Materials Science and Engineering B51 (1998) 114 – 117
Temperature effects in semiconductor quantum dot lasers S. Fafard a,*, K. Hinzer a,c, A.J. Springthorpe b, Y. Feng a, J. McCaffrey a, S. Charbonneau a, E.M. Griswold b a
Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ont. K1A 0R6, Canada b Ad6anced Technology Lab, Nortel Technology, Ottawa, Ont. K1Y 4H7, Canada c Physics Department, Uni6ersity of Ottawa, Ottawa, Ont. K1N 6N5 Canada
Abstract Self-assembled quantum dots (QDs) of highly strained InAlAs have been grown by molecular beam epitaxy (MBE) in separate confinement p-i-n heterostructres on (001) GaAs substrates. At low temperatures, the lasing threshold currents for red-emitting QD lasers are found to be more temperature-independent than for quantum well (QW) lasers. At higher temperatures, the temperature dependence of the threshold currents is governed mainly by the depth of the separate confinement region which was designed to obtain QD lasers capable of room temperature emission with simple broad area laser devices having external efficiencies of 13% at low temperatures. Crown copyright © 1998 Published by Elsevier Science S.A. All rights reserved. Keywords: Quantum dot; Laser diode; Nano-optics; Self-assembled; Molecular beam epitaxy; Quantum well; Indium aluminium arsenide; spontaneous emission
1. Introduction Semiconductor laser diodes rank among the most important devices based on low dimensional structures because their applications range from telecommunications, to barcode scanning, including optical storage, image recording, displays for entertainment and instrumentation. Commercial devices are presently based on quantum wells (QWs) relying on two-dimensional (2D) density-of-states and are capable of high powers and efficiencies at a variety of wavelengths in the visible and infrared. It has been shown recently that self-assembled quantum dots (QDs) can be used to obtain laser diodes based on high quality zero-dimensional (0D) structures with emission in the visible  and in the infrared [2–7] with threshold currents and efficiencies comparable or better to the ones obtained with 2D QWs. The self-assembled QDs are readily obtained with the current epitaxy techniques, without any additional processing required, and feature unique properties such as temperature-independent lifetimes and sharp temperature-independent homogeneous linewidths . It is therefore interesting to investigate the effect of temperature on * Corresponding author. Tel.: +1 613 9936018; fax: + 1 613 9528701; e-mail: [email protected]
actual QD laser diodes and understand how the thermal energy will affect the device performances. In this study, we investigate QD lasers grown with various separate confinement heterostructures and systematically compare the results with control QW structures. This allows us to understand the temperature dependence of the threshold current which is found to depend mainly on the potential depth of the separate confinement region for temperatures above 125 K. We therefore design the separate confinement layers for higher temperature operations, and obtain red-emitting QD lasers operating at room temperature.
2. Experimental The QDs were obtained using the spontaneous island formation in the initial stages of the Stranski-Krastanow growth mode during the molecular beam epitaxy (MBE) of highly strained Al0.36In0.64As on AlGaAs layers grown on n + (100) GaAs substrate . Fig. 1(a) shows a cross-section view of such Al0.36In0.64As QDs embedded in Al0.25Ga0.75As. The resulting QD have the shapes of an hemispherical cap with a base diameter of 20–25 nm. The laser structure consists of a thick ( 2 mm) n + Alx Ga1 − x As contact layer, followed by
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an Aly Ga1 − y As bottom cladding layer with a lower doping and with y Bx to form a two-step separate confinement cladding region, followed by the active region. The active region is made of a 16-nm undoped Al0.25Ga0.75As on each side of the Al0.36In0.64As QD layer(s). Several samples with one or five QD layers have been investigated with thicknesses between four and five monolayers (ML) of Al0.36In0.64As deposited at 530°C. This is followed by the symmetric step-graded cladding and contact layers with p-doping, and terminated with a p + GaAs cap. Control laser diode structures were also grown using the same structure but with active regions incorporating QWs instead of QDs by either using thinner Al0.36In0.64As layers and thus stopping the growth prior to the island formation, or growing an unstrained GaAs layer 5.0 nm thick. For example, Fig. 1(b) shows the transmission electron microscope (TEM) cross-section view of a stack of five QD layers, grown with 4.5 ML of Al0.36In0.64As separated by barriers of 8.0 nm of Al0.25Ga0.75As for the spacer layers. The TEM cross-section view of a control sample obtained with 2.5 ML of Al0.36In0.64As is shown in Fig. 1(c) and clearly evidence the resulting stack of five QW layers in latter case. The TEM micrographs also show that the growth front returns to a planar mode after only a few nanometers of Al0.25Ga0.75As above the QDs to produce an atomically flat interface. The p-i-n structures were then processed into broadarea lasers with two cleaved uncoated facets to form the laser cavities, here between 400 and 1250 mm in length, and metallization widths of 40, 60, or 100 mm. Ohmic contacts were formed on the n + and p + sides, and the resulting broad area laser diodes were pump electrically using pulsed excitation.
