- Email: [email protected]
Some aspects of GaN growth on GaAs(100) substrates using molecular beam epitaxy with an RF activated nitrogen-plasma source
N
ELSEVIER
.........
CRYSTAL GROWTH
Journal of Crystal Growth 155 (1995) 157-163
Some aspects of GaN growth on GaAs(100) substrates using molecular beam epitaxy with an RF activated nitrogen-plasma source S.E. Hooper a,*, C.T. Foxon a T.S. Cheng a, L.C. Jenkins a D.E. Lacklison J.W. Orton b, T. Bestwick c, A. Kean c, M. Dawson c, G. Duggan c
b
a Department of Physics, University of Nottingham, University Park, Nottingham, NG7 2RD, UK b Department of Electrical and Electronic Engineering, University of Nottingham, Nottingham, UK c Sharp Laboratories of Europe Ltd., Oxford Science Park, Oxford OX4 4GA, UK Received 16 January 1995; manuscript received in final form 4 April 1995
Abstract
We have investigated how supplying active nitrogen from an RF activated plasma sourcc under various plasma conditions influences certain aspects of the growth of GaN films on GaAs(100) substrates, using molecular beam epitaxy. In the first instance, the quantity of active nitrogen generated by the source was found to have a strong dependence on both the RF power and amount of nitrogen gas supplied to the plasma. In addition, the degree of optical discharge from the plasma was observed to give a semi-quantitative measure of active nitrogen. No observable loss of nitrogen from the sample surface in the temperature range 450 to 680°C was found during GaN growth. Scanning electron microscopy on the cleaved edges of the GaN/GaAs(100) samples showed the GaN layer to be polycrystalline with a columnar nature typical of a highly lattice mismatched material system. X-ray diffraction measurements indicated that the GaN layers were entirely wurtzite in structure, with the full width at half maximum of the GaN (0002) reflection in the range 9 to 11.5 arcmin. A broad peak centred at around 3.4 eV was recorded using room temperature photoluminescence measurements on the layers.
1. I n t r o d u c t i o n
Gallium nitride in its wurtzite form has a direct band-gap of around 3.4 eV at room temperature [1]. Such a wide band-gap makes it a promising material system for semiconductor device applications emitting in the blue-ultraviolet region of the electromagnetic spectrum.
* Corresponding author. Fax: + 4 4 115 9515180.
In recent years, many studies have investigated the growth by molecular beam epitaxy (MBE) of GaN on a variety of different substrates, including sapphire [2-6], Si [7,8] and GaAs [9-11]. Gallium arsenide offers several advantages as a substrate material for I I I - V nitride growth, one of these being that the cubic nature of GaAs makes it possible to grow GaN in either its zincblende [9,10], or wurtzite phase [11]. For the epitaxial growth of GaN by MBE it is necessary to supply a molecular beam of reactive
0(J22-0248/95/$0950 © 1995 Elsevier Science B.V. All rights reserved SSDI [ ) 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 2 1 6 - 2
S.E. Hooper et aL /Journal of Crystal Growth 155 (1995) 157-163
158
nitrogen. Since simple nitrogen gas is itself inactive it must be activated by using one of several commercially available sources. These sources include the Kaufrnan ion gun [4,6,10], the electron cyclotron resonance microwave plasma (ECR) [3,7,9], and the radio frequency (RF) activated glow discharge plasma. The ECR source is currently the most popular method of supplying active nitrogen, making it reasonably well characterised. Whereas, use of the RF activated plasma source in III-V nitride growth is much less documented and therefore needs to be addressed, although it has been used as an effective source of p-type doping for MBE grown II-VI semiconductors [12-14]. Consequently, the aim of the work presented in this paper, was to investigate how varying the plasma source parameters affected certain aspects of the growth of GaN on GaAs(100) substrates. In particular, the growth rate of GaN has been studied under various plasma conditions.
Information on the structural (using X-ray diffraction and scanning electron microscopy) and optical nature (using room temperature photoluminescence) of the layers is also presented.
2. Experimental details All material growth was carried out in a VarJan Mod Gen II solid source MBE system. The growth chamber is pumped by a combination .of an ion pump, a cryopump and a liquid nitrogen filled cryopanel surrounding the substrate, to a base pressure of ~ 10-1° Torr. During the GaN growth only the cryopump and cryopanel were employed. The growth chamber is also equipped with reflection high energy electron diffraction (RHEED) and a quadrupole mass spectrometer. Attached to one of the effusion cell ports is the RF activated nitrogen plasma source. This source is an Oxford Applied Research Model
ACTIVE NITROGEN
SUPPLY
N2GASOUT
C O O L I N G L I Q U I D IN
BN CRUCIBLE
PROPO~I'I I O N A I VAI V E
LIQUIDN 2
FEEDBACK MULTIHOLE APERTURE PLATE TO UHV PUMP VACUUM SYSTEM PIIOTO DIODE
\
CABLE
PHASE SEPARATOR
Fig. I. Schematic diagram of the R F activated nitrogen plasma source arrangement.
