A combination of high operation temperatures and small sizes of diode lasers directly grown on silicon substrates is essential for their application in future photonic integrated circuits. In this work, we report on electrically-pumped III-V microdisk lasers monolithically grown on Si substrates with active regions of two kinds: either an InGaAs/GaAs quantum well (QW) or InAs/InGaAs/GaAs quantum dots (QDs). Microdisk resonators were defined using photolithography and plasma chemical etching. The active region diameter was varied from 11 to 31 µm. Microlasers were tested without external cooling at room and elevated temperatures.
The QW laser structure was epitaxially grown by MOCVD on silicon (100) with an intermediate MBE-grown Ge buffer. Under pulsed injection (0.5-µs-long injection pulses with 150 Hz repetition rate), lasing is achieved in QW microlasers with diameters of 23-31 µm with a minimal threshold current density of 28 kA/cm^2. Quasi-single mode lasing (SMSR is up to 20 dB) is observed with emission wavelength around 988 nm. To the best of our knowledge, this is the first quantum well electrically-pumped microdisk lasers monolithically deposited on (001)-oriented Si substrate. 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. The sidewall passivation can be helpful to reduce the threshold current.
As compared to QWs, quantum dots demonstrate reduced sensitivity to threading dislocations and other crystalline defects as well as to sidewall recombination owing to a suppressed lateral transport of charge carriers which prevents their diffusion towards non-radiate recombination centers. The QD laser structure was directly grown by MBE on Si (001) substrate with 4° offcut to the  plane. QD microlasers were tested at room temperature in CW regime with a DC current varied from 0 to 50 mA and at elevated temperatures under CW and pulsed excitation (0.5-µs-long injection pulses with 10 kHz repetition rate). The InAs/InGaAs QDs active region provides the wavelengths in the 1.32–1.35 µm spectral interval. At room temperature, lasing is achieved in microlasers with diameters of 14-30 µm with a minimal threshold current density of 600 A/cm2 (compare with that of 427 A/cm2 in edge-emitting laser). The threshold current density and specific thermal resistance of 0.004 °C×cm^2/mW are comparable to those of high-quality QD microdisk lasers on GaAs substrates. Lasing wavelength demonstrates low sensitivity to current-induced self-heating. Lasing is single mode (SMSR 20 dB) with a dominant mode linewidth as narrow as 30 pm. Under CW excitation lasing sustains up to 60 °C in microlasers with diameter of 30 µm. Because of self-heating, an actual temperature of the active region is close to 100°C. Under pulsed excitation, the maximal lasing temperature is 110°C. To our best knowledge, these are the smallest microlasers on silicon operating at such elevated temperatures ever reported. Up to 90°C lasing proceeds on the ground state optical transition of QDs with wavelength about 1.35 µm. At higher temperatures, lasing wavelength jumps to the excited state transition.
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