Deep Ultraviolet Optoelectronic Devices Research Articles

Controlled exfoliation of wafer-scale single-crystalline AlN film on MOCVD-grown layered h-BN

In this work, we present a stress-free AlN film with improved crystal quality assisted by h-BN and demonstrate the mechanical exfoliation of wafer-scale single-crystal AlN freestanding membrane and reveal the controllable exfoliation mechanism of AlN. Uniform and continuous wafer-scale h-BN is directly grown on c-plane sapphire using a flow modulation epitaxy mode by metal-organic chemical vapor deposition. The nucleation and evolution processes of quasi-van der Waals epitaxy (QvdWE) of AlN on h-BN are revealed. It is found that O2-plasma-treated h-BN can effectively promote the nucleation islands of AlN and contribute to the release of biaxial stress and the reduction of dislocation density in the epilayers. Eventually, the QvdWE growth of a stress-free AlN film (0.08 GPa) is achieved, and wafer-scale mechanical exfoliation of the AlN membrane has been realized. This work provides an effective strategy for the quality improvement of III-nitride films and paves the way for the vertical structure and flexible deep-ultraviolet optoelectronic devices.

Growth optimization, optical, and dielectric properties of heteroepitaxially grown ultrawide-bandgap ZnGa2O4 (111) thin film

Ultrawide bandgap ZnGa2O4 (ZGO) thin films were grown on sapphire (0001) substrates at various growth temperatures with a perspective to investigate the electrical and optical characteristics required for high-power electronic applications. Due to the variation in the vapor pressure of Zn and Ga, severe loss of Zn was observed during pulsed laser deposition, which was solved by using a zinc-rich Zn0.98Ga0.02O target. A pure phase single-crystalline ZGO thin film was obtained at a deposition temperature of 750 °C and an oxygen pressure of 1 × 10−2 Torr. The out-of-plane epitaxial relationship between the sapphire and ZGO thin film was obtained from φ-scan. The x-ray rocking curve of the ZGO thin film grown at 750 °C exhibits a full width at half maximum of ∼0.098°, which indicates a good crystalline phase and quality of the thin film. Core-level x-ray photoelectron spectroscopy of ZGO grown at 750 °C indicated that Zn and Ga were in the 2+ and 3+ oxidation states, respectively, and the atomic ratio of Zn/Ga was estimated to be ∼0.48 from the fitted values of Zn-2p3/2 and Ga-2p3/2. The high-resolution transmission electron microscopy images revealed a sharp interface with the thickness of the ZGO film of ∼265 nm, and the signature of minor secondary phases was observed. The bandgap of the ZGO film at different growth temperatures was calculated from the ultraviolet-diffuse reflectance spectroscopy spectra, and its value was obtained to be ∼5.08 eV for the 750 °C grown sample. The refractive index (n) and the extinction coefficient (k) were determined to be ∼1.94 and 0.023 from the ellipsometric data, respectively, and the real dielectric function (ɛr) was estimated to be ∼6.8 at energy 5 eV. The ultrawide bandgap and dielectric function of ZGO recommend its possible potential applications in deep-ultraviolet optoelectronic devices and high-power electronics.

Fabrication of a freestanding AlN substrate via HVPE homoepitaxy on a PVT-AlN substrate

Hydride vapor phase epitaxy (HVPE) is employed for the homoepitaxial development of AlN thick films on AlN substrates grown via physical vapor transport (PVT). A freestanding AlN substrate with a 200 μm thickness is then obtained by mechanically grinding away the PVT-AlN substrate. The as-grown HVPE AlN layer has a smooth surface with long parallel atomic steps. The freestanding HVPE-AlN substrate is crack-free and stress-free. In comparison to PVT-AlN substrate, HVPE-AlN substrate not only has better crystal quality but also substantially lower C, O, and Si impurity concentrations. The deep ultraviolet (DUV) transmittance of the 200 μm thick freestanding AlN substrate is as high as 66% at 265 nm. This performance aligns perfectly with the demands of AlGaN-based DUV optoelectronic devices.

