- Email: [email protected]
Characterization of oxygen-related defects in p-Al0.3Ga0.7As by DLTS
Journal of Crystal Growth 210 (2000) 242}246
Characterization of oxygen-related defects in p-Al Ga As by DLTS 0.3 0.7 H. Ishii!,*, T. Shinagawa!, S. Tanaka!, T. Okumura" !Yokohama R&D Laboratories, The Furukawa Electric Co. Ltd., 2-4-3 Okano, Yokohama 220-0073, Japan "Department of Electrical Engineering, Tokyo Metropolitan University, Hachiohji 192-0397, Japan
Abstract The oxygen-related defects in undoped Al Ga As (p-type) were characterized by deep-level transient spectroscopy 0.3 0.7 (DLTS). We prepared pn`-junction diodes by the metal organic chemical vapor deposition (MOCVD) technique in order to study both majority- and minority-carrier emission properties of the defects. Under majority-carrier "lling condition, just one hole-emission peak (E) was observed around 450 K in addition to DX centers in the n`-layer. When the minority-carrier injection pulse was applied, an electron-emission peak (C) appeared around 300 K in accordance with the disappearance of peak E. The defect related to the peak E acts as an e$cient recombination center which dominates the current}voltage characteristics of the diodes. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 71.55.Eq Keywords: AlGaAs; DLTS; MOCVD; Recombination center; Oxygen
1. Introduction GaAs epitaxial wafers with an undoped AlGaAs bu!er layer grown by metal organic chemical vapor deposition (MOCVD) are widely used to fabricate GaAs metal-semiconductor "eld e!ect transistors (MESFETs) [1] in the "eld of microwave power application. It is well known that the AlGaAs grown by MOCVD is likely to be contaminated by oxygen because of high reactivity of aluminum with oxygen, and thus the quality of the epitaxial layer, such as luminescence e$ciency [2], is seriously
* Corresponding author. Tel.: #81-45-311-1219; fax: #8145-314-5190. E-mail address: [email protected] (H. Ishii)
in#uenced by oxygen contamination. On the other hand, oxygen-ion implantation into n-AlGaAs followed by annealing forms a stable layer of high resistivity, which is attributed to the creation of a deep acceptor level compensating for shallow donors [3]. While high resistivity of the AlGaAs bu!er layer is advantageous for the fabrication of MESFETs, there could be some drawbacks due to the deep levels related to oxygen in the crystal, which depreciate the performance of MESFETs, for example, drain-current instability [4]. Recently, we reported that the oxygen-related electron trap originating from AlGaAs bu!er layer is responsible for the drain-current transient in the MESFETs [5]. In addition to the electron trap, we observed a hole-trap-like transient for the puri"ed epi/sub interface sample, which notably contains
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 8 8 - 0
H. Ishii et al. / Journal of Crystal Growth 210 (2000) 242}246
the oxygen impurity in neither the AlGaAs bu!er layer nor the epi/sub interface. Yet, the origin of the hole-trap-like transient has not been clear to date. The purpose of this work is to realize how these complex phenomena are caused in the current transient of MESFETs by the deep levels related to the oxygen in AlGaAs. In order to investigate the nature of deep levels in AlGaAs, we fabricated a pn` diode sample, which enables us to characterize the mid-gap level under minority-carrier injection conditions simulated to the bu!er layer of MESFETs.
2. Experimental procedure The epitaxial layers were grown by MOCVD system. Trimethylgallium (TMGa) and trimethylaluminum (TMAl) and arsine (AsH ) were used as 3 the sources of Ga, Al and As, respectively. SiH and 4 CBr were used as the doping sources of n-type and 4 p-type layer, respectively. The structure of the pn` sample consists of a 500 nm n`GaAs layer doped with Si to 4]1018 cm~3 followed by 200 nm n`Al Ga As 0.3 0.7 with Si to 2]1018 cm~3, 200 nm undoped (p)Al Ga As, 100 nm p`Al Ga As with C to 0.3 0.7 0.3 0.7 1]1019 cm~3, and "nally, a 100 nm p`GaAs cap with C to 4]1019 cm~3. The undoped AlGaAs layer grown at low ¹ (5903C) shows a p-type 46" conductivity, whose net density of ionized impurity measured by C}< method was 1.2]1017 cm~3. SIMS measurements revealed that this layer contains C and O with concentrations of 1.7]1017 and 8.4]1017 cm~3, respectively. In the n`AlGaAs layer grown at high ¹ (6903C), C and O concen46" trations were below the detection limit of SIMS. Mesa diodes were fabricated for the electrical characterization. The diameter of mesas is 220 lm, and non-alloyed Ti/Au electrodes and alloyed AuGe/Ni/Au electrodes were fabricated for p-type and n-type ohmic contacts, respectively. The ideality factor (n-value) of forward current}voltage characteristics of the diodes was 2, which means that the recombination current is dominant in this diode. The deep-level transient spectroscopy (DLTS) measurements were performed using BIO-RAD
243
DL8000 system. The temperature was scanned between 85 and 550 K.
