Strained InGaAs Quantum Wells Research Articles

An InGaAs Vertical-Cavity Surface-Emitting Laser Emitting at 1130 nm for Silicon Photonics Application

A highly strained InGaAs quantum well (QW) vertical-cavity surface-emitting laser (VCSEL) with low threshold current density, high efficiency and output power emissions around 1130 nm was grown by MOCVD. Its static characteristics at room temperature and high operation temperature were studied in detail. The 7 μm oxide aperture device exhibits a threshold current of 0.68 mA, corresponding to a threshold current density of 1.7 kA/cm2. The slope efficiency is 0.43 W/A and the maximum output power is 3.3 mW. Continuous-wave (CW) operation in the 10–80 °C temperature range is observed. The slope efficiency is almost constant at 10–80 °C. The threshold current becomes lower at high temperatures thanks to the alignment between gain peak and cavity mode. The 3 μm oxide aperture device’s lasing in single mode with the RMS spectral width of 0.163 nm and orthogonal polarization suppression ratio (OPSR) is ~15 dB at 25 °C. The small-signal response analysis indicates that reducing the parasitics of the device and refining the fabrication process will improve the dynamics response characteristics. These results indicate that the 1130 nm GaAs-based VCSEL with highly strained InGaAs QWs is expected to be used as source for silicon photonics.

Selective Area Epitaxy of Highly Strained InGaAs Quantum Wells (980-990 nm) in Ultrawide Windows Using Metalorganic Chemical Vapor Deposition.

We employed the selective-area-epitaxy technique using metalorganic chemical vapor deposition to fabricate and study samples of semiconductor heterostructures that incorporate highly strained InGaAs quantum wells (980-990 nm emission wavelength). Selective area epitaxy of InGaAs quantum wells was performed on templates that had a patterned periodic structure consisting of a window (where epitaxial growth occurred) and a passive mask (where epitaxial growth was suppressed), each with a width of 100 µm for every element. Additionally, a selectively grown potential barrier layer was included, which was characterized by an almost parabolic curvature profile of the surface. We conducted a study on the influence of the curvature profile of the growth surface on the optical properties of InGaAs quantum wells and the spatial distribution of composition in an ultrawide window. Our results showed that, under fixed selective-area-epitaxy conditions, the composition of the InxGa1-xAs and the wavelength of the quantum-well emission changed across the width of the window. Our study demonstrates that increasing the curvature profile of the growth surface of highly strained quantum wells leads to a transition in the photoluminescence wavelength distribution profile across the window, from quasi-parabolic to inverted parabolic.

3D Bragg Coherent Diffraction Imaging of Extended Nanowires: Defect Formation in Highly Strained InGaAs Quantum Wells.

InGaAs quantum wells embedded in GaAs nanowires can serve as compact near-infrared emitters for direct integration onto Si complementary metal oxide semiconductor technology. While the core-shell geometry in principle allows for a greater tuning of composition and emission, especially farther into the infrared, the practical limits of elastic strain accommodation in quantum wells on multifaceted nanowires have not been established. One barrier to progress is the difficulty of directly comparing the emission characteristics and the precise microstructure of a single nanowire. Here we report an approach to correlating quantum well morphology, strain, defects, and emission to understand the limits of elastic strain accommodation in nanowire quantum wells specific to their geometry. We realize full 3D Bragg coherent diffraction imaging (BCDI) of intact quantum wells on vertically oriented epitaxial nanowires, which enables direct correlation with single-nanowire photoluminescence. By growing In0.2Ga0.8As quantum wells of distinct thicknesses on different facets of the same nanowire, we identified the critical thickness at which defects are nucleated. A correlation with a traditional transmission electron microscopy analysis confirms that BCDI can image the extended structure of defects. Finite element simulations of electron and hole states explain the emission characteristics arising from strained and partially relaxed regions. This approach, imaging the 3D strain and microstructure of intact nanowire core-shell structures with application-relevant dimensions, can aid the development of predictive models that enable the design of new compact infrared emitters.

Peculiarities of growing InGaAs/GaAs/AlGaAs laser structures by MOCVD on Ge/Si substrates

The growth of InGaAs/GaAs/AlGaAs laser structures by MOCVD at low pressures on Si(001) substrates with a Ge epitaxial metamorphic buffer layer of various thicknesses is studied. The results of the influence of the growth temperature and incorporation of additional AlAs layers at the boundary with the Ge/Si(001) substrate on the crystalline and optical quality of the formed III–V structures are presented. It is shown that the incorporation of the AlAs/GaAs/AlAs lattice at the initial growth stages of III–V heterostructures on Ge buffer layers grown on exact Si(001) substrates makes it possible to considerably decrease the density of threading defects and, consequently, form effectively emitting laser structures. The possibility of growing strained InGaAs quantum wells on Si(001) substrates allowing stimulated radiation in a wavelength region longer than 1100 nm is shown.

