- Email: [email protected]
GaAs-cluster hybrid layers by MOVPE
ARTICLE IN PRESS
Journal of Crystal Growth 272 (2004) 772–777 www.elsevier.com/locate/jcrysgro
Te-co-doping experiments in ferromagnetic Mn(Ga)As/GaAscluster hybrid layers by MOVPE M. Lampalzer, S. Nau, C. Pietzonka, W. Treutmann, K. Volz, W. Stolz Department of Physics, Material Sciences Center, Philipps-University Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany Available online 13 October 2004
Abstract Hybrid structures consisting of ferromagnetic Mn(Ga)As clusters, which are embedded defect-free in a p-GaAs:Mn matrix, are realized by epitaxial growth using metalorganic vapour-phase epitaxy (MOVPE). The successful Te-co-doping of the GaAs:Mn matrix leads to a change from p- to n-type carrier transport. This behaviour enables the growth of structures for studying electron spin-injection effects. First laser devices including a co-doped hybrid structure in the ntype region of the contact of the device are realized. Investigations by SQUID magnetometer show that the ferromagnetic properties of the Mn(Ga)As clusters are not influenced by the Te co-doping of the surrounding GaAs matrix. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Pp; 78.66.Fd; 81.15.Gh Keywords: A1. Cluster; A3. Metalorganic vapor phase epitaxy; B1. MnAs; B2. Magneto-optic materials; B2. Semiconducting III–V materials; B3. Laser diodes
1. Introduction For applications in spin-(opto-)electronics the key challenge is to get a room temperature ferromagnetic material for easy integration in common III/V-semiconductors. There are various investigations on dilute-magnetic semiconductors (DMS) mainly by molecular beam epitaxy (MBE) [1–5] but in recent years also by metalorganic Corresponding author. Tel.: +49 6421 28 2 70 40; fax: +49 6421 28 2 89 35. E-mail address: [email protected] (M. Lampalzer).
vapour-phase epitaxy (MOVPE) [6–8]. In typical III/V-semiconductors magnetic ions like Mn act as an acceptor dopant. The short spin dephasing times [9] of p-type materials, thus, demand in particular n-type electron carrier transport in the active device structure for room temperature applications. In our approach we are using a MOPVE grown hybrid material, consisting of ferromagnetic Mn(Ga)As clusters defect-free embedded in a GaAs:Mn matrix [10]. In this study we focus on the Te-co-doping of the GaAs:Mn matrix to n-type carrier transport combined with ferromagnetism of the hybrid layer at room
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.068
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
temperature. The Te incorporation is investigated by means of secondary ion mass spectrometry (SIMS), the carrier transport by van-der-Pauw Hall measurement. The magnetic measurements are performed by SQUID magnetometer.
2. Experimental procedure (GaMn)As samples have been grown in a standard commercial horizontal MOVPE reactor system (AIX 200, Aixtron Corp.) using H2-carrier gas at a reduced reactor pressure of 50 hPa. As precursors we employed triethylgallium (TEGa), tertiarybutylarsine (TBAs), bis(methylcyclopentadienyl)manganese (MeCp)2Mn and diethyltelluride (DETe). The (GaMn)As layers were grown on a GaAs (1 0 0) substrate, with a typical V/III ratio of 5. To enable detailed investigations of the incorporation of Te and Mn using SIMS a specific layer sequence with a nominal Mn/Ga ratio in gas phase of 0.5%, leading to minimal Mn segregation, and various Te-doping concentrations and growth temperatures was deposited. By increasing the Mn/Ga ratio to values of 24% and above, we achieve the formation of hybrid layers consisting of defect-free embedded Mn(Ga)As clusters in a surrounding GaAs:Mn matrix [10–12]. In this GaAs:Mn matrix, Mn-atoms are only incorporated up to the solubility limit in the range of 2 1019 cm3–4 1019 cm3. The SIMS-analysis was performed by RTG Mikroanalyse GmbH Berlin. The magnetic properties were measured in a SQUID magnetometer (quantum design).
