Net Doping Concentration Research Articles

Localized Epitaxial Growth of 402 V Breakdown Voltage Quasi‐Vertical GaN‐on‐Si p‐n Diode on 200 mm‐Diameter Wafers

Localized epitaxy of gallium nitride (GaN) on silicon (Si) wafers is an efficient way to relax elastically the tensile stress generated in the GaN layer after growth, allowing epitaxy of thick layers for the fabrication of vertical power devices operating at high voltage. In this study, a 4.7 μm‐thick GaN layer is grown by metal–organic vapor phase epitaxy on 200 mm‐diameter Si wafers for the fabrication of quasi‐vertical Schottky and p‐n diodes. The uniformity of the doping concentration in the layer is mapped spatially by scanning spreading resistance microscopy, while scanning capacitance microscopy illustrates the differently doped regions in the p‐n diode. The net doping concentration is extracted by capacitance–voltage (C–V) measurements and it is found to be about 3 × 1016 cm−3. On a 140 μm‐diameter quasi‐vertical p‐n diode, destructive breakdown occurs at 402 V, with no periphery protection on the device, demonstrating that localized epitaxy of GaN on Si has great potential for vertical high‐power devices.

Nitrogen-displacement-related recombination centers generated by electron beam irradiation in n-type and p-type homoepitaxial GaN layers

Recombination centers originating from point defects generated by the displacement of N atoms in n-type and p-type GaN were investigated by analyzing Shockley–Read–Hall (SRH) recombination currents in homoepitaxial GaN p–n junctions. These defects were intentionally generated by electron beam (EB) irradiation at 137 keV. The net doping concentrations in p+–n junction diodes were not changed following irradiation although the levels in p–n+ junction diodes decreased as the EB fluence was increased. The SRH recombination current also increased with increases in the fluence. This work additionally evaluated the relationship between recombination lifetimes and trap concentrations obtained by deep level transient spectroscopy.

The role of carbon and C-H neutralization in MOCVD β-Ga2O3 using TMGa as precursor

In this Letter, the role of background carbon in metalorganic chemical vapor deposition (MOCVD) β-Ga2O3 growth using trimethylgallium (TMGa) as the Ga precursor was investigated. The quantitative C and H incorporations in MOCVD β-Ga2O3 thin films grown at different growth rates and temperatures were measured via quantitative secondary ion mass spectroscopy (SIMS). The SIMS results revealed both [C] and [H] increase as the TMGa molar flow rate/growth rate increases or growth temperature decreases. The intentional Si incorporation in MOCVD β-Ga2O3 thin films decreases as the growth rate increases or the growth temperature decreases. For films grown at relatively fast growth rates (GRs) (TMGa > 58 μmol/min, GR > 2.8 μm/h) or relatively low temperature (<950 °C), the [C] increases faster than that of the [H]. The experimental results from this study demonstrate the previously predicted theory—H can effectively passivate the compensation effect of C in n-type β-Ga2O3. The extracted net doping concentration from quantitative SIMS {[Si]-([C]-[H])} agrees well with the free carrier concentration measured from Hall measurement. The revealing of the role of C compensation in MOCVD β-Ga2O3 and the effect of H incorporation will provide guidance on designing material synthesis for targeted device applications.

Homoepitaxial growth of (100) Si-doped β-Ga2O3 films via MOCVD

Homoepitaxial growth of Si-doped β-Ga2O3 films on semi-insulating (100) β-Ga2O3 substrates by metalorganic chemical vapor deposition (MOCVD) is studied in this work. By appropriately optimizing the growth conditions, an increasing diffusion length of Ga adatoms is realized, suppressing 3D island growth patterns prevalent in (100) β-Ga2O3 films and optimizing the surface morphology with [010] oriented stripe features. The slightly Si-doped β-Ga2O3 film shows smooth and flat surface morphology with a root-mean-square roughness of 1.3 nm. Rocking curves of the (400) diffraction peak also demonstrate the high crystal quality of the Si-doped films. According to the capacitance–voltage characteristics, the effective net doping concentrations of the films are 5.41 × 1015 – 1.74 × 1020 cm−3. Hall measurements demonstrate a high electron mobility value of 51 cm2/(V·s), corresponding to a carrier concentration of 7.19 × 1018 cm−3 and a high activation efficiency of up to 61.5%. Transmission line model (TLM) measurement shows excellent Ohmic contacts and a low specific contact resistance of 1.29 × 10-4 Ω·cm2 for the Si-doped film, which is comparable to the Si-implanted film with a concentration of 5.0 × 1019 cm−3, confirming the effective Si doing in the MOCVD epitaxy.

