1. Introduction

Microdisk laser cavities with high-Q whispering gallery mode (WGM) resonances are effective for low threshold lasing and enhanced spontaneous emission. For microcavity lasers with large surface to volume ratio, the surface recombination is an important issue. The quantum dot (QD) embedded microdisk lasers are the candidates of ultralow threshold lasers due to the strong carrier confinement which suppresses the surface recombination. The room temperature lasing of InAs QD microdisk lasers has been achieved by optical pumping [1,2]. In recent years, the integration between current-injection quantum-well microdisk lasers and silicon-on-insulator (SOI) waveguide has also been reported [3] and shows that microdisk laser devices can be exploited as an efficient laser source on SOI platform. Current-injection QD microdisk laser devices have also been proposed. However, the experimentally demonstrated operation temperature is limited to 5K [4]. In this study, our laser devices are buried in benzocyclobutene (BCB) cladding [5]. The benefits of this device planarization are the better thermal conductivity and the less fragility. The microdisk pedestal diameter is designed to be 2 μm smaller than the microdisk diameter so that injected carriers can diffuse to the disk perimeter more effectively. The microdisk pedestal diameter control can also serve as an efficient mechanism to select fundamental whispering gallery modes with radial number equal to one while other higher order modes experience greater loss. The scanning electron micrograph (SEM) of this planarized microdisk laser structure with microdisk diameter of 10.5 μm is shown in Fig. 1 . Room-temperature ground-state lasing from these BCB-cladding QD microdisk lasers is achieved, which demonstrates their potential for realistic applications.

 

Fig. 1 The cross-section SEM image of a current-injection microdisk laser device. The microdisk diameter is 10.5 μm.

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2. Device Fabrication and Experiment

The sample in this work is grown by molecular beam epitaxy (MBE) on n-GaAs substrate. The active layer consists of five-stacked In_0.5Ga_0.5As QDs layers with 10 nm-thick GaAs spacers. The total thickness including the 80 nm optical confinement layers on both sides is near 200 nm. The cladding layers are p-doped and n-doped Al_0.8Ga_0.2As, and the 80 nm p-GaAs is grown on top as contact layer. To fabricate microdisks, first we deposit a thin SiO₂ layer by plasma enhanced chemical vapor deposition (PECVD) on the contact layer surface. The microdisk patterns are defined by electron-beam lithography on the SiO₂ layer. The circular patterns are transferred to the SiO₂ layer by reactive ion etching (RIE). And then, instead of using metal masks for dry etching [5], the SiO₂ patterns are used as the etching mask for the following two-step wet etching processes, avoiding any ion-induced damage caused by dry etching processes [6,7]. The first HBr-based solution unselectively etches both GaAs and Al_0.8Ga_0.2As layers. The actual microdisk diameters are 6.5 μm and 10.5 μm, respectively, which are 1.5 μm smaller than the SiO₂ pattern sizes defined by electron-beam lithography. Although the etching surface from dry etching process is more vertical, but the edge surface is more rough and the surface scattering is higher. Next we selectively etch the Al_0.8Ga_0.2As upper and lower cladding layers by dilute HF solution to form the pedestal structure. The lateral etching depth beneath the microdisk is 1 μm so that the microdisk pedestal diameter is 2 μm smaller than the microdisk and injected carriers can diffuse to the disk perimeter more effectively, The etching depth of 1 μm is enough to suppress the scattering loss through the pedestal for the WGMs with lowest radial number, as predicted by our theoretical calculation based on an effective-index model [8] for the field distribution shown in Fig. 2 . After the etching processes, the BCB polymer from Dow Chemical is spin-coated, cured at 250 °C with flowing nitrogen. Next, we etch the BCB polymer by RIE to expose the top contact layer for the p-side metal contact by deposition of Ti/Pt/Au on the top. The Au/Ge/Ni alloy and Au are deposited on the back to form the n-side metal contact. The cross-section image of a fabricated current-injection microdisk laser is shown in Fig. 1.

 

Fig. 2 The field intensity distribution along the radial direction of a microdisk catity. The diameter of the microdisk is 10.5 μm. The radial numbers (m) of each mode are indicated in the figure.

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For the investigation of laser properties, we collect the scattering light from the microdisk edge by an objective lens. The spectra are analyzed by a monochromator with a 600-groove/mm grating, followed by a single photon counting avalanche photodiode and a photon counting instrument, to detect the luminescence. The sample temperature is controlled in a cryostat cooled by liquid nitrogen. To avoid the sample heating, we inject the devices by electric pulses with duty cycle at 1%.