3. Results Fig. 1 shows the DLTS spectra which were measured using 1 Hz single pulse of a constant pulse height of 1 V with several bias points between !6 and 0 V. The rate window was 2 s~1. Five distinct peaks are observed in the "gure, and they are labeled A}E from lower to higher temperature region. Table 1 summarizes the activation energy for carrier emission and the capture cross section for each peak calculated from the Arrhenius plots. Only majority-carrier emissions are observed in the spectra measured at an applied bias below !1 V. The 150 K peak labeled as A is observed at all bias points and can be attributed to the electron emission from the DX center in n`AlGaAs. The peak B around 220 K could not be identi"ed, but it may be due to the electron emission from the DX
Fig. 1. The DLTS spectra scanned between 85 and 550 K. The pulse sequence is 1 Hz single pulse of a constant pulse height of 1 V. The spectra on the bias points of !6, !1 and 0 V are shown. Inset graph shows the DLTS spectra using the pulse height of 0.2 V. The bias points of the range between !1 and 0.6 V were applied. Five distinct peaks are labeled A}E in the "gure.
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H. Ishii et al. / Journal of Crystal Growth 210 (2000) 242}246
Table 1 The trap parameters for the peaks measured by DLTS Label
A
B
C
D
E
Peak temperature (K) Emission energy (ev) Cross section! (cm2) Concentration" (cm~3)
162.5 0.36 9.8]10~14 *
221.3 0.47 3.9]10~14 *
312.1 0.68 3.0]10~14 9.8]1015
372.3 0.77 8.1]10~17 *
450.6 1.03 3.6]10~15 8.3]1016
!The capture cross sections as temperaturePR for electron (A, B, C) and for hole (D, E). "Estimated by taking the so-called j e!ect [18] into account.
center. The details of another majority-carrier emission peak E will be discussed later. The signals due to minority-carrier emission start to appear, when the polarity of the bias pulse comes into the forward direction, i.e., the injection-pulse sequence. The peak E at 450 K could be clearly distinguished from the electron emission from EL2 [6] in n`AlGaAs, because we could not observe the photo-capacitance quenching e!ect which is regarded as a "nger print of EL2 [7]. The emission activation energy of the peak E (1.03 eV) is apparently close to the level of the oxygen-related defect (0.99 eV) reported by Ando et al. [8]. They observed it in p`n sample of Al Ga As, and iden0.4 0.6 ti"ed with the electron emission from the defect. However, the peak E is not similar to the electron emission from the oxygen-related defect as reported by Ando. We assigned the peak E to the hole emission from defects in undoped p-AlGaAs, since the peak E disappears by applying large reverse bias (!6 V) at which the depletion-layer edge reaches through the p`AlGaAs region on top. Nevertheless, the peak A (majority-carrier emission) from the DX center in the n`AlGaAs region was detected even under a reverse bias of !6 V. If the peak E were due to electron emission due to the defect in the n`AlGaAs, it could be observed at !6 V just like the DX center. Therefore, the peak E is most similar to the hole emission (0.87 eV) in n-Al Ga As reported by Bhattacharya et al. [9] 0.3 0.7 using optical DLTS method. They used the sample grown by MOCVD at low temperature, which is similar to the growth condition used in the present work. The minority-carrier as well as majority-carrier emission peaks C and D appear at 300 and 350 K,
respectively, while the dominant majority-carrier peak E disappears under minority-carrier injection. The behavior of peaks C (0.68 eV) and D (0.77 eV) in this study is similar to the current transient observed in MESFETs [5], where we observed electron emission with an activation energy of 0.66 eV and hole emission of 0.85 eV. The peak C is attributed to the electron emission in the undoped p-AlGaAs, and could be identical to the oxygenrelated peak detected in n-type AlGaAs [9}14]. In addition, we observed the same emissions as the peak C in the Schottky barrier-diode samples with an n-Al Ga As layer grown in the same growth 0.3 0.7 chamber. Under minority-carrier injection, the peak E seems to change into the peak D around 350 K. In order to investigate in detail the behavior of the peaks D and E, the DLTS spectra were measured at various bias points between !1 and 0.6 V using the "lling pulse with a "xed amplitude of 0.2 V. The transformation from the peak E to D is clearly seen in the inset spectra of Fig. 1.
