Anomalous disorder-related phenomena in InGaN/GaN multiple quantum well heterosystems

The influences of InGaN/GaN multiple quantum well (MQW) heterostructures with InGaN/GaN and GaN barriers on carrier confinement were investigated. The degree of disordering over a broad range of temperatures from 20 to 300 K was considered. The optical and electrical properties were strongly influenced by structural and compositional disordering of the InGaN/GaN MQW heterostructures. To compare the degree of disordering we examined the temperature dependence of the luminescence spectra and electrical conductance contingent on the Berthelot-type mechanisms in the InGaN/GaN MQW heterostructures. We further considered carrier transport in the InGaN/GaN disordered systems, probability of carrier tunneling, and activation energy of the transport mechanism for devices with InGaN/GaN and GaN barriers. The optical properties of InGaN/GaN disordered heterosystems can be interpreted from the features of the absorption spectra. The anomalous temperature-dependent characteristics of the disordered InGaN/GaN MQW structures were attributable to the enhancement of the exciton confinement.     

Electron probe microanalysis and local cathodoluminescence studies of multilayer heterostructures based on InGaN/GaN

This paper presents the results of the study on multilayer heterostructures based on InGaN/GaN grown by MOCVD. A new procedure to measure the composition and depth of InGaN quantum wells and AlGaN barrier layers is developed. Features of luminescence from different depths of the multilayer heterostructure are analyzed.     

Fang–Howard wave function modelling of electron mobility in AlInGaN/AlN/InGaN/GaN double heterostructures

To study the electron transport properties in InGaN channel-based heterostructures,a revised Fang-Howard wave function is proposed by combining the effect of GaN back barrier.Various scattering mechanisms,such as dislocation impurity(DIS) scattering,polar optical phonon(POP) scattering,piezoelectric field(PE) scattering,interface roughness(IFR) scattering,deformation potential(DP) scattering,alloy disorder(ADO) scattering from InGaN channel layer,and temperature-dependent energy bandgaps are considered in the calculation model.A contrast of AlInGaN/AlN/InGaN/GaN double heterostructure(DH) to the theoretical AlInGaN/AlN/InGaN single heterostructure(SH) is made and analyzed with a full range of barrier alloy composition.The effect of channel alloy composition on InGaN channel-based DH with technologically important Al(In,Ga)N barrier is estimated and optimal indium mole fraction is 0.04 for higher mobility in DH with Al_(0.4)In_(0.07)Ga_(0.53)N barrier.Finally,the temperature-dependent two-dimensional electron gas(2 DEG) density and mobility in InGaN channel-based DH with Al_(0.83)In_(0.13)Ga_(0.0)4 N and Al_(0.4)In_(0.07)Ga_(0.53)N barrier are investigated.Our results are expected to conduce to the practical application of InGaN channel-based heterostructures.     

Piezoelectric Relaxation of Two-Dimensional Electron Gas in Heterostructures with InGaN/GaN Quantum Wells

Russian Physics Journal - The article is devoted to the study of the mechanism of piezoelectric scattering of charge carriers in InGaN/AlGaN/GaN heterostructures with one filled size quantization...     

Mechanisms for spontaneous and stimulated recombination in multiple quantum wells of InGaN/GaN heterostructures on silicon substrates

With the aim of establishing the mechanisms for spontaneous recombination and lasing, we have studied InGaN/GaN multiple quantum well heterostructures emitting in the 450 nm region and grown by organometallic vapor-phase epitaxy on silicon substrates using several mechanical stress-reducing AlN/AlGaN inserts. Photoluminescence (PL) excitation spectroscopy, the non-monoexponential nonequilibrium carrier relaxation kinetics, and x-ray diffractometry data indicate significant inhomogeneity of the InGaN solid solution in quantum wells of these structures. The dependences of the position of the photoluminescence spectra on the excitation level and the temperature, the large shift in the photoluminescence, gain, and lasing spectra relative to the absorption edge allow us draw the conclusion that the dominant contribution to spontaneous and stimulated recombination comes from nonequilibrium charge carriers localized in indium-rich InGaN clusters. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 94–101, January–February, 2008.     

