Thin AlN Buffer Layer Research Articles

Study of Zinc Oxide Films Growth on Aluminum Nitride and Silicon Substrates: Structural, Optical and Electrical Properties

ZnO films were deposited by magnetron sputtering using RF power supply, in order to study the effect of substrates on quality of the prepared films. Then, growth of the ZnO films on thin AlN buffer layer and Si(100) substrates were characterized using different techniques. The surface morphology was investigated by means of scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The structural properties were investigated via X-ray diffraction (XRD) patterns, Rocking Curve as well as Pole figures. The ZnO films were textured and they had preferred orientation (002) and the crystallinity was better for ZnO/Si in the used growth conditions. The XRD results were confirmed by HRTEM. Optical properties were analyzed by photoluminescence (PL), as well as electrical characteristics were performed by C-V and I-V measurements. The dispersion orientation of these films, as indicated via the FWHM (rocking curves), is small for thin ZnO/Si. These results are considered as hopeful for piezoelectric applications.

Ultrathin GaN film and AlGaN/GaN heterostructure grown on thick AlN buffer by MOCVD

High-quality AlGaN/GaN/AlN heterostructures with thin GaN channel and thick AlN buffer layer were grown by metal organic chemical vapor deposition (MOCVD) on SiC substrate. By analyzing growth modes of GaN films on AlN buffer layers with different thicknesses, it is revealed that film-forming point of GaN grown on AlN buffer increases with the increase in AlN buffer thickness. Accordingly, new growth model of GaN on AlN buffer was proposed, which shows that there is an optimal matching value between Ga source flow rate and AlN thickness when GaN is grown on AlN buffer of different thicknesses. Under optimal conditions, AlGaN/GaN/AlN heterostructures with 120 nm thin GaN channel layer and thick AlN buffer show excellent carrier-limited domain, high crystalline quality, and good transport properties. Results in this work would be useful for preparing high-quality heterostructures on AlN buffer and high electron mobility transistor (HEMT) devices. Moreover, these findings can also be applied to the growth of other hyperfine structures (quantum wells, superlattices, and digital alloys) in the future.

Preferential sublimation along threading dislocations in InGaN/GaN single quantum well for improved photoluminescence

InGaN/GaN single quantum wells were grown by molecular beam epitaxy on the silicon substrate onto thin AlN and GaN buffer layers. The InGaN/GaN structure is porosified using a combination of SixNy nanomasking and sublimation and compared with a non-porous reference. The photoluminescence efficiency at room temperature of the porosified sample is improved by a factor reaching 40 compared with the reference sample. Plan-view and cross-sectional transmission electron microscopy images reveal that the remaining material is free of dislocation cores. The regions around dislocations are, thus, preferentially sublimated. This explains the strong photoluminescence improvement of nanoporous InGaN/GaN samples.

Vertical semiconductor deep ultraviolet light emitting diodes on a nanowire-assisted aluminum nitride buffer layer

Vertical light-emitting diodes (LEDs) have many advantages such as uniform current injection, excellent scalability of the chip size, and simple packaging process. Hitherto, however, technologically important semiconductor aluminum gallium nitride (AlGaN) deep ultraviolet (UV) LEDs are mainly through lateral injection. Herein, we demonstrate a new and practical path for vertical AlGaN deep UV LEDs, which exploits a thin AlN buffer layer formed on a nanowire-based template on silicon (Si). Such a buffer layer enables in situ formation of vertical AlGaN deep UV LEDs on Si. Near Lambertian emission pattern is measured from the top surface. The decent reflectivity of Si in the deep UV range makes such a configuration a viable low-cost solution for vertical AlGaN deep UV LEDs. More importantly, the use of such a thin AlN buffer layer can allow an easy transfer of device structures to other carrier wafers for vertical AlGaN deep UV LEDs with ultimately high electrical and optical performance.

