1. Introduction

A capability of high-temperature low-threshold lasing demonstrated for GaAs-based microdisk lasers [1–3] makes them very promising for the on-chip light emitters [4, 5]. In particular, a low threshold current of 0.45 mA has been achieved at room temperature from a device of 6.5 μm in diameter with an InAs/InGaAs quantum dot active region grown on GaAs substrate [1]. With similar structure, lasing has been achieved at 100°C with a threshold current of 13.8 mA in a 31-µm microdisk laser [3].

Despite a significant progress in III-V-Si macrolasers demonstrated in the past years [6–10], there are only few publications on realization of III-V microlasers monolithically grown on silicon [11–14]. Free-standing InGaAs needle-shaped microlasers grown on silicon exhibited lasing at room temperature upon optical pumping [11]. Lasing operation up to room temperature was demonstrated in optically pumped In(Ga)As/(Al)GaAs suspended microdisk lasers grown on V-groove patterned (001) silicon [14]. Although the above examples of the optically-pumped microlasers represent major advancements toward the commercialization of fully integrated silicon photonics, injection (i.e. electrically-pumped) micro-devices are preferred for practical application.

Very recently the first electrically pumped micro-ring lasers epitaxially grown on (001) silicon have been demonstrated [15]. Those microlasers, similar to a majority of III-V macrolasers monolithically integrated to silicon, exploit quantum dots owing to their reduced sensitivity to threading dislocations and other crystalline defects [16, 17]. Nevertheless, quantum wells also have a potential for application in III-V-on-silicon macro-lasers and micro-lasers, as it will be demonstrated in this work. Quantum wells are typically characterized by high optical gain and high direct modulation bandwidth, which can be important in view of further miniaturization of microlasers and their future application.

We have recently fabricated InGaAs/GaAs quantum well laser structures using metal organic chemical vapor deposition (MOCVD) on an exact Si(001) substrate with a relaxed Ge buffer. Details of growth of Ge/Si(001) virtual substrate can be found in [18]. Edge-emitting laser has been demonstrated with a threshold current density of 5.5 kA/cm² (room temperature, pulsed mode) [19]. Using the similar laser structure, we have succeeded in the fabrication of microdisk laser resonators capable of operating at room temperature without external cooling. The results are presented in this work. To the best of our knowledge, this is the first demonstration of an injection quantum well microdisk laser fabricated of III-V materials monolithically grown on silicon (001) substrate.

2. Microlasers description and electrical characterization

A Ge/Si virtual substrate was fabricated using the two-step MBE growth with a Riber SIVA-21 machine equipped with e-beam evaporators for Ge and Si deposition. A standard exact (001)-oriented Si wafer (offcut angle < 0.5°) was used. First, a 50-nm-thick Ge layer was deposited at low temperature of 275°C in order to facilitate the two-dimensional growth mode and suppress the island formation. At this stage, the strain relaxation occurs via formation of a large number of misfit dislocations, while the surface remains flat. At the second stage, the rest part of the Ge layer (~1-µm-thick) was grown at high temperature of 600°C in order to improve the crystalline quality. Then, a cyclic annealing at 850°C/550°C was used with 2 min exposure at each temperature for 5 times. After the thermal cycling, the Ge-on-Si layer is characterized by low surface roughness (RMS is below 1 and 0.8 nm over 30 × 30 μm² and 5 × 5 μm² scan areas, respectively). Threading dislocation density is reduced from (3–4) × 10⁸ cm⁻² (typical for un-annealed samples of the same thickness) down to ~10⁷ cm⁻². High crystalline quality of the Ge/Si virtual substrates has also been confirmed by a low width (~0.05°) of the Ge(004) X-ray diffraction peak.

Upon standard chemical cleaning, the virtual Ge/Si substrate was placed into a MOCVD AIX 200RF reactor where it was annealed at 670°C for 5 min under a hydrogen/arsine flow. A III-V part of the laser heterostructure was grown under reduced pressure (100 mbar) at a rate of 1 nm/s using trimethylgallium, trimethylaluminum, trimethylindium, and arsine as elemental sources, silane and carbon tetrachloride as dopant sources. Scanning electron microscopy (SEM) image of the whole laminated structure is shown in Fig. 1(a). The layer sequence was started with an AlAs (10 nm) / GaAs (50 nm) / AlAs (10 nm) buffer followed by a 2.5-µm-thick n-type doped (2 × 10¹⁸ cm⁻³) GaAs current spread layer. Flat interfaces and absence of threading dislocations (at least, within a ~2-µm-large observation area) is revealed by transmission electron microscopy (TEM) micrograph taken with a JEOL JEM-2100F microscope operated at 200 kV, Fig. 1(b).

 

Fig. 1 Cross-section SEM (a) and TEM (b,c) images of the epitaxial structure.

