Temperature effect on zinc oxysulfide-Zn(O,S) films synthesized by atomic layer deposition for Cu(In,Ga)Se2 solar cells

Abstract

Thin films of Zn(O,S) were deposited by atomic layer deposition from diethylzinc, water (H2O), and hydrogen sulfide (H2S). First, a study on the influence of the H2S/(H2O+H2S) pulse ratio from pure ZnO to pure ZnS was performed at deposition temperature Tdep=120 and 200 °C. Zn(O,S) films had higher S content than expected, and this effect was stronger at Tdep=200 °C. Then, Zn(O,S) films have been synthesized over the range of temperature 120–220 °C at the constant H2S/(H2O+H2S) pulse ratio of 9%. For Tdep<180 °C, high and almost constant S content has been measured in the films. The significant increase of the S/(O+S) atomic ratio for Tdep>180 °C confirmed that exchange reactions occurred between the Zn(O,S) growing films and H2S. The grazing incidence x-ray diffraction patterns showed Zn(O,S) films with hexagonal wurtzite structures and with an optimum crystallization for temperatures Tdep=160–180 °C. Indeed, in this temperature range, well crystallized and large grains were obtained which was in good correlation with the film morphology determined by scanning electron microscope; and Hall effect measurements revealed low resistivities, high carrier concentrations (>1019 cm−3), and low mobilities. From these results, the authors propose the existence of a temperature range where the properties undergo significant changes while the atomic composition remains constant.

Exploring the local electronic structure and geometric arrangement of ALD Zn(O,S) buffer layers using X-ray absorption spectroscopy

“…Allowing atomic scale control of film thickness and composition, atomic layer deposition (ALD) enables the necessary finetuning of the Zn(O,S) system. 10,[14][15][16][17] ALD was used to deposit Zn(O,S) by alternating sequences of diethylzinc (DEZ), H 2 O and H 2 S exposures, where Zn(O,S) formation was suggested to proceed via the incorporation of sulfur into the ZnO matrix, causing changes to the local bonding environment. 7,18,19 Samples named ZnO and ZnS served as our references.…”

Growth Mechanism and Film Properties of Atomic-Layer-Deposited Titanium Oxysulfide

“…For ZnO x S y , this reaction has been reported to have a negative Gibbs energy of −75.5 kJ/mol and is thus thermodynamically favorable. 14 Similarly, Choi et al showed through a density functional theory (DFT) study that for SnO x S y , the exchange of a surface OH group with H 2 S is exothermic (−46.0 kJ/mol). 6 The supercycle-based approach to prepare ALD TiO x S y films used in this work combines ALD TiO x with an ALD TiS x recipe that was recently developed in our group for the synthesis of 2D TiS x -based films.…”

“…For ZnO x S y , this reaction has been reported to have a negative Gibbs energy of −75.5 kJ/mol and is thus thermodynamically favorable . Similarly, Choi et al showed through a density functional theory (DFT) study that for SnO x S y , the exchange of a surface OH group with H 2 S is exothermic (−46.0 kJ/mol) …”

“…Moreover, ALD can yield a very high level of control over the film composition, which is most commonly achieved by combining multiple ALD processes in a supercycle fashion . Nevertheless, although this supercycle method has been successfully applied to prepare a wide variety of material systems, there exist many examples in the literature which show that there can be various “growth effects” when combining ALD cycles into a supercycle scheme. Examples include a reduced growth rate, a nucleation delay, or even etching when alternating the ALD processes within the supercycle.…”

“…For ALD metal oxysulfides, in particular, exchange reactions involving oxygen and sulfur have been shown to affect the film composition strongly. Specifically, ALD ZnO x S y and SnO x S y films have been reported to preferentially incorporate S due to the following exchange reactions. ,− …”