High-power diode lasers at 1178 nm with high beam quality and narrow spectra.

High-power distributed Bragg reflector tapered diode lasers (DBR-TPLs) at 1180nm were developed based on highly strained InGaAs quantum wells. The lasers emit a nearly diffraction-limited beam with more than two watts with a narrow spectral width. These features are believed to make this type of diode laser a key component for the manufacturing of miniaturized laser modules in the yellow and orange spectral range by second-harmonic generation to cover a spectral region currently not accessible with direct emitting diode lasers. Future applications might be the laser-cooling of sodium, high-resolution glucose-content measurements, as well as spectroscopy on rare earth elements.

High Brightness, Narrow Bandwidth DBR Diode Lasers at 1120 nm

In this letter, we report on monolithic distributed Bragg reflector ridge waveguide diode lasers. The lasers feature highly strained InGaAs quantum wells and fifth order surface gratings for a stabilized emission wavelength ~1120 nm. Output powers up to 1 W and a maximum conversion efficiency of ~34% were achieved in a spatial and spectral single-mode. In a preliminary reliability test, at 0.4 W a lifetime of could be demonstrated. Therefore, the laser sources should allow for an efficient non-linear frequency doubling to 560 nm.

Butterfly packaged high‐speed and low leakage InGaAs quantum well photodiode for 2000nm wavelength systems

An edge-coupled high-speed photodiode based on strained InGaAs quantum wells for detection at 2000nm is demonstrated. The fabricated device shows a leakage current as low as 120nA at −3V bias voltage and a responsivity of around 0.3A/W at 2000nm. The high-speed butterfly packaging of the device is presented which shows a 3dB bandwidth of more than 10GHz. The device shows similar high-speed performance at 1550nm.

InP-based InGaAs/InAlGaAs digital alloy quantum well laser structure at 2 μm

The structural and optical characteristics of InP-based compressively strained InGaAs quantum wells have been significantly improved by using gas source molecular beam epitaxy grown InAs/In0.53Ga0.47As digital alloy triangular well layers and tensile In0.53Ga0.47As/InAlGaAs digital alloy barrier layers. The x-ray diffraction and transmission electron microscope characterisations indicate that the digital alloy structures present favourable lattice quality. Photo-luminescence (PL) and electroluminescence (EL) measurements show that the use of digital alloy barriers offers better optical characteristics than that of conventional random alloy barriers. A significantly improved PL signal of around 2.1 μm at 300 K and an EL signal of around 1.95 μm at 100 K have been obtained.

Intersubband absorption in modulation doped heterostructures

Intersubband absorption is reported in a new modulation doped structure using strained InGaAs quantum wells (QWs) that support transistor operation. Well defined absorption peaks (1000 to 1700 cm−1) from 8 to 11.5 μm have been obtained using either n-type or p-type modulation doped wells. Both n and p well responses show strong polarization dependence with maximum values at incident angles of 65°–70° and peak positions which are adjusted by the QW parameters. The p well shows a double peaked response with a peak separation of about 1.5 μm which results from heavy and light hole contributions. The absorption data are compared with a theoretical model developed on the basis of variable k conservation and excellent agreement is obtained. It is shown that a mass difference between the upper and lower states is essential to predict a finite spectral width of the response.

Active Region Design for High-Speed 850-nm VCSELs

Higher speed short-wavelength (850 nm) VCSELs are required for future high-capacity, short-reach data communication links. The modulation bandwidth of such devices is intrinsically limited by the differential gain of the quantum wells (QWs) used in the active region. We present gain calculations using an 8-band k·p Hamiltonian which show that the incorporation of 10% In in an InGaAs/AlGaAs QW structure can approximately double the differential gain compared to a GaAs/AlGaAs QW structure, with little additional improvement achieved by further increasing the In composition in the QW. This improvement is confirmed by extracting the differential gain value from measurements of the modulation response of VCSELs with optimized InGaAs/AlGaAs QW and conventional GaAs/AlGaAs QW active regions. Excellent agreement is obtained between the theoretically and experimentally determined values of the differential gain, confirming the benefits of strained InGaAs QW structures for high-speed 850-nm VCSEL applications.

