Robert M. Farrell

Materials and growth issues for high-performance nonpolar and semipolar light-emitting devices

Growth of InGaN/GaN light-emitting devices on nonpolar or semipolar planes offers a viable approach to reducing or eliminating the issues associated with polarization-related electric fields present in c-plane III-nitride heterostructures. Although progress in device performance has been rapid since the introduction of high-quality free-standing nonpolar and semipolar GaN substrates, a full appreciation of the materials challenges unique to nonpolar and semipolar III-nitride semiconductors has been slower to emerge. Only recently have researchers begun to understand issues such as the origins of the pyramidal hillocks typically observed on nominally on-axis m-plane GaN films, the effects of m-plane substrate misorientation on surface morphology and device performance, the mechanics of anisotropic cracking in tensile strained m-plane AlGaN films, the formation of basal-plane stacking faults in long-wavelength m-plane InGaN quantum wells, and the mechanisms for stress relaxation in semipolar AlGaN and InGaN films. In this paper, we review the materials and growth issues unique to high-performance nonpolar and semipolar light-emitting devices grown on high-quality free-standing GaN substrates and provide an outlook for the opportunities and challenges that lie ahead. (Some figures in this article are in colour only in the electronic version)

Demonstration of Nonpolar m-Plane InGaN/GaN Laser Diodes

The first nonpolar m-plane (1-100) nitride laser diodes (LDs) have been realized on low extended defect bulk m-plane GaN substrates. The LDs were grown by metal organic chemical vapor deposition (MOCVD) using conditions similar to that of c-plane device growth. Broad area lasers were fabricated and tested under pulsed conditions. Lasing was observed at duty cycles as high as 10%. These laser diodes had threshold current densities (Jth) as low as 7.5 kA/cm2. Stimulated emission was observed at 405.5 nm, with a spectral line-width of 1 nm.

Demonstration of a semipolar (101¯3¯) InGaN∕GaN green light emitting diode

We demonstrate the growth and fabrication of a semipolar (101¯3¯) InGaN∕GaN green (∼525nm) light emitting diode (LED). The fabricated devices demonstrated a low turn-on voltage of 3.2V and a series resistance of 14.3Ω. Electroluminescence measurements on the semipolar LED yielded a reduced blueshifting of the peak emission wavelength with increasing drive current, compared to a reference commercial c-plane LED. On-wafer measurements yielded an approximately linear increase in output power with drive current, with measured values of 19.3 and 264μW at drive currents of 20 and 250mA, respectively. The external quantum efficiency did not decrease appreciably at high currents. Polarization anisotropy was also observed in the electroluminescence from the semipolar green LED, with the strongest emission intensity parallel to the [12¯10] direction. A polarization ratio of 0.32 was obtained at a drive current of 20mA.

High internal and external quantum efficiency InGaN/GaN solar cells

High internal and external quantum efficiency GaN/InGaN solar cells are demonstrated. The internal quantum efficiency was assessed through the combination of absorption and external quantum efficiency measurements. The measured internal quantum efficiency, as high as 97%, revealed an efficient conversion of absorbed photons into electrons and holes and an efficient transport of these carriers outside the device. Improved light incoupling into the solar cells was achieved by texturing the surface. A peak external quantum efficiency of 72%, a fill factor of 79%, a short-circuit current density of 1.06 mA/cm2, and an open circuit voltage of 1.89 V were achieved under 1 sun air-mass 1.5 global spectrum illumination conditions.

Continuous-wave Operation of AlGaN-cladding-free Nonpolar m-Plane InGaN/GaN Laser Diodes

We demonstrate continuous-wave (CW) operation of nonpolar m-plane InGaN/GaN laser diodes without Al-containing waveguide cladding layers. Thick InGaN quantum wells (QWs) are used to generate effective transverse optical mode confinement, eliminating the need for Al-containing waveguide cladding layers. Peak output powers of more than 25 mW are demonstrated with threshold current densities and voltages of 6.8 kA/cm2 and 5.6 V, respectively. The unpackaged and uncoated laser diodes operated under CW conditions for more than 15 h.

High quantum efficiency InGaN/GaN multiple quantum well solar cells with spectral response extending out to 520 nm

We demonstrate high quantum efficiency InGaN/GaN multiple quantum well (QW) solar cells with spectral response extending out to 520 nm. Increasing the number of QWs in the active region did not reduce the carrier collection efficiency for devices with 10, 20, and 30 QWs. Solar cells with 30 QWs and an intentionally roughened p-GaN surface exhibited a peak external quantum efficiency (EQE) of 70.9% at 390 nm, an EQE of 39.0% at 450 nm, an open circuit voltage of 1.93 V, and a short circuit current density of 2.53 mA/cm2 under 1.2 suns AM1.5G equivalent illumination.

AlGaN-Cladding Free Green Semipolar GaN Based Laser Diode with a Lasing Wavelength of 506.4 nm

We demonstrate electrically driven InGaN based laser diodes (LDs), with a simple AlGaN-cladding-free epitaxial structure, grown on semipolar (2021) GaN substrates. The devices employed In0.06Ga0.94N waveguiding layers to provide transverse optical mode confinement. A maximum lasing wavelength of 506.4 nm was observed under pulsed operation, which is the longest reported for AlGaN-cladding-free III-nitride LDs. The threshold current density (Jth) for index-guided LDs with uncoated etched facets was 23 kA/cm2, and 19 kA/cm2 after application of high-reflectivity (HR) coatings. A characteristic temperature (T0) value of ~130 K and wavelength red-shift of ~0.05 nm/K were confirmed.

AlGaN-Cladding-Free Nonpolar InGaN/GaN Laser Diodes

We have previously demonstrated the AlGaN-cladding-free (ACF) laser diode (LD) concept over a wide visible spectral range and various nonpolar/semipolar crystallographic orientations. The benefits of this ACF epitaxial design include lower operating voltages and higher production yields. Nonpolar LDs have been demonstrated out to 500nm1 with semipolar reaching as long as 534nm2. No results have been published on nonpolar/semipolar orientations for short wavelength LDs. In this paper we report on the short wavelength limits of this epitaxial design on nonpolar bulk m-plane GaN substrates.

Formation and reduction of pyramidal hillocks on m-plane {11¯00} GaN

Surface morphology and hillock reduction were studied on m-plane {11¯00} n-type GaN films and light emitting diode structures grown by metal organic chemical vapor deposition on low defect-density m-plane GaN substrates. For nominally on-axis m-plane films, predominantly pyramidal hillocks were observed, which were composed of two faces symmetrically inclined by 0.1°–0.25° to the ±[112¯0] a direction and two faces inclined by 0.5°–0.95° to the [0001¯] c− and the [0001] c+ directions, respectively. All faces of the pyramidal hillocks for the nominally on-axis GaN films had clearly defined step-terrace structures. Gradual changes in nominal miscut angles from 0° to 10° along the a and the c− directions succeeded in a continuous hillock reduction yielding atomically flat surfaces.

Effects of built-in polarization on InGaN-GaN vertical-cavity surface-emitting lasers

We investigate the effect of built-in spontaneous and piezoelectric polarization on the internal device physics of current-injected GaN-based vertical-cavity surface-emitting lasers (VCSELs) with strained InGaN quantum wells. Advanced device simulation is applied to a previously manufactured device design featuring dielectric mirrors and an indium-tin-oxide current injection layer. Contrary to common perception, we show: 1) that only a small fraction of the built-in quantum-well polarization is screened at typical injection current densities and 2) that the polarization of the AlGaN electron stopper layer has a strong effect on the VCSEL threshold current which can be partly compensated for by higher p-doping.