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Correlation between the photoluminescence (PL) energy at ambient pressure and the pressure coefficient of photoluminescence is studied in quantum wells (QWs) based on nitride alloys, such as InGaN/GaN, GaN/AlGaN, and GaN/InAlN, grown along the polar direction [0001] of the wurtzite structure. Analyzing previously published and new experimental data, we have found that for InGaN/GaN QWs independent of In content (in the range between 6% and 25%) and also QW number and QW width, a linear relationship between these two parameters occurs. The presented experimental results are in agreement with numerical calculations carried out in the framework of the kvector ·pvector method with excitonic effects, provided that nonlinear piezoelectricity and nonlinear elasticity are taken into account. The performed analytical analysis indicates that the slope of the linear relationship between the pressure coefficient of photoluminescence and the photoluminescence energy at ambient pressure is determined by the logarithmic derivative of the built-in electric field with respect to pressure. Then, we show that the pressure coefficient of photoluminescence depends linearly on the photoluminescence energy at ambient pressure also in GaN/AlGaN and GaN/InAlN QWs. In GaN/AlGaN QWs, the slope of this dependence slightly decreases with Al content in the barriers. For GaN/InAlN QWs, we predict an unusual dependence of this slope on In content, which is associated with the vanishing built-in electric field in structures with 30% of In. For all studied nitride systems, a reasonable agreement between the experimental and theoretical results is achieved when the effects of nonlinear piezoelectricity and nonlinear elasticity are taken into account. [ABSTRACT FROM AUTHOR]

High quality GaN/AlGaN multiquantum well (QW) structures were grown by ammonia molecular beam epitaxy along the (0001) polar and [formula] nonpolar directions. Each sample contains three QWs with thicknesses of 2, 3, and 4 nm as well as 10 nm Al0.30Ga0.70N barriers. The measured photoluminescence (PL) spectrum consists of three peaks originating from the radiative recombination of excitons in individual QWs. In the nonpolar sample, the energy positions (EPL) of the observed peaks are separated because of the quantum confinement effect, whereas in the polar sample an additional redshift is induced by the quantum confined Stark effect. The dependence of EPL on QW width was used to estimate the built-in electric field magnitude in the latter sample to be about 2 MV/cm. Hydrostatic pressure studies of the PL in both samples gave qualitatively different results. In the polar sample, the pressure shift of EPL, dEPL/dp decreases significantly with QW width. The important finding is derived from the observation of a QW width independent dEPL/dp in the nonpolar sample. It shows that for GaN/Al0.30Ga0.70N, the quantum confinement remains practically independent of the applied hydrostatic pressure. This result reveals that in the polar sample, the variation in dEPL/dp with the QW width is due to the pressure-induced increase in the built-in electric field Fint. Thus, a more quantitative analysis of the latter effect becomes justified. We found that the Fint increases with pressure with a rate of about 80 kV(cm GPa)-1. [ABSTRACT FROM AUTHOR]

Light emission of nitride single quantum well light-emitting diodes operating in the range from 395 to 440 nm, grown by metal-organic vapor phase epitaxy, was investigated by means of photoluminescence (PL) and electroluminescence (EL) spectroscopies as functions of temperature and current injection level. The indium content of the active layers was varied via the growth temperature, which ranged from 770 to 810 °C. It was found that samples with higher indium contents (grown at lower temperatures) exhibit larger luminescence full widths at half maximum (FWHMs) and higher EL intensities. The larger FWHM points to a larger amplitude of the potential profile fluctuations, which suggests that these fluctuations may be useful for the increase in the device output power. In agreement with this result, the performed examination by PL measurements shows large thermal stability of the luminescence intensity for the lowest growth temperature sample. On the other hand, a nontrivial dependence of thermal stability on growth temperature suggests the additional involvement of different nonradiative recombination center concentrations in the individual samples, for example, because of dissimilar substrate qualities. [ABSTRACT FROM AUTHOR]

