N-polar GaN Research Articles

Surface morphology evolution of N-polar GaN on SiC for HEMT heterostructures grown by plasma-assisted molecular beam epitaxy

The surface morphology evolution of N-polar GaN with growth time was investigated and compared with Ga-polar GaN. N-polar GaN directly grown on SiC substrates was found to have slower 3D-to-2D growth transformation and less coalescence than the Ga-polar counterpart, resulting in rougher surface morphology, whereas the AlN nucleation layer accelerated 3D-to-2D transformation, resulting in smoother surface morphology. N-polar GaN was found to have mound-type surface morphology with clustered atomic steps, unlike the regular screw-type dislocation-mediated step-flow growth observed for Ga-polar GaN. This was explained by the lower diffusion of adatoms on the N-polar surface due to its higher surface energy and higher Ehrlich–Schwoebel barrier. In addition, the increased III/V ratio in N-polar GaN growth was found to reduce the surface roughness from 2.4 nm to 1 nm. Without Si doping, the N-polar GaN high electron mobility transistor (HEMT) heterostructures grown under optimized conditions with smoother surface morphologies exhibited a sheet carrier density of 0.91 × 1013 cm−2 and a mobility of 1220 cm2 (V s)−1. With Si δ-doping, the sheet carrier density was increased to 1.28 × 1013 cm−2 while the mobility was reduced to 1030 cm2 (V s)−1. These results are comparable to the state-of-the-art data of plasma-assisted molecular beam epitaxy-grown N-polar GaN HEMT heterostructures on SiC substrates.

Record RF Power Performance at 94 GHz From Millimeter-Wave N-Polar GaN-on-Sapphire Deep-Recess HEMTs

In this article, N-polar GaN-on-sapphire deep-recess metal–insulator–semiconductor (MIS)-high-electron-mobility transistors (HEMTs) with a breakthrough performance at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula> -band are presented. Compared with prior N-polar GaN MIS-HEMTs, a thin GaN cap layer and atomic layer deposition (ALD) ruthenium (Ru) gate metallization were used along with high-quality GaN-on-sapphire epitaxy from Transphorm Inc. Before SiN passivation, 94 GHz large signal load–pull shows that the transistor obtains a record-high 9.65 dB linear transducer gain and demonstrated 42% power-added efficiency (PAE) with associated 4.4 W/mm of output power density at 12 V drain bias. By biasing the drain at 8 V, the device shows an even higher PAE of 44% with an associated 2.6 W/mm of output power density. After SiN passivation, the fabricated N-polar GaN-on-sapphire HEMTs show a high PAE of 40.2% with an associated 4.85 W/mm of output power density. Furthermore, a very high output power density of 5.83 W/mm with 38.5% PAE is demonstrated at a 14 V drain bias. This power performance shows significant efficiency improvement over previous N-polar GaN-on-SiC and demonstrates a combined efficiency and power density beyond what has been reported for Ga-polar devices, in spite of the low-thermal-conductivity sapphire substrate. This shows that N-polar GaN-on-sapphire technology is an attractive candidate for millimeter-wave power amplifier applications with simultaneous high efficiency and power density.

Selective-area growth of GaN and AlGaN nanowires on N-polar GaN templates with 4° miscut by plasma-assisted molecular beam epitaxy

In this study, selective area growth (SAG)s of AlGaN/GaN nanowires (NWs) on miscut N-polar GaN templates were studied and compared with that grown on Ga-polar templates using plasma-assisted molecular beam epitaxy (MBE). The SAG of N-polar AlGaN/GaN NWs demonstrated higher growth rate and more growth selectivity than that of Ga-polar AlGaN/GaN NWs. Additionally, lateral growth rate of N-polar NWs was significantly lower than Ga-polar NWs. Compared to the pyramidal shaped top surfaces of Ga-polar NWs, the N-polar GaN NWs had flat head top surfaces. The combination of higher growth selectivity, larger growth window, negligible lateral growth, and flat head, makes N-polar (Al,Ga)N NWs attractive for a variety of applications including UV LEDs and detectors as well as quantum sensing.

N-polar GaN/AlGaN/AlN high electron mobility transistors on single-crystal bulk AlN substrates

Recent observation of high density polarization-induced two-dimensional electron gases in ultra-thin N-polar GaN layers grown on single-crystal AlN has enabled the development of N-polar high electron mobility transistors (HEMTs) on AlN. Such devices will take advantage of thermal and power handling capabilities of AlN, while simultaneously benefitting from the merits of N-polar structures, such as a strong back barrier. We report the experimental demonstration of N-polar GaN/AlGaN/AlN HEMTs on single-crystal AlN substrates, showing an on-current of 2.6 A/mm with a peak transconductance of 0.31 S/mm. Small-signal RF measurements revealed speeds exceeding ft/fmax = 68/100 GHz. These results pave the way for developing RF electronics with excellent thermal management based on N-polar single-crystal AlN.

