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Area-selective regrowth followed by AsH3 surface treatment and its application for ultra-shallow GaAs sidewall tunnel junctions
Applied Surface Science 216 (2003) 549–553
Area-selective regrowth followed by AsH3 surface treatment and its application for ultra-shallow GaAs sidewall tunnel junctions Takeo Ohnoa, Yutaka Oyamaa,b,*, Kenji Tezukaa, Ken Sutoa,b, Jun-ichi Nishizawab a
Department of Materials Science, Graduate School of Engineering, Tohoku University, Aramaki Aza Aoba02, Sendai 980-8579, Japan b Semiconductor Research Institute of Semiconductor Research Foundation, Kawauchi Aoba, Sendai 980-0862, Japan
Abstract Low-temperature (290 8C) area-selective regrowth (ASR) by the intermittent injection of triethylgallium (TEGa) and arsine (AsH3) in an ultra-high vacuum (UHV) was applied for the fabrication of ultra-shallow sidewall GaAs tunnel junctions with the junction area in the order of 108 cm2. Fabricated tunnel junctions have shown the record peak current density up to 35,000 A/ cm2 at 100 mm long strip structure. It is shown that the tunnel junction characteristics are seriously dependent on the sidewall orientation and the regrown interface quality, which was determined by the surface treatment conditions under AsH3 just prior to regrowth. The junction characteristics and AsH3 surface treatment effects are discussed in view of the orientation dependence of Be doping and control of surface stoichiometry. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Semiconductor homo- and hetero-interfaces; GaAs; Regrowth; Selective epitaxy; Tunnel junction; Sidewall
1. Introduction High quality tunnel junction is the most important semiconductor device segments for the application of mm and sub-mm wave source and detectors. Among the semiconductor sub-mm wave sources, the tunnel injection transit time effect diode (TUNNETT) [1] is one of the most promising sub-mm wave source up to terahertz region. For TUNNETT oscillators, the most important factor is the high quality tunnel junction in order to achieve high oscillation frequency and output power. In addition, the reduction of parasitics improves the device
*
Corresponding author. Tel.: þ81-22-217-7328; fax: þ81-22-217-7329. E-mail address: [email protected] (Y. Oyama).
performances on the basis of the impedance matching as an oscillation circuits [2]. In this work, low-temperature area-selective regrowth (ASR) was applied to fabricate the ultrashallow pþnþ GaAs tunnel junctions on the shallow sidewall surface [3]. By using ASR process, the junction area can be easily reduced to be in the order of 108 cm2 without photolithography limit, and wide contact pad on the top can reduce the parasitic resistance, which cannot be achieved by the conventional vertical structure. This paper reports the tunneling characteristics of sidewall pþnþ GaAs tunnel junctions. The tunnel junctions have shown the record peak current density up to 35,000 A/cm2. It is shown that the tunnel junction characteristics are seriously dependent on the sidewall orientation. The effects of AsH3 surface
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00506-3
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T. Ohno et al. / Applied Surface Science 216 (2003) 549–553
treatment just prior to the regrowth are also shown in view of the control of junction interface characteristics and surface stoichiometry.
2. Experimental The tunnel junctions were fabricated by the areaselective molecular layer epitaxy (MLE) [4] using intermittent supply of triethylgallium (TEGa) and arsine (AsH3) in an ultra-high vacuum (UHV). First, Te and S co-doped nþ-GaAs was grown on {1 0 0} oriented intentionally undoped semi-insulating (SI) GaAs at 360 8C. Epitaxial layer thickness of nþ-GaAs is 50 nm, which determines the junction area of sidewall tunnel junctions. Diethyltelluride (DETe) and diethylsulfur (DES) are used for the gas source as donor doping. In order to achieve high impurity concentration, DETe was exposed on the gallium stabilized surface (mode AG) [5] and DES was introduced on the arsenic stabilized surface (mode AA) during MLE growth cycles. Te and S co-doping [6] is effective to reduce the lattice strain in a heavily donor doped layer with the carrier concentration of 2 1019 cm3. Then, SiN was deposited for the masking layer. Windows were opened for ASR by using conventional photolithography process and the reactive ion etching (RIE) followed by the final slight wet etching of remained thin SiN. Through SiN windows, the sidewall mesa was formed by H2SO4-based solution with the depth of 60 nm. The patterned substrates with nþ layer were set in the UHV growth chamber for regrowth, and the surface treatment was carried out at 350 8C for 30– 120 min under AsH3 pressure of 1:1 101 Pa (8 104 Torr). In this report, the effects of AsH3 treatment time (tFD) are investigated. The ASR of pþGaAs:Be was carried out at 290 8C using Be(MeCp)2, which was introduced by mode AG during MLE cycles [7] to achieve high carrier concentration of 8 1019 cm3. When the pþ-GaAs:Be was regrown, the duration of TEGa supply was 1 or 3 s. Non-alloyed Ti/Au contacts were formed by the evaporation using conventional lift-off process. From the FE-SEM observations of deep etched trench (depth ¼ 200 nm), the sidewall orientations for normal mesa, 458 inclined configuration and reverse mesa were determined to be nearly {1 1 1}A, {1 1 0}
Fig. 1. Schematic drawings of the cross-sectional view of the fabricated sidewall tunnel junction. Ti/Au non-alloyed contact pad has 100 mm in length.
