBr2 (1-θ ) - k4CH3GaO3 ⋅H3PO3

(16)

The flux boundary for InP is described as:

DBr2 ∇CBr2 ⋅ n = −3k5CBr2 (1-θ )

(17)


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DBr- ∇CBr- ⋅ n = 6k5CBr2 (1-θ )

(18)

DH3InO3 ⋅H3PO3 ∇CH3InO3 ⋅H3 PO3 ⋅ n = k5CBr2 (1-θ ) - k6CH3InO3 ⋅H3 PO3

(19)

where θ is the surface coverage of oxide species on semiconductor. For the surface reactions module, the mass transport in an axisymmetric cylindrical coordinates is governed by: ∂θ 2 (20) = Ds ∇ θ + Rs ∂t 2 -1 where θ and Ds (cm s ) are the surface coverage and surface diffusion coefficient of the oxide

species on semiconductor surface. Here we consider the diffusion coefficient of oxide species is zero. Rs is the surface reaction rate in equation 3, 5 and 7, which is related to the rate constant of the generation reaction of oxide species. Thus, Rs for GaAs is defined as:

Rs,GaAs =

∂ (1-θ ) ∂t

Rs,Ga 2O3 ⋅As2O3 =

∂t

∂ (1-θ )

1 k2 CH3GaO3 ⋅H3AsO3 3

(22)

∂θ

Rs for InP is defined as: ∂ (1-θ ) ∂t

=

= − β k4CH3GaO3 ⋅H3PO3

∂t

Rs,Ga 2O3 ⋅Ga(PO3 )3 =

Rs,InP =

(21)

∂θ

Rs for GaP is defined as:

Rs,GaP =

= −α k2CH3GaO3 ⋅H3AsO3

∂t

=

1 k4 CH3GaO3 ⋅H3PO3 3

= −γ k6CH3InO3 ⋅H3PO3

(23) (24)

(25)

∂θ 1 (26) Rs,In 2O3 ⋅In(PO3 )3 = = k6CH3InO3 ⋅H3PO3 ∂t 3 where α, β and γ are the site occupancy number for oxides species on GaAs, GaP and InP,

respectively. The tip feedback current is calculated by:


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∂C a (27) itip = ∫ nFDBr- Br 2π RdR, n=1 0 ∂Z The normalized feedback current is defined by itip / iss, where * for RG = 2. iss = 4.43nFDBr- aCBr -

To correlate the etching kinetics to etching topography, an arbitrary Lagrange-Eulerian (ALE) simulation combined with the modules established above is applied. A deformed geometry condition is introduced in which the dotted lines I and II on the semiconductors (Figure 1) represent the change of etching topography with the ongoing etching processes, d represents the initial distance between tip and substrate before etching, and h is the depth of etching pits. The movement of boundaries is defined as followed. For the boundaries 2, 4, 5, 6 and 7, no etching reaction occurs. Thus, these boundaries are fixed in both Z and R dimension. For the axisymmetric boundary 1, the movement is fixed in R dimension but free in Z dimension. For the interfacial etching boundary 3, the movement in R dimension is fixed, but the movement in Z dimension is defined by the normal velocity (ν Z ) to semiconductor surface. ν Z is related to the etching rate constant of semiconductors: ν Z ,GaAs = k1 (1 − θ ) CBr ρ GaAs

(28)

ν Z ,GaP = k3 (1 − θ ) CBr ρ GaP

(29)

ν Z ,InP = k5 (1 − θ ) CBr ρ InP

(30)

