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Heavy doping effects in Mg-doped GaN

Peter Kozodoy,a) Huili Xing, Steven P. DenBaars, and Umesh K. MishraDepartment of Electrical and Computer Engineering, University of California, Santa Barbara,California 93106

A. Saxler, R. Perrin, S. Elhamri,b) and W. C. MitchelAir Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLPO,Wright-Patterson AFB, Ohio 45433-7707

Received 23 August 1999; accepted for publication 2 November 1999

The electrical properties of p-type Mg-doped GaN are investigated through variable-temperature

Hall effect measurements. Samples with a range of Mg-doping concentrations were prepared by

metalorganic chemical vapor phase deposition. A number of phenomena are observed as the dopant

density is increased to the high values typically used in device applications: the effective acceptor

energy depth decreases from 190 to 112 meV, impurity conduction at low temperature becomes

more prominent, the compensation ratio increases, and the valence band mobility drops sharply. The

measured doping efficiency drops in samples with Mg concentration above 21020 cm3. 2000

American Institute of Physics. S0021-8979 00 04304-8

INTRODUCTION

Mg-doped p-type GaN is of critical importance for a

host of nitride-based devices including light emitting diodes,

lasers, photodetectors, and bipolar transistors.1 Because of

the deep nature of the Mg acceptor very high doping levels

approximately 1020 cm3) are frequently used in device

structures. Heavy doping effects may be important at these

high doping levels, leading to such phenomena as valence

band-tail states and impurity band formation.2

Temperature-dependent Hall effect measurements have

been performed on Mg-doped GaN by a number of groups,

yielding a range of activation energies for the Mg dopant

between 125 and 215 meV.3 8 This wide spread in experi-

mental data may be at least partially explained by a variationin doping level, and consequently in the importance of heavy

doping effects, in the various samples. In this work we

present a careful examination of the electrical properties of

Mg-doped GaN as a function of dopant concentration. A

wide range of dopant densities are studied in order to inves-

tigate the evolution of high-doping effects both below and

above the optimal dopant concentration. Temperature-

dependent Hall effect measurements are employed to analyze

the electrical properties of each sample.

EXPERIMENT

The GaN samples were grown by metalorganic chemicalvapor deposition on c-plane sapphire substrates. Trimethyl-

gallium TMGa , biscyclopentadienyl-magnesium (Cp2Mg),

and ammonia (NH3) were used as precursors. The layer

structure used was that of a pn junction: a base layer of

approximately 3 m of n-type GaN was first deposited, fol-

lowed by a top layer of Mg-doped GaN 0.5 or 1.0 m thick.

The Mg-doping level was varied by changing the flow of

Cp2Mg during growth from 24 to 428 nmol/min. In allsamples the TMGa molar flow was 19.3 mol/min. Second-

ary ion mass spectroscopy SIMS measurements were per-

formed on the samples in order to measure the chemical Mg

concentration, which scaled roughly with Cp2Mg flow and

varied between 21019 and 81020 cm3. The samples

with the highest magnesium concentration are overdoped,

i.e., the magnesium concentration is so high that the material

qualities have begun to degrade. In the most heavily doped

sample sample F the Mg concentration is estimated to be

about 2% of the Ga concentration, and a severely degraded

surface morphology consisting of densely packed hexagonal

pyramids was observed.The samples were prepared for Hall effect measurements

using lithographically defined van der Pauw structures.

Variable-temperature Hall effect measurements were per-

formed using magnetic fields between 1 and 2 T. During the

measurements the applied voltage was kept below 3 V to

minimize any leakage through the underlying n-type layer.

The hole concentration p was obtained from the Hall

constants RH using prH/qRH with the Hall scattering fac-

tor rH assumed to be of value unity. In Fig. 1 the calculated

hole concentration is plotted as a function of the inverse

temperature for each of the samples. An activation energy

dependence is clearly evident in the high-temperature regimefor all of the samples. In all but the most lightly doped

sample the measured hole concentration increases again at

low temperature, indicating the onset of hopping or impurity-

band conduction. In the most heavily doped samples this

impurity conduction is clearly an important component of

the total room temperature conductivity.

The acceptor activation energy (EA), acceptor concen-

tration (NA), and compensating donor concentration (ND)

were extracted by fitting the high-temperature data to the

formula

a Electronic mail: [email protected] Permanent address: Department of Physics, University of Dayton, Dayton,

OH 45469.

JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 4 15 FEBRUARY 2000

18320021-8979/2000/87(4)/1832/4/$17.00 2000 American Institute of Physics

Downloaded 23 Aug 2006 to 129.74.159.226. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp


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p pND

NANDp

NV

gexp

EA

kT, 1

where k is the Boltzmann constant, T is the temperature, g is

acceptor degeneracy which is assumed to be equal to four,

and NV is the effective valence band density of states

NV2 2m h*kT3/2/h 3, 2

where h is the Planck constant. The hole effective mass m h*in GaN is not well known. Early reports suggested values

around m h*0.8 m0 ,9 while more recent results have yielded

larger values of 1.1m 010 and 2.2 m 0 .