Two dimensional electron gas in a hybrid GaN/InGaN/ZnO heterostructure with ultrathin InGaN channel layer

We investigated the influence of an ultrathin InGaN channel layer on two-dimensional electron gas (2DEG) properties in a newly proposed hybrid GaN/In_xGa_1−xN/ZnO heterostructure using numerical methods. We found that 2DEG carriers were confined at InGaN/ZnO and GaN/InGaN interfaces. Our calculations show that the probability densities of 2DEG carriers at these interfaces are highly influenced by the In mole fraction of the InGaN channel layer. Therefore, 2DEG carrier confinement can be adjustable by using the In mole fraction of the InGaN channel layer. The influence of an ultrathin InGaN channel layer on 2DEG carrier mobility is also discussed. Usage of an ultrathin InGaN channel layer with a low indium mole fraction in these heterostructures can help to reduce the short-channel effects by improvements such as providing 2DEG with higher sheet carrier density which is close to the surface and has better carrier confinement.     

Low-energy electron-excited nanoluminescence studies of GaN and related materials

We have used low-energy electron-excited nanoluminescence (LEEN) spectroscopy combined with ultrahigh vacuum surface analysis techniques to obtain electronic bandgap, confined state and deep-level trap information from III nitride compound semiconductor surfaces and their buried interfaces on a nanometer scale. Localized states are evident at GaN/InGaN quantum wells, GaN ultrathin films, AlGaN/GaN pseudomorphic heterostructures, and GaN/Al₂O₃ interfaces that are sensitive to the chemical composition, bonding and atomic structure near interfaces, and in turn to the specifics of the epitaxial growth process. Identification of electrically active defects in these multilayer nanostructures provides information to optimize interface growth and control local electronic properties.     

Modelling of bandgap and band offset properties in III-N related heterostructures

Reliable and precise knowledge about the strain and composition effects on the electronic properties is crucial for the optimization of III-nitride based heterostructures for electronic and optoelectronic device applications. Using the nearest neighbor semi-empirical tight binding formalism, we investigated the composition effects on bandgaps and offsets in pseudomorphic InGaN/GaN and GaAsN/GaAs heterostructures with zinc-blende structures. The strain effects are incorporated afterwards in a semi-empirical way. The model should be useful in understanding the effects of composition and strain on heterostructure energy band properties.     

Coherent phonons, nanoseismology and THz radiation in InGaN/GaN heterostructures

The ultrafast optical photoexcitation of hot electrons and holes in semiconductors by femtosecond laser pulses can trigger coherent phonon oscillations. We discuss the huge coherent acoustic phonons which have been generated in InGaN/GaN heterostructures and epilayers and how they might be used in imaging of surfaces and interfaces in nanostructures. We also discuss the THz radiation emitted from these phonons.     

Numerical investigation on the enhanced carrier collection efficiency of Ga-face GaN/InGaN p-i-n solar cells with polarization compensation interlayers

The impact of the polarization compensation InGaN interlayer between the heterolayers of Ga-face GaN/InGaN?p-i-n solar cells is investigated numerically. Because of the enhancement of carrier collection efficiency, the conversion efficiency is improved markedly, which can be ascribed to both the reduction of the polarization-induced electric field in the InGaN absorption layer and the mitigation of potential barriers at heterojunctions. This beneficial effect is more remarkable in situations with higher polarization, such as devices with a lower degree of relaxation or devices with a higher indium composition in the InGaN absorption layer.     

A study of indium incorporation in In-rich InGaN grown by MOVPE

InGaN/GaN heterostructures have been deposited onto (0 0 0 1) sapphire by our home-made low pressure MOVPE with different growth parameters. It has been noted that the indium incorporation depends by a complex way on a number of factors. In this work, the effect of substrate temperature, trimethylindium input flow and V/III ratio on the indium incorporation has been investigated. Finally, by optimizing the growth parameters, we made a series of single-phase InGaN samples with indium content from 10% up to 45%.     