2

2

2

where, ρ is the molar density of the semiconductors ( ρ

GaAs

= 3.71 × 104, ρ = 4.10 × 104, ρ = GaP InP

3.28 × 104 mol m-3). 3. EXPERIMENTAL SECTION


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Chemicals and materials. Silicon doped GaAs (100) wafers with doping level of 0.8 ~ 2.3 × 1018 cm-3 were purchased from China Crystal Technologies Co., China. Sulfur doped GaP (100) wafers with doping level of 2.0 ~ 8.0 × 1017 cm-3, and sulfur doped InP (100) wafers with doping level of 1.0 ~ 3.0 × 1018 cm-3 were purchased from Hefei Kejing Materials Technology Co., China. Before experiment, semiconductors were rinsing with acetone and deionized water. All chemicals (KBr, H2SO4, 2, 2’-bipyridine) are analytical grade or better and provided by Sinopharm Co., China. All aqueous solutions were prepared with deionized water (18.2 MΩ cm-1, Milli-Q, Millipore Corp.). Instrumentation and Procedures. Both the etching and approaching experiments were performed with a CHI 920c SECM workstation (CH Instruments Co., USA) equipped with a sensitive force sensor at the Z axis stepping motor. Three-electrode system was adopted with a 25μm-diameter Pt ultrmicroelectrode (RG = 2) as the working electrode, a Pt wire (500-μm-diameter) as the counter electrode and an Ag/AgCl reference electrode. The etching experiments on semiconductors were performed in the aqueous solution containing 0.01 mol/L KBr and 0.5 mol/L H2SO4. Firstly, the tip was contacted with the substrate with a contact force of 1.0 mN indicated by the force senor. Secondly, the tip was withdrawn 2 μm from the substrate and held at this place during the etching processes. The tip potential was biased at 1.1 V to generate Br2 and to perform the etching processes for a certain time. After that, the tip was moved to another place to start a new etching experiment. At least three parallel etching experiments were performed to obtain the average profiles to pits (Figure S1). For the current feedback curves, the tip was firstly contacted with the substrate with a contact force of 1.0 mN and then withdrawn 100 μm from the substrate. Secondly, the tip was moved to the substrate with a potential bias of 1.1 V. To investigate the surface passivation effect, the approach curves were recorded after etching for a certain time as


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discribed above. Since the surface passivation of n-InP was serious, 0.05 mol/L 2, 2’-bipyridine was added in the etching solution as a ligand for In3+ to improve its solubility. Confocal laser scanning microscope (VK-X200, KEYENCE Co.) were employed to characterize the depths and profiles of the etching pits. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an Omicron Sphera II hemispherical electron energy analyzer (Monochromatic Al Kα with 1486.6 eV operating at 15 kV and 300 W) to characterize the components of passivation layer on semiconductor surface. The etching process was simulated by commercial finite element package COMSOL Multiphysics 5.0 (Comsol AB, Sweden). 4. RESULTS AND DISCUSSION 4.1 Coupling effect between ke and kp The simulation results including the normalized tip current, depth of pits and surface coverage of the oxide species with the function of etching time under different etching rate (ke) and passivation rate (kp) are shown in Figure 2. Larger ke leads to larger normalized tip current, which is attributed to the enhanced flux of Br- on the Pt tip by the etching reaction on substrate. At the beginning of etching time, the concentration of oxide precursors (H3GaO3, H3AsO3, H3InO3, etc.) is relative small and the decomposition rates of them are negligible. Therefore, the normalized tip current under different kp are almost the same with a value of 1.8 (Figure 2d). However, with the etching progress going on, the concentration of these precursors will increase and the increase of kp will make the tip current decreased significantly. The correlations between removal rate (defined by the ratio of etching depth over etching time) and the two kinetic rates (ke and kp) are shown in Figure 2b and 2e, respectively. When ke and kp are both relatively small with a value of 0.001 cm/s, the etching depth is relative small (< 500 nm) in 500 s (square line in Figure 2b). In this situation,


