Supercritical Fluid Electrodeposition of Elemental Germanium onto Titanium Nitride Substrates

Abstract

We report the electrodeposition of germanium from supercritical difluoromethane (sc-CH 2 F 2 ) at 19 MPa and 358 K using [N n Bu 4 ] [GeCl 3 ]. Voltammetry shows a classic nucleation loop on the first anodic scan with a high nucleation overpotential for germanium on TiN. In all cases the deposition appears to be kinetically limited by a coupled, potential independent, chemical step. Films of germanium were deposited at a range of potentials. At high overpotentials the films were dendritic and poorly adherent. At lower overpotentials, below −2 V vs. Ag|LaF 3 , the films are smoother and more homogeneous. Analysis of the films by energy dispersive X-ray (EDX) spectroscopy shows the presence of germanium with some chloride impurity. Raman spectroscopy confirms the deposition of amorphous germanium. Plating by pulsing to −1.9 V vs. Ag|LaF 3 for 100 ms and then growth at −1.5 V vs. Ag|LaF 3 , was found to produce the best films. On annealing at 700 • C under an Ar atmosphere for 1 hour the as-deposited amorphous germanium film is converted into crystalline germanium, as determined by X-ray diffraction (XRD) and Raman spectroscopy. Since the discovery of semiconductor effects in crystalline germanium over half a century ago, 1 germanium based thin films and structures have received considerable interest. These include a number of advanced applications from doping in optics components 2 to high speed electronics, 3 and from Li-ion batteries 4 to third generation photovoltaics. 5Thin films of germanium are typically formed via vacuum processes such as chemical vapor deposition 6,7 or molecular beam epitaxy. 8 In comparison, the non-aqueous electrodeposition of germanium thin films is an attractive alternative as it has the advantages of high material efficiency, directly templated growth and fine control over the evolving structure. 9Over the previous decade Endres and his colleagues have pioneered the electrodeposition of germanium thin films from ionic liquids. [10][11][12][13][14] In this context ionic liquids have a number of advantages over conventional aqueous based solvents; they are chemically stable, they can be purified to remove impurities such as oxygen and water which can decompose the germanium precursors, and they have a wide electrochemical potential window. Initial work in ionic liquids focused on electrodeposition from the Ge(IV) reagent GeI 4 . The electrodeposition was investigated using in situ scanning tunnelling microscopy (STM) and the plated films were shown to be extremely thin (of the order of 12-16 nm) and the plating rate very slow. This self-limiting electrodeposition was attributed to the low conductivity of the deposited germanium film and complex germanium surface chemistry, possibly involving formation of a hydride, fluoride, iodide or hydroxide surface state.12 Germanium thin films have also been electrodeposited from the corresponding chloride and bromide precursors GeX 4 (X = Cl 11 or Br 10 ) in ionic liquids. Again plating rates were low and only very thin films were produced. A re...

Electrodeposition of Protocrystalline Germanium from Supercritical Difluoromethane

“…Note that both plating rates are considerably larger than that obtained previously for the equivalent chloride containing electrolyte (ca. 0.4 μm hr −1 ) and are comparable with deposition rates reported for ionic liquids at room temperature . However, the deposition rate is lower than that reported for the deposition at elevated temperature from ionic liquid reported by Wu et al .…”

“…The redox potentials for the onset of reduction of the three tri‐halides follow the trend for the Ge−X bond strength [GeCl 3 ] − −0.71 V, [GeBr 3 ] − −0.28 V, [GeI 3 ] − −0.23 V (all vs. Ag|LaF 3 ), with the iodide being the most easily reduced. This trend is consistent with earlier studies of these complexes in liquid dichloromethane …”

“…Electrodeposition of germanium was carried out from supercritical difluoromethane at 19.1 MPa and 358 K, containing either 16 m m [EMIM][GeI 3 ] with 60 m m [EMIM][BF 4 ] or 16 m m [N n Bu 4 ][GeI 3 ] with 60 m m [N n Bu 4 ][BF 4 ]. In both cases, deposition using the [GeI 3 ] − anion was around ten times faster than that previously reported using [GeCl 3 ] − , and deposition rates of 2–3 μm h −1 were achieved.…”

“…Deposits formed from GeCl 4 in scCH 2 F 2 at a high over potential were found to contain amorphous germanium with significant chloride contamination. Subsequently, we studied the deposition of germanium from [N n Bu 4 ][GeCl 3 ] in scCH 2 F 2 and reported an improvement in the quality of the germanium thin films . The predominantly amorphous germanium films that were produced could be converted to the crystalline phase through a thermal post‐deposition treatment.…”

Electrodeposition of germanium-containing precursors for Cu2(Sn,Ge)S3 thin film solar cells

“…These works showed that despite being possible to electrodeposit germanium from ionic liquids, there were several issues: the germanium sources are poorly soluble in the ionic liquids, non-standard experimental setups had to be used to cope with the volatility of the most common germanium compounds and deposition rates are low under standard conditions. In another approach, Bartlett et al electrodeposited amorphous germanium from liquid CH 3 [20,21]. Albeit successful, this approach presents some drawbacks: on the one hand, the need of a high pressure cell, which increases the complexity of the experiment and decreases output.…”