Effective Acceptor Concentration Research Articles

Mg activation anneal of the p-GaN body in trench gate MOSFETs and its effect on channel mobility and threshold voltage stability

This work addresses the impact of the Mg activation anneal step and the resulting acceptor concentration on the channel mobility and VT stability of vertical MOSFETs. Increasing the annealing time with N2 only ambient and the annealing temperature with O2 in the ambient is shown to be effective in increasing the channel acceptor concentration. When the effective acceptor concentration is increased, the mobility is degraded due to a transition in the main scattering mechanism from Coulomb to surface roughness scattering. Degradation of the on-state current and maximum transconductance at high operating temperatures was linked to bulk mobility degradation of the drift layer due to lattice scattering. The two Mg activation annealing conditions considered here show different trends with regard to the threshold voltage stability, while N2 only ambient did not impact this parameter, including O2 increased threshold voltage instability. It is shown that increasing the Mg chemical concentration in the p-GaN layer degrades channel mobility and threshold voltage stability, irrespectively of the effective acceptor concentration, providing evidence for degradation of the channel/dielectric interface characteristics with higher Mg chemical concentration. This study shows that it is possible to achieve very low threshold voltage hysteresis and high channel mobility by reducing the Mg chemical concentration while maintaining high effective acceptor concentration. These results provide key insights for the development of vertical GaN FETs.

Effects of sintering additives on defect chemistry and hydration of BaZr0.4Ce0.4(Y,Yb)0.2O3−δ proton conducting electrolytes

The effects of NiO, ZnO, and CuO sintering additives (0.5, 1.0, or 2.0 wt%) on the sintering behaviour, effective acceptor concentration, and hydration thermodynamics are examined for BaZr0.4Ce0.4Y0.1Yb0.1O3−δ and BaZr0.4Ce0.4Y0.2O3−δ proton conducting electrolytes. Thermogravimetry of hydration shows that the sintering additives – except for 0.5 wt% CuO – lead to a decrease in the effective acceptor concentration, and the decrease per mole sintering additive is the largest for NiO. The absence of typical secondary phases such as BaY2NiO5 and the homogeneous distribution of Ni determined from elemental mapping imply that sintering additives dissolve into the perovskite lattice. Exsolution of metallic Ni and Cu upon reduction leads to a substantial recovery of the effective acceptor concentration. Subsequent oxidation is accompanied by a repeated decrease in the effective acceptor concentration as the sintering additives appear to re-dissolve. Defect chemical reactions are proposed to explain the observed results and these are supported by energetics from first principles calculations. Overall, NiO has the highest positive impact on densification and grain growth, and a relatively small amount of 0.5 wt% NiO or CuO is preferable to optimise both sintering and hydration.

Process engineering of GaN power devices via selective-area p-type doping with ion implantation and ultra-high-pressure annealing

P-type doping in selected areas of gallium nitride (GaN) using magnesium (Mg)-ion implantation and subsequent ultra-high-pressure annealing (UHPA) are investigated to improve the performance of vertical GaN power devices. UHPA allows a high-temperature process without decomposition of the GaN surface and virtually complete activation of the implanted Mg ions in GaN. In the present paper, we provide an overview of recent challenges in making UHPA more realistic as an industrial process. Instead of UHPA at more than 1400 °C for a short duration, prolonged UHPA at 1300 °C demonstrates a comparable acceptor activation of Mg-ion-implanted GaN. This can reduce the annealing pressure to approximately 300 MPa and enlarge the processable wafer diameter. The second challenge is controlling the doping profiles in the lateral and vertical directions. We demonstrate fine patterning of the p-type regions, which indicates the limited lateral diffusion of Mg through UHPA. However, controlling the vertical doping profile is challenging. The nitrogen vacancies formed by ion implantation reduce the effective acceptor concentration near the surface, which can be compensated for by sequential nitrogen ion implantation. Defect-assisted Mg diffusion to the deeper region causes a redistribution of the Mg atoms and should be considered in the design of a device. Such anisotropic diffusion of Mg to the c-axis has potential applications in the fabrication of unique vertical device structures such as super junctions.

