GaN Samples Research Articles

Effect of local fields on the Mg acceptor in GaN films and GaN substrates

The electron paramagnetic resonance (EPR) spectrum of the Mg acceptor is studied in a variety of GaN samples, including mm-thick free-standing substrates and sub-micron heteroepitaxial films. The former allows a view of the acceptor unique for EPR—in an environment with less than 107 cm−3 dislocations and doping densities ranging from 4 × 1016 to 6 × 1018 cm−3. By probing Mg in a broad range of samples in one study a new feature of the acceptor emerges. The EPR data reveal an anisotropic line shape that reflects a change in the crystal field in the vicinity of the Mg acceptor. This feature must be accommodated by any of the models proposed for the Mg acceptor. Here, we show that one such previously proposed model agrees well with the EPR data obtained from the wide variety of samples studied. The work implies that Mg-doped GaN contains a common Mg-related defect which can be affected by local crystal fields established during growth.

Detection of Streptavidin-Biotin Complexes Using a Highly Sensitive AlGaN/GaN-Based Extended-Gate MISHEMT-Type Biosensor

In this paper, we propose an AlGaN/GaN-based extended-gate metal-insulator-semiconductor high electron mobility transistor (MISHEMT)-type biosensor for detecting streptavidin-biotin complexes. We measure the drain current of the fabricated sensor, which varies depending on the antibody-antigen reaction of streptavidin with biotin molecules. To confirm the immobilization of biotin polyethylene glycol (PEG) thiol, we analyze the Au surface of a GaN sample using X-ray photoelectron spectroscopy (XPS). The proposed biosensor shows higher sensitivity than Si-based extended-gate metal oxide semiconductor field effect transistor (MOSFET)-type biosensor. In addition, the proposed AlGaN/GaN-based extended-gate MISHEMT-type biosensor exhibits better long-term stability, compared to the conventional AlGaN/GaN-based MISHEMT-type biosensor.

Quantitative secondary ion mass spectrometric analysis of secondary ion polarity in GaN films implanted with oxygen

Quantitative analyses of N and O ions in GaN thin films implanted with oxygen ions (16O+) were conducted by secondary ion mass spectrometry (SIMS). Positive (CsM+) and negative secondary ions extracted by Cs+ primary ion bombardment were analyzed for oxygen quantitative analysis. The oxygen depth profiles were obtained using two types of primary ion beams: a Gaussian-type beam and a broad spot beam. The oxygen peak concentrations in GaN samples were from 3.2 × 1019 to 7.0 × 1021 atoms/cm3. The depth profiles show equivalent depth resolutions in the two analyses. The intensity of negative oxygen ions was approximately two orders of magnitude higher than that of positive ions. In contrast, the O/N intensity ratio measured using CsM+ molecular ions was close to the calculated atomic density ratio, indicating that the SIMS depth profiling using CsM+ ions is much more effective for the measurements of O and N ions in heavy O-implanted GaN than that using negative ions.

The importance of structural inhomogeneity in GaN thin films

This paper describes two types of MOCVD-grown n-type GaN layers (Samples A and B) with similar carrier concentration but behaved differently under galvanic photo-etching. In order to understand this behavior, Transmission Electron Microscopy (TEM) for cross-section and plan-view samples, Secondary Ion Mass Spectroscopy (SIMS) and photoluminescence (PL) techniques were applied. SIMS studies showed that Si, C and O are approximately at the same concentration in both samples, but Sample B also contained Fe and Mg. Both GaN samples were grown on sapphire substrate with Ga growth polarity, which was confirmed by Convergent Beam Electron Diffraction (CBED). Despite a smaller layer thickness in Sample B, the density of edge dislocations is almost one order of magnitude lower than in Sample A. In addition, planar defects formed in this sample in the transition area between the undoped buffer and Si doped layers resulted in a substantial decrease in the density of screw dislocations at the sample surface. These planar defects most probably gave rise to the PL lines observed at 3.42eV and 3.32eV. The new PL lines that only appeared in Sample B might be related to Mg impurities found in this sample. There were no detectable gettering of these impurities at dislocations using different diffraction conditions. However, Fe rich platelets were found only in Sample B due to the presence of Fe as well as hexagonal features, similar to defects reported earlier in highly Mg-doped GaN. These structural and chemical non-uniformities between the two GaN samples can explain their different etching behaviors. This paper demonstrates that samples with similar carrier concentrations do not necessarily ensure similar structural and optical properties and that additional material characterization are needed to ensure that devices built on such samples have similar performance.