Temperature-dependent current transport in quasi-vertical Pt/AlN/Al0.6Ga0.4N heterostructure Schottky barrier diodes with significant improved forward characteristic

In this letter, quasi-vertical Schottky barrier diodes (SBDs) based on AlN/Al0.6Ga0.4N heterostructure are fabricated and temperature-dependent current transport is systematically investigated. Benefited from the AlN/Al0.6Ga0.4N heterostructure with high net doping concentration of ∼1 × 1018 cm−3 in Al0.6Ga0.4N, the device demonstrates lower ideality factor of 2.95, high on/off current ratio of ∼108, the on-current of above 40 A cm−2 which is two order magnitude higher than that of the state-of-the-art AlN SBDs. The barrier inhomogeneity is characterized by the model of Gaussian distribution, and the reverse current observed is identified to be traps-related leakage process such as Poole–Frenkel emission and trap assisted tunneling. This work reveals the heterostructure engineering is an alternative pathway to overcome the difficulty of AlN-based SBDs.

Influence of residual dopants to net doping concentration in N-type 4H-SiC films grown using high-speed wafer rotation vertical CVD method

In this study, the influence of residual dopants on the net doping concentration in n-type 4H-SiC epitaxial films grown at different N2 flow rates and C/Si ratios were investigated. By reducing the N2 flow rate, the influence of the residual donors on the net doping concentration was observed to become dominant for the films grown at low C/Si ratios and that of the residual acceptors on the net doping concentration becomes dominant for the films grown at high C/Si ratios. For the films grown at the middle C/Si ratio, an apparent proportional relation due to the compensation balance between the residual and intentional donors and the residual acceptors was observed in the N2 flow rate dependence of the net doping concentration. Furthermore, the decay curve of the net doping concentration observed in the C/Si ratio dependence is affected by the compensation balance between the intentional dopant concentration and the residual dopant concentration.

Vertical GaN-on-GaN nanowire Schottky barrier diodes by top-down fabrication approach

An optimized top-down approach was utilized to fabricate vertical GaN-on-GaN nanowire Schottky barrier diodes (NWSBDs) in this letter. The suppressed reverse leakage current and enhanced breakdown voltage (BV) of the vertical GaN NWSBDs are attributed to the reduced electric field at the interface of the Schottky junction achieved through the dielectric reduced surface field technique. As-fabricated NWSBD delivers a low turn-on voltage of 0.80 V, a near-unity ideality factor of 1.04, along with a soft BV of 480 V. The measured soft BV is comparable with the avalanche BV of the p–n diode with a similar net doping concentration in the drift region.

Selective-area growth study of GaN micropillars for quasi-vertical Schottky diodes

In this work, metal organic vapor phase epitaxy (MOVPE) is employed for selective-area growth (SAG) of undoped and lightly n-doped GaN micropillars on masked GaN-on-sapphire templates. In micropillar geometry, the limits of GaN drift layer thickness in hetereoepitaxial Schottky diodes are expected to be significantly pushed upwards. This is an important step towards the realization of GaN-based quasi-vertical power devices with high breakdown voltage (V br) on low-cost foreign substrates. Micropillar growth evolution and the impact of growth rate and V/III ratio on micropillar morphology and parasitic GaN deposition on AlOx hard masks are investigated. By using a combination of low growth rate and high V/III ratio, 12–22 µm high micropillars with planar pillar tops are grown in circular mask openings with radii of 35–100 µm. The threading dislocation density of the highest pillars is (1.4 ± 0.3) × 107 cm−2. Schottky diodes on micropillars with lowest net doping concentration (N D − N A) of 1.3 × 1016 cm−3 exhibit excellent forward characteristics with ideality factors , on/off-ratios up to 1010, and on-resistances R on 1 m cm−2.

