- Email: [email protected]
MOVPE growth and characterization of Mg-doped GaN
Journal of Crystal Growth 195 (1998) 265—269
MOVPE growth and characterization of Mg-doped GaN P. Kozodoy*, S. Keller, S.P. DenBaars, U.K. Mishra Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA
Abstract Mg-doping of GaN films has been investigated as a function of growth rate, reactor pressure and N to Ga flow ratio (V/III ratio). The nominal doping level, determined by the Mg to Ga flow ratio, was kept constant. Secondary-ion-massspectroscopy (SIMS) measurements indicate that more Mg is incorporated at low pressure; the incorporation efficiency is independent of the other variables. The electrical properties of the films grown at atmospheric pressure appear independent of growth rate and V/III ratio, displaying an average hole concentration of 7.4;10 cm\ and mobility of 9.3 cm/V s. However, films grown at a reduced pressure of 76 Torr exhibit a strong dependence on V/III ratio; the conductivity is observed to improve by an order of magnitude when the V/III ratio is raised by a factor of four. We observe a simultaneous increase in hole concentration and mobility in these films suggesting that a high degree of compensation, probably due to nitrogen vacancies, is present in the films grown at low V/III ratios. Finally, roomtemperature photoluminescence (PL) measurements were performed on the samples. The PL spectra are dominated by a broad peak at 2.8 eV; the peak intensity is found to increase with increasing V/III ratio. 1998 Elsevier Science B.V. All rights reserved. PACS: 61.72.V; 81.15.G Keywords: GaN; Mg; p-Type; Compensation; Vacancy; MOVPE
1. Introduction The successful achievement of Mg-doping of GaN for p-type conduction [1,2] has enabled rapid progress in the MOVPE growth of high quality optoelectronic devices [3]. However, due to the deep nature of the Mg acceptor [4], it is necessary
* Corresponding author. Fax: #1 805 893 3262; e-mail: [email protected].
to incorporate large quantities of Mg in the film in order to obtain low resistivity layers. The electrical characteristics of heavily Mg-doped GaN films have been shown to be quite sensitive to the MOVPE growth conditions [5—7], so a careful exploration of the growth parameter space is needed. Several studies in the literature have examined the effect of Mg source flow on the electrical properties of the resulting films [8,9], but other parameters such as pressure and N to Ga flow ratio (V/III ratio) have received only limited attention.
0022-0248/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 6 7 6 - 9
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P. Kozodoy et al. / Journal of Crystal Growth 195 (1998) 265–269
Recent work [7,10] indicates that compensation by nitrogen vacancies plays an important role in the p-type doping of GaN, suggesting that these growth parameters will have a significant impact on the electrical characteristics of p-type GaN. In this paper we study the effect of reactor pressure, growth rate and V/III ratio on the electrical and optical properties of Mg-doped GaN.
2. Experimental procedure The GaN films were deposited on c-plane sapphire substrates using a horizontal two-flow MOVPE reactor system [11]. Growth precursors were trimethylgallium (TMGa), ammonia (NH ) and biscyclopentadienyl-magnesium (Cp Mg). Each growth consisted of a p—i—n layer structure; the growth conditions for the top (Mg-doped) layer have been varied in this study. The bottom n-type layers were Si-doped using disilane and grown approximately 2 lm thick. This was capped with 0.5 lm undoped highly-resistive GaN, and finally the 0.5 lm thick Mg-doped layer. This p—i—n structure is useful for making electrical measurements on the Mg-doped layer because the resulting p—n junction has a high reverse breakdown voltage; as a consequence a high bias can be applied between two p-contacts and only hole current will flow between them, enabling accurate Hall measure-
ments. In practice the applied voltage was kept below 10 V; the resulting current was typically below 100 lA. This layer structure also avoids any structural changes to the film which may occur if Mg doping is employed from the beginning of the growth on sapphire. The Mg-doped GaN layers have been characterized through Hall effect measurements, secondary ion mass spectroscopy (SIMS) and photoluminescence measurements. The growth parameters which were varied in this study include reactor pressure, growth rate and the molar flow ratio of NH to TMGa#Cp Mg (V/III ratio). Growths have been performed at both atmospheric pressure and at a reduced pressure of 76 Torr, at growth rates ranging from 0.5 lm/h to 2 lm/h and V/III ratios between 6900 and 27 700. The molar flow ratio of TMGa to Cp Mg was 142; this was kept constant so that the nominal doping concentration is identical for all the growths. The total flow through the reactor was kept constant in all the runs and is approximately 10 standard liters per minute (slpm). The Mg-doped layers were grown in a H ambient and a post-growth anneal ing step was employed to activate the dopants. The anneal was performed in a rapid thermal annealer at 950°C for 3 min in a N ambient. The ten samples grown for this study are divided into three series; the growth conditions and electrical data are summarized in Table 1. In series 1 and series 2 the GaN growth rate and the V/III ratio
