Defects In n-GaN Research Articles

Nitrogen vacancies in GaN templates and their critical role on the luminescence efficiency of blue quantum wells.

In InGaN-based LEDs, an InGaN layer underlying active region has been widely used to improve the luminescence efficiency of the quantum wells (QWs). It has been reported recently that the role of InGaN underlayer (UL) is to block the diffusion of point defects or surface defects in n-GaN into QWs. The type and the source of the point defects need further investigations. In this paper, using temperature-dependent photoluminescence (PL) measurements, we observe emission peak related to nitrogen vacancies (VN) in n-GaN. In combination with secondary ion mass spectroscopy (SIMS) measurement and theoretical calculation, it is found that VN concentration in n-GaN is as high as about 3 × 1018 cm-3 in n-GaN grown with low growth V/III ratio and can be suppressed to about 1.5 × 1016 cm-3 by increasing growth V/III ratio. Luminescence efficiency of QWs grown on n-GaN under high V/III ratio is greatly improved. These results indicate high density of nitrogen vacancies are formed in n-GaN layer grown under low V/III ratio and diffuse into quantum wells during epitaxial growth and reduce the luminescence efficiency of the QWs.

(Invited) DLTS Studies of Defects in n-GaN

Introduction There exist various defects in GaN depending on growth methods or growth conditions or substrates used for the epitaxial growth. There are point defects, such as vacancy-related defects, antisite defects, interstitials as well as line defects, i.e. dislocations. These affect the performance of GaN-based devices, since they create deep levels in the bandgap and act as traps and generation-recombination centers. Foreign atoms, such as carbon and iron, also create deep levels. It is important to characterize these defects and identify their origins to realize the high performance of GaN-based devices. Current collapse in AlGaN/GaN high electron mobility transistors is well known to be caused by trapping of carriers [1]. In this work, we present our results on the characterization of traps in Si-doped n-GaN grown by metal organic chemical vapor deposition (MOCVD) on different substrates, that is, sapphire, n+-GaN, SiC and Si [2]. Experimental Lateral and vertical Schottky diodes were fabricated using Pt/Au or Ni/Au as a Schottky contact and Ti/Al/Ni/Au as an ohmic contact for n-GaN on sapphire, Si and SiC substrates and for n-GaN on n+-GaN substrates, respectively. Carrier concentrations determined from capacitance-voltage measurements at room temperature are in the range from 1016 to 1017 cm-3. Deep level transient spectroscopy (DLTS) using bias pulses has been applied to estimate electron traps for fabricated Schottky diodes. Minority carrier transient spectroscopy (MCTS) using above-band-gap light pulses has been used to characterize hole traps. Results and Discussion Temperature-scan DLTS and MCTS have been performed in the temperature range from 90 to around 400 K. DLTS measurements detected four electron traps labeled E1 (Ec-0.24 eV), E3 (Ec-0.57 eV), E6 (Ec-0.40 eV) and E7 (Ec-0.73 eV). E1 and E3 might be ascribed to VGa-VN pairs and NGa antisite defects, respectively. E6 and E7 are observed for n-GaN on SiC and Si, suggesting their correlation to dislocation-related defects. MCTS measurements detected three hole traps labeled H1 (Ev+0.86 eV), H2 (Ev+0.25 eV) and H3 (Ev+0.25 eV). Trap E3 and H1 concentrations are as high as 1016 cm-3 and show a large variation of trap concentration on each wafer. Moreover, trap H1 concentration increases in n-GaN intentionally doped with carbon, suggesting its correlation to carbon-related defects, possibly in addition to VGa-related defects. We have studied a distribution of trap E3 and H1 concentration for a quarter of 2-inch n-GaN on n+-GaN using isothermal DLTS and MCTS measurements at room temperature, respectively. A variation of trap E3 concentration over four orders of magnitude is found in the range from ~1012 to ~1016 cm-3 with the average concentration of 4.6x1014 cm-3, while trap H1 shows a relatively small variation in the range from~1015 to ~1016 cm-3 with the average concentration of 5.9x1015 cm-3. Average trap concentration of trap H1 is higher in n-GaN doped with carbon of 6x1016 cm-3 and is found to be 4.2x1016 cm-3. It is thought that the increased concentration of trap H1 in carbon-doped GaN corresponds to the concentration of carbon-related defects. In this context, we will present a method to distinguish between VGa-related and carbon-related defects with similar thermal emission activation energies by a combination of MCTS and optical deep level transient spectroscopy (ODLTS) using below-band-gap light pulses. Further, we will discuss on the identification of these traps by varying MOCVD growth conditions, namely, V/III ratios, growth pressures and growth temperatures. Acknowledgments This work has been performed as an MEXT-Supported Program for the Strategic Research Foundation at Private Universities (2010-2014).

