Ultrathin AlN Layers Research Articles

Electrical Spin Injection into the 2D Electron Gas in AlN/GaN Heterostructures with Ultrathin AlN Tunnel Barrier

AbstractThe spin injection into 2D electron gas (2DEG) in AlN/GaN heterostructures is studied by magneto‐transport measurements. An ultrathin AlN layer at the hetero‐interface acts as a barrier to form high‐quality 2DEG in the triangular quantum well and a tunneling barrier for the spin injection to overcome the conductance mismatch issue. In this study, Hanle signals and inversed Hanle signals are observed, proving that the spin injection is achieved in the 2DEG in the AlN/GaN heterostructure rather than in the interfacial states. The spin‐relaxation time in 2DEG at 8 K is found to be as long as 860 ps, which almost keeps constant with bias and decreases with increasing temperature. The spin‐relaxation process is illustrated as Rashba spin‐orbit coupling dominated D'yakonov Perel’ mechanisms above 8 K. These results show the promising potential of 2DEG in AlN/GaN heterostructures for spin field‐effect transistor applications.

Engineering the Berreman mode in mid-infrared polar materials.

We demonstrate coupling to and control over the broadening and dispersion of a mid-infrared leaky mode, known as the Berreman mode, in samples with different dielectric environments. We fabricate subwavelength films of AlN, a mid-infrared epsilon-near-zero material that supports the Berreman mode, on materials with a weakly negative permittivity, strongly negative permittivity, and positive permittivity. Additionally, we incorporate ultra-thin AlN layers into a GaN/AlN heterostructure, engineering the dielectric environment above and below the AlN. In each of the samples, coupling to the Berreman mode is observed in angle-dependent reflection measurements at wavelengths near the longitudinal optical phonon energy. The measured dispersion of the Berreman mode agrees well with numerical modes. Differences in the dispersion and broadening for the different materials is quantified, including a 13 cm-1 red-shift in the energy of the Berreman mode for the heterostructure sample.

Modification of strain and optical polarization property in AlGaN multiple quantum wells by introducing ultrathin AlN layer

The effects of ultrathin AlN insertion layers on the strain status, as well as optical properties of AlGaN multiple quantum wells (MQWs), were studied. A large stress variation of about -1.46 GPa can be achieved by introducing two ultrathin AlN layers at each interface between the quantum well and the barrier, thereby resulting in the fact that the degree of polarization is increased from 17.8% to 22.3% in traditional MQWs. In addition, the quantum well emission are found to become symmetric and narrower due to the suppression of compositional fluctuation. These results provide a simple technique to modify the strain field of MQWs so as to improve transverse-electric polarized emission for deep ultraviolet light emitting diodes.

Interfacial and electrical properties of Al2O3/GaN metal–oxide–semiconductor junctions with ultrathin AlN layer

Ultrathin AlN layer deposited by atomic layer deposition (ALD) was employed in Al2O3/GaN metal–oxide–semiconductor (MOS) capacitors, and their interfacial and electrical properties were investigated using X-ray photoelectron spectroscopy (XPS) and current–voltage (I–V) and capacitance–voltage (C–V) measurements. XPS analyses revealed that the diffusion of N atoms into Al2O3 and the degradation of Al2O3 film quality were significant for the thickest Al2O3 (10 nm). The sample with a 10-nm-thick Al2O3 layer produced the highest leakage current and trap density. These results may result from the deteriorated interface characteristics near the AlN layer caused by long growth time. Therefore, it is suggested that the Al2O3 thickness (and optimal growth time) is a key factor in Al2O3/AlN/GaN MOS capacitors.

