High‐Mobility p‐Type Transistor Based on a Spin‐Coated Metal Telluride Semiconductor

Abstract

Recently, we demonstrated the solution deposition of ultrathin, high-mobility metal chalcogenide films using hydrazine or hydrazine/water mixtures as solvent. [1,2] The process involves forming soluble hydrazinium-based precursors of targeted metal chalcogenide semiconductors (e.g., SnS 2-x Se x ), spin-coating films of the precursors, and thermally decomposing the precursor films at low temperature to form the desired metal chalcogenides. Additionally, indium(III) selenide films have been spin-coated, using the corresponding hydrazinium precursor and mixed ethanolamine/dimethyl sulfoxide (DMSO) solvents rather than hydrazine.[3] Thin-film field-effect transistors (TFTs) have been prepared, based on the solution-deposited semiconducting chalcogenides, yielding n-type channels with mobilities in excess of 10 cm 2 V -1 s -1 -approximately an order of magnitude better than previous results for spin-coated or drop-cast semiconductors.[4-8] The higher mobilities may extend the application range for spin-coated films to higher-end devices than currently envisioned for analogous organic-based systems (e.g., very large area electronics, logic applications).In each of the previously reported spin-coated TFT channel layers, [1][2][3] the semiconductor has been a metal sulfide or selenide and the resulting electronic transport has been n-type. To further investigate the hydrazine-based precursor route to chalcogenide semiconductors and films, it is important to explore beyond the sulfides and the selenides and to identify high-mobility p-type systems to complement the n-type examples already demonstrated. Solar cells, for example, rely on the formation of a junction between n-and p-type layers (to drive charge separation at the interface) and the availability of both n-and p-type transistors is also critical for the realization of complementary metal-oxide semiconductor (CMOS) technology. Recently, we demonstrated the deposition of p-type ZnTe films from a hydrazine-based solution.[9] However, initial results employing these telluride-based films in transistors did not yield high mobility, perhaps because of poor grain structure or deleterious surface states in the ultrathin films. Films of p-type CuInSe 2 have similarly been deposited from a hydrazine-based solution, although again the performance of the devices was not in the range of the previous n-type systems (< 1 cm 2 V -1 s -1 ).

A High‐Efficiency Solution‐Deposited Thin‐Film Photovoltaic Device

“…Optimization of the solutionprocessed PV devices has just begun and additional improvement in device efficiency can be expected as absorber layer composition is further refined, both using uniform and graded compositional profiles. Additionally, as previously demonstrated, [22][23][24] the ability to use hydrazine as a solvent for a wide range of metal chalcogenides (including Sb 2 Se 3 and CuInTe 2 ) suggests that the approach should be more generally applicable to other prospective metal-chalcogenide-based absorbers. The use of single, molecular-based (rather than nanoparticlebased) precursor solutions to achieve high-quality absorber layers should also be extendable to processes in which hydrazine is fully or partially replaced with other solvents.…”

“…[12,21] In an effort to significantly reduce fabrication costs relative to vacuum-based approaches (e.g., no vacuum requirements, less energy intensive deposition, better materials utilization efficiency), as well as known solution-based approaches (e.g., fewer processing steps, no high-temperature selenization treatment, more facile Ga incorporation), we are pursuing a simple molecular-based approach for solution-depositing CIGS layers. The targeted hydrazine-based deposition process relies on forming a soluble molecular-based precursor comprising metal chalcogenide anions separated by hydrazinium (i.e., N 2 H 5 þ ) cations, analogous to that described previously for the deposition of ultrathin (nanometer-scaled) films of chalcogenides for transistor applications, [22][23][24] but with the added requirement of orders of magnitude thicker (micrometer-scaled) films and more complex stoichiometry control. Deposition involves three steps: (i) dissolve the elements in a hydrazine-based solution at room temperature, (ii) process the precursor solution into thin-film form, and (iii) heat-treat in an inert atmosphere to yield the CIGS layer.…”

Solution Processing of Chalcogenide Semiconductors via Dimensional Reduction

“…Demonstration of p-type films using the hydrazine-based process was pursued using the ternary chalcopyrite systems, CuInTe 2 and CuInSe 2 . [44,46] CuInTe 2 proved to be a particularly interesting example because the zinc-blende analog is not substantially soluble in hydrazine, thereby requiring the development of a modified approach for deposition, and it also represents a first example of deposition of a telluride-based system using the hydrazine-based approach (another example is ZnTe). [43] Crystals of CuInTe 2 also offer a band gap of %1 eV and relatively high Hall mobilities (p-type transport), ranging from 30 to 150 cm 2 V À1 s À1 , depending upon preparation conditions.…”

“…[61][62][63] Since CuInTe 2 does not significantly dissolve in hydrazine, a new stepwise process was devised for dissolution, which involves separate indium-and copper-based precursor solutions. [46] One component, In 2 Te 3 , readily goes into solution at a concentration of 0.05 M in hydrazine. The other prospective component, Cu 2 Te, is not substantially soluble in hydrazine.…”

“…[39] The simple approach described in this report generally employs additional chalcogen (S, Se, Te), typically in anhydrous hydrazine, to directly improve the solubility and film forming properties of selected metal chalcogenides, including SnSe 2-x S x , In 2 Se 3 , GeS 2 , GeSe 2 , Cu 2 S, Sb 2 Se 3 , Sb 2 Te 3 , CuInSe 2 , Cu(In,Ga)Se 2-x S x , and Ga 2 Se 3 . [40][41][42][43][44][45][46][47] Dissolution generally proceeds by formation of metal chalcogenide anions accompanied by hydrazinium cations. The additional chalcogen added to the hydrazine facilitates the disruption of the metal chalcogenide framework by breaking up M-X-M linkages, in analogy to the dimensional reduction examples described above.…”