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(1 1 1)Si heterostructure
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Physica E 21 (2004) 787 – 792 www.elsevier.com/locate/physe
Photocurrent spectroscopy of a (0 0 0 1)GaN/AlGaN/(1 1 1)Si heterostructure Y. Kuroiwa, Y. Honda, N. Sawaki∗ Department of Electronics, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
Abstract A crack-free GaN/AlGaN sample was grown on (1 1 1) Si by selective area metal-organic vapor-phase epitaxy method using an AlGaN intermediate layer. The electrical properties of the GaN/AlGaN/Si heterojunction diode were investigated with photocurrent spectroscopy at 77 K. It was found that the current–voltage characteristics depend on the thickness/composition of the intermediate layer. The photocurrent spectra indicated that the energy band of the n-type-doped Si substrate is lifted at the heterointerface, while it is 2at in case of the sample grown on a p-type-doped Si. The depletion of the energy band in the n-Si at the heterointerface is attributed to the di3usion of Al during the growth. ? 2003 Elsevier B.V. All rights reserved. PACS: 73.40.Kp; 68.35.Fx; 81.15.Gh Keywords: GaN; Heterostructure; Photocurrent spectroscopy; MOVPE
1. Introduction The growth of GaN on a silicon substrate has been attempted by many authors [1–3]. Most of the trials are to replace the sapphire substrate because of the limited area and high cost. By virtue of the chemical and physical hardness of the III-nitride, the growth of highly quali@ed GaN on an Si substrate will open another possibility in the power electronics. Since the direct growth of GaN on an Si substrate is not so easy, a thin AlGaN intermediate layer has been adopted in the growth processes. Because of the di3erent energy band gaps of AlGaN, GaN and Si, the electron transport across the heterointerface will be subject to the energy band diagram of the heterojunction, which has, ∗ Corresponding author. Tel.:+81-52-789-3321; fax:+81-52789-3157. E-mail address: [email protected] (N. Sawaki).
however, not been known in detail till now. In this article, we will report on the electronic and electrical properties of the GaN/AlGaN/Si heterojunction for the @rst time. We will study the potential pro@le of the GaN/AlGaN/Si heterostructure with photocurrent (PC) spectroscopy. 2. Experimental methods Usually, the GaN layer on an Si substrate grown by metal-organic vapor-phase epitaxy (MOVPE) has cracks because of the large di3erence of the lattice constants and the thermal expansion coeGcients [1–3]. In order to get reliable data on the electrical characteristics, we made a crack-free GaN by limiting the size of the grown layer with selective area growth (SAG) method [4]. On an n-type-doped (1 1 1) Si substrate, we deposited an SiO2 @lm and made square openings
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.125
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Y. Kuroiwa et al. / Physica E 21 (2004) 787 – 792
2
1 b
Current (mA)
a
Fig. 1. SEM top image (a) and cross-sectional REM image (b) of a sample.
(windows) separated by a stripe mask of 10 m wide. The window area was 300 m × 300 m. The growth of GaN was performed only on the window using a 50 nm-Alx Ga1−x N (0 ¡ x 6 1) intermediate layer. The thickness of the GaN layer was typically 0:8 m. The grown layer was nominally non-doped but showed n-type conduction (carrier density 6:1 × 1017 cm−3 at 300 K). The surface of the sample was smooth and mirror 2at except weak-ridge growth at the boundary of the sample. A thin Al/Ti @lm (200 m × 200 m) was deposited on the surface of the GaN followed by heat treatment to get an ohmic contact. The back contact was taken on the back surface of the Si substrate by evaporating Au/Sb followed by heat treatment. For the sake of comparison, similar samples were made on a p-type-doped Si substrate. Typical picture of the sample is shown in Fig. 1. The top image in Fig. 1(a) shows that we have no cracking in the sample. The re2ection electron microscope (REM) image shown in Fig. 1(b) indicates that the intermediate layer is not uniform but shows irregular structure. The current–voltage (I –V ) characteristics and the PC spectra were measured at 77 K for samples grown on various thicknesses and Al composition of the intermediate layer. 3. I –V characteristics Fig. 2 shows typical I –V characteristics obtained for GaN/AlN/n-Si diodes made with di3erent AlN intermediate layer thickness. Though the samples were of n-GaN/n-Si diode, they exhibited asymmetric non-linear behavior typical to a heterojunction diode. The current should be determined by the transport of
c
d
d
0 c b -1
a
-2 -6
-4
-2
0 2 Voltage (V)
4
6
Fig. 2. I –V characteristic of GaN/AlN/n-Si diodes. Nominal thickness of the AlN intermediate layer is (a) 10, (b) 20, (c) 30, or (d) 40 nm.
