Net Acceptor Concentration Research Articles

Simulation analyses of carrier dynamics in npn-type GaN-heterojunction bipolar transistors with different hole-concentration p-base layers

In this study, the operation of npn-type GaN-based heterojunction bipolar transistors with different net acceptor concentrations in p-base regions was simulated. It was confirmed that there is a critical net acceptor concentration (NA-ND) depending on the thickness of the base region and that if the NA-ND is lower than the critical value, the collector current may anomalously increase, regardless of base current injection. This phenomenon is caused by the punch-through process via the depletion layer extending from the collector–base junction. The effect of the valence band energy offset at the emitter-base heterojunction (ΔEV) on the current gain (β) was also investigated, and the results showed that β peaks when ΔEV is 0.22–0.30 eV. This is determined by the balance between the hot-electron injection and thermal diffusion processes in the electron transport from the emitter to the base.

Study of Magnesium Activation Effect on Pinch-Off Voltage of Normally-Off p-GaN HEMTs for Power Applications

The role of the magnesium (Mg) doping and its electrical activation on the off-state of p-GaN/AlGaN/GaN HEMTs has been investigated in this work. Firstly, the effect of different Mg doping profiles has been studied via the help of Technology Computer-Aided Design (TCAD) simulations, with the objective of analyzing the band diagrams of the structure. Then, it has been shown how experimental Capacitance–Voltage measurements can be useful to obtain information on the net acceptor concentration in the p-GaN. As a result, devices with an undoped (magnesium-free) GaN gate have been experimentally compared to devices whose p-GaN gate has been activated via a reference annealing process. Finally, results on a device characterized by an improved p-GaN activation have been presented and compared, showing improvements on several parameters of both off- and on-state, thus underlining the key role of the Mg activation process in the overall performances of normally-off GaN HEMTs.

Nonradiative Recombination Dominates Voltage Losses in Cu(In,Ga)Se2 Solar Cells Fabricated using Different Methods

Voltage losses reduce the photovoltaic conversion efficiency of thin‐film solar cells and are a primary efficiency limitation in Cu(In,Ga)Se2. Herein, voltage loss analysis of Cu(In,Ga)Se2 solar cells fabricated at three institutions with variation in process, bandgap, absorber structure, postdeposition treatment (PDT), and efficiency is presented. Nonradiative voltage losses due to Shockley–Read–Hall charge carrier recombination dominate and constitute >75% of the total compared to <25% from radiative voltage losses. The radiative voltage loss results from nonideal absorption and carriers in band tails that stem from local composition‐driven potential fluctuations. It is shown that significant bulk lifetime improvements are achieved for all alkali PDT processed absorbers, chiefly associated with reductions in nonradiative recombination. Primary voltage loss contributions (radiative and nonradiative) change little across fabrication processes, but variation in submechanisms (bulk lifetime, net acceptor concentration, and interface recombination) differentiate nonradiative loss pathways in this series of solar cells.

Large critical field of Li-doped NiO investigated by p+-NiO/n+-Ga2O3 heterojunction diodes

Critical electric fields (E C) of lithium-doped p+-nickel oxide (NiO) were investigated by the capacitance (C)–voltage (V) and current (I)–V measurements using p+-NiO/n+-gallium oxide (Ga2O3) heterojunction diodes. The E C was estimated by device simulations using the net acceptor concentrations (N A) obtained from C–V measurements and breakdown voltages obtained from reverse I–V characteristics. The E C of NiO depended on the N A of the NiO and ranged from 5.4 to 10.1 MV cm−1. Large E C was obtained for high N A. NiO was confirmed to be one of the promising p-type oxides to realize high-power p-n heterojunction devices with Ga2O3 due to the high E C.

Tuning the p-type doping of GaN over three orders of magnitude via efficient Mg doping during halide vapor phase epitaxy

The precise control of Mg concentration ([Mg]) in p-type GaN layers from 2.3 × 1016 to 2.0 × 1019 cm−3 was demonstrated by halide vapor phase epitaxy (HVPE) on n-type GaN (0001) freestanding substrates. [Mg] in GaN layers could be controlled well by varying the input partial pressure of MgCl2 formed by a chemical reaction between MgO solid and HCl gas under the thermodynamic equilibrium condition. In the sample with [Mg] of 2.0 × 1019 cm−3, a step-bunched surface was observed because the surface migration of Ga adatoms was enhanced by the surfactant effect of Mg atoms. The samples show high structural qualities determined from x-ray rocking curve measurements. The acceptor concentration was in good agreement with [Mg], indicating that almost all Mg atoms act as acceptors. The compensating donor concentrations in the samples were higher than the concentrations of Si, O, and C impurities. We also obtained the Mg acceptor level at a sufficiently low net acceptor concentration of 245 ± 2 meV. These results show that the HVPE method is promising for fabricating GaN vertical power devices, such as n-channel metal–oxide–semiconductor field-effect transistors.

