Increase In Threshold Current Density Research Articles

InAs–GaAs Quantum Pyramid Lasers: In Situ Growth, Radiative Lifetimes and Polarization Properties

We have realized injection lasers based on InAs–GaAs and InGaAs–GaAs quantum pyramids (QPs) with a lateral size ranging from 80 to 140 Å. The structures with relatively small dots (∼80 Å) exhibit properties predicted earlier for quantum dot (QD) lasers such as low threshold current densities (below 100 Acm-2) and ultrahigh characteristic temperatures (T0=350–425 K). For operation temperatures above 100–130 K, T0 decreases and the threshold current density increases (up to 0.95–3.3 kAcm-2 at room temperature) due to carrier evaporation from QPs. Larger InAs QPs (∼140 Å) providing better carrier localization exhibit saturation of the ground-state emission and enhanced nonradiative recombination rate at high excitation densities. The radiative lifetime shows a weak dependence on the dot size in the range 80–140 Å being close to ∼1.8–2 ns, respectively. A significant decrease in radiative lifetime is realized in vertically coupled quantum dots formed by a QP shape-transformation effect. The final arrangement corresponds to a three-dimensional tetragonal array of InAs islands inserted in a GaAs matrix each composed of several vertically merging InAs parts. We achieved injection lasing in such an array for the first time.

Evaluating the effects of optical and carrier losses in etched-post vertical cavity lasers

We demonstrate the combined effects of optical scattering loss and surface recombination (or carrier diffusion) on the performance and scalability of etched-post vertical cavity lasers (VCLs). The size dependence of optical losses and threshold gain are determined from pulsed measurements of external quantum efficiency. Deeper etch depths result in a stronger radial dependence of the threshold gain, which quickly increases the threshold current density. With optical loss accounted for, pulsed threshold current density measurements give the extra information needed for evaluating carrier loss. Surface recombination or carrier diffusion also results in threshold current density increases, but scalability is ultimately limited by the ability of the active region to provide enough gain for smaller size, higher optical loss devices. Even with these losses, three-quantum-well VCLs with shallow etches have threshold currents as low as 420 μA.

Second quantized state lasing and gain spectra measurements in n-type modulation doped GaAs-AlGaAs quantum-well lasers

The emission characteristics of n-type modulation doped GaAs-AlGaAs quantum-well lasers are studied for constant doping density and stepped doping density laser cores. Constant doping density cores are found to have a shift to shorter wavelength with increasing doping density but suffer from a corresponding large increase in threshold current density. Stepped doping density cores exhibit clear wavelength shifting from the first to second quantized state transitions with increased doping near the quantum well while maintaining low threshold current densities. Threshold current densities of 440 A/cm/sup 2/ are measured for second quantized state lasing in stepped core lasers. Gain spectra are measured for the stepped doping density core devices and modulation doping is shown to improve the gain bandwidth by 50% over undoped devices. >

Prospects for truly blue ZnSe/Zn1−uMguSvSe1−v/Zn1−xMgxSy Se1−y semiconductor diode lasers

The electrical and optical operation of separate confinement heterostructure quantum-well lasers of ZnSe/Zn1−uMguSvSe1−v/Zn1−xMgxSy Se1−y (blue laser) and Zn0.8Cd0.2Se/ZnS0.06Se0.94/ Zn1−zMgzSuSe1−u (blue-green laser) was studied by numerical simulation. The physical model was based on the self-consistent solution of Poisson’s equation, the current continuity equation, the photon rate equation, and the scalar wave equation. The band structure of strained quantum well was calculated using k⋅p theory, and the gain model was based on the interband electron-hole pair recombination without including excitonic effects. Good agreement was obtained between the experimental and the calculated output characteristics of Zn0.8Cd0.2Se/ZnSe0.06Se0.94/ Zn1−zMgzSuSe1−u blue-green lasers over a wide temperature range from 85 to 394 K. Further extension of these calculations to the ZnSe/Zn1−uMguSvSe1−v/Zn1−xMgxSy Se1−y structures demonstrated a considerable increase in threshold current density of blue lasers, due to the insufficient level of doping in wide-band p-Zn1−xMgxSySe1−y cladding layers. This would likely impose more stringent conditions for realization of continuous blue laser emission compared to their blue-green laser counterparts.