Progress on and challenges of p-type formation for GaN power devices

The fabrication processes of p-type regions for vertical GaN power devices are investigated. A p-type body layer in a trench gate metal-oxide-semiconductor field-effect transistor requires precise control of the effective acceptor concentration, which is equal to the difference between the Mg acceptor concentration (Na) and the compensating donor concentration (Nd). The carbon atoms incorporated during growth via metalorganic vapor phase epitaxy substitute nitrogen sites (CN) and function as donor sources in a p-type GaN layer. Since interstitial H atoms (Hi) also compensate holes, their removal from an Mg-doped layer is crucial. Extended anneals to release H atoms cause the formation of extra hole traps. The p+ capping layer allows effective and rapid removal of H atoms from a p-type body layer owing to the electric field across the p+/p– junction. On the other hand, selective area p-type doping via Mg ion implantation is needed to control the electrical field distribution at the device edge. Ultrahigh-pressure annealing (UHPA) under a nitrogen pressure of 1 GPa enables post-implantation annealing up to 1753 K without thermal decomposition. Cathodoluminescence spectra and Hall-effect measurements suggest that the acceptor activation ratio improves dramatically by annealing above 1673 K as compared to annealing at up to 1573 K. High-temperature UHPA also induces Mg atom diffusion. We demonstrate that vacancy diffusion and the introduction of H atoms from the UHPA ambient play a key role in the redistribution of Mg atoms.

Effect of NiO addition on proton uptake of BaZr1-xYxO3-x/2 and BaZr1-xScxO3-x/2 electrolytes

The effect of Ni added in the “solid state reactive sintering” process for Sc- and Y-doped BaZrO3 proton conducting electrolytes is investigated as function of Ni and (Sc, Y) acceptor concentration. Thermogravimetry shows that NiO addition significantly decreases the effective acceptor concentration and thus proton uptake. Samples with substitutional Ni on the Zr site exhibit a distinctly different hydration behavior. Thus the effect of the added Ni is assigned to the withdrawal of Ba from the perovskite lattice with corresponding partial site change of the acceptor dopant, rather than to Ni incorporation into the BaZrO3.

Quantitative investigation of the lateral diffusion of hydrogen in p-type GaN layers having NPN structures

This work quantitatively examined lateral diffusion of hydrogen in the (0001) in-plane direction in a Mg-doped p-type GaN layer sandwiched between n-type layers, based on the preparation and annealing of circular mesa structures having different radius values. Capacitance–voltage analyses and secondary ion mass spectrometry confirmed a decrease in the effective acceptor concentration along with an increase in the residual hydrogen concentration with increasing radius values, indicating hydrogen desorption through the sidewall and lateral diffusion. Considering the surface barrier to desorption, we estimated a lateral diffusion coefficient of 7 × 10−8 cm2 s−1 at 1123 K, reasonably reproducing the effect of the mesa radius.

Tuning of transport properties of the double-step chemical bath deposition grown zinc oxide (ZnO) nanowires by controlled annealing: An approach to generate p-type ZnO nanowires

ZnO nanowires/p-Si heterojunction diodes are fabricated by employing low temperature double-step chemical bath deposition technique. The grown samples are initially annealed at different temperatures in the range of 300 °C to 600 °C at 10 psi argon atmosphere for 30 min. Field emission scanning electron microscope images confirmed the growth of vertically oriented ZnO nanowires with average nanowire diameter and height of 183–190 nm and ~1.4 μm, respectively. The x-ray diffraction study shows that the samples are poly-crystalline with hexagonal wurtzite structure and ZnO nanowires are vertically c-axis oriented. The photoluminescence study indicates the presence of oxygen vacancy in as-grown and 300 °C annealed sample, whereas, the 500 °C annealed ZnO nanowires show the existence of oxygen interstitials. Heterojunction diodes are fabricated on the as-grown, 300 °C, 400 °C, 450 °C, 460 °C, 475 °C, 490 °C, 500 °C, 550 °C and 600 °C annealed nanowires for the measurements of current-voltage and capacitance-voltage characteristics. Both the results indicate an electron dominated transport for the as-grown to 490 °C samples and hole transport for ≥500 °C annealed sample. The acceptor or p-type dopant formation temperature is observed to be 500 °C, which shows an effective acceptor concentration of 1.15 × 1015 cm−3 (at 10 kHz) and 1.74 × 1015 cm−3 (at 1 MHz). Thus the work indicates a possible route for converting the n-type ZnO nanowires to p-type of nature.

