Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

GaN community has recently recognized that it is imperative that the extended, and point defects in GaN and related materials, and the mechanisms for their formation are understood. This is a first and an important step, which must be followed by defect reduction before full implementation of this material and its allied binaries/ternaries in devices. This review is based on a recent concerted effort to establish benchmarks as far as defects are concerned, and identify the basic issues involved. Samples were analyzed for extended defects by TEM and chemical etches, for polarity by electric force microscopy and convergent beam electron diffraction (CBED), for point defects by DLTS, for optical quality and deep defects by photoluminescence (PL), for vacancies by positron annihilation, for donor and acceptor like states within the gap by ODMR and EPR, and for carrier transport targeted for defects and impurities by variable temperature and magnetic field-dependent Hall measurements. Hydride VPE samples grown at Lincoln Laboratories with 1.5, 5.5 and 55 μm thicknesses were investigated for defects by TEM, and their polarity was found to be Ga-polarity, as expected, by CBDE combined with simulations. The density of misfit dislocations at the substrate/EPI interface was determined to be on the order of 10 13 cm -2 based on high-resolution electron microscopy images. The threading dislocation density decreased gradually with distance from the interface, reaching a value of about 10 8 cm -2 at the surface of a 55 μm film. A 200 μm thick laser separated and free-standing HVPE grown GaN template grown at Samsung was also characterized similarly. The free surface and substrate sides were confirmed to be Ga- and N-polarity, respectively. The density of dislocations near the N-face was determined to be, in order, (3 ± 1) x 10 7 and (4 ± 1) x 10 7 by cross-sectional TEM and plan-view TEM, respectively. Identical observations on the Ga-face revealed the defect concentration to be less than 1 x 10 7 cm -2 by plan-view TEM and 5 x 10 5 cm -2 by cross-sectional TEM. Defects in a 10 μm thick GaN layer grown by HVPE at Lincoln Laboratory have been investigated by photo electrochemical (PEC) etching, and by wet etching in hot H 3 PO 4 acid and molten KOH. Threading vertical wires (i.e. whiskers) and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic force microscopy, one finds the density of whisker-like features to be 2 x 10 9 cm -2 , the same value found for the etch-pit density on samples etched with both H 3 PO 4 and molten KOH. Values agree well with TEM results. A free standing GaN template has been characterized for its structural and optical properties using X-ray diffraction, defect delineation etch followed by imaging with atomic force microscopy (AFM). The Ga-face and the N-face of the c-plane GaN exhibited a wide variation in terms of the defect density. The defect concentrations on Ga- and N-faces were about 5 x 10 5 cm -2 for the former and about 1 x 10 7 cm -2 for the latter, again in good agreement with TEM results mentioned above. High resolution X-ray rocking curves (omega scans) were measured. The [0 0 2] symmetric and [104] asymmetric peaks in 10 μm thick HVPE films had FWHM values between 5.8 and 7.9 arcmin, and 3.9 and 5.2 arcmin, respectively. The Samsung template investigated had wide diffraction peaks (20.6 and 24 arcmin for [0 0 2] and [1 0 4] diffractions, respectively) on the Ga-face, similar for the N-face, when a 2 mm slit size was used. When the slit size was reduced to 0.02 mm, the Ga- and N-face [0 0 2] peaks reduced to 69 and 160 arcsec. A bowing radius of 1.2 m was calculated to account for increased broadening with wider slits.

Comprehensive characterization of hydride VPE grown GaN layers and templates

We fabricated high standoff voltage (450 V) Schottky rectifiers on hydride vapor phase epitaxy grown GaN on sapphire substrate. Several Schottky device geometries were investigated, including lateral geometry with rectangular and circular contacts, mesa devices, and Schottky metal field plate overlapping a SiO2 layer. The best devices were characterized by an ON-state voltage of 4.2 V at a current density of 100 A/cm2 and a saturation current density of 10^–5 A/cm2 at a reverse bias of 100 V. From the measured breakdown voltage we estimated the critical field for electric breakdown in GaN to be (2.2 ± 0.7) × 10^6 V/cm. This value for the critical field is a lower limit since most of the devices exhibited abrupt and premature breakdown associated with corner and edge effects.

/pdf/high-voltage-450-v-gan-schottky-rectifiers-53qrztaqeq.pdf

High voltage (450 V) GaN Schottky rectifiers

Defects may cause compensation or act as recombination centers in Mg-doped GaN. Using hybrid functional calculations, we investigate the effects of nitrogen vacancies (VN) and Mg-vacancy complexes (MgGa-VN) on the electrical and optical properties of GaN. We find that MgGa-VN are compensating centers in p-type but electrically inactive in n-type GaN. They give rise to a broad emission at 1.8 eV, explaining the red luminescence that is frequently observed in Mg-doped GaN, regardless of the Fermi level. Nitrogen vacancies are also compensating centers in p-type GaN and likely contribute to the yellow luminescence that has been observed in Mg-doped GaN.

/pdf/role-of-nitrogen-vacancies-in-the-luminescence-of-mg-doped-4gq2k2r2yh.pdf

Role of nitrogen vacancies in the luminescence of Mg-doped GaN

Defects in GaN layers grown by hydride vapor-phase epitaxy have been investigated by photoelectrochemical (PEC) etching, and by wet etching in hot H3PO4 acid and molten potassium hydroxide (KOH). Threading vertical wires (i.e., whiskers) and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic-force microscopy, we find the density of “whisker-like” features to be 2×109 cm−2, the same value found for the etch-pit density on samples etched with both H3PO4 and molten KOH. This value is comparable to the dislocation density obtained in similar samples with tunneling electron microscopy, and is also consistent with the results of Youtsey and co-workers [Appl. Phys. Lett. 73, 797 (1998); 74, 3537 (1999)].

/pdf/dislocation-density-in-gan-determined-by-1sirk1hc5y.pdf

Dislocation density in GaN determined by photoelectrochemical and hot-wet etching

The electronic and structural properties of GaN were investigated for heteroepitaxial layers grown by hydride vapor phase epitaxy. Uniform film nucleation on the sapphire substrates was facilitated by a GaCl pretreatment. The films were all unintentionally doped n type. Variable temperature Hall effect measurements reveal electron concentrations as low as 2×1017 cm−3 and electron mobilities as high as 460 cm2/V s at 300 K. The films exhibit bound exciton photoluminescence lines with a full width at half‐maximum (FWHM) of 2.42 meV at 2 K. Transmission electron microscopy studies of the GaN/sapphire interface reveal a ∼200 nm thick, highly defective GaN layer consisting predominantly of stacking faults. The excellent quality of these GaN films is attributed to this ‘‘auto‐buffer’’ layer which enables growth of GaN cells with a dislocation density of ∼3×108 cm−2 after ∼12 μm of film growth.

Electronic and structural properties of GaN grown by hydride vapor phase epitaxy

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Electronic and structural properties of GaN grown by hydride vapor phase epitaxy