Mg In GaN Research Articles

Dissolution of Mg-enriched defects in implanted GaN and increased p-type dopant activation

Annealing Mg-implanted homoepitaxial GaN at temperatures above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects, in the form of inversion domains, contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N2 overpressure), which results in a low dopant activation efficiency. Triple-axis x-ray data reveal that implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that strain-inducing point defects formed during implantation have reconfigured and inversion domains are formed. However, annealing at temperatures of 1400–1500 °C (one GPa N2 overpressure) eliminates the presence of the inversion domains. While residual defects, such as dislocation loops, still exist after annealing at and above 1400 °C, chemical analysis at multiple dislocation loops shows no sign of Mg segregation. Meanwhile, an overall decreasing trend in the dislocation loop density is observed after annealing at the higher temperatures and longer times. Additionally, once inversion domains are formed and the samples are cooled to room temperature, they are shown to dissolve with subsequent annealing above 1400 °C. While such defects have been observed before, the important finding that such defects can be dissolved with a short, higher temperature step is key. Earlier work [Breckenridge et al., J. Appl. Phys. Lett. 118, 022101 (2021)] addressing electrical measurements of these types of samples showed that annealing at 1400 °C leads to a dopant activation efficiency that is an order of magnitude higher than that observed at 1300 °C. This work complements earlier work by identifying the inversion domains, which incorporate Mg, and points to the benefits, in terms of defect density and p-type dopant activation, of using higher temperature (>1400 °C) annealing cycles to activate Mg in GaN, even if the Mg-containing inversion domains had been formed during lower temperature annealing.

(Invited) Understanding Mg-Related Defects for Vertical GaN p-n Junction Structures Via p-Type Ion Implantation

Vertical GaN power devices have emerged to become promising candidates for next-generation high power applications due to superior material properties such as high breakdown voltage, low on-resistance, and high mobility compared to devices based on Si and SiC. GaN-based p-n junction switching devices enable higher voltage power with significantly higher efficiencies with added advantages of systems with reduced size and weight. A technological limitation of GaN, however, has been the inability to achieve high p-type doping in a planar, vertical device. Here, we will focus on recent developments to achieve high p-type efficiency though ion implantation, novel high temperature annealing schemes, and the importance of defects and morphology in native substrates and epitaxial layers.Mg-implanted homoepitaxial GaN annealed at temperatures at or above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects in the form of inversion domains contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N2 overpressure), which results in a low dopant activation efficiency. While residual defects, such as dislocation loops, still exist after annealing at and above 1400 °C, chemical analysis at multiple dislocation loops shows no sign of Mg segregation. Meanwhile, an overall decreasing trend in the dislocation loop size and density is observed after annealing at the higher temperatures and longer times. Earlier work addressing electrical measurements of these types of samples showed that annealing at ~1400 °C leads to a dopant activation efficiency that is an order of magnitude higher than that observed at 1300 °C. This work complements the earlier work by identifying the microscopic defects (inversion domains) which incorporate Mg, and points to the benefits, in terms of defect density and p-type dopant activation, of using higher temperature (1400 °C) annealing cycles to activate Mg in GaN.

(Invited) In-Situ MOCVD Etching of GaN Using XeF2 for Selective-Area-Epitaxial-Regrowth of p-Type GaN for High Voltage PN Diodes

