Klaus Vogel

Simple method for examining sulphur passivation of facets in InGaAs–AlGaAs (λ=0.98 μm) laser diodes

The effect of (NH4)2Sx treatment of the facet of InGaAs/AlGaAs ridge waveguide (RW) laser diodes on the nonradiative current and catastrophic optical damage (COD) level is reported. Using the power–voltage–current (P–V–I) characteristics of the electroluminescence at low injection levels, changes in the density of surface states at the laser facets are described.

Crystalline perfection in GaInAsP/GaAs laser structures with GaInP or AlGaAs cladding layers

Abstract The crystalline perfection of GaInAsP layers grown lattice-matched to GaAs by metal-organic vapour phase epitaxy (MOVPE) has been studied using cathodoluminescence (CL) and transmission electron microscopy (TEM). The tendency for phase separation observed for single layers with composition close to the miscibility gap is further enhanced in broad-area laser diodes, where even GaInAs grown on top of a decomposed layer can show decomposition. The crystalline perfection of the layer structure is correlated to the optoelectronic properties of the laser diodes. For buried laser structures, the presence of different growth facets leads to additional problems due to composition fluctuations that can result in defect formation when GaInAsP is used for the regrowth.

Conductively Cooled 1 kW—QCW Diode Laser Stacks Enabling Simple Fiber Coupling

A new mini-bar design densely packed with 13 emitters in a 1.6 mm aperture has been developed. These mini-bars deliver >200 W output. With a very low divergence of 95% power enclosed in 25° they are suitable for fast axis collimation (FAC) lenses with large working distance. The mini-bars are mounted on compact CuW carriers, which are further integrated into a custom stack assembly where they are conductively cooled from both sides. This completely AuSn soldered stack consists of only coefficient of thermal expansion matched materials and was 100 times temperature cycled from -40°C to 85°C, without any change in the PUI characteristic, the spectrum and the far field. The beam propagation factor M 2 of a 12-layer stack is ¿130 in the vertical direction (after FAC) and ¿210 in the lateral direction. Thus, the stack can be coupled to a 1.2 mm multi-mode fiber (M 2 = 260) with a coupling efficiency of 90% using only low-cost lenses.

Influence of current spreading on the transparency current density of quantum-well lasers

We investigate the dependence of the threshold current of broad-area lasers on the stripe width. A comparison of differently deep-etched devices fabricated from the same wafer reveals that the stripe width dependence of the threshold current is caused by the current spreading effect. We propose a simple method to obtain a transparency current density characterizing a particular epitaxial structure without being influenced by current spreading.

Comparison of Ti/Pt/Au and Ti/Ru/Au contact systems to p-type InGaP

The electrical and metallurgical properties of vacuum-evaporated Ti/Pt/Au and Ti/Ru/Au contacts to moderately doped p-type InGaP and their thermal stability at higher temperatures are reported. Ohmic behaviour and typical contact resistivities of 4 - and 6 - could be obtained only for contacts annealed between 500 and . When subjected to an aging test at for 50 h in , the contacts originally annealed at exhibited good electrical stability and only a minor increase of the specific contact resistance was observed during the first 25 h. SEM observation of surface morphology and SIMS in-depth profile measurements showed that the Ru layer, incorporated between the Ti contact layer and a thick Au top layer, proved to be a more effective and thermally more stable diffusion barrier than the Pt layer. As a result, the standard Ti/Pt/Au contacts were metallurgically stable up to , while the newly proposed Ti/Ru/Au contact scheme exhibited a remarkable stability even after annealing at .

Continuous-wave vertical-cavity surface-emitting lasers with emission wavelengths near 650 nm

Red VCSELs for emission wavelengths near 650nm find applications in emerging technologies such as plastic-fiber-based data communication. However, these devices are challenging due to low band offsets and high electrical and thermal resistivity of especially the p-DBR. The paper presents the optimization of p-DBR and QW design for the reduction of the series resistance and the threshold current density. VCSEL structures were grown using MOVPE and processed to air-post mesas. The resistance of the p:DBR mirrors was optimized using different dopants and interfaces. By changing from Zn doping and abrupt interfaces to the dopant C and introducing graded interfaces the differential resistance decreased. Due to a relative shift across the wafer between the DBR stop-band and gain peak wavelength defined by the MQW active region, devices are available with lasing wavelengths between 638nm and 662nm in pulsed-mode operation. Threshold current densities of 3.6kA/cm2 at 650nm are measured. For improving device parameters a current aperture was processed by selective wet oxidation of AlxGa1-xAs with varying x. Cw laser operation is achieved for wavelengths between 644nm and 657nm at 10°C ambient temperature. With threshold currents under 4mA maximum cw output powers of 160µW are obtained at wavelengths of 657nm and 650 nm.