electrons in any case. The positive bias was applied to the n-Si. We found that, if the AlN layer is thicker than 30 nm, we have no injection of electrons into the GaN from the n-Si. If it is thinner than 20 nm, we have enough injection of electrons and the onset voltage is of the order of 0:5 V. In the positive bias region, we get injection of electrons from the n-GaN into the Si, irrespective of the nominal thickness of the AlN intermediate layer. The re2ection electron microscope (REM) images as shown in Fig. 1(b) indicated the irregular structure of the AlN intermediate layers suggesting the presence of small area with very thin AlN. The electron might be injected through such a portion via tunneling process. The sharp switch of the characteristics in Fig. 2 is attributed to the formation of thick barrier layer by a long-AlN growth time. In order to con@rm the assumption, we tested the sample grown on various Alx Ga1−x N intermediate layers. In these cases, because of the microscopic non-uniformity of the composition, we might have a part made of GaN at the heterointerface. Typical results are displayed in Fig. 3. Even if the thickness of the intermediate layer was as thick as 40 nm, we achieved clear increase of the current in the negative bias region (electron injection from the n-Si into the GaN). We might conclude that the potential barrier for electrons at the
Y. Kuroiwa et al. / Physica E 21 (2004) 787 – 792
2
2
1.5
a 1
b
c
a
0
Current (mA)
Current (mA)
789
a
b
1
0.5 b
b
-1
0 a c
-0.5 -10
-2 -8
-6
-4
-2 0 2 Voltage (V)
4
6
8
Fig. 3. The I –V characteristics of GaN/AlGaN/n-Si diodes. The intermediate layer is made of (a) 40 nm-AlN, (b) 5 nm-AlN and 35 nm-Al0.17Ga0.83N, or (c) 40 nm-Al0.17Ga0.83N.
heterointerface is lowered substantially by the inclusion of Ga in the intermediate layer. The sharp rise of the current at V = −0:5 V shown in Fig. 2 indicates the presence of built-in potential on the surface of the n-Si and/or it might be attributed to the conduction band o3set at the GaN/AlGaN/Si interface. Since the value −0:5 V was also obtained in a sample with low-Al composition in the intermediate layer, the band o3set at the AlGaN/Si heterointerface will be ruled out in the phenomenon. Apparently, it should be determined by the GaN/n-Si junction. In Figs. 2 and 3, the gradual increase of the injection current at the positive biases depended on the structure/thickness of the intermediate layer. The behavior is rather complicated and could not be explained by a simple model with multi-layer structure. This indicates the presence of a built-in @eld on the surface of the AlGaN as well as the GaN near the heterointerface. For the sake of comparison, similar structure was made on a p-type-doped Si substrate. Fig. 4 shows typical results, where we deposited a 40 nm-Alx Ga1−x N as the intermediate layer. In the negative bias region, we did not get high-current density in contrast to the cases in samples made on n-Si, irrespective of the alloy composition of the intermediate layer. In the positive bias region, on the other hand, we got high current that was almost independent of the conduction type
-5
0 Voltage (V)
5
10
Fig. 4. The I –V characteristics of GaN=Alx Ga1−x N=p-Si diodes. The Al composition of the intermediate layer is (a) x = 0:25, (b) x = 0:5.
