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RL (1.81eV) band in undoped GaN

RL (1.81eV) band in undoped GaN

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Table 2 Parameters of defect-related PL bands in GaN

PL band

ħωmax (eV)

E0 (eV)

EA (eV)

CnA (cm3 s21)



UVL



3.27



3.27



0.2



BL2



3.02



3.34



0.15



BL

AL



2.88

2.56



3.10

$3.0



0.4

0.5



GL



2.4



2.9



0.54



GL2



2.35



2.85



0.43



YL



2.2



2.64



0.86



YL



2.1



2.57



0.9



RL

RL2



1.81

$1.8



2.36

$2.5



1.13



8 Â 10



À13



2 Â 10



À13



1 Â 10



À13



3 Â 10



À12



1 Â 10



À13



2 Â 10



À14



CpA (cm3 s21)



7 Â 10



À8



7 Â 10



À7



2 Â 10



À7



6 Â 10



À7



ħΩe (meV)



ħΩg (meV)



Se



Sg



Identity



60



3



4.2



ZnGa



41



50



8.5



10.4



CN



23



21



27



24



VN



52



57



7.4



8.1



CNON



52



59



7.4



8.4



CN



$30



$20



$20



91

36, 91

36, 91

38



38, 91

$30



332



Michael A. Reshchikov



Figure 4 Main defect-related PL bands in undoped GaN at T ¼ 18 K.



modes. The ZPL at 2.36 eV has been assigned to transitions from a shallow

donor to a deep acceptor level located at 1.13 eV above the valence band.

Such an assignment is supported by the transformation of the fine structure

with increasing temperature (Fig. 5B): the donor–acceptor pair (DAP)-type

lines decrease, while the eA-type lines emerge at T > 40 K.

The identity of the deep acceptor responsible for the RL band is

unknown. Wang et al. (2007) observed the RL band with the same position

and shape (yet without the fine structure) in GaN grown by HVPE, and they

associated this band with the presence of C and O (with concentrations of

about 1019 cmÀ3). However, their attribution of the RL band to transitions

from a deep donor, VNCN, to a deep acceptor, VGaON, is inconsistent with

the first-principles calculations for the VGaON acceptor (Section 2) and contradicts the above conclusion that the donor involved in the DAP transition

is shallow.

4.3.2 RL2 (1.8 eV) and GL2 (2.35 eV) bands in Ga-rich,

high-resistivity GaN

An RL band with a maximum at about 1.8 eV was observed in highresistivity GaN grown by MBE in Ga-rich conditions (Reshchikov and

Morkoc¸, 2005). It is labeled RL2, to distinguish it from the RL band

described in Section 4.3.1, because the properties of the RL and RL2 bands

are very different. The RL2 band commonly appears together with a green

PL band, which has a maximum at 2.35 eV and is labeled GL2 (Fig. 6).



333



Point Defects in GaN



A



B



Figure 5 PL spectra from undoped GaN grown by HVPE at Pexc ¼ 1 mW cmÀ2. (A) PL

spectra at 18 and 200 K. (B) Fine structure of the RL band at 18 and 40 K. Reproduced

with permission from Reshchikov et al. (2014b), Copyright 2014, AIP Publishing LLC.



McNamara et al. (2013) have found that the RL2 and GL2 bands can also be

observed in the near-surface layer of high-purity freestanding GaN mechanically polished and containing a great number of defects in the top 1-μmthick layer (sample 1412.3 in Fig. 6).

The RL2 band has an FWHM of 300 meV at low temperatures. It is

quenched at temperatures above 100 K with an activation energy of about

100 meV, and its intensity decreases by two orders of magnitude by

200 K. The position of the maximum of the RL2 band varies between

1.75 and 1.9 eV in different samples. However, the temperature behavior

of PL is reproducible for all the samples containing this PL band. The



334



Michael A. Reshchikov



Figure 6 The RL2 and GL2 bands in GaN at T ¼ 18 K and Pexc ¼ 1 mW cmÀ2. The RL2

band has a maximum at 1.80, 1.75, and 1.90 eV, and the GL2 band has a maximum

at 2.34, 2.31, and 2.40 eV in MBE GaN (samples svt591 and svt385), and HVPE GaN (sample 1412.3), respectively. The dashed lines show fits for sample svt385 using Eq. (7) with

the following parameters: Se ¼ 23, ℏΩe ¼ 30meV, ℏωmax ¼ 1:75eV, E0 ¼ 2.48 eV (RL2

band), and Se ¼ 26.5, ℏΩe ¼ 23meV, ℏωmax ¼ 2:34eV, E0 ¼ 2.84 eV (GL2 band).



RL2 band can be excited resonantly with photons having energy higher than

2.7 eV. The shape and position of the RL2 band are consistent with the

shape and position of its PLE spectrum, provided that the ZPL is located

at about 2.5 eV. The activation energy of $0.1 eV is most probably associated with the thermal emission of electrons from an excited state to the conduction band. The decay of the RL2 band after the laser pulse is exponential

at low temperatures, which can be explained by an internal transition from

an excited state (a level at 0.1 eV below the conduction band) to the ground

state (a level at $0.9 eV above the valence band).

