The 2 μm spectral window is a host of a variety of applications in communication and sensing. However, the development of nanoscale emitters for this window has largely stagnated. Here, we present our growth, fabrication, and testing of InAs quantum dash (Qdash) lasers on InP for 2 μm photonics. The samples were grown by molecular beam epitaxy. Ridge waveguide lasers emitted at 1.97 μm, coinciding with the ground-state emission from the Qdash ensemble. A low threshold current density of 131 A/cm2 per Qdash layer was calculated under pulsed mode conditions. Good thermal stability was observed up to 50 °C, with a characteristic temperature of 44 K. InAs Qdash on InP is promising 0D laser gain materials for 2 μm communication and sensing.
We demonstrate the metamorphic growth of InAs1-xSbx (x = 0.08 – 0.68) layers on GaAs substrate with an optimized InAs-based intermediate buffer layer by molecular beam epitaxy. The broad range of group-V flux ratio is applied to investigate the effect between Sb incorporation and material quality. We find that high Sb compositions significantly roughen surface morphology, and that optimized growth temperature is crucial to prevent phase separation and surface segregation of Sb atoms. In addition, we achieve a high degree of strain relaxation (~94%) in the metamorphic InAsSb layers even with a 58% Sb composition. This result indicates that our InAs/GaAs virtual substrate is suitable for the growth of an almost fully relaxed InAsSb layer. Also, we investigate a threading dislocation density (TDD) trend with the broad range of Sb compositions, and a drastically increasing TDD trend (> 200 times) was observed. Finally, we report a narrow bandgap of 0.13 eV at 10 K of the InAs0.42Sb0.58 layer, which is promising for the detection of longwavelength infrared radiation. This InAsSb layer on GaAs substrate opens up possibility for mid-wavelength and long-wavelength infrared optoelectronics applications.
We have grown GaAs quantum dots (QDs) in Al0.3Ga0.7As matrix by droplet epitaxy for application in single photon
sources. This growth method enables the formation of QDs without strain, with emission wavelengths of around 700 nm
within the optimal detection range of cost effective silicon detector, and with reduced surface density of several tens to a
few QDs per μm2 for easier isolation of single QDs. The optical properties of QDs were envisaged by exciton and
biexciton emission peaks identified from power dependent and time-resolved micro-photoluminescence (μ-PL)
measurements. The possibility of fabricating photonic crystal (PC) resonator including a single QD was shown by
obtaining precise spectral and spatial information from a few QDs in a mesa structure, utilizing cathodoluminescence
(CL) measurements.
We have tuned the lasing wavelength of a quantum dot laser diode (QDLD) by a thermal treatment. The InGaAs QDLD
structure for 980 nm wavelength applications was grown by molecular beam epitaxy using the Stranski-Krastanov
growth mode. The room temperature photoluminescence (PL) of a QDLD showed the ground state (GS) and excited
state (ES) at the wavelengths at 993 and 946 nm, respectively. The 100 μm-wide and 4 mm-long broad area QDLD
showed the lasing wavelength of 963 nm attributed to the ES of QDs with higher gain. After the thermal treatment at
800 °C for 3 minutes with 300 nm-thick SiO2 capping layers, the PL intensity of the GS increased, which caused the
enhanced GS gain. The enhanced GS gain is thought to the attribution to the decreased carrier trapping due to the
defects quenching. As a result, we could control the lasing wavelength of the QDLD from a wavelength of 963 nm to a
wavelength of 980 nm. Moreover, the performances of these QDLDs have been discussed. This post-growth technique
can be used to enhance the performances of the optoelectronic devices based on quantum dot.
We demonstrated the room temperature lasing of GaAs-based 1.3 μm quantum-dot laser diode (QDLD) grown by atomic layer epitaxy (ALE). The active region of a QDLD consists of 3-stacked InAs quantum-dots (QDs) in an In0.15Ga0.85As quantum well (dots-in-a-well: DWELL), which was grown by molecular beam epitaxy (MBE). For advanced performances of QDLD, the high-growth-temperature spacer layer and p-type modulation doping were applied to QDLD active region. We fabricated ridge waveguide structure LDs which had 10 ~ 50 μm ridge width with several cavity lengths and applied a high reflection (HR) coating on one-sided mirror facet. The threshold current density was 95 A/cm2 under a pulsed operation and 247 A/cm2 under a CW operation, respectively. The lasing wavelength was 1.31 μm under a pulsed operation condition and 1.32 μm under a CW operation at room temperature. The QDLD showed a simultaneous lasing and a state switching to the higher-order state. The lasing wavelength switching from the ground state to the excited state depends on the cavity length, the injection current and operating temperature.
Optically pumped terahertz emission has been observed in a wide range of semiconductors. We show that InAs quantum dots on GaAs can be used to significantly enhance terahertz emission compared with a bare GaAs surface.
