We report a comparative spectroscopic study on the thin films of epitaxial aluminum nitride (AlN) on basal plane sapphire (Al2O3) substrates grown in hydrogen (H2) and nitrogen (N2) gas reaction environments. AlN films of similar thicknesses (~3.0 µm) were grown by metal-organic chemical vapor deposition (MOCVD) for comparison. The impact of the gas environment on the AlN epilayers was characterized using high-resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS), Raman scattering (RS), secondary ion mass spectroscopy (SIMS), cathodoluminescence (CL), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The study showed that AlN layers grown in a N2 environment have 50% less stress (~0.5 GPa) and similar total dislocation densities (~109/cm2) as compared to the films grown in a H2 environment. On the other hand, AlN films grown in a H2 gas environment have about 33% lesser carbon and 41% lesser oxygen impurities than films grown in a N2 growth environment. The possible mechanisms that influenced the structural quality and impurity incorporation for two different gas environments to grow AlN epilayers in the MOCVD system on sapphire substrates were discussed.

Ultra-wide bandgap AlGaN has attracted recent attention as a promising channel material for next-generation high electron mobility transistors (HEMTs) for RF power due to its high critical field, excellent transport properties, and potential for operation in extreme environments. However, the effects of temperature on the transport properties are not fully understood. Here, Al0.62Ga0.38N/Al0.45Ga0.55N HEMTs have been fabricated and characterized up to 150 °C at DC and RF to evaluate the effect of temperature on electron mobility and carrier velocity. Measured results indicate that both mobility and carrier velocity exhibit modest dependence on temperature, suggesting that AlGaN channel HEMTs are promising for future RF power applications.

We report a 1.8 times improved light extraction efficiency for truncated cone AlGaN DUV (275 nm emission) micropixel LEDs when the pixel size was reduced from 90 to 5 μm. This is a direct consequence of reducing the re-absorption of sideways-travelling photons, as the lateral absorption length was measured to be 15 μm using cathodoluminescence. We also investigated the effect of an Al2O3/Al sidewall coating on the device performance which led to an on-wafer pulsed-mode brightness of 10.3 kW cm−2 from a single 5 μm truncated cone micropixel.

In this paper, we report a study of the degradation of AlGaN-based 280 nm LEDs, which were grown on sapphire substrates using migration-enhanced metalorganic chemical vapor deposition process (MEMOCVD). Electroluminescence (EL), atomic force microscopy (AFM), cathodoluminescence (CL), and scanning electron microscopy (SEM) observations showed that the degradation of deep UV LEDs generally fell into two categories: catastrophic degradation and gradual degradation. The catastrophic degradation was found to be mostly caused by the non-uniformity of surface morphology. The gradual power reduction had two characteristic time constants indicating two possible degradation mechanisms as found from temperature and bias dependent LED power degradation measurements. The faster time constant was bias dependent and virtually constant with temperature whereas the second time constant (slower) varied exponentially with junction temperature. For this temperature dependent part, the activation energies of degradation were determined to be 0.23 eV and 0.27 eV under injected current density of 100 A/cm(2) and 200 A/cm(2) respectively.