11 The higher value of

m h*2.2 m 0 was used in these calculations because it yields

an improved agreement between the extracted acceptor con-

centration and the SIMS measurements of Mg concentration.

We note that the model used to fit the data is quite

simple; Eq. 1 assumes a single acceptor level and Eq. 2

assumes a parabolic valence band. The formation of a broad

impurity band, or of valence band-tail states, is therefore

beyond the scope of this model. If these effects play an im-

portant role in the heavily doped samples, then the fitting

procedure employed may provide results that are somewhatinaccurate.

RESULTS AND DISCUSSION

Figure 2 and Table I summarize the parameters that pro-

vide the best fit between Eq. 1 and the temperature-

dependent Hall effect measurements. The measured activa-

tion energy for the Mg acceptor decreases as the doping is

increased, going from 190 meV in sample A to 112 meV in

sample E. The material quality begins to degrade before the

Mott transition can be reached and the activation energy rises

again in the severely overdoped sample.

The extracted values of NA and ND are also plotted in

Fig. 2. While the activation energy can be extracted quite

accurately from the least-squares fit to Eq. 1 , there is some

uncertainty in the acceptor and compensating donor concen-trations. This is due not only to the assumption of the hole

mass, but also to the nature of the data fit especially in the

more heavily doped samples where there is little indication

of saturation in the hole concentration measured at high tem-

perature .

Nonetheless, the extracted concentrations match closely

with expected values. The acceptor concentration rises as the

dopant density is increased, in good agreement with the Mg

concentration as measured by SIMS. The highest acceptor

concentration around 21020 cm3) was obtained in

sample D. When the Mg concentration is increased beyond

this point the measured acceptor concentration is actually

reduced, which may indicate that much of the Mg is notincorporating in the desired substitutional site. We also note

that the calculated compensation level (ND /NA) is observed

to rise dramatically as the doping level is increased.

The measured decrease in acceptor activation energy at

high doping levels explains the variation in published results

for the depth of the Mg acceptor. This phenomenon is well

known from conventional semiconductors2,12 and may have

various causes including: the formation of a broad Mg ac-

ceptor band extending toward the valence band edge, the

creation of valence band-tail states extending into the forbid-

den gap, screening of the acceptor potential by free carriers,

and binding energy reduction through a Coulomb interaction

FIG. 1. Hole concentration measured as a function of temperature on Mg-

doped GaN samples. For clarity of presentation the data have been divided

between two separate plots; note that the scale differs on the two plots. The

solid lines represent fits to Eq. 1 .

FIG. 2. Concentration data extracted from the Hall effect and SIMS mea-surements are presented in the top plot. In the lower plot, the solid triangles

represent the measured activation energy and the open triangles represent

that predicted by Eq. 3 .

1833J. Appl. Phys., Vol. 87, No. 4, 15 February 2000 Kozodoy et al.



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between valence-band holes and ionized acceptors.

Gotz et al.7 have suggested an important role for the last

of these effects in Mg-doped GaN. In this case the acceptor

activation energy may be written as

EA NA EA,0f

q2

4 NA

1/3, 3

where EA ,0 is the inherent acceptor energy observed at very

low doping concentrations, (NA

)(1/3) is the average dis-

tance between ionized acceptors, q is the electronic charge,

is the dielectric constant assumed to be 9.5 0), and f is a

geometric factor of value (2/3)(4/3)1/3. 7 We note that

this formulation neglects the repulsive potential of the ion-

ized compensating donors, an assumption which is some-

what justified because the free holes are likely to concentrate

around the ionized acceptors and avoid the donors.12

Gotz suggests that the acceptor energy in Eq. 1 be

re-evaluated at each temperature depending on the ionized

acceptor concentration. However, in this case we are justified

in using a simpler approach in which a single temperature-

independent activation energy is used for each sample. This

is because the ionized acceptor concentration is determinedmainly by the number of compensating donors, not the num-

ber of free holes, across the great majority of the temperature

range used for data fitting. We therefore use Eq. 1 to fit the

Hall effect data and then compare the measured acceptor

energy to that predicted by Eq. 3 using NAND . Assum-

ing an inherent activation energy of EA ,0220 meV, a very

close agreement is obtained between the measured activation

energy and that predicted by Eq. 3 , as shown in Fig. 2. In

fact, the fit is surprisingly good considering the uncertainty

in the values of ND .)

The compensation is believed to be due to a native donor

and/or a Mg-related state. Several authors have suggested

that nitrogen vacancies play a role in the compensation of

p-type GaN.1315 We have shown in other work that the com-

pensation level in Mg-doped GaN may be adjusted by prop-erly tailoring the growth conditions,14 indicating that this is a

parameter which will be dependent, at least partially, on the

choice of growth conditions and the details of reactor design.