and {1 1 1}B, respectively. Fig. 1 shows the schematic drawings of the cross-sectional of the fabricated sidewall tunnel junction diodes. The impurity concentration and its profile were measured by secondary ion mass spectroscopy analysis (SIMS) with 1 keV–20 nA Csþ beam as a primary ion. Current density–voltage (J–V) characteristics were measured at nominal room temperature (RT) and 77 K.
3. Results Fig. 2 shows the J–V characteristics of GaAs sidewall tunnel junctions on various sidewall orientations. The tunnel junction on normal mesa orientation have shown the record peak current density (Jp) up to 31,000 A/cm2 at RT with the peak-to-valley current ratio (Jp /Jv) of 2.1. Jp of 458 inclined configuration and that of reverse mesa orientation are 5200 and 2100 A/cm2, respectively. Table 1 summarizes the
Fig. 2. J–V characteristics of regrown GaAs tunnel junctions on various sidewall orientations.
T. Ohno et al. / Applied Surface Science 216 (2003) 549–553
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Table 1 Regrown sidewall tunnel junction characteristics
Jp(RT) (A/cm2) Jv(RT) (A/cm2) Jp(RT)/Jv(RT) Jv(77 K) (A/cm2) Jv(RT)/Jv(77 K) Vp(RT) (mV) Vv(RT) (mV) Vp(RT)/Jp(RT) (O cm2) Zero-bias specific resistivity (O cm2) Negative differential conductance (S)
Normal mesa {1 1 1}A
458 inclined configuration {1 1 0}
Reverse mesa {1 1 1}B
31000 14900 2.1 6920 2.1 214 414 6.9 106 5.3 106 1.4 105
5200 3110 1.7 1480 2.1 130 282 2.5 105 1.0 105 7.1 105
2100 1530 1.4 550 2.8 136 252 6.5 105 2.9 105 2.1 104
present tunnel diode characteristics at RT and 77 K. It is shown that the peak, valley current density (Jv) and resultant peak-to-valley ratio have shown strong mesa orientation dependences. Fig. 3 shows Jp, Jv and Jp /Jv as a function of flush desorption time (tFD). Under the present regrowth conditions with the TEGa introduction time of 3 s in
MLE, all junctions on {1 1 1}A sidewall have shown Esaki characteristics at RT. However, junctions on {1 1 1}B and {1 1 0} sidewall have shown ohmiclike characteristics due to the large excess current at RT. Jp/Jv of tunnel junctions on normal mesa shows monotonic increase from 1.8 to 4.2 when tFD becomes longer. It is shown that long AsH3 treatment prior to regrowth improves the tunnel junction characteristics. Jp at tFD ¼ 120 min was twice as small as 30 and 60 min, but these extremely high Jp (highest value is 35,000 A/cm2) are at least 10–20 times higher than that reported previously [8]. Jv shows monotonic decrease for longer tFD. This is due to the reduction of deep level density at the junction interface.
