Background Electron Concentration Research Articles

Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of Au∕GaN Schottky contacts

Under identical preparation conditions, Au∕GaN Schottky contacts were prepared on two kinds of GaN epilayers with significantly different background electron concentrations and mobility as well as yellow emission intensities. Current-voltage (I-V) and variable-frequency capacitance-voltage (C-V) characteristics show that the Schottky contacts on the GaN epilayer with a higher background carrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-V characteristics. This is attributed to the presence of deep level centers. Theoretical simulation of the low-frequency C-V curves leads to a determination of the density and energy level position of the deep centers.

Electrical characteristics of bulk GaN-based Schottky rectifiers with ultrafast reverse recovery

A vertical Schottky diode rectifier was fabricated using a bulk n−GaN wafer. Pt Schottky contacts were prepared on the Ga face and full backside ohmic contact was prepared on the N face by using Ti∕Al. The root mean square surface roughnesses of the Ga and N faces are 0.61 and 4.7nm, respectively. A relatively high breakdown field of 5.46kV∕cm was achieved with no additional edge termination. The breakdown field decreases as the size of the device increases. The background electron concentration of the bulk GaN wafer was low (5×1015cm−3), which may lead to a relatively high breakdown field even with no special edge termination. The forward turn-on voltage was as low as 2.4V at the current density of 100A∕cm2. The device exhibited an ultrafast reverse recovery characteristics (reverse recovery time <20ns).

Correlation of in-situ reflectance spectra and resistivity of GaN/Al2O3 interfacial layer in metalorganic chemical vapor deposition

The correlation between the resistivity of an undoped GaN/Al2O3 interfacial layer and in-situ reflectance spectrum in metalorganic chemical vapor deposition and the mechanism of this correlation were investigated. The first minimum reflectance during the initial high-temperature GaN growth was found to be a good indicator of the resistivity of the GaN buffer. The background electron concentration and mobility were both higher in the samples with higher indicative reflectance at that point. The resistivity of the GaN buffer layer was predominantly determined by an ∼0.25-µm-thick layer near the GaN/Al2O3 interface. Atomic force microscope (AFM) and high-resolution x-ray diffraction (HRXRD) results showed that the samples with higher indicative reflectance had smaller sized but higher density nuclei before the high-temperature GaN growth and lower screw threading dislocation (TD) density in the initially grown GaN. The difference in the background electron concentration and mobility of the interfacial layer was related to the relatively higher concentration of the O and Al diffused from Al2O3, which is also dependent on the size and density of the nuclei. These differences were found not to affect the structural and electrical properties or the surface morphology of AlGaN/GaN high electron-mobility transistors (HEMTs, except for the buffer conduction) when the GaN buffer is thick enough (e.g., ∼2.5 µm).

Transport properties of undoped and NH3-doped polycrystalline SnO2 with low background electron concentrations

A series of polycrystalline 1 μm thick SnO2 films were deposited onto borosilicate glass substrates by atmospheric pressure chemical vapor deposition. Unintentionally doped as-grown SnO2 layers had electron concentrations and mobility of 2–4×1017cm−3 and 25–30cm2∕Vs, respectively. Potential barriers and trap concentrations were calculated to be 30 meV and 2.3×1012cm−2, respectively. Upon N2∕vacuum annealing at 670 K for 15–20 min, the potential barrier height decreased to 8 meV and the electron mobility increased to 58cm2∕Vs. When doped with ammonia, the mobility of as-grown samples decreased to 0.5cm2∕Vs. The magnitude of the potential barriers varied, with ammonia doping, from 175 to 31 meV with trap densities of 4.7–1.2×1012cm−3, respectively. Upon vacuum∕N2 annealing at 670 K for 20 min, the electron mobilities of ammonia doped films recovered to 50–71cm2∕Vs, whereas the height of the potential barriers decreased to 3–4 meV with trap concentrations of 8–9×1011cm−2. The observed changes in the electrical properties are well described by a double back-to-back Schottky barrier model.

