p-GaN Substrates Research Articles

Fabrication, properties, and photodetector of β-(AlxGa1-x)2O3/GaN heteroepitaxial films grown by MOCVD

In this paper, β-(AlxGa1-x)2O3 heteroepitaxial films with different Al content were successfully prepared on p-GaN substrates by MOCVD method. When the “x” values are in a range of 0–0.38, the (AlxGa1-x)2O3 grown on p-GaN is an epitaxial film, maintaining the lattice structure of β-Ga2O3, and presenting an adjustable bandgap of 5.0–5.93 eV. When x = 0.45, the film is amorphous, and present a low bandgap of ∼5.73 eV due to the band tail formed by a large number of defects. φ scanning, HRTEM and SAED results showed that the prepared film was a six-fold domain structure and had an epitaxial relationship with the substrate of β-(AlxGa1-x)2O3 (2‾01) || GaN (0001) with β-(AlxGa1-x)2O3 [102] || GaN <1-100>. The photodetector (PD) based on β-(AlxGa1-x)2O3/GaN p-n junction presents high photoresponsivity (0.045 A/W) at 218 nm. The wavelength is far lower than the optimal responsive wavelength (254 nm) of β-Ga2O3 in previous reports, meaning that β-(AlxGa1-x)2O3 can be used to fabricate shorter wavelength UV photodetector.

UV–visible dual-band photodetector based on an all-inorganic Mn-doped CsPbCl3/GaN type-II heterojunction

The all-inorganic perovskite CsPbCl3 has attracted significant attention for its excellent carrier mobility and high optical absorption coefficient, demonstrating great potential for use as a high-performance photodetector. However, the severe toxicity of lead and its narrow-band response in the UV region pose a huge challenge to the application of the perovskite. In this study, we reduce the toxicity of CsPbCl3 by substituting Mn atoms for Pb atoms and introduce a wide-bandgap semiconductor p-GaN to improve the photoresponse of CsPbCl3 in the UV region. Mn-doped CsPbCl3/GaN heterojunction is prepared by transferring the synthesized Mn-doped CsPbCl3 precursor solution onto p-GaN substrates via the spin-coating method. The morphology and optical properties of the heterojunction are characterized in various ways, and the current–voltage characteristics and optical response properties of the Mn-doped CsPbCl3/GaN heterojunction photodetector are also measured. The rectification behavior and UV–visible dual-band response of the heterojunction are demonstrated, with the response in the UV region extending to 320 nm. Furthermore, the calculations, which are based on density functional theory, confirm that the optical absorption of the Mn-doped CsPbCl3/GaN heterojunction is significantly enhanced compared with that of the isolated material. Meanwhile, a type-II energy band arrangement is found at the heterojunction interface. The results of this study suggest that Mn-doped CsPbCl3/GaN heterojunctions are potential candidates for use as UV–visible dual-band photodetectors.

Niobium oxides films on GaN: Photoelectron spectroscopy study

Non-stoichiometric niobium oxides were grown on p-GaN(0001) substrates using physical vapor deposition of Nb under ultrahigh vacuum (UHV) enriched with oxygen. The film consisted mainly of NbOx and Nb2O5 phases. NbO2 compound as well as trace amounts of NbO and Nb in metallic were also present in the film. Then, in order to eliminate non-insulating fractions, the method of oxygen-ion implantation followed by annealing was used. This resulted in a formation of insulator films composed almost only of Nb2O5 compound. The surface of the cleaned substrate and niobium oxides/GaN interfaces (prior and after an O-ion implantation) were characterized in situ by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The result allowed to determine band offsets at the Nb2O5/GaN interface.

Preparation of high performance Ga2O3 based ultraviolet photodetector by CVD

Here, β-Ga2O3 films have been grown on c-Al2O3 and p-GaN substrates by chemical vapor deposition method to fabricate solar-blind ultraviolet photodetectors with MSM-type planar structure and pn-junction vertical structure respectively. The β-Ga2O3 films grown with domain growth mode exhibit high crystallinity and flat surface, so the fabricated MSM-type photodetector shows spectral peak responsivity of 1.83 A/W at 254 ​nm and photo-to-dark current ratio of 104 under 25 ​V bias with 20% oxygen fraction and 120 sccm flow rate. On this basis, the constructed vertical structured β-Ga2O3/p-GaN heterojunction photodetector shows peak responsivity of 1.99 A/W under 25 ​V for the junction effect between β-Ga2O3 and p-GaN. Meanwhile, spectral responsivity peak of 1.36 ​× ​10−4 A/W at 0V claims significant self-powered effect induced by build-in electric field between β-Ga2O3 and p-GaN. These results confirm that high performance β-Ga2O3 solar-blind ultraviolet photodetectors can be obtained by simple chemical vapor deposition method.

