GaN Films Research Articles

Structure and luminescence properties of Eu3+ and Dy3+ implanted GaN films

X-Ray diffraction (XRD), Raman scattering , Cathodoluminescence measurements (CL) and Photoluminescence (PL) were employed to characterize the structure and optical properties of Dysprosium (Dy) and Europium (Eu) implanted GaN films grown by MOCVD. For GaN: Dy 3+ samples, Dy 3+ implantation leads to strain and radiation damage increasing with Dy 3+ implanted fluence . From the CL spectra, the characteristic transitions from the excited 4 F 9/2 level of Dy 3+ to 6 H 15/2 (485 nm), 6 H 13/2 (583 nm) and 6 H 11/2 (671 nm) were observed. For Dy 3+ and Eu 3+ co-doped GaN, considering the doping effects of Dy 3+ , we proposed that there might exist a resonance energy transfer process from Eu 3+ ions 7 F 0 → 5 D 2 to Dy 3+ ions 4 F 9/2 → 6 H 15/2 , which is dominated by the electric dipole-dipole interaction. • Eu 3+ , Dy 3+ co-implanted GaN crystalline thin films were prepared for the first time. • Energy transfer processes between Eu 3+ and Dy 3+ were proposed. • Dy implantation leads to strain and radiation damage increasing with Dy dose.

Growth of continuous GaN films on ZnO buffer layer by chemical vapor deposition for ultraviolet photodetector

Growth of continuous GaN films on ZnO buffer layer by chemical vapor deposition for ultraviolet photodetector

Van der Waals epitaxy of nearly single-crystalline nitride films on amorphous graphene-glass wafer.

Van der Waals epitaxy provides a fertile playground for the monolithic integration of various materials for advanced electronics and optoelectronics. Here, a previously unidentified nanorod-assisted van der Waals epitaxy is developed and nearly single-crystalline GaN films are first grown on amorphous silica glass substrates using a graphene interfacial layer. The epitaxial GaN-based light-emitting diode structures, with a record internal quantum efficiency, can be readily lifted off, becoming large-size flexible devices. Without the effects of the potential field from a single-crystalline substrate, we expect this approach to be equally applicable for high-quality growth of nitrides on arbitrary substrates. Our work provides a revolutionary technology for the growth of high-quality semiconductors, thus enabling the hetero-integration of highly mismatched material systems.

Effects in the Optical and Structural Properties Caused by Mg or Zn Doping of GaN Films Grown via Radio-Frequency Magnetron Sputtering Using Laboratory-Prepared Targets

GaN films doped with Mg or Zn were obtained via radio-frequency magnetron sputtering on silicon substrates at room temperature and used laboratory-prepared targets with Mg-doped or Zn-doped GaN powders. X-ray diffraction patterns showed broadening peaks, which could have been related to the appearance of nano-crystallites with an average of 7 nm. Scanning electron microscopy and transmission electron microscopy showed good adherence to silicon non-native substrate, as well as homogeneity, with a grain size average of 0.14 µm, and 0.16 µm for the GaN films doped with Zn or Mg, respectively. X-ray photo-electron spectroscopy demonstrated the presence of a very small amount of magnesium (2.10 mol%), and zinc (1.15 mol%) with binding energies of 1303.18, and 1024.76 eV, respectively. Photoluminescence spectrum for the Zn-doped GaN films had an emission range from 2.89 to 3.0 eV (429.23–413.50 nm), while Mg-doped GaN films had an energy emission in a blue-violet band with a range from 2.80 to 3.16 eV (443.03–392.56 nm). Raman spectra showed the classical vibration modes A1(TO), E1(TO), and E2(High) for the hexagonal structure of GaN.