We have investigated the characteristics of GaAs-based 1.3 μm quantum-dot laser diode (QDLD) with Al0.7Ga0.3As cladding layers. The active region of QDLD consists of 3-stacked InAs quantum-dots (QDs) in an In0.15Ga0.85As quantum well (dots-in-a-well: DWELL), which was grown by molecular beam epitaxy (MBE). For advanced performances of QDLD, the high-growth-temperature spacer layer and p-type modulation doping were applied to QDLD active region. We fabricated ridge waveguide structure LDs which had 10 ~ 50 μm ridge width with several cavity lengths and applied a high reflection (HR) coating on one-sided mirror facet. The threshold current density was 155 and 95 A/cm2 for a 2000 μm-long as-cleaved and a 1500 μm-long HR coated LDs, respectively. The lasing wavelength was 1.31 μm from the ground state transition, under a pulsed operation condition (0.1%) at room temperature. The QDLD showed simultaneous lasing at 1.31 μm and 1.23 μm from the ground state (GS) and the excited state (ES), respectively. The lasing wavelength switching from the GS to the ES depends on the cavity length, the injection current and operating temperature.
We have investigated the device characteristics of quantum dot infrared photo detector (QDIP) utilizing InAs QDs in an In0.15Ga0.85As quantum well structure. Device characteristics, such as dark current, photoluminescence (PL), and photocurrent spectra, have been measured. Two peak positions were measured at 163 and 219 meV in photocurrent spectrum. The photo-current of the peak at 163 meV was larger than that at 219 meV. The full width at half maximum (FWHM) of the peak at 163 meV was 18 meV, which was attributed to bound-to-bound transition. In0.15Ga0.85As layers were believed to contribute to induce bound-to-bound transition energy (163 meV). The activation energies of electrons in an InGaAs QDs were determined to be 171 meV and 221 meV from temperature-dependent integrated PL intensities. These activation energies from PL measurement are quite well matched to peak IR detection energies of 163 meV and 219 meV from the photo-current spectrum. This result implies that one can estimate the peak IR detection wavelength of QDIP from PL measurements of QDIP structure before its fabrication and measurement.
In this study, we have studied the thermal treatment effect not only on the optical and structural properties of QDIP structure but also device performance of the QDIP. The thermal treatment of InAs/GaAs QDIP structure have been carried out at the temperature range from 650oC and 850oC with SiO2 capping layer for 1 minute under the N2-gas ambient. After the thermal treatment, the structure was processed to QDIP and its device characteristics such as dark current and IR photo-response were measured. Results show that the photoluminescence (PL) peak was blue-shifted from 1288nm to 1167nm while the peak of photo-currents spectrum was red-shifted from 7.6 um to 7.8 um after the thermal treatment. It is also noted that the thermally treated sample showed the increase of photo-currents, which resulted in the increase of detectivity.
Influence of quantum dot growth on the electrical properties of Au/GaAs Schottky diode structures containing self-assembled InAs quantum dots fabricated via atomic layer molecular beam epitaxy is investigated. Current-voltage characteristics and low frequency noise measurements were performed and analyzed. Employing four different structures; containing single quantum dot layer, without quantum dot layer for a reference, thicker capping layer with single quantum dot layer, three quantum dot layers, we find the diode containing single quantum dot layer show largest leakage current and all the dots show 1/f behavior in low frequency noise characteristics. Current dependence of the noise current power spectral density shows that all the dots have linear current dependence at low bias which is explained by the mobility and diffusivity fluctuation. The Hooge parameter was determined to be in the range of 10-7 to 10-8. At high bias, the diodes containing quantum dot layer(s) show IFβ dependence with the value of β larger than 2 (3.9, and 2.7), and the diode without quantum dot layer and thicker capping layer show the value of β smaller than 2 (1.6). The deviation of the values of β from two is explained by the random walk of electrons involving interface states at the metal-semiconductor Schottky barrier interface via barrier height modulation. It seems that the growth of quantum dots induces generation of the interface states with its density increasing towards the conduction band edge. The value of β smaller than 2 means that the interface states density is increasing towards the midgap. Typical value of the interface states density was found to be on the order of 1011 to 1012cm2/Vs.
Intermixing effects of MOCVD (metal organic chemical vapor deposition) grown InGaAs SAQDs (self-assembled quantum dots) covered with SiO2 and SiNx-SiO2 dielectric capping layers were investigated. The intermixing of SAQDs was isothermally performed at 700°C by varying annealing time under the N2-gas ambient. It was confirmed from the PL measurement after the thermal annealing that, the emission energy of SAQDs was blue-shifted by 190 meV, the FWHM (full width at half maximum) was narrowed from 76 meV to 47 meV and the PL intensity was increased. SiNx-SiO2 double capping layer have been found to induce larger PL intensity after the thermal annealing of SAQDs compared to SiO2 single capping layer. The results can be implemented for increasing quantum efficiency and tuning the detection wavelength in quantum dot infrared photodetector (QDIP).