Table 1 Growth parameters and electrical data for the samples under study. The samples have been divided into three series Series
Sample
Pressure (Torr)
Flow TMG (lmol/min)
Flow NH (slpm)
V/III ratio
Hole concentration (cm\)
Mobility (cm/V s)
1
105PA 130PD 426PA 818PC
760 760 760 760
38.4 38.4 19.3 9.6
6.0 6.0 6.0 6.0
6900 6900 13 800 27 700
8.2;10 7.3;10 6.9;10 7.2;10
10.2 8.0 10.7 7.4
2
714PD 814PC 818PA
76 76 76
38.4 19.3 9.6
6.0 6.0 6.0
6900 13 800 27 700
1.1;10 9.4;10 2.5;10
6.4 12.1 19.6
3
114PC 114PB 114PA
76 76 76
9.6 9.6 9.6
1.5 3.0 6.0
6900 13 800 27 700
6.1;10 1.1;10 2.7;10
9.7 14.5 19.2
P. Kozodoy et al. / Journal of Crystal Growth 195 (1998) 265–269
were varied simultaneously by changing the TMGa flow (and also the Cp Mg flow) while keeping the NH flow constant at 6 slpm. Series 1 was grown at atmospheric pressure and series 2 was grown at 76 Torr. In these growths, the TMGa flow was varied from 9.6 to 38.4 lmol/min corresponding to a growth rate variation from 0.5 to 2.0 lm/h (the growth rate is independent of pressure). Series 3 was grown at 76 Torr and the growth rate was kept constant at 0.5 lm/h; in this group the V/III ratio was varied by reducing the NH flow (H was added in order to keep the total flow constant). Two of the growths are repeats and were included in order to check consistency (105PA and 130PD, 818PC and 114PA).
267
Fig. 1. Resistivity as a function of growth rate for the samples in series 1 and series 2. The NH flow is kept constant in these growths, so both the growth rate and V/III ratio vary simultaneously.
3. Results and discussion SIMS measurements have been performed in order to correlate the atomic Mg concentration with growth conditions. The Mg concentration appears to be quite independent of growth rate and V/III ratio, but is sensitive to the total pressure during growth. All the films grown at atmospheric pressure yield a Mg concentration of approximately 4;10 cm\, while those grown at 76 Torr contain about 1;10 cm\. We note that the SIMS measurements yield only the chemical concentration and that not all of the Mg atoms may be active as acceptors (especially at the higher concentration). The increase in Mg incorporation efficiency at low pressure is tentatively attributed to a decline in parasitic gas phase reactions. Evidence for the formation of an adduct with NH has been present ed for growth using the chemically similar bismethylcyclopentadienyl-magnesium [12], and it seems likely that the same effect is occurring here. Based on the SIMS data we calculate that Mg incorporation efficiency is 13% of the Ga incorporation efficiency at 760 Torr and 32% at 76 Torr. The electrical characteristics of the samples have been investigated through room temperature Hall effect measurements; the results are summarized in Table 1. The samples grown at atmospheric pressure show very little variation; the average hole concentration in these samples is 7.4;10 cm\
while the mobility varies between 7.4 and 10.7 cm/V s. At 76 Torr, however, the electrical properties demonstrate a dramatic dependence on the growth conditions. By reducing the growth rate and increasing the V/III ratio, the resistivity of the film is reduced by almost an order of magnitude. This data for the first two growth series is summarized in Fig. 1. Fig. 2 shows the hole concentration and mobility for low-pressure-grown films as a function of the V/III ratio. The data are quite similar for the two series, indicating that it is the V/III ratio and not simply the growth rate which is controlling the electrical characteristics. We note that both the hole concentration and the mobility increase with increasing V/III ratio, combining to give the large reduction in resistivity seen in Fig. 1. The simultaneous increase in hole mobility and concentration suggests that the improved electrical characteristics are due to a reduction in compensation rather than an increase in acceptor concentration. This interpretation is supported by the SIMS data, which shows no significant difference in Mg concentration for any of the films measured in Fig. 2. The most likely candidate for the compensation mechanism is the formation of nitrogen vacancies, which are widely believed to behave as donors. Recent publications have suggested that the formation of nitrogen vacancies is much more likely in p-type GaN than in n-type GaN due to the
268
P. Kozodoy et al. / Journal of Crystal Growth 195 (1998) 265–269
Fig. 2. Electrical characteristics of samples grown at low pressure (76 Torr). The dotted lines are intended solely as eye-guides. In series 2 the V/III ratio was varied by keeping the NH flow constant and changing the TMGa flow; in series 3 the TMGa flow was kept constant and the NH flow was varied.