Investigation and optimization of N-polar GaN porosification for regrowth of smooth hillocks-free GaN films

This work investigates the process of planar electrochemical etching of pores in n-type nitrogen-polar GaN and the effect of pore morphology on regrown GaN film surface quality. An increase in the anodization voltage was found to increase the pore diameter and reduce the density of pores with inclined sidewalls near the surface of the porosified films. Simultaneously, a decrease in the hexagonal hillock size and number following GaN regrowth was observed. It is proposed that vertical pore sidewalls are essential to demonstrate high quality film coalescence. For smooth hillock-free 100 nm GaN regrowth, an optimal bias of 17 and 13 V for Ti-contacted N-polar GaN:Si with a Si doping of 4.5 × 1018 and 8 × 1018 cm−3, respectively, was found.

Photoluminescence and scintillation properties GaN

Photoluminescence (PL) and scintillation properties of GaN film deposited on Al2O3 substrate were evaluated. In PL and X-ray induced scintillation spectra, two emission bands at 400 and 550 nm were observed. The X-ray induced scintillation decay curve demonstrated two decay components of 1.6 and 8.4 ns. Upon 55Fe 5.9 keV X-ray and 241Am 5.5 MeV α-ray excitation, the GaN film showed a higher scintilaltion light yield than ZnO, which was a common semiconductor scintillator.

Effect of MOVPE growth conditions on AlN films on annealed sputtered AlN templates with nano-striped patterns

The metalorganic vapor phase epitaxial (MOVPE) growth of AlN films was performed on face-to-face annealed sputtered AlN templates (FFA Sp-AlN) having a nano-striped pattern with a period of 300 nm and depth of 120 nm. Then, the effect of MOVPE growth conditions on the crystallinity was elucidated. The faceted structure clearly changed with the growth temperature (Tg), following the same trend as the epitaxial lateral overgrowth of GaN film on micro-patterned GaN. High Tg levels enhanced the lateral growth, and thus with a high Tg of 1300 °C, the coalescence in the early growth stage resulted in AlN films with coalesced surfaces and a thickness of 1 µm. For the AlN film grown at a Tg of 1300 °C, the threading dislocation density (TDD) was estimated to be 6.0 × 108 cm−2. This value proved that coalesced AlN films on nano-patterned FFA Sp-AlN templates with low TDDs could be achieved with the appropriate MOVPE growth conditions. The compressive strain in the AlN film on nano-patterned FFA Sp-AlN was lower than that of the film grown on FFA Sp-AlN without a pattern.

The Effect of RF Sputtering Temperature Conditions on the Structural and Physical Properties of Grown SbGaN Thin Film

By using a single ceramic SbGaN target containing a 14% Sb dopant, Sb0.14GaN films were successfully grown on n-Si(100), SiO2/Si(100), and quartz substrates by an RF reactive sputtering technology at different growth temperatures, ranging from 100 to 400 °C. As a result, the structural characteristics, and optical and electrical properties of the deposited Sb0.14GaN films were affected by the various substrate temperature conditions. By heating the temperature deposition differently, the sputtered Sb0.14GaN films had a wurtzite crystal structure with a preferential (101¯0) plane, and these Sb0.14GaN films experienced a structural distortion and exhibited p-type layers. At the highest depositing temperature of 400 °C, the Sb0.14GaN film had the smallest bandgap energy of 2.78 eV, and the highest hole concentration of 8.97 × 1016 cm−3, a conductivity of 2.1 Scm−1, and a high electrical mobility of 146 cm2V−1s−1. The p-Sb0.14GaN/n-Si heterojunction diode was tested at different temperatures, ranging from 25 to 150 °C. The testing data showed that the change of testing temperature affected the electrical characteristics of the diode.

Effect of ultraviolet and room lights on porous GaN films using photo-assisted electrochemical etching

We fabricated high reflectivity GaN/nanoporous (NP) GaN distributable Bragg reflector (DBR) structures by electrochemical etching of HF solution in ultraviolet (UV)- and room-light. Compared with in room-light, the DBR fabricated in UV-light has higher reflectivity. To elucidate the difference of these two DBRs, the effect of lights on the etching breakdown voltage, pore morphology, crystal quality and photoluminescence (PL) properties was studied in detail. By comparison, the NP-GaN film obtained under the UV-light has lower breakdown voltage, higher porosity, higher crystal quality and bigger red-shift of PL position, meaning that the DBR obtained in the UV-light has higher quality and reflectivity.

