K. R. Evans

AlGaAs/GaAs double barrier diodes with high peak‐to‐valley current ratio

We report the largest peak‐to‐valley current (PVC) ratios to date from AlGaAs/GaAs double barrier (either alloy barrier or superlattice barrier) diodes. PVC ratios as high as 3.6 and 21.7 were obtained from an AlAs/GaAs superlattice barrier structure at 300 and 77 K, respectively. In an alloy barrier structure with x=0.42 (x=0.3), PVC ratios of 3.9 (2.2) and 14.3 (7.0) were observed at 300 and 77 K, respectively. We attribute these excellent results to a ‘‘two‐step’’ spacer layer incorporated in the devices studied which facilitated the growth of high material quality.

Optical Properties of GaN Grown on ZnO by Reactive Molecular Beam Epitaxy

High quality wurtzite GaN epilayers have been grown on ZnO(0001) substrates by reactive molecular beam epitaxy. Photoluminescence and reflectivity measurements point to high quality presumably due to the near match of both the crystal lattice parameter and the stacking order between GaN and ZnO. In addition, the good films lack the characteristic yellow photoluminescence band. Any misorientation of the GaN epilayer planes with respect to the ZnO substrate is not detectable with polarized reflectivity. The x-ray double crystal diffraction measurements indicate this misorientation is much smaller than those for GaN epilayers on SiC and Al2O3 .

InGaN staircase electron injector for reduction of electron overflow in InGaN light emitting diodes

Ballistic and quasiballistic electron transport across the active InGaN layer are shown to be responsible for electron overflow and electroluminescence efficiency droop at high current levels in InGaN light emitting diodes both experimentally and by first-order calculations. An InGaN staircase electron injector with step-like increased In composition, an “electron cooler,” is proposed for an enhanced thermalization of the injected hot electrons to reduce the overflow and mitigate the efficiency droop. The experimental data show that the staircase electron injector results in essentially the same electroluminescence performance for the diodes with and without an electron blocking layer, confirming substantial electron thermalization. On the other hand, if no InGaN staircase electron injector is employed, the diodes without the electron blocking layer have shown significantly lower (three to five times) electroluminescence intensity than the diodes with the blocking layer. These results demonstrate a feasible method for the elimination of electron overflow across the active region, and therefore, the efficiency droop in InGaN light emitting diodes.

Sb-surface segregation and the control of compositional abruptness at the GaAsSbGaAs interface

Abstract Quantitative measurements of Sb surface segregation in pseudomorphic GaAs 0.8 Sb 0.2 layers grown on GaAs (001) using molecular beam epitaxy (MBE) were performed in situ using the line-of-sight mass spectrometry (MS) technique. It was observed that substantial surface accumulation of Sb occurs under normal growth conditions, giving rise to compositional broadening of the GaAsSb on GaAs interface. This compositional grading is found to strongly depend on the growth temperature as well as the presence of Sb at the GaAs film surface prior to the initiation of GaAsSb layer growth. We report the effect to substrate temperature and shutter sequence on the evolution of Sb surface accumulation near the interface and demonstrate that a compositionally abrupt interface can be formed by populating the GaAs surface with an amount of Sb equivalent to that expected on the GaAsSb surface during growth at steady state. Additionally, compositional abruptness at the GaAs on GaAsSb interface can be achieved by a “chemical flashoff” of the surface Sb population during exposure to an incident As 2 flux.

Intersubband infrared absorption in a GaAs/Al0.3Ga0.7As quantum well structure

The linewidth, total integrated area, and peak position (ν0) of the intersubband transition (IT) in a GaAs/Al0.3Ga0.7As multiple quantum well, with doping in the barrier, are studied as a function of temperature using the infrared absorption technique. From the temperature dependence of the linewidth and the configuration coordinate model we find that the electrons in the GaAs well are weakly coupled to the GaAs normal optical phonon mode. The electron density (σ) in the quantum well is extracted from the total integrated area of the IT. From the temperature‐dependence of σ we conclude that the Fermi energy is also temperature dependent and that at 5 K it is about 36 meV above the ground state energy. We also find that ν0 increases as the temperature decreases. We calculated the absorption spectrum for the quantum well in a nonparabolic‐anisotropic envelope function approximation including temperature‐dependent effective masses, nonparabolicity, conduction‐band offsets, the Fermi level, and line shape bro...