4. Discussion In this section, we will discuss the mechanism involved in the remarkable shift of the peak E toward lower temperatures due to minority-carrier injection. It is well known that the presence of leakage current can drastically a!ect the DLTS spectra [15]. When the carrier capture takes place simultaneously with the carrier emission, the time constant for the change of the charge state at a trap level decreases; a `neta emission rate increases with an
H. Ishii et al. / Journal of Crystal Growth 210 (2000) 242}246
Fig. 2. The behavior of q (j, h) at 400 K and *C/C (m, n) related to the injection current. The open symbols correspond to the "lling pulse measurements (without injection). The solid line and the dashed line are theoretical curves for q and *C/C, respectively.
additional rate term. In this case, the *C/C also decreases in proportion to the time constant [15]. Fig. 2 shows the relationship between the time constant of the `neta hole emission from the trap E and the diode current observed at each bias point. The values of *C/C estimated from the peak intensity of DLTS spectra are also shown in the "gure. The rate of decrease in *C/C owing to the carrier injection is remarkably small compared with that of the time constant. If the trap can capture minority carrier (electron) as well as (the) majority carrier (hole), the `neta emission time constant (q) and the electron occupation function at a hole trap, like the peak E, in the steady state ( f ) are given by T= q"1/(e #pv p #nv p ), (1) p 5)p p 5)n n f "(e #nv p )/(e #pv p #nv p ), (2) T= p 5)n n p 5)p p 5)n n where e is the thermal emission rate for hole, n and p p are the density of injected electron and hole, v and v are the thermal velocities for electron 5)n 5)p and hole, and p and p are the cross section for n p electron and hole, respectively. Under a rather low forward bias voltage, the `neta emission time-
245
Fig. 3. The dependence of the activation energy for the peak E (j) in the DLTS spectra as well as the diode current (n) on the applied bias voltage.
constant q decreases mainly due to the term pv p 5)p p in Eq. (1), and f decreases. With the increase in T= the forward bias voltage and the concomitant increase in nv p , q decreases further, while f does 5)n n T= not decrease. The experimental results shown in Fig. 2 are qualitatively accounted for by such a recombination process at the deep level. It is in good agreement with the forward current}voltage characteristics in which the recombination current is dominant (n"2). Fig. 3 shows the dependence of the activation energies for the peak E in the DLTS spectra as well as the diode current on the applied bias voltage. Both are in good agreement in the forward bias region, and hence it is a natural consequence that the defect E could be responsible for the recombination current of the pn` diode used in this work. Very recently, Taguchi and Kageshima [16] using "rst-principles calculations revealed that oxygen in GaAs and AlAs shows a negative ; nature. According to their calculations for Fermilevel e!ect, the most stable state of oxygen in Al Ga As is considered to be the double 0.3 0.7 negative-charged state in n-type region, and the neutral charged state in p-type region, respectively.
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H. Ishii et al. / Journal of Crystal Growth 210 (2000) 242}246
Because the trap E is thought to be the principle electronic state related to oxygen defects in pAlGaAs, the level of the trap E could be regarded as the neutral state of oxygen. The electron-emission energy from the trap C is too small to regard the trap C as the double-charged oxygen which ought to be stabilized by lattice relaxation due to the negative ; property. The electron-emission energy for the trap C may be also reduced by the carrier capture same as the trap D. Alternatively, the trap C may be a di!erent type of defect related to oxygen [17] from trap E, because the density of trap E estimated by taking the so-called j e!ect [18] into account is by one order of magnitude smaller than the oxygen concentration measured by SIMS. Further examination will be needed to realize the whole nature of oxygen defects in AlGaAs.
