Strained Layer Multi-quantum Well Research Articles

G-PON용 높은 전광변환효율을 갖는 1.31 um 비냉각 DFB-LD

[ <TEX>$-40^{\circ}C$</TEX> ]에서 <TEX>$85^{\circ}C$</TEX>의 온도에서 냉각장치 없이 동작하는 1.31 um 비냉각 DFB-LD가 유기 금속 화학 증착법에 의해 성장되었다. 높은 전광변환효율을 갖는 레이저의 제작은 스트레인이 인가된 다중양자우물 구조의 최적화를 통해 가능하며, 특히 스트레인의 양, 양자 우물의 두께, 전위장벽의 두께, 양자 우물의 수, 활성층의 폭에 주로 영향을 받는다. 본 연구에서 제작된 DFB-LD는 <TEX>$25^{\circ}C$</TEX>와 <TEX>$85^{\circ}C$</TEX>에서 전광변환효율은 0.38[mW/mA]와 0.26[mW/mA], 발진개시전류는 각각 7.1[mA]와 19.8[mA]의 값을 가졌다. A Strained Layer Multiquantum-Well (SL-MQW) distributed feedback laser at a wavelength of 1.31 um operating from <TEX>$-40^{\circ}C$</TEX> to <TEX>$85^{\circ}C$</TEX> without any cooling is grown by metal-organic chemical vapor deposition (MOCVD). Lasers with high slope efficiency are achieved through careful optimization of a SL-MQW active layer, especiallyoptimizing the amount of strain, the well thickness, the barrier thickness, the number of wells, and the active layer width. In this paper, we obtain the slope efficiencies of 0.38[mW/mA] and 0.26 [mW/mA] at <TEX>$25^{\circ}C$</TEX> and <TEX>$85^{\circ}C$</TEX>, respectively. Threshold currents are 7.1[mA] and 19.8[mA] at <TEX>$25^{\circ}C$</TEX> and <TEX>$85^{\circ}C$</TEX>, respectively.

Effect of Sb pre-deposition on the compositional profiles in MOVPE-grown InAsSb/InAs(1 1 1) multi-quantum wells

This paper describes the effect of Sb pre-deposition on the composition modulation in InAsSb/InAs strained-layer multi-quantum wells. The multi-quantum-well structures, grown by metal–organic vapor-phase epitaxy (MOVPE) on InAs (1 1 1) substrates, had periods of ≈20 nm and nominal InAs 0.8Sb 0.2 layer thicknesses of ≈4.5 nm. Sb surface coverages from 0 to 1.4 monolayers (ML) were deposited on the InAs surfaces during growth interruptions prior to alloy layer growth. With no pre-deposition, kinematical simulations of measured θ–2 θ X-ray diffraction (XRD) patterns yielded best fits for exponential composition profiles as expected for Sb segregation. The 1/ e Sb segregation lengths were 1.2–1.6 nm. When ≈0.8 ML of Sb was pre-deposited, XRD simulations indicated sharper InAsSb-on-InAs interfaces. Sb coverages of ⩾1 ML broadened the XRD superlattice reflections, indicating that interface roughness increased.

Optical anisotropy investigation in multi-quantum wells by two-photon spectroscopy

The light polarization dependence of two-photon absorption (TPA) near half the band gap has been experimentally investigated in strained-layer multi-quantum wells. A detailed analysis of their TPA photoluminescence spectra, measured for two different polarization configurations of the exciting laser beam (perpendicular and parallel to the QW growth axis z), permitted us to sensitively probe the exciton anisotropy in quasi-bidimensional (Q2D) heterostructures. Fundamental and excited exciton states of different symmetry have been selectively detected by exploiting the strong anisotropic selection rules for TPA processes. In particular, the polarization along the z axis allowed us to obtain the first clear experimental evidence of higher-order light-hole excitons. Our experimental results are in good agreement with theoretical predictions of an analytical model of TPA in Q2D exciton systems, demonstrating the usefulness of a simplified approach for the case of photon energies close to half the band gap.

The effect of varying barrier height on the operational characteristics of 1.3-μm strained-layer MQW lasers

This paper presents an empirical study of the effects that barrier layer composition has on the operational characteristics of 1.3-/spl mu/m-wavelength InGaAsP-InP multiquantum-well (MQW) strained-layer ridge-waveguide lasers. A systematic empirical investigation of how this design choice affects practical device operation was undertaken by examining threshold current, efficiency, and modal gain as a function of temperature in five different laser structures. The results of these studies indicate that small barrier heights improve device performance, despite the loss of electronic confinement in the shallow conduction band quantum wells. Indeed, it appears that carrier uniformity in the MQW structure may be improved by carrier redistribution due to thermal or tunneling effects, which in turn enhances the operation of the low barrier height structures.