S.E. Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
CAR25 [15] which operates by means of an electrical discharge created from inductively-coupled RF excitation at 13.56 MHz, and is liquid nitrogen cooled. A schematic diagram of the source set-up is shown in Fig. 1. A beam aperture having 37 x 0.5 mm diameter hole arrays was used. The nitrogen gas used to supply the source is taken from liquid nitrogen boil-off and purified by an active metal getter. Gas flow into the source is regulated by means of a metal sealed MKS mass flow control valve. The plasma glow discharge is monitored by means of an optical emission detector (OED) consisting of a Si photodiode with a spectral peak response at about 850 nm and an integrated circuit current to voltage converter that gives an output voltage ( O E D voltage) proportional to illumination. A more detailed analysis of the optical emission spectrum from the plasma has been carried out using a monochrom a t o r - p h o t o d e t e c t o r system. This analysis has revealed that the optical spectrum was very strongly dominated by emission lines from atomic nitrogen, relative to the molecular lines. A study by Vaudo et al. [16] has also reported spectral domination by atomic nitrogen from a similar RF activated source, and goes on to compare it with that of an ECR source. Fig. 2 shows that two parameters determine the plasma glow discharge or O E D voltage, these are the R F power and the
/ / ./E~-
--0
3
159
amount of nitrogen gas supplied to the source, i.e. the pressure of nitrogen in the M B E growth chamber. For this study, the RF power was varied between 300 and 550 W, although it is capable of running continuously at 600 W, whilst a nitrogen gas pressure (as measured in the MBE growth chamber) of 1.3 x 10 -5 to 2 × 10 -5 Torr was used. On axis, semi-insulating GaAs(100) substrates supplied by MCP Technology were used for all growth experiments. No chemical treatment of these substrates was made prior to their insertion into the MBE system. The substrates were mounted using In-free molybdenum holders, and were heated up to ~ 400°C in the preparation chamber before transfer into the growth chamber. The GaN samples reported in this article were grown at substrate temperatures of between 450 and 680°C, and characterised, ex-situ, using X-ray diffraction (XRD), reflectivity measurements, room temperature photoluminescence (PL) and scanning electron microscopy (SEM). The samples were not intentionally doped. The X-ray data were taken using a Philips PW3710 X P E R T diffractometer equipped with a Cu K a radiation source. The scans were taken in the standard O/219 mode. The thicknesses, and hence growth rates, of the GaN films were determined by an optical interference method. A value of 2.3 was used for the refractive index of GaN in the thickness calculations. The PL measurements were made using a Kimmon helium cadmium laser which has an output of 8 mW at 325 nm. An Hitachi S-4000 was used for SEM observations in this study.
L! . O'"'"
3. Experimental results and discussion
@.--
3.1. Growth rate study "0×10 ~
15×10 s
20x10 s
25x10 s
Nitrogen supDly (Tort) Fig. 2. A plot of the O E D output voltage from the plasma discharge as a function of RF power and nitrogen gas pressure.
On insertion into the MBE growth chamber, the GaAs substrates were heated under an active nitrogen flux in order to thermally remove the surface oxide. The plasma source was set at an
160
S.E. Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
RF power and nitrogen gas pressure of 470 W and 1.7 × 10 -5 Torr, respectively, during oxide removal. The GaAs surface oxide desorbed at a temperature of approximately 600°C, revealing a R H E E D pattern with a ring-like appearance with the incident electron beam along the [110] direction, and diffuse spots in the orthogonal direction. This R H E E D pattern can be associated with the formation, during nitridation, of a thin ( < 10 nm) layer of GaN, measured using in-situ Auger electron spectroscopy (AES). In order to study how changing the plasma source parameters and substrate temperature effect the GaN growth rate, the RF power, amount of nitrogen gas and growth temperature were systematically varied whilst keeping a constant Ga flux of ~ 3 x 1014 atoms cm -2 s -1, and a deposition time of 1.5 h. In the first instance, the substrate temperature and nitrogen gas pressure were fixed at 600°C and 1.3 x 10 -5 Torr, respectively, whilst the R F power was varied between 300 and 500 W. It should be noted here that during all GaN growth a R H E E D pattern identical to that of the nitrided GaAs surface was observed throughout, and no R H E E D intensity oscillations were recorded. The GaN growth rate (as calculated from the reflectivity measurements) was found to increase linearly with increasing RF power, indicating that the amount of active nitrogen generated by the plasma source was directly related to the degree of RF power supplied to it. To study the GaN growth rate as a function of the amount of nitrogen gas supplied to the plasma, the R F power and substrate temperature were fixed at 400 W and 600°C, respectively, whilst the nitrogen gas pressure was varied between 1.3 x 10 -5 and 2 X 10 -5 Torr. A monotonic dependence of the growth rate on the nitrogen gas pressure was observed, again, this is indicative of an increase in the amount of active nitrogen supplied by the source. However, the GaN growth rate varied markedly across the sample surface as the amount o f nitrogen gas was increased. For the type of beam aperture used in the source, the beam of active nitrogen supplied by the plasma seemed to be focused towards the centre of the sample, particularly when high nitrogen gas pressures were used.
