GaN structure

ARTICLE IN PRESS Physica B 404 (2009) 4907–4910

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Deep-level transient spectroscopy studies of light-emitting diodes based on multiple-quantum-well InGaN/GaN structure Mikhail M. Sobolev , Natalia M. Shmidt Ioffe Physical Technical Institute 26 Politekhnicheskaya ul., 194021 St. Petersburg, Russia

a r t i c l e in fo

Keywords: InGaN/GaN Quantum-well superlattice Wannier-Stark effect

abstract Capacitance–voltage (C–V) and deep-level transient (DLTS) spectroscopies has been applied to study the carrier emission from states of the quantum-well superlattice (QWSL) in a p–n InGaN/GaN heterostructure. Changes in the DLTS spectra of this structure strongly depend on bias-on–bias-off cooling conditions and the applied bias Ub. It is believed that these changes are determined by (i) builtin piezoelectric field that leads to spatial separation of the electron and hole ground-state wave functions of the SL, defined as a manifestation the quantum-confined Stark effect and (ii) localization of the electron wave functions in separate QWs, named the Wannier-Stark localization. & 2009 Elsevier B.V. All rights reserved.

1. Introduction The industry of blue light-emitting diodes (LEDs) based on multiple-quantum-well (MQW) InGaN/GaN structures has been intensively developing during the last decade [1–6]. The substantial increase in the external quantum efficiency (QE) is due to application of InGaN/GaN short-period superlattices (SLs). However, the InGaN/GaN SL has practically not been studied at all. It is believed that this variant has been implemented in commercial LEDs with high QE (Cree). It is a common knowledge that electron wave functions in the SLs may be completely delocalized, with their energies distributed in minibands because of the pronounced coupling between the QWs [7]. The Wannier-Stark (WS) effect in quantum-well (QW) SLs has been predicted and observed [7–10]. As a low uniform electric field is applied to an SL along its growth direction, the energy degeneracy of the miniband energies is lifted, which gives rise to a set of discrete levels, named states of the WS ladder. These states extend over several SL periods with a localization length L. Under optical excitation, new transitions involving the states of the WS ladder become possible. As the electric field increases, the overlapping of the electron wave functions is suppressed and electrons may become localized in separate QWs. Then the system resembles a multiple-QW sample. The interband optical transitions become vertical, which leads to an increase in the oscillator strength. This effect is known as the WS localization [7–10]. The induced localization is usually studied optically by measuring the absorption caused by transitions

 Corresponding author. Postal Address: Ioffe Physical Technical Institute, Russian Academy of Sciences, 26 Politekhnicheskaya, St. Petersburg 194021, Russian Federation. Tel.: +7 812 292 7382; fax: +7 812 247 1017. E-mail address: [email protected] (M.M. Sobolev).

0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.268

between the hole and electron states of isolated wells. Earlier [11–20], we have already studied systems of vertically correlated InAs quantum dots (VCQD) in InAs/GaAs p–n structures [11–18] and a quantum-dot superlattice (QDSL) in a Ge/Si p–n heterostructure [19,20] using the DLTS technique. In contrast to the interband spectroscopy, this method enables not only joint, but also separate analysis of the behavior of electrons and holes in semiconductor InAs/GaAs and Ge/Si p–n structures. This method can be used to study the dependence of the position and amplitude of DLTS peaks on the optical illumination intensity and on the amplitude Ur of the reverse bias pulse indicating the occurrence of quantum-confined Stark effect and to correlate the observed DLTS peaks to quantum states of the superlattice and deep-level defects. In this communication, we report on a detailed study by C–V spectroscopy and DLTS of the carrier emission from MQW states in p–n InGaN/GaN heterostructure under varied recording voltage pulse Ub and filling voltage pulse Uf and biason–bias-off cooling conditions (Ubaa0, Uba ¼ 0).

