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Electroluminescence in GaN
JOURNAL OF LUMINESCENCE 4(1971) 63-66 © North-Holland Publshing Co.
LETTER TO THE EDITOR ELECTROLUMINESCENCE IN GaN J. I. PANKOVE, E. A. MILLER, D. RICHMAN and J. E. BERKEYHEISER RCA Laboratories, Princeton, New Jersey, U.S.A. Received 7 May 1971 Electroluminescence in Zn-doped GaN peaks at 2.6 eV. It is attributed to the injection of holes generated by breakdown at internal boundaries.
Although luminescence has been induced in GaN by electron-beam and optical excitation’ 5), this is the first report of electroluminescence in this material. The measurements were made at room temperature. The GaN was grown by the vapor transport technique6) on a sapphire substrate. The semiconductor was doped with Zn during growth. The presence of Zn gives a reddish tinge to the otherwise colorless material and strongly compensates the unintentionally present donors thus making the
U)
z
/
/ I
EL I—
*
I
\PL
I
I
z w I-
w 0 z w 0
I
I
(I)
/
z
/ /F
-J -—--
3.0
2.0 ‘—hi’ (eV)
Fig. I. Electroluminescence (EL) and photoluminescence (PL) spectra of Zn-doped GaN. 63
64
i. i. PANKOVE ET AL.
material semi-insulating. In the absence of Zn, the electron concentration is about 2 x 1018 cm3. Photoluminescent spectra of various specimens indicate that Zn forms a deep acceptor about 0.8 eV above the valence band. In a partly compensated specimen, electroluminescence was obtained at room temperature by passing a dc current between two point contacts. Blue light was emitted near one or both contacts. The corresponding emission spectrum shown in fig. I consists of a broad peak at 2.6 eV; its width at half maximum is 290 meV. Reversing the polarity of the current does not change substantially the emission spectrum although its intensity may be different than with the first polarity. The shape and position of the emission spectrum do not change as the intensity of the current is varied over one order of magnitude. In this material, the photoluminescent spectrum obtained by excitation with a UV laser peaks at 2.4 eV and its width is about 350 meV at room temperature (fig. 1). No near-gap luminescence is obtained with either optical or electrical excitation in Zn-doped GaN. This contrasts to the photoluminescence of “undoped” material which peaks at about 3.4 eV.4). Microscopic observation under high magnification reveals that the light is generated at many spots near the interface to the sapphire substrate. The morphology of most light spots seems to correlate in shape and distribution with grain boundaries which manifest themselves in the texture of the free surface through the angular shape of hillocks and valleys. When the polarity of the current is reversed, different spots light up. However, some of the new spots are very near to and similar in shape to spots seen with the first polarity. At those spots which light up at nearly the same place with both bias polarities, when the polarity is reversed the light appears to switch from one side to the other side of a grain boundary; in this case, the light appears from the side nearest the negative electrode. The current—voltage characteristic is typical of that expected from avalanche breakdown at many grain boundaries in series and parallel forming n—i—n transitions. The breakdown portion of the 1(V) characteristic is very rounded and occurs between 60 and 100 V depending on position and spacing of the two probes. The 1(V) characteristic is usually, but not always, symmetrical. The light intensity varies approximately as the ~ power of the current over at least two orders of magnitude. The light intensity at 0.2 mA is bright enough to be easily seen in a well-lit room. Under pulsed conditions, the time constant for turn on and turn off of luminescence is less than 10 ns at low currents (< 0.2 mA). At high currents, the fast rise is followed by a slower rise with a time constant of about 1 ~~s;similarly, the fast decay is followed by a slower decay with a 1 ~ts time constant.
EL.ECTROLUMINESCENCE IN
GaN
65
hi,
I
/
(b)
/
I
+
-
I’
V
II I Fig. 2. Model for the mechanism of electroluminescence at a grain boundary (GB): (a) no bias applied; (b) with bias.
A model for the present electroluminescence mechanism7) is shown in fig. 2. Accordingly, the crystal would consist of many identically oriented microcrystals. The Zn acceptors would segregate at the grain boundaries forming a plurality of n—i—n transitions. When a current is passed through the material, electrons tunnel from interfacial acceptors to the conduction band at some grain boundary with a subsequent avalanche breakdown in the high field region which is biased positively with respect to the grain boundary. The holes thus generated are injected into the negatively-biased side of the boundary where radiative recombination to the acceptor levels occurs, in agreement with our observations. We wish to thank R. T. Smith for the X-ray diffraction, Mrs. M. Harvey for mounting the specimen and Drs. H. Kressel, D. Redfield and J. J. Tietjen for helpful discussions.
66
J. I. PANKOVE PT AL.
References 1) 2) 3) 4)
H. G. Grimmeiss and H. Koelmans, Z. Naturforsch. 14a (1959) 264. H. G. Grimmeiss, R. Groth and J. Maak, Z. Naturforsch. 15a (1960) 799. M. R. Lorenz and B. B. Binowski, J. Electrochem. Soc. 109 (1962) 24. J. I. Pankove, J. E. Berkeyheiser, I-I. P. Maruska and J. Wittke, Solid State Commun. 8 (l970~1051. 5) H. G. Grimmeiss and B. Monemar, J. AppI. Phys. 41(1970) 4054. 6) H. P. Maruska and J. J. Tietjen, AppI. Phys. Letters 15 (1969) 327. 7) A similar type of mechanism has been proposed by A. G. Fisher, in: Luminescence of Inorganic Semiconductors, Ed. P. Goldberg (Academic Press, New York, 1966) p. 576.
