We have performed a detailed investigation of the photoluminescence pressure dependence of heteroepitaxial GaN thin films on sapphire substrates. A comparison between as grown GaN on sapphire and free-standing GaN membranes, created using a laser assisted substrate liftoff process, revealed that the presence of the sapphire substrate leads to an energy gap pressure coefficient reduction of approximately 5%. This result agrees with the numerical simulations presented in this article. We established that the linear pressure coefficient of free-standing GaN is 41.4±0.2 meV/GPa, and that the deformation potential of the energy gap is −9.36±0.04 eV. Our results also suggest a new, lower value of the pressure derivative for the bulk modulus of GaN (B′=3.5).

The energies of photo-and electroluminescence transitions in In x Ga 1Ϫx N quantum wells exhibit a characteristic ''blueshift'' with increasing pumping power. This effect has been attributed either to band-tail filling, or to screening of piezoelectric fields. We have studied the pressure and temperature behavior of radiative recombination in In x Ga 1Ϫx N/GaN quantum wells with xϭ0.06, 0.10, and 0.15. We find that, although the recombination has primarily a band-to-band character, the excitation-power induced blueshift can be attributed uniquely to piezoelectric screening. Calculations of the piezoelectric field in pseudomorphic In x Ga 1Ϫx N layers agree very well with the observed Stokes redshift of the photoluminescence. The observed pressure coefficients of the photoluminescence ͑25-37 meV/GPa͒ are surprisingly low, and, so far, their magnitude can only be partially explained.

We have studied the pressure and temperature dependence of the absorption edge of a 4-μm-thick layer of the alloy Ga0.92In0.08As0.985N0.015. We have measured the hydrostatic pressure coefficient of the energy gap of this alloy to be 51 meV/GPa, which is more than a factor two lower than that of GaAs (116 meV/GPa). This surprisingly large lowering of the pressure coefficient is attributed to the addition of only ∼1.5% nitrogen. In addition, the temperature-induced shift of the edge is reduced by the presence of nitrogen. We can explain this reduction by the substantial decrease of the dilatation term in the temperature dependence of the energy gap.

A correlation of the local indium concentration measured on an atomic scale with luminescence properties of In x Ga 1-x N quantum wells reveals two different types of recombination mechanisms. A piezoelectric-field based mechanism is shown to dominate in samples with thick wells (L > 3 nm) of low indium concentration (x < 0.15-0.20). Spatial indium concentration fluctuations dominate luminescence properties in samples of higher indium concentrations in thinner wells. Quantum confinement is shown to have a major effect on the radiative recombination energy. A model is presented that relates the experimentally measured nano scale structural and chemical properties of quantum wells to the characteristics of the luminescence.

Continuous-wave and time-resolved photoluminescence of InGaN quantum wells are measured as a function of applied biaxial strain, which provides a unique means of altering the built-in polarization field in these structures. The direction and magnitude of the shift of the luminescence-peak energy are quantitatively analyzed within an analytical carrier separation model. It is found that the presently used piezoelectric coefficients of InGaN are not entirely consistent with our experimental results. Instead, consistent interpretation of our data requires the e13 and e33 piezoelectric coefficients of InN to be ∼15% larger than the commonly accepted values. Our analysis allows the assignment of an effective carrier-separation parameter to each investigated quantum-well sample, which quantifies the shift of the luminescence peak energy with the change in the polarization field. The effective carrier separation is found to be zero for narrow quantum wells (&lt;1.5 nm) and asymptotically approaches the full quantum well width for increasing well width. However, heavy doping or increased indium content are found to reduce the effective carrier separation, which is ascribed to screening of the polarization field or localization effects, respectively. A reduction of the carrier lifetime with the application of strain supports the carrier separation model and allows the derivation of a quantity related to the change of the wave function shape with the polarization field.

Gallium nitride epitaxial layers were grown on sapphire by molecular-beam epitaxy using nitridated gallium metal films as buffer layers. The mechanical properties of the buffer layers were investigated and correlated with their chemical composition as determined by synchrotron radiation photoelectron spectroscopy. Biaxial tension experiments were performed by bending the substrates in a pressure cell designed for simultaneous photoluminescence measurements. The shift of the excitonic luminescence peak was used to determine the stress induced in the main GaN epilayer. The fraction of stress transferred from substrate to main layer was as low as 27% for samples grown on nitridated metal buffer layers, compared to nearly 100% for samples on conventional low-temperature GaN buffer layers. The efficiency of stress relief increased in proportion to the fraction of metallic Ga in the nitridated metal buffer layers. These findings suggest GaN films containing residual metallic Ga may serve as compliant buffer layers for heteroepitaxy.

Direct application of biaxial strain to GaN and InGaN/GaN multiple quantum-well ͑MQW͒ structures is achieved through the use of a specially designed pressure cell. The photoluminescence of the samples is measured as a function of the applied biaxial strain. The luminescence of the GaN sample redshifts with tensile strain in a manner that agrees quantitatively with the expected shrinkage of the energy gap. The luminescence of the InGaN MQW structures shows a smaller than expected redshift for one sample, and a blueshift for another. This blueshift agrees with calculations based on the built-in electric field and the piezoelectric effect in a quantum well in which the radiative recombination is dominated by the quantum-confined Stark effect. 320 PRB 62 N. A. SHAPIRO et al. RAPID COMMUNICATIONS R16

We demonstrate that the use of pure gallium (Ga) as a buffer layer results in improved crystal quality of GaN epilayers grown by plasma-assisted molecular beam epitaxy on c-plane sapphire. The resulting epilayers show electron Hall mobilities as high as 400 cm 2 /Vs at a background carrier concentration of 4 x 10 17 cm -3 , an outstanding value for an MBE-grown GaN layer on sapphire. Structural properties are also improved; the asymmetric (101) X-ray rocking curve width is drastically reduced with respect to that of the reference GaN epilayer grown on a low-temperature GaN buffer layer. Nitrided Ga metal layers were investigated for different Ga deposition time. These layers can be regarded as templates for the subsequent GaN main layer growth. It was found that there is an optimum Ga metal layer deposition time for improving the electron mobility in the epilayer. Heating of the Ga metal layer to the epilayer growth temperature under nitrogen plasma is found to be sufficient to produce highly oriented GaN crystals. However, nonuniform surface morphology and incomplete surface coverage were observed after nitridation of comparatively thick Ga metal layers. This is shown to be the reason for the decreasing electron mobility of the epilayers as the Ga metal layer thickness exceeds the optimum value.