(Invited) DLTS Studies of Defects in n-GaN

We have studied electron and hole traps in Si-doped n-GaN grown by metal organic chemical vapor deposition on n+-GaN, sapphire, SiC and Si substrates. DLTS using bias pulses, MCTS using above-band-gap light pulses and ODLTS using below-band-gap light pulses have been employed for fabricated Schottky diodes. The energy levels and capture cross sections of four electron traps and three hole traps are reported. A variation of dominant traps in concentration between wafers and among diodes on each wafer is observed, which is discussed based on the dependence of trap concentrations on the growth conditions.

As-grown deep-level defects in n-GaN grown by metal–organic chemical vapor deposition on freestanding GaN

Traps of energy levels Ec −0.26 and Ec −0.61 eV have been identified as as-grown traps in n-GaN grown by metal–organic chemical vapor deposition by using deep level transient spectroscopy of the Schottky contacts fabricated by resistive evaporation. The additional traps of Ec −0.13 and Ec −0.65 eV have been observed in samples whose contacts are deposited by electron-beam evaporation. An increase in concentration of the Ec −0.13 and Ec −0.65 eV traps when approaching the interface between the contact and the GaN film supports our argument that these traps are induced by electron-beam irradiation. Conversely, the depth profiles of as-grown traps show different profiles between several samples with increased or uniform distribution in the near surface below 50 nm. Similar profiles are observed in GaN grown on a sapphire substrate. We conclude that the growth process causes these large concentrations of as-grown traps in the near-surface region. It is speculated that the finishing step in the growth process should be an essential issue in the investigation of the surface state of GaN.

Radiation-produced defects in n-GaN

Radiation-produced defects in doped and nominally undoped n-GaN with charge carrier concentrations in a range of 4×10 16 cm −3 to about 2×10 18 cm −3 are investigated. Layers of n-GaN are irradiated with fast electrons at 0.9 MeV, 60Co gamma rays and protons at 150 keV. The production rates of radiation defects in n-GaN are estimated and compared with literature data. Annealing processes of radiation defects in n-GaN in the temperature range 100–700 °C are also discussed. The annealing behavior appears to be complicated. Two temperature intervals, from T=100 to 400 °C and from T=500 to 700 °C, are characteristic for modification and annealing processes of radiation-produced defects in n-GaN.

60Co gamma-irradiation-induced defects in n-GaN

Transient capacitance measurements of Schottky diodes fabricated on nominally undoped n-type GaN exposed to 60Co gamma irradiation indicate the introduction of two defect levels with thermal activation energies of 89±6 and 132±11 meV. While the emission characteristics of these defects manifest significant broadening, their parameters are consistent with reported electron-irradiation-induced nitrogen-vacancy related centers. Three deep-level defects present before irradiation exposure with activation energies of 265, 355, and 581 meV were found to remain unaffected for cumulative gamma-ray doses up to 21 Mrad(Si).

Radiation induced defects in n-GaN, an overview

We determine, using deep level transient spectroscopy, the electronic and annealing properties of defects introduced in MOVPE grown n-GaN by high energy electron, helium-ion and proton irradiation. As in other semiconductors these three particles introduce electrically active defects in n-GaN. There are 6 electron defects (ER1-ER6) introduced in the hexagonal n-GaN. A deep lying defect (1.04 eV), ER6, was detected and characterised for the first time. Defect ER3 which exhibits a field dependent emission rate has been reported to be the nitrogen vacancy. Defect ER2 appears to have a higher introduction rate when using protons. The deeper lying defects ER4-ER6 which are positioned reasonably close to one another have peak temperature positions which differ depending on the incident particle mass.