Near-lattice-matched GaN/AlInN multiple quantum wells (MQWs) are investigated by means of the diamond anvil cell high-pressure technique. The hydrostatic pressure dependence of the photoluminescence dEPL/dp and the variation in the PL peak energy with the QW width for different pressures were measured. Taking into account the influence of a large Stokes shift on the correct determination of the built-in electric field value, we find that the electric field, equal to 4.2±1.1 MV/cm at ambient pressure, increases with pressure at a rate of 0.29 MV/(cm GPa). This value is in reasonable agreement with the theoretically predicted value, based on nonlinear elasticity, of 0.17 MV/(cm GPa). Interestingly, the observed behavior is very similar to strongly mismatched GaN/AlGaN QWs with a similar band offset, indicating that in GaN/AlInN QWs there is still a pressure dependence of piezoelectric effects, in spite of ambient-pressure lattice matching. [ABSTRACT FROM AUTHOR]

The hydrostatic pressure dependence of the polarization-induced interface charge in an AlGaN/GaN heterostructure is investigated via capacitance-voltage (C-V) characterization of the two-dimensional electron gas (2DEG) at the heterojunction. The linear pressure coefficient of (3.3±0.4)×1011 cm-2 GPa-1 of the polarization-induced interface charge as obtained via C-V measurements of the investigated (28 nm Al0.26Ga0.74N)/GaN heterostructure agrees well with existing theoretical and experimental data (obtained via Hall-effect measurements). It is demonstrated that C-V characterization can be applied even when Hall-effect measurements of the 2DEG are inconclusive because of parallel conduction via the underlying GaN layer. In addition, we discuss the influence on the 2DEG concentration of interface trap states, which introduce a dependence of the measured 2DEG concentration on C-V test frequency. It is shown, however, that this effect does not influence the value of the pressure coefficient of the polarization-induced interface charge as determined via C-V characterization. Collaterally, the applicability of capacitance measurements of AlGaN/GaN heterostructures for pressure sensing applications is confirmed. [ABSTRACT FROM AUTHOR]

We have performed a study of excitation power-dependent spectra of GaN/AlGaN single quantum wells (QWs). First, the experimental "blueshift" of the emission energy, due to screening of internal piezoelectric fields, was compared with the model calculations based on self-consistent solution of Schroedinger and Poisson equations. We found that, even for the highest applied levels of excitation power (2.5 MW/cm²), only 0.5 × 10[sup 12] cm[sup -2] carriers were present in the QW layers. Second, we analyzed the evolution of power-dependent spectra of two single QW having different widths. For the thinner QW (2.1 nm), the peak corresponding to a QW photoluminescence (PL) emission dominates the entire spectrum in the whole range of the used excitation power. In the case of the wider QW (4.4 nm), for sufficiently high excitation power, we observe the effect of PL quenching. Using the rate equation model we show that the observed effect of the PL quenching can be associated with the reduction of exciton binding energy due to the many body interactions in the QW. [ABSTRACT FROM AUTHOR]

Contactless electroreflectance (CER) has been applied to study optical transitions between the ground and excited states in polar and a-plane nonpolar 2 nm wide GaN/Al0.12Ga0.88N quantum well (QW) structures. In addition to the fundamental transition, CER features related to optical transitions between excited states were clearly observed for the polar QW structure whereas such features were not observed for the nonpolar QW structure. This experimental result clearly shows that the polarization-related electric field leads to a quantum confinement of some extra states in the polar QW system. Such states are not confined in the nonpolar QW and, therefore, optical transitions between them are not detected, i.e., only the fundamental transition is observed in CER spectrum of the nonpolar QW. [ABSTRACT FROM AUTHOR]

The potential fluctuations in III-nitride quantum wells lead to many effects like emission broadening and S-shape energy vs. temperature dependence. The best description of the energy dependence comes from calculations based on Gaussian density of states. However, in most of the published reports, changes of carrier lifetime with energy and temperature are not taken into account. Since experimental evidence shows that lifetime significantly depends on energy and temperature, here we propose a model that describes two basic parameters of luminescence: lifetime of carries and emission energy as a function of temperature in the case of quantum wells and layers that are characterized by potential fluctuations. Comparison of the measured energy and lifetime dependences on temperature in specially grown InGaN/GaN quantum wells and InAlGaN layer shows very good agreement with the proposed theoretical approach. [ABSTRACT FROM AUTHOR]

