F. Salomonsson

Structural effects of the thermal treatment on a GaInNAs/GaAs superlattice

We have studied structural changes that occur during annealing of GaInNAs/GaAs multiple quantum wells grown by metalorganic vapor-phase epitaxy (MOVPE). Different thermal treatments led to an improved room-temperature photoluminescence (PL) intensity, but also to room-temperature PL peak splitting. This splitting is related to the appearance of compositional clustering as displayed by transmission electron microscopy (TEM). In addition to this, interfacial layers on each side of the wells have also been observed by TEM and their composition is discussed on the basis of high resolution x-ray diffraction studies. It is suggested that the interface layers are indium deficient, but enriched in nitrogen, degrading the optical quantum well performance and indicating a need for improved switching sequences in the MOVPE growth.

Doping-induced losses in AlAs/GaAs distributed Bragg reflectors

We have studied n- and p-type doping-induced performance degradation of AlAs/GaAs distributed Bragg reflectors (DBRs) for applications in vertical cavity lasers (VCLs). Based on high-accuracy optical reflectance and triple-axis x-ray diffraction measurements on a variety of differently doped DBR structures grown by metalorganic vapor-phase epitaxy, a fitting procedure was employed to extract the doping-dependent optical loss. A striking observation is that the reflectance of these DBRs is much more sensitive to n- than p-type doping incorporation. While in the latter case the loss can be well accounted for by intervalence-band and free-carrier absorption, additional loss mechanisms must be considered for n-type DBRs. We relate the losses to doping-enhanced interdiffusion effects resulting in increased interface scattering. These findings should have important consequences for the design of VCLs, demonstrating the importance of reduced n-type doping concentrations and/or growth temperatures, or the applica...

Defects, structure, and chemistry of InP–GaAs interfaces obtained by wafer bonding

We have examined the crystallographic structure of GaAs/InP interfaces obtained by wafer fusion following different procedures. Plan-view and cross-sectional transmission electron microscopy reveal that the interface is not only composed of a regular array of two sets of edge dislocations and is more complex than generally supposed. If a twist is created due to misalignment of the two substrates, the dislocations are not edge dislocations but also have a screw component. Dislocations for which the Burgers vectors have a component normal to the interface are also present. Those dislocations probably result from steps and some of them accommodate the tilt between the two substrates. Inclusions and voids as well as a low number of volume dislocations are present in all the samples. The observed volume dislocation density near the interface lies in the 105–107 cm−2 range and these volume dislocations may be associated with thermal mismatch. The origin of all these defects is discussed.

All-Epitaxial Single-Fused 1.55 µm Vertical Cavity Laser Based on an InP Bragg Reflector

We have realised an all-epitaxial 1.55 µm vertical cavity laser by employing a single wafer-fusion step. The laser structure is fabricated by fusing a 32-period p-doped (C) AlGaAs/GaAs top mirror onto a half-cavity structure consisting of a 50-period n-doped (Si) GaInAsP/InP bottom mirror and a 9 quantum well GaInAsP-active material. Laser mesas are fabricated using a wet etching procedure for the top mirror. The top mirror also contains an AlAs layer for oxidation for current confinement. The lasers operate pulsed at temperatures up to 40°C and at pulse lengths of 10 µs up to 5°C. The minimum threshold current density at room temperature is 1.8 kA/cm2 for a device diameter of 55 µm. Compared to nonoxidised laser diodes, the threshold current is markedly decreased in oxidised laser diodes.

Systematics of Electrical Conductivity across InP to GaAs Wafer-Fused Interfaces

We report on the electrical and compositional characterization of wafer-fused isotype heterojunctions between Zn-, C- or Si-doped GaAs and Zn- or Si-doped InP. The junctions were characterized by current-voltage and secondary ion mass spectrometry (SIMS) measurements. It is demonstrated that very low-resistive junctions can be obtained in each case, but also that there is a strong influence from the detailed sample structure and processing conditions. SIMS was used to monitor the doping concentration across the interface as well as the impurity concentrations of oxygen, carbon and iron.

