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Metallic p-type GaAs and InGaAs grown by MOMBE
Journal of Crystal Growth 105 (1990) 359—365 North-Holland
359
METALLIC p-TYPE GaAs AND InGaAs GROWN BY MOMBE Makoto KONAGAI, Takumi YAMADA, Takeshi AKATSUKA, Shinji NOZAKI, Ryuji MIYAKE, Koki SAITO, Taichi FUKAMACHI, Eisuke TOKUMITSU and Kiyoshi TAKAHASHI Department of Electrical and Electronic Engineering Tokyo Institute of Technology, 2-12-I, Ohokayama, Meguro-ku, Tokyo 152, Japan
Heavily carbon-doped p-GaAs layers with a hole Concentration of 1.5 X 1021 _3.4 X 1018 cm were grown by metalorganic molecular beam epitaxy (MOMBE). The carrier concentration agrees well with the carbon concentration measured by SIMS, which suggests 100% of electrical activation of the incorporated carbon as an acceptor. The p—n diodes with carbon-doped p-GaAs show good rectification. The lattice constant of GaAs decreases with increasing carbon concentration. The carbon-doped InGaAs with a 3, lattice-matched with a GaAs substrate, was obtained for the first time. The effective bandgap hole concentration of 2.6 x 1020 cm narrowing in heavily doped GaAs was also studied by photoluminescence. The measured bandgap with a hole concentration of 1020 cm3 is about 100 meV lower than that of intrinsic GaAs. Finally, the static and high-frequency characteristics of heterojunction bipolar transistors with carbon-doped p-GaAs were calculated.
1. Introduction In conventional molecular beam epitaxy (MBE), beryllium (Be) is commonly used as a p-type dopant. The maximum carrier concentration obtamed by MBE using Be is 2 X 1020 and 5 x 1020 cm3 for GaAs [1] and InGaAs [2], respectively, However, thermal diffusion and interstitial incorporation of Be have become serious problems, especially at high doping levels. The beryllium atoms incorporated into MBE GaAs films are substitutional as an acceptor for the doping lower than 5 x iO’~cm~3and interstitial for higher doping. The interstitially incorporated beryllium degrades the film quality [3]. To solve these problems, use of carbon (C) as a new p-type dopant has been investigated [4—6].In this work, metalorganic molecular beam epitaxy (MOMBE) was used to grow C-doped GaAs. Since C is an amphoteric impurity, the site distribution ratio is determined by the surface stoichiometry during growth in the conventional MBE growth. However, in the MOMBE growth using trimethylgallium and As 4, incomplete dissociation of Ga alkyl molecules promotes carbon incorporation, and C is preferentially incorporated as an acceptor on As sites even at high doping levels. The properties of C as an acceptor are much more suitable than 0022-0248/90/$03.50 © 1990
—
those of Be for device applications such as heterojunction bipolar transistors (HBTs), which require very thin p-GaAs layers of ultra low resistivity. In this paper, first, we present the characterization results of C-doped metaffic p-GaAs grown by MOMBE. Next, p—n junctions and heavily Cdoped InGaAs lattice-matched with the GaAs substrate are described. Finally, theoretical analysis of heavy doping effects on the current gain and high-frequency performance of AlGaAs/ GaAs HBTs are presented.
2. Experiment Two MOMBE systems were used. One is a home-made system to grow heavily C-doped pGaAs, which has been already described in detail [6], and the other is V-80H, made by VG Semicon (fig. 1) which was used to prepare p—n junctions and InGaAs layers. In the MOMBE growth of GaAs and InGaAs, trimethylgallium (TMG), elemental indium and elemental arsenic were used as sources for Ga, In and As4, respectively. A mixture of elemental gallium and TMG was also used as a Ga source for p-GaAs with hole concentrations of less than 1020 cm Although in the conventional MOMBE growth of GaAs, hydrogen
Elsevier Science Publishers B.V. (North-Holland)
~.
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M. Konagai et 01.