PN diodes with multi-kilovolt breakdown voltages have been demonstrated to great than 6 kV in GaN, validating the device potential predicted by the intrinsic material properties of III-nitride semiconductors. However, power control circuits also require switching transistors such as junction field effect transistors (JFETs) and practical diodes such as merged PIN Schottky (MPS) diodes. Such devices usually employ selective areas of p-type semiconductor surrounded by n-type material and require the formation of PN junctions with low reverse leakage current. Selective-area p-type doping in Si and SiC based power devices is typically achieved using implantation and thermal annealing to form the p-type areas. Ion implantation of Mg in GaN has demonstrated p-type conductivity, but this process requires specialized equipment for the high-pressure and high-temperature activation of Mg dopants. We have investigated selective-area-regrowth (SArG) to epitaxially grow p-type GaN as an alternative to dopant implantation. Successful PN diode formation by SArG requires a process to remove residual crystalline damage resulting from the ex-situ inductively coupled plasma (ICP) etch typically used to form the p-well, as well as a method to remove the elevated level of Si found at the regrowth interface of a surface that had been exposed to air. For the case of SArG of p-GaN on air-exposed, blanket ICP etched n-type GaN, we present the novel use of a fluorine-based precursor (XeF2) for in-situ etching of GaN in the MOCVD chamber. Unlike chlorine (TBCl and CCl4) and bromine (CBr4) -based precursors we have studied; we obtained a smooth surface following in-situ fluorine etching of air-exposed GaN. Schottky barrier diodes (SBDs) formed by shadow mask evaporation on in-situ fluorine etched n-GaN that had been previously ICP etched showed reverse leakage currents to -40 V that are equal to those of SBDs formed on as-grown n-GaN layers. The in-situ fluorine/ICP etched Schottky diodes had reverse leakage currents more than 3 orders of magnitude lower than those formed on n-GaN layers that had only experienced ICP etching. This suggests that the in-situ fluorine etch was effective at removing the residual damage from the ICP etch process. Furthermore, we discuss the use of in-situ fluorine etching to reduce the concentration of Si at the regrowth interface required to form PN junctions with low reverse leakage current. This work was supported in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES program directed by Dr. Isik Kizilyalli, and in part by the US Department of Energy (DOE) Vehicle Technologies Office (VTO) under the Electric Drive Train Consortium. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the presentation do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Defect-mediated diffusion of implanted Mg in GaN: Suppressing dopant redistribution by sequential thermal and microwave annealing

The diffusion behavior of Mg in Mg/N co-implanted GaN is investigated in response to a set of annealing conditions and methodologies, namely, 1000 °C/30 min thermal anneal, by high-temperature pulsed gyrotron microwave annealing at 1420 or 1500 °C, or by thermal and microwave annealing, sequentially. After 1000 °C annealing, the diffusion of Mg in GaN is found to be negligible, as measured by secondary ion mass spectrometry. Annealing by gyrotron microwave annealing alone induces the diffusion of Mg at a rate on the order of 10−12 cm2/s. However, the use of a thermal anneal before microwave gyrotron annealing reduces this rate by an order of magnitude to 10−13 cm2/s. We find that a model that considers Mg diffusion from an inhomogeneous medium that contains a defect-rich implanted region near-surface to a relatively pristine region below the implant range better explains the observed diffusion behavior than a conventional model that assumes a homogeneous medium. By analyzing the diffusion behavior using the Boltzmann–Matano method, we present a discussion of reduction in [VGa] by thermal annealing at 1000 °C, leading to a suppressed diffusion coefficient during subsequent high-temperature annealing relative to diffusion after 1420/1500 °C annealing alone. This effect holds potential for improvement in the precision of selectively doped regions for future applications based on the (Al)GaN material system. An improved field profile control in real devices can increase the breakdown and current-handling capabilities in power electronic applications.

Pressure Effect on Diffusion of Native Defects and Mg Impurity in Mg‐Ion‐Implanted GaN during Ultrahigh‐Pressure Annealing

Herein, transmission electron microscopy direct observations are used to examine the time evolution of dislocation loops in Mg‐ion‐implanted GaN during annealing in N2 atmospheres of 2.0 and 0.3 GPa. It is indicated in the results that the diffusion of the native defects, for both interstitials and vacancies, is retarded during annealing at the higher pressure. Furthermore, secondary ion mass spectrometry shows that annealing at the higher pressure retards the migration of Mg and increases Mg‐acceptor concentration in the ion‐implanted region. These findings provide a design principle of the postimplantation annealing process to activate ion‐implanted Mg in GaN.

Selectively patterned Mg-doped GaN by SiNx-driven hydrogen injection

We demonstrate a method to achieve selectively patterned Mg-doped GaN layers using hydrogen drive-in through plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx) films. Activated Mg-doped GaN layers were selectively deactivated by patterned PECVD SiNx films with low-temperature annealing and showed high-resistive behavior. Spatially resolved photoluminescence measurements were used to optically verify the deactivation of Mg acceptors and showed distinct features corresponding to activated and deactivated Mg in GaN. The method suggested here provides a simple and effective method to achieve patterned Mg-doped GaN regions without thermal and plasma damage, which could cause degradation of device performance. The proposed method could provide a way to achieve future high-performance GaN lateral and vertical devices that rely on laterally patterned doping.