3. Results and discussion In the following, Te-co-doping experiments of the GaAs:Mn matrix as analysed by SIMS and electrical measurements are presented and discussed. Then the first realization of a simple laser device structure is described in which the ferromagnetic Mn(Ga)As clusters are embedded defectfree in the n-type contact layer of the device. Finally, the influence of Te-co-coping on the magnetic properties of the Mn(Ga)As clusters is studied.
773
3.1. Te incorporation in the GaAs:Mn matrix 3.1.1. SIMS investigations To investigate the Te incorporation, a multilayer structure consisting of GaAs:Mn layers with additional Te co-doping of five selected Te/Ga ratios from 0.5 104 to 8 104 was grown at deposition temperatures of 500 and 600 1C. The SIMS profiles of this structure for a growth temperature of 500 1C are shown in Fig. 1. The nominal Mn/Ga ratio in gas phase was fixed at 0.5% to maximize the Mn incorporation up to the solubility limit in GaAs of these growth conditions. Under these conditions no Mn(Ga)As clusters are formed. To compare the Te incorporation in the GaAs matrix with and without additional Mn supply, for each of the layers the Mn source was switched on and off (as indicated by the step-line in Fig. 1). As mentioned before, the Mn/Ga ratio was selected high enough to incorporate Mn up to the solubility limit. Therefore, for the lower Te concentrations where the amplification of the Mn incorporation plays a less important role, Mn segregation occurs. As a consequence, the Mn profile measured by SIMS is broadened and follows not exactly the switching of the Mn source. The left axis in Fig. 1 represents the atomic concentration of Te- and Mn-atoms per cm3 as determined by SIMS. The epitaxial layer structure starts on the right side with a 250 nm buffer layer grown at 650 1C. For the growth of this layer the Mn was not yet switched to the reactor. The value of 8 1017 Mn-atoms/cm3 determines the typical Mn background in the reactor in a growth series of hybrid structures. It is important to remark that a change of the easy removable parts like susceptor and liner tube will also remove this background for the growth of high-purity GaAs layers. During a As stabilized growth interruption the temperature was decreased to 500 1C. The Mn source was switched on to grow a first GaAs:Mn layer in the next step. In the following layers the Te incorporation level follows directly the selected Te/Ga ratio. The Te incorporation is always roughly three times higher as compared to the nominal Te/Ga ratio in the gas phase. This behaviour is presumably caused by a higher decomposition rate of DETe as compared
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
Tsubstrate = 500°C
SIMS: concentration [ 1/cm3 ]
10
10 Mn off
10 10
Mn 10
-4
18
2*10
10 10
10
-4
-5
17
Te 10
-3
19
Te/Ga 10
-2
Mn on
20
16
10
Te (SIMS) Mn (SIMS) Te/Ga ratio (gas phase)
10
ppartTe / ppartGa
774
-6
-7
15
0.0 0.2 0.4 0.6
0.8
1.0 1.2
1.4 1.6
1.8
2.0
depth [µm] Fig. 1. SIMS profile of a GaAs:Te,Mn layer sequence with various Te/Ga ratios from 0.5 104 to 8 104. For the Te/Ga ratio of 2 104 the level of the Te-concentration is comparable to Mn incorporation.
to TEGa at 500 1C. The Te incorporation is independent on the Mn supply. But there is an increase of the Mn incorporation by adding Te. At the highest Te/Ga ratios of 103, the incorporated Te concentration exceeds the Mn level by a factor of 2. For deposition temperatures of 600 1C, the overall incorporated Te level is significantly lower only reaching about 50% of the Mn level. In addition, strong interactions in the incorporation behaviour of the two elements are observed. 3.1.2. Hall measurements To investigate the electrical properties van-derPauw Hall measurements of five individual layers were performed. The Te/Ga ratios and the constant Mn/Ga ratio of 0.5% in gas phase in these layers are identical to the levels used in the structure for SIMS investigation at a substrate temperature of 550 1C. Without the formation of clusters, these layers are non-magnetic. Under these growth conditions, the typical p-type carrier concentration at room temperature is
2.4 1018 cm3. By adding slightly Te (Te/Ga ratio 0.5 104) at first the hole density increases to 5.3 1018. This is identical to the observed effect in the SIMS layers, where an increasing of the Mn incorporation by adding Te was detected. At the same time, the mobility decreases from 95 to 43 cm2 V1 s1 primarily due to compensation effects. At higher Te/Ga ratios of 2 104, the intended change to n-type carrier transport is observed with measured n-type carrier concentration from 5.9 1018 to 2.6 1019. The mobility increases from 75 to 520 cm2 V1 s1 with increasing Te/Ga ratio. In the intermediate region (around Te=Ga ¼ 1 104 ), the experimental accuracy of the determination of carrier concentration and mobility is significantly worse due to the change in the sign of the Hall constant (data points are displayed in brackets). The change from p-type to n-type carrier transport takes place at the Te/ Ga ratio of 2 104. At this level, the Te concentration is comparable to Mn incorporation as verified by the SIMS analysis (Fig. 2).