of the Si substrate. That is to say, the current in the positive bias region is determined absolutely by the injection of electrons from the nominally non-doped n-GaN into the Si substrate. That is, the hole injection from the p-Si is not the case. As described earlier, the current in the positive biases depending on the composition of the AlGaN layer should re2ect the built-in potential in the GaN and/or the potential barrier at the AlGaN/GaN heterointerface. We tested two samples grown on n-Si and p-Si using a 40 nm thick Al0:5 Ga0:5 N alloy as the intermediate layer. We found that the threshold voltage in a sample made on a p-type-doped Si was 2:5 V lower than that made on a n-type-doped Si. The di3erence is attributed to the di3erent formation of the depletion layers at the heterojunction. In case of the sample on an n-Si, we have depletion layers in Si and GaN. The bias voltage should be applied on both the layers. In case of the sample on a p-Si we might rule out the depletion layer for electrons in the Si surface. If this is the case, we will have a low-threshold voltage in agreement with the observation. 4. PC spectra In order to determine the built-in @eld (depletion layer) near the heterointerface, the PC spectra were
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Y. Kuroiwa et al. / Physica E 21 (2004) 787 – 792
1.5
6
1 +0.4V
Photocurrent (µA)
Photocurrent (µA)
4 2 0 0 -2
-0.4
(a)
0.5 0 0 -2.8 -0.5 -1
-4 -6 300
+2.8V
350
400 450 Wavelenrth (nm)
-1.5 700
500 (b)
800 900 1000 Wavelength (nm)
1100
Fig. 5. The PC spectra of a GaN/Al0.17Ga0.83N/n-Si diode at di3erent bias voltages: (a) 300 ¡ ¡ 500 nm, (b) 700 ¡ ¡ 1150 nm. The numbers in the @gure are the applied DC bias voltages.
investigated at 77 K. The light of a tungsten lump was monochromated and illuminated on the sample in liquid N2 . The PC was processed by a lock-in detection system under various bias voltages. The PC increased linearly as the increase of the bias voltage followed by saturation at high biases. Fig. 5 shows the typical spectra obtained in the linear region for a GaN/AlGaN/n-Si diode. The sign of the current coincides with those shown in the previous @gures. The sign of the PC for = 400–850 nm was positive irrespective of the sign of the bias, showing the bend-up of the potential pro@le or the strong depletion of the n-Si near the heterointerface. As the origin of the strong depletion, we should remind the introduction of defects/impurities at/near the heterointerface during the growth processes. It is partly attributed to the di3usion of Al or Ga into the Si, which should behave as acceptors to compensate the n-type conduction in the Si. The negative component appearing at = 360–400 nm (sub-threshold wavelength region of the GaN band gap), on the other hand, suggests the bend-up of the potential in the GaN near the heterointerface. This is attributed to the introduction of surface states at the heterointerface and/or to the piezo-electric @eld in the GaN/AlGaN heterostructure (the contribution of the AlGaN layer itself to the PC will be ruled out because of the wide energy band gap). Since the sign of this component obeys that of