Probing and controlling oxygen impurity diffusion in h-BN semi-bulk crystals

Combining its unique features of ultrawide bandgap (UWBG) and two-dimensional nature, h-BN has been explored for emerging applications such as deep ultraviolet optoelectronic devices and single photon emitters. One of the unusual applications of h-BN is for solid-state neutron detectors by utilizing the property of high thermal neutron capture cross section of B-10 as well as its UWBG properties. Although a record high detection efficiency of 59% has been attained by h-BN detectors, the understanding/minimization of defects and impurities is still needed to further advance the h-BN material and detector technologies. We report metal organic chemical vapor deposition growth and oxygen (O) impurity diffusion in thick h-BN. The diffusion coefficient (D) of O impurities has been measured via the evolution of an oxygen related emission with the etching depth, providing a value of D of ∼ 2 × 10−13 cm2/s at 1450 °C and supporting the interpretation that oxygen in h-BN is a substitutional donor. A multiple-buffer-layer approach was employed to mitigate to a certain degree the issue of oxygen diffusion from sapphire substrate during growth. It was demonstrated that the performance of h-BN neutron detectors fabricated from the wafer incorporating multiple buffer layers was significantly improved, as manifested by the enhanced thermal neutron detection efficiency. The advancement of the crystal growth technology of h-BN semi-bulk crystals creates applications in optoelectronic and power electronic devices utilizing the UWBG semiconductor properties of h-BN, while high efficiency h-BN neutron detectors have the potential to supplant the traditional He-3 gas detectors in various application areas by offering the obvious advantages of UWBG semiconductor technologies.

Layer-selective growth of 2D hexagonal boron nitride using two-step chemical vapor deposition

Hexagonal boron nitride (h-BN) has recently become a promising and popular material due to its wide applications, such as deep ultraviolet optoelectronic devices, novel nonvolatile memory technology, room temperature quantum emitter, and dielectric in devices based on two-dimensional (2D) materials. However, realizing uniform h-BN thin films thicker than monolayer remains a major challenge, which is important in both fundamental and practical studies. In this work, we report the selective growth of large-area bilayer h-BN using a two-step chemical vapor deposition (CVD) approach. Through constructing a confined space in the Cu foils/sapphire stack, controlled synthesis of bilayer h-BN is achieved. Systematic characterization verified the formation of bilayer h-BN and the mechanism of CVD growth is also studied. We find that the in situ annealing between the two steps could efficiently facilitate the selective growth, and the local oxidation and reduction process could create preferred nucleation sites for uniform bilayer h-BN. Our results could enrich the understanding of the synthesis mechanism and lay the foundation for controllable synthesis of high-quality h-BN.

Deep UV transparent conductive Si-doped Ga2O3 thin films grown on Al2O3 substrates

β-Ga2O3 is attracting considerable attention for applications in power electronics and deep ultraviolet (DUV) optoelectronics owing to the ultra-wide bandgap of 4.85 eV and amendable n-type conductivity. In this work, we report the achievement of Si-doped β-Ga2O3 (Si:β-Ga2O3) thin films grown on vicinal α-Al2O3 (0001) substrates with high electrical conductivity and DUV transparency of promising potential as transparent electrodes. The use of Al2O3 substrates with miscut angles promotes step-flow growth mode, leading to substantial improvement of crystalline quality and electrical properties of the Si:β-Ga2O3 films. A high conductivity of 37 S·cm−1 and average DUV transparency of 85% have been achieved for 0.5% Si-doped film grown on a 6° miscut substrate. High-resolution x-ray and ultraviolet photoemission spectroscopy were further used to elucidate the surface electronic properties of the grown Si:β-Ga2O3 films. An upward surface band bending was found at the surface region of Si:β-Ga2O3 films. Interestingly, all the Si:β-Ga2O3 films have a very low work function of approximately 3.3 eV, which makes Si:β-Ga2O3 suitable materials for efficient electron injection. The present Si:β-Ga2O3 films with high conductivity, DUV transparency, and low work function would be useful as the DUV transparent electrode to develop advanced DUV optoelectronic devices.

Nanoscale phase separation on an AlGaN surface characterized by scanning diffusion microscopy.