Comparative Study on Structural, Optical, and Electrical Properties of ZnO Thin Films Prepared by PLD and Sputtering Techniques

ZnO thin films were formed on c-plane sapphire and p-GaN substrates by pulsed laser deposition (PLD) and radio frequency (RF) magnetron sputtering techniques. XRD analysis including omega scan depicted the formation of highly textured wurtzite ZnO with c-axis. The texture was primarily introduced by the substrate effects as the planes lying at oblique angles also exhibited six-fold symmetry during phi scan. Atomic force microscopy exhibited the surface roughness of 4.33 and 12.99 nm for PLD and sputtered ZnO films, respectively. In photoluminescence (PL) measurements, a strong UV emission was observed at 3.30 eV for both ZnO films. However, deep-level emission was observed at around 2.61 eV in PLD film, but it had a wide range from 2.61 to 2.29 eV in case of sputter-deposited film. From the transmission spectra, the optical band gap values were found to be 3.29 and 3.28 eV for PLD and sputtered ZnO films, respectively. Hall measurement revealed the resistivity values of 0.0792 and 0.4832 Ω cm and carrier concentrations of 2.28 × 1018 and 1.73 × 1018 cm–3 for respective PLD and sputtered films. I(V) current–voltage curves clearly demonstrated the n-ZnO|p-GaN hetero-junction with turn-on voltage of 3.8 and 5.2 V for PLD and sputtered samples, respectively.

Comparative Studies between Porous Silicon and Porous P-Type Gallium Nitride Prepared Using Alternating Current Photo-Assisted Electrochemical Etching Technique

Porous n-type Si and porous p-type GaN nanostructures were fabricated using alternating current photo-assisted electrochemical (ACPEC) etching in 1:4 volume ratio of hydrofluoric acid (HF) and ethanol (C2H5OH) for a duration of 30 min. The proposed approach to this work was to study pore formation on the Si and p-GaN substrates in the aspects of morphological and structural changes. The morphological and structural properties of porous Si and porous p-type GaN samples have been studied using field emission scanning electron microscopy (FESEM) measurement, energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and high-resolution X-ray diffraction (HR-XRD) in comparison to the respective as-grown sample. FESEM analysis revealed that uniform pore size with triangular-like shape was formed in porous Si sample while circular-like shape pores were formed in the porous p-type GaN sample. AFM measurement revealed that the root-mean-square surface roughness of porous Si and porous p-type GaN was 6.15 nm and 5.90 nm, respectively. Detailed investigation will be presented in this work to show that ACPEC etching technique is a viable technique to produce porous nanostructures in different substrates.

Facile approaches to prepare n-ZnO/(i-ZnO)/p-GaN heterojunction light-emitting diodes with white-light-electroluminescence

White-light-emitting diode (LED) with effective energy conservation and long service life could be employed in numerous applications. In this study, the high-performance n-ZnO films were first prepared via pulsed laser deposition on p-GaN substrates and then the n-ZnO/p-GaN heterojunction LED was fabricated. This LED exhibits blue and yellow light emission, and their emission intensities can be tuned by adjustment of the fabrication parameters (e.g. oxygen pressure) and/or by introduction of a semi-insulating i-ZnO layer to form a p-GaN/i-ZnO/n-ZnO heterojunction. Thus, a facile approach has been proposed for the preparation of white LED.