the applied bias voltage, the bend-up (depletion) is not so strong. The PC at ¡ 355 nm should re2ect the potential pro@le in the GaN epitaxial layer or the top surface of the sample. The positive sign of the PC in this short wavelength region suggests the presence of a depletion layer at the top surface of the GaN, which is attributed to the defects or the presence of Shottkey junction due to the ‘ohmic’ contact. Since the sign of the PC obeyed that of the applied bias voltage in this region, we may conclude that the depletion was not so strong at the top surface. The PC at long wavelength region shown in Fig. 5(b) is somewhat complicated. In this region the signal-to-noise ratio was not good, but we recognize the increase in the absorption at 1100 nm due to the energy band gap of Si. In Fig. 5, we found a broad peak at = 1000 nm. Similar peak was found in all the samples measured, which is attributed to the formation of an amorphous-like layer near the AlGaN/Si interface in accordance with the TEM observation [5]. The PC in a sample made on a p-type-doped Si was measured for comparison use. Fig. 6 shows the typical result. Under zero bias condition, we have hardly observed the PC. This suggests that we have no depletion layer in the Si. At wavelengths = 400–850 nm we achieved ‘negative’ PC by negative bias voltages. This is very contrast to the results shown for samples on n-Si. Thus, we may conclude that we have a 2at
Y. Kuroiwa et al. / Physica E 21 (2004) 787 – 792
791
0.1
+6.0V
0.05
Photocurrent (µA)
Photocurrent (µA)
0.04
0 0
-0.05
-6.0
0.02 3.0 and +0V 0 -0.02
-6.0
-0.04 -0.1
360 380 400 420 440 460 480 Wavelength (nm)
(a)
800 (b)
850
900
950
1000
Wavelength (nm)
Fig. 6. The PC spectra of a GaN/Al0.5Ga0.5N/p-Si diode at di3erent bias voltages: (a) 300 ¡ ¡ 500 nm, (b) 700 ¡ ¡ 1150 nm.
Concentration (au)
1E21 Si
1E20 1E19
Ga
1E18 All 1E17
Si
All
1E16 0.0
0.2
0.4
0.6 0.8 1.0 Depth (µm)
1.2
1.4
1.6
at 1000◦ C is only 15 min. But it is long enough for the Al to get remarkable di3usion. As a result, the Al diffused into Si would compensate the n-type doping of the substrate. This will in turn make the depletion of the Si there and enhance the bent-up of the potential. The di3usion of Al would not in2uence the conductivity type of p-Si. Actually, we have found no depletion in case of a diode made on p-Si. Thus the di3usion of Al should dominate the potential variation at the heterointerface. The introduction of defects at the heterointerface has less importance at the moment.
Fig. 7. SIMS pro@le of a GaN/AlN/Si sample.
potential pro@le in the Si near the heterointerface. The absence of the depletion layer in the p-type Si is in agreement with the small threshold voltage in the I –V characteristics as described earlier. 5. Secondary ion mass spectroscopy (SIMS) analyses The SIMS was performed to detect the distribution of the Al, Ga and Si in the GaN/AlN/Si sample and the results are displayed in Fig. 7. The peak of Al concentration observed at 0:8 m represents the AlN intermediate layer. In Fig. 7, we @nd that the distribution of the Ga in the Si layer shows sharp drop while that of Al exhibits a long tail. This shows the occurrence of the di3usion of Al into the Si during the growth process. The total process time for the growth
6. Summary The energy band diagrams of GaN/AlGaN/n-Si and GaN/AlGaN/p-Si heterojunction diodes have been investigated by measuring the I –V characteristics and the PC spectra. The characteristics were very sensitive to the structure/composition of the AlGaN intermediate layer. It was found that there is strong depletion in the n-Si at the heterointerface, while no depletion was found out in the p-Si. The depletion in the n-Si is attributed to the di3usion of Al during the growth process. Acknowledgements This work is supported partly by the Grant-in-Aid from the Japan Society for Promotion of Science
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(JSPS). The SIMS measurements were due to Dr. X. Chen and Dr. M. Ishiko of Toyota Central Research Laboratory. References [1] T. Takeuchi, H. Amano, K. Hiramatsu, N. Sawaki, I. Akasaki, J. Crystal Growth 115 (1991) 634.
