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Investigation of GaInN films and development of double-hetero (DH) structures for blue and green light emitters
Journal of Crystal Growth 203 (1999) 340}348
Investigation of GaInN "lms and development of double-hetero (DH) structures for blue and green light emitters M. Schwambera , O. Schoen, B. Schineller , D. Schmitz, M. Heuken * Institut fu( r Halbleitertechnik, RWTH Aachen, Templergraben 55, D-52056 Aachen, Germany AIXTRON AG, Kackertstr. 15}17, D-52072 Aachen, Germany Received 15 July 1998; accepted 30 December 1998 Communicated by J.B. Mullin
Abstract GaInN single layers were investigated in order to optimise their photoluminescence properties and to "nd dependencies on the main growth parameters such as growth temperature, In/(In#Ga) gas phase ratio and total pressure. Several approaches for capping GaInN/GaN with another GaN layer in order to develop high quality double-hetero (DH) structures were presented. Using an optimised interfacing technique we obtain device quality DH structures with state of the art composition uniformity across a 2 wafer. Further investigation of these DH structures results in high quality single quantum well (SQW) and multi quantum well (MQW) structures. 1999 Elsevier Science B.V. All rights reserved. Keywords: MOVPE; GaN; Heterostructures; SQW; AQW; Nitrides
1. Introduction Since GaN and its related alloys, InGaN and AlGaN, continue to increase in importance for optoelectronic and high power applications the demand for reliable and e$cient mass production systems rises inexplorably. Due to increasing interest in the production of GaN-based devices we have used an AIXTRON single wafer horizontal tube reactor for the fabrication of GaInN/GaN heterostructures. It has been reported in Ref. [1]
* Corresponding author. AIXTRON AG, Kackertstr. 15}17, D-52072 Aachen, Germany. Tel.: #49-241-89090; fax: #49241-890940. E-mail address: [email protected] (M. Heuken)
that these results can be transferred to multiwafer Planetary Reactors威 to shorten development times in a production facility. GaInN is playing a key role for the fabrication of blue and green light emitters, because it is used as the active zone in these optical devices. Therefore the development of GaInN single and double heterostructures is very important and a close inspection of GaInN growth conditions has to be carried out. Since growth of such epitaxial "lms requires high #exibility in both process pressure and deposition temperature, carefully adapted designs of the reactor chambers were employed. Optimal reactor geometries have been found by extensive modelling [2] of their thermal and chemical properties to guarantee laminar #ow and controlled reactant depletion at temperatures up to 12003C and for total pressures from 50 mbar
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to 1 bar. Using Gas Foil Rotation威 of the substrates these reactors allow the growth of highly uniform layers with excellent physical properties (close to the theoretical limits). The properties of doped material from AIXTRON machines have been published in Ref. [3]. The quality of single ternary layers of the AlGaN and InGaN on GaN have been reported previously [3,4]. This study is focused on methods to grow double-hetero (DH) structures of ternary material embedded between bu!er and cap layers of GaN. A close inspection of the growth of such structures is necessary for the development of SQW and MQW devices in the nitride system because of the drastic changes in process conditions used for each single layer. Especially GaN/GaInN structures pose a challenge as temperatures have to be changed by several hundred 3C, molar #ows di!er by an order of magnitude and even total pressure may have to be altered.
2. Experimental procedure The samples presented here were grown in an AIX 200RF horizontal tube reactor, using TEGa/ TMGa, TMIn, NH , SiH and Cp Mg as precur sors. All samples contain a GaN bu!er layer with a GaN nucleation layer grown at 5603C on c-plane sapphire wafers. The GaN nucleation layer is nominally 25 nm thick. For the GaN bu!er layer growth the following parameters have been found optimal: ¹ "11203C and r"1 lm/h, resulting in layers with background carrier concentrations down to 7;10 cm\. We investigated the dependencies of the GaInN growth on deposition temperature, pressure and In/(In#Ga) gas phase ratio before further developments resulting in DH, SQW and MQW structures were made. The optical, electrical and structural properties were characterised by X-ray di!ractometry (XRD), room temperature (RT) and low temperature (LT), photoluminescence (PL), secondary ion mass spectroscopy (SIMS) and electrical measurements. Statistical evaluations usually take the entire 2 wafer area (without edge exclusion) into account. The composition of the GaInN was estimated by the splitting of the GaN and
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InGaN layer peaks in the h/2h mode of X-ray di!raction measurements. All layers with more than 5% InN in the solid show a clear splitting as measured on a Bede DCC double crystal di!ractometer. For all temperatures cited in this discussion it has to be noted that the AIX 200 RF reactors are equipped with a thermocouple in the centre of the susceptor for temperature control. Thus the actual temperature at the surface of the substrate will be lower by 50 to 1003C than the indicated value, depending on process conditions.
