- Email: [email protected]
(Al,Ga)As multi-quantum-well structures
ARTICLE IN PRESS
Journal of Crystal Growth 278 (2005) 219–223 www.elsevier.com/locate/jcrysgro
Electrical and optical properties of (Ga,In)(As,N)/(Al,Ga)As multi-quantum-well structures R. Hey, Y.-J. Han, M. Giehler, M. Ramsteiner, H.T. Grahn, K.H. Ploog Paul-Drude-Institut fu¨r Festko¨rperelektronik, Hausvogteiplatz 5-7, Berlin 10117, Germany Available online 1 February 2005
Abstract The electrical properties in highly doped, weakly strained (In,Ga)(As,N)/(Al,Ga)As MQWs are studied by Hall measurements at various temperatures. With increasing N content in the well and Al content in the barrier, the electron mobility is strongly reduced. Magnetoresistance measurements are performed at low temperatures revealing carrier localization induced by the incorporation of N. From Raman spectroscopy, a preferential incorporation of N at the interface with the (Al,Ga)As barrier is found. Intersubband absorption experiments show only a small shift of the signal with increasing N content, but a strongly reduced and broadened absorption peak. r 2005 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 73.63.Hn; 78.66.Fd; 78.30.Fs Keywords: A1. Low-dimensional structures; A3. Molecular beam epitaxy; B2. Semiconducting III–V materials
1. Introduction It is well established that the incorporation of small amounts of nitrogen into III–V compound semiconductor structures remarkably modifies their electronic properties (see Ref. [1] for a more comprehensive review). Therefore, dilute nitride III–V semiconductor alloys are extensively investigated, in particular due to their strong nonlinear band-gap reduction [2], which facilitates their Corresponding author. Tel.: +49 30 20377354;
fax: +49 30 20377201. E-mail address: [email protected] (R. Hey).
application as interband light emitters for telecommunication in the 1.3 and 1.55 mm spectral range [3]. The incorporation of small amounts of nitrogen into heterojunctions predominantly affects the discontinuity of the conduction band DEC, while leaving the discontinuity of the valence band mostly unaffected [4]. A larger DEC in GaAs-based materials systems such as Ga(As,N) and (Ga,In)(As,N) may allow to diversify the performance of intersubband-based devices. However, the nitrogen level is resonant with the conduction band leading to an anticrossing [5]. The impact of the resonant coupling between these levels on the intersubband properties remains still unclear. Recently, first results on investigations
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.12.058
ARTICLE IN PRESS 220
R. Hey et al. / Journal of Crystal Growth 278 (2005) 219–223
of intersubband transitions are reported for (In,Ga)(As,N)/GaAs multi-quantum wells (MQW) with a high In content, showing that the intersubband energies scale with the N concentration [6], and double-quantum-well infrared detectors in the Ga(As,N)/(Al,Ga)As materials system [7]. However, the structural, optical, and electrical properties deteriorate with increasing N incorporation. We report on the electrical and optical properties of dilute nitrogen (Ga,In)(As,N)/(Al,Ga)As MQW structures.
2. Experimental techniques The samples are grown by molecular-beam epitaxy on GaAs(0 0 1) substrates at 450 1C at growth rates of 0.7 mm/h for the GaAs component and with a III/V-BEP-ratio of 30 using an ADDONs RF plasma-assisted N-source powered at 100 W for a N flux in the range of about 0.005 sscm, which is the lower limit for stable operation. The MQW structures consist of 25 or 50 periods of 6-nm-thick wells and 10-nm-thick barriers. The MQW structures are doped n-type by Si to a level of about 2 1012 cm2 per well. The N and In composition in the wells are varied up to 1% and 5%, respectively, for weakly strained MQWs. The Al mole fraction in the barriers is 0 or 0.33. For selected samples, rapid thermal annealing (RTA) has been performed at 700 through 900 1C for 60 s. Photoluminescence (PL) measurements are carried out at 5 K. Hall-bar structures are fabricated for measuring the transport properties at various temperatures. Raman investigations are performed on MQWs with different Al content in the barriers as well as on bulk layers with different mole fractions of Al and N. Intersubband absorption (ISA) spectra are recorded using polarized light by putting the samples in a waveguide geometry with 451 facets and 7 internal reflections.