[2] A. Strittmater, A. Krost, J. Blasing, D. Bimberg, Phys. Status Sol. A 176 (1999) 611. [3] N.P. Kobayashi, J.T. Kobayashi, P.D. Dapkus, W.J. Choi, A.E. Bond, X. Zhang, D.H. Rich, Appl. Phys. Lett. 71 (1997) 3569. [4] Y. Honda, Y. Kuroiwa, M. Yamaguchi, N. Sawaki, Appl. Phys. Lett. 80 (2002) 222. [5] S. Tanaka, Y. Kawaguchi, N. Sawaki, M. Hibino, K. Hiramatsu, Appl. Phys. Lett. 76 (2000) 2701.
Physica E 21 (2004) 787 – 792 www.elsevier.com/locate/physe
Photocurrent spectroscopy of a (0 0 0 1)GaN/AlGaN/(1 1 1)Si heterostructure Y. Kuroiwa, Y. Honda, N. Sawaki∗ Department of Electronics, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
Abstract A crack-free GaN/AlGaN sample was grown on (1 1 1) Si by selective area metal-organic vapor-phase epitaxy method using an AlGaN intermediate layer. The electrical properties of the GaN/AlGaN/Si heterojunction diode were investigated with photocurrent spectroscopy at 77 K. It was found that the current–voltage characteristics depend on the thickness/composition of the intermediate layer. The photocurrent spectra indicated that the energy band of the n-type-doped Si substrate is lifted at the heterointerface, while it is 2at in case of the sample grown on a p-type-doped Si. The depletion of the energy band in the n-Si at the heterointerface is attributed to the di3usion of Al during the growth. ? 2003 Elsevier B.V. All rights reserved. PACS: 73.40.Kp; 68.35.Fx; 81.15.Gh Keywords: GaN; Heterostructure; Photocurrent spectroscopy; MOVPE
1. Introduction The growth of GaN on a silicon substrate has been attempted by many authors [1–3]. Most of the trials are to replace the sapphire substrate because of the limited area and high cost. By virtue of the chemical and physical hardness of the III-nitride, the growth of highly quali@ed GaN on an Si substrate will open another possibility in the power electronics. Since the direct growth of GaN on an Si substrate is not so easy, a thin AlGaN intermediate layer has been adopted in the growth processes. Because of the di3erent energy band gaps of AlGaN, GaN and Si, the electron transport across the heterointerface will be subject to the energy band diagram of the heterojunction, which has, ∗ Corresponding author. Tel.:+81-52-789-3321; fax:+81-52789-3157. E-mail address: [email protected] (N. Sawaki).
however, not been known in detail till now. In this article, we will report on the electronic and electrical properties of the GaN/AlGaN/Si heterojunction for the @rst time. We will study the potential pro@le of the GaN/AlGaN/Si heterostructure with photocurrent (PC) spectroscopy. 2. Experimental methods Usually, the GaN layer on an Si substrate grown by metal-organic vapor-phase epitaxy (MOVPE) has cracks because of the large di3erence of the lattice constants and the thermal expansion coeGcients [1–3]. In order to get reliable data on the electrical characteristics, we made a crack-free GaN by limiting the size of the grown layer with selective area growth (SAG) method [4]. On an n-type-doped (1 1 1) Si substrate, we deposited an SiO2 @lm and made square openings
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.125
788
Y. Kuroiwa et al. / Physica E 21 (2004) 787 – 792
2
1 b
Current (mA)
a
Fig. 1. SEM top image (a) and cross-sectional REM image (b) of a sample.