Anomalous temperature dependence of PL characteristics in ordered InGaAsP strained layer multi-quantum well structure

Clarification of anomalous photoluminescence (PL) characteristics for InGaAsP compressively strained layer multi-quantum well (SL-MQW) structures is studied to realize highly strained SL-MQW lasers with a large number of well layers. PL characteristics such as an increase in PL linewidth and a small temperature dependence of the PL peak wavelength are observed in the SL-MQW structures in which a structure of CuPt-type atomic ordering is observed. Elimination of the ordered structure as well as reduction of the PL linewidth have been confirmed with increasing growth temperature. Although formation of an interfacial undulation (lateral modulation) between SL-MQW layers is also suppressed by the increase in growth temperature, the formation has ultimately limited the maximum strain in SL-MQW. We suggest that the anomalous PL characteristics result from generation of ordering, dominant when the growth temperature is lower, as well as from interfacial undulation, dominant in highly strained SL-MQW when the growth temperature is increased.

Degradation of PL characteristics in strained layer multi-quantum well structure with atomic ordering structure

Clarification of the degradation mechanism of photoluminescence (PL) charac-teristics of InGaAsP compressively strained layer multi-quantum well (SL-MQW) structures on an InP substrate is studied to realize highly strained SL-MQW lasers with a large number of well layers. An unusual temperature-dependence of the PL peak wavelength is observed in the SL-MQW structure: the shift of the PL peak wavelength is reduced with change in measurement temperature. The degradation of PL characteristics is evaluated by the reduc-tion in the PL shift as well as an increase in PL linewidth and a decrease in PL peak intensity. The extent of the PL degradation increases with an increase in the number of well layers and an increase in strain. In the SL-MQW structure with degraded PL characteristics, the structure of CuPt-type atomic ordering is clearly observed for the first time in each barrier layer by high-resolution transmission electron microscope (HR-TEM) measurements. In the SL-MQW structure, neither dislocation nor three-dimensional nucleation is observed by TEM measurements. By increasing the growth temperature, elimination of the ordering structure as well as improvement in the PL characteristics have been confirmed even in the SL-MQW structure with a large amount of strain and a large number of well layers.

Single-mode operation over a wide temperature range in 1.3 μm InGaAsP/InP distributed feedback lasers

An approach for single mode operation of 1.3 /spl mu/m distributed feedback (DFB) lasers with a large side mode suppression ratio over a wide temperature range of -40/spl deg/C to 100/spl deg/C, is reported. The lasers utilize an optimized strained-layer multiquantum well (MQW) active region in combination with index/gain-coupling and detuning effect. A high characteristic temperature T/sub 0/ (90-100 K) was obtained in 1.3 /spl mu/m InGaAsP/InP strained-layer MQW Fabry-Perot lasers when the number of QWs exceeded 10. In gain-coupled DFB lasers, a very low temperature dependence of the threshold current has been obtained when there is no detuning or positive detuning of the lasing wavelength at room temperature with respect to the material gain peak. An infinite T/sub 0/ can be realized over certain temperature ranges, in which the threshold current exhibits a minimum, depending on the amount of detuning. The physical mechanism responsible for the appearance of this minimum, as well as the high side mode suppression ratio, are explained theoretically.

New Structure of 1.3 µm Strained-Layer Multi-Quantum Well Complex-Coupled Distributed Feedback Lasers

A new structure of 1.3 µm strained-layer multi-quantum well (MQW) complex-coupled distributed feedback lasers having an InAsP absorptive grating is demonstrated. In this structure, the InAsP absorptive grating and the strained-layer MQW active layer can be fabricated by a metalorganic vapor phase epitaxy process involving only one step. Furthermore, the fabricated laser has a low threshold current (16.8 mA) and a high efficiency (0.45 mW/mA).

