Localized exciton dynamics in InGaN quantum well structures

doi:10.1016/S0169-4332(01)00907-2

Applied Surface Science

Volume 190, Issues 1–4, 8 May 2002, Pages 330-338

https://doi.org/10.1016/S0169-4332(01)00907-2 Get rights and content

Abstract

InGaN multiple quantum well laser diode (LD) wafer that lased at 400 nm was shown to have the InN mole fraction, x, of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit. However, the wells exhibited a Stokes-like shift (SS) of 49 meV and an effective localization depth E₀ was estimated by time-resolved photoluminescence (TRPL) measurement to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells is estimated to be as small as 286 kV/cm, SS is considered to originate from an effective bandgap inhomogeneity. Because the well thickness fluctuation was insufficient to produce SS or E₀, the exciton localization is considered to be an intrinsic phenomenon in InGaN material. Indeed, bulk cubic In_0.1Ga_0.9N, which does not suffer any polarization field or thickness fluctuation effect, exhibited an SS of 140 meV at 77 K and similar TRPL results. The origin of the localization is considered to be due to the large bandgap bowing and In clustering in InGaN material. Such shallow and low density localized states are leveled by injecting high density carriers under the lasing conditions for the 400 nm LDs.

Introduction

In_xGa_1−xN quantum wells (QWs) are attracting attention because they serve as an active region [1], [2], [3] of UV to visible light emitting diodes (LEDs) and purple laser diodes (LDs). Since they exhibit an efficient emission with external quantum efficiency, η_ext, up to 20% at 470 nm in spite of large threading dislocation (TD) density up to 10¹⁰ cm⁻² [4], optical properties of InGaN QWs have been investigated intensively. Internal electric field, F, due to spontaneous and piezoelectric polarization [5], [6] has been shown to modify the QW energy states through quantum-confined Stark effect (QCSE) [7]; redshift of the emission peak compared to unperturbed QW resonance energy [5], [8] and reduction of electron–hole wavefunction overlap (oscillator strength) [6], [9]. Coulomb screening due to carrier injection [5], [8] or impurity doping [9] recovers the overlap. A predominant reason for the efficient emission has been proposed [8], [9] to be due to short quasi-diffusion length [10] of carriers, which is caused by the presence of quantum-disk (Q-disk)-size [11] effective bandgap inhomogeneity [8], [9], [10], [12] or quantum dots (QDs) [13], [14], [15]. However, degree of the compositional fluctuation and structural quality of practical InGaN multiple quantum well (MQW) LD structures exhibiting the lasing wavelengths between 390 and 410 nm, which are proper wavelengths to obtain low threshold current density and long-lived cw lasers, are not fully understood yet.

In this article, results of structural and compositional analyses on InGaN MQW LD structure that lased at around 400 nm are shown in addition to the results of static and time-resolved (TR) photoluminescence (PL) spectroscopy to clarify the spontaneous emission mechanisms in terms of QW exciton [7], [16] localization due to large bandgap bowing in InGaN alloys. To exclude the disturbance due to F and thickness fluctuation effect usually seen in hexagonal QW structures, bulk cubic In_0.1Ga_0.9N film and MQWs were also examined.

Section snippets

Experimental

In_xGa_1−xN MQW LD wafer that lased at 400 nm was grown by two-flow metal organic vapor phase epitaxy on sapphire (0 0 0 1) substrates. It consisted of a 30 nm thick GaN low temperature nucleation layer, a 3.5 μm thick GaN:Si template, a 0.16 μm thick InGaN compliance layer, a 0.3 μm thick AlGaN:Si cladding, a 0.1 μm thick GaN:Si waveguide, an MQW, a 20 nm thick AlGaN:Mg electron-overflow blocking layer, a 0.1 μm thick GaN:Mg, a 0.3 μm thick AlGaN:Mg and a 0.18 μm thick GaN:Mg layer. The MQW consisted of 10

Structural and optical properties of h-InGaN MQW LD wafer

The MQW LD structure had atomically flat and abrupt interfaces, as shown in HR-TEM bright-field images in Fig. 1. The upper limit of the thickness fluctuation (sum of the two interfaces) of InGaN wells is not greater than two monolayers (2 ML; 0.52 nm) over a few micrometers considering possible defocusing effects.

Well-resolved superlattice (SL) satellite peaks due to InGaN MQW are found in the HR-XRD pattern, as shown in Fig. 2. The entire structure is confirmed to be pseudomorphically grown on

Conclusions

In_0.06Ga_0.94N MQW LD structure that lased at 400 nm was shown to have atomically flat interfaces and very small compositional inhomogeneity. However, excitons were confirmed to be localized within the DOS which might have an exponential-tail type distribution. Although the dimensionality of the localization is not yet resolved, the variation of τ_r for c-InGaN implied that lateral confinement is weak at 300 K. Q-disks might be responsible. However, such shallow and low density localized states

Acknowledgements

The authors are grateful to Dr. Alex Zunger and Dr. M. Sugawara for stimulating discussions. They wish to thank Dr. Y. Ishida, N. Ohtake, T. Kuroda, M. Sugiyama and T. Kitamura for help in experiments. They are thankful to Professor F. Hasegawa for continuous encouragements. This work was supported in part by the Ministry of Education, Science, Sports, and Culture of Japan (Grant-in-Aid for Scientific Research No. 11750268), Ogasawara Foundation for the Promotion of Science and Engineering and

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Localized exciton dynamics in InGaN quantum well structures

Abstract

Introduction

Section snippets

Experimental

Structural and optical properties of h-InGaN MQW LD wafer

Conclusions

Acknowledgements

Phys. Rev. B

Appl. Phys. Lett.

Jpn. J. Appl. Phys.

Jpn. J. Appl. Phys.

Nature

Jpn. J. Appl. Phys.

Phys. Rev. B

Phys. Stat. Sol. (b)

Phys. Rev. Lett.

Phys. Rev. B

Appl. Phys. Lett.

Mater. Sci. Eng. B

MRS Internet J. Nitride Semicond. Res.

Appl. Phys. Lett.

J. Appl. Phys.

Appl. Phys. Lett.

Jpn. J. Appl. Phys.

Phys. Rev. Lett.

Appl. Phys. Lett.

Phys. Rev. B