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A-axis GaN/AlN/AlGaN Core-shell Heterojunction Microwires as Normally-off High Electron Mobility Transistors Weidong Song, Rupeng Wang, Xingfu Wang, Dexiao Guo, Hang Chen, Yuntao Zhu, Liu Liu, Yu Zhou, Qian Sun, Li Wang, and Shuti Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12986 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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ACS Applied Materials & Interfaces

A-axis GaN/AlN/AlGaN Core-shell Heterojunction Microwires as Normally-off High Electron Mobility Transistors Weidong Song,

†

‡

†

†

†

Yuntao Zhu, Liu Liu, Yu Zhou, †

†

Rupeng Wang, ‡ Xingfu Wang, ‡ §

†

Dexiao Guo, Hang Chen,

†

Qian Sun, Li Wang and Shuti Li * §

⊥

†

Guangdong Engineering Research Center of Optoelectronic Functional Materials and

Devices, South China Normal University, Guangzhou 510631, PR China. §

Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech

and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, PR China. ⊥

School of Materials Science and Engineering, Nanchang University, Nanchang,

330031, PR China.

*

Corresponding author:

E-mail address: [email protected]

Keywords: GaN microwire, GaN/AlN/AlGaN, heterojunction, electron gas, normally-off, HEMT

1

ACS Paragon Plus Environment


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Abstract

Micro/nanowire-based devices have been envisioned as a promising new route towards improved electronics and opto-electronics applications, which attracts considerable research interests. However, suffering from applicable strategies to synthesize uniform core-shell structures to meet the requirement for the investigations of electrical transport behaviors along length direction or HEMT devices, heterojunction wire-based electronics have been explored limitedly. In the present work, GaN/AlN/AlGaN core-shell heterojunction microwires on patterned Si substrates were synthesized without any as-synthesized

microwires

had

low

catalyst

dislocation,

via sharp

MOCVD. The and

uniform

heterojunction interfaces. Electrical transport performances were evaluated by fabricating HEMTs on the heterojunction microwire channels. Results demonstrated that normally-off operation was achieved with a threshold voltage of 1.4 V, a high on/off current ratio of 108, a transconductance of 165 mS/mm and a low subthreshold swing of 81 mV/dec. The normally-off operation may attribute to the weak polarization along semi-polar facets of the heterojunction, which leads to weak constrain of 2DEG.

1. INTRODUCTION In addition to significant success have been achieved in electro-optic field, GaN power electronics are envisioned as strategic frontier technologies in the next 2


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generation.1 However, doped GaN suffers from carrier mobility degradation due to the scatterings of charged dopant centres, which restricts their high power processing ability.2-4 To overcome this issue, HEMTs based on modulation doped AlGaN/GaN heterojunctions have been studied for more than two decades.5 Since it enables spatially separating carriers from ionic impurities to reduce/eliminate scatterings, high density 2DEG confined in the quantum well at the local heterointerface possesses significantly enhanced mobility, which is desirable for devices working at high speed with low noise.2,

6-7

Moreover,

AlGaN/GaN heterostructures own stronger intrinsic polarizations leading to the 2DEG value with 12 times better than GaAs or InP HEMTs,8 which makes them more competitive for microwave and mm-wave applications.9 Up to now, the investigation on planar HEMTs has yielded a large number of research results. However, far fewer synchronous achievements have been made in AlGaN/GaN wire-based electronics, although micro/nanowire-based devices have been considered as a promising new route towards improved electronic applications.10-12 Moreover, micro/nano wires possess desirable intrinsic properties such as small footprints and improved strain relaxation, which provides exciting new degree of freedom as compared to their conventional planar counterparts.10, 13-15 So far III-nitride micro/nanowires have been grown/synthesized through various approaches, either the top-down or bottom-up process. The inevitable 3