5. Summary The deep levels in undoped (p)-Al Ga As 0.3 0.7 containing oxygen impurity were investigated. Under majority-carrier "lling condition, just one hole emission of 1.03 eV was observed. Applying the minority-carrier injection pulse, an electron-emission peak of 0.68 eV appeared and the hole emission disappeared in accordance with the large lowering shifts of emission temperature. This temperature shift of hole emission under the injection was caused by the recombination process via the hole trap. The defect related to this trap acts as an e$cient recombination center which dominates the diode current. This trap could be responsible for the hole-trap-like transient observed in draincurrent characteristics of MESFETs with AlGaAs bu!er layer.
Acknowledgements The authors thank K. Kato for sample preparation, H. Maruya for SIMS measurements, Y. Shiina and N. Ueda for useful discussions. They thank J. Kikawa for his continuous support.
References [1] D.B. Gibod, J.P. Andre, P. Baudet, J.P. Hallais, IEEE Trans. Electron Dev. ED-27 (1980) 1141. [2] G.B. Stringfellow, G. Hom, Appl. Phys. Lett. 34 (1979) 794. [3] S.J. Pearton, M.P. Iannuzzi, C.L. Reynolds Jr., L. Peticolas, Appl. Phys. Lett. 52 (1988) 395. [4] T. Itoh, H. Yanai, IEEE Trans. Electron Dev. ED-27 (1980) 1037. [5] S. Tanaka, H. Ishii, Jpn. J. Appl. Phys. 38 (1999) 22. [6] N.M. Johnson, R.D. Burnham, D. Fehete, R.D. Yingling, in: J. Narayam, T.Y. Tan (Eds.), Defects in Semiconductors, North-Holland, Amsterdam, 1981, p. 481. [7] A.B. Cherifa, R. Azoulay, A. Nouailhat, G. Guillot, Inst. Phys. Conf. Ser. 91 (1987) 235. [8] K. Ando, C. Amano, H. Sugiura, M. Yamaguchi, A. Saletes, Jpn. J. Appl. Phys. 26 (1987) L266. [9] P.K. Bhattacharya, T. Matsumoto, S. Subramanian, J. Cryst. Growth 68 (1984) 301. [10] H.C. Casey Jr., A.Y. Cho, D.V. Lang, E.H. Nicollian, P.W. Foy, J. Appl. Phys. 50 (1979) 3484. [11] R.H. Wallis, Inst. Phys. Conf. Ser. 56 (1981) 73. [12] S.R. McAfee, W.T. Tsang, D.V. Lang, J. Appl. Phys. 52 (1981) 6165. [13] K. Akimoto, M. Kamada, K. Taira, M. Arai, N. Watanabe, J. Appl. Phys. 59 (1986) 2833. [14] M.O. Watanabe, Y. Ahizawa, N. Sugiyama, T. Nakanishi, Inst. Phys. Conf. Ser. 83 (1986) 105. [15] T. Okumura, M. Hoshino, Semi-Insulating III}V Materials, North-Holland, Tokyo, 1986, p. 409. [16] A. Taguchi, H. Kageshima, Phys. Rev. B 60 (1999) 5383. [17] M.S. Goorsky, T.F. Kuech, F. Cardone, P.M. Mooney, G.J. Scilla, Appl. Phys. Lett. 58 (1991) 1979. [18] Y. Zohta, M.O. Watanabe, J. Appl. Phys. 53 (1982) 1809.