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both the passivation effect and the change of tip-substrate distance are negligible. Hence the relationship between the etching depth and etching time is almost linear. With the increase of ke, the surface coverage of oxide species increases gradually (Figure 2c) and etching depth increases significantly. The depth~t curves deviated the linear relationship obviously (Figure 2b) because of the serious passivation of the semiconductor surface, i.e., an increased kp (Figure 2e). When the etching time is less than 200 s, the etching depth at different kp are almost the same. When the etching time is more than 200 s, the depth~t curves deviate remarkably from the linear relationship without passivation effect (i.e., kp = 0). The surface coverage of oxide species increases dramatically with the increased kp (Figure 2f). In conclusion, at the beginning the removal rate is mainly determined by ke. However, with the etching progress going on (e.g., 200 seconds’ after) the passivation becomes serious gradually which is depended on kp kinetically. 4.2 Etching kinetics of n-GaAs: fast ke vs. slow kp The topography of etching pits obtained with different etching time are shown in Figure 3a. As discussed in section 4.1, at the beginning the etching rate is mainly dependent on ke. Therefore, the depth~t curve in Figure 3c before 200 s is used to get k1 and results in a value of 0.015 cm/s. Here we define the depth~t curve in which the passivation effect is negligible (k2 = 0) as the ideal depth~t curve (black solid line in Figure 3c). The experimental depth~t curve obtained by SECM fits well to the ideal curve before 200s, and deviates from it after 200 s, which proves the existence of passivation effect. This deviation on GaAs is not serious, which indicates the passivation rate constant in equation 3 is slow. Simulation results show k2 with a value of 0.001 cm/s (α = 8), and θ is increased to 0.65 when the etching time reaches 500 s (red circles in Figure 2f). Generally, H3GaO3 is easy to dissolve in acid solution as Ga3+. H3AsO3 can also dissolve in acid solution as AsO33-. Therefore, the decomposition of H3GaO3 and H3AsO3 to form Ga2O3, As2O3 and other


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oxides are not so serious. Consequently, the etching of n-GaAs has a faster ke and a slower kp, and the passivation effect is negligible. Using the obtained kinetic parameters, the cross-sectional profiles simulated by the deformed geometry module are well in accordance with the experimental results (Figure 3b), which indicates the reasonability of the kinetic model described in Section 2. 4.3 Etching kinetics of n-GaP: slow ke vs. fast kp Similar experiments were also performed on n-GaP. The topography and corresponding crosssectional profiles of the pits obtained with different etching time are shown in Figure S2. The etching depth on GaP in 100 s is only 0.05 μm, much smaller than that on GaAs (0.92 μm), indicating that k3 is much slower than k1. However, the depth~t curve deviates from the ideal curve more seriously compared with n-GaAs (Figure 4a), which indicates k4 is much faster than k2. Fitting to the simulated curves, k3 is obtained with a value of 0.0004 cm/s, and k4 is obtained with a value of 0.1 cm/s (β = 100). Using the kinetic parameters obtained above, the cross-sectional profiles simulated are well in accordance with the experimental results (Figure S2). In conclusion, n-GaP has a faster kp and a slower ke, indicating a serious surface passivation effect. 4.4 Etching kinetics of n-InP: fast ke vs. fast kp Experiments performed on n-InP wafer demonstrate a fast etching kinetics vs. a fast passivation kinetics. The cross-sectional profiles of pits obtained with different etching time are shown in Figure S3. The depth on InP wafer at 100 s (0.5 μm) is smaller than that on GaAs (0.92 μm), which indicates k5 is slower than k1 but much faster than k3. In contrast to n-GaAs, the experimental depth~t curve of n-InP deviates from the ideal curve after only 50 seconds’ etching (Figure 4b), which predicts k6 is much faster than k2. Simulation results give k5 with a value of 0.006 cm/s, and k6 with a value of 0.1 cm/s (γ = 30). The cross-sectional profiles simulated using the obtained kinetics parameters are in harmonious accordance with the experimental profiles (Figure S3). The


