InGaAs submonolayer quantum-dot photonic-crystal LEDs for fiber-optic communications
Introduction
High brightness (HB) light-emitting diodes (LEDs) have drawn a lot of attentions because of their applications in mobile electronics, flat panel displays, automobiles, traffic signals, large outdoor displays, and general lighting [1]. More recently, photonic-crystal light-emitting diodes (PhC-LEDs) have achieved higher external quantum efficiency [2], [3], as compared to conventional LEDs. For fiber-optic applications, LEDs made with smaller light-emitting apertures are needed because of their smaller divergence angles and better optical fiber coupling efficiency. For LEDs made with small oxide-confined structure, the device may have reliability problem, similar to that of vertical-cavity surface-emitting lasers (VCSELs) [4]. For LEDs made with integrated lenses, the deep wet chemical etch to form lenses is difficult to control uniformity in lens curvature and diameter. Small-aperture LEDs are designed for small light beam emission at lower currents for better optical fiber coupling. The current spreading can be improved for devices with smaller aperture, as compared to the large-aperture LEDs (⩾300 μm in diameter). Higher coupling efficiency can be achieved with the emitted light beam size smaller than core diameter of the optical fiber. Moreover, epitaxially grown distributed Bragg reflectors (DBRs) can be used to form microcavity for enhanced light emission [5], [6]. The absorption of the spontaneous emission by the substrate can be minimized. Also, microhole array LEDs have demonstrated enhanced light output recently [7]. For shorter-wavelength emission, InGaAs/GaAs submonolayer (SML) QD embedded in a GaAs matrix shows luminescence peaks and high-power lasing performance in the 0.92–1 μm range [8], [9]. The advantages of InGaAs SML QDs include better growth uniformity, narrower gain spectrum, higher differential gain, and lower threshold current density [8], [9]. In this paper, we report our results on the InGaAs SML QD PhC-LEDs with in the 980 nm range. The photonic-crystal (PhC) structure in this work is the arrangement of etched holes as a triangular photonic-crystal structure with larger lattice constant (5 μm) and etched hole diameters (2–2.5 μm) [2], [3]. Two-dimensional (2-D) photonic-crystal etched holes were formed within the p-type ohmic contact ring for light extraction and collimation. The collimation of the output light beam can be further improved by using the sidewalls within the etched holes for light deflection. A maximum continuous-wave (CW) output power of over 0.34 mW has been obtained. High-resolution imaging studies show that the device emits light beams which mainly through the etched holes.
Section snippets
Experiments
The epitaxial layers of the InGaAs SML QD LED wafers were grown on 3-in. n+-GaAs(0 0 1) substrates by molecular beam epitaxy (MBE) in a Riber 49 chamber. The bottom distributed Bragg reflector (DBR) consists of a 33-pair n-type (Si-doped) quarter-wave stack (λ/4) of Al0.9Ga0.1As/GaAs. The top DBR consists of a 20-pair p-type (carbon-doped) Al0.9Ga0.1As/GaAs quarter-wave stack. Above the top DBR, is a heavily doped p-type GaAs contact layer. The undoped 1λ cavity contains three 8 nm InGaAs SML QD
Results and discussion
Fig. 2 shows the CW light–current (L–I) output of the InGaAs SML QD PhC-LEDs. The lattice constant (Λ) is 5 μm and the hole diameter (α) is 2.5 μm for both the 68- and 78-μm-mesa devices. The PhC-LEDs show maximum output powers of 0.19 and 0.34 mW for the 68- and 78-μm-mesa devices, respectively. The device emits a higher output power with larger oxide aperture and more etched holes. The lower output power of the devices is due to smaller oxide apertures for light emission (approximately 36 and 48
Conclusions
In conclusion, we report InGaAs SML QD PhC-LEDs for fiber-optic communications. A maximum CW output power of 0.34 mW has been obtained. The present results indicate that a PhC-LED using an oxide layer for current confinement and a photonic-crystal structure for light extraction is an alternative approach to achieve small light beam output for optical fiber coupling.
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