The metal oxide semiconductor heterostructure field effect transistors (MOSHFETs) based on AlxGa1-xN materials with the high aluminum composition is a promising choice for high-power, high-temperature harsh environment applications. In the present work the temperature stability of MOSHFETS with high-k ZrO2, Al2O3 gate dielectrics has been studied. Our data show these high-k dielectrics introduce negative fixed charges (Qox) as high as 1-3×1013 cm-2 depleting 2DEG density of 2×1013 cm-2 causing a positive shift of threshold voltage (VTH) compared to that for HFET which is an important feature for realizing enhanced (E-) mode MOSHFET. ZrO2 possesses higher Qox resulting in stronger positive VTH shift in devices, while devices with Al2O3 demonstrate lower gate leakage. To take advantages of both oxides, UWBG Al0.4Ga0.6N channel E-mode MOSHFET has been fabricated. E-mode device was realized using the hybrid oxide (ZrO2/Al2O3) combined with gate recess. To separate effects of dielectric charges and damage from recess process on device performance depletion (D-) mode and E-mode devices were fabricated on the same wafer simultaneously. D-mode devices were protected during gate recess step to avoid additional damage. Thermally induced VTH instability of these MOSHFETs were studied and potential mechanism for this VTH instability is discussed. Experimental: Figure 1(a) shows the pseudomorphic device structure that was grown using metalorganic chemical vapor deposition on AlN/sapphire templates. A graded composition (AlxGa1-xN, x=1-0.4) back barrier reduces internal stress and improves gate control in the devices.[i] , [ii] The top 200 Å thick graded n-AlxGa1-xN (x from 0.6 to 0.3) layer assists with the formation of ohmic contacts resistance as low as 1.7 Ω-mm. The n-doping of this layer compensates the positive charges resulting from the reverse composition grading.[iii] The 2DEG sheet resistance was ~1900 ohm/□. Device processing details published elsewhere.[iv] The fixed gate-length LG ≈ 2.0 μm, source to drain spacing, LSD=6 μm, were used for regular devices with 15 μm channel width, while for precise C(V,T,f) measurements we use a test structure with gate area 200x80 μm2 . Results and discussion: The combination of gate recessing and hybrid oxide resulted in threshold-voltage (VTH) shift of +12.2 V from D to E-mode device (Figure 1(b)). The gate leakage current in both D and E-mode MOSHFETs is ~103 smaller than that of MOSHFETs using singe Al2O3 or ZrO2 layer (Figure 2(a)) which allows us to apply gate voltage as high as +12 V. The peak DC currents for D and E-mode devices were found to be 1.1 A/mm and 0.48 A/mm respectively while in the pulse mode it was 1.3 A/mm and 0.53 A/mm. ON/OFF ratio as high as 3×108 was achieved which is higher than ~2 orders of magnitude than that for Al2O3 or ZrO2. We then performed temperature dependent threshold and gate leakage study of our fabricated MOSHFETs. Figure 2(b) shows the temperature dependent gate leakage characteristics of MOSHFETs of this study. It has been found that, in the D-mode MOSHFET, the VTH experiences positive VTH shift of + 1.7 V from RT to 150 °C; for the E-mode MOSHFET the shift is negative: -2.9 V (Figure 3(a)). The VTH shift for similar device having Schottky gate (no dielectric) (HFET) is significantly smaller, +0.2 V. This shows that the VTH shift is mainly due to the charges in dielectric or at dielectric-barrier interface. In the E-mode devices, the effective channel mobility and VTH was additionally affected by radiative defects introduced during gate-recess step. The mobility (µ) in D-mode devices decreases with temperature while it increases for the E-mode devices. We estimated temperature dependent interface state density (DIT(T)) and Qox (T) using frequency- dependent C-V measurements as shown in Figure 3(b). Our analysis show that in D-mode device, Qox(T) dominates over interface charges. These are fixed negative charges which deplete the channel giving a total VTH shift of +1.7 V from RT to 150°C. The extracted SS value for D and E-mode devices were 99 mV/decade and 134 mV/decade, indicating an increased density of interface traps (DIT) at the recessed interface in the E-mode device. Larger DIT value in E-mode devices caused by a radiation damage from the barrier recessing process and becomes comparable with Qox. Thus the temperature effect on Qox is compensated by DIT changes, increasing SS and making the VTH(T) more negative. [i]) G. Simin et al., Jpn. J. Appl. Phys., 40, L1142 (2001). [ii]) C. Ren et al., J. Semicond. 36, 014008-1 (2015). [iii]) S. Bajaj et al., Appl. Phys. Lett. 109, 133508-1 (2016). [iv]) S. Mollah et al., Appl. Phys. Lett. 117, 232105 (2020). Figure 1