AlScO3 perovskite—An ∼8 eV bandgap oxide predicted to exhibit low small hole polaron ionization energies and p-type conductivity at elevated temperatures

We investigate electronic structure and dopability of an ultrawide bandgap (UWBG) AlScO3 perovskite, a known high-pressure and long-lived metastable oxide. From first-principles electronic structure calculations, HSE06(+G0W0), we find this material to exhibit an indirect bandgap of around 8.0 eV. Defect calculations point to cation and oxygen vacancies as the dominant intrinsic point defects limiting extrinsic doping. While acceptor behaving Al and Sc vacancies prevent n-type doping, oxygen vacancies permit the Fermi energy to reach ∼0.3 eV above the valence band maximum, rendering AlScO3 p-type dopable. Furthermore, we find that both Mg and Zn could serve as extrinsic p-type dopants. Specifically, Mg is predicted to have achievable net acceptor concentrations of ∼1017 cm−3 with ionization energy of bound small hole polarons of ∼0.49 eV and free ones below 0.1 eV. These values place AlScO3 among the UWBG oxides with lowest bound small hole polaron ionization energies, which, as we find, is likely due to large ionic dielectric constant that correlates well with low hole polaron ionization energies across various UWBG oxides.

Hole capture cross section of the Al acceptor level in 4H-SiC

We performed current deep level transient spectroscopy (I-DLTS) on Al-doped p-type 4H-SiC epilayers. The epilayers exhibited a peak at 100–120 K in the I-DLTS spectra. The correspondence of the deep-level concentrations with the net acceptor concentration implies that the peak originates from the Al acceptor level. The observed activation energy of the peak was low compared to the Al acceptor level reported by Hall measurements owing to the Poole-Frenkel effect. We estimated capture cross section σ for holes of the Al acceptor level at the peak temperature based on the I-DLTS peak height dependence on the injection pulse widths and analyzed the temperature dependence of σ for holes.

Gate Reliability and its Degradation Mechanism in the Normally OFF High-Electron-Mobility Transistors With Regrown p-GaN Gate

Gate reliability and its degradation mechanism were studied for the normally off high-electron-mobility transistors (HEMTs) with regrown p-GaN gate and AlN/SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> stack passivation. Through comparing the forward leakage current in two designed structures, the conduction mechanism is determined to be Fowler-Nordheim tunneling whose current is independent of temperature when the Pd/p-GaN Schottky junction is under high electric field. Even for the regrown p-GaN gates, this Schottky junction fails at first, resulting in an abrupt increase in gate current. The maximum gate operation voltage with a failure rate of 1% for 10-year lifetime is estimated to be about 6.87 and 6.07 V at room temperature by adopting power law and exponential law as extrapolation fitting, respectively. Degradation process monitoring reveals that the net acceptor concentration N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</sub> extracted through C- V fitting presents an apparent decreasing trend from 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> to 1.1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> with the stress time increased from 30 to 2600 s. This is assumed to be related with the defects generation in the Schottky depletion region under high tunneling current and high electric field. These analyses show the feasibility of the normally off HEMTs with a regrown p-GaN gate and AlN/SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> stack passivation for practical applications, giving directions for further improving the gate breakdown voltage and lifetime.

Effect of rapid thermal annealing on the electrical properties of dilute GaAsPN based diodes