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damages of materials and crystal structures as well as surface defects, however, are caused by dry or wet etching during the top-down process,16 which brings about the degradation of device performance. Therefore a large body of investigations on III-nitride wires is based on bottom-up strategy, which provides a better flexibility in controlling the device architectures. And different synthesis techniques including chemical vapor deposition (CVD),17 metalorganic chemical vapor deposition (MOCVD)18-20 and molecular beam epitaxy (MBE)21-22 have been developed within this category. However, most of the reported III-nitride wires are vertically self-assembled growth along [0001] crystallization direction on substrates, which results in the shell epilayers deposited on non-polar sidewalls when growing core-shell structures or the heterojunctions formed on axial direction.23-24 As for the former, the reduced polarization electric field along non-polar crystal orientation is advantageous to improve quantum efficiency for light emitting devices, but it imposes restrictions on applications for HEMT devices, where carrier gas is in close relationship with the polarity. For the latter situation, such wires cannot meet the requirement for HEMT devices that the orientations between heterojunctions and electrical transport should be perpendicular. One the other hand, typical GaN HEMTs and related devices are normally on, which means that they conduct current at zero gate voltage.25-27 However, it is more desirable in practical applications to have normally-off devices that do not conduct 4


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current at zero gate voltage to avoid damages to the device and reduce circuit design complexity.28 In order to deplete the conduction channel, an easier implementation as developing channels on non/semi-polar growth surfaces29-31 is appealing when considering much complex and ongoing improved processing techniques, such as partially recessed AlGaN barrier layer,28 totally recessed AlGaN with hybrid MIS-HEMT structures,32 p-(Al)GaN cap layer33 or fluorine plasma treatment,34 usually employed traditionally. Moreover, vertically grown wires pose considerable challenges on fabricating integrated devices on account of the incompatibility with conventional planar process.35 Therefore, significant efforts have been dedicated to the synthesis of wires on patterned substrates featuring highly controlled position, size and uniformity in many groups,19, 36-38 which is also desirable for the investigation of III-nitride heterojunction wire-based electronics. In the present work, we demonstrate the synthesis of GaN/AlN/AlGaN core-shell heterojunction microwires (MWs) by MOCVD. The heterostructure MWs, distinguished from those vertical grown wires, were synthesized horizontally on patterned Si substrates without any catalyst. The MW featured trapezoidal cross-sectional morphologies that encompassed by two polar and two semi-polar facets. HAADF-STEM results exhibited that the as-grown MWs had low dislocation, sharp and uniform heterojunction interfaces. Normally-off HEMT

devices

were

fabricated

from

the

heterojunction

5


MWs

and


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corresponding electrical performances were assessed. A high on/off current ratio of 108, a transconductance of 165 mS/mm and a low subthreshold swing of 81 mV/dec were obtained from the metal oxide semiconductor HEMTs (MOS-HEMTs) with a thin GaN capping layer and SiO2 dielectric. The normally-off operation with threshold voltage of 1.4 V achieved in the MOS-HEMT may attribute to the weak polarization along semi-polar facets of the heterojunction, which results in weak constrain of 2DEG.

2. EXPERIMENTAL 2.1 Fabrication of Heterostructure MWs on Patterned Si The fabrication process was started by depositing a 150 nm SiO2 thin film on Si substrate via plasma-enhanced chemical vapor deposition (PECVD). Then, rectangular stripes (5 µm space/ 5 µm width) along the direction of Si were achieved by lithographic and wet chemical etching. Afterwards, inverted trapezoidal growth grooves with depth about 1 µm were fabricated by anisotropic etching in KOH (40 wt%) for 18 minutes at a constant temperature of 40 ℃. Prior to growth, the hydrofluoric acid solution was used to yield an oxide-free hydrogen-passivated Si surface. After that, substrates were loaded into MOCVD reactor (Thomas Swan Scientific Equipment Ltd.) for epitaxial growth. Trimethylgallium (TMGa), trimethylaluminum (TMAl), and ammonia (NH3) are used as Ga, Al, and N sources respectively. A thin AlN buffer layer 6