AlGaN is an important material for deep ultraviolet optoelectronic devices and electronic devices. The phase separation on the AlGaN surface means small-scale compositional fluctuations of Al, which is prone to degrade the performance of devices. In order to study the mechanism of the surface phase separation, the Al0.3Ga0.7N wafer was investigated by the scanning diffusion microscopy method based on the photo-assisted Kelvin force probe microscope. The response of the surface photovoltage near the bandgap was quite different for the edge and the center of the island on the AlGaN surface. We utilize the theoretical model of scanning diffusion microscopy to fit the local absorption coefficients from the measured surface photovoltage spectrum. During the fitting process, we introduce as and ab parameters (bandgap shift and broadening) to describe the local variation of absorption coefficients α(as, ab, λ). The local bandgap and Al composition can be calculated quantitatively from the absorption coefficients. The results show that there is lower bandgap (about 305 nm) and lower Al composition (about 0.31) at the edge of the island, compared with those at the center of the island (about 300 nm for bandgap and 0.34 for Al composition). Similar to the edge of the island, there is a lower bandgap at the V-pit defect which is about 306 nm corresponding to the Al composition of about 0.30. These results mean Ga enrichment both at the edge of the island and the V-pit defect position. It proves that scanning diffusion microscopy is an effective method to review the micro-mechanism of AlGaN phase separation.

Room-Temperature Preparation of Large-Area Transparent Two-Dimensional ZnO-Doped Ga2O3 Nanostructure-Based Layers: Implications for Optoelectronic Nanodevices

Herein, we demonstrated the controllable synthesis of a centimeter-scale two-dimensional (2D) ZnO-doped Ga2O3 nanostructure layer by a liquid Ga–Zn alloy printing strategy at near room temperature. Different from the liquid Ga–In and Ga–In–Sn alloys, the surface oxidation behavior of a liquid Ga–Zn alloy follows an obvious competition and cooxidation characteristics instead of the dominant oxidation characteristic of Ga, which could be effectively used to precisely tailor the Zn content of 2D Ga2O3 films. With an increase of the nominal Zn content in the Ga–Zn alloy from 0 to 8 atom %, the real Zn content of 2D ZnO-doped Ga2O3 films gradually increases and finally reaches a maximum saturated value of 16–18 atom % at the eutectic component of 3.87 atom %. Correspondingly, the transmittance and band gap of 2D ZnO-doped Ga2O3 films could also be tuned by changes of the Zn content and crystallinity. The method proposed in this work provides a general route toward the doping synthesis of a diverse nonlayered 2D structure, which will shed light on the applications of various displays and deep-ultraviolet optoelectronic devices.

Effects of nitrogen flux and RF sputtering power on the preparation of crystalline a-plane AlN films on r-plane sapphire substrates

A-plane aluminum nitride (AlN) with high quality is crucial to fabricate high-performance non-polar deep-ultraviolet optoelectronic devices. In this work, we prepared crystalline a-plane AlN films on r-plane sapphire substrates by combining reactive magnetron sputtering and high temperature annealing (HTA). The effects of N2 flux and radio frequency (RF) sputtering power on the crystal quality, the surface morphology and the in-plane stress state of a-plane AlN films were comprehensively investigated. The results suggest that the properties of high temperature annealed a-plane AlN (HTA-AlN) films positively depend on the initial states of the sputtered AlN (SP-AlN) films. Increasing the N2 flux or the RF sputtering power can improve the crystalline quality of SP-AlN films by reducing the kinetic energy of deposited particles, which facilitates a-plane AlN deposition. A higher N2 flux smoothens the surface morphology due to the relieved bombardment effect, which is confirmed by the enlarged in-plane tensile stress state. However, a higher sputtering power leads to a rougher surface because of the accelerated deposition rate. With optimized sputtering parameters, a high-quality a-plane HTA-AlN template was obtained with full width at half maximum values of (11–20) plane x-ray rocking curves as low as 1188 and 1224 arcsec along [0001] and [1–100] directions, respectively. The surface presents an ordered stripe-like morphology with a root-mean-square value of 0.79 nm. Our work provides a convenient and effective strategy to prepare high quality a-plane AlN templates and accelerate the versatile application of non-polar deep-ultraviolet light-emitting diode devices.