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
3.2. Laser device structure
775
usual pn-I/V characteristics as well as light emission of the diode. This prooves the successful injection of electrons to the active quantum well region. Broad area laser devices were processed and measured at room temperature. The current–power curve is displayed in Fig. 4. Clear laser operation is obtained, although, due to various non-optimized conditions the device characteristics can expected to be improved in future studies. Despite these non-optimized conditions, however, the achieved laser activity clearly demonstrates the suitability of the Te-co-doped layer for n-type carrier transport in the ferromagnetic hybrid structures at room temperature.
In the last paragraph we investigated nonferromagnetic samples with Mn/Ga ratios just below the threshold for Mn(Ga)As cluster formation. Due to the solubility limit of Mn in GaAs of about 2 1019 cm3 (determined by SIMS) this level is also maintained in the cluster containing hybrid layers with Mn/Ga ratios of 24% and above. As a consequence, the Mn concentration in the GaAs:Mn matrix is identical in both types of layers. However, the interpretation of the magneto-transport experiments in the hybrid layer structures containing ferromagnetic Mn(Ga)As clusters is much more complicated [13]. Therefore, we have grown a test structure to prove the electron transport properties by integrating a codoped n-type hybrid layer (GaAs:Te,Mn matrix with Mn(Ga)As clusters) in the n-region of a lightemitting diode structure on a p-doped GaAssubstrate. The layer sequence is schematically shown in Fig. 3 including a Ga85In15As-QW in the active region. This layer structure shows the
3.3. Te-co-doped hybrid structures: comparison of the magnetic properties
=
To investigate the magnetic properties of the Te-co-doped hybrid structures, SQUID measurements of Mn(Ga)As/GaAs:Mn and hybrid layers grown at 550 1C with and without with
1020
conc: p-type n-type
@ 293 K
( )
104
) ba r m
*1 0
-6
*1 0
p-type
=
1.
1018
103
35
Te
/G
a2
-3
conc. [ cm ]
-4
n-type
(p
pa
rt
102
mobility: p-type n-type
1017
mobility [ cm2 / Vs ]
1019
101
( )
=
1016 "0"
100 -4
10
-3
10
ppart Te/ ppart Ga Fig. 2. Van-der-Pauw Hall measurements, indicating a change from p- to n-type carrier transport for Te/Ga ratios X2 104.
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
776
6 (GaMn)As:Te Mnepitaxy
@ 298 K
10 nm GaAs:Te
4
500 nm (Al30Ga70)As:Te 50 nm GaAs
2 M [arb. units]
transfer 50 nm GaAs 10 nm (Ga85In15)As 100 nm GaAs
0
-2
1300 nm (Al30Ga70)As:C
Mnfree epitaxy
Mn(Ga)As cluster
300 nm (Al30Ga70)As:C (graded)
-4
p-type GaAs-matrix n-type GaAs-matrix
300 nm GaAs:C buffer-layer -6 -0.3
p-substrate Fig. 3. Laser device structure as a test to prove the electron transport properties. Due to the high Mn background doping concentration, the LED/LD-structures have been grown in two epitaxial systems connected by a common N2 glove box. The transfer between the two systems is indicated by the black line.
12 opt. power [mW]
150
11 10
U
8 7
100
6 5
U [V]
opt. power [mW]
9
4 50
-0.2
-0.1
0.0 µ0H [T]
0.1
0.2
0.3
Fig. 5. Comparison of the hysteresis curves of two hybrid layers with p-type GaAs:Mn matrix and n-type GaAs:Mn,Te matrix measured by SQUID magnetometer at room temperature.