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was grown on the two opposite side facets at 900 ℃ at first. Then, the GaN core was selectively grown at 1020 ℃ and 400 mbar for 800 s. The AlN layer growth was carried out at 1050 ℃ and 75 mbar for 40 s. Subsequently, undoped AlGaN barrier layer was deposited for 25 s. A Si-doped AlGaN barrier was grown for 90 s using SiH4 as n-type doping source. The Al content was estimated to be about 30% according to planar growth parameters. The GaN cap layer was grown at 1050 ℃ for 30 s with other conditions equally. 2.2 Device Fabrication The GaN/AlN/AlGaN MWs were first released from the Si substrates by ultrasonication in isopropyl alcohol for 10-30 s after etching by acid solution (HNO3:HF=5:2), before which they were immersed in BOE solution to eliminate SiO2 mask layer. Then the solution containing MWs was dripped on a new substrate with 200 nm SiO2 insulating layer by spin coating. The MWs was then put into plasma cleaning apparatus for 15 minutes to clean the surface. Thereafter, Ohmic contacts were prepared by electron beam evaporating multilayered Ti/Al/Ti/Au (25/150/25/50 nm) followed by rapid thermal annealing at 800 °C for 30 s in a N2 ambient. The Schottky gate metallization was consisted of a Ni/Au (150/50 nm) double layer, while For MOS-HEMTs, a SiO2 with thickness of nominal 25 nm (referred to planar process) was deposited by PECVD at 300 ℃ before gate metal deposition.

7



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3. RESULTS AND DISCUSSION In our previous works,19, 39-40 high-quality GaN micro/nanowires and corresponding InGaN/GaN heterostructures on patterned Si substrates were prepared by MOCVD growth technique, which exhibited excellent optical and optoelectronic performances. In this work, we further explored the deposition of shell layers consisting of an AlN layer and an AlGaN barrier on the GaN MW templates and their electrical transport performances. The growth parameters were described in detail in the experimental. The MWs grown on patterned Si (100) are schematically illustrated in Figure 1a and GaN/AlN/AlGaN epitaxial growth relationship along direction on Si (111) sidewalls is shown clearly in the enlarged inset. Heterostructure MWs with smooth surfaces are distributed uniformly on Si as shown in Figure 1b and Figure S1 (the supporting information). The length of MWs can be controlled from several tens of micrometers to several hundreds of micrometers by the growth pattern and the length direction of —

MWs is along the non-polar direction , as a representative scanning electron microscopy (SEM) image (Figure 1c) indicates. The inset in Figure 1c shows released MWs from patterned Si substrates. The non-polar direction was confirmed by XRD pattern shown in Figure S2 in the supporting information. Trapezoidal shape of the cross-sectional MW is displayed clearly in the SEM image. It is worth to note that, by adjusting the Si pattern and tuning growth parameters, such as growth temperature, time and V/III ratio, the morphology 8


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and chemical composition of MWs can be controlled precisely and effectively. The trapezoidal cross-sectional MWs are encompassed by two {0001} polar —

—

facets and two semi-polar facets ({1101}, {1101}). The low index semi-polar planes are spontaneously formed and are more favorable for smooth GaN templates growth,33 after which are developed as platforms for shell layers growth. Foreseeably, GaN grown on Si should have a high dislocation density because of the large lattice mismatch between GaN and Si (~17%), thus seriously hindering the progress of GaN based device integrating on low-cost and compatible Si techniques. Despite the use of AlGaN transition layer or AlN buffer layer technology significantly improves the crystal quality, it still hardly comes up to the crystal quality on Al2O3 substrate.41-42 However, our previous works found that most of dislocations start to bend or annihilate at the initial growth stage and finally nanowires with few defects were obtained by adjusting the pattern on the Si and tuning growth parameters.19 Note that high-quality GaN MW templates set the foundation for further depositing high-quality AlN/AlGaN shell layers. Figure 1d1-1d3 shows the cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of an as-fabricated MW. The contrast difference reveals the abrupt Si/AlN and AlN/GaN interfaces in the Figure 1d1. Most of dislocations are developed at the initial growth stage and then turned to the direction 9



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perpendicular to the growth direction of GaN,