3. Results and Discussion

From the previous photoluminescence measurement of our QD sample, the emission of QD ground states is around 1060 nm at room temperature. The mode spectrum and L-I curve of a 6.5-μm-diameter microdisk is shown in Fig. 3 . From the spectrum in Fig. 3(a), only one lasing WGM is observed. The WGM wavelength is centered at 1061 nm, which corresponds to the TE(1,53) mode in a cylindrical resonator, where the m and n in TE(m,n) represent the radial and azimuthal numbers, respectively. There is no noticeable wavelength shift observed with increasing current injection during the L-I curve measurement in Fig. 3(b). The linewidth of WGM obtained from the spectrum is 0.28 nm, which corresponds to the quality factor of ~3800. This WGM linewidth is limited by the measurement system resolution, and the actual quality factor should be higher than that value. The threshold current is 0.45 mA, extracted from Fig. 3(b).

 

Fig. 3 (a) The mode spectrum of a lasing 6.5-μm-diameter microdisk. The WGM linewidth is 0.28 nm. (b) The L-I curve of the 6.5-μm-diameter microdisk, the threshold current is 0.45 mA.

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Next we measure the lasing spectra under various temperatures. The lasing WGM wavelengths of the 6.5-μm-diameter microdisk device at different temperatures are summarized in Fig. 4(a) . From 80 K to 300 K, there are three different WGM resonances involved, all from QD ground states. Different WGM resonant modes are excited at different temperatures due to the QD bandgap shrinkage with increasing temperature by 0.21 nm/K, as determined from the photoluminescence study of the original unprocessed QD sample. At low temperatures, there are two WGMs lasing simultaneously. At temperature higher than 180 K, the lasing emission is single mode, except at 270 K when the mode hopping happens. Each WGM shifts to longer wavelength with increasing temperature by 0.07 nm/K. In order to compare with these results, we also fabricate another air-cladding microdisk sample without the BCB planarization step for optical pumping. The WGM wavelengths of the air-cladding sample also show the redshift phenomenon with average redshift 0.11 nm/K, which is higher than that of the BCB-cladding microdisks. The redshift behavior is due to the increase of modal effective refractive index with increasing temperature. The redshift amount of the BCB-cladding device is smaller because its increase of modal effective refractive index is partially compensated by the negative temperature dependence of the refractive index of BCB polymer, which was also observed in GaAsInP microdisk injection lasers [5]. Therefore, the temperature stability of emission wavelength is largely enhanced which is an important factor in a wavelength-division multiplexing system. For the comparison, another microdisk device with diameter of 10.5 μm is also measured, which operates at higher threshold current of 0.64 mA at room temperature. Figure 4(a) shows its lasing wavelengths at various temperatures. Due to the larger cavity size, there are more lasing modes appearing in the measured temperature range; all of them are attributed to the TE polarization and m = 1 family. None of the m = 2 lasing modes appear in these two devices, which indicates that the m = 1 WGMs are well supported while the WGMs with higher radial numbers suffer greater scattering loss through the pedestal.

 

Fig. 4 (a) The lasing WGM wavelengths of microdisk lasers and (b) their threshold current at various temperatures.

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The threshold currents obtained from microdisks with sizes of 6.5 and 10.5 μm at various temperatures are depicted in Fig. 4(b). For both devices, the minimum threshold currents are achieved around 150 K. Between 80 K and 150 K, both devices show a negative characteristic temperature T ₀, defined by I_th = I ₀exp(T/T ₀). From 150 K to room temperature, the temperature-dependent threshold current can be fitted with one single positive T ₀ which is 107 K for the 6.5 μm device and 98 K for the 10.5 μm one. The transition from negative T ₀ to positive T ₀ is due to the carrier redistribution in QDs from nonequilibrium towards equilibrium [9]. This transition temperature is around 150 K which is higher than that of the edge-emitting lasers fabricated from the same wafer.

The conventional edge-emitting QD lasers also show similar negative characteristic temperature phenomenon, but with lower transition temperature from negative T ₀ to positive T ₀. Figure 5 shows the temperature-dependent threshold current of a conventional 1.5 mm long and 50 μm wide edge-emitting QD laser fabricated from the same wafer with ground-state lasing. The transition temperature from negative T ₀ to positive T ₀ is about 105 K, which is much lower than that observed 150 K in microdisk lasers. This phenomenon can be explained by simulations of the temperature-dependent QD gain spectra based on a rate equation model [10]. In order to account for the thermal effect, we use energy-dependent time constants in our rate equation model to describe the thermal escape mechanism between four energy states, including separate confinement heterostructure, wetting layer, QD excited states, and QD ground states. We express the time constants corresponding to the carrier escape from QD as a Boltzmann function,τ = τ ₀exp(-ΔE_d/kT) [11], where τ ₀ is a constant, ΔE_d represents the energy difference between the energy states, and kT is the product of the Boltzmann constant and the temperature, respectively. The QD homogeneous broadening effect is also an important parameter correlated to temperature. The homogeneous broadening can be described by a Lorenz function. The full width at half maximum (FWHM) of the Lorenz function is based on the experimental results [12], which broadens linearly by 0.02 meV/K above 100 K, and 0.006 meV/K below 100 K. The Gaussian-shaped inhomogeneous broadening function of QD distribution is also taken into consideration in our gain model.

