Thin InGaN Layers Research Articles

Indium-rich island structures formed by in-situ nanomasking technology

A new method of forming InGaN/GaN nanostructures emitting in the long-wavelength green spectral range is proposed and implemented. The method is based on growing a thin InGaN layer over nanosized pits formed in situ by etching a locally AlN-masked GaN layer in a hydrogen-containing atmosphere.

Theoretical modeling for quantum-confined Stark effect due to internal piezoelectric fields in GaInN strained quantum wells

Recent experimental investigations revealed that the biaxial stress in thin InGaN layers grown on thick GaN layer induces a large piezoelectric field along [0001] orientation that causes red-shift in optical transitions and reduction in oscillator strengths because of spatial separation of the electron and hole wave functions. In this Letter based on theoretical modeling we determined the well width z-dependent effect on red-shifted quantum-confined Stark effect (QCSE) in GaN/In x Ga 1 − x N ( x = 0.13 ) strained quantum well structures. Analyses are based on the solution of Schrödinger equation in a finite well including the internal piezoelectric electric field ( F) due to the strained polarization as the perturbation potential. Our theoretical results show: (1) the red-shift in optical transition has a quadratic well-width form as it is for infinite wells (Davies, 1998) [1], (2) assuming the model based on a carrier effective mass dependence on the width of quantum wells, m ∗ ( z ) , fits the experimental data (Takeuchi et al., 1997) [2] much more accurate compare to the model with constant effective mass, m ∗ .

GaN/InGaN heterostructures grown by ammonia MBE with a wetting metal indium layer

We have studied the influence of conditions used for the growth of InGaN layers by the molecular beam epitaxy (with ammonia as a source of nitrogen) on the properties of GaN/InGaN heterostructures. It is established tat the temperature dependence of the critical flux of indium leading to the formation of droplets for the growth of InGaN on GaN is exponential and is determined by the enhanced desorption of indium from the surface of a growing layer with increasing substrate temperature. The growth of InGaN layers on GaN, even at the maximum possible flux of indium not accompanied by the droplet formation, leads to an extended profile of indium distribution in InGaN layers. It is shown that, in order to obtain sharp heteroboundaries and increase the content of indium in thin InGaN layers, it is necessary to create and maintain the so-called “wetting” film of metallic indium on the substrate surface prior to and in the course of InGaN layer growth. Using the obtained results, basic conditions are established for growing thin InGaN layers for the active elements of light-emitting devices operating in the blue-violet spectral range.

InGaN quantum well epilayers morphological evolution under a wide range of MOCVD growth parameter sets

AbstractThis study exemplifies the use of TappingModeTM atomic force microscopy (AFM) surface morphology imaging to investigate and optimise the metalorganic chemical vapour deposition (MOCVD) growth conditions and post‐growth stability of thin (<40 Å) InGaN layers with direct implications to the structural and optical properties of blue (460 nm) and green (520 nm) LEDs. InGaN epilayers less than 40 Å thick of ∼20% solid phase indium were produced on thick (3‐4 µm) 2″ GaN templates grown on (0001) c ‐plane sapphire substrates. The morphological evolution of the InGaN material was studied utilising a DI3100 AFM tool. Surface morphology and its correlation with photoluminescence and X‐ray diffraction results will be discussed for every set of conditions employed. More specifically, the post‐growth ambient exposure and thermal stability of the uncapped InGaN epilayers were investigated. In addition, the initial stage of subsequent GaN growth, which is an essential step towards the manufacture of LED active regions, was examined. Based on the above findings, a flexible MOCVD growth parameter space and improved LED constituent layer sequencing techniques have been established leading to more efficient and stable LED devices. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of the Morphology and In Distribution of Capped and Uncapped InGaN Layers by Transmission Electron Microscopy

We have compared and analysed the morphology of capped and uncapped thin InGaN layers by transmission electron microscopy. The samples were grown under the same conditions by metal-organic chemical vapour deposition. The capped layer appears to be homogenous in thickness whereas the uncapped one is characterized by the presence of large islands. Moreover, the In distribution was determined in both samples by quantitative high-resolution transmission electron microscopy. The average indium concentration was found to be comparable. Fluctuations of the In concentration on a large scale occur in both cases whereas fluctuations on a scale of a few nanometers are more pronounced in the capped layer.