motion of the fermi level [10], and that the concentration of these vacancies can be minimized by increasing the NH flow during growth [7]. During growth at atmospheric pressure the absolute NH over-pressure is quite high (approxi mately 0.6 atm) so that nitrogen vacancy formation is minimized, and no strong dependence on V/III ratio is observed within the range that has been investigated. At a reduced pressure of 76 Torr, however, the NH over-pressure is ten times lower and in this regime we believe that the V/III ratio during growth has a considerable impact on the nitrogen vacancy concentration. According to this model, growth at low pressure and low V/III ratio encourages the formation of compensating nitrogen vacancies and degrades the electrical characteristics of the film. The possibility of compensation through incorporation of an external impurity was checked by SIMS measurements of C, O, H and Si. Only the carbon concentration was observed to increase at reduced pressure and V/III ratio. However, the carbon is not believed to incorporate as a donor, so it is not a candidate for the compensating species [10]. Another possible mechanism for the degradation in electrical properties is the formation at low NH overpressure of Mg-complexes or clusters which do not act as shallow acceptors.
Photoluminescence measurements of Mg-doped GaN have been studied extensively in the literature [13—15]. Two peaks are commonly identified, one at approximately 3.2 eV and a broader “blue band” centered at 2.8 eV. The origin of these peaks remains unclear but the former has often been attributed to recombination from the conduction band to the acceptor level [14], while the latter (which dominates in heavily-doped samples) has been ascribed to Mg complexes [15] or a donor—acceptor transition involving the Mg acceptor and a compensating nitrogen vacancy [14]. Given these proposed explanations for the Mg-related luminescence, a correlation between photoluminescence properties and the degree of compensation is expected. We have performed room temperature photoluminescence measurements on the annealed samples using a 325 nm HeCd laser with an excitation density of 40 W/cm. The spectra are dominated by the broad level centered at approximately 2.8 eV; the peak at 3.2 eV is discernible only when the 2.8 eV luminescence is very weak. In Fig. 3 the intensity of the 2.8 eV peak is plotted as a function of V/III ratio during growth. The PL becomes more intense as the V/III ratio is increased from 6900 to 13 800, but appears to saturate as the V/III ratio is raised higher. The same trend is evident regardless of growth pressure, although the
Fig. 3. Intensity of the 2.8 eV PL peak as a function of V/III ratio. The dotted lines are intended solely as eye-guides.
P. Kozodoy et al. / Journal of Crystal Growth 195 (1998) 265–269
low-pressure-grown samples exhibit considerably higher luminescence intensity (due to the higher Mg content in these films). The trend toward increased intensity at higher V/III ratio appears to contradict the hypothesis that this photoluminescence peak is associated with the compensating mechanism. However the data may simply reflect a reduction in the efficiency of competing nonradiative recombination paths due to improved crystal quality at high V/III ratios. We note that even at the highest V/III ratio the hole concentration in the low-pressure grown films is lower than in the atmospheric-pressure films, despite the higher Mg concentration. Therefore a large portion of the Mg atoms in these heavily doped films remain inactive, possibly as a result of more than one mechanism (such as complex formation or the presence of various compensating donors). A clearer understanding of both the 2.8 eV luminescence and the electrical properties of the heavily Mg-doped films requires more detailed growth experiments and photoluminescence measurements; these are the subjects of ongoing research efforts.