Atomic resolution analysis of extended defects and Mg agglomeration in Mg-ion-implanted GaN and their impacts on acceptor formation

We carried out atomic-scale observations of Mg-ion-implanted GaN by transmission electron microscopy (TEM) and atom probe tomography (APT) to clarify the crystallographic structures of extended defects and Mg agglomerations that form during post-implantation annealing. The complementary TEM and APT analyses have shown that Mg atoms agglomerate at dislocations that bound extended defects. The concentration of Mg is higher at the dislocations with a larger Burgers vector. This indicates that Mg agglomeration is caused by the pressure at the dislocations. Mg concentration in highly Mg-rich regions is 1 at. %, which exceeds the solubility limit of Mg in GaN. We investigated isothermal and isochronal evolution of the defects by TEM, cathodoluminescence analysis, and positron annihilation spectroscopy. The results indicated that the intensity of donor–acceptor pair emission increases with the annealing temperature and duration and reaches a maximum after elimination of the extended defects with highly Mg-rich regions. These results strongly suggest that such extended defects reduce the acceptor formation and that they as well as the previously reported compensating centers, such as N-related vacancies, can inhibit the formation of p-type GaN. The mechanism by which the extended defects reduce acceptor formation is discussed.

(Invited) Prospects for Magnesium Diffusion Doping of GaN

Ex-situ p-type doping of GaN has been a long-standing challenge and prevented adoption of numerous devices. While regrowth of p-GaN on etched surfaces and ion implantation have made progress, technical challenges remain. Diffusion of Mg into GaN has received considerably less attention, primarily due to the extremely low diffusivity of Mg in GaN as well as the instability of GaN at the temperatures required (typically >1000oC), as well as its propensity to form compensating defects. Recent work at LLNL on GaN grown both on sapphire and n-type GaN substrates has demonstrated the potential for diffusion of Mg at temperatures as low as 700oC. This is theorized to be through enhanced formation of Gallium vacancies, which provide a pathway for rapid diffusion into GaN. Use of a solid Mg source, such as MgF2, capped by a metal has been shown to be effective, with the diffusivity dependent on the specific metal cap (Figure 1).This talk will discuss the chemical thermodynamics of this process, termed Gallidation Assisted Impurity Diffusion or GAID, methods for control, current challenges, and device results including enhanced barrier height Schottky contacts resulting from the process. Prospects for the future of the technique and its applicability to new device concepts will be briefly addressed.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, LLNL-ABS-821793. This support does not constitute an express or implied endorsement on the part of the Government. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, PNDIODES program monitored by Dr. Isik Kizilyalli. Figure 1

Lattice Location Studies of the Amphoteric Nature of Implanted Mg in GaN

AbstractDespite the renewed interest in ion implantation doping of GaN, efficient electrical activation remains a challenge. The lattice location of 27Mg is investigated in GaN of different doping types as a function of implantation temperature and fluence at CERN's ISOLDE facility. The amphoteric nature of Mg is elucidated, i.e., the concurrent occupation of substitutional Ga and interstitial sites: following room temperature ultra‐low fluence (≈2 × 1010 cm−2) implantation, the interstitial fraction of Mg is highest (20–24%) in GaN pre‐doped with stable Mg during growth, and lowest (2–6%) in n‐GaN:Si, while undoped GaN shows an intermediate interstitial fraction of 10–12%. Both for p‐ and n‐GaN prolonged implantations cause interstitial 27Mg to approach the levels found for undoped GaN. Implanting above 400 °C progressively converts interstitial Mg to substitutional Ga sites due to the onset of Mg interstitial migration (estimated activation energy 1.5–2.3 eV) and combination with Ga vacancies. In all sample types, implantations above a fluence of 1014 cm−2 result in >95% substitutional Mg. Ion implantation is hence a very efficient method to introduce Mg into substitutional Ga sites, i.e., challenges toward high electrical activation of implanted Mg are not related to lack of substitutional incorporation.