Experimental Demonstration of n- and p-channel GaN-MOSFETs toward Power IC Applications

We report on the systematic demonstrations of the depletion-mode p-channel and enhancement-mode n-channel GaN metal-oxide field-effect transistors (MOSFETs) based on homoepitaxial p-GaN substrates. The world's first depletion-mode p-channel GaN-MOSFET fabricated in this work shows a good Ion/Ioff characteristic (∼104) and an effective channel hole mobility of approximately 0.2 cm2/V·s at room temperature. For the first time, it is proved that the p-channel GaN-MOSFETs can work independently without the necessity of two-dimension hole gas (2DHG) sources of heterostructures. Meanwhile, the maximum effective electron mobility of n-channel GaN-MOSFETs is approximately 105 cm2/V·s. The post deposition annealing (PDA) treatment of Al2O3 dielectrics is also verified in both n and p-channel GaN-MOSFETs and 700°C is found to be the optimized condition on improving the effective hole/electron mobility. The findings in this work provide a valuable information in the design of novel power electronics taking advantage of p-type doped GaN.

Fabrication of tunable n-Zn1-xCdxO/p-GaN heterojunction light-emitting diodes

Zn1-xCdxO films with different Cd contents obtained by controlling the growth oxygen pressure (Op) were deposited on both c-sapphire (Al2O3) substrates and p-GaN substrates, by means of the pulsed laser deposition (PLD) method. Photoluminescence (PL) measurement revealed that the variation of Op influenced the amount of Cd doping into Zn1-xCdxO films deposited on Al2O3, leading tunable luminescence from the ultraviolet (UV) to blue emission extended to the green band. Then all the fabricated n-Zn1-xCdxO/p-GaN heterojunction possessed good ohmic contacts and exhibited typical rectifying characteristic of the diode. Indeed, the high luminescence from the ZnCdO layer could be attained by inserting a MgO insulator to the heterojunction interface. The fabrication of tunable n-Zn1-xCdxO/p-GaN heterojunction light-emitting diodes is available which was supported by the results of electroluminescence (EL) experiment.

Measuring the absorption of TiN metallic films using cathodoluminescence of GaN films

Although TiN thin metallic film has attractive prospects in plasma applications, its optical properties still remain less well explored. Here we have fabricated TiN thin metallic film with an average of about 50 nm thickness by pulsed laser deposition on p-GaN substrates at 650°C. The morphology and crystal structure are characterized by atomic force microscopy and X-ray diffraction. The results exhibit a smooth surface and an obviously preferred (111) orientation. A combination of cathodoluminescence spectroscopy and a cross-sectional scanning electron microscopy is developed to study the absorption behavior of TiN thin metallic film systematically. The absorption coefficients of TiN thin metallic film obtained here are in good agreement with the values obtained by the spectroscopic ellipsometry.

Improvement of Electrical Properties of p-type GaN and Au Contact Interface

In order to improve electrical conductivity of Mg-doped p-GaN, the enhancement of H release from Mg-doped p-GaN were attempted by applying current flow and electrical field through the p-GaN substrates during low temperature annealing. The microstructure and electrical properties after annealing were then investigated by transmission electron microscopy observation, direct current conduction test and Hall effect measurement. The results reveal that no reaction occurs between deposited Au film and GaN substrate during annealing at 573 K for 3600 s. The electrical conductivity does not show any improvement only by applying electrical field or by annealing at 573 K for 3600 s without current flow. It is likely due to the limited mobility of H within p-GaN. However, by applying current flow under the constant voltage of 30 V during annealing at 573 K for 3600 s, a significant improvement of the electrical conductivity has been observed. By applying electrical field and current flow during annealing at 573 K for 3600 s, the H release from p-GaN was enhanced and Mg was activated, resulting in increasing carrier concentration of acceptor and improving the electrical conductivity.

Highly ordered ZnO nanostructure arrays: Preparation and light-emitting diode application

In this paper, ordered arrays of various ZnO nanostructures were fabricated on both Si and p-GaN substrates via a hydrothermal process by the self-assembled nanosphere lithography (NSL). The morphology and orientation evolution of ZnO nanostructures were well-monitored by varying seed-layer-thickness, solution concentration, and underlying substrate. On this basis, the heterojunction light emitting diode (LED) based on the highly oriented ZnO nanocone array on the p-GaN substrate was fabricated and studied in detail. It is found that the ZnO nanostructure-based LED exhibits a much stronger ultraviolet (UV) emission peaked at 388 nm compared with its counterparts based on ZnO film, which is attributed to the low density of interfacial defects, improved carrier injection efficiency through the nano-sized junction, and excellent waveguiding property of the ZnO nanostructure array.

