The work on Au-Ge nanoparticles (Nps) carried out so far by us has been successfully applied to devices like Bilayer, Trilayer, Hepta-layer Quantum dot infrared photodetectors (QDIP). An improved photonic response is achieved for the devices in terms of responsivity, photocurrent, responsivity, absorption and scattering. The dedicated standard recipe to get Nano-particles of different materials (Metals, Semiconductor) on distinguished substrate are revealed. It has been observed that the processes are repeated multiple time at the condition where desirable plasmonic condition does not match. Here the process has been optimized with multiple repeated annealing of Gold (Au) and Gold-Germanium (Au88Ge12) that shows the consistent pattern of reflectance where each anneal modifies the refractive index in same order with variable thickness of annealed film. This technique dilutes the constraints of fresh sample preparation whenever the nanoparticle response is dull, then the induced variation in size and volume of particle along with tuned distribution will become suitable.
Modulation doping or localization of carriers in the detector or solar cell structure is an interesting technique which has piqued the interest of researchers. In this study, we demonstrate the effect of modulation doping on InAs/GaAs p-i-p QDIP grown on semi-insulating GaAs substrate using MBE. The active region consists of 10 layers of 2.7 ML InAs quantum dots followed up with 60 nm GaAs capping layer. In the GaAs capping, a modulation p-doping of 3 nm was introduced at 7, 12 and 17 nm from the InAs dot layer thus forming sample A, B and C, respectively. The ground state emission peak at 19 K from photoluminescence (PL) spectroscopy was measured at 1055.5, 1057.5 and 1062 nm for sample A, B and C respectively. Activation energies calculated from temperature dependent PL spectra were 157.57, 167.18 and 146.63 meV for the respective samples. The fabricated single pixel detectors exhibited spectral response peak from 1 to 3.5 μm in short wave infrared (SWIR) region for all the samples. The spectral response peaks observed were at 2.01 and 2.43 μm for device A, at 1.83 μm for device B and at 1.77 μm for device C. Highest operating temperature obtained from device A, B and C were 100K, 150K and 200K, respectively. The peak responsivities observed at 100K were 0.503, 0.154 and 0.33 A/W for the device A, B and C, respectively. Optimizing the position of localized carriers introduced in the active region can achieve the tunability in detection peak.
Quantum dots based infrared photodetectors (QDIPs) having intra-valence band transitions and holes as majority carriers have been explored in this work. Here, we are demonstrating the effect of modulation doping on p-i-p QDIP (InAs/GaAs) grown using molecular beam epitaxy (MBE). The active region of the detector consists of 10 layers of selfassembled InAs quantum dots (2.7 ML) capped with GaAs layers and embedded in between p-type (beryllium-doped) GaAs layers. The performance of InAs/GaAs p-i-p QDIP (device A) was compared with modulation doped InAs/GaAs QDIP (device B). In the case of device B, modulation doping with p-type GaAs was introduced after growing 7nm of GaAs capping. The ground state emission peak at 10 K from photoluminescence spectroscopy was measured at 1060.5 nm and 1055.5 nm with a thermal activation energy of 222.93 meV and 157.57 meV for sample A and B, respectively. The measured dark current density at 75 K was 0.448 and 1.012 A/cm2 at -1 V for device A and B, respectively. Spectral response peak in short wave infrared region (1.5 to 2.5 μm) were observed from both devices but in the case of device B, the spectral peaks were visible in mid wave infrared regime as well. At 75 K, the peak responsivity value measured was 35.11 A/W (at -1.5 V) and 0.333 A/W (at -1.5 V bias) for device A and B, respectively. High temperature of operation upto 200 K was observed from Device A whereas Device B exhibited response up to 125 K. Modulation doping close to the InAs quantum dots deteriorates the device performance.