On carrier spillover in c- and m-plane InGaN light emitting diodes

The internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) in InGaN light-emitting diodes (LEDs) emitting at 400 nm with and without electron blocking layers (EBLs) on c-plane GaN and m-plane GaN were investigated in order to shed some light on any effect of polarization charge induced field on efficiency killer carrier spillover. Without an EBL the EQE values suffered considerably (by 80%) for both orientations, which is clearly attributable to carrier spillover. Substantial carrier spillover in both polarities, therefore, suggests that the polarization charge is not the major factor in efficiency degradation observed, particularly at high injection levels. Furthermore, the m-plane variety with EBL did not show any discernable efficiency degradation up to a maximum current density of 2250 A cm−2 employed while that on c-plane showed a reduction by ∼40%. In addition, IQE of m-plane LED structure determined from excitation power dependent photoluminescence was ∼80% compared to 50%...

High Electron Velocity Submicrometer AlN/GaN MOS-HEMTs on Freestanding GaN Substrates

AlN/GaN heterostructures with 1700-cm²/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (L_G) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. L_G = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency f_T of 165 GHz, a maximum frequency of oscillation f_max of 171 GHz, and intrinsic average electron velocity v_e of 1.5 ×10⁷ cm/s. The 40-GHz load-pull measurements of L_G = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high v_e, and high RF performance in AlN/GaN transistors.

Negative persistent photoconductivity in the Al0.6Ga0.4Sb/InAs quantum wells

We have measured the Shubnikov–de Haas (SdH) effect in the Al0.6Ga0.4Sb/InAs quantum wells under the negative persistent photoconductivity (NPPC) conductions. By illuminating the sample at low‐temperature, the carrier concentration of the two‐dimensional electron gas in the InAs well was reduced from 5.8 to 3.6×1011 cm−2 and the corresponding quantum lifetime increases from 0.16 to 0.21 ps. The electrons which escaped from the InAs well were captured by the ionized deep donors in the Al0.6Ga0.4Sb layers. The effective mass is equal to (0.0317±0.0005)m0. We also propose, based on the SdH data, that the illumination of the sample with the ionized deep donors at low temperature will exhibit the NPPC effect.

Effect of hot phonon lifetime on electron velocity in InAlN/AlN/GaN heterostructure field effect transistors on bulk GaN substrates

We report on electron velocities deduced from current gain cutoff frequency measurements on GaN heterostructure field effect transistors (HFETs) with InAlN barriers on Fe-doped semi-insulating bulk GaN substrates. The intrinsic transit time is a strong function of the applied gate bias, and a minimum intrinsic transit time occurs for gate biases corresponding to two-dimensional electron gas densities near 9.3×1012 cm−2. This value correlates with the independently observed density giving the minimum longitudinal optical phonon lifetime. We expect the velocity, which is inversely proportional to the intrinsic transit time, to be limited by scattering with non equilibrium (hot) phonons at the high fields present in the HFET channel, and thus, we interpret the minimum intrinsic transit time in terms of the hot phonon decay. At the gate bias associated with the minimum transit time, we determined the average electron velocity for a 1.1 μm gate length device to be 1.75±0.1×107 cm/sec.

Internal quantum efficiency of c-plane InGaN and m-plane InGaN on Si and GaN

We investigated internal quantum efficiency (IQE) of polar (0001) InGaN on c-sapphire, and (11¯00) nonpolar m-plane InGaN on both m-plane GaN and specially patterned Si. The IQE values were extracted from the resonant photoluminescence intensity versus the excitation power. Data indicate that at comparable generated carrier concentrations the efficiency of the m-plane InGaN on patterned Si is approximately a factor of 2 higher than that of the highly optimized c-plane layer. At the highest laser excitation employed (∼1.2×1018 cm−3), the IQE of m-plane InGaN double heterostructure on Si is approximately 65%. We believe that the m-plane would remain inherently advantageous, particularly at high electrical injection levels, even with respect to highly optimized c-plane varieties. The observations could be attributed to the lack of polarization induced field and the predicted increased optical matrix elements in m-plane orientation.