AlN Cap Research Articles

AlGaN ultraviolet metal-semiconductor-metal photodetectors grown on Si substrates

AlGaN ultraviolet metal-semiconductor-metal photodetectors (PDs) with low temperature (LT)-AlN and LT-GaN cap layers were prepared on Si substrates. Unlike PDs prepared on sapphire substrates, no markedly reduction in dark current was observed from the PD with LT-GaN cap layer. With an incident wavelength of 305nm and an applied bias of 5V, it was found that peak responsivities were 0.02, 0.005 and 0.007A/W for the PDs with LT-AlN cap layer, with LT-GaN cap layer and without cap layer, respectively. The corresponding detectivities were 2.2×1010, 1.36×1010 and 1.55×1010cmHz0.5W−1, respectively.

Variations in the Effects of Implanting Al at Different Concentrations into SiC

SiC samples implanted at 600°C with 1018, 1019, or 1020 cm-3 of Al to a depth of ~ 0.3 μm and annealed with a (BN)AlN cap at temperatures ranging from 1300 – 1700°C were studied. Some of the samples have been co-implanted with C or Si. They are examined using Hall, sheet resistivity, CL, EPR, RBS, and TEM measurements. In all instances the sheet resistance is larger than a comparably doped epitaxial layer, with the difference being larger for samples doped to higher levels. The results suggest that not all of the damage can be annealed out, as stable defects appear to form, and a greater number or more complex defects form at the higher concentrations. Further, the defects affect the properties of the Al as no EPR peak is detected for implanted Al, and the implanted Al reduces the AlSi peak intensity in bulk SiC. CL measurements show that there is a peak near 2.9941 eV that disappears only at the highest annealing temperature suggesting it is associated with a complex defect. The DI peaks persist at all annealing temperatures, and are possibly associated with a Si terminated partial dislocation. TEM analyses indicate that the defects are stacking faults and/or dislocations, and that these faulted regions can grow during annealing. This is confirmed by RBS measurements.

Nitride-based ultraviolet Schottky barrier photodetectors with LT-AlN cap layers

Abstract We present the characteristics of novel GaN-based ultraviolet (UV) Schottky barrier photodetectors (PDs) with a low-temperature (LT-)AlN cap layer. Comparing them with conventional Schottky barrier PDs, it was found that we achieved smaller dark current and larger UV to visible rejection ratio from the PDs with the LT-AlN cap layer. The dark leakage current for the Schottky barrier PDs with the LT-AlN cap layer was shown to be about four orders of magnitude smaller than that for the conventional Schottky barrier PDs. With −5 V applied bias, the measured responsivity and UV to visible rejection ratio are 0.16 A /W and 7.74×102 for the Schottky barrier PDs with the LT-AlN cap layer, respectively. This result can be attributed to the thicker and higher potential barrier when the LT-AlN cap layer was inserted.

The structure of crystallographic damage in GaN formed during rare earth ion implantation with and without an ultrathin AlN capping layer

Abstract The crystallographic nature of the damage created in GaN implanted by rare earth ions at 300 keV and room temperature has been investigated by transmission electron microscopy versus the fluence, from 7×10 13 to 2×10 16 at/cm 2 , using Er, Eu or Tm ions. The density of point defect clusters was seen to increase with the fluence. From about 3×10 15 at/cm 2 , a highly disordered ‘nanocrystalline layer’ (NL) appears on the GaN surface. Its structure exhibits a mixture of voids and misoriented nanocrystallites. Basal stacking faults (BSFs) of I 1 , E and I 2 types have been noticed from the lowest fluence, they are I 1 in the majority. Their density increases and saturates when the NL is observed. Many prismatic stacking faults (PSFs) with Drum atomic configuration have been identified. The I 1 BSFs are shown to propagate easily through GaN by folding from basal to prismatic planes thanks to the PSFs. When implanting through a 10 nm AlN cap, the NL threshold goes up to about 3×10 16 at/cm 2 . The AlN cap plays a protective role against the dissociation of the GaN up to the highest fluences. The flat surface after implantation and the absence of SFs in the AlN cap indicate its high resistance to the damage formation.