Study of Optical and Electrical Properties of Ag/ITO Multi-Layer Transparent Electrodes

The samples of Ag/ITO multilayer films with different Ag insert layer thickness (0.5, 2, 4 nm) were prepared on sapphire and p-GaN substrates. The effects of the Ag layer thickness, annealing temperature and annealing time on the transmittance, sheet resistance and specific contact resistance of Ag/ITO films were investigated. The experiment results show that the transmittance is obviously affected by Ag insert layer thickness. The Current–voltage (I–V) measurements indicate that the sheet resistance and specific contact resistance of Ag/ITO film on p-GaN are lower than those of single ITO film. The samples with Ag(0.5nm)/ITO film on p-GaN produce the low specific contact resistance of ~1.386×10-4Ω•cm2 , low sheet resistance of ~11Ω/sq and high transmittance of ~ 90% at 455nm when the samples are annealed at 600°C for 10 minutes.

Ultraviolet electroluminescence from ordered ZnO nanorod array/p-GaN light emitting diodes

The highly ordered and aligned ZnO nanorod arrays were grown on p-GaN substrates via a facile hydrothermal process assisted by the inverted self-assembled monolayer template, from which the ZnO nanorod/p-GaN heterojunction light emitting diodes (LEDs) were fabricated. The ZnO nanorod-based LEDs exhibit a stronger ultraviolet emission of 390 nm than the ZnO film-based counterpart, which is attributed to the low density of interfacial defects, the improved light extraction efficiency, and carrier injection efficiency through the nano-sized junctions. Furthermore, the LED with the 300 nm ZnO nanorods has a better electroluminescence performance compared with the device with the 500 nm nanorods.

Effect of doping precursors on the optical properties of Ce-doped ZnO nanorods

We have investigated the use of different cerium precursors for fabrication of Ce doped ZnO nanorods by electrodeposition. ZnO nanorods fabricated with different Ce precursors had similar morphology, but very different optical properties. The influence of annealing in oxygen at different temperatures on the optical properties of Ce doped ZnO nanorods was also studied. The as-grown samples exhibited very different ratios of ultraviolet to visible (defect) emission. The relative contributions of green and orange–red emissions in the emission spectra of annealed samples were also dependent on the precursor used, indicating that the types and concentrations of native defects were dependent on the precursor used. To explore the possibility of applications of these nanorods in ZnO-based devices, nanorods were also grown on p-GaN substrates. Two different Ce precursors in this case resulted in different current–voltage curves, as well as different electroluminescence and photoluminescence spectra of the devices. Obtained results are discussed.

High Aspect Ratio Ternary Zn1–xCdxO Nanowires by Electrodeposition for Light-Emitting Diode Applications

We present a combined experimental and computational approach to study Zn1–xCdxO nanowires (NWs) and their integration in light-emitting diode (LED) structures. Self-standing Zn1–xCdxO NWs have been electrodeposited on fluorine-doped tin oxide and p-GaN substrates. The electrochemical behavior has been studied, and the reaction mechanism is discussed. Low-dimensional Zn1–xCdxO structures have been obtained for CdCl2 concentrations in the deposition bath lower than 6 μM whereas at higher concentration it is admixed with crystallized CdO and the aspect ratio of the wires is decreased. According to scanning electron microscopy observations, the Zn1–xCdxO NWs have a higher aspect ratio (>30) than pure ZnO NWs (∼20) grown in similar conditions. Analyses show that the ZnO is doped with cadmium incorporated within ZnO NWs and that Cd doping increases with increasing Cd(II) content in the deposition bath. X-ray diffraction studies show increased lattice parameters in Cd-alloyed ZnO NWs. Photoluminescence studies on pure ZnO and Zn1–xCdxO NWs show the near band-edge emission red shifted by 3–7 nm as a function of Cd(II) concentration (4 or 8 μM in the electrolyte). The structural and optical properties of the prepared Zn1–xCdxO materials have been interpreted using density functional theory (DFT) to computationally simulate the effect of Cd substitution for Zn in the ZnO lattice. DFT calculations show that the crystal lattice parameters increase with the partial replacement of Zn atoms by Cd and that the band gap enlargement is due to the increased lattice parameters. We demonstrate the possibility to tailor the electroluminescence emission wavelength by cadmium doping in ZnO nanowires integrated in Zn1–xCdxO NWs/p-GaN heterojunction based LED structures. Reported results are of great interest for the research on band gap engineering of low-dimensional zinc oxide by doping/alloying NWs and for wavelength-tunable LED applications.

