Heterostructure Lasers Research Articles

Comprehensive Analysis of Electric Properties of Oxide-Confined Vertical-Cavity Surface-Emitting Lasers

We present a comprehensive description of electric properties of vertical–cavity surface–emitting lasers (VCSELs). A complete drift–diffusion model is developed and applied for carrier transport in a multilayer semiconductor laser heterostructure with a p–n junction. We address the impact of interface grading in distributed Bragg reflectors (DBRs), modulation doping of the DBRs and surrounding layers of the quantum well as well as drastically material–dependent carrier mobilities and recombination constants. Solution of the drift–diffusion model yields spatial profiles of the nonequilibrium carrier concentrations and current. The focus is made on oxide–confined GaAs/AlGaAs VCSELs. We evaluate both depletion and diffusion capacitance of the device and show that both contributions to the capacitance as well as the differential series resistance critically depend on the injection current and VCSEL chip design such that, in general, VCSEL cannot be properly modeled by an equivalent circuit approximation. Solution for the current profile demonstrates significant enhancement of the current density at the edges of the oxide–confined aperture (current crowding). We show that, as long as the mesa diameter is small, the effective RC–product can be kept low at small aperture diameters.

Spectral Shift of Quantum-Cascade Laser Emission under the Action of Control Voltage

Spectral redistribution of the intensity of short- and long-wavelength emission components within gain bandwidth of a 7- to 8-μm quantum-cascade laser under the action of control voltage is demonstrated. As the voltage was increased from 10.5 to 18.2 V, the wavelength of maximum laser emission intensity shifted by approximately 200 nm. The maximum bandwidth of laser gain was about 300 nm (at a temperature of 80 K). The quantum-cascade laser heterostructure was grown by molecular beam epitaxy. The laser active region design was based on double-phonon depopulation of the lower level as implemented on In0.53Ga0.47As/In0.52Al0.48As heteropair of solid alloys lattice-matched with an InP substrate.

Negative terahertz conductivity and amplification of surface plasmons in graphene–black phosphorus injection laser heterostructures

We propose and evaluate the heterostructure based on the graphene-layer (GL) with the lateral electron injection from the side contacts and the hole vertical injection via the black phosphorus layer (PL) (p$^+$PL-PL-GL heterostructure). Due to a relatively small energy of the holes injected from the PL into the GL (about 100 meV, smaller than the energy of optical phonons in the GL which is about 200 meV), the hole injection can effectively cool down the two-dimensional electron-hole plasma in the GL. This simplifies the realization of the interband population inversion and the achievement of the negative dynamic conductivity in the terahertz (THz) frequency range enabling the amplification of the surface plasmon modes. The later can lead to the plasmon lasing. The conversion of the plasmons into the output radiation can be used for a new types of the THz sources.

Stimulated Emission at a Wavelength of 2.86 μm from In(Sb, As)/In(Ga, Al)As/GaAs Metamorphic Quantum Wells under Optical Pumping

Metamorphic laser heterostructures In(Sb, As)/In0.81Ga0.19As/In0.75Al0.25As with InSb/InAs/InGaAs composite quantum wells based on submonolayer InSb insertions in a 10-nm InAs layer have been grown by molecular beam epitaxy on GaAs (001) substrates. Stimulated emission at a wavelength of λ ~ 2.86 μm at temperatures of 10–60 K at optical pumping has been demonstrated in such structures without an optical cavity. The threshold pump power density is about 5 kW/cm2 at a temperature of 10 K.

High-Power Quantum-Cascade Lasers Emitting in the 8-μm Wavelength Range

Molecular beam epitaxy techniques were used to grow a quantum-cascade laser (QCL) heterostructure based on an In0.53Ga0.47As/Al0.48In0.52As heteropair lattice-matched with an InP substrate. InP layers were used to form an optical waveguide. The obtained QCL heterostructure ensured room-temperature lasing in the 8-μm wavelength range at a maximum output optical power of 0.45 W from one facet in a standard ridge geometry of the Fabry–Perot cavities formed by cleaved facets.