The external H-field was orientated along GaAs[0 1 1]. Both layers show a clear hysteresis loop at 298 K with a coercitive field of approximately 2072 mT, indicating that both type of hybrid layers are suitable to achieve room temperature ferromagnetism. In addition, the two hysteresis loops are nearly identical. This prooves that the magnetic properties are not influenced by the Te-co-doping process.
3 2
4. Summary
1 0 0
500
1000
0 1500 2000 2500 3000 I [mA]
Fig. 4. Current–power curve (pulsed operation) of the processed broad area laser devices, measured at room temperature.
Te-co-doping were performed. Whether Te is incorporated not only in the GaAs:Te,Mn matrix but also in the Mn(Ga)As clusters is unclear at present. A comparison of the hysteresis curve of two hybrid layers with p-type GaAs:Mn matrix and n-type GaAs:Mn,Te matrix is shown in Fig. 5.
We have presented the successful Te-co-doping of the p-GaAs:Mn-matrix leading to a change from p-type to n-type carrier transport. The room temperature ferromagnetic properties of the hybrid structures are not influenced by the Te-codoping. In addition to Mn incorporation, SIMSprofiles show well-controlled incorporation of the Te for substrate temperatures of 500 1C. A first successful laser-structure verifies the achieved ntype carrier transport by including a co-doped hybrid layer in the n-type contact region of the device.
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
Acknowledgement Fruitful discussions with Th. Hartmann, P.J. Klar and W. Heimbrodt are gratefully acknowledged. This work has been supported by the Deutsche Forschungsgemeinschaft (DFG), the Federal Ministry of Education and Research (BMBF) and the Interdisciplinary Research Center on ‘‘Optodynamics’’. References [1] H. Ohno, J. Crystal Growth 251 (2003) 285. [2] R. Moriya, H. Munekata, J. Appl. Phys. 93 (2003) 4603. [3] M. Tanaka, J. Crystal Growth 201/202 (1999) 660. [4] M. Tanaka, Semicond. Sci. Technol. 17 (2002) 327. [5] L. Da¨weritz, F. Schippan, A. Trampert, M. Ka¨stner, G. Behme, Z.M. Wang, M. Moreno, P. Schu¨tzendu¨be, K.H. Ploog, J. Crstal Growth 227–228 (2001) 834.
777
[6] Th. Hartmann, M. Lampalzer, W. Stolz, K. Megges, J. Lorberth, P.J. Klar, W. Heimbrodt, Thin Solid Films 364 (2000) 209. [7] S. Hara, M. Lampalzer, T. Torunski, K. Volz, W. Treutmann, W. Stolz, J. Crystal Growth 261 (2004) 330. [8] S.J. May, A.J. Blattner, B.W. Wessels, Physica B 340–342 (2003) 870. [9] M. Oestreich, M. Bender, J. Hu¨bner, D. Ha¨gele, W.W. Ru¨hle, Th. Hartmann, P.J. Klar, W. Heimbrodt, M. Lampalzer, K. Volz, W. Stolz, Semicond. Sci. Technol. 17 (2002) 285. [10] K. Volz, M. Lampalzer, A. Schaper, J. Zweck, W. Stolz, Inst. Phys. Conf. Ser. 169 (2001) 211. [11] M. Lampalzer, K. Volz, W. Treutmann, S. Nau, T. Torunski, K. Megges, J. Lorberth, W. Stolz, Inst. Phys. Conf. Ser. 170 (3) (2002) 249. [12] M. Lampalzer, K. Volz, W. Treutmann, S. Nau, T. Torunski, K. Megges, J. Lorberth, W. Stolz, J. Crystal Growth 248 (2003) 474. [13] S. Ye, P.J. Klar, Th. Hartmann, W. Heimbrodt, M. Lampalzer, S. Nau, T. Torunski, W. Stolz, T. Kurz, H.-A. Krug von Nidda, A. Loidl, Appl. Phys. Lett. 83 (2003) 3927.