UV-Raman scattering study of lattice recovery by thermal annealing of Eu+ -implanted GaN layers

Lattice recovery of Eu-implanted GaN has been studied by means of Raman scattering under UV excitation. GaN epilayers implanted at 300 keV with doses ranging from 2 x 10(14) to 4 x 10(15) cm(-2) and subsequently annealed at 1000 degrees C for 20 min show an increasing degree of disorder as the implantation dose increases. Higher temperature annealings up to 1300 degrees C were also investigated in samples having an AlN capping layer. Disorder related modes, observed in samples annealed at 1000 degrees C, disappear at higher annealing temperatures, indicating an incomplete lattice recovery at 1000 degrees C. The Raman scattering spectra show resonant A(1)(LO) multiphonon scattering up to sixth order, whose relative intensities depend on the implantation dose. The intensity ratios between multiphonon peaks observed for the highest implantation doses suggest a spread of the resonance, which could be related to a heterogeneous strain distribution, also indicative of incomplete lattice recovery. (c) 2006 Elsevier Ltd. All rights reserved.

Rare Earth Ion Implantation in GaN: Damage Formation and Recovery

Rare earth ions implanted GaN has been investigated by transmission electron microscopy versus the fluence, using Er, Eu or Tm ions at 150 keV or 300 keV and at room temperature. Point defect clusters and stacking faults are generated from low fluences (7 × 10 at/cm), their density increases with the fluence up to the formation of a highly disordered layer at the surface. This highly disordered layer is observed from a threshold fluence of 3×10 at/cm at 150 keV and 3×10 at/cm at 300 keV, and appears to be composed of voids and misoriented nanocrystallites. Its thickness rapidly increases with the fluence, and then saturates. Both basal and prismatic stacking faults were observed. Basal stacking faults are I1 in majority, but E or I2 have also been identified. I1 basal stacking faults propagate easily through GaN by folding from basal to prismatic planes. Channelling implantation, increasing the implantation temperature from room temperature to 500◦C, or implanting through a 10 nm thick AlN cap reduce the crystallographic damage, particularly by retarding the formation of the highly disordered layer. Implanting through the AlN cap allows the highly disordered layer formation threshold fluence to be increased by one order of magnitude, as well as the annealing at high temperature (1300◦C) which brings about a strong optical activation of the rare earths.

Annealing Effect and Suppression of Hydration of La2O3 Thin Films

Lanthanum oxide thin films were deposited using alternate injections of La(iPrCp)3 and H2O. The deposited films were hydrated easily when exposed to air, and Si diffusion during PDA in the La2O3 film was remarkable. An AlN capping layer on the La2O3 films was effective to prevent hydration. Annealing in NH3 gas increased thermal stability and the dielectric constant due to nitrogen incorporation. The film annealed in NH3 gas at 800oC showed an EOT of 2.2nm and a leakage current of 1.7 x 10-8 A/cm2 at -1 V. The amount of Si atoms diffused into the La2O3 films decreased in the sequence; O2-annealed, NH3-annealed and pre-nitrided samples. A pre-nitridation to form a SiN reaction barrier was effective to increase thermal stability.

Behaviour of the AlN cap during GaN implantation of rare earths and annealing

AbstractThe structural evolution of 10 nm AlN caps grown on GaN layers has been investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) after growth, and implantation by rare earths ions and annealing at high temperature. The AlN cap relaxation is shown to take place by pinholes formation. During annealing at 1300 °C, pinhole‐free areas of the AlN cap protect the GaN layer and allow efficient optical activation of the rare earths dopants. Through pinholes, an explosive dissociation of the GaN layer occurs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical and optical activation studies of high dose Si-implanted Al 0.18Ga 0.82N

Both electrical and optical activation studies of Si-implanted Al 0.18Ga 0.82N have been made as a function of anneal time and anneal temperature to obtain maximum possible electrical activation efficiency. Silicon ions were implanted at 200 keV with doses of 5×10 14 and 1×10 15 cm −2, and the samples were annealed from 1100 to 1250 ∘C for 5–25 min with a 500 Å thick AlN cap in a nitrogen environment. The electrical activation efficiency and Hall mobility increase with anneal time and anneal temperature. Nearly 100 and 95% electrical activation efficiencies were obtained for Si-implanted Al 0.18Ga 0.82N with doses of 5×10 14 and 1×10 15 cm −2 and annealing at 1250 and 1200 ∘C for 25 min, respectively. The photoluminescence measurements show an excellent implantation damage recovery after annealing at these optimum anneal conditions, showing a strong near band emission. These optical results correlate well with the electrical results.

