Design and technology of vertical-cavity surface-emitting lasers with nonconducting epitaxial mirrors

Design and technology problems in the fabrication of vertical-cavity surface-emitting lasers (VCSELs) equipped with nonconducting distributed Bragg reflectors (DBRs) and fabricated using molecular-beam epitaxy are considered. VCSELs with an active region were formed on the basis of InGaAs quantum wells and incorporated an AlGaAs/GaAs bottom DBR and an oxidized AlGaO/GaAs top DBR; the diameter of the oxidized aperture was equal to 7–12 μm. The devices exhibit a continuous-wave lasing at room temperature with threshold currents of 0.5–1.5 mA, a differential efficiency as high as 0.5 mW/mA, and a highest output power of 3 mW. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 10, 2003, pp. 1265–1269. Original Russian Text Copyright ? 2003 by Maleev, Kovsh, Zhukov, Vasil’ev, Mikhrin, Kuz’menkov, Bedarev, Zadiranov, Kulagina, Shernyakov, Shulenkov, Bykovskii, Solov’ev, M?ller, Ledentsov, Ustinov.     

1.55-μm buried-heterostructure VCSELs with InGaAsP/lnP-GaAs/AlAsDBRs on a GaAs substrate

We demonstrate 1.55-μm buried-heterostructure (BH) vertical-cavity surface-emitting lasers (VCSELs) on a GaAs substrate. Thin-film wafer-fusion technology enables InP-based BH VCSELs to be fabricated on GaAs/AlAs distributed Bragg reflectors. Detailed investigations of the device resistance are also described. As a result of introducing BH and obtaining low device resistance, the threshold current density under CW operation shows the independence of mesa size due to a strong index guide and small noneffective current. A 5-μm VCSEL exhibits a record threshold current of 380 μA at 20°C. This VCSEL also operates with single transverse mode up to the maximum optical output power     

Electrically-pumped vertical-cavity lasers with Al/sub x/O/sub y/-GaAs reflectors

We have fabricated the first electrically-pumped vertical-cavity surface-emitting lasers (VCSELs) which use oxide-based distributed Bragg reflectors (DBRs) on both sides of the gain region. They require a third the epitaxial growth time of VCSELs with semiconductor DBRs. We obtain threshold currents as low as 160 /spl mu/A in VCSELs with an active area of 8 /spl mu/m/spl times/8 /spl mu/m using a two quantum well InGaAs-GaAs active region. By etching away mirror pairs from the top reflector, quantum efficiencies as high as 61% are attained, while still maintaining a low threshold current of 290 /spl mu/A.     

Design and fabrication technology for arrays for vertical-cavity surface-emitting lasers

The design and fabrication technology for integrated arrays for vertical-cavity surface-emitting lasers with bottom AlGaAs/GaAs semiconductor and top AlGaO/GaAs distributed Bragg reflectors is suggested. Arrays containing 8×8 lasers with an active region based on two InGaAs quantum wells were produced. Individual emitters with an oxidized aperture of 8–10 μm in diameter demonstrate cw lasing at 960–965 nm at room temperature, with a threshold current of 1.0–2.5 mA, external efficiency of up to 0.4 mW/mA, and a maximum output power of over 2 mW.     

Red Light Vertical-Cavity Surface-Emitting Lasers

Vertical-cavity surface-emitting lasers (VCSELs) have entered into commercial market over the last few years. The paper describes the progress of visible (red) VCSELs in particular. The Basic experimental structures are reviewed, with emphasis on distributed Bragg reflectors, gain media, as well as detuning.lt also points out some technical issues that must still be resolved. Finally, the polarization of VCSELs devices is discussed.     

Design, fabrication, and performance of infrared and visiblevertical-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     

GaAs/AlGaAs multiple quantum well GRIN-SCH vertical cavity surfaceemitting laser diodes

Vertical cavity surface emitting lasers (VCSELs) with GaAs/AlGaAs multiple quantum well (20 wells) graded-index separate-confinement-heterostructure (GRIN-SCH) active regions are discussed. The VCSEL structures, which also contained two Al_xGa_1-xAs/Al_yGa_1-yAs distributed Bragg reflectors, were grown by molecular beam epitaxy and had a threshold current and current density at room temperature pulsed excitation of 16 mA and 14 kA/cm², respectively, near 0.85-μm wavelength. Both single-longitudinal and fundamental transverse mode emission characteristics were observed with a light output greater than 3 mW and a slope efficiency of 0.12 mW/mA     

