Thick GaN Epitaxial Layers Research Articles

Performance of semi-insulating metal-semiconductor-metal GaN prototype devices as ionizing radiation detector

We describe the fabrication and characterization of semi-insulating GaN devices for the detection of ionizing radiation with applications in high radiation environment. We present the DC characterization and the signal response of such devices from Am-241 α-source. The detector prototypes show up to 80% charge collection efficiency with bias voltages as low as -40V. Wide band gap semiconductors like synthetic diamond, GaN, SiC have gained immense importance in the field of charged particle detection due to their low intrinsic noise and high radiation tolerance. The ability of suppressing the thermal noise generated by intrinsic carriers, makes the fabrication of these devices simpler when compared to the multi-step lithography required for doping of traditional silicon-based solid-state detectors. We have made interdigitated metal-semiconductor-metal(MSM) device on a 3μm in thick GaN epitaxial layer grown on sapphire substrate by MOCVD technique. We employed Ni/Pt/Au metal stack for Schottky contact with finger width of 4μm, spacing of 8μm and finger length of 120μm. Details of the experimental set-up for fabrication and characterization will be presented.

Reliability limitations from crystal defects in thick GaN epitaxial layers

The vertical power switching devices made from GaN and related nitride semiconductors contain a high density of crystal defects, especially in the thick epitaxial drift layers. Most of these defects are present initially in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects play on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful high voltage environments. This paper provides the progress of characterization of thick GaN power semiconductor material epitaxial layers and the potential impact crystal defects may have on high-density power switching electronics. The investigation also presents simulated results of the electric field distribution through thick epitaxial (greater than 10 mm) region.

Research on epitaxial of 250 nm high quality GaN HEMT based on AlN surface leveling technology

GaN HEMT epitaxial materials with different buffer layer thicknesses were grown on 4H-SiC substrates by metal-organic chemical vapor deposition (MOCVD). The interface quality between AlN and GaN was improved based on AlN surface planarization technology. A high quality 250 nm thick GaN epitaxial layer was grown. The (0 0 2) plane and the (1 0 2) plane rocking curve have a full width at half maximum of 81 arc seconds and 209 arc seconds, respectively, and the electron mobility of the heterojunction two-dimensional electron gas reaches 2238 cm2/V·s. The surface topography and electrical properties of the material were measured by atomic force microscopy (AFM) and non-contact Hall tester. It was found that the surface smoothness of the GaN buffer layer and the electron mobility of 2DEG did not change significantly, and the X-rays diffractometer was used to analyze the crystal quality and stress of the GaN epitaxial layer.

(Invited) Nanostructures and Crystal Defects in Thick GaN and SiC Epitaxial Layers for Power Electronic Switches

