P-type GaN Films Research Articles

Effect of growth temperature on the properties of p-type GaN grown by plasma-assisted molecular beam epitaxy

p-type GaN films were grown at two different temperatures, 650 and 700°C, by plasma-assisted molecular beam epitaxy (PAMBE) using a radio frequency (RF) plasma source. High hole concentrations of the order of 10 18 cm −3 were achieved for both films without any post-growth treatment. For (0 0 0 2) diffraction from the p-type films grown at 700°C, the full-width at half-maximum (FWHM) of the double-crystal X-ray rocking curve was 7.8 arcmin, the smallest ever reported for p-type GaN films grown on sapphire substrates. The room-temperature photoluminescence (PL) measurements on both films showed that band edge emission at 365 nm dominated the PL spectra of the films grown at 700°C, while emission at 413 nm, associated with deep Mg complexes, dominated those of the films grown at 650°C. The films grown at 700°C showed better crystalline quality and optical properties than those grown at 650°C.

Deep levels and persistent photoconductivity in GaN thin films

Photocurrent decay in GaN thin films was studied in the time span from a few seconds to several days. The persistent photoconductivity (PPC) behavior was observed not only in Mg-doped p-type GaN films but also in undoped n-type GaN films. The photoconductivity spectra and the photocurrent response time were measured using a weak probe light at several times after the samples had been kept in the dark. During the relaxation, the photocurrent due to the subband-gap probe light decreased more than the photocurrent due to the UV probe light. It is suggested that metastable centers at 1.1, 1.40, and 2.04 eV above the valence band edge are responsible for the PPC behavior in Mg-doped GaN, and that Ga vacancy is the candidate for PPC effect in n-type GaN.

New dopant precursors for n-type and p-type GaN

Tetraethylsilane (TeESi) and bis(ethylcyclopentadienyl)Mg (ECp2Mg) were employed as Si and Mg dopant precursors for MOVPE growth of n-type and p-type GaN films, respectively. In Si doping, the electron concentration was observed to increase with the increase of the TeESi flow rate. The temperature dependence of the Hall mobility showed good agreement with n-type GaN films grown using different dopant precursors (SiH4, GeH4, Si2H6). The donor activation energy was estimated to be 27 meV, which is almost the same as the literature values. In Mg doping, we also found that the Mg concentration increases as the ECp2Mg flow rate increases. All of Mg-doped samples in this study showed p-type conduction after annealing. The acceptor activation energy was estimated to be 170 meV, which was close to the reported values.

Photoluminescence of Strain-Engineered MBE-Grown GaN and InGaN Quantum Well Structures

ABSTRACTThe paper describes the influence of strain on the optical quality of GaN films grown by MBE on c-plane sapphire. The photoluminescence (PL) line width of the donor-bound exciton can be designed to be as narrow as 1.2 meV by actively utilizing hydrostatic and biaxial stress components. Unstrained p-type Mg-doped GaN films exhibit comparably narrow near band edge transitions. A sharp PL line at 3.261 eV in some of our films is identified as the donor bound exciton of the cubic phase. The formation of these cubic inclusions can be stimulated by a high III/V flux ratio at the growth temperature of T = 725°C. The PL spectrum of an InGaN multi quantum well structure is significantly broadened compared with the spectra of single quantum well structures. Combination of PL and TEM indicates that this effect relates to a progressive increase of the quantum well widths and their spacing along the growth direction. It is argued that strain affects the growth rate and the incorporation of Indium into the quantum well structures.

Optical characteristics of p-type GaN films grown by plasma-assisted molecular beam epitaxy

Using a molecular beam epitaxy system equipped with an inductively coupled radio frequency nitrogen plasma source, p-type GaN films were grown on sapphire substrates with no postgrowth treatment. Uniformity of the surface morphology and spatial homogeneity of the luminescence of the films were investigated using scanning electron microscopy and cathodoluminescence (CL) imaging, respectively. By examining the dependence of photoluminescence on the excitation laser power density at 6 and 300 K, three different emissions having different origins were identified. A blue emission at ∼3.25 eV is associated with shallow Mg impurities, while two different lower-energy emissions at ∼2.43 and ∼2.87 eV are associated with deep Mg complexes. The spatial distributions of the shallow and deep Mg impurities dominating the optical properties of the p-type GaN films were also examined along the growth direction by low- and room-temperature CL using an electron beam with a range of penetration depths