4. Conclusions The electrical and optical properties of MOVPE grown Mg-doped GaN have been investigated as a function of reactor pressure, growth rate and V/III ratio. For growth at atmospheric pressure the electrical properties of the films are relatively constant, but at low growth pressure a high V/III ratio must be maintained in order to achieve low resistivity p-type films. Compensation by nitrogen vacancy formation is proposed as the origin of this effect. Room temperature photoluminescence of these films is dominated by a broad peak at approximately 2.8 eV; the intensity of this peak is found to increase with increasing V/III ratio.
269
Acknowledgements This work was supported by DARPA through a contract supervised by Dr. Robert F. Leheny and by AFOSR through a contract supervised by Dr. Gerald Witt.
References [1] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (Part 2) (1989) L2112. [2] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (Part 2) (1992) L139. [3] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Heidelberg, 1997. [4] H. Nakayama, P. Hacke, M.R.H. Khan, T. Detchprohm, K. Hiramatsu, N. Sawaki, Jpn. J. Appl. Phys. 35 (Part 2) (1996) L282. [5] C.J. Eiting, P.A. Grudowski, R.D. Dupuis, Electron. Lett. 33 (1997) 1987. [6] N. Akutsu, H. Tokunga, I. Waki, A. Yamaguchi, K. Matumoto, Mater. Res. Soc. Symp. Proc. 482 (1998) 113. [7] S.N. Mohammad, A.E. Botchkarev, A. Salvador, W. Kim, O. Aktas, H. Morkoc, Philos. Magn. B 76 (1997) 131. [8] H. Lu, I. Bhat, Mater. Res. Soc. Symp. Proc. 395 (1996) 497. [9] C.J. Eiting, P.A. Grudowski, J. Park, D.J.H. Lambert, B.S. Shelton, R.D. Dupuis, J. Electrochem. Soc. 144 (1997) L219. [10] J. Neugebauer, C.G. Van de Walle, Mater. Res. Soc. Symp. Proc. 395 (1996) 645. [11] B.P. Keller, S. Keller, D. Kapolnek, W.-N. Jiang, Y.-F. Wu, H. Masui, X.H. Wu, B. Heying, J.S. Speck, U.K. Mishra, S.P. DenBaars, J. Electron. Mater. 24 (1995) 1707. [12] S. Haffouz, B. Beaumont, M. Leroux, M. Laugt, P. Lorenzini, P. Gibart, L.G. Hubert-Pfalzgraf, MRS Internet J. Nitride Semicond. Res. 2 (1997) 37. [13] E. Oh, H. Park, Y. Park, Appl. Phys. Lett. 72 (1998) 70. [14] U. Kaufmann, M. Kunzer, M. Maier, H. Obloh, A. Ramakrishnan, B. Santic, P. Schlotter, Appl. Phys. Lett. 72 (1998) 1326. [15] J.M. Myoung, K.H. Shim, C. Kim, O. Gluschenkov, K. Kim, S. Kim, D.A. Turnbull, S.G. Bishop, Appl. Phys. Lett. 69 (1996) 2722.