3. Results 3.1. Investigation of GaInN/GaN heterostructures In previous investigations the dependence of In incorporation was studied with respect to growth temperature and In precursor partial pressure. The highest material quality was found for layers grown at the highest possible temperature for a certain emission wavelength [5]. This was also observed by Scholz et al. [6]. For samples grown with a constant In/(In#Ga) gas phase mole ratio we found a decreasing In incorporation with increasing growth temperature (Fig. 1a). This dependency has been observed by several authors, recently by HaK rle et al. [7], Lee et al. [8] and Scholz et al. [9]. As expected the In incorporation increased with increase in gas phase mole ratio In/(In#Ga). This evaluation can be used to adjust certain gas phase ratios to grow "lms with a desired In content by using a linear "t restricted to the developed boundaries. The In incorporation dependency on the growth pressure is shown in Fig. 1b. For this evaluation growth temperature and the indium gas phase mole ratio were held at constant values of ¹ "8503C " and In/(In#Ga)"80%. It is shown that with increase in pressure the incorporation of In also increased. PL measurements show clear advantages for the high pressure process. The optical properties of the GaInN "lms improve with higher total pressure. This can be veri"ed by comparing the intensities and the FWHM of the GaInN related RT (300 K) and LT (12 K) PL peaks shown in
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Fig. 1. In incorporation dependency on growth temperature and In/(In#Ga) gas phase ratio (a) and on growth pressure at constant growth temperature and In/(In#Ga) gas phase ratio (b).
Fig. 2. The intensity of the GaInN related peak measured at RT increases when using a higher total pressure. Additionally lower FWHM were found in the LT PL spectra of samples grown at higher pressures with the smallest width of 9.3 nm achieved when the sample was grown at 200 mbar, resulting in a 14.7% In content. Thus the best optical properties were obtained by using high total pressure. The high quality of this "lm is also con"rmed by the missing yellow emission at LT PL using low excitation energies. Indications for an e!ect similar to the `composition pulling e!ecta [10}12] were found in secondary ion mass spectroscopy (SIMS) measurements. Fig. 3 shows the SIMS measurements of 200 nm thick GaInN "lms grown on GaN. The Ga and In related signal (proportional to the content of the respective atoms) versus the sputtering time (proportional to the depth) are shown starting with the surface on the left and ending with the GaInN/GaN interface indicated by the drastic decrease of In atoms. The composition pulling e!ect causes an increasing In content with layer thickness. Evidence is given by comparing the In concentrations at the surface (left) and the interface (right) layers (Fig. 3a). This means that indium atoms are excluded from GaInN at the initial growth stage to reduce the deformation energy due
Fig. 2. Comparison of the optical properties of GaInN/GaN SH structures grown using di!erent total pressures.
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Fig. 3. SIMS measurements of GaInN layers (with indication of composition pulling e!ect on the left).
to the lattice mismatch. Fig. 3b shows a measurement of a GaInN sample with low In content of about 8%. This sample has a nearly constant indium concentration in the cross section, thus the composition pulling e!ect becomes less dominant. This was established using additional SIMS measurements which are summarised in Fig. 4. To have an indicator for the e!ect, we de"ned the Ga/In deviation ratio shown there. The values for the Ga/In minimum and maximum ratios were taken from the locations marked by the vertical lines in the SIMS measurements shown in Fig. 3. Comparing the composition pulling e!ect (Ga/In deviation) between samples with di!erent In content, there is clear evidence that the e!ect becomes less dominant with decreasing In content due to the decreased lattice mismatch, as expected. This behaviour was also found by Kawaguchi et al. [10]. Discrepancies between PL and X-ray measurements for the evaluation of the In composition are shown in Fig. 5. It shows the dependency of the band gap energy estimated by PL measurements on the In content of the examined samples. It is evident that the PL positions of the bulk SH structures (circles) did not follow the relation [13]
Fig. 4. Comparison of composition pulling e!ect.
E (x)"xE ( 3N)#(1!x)E (GaN)!bx(1!x) (1) using a bowing parameter of b"1 eV, E (InN)"1.95 eV and E (GaN)"3.43 eV. The E
Fig. 5. Inconsistency between XRD and PL measurements. Full line represents theoretical data with b"1 eV.
