Doped GaN Research Articles

Realization of p-type gallium nitride by magnesium ion implantation for vertical power devices

Implementing selective-area p-type doping through ion implantation is the most attractive choice for the fabrication of GaN-based bipolar power and related devices. However, the low activation efficiency of magnesium (Mg) ions and the inevitable surface decomposition during high-temperature activation annealing process still limit the use of this technology for GaN-based devices. In this work, we demonstrate successful p-type doping of GaN using protective coatings during a Mg ion implantation and thermal activation process. The p-type conduction of GaN is evidenced by the positive Seebeck coefficient obtained during thermopower characterization. On this basis, a GaN p-i-n diode is fabricated, exhibiting distinct rectifying characteristics with a turn-on voltage of 3 V with an acceptable reverse breakdown voltage of 300 V. Electron beam induced current (EBIC) and electroluminescent (EL) results further confirm the formation of p-type region due to Mg ion implantation and subsequent thermal activation. This repeatable and uniform manufacturing process can be implemented in mass production of GaN devices for versatile power and optoelectronic applications.

Impact of remote plasma oxidation of a GaN surface on photoluminescence properties

The impact of oxidation of a GaN surface changing the oxidation temperatures and the oxidation time using remote oxygen plasma on photoluminescence (PL) properties are investigated with a well-controlled photon flux for the excitation. The PL intensities related to transition at the band edges obviously decreased after the oxidation. From the detailed spectra analysis of PL combining the results obtained by photoelectron yield spectroscopy and X-ray photoelectron spectroscopy, we found that oxygen doping in GaN occurs during plasma oxidation, resulting in PL intensity reduction. These results suggest that, to precisely control the dopant concentration, attention should be paid to the oxidation process in the GaN device fabrication process.

Influence of intrinsic or extrinsic doping on lattice locations of carbon in semi-insulating GaN

We demonstrate that different doping approaches can significantly influence the lattice locations of carbon (C) in GaN grown by MOCVD. For intrinsically doped GaN with TMGa as the C source, an annealing induced switching process of C atoms from Ga sites to N sites is observed, revealing the existence of substitutional C atoms on both Ga and N sites. While for extrinsically doped GaN with propane as the C precursor, C atoms incorporate almost on N sites. From the viewpoint of growth dynamics, we propose a mechanism for C incorporation into different lattice locations.

Highly efficient p-type doping of GaN under nitrogen-rich and low-temperature conditions by plasma-assisted molecular beam epitaxy

Highly efficient and reproducible p-type doping of GaN under nitrogen-rich and low-growth-temperature conditions was demonstrated with the plasma-assisted molecular beam epitaxy technique. The low-temperature range is approximately below 650 °C and refers to growth temperatures at which the thermal desorption of any excess Ga is negligibly slow. The Mg and hole concentrations obtained with the N-rich condition were more than one order of magnitude higher than those obtained with the Ga-rich condition while keeping all other conditions identical. The Mg doping under such N-rich conditions was also found to show Mg-mediated suppression of background impurities, good epitaxy quality on GaN templates, and relatively low surface roughness. Over the investigated growth temperature range from 580 °C to 650 °C, the Mg incorporation efficiency under the N-rich condition was found to be close to unity (70%-80%) and independent of the growth temperature. High hole concentrations of up to 2×1019 cm-3 and activation efficiencies of up to 16.6% were obtained. The result rules out the Mg surface sticking probability as the limiting mechanism for Mg incorporation in this temperature range, as it would be temperature dependent. Instead, the Mg incorporation rate was more likely governed by the availability of substitutional sites for Mg on the surface, which should be abundant under the N-rich growth conditions. Excellent diode characteristics and electroluminescence results were observed when this p-type doping method was employed in the growth of full device structures.

