Hemant Ghadi

More than one order enhancement in peak detectivity (D*) for quantum dot infrared photodetectors implanted with low energy light ions (H−)

In(Ga)As/GaAs-based quantum dot infrared photodetectors (QDIPs) have emerged as one of the most suitable devices for infrared detection. However, quantum dot devices suffer from lower efficiencies due to a low fill-factor (∼20%–25%) of dots. Here, we report a post-growth technique for improving the QDIP performance using low energy light ion (H−) implantation. At high bias, there is evidence of suppression in the field-assisted tunneling component of the dark current. Enhancement in peak detectivity (D*), a measure of the signal-to-noise ratio, by more than one order, from ∼109 to 2.44 × 1010 cm Hz1/2/W was obtained from the implanted devices.

Comprehensive study on molecular beam epitaxy-grown InAs sub-monolayer quantum dots with different capping combinations

Here the authors report a comprehensive study on InAs sub-monolayer quantum dots with different capping layers. After performing systematic optimization of InAs deposition and GaAs thickness, they grew three samples, namely A, B and C, using solid-state molecular beam epitaxy with identical architecture but different capping materials (2 nm of GaAs, InGaAs-GaAs, and InAlGaAs-GaAs, respectively). Photoluminescence emission peaks due to the ground state transition from the dots were observed at 898, 917, and 867 nm for samples A, B, and C, respectively. Narrow full-width half-maxima (19–32 meV) of the emission peaks indicates high uniformity of dot size distribution. Using the conventional Arrhenius plot, the authors calculated the thermal activation energies from temperature-dependent photoluminescence experiment for samples A, B, and C as 49, 112, and 109 meV, respectively. To complete the study, single-pixel photodetectors were fabricated from samples A, B, and C and temperature-dependent dark current va...

Effect of barrier thickness on structural, optical, and spectral behaviors of vertically strain coupled InAs/GaAs quantum dot infrared photodetectors

The optical, electrical, and spectral properties of a strain coupled InAs quantum dot detector with a fixed quaternary capping of InAlGaAs and variable GaAs barrier thickness were investigated along with an equivalent uncoupled structure. Self-assembled quantum dots with a multimodal dot size distribution were achieved owing to vertical strain coupling. Strain and electronic coupling were utilized to improve the optical and electrical performance of the fabricated quantum dot infrared photodetector. The peak spectral response was tuned by varying barrier thickness, and a blue shift (almost 1 μm) was observed by increasing the capping thickness from sample A (90 A capping) to E (500 A capping). High responsivity and detectivity (∼1010 cm Hz1/2/W) were observed for all coupled samples as compared to the uncoupled sample. All coupled samples showed high thermal stability in the photoluminescence peak with high-temperature annealing.

AuGe surface plasmon enhances photoluminescence of the InAs/GaAs bilayer quantum dot heterostructure

We report an improvement in the photoluminescence of a GaAs-capped InAs/GaAs bilayer quantum dot (QD) heterostructure by AuGe nanoparticle deposition on the surface of a thin capped layer. Scanning electron microscopy confirmed the formation of AuGe nanoparticles on the surface at temperatures ranging from 300 to 700 °C. Optical absorption spectroscopy revealed the plasmon resonance peak of AuGe nanoparticles at around 670 nm for the sample annealed at 300 °C, confirming the presence of the plasmonic effect. Raman spectroscopy revealed a QD phonon peak at ∼238.5 cm−1 for the sample annealed at 300 °C, indicating InAs QDs in the heterostructure. Compared to the uncovered sample, enhancements were observed in the PL spectra of the AuGe-deposited samples annealed at 300 °C and 400 °C (with enhancement factors of 2.58 and 2.18, respectively). The observed enhancement is attributed to photon trapping by scattering from the cross section of the dipole radiation field. Increasing the annealing temperature from 300 °C to 700 °C blue-shifted the photoluminescence peaks at 18 K because of In/Ga inter-diffusion. A decrease in activation energy was observed with the increase in annealing temperature from 300 °C to 700 °C, attributed to poor confinement potential and high electron concentration at the sample surface. Our findings contribute to the realization of high-efficiency plasmonic-based InAs QD detectors for optical communication in the 1300 nm optical window.