Strain-enhanced electron mobility anisotropy in In xGa 1− xAs/InP two-dimensional electron gases

We systematically investigated electron mobility anisotropy in compressively strained, lattice-matched, and tensilely strained InGaAs quantum wells (QWs) grown on InP (0 0 1) by using Hall-bar devices with various current-flowing directions. Anisotropy of electron mobility, the highest along the [1 1 ¯ 0] direction and the lowest along [1 1 0], is systematically observed in all QWs, and well-fitted with a sinusoidal function of the current-flowing direction angle. The mobility anisotropy is minimum in the lattice-matched case and enhanced by both compressive and tensile strains in the QWs. We consider that random piezoelectric scattering, which is enhanced by the average normal strain in the QW, has anisotropy and plays an important role for the observed results.

Gain characteristics of the InGaAs strained quantum wells with GaAs, AlGaAs, and GaAsP barriers in vertical-external-cavity surface-emitting lasers

InGaAs strained quantum wells (QWs) with GaAs, AlGaAs, and GaAsP barriers are widely used in optically pumped vertical-external-cavity surface-emitting lasers operating at 1 μm wavelength band. Compared with the reported data, the model-solid theory, which is more suitable for the studied materials, is selected to calculate the band offset. The band structures and the gain characteristics of the three different QWs are computed and compared, and the theoretical results are in good agreement with the recent experimental reports. The numerical simulation shows that the QW with the GaAs barrier has the highest absorption but the lowest peak gain, while for the AlGaAs barrier, it has the lowest absorption but the highest peak gain, and for the GaAsP barrier, it has a moderate absorption and peak gain. GaAsP is the most appropriate candidate for the barrier of InGaAs strained QW when the low-threshold, large-gain, and high-temperature characteristics are demanded simultaneously.

32 Gbit/s multimode fibre transmission using high-speed, low current density 850 nm VCSEL

Error free transmission over multimode fibre at data rates up to 32 Gbit/s at 25°C and 25 Gbit/s at 85°C using an oxide confined 850 nm VCSEL biased at a current density of 11–14 kA/cm2 is demonstrated. The VCSEL is optimised for high-speed by reducing capacitance and self-heating and by using strained InGaAs quantum wells for high differential gain.

Optical Anisotropy of (11n)-Oriented InGaAs Strained Quantum Wells with Finite Barrier Potential Calculated with Mixing Effects of the Spin–Orbit Split-Off Band

In-plane optical anisotropy was calculated for InGaAs strained quantum wells (QWs) with finite barrier potential on high-index (11n)-oriented GaAs substrates. The mixing effects of the spin–orbit split-off (SO) band were included by using 6 ×6 Hamiltonian based on the Luttinger–Kohn model. Calculated results reveal that optical anisotropy of the narrow QW is strongly dependent on barrier potential height, because the SO band mixing is enhanced by the strong confinement. Substrate off-angle dependence of photoluminescence intensity anisotropy observed in the (11n)A (n = 3, 4, and 5) In0.08Ga0.92As/GaAs strained QWs is well explained by the calculated curve with taking into account of the SO band mixing.

High-power laser diodes emitting light above 1100 nm with a small vertical divergence angle of 13°

Laser diodes with highly strained InGaAs quantum wells, emitting at 1130 nm, embedded in a GaAs waveguide were investigated. This Letter reviews the design of the vertical structure for enclosing high output power in angles smaller than 18 degrees . Example designs were processed to 200 microm stripe-width lasers with an 8-mm-long optical cavity. When these are mounted on C mounts, they give an output power of 38 W under quasi-cw operation from a single emitter.

Vacancy-mediated intermixing in InAs/InP(001) quantum dots subjected to ion implantation

We have investigated the influence of defects emanating from phosphorus implantation damage in the InP capping layer on postgrowth thermally induced intermixing in self-assembled InAs/InP(001) quantum dots (QDs). Photoluminescence (PL) spectra from as-grown samples could be described as the superposition of separate PL peaks where each peak corresponded to emission from an ensemble of QDs with a particular height ranging from 4 to 13 ML. Blueshift of up to 270 meV and significant bandwidth broadening were observed in the PL spectra after ion implantation with a fluence of 5×1011−1014 cm−2 and subsequent annealing at temperatures ranging from 450 to 600 °C. From the analysis of the evolution of the QD peaks upon intermixing, which revealed the coexistence of intact QD PL and a broad PL feature related to heavily intermixed QDs, it was suggested that the bandwidth broadening resulted from spatial inhomogeneity in the compositional intermixing. In order to better understand the mechanism responsible for the ion-implantation-induced intermixing, samples capped with a stack of compressively strained In0.75Ga0.25As/InP quantum wells (QWs) were prepared to trap vacancies released by the implantation damage while not inhibiting the effect of the interstitials. Both blueshift and bandwidth broadening were suppressed in samples containing the strained InGaAs QWs, whereas the evolution of the PL spectra from the QDs behaves as expected for interstitial-mediated intermixing. The vacancies were thus believed to be trapped in the QWs and indicated that intermixing in ion-implanted InP capped samples is mediated by vacancies. The shape of the QDs changed from a truncated pyramid in the as-grown state to a double convex lens structure after intermixing as confirmed by cross-sectional scanning transmission electron microscopy imaging. Furthermore, the change in shape and compositional intermixing of the QDs were attributed to vacancy trapping in the vicinity of the QDs as based on atomistic strain calculations.