4. Discussions The sidewall tunnel junctions have shown Jp(RT) / Jv(RT) of 1.4–2.1 (Table 1). In heavily donor doped nþGaAs, it has been reported that the donor-defect complex (DX) center [9] is formed, and the excess current is dominated by the leakage current via states. In this case, the Jv is expressed as [10] sffiffiffiffiffiffiffiffi ! p emx Jv ADx exp ½Egap þ 0:35ðxn þ xp Þ
2hq Neff
Fig. 3. (a) Peak-to-valley current ratio and (b) peak and valley current density at RT as a function of tFD. Sidewall orientation is normal mesa.
where the valley voltage is equal to 0:65ðxn þ xp Þ, and Dx is the volume density of occupied levels at energy Ex above the top of the valence band and A is a constant under the assumption of mx ¼ meff . Neff
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T. Ohno et al. / Applied Surface Science 216 (2003) 549–553
and meff are the effective density of states and the tunneling effective mass, which are given by Neff ¼
pn ; pþn
meff ¼
mlh me mlh þ me
Therefore, the Jv is dependent on the doping concentration (n and p). Thus, it is considered that one of the reasons for low Jp /Jv is the very high effective density of states Neff in the present sidewall tunnel junctions. In view of the mechanism for the strong sidewall orientation dependences on Jp, the Be-doping characteristics of the present epitaxial technique are investigated by using various surface orientations at the same epitaxial run under the identical conditions for device epitaxy. Be concentrations on each oriented GaAs is in the order of f1 1 1gA > f1 1 1gB > f1 1 0g. The reason for the highest Jp on normal mesa orientation, which is nearly {1 1 1}A, can be understood partially in view of the Be doping characteristics. However, the reason for Jp f1 1 0g > Jp f1 1 1gB is still not clear. The Jp is affected by many parameters. After Kane [11] and Beji et. al. [12], the peak current density for the direct tunneling is obtained as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 q5 meff Neff V0 Jp ¼ 2 eEgap 36ph 0 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 3 eEgap meff p ADðqVp Þ exp@ V 2h q3 Neff 0 where D is an integral in energy units, qV0 the built-in potential energy, q the electron charge, h the reduced Plank’s constant, Egap the band-gap energy, and e is the dielectric constant. In addition, in case of trap assisted or phonon assisted inelastic tunneling process, the peak current density is affected seriously by many parameters like trap density, occupation probability of traps [13] and deformation potential [14], etc. The orientation dependence on effective tunneling mass [15] is also one of the important parameters, which determines the excess current in the tunnel diodes. Actually, from Table 1, the valley current density ratios Jv(RT) /Jv(77 K) between RT and 77 K are 2.1 for normal mesa and 458 inclined configuration. However, that for reverse mesa, on which Jp and Jp/Jv are lowest, shows larger temperature dependence and to be 2.8. This indicates that the excess current, i.e. the valley current is dominated by the tunneling through
Fig. 4. Be profiles of regrown pþnþ tunnel junction on {1 0 0} surface as a function of AsH3 treatment time.
the midgap levels in the tunnel junction, especially on reverse mesa orientation. In order to investigate the reason for greatly high Jp and the effects of AsH3 treatment on the regrown interface features, SIMS measurements were applied to measure the impurity profile and interface pile-up of regrown interface. Fig. 4 shows the Be profiles of regrown pþnþ tunnel junction on {1 0 0} surface as a function of tFD. It is shown that the long AsH3 treatment enhances the steepness of Be profile, and that the Be interface pile-up increases as the tFD is longer. Be interface pile-up is 9 1019 , 1:2 1020 and 1:5 1020 cm3 for tFD ¼ 30, 60 and 120 min, respectively. These results of improved steepness of Be profile and the enhanced Be interface pile-up concentration at regrown interface are the major reasons for the extremely high Jp of sidewall tunnel junctions. The tFD dependence of Jv change between RT and 77 K (Jv(RT)/Jv(77 K)) were investigated. The value of Jv(RT)/Jv(77 K) is 2.3, 2.6 and 1.6 for tFD ¼ 30, 60 and 120 min, respectively. It is noticed that Jv(RT)/Jv(77 K) is not so change for tFD ¼ 30, 60 min, but AsH3 treatment for 120 min reduced the value of Jv(RT)/ Jv(77 K). Here, Jv(RT)/Jv(77 K) is almost proportional to the temperature dependence of Dx, because n and p in degenerated semiconductor will be almost temperature-independent. Thus, for simplicity, if the monovalent donor-type deep level is formed in the narrow depletion layer of tunnel junction, the temperature dependence of the Dx is expressed as [16] Dx ¼
Nd 1 þ b exp½ðEc Ed fÞ=kT
T. Ohno et al. / Applied Surface Science 216 (2003) 549–553
where Nd is the deep level density, Ec Ed is the activation energy of deep levels, f is the Fermi energy, b is the deep level spin degeneracy and k is the Boltzmann constant. From this expression, it is concluded that the larger Jv(RT)/Jv(77 K) value indicates the deeper activation energy of defect levels. Thus, it is concluded that the sufficient AsH3 treatment with long tFD induces shallower deep levels, which will be followed by the less strained atomic configuration around the defects. As reported previously [17], it has been shown that this reduction of excess current does not come from the removal of surface oxides and/or carbides by AsH3 treatment, but that the controlled surface stoichiometry reduces the non-stoichiometry related defects at the regrown interface. Thus, it is considered that the surface stoichiometry control at the regrown interface reduces the deep levels at the tunnel junction interface, thus the valley current density reduces as the longer AsH3 treatment time.