We report the growth evolution of high-quality epitaxial aluminum nitride using nitrogen as carrier gas on the sapphire substrate using MOCVD. A series of samples were grown with thickness varying from 1 to 4 μm utilizing a two-step process without any interlayer. SEM/AFM data showed voids up to 3 µm thickness along the growth direction, aiding to grow thicker layers and achieve low dislocation densities. For the 4 μm thick sample, XRD studies showed the FWHM of 289 arcsec of the rocking curve for (10ī2) plane and total calculated dislocation density 1.1 × 109 cm−2 using Williamson and Hall procedure. Raman's study showed the E2 (high) phonon peak linewidth of 3.4 cm−1 at around 659 cm−1, showing significantly low compressive stress of 0.59 GPa for 4 µm thick AlN. This study shows a simplified and economical method to grow high-quality AlN using N2 as a carrier gas.

Enhancement‐mode ultrawide‐bandgap Al0.65Ga0.35N/Al0.4Ga0.6N metal–insulator–semiconductor heterojunction field‐effect transistors are demonstrated using fluorine‐plasma treatment for threshold voltage control and Al2O3 as gate dielectric. The device exhibits a threshold voltage of 5 V, a maximum drain current density of 105 mA mm−1, and a transconductance of 19 mS mm−1. In addition, the capability to achieve low off‐state current density at 3–4 × 10−9 mA mm−1, an exceptionally low gate leakage current density of 1.4 × 10−8 mA mm−1 even at a high forward gate bias of VGS = 12 V, and a current on/off ratio >1010 is shown. Small signal measurement shows that the device has a unity current gain cutoff frequency fT of 3.8 GHz and power gain cutoff frequency fmax of 4.5 GHz.

Ultra-wide bandgap semiconductors have attracted much interest over the last decade. These materials have strong chemical bonding that led to high temperature tolerance, high voltage and power handling capabilities as described by the Baliga figure of merit. Al-rich III-nitride materials are expected to have the best performance over the industry standard Si, and newer options such as SiC and GaN. Continuous improvements dictate the miniaturization of electronic devices, leading to increasingly higher power densities in smaller footprints, leading to heating that causes system failure. Thus, the thermal management is the limiting task for continuous performance scaling. In past we have reported on deep ultraviolet light emitting diodes (LEDs) and Al x Ga 1-x N (x>0.4) channel Heterojunction Field-Effect Transistors (HFETs) on 2-3 µm thick MOCVD grown high quality AlN/sapphire templates. For template thicknesses over 5 µm either sapphire or AlN patterning is needed to avoid stress related cracking. In this research, we for the first-time report on the direct MOCVD growth and characterization of crack-free, low-impurity AlN templates in excess of 16 μm on basal plane sapphire substrates without doing any external processing. Increasing the thickness improves thermal management and enables an easier sapphire substrate’s laser liftoff for increased DUV light extraction. It would also be used as a heat spreader in power electronics to minimize the operating temperature to maximize power output to industrial loads such as electric vehicles, and large turbines in manufacturing and transportation. All growths were carried out in custom MOCVD reactors with a fast-metalorganic switching manifold using a modified epitaxy procedure in which the Al- and N-precursors (Trimethyl Aluminum and NH 3 ) are pulsed for increased surface mobility. At the initial pulsed growth stage, the growth-temperature and precursor-flow rates are adjusted to yield air-pocketed (voids) rough AlN layers. Most of these high defect materials are located near the AlN/sapphire interface. As the AlN thickness increases in the subsequent growths, the layers turned out smooth and the number of defects is significantly reduced due to dislocation bending/annihilation and cracking is avoided due to strain relief from the voids at the interface. All template growths were carried out at temperatures ~ 1200 - 1300 °C and 40 torr. From X-ray diffraction (XRD), Reciprocal space map (RSM), and Transmission Electron Microscopy (TEM) analysis of these thick AlN templates, we established that they were fully relaxed. Cross-section TEM data clearly shows the presence of voids at ~ 0.4 – 0.9 μm from the AlN/sapphire interface. There is a high density of dislocations present near the void region. Above this region, at least one order of magnitude lower dislocations is observed at the AlN smooth layer region. From the room-temperature Cathodoluminescence (CL) data we see an increase in the band edge emission and a decrease in the defect related long-wavelength signal with increasing AlN template thicknesses. The CL spectra were measured in the edge emission pump geometry on cleaved bars due to the transverse-magnetic polarized nature of the band-edge emission. The monochromatic CL imaging data also confirmed a highly defective region at the AlN/sapphire interface which is also the origin of the long wavelength (below bandgap) CL emission. The Atomic Force Microscopy (AFM) surface scans of the templates showed the surface roughness ~ 0.15 nm to 0.25 nm (for a 5 µm x 5 µm scan). The (102) off axis X-ray linewidths for all these samples ranged from 280 to 330 arc-secs. XRD and Raman experiments confirmed a residual compressive strain in AlN templates compared to the bulk sample. The measured stress ranges from -0.6 GPa to -1.2 GPa for all the templates, reasonable agreement with previous reports. We have found no obvious thickness dependence of the stresses measured from both XRD and Raman, an indication of preserving similar stresses in thick and thin templates. The thermal conductivity of our high-quality 16 µm thick template and the bulk AlN are measured at different temperatures by Time-domain Thermoreflectance (TDTR) technique. The thermal conductivity of the thick AlN at room temperature is 320 Wm -1 K -1 , or ~10% higher than that of the bulk AlN (~285 W/m-K). At low temperatures (120 K), it exceeds that of bulk AlN by more than 40%. In summary, these templates showed thermal conductivity as high or better than high-cost bulk AlN substrates, demonstrating for the first time that sapphire-based power electronics could be effectively managed thermally. Figure 1

