Separate Confinement Heterostructure Region Research Articles

Degradation Mechanisms of Heterogeneous III-V/Silicon 1.55- $\mu \text{m}$ DBR Laser Diodes

This paper reports an extensive analysis of the degradation processes of heterogeneous III-V/Silicon infrared laser diodes designed for integrated telecommunications and interconnects. By submitting the devices to a series of constant current stress tests, a gradual degradation of the main device parameters was observed. In particular, in every stress scenario the devices under test showed: 1) an increase in the threshold current; 2) a decrease of the turn-ON voltage; and 3) an increase in the apparent carrier concentration within the space charge region. The variation of the electrical parameters (turn-ON voltage and apparent charge concentration) was found to be significantly correlated to the optical degradation for long stress times; the results support the hypothesis that degradation originates from an increase in the non-radiative recombination rate, possibly due to the diffusion of defects toward the active region of the devices. In order to investigate the physical origin of those defects, a capacitance deep level transient spectroscopy analysis was performed. The results indicate the presence of five different deep levels, with a main trap located around 0.43 eV above the valence band energy. This trap is compatible with an interface defect located between the In 0.53 Al x Ga 0.47-x As separate confinement heterostructure region and the InP layer.

Thermal dependence of the optical gain and threshold current density of GaInNAs/GaAs/AlGaAs quantum well lasers

The thermal dependence of the optical gain and threshold current density of GaInNAs/GaAs/AlGaAs quantum well (QW) lasers with the emission wavelength of 1.3 μm have been theoretically investigated. The optical gain is found to decrease with the increase of the temperature due to the hole leakage in the separate confinement heterostructure (SCH) region. The distribution of unconfined electrons in the SCH region is negligible while the distribution of unconfined holes in the SCH region plays an important role in the leakage process, reflecting the smaller bandgap discontinuity in the valence band than in the conduction band. The threshold current density increases with the increase of temperature, which is dominated by the monomolecular recombination process. Our calculated threshold current densities with temperatures are in agreement with the available experimental results.

The Effect of Temperature on the Operation of Quantum Well Laser: A Simulation Study Based on Three-Level Rate Equations

A temperature-dependent quantum well laser equivalent circuit model based on three-level rate equations is presented. The model is simulated using an ORCAD PSPICE circuit simulator. The characteristic properties such as threshold current, delay, and frequency response are simulated and compared with the available results. Included are temperature effect, leakage current, and heat dissipation effect on the chip. Threshold current increases, whereas turn on delay and optical output decrease with increasing temperature. An increase in temperature also reduces the operating frequency and resonance peak shifts to higher frequency. It is also observed that the modulation response of a QW laser with shorter separate confinement heterostructure region is more sensitive to the temperature variation.

Growth and characterization of compressive-strain GaInAsP/InP multiple-quantum-well laser diodes with the tensile-strain GaInP quantum barrier

In this article, we report on the growth of 1.3-μm-compressive-strain GaInAsP/InP multiple-quantum-well laser diodes (CS-MQW LDs) with a tensile-strain GaInP quantum barrier (GaInP-QB) in the separate-confinement-heterostructure (SCH) regions. Observed via the photoluminescence (PL) spectra, the optimum Ga composition of GaxIn1−xP-QB, GaInP-QB thickness, and pair number of the GaxIn1−xP-QB are 0.09, 5 nm thick and one pair, respectively. The optimum GaInP-QB structure in the p-side SCH region exhibits the narrowest PL full width at half maximum of 43.1 meV, the lowest threshold current of 23 mA, and the highest characteristic temperature of 52 K for the as-cleaved LDs with a 600-μm-cavity length and a 3.5-μm-wide ridge stripe. This low threshold current is better than that of the LDs with GaInP-QB in the n-side SCH region and that of the conventional LDs without GaInP-QB.

Lattice Temperature Model and Temperature Effects in Oxide-Confined VCSEL’s

A lattice temperature model is derived for oxide-confined vertical cavity surface emitting lasers (VCSELs) based on carrier transport and the conservation of energy. Peltier heat is caused by the bandedge and quasi-Fermi level discontinuities at a heterojunction. However, considering the device size, Peltier heat needs to be distributed and is not just generated at the interface, otherwise, an anomolous spike in temperature will occur. We have developed a novel treatment to model the Peltier heat at a heterojunction by use of a Monte Carlo simulation. Peltier heat is found to be a major heat contributor, and it results in a rapid and high temperature rise in the separate confinement heterostructure (SCH) region of the laser diode. We have also shown that the carrier thermal conductivities for materials with high mobilities must be included at high carrier densities because they contribute to additional spreading of the thermal energy. Subsequently, this lattice temperature model is coupled self-consistently to electronic and optical solvers to form a complete simulator for VCSELs. Self-heating causes a fast temperature rise when the VCSEL is operated under continuous wave conditions, causing resonant wavelength changes and an eventual thermal rollover. The resonant wavelength shift has been shown to be caused mainly by the heating of the distributed Bragg reflectors even though the peak temperature occurs within the SCH region. Possible physical factors causing the thermal rollover have also been examined with our complete simulator. The Auger recombination process is found to be one of the main factors causing the thermal rollover in 980 nm oxide-confined VCSELs while the photon lifetime is a factor in determining the position of the thermal rollover. We have also achieved a very good match between our simulated results and experimental data.

