AlN Nucleation Research Articles

AlN/AlGaN/GaN buffer optimization on silicon (111): bow and crystal quality control for Si‐CMOS fabs

AbstractIn GaN‐on‐silicon there are many challenges which are currently encountered when making this technology compatible with standard Si‐CMOS fabs especially the bow. Bringing this technology to CMOS fabs can potentially drive the costs down, in addition to the cost advantage expected from large wafer sizes. We report here on the impact of AlN nucleation layer on crystal quality and bow. The focus will be on V/III ratios and pre‐dose conditions for AlN, which have considerable effect on the AlN/AlGaN/GaN buffer structures. We will also discuss how AlN thickness can be used to tune the bow. Finally, a brief description on the effect of surface pits will be presented.(© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystallographic tilt in GaN-on-Si (111) heterostructures grown by metal–organic chemical vapor deposition

We report on the studies of crystallographic tilt induced by miscut of the Si (111) substrate in GaN-on-Si (111) heterostructures grown by metal–organic chemical vapor deposition. By employing high-resolution X-ray diffraction, we found that the onset of crystallographic tilt occurred at the interface between the AlN nucleation layer and the Si (111) substrate. The orientation of the GaN overlayer always follows that of the AlN nucleation layer irrespective of its quality and miscut of the substrates. The resultant GaN [0002] is tilted toward GaN (11−20) and (10−10) atomic planes for the miscuts of Si (111) toward Si [1−10] and [11−2], respectively. In both cases, the misorientation of GaN (0002), i.e., the tilt of GaN [0002] from the surface normal direction, is in the same direction of the miscut of Si (111). The misorientation angle of the GaN epilayer is generally smaller than the miscut angle of the substrate. However, the crystallographic tilt, i.e., the angle formed between GaN [0002] and Si [111], is always much larger than the Nagai tilt. These observations are attributable to misfit dislocations that are anisotropically generated at the AlN/Si (111) interface. This mechanism is discussed based on recent microscopic observations of in-plane misfit dislocations at the interface near the atomic step edges.

Mixed Phases at the Bottom Interface of Si‐Doped AlGaN Epilayers of Optoelectronic Devices

This paper presents an analysis of crystalline structures of Si‐doped Al0.4Ga0.6N layers grown on not‐intentionally doped AlGaN buffer layer with an AlN nucleation layer by metal organic chemical vapor deposition. Weak cubic Al0.4Ga0.6N (002) and (103) reflection peaks are observed in high‐resolution XRD θ/2θ scans and cubic Al0.4Ga0.6N (LO) mode in Raman scattering spectroscopy. These cubic subgrains are localized at the bottom interface of Si‐doped layer due to the pulsed lower growth temperature and rich hydrogen atmosphere at the start of silane injection. Their appearance has no direct relationship with the buffer and nucleation layer. This study is helpful not only to understand fundamental properties of high aluminum content Si‐doped AlGaN alloys but also to provide specific guidance on the fabrication of multilayer optoelectronic devices where weak cubic subgrains potentially occur and exert complicated influences on the device performance.

Quality Improvement of GaN on Si Substrate for Ultraviolet Photodetector Application

GaN is grown on an Si substrate using metal-organic vapor-phase epitaxy. Compared with the full width at half maximum values from X-ray diffraction patterns and photoluminescence spectra of conventional GaN on the Si substrate, those of GaN on the Si substrate with the insertion of various-temperature AlN nucleation layers and Al0.3Ga0.7N/GaN superlattice intermediate layers are reduced by 34.9% and 25.6%, respectively. In addition, Raman spectra show that residual stress on the GaN epilayers decreased by 0.35 GPa. The c-lattice parameter of the GaN epilayer is 5.1844 Å, which is close to that of an unstrained GaN layer. Ultraviolet metal-semiconductor-metal photodetectors are fabricated on an almost-crack-free GaN surface. The dark current of a photodetector on the Si substrate is 2.4 ×10-11 A at a 9 V applied bias, which is one order of magnitude smaller than that of a photodetector on a conventional sapphire substrate. The maximum quantum efficiency value of a photodetector on the Si substrate is ~ 97% with an incident light wavelength of 360 nm and a 9 V applied bias.

Occurrence and elimination of in-plane misoriented crystals in AlN epilayers on sapphire via pre-treatment control

AlN epilayers are grown directly on sapphire (0001) substrates each of which has a low temperature AlN nucleation layer. The effects of pretreatments of sapphire substrates, including exposures to NH3/H2 and to H2 only ambients at different temperatures, before the growth of AlN epilayers is investigated. In-plane misoriented crystals occur in N-polar AlN epilayers each with pretreatment in a H2 only ambient, and are characterized by six 60°-apart peaks with splits in each peak in (101̄2) phi scan and two sets of hexagonal diffraction patterns taken along the [0001] zone axis in electron diffraction. These misoriented crystals can be eliminated in AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient. AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient are Al-polar. Our results show the pretreatments and the nucleation layers are responsible for the polarities of the AlN epilayers. We ascribe these results to the different strain relaxation mechanisms induced by the lattice mismatch of AlN and sapphire.