Spontaneous Radiative Efficiency and Gain Characteristics of Strained-Layer InGaAs–GaAs Quantum-Well Lasers

The optical gain spectra, unamplified spontaneous emission spectra, and spontaneous radiative efficiency are extracted from the measurement of amplified spontaneous emission (ASE) on a single pass, segmented contact 0.98-mum-emitting aluminum-free InGaAs-InGaAsP-GaAs quantum-well (QW) laser diode. These measurements provide a baseline for which to compare higher strain InGaAs QW lasers emitting near 1.2 mum. The peak gain-current relationship is extracted from gain spectra and the peak gain parameter go is found to agree within 25% of the value extracted using conventional cavity length analysis for 0.98-mum-emitting devices. The spontaneous radiative current is extracted using the fundamental connection between gain and unamplified spontaneous emission, which in turn gives an estimate of the amount of nonradiative recombination in this material system. The spontaneous radiative efficiency, the ratio of spontaneous radiative current to total current, at room temperature of 0.98-mum-emitting InGaAs QW laser material is found to be in the range of 40%-54%, which is 2.5-3.5 times larger than that of highly strained InGaAs QW laser emitting near lambda = 1.2 mum. Whereas the gain parameter, g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = dg/d(ln j), was measured to be 1130 and 1585 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for the 0.98-mum- and 1.2-mum-emitting materials, respectively. From the calculated below threshold current injection efficiency of 75%-85%, we deduce that the internal radiative efficiency of the QW material is ~ 20% higher than the ratio of internal radiative current to external injected current extracted directly from ASE measurements.

Transistor laser with emission wavelength at 1544nm

Data are presented demonstrating continuous wave laser operation at −185°C of an InP–InAlGaAs–InAlAs double heterojunction bipolar transistor with strained InGaAs quantum wells incorporated in the p-type base region. The laser exhibits a peak wavelength λ∼1544nm when biased in the forward active mode in the common-emitter configuration. A threshold current IB=10mA is observed.

Optical anisotropy of strained quantum wells on high index substrates

AbstractWe investigate anisotropic properties of strained InGaAs quantum wells on the high index (11n)A (n = 3,4,5) GaAs substrates. The polarization dependence of photoluminescence of strained InGaAs quantum wells were measured, and optical anisotropy was observed. We calculated anisotropy of the optical transition matrix elements based on the 4x4 and 6x6 Luttinger model for the quantum wells with uniaxial strain. The calculated anisotropy taking into account of split‐off band qualitaively agree with our experiment. This indicates that the band mixing of split‐off band into the heavy‐hole band largely contributes to the the optical anisotropy in the strained InGaAs quantum wells on the high index substrates. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Noise, distortion and dynamic range of single mode 1.3 µm InGaAs vertical cavity surface emitting lasers for radio-over-fibre links

The analogue modulation characteristics, including second order harmonic and third order intermodulation distortion, relative intensity noise (RIN) and spurious free dynamic range (SFDR), of single mode, GaAs-based 1.28 μm vertical cavity surface emitting laser (VCSELs) with highly strained InGaAs quantum wells have been investigated. The VCSELs utilise an oxide aperture for current and optical confinement and an inverted surface relief (SR) for suppression of higher order transverse modes. The inverted SR structure also has the advantage of suppressing oxide modes that, otherwise, appear in VCSELs with a large detuning of the cavity resonance with respect to the gain peak, which is needed to extend the emission wavelength. RIN levels comparable with those of single mode VCSELs emitting at 850 nm are demonstrated, with values from 140 to 150 dB/Hz in the 2 5 GHz range. SFDR values of 100 and 95 dBHz2/3 are obtained at 2 and 5 GHz, respectively. These values are in the range of those required in radio-over-fibre links.