Electrical characterization of growth-induced defects in n-GaN

We report the electronic properties, obtained by deep level transient spectroscopy (DLTS), of electron trap defects in n-GaN, grown by hydride vapour phase epitaxy (HVPE), metal-organic vapour phase epitaxy (MOVPE), reactive molecular beam epitaxy (RMBE) and, for the first time, epitaxial lateral overgrowth (ELO) MOVPE. Two prominent traps with energy levels at EC − 0.27 eV and EC − 0.61 eV, respectively, are introduced during all four methods. Their filling kinetics suggest that they are extended defects. On the other hand, the levels at EC − 0.65 eV and EC − 0.21 eV are only detected in HVPE and RMBE grown GaN, respectively, and appear to be characteristic of these methods.

Carrier Trapping and Recombination at Point Defects and Dislocations in MOCVD n-GaN

We present the results of an approach that allows the analysis of grown-in defects in n-GaN grown by metal organic vapor phase epitaxy by relating the presence of various structural defects to the concentration and properties of distinct deep levels. Transmission electron microscopy and electron beam induced current (EBIC) microscopy together with post-growth hydrogenation are used to determine the threading dislocation density (TDD) and observe their local electrical activity. In conjunction, deep level optical and transient spectroscopies (DLOS and DLTS, respectively) are used to detect deep levels, determine their concentration and analyze their carrier trapping kinetics for these films. The comparison of EBIC analysis with trap spectra prior and after hydrogenation establishes a strong correlation between two specific levels, at Ec — Et = 0.58 and 1.35 eV, and recombination centers distributed in the field of the GaN films. Further, EBIC analysis shows that, independent of hydrogenation, TDs behave as strong recombination centers, and indicates that there must be a deep level associated with these regions. A level observed at Ec — 2.64/Ev + 0.87 eV is a good candidate to account for the electrical activity of the TDs because it captures both electrons and holes, which is characteristic of recombination centers. This is supported by DLTS analysis of the trapping kinetics for this level that reveals a behavior characteristic of a linear arrangement of point defects likely found along the TDs.

Electrical Characterization of Sputter Deposition Induced Defects in n-GaN

We have used current-voltage (I-V) measurements to assess and compare the electrical characteristics of resistively evaporated and sputter deposited Au Schottky contacts on epitaxially grown GaN. These I-V measurements revealed that resistively deposited Au contacts exhibited excellent rectification properties: high barrier height, low reverse current and good ideality factor (n = 1.04). In contrast, sputter deposited contacts had poor characteristics: low barrier height, high reverse current and non-linear forward I-V characteristics. The cause of this is thought to be defects introduced at and near the surface during sputter deposition. Deep level transient spectroscopy (DLTS) showed that at least four defects, with energy levels at 0.22±0.02 eV, 0.30±0.01 eV, 0.40±0.01 eV and 0.45±0.10 eV below the conduction band, were introduced in the GaN during sputter deposition. The first of these defects has similar electronic properties as a radiation induced defect in GaN, speculated to be the nitrogen vacancy, while the second appears to be the same as a defect in the as-grown material. The latter two defects have not previously been observed in as-grown or processed epitaxial GaN.

Electrical Characterization of Sputter Deposition Induced Defects in n-GaN

AbstractWe have used current-voltage (I-V) measurements to assess and compare the electrical characteristics of resistively evaporated and sputter deposited Au Schottky contacts on epitaxially grown GaN. These I-V measurements revealed that resistively deposited Au contacts exhibited excellent rectification properties: high barrier height, low reverse current and good ideality factor (n = 1.04). In contrast, sputter deposited contacts had poor characteristics: low barrier height, high reverse current and non-linear forward I-V characteristics. The cause of this is thought to be defects introduced at and near the surface during sputter deposition. Deep level transient spectroscopy (DLTS) showed that at least four defects, with energy levels at 0.22±0.02 eV, 0.30±0.01 eV, 0.40±0.01 eV and 0.45±0.10 eV below the conduction band, were introduced in the GaN during sputter deposition. The first of these defects has similar electronic properties as a radiation induced defect in GaN, speculated to be the nitrogen vacancy, while the second appears to be the same as a defect in the as-grown material. The latter two defects have not previously been observed in as-grown or processed epitaxial GaN.