Bulk GaN Substrates Research Articles

Peculiarities of plastic relaxation of (0001) InGaN epilayers and their consequences for pseudo-substrate application

In this paper, we study the plastic relaxation of InGaN layers deposited on (0001) GaN bulk substrates and (0001) GaN/sapphire templates by molecular beam epitaxy. We demonstrate that the InGaN layers relax by the formation of (a+c)-type misfit dislocations gliding on pyramidal planes in the slip system ⟨112¯3⟩{112¯2} down to the interface where they form a trigonal dislocation network. Combining diffraction contrast and large angle convergent beam electron diffraction analyses performed using a transmission electron microscope, we reveal that all (a+c)-type dislocations belonging to one subset of the network exhibit Burgers vectors with the same c-component. This relaxation mechanism leads to partially relaxed InGaN layers with smooth surfaces and threading dislocation densities below 109 cm−2. Such layers are of potential interest as pseudo-substrates for the growth of InGaN heterostructures.

High Performance Vertical GaN-on-GaN p-n Power Diodes With Hydrogen-Plasma-Based Edge Termination

This letter reports the first implementation of a hydrogen-plasma-based edge termination technique (HPET) in vertical GaN p-n power diodes grown on bulk GaN substrates using metalorganic chemical vapor deposition. The device with a 9- $\mu \text{m}$ -thick drift layer exhibited a high breakdown voltage ( ${V}_{\textsf {bd}}$ ) of 1.57 kV, a low ON-resistance ( ${R}_{\mathrm{ ON}}$ ) of 0.45 $\text{m}\Omega ~ \cdot $ cm2 (or 0.70 $\text{m}\Omega ~\cdot $ cm2 with current spreading considered) and a high Baliga’s figure-of-merit ( ${V}^{\textsf {2}}_{\textsf {bd}}/{R}_{\mathrm{ ON}}$ ) of 5.5 GW/cm2 (or 3.6 GW/cm2) without passivation or field plate, which are close to the theoretical limit of GaN. This technique enabled a significant reduction in leakage current (~106 times at −300 V) and a huge enhancement in ${V}_{\textsf {bd}}$ (from ~300 V to 1.57 kV). Furthermore, the device showed good forward characteristics with a turn-ON voltage of 3.5 V, an ON-current of ~2 kA/cm2 (or 1.3 kA/cm2), an ON/OFF ratio of ~109, and an ideality factor of 1.4. This work shows the HPET can serve as an effective, low cost, and easy-to-implement edge termination technique for high voltage and high power GaN p-n power diodes.

Improved Dynamic RON of GaN Vertical Trench MOSFETs (OG-FETs) Using TMAH Wet Etch

This letter reports on the dynamic $R_{\text{ON}}$ performance of large-area GaN vertical trench MOSFETs (OG-FETs) fabricated on bulk GaN substrates. OG-FETs demonstrated excellent DC performance with a breakdown voltage of 900 V and a $R_{\text{ON}}$ of 4.1 $\Omega$ (8.2 $\text{m}\Omega \cdot \text {cm}^{2}$ ). However, damage to the trench sidewalls caused by RIE dry etching led to poor dynamic $R_{\text{ON}}$ performance. An improved dynamic $R_{\text{ON}}$ performance was achieved by following the RIE dry etch with a TMAH wet etch. With this process combination, the dynamic $R_{\text{ON}}$ was reduced by more than 10 times compared with the dynamic $R_{\text{ON}}$ in devices fabricated by dry etch only.

Experimental characterization of impact ionization coefficients for electrons and holes in GaN grown on bulk GaN substrates

Epitaxial p-i-n structures grown on native GaN substrates have been fabricated and used to extract the impact ionization coefficients in GaN. The photomultiplication method has been used to experimentally determine the impact ionization coefficients; avalanche dominated breakdown is confirmed by variable-temperature breakdown measurements. To facilitate photomultiplication measurements of both electrons and holes, the structures include a thin pseudomorphic In0.07Ga0.93N layer on the cathode side of the drift layer. Illumination with 193 nm and 390 nm UV light has been performed on diodes with different intrinsic layer thicknesses. From the measured multiplication characteristics, the impact ionization coefficients of electrons (α) and holes (β) were determined for GaN over the electric field range from 2 MV/cm to 3.7 MV/cm. The results show that for transport along the c-axis, holes dominate the impact ionization process at lower electric field strengths; the impact ionization coefficient of electrons becomes comparable to that of holes (β/α<5) for electric field strengths above 3.3 MV/cm.

