Long-wavelength (1.3-1.55µm) vertical cavity surface emitting lasers (VCSEL) are the most promising low-cost light sources for local-area and long-haul fiber-based optical communication systems and optical inter- connection. Contrary to edge-emitting lasers, VCSELs are best suited for wafer-scale production and testing, 2-D array fabrication and optical coupling to fibers due to their low-diffraction beam output. In addition, it has also been shown that the small modal volume and high photon density in a VCSEL can lead to high modulation bandwidth at lower currents. VCSELs require highly reflective Distributed Bragg Reflector (DBR) mirrors and low thermal resistance. These requirement have presented researchers in this area with the toughest challenge of finding the appropriate device design needed for making high performance long wavelength VCSELs reliably and cost-effectively.
As with many other devices, the development of long wavelength (l = 1.55 µm) InP-based vertical cavity surface emitting lasers (VCSELs) has been less rapid than the GaAs-based devices. Aside from the relative complexities in processing InP-based devices, there are three reasons for this that can be cited: Auger recombination and intervalence band absorption, less applicability of the selective wet oxidation of Al-bearing compounds, which has been so effective in 1µm VCSEL technology, and the difficulty in fabricating high-reflectivity semiconductor Bragg mirrors lattice matched to InP. It is the last factor which poses as the most serious obstacle. Because of the small refractive index difference between InP and InGaAsP (or InGaAlAs) lattice matched to it, as many as 45 pairs of l/4 layers are required to get a high enough reflectivity [1]. This presents both epitaxy and processing challenges.
The most successful design is the 1.54 µm VCSEL that employs two fused GaAs/AlGaAs DBR mirrors and a InP-based active region with strain-compensated InGaAsP active region and, has achieved continuous-wave (CW) operation above room-temperature[2]. This structure overcomes the mirror problem but still suffers from key disadvantages which include the precise control of the resonant cavity wavelength determined by the thickness of the spacer layer, the active region and the separate DBR mirror layers. In addition, the wafer fusion technique requires multiple substrates and is difficult to achieve on a full wafer. Other long wavelength VCSELs that have achieved similar success involve a combination of fused GaAs-based and dielectric mirrors [3] or two dielectric top and bottom mirrors [4]. More recent approaches that eliminate the fusion step and show great promise include a Sb-based VCSEL structure with AlAsSb/AlGaAsSb DBR mirrors [5] and the metamorphic growth of GaAs/AlAs Bragg mirrors on InP by GaAs source molecular beam epitaxy (MBE) [6]. Also recently, the first 1300nm VCSEL (also utilizing fusion and internal photo-pumping) operating CW up to 80°C and compatible with commercial datacom performance requirements has been reported[7].
Clearly, there is a need for an alternate mirror technology that takes advantage of the intrinsic optical, electrical and thermal properties of the GaAs/AlGaAs system yet at the same time, can be made compatible with a 2 inch wafer process.
It has been demonstrated that epitaxial growth on finite substrates can dramatically reduce the density of misfit dislocations present in the starting substrate [8] or from the strain relaxation process once the critical thickness is reached during strained layer epitaxy [9]. If, in addition, sources of dislocation nucleation at the mesa edge can be minimized, the epitaxial overlayer on the mesa can be relatively defect-free and linear dislocation densities are less than 100 cm-1 along one of the <100> directions.
Because the lateral dimensions of low threshold current VCSELs vary from 2-10 µm and the top distributed Bragg reflector (DBR) l/4 stack is of similar dimension, we investigated the direct growth of GaAs- based DBR directly onto a patterned InP-based VCSEL hetero- structure [10]. Careful photo- luminescence and TEM studies confirmed that there are no propagating defects in the GaAs/AlxGa1-xAs DBR grown on the patterned VCSEL sample, specifically in the MQW region. Based on these findings a novel InP-based 1.55mm patterned VCSEL was proposed.
