Gilberto A. Umana-Membreno

/sup 60/Co gamma irradiation effects on n-GaN Schottky diodes

The effect of /spl gamma/-ray exposure on the electrical characteristics of nickel/n-GaN Schottky barrier diodes has been investigated using current-voltage (I-V), capacitance-voltage (C-V), and deep-level transient spectroscopy (DLTS) measurements. The results indicate that /spl gamma/-irradiation induces an increase in the effective Schottky barrier height extracted from C-V measurements. Increasing radiation dose was found to degrade the reverse leakage current, whereas its effect on the forward I-V characteristics was negligible. Low temperature (/spl les/50) post-irradiation annealing after a cumulative irradiation dose of 21 Mrad(Si) was found to restore the reverse I-V characteristics to pre-irradiation levels without significantly affecting the radiation-induced changes in C-V and forward I-V characteristics. Three shallow radiation-induced defect centers with thermal activation energies of 88 104 and 144 meV were detected by DLTS with a combined production rate of 2.12 /spl times/ 10/sup -3/ cm/sup -1/. These centers are likely to be related to nitrogen-vacancies. The effect of high-energy radiation exposure on device characteristics is discussed taking into account possible contact inhomogeneities arising from dislocations and interfacial defects. The DLTS results indicate that GaN has an intrinsically low susceptibility to radiation-induced material degradation, yet the effects observed in the Schottky diode I-V and C-V characteristics indicate that the total-dose radiation hardness of GaN devices may be limited by susceptibility of the metal-GaN interface to radiation-induced damage.

Temperature-Dependent Characterization of AlGaN/GaN HEMTs: Thermal and Source/Drain Resistances

This paper shows the application of simple dc techniques to the temperature-dependent characterization of AlGaN/ GaN HEMTs in terms of the following: 1) thermal resistance and 2) ohmic series resistance (at low drain bias). Despite their simplicity, these measurement techniques are shown to give valuable information about the device behavior over a wide range of ambient/channel temperatures. The experimental results are validated by comparison with independent measurements and numerical simulations.

Characterisation of Multiple Carrier Transport in Indium Nitride Grown by Molecular Beam Epitaxy

Transport properties of two distinct electron species in indium nitride grown by molecular beam epitaxy (MBE) have been measured. Variable field Hall and resisitivity voltages were used in a quantitative mobility spectrum analysis (QMSA) to extract the concentrations and mobilities of the two electron species, attributed to the bulk electrons and a surface accumulation layer. Single magnetic field data corresponds to neither electron species. The bulk electron distribution has an extracted average mobility of 3570 cm2/(V s) at 300 K, which rises to over 5100 cm2/(V s) at 150 K. Bulk electron concentration in the sample is 1.5 ×1017 cm-3 . The surface electrons have a higher sheet charge density and an order of magnitude lower average mobility than those in the bulk.

Ion versus pH sensitivity of ungated AlGaN/GaN heterostructure-based devices

We have investigated the pH and ion sensitivity of AlGaN/GaN heterostructure devices; these devices are sensitive to the ion concentration rather than to the pH of the solution. Sheet resistance as a function of pH for calibrated pH solutions and dilute NaOH, HCl, KOH, and NaCl showed an increase as a function of ionic concentration, regardless of whether the pH was acidic, basic, or neutral. An increase in resistance corresponds to accumulation of negative ions at the AlGaN surface, indicating device selectivity toward the negative ions. We attribute this to the formation of a double layer at the liquid/semiconductor interface.

A method of removing the valence band discontinuity in HgCdTe-based nBn detectors

A method is described where the valence band discontinuity in HgCdTe-based nBn detectors will be eliminated. The method relies on doping modulation technique, where grading the material composition and doping concentration of the barrier layer at the same time will lead to elimination of the valence band discontinuity in HgCdTe-based nBn detectors. The method is not limited to the nBn structure and can be applied to any barrier detector structure with xBx (with x = n, p) to eliminate the energy band discontinuity in the valence band or conduction band.

Engineering the Bandgap of Unipolar HgCdTe-Based nBn Infrared Photodetectors

Design of practically realizable unipolar HgCdTe nBn photodetectors has been studied in detail by numerical analysis. The simulations reported herein reveal that, by optimization of barrier doping, dark current levels can be reduced and collection efficiency substantially improved. It is shown that p-type doping of the barrier layer can significantly reduce the effective potential barrier arising from the valence band offset between the absorber and barrier regions, thus enabling HgCdTe nBn detector operation under near zero-bias conditions. However, relatively high electric fields in the space charge regions near the barrier/absorber interface result in enhanced trap-assisted Shockley–Read–Hall thermal generation. Our calculations indicate that nBn HgCdTe detectors with barriers engineered by use of HgTe/Hg0.05Cd0.95Te superlattices have, potentially, substantially better valence band alignment without the need for p-type doping.

