Joachim Piprek

Origin of efficiency droop in GaN-based light-emitting diodes

The efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.

What limits the maximum output power of long-wavelength AlGaInAs/InP laser diodes?

We analyze the high-temperature continuous-wave performance of 1.3-/spl mu/m AlGaInAs/InP laser diodes grown by digital alloy molecular-beam epitaxy. Commercial laser software is utilized that self-consistently combines quantum-well bandstructure and gain calculations with two-dimensional simulations of carrier transport, wave guiding, and heat flow. Excellent agreement between simulation and measurements is obtained by careful adjustment of material parameters in the model. Joule heating is shown to be the main heat source; quantum-well recombination heat is almost compensated for by Thomson cooling. Auger recombination is the main carrier loss mechanism at lower injection current. Vertical electron escape into the p-doped InP cladding dominates at higher current and causes the thermal power roll-off. Self-heating and optical gain reduction are the triggering mechanisms behind the leakage escalation. Laser design variation is shown to allow for a significant increase in the maximum output power at high temperatures.

Thermionic emission cooling in single barrier heterostructures

Nonisothermal transport in InGaAsP-based heterostructure integrated thermionic coolers is investigated experimentally. Cooling on the order of a degree over 1 μm thick barriers has been observed. This method can be used to enhance thermoelectric properties of semiconductors beyond what can be achieved with the conventional Peltier effect.

Band gap bowing and refractive index spectra of polycrystalline AlxIn1−xN films deposited by sputtering

The AlGaInN semiconductor system is currently of high interest for applications in blue light emitting devices. AlInN is a prospective material for lattice matched confinement layers. We measure the refractive index as well as the band gap across the entire compositional range of high-quality polycrystalline AlInN samples. Strong band gap bowing is observed.

Material parameters of quaternary III - V semiconductors for multilayer mirrors at wavelength

Nine quaternary (Al,Ga,In) - (P,As,Sb) semiconductor compounds lattice matched to InP are investigated theoretically. Direct bandgap, refractive index at wavelength, and thermal conductivity are calculated as a function of the composition. These material properties are important, e.g. in distributed Bragg reflectors of vertical-cavity lasers. The alloy systems AlGaAsSb, AlGaPSb and GaInPSb are found to promise better performance of those mirrors than the common InGaAsP system.

Minimum temperature sensitivity of 1.55 μm vertical-cavity lasers at −30 nm gain offset

Double-fused vertical-cavity surface-emitting lasers (VCSELs) have demonstrated the highest temperature performance of any 1.5 μm VCSEL, but further optimization is needed to reduce their temperature sensitivity. We present and analyze threshold current measurements of these devices between −90 °C and 30 °C stage temperature. Despite a zero gain peak offset from the emission wavelength at room temperature, the pulsed threshold current has its minimum near −50 °C corresponding to about −30 nm gain offset. This is in contrast to a common VCSEL design rule. Temperature effects on the optical gain of the strain-compensated InGaAsP/InP active region are found to be the main cause for the disagreement. A design rule modification is proposed. Numerical simulation of an optimized 1.55 μm VCSEL shows that gain offset improvements are counteracted by loss mechanisms.

64/spl deg/C continuous-wave operation of 1.5-/spl mu/m vertical-cavity laser

We report on 64/spl deg/C continuous-wave (CW) operation of a 1.5-/spl mu/m vertical-cavity laser. This laser consists of two fused AlGaAs-GaAs mirrors with a strain-compensated InGaAsP-InP MQW active region. Selective lateral oxidation is used for current confinement. Minimum room-temperature threshold current is as low as 0.8 mA, and maximum CW output power is as high as 1 mW at 15/spl deg/C. Pulsed operation is achieved up to 100/spl deg/C. Current spreading losses and device heating are analyzed in detail. Dynamic parameters such as maximum 3-dB parameters such as maximum, 3-dB bandwidth (4.7 GHz), alpha factor (4.0), and linewidth (39 MHz) are also investigated.

Design and analysis of double-fused 1.55-/spl mu/m vertical-cavity lasers

Detailed design and experimental characterization of three generations of double-fused vertical-cavity lasers are described. The result of this design evolution is the first above-room-temperature continuous-wave operation of long-wavelength vertical-cavity lasers. Threshold currents of 2.3 mA and yields greater than 90% have been obtained.

Self-consistent analysis of high-temperature effects on strained-layer multiquantum-well InGaAsP-InP lasers

We present a comprehensive evaluation of the temperature effects on the threshold current and the slope efficiency of 1.55 /spl mu/m Fabry-Perot ridge-waveguide lasers between 20/spl deg/C and 120/spl deg/C. Experimental results are analyzed using the commercial laser simulator PICS3D. The software self-consistently combines two-dimensional carrier transport, heat flux, strained quantum-well gain computation, and optical waveguiding with a longitudinal mode solver. All relevant physical mechanisms are considered, including their dependence on temperature and local carrier density. Careful adjustment of material parameters leads to an excellent agreement between simulation and measurements at all temperatures. At lower temperatures, Auger recombination controls the threshold current and the differential internal efficiency. At high temperatures, the vertical electron leakage from the separate confinement layer mainly limits the laser performance. The increase of internal absorption is less important. However, all these carrier and photon loss enhancements with higher temperature are mainly triggered by the reduction of the optical gain due to wider Fermi spreading of electrons.

High-power 1320-nm wafer-bonded VCSELs with tunnel junctions

A new long-wavelength vertical-cavity surface-emitting laser structure is described that utilizes AlGaAs-GaAs mirrors bonded to AlInGaAs-InP quantum wells with an intracavity buried tunnel junction. This structure offers complete wavelength flexibility in the 1250-1650 nm fiber communication bands and reduces the high free-carrier losses and bonded junction voltage drops in previous devices. The intracavity contacts electrically bypass the bonded junctions to reduce threshold voltage. N-type current spreading layers and undoped AlGaAs mirrors minimize optical losses. This has enabled 134/spl deg/C maximum continuous-wave lasing temperature, 2-mW room-temperature continuous-wave single-mode power, and 1-mW single-mode power at 80/spl deg/C, in various devices in the 1310-1340 nm wavelength range.