Characterization of carrier concentration and mobility of GaN bulk substrates by Raman scattering and infrared reflectance spectroscopies

n-Type GaN bulk substrates with carrier concentrations (n) of 1016–1020 cm−3 were characterized by Raman scattering (350–5000 cm−1) and infrared reflectance (400–5000 cm−1) spectroscopies. Experimental spectra were fitted with the curves calculated from the dielectric function and carrier concentration and mobility of the GaN bulk. The obtained values agree well with the values from Hall-effect measurements for n of 1016–1019 cm−3 in Raman scattering measurements and for n of 1018–1020 cm−3 in infrared reflectance measurements.

Structural, Electrical and Optical Properties of Sputtered-Grown InN Films on ZnO Buffered Silicon, Bulk GaN, Quartz and Sapphire Substrates

Indium nitride (InN) films were grown on Si (111), bulk GaN, quartz and sapphire substrates by radio frequency magnetron sputtering. Prior to the film deposition, a zinc oxide (ZnO) buffer layer was deposited on all the substrates. The x-ray diffraction patterns of InN films on ZnO-buffered substrates indicated c-plane-oriented films whereas the Raman spectroscopy results indicated A1 (LO) and E2 (high) modes of InN on all the substrates. The crystalline quality of InN was found to be better on sapphire and quartz than on the other substrates. The surface roughness of InN was studied using an atomic force microscope. The results indicated higher surface roughness of the film on sapphire as compared to the others; however, roughness of the film was lower than 8 nm on all the substrates. The electrical properties indicated higher electron mobility of InN (20.20 cm2/Vs) on bulk GaN than on the other substrates. The optical band gap of InN film was more than 2 eV in all the cases and was attributed to high carrier concentration in the film.

Large-Area <italic>In-Situ</italic> Oxide, GaN Interlayer-Based Vertical Trench MOSFET (OG-FET)

We present a large-area in-situ oxide, GaN interlayer-based vertical trench MOSFET (OG-FET) with a metal organic chemical vapor deposition regrown 10-nm unintentional-doped-GaN interlayer as the channel and 50-nm in-situ Al2O3 as the gate dielectric. The threshold voltage of the device on bulk GaN substrate was 1 V measured at $\text{I}_\mathrm {on}/\text{I}_\mathrm {off}= 10^{7}$ . The OG-FET with an area scaled to 0.2 mm2 demonstrated a breakdown voltage ( $\text{V}_\mathrm {BR}$ ) of 320 V and an on-state resistance ( $\text{R}_\mathrm {on}$ ) of $3.8~\Omega $ (specific on-state resistance: $\text{R}_\mathrm {on,sp}= 7.6\,\,\text{m}\Omega \,\,\cdot $ cm2). For the single unit cell OG-FET from the same sample, $\text{V}_\mathrm {BR}$ was as high as 700 V (measured at $\text{V}_\mathrm {GS}=-10$ V), corresponding to a breakdown electric field of 1.4 MV/cm and $\text{R}_\mathrm {on,sp}$ of 0.98 $\text{m}\Omega ~\cdot $ cm2. On our control sample, which was grown on sapphire substrate, a 1-A current was measured as well. These results show the potential of the OG-FET in power conversion applications.