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di!erence between the PL measurements and the predicted values increased with increasing indium concentration. This clearly seems to be related to the composition pulling e!ect. A probable explanation for this is that the PL reveals the peak associated with the GaInN photoluminescence with the highest In content, because the radiative recombination takes place at the location of the lowest band gap (surface) whereas XRD measurements yield average values for the whole "lm. Other explanations for the discrepancy of PL measurements and the predicted values for the band gap can be found in Ref. [6]. Nevertheless PL measurements of optimised GaInN layers grown at elevated total pressure (triangles) show good agreement with Eq. (1). By now larger bowing factors have been published by Van de Walle et al. [14] with values of b up to 4.8 eV. Using these elevated bowing factors in Eq. (1) would result in a better "tting between our measurements and the predicted values. We achieved very good compositional uniformity over the whole wafer as demonstrated by the PL mapping shown in Fig. 6 of a GaInN "lm with a peak wavelength standard deviation of 1.11 nm. The "lm thickness of the GaInN layer was 200 nm. The growth temperature was adjusted to 9003C
resulting in an In content of about 8%. Thus with thickness homogeneities of less than 1% standard deviation the main speci"cation requirements for mass production are ful"lled.
3.2. Thermal stability investigation of DH structures Since the straightforward stacking of the three single layers GaN bu!er, GaInN, GaN cap } grown at di!erent growth temperature, total pressure and carrier gas than the InGaN } was found to result in greyish layers that could be characterised neither by X-ray di!raction (XRD) nor by PL, an intermediate cap of GaN grown using GaInN process conditions was introduced in order to protect the GaInN from dissolution while heating up to the standard growth temperature of GaN. To test the thermal stability of the DH structure thus formed by a 2 lm GaN bu!er, 50 nm GaInN layer and 100 nm GaN low temperature (LT) cap, such samples were exposed to a series of anneals at process conditions for GaN growth (low pressure, NH stabilisation) at increasing temperatures. A series of four anneal runs at 900, 1000, 1050 and 11003C, respectively were performed. Each
Fig. 6. PL mapping of Ga In N (200 nm) grown at 9003C on GaN (1.8 lm).
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sample, being left in the reactor at elevated temperature for 5 min while stabilising the surface with ammonia using N as the main carrier gas, was individually examined for surface changes. Samples exposed to 1050 and 11003C show a grey colour on the surface which is attributed to the dissolution of the GaInN layer and formation of In droplets as nitrogen evaporates out of the layer. Layers exposed to temperatures between 900 and 10003C did not show any such surface change. This indicates that the GaInN layer will not signi"cantly decompose in less than 5 min at 10003C when capped with at least 100 nm of low temperature GaN. Growth at higher temperatures should increase the durability of the InGaN layer, but at growth temperatures above 11003C some gradual degradation of the ternary material is still expected. 3.3. Growth and optimisation of GaN/GaInN/GaN double heterostructures Best growth parameters from GaInN single-hetero (SH) structure experiments were used for the active layer in the double-hetero (DH) structures. The growth was optimised by adjusting the growth rate (1 nm/min), the growth temperature (8603C)
345
and the layer thickness (50 nm) to minimise the composition pulling e!ect. The critical step during the growth is the interface of GaInN and the GaN cap due to the large growth temperature gap between both layer growths. During the heat up to GaN cap growth conditions indium di!uses out of the InGaN. A suitable process has been found by continuously growing during the heat up with GaInN growth conditions into the GaN cap layer without a growth stop at the interface. This results in high quality DH structures. Using this optimised interface process a DH structure consisting of 1.6 lm GaN : Si bu!er (doped to contain nominally N "8;10 cm\), 50 nm GaInN (nominally " 10% InN in the solid) and 20 nm GaN cap (thin to limit absorption of the PL laser line) has been grown. Samples have been examined using RT PL at a low excitation density, using the Philips MCS PL system. Fig. 7 shows RT and LT PL spectra taken at the central point of the wafer, showing clear lines for the GaN and GaInN band edge related emissions. The yellow luminescence visible in the RT spectrum (Fig. 7a; modulated with an interference pattern due to the bu!er layer thickness) underlines the low excitation density used for
Fig. 7. RT and LT (20 K) PL spectrum of GaN (20 nm)/GaInN (50 nm)/GaN : Si (1.6 lm, N "8;10 cm\). "
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Fig. 8. RT PL mapping of the GaInN related peak wavelength for the 2 wafer of the sample shown in Fig. 9, average wavelength is 394.3 nm, standard deviation is 1.1 nm.
this measurement. At higher excitation densities it is negligible. The GaInN band edge related peak appears at 3.16 eV with a FWHM of 13.4 nm. The LT spectrum (Fig. 7b) is featureless except for the GaN and GaInN band edge related emissions demonstrating excellent material quality. The InGaN related peak at 391 nm is now two orders of magnitude higher than the yellow PL peak and about four times more intense than the GaN band edge. This re#ects the high optical quality of this DH structure. To assess the uniformity of the GaInN composition the wafer was scanned on the Philips MCS wafer mapper at room temperature. In Fig. 8 the mapping of the entire 2 wafer is shown displaying an average wavelength of 394.3 nm with a standard deviation of 1.1 nm.