Comparisons of surface and optical properties of the heavily carbon-doped GaN nanocrystalline films deposited by thermionic vacuum arc method

In this paper, heavily C-doped GaN samples were deposited on glass and PET substrates by the thermionic vacuum arc (TVA) method, for the first time. Microstructure, surface and optical properties of the carbon-doped GaN samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy, atomic force microscopy, and UV–Vis spectrophotometer. According to XRD pattern, fullerene (C60) structures were able to deposit in doped samples, for the first time. The fullerene structures were detected at heavily carbon-doped GaN. The obtained band gap changes of heavily carbon-doped GaN samples are determined. The band gap of CGaN on PET substrate decreased to a lower value, about 100 meV. The band gaps were shifted to the lowest wavelengths towards the edge of the UV region by heavily doped carbon. (002) and (004) peaks for GaN were also detected.

Zero-phonon line and fine structure of the yellow luminescence band in GaN

The yellow luminescence band was studied in undoped and Si-doped GaN samples by steady-state and time-resolved photoluminescence. At low temperature (18 K), the zero-phonon line (ZPL) for the yellow band is observed at 2.57 eV and attributed to electron transitions from a shallow donor to a deep-level defect. At higher temperatures, the ZPL at 2.59 eV emerges, which is attributed to electron transitions from the conduction band to the same defect. In addition to the ZPL, a set of phonon replicas is observed, which is caused by the emission of phonons with energies of 39.5 meV and 91.5 meV. The defect is called the YL1 center. The possible identity of the YL1 center is discussed. The results indicate that the same defect is responsible for the strong YL1 band in undoped and Si-doped GaN samples.

Diffusion of oxygen in bulk GaN crystals at high temperature and at high pressure

Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by Halide Vapor Phase Epitaxy (HVPE) are reported. Oxygen concentration profiles were studied in as-grown GaN crystals and also after annealing of crystals at temperatures up to 3400K and pressures up to 9GPa. Investigated crystals contained large conical defects i.e. pinholes of significantly higher oxygen concentration (NO=(2–4)×1019cm−3) than that in the bulk matrix (NO<1×1017cm−3). The pinholes were revealed by a photo-etching method in as-grown and annealed GaN samples. Confocal micro-Raman spectroscopy was applied to measure the profiles of free electron concentration, which directly corresponds to the concentration of oxygen impurity. Lateral scanning across the interfaces between pinholes and matrix in the as-grown HVPE GaN crystals showed sharp step-like carrier concentration profiles. Annealing at high temperature and high pressure resulted in the diffusion blurring of the profiles. Analysis of obtained data allowed for the first time for estimation of oxygen diffusion coefficients DO(T, P). The obtained values of DO(T, P) are anomalously small similarly to the values obtained by Harafuji et al. by molecular dynamic calculations for self-diffusion of nitrogen. Whereas oxygen and nitrogen are on the same sublattice it could explain the similarity of their diffusion coefficients.

Deep-level transient spectroscopy of low-free-carrier-concentration n-GaN layers grown on freestanding GaN substrates: Dependence on carbon compensation ratio

Electron traps in n-GaN layers with a relatively low-free-carrier-concentration of approximately 1 × 1016 cm−3 were characterized by deep-level transient spectroscopy. Sample layers were grown by metal organic chemical vapor deposition with a thickness of 12 µm on freestanding GaN substrates, and were doped with both silicon and carbon. The measurement results showed a reduction in the density of carbon-related electron traps at an energy level of EC −0.40 eV in GaN on GaN samples, compared with GaN on SiC samples. It was also observed that the doping of carbon significantly suppressed electron traps at EC −0.61 eV, which was associated with the nitrogen antisite. Consequently, the possibility of minimizing all of the electron traps located between EC −0.19 and −0.89 eV in n-GaN was demonstrated by controlling the carbon doping in the nitrogen site.