5. Conclusion Ultra-shallow sidewall GaAs tunnel junctions were fabricated by the low-temperature (290 8C) ASR using the intermittent injection of precursors in an ultra-high vacuum. The tunnel junctions on the normal mesa orientation have shown the record peak current density up to 35,000 A/cm2 at 100 mm long strip structure. High concentration Be pile-up at the regrown interface is one of the possible reasons for extremely high peak current density under the assumption that the interface pile-up phenomenon is similar for all sidewall orientations. Tunnel junction characteristics have been improved by AsH3 surface treatment just prior to regrowth due to the improved steepness in Be doping profile, interface pile-up and reduction of stoichiometry-dependent deep states at the regrown interface. Crystallographic orientation dependence of Be doping characteristics and the
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midgap states in the tunnel junctions are the important factor for the high quality sidewall tunnel junction characteristics. References [1] J. Nishizawa, Y. Watanabe, Sci. Rep. Res. Inst. Tohoku Univ. 10 (1958) 91; K. Motoya, J. Nishizawa, Topics in Millimeter Wave Technology, vol. 2, Academic Press, New York, 1988, p. 1 (Chapter 1). [2] J. Nishizawa, H. Makabe, F. Matsumoto, P. Plotka, T. Kurabayashi, Electron. Lett. 38 (2002) 660. [3] Y. Oyama, T. Ohno, K. Tezuka, K. Suto, J. Nishizawa, Appl. Phys. Lett. 81 (2002) 2563. [4] J. Nishizawa, Y. Kokubun, Extended Abstracts of 16th Conference on Solid State Devices and Materials, Kobe, Japan, 1984, p. 1; P. Plotka, T. Kurabayashi, Y. Oyama, J. Nishizawa, Appl. Surf. Sci. 82–83 (1994) 91. [5] Y. Oyama, J. Nishizawa, K. Seo, K. Suto, J. Cryst. Growth 212 (2000) 402. [6] Y. Oyama, K. Tezuka, K. Suto, J. Nishizawa, J. Cryst. Growth 246 (2002) 15. [7] T. Ohno, Y. Oyama, K. Suto, J. Nishizawa, submitted for publication. [8] S. Ahmed, M.R. Melloch, E.S. Harmon, D.T. McInturff, J.M. Woodall, Appl. Phys. Lett. 71 (1997) 3667. [9] E. Munoz, E. Calleja, in: E. Munoz (Ed.), DX Centers-donors in AlGaAs and Related Compounds, vol. 1089, Scitec, Switzerland, 1994, p. 135. [10] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley/Interscience, New York, 1981. [11] E.O. Kane, J. Appl. Phys. 32 (1961) 83. [12] L. Beji, B. el Jani, P. Gilbart, J.C. Portal, P. Basmaji, J. Appl. Phys. 83 (1998) 5573. [13] F. Jimenez-Molinos, A. Palma, F. Gamiz, J. Banqueri, J.A. Lopez-Villanueva, J. Appl. Phys. 90 (2001) 3396. [14] C. Rivas, R. Lake, G. Klimech, W.R. Frensley, M.V. Fischetti, P.E. Thompson, S.L. Rommel, P. Berger, Appl. Phys. Lett. 78 (2001) 814. [15] L.F. Luo, R. Beresford, W.I. Wang, E.E. Mendez, Appl. Phys. Lett. 54 (1989) 2133. [16] J.S. Blakemore, Semiconductor Statistics, Pergamon Press, Oxford, 1962. [17] J. Nishizawa, Y. Oyama, P. Plotka, H. Sakuraba, Surf. Sci. 348 (1996) 105.
