Controlling hydrogen induced n-type doping in InGaZnO₄ films.

 

Semiconducting oxides (SCO) are commonly used in electronic devices, such as thin-film-transistors (TFTs) in optical displays and are now becoming increasingly important in memory devices. Because most SCOs are exclusively n-type, inversion of TFT channels is absent and enables very low off-currents under negative gate biasing. Especially when the lack of mobile holes is combined with an amorphous morphology is exploited, the channel current leakage drops below the gate dielectric leakage, enabling the TFT to control charge storage in dynamic random-access memories (DRAM). 

 

We are exploiting Indium-Gallium-Zinc-Oxide (IGZO) as a SCO to reach DRAM retention times in the order of 10⁴ seconds. Unfortunately, the reliability of OSC TFT’s is dominated by the free electron concentration which remains difficult to control due to shifts in the ionic balance. In IGZO the bonding is also predominantly ionic in nature. The ionic charge balance between cations and anions is easily disturbed by introduction hydrogen and the easy reduction of the oxide, causing oxygen deficiency.

 

In this work we investigate how the shift of ionic charge balance can be mitigated by the formation of covalent bonds. First, we show how large amounts of hydrogen are trapped to excess oxygen during hydrogen annealing without increasing the free electron concentration. The excess oxygen concentrations in IGZO are controlled by addition of oxygen in the physical vapor deposition (PVD) process. Substrate free Rutherford Back Scattering (RBS) shows that this can be as high as 3 at.%. Hydrogen concentrations profiles are obtained from calibrated ToFSIMS and show increased concentrations matching to layers with higher oxygen contents (see fig. 1). With using a crystalline template of Ga₂ZnO₄, the IGZO films are no longer amorphous, but poly-crystalline with a spinel crystal structure. This allows to investigate the oxygen interstitial interaction with hydrogen in larger detail and to establish a model for amorphous phase. 

 

Next, the addition of MgO to IGZO is explored. Mg has a stronger tendency to bind hydrogen, deactivating the electron doping properties. Moreover, replacing Zn for Mg improves stability of the spinel phase. We will report on the composition window of Mg/Zn at which amorphous/spinel transition occurs in InGa(Zn/Mg)O₄ and discuss the correlation with free carrier concentrations induced by hydrogen doping.