Changes in surface composition of GaN by impurity doping

Changes in the surface composition of GaN films by p- and n-type doping were studied using both Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). It was found that the surface composition of the GaN films was affected by impurity doping, i.e. the surface of the p-type GaN film was enriched with gallium and that of the n-type GaN film with nitrogen, compared with that of the undoped GaN film. The surface composition of the GaN films by AES agrees with that by XPS after taking the contamination layer thickness into account. The experimental results thus obtained were discussed by taking account of the formation energies of native defects in p- and n-type GaN.

Growth of SiC and III–V nitride thin films via gas-source molecular beam epitaxy and their characterization

Silicon carbide (SiC) and aluminum nitride (AlN) thin films have been grown on 6HSiC(0001) substrates by gas-source molecular beam epitaxy (GSMBE) at 1050°C. Step flow, step bunching and the deposition of 6HSiC occurred at the outset of the exposure of the (1 × 1) vicinal substrate surface to C 2 H 4 Si 2 H 6 gas flow ratios of 1, 2 and 10. Subsequent deposition resulted in step flow and continued growth of 6H films or formation and coalescence of 3CSiC islands using the gas flow ratio of one or the ethylene-rich ratios, respectively. The (3 × 3) surface reconstruction observed using the former ratio is believed to enhance the diffusion lengths of the adatoms, which in turn promotes step flow growth. Essentially atomically flat monocrystalline AlN surfaces were obtained using on-axis substrates. Island-like features were observed on the vicinal surface. The coalescence of the latter features at steps gave rise to inversion domain boundaries (IDBs) as a result of the misalignment of the Si C bilayer steps with the AlN bilayers in the growing film. The quality of thicker AlN films is strongly influenced by the concentration of IDBs. Undoped, highly resistive (10 2 Ω · cm) and Mg-doped, p-type (0.3 Ω · cm) monocrystalline GaN films having a thickness of 0.4–0.5 μm have also been grown via the same technique on AlN buffer layers without post-processing annealing.

High Quality P-Type Gan Films Grown By Plasma-Assistedmolecular Beam Epitaxy

Abstractp-type GaN films were grown on a (0001) sapphire substrate by the plasma-assisted molecular beam epitaxy. A low-contamination, high-power efficiency inductively coupled radio frequency plasma source was used, which was developed at the University of Illinois. Using an MBE system equipped with this plasma source, high-quality p-type GaN films were grown without post-growth treatment. X-ray rocking curve measurements for (0002) diffraction showed a full width at half maximum of less than 7 arcmin. The highest room-temperature hole concentration obtained was 1.4× 1020 cm−3, and for the same sample, the mobility was 2.5 cm2/Vs It is believed that the Mott-Anderson transition occurred in this sample resulting in a metallictype conductivity in the impurity subband. Low-temperature photoluminescence had a blue emission band and no deep-level transitions, indicating the high quality of the grown films. Uniformity of the Mg doping was confirmed by secondary ion mass spectrometry.

High quality GaN grown at high growth rates by gas-sourcemolecular beam epitaxy

High quality GaN has been grown by gas-source molecular beam epitaxy (MBE) using ammonia as the nitrogen source. A growth rate as high as 1 /spl mu/m/h, which is an order of magnitude higher than previously reported for the growth of GaN by MBE, has been achieved. Strong reflection high-energy electron diffraction (RHEED) intensity oscillations have been observed during the growth, making in situ thickness monitor and control as thin as one monolayer possible. The undoped GaN demonstrated an unintentional n-type carrier density of 2/spl times/10/sup 17/cm/sup -3/ and an electron Hall mobility of 110 cm/sup 2//V.s, the best ever achieved by MBE. For Mg-doped p-type GaN films a hole density of 4/spl times/10/sup 17/ cm/sup -3/ and hole mobility of 15 cm/sup 2//V.s were achieved without post-growth annealing. Low temperature photoluminescence of as-grown materials was dominated by band-edge emissions, indicative of high quality materials which are promising for applications to blue light emitters and high-temperature electronic devices.