MOVPE growth and characterization of Mg-doped GaN P. Kozodoy*, S. Keller, S.P. DenBaars, U.K. Mishra Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA
Abstract Mg-doping of GaN films has been investigated as a function of growth rate, reactor pressure and N to Ga flow ratio (V/III ratio). The nominal doping level, determined by the Mg to Ga flow ratio, was kept constant. Secondary-ion-massspectroscopy (SIMS) measurements indicate that more Mg is incorporated at low pressure; the incorporation efficiency is independent of the other variables. The electrical properties of the films grown at atmospheric pressure appear independent of growth rate and V/III ratio, displaying an average hole concentration of 7.4;10 cm\ and mobility of 9.3 cm/V s. However, films grown at a reduced pressure of 76 Torr exhibit a strong dependence on V/III ratio; the conductivity is observed to improve by an order of magnitude when the V/III ratio is raised by a factor of four. We observe a simultaneous increase in hole concentration and mobility in these films suggesting that a high degree of compensation, probably due to nitrogen vacancies, is present in the films grown at low V/III ratios. Finally, roomtemperature photoluminescence (PL) measurements were performed on the samples. The PL spectra are dominated by a broad peak at 2.8 eV; the peak intensity is found to increase with increasing V/III ratio. 1998 Elsevier Science B.V. All rights reserved. PACS: 61.72.V; 81.15.G Keywords: GaN; Mg; p-Type; Compensation; Vacancy; MOVPE
1. Introduction The successful achievement of Mg-doping of GaN for p-type conduction [1,2] has enabled rapid progress in the MOVPE growth of high quality optoelectronic devices [3]. However, due to the deep nature of the Mg acceptor [4], it is necessary
* Corresponding author. Fax: #1 805 893 3262; e-mail: [email protected].
to incorporate large quantities of Mg in the film in order to obtain low resistivity layers. The electrical characteristics of heavily Mg-doped GaN films have been shown to be quite sensitive to the MOVPE growth conditions [5—7], so a careful exploration of the growth parameter space is needed. Several studies in the literature have examined the effect of Mg source flow on the electrical properties of the resulting films [8,9], but other parameters such as pressure and N to Ga flow ratio (V/III ratio) have received only limited attention.
0022-0248/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 6 7 6 - 9
266
P. Kozodoy et al. / Journal of Crystal Growth 195 (1998) 265–269
Recent work [7,10] indicates that compensation by nitrogen vacancies plays an important role in the p-type doping of GaN, suggesting that these growth parameters will have a significant impact on the electrical characteristics of p-type GaN. In this paper we study the effect of reactor pressure, growth rate and V/III ratio on the electrical and optical properties of Mg-doped GaN.
2. Experimental procedure The GaN films were deposited on c-plane sapphire substrates using a horizontal two-flow MOVPE reactor system [11]. Growth precursors were trimethylgallium (TMGa), ammonia (NH ) and biscyclopentadienyl-magnesium (Cp Mg). Each growth consisted of a p—i—n layer structure; the growth conditions for the top (Mg-doped) layer have been varied in this study. The bottom n-type layers were Si-doped using disilane and grown approximately 2 lm thick. This was capped with 0.5 lm undoped highly-resistive GaN, and finally the 0.5 lm thick Mg-doped layer. This p—i—n structure is useful for making electrical measurements on the Mg-doped layer because the resulting p—n junction has a high reverse breakdown voltage; as a consequence a high bias can be applied between two p-contacts and only hole current will flow between them, enabling accurate Hall measure-
ments. In practice the applied voltage was kept below 10 V; the resulting current was typically below 100 lA. This layer structure also avoids any structural changes to the film which may occur if Mg doping is employed from the beginning of the growth on sapphire. The Mg-doped GaN layers have been characterized through Hall effect measurements, secondary ion mass spectroscopy (SIMS) and photoluminescence measurements. The growth parameters which were varied in this study include reactor pressure, growth rate and the molar flow ratio of NH to TMGa#Cp Mg (V/III ratio). Growths have been performed at both atmospheric pressure and at a reduced pressure of 76 Torr, at growth rates ranging from 0.5 lm/h to 2 lm/h and V/III ratios between 6900 and 27 700. The molar flow ratio of TMGa to Cp Mg was 142; this was kept constant so that the nominal doping concentration is identical for all the growths. The total flow through the reactor was kept constant in all the runs and is approximately 10 standard liters per minute (slpm). The Mg-doped layers were grown in a H ambient and a post-growth anneal ing step was employed to activate the dopants. The anneal was performed in a rapid thermal annealer at 950°C for 3 min in a N ambient. The ten samples grown for this study are divided into three series; the growth conditions and electrical data are summarized in Table 1. In series 1 and series 2 the GaN growth rate and the V/III ratio
