S. J. Hwang

Temperature dependence of interband transitions in GaN grown by metalorganic chemical vapor deposition

The interband transitions in single‐crystal GaN films grown by metalorganic chemical vapor deposition (MOCVD) have been studied as a function of temperature (15≤T≤300 K) by reflectance and photoluminescence measurements. At low temperatures, well‐resolved spectral features corresponding to the GaN band structure were observed. The energies of the excitonic interband ΓV9−ΓC7,ΓV7 (upper band)−ΓC7 and ΓV7(lower band)−ΓC7 transitions are found to be 3.485, 3.493, and 3.518 eV at 15 K, respectively, for the MOCVD GaN. The spectral features are broadened and shift to lower energy as temperature increases. At room temperature (300 K), the ΓV9−ΓC7and ΓV7 (upper band) −ΓC7 transition energies of this wide band‐gap material are determined to be 3.420 and 3.428 eV, respectively. The temperature dependence of these two transitions have been determined using the Varshni empirical relation. Our results yield E0(T)=3.486–8.32×10−4 T2/(835.6+T) eV for the ΓV9−ΓC7 transition and E0(T)=3.494–10.9×10−4 T2/(1194.6+T) eV for ...

Time-resolved study of yellow and blue luminescence in Si- and Mg-doped GaN

Time-resolved photoluminescence has been employed to study the donor-acceptor pair recombination kinetics of the yellow (∼2.3 eV) and blue (∼2.8 eV) luminescence bands in Si- and Mg-doped GaN layers, respectively. As the Si doping concentration in Si-doped GaN increases, the lifetime τ1/e of the yellow luminescence decreases, indicating that a shallow Si donor is the origin of the yellow luminescence. The blue luminescence is most likely due to a shallow Mg acceptor and a deep donor composed of a Mg acceptor-nitrogen vacancy complex, as seen by the independence of τ1/e on the Mg concentration measured by secondary ion mass spectroscopy in the range (2.5–6.0)×1019 cm−3. As the temperature is increased from 10 to 300 K, the lifetimes for the yellow and blue luminescence remain nearly constant, indicating that the distribution of electrons and holes bound to donors and acceptors does not change much with increasing temperature.

Intrinsic exciton transitions in GaN

Intrinsic excitonic transitions in GaN have been studied using a variety of spectroscopic measurements. Sharp spectral structures associated with intrinsic free excitons could be observed in photoluminescence, reflection, and absorption spectra. The energy positions of excitonic transitions in GaN epitaxial layers were found to be influenced by the residual strain resulting from lattice-parameter and thermal-expansion mismatches between the epilayers and the substrates. The values of the four principal deformation potentials of wurtzite GaN were derived by using the strain tensor components determined by x-ray measurements. The observation of spectral features involving the emission of LO phonons in absorption and photoluminescence excitation spectra at energies above exciton resonances indicate that a phonon-assisted indirect excitation process, which simultaneously generates a free exciton and a LO phonon, is a very significant and efficient process in GaN. The lifetime of the free excitons is found to ...

Photoluminescence of zinc‐blende GaN under hydrostatic pressure

Photoluminescence spectra of cubic GaN grown on a GaAs substrate by molecular beam epitaxy have been studied as a function of hydrostatic pressure at 10 K. The spectra are abundant in emission structures arising from a variety of radiative recombination processes, such as free‐electron–bound‐hole and donor‐acceptor pair transitions. These emission peaks shift to higher energy with increasing pressure, providing a measure of the pressure coefficient of the band gap of cubic GaN. In addition, a spectral feature, which is superimposed on the other emission peaks and not observable at atmospheric pressure, becomes gradually resolvable as pressure increases. The difference of pressure dependence of this emission from the others suggests that it is associated with a deep center.

Effect of hydrostatic pressure on strained CdSe/ZnSe single quantum wells

The effect of hydrostatic pressure on the quantum confined transition in CdSe/ZnSe single quantum wells grown by molecular beam epitaxy has been studied by low‐temperature photoluminescence measurements. Samples with layer thicknesses of CdSe from 1 to 4 monolayers were used. Strong excitonic emissions associated with the lowest Γ‐Γ interband transitions were observed in these highly strained quantum well samples. The pressure coefficients of the interband transitions are found to depend on well thickness with the numerical value decreasing as the well width increases. Pronounced sublinear pressure dependence of the excitonic emissions was observed in the samples with 3 and 4 monolayer CdSe wells, indicating the degradation of the samples due to strain relaxation. Our results suggest that the critical thickness for the CdSe layer pseudomorphically grown on ZnSe is less than 4 monolayers.