Effect of Ge doping on growth stress and conductivity in AlxGa1-xN

Silicon (Si) is a common n-type donor in AlxGa1-xN; however, it induces bending of edge-type threading dislocations which can generate tensile stress in the film leading to the formation of channeling cracks in thick layers. Germanium (Ge) has previously been investigated as an alternative to Si for n-type doping of GaN, but its impact on film stress in AlxGa1-xN has not been investigated in detail. In this study, we employ in situ wafer curvature measurements combined with postgrowth characterization to investigate Ge doping of AlxGa1-xN (x = 0–0.62) layers grown on 6H-SiC by metalorganic chemical vapor deposition. It was found that Ge doping (n ∼ 1.6 × 1019 cm−3) of Al0.30Ga0.70N does not induce tensile stress during growth in contrast to that observed with a similar level of Si doping. In addition, the average inclination angle of edge dislocations was similar for undoped and Ge doped films indicating that Ge does not promote surface-mediated dislocation climb. High n-type doping was achieved in Ge doped AlxGa1-xN for lower Al fraction range (x < 0.5), but resistivity increased and carrier density decreased significantly for higher Al fractions. The results demonstrate Ge doping as a viable alternative to Si doping of AlxGa1-xN (x < 0.5) for achieving thick, crack-free layers.

Germanium doping of cubic GaN grown by molecular beam epitaxy

We present a study of germanium as an alternative to silicon for n-type doping of cubic GaN. We find that Ge is a well-suited donor impurity. Our layers were grown by plasma-assisted molecular beam epitaxy on 3C-SiC/Si (001) substrates. Germanium-doped layers were fabricated with donor concentrations ranging over several orders of magnitude up to 3.7 × 1020 cm−3. For comparison, silicon-doped layers with donor concentrations of up to 3.8 × 1019 cm−3 were also grown. Incorporation of germanium into the cubic GaN layers was verified by time-of-flight secondary ion mass spectrometry. The crystalline quality of our layers was analyzed using high-resolution x-ray diffraction. Germanium- as well as silicon-doped layers with donor concentrations above 1019 cm−3 exhibited an increase of the dislocation density with increasing dopant concentration. The surface topography of our layers was investigated by atomic force microscopy. Comparable values for the surface roughness were measured for germanium- as well as silicon-doped layers. Optical properties were investigated by photoluminescence spectroscopy at 13 K. Doping with silicon resulted in a spectrally slightly narrower luminescence than doping with germanium. Donor concentrations and carrier mobilities were determined by Hall effect measurements at room temperature and we observe 20% higher electron mobilities for Ge-doping compared to Si-doping in the case of high dopant concentrations.

Trap‐Related Breakdown and Filamentary Conduction in Carbon Doped GaN

A breakdown phenomenon is studied in thin carbon doped GaN layers (GaN:C; carbon concentration 1019 cm−3) embedded between a top metal electrode and bottom n‐doped GaN (n‐GaN). When slowly sweeping positive bias V at the top electrode up and down, a hysteresis is found with transitions to on‐ and off‐states at voltages and (<Vbd,up), respectively, and on‐ to off‐current ratios exceeding 103. Breakdown at occurs at an electric field of about 0.5 MV cm−1 in GaN:C. For , transition to on‐state is time‐dependent with random time‐to‐breakdown in the ms to 10s range. Electroluminescence measurements in on‐state show conduction via multiple standing or slowly moving current filaments (CFs) which number and size increases with total current. The origin of an S‐shape I–V curve leading to hysteresis behavior is discussed in terms of trap‐related nonlinear generation‐recombination processes. Formation of multiple CFs is explained by spontaneous CF formation in a homogeneous spatially extended system composed of an active medium with S‐shape I–V curve (here GaN:C) connected in series with a passive layer (here n‐GaN). Unlike previous studies in various III‐N opto‐electronic systems, extended defects are supposed not to be responsible for the CF origin but just for their pinning.

Evidence of Mg Segregation to Threading Dislocation in Normally-Off GaN-HEMT

In this article, the segregation of Mg to threading dislocation is demonstrated in a commercial normally-off GaN High Electron Mobility Transistor using atom probe tomography. The diffusion of Mg from a doped p-type GaN gate through the AlGaN buffer to the GaN substrate is demonstrated.

Electrical characterization of HVPE GaN containing different concentrations of carbon dopants