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passivation effect is so serious that the etching process is almost stopped when the etching time is longer than 300 s (symbols in Figure 4b). 4.5 The components and depassivation of oxide film on n-InP To investigate what happens on n-InP surface at different etching time, the feedback curves are obtained on n-InP surface immediately after etching for a certain time (Figure 4c). After 100seconds’ etching, a positive feedback current is still observed on n-InP surface. However, after 250-seconds’ etching, a very weak positive feedback current is obtained and followed with a negative feedback current, indicating that the n-InP surface is passivized partially by the oxides film. Furthermore, after 500-seconds’ etching, a negative feedback current is observed, which indicates the surface is passivized completely. The SEM image of n-InP surface after etching for 500 s is shown in Figure 5a. An oxide layer with thickness of 0.9 μm is observed on n-InP surface (Figure 5b), which hinders the etching progress. The P 2p and In 3d bonding states of the XPS spectra of the n-InP surface after 500 seconds’ etching are shown in Figure 5e-(1) and Figure 5f(1). The main components of the oxide layer are identified as In2O3, In(PO3)3 and InPO4. Figure 5c shows the SEM image of the sample in Figure 5a after being immersed in 0.005 mol/L Br2 and 0.5 mol/L H2SO4 solution for 10 min. The oxide layer becomes smoother, indicating the dissolution of the oxides in the acid electrolyte, which was also proved by the XPS spectra (Figure 5e-(2), Figure 5f-(2)). The intensity of peaks at 133 eV, attributed to phosphide and phosphite, decrease dramatically after 10 minutes’ immersion in 0.005 mol/L Br2 and 0.5 mol/L H2SO4 solution. As for the control XPS spectra in which a pristine n-InP workpiece was immersed in same solution for 10 minutes, little oxides was observed on n-InP surface (Figure 5e-(3), Figure 5f-(3)). The SEM images of GaAs and GaP workpiece after 500 seconds’ etching were shown in Figure S4. The results illustrate that the surface passivation of n-InP is more serious than that of GaAs and


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GaP although the depassivation will occur when the oxides film is exposure to the bulk solution. This is actually caused by the limited solubility of the etching products in the ultrathin electrolyte layer between tip and substrate, which correlates to the coupling effect of the etching and passivation processes. Since both etching and passivation processes are kinetically fast, InP present the most serious surface passivation. For GaP, the surface passivation layer is hardly observed because of the slow etching rate. An effective way to avoid precipitation of the oxides is to add suitable ligands to form cation complexes with better solubility. Ligands with oxygen-containing and amine-containing electron donor can coordinate with rare-earth metal cations effectively.44-47 Therefore, we added 0.05 mol/L 2, 2’-bipyridine in the electrolyte to perform the etching processes by SECM. From the results shown in Figure 4d, the etching progress kept going for 500 s without serious surface passivation. The pit after 500 seconds’ etching was deepenned significantly from 1.0 to 2.0 μm comparing with the results without adding 2, 2’-bipyridine. The SEM image of n-InP surface after 500 seconds’ etching with the addition of 2, 2’-bipyridine shows that no apparent scale-like oxide layer on the surface (Figure 5d). That means 2, 2’-bipyridine can coordinate with In3+ to avoid effectively the precipitations of In2O3, In(PO3)3 and InPO4 etc. We also simulated the influence of complex reaction between In3+ and 2, 2’-bipyridine on the etching rate (Figure 4d). The simulated depth~t curve fits well to the experiments results when khomo is set with a value of 1×105 dm3 mol-1 s-1 (red line in Figure 4d). However, the experimental depth~t curve still deviates from the ideal curve, which indicates the passivation effect is not eliminated completely by 2, 2’-bipyridine. 4.6 Correlating the kinetic rates with the topographies of etching pits Because of the coupling effect between ke and kp, we correlate the topography of the pits obtained at 100 seconds’ etching with the etching kinetic rate ke (Figure 6). Table 1 lists the