The origin of the red emission in n-ZnO nanotubes/p-GaN white light emitting diodes

In this article, the electroluminescence (EL) spectra of zinc oxide (ZnO) nanotubes/p-GaN light emitting diodes (LEDs) annealed in different ambients (argon, air, oxygen, and nitrogen) have been investigated. The ZnO nanotubes by aqueous chemical growth (ACG) technique on p-GaN substrates were obtained. The as-grown ZnO nanotubes were annealed in different ambients at 600°C for 30 min. The EL investigations showed that air, oxygen, and nitrogen annealing ambients have strongly affected the deep level emission bands in ZnO. It was concluded from the EL investigation that more than one deep level defect is involved in the red emission appearing between 620 and 750 nm and that the red emission in ZnO can be attributed to oxygen interstitials (Oi) appearing in the range from 620 nm (1.99 eV) to 690 nm (1.79 eV), and to oxygen vacancies (Vo) appearing in the range from 690 nm (1.79 eV) to 750 nm (1.65 eV). The annealing ambients, especially the nitrogen ambient, were also found to greatly influence the color-rendering properties and increase the CRI of the as - grown LEDs from 87 to 96.

Fabrication of a White-Light-Emitting Diode by Doping Gallium into ZnO Nanowire on a p-GaN Substrate

This study evaluated a process for fabricating white light emitting diodes (LEDs) by doping Ga into ZnO nanowires (ZnO:Ga NWs) on p-GaN substrates. Vertically aligned ZnO:Ga NWs were grown by thermal chemical vapor deposition to 0.7 μm in length and 50−300 nm in diameter. The white light LED was successfully fabricated by forming an n-p-n heterojunction on an ITO/glass substrate. The electroluminescence (EL) emission peak was 500 nm, and the broad band fwhm intensity was 200 nm. Finally, photographs show a white light from the ZnO:Ga LED.

Near ultraviolet light emitting diode composed of n-GaN∕ZnO coaxial nanorod heterostructures on a p-GaN layer

The authors report on the fabrication and characteristics of near ultraviolet nanorod light emitting diodes (LEDs) composed of n-GaN∕ZnO nanorod heterostructures on p-GaN substrates. The nanorod LEDs consist of the vertically aligned n-GaN∕ZnO coaxial nanorod arrays grown on a p-GaN substrate. The LEDs demonstrated strong near ultraviolet emission at room temperature. The nanorod LEDs were turned on a forward-bias voltage of 5V, and exhibited a large light emitting area. From electroluminescent spectra, dominant emission peaks were observed at 2.96 and 3.24eV for an applied current of 2mA. The origins of the strong and large area light emission are also discussed in terms of enhanced carrier injection from n-GaN nanostructures to p-GaN substrates.

Effect of Au distribution in NiO/Au film on the ohmic contact formation to p-type GaN

The distribution of Au and NiO in NiO/Au ohmic contact on p-type GaN was investigated in this work. Au (5 nm) films were deposited on p-GaN substrates by magnetron sputtering. Some of the Au films were preheated in N2 ambient to agglomerate into semi-connected structure (abbreviated by agg-Au); others were not preheated and remained the continuous (abbreviated by cont-Au). A NiO film (5 nm) was deposited on both types of samples, and all samples were subsequently annealed in N2 ambient at the temperatures ranging from 100 to 500 °C. The surface morphology, phases, and cross-sectional microstructure were investigated by scanning electron microscopy, glancing incident angle x-ray diffraction, and transmission electron microscopy. I-V measurement on the contacts indicates that only the 400 °C annealed NiO/cont-Au/p-GaN sample exhibits ohmic behavior and its specific contact resistance (ρc) is 8.93 × 10−3 Ω cm2. After annealing, Au and NiO contact to GaN individually in the NiO/agg-Au/p-GaN system while the Au and NiO layers become tangled in the NiO/cont-Au/p-GaN system. As a result, the highly tangled NiO-Au structure shall be the key to achieve the ohmic behavior for NiO/cont-Au/p-GaN system.