Influence of Grain Size on Impedance Spectra and Resistance Degradation Behavior in Acceptor (Mg)‐Doped BaTiO3Ceramics

The effect of acceptor concentration in (Mg)‐doped BaTiO3on the resistance degradation behavior was contrasted between coarse‐ and fine‐grain samples with ∼90 and ∼0.8 μm in size, respectively. Both cases showed similar trends that the time to degradation decreased systematically with the increase of acceptor concentration, however the absolute rates are very different for a given dopant concentration. Although the grain size and grain boundaries are important, it is also shown through an impedance analysis that the degradation behavior depends on the grain conductivity (σg) and ionic transference number (tion) as evaluated by the Warburg impedance. Under the condition of the same nominal acceptor concentration, fine‐grain samples showed much lower grain conductivity (σg) but little differences in the grain boundary conductivity (σgb) than that of coarse‐grain samples. The ionic transference number (tion) was also much smaller in fine‐grain samples. These results show that the actual effective acceptor concentration on being ionically compensated to enhance oxygen vacancies becomes smaller with the decrease of grain size. This fact coupled to the increase of the number of grain boundaries accounts for the improved degradation behavior with the decrease of grain size.

Analysis of degradation mechanism of GaN blue light emitting diode by the characteristics of capacitance and conductance

A study of the capacitance-voltage characteristics of GaN light emitting diode（LED） with a view to reveal its degradation mechanism is presented in this article by forward alternating current small-signal method. Combined with series resistance， ideality factor and tunneling current measurement， the properties of negative capacitance and conductance are discussed. The threshold voltage of negative capacitance during degradation is qualitatively analysed. The decreased negative capacitance during aging may be due to the decrease in the effective acceptor concentration and radiative recombination rate， and the increase of defects and non-radiative recombination centers resulting in the enhanced capture effect of carriers. The tunneling effect leads to the obvious increase of conductance under reversed and low-forward bias voltage. The decreased conductance during aging may be due to the series resistance. On the basis of the capacitance-voltage characteristic， light output， current-voltage curves with the degradation mechanism of LED， the characteristics of negative capacitance and conductance can be used as important evidence for the degradation analysis of LED as proved by experiment and theory.

Reversible Degradation of Ohmic Contacts on p-GaN for Application in High-Brightness LEDs

This paper analyzes the high-temperature long-term stability of ohmic contacts on p-type gallium nitride (p-GaN). The contributions of the ohmic contacts and semiconductor material degradation are separated by adopting the transmission line method (TLM). Before stress, the current-voltage (I-V) curves measured at the pads of the TLMs showed a linear shape, indicating a good ohmic behavior of the contacts. Thermal treatment at 250degC was found to induce the worsening of the electrical characteristics of the contacts: identified degradation modes consist of a shift of the I-V curves toward higher voltages and strong nonlinearity of the characteristics around zero. This paper shows that the high-temperature instabilities of ohmic contacts on p-GaN are related to the interaction between the device surface and the plasma-enhanced chemical vapor deposition SiN passivation layer. Hydrogen contained in the passivation layer is supposed to play an important role in the degradation process: the interaction with the acceptor dopant at the metal/semiconductor interface induces the decrease of the effective acceptor concentration. As a consequence, both the ohmic contact characteristics and the semiconductor sheet resistance are worsened.

High-Temperature Annealing Behavior of p-Type Doping Characteristics in Mg-Doped GaN

We report on the high-temperature annealing behavior of p-type doping characteristics in Mg-doped GaN grown by metallorganic chemical-vapor deposition. After activation of Mg dopants by a first 700°C anneal, the second annealing was sequentially performed with encapsulation layers at temperatures between 1000 and 1300°C. From room-temperature capacitance-voltage measurements, the effective acceptor concentration is seen to increase monotonically with the rise of the annealing temperature up to 1200°C. In addition, both thermal admittance and current deep-level transient spectroscopic measurements show a gradual decrease in depth of the acceptor levels from 136 to 117 meV with an increase of annealing temperature up to 1200°C, in which the diffusion of Mg atoms cannot be observed by secondary ion mass spectrometry measurements. Therefore, the enhanced p-type doping characteristics that we observed turn out to be thermally stable even after the high-temperature annealing process. © 2004 The Electrochemical Society. All rights reserved.