Far-IR investigation of thin InGaN layers

We have measured the reflection spectra of ultra-thin In x Ga 1− x N/GaN layers on sapphire substrates with thick GaN buffer layer at temperatures of 300, 200 and 90 K. We could not observe any signal associated with these layers. Comparing the measured spectra with the calculated ones for well-mixed In x Ga 1− x N layers, we concluded that our samples had no mixed state. We also concluded that the separated In and N atoms did not form InN layers. The reason for the fact that we did not observe a response from these layers is the inhomogeneous broadening of the In x Ga 1− x N reflection line. We have calculated the absorption spectra of small parallelepiped-like InN particles in the GaN matrix. It has shown that the absorption line width for such small parallelepiped-like particles is bigger than that for the bulk InN material. We concluded that our samples contained separated In and demonstrated formation of InN clusters.

Novel type of ohmic contacts to p-doped GaN using polarization fields in thin InxGa1−xN capping layers

Thin p-doped InGaN layers on p-doped GaN were successfully used to demonstrate a new type of low-resistance ohmic contact. A significant reduction of specific contact resistance can be achieved by increasing the free-hole concentration and the probability for hole tunneling through the Schottky barrier as a consequence of polarization-induced band bending. As obtained from the transmission-line method, the specific contact resistances of Ni (10 nm)/Au (30 nm) contacts deposited on InGaN capping layers were 1.2 × 10-2 Ωcm2 and 6 × 10-3 cm2 for capping layer thicknesses of 20 nm and 2 nm, respectively.

Ohmic contacts to p-type GaN mediated by polarization fields in thin InxGa1−xN capping layers

Low-resistance ohmic contacts are demonstrated using thin p-type InGaN layers on p-type GaN. It is shown that the tunneling barrier width is drastically reduced by polarization-induced electric fields in the strained InGaN capping layers resulting in an increase of the hole tunneling probability through the barrier and a significant decrease of the specific contact resistance. The specific contact resistance of Ni (10 nm)/Au (30 nm) contacts deposited on the InGaN capping layers was determined by the transmission line method. Specific contact resistances of 1.2×10−2 Ω cm2 and 6×10−3 Ω cm2 were obtained for capping layer thicknesses of 20 nm and 2 nm, respectively.

Electroabsorption Spectroscopy ? Direct Determination of the Strong Piezoelectric Field in InGaN/GaN Heterostructure Diodes

We report on room temperature electroabsorption measurements on (In)GaN heterostructure p-i-n diodes. This technique allows a precise determination of the strong changes of internal fields at the InGaN/GaN hetero interface due to differences in the spontaneous polarisation and strain-induced piezoelectric fields. The measured fields vary between 1 and 1.4 MV/cm for thin InGaN layers with an In content ranging from approximately 7% to 9%. Absorption spectroscopy is also well suited to investigate the optical properties of GaN layers. In contrast to Si-doped or intrinsic material Mg-doped GaN films reveal a pronounced absorption tail below the band edge with absorption coefficients of about 30 cm -1 at typical emission wavelengths (λ 400 nm) of InGaN laser diodes.

Electroabsorption Spectroscopy — Direct Determination of the Strong Piezoelectric Field in InGaN/GaN Heterostructure Diodes

We report on room temperature electroabsorption measurements on (In)GaN heterostructure p-i-n diodes. This technique allows a precise determination of the strong changes of internal fields at the InGaN/GaN hetero interface due to differences in the spontaneous polarisation and strain-induced piezoelectric fields. The measured fields vary between 1 and 1.4 MV/cm for thin InGaN layers with an In content ranging from approximately 7% to 9%. Absorption spectroscopy is also well suited to investigate the optical properties of GaN layers. In contrast to Si-doped or intrinsic material Mg-doped GaN films reveal a pronounced absorption tail below the band edge with absorption coefficients of about 30 cm -1 at typical emission wavelengths (λ 400 nm) of InGaN laser diodes.

