InP-based Material System Research Articles

Introduction to JSTQE Special Issue on Foundry Enabled Photonic Integrated Circuits

The papers in this special section focus on foundry enabled photonic integrated circuits. This research highlights the recent progress and trends in foundries and associated technologies to develop volume manufacturing of photonic integrated circuits (PICs). As integrated photonics technology matures and potential market demands increase, foundries become critical for volume production. In recent years the developments in the offerings of foundry services and multi-project wafer (MPW) runs, in both silicon and InP based material systems, enable a path toward volume production of photonic integrated circuits. Fabrication in the foundry environment will also lead to more highly integrated circuits, performance improvements and lower cost.

Single-photon emission of InAs/InP quantum dashes at 1.55 μm and temperatures up to 80 K

We report on single photon emission from a self-assembled InAs/InGaAlAs/InP quantum dash emitting at 1.55 μm at the elevated temperatures. The photon auto-correlation histograms of the emission from a charged exciton indicate clear antibunching dips with as-measured g(2)(0) values significantly below 0.5 recorded at temperatures up to 80 K. It proves that the charged exciton complex in a single quantum dash of the mature InP-based material system can act as a true single photon source up to at least liquid nitrogen temperature. This demonstrates the huge potential of InAs on InP nanostructures as the non-classical light emitters for long-distance fiber-based secure communication technologies.

Single photon emission at 1.55 μm from charged and neutral exciton confined in a single quantum dash

We investigate charged and neutral exciton complexes confined in a single self-assembled InAs/InGaAlAs/InP quantum dash emitting at 1.55 μm. The emission characteristics have been probed by measuring high-spatial-resolution polarization-resolved photoluminescence and cross-correlations of photon emission statistics at T = 5 K. The photon auto-correlation histogram of the emission from both the neutral and charged exciton indicates a clear antibunching dip with as-measured g(2)(0) values of 0.18 and 0.31, respectively. It proves that these exciton complexes confined in single quantum dashes of InP-based material system can act as true single photon emitters being compatible with standard long-distance fiber communication technology.

InGaAs/GaAsSb/InP terahertz quantum cascade lasers

The development of In0.53Ga0.47As/GaAs0.51Sb0.49 terahertz quantum cascade lasers is reviewed, starting with the first demonstration, through growth direction dependent performance issues, to high performance devices. This InP-based material system is an attractive alternative to the almost exclusively used GaAs/AlxGa1-xAs. Devices achieve maximum operating temperatures of 142 K and exhibit broadband lasing over a range of 660 GHz. A special focus has to be put on the growth direction related interface asymmetry for this material system. Symmetric active region designs are an elegant technique to investigate such asymmetries. A significant impact on the device performance is observed and attributed to interface roughness scattering.

Room temperature continuous wave operation of λ ∼ 3–3.2 μm quantum cascade lasers

We demonstrate quantum cascade lasers emitting at wavelengths of 3–3.2 μm in the InP-based material system. The laser core consists of GaInAs/AlInAs using strain balancing technique. In room temperature pulsed mode operation, threshold current densities of 1.66 kA/cm2 and 1.97 kA/cm2, and characteristic temperatures (T0) of 108 K and 102 K, are obtained for the devices emitting at 3.2 μm and 3 μm, respectively. Room temperature continuous wave operation is achieved at both wavelengths.

Gas chopping etching process for InP based nanostructures with high aspect ratios

The authors report a novel etching process for the fabrication of nanostructured III-V semiconductors with ultra-high aspect ratios. For the very first time, a time multiplexed etching process (gas chopping process) for an InP based material system has been successfully developed and demonstrated. The repetition of a Cl2/H2/Ar based etching step together with passivation and removal steps ensures very deep etching with straight walls without general restrictions by the etch depth. The newly developed process is very promising for the fabrication of deeply etched feedback gratings for lasers or nanophotonic devices requesting high aspect ratios, like lasers with surface defined distributed Bragg reflector/distributed feedback gratings or deeply etched photonic crystals. This new process allows the formation of vertical profiles with smooth surfaces and reduced footing effect. Using this gas chopping process for InP, a high mask selectivity of 15, good etching rate of 95 nm/min, and record values of the aspect ratio up to 41 could be demonstrated.

Room Temperature Optically Pumped 2.56-$\mu{\rm m}$ Lasers With “W” Type InGaAs/GaAsSb Quantum Wells on InP Substrates

The optically pumped laser using InGaAs/GaAsSb W-type quantum wells is demonstrated with a threshold power density ~ 40 kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The L-L curve and the dramatic line width shrinkage above threshold confirm, for the first time, mid-infrared lasing in this structure on InP substrates. The lasing wavelength at 2.56 μm is the longest lasing wavelength at room temperature for the interband transition of InP-based material system.

