Thermal Admittance Spectroscopy Research Articles

Non-metastable recombination induced reactions involving hydrogen in SiC

ABSTRACTA brief survey of some of the recent results of recombination-induced defect reactions involving hydrogen in 4H and 6H-SiC is given. A variety of outcomes of such reactions have been observed in hydrogenated 4H and 6H-SiC polytypes under optical excitation at reduced temperature. A few different non-metastable hydrogen-defect complexes can form, including hydrogen complexes with Al and B acceptors, hydrogen complex with Si vacancy, as well as some other non-identified complexes. Electrical measurements indicated strong recombination-induced passivation of the electrical activity of aluminum and boron acceptors in SiC. This passivation resulted in the reduction of the net free hole concentration and even inversion of the conductivity type. It is suggested that optical excitation causes a long-range migration of hydrogen followed by its capture in one or another kind of defect complexes. Further insight in the formation of specific complexes as a result of recombination-induced defect reactions is provided by thermal admittance spectroscopy. Energy gap levels in the regions that remained p-type after hydrogenation as well as in the regions where the conductivity was inverted by the recombination-induced passivation are investigated.

Deep Defects in Fe-Doped GaN Layers Analysed by Electrical and Photoelectrical Spectroscopic Methods

ABSTRACTThe electrical activity of iron in Fe- doped, and in Si and Mg co-doped GaN layers grown on sapphire substrates by metal organic vapor phase epitaxy was studied as shown by temperature dependent Hall Effect (TDH) measurements. In all samples iron doping generates an acceptor defect, which compensates donors in n-type GaN. Furthermore, iron doping causes strong potential inhomogeneities, which decrease the Hall mobility in the layers. To verify, if iron creates only hole traps, defects in n-type Si:Fe and Fe doped samples were investigated. The well known dominant electron traps in n-type GaN at 520 – 550 meV and 480 meV were found by deep level transient spectroscopy and thermal admittance spectroscopy, respectively. A high Fe-doped GaN layer shows a low p-type conductivity dominated by the iron acceptor. An activation energy of EV+ 460 meV was determined by TDH indicating, that the iron acceptor correlates with this defect level.

Electrical Characterization of InGaN/GaN Multiple‐Quantum‐Well Structures by Thermal Admittance Spectroscopy

Experimental results of electrical characterization of InGaN/GaN multiple-quantum-well electroluminescence test structures obtained by thermal admittance spectroscopy are presented. The studied GaN : Mg/5 × (InGaN/GaN)/GaN:Si structures were grown on c-plane sapphire substrate by metal-organic vapor phase epitaxy. Admittance measurements were conducted from room temperature down to 125 K for a wide frequency range and for different applied bias voltages. Analysis of the capacitance versus frequency curves shows the presence of several cutoff frequencies which originate from the response of equivalent RC series circuits and give peaks in the conductance divided by angular frequency. The dependence of the position and the amplitude of these peaks on temperature is discussed.

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 Be-implanted GaN

We have investigated electrically the acceptor levels that are present in Be-implanted GaN. Slight p-type conductivity was attained in undoped GaN films by Be implantation and subsequent annealing at 1050 °C with a SiO2 encapsulation layer. Capacitance-frequency measurements showed a typical dispersion effect characteristic of deep acceptors in fabricated Schottky diodes. Thermal admittance spectroscopy measurements revealed a discrete deep level located at ∼231 meV above the valence band. This energy level is in reasonable agreement with the frequency dependence of the capacitance in view of the impurity transition frequency. Therefore, this energy level can most probably be assigned to a Be-related deep acceptor.

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.