The effect of rapid thermal annealing on the electrical properties of p++GaP/p−GaAsPN/n+GaP diodes were investigated by using current-voltage (I-V), capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) techniques in the temperature range from 100 K to 440 K. It was observed that rapid thermal annealing treatment improves the electrical characteristics of as-grown structures. The annealed samples showed an ideality factor lower than the as-grown samples for all temperatures. The ideality factor values from I-V characteristics has changed between 6.8 and 1.9 in the temperature range of 110–430 K for as grown diode, and between 6.3 and 1.44 in the temperature range 100–400 K for the annealed diode. On the other hand, the barrier height increases and the ideality factor decreases with increasing temperature for all samples. The barrier height values has changed between 0.29 eV and 0.71 eV in the temperature range of 190–430 K for as grown diode, and between 0.38 eV and 0.77 eV in the temperature range 180–420 K for the annealed diode. High values of barrier heights were observed in the annealed samples due to the barrier height in-homogeneities at the p-i-n junction. The net acceptor concentration was calculated to be 1.18 × 1018 cm−3 and 2.11 × 1018 cm−3 for the as-grown and annealed GaAsPN layers, respectively. The net acceptor concentration increases by and the leakage current of the GaAsPN/GaP p-i-n junction decreases by 1–2 orders after RTA. DLTS and Laplace-DLTS measurements reveal three hole traps, H1an(0.06 eV), H2an(0.065 eV) and H3(0.23 eV) in the annealed samples as compared with two hole traps, H4ag(0.07 eV) and H5(0.25 eV) in the as-grown samples. After rapid thermal annealing an extra shallow trap is created.

Alkali treatment for single-stage co-evaporated thin CuIn0.7Ga0.3Se2 solar cells

KF and NaF treatments were done for single-stage co-evaporated CuIn0.7Ga0.3Se2. The absorber layers were grown on a substrate with an alkali barrier layer and NaF was either added before or after absorber layer growth. No differences were found on the device performance amongst the procedures to add Na. This is expected if the single-stage process does not have a copper rich stage or a Ga gradient, which is likely since there was no change of the elemental fluxes during absorber layer growth and no Ga profile was measured. KF was added by post-deposition only. Current-voltage characteristics were measured and net doping concentrations were determined from capacitance-voltage measurements (CV). We see an improvement of the open-circuit voltage (Voc) with increasing KF amount, and a marginal increase of the fill factor. CV measurements showed increasing net acceptor concentration with increasing KF amount. Time resolved photoluminescence (PL) showed an increased decay time for KF treated cells and the PL peak shape changed. Without KF treatment the PL peak is symmetric, after KF treatment a further peak appears at higher energy in the PL spectrum. This higher energy peak increases in intensity with increasing KF concentration. The same effects were seen in a sample without Na, but here the Voc was limited due to large tailing. Hence both Na and K are required for good cell efficiencies.

Significance of metastable acceptors in Cu(In,Ga)Se2 solar cells in accelerated lifetime testing

The significance of metastable acceptors in Cu(In,Ga)Se2 (CIGS) solar cells in accelerated lifetime testing has been investigated. Dry heating under light irradiation improves the conversion efficiency of CIGS solar cells at the early stage, which slightly decreases after long-term testing. However, after dry heating in the dark, the conversion efficiency significantly decreases. The net acceptor concentration considerably decreases after dark heating, and the reduction in the number of metastable acceptors results in decreases in open-circuit voltage and fill factor. After heat illumination treatment, the net acceptor concentration increases because of the metastable acceptors in CIGS layers, and the open-circuit voltage and fill factor partly recover. Therefore, the concentrations of metastable acceptors should be controlled by light irradiation or current injection with positive bias to evaluate correctly the degradation rates during accelerated lifetime testing.

Low p-type contact resistance by field-emission tunneling in highly Mg-doped GaN

Mg-doped GaN with a net acceptor concentration (NA-ND) in the high 1019 cm−3 range was grown using ammonia molecular-beam epitaxy. Electrical properties of NiO contact on this heavily doped p-type GaN were investigated. A potential-barrier height of 0.24 eV was extracted from the relationship between NA-ND and the specific contact resistivity (ρc). We found that there is an optimum NA-ND value of 5 × 1019 cm−3 for which ρc is as low as 2 × 10−5 Ω cm2. This low ρc is ascribed to hole tunneling through the potential barrier at the NiO/p+-GaN interface, which is well accounted for by the field-emission model.

Vacancy-type defects in Mg-doped GaN grown by ammonia-based molecular beam epitaxy probed using a monoenergetic positron beam

Vacancy-type defects in Mg-doped GaN were probed using a monoenergetic positron beam. GaN films with a thickness of 0.5–0.7 μm were grown on GaN/sapphire templates using ammonia-based molecular beam epitaxy and characterized by measuring Doppler broadening spectra. Although no vacancies were detected in samples with a Mg concentration [Mg] below 7 × 1019 cm−3, vacancy-type defects were introduced starting at above [Mg] = 1 × 1020 cm−3. The major defect species was identified as a complex between Ga vacancy (VGa) and multiple nitrogen vacancies (VNs). The introduction of vacancy complexes was found to correlate with a decrease in the net acceptor concentration, suggesting that the defect introduction is closely related to the carrier compensation. We also investigated Mg-doped GaN layers grown using In as the surfactant. The formation of vacancy complexes was suppressed in the subsurface region (≤80 nm). The observed depth distribution of defects was attributed to the thermal instability of the defects, which resulted in the introduction of vacancy complexes during the deposition process.