Optically pumped InGaN/GaN MQW lift-off lasers grown on silicon substrates

Chemical etching and removal of the silicon substrate was used for the creation of optically pumped lift-off InGaN/GaN multiple quantum well (MQW) lasers from heterostructures grown on silicon substrate by MOVPE. Luminescence and laser properties of these heterostructures on silicon substrates as well as those of MQWs lifted-off from their substrate by chemical etching were investigated. The lowest value of the lasing threshold of the lift-off lasers at room temperature was about 205 kW/cm² for a laser wavelength of 463 nm and about 360 kW/cm² for a wavelength of 475 nm. It was shown theoretically that the reduction of internal losses, caused by the absence of absorption in the substrate (resulting from its removal) is most significant for the high order modes having lower values of mirror losses and can lead to a 50% reduction of the threshold (or material gain in InGaN necessary to achieve the threshold).     

Effects of the strain relaxation of an AlGaN barrier layer induced by various cap layers on the transport properties in AlGaN/GaN heterostructures

The strain relaxation of an AlGaN barrier layer may be influenced by a thin cap layer above, and affects the transport properties of AlGaN/GaN heterostructures. Compared with the slight strain relaxation found in AlGaN barrier layer without cap layer, it is found that a thin cap layer can induce considerable changes of strain state in the AlGaN barrier layer. The degree of relaxation of the AlGaN layer significantly influences the transport properties of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. It is observed that electron mobility decreases with the increasing degree of relaxation of the AlGaN barrier, which is believed to be the main cause of the deterioration of crystalline quality and morphology on the AlGaN/GaN interface. On the other hand, both GaN and AlN cap layers lead to a decrease in 2DEG density. The reduction of 2DEG caused by the GaN cap layer may be attributed to the additional negative polarization charges formed at the interface between GaN and AlGaN, while the reduction of the piezoelectric effect in the AlGaN layer results in the decrease of 2DEG density in the case of AlN cap layer.     

Strain relaxation and optical properties of etched In_0.19Ga_0.81 N nanorod arrays on the GaN template

InGaN/GaN epilayers,which are grown on sapphire substrates by the metal-organic chemical-vapour deposition(MOCVD) method,are formed into nanorod arrays using inductively coupled plasma etching via self-assembled Ni nanomasks.The formation of nanorod arrays eliminates the tilt of the InGaN(0002) crystallographic plane with respect to its GaN bulk layer.Photoluminescence results show an apparent S-shaped dependence on temperature.The light extraction efficiency and intensity of photoluminescence emission at low temperature of less than 30 K for the nanorod arrays are enhanced by the large surface area,which increases the quenching effect because of the high density of surface states for the temperature above 30 K.Additionally,a red-shift for the InGaN/GaN nanorod arrays is observed due to the strain relaxation,which is confirmed by reciprocal space mapping measurements.     

Strain relaxation and optical properties of etched In0.19Ga0.81N nanorod arrays on the GaN template

InGaN/GaN epilayers,which are grown on sapphire substrates by the metal-organic chemical-vapour deposition(MOCVD) method,are formed into nanorod arrays using inductively coupled plasma etching via self-assembled Ni nanomasks.The formation of nanorod arrays eliminates the tilt of the InGaN(0002) crystallographic plane with respect to its GaN bulk layer.Photoluminescence results show an apparent S-shaped dependence on temperature.The light extraction efficiency and intensity of photoluminescence emission at low temperature of less than 30 K for the nanorod arrays are enhanced by the large surface area,which increases the quenching effect because of the high density of surface states for the temperature above 30 K.Additionally,a red-shift for the InGaN/GaN nanorod arrays is observed due to the strain relaxation,which is confirmed by reciprocal space mapping measurements.     