Improvement of critical temperature of niobium nitride deposited on 8-inch silicon wafers thanks to an AlN buffer layer

In this paper, we study the crystalline properties and superconducting critical temperature of ultra-thin (5–9 nm) NbN films deposited on 8-inch silicon wafers by reactive sputtering. We show that the deposition of NbN on a thin (10–20 nm) AlN buffer layer, also synthesized by reactive sputtering, improves the critical temperature by several Kelvin, up to 10 K for 9 nm NbN on 20 nm AlN. We correlate this improvement to the higher-crystalline quality of NbN on AlN. While NbN deposited directly on silicon is polycrystalline with randomly oriented grains, NbN on AlN(0001) is textured along (111), due to the close lattice match. The superconducting properties of the NbN/AlN stack are validated by the demonstration of fibre-coupled normal-incidence superconducting nanowire single photon detectors. The whole fabrication process is CMOS compatible, with a thermal budget compatible with the integration of other passive and active components on silicon. These results pave the way for the integration of a large number of surface or waveguide-integrated detectors on large-scale silicon wafers. Furthermore, as AlN is transparent over a broad wavelength range from the visible to the near-infrared, the optimized superconducting NbN/AlN stack can be used for a wide variety of applications, from imaging to quantum communications and quantum computing.

GaN Heteroepitaxy on Strain-Engineered (111) Si/Si1−xGex

The metalorganic chemical vapor deposition growth of GaN on strained Si/Si1−xGex epilayers on (111) Si substrates was investigated. A multi-beam optical stress sensor (MOSS) was used for in situ stress measurements during growth of the entire heterostructure. MOSS was initially used to measure the extent of stress relaxation during the growth of constant-composition Si1−xGex layers on Si. The results compared favorably to that obtained by post-growth high-resolution x-ray diffraction. MOSS was also used to monitor stress during the growth of thin, tensile-strained Si on relaxed Si0.95Ge0.05/compositionally graded epilayers. The tensile-strained Si/Si1−xGex epilayers were then used as virtual substrates for the growth of GaN epilayers using a thin (90 nm) AlN buffer layer. GaN grown on tensile-strained Si exhibited a higher initial compressive stress and reduced final tensile stress compared to films grown directly on (111) Si, resulting in a lower crack density in the GaN along with a reduced density of threading dislocations. These results suggest that strain engineering of the Si surface prior to growth may provide an alternative method to improve the quality of GaN grown on (111) Si.

Vertical GaN nanocolumns grown on graphene intermediated with a thin AlN buffer layer

We report on the self-assembled growth of high-density and vertically-oriented n-doped GaN nanocolumns on graphene by radio-frequency plasma-assisted molecular beam epitaxy. Graphene was transferred to silica glass, which was used as the substrate carrier. Using a migration enhanced epitaxy grown AlN buffer layer for the nucleation is found to enable a high density of vertical GaN nanocolumns with c-axis growth orientation on graphene. Furthermore, micro-Raman spectroscopy indicates that the AlN buffer reduces damage on the graphene caused by impinging active N species generated by the radio-frequency plasma source during the initial growth stage and nucleation of GaN. In addition, the grown GaN nanocolumns on graphene are found to be virtually stress-free. Micro-photoluminescence measurements show near band-edge emission from wurtzite GaN, exhibiting higher GaN bandgap related photoluminescence intensity relative to a reference GaN bulk substrate and the absence of both yellow luminescence and excitonic defect emission. Transmission electron microscopy reveals the interface of GaN nanocolumns on graphene via a thin AlN buffer layer. Even though the first few monolayers of AlN on top of graphene are strained due to in-plane lattice mismatch between AlN and graphene, the grown GaN nanocolumns have a wurtzite crystal structure without observable defects. The results of this initial work pave the way towards realizing low-cost and high-performance electronic and optoelectronic devices based on III-N semiconductors grown on graphene.