temperature range for which the threshold has little temperature dependence extends to 125 K. Curve (ii) is for the sample of Fig. 1(b) with a correlated stack of five layers clearly showing vertical self-assembling of the QDs, and with an estimated dot density of 330 mm − 2 per layer. Curve (iii) and (iv) are for a sample with a single QD layer deposited with 5.0 ML, with laser cavities of 60× 400 mm and 40 × 1250 mm, respectively. The results obtained with the QD lasers can be compared with curve (v) which is the one obtained with the control QW of Fig. 1(c) having a stack of five QWs, with a well thickness of 2.5 ML, also grown with the deeper separate confinement (Al0.35Ga0.65As/ Al0.70Ga0.30As). At temperatures below 125 K, the threshold of the QW laser (curve (v)) has a more pronounced temperature dependence than the one of QD lasers (curves (ii)–(iv)). At higher temperatures however, the thresholds of the QW and QD lasers are somewhat comparable, allowing for room temperature operation at a few kA cm − 2. This suggests that at higher temperatures, when the thermionic emission rates of the injected carriers out of the QD or QW potential becomes important, the lasing threshold is governed by the depth of the separated confinement
3. Results Fig. 2 compares the temperature dependence of the threshold current density (Jth) for various QD lasers (curve (i)–(iv)), for the control QW sample of Fig. 1(c) (curve (v)), and for a 5.0 nm GaAs QW (curve (vi)). Curve (i) is for a laser with a single QD layer emitting at 1.75 eV (707 nm) which was designed with Al0.30Ga0.70As/Al0.33Ga0.67As separate confinement layers having bandgaps of 1.94 and 1.99 eV, respectively. For temperatures below 50 K, the lasing threshold is almost independent of the temperature, but for higher temperatures the threshold increases rapidly because of the shallow separate confinement structure used here with respect to the lasing energy. This is evidenced by comparing the results of curve (i) with the one obtained with curves ((ii)– (vi)) in which cases the lasers are grown with deeper Al0.35Ga0.65As/Al0.70Ga0.30As separate confinement layers. For the QD lasers utilizing the deeper separate confinement layers (curve (ii) – (iv)), the
Fig. 1. Transmission electron microscope (TEM) cross-section view of the active region for Al0.36In0.64As QD lasers (a) and (b) with 4.5 ML deposited, and (c) for a laser with a stack of 5 Al0.36In0.64As QW with 2.5 ML deposited.
S. Fafard et al. / Materials Science and Engineering B51 (1998) 114–118
Fig. 2. Temperature dependence of the threshold current density for various QD lasers and control QW lasers with Alx Ga1 − x As/ Aly Ga1 − y As/Al0.25Ga0.75As separate confinement layers. (i) Single layer QD with shallow separate confinement; (ii) correlated stack of five QD layers as seen in Fig. 1(b); (iii) single layer QD; (iv) single layer QD, 40 ×1250 mm cavity; (v) stack of five Al0.36In0.64As QW layers (2.5 ML thick) as seen in Fig. 1(c); (vi) single GaAs QW 5.0 nm thick, 40 × 400 mm cavity. All the devices are broad area FabryPerot lasers with bare cleaved facets and with a cavity of 60 ×400 mm except for (iv) and (vi) which have the cavity indicated above.