Local mismatch and noise investigation for pre and post multilayer pHEMTs

The key objective of this study is to investigate the local mismatch of pre and post multilayer structure on the active devices. Five pre and post multilayer structure pHEMTs on the same wafer within the same cell between adjacent devices are considered. As the study of local mismatch ensures good yields and a way to gain insights about the technology various comparisons are made including the effects of multilayer structuring. The threshold voltage, built-in potential and the net doping concentration of the 2-DEG of the devices are extracted through capacitance-voltage data. The underlying electrical parameter of the transistors as well as the RF figure of merit has been analyzed. The microwave noise related parameters namely minimum noise figure, associate gain, noise resistance, magnitude of the optimum reflection are also discussed and investigated. Centre-to-edge mismatch results in minor variation in performance between devices. These studies would help within the advancement of solid, proficient and low cost generation of future compact structure in 3-D multilayer MMICs.

Ultra-thin passivation layers in Cu(In,Ga)Se2 thin-film solar cells: full-area passivated front contacts and their impact on bulk doping

In the search for highly transparent and non-toxic alternative front layers replacing state-of-the-art CdS in Cu(In,Ga)Se2 thin-film solar cells, alternatives rarely exceed reference devices in terms of efficiency. Full-area ultra-thin aluminium oxide tunnelling layers do not require any contact patterning and thus overcome the main drawback of insulating passivation layers. Even a few monolayers of aluminium oxide can be deposited in a controlled manner by atomic layer deposition, they show excellent interface passivation properties, low absorption, and suitable current transport characteristics on test devices. Depositing a ZnO-based transparent front contact, however, results in extremely poor solar cell performance. The issue is not necessarily a low quality of the alternative front layer, but rather the intricate relation between front layer processing and electronic bulk properties in the absorber layer. We identify three challenges critical for the development of novel front passivation approaches: (i) both Cd and Zn impurities beneficially reduce the high native net dopant concentration in the space charge region, (ii) sputter deposition of ZnO damages the passivation layer resulting in increased interface recombination, (iii) thermal treatments of devices with ZnO layer result in substantial Zn diffusion, which can penetrate the full absorber thickness already at moderate temperatures.

Current-voltage and capacitance study of light-induced metastabilities in CuZnSnSSe thin film solar cells

White, red and blue light-induced metastabilities in Cu2ZnSnS4(Se4) solar cells were investigated by temperature dependent current-voltage measurements, drive level capacitance profiling, impedance and thermal admittance spectroscopy. A set of devices were studied where white and blue light soaking at room temperature led to degradation of the device performance, while after red light soaking the solar cell efficiency did not change.We observed a significant effect of light soaking on capacitance data measured in both low and high-temperature ranges for these devices. In particular, the net doping concentration extracted from drive-level capacitance profiling substantially increased after light soaking treatments.Low and high-temperature capacitance steps observed in the reference capacitance-frequency spectra were assigned to Fermi level pinning and bulk defects, correspondingly. Light soaking with different-wavelength light led to a shift of both steps toward the high-frequency range, and hence a decrease in the thermal admittance activation energies.A low-frequency ‘inductive’ loop was detected in the impedance spectra after light soaking, regardless of wavelength. It was proposed that the appearance of the ‘inductive’ loop is due to the formation of a negative electric field at the highly defected CdS/Cu2ZnSnS4(Se4) hetero-interface. This result also leads us to conclude that such electric field is responsible for the metastable behaviour of these devices at room temperature, while the low temperature metastable changes might have a different origin.We also discuss the methodology for electrical characterization of the metastable solar cells in detail.

Can we see defects in capacitance measurements of thin‐film solar cells?

AbstractThermal admittance spectroscopy and capacitance‐voltage measurements are well established techniques to study recombination‐active deep defect levels and determine the shallow dopant concentration in photovoltaic absorbers. Applied to thin‐film solar cells or any device stack consisting of multiple layers, interpretation of these capacitance‐based techniques is ambiguous at best. We demonstrate how to assess electrical measurements of thin‐film devices and develop a range of criteria that allow to estimate whether deep defects could consistently explain a given capacitance measurement. We show that a broad parameter space, achieved by exploiting bias voltage, time, and illumination as additional experimental parameters in admittance spectroscopy, helps to distinguish between deep defects and capacitive contributions from transport barriers or additional layers in the device stack. On the example of Cu(In,Ga)Se2 thin‐film solar cells, we show that slow trap states are indeed present but cannot be resolved in typical admittance spectra. We explain the common N1 signature by the presence of a capacitive barrier layer and show that the shallow net dopant concentration is not distributed uniformly within the depth of the absorber.