The laser heterostructure, shown in the schematic cross-section of Fig. 1, is grown in two steps. The first part is grown on n+-InP substrate by metal- organic vapor phase epitaxy (MOVPE) and consists of a 45.5 period lattice- matched n-doped InP/InGaAsP DBR, a l-cavity with strain-compensated InGaAsP/InP MQW active region and InP spacer layer. Directly over the cavity, is an In0.52Al0.48As layer which is partially oxidized to form a dielectric aperture. It is followed by a l/4 p+-InP layer and a l/4 p+-InGaAsP (l=1.2µm) contact layer. The heterostructure is patterned into 2µm high and 20-40µm diameter mesas. The patterned substrate is then selectively oxidized in a clean horizontal tube furnace at 475ºC in a water vapor ambient. The exposed InAlAs layer is laterally oxidized to form the desired active area size in the range of 6-26µm. Our aim was to partially oxidize the current confining In0.52Al0.48As layer prior to growth of the top DBR mirror. Oxidation experiments with InAlAs in InP-based heterostructures grown by MOVPE revealed that wet-oxidation at 475ºC produced the best results [11]. More importantly, InAlAs sandwiched between InP layers had higher rates of oxidation than InAlAs sandwiched between InGaAs layers in the temperature range of 475-525 ºC. We believe that the As rich layers in the InGaAs/InAlAs heterostructure also inhibit the desorption of As during the oxidation process.
Figure 1. Schematic cross-section of the 1.55 mm
Patterned VCSEL with regrown
GaAs-based top DBR mirror.
A 5-period stack of Al0.95Ga0.05As/GaAs bilayers is then grown on the patterned substrate by molecular beam epitaxy (MBE) for the formation of a high-contrast DBR stack consisting of oxidized AlGaAs and GaAs. The regrown DBR is dry-etched (5-20µm diameter), to expose the top surface of the highly doped InGaAsP contact layer. Oxidation at 425°C for 45 minutes formed a stable AlxOy/GaAs top DBR. The VCSEL fabrication was completed using standard photolithography and metallization techniques.
The room-temperature pulsed light-current and voltage-current characteristics for a 26µm device are shown in Fig. 2(a). Devices were tested with no heat sinking. Threshold currents as low as 12mA were observed for 26µm devices. The emission wavelength was 1.542µm with a FWHM of 0.6nm, as obtained from a 10µm device (Fig. 2(b)). Threshold current varied from one area to another due to thickness non-uniformity across the wafer causing variation in the bottom reflectivity. This is especially true with MOVPE grown wafers. Additional L-I curves were collected from cooled devices of varying size. Figure 3 shows the threshold current dependence on diameter for devices tested under a pulsed bias source at 15ºC.
Figure 2. (a) Light vs current characteristics
of 26µm devices operating at
room temperature (pulsed) and (b) optical spectrum of 10µm device (Va=6V,
I>Ith)
Figure 3. Threshold current vs device diameter at 15ºC (under pulsed bias conditions)
We have demonstrated a room temperature, pulsed-biased single-substrate InP-based 1.55µm vertical cavity laser. The device is selectively oxidized using an InAlAs layer. With further optimization of the growth uniformity, cavity design, and gain-offset, lower threshold currents and lower temperature sensitivity can be expected.
The authors acknowledge the help provided by K. Kamath and X. Zhang. This work is supported by the Office of Naval Research.
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* Solid-State Electronics Laboratory, Department of Electrical Engineering and Computer Science, University of Michigan, MI 48105-2122
** Corning Inc., Corning, NY 14831-0001
Coupled Resonator Vertical Cavity Laser Diodes
Toward Quantum Entanglement In A Quantum-Dot Nanocavity
Epitaxial long wavelength DBRs on InP: AlAsSb or lateral oxidation
High Temperature 1300 nm VCSELs for single-mode fiber-optic communication
Manufacturing of Oxide VCSEL at Hewlett Packard
Tapered-apertures for high-efficiency miniature VCSELs
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