Identification of rhenium donors and sulfur vacancy acceptors in layered MoS2 bulk samples

MoS2 monolayers, a two-dimensional (2D) direct semiconductor material with an energy gap of 1.9 eV, offer many opportunities to be explored in different electronic devices. Defects often play dominant roles in the electronic and optical properties of semiconductor devices. However, little experimental information about intrinsic and extrinsic defects or impurities is available for this 2D system, and even for macroscopic 3D samples for which MoS2 shows an indirect bandgap of 1.3 eV. In this work, we evaluate the nature of impurities with unpaired spins using electron paramagnetic resonance (EPR) in different geological macroscopic samples. Regarding the fact that monolayers are mostly obtained from natural crystals, we expect that the majority of impurities found in macroscopic samples are also randomly present in MoS2 monolayers. By EPR at low temperatures, rhenium donors and sulfur vacancy acceptors are identified as the main impurities in bulk MoS2 with a corresponding donor concentration of about 108–12 defects/cm2 for MoS2 monolayer. Electrical transport experiments as a function of temperature are in good agreement with the EPR results, revealing a shallow donor state with an ionization energy of 89 meV and a concentration of 7 × 1015 cm−3, which we attribute to rhenium, as well as a second deeper donor state with ionization energy of 241 meV with high concentration of 2 × 1019 cm−3 and net acceptor concentration of 5 × 1018 cm−3 related to sulfur vacancies.

Backward diodes using heavily Mg-doped GaN growth by ammonia molecular-beam epitaxy

We grew heavily Mg-doped GaN using ammonia molecular-beam epitaxy. The use of low growth temperature (740 °C) allows decreasing the incorporation of donor-like defects (&amp;lt;3 × 1017 cm−3) responsible for p-type doping compensation. As a result, a net acceptor concentration of 7 × 1019 cm−3 was achieved, and the hole concentration measured by Hall effect was as high as 2 × 1019 cm−3 at room temperature. Using such a high Mg doping level, we fabricated GaN backward diodes without polarization-assisted tunneling. The backward diodes exhibited a tunneling-current density of 225 A/cm2 at a reverse bias of −1 V at room temperature.

New perspective on the performance stability of CdTe solar cells

Cu is a well-known cause for degradation of CdTe solar cells, yet, the exact mechanisms remain subject of discussion. We have chosen a novel approach to study CdTe solar cell stability by preparing CdTe solar cells in substrate configuration and applying a minimum amount of Cu to CdTe after recrystallization. These cells were exposed to an accelerated stress test. After 1000h of operation at maximum power point under 1 sun illumination intensity at 80°C substrate temperature such devices showed 90% of their initial power output indicating good performance stability. In order to investigate the remaining degradation and understand the role of Cu the Cu distribution in the multilayer stack was analyzed by SIMS before and after accelerated stress testing. We found that neither Cu loss in the back contact nor Cu accumulation in the CdS layer, which are often associated with degradation of CdTe solar cells, occurred in our case. C–V measurements revealed a decrease in net acceptor concentration by 70% and SCAPS simulations confirmed that this decrease can fully explain the observed performance losses of the solar cells. Time resolved photoluminescence measurements are consistent with the assumption that net acceptor loss upon stress is caused by a decay of acceptor type defects rather than by the formation of compensating donors. Acceptor loss can be explained by a redistribution of Cu inside the CdTe layer, possibly from within the grains to grain boundaries. The results demonstrate that by the use of a suitable Cu doping process that reduces Cu content at the back contact efficiency degradation due to Cu redistribution from back to front contact can be overcome. However, the observed acceptor loss appears in either configuration and seems to be independent of Cu movement from back to front or vice versa.