Optimization of the Number of Stacks in the Submonolayer Quantum Dot Heterostructure for Infrared Photodetectors

We studied the optical, electrical, and spectral properties of InAs submonolayer quantum dot infrared photodetectors with different number of stacks. Three samples with 4, 6, and 8 dot stacks were grown by molecular beam epitaxy under identical conditions. Increasing the number of stacks results in a gradual shift in the photoluminescence ground-state transition energy of the samples from 1.195 to 1.111 eV. Cross-sectional transmission electron microscopy images confirm increase in dot size with increasing number of stacks from 4 to 8. Samples with 4 and 6 stacks measured moderately uniform dot size distribution and with further increasing the number of stacks 4 to 8 variations in dot sizes along with improper dot size formation were observed. The activation energy of the samples was measured by both optical and electrical methods increase with increasing number of dots. All photodetectors exhibit a photocurrent peak in the range of 7.3-7.8 μm at 77 K at an applied bias of -1 V. Highest peak responsivity value of 0.04523 A/W at 77 K was observed from the 6 stacked sample, which was highest among the three samples. It also exhibited highest detectivity of 5E9 Jones with lowest noise current density among the others. The sample with 6 dot stacks is the best as it exhibited lowest dark current density of 6.1 (10-7 A/cm2 and highest operating temperature of 110 K).

Enhancement in Peak Detectivity and Operating Temperature of Strain-Coupled InAs/GaAs Quantum Dot Infrared Photodetectors by Rapid Thermal Annealing

We report the effects of rapid thermal annealing on the optical, structural, and device properties of 30 layer strain-coupled InAs/GaAs quantum dot infrared photodetectors. Stability in the photoluminescence peak is observed for annealing up to 800°C, which has not been previously reported. Cross-sectional transmission electron microscopy images show preservation of quantum dots is observed up to 800°C. Device with total capping thickness of 150 nm annealed at 750°C exhibit a fivefold enhancement in spectral intensity compared to as-grown devices and increase in the temperature of detector operation is observed from 100 to 140 K from the same device. The annealed devices exhibited a single-order enhancement in peak detectivity compared to as-grown quantum dot infrared photodetector.

Highly efficient InAs/InGaAs quantum dot-in-a-well heterostructure validated with theoretically simulated model

Quantum dots based devices suffer certain disadvantages like low quantum efficiency, smaller absorption co-efficient and high dark current. Dot-in-a- well heterostructures (DWELL) offers tuning of detection peak wavelength, low dark current and higher operating temperature with optimized quantum well thickness. In this report, we correlate the optoelectronics properties of 3 different DWELL structures namely samples A, B and C having SRL thickness of 4, 6 and 8 nm, respectively with concentration-dependent theoretical model. A blue shift of around 24 nm with a decrease in PL intensity is observed as the capping thickness increases above 6 nm. Full width at Half Maximum (FWHM) decreases from B to C. These are attributed to the presence of large number of defect states and the formation of InGaAs wells with dissolution of dots in C. Low temperature PL measurement at 2.54W/cm2 and Photoluminescence Excitation (PLE) spectrum validate the presence of InGaAs wells peak at 1094.4nm for C. All samples exhibited peak spectral response at 7.56 μm. A concentration-dependent theoretical model using the Schrödinger equation was developed to calculate ground-state and inter-sub band energy-levels. The developed model shows great agreement with experimentally observed peaks from PL, PLE and spectral response. The same model was used to calculate the energy levels for InGaAs well. Based on the InGaAs experimental peak at 1.134 eV, average In concentration in the well was calculated to be around 30%.

Co-relation of theoretical simulation with experimental results for InAs quantum-dot heterostructures with different capping material

Quantum dot(QD) based devices are capped with various strain reducing layers in order to improve quality of dots by low surface diffusion and increase detection to long wavelength infrared region(LWIR). We present a model for the effect of various strain reducing layers on quantum dot heterostructures and study the corresponding variation in optoelectronic properties viz. photoluminescence (PL), photoluminescence excitation (PLE) and device characteristics of the samples. Schrödinger equation was used in the concentration dependent model in order to calculate ground state and inter-sub band energy-levels. Three InAs QD (2.7 ML) samples with different capping GaAs (Sample A), 6 nm In0.15Ga0.85As (Sample B) and 6 nm In0.15Ga0.85As DWELL (Sample C with 2 nm pseudomorphic layer of In0.15Ga0.85As) were grown. Low temperature (8 K) PL spectra exhibits ground state peak at 1112.62, 1150 and 1166.93 nm for samples A, B and C, respectively. PLE measured at 8 K exhibited first and second excited state peaks at 1046.15 nm, 991.59 nm for Sample A, 1079nm and 1003nm for Sample B and 1095.55nm and 1034.51nm for Sample C. Highest absolute area measured using temperature dependent PL (photocurrent) was observed for sample B which can be justified by increment in quantum dots formation thus resulting higher quantum yield. Single pixel detectors were fabricated and sample B yielded lowest dark current density at 80 K. A multicolor spectral response was observed from sample B with corresponding peaks at 5.13 and 7.53 μm. The calculated energy levels are in good agreement with experimental results (PL and PLE). Spectral response peaks observed from all samples were successfully matched to the energy levels calculated from the simulation.

Effect of time varying phosphorus implantation on optoelectronics properties of RF sputtered ZnO thin-films

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.

Temperature-dependent phosphorous dopant activation in ZnO thin film deposited using plasma immersion ion implantation

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.