Self-compensation due to point defects in Mg-doped GaN

Using hybrid density functional theory, we address point defects susceptible to cause charge compensation upon Mg doping of GaN. We determine the free energy of formation of the nitrogen vacancy and of several Mg-related defects. The entropic contribution as a function of temperature is determined within the quasiharmonic approximation. We find that the Mg interstitial shows a noticeably lower free energy of formation than the Mg substitutional to Ga in p-type conditions. Therefore, the Mg impurity is amphoteric behaving like an acceptor when substitutional to Ga and like a double donor when accommodated in an interstitial position. The hybrid-functional results are then linked to experimental observations by solving the charge neutrality equations for semiconductor dominated by impurities. We show that a thermodynamic equilibrium model is unable to account for the experimental hole concentration as a function of Mg doping density, due to nitrogen vacancies and Mg interstitials acting as compensating donors. To explain the experimental result, which includes a dropoff of the hole concentration at high Mg densities, we thus resort to nonequilibrium models. We show that either nitrogen vacancies or Mg interstitials could be at the origin of the self-compensation mechanism. However, only the model based on interstitial Mg donors provides a natural mechanism to account for the sudden appearance of self-compensation. Indeed, the amphoteric nature of the Mg impurity leads to Fermi-level pinning and accounts for the observed dropoff of the hole concentration of GaN samples at high Mg doping. Our work suggests that current limitations in p-type doping of GaN could be overcome by extrinsically controlling the Fermi energy during growth.

Optical and structural properties of microcrystalline GaN on an amorphous substrate prepared by a combination of molecular beam epitaxy and metal–organic chemical vapor deposition

Microscale platelet-shaped GaN grains were grown on amorphous substrates by a combined epitaxial growth method of molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). First, MBE GaN was grown on an amorphous substrate as a pre-orienting layer and its structural properties were investigated. Second, MOCVD grown GaN samples using the different growth techniques of planar and selective area growth (SAG) were comparatively investigated by transmission electron microscopy (TEM), cathodoluminescence (CL), and photoluminescence (PL). In MOCVD planar GaN, strong bound exciton peaks dominated despite the high density of the threading dislocations (TDs). In MOCVD SAG GaN, on the other hand, TDs were clearly reduced with bending, but basal stacking fault (BSF) PL peaks were observed at 3.42 eV. The combined epitaxial method not only provides a deep understanding of the growth behavior but also suggests an alternative approach for the growth of GaN on amorphous substances.

Electron band bending of polar, semipolar and non-polar GaN surfaces

The magnitudes of the surface band bending have been determined by X-ray photoelectron spectroscopy for polar, semipolar, and non-polar surfaces of wurtzite GaN crystals. All surfaces have been prepared from crystalline GaN samples grown by the hydride-vapour phase epitaxy and separated from sapphire substrates. The Ga 3d core level peak shifts have been used for band bending determination. Small band bending magnitudes and also relatively small difference between the band bendings of the surfaces with opposite polarity have been found. These results point to the presence of electron surface states of different amounts and types on surfaces of different polarity and confirm the important role of the electron surface states in compensation of the bound surface polarity charges in wurtzite GaN crystals.

Study of optical properties of bulk GaN crystals grown by HVPE

We investigated the optical properties of a series of GaN samples sliced from the same bulk crystal grown using hydride vapor phase epitaxy. The high crystalline quality of the samples was evaluated using cathodoluminescence measurements, and the dislocation density ranged from 2.4 × 106 to 2.3 × 105 cm−2. The impurity concentration was determined using secondary-ion mass spectroscopy, and photoluminescence (PL) measurements were conducted in the range of 3–300 K. We did not find a correlation between the O or C impurities and the weak yellow luminescence (YL) band. As the dislocation density decreased, the intensity of the band edge emission increased and that of the YL band decreased. A competition between the two-electron satellite lines correlated to Si and the YL band was also observed in the low-temperature PL spectra, which demonstrated that the Si impurity also plays an important role in the weak YL band of these GaN samples. These results indicate that the Si donors around the dislocations, as reasonable sources of shallow donors, will recombine with possible deep acceptors and finally respond with the YL.