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which are indicated by

the small black arrows in Figure 1d2 and Figure 1d3. This should ascribe to the effect of the strong interplay between defect engineering and stress management for GaN grown on Si.43 The inclination of the threading dislocations effectively generates in-plane misfit dislocation segments, which results in a partial relaxation of the accumulated compressive strain within the AlN/GaN layer.44 The compressive strain within the AlN/GaN structure, induced by lattice mismatch, facilitates the threading dislocations bending at a larger angle, as well as their annihilation through dislocation interaction. The resulting crack-free, low dislocation GaN on Si becomes the template for the high-quality thin AlN/AlGaN shell layer coating. The high-resolution transmission electron microscope (HRTEM) image (Figure 1e) and the corresponding area electron diffraction (SEAD) pattern (Figure 1f) taken from the cross-sectional GaN core are presented to confirm the wurtzite single crystal structure of GaN without noticeable defects. Figure 2a demonstrates a magnified TEM image that clearly shows a dark contrast line (AlN layer) sandwiched by the top AlGaN layer and bottom GaN layer. The thickness of AlGaN and AlN is estimated to be 50 nm, 5 nm along the polar facet, respectively, as is shown in the lattice-resolved HAADF-STEM image in Figure 2b; while the thickness of AlGaN and AlN along semi-polar facets is estimated to be 8 nm, 0.8 nm, respectively, which is 10


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consistent with our previous work19 indicating that the growth rate of polar facets is about 6 times faster than that of two semi-polar facets. Compared to GaN/AlGaN

heterojunctions,

the

insert

of

an

AlN

interlayer

in

GaN/AlN/AlGaN heterojunctions significantly improves the conductance as the reduced electrons scattering as well as larger conduction band discontinuity as demonstrated in our previous work.2 Interfaces between GaN/AlN/AlGaN are atomically uniform and sharp without obvious boundary defects and —

dislocation. Further, the EDX line profile (Figure 2c) scanned along direction confirms Al, Ga, N elements and the abrupt change of Al elements due to AlGaN/AlN shell layer deposition is observed obviously. HEMTs fabricated on GaN/AlN/AlGaN core-shell nanowires reported by Yat Li etc. have demonstrated on/off current ratio as large as 107 and high transconductance of 420 mS/mm, as listed in Table 1.3 However, it is believed that microwires are preferable for high power device applications on account of their larger current carrying capability as compared to nanowires. Moreover, microwires provide a promising intermediate solution to avoid excessive surface sensitivity, which is reported to be the origin of Fermi-level pinning to prevent Ohmic contact conduction at nanowire surface.14 In the following work, direct-current

electrical

transport

properties

were

evaluated

on

the

heterojunction MW HEMTs by a probe station, which was sourced and collected by a dual-channel precision source/measure unit (Model B2902, 11



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Keysight

Technologies,

Inc.).

Firstly,

Schottky

gated

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MW

HEMTs

(SG-HEMT) were fabricated using Ni/Au stack as gate metal. For devices preparation, the gate length, gate–source distance, and gate–drain distance are 8 µm, 5 µm, and 7 µm respectively (Figure S3 in the supporting information). Figure 3a shows the transfer and transconductance characteristic curves that reflect the ability to control drain-source current by gate voltage (Vgs). Normally-off operation with a threshold voltage of about 0.5 V is observed. When the Vgs increases to nearly 0.6 V, the drain current increases exponentially to 8.6 mA/mm and attains saturation at Vgs = 2.5 V, confirming the process of accumulating electron carriers. In this course, the fastest increasing position is at about 1.3 V, which means the

maximal

transconductance position as well. The maximum transconductance of ~ 4 mS/mm (total gate width ~ 3.1 µm) and on/off current ratio of 105（Ion = 69 µA, Ioff = 520 pA）are obtained. Figure 3b exhibits the output drain-current (Ids) versus drain-source voltage (Vds) characteristics, which shows the typical n-type behavior with saturated Ids current at 8.5 mA/mm. The threshold voltages along with the saturated output currents of the SG-HEMTs are relatively low and Schottky gate leakage current is often very large, in agreement with planar HEMTs.27, 45 To improve gate leakage, MOS-HEMTs were prepared by depositing SiO2 gate dielectric before gate metallization. Gate dielectric are often introduced 12


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aiming at effectively blocking the gate leakage current and suppressing the surface leakage current by passivating the interface trap density and surface states at the AlGaN surface.25,

