Multicycle Rapid Thermal Annealing Research Articles

Research On Mg Doping in Nitride Semiconductor Materials

Summarized the achievements of various model research directions in nitride Mg doped semiconductors in this thesis. Due to the Internal properties of Mg acceptors which resulting the low ionization rate of acceptor, and the low hole mobility in heavily Mg doped nitrides, the conductivity of Mg doped nitrides is limited. At present, research is divided into three categories of models, one is a new type of polarization model. This model adopts a built-in electron polarization ionization acceptor dopant in the bulk uniaxial semiconductor crystal, and provides an attractive solution for the problem of P-type and n-type doping in broadband gap semiconductors. The other two are the traditional thermal activation model. There are mainly ultra-high-pressure-annealing (UHPA) and multicycle rapid thermal annealing (MRTA) respectively. Both of these schemes can activate more Mg impurities, resulting in higher conductivity in nitrides. The principles, advantages and disadvantages, and future development prospects will be explained of these three models in this thesis.

Novel Codoping Moiety to Achieve Enhanced P‐Type Doping in GaN by Ion Implantation

Codoping of gallium nitride for improved acceptor ionization has long been theorized; however, reduction to practice proves difficult via growth. Herein, implementation of codoping via ion implantation and symmetric multicycle rapid thermal annealing utilizing magnesium codoped with silicon or oxygen is demonstrated. Results show enhanced photoluminescence with both donor species but with an order of magnitude greater increase with concurrent p‐type hall for codoping with oxygen. Furthermore, the addition of nitrogen to balance stoichiometry suppresses defect photoluminescence signals. The incorporation of the donor and nitrogen demonstrates defect reduction beyond magnesium, only implants despite the additional implant dose and resultant damage with coimplantation. The enhanced hole concentrations evident with oxygen incorporation reveal important considerations for device design given unintentional doping during growth and future incorporation of ion implantation capabilities.

Efficient Activation and High Mobility of Ion-Implanted Silicon for Next-Generation GaN Devices

Selective area doping via ion implantation is crucial to the implementation of most modern devices and the provision of reasonable device design latitude for optimization. Herein, we report highly effective silicon ion implant activation in GaN via Symmetrical Multicycle Rapid Thermal Annealing (SMRTA) at peak temperatures of 1450 to 1530 °C, producing a mobility of up to 137 cm2/Vs at 300K with a 57% activation efficiency for a 300 nm thick 1 × 1019 cm−3 box implant profile. Doping activation efficiency and mobility improved alongside peak annealing temperature, while the deleterious degradation of the as-grown material electrical properties was only evident at the highest temperatures. This demonstrates efficient dopant activation while simultaneously maintaining low levels of unintentional doping and thus a high blocking voltage potential of the drift layers for high-voltage, high-power devices. Furthermore, efficient activation with high mobility has been achieved with GaN on sapphire, which is known for having relatively high defect densities but also for offering significant commercial potential due to the availability of cheap, large-area, and robust substrates for devices.

P-Conductivity in Co-Implanted and Gyrotron Microwave-Annealed GaN: Optical and Electrical Studies