We report the growth of crack-free 4 μm thick Aluminum Nitride (AlN) layers in a custom build vertical cold wall metal–organic chemical vapor deposition (MOCVD) reactor using N2 carrier gas on 0.2° offcut sapphire substrate without any additional substrate preprocessing steps. The growth process includes a low-temperature pulsed rough buffer layer followed by a high-temperature layer with continuous growth without any interlayer. The structural properties of the AlN were analyzed using atomic force microscopy (AFM), X-ray diffraction (XRD), and Raman spectroscopy. The AFM image of the 4 µm AlN layer shows an atomically smooth 2-dimensional surface with terrace-like steps. The dislocation density of 1 × 109 cm−2 was calculated using Williamson and Hall process for a 4 µm AlN sample. Additionally, strain calculation from XRD and stress calculation from Raman spectroscopy of AlN grown with N2 carrier gas are discussed.

We report on 193 nm excimer laser-based liftoff (LLO) of Al0.26Ga0.74N/GaN high electron mobility transistors (HEMTs) with thick (t > 10 μm) AlN heat spreading buffer layers grown over sapphire substrates. The use of the thick AlN heat spreading layer resulted in thermal resistance (Rth) of 16 K mm/W for as-fabricated devices on sapphire, which is lower than the value of ∼25–50 K mm/W for standard HEMT structures on sapphire without the heat-spreaders. Soldering the LLO devices onto a copper heat sink led to a further reduction of Rth to 8 K mm/W, a value comparable to published measurements on bulk SiC substrates. The reduction in Rth by LLO and bonding to copper led to significantly reduced self-heating and drain current droop. A drain current density as high as 0.9 A/mm was observed despite a marginal reduction of the carrier mobility (∼1800 to ∼1500 cm2/V s). This is the highest drain current density and mobility reported to-date for LLO AlGaN/GaN HEMTs.