High speed QWIP FPAs on InP substrates

Quantum well infrared photodetector (QWIP) technology has allowed the realization of low cost staring focal plane arrays (FPAs). However, AlGaAs/(In)GaAs QWIP FPAs suffer from low quantum and conversion efficiencies under high frame rate and/or low background conditions. We extensively discuss the effect of sensor gain on the FPA performance under various operating conditions, and highlight the superiority of the InP/InGaAs material system with respect to AlGaAs/GaAs for high speed/low background thermal imaging applications. InP/InGaAs QWIPs, providing a bias adjustable gain in a wide range, offer the flexibility of adapting the FPA to strict operating conditions. We also present an experimental comparison of large format AlGaAs/GaAs and (strained) InP/InGaAs QWIP FPAs under different operating conditions. A 640 × 512 QWIP FPA constructed with the 40-well strained InP/In 0.48Ga 0.52As material system displays a cut-off wavelength of 9.7 μm ( λ p = 8.9 μm) with a BLIP temperature higher than 65 K ( f/2), and a peak quantum efficiency as high as 12% with a broad spectral response (Δ λ/ λ p = 17%). The conversion efficiency of the FPA pixels is as high as 20% under large bias (4 V) where the detectivity is reasonably high (∼3 × 10 10 cm Hz 1/2/W, f/2, 65 K). While providing a considerably higher quantum efficiency than the pixels of a similar AlGaAs/GaAs FPA, the InP/InGaAs QWIP provides similar NETD values with much shorter integration times and, being less sensitive to the read noise, successfully operates with sub-millisecond integration times. The results clearly demonstrate that InP based material systems display high potential for single- and dual-band QWIP FPAs by overcoming the limitations of the standard GaAs based QWIPs under high frame rate and/or low background conditions.

QWIP focal plane arrays on InP substrates for single and dual band thermal imagers

Alternative material systems on InP substrate provide certain advantages for mid-wavelength infrared (MWIR), long-wavelength infrared (LWIR) and dual band MWIR/LWIR quantum well infrared photodetector (QWIP) focal plane arrays (FPAs). While InP/InGaAs and InP/InGaAsP LWIR QWIPs provide much higher responsivity when compared to the AlGaAs/GaAs QWIPs, AlInAs/InGaAs system facilitates completely lattice matched single band MWIR and dual band MWIR/LWIR FPAs. We present an extensive review of the studies on InP based single and dual band QWIPs. While reviewing the characteristics of InP/InGaAs and InP/InGaAsP LWIR QWIPs at large format FPA level, we experimentally demonstrate that the cut-off wavelength of AlInAs/InGaAs QWIPs can be tuned in a sufficiently large range in the MWIR atmospheric window by only changing the quantum well (QW) width at the lattice matched composition. The cut-off wavelength can be shifted up to ∼5.0 μm with a QW width of 22 Å in which case very broad spectral response (Δ λ/ λ p = ∼30%) and a reasonably high peak detectivity are achievable leading to a noise equivalent temperature difference as low as 14 mK ( f/2) with 25 μm pitch in a 640 × 512 FPA. We also present the characteristics of InP based two-stack QWIPs with wavelengths properly tuned in the MWIR and LWIR bands for dual color detection. The results clearly demonstrate that InP based material systems display high potential for dual band MWIR/LWIR QWIP FPAs needed by third generation thermal imagers.

Al(In)As–(Ga)InAs strain-compensated active regions for injectorless quantum cascade lasers

We present a new design for quantum cascade lasers (QCLs) without the typically used injector between two consecutive active stages. The lasers are realized with the InP-based material system AlInAs/GaInAs. With additional AlAs and InAs layers a significant optimization of the structure can be realized. In this improved structure the possibility of electrons escaping into the quasi-continuum is drastically reduced by the AlAs-blocking layer. On the other hand, InAs, a material with a very low effective mass, significantly prolongs the carrier lifetime, enhancing the population inversion and increasing the dipole matrix element of the transition. Both inserted layers result in an overall improvement of the device properties, basically the threshold current density ( j th), maximum operating temperature ( T max), output power, slope efficiency and characteristic temperature T 0. With high reflection coated facets a record threshold current density as low as 450 A/cm 2 at 300 K was achieved in the pulsed mode.