Characterization of oxygen-related defects in p-Al Ga As by DLTS 0.3 0.7 H. Ishii!,*, T. Shinagawa!, S. Tanaka!, T. Okumura" !Yokohama R&D Laboratories, The Furukawa Electric Co. Ltd., 2-4-3 Okano, Yokohama 220-0073, Japan "Department of Electrical Engineering, Tokyo Metropolitan University, Hachiohji 192-0397, Japan
Abstract The oxygen-related defects in undoped Al Ga As (p-type) were characterized by deep-level transient spectroscopy 0.3 0.7 (DLTS). We prepared pn`-junction diodes by the metal organic chemical vapor deposition (MOCVD) technique in order to study both majority- and minority-carrier emission properties of the defects. Under majority-carrier "lling condition, just one hole-emission peak (E) was observed around 450 K in addition to DX centers in the n`-layer. When the minority-carrier injection pulse was applied, an electron-emission peak (C) appeared around 300 K in accordance with the disappearance of peak E. The defect related to the peak E acts as an e$cient recombination center which dominates the current}voltage characteristics of the diodes. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 71.55.Eq Keywords: AlGaAs; DLTS; MOCVD; Recombination center; Oxygen
1. Introduction GaAs epitaxial wafers with an undoped AlGaAs bu!er layer grown by metal organic chemical vapor deposition (MOCVD) are widely used to fabricate GaAs metal-semiconductor "eld e!ect transistors (MESFETs) [1] in the "eld of microwave power application. It is well known that the AlGaAs grown by MOCVD is likely to be contaminated by oxygen because of high reactivity of aluminum with oxygen, and thus the quality of the epitaxial layer, such as luminescence e$ciency [2], is seriously
* Corresponding author. Tel.: #81-45-311-1219; fax: #8145-314-5190. E-mail address: [email protected] (H. Ishii)
in#uenced by oxygen contamination. On the other hand, oxygen-ion implantation into n-AlGaAs followed by annealing forms a stable layer of high resistivity, which is attributed to the creation of a deep acceptor level compensating for shallow donors [3]. While high resistivity of the AlGaAs bu!er layer is advantageous for the fabrication of MESFETs, there could be some drawbacks due to the deep levels related to oxygen in the crystal, which depreciate the performance of MESFETs, for example, drain-current instability [4]. Recently, we reported that the oxygen-related electron trap originating from AlGaAs bu!er layer is responsible for the drain-current transient in the MESFETs [5]. In addition to the electron trap, we observed a hole-trap-like transient for the puri"ed epi/sub interface sample, which notably contains
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 8 8 - 0
H. Ishii et al. / Journal of Crystal Growth 210 (2000) 242}246
the oxygen impurity in neither the AlGaAs bu!er layer nor the epi/sub interface. Yet, the origin of the hole-trap-like transient has not been clear to date. The purpose of this work is to realize how these complex phenomena are caused in the current transient of MESFETs by the deep levels related to the oxygen in AlGaAs. In order to investigate the nature of deep levels in AlGaAs, we fabricated a pn` diode sample, which enables us to characterize the mid-gap level under minority-carrier injection conditions simulated to the bu!er layer of MESFETs.
2. Experimental procedure The epitaxial layers were grown by MOCVD system. Trimethylgallium (TMGa) and trimethylaluminum (TMAl) and arsine (AsH ) were used as 3 the sources of Ga, Al and As, respectively. SiH and 4 CBr were used as the doping sources of n-type and 4 p-type layer, respectively. The structure of the pn` sample consists of a 500 nm n`GaAs layer doped with Si to 4]1018 cm~3 followed by 200 nm n`Al Ga As 0.3 0.7 with Si to 2]1018 cm~3, 200 nm undoped (p)Al Ga As, 100 nm p`Al Ga As with C to 0.3 0.7 0.3 0.7 1]1019 cm~3, and "nally, a 100 nm p`GaAs cap with C to 4]1019 cm~3. The undoped AlGaAs layer grown at low ¹ (5903C) shows a p-type 46" conductivity, whose net density of ionized impurity measured by C}< method was 1.2]1017 cm~3. SIMS measurements revealed that this layer contains C and O with concentrations of 1.7]1017 and 8.4]1017 cm~3, respectively. In the n`AlGaAs layer grown at high ¹ (6903C), C and O concen46" trations were below the detection limit of SIMS. Mesa diodes were fabricated for the electrical characterization. The diameter of mesas is 220 lm, and non-alloyed Ti/Au electrodes and alloyed AuGe/Ni/Au electrodes were fabricated for p-type and n-type ohmic contacts, respectively. The ideality factor (n-value) of forward current}voltage characteristics of the diodes was 2, which means that the recombination current is dominant in this diode. The deep-level transient spectroscopy (DLTS) measurements were performed using BIO-RAD
243
DL8000 system. The temperature was scanned between 85 and 550 K.