Theoretical assessment of electronic transport in InN

Among the group-III nitrides, InN displays markedly unusual electronic transport characteristics due to its smaller effective mass, high peak velocity and high background electron concentration. First, a non-local empirical pseudopotential band structure of InN is obtained in the light of recent experimental and first-principles results. This is utilized within an ensemble Monte Carlo framework to illuminate the interesting transport properties. It is observed that InN has a peak velocity which is about 75% higher than that of GaN while at higher fields its saturation velocity is lower than that of GaN. Because of the strongly degenerate regime brought about by the high background electron concentration, the electron–electron interaction is also investigated, but its effect on the steady-state and transient velocity–field characteristics is shown to be negligible. Finally, hot phonon generation due to excessive polar optical phonon production in the electron scattering and relaxation processes is accounted for. The main findings are the appreciable reduction in the saturation drift velocity and the slower recovery from the velocity overshoot regime. The time evolution of the hot phonon distribution is analysed in detail and it is observed to be extremely anisotropic, predominantly along the electric force direction.

Characterization of N-doped ZnO layers grown on (0001) GaN/Al2O3 substrates by molecular beam epitaxy

Abstract We investigated the optical properties and electrical properties of N-doped ZnO layers grown on (0 0 0 1) GaN/Al 2 O 3 substrates by molecular beam epitaxy, employing 10 K photoluminescence (PL) measurements, current–voltage ( IV ) measurements, capacitance–voltage ( CV ) measurements, and 100 K photocapacitance (PHCAP) measurements. 10 K PL spectra showed that excitonic emission is dominant in N-doped ZnO layers grown after O-plasma exposure, while overall PL emission intensity is significantly reduced and deep level emission at around 2.0 ∼ 2.2 eV is dominant in N-doped ZnO layers grown after Zn exposure. IV and CV measurements showed that N-doped ZnO layers grown after Zn exposure have better Schottky diode characteristics than O-plasma exposed samples, and an N-doped ZnO layer grown at 300 °C after Zn exposure has best Schottky diode characteristics. This phenomenon is presumably due to lowered background electron concentration induced by the incorporation of N. PHCAP measurements for the N-doped ZnO layer revealed several midgap trap centers at 1.2 ∼ 1.8 eV below conduction band minimum.

The growth of N-face GaN by MOCVD: effect of Mg, Si, and In

The MOCVD growth of N-face GaN on Ga-face GaN has been investigated using excess Mg to produce inversion of the polar axis. Characterization of the inverted material by AFM, TEM, optical microscopy and chemo-mechanical polishing indicates complete inversion and the ability to grow N-face films. Growth of N-face GaN is accompanied by higher oxygen incorporation as well as higher background electron concentrations than growth of the Ga-face material under similar conditions. Silicon can be incorporated during the growth of the N-face material and acts as an n-type dopant as is the case for growth of the Ga-face. InGaN layers can be deposited on the N-face and exhibit typical emission in the 380–450 nm range with an additional emission in the red likely caused by defect levels in the InGaN. Conditions for growing high-quality inverted layers are discussed.

Effect of native defects on electrical and optical properties of undoped polycrystalline GaN

Polycrystalline GaN (poly-GaN) films were grown on silica glass substrates by metal-organic chemical vapor deposition. An n-type conductivity in all the undoped poly-GaN films was observed and background impurities such as silicon and carbon were detected. The Si impurity was found to be diffused from the silica substrate and thought to be a dominant source of the n-type conductivity. It was also assumed that the C impurity as a compensating acceptor slightly affected the background electron concentration. It was thought that deep level emissions around 2.2 eV were associated with gallium vacancies and carbon impurity complexes at grain boundaries of the poly-GaN.