Wide gap p-type NiO-Ga2O3 alloy via electronic band engineering

Gallium oxide (Ga2O3) has gained significant interest in recent years due to its wide bandgap and related unique properties, making it suitable for many high power and deep ultra-violet optoelectronic and photodetection devices. Nevertheless, Ga2O3 can only be doped effectively n-type, and its full potential in device applications is severely limited by the lack of a reliable p-type material. Here, we report on modifying electronic bands of Ga2O3 through alloying with NiO to achieve p-type conducting Ga2O3-NiO alloy (NixGa1−xO) thin films. We find that room temperature sputter-deposited stoichiometric and O-rich alloys with low Ni content (x < 0.22) have an amorphous structure. In contrast, films with higher Ni content (x ≳ 0.22) are polycrystalline with the rocksalt (RS) NiO structure. O-rich RS-alloys are p-type with resistivity ~20 Ω-cm (for x~0.6) and decreases to< 10 Ω-cm with increasing x. Optically, p-type O-rich films with x ≥ 0.46 have strong sub-gap absorption in the low energy region (˂ 3.5 eV) due to a high concentration of Ni vacancies VNi and this results in a low transmittance which also decreases with x from ~70 % (x = 0.3) to ~ 40 % (x = 1). The band gap Eg of the alloy films exhibits a wide tunability with a monotonic decrease with increasing x from 4.98 (x = 0) to 3.53 (x = 1). The Eg for the RS alloys follows the virtual crystal approximation with a small bandgap bowing of 0.36 eV and an extrapolated Eg of 4.6 eV for RS Ga2O3. Furthermore, we find that the amorphous and RS crystalline alloys have a type II (staggered) band offset (ΔEV~1.8 eV and ΔEC~1.4 eV) with a stepwise upshift of the valence band maximum (VBM) position from ~8–6.2 eV below the vacuum level at the amorphous to RS transition (x~0.2). This significant uplift of the VBM is believed to be responsible for the measured p-type conductivity of the O-rich RS alloys. It also suggests that deep acceptors in Ga2O3 would become shallow in these alloys and hence effective acceptor doping to further improve their p-type conductivity is possible.

Effects of substrate pretreatment and annealing processes on AlN thin films prepared by EVPE

Herein, a novel process of substrate pretreatment and high temperature annealing for elementary source vapor phase epitaxy (EVPE) is shown to be capable of optimizing the surface flatness of the AlN single crystal thin film, thus increasing its suitability for application in deep ultraviolet optoelectronic devices. Specifically, the increased surface smoothness/flatness of the so-prepared AlN samples are confirmed by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). In addition, the crystal quality, stress conditions and luminescence performance are significantly improved.

Origin of the discrepancy between the fundamental and optical gaps and native defects in two dimensional ultra-wide bandgap semiconductor: Gallium thiophosphate

Ultra-wide bandgap (UWBG) semiconductors have great potential for high-power electronics, radio frequency electronics, deep ultraviolet optoelectronic devices, and quantum information technology. Recently, the two-dimensional UWBG GaPS4 was first applied to the solar-blind photodetector in experiments, which was found to have remarkable performance, such as high responsivity, high quantum efficiency, etc., and promising applications in optoelectronic devices. However, the knowledge of monolayer (ML) GaPS4 for us is quite limited, which hinders its design and application in optoelectronic devices. Here, we focus on the properties of electronic structure and intrinsic defects in ML GaPS4 by first-principles calculations. We confirmed that the fundamental gap of ML GaPS4 is 3.87 eV, while the optical gap is 4.22 eV. This discrepancy can be attributed to the inversion symmetry of its structure, which limits the dipole transitions from valence band edges to conduction band edges. Furthermore, we found that intrinsic defects are neither efficient p-type nor n-type dopants in ML GaPS4, which is consistent with experimental observations. Our results also show that if one expects to achieve p-type ML GaPS4 by selecting the appropriate dopant, P-rich conditions should be avoided for the growth process, while for achieving n-type doping, S-rich growth conditions are inappropriate. This is because due to the low strain energy, PS(c)+ has very low formation energy, which leads to the Fermi levels (EF) pinning at 0.35 eV above the valence band maximum and is not beneficial to achieve p-type ML GaPS4 under the P-rich conditions; the large lattice relaxation largely lowers the formation energy of SGa−, which causes the EF pinning at 0.72 eV below the conduction band minimum and severely prevents ML GaPS4 from being n-type doping under the S-rich conditions. Our studies of these fundamental physical properties will be useful for future applications of ML GaPS4 in optoelectronic devices.