We report a recessed-gate enhancement-mode Al 2 O 3 -ZrO 2 /Al 0.6 Ga 0.4 N/Al 0.4 Ga 0.6 N metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET) with drain current as high as 0.48 A mm −1 at a gate-source voltage of +12 V. This was enabled by a pseudomorphic HFET structure with graded back barrier for strain management and to screen the growth interface from the channel. The device exhibited a threshold-voltage ( V TH ) of 2.75 ± 0.57 V with absolute maximum V TH = 3.6 V, a +12.2 V shift from that for a depletion-mode MOSHFET fabricated on the same wafer. A 3-terminal breakdown voltage of 700 V was measured in the off-state, showing the viability of E-mode UWBG AlGaN for power electronics.

We present a study of the light output power and the thermal impedance of 281 nm emission AlGaN based micropixel LEDs. A modular interconnected micropixel array design is presented which enables dense packing with area and power scalability. We study 5–15 μ m diameter stand-alone devices and parallel-connected micropixel arrays with 5 μ m interpixel gaps. A standalone 5 μ m pixel emits 291 W cm −2 at 10.2 kA cm −2 DC-drive. A power as high as 23 mW (361 W cm −2 ) was measured at a pulsed-pump current of 800 mA (∼15 kA cm −2 ) for an interconnected array. These are the smallest and brightest DUV LEDs to date.

Aluminum nitride (AlN) has garnered much attention due to its intrinsically high thermal conductivity. However, engineering thin films of AlN with these high thermal conductivities can be challenging due to vacancies and defects that can form during the synthesis. In this work, we report on the cross-plane thermal conductivity of ultra-high-purity single-crystal AlN films with different thicknesses (∼3-22 μm) via time-domain thermoreflectance (TDTR) and steady-state thermoreflectance (SSTR) from 80 to 500 K. At room temperature, we report a thermal conductivity of ∼320 ± 42 W m-1 K-1, surpassing the values of prior measurements on AlN thin films and one of the highest cross-plane thermal conductivities of any material for films with equivalent thicknesses, surpassed only by diamond. By conducting first-principles calculations, we show that the thermal conductivity measurements on our thin films in the 250-500 K temperature range agree well with the predicted values for the bulk thermal conductivity of pure single-crystal AlN. Thus, our results demonstrate the viability of high-quality AlN films as promising candidates for the high-thermal-conductivity layers in high-power microelectronic devices. Our results also provide insight into the intrinsic thermal conductivity of thin films and the nature of phonon-boundary scattering in single-crystal epitaxially grown AlN thin films. The measured thermal conductivities in high-quality AlN thin films are found to be constant and similar to bulk AlN, regardless of the thermal penetration depth, film thickness, or laser spot size, even when these characteristic length scales are less than the mean free paths of a considerable portion of thermal phonons. Collectively, our data suggest that the intrinsic thermal conductivity of thin films with thicknesses less than the thermal phonon mean free paths is the same as bulk so long as the thermal conductivity of the film is sampled independent of the film/substrate interface.

Due to their superior breakdown fields compared with GaN and SiC and high thermal conductivity, AlxGa1−xN (x > 0.4) channel high‐electron‐mobility transistors (HEMTs) will find applications in extreme environments such as power electronics. Herein, the high‐temperature operation of ultrawide‐bandgap (UWBG) Al0.65Ga0.35N/Al0.4Ga0.6N metal oxide semiconductor heterostructure field effect transistors (MOSHFETs) with atomic layer‐deposited (ALD) high‐k gate dielectrics TiO2, Al2O3, and ZrO2 is reported. As compared with similar geometry HFETs, these devices exhibit a simultaneous reduction in gate‐leakage current by ≈104 and a positive shift of the threshold voltage as much as 4 V. This positive threshold shift indicates the introduction of negative charges at the oxide/barrier interface and within the thin oxide, attributed to the pre‐ALD plasma treatment. The gate leakage increases weakly with temperature up to 250 °C, whereas the peak drain currents decrease from ≈0.5 to 0.3 A mm−1. An analysis of the C–V and I–V characteristics reveals that this drain current decrease is due to a reduction in channel electron mobility. The potential mechanisms responsible for this are discussed. Up to the measured temperature of 250 °C, the devices withstand repeated temperature cycles without catastrophic degradation or breakdown, underscoring the promise of these materials.