(Invited) Moderate Temperature Mg Diffusion Doping of GaN

Ex-situ p-type doping of GaN has been a long-standing challenge and prevented adoption of numerous devices. While regrowth of p-GaN on etched surfaces and ion implantation have made progress, technical challenges remain. Diffusion of Mg into GaN has received considerably less attention, primarily due to the extremely low diffusivity of Mg in GaN as well as the instability of GaN at the temperatures required (typically >1000oC), as well as its propensity to form compensating defects. Here, we report on a new technique for diffusion doping of GaN with Mg at relevant temperatures (under 1000oC). This approach uses a bi-layer structure, with a solid Mg source in contact with the GaN combined with a metal capping layer. Using this approach, we have measured significant diffusion of Mg into GaN by Secondary Ion Mass Spectrometry at temperatures as low as 800oC and time on the order of minutes (Figure 1) with concentrations exceeding 1019 cm-3 and Mg extending >100 nm into the GaN film. Room temperature Hall effect measurements confirm p-type conductivity under a limited set of diffusion conditions. In other cases, compensation of the diffused Mg by formation of defects during diffusion resulted in highly resistive layers. Photo-Hall measurements using sub-band gap light of different wavelengths was used to excite both holes and electrons, depending on the wavelength. Samples with higher measured Mg concentration show correspondingly increased hole density under illumination with 2 eV light. These results strongly imply the diffusion of Mg through Ga sites, but with corresponding formation of compensating defects as the material becomes more p-type. Density functional theory (DFT) calculations on the diffusivity of Mg through various pathways as well as the formation energies of various compensating defects suggest a path towards achieving higher Mg concentrations with high free hole concentrations.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, LLNL-ABS-810935. This support does not constitute an express or implied endorsement on the part of the Government. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, PNDIODES program monitored by Dr. Isik Kizilyalli.Figure 1: Mg profiles after diffusion at 800oC for 5 minutes Figure 1

Influence of Mg doping on In adsorption and In incorporation in (In,Ga)N superlattices

We present a detailed investigation of the mechanisms at play for the incorporation of In and Mg on the GaN(0001) surface during plasma-assisted molecular beam epitaxy. First, we have studied the kinetics of In desorption in the presence of Mg either without or with N supply from the plasma cell by quadrupole mass spectrometry in the line of sight. Second, we have explored the effect of Mg doping at a different time along the cycle of (InN/GaN) supply repeated 10 times to form (In,Ga)N/GaN superlattices (SLs). By the complementary ex situ investigation of these SLs by x-ray diffraction and secondary ion mass spectrometry, we found that in the monolayer-thick (In,Ga)N layer, the In content was maximized when Mg was not supplied simultaneously to In, but it drastically decreased otherwise. In contrast, the Mg concentration strongly increased in the (In,Ga)N monolayers compared to the GaN barriers. We attribute this finding to the surfactant effect of In for Mg, which decreases the binding energy of Mg in GaN in the presence of N.

Strategies for p-type doping of ZnGeN2

ZnGeN2 has been proposed as an attractive semiconductor for a number of applications, but doping is largely unexplored. We examine the behavior of Li, Cu, Al, Ga, In, and C as candidate acceptors using hybrid density functional theory. Cu, In, and C give rise to deep acceptor levels, but Li, Al, or Ga could potentially lead to p-type conductivity. Al is particularly attractive since it has an ionization energy of 0.24 eV, comparable to Mg in GaN. However, self-compensation due to wrong-site incorporation is a serious issue. We demonstrate that co-doping with hydrogen can be used to overcome this problem.

Origin of Blue Luminescence in Mg -Doped GaN

In semiconductor science, Mg is used as a dopant in as-grown $n$-type GaN, not only to compensate the native electrons, but also to attain a reasonable hole concentration. However, Mg doping leads to defects that emit blue light, and the precise origin of this emission process needs to be understood, so that it can be eliminated. Here a detailed experimental and theoretical analysis identifies the formation of defect complexes consisting of interstitial and substitutional Mg in GaN. This study, performed on a large-surface-area network of GaN nanowalls exhibiting enhanced band-edge emission, also suggests growth conditions for fabricating $p$-type GaN with a high hole concentration.

Accuracy of GW for calculating defect energy levels in solids

The accuracy of $GW$ in the determination of defect energy levels is assessed through calculations on a set of well-characterized point defects in semiconductors: the As antisite in GaAs, the substitutional Mg in GaN, the interstitial C in Si, the Si dangling bond, and the Si split-vacancy complex in diamond. We show that the $GW$ scheme achieves a reliable description of charge transition levels, but the overall accuracy is comparable to that of hybrid-functional calculations.

Lattice Location of Mg in GaN: A Fresh Look at Doping Limitations.