3. Results and discussion From X-ray diffraction and secondary-ionmass-spectrometry measurements, we obtained a
ratio of 10:1 (N-shutter opened and closed) for the N incorporation leading to an unintentional N concentration in the barrier of about 0.1%. N concentrations below 0.6% are realized by periodically opening and closing the shutter of the N source. The incorporation of N into the group-V sublattice causes the formation of electrically active defects, which decrease the dopant activity and transport properties such as carrier lifetime [8], mobility [9], and diffusion length [10]. In general, we observed that a RTA treatment up to 800 1C of samples with different barrier compositions improves their mobility, e.g. from 630 to 1000 cm2/Vs (at slightly increasing carrier concentrations). For RTA temperatures above 800 1C, we found a strong decrease of the carrier concentration. We assume that this temperature dependence is due to an impurity-induced disorder process of the highly doped material. The improvement of the conductivity is better for GaAs barriers and decreases with increasing Al mole fraction in the barriers. In the following, we concentrate on the investigation of the lateral transport of as-grown samples. The composition, the mobility, and the carrier concentration of the samples are listed in Table 1. Fig. 1 displays the carrier mobilities versus temperature for MQWs with different mole fractions of In, N, and Al. As a reference, the mobility of an (In0.03Ga0.97)As/(Al0.33Ga0.67)As MQW amounts to 20000 cm2/Vs at low temperatures and decreases for higher temperatures due to optical-phonon scattering. For an (In0.03Ga0.97)(As0.99N0.01)/(Al0.33Ga0.67)As MQW, a drastic reduction of the mobility is measured with a main value of about 170 cm2/Vs at room temperature (RT). Even for the MQW with unintentional N incorporation in the barriers and wells—grown with the plasma source turned on and the shutter closed—the mobility already drops to 430 cm2/Vs at RT (not shown). Similar mobility reductions are observed for Al-free barriers (In0.03Ga0.97)(As0.99N0.01)/GaAs as well as strained (In0.1Ga0.9)(As0.99N0.01)/GaAs and (In0.15Ga0.85)(As0.99N0.01)/GaAs MQWs, resulting in mobilities of 350, 750, and 1050 cm2/Vs at RT, respectively. The results clearly show that in
ARTICLE IN PRESS R. Hey et al. / Journal of Crystal Growth 278 (2005) 219–223 Table 1 xN and xIn denote the mole fractions of N and In in the wells, xAl the mole fraction of Al in the barrier, N the number of wells, D the type of doping (b: barrier, w: well), n the electron concentration in the well, and m the carrier mobility at RT for as-grown MQWs (A–L) and a bulk layer (M) #
N
Well
D
n
m
1012cm2
cm2/Vs
xN
xIn
xAl
A B C
0.008 0.006 0.006
0.01 0.10 0.15
0 0 0
25 12 5
b b b
1.1 2.3 3
350 750 1050
D E
0 0
0.03 0.05
0.33 0.33
25 50
b w
1.6 2.5
6200 1870
F G H I J K L
0.001 0.012 0.008 0.005 0.005 0.01 0.01
0.05 0.03 0.05 0.05 0.05 0.05 0.05
0.33 0.33 0.33 0.33 0.33 0.33 0.33
50 25 50 50 50 25 25
w b w w w b b
2.2 0.7 1.6 1.1 1.6
430 170 130 170 120
1
280
M
0.01
0
0.33
2
Mobility (cm /Vs)
10000
D
221
dominant process, samples J and K have been grown, where the N source was turned on and off during the growth of the well leaving a 1-nm-thick (In,Ga)As spacer layer adjacent to the (Al,Ga)As barrier. As shown below by Raman experiments, the barriers are completely free of N. However, no improvement of the mobility is detected. Therefore, we conclude that the mobilities are inherently limited by the dilute (In,Ga)(As,N) well. For larger In mole fractions, higher mobilities can be obtained, but at the expense of a larger lattice strain. Some of the samples even show localized carrier transport behaviour. In this case, the resistance exponentially increases with decreasing temperature. In Fig. 2, we show the negative magnetoresistance for two samples A and E, indicating strong carrier localization. This is caused by the potential fluctuations, which are created by N-related defects as discussed in Ref. [11]. The optical characterization is performed by PL, Raman, and infrared (IR) intersubband absorption spectroscopy. Fig. 3 shows representative low-temperature PL spectra of as-grown samples with 0.1, 0.5, and 0.8% N in doped (In0.05Ga0.95)(As,N)/(Al0.33Ga0.67)As MQWs. No N-related emission is detected for the MQW
C
1000
B
300
A G
100
100
Temperature (K) Fig. 1. Mobility versus temperature of as-grown N-free Ga0.95In0.05As/Al0.33Ga0.67As MQW (D), GaxIn1x(As,N)/ GaAs MQWs with different In mole fractions (A, B, C), and a Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQW (G).