Highly stable conductively cooled 1 kW - QCW diode laser stacks with high duty cycle

A new mini-bar design with densely packed 13 emitters in 1,6 mm aperture has been developed These mini-bars deliver > 200 W output. With a very low divergence of 95% power enclosed in 25deg they are suitable for fast axis collimation (FAC) lenses with large working distance. The mini-bars are mounted on compact CuW carriers, which are further integrated into a custom stack assembly where they are conductively cooled from both sides. This completely AuSn soldered stack consists of only CTE matched materials and was 100 times temperature cycled from -40degC...85degC without any change. The beam parameter product (95% power) of a 12 layer Stack is in the vertical direction (after FAC) les45 mmmiddotmrad and in the lateral direction les75 mmmiddotmrad. Thus the stack can be coupled to an 1.2 mm multi mode fibre with a coupling efficiency of 90% using only low-cost lenses.

High-power diode lasers based on InGaAsP spacer and waveguide layers with AlGaAs cladding layers

Broad-area stripe laser diodes based on GaAs and Al-free spacer and Al-free spacer and waveguide layers were studied and compared to conventional AlGaAs-laser diodes. The structures were grown by low pressure MOVPE (metal organic vapor phase epitaxy). For the active region InGaAs-single quantum wells with an emission wavelength of 915 nm were used. Threshold current density, internal loss and internal efficiency are comparable for the three structures under study. The use of a quaternary spacer layer instead of GaAs improves the performance for laser diodes with a low divergence ((Theta) perpendicular equals 20 degree(s) FWHM). Using a non-optimized facet coating procedure about 1.2 W cw output power at a vertical divergence of (Theta) perpendicular equals 26.5 degree(s) are obtained from a 50 micrometers wide stripe laser with Al-free waveguides.

Related Electrical and Metallurgical Properties of Pd/Zn/Pd/Ge and Pd/Zn/Pd/Au Contact Systems to p-InGaP

The comparison of electrical and metallurgical properties of vacuum-evaporated Ge/Pd(Zn) and Au/Pd(Zn) contact metallizations to moderately doped p-type InGaP epitaxial layers (p ≈ 2 × 1018 cm—3) is presented. The Pd/Zn/Pd/Ge contacts exhibited Ohmic behaviour already from 400 °C and their specific contact resistance dropped to 4 × 10—5 Ω cm2 when annealed at 490 °C. Cross-sectional transmission electron microscopy and energy dispersive X-ray spectrometry demonstrated that a relatively sharp and nonspiking contact–InGaP interface has still been preserved at this high temperature. On the other hand, regardless of the excellent electrical properties (ϱc = 5 × 10—6 Ω cm2 at 490 °C) of the Pd/Zn/Pd/Au contacts, strong metallurgical reactions at the contact–InGaP interface at temperatures ≥450 °C resulted in an extensive decomposition and melting of the interface area with deep protrusions into the InGaP.

Development of 650 nm-emitting VCSELs for cw operation

This paper reports on the optimization of red vertical-cavity surface-emitting laser (VCSEL) diodes grown by metalorganic vapour phase epitaxy. The VCSEL structure has an GaInP/AlGaInP multiquantum well active zone sandwiched between AlGaAs/AlAs distributed Bragg reflectors (DBRs). We present results on the optimization of the electrical resistance of the p:DBR and the wavelength tuning. Using these optimized parameters pulsed laser operation is demonstrated for wavelengths between 639 nm and 662 nm. At 650 nm the threshold current density is 3.7 kA/cm/sup 2/. Cw laser operation is achieved at room temperature. With wet oxidized VCSELs maximum cw output powers of 160 /spl mu/W are obtained at a wavelength of 656 nm and 10/spl deg/C.