Influence of proximity effects in electron-beam lithography on the optical properties of planar photonic-crystal waveguides

To measure the influence of proximity effects in electron-beam lithography on the optical properties of planar photonic crystal (PPC) waveguides we propose a PPC structure called the “PECmeter.” The PECmeter consists of nearly identical PPC waveguides which only differ in the number of rows of holes along the waveguide. The difference in the number of rows does not influence the modal properties directly but changes the diameter of the holes neighboring the waveguide through the proximity effect. The operation principle of the PECmeter is demonstrated using energy-intensity simulations of a W3 waveguide (three missing rows of holes) mini stop band. The principle is confirmed experimentally with structures fabricated in the InP-based material system and measured by the end-fire transmission technique. The results clearly show that the application of proximity-effect correction (PEC) is crucial for the fabrication of PPC waveguides. We demonstrate that when using the midpoint-equalization PEC method a near-to-perfect correction with sub-nm hole-radius uniformity can be achieved. We show the PECmeter to be sensitive enough to detect hole-radius changes as small as ΔR=0.4 nm.

Growth of InAs-containing quantum wells for InP-based VCSELs emitting at 2.3 μm

Vertical-cavity surface-emitting lasers are attractive light sources particularly for gas-sensing applications. Using the AlGaInAs/InP material system, an emission wavelength ranges from 1.3 to 2.05 μm has already been achieved [G. Boehm, M. Ortsiefer R. Shau, J. Rosskopf, C. Lauer, M. Maute, F. Köhler, F. Mederer, R. Meyer, M.-C. Amann, in: Proceedings of MBE XII, J. Crystal Growth (2002) pp. 748–753]. Within this range, absorption lines of gases like H 2S (1590 nm), CH 4 (1654 nm), H 2O (1877 nm) and CO 2 (2004 nm) are detectable [G. Totschnig, M. Lackner, R. Shau, M. Ortsiefer, J. Rosskopf, M.-C. Amann, F. Winter, Meas. Sci. Technol. 14 (2003) 472]. The aim of this work is a further expansion of the emission wavelength up to 2.3 μm, exploiting the well-known InP-based material system and the fabrication technique of buried tunnel junction (BTJ) VCSELs [R. Shau, M. Ortsiefer, J. Rosskopf, G. Boehm, C. Lauer, M. Maute, M.-C. Amann, in: Proceedings of SPIE, 5364 (2004) 1–15] to reach the technologically important absorption lines of carbon monoxide (CO) around 2.33 μm.

Three-dimensional calculation of propagation losses in photonic crystal waveguides

Overall losses of optical waveguides, patterned in a two-dimensional photonic crystal in InP-based material system, are determined through the calculation of propagation properties by means of a full three-dimensional finite difference time domain code. Intrinsic origins of the losses related to injection, radiation and diffraction phenomena are more specially addressed, qualitatively and quantitatively. Various configurations including thick substrate and membrane-like approaches are tested. The results, compared to the best published experiments, validate our approach as a predicting tool and give trends for the design of competitive devices.

Wide and continuous wavelength tuning of microcavity devices for optoelectronic applications

Ultra-widely tunable microcavity devices implemented by surface micromachining are studied. We model, fabricate, and characterize 1.55-μm vertical-resonator-based optical filters and vertical cavity surface emitting lasers (VCSELs) capable of wide, monotonic, and kink-free tuning by a single control parameter. Our devices are comprised of single or multiple horizontal air gaps in the dielectric and InP-based material system. Distributed Bragg mirrors with multiple air gaps are implemented. Due to the high refractive index contrast between air (n = 1) and InP (n = 3.17), only three periods are sufficient to guarantee a reflectivity exceeding 99.8% and offer an enormous stop-band width exceeding 500 nm. Unlike InGaAsP/InP or dielectric mirrors, they ensure short penetration depth of the optical intensity field in the mirrors and low absorption values. Stress control of the suspended membrane layers is of utmost importance for the fabrication of these devices. By controlling the stress, we are able to fabricate InP membranes that are extremely thin (357 nm thick) and at the same time flat (radius of curvature above 5 mm). Micromechanical single parametric actuation is achieved by both thermal and electrostatic actuation. Filter devices with a record tuning more than 142 nm with 3.2 V are presented.

Nanofabrication of two-dimensional photonic crystal mirrors for 1.5 μm short cavity lasers

We have developed a fabrication scheme for two-dimensional (2D) triangular photonic crystals (PCs) on InP-based material systems involving high resolution electron beam lithography, pattern transfer to a SiO2 etch mask, and a Cl2/Ar electron cyclotron resonance reactive ion etch step yielding PCs with periods of a=300–450 nm and air fill factors of f=18%–63%. These PCs are employed as high reflectivity mirrors for 1.5 μm short cavity lasers, which are key components in future highly integrated PC based photonic circuits. We have fabricated lasers with 2 PC mirrors and cavity lengths down to 100 μm. Threshold currents as low as 7.6 mA were achieved for the shortest lasers with 2 PC mirrors. The short laser cavity results in a large Fabry–Pérot mode spacing and mode competition leads to single mode lasing over a reasonably large current range up to 4.5× threshold. Lasers with one PC back mirror and a cleaved output facet show a higher threshold current of 13 mA and a maximum output power of more than 4 mW. The variation of laser performance with different cavity lengths is presented and discussed.

