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We study the basic mechanism for limiting the maximum operating temperature (Tm) of InP‐based multiple‐quantum‐well (MQW) lasers emitting at a wavelength of 1.3 μm. It is shown that laser operation is characterized in terms of the temperature dependence of threshold gain (gth) by introducing a critical temperature (Tc): gth exhibits a linear relationship with temperature below Tc, while it superlinearly increases with increasing temperature above Tc. This rapid increase in gth leads to a marked increase in threshold current and a significant reduction in differential quantum efficiency above Tc. We indicate that Tc exhibits a direct correlation with Tm: the higher Tc, the higher Tm. In the temperature range above Tc, laser operation moves into a loss‐multiplication regime, where a considerable portion of the injected carriers brings about a significant increase in internal loss rather than gain due to their pileup in the separate confinement heterostructure layers. We demonstrate that an anomalous increas...

By taking into account the temperature dependence of the intrasubband relaxation times for electrons and holes, we study the temperature sensitivity of the optical gain and of the gain saturation coefficient for 1.3‐μm InP‐based strained‐layer (SL) quantum‐well (QW) lasers. The band structures are obtained by the self‐consistent numerical solution of the Poisson equation, the scalar effective‐mass equation for the conduction band, and the multiband effective‐mass equation for the valence band. The intrasubband relaxation times are then calculated within the fully dynamic random phase approximation including carrier‐carrier and carrier‐phonon interactions on an equal basis. We demonstrate that the temperature dependence of the intrasubband relaxation times plays a key role in determining the temperature sensitivity of the optical gain and of the gain saturation coefficient of InP‐based SL‐QW lasers.

Basic design principles are formulated for increasing the differential gain of 1.55 μm InGaAsP/InP compressive‐strained multiple‐quantum‐well (MQW) lasers. An InGaAsP quaternary active layer can provide more freedom of design compared to an InGaAs ternary active layer since the amount of strain and the quantum‐well thickness can be independently determined in an InGaAsP material system for a given emission wavelength. Compressive strain ranging from 1% to 1.5% is large enough to reduce the density of states below one‐third of that of unstrained wells. No further reduction is expected even if more compressive strain is applied. When the well thickness is, in turn, determined, it is essential to incorporate a trade‐off between the conduction and valence subband energy spacings and the squared optical matrix elements. The extra enhancement of differential gain in MQW structures with a modulation p doping is also studied. By designing MQW structures according to these principles, differential gain can be incr...

The current blocking mechanism in InP buried heterostructure (BH) lasers is analyzed with a self‐consistent, two‐dimensional numerical analysis method. It has been clarified that quasi‐Fermi potentials which are kept constant in the active region under lasing oscillation play a key role in determining the current blocking capability. Due to this potential pinning effect, the junction voltages at each constituent p‐n junction in the blocking regions are maintained lower than the electrical turn‐on voltage, which realizes an excellent current blocking in BH lasers.

We study the critical behavior of 1.3 μm InP-based strained-layer (SL) multiple-quantum-well (MQW) lasers at elevated temperatures. We show that, under the critical injection condition where the carrier density in the quantum wells reaches the maximum possible without causing any extra pile-up of carriers in the separate heterostructure confinement regions, an InP-based SL-MQW system exhibits an absorption-to-gain phase transition at some critical temperature (Tc). The characteristic feature of this phase transition shows excellent agreement with Landau theory of second-order phase transitions. It is demonstrated that Tc is a significant and meaningful quantity not only for laser design but also for characterizing the nature of an InP-based SL-MQW system in terms of condensed matter physics.

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.