X-ray absorption study of defects in reactively sputtered GaN films displaying large variation of conductivity

Undoped GaN films were epitaxially grown on c-sapphire by rf magnetron reactive sputtering of GaAs at different N2 percentages in Ar–N2 sputtering atmosphere. These films display a large increase in conductivity from ∼10−6 Ω−1cm−1 to ∼102 Ω−1cm−1 with decrease of N2 in sputtering atmosphere from 100% to 10%. X-ray photoelectron spectroscopy reveals a monotonous decrease of N/Ga ratio, concurrently with increase of uncoordinated/interstitial Ga in the films, with decrease of N2 percentage in sputtering atmosphere. Extended x-ray absorption fine structure measurements and x-ray absorption near edge structure (XANES) simulation at Ga K-edge demonstrate the absence of N vacancies in sputtered GaN films and point towards the presence of Ga vacancies, which increase with decrease of N2 percentage in sputtering atmosphere. XANES measurements at O K-edge reveal that O interstitials are present in most of the films, except for the film grown at low N2 percentages (∼10%), in which case, O substitution on N-site may also take place. A monotonous increase of n-type conductivity is observed with increase of Ga interstitials, which suggests that Ga interstitials are the main source of n-type conductivity in sputtered GaN films. The low conductivity of the films grown at high N2 percentages (≳75%) is attributed to the limited presence of Ga interstitials and a strong compensation, dominated by N and O interstitials. The high conductivity GaN films grown at low N2 percentages may be partly contributed by O substituted on N site, along with Ga vacancies and possibly VGa-ON complexes, as the compensating defects.

Twinned growth of ScN thin films on lattice-matched GaN substrates

Scandium nitride (ScN) has attracted significant interest in recent years for diverse electronic and thermoelectric applications. Most ScN growth utilizes MgO and Al2O3 as substrates that lead to extended defects such as dislocations and grain boundaries. (0001) GaN exhibits less than 0.1% lattice mismatch with (111) ScN, and therefore, should result in single-crystalline ScN film growth. However, not much attention is devoted to understanding the microstructure of ScN on GaN substrates. In this work, we present (111) oriented lattice matched ScN film growth on (0001) GaN substrates. X-ray pole figure exhibited six spots for the (002) ScN superimposed on that of (101) GaN separated by 60° corresponding to twin domains of threefold symmetry for (111) ScN, which are furthermore directly imaged by lattice resolved microscopy images. The presence of twins is attributed to vertical dislocations originating at the GaN/Al2O3 interface and extending throughout the GaN film to its interface with ScN.

Effect of backside dry etching on the device performance of AlGaN/GaN HEMTs

AlGaN/GaN heterojunction-based high-electron-mobility transistors (HEMTs) have significant advantages of high carrier concentration, high electron mobility, and large breakdown voltage, and show promising potential as power devices. Being widely used in semiconductor manufacturing, dry etching process is capable of fabricating microstructures and thinning substrate from backside, which is good for developing flexible devices. Here, we investigate the effect of backside dry etching of Si substrate on the physical and electrical properties of AlGaN/GaN HEMTs. The physical properties were characterized by scanning electron microscope, Raman spectra, and x-ray diffraction (XRD). After the dry etching process, the peak red-shift of GaN E 2 mode indicates an increase of tensile stress, and the XRD rocking curve of GaN film shows to a certain extent decreased dislocation density. Furthermore, the maximum saturation current density and maximum transconductance of the HEMTs are improved by 21.1% and 25.5%, respectively. The approach of backside dry etching for thinning Si substrate would contribute to the optimization of GaN heterojunction-based devices, and also provide inspirations for the development of flexible and robust power devices.