Table 1 Growth parameters and electrical data for the samples under study. The samples have been divided into three series Series
Sample
Pressure (Torr)
Flow TMG (lmol/min)
Flow NH (slpm)
V/III ratio
Hole concentration (cm\)
Mobility (cm/V s)
1
105PA 130PD 426PA 818PC
760 760 760 760
38.4 38.4 19.3 9.6
6.0 6.0 6.0 6.0
6900 6900 13 800 27 700
8.2;10 7.3;10 6.9;10 7.2;10
10.2 8.0 10.7 7.4
2
714PD 814PC 818PA
76 76 76
38.4 19.3 9.6
6.0 6.0 6.0
6900 13 800 27 700
1.1;10 9.4;10 2.5;10
6.4 12.1 19.6
3
114PC 114PB 114PA
76 76 76
9.6 9.6 9.6
1.5 3.0 6.0
6900 13 800 27 700
6.1;10 1.1;10 2.7;10
9.7 14.5 19.2
P. Kozodoy et al. / Journal of Crystal Growth 195 (1998) 265–269
were varied simultaneously by changing the TMGa flow (and also the Cp Mg flow) while keeping the NH flow constant at 6 slpm. Series 1 was grown at atmospheric pressure and series 2 was grown at 76 Torr. In these growths, the TMGa flow was varied from 9.6 to 38.4 lmol/min corresponding to a growth rate variation from 0.5 to 2.0 lm/h (the growth rate is independent of pressure). Series 3 was grown at 76 Torr and the growth rate was kept constant at 0.5 lm/h; in this group the V/III ratio was varied by reducing the NH flow (H was added in order to keep the total flow constant). Two of the growths are repeats and were included in order to check consistency (105PA and 130PD, 818PC and 114PA).
267
Fig. 1. Resistivity as a function of growth rate for the samples in series 1 and series 2. The NH flow is kept constant in these growths, so both the growth rate and V/III ratio vary simultaneously.
3. Results and discussion SIMS measurements have been performed in order to correlate the atomic Mg concentration with growth conditions. The Mg concentration appears to be quite independent of growth rate and V/III ratio, but is sensitive to the total pressure during growth. All the films grown at atmospheric pressure yield a Mg concentration of approximately 4;10 cm\, while those grown at 76 Torr contain about 1;10 cm\. We note that the SIMS measurements yield only the chemical concentration and that not all of the Mg atoms may be active as acceptors (especially at the higher concentration). The increase in Mg incorporation efficiency at low pressure is tentatively attributed to a decline in parasitic gas phase reactions. Evidence for the formation of an adduct with NH has been present ed for growth using the chemically similar bismethylcyclopentadienyl-magnesium [12], and it seems likely that the same effect is occurring here. Based on the SIMS data we calculate that Mg incorporation efficiency is 13% of the Ga incorporation efficiency at 760 Torr and 32% at 76 Torr. The electrical characteristics of the samples have been investigated through room temperature Hall effect measurements; the results are summarized in Table 1. The samples grown at atmospheric pressure show very little variation; the average hole concentration in these samples is 7.4;10 cm\
while the mobility varies between 7.4 and 10.7 cm/V s. At 76 Torr, however, the electrical properties demonstrate a dramatic dependence on the growth conditions. By reducing the growth rate and increasing the V/III ratio, the resistivity of the film is reduced by almost an order of magnitude. This data for the first two growth series is summarized in Fig. 1. Fig. 2 shows the hole concentration and mobility for low-pressure-grown films as a function of the V/III ratio. The data are quite similar for the two series, indicating that it is the V/III ratio and not simply the growth rate which is controlling the electrical characteristics. We note that both the hole concentration and the mobility increase with increasing V/III ratio, combining to give the large reduction in resistivity seen in Fig. 1. The simultaneous increase in hole mobility and concentration suggests that the improved electrical characteristics are due to a reduction in compensation rather than an increase in acceptor concentration. This interpretation is supported by the SIMS data, which shows no significant difference in Mg concentration for any of the films measured in Fig. 2. The most likely candidate for the compensation mechanism is the formation of nitrogen vacancies, which are widely believed to behave as donors. Recent publications have suggested that the formation of nitrogen vacancies is much more likely in p-type GaN than in n-type GaN due to the
268
P. Kozodoy et al. / Journal of Crystal Growth 195 (1998) 265–269
Fig. 2. Electrical characteristics of samples grown at low pressure (76 Torr). The dotted lines are intended solely as eye-guides. In series 2 the V/III ratio was varied by keeping the NH flow constant and changing the TMGa flow; in series 3 the TMGa flow was kept constant and the NH flow was varied.