Optical properties of highly strained CdSe/ZnSe quantum wells

We present a study of the optical properties of highly strained CdSe/ZnSe quantum well system. A variety of CdSe/ZnSe samples containing single quantum well or multiple quantum wells grown by molecular beam epitaxy has been studied by using low‐temperature photoluminescence (PL), photoluminescence excitation, and photoreflectance measurements. The strong PL signals associated with excitonic emissions from the samples show that the CdSe/ZnSe heterostructure system is promising in the development of laser diodes and light‐emitting diodes operating in the blue‐green range. Linewidth narrowing of PL spectra with decreasing well width is observed and attributed to alloy formation at the interface due to lateral interdiffusion. The PL signal intensities and the pressure coefficients of interband transitions are also found to depend on the well width, which can be explained in terms of strain relaxation induced misfit dislocations and the critical thickness in the heterostructure system. Our results suggest that...

Effect of the number of wells on optical and structural properties in InGaN quantum well structures grown by metalorganic chemical vapor deposition

High-quality InGaN quantum well (QW) structures with one, two, three, five, and seven wells were grown by metalorganic chemical vapor deposition. The effect of the number of InGaN QWs on the structural and optical properties was studied by high-resolution x-ray diffraction (HRXRD), atomic force microscopy, low excitation density photoluminescence (PL), high excitation density pulsed PL, and PL excitation (PLE). The 10 K PLE band edge of all the samples is almost same, but the 10 K PL peaks of the InGaN QWs initially blueshifts, and then redshifts as the number of wells increases. HRXRD reciprocal space mapping and high excitation pulsed PL show that this anomalous peak shift is due mainly to potential fluctuations, rather than the piezoelectric field. The degree of potential fluctuations varies with dislocation density, which could be affected by growth interruption, the deposition of strained layers, and the accumulated strain energy in InGaN QW structures.

Interband transitions in InAsxP1−x/InP strained multiple quantum wells

We report the optical study of the interband transitions in InAsxP1−x/InP strained‐layer multiple quantum wells grown by gas‐source molecular beam epitaxy. Low‐temperature photoluminescence, photoluminescence excitation, and room temperature photomodulated transmission measurements were performed to investigate optical interband transitions. In addition to transitions associated with the heavy‐hole and the light‐hole bands, a transition involved with the spin‐orbit split‐off band was observed. We also observed spectral linewidth broadening due to compositional inhomogeneity and layer‐thickness fluctuations from the sample using short‐period superlattices as the well materials. Calculations based on the envelope‐function approximation and phenomenological deformation potential theory, including both band nonparabolicity and strain‐induced valence‐band mixing, were compared with experimental data to identify the optical transitions between quantized states in the wells. We found good agreement between theor...

Valence band offset of GaAs/GaAs0.68P0.32 multiple quantum wells

Low‐temperature photoluminescence measurements have been performed under hydrostatic pressure on GaAs/GaAs0.68P0.32 strained multiple quantum well samples grown by gas‐source molecular beam epitaxy. The pressure induced crossover of the first confined electron state in the GaAs wells against the conduction band (001) X minima in the GaAs0.68P0.32 barriers has been observed, which allows a direct spectroscopic determination of the valence band offset for the heterostructure. As the result we obtain the unstrained valence band offset as 0.09±0.02 eV, which corresponds to an approximate 77:23 distribution of the energy gap difference in the conduction and valence bands, respectively, for the GaAs/GaAs0.68P0.32 system.

Optical Properties and Lasing in (In, Al)GaN Structures

We achieved low-threshold ultra-violet lasing in optically pumped GaN/AlGaN separate confinement heterostructures over a wide temperature range. Lasing modes of a single microcavity were examined from 20 to 300 K and gain mechanisms were compared to those of a thick GaN epilayer. We have also systematically studied InGaN/(In)GaN multiple quantum wells as a function of well and barrier thickness. We demonstrate that the stimulated emission threshold and photoluminescence (PL) decay time are strongly dependent on the well and barrier thickness. The experimental results indicate that the enhanced optical quality of samples with larger barrier thicknesses can he readily applied to the fabrication of InGaN/(In)GaN laser diodes.