Gain-current characteristics of strained AlGaInP quantum well lasers

Single quantum well AlGaInP lasers, each having a 65 Å width well have been fabricated with GaxIn1−xP well compositions from x=0.43 to 0.54 grown on substrates orientated 10° off the (100) axis. The threshold currents have been measured from 120 K to room temperature and the samples show a systematic variation with x. At low temperature the results are in good agreement with our calculated gain-current curves and we show that at room temperature the rapid increase in threshold current density with decreasing length is due to a thermally activated leakage current rather than gain saturation.

Study of growth temperature in gas-source molecular-beam epitaxy growth of InGaAs/GaAs quantum well lasers

Strained-layer InGaAs/GaAs single quantum well lasers were grown by gas-source molecular-beam epitaxy at temperatures between 430 and 610 °C. The optimum growth temperature for the quantum wells of the lasers was found to be about 520 °C, as deduced from the measurement of threshold current density of the lasers and room-temperature photoluminescence (PL) of the quantum wells. If the growth temperature is lower than 520 °C, the threshold current density dramatically increases due to the formation of nonradiative carrier recombination centers in the quantum well. These recombination centers originate from lattice mismatch defects and alloy disordering. If the growth temperature is higher than 520 °C, indium desorption and segregation become increasingly severe. Thus, at high growth temperature, the InGaAs/GaAs interfaces become rough and the threshold current density increases. In addition, rapid thermal annealing was observed to remove most of the nonradiative recombination centers from the quantum wells and to improve the threshold current density of the lasers and PL intensity of the quantum wells.

High on/off ratio bistable operation of InGaAs/InAlAs MQW lasers using resonant tunneling effect

The structural dependence of the resonant tunneling characteristic in InGaAs/InAlAs MQW diodes is studied. It is found that the threshold current density for negative differential resistance caused by resonant tunneling effect increases with a decrease in the barrier width, well width and well number. In particular, the threshold current density increases by an order of one and a half when the barrier width decreases from 67 to 50 Å. Bistable operation with quite a high light output on/off ration (800:1) is obtained for the InGaAs/InAlAs MQW laser diodes with 50 Å width barriers using the resonant tunneling effect.

Waveguiding, spectral, and threshold properties of a stripe geometry single quantum well laser

In this paper we report and analyze some interesting properties observed for gain-guided GB-SQW lasers. For narrow stripe widths ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\leq10 \mu</tex> m) and thin active layers (≤150 Å) we observe a long delay time for lasing and abnormal increase in threshold current density. To explain this behavior we developed a model of waveguiding that takes into account the effect of heat profile generated along the junction plane due to carrier injection, and good agreement with experimental results is obtained.

Wavelength multiplexing DH AlGaAs injection laser source with a graded composition along the active layer

The use of DH (AlGa)As wafers with graded composition along the active layer for creation of multiwavelength laser source for a wavelength-division-multiplexing systems is proposed. The possibility of obtaining such structures by the LPE method under the condition of isothermal mixing is investigated. Electroluminescence measurements show that significant variation of emission wavelength along the structure does not cause the increase in threshold current density above a common level. The source fabricated is a set of monolithically integrated DBR lasers. Similarity of experimental dispersion of the effective guide indexes and that of refractive indexes of guide material facilitates the choice of DBR grating periods.

Suppression of output nonlinearities in double-heterostructure lasers by use of misaligned mirrors

Nonlinearity (kinks) in the dependence of light output power on current has been effectively eliminated by the use of mirrors aligned at a small angle from the direction perpendicular to the stripe or filament. Such a structure increases confinement of the lasing filament and causes an increase in threshold current density that is smaller than that produced by stripe narrowing.

Double heterostructure Pb1-xSnx Te–PbTe lasers with cw operation at 77 K

Diode lasers that operate cw at 77 K have been made from a Pb0.88Sn0.12 Te–PbTe double heterostructure grown by liquid phase epitaxy. Junction diffusion during growth has been prevented by the use of Tl-doped PbTe substrates. The wavelength emitted in cw operation has been temperature tuned from 10.5 μm at 12 K to less than 8.2 μm at 80 K, a range of nearly 280 cm−1. The cw threshold current density increases from 1.6 × 103 A/cm2 at 12 K to 4.2 × 103 A/cm2 at 77 K. The measured cw power is 10 mW in four modes at 12 K (1.8 × 104 A/cm2) and 1.2 mW in a single mode at 77 K (7.5 × 103 A/cm2).