Ultranarrow-waveguide AlGaAs/GaAs/InGaAs lasers

We have designed, fabricated and studied ultranarrow-waveguide heterostructure lasers emitting in the spectral range 1000 – 1100 nm. The lasers have been characterised by current – voltage, light – current, far-field intensity distribution and internal optical loss measurements. The ultranarrow-waveguide lasers have been shown to have a threshold current density of ∼75 A cm−2, internal quantum efficiency near 100 % and internal optical loss near the lasing threshold under 1 cm−1, which corresponds to the level of standard heterostructures. We have demonstrated the possibility of obtaining up to 5 W of output power in continuous mode and up to 30 W in pulsed mode, with a beam convergence (FWHM) of 17.8°. The slope of the internal optical loss as a function of pump current for the ultranarrow-waveguide lasers can be markedly lower than that in lasers with a standard design, but internal quantum efficiency drops to 40 % with increasing pump current. The use of barrier layers in ultranarrow-waveguide lasers makes it possible to substantially reduce the drop in internal quantum efficiency.

Specific Features of Closed-Mode Formation in Rectangular Resonators Based on InGaAs/AlGaAs/GaAs Heterostructures for High-Power Semiconductor Lasers

This study is concerned with the specific features of how high-Q closed modes operating on the total-internal-reflection effect in large-size (up to hundreds or thousands of wavelengths) rectangular resonators based on InGaAs/GaAs/AlGaAs laser heterostructures are formed. The specific features of the spectral composition and spatial configurations of closed-mode configurations are experimentally examined. The presence of frequency combs in the spectra is demonstrated and their correspondence to the separate spatial configurations of closed modes is shown. The effect of changing the pumping level and the temperature on the mode composition of the emission is also considered.

Особенности формирования замкнутых модовых структур в прямоугольных резонаторах на основе гетероструктур InGaAs/AlGaAs/GaAs для мощных полупроводниковых лазеров

The work is devoted to the study of the formation features of high-quality closed modes based on total internal reflection effect in rectangular resonators of large (up to thousands wavelengths) size based on InGaAs/GaAs/AlGaAs laser heterostructures. Features of the spectral composition and spatial configurations of closed mode structures are experimentally investigated. The presence of frequency combs in the spectra was demonstrated and their correspondence to separate spatial configurations of closed modes was shown. The effect of a change in pumping and temperature on the mode composition is also considered.

The effect of AlGaAs/GaAs laser heterostructure ionic surface cleaning on the structure and properties of AlN films deposited by the method of reactive ion-plasma sputtering

The paper presents studies of the structural and optical properties of AlN thin films grown on an AlGaAs/GaAs semiconductor laser heterostructure with the use of preliminary ionic surface cleaning and without it. After cleavage of the heterostructure, a natural surface oxide forms on the facet. In the paper, the ionic etching regime providing the removal of the surface oxide layer without significant defects in the semiconductor heterostructure is determined.

Low-Loss Regrowth-Free Long Wavelength Quantum Cascade Lasers

Optical power output is the most sought-after quantity in laser engineering. This is also true for quantum cascade lasers operating especially at long wavelengths. Buried heterostructure cascade lasers with epitaxial regrowth have typically shown the lowest loss due to high current confinement as well as superior lateral thermal conductivity at the expense of complexity and cost. Among the many factors affecting optical output are the widely used passivating materials such as Si3N4 and SiO2. These materials have substantial optical absorption in the long wavelength infrared, which results in optical loss reducing the output of the laser. In this letter, we report on quantum cascade lasers with various waveguide widths and cavity lengths using both PECVD grown Si3N4 and e-beam evaporated HfO2 as passivating material on the same structure. Their slope efficiency was measured, and the cavity losses for the two lasers were calculated. We show that HfO2 passivated lasers have approximately 5.5 cm−1 lower cavity loss compared to Si3N4 passivated lasers. We observe up to 38% reduction in lasing threshold current, for lasers with HfO2 passivation. We model the losses of the cavity due to both insulator and metal contacts of the lasers using Comsol Multiphysics for various widths. We find that the loss due to absorption in the dielectric is a significant effect for Si3N4 passivated lasers and lasers in the 8–12- $\mu \text{m}$ range may benefit from low loss passivation materials such as HfO2. Our results suggest that low-loss long wavelength quantum cascade lasers can be realized without epitaxial overgrowth.