3. Results Fig. 1 shows the DLTS spectra which were measured using 1 Hz single pulse of a constant pulse height of 1 V with several bias points between !6 and 0 V. The rate window was 2 s~1. Five distinct peaks are observed in the "gure, and they are labeled A}E from lower to higher temperature region. Table 1 summarizes the activation energy for carrier emission and the capture cross section for each peak calculated from the Arrhenius plots. Only majority-carrier emissions are observed in the spectra measured at an applied bias below !1 V. The 150 K peak labeled as A is observed at all bias points and can be attributed to the electron emission from the DX center in n`AlGaAs. The peak B around 220 K could not be identi"ed, but it may be due to the electron emission from the DX
Fig. 1. The DLTS spectra scanned between 85 and 550 K. The pulse sequence is 1 Hz single pulse of a constant pulse height of 1 V. The spectra on the bias points of !6, !1 and 0 V are shown. Inset graph shows the DLTS spectra using the pulse height of 0.2 V. The bias points of the range between !1 and 0.6 V were applied. Five distinct peaks are labeled A}E in the "gure.
244
H. Ishii et al. / Journal of Crystal Growth 210 (2000) 242}246
Table 1 The trap parameters for the peaks measured by DLTS Label
A
B
C
D
E
Peak temperature (K) Emission energy (ev) Cross section! (cm2) Concentration" (cm~3)
162.5 0.36 9.8]10~14 *
221.3 0.47 3.9]10~14 *
312.1 0.68 3.0]10~14 9.8]1015
372.3 0.77 8.1]10~17 *
450.6 1.03 3.6]10~15 8.3]1016
!The capture cross sections as temperaturePR for electron (A, B, C) and for hole (D, E). "Estimated by taking the so-called j e!ect [18] into account.
center. The details of another majority-carrier emission peak E will be discussed later. The signals due to minority-carrier emission start to appear, when the polarity of the bias pulse comes into the forward direction, i.e., the injection-pulse sequence. The peak E at 450 K could be clearly distinguished from the electron emission from EL2 [6] in n`AlGaAs, because we could not observe the photo-capacitance quenching e!ect which is regarded as a "nger print of EL2 [7]. The emission activation energy of the peak E (1.03 eV) is apparently close to the level of the oxygen-related defect (0.99 eV) reported by Ando et al. [8]. They observed it in p`n sample of Al Ga As, and iden0.4 0.6 ti"ed with the electron emission from the defect. However, the peak E is not similar to the electron emission from the oxygen-related defect as reported by Ando. We assigned the peak E to the hole emission from defects in undoped p-AlGaAs, since the peak E disappears by applying large reverse bias (!6 V) at which the depletion-layer edge reaches through the p`AlGaAs region on top. Nevertheless, the peak A (majority-carrier emission) from the DX center in the n`AlGaAs region was detected even under a reverse bias of !6 V. If the peak E were due to electron emission due to the defect in the n`AlGaAs, it could be observed at !6 V just like the DX center. Therefore, the peak E is most similar to the hole emission (0.87 eV) in n-Al Ga As reported by Bhattacharya et al. [9] 0.3 0.7 using optical DLTS method. They used the sample grown by MOCVD at low temperature, which is similar to the growth condition used in the present work. The minority-carrier as well as majority-carrier emission peaks C and D appear at 300 and 350 K,
respectively, while the dominant majority-carrier peak E disappears under minority-carrier injection. The behavior of peaks C (0.68 eV) and D (0.77 eV) in this study is similar to the current transient observed in MESFETs [5], where we observed electron emission with an activation energy of 0.66 eV and hole emission of 0.85 eV. The peak C is attributed to the electron emission in the undoped p-AlGaAs, and could be identical to the oxygenrelated peak detected in n-type AlGaAs [9}14]. In addition, we observed the same emissions as the peak C in the Schottky barrier-diode samples with an n-Al Ga As layer grown in the same growth 0.3 0.7 chamber. Under minority-carrier injection, the peak E seems to change into the peak D around 350 K. In order to investigate in detail the behavior of the peaks D and E, the DLTS spectra were measured at various bias points between !1 and 0.6 V using the "lling pulse with a "xed amplitude of 0.2 V. The transformation from the peak E to D is clearly seen in the inset spectra of Fig. 1.