44, 46

Figure 3c shows the transfer and

transconductance characteristic curves of the MOS-HEMT. The threshold voltage is increased from 0.5 V to 1.6 V compared to that of SG-HEMT. The maximum transconductance of 103 mS/mm, on/off current ratio of 107 (Ion = 3.1 mA, Ioff = 320 pA) are enhanced by a factor of ~ 26 and 100 respectively. The output performance (Figure 3d) exhibits that the maximum drain current of 272 mA/mm (Ids = 3.1 mA) is improved by about 32 times than that of the SG-HEMT. All these performance characteristics confirm the effectiveness of employing SiO2 dielectric. In fact, as reported by A. Khan et al, SiO2 prepared by PECVD on AlGaN exhibited good interface quality to reduce the gate leakage, which led to a saturation current of as large as 600 mA/mm on the MOS-HEMT using PECVD SiO2 as dielectric.47 Further, a thin (~ 5 nm) unintentionally doped GaN (u-GaN) cap layer was grown on GaN/AlN/AlGaN MWs, which was expected to further reduce the leakage current and to improve gate control performances. A threshold voltage of 1.4 V is achieved, slightly lower than that without capping layer, as indicated in Figure 4a. The data show that the maximum transconductance of 511 µS (165 mS/mm) is obtained in the u-GaN cap scheme and a large on/off current ratio of 108 (Ion = 5.5 mA, Ioff = 46 pA), an order of magnitude higher than that 13



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before GaN capping layer deposited. The large on/off current ratio is resulted from the increased on-state current and reduced off-state current and it is the best result, to our knowledge, among wire-based Ⅲ-nitride HEMTs reported in literatures. In Figure 4b, the Ids-Vds data are shown from Vgs = 0.8 V to Vgs = 2.4 V with a step of 0.2 V. The maximum Ids = 532 mA/mm (Ids = 5.86 mA) was obtained, which was almost twice higher than 272 mA/mm achieved on our MOS-HEMT without the u-GaN cap layer (Figure 3d). The subthreshold swing (SS), S factor, is an important parameter which explains how effectively a device can be turned off as the logic switch in the subthreshold state. As is presented in Figure 4c, by adding the u-GaN cap layer, the HEMTs display an outstanding subthreshold behavior with 81 mV/dec at Vds = 10 V, which shows excellent gate control characteristic near the cut-off voltage and the value is close to the superior result obtained on planar HEMTs.48 These improved properties are obviously attributed to the low leakage current by using the u-GaN cap. With the u-GaN it enables a lower ideality factor and a higher Schottky barrier height, as it has been reported that by designing a InGaN,49 InGaAs,50 GaN51 cap layer to enhance the Schottky barrier height in literatures. It was suggested that an electric field pointing from the surface of the GaN towards the AlGaN layer was formed by the negative polarization charge at the interface. As a result, the electric field bends the energy band of GaN cap upwards, which reduces of the thermal emission leakage current. One the other 14


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hand, the decreased ideal factor due to the introduction of GaN cap layer implies the suppression of the tunneling current.51 These two effects lead to the decrease of the total leakage. Additionally, the extracted field-effect mobility is estimated to be 1153 and 1145 cm2 V-1 s-1 at Vds of 10 V for MOS-HEMT and MOS-HEMT with u-GaN cap structures respectively (the supporting information). The relative low mobility compared to film counterparts may due to the thick AlN (5 nm) that formed a heterojunction rather than acted as an interlayer with GaN along polar facets.52 For a clear comparison, device performance

parameters

in

terms

of

a

threshold

voltage,

on/off

ratio,

transconductance, subthreshold swing, behavior (i.e. normally-off or on) and field-effect mobility are listed in Table 1, while GaN/AlN/AlGaN core-shell nanowire HEMT in reference 3 is referred. It is worth to point out that all the three heterojunction MW HEMTs reveal normally-off behavior, which is distinguished from the usually normally-on operation obtained on AlGaN/GaN heterojunction HEMTs, ascribed from their high 2DEG concentration induced by strong spontaneous and piezoelectric polarization. In fact, how to achieve normally-off HEMTs based on III-nitride materials, necessary to reduce circuit design complexity and achieve a fail-safe, has attracted considerable investigation interests. To realize normally-off devices, two strategies are often under researched: (1) to realize the control of 2DEG through modifying epitaxial growth techniques or device processing, 15