Area-selective regrowth followed by AsH3 surface treatment and its application for ultra-shallow GaAs sidewall tunnel junctions Takeo Ohnoa, Yutaka Oyamaa,b,*, Kenji Tezukaa, Ken Sutoa,b, Jun-ichi Nishizawab a
Department of Materials Science, Graduate School of Engineering, Tohoku University, Aramaki Aza Aoba02, Sendai 980-8579, Japan b Semiconductor Research Institute of Semiconductor Research Foundation, Kawauchi Aoba, Sendai 980-0862, Japan
Abstract Low-temperature (290 8C) area-selective regrowth (ASR) by the intermittent injection of triethylgallium (TEGa) and arsine (AsH3) in an ultra-high vacuum (UHV) was applied for the fabrication of ultra-shallow sidewall GaAs tunnel junctions with the junction area in the order of 108 cm2. Fabricated tunnel junctions have shown the record peak current density up to 35,000 A/ cm2 at 100 mm long strip structure. It is shown that the tunnel junction characteristics are seriously dependent on the sidewall orientation and the regrown interface quality, which was determined by the surface treatment conditions under AsH3 just prior to regrowth. The junction characteristics and AsH3 surface treatment effects are discussed in view of the orientation dependence of Be doping and control of surface stoichiometry. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Semiconductor homo- and hetero-interfaces; GaAs; Regrowth; Selective epitaxy; Tunnel junction; Sidewall
1. Introduction High quality tunnel junction is the most important semiconductor device segments for the application of mm and sub-mm wave source and detectors. Among the semiconductor sub-mm wave sources, the tunnel injection transit time effect diode (TUNNETT) [1] is one of the most promising sub-mm wave source up to terahertz region. For TUNNETT oscillators, the most important factor is the high quality tunnel junction in order to achieve high oscillation frequency and output power. In addition, the reduction of parasitics improves the device
*
Corresponding author. Tel.: þ81-22-217-7328; fax: þ81-22-217-7329. E-mail address: [email protected] (Y. Oyama).
performances on the basis of the impedance matching as an oscillation circuits [2]. In this work, low-temperature area-selective regrowth (ASR) was applied to fabricate the ultrashallow pþnþ GaAs tunnel junctions on the shallow sidewall surface [3]. By using ASR process, the junction area can be easily reduced to be in the order of 108 cm2 without photolithography limit, and wide contact pad on the top can reduce the parasitic resistance, which cannot be achieved by the conventional vertical structure. This paper reports the tunneling characteristics of sidewall pþnþ GaAs tunnel junctions. The tunnel junctions have shown the record peak current density up to 35,000 A/cm2. It is shown that the tunnel junction characteristics are seriously dependent on the sidewall orientation. The effects of AsH3 surface
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00506-3
550
T. Ohno et al. / Applied Surface Science 216 (2003) 549–553
treatment just prior to the regrowth are also shown in view of the control of junction interface characteristics and surface stoichiometry.
2. Experimental The tunnel junctions were fabricated by the areaselective molecular layer epitaxy (MLE) [4] using intermittent supply of triethylgallium (TEGa) and arsine (AsH3) in an ultra-high vacuum (UHV). First, Te and S co-doped nþ-GaAs was grown on {1 0 0} oriented intentionally undoped semi-insulating (SI) GaAs at 360 8C. Epitaxial layer thickness of nþ-GaAs is 50 nm, which determines the junction area of sidewall tunnel junctions. Diethyltelluride (DETe) and diethylsulfur (DES) are used for the gas source as donor doping. In order to achieve high impurity concentration, DETe was exposed on the gallium stabilized surface (mode AG) [5] and DES was introduced on the arsenic stabilized surface (mode AA) during MLE growth cycles. Te and S co-doping [6] is effective to reduce the lattice strain in a heavily donor doped layer with the carrier concentration of 2 1019 cm3. Then, SiN was deposited for the masking layer. Windows were opened for ASR by using conventional photolithography process and the reactive ion etching (RIE) followed by the final slight wet etching of remained thin SiN. Through SiN windows, the sidewall mesa was formed by H2SO4-based solution with the depth of 60 nm. The patterned substrates with nþ layer were set in the UHV growth chamber for regrowth, and the surface treatment was carried out at 350 8C for 30– 120 min under AsH3 pressure of 1:1 101 Pa (8 104 Torr). In this report, the effects of AsH3 treatment time (tFD) are investigated. The ASR of pþGaAs:Be was carried out at 290 8C using Be(MeCp)2, which was introduced by mode AG during MLE cycles [7] to achieve high carrier concentration of 8 1019 cm3. When the pþ-GaAs:Be was regrown, the duration of TEGa supply was 1 or 3 s. Non-alloyed Ti/Au contacts were formed by the evaporation using conventional lift-off process. From the FE-SEM observations of deep etched trench (depth ¼ 200 nm), the sidewall orientations for normal mesa, 458 inclined configuration and reverse mesa were determined to be nearly {1 1 1}A, {1 1 0}
Fig. 1. Schematic drawings of the cross-sectional view of the fabricated sidewall tunnel junction. Ti/Au non-alloyed contact pad has 100 mm in length.