Characterization of SiC Implanted with B or Al Using Thermal Admittance Spectroscopy

AbstractWe have measured the thermal activation energies of electrically active defects in 4H- and 6H-SiC implanted with either Al or B ions, using Thermal Admittance Spectroscopy. The net acceptor concentrations were monitored using room temperature low frequency Capacitance-Voltage measurements. The substrates were n/n+ epilayers. The implantations plus annealing produced p-type layers that were acceptable for characterization. The specimens were annealed in Ar at 1600 ° C, after which Ni Schottky diodes were fabricated on the specimens. Annealing times were 5, 15, 30, and 60 min. In some of the specimens, a shallow level was found that did not correspond to known levels. As the annealing progressed, energy shifts were noted for some of the detected levels. In some specimens, the implanted p-type impurity and the n-type residual dopants in the substrate were simultaneously detected. Measurements of electrically active ptype species were compared to “control” specimens implanted with Ar. From this comparison, we conclude that at least one shallow donor level is introduced into the bandgap by the implantation process, and is not annealed out. The defects associated with the implantation may affect actual device performance of diodes by destabilizing the lattice occupation of the implanted dopant atoms (energy shift with annealing), and act as lifetime killers.

Electrical characterization of magnesium implanted gallium nitride

Gallium nitride layers grown by molecular beam epitaxy on c axis oriented sapphire substrates were implanted with 180 keV magnesium ions with ion doses between 1×1014 and 1×1016 cm−2. The implantation induced defect states were investigated by temperature dependent conductivity (TDC) as well as by thermal and optical admittance spectroscopy (TAS, OAS) measurements. Dominant carrier emissions having thermal activation energies between 360 and 800 meV were found in TAS and TDC. These states are assigned to implantation induced electron traps since they do not appear in the nonimplanted reference sample. Defect states with similar transition energies were also observed in OAS resulting in an enhancement of defect-to-band transitions in the near band-gap region around 3.45 eV, in the blue band around 3.0 eV, as well as in the midgap range for photon energies between 2.5 and 1.80 eV, respectively. In addition, new transitions were found at 2.1 and 1.95 eV. Furthermore, transitions from implantation induced shallow states were observed, i.e., the magnesium acceptor as well as a new donor level at about 70 meV, tentatively discussed as nitrogen vacancy. The critical ion dose for amorphization was determined to be between 5×1015 and 1×1016 Mg+ cm−2 using x-ray diffraction.

Investigation of Defect Levels in Mg-Doped GaN Schottky Structures by Thermal Admittance Spectroscopy

Schottky structures based on Mg-doped GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrate are studied by thermal admittance spectroscopy from 90 K to room temperature. Evidence of two impurity levels results from the analysis of the observed peaks in the conductance curves, whose positions and strengths are temperature dependent. The experimental results are analyzed within a detailed theoretical study of the steady-state and small-signal electrical characteristics of the structure. 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.

Investigation of Defect Levels in Mg-Doped GaN Schottky Structures by Thermal Admittance Spectroscopy

Schottky structures based on Mg-doped GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrate are studied by thermal admittance spectroscopy from 90 K to room temperature. Evidence of two impurity levels results from the analysis of the observed peaks in the conductance curves, whose positions and strengths are temperature dependent. The experimental results are analyzed within a detailed theoretical study of the steady-state and small-signal electrical characteristics of the structure. 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.

Current deep-level transient spectroscopy investigation of acceptor levels in Mg-doped GaN

The current deep-level transient spectroscopy (I-DLTS) technique was used to investigate acceptor levels in Mg-doped GaN grown by metalorganic vapor phase epitaxy. For activation of the Mg dopants, rapid thermal annealing was performed with a SiO2 encapsulation layer at 850 °C in N2. I-DLTS measurements on the Schottky diode fabricated revealed a discrete deep level located ∼112 meV above the valence band, corresponding to the energy level measured by conventional thermal admittance spectroscopy. This energy level is also in good agreement with the frequency dependence of the capacitance in view of the characteristic frequency. Therefore, this energy level can most probably be attributed to the Mg acceptor state itself.

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.