Surface studies and photoluminescence of GaN epitaxial layers grown by MOCVD technique for applications of solar cells and light emitting devices

This article presents theoretical and experimental results of n-type GaN semiconductor. In the first part, the variation of surface potential of GaN with varying carrier concentration in the range from 1015 to 1019 cm−3 is determined as well as the change in the electric field with variation of surface potential is also estimated. In addition, the change in the electric field (F) with effect of irradiation of laser beam on the GaN sample, while measuring photoluminescence, is found by developing appropriate theoretical models using known activation energies of traps, which are obtained by the experimental photoluminescence (PL) measurements. In the second part, the experimental results of n-type GaN such as growth by MOCVD, electrical, structural and optical properties are presented. The un-doped GaN thin films have been successfully deposited onto c-sapphire substrates by MOCVD technique with different recipes. It is observed that the carrier concentration of the films varies with varying deposition parameters. The PL studies are systematically carried out on these samples at low temperature. The correlation between PL results and the electrical data such as carrier concentration (n) or mobility (μ) is analyzed for GaN thin films. Based on the activation energies of donor–acceptor pair (DAP) and the yellow band emissions obtained from the PL measurements, the possible transition energy levels in the energy band are hypothetically proposed. The highly resolved phonon replicas in PL spectra are observed for high carrier density samples. The n and μ are found to be in the range of 1017–1018 cm−3 and 65–120 cm2/V−s, respectively. The XRD analysis reveals that the samples show preferred orientation and hexagonal structure. On the other hand, the nature of the samples changes from epitaxy to polycrystalline with increasing carrier concentration in the samples.

Deep levels in as-grown and electron-irradiated n-type GaN studied by deep level transient spectroscopy and minority carrier transient spectroscopy

Development of high performance GaN-based devices is strongly dependent on the possibility to control and understand defects in material. Important information about deep level defects is obtained by deep level transient spectroscopy and minority carrier transient spectroscopy on as-grown and electron irradiated n-type bulk GaN with low threading dislocation density produced by halide vapor phase epitaxy. One hole trap labelled H1 (EV + 0.34 eV) has been detected on as-grown GaN sample. After 2 MeV electron irradiation, the concentration of H1 increases and at fluences higher than 5 × 1014 cm−2, a second hole trap labelled H2 is observed. Simultaneously, the concentration of two electron traps, labelled T1 (EC – 0.12 eV) and T2 (EC – 0.23 eV), increases. By studying the increase of the defect concentration versus electron irradiation fluence, the introduction rate of T1 and T2 using 2 MeV- electrons was determined to be 7 × 10−3 cm−1 and 0.9 cm−1, respectively. Due to the low introduction rate of T1, it is suggested that the defect is associated with a complex. The high introduction rate of trap H1 and T2 suggests that the defects are associated with primary intrinsic defects or complexes. Some deep levels previously observed in irradiated GaN layers with higher threading dislocation densities are not detected in present investigation. It is therefore suggested that the absent traps may be related to primary defects segregated around dislocations.

The impact of RF-plasma power in carrier relaxation dynamics of unintentional doped GaN epitaxial layers grown by MBE

In this work, unintentionally doped GaN samples were prepared on GaN template by radio frequency (RF)-plasma MBE technique using two different RF-plasma powers. Photoluminescence (PL), steady state photoconductivity (PC) and ultrafast optical pump-probe spectroscopy measurements have been carried out to characterize the samples. The effect of RF-plasma power towards unintentional doping and giving rise to yellow luminescence (YL) is discussed. Our PC measurements show relatively faster decay for sample grown with higher RF-plasma power. In addition, the ultrafast optical pump-probe spectroscopy results show the presence of various defect levels with different relaxation times. A faster ultrafast relaxation time from the conduction band to the closest defect level and conduction band to the next defect level was observed for the sample grown with higher plasma power. A comparatively low defect density and faster carrier relaxation observed in higher RF-plasma power grown samples is caused by lower impurities and gallium vacancies. The results imply that RF-plasma power is very important parameter for the growth of epitaxial GaN films and undesirable impurities and gallium vacancies might get incorporated in the epitaxial GaN films.

Effect of light Si doping on the properties of GaN

An obvious increase in electron mobility and yellow luminescence (YL) band intensity was found in light Si doping GaN. For a series of GaN samples with different doping concentration, the dislocation density is almost the same. It is inferred that the abrupt increase in mobility and YL intensity does not originate from the change of dislocation density. The mobility behavior is attributed to the screening of scattering by dislocation and increase of ionized impurity scattering with the increase of Si doping concentration. At lower doping level, the screening of dislocation scattering is dominant, which results in the increase in carrier mobility. At higher doping level, the increase in ionized impurity scattering leads to the decrease in carrier mobility. Higher mobility causes longer diffusion length of nonequilibrium carrier. More dislocations will participate in the recombination process which induces stronger YL intensity in light Si doping GaN.