and {1 1 1}B, respectively. Fig. 1 shows the schematic drawings of the cross-sectional of the fabricated sidewall tunnel junction diodes. The impurity concentration and its profile were measured by secondary ion mass spectroscopy analysis (SIMS) with 1 keV–20 nA Csþ beam as a primary ion. Current density–voltage (J–V) characteristics were measured at nominal room temperature (RT) and 77 K.
3. Results Fig. 2 shows the J–V characteristics of GaAs sidewall tunnel junctions on various sidewall orientations. The tunnel junction on normal mesa orientation have shown the record peak current density (Jp) up to 31,000 A/cm2 at RT with the peak-to-valley current ratio (Jp /Jv) of 2.1. Jp of 458 inclined configuration and that of reverse mesa orientation are 5200 and 2100 A/cm2, respectively. Table 1 summarizes the
Fig. 2. J–V characteristics of regrown GaAs tunnel junctions on various sidewall orientations.
T. Ohno et al. / Applied Surface Science 216 (2003) 549–553
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Table 1 Regrown sidewall tunnel junction characteristics
Jp(RT) (A/cm2) Jv(RT) (A/cm2) Jp(RT)/Jv(RT) Jv(77 K) (A/cm2) Jv(RT)/Jv(77 K) Vp(RT) (mV) Vv(RT) (mV) Vp(RT)/Jp(RT) (O cm2) Zero-bias specific resistivity (O cm2) Negative differential conductance (S)
Normal mesa {1 1 1}A
458 inclined configuration {1 1 0}
Reverse mesa {1 1 1}B
31000 14900 2.1 6920 2.1 214 414 6.9 106 5.3 106 1.4 105
5200 3110 1.7 1480 2.1 130 282 2.5 105 1.0 105 7.1 105
2100 1530 1.4 550 2.8 136 252 6.5 105 2.9 105 2.1 104
present tunnel diode characteristics at RT and 77 K. It is shown that the peak, valley current density (Jv) and resultant peak-to-valley ratio have shown strong mesa orientation dependences. Fig. 3 shows Jp, Jv and Jp /Jv as a function of flush desorption time (tFD). Under the present regrowth conditions with the TEGa introduction time of 3 s in
MLE, all junctions on {1 1 1}A sidewall have shown Esaki characteristics at RT. However, junctions on {1 1 1}B and {1 1 0} sidewall have shown ohmiclike characteristics due to the large excess current at RT. Jp/Jv of tunnel junctions on normal mesa shows monotonic increase from 1.8 to 4.2 when tFD becomes longer. It is shown that long AsH3 treatment prior to regrowth improves the tunnel junction characteristics. Jp at tFD ¼ 120 min was twice as small as 30 and 60 min, but these extremely high Jp (highest value is 35,000 A/cm2) are at least 10–20 times higher than that reported previously [8]. Jv shows monotonic decrease for longer tFD. This is due to the reduction of deep level density at the junction interface.