N-type doping induced losses in 1.3/1.55 /spl mu/m distributed Bragg reflectors

The effect of n-type doping on the peak reflectivity of InGaAsP/InP as well as GaAs/AlAs distributed Bragg reflectors for long-wavelength vertical-cavity lasers has been investigated. A variety of mirrors with different doping levels were grown in both material systems using metal organic vapour phase epitaxy. The reflectance of the structures was measured with high accuracy employing two independent measurement techniques. While nominally undoped DBRs exhibit an expected reflectivity in excess of 99.9%, doping is found to induce significant losses resulting in up to 0.6% reduced reflectance.

Temperature-dependent performance of 1.55 /spl mu/m vertical-cavity lasers with InGaAsP/InP bottom mirror

We have fabricated and evaluated a long-wavelength vertical-cavity laser (VCL) based on an epitaxially integrated InP distributed Bragg reflector (DBR) under continuous wave (CW) and pulsed conditions. We conclude that, for an InP-DBR-down configuration, the high temperature performance is limited by the heat conductivity of the bottom mirror. The highest operating temperature for CW and pulsed condition is 17/spl deg/C and 101/spl deg/C respectively, indicating a substantial self-heating for CW. To investigate the prospect for improved performance in other mounting configurations, we have applied a two-dimensional finite element analysis to the heat transfer problem. It is suggested that for top-side-down mounting with the AlGaAs/GaAs DBR closest to the heat sink, a performance comparable to that of so called double-fused VCLs could be possible.

Novel Technologies for 1.55µm Vertical Cavity Lasers

We report on three novel vertical-cavity laser (VCL) structures for 1.55-?m operation. Two of the structures utilize an n-type GalnAsP/ InP Bragg mirror combined with an Al(Ga)As/GaAs mirror using either wafer fusion or metamorphic epitaxial growth. The third employs two wafer-fused AlGaAs/GaAs mirrors, in which lateral current confinement is obtained by localized fusion of the p mirror. All three VCLs use strained GalnAsP quantum wells as active material and achieve continuous-wave (cw) operation at room temperature or above. The single fused VCL operates up to 17 and 101°C in continuous-wave and pulsed mode, respectively. The monolithic VCL-structure with a metamorphic GaAs/AlAs n-type mirror uses a reverse-biased tunnel junction for current injection. This laser achieves record high output power (1 mW) at room temperature and operates cw up to 45°C. The double fused VCLs with a 10x 10-?m2 active area operate cw up to 30°C with threshold current as low as 2.5 mA and series resistance of 30 H. The emission spectra exhibit a single lasing mode polarized with 30-dB extinction ratio and a spectral linewidth of 150 MHz.

Long wavelength vertical cavity lasers

We report on three novel vertical cavity laser (VCL) structures for 1.55μm operation. Two of the VCL structures utilize an n-type GaInAsP/InP Bragg mirror combined with an Al(Ga)As/GaAs mirror using either wafer-fusion or metamorphic epitaxial growth. The third VCL employs two wafer fused AlGaAs/GaAs mirrors, in which lateral current confinement is obtained by localized fusion of the p-mirror. All three VCLs use strained GaInAsP quantum wells as active material and achieve continuous-wave (CW) operation at room-temperature or above. The single fused VCL operates up to 17°C and 101°C in continuous-wave and pulsed mode, respectively. The monolithic VCL-structure with a metamorphic GaAs/AlAs n-type mirror uses a reversed biased tunnel junction for current injection. This laser achieves record high output power (1mW) at room temperature and operates CW up to 45°C. The double fused VCLs with a 10×10μm 2 active area operate CW up to 30°C with threshold current as low as 2.5mA and series resistance of 30 Ohms. The emission spectra exhibit a single lasing mode polarized with 30 dB extinction ratio and a spectral linewidth of 150 MHz.

Systematics of electrical conductivity across InP to GaAs wafer fused interfaces

We report on the electrical and compositional characterization of wafer fused isotype heterojunctions between Zn, C or Si doped GaAs and Zn or Si doped InP. The junctions were characterized with current-voltage and secondary ion mass spectrometry (SIMS) measurements. It is demonstrated that very low-resistive junctions can be obtained in each case, but also that there is a strong influence from the detailed sample structure and processing conditions. SIMS was used to monitor the doping concentration across the interface as well as the impurity concentration of oxygen, carbon and iron.