O
o ~3
o
b( i
?5
~3 '
,i ~) ",~tp,J: ',,' Fig. 3. GaN growth rate dependence on the OED output voltage, using a growth temperature in the range 600 to 620°C.
As discussed earlier and illustrated in Fig. 2, the O E D voltage output from the plasma discharge is dependent on both the R F power and the nitrogen gas pressure. It was therefore interesting to investigate how the GaN growth rate varied with O E D voltage. For this study, the growth temperature was kept constant at around 600 to 620°C. The results, illustrated in Fig. 3, show that the growth rate increased linearly at low values of O E D voltage ( < 0.7 V) and then saturated. A simple calculation can be performed to show that the maximum possible growth rate of GaN, assuming a G a - N bond length of 4.54 ,~ and a Ga flux of 3 × 1014 atoms cm -2 s -1 is approximately 0.26/zm h -1, which is identical to the growth rate saturation value shown in Fig. 3. Hence, it was likely that below this saturation value there was a surplus of Ga atoms (or a shortage of active nitrogen), whilst, above the saturation value there was a surplus of active nitrogen, which will be lost by desorption. Indeed, Normarski interference optical microscopy of the samples has revealed the presence of surface Ga droplets for layers grown with an O E D output of less than ~ 0.7 V, as indicated in Fig. 4. For layers grown with an O E D output greater than ~ 0.7 V no surface Ga droplets were observed. The O E D output voltage may therefore, it seems, be used as a semi-quantitative measure
S.E. Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
161
?
4
6
'{)E~ ~)btpJt (V) Fig. 4. T h e density of surface Ga droplets as a function of the O E D output voltage.
of the amount of active nitrogen supplied by the plasma source. In addition, electron probe micro-analysis (EPMA) measurements on samples of Ga(AsN), grown using the same experimental set-up as described previously in Section 2, have shown that the amount of nitrogen incorporated into the Ga(AsN) films was directly proportional to the plasma O E D voltage output used during the growth [17]. One has to be very careful however in using the O E D output to determine the active nitrogen flux, since its value has an element of dependency on the exact position of the Si photodiode in the R F source. Regular calibrations of the GaN growth rate as a function of O E D output are therefore necessary. A constant R F power and nitrogen gas pressure of 470 W and 1.7 × 10 -5 Torr, respectively, ensuring an O E D voltage equal to the saturation value ( ~ 0.7 V), was used to investigate the influence of growth temperature on the GaN growth rate. The growth temperature was varied in the range 450 to 680°C, and the subsequent measured growth rates are plotted in Fig. 5. Clearly, the rate of GaN growth exhibited little or no dependence on the substrate temperature used. This is an important observation, since it indicates that there was no significant loss of nitrogen or gallium from the GaN surface up to a temperature of 680°C.
Fig. 5. GaN growth rate dependence on the growth temperature, using a constant R F power of 470 W and a nitrogen gas pressure of 1.7 / 10 -5 Torr.
3.2. Structural and optical details Scanning electron microscope (SEM) scans were made on the cleaved edges of a selection of the samples described in the previous section of this paper. In general, all of the scans were observed to be similar in nature, and a typical example is shown in Fig. 6. Here, the GaN layer consisted of connected columns which are typical of a highly mismatched material system [18,19]. However, the GaN appeared not to be epitaxial
Fig. 6. A typical SEM scan of the cleaved edge of a G a N / G a A s ( 1 0 0 ) s~mple.
S.E. Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
162
with the GaAs substrate, since, it traverses the trenches present on the substrate surface, i.e. it did not grow into the trenches. It is difficult to explain how these trenches were formed on the GaAs surface, as they are not believed to be a result of thermally removing the substrate oxide without an overpressure of As atoms. All of the GaN/GaAs(100) samples described in the previous section have been studied using X-ray diffraction (XRD). All samples exhibited only a wurtzite GaN layer peak, as well as the substrate peak, i.e. no zincblende GaN was present. A typical XRD scan is shown in Fig. 7, here, the wurtzite GaN peak from the (0002) reflection is identifiable at 34.6 °. In contrast, previous reports [10,11,20] have observed GaN to grow in a cubic phase on the nitrided GaAs(100) surface. However, Cheng et al. [21] have shown that either cubic, wurtzite or a mixed phase of GaN can be grown on GaAs(100) depending on whether a flux of As impinges on the sample surface during nitridation and growth. We believe, in our case, that by nitriding the GaAs substrate with no As flux present, a thin wurtzite GaN template layer was formed. The full width at half maximum (FWHM) of the GaN(0002) reflection peaks were in the range 9 to 11.5 arcmin, with the lowest values being measured for the thickest layers ( ~ 0.5/~m). Finally, all the GaN/GaAs(100) samples described in the previous section were analyzed using room temperature photoluminescence (PL). A typical PL spectra, measured at room tempera-
t~o..t,]"
GoA=
~200)
.
..~
=
o
144OO
IO00O
¢/) r"
36OO
i
4oo
oo
'
-
~
Zel
Diffraction angle 2e (degrees) Fig. 7. A typical X R D pattern of a G a N / G a A s ( I 0 0 ) s a m p l e .