Behavior of Etching Process on Formation of Porous Polycrystalline GaN Layer through Electroless Etching

At the first stage of this work, growth of polycrystalline GaN layer on GaAs (100) substrate using RF-sputtering was demonstrated at room temperature, with and without the insertion of AlN as a buffer layer. Diffraction pattern from XRD measurement of the samples showed the evidence of GaN in and (0002) orientations. It was found that the FWHM of the GaN for both samples, with and without the AlN buffer layer were 0.17°. However, the FWHM of the GaN (0002) peak for the sample with the AlN buffer layer was 0.08°, while without the AlN buffer layer was 0.16°. Next, both samples were fabricated into porous structure through electroless etching for 3 and 5 minutes, respectively. The cubic inverse truncated pyramid-like porous structure was observed. Detailed inspection found that the etching rate reduced as the etching time increased. More interestingly, the 3 minutes etched-porous on the GaN sample without the AlN buffer layer showed better porosity compared to its counterparts. It is proposed that low properties of GaN sample tends to have high etching rate and better porosity profile in term of pores distribution and density and this is due to weak Ga-N bonding.

Photoelectrochemical Property Differences between NiO Dots and Layer on n-Type GaN for Water Splitting

The n-type GaN photoelectrode is suitable for hydrogen generation from water because it can split water without bias. However, the n-type GaN photoelectrode has the anodic corrosion problem caused by the etching of the GaN. This problem can be resolved by NiO loading on GaN. The reported method of NiO loading on GaN surface was carried out by coating the surface by diluted metal organic decomposition (MOD) and high-temperature annealing. Since no NiO loading methods using Ni(OH)2 dispersed solution and low-temperature annealing have been reported, in this study, we attempted NiO loading Ni(OH)2 dispersed solution. Experimental results confirmed that the method can be easily performed and effective. In addition, we also compared the photoelectrochemical properties of the n-type GaN samples obtain by Ni(OH)2 loaded using four different Ni(OH)2 synthetic methods, loading by NiO-MOD, and no-NiO loading. The photocurrent densities of all samples loaded using Ni(OH)2 dispersed solution increased compared to samples without NiO and with NiO-MOD. The photocurrent densities and stabilities were affected by the NiO shape and density which were found to be related with the method of synthesizing the Ni(OH)2 dispersed solutions. The effects of NiO shape and density on the photocurrent density and stability are also discussed.

Effect of Polarity on Photoelectrochemical Properties of Polar and Semipolar GaN Photoanode

In this study, the photoelectrochemical (PEC) properties of polar GaN were compared to those of semipolar GaN. GaN samples were grown on sapphire by metal–organic chemical vapor deposition (MOCVD). Various electrolytes, namely, aqueous solutions of H2SO4 (pH 0.2), Na2SO4 (pH 6.0), and NaOH (pH 14.0), were used for investigating the effect of pH. The band-edge potentials of polar and semipolar GaN calculated from the Mott–Schottky plots indicates that these potentials can split water and followed the Nernst equation for each electrolyte. Further, the PEC properties of GaN with different polarities were investigated under 100 mW illumination. The photocurrent density of polar GaN was found to be higher than that of semipolar GaN. This result was attributed to the polarization effect of polar GaN. Interestingly, all GaN samples showed a divergent trend in the two-electrode method, similar to a real device.

Selective p-type Doping of GaN:Si by Mg Ion Implantation and Multicycle Rapid Thermal Annealing

Selective p-type doping of Si-doped GaN by Mg ion implantation and multicycle rapid thermal annealing (MRTA) is demonstrated. Samples of GaN were capped by AlN and annealed in N2 overpressure at temperatures up to 1330°C. MRTA resulted in a lower resistivity films, compared to a conventionally-annealed reference sample. Mg acceptor activation for the MRTA annealed sample was calculated using a two-layer corrected Hall method to be about 1% at room temperature. Low temperature photoluminescence revealed donor-acceptor bound electron-hole pair recombination near 3.2 eV, indicating activation of Mg upon MRTA annealing.