Photoelectrochemical Etching of InAs

Photoelectrochemieal etching of n-InAs (Eg = 0.36 eV) is demonstrated. Although the concentration of thermally generated minority carriers and saturation current are high compared to larger bandgap semiconductors, photocurrent to dark current ratios as high as 4:1 were obtained at low temperature (2°C) and at potentials near the flatband potential. A surface film primarily composed of arsenic oxide was formed during oxidative decomposition of the semiconductor and plays a role in the rate oi dissolution and in the current-voltage response. In 0.2M H2SO4, 6 electrons per InAs were involved in the oxidation dissolution, while only 4 electrons per InAs were found in 0.2M HCI because of the formation of In(I)-chloride species. Interest in the processing of III-V semiconductors has grown as the variety of devices using the materials has increased. III-V semiconductors are particularly well suited for use in optoelectronic devices such as light-emitting diodes (LEDs), photodetectors, and lasers because of their direct bandgap. InAs has been used for tunnel diode heterostructures in microwave applications such as high speed digital switches and frequency locking circuits. 1-5 InAs has a high carrier mobility and large offset of the conduction and valence bands from other III-V materials that makes it ideal for use in quantum-well structures. These types of devices present many opportunities for photoelectrochemical (PEC) etching of InAsfi PEC processing is the selective reaction of an illuminated semiconductor in contact with an electrolyte. N-type semiconductors can be oxidized selectively by photo-induced hole-initiated oxidation of the lattice leading to the dissolution of the semiconductor in the illuminated regions. The surface of a p-type semiconductor can be reduced selectively by using photogenerated electrons in the conduction band. PEC etching has several advantages over more conventional processing techniques; 6 (i) PEC processing uses low energy reactants (photons whose energy is greater than the semiconductor bandgap) as opposed to the high energy plasmas used in dry etching, so that damage-free processing can be achieved; (it) the rate of reaction is directly proportional to the illumination intensity so that spatial variations in the light intensity lead to the. same spatial variations in the etch rate; (iii) different materials can be etched selectively by using a light source with a wavelength specific to the particular bandgap. This is particularly useful for processing multilayered heterostruetures; (iv) the processing can be made specific to the dopant type (n or p) because photogenerated minority carriers are one of the reactants; (v) the electrical current is directly proportional to the etch rate so that the progress of the etch can be monitored easily. The reaction for InAs can be expressed as InAs + (6 x)h ÷ -~ In 3÷ + As 3. + xe[1] In photoetching n-type semiconductors, photogenerated holes migrate to the semiconductor-solution interface where they represent a missing electron in the bonding orbital of the crystal, thus weakening the bond of the semiconductor. This leads to the dissolution of the semiconductor crystal. Photoelectrochemical etching has been used to produce structures in n-GaAs 7-10 and n-InP n-13 devices. Integral lenses have been fabricated on InP LEDs, 14 and PEC etching has been used to make diffraction gratings in n-InP with periods as fine as 235 nm for distributed feedback lasers. 15 PEC etching also has been demonstrated for pGaAs and p-InP. The illumination of a p-type semiconductor biased negative of the fiatband potential, leads to a high concentration of electrons at the semiconductor surface. A * Electrochemical Society Active Member. a Present address: IBM Corporation, Hopewell Junction, NY 12533-6531. 1274 two-step etching procedure was investigated because the reduction of the III-V semiconductors does not lead to the formation of soluble products. 14 The PEC etch rate and spatial resolution can be limited by the dissolution rate of the reaction products, or by the thermal generation of charge carriers. Either may cause dissolution of the semiconductor in nonilluminated areas. Thermally generated carriers are especially important with the smaller bandgap semiconductors, such as InAs. Previously, the small photocurrents observed for InAs were attributed to its small bandgap. 16 The feasibility of PEC etching InAs is investigated here, focusing particularly on the dissolution of the products and the effect of thermally generated carriers.