Elemental diffusion study of Ge/Al2O3 and Ge/AlN/Al2O3 interfaces upon post deposition annealing

The thermal stability of Ge/Al2O3 and Ge/AlN/Al2O3 stacks has been systematically studied upon post deposition annealing (PDA) at 400, 500 and 600 °C with nitrogen gas flow. X-ray photoelectron spectroscopy (XPS) with the incident photon energy of 1486.7 eV and synchrotron radiation photoemission spectroscopy (SRPES) with the incident photon energy of 720, 500 and 200 eV have been used to characterize the interface chemistry and the diffusion of Ge-oxides. More aggressive “clean-up” effect takes place with a higher substrate temperature during the atomic layer deposition (ALD) process for the growth of Al2O3 and AlN thin films. A competitive process among the Ge-oxides growth at the Ge/high-k dielectrics interface, the Ge-oxides diffusion, and GeO desorption has been suggested upon PDA treatments. The effective suppression for formation of Ge-oxides by an ultrathin AlN layer has been observed for the samples before PDA and after PDA at 400 °C. Ge-oxide diffusion in proximity to the gate oxide surface has been characterized from the Ge2p3/2 spectra by XPS after PDA at 400 °C for Ge/Al2O3 and Ge/AlN/Al2O3 stacks. The diffusion mechanism is hypothesized by diffusion of oxygen vacancy. Moreover, a significant desorption of GeO occurs after PDA at 600 °C for the AlN passivated sample.

Passivation of InGaAs interface states by thin AlN interface layers for metal-insulator-semiconductor applications

The passivation of InGaAs by thin AlN layers allows a significant reduction of the interface state density compared to that of the widely used Al2O3/InGaAs structure. The influence of the AlN layer thickness on the interface electrical properties, as well as the role of the post-deposition annealing, was carefully examined. Ultrathin AlN layers (∼1 nm) provide high quality interfacial electrical properties after a mild anneal (400 °C). Thick AlN passivation layers require annealing at higher temperature (500 °C) to achieve low interface states density. Possible explanations of the observed trend are suggested.

Improved interface properties of GaN metal-oxide-semiconductor device with non-polar plane and AlN passivation layer

Utilizing a non-polar plane substrate and an ultra-thin AlN passivation layer results in significantly improved interface properties of a GaN metal-oxide-semiconductor (MOS) device. After depositing an Al2O3 gate dielectric layer on GaN substrates with polar c-plane and non-polar m-plane surfaces, it is found that the devices on the non-polar surface show much better interface properties than those on the polar surface. To further improve the interface properties, an amorphous ultra-thin AlN layer is deposited on the substrate before the Al2O3 deposition. The interface properties of both devices on the c-plane and m-plane are dramatically improved by the AlN passivation layer. The interface trap density of the Al/Al2O3/AlN/GaN MOS capacitor on the non-polar surface is reduced by two orders of magnitude compared to that on the polar surface.

Electron beam induced current microscopy investigation of GaN nanowire arrays grown on Si substrates

We report on the electron beam induced current (EBIC) investigation of GaN nanowires grown on n-doped Si (111) substrates. The objective of this study is to acquire information about the modifications of the substrate properties induced by the wire growth. We show that the growth procedure using deposition of an ultra-thin AlN layer prior to the nanowire growth step leads to the formation of a p-n junction in the Si substrate with a high surface conductivity. The induced p-n junction exhibits a photoresponse over the spectral range from 360nm to 1100nm. The properties of the induced p-n junction are investigated on the cross section and in a top view configuration with EBIC microscopy. For a localized contact of the GaN nanowires, the collection range in Si extends over a few millimeters. The treatment of the surface using reactive ion etching with a CHF3 plasma leads to the inhibition of the surface conductivity and to the appearance of an S-shape in the current-voltage characteristics under illumination. The conversion efficiency of the plasma-treated sample under AM1.5G solar spectrum is estimated to be in the 2.1–2.7% range.

Properties of atomic-layer-deposited ultra-thin AlN films on GaAs surfaces

Abstract Properties and passivation effect of ultra-thin AlN films fabricated on InGaAs/GaAs near-surface quantum wells by plasma-enhanced atomic layer deposition are investigated. The role of the coating on the surface is studied by examining the electric field build-up by photoreflectance. Photoluminescence confirms the passivation effect with ultra-thin layers and the reduced electric fields with thicker AlN layers. Atomic force microscopy shows that an ultra-thin AlN layer does not substantially alter the surface morphology.

