Atmospheric pressure chemical vapor deposition of undoped zinc oxide films from a zinc amide precursor

Abstract

Homoleptic transition-metal and main group amide complexes, M(NRR′)n (where R and R′ are alkyl groups, such as CH3 or CH2CH3), in combination with ammonia have been demonstrated to be practical precursors for the chemical vapor deposition (CVD) of transition-metal and main group nitride films [1–9]. The favorable physical properties and ease of synthesis of the amide complexes have stimulated limited studies on using amide complexes as precursors to oxide films as well; for example, Si[N(CH3)2]4, Sn[N(CH3)2]4 [10], Zr[N(CH2CH3)2]4 [11], and Ta[N(CH3)2]5 [12] in combination with O2 have been shown to give the corresponding metal oxide thin films at low substrate temperatures. Prompted by these observations, we set out to investigate the use of a homoleptic zinc amide complex as a precursor to ZnO films. We report here the results of using bis(2,2,6,6-tetramethylpiperidino)zinc, Zn(tmp)2, as the ZnO precursor in an atmospheric pressure CVD process. In previous work on the CVD of ZnO films, the most common precursors have been the pyrophoric dialkyl zinc compounds dimethylzinc, Zn(CH3)2, and diethylzinc, Zn(CH2CH3)2 [13–22]. To avoid the handling problems associated with the use of the pyrophoric compounds, alternative zinc sources such as zinc acetate, Zn(O2CCH3)2 [23–25], zinc acetylacetonate, Zn[CH3C(O)CHC(O)CH3]2 [26–30], zinc 2-ethylhexanoate, Zn[O2CCH(CH2CH3)(CH2)3CH3]2 [31], methyl zinc isopropoxide, [(CH3)Zn{OCH(CH3)2}]4, methyl zinc tert-butoxide, [(CH3)Zn{OC(CH3)3}]4 [32], and ethyl zinc diethylamide, (CH3CH2)Zn[N(CH2CH3)2] [33], have also been examined. The precursor used in this study, Zn(tmp)2, a volatile solid (sublimation 75–80 ◦C/0.01 torr; reported 135 ◦C/0.01 torr [34]), was synthesized in approximately 60% yield from ZnCl2 and two equivalents of the lithium salt of 2,2,6,6-tetramethylpiperidine by using a modification of an established procedure [34] and purified by crystallization from a cold (−35 ◦C) hexanes solution. Although Zn(tmp)2 is air sensitive, it is not pyrophoric, in contrast to the dialkyl zinc compounds used commonly as precursors to zinc oxide. Film depositions were carried out in an atmospheric-pressure rectangular-shaped glass deposition reactor [5]. The flow rate of argon (UHP grade) carrier gas through the Zn(tmp)2 container was 300 sccm, and the oxygen (extra dry grade; 200 sccm) reactant was diluted with argon (350 sccm). The precursor container and the feed line temperatures were maintained at 70 and 90 ◦C, respectively. After the line from the precursor container was shut off, the films were left under the O2/Ar flow for 5 min while maintaining the deposition temperature. The flow was then switched to argon and the sample was allowed to cool quickly. Zn(tmp)2 and dry oxygen reacted at substrate temperatures of 250–400 ◦C to give shiny films that adhered well to silicon, glass and quartz substrates. Table I lists selected film growth rates and composition data. The growth rates increased with deposition temperatures up to 350 ◦C. The rate decrease at 400 ◦C was due to a flaw in the reactor design that forced most of the film to deposit prior to reaching the substrate. A growth rate of ≈170 A/min was observed when the precursor container and feed line temperatures were increased from the normal values to 85 and 100 ◦C, respectively. The observed growth rates are much lower than those found in other atmospheric pressure CVD studies (e.g., the precursors Zn(O2CCH3)2 and Zn[CH3C(O)CHC(O)CH3]2 gave growth rates of ≈6000 A/min at 500 ◦C [25] and 2200 A/min at 600 ◦C [27], respectively). The film stoichiometries (determined by Rutherford backscattering spectrometry (RBS)) depended on the deposition temperature with the O/Zn ratio decreasing with increasing deposition temperature. Carbon and nitrogen peaks were not observed in the RBS spectra