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects play on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful environments. This paper provides a review of the current state-of-the-art of SiC and GaN power semiconductor material technology, and the potential impact crystal defects may have on high-density power switching electronics. Group III-Nitride materials suffer from both extended and point defects, each of which will challenge the material’s application in both vertical and lateral power devices. The extended defects include vertical threading dislocations of both edge and screw type. The latter defects have been shown to be correlated to leakage in vertical two terminal device structures while the influence of the former is still undetermined and remains a critical research issue. Channel surfaces in vertical three terminal devices will also degrade due to vertical threading dislocations. These extended defects occur in all epitaxial layers grown on c-plane substrates (the predominant and largest area substrate type) and are the result of the lack of a high quality substrate bulk material as well as substrate surface. The present substrate technology after multiple efforts (either GaN, SiC or Si) still have a high defect density indicating that low-defect seeds may be non-existing. Point defects influence the background carrier concentration in low-doped layers and various recombination processes. The intrinsic nature of III-N materials makes nitrogen-vacancies a predominant point defect that automatically dopes the crystal n-type, but there are impurities (oxygen and carbon) from most growth environments that also contribute to conductivity. These defects are not well understood and not well controlled. Such defects must be minimized in order to realize thick epitaxial layers with drift regions that will support both high blocking voltages and low on-state resistances. A wide range of materials characterization methods have been applied to further understand these impurities. The semiconductor and dielectric materials used in the construction of power electronics switching devices employed for energy efficiency applications experience extreme electrical and thermal stresses during the on-state as well as when switching high voltages and high currents. For example, in a bipolar type power semiconductor switching device in order to achieve significant conductivity modulation and reduce the drift-region resistance, high-level injection of minority carriers is typical. If the semiconductor contains a high density of crystal defects, as is the case with WBG semiconductors, minority carrier recombination phenomenon can be adversely affected. For example, it has been shown more than a decade ago [3, 4] that basal plane dislocations (BPDs) present in SiC are excited by the energy released from minority carrier recombination process; the result is excess BPD generation and its glide within the semiconductor material that leads to an increase in the drift-region resistance and thermal run-away in a forward biased bipolar junction diode The reported experimental data for high-voltage SiC semiconductor strongly suggests that material defects have profound impact on its characteristics under extreme environments. Since GaN semiconductor has even higher density of material defects, this problem is further exacerbated especially in thick epitaxial GaN layers which are required for power switches beyond 5kV. The integration of the pillar junction scheme with the trench-gate GaN MOSFET processes developed at the University of Maryland will be reported. The thick pillar junction epitaxial layers have been grown by a commercial vendor, using a windowed GaN growth process and/or selective deep etching, followed by GaN regrowth to fabricate columns with opposite carrier polarities. The doping density of the n--pillars under the gate has been specified with background carrier concentration n ≤ 1x1016 cm-3, while the p-pillars will have a higher doping density (p ≥ 1x1018 cm-3) to maximize depletion in the n--pillar. The p-body and n+-source layers are grown by subsequent epitaxial growth across the whole structure. Our current research into optimized trench gate fabrication and gate dielectric processing has been used to fabricate gate trenches into the structure. These devices can be compared to traditional trench MOSFETs with extremely thick blocking layers of comparable thickness to the pillar junction layer in order to determine the role of defects in thick GaN epi layers in terms of limiting device performance and reliability.

(Invited) Nanostructures and Crystal Defects in Thick GaN and SiC Epitaxial Layers for Power Electronic Switches

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects paly on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful environments. This paper provides a review of the current state-of-the-art of SiC and GaN power semiconductor material technology, and the potential impact crystal defects may have on high-density power switching electronics. A review of silicon technology development and manufacturing evolution is made to draw a parallel between silicon and wide bandgap (WBG) semiconductor power electronics.

Electrothermal evaluation of thick GaN epitaxial layers and AlGaN/GaN high-electron-mobility transistors on large-area engineered substrates

AlGaN/GaN high-electron-mobility transistor (HEMT) device layers were grown by metal organic chemical vapor deposition (MOCVD) on commercial engineered QST™ substrates to demonstrate a path to scalable, cost-effective foundry processing while supporting the thick epitaxial layers required for power HEMT structures. HEMT structures on 150 mm Si substrates were also evaluated. The HEMTs on engineered substrates exhibited material quality, DC performance, and forward blocking performance superior to those of the HEMT on Si. GaN device layers up to 15 µm were demonstrated with a wafer bow of 1 µm, representing the thickest films grown on 150-mm-diameter substrates with low bow to date.

Effect of the Ti-Nanolayer Thickness on the Self-Lift-off of Thick GaN Epitaxial Layers

The effect of the type of substrate, sapphire substrate (c- and r-orientation) or GaN/Al2O3 template (c- and r-orientations), on the nitridation of an amorphous titanium nanolayer is shown. The effect of the titanium-nanolayer thickness on thick GaN epitaxial layer self-separation from the substrate is revealed. The titanium-nanolayer thickness at which thick GaN layer is reproducibly self-separated is within 20–40 nm.

