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shown in our previous paper [18] for the ideal c-GaN, the above model provides the energy band structure in excellent agreement with results of more accurate but time consuming methods, such as the full potential augmented plane wave and full potential LMTO methods. RESULTS AND DISCUSSION

1.0

The TB-LMTO calculations of the ideal c-GaN (see [18]) give

20-

10-

10

2N"

the direct LDA gap of 1.92eV,

2r,.Go

band (VB) composed of N2p

with the upper part of the valence

0 .14% 0

2

r

•20-

i•

hybridized with Ga4p Both Ga4s and N2p states

.states > -states.

I00 *

'u(W ION a

contribute to the states near the

i limull

: 1*G,

,

V

set

_

0

so-

"10

20

4

--

a.8

Torm

,

,

0

0

4

E.wgy(eW)

(a)

(b) 4 1 2,Ndii

subband merges with the VB edge for both cases. The Fermi

I0 rat

level crosses the maximum of each impurity band and results in the creation of holes in the VB. Both Be and Mg dopings may

k•

0. 10

result in the p-type conductivity of

2

c-GaN. But the energy and spatial localization of Be and Mg states very different. Be2p states are hybridized with the whole VB in

00 o. 82 -

1

">

-8_2are

o"

o .

I

•1o-

I

lNud

,

,gli1

4-,

rather uniform way (Fig. l(a)),

lulNul.l

S

A4distant

0

4

0

Enerw(ev) E Eki

0

,atom

l

S0

_,

0

TOM

0-

0 Enry(evW)

(d)

V-4

conduction band (CB) edge. andFigs. l(a) and (b) show total and shell-projected density of states (DOS) obtained in the 64supercell TB-LMTO calculations for the Be and Mg impurities substituting a Ga atom respectively. A narrow impurity

TW

0-.

Ene (W)

(0)

Fig.1. Total and shell-projected densities of states for Be (a), impurities at Ga sites, and Si impurity at Ga (c) (d) sites. Shell-projected densities are given for four coordinational spheres around impurity.

632

mix noticeably with the states of shell of atoms from the impurity (up to 4th N and Ga). Mg states (Fig. l(b)) are essentially shifted to the upper part of the VB and form the sharp partially occupied band at the VB edge. This is in line with the recent photoluminescence data

[7], from which the Mg acceptor

binding energy was estimated to be 0.25eV (wurtzite phase). The analysis of shell-projected DOS showed that Mg acceptor states (of Mg2p character) are much more localized in space than the ones of Be

(A)

impurity: for doping Mg, atoms of the 4-th shell sphere practically do not feel the presence of the impurity. The spatial extensions of impurity states can be seen by calculating the charge density maps. In Fig. 2, the maps of the total charge density for the c-GaN with a Be (Fig. 2(a)) and a Mg (Fig. 2(c)) impurities are presented. All the charge distributions are given in the (110) plane. As is seen, the chemical bonding of both impurities in the host crystal is of highly ionic characteriThe charge density maps for impurity levels at the VB edge (Figs. 2(b) and (d)) show that both the Be and the Mg atoms polarize strongly the states of many shells of neighboring atoms. Such a polarization is especially strong in the Be impurity case. Thes