Thin GaN Epilayers Research Articles

Crystal quality and surface structure tuning of semi-polar (11–22) GaN on m-plane sapphire via in-situ multiple ammonia treatment

The crystal quality and morphological properties of semi-polar (11–22) gallium GaN were enhanced by introducing the In-Situ Multiple Ammonia Treatment (I-SMAT) method. On- and off-axis x-ray rocking curve (XRC) analysis reveals that I-SMAT would reduce several types of crystal defect namely prismatic stacking faults, basal stacking fault type I and II, and partial/perfect dislocations. By implementing an optimized flow of ammonia (NH3) and number of treated GaN pairs, the arrowhead-like features of semi-polar (11–22) GaN was effectively reduced the root mean square roughness from 4.52 to 3.07 nm. High atomic force microscopy (AFM) magnification scan demonstrated the alteration of the grain structure on the surface to become more compact and orderly arranged resulting in narrowing/shallowing of the interfacial valleys between grains. Great influence of employed I-SMAT pairs and NH3 flux was observed from XRC and AFM analysis. Conversely, excessive NH3 flux during this approach would roughen the structural and morphological properties of the semi-polar epilayer whereby the alternating thin GaN epilayer would undergo the selective-area etching to the extreme.

Erbium doped GaN synthesized by hydride vapor-phase epitaxy

New and improved gain materials for solid-state high power and high energy lasers are highly sought. GaN satisfies many of the criteria of an excellent lasing gain medium including its higher thermal conductivity and lower thermal expansion coefficient than traditional gain materials such as yttrium aluminum garnet (YAG) crystals doped with neodymium emitting 1.06 μm wavelength. Single crystals of erbium doped GaN (Er:GaN) thick layers have been successfully synthesized by hydride vapor-phase epitaxy. By utilizing a thin GaN epilayer grown on c-plane sapphire using metal organic chemical vapor deposition as a template and varying growth parameters including the NH3 flow rate, thick layers of Er:GaN which exhibit the desired Er3+ related emission at 1.54 μm window at room temperature have been realized for the first time. The work opens up the feasibility to utilize Er:GaN bulk crystals as a gain medium for next generation solid-state high power and high energy lasers operating in the eye-safer 1.5 μm window.

Thermal Properties of AlGaN/GaN HFETs on Bulk GaN Substrates

Micro-Raman thermography, microphotoluminescence spectroscopy, and thermal simulation were used to study the thermal properties of AlGaN/GaN heterostructure field-effect transistors grown on semi-insulating bulk GaN substrates. A bulk GaN thermal conductivity of 260 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{W}\cdot\hbox{m}^{-1} \cdot\hbox{K}^{-1}$</tex></formula> was determined from temperature measurements on operating devices in combination with finite-difference thermal simulations. This is significantly higher than typical thin GaN epilayer thermal conductivities, due to a lower dislocation density in bulk GaN. Despite the thermal conductivity of bulk GaN being lower than that of SiC, transistors on bulk GaN exhibited a similar thermal resistance as GaN-on-SiC devices, attributed to the absence of a thermal boundary resistance between the device epilayers and substrate for GaN-on-GaN devices.

Investigation of different mechanisms of GaN growth induced on AlN and GaN nucleation layers

The evolution of GaN growth on AlN and GaN nucleation layers is compared through morphological and structural analyses, including ion beam analysis. By using AlN nucleation layer grown at high temperature, improved crystalline quality is exhibited by 300 nm thin GaN epilayers. GaN (002) x-ray rocking curve as narrow as 168 arc sec and atomic-step surface morphology characterize such a thin GaN film on AlN. Defects are strongly confined into the first 50 nm of growth, whereas a fast laterally coherent growth is observed when increasing thickness, as an effect of high temperature AlN surface morphology and Ga adatom dynamics over this template.

Defect reduction in GaN epilayers grown by metal-organic chemical vapor deposition with in situ SiNx nanonetwork

Line and point defect reductions in thin GaN epilayers with single and double in situ SiNx nanonetworks on sapphire substrates grown by metal-organic chemical vapor deposition were studied by deep-level transient spectroscopy (DLTS), augmented by x-ray diffraction (XRD), and low temperature photoluminescence (PL). All samples measured by DLTS in the temperature range from 80to400K exhibited trap A (peak at ∼325K) with an activation energy of 0.55–0.58eV, and trap B (peak at ∼155K) with an activation energy of 0.21–0.28eV. The concentrations of both traps were much lower for layers with SiNx nanonetwork compared to the reference sample. The lowest concentration was achieved for the sample with 6min deposition SiNx nanonetwork, which was also lower than that for a sample prepared by conventional epitaxial lateral overgrowth. In concert with the DLTS results, PL and XRD linewidths were reduced for the samples with SiNx network indicating improved material quality. Consistent trend among optical, structural, and DLTS results suggests that SiNx network can effectively reduce both point and line defects.