(windows) separated by a stripe mask of 10 m wide. The window area was 300 m × 300 m. The growth of GaN was performed only on the window using a 50 nm-Alx Ga1−x N (0 ¡ x 6 1) intermediate layer. The thickness of the GaN layer was typically 0:8 m. The grown layer was nominally non-doped but showed n-type conduction (carrier density 6:1 × 1017 cm−3 at 300 K). The surface of the sample was smooth and mirror 2at except weak-ridge growth at the boundary of the sample. A thin Al/Ti @lm (200 m × 200 m) was deposited on the surface of the GaN followed by heat treatment to get an ohmic contact. The back contact was taken on the back surface of the Si substrate by evaporating Au/Sb followed by heat treatment. For the sake of comparison, similar samples were made on a p-type-doped Si substrate. Typical picture of the sample is shown in Fig. 1. The top image in Fig. 1(a) shows that we have no cracking in the sample. The re2ection electron microscope (REM) image shown in Fig. 1(b) indicates that the intermediate layer is not uniform but shows irregular structure. The current–voltage (I –V ) characteristics and the PC spectra were measured at 77 K for samples grown on various thicknesses and Al composition of the intermediate layer. 3. I –V characteristics Fig. 2 shows typical I –V characteristics obtained for GaN/AlN/n-Si diodes made with di3erent AlN intermediate layer thickness. Though the samples were of n-GaN/n-Si diode, they exhibited asymmetric non-linear behavior typical to a heterojunction diode. The current should be determined by the transport of
c
d
d
0 c b -1
a
-2 -6
-4
-2
0 2 Voltage (V)
4
6
Fig. 2. I –V characteristic of GaN/AlN/n-Si diodes. Nominal thickness of the AlN intermediate layer is (a) 10, (b) 20, (c) 30, or (d) 40 nm.
electrons in any case. The positive bias was applied to the n-Si. We found that, if the AlN layer is thicker than 30 nm, we have no injection of electrons into the GaN from the n-Si. If it is thinner than 20 nm, we have enough injection of electrons and the onset voltage is of the order of 0:5 V. In the positive bias region, we get injection of electrons from the n-GaN into the Si, irrespective of the nominal thickness of the AlN intermediate layer. The re2ection electron microscope (REM) images as shown in Fig. 1(b) indicated the irregular structure of the AlN intermediate layers suggesting the presence of small area with very thin AlN. The electron might be injected through such a portion via tunneling process. The sharp switch of the characteristics in Fig. 2 is attributed to the formation of thick barrier layer by a long-AlN growth time. In order to con@rm the assumption, we tested the sample grown on various Alx Ga1−x N intermediate layers. In these cases, because of the microscopic non-uniformity of the composition, we might have a part made of GaN at the heterointerface. Typical results are displayed in Fig. 3. Even if the thickness of the intermediate layer was as thick as 40 nm, we achieved clear increase of the current in the negative bias region (electron injection from the n-Si into the GaN). We might conclude that the potential barrier for electrons at the
Y. Kuroiwa et al. / Physica E 21 (2004) 787 – 792
2
2
1.5
a 1
b
c
a
0
Current (mA)
Current (mA)
789
a
b
1
0.5 b
b
-1
0 a c
-0.5 -10
-2 -8
-6
-4
-2 0 2 Voltage (V)
4
6
8
Fig. 3. The I –V characteristics of GaN/AlGaN/n-Si diodes. The intermediate layer is made of (a) 40 nm-AlN, (b) 5 nm-AlN and 35 nm-Al0.17Ga0.83N, or (c) 40 nm-Al0.17Ga0.83N.
heterointerface is lowered substantially by the inclusion of Ga in the intermediate layer. The sharp rise of the current at V = −0:5 V shown in Fig. 2 indicates the presence of built-in potential on the surface of the n-Si and/or it might be attributed to the conduction band o3set at the GaN/AlGaN/Si interface. Since the value −0:5 V was also obtained in a sample with low-Al composition in the intermediate layer, the band o3set at the AlGaN/Si heterointerface will be ruled out in the phenomenon. Apparently, it should be determined by the GaN/n-Si junction. In Figs. 2 and 3, the gradual increase of the injection current at the positive biases depended on the structure/thickness of the intermediate layer. The behavior is rather complicated and could not be explained by a simple model with multi-layer structure. This indicates the presence of a built-in @eld on the surface of the AlGaN as well as the GaN near the heterointerface. For the sake of comparison, similar structure was made on a p-type-doped Si substrate. Fig. 4 shows typical results, where we deposited a 40 nm-Alx Ga1−x N as the intermediate layer. In the negative bias region, we did not get high-current density in contrast to the cases in samples made on n-Si, irrespective of the alloy composition of the intermediate layer. In the positive bias region, on the other hand, we got high current that was almost independent of the conduction type
-5
0 Voltage (V)
5
10
Fig. 4. The I –V characteristics of GaN=Alx Ga1−x N=p-Si diodes. The Al composition of the intermediate layer is (a) x = 0:25, (b) x = 0:5.