Vacancy-type defects in bulk GaN grown by oxide vapor phase epitaxy probed using positron annihilation

Defects in bulk GaN grown by the oxide vapor phase epitaxy (OVPE) method were probed by using positron annihilation spectroscopy. Measurements of the Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons revealed that the major defect species in OVPE-GaN was Ga vacancy-type defects coupled with oxygen atoms such as VGa(ON)n and VGaVN(ON)n. The formation of vacancy-oxygen complexes was attributed to the high concentration of oxygen incorporated in the samples during their growth process. A correlation between such defect complexes and sample transparencies was observed. An 8-μm-thick epitaxial film grown on OVPE-GaN by using metalorganic vapor phase epitaxy was also characterized. The vacancy concentration in the GaN film was found to be under or close to the detection limit of positron annihilation (1015 cm−3), suggesting that an epitaxial layer can be grown on without being influenced by vacancies in the substrates.

Electrode‐Dependent Electrical Properties of Detection‐Band Tunable Ultraviolet Photodetectors Based on Ga2O3/GaN Heterostructures

The polycrystal Ga2O3 on GaN was realized by high‐temperature oxidization of the GaN films with oxidation time 1 and 10 h, and the metal‐semiconductor‐metal photodetectors based on the Ga2O3/GaN heterostructure were fabricated by using Ti and Au metal electrodes. A comparative study of different electrodes and oxidation time on the performance of ultraviolet photodetector is performed. Among the devices prepared by 1 h oxidation, it is found that the device fabricated with Au electrode shows a lower dark‐current and higher photocurrent than the one with Ti electrode. This is mainly due to the higher Schottky barrier height of sample with Au electrode. As for the device that experienced 10 h oxidation with Au electrode, the solar‐blind responsivity with the full width at half maxima of 12 nm was obtained, which can be attributed to thicker Ga2O3 film. The devices exhibit excellent performance, including low dark‐current and satisfactory responsivity, even under ultra‐weak signal. The authors attribute these excellent properties to the Ga2O3 film, as it has a large grain size and a porous structure. The results provide a simple method that is low‐cost and high‐output with large‐area production of high‐performance ultraviolet photodetectors.

Demonstration of epitaxial growth of strain-relaxed GaN films on graphene/SiC substrates for long wavelength light-emitting diodes

Strain modulation is crucial for heteroepitaxy such as GaN on foreign substrates. Here, the epitaxy of strain-relaxed GaN films on graphene/SiC substrates by metal-organic chemical vapor deposition is demonstrated. Graphene was directly prepared on SiC substrates by thermal decomposition. Its pre-treatment with nitrogen-plasma can introduce C–N dangling bonds, which provides nucleation sites for subsequent epitaxial growth. The scanning transmission electron microscopy measurements confirm that part of graphene surface was etched by nitrogen-plasma. We study the growth behavior on different areas of graphene surface after pre-treatment, and propose a growth model to explain the epitaxial growth mechanism of GaN films on graphene. Significantly, graphene is found to be effective to reduce the biaxial stress in GaN films and the strain relaxation improves indium-atom incorporation in InGaN/GaN multiple quantum wells (MQWs) active region, which results in the obvious red-shift of light-emitting wavelength of InGaN/GaN MQWs. This work opens up a new way for the fabrication of GaN-based long wavelength light-emitting diodes.

Nanoindentation of AlN, GaN, and AlGaN films grown by HVPE on SiC/Si hybrid substrates

This paper presents an experimental study of structural and mechanical characteristics of thin films of AlN, GaN, and AlGaN grown on hybrid SiC/Si substrates by the method of HVPE. The surface roughness of the films and substrates has been measured. The thickness and molecular composition of the films have been determined by spectral ellipsometry. An analysis of elastic deformation occurring in the films as a result of indentation has been made. The values of hardness and Young’s modulus of the films have been calculated.