motion of the fermi level [10], and that the concentration of these vacancies can be minimized by increasing the NH flow during growth [7]. During growth at atmospheric pressure the absolute NH over-pressure is quite high (approxi mately 0.6 atm) so that nitrogen vacancy formation is minimized, and no strong dependence on V/III ratio is observed within the range that has been investigated. At a reduced pressure of 76 Torr, however, the NH over-pressure is ten times lower and in this regime we believe that the V/III ratio during growth has a considerable impact on the nitrogen vacancy concentration. According to this model, growth at low pressure and low V/III ratio encourages the formation of compensating nitrogen vacancies and degrades the electrical characteristics of the film. The possibility of compensation through incorporation of an external impurity was checked by SIMS measurements of C, O, H and Si. Only the carbon concentration was observed to increase at reduced pressure and V/III ratio. However, the carbon is not believed to incorporate as a donor, so it is not a candidate for the compensating species [10]. Another possible mechanism for the degradation in electrical properties is the formation at low NH overpressure of Mg-complexes or clusters which do not act as shallow acceptors.
Photoluminescence measurements of Mg-doped GaN have been studied extensively in the literature [13—15]. Two peaks are commonly identified, one at approximately 3.2 eV and a broader “blue band” centered at 2.8 eV. The origin of these peaks remains unclear but the former has often been attributed to recombination from the conduction band to the acceptor level [14], while the latter (which dominates in heavily-doped samples) has been ascribed to Mg complexes [15] or a donor—acceptor transition involving the Mg acceptor and a compensating nitrogen vacancy [14]. Given these proposed explanations for the Mg-related luminescence, a correlation between photoluminescence properties and the degree of compensation is expected. We have performed room temperature photoluminescence measurements on the annealed samples using a 325 nm HeCd laser with an excitation density of 40 W/cm. The spectra are dominated by the broad level centered at approximately 2.8 eV; the peak at 3.2 eV is discernible only when the 2.8 eV luminescence is very weak. In Fig. 3 the intensity of the 2.8 eV peak is plotted as a function of V/III ratio during growth. The PL becomes more intense as the V/III ratio is increased from 6900 to 13 800, but appears to saturate as the V/III ratio is raised higher. The same trend is evident regardless of growth pressure, although the
Fig. 3. Intensity of the 2.8 eV PL peak as a function of V/III ratio. The dotted lines are intended solely as eye-guides.
P. Kozodoy et al. / Journal of Crystal Growth 195 (1998) 265–269
low-pressure-grown samples exhibit considerably higher luminescence intensity (due to the higher Mg content in these films). The trend toward increased intensity at higher V/III ratio appears to contradict the hypothesis that this photoluminescence peak is associated with the compensating mechanism. However the data may simply reflect a reduction in the efficiency of competing nonradiative recombination paths due to improved crystal quality at high V/III ratios. We note that even at the highest V/III ratio the hole concentration in the low-pressure grown films is lower than in the atmospheric-pressure films, despite the higher Mg concentration. Therefore a large portion of the Mg atoms in these heavily doped films remain inactive, possibly as a result of more than one mechanism (such as complex formation or the presence of various compensating donors). A clearer understanding of both the 2.8 eV luminescence and the electrical properties of the heavily Mg-doped films requires more detailed growth experiments and photoluminescence measurements; these are the subjects of ongoing research efforts.
4. Conclusions The electrical and optical properties of MOVPE grown Mg-doped GaN have been investigated as a function of reactor pressure, growth rate and V/III ratio. For growth at atmospheric pressure the electrical properties of the films are relatively constant, but at low growth pressure a high V/III ratio must be maintained in order to achieve low resistivity p-type films. Compensation by nitrogen vacancy formation is proposed as the origin of this effect. Room temperature photoluminescence of these films is dominated by a broad peak at approximately 2.8 eV; the intensity of this peak is found to increase with increasing V/III ratio.
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Acknowledgements This work was supported by DARPA through a contract supervised by Dr. Robert F. Leheny and by AFOSR through a contract supervised by Dr. Gerald Witt.
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