1.5 μm wavelength compressively strained GaInAs/AlGaInAs multiquantum-well lasers grown by molecular-beam epitaxy with high differential gain and low threshold current density

1.5 μm GaInAs/AlGaInAs multiquantum-well (MQW) lasers with 1% compressively strained quantum wells were grown by molecular-beam epitaxy. The effective differential gain (g0) determined from the squared relaxation oscillation frequency versus output power relations is a high value of 9.3×10−16 cm2 in long-wavelength lasers. On the other hand, the effective transparent carrier density (n0) of strained-layer MQW lasers determined from the measurement of the spontaneous carrier lifetime was found to be very high, which is different from the theory of strain effects. However, by taking the carrier transport effect into account, it was shown that (1) for the strained MQW lasers the intrinsic transparent carrier density is lower than that of the lattice-matched MQW lasers, and (2) the intrinsic value of g0 is estimated to be 28–56×10−16 cm2, which is close to the theoretically predicted value. By improving the laser structure to have better carrier transport, much higher effective differential gain and lower effective transparent carrier density can be expected.

High-power and high-speed performance of 1.3-μm strained MQW gain-coupled DFB lasers

In-phase gain-coupled distributed feedback (DFB) lasers with etched quantum-well active-layers emitting at 1.3-/spl mu/m wavelength have been fabricated for the first time. High optical output power up to 50 mW was achieved while single-mode operation was maintained at the longer wavelength side of the Bragg stop band because the in-phase gain coupling effect favors this wavelength. With the optimization of the strained-layer multi-quantum wells (MQW) in the active region, a large effective differential gain of 7.5-19/spl times/10/sup -16/ cm/sup 2/, a high modulation bandwidth of 18 GHz, and high temperature operation up to 100/spl deg/C were obtained when the DFB wavelength was not detuned from the material gain peak.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical transitions between light hole subbands in InGaAs/InP strained layer multiquantum wells

The observation of light hole intersubband absorption in both p-doped and photoexcited undoped strained InxGa1−xAs/InP (x≂0.35) quantum-well structures is reported. The absorption is polarized along the growth direction and is in agreement with calculations which show that the strain causes the light hole level to be first occupied upon p-doping or photoexcitation. Both impurity bound and free holes transitions are identified.

Design and optimization of strained-layer-multiquantum-well lasers for high-speed analog communications

We report on how the contributions from spatial hole burning, gain suppression, and relaxation oscillations to the chirp and harmonic distortion of SL-MQW DFB lasers can be calculated and minimized. It is shown how, by taking into account the specific properties of strained-layer-multiquantum-well (SL-MQW) lasers, simple solutions of the rate equations point the way to a chirp reduction and an increase of the useful bandwidth for analog communications. In such lasers, the absorption is only weakly dependent on the carrier density and therefore the harmonic distortion at lower modulation frequencies is mainly caused by spatial hole burning. Our numerical simulations indicate that in many cases this distortion is seduced by the same measures that reduce the chirp and increase the bandwidth. >

Enhanced relaxation oscillation frequency of 1.3 /spl mu/m strained-layer multiquantum well lasers

Dependence of relaxation oscillation frequency (f/sub r/) on the bandgap wavelength of InGaAsP barrier layers (/spl lambda//sub g//sup b/) and number of quantum wells (N/sub w/) were investigated for the first time, for 1.3 /spl mu/m InGaAsP/InGaAsP compressively strained multiquantum well (MQW) lasers. 1.3 times higher f/sub r/ was confirmed for strained-layer MQW lasers with large N/sub w/ (N/sub w//spl ges/7) and wide bandgap barrier layers (/spl lambda//sub g//sup b/=1.05 /spl mu/m) at the same injection level, compared with unstrained MQW lasers having the same well thicknesses and the same emitting wavelength. This enhancement mainly results from increased differential gain due to strain effects separated from the quantum-size effect. >

Transfer-matrix dynamic model of partly gain-coupled 1.55 μm DFB lasers with a strained-layer MQW active grating

A dynamic model for partly gain-coupled 1.55 /spl mu/m MQW DFB lasers consisting of etched strained-layer multiquantum wells is presented. For the modulation and noise characteristics of DFB lasers, analytical expressions which take into account both the longitudinal distribution of laser parameters and carrier transport effects are derived for the first time using the transfer-matrix method. As a numerical example, the relaxation oscillation frequency is compared to experimental results, and reasonable agreements are obtained between the theory and experiment. >

Absorption spectroscopy on room temperature excitonic transitions in strained layer InGaAs/InGaAlAs multiquantum-well structures

The physical properties (transition energy, oscillator strength, linewidth, binding energy, and reduced effective mass) of room temperature excitons in compressively strained InGaAs/InGaAlAs multiquantum-well (MQW) structures as a function of the well width have been investigated for the first time by both absorption measurements and photomodulated transmittance measurements. Photomodulated transmittance spectroscopy has been successfully applied to clearly reveal critical transition points. Measured transition energies are in good agreement with a model which includes the heavy hole and light hole splitting due to the strain. For well widths of 2.5–7.5 nm, oscillator strengths are smaller for the strained layer MQWs than for the lattice-matched MQWs by 35%–45%. This is due to the larger exciton radius for the strained MQWs resulting from smaller in-plane reduced effective masses (0.031–0.038m0), which are 65% of those of the lattice-matched MQWs.