/ Metallic p-type GaAs and InGaAs grown by MOMBE
is widely used as a carrier gas, helium carrier gas was used in our experiments, because hydrogen reacts with alkyls and reduces the carbon incorporation, as we have previously reported [7]. The hole concentration is found to weakly depend on V/Ill ratio. The hole concentration slightly increases with decreasing V/Ill ratio. The V/Ill ratio was set to less than 1 for low growth temperature (450°C)to obtain specular surfaces [6]. p—n Junctions were formed by growing 0.5 ~sm thick C-doped p-GaAs on 1 j~mSi-doped MBEgrown GaAs by MOMBE. The electron concentration of the n-GaAs was fixed to 1017 cm3, and the hole concentration of the p-GaAs was varied from 3 x i0’~to 1 x 1020 cm3. The growth ternperature of all layers was 580°C.Diodes with area of 9 )< 10 cm2 were fabricated by standard photolithography. Ohmic contacts to p- and nGaAs layers were formed by alloying Au—1%Be at 450 °C for 5 mm and Au—7.4%Ge/Ni/Au at 350°C for 3 mm, respectively. Forward and reverse I—V characteristics of the fabricated diodes were measured with an HP4145, at 77 and 300 K.
3. Properties of metallic p-GaAs 3.1. Hole concentration and mobility
Fig. 2 shows the substrate temperature dependence of the hole concentration for (100) and (111)B GaAs substrates. It shows a strong dependence of the hole concentration on growth temperature. The hole concentration exponentially increases with increasing the inverse of growth ternperature. This tendency is well explained by the carbon incorporation mechanism proposed by us [6]. The sample grown on (100) GaAs at 450°C has a maximum hole concentration of 1.5 >< 1021 cm3. The hole mobility is 22 cm2/V s. corresponding to a resistivity of 1.9x104 ~2 cm. This is the lowest resistivity ever reported for p-GaAs epitaxial layers. The holes in the heavily C-doped p-GaAs do not freeze out even at 4.2 K, and its resistivity monotonically decreases with decreasing temperature. Since the temperature dependence of the resistivity is similar to that of metal resistivity, we call it metallic p-GaAs. .
Deposition chamber
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__________
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M. Konagai et a!.
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concentration dependence of the lattice constant of C-doped GaAs epilayers.
-
Such a lattice mismatch may generate misfit or
-
threading dislocations and degrade the crystallinity of p-GaAs. In order to reduce the lattice mismatch at high doping levels, we grew C-doped InGaAs layers, which are discussed later.
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700 600 500 400 Substrate temperature (°C) Fig. 2. Substrate temperature dependence of the hole concentration.
3.2. SIMS analysis
The carbon concentration of the p-GaAs with a hole concentration of 1 x 1021 cm was measured by SIMS. It agrees well with the hole concentration, which suggests that 100% of the incorporated carbon atoms act as an acceptor. This result also supports that carbon is an ideal p-type dopant for GaAs.
3.4. Bandgap narrowing Photoluminescence (PL) spectra of C-doped GaAs were measured at 4.2 K. The effective bandgap narrowing was estimated using the spectra in the same way as discussed in ref. [8] and the results are plotted in fig. 4. The measured bandgap with a hole concentration of 1020 cm ~ is about 100 meV lower than that of intrinsic GaAs. The effective bandgap narrowing is caused by heavy doping. We calculated the effective band~10’ a E
___________________________ ~ I p—GaAs
3.3. Lattice constant S
A double-crystal X-ray diffraction study of Cdoped GaAs was carried out to investigate the variation of the lattice constant at the various doping levels. Fig. 3 shows the hole concentration dependence of the lattice constant. The lattice constant decreases with increasing the carrier concentration, showing good agreement with the values calculated from Vegard’s law in the GaAs—C alloy system (solid line). The lattice mismatch between the GaAs layer with a hole concentration of 1 x 1021 cm3 and GaAs substrate is 0.5%.
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362
M. Konagai ci al.
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Metallic p-type GaAs and InGaAs grown by MOMBE
gap narrowing as a function of an impurity concentration, using the functions of the density of states resulting from the work of Kane [9] and Morgan [10]. The screening length is a critical parameter in the calculations, and we adopted the screening length function in ref. [11]. Fig. 5 shows the values of the density of states as a function of the impurity concentration. The impurity band and band tailing shown in fig. 5 result in the effective bandgap narrowing. The solid line in fig. 4 is the theoretically calculated hole-concentration dependence of the effective bandgap narrowing. The theoretically calculated values of the effective bandgap narrowing show good agreement with the values determined from the PL measurements.