Effects of KrF excimer laser irradiation on metal contacts to n-type and p-type GaN

Electrical properties of metal contacts on laser-irradiated n-type and p-type GaN surfaces were investigated using current–voltage, capacitance–voltage, and synchrotron radiation photoemission spectroscopy. After the irradiation of a KrF excimer laser pulse (600 mJ/cm2 at 248 nm for 38 ns) onto Si-doped GaN, a nonalloyed Ti/Al metallization formed an ohmic contact with the specific contact resistivity of 1.7×10−6 Ω cm2. The laser irradiation decomposed GaN into metallic Ga and nitrogen gas. The decomposed metallic Ga reacted with oxygen in air to form a Ga oxide layer with the thickness of ∼40 Å, producing a large number of N vacancies near the surface. The formation of a degenerated n-type GaN layer resulted in the low contact resistivity. For Mg-doped GaN, the laser irradiation increased the effective acceptor concentration. Simultaneously, the activation efficiency of Mg dopants was enhanced by the photon-assisted breaking of Mg–H bonds and/or the removal of hydrogen atoms in the presence of oxygen, producing the p-type GaN with an increased hole concentration. As a result, the contact resistivity of an oxidized Ni/Au contact could be reduced from 1.3×10−3 to 3.6×10−4 Ω cm2.

Electrical Properties of Acceptor Levels in Mg‐Doped GaN

Thermal admittance and current deep-level transient spectroscopy techniques have been applied to Schottky diodes fabricated on Mg-doped GaN grown by metalorganic chemical vapour deposition to investigate the dependence of the Mg acceptor levels on the annealing temperature. Both measurements revealed two deep acceptor levels with activation energies of ∼135 and ∼160 meV above the valence band. The former level is only seen when the samples were annealed at temperatures between 650 °C and 700 °C, and its presence corresponds with a significant increase in effective acceptor concentration, as confirmed by capacitance–voltage measurements. Therefore, this acceptor level is considered to dominate the electrical activation of Mg in GaN.

Electrical characterization of acceptor levels in Mg-doped GaN

Thermal admittance and current deep-level transient spectroscopy techniques have been applied to Schottky diodes fabricated on Mg-doped GaN grown by metalorganic chemical vapor deposition to investigate the dependence of the Mg acceptor levels on the annealing temperature. Both measurement techniques revealed two deep acceptor levels with activation energies at ∼135 and ∼160 meV above the valence band. The former level was only seen when the samples were annealed at temperatures between 650 and 700 °C, and its presence corresponds with a significant increase in effective acceptor concentration, as confirmed by capacitance–voltage measurements. Therefore, this acceptor level is considered to dominate the electrical activation of Mg in GaN.

On the mechanism of time-dependent spectral diffusion in binary solid solutions: The 1,4-dibromonaphthalene and 1-bromo-4-chloronaphthalene system

The mechanism of the spectral diffusion process has been studied in the presence of both orientational and substitutional disorder with the help of a simple model calculation. The systems of study are different compositions of solid solutions of 1-bromo-4-chloronaphthalene and 1,4-dibromonaphthalene. The isotropic exchange mechanism for different dimensions are considered in detail. The calculation points to a two-dimensional excitation exchange interaction mechanism. Furthermore, the switching of spectral diffusion occurs at a critical effective acceptor concentration.

Unified charge control model and subthreshold current in heterostructure field-effect transistors

A unified analytical charge control model covering the entire range of gate voltages from below and above threshold is developed for heterojunction field-effect transistors (HFETs). This model is based on a new interpretation of the quantized energy levels for the two-dimensional electron gas. It reduces to a classical charge sheet model in the limit of low surface field. The model is used to interpret the experimental data for the subthreshold regime of HFETs. The results indicate wide range variation of the effective acceptor concentration after device fabrication processing in the unintentionally doped GaAs buffer layer.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>