Five stacks of InAs quantum dots (QDs) with InGaAsP barriers were grown on (100) InP and luminescence characteristics were analyzed. Cross-sectional transmission electron microscopy shows that small dots with a lateral size of ∼30 nm and a height of ∼3 nm are formed with an areal density of ∼5×1010 cm−2. The QDs emit strong photoluminescence (PL) peaks in the range of 1.4–1.6 μm that can be controlled by nominal InAs thickness. The integrated PL intensity from QDs stays very high at room temperature as much as 20% of that at 10 K. At weak excitation, the carrier lifetimes are measured to be almost the same across the whole PL band at low temperature with a value of ∼4 ns and they remain at that value at room temperature. These characteristics strongly evidence that individual QDs are well isolated and have a strong carrier confinement at room temperature.
We have investigated the effects of surface passivation on off-state leakage current and current collapse effects of high-voltage GaN-on-Si heterojunction field effect transistors (HFETs) by using low pressure chemical vapor deposition (LPCVD) of silicon nitride (SiN x ). In this work, the metaloxide-semiconductor (MOS) structure-based HFETs are realized on AlGaN/GaN epitaxy grown silicon substrates by metal-organic chemical vapor deposition (MOCVD). For a comparative study, we have fabricated two types of HFETs, standard and modified MOS-HFETs. In the modified MOS-HFETs process, the surface passivation layer of SiN x is deposited by LPCVD after the mesa isolation step, while the gate is deposited and self-aligned in the trench etched in LPCVD-SiN x layer using inductively coupled plasma reactive ion etching (ICP-RIE). The high temperature deposition of LPCVD-SiN x prevents the degradation caused by the ohmic annealing and other process-induced surface damage. Compared to the standard MOS structure, the modified MOS-HFET devices exhibit 10 times lower off-state leakage currents within high voltage range (0-800 V) and significantly alleviated current collapse effects simultaneously.
Continuous-wave operation at room temperature from InGaAs∕InGaAsP∕InP quantum dot (QD) laser diodes (LD) has been achieved. A ridge waveguide QD LD with 7 QD-stacks in the active region lases at 1.503μm at 20°C and that with 5 QD-stacks lases at 1.445μm at room temperature. The shift in lasing wavelength is believed to be due to the difference in the quantized energy states involved in producing gain for lasing. With smaller number of QD stacks and shorter cavity length, the lasing wavelength shifts to shorter wavelength indicating that more of higher excited states are involved in producing gain. By increasing the number of QD stacks to 15, lasing at 1.56μm has been achieved under pulsed mode.
Ground-state energy of InAs quantum dots (QDs) in the GaAs matrix can be changed significantly by introducing a thin AlAs layer (1 nm). The photoluminescence (PL) peak position of the QDs grown directly on the thin AlAs layer is blueshifted by 171 meV from that of the QDs grown without the AlAs layer. QDs grown on an additional GaAs thin layer on top of the AlAs layer have PL peaks systematically redshifted to lower energy as the GaAs layer becomes thicker. Time-resolved PL shows that the QDs have similar lifetimes, attesting to the fact that all the QDs grown in this way are of high quality, although the energy level change is large and a thin AlAs layer is introduced.
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