A key step to the development of future (Al)GaN-based power devices is the ability to form selective-area p-type regions, and a promising method of accomplishing this is through ion implantation of the dominant acceptor dopant, Mg. Ion implantation in GaN induces lattice damage and creates point defects, including vacancies and interstitials. To remove this damage, heal defects, and activate the implanted dopants requires temperatures >1000 °C, beyond the thermodynamic equilibrium stability of GaN [1]. Removal of point defects, especially donor-like nitrogen vacancies (VN) in the material can be accomplished through high temperature annealing and may be aided by co-implantation with N [2]. It is speculated that compensation of implant-induced VN may be enhanced during the annealing process by diffusion of implanted Ni to the corresponding site in the N sublattice, however evidence for such compensation through suppression of VN-related photoluminescence has not been reported in p-type material. Methods of high-temperature nonequilibrium annealing such as symmetric multicycle rapid thermal annealing (SMRTA), microwave annealing, and laser annealing have been reported to achieve p-type material, however activation of Mg-implanted material remains an area of active study.In this study, we report our latest results in obtaining p-type conductivity in implanted GaN by repeated, short-duration thermal cycles using a high-power (kW) gyrotron microwave source and systematic study of Mg implantation and Mg+N co-implantation into GaN and the resulting defect microstructure [3]. We examine the effect of co-implantation to depths of 250 nm and 500 nm, and Mg:N concentration ratios of 1 and 2 with [Mg]=1019 cm-3. Implanted MOCVD-grown u-GaN is capped with AlN and annealed for <60 sec in 1.5–3 sec heating cycles at 1350 °C using a gyrotron under relatively low N2 overpressure of 3 MPa.The optical and electrical properties of the resulting films are characterized by temperature-dependent photoluminescence (PL) and I-V measurements of vertical p-i-n diodes. Highly resistive Mg+N co-implanted and annealed material shows dominant luminescence of the VN-related GL2 band at 2.37 eV and relatively lower-intensity acceptor-related UVL and UVL* bands (at 3.05–3.27 eV). However, implanted/annealed samples showing p-n diode behavior (Vth=3–3.1 V at room temperature) and hole concentrations in the range of 1015–1017 cm-3 show dominant/high UVL/UVL* bands and YL band centered at 2.30 eV. The YL is attributed to unintentionally introduced CN-ON complexes and is commonly observed in GaN grown by MOCVD but is typically not observed in implanted/annealed GaN. Additionally, samples implanted only with Mg and activated under the same annealing conditions show persistent dominance of the GL2 band in this range.This work is funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy under the PNDIODES programs directed by Dr. Isik Kizilyalli.[1] G. Alfieri, et al. J. Appl. Phys., 123, 205303 (2018)[2] R. Tanaka et al. Japan. J. Appl. Phys., 59 , SGGD02 (2020)[3] V. Meyers et al. manuscript submitted for publication (May 2020) Figure 1

(Invited) Ion Implantation and Activation of n- and p-Type Dopants in GaN

GaN-based power switching devices are of significant interest for high efficiency power conversion circuits in medium voltage applications. However, there are still significant challenges preventing mass production and widespread adoption. Recently, GaN-based vertical and lateral power devices have attracted significant interest due to promising device results coupled with progress in native substrate, epitaxial growth, and processing technology developments. However, there are two significant limiting factors for vertical GaN power devices at present. The first issue is substrate and epitaxial layer uniformity and reliability, and the second is the lack of a planar selective-area doping process. While p-type epitaxial growth capability and the ability to stack layers are quite mature from the LED industry, power devices require planar selective-area doping, which is a routine step by ion implantation in the Si and SiC device industry. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge as implant activation typically requires annealing at temperatures ~ 2/3 of the melting point, which is a region where the GaN crystal is unstable and readily decomposes to Ga + N2 at atmospheric pressure. The NRL-developed symmetric multicycle rapid thermal annealing (SMTRA) technique has been the most successful, demonstrating electrical activation of up to ~10% of the implanted Mg dopant atoms using a combination of a temporary, thermally stable capping layer, annealing in a moderate nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. Advances in the SMRTA process development, including optimization of capping processes, implantation profile, and processing steps such as metallization and surface treatments will be reviewed here. In addition, the evaluation of components of the SMRTA process, such as the cap and overpressure anneal, have been applied to Si implantation for n-type doping and will be presented here. This is a critical step to reduce contact resistance in lateral GaN structures as well as form the source regions of vertical devices that is typically accomplished by selective area regrowth. The use of ion implantation and activation utilizes fewer process steps and avoids impurity incorporation at sidewalls compared to the selective area regrowth process, and has been shown to yield lower contact resistance as well. This presentation will review advances in both Mg and Si ion implantation and assess the viability of this process for practical device fabrication.

Polarity dependent implanted p-type dopant activation in GaN

Here we present a direct comparison between the activation of implanted Mg ions in N–polar and Ga–polar substrates to produce p-type GaN via symmetric multicycle rapid thermal annealing (SMRTA). Physical dopant activation was achieved by annealing in moderate nitrogen pressures (3.3 MPa) in conjunction with a bi-layer cap. Photoluminescence shows activation was more readily achieved for N–polar films with measured UV luminescence up to ∼15× as intense as yellow/green luminescence compared to ∼2.4× for Ga–polar films. The greater activation of N–polar material was primarily due to a higher thermal stability compared to Ga–polar films. This demonstration of implanted Mg activation by SMRTA enables a facile route toward next generation vertical devices.