Journal of Crystal Growth 272 (2004) 772–777 www.elsevier.com/locate/jcrysgro
Te-co-doping experiments in ferromagnetic Mn(Ga)As/GaAscluster hybrid layers by MOVPE M. Lampalzer, S. Nau, C. Pietzonka, W. Treutmann, K. Volz, W. Stolz Department of Physics, Material Sciences Center, Philipps-University Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany Available online 13 October 2004
Abstract Hybrid structures consisting of ferromagnetic Mn(Ga)As clusters, which are embedded defect-free in a p-GaAs:Mn matrix, are realized by epitaxial growth using metalorganic vapour-phase epitaxy (MOVPE). The successful Te-co-doping of the GaAs:Mn matrix leads to a change from p- to n-type carrier transport. This behaviour enables the growth of structures for studying electron spin-injection effects. First laser devices including a co-doped hybrid structure in the ntype region of the contact of the device are realized. Investigations by SQUID magnetometer show that the ferromagnetic properties of the Mn(Ga)As clusters are not influenced by the Te co-doping of the surrounding GaAs matrix. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Pp; 78.66.Fd; 81.15.Gh Keywords: A1. Cluster; A3. Metalorganic vapor phase epitaxy; B1. MnAs; B2. Magneto-optic materials; B2. Semiconducting III–V materials; B3. Laser diodes
1. Introduction For applications in spin-(opto-)electronics the key challenge is to get a room temperature ferromagnetic material for easy integration in common III/V-semiconductors. There are various investigations on dilute-magnetic semiconductors (DMS) mainly by molecular beam epitaxy (MBE) [1–5] but in recent years also by metalorganic Corresponding author. Tel.: +49 6421 28 2 70 40; fax: +49 6421 28 2 89 35. E-mail address: [email protected] (M. Lampalzer).
vapour-phase epitaxy (MOVPE) [6–8]. In typical III/V-semiconductors magnetic ions like Mn act as an acceptor dopant. The short spin dephasing times [9] of p-type materials, thus, demand in particular n-type electron carrier transport in the active device structure for room temperature applications. In our approach we are using a MOPVE grown hybrid material, consisting of ferromagnetic Mn(Ga)As clusters defect-free embedded in a GaAs:Mn matrix [10]. In this study we focus on the Te-co-doping of the GaAs:Mn matrix to n-type carrier transport combined with ferromagnetism of the hybrid layer at room
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.068
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
temperature. The Te incorporation is investigated by means of secondary ion mass spectrometry (SIMS), the carrier transport by van-der-Pauw Hall measurement. The magnetic measurements are performed by SQUID magnetometer.
2. Experimental procedure (GaMn)As samples have been grown in a standard commercial horizontal MOVPE reactor system (AIX 200, Aixtron Corp.) using H2-carrier gas at a reduced reactor pressure of 50 hPa. As precursors we employed triethylgallium (TEGa), tertiarybutylarsine (TBAs), bis(methylcyclopentadienyl)manganese (MeCp)2Mn and diethyltelluride (DETe). The (GaMn)As layers were grown on a GaAs (1 0 0) substrate, with a typical V/III ratio of 5. To enable detailed investigations of the incorporation of Te and Mn using SIMS a specific layer sequence with a nominal Mn/Ga ratio in gas phase of 0.5%, leading to minimal Mn segregation, and various Te-doping concentrations and growth temperatures was deposited. By increasing the Mn/Ga ratio to values of 24% and above, we achieve the formation of hybrid layers consisting of defect-free embedded Mn(Ga)As clusters in a surrounding GaAs:Mn matrix [10–12]. In this GaAs:Mn matrix, Mn-atoms are only incorporated up to the solubility limit in the range of 2 1019 cm3–4 1019 cm3. The SIMS-analysis was performed by RTG Mikroanalyse GmbH Berlin. The magnetic properties were measured in a SQUID magnetometer (quantum design).
3. Results and discussion In the following, Te-co-doping experiments of the GaAs:Mn matrix as analysed by SIMS and electrical measurements are presented and discussed. Then the first realization of a simple laser device structure is described in which the ferromagnetic Mn(Ga)As clusters are embedded defectfree in the n-type contact layer of the device. Finally, the influence of Te-co-coping on the magnetic properties of the Mn(Ga)As clusters is studied.