Radioactive ^{27}Mg (t_{1/2}=9.5 min) was implanted into GaN of different doping types at CERN's ISOLDE facility and its lattice site determined via β^{-} emission channeling. Following implantations between room temperature and 800 °C, the majority of ^{27}Mg occupies the substitutional Ga sites; however, below 350 °C significant fractions were also found on interstitial positions ∼0.6 Å from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped p type with 2×10^{19} cm^{-3} stable Mg during epilayer growth, and lowest in Si-doped n-GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above 350 °C converts interstitial ^{27}Mg to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0eV.

Mg incorporation in GaN grown by plasma-assisted molecular beam epitaxy at high temperatures

The influence of growth conditions on the incorporation and activation of Mg in GaN grown by plasma-assisted molecular beam epitaxy at high growth temperature (>700°C) is presented. It is found that the highest Mg incorporation with optimized electrical properties is highly sensitive both to the Mg/Ga flux ratio and III/V flux ratio. A maximum Mg activation of ~5% can be achieved at a growth temperature of 750°C. The lowest resistivity achieved is 0.56Ω-cm which is associated with a high hole mobility of 6.42cm2/V-s and a moderately high hole concentration of 1.7×1018cm−3. Although the highest hole concentration achieved in a sample grown under a low III/V flux ratio and a high Mg/Ga flux ratio reaches 7.5×1018cm−3, the mobility is suffered due to the formation of defects by the excess Mg. In addition, we show that modulated beam growth methods do not enhance Mg incorporation at high growth temperature in contrast to those grown at a low temperature of 500°C (Appl. Phys. Lett. 93, 172112, Namkoong et al., 2008 [19]).

(Invited) From MRTA to SMRTA: Improvements in Activating Implanted Dopants in GaN

GaN and related compounds have received a great deal of attention from the research community due to their tunable direct bandgap, radiation hardness, and a favorable Baliga figure of merit compared to SiC and Si. P-type GaN is a challenging material and has been the focus of many studies because it has many potential applications in power electronics and optoelectronics. P-type dopant implantation and activation adds additional complexity to the synthesis of p-type GaN. The ability to implant and activate p-type species in GaN is crucial for devices that require selective area doping, such as many vertical devices. The implantation and activation of Mg in GaN is difficult because of the high temperatures required to activate the implanted dopants. Temperatures of at least 1200 ˚C are necessary to achieve Mg activation. At these high temperatures, GaN decomposes, and N vacancies, which are donors, form. Thus, for p-type dopant activation, it is necessary to avoid any compensating decomposition or defects. Advanced annealing processes, such as the Multicycle Rapid Thermal Annealing (MRTA) are required for Mg activation. In this research, we compare the previously reported MRTA technique with a new method termed Symmetrical Multicycle Rapid Thermal Annealing (SMRTA). The MRTA technique includes a conventional anneal followed by rapid heating and cooling pulses (1). MRTA has previously been shown to activate ~8% of Mg implanted GaN (2). The SMRTA technique differs from the MRTA process by introducing an additional conventional anneal after the rapid heating and cooling pulses to improve the crystal quality and thus repeatability of the process. GaN films grown on sapphire substrates using MOCVD were implanted with a previously described implantation profile using a three-layer AlN capping structure (3, 4). One of the implanted samples was annealed using the MRTA technique, which included a conventional anneal at 1000 ˚C for 30 minutes followed by 40 rapid heating and cooling pulses with a peak temperature of 1350 ˚C. A second sample was annealed using the same MRTA process with the exception of an additional conventional anneal at 1000 ˚C for 30 minutes after the rapid heating and cooling pulses. Characterization performed on the samples includes atomic force microscopy for assessing the surface morphology and Raman spectroscopy for monitoring the crystal quality. AFM measurements before and after the SMRTA treatment revealed that the surface morphology and surface roughness were unchanged after the annealing process. The RMS roughnesses before and after annealing were 1.55 and 1.65 nm, respectively. Thus, the three-layer cap which has been previously shown to protect the underlying GaN film during the MRTA process also adequately protects the surface during the SMRTA process (3). To determine crystal structure changes during the annealing processes, the full widths at half maximum (FWHM) of the Raman E2 modes were compared before and after the SMRTA process. After the MRTA process, the E2 Raman mode FWHM decreases from 6.19 cm-1 (as implanted) to 5.4 cm-1 (MRTA annealed). The SMRTA process results in a further decrease in the E2 FWHM to 5.2 cm-1. This decrease in the FWHM indicates an improvement in the crystalline quality of the implanted GaN after the MRTA process and an even further improvement after the SMRTA process. The additional crystalline improvement afforded by the conventional anneal that distinguishes the MRTA and SMRTA processes is attributed to the removal of stresses and associated defects that form in the GaN layer during the rapid heating and cooling pulses. Electrical data for SMRTA annealed devices will also be presented. The additional improvement in crystal quality provided by the SMRTA process will be a key enabling step for future Mg-implanted devices.

