Arjun Mandal

Effect of heavy ion implantation on self-assembled single layer InAs/GaAs quantum dots

We report the degradation in photoluminescence efficiency of GaAs/(InAs/GaAs) quantum dot (QD) heterostructures subjected to 20 to 50?keV sulfur implantation. Sulfur ions of fluence ranging from 2.5 ? 1013 to 2 ? 1015?ions?cm?2 were used for implantation. Implantation resulted in shift in photoluminescence emission towards lower wavelength and degradation in photoluminescence efficiency. X-ray diffraction analysis revealed reduction in crystalline quality of GaAs cap layer and an amorphous layer was accomplished with 50?keV sulfur implantation, with a fluence of 2.5 ? 1014?ions?cm?2. The amorphous layer was formed due to the overlap of defect clusters created during implantation, as a result of exceeding the critical nuclear energy density deposited in the GaAs system. Cross-sectional transmission electron microscopy revealed damage zones in the cap layer and deformation of QDs upon 50?keV sulfur implantation. Creation of damaged/amorphous GaAs layer probably increased the compressive strain in InAs/GaAs QDs, which resulted in change in energy gap of QDs and blue shift in photoluminescence emission. Implantation resulted in decrease in activation energy from 111?meV (20?keV) to 10?meV (50?keV S). Increase in implantation energy created defects/damage profile at a depth in the vicinity of the QDs. Non-radiative recombination of carriers through these defects might be the possible reason for the degradation of photoluminescence efficiency.

Magneto-optical Kerr effect spectroscopy based study of Landé g-factor for holes in GaAs/AlGaAs single quantum wells under low magnetic fields

Results from a magneto-optical Kerr effect (MOKE) spectroscopy study of ground state heavy-hole and light-hole excitons in GaAs/Al0.3Ga0.7As single quantum wells, with widths ranging from 4.3 nm to 14 nm, are presented. A novel setup and signal analysis procedure was adopted whereby polar MOKE measurements in magnetic fields up to 1.8 T could be performed with a conventional H-frame electromagnet. A first principles simulation based procedure used for simultaneously analyzing both the measured Kerr rotation and Kerr ellipticity spectral lineshapes is described in detail. The Zeeman splitting obtained from the above analysis helped to determine the longitudinal Lande g-factors. The hole g-factors were found to vary with well width, ranging from −0.6 to 1.1 for heavy-holes and 6.5 to 8.6 for light-holes. While the heavy-hole g-factor values are in fair agreement with values expected from k·p perturbation theory, no evidence is found to support theoretical predictions of possible well width dependent giant l...

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.

Influence of Li implantation on the optical and electrical properties of ZnO film

ZnO has been a subject of intense research in the optoelectronics community owing to its wide bandgap (3.3eV) and large exciton binding energy (60meV). However, difficulty in doping it p-type posts a hindrance in fabricating ZnObased devices. In an attempt to make it p-type we have studied Li-implanted (Energy=40keV, dose=5x1013cm-2) <002> ZnO films grown over <001> sapphire substrates by Pulsed Laser Deposition technique at 400°C (sample A). The samples were subsequently subjected to Rapid Thermal Annealing at 650° and 750°C (samples B and C) for 30 seconds. Room temperature Photoluminescence study of as-deposited sample reveal consistent Donor-bound exciton (D0X) peak at 3.3eV, which shifts to 3.298eV, 3.298eV, and 3.289eV for samples A, B and C respectively. This data validates the n-type conductivity of the samples with a carrier concentration and Hall mobility of 8.68x1019cm-3, 1.13x1019cm-3 and 2.9x1020cm-3 and 2.14cm2/V-sec, 35.2cm2/V-sec, 16.9cm2/V-sec for samples A, B and C respectively. The reduced energy of D0X peak is probably due to strain variations occurred during the various processing steps. While the higher carrier concentration in sample C can be attributed to aggregated vacancy clusters at high temperature annealing. Since Li acts as an acceptor for ZnO, so a free electron-acceptor (FA) peak at 3.227eV, 3.217eV and 3.225eV in samples A, B and C is evident. A third peak at 3.128eV may be due to the donor-acceptor pair, a reason for a lower energy FA peak for sample B.

Structural and electrical properties of rectifying p-ZnO/n+-InP heterojunction

ZnO thin films were deposited over n+ InP (100) substrates by Pulsed Laser Deposition (PLD) technique at 400°C temperature in an oxygen ambient of 75 mTorr followed by Rapid Thermal Annealing (RTA) at the temperatures 450°C, 550°C and 650°C respectively. XRD results revealed that the full width at half maxima (FWHM) of the annealed samples were narrower (0.1836°) compared to that of the as grown sample (0.3264°) for the c-axis oriented ZnO (002) films. A lower strain (~ -0.23%) and less biaxial compressive stress (~ -1.063 GPa) were observed for the annealed samples. AFM images depicted lowest surface roughness of 7.257 nm (root-mean-square) for the film annealed at 550°C. A high absorption coefficient of 28.12 μm-1 was calculated around 380 nm wavelength from the UV/VIS spectroscopy in reflection mode for the as-grown sample. The optical band gap was calculated to be about 3.23 eV. p-type ZnO film, grown under same condition (annealed at 550°C) over semi insulating InP (100) substrates had a high hole concentration of 2.95X1019 cm-3 and Hall mobility of 8.63 cm2/V-s at room temperature. Current Rectification Ratio (IF/IR) |V|=1.5 of 17.2 was measured from the I-V characteristics of the p-ZnO/n+-InP heterojunction diode fabricated with the ZnO film annealed at 550°C.

Impact of phosphorus ion implantation on the material and optical properties of InAs/GaAs quantum dots

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.

Low-Energy Ion Implantation Over Single-Layer InAs/GaAs Quantum Dots

This chapter deals with the impact of both low-energy heavy ion (sulphur) and light ion (hydrogen) implantation over single-layer InAs/GaAs QDs. The material and structural properties of both un-implanted and implanted QDs are discussed, along with the results achieved through different characterizations. Sulphur (S−) ion implantation caused degradation of material quality whereas hydrogen (H−) ion implantation improved the material properties of InAs/GaAs QDs. The main purpose of this study was to optimize the particular ion and its energy and fluence range for experiencing the impact of ion implantation further on In(Ga)As/GaAs QD-based device structures as discussed in the following chapters.

Introduction to Quantum Dots

This chapter deals with the basics of zero-dimensional quantum structures, i.e. quantum dots. An abridged explanation of its electronic properties is mentioned in this chapter. Different fabrication techniques for growing quantum dots are also chalked out in short. The advantages and disadvantages of self-assembled quantum dots are described in detail. Various in-situ and ex-situ techniques along with importance of different capping layers for improving electronic properties of self-assembled quantum dots are also referred in this chapter.