Study of Schottky diodes based on ultrawide-band gap semiconductors

In this thesis, Ni/β-Ga2O3 Schottky barrier diodes (SBDs) deposited by confined magnetic field based sputtering (CMFS) and Electron-beam evaporation (EBE), are modelled using SILVACO-Atlas and compared to experimental measurements. Firstly, Forward and reverse current of CFMS SBDs were studied. A model was developed to understand the role of Ni atoms diffusion in the surface of β-Ga2O3. In this model, Ni out diffusion combines with oxygen to form a new (NixGa1−x)2O3 interfacial layer. This new compound is modelled as a semiconductor with different energy gap and affinity and less defects since Ni diffusion compensates Ga vacancy related defects. A good agreement between simulation and measurement for forward at high and low temperatures and reverse current with the consideration of band-to-band (BBT) and impact ionization for the reverse current. The achieved agreement demonstrates the soundness of the proposed model. In addition, temperature dependent SBD characteristics were studied. At room temperature, the deviation of SBD parameters from the ideal case is due to the effect of interfacial states due to plasma and Ar bombardment. It was found that the Schottky barrier height

Hole diffusion effect on the minority trap detection and non-ideal behavior of NiO/β-Ga2O3 heterojunction

A NiO/β-Ga2O3 heterojunction was fabricated by sputtering a highly p-doped NiO layer onto β-Ga2O3. This heterojunction showed a low leakage current and a high turn-on voltage (Von) compared to a Ni/β-Ga2O3 Schottky barrier diode. The extracted Von from the NiO/β-Ga2O3 heterojunction's forward current-voltage characteristics was ∼1.64 V, which was lower than the extracted built-in potential voltage (Vbi) obtained from the capacitance-voltage curve. To explain this difference, deep level transient spectroscopy and Laplace-deep level transient spectroscopy were employed to study majority and minority traps in β-Ga2O3 films. A minority trap was detected near the surface of β-Ga2O3 under a reverse bias of −1 V but was not observed at −4 V, indicating its dependence on hole injection density. Using Silvaco TCAD, the hole diffusion length from P+-NiO to β-Ga2O3 was determined to be 0.15 μm in equilibrium, which is increased with increasing forward voltage. This finding explained why the trap level was not detected at a large reverse bias. Moreover, hole diffusion from NiO into β-Ga2O3 significantly affected the β-Ga2O3 surface band bending and impacted transport mechanisms. It was noted that the energy difference between the conduction band minimum (CBM) of β-Ga2O3 and the valence band maximum (VBM) of NiO was reduced to 1.60 eV, which closely matched the extracted Von value. This supported the dominance of direct band-to-band tunneling of electrons from the CBM of β-Ga2O3 to the VBM of NiO under forward bias voltage

Control of Ni/β-Ga2O3 Vertical Schottky Diode Output Parameters at Forward Bias by Insertion of a Graphene Layer

Controlling the Schottky barrier height (ϕB) and other parameters of Schottky barrier diodes (SBD) is critical for many applications. In this work, the effect of inserting a graphene interfacial monolayer between a Ni Schottky metal and a β-Ga2O3 semiconductor was investigated using numerical simulation. We confirmed that the simulation-based on Ni workfunction, interfacial trap concentration, and surface electron affinity was well-matched with the actual device characterization. Insertion of the graphene layer achieved a remarkable decrease in the barrier height (ϕB), from 1.32 to 0.43 eV, and in the series resistance (RS), from 60.3 to 2.90 mΩ.cm2. However, the saturation current (JS) increased from 1.26×10−11  to 8.3×10−7(A/cm2). The effects of a graphene bandgap and workfunction were studied. With an increase in the graphene workfunction and bandgap, the Schottky barrier height and series resistance increased and the saturation current decreased. This behavior was related to the tunneling rate variations in the graphene layer. Therefore, control of Schottky barrier diode output parameters was achieved by monitoring the tunneling rate in the graphene layer (through the control of the bandgap) and by controlling the Schottky barrier height according to the Schottky–Mott role (through the control of the workfunction). Furthermore, a zero-bandgap and low-workfunction graphene layer behaves as an ohmic contact, which is in agreement with published results

Schottky contact diameter effect on the electrical properties and interface states of Ti/Au/p-AlGaAs/GaAs/Au/Ni/Au Be-doped p-type MBE Schottky diodes

Schottky diodes based on Be-doped p-type AlGaAs were grown by molecular beam epitaxy (MBE) and their current-voltage (I-V) and capacitance-voltage (C-V) characteristics measured. The Schottky and Ohmic contacts are Ti/Au and Au/Ni/Au, respectively. The effect of the Schottky contact diameter on I-V and C-V characteristics was studied. To elucidate this effect, the Schottky diode figures of merits and interface states are extracted from I-V and C-V characteristics, respectively. It was found that interface states density increases with increasing Schottky contact diameter then saturates beyond 400 µm. The frequency dependence of the C-V characteristics was also related to these interface states. The results of this present study can help choosing the right Schottky contact dimensions

On the nature of majority and minority traps in β-Ga2O3: A review

In the last decade, researchers and commercial companies have paid great attention to ultrawide bandgap semiconductors especially gallium oxide (Ga2O3). Ga2O3 has very interesting properties such as a bandgap higher than 4.8 eV, high electrical breakdown field and easy to control the doping density. For example, vacancies and impurities play an important role in controlling the n-type conductivity of this material and hence improving the device performance. This review paper discusses mostly the point defects in Ga2O3 and the sources of majority and minority deep levels (traps) in Ga2O3 characterized using different methods such as deep level transient spectroscopy (DLTS), optical DLTS (ODLTS), deep level optical spectroscopy (DLOS) and other techniques. Majority traps such as E1, E2*, E2 and E3 with energies of about 0.56, 0.75, 0.79 and 1.05 eV below the conduction band maximum (CBM), respectively, are the most observed in Ga2O3. These traps are mostly related to impurities such as iron (Fe), silicon (Si), titanium (Ti) and other impurities, or alternatively to gallium or oxygen vacancies. Minorities traps H1, H2 and H3 with energies of about 0.2, 0.3 and 1.3 eV, respectively, above the valence band maximum (VBM) are the most known defects that are related to vacancies. These minorities traps are usually extracted using optical techniques because of the very low hole density in Ga2O3

Ultrahigh Photoresponsivity of W/Graphene/β-Ga₂O₃ Schottky Barrier Deep Ultraviolet Photodiodes

The integration of graphene with semiconductor materials has been studied for developing advanced electronic and optoelectronic devices. Here, we propose ultrahigh photoresponsivity of β-Ga2O3 photodiodes with a graphene monolayer inserted in a W Schottky contact. After inserting the graphene monolayer, we found a reduction in the leakage current and ideality factor. The Schottky barrier height was also shown to be about 0.53 eV, which is close to an ideal value. This was attributed to a decrease in the interfacial state density and the strong suppression of metal Fermi-level pinning. Based on a W/graphene/β-Ga2O3 structure, the responsivity and external quantum efficiency reached 14.49 A/W and 7044%, respectively. These values were over 100 times greater than those of the W contact alone. The rise and delay times of the W/graphene/β-Ga2O3 Schottky barrier photodiodes significantly decreased to 139 and 200 ms, respectively, compared to those obtained without a graphene interlayer (2000 and 3000 ms). In addition, the W/graphene/β-Ga2O3 Schottky barrier photodiode was highly stable, even at 150 °C