It is shown that in polar InGaN QWs emitting in the blue-green spectral region a Stokes shift between spontaneous emission (SE) and optical transition observed in contactless electroreflectance (CER) spectrum (absorption-like technique) can be observed even at room temperature, despite the fact that the SE is not associated with localized states. Time resolved photoluminescence measurements clearly confirm that the SE is strongly localized at low temperatures whereas at room temperature the carrier localization disappears and the SE can be attributed to the fundamental transition in this QW. The Stokes shift is observed in this QW system because of the large built-in electric field, i.e., the CER transition is a superposition of all optical transitions with non-zero electron-hole overlap integrals and, therefore, the energy of this transition does not correspond to the fundamental transition of InGaN QW. Lasing from this QW has been observed at the wavelength of 475 nm, whereas the SE was observed at 500 nm. The 25 nm shift between the lasing and SE is observed because of a screening of the built-in electric field by photogenerated carriers. However, our analysis shows that the built-in electric field inside the InGaN QW region is not fully screened under the lasing conditions. [ABSTRACT FROM AUTHOR]

Quantitative high resolution transmission electron microscopy studies of intentionally grown 1InN/n GaN short-period superlattices (SLs) were performed. The structures were found to consist of an InxGa1-xN monolayer with an Indium content of x = 0.33 instead of the intended x = 1. Self-consistent calculations of the band structures of 1In0.33Ga0.67N/n GaN SLs were carried out, including a semi-empirical correction for the band gaps. The calculated band gap, Eg, as well as its pressure derivative, dEg /dp, are in very good agreement with the measured photoluminescence energy, E PL, and its pressure derivative, dE PL /dp, for a series of 1In0 .33Ga0.67N/n GaN samples with n ranging from 2 to 40. This resolves a discrepancy found earlier between measured and calculated optical emission properties, as those calculations were made with the assumption of a 1InN/n GaN SL composition. [ABSTRACT FROM AUTHOR]

Near-lattice-matched GaN/AlInN quantum wells are investigated by means of contactless electroreflectance (CER) and temperature-dependent photoluminescence (PL). Large Stokes shifts, up to 400 meV, between PL peak energies and CER resonances are identified. This Stokes shift is attributed to large potential profile fluctuations (PPFs) in the AlInN barriers. Further evidence for such PPFs and for the additional influence of QW width fluctuations is provided by temperature-dependent PL measurements, demonstrating large PL halfwidths and clear “S-shape” behavior. The influence of a large Stokes shift on the correct determination of the value of the built-in electric field is discussed, and it is shown that PL measurements may lead to a significant overestimation of the built-in electric field in GaN/AlInN QWs. [ABSTRACT FROM AUTHOR]

This letter concerns experiments on optically pumped GaN/AlGaN separate-confinement heterostructure laser structures grown by plasma assisted molecular beam epitaxy. The structures were grown along the (1120) nonpolar crystallographic direction on a bulk GaN substrate. Different widths of GaN quantum wells were applied in the studied structures. Laser action is clearly demonstrated by the spontaneous emission saturation, abrupt line narrowing, and strong TE polarization of output light. A lasing threshold was reached at an excitation power density of 260 kW/cm2 for a 700-μm-long cavity at room temperature. [ABSTRACT FROM AUTHOR]

By using an atypically wide quantum well (95 Å) in the active layer of InGaN violet light emitting laser diode, we managed to fabricate a device characterized by very high thermal stability of the threshold current. The characteristic T0 temperature was measured to be 302 K, which is the highest reported value up to date. After thermal cycling of the device, T0 drops down to the lower value of 220 K. The very high value of T0 in our devices is accompanied by anomalous temperature behavior of the device slope efficiency. The slope efficiency improves with increasing temperature, reaches a maximum and then gradually decreases. This behavior we interpret as the competition between a regular increase of the thermal carrier escape and an improvement of carrier capture efficiency with an opposite temperature dependence. The latter mechanism we tentatively attribute to the temperature quenching of the ballistic transport related carrier leakage from the active region of the laser diode. [ABSTRACT FROM AUTHOR]

Nonpolar multiple quantum wells (MQWs) have been grown by plasma assisted molecular beam epitaxy on bulk GaN crystals oriented along the s1120d direction. The photoluminescence intensity of the nonpolar MQWs was significantly higher than that found for the polar samples, both at low s10 Kd and room temperature. This is a consequence of the lack of built-in electric field in samples grown along the s1120d direction. Clearly resolved spectra of the excitons have been observed in the studied MQWs. Studies of these excitonic structures, by means of polarization and temperature measurements enabled us to assign the observed lines to free and bound excitons in GaN quantum wells. [ABSTRACT FROM AUTHOR]