Luminescence Study for Band Discontinuity in Free-Standing CdZnS/ZnS Strained Layer Multi-Quantum Wells

We present the results of luminescence studies for free-standing CdZnS/ZnS strained layer multi-quantum wells (MQWs). PL measurements of the CdZnS/ZnS MQW at 1.4 K show intense, sharp excitonic emission in blue-ultraviolet spectral region. The band discontinuity is estimated by an analysis of the luminescence results in terms of a finite-potential well model. A free-standing MQW is assumed. The calculated zero-stress band discontinuity is in extremely good agreement with the value from Harrison’s model.

Room-temperature operation of MOCVD-grown GaInAs/InP strained-layer multiquantum well lasers in 1.8 μm range

The first successful room-temperature pulsed operation is reported of InGaAs strained layer multiquantum well (SLMQW) injection lasers grown by MOVPE on InP substrates in the 1.8 μm range. The threshold current density and the external differential quantum efficiency of the 10 μm wide ridge waveguide lasers were 2.5 kA/cm2 (cavity length = 1mm) and 5% (cavity length = 400μm), respectively. Broad-area lasers, 100 μm wide and 1 mm long, had a reverse leakage current of less than 10 μA at − 1V indicating high quality of the epitaxial layers.

High quality In0.15Ga0.85As/AlxGa1−xAs strained multi quantum wells grown by metalorganic vapor phase epitaxy

We have investigated the growth of In0.15Ga0.85As/AlxGa1−xAs strained layer multi quantum wells with the metalorganic vapor phase epitaxy technique. X-ray rocking curves, photoluminescence, and photocurrent measurements clearly demonstrate the influence of the growth temperature on the quality of the deposited layers in this material system. The optimum temperature is found to be at about 650 °C. Photocurrent measurements on diode structures with these strained multi quantum wells as active layer show a clear shift of the exciton absorption peaks due to the quantum confined Stark effect. When the results are compared to the shifts seen for InGaAs/GaAs quantum wells, one observes an enhanced quantum exciton oscillator strength when high transverse electric fields are applied across the multi quantum wells. This property makes the InGaAs/AlGaAs material combination very suitable for use in vertical optical modulators.

Room-temperature operation of MOCVD-grown GaInAs/InP strained-layer multiquantum-well lasers in the 1.8- mu m range

Summary form only given. The authors report the first successful room-temperature pulsed operation of InGaAs strained-layer multiquantum-well (SL-MQW) injection lasers grown by MOVPE (metal-organic vapor-phase epitaxy) on InP substrates in the 1.8- mu m range. The threshold current density and external differential quantum efficiency of 10- mu m-wide ridge waveguide lasers were 2.5 kA/cm/sup 2/ and 5% for 1-mm and 400- mu m-long cavities, respectively. The laser structure consists of four 35-AA-wide In/sub 0.75/Ga/sub 0.25/As SL-QWs separated by 65-AA-wide barrier layers of lattice-matched InGaAs. The MQW is further sandwiched between 150 nm of InGaAsP ( lambda =1.3 mu m) and 2 mu m of InP on both sides. >

MBE growth of InAs/AlInAs strained-layer multi quantum wells for optical device applications

InAs/ Al 0.48 In 0.52 As strained-layer single quantum wells and strained-layer multi quantum wells have been grown by molecular beam epitaxy (MBE) on (100)-InP substrates. Although there is highly biaxial compressive strain in this material, high-crystalline quality without misfit dislocations is demonstrated by transmission electron microscopy (TEM) and low temperature photoluminescence (PL) measurements. PL peaks have been successfully observed from the MQWs with well width L z of 9–40 Å. As for the sample with L z = 9 Å, a large energy shift of 750 meV from the band-gap of bulk InAs was confirmed. A laser diode with InAs/AlInAs MQW active layer was also fabricated and the stimulated emission at 1.15 μm wavelength was confirmed at 77 K for the first time.