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BL (2.9eV) band in undoped and Zn-doped GaN

BL (2.9eV) band in undoped and Zn-doped GaN

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Point Defects in GaN



Figure 9 YL and GL bands in HVPE-grown freestanding GaN. (A) Transformation of the

YL band into the GL band with increasing excitation intensity at T ¼ 16 K. (B) PL spectra

at selected time delays after a laser pulse at T ¼ 100 K. Lines are calculated using Eq. (7)

with the following parameters: Se ¼ 8.5, ℏΩe ¼ 41meV, ℏωmax ¼ 2:4eV, and E0 ¼ 2.91 eV

(curves 1 and 2 for the GL band), and Se ¼ 7.4, ℏΩe ¼ 52meV, ℏωmax ¼ 2:18eV, and

E0 ¼ 2.63 eV (curve 3 for the YL band).

GaN grown by the HVPE and MOCVD methods where Zn is introduced as

a Ga source contaminant or due to a memory effect. The BL band is

asymmetric, with a characteristic fine structure on the high-energy side.

A sharp peak at 3.10 eV, identified as the ZPL, is followed by two sets of

phonon replicas: an LO phonon mode (91 meV) and a local or quasi-local

phonon mode (36 meV) (Reshchikov and Morkoc¸, 2005; Reshchikov


Michael A. Reshchikov



Figure 10 (A) Low-temperature PL spectra from undoped GaN (sample th1011), lightly

Zn-doped ([Zn] ¼ 1.7 Â 1017 cmÀ3) GaN (sample ap274), and heavily Zn-doped ([Zn] ¼

5 Â 1018 cmÀ3) GaN (sample s451). The BL band has a ZPL at 3.10 eV. (B) The temperature dependence of the BL band quantum efficiency for undoped GaN (sample th1011),

Zn-doped ([Zn] ¼ 2 Â 1019 cmÀ3) GaN (sample s452), and GaN codoped with Si and Zn

([Si] ¼ 1019 cmÀ3, [Zn] % 6 Â 1017 cmÀ3) GaN (sample 1142).

et al., 2000). In GaN samples lightly doped with Zn, the BL band has the

same asymmetrical shape and traces of the phonon structure (Fig. 10A).

In GaN heavily doped with Zn, the fine structure disappears, and the PL

band broadens. This may be in part due to potential fluctuations in highresistivity GaN:Zn.

In conductive n-type GaN (both undoped and lightly doped with Zn),

the BL band is quenched at temperatures above 200 K, with the activation

Point Defects in GaN


energy of about 300–350 meV (Fig. 10B). The quenching is explained by

the thermal emission of holes from the ZnGa acceptor to the valence band

and their recapture by other recombination channels. When the QE of the

BL band is very high (over 90%), the quenching region shifts to higher temperatures, because the ZnGa acceptors efficiently recapture the emitted holes

(Reshchikov et al., 2012). According to Eq. (3), the shift of the quenching

region to higher temperatures can also be caused by shorter PL lifetime in

GaN with a higher concentration of free electrons.

For high-resistivity GaN:Zn (sample s452 in Fig. 10B), the quenching of

PL is very different: the BL band intensity abruptly drops at a characteristic

temperature T*, which shifts to higher temperatures as the excitation intensity increases; i.e., the quenching is tunable by the excitation intensity. This

abrupt and tunable quenching is explained by a sudden redirection of the

recombination flow from the radiative channel (via the ZnGa acceptor) to

a nonradiative channel (via some unknown deep donor) at temperatures

when the thermal emission of holes from the ZnGa acceptor to the valence

band becomes significant (Reshchikov, 2014b; Reshchikov et al., 2011). At

T < T*, the nonradiative donor is saturated with photogenerated electrons

due to its large electron-capture cross-section. This causes a population

inversion in the system (higher-energy levels in the gap are almost

completely filled with electrons, while lower-energy levels have a deficit

of electrons), a buildup of electrons in the conduction band, and a conversion of conductivity from p-type to n-type under UV illumination

(Reshchikov, 2014d). At T % T*, the concentration of the thermally emitted holes becomes approximately equal to the concentration of free holes

generated by light. The additional holes are captured by the deep donor

and unblock the nonradiative channel. A small change in temperature near

T* results in the abrupt reduction of the concentration of free electrons and

electrons bound to the deep donor. At T > T*, the system returns to the

thermal equilibrium population, and conductive n-type converts into

high-resistivity n-type or even p-type. T* shifts to higher temperatures as

the excitation intensity increases, because more thermal holes are needed

to keep the “balance” with photogenerated holes. The abrupt and tunable

quenching can be observed not only for Zn-doped GaN but also for other

high-resistivity semiconductors (Reshchikov, 2014b). Two-step tunable

quenching is sometimes observed and is explained in a similar way

(Reshchikov, 2012b).