Graphene-driving strain engineering to enable strain-free epitaxy of AlN film for deep ultraviolet light-emitting diode

The energy-efficient deep ultraviolet (DUV) optoelectronic devices suffer from critical issues associated with the poor quality and large strain of nitride material system caused by the inherent mismatch of heteroepitaxy. In this work, we have prepared the strain-free AlN film with low dislocation density (DD) by graphene (Gr)-driving strain-pre-store engineering and a unique mechanism of strain-relaxation in quasi-van der Waals (QvdW) epitaxy is presented. The DD in AlN epilayer with Gr exhibits an anomalous sawtooth-like evolution during the whole epitaxy process. Gr can help to enable the annihilation of the dislocations originated from the interface between AlN and Gr/sapphire by impelling a lateral two-dimensional growth mode. Remarkably, it can induce AlN epilayer to pre-store sufficient tensile strain during the early growth stage and thus compensate the compressive strain caused by hetero-mismatch. Therefore, the low-strain state of the DUV light-emitting diode (DUV-LED) epitaxial structure is realized on the strain-free AlN template with Gr. Furthermore, the DUV-LED with Gr demonstrate 2.1 times enhancement of light output power and a better stability of luminous wavelength compared to that on bare sapphire. An in-depth understanding of this work reveals diverse beneficial impacts of Gr on nitride growth and provides a novel strategy of relaxing the vital requirements of hetero-mismatch in conventional heteroepitaxy.

Low-Temperature Direct Growth of Few-Layer Hexagonal Boron Nitride on Catalyst-Free Sapphire Substrates.

Wide-band-gap layered semiconductor hexagonal boron nitride (h-BN) is attracting intense interest due to its unique optoelectronic properties and versatile applications in deep ultraviolet optoelectronic and two-dimensional electronic devices. However, it is still a great challenge to directly grow high-quality h-BN on dielectric substrates, and an extremely high substrate temperature or annealing is usually required. In this work, high-quality few-layer h-BN is directly grown on sapphire substrates via ion beam sputtering deposition at a relatively low temperature of 700 °C by introducing NH3 into the growth chamber. Such low growth temperature is attributed to the presence of abundant active N species, originating from the decomposition of NH3 under ion beam irradiation. To further tailor the properties of h-BN, carbon was introduced into the h-BN layer by simultaneously introducing CH4 and NH3 during the growth process, indicating the wide applicability of this approach. Moreover, a deep ultraviolet (DUV) photodetector is also fabricated from a C-doped h-BN layer and exhibits superior performance compared with an intrinsic h-BN device.

Improvement of Crystal Quality of AlN Films with Different Polarities by Annealing at High Temperature.

High-quality AlN film is a key factor affecting the performance of deep-ultraviolet optoelectronic devices. In this work, high-temperature annealing technology in a nitrogen atmosphere was used to improve the quality of AlN films with different polarities grown by magnetron sputtering. After annealing at 1400–1650 °C, the crystal quality of the AlN films was improved. However, there was a gap between the quality of non-polar and polar films. In addition, compared with the semi-polar film, the quality of the non-polar film was more easily improved by annealing. The anisotropy of both the semi-polar and non-polar films decreased with increasing annealing temperature. The results of Raman spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy revealed that the annihilation of impurities and grain boundaries during the annealing process were responsible for the improvement of crystal quality and the differences between the films with different polarities.