In high-resistivity GaN grown under Ga-rich conditions, a green PL

band with a maximum at 2.35 eV is observed. It has unique properties

and is called the GL2 band to distinguish it from the GL band with a maximum at 2.4 eV in high-purity GaN grown by HVPE. The GL2 band is relatively narrow (FWHM is 230 meV) for a deep defect, and its shape can be

fit using Eq. (7) with E0 ¼ 2.85 eV and Se ¼ 26.5 (Reshchikov et al., 2014a).

Such high Huang–Rhys factors (Sg ¼ 24, Se ¼ 26.5) and small characteristic

phonon energies (ℏΩe ¼ 23meV and ℏΩg ¼ 21meV) are typical for deep

donors (Alkauskas et al., 2012).

Reshchikov et al. (2014a) have attributed the GL2 band to the isolated

nitrogen vacancy, VN. While for a majority of PL bands in n-type GaN, the



Point Defects in GaN



335



PL decay after a pulsed excitation is nonexponential at low temperatures

because DAP-type transitions dominate, and the separations in these pairs

are random, the decay of the GL2 band is exponential for a wide range

of temperatures (15–100 K), with a characteristic PL lifetime of about

0.3 ms. To explain this unusual behavior, Reshchikov et al. (2014a)

suggested that the observed GL2 band is caused by an internal transition,

whereby the weakly localized electron collapses to the localized orbital,

and the center converts from V2N+ to VN+ . The PL quenching with an activation energy of $100 meV in the temperature range of 100–200 K is attributed to the thermal emission of electrons from the 0/+ level of VN to the

conduction band, whereas the PL quenching with an activation energy of

$400 meV at higher temperatures is attributed to the thermal emission of

holes from the +/2+ level to the valence band.

4.3.3 YL (2.2 eV) band in MOCVD and MBE GaN

The YL band with a maximum at about 2.2 eV is the most common defectrelated PL band in n-type GaN. Recent studies show that there are at least

two YL bands originating from different defects. In this section, the YL band

in MOCVD or MBE GaN will be considered, while a similar in shape and

position YL band observed in HVPE GaN samples will be discussed in

Section 4.3.4. Figure 7 shows the YL band in undoped and Si-doped

GaN grown by MOCVD and MBE. It has the same shape and position

in different samples. The YL band shape is fit using Eq. (7) with Se ¼ 7.4,

ℏωmax ¼ 2:21eV, and E0 ¼ 2.64 eV. The inset of Fig. 7 shows the highenergy side of the YL band, where the onset at 2.61 eV can be identified

as the position of the ZPL.

The characteristic excitation band maximum for the YL band was estimated to be at 3.19 eV (Ogino and Aoki, 1980) or 3.32 eV (Reshchikov

et al., 2002b) from the analysis of the shape of the PL excitation spectrum.

This energy corresponds to transition AB in the CC diagram shown in

Fig. 3A. The transition level of the YL-related defect at low temperature

can be estimated as 0.85–0.89 eV by taking the difference between the

bandgap and the ZPL. It can also be found from the thermal quenching

of the YL band.

The thermal quenching of the YL band begins at temperatures above

400 K (Fig. 8) and can be described with Eq. (3). For the majority of

GaN samples grown by MOCVD and MBE (undoped and Si-doped),

the activation energy of the defect responsible for the YL band is

0.85–0.9 eV (Reshchikov and Morkoc¸, 2005). However, in some samples,



336



Michael A. Reshchikov



Figure 7 The YL band in undoped and Si-doped GaN grown by MBE and MOCVD. The

symbols are experimental points (every 10th point is shown): empty circles—MOCVD

GaN:Si (n ¼ 7 Â 1018 cmÀ3 at room temperature); empty triangles—MOCVD GaN

(n ¼ 2 Â 1016 cmÀ3); filled triangles—MBE GaN (n ¼ 3 Â 1016 cmÀ3). The line is calculated

using Eq. (7) with the following parameters: Se ¼ 7.4, ℏΩe ¼ 56meV, ℏωmax ¼ 2:21eV,

and E0 ¼ 2.64 eV. The inset shows a high resolution of the high-energy edge with an

apparent ZPL at 2.61 eV for the MOCVD GaN:Si sample (n ¼ 7 Â 1018 cmÀ3).



Figure 8 The temperature dependence of the YL intensity for MOCVD GaN,

n ¼ 2 Â 1016 cmÀ3 (empty triangles) and MOCVD GaN:Si, n ¼ 1 Â 1019 cmÀ3 (filled circles). The solid and dashed lines are calculated using Eq. (7) with the following parameters: EA ¼ 0:85eV, σ pA ¼ 7 Â 10À14 cm2 , η0 ¼ 0:05, and τ ¼ 0:35ms (curve 1 for undoped

GaN); EA ¼ 0:45eV, σ pA ¼ 4 Â 10À17 cm2 , η0 ¼ 0:05, and τ ¼ 14μs (curve 2 for Si-doped

GaN); and mp ¼ 0:8m0 and g ¼ 2 for both. The inset shows normalized low-temperature

PL spectra for these samples.



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