W. W. Chow

Design, fabrication, and performance of infrared and visible vertical-cavity surface-emitting lasers

This paper discusses the issues involving the design and fabrication of vertical-cavity surface-emitting lasers (VCSELs). A review of the basic experimental structures is given, with emphasis on recent developments in distributed Bragg reflectors, gain media, as well as current and optical confinement techniques. The paper describes present VCSEL performance, in particular, those involving selective oxidation and visible wavelength operation.

Room-temperature direct current operation of 290 nm light-emitting diodes with milliwatt power levels

Ultraviolet light-emitting diodes (LEDs) have been grown by metalorganic vapor phase epitaxy using AlN nucleation layers and thick n-type Al0.48Ga0.52N current spreading layers. The active region is composed of three Al0.36Ga0.64N quantum wells with Al0.48Ga0.52N barriers for emission at 290 nm. Devices were designed as bottom emitters and flip-chip bonded to thermally conductive submounts using an interdigitated contact geometry. The ratio of quantum well emission to 330 nm sub-band gap emission is as high as 125:1 for these LEDs. Output power as high as 1.34 mW at 300 mA under direct current operation has been demonstrated with a forward voltage of 9.4 V. A peak external quantum efficiency of 0.18% has been measured at an operating current of 55 mA.

Scalability of small-aperture selectively oxidized vertical cavity lasers

We analyze the threshold properties of small area selectively oxidized vertical cavity lasers. Agreement for threshold gain versus laser size is found using the experimental intrinsic threshold voltage matched with a gain theory, as compared to a two-dimensional optical cavity simulation. Our analysis indicates the increasing threshold current density of small area lasers arises from both increasing threshold gain and the concomitant increasing leakage current. We further show that the optical loss can be reduced for lasers with areas as small as 0.25 μm2 while maintaining sufficient transverse optical confinement by displacing the apertures longitudinally away from the cavity and reducing the oxide thickness.

PIEZOELECTRIC EFFECTS ON THE OPTICAL PROPERTIES OF GAN/ALXGA1-XN MULTIPLE QUANTUM WELLS

Piezoelectric effects on the optical properties of GaN/AlGaN multiple quantum wells (MQWS) have been investigated by picosecond time-resolved photoluminescence (PL) measurements. For MQWS with well thickness 30 and 40 the excitonic transition peak positions at 10 K in continuous wave (CW) spectra are red-shifted with respect to the GaN epilayer by 17 meV and 57 meV, respectively. The time-resolved PL spectra of the 30 and 40 well MQWS reveal that the excitonic transition is in fact blue-shifted at early delay times due to quantum confinement of carriers. The spectral peak position shifts toward lower energies as the delay time increases and becomes red-shifted at longer delay times. We have demonstrated that the results described above is due to the presence of the piezoelectric field in the GaN wells of GaN/AlGaN MQWS subject to elastic strain together with screening of the photoexcited carriers. By comparing experimental and calculation results, we conclude that the piezoelectric field strength in GaN/Al.15G~.85N MQWS has a lower limit value of about 560 kV/cm: The electron and hole wave function distributions have also been obtained. The implication of our findings on the practical applications of GaN based optoelectronic devices is also discussed.

Laser Gain and Threshold Properties in Compressive-Strained and Lattice-Matched GaInNAs/GaAs Quantum Wells

The optical gain spectra for compressive-strained and lattice-matched GaInNAs/GaAs quantum wells are computed using a microscopic laser theory. From these spectra, the peak gain and carrier radiative decay rate as functions of carrier density are determined. These dependences allow the study of lasing threshold current density for different GaInNAs/GaAs laser structures.

Theoretical study of room temperature optical gain in GaN strained quantum wells

The determination of gain properties in group III nitride quantum wells is complicated by the incomplete knowledge of band structure properties, and the need for a consistent treatment of many‐body Coulomb effects. This letter describes an approach that involves a first‐principles band structure calculation, the results of which are incorporated into a microscopic laser theory where many‐body Coulomb effects are treated in a consistent manner. Using this approach, we investigate quantum well structures composed of alloys of GaN, AlN, and InN, in particular, GaN–AlInN, which has high confinement potentials in both strained and unstrained configurations.

Determination of the Piezoelectric Field in InGaN Quantum Wells

In many studies, the value of the experimentally determined internal piezoelectric field has been reported to be significantly smaller than theoretical values. We believe this is due to an inappropriate approximation for the electric field within the depletion region, which is used in the analysis of experimental data, and we propose an alternative method. Using this alternative, we have measured the strength of the internal field of InGaN p-i-n structures, using reverse bias photocurrent absorption spectroscopy and by fitting the bias dependent peak energy using microscopic theory based on the screened Hartree-Fock approximation. The results agree with those using material constants interpolated from binary values.

Time evolution of the screening of piezoelectric fields in InGaN quantum wells

We have measured the time response of the emission spectra of In 0.07Ga0.93N quantum wells with widths of 2, 3, and 4nm in GaN following pulsed optical excitation. We observe a blue shift of the emission peak during the excitation and a subsequent red shift as the carriers recombine in the 3- and 4-nm wells, and a negligible shift for the 2-nm well. Using a comprehensive theory we are able to fit both the time evolution of the peak emission energy and the integrated emission intensity. The shift of the emission peak (by about 17 meV) arises from the balancing of the change in screening of the internal piezoelectric field as the carrier density changes and bandgap renormalization. We have projected the calculations to quantify the degree of screening at typical threshold carrier densities. At transparency we estimate carrier densities of 4.3times1016 m-2 and 4.8times1016 m-2 for the 4- and 3-nm wells, respectively, which reduce the internal piezoelectric field in the well to 0.97times108 (4 nm) and 1.03times10 8 (3 nm) Vmiddotm-1 compared with the unscreened value of about 1.23times108 Vmiddotm-1. Thus, a substantial field remains in these wells under laser conditions. We find that this partially screened field is beneficial in reducing the threshold current compared with that of a square well for modal gains up to about 150 cm-1

High single-mode power observed from a coupled-resonator vertical-cavity laser diode

We report a monolithic coupled-resonator vertical-cavity laser with an ion-implanted top cavity and a selectively oxidized bottom cavity which exhibits single fundamental-mode operation. The output powers are as high as 6.1 mW with side mode suppression ratios greater than 30 dB. The sizes of the implant and oxide current apertures are shown to be important for demonstrating the required selectivity for the fundamental lasing mode. With a fixed bias current on the implant cavity and increasing oxide cavity current, mode switching from single-mode operation to multimode operation and back to single-mode operation was observed. The intensities of the fundamental and first transverse modes were calculated by solving a set of multimode rate equations. The calculation indicates that the observed mode switching can be identified with changes in the optical length of the oxide cavity with increasing pump current. The observed mode dynamics are unique to coupled-resonator vertical-cavity lasers.

Growth and photoluminescence studies of Al-rich AlN∕AlxGa1−xN quantum wells

Both a-plane and c-plane AlN∕Al0.65Ga0.35N quantum wells (QWs) have been grown by metal organic chemical vapor deposition and their photoluminescence (PL) emission properties were studied and compared. It was found that the low temperature PL characteristics of a-plane QWs are primarily governed by the quantum size effect, whereas those of c-plane QWs are significantly affected by the polarization fields. The PL decay time was found to be only weakly dependent on the well width Lw for a-plane QWs, whereas a strong dependence of the PL decay time on Lw was observed for c-plane QWs. Moreover, Lw dependence studies also revealed that structures with Lw>2nm and Lw≈2nm provide highest emission efficiency in a-plane and c-plane AlN∕Al0.65Ga0.35N QWs, respectively.