Direct AFM Observation of Strain Effects on MOCVD-Grown GaN Epilayer Surface Morphology

ABSTRACTIn this study, we investigate the dependence of GaN surface morphology on the absolute strain values for thin (&lt;10 μm) epitaxial films grown by MOCVD on c-plane sapphire substrates of various miscut angles towards the m-plane. Results indicate an excellent correlation between the surface roughness observed employing an AFM tool and epilayer strain values. An overall increase of surface roughness (decrease of atomic terrace width) is found with decreasing compressive strain (epilayer vs. bulk value). In addition, sapphire substrates with increasing miscut angle (0.30 deg) appear to relax the inherent, built-in strain differently in the vertical (growth) direction when compared to just (0.00 deg) substrates. Strain relaxation by typical V-shaped, hexagonal pits is directly imaged through the comparison of surface features inside and outside of pits in the thin GaN epilayer films.

Effects of internal electric field and carrier density on transient absorption spectra in a thin GaN epilayer

Carrier dynamics in a thin GaN epilayer with a thickness of $0.15 \ensuremath{\mu}\mathrm{m}$ almost coherently grown on ${\mathrm{Al}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$ was investigated using transient absorption spectroscopy above and below the band gap. The absorption peak of excitons was observed at 3.521 eV without a pump beam. Just after the photoexcitation above the band gap, the bleaching occurred and the absorption peak of excitons disappeared due to high-density photogenerated carriers. At about 30 ps, the absorption peak was restored at 3.527 eV, as the carriers were decreased owing to radiative and/or nonradiative recombination. After that, the absorption spectra were larger than that without a pump beam. This is because excitonic absorption suppressed by both Franz-Keldysh and Stark effects is restored by photoinduced carriers as a result of the screening of the internal electric field.

Structural properties of GaN epilayers directly grown on on-axis 6H-SiC(0 0 0 1) by plasma-assisted MBE

We report on the structural properties of a series of thin GaN epilayers directly grown on on-axis 6H-SiC(0 0 0 1) by plasma-assisted MBE. X-ray measurements show that the crystalline perfection of the layers steadily improves with film thickness. In fact, the density of threading defects detected by transmission electron microscopy is found to decrease drastically with the distance away from the GaN/SiC interface, finally reaching a value of less than 5×10 9 cm −2 at a layer thickness of 0.5 μm. The formation mechanisms of the threading dislocations in the GaN films are discussed in consideration of the specific GaN/SiC interface structure.

Structural characterization of thin GaN epilayers directly grown on on-axis 6H–SiC(0001) by plasma-assisted molecular beam epitaxy

The structural properties of a series of thin (0.2–0.6 μm) GaN epilayers directly grown on 6H–SiC(0001) by plasma-assisted molecular beam epitaxy are studied. X-ray reciprocal space maps of the GaN(0002) reflection reveal negligible inhomogeneous strain within the layer but a comparatively large orientational spread of the GaN c axis. X-ray rocking curve measurements show, however, that this mosaicity steadily decreases with film thickness. In fact, the density of threading defects detected by transmission electron microscopy is found to decrease drastically with the distance away from the GaN/SiC interface, finally reaching a value of less than 5×109 cm−2 at a layer thickness of 0.5 μm. The formation mechanisms of the threading dislocations in the GaN films are discussed in consideration of the specific GaN/SiC interface structure.

Temperature-dependent absorption measurements of excitons in GaN epilayers

Optical absorption measurements were performed on a series of thin GaN epilayers. Sharp spectral features were observed due to the 1s A and B exciton transitions. Using polarization dependent absorption, the C exciton transition was identified. A broad absorption feature was observed at ∼3.6 eV, which is attributed to indirect exciton-phonon absorption. The excitonic structure was found to persist well above room temperature. A fit to the Varshni formula yielded a temperature dependence of E(T)=E(T=0)−11.8×10−4T2(1414+T) eV for the A and B excitons. The exciton absorption linewidth was studied as a function of temperature, indicating that GaN exhibits very large exciton-phonon coupling.

Luminescence and absorption of GaN films under high excitation

The samples studied are thin GaN epilayers grown by metalorganic vapor phase epitaxy (MOVPE). At liquid helium temperature, the linear luminescence, transmission and reflection spectra of the films has been recorded, as well as the nonlinear emission spectra under high UV excitation. By using the variable strip length method, we have measured the gain coefficient as a function of the excitation intensity and the temperature. Pump and probe experiments have been performed in the femtosecond regime, showing crossed two-photon absorption without long lasting response. This allows to envisage the use of GaN for the characterization femtosecond pulses in the blue spectral range.

Far infrared spectroscopy of thin epitaxial layers of GaN deposited by molecular beam epitaxy on GaP substrates

The far infrared optical properties of a selection of thin GaN epilayers on GaP substrates have been investigated at room temperature by oblique incidence reflection spectroscopy in p- and s-polarization. The GaN layers were deposited in the cubic, wurtzite, or mixed (cubic and wurtzite) phase by molecular beam epitaxy. The GaN and GaP reststrahlen regions are well separated in frequency and in each case a good theoretical fit to the measured spectrum is obtained. The model uses damped simple harmonic oscillators to describe the dielectric response of the phonons in the two materials, and the frequencies of TO and LO phonons in the GaN layers are determined from the model fit. The frequency of an interference feature in the GaP reststrahl region provides a useful probe of the film thickness, and the feature is found to change after cooling the sample to 77 K, indicating that the films are not stable. A weak GaAs-like feature, which correlates well with the use of As during growth, is seen in the mixed phase sample.