Step‐Flow Growth of InN on N‐Polarity GaN Template by Molecular Beam Epitaxy with a Growth Rate of 1.3 μm/h

The effects of substrate temperature and surface stoichiometry on the growth behavior of InN were investigated in molecular beam epitaxy with in situ monitoring by reflection high-energy electron diffraction and spectroscopic ellipsometry. InN was grown on nitrided sapphire substrate or N-polarity GaN template. For both cases, InN layers were found in N polarity by coaxial impact collision ion scattering spectroscopy. At growth temperatures ranging from 470 to 590 °C, the N-rich condition was favorable for stable InN growth. Under the In-limited growth condition, the step-flow growth of InN was achieved on N-polarity GaN template at 580 °C with a growth rate of 1.3 μm/h. The FWHMs of X-ray rocking curves around InN (002) and (102) reflections were about 200–250 arcsec and 950–1100 arcsec, respectively. The InN layer grown with N polarity on GaN showed a Hall mobility as high as 800 cm2/V · s with a background electron concentration of 2.1 × 1019 cm—3 at room temperature.

Optical Characterization of High Quality GaN Produced by High Rate Magnetron Sputter Epitaxy

ABSTRACTThe thick films of GaN were investigated using X-ray diffraction, micro-Raman spectroscopy and photoluminescence spectroscopy. The thick films of GaN were prepared on (0001) sapphire using high rate magnetron sputter epitaxy with growth rates as high as 10–60 m/min. The width of the X-ray rocking curve ((0002) reflection) for the sample produced by this method is ∼300 arc-sec. Only the allowed modes were observed in the polarized Raman spectra. The background electron concentration is lower than 3×1016 cm−3, which was determined from the Raman spectra. The phonon lifetime determined from Raman E2(2) mode was 1.6 ps, which is comparable to that of bulk single crystal GaN grown by sublimation (1.4 ps). The full-width-at-half-maximum of the near band-edge photoluminescence peak obtained at 77K is ∼100 meV.

Characteristics of GaN grown by metalorganic chemical vapor deposition using trimethylgallium and triethylgallium

Metalorganic chemical vapor deposition growth of GaN films using trimetylgallium (TMGa) and triethylgallium (TEGa) sources exhibited the different growth mechanisms and properties of GaN films. In situ normal incidence reflectance measurements during the growth of GaN with TEGa showed an increased coalescence time and a lower growth rate compared with TMGa, resulting in defect free columnar domains, a lower density of dislocation at the domain boundary, a rough surface, and good electrical properties. These results indicate that the growth mode for TEGa-grown GaN slowly changes from three-dimensional to quasi-two dimensional lateral growth via a coalescence stage and that the roughening and coalescence processes are repeated with increasing film thickness. A broad yellow emission peak was also observed at around 550 nm, in the case of the TEG-grown GaN. This yellow emission can be attributed to an increase in the concentration of nitrogen vacancy-related complexes or extended defects in the TEGa-grown GaN. This situation led also to an increase in the background electron concentration and a rough surface, compared with those of TMG-grown GaN.

Gas source molecular beam epitaxy of high quality AlxGa1−xN (0⩽x⩽1) on Si(111)

Layers of AlxGa1−xN, with 0⩽x⩽1, were grown on Si(111) substrates by gas source molecular beam epitaxy with ammonia. We show that the initial formation of the Si–N–Al interlayer between the Si substrate and the AlN layer, at a growth temperature of 1130–1190 K, results in very rapid transition to two-dimensional growth mode of AlN. The transition is essential for subsequent growth of high quality GaN, AlxGa1−xN, and AlGaN/GaN superlattices. The undoped GaN layers have a background electron concentration of (2–3)×1016 cm−3 and mobility up to (800±100) cm2/V s, for film thickness ∼2 μm. The lowest electron concentration in AlxGa1−xN, with 0.2<x<0.6, was ∼(2–3)×1016 cm−3 for 0.5–0.7-μm-thick film. Cathodoluminescence and optical reflectance spectroscopy were used to study optical properties of these AlxGa1−xN layers. We found that the band gap dependence on composition can be described as Eg(x)=3.42+1.21x+1.5x2. p–n junctions have been formed on crack-free layers of GaN with the use of Mg dopant. Light emitting diodes with peak emission wavelength at 3.23 eV have been demonstrated.