Figure 3 presents the hole mobility measured as a func-

tion of temperature. The highest mobility recorded is

62 cm2/V s for the lightly doped sample at T146 K. As the

doping level is increased the peak mobility begins to drop

rapidly due to the high concentration of ionized species re-

sulting from the higher doping level and compensation ratio.

In the most heavily doped sample the hole mobility is ob-

served to increase again slightly, most likely a consequence

of the reduced ionized dopant density in this sample. In most

samples the measured mobility exhibits a precipitous drop atlow temperature; this is due to the onset of hopping conduc-

tion, which is characterized by a very low mobility. The

factors determining the exact temperature dependence of the

valence-band mobility are not fully understood at this

pointa more complete analysis of the mobility measure-

ments is currently underway.

CONCLUSIONS

In conclusion, we have examined the electrical proper-

ties of Mg-doped GaN as a function of doping level. As the

doping level is increased a number of heavy doping effects

are observed, including an increase in both impurity conduc-

tion and degree of compensation. The increased compensa-

tion appears to drive other effects such as a pronounced re-

duction in acceptor activation energy and lower hole

mobility values. The measured acceptor concentration in-

creases with Mg concentration up to the optimum doping

level of 21020 cm3. Beyond this point the doping effi-

ciency drops and at very high doping levels severe morpho-

logical changes are observed.

ACKNOWLEDGMENTS

This work was supported by the Air Force Office of

Scientific Research through a contract monitored by Dr. Ger-

ald Witt and by the Office of Naval Research through a con-

tract monitored by Dr. John Zolper. The authors are grateful

to J. Antoszewski, J. M. Dell, and L. Faraone at the Univer-

sity of Western Australia for preliminary measurements and

useful discussions and to S. Davidson of Wright-Patterson

AFB for technical assistance.

1 O. Ambacher, J. Phys. D 31, 2653 1998 .2 E. F. Schubert, Doping in IIIV Semiconductors Cambridge University

Press, Cambridge, 1993 .3 T. Tanaka, A. Watanabe, H. Amano, Y. Kobayashi, I. Akasaki, S.

Yamazaki, and M. Koike, Appl. Phys. Lett. 65, 593 1994 .

TABLE I. Summary of fitting results from Hall effect measurements. The

Mg concentration measured by SIMS is also listed.

Sample

Cp2Mg flow

nmol/min EA meV NA (cm3) ND (cm

3) Mg (cm3)

A 24.4 190 1.81019 1.11018 1.61019

B 42.3 174 4.61019 3.01018 41019

C 76.5 152 1.41020 1.21019 81019

D 135 118 2.21020 4.11019 21020

E 228 112 8.6

1019

2.7

1019

3

1020

F 428 165 7.61018 5.01018 81020

FIG. 3. Mobility measured as a function of temperature on Mg-doped GaN

samples.

1834 J. Appl. Phys., Vol. 87, No. 4, 15 February 2000 Kozodoy et al.



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4 W. Gotz, N. M. Johnson, J. Walker, D. P. Bour, and R. A. Street, Appl.

Phys. Lett. 68, 667 1996 .5 H. Nakayama, P. Hacke, M. R. H. Khan, T. Detchprohm, T. D. Kazumasa,

K. Hiramatsu, and N. Sawaki, Jpn. J. Appl. Phys., Part 2 35, L282 1996 .6 C. J. Eiting, P. A. Grudowski, and R. D. Dupuis, Electron. Lett. 33, 1987

1997 .7 W. Gotz, R. S. Kern, C. H. Chen, H. Liu, D. A. Steigerwald, and R. M.

Fletcher, Mater. Sci. Eng., B 59, 211 1999 .8 W. Kim, A. E. Botchkarev, A. Salvador, G. Popovidi, H. Tang, and H.

Morkoc, J. Appl. Phys. 82, 219 1997 .9

J. J. Pankove, S. Bloom, and G. Harbecke, RCA Rev. 36, 163 1975 .

10 M. Suzuki and T. Uenoyama, Jpn. J. Appl. Phys., Part 1 34, 3442 1995 .11 J. S. Im, A. Moritz, F. Steuber, V. Harle, F. Scholz, and A. Hangleiter,

Appl. Phys. Lett. 70, 631 1997 .12 P. P. Debye and E. M. Conwell, Phys. Rev. 93, 693 1954 .13 U. Kaufmann, M. Kunzer, M. Maier, H. Obloh, A. Ramakrishnan, B.

Santic, and P. Schlotter, Appl. Phys. Lett. 72, 1326 1998 .14 P. Kozodoy, S. Keller, S. P. DenBaars, and U. K. Mishra, J. Cryst. Growth

195, 265 1998 .15 C. G. Van de Walle, C. Stampfl, and J. Neugebauer, J. Cryst. Growth

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00_kozodoy_heavy doping effects in mg doped gan

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