Photoconductance measurements and Stokes shift in InGaN alloys

We report the results of photoluminescence (PL), transmission and photoconductance (PC) studies of In x Ga 1− x N alloys grown by metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). First, we demonstrate that PC measurements allow to determine the gap energy and give the same value as transmission experiments in the case of thin InGaN layers. Second, we have performed PL and PC measurements in thin InGaN films. The comparison between the PL peak and PC measurements gives the Stokes shift. We study the Stokes shift as a function of temperature, and explained it in terms of localization and Burstein−Moss effect. Finally, we show that our measurements can be extended to the case of InGaN/GaN quantum dots.

Low-Resistance Electrical Contacts to p-Type GaN by Using InxGa1-xN Cap Layers

AbstractA novel type of low-resistance ohmic contacts is demonstrated utilizing polarization-induced electric fields in thin p-type InGaN layers on p-type GaN. An increase of the hole tunneling probability through the barrier and a concomitant significant decrease of the specific contact resistance can be attributed to a reduction of the tunneling barrier width in the InGaN capping layers due to the polarization-induced electric fields. The specific contact resistance of Ni (10 nm) / Au (30 nm) contacts deposited on the InGaN capping layers was determined by the transmission line method. Specific contact resistances of 1.2 × 10-2 Ω cm2 and 6 × 10-3 & cm2 were obtained for capping layer thicknesses of 20 nm and 2 nm, respectively.

Photoconductance measurements on thin InGaN layers

We report photoluminescence (PL), transmission and photoconductance (PC) studies of InxGa1−xN and GaN alloys grown by metalorganic chemical vapor deposition. We demonstrate that PC measurements can provide reliable gap estimates for a large range of film thicknesses in contrast to transmission or photoreflectance experiments. We show that PC measurements give the expected classical value for thick GaN layers by using an elementary model for absorption. They also give the same value as transmission experiments in the case of thin InGaN layers. We have performed PL and PC measurements in InGaN layers. The comparison between the PL peak and PC measurements gives a direct access to the Stokes shift. We study the Stokes shift as a function of temperature. Depending on the particular film, the Stokes shift can be explained in terms of localization or the Burstein–Moss effect. Indeed, Hall measurements show a high electron concentration in some of the films. Thus, our measurements show that very homogeneous InGaN layers with high indium content (22%) can be obtained with a very small Stokes shift and with a very narrow PL peak. In addition, we give the temperature dependence of the InGaN band-gap.

Dislocation reduction in GaN films through selective island growth of InGaN

Dislocation reduction in GaN films grown on sapphire and silicon substrates was achieved by inserting thin InGaN layers grown in a selective island growth mode after partial passivation of the GaN surface with a submonolayer of silicon nitride. We show that this technique is most effective at reducing the pure edge dislocation density when it is high (i.e., >1010 cm−2) and less when the density is in the 108–109 cm−2 range. Thus, the structural quality of typically highly dislocated GaN on silicon films could be significantly improved, visible in a reduction of the (0002) full width at half maximum (FWHM) from 1300 arcsec for ordinary GaN on silicon to 800 arcsec for GaN films with silicon nitride/InGaN interlayers. In the case of GaN layers grown on sapphire (dislocation density ∼109 cm−2), the method resulted mainly in a reduction of the FWHM of the (101̄2) and (202̄1) diffraction peaks.

Effect of the growth rate and the barrier doping on the morphology and the properties of InGaN/GaN quantum wells

InGaN/GaN single and multi quantum wells have been grown by metal-organic chemical vapor deposition, varying the growth rate of well and barrier layers as well as the Si-doping of the GaN barriers. Separately, the effect of these growth parameters on the surface morphology of thin GaN and InGaN layers grown under the same conditions had been studied. The surface morphology of the layers strongly influenced the structural properties of the multi quantum wells. The optical properties seemed to be less affected by the observed layer thickness fluctuations in the 200–500 nm range rather than by variations in the indium composition on a shorter length scale.