RS (kΩ)
E 10
200
1.0
A
(Ga,In)(As,N)/(Al,Ga)As MQWs the mobility is strongly reduced, resulting in a low electron mean free path, which is comparable to the average separation of a few nanometres of N atoms in the well. In order to suppress carrier scattering at the (In,Ga)(As,N)/(Al,Ga)As interfaces as the
0.8
0
2 B (T)
4
Fig. 2. Sheet resistance versus magnetic field of an as-grown Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQW (E) and a Ga0.95 In0.05(As,N)/GaAs MQW (A) for a temperature of 1.4 K.
ARTICLE IN PRESS R. Hey et al. / Journal of Crystal Growth 278 (2005) 219–223
222 40000 J
30000
2x M
20000
10000 F 0 800
850
900
950
1000
Wavelength (nm) Fig. 3. Low-temperature photoluminescence spectra of asgrown, n-type Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQWs with different N mole fraction in the wells excited with 1.8 mW at 532 nm for a temperature of 5 K.
Intensity (arb. units)
PL Intensity (arb. units)
H
J
F
containing 0.1% N in the barriers and the wells. The PL emission wavelength increases with increasing N concentration. Different vibrational modes of N on an As-site (NAs) in (Al,Ga)As of as-grown (In,Ga)(As,N)/ (Al,Ga)As MQWs are clearly detected by Raman spectroscopy. Fig. 4 shows Raman spectra of a 1-mm-thick Al0.33Ga0.67As0.99N0.01 layer (M) and three Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQWs with varying xN ¼ 0.005 (J), 0.001 (F), and 0.01 (K). Samples J and F contain an unintentional amount of N (xN ¼ 0.0007) in the barriers. Because the plasma source was turned off during barrier growth of sample K, the barriers are free of N. The N-related local vibration modes at 401, 453, and 498 cm1 in samples M, J, and F, respectively, are due to NAs in (Al,Ga)As. Comparing the Raman line intensities of the bulk sample with the MQW samples, we conclude that the N concentration at the barrier-well interface is increased compared to the one in the (Al,Ga)As barrier due to a preferential bonding of N to Al [12]. The N-related Raman signal is strongly enhanced by resonant excitation, which allows in particular the detection of very small N concentrations. Fig. 5 shows the IR-transmittance spectra for different MQW structures. The MQW without N shows a strong intersubband absorption (ISA) signal around 1300 cm1. A large asymmetric
15 x K
400
450
500
550
Raman Shift (cm–1) Fig. 4. Raman spectra of a 1 mm thick Al0.33Ga0.67As0.99N0.01 layer (M) and three Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQWs with varying xN ¼ 0.005 (J), 0.001 (F), and 0.01 (K). Samples J and F contain an unintentional amount of N (xN ¼ 0.0007) in the barriers, sample K has N-free barriers.
broadening and decrease of the absorption peak is observed with increasing xN. Even the MQW with the lowest N concentration (sample F) already displays a decreased ISA signal in comparison to the sample without N. This decrease seems to be related to the measured reduction of the carrier mobility and may question the suitability of (Ga,In)(As,N)/Al0.33Ga0.67As MQWs for optoelectronic devices based on intersubband transitions. Furthermore, the dependence of the ISA wavelength on xN is weaker than expected from the level spacing due to the band-gap reduction. Further studies are necessary to understand the influence of the N-related electronic level on the intersubband levels of a quantum well with a low In content.
ARTICLE IN PRESS R. Hey et al. / Journal of Crystal Growth 278 (2005) 219–223
the (In,Ga)(As,N) wells. Higher mobilities are only obtained in strained MQWs with higher In mole fraction. From Raman spectroscopy, we conclude that N is preferentially incorporated at the interface with the (Al,Ga)As barrier. ISA experiments reveal only a small wavelength shift of the signal with increasing N content, but a strong reduction in intensity and broadening of the absorption peak.