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4. C-doped p-InGaAs lattice-matched to a GaAs substrate The lattice constant of p-GaAs decreases with increasing the carrier concentration as discussed earlier. In order to match the lattice constant of the heavily C-doped layer with that of GaAs substrate, we have grown heavily C-doped InGaAs. Fig. 6 shows the dependence of the lattice constant and the hole concentration of the C-doped p-InGaAs on the indium flux. Heavily C-doped InGaAs with a hole concentration of 2.6 x 1020 cm3 and resistivity of 6.5 x i0~ ~2 cm, lattice-
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Fig. 6. The hole concentration and the lattice constants as a function of the indium flux.
matched with the GaAs substrate, was successfully obtained. The carrier concentration, however, decreases with increasing indium content. The carbon concentration, decreasing with increasing indium content, found in preliminary SIMS results, suggests that the decrease of the hole concentration may not be due to increased compensation, but to decreased carbon incorporation in InGaAs. Further verification is under way. 5. p—n Diodes with C-doped p-GaAs
GaAs:C 300K
A cross-section of theis fabricated with C-doped p-GaAs shown in p—n fig. 7.diodes The current at 0.8 V under forward bias is by five
p V
20
orders of magnitude larger than that under reverse bias, which clearly shows rectification, as seen in fig. 8. The forward I— V characteristics do not
3
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depend on the hole concentration of p-GaAs and show the ideality factor of 1.8, which is attributed to surface leakage at the diode perimeter rather
0.2
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than the recombination current in the depletion region caused by the lattice mismatch between
Fig. 5. Calculated density of states as a function of the carrier concentration,
nGaAs and p-GaAs. The reverse breakdown voltage decreases from 18 to 16 V with increasing the hole concentration from 3 x 1011 to 1 x 1020 cm
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M. Konagai et al. Device area
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Metallicp-type GaAs and InGaAs grown by MOMBE
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Fig. 7. Cross-section of the fabricated p—n diodes with C-doped p-GaAs Layer
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Since the reverse breakdown voltage becomes lower at 77 K for all the hole concentrations from 3 x 1018 to 1 >< 1020 cm the breakdown is considered to be caused by avalanche multiplication of carriers due to high electric field within the depletion region rather than by tunneling of carriers. In the doping range of p-GaAs studied for the p—n diodes, there seems to be no significant de~,
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tics of an AlGaAs/GaAs HBT with a base doping of 1021 cm3 were calculated [12]. The transport of electrons in p-GaAs was estimated using a Monte Carlo method, taking account of the electron—hole, hole—plasmon and screened LO-phonon scatterings. The drift velocity of the electrons injected into the heavily doped p-GaAs as a function of the electric field is plotted in fig. 9. For hole concentrations between 1019 and 1020 cm ~ negative differential resistance is not observed at electric fields up to 10 kY/cm because of the hole—plasmon scattering. On the other hand, for a holeenergy concentration 1 x 1021 the plasmon becomes of larger than cm3, the F—L valley separation of 0.33 eV, and thus, even at lower electric fields, most electrons transfer to the 2 I
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6. Predictions of HBT performances with metallic p-GaAs base We have already reported that C is a more appropriate dopant than Be for thin heavily doped p-GaAs layers, particularly for the base layer of HBTs. The static and high-frequency characteris-
-
/
10
Fig. 8.
lattice mismatch between p- and n-GaAs. However, further increase of the hole concentration in p-GaAs of p—n diodesDiodes is expected degrademore the I—V characteristics. withtomuch
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gradation of the I—V characteristics due to the
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characteristics of the p—n diode with a hole concentration of 1020 cm3 in the p-GaAs.
10
Electric Field (kV/ cm)
Fig. 9. Drift velocity of minority electrons as a function of the electric field in p-type GaAs.