Effect of C and B sequential implantation on the B acceptors in 4H–SiC

We have systematically investigated the effect of C and B sequential coimplantation on B-related acceptors and deep levels in 4H–SiC using thermal admittance spectroscopy. By increasing the concentration of coimplanted C, the density of deep levels decreased and was completely suppressed for a C and B ratio of 1:1. Moreover, the density and ionization energy of B acceptors increased and decreased, respectively, with increasing C concentration. However, we found that excess C content leads to the formation of a complex defect. Capacitance–voltage results also support the expected increase in the free hole concentration with increasing concentration of the coimplanted C atoms, which is followed by a decrease in the concentration under C-rich conditions. This is in reasonable agreement with the behavior of the B acceptors and deep defect levels. Therefore, the concentration of coimplanted C atoms is considered to be very sensitive to the formation of the B acceptor levels.

Detailed deep trap analysis in Mg-doped p-type GaN layers grown by MOVPE

p-Type Mg-doped GaN layers grown by metal organic vapor phase epitaxy on undoped GaN buffer layers on (0001) sapphire substrates were investigated by thermal admittance spectroscopy, temperature dependence conductivity and thermally stimulated currents. A detailed trap characterization in the p-type GaN:Mg as well as in the undoped buffer layers shows that the TAS peaks observed from the p-type samples are caused by Mg-defects. Furthermore, the undoped buffer layer mainly dominates the optical induced deep defect-to-band transitions as measured by photoelectrical optical admittance spectroscopy. An increasing Mg-doping level introduces an enhancement of a deep defect signal at about E G-(280–360) meV. These results show the complex nature of the Mg defect and the introduction of further deep defects by the doping process.

Semiconductor impurity parameter determination from Schottky junction thermal admittance spectroscopy

The problem of interpretation of thermal admittance spectroscopy data for semiconductor impurity parameter extraction is considered. Traditional analysis predicts that the Arrhenius plot for conductance peak temperatures is a straight line with the slope proportional to impurity activation energy and the intercept determining its capture cross section. Using a general model of the Schottky junction admittance we show that conductance peak positions strongly depend on the impurity bulk occupation number and potential distribution in the space-charge region, and, as a result, the Arrhenius plot is nonlinear for some semiconductor parameters and experimental conditions, in particular for relatively shallow impurities. In this case, the traditional linear approximation of the Arrhenius plot yields inaccurate values of activation energy and capture cross section. We propose a more accurate procedure for admittance spectroscopy data analysis involving least-squares fitting using the general and the small-signal models of the junction admittance. Although much more computationally intensive, the general model is shown to provide a better fit to the data at low temperatures, where the small-signal approximation is invalid. This approach is applied for an example admittance data and yields a better fit of the theoretical curve to the data and an improved value of activation energy for the nitrogen donor in 6H-SiC.

Admittance spectroscopy of efficient CuInS2 thin film solar cells

Thermal admittance spectroscopy is applied to characterize the trap properties of high-efficiency CuInS2/CdS/ZnO thin film solar cells. To that aim, the capacitance spectrum is examined in detail. Nontrap capacitance contributions like freeze-out of free carriers and series resistance are discussed. The dependence of the resistance on the sample area is used to identify its physical origin. Simulation results of a transmission line model are in good agreement with the spreading resistance of the ZnO window layer. Defect spectra, i.e., the distribution of the deep trapping states in the band gap, are extracted from the admittance spectra, using a method established by T. Walter, R. Herberholz, C. Müller, and H. W. Schock [J. Appl. Phys, 80, 4411 (1996)]. Arrhenius data of the traps are drawn best from the spectrum of conductance versus temperature. The defect spectra do not depend on the buffer layer. Generally, they show a high constant background trap density with some broad peaks. Two new trap levels at 0.3 and 0.5 eV are found, confirmed by deep level transient spectroscopy and identified as majority carrier traps in the bulk of the p-type CuInS2 absorber. An increasing density of the trap at 0.5 eV correlates with a decreasing open-circuit voltage.