Ideal phonon-scattering-limited mobility in inversion channels of 4H-SiC(0001) MOSFETs with ultralow net doping concentrations

The phonon-limited mobility in 4H-silicon carbide (SiC) inversion channels was precisely evaluated by employing ultralow net doping concentrations. The measured mobility in the inversion channels of these samples was comparable to the electron mobility in bulk 4H-SiC, and the temperature dependence indicated that the mobility can be ascribed to phonon-scattering-limited mobility. The strong dependence of the mobility on the net doping concentration cannot be explained by Coulomb scattering by dopant impurities. This indicates the existence of scattering origins at the SiO2/SiC interface. Comparison of dry oxidized samples and samples subjected to postoxidation annealing in nitric oxide revealed that the scattering origins were not attributable to trapped electrons at the SiO2/SiC interface states, although the nature of the scattering origins remains unclear.

(Invited) High-Voltage β-Ga2O3 vertical Power Diodes and Transistors with Fin Channels

For high voltage and high current applications, vertical power devices are preferred over the lateral counterparts, due to the unaltered footprint with elongated drift region width and the generally higher current density. The development of vertical power devices relies on the ability to epitaxially grow thick drift layers with low defect density and precisely-controlled net doping concentration, often on the order of 1016 cm-3 or lower. One of the unique advantages of β-Ga2O3 over SiC and GaN is the availability of melt-growth methods for the production of high quality single-crystal substrates, which provide excellent platform for homoepitaxy. Recently, high quality thick epitaxial layers grown by halide vapor phase epitaxy (HVPE) have been developed, and β-Ga2O3 vertical power devices have seen rapid developments. In this work, we discuss our recent advancements on high-voltage β-Ga2O3 vertical power diodes and transistors with fin channels. The ultra-high critical breakdown field of 6-8 MV/cm in β-Ga2O3 is another key advantage of this material. While the high critical field leads to a high projected Baliga’s figure-of-merit, it also brings tremendous challenge to the management of the electric field in power devices, especially with the absence of native p-type doping up till now. One example is the Schottky barrier diodes, in which the electric field at the top surface must be controlled to be much lower than the critical field in order to avoid excessive tunneling current. Another example is the device periphery, where proper edge termination designs are required to avoid premature breakdown due to field crowding. To suppress the electrical field at the surface, we adopted fin channels with metal-insulator-semiconductor (MIS)-based sidewall in Schottky barrier diodes. We show that the charge-coupling effect in these structures allows for effective reduction of the electric field at the top surface, therefore effectively reducing the leakage current due to tunneling. The gated fin channels with sub-micron channel width also allow for the realization of normally-off vertical transistors. A general trend of the electric field peak at the bottom corners of a fin channel is discovered, which provides important guidance for the design of vertical fin channel devices. With the help of field-plating at the device periphery, we show that the breakdown voltage of the fin channel transistors can be much improved. With these field management techniques, we achieved state-of-the-art figure-of-merits in both the fin channel diodes as well as transistors.

Reduction in reverse leakage current of diamond vertical Schottky barrier diode using SiNX field plate structure

Abstract Diamond vertical Schottky barrier diodes (SBDs) with SiNX field-plate (FP) structure has been investigated. Ti/Au and Zr/Ni/Au metal stacks are used as ohmic and Schottky metal, respectively. The forward current density of SBDs with and without FP are 1600 and 3300 A/cm2 at −10 V, respectively. The depletion layer’s thickness and net doping concentration are 380 nm and 1.4 × 1016 cm−3, respectively, as extracted from the capacitance-voltage measurement. The reverse leakage current of SBDs with and without FP are 1.8 × 10−6 and 6.3 A/cm2 at 100 V, respectively, indicating that the FP technique can significantly suppress reverse leakage current at the Schottky junction edge.

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.

Interdiffusion and Doping Gradients at the Buffer/Absorber Interface in Thin-Film Solar Cells.

