Vertical structure GaN power devices fabricated on freestanding GaN substrates have high potentials in ideally efficient energy conversion systems. This letter describes successful acquisition of ...high avalanche capabilities for high breakdown voltage GaN p-n junction diodes. We have already reported the high avalanche capability by applying a punch-through structure in p-GaN layer, however; the structure caused a drawback of increase in on-resistance and turn-on voltage. By applying a newly developed two-step mesa structure consisting of the first inner mesa with partially thinned p-GaN layer and the second outer mesa etched to n-GaN drift layer, the high avalanche capabilities with reversible current-voltage characteristics have been realized at high breakdown voltages of 4.7-4.8 kV without sacrificing the forward I-V characteristics. The two-step mesa structure transferred the position of the peak electric field in the p-n junction from the dry-etch damaged second outer mesa to the first mesa covered by the thinned p-GaN layer, which could lead the mild breakdown. The high avalanche capability was obtained with good reproducibility regardless of the anode electrode diameter. This structure can contribute to the construction of robust power systems.
This letter describes a new two-step electrode process on p-GaN and characteristics of GaN p-n junction diodes on free-standing GaN substrates with low specific ON-resistance R οn and high breakdown ...voltage V B . We develop a two-step process for anode electrodes in order to avoid plasma damage to the p + -GaN contact layer during the sputtering process. The specific ON-resistance is further improved due to a new low-damage process. The breakdown voltage of the diodes with the field-plate (FP) structure is over 1100 V, and the leakage current was low, i.e., in the range of 10 -9 A. The specific ON-resistance of the diodes of 50 μm in diameter with the FP structure was 0.4 mΩ · cm 2 . Baliga's figure of merit (V B 2 /R on ) of 3.0 GW/cm 2 is obtained. These are the best values ever reported among those achieved by GaN p-n junction diodes on free-standing GaN substrates.
By eliminating the influence of surface recombination, the reported consideration that on‐resistance of GaN p+n diodes fabricated on free‐standing substrates should be limited by nonradiative ...recombination around dislocation lines is validated. The validation is based on analyses of 1) bulk nonradiative recombination current densities (Jnr), determined from the y intercept of forward‐current density/inverse of anode radius plots, of diodes fabricated on two kinds of vapor‐phase epitaxially grown freestanding substrates (i.e., maskless 3D M‐3D, dislocation density Ndis ≤ 4 × 105 cm−2 and void‐assisted separation VAS, Ndis = 1–4 × 106 cm−2) and 2) distributions of two‐photon photoluminescence (2PPL) from an n‐GaN layer identifying dislocations as dark spots. Based on the extracted values of Jnr, the Shockley–Read–Hall lifetime of GaN p+n diodes on M‐3D and VAS substrates, τSRHM‐3Dand τSRHVAS are, respectively, estimated to be 11 and 2 ns. Under the assumption of high excitation for the 2PPL measurement, the use of τSRHM‐3D of 11 ns reproduces the 2PPL data with the effective dislocation radius ranging from 0.2 to 2 nm.
The reported limitation on the on‐resistance of GaN p+n diodes is mainly validated based on bulk nonradiative recombination current densities determined from the y intercept of forward‐current density/inverse of anode radius plots of diodes on maskless 3D (M‐3D; dislocation density Ndis ≤ 4 × 105 cm−2) and void‐assisted separation (VAS; Ndis = 1–4 × 106 cm−2) substrates.
Impurity incorporation during vapor-phase epitaxy on stepped surfaces was modeled by classifying rate-limiting processes into i) surface diffusion, ii) step kinetics, and iii) segregation. Examples ...were shown for i) desorption-limited Al incorporation during chemical vapor deposition (CVD) of (0001) SiC, ii) preferential desorption of C atoms from kinks during CVD of Al-doped (000-1) SiC, and iii) segregation-limited C incorporation during metalorganic vapor-phase epitaxy of (0001), (000-1), and (10-10) GaN.
The influence of surface recombination on forward current-voltage (I F -V F ) characteristics of gallium nitride (GaN) p + n diodes formed on GaN free-standing substrates was both experimentally and ...computationally investigated. In the temperature range of 373-273 K, the surface-recombination velocity 5 of 0.5-μm n GaN overetched circular-mesa diodes was derived, respectively, as 2-10 × 10 7 and 1 × 10 7 cm/s for diodes with and without a mesa-field-plate termination structure consisting of dielectric and metal layers. It was shown that IF (V F <; 2.9 V) is dominated by carrier recombination at the side surface of the etched n GaN. The I F -V F characteristics of the fabricated diodes were compared with the reported GaN p + n diodes with almost-zero overetched depth of n - GaN. The large I F of the latter diodes is attributed to enhanced photon recycling through photon reflection at the metal mirror. Moreover, reducing S to less than 10 5 cm/s is considered indispensible for achieving similar enhanced photon recycling in overetched terminated diodes.
Inductively coupled plasma dry etching has been proceeded to form high breakdown-voltage mesa-structured GaN p-n junction diodes for power electronics applications. Occurrences of crystalline damages ...were inevitable during the etching by accelerated reactive ions. However; the damages were removed by thermal treatment at 850°C, which lead low specific on-resistances (Ron<1mΩcm2) and high breakdown voltages (VB>4.2kV) of the diode.