Demonstration of N-Polar GaN MIS-HEMT with High-k Atomic Layer Deposited HfO2 as Gate Dielectric

Demonstration of N-Polar GaN MIS-HEMT with High-k Atomic Layer Deposited HfO2 as Gate Dielectric

Electrical properties of N-polar Si-doped GaN prepared by pulsed sputtering

We have demonstrated the homoepitaxial growth of N-polar GaN and its Si doping by pulsed sputtering deposition (PSD). Enhanced surface migration by a pulsed supply of precursors enabled the step-flow growth on N-polar GaN bulk substrates even with small miscut angles. The relationship between electron concentration and mobility in N-polar GaN follows the Caughey–Thomas relationship for Ga-polar GaN, which indicates PSD N-polar GaN has a low concentration of scattering centers. N-polar heavily Si-doped GaN film yielded a record-low resistivity of 1.6 × 10−4 Ωcm with an electron concentration of 3.6 × 1020 cm−3 and mobility of 109 cm2V−1s−1, comparable to the best data for Ga-polar GaN. The high electron mobility can be attributed to the reduced concentration of compensating acceptors, which is also consistent with its optical measurements. Moreover, optical measurements show that the Burstein–Moss effect raises the Fermi level by 0.2 eV. These results show that heavily Si-doped N-polar GaN prepared by pulsed sputtering is promising for future applications such as the source/drain of high-performance N-polar GaN HEMTs.

N-polar GaN HEMT with HfO2 as Gate Dielectric for mm-wave Applications

N-polar GaN HEMT with HfO2 as Gate Dielectric for mm-wave Applications

N-Polar growth of nitride semiconductors with MOVPE and its applications

All white LEDs that largely contribute to the energy saving as well as high electron mobility transistors (HEMTs) powering the base stations of cellular phone networks are made from only Ga-polar materials epitaxially grown along the c-axis. N-polar material, which has the opposite crystal-direction, is considered having high potential for expanding the application field. In this paper, the present status of N-polar epitaxial growth in our lab. is reviewed. Especially, the growth mechanism of N-polar GaN on the widely used sapphire substrate is investigated in detail by using transmission electron microscopy. Finally, as device applications, solar cells, red LEDs, and inverted HEMT are described.

Fabrication of freestanding GaN nanobelts with enhanced ultraviolet photoresponse performance

High-quality single component freestanding p-type and n-type GaN nanobelts were prepared by an electrochemical (EC) lift-off technique and subsequently transferred onto SiO2/Si substrates for the fabrication of high-performance ultraviolet photodetectors (UV PDs). The fabricated p-type nitrogen-polar (N-polar) GaN nanobelt UV PDs exhibited high photoresponsivity (7416.7 A/W) with a high photocurrent of 311 μA and the n-type devices presented high detectivity (1.44 × 1014 Jones) with a low dark current of 0.351 nA. Under dark condition, the thermionic field emission (TFE) and thermionic emission (TE) were found to be responsible for the current transport mechanisms of the p-type and n-type GaN nanobelt UV PDs, respectively. In addition, the enhancement of photoresponse performances in the N-polar GaN nanobelt PDs is not only attributed to their large surface-to-volume ratio but also to the strong built-in electric field. This work suggests that the rational combination of N-polar and surface effects of the freestanding GaN nanobelts is a viable approach to achieving high-performance GaN-based optoelectronic devices.

High-quality N-polar GaN optimization by multi-step temperature growth process

We report growth optimization of Nitrogen (N)-polar GaN epitaxial layers by hot-wall metal–organic vapor phase epitaxy on 4H-SiC (0001̄) with a misorientation angle of 4° towards the [112̄0] direction. We find that when using a 2-step temperature process for the N-polar GaN growth, step bunching is persistent for a wide range of growth rates (7 nm/min to 49 nm/min) and V/III ratios (251 to 3774). This phenomenon is analyzed in terms of anisotropic step-flow growth and the Ehrlich–Schwöebel barrier, and their effects on the surface step height and step width. The N-polar GaN growth is further optimized by using 3-step and 4-step temperature processes and the layers are compared to those using the 2-step temperature process in terms of surface morphology and defect densities. It is shown that a significantly improved surface morphology with a root mean square of 1.4 nm and with low dislocation densities (screw dislocation density of 2.8 × 108 cm−2 and edge dislocation density of 1.3 × 109 cm−2) can be achieved for 4-step temperature process. The optimized growth conditions allow to overcome the step-bunching problem. The results are further discussed in view of Ga supersaturation and a general growth strategy for high-quality N-polar GaN growth is proposed.

