Nripendra N. Halder

Lead-free epitaxial ferroelectric material integration on semiconducting (100) Nb-doped SrTiO3 for low-power non-volatile memory and efficient ultraviolet ray detection

We report lead-free ferroelectric based resistive switching non-volatile memory (NVM) devices with epitaxial (1-x)BaTiO3-xBiFeO3 (x = 0.725) (BT-BFO) film integrated on semiconducting (100) Nb (0.7%) doped SrTiO3 (Nb:STO) substrates. The piezoelectric force microscopy (PFM) measurement at room temperature demonstrated ferroelectricity in the BT-BFO thin film. PFM results also reveal the repeatable polarization inversion by poling, manifesting its potential for read-write operation in NVM devices. The electroforming-free and ferroelectric polarization coupled electrical behaviour demonstrated excellent resistive switching with high retention time, cyclic endurance, and low set/reset voltages. X-ray photoelectron spectroscopy was utilized to determine the band alignment at the BT-BFO and Nb:STO heterojunction, and it exhibited staggered band alignment. This heterojunction is found to behave as an efficient ultraviolet photo-detector with low rise and fall time. The architecture also demonstrates half-wave rectification under low and high input signal frequencies, where the output distortion is minimal. The results provide avenue for an electrical switch that can regulate the pixels in low or high frequency images. Combined this work paves the pathway towards designing future generation low-power ferroelectric based microelectronic devices by merging both electrical and photovoltaic properties of BT-BFO materials.

Integration of lead-free ferroelectric on HfO2/Si (100) for high performance non-volatile memory applications

We introduce a novel lead-free ferroelectric thin film (1-x)BaTiO3-xBa(Cu1/3Nb2/3)O3 (x = 0.025) (BT-BCN) integrated on to HfO2 buffered Si for non-volatile memory (NVM) applications. Piezoelectric force microscopy (PFM), x-ray diffraction, and high resolution transmission electron microscopy were employed to establish the ferroelectricity in BT-BCN thin films. PFM study reveals that the domains reversal occurs with 180° phase change by applying external voltage, demonstrating its effectiveness for NVM device applications. X-ray photoelectron microscopy was used to investigate the band alignments between atomic layer deposited HfO2 and pulsed laser deposited BT-BCN films. Programming and erasing operations were explained on the basis of band-alignments. The structure offers large memory window, low leakage current, and high and low capacitance values that were easily distinguishable even after ~106 s, indicating strong charge storage potential. This study explains a new approach towards the realization of ferroelectric based memory devices integrated on Si platform and also opens up a new possibility to embed the system within current complementary metal-oxide-semiconductor processing technology.

Role of ultra thin pseudomorphic InP layer to improve the high-k dielectric/GaAs interface in realizing metal-oxide-semiconductor capacitor

In this article, we report GaAs metal-oxide-semiconductor (MOS) capacitors with a metal organic chemical vapor deposited ultrathin (1.5 nm) pseudomorphic InP interface passivation layer (IPL) and a thin (5 nm) ZrO2 high-k dielectric. Reduction of the surface states on InP passivated GaAs surfaces was observed from the photoluminescence study. The x-ray photoelectron spectra confirmed the dramatic reduction of GaAs native oxides (Ga-O and As-O) from the interface of ZrO2 and p-GaAs, implying that the Fermi level at the high-k/GaAs interface can be unpinned with good interface quality. As a result, very low values of interface trap density (1.1 × 1011 cm−2 eV−1) and hysteresis (8.21 mV) were observed. The same was done for directly deposited ZrO2 on GaAs surface to understand the efficacy of InP interface passivation layer on GaAs MOS devices. A systematic capacitance-voltage and current density-voltage studies were performed on bothAl/ZrO2/InP/p-GaAs and Al/ZrO2/p-GaAs structures. It was found that inserti...

Effect of band offset on carrier transport and infrared detection in InP quantum dots/Si nano-heterojunction grown by metalorganic chemical vapor deposition technique

Epitaxy of III-V semiconductors on Si gets recent interest for next generation system on heterogeneous chip on wafer. The understanding of band offset is thus necessary for describing the charge transport phenomenon in these heterojunctions. In this work, x-ray photoemission spectroscopy has been used to determine the band offsets in a heterojunction made of InP quantum dots on Si. The valence and conduction band offset was found to be 0.12 eV and 0.35 eV, respectively, with a type-II band lineup. Deviation from theoretical prediction and previously published reports on quasi similar systems have been found and analyzed on the basis of the effect of strain, surface energy, shift in the electrostatic dipole and charge transfer at the interface. The carrier transport mechanisms along with different device parameters in the heterojunction have been studied for a temperature range of 180–300 K. This heterojunction is found to behave as an efficient infrared photodetector with an ON/OFF ratio of 21 at a reverse bias of 2 V. The corresponding rise and decay time was found to be 132 ms and 147 ms, respectively.