The improvement in performance of In0.5Ga0.5As Quantum Dot Infrared Photodetector (QDIP) has been investigated by introducing Al in the In0.15Ga0.85As capping layer. The QDIPs are consist of ten uncoupled In0.5Ga0.5As dot layers with 3/47 nm Inx(Aly(~x))Ga1-x-yAs/GaAs capping. The monolayer coverage for both QDIPs is 6, which accommodate an overgrowth percentage of 59%. A FWHM (46.59 nm) and higher activation energy (267 meV) has been obtained for the ground state photoluminescence emission due to the incorporation of Al in the InGaAs capping layer. This indicates better carrier confinement and homogeneous dot size distribution in the quaternary (In0.21Al0.21Ga0.58As) capped QD structure with respect to the ternary (In0.15Ga0.85As) capped QD. A six order reduction in dark current density has been obtained in the InGaAs QDIP due to the incorporation of Al in the capping layer. The narrow spectral width of 0.07 for the transition peak at 7.8 μm represents the homogeneous dot size distribution in the InAlGaAs/GaAs capped QDIP heterostructure.
Strain-coupling in Quantum Dot Infrared Photodetector (QDIP) structures has been used as a strategy to achieve higher absorption efficiency and better device characteristics. Improvement in device characteristics due to the incorporation of strain-coupled QD layers has been divulged in this report. A 3/47 nm In0.15Ga0.85As/GaAs capped conventional uncoupled QDIP has been compared with QDIP having 3/12 nm In0.15Ga0.85As/GaAs capped strain-coupled QDs. The single pixel photodetector fabricated from the coupled structure has a lower dark current density (7.9×10-4 A/cm2) compared to the uncoupled structure (12.17 A/cm2) at Vbias = -1 V and 300 K, which attributes a lower sensitivity to the thermalization effect in the former one. The strain-coupled QD heterostructure has photoluminescence peak at longer wavelength and lower full width at half maxima (24.86 nm), which indicates homogeneous dot size distribution. The surface chemical potential is less near the QDs due to the strain-relaxation. Hence, the lower lying dots forge the preferential nucleation sites for the upper QDs and it inhibits the inhomogeneous broadening occurs due to dot size fluctuation. The rocking-curve analysis from HRXRD measurement shows higher average strain in the strain-coupled QDIP (9.27×10-4) compared to the uncoupled one (5.42×10-4), which probably happens due to the accumulation of longitudinal strain from the lower QD layer towards the upped QD. The mid-infrared spectral response obtained from the strain-coupled QDIP has low spectral width.
Quantum dot infrared photodetectors (QDIPs) with different dot materials have been investigated in this study to analyze the optical, structural and electrical behavior. The InAs and In0.5Ga0.5As QDIPs comprise ten vertically-stacked uncoupled quantum dot (QD) layers with In0.15Ga0.85As/GaAs capping, whereas the overgrowth percentage in both the dot materials has been kept similar (~59%). The InGaAs QDIP has a red shifted photoluminescence spectra compared to the InAs QDIP along with a lower full width at half maxima (FWHM) and higher activation energy. This attributes the formation of dots with larger size and higher vertical barrier potential in the InGaAs QDIP heterostructure. The lattice mismatch between the dot and its underlying/capping layer is less in the InGaAs QDs, which has been observed from the HRXRD rocking-curve analysis. The average strain obtained in the InGaAs QD is less compared to the InAs QD. Moreover, a reduced dark current density has been obtained in the InGaAs QDIP compared to the InAs QDIP at room temperature. Both the QDIPs have their spectral response in the mid-infrared range. However, the InGaAs QDIP has peaks with lower FWHM due to minimized dot size dispersion in the structure.