Effect of carrier localization on performance of coupled large optical cavity diode lasers

Results are presented on a comparative study of InGaAs/GaAs/AlGaAs coupled large optical cavity (CLOC) laser heterostructures emitting at either 1.03 or 0.98 μm. The smaller energy of carrier localization in the active region of the latter structure leads to stronger carrier leakage out of the quantum well that in its turn results in a higher internal optical loss, higher threshold current density and more pronounced temperature sensitivity. In continuous wave regime, the maximal output power of 8 W in the 0.98-μm lasers is found to be limited by the thermal rollover, whereas in the longer wavelength laser it is 10.8 W being limited by the catastrophic optical mirror damage.

Growth and optical characterization of 7.5 μm quantum-cascade laser heterostructures grown by MBE

We present the results on growth, characterization and optical properties of quantum-cascade laser heterostructures grown by molecular-beam epitaxy. The double-phonon resonance design for forming of active region together with thick InP-based top cladding was used to increase the quantum-cascade laser performance. The results of electroluminescence studies of 4-cleaved samples are presented. The room temperature lasing at 8.0 um with threshold current density about 1.98 kA/cm2 was achieved.

On the Application of Strain-Compensating GaAsP Layers for the Growth of InGaAs/GaAs Quantum-Well Laser Heterostructures Emitting at Wavelengths above 1100 nm on Artificial Ge/Si Substrates

Stressed InGaAs/GaAs quantum-well laser structures are grown by gas-phase epitaxy from organometallic compounds on GaAs substrates and artificial Ge/Si substrates based on Si(001) with an epitaxial metamorphic Ge layer. To suppress the relaxation of elastic stresses during the growth of InGaAs quantum wells with a high fraction of In, strain-compensating GaAsP layers are applied. The structural and radiative properties of the samples grown on substrates of various types are compared. Stimulated radiation at wavelengths up to 1.24 μm at 300 K is obtained for structures grown on GaAs substrates and at wavelengths up to 1.1 μm at 77 K, for structures grown on Ge/Si substrates.

External cavity type-I quantum well cascade diode lasers with a tuning range of 440 nm near 3 μm.

A three-stage cascade GaSb-based diode laser heterostructure with an enhanced optical gain spectral bandwidth was designed and fabricated. The gain broadening was achieved by varying the thickness of the type-I quantum wells in different stages of the cascade active region from 10 to 14nm. The structures were processed into bent ridge gain chips with virtually eliminated feedback from the anti-reflection-coated angled facet. The external cavity devices based on a novel gain chip design demonstrated a record wide tuning range from 2.79 to 3.23μm in a Littrow cavity configuration at 20°C.

Influence of waveguide strain and surface morphology on AlGaN-based deep UV laser characteristics

The waveguide strain and the surface morphologies of AlGaN-based laser heterostructures emitting in the deep UV spectral range have been investigated. In particular, the impact of the AlGaN heterostructure design on the strain relaxation as well as the effect of the growth temperature on the surface morphology were explored. We found strain-induced plastic relaxation for laser heterostructures with 130 nm thick Al0.45Ga0.55N waveguide layers, whereas pseudomorphic growth was obtained for laser heterostructures with Al0.70Ga0.30N waveguide layers. Optically pumped lasing near 280 nm was demonstrated for the coherently grown laser heterostructures. A strong correlation of the surface morphology with the waveguide growth conditions was also observed. Low growth temperatures of 900 °C lead to a high density of V-pits originating from dislocations. By increasing the growth temperatures to 1070 °C the V-pit density significantly decreases, resulting in a more than two-fold reduction of the threshold power density of optically pumped lasers.

HgCdTe based quantum well heterostructures for long-wavelength lasers operating in 5 - 15 THz range

Stimulated emission (SE) at wavelengths up to 20.3 μm (14.7 THz) is demonstrated from HgCdTe based quantum well heterostructures. The mitigation of Auger processes resulting from «graphene-like» energy spectrum in structures under study is exemplified. By studying the carrier lifetime via time resolved photoconductivity and transmission pump-probe measurements we show that further increase in SE wavelength is feasible and discuss the promising routes towards the 5 – 15 THz frequency domain, where quantum cascade lasers are scarce and HgCdTe lasers may be competitive to the prominent emitters.

Quantum-Cascade Lasers Generating at the 4.8-μm Wavelength at Room Temperature

We report on room-temperature generation at the 4.8-μm wavelength in a lattice-matched structure of a quantum cascade laser (QCL) grown on indium phosphide (InP) substrate. Laser heterostructures grown by molecular beam epitaxy (MBE) comprised 30 cascades and were designed to generate at the 4.80 μm wavelength corresponding to an atmospheric transparency window. Experiments demonstrated effective lasing at temperatures from 80 to 300 K on a wavelength coinciding with the calculated value, which confirmed the high quality of interfaces, high precision of layer thicknesses, and high accuracy of active region doping.