1.5
K
Transmittance
223
1.0 I
Acknowledgements The authors gratefully acknowledge sample growth by M. Ho¨ricke, Hall-bar preparation by A. Riedel, and the support in the magnetotransport studies by K.-J. Friedland.
0.5
References
F
E 0.0 1000
2000
3000 –1
Wavenumber (cm ) Fig. 5. Transmittance (p-polarized intensity normalized to spolarized intensity) of doped Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQWs with varying xN ¼ 0.0 (E), 0.001 (F), 0.005 (I), and 0.01 (K). Sample I and F contain an unintentional amount of N (xN ¼ 0.0007) in the barriers. The plasma source was turned off during barrier growth of sample K.
4. Summary The carrier mobility in highly doped, weakly strained (In,Ga)(As,N)/(Al,Ga)As MQWs decreases with increasing N content in the wells in the dilute nitrogen concentration regime and Al content in the barriers by about two orders of magnitude to a level of about 120–200 cm2/Vs at RT. In addition, we observed carrier localization attributed to N-induced potential fluctuations in
[1] I.A. Buyanova, W.N. Chen, B. Monemar, MRS Int. J. Nitride Semicond. Res. 6 (2001) 2. [2] M. Weyers, M. Sato, H. Ando, Jpn. J. Appl. Phys. Part 1 31 (1992) L853. [3] M. Kondow, K. Uomi, A. Niva, T. Kitatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. Part 1 35 (1996) 1273. [4] P. Krispin, S.G. Spruytte, J.S. Harris, K.H. Ploog, J. Appl. Phys. 90 (2001) 2405. [5] W. Shan, W. Walukiewicz, J.W. Ager, E.E. Haller, J.F. Geisz, D.J. Friedman, J.M. Olsen, S.R. Kurtz, Phys. Rev. Lett. 82 (1999) 1221. [6] J.-Y. Duboz, J.A. Gupta, M. Byloss, G.C. Aers, H.C. Liu, Z.R. Wasilewski, Appl. Phys. Lett. 81 (2002) 1836. [7] E. Luna, M. Hopkinson, J.M. Ulloa, A. Guzman, E. Munoz, Appl. Phys. Lett. 83 (2003) 3111. [8] J.F. Geisz, D.J. Friedman, Semicond. Sci. Technol. 17 (2002) 769. [9] S.R. Kurtz, N.A. Modine, E.D. Jones, A.A. Allerman, J.F. Klem, Semicond. Sci. Technol. 17 (2002) 843. [10] S.R. Kurtz, A.A. Allerman, E.D. Jones, J.M. Gee, J.J. Bans, B.E. Hammons, Appl. Phys. Lett. 74 (1999) 729. [11] J. Teubert, P.J. Klar, W. Heimbrodt, K. Volz, W. Stolz, P. Thomas, G. Leibiger, V. Gottschalch, Appl. Phys. Lett. 84 (2004) 747. [12] J. Wagner, T. Geppert, K. Ko¨hler, P. Ganser, M. Meier, Appl. Phys. Lett. 83 (2003) 2799.
Journal of Crystal Growth 278 (2005) 219–223 www.elsevier.com/locate/jcrysgro
Electrical and optical properties of (Ga,In)(As,N)/(Al,Ga)As multi-quantum-well structures R. Hey, Y.-J. Han, M. Giehler, M. Ramsteiner, H.T. Grahn, K.H. Ploog Paul-Drude-Institut fu¨r Festko¨rperelektronik, Hausvogteiplatz 5-7, Berlin 10117, Germany Available online 1 February 2005
Abstract The electrical properties in highly doped, weakly strained (In,Ga)(As,N)/(Al,Ga)As MQWs are studied by Hall measurements at various temperatures. With increasing N content in the well and Al content in the barrier, the electron mobility is strongly reduced. Magnetoresistance measurements are performed at low temperatures revealing carrier localization induced by the incorporation of N. From Raman spectroscopy, a preferential incorporation of N at the interface with the (Al,Ga)As barrier is found. Intersubband absorption experiments show only a small shift of the signal with increasing N content, but a strongly reduced and broadened absorption peak. r 2005 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 73.63.Hn; 78.66.Fd; 78.30.Fs Keywords: A1. Low-dimensional structures; A3. Molecular beam epitaxy; B2. Semiconducting III–V materials
1. Introduction It is well established that the incorporation of small amounts of nitrogen into III–V compound semiconductor structures remarkably modifies their electronic properties (see Ref. [1] for a more comprehensive review). Therefore, dilute nitride III–V semiconductor alloys are extensively investigated, in particular due to their strong nonlinear band-gap reduction [2], which facilitates their Corresponding author. Tel.: +49 30 20377354;
fax: +49 30 20377201. E-mail address: [email protected] (R. Hey).