4. Discussions The sidewall tunnel junctions have shown Jp(RT) / Jv(RT) of 1.4–2.1 (Table 1). In heavily donor doped nþGaAs, it has been reported that the donor-defect complex (DX) center [9] is formed, and the excess current is dominated by the leakage current via states. In this case, the Jv is expressed as [10] sffiffiffiffiffiffiffiffi ! p emx Jv ADx exp ½Egap þ 0:35ðxn þ xp Þ
2hq Neff
Fig. 3. (a) Peak-to-valley current ratio and (b) peak and valley current density at RT as a function of tFD. Sidewall orientation is normal mesa.
where the valley voltage is equal to 0:65ðxn þ xp Þ, and Dx is the volume density of occupied levels at energy Ex above the top of the valence band and A is a constant under the assumption of mx ¼ meff . Neff
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T. Ohno et al. / Applied Surface Science 216 (2003) 549–553
and meff are the effective density of states and the tunneling effective mass, which are given by Neff ¼
pn ; pþn
meff ¼
mlh me mlh þ me
Therefore, the Jv is dependent on the doping concentration (n and p). Thus, it is considered that one of the reasons for low Jp /Jv is the very high effective density of states Neff in the present sidewall tunnel junctions. In view of the mechanism for the strong sidewall orientation dependences on Jp, the Be-doping characteristics of the present epitaxial technique are investigated by using various surface orientations at the same epitaxial run under the identical conditions for device epitaxy. Be concentrations on each oriented GaAs is in the order of f1 1 1gA > f1 1 1gB > f1 1 0g. The reason for the highest Jp on normal mesa orientation, which is nearly {1 1 1}A, can be understood partially in view of the Be doping characteristics. However, the reason for Jp f1 1 0g > Jp f1 1 1gB is still not clear. The Jp is affected by many parameters. After Kane [11] and Beji et. al. [12], the peak current density for the direct tunneling is obtained as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 q5 meff Neff V0 Jp ¼ 2 eEgap 36ph 0 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 3 eEgap meff p ADðqVp Þ exp@ V 2h q3 Neff 0 where D is an integral in energy units, qV0 the built-in potential energy, q the electron charge, h the reduced Plank’s constant, Egap the band-gap energy, and e is the dielectric constant. In addition, in case of trap assisted or phonon assisted inelastic tunneling process, the peak current density is affected seriously by many parameters like trap density, occupation probability of traps [13] and deformation potential [14], etc. The orientation dependence on effective tunneling mass [15] is also one of the important parameters, which determines the excess current in the tunnel diodes. Actually, from Table 1, the valley current density ratios Jv(RT) /Jv(77 K) between RT and 77 K are 2.1 for normal mesa and 458 inclined configuration. However, that for reverse mesa, on which Jp and Jp/Jv are lowest, shows larger temperature dependence and to be 2.8. This indicates that the excess current, i.e. the valley current is dominated by the tunneling through
Fig. 4. Be profiles of regrown pþnþ tunnel junction on {1 0 0} surface as a function of AsH3 treatment time.
the midgap levels in the tunnel junction, especially on reverse mesa orientation. In order to investigate the reason for greatly high Jp and the effects of AsH3 treatment on the regrown interface features, SIMS measurements were applied to measure the impurity profile and interface pile-up of regrown interface. Fig. 4 shows the Be profiles of regrown pþnþ tunnel junction on {1 0 0} surface as a function of tFD. It is shown that the long AsH3 treatment enhances the steepness of Be profile, and that the Be interface pile-up increases as the tFD is longer. Be interface pile-up is 9 1019 , 1:2 1020 and 1:5 1020 cm3 for tFD ¼ 30, 60 and 120 min, respectively. These results of improved steepness of Be profile and the enhanced Be interface pile-up concentration at regrown interface are the major reasons for the extremely high Jp of sidewall tunnel junctions. The tFD dependence of Jv change between RT and 77 K (Jv(RT)/Jv(77 K)) were investigated. The value of Jv(RT)/Jv(77 K) is 2.3, 2.6 and 1.6 for tFD ¼ 30, 60 and 120 min, respectively. It is noticed that Jv(RT)/Jv(77 K) is not so change for tFD ¼ 30, 60 min, but AsH3 treatment for 120 min reduced the value of Jv(RT)/ Jv(77 K). Here, Jv(RT)/Jv(77 K) is almost proportional to the temperature dependence of Dx, because n and p in degenerated semiconductor will be almost temperature-independent. Thus, for simplicity, if the monovalent donor-type deep level is formed in the narrow depletion layer of tunnel junction, the temperature dependence of the Dx is expressed as [16] Dx ¼
Nd 1 þ b exp½ðEc Ed fÞ=kT
T. Ohno et al. / Applied Surface Science 216 (2003) 549–553
where Nd is the deep level density, Ec Ed is the activation energy of deep levels, f is the Fermi energy, b is the deep level spin degeneracy and k is the Boltzmann constant. From this expression, it is concluded that the larger Jv(RT)/Jv(77 K) value indicates the deeper activation energy of defect levels. Thus, it is concluded that the sufficient AsH3 treatment with long tFD induces shallower deep levels, which will be followed by the less strained atomic configuration around the defects. As reported previously [17], it has been shown that this reduction of excess current does not come from the removal of surface oxides and/or carbides by AsH3 treatment, but that the controlled surface stoichiometry reduces the non-stoichiometry related defects at the regrown interface. Thus, it is considered that the surface stoichiometry control at the regrown interface reduces the deep levels at the tunnel junction interface, thus the valley current density reduces as the longer AsH3 treatment time.