Selective growth of N-polar InN through an in situ AlN mask on a sapphire substrate

Selective growth of N-polar InN by exploiting the ∼100° gap between the upper limits of the growth temperatures of In- and N- polarities has been introduced. An InN epilayer grown at a temperature in this gap on a sapphire substrate covered with an ultrathin AlN layer has demonstrated N-polarity. The dislocation density of such-grown InN layer has been significantly reduced (one order of magnitude lower than the conventional InN epilayers). The results have demonstrated great potential for improving the crystalline quality of hetero-epitaxial InN films. This concept can be easily adopted for other substrates.

Oxygen incorporation in homoepitaxial N-polar GaN grown by radio frequency-plasma assisted molecular beam epitaxy: Mitigation and modeling

We have investigated the effect of gallium deposition and desorption cycles and ultrathin (15 Å) AlN layers on the oxygen impurity concentrations in homoepitaxial N-polar GaN layers. Secondary ion mass spectroscopy (SIMS) indicates that three Ga deposition and desorption cycles reduce the total oxygen by 70%–80%, while ten cycles reduces the total oxygen by more than 90%. We present a model of surface segregation and incorporation which accurately captures the distribution of oxygen in these layers. By fitting the model to the SIMS data, we are able to determine the fraction of an oxygen layer adsorbed on a GaN surface which segregates upon initiation of epitaxial GaN growth. Under the conditions investigated, we find this fraction to be 80%.

Homoepitaxial N-polar GaN layers and HEMT structures grown by rf-plasma assisted molecular beam epitaxy

The authors have investigated the growth and structural and electrical properties of homoepitaxial GaN layers and GaN/AlGaN heterostructures grown on free-standing, hydride vapor phase epitaxy grown, N-polar GaN:Fe substrates by rf-plasma molecular beam epitaxy. Secondary-ion mass spectroscopic analysis of unintentionally doped and Be-doped N-polar GaN layers indicate that oxygen is the dominant impurity in all layers and is largely insensitive to growth temperature in the range investigated (675 °C < TS < 760 °C). Transmission electron microscopy (TEM) indicates that threading dislocations are generated at the regrowth interface in these samples; in contrast to homoepitaxial growth on Ga-polar GaN, and that the density of threading dislocations diminishes as the growth temperature increases. However, examination by TEM indicates that threading dislocations are not generated at the regrowth interface of samples subjected to pregrowth substrate surface cleaning by gallium deposition and desorption and subsequent growth of ultrathin (15 Å) initial AlN layers. N-polar GaN/AlGaN heterostructures grown on Be-doped homoepitaxial N-polar GaN buffers exhibit low buffer leakage and Hall mobilities up to 1680 cm2/Vs at sheet densities of 1.3 × 1013 cm−2. High electron mobility transistors have been fabricated on these structures; drain current densities over 700 mA/mm and breakdown voltages as high as 70 V have been measured.

First principles calculation of polarization induced interfacial charges in GaN/AlN heterostructures

We propose a method to calculate polarization induced interfacial charges in semiconductor heterostructures using classical electrostatics applied to real-space band diagrams from first principles calculations and apply it to GaN/AlN heterostructures with ultrathin AlN layers (4–6 monolayers). We show that the calculated electric fields and interfacial charges are independent of the exchange-correlation functionals used (local density approximation and hybrid). We also find the calculated interfacial charge of (6.8±0.4)×1013 cm−2 to be in excellent agreement with experiments and the value of 6.58×1013 cm−2 calculated from bulk polarization constants, validating the use of bulk constants even for very thin films.