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topographic parameters and ke for n-GaAs, n-InP and n-GaP. The results demonstrate that, with the increased ke, the etching depth increases and the pit diameter decreases. When ke increases from 0.0004 to 0.015 cm/s, the depth increases from 0.05 to 0.94 μm, and the diameter decreases from 80.34 to 34.41 μm. The results indicate a larger ke helps to improve both the etching efficiency and etching accuracy. In the etching process, semiconductor substrate works as an electrode to collect bromine during the etching process, which is similar to the tip generation/substrate collection (TG/SC) mode in SECM with an infinite substrate. In TG/SC mode, a large substrate kinetics always leads to higher collection efficiency, which means smaller part of lateral diffusion and larger part of normal diffusion to substrate. As for the semiconductor substrate in the etching process, larger ke results in higher etching efficiency and resolution. Meanwhile, larger kp leads to the slowdown or even the cease of etching progress. 5. CONCLUSIONS Combining SECM experiments and FEM simulations, we investigated the coupling effect of surface passivation on the electrochemically induced chemical etching process of III-V semiconductors. We found there is an obvious surface passivation on the III-V semiconductor substrate, caused by the decomposition and precipitation of the etching products because of their limited solubility in the ultrathin electrolyte solution between the tip and the substrate. The coupling effect between the etching and passivation processes is determined by the relative kinetic rate of them. The passivation effect will be more serious if the etching and passivation rates are both fast. Although the passivation film can be dissolved gradually when exposure to the bulk solution, the effective way to avoid surface passivation is to add ligand to improve the solubility of the produced cations, e.g., 2, 2’-bipyridine as the ligand for In3+. The etching rate and passivation rate of n-GaAs, n-InP and n-GaP are obtained to correlate with the topography of the etching pits,


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respectively. The harmonious accordance between the dynamic simulations and the experimental results elucidates the reasonability of the coupling model, which will be instructive in the ECM processes of III-V semiconductor.

AUTHOR INFORMATION Corresponding Author * D.Z.: e-mail, [email protected]; Tel: +865922185797; Fax: +865922181906. Present Addresses †Department of Chemistry, College of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China. E-mail: [email protected] Author Contributions J. Zhang and J. Lai contributed equally to the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information The parallel etching experiments, the confocal laser microscopy images and cross section profiles of the etching pits are available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT


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The financial support of the National Natural Science Foundation of China (21327002, 91323303, 51605404, 21573054 and 21321062), and the State Key Laboratory for Manufacturing Systems Engineering (Xi‘an Jiaotong University) are appreciated. ABBREVIATIONS SECM, Scanning Electrochemical Microscopy; FEM, finite element method; ECM, electrochemical microfabriction; 3D-MNS, three-dimensional micro-nanostructures; CELT, confined etchant layer technique; ALE, arbitrary Lagrange-Eulerian; XPS, X-ray photoelectron spectroscopy; TG/SC, tip generation/substrate collection. REFERENCES 1.

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Towards

Bipyridines,

Terpyridines,

Imino-Substituted

Pyridines

and

Bis(imino)Acenaphthenes. New J. Chem. 2004, 28, 207-213. 47. Evers, A.; Hancock, R. D.; Martell, A. E.; Motekaitis, R. J., Metal Ion Recognition in Ligands with Negatively Charged Oxygen Donor Groups. Complexation of Iron(Ⅲ), Gallium(Ⅲ ), Indium(Ⅲ), Aluminum(Ⅲ), and Other Highly Charged Metal Ions. Inorg. Chem. 1989, 28, 2189-2195.


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FIGURES Axis of symmetry Pt wire Glass

Bulk solution 6

rglass 5 a 7 2 d

4

1

rsub

Z 3 R h

Ⅰ

Ⅱ

GaAs, InP or GaP substrate

Figure 1. Schematic diagram of the axisymmetric cylindrical geometry used in the simulation. The numbers in bold represent the boundaries as defined in the text. Boundary 3 represents the initial shape of semiconductor substrate. Boundary I and II with dash line represents the crosssection profile of the etching pit in the deformed geometry model.


