C. J. Sun

Enhancement and depletion mode GaN/AlGaN heterostructure field effect transistors

We report on the fabrication and characterization of Al0.1Ga0.9N/GaN heterojunction field effect transistors, both an enhancement mode and a depletion mode with a low pinchoff voltage, suitable for digital integrated circuit applications. For an enhancement mode device with a 1 μm gate length and 5 μm drain‐to‐source separation, the dc transconductance is around 23 mS/mm. Connecting the enhancement mode device as a switching transistor and a depletion mode device as a load, we demonstrate an AlGaN/GaN inverter.

Fundamental optical transitions in GaN

A coherent picture for the band structure near the Γ point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time‐resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.

Metastability and persistent photoconductivity in Mg‐doped p‐type GaN

Electrical properties of Mg‐doped p‐type GaN grown by metalorganic chemical vapor deposition have been investigated by Hall effect and conductivity measurements. Metastability and persistent photoconductivity effects have been observed in GaN. It was found that at low temperatures, it takes several hours for the free hole concentration to reach its equilibrium value in the dark as well as in the photoexcited state, implying a bistable nature of impurities in p‐type GaN. Temperature dependence of these behaviors have been studied, from which the energy barrier for free hole capture by ionized impurities as well as between the metastable and the stable states of neutral impurities have been obtained.

High quality GaN–InGaN heterostructures grown on (111) silicon substrates

We report on the low pressure metal organic chemical vapor deposition of single crystal, wurtzitic layers of GaN and GaN/InGaN heterostructures on (111) GaAs/Si composite substrates. The structural, optical, and electrical properties of the epitaxial layers are evaluated using x‐ray diffraction, transmission electron microscopy, photoluminescence, and measurements of minority carrier diffusion length. These measurements demonstrate high quality of GaN grown on the composite substrate.

Cleaved cavity optically pumped InGaN–GaN laser grown on spinel substrates

We report an optically pumped multiple‐quantum‐well laser of InGaN–GaN grown on cubic, (111)‐oriented spinel substrates. The laser cavity is formed by cleaving. Atomic force microscopy shows that the cleaved GaN and spinel facets are of similar flatness. The onset of lasing is clearly demonstrated by the saturation of spontaneous emission, abrupt line narrowing, and the highly polarized light output. A lasing threshold power of 140 kW/cm2 is measured in a 400‐μm‐long cavity at 150 K.

Quantum shift of band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes

We report on the band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes with varying widths and barrier thicknesses of the quantum wells. In these devices, we observe that the stimulated emission peak wavelength shifts to shorter values with decreasing well thickness. From the comparison of the results of the quantum mechanical calculations of the subbands energies with the measured data, we estimate the effective conduction- and valence-band discontinuities at the GaN–In0.13Ga0.87N heterointerface to be approximately 130–155 and 245–220 meV, respectively. We also discuss the effect of stress on the estimated values of band discontinuities.

High transconductance heterostructure field‐effect transistors based on AlGaN/GaN

We report on the fabrication and characterization of Al0.15Ga0.85N/GaN heterostructure field‐effect transistors (HFETs) with transconductance as high as 120 mS/mm and saturated current density of 0.35 A/mm for a device with a gate length and width of 1 and 100 μm. This represents one of the best results for such device. A comparison of the maximum transconductance of devices on wafers with different channel conductance is presented to analyze the factors limiting the performance. Our data indicates the series resistance between the source and drain to be the limiting factor for the maximum dc transconductance.

Two‐dimensional electron gas in GaN–AlGaN heterostructures deposited using trimethylamine‐alane as the aluminum source in low pressure metalorganic chemical vapor deposition

In this letter, we report the fabrication of high quality GaN–Al0.1Ga0.9N heterostructures using trimethylamine‐alane as the aluminum source. The two‐dimensional electron gas (2DEG) with mobility values as high as 5000 cm2/V s at 150 K is demonstrated. We also present the results of our calculations of the 2DEG mobility at the GaN–Al0.1Ga0.9N heterointerface. Our calculations show that the mobility enhancement in the 2DEG is related to a much larger volume carrier concentration (compared to bulk GaN) and, therefore, to a larger Fermi energy and to a more effective screening of impurity and piezoelectric scattering.

Neutral‐donor‐bound exciton recombination dynamics in GaN grown by metalorganic chemical vapor deposition

Neutral‐donor‐bound exciton recombination (I2) dynamics have been studied by photoluminescence in an unintentionally doped n‐type GaN epitaxial layer grown by metalorganic chemical vapor deposition. The luminescence emission line shape, peak position, and intensity as functions of temperature have been measured. In particular, time‐resolved emission spectroscopy has been employed to study the dynamic processes of the bound exciton recombination, from which the temperature and the emission energy dependencies of the recombination lifetime of this transition have been obtained.

Free excitonic transitions in GaN, grown by metal‐organic chemical‐vapor deposition

The dynamic processes of the free excitonic transitions in GaN grown by metal‐organic chemical‐vapor deposition (MOCVD) have been studied. The recombination lifetimes of the A and B excitons have been measured at different temperatures and excitation intensities, from which radiative recombination lifetimes of about 0.35 and 0.3 ns for the A and B excitons, respectively, have been obtained. An increase in excitation power has resulted in a drastic enhancement in the radiative decay rate as well as in the exciton photoluminescence quantum yield, suggesting the excitonic transitions may provide gain for laser actions in GaN. The high quality as well as high purity of the investigated MOCVD sample has been demonstrated by the observations of (1) the free A‐ and B‐excitonic transitions, (2) excited states of the free excitons, (3) narrow free excitonic emission linewidths (1.7 meV at 10 K), (4) low electron concentration, and (5) high electron mobilities (∼600 cm2/V s).