Fabrication and modelling of MgZnO/Zno based heterostructures to realize 2D confinement of electron gas for HEMT application

Abstract

ZnO has emerged as the subject of research for many electronic and optoelectronic applications in recent years because it is nontoxic, abundant, chemically stable, and biocompatible. Due to its direct wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, many researchers have explored it for devices like light emitting diode, photodetectors, lasers, and solar cells, etc. Due to the large spontaneous and piezoelectric polarization, ZnO based heterostructures are also being explored for two-dimensional electron gas (2DEG) density and applications in heterostructure field-effect transistors (HFETs). This is mainly due to the advantages it presents, such as high saturation velocity, low lattice mismatch, high conduction band offset, endurance to radiation damage, availability of native substrates, amenability to wet chemical processing, and feasibility of achieving high-quality films. In MgxZn1-xO alloy, the energy band gap and degree of polarization can be fine-tuned by varying the Mg molar composition. Moreover, as compared to othersemiconductor materials such as GaN, SiC, and AlN, ZnO offers some fundamental advantages such as high breakdown strength and high-temperature operation.In this research work, the 2DEG was realized in MgZnO/ZnO material system using Dual Ion Beam Sputtering (DIBS) system. The analytical model for the estimation of 2DEG density was also developed. My findings on MgZnO/ZnO material system finds application on the development of ZnO based heterostructure field effect transistorsFirst of all, an analytical model for the estimation of 2DEG density was developed for graded MgZnO/ZnO heterostructures. Here, graded means MgZnO layers of different thickness and different Mg molar compositions deposited one over the other on the ZnO buffer layer. The developed model was based on the continuity of the electric field at the interfaces of different layers, dominant piezoelectric and spontaneous polarization components in different layers, Mg composition, and layers thickness. The developed generic model can be easily reduced for relatively simpler structures. Calculated 2DEG density (ns) values suggest that the use of graded MgZnO layer significantly increases (2.3 times higher) the 2DEG density in MgZnO/ZnO heterostructure but at the expense of an increased threshold voltage (VOFF). A graded structure with a lower thickness of layers was proposed, to achieve a compromise between high 2DEG density and VOFF, based on the developed model.After the development of an analytical model for the estimation of 2DEG density, the study of contact properties on MgZnO (barrier layer) layer was done with the aim of fabricating a good Schottky contact. For this, Mg0.05Zn0.95O (MZO) film of 400 nm thickness was deposited using DIBS at 300 °C and 80 % Oxygen partial pressure. The p-Si (111) was used as the substrate. Various material characterizations including Hall, XRD, EDX, and spectroscopic ellipsometry were performed to investigate the film properties. Finally, Schottky contact was fabricated using DIBS at room temperature by depositing 100 nm thick and 12.38 μm2 Gold contact using shadow masking techniques. The properties of Schottky contact was measured using temperature dependent I-V and C-V measurement. The Hall measurement demonstrates n-type conduction with resistivity, carrier concentration, and mobility of the MgZnO film as 0.12 Ω-cm, 6.05 × 1017cm-3, and 85.12 cm2V-1s-1, respectively at room temperature. XRD measurement demonstrates (002) crystal orientation of the film. The apparent Schottky barrier height (SBH) and the ideality factor obtained from the I-V measurements were observed to increase and reduce, respectively, with increasing measurement temperature. That anomalous observation in the behavior of the SBH was in good agreement with the predictions of a double Gaussian distribution (DGD) of the inhomogeneous SBH at a metal-semiconductor (MS) interface. The values of the SBH as determined from C-V measurements were expectedly higher than those extracted from I-V measurements. The DGD model was observed to fit the experimentally obtained data for temperature dependent SBH with mean values of the SBH as 0.95 and 0.54 eV and standard deviations as 0.131 and 0.072 eV in the temperature range of 160-300 K and 80-160 K, respectively. The larger value of the SBH standard deviation confirms more SBH inhomogeneity at the MS interface, and those inhomogeneities were attributed to the presence of deep level or surface level interface states. The calculated interface states density was seen to vary from 6.46 × 1014 eV-1cm-2 at EC-0.27 eV to 1.58 × 1014 eV-1cm-2 at EC-0.74 eV, where EC is thebottom of a conduction band at 300 K. After the successful fabrication of Schottky contact on barrier layer MZO film, 2DEG density was successfully realized in MgZnO/ZnO based heterostructures fabricated by DIBS. All the layers of the heterostructures were deposited in the same ambient, i.e., at 300 °C and 60 % Oxygen partial pressure. The effect of variation of Mg composition, barrier layer thickness, and cap layer thickness on 2DEG density was studied. The value of ns was found to be increasing with Mg composition for bilayer (MgZnO/ZnO) and capped (ZnO/MgZnO/ZnO) heterostructures, attaining values of 1.11 × 1014 cm-2 and 1.13 × 1014 cm-2 for x = 0.3 for the bilayer and capped heterostructure, respectively, from ~5 × 1012 cm-2 for the bare ZnO. The value of ns initially increases and then saturates with the increase of barrier layer thickness in bilayer heterostructure keeping the Mg molar composition constant in the barrier layer. For the capped heterostructurethe ns initially decreases and then saturates with the increase of cap layer thickness, provided, all other parameters are kept constant. The source of electrons in 2DEG was also investigated, and the distinct roles of donor and acceptor states along with interface charges in 2DEG formation were analyzed. It was found that in case of sputter deposited samples, the dominant source of 2DEG electrons is interface charge. The fabrication of source and drain contacts on top of MgZnO/ZnO heterostructures was also attempted by lithography and lift-off process for transistor application. The source and drain contacts on top of MgZnO/ZnO heterostructures were fabricated and characterized successfully by lift-off process which involves three lithography steps. The Au/Ti was used for source and drain ohmic contacts in which first of all 10 nm of Ti was deposited and then 200 nm of Au. To prove the ohmic nature of these contacts the current vs. voltage measurement was taken and was found to be linear. The gate contact of Pt/Ti was also attempted on top of MgZnO/ZnO heterostructure using lift-off. But the two lift-off attempts of gate metallization failed and hence remains as future course of action to realize reliable gate.