High-quality GaN grown by gas-source MBE

High-quality GaN epilayers were consistently obtained using a home-made gas-source MBE system on sapphire substrates. Room-temperature electron mobility of the grown GaN film is 300 cm 2/V s with a background electron concentration as low as 2×10 17 cm −3. The full-width at half-maximum of the GaN (0 0 0 2) double-crystal X-ray rocking curve is 6 arcmin. At low temperature (3.5 K), the FWHM of the near-band-edge photoluminescence emission line is 10 meV. Furthermore, using piezoelectric effect alone with the high-quality films, two-dimensional electron gas was formed in a GaN/AlN/GaN/sapphire structure. Its room-temperature and low-temperature (77 K) electron mobility is 680 cm 2/V s and 1700 cm 2/V s, and the corresponding sheet electron density is 3.2×10 13 and 2.6×10 13 cm −2, respectively.

Hydrogen behavior in GaN epilayers grown by NH 3-MBE

Hydrogen behavior in unintentionally doped GaN epilayers on sapphire substrates grown by NH 3-MBE is investigated. Firstly, we find by using nuclear reaction analysis (NRA) that with increasing hydrogen concentration the background electron concentration increases, which suggests that there exists a hydrogen-related donor in undoped GaN. Secondly, Fourier transform infrared (FTIR) absorption and X-ray photoelectron spectroscopy (XPS) reveal further that hydrogen atom is bound to nitrogen atom in GaN with a local vibrational mode at about 3211 cm −1. Hence, it is presumed that the hydrogen-related complex Ga … H–N is a hydrogen-related donor candidate partly responsible for high n-type background commonly observed in GaN films. Finally, Raman spectroscopy results of the epilayers show that in addition to the expected compressive biaxial strain, in some cases GaN films suffer from serious tensile biaxial strain. This anomalous behavior has been well interpreted in terms of interstitial hydrogen lattice dilation.

GaN layer growth optimization for high power devices

In this work, a novel method for GaN layer optimization — statistical multi-parameter design of experiments (DOE) — is presented. According to the statistical model obtained, increasing the buffer layer V/III ratio is beneficial for minimizing the full width at half maximum (FWHM) of the X-ray diffraction rocking curve for the (002) reflection. Statistical models were also obtained for background electron concentration and room temperature Hall mobility, but further data analysis and electrical measurements lead us to the conclusion that those models are disturbed by the presence of a highly conductive layer near the GaN/sapphire interface. Inclusion of an Al x Ga (1− x) N isolation layer results in a reduction of two orders of magnitude in the measured background concentration, as well as a significant increase in Hall mobility, without degradation of the crystalline quality.

Dissociation of Al2O3(0001) substrates and the roles of silicon and oxygen in n-type GaN thin solid films grown by gas-source molecular beam epitaxy

Unintentionally doped and silicon doped GaN films prepared by molecular beam epitaxy using ammonia are investigated. Hall, secondary ion mass spectroscopy (SIMS), photoluminescence, and x-ray data are utilized for analysis of sources of autodoping of GaN epitaxial films in an effort to identify whether the n-type background electron concentration is of impurity origin or native defect origin. We identify and quantify an anomalous relationship between the Si doping concentration and free carrier concentration and mobility using temperature dependent Hall measurements on a series of 2.0-μm-thick GaN(0001) films grown on sapphire with various Si doping concentrations. SIMS is used to identify oxygen as the origin of the excess free carriers in lightly doped and undoped GaN films. Further, the source of the oxygen is positively identified to be dissociation of the sapphire substrate at the nitride-sapphire interface. Dissociation of SiC at the nitride-carbide interface is also observed. Finally, SIMS is again utilized to show how Si doping can be utilized to suppress the diffusion of the oxygen into the GaN layer from the sapphire substrate. The mechanism of suppression is believed to be formation of a Si–O bond and a greatly reduced diffusion coefficient of the subsequent Si–O complex in GaN.