Shallow and Deep Level Defects in GaN

ABSTRACTShallow and deep electronic defects in MOCVD-grown GaN were characterized by variable temperature Hall effect measurements, deep level transient spectroscopy (DLTS) and photoemission capacitance transient spectroscopy (O-DLTS). Unintentionally and Si-doped, n-type and Mg-doped, p-type GaN films were studied. Si introduces a shallow donor level into the band gap of GaN at ∼Ec - 0.02 eV and was found to be the dominant donor impurity in our unintentionally doped material. Mg is the shallowest acceptor in GaN identified to date with an electronic level at ∼Ev + 0.2 eV. With DLTS deep levels were detected in n-type and p-type GaN and with O-DLTS we demonstrate several deep levels with optical threshold energies for electron photoemission in the range between 0.87 and 1.59 eV in n-type GaN.

P-type Mg-doped GaN grown by molecular beam epitaxy using ammonia as the nitrogen source

ABSTRACTThe electrical and luminescent properties of Mg-doped GaN films grown by molecular beam epitaxy (MBE) using ammonia as the nitrogen source have been investigated. Due to their different growth environments, the Mg-doped GaN films grown by MBE using ammonia exhibited properties that were different from similar films grown by metal-organic chemical vapor deposition (MOCVD). It has been found that the introduction of positive charges during growth is important in the achievement of p-type Mg-doped GaN grown by MBE using ammonia. With the introduction of a moderate nitrogen plasma, we have achieved p-type Mg-doped GaN films with a hole density of 4×1017 cm−3 and a mobility of 15 cm2/V-s at room temperature.

Schottky barrier photodetector based on Mg-doped p-type GaN films

In this letter we report the fabrication and characterization of Schottky barrier photodetectors on p-type GaN films. These films were grown over basal plane sapphire substrates using low pressure metalorganic chemical vapor deposition and magnesium as the p-type dopant. The current-voltage and capacitance-voltage characteristics were measured for Ti/Au Schottky barriers for a film with a p doping of 7×1017 cm−3. We measured a 1.5 V forward turn on and a 3 V reverse breakdown. The zero bias responsivity of a detector with 1 mm2 area was measured to be 0.13 A/W. For these photovoltaic detectors, the photoresponse was nearly constant from 200 to 365 nm and fell sharply by several orders of magnitude for wavelengths above 365 nm.

Deposition of highly resistive, undoped, and p-type, magnesium-doped gallium nitride films by modified gas source molecular beam epitaxy

Undoped, highly resistive (≳102 Ω cm) and Mg-doped, p-type (0.3 Ω cm) monocrystalline GaN films with a thickness of (4–5)×103 Å have been achieved via modified gas source molecular beam epitaxy on AIN buffer layers without the post-processing procedures of either electron beam irradiation or annealing necessary in chemical vapor deposition (CVD) derived films. The carrier concentrations and mobilities could not be accurately measured in the undoped films; values to 1018 cm−3 and 10 cm2/V s, respectively, have been achieved in the p-type films. These results indirectly support the hypothesis of Mg-H complexes as the cause of the difficulty in achieving highly conducting p-type materials using CVD techniques.

Hole Compensation Mechanism of P-Type GaN Films

Low-resistivity p-type GaN films, which were obtained by N2-ambient thermal annealing or low-energy electron-beam irradiation (LEEBI) treatment, showed a resistivity as high as 1×106 Ω·cm after NH3-ambient thermal annealing at temperatures above 600°C. In the case of N2-ambient thermal annealing at temperatures between room temperature and 1000°C, the low-resistivity p-type GaN films showed no change in resistivity, which was almost constant between 2 Ω·cm and 8 Ω·cm. These results indicate that atomic hydrogen produced by NH3 dissociation at temperatures above 400°C is related to the hole compensation mechanism. A hydrogenation process whereby acceptor-H neutral complexes are formed in p-type GaN films was proposed. The formation of acceptor-H neutral complexes causes hole compensation, and deep-level and weak blue emissions in photoluminescence.