application as interband light emitters for telecommunication in the 1.3 and 1.55 mm spectral range [3]. The incorporation of small amounts of nitrogen into heterojunctions predominantly affects the discontinuity of the conduction band DEC, while leaving the discontinuity of the valence band mostly unaffected [4]. A larger DEC in GaAs-based materials systems such as Ga(As,N) and (Ga,In)(As,N) may allow to diversify the performance of intersubband-based devices. However, the nitrogen level is resonant with the conduction band leading to an anticrossing [5]. The impact of the resonant coupling between these levels on the intersubband properties remains still unclear. Recently, first results on investigations
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.12.058
ARTICLE IN PRESS 220
R. Hey et al. / Journal of Crystal Growth 278 (2005) 219–223
of intersubband transitions are reported for (In,Ga)(As,N)/GaAs multi-quantum wells (MQW) with a high In content, showing that the intersubband energies scale with the N concentration [6], and double-quantum-well infrared detectors in the Ga(As,N)/(Al,Ga)As materials system [7]. However, the structural, optical, and electrical properties deteriorate with increasing N incorporation. We report on the electrical and optical properties of dilute nitrogen (Ga,In)(As,N)/(Al,Ga)As MQW structures.
2. Experimental techniques The samples are grown by molecular-beam epitaxy on GaAs(0 0 1) substrates at 450 1C at growth rates of 0.7 mm/h for the GaAs component and with a III/V-BEP-ratio of 30 using an ADDONs RF plasma-assisted N-source powered at 100 W for a N flux in the range of about 0.005 sscm, which is the lower limit for stable operation. The MQW structures consist of 25 or 50 periods of 6-nm-thick wells and 10-nm-thick barriers. The MQW structures are doped n-type by Si to a level of about 2 1012 cm2 per well. The N and In composition in the wells are varied up to 1% and 5%, respectively, for weakly strained MQWs. The Al mole fraction in the barriers is 0 or 0.33. For selected samples, rapid thermal annealing (RTA) has been performed at 700 through 900 1C for 60 s. Photoluminescence (PL) measurements are carried out at 5 K. Hall-bar structures are fabricated for measuring the transport properties at various temperatures. Raman investigations are performed on MQWs with different Al content in the barriers as well as on bulk layers with different mole fractions of Al and N. Intersubband absorption (ISA) spectra are recorded using polarized light by putting the samples in a waveguide geometry with 451 facets and 7 internal reflections.