We derive a basic design rule for highly-efficient operation of 1.3-/spl mu/m InP-based strained-layer (SL) multiple-quantum-well (MQW) lasers at elevated temperatures on the basis of a self-consistent numerical approach including the Poisson equation and effective-mass equations. Following the derived design rule, high-efficiency (0.55 W/A at 363 K) and high-power (over 35 mW at 363 K) InP-based SL-MQW lasers have been fabricated. >

High‐temperature operation of InAsP‐based laser diodes at 1.3 μm has been realized according to guidelines of a large conduction‐band‐offset material (ΔEc) with a large optical confinement factor (Γ). Using photoluminescence excitation spectroscopy measurements, it was found that the conduction‐band offset of InAs0.52P0.48P/InGaAsP is the half of the band‐gap energy difference (0.5 ΔEg), which is larger than that of conventional quaternary material systems. A strain‐compensation growth technique enabled the fabrication of a large number of wells for large Γ. For broad‐area laser diodes, the maximum operating temperature increased as the number of wells increased from 4 to 15. In buried heterostructure lasers with ten wells, with high‐reflectivity coating on both facets, continuous‐wave lasing operation at temperatures up to 150 °C was achieved with a characteristic temperature of 59 K (30

Recent temperature sensitivity measurements of Auger recombination effects in compressively strained quantum‐well structures, which are seemingly inconsistent, are explained in terms of calculation results from a Monte Carlo analysis. In this analysis, realistic valence subband structures obtained using a 4×4 k⋅p method and Fermi–Dirac statistics are employed, where the conduction‐hole‐hole‐splitoff process is also assumed to be the dominant recombination process. It is found that at low carrier density levels and high temperature—often the conditions under which the Auger recombination coefficient is measured—the temperature sensitivity of the Auger recombination coefficient is expected to be low. A qualitative description of this behavior is also given.

The gain saturation coefficients of tensile‐strained, lattice‐matched, and compressive‐strained InGaAs/InGaAsP quantum‐well lasers (QWLs) are calculated from intrasubband relaxation times. The intrasubband relaxation times are in turn obtained within the random‐phase approximation including carrier–carrier and carrier–polar‐optical phonon interactions at room temperature. The effects of strain on the band structures are included by taking into account the strain‐dependent coupling among heavy‐hole, light‐hole, and spin‐orbit split‐off subbands on the basis of the multiband effective‐mass theory. It is demonstrated that the gain saturation coefficient in tensile‐strained QWLs is less sensitive to the amount of strain than in compressive‐strained QWLs where it markedly increases with strain.

The possibilities of extreme reduction in the linewidth enhancement factor α and of chirpless operation are theoretically investigated in InGaAs/InGaAsP quantum‐well lasers. First, the condition for lasing operation at the peak of the differential gain spectrum is derived. Lasing characteristics are then calculated taking into account the effects of strain on the valence subband structure. It is shown that by appropriate application of detuning and p‐type modulation doping to a compressive‐strained quantum well, it is possible to reduce the α factor to virtually zero while keeping the laser operating in the region of positive gain.

InP-based strained-layer quantum-well (SL-QW) lasers have become promising devices for various optical communication systems, such as high-speed, long-haul transmission systems and fiber-in-the-loop (FITL) systems [1]. This paper describes a fundamental analysis method for designing high-performance InP-based SL-QW lasers. We formulate basic design principles for minimizing the threshold current density by taking full advantage of the characteristic features of strain-induced changes in the valence-band structures. We also demonstrate that a more advanced numerical approach is crucial for an analysis of the high-temperature characteristics. Finally, we discuss some design rules for integrated SL-QW lasers applied in FITL systems.

InP-based strained-layer quantum-well (SL-QW) lasers have attracted much interest since they are among the most promising light sources for high-speed, long-haul optical transmission system. When designing high-speed lasers, one of the important criteria is to increase the differential gain large enough to overcome the gain saturation because the higher gain saturation is one of the major hurdles limiting the modulation bandwidth of QW lasers [1], [2]. Since the demonstration of the high potential for improving differential gain by the application of strain (both tensile and compressive) [3], extensive studies have been carried out to clarify the effects of strain on the resonance frequency of SL-QW lasers. However, the improvement of the actual modulation bandwidth is not guaranteed at all even if the resonance frequency is increased by the application of strain. This is mainly because the carrier transport, escape and capture processes [4], [5] also play a dominant role in determining the bandwidth as well. To properly evaluate the actual modulation bandwidth, it is necessary to take into account these carrier-transport-related effects as well as the strain effects. In this paper, we analyze the modulation bandwidth of InP-based SL-QW lasers on the basis of the unified formalism which includes both the strain effects and the carrier-transport-related effects. We demonstrate here that both effects compete with each other for determining the modulation bandwidth and that the tensile strain exerts a more pronounced impact on the modulation bandwidth of InP-based SL-QW lasers under the strain-limited situation.