4. Development of SQW and MQW structures Further development of the above described structures was made resulting in SQW and MQW structures. Fig. 9a shows the LT PL of a GaN/
GaInN/GaN : Si SQW structure with a well width of 4 nm. The same growth conditions used for the sample discussed in Fig. 3 were used for the GaInN layer. The PL shows a strong GaN near band edge peak and a GaInN peak with nearly half the intensity of the GaN related peak. The high quality of this structure can be veri"ed by comparing the emission intensities of the GaN and GaInN peaks with the low yellow emission intensity in this measurement using low excitation energies. The intensity of the GaInN peak is about eight times higher than the GaN emission. Additionally a blue shift of the GaInN peak of about 10 meV can be found compared to Fig. 7, due to the quantisation e!ect. The same structure was used for a MQW structure which was formed by a triple repetition of a 7 nm Ga In N barrier layer and a 4 nm Ga In N well layer. In Fig. 9b the LT PL of this sample is shown. As expected the intensity of the GaInN well related emission increased resulting in a 100 fold increase in the emission intensity compared to the GaN emission intensity which re#ects the high optical quality of this structure even at room temperature. Also a blue shift of the GaInN
M. Schwambera et al. / Journal of Crystal Growth 203 (1999) 340}348
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Fig. 9. LT (12 K) PL of GaN/Ga In N/Ga In N/ Ga In N/GaN : Si SQW structure and of GaN/3; +Ga In N/Ga In N,/Ga In N/GaN : Si MQW structure.
peak of about 25 meV can be found compared to Fig. 7. The di!erence in the value for the SQW may be due to di!erent strain e!ects in the MQW resulting in a di!erent In content in the layer or piezo electric e!ects causing the change in emission wavelength.
tures results in high quality SQW and MQW structures, which underlines the suitability of the developments for the mass production of blue and green light emitters.
References 5. Conclusions This paper reports on the investigation of selected GaN heterostructures. Single-hetero GaInN/ GaN layers have been optimised for small FWHM of the RT PL peaks for a given In incorporation by introducing a high temperature (8603C), high In versus total MO gas phase (80%) process. Methods of capping GaInN layers to prevent dissolution when preparing to grow GaN top layers have been investigated. A suitable cap for overgrowth up to 10003C has been demonstrated. Using an optimised capping technique DH structures of device quality have been produced, resulting in samples with state of the art composition uniformity across full 2 wafers. Further developments of these DH struc-
[1] M. Deschler, R. Beccard, B. Wachtendorf, D. Schmitz, H. Juergensen, Mater. Sci. Eng. B 50 (1997) 1. [2] M. Dauelsberg, L. Kadinski, Yu.N. Makarov, E. Woelk, G. Strauch, D. Schmitz, H. Juergensen, Inst. Phys. Ser. 142 (5) (1996) 887. [3] C. von Eichel-Streiber, O. Schoen, R. Beccard, D. Schmitz, M. Heuken, H. Juergensen, J. Crystal Growth 189/190 (1998) 344. [4] R. Beccard, O. Schoen, B. Wachtendorf, D. Schmitz, H. Juergensen, E. Woelk, Electron. Mat. (Special Issue) 26 (10) (1997) 1123. [5] O. Schoen, M. Schwambera, B. Schineller, D. Schmitz, M. Heuken, H. Juergensen, J. Crystal Growth 195 (1998) 297. [6] F. Scholz, J. O!, A. Sohmer, V. Syganov, A. Doernen, O. Ambacher, J. Crystal Growth 189/190 (1998) 8. [7] V. Haerle, H. Bolay, F. Steuber, F. Scholz, V. Syganow, G. Frankowsky, A. Hangleiter, Int. Symp. on Blue Laser and Light Emitting Diodes, Chiba Univ. Japan, Tu-12, 1996, pp. 62}65.