Surface-Grating Distributed Bragg Reflector Lasers with Deeply Etched Grooves Formed by Reactive Beam Etching

Two surface-grating distributed Bragg reflector (DBR) lasers, one using a 1.55-µm-wavelength InP-based material system and one using a 0.98-µm-wavelength GaAs/AlGaAs material system, were made using the same reactive beam etching technique following single-step growth of the layer structure for the lasers. A 800-nm-deep grooved grating with a 240 nm period for 1.55-µm-wavelength first-order diffraction and one with a 150 nm period for 0.98-µm-wavelength first-order diffraction were formed using electron-beam lithography and Br2-N2 mixed-gas reactive beam etching. Single-longitudinal-mode operation with a side-mode suppression ratio of over 27 dB was obtained for both lasers; the output power was more than 15 mW for the 0.98-µm one. Between 15 and 55°C, the measured temperature dependence of the lasing wavelength for both lasers was about 0.1 nm/°C, which is one-fifth of the dependence of a Fabry-Perot-type laser. The performance of these lasers indicates the wide applicability of the present etching technique to the fabrication of various wavelength-selective devices.

Exploitation of surface selective growth in metalorganic growth technologies for device applications

Surface selective growth (SSG) is gaining importance for the realization of complex device concepts. This review article addresses basic SSG mechanisms both in metalorganic vapor phase epitaxy (MOVPE) and in metalorganic molecular beam epitaxy (MOMBE/CBE). It is described how these mechanisms can be exploited for the fabrication of complex devices focusing on the InP based material system. Additionally it is shown that with novel precursors new effects can be observed in SSG. The applications of SSG are highlighted by a discussion on selected devices. A critical comparison reveals that SSG in MOVPE can find some applications, but MOMBE offers higher flexibility, moreover a higher yield than in MOVPE can be expected. Therefore, MOMBE appears to be the preferred technology for SSG. However, since MOMBE is not established in production, a substantial R&D back-up is required for an industrial application of SSG in MOMBE.

Polarization insensitive waveguide grating routers in InP

We demonstrate a 16-channel waveguide grating router (WGR) in the InP-based material system. The channel spacing of the WGRs is 100 GHz and the maximum interchannel crosstalk is -28 dB. The polarization related (TE-TM modal) wavelength shift is approximately 0.08 nm and the polarization dependent variation in peak signal is less than 0.2 dB. 2.60-/spl mu/m-wide /spl times/1.90-/spl mu/m-thick raised-stripe InGaAsP Quaternary (/spl lambda//sub g/=1.17 /spl mu/m) waveguides on an n-type InP substrate make up the basis paths of the WGR. In this letter, we describe the design, fabrication and the results of measurement of the 16-channel polarization-insensitive WGR.

Fabrication and characterization of an InAlAs/InGaAs/InP ring oscillator using integrated enhancement- and depletion-mode high-electron mobility transistors

The fabrication and characterization of an 11-stage ring oscillator utilizing integrated enhancement- and depletion-mode (E/D-mode) high-electron mobility transistors (HEMT's) in the lattice-matched InAlAs/InGaAs/InGaAs material system is demonstrated. The 0.5-μm gate length depletion-mode HEMT's (D-HEMT's) used in the circuit exhibit a threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) of -365 mV with a standard deviation of 19 mV, while the enhancement-mode HEMT's (E-HEMT's) with identical gate length display a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> of 195 mV with a standard deviation of only 9 mV. The unity current gain cutoff frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) for both devices is 70 GHz. The extremely high uniformity of the threshold voltages of these devices allowed for the implementation of a ring oscillator utilizing direct coupled FET logic (DCFL). At a supply voltage of 0.4 V, a room temperature propagation delay time (/spl tau//sub pd/) of 22.4 ps/stage, and a corresponding power dissipation of 120 μW/stage is measured, yielding a power delay product (PDP) of 2.65 fJ/stage. To the best of the authors knowledge, this is the first demonstration of a circuit employing E/D-HEMT technology in the lattice-matched InP-based material system.

InP-based high-performance monostable-bistable transition logic elements (MOBILEs) using integrated multiple-input resonant-tunneling devices

MOBILEs (monostable-bistable transition logic elements), which have the advantages of multiple-input and multiple-function, are demonstrated in InP-based material system using monolithic integration of resonant-tunneling diodes and high electron mobility transistors. The high peak current density, high peak-to-valley ratio, and high transconductance, which are required for high-performance MOBILEs, are demonstrated in this InP-based material system. A fabricated MOBILE with three-input gates having 1:2:4 width ratio can perform weighted-sum threshold logic operation, and has a wide range of applications in new computing architectures, such as neural networks.