Defect Characterization of Multicycle Rapid Thermal Annealing Processed p-GaN for Vertical Power Devices

Post-implantation damage and defect characterization of multicycle rapid thermal annealing (MRTA) activated Mg+ implanted (0001) GaN on (110) sapphire substrate was investigated using high resolution X-ray scattering and transmission electron microscope techniques. The initial implant-induced isolated defects produced a pseudomorphic strain that matches the calculated recoil damage profile in the GaN - along the [0001] direction. Triple axis X-ray diffraction showed removal of the implant-induced elastic strain during two different MRTA annealing processes. An improvement in crystalline quality after the different MRTA processes was also noted, indicating that the MRTA processes can reduce crystalline strain and defects in as-grown material. For the different MRTA processes, an improvement in electrical conductivity was correlated with a lower (0004) triple axis rocking curve width and with a reduced presence of extended defects observed from electron microscopy measurements. The extended defects were identified to be circular defects with an observable screw component. The defect density was on the order of 1010 cm−2. While both MRTA processes led to a high activation efficiency, a higher residue defect density was correlated with a greater X-ray scattering rocking curve width of the symmetric (0004) reflection, which was further correlated with a lower hole mobility (20 cm2/V⋅s vs 40 cm2/V⋅s).

Effect of high temperature, high pressure annealing on GaN drift layers for vertical power devices

In this work, we evaluate the effect of the novel symmetric multicycle rapid thermal annealing (SMRTA) process on both Mg ion implanted and non-implanted thick unintentionally doped GaN drift layers for vertical power devices. The typical p-type behavior and restoration of implant damage are observed in implanted samples, but on non-implanted samples a reduction in background carrier concentration and associated reduction in leakage current and increase in breakdown voltage is observed. This indicates that the capping/annealing process itself is not detrimental to the crystal, and the annihilation of native point defects in the process has beneficial effects for device structures.

(Invited) Novel Implantation Processing and Characterization for Scalable GaN Power Devices

Novel ion implantation processing has been introduced to enable p-type doping for high performance vertical GaN p-n power devices. Multicycle Rapid Thermal Annealing (MRTA) leads to state-of the-art p-type doping (> 8% dopant ionization); high n-type doping can also be achieved. The implantation process is also compatible with a tapered implanted junction terminal extension approach to achieve effective edge termination necessary to provide breakdown voltages over 90% of the theoretical maximum value at minimal cost and processing complexity. MRTA, with ambient control (N2 ambient at 200 psi), pulsed annealing up to ~1400 °C, and an appropriate dielectric encapsulant, is utilized to further improve activation efficiency and, ultimately, p-n junction performance through selective ion implantation and processing optimization. Here, the focus is on MRTA activation of Mg+ implantation, with comparison to the potential effectiveness of other p-type dopants, to assess the impact of the crystalline nature of the substrate and to optimize the encapsulant mask, in order to promote the highest dopant activation. This fundamental study of the role of defects in material produced by GaN epitaxial deposition is expected to lead to a pathway to high activation efficiency through a mechanistic understanding of the relationship between processing methods and performance. The critical link between defects and implant activation will be discussed through the use of novel structural materials characterization techniques as well as standard electrical and optical techniques. Electron microscopy defect analysis, determining the role of implanted and annealed defects, and providing localized strain measurements helps assess defect evolution as a function of implant and annealing parameters. X-ray based reciprocal space mapping as a function of the implant and subsequent annealing will be shown to correlate strain and defect formation with dopant activation in order to parameterize the MRTA process non-destructively. This information leads to the production of p-n junction devices that are scalable, provide high surge currents, and high ideality factors. Understanding point defect concentrations, dislocation types and concentrations, and residual stress will lead to effective p-type dopant activation and high device performance.