(Invited) Shallow Donor Impurities in GaN

Significant advances have been achieved during the last three decades in heteroepitaxial, homoepitaxial, and bulk growth of nitride semiconductors. Despite this remarkable progress, the full realization of the functionality of this material system is often limited by the presence of pervasive residual impurities inherent to the high-temperature growth process, and/or by the presence of lattice point and extended defects. A number of important fundamental aspects of these properties must be understood and controlled in order to realize optimum device performance.We review here on a systematic investigation, employing a combination of defect-sensitive techniques, carried out on freestanding HVPE-GaN films to identify the chemical nature of the shallow donors. High-resolution, variable temperature PL experiments were performed in the spectral region associated with recombination processes involving the ground and excited states of the neutral donor bound excitons and their two-electron-satellites in GaN substrates and homoepitaxial films. High-sensitivity SIMS and high-resolution FTIR measurements unambiguously identify Si and O shallow donors with ground state binding energies of 30.18 meV and 33.20 meV, respectively. These binding energies are in excellent agreement with values obtained by the analyses of the two-electron-satellite PL spectra considering the participation of ground and excited state donor bound excitons. High-resolution PL studies of homoepitaxial GaN films confirm the SIMS and the FTIR impurity identifications. These results were conveniently employed to verify the incorporation and activation of shallow donors in high crystalline quality GaN substrates.This work supported by the Office of Naval Research

Temperature Effect of van der Waals Epitaxial GaN Films on Pulse-Laser-Deposited 2D MoS2 Layer

Van der Waals epitaxial GaN thin films on c-sapphire substrates with a sp2-bonded two-dimensional (2D) MoS2 buffer layer, prepared by pulse laser deposition, were investigated. Low temperature plasma-assisted molecular beam epitaxy (MBE) was successfully employed for the deposition of uniform and ~5 nm GaN thin films on layered 2D MoS2 at different substrate temperatures of 500, 600 and 700 °C, respectively. The surface morphology, surface chemical composition, crystal microstructure, and optical properties of the GaN thin films were identified experimentally by using both in situ and ex situ characterizations. During the MBE growth with a higher substrate temperature, the increased surface migration of atoms contributed to a better formation of the GaN/MoS2 heteroepitaxial structure. Therefore, the crystallinity and optical properties of GaN thin films can obviously be enhanced via the high temperature growth. Likewise, the surface morphology of GaN films can achieve a smoother and more stable chemical composition. Finally, due to the van der Waals bonding, the exfoliation of the heterostructure GaN/MoS2 can also be conducted and investigated by transmission electron microscopy. The largest granular structure with good crystallinity of the GaN thin films can be observed in the case of the high-temperature growth at 700 °C.

Removal of GaN film over AlGaN with inductively coupled BCl3/Ar atomic layer etch

Atomic layer etching (ALE) of thin film GaN (0001) is reported in detail using sequential surface modification by BCl3 adsorption and removal of the modified surface layer by low energy Ar plasma exposure in a reactive ion etching system. The estimated etching rate of GaN is ∼ 0.74 nm/cycle. The GaN is removed from the surface of AlGaN after 135 cycles. To study the mechanism of the etching, the detailed characterization and analyses are carried out, including scanning electron microscope (SEM), x-ray photoelectron spectroscopy (XPS), and atomic force microscope (AFM). It is found that in the presence of GaCl x after surface modification by BCl3, the GaCl x disappears after having exposed to low energy Ar plasma, which effectively exhibits the mechanism of atomic layer etch. This technique enables a uniform and reproducible fabrication process for enhancement-mode high electron mobility transistors with a p-GaN gate.

HVPE fabrication of GaN sub-micro pillars on preliminarily treated Si(001) substrate

The work reports on the chloride vapour phase epitaxy growth of a GaN film on the preliminarily treated silicon substrates of Si(001) with a 3° miscut to (011) through the buffer layer of AlN. It was demonstrated that the use of the proposed technology resulted in the formation of a transition sub-layer in the Si substrate with the thickness of ~170–200 nm, and subsequent growth on this layer led to the formation of pillar-like GaN grains with a lateral size of ~500 nm involving the thin interlayer of the AlN phase between the pillars. The epitaxial GaN film was characterised by the low value of the residual stresses calculated using the data of XRD, Raman, and PL measurements. This idea was supported by the intense photoluminescence and the short life time of the photons in the epitaxial layer. The obtained data will be utilised as an important material for the understanding of the basic foundations in the physics of GaN/AlN/Si nano-heterostructures, thus facilitating their potential applications in optoelectronics.