Effects of growth temperature on Mg-doped GaN epitaxial films grown by plasma-assisted molecular beam epitaxy

A series of Mg-doped GaN films were grown by plasma-assisted molecular beam epitaxy at different temperatures and the resulting surface morphology, crystallinity, and electrical properties were examined. Although the films were grown under N-rich conditions which usually do not give rise to high quality films, very smooth surfaces were obtained at high temperatures (Ts⩾650 °C) when doped with Mg. From the information on grain size measured with an atomic force microscope, the activation energy for Ga diffusion was determined to be ∼1.0 eV. This low value is considered to be responsible for promoting the diffusion of Ga atoms on the growing surface and facilitating two-dimensional growth at high temperatures. It was found, with no surprise, that the concentration of Mg incorporated into the film depends on the growth temperature and that the type of electrical conduction in the films is determined by the competition between the background electron concentration and Mg-doping. Mg-doped GaN films grown at temperatures between 650 °C and 700 °C exhibited the p-type electrical properties with smooth surfaces (root-mean-square roughness ∼2 nm) and good crystallinity (full width at half maximum ⩽20 arcmin).

Gas source molecular beam epitaxy of high quality AlGaN on Si and sapphire

ABSTRACTWe report the results of epitaxial growth experiments on AlxGa1−xN (0≤ x ≤ 1) on Si(111) and sapphire substrates aimed at understanding the origin and elimination of cracking. We describe growth procedures resulting in thick layers of AlxGa1−xN, grown by gas source molecular beam epitaxy with ammonia, that are free of cracks. In GaN layers with the thickness of ∼2.5 µm, we find the background electron concentration of (1-2)×1016 cm−3 and mobility of (800±100) cm2/Vs. In AlxGa1−xN (0.2 < x < 0.6) with the film thickness of 0.5-0.7 µm the electron concentration of (2-3)×1016 cm−3 is obtained. Low background concentrations in GaN allow for formation of p-n junctions by doping with Mg. Light emitting diodes with the peak emission at 380 nm have been demonstrated.

Multiparameter Statistical Design of Experiments for GaN Growth Optimization

A statistical multi-parameter Design of Experiments for growth optimization of GaN is presented. According to the obtained statistical model, increasing the buffer layer V/III ratio is beneficial for minimizing the FWHM of the X-ray diffraction rocking curve for the (002) reflection. Statistical models were obtained also for background electron concentration and Hall mobility, but further electrical measurements lead us to the conclusion that those models are disturbed by the presence of a highly conductive layer near the low temperature buffer layer. Inclusion of an AlxGa1—xN isolation layer shows a reduction by two orders of magnitude in the measured background concentration, as well as a significant increase in Hall mobility, without degradation of the crystalline quality.

Growth and Characterization of High-Quality InAs0.86Sb0.05P0.09 Alloy by Liquid-Phase Epitaxy

High-quality InAs0.86Sb0.05P0.09 epitaxial layers lattice-matched to InAs substrates were grown by liquid-phase epitaxy employing a supercooling technique. The epitaxial layers exhibit a shiny and mirror-like surface morphology and a flat interface. The energy gap of InAs0.86Sb0.05P0.09 epitaxial layers is 0.4 eV estimated by Fourier-transform infrared reflectance. The InAs0.86Sb0.05P0.09 diodes have an ideality factor of 1.1 and a breakdown voltage of 3.5 V. The background electron concentration, measured by capacitance-voltage measurements, is 1.1×1016 cm-3 for the undoped InAsSbP layers.