/ \ 5 o
.Z"
/
3
\
/
2
/
/
/ /
1
/ .9
3.0
3.1
3.2
\
/ 3.3
3:4
3.5
3.6
Photon Energy (eV)
Fig. 8. A typical room G a N / G a A s ( 1 0 0 ) sample.
temperature
PL
spectra of a
ture, is shown in Fig. 8, consisting of a single broad peak centred around 3.4 eV.
4. Conclusions In this study we have investigated how supplying active nitrogen from an RF activated plasma source influences certain aspects of GaN films grown by MBE on GaAs(100) substrates. In particular, the GaN growth rate, recorded using reflectivity measurements, for various settings of the plasma source parameters and substrate temperature have been studied. In the first instance, the GaN growth rate (or amount of active nitrogen supplied by the source) was found to have a strong dependence on both the RF power and amount of nitrogen gas supplied. In addition, the OED voltage output from the plasma discharge was observed to give a semi-quantitative measure of the amount of active nitrogen being produced by the source. Varying the temperature of GaN growth between 450 and 680°C did not cause any change in the growth rate, consistent with no significant loss of nitrogen or gallium from the sample surface. SEM images of the cleaved edges of the G a N / G a A s samples showed the GaN layer to consist of connected columns or preferentially oriented polycrystalline grains. The GaN layers exhibited an entirely wurtzite structure using XRD measurements in the O/2~9
S.E~ Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
mode, with the FWHM of the GaN(0002) reflection being 9 to 11.5 arcmin. A broad peak centred at around 3.4 eV was recorded using room temperature PL measurements on the layers.
Acknowledgements The technical contributions of Mr. B. Hill and Mr. A. Vickers are readily acknowledged.
References [1] S. Strite and H. Morkoq, J. Vac. Sci. Technol. B 10 (1992) 1237. [2] M.E. Lin, B.N. Sverdlov and H. Morko~, J. Appl. Phys. 74 (1993) 5038. [3] C.R. Eddy, Jr, T.D. Moustakas and J. Scanlon, J. Appl. Phys. 73 (1993) 448. [4] R.C. Powell, N.E. Lee, Y.-W. Kim and J.E. Greene, J. Appl. Phys. 73 (1993) 189. [5] C. Wang and R.F. Davis, Appl. Phys. Lett. 63 (1993) 990. [6] M. Rubin, N. Newman, J.S. Chan, T.C. Fu and J.T. Ross, Appl. Phys. Lett. 64 (1994) 64. [7] T. Lei, T.D. Moustakas, R.J. Graham, Y. He and S.J. Berkowitz, J. Appl. Phys. 71 (1992) 4933.
163
[8] T. Lei, M. Fanciulli, R.J. Molnar, T.D. Moustakas, R.J. Graham and J. Scanlon, Appl. Phys. Lett. 59 (1991) 944. [9] M.E. Lin, G. Xue, G.L. Zhou, J.E. Greene and H. Morko~, Appl. Phys. Lett. 63 (1993) 932. [10] Z.Q. He, X.M. Ding, X.Y. Hou and X. Wang, Appl. Phys. Lett. 64 (1994) 315. [11] H. Okumura, S. Misawa and S. Yoshida, Appl. Phys. Lett. 59 (1991) 1058. [12] J. Qiu, H. Cheng, J.M. DePuydt and M.A. Hasse, J. Crystal Growth 127 (1993) 279. [13] J. Simpson, S.Y. Wang, I. Hauksson, H. Stewart, S.J.A. Adams, K.A. Prior and B.C. Cavenett, J. Crystal Growth 127 (1993) 327. [14] H. Stewart, J. Simpson, S.Y. Wang, I. Hauksson, S.J.A. Adams, K.A. Prior and B.C. Cavenett, J. Crystal Growth 127 (1993) 379. [15] Oxford Applied Research, Crawley Mill, Witney, Oxfordshire OX8 5TJ, UK. [16] R.P. Vaudo, J.W. Cook, Jr and J.F. Schetzina, J. Vac. Sci. Technol. B 12 (1993) 1232. [17] C.T. Foxon, T.S. Cheng, S.V. Novikov, D.E. Lacklison, L.C. Jenkins, D. Johnston, J.W. Orton, S.E. Hooper, N. Baba-Ali, T.L. Tansley and V.V. Tret'yakov, J. Crystal Growth 150 (1995) 892. [18] K. Hiramatsu, S. ltoh, H. Amano, I. Akasaki, N. Kuwano, T. Shiraishi and K. Oki, J. Crystal Growth 115 (1991)628. [19] I. Akasaki, H. Amano, H. Murakami, M. Sassa, H. Kato and K. Manabe, J. Crystal Growth 128 (1993) 379. [20] M.J. Paisly, Z. Sitar, J.B. Posthill and R.F. Davis, J. Vac. Sci. Technol. A 7 (1989) 701. [21] T.S. Cheng, L.C. Jenkins, S.E. Hooper, C.T. Foxon, J.W. Orton and D.E. Lacklison, Appl. Phys. Lett (1995), to be published.