Structure and role of ultrathin AlN layers for improving optical activation of rare earth implanted GaN

In this work we carry out characterization of GaN implanted with Tm and Eu ions by scanning and transmission electron microscopy. Structural investigation is supported by optical measurements. We have investigated samples implanted at room temperature with different energies (150-300 keV) and fluences (1-7 x 10 15 at/cm 2 ). High temperature annealing was performed at 1200 °C and 1300 °C with the GaN surface protected by a 10nm thick AlN cap grown by MOCVD prior to the implantation. Direct implantation in GaN, results in the formation of a high density of stacking faults in the damaged surface layer and a nanocrystalline surface layer when a critical dose is exceeded. Implanting through the AlN capping layer prevents the formation of the nanocrystalline layer even for high fluences but the stacking faults are still generated. The AlN cap also protects the surface during high temperature annealing.

Step-by-step capping and strain state ofGaN∕AlNquantum dots studied by grazing-incidence diffraction anomalous fine structure

The investigation of small size embedded nanostructures, by a combination of complementary anomalous diffraction techniques, is reported. GaN Quantum Dots (QDs), grown by molecular beam epitaxy in a modified Stranski-Krastanow mode, are studied in terms of strain and local environment, as a function of the AlN cap layer thickness, by means of grazing incidence anomalous diffraction. That is, the X-ray photons energy is tuned across the Ga absorption K-edge which makes diffraction chemically selective. Measurement of \textit{hkl}-scans, close to the AlN (30-30) Bragg reflection, at several energies across the Ga K-edge, allows the extraction of the Ga partial structure factor, from which the in-plane strain of GaN QDs is deduced. From the fixed-Q energy-dependent diffracted intensity spectra, measured for diffraction-selected iso-strain regions corresponding to the average in-plane strain state of the QDs, quantitative information regarding composition and the out-of-plane strain has been obtained. We recover the in-plane and out-of-plane strains in the dots. The comparison to the biaxial elastic strain in a pseudomorphic layer indicates a tendency to an over-strained regime.

Electrical characterization of Si-ion implanted AlxGa1-xN annealed at lower temperatures

Electrical activation studies of Si-implanted Al x Ga 1-x N (x = 0.1 and 0.18) grown on sapphire substrate have been made as a function of anneal time, anneal temperature, and ion dose. Silicon implantation was done at room temperature with a dose ranging from 5 x 10 13 to 5 x 10 15 cm -2 at 200 keV. The samples were proximity cap annealed from 1100 to 1250 °C for 5-40 min with a 500 A-thick AlN cap in a nitrogen environment. The electrical activation efficiencies of 84% and 75% were obtained for the Al 0.1 Ga 0.9 N after annealing at 1200 °C for 40 min for a dose of 5 × 10 13 and 1 × 10 14 cm -2 , respectively. For the Al 0.18 Ga 0.82 N, nearly 100% activation for a dose of 5 × 10 14 cm -2 and 94% for a dose of 1 x 10 15 cm -2 were achieved after annealing at 1250 °C and 1200 °C for 20 min, respectively. Both the activation efficiency and mobility increase with anneal time and anneal temperature, indicating an improved implantation damage recovery. The highest mobility obtained at room temperature is 89 cm 2 /Vs for the Al 0.1 Ga 0.9 N samples having doses of 5 × 10 13 and 1 x 10 14 cm -2 and annealed at either 1200 °C for 40 min or 1250 °C for 20 min.