High‐voltage RESURF AlGaN/GaN high electron mobility transistor with back electrode

A novel reduced surface field (RESURF) AlGaN/GaN high electron mobility transistor (HEMT) with back electrode is proposed. The back electrode is electrically grounded and attached to the AlN nucleation layer after the substrate is removed. The back electrode, which attracts the electric field lines at the AlGaN/GaN interface by inducing negative charges, leads to a more uniform horizontal electric field along the channel and, hence, a significant improvement in breakdown voltage. Meanwhile, there is negligible negative impact on the ON-state resistance. Numerical simulation demonstrates a breakdown voltage of 1701 V and an ON-state resistance of 10.45 Ω mm with a gate-drain spacing of 6 μm for the proposed device, compared to a breakdown voltage of 1118 V and an ON-state resistance of 10.34 Ω mm for conventional RESURF AlGaN/GaN HEMT.

Polarity control of AlN layers grown on sapphire substrates by oxygen doping during AlN nucleation

We report on the polarity control of the crystal polarity of AlN layers grown on sapphire substrates by oxygen doping during AlN nucleation. Oxygen concentration amongst AlN nuclei as well as the resulting impact on surface morphology and crystalline quality was investigated using isotopic oxygen as a dopant. We observed that the density of AlN nuclei increased with increasing oxygen concentration, and that AlN films with Al-polar can be obtained with an oxygen concentration of over 1021cm−3 in the AlN nucleation layer. The results agree well with the surface morphologies and (101̄2) full width at half maximum values of the X-ray ω-rocking curve of AlN overgrowth layers, indicating that the proposed method with the optimized oxygen concentration can be used to control the polarity and obtain AlN films with a smooth surface and good crystalline quality.

Influence of the growth temperature of AlN nucleation layer on AlN template grown by high-temperature MOCVD

We studied the influence of the growth temperature of AlN nucleation layer (TNL) on the AlN template grown by high-temperature metal-organic chemical vapor deposition (HT-MOCVD). The AlN templates were characterized by high-resolution X-ray diffractometer, atomic force microscopy and room-temperature Raman scattering spectrometer. The results revealed that the TNL had a direct influence on the quality of the AlN template. By optimizing the TNL at 950°C, we obtained a high-quality AlN template with the full width at half maxima for the (0002) and (10–12) planes of 90″ and 612″, respectively. The AlN template also presented atomic level step with a root mean square (RMS) roughness of 0.133nm. In addition, it performed excellent single crystallographic orientation along the c-axis. The growth evolution of AlN nucleation layer at different TNL was also explained in detail.

Epitaxial growth of AlN films via plasma-assisted atomic layer epitaxy

Thin AlN layers were grown at 200–650 °C by plasma assisted atomic layer epitaxy (PA-ALE) simultaneously on Si(111), sapphire (112¯0), and GaN/sapphire substrates. The AlN growth on Si(111) is self-limited for trimethyaluminum (TMA) pulse of length > 0.04 s, using a 10 s purge. However, the AlN nucleation on GaN/sapphire is non-uniform and has a bimodal island size distribution for TMA pulse of ≤0.03 s. The growth rate (GR) remains almost constant for Tg between 300 and 400 °C indicating ALE mode at those temperatures. The GR is increased by 20% at Tg = 500 °C. Spectroscopic ellipsometry (SE) measurement shows that the ALE AlN layers grown at Tg ≤ 400 °C have no clear band edge related features, however, the theoretically estimated band gap of 6.2 eV was measured for AlN grown at Tg ≥ 500 °C. X-ray diffraction measurements on 37 nm thick AlN films grown at optimized growth conditions (Tg = 500 °C, 10 s purge, 0.06 s TMA pulse) reveal that the ALE AlN on GaN/sapphire is (0002) oriented with rocking curve f...

AlGaN Ultraviolet A and Ultraviolet C Photodetectors with Very High Specific Detectivity D*

The development of AlGaN pin photodetectors sensitive in the UV range with different narrow band active regions is reported in this paper. Structures were grown by metalorganic vapor phase epitaxy on (0001) sapphire substrates using three-dimensional GaN as well as high temperature AlN nucleation. Very high specific detectivities of 1×1014 cm Hz0.5 W-1 can be achieved based on optimized growth conditions of undoped and doped AlGaN layers with an Al-content ranging from 0% up to 100%. The crack-free AlGaN layers have edge dislocation densities in the range of 5×109 cm-2. Based on the two different nucleation types, pin layer structures were grown and fabricated to UV-A (320 to 365 nm) and UV-C (< 280 nm) photodetectors. The electro-optical performance of these photodetectors measured on-wafer will be presented in this paper, supplemented by the data of a single photodetector chip mounted in a TO 18 package.