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such as by adopting thin AlGaN barrier, energy band engineering and trench gate; 30, 26, 53 (2) by interconnecting normally-on GaN devices with normally-off Si devices.54 However, the 2DEG generated in the present GaN/AlN/AlGaN core-shell heterojunction MWs is induced by polarization effects along a polar facet and two semi-polar facets together and the two semi-polar facets take up most of the contacts due to the unique cross-sectional morphology. Therefore, the HEMT device can be roughly considered as two semi-polar faceted conducting channels in parallel with a polar faceted channel, as schematically illustrated in Figure 5. For strained AlGaN grown on semi-polar GaN, the total electrical polarization is intensively dependent on the semi-polar plane orientation deviated from c-plane.26 As the angle approaches to ~ 62°in the —

present case ({1101} to {0001}), the total polarization decreases significantly close to zero. Because of the weakened polarization along the two semi-polar facets, much less positive net polarization charges and thus low 2DEG density would induce at the two interfaces, along with the urgently lowered conductance.16 On the other hand, though the conductance is enhanced resulted from total polarization induced at the polar facet, the thick AlGaN barrier layer, as presented in Figure 2, inevitably increase the access resistance and reduce the gate control ability. Therefore, it is understandable that normally-off operation of the HEMT is the result of the overall effects.

16


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4. CONCLUSION In conclusion, by selectively growth of GaN templates on side facets of patterned Si, high-quality GaN/AlN/AlGaN core/shell heterojunction MW arrays were further synthesized

by

MOCVD.

The

heterojunction

MWs

featured

trapezoidal

cross-sectional morphologies with smooth surfaces and sharp atomic interfaces. By fabricating HEMTs using the microwires as channels, electrical transport properties were assessed. Three HEMT devices were designed with experimental data showing that obvious electrical improvements are obtained with the SiO2 gate dielectric and GaN cap layer to reduce gate leakage. All devices exhibited normally-off operation with excellent electrical properties. A threshold voltage of 1.4 V, a high on/off current ratio of 108, a transconductance of 165 mS/mm and a low subthreshold swing of 81 mV/dec were demonstrated. The normally-off operation may attribute to the weak polarization along semi-polar facets, which is explained by the energy band diagram.

Corresponding Author * E-mail address: [email protected]

Author Contributions ‡These authors contributed equally.

17



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Supporting Information.

Fiure S1, cross-sectional optical image of microwire

arrays on Si; Figure S2, XRD pattern of GaN microwires; Figure S3, optical image of a typical HEMT device.

ACKNOWLEDGMENT

This work is supported by the National Natural Science Foundation of China (Grant Nos. 11474105 and 61404156), the Science and Technology Program of Guangdong Province, China (Grant Nos. 2015B090903078 and 2015B010105011), the Science and Technology Project of Guangzhou City (No. 201607010246), the Innovation Project of Graduate School of South China Normal University, and the Program for Changjiang Scholars and Innovative Research Team in Universities of China (Grant No. IRT13064).

REFERENCES (1) Fletcher, A. S. A.; Nirmal, D. A survey of Gallium Nitride HEMT for RF and high power applications. Superlattice Microst. 2017, 109, 519-537. (2) Wang, X.; Yu, R.; Jiang, C.; Hu, W.; Wu, W.; Ding, Y.; Peng, W.; Li, S.; Wang, Z. L. Piezotronic Effect Modulated Heterojunction Electron Gas in AlGaN/AlN/GaN Heterostructure Microwire. Adv. Mater. 2016, 28, 7234-7242. (3) Li, Y.; Xiang, J.; Qian, F.; Gradecak, S.; Wu, Y.; Yan, H.; Yan, H.; Blom, D. A.; Lieber, C. M. Dopant-free GaN/AlN/AlGaN radial nanowire heterostructures as high electron mobility transistors. Nano Lett. 2006, 6, 1468-1473. 18