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[8] C.R. Lee, S.J. Son, I.H. Lee, J.Y. Leem, S.K. Noh, J. Crystal Growth 182 (1997) 6. [9] F. Scholz, V. Haerle, F. Steuber, A. Sohmer, H. Bolay, V. Syganow, A. Doernen, J.-S. Im, A. Hangleiter, J.Y. Duboz, P. Galtier, E. Rosencher, O. Ambacher, D. Brunner, H. Lakner, Mater. Res. Soc. Symp. Proc. 449 (1997) [10] Y. Kawaguchi, M. Yamaguchi, N. Sawaki, K. Hiramatsu, W. Taki, N. Kuwano, K. Oki, T. Zheleva, R.F. Davis Record of the 16th Electronic Mater. Symp., Minoo, 9}11 July 1997, SB-9.
[11] H. Sato, T. Sugahara, Y. Naoi, S. Sakai, ICNS'97, M 2-7, Tokushima, 27}31 October 1997, pp. 28}29. [12] W. Taki, H. Tsuda, K. Tsukamoto, N. Kuwano, K. Oki, Y. Kawaguchi, M. Shimizu, K. Hiramatsu, ICNS'97, P135, Tokushima, 27}31 October 1997, pp. 102}103 [13] S. Nakamura, T. Mukai, M. Senoh, S. Nagahama, N. Iwasa, J. Appl. Phys. 74 (1993) 3911. [14] C.G. Van de Walle, M.D. McCluskey, C.P. Master, L.T. Romano, N.M. Johnson, EMRS Symp. L, VII.3, p. L-10, Strasbourg 1998, to be published in a special issue.
Investigation of GaInN "lms and development of double-hetero (DH) structures for blue and green light emitters M. Schwambera , O. Schoen, B. Schineller , D. Schmitz, M. Heuken * Institut fu( r Halbleitertechnik, RWTH Aachen, Templergraben 55, D-52056 Aachen, Germany AIXTRON AG, Kackertstr. 15}17, D-52072 Aachen, Germany Received 15 July 1998; accepted 30 December 1998 Communicated by J.B. Mullin
Abstract GaInN single layers were investigated in order to optimise their photoluminescence properties and to "nd dependencies on the main growth parameters such as growth temperature, In/(In#Ga) gas phase ratio and total pressure. Several approaches for capping GaInN/GaN with another GaN layer in order to develop high quality double-hetero (DH) structures were presented. Using an optimised interfacing technique we obtain device quality DH structures with state of the art composition uniformity across a 2 wafer. Further investigation of these DH structures results in high quality single quantum well (SQW) and multi quantum well (MQW) structures. 1999 Elsevier Science B.V. All rights reserved. Keywords: MOVPE; GaN; Heterostructures; SQW; AQW; Nitrides
1. Introduction Since GaN and its related alloys, InGaN and AlGaN, continue to increase in importance for optoelectronic and high power applications the demand for reliable and e$cient mass production systems rises inexplorably. Due to increasing interest in the production of GaN-based devices we have used an AIXTRON single wafer horizontal tube reactor for the fabrication of GaInN/GaN heterostructures. It has been reported in Ref. [1]
* Corresponding author. AIXTRON AG, Kackertstr. 15}17, D-52072 Aachen, Germany. Tel.: #49-241-89090; fax: #49241-890940. E-mail address: [email protected] (M. Heuken)
that these results can be transferred to multiwafer Planetary Reactors威 to shorten development times in a production facility. GaInN is playing a key role for the fabrication of blue and green light emitters, because it is used as the active zone in these optical devices. Therefore the development of GaInN single and double heterostructures is very important and a close inspection of GaInN growth conditions has to be carried out. Since growth of such epitaxial "lms requires high #exibility in both process pressure and deposition temperature, carefully adapted designs of the reactor chambers were employed. Optimal reactor geometries have been found by extensive modelling [2] of their thermal and chemical properties to guarantee laminar #ow and controlled reactant depletion at temperatures up to 12003C and for total pressures from 50 mbar
0022-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 1 2 4 - 4
M. Schwambera et al. / Journal of Crystal Growth 203 (1999) 340}348
to 1 bar. Using Gas Foil Rotation威 of the substrates these reactors allow the growth of highly uniform layers with excellent physical properties (close to the theoretical limits). The properties of doped material from AIXTRON machines have been published in Ref. [3]. The quality of single ternary layers of the AlGaN and InGaN on GaN have been reported previously [3,4]. This study is focused on methods to grow double-hetero (DH) structures of ternary material embedded between bu!er and cap layers of GaN. A close inspection of the growth of such structures is necessary for the development of SQW and MQW devices in the nitride system because of the drastic changes in process conditions used for each single layer. Especially GaN/GaInN structures pose a challenge as temperatures have to be changed by several hundred 3C, molar #ows di!er by an order of magnitude and even total pressure may have to be altered.