Strain Effect on Photoluminescences from InGaN MQWs with Different Barriers Grown by MOCVD

InGaN/GaN MQWs, InGaN/AlGaN MQWs and InGaN/AlInGaN MQWs are grown on （0001） sapphire substrates by MOCVD. Membrane samples are fabricated by laser lift-off technology. The photoluminescence spec-ra of membranes show a blue shift of peak positions in InGaN/GaN MQWs, a red shift of peak positions in InGaN/AlGaN MQWs and no shift of peak positions in InGaN/AIlnGaN MQWs from those of samples with substrates. Different changes in Raman scattering spectra and HR-XRD （0002） profile of InGaN/AlInGaN MQWs, from those of InGaN/GaN MQWs and InGaN/AlGaN MQWs, are observed. The fact that the strain changes differently among InGaN MQWs with different barriers is confirmed. The AIlnGaN barrier could adjust the residual stress for the least strain-induced electric field in InGaN/AIlnGaN quantum wells.     

The effect of junction temperature on the optoelectrical properties of InGaN/GaN multiple quantum well light-emitting diodes

Thermal effects on the optoelectrical characteristics of green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been investigated in detail for a broad temperature range, from 30 °C to 100 °C. The current-dependent electroluminescence (EL) spectra, current–voltage (I–V) curves and luminescence intensity–current (L–I) characteristics of green InGaN/GaN MQW LEDs have been measured to characterize the thermal-related effects on the optoelectrical properties of the InGaN/GaN MQW LEDs. The experimental results show that both the forward voltages decreased with a slope of ?3.7 mV/K and the emission peak wavelength increased with a slope of +0.02 nm/K with increasing temperature, indicating a change in the contact resistance between the metal and GaN layers and the existence of a band gap shrinkage effect. The junction temperature estimated from the forward voltage and the emission peak shift varied from 25.6 to 14.5 °C and from 22.4 to 35.6 °C, respectively. At the same time, the carrier temperature decreased from 371.2 to 348.1 °C as estimated from the slope of high-energy side of the emission spectra. With increasing injection current, there was found to be a strong current-dependent blueshift of ?0.15 nm/mA in the emission peak wavelength of the EL spectra. This could be attributed to not only the stronger band-filling effect but also the enhanced quantum confinement effect that resulted from the piezoelectric polarization and spontaneous polarization in InGaN/GaN heterostructures. We also demonstrate a helpful and easy way to measure and calculate the junction temperature of InGaN/GaN MQW LEDs.     

引入n型InGaN/GaN超晶格层提高量子阱特性研究

利用金属有机物化学气相淀积技术在蓝宝石衬底上生长了InGaN/GaN量子阱结构. 研究了引入n型InGaN薄层或InGaN/GaN超晶格层的量子阱特性,结果表明通过引入n型InGaN薄层或InGaN/GaN超晶格层缓解了量子阱有源区中的应力,改善了多量子阱表面形貌,减少了V型缺陷密度,而且提高了多量子阱的光致发光强度,从而也改进了LED的发光效率. 关键词： InGaN/GaN多量子阱 原子力显微镜 X射线双晶衍射 光致发光     

Resonance Bragg structure with double InGaN quantum wells

The effect of exciton-polariton resonance on the optical properties of periodic heterostructures with double InGaN quantum wells in a GaN matrix has been studied. It has been found that the light reflection is amplified at the frequency corresponding to the exciton energy when it coincides with the frequency of the Bragg resonance. This effect is observed to be twice as large as that in a similar system of single quantum wells.     

Growth and characterization of InGaN nanodots hybrid with InGaN/GaN quantum wells

Uniform InGaN nanodots were successfully grown on SiO₂ pretreated GaN surface. It was found that the InGaN nanodots were 20?nm in diameter and 5?nm in height, approximately. After the growth of two periods of InGaN/GaN quantum wells on the surface of InGaN nanodots, nanodot structure still formed in the InGaN well layer caused by the enhanced phase separation phenomenon. Dual-color emissions with different behavior were observed from photoluminescence (PL) spectrum of InGaN nanodots hybrid with InGaN/GaN quantum wells. A significant blueshift and a linewidth broadening were measured for the low-energy peak as the increase of PL excitation power, while a slight blueshift and a linewidth narrowing occurred for the high-energy peak. Accordingly, these two peaks were assigned to be from the In-rich nanodots and quantized state transition from the InGaN/GaN quantum wells with indium content, respectively.