Selective and Nonselective Wet Etching of Zn^sub 0.9^Mg^sub 0.1^O/ZnO

Wet engrave rates at 25?°C for Zn^sub 09^Mg^sub 01^O grown upon sapphire substrates by pulsed laser deposition (PLD) were in the range 300-1100 nm min^sup -1^ with HCl/H2O (5 ?— 10^sup -3^-2 ?— 10^sup -2^ M) and 120-300 nm min^sup -1^ with H^sub 3^PO^sub 4^/H2O (5 ?— 10^sup -3^-2 ?— 10^sup -2^ M) one as well as the other of these dilute mixtures exhibited diffusion-limited etching, with thermal activation energies of 2-3 kCal mol^sup -1^. By sharp contrast, the engrave rates for ZnO also grown upon sapphire by PLD were abundant slower in similar solutions, with rates of 12-50 nm min^sup -1^ in HCl/H2O (001-12 M) and 12-54 nm min^sup -1^ in H^sub 3^PO^sub 4^/H2O (002-015 M) The etching was reaction limited above the temperature range 25-75?°C, with activation energies shut up to 6 kCal . mol^sup -1^ The resulting selectivity of Zn^sub 09^Mg^sub 01^O above ZnO can be a high as ~400 with HCl and ~30 with H^sub 3^PO^sub 4^

Key words: ZnO, etching, selectivity

INTRODUCTION

ZnO has a drawn out list of applications in gas sensing, in varistors, in piezoelectric transducers, as a light transmitting electrode in optoelectronic devices, in electro-optic modulators, and as a sunscreen1 freshly major advances in the areas of conductivity superintendence and availability of high-quality dimensions ZnO substrates has refocused attention upon ZnO for ultraviolet light emitters and transparent electronics. ZnO can be grown at relatively depressed temperatures on cheap substrates similar as glass, and it has a larger exciton binding efficiency (~60 meV) than GaN (~25 meV) (enabling the fabrication of vertical geometry devices with depressed threading dislocation densities) and has simpler processing relative to GaN for which convenient wet etches are not available.2-22 novel reports of ZnO-based light emitting forms have included homojunctions with depressed emission intensity;23-25 hybrid structures involving heterostructures with SiC, GaN/sapphire, AlGaN/ sapphire, or Cu^sub 2^O;26-31 and ZnO pin homojunction diodes grown upon ScMgAlO^sub 4^ substrates.32,33 Another important factor in the design of ZnO-based light-emitting diodes (LED) is the realization of bandgap engineering to create barrier layers and quantum wells in heterostructures. With esteem to higher bandgaps, an increase up to 40 eV has been achieved by the agency of the incorporation of Mg in the ZnO layer while still maintaining the wurtzite structure34 Ternary ZnCdO looks to be an appropriate candidate for narrow bandgap because of the smaller bandgap of CdO (23 eV)35 In addition, the advances in increase of ZnO nanorods and nanowires have intimateed that these may have applications to bio-detection22 because of their large surface areas.

In greatest in quantity device fabrication schemes, wet etching is extremityed for isolation or mesa formation. ZnO is readily etched in many acid solutions, including HNO^sub 3^/HCl and HF36-39 In greatest in quantity cases, the etching is reaction limited, with activation energies of >6 kCal mol^sup -1^. In preliminary work, we have lay the foundation of that the etching of the ZnO is powerfully dependent on material quality. If the ZnO is true thin, the wet etch rates are high in all acid solutions. A particular question encountered with the wet etching of ZnO/AlGaN-based L manner of makings was the presence of a true significant undercut (as much as around 10 ?µm)30 which occurr mainly at the extremity of the selective removal of the ZnO from the underlying AlGaN.

In this paper, we report upon the selective etching of Zn^sub 09^Mg^sub 01^O relative to ZnO, the two grown with similar thicknesses upon sapphire substrates by pulsed laser deposition (PLD) to render certain similar crystal quality. Selectivities above 400 can be achieved with HCl at high dilution factors with water.

EXPERIMENTAL

The films were grown through PLD on c-plane Al^sub 2^O^sub 3^ substrates. Phosphorus-doped (Zn^sub 09^Mg^sub 01^)O targets were fabricated using high-purity ZnO (999995%) and MgO (99998%) with P^sub 2^O^sub 5^ (99998%) serving as the doping agent. The ablation targets were fabricated with a phosphorus doping horizontal of 2 at.% (which can bring forward p-type conductivity if the films are annealed after growth40) and a KrF excimer laser with a repetition rate of 1 Hz and pulsation energy density of 1-3 J/cm^sup 2^ was used as the ablation source. The target to substrate distance was 4 cm The ZnO extension chamber exhibits a base crushing of 10^sup -6^ torr. Film extension was performed at 500?°C for ZnMgO or 400?°C for ZnO in an oxygen compressing of 50-150 mTorr. Both impressed signs of films were ~0.5-?µm thick with an electron concentration of 10^sup 17^ cm^sup -3^ and mobility of 25 cm^sup 2^ V^sup -1^ s^sup -1^ for the ZnO and a mixed conductivity (indeterminate conductivity mark with carrier density around 10^sup 16^ cm^sup -3^) for the ZnMgO layers, as determined from van der Pauw Hall measurements.

The wet etching was performed with HCl/H2O or H^sub 3^PO^sub 4^/H2O solutions as a function of the one and the other concentration and temperature. A photoresist mask (AZ 1045) was used for creating features whose profundity was measured by stylus profilometry after postetch removal of the mask in acetone.