(Invited) Vertical GaN Devices Enabled By Selective Area P-Type Doping

A key element for enabling high performance vertical GaN power electronic devices is the capability to realize selective area p-type doping. The ability to arbitrarily define p-type regions through ion implantation eliminates the need for epitaxial regrowth on etched surfaces, which is currently a well-known technological roadblock. However, activation of implanted Mg is also a challenge, since activation of the Mg acceptors requires annealing at temperatures above the thermodynamic stability limits of GaN in atmosphere. Implementing the symmetric multi cycle rapid thermal annealing (SMTRA) technique has been shown to activate up to ~10% of the implanted Mg dopant atoms. This technique includes a temporary thermally stable capping layer, annealing in a nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. Vertical GaN junction barrier Schottky (JBS) diodes and Schottky barrier diodes (SBDs) with implanted junction termination extension (JTE) are demonstrated using the SMRTA process, as are process module development toward trench MOSFET devices.

Thermal Evolution of Implantation Damages in Mg-Implanted GaN Layers Grown on Si

Localized p-doping is a keystone in the development of many GaN based devices that need selectively doped pocket area such as MOSFETs or PIN diodes. Electrical activation of implanted Mg has been obtained lately with GaN grown on sapphire substrate using an AlN based protective layer combined with multi cycle of rapid thermal annealing at very high temperature (>1300 °C) proving the feasibility and interest of the method [1]. The use of Si (111) substrate allows a better integration of the process on microelectronic production line. However the growth of GaN on Si (111) is less mature and necessitates a buffer usually composed of a complex staking based on AlN intermediate layers [2]. The difference in lattice parameters and thermal extension coefficient in the stacking may make the samples more unstable during the high thermal budget required for doping activation. In the last few years, several groups focused their research on the prevention of GaN surface degradation at high temperature [3] and we have shown that a combination of in situ epi deposited AlN and SiN thin layers is very efficient to avoid this phenomena [4]. Here, we propose to study the impact of high thermal annealing on the evolution of the damage in Mg-implanted GaN (on Si). Indeed, comprehensive studies of the GaN healing mechanism in this kind of structures are not yet reported. We use samples of GaN layer grown by MOCVD on Si (111) using an AlN based buffer layer and protected by an AlN/SiNx cap layer. These samples are implanted with Mg fluences ranging from 1013 to 1015 at/cm² corresponding to maximum effective concentration between 5.1017 and 5.1019 at/cm3 in the implanted layer. 400 °C -1100 °C anneals are proceeded in controlled N2 atmosphere. X-Ray Diffraction (XRD) and Photoluminescence (PL) measurements point out that ion implantation induced damage are healed thanks to different mechanisms: both lattice strain relaxation and point defect annihilation can occur during annealing with different kinetics. Indeed as we can see on Fig.1a, on samples implanted with low to medium fluence (up to 1014 at/cm²), relatively low temperature anneals (< 450 °C) are enough to obtain a near complete correction of the lattice deformation. However, surprisingly, the PL spectra of 400 °C annealed samples shown on Fig1.b indicates that a lot of non-radiative defects are still present and shutting down the PL intensity. The spectra of the sample annealed at 1100°C exhibits bands characteristic of Mg in Ga site (Mg(Ga)) which proves the necessity of a higher thermal treatment to reduce non radiative defects concentration and promote Mg incorporation in the GaN lattice. As shown on Fig 2.a, an increase of the fluence (1015 at/cm²) leads to an increase of the induced strain but also of the temperature necessary to fully relax it (1100°C). However, the PL spectra (Fig 2.b) indicates that even after such a high thermal treatment, the PL intensity of the bands associated with Mg(Ga) is weaker than for medium fluence despite the higher Mg concentration in the matrix [4]. We assume that this behavior may be due to residual damage. This is supported by Transmission Electron Microscopy images of the layer (not shown here) that evidence remaining defective pockets after a 1 h 1100 °C in N2 annealing. These results give insight on the healing mechanism occurring during the activation anneal of Mg implanted in GaN. They also highlight that damage management is a critical parameter for GaN (on Si) p-doping via Mg implantation. To reach this objective, we will show that it is necessary to implement further specific processes to limit the implantation induced damage and to improve the damage recovery efficiency. 1. Tadjer, M.J., et al., Selective p-type Doping of GaN:Si by Mg Ion Implantation and Multicycle Rapid Thermal Annealing. ECS Journal of Solid State Science and Technology, 2015. 5(2): p. P124-P127. 2. Charles, M., et al., The effect of AlN nucleation temperature on inverted pyramid defects in GaN layers grown on 200 mm silicon wafers. Journal of Crystal Growth, 2017. 464: p. 164-167. 3. El-Zammar, G., et al., Surface state of GaN after rapid-thermal-annealing using AlN cap-layer. Applied Surface Science, 2015. 355: p. 1044-1050. 4. Lardeau-Falcy, A., et al., Capping stability of Mg-implanted GaN layers grown on silicon. physica status solidi (a), 2016. Figure 1