773
3.1. Te incorporation in the GaAs:Mn matrix 3.1.1. SIMS investigations To investigate the Te incorporation, a multilayer structure consisting of GaAs:Mn layers with additional Te co-doping of five selected Te/Ga ratios from 0.5 104 to 8 104 was grown at deposition temperatures of 500 and 600 1C. The SIMS profiles of this structure for a growth temperature of 500 1C are shown in Fig. 1. The nominal Mn/Ga ratio in gas phase was fixed at 0.5% to maximize the Mn incorporation up to the solubility limit in GaAs of these growth conditions. Under these conditions no Mn(Ga)As clusters are formed. To compare the Te incorporation in the GaAs matrix with and without additional Mn supply, for each of the layers the Mn source was switched on and off (as indicated by the step-line in Fig. 1). As mentioned before, the Mn/Ga ratio was selected high enough to incorporate Mn up to the solubility limit. Therefore, for the lower Te concentrations where the amplification of the Mn incorporation plays a less important role, Mn segregation occurs. As a consequence, the Mn profile measured by SIMS is broadened and follows not exactly the switching of the Mn source. The left axis in Fig. 1 represents the atomic concentration of Te- and Mn-atoms per cm3 as determined by SIMS. The epitaxial layer structure starts on the right side with a 250 nm buffer layer grown at 650 1C. For the growth of this layer the Mn was not yet switched to the reactor. The value of 8 1017 Mn-atoms/cm3 determines the typical Mn background in the reactor in a growth series of hybrid structures. It is important to remark that a change of the easy removable parts like susceptor and liner tube will also remove this background for the growth of high-purity GaAs layers. During a As stabilized growth interruption the temperature was decreased to 500 1C. The Mn source was switched on to grow a first GaAs:Mn layer in the next step. In the following layers the Te incorporation level follows directly the selected Te/Ga ratio. The Te incorporation is always roughly three times higher as compared to the nominal Te/Ga ratio in the gas phase. This behaviour is presumably caused by a higher decomposition rate of DETe as compared
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
Tsubstrate = 500°C
SIMS: concentration [ 1/cm3 ]
10
10 Mn off
10 10
Mn 10
-4
18
2*10
10 10
10
-4
-5
17
Te 10
-3
19
Te/Ga 10
-2
Mn on
20
16
10
Te (SIMS) Mn (SIMS) Te/Ga ratio (gas phase)
10
ppartTe / ppartGa
774
-6
-7
15
0.0 0.2 0.4 0.6
0.8
1.0 1.2
1.4 1.6
1.8
2.0
depth [µm] Fig. 1. SIMS profile of a GaAs:Te,Mn layer sequence with various Te/Ga ratios from 0.5 104 to 8 104. For the Te/Ga ratio of 2 104 the level of the Te-concentration is comparable to Mn incorporation.
to TEGa at 500 1C. The Te incorporation is independent on the Mn supply. But there is an increase of the Mn incorporation by adding Te. At the highest Te/Ga ratios of 103, the incorporated Te concentration exceeds the Mn level by a factor of 2. For deposition temperatures of 600 1C, the overall incorporated Te level is significantly lower only reaching about 50% of the Mn level. In addition, strong interactions in the incorporation behaviour of the two elements are observed. 3.1.2. Hall measurements To investigate the electrical properties van-derPauw Hall measurements of five individual layers were performed. The Te/Ga ratios and the constant Mn/Ga ratio of 0.5% in gas phase in these layers are identical to the levels used in the structure for SIMS investigation at a substrate temperature of 550 1C. Without the formation of clusters, these layers are non-magnetic. Under these growth conditions, the typical p-type carrier concentration at room temperature is
2.4 1018 cm3. By adding slightly Te (Te/Ga ratio 0.5 104) at first the hole density increases to 5.3 1018. This is identical to the observed effect in the SIMS layers, where an increasing of the Mn incorporation by adding Te was detected. At the same time, the mobility decreases from 95 to 43 cm2 V1 s1 primarily due to compensation effects. At higher Te/Ga ratios of 2 104, the intended change to n-type carrier transport is observed with measured n-type carrier concentration from 5.9 1018 to 2.6 1019. The mobility increases from 75 to 520 cm2 V1 s1 with increasing Te/Ga ratio. In the intermediate region (around Te=Ga ¼ 1 104 ), the experimental accuracy of the determination of carrier concentration and mobility is significantly worse due to the change in the sign of the Hall constant (data points are displayed in brackets). The change from p-type to n-type carrier transport takes place at the Te/ Ga ratio of 2 104. At this level, the Te concentration is comparable to Mn incorporation as verified by the SIMS analysis (Fig. 2).