Semiconductor industry thrives on the principle of continuous improvement and it has come a long way relying on that. Zinc oxide (ZnO) semiconductors is one such candidate whose bandgap can be tuned by assimilation of Mg thus making it a promising candidate for various optoelectronic applications. But as in case with ZnO, zinc magnesium oxide (ZnMgO) too has difficulty in achieving p-type conductivity due to native donor defects. For achieving p-type conductivity in such materials, co-doping technique seems to be the most viable solution as it improves the acceptor solubility and also lowers acceptor energy levels. In this report, we have studied the effect of boron doping on the optical and structural properties of phosphorus doped Zn0.85Mg0.15O thin film. Plasma immersion ion implantation (PIII) technique was used to dope RF sputtered Zn0.85Mg0.15O film with phosphorus for 70 s followed by boron doping for 5 s. The sample was further annealed at 1000oC in oxygen ambience for 10s. Low temperature photoluminescence (PL) spectra exhibited improvement in acceptor type behaviour with free acceptor (FA) peak at around 3.55 eV and near band edge (NBE) emission was further improved with the presence of free exciton (FX) peak at around 3.65 eV. These peaks were absent in phosphorus doped sample. High resolution x-ray diffraction (HRXRD) showed <002< orientation for codoped samples. X-ray photoelectron spectroscopy (XPS) confirmed the presence of boron and increment in phosphorus concentration with co-doping.
Inherent properties of wide bandgap (3.37 eV) and high exciton binding energy (60 meV) have helped zinc oxide (ZnO) to claim its potential in the area short-wavelength optoelectronic devices. Furthermore, it exhibits n-type conductivity due to presence of native defects which has restricted its effective utilization in junction devices. Doping ZnO to achieve p-type conductivity has been an area of interest over the last couple of decades. Taking into consideration the limitations imposed by mono-dopant on the p-type behaviour achieved, co-doping has emerged out to be promising technique with the advantage of increasing dopant solubility and reducing ionization energy. In this report we have studied the enhancement in properties of phosphorus doped ZnO thin film with boron as a co-dopant. Doping was done using plasma immersion ion implantation (PIII) technique where phosphorus was implanted for 70 s and subsequently boron for 10s followed by annealing at 800oC for 10 s in oxygen ambient. Low temperature photoluminescence (PL) spectra showed improvement in the acceptor behaviour with donor-acceptor pair (DAP) and free acceptor (FA) peaks observed at around 3.24 and 3.31 eV, respectively for co-doped sample as compared to phosphorus doped sample which did not show these peaks. High resolution x-ray diffraction (HRXRD) showed c-axis (<002<) orientation of the film with increase in peak intensity and angle for the co-doped sample. For co-doped sample, a blue-shift was observed for E2H peaks in Raman spectra with increase in peak intensities suggesting an improvement in the film crystallinity.
In the present work we are introducing heterogeneously coupled InAs stranski-krastanov and submonolayer quantum dot as an active material for quantum dot based infrared photodetector. Initially, we have optimized the basic SK on SML heterostructure. The thickness of the GaAs barrier layer is varied from 2.5 to 7.5 nm to tune the vertical coupling between seed SML and top SK QDs. PL and PLE response confirms the carrier tunneling between these heterogeneous QDs. The vertical alignment of SML and SK QDs is shown in Cross sectional TEM images. The sample with 7.5 nm barrier layer is incorporated into a N-I-N based quantum dot infrared photodetector, which shows broader spectral response than standard SK QD based IR detectors.
ZnO has potential application in the field of short wavelength devices like LED’s, laser diodes, UV detectors etc, because of its wide band gap (3.34 eV) and high exciton binding energy (60 meV). ZnO possess N-type conductivity due to presence of defects arising from oxygen and zinc interstitial vacancies. In order to achieve P-type or intrinsic carrier concentration an implantation study is preferred. In this report, we have varied phosphorous implantation time and studied its effect on optical as well structural properties of RF sputtered ZnO thin-films. Implantation was carried out using Plasma Immersion ion implantation technique for 10 and 20 s. These films were further annealed at 900°C for 10 s in oxygen ambient to activate phosphorous dopants. Low temperature photoluminescence (PL) spectra measured two distinct peaks at 3.32 and 3.199 eV for 20 s implanted sample annealed at 900°C. Temperature dependent PL measurement shows slightly blue shift in peak position from 18 K to 300 K. 3.199 eV peak can be attributed to donoracceptor pair (DAP) emission and 3.32 eV peak corresponds to conduction-band-to-acceptor (eA0) transition. High resolution x-ray diffraction revels dominant (002) peak from all samples. Increasing implantation time resulted in low peak intensity suggesting a formation of implantation related defects. Compression in C-axis with implantation time indicates incorporation of phosphorus in the formed film. Improvement in surface quality was observed from 20 s implanted sample which annealed at 900°C.