GaN-on-insulator technology for high-temperature electronics beyond 400 °C

Lattice-matched InAlN/GaN high electron mobility transistors (HEMTs) have been prepared in a silicon-on-insulator (SOI)-like configuration. Here, this implies an ultrathin body 50 nm GaN channel/50 nm AlN nucleation layer material structure on sapphire with the active areas confined by mesa etching, resulting in semi-enhancement mode device characteristics. In contrast to conventional technologies, the device characteristics (maximum drain current, threshold voltage and 1 MHz large signal operation) change only within less than approx. 10% up to 600 °C compared to room temperature (RT). The current on/off ratio decreases from 1010 at RT to 106 at 600 °C, due to residual defect activation. These first results of ultrathin body GaN-on-sapphire-based materials and device technology may indicate that essential improvements in the temperature-handling capability of electronic device structures beyond what is common at present may be possible with only limited sacrifice of device performance.

Ultrathin Body InAlN/GaN HEMTs for High-Temperature (600$^{\circ} {\rm C}$) Electronics

Lattice matched 0.25-μm gatelength InAlN/GaN high electron mobility transistors are realized in an ultrathin body mesa technology (50-nm AlN nucleation layer/50-nm GaN buffer) on sapphire. At room temperature, the maximum output current density is I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> =0.4A/mm, the threshold voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> =-1.4 V with an associated subthreshold voltage swing of 73 mV/dec and a leakage current ≈ 1 pA (for W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> =50 μm) and thus a current on/off ratio of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> . At 600°C, the maximum drain current, threshold voltage, and transconductance are nearly unchanged. The current on/off ratio is still approximately 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> . First 1-MHz class A measurements with ±2.0 V peak-to-peak signal amplitude have resulted in 109-mW/mm output power at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> =8.75 V.

MOVPE growth of semipolar III-nitride semiconductors on CVD graphene

We report the growth and characterization of group III-nitride semiconductor layers on graphene grown by chemical vapour deposition. GaN, AlGaN alloys, and InN layers are grown using an AlN nucleation and a buffer layer. We investigate the effect of varying growth temperature and V/III ratio and show that under optimized growth conditions preferentially semipolar (101¯1) oriented nitride layers can be obtained. These layers, though polycrystalline, are highly oriented and show strong room temperature photoluminescence, thus showing graphene to be a novel substrate for the growth of III-nitride materials.

Plasma‐assisted molecular beam epitaxy of AlGaN heterostructures for deep‐ultraviolet optically pumped lasers

AbstractThe paper reports on elaboration of plasma‐assisted molecular beam epitaxy (MBE) of AlxGa1 − xN‐based quantum‐well (QW) structures with high Al content (up to 50% in the QW) grown directly on c‐sapphire. Different elements of the structure design are considered consecutively in detail along with the advanced growth approaches developed for each element. Special attention is paid to the growth conditions of (i) AlN nucleation layers with suppressed generation of threading dislocations (TDs), (ii) 2‐µm thick AlN buffer layers with atomically smooth droplet‐free morphology (rms = 0.46 nm) grown under the strongly metal‐rich conditions, (iii) cladding and waveguide AlGaN layers also possessing the atomically smooth droplet‐free morphology that is ensured by the accurately established phase diagram of metal(Ga)‐rich growth conditions within the temperature range 660–780 °C. Employing several 3‐nm thick strained GaN insertions in the AlN buffer layer and a AlGaN/AlN superlattice (SL) on top of it is shown to result in a significant decrease of TD's density down to 108–109 cm−2 in the top QW region fabricated by a submonolayer digital alloying (SDA) technique. Finally, advanced AlGaN‐based QW structures are presented, which demonstrate optically pumped lasing within the deep‐ultraviolet (UV) wavelength range with the threshold power density below 600 kW cm−2 (at 289 nm).

Impact of AlN nucleation layer on strain in GaN grown on 4H-SiC substrates

The impact of AlN nucleation layers on the strain evolution in a subsequent GaN layer was investigated by in-situ wafer curvature measurement. It is shown that growth temperature and thickness of the AlN nucleation layer strongly influence the built-in strain and crystalline quality of GaN. A two-step AlN growth procedure leads to a decrease of compressive strain as well as the reduction of the total dislocation density (6–9×108cm−2) in the overgrown GaN layer. TEM analysis reveals different relaxation mechanisms of GaN in v-pits and on flat surfaces of AlN. With a two-step AlN nucleation layer a low wafer bow can be achieved together with a low dislocation density.