In situ resonant x-ray study of vertical correlation and capping effects during GaN∕AlN quantum dot growth

Grazing incidence anomalous x-ray scattering was used to monitor in situ the molecular beam epitaxy growth of GaN∕AlN quantum dots (QDs). The strain state was studied by means of grazing incidence multiwavelength anomalous diffraction (MAD) in both the QDs and the AlN during the progressive coverage of QDs by AlN monolayers. Vertical correlation in the position of the GaN QDs was also studied by both grazing incidence MAD and anomalous grazing incidence small angle scattering as a function of the number of GaN planes and of the AlN spacer thickness. In a regime where the GaN QDs and the AlN capping are mutually strain influenced, a vertical correlation in the position of QDs is found with as a side effect an average increase in the QD width.

Electrical and optical characterization studies of lower dose Si-implanted AlxGa1−xN

Electrical and optical activation studies of lower dose Si-implanted AlxGa1−xN (x=0.14 and 0.24) have been made systematically as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1×1013 cm−2 to 1×1014 cm−2 at room temperature. The samples were proximity cap annealed from 1,100°C to 1,350°C with a 500-A-thick AlN cap in a nitrogen environment. Nearly 100% electrical activation efficiency was obtained for Al0.24Ga0.76N implanted with a dose of 1 × 1014 cm−2 after annealing at an optimum temperature around 1,300°C, whereas for lower dose (≤5×1013 cm−2) implanted Al0.24Ga0.76N samples, the electrical activation efficiencies continue to increase with anneal temperature up through 1,350°C. Seventy-six percent electrical activation efficiency was obtained for Al0.14Ga0.86N implanted with a dose of 1 × 1014 cm−2 at an optimum anneal temperature of around 1,250°C. The highest mobilities obtained were 89 cm2/Vs and 76 cm2/Vs for the Al0.14Ga0.86N and Al0.24Ga0.76N, respectively. Consistent with the electrical results, the photoluminescence (PL) intensity of the donor-bound exciton peak increases as the anneal temperature increases from 1,100°C to 1,250°C, indicating an increased implantation damage recovery with anneal temperature.

Diffusion of Mn in gallium arsenide

The aim of this work is to compare diffusion coefficient and shape of the manganese atomic profile in GaAs treated under different annealing conditions. Diffusion was performed from implanted Mn as well as from external source of Mn. Manganese implanted GaAs, GaAs:Zn and GaAs:Te bulk samples were investigated. Implantation was performed at room temperature to a dose of 10 16/cm 2 at an energy of 150 keV. The samples, were protected with AlN layers, and annealed at 800 °C with RTA (rapid thermal annealing) method as well as at 700 and 900 °C in a sealed quartz ampoule. After removing the AlN films the extent of diffusion of the species was characterized using the secondary ion mass spectrometry (SIMS) technique. The depth profiles of in-diffused manganese in RTA experiment strongly indicate that the diffusion coefficient D is concentration-dependent. For both: quartz ampoule and RTA annealings of implanted samples, the Mn diffusivity was found larger when GaAs was annealed with the AlN cap than that annealed without a cap. Over 10 times shallower diffusion range in uncovered samples than those encapsulated with AlN is interpreted in terms of generation of additional vacancies in the Ga sub-lattice. Mn atoms incorporate in Ga sites lowering thus the diffusion coefficient. In case of diffusion from external source into differently doped GaAs, the largest diffusion coefficient was found for GaAs:Zn. This result indicates highest Mn diffusivity in the sample with a low Fermi level which provides lowest concentration of ionized Ga vacancies. Both results confirm an interstitial diffusion mechanism and decreasing diffusion coefficient with increasing gallium vacancy concentration. The Boltzmann–Matano analysis was employed to evaluate the concentration-dependent diffusion coefficient of Mn in GaAs.

Failure mechanism of AlN nanocaps used to protect rare earth-implanted GaN during high temperature annealing

The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300°C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1–2μm which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates.