2. Experimental procedure The samples presented here were grown in an AIX 200RF horizontal tube reactor, using TEGa/ TMGa, TMIn, NH , SiH and Cp Mg as precur sors. All samples contain a GaN bu!er layer with a GaN nucleation layer grown at 5603C on c-plane sapphire wafers. The GaN nucleation layer is nominally 25 nm thick. For the GaN bu!er layer growth the following parameters have been found optimal: ¹ "11203C and r"1 lm/h, resulting in layers with background carrier concentrations down to 7;10 cm\. We investigated the dependencies of the GaInN growth on deposition temperature, pressure and In/(In#Ga) gas phase ratio before further developments resulting in DH, SQW and MQW structures were made. The optical, electrical and structural properties were characterised by X-ray di!ractometry (XRD), room temperature (RT) and low temperature (LT), photoluminescence (PL), secondary ion mass spectroscopy (SIMS) and electrical measurements. Statistical evaluations usually take the entire 2 wafer area (without edge exclusion) into account. The composition of the GaInN was estimated by the splitting of the GaN and
341
InGaN layer peaks in the h/2h mode of X-ray di!raction measurements. All layers with more than 5% InN in the solid show a clear splitting as measured on a Bede DCC double crystal di!ractometer. For all temperatures cited in this discussion it has to be noted that the AIX 200 RF reactors are equipped with a thermocouple in the centre of the susceptor for temperature control. Thus the actual temperature at the surface of the substrate will be lower by 50 to 1003C than the indicated value, depending on process conditions.
3. Results 3.1. Investigation of GaInN/GaN heterostructures In previous investigations the dependence of In incorporation was studied with respect to growth temperature and In precursor partial pressure. The highest material quality was found for layers grown at the highest possible temperature for a certain emission wavelength [5]. This was also observed by Scholz et al. [6]. For samples grown with a constant In/(In#Ga) gas phase mole ratio we found a decreasing In incorporation with increasing growth temperature (Fig. 1a). This dependency has been observed by several authors, recently by HaK rle et al. [7], Lee et al. [8] and Scholz et al. [9]. As expected the In incorporation increased with increase in gas phase mole ratio In/(In#Ga). This evaluation can be used to adjust certain gas phase ratios to grow "lms with a desired In content by using a linear "t restricted to the developed boundaries. The In incorporation dependency on the growth pressure is shown in Fig. 1b. For this evaluation growth temperature and the indium gas phase mole ratio were held at constant values of ¹ "8503C " and In/(In#Ga)"80%. It is shown that with increase in pressure the incorporation of In also increased. PL measurements show clear advantages for the high pressure process. The optical properties of the GaInN "lms improve with higher total pressure. This can be veri"ed by comparing the intensities and the FWHM of the GaInN related RT (300 K) and LT (12 K) PL peaks shown in
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Fig. 1. In incorporation dependency on growth temperature and In/(In#Ga) gas phase ratio (a) and on growth pressure at constant growth temperature and In/(In#Ga) gas phase ratio (b).
Fig. 2. The intensity of the GaInN related peak measured at RT increases when using a higher total pressure. Additionally lower FWHM were found in the LT PL spectra of samples grown at higher pressures with the smallest width of 9.3 nm achieved when the sample was grown at 200 mbar, resulting in a 14.7% In content. Thus the best optical properties were obtained by using high total pressure. The high quality of this "lm is also con"rmed by the missing yellow emission at LT PL using low excitation energies. Indications for an e!ect similar to the `composition pulling e!ecta [10}12] were found in secondary ion mass spectroscopy (SIMS) measurements. Fig. 3 shows the SIMS measurements of 200 nm thick GaInN "lms grown on GaN. The Ga and In related signal (proportional to the content of the respective atoms) versus the sputtering time (proportional to the depth) are shown starting with the surface on the left and ending with the GaInN/GaN interface indicated by the drastic decrease of In atoms. The composition pulling e!ect causes an increasing In content with layer thickness. Evidence is given by comparing the In concentrations at the surface (left) and the interface (right) layers (Fig. 3a). This means that indium atoms are excluded from GaInN at the initial growth stage to reduce the deformation energy due
Fig. 2. Comparison of the optical properties of GaInN/GaN SH structures grown using di!erent total pressures.