The influence of V/III ratio on GaN grown on patterned sapphire substrate with low temperature AlN buffer layer by hydride vapor phase epitaxy

In this paper, 2–4 in. GaN wafers with high crystal quality were obtained with optimized V/III ratio of source gases on the patterned sapphire substrate (PSS) covered by a sputtered thin AlN buffer layer by using hydride vapor phase epitaxy (HVPE) growth technology. It was found that the morphology and crystal quality of the top GaN layer were significantly different with the variation of V/III ratio of source gases in initial nucleation growth step. Transparent single crystalline GaN layer could be obtained under relatively low V/III ratio of source gases, while it transferred to yellow and polycrystalline structure with increasing of V/III ratio of source gases. This is mainly attributed to the significant transition of GaN nucleation process with different crystallographic plane of AlN buffer layer, and corresponding variation of GaN growth mode at different V/III ratio of source gases, supported by surface, lateral SEM characterization, high-resolution X-ray diffraction (XRD) measurement, and detailed growth mechanism analysis. Moreover, the crystal quality of GaN grown under optimized condition can be further improved by increasing the thickness of GaN. It would offer a simple way to obtain GaN templates with high crystal quality at relatively low cost.

Nitrogen-Polar (000 1 ¯ ) GaN Grown on c-Plane Sapphire with a High-Temperature AlN Buffer.

We demonstrate growing nitrogen-polar (N-polar) GaN epilayer on c-plane sapphire using a thin AlN buffer layer by metalorganic chemical vapor deposition. We have studied the influence of the AlN buffer layer on the polarity, crystalline quality, and surface morphology of the GaN epilayer and found that the growth temperature of the AlN buffer layer played a critical role in the growth of the GaN epilayer. The low growth temperature of the AlN buffer results in gallium-polar GaN. Even a nitridation process has been conducted. High growth temperature for an AlN buffer layer is required to achieve pure N-polarity, high crystalline quality, and smooth surface morphology for a GaN epilayer.

Evaluation of AlGaN/GaN high electron mobility transistors grown on ZrTi buffer layers with sapphire substrates

AlGaN/GaN high electron mobility transistors (HEMTs) have been grown on sapphire substrates, using ZrTi buffer layers to provide in-plane lattice-matching to hexagonal GaN. X-ray diffraction (XRD) as well as cross-section transmission electron microscopy (TEM) were used to assess the quality of the HEMT structure. The XRD 2θ scans showed full-width-at-half-maximum values of 0.16°, 0.07°, and 0.08° for ZrTi alloy, GaN buffer layer, and the entire HEMT structure, respectively. TEM studies of the GaN buffer layer and the AlN/ZrTi/AlN stack showed the importance of growing thin AlN buffer layers on the ZrTi layer prior to growth of the GaN buffer layer. The density of threading dislocations in the GaN channel layer of the HEMT structure was estimated to be in the 108 cm−2 range. The HEMT device exhibited a saturation drain current density of 820 mA/mm, and the channel of the fabricated HEMTs could be well modulated. A cutoff frequency (fT) of 8.9 GHz and a maximum frequency of oscillation (fmax) of 17.3 GHz were achieved for HEMTs with gate dimensions of 1 × 200 μm.

Deep-UV nitride-on-silicon microdisk lasers.

Deep ultra-violet semiconductor lasers have numerous applications for optical storage and biochemistry. Many strategies based on nitride heterostructures and adapted substrates have been investigated to develop efficient active layers in this spectral range, starting with AlGaN quantum wells on AlN substrates and more recently sapphire and SiC substrates. Here we report an efficient and simple solution relying on binary GaN/AlN quantum wells grown on a thin AlN buffer layer on a silicon substrate. This active region is embedded in microdisk photonic resonators of high quality factors and allows the demonstration of a deep ultra-violet microlaser operating at 275 nm at room temperature under optical pumping, with a spontaneous emission coupling factor β = (4 ± 2) 10−4. The ability of the active layer to be released from the silicon substrate and to be grown on silicon-on-insulator substrates opens the way to future developments of nitride nanophotonic platforms on silicon.