A comprehensive study of the electrical characteristics in Schottky diodes made of GaN:C grown by hydride vapour phase epitaxy (HVPE) technology has been reported. The Schottky junctions were made of an Ni/Au (25/200 nm) metal stack and ohmic contacts were fabricated by Ti/Al/Ni/Au (30/90/20/100 nm) e-beam deposition of the metal thin-films followed by rapid thermal annealing. A good quality of the fabricated Schottky diodes has been proved by considering the transient shape using a pulsed technique of barrier evaluation under linearly increasing voltage (BELIV). The concentrations of equilibrium carriers of 1 × 1011 cm−3 and of 2 × 1010 cm−3 have been evaluated for relatively low and high carbon density doped samples, respectively, using photo-capacitance characteristics dependent on excitation intensity. The effective mobility of carriers of μeff = 610 cm2/Vs has been estimated by considering the serial resistance variations dependent on excitation density, through analysis of delay times that appeared in the formation of peaks within BELIV transients. Optical deep level transient spectroscopy has shown that thermal emission from the rather shallow centres prevails. The centres, ascribed to vacancies as well as to carbon on Ga sites, have been identified. It has been demonstrated that Schottky junctions made of heavy carbon doped (NC ≥ 1018 cm−3) HVPE GaN material are able to withstand voltages of ≥300 V.

Hysteretic Photochromic Switching (HPS) in Doubly Doped GaN(Mg):Eu-A Summary of Recent Results.

Europium is the most-studied and least-well-understood rare earth ion (REI) dopant in GaN. While attempting to increase the efficiency of red GaN light-emitting diodes (LEDs) by implanting Eu+ into p-type GaN templates, the Strathclyde University group, in collaboration with IST Lisbon and Unipress Warsaw, discovered hysteretic photochromic switching (HPS) in the photoluminescence spectrum of doubly doped GaN(Mg):Eu. Our recent work, summarised in this contribution, has used time-, temperature- and light-induced changes in the Eu intra-4f shell emission spectrum to deduce the microscopic nature of the Mg-Eu defects that form in this material. As well as shedding light on the Mg acceptor in GaN, we propose a possible role for these emission centres in quantum information and computing.

Buried channel normally-off AlGaN/GaN MOS-HEMT with a p-n junction in GaN buffer

A buried channel normally-off AlGaN/GaN MOS-HEMT with gate recess etching is reported. The buried channel operation is achieved by in situ doping of GaN to introduce a p-n junction in the GaN buffer. The fabricated buried channel MOS-HEMT with 12.5 nm atomic layer deposition (ALD) Al2O3 gate dielectric featured a threshold voltage of 1.3 V with a drain saturation current of 287 mA mm−1 for a device with 3.5 μm long gate length and 11 μm source-drain spacing. The field effect mobility improved from 25 cm2/Vs for a reference device to 142 cm2/Vs for the buried channel device. Due to the presence of the p-n junction depletion region in the GaN buffer, the leakage in the off state decreased by about 4 orders of magnitude (4 nA mm−1 compared to 76 uA mm−1 for the reference device). The buried channel device also gives better breakdown characteristics, with a breakdown voltage of 158 V compared to 98 V for the reference device.

(Invited) GaN Buried Channel Normally Off MOSHEMT: Design Optimization and Experimental Integration on Silicon Substrate

AlGaN/GaN High Electron Mobility Transistors (HEMTs) find application in power electronics systems as high-frequency power switches. Low on-state resistance and high breakdown voltage in the off-state are the key features of AlGaN/GaN HEMTs. Achieving these qualities with a normally-off operation is a challenge. Gate recess etching is the most widely used technique to realize a normally off AlGaN/GaN HEMT. On-state current of gate recess etched devices are limited by Scattering of electrons at the oxide-GaN interface. Free carriers in the buffer layers and nonuniform electric field distribution between gate and drain electrode limits the breakdown voltage of these devices. We address these issues by implementing a buried channel device architecture in normally-off AlGaN/GaN MOS-HEMTs. Buried channel architecture has been reported in Si, SiC and InGaAs devices. With this architecture, the on-state characteristics improve significantly since the conduction channel is formed away from the oxide-semiconductor interface, thus avoiding interface scattering. The off-state characteristics improve due to the presence of depletion region associated with a p-n junction in GaN buffer. The p-n junction in the buffer also helps to achieve uniform electric field distribution between gate and drain. The schematic stack of the proposed buried channel normally-off AlGaN/GaN MOS HEMT is shown in fig. 1(a). Atlas silvaco simulator has been used to perform electrostatic simulation on the proposed buried channel device architechure. The thickness and doping concentrations of p-type and n-type regions in the GaN buffer are the important design parameters to be optimized. Fig. 1(b) and fig 1(c) show the electron concentration colour plot of the optimized device stack at VG = 0 V and at VG > VT respectively. The carriers bellow the gate electrode are depleted at VG = 0 V as shown in fig. 1(b) and the depletion region associated with p-n junction in the GaN buffer ensures low leakage current at off state. When VG > VT, a buried channel is formed below gate electrode as shown in the fig. 1(c), ensuring a buried conduction in the proposed device architecture. The device stack growth was carried out in a horizontal flow AIXTRON MOCVD reactor on a 2' silicon substrate. Standard HEMT fabrication procedure with Ti/Al/Ni/Au as source/drain contact and recessed Ni/Au gate contact with ALD Al2O3 dielectric was followed for the realization of the device. Performance of a buried channel device is compared with a reference device here. As shown in Fig. 2, VT of the device fabricated on buried channel stack and reference stack were 1.3 V and 1.7 V respectively, determined by extrapolating the linear region method. The lower VT of buried channel device compared to reference device is due to the presence of intentionally n-doped (1.5x1017 cm-3) region below UID GaN (1014 cm-3). This can be increased appropriately by increasing the p-type GaN doping, or reducing n-type doping. The field effect mobility of the buried channel device is 142 cm2/Vs compared to 25 cm2/Vs in case of reference device. More than five times increase in the field effect mobility is because of lesser interface scattering effect on electron transport in buried channel device. The off-state drain leakage current in devices on buried channel stack is four orders of magnitude lower than that on reference stack (4 nA/mm for buried channel device compared to 76 uA/mm for reference). The buried channel device has a breakdown voltage of 158V compared to 98V for reference device. The matching of calibrated device simulation and experimental results validate the advantages of buried channel architecture. Figure 1