Effects of InxGa1-xAs strain relaxation layers on the optical and structural properties of InAs quantum dots (QDs) were studied systemically. 300 K-photoluminescence (PL) shows that PL peak energy of the QDs is blue-shifted in GaAs/InAs QDs/5 nm-thick In0.1Ga0.9As structure compared to GaAs/InAs QDs/GaAs structure. This is attributed to the intermixing of materials between the QDs and the InGaAs layer below the QDs, whereas capping of a 5 nm-thick In0.1Ga0.9As layer leads to red shift due to strain relaxation effect. As thickness of InxGa1-xAs capping layer (TI) increases, 300 K-PL peaks experience red shift below TI < ~7 nm. Unlikely, TI above 7 nm results in blue shift. Considering average height of the QDs is ~ 7 nm, this is attributed to intermixing of material between the QDs and InGaAs capping layers. The blue shift in x = 0.2 over TI > ~7 nm is relatively smaller compared to that in x = 0.1. It is noteworthy that strain difference between the InAs QDs and the InxGa1-xAs is smaller in x = 0.2 rather than in x = 0.1. Finally, InAs QDs are sandwiched by asymmetric thickness (7.5 nm-thick capping InGaAs, 0, 1.2, and 2.5 nm-thick bottom InGaAs) of In0.2Ga0.8As layers. 300 K-PL spectrum shows that 1.2 nm-thick bottom InGaAs leads to the longest wavelength (1306 nm) among this sample set. This is attributed to reduced barrier height and ignorable accumulated strain effect in thin bottom InGaAs layers. In this report, we justify merit of dots in an asymmetric well structure over conventional dots in a symmetric well structure and strain relaxation structure for the control of PL peak energy.
Optical response of both the gate current and the drain current in p-channel InGaP/GaAs/InGaAs double heterojunction pseudomorphic MODFET is reported and analytic models are presented. Based on quantum nature of the two-dimensional carrier statistics in the channel and a new model for the gate current, the overall current variation under optical illumination is explained. The results show power law relation between the current variation and the optical intensity. Near-threshold region in saturation region is found to be most sensitive to the optical intensity variation
Three types of thin layer were inserted between 1st and 2nd separate confinement heterostructure (SCH) layer of 1.55 μm InGaAaP/InGaAs multi-quantum well (MQW) laser diodes. The three types were Type A (p-InGaAsP, 1x1017/cm3), Type B (p-InGaAsP, 2x1018/cm3), and Type C (p-InP, 2x1018/cm3), respectively. It was shown that the light-current (L-I) characteristics for those three types were similar, while the characteristic temperature (T0) was higher for type B than others.
In this paper, we propose a new technique to suppress the non- linearity of multiple quantum well (MQW) electro-absorption (EA) modulator, mainly due to an exponential-like transmission characteristics of EA modulator and non-linearity of quantum confined stark effect (QCSE), by intermixing MQW absorption region. Optical properties and its dependence on applied bias voltages of intermixed InGaAs/InGaAsP MQW absorption region, such as transition energy and gain (or absorption) spectrum have been calculated by solving Luttinger-Kohn Hamiltonian. It has been shown that the transfer function of a MQW-EA modulator can be tailored by introducing differently intermixed regions along the waveguide direction. It has been also shown that proposed technique can suppress IMD2 (2nd order intermodulation distortion) by 39.6 dB and enhance spurious free dynamic range (SFDR) by a 3.6 dB by choosing proper combination of interdiffusion lengths and waveguide lengths.
A new model for electrical low frequency noise in semiconductor heterostructure laser diodes is developed based on number fluctuation theory. The model includes carrier number fluctuation mechanisms such as thermal activation, tunneling and random walk involving bulk traps and interface traps at the heterojunction interface. Noise sources in heterostructure semiconductor laser diodes can be divided into three parts, namely, series resistance including ohmic contacts, p-n junction and the heterojunction. The traps located at the interface and or at the bulk of the barrier layer can induce the modulation of barrier height which in turn results in the current fluctuation. Noise generation mechanisms for p-n junction is reviewed. Correlation between electrical and optical noise is also discussed.
We present on the experimental results of the NpnP optoelectronic switching device with a significant optical sensitivity. This structure consists of a novel fully depleted optical thyristor (DOT) with a bottom mirror layers. We measure the emission efficiency for various sizes and injected currents. This device shows 20 percent and 45 percent enhancement in switching voltage change and spontaneous emission efficiency, respectively, which is very important for the sensitivity and the low power consumption of DOT.
Impurity free intermixing of In0.22Ga0.78As/GaAs quantum wells in a semiconductor laser has been carried out by rapid thermal annealing various width ridges capped with SiNx film. We have observed greater blue-shifting of the lasing wavelength for wider ridge lasers, caused by greater degree of quantum well intermixing. This dependence of quantum well intermixing on the ridge width has been explained by the redistribution of stress fields created by the SiNx capping film on the patterned structure.
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