ELSEVIER
.........
CRYSTAL GROWTH
Journal of Crystal Growth 155 (1995) 157-163
Some aspects of GaN growth on GaAs(100) substrates using molecular beam epitaxy with an RF activated nitrogen-plasma source S.E. Hooper a,*, C.T. Foxon a T.S. Cheng a, L.C. Jenkins a D.E. Lacklison J.W. Orton b, T. Bestwick c, A. Kean c, M. Dawson c, G. Duggan c
b
a Department of Physics, University of Nottingham, University Park, Nottingham, NG7 2RD, UK b Department of Electrical and Electronic Engineering, University of Nottingham, Nottingham, UK c Sharp Laboratories of Europe Ltd., Oxford Science Park, Oxford OX4 4GA, UK Received 16 January 1995; manuscript received in final form 4 April 1995
Abstract
We have investigated how supplying active nitrogen from an RF activated plasma sourcc under various plasma conditions influences certain aspects of the growth of GaN films on GaAs(100) substrates, using molecular beam epitaxy. In the first instance, the quantity of active nitrogen generated by the source was found to have a strong dependence on both the RF power and amount of nitrogen gas supplied to the plasma. In addition, the degree of optical discharge from the plasma was observed to give a semi-quantitative measure of active nitrogen. No observable loss of nitrogen from the sample surface in the temperature range 450 to 680°C was found during GaN growth. Scanning electron microscopy on the cleaved edges of the GaN/GaAs(100) samples showed the GaN layer to be polycrystalline with a columnar nature typical of a highly lattice mismatched material system. X-ray diffraction measurements indicated that the GaN layers were entirely wurtzite in structure, with the full width at half maximum of the GaN (0002) reflection in the range 9 to 11.5 arcmin. A broad peak centred at around 3.4 eV was recorded using room temperature photoluminescence measurements on the layers.
1. I n t r o d u c t i o n
Gallium nitride in its wurtzite form has a direct band-gap of around 3.4 eV at room temperature [1]. Such a wide band-gap makes it a promising material system for semiconductor device applications emitting in the blue-ultraviolet region of the electromagnetic spectrum.
* Corresponding author. Fax: + 4 4 115 9515180.
In recent years, many studies have investigated the growth by molecular beam epitaxy (MBE) of GaN on a variety of different substrates, including sapphire [2-6], Si [7,8] and GaAs [9-11]. Gallium arsenide offers several advantages as a substrate material for I I I - V nitride growth, one of these being that the cubic nature of GaAs makes it possible to grow GaN in either its zincblende [9,10], or wurtzite phase [11]. For the epitaxial growth of GaN by MBE it is necessary to supply a molecular beam of reactive
0(J22-0248/95/$0950 © 1995 Elsevier Science B.V. All rights reserved SSDI [ ) 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 2 1 6 - 2
S.E. Hooper et aL /Journal of Crystal Growth 155 (1995) 157-163
158
nitrogen. Since simple nitrogen gas is itself inactive it must be activated by using one of several commercially available sources. These sources include the Kaufrnan ion gun [4,6,10], the electron cyclotron resonance microwave plasma (ECR) [3,7,9], and the radio frequency (RF) activated glow discharge plasma. The ECR source is currently the most popular method of supplying active nitrogen, making it reasonably well characterised. Whereas, use of the RF activated plasma source in III-V nitride growth is much less documented and therefore needs to be addressed, although it has been used as an effective source of p-type doping for MBE grown II-VI semiconductors [12-14]. Consequently, the aim of the work presented in this paper, was to investigate how varying the plasma source parameters affected certain aspects of the growth of GaN on GaAs(100) substrates. In particular, the growth rate of GaN has been studied under various plasma conditions.
Information on the structural (using X-ray diffraction and scanning electron microscopy) and optical nature (using room temperature photoluminescence) of the layers is also presented.
2. Experimental details All material growth was carried out in a VarJan Mod Gen II solid source MBE system. The growth chamber is pumped by a combination .of an ion pump, a cryopump and a liquid nitrogen filled cryopanel surrounding the substrate, to a base pressure of ~ 10-1° Torr. During the GaN growth only the cryopump and cryopanel were employed. The growth chamber is also equipped with reflection high energy electron diffraction (RHEED) and a quadrupole mass spectrometer. Attached to one of the effusion cell ports is the RF activated nitrogen plasma source. This source is an Oxford Applied Research Model
ACTIVE NITROGEN
SUPPLY
N2GASOUT
C O O L I N G L I Q U I D IN
BN CRUCIBLE
PROPO~I'I I O N A I VAI V E
LIQUIDN 2
FEEDBACK MULTIHOLE APERTURE PLATE TO UHV PUMP VACUUM SYSTEM PIIOTO DIODE
\
CABLE
PHASE SEPARATOR
Fig. I. Schematic diagram of the R F activated nitrogen plasma source arrangement.