Effect of coalescence layer thickness on the properties of thin‐chip InGaN/GaN light emitting diodes with embedded photonic quasi‐crystal structures

AbstractThe simulation, fabrication and optical characterization of InGaN/GaN MQW‐LEDs grown by MOVPE over embedded photonic quasi‐crystals (PQCs) are reported. Fully coalesced GaN layers as thin as 140 nm were grown over 1200 nm high air‐gap PQCs using an intermittent pulsed/normal growth method. Simulations and angle‐resolved photoluminescence measurements reveal there are strong interactions between the embedded PQC and a wide range of trapped modes as well as the dominant low order mode. The interaction with a dominant low order mode is most pronounced when the LED cavity above the embedded PQC has fewer guided modes. magnified image 1200 nm high embedded air‐gap PQC overgrown with a 280 nm thick coalesced GaN epitaxial layer by MOVPE.

RBS/Channeling and TEM Study of Damage Buildup in Ion Bombarded GaN

A systematic study on structural defect buildup in 320 keV Ar-ion bombarded GaN epitaxial layers has been reported, by varying ion fluences ranged from 5 × 10 to 1 × 10 at./cm. 1 μm thick GaN epitaxial layers were grown on sapphire substrates using the metal-organic vapor phase epitaxy technique. Rutherford backscattering/channeling with 1.7 MeVHe beam was applied for analysis. As a complementary method high resolution transmission electron microscopy has been used. The later has revealed the presence of extended defects like dislocations, faulted loops and stacking faults. New version of the Monte Carlo simulation code McChasy has been developed that makes it possible to analyze such defects on the basis of the bent channel model. Damage accumulation curves for two distinct types of defects, i.e. randomly displaced atoms and extended defects (i.e. bent channel) have been determined. They were evaluated in the frame of the multistep damage accumulation model, allowing numerical parameterization of defect transformations occurring upon ion bombardment. Displaced atoms buildup is a three-step process for GaN, whereas extended defect buildup is always a two-step process.

Growth of high-quality InGaN/GaN LED structures on (1 1 1) Si substrates with internal quantum efficiency exceeding 50%

GaN-based light-emitting-diodes (LEDs) on (1 1 1) Si substrates with internal quantum efficiency (IQE) exceeding 50% have been successfully grown by metal organic vapor phase epitaxy (MOVPE). 3.5 μm thick crack-free GaN epitaxial layers were grown on the Si substrates by the re-growth method on patterned templates. Series of step-graded Al x Ga 1− x N epitaxial layers were used as the buffer layers to compensate thermal tensile stresses produced during the post-growth cooling process as well as to reduce the density of threading dislocations (TDs) generated due to the lattice mismatches between III-nitride layers and the silicon substrates. The light-emitting region consisted of 1.8 μm thick n-GaN, 3 periods of InGaN/GaN superlattice, InGaN/GaN multiple quantum wells (MQWs) designed for a peak wavelength of about 455 nm, an electron blocking layer (EBL), and p-GaN. The full-widths at half-maximum (FWHM) of (0 0 0 2) and (1 0 −1 2) ω-rocking curves of the GaN epitaxial layers were 410 and 560 arcsec, respectively. Cross-sectional transmission electron microscopy (TEM) investigation revealed that the propagation of the threading dislocations was mostly limited to the interface between the last Al x Ga 1− x N buffer and n-GaN layers. The density of the threading dislocations induced pits of n-GaN, as estimated by atomic force microscopy (AFM), was about 5.5×10 8 cm −2. Temperature dependent photoluminescence (PL) measurements with a relative intensity integration method were carried out to estimate the internal quantum efficiency (IQE) of the light-emitting structures grown on Si, which reached up to 55%.