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
3.2. Laser device structure
775
usual pn-I/V characteristics as well as light emission of the diode. This prooves the successful injection of electrons to the active quantum well region. Broad area laser devices were processed and measured at room temperature. The current–power curve is displayed in Fig. 4. Clear laser operation is obtained, although, due to various non-optimized conditions the device characteristics can expected to be improved in future studies. Despite these non-optimized conditions, however, the achieved laser activity clearly demonstrates the suitability of the Te-co-doped layer for n-type carrier transport in the ferromagnetic hybrid structures at room temperature.
In the last paragraph we investigated nonferromagnetic samples with Mn/Ga ratios just below the threshold for Mn(Ga)As cluster formation. Due to the solubility limit of Mn in GaAs of about 2 1019 cm3 (determined by SIMS) this level is also maintained in the cluster containing hybrid layers with Mn/Ga ratios of 24% and above. As a consequence, the Mn concentration in the GaAs:Mn matrix is identical in both types of layers. However, the interpretation of the magneto-transport experiments in the hybrid layer structures containing ferromagnetic Mn(Ga)As clusters is much more complicated [13]. Therefore, we have grown a test structure to prove the electron transport properties by integrating a codoped n-type hybrid layer (GaAs:Te,Mn matrix with Mn(Ga)As clusters) in the n-region of a lightemitting diode structure on a p-doped GaAssubstrate. The layer sequence is schematically shown in Fig. 3 including a Ga85In15As-QW in the active region. This layer structure shows the
3.3. Te-co-doped hybrid structures: comparison of the magnetic properties
=
To investigate the magnetic properties of the Te-co-doped hybrid structures, SQUID measurements of Mn(Ga)As/GaAs:Mn and hybrid layers grown at 550 1C with and without with
1020
conc: p-type n-type
@ 293 K
( )
104
) ba r m
*1 0
-6
*1 0
p-type
=
1.
1018
103
35
Te
/G
a2
-3
conc. [ cm ]
-4
n-type
(p
pa
rt
102
mobility: p-type n-type
1017
mobility [ cm2 / Vs ]
1019
101
( )
=
1016 "0"
100 -4
10
-3
10
ppart Te/ ppart Ga Fig. 2. Van-der-Pauw Hall measurements, indicating a change from p- to n-type carrier transport for Te/Ga ratios X2 104.
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
776
6 (GaMn)As:Te Mnepitaxy
@ 298 K
10 nm GaAs:Te
4
500 nm (Al30Ga70)As:Te 50 nm GaAs
2 M [arb. units]
transfer 50 nm GaAs 10 nm (Ga85In15)As 100 nm GaAs
0
-2
1300 nm (Al30Ga70)As:C
Mnfree epitaxy
Mn(Ga)As cluster
300 nm (Al30Ga70)As:C (graded)
-4
p-type GaAs-matrix n-type GaAs-matrix
300 nm GaAs:C buffer-layer -6 -0.3
p-substrate Fig. 3. Laser device structure as a test to prove the electron transport properties. Due to the high Mn background doping concentration, the LED/LD-structures have been grown in two epitaxial systems connected by a common N2 glove box. The transfer between the two systems is indicated by the black line.
12 opt. power [mW]
150
11 10
U
8 7
100
6 5
U [V]
opt. power [mW]
9
4 50
-0.2
-0.1
0.0 µ0H [T]
0.1
0.2
0.3
Fig. 5. Comparison of the hysteresis curves of two hybrid layers with p-type GaAs:Mn matrix and n-type GaAs:Mn,Te matrix measured by SQUID magnetometer at room temperature.