S.E. Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
CAR25 [15] which operates by means of an electrical discharge created from inductively-coupled RF excitation at 13.56 MHz, and is liquid nitrogen cooled. A schematic diagram of the source set-up is shown in Fig. 1. A beam aperture having 37 x 0.5 mm diameter hole arrays was used. The nitrogen gas used to supply the source is taken from liquid nitrogen boil-off and purified by an active metal getter. Gas flow into the source is regulated by means of a metal sealed MKS mass flow control valve. The plasma glow discharge is monitored by means of an optical emission detector (OED) consisting of a Si photodiode with a spectral peak response at about 850 nm and an integrated circuit current to voltage converter that gives an output voltage ( O E D voltage) proportional to illumination. A more detailed analysis of the optical emission spectrum from the plasma has been carried out using a monochrom a t o r - p h o t o d e t e c t o r system. This analysis has revealed that the optical spectrum was very strongly dominated by emission lines from atomic nitrogen, relative to the molecular lines. A study by Vaudo et al. [16] has also reported spectral domination by atomic nitrogen from a similar RF activated source, and goes on to compare it with that of an ECR source. Fig. 2 shows that two parameters determine the plasma glow discharge or O E D voltage, these are the R F power and the
/ / ./E~-
--0
3
159
amount of nitrogen gas supplied to the source, i.e. the pressure of nitrogen in the M B E growth chamber. For this study, the RF power was varied between 300 and 550 W, although it is capable of running continuously at 600 W, whilst a nitrogen gas pressure (as measured in the MBE growth chamber) of 1.3 x 10 -5 to 2 × 10 -5 Torr was used. On axis, semi-insulating GaAs(100) substrates supplied by MCP Technology were used for all growth experiments. No chemical treatment of these substrates was made prior to their insertion into the MBE system. The substrates were mounted using In-free molybdenum holders, and were heated up to ~ 400°C in the preparation chamber before transfer into the growth chamber. The GaN samples reported in this article were grown at substrate temperatures of between 450 and 680°C, and characterised, ex-situ, using X-ray diffraction (XRD), reflectivity measurements, room temperature photoluminescence (PL) and scanning electron microscopy (SEM). The samples were not intentionally doped. The X-ray data were taken using a Philips PW3710 X P E R T diffractometer equipped with a Cu K a radiation source. The scans were taken in the standard O/219 mode. The thicknesses, and hence growth rates, of the GaN films were determined by an optical interference method. A value of 2.3 was used for the refractive index of GaN in the thickness calculations. The PL measurements were made using a Kimmon helium cadmium laser which has an output of 8 mW at 325 nm. An Hitachi S-4000 was used for SEM observations in this study.
L! . O'"'"
3. Experimental results and discussion
@.--
3.1. Growth rate study "0×10 ~
15×10 s
20x10 s
25x10 s
Nitrogen supDly (Tort) Fig. 2. A plot of the O E D output voltage from the plasma discharge as a function of RF power and nitrogen gas pressure.
On insertion into the MBE growth chamber, the GaAs substrates were heated under an active nitrogen flux in order to thermally remove the surface oxide. The plasma source was set at an
160
S.E. Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
RF power and nitrogen gas pressure of 470 W and 1.7 × 10 -5 Torr, respectively, during oxide removal. The GaAs surface oxide desorbed at a temperature of approximately 600°C, revealing a R H E E D pattern with a ring-like appearance with the incident electron beam along the [110] direction, and diffuse spots in the orthogonal direction. This R H E E D pattern can be associated with the formation, during nitridation, of a thin ( < 10 nm) layer of GaN, measured using in-situ Auger electron spectroscopy (AES). In order to study how changing the plasma source parameters and substrate temperature effect the GaN growth rate, the RF power, amount of nitrogen gas and growth temperature were systematically varied whilst keeping a constant Ga flux of ~ 3 x 1014 atoms cm -2 s -1, and a deposition time of 1.5 h. In the first instance, the substrate temperature and nitrogen gas pressure were fixed at 600°C and 1.3 x 10 -5 Torr, respectively, whilst the R F power was varied between 300 and 500 W. It should be noted here that during all GaN growth a R H E E D pattern identical to that of the nitrided GaAs surface was observed throughout, and no R H E E D intensity oscillations were recorded. The GaN growth rate (as calculated from the reflectivity measurements) was found to increase linearly with increasing RF power, indicating that the amount of active nitrogen generated by the plasma source was directly related to the degree of RF power supplied to it. To study the GaN growth rate as a function of the amount of nitrogen gas supplied to the plasma, the R F power and substrate temperature were fixed at 400 W and 600°C, respectively, whilst the nitrogen gas pressure was varied between 1.3 x 10 -5 and 2 X 10 -5 Torr. A monotonic dependence of the growth rate on the nitrogen gas pressure was observed, again, this is indicative of an increase in the amount of active nitrogen supplied by the source. However, the GaN growth rate varied markedly across the sample surface as the amount o f nitrogen gas was increased. For the type of beam aperture used in the source, the beam of active nitrogen supplied by the plasma seemed to be focused towards the centre of the sample, particularly when high nitrogen gas pressures were used.
O
o ~3
o
b( i
?5
~3 '
,i ~) ",~tp,J: ',,' Fig. 3. GaN growth rate dependence on the OED output voltage, using a growth temperature in the range 600 to 620°C.