Effects of carrier transport on the transverse-mode selection of index-guided vertical-cavity surface-emitting lasers

We study the transverse mode selection of a weakly index-guided vertical-cavity surface-emitting laser (VCSEL), based on an extension of the model of Valle et al. [Opt. Commun. 115 (1995) 297]. In the model we incorporate carrier capture and escape processes between the separate confinement heterostructure (SCH) layer and the quantum well (QW) active region of a VCSEL. These effects are characterized with phenomenological capture and escape times, τ cap, τ esc. We show analytically and by numerical simulations that when the carrier loss in the SCH region is taken into account, carrier capture and escape critically affects the transverse-mode selection of a VCSEL. A large ratio between τ cap and τ esc diminishes the number of photons in the QWs, leading to a decrease of the spatial-hole burning, which in turn benefits emission on the fundamental transverse mode. On the contrary, a small ratio between τ cap and τ esc increases the number of photons in the QWs, leading to a large spatial-hole burning which in turn favors higher-order transverse modes. If the carrier loss in the SCH region is neglected the total number of photons in the QW region is independent of τ cap and τ esc. In this case carrier capture and escape play no fundamental role in the transverse-mode selection of the VCSEL, but modify the transient dynamics of the relaxation oscillations.

Temperature dependence of intrinsic recombination coefficients in 1.3 μm InAsP/InP quantum-well semiconductor lasers

In this letter, we report on the temperature dependence of the intrinsic recombination coefficients in long-wavelength quantum-well lasers. Unlike previous studies, we obtain the intrinsic recombination coefficients from carrier lifetime measurements with a correction for the carrier population in the barrier and separate confinement heterostructure region. Our results show that this carrier population not only affects the value of the recombination coefficients obtained but also their temperature dependence. We measure a significant increase in the intrinsic Auger coefficient with temperature indicating that the frequently reported temperature insensitivity of this coefficient is likely due to carriers spilling out of the wells at elevated temperatures and not an intrinsic property of the Auger process.

Nonlinear analysis of quantum-well lasers with the effects of carrier transport

An improved nonlinear quantum-well (QW) laser model, which takes into account the effects of quantum capture and escape processes, is presented, based on laser rate equations and Volterra theory. This model is further expanded by proper parameter transform to include the effect of carrier diffusion in the separate confinement heterostructure region. Various QW laser distortions have been evaluated using this model and compared with the results obtained from the previous model where the transport effects are absent. The results shows that the effect of transport processes on laser dynamic nonlinearity can be significant.

1.3-μm InP-based quantum-well lasers with n-doped separate confinement heterostructure layers for high-temperature operation

We present a novel approach for reducing the light output power penalty in 1.3-μm InP-based strained-layer (SL) MQW lasers at elevated temperatures. It is shown that n-type doping in the separate confinement heterostructure (SCH) layers increases the barrier height in the valence band profiles effectively, which makes it possible to suppress the pile-up of holes in the SCH region under high-temperature, high-injection conditions. One significant impact of this approach is that the power penalty can be reduced to one half of that in conventional SL-MQW lasers with undoped SCH. We show that SL-MQW structures with n-doped SCH have a great potential for realizing a low power penalty as well as high efficiency in InP-based MQW lasers at elevated temperatures.

Mode analysis of semiconductor lasers using lateral confinement by native-oxide layers

We have analyzed the lateral mode behavior of 980 nm InGaAs-AlGaAs lasers, where the waveguiding is carried out by oxidized AlAs layers beside a current aperture. The effective refractive-index step between the active and oxidized region has been calculated. We have found that the effective refractive-index step can be adjusted exactly by the thickness of the AlAs layer and its distance to the graded-index separate-confinement heterostructure region. We have compared this native-oxide-confined laser to a standard ridge-waveguide laser with respect to the lateral index step. Furthermore, we suggest a structure for a lateral separate confinement of carriers and optical waves. Experimental results are presented which are in good agreement with the numerical simulations.