Very low concentrations of Zn can cause the BL band (Mohajerani et al,

2013). The concentration of the ZnGa acceptors can be controlled by the


Michael A. Reshchikov

Figure 11 (A) The concentration of the ZnGa acceptors determined from PL as a function of the diethylzinc flow in the MOCVD growth. The PL measurements and analysis

were independently carried out at VCU and at TUBS (Germany). (B) The dependence of

the BL band quantum efficiency on the concentration of the ZnGa acceptors.

diethylzinc (DEZn) flow in the MOCVD growth (Fig. 11A). The BL intensity increases linearly with the concentration of ZnGa up to 1016 cmÀ3, when

it reaches a very high QE (more than 20%). The BL intensity is nearly independent with a further increase in Zn doping and even decreases for very

high doping levels (Fig. 11B).

The Zn-related BL band should not be confused with the BL2 band

(Section 4.3.6) or with the blue band (labeled BLMg) in Mg-doped

Point Defects in GaN


GaN. The blue band in GaN heavily doped with Mg is attributed to transitions from a deep donor ($0.4 eV below the conduction band) to the

shallow MgGa acceptor (Kamiura et al., 2005; Kaufmann et al., 1999;

Reshchikov et al., 1999). Such an attribution explains the very large shift

of the blue band with increasing excitation intensity (from $2.7 to

$3.0 eV), along with other properties of this band. The deep donor is

sometimes attributed to the VNMgGa complex. However, it is known that

the BLMg band can be greatly enhanced by thermal annealing, whereas the

concentration of the VNMgGa complexes significantly decreases after

annealing at T > 500 °C (Hautakangas et al., 2003). Since the BLMg band

is commonly observed in GaN:Mg grown by MOCVD and rarely

observed in the MBE-grown GaN:Mg, one may expect that the deep

donor involved in the BLMg band contains hydrogen as a part of the

defect. Such an assumption is supported by the annealing studies in

H plasma, according to which the BLMg band is greatly enhanced by

remote plasma treatment, with plasma containing atomic hydrogen

(Kamiura et al., 2005).

4.3.6 BL2 (3.0 eV) band in high-resistivity GaN

A broad blue band with a maximum at 3.0 eV (labeled BL2) can be found in

PL spectra from high-resistivity GaN. It is often observed for C-doped GaN

grown by MOCVD, along with the YL band (Polyakov et al., 1996; Seager

et al., 2002, 2004). It can also be seen in undoped or Fe-doped highresistivity GaN grown by MOCVD or HVPE (Reshchikov and Morkoc¸,

2006; Reshchikov et al., 2006b). In some GaN samples (undoped,

C-doped, and Fe-doped), the BL2 band has a fine structure with the

ZPL at 3.33–3.34 eV followed by a few phonon replicas with the dominant

phonon modes having energies of 36 and 91 meV (Fig. 12A). From the position of the ZPL, it was suggested that the BL2 band is caused by transitions of

electrons from the conduction band or from an excited state near it to a level

located at 0.15 eV above the valence band (Reshchikov and Morkoc¸, 2006).

In agreement with this assumption, the BL2 band is quenched above 75 K

with an activation energy of about 150 meV (Reshchikov et al., 2006b;

Seager et al., 2004).

An important feature of the BL2 band is its bleaching during continuous

UV illumination (Reshchikov et al., 2006b). Simultaneously with this

bleaching, the YL band intensity rises (Fig. 12B). It was suggested that

the BL2 band is associated with a defect complex containing hydrogen,

and the bleaching is caused by a recombination-assisted dissociation of this


Michael A. Reshchikov



Figure 12 The BL2 band in high-resistivity GaN. (A) The fine structure of the BL2 band

with the ZPL at 3.33 eV. (B) Evolution of the PL spectrum in MOCVD-grown GaN measured at Pexc % 1 mW cmÀ2 after exposure of the sample to the laser light with

Pexc ¼ 0.3 W cmÀ2 for selected times.

complex under UV exposure (Reshchikov and Morkoc¸, 2005). It is tempting to assign the BL2 band to the CN–H or CNON–H complexes, which

dissociate under UV irradiation to form the CN or CNON defect. The

released hydrogen remains in the crystal lattice and can be trapped back

by the carbon-containing defects after keeping the sample in dark at high

temperatures for an extended period of time. Attribution of the BL2 band

to a complex containing carbon and hydrogen is consistent with the fact that

the BL2 band is observed in GaN grown by either MOCVD or HVPE, the

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BL (2.9eV) band in undoped and Zn-doped GaN

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