Study of dry etched N-polar (Al)GaN surfaces obtained by inductively coupled plasma etching

We report the Cl-based inductively coupled plasma etching of N-polar Al(Ga)N layers obtained from layer transfer. It is found that debris appeared on the etched N-polar surface after exposing in air for a short period whereas the etched Al-/Ga-polar surface was clean and smooth. The debris can be completely self-vanished on the N-polar Al0.4Ga0.6N surface after exposing in air for a few hours but still remained on the N-polar GaN surface even after over 1 month. The surface chemical analysis results suggested that the debris is the result of Cl-related byproduct generated during the etching process. Byproducts like Al(Ga)Clx and its derivatives are believed to cover on the N-polar surface after the inductively coupled plasma etching and increase the etched surface roughness significantly. The formation and disappearance of debris are attributed to the formation of Al(Ga)Clx⋅ 6H2O crystals when Al(Ga)Clx absorbs moisture in the air and its spontaneous decomposition on the N-polar surface, respectively. Adding O2/SF6 in the process helps remove Al(Ga)Clx byproducts but at the cost of roughened surface/reduced etch rate. With an additional cleaning process after etching, an uniform and smooth N-polar GaN surface with a low root-mean-square surface roughness of 0.5–0.6 nm has been successfully obtained at a reasonable etch rate (∼150 nm/min). The results can provide valuable guidance for the fabrication of high-performance N-polar GaN devices.

N-polar GaN evolution on nominally on-axis c-plane sapphire by MOCVD Part-II: Microstructural investigation

The present work is part-II of a study that addresses in detail the microstructural features of nitrogen-polar gallium nitride (N-polar GaN) deposited on c-plane sapphire wafers under the metalorganic chemical vapor deposition conditions described in the part-I work. The growth process was interrupted at various stages and characterized the depositions using transmission electron microscopy. The plan view diffraction study in combination with the Moiré fringe analysis revealed the presence of rotational domains of 4° in the low-temperature GaN nucleation layer. A crystallographic basis for the occurrence of these rotational domains is proposed based on the arrangement of surface atoms in the underlying sapphire substrate. Pure a dislocations are found to accommodate the twist between the rotational domains. In addition, Ga-polar inversion domains are observed in the matrix of N-polar GaN, and they originate at atomic level steps induced by the initial thermal cleaning and nitridation process of sapphire wafers at 1100°C. The rotational domains, the accommodating a dislocations and the inversion domains are replicated into the N-polar GaN epilayers.

Role of low temperature Al(Ga)N interlayers on the polarity and quality control of GaN epitaxy

Polarity control is essential for lateral polarity heterostructures. N-polar GaN films were directly grown on high temperature AlN buffer layer. A thin low temperature AlN interlayer could invert the polarity of subsequent GaN layer to Ga-polarity but a low temperature GaN interlayer with the same condition could not do it rather deteriorate the quality of GaN film. In addition, a relationship between the yellow luminescence band and the surface roughness was discovered. The blue luminescence band was found only on the inverted Ga-polar GaN film and was connected to screw dislocation density.

Electrical properties and energy band alignment of SiO2/GaN metal-oxide-semiconductor structures fabricated on N-polar GaN(0001¯) substrates

The interface properties and energy band alignment of SiO2/GaN metal-oxide-semiconductor (MOS) structures fabricated on N-polar GaN(0001¯) substrates were investigated by electrical measurements and synchrotron-radiation x-ray photoelectron spectroscopy. They were then compared with those of SiO2/GaN MOS structures on Ga-polar GaN(0001). Although the SiO2/GaN(0001¯) structure was found to be more thermally unstable than that on the GaN(0001) substrate, excellent electrical properties were obtained for the SiO2/GaN(0001¯) structure by optimizing conditions for post-deposition annealing. However, the conduction band offset for SiO2/GaN(0001¯) was smaller than that for SiO2/GaN(0001), leading to increased gate leakage current. Therefore, caution is needed when using N-polar GaN(0001¯) substrates for MOS device fabrication.

Low resistivity, p-type, N-Polar GaN achieved by chemical potential control

We report on low resistivity (1.1 Ω cm) in p-type bulk doping of N-polar GaN grown by metalorganic chemical vapor deposition. High nitrogen chemical potential growth, facilitated by high process supersaturation, was instrumental in reducing the incorporation of compensating oxygen as well as nitrogen-vacancy-related point defects. This was confirmed by photoluminescence studies and temperature-dependent Hall effect measurements. The suppressed compensation led to an order of magnitude improvement in p-type conductivity with the room-temperature hole concentration and mobility measuring 6 × 1017 cm−3 and 9 cm2 V−1 s−1, respectively. These results are paramount in the pathway towards N-polar GaN power and optoelectronic devices.