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Figure captions Table 1. Comparison of performance parameters of GaN/AlN/AlGaN MW HEMTs with different structures to AlGaN/AlN/GaN nanowire HEMT reported in the reference. Figure 1. (a) Schematic illustration of heterojunction MWs grown on the patterned Si substrate; the inset is a magnified drawing to illustrate the epitaxial growth relationship. (b) A typical SEM image of uniformly aligned MW arrays on Si (100). (c) SEM image of transferred MWs on a fresh Si substrate. The inset shows released MWs from patterned Si. (d1) Cross-sectional HAADF-STEM image of the heterostructured MW and (d2) and (d3) are corresponding magnified illustrations from the labeled areas. (e) HRTEM image of the MW and (f) corresponding SAED pattern taken from the GaN core. Figure 2. (a) HAADF-STEM image of the heterojunction MW. (b) HRTEM atomic image to clearly illustrate sharp interface and smooth atomic distribution of GaN/AlN/AlGaN collected from the labeled region. (c) The EDX line profiles for Ga —

(red), Al (blue) and N (orange) elements scanned along (0001) direction.

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Figure 3. (a) Transconductance and drain current curves of the SG-HEMTs Vds = 10 V. (b) Drain current versus Vds. Vgs varies from 0.4 V to 2.4 V with a step of 0.4 V. (c) Transconductance and drain current of the MOS-HEMTs with SiO2 dielectric at Vds = 10 V. (d) Drain current versus Vds. Vgs varies from 1.2 V to 2.7 V with a step of 0.3 V. Figure 4. (a) Transconductance and drain current curves of the MOS-HEMT with u-GaN cap at Vds = 10 V. (b) Drain current versus Vds. Vgs varies from 0.8 V to 2.4 V with a step of 0.2 V. (c) Exponential dependence of drain current (Log (Ids )) on Vgs. Figure 5. Schematic diagram of semi-polar and polar channels in parallel and corresponding energy band structures to illustrate the underlying mechanisms.

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Table 1. Comparison of performance parameters of GaN/AlN/AlGaN MW HEMTs with different structures to AlGaN/AlN/GaN nanowire HEMT reported in the reference.

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Figure 1. (a) Schematic illustration of heterojunction MWs grown on the patterned Si substrate; the inset is a magnified drawing to illustrate the epitaxial growth relationship. (b) A typical SEM image of uniformly aligned MW arrays on Si (100). (c) SEM image of transferred MWs on a fresh Si substrate. The inset shows released MWs from patterned Si. (d1) Cross-sectional HAADF-STEM image of the heterostructured MW and (d2) and (d3) are corresponding magnified illustrations from the labeled areas. (e) HRTEM image of the MW and (f) corresponding SAED pattern taken from the GaN core.

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Figure 2. (a) HAADF-STEM image of the heterojunction MW. (b) HRTEM atomic image to clearly illustrate sharp interface and smooth atomic distribution of GaN/AlN/AlGaN collected from the labeled region. (c) The EDX line profiles for Ga —

(red), Al (blue) and N (orange) elements scanned along (0001) direction.

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Figure 3. (a) Transconductance and drain current curves of the SG-HEMTs Vds = 10 V. (b) Drain current versus Vds. Vgs varies from 0.4 V to 2.4 V with a step of 0.4 V. (c) Transconductance and drain current of the MOS-HEMTs with SiO2 dielectric at Vds = 10 V. (d) Drain current versus Vds. Vgs varies from 1.2 V to 2.7 V with a step of 0.3 V.

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Figure 4. (a) Transconductance and drain current curves of the MOS-HEMT with u-GaN cap at Vds = 10 V. (b) Drain current versus Vds. Vgs varies from 0.8 V to 2.4 V with a step of 0.2 V. (c) Exponential dependence of drain current (Log (Ids )) on Vgs.

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Figure 5. Schematic diagram of semi-polar and polar channels in parallel and corresponding energy band structures to illustrate the underlying mechanisms.

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Abstract Graphic Catalyst-free grown GaN/AlN/AlGaN core-shell heterojunction microwires on patterned Si by MOCVD are characterized in detail and corresponding electrical performances by fabricating HEMTs are evaluated.

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AlGaN CoreâShell ... - ACS Publications

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