I. Introduction Strained layer quantum well lasers (SL-QWLs) have attracted much interest since the application of band structure engineering [1], [2] can improve lasing properties such as lower threshold currents and larger differential gains. It has been expected that the larger differential gain due to strain can enhance modulation bandwidths of QWLs [3], However, the highest bandwidths found so far are much smaller than expected. One reason for this is that gain saturation also influences the modulation bandwidths of QWLs as well as the differential gain [4], [5]. We have alreiidy shown that the gain saturation coefficient increases with compressive strain in GaAs-based SL-QWLs due to an increase of the intra-subband relaxation times [6]. Recent experimental work has also shown that the gain saturation coefficient increases with compressive strain in InGaAs/InGaAsP SL-QWLs [7]. It is, therefore, necessary to take into account the effects of strain not only on differential gains but also on gain saturation coefficients to properly evaluate the enhancement of modulation bandwidths in SL-QWLs. Strained layer quantum well lasers (SL-QWLs) have attracted much interest since the application of band structure engineering [1], [2] can improve lasing properties such as lower threshold currents and larger differential gains. It has been expected that the larger differential gain due to strain can enhance modulation bandwidths of QWLs [3], However, the highest bandwidths found so far are much smaller than expected. One reason for this is that gain saturation also influences the modulation bandwidths of QWLs as well as the differential gain [4], [5]. We have alreiidy shown that the gain saturation coefficient increases with compressive strain in GaAs-based SL-QWLs due to an increase of the intra-subband relaxation times [6]. Recent experimental work has also shown that the gain saturation coefficient increases with compressive strain in InGaAs/InGaAsP SL-QWLs [7]. It is, therefore, necessary to take into account the effects of strain not only on differential gains but also on gain saturation coefficients to properly evaluate the enhancement of modulation bandwidths in SL-QWLs. The purpose of this paper is to clarify the relationship between gain saturation coefficients and the amount of strain in InP-based SL-QWLs. The gain saturation coefficients of a tensile-strained, a lattice-matched and a compressive-strained InGaAs/InGaAsP quantum well laser are analyzed on the basis of spectral hole burning theory. The intra-subband relaxation times are calculated by taking into account the strain effects within the common framework of carrier-carrier and carrier-phonon interactions. We demonstrate here that the gain saturation coefficient in tensile-strained QWLs remains constant independent of the amount of strain, while it increases with strain in compressive-strained QWLs.

The relationship between crystallization processes in the formation of polycrystalline-silicon (poly-Si) films and trap state densities at grain boundaries is described. Three different crystallization techniques were used to obtain poly-Si films: 1) LPCVD, 2) solid-phase crystallization, and 3) laser recrystallization. Trap state densities in laser-recrystallized poly-Si are 9.2-9.6 × 1011cm-2, regardless of grain size. These values are half of those in LPCVD and solid-phase crystallized poly-Si. It is indicated quantitatively that laser-induced melting and the subsequent solidification process exert a significant influence on the electrical activity of silicon grain boundaries.

Metal-gate thin-film transistors (TFT's) have been fabricated in layers of laser-recrystallized polycrystalline silicon on fused quartz substrates at processing temperatures below 625°C. Tantalum pentoxide (Ta 2 O 5 ) was used as a gate insulator instead of a conventional thermally grown silicon dioxide (SiO 2 ). Ta 2 O 5 gate insulator was deposited onto the recrystallized silicon layer at room temperature, using an RF-magnetron sputtering system. The reactive ion etching method, using CF 4 as a reactive gas, was employed in patterning deposited Ta 2 O 5 . These TFT's have exhibited p-channel depletion-mode characteristics with a threshold voltage of 2.5 V and a transconductance of 70 µS at V g = - 2 V. An on-off current ratio exceeding 105has been obtained.