region relative to the laser emission energy, with little dependence on the dimensionality of the potential. A 5.0 nm GaAs laser structure also based on the Al0.35Ga0.65As/Al0.70Ga0.30As separate confinement was studied to confirm the results (see curve (vi)). Indeed, the lasing threshold of such a standard QW structure clearly display a continuous temperature dependence similar to the one found with the previous control QW laser of curve (v). In the latter case however, the overall temperature dependence is lower because the 5.0 nm GaAs QW laser emit at a lower energy of 1.579 eV (i.e. an energy 190 meV lower than the other lasers of curves (i) – (v)). Therefore, for curve (vi), the deeper potential of the GaAs QW with respect to the Al0.25Ga0.75As barrier will reduce the thermionic emission rates of the injected carriers, resulting in lower thresholds. The efficiencies of the broad area QD lasers have been investigated in the temperature range where the threshold has a weak temperature dependence. For example the efficiency of the single layer QD laser of curve (iii) of Fig. 2 was investigated at 77 K by monitoring the output power emitted from one of the cleaved facets as a function of injected current for pulsed excitation with a duty cycle of a fraction of a percent. Fig. 3 shows the external efficiency for injection currents up to 0.5 A. The threshold current was 210 A cm − 2 and the differential efficiency above
threshold 0.2 mW mA − 1. The net external quantum efficiency (for both facet), is 13%. This value can be compared with curve (ii) of Fig. 3 which is the result obtained with the standard 5.0 nm GaAs QW (curve (vi) of Fig. 2). Clearly, the external efficiencies obtained with the QD lasers are comparable with the ones obtained with standard QW lasers (here in the case of Fig. 3 the QD laser efficiency slightly exceed the QW laser efficiency). This is very encouraging for device applications based on 0-D structures. The room temperature spectral output of the single layer QD laser (curve (iii) of Fig. 2, and curve (i) of Fig. 3) is shown in Fig. 4. The spectrum below threshold (curve (i)) shows a Gaussian lineshape with a full-width-half-maximum of 64 meV, caused by the inhomogeneous broadening in the QD ensemble. Curve (ii) is taken slightly above threshold at 4.4 kA cm − 2 at 300 K, and reveals the stimulated emission at 722 nm.
4. Conclusions Red-emitting QD lasers have been obtained by MBE using self-assembled growth. At low temperatures, the lasing threshold currents measured with the QD lasers are found to be more temperature-independent than the ones measured with control QW lasers. Also, the external efficiencies found with the QDs were comparable or even better than the one measured on a standard GaAs QW. At higher temperatures, the temperature
Fig. 3. Comparison between the external electrical to optical efficiency (oext) measured with a single layer QD laser (i) and a standard 5.0 nm GaAs QW laser (ii) at 77 K. The devices are broad area Fabry-Perot lasers with bare cleaved facets and with a cavity of 60 × 400 mm. The separate confinement and barrier layers are Al0.70Ga0.30As/Al0.35Ga0.65As/Al0.25Ga0.75As. Both curves are corrected for the power emitted from rear facet and for the thermal losses in the residual contact resistance .
S. Fafard et al. / Materials Science and Engineering B51 (1998) 114–117
Acknowledgements This work was partly supported by the Natural Science and Engineering Research Council of Canada (NSERC).
Fig. 4. Room temperature spectral output from a single layer Al0.36In0.64As QD laser embedded in Al0.70Ga0.30As/Al0.35Ga0.65As/ Al0.25Ga0.75As. Broad area Fabry-Perot lasers with bare cleaved facets and with a cavity of 60 ×400 mm. Before and after threshold for J = 4.0 kA cm − 2 (i); and J= 4.4 kA cm − 2 (ii).
dependence of the threshold currents is governed mainly by the confining potentials at the emission energy, and by the depth of the separate confinement region. We obtained QD lasers capable of room temperature emission with simple broad area laser devices having threshold current densities of a few kA cm − 2. QD lasers operating at longer wavelengths (for example at 1.3–1.5 mm ) should therefore have lower thresholds.
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