Electrical Properties of Silicon Doped GaN Activated By Ion Implantation

One major goal of GaN based technology studies is the fabrication of rugged power switching devices. While this is best achieved through vertical GaN technology, the process development of GaN substrates is not yet mature, and the substrates for lateral power switch are more cost effect than GaN. A common way of doping lateral GaN devices is through ion implantation followed by rapid thermal annealing for ion activation. Activation of most p-type dopants requires temperatures above 1300 °C; however, silicon will activate as an n-type dopant at lower temperatures. This research reports on the electrical properties of devices such as junction field effect transistors and Van der Pauw structures containing silicon doped GaN activated by ion implantation.

Analysis of defect structure in GaN epilayers doped with implanted Si+ by RBS/c method

In this work, the Si+ doping of undoped GaN (n ∼1 × 1016 cm−3) on sapphire by a single step implantation or sequential implantation/recrystallization processes, to a total fluence of 1 × 1016 at/cm2 and subsequent activation annealing at 1100 °C for 2 min, were demonstrated and analysed. We proved that the multicycle implantation/recrystallization process is better in terms of structural and electrical parameters of implanted region than conventional single step implantation. Rutherford backscattering spectrometry in channeling geometry (RBS/c), McChasy simulations and atomic force microscopy (AFM) studies showed, that the proposed process does not deteriorate surface morphoplogy and leads to a significantly lower defect concentrations. Low resistivity ohmic contacts, with RC < 0.1 Ω mm, to low sheet resistivity Si-implanted GaN region (RSH = (36.9 ± 0.7) Ω/sq.), were obtained with the use of sequential implantation with 30 s recrystallization annealing.

Quantitative characterization of semiconductor structures with a scanning microwave microscope.

In this work, our earlier method for measuring resistance Rsh of semiconductor films with a near-field scanning microwave microscope [A. N. Reznik and S. A. Korolyov, J. Appl. Phys. 119, 094504 (2016)] is studied in a 0.1 kΩ/sq < Rsh < 15 kΩ/sq range. The method is based on a microscope model in the form of a monopole or dipole antenna interacting with an arbitrary layered structure. The model fitting parameters are determined from the data yielded by calibration measurements on a system of etalon samples. The performance of the method was analyzed experimentally, using strip-probe and coaxial-probe microscopes in the frequency range of 1-3 GHz. For test structures, we used doped GaN films on the Al2O3 substrate and also transistor structures based on the AlGaN/GaN heterojunction and AlGaAs/GaAs/InGaAs/GaAs/AlGaAs quantum well with a conducting channel. The obtained microwave microscope data were compared with the results of measurements by the van der Pauw method. At the first stage of the experiment, the calibration etalons were bulk homogeneous samples with different permittivity/conductivity values. In this case, satisfactory agreement between the microscope and the van der Pauw data was obtained with a strip probe on all tested samples in the entire range of Rsh. With a coaxial probe, such accordance was observed only in high-ohmic samples with Rsh > 1 kΩ/sq. The use of GaN film structures as a calibration system helped to increase the accuracy of the coaxial-probe-aided measurement of Rsh to a level of ∼10%.