The external H-field was orientated along GaAs[0 1 1]. Both layers show a clear hysteresis loop at 298 K with a coercitive field of approximately 2072 mT, indicating that both type of hybrid layers are suitable to achieve room temperature ferromagnetism. In addition, the two hysteresis loops are nearly identical. This prooves that the magnetic properties are not influenced by the Te-co-doping process.
3 2
4. Summary
1 0 0
500
1000
0 1500 2000 2500 3000 I [mA]
Fig. 4. Current–power curve (pulsed operation) of the processed broad area laser devices, measured at room temperature.
Te-co-doping were performed. Whether Te is incorporated not only in the GaAs:Te,Mn matrix but also in the Mn(Ga)As clusters is unclear at present. A comparison of the hysteresis curve of two hybrid layers with p-type GaAs:Mn matrix and n-type GaAs:Mn,Te matrix is shown in Fig. 5.
We have presented the successful Te-co-doping of the p-GaAs:Mn-matrix leading to a change from p-type to n-type carrier transport. The room temperature ferromagnetic properties of the hybrid structures are not influenced by the Te-codoping. In addition to Mn incorporation, SIMSprofiles show well-controlled incorporation of the Te for substrate temperatures of 500 1C. A first successful laser-structure verifies the achieved ntype carrier transport by including a co-doped hybrid layer in the n-type contact region of the device.
ARTICLE IN PRESS M. Lampalzer et al. / Journal of Crystal Growth 272 (2004) 772–777
Acknowledgement Fruitful discussions with Th. Hartmann, P.J. Klar and W. Heimbrodt are gratefully acknowledged. This work has been supported by the Deutsche Forschungsgemeinschaft (DFG), the Federal Ministry of Education and Research (BMBF) and the Interdisciplinary Research Center on ‘‘Optodynamics’’. References [1] H. Ohno, J. Crystal Growth 251 (2003) 285. [2] R. Moriya, H. Munekata, J. Appl. Phys. 93 (2003) 4603. [3] M. Tanaka, J. Crystal Growth 201/202 (1999) 660. [4] M. Tanaka, Semicond. Sci. Technol. 17 (2002) 327. [5] L. Da¨weritz, F. Schippan, A. Trampert, M. Ka¨stner, G. Behme, Z.M. Wang, M. Moreno, P. Schu¨tzendu¨be, K.H. Ploog, J. Crstal Growth 227–228 (2001) 834.
777
[6] Th. Hartmann, M. Lampalzer, W. Stolz, K. Megges, J. Lorberth, P.J. Klar, W. Heimbrodt, Thin Solid Films 364 (2000) 209. [7] S. Hara, M. Lampalzer, T. Torunski, K. Volz, W. Treutmann, W. Stolz, J. Crystal Growth 261 (2004) 330. [8] S.J. May, A.J. Blattner, B.W. Wessels, Physica B 340–342 (2003) 870. [9] M. Oestreich, M. Bender, J. Hu¨bner, D. Ha¨gele, W.W. Ru¨hle, Th. Hartmann, P.J. Klar, W. Heimbrodt, M. Lampalzer, K. Volz, W. Stolz, Semicond. Sci. Technol. 17 (2002) 285. [10] K. Volz, M. Lampalzer, A. Schaper, J. Zweck, W. Stolz, Inst. Phys. Conf. Ser. 169 (2001) 211. [11] M. Lampalzer, K. Volz, W. Treutmann, S. Nau, T. Torunski, K. Megges, J. Lorberth, W. Stolz, Inst. Phys. Conf. Ser. 170 (3) (2002) 249. [12] M. Lampalzer, K. Volz, W. Treutmann, S. Nau, T. Torunski, K. Megges, J. Lorberth, W. Stolz, J. Crystal Growth 248 (2003) 474. [13] S. Ye, P.J. Klar, Th. Hartmann, W. Heimbrodt, M. Lampalzer, S. Nau, T. Torunski, W. Stolz, T. Kurz, H.-A. Krug von Nidda, A. Loidl, Appl. Phys. Lett. 83 (2003) 3927.