of the Si substrate. That is to say, the current in the positive bias region is determined absolutely by the injection of electrons from the nominally non-doped n-GaN into the Si substrate. That is, the hole injection from the p-Si is not the case. As described earlier, the current in the positive biases depending on the composition of the AlGaN layer should re2ect the built-in potential in the GaN and/or the potential barrier at the AlGaN/GaN heterointerface. We tested two samples grown on n-Si and p-Si using a 40 nm thick Al0:5 Ga0:5 N alloy as the intermediate layer. We found that the threshold voltage in a sample made on a p-type-doped Si was 2:5 V lower than that made on a n-type-doped Si. The di3erence is attributed to the di3erent formation of the depletion layers at the heterojunction. In case of the sample on an n-Si, we have depletion layers in Si and GaN. The bias voltage should be applied on both the layers. In case of the sample on a p-Si we might rule out the depletion layer for electrons in the Si surface. If this is the case, we will have a low-threshold voltage in agreement with the observation. 4. PC spectra In order to determine the built-in @eld (depletion layer) near the heterointerface, the PC spectra were
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Y. Kuroiwa et al. / Physica E 21 (2004) 787 – 792
1.5
6
1 +0.4V
Photocurrent (µA)
Photocurrent (µA)
4 2 0 0 -2
-0.4
(a)
0.5 0 0 -2.8 -0.5 -1
-4 -6 300
+2.8V
350
400 450 Wavelenrth (nm)
-1.5 700
500 (b)
800 900 1000 Wavelength (nm)
1100
Fig. 5. The PC spectra of a GaN/Al0.17Ga0.83N/n-Si diode at di3erent bias voltages: (a) 300 ¡ ¡ 500 nm, (b) 700 ¡ ¡ 1150 nm. The numbers in the @gure are the applied DC bias voltages.
investigated at 77 K. The light of a tungsten lump was monochromated and illuminated on the sample in liquid N2 . The PC was processed by a lock-in detection system under various bias voltages. The PC increased linearly as the increase of the bias voltage followed by saturation at high biases. Fig. 5 shows the typical spectra obtained in the linear region for a GaN/AlGaN/n-Si diode. The sign of the current coincides with those shown in the previous @gures. The sign of the PC for = 400–850 nm was positive irrespective of the sign of the bias, showing the bend-up of the potential pro@le or the strong depletion of the n-Si near the heterointerface. As the origin of the strong depletion, we should remind the introduction of defects/impurities at/near the heterointerface during the growth processes. It is partly attributed to the di3usion of Al or Ga into the Si, which should behave as acceptors to compensate the n-type conduction in the Si. The negative component appearing at = 360–400 nm (sub-threshold wavelength region of the GaN band gap), on the other hand, suggests the bend-up of the potential in the GaN near the heterointerface. This is attributed to the introduction of surface states at the heterointerface and/or to the piezo-electric @eld in the GaN/AlGaN heterostructure (the contribution of the AlGaN layer itself to the PC will be ruled out because of the wide energy band gap). Since the sign of this component obeys that of