GaAs surface passivation by plasma-enhanced atomic-layer-deposited aluminum nitride

A low-temperature passivation method for GaAs surfaces is investigated. Ultrathin AlN layers are deposited by plasma-enhanced atomic-layer-deposition at 200 ° C on top of near-surface InGaAs/GaAs quantum well structures. A significant passivation effect is seen as shown by up to 30 times higher photoluminescence intensity and up to seven times longer lifetime compared to uncoated reference samples. The improved optical properties are accompanied by a redshift of the quantum well photoluminescence peak likely caused by a combination of the nitridation of the GaAs capping layer and a surface coupling effect.

Controlled Growth of High‐Quality ZnO‐Based Films and Fabrication of Visible‐Blind and Solar‐Blind Ultra‐Violet Detectors

AbstractZnO is a wide‐bandgap (3.37 eV at room temperature) oxide semiconductor that is attractive for its great potential in short‐wavelength optoelectronic devices, in which high quality films and heterostructures are essential for high performance. In this study, controlled growth of ZnO‐based thin films and heterostructures by molecular beam epitaxy (MBE) is demonstrated on different substrates with emphasis on interface engineering. It is revealed that ultrathin AlN or MgO interfacial layers play a key role in establishing structural and chemical compatibility between ZnO and substrates. Furthermore, a quasi‐homo buffer is introduced prior to growth of a wurtzite MgZnO epilayer to suppress the phase segregation of rock‐salt MgO, achieving wide‐range bandgap tuning from 3.3 to 4.55 eV. Finally, a visible‐blind UV detector exploiting a double heterojunction of n‐ZnO/insulator‐MgO/p‐Si and a solar‐blind UV detector using MgZnO as an active layer are fabricated by using the growth techniques discussed here.

Improved Current Gain in GaN/SiC Heterojunction Bipolar Transistors by Insertion of Ultra-Thin AlN Layer at Emitter-Junction

GaN/SiC Heterojunction Bipolar Transistors (HBTs) with ultra-thin AlN insertion layers at the n-GaN/p-SiC emitter junction are proposed to improve carrier injection efficiency. The current-voltage characteristics of n-GaN/AlN/p-SiC heterojunctions have exhibited very small reverse leakage and good rectification. The capacitance-voltage measurement have revealed that the conduction band offset between n-GaN and p-SiC has been reduced from -0.74 eV to -0.54 eV by insertion of AlN, indicating that the GaN/AlN/SiC heterojunction may show better electron-injection efficiency. A significantly improved common-base current gain (α~0.2) is obtained for GaN/AlN/SiC HBTs with initial N* pre-irradiation, while it was very low (α~0.001) for GaN/SiC HBTs without AlN layers.

Atomic layer deposition of insulating nitride interfacial layers for germanium metal oxide semiconductor field effect transistors with high-κ oxide/tungsten nitride gate stacks

Atomic layer deposition (ALD) was used to deposit passivating interfacial nitride layers between Ge and high-κ oxides. High-κ oxides on Ge surfaces passivated by ultrathin (1–2nm) ALD Hf3N4 or AlN layers exhibited well-behaved C-V characteristics with an equivalent oxide thickness as low as 0.8nm, no significant flatband voltage shifts, and midgap density of interface states values of 2×1012cm−1eV−1. Functional n-channel and p-channel Ge field effect transistors with nitride interlayer/high-κ oxide/metal gate stacks are demonstrated.

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.

Barrier height control for electron field emission by growing an ultra-thin AlN layer on GaN/Mo

We report on a barrier height control on electron field emission characteristics of GaN. It has been found that growing an ultra-thin AlN layer is effective to improve the field emission characteristics, lower threshold electric field and higher emission current density, of polycrystalline GaN on Mo. The Fowler–Nordheim plots of GaN/Mo with and without the ultra-thin AlN layer showed that the growth of the AlN layer effectively lowers the barrier height for field emission. The decrease in the barrier height results from the decrease in the effective electron affinity of GaN by the growth of the ultra-thin AlN layer and not from the negative electron affinity of AlN itself.