Dynamics of low temperature excitons in Fe-doped GaN

The recombination processes of excitons in Fe-doped GaN have been characterized by time-resolved photoluminescence measurement. The photoluminescence shape at different excitation powers revealed that the hole capture process by Fe2+ centers has always existed in doped GaN. Decreasing of the fast component in the biexponential decay curves with increasing iron concentration indicates the enhancive contribution of the hole capture process. Furthermore, the structures of an iron-related acceptor and complex bound exciton were confirmed by the dependence of lifetime constants on the localization energy. Also, the extended wave function of the hole from the complex bound exciton will enable spin coupling between isolated iron ions.

High-performance ZnS/GaN heterostructure photoanode for photoelectrochemical water splitting applications

We present a study of ZnS/GaN heterostructure photoanodes fabricated by using ZnS thin films deposited on GaN by atomic layer deposition and a significantly enhanced photoelectrochemical (PEC) water splitting performance was demonstrated. The PEC performance of the photoanodes was investigated for various ZnS thicknesses and GaN doping concentrations. The photocurrent density of the ZnS/GaN photoanode at zero bias was enhanced by a factor of 1.75 compared to that for the reference GaN structure. Furthermore, significantly enhanced photoanode stability was observed with the optimized ZnS coating. The high performance of the ZnS/GaN photoanode is attributed to the type-II band alignment of the heterojunction, which forms a potential barrier for electron injection to the electrolyte while facilitating hole transfer. Therefore, using a ZnS cocatalyst coated on GaN is a promising technique to fabricate photoanodes for PEC-based solar energy harvesting.

Growth rate independence of Mg doping in GaN grown by plasma-assisted MBE

Doping of Ga(Al)N layers by plasma-assisted molecular beam epitaxy in Ga-rich conditions on c-plane bulk GaN substrates was studied. Ga(Al)N samples, doped with Mg or Si, grown using different growth conditions were compared. In contrast to Si doped layers, no change in the Mg concentration was observed for layers grown using different growth rates for a constant Mg flux and constant growth temperature. This effect enables the growth of Ga(Al)N:Mg layers at higher growth rates, leading to shorter growth time and lower residual background doping, without the need of increasing Mg flux. Enhancement of Mg incorporation for Al containing layers was also observed. Change of Al content from 0% to 17% resulted in more than two times higher Mg concentration.

GaN codoping and annealing on the optoelectronic properties of SnO2 thin films

Transparent GaN doped SnO2 thin films were deposited on glass substrates by e-beam evaporation with GaN:SnO2 targets of various GaN weight ratios. The effects of doping level and annealing temperature on the optoelectronic properties of GaN codoped SnO2 thin films were investigated. A conversion from n-type conduction to p-type was observed for GaN doped thin films upon annealing at 440 °C regardless doping level. However, it converted back to n-type conduction at various higher temperatures depending on GaN doping levels. Hole concentration for p-type GaN:SnO2 thin films could be achieved as high as 1.797 × 1019 cm−3 because of the codoping of Ga and N. Hall measurements showed that upon proper thermal treatments, Ga3+-Sn4+ and N3−-O2− substitution reactions occurred in the thin films, which regulated the polarity of conduction and carrier concentration. The formation of No substitutions in the GaN:SnO2 thin films and decomposition of them at certain higher temperature were mainly responsible for the n-p-n conduction transition.

Thermal quenching of the yellow luminescence in GaN

We observed varying thermal quenching behavior of the yellow luminescence band near 2.2 eV in different GaN samples. In spite of the different behavior, the yellow band in all the samples is caused by the same defect—the YL1 center. In conductive n-type GaN, the YL1 band quenches with exponential law, and the Arrhenius plot reveals an ionization energy of ∼0.9 eV for the YL1 center. In semi-insulating GaN, an abrupt and tunable quenching of the YL1 band is observed, where the apparent activation energy in the Arrhenius plot is not related to the ionization energy of the defect. In this case, the ionization energy can be found by analyzing the shift of the characteristic temperature of PL quenching with excitation intensity. We conclude that only one defect, namely, the YL1 center, is responsible for the yellow band in undoped and doped GaN samples grown by different techniques.