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 2. (a-c) the simulation results of normalized tip current, depth of pits and surface coverage of oxide species at different etching time with different ke when kp is kept constant with a value of 0.001 cm/s. (d-f) the simulation results of normalized tip current, depth of pits and surface coverage of oxide species at different etching time with different kp when ke is kept constant with a value of 0.015 cm/s. The initial tip-substrate distance is 2 μm. The electrolyte is 0.01 mol/L KBr and 0.5 mol/L H2SO4.


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(a)

(b)

(c)

Figure 3. (a) The confocal laser microscopy image of etching pits on n-GaAs obtained with different etching time. The etching time in (a) is 100, 200, 300, 400 and 500 s from left to right with an initial tip-substrate distance of 2 μm. (b) The experimental cross-section profiles (solid line) of the etching pits shown in (a). The circle symbols are the simulated cross-section profiles. (c) The simulated relationship between the etching depth and etching time (solid line). The symbols are the experimental relationship between the etching depth and etching time shown in (a). The simulation is performed with k1 = 0.015 cm/s and k2 = 0.001 cm/s. The electrolyte is 0.01 mol/L KBr and 0.5 mol/L H2SO4.


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(a)

(b)

(c)

(d)

Figure 4. (a) The simulated relationship between the etching depth and etching time (solid line) for n-GaP with a constant k3 of 0.0004 cm/s and different k4. The symbols are the experimental results shown in Figure S2. (b) The simulated relationship between the etching depth and etching time (solid lines) for n-InP with a constant k5 of 0.006 cm/s and different k6. The symbols are the experimental results shown in Figure S3. (c) The feedback curves obtained on n-InP wafer immediately after different etching time. (d) The simulated relationship between the etching depth and etching time (solid lines) for n-InP with constant k4 of 0.1 cm/s, conatant k5 of 0.006 cm/s and different khomo when 0.05 mol/L 2, 2’-bipyridine is added in the electrolyte. The symbols are the experimental results with (red circle) or without (cyan circle) 0.05 mol/L 2, 2’-bipyridine.


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(a)

(b)

(c)

(d)

(e)

(f)

Page 28 of 29

Figure 5. (a) The SEM image of n-InP surface after etching for 500 s. The electrolyte is 0.01 mol/L KBr and 0.5 mol/L H2SO4. A cylinder Pt electrode with 200 μm diameter is used in the etching process. (b) The cross-sectional SEM image of (a). (c) The SEM image of (a) after immersed in 0.005 mol/L Br2 and 0.5 mol/L H2SO4 solution for 10 min. (d) The SEM image of n-InP surface after etching for 500 s when 0.05 mol/L 2, 2’-bipyridine is added in the electrolyte. (e), (f) The Xray photoelectron spectra of n-InP showing the P 2p and In 3d bonding states, where curve (1) is obtained with the sample shown in Figure 5a, curve (2) is obtained with the sample shown in Figure 5c, and Curve (3) is obtained with the pristine (i.e., un-etched) n-InP after immersed in 0.005 mol/L Br2 and 0.5 mol/L H2SO4 solution for 10 min.


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Figure 6. The correlation of the experimental depths and diameters of the etching pits obtained on GaAs, InP and GaP to the logarithm of kinetic etching rate (logke). The etching time is 100 s. The electrolyte solution contains 0.01 mol/L KBr and 0.5 mol/L H2SO4. Table 1. Correlation between ke and topography of pits with a etching time of 100 s. Sample

log(ke / cm s-1)[a]

Depth / μm

Diameter / μm

n-GaAs

-1.82

0.94

80.34

n-InP

-2.22

0.50

54.27

n-GaP

-3.40

0.05

34.41

TOC Graphic

Br2

2Br− 2Br− + Prod.

ke

Br2

Passivation film kp Semiconductor


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Etching Kinetics of IIIâV Semiconductors Coupled with Surface

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