Control of threading dislocation density at the initial growth stage of AlN on c-sapphire in plasma-assisted MBE

Crystalline properties of (1–2)-μm-thick AlN buffer layers grown by plasma-assisted molecular-beam epitaxy (PA MBE) on c-Al2O3 substrates with different AlN nucleation layers have been studied. The best quality layers are obtained on 50-nm-thick nucleation AlN layers grown by a migration enhanced epitaxy (MEE) at substrate temperature of 780°C. In this case the buffer layers possess the lowest FWHM values of the symmetric AlN(0002) and skew symmetric AlN(10–15) x-ray rocking curve peaks of 469 and 1025arcsec, respectively, which correspond to the screw and edge threading dislocation densities of 4.7×108cm−2 and 5.9×109cm−2. This improvement seems to be related with the larger diameter of the flat-top grains in the AlN nucleation layers grown in the MEE mode at high substrate temperatures.

GaN-Based LEDs With an HT-AlN Nucleation Layer Prepared on Patterned Sapphire Substrate

We report the growth and fabrication of GaN-based light-emitting diodes (LEDs) with a high-temperature (HT) AlN nucleation on patterned sapphire substrate. It was found that the undercut sidewalls were only formed for the HT-AlN LED through defect selective etching. At 1-A current injection, the output power of the LED with HT-AlN nucleation was 12% higher than that of an LED with a conventional low temperature GaN nucleation layer.

In situ observation of two-step growth of AlN on sapphire using high-temperature metal–organic chemical vapour deposition

We studied the two-step growth of AlN using high-temperature (HT) metal–organic chemical vapour deposition (MOCVD) using a 405 nm short-wavelength in situ monitoring system. First, an AlN nucleation layer (NL) was grown on sapphire at 950 °C before deposition of the HT-AlN film at 1300 °C. The 405 nm wavelength in situ reflectance-transient curves revealed the evolution of AlN growth. The reflectance intensity of the 405 nm signal first decreased during AlN growth and then became stronger until it reached a steady state at large and equal amplitude, revealing that the AlN growth underwent a transition from nuclei to nuclei decomposed island recovery to quasi-layer-by-layer growth. The effect of different initial growth conditions on the AlN growth mode was also studied using the in situ monitoring system. The growth mechanism for the films was proposed based on the 405 nm in situ reflectance curves and atomic force microscopy observations. By optimizing the NL growth conditions, we obtained a high-quality AlN/sapphire template with full width at half maxima for the (0002) and (10−12) planes of 60 arcsec and 550 arcsec, respectively. Finally, a high performance PIN-AlGaN detector was fabricated on the AlN template, which further demonstrated its high quality.

Reducing dislocations of thick AlGaN epilayer by combining low-temperature AlN nucleation layer on c-plane sapphire substrates

In this study, we have reported on growth of thick AlGaN layer on the c-plane sapphire substrate with low-temperature AlN (LT-AlN) nucleation layer by low-pressure metal–organic chemical vapor deposition (LPMOCVD). High resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements have been employed to study the crystal quality, threading dislocation density, surface morphology, and optical properties of thick AlGaN layer. Results indicate that the insertion of LT-AlN nucleation layer between sapphire substrate and high-temperature AlN nucleation layer effectively improves the thick AlGaN crystal quality, reduces the surface roughness and eliminates the threading dislocation density.

Mechanism of Si outdiffusion in plasma‐assisted molecular beam epitaxy of GaN on Si

AbstractThe mechanism of Si outdiffusion related to the formation of Si‐rich GaN clusters during the growth of GaN‐on‐Si by plasma‐assisted molecular beam epitaxy (PAMBE) has been identified. GaN thin films were grown, on high resistivity Si (111) substrates at Ga‐rich conditions to favour the 2D step‐flow growth mode. Hall‐effect measurements revealed very high electron background concentrations, ranging from 2 x 1018 cm‐3 to 3 x 1019 cm‐3, with degraded carrier mobility in the range of 5‐30 cm2/Vs. SEM examination of surfaces and cross‐sections of the samples revealed the presence of hollow GaN clusters on the film surface and voids at the Si/III‐nitride interface. EDX spot analysis revealed dissolution of Si material in the GaN clusters. Growth of GaN on Si under stoichiometric conditions resulted in films without GaN clusters, which exhibited a serious reduction in carrier concentration, down to 1.4 x 1016 cm‐3, with a simultaneous increase of the carrier mobility to 137 cm2/Vs. C‐V carrier profile measurements were consistent with Hall‐effect measurements and revealed a uniform electron concentration in the GaN film, down to the AlN nucleation layer (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)