5. Conclusion Ultra-shallow sidewall GaAs tunnel junctions were fabricated by the low-temperature (290 8C) ASR using the intermittent injection of precursors in an ultra-high vacuum. The tunnel junctions on the normal mesa orientation have shown the record peak current density up to 35,000 A/cm2 at 100 mm long strip structure. High concentration Be pile-up at the regrown interface is one of the possible reasons for extremely high peak current density under the assumption that the interface pile-up phenomenon is similar for all sidewall orientations. Tunnel junction characteristics have been improved by AsH3 surface treatment just prior to regrowth due to the improved steepness in Be doping profile, interface pile-up and reduction of stoichiometry-dependent deep states at the regrown interface. Crystallographic orientation dependence of Be doping characteristics and the
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midgap states in the tunnel junctions are the important factor for the high quality sidewall tunnel junction characteristics. References [1] J. Nishizawa, Y. Watanabe, Sci. Rep. Res. Inst. Tohoku Univ. 10 (1958) 91; K. Motoya, J. Nishizawa, Topics in Millimeter Wave Technology, vol. 2, Academic Press, New York, 1988, p. 1 (Chapter 1). [2] J. Nishizawa, H. Makabe, F. Matsumoto, P. Plotka, T. Kurabayashi, Electron. Lett. 38 (2002) 660. [3] Y. Oyama, T. Ohno, K. Tezuka, K. Suto, J. Nishizawa, Appl. Phys. Lett. 81 (2002) 2563. [4] J. Nishizawa, Y. Kokubun, Extended Abstracts of 16th Conference on Solid State Devices and Materials, Kobe, Japan, 1984, p. 1; P. Plotka, T. Kurabayashi, Y. Oyama, J. Nishizawa, Appl. Surf. Sci. 82–83 (1994) 91. [5] Y. Oyama, J. Nishizawa, K. Seo, K. Suto, J. Cryst. Growth 212 (2000) 402. [6] Y. Oyama, K. Tezuka, K. Suto, J. Nishizawa, J. Cryst. Growth 246 (2002) 15. [7] T. Ohno, Y. Oyama, K. Suto, J. Nishizawa, submitted for publication. [8] S. Ahmed, M.R. Melloch, E.S. Harmon, D.T. McInturff, J.M. Woodall, Appl. Phys. Lett. 71 (1997) 3667. [9] E. Munoz, E. Calleja, in: E. Munoz (Ed.), DX Centers-donors in AlGaAs and Related Compounds, vol. 1089, Scitec, Switzerland, 1994, p. 135. [10] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley/Interscience, New York, 1981. [11] E.O. Kane, J. Appl. Phys. 32 (1961) 83. [12] L. Beji, B. el Jani, P. Gilbart, J.C. Portal, P. Basmaji, J. Appl. Phys. 83 (1998) 5573. [13] F. Jimenez-Molinos, A. Palma, F. Gamiz, J. Banqueri, J.A. Lopez-Villanueva, J. Appl. Phys. 90 (2001) 3396. [14] C. Rivas, R. Lake, G. Klimech, W.R. Frensley, M.V. Fischetti, P.E. Thompson, S.L. Rommel, P. Berger, Appl. Phys. Lett. 78 (2001) 814. [15] L.F. Luo, R. Beresford, W.I. Wang, E.E. Mendez, Appl. Phys. Lett. 54 (1989) 2133. [16] J.S. Blakemore, Semiconductor Statistics, Pergamon Press, Oxford, 1962. [17] J. Nishizawa, Y. Oyama, P. Plotka, H. Sakuraba, Surf. Sci. 348 (1996) 105.