An initial study of losses in n‐AlxGa1−xN planar waveguides at λemission ≈ 280 nm using monolithically integrated AlxGa1−xN multiple quantum wells (MQWs)‐based light‐emitting diodes and detectors is presented. The epilayer structure for the integrated devices is grown on an AlN (3.5 μm thick) template over sapphire substrates. Emitter–detector optical coupling and the directional independence of radiation within the epistructure are experimentally established. A model for estimating the attenuation coefficient under these conditions is developed. The attenuation coefficient for a planar n‐Al0.65Ga0.35N waveguide is measured to be 5–6 cm−1, and it primarily arises from the free‐carrier absorption rather than surface roughness‐dependent Rayleigh scattering.

We report an Ultrawide Bandgap Al0.4Ga0.6N channel metal-oxide-semiconductor heterostructure field effect transistor with drain currents exceeding 1.33 A mm(-1)(pulse) and 1.17 A mm(-1)(DC), around a 2-fold increase over past reports. This increase was achieved by incorporating a hybrid barrier layer consisting of an AlN spacer,n-doped Al0.6Ga0.4N barrier and a thin reverse graded AlxGa1-xN (xfrom 0.60 to 0.30) cap layer. To enhance current spreading, a "perforated" channel layout comprising of narrow channel sections separated by current blocking islands was used. A composite ALD deposited ZrO2/Al(2)O(3)film was used as gate dielectric. A breakdown field above 2 MV cm(-1)was measured.

We report on the demonstration of high current density in Al 0.65 Ga 0.35 N/Al 0.4 Ga 0.6 N heterojunction field effect transistors with micro-channels enabled by enhanced contact injection. Devices with a gate length of 100 nm exhibited a maximum current density of 910 mA/mm and a maximum transconductance of 140 mS/mm. A current gain cut off frequency of 20 GHz and maximum oscillation frequency of 36 GHz were obtained. Large-signal load-pull characterization of the transistors showed output power density of 2.7 W/mm at 10 GHz. The current density and output power density represent the state-of-art performance for high Al-composition AlGaN channel transistors.

We report on a study of UVC photonics integrated circuit consisting of monolithically integrated AlxGa1-xN multiple quantum wells based light-emitting diodes, detectors and channel waveguides on sapphire substrates. The waveguide stack consisted of a 1.5 mu m thick n-Al0.65Ga0.35N waveguide over an AlN (3.5 mu m thick) clad layer. Using the integrated devices, we estimated the multi-mode ridge waveguide losses to be 23 cm(-1) at lambda(emission) similar to 280 nm. We also measured that approximately 80% of the guided light was confined in the n(+)-Al0.65Ga0.35N layer, 7% in the underlying AlN cladding and the remaining 13% in the double-side polished sapphire substrate.

We present the temperature-dependent electrical characteristics of high-current depletion (D-mode) and barrier-recessed enhancement-mode (E-mode) ultrawide bandgap (UWBG) AlxGa1−xN channel insulated gate heterojunction field-effect transistors fabricated on the same wafer. The key motivation is the higher Baliga figure of merit for devices with the UWBG AlGaN channel and their strong potential for use in high-power, high-temperature harsh environmental applications. Over a temperature range of 125 °C, the VTH shifted in the opposite direction for D- and E-mode devices with a rate of +13.5 mV/K and −23 mV/K, respectively, giving an overall shift of +1.7 V and −2.9 V. This was attributed to changes in the fixed and trapped charge densities in the dielectric and at the dielectric–AlGaN barrier interface. A single deep sub-bandgap trap level was sufficient to explain the threshold shifts in both devices. The effective channel mobility in the E-mode devices was argued to be limited by charge scattering, arising from the same charges introduced during barrier recessing that shifted VTH.