(Invited) Novel Implantation Processing and Characterization for Scalable GaN Power Devices

Novel ion implantation processing enables p-type doping for high performance vertical GaN p-n power devices. Multicycle Rapid Thermal Annealing with an N2 ambient at 200 psi, pulsed annealing up to ~1400 °C, and an appropriate dielectric encapsulant, is utilized to improve activation efficiency and, ultimately, p-n junction performance through selective ion implantation and processing optimization. Here, the focus is on analysis of MRTA activated, Mg+ implantation into GaN deposited on sapphire substrates. The link between defects and implant activation will be assessed through the use of novel structural materials characterization techniques. X-ray based reciprocal space mapping as a function of the implant and subsequent annealing will be shown to correlate strain and defect formation with dopant activation. Electron microscopy defect analysis is demonstrated to provide localized strain measurements after annealing. Understanding the evolution of implant-induced point defect concentrations and dislocation formation, will lead to effective p-type dopant activation and high device performance.

Vertical GaN Junction Barrier Schottky Diodes

Vertical GaN junction barrier Schottky (JBS) diodes are demonstrated. The JBS p-type grid was formed by Mg-implantation into a 10 μm thick unintentionally doped GaN homoepitaxial drift layer, grown by metal organic chemical vapor deposition (MOCVD). Then, symmetrical multi-cycle rapid thermal annealing (SMRTA) repaired implantation damage and activated the Mg ions. This process includes deposition of an AlN capping layer, annealing in a nitrogen overpressure, and rapid heating and cooling pulsed annealing. In addition, PiN diodes and Schottky barrier diodes (SBDs) were fabricated on the same substrate for comparison. The JBS diode demonstrates forward conduction characteristics dominated by the Schottky barrier (turn-on voltage ∼0.5 V), and reverse breakdown voltage comparable to the PiN diode (−610 V).

Improvements in the Annealing of Mg Ion Implanted GaN and Related Devices

The activation of ion implanted p-type dopants in GaN is notoriously difficult as the extremely high temperatures required to activate implanted Mg also damage the GaN crystal. In this paper, we present refinements to our novel annealing process (symmetric multicycle rapid thermal annealing) to reduce surface damage and contamination responsible for elevated leakage currents and non-ideal diode behavior. Furthermore, we apply the technique to Mg-implanted bulk GaN substrates to enable vertical power device structures, demonstrating rectifying p-i-n junctions. In addition, the technique was applied for edge termination in both p-i-n and Schottky barrier diodes, realizing floating guard ring and junction termination extension structures. The processes demonstrated here represents a key enabling step for future GaN-based power devices.

(Invited) Improved Vertical GaN Diodes with Mg Ion Implanted Junction Termination Extension

GaN is an ideal material for power applications due to its high breakdown field, high switching speed, and low switching losses. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge. Dopant implantation and activation is crucial for many devices since it is the most straightforward method of selective lateral doping in GaN. Devices such as the current aperture vertical electron transistor (CAVET), Schottky diodes with guard ring termination or grayscale junction termination extension, and vertical p-i-n diodes will be enhanced and enabled by the ability to selectively dope GaN without the need for etching or regrowth. We have previously reported on the first implanted p-i-n diode fabricated on a bulk GaN substrate. Here, we present improved GaN Schottky barrier diodes (SBDs) and p-i-n diodes with edge termination formed by either a field plate or junction termination extension (JTE) achieved by ion implanted and symmetric multicycle rapid thermal annealing (SMRTA). The devices with JTE exhibited substantially reduced leakage currents and improved turn-on characteristics. This is attributed to the elimination of the plasma process steps associated with the deposition and patterning of the field oxide layer required in a field plate process. The breakdown characteristics were studied by electroluminescence imaging, and is indicative of avalanche behavior. The realization of vertical GaN devices with low reverse leakage and ion implanted termination regions represents a key step for future power electronic devices.