An accurate determination of the net dopant concentration in photovoltaic absorbers is critical for understanding and optimizing solar cell performance. The complex device structure of multilayered thin-film solar cells poses challenges to determine the dopant concentration. Capacitance-voltage ( C- V) measurements of Cu(In,Ga)Se2 thin-film solar cells typically yield depth-dependent apparent doping profiles and are not consistent with Hall measurements of bare absorbers. We show that deep defects cannot fully explain these discrepancies. We instead find that the space charge region capacitance follows the model of a linearly graded junction in devices containing a CdS or Zn(O,S) buffer layer, indicating that elemental intermixing at the absorber/buffer interface alters the dopant concentration within the absorber. For absorbers covered with MgF2, C- V measurements indeed agree well with Hall measurements. Photoluminescence measurements of Cu(In,Ga)Se2 absorbers before and after deposition of a CdS layer provide further evidence for a significant reduction of the near-surface net dopant concentration in the presence of CdS. We thus demonstrate that interdiffusion at the absorber/buffer interface is a critical factor to consider in the correct interpretation of doping profiles obtained from C- V analysis in any multilayered solar cell and that the true bulk dopant concentration in thin-film devices might be considerably different.

Inhomogeneities in Cu(In,Ga)Se2 Thin Films for Solar Cells: Band‐Gap Versus Potential Fluctuations

Inhomogeneities in Cu(In,Ga)Se2 thin films have been reported to lead to band‐gap or electrostatic potential fluctuations, which may reduce the photovoltaic performance of the corresponding solar cells via enhanced recombination. The issue of where these inhomogeneities occur in the Cu(In,Ga)Se2 absorber has so far not been discussed in detail in literature. The present work gives an overview of spatial variations in composition, net doping, and lifetime on various scales, also with respect to their occurrences at interfaces and extended structural defects. Impacts of these spatial variations on the device performance of the corresponding solar cells are discussed. It can be shown that compositional inhomogeneities possibly affecting the device performance are only present at (partial) dislocations and at the Cu(In,Ga)Se2/buffer interface, and that inhomogeneous distributions of excess charges at line/planar defects as well as of net doping concentrations affect considerably the potential landscape within Cu(In,Ga)Se2 thin films.

Schottky Diodes on ZnO Thin Films Grown by Plasma-Enhanced Atomic Layer Deposition

Enhancement of the properties of zinc oxide (ZnO)-based Schottky diodes has been explored using a combination of plasma-enhanced atomic layer deposition (PE-ALD) ZnO thin films and silver oxide Schottky contacts deposited by reactive radio-frequency sputtering. The electrical properties of the ZnO thin films were systematically tuned by varying the deposition temperature and oxygen plasma time during PE-ALD to optimize the performance of the diode. Low temperature (80 °C) coupled with relatively long oxygen plasma time (>30 s) PE-ALD is the key to produce ZnO films with net doping concentration lower than 1017 cm−3. Under the optimal deposition conditions identified, the diode shows an ideality factor of 1.33, an effective barrier height of 0.80 eV, and an ON/OFF ratio of ${3.11} \times {10}^{{5}}$ .

Effect of 60Co γ-ray irradiation on electrical properties of Ti/Au/GaAs1−xNx Schottky diodes

Current-voltage (I-V), capacitance-voltage-frequency (C-V-f) and conductance-voltage-frequency (G/ω-V-f) measurements at room temperature are used to study 50 kGy 60Co γ-ray electrical properties irradiation dependence of Ti/Au/GaAs1−xNx Schottky diodes with 0.2%; 0.4%; 0.8% and 1.2% nitrogen dilution. This γ-ray irradiation induces a permanent damage that has increased ideality factor and series resistance for all samples. It was accompanied by a decrease in Schottky barrier height with nitrogen content up to 0.4%N and remained constant thereafter. Radiation was also found to degrade the reverse leakage current.At high frequency (1 MHz), capacitance and conductance decreased after radiation due to a decrease in net doping concentration. Interface state density and series resistance were determined from C-V-f and G/ω-V-f characteristics using Hill-Coleman methods. Interface states density exponentially decreased with increasing frequency confirming the behavior of interface traps response to ac signal.Series resistance increases after irradiation is attributed to carrier's removal effect and mobility degradation. It has two peaks in the accumulation and inversion region for some diodes (0.4%N, 0.8%N). γ-ray irradiation produced traps levels and recombination centers that reduce relaxation time. An increase in %N content can impede irradiation damage with even some compensation when the percent of diluted nitrogen is high (1.2%N).