Effect of C/B sequential implantation on the B acceptors in 4H–SiC

Boron (B) is one of the promising group-IIIA acceptor impurities for SiC and forms a shallow acceptor level (285–390 meV) and deep level (540–720 meV). The B-related deep defect can lead to the degradation of a devices electrical characteristics over the long terms, therefore, a suitable process is needed to suppress the B-related deep defect level, known as the D-center. The C/B sequential implantation technique, a site-competition effect, was applied to suppress the formation of the D-center. The thermal admittance spectroscopy was used to detect the electrically active shallow and deep defect levels introduced by the C/B sequential implantation of 4H–SiC. The C/B sequential implantation was found to be effective in suppressing the formation of the D-center. However, the concentration of the co-implanted C-atoms is very sensitive as the increasing C content leads to the decrease in the active hole concentration by quenching the shallow acceptor level due to the formation of a deep defect level at about 428 meV. A complex microstructure in which C-atom bonds with the B atom at the Si-lattice site was proposed to account for the experimentally observed new B-related deep defect level.

GaN/SiC heterojunctions grown by LP–CVD

Heterojunction bipolar transistors (HJBT) on the basis of GaN/SiC heterostructures have several advantages over group III nitride heterostructures grown on sapphire. For example 6H–SiC has less of a thermal and structural mismatch to GaN than sapphire has. Furthermore there is not the problem of optical recombination in a highly doped base region as there is for the group III nitrides on account of their direct bandgap. However, despite the well acknowledged potential of the GaN/SiC material system there are still unanswered questions relating to the technology used as well as to some fundamental properties of GaN/SiC heterostructures. Therefore we investigated epitaxial growth and physical properties of n-GaN/ p-SiC heterojunctions with respect to their significance to n-GaN/ p-SiC/ n-SiC HJBT. We grew n-type GaN ( n=10 18 cm −3) on p-type ( p=2×10 18 cm −3) and n-type ( n=4×10 18 cm −3) 6H–SiC substrates in a horizontal hot wall reactor. This approach is very similar to the more common HVPE. Instead of synthesizing GaCl in situ from HCl and metallic Ga we used GaCl 3 as the Ga precursor. All our experiments were carried out at low pressures around 1 mbar resulting in a good homogeneity. As it is common for the more usual HVPE we grew GaN without a buffer layer. From thermal admittance spectroscopy (TAS) as well as temperature dependent I– V characteristics we gained knowledge about deep level defects and the role of interface traps. The microstructure of the interface was investigated by transmission electron microscopy (TEM). Furthermore we present details about device processing by ion beam sputter etching with carbon dioxide (CO 2) as the working gas.

Electrothermal analysis of SiC power devices using physically-based device simulation

A comprehensive electrothermal transport model is presented which covers, on the continuous-field level, several physical effects particularly relevant to SiC devices. The effect of anisotropic transport properties and impurity kinetics on the device performance is demonstrated. Based on deep level transient spectroscopy and thermal admittance spectroscopy measurements, the impact on the stationary and transient behavior of SiC devices is evaluated.

Fermi Level Pinning at GaN-interfaces: Correlation of electrical admittance and transient spectroscopy

In GaN layers grown by molecular beam epitaxy as well as metal organic vapor phase epitaxy significant differences were found in the appearance of deep defects detected by thermal admittance spectroscopy as compared for deep level transient spectroscopy measurements. While, thermal admittance spectroscopy measurements which were made under zero bias conditions only show thermal emissions at activation energies between 130 and 170 meV, further deep levels existing in these GaN layers were evidenced by transient spectrocopy. This discrepancy is explained by a pinning effect of the Fermi level at the metal / GaN interface induced by high a concentration of the deep levels showing up in thermal admittance spectroscopy. We compare our results with a GaAs:Te Schottky- diode as a refernec sample. Here, both spectroscopic methods give exactly the same deep level emissions.