As discussed earlier and illustrated in Fig. 2, the O E D voltage output from the plasma discharge is dependent on both the R F power and the nitrogen gas pressure. It was therefore interesting to investigate how the GaN growth rate varied with O E D voltage. For this study, the growth temperature was kept constant at around 600 to 620°C. The results, illustrated in Fig. 3, show that the growth rate increased linearly at low values of O E D voltage ( < 0.7 V) and then saturated. A simple calculation can be performed to show that the maximum possible growth rate of GaN, assuming a G a - N bond length of 4.54 ,~ and a Ga flux of 3 × 1014 atoms cm -2 s -1 is approximately 0.26/zm h -1, which is identical to the growth rate saturation value shown in Fig. 3. Hence, it was likely that below this saturation value there was a surplus of Ga atoms (or a shortage of active nitrogen), whilst, above the saturation value there was a surplus of active nitrogen, which will be lost by desorption. Indeed, Normarski interference optical microscopy of the samples has revealed the presence of surface Ga droplets for layers grown with an O E D output of less than ~ 0.7 V, as indicated in Fig. 4. For layers grown with an O E D output greater than ~ 0.7 V no surface Ga droplets were observed. The O E D output voltage may therefore, it seems, be used as a semi-quantitative measure
S.E. Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
161
?
4
6
'{)E~ ~)btpJt (V) Fig. 4. T h e density of surface Ga droplets as a function of the O E D output voltage.
of the amount of active nitrogen supplied by the plasma source. In addition, electron probe micro-analysis (EPMA) measurements on samples of Ga(AsN), grown using the same experimental set-up as described previously in Section 2, have shown that the amount of nitrogen incorporated into the Ga(AsN) films was directly proportional to the plasma O E D voltage output used during the growth [17]. One has to be very careful however in using the O E D output to determine the active nitrogen flux, since its value has an element of dependency on the exact position of the Si photodiode in the R F source. Regular calibrations of the GaN growth rate as a function of O E D output are therefore necessary. A constant R F power and nitrogen gas pressure of 470 W and 1.7 × 10 -5 Torr, respectively, ensuring an O E D voltage equal to the saturation value ( ~ 0.7 V), was used to investigate the influence of growth temperature on the GaN growth rate. The growth temperature was varied in the range 450 to 680°C, and the subsequent measured growth rates are plotted in Fig. 5. Clearly, the rate of GaN growth exhibited little or no dependence on the substrate temperature used. This is an important observation, since it indicates that there was no significant loss of nitrogen or gallium from the GaN surface up to a temperature of 680°C.
Fig. 5. GaN growth rate dependence on the growth temperature, using a constant R F power of 470 W and a nitrogen gas pressure of 1.7 / 10 -5 Torr.
3.2. Structural and optical details Scanning electron microscope (SEM) scans were made on the cleaved edges of a selection of the samples described in the previous section of this paper. In general, all of the scans were observed to be similar in nature, and a typical example is shown in Fig. 6. Here, the GaN layer consisted of connected columns which are typical of a highly mismatched material system [18,19]. However, the GaN appeared not to be epitaxial
Fig. 6. A typical SEM scan of the cleaved edge of a G a N / G a A s ( 1 0 0 ) s~mple.
S.E. Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
162
with the GaAs substrate, since, it traverses the trenches present on the substrate surface, i.e. it did not grow into the trenches. It is difficult to explain how these trenches were formed on the GaAs surface, as they are not believed to be a result of thermally removing the substrate oxide without an overpressure of As atoms. All of the GaN/GaAs(100) samples described in the previous section have been studied using X-ray diffraction (XRD). All samples exhibited only a wurtzite GaN layer peak, as well as the substrate peak, i.e. no zincblende GaN was present. A typical XRD scan is shown in Fig. 7, here, the wurtzite GaN peak from the (0002) reflection is identifiable at 34.6 °. In contrast, previous reports [10,11,20] have observed GaN to grow in a cubic phase on the nitrided GaAs(100) surface. However, Cheng et al. [21] have shown that either cubic, wurtzite or a mixed phase of GaN can be grown on GaAs(100) depending on whether a flux of As impinges on the sample surface during nitridation and growth. We believe, in our case, that by nitriding the GaAs substrate with no As flux present, a thin wurtzite GaN template layer was formed. The full width at half maximum (FWHM) of the GaN(0002) reflection peaks were in the range 9 to 11.5 arcmin, with the lowest values being measured for the thickest layers ( ~ 0.5/~m). Finally, all the GaN/GaAs(100) samples described in the previous section were analyzed using room temperature photoluminescence (PL). A typical PL spectra, measured at room tempera-
t~o..t,]"
GoA=
~200)
.
..~
=
o
144OO
IO00O
¢/) r"
36OO
i
4oo
oo
'
-
~
Zel
Diffraction angle 2e (degrees) Fig. 7. A typical X R D pattern of a G a N / G a A s ( I 0 0 ) s a m p l e .