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An active region was placed in the center of 0.8-µm-thick GaAs waveguiding layer confined with n- and p-type doped (5 × 10¹⁷ cm⁻³) 1-µm-thick Al_0.3Ga_0.7As cladding layers. The active region was formed by the deposition of three In_xGa_1-xAs layers (TEM image is depicted in Fig. 1(c)) having an average InAs mole fraction x = 17% and thickness of 10 nm. This combination of parameters results in a uniform two-dimensional quantum well. The laser structure was terminated with 0.5-µm-thick GaAs contact layers heavily doped with carbon atoms (2 × 10¹⁹ cm⁻³). Fabry-Perot lasers made of the similar epitaxial structure revealed the threshold current density of 0.5 and 5.5 kA/cm² at 77 and 300 K, respectively [19]. Threading dislocation density in the laser active region was estimated to be below 10⁷ cm⁻²; antiphase boundary density was about 0.3 µm⁻¹.

Microdisk resonators were defined using photolithography and plasma chemical etching with a SemiTEq STE ICPe68L machine. The etching depth was about 3 µm with sidewall vertical angle of about 83-86°. The microdisk diameter D was varied from 11 to 31 µm as measured in the active region plane. A p-ohmic contact formed with AgMn/NiAu metallization has a circular shape with a diameter Dcap being 4 µm smaller as compared to the microdisk diameter Ditself. A common AuGe/Ni/Au n-ohmic contact was placed onto the etched surface between the mesas. A micrograph of the microdisk array is shown in Fig. 2.

 

Fig. 2 SEM image of a microdisk array. Outer diameter D within a raw is indicated.

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Microlasers were put on a copper heatsink without soldering to be tested at room temperature (about 25-27°C). No external cooling (e.g. fan) was used. Current-voltage (I-V) curves were measured in the DC regime. A typical diode behavior is observed as it is shown in Fig. 3 for microlasers of two different sizes. Turn-on voltage is about 1.2 V, which is in accord with the expected optical transition energy. Series resistance increases with decreasing the microdisk diameter remaining below 100 Ω.

 

Fig. 3 I-V characteristic (solid lines) and its linear fit (dashed line) for two microdisk lasers of different diameters.

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Electroluminescence was excited with 0.5-µs-long injection pulses with 150 Hz repetition rate. Electroluminescence signal was collected with a piezoelectrically adjustable × 10 Olympus LMPlan IR objective. A Horiba FHR 1000 monochromator in combination with a Horiba Symphony Si CCD array was used for spectral detection. The spectral resolution was about 30 pm. The detector spectral response was taken into account when a spectrum shape is analyzed. In addition to a broad spontaneous emission spectrum, which covers a spectral interval from ~0.85 to 1 µm, a series of narrow lines becomes apparent at sufficiently high injection.

3. Electroluminescence results

For a microlaser with D = 31 µm, which spectra are presented in Fig. 4, three lines (985.66, 988.47, 991.49 nm) can be resolved as injection current reaches ~0.25 A. Suggesting a whispering gallery mode nature of these lines, a free spectral range of 2.8…3 nm gives an effective mode index of 3.35…3.58. Dependence of an integrated intensity of the dominant mode (at 988.47 nm) on the injection current (Fig. 5) demonstrates a pronounced knee, which implies an onset of lasing with a threshold current of 0.31 A. Laser operation is further confirmed by narrowing the spectral linewidth from 81 pm at minimal current, at which the mode can be resolved, down to 35 pm at the threshold (λ/Δλ ≈28 000) mostly limited by our spectral resolution. The corresponding spectrum is depicted in Fig. 4 (inset).

 

Fig. 4 Series of EL spectra taken at different currents for D = 31 µm microdisk laser. Spectra are vertically shifted by 10 dB for clarity. Inset: close-up spectrum near the threshold (312 mA).

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Fig. 5 Integrated EL intensity of dominant mode against injection current. Inset: threshold current as a function of the microdisk diameter (dashed line: the smallest threshold current density).

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It is worth mentioning that even at the threshold the peak intensity of the dominant cavity mode is an order of magnitude higher as compared to the background and the side modes. With further growth of the injection current, side mode suppression ratio increases up to about 20 dB. At higher currents, there is an excitation of new lasing modes accompanied by quenching of the initial mode.

Figure 5 also depicts two light-current curves measured for microdisk lasers of smaller diameter (27 and 23 µm). Again, a well-distinguished lasing threshold is observed at 160 and 131 mA, respectively. The threshold current as a function of the microdisk diameter data are summarized in the inset of Fig. 5. The smallest threshold current density was estimated to be 28 kA/cm² (D = 27 µm). It is about 5 times larger than the threshold current density measured in the edge-emitting laser made of similar structure [19]. Increase of the threshold current density in microdisk lasers is presumably caused by nonradiative recombination at sidewalls. The sidewall recombination is probably the reason why we did not achieve to date room temperature lasing in microdisk resonators with diameter of 21 µm and smaller.

In conclusion, we demonstrated the first operation of electrically-pumped quantum well microdisk lasers monolithically grown on a silicon substrate with a quantum well active region. Quasi-single mode lasing is observed at room temperature under pulsed excitation. The electrical parameters exhibit no peculiarities (turn-on voltage corresponds to the optical transition energy), whereas the threshold current density is several times higher compared to the edge-emitting laser. The minimal microdisk diameter at which lasing was achieved is 23 µm. The minimal absolute value of the threshold current is 130 mA. The surface passivation procedure will be probably helpful to reduce the threshold current.