Dominant mechanisms for the temperature sensitivity of 1.3 μm InP-based strained-layer multiple-quantum-well lasers

We study the dominant mechanism for the temperature sensitivity of the differential quantum efficiency and threshold current of 1.3 μm InP-based strained-layer multiple-quantum-well (MQW) lasers. We show that the temperature dependence of both properties is separated into two regions at critical temperature c). Below c, the temperature dependence of internal loss in the QWs plays a very important role in determining the temperature sensitivity of the differential quantum efficiency. On the other hand, above c, its temperature sensitivity is affected more by the internal loss in the separate confinement heterostructure (SCH) region. Excellent correlation is observed between the spillover of holes into the SCH region and c. It is also shown that the Auger recombination current plays a more significant role in determining the temperature dependence of threshold current below c. However, above c, electrostatic band-profile deformation, which causes a significant increase in loss and radiative recombination current in the SCH region, plays the more dominant role than the Auger recombination current.

A time-domain model for high-speed quantum-well lasers including carrier transport effects

Carrier transport plays an important role and can significantly affect the ultra-fast properties of quantum-well (QW) lasers. We present a detailed multi-mode time-domain large-signal dynamic model including the effects of carrier transport, suitable for the high-speed QW lasers. It is based on the well-proven transmission-line laser modelling technique with the addition of a multilevel system of coupled rate equations. Simulated results from studies of both the static and small-signal properties are compared with measurements from another laboratory. Our model can accurately predict the modulation-bandwidth discontinuity in QW laser structures with large separate-confinement-heterostructure (SCH) regions. We use large-signal simulations to predict increased damping of transient responses and larger turn-on delays caused by the effects of carrier transport. Our large-signal simulations also show that an increase in the turn-on delay times is expected in QW structures with large carrier transport times across the SCH region, whereas the inter-well transport times do not affect the turn-on delay times significantly. >

Comparison of two- and three-level rate equations in the modeling of quantum-well lasers

By explicitly including the effect of gateway states a description of the carrier transport process in quantum-well lasers is possible. These are states spatially localized to a quantum well, but which are energetically above the well and aid in the transfer of carriers from the unconfined 3-D barrier region to the 2-D well region, and vice versa. Analytical solutions for the single quantum-well case, using the ambipolar transport approximation, serve to highlight the dominant transport mechanisms in quantum-well lasers and to unify previously published two- and three-level rate equation models (Nagarajan et al, Kan et al, and Tessler et al.) in the limits of vanishingly small carrier capture and diffusion free transport across the separate confinement heterostructure (SCH) region. Although the two-level effective capture model has been successful at modeling wide SCH region quantum-well lasers, in the model developed in this work the physical mechanisms governing carrier transport (diffusion and local carrier capture) are treated separately. As a result the two-level system equations correspond to the more general model detailed here at the phenomenological level only. The implications this has on dynamical parameter estimation from measurement data are highlighted. >

Structure-dependence of the chirp-to-power ratio spectrum for GaAs external cavity lasers

The chirp-to-power ratio (CPR) spectrum has been modeled for GaAs quantum well external cavity lasers. The model includes effects of transverse carrier spatial hole burning, finite carrier transport and capture time from the separate confinement-heterostructure (SCH) region to the quantum well region, and intrinsic material gain compression. The model explains the measured difference of the phase of the CPR at low modulation frequencies between GaAs quantum well and channel-substrate planar (CSP) lasers both in extended cavities. Our results indicate that the carrier effect in the SCH region can make a major contribution to the CPR. >

On the effects of carrier diffusion and quantum capture in high speed modulation of quantum well lasers

It is shown that, the combined effects of carrier diffusion in the separate confinement heterostructure region and quantum capture into the quantum well must be considered together when evaluating the limits on the modulation bandwidth of quantum well lasers, despite the fact that quantum capture is considerably faster than carrier diffusion. The importance of quantum capture may be observed by noting that the modulation bandwidth is adversely affected even for quantum capture times as short as 0.2 ps, or if the ratio of the quantum capture time to quantum escape time is large (≳0.01). Therefore, the bandwidth limitation may be caused by electron transport rather than hole transport since quantum capture of holes is faster than that of electrons.

Temperature dependence of damping in high-speed quantum-well lasers

The temperature dependence of damping in quantum-well lasers has been investigated. The lasers with large K factors at room temperature show a rapid increase of the K factor with temperature. The temperature dependence of the K factor is larger for lasers with wider separate confinement heterostructure region, fewer wells, and narrower quantum-well width. The carrier transport in quantum-well structures is responsible for the excess increase of the K factor at high temperatures.

Effects of carrier transport on high-speed quantum well lasers

We present a model for the dynamic response of quantum well lasers which shows that the carrier transport across the separate confinement heterostructure region and the barriers can be critical in determining the modulation bandwidth. We also show that, depending on the particular quantum well laser structure, a large part of the experimentally measured reduction in modulation bandwidth is due to transport factors.