/ \ 5 o
.Z"
/
3
\
/
2
/
/
/ /
1
/ .9
3.0
3.1
3.2
\
/ 3.3
3:4
3.5
3.6
Photon Energy (eV)
Fig. 8. A typical room G a N / G a A s ( 1 0 0 ) sample.
temperature
PL
spectra of a
ture, is shown in Fig. 8, consisting of a single broad peak centred around 3.4 eV.
4. Conclusions In this study we have investigated how supplying active nitrogen from an RF activated plasma source influences certain aspects of GaN films grown by MBE on GaAs(100) substrates. In particular, the GaN growth rate, recorded using reflectivity measurements, for various settings of the plasma source parameters and substrate temperature have been studied. In the first instance, the GaN growth rate (or amount of active nitrogen supplied by the source) was found to have a strong dependence on both the RF power and amount of nitrogen gas supplied. In addition, the OED voltage output from the plasma discharge was observed to give a semi-quantitative measure of the amount of active nitrogen being produced by the source. Varying the temperature of GaN growth between 450 and 680°C did not cause any change in the growth rate, consistent with no significant loss of nitrogen or gallium from the sample surface. SEM images of the cleaved edges of the G a N / G a A s samples showed the GaN layer to consist of connected columns or preferentially oriented polycrystalline grains. The GaN layers exhibited an entirely wurtzite structure using XRD measurements in the O/2~9
S.E~ Hooper et al. /Journal of Crystal Growth 155 (1995) 157-163
mode, with the FWHM of the GaN(0002) reflection being 9 to 11.5 arcmin. A broad peak centred at around 3.4 eV was recorded using room temperature PL measurements on the layers.
Acknowledgements The technical contributions of Mr. B. Hill and Mr. A. Vickers are readily acknowledged.
References [1] S. Strite and H. Morkoq, J. Vac. Sci. Technol. B 10 (1992) 1237. [2] M.E. Lin, B.N. Sverdlov and H. Morko~, J. Appl. Phys. 74 (1993) 5038. [3] C.R. Eddy, Jr, T.D. Moustakas and J. Scanlon, J. Appl. Phys. 73 (1993) 448. [4] R.C. Powell, N.E. Lee, Y.-W. Kim and J.E. Greene, J. Appl. Phys. 73 (1993) 189. [5] C. Wang and R.F. Davis, Appl. Phys. Lett. 63 (1993) 990. [6] M. Rubin, N. Newman, J.S. Chan, T.C. Fu and J.T. Ross, Appl. Phys. Lett. 64 (1994) 64. [7] T. Lei, T.D. Moustakas, R.J. Graham, Y. He and S.J. Berkowitz, J. Appl. Phys. 71 (1992) 4933.
163
[8] T. Lei, M. Fanciulli, R.J. Molnar, T.D. Moustakas, R.J. Graham and J. Scanlon, Appl. Phys. Lett. 59 (1991) 944. [9] M.E. Lin, G. Xue, G.L. Zhou, J.E. Greene and H. Morko~, Appl. Phys. Lett. 63 (1993) 932. [10] Z.Q. He, X.M. Ding, X.Y. Hou and X. Wang, Appl. Phys. Lett. 64 (1994) 315. [11] H. Okumura, S. Misawa and S. Yoshida, Appl. Phys. Lett. 59 (1991) 1058. [12] J. Qiu, H. Cheng, J.M. DePuydt and M.A. Hasse, J. Crystal Growth 127 (1993) 279. [13] J. Simpson, S.Y. Wang, I. Hauksson, H. Stewart, S.J.A. Adams, K.A. Prior and B.C. Cavenett, J. Crystal Growth 127 (1993) 327. [14] H. Stewart, J. Simpson, S.Y. Wang, I. Hauksson, S.J.A. Adams, K.A. Prior and B.C. Cavenett, J. Crystal Growth 127 (1993) 379. [15] Oxford Applied Research, Crawley Mill, Witney, Oxfordshire OX8 5TJ, UK. [16] R.P. Vaudo, J.W. Cook, Jr and J.F. Schetzina, J. Vac. Sci. Technol. B 12 (1993) 1232. [17] C.T. Foxon, T.S. Cheng, S.V. Novikov, D.E. Lacklison, L.C. Jenkins, D. Johnston, J.W. Orton, S.E. Hooper, N. Baba-Ali, T.L. Tansley and V.V. Tret'yakov, J. Crystal Growth 150 (1995) 892. [18] K. Hiramatsu, S. ltoh, H. Amano, I. Akasaki, N. Kuwano, T. Shiraishi and K. Oki, J. Crystal Growth 115 (1991)628. [19] I. Akasaki, H. Amano, H. Murakami, M. Sassa, H. Kato and K. Manabe, J. Crystal Growth 128 (1993) 379. [20] M.J. Paisly, Z. Sitar, J.B. Posthill and R.F. Davis, J. Vac. Sci. Technol. A 7 (1989) 701. [21] T.S. Cheng, L.C. Jenkins, S.E. Hooper, C.T. Foxon, J.W. Orton and D.E. Lacklison, Appl. Phys. Lett (1995), to be published.