the applied bias voltage, the bend-up (depletion) is not so strong. The PC at ¡ 355 nm should re2ect the potential pro@le in the GaN epitaxial layer or the top surface of the sample. The positive sign of the PC in this short wavelength region suggests the presence of a depletion layer at the top surface of the GaN, which is attributed to the defects or the presence of Shottkey junction due to the ‘ohmic’ contact. Since the sign of the PC obeyed that of the applied bias voltage in this region, we may conclude that the depletion was not so strong at the top surface. The PC at long wavelength region shown in Fig. 5(b) is somewhat complicated. In this region the signal-to-noise ratio was not good, but we recognize the increase in the absorption at 1100 nm due to the energy band gap of Si. In Fig. 5, we found a broad peak at = 1000 nm. Similar peak was found in all the samples measured, which is attributed to the formation of an amorphous-like layer near the AlGaN/Si interface in accordance with the TEM observation [5]. The PC in a sample made on a p-type-doped Si was measured for comparison use. Fig. 6 shows the typical result. Under zero bias condition, we have hardly observed the PC. This suggests that we have no depletion layer in the Si. At wavelengths = 400–850 nm we achieved ‘negative’ PC by negative bias voltages. This is very contrast to the results shown for samples on n-Si. Thus, we may conclude that we have a 2at
Y. Kuroiwa et al. / Physica E 21 (2004) 787 – 792
791
0.1
+6.0V
0.05
Photocurrent (µA)
Photocurrent (µA)
0.04
0 0
-0.05
-6.0
0.02 3.0 and +0V 0 -0.02
-6.0
-0.04 -0.1
360 380 400 420 440 460 480 Wavelength (nm)
(a)
800 (b)
850
900
950
1000
Wavelength (nm)
Fig. 6. The PC spectra of a GaN/Al0.5Ga0.5N/p-Si diode at di3erent bias voltages: (a) 300 ¡ ¡ 500 nm, (b) 700 ¡ ¡ 1150 nm.
Concentration (au)
1E21 Si
1E20 1E19
Ga
1E18 All 1E17
Si
All
1E16 0.0
0.2
0.4
0.6 0.8 1.0 Depth (µm)
1.2
1.4
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at 1000◦ C is only 15 min. But it is long enough for the Al to get remarkable di3usion. As a result, the Al diffused into Si would compensate the n-type doping of the substrate. This will in turn make the depletion of the Si there and enhance the bent-up of the potential. The di3usion of Al would not in2uence the conductivity type of p-Si. Actually, we have found no depletion in case of a diode made on p-Si. Thus the di3usion of Al should dominate the potential variation at the heterointerface. The introduction of defects at the heterointerface has less importance at the moment.
Fig. 7. SIMS pro@le of a GaN/AlN/Si sample.
potential pro@le in the Si near the heterointerface. The absence of the depletion layer in the p-type Si is in agreement with the small threshold voltage in the I –V characteristics as described earlier. 5. Secondary ion mass spectroscopy (SIMS) analyses The SIMS was performed to detect the distribution of the Al, Ga and Si in the GaN/AlN/Si sample and the results are displayed in Fig. 7. The peak of Al concentration observed at 0:8 m represents the AlN intermediate layer. In Fig. 7, we @nd that the distribution of the Ga in the Si layer shows sharp drop while that of Al exhibits a long tail. This shows the occurrence of the di3usion of Al into the Si during the growth process. The total process time for the growth
6. Summary The energy band diagrams of GaN/AlGaN/n-Si and GaN/AlGaN/p-Si heterojunction diodes have been investigated by measuring the I –V characteristics and the PC spectra. The characteristics were very sensitive to the structure/composition of the AlGaN intermediate layer. It was found that there is strong depletion in the n-Si at the heterointerface, while no depletion was found out in the p-Si. The depletion in the n-Si is attributed to the di3usion of Al during the growth process. Acknowledgements This work is supported partly by the Grant-in-Aid from the Japan Society for Promotion of Science
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(JSPS). The SIMS measurements were due to Dr. X. Chen and Dr. M. Ishiko of Toyota Central Research Laboratory. References [1] T. Takeuchi, H. Amano, K. Hiramatsu, N. Sawaki, I. Akasaki, J. Crystal Growth 115 (1991) 634.
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