364
/ Metallic p-type
M. Konagai ci a!.
cantly and decreases the current gain. In order to obtain a current gain of 10. it is necessary to reduce the base width to less than 300 A. Moreover, the cutoff frequencies of HBTs with the base doping of 1021 cm3 were estimated. The base transit time was calculated by Monte Carlo simulation, and for other delay times we used the
Table 1 Simulated HBT structure Layer
Thickness (A)
Type
Doping 3) (cm
AlAs fraction
Emitter
1200/300
n
5x1017
0.3/0.3—0
Base Collector Buffer
WB 5000 1500
p n n
1 x1021 5 X 10~ 5x1017
0 0 0
GaAs and InGaAs grown by MOMBE
_______________________________________________
parameters reported in ref. [2]. By reducing the base width to 200—300 A. fT 100 GHz and frnax 300 GHz are expected. =
upper valleys before they lose energy by the hole—plasmon scattering. The electron mobility is estimated to be 300 cm2/V. s for a hole concentration of 1 x 1021 cm3. Based on the calculated electron mobilities in p-GaAs, the static characteristics of AlGaAs/ GaAs HBTs with a base doping of 1021 cm3 were estimated, including the effects of bandgap narrowing. In addition to the Shockley—Read—Hall recombination, the Auger recombination was also taken into account. The calculation leads to an electron lifetime of 9.9 ps in the metallic p-type GaAs with a hole concentration of 1021 cm3. The HBT structure parameters used in the simulation are listed in table 1. Fig. 10 shows the common-emitter current gain versus collector current density for various base widths WB. Consideration of the effects of the bandgap narrowing in the base increases the current gain by a factor of 10 at this doping level. However, the Auger recombination increases the base current signifi-
=
7. Conclusions We have successfully grown metallic p-type GaAs by MOMBE. The maximum hole concentralion we achieved up to now is 1.5 x 1021 cm Even at a heavy doping level over 1021 cm ~ the electrical activation of the incorporated carbon as an acceptor is about 100%. The p—n diodes with C-doped p-GaAs show good rectification. However, a decrease in the lattice constant of GaAs with increasing hole concentration causes a lattice-mismatch problem. In order to solve the problem, we have also grown heavily C-doped InGaAs, lattice-matched with a GaAs substrate. for the first time. The bandgap narrowing effect due to heavy doping was also studied. Finally, we numerically simulated static and high-frequency characteristics of AlGaAs/GaAs HBTs with Cdoped metallic p-GaAs base layers. By reducing the base width to 200—300 A, fmax 300 GHz is expected. =
_______________________________________ 7C 60
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References
Wa 1 OOA 50
[1] J.L. Lievin and F. Alexandre, Electron. Letters 21(1985)
C
413. [2] R.A. Hanmi, M.B. Panish, RN. Nottenburg, Y.K. Chen
4~ a -
and D.R. Humphrey, AppI. Phys. Letters 54 (1989) 2586. [3] Y.C. Pao, T. Hierl and T. Cooper, J. AppI. Phys. 60 (1986)
30
0
200 20
~
_.__
300 10
(3 i02
~_.
I
I
1
i02 1O4 Collector current (A/cm0
I
10~
Fig. 10. Common-emitter current gain versus collector current density with the base width WB as a parameter.
201. [4] T. Yamada, E. Tokumitsu, K. Saito, T. Akatsuka, M. Miyauchi. M. Konagai and K. Takahashi, J. Crystal Growth 95 (1989) 145. [5] K. Saito, E. Tokumitsu. T. Akatsuka, M. Miyauchi, T. Yamada, M. Konagai and K. Takahashi, in: Proc. 15th Intern. Symp. on GaAs and Related Compounds, Atlanta, GA, 1988, Inst. Phys. Conf. Ser. 96, Ed. J.S. Harris (Inst. Phys., London—Bristol, 1989) P. 69.
M. Konagai et a!. / Metallicp-type GaAs and InGaAs grown by MOMBE [6] M. Konagai, T, Yamada, T. Akatsuka, K. Saito, E, Tokumitsu and K. Talcahashi, J. Crystal Growth 98 (1989) 167. [7] E. Tokumitsu, Y. Kudou, M. Konagai and K. Takahashi, Japan. J. AppI. Phys. 24 (1985) 292. [8] N.A. Titkov, E.I. Chaikina, EM. Komova and N.G. F.rmakova, Soviet Phys..Semicond. 15 (1981) 198.
365
[91E.O. Kane, Phys. Rev. 131 (1963) 79. [101 TN. Morgan, Phys. Rev. 139 (1965) A343. [111 J.W. Slotboom, Solid-State Electron. 20 (1977) 279. [121 K. Saito, T. Yamada, T. Akatsuka, T. Fukamachi, E. Tokumitsu, M. Konagai and K. Takahashi, Japan. J. Appl. Phys. 28 (1989) L2081.