3. Results and discussion From X-ray diffraction and secondary-ionmass-spectrometry measurements, we obtained a
ratio of 10:1 (N-shutter opened and closed) for the N incorporation leading to an unintentional N concentration in the barrier of about 0.1%. N concentrations below 0.6% are realized by periodically opening and closing the shutter of the N source. The incorporation of N into the group-V sublattice causes the formation of electrically active defects, which decrease the dopant activity and transport properties such as carrier lifetime [8], mobility [9], and diffusion length [10]. In general, we observed that a RTA treatment up to 800 1C of samples with different barrier compositions improves their mobility, e.g. from 630 to 1000 cm2/Vs (at slightly increasing carrier concentrations). For RTA temperatures above 800 1C, we found a strong decrease of the carrier concentration. We assume that this temperature dependence is due to an impurity-induced disorder process of the highly doped material. The improvement of the conductivity is better for GaAs barriers and decreases with increasing Al mole fraction in the barriers. In the following, we concentrate on the investigation of the lateral transport of as-grown samples. The composition, the mobility, and the carrier concentration of the samples are listed in Table 1. Fig. 1 displays the carrier mobilities versus temperature for MQWs with different mole fractions of In, N, and Al. As a reference, the mobility of an (In0.03Ga0.97)As/(Al0.33Ga0.67)As MQW amounts to 20000 cm2/Vs at low temperatures and decreases for higher temperatures due to optical-phonon scattering. For an (In0.03Ga0.97)(As0.99N0.01)/(Al0.33Ga0.67)As MQW, a drastic reduction of the mobility is measured with a main value of about 170 cm2/Vs at room temperature (RT). Even for the MQW with unintentional N incorporation in the barriers and wells—grown with the plasma source turned on and the shutter closed—the mobility already drops to 430 cm2/Vs at RT (not shown). Similar mobility reductions are observed for Al-free barriers (In0.03Ga0.97)(As0.99N0.01)/GaAs as well as strained (In0.1Ga0.9)(As0.99N0.01)/GaAs and (In0.15Ga0.85)(As0.99N0.01)/GaAs MQWs, resulting in mobilities of 350, 750, and 1050 cm2/Vs at RT, respectively. The results clearly show that in
ARTICLE IN PRESS R. Hey et al. / Journal of Crystal Growth 278 (2005) 219–223 Table 1 xN and xIn denote the mole fractions of N and In in the wells, xAl the mole fraction of Al in the barrier, N the number of wells, D the type of doping (b: barrier, w: well), n the electron concentration in the well, and m the carrier mobility at RT for as-grown MQWs (A–L) and a bulk layer (M) #
N
Well
D
n
m
1012cm2
cm2/Vs
xN
xIn
xAl
A B C
0.008 0.006 0.006
0.01 0.10 0.15
0 0 0
25 12 5
b b b
1.1 2.3 3
350 750 1050
D E
0 0
0.03 0.05
0.33 0.33
25 50
b w
1.6 2.5
6200 1870
F G H I J K L
0.001 0.012 0.008 0.005 0.005 0.01 0.01
0.05 0.03 0.05 0.05 0.05 0.05 0.05
0.33 0.33 0.33 0.33 0.33 0.33 0.33
50 25 50 50 50 25 25
w b w w w b b
2.2 0.7 1.6 1.1 1.6
430 170 130 170 120
1
280
M
0.01
0
0.33
2
Mobility (cm /Vs)
10000
D
221
dominant process, samples J and K have been grown, where the N source was turned on and off during the growth of the well leaving a 1-nm-thick (In,Ga)As spacer layer adjacent to the (Al,Ga)As barrier. As shown below by Raman experiments, the barriers are completely free of N. However, no improvement of the mobility is detected. Therefore, we conclude that the mobilities are inherently limited by the dilute (In,Ga)(As,N) well. For larger In mole fractions, higher mobilities can be obtained, but at the expense of a larger lattice strain. Some of the samples even show localized carrier transport behaviour. In this case, the resistance exponentially increases with decreasing temperature. In Fig. 2, we show the negative magnetoresistance for two samples A and E, indicating strong carrier localization. This is caused by the potential fluctuations, which are created by N-related defects as discussed in Ref. [11]. The optical characterization is performed by PL, Raman, and infrared (IR) intersubband absorption spectroscopy. Fig. 3 shows representative low-temperature PL spectra of as-grown samples with 0.1, 0.5, and 0.8% N in doped (In0.05Ga0.95)(As,N)/(Al0.33Ga0.67)As MQWs. No N-related emission is detected for the MQW
C
1000
B
300
A G
100
100
Temperature (K) Fig. 1. Mobility versus temperature of as-grown N-free Ga0.95In0.05As/Al0.33Ga0.67As MQW (D), GaxIn1x(As,N)/ GaAs MQWs with different In mole fractions (A, B, C), and a Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQW (G).
RS (kΩ)
E 10
200
1.0
A
(Ga,In)(As,N)/(Al,Ga)As MQWs the mobility is strongly reduced, resulting in a low electron mean free path, which is comparable to the average separation of a few nanometres of N atoms in the well. In order to suppress carrier scattering at the (In,Ga)(As,N)/(Al,Ga)As interfaces as the
0.8
0
2 B (T)
4
Fig. 2. Sheet resistance versus magnetic field of an as-grown Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQW (E) and a Ga0.95 In0.05(As,N)/GaAs MQW (A) for a temperature of 1.4 K.