Self-organization of dislocation-free, high-density, vertically aligned GaN nanocolumns involving InGaN quantum wells on graphene/SiO2 covered with a thin AlN buffer layer

We demonstrated the self-organization of high-density GaN nanocolumns on multilayer graphene (MLG)/SiO2 covered with a thin AlN buffer layer by RF-plasma-assisted molecular beam epitaxy. MLG/SiO2 substrates were prepared by the transfer of CVD graphene onto thermally oxidized SiO2/Si [100] substrates. Employing the MLG with an AlN buffer layer enabled the self-organization of high-density and vertically aligned nanocolumns. Transmission electron microscopy observation revealed that no threading dislocations, stacking faults, or twinning defects were included in the self-organized nanocolumns. The photoluminescence (PL) peak intensities of the self-organized GaN nanocolumns were 2.0–2.6 times higher than those of a GaN substrate grown by hydride vapor phase epitaxy. Moreover, no yellow luminescence or ZB-phase GaN emission was observed from the nanocolumns. An InGaN/GaN MQW and p-type GaN were integrated into GaN nanocolumns grown on MLG, displaying a single-peak PL emission at a wavelength of 533 nm. Thus, high-density nitride p–i–n nanocolumns were fabricated on SiO2/Si using the transferred MLG interlayer, indicating the possibility of developing visible nanocolumn LEDs on graphene/SiO2.

In situ temperature measurements for selective epitaxy of GaN nanowires

AbstractWe demonstrate with spatially resolved, in situ temperature measurements and ex situ reflectance measurements that differences in appearance for masked and unmasked surfaces on patterned growth substrates arise from wavelength‐dependent emissivity variations and not from actual temperature differences. For this study, Si(111) substrates were coated with thin AlN buffer layers (about 40 nm thick) and then further coated with a SiNx mask layer (about 60 nm thick). “Blackbody” emission spectra were collected from homogeneous regions at growth temperatures near those that allow the selective epitaxy of catalyst‐free GaN nanowires on the AlN by molecular beam epitaxy. The data were acquired with a commercial in situ diagnostic system that incorporates optical fiber collection to limit the sampling area to a circle about 5 mm in diameter on the substrate. The emittance spectra are estimated from room‐temperature reflectance measurements. The implication for selectivity mechanisms and temperature measurements in molecular beam epitaxy in general are also discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A RHEED investigation of self‐assembled GaN nanowire nucleation dynamics on bare Si and on Si covered with a thin AlN buffer layer

Abstractmagnified imageBased on reflection high energy electron diffraction (RHEED) experiments, we show that the nucleation delay of GaN nanowire growth by plasma‐assisted molecular beam epitaxy on nitrided Si(111) and on AlN/Si(111) exhibit similar behaviors in function of the substrate temperature and of the Ga flux. On bare Si(111), a finite delay, estimated to be one minute in our system and under our set of growth conditions, is necessary to form and subsequently amorphize crystalline β‐Si3N4 before GaN can start to form. Although the amorphization time is found to be independent of the substrate temperature, this process adds a supplemental delay within the overall delay in the growth of GaN nanowires on bare Si(111), compared to growth on AlN/Si(111) or on amorphous silicon nitride on Si(111). (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Selective-area catalyst-free MBE growth of GaN nanowires using a patterned oxidelayer

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, byrf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. Theoxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate.Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography.The selectivity of growth has been studied varying the substrate temperature, galliumbeam equivalent pressure and patterning layout. Adjusting the growth parameters, GaNNWs can be selectively grown in the holes of the patterned oxide with completesuppression of the parasitic growth in between the holes. The occupation probability of ahole with a single or multiple NWs depends strongly on its diameter. The selectively grownGaN NWs have one common crystallographic orientation with respect to theSi(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD)measurements. Based on the experimental data, we present a schematic model ofthe GaN NW formation in which a GaN pedestal is initially grown in the hole.

Growth of InGaN-based laser diode structure on silicon (111) substrate

This paper reports growth and material characterization of InGaN-based laser diode (LD) structure grown on Si(111) substrate by metal organic chemical vapor deposition (MOCVD) with a high-temperature-grown thin AlN buffer layer and an AlN/GaN multi-layer (ML) intermediate layers. Two types of LD structures on Si were prepared, (a) with bulk n-and p-type Al0.02Ga0.98N cladding layer, and (b) with SLS n- and p-type Al0.06Ga0.94N/GaN cladding layer. Active layer with 3 periods of In0.10Ga0.90N/In0.03Ga0.97N multi-quantum-well (MQW) is sandwiched between the cladding layers. A standard LD structure (c) with identical MQW grown on GaN(0001) free-standing substrate was also prepared for comparison. Mirrorlike and meltback-etching free surfaces have been achieved for both of the samples grown on Si. High-resolution x-ray diffraction (HRXRD) ω/2θ scan reveals that a good quality of MQW has been achieved for both of the samples grown on Si. Temperature-dependence PL measurement reveals that LD structure grown on Si with SLS cladding layer has higher internal quantum efficiency ηint than that of the sample with bulk cladding layer.