High power AlGaN/GaN HFETs on 4 inch Si substrates

AbstractIn this paper, we successfully demonstrate an AlGaN/GaN HFET with a high breakdown voltage on 4‐inch Si substrates. In order to obtain the high breakdown voltage and to improve the crystalline quality of GaN layers, a thick GaN epitaxial layer including a buffer layer was grown. The breakdown voltage and the maximum drain current were achieved to be over 1.3 kV and 120 A, respectively. Furthermore, the suppression of a current collapse phenomenon was examined. The on‐resistance was not significantly increased up to a high drain off‐bias‐stress of 900 V. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cross‐sectional and surface Raman mapping of thick GaN layers

AbstractRaman scattering spectroscopy was utilized for investigation of the structural properties of thick GaN layers. These layers with thickness ∼ 40 μm have been grown by HVPE technique on the sapphire substrates. The investigations have been focused on the strain distribution in GaN layer cross‐section as a function of distance from an interface sapphire/GaN and mapping of the surface and of the inner layer, near the sapphire/GaN interface. From the observed phonon shifts in the Raman spectra strain differences lower than 6.4×10–4 corresponding to stress differences of 240 MPa were estimated across the thick GaN epitaxial layer. The measurements exhibit that strain in the layer causes changes in the Raman spectra and allow determining the relaxation process in the crystal. The obtained results confirmed, that the mode frequencies in the measured Raman spectra in both directions (parallel or perpendicular to the growth direction) for layer thicknesses over 30 µm are comparable with typical values for bulk material and match the low strain in the structure due to relaxation processes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spatial variation of optical and structural properties of ELO GaN directly grown on patterned sapphire by HVPE

A 275 µm thick GaN layer was directly grown on the SiO2-prepatterned sapphire in a home-built vertical hydride vapour phase epitaxy (HVPE) reactor. The variation of optical and structure characteristics were microscopically identified using spatially resolved cathodeluminescence and micro-Raman spectroscopy in a cross section of the thick film. The D 0 XA line with the FWHM of 5.1 meV and etch-pit density of 9 × 106 cm−2 illustrated high crystalline quality of the thick GaN epitaxial layer. Optically active regions appeared above the SiO2 masks and disappeared abruptly due to the tapered inversion domains at 210–230 µm thickness. The crystalline quality was improved by increasing the thickness of the GaN/sapphire interface, but the region with a distance of 2 µm from the top surface revealed relatively low quality due to degenerate surface reconstruction by residual gas reaction. The x-ray rocking curve for the symmetric (0 0 2) and asymmetric (1 0 2) reflections also showed good quality and a small wing tilt of the epitaxial lateral overgrowth (ELO) GaN.

Improvement of diodes performance with a multiple-pair buffer layer by MOCVD

The GaN homo-junction light-emitting diodes (LEDs) with a multiple-pair buffer layer (MBL) were grown by metalorganic vapor phase epitaxy on sapphire substrates. Each pair of the buffer layer consists of a 300 Å thick GaN nucleation layer grown at a low temperature of 525°C and a 4 m thick GaN epitaxial layer grown at a high temperature of 1025°C. It is found that the GaN LEDs with a three-pair buffer layer will exhibit a low turn-on voltage, stronger electroluminescence intensity and higher light-output power as compared to the conventional growth without a buffer layer. This is attributed to the effective reduction in the propagation of defects and dislocations near the p– n junction for the LEDs with MBL.

Electrical and optical characteristics of the GaN light-emitting diodes with multiple-pair buffer layer

The GaN homo-junction light-emitting diodes (LEDs) with multiple-pair buffer layer (MBL) were grown by metalorganic vapor phase epitaxy on sapphire substrates. Each pair of the buffer layer consists of a 300 Å thick GaN nucleation layer grown at a low temperature of 525°C and a 4 μm thick GaN epitaxial layer grown at a high temperature of 1025°C. As compared to the conventional growth without buffer layer, the GaN LEDs with MBL will exhibit a low turn-on voltage, stronger electroluminescence intensity, and higher light output power. It is attributed to the effective reduction in the propagation of defects and dislocations near the p–n junction for the LEDs with MBL.

