High-temperature lasing in a microring laser with an active region based on InAs/InGaAs quantum dots

Lasing at a wavelength of >1.3 ??m has been achieved at temperatures of up to 380 K in a ring microlaser (diameter of 6 ??m) with an active region based on InAs/InGaAs quantum dots.     

Optical studies of a two-dimensional photonic crystal with the InAs/InGaAs quantum-dot structure as an active region

A two-dimensional semiconductor photonic crystal with a hexagonal lattice of submicrometer holes is produced by etching the GaAs/AlGaAs planar structure containing the InAs/InGaAs quantum dots in the waveguide layer. By analyzing the reflectance spectra at variable angles of incidence and polarizations of light, the photonic band structure is determined. The Fano resonance peaks revealed in the reflectance spectra at the TM (TE) polarization along the Γ-K (Γ-M) symmetry direction are due to the resonance interaction of optically active photonic bands with the incident light. The band structure of the radiation leakage modes is investigated by studying the angular dependence of the photoluminescence intensity. A threefold increase in the photoluminescence intensity revealed at the resonance frequency of the photonic crystal is attributed to the Purcell effect.     

Room-temperature optical absorption in the InAs/GaAs quantum-dot superlattice under an electric field

Electroluminescence and absorption spectra of a ten-layer InAs/GaAs quantum dot (QD) superlattice built in a two-section laser with sections of equal length is experimentally studied at room temperature. The thickness of the GaAs spacer layer between InAs QD layers, determined by transmission electron microscopy, is ∼6 nm. In contrast to tunnel-coupled QDs, QD superlattices amplify the optical polarization intensity and waveguide absorption of the TM mode in comparison with the TE mode. It is found that variations in the multimodal periodic spectrum of differential absorption of the QD superlattice structure are strongly linearly dependent on the applied electric field. Differential absorption spectra exhibit the Wannier-Stark effect in the InAs/GaAs QD superlattice, in which, in the presence of an external electric field, coupling of wave functions of miniband electron states is suppressed and a series of discrete levels called the Wannier-Stark ladder states are formed.     

Localization of holes in an InAs/GaAs quantum-dot molecule

Deep-level transient spectroscopy is used to study the emission of holes from the states of a vertically coupled system of InAs quantum dots in p-n InAs/GaAs heterostructures. This emission was considered in relation to the thickness of a GaAs interlayer between two layers of InAs quantum dots and to the reversebias voltage U_r. It is established that hole localization at one of the quantum dots is observed for a quantum-dot molecule composed of two vertically coupled self-organized quantum dots in an InAS/GaAs heterostructure that has a 20-Å-thick or 40-Å-thick GaAs interlayer between two layers of InAs quantum dots. For a thickness of the GaAs interlayer equal to 100 Å, it is found that the two layers of quantum dots are incompletely coupled, which results in a redistribution of the hole localization between the upper and lower quantum dots as the voltage U_r applied to the structure is varied. The studied structures with vertically coupled quantum dots were grown by molecular-beam epitaxy using self-organization effects.     

Reduced temperature sensitivity of lasing wavelength in near-1.3 /spl mu/ InAs/GaAs quantum-dot laser with stepped composition strain-reducing layer

An InGaAs strain-reducing capping layer with a stepped composition is shown to significantly reduce the temperature sensitivity of the lasing wavelength in a 1.3 mum InAs/GaAs quantum-dot laser. With this technique, the sensitivity is reduced from 0.48 nm/K for a laser with standard capping layer to 0.11 nm/K for the new design over the temperature range 20-130degC     

Modulation-doped field-effect transistors with an 8-nmInGaAs/InAs/InGaAs quantum well

We show that the channel thickness of modulation-doped field-effect transistors (MODFETs) based on an InAlAs/InGaAs heterojunction can be reduced from 15 to 8 nm without any degradation of the main DC and RF figures of merits. Furthermore, short-channel effects, which are pronounced in sub-0.1-μm devices, can be effectively suppressed by this thin-channel design. The retainment of high performance and alleviation of short-channel effects are attributed to the excellent two-dimensional electron gas (2-DEG) confinement by the inserted InAs layer. The successful channel thinning opens up the possibility of employing high-quality thin-channel structures for MODFETs with gate lengths below 0.05 μm     

Small-signal field effect in GaAs/InAs quantum-dot heterostructures

InAs quantum dots (QDs), incorporated into the space-charge region of an epitaxial n-GaAs film at different distances (5?C300 nm) from the surface, decrease the potential barrier for the electrons located in n-GaAs. For tunnel-thin coating layers this decrease is related to tunneling through QD energy levels. For thick layers this decrease is caused by negative charging of the QDlevels and defects located near QDs. The decrease in the barrier increases the efficiency of electron capture by surface states and shifts the frequency dispersion of mobility under the field effect, related to this capture, toward higher frequencies. When QDs are incorporated near the barrier??s base, they manifest themselves in the relaxation of the small-signal field effect. Some parameters of the QD levels are determined. Defect formation is revealed in the layers adjacent to QDs.     