TEM investigation of Tm implanted GaN, the influence of high temperature annealing

Abstract In this work we carry out characterization of GaN implanted with Tm ions by transmission electron microscopy. We have investigated samples implanted at room temperature with different energies (150–300 keV) and fluences (1 × 1014–4.9 × 1015 Tm/cm2). High temperature annealing was performed at 1000 °C and 1200 °C with the GaN surface protected by a 10 nm thick AlN cap grown by MOCVD prior to the implantation or by a proximity cap. The influence of fluence, energy, annealing temperature and AlN cap on the structural quality of the layers is discussed. Direct implantation in GaN, results in the formation of a nanocrystalline surface layer when a critical dose is exceeded. Moreover, there is a generation of basal stacking faults. Implanting through the AlN capping layer prevents the formation of the nanocrystalline layer even for high fluences but a high density of stacking faults is still observed after implantation. The AlN cap also protects the surface during high temperature annealing, showing no signs of surface dissociation after annealing at 1200 °C. At this temperature a significant reduction of the number of stacking faults is observed.

High temperature annealing of rare earth implanted GaN films: Structural and optical properties

GaN epilayers grown by MOCVD were implanted with Tm and Eu under different implantation conditions, in order to optimize the implantation parameters of fluence, implantation temperature and energy. Rutherford backscattering spectrometry in the channelling mode (RBS/C), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL) and room temperature cathodoluminescence (CL) were used to study structural and optical properties of the samples. Different annealing methods are compared. According to RBS/C measurements, Tm at low fluences is found to incorporate entirely on substitutional Ga-sites while for higher fluences the substitutional fraction decreases. Increasing the fluence leads to increasing damage levels and finally to the formation of a partially amorphous, nanocrystalline surface layer. The luminescence intensity increases with fluence at first. However, for fluences in excess of 2 × 1015 Tm/cm2 it decreases again, showing a strong correlation between structural and optical properties. Implanting at higher temperature inhibits the formation of the nanocrystalline layer and increases both the substitutional fraction and the luminescence intensity. We also found that the presence of a thin AlN cap protects the sample during post-implant high temperature annealing and prevents the formation of a nanocrystalline surface layer during the implantation even for high fluences. The intensity of Eu-related CL near 622 nm at room temperature increases by a factor of 40 within the studied annealing range from 1000 to 1300 °C.

Selectively excited photoluminescence from Eu-implanted GaN

The intensity of Eu-related luminescence from ion-implanted GaN with a 10nm thick AlN cap, both grown epitaxially by metal organic chemical vapor deposition (MOCVD) is increased markedly by high-temperature annealing at 1300°C. Photoluminescence (PL) and PL excitation (PLE) studies reveal a variety of Eu centers with different excitation mechanisms. High-resolution PL spectra at low temperature clearly show that emission lines ascribed to D05-F27 (∼622nm), D05-F37 (∼664nm), and D05-F17 (∼602nm) transitions each consist of several peaks. PL excitation spectra of the spectrally resolved components of the D05-F27 multiplet contain contributions from above-bandedge absorption by the GaN host, a GaN exciton absorption at 356nm, and a broad subedge absorption band centred at ∼385nm. Marked differences in the shape of the D05-F27 PL multiplet are demonstrated by selective excitation via the continuum/exciton states and the below gap absorption band. The four strongest lines of the multiplet are shown to consist of two pairs due to different Eu3+ centers with different excitation mechanisms.

Electrical and optical activation studies of Si-implanted GaN

Comprehensive and systematic electrical and optical activation studies of Si-implanted GaN were made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1×1013 cm−2 to 5×1015 cm−2 at room temperature. The samples were proximity-cap annealed from 1050°C to 1350°C with a 500-A-thick AlN cap in a nitrogen environment. The optimum anneal temperature for high dose implanted samples is approximately 1350°C, exhibiting nearly 100% electrical activation efficiency. For low dose (≤5×1014 cm−2) samples, the electrical activation efficiencies continue to increase with an anneal temperature through 1350°C. Consistent with the electrical results, the photoluminescence (PL) measurements show excellent implantation damage recovery after annealing the samples at 1350°C for 20 sec, exhibiting a sharp neutral-donor-bound exciton peak along with a sharp donor-acceptor pair peak. The mobilities increase with anneal temperature, and the highest mobility obtained is 250 cm2/Vs. The results also indicate that the AlN cap protected the implanted GaN layer during high-temperature annealing without creating significant anneal-induced damage.