Low threshold current 1.3-/spl mu/m GaInNAs VCSELs grown by MOVPE

The structure of the conventional contact 1.3-/spl mu/m GaInNAs-GaAs vertical-cavity surface-emitting lasers (VCSELs) was optimized and low threshold current 1.3-/spl mu/m GaInNAs VCSELs grown by metal-organic vapor-phase epitaxy were reported. The idea is to optimize the active region, the doping profiles, and the pairs of p-distributed Bragg reflectors, and the detuning between the emission wavelength and the photoluminescence gain peak wavelength. The continuous-wave 1.0-mA threshold current was achieved for the single-mode VCSEL. For the multiple-mode VCSELs, the below 2-mA threshold currents at 5/spl deg/C-85/spl deg/C , the 1.13-mA threshold current at 55/spl deg/C, and 1.52-mA threshold current at 85/spl deg/C are the best results for 1.3-/spl mu/m GaInNAs VCSELs.     

X-Ray diffraction analysis of bandgap-engineered distributed bragg reflectors

Compositionally graded interfaces between constitutive layers of the distributed Bragg reflectors (DBRs) in vertical cavity surface emitting lasers (VCSELs) greatly reduce operating voltages. In this paper, we demonstrate the use of high-resolution x-ray diffraction (HRXRD) in the characterization of a bandgap-engineered DBR. The final structure of the DBR is determined by comparing the Bragg peak intensities and locations in the x-ray rocking curve measurements with those of dynamical x-ray diffraction simulations. The aluminum concentration of the graded interface region is determined as a function of distance to within 2%. The width of the interface is accurate to within 1.5 nm. Such sensitivity demonstrates the utility of the non-invasive x-ray technique in characterizing not only the DBR, but also the entire VCSEL structure.     

Blue-green optically pumped GaN-based vertical cavity surface emitting laser

Blue-green GaN-based vertical cavity surface emitting lasers (VCSELs) were fabricated with two dielectric Ta₂O₅/ SiO₂ distributed Bragg reflectors. Lasing action was observed at a wavelength of 498.8 nm at room temperature under optical pumping. Threshold energy density and emission linewidth were 189 mJ/cm² and 0.15 nm, respectively. The result demonstrates that blue-green VCSELs can be realised using III-nitride semiconductors.     

Optoelectronic device simulation of Bragg reflectors and theirinfluence on surface-emitting laser characteristics

This paper presents a simulation analysis of distributed Bragg reflectors (DBRs) and their affect on the characteristics of vertical-cavity surface-emitting lasers (VCSELs). The SimWindows semiconductor device simulator models the close interaction between electrical, optical, and thermal processes present in VCSELs. This simulator is used to examine the electrical characteristics of some simple DBR designs. Due to the different transport characteristics of electrons and holes, these results will show that n-type DBR designs must be different than p-type designs in order to achieve the best operating characteristics for the overall laser. This analysis will demonstrate the improvement in the characteristics by comparing the simulation results of a standard VCSEL with the results of a VCSEL using improved DBR designs     

Threshold analysis of vertical-cavity surface-emitting lasers with intracavity contacts

A numerical analysis of vertical-cavity surface-emitting lasers (VCSELs) incorporating intracavity contacts and distributed Bragg reflectors (DBRs) is presented. The model considers polarization dependent reflection at the DBRs, current spreading, and nonuniform carrier density distribution self consistently. Analytic expressions for the current spreading and the corresponding series resistance for VCSELs incorporating intracavity contacts are derived. It is shown that current spreading strongly affects the lateral gain profile, the threshold current density, the transverse mode shape and the transverse mode discrimination through the creation of intracavity optical phase and gain apertures. The series resistance and the depth of the dip in the current density distribution are used as figures-of-merit to provide guidelines for device optimization, as illustrated by means of two examples of long wavelength VCSEL designs.     

Selectively etched tunnel junction for lateral current and optical confinement in InP-based vertical cavity lasers

We demonstrate a thin, selectively lateral-etched, AlIn(Ga)As tunnel-junction (TJ) layer as a current and optical confinement aperture in the InP-based long-wavelength vertical cavity surface-emitting lasers (VCSELs). A high etch selectivity was demonstrated by etching the aperture a distance of several microns without affecting the surrounding InP etch-resistant layer. Edgeemitting lasers enclosing the TJ aperture showed high injection efficiency and low current spreading underneath the aperature. Single-mode continuous-wave operation of a 1.55-μm VCSEL was demonstrated successfully with a room-temperature differential efficiency of 21% using a 6-μm-wide TJ aperature.     