Design of Band Engineered HgCdTe nBn Detectors for MWIR and LWIR Applications

In this paper, we present a theoretical study of mercury cadmium telluride (HgCdTe)-based unipolar n-type/barrier/n-type (nBn) infrared (IR) detector structures for midwave IR and longwave IR spectral bands. To achieve the ultimate performance of nBn detectors, a bandgap engineering method is proposed to remove the undesirable valence band discontinuity that is currently limiting the performance of conventional HgCdTe nBn detectors. Our proposed band engineering method relies on simultaneous grading of the barrier composition and doping density profiles, leading to efficient elimination of the valence band discontinuity. This allows the detector to operate at |Vbias| <;50 mV, rendering all tunneling-related dark current components insignificant and allowing the detector to achieve the maximum possible diffusion current limited performance.

Phonon limited transport in graphene nanoribbon field effect transistors using full three dimensional quantum mechanical simulation

This paper present a study of carrier transport in graphene nanoribbon (GNR) transistors using three-dimensional quantum mechanical simulations based on a real-space approach of the non-equilibrium Greens function formalism in the ballistic and dissipative limit. The carrier transport parameters are determined in the presence of electron-phonon scattering, and its influence on carrier mobility including both optical phonons (OPs) and acoustic phonons (APs). The performances of GNR field effect transistors (GNRFETs) are investigated in detail considering the third nearest neighbour tight-binding approximation. The low-field mobility is extracted in the presence of AP and OP as a function of nanoribbon width and length, from which the diffusive/ballistic limit of operation in GNRFETs is determined.This paper present a study of carrier transport in graphene nanoribbon (GNR) transistors using three-dimensional quantum mechanical simulations based on a real-space approach of the non-equilibrium Greens function formalism in the ballistic and dissipative limit. The carrier transport parameters are determined in the presence of electron-phonon scattering, and its influence on carrier mobility including both optical phonons (OPs) and acoustic phonons (APs). The performances of GNR field effect transistors (GNRFETs) are investigated in detail considering the third nearest neighbour tight-binding approximation. The low-field mobility is extracted in the presence of AP and OP as a function of nanoribbon width and length, from which the diffusive/ballistic limit of operation in GNRFETs is determined.

Performance Modeling of Bandgap Engineered HgCdTe-Based n B n Infrared Detectors

In this paper, we present a theoretical study of a band engineered detector design, which significantly improves the performance of mercury cadmium telluride (HgCdTe)-based unipolar n-type/barrier/n-type (nBn) infrared (IR) detectors for the midwave IR and longwave IR spectral bands. This band engineered nBn detector is based on the assumption that the valence band offset, which is normally present between the barrier and n-type absorber layer, can be eliminated using a bandgap engineering approach. The valence offset is the key issue that currently limits the performance of HgCdTe-based nBn detectors. Eliminating the valence band offset allows the nBn detectors to operate at |VBias| <; 50 mV, thus rendering insignificant all tunneling-related dark current components and allowing the detector to achieve the maximum possible diffusion current limited performance. The developed model allows the device performance to be optimized by an appropriate design of the conduction band barrier to block the flow of majority carrier electrons, while allowing minority carrier holes photogenerated in the absorber layer to reach the contact layer unimpeded. Furthermore, because of the absence of tunneling-related dark currents, it is shown that band engineered nBn detector architecture exhibits a better performance at maximum allowed absorber layer doping density compared with conventional nBn detector.

Transport Studies of AlGaN/GaN Heterostructures of Different Al Mole Fractions With Variable

The influence of passivation by silicon nitride (SiNx) of different stress states (compressive, neutral, and tensile) is presented as a function of varying Al mole fraction (x). All types of SiNx passivant induced, as expected, an increase in 2-D electron gas (2DEG) concentration. In addition, however, the 2DEG mobility increased after passivation for the low-x (0.15) sample, and the more tensile the film stress is, the greater the relative increase. This led to a very highly measured 2DEG mobility of 2360 cm2V-1s-1 at 300 K. In higher x samples, however, mobility was decreased after passivation, increasingly so as x increased, and to a varying extent with different stresses. It is apparent from the results that there is a complex relationship between the stress in the SiNx layer, the mole fraction, and the transport properties of the 2DEG. Thus, tailoring of the passivant deposition conditions, and not simply passivant dielectric choice, to optimize transport properties, is critical for a given passivant, deposition tool, and Al mole fraction, to maximize device performance.