Low-threshold high-T/sub 0/ 1.3-/spl mu/m InAs quantum-dot lasers due to p-type modulation doping of the active region

P-type doping is used to demonstrate high-To, low-threshold 1-3 /spl mu/m InAs quantum-dot lasers. A 5-/spl mu/m-wide oxide confined stripe laser with a 700-/spl mu/m-long cavity exhibits a pulsed T/sub 0/ = 213 K (196 K CW) from 0/spl deg/C to 80/spl deg/C. At room temperature, the devices have a CW threshold current of /spl sim/4.4 mA with an output power over 15 mW. The threshold at 100/spl deg/C is 8.4 mA with an output power over 8 mW.     

High-performance double-modulation-doped InAlAs/InGaAs/InAs HFETs

We demonstrate the improvement of double-sided-doped InAlAs/InGaAs MODFETs by inserting a thin InAs layer in the center of the conventional InGaAs channel. A maximum extrinsic transconductance of 1.4 S/mm is achieved for 0.13-μm devices. The current gain cutoff frequency of this device is as high as 265 GHz. Delay time analysis shows a significant improvement in the effective saturated velocity, from 2.4×10⁷ cm/s for LM devices to 3.1×10⁷ cm/s for InAs devices. We believe the superior performance of this device is primarily due to the reduction of scattering from donor layers, especially under the channel, and the interface roughness, which is achieved by inserting a 4-nm InAs layer in the channel     

Kink effect in an InAs inserted-channel InAlAs/InGaAs inverted HEMTat low temperature

The authors have investigated the kink effect In an InAs-inserted-channel InAlAs/InGaAs inverted HEMT at low temperature. The kink effect was not observed at both 77 and 300 K, but it appeared at 4.2 K. It is shown that the kink effect is caused at low drain voltages by the suppression of the drain current due to an increase in the source access resistance and at higher drain voltages by the increase in the drain current due to holes generated by impact ionization     

Photoluminescence of strain-induced coupled InGaAs/GaAs quantum-dot pairs

Coupled quantum dot-pairs were fabricated by growing InP self-assembled islands as stressors on InGaAs/GaAs double quantum wells. State filling in the photoluminescence spectra was used to resolve the quantum states in the coupled dots. The total strain field below the stressor decays exponentially with a penetration depth of about 25 nm, within which a dot-pair can be fabricated. Strong coupling is observed at a barrier width less than 4 nm separating the dot-pair. By increasing the indium composition in the lower well in order to match its dot level with one in the upper dot with identical quantum numbers, resonant coupling between the electron states with identical quantum numbers in the two dots can be achieved. Decoupling of the hole states and exchange of the electron bonding states from dominating the upper dot to the lower one are clearly resolved from the state energies and their spacings.     

State filling in InAs quantum-dot laser structures

We have measured the passive modal absorption, modal gain, and spontaneous emission spectra of a quantum-dot system where the inhomogeneous broadening is sufficiently small so that the ground- and excited-state transitions can be spectrally resolved. Absorption by ground- and excited-state transitions is in the ratio 1:1.88 which is close to the ratio of 2 expected for dots with similar dimensions in two directions. The absorption cross section per dot is measured to be 1.1 /spl times/ 10/sup -14/ cm/sup 2/. Optical gain from the ground-state saturates with current at a maximum value of one third of that predicted from the measured absorption if the system is fully inverted. The measured population inversion factor spectrum shows that the carrier distributions cannot be described only by a single global Fermi distribution and that the system is not in overall equilibrium. However, using parameters obtained by fitting the absorption spectrum, we find that for these particular samples the inversion factor spectrum can be described by a possible model where the ground- and excited-state occupancies are each described by a Fermi distribution but with different quasi-Fermi energy separations. We speculate that photon mediation within the homogeneous linewidth could be one possible process which establishes quasi-equilibrium within each of the ground- and excited-state inhomogeneous distributions.     