 

Fig. 5 The temperature-dependent threshold current of a conventional QD edge-emitting laser fabricated from the same wafer.

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The temperature-dependent threshold current density from our simulation is shown in Fig. 6(a) . It is clear that when the loss level of the QD laser increases, the transition from negative T ₀ to positive T ₀ occurs at lower temperature. At low loss level of 5 cm⁻¹, the transition temperature is 150 K. For higher loss level of 15 cm⁻¹, the transition temperature is around 100 K. The reason of the different transition temperature can be explained by the gain spectra shown in Fig. 6(b). Two of the gain spectra correspond to the lower injection level with peak gain value of 5 cm⁻¹ but under different temperature. The FWHM of the gain spectra at 100 and 150 K are 15.3 nm (18.3 meV) and 14.5 nm (16.9 meV), respectively. The gain spectrum at 100 K is wider, due to random population of carriers among QDs at this temperature. At 150 K, carriers will be re-distributed towards the Fermi distribution due to higher thermal energy, and results in a narrower gain spectrum. In this case, the necessary current to achieve a modal gain of 5 cm⁻¹ is lower at 150 K. For the other two gain spectra with higher peak gain value of 15 cm⁻¹, on the other hand, the FWHM are 17.1 nm (20.4 meV) at 100 K, and 17.4 nm (20.4 meV) at 150 K. The injection condition is closer to the QD ground-state saturation gain, which is set to about 20 cm⁻¹ in our simulation. The influence of thermal equilibrium to the QD gain spectral width is less important at the loss level of 15 cm⁻¹, since the injection is so high that almost all inhomogenously broadened QDs with different sizes are occupied. In this case, the injected current to achieve the gain level of 15 cm⁻¹ is higher at 150 K, due to the thermionic evaporation of carriers out of QDs at higher temperature. According to the discussion above, QD lasers under different loss levels can have different threshold behaviors with temperature, due to the different injection levels.

 

Fig. 6 (a) The temperature-dependent threshold current density with different loss levels. (b) The gain spectra of the QD lasers with different loss levels operating at various temperatures.

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In comparison with the temperature-dependent threshold behavior of the conventional QD edge-emitting lasers fabricated from the same wafer, the transition temperature from negative T ₀ to positive T ₀ for microdisk lasers is much higher. This suggests that the loss level in our microdisk laser is low due to the excellently confined cavities. It is interesting to note that the characteristic temperature observed in our microdisk lasers in Fig. 4(b) is much higher than that of the conventional QD edge-emitting laser, about 63 K from Fig. 5. At lower loss level, higher energy levels in QD ground states are not filled with carriers so that the thermal escape of carriers out of QDs is reduced and a higher characteristic temperature is observed. Similar phenomena are also reported in the literature with different cavity length or with high-reflection facets [13,14]. This also suggests that the loss level of our microdisk lasers is lower than the conventional edge-emitting lasers. Therefore, when well designed and fabricated, the threshold temperature stability of microdisk lasers is potentially superior to that of conventional edge-emitting lasers.

4. Conclusion

The QD microdisk injection laser devices with BCB polymer cladding are designed, fabricated, and characterized in this work. Room temperature lasing with single-mode emission from quantum-dot ground states is demonstrated. The lowest threshold current measured at room temperature is 0.45 mA, from a 6.5-μm-diameter microdisk. In temperature dependent experiments, the redshift amount of the WGM wavelength in the BCB-cladding device is as small as 0.07 nm/K due to the negative temperature dependence of the refractive index of BCB polymer. The negative characteristic temperature of QD microdisk lasers is also observed in the experiments. The transition temperature from negative T ₀ to positive T ₀ is around 150 K. With the help of simulations, this experimental result indicates the high quality factor of our microdisk cavity. Single-mode current-injection QD microdisk lasers with very low threshold current and high temperature stability of wavelength and threshold can serve as very suitable light sources in a compact integrated optical communication system.