Marianne Germain

Thermal admittance spectroscopy of Mg-doped GaN Schottky diodes

Thermal admittance spectroscopy measurements at temperatures ranging from room temperature to 90 K are performed on Schottky structures based on Mg-doped GaN layers grown by metalorganic vapor phase epitaxy on sapphire. The analysis of the experimental data is made by a detailed theoretical study of the steady-state and small-signal electrical characteristics of the structures. Numerical simulations are based on the solution of the basic semiconductor equations for the structure consisting of two Schottky diodes connected back to back by a conduction channel formed by the GaN layer. The description explicitly includes the Mg-related acceptor level, with its temperature- and position-dependent incomplete occupation state, leading to a dynamic exchange with the valence band. It fully reproduces the variations with temperature of the capacitance–frequency and conductance over frequency curves, allowing to give for all temperature ranges the origin of the various contributions to the junction capacitance and of the microscopic mechanisms responsible for the capacitance–frequency cutoff. Series resistance effects are shown to be dominant at temperatures above 230 K, whereas the Mg-related acceptor level governs the electrical behavior below 230 K. The existence of a second acceptor level with an activation energy of several tens of meV is revealed from the analysis of the characteristics at low temperature. An optimized fitting procedure based on the comparison of the electrical characteristics obtained from the numerical simulations to the experimental data allows one to determine the microscopic parameters describing the structure, among which the acceptor activation energies, thermal capture cross sections, concentrations, and the Schottky contact barrier heights are the most important ones. The obtained activation energy of the Mg-acceptor level of 210 meV is by a factor of 2 larger than that obtained from a classical Arrhenius plot, showing that a complete description of Mg-doped GaN junctions requires the correct treatment of the Mg level, acting as a dopant and as deep impurity, as well as the inclusion of series resistance effects.

1900 V, 1.6 mΩ cm2 AlN/GaN-on-Si power devices realized by local substrate removal

We demonstrate a high-voltage low on-resistance AlN/GaN/AlGaN double heterostructure grown by metal–organic chemical vapor deposition on a silicon (111) substrate using a total buffer thickness of less than 2 µm. A fully scalable local substrate removal technique was developed to dramatically enhance the off-state breakdown voltage of the transistors. The three-terminal breakdown voltage of these devices using a gate–drain distance of 15 µm increased significantly, from 750 V to 1.9 kV, after local substrate removal. The high two-dimensional electron gas carrier density (2.3 × 1013 cm−2) associated with the low sheet resistance enables a record combination of a specific on-resistance (1.6 mΩ cm2) and high breakdown voltage for GaN-on-Si transistors.

Competition between deep impurity and dopant behavior of Mg in GaN Schottky diodes

The effect of the deep acceptor Mg on the electrical characteristics of p-doped GaN Schottky diodes is analyzed. The theoretical study is based on the numerical resolution of the basic semiconductor equations, including the continuity equation for the Mg-related acceptor level. It gives the steady-state and small-signal analysis of p-doped GaN:Mg Schottky diodes, yielding as final result the frequency dependent capacitance and conductance of the structure. It is shown that the low-frequency characteristics are determined by the carrier exchange between the Mg related impurity level and the valence band, whereas above the impurity transition frequency, the hole modulation of the depletion layer edge governs the electrical response. Detailed results are shown on the effect of temperature, applied steady-state voltage and series resistance. The study of two back-to-back connected GaN Schottky diodes reveals the appearance of typical features in the electrical characteristics, depending on the respective Scho...

Indium-Rich InGaN Films Grown on Ge Substrateby Plasma-Assisted Molecular Beam Epitaxy for Solar Water Splitting

Indium-rich InGaN films were grown on Ge(111) substrate by plasma-assisted molecular beam epitaxy (PAMBE). The influence of the indium flux on the structural properties, surface morphology, and photocurrent for water splitting has been investigated. Before the InGaN growth, 20xa0nm of GaN was deposited as a buffer layer. A streaky reflection high-energy electron diffraction (RHEED) pattern was observed for the GaN buffer growth. At the onset of InGaN growth, the streaks became spotty, indicating roughening of the surface and three-dimensional (3D) growth due to the lattice mismatch between GaN and InN. The indium composition in the InGaN structure was roughly fit to be around 50% from x-ray diffraction (XRD) ω–2θ measurements. Growth with excess indium supply led to segregation of metal indium on the surface. During cooling down, this metal indium transformed partially into InN. The crystal quality of the InGaN film decreased with increase of the indium flux. The photocurrent of the InGaN films used as photoelectrodes for water splitting also decreased with increase of the indium flux.