High performance of 1.55-μm buried-heterostructurevertical-cavity surface-emitting lasers

We report a record low threshold current of 1.55-μm vertical-cavity surface-emitting laser (VCSEL). Thin-film wafer-fusion technology enables InP-based buried heterostructure VCSELs to be fabricated on GaAs-AlAs distributed Bragg reflectors. Threshold current density is independent of mesa size, and a 5-μm VCSEL exhibits a threshold current as low as 380 μA at 20°C and a single transverse mode up to the maximum optical output power under continuous-wave operation     

IaAsP/InGaAsP quantum-well 1.3 μm vertical-cavity surface-emitting lasers

1.3 μm vertical-cavity surface-emitting lasers based on a novel gain media consisting of InAsP/InGaAsP strain-compensated multiple quantum wells are reported. SiO₂/TiO₂/ dielectric thin-film pairs and wafer-bonded GaAs/Al(Ga)As distributed Bragg reflectors are used as the top and bottom cavity mirrors, respectively. The device with a 5 μm-diameter selectively etched tunnel-junction aperture exhibits submilliampere threshold current as low as 0.54 mA and single-transverse mode emission. Maximum output optical power of 1.9 mW was observed in multimode lasing devices.     

Performance of gain-guided surface emitting lasers withsemiconductor distributed Bragg reflectors

The performance limitations of gain-guided vertical cavity surface emitting lasers (VCSELs) which use epitaxially grown semiconductor distributed Bragg reflectors (DBRs) are discussed. The light-current ( L-I) characteristics and emission wavelength of such lasers are examined as a function of temperature and time under continuous wave (CW) and pulsed operation. The authors observed a sharp roll-over in the CW L-I characteristics which limits the maximum output power. The threshold current under CW operation is found to be lower than that obtained under pulsed conditions. Several microseconds long delay in lasing turn-on is also observed. It is shown quantitatively that these anomalies are a consequence of severe heating effects. It is shown that reduction of the series resistance and threshold current density can lead to significant improvements in the power performance of VCSELs     

GaAs-AlxGa1-xAs integrated twin-guide lasers with distributed Bragg reflectors

A GaAs-AlxGa1-xAs integrated twin-guide (ITG) laser with distributed Bragg reflectors (DBR) was fabricated. The DBR was in the form of a third-order grating which was chemically etched on the surface of the output waveguide. The ITG lasers with distributed Bragg reflectors (DBR-ITG lasers) operating at room temperature showed single mode oscillation up to 1.5 times the threshold current and the half-side external quantum efficiency was 1 ∼ 2 percent.     

Vertical-cavity surface-emitting lasers with spatially adjustableDBR reflectivity to enable free-space photonic repeaters

We have created bidirectional high-performance top- and bottom-emitting vertical-cavity surface-emitting lasers (VCSELs) from a single epitaxial wafer. Using a spatially selective oxidation of AlAs layers in hybrid distributed Bragg reflectors (DBRs) of a VCSEL structure, devices processed for top emission had threshold currents of 100-110 μA with slope efficiencies of 18%-20%. Devices from the same wafer processed for bottom substrate emission had thresholds of 110-120 μA and slope efficiencies of 17% and 18%. This technology will greatly simplify the use of VCSEL's in large three-dimensional (3-D) parallel interconnects, photonic repeaters, and smart pixels     

InP-based all-epitaxial 1.3-/spl mu/m VCSELs with selectively etched AlInAs apertures and Sb-based DBRs

We report, for the first time, InP-based all-epitaxially grown 1.3-/spl mu/m vertical-cavity surface-emitting lasers with lattice-matched Sb-based distributed Bragg reflectors and AlInAs etched apertures. The minimum threshold current and voltage under pulsed operation were 3 mA and 2.0 V, respectively. The thermal impedance was as low as 1.2 K/mW without heat sinking. Implementation of the AlInAs etched aperture was quite effective in improving the injection efficiency and reducing the internal loss, resulting in improved differential efficiency.     

GaAs-based 1.53 μm GaInNAsSb vertical cavity surface emitting lasers

Electrically-pumped GaAs-based 1.53 mum vertical cavity surface emitting lasers operating in pulsed mode at room temperature and continuous wave (CW) up to 20degC are reported for the first time. The lasers employ a GaInNAsSb/GaNAs multiple quantum well active region, a selectively oxidised AlAs aperture and p- and n-doped Al(Ga)As/GaAs distributed Bragg reflectors. Typical devices have room-temperature pulsed threshold current densities of 8.3 kA/cm and threshold voltages of 5.5 V. CW threshold currents as low as 2.87 mA for a 7 mum aperture device were observed at 15degC.