M. Schwambera et al. / Journal of Crystal Growth 203 (1999) 340}348
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Fig. 3. SIMS measurements of GaInN layers (with indication of composition pulling e!ect on the left).
to the lattice mismatch. Fig. 3b shows a measurement of a GaInN sample with low In content of about 8%. This sample has a nearly constant indium concentration in the cross section, thus the composition pulling e!ect becomes less dominant. This was established using additional SIMS measurements which are summarised in Fig. 4. To have an indicator for the e!ect, we de"ned the Ga/In deviation ratio shown there. The values for the Ga/In minimum and maximum ratios were taken from the locations marked by the vertical lines in the SIMS measurements shown in Fig. 3. Comparing the composition pulling e!ect (Ga/In deviation) between samples with di!erent In content, there is clear evidence that the e!ect becomes less dominant with decreasing In content due to the decreased lattice mismatch, as expected. This behaviour was also found by Kawaguchi et al. [10]. Discrepancies between PL and X-ray measurements for the evaluation of the In composition are shown in Fig. 5. It shows the dependency of the band gap energy estimated by PL measurements on the In content of the examined samples. It is evident that the PL positions of the bulk SH structures (circles) did not follow the relation [13]
Fig. 4. Comparison of composition pulling e!ect.
E (x)"xE ( 3N)#(1!x)E (GaN)!bx(1!x) (1) using a bowing parameter of b"1 eV, E (InN)"1.95 eV and E (GaN)"3.43 eV. The E
Fig. 5. Inconsistency between XRD and PL measurements. Full line represents theoretical data with b"1 eV.
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di!erence between the PL measurements and the predicted values increased with increasing indium concentration. This clearly seems to be related to the composition pulling e!ect. A probable explanation for this is that the PL reveals the peak associated with the GaInN photoluminescence with the highest In content, because the radiative recombination takes place at the location of the lowest band gap (surface) whereas XRD measurements yield average values for the whole "lm. Other explanations for the discrepancy of PL measurements and the predicted values for the band gap can be found in Ref. [6]. Nevertheless PL measurements of optimised GaInN layers grown at elevated total pressure (triangles) show good agreement with Eq. (1). By now larger bowing factors have been published by Van de Walle et al. [14] with values of b up to 4.8 eV. Using these elevated bowing factors in Eq. (1) would result in a better "tting between our measurements and the predicted values. We achieved very good compositional uniformity over the whole wafer as demonstrated by the PL mapping shown in Fig. 6 of a GaInN "lm with a peak wavelength standard deviation of 1.11 nm. The "lm thickness of the GaInN layer was 200 nm. The growth temperature was adjusted to 9003C
resulting in an In content of about 8%. Thus with thickness homogeneities of less than 1% standard deviation the main speci"cation requirements for mass production are ful"lled.
3.2. Thermal stability investigation of DH structures Since the straightforward stacking of the three single layers GaN bu!er, GaInN, GaN cap } grown at di!erent growth temperature, total pressure and carrier gas than the InGaN } was found to result in greyish layers that could be characterised neither by X-ray di!raction (XRD) nor by PL, an intermediate cap of GaN grown using GaInN process conditions was introduced in order to protect the GaInN from dissolution while heating up to the standard growth temperature of GaN. To test the thermal stability of the DH structure thus formed by a 2 lm GaN bu!er, 50 nm GaInN layer and 100 nm GaN low temperature (LT) cap, such samples were exposed to a series of anneals at process conditions for GaN growth (low pressure, NH stabilisation) at increasing temperatures. A series of four anneal runs at 900, 1000, 1050 and 11003C, respectively were performed. Each
Fig. 6. PL mapping of Ga In N (200 nm) grown at 9003C on GaN (1.8 lm).