A study of internal electric field contribution to the light emission mechanism of InAlGaN based multiquantum wells was performed. To determine the magnitude of the built-in electric field we employed: (i) theoretical estimation of the piezoelectric and spontaneous polarizations, (ii) analysis of the emission energy as a function of the quantum well width, (iii) hydrostatic pressure experiments, and finally (iv) measurements of photoluminescence decay. Performed calculations gave high magnitude of the built-in electric field. On the contrary, independently of the quantum well width the pressure shift of the light emission energy and the photoluminescence decay time showed almost constant values. These observations are interpreted as evidence of a lack of the built-in electric field in the used quaternary quantum wells. Possible reasons for the controversies between theory and experiment are suggested. [ABSTRACT FROM AUTHOR]

We have studied the influence of hydrostatic pressure on the light emission from cubic In[sub 0.1]Ga[sub 0.9]N. A qualitative difference between pressure dependence of photoluminescence peak energies for cubic and wurtzite symmetry InGaN/GaN quantum wells (QWs) was found. Cubic samples revealed magnitude of dE[sub E]/dP of 26-30 meV/GPa, practically independent of the QW width. Previous studies of the hexagonal InGaN/GaN structures showed that with increasing QW width dE[sub E]/dP changed between about 30 meV/GPa and 0 meV/GPa. This different behavior of two types of QWs can be explained by the lack of built-in electric field (along growth direction) in case of cubic structures. To describe pressure evolution of the optical transitions in cubic InGaN/GaN QWs and thick epitaxial layer, we use a simple k×p model based on the linear theory of elasticity. To reproduce the experimental data, it is necessary to invoke presence of In-rich fluctuations in the cubic In[sub 0.1]Ga[sub 0.9]N samples. [ABSTRACT FROM AUTHOR]

Studies the phase separation that occurs in In[sub 0.33]Ga[sub 0.67]N/GaN multiple-quantum-well structures after annealing at high nitrogen pressures. Superlattice peaks showed by the x-ray diffraction (XRD) spectra of the as-grown samples; Transmission electron microscopy (TEM) images of the annealed samples; TEM and XRD measurements.

We have studied the influence of hydrostatic pressure on the light emission from a strained GaN/AlGaN multiquantum well system. We have found that the pressure coefficients of the photoluminescence peak energies are dramatically reduced with respect to that of GaN energy gap and this reduction is a function of the quantum well thickness. The decrease of the light emission pressure coefficient may be as large as 30% for a 32 monolayer (8 nm) thick quantum well. We explain this effect by the hydrostatic-pressure-induced increase of the piezoelectric field in quantum structures. Model calculations based on the k×p method and linear elasticity theory reproduce the experimental results well, demonstrating that this increase may be explained by small anisotropy of the wurtzite lattice of GaN and a specific interplay of elastic constants and values of the piezoelectric tensor. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Interdiffusion of In and Ga is observed in InGaN/GaN multiple quantum wells for annealing temperatures of 1300–1400 °C. Hydrostatic pressures of up to 15 kbar were applied to prevent surface decomposition. In as-grown material, x-ray diffraction spectra show InGaN diffraction peaks up to the fourth order. After annealing at 1400 °C for 15 min, only the zero-order peak is observed, as a result of compositional disordering of the quantum well superlattice. Transmission electron microscopy confirms that the superlattice is completely disordered after annealing at 1400 °C for 15 min. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Spontaneous and stimulated luminescence in highly excited InGaN/GaN multiple quantum wells grown by MOCVD over sapphire and bulk GaN substrates is reported. By using high-power laser pulses of 20-ps duration, free-carrier densities as high as were achieved, which resulted in almost screened built-in field. Therefore, dynamics of spontaneous and stimulated emission due solely to band potential fluctuations were revealed. In comparison with the heteroepitaxial counterparts, structures grown over bulk GaN exhibited strong stimulated emission within the first 100ps of relaxation, a reduced intensity of spontaneous emission on the later stage of relaxation, and domination of delocalized carriers in the spontaneous spectra. The obtained results imply a significant reduction of band potential fluctuations in the InGaN/GaN structures grown over bulk GaN. [Copyright &y& Elsevier]