Semipolar Structures Research Articles

Carrier Transport in InGaN MQWs of Aquamarine- and Green-Laser Diodes

We studied experimentally and theoretically the substrate-orientation impact on carrier transport and capture in InGaN multiple quantum well (MQW) laser diodes (LDs) with emission in the aquamarine-green spectral range. A new simulation approach was developed to analyze this behavior of LEDs and LDs emitting at these wavelengths. We show that due to deep carrier confinement, the thermal escape from a QW in such devices is negligible. The carrier distribution among QWs is therefore determined by the carrier transport and capture rates. We also show that the ballistic transport mechanism is dominant in this type of MQW active region. In c-plane structures, this mechanism is tunneling-assisted, and therefore, the transport is much slower than in nonpolar and semipolar structures. Because of this, a strong carrier injection nonuniformity observed in c-plane LDs, causes the threshold current increase when number of QWs is >;2. This effect is not observed in semipolar LDs because the carrier transport rate is faster than the capture rate.

Strain effects on indium incorporation and optical transitions in green‐light InGaN heterostructures of different orientations

AbstractStrain effect on indium incorporation and optical transitions in bulk InGaN and GaN/InGaN/GaN quantum wells (QWs) coherently grown on GaN substrates with different orientations of hexagonal axis is studied by simulation. The strain modification in the nonpolar and semipolar structures, as compared to polar ones, is found to result in both a higher indium percentage in the InGaN alloy and a larger materials bandgap, producing opposite trends in variation of the optical transition energy (emission wavelength) with the crystal orientation. The interplay between the effects is discussed in view of development of green‐light emitters. A possible way for controlling the strain in the InGaN layers and QWs and thus the emission wavelength is considered and tested by modelling.Optical transition energy of the same single InGaN QW grown on either GaN or relaxed In0.08Ga0.92N sublayer as a function of the crystal C‐axis inclination to epitaxial layer plane.magnified imageOptical transition energy of the same single InGaN QW grown on either GaN or relaxed In0.08Ga0.92N sublayer as a function of the crystal C‐axis inclination to epitaxial layer plane.

Different behavior of semipolar and polar InGaN/GaN quantum wells: Pressure studies of photoluminescence

AbstractPhotoluminescence of (11–22) semipolar and (0001) polar InGaN/GaN quantum wells (QWs) was studied as a function of exciting laser intensity and hydrostatic pressure. A large photoluminescence energy shift with pressure (dEPL/dp) was observed in semipolar QWs comparing to polar ones. In addition, a negligible blue‐shift as a function of exciting laser intensity gives a strong argument for the absence of a built‐in electric field in the studied semipolar structures. These results suggest that In‐segregation effects play an important role in semipolar QWs.

Synthesis and solution properties of poly(methacrylate)s with semipolar structures on the side chains

AbstractPoly{2‐(N,N‐dimethylamino)ethyl methacrylate [poly(DMMA)]}, which was prepared by radical polymerization initiated with dimethyl 2,2‐azobis(2‐methylpropionate), was reacted with hydrogen peroxide, diethyl sulfate, and chloroacetic acid to yield poly[N,N‐dimethyl‐N‐(2‐methacryloyloxyethyl)amine N‐oxide] [poly(DMANO)], poly[N‐ethyl‐N,N‐dimethyl‐N‐(2‐methacryloyloxyethyl)ammonium ethyl sulfate] [poly(EDMES)], and poly[N,N‐dimethyl‐N‐(2‐methacryloyloxy)ethylammonioacetate] [poly(DMEAA)] as ion‐containing water‐soluble polymers, respectively. The solution properties of these charged polymers were compared via the reduced viscosities of these three charged polymers in aqueous solutions as a function of the concentration. Poly(EDMES) showed typical polyelectrolyte behavior, and the other two polymers [poly(DMANO) and poly(DMEAA)] exhibited antipolyelectrolyte behavior. Furthermore, the antipolyelectrolyte behavior was different for poly(DMANO) and poly(DMEAA); that is, poly(DMANO) was less dependent on small electrolytes. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 129–141, 2005