(Invited) Improved Vertical GaN Diodes with Mg Ion Implanted Junction Termination Extension

Improved GaN Schottky barrier diodes (SBDs) and p-i-n diodes with edge termination formed by either a field plate or junction termination extension (JTE) achieved by ion implanted and symmetric multicycle rapid thermal annealing (SMRTA) are demonstrated. The devices with JTE exhibited substantially reduced leakage currents and improved turn-on characteristics. This is attributed to the elimination of the plasma process steps. The breakdown characteristics were studied by electroluminescence imaging, and is indicative of avalanche behavior. The realization of vertical GaN devices with low reverse leakage and ion implanted termination regions represents a key step for future power electronic devices.

Defect reduction in MBE-grown AlN by multicycle rapid thermal annealing

Multicycle rapid thermal annealing (MRTA) is shown to reduce the defect density of molecular beam epitaxially grown AlN films. No damage to the AlN surface occurred after performing the MRTA process at 1520°C. However, the individual grain structure was altered, with the emergence of step edges. This change in grain structure and diffusion of AlN resulted in an improvement in the crystalline structure. The Raman E2 linewidth decreased, confirming an improvement in crystal quality. The optical band edge of the AlN maintained the expected value of 6.2 eV throughout MRTA annealing, and the band edge sharpened after MRTA annealing at increased temperatures, providing further evidence of crystalline improvement. X-ray diffraction shows a substantial improvement in the (002) and (102) rocking curve FWHM for both the 1400 and 1520°C MRTA annealing conditions compared to the as-grown films, indicating that the screw and edge type dislocation densities decreased. Overall, the MRTA post-growth annealing of AlN lowers defect density, and thus will be a key step to improving optoelectronic and power electronic devices.

Improved Vertical GaN Schottky Diodes with Ion Implanted Junction Termination Extension

The realization of selectively implanted p-type regions in GaN as well as an understanding of processing effects that cause carrier type conversion are key enabling steps for vertical GaN devices. Here, GaN Schottky barrier diodes (SBDs) with edge termination formed by either a field plate or junction termination extension (JTE) achieved by ion implanted and symmetric multicycle rapid thermal annealing (SMRTA) are presented. The devices with JTE exhibited substantially reduced leakage currents and improved turn-on characteristics. This is attributed to the elimination of the plasma process steps associated with the deposition and patterning of the field oxide layer required in a field plate process. The breakdown characteristics were studied by electroluminescence imaging, and is indicative of avalanche behavior. The realization of vertical GaN devices with low reverse leakage and ion implanted termination regions represents a key step for future power electronic devices.

Selective p-type Doping of GaN:Si by Mg Ion Implantation and Multicycle Rapid Thermal Annealing

Selective p-type doping of Si-doped GaN by Mg ion implantation and multicycle rapid thermal annealing (MRTA) is demonstrated. Samples of GaN were capped by AlN and annealed in N2 overpressure at temperatures up to 1330°C. MRTA resulted in a lower resistivity films, compared to a conventionally-annealed reference sample. Mg acceptor activation for the MRTA annealed sample was calculated using a two-layer corrected Hall method to be about 1% at room temperature. Low temperature photoluminescence revealed donor-acceptor bound electron-hole pair recombination near 3.2 eV, indicating activation of Mg upon MRTA annealing.

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

The implantation and activation of dopants in GaN is a key enabling step for future devices requiring selective area doping. High temperatures (over 1300 ˚C) are required to activate p-type dopants in GaN. These high temperatures are above the decomposition temperature of GaN, which necessitates advanced annealing processes. One such process, the multicycle rapid thermal annealing (MRTA) process, has enabled the implantation and activation of p-type dopants in GaN. In this research, we demonstrate the symmetrical multicycle rapid thermal annealing (SMRTA) annealing process. The SMRTA annealing process includes an extra conventional annealing step after the rapid heating and cooling cycles and is shown to improve the crystalline structure of annealed GaN compared to the MRTA process.