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sample, being left in the reactor at elevated temperature for 5 min while stabilising the surface with ammonia using N as the main carrier gas, was individually examined for surface changes. Samples exposed to 1050 and 11003C show a grey colour on the surface which is attributed to the dissolution of the GaInN layer and formation of In droplets as nitrogen evaporates out of the layer. Layers exposed to temperatures between 900 and 10003C did not show any such surface change. This indicates that the GaInN layer will not signi"cantly decompose in less than 5 min at 10003C when capped with at least 100 nm of low temperature GaN. Growth at higher temperatures should increase the durability of the InGaN layer, but at growth temperatures above 11003C some gradual degradation of the ternary material is still expected. 3.3. Growth and optimisation of GaN/GaInN/GaN double heterostructures Best growth parameters from GaInN single-hetero (SH) structure experiments were used for the active layer in the double-hetero (DH) structures. The growth was optimised by adjusting the growth rate (1 nm/min), the growth temperature (8603C)
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and the layer thickness (50 nm) to minimise the composition pulling e!ect. The critical step during the growth is the interface of GaInN and the GaN cap due to the large growth temperature gap between both layer growths. During the heat up to GaN cap growth conditions indium di!uses out of the InGaN. A suitable process has been found by continuously growing during the heat up with GaInN growth conditions into the GaN cap layer without a growth stop at the interface. This results in high quality DH structures. Using this optimised interface process a DH structure consisting of 1.6 lm GaN : Si bu!er (doped to contain nominally N "8;10 cm\), 50 nm GaInN (nominally " 10% InN in the solid) and 20 nm GaN cap (thin to limit absorption of the PL laser line) has been grown. Samples have been examined using RT PL at a low excitation density, using the Philips MCS PL system. Fig. 7 shows RT and LT PL spectra taken at the central point of the wafer, showing clear lines for the GaN and GaInN band edge related emissions. The yellow luminescence visible in the RT spectrum (Fig. 7a; modulated with an interference pattern due to the bu!er layer thickness) underlines the low excitation density used for
Fig. 7. RT and LT (20 K) PL spectrum of GaN (20 nm)/GaInN (50 nm)/GaN : Si (1.6 lm, N "8;10 cm\). "
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Fig. 8. RT PL mapping of the GaInN related peak wavelength for the 2 wafer of the sample shown in Fig. 9, average wavelength is 394.3 nm, standard deviation is 1.1 nm.
this measurement. At higher excitation densities it is negligible. The GaInN band edge related peak appears at 3.16 eV with a FWHM of 13.4 nm. The LT spectrum (Fig. 7b) is featureless except for the GaN and GaInN band edge related emissions demonstrating excellent material quality. The InGaN related peak at 391 nm is now two orders of magnitude higher than the yellow PL peak and about four times more intense than the GaN band edge. This re#ects the high optical quality of this DH structure. To assess the uniformity of the GaInN composition the wafer was scanned on the Philips MCS wafer mapper at room temperature. In Fig. 8 the mapping of the entire 2 wafer is shown displaying an average wavelength of 394.3 nm with a standard deviation of 1.1 nm.
4. Development of SQW and MQW structures Further development of the above described structures was made resulting in SQW and MQW structures. Fig. 9a shows the LT PL of a GaN/
GaInN/GaN : Si SQW structure with a well width of 4 nm. The same growth conditions used for the sample discussed in Fig. 3 were used for the GaInN layer. The PL shows a strong GaN near band edge peak and a GaInN peak with nearly half the intensity of the GaN related peak. The high quality of this structure can be veri"ed by comparing the emission intensities of the GaN and GaInN peaks with the low yellow emission intensity in this measurement using low excitation energies. The intensity of the GaInN peak is about eight times higher than the GaN emission. Additionally a blue shift of the GaInN peak of about 10 meV can be found compared to Fig. 7, due to the quantisation e!ect. The same structure was used for a MQW structure which was formed by a triple repetition of a 7 nm Ga In N barrier layer and a 4 nm Ga In N well layer. In Fig. 9b the LT PL of this sample is shown. As expected the intensity of the GaInN well related emission increased resulting in a 100 fold increase in the emission intensity compared to the GaN emission intensity which re#ects the high optical quality of this structure even at room temperature. Also a blue shift of the GaInN
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Fig. 9. LT (12 K) PL of GaN/Ga In N/Ga In N/ Ga In N/GaN : Si SQW structure and of GaN/3; +Ga In N/Ga In N,/Ga In N/GaN : Si MQW structure.
peak of about 25 meV can be found compared to Fig. 7. The di!erence in the value for the SQW may be due to di!erent strain e!ects in the MQW resulting in a di!erent In content in the layer or piezo electric e!ects causing the change in emission wavelength.
tures results in high quality SQW and MQW structures, which underlines the suitability of the developments for the mass production of blue and green light emitters.
References 5. Conclusions This paper reports on the investigation of selected GaN heterostructures. Single-hetero GaInN/ GaN layers have been optimised for small FWHM of the RT PL peaks for a given In incorporation by introducing a high temperature (8603C), high In versus total MO gas phase (80%) process. Methods of capping GaInN layers to prevent dissolution when preparing to grow GaN top layers have been investigated. A suitable cap for overgrowth up to 10003C has been demonstrated. Using an optimised capping technique DH structures of device quality have been produced, resulting in samples with state of the art composition uniformity across full 2 wafers. Further developments of these DH struc-
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