Effect of thermal interaction between bulk GaN substrates and corral sapphire on blue light emission InGaN/GaN multi-quantum wells by MOCVD

The InGaN/GaN multi-quantum wells, growth on bulk GaN substrate were studied for blue light emission. Growth temperature plays a key role determining the peak wavelength of a quantum well. The study was carried out by growing quantum wells, MQWs on the whole sapphire at 716 °C and observed peak wavelength at 463 nm. While the bulk GaN substrate with sapphire corral grown at 703 °C and observed a blueshift at 433 nm peak wavelength. These results contradict that of typical observation of wavelength emission inversely proportional to the growth temperature. On the other hand, the growth of GaN-sapphire and GaN-silicon at similar conditions emits 435 nm and 450 nm respectively. The heat interaction of bulk GaN substrates surrounded by the sapphire corral exhibits different growth conditions in multi-quantum wells when compared to that of a whole sapphire substrate (absence of bulk GaN). The predicated surface temperature of bulk GaN substrate is 10 °C–15 °C of more than the corral sapphire. This observation may link to the difference in the thermal distribution of the growth surface corresponding to the different thermal conductivity ratio. The photoluminescence and computational techniques were used to understand in-depth of the heat interaction.

(Invited) Current-Induced Degradation in Bulk GaN Vertical Schottky Diodes

The recent availability of high quality bulk GaN substrates with low threading dislocation density has made vertical power device structures an attractive possibility. Due to the low probability of a threading dislocation occurring within the active area of devices grown upon bulk GaN, reliability is driven by bulk or surface effects. Past work upon the reliability of Ni-GaN Schottky interfaces has suggested that interdiffusion of the Ni into the GaN can occur under thermal stress, adversely affecting the effective energy height of the barrier and increasing the ohmic nature of the interface. Conditions similar to such thermal stressing can be found at typical operating points in high current power devices used in power electronic applications. Our work examines the influence of high current density stress upon the reliability of vertical bulk GaN Schottky diodes. The vertical diodes studied were fabricated with Pd Schottky metallization with an Au ohmic overlayer. To study the effects of surface chemistry, reliability is compared between devices fabricated with different acidic and basic surface preparations, using HCl and KOH respectively. The fabricated devices were subject to life testing under varying levels of constant current density, up to 10 kA/cm^2. The lifetesting system was designed as a stress-measure-stress system, allowing for the evaluation of device parameters in-situ during testing. Electrical parameters indicating the state of the Schottky barrier were evaluated continuously during the measurement points of testing. In addition, a measurement of barrier inhomogeneity, evaluated using the Tung model, was also used as an indication of the condition of the Schottky interface. The microstructure of the interface before and after stress was examined using S/TEM imaging of representative lamellae samples of the stressed and unstressed devices. EDS imaging was used to determine the degree of diffusion between the Schottky metallization and the GaN substrate. Results of microstructure imaging are correlated with the electrical measurements taken to determine the responsible mechanisms for device degradation.

431 kA/cm2 peak tunneling current density in GaN/AlN resonant tunneling diodes

We report on the design and fabrication of high current density GaN/AlN double barrier resonant tunneling diodes grown via plasma assisted molecular-beam epitaxy on bulk GaN substrates. A quantum-transport solver was used to model and optimize designs with high levels of doping and ultra-thin AlN barriers. The devices displayed repeatable room temperature negative differential resistance with peak-to-valley current ratios ranging from 1.20 to 1.60. A maximum peak tunneling current density (Jp) of 431 kA/cm2 was observed. Cross-gap near-UV (370–385 nm) electroluminescence (EL) was observed above +6 V when holes, generated from a polarization induced Zener tunneling effect, recombine with electrons in the emitter region. Analysis of temperature dependent measurements, thermal resistance, and the measured EL spectra revealed the presence of severe self-heating effects.

Proton irradiation effects on minority carrier diffusion length and defect introduction in homoepitaxial and heteroepitaxial n-GaN