Enhanced direct-gap light emission from Si-capped n+-Ge epitaxial layers on Si after post-growth rapid cyclic annealing: impact of non-radiative interface recombination toward Ge/Si double heterostructure lasers: erratum.

We report an error in Fig. 13(a) of our article [Opt. Express25, 21286 (2017)], and the correction is presented. Typographical errors in the text are also corrected.

(Invited) Epitaxy of Direct Bandgap Group IV Si-Ge-Sn Alloys towards Heterostructure Light Emitters

Silicon (Si)-based group IV materials form the backbone of our digital society, being found in every smartphone and tablet. They, however, lack the possibility to efficiently emit and detect light, because of their indirect bandgap. A truly Si-compatible direct bandgap material allows merging of state-of-the-art CMOS technology on-chip with photonic components to tackle bottlenecks of modern electronics by radically reducing the required power consumption or increasing the available data bandwidth [1]. Alloying the group IV elements germanium (Ge) and tin (Sn) has turned out to be a viable solution, as a direct bandgap semiconductor – even suitable for lasing – is formed for relaxed epilayers with Sn incorporation above ∼9 at.% [2]. The value of the bandgap can further be controlled by adding Si into GeSn, which can be exploited for carrier confinement inside heterostructures [3]. In this contribution, we discuss the epitaxy of various Sn-based group IV alloys. Different heterostructure designs combine direct bandgap active layers, ternary barriers and a GeSn buffer technology. In Figure 1a, a TEM micrograph of a SiGeSn/GeSn/SiGeSn double heterostructure (DHS) is presented, overlapped with the elemental distribution taken by energy-dispersive X-Ray spectroscopy (EDX). A Sn incorporation of roughly 14.5 at.% ensures a direct bandgap inside the active region. Another approach facilitates direct bandgap GeSn multi quantum wells (MQW), sandwiched between SiGeSn barriers. Atom Probe Tomography (APT) allows to obtain precise elemental distribution within this complex heterostructure stack, as is demonstrated in Figure 1b. Techniques such as temperature-dependent photoluminescence are used to compare the optical properties of the grown heterostructures. Moreover, the impact of quantum confinement on MQW emission is revealed, backed up by bandstructure calculations. Optically pumped microdisk resonators are formed to demonstrate lasing from different types of heterostructures, as shown in Figure 1c. As depicted in the inset, the superiority of MQW structures over DHSs (and bulk GeSn layers) is demonstrated, leading to an order of magnitude reduced lasing thresholds at cryogenic temperatures. In the next step of integrated light emitters, carriers need to be injected electrically, rather than optically. We will show first experiments on epitaxially grown p-i-n heterostructure diodes. In Figure 1d, incorporation of phosphorus and boron for the formation of n- and p-doped carrier injection layers is shown by SIMS. We will further point towards possible improvements that may pave the way towards future energy efficient opto-electronic integrated circuits (OEICs). [1] L. Vivien, Computer technology: Silicon chips lighten up, Nature. 528 (2015) 483–484. [2] S. Wirths, R. Geiger et al., Lasing in direct-bandgap GeSn alloy grown on Si, Nat. Photonics. 9 (2015) 88–92. [3] N. von den Driesch et al., Advanced GeSn/SiGeSn Group IV Heterostructure Lasers, Adv. Sci. (2018). doi:10.1002/advs.201700955. Figure 1

Room Temperature Lasing of Multi-Stage Quantum-Cascade Lasers at 8 μm Wavelength

Room-temperature lasing at a wavelength of 8 μm in multistage quantum-cascade lasers pumped by current pulses is demonstrated. A quantum-cascade laser heterostructure based on the In0.53Ga0.47As/Al0.48In0.52As alloy heteropair, matched to an InP substrate, is grown by molecular-beam epitaxy and consists of 50 identical cascades placed in a waveguide with air as the top cladding. A threshold current density of ~5.1 kA/cm2 at a temperature of 300 K is obtained in ridge lasers with a cavity length of 1.4 mm and a ridge width of 24 μm.