GaAs metal-oxide-semiconductor based nonvolatile memory devices embedded with ZnO quantum dots

Ultrathin InP passivated GaAs non-volatile memory devices were fabricated with chemically synthesized 5 nm ZnO quantum dots embedded into ZrO2 high-k oxide matrix deposited through metal organic chemical vapor deposition. In these memory devices, the memory window was found to be 6.10 V and the obtained charge loss was only 15.20% after 105 s. The superior retention characteristics and a wide memory window are achieved due to presence of ZnO quantum dots between tunneling and control oxide layers. Room temperature Coulomb blockade effect was found in these devices and it was ascertained to be the main reason for low leakage. Electronic band diagram with program and erase operations were described on the basis of electrical characterizations.

Charge storage properties of InP quantum dots in GaAs metal-oxide-semiconductor based nonvolatile flash memory devices

Metal organic vapor phase epitaxially grown 5 nm InP quantum dots (QDs) were embedded as charge storage elements between high-k control and tunneling dielectric layers in GaAs metal-oxide-semiconductor based nonvolatile memory devices. The QDs trap more electrons resulting in a large memory window (6.3 V) along with low leakage due to Coulomb blockade effect. 16.5% charge loss was found even after 105 s indicating its good charge storing potential. The programming and erasing operations were discussed with proposed band diagram.

Graded barrier AlGaN/AlN/GaN heterostructure for improved 2-dimensional electron gas carrier concentration and mobility

AbstractThis paper presents an approach of compositional grading of the barrier in AlGaN/GaN quantum well heterostructure to achieve high two dimensional electron gas (2DEG) carrier concentration and mobility for RF power amplifier applications. Plasma assisted Molecular Beam Epitaxy (PAMBE) has been used to grow compositionally graded AlGaN/GaN and AlGaN/AlN/GaN heterostructures. In-situ cathodoluminescence (CL) and ex-situ high resolution x-ray diffraction (HRXRD) along with high resolution transmission electron microscopy (HRTEM) techniques were used to study the compositions and thicknesses of grown heterostructures. Ohmic contact formation for all the samples were found to be challenging due to unusual surface behavior and thus addressed with three different metallization schemes. The graded AlGaN/GaN and AlGaN/AlN/GaN heterostructures show 2DEG carrier concentrations of 2.0 × 1013 cm−2 and 2.3 × 1013 cm−2 with carrier mobility of 764 cm2v−1s−1 and 960 cm2v−1s−1, respectively at room temperature. A performance index has been proposed to correlate the obtained results with its suitability for particular RF applications.

Effect of band alignment on photoluminescence and carrier escape from InP surface quantum dots grown by metalorganic chemical vapor deposition on Si

A detailed analysis of photoluminescence (PL) from InP quantum dots (QDs) grown on Si has been carried out to understand the effect of substrate/host material in the luminescence and carrier escape process from the surface quantum dots. Such studies are required for the development of monolithically integrated next generation III-V QD based optoelectronics with fully developed Si microelectronics. The samples were grown by atmospheric pressure metalorganic chemical vapor deposition technique, and the PL measurements were made in the temperature range 10–80 K. The distribution of the dot diameter as well as the dot height has been investigated from atomic force microscopy. The origin of the photoluminescence has been explained theoretically. The band alignment of InP/Si heterostructure has been determined, and it is found be type II in nature. The positions of the conduction band minimum of Si and the 1st excited state in the conduction band of InP QDs have been estimated to understand the carrier escape p...

Anomalous diffusion of Ga and As from semi-insulating GaAs substrate into MOCVD grown ZnO films as a function of annealing temperature and its effect on charge compensation

The diffusion behavior of arsenic (As) and gallium (Ga) atoms from semi-insulating GaAs (SI-GaAs) into ZnO films upon post-growth annealing vis-a-vis the resulting charge compensation was investigated with the help of x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy. The films, annealed at 600 oC and 700 oC showed p-type conductivity with a hole concentration of 1.1 × 1018 cm−3 and 2.8 × 1019 cm−3 respectively, whereas those annealed at 800 oC showed n-type conductivity with a carrier concentration of 6.5 × 1016 cm−3. It is observed that at lower temperatures, large fraction of As atoms diffused from the SI-GaAs substrates into ZnO and formed acceptor related complex, (AsZn–2VZn), by substituting Zn atoms (AsZn) and thereby creating two zinc vacancies (VZn). Thus as-grown ZnO which was supposed to be n-type due to nonstoichiometric nature showed p-type behavior. On further increasing the annealing temperature to 800 oC, Ga atoms diffused more than As atoms and substitute Zn atoms...

Fabrication and characterization of p-Si/n-ZnO heterojunction ultraviolet photodetector

This work deals with the fabrication and characterization of ultraviolet (UV) photodetectors (PDs) based on Si(p)/ZnO(n) heterojunction diodes. Zinc oxide films were grown by metal-organic chemical vapor deposition technique at a growth temperature of 450 °C. The non-stoichiometric nature with oxygen vacancy implies the n-type conductivity of as-grown ZnO. The optical bandgap was found to be 3.37 eV which was direct in nature. The ideality factor of the diode was calculated as 3.57. At a reverse bias of 3V the detector showed an on/off ratio of 16. The rise and decay time of transient response were 242 ms and 354 ms, respectively.