4. Discussion In this section, we will discuss the mechanism involved in the remarkable shift of the peak E toward lower temperatures due to minority-carrier injection. It is well known that the presence of leakage current can drastically a!ect the DLTS spectra [15]. When the carrier capture takes place simultaneously with the carrier emission, the time constant for the change of the charge state at a trap level decreases; a `neta emission rate increases with an
H. Ishii et al. / Journal of Crystal Growth 210 (2000) 242}246
Fig. 2. The behavior of q (j, h) at 400 K and *C/C (m, n) related to the injection current. The open symbols correspond to the "lling pulse measurements (without injection). The solid line and the dashed line are theoretical curves for q and *C/C, respectively.
additional rate term. In this case, the *C/C also decreases in proportion to the time constant [15]. Fig. 2 shows the relationship between the time constant of the `neta hole emission from the trap E and the diode current observed at each bias point. The values of *C/C estimated from the peak intensity of DLTS spectra are also shown in the "gure. The rate of decrease in *C/C owing to the carrier injection is remarkably small compared with that of the time constant. If the trap can capture minority carrier (electron) as well as (the) majority carrier (hole), the `neta emission time constant (q) and the electron occupation function at a hole trap, like the peak E, in the steady state ( f ) are given by T= q"1/(e #pv p #nv p ), (1) p 5)p p 5)n n f "(e #nv p )/(e #pv p #nv p ), (2) T= p 5)n n p 5)p p 5)n n where e is the thermal emission rate for hole, n and p p are the density of injected electron and hole, v and v are the thermal velocities for electron 5)n 5)p and hole, and p and p are the cross section for n p electron and hole, respectively. Under a rather low forward bias voltage, the `neta emission time-
245
Fig. 3. The dependence of the activation energy for the peak E (j) in the DLTS spectra as well as the diode current (n) on the applied bias voltage.
constant q decreases mainly due to the term pv p 5)p p in Eq. (1), and f decreases. With the increase in T= the forward bias voltage and the concomitant increase in nv p , q decreases further, while f does 5)n n T= not decrease. The experimental results shown in Fig. 2 are qualitatively accounted for by such a recombination process at the deep level. It is in good agreement with the forward current}voltage characteristics in which the recombination current is dominant (n"2). Fig. 3 shows the dependence of the activation energies for the peak E in the DLTS spectra as well as the diode current on the applied bias voltage. Both are in good agreement in the forward bias region, and hence it is a natural consequence that the defect E could be responsible for the recombination current of the pn` diode used in this work. Very recently, Taguchi and Kageshima [16] using "rst-principles calculations revealed that oxygen in GaAs and AlAs shows a negative ; nature. According to their calculations for Fermilevel e!ect, the most stable state of oxygen in Al Ga As is considered to be the double 0.3 0.7 negative-charged state in n-type region, and the neutral charged state in p-type region, respectively.
246
H. Ishii et al. / Journal of Crystal Growth 210 (2000) 242}246
Because the trap E is thought to be the principle electronic state related to oxygen defects in pAlGaAs, the level of the trap E could be regarded as the neutral state of oxygen. The electron-emission energy from the trap C is too small to regard the trap C as the double-charged oxygen which ought to be stabilized by lattice relaxation due to the negative ; property. The electron-emission energy for the trap C may be also reduced by the carrier capture same as the trap D. Alternatively, the trap C may be a di!erent type of defect related to oxygen [17] from trap E, because the density of trap E estimated by taking the so-called j e!ect [18] into account is by one order of magnitude smaller than the oxygen concentration measured by SIMS. Further examination will be needed to realize the whole nature of oxygen defects in AlGaAs.
5. Summary The deep levels in undoped (p)-Al Ga As 0.3 0.7 containing oxygen impurity were investigated. Under majority-carrier "lling condition, just one hole emission of 1.03 eV was observed. Applying the minority-carrier injection pulse, an electron-emission peak of 0.68 eV appeared and the hole emission disappeared in accordance with the large lowering shifts of emission temperature. This temperature shift of hole emission under the injection was caused by the recombination process via the hole trap. The defect related to this trap acts as an e$cient recombination center which dominates the diode current. This trap could be responsible for the hole-trap-like transient observed in draincurrent characteristics of MESFETs with AlGaAs bu!er layer.
Acknowledgements The authors thank K. Kato for sample preparation, H. Maruya for SIMS measurements, Y. Shiina and N. Ueda for useful discussions. They thank J. Kikawa for his continuous support.
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