Optical and electrical properties of MOVPE-grown ZnSe:N using triallylamine as a nitrogen precursor

Photoluminescence (PL), C-V and I-V characteristics of MOVPE-grown ZnSe :N doped using triallylamine (TAN) have been investigated. In our experiments, the use of TAN as dopant source does not lead to the incorporation of nitrogen into the lattice at the VI/II ratio optimized for the growth of undoped ZnSe. At smaller VI/II ratios a high concentration of donor states (10 17 -10 18 cm -3 ) appears leading to the formation of a broad PL band at 2.791-2.792 eV caused by a transition from the states formed by an overlap of the conduction band and donor states to the valence band. The acceptor states with an activation energy of 80 meV are most likely formed by oxygen contamination of TAN.

Correlation of carbon doping variations with the vertical breakdown of GaN-on-Si for power electronics

Abstract In this work gallium nitride (GaN) grown on silicon substrates was investigated in order to determine critical defects responsible for differences in the vertical breakdown of HEMT structures. Cathodoluminescence studies at the SEM revealed a direct correlation between the intensity of the blue luminescence (BL) band and the carbon doping concentration. Observing this, carbon depletion zones were found around threading dislocations in the active GaN layer, as well as a deep depletion in growth columns concluded from a reduction of the BL intensity. Using the given results a model of a defect, here called the deep carbon depletion (DCD), is proposed to explain the correlation between carbon variations and the vertical breakdown.

Efficiency Enhancement of Harmonic-Tuned GaN Power Amplifier Using Doherty like Load Modulation

In this paper, the Doherty like load modulation has been implemented in second-harmonic-tuned gallium nitride (GaN) power amplifiers (PAs) to further improve their performance. Both harmonic tuning and load modulation are based on an accurate large-signal model extracted for the GaN devices. Two devices of identical size are used. In previous research works, the use of load modulation on two identical size devices always results in decreases in output power, gain, or even linearity, although improvements in efficiency are shown. Besides that, the influence of the offset line on linearity is not covered. In this experiment, by optimizing the load modulation and properly designing the offset line, a power added efficiency (PAE) of 65%, a linear gain of 16 dB, and an output power of 40.5 dBm have been achieved at 2 GHz. Compared with those of a balanced power amplifier, the load modulated PA shows superior power performance both in efficiency (more than 10% improvement in PAE) and in linearity. Moreover, its output power and gain maintain as high as that of the balanced PA.

Measurements of transient photocapacitance and photocurrent on MOVPE-grown Au/ZnSe/GaAs heterostructures

The results of transient photocapacitance and photocurrent measurements performed on MOVPE-grown Au/ZnSe/GaAs heterostructures are reported for different photon energies and intensities of the monochromatic radiation. Thresholds in both spectra are observed at 1.4 eV. We attribute the photocapacitance and the photocurrent to the generation of electron-hole pairs in GaAs followed by trapping of the holes on interface states at the heterojunction between ZnSe and GaAs. In our interpretation, the stationary photocurrent is due to recombination of the trapped holes with electrons of the ZnSe conduction band.

Electrical characterization of doped ZnSe-based heterostructures grown by MOVPE

The electrical characterization of Cl- or N-doped layers in the ZnMgSSe material system is reported. We used biscyclopentadienylmagnesium, dimethylzinc-triethylamine, tertiary-butylthiol, diisopropylselenide and ditertiarybutylselenide to grow layers by metalorganic vapor phase epitaxy at various low growth temperatures (330-440°C). Capacitance-voltage profiling, current-voltage, Hall, and transient photocapacitance measurements in conjunction with photoluminescence spectra were used to verify the layer properties. With intentional chlorine doping using 1-chlorobutane, free electron concentrations of up to 2 X 10 18 cm -3 in ZnSe and 1 X 10 17 cm -3 in ZnS x Se 1 -x (x= 4%) have been achieved. All C-V measurements on ZnSe:N doped with bistrimethylsilylamidozinc, trimethylsilylazide, or triallylamine show n-type or semi-insulating behavior, although PL spectra show acceptor-bound excitons and DAP recombinations. Compensation due to incorporated hydrogen originating from the precursor is assumed, besides a background chlorine contamination. However, Hall measurements of several samples doped with BTM and TAN indicate p-type conduction. Maximum hole concentrations of 5 X 10 17 cm -3 and Hall mobilities of 30 cm 2 /V. s were measured. Transient photocapacitance measurements confirm the presence of traps with very long time response, probably at the Au/ZnSe interface.

Investigations of Critical Structural Defects in Active Layers of GaN-on-Si for Power Electronic Devices

The influence of structural defects in the active layer of GaN-on-Si substrates on the vertical leakage current was studied. The structural defects were analyzed by analytical scanning electron microscopy by means of cathodoluminescence (CL). The leakage current was determined by vertical I-V measurements.Two possibilities were found, which give potential explanations for the variations of the vertical leakage current: i) Threading dislocations, which may partially form leakage paths, were detected by CL imaging. ii) Variations of the carbon doping, which is used to tune GaN to a semi insulating material were revealed by CL spectroscopy.