InGaN based micro light emitting diodes featuring a buried GaN tunnel junction

GaN tunnel junctions (TJs) are grown by ammonia molecular beam epitaxy. High doping levels are achieved with a net acceptor concentration close to ∼1020 cm−3, thanks to the low growth temperature. This allows for the realization of p-n junctions with ultrathin depletion width enabling efficient interband tunneling. n-p-n structures featuring such a TJ exhibit low leakage current densities, e.g., &amp;lt;5 × 10−5 A cm−2 at reverse bias of 10 V. Under forward bias, the voltage is 3.3 V and 4.8 V for current densities of 20 A cm−2 and 2000 A cm−2, respectively. The specific series resistance of the whole device is 3.7 × 10−4 Ω cm2. Then micro-light emitting diodes (μ-LEDs) featuring buried TJs are fabricated. Excellent current confinement is demonstrated together with homogeneous electrical injection, as seen on electroluminescence mapping. Finally, the I-V characteristics of μ-LEDs with various diameters point out the role of the access resistance at the current aperture edge.

High temperature annealing effects on deep-level defects in a high purity semi-insulating 4H-SiC substrate

Effects of high-temperature annealing on deep-level defects in a high-purity semi-insulating 4H silicon carbide substrate have been studied by employing current-voltage, capacitance-voltage, junction spectroscopy, and chemical impurity analysis measurements. Secondary ion mass spectrometry data reveal that the substrate contains boron with concentration in the mid 1015 cm−3 range, while other impurities including nitrogen, aluminum, titanium, vanadium and chromium are below their detection limits (typically ∼1014 cm−3). Schottky barrier diodes fabricated on substrates annealed at 1400–1700 °C exhibit metal/p-type semiconductor behavior with a current rectification of up to 8 orders of magnitude at bias voltages of ±3 V. With increasing annealing temperature, the series resistance of the Schottky barrier diodes decreases, and the net acceptor concentration in the substrates increases approaching the chemical boron content. Admittance spectroscopy results unveil the presence of shallow boron acceptors and deep-level defects with levels in lower half of the bandgap. After the 1400 °C annealing, the boron acceptor still remains strongly compensated at room temperature by deep donor-like levels located close to mid-gap. However, the latter decrease in concentration with increasing annealing temperature and after 1700 °C, the boron acceptor is essentially uncompensated. Hence, the deep donors are decisive for the semi-insulating properties of the substrates, and their thermal evolution limits the thermal budget for device processing. The origin of the deep donors is not well-established, but substantial evidence supporting an assignment to carbon vacancies is presented.

Strong temperature dependence of the Hall factor of p-type CoSb3: A re-analysis incorporating band nonparabolicity

The Hall factor has been calculated as a function of temperature for p-type CoSb3 samples reported by Hui et al. [J. Appl. Phys. 115, 103704 (2014)] in addition to those reported by Caillat et al. [J. Appl. Phys. 80, 4442 (1996)], incorporating the valence band nonparabolicity. For the nominally undoped p-type samples, reported by Caillat et al., the Hall factor was calculated to increase with temperature from about 1.6 around room temperature to about as large as 5 at 900 K. Owing to the incorporation of the Hall factor, the net acceptor concentration has been corrected to be larger by about 1.6 times. Furthermore, due to the strong temperature dependence of the Hall factor, the band-gap energy deduced through fitting the temperature dependence of the Hall coefficient has been corrected to be smaller by about 0.03 eV than the values deduced when assuming a constant Hall factor of unity. It has also been shown that, even when the temperature dependence of the Hall factor is considered, the incorporation of the second valence band into the calculation is necessary to explain the anomalous increase of the Hall coefficient with temperature in the Sn-doped p-type CoSb3 sample reported by Hui et al.

Incorporation of Na in Cu(In,Ga)Se&lt;inline-formula&gt;&lt;tex-math&gt;$_{\bf 2}$&lt;/tex-math&gt;&lt;/inline-formula&gt; Thin-Film Solar Cells: A Statistical Comparison Between Na From Soda-Lime Glass and From a Precursor Layer of NaF

The presence of Na in Cu(In,Ga)Se 2 layers increases the electrical performance of this type of thin-film solar cell. A detailed comparison of incorporating Na in the CIGS layer by two different methods is performed by evaluating several hundred devices fabricated under similar conditions. The first method is based on the conventionally used Na diffusion from the soda-lime glass substrate, whereas the second method is based on a NaF precursor layer deposited on a Mo-coated alkali-free glass substrate. The sample where Na is introduced by using a NaF precursor layer shows an orientation weighted toward (2 0 4)/(2 2 0) and a net acceptor concentration of 3.4 × 10 16 cm -3 , while SLG shows a (1 1 2) orientation with a 2.9 × 10 16 cm -3 acceptor concentration. Both sample types show close identical elemental depth profiles, morphology, and electrical performance.