359
METALLIC p-TYPE GaAs AND InGaAs GROWN BY MOMBE Makoto KONAGAI, Takumi YAMADA, Takeshi AKATSUKA, Shinji NOZAKI, Ryuji MIYAKE, Koki SAITO, Taichi FUKAMACHI, Eisuke TOKUMITSU and Kiyoshi TAKAHASHI Department of Electrical and Electronic Engineering Tokyo Institute of Technology, 2-12-I, Ohokayama, Meguro-ku, Tokyo 152, Japan
Heavily carbon-doped p-GaAs layers with a hole Concentration of 1.5 X 1021 _3.4 X 1018 cm were grown by metalorganic molecular beam epitaxy (MOMBE). The carrier concentration agrees well with the carbon concentration measured by SIMS, which suggests 100% of electrical activation of the incorporated carbon as an acceptor. The p—n diodes with carbon-doped p-GaAs show good rectification. The lattice constant of GaAs decreases with increasing carbon concentration. The carbon-doped InGaAs with a 3, lattice-matched with a GaAs substrate, was obtained for the first time. The effective bandgap hole concentration of 2.6 x 1020 cm narrowing in heavily doped GaAs was also studied by photoluminescence. The measured bandgap with a hole concentration of 1020 cm3 is about 100 meV lower than that of intrinsic GaAs. Finally, the static and high-frequency characteristics of heterojunction bipolar transistors with carbon-doped p-GaAs were calculated.
1. Introduction In conventional molecular beam epitaxy (MBE), beryllium (Be) is commonly used as a p-type dopant. The maximum carrier concentration obtamed by MBE using Be is 2 X 1020 and 5 x 1020 cm3 for GaAs [1] and InGaAs [2], respectively, However, thermal diffusion and interstitial incorporation of Be have become serious problems, especially at high doping levels. The beryllium atoms incorporated into MBE GaAs films are substitutional as an acceptor for the doping lower than 5 x iO’~cm~3and interstitial for higher doping. The interstitially incorporated beryllium degrades the film quality [3]. To solve these problems, use of carbon (C) as a new p-type dopant has been investigated [4—6].In this work, metalorganic molecular beam epitaxy (MOMBE) was used to grow C-doped GaAs. Since C is an amphoteric impurity, the site distribution ratio is determined by the surface stoichiometry during growth in the conventional MBE growth. However, in the MOMBE growth using trimethylgallium and As 4, incomplete dissociation of Ga alkyl molecules promotes carbon incorporation, and C is preferentially incorporated as an acceptor on As sites even at high doping levels. The properties of C as an acceptor are much more suitable than 0022-0248/90/$03.50 © 1990
—
those of Be for device applications such as heterojunction bipolar transistors (HBTs), which require very thin p-GaAs layers of ultra low resistivity. In this paper, first, we present the characterization results of C-doped metaffic p-GaAs grown by MOMBE. Next, p—n junctions and heavily Cdoped InGaAs lattice-matched with the GaAs substrate are described. Finally, theoretical analysis of heavy doping effects on the current gain and high-frequency performance of AlGaAs/ GaAs HBTs are presented.
2. Experiment Two MOMBE systems were used. One is a home-made system to grow heavily C-doped pGaAs, which has been already described in detail [6], and the other is V-80H, made by VG Semicon (fig. 1) which was used to prepare p—n junctions and InGaAs layers. In the MOMBE growth of GaAs and InGaAs, trimethylgallium (TMG), elemental indium and elemental arsenic were used as sources for Ga, In and As4, respectively. A mixture of elemental gallium and TMG was also used as a Ga source for p-GaAs with hole concentrations of less than 1020 cm Although in the conventional MOMBE growth of GaAs, hydrogen
Elsevier Science Publishers B.V. (North-Holland)
~.
360
M. Konagai et 01.
/ Metallic p-type GaAs and InGaAs grown by MOMBE
is widely used as a carrier gas, helium carrier gas was used in our experiments, because hydrogen reacts with alkyls and reduces the carbon incorporation, as we have previously reported [7]. The hole concentration is found to weakly depend on V/Ill ratio. The hole concentration slightly increases with decreasing V/Ill ratio. The V/Ill ratio was set to less than 1 for low growth temperature (450°C)to obtain specular surfaces [6]. p—n Junctions were formed by growing 0.5 ~sm thick C-doped p-GaAs on 1 j~mSi-doped MBEgrown GaAs by MOMBE. The electron concentration of the n-GaAs was fixed to 1017 cm3, and the hole concentration of the p-GaAs was varied from 3 x i0’~to 1 x 1020 cm3. The growth ternperature of all layers was 580°C.Diodes with area of 9 )< 10 cm2 were fabricated by standard photolithography. Ohmic contacts to p- and nGaAs layers were formed by alloying Au—1%Be at 450 °C for 5 mm and Au—7.4%Ge/Ni/Au at 350°C for 3 mm, respectively. Forward and reverse I—V characteristics of the fabricated diodes were measured with an HP4145, at 77 and 300 K.
3. Properties of metallic p-GaAs 3.1. Hole concentration and mobility
Fig. 2 shows the substrate