Stable mode-locked operation up to 80 /spl deg/C from an InGaAs quantum-dot laser

We demonstrate the mode-locked operation of a two-section quantum-dot laser in a broad temperature range. Stable mode-locking was observed at temperatures ranging from 20 /spl deg/C to 70 /spl deg/C, with signal-to-noise ratios well over 20 dB and a -3-dB linewidth smaller than 80 kHz. In the temperature range between 70 /spl deg/C and 80 /spl deg/C, the mode-locking was less stable. It was found that in order to provide stable mode-locked operation with increasing temperature, the reverse bias on the absorber section must be reduced accordingly.     

Linewidth enhancement factor in InGaAs quantum-dot amplifiers

We report systematic measurements of the linewidth enhancement factor (LEF) in an electrically pumped InGaAs quantum-dot (QD) amplifier in the temperature range from 50 K to room temperature. At injection currents below transparency, the value of the linewidth enhancement factor of the ground-state interband (excitonic) transition is between 0.4 and 1, and increases with increasing carrier density. Additionally, we investigate the spectral dependence of the LEF by tuning the wavelength of our optical probe from below resonance with the ground state of the QDs up to resonance with the first optically active excited-state transition. We find a decrease of the LEF with increasing photon energy at all investigated temperatures.     

Excited-state gain dynamics in InGaAs quantum-dot amplifiers

The ultrafast gain recovery dynamics of the first excited state (ES) is studied in an electrically pumped InGaAs quantum-dot amplifier at room temperature and compared with the ground-state (GS) gain dynamics. Pump-probe differential transmission experiments are performed in heterodyne detection and the gain dynamics are investigated as a function of injection current. An ultrafast (<200 fs) initial gain recovery of both GS and ES transition is found, promising for optical signal processing at high bit rates. The obtained results suggest the occurrence of a fast recovery of the state occupation mediated by carrier-carrier scattering as long as a reservoir of carriers in the ESs and wetting layer is present.     

Recombination mechanisms in 1.3-/spl mu/m InAs quantum-dot lasers

We measure, in real units, the radiative and total current density in high performance 1.3-/spl mu/m InAs quantum-dot-laser structures. Despite very low threshold current densities, significant nonradiative recombination (/spl sim/80% of the total recombination) occurs at 300 K with an increasing fraction at higher current density and higher temperature. Two nonradiative processes are identified; the first increases approximately linearly with the radiative recombination while the second increases at a faster rate and is associated with the loss of carriers to either excited dot states or the wetting layer.     

InAs/InGaAs数字合金应变补偿量子阱激光器

采用气态源分子束外延在InP衬底上生长InAs/InGaAs数字合金应变补偿量子阱激光器.有源区的多量子阱结构由压应变的InAs/In_(0.53)Ga_(0.47)As数字合金三角形势阱和张应变的In_(0.43)Ga_(0.57)As势垒构成.X射线衍射测试表明赝晶生长的量子阱结构具有很高的晶格质量.在100K、130mA连续波工作模式下,激光器的峰值波长达到1.94μm,对应的阈值电流密度为2.58 kA/cm~2.随着温度升高,激光器的激射光谱出现独特的蓝移现象,这是由于激光器结构中相对较高的内部吸收和弱的光学限制引起最大增益函数斜率降低所导致的.     

Self-organized nanoscale InP islands in an InGaP/GaAs host and InAs islands in an InGaAs/InP host

Arrays of strained nanoscale InP islands in an In_0.49Ga_0.51P host on a GaAs(100) substrate and InAs islands in a In_0.53Ga_0.47As host on an InP(100) substrate are obtained by metalorganic vapor-phase epitaxy (MOVPE). Their structural and photoluminescence properties are investigated. It is shown that the nanoscale islands that are formed measure 80 nm (InP/InGaP) and 25–60 nm (InAs/InGaAs). The photoluminescence spectra of the nanoscale islands display bands in the wavelength ranges 0.66–0.72 and 1.66–1.91 μm at 77 K with maxima whose position does not vary as the effective thickness of InP and InAs increases. The radiation efficiency of the nanoscale InP islands is two orders of magnitude greater than the luminescence intensity of the InAs islands. Fiz. Tekh. Poluprovodn. 33, 858–862 (July 1999)     

Optical detection of asymmetric quantum-dot molecules in double-layer InAs/GaAs structures

Self-assembled quantum dots (QDs) in double-layer InAs/GaAs structures are studied by resonant photoluminescence and photoluminescence excitation spectroscopy. A weakly correlated (50%) double-layer system with an array of vertically coupled QDs (asymmetric quantum-dot molecules) was formed in a structure consisting of the 1.8-monolayer-thick first and the 2.4-monolayer-thick second InAs layers separated by 50 monolayers of GaAs. The nature of discrete quantum states in this system was studied and resonances corresponding to vertically coupled QDs were clearly observed for the first time.