LETTER TO THE EDITOR ELECTROLUMINESCENCE IN GaN J. I. PANKOVE, E. A. MILLER, D. RICHMAN and J. E. BERKEYHEISER RCA Laboratories, Princeton, New Jersey, U.S.A. Received 7 May 1971 Electroluminescence in Zn-doped GaN peaks at 2.6 eV. It is attributed to the injection of holes generated by breakdown at internal boundaries.
Although luminescence has been induced in GaN by electron-beam and optical excitation’ 5), this is the first report of electroluminescence in this material. The measurements were made at room temperature. The GaN was grown by the vapor transport technique6) on a sapphire substrate. The semiconductor was doped with Zn during growth. The presence of Zn gives a reddish tinge to the otherwise colorless material and strongly compensates the unintentionally present donors thus making the
U)
z
/
/ I
EL I—
*
I
\PL
I
I
z w I-
w 0 z w 0
I
I
(I)
/
z
/ /F
-J -—--
3.0
2.0 ‘—hi’ (eV)
Fig. I. Electroluminescence (EL) and photoluminescence (PL) spectra of Zn-doped GaN. 63
64
i. i. PANKOVE ET AL.
material semi-insulating. In the absence of Zn, the electron concentration is about 2 x 1018 cm3. Photoluminescent spectra of various specimens indicate that Zn forms a deep acceptor about 0.8 eV above the valence band. In a partly compensated specimen, electroluminescence was obtained at room temperature by passing a dc current between two point contacts. Blue light was emitted near one or both contacts. The corresponding emission spectrum shown in fig. I consists of a broad peak at 2.6 eV; its width at half maximum is 290 meV. Reversing the polarity of the current does not change substantially the emission spectrum although its intensity may be different than with the first polarity. The shape and position of the emission spectrum do not change as the intensity of the current is varied over one order of magnitude. In this material, the photoluminescent spectrum obtained by excitation with a UV laser peaks at 2.4 eV and its width is about 350 meV at room temperature (fig. 1). No near-gap luminescence is obtained with either optical or electrical excitation in Zn-doped GaN. This contrasts to the photoluminescence of “undoped” material which peaks at about 3.4 eV.4). Microscopic observation under high magnification reveals that the light is generated at many spots near the interface to the sapphire substrate. The morphology of most light spots seems to correlate in shape and distribution with grain boundaries which manifest themselves in the texture of the free surface through the angular shape of hillocks and valleys. When the polarity of the current is reversed, different spots light up. However, some of the new spots are very near to and similar in shape to spots seen with the first polarity. At those spots which light up at nearly the same place with both bias polarities, when the polarity is reversed the light appears to switch from one side to the other side of a grain boundary; in this case, the light appears from the side nearest the negative electrode. The current—voltage characteristic is typical of that expected from avalanche breakdown at many grain boundaries in series and parallel forming n—i—n transitions. The breakdown portion of the 1(V) characteristic is very rounded and occurs between 60 and 100 V depending on position and spacing of the two probes. The 1(V) characteristic is usually, but not always, symmetrical. The light intensity varies approximately as the ~ power of the current over at least two orders of magnitude. The light intensity at 0.2 mA is bright enough to be easily seen in a well-lit room. Under pulsed conditions, the time constant for turn on and turn off of luminescence is less than 10 ns at low currents (< 0.2 mA). At high currents, the fast rise is followed by a slower rise with a time constant of about 1 ~~s;similarly, the fast decay is followed by a slower decay with a 1 ~ts time constant.
EL.ECTROLUMINESCENCE IN
GaN
65
hi,
I
/
(b)
/
I
+
-
I’
V
II I Fig. 2. Model for the mechanism of electroluminescence at a grain boundary (GB): (a) no bias applied; (b) with bias.
A model for the present electroluminescence mechanism7) is shown in fig. 2. Accordingly, the crystal would consist of many identically oriented microcrystals. The Zn acceptors would segregate at the grain boundaries forming a plurality of n—i—n transitions. When a current is passed through the material, electrons tunnel from interfacial acceptors to the conduction band at some grain boundary with a subsequent avalanche breakdown in the high field region which is biased positively with respect to the grain boundary. The holes thus generated are injected into the negatively-biased side of the boundary where radiative recombination to the acceptor levels occurs, in agreement with our observations. We wish to thank R. T. Smith for the X-ray diffraction, Mrs. M. Harvey for mounting the specimen and Drs. H. Kressel, D. Redfield and J. J. Tietjen for helpful discussions.
66
J. I. PANKOVE PT AL.
References 1) 2) 3) 4)
H. G. Grimmeiss and H. Koelmans, Z. Naturforsch. 14a (1959) 264. H. G. Grimmeiss, R. Groth and J. Maak, Z. Naturforsch. 15a (1960) 799. M. R. Lorenz and B. B. Binowski, J. Electrochem. Soc. 109 (1962) 24. J. I. Pankove, J. E. Berkeyheiser, I-I. P. Maruska and J. Wittke, Solid State Commun. 8 (l970~1051. 5) H. G. Grimmeiss and B. Monemar, J. AppI. Phys. 41(1970) 4054. 6) H. P. Maruska and J. J. Tietjen, AppI. Phys. Letters 15 (1969) 327. 7) A similar type of mechanism has been proposed by A. G. Fisher, in: Luminescence of Inorganic Semiconductors, Ed. P. Goldberg (Academic Press, New York, 1966) p. 576.