Transfer-free graphene-guided high-quality epitaxy of AlN film for deep ultraviolet light-emitting diodes

We report on quasi-van der Waals epitaxy of high-quality AlN film guided by transfer-free graphene (Gr) with low wrinkle density (LWD-Gr) on sapphire. The surface wrinkle density of LWD-Gr is greatly reduced by 94% compared to conventional Gr with high wrinkle density (HWD-Gr). Also noteworthy is that AlN nucleation islands grown on LWD-Gr exhibit better crystalline orientation consistency and more feasibility to coalesce with each other, thus forming AlN film with lower stress and dislocation density. Finally, 285 nm deep ultraviolet (DUV) light-emitting diodes are fabricated on the high-quality AlN template with LWD-Gr, which shows stronger electroluminescence intensity than its counterpart without and with HWD-Gr. The insights granted by this research pave a new pathway for improving the performance of nitride-based DUV optoelectronic devices through Gr engineering.

Band alignment analysis of CuGaO2/β-Ga2O3 heterojunction and application to deep-UV photodetector

Single-oriented CuGaO2 films have been successfully grown on β-Ga2O3 (2¯01) substrate by reactive deposition epitaxy. The energy band offsets and alignment at CuGaO2/β-Ga2O3 heterojunction are investigated by x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). A type-II band alignment is identified at CuGaO2/β-Ga2O3 heterojunction with valence band offset (VBO) of 1.63 eV and conduction band offset (CBO) of 0.45 eV. The CuGaO2/β-Ga2O3 heterojunction based ultraviolet photodetector is prepared which exhibits an obvious ultraviolet (UV) photoresponse at zero bias voltage. The combination between β-Ga2O3 and wide bandgap delafossite oxide semiconductor may open up possibilities for next generation self-power deep UV optoelectronic devices in future.

Bandgap bowing parameter and alloy fluctuations for β-(AlxGa1−x)2O3 alloys for x ≤ 0.35 determined from low temperature optical reflectivity

A bandgap bowing parameter of 0.4 ± 0.2 eV for β-(AlxGa1−x)2O3 alloys, with Al compositions (x) up to 0.35, has been determined from the bandgap obtained from low temperature optical reflectivity, which suppresses the effect of electron–phonon interaction on the bandgap. A length scale of inhomogeneity of 0.21 ± 0.03 times of the electron–hole mean free path length has been estimated for β-(AlxGa1−x)2O3 alloys. The unit cell of β-(AlxGa1−x)2O3 alloys compresses, and the lattice parameters vary linearly with Al substitution. Our results provide insight into bandgap engineering and alloy disorder for β-(AlxGa1−x)2O3 alloys, which are an important material system for applications in deep ultraviolet opto-electronic devices.

Interface and optical properties of Zn1−xMgxO films with Mg content of more than 70% grown on the (12̄10)-ZnO substrates

Fabricating Zn1−xMgxO films with a high Mg content is key to their applications in deep-ultraviolet optoelectronic devices. In this work, we report the preparation of Zn1−xMgxO films on (12̄10)-ZnO substrates by molecular beam epitaxy. The Zn1−xMgxO/(12̄10)-ZnO structure is revealed by x-ray diffraction and high-resolution transmission electron microscopy. Remarkably, no cubic MgO is observed for films with 74.6% Mg content; the film shows mainly the wurtzite structure with some intermediate phases at the interface. Photoluminescence spectra show that the film exhibits good optoelectronic properties with a bandgap of 4.6 eV. This work provides a new avenue for the fabrication of deep-ultraviolet Zn1−xMgxO films.

Origination and evolution of point defects in AlN film annealed at high temperature

While high temperature annealing has been proven to be an effective strategy to reduce threading dislocation density of AlN film, the point defect induced near-ultraviolet emission increases dramatically with the increase in annealing temperature, and thus limits its application in deep ultraviolet optoelectronic devices. Herein, the origination and evolution of point defects in high-temperature annealed AlN are studied and clarified by photoluminescence spectroscopy, secondary ion mass spectrometry, positron annihilation and first-principles calculation. We have confirmed that (1) the annealing induces the increased O impurity concentration by two orders of magnitude; (2) The increase of O impurity concentration in AlN after high temperature annealing is the key factor that causes the evolution of point defects and the enhancement of near ultraviolet defect peak; (3) the formation of more O content [VAl-n(ON)] and VN at different annealing temperatures are responsible for photoluminescence evolution. Present work reveals the formation mechanism of point defects in AlN and provides further support for improving the quality of AlN.