High band gap (3.34 eV) and large exciton binding energy (60 meV) at room temperature facilitates ZnO as a useful candidate for optoelectronics devices. Presence of zinc interstitial and oxygen vacancies results in n-type ZnO film. Phosphorus implantation was carried out using plasma immersion ion implantation technique (2kV, 900W) for constant duration (50 s) on RF sputtered ZnO thin films (Sample A). For dopant activation, sample A was subjected to Rapid Thermal Annealing (RTA) at 700, 800, 900 and 1000°C for 10 s in Oxygen ambient (Sample B, C, D, E). Low temperature (18 K) photoluminescence measurement demonstrated strong donor bound exciton peak for sample A. Dominant donor to acceptor pair peak (DAP) was observed for sample D at around 3.22 eV with linewidth of 131.3 meV. High resolution x-ray diffraction measurement demonstrated (001) and (002) peaks for sample A. (002) peak with high intensity was observed from all annealed samples. Incorporation of phosphorus in ZnO films leads to peak shift towards higher 2θ angle indicate tensile strain in implanted samples. Scanning electron microscopy images reveals improvement in grain size distribution along with reduction of implantation related defects. Raman spectra measured A1(LO) peak at around 576 cm-1 for sample A. Low intensity E2 (high) peak was observed for sample D indicating formation of (PZn+2VZn) complexes. From room temperature Hall measurement, sample D measured 1.17 x 1018 cm -3 carrier concentration with low resistivity of 0.464 Ω.
In this work, we investigated the effects of phosphorus ions implantation on InAs/GaAs QDs by varying the fluences from 8× 1011 to 1×1013 ions/cm2 at a fixed energy of 50 keV. Temperature dependent photoluminescence (PL) study shows a suppression of emission efficiency with the increase of fluence of implanted ions, attributed to the generation of defects/dislocations near around QDs acting as trapping centers for photocarriers. All the implanted samples demonstrated degradation in activation energy from 184 meV (as-grown) to 73 meV (highest fluence sample) indicating weaker carrier confinement in QDs. Implantation also resulted 40 nm blue shift in PL emission wavelength which is caused due to the atomic intermixing between QDs and surrounding materials. Rocking curves plotted from the double crystal X-ray diffraction study, depict a vanishing trend of satellite peaks with the increase of fluence of implanted ions, resulting from the loss of interface sharpness due to interdiffusion.
The hydrostatic (εhy) and biaxial (εbi) strain in lateral (x) and growth (z) direction have been computed and compared for InAs quantum dot (QD) with different capping. The capping layers are: GaAs, InGaAs/GaAs, InAlGaAs/GaAs, InGaAs/InAlGaAs/GaAs, InAlGaAs/InGaAs/GaAs, and the total thickness is kept constant for all QD structures. The strain distribution is mainly confined within the dot and dies down towards the capping layer. The movement of conduction band edges is controlled by hydrostatic strain. QDs capped with InAlGaAs/InGaAs/GaAs and InAlGaAs/GaAs shows lower magnitude of εhy, which indicates better carrier confinement as compared to other capping. The electrostatic potential obtained for the InAlGaAs capped QDs is larger (~0.5 V) than other structures. The valence band splitting into the heavy hole and light hole depends on the biaxial strain. It is observed that GaAs and InAlGaAs/InGaAs/GaAs capping has the smallest and largest values of εbi respectively in the growth direction. The GaAs capped QD structure has a smaller εbi, which would increase the energy of the ground state hole, leading to blue shift in photoluminescence spectrum. However, the ground hole state has a lower energy due to larger εbi in InAlGaAs/InGaAs/GaAs capped QD, which results in a red shift in the photoluminescence spectrum (~1.35 μm at 300 K). Nonetheless, InAlGaAs capped QDs shows better results in the lateral direction also. Thus, based on the strain profile, QDs capped with InAlGaAs as the first capping layer is the optimized structure which can be useful for various optoelectronic applications.
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