A study on MOCVD growth window for high quality N-polar GaN for vertical device applications

Lateral N-polar GaN devices have outperformed Ga-polar GaN counterparts in mm-wave applications. A vertical N-polar GaN device structure with higher breakdown voltage is expected to further improve the output power density. This paper is aimed at understanding the growth conditions for such a vertical N-polar GaN device stack. A vertical device stack requires high-quality n-GaN layer with low unintentional dopant concentration and high electron mobility for better ON resistance–breakdown voltage performance. A systematic study by varying reactor temperature, pressure and NH3 flow rate is performed in the metal organic chemical vapor deposition growth of N-polar GaN for vertical device structure. Structural quality of the sample is increased with increase growth temperature and higher NH3 flow rate, yet an increase in surface roughness is observed for highest temperature used. The N-polar GaN grown at the optimized conditions exhibited a x-ray diffraction rocking curve full width half maximum of symmetric (0002) and skew-symmetric (20 ) scans of 358″ and 569″ respectively with a low oxygen and carbon concentration of 6 × 1016 cm−3 and 5 × 1016 cm−3, respectively. The optimized sample exhibited a low screw dislocation density of 2.5 × 108 cm−2 on sapphire substrate. A gradual increase in the bulk mobility of electrons with decrease in screw dislocation density in N-polar GaN is also reported.

N-polar GaN p-n junction diodes with low ideality factors

High-quality N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy. The room-temperature current–voltage characteristics reveal a high on/off current ratio of >1011 at ±4 V and an ideality factor of 1.6. As the temperature increases to 200 °C, the apparent ideality factor gradually approaches 2. At such high temperatures, Shockley–Read–Hall recombination times of 0.32–0.46 ns are estimated. The measured electroluminescence spectrum is dominated by a strong near-band edge emission, while deep level and acceptor-related luminescence is greatly suppressed.

Schottky contacts to N-polar GaN with SiN interlayer for elevated temperature operation

In this Letter, we unveil the high-temperature limits of N-polar GaN Schottky contacts enhanced by a low-pressure chemical vapor deposited (LPCVD) SiN interlayer. Compared to conventional Schottky diodes, the insertion of a 5 nm SiN lossy dielectric interlayer in-between Ni and N-polar GaN increases the turn-on voltage (VON) from 0.4 to 0.9 V and the barrier height (ϕB) from 0.4 to 0.8 eV. This modification also reduces the leakage current at zero bias significantly: at room temperature, the leakage current in the conventional Schottky diode is &amp;gt;103 larger than that observed in the device with the SiN interlayer, while at 200 °C, this ratio increases to 105. Thus, the rectification ratio (ION/IOFF) at ±1.5 V reduces to less than one at 250 °C for the conventional Schottky diode, whereas for SiN-coated diodes, rectification continues until 500 °C. The I–V characteristics of the diode with an SiN interlayer can be recovered after exposure to 400 °C or lower. Contact degradation occurs at 500 °C, although devices are not destroyed yet. Here, we report N-polar GaN Schottky contact operation up to 500 °C using an LPCVD SiN interlayer.

N-polar GaN Film Epitaxy on Sapphire Substrate without Intentional Nitridation.

High-temperature nitridation is commonly thought of as a necessary process to obtain N-polar GaN films on a sapphire substrate. In this work, high-quality N-polar GaN films were grown on a vicinal sapphire substrate with a 100 nm high-temperature (HT) AlN buffer layer (high V/III ratio) and without an intentional nitriding process. The smallest X-ray full width at half maximum (FWHM) values of the (002)/(102) plane were 237/337 arcsec. On the contrary, N-polar GaN film with an intentional nitriding process had a lower crystal quality. In addition, we investigated the effect of different substrate treatments 1 min before the high-temperature AlN layer’s growth on the quality of the N-polar GaN films grown on different vicinal sapphire substrates.

Interface Recombination in Ga- and N-Polar GaN/(Al,Ga)N Quantum Wells Grown by Molecular Beam Epitaxy

We explore and systematically compare the morphological, structural and optical properties of GaN/(Al,Ga)N multiple quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PA-MBE) on freestanding GaN$(0001)$ and GaN$(000\bar{1})$ substrates. Samples of different polarity are found to be comparable in terms of their morphological and structural perfection and exhibit essentially identical quantum well widths and Al content. Regardless of the crystal orientation, the exciton decay in the MQWs at 10 K is dominantly radiative and the photoluminescence (PL) energy follows the quantum confined Stark effect (QCSE) for different quantum well widths. A prominent free-to-bound transition involving interface shallow donors is, however, visible for the N-polar MQWs. At room-temperature, in contrast, the exciton decay in all samples is dominated by nonradiative recombination taking place at point defects, presumably Ca or V N located at the bottom QW interface. Remarkably, the N-polar MQWs exhibit a higher PL intensity and longer decay times than the Ga-polar MQWs at room-temperature. This improved internal quantum efficiency is attributed to the beneficial orientation of the internal electric field that effectively reduces the capture rate of minority carriers by interface trap states.