Diffusion of Mg dopant in metal-organic vapor-phase epitaxy grown GaN and AlxGa1−xN

Diffusion of the p-type dopant Mg in GaN and AlxGa1−xN which is accompanied by segregation and affected by transient effects in metal-organic vapor-phase epitaxy reactors is investigated. We have grown 110 nm thick Mg doped GaN and Al0.1Ga0.9N layers on top of undoped GaN and Al0.1Ga0.9N layers, respectively, in a temperature range between 925 °C and 1050 °C where we placed special emphasis on the lower temperature limit without diffusion to allow separation of Mg transients, diffusion, and segregation. Hereby, AlxGa1−xN layers enable monitoring of the resolution limit by secondary ion mass spectrometry analyses for the respective samples; therefore, thin AlxGa1−xN marker layers are incorporated in the thick GaN layers. We found an upper limit of 1.25 × 1019 cm−3 for diffusing Mg atoms in both sample types. Owing to the marked influence of Mg segregation in Al0.1Ga0.9N, diffusion is only seen by using a GaN cap on top of the Al0.1Ga0.9N layer sequence. Diffusion in Al0.1Ga0.9N is shown to be increased by about 25%−30% compared to GaN. Post growth annealing experiments under conditions equivalent to those used for growth of the Mg doped samples showed negligible diffusion. Comparing the results to well established findings on other doped III-V compounds, diffusion is explained by an interstitial-substitutional mechanism with a diffusion coefficient, which is concentration dependent. Analysis of the temperature dependent diffusivity revealed an activation energy of 5.0 eV for GaN:Mg and 5.2 eV for Al0.1Ga0.9N:Mg.

Enhanced electroluminescence intensity of InGaN∕GaN multi-quantum-wells based on Mg-doped GaN annealed in O2

In Ga N ∕ Ga N multi-quantum-well blue (461±4nm) light emitting diodes with higher electroluminescence intensity are obtained by postgrowth thermal annealing at 720°C in O2-ambient. Based on our first-principle total-energy calculations, we conclude that besides dissociating the Mg–H complex by forming H2O, annealing in O2 has another positive effect on the activation of acceptor Mg in GaN. Mg can be further activated by the formation of an impurity band above the valence band maximum of host GaN from the passivated MgGa–ON complex. Our calculated ionization energy for acceptor Mg in the passivated system is about 30meV shallower than that in pure GaN, in good agreement with previous experimental measurement. Our model can explain that the enhanced electroluminescence intensity of InGaN∕GaN MQWs based on Mg-doped p-type GaN is due to a decrease in the ionization energy of Mg acceptor with the presence of oxygen.

Electrical Activation and Carrier Compensation in Si and Mg Implanted GaN by Scanning Capacitance Microscopy

We studied the effect of the annealing temperatures (in the 1100-1200°C temperature range) on the electrical activation of ion implanted Si and Mg in GaN. Si + ions were implanted at multiple energies (from 80keV to 180keV) and with a total fluence up to 27x10 14 cm -2 in heteroepitaxial GaN films on sapphire. Some samples were subjected to conventional low ramp rate processes at 1100°C and 1200°C in N 2 ambient. A fast ramp rate pre-annealing in N 2 ambient was carried out on other samples before those low-ramp rate processes at 1100°C and 1200°C. The calibrated Scanning Capacitance Microscopy (SCM) was applied to determine the active (i.e. substitutional) Si depth profile. The rapid pre-annealing process leads to an increase from 36% to 63% in the substitutional Si dose. The Mg ions were implanted with energy of 50keV and fluence of 5x10 14 cm -2 in n' GaN layer on sapphire. Two fast ramp rate annealing process were performed. However, only an annealing at 1200°C for 30s in N 2 allows to observe, by means of SCM, the formation of a p layer.