Effect of the MgO substrate on the growth of GaN

We investigate the effect of the MgO substrate on the growth of GaN by molecular beam epitaxy with radio frequency plasma source. MgO substrate is advantageous for its closer lattice constant to GaN. However, epitaxial GaN layer grown on MgO (1 1 1) substrate is often inclined about 2 ∘ toward a particular a-axis of the GaN. This inclination is found to be caused by the domain structure of the MgO substrates. On the tilted domains of the substrate, the strain is effectively relaxed through the inclination. Another difficulty of MgO substrate is the diffusion of Mg atoms into the GaN layer. This Mg diffusion from the MgO substrate has successfully been suppressed by introducing the thin low-temperature grown AlN buffer layer. The AlN buffer also improves the crystalline quality of the GaN layer.

From nucleation to growth of catalyst-free GaN nanowires on thin AlN buffer layer

We demonstrate that thin AlN buffer layers improve the orientation of GaN nanowire grown on Si(111). The deposited GaN initially forms into islands which act as a seed for the wires. By raising substrate temperature, the actual amount of grown material decreases but wire density increases and well-separated wires are achieved. Fast wire length growth rate at high growth temperature is assigned to an enhancement of adatom diffusion. The upper limit of length growth rate is determined by the supply rate of active nitrogen.

Effects of a compositionally graded buffer layer on stress evolution during GaN and AlxGa1−xN MOCVD on SiC substrates

Abstract In situ substrate curvature measurements were used to monitor stress evolution during metal organic chemical vapor deposition (MOCVD) of Al x Ga 1− x N epilayers ( x =0, 0.5, 0.7) on 6H-SiC (0 0 0 1) substrates that were grown using compositionally graded AlN to Al x Ga 1− x N buffer layers. The results were compared to identical growths carried out using thin (∼80 nm) AlN buffer layers in order to assess the impact of the graded layer on the mean film stress and structural properties of Al x Ga 1− x N. Compositionally graded (AlN–GaN) layers effectively increased the mean compressive stress in GaN epilayers grown on SiC compared to samples grown on AlN buffer layers, with the mean stress in the GaN layer increasing with buffer layer thickness. Increasing buffer thickness correlated with a decrease in the threading dislocation density measured by plan-view-TEM. In contrast, the mean stress in high Al-fraction Al x Ga 1− x N epilayers ( x =0.5 and 0.7) was not significantly altered by the use of AlN to Al x Ga 1− x N graded buffer layers and in some cases, the graded layer leads to a decrease in mean compressive stress in the films. The differences in stress evolution are explained in terms of the available compressive strain energy, which is suggested to be responsible for dislocation bending observed in the GaN graded layer samples.

High resolution X‐ray diffraction analyses of ion‐implanted GaN/AlN/Si heterostructures

AbstractHigh resolution triple‐axis X‐ray diffraction has been used to characterize the effects of nitrogen ion implantation into thin (15 nm and 55 nm) AlN buffer layers on the resultant structural quality of nominally 2 μm epitaxial GaN layers grown on silicon substrates. The GaN symmetric (0002) and the asymmetric (10$ \bar 1 $4) and (10$ \bar 1 $5) Bragg reflections shows that in all cases, the extent of the off‐peak kinematic diffuse scattering was reduced when growth was performed on the ion‐implanted AlN buffer layers. Analysis of the log I – log q dependence of the kinematic intensity shows linear behavior but with slopes changing from –4.3 to –3.7 (in the case of the (0002) reflection) in the unimplanted and ion‐implanted buffer layers, respectively, indicating a change in the defect character in the layers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)