Electrochemical nanostructuring of (111) oriented GaAs crystals: from porous structures to nanowires

Abstract

A comparative study of the anodization processes occurring at the GaAs(111)A and GaAs(111)B surfaces exposed to electrochemical etching in neutral NaCl and acidic HNO₃ aqueous electrolytes is performed in galvanostatic and potentiostatic anodization modes. Anodization in NaCl electrolytes was found to result in the formation of porous structures with porosity controlled either by current under the galvanostatic anodization, or by the potential under the potentiostatic anodization. Possibilities to produce multilayer porous structures are demonstrated. At the same time, one-step anodization in a HNO₃ electrolyte is shown to lead to the formation of GaAs triangular shape nanowires with high aspect ratio (400 nm in diameter and 100 µm in length). The new data are compared to those previously obtained through anodizing GaAs(100) wafers in alkaline KOH electrolyte. An IR photodetector based on the GaAs nanowires is demonstrated.

Keywords: anodization; crystallographically oriented pores; gallium arsenide (GaAs); nanowires; neutral electrolyte; photocurrent; porous GaAs.

Figure 1

SEM images in cross section of porous GaAs layers for three different conditions of anodization in 1.75 M NaCl: (A) galvanostatic regime on the GaAs(111)A surface; (B) galvanostatic regime on the GaAs(111)B surface; (C) potentiostatic regime on the GaAs(111)B surface. The substrate is on the right side of the images. The inserted plots represent the applied currents (A, B) and the applied potential (C) during anodization.

Figure 2

(A–C) SEM images in cross section at higher magnification of the porous layers obtained by anodization of the GaAs(111)B surface in galvanostatic regime, corresponding to the images illustrated in Figure 1B. (D, E) SEM images of samples anodized in potentiostatic regime, corresponding to images illustrated in Figure 1C. (F) Evolution of the current during anodization in potentiostatic regime of the GaAs(111)B surface at applied voltages of 3, 2 and 1 V.

Figure 3

(A) SEM images of the formation of interrupted GaAs nanowires on the (111)B surface anodized in NaCl electrolyte at 6 V. (B) The polarization curves measured at a scan rate of 50 mV·s⁻¹ at the beginning of the anodization process of a GaAs(111)B crystal in 1 M HNO₃ (curve 1) and 1.75 M NaCl (curve 2).

Figure 4

(A) SEM image in cross section of a GaAs(111)B sample anodized at 3 V for 20 min in 1 M HNO₃. (B, C) Top-view SEM images.

Figure 5

PL spectra of bulk (curve 1) and anodized (curve 2) GaAs samples measured at a temperature of 10 K.

Figure 6

XRD pattern of the anodized GaAs(111)B sample.

Figure 7

(A) Optical microscopy image of the opened regions in the photoresist on the glass substrate for deposition of the metal contacts on the selected GaAs nanowire. The inset in (A) shows a photo of five contacted GaAs nanowires on the same glass substrate. (B) Photocurrent build-up and relaxation of the photodetector measured for an IR illumination density of 800 mW·cm⁻².

Figure 8

Current–voltage characteristics measured in dark (curve 1) and under IR illumination with power density of 800 mW·cm⁻² (curve 2) for the GaAs nanowire photodetector with the design shown in Figure 7.