Crystallographic Effects on the Photoelectrochemical Etching of Gratings in Compound Semiconductors
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CRYSTALLOGRAPHIC EFFECTS ON THE PHOTOELECTROCHEMICAL ETCHING OF GRATINGS IN COMPOUND SEMICONDUCTORS MICHAEL M. CARRABBA, NGUYET M. NGUYEN AND R. DAVID RAUH EIC Laboratories, Inc., Ill Downey Street, Norwood, MA02062 ABSTRACT Grooves have been etched photoelectrochemically into a (100) n-GaAs surface using a 100 cycle/mm grating mask to define the illuminated area. The degree of mask undercutting and the shape of the groove bottom are shown to be functions of doping density and crystallographic orientation. High doping density gives rise to reduced undercutting, presumably due to a lesser extent of surface diffusion of photogenerated holes. INTRODUCTION Vertical chemical etching technology of III-V semiconductors has generally lagged behind that of Si. This is because the highest etch rate ratios achieved for the crystal faces of III-V materials are not nearly as high as for Si, and also because the [111] faces of compound semiconductors are polar and often etch at different rates [1]. Photoelectrochemical processing has the potential for a highly directional etching, since the reaction zone is controlled by the light flux. Indeed, a variety of deep via holes and grooves have been demonstrated using UVlaser-induced photochemical etching, although a free radical rather than an electrochemlc-c-a-mechanism may be involved [2]. Ostermayer and co-workers have characterized in detail photoelectrochemical etching of shallow phase gratings in n-InP and n-GaInAsP in which the etching rate is proportional to the photocurrent [3-5]. Cheng and Kohl demonstrated photoelectrochemical etching of grooves in n-InP through a metallized 10-20 pm grating pattern, which also served as the control electrode [6]. A certain amount of undercutting and trenching was observed which was proportional to light intensity, and which was ascribed to diffraction effects arising from the collimated 633 nm source [6]. We report here that the vertical photoelectrochemical etching of n-GaAs, a typical III-V compound, is very much a function of material variables, particularly crystallographic orientation and doping density. These parameters dominate the degree of undercutting and shape of deeply etched grooves. EXPERIMENTAL The details of the experimental arrangement have been reported elsewhere [7-9]. Briefly, a 3-electrode electrochemical cell was employed, with a standard calomel reference electrode and a Pt counter electrode. The roughened back of the n-GaAs working electrode was ohmically contacted with a In/Ga eutectic, while the polished front side was exposed to the electrolyte solution. The electrolyte was 0.5M Tiron (1,2-dihydroxybenzene-3,5-disulfonic acid) [10], a Ga+ý complexant with a solution pH of 3.4. Alternatively, iM KCl was employed, although Tiron gave somewhat smoother profiles. Both electrolytes could support smooth, continuous photoelectrochemical dissolution of n-GaAs at rates >100 mA/cm2 without passivation. Photoelectrochemical etching was typically conducted at ~O.5V vs. SCE, at which photon-limited photocurrents were observe
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