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Deposition and Characterization of Post-growth annealed ZnSnN2 thin films.


In order to cater to modern day photovoltaic and solar needs, semiconductor materials which are earth abundant with a direct bandgap (Eg) around 1.5eV are necessary[1,2] . Of the commonly known semiconductor materials like Si, Ge or binary III-V (Where III = B, A, Ga, In and V = N, P, As, Sb) and II-VI (Where II = Zn, Cd; and VI = O, S, Se, Te), only a few candidates consist of bandgaps which are in the range 1.0–2.0 eV. The requirement for efficient, high quality photovoltaics energy conversion and opto-electronic devices which can be an alternative to the current available semiconductor materials inspires the search for other earth abundant materials [3]. One way to search for these new materials is by studying ternary or multi-ternary semiconductor materials which have more opto-electronic properties.

The Zn-IV-N2 group of semiconductor materials has been attracting a lot of attention following the synthesis of Sn containing material, ZnSnN2[4-5]. These novel Zn-IV-N2 group of semiconductor materials can act as a potential earth abundant alternative to InGaN and other thin film solar cell materials such as CdTe and CuInGaSe2 for use in light generation and light absorption applications. For materials such as Zn(Ge,Sn)N2 alloys, absorption edges which are in the range of 2.0 – 3.1eV have been shown already[6]. And an even larger spectral distribution from the infrared region to the ultraviolet region is expected for Zn(Si,Sn)N2 alloys. Even wider spectral coverage from the infrared to the ultraviolet is predicted for Zn(Si,Sn)N2 alloys[7]. Of all the potential options, the ZnSnN2 material exhibits properties such as large optical absorption coefficient and a tunable bandgap range among others that make it a really good potential material as the absorber layer for the next generation PV devices.

One of the major challenges facing this material is the discrepancies between the measured and calculated bandgap values. The calculated bandgap values are in the range of 0.35 – 2.64 eV depending on the crystal structure assumed[39] . Whereas the value of the experimental bandgap lies between 1.7 -2.1 eV [40]. One of the reasons for this inability to converge on a bandgap value has been attributed to the bandgap filling due to the degenerate carrier concentration of the material[38] (around 1021 – 1022 per cm-3).

In this dissertation, issues related to the carrier concentration in ZnSnN2 material are studied. Most of the work done focuses on efforts to reduce the carrier concentration in ZnSnN2 films to around 1018 per cm-3 range. The growth method used for growing the ZnSnN2 films was RF sputtering using Zn-Sn cathodes containing Zn-Sn in the ratio 3:1. RF Sputtering was chosen because it produces films that are more uniform than the films fabricated using other methods like vapor-liquid-solid plasma assisted growth or CVD[41]. The films were grown on c-plane Sapphire and (0001) GaN substrates. The so obtained films were characterized using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Hall Effect measurement.

The films were later subjected to rapid thermal annealing (RTA) at various temperatures to study the effect of rapidly heating the samples to high temperatures, holding them for a given time and cooling them quickly. The samples subjected to RTA were characterized by XRD, SEM and Hall Effect measurements. The results indicated a significance decrease in the electron concentration of the material after the annealing process which could make ZnSnN2 an interesting alternative as an earth abundant semiconductor material for future absorber layers.

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