Effects of multiple buffer layers on structural electronic properties of GaN growth by atmospheric pressure Organometallic Vapor Phase Epitaxy

High-quality GaN epitaxial films have been grown on sapphire by organometallic vapor phase epitaxy using multiple-pair buffer layers. Each pair of buffer layers consists of a thin GaN nucleation layer grown at a low temperature around 500°C and a thick GaN epitaxial layer around 4 μm thick grown at a high temperature around 1000°C. The sample with four-pair buffer layers showed much improved GaN epitaxial films, as compared to the sample with only one-pair of buffer layers. The better qualities include narrower full width at half maximum of 150 arc-s and stronger intensity in double-crystal X-ray diffraction, higher electron mobility of 420 cm 2 V-s −1, lower background concentration of 3×10 17 cm −3, and lower etch-pit density of mid-10 5 cm −2.

Effectiveness of multiple-pair buffer layer to improve the GaN layers grown by metalorganic chemical vapor deposition

High-quality GaN epitaxial layers with a multiple-pair buffer layer have been grown on sapphire substrates in a separate-flow reactor by metalorganic chemical vapor deposition. Each pair of buffer layer consists of a 300 Å thick GaN nucleation layer grown at a low temperature of 525 °C and a 1–4 μm thick GaN epitaxial layer grown at a high temperature of 1000 °C. The GaN samples with a multiple-pair buffer layer are characterized by double-crystal x-ray diffraction (DC-XRD), Hall method and photoluminescence (PL) at 300 K, and etch-pit density measurements. The optimized condition to obtain the best quality of GaN epitaxial layers is to grow the four-pair buffer layer with a pair thickness of 4 μm. The GaN samples with the optimized buffer layer exhibit a narrow full width at half maximum (FWHM) of 150 arcsec and a strong intensity in DC-XRD, a high electron mobility of 450 cm2/V s, a low background concentration of 3×1017 cm−3, a low etch-pit density of mid-105 cm−2, and a narrow FWHM of 56 meV in PL spectrum.

High rate GaN epitaxial growth by sublimation sandwich method

Thick GaN epitaxial layers were grown by the sublimation “sandwich method” (SSM) on SiC substrates at temperatures from 1100°C to 1250°C in ammonia flow. Metallic Ga or GaN powder was used as the vapor source. The possibility of growing of monocrystalline GaN layers with growth rates as high as 1 mm/h was demonstrated. The dependence of the growth kinetics on temperature, source to substrate distance and input ammonia flow rate was studied. Various characterization techniques show the high quality of the GaN layers.

Pressure-dependent photoluminescence study of InxGa1−xN

We present the results of pressure-dependent photoluminescence (PL) studies of single-crystal InxGa1−xN (0⩽x<0.15) films grown on top of thick GaN epitaxial layers by metalorganic chemical vapor deposition with sapphire as substrates. PL measurements were performed at 10 K as a function of applied hydrostatic pressure using the diamond-anvil-cell technique. The luminescence emissions from the InxGa1−xN epifilms were found to shift linearly toward higher energy with increasing pressure. By examining the pressure dependence of the PL spectra, the pressure coefficients for the emission structures associated with the direct band gap of InxGa1−xN were determined. The values of the pressure coefficients were found to be 3.9×10−3 eV/kbar for In0.08Ga0.92N and 3.5×10−3 eV/kbar for In0.14Ga0.86N.

Optical transitions in InxGa1−xN alloys grown by metalorganic chemical vapor deposition

We present the results of optical studies of InxGa1−xN alloys (0<x<0.2) grown by metalorganic chemical vapor deposition on top of thick GaN epitaxial layers with sapphire as substrates. Photoluminescence (PL) and photoreflectance measurements were performed at various temperatures to determine the band gap and its variation as a function of temperature for samples with different indium concentrations. Carrier recombination dynamics in the alloy samples were studied using time-resolved luminescence spectroscopy. While the measured decay time for the alloy near-band-edge PL emissions was observed to be generally around a few hundred picoseconds at 10 K, it was found that the decay time decreased rapidly as the sample temperatures increased. This indicates a strong influence of temperature on the processes of trapping and recombination of excited carriers at impurities and defects in the InGaN alloys.