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Current relaxations in AlGaN/GaN high electron mobility transistors (HEMTs) often show a broad spread of relaxation times. These are commonly linked to the ionization energies of the traps in different regions of the devices and the relaxations are assumed to be exponential. To explain the observed spread of parameters, the presence of multiple centers is assumed. However, in actual spectra, only a few main peaks in the lifetimes distributions are observed, with considerable broadening of the peaks. In this paper, the authors examine the possible origin of the relaxation time broadening, including the presence of disorder giving rise to extended exponential decays and to physical broadening of discrete levels into bands. The latter is modeled by Gaussian broadening of the logarithm of relaxation time. The authors demonstrate the analysis of the peak positions and widths of the first derivative of the current transient by the logarithm in time, which is quite useful in deriving the relevant broadening parameters. They illustrate the approach for current relaxations in HEMTs for different pulsing modes.

The dependence of barrier height inhomogeneity on the gate metal has been investigated for the AlGaN/GaN Schottky diode. The analysis from the electroreflectance spectroscopy measurement for different types of Schottky gate metals tried (in this case, Au, Pt, Pd, and Ni) reveals that the surface donor states of AlGaN/GaN heterostructure strongly depends on the type of Schottky gate metals used, which suggests that barrier height inhomogeneity is strongly dependent on the gate metal. The X-ray photoelectron spectroscopy also reveals a strong correlation between the barrier height inhomogeneity and the gate metal type.

Improvements of external quantum efficiency of InGaN-based blue light-emitting diodes at high current density using GaN substrates

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AlGaN ∕ GaN multiquantum barriers (MQBs) were introduced into violet AlInGaN laser diodes with an InGaN multiquantum-well structure, resulting in drastic improvements in lasing performance. Comparing with conventional AlGaN single electron blocking layer (EBL), lower threshold current of 32mA and higher slope efficiency of 1.12W∕A at room temperature has been achieved by using the AlGaN∕GaN multiquantum barrier. This improvement implies that p-type AlGaN∕GaN MQBs are more effective in suppressing the overflow of electrons than p-type AlGaN single EBL. Effective barrier heights of the MQBs should be higher than the single EBL due to the quantum effect of MQBs and the enhancement of p-type doping efficiency. Additionally, the effect of strain on InGaN multiquantum wells from the single EBL can be reduced by using the AlGaN∕GaN MQBs structure.

We demonstrated the long wavelength (485nm) lasing of InGaN laser diodes under continuous wave condition at room temperature over 10mW. Two InGaN laser structures were adapted with different indium composition for InGaN optical confinement layers (OCLs) below quantum wells. The blue shift of electroluminescence (EL) was reduced in InGaN laser diodes grown on 3% In concentration in InGaN OCL compared with 1.5% In concentration in InGaN OCL. The EL peak for laser diode with 3% In concentration in InGaN OCL occurs at longer wavelength for all current levels compared to the laser with 1.5% In concentration in InGaN OCL. In addition, the laterally nonuniform InGaN wells grown on 1.5% In concentration in InGaN OCL was verified by the cross-sectional view of InGaN active layer using high-resolution transmission electron microscopy.

We investigated the properties of nonpolar a-plane InGaN∕GaN multiple-quantum wells (MQWs) grown on maskless lateral epitaxial overgrowth (LEO) a-plane GaN∕r-sapphire. Many surface defects with asymmetric V-shape were observed on a-plane InGaN MQWs grown on the defective regions which were seed and coalescence regions. In the low defect regions, the surface defect density of a-plane InGaN MQWs was ∼1.0×107∕cm2, which was higher than that of conventional c-plane LEO-GaN, by measuring atomic force microscope and scanning tunneling electron microscope. The cathode luminescence intensity distribution of a-plane InGaN MQWs was significantly dependent on the distribution of surface asymmetric V-defect. Therefore, we suggest that the optical properties of a-plane InGaN active layer were affected by the asymmetric V-defects which were generated by interaction between the epitaxial defects and the limit of InGaN growth kinetics.