ARTICLE IN PRESS R. Hey et al. / Journal of Crystal Growth 278 (2005) 219–223
222 40000 J
30000
2x M
20000
10000 F 0 800
850
900
950
1000
Wavelength (nm) Fig. 3. Low-temperature photoluminescence spectra of asgrown, n-type Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQWs with different N mole fraction in the wells excited with 1.8 mW at 532 nm for a temperature of 5 K.
Intensity (arb. units)
PL Intensity (arb. units)
H
J
F
containing 0.1% N in the barriers and the wells. The PL emission wavelength increases with increasing N concentration. Different vibrational modes of N on an As-site (NAs) in (Al,Ga)As of as-grown (In,Ga)(As,N)/ (Al,Ga)As MQWs are clearly detected by Raman spectroscopy. Fig. 4 shows Raman spectra of a 1-mm-thick Al0.33Ga0.67As0.99N0.01 layer (M) and three Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQWs with varying xN ¼ 0.005 (J), 0.001 (F), and 0.01 (K). Samples J and F contain an unintentional amount of N (xN ¼ 0.0007) in the barriers. Because the plasma source was turned off during barrier growth of sample K, the barriers are free of N. The N-related local vibration modes at 401, 453, and 498 cm1 in samples M, J, and F, respectively, are due to NAs in (Al,Ga)As. Comparing the Raman line intensities of the bulk sample with the MQW samples, we conclude that the N concentration at the barrier-well interface is increased compared to the one in the (Al,Ga)As barrier due to a preferential bonding of N to Al [12]. The N-related Raman signal is strongly enhanced by resonant excitation, which allows in particular the detection of very small N concentrations. Fig. 5 shows the IR-transmittance spectra for different MQW structures. The MQW without N shows a strong intersubband absorption (ISA) signal around 1300 cm1. A large asymmetric
15 x K
400
450
500
550
Raman Shift (cm–1) Fig. 4. Raman spectra of a 1 mm thick Al0.33Ga0.67As0.99N0.01 layer (M) and three Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQWs with varying xN ¼ 0.005 (J), 0.001 (F), and 0.01 (K). Samples J and F contain an unintentional amount of N (xN ¼ 0.0007) in the barriers, sample K has N-free barriers.
broadening and decrease of the absorption peak is observed with increasing xN. Even the MQW with the lowest N concentration (sample F) already displays a decreased ISA signal in comparison to the sample without N. This decrease seems to be related to the measured reduction of the carrier mobility and may question the suitability of (Ga,In)(As,N)/Al0.33Ga0.67As MQWs for optoelectronic devices based on intersubband transitions. Furthermore, the dependence of the ISA wavelength on xN is weaker than expected from the level spacing due to the band-gap reduction. Further studies are necessary to understand the influence of the N-related electronic level on the intersubband levels of a quantum well with a low In content.
ARTICLE IN PRESS R. Hey et al. / Journal of Crystal Growth 278 (2005) 219–223
the (In,Ga)(As,N) wells. Higher mobilities are only obtained in strained MQWs with higher In mole fraction. From Raman spectroscopy, we conclude that N is preferentially incorporated at the interface with the (Al,Ga)As barrier. ISA experiments reveal only a small wavelength shift of the signal with increasing N content, but a strong reduction in intensity and broadening of the absorption peak.
1.5
K
Transmittance
223
1.0 I
Acknowledgements The authors gratefully acknowledge sample growth by M. Ho¨ricke, Hall-bar preparation by A. Riedel, and the support in the magnetotransport studies by K.-J. Friedland.
0.5
References
F
E 0.0 1000
2000
3000 –1
Wavenumber (cm ) Fig. 5. Transmittance (p-polarized intensity normalized to spolarized intensity) of doped Ga0.95In0.05(As,N)/Al0.33Ga0.67As MQWs with varying xN ¼ 0.0 (E), 0.001 (F), 0.005 (I), and 0.01 (K). Sample I and F contain an unintentional amount of N (xN ¼ 0.0007) in the barriers. The plasma source was turned off during barrier growth of sample K.
4. Summary The carrier mobility in highly doped, weakly strained (In,Ga)(As,N)/(Al,Ga)As MQWs decreases with increasing N content in the wells in the dilute nitrogen concentration regime and Al content in the barriers by about two orders of magnitude to a level of about 120–200 cm2/Vs at RT. In addition, we observed carrier localization attributed to N-induced potential fluctuations in
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