Louis J. Guido

Many emerging GaN electronic and optoelectronic devices comprise p-GaN layers buried below n-GaN or AlGaN. The activation of these buried p-GaN layers usually relies on the lateral hydrogen diffusion through the etched mesa sidewalls, which is known to induce nonuniform acceptor distributions. However, the acceptor profile, electric field ( ${E}$ -field) blocking capability, and leakage current mechanisms of the sidewall activated p-GaN layer have not been fully understood. This work addresses these knowledge gaps by fabricating vertical GaN p-n diodes with a thick ( $3.8~\mu \text{m}$ ) p-GaN. Two activation schemes were performed to allow the hydrogen diffusion through sidewalls and the top surface. For the sidewall activation, an analytical model was developed to depict the spatial distribution of the activated acceptor and the temporal evolution of this distribution with the increased annealing time. This model was validated using the ${C}$ – ${V}$ characteristics of the fabricated diodes with various radii. Under reverse biases, the breakdown ${E}$ -field and leakage current of the sidewall-activated diodes were found to be determined only by the edge area with the highest activation efficiency. The leakage mechanism agrees with the trap-assisted tunneling (TAT) model, and the peak junction ${E}$ -field can exceed 3 MV/cm, both being similar to those of surface-activated diodes. These results provide critical information for the design and processing of advanced GaN devices with the buried p-GaN.

Schottky diodes were formed by oxidizing Ru thin films deposited on n-type GaN at 400, 500, and 600 °C in normal laboratory air, and their electrical behavior was compared to that of a Ru/n-GaN reference device. The GaN epitaxial layers were grown via metalorganic chemical vapor deposition. The ruthenium films were deposited by electron beam evaporation. The Schottky barriers were characterized via current vs voltage (IV) and deep-level transient spectroscopy (DLTS) measurements between 70 and 400 K. The temperature dependent forward bias IV characteristics were fit, and the extracted temperature dependence of the effective barrier height for each device was shown to be caused by inhomogeneity at the metal/semiconductor interface. It was found that barrier inhomogeneity could be well described by a modified log-normal distribution. In reverse bias, it was shown that the low-energy tail of the barrier distribution is an important factor in determining leakage current. Favorable results occur for diodes oxidized at 400 and 500 °C, but raising the oxidation temperature to 600 °C results in a drastic increase in leakage current. DLTS measurements reveal one electron trap at E_C − 0.57 eV in each of the samples. It was found that the concentration of this 0.57 eV trap increases substantially at 600 °C and that trap-assisted tunneling likely contributes an additional pathway for reverse leakage current.

Carbon, a compensator in GaN, is an inherent part of the organometallic vapor phase epitaxy (OMVPE) environment due to the use of organometallic sources. In this study, the impact of growth conditions are explored on the incorporation of carbon in GaN prepared via OMVPE on pseudo-bulk GaN wafers (in several cases, identical growths were performed on GaN-on-Al₂O₃ templates for comparison purposes). Growth conditions with different growth efficiencies but identical ammonia molar flows, when normalized for growth rate, resulted in identical carbon incorporation. It is concluded that only trimethylgallium which contributes to growth of the GaN layer contributes to carbon incorporation. Carbon incorporation was found to decrease proportionally with increasing ammonia molar flow, when normalized for growth rate. Ammonia molar flow divided by growth rate is proposed as a reactor independent predictor of carbon incorporation as opposed to the often-reported input V/III ratio. A low carbon concentration of 7.3 × 10¹⁴ atoms/cm³ (prepared at a growth rate of 0.57 µm/h) was obtained by optimizing growth conditions for GaN grown on pseudo-bulk GaN substrates.

The current versus voltage (I-V) characteristics of a Ni/GaN Schottky diode are measured from 50 to 400 K and the temperature dependence of the extracted barrier heights and ideality factors is described as a consequence of lateral inhomogeneity at the metal-semiconductor (M-S) interface. It is shown that by invoking a modified log-normal distribution of barrier heights at the M-S interface, the extracted barrier height temperature dependence can be well explained. Further, it is shown that this approach can describe the voltage dependence of the lateral barrier distribution revealing that for effective barrier height values calculated at increasingly higher voltages, the distribution begins to converge on a single value of 0.77 eV. This value is in good agreement with the flat-band barrier height of 0.77 ± 0.02 eV extracted from capacitance-voltage (C-V) measurements on the same device. The same procedure is used to describe the parallel conduction path apparent at low temperatures, revealing its behavior is indicative of an additional Schottky region with an increased density of low barriers which are more heavily perturbed by external bias. Finally, the model is successfully applied to previously published work on various Schottky diodes structures.

High-performance vertical GaN-based p-n junction diodes fabricated using bandgap selective photoelectrochemical etching-based epitaxial liftoff (ELO) from bulk GaN substrates are demonstrated. The epitaxial GaN layers and pseudomorphic InGaN release layer were grown by MOCVD on bulk GaN substrates. A comparison study was performed between devices after liftoff processing (after transfer to a Cu substrate) and nominally identical control devices on GaN substrates without the buried release layer or ELO-related processing. ELO and bonded devices exhibit nearly identical electrical performance and improved thermal performance, compared with the control devices on full-thickness GaN substrates. The breakdown voltage, ideality factor, and forward turn-ON performance were found to be nearly identical, indicating that the transfer process does not degrade the quality of the p-n junctions. The devices exhibit turn-ON voltages of 3.1 V at a current density of 100 A/cm ² , with a specific ON-resistance ( ${R} _{ \mathrm{\scriptscriptstyle ON}}$ ) of 0.2–0.5 $\text{m}\Omega \cdot \text {cm}^{{2}}$ at 5 V and a breakdown voltage ( ${V} _{\text {br}}$ ) of 1.3 kV. Both optical and electrical characterization techniques show that the thermal resistance of ELO devices bonded to a Cu carrier is approximately 30% lower than that for control devices on GaN substrates.

A semiconductor device which converts electrical energy into electromagnetic radiation, sued for lig…

An AlxGa1−xAs p-n junction light emitting diode, with a thin In2O3 current spreading layer, on the top side, and an indirect band-gap AlAs-Al0.6Ga0.4As distributed Bragg reflector, on the bottom side, has been fabricated and characterized under continuous-wave, room-temperature operation. These relatively simple modifications yield a 15-fold increase in electrical-to-optical power conversion efficiency at λ∼675 nm, and a significant reduction in active region heating—in comparison with standard devices prepared and tested as controls.

The relationship between the magnitude of the piezoelectric field and the degree of lattice constant mismatch is investigated via low‐temperature photoluminescence measurements of the quantum‐confined Stark effect for a series of (111)B Al_0.15Ga_0.85As‐In_yGa_1−yAs pseudomorphic quantum well heterostructures. The experimental strain‐induced electric field values agree well with theoretical calculations for indium mole fractions in the range 0.037≤y≤0.09. In addition, an anomalous saturation of the photoluminescence transition energy is observed at values of applied voltage greater than that required to nullify the piezoelectric field, despite the indication from separate electroreflectance measurements that the net electric field within the quantum well reverses polarity under similar electrical biasing conditions.