Inherent advantages of wide bandgap materials make GaN-based devices attractive for power electronics and applications in radiation environments. Recent advances in the availability of wafer-scale, bulk GaN substrates have enabled the production of high quality, low defect density GaN devices, but fundamental studies of carrier transport and radiation hardness in such devices are lacking. Here, we report measurements of the hole diffusion length in low threading dislocation density (TDD), homoepitaxial n-GaN, and high TDD heteroepitaxial n-GaN Schottky diodes before and after irradiation with 2.5 MeV protons at fluences of 4–6 × 1013 protons/cm2. We also characterize the specimens before and after irradiation using electron beam-induced-current (EBIC) imaging, cathodoluminescence, deep level optical spectroscopy (DLOS), steady-state photocapacitance, and lighted capacitance-voltage (LCV) techniques. We observe a substantial reduction in the hole diffusion length following irradiation (50%–55%) and the introduction of electrically active defects which could be attributed to gallium vacancies and associated complexes (VGa-related), carbon impurities (C-related), and gallium interstitials (Gai). EBIC imaging suggests long-range migration and clustering of radiation-induced point defects over distances of ∼500 nm, which suggests mobile Gai. Following irradiation, DLOS and LCV reveal the introduction of a prominent optical energy level at 1.9 eV below the conduction band edge, consistent with the introduction of Gai.

Structural study of GaN grown on nonpolar bulk GaN substrates with trench patterns

The selective-area growth of GaN (SAG-GaN) films on nonpolar bulk GaN substrates with trench patterns was performed by metalorganic vapor phase epitaxy. We investigated the transformation of SAG-GaN facet structures by changing the direction of the trench patterns. Anisotropic SAG-GaN structures with the () facet on the (0001) plane sidewall, the () facet on the top surface, and the () facet on the () plane sidewall appeared for trench patterns along the a-axis. The width of () facets decreased whereas () facets expanded with increasing off angle of trench patterns from the a-axis. On the other hand, hexagonal facet structures with {} facets appeared for trench patterns along the c-axis. () facets on the top surface expanded with increasing off angle of trench patterns from the c-axis, similar to the results of using trench patterns along the a-axis. Basal-plane stacking faults were annihilated by using trench-patterned substrates. Low basal-plane stacking fault (BSF) density and faster coalescence were obtained for the off angle of trench patterns of around 6° from the c-axis.

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

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

Impact of Trench Dimensions on the Device Performance of GaN Vertical Trench MOSFETs

In this letter, we have examined the impact of trench dimensions on the breakdown voltage and ON-resistance of trench MOSFETs fabricated on sapphire and bulk GaN substrates. Contrary to simulation studies, the breakdown voltage decreased with an increase in trench dimensions in devices fabricated on sapphire substrates. However, such breakdown voltage dependence with trench dimensions was not observed in devices fabricated on bulk GaN substrates of the same area. The observed trend on GaN on sapphire devices was associated with the equivalently reduced number of dislocations per device area. These results give an insight into how dislocations could affect breakdown voltage in power MOSFETs.

First Demonstration of AlSiO as Gate Dielectric in GaN FETs; Applied to a High Performance OG-FET

Gate dielectric plays an integral role in advancing the performance and reliability of GaN-based transistors. Si-alloying of aluminum oxide (Al2O3) dielectrics have been shown to provide a promising route to improve gate dielectric properties in GaN. In this letter, we report on the first demonstration of a GaN FET with aluminum silicon oxide (AlSiO) as the gate dielectric. Vertical normally-off GaN MOSFETs were fabricated on bulk GaN substrate. Excellent dc performance was achieved with a breakdown voltage of 1.2 kV, an ON-resistance of 2 $\text{m}\Omega $ .cm2, and a threshold voltage of 1.5 V (defined at $I_{\textsf {DS}} =1\,\,\mu \text{A}$ /mm). A high breakdown electric-field of 2.3 MV/cm was calculated in these devices. In addition to vertical GaN MOSFET results, a comparative study of Al2O3 and AlSiO-based in situ GaN MOS capacitors, including time-dependent dielectric breakdown characteristics is also presented.

Near-UV electroluminescence in unipolar-doped, bipolar-tunneling GaN/AlN heterostructures

Cross-gap light emission is reported in n-type unipolar GaN/AlN double-barrier heterostructure diodes at room temperature. Three different designs were grown on semi-insulating bulk GaN substrates using molecular beam epitaxy (MBE). All samples displayed a single electroluminescent spectral peak at 360 nm with full-width at half-maximum (FWHM) values no greater than 16 nm and an external quantum efficiency (EQE) of ≈0.0074% at 18.8 mA. In contrast to traditional GaN light emitters, p-type doping and p-contacts are completely avoided, and instead, holes are created in the GaN on the emitter side of the tunneling structure by direct interband (that is, Zener) tunneling from the valence band to the conduction band on the collector side. The Zener tunneling is enhanced by the high electric fields (~5 × 106 V cm−1) created by the notably large polarization-induced sheet charge at the interfaces between the AlN and GaN.

New Tunneling Features in Polar III-Nitride Resonant Tunneling Diodes

Double barrier GaN/AlN resonant tunneling heterostructures have been grown by molecular beam epitaxy on the (0001) plane of commercially available bulk GaN substrates. Resonant tunneling diodes were fabricated; room temperature current-voltage measurements reveal the presence of a negative differential conductance region under forward bias with peak current densities of ~6.4 $kA/cm^2$ and a peak to valley current ratio of ~1.3. Reverse bias operation presents a characteristic turn-on threshold voltage intimately linked to the polarization fields present in the heterostructure. An analytic electrostatic model is developed to capture the unique features of polar-heterostructure-based resonant tunneling diodes; both the resonant and threshold voltages are derived as a function of the design parameters and polarization fields. Subsequent measurements confirm the repeatability of the negative conductance and demonstrate that III-nitride tunneling heterostructures are capable of robust resonant transport at room temperature.

P–n junction diodes with polarization induced p-type graded InxGa1–xN layer

In this study, p–n junction diodes with polarization induced p-type layer are demonstrated on Ga polar (0001) bulk GaN substrates. A quasi-p-type region is obtained by linearly grading the indium composition in un-doped InxGa1–xN layers from 0% to 5%, taking advantage of the piezoelectric and spontaneous polarization fields which exist in group III-nitride heterostructures grown in the typical (0001) or c-direction. The un-doped graded InxGa1–xN layers needed to be capped with a thin Mg-doped InxGa1–xN layer to make good ohmic contacts and to reduce the on-resistance of the p–n diodes. The Pol-p–n junction diodes exhibited similar characteristics compared to reference samples with traditional p-GaN:Mg layers. A rise in breakdown voltage from 30 to 110 V was observed when the thickness of the graded InGaN layer was increased from 100 to 600 nm at the same grade composition.

Origin of temperature-induced luminescence peak shifts from semipolar (112¯2) InxGa1−xN quantum wells

Observed temperature-induced peak shifts in photoluminescence (PL) originating from ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ single quantum wells grown on semipolar $(11\overline{2}2)$ GaN bulk substrates are discussed in terms of a numerical exciton hopping model based on the Monte Carlo method. The experimentally observed PL peak shifts cannot be reproduced by conventional simulation models developed for the polar (0001) plane, where the recombination lifetime is assumed to be temperature independent, and blue-shifts in PL signal are found to be induced by exciton thermal repopulation. Therefore, we incorporate temperature-dependent radiative and nonradiative recombination processes into the model, in which temperature-induced reductions in recombination lifetimes effectively limit the exciton motion. This model is found to be a much better fit for the data, indicating that, in addition to the thermal repopulation process, the reduction of lifetime with increasing temperature can also contribute to the PL blue-shift because shortened lifetimes suppress the exciton motion and therefore produce smaller Stokes shifts. We propose that the dominant factor responsible for the PL blue-shifts depends on the degree of potential fluctuation.

Bulk GaN substrate with overall dislocation density on the order of 105/cm2 fabricated by hydride vapor phase epitaxy

In this study, a combined facet and flattening (FF) growth technique was implemented to fabricate GaN substrates by hydride vapor phase epitaxy. By changing the growth conditions, i.e., the growth temperature and V/III ratio, it was found that facet growth was promoted with a high V/III ratio and low temperature and planar growth was promoted with low V/III ratios and high temperature. We introduce a FF growth technique involving further reduction of the dislocation density using facet growth as the first step and flattening growth of the GaN layer as the second step. To further reduce dislocation density, we also finally demonstrate a multiple-step growth technique based on FF growth and succeeded in producing GaN substrates with overall dislocation densities on the order of 105cm−2.