2. Results and discussion The MQW InGaN/GaN structures under study were grown by metal-organic chemical vapor deposition (MOCVD) under standard conditions. According to the wafer manufacturer, the structure consisted of an n+-GaN layer, a five period InGaN/GaN (3/2 nm) superlattice (SL), and a p+-GaN contact layer. The spatially localized states in QWs, deep-level defects, and the carrier distribution profile in the heterostructures were studied by DLTS and C–V methods with a BIORAD DL4600 spectrometer. Prior to each DLTS measurement, a sample was subjected to isochronous annealing at Ta ¼ 300 K under one of the following

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M.M. Sobolev, N.M. Shmidt / Physica B 404 (2009) 4907–4910

conditions: (i) voltage pulse Uba40 and (ii) zero bias Uba ¼ 0. After that the sample was cooled to 80 K. Then, we started the DLTS measurements in the dark, or if specified otherwise, under exposure to white light. For this structure, we measured I–V curves, which showed that (a) at low biases Uo1 V, the forward and reverse currents have very close values; (b) at intermediate biases 1 VoUo2:5 V, the difference between the forward and reverse currents is noticeable (Fig. 1). We measured the C–V characteristics of an InGaN/GaN p–n heterostructure with 5 layers of Ge QWs. The dependences of the capacitance on the bias voltage Ub exhibit for QW and QD structures a typical space-charge behavior [11–18]. There are two plateaus in the C–V characteristics for the QW structure, which can be attributed to release of electrons accumulated in QW states [11–18]. The carrier density profiles (n*(Ub)) were determined from these C–V characteristics. It follows from (Fig. 2) that the n*(Ub) profiles of the MQW structure demonstrate two peaks: at UbE0.75 V and at UbE2.15 V. The appearance of these peaks related to emptying of quantum states of the wells. From the C–V data, we determined the ranges of bias voltages at which signals associated with the emission of carriers from the MQW states

should be observable in the DLTS spectra. These measurements have shown that, when recording the DLTS signal at Ub ¼ 0, the MQW lie in the space-charge region. Thus, when performing DLTS measurements, it is necessary to analyze the carrier emission from QWs by varying Ub and Uf in the range of positive values. The DLTS study of the carrier emission from quantum states of QWs in p–n InGaN/GaN heterostructures as a function of the recording voltage pulse amplitude Ub demonstrated that three peaks are observed for a multilayer InGaN/GaN heterostructure with QWs: two DLTS peaks (E1 and E2) with a positive sign and related to traps for majority carriers (electrons), and one peak (H1) with a negative sign, related to traps for minority carriers (hole) (Fig. 3). To identify the origin of the levels responsible for these DLTS peaks, we studied how the DLTS spectra depend on the preliminary isochronous annealing conditions (with Uba40 or Uba ¼ 0) (Fig. 4). For all the DLTS peaks, we observed a strong shift in the position of the DLTS peaks toward higher temperatures (E1-E1*, E2-E2* and H1-H2*) and a change of their amplitudes in the case of annealing at Ub ¼ 0, which is due to a shift of the quasi-Fermi level for electrons and holes as a result of recharging of deep-level defects in the GaN layer and a change in

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Fig. 1. (1) Forward and (2) reverse current–voltage (I–V) curves of an InGaN/GaN LED.

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Fig. 3. DLTS spectra of a InGaN/GaN heterostructure with a 5layer MQW, measured under filling voltage pulse Uf ¼ 0.01 V and varied recording voltage pulse Ub. V: 12.45, 22.27, 32.23, and 42.17. The rate window is 200 s1.

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Fig. 2. Apparent electron density profile n*(Ur) of p-n InGaN/GaN heterostructure with 5-layer QWs, obtained from 1 MHz C–V measurements at 82 K after preliminary isochronous annealing at Ta ¼ 300 K and cooling under: (1) Uba40 and (2) Uba ¼ 0.

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Fig. 4. DLTS spectra of a InGaN/GaN heterostructure with a 5-layer MQW, measured under Uf ¼ 0.01 V and Ub ¼ 2.27 V after a preliminary isochronous annealing at Ta ¼ 300 K under: (1) Uba40 and (2) Uba ¼ 0.

ARTICLE IN PRESS M.M. Sobolev, N.M. Shmidt / Physica B 404 (2009) 4907–4910

170 160

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2.0 amplitude of DLTS signal (pF)

the population of the QW energy states [11–18]. Earlier, we have already reported [11–18] that one of the distinctive features applicable to identification of the DLTS peaks with QD and QW states is the dependence of the positions of these peaks on the preliminary isochronous annealing conditions (Ubaa0, Uba ¼ 0). For all of the three DLTS peaks, such a change is observed upon isochronous annealing. This effect is attributed to the emission of carriers from the localized states in QWs. The shift in the position of the DLTS peaks is due to the built-in piezoelectric field leading to spatial separation of the electron and hole ground-state wave functions and to formation of a dipole [1–4]. This effect of the electric field on QWs is known as the quantum-confined Stark effect (QCSE). The spatial separation of injected carriers induces an electrostatic potential that screens the piezoelectric field. The surface density profiles of electrons and holes trapped by the SL states show the carrier localization with one maximum shifted with respect to the other. It is found that the positions of the maxima for all the three DLTS peaks depend on the amplitude Ub of the bias pulse applied to the sample in the DLTS measurements (Fig. 3). We observed that, as the bias Ub increases, E1, E2, and H1 peaks in the DLTS spectra are markedly shifted to lower temperatures. We plotted for E1, E2, and H1 peaks the dependences of the positions of their maxima on the temperature scale on the bias voltage Ub at which the peaks were measured (Fig. 5). When the bias Ub is raised from 2.05 to 2.45 V, the positions of E1, E2, and H1 peaks shift from 96 to 88 K, from 171 to 108 K, and from 140 to 108 K, respectively. The thermal activation energies Ea associated with carrier emission from traps were estimated for E1, E2, and H1 peaks by processing the Arrhenius dependence and using a simulation method based on fitting of the theoretical and experimental DLTS curves. These energies are EaE50, 150, and 110 meV, respectively, at Ur ¼ 2.23 V. Using the DLTS spectra, we determined the dependence of the amplitudes of the signals associated with these levels on the bias voltage Ub (Fig. 6) which, in that instance, was proportional the surface density of electrons and holes trapped by the quantum states of the MQW InGaN/GaN. As shown in Fig. 6, the surface density profiles of the electrons and holes trapped on the E2 and H1 levels demonstrate their localization with one maximum displaced with respect to the other. Possibly, this is manifestation of the spatial separation of the electron and hole ground-state wave functions by the electric field. Let us consider the experimental results obtained in C–V and DLTS measurements. First, the I–V curves show that the forward

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and reverse currents only slightly depend on voltage up to 2.3 V (Fig. 1), which may be due to properties p–i–n structure and to the presence of two junctions, one of which becomes revere-biased, depending on the polarity of the bias voltage applied to the structure. Second, the p–n a structure under study has an n-type base layer within which the MQW InGaN/GaN structure characterized by two maxima in the carrier density profiles n*(Ub) is contained (Fig. 2), is related to emptying of quantum states of the wells. Third, the DLTS spectra (Fig. 3) contain peaks related to the electron and hole ground-states (E2 and H1) whose surface density is localized, approximately, in the middle of the n–type layer. This MQW InGaN/GaN structure (Fig. 6) is characterized by a spatial separation of the electron and hole ground-state wave functions by the piezoelectric field. Fourth, for the energy levels of the E2 and H1 states, there is a strong linear shift in the dependence on the bias voltage Ub applied to the sample in the DLTS measurements (Fig. 5). The results obtained can be understood on the assumption that the structure under study contains not simply an MQW InGaN/GaN structure, but an SL in which minibands are formed for the electron and hole ground-states (E2 and H1) and also for the excited electron states (E1). The presence of a strong built-in piezoelectric field results in that the energy degeneracy of the miniband energies is lifted, which gives rise to a discrete WS ladder spectrum. The energy difference between neighboring states is equal to the change in the electric field potential over a superlattice period. This difference between the neighboring states is insufficient for the DLTS spectra to be resolved, and, consequently, we observe only three broad DLTS peaks related to the electron and hole ground-states (E2 and H1) and to the excited electron states (E1) (Fig. 3). The spatial extent of these states is given by the localization length. It was found experimentally and is shown in Fig. 6.

120 110

3. Conclusions

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Fig. 5. Dependence of the temperature positions of the maxima of DLTS peaks E1, E2, and H1 on the bias voltage Ub.

Thus, an experimental study by C–V and DLTS methods demonstrated that LEDs based on a p–n heterostructure contain an InGaN/GaN SLQW. In the SLQW, the energy degeneracy of the miniband energies is lifted owing to the presence of the built-in piezoelectric field inherent in nitrides with a wurtzite phase and discrete WS states localized in separate QWs are formed, which is typical of the effect of the WS localization.

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M.M. Sobolev, N.M. Shmidt / Physica B 404 (2009) 4907–4910

Acknowledgments This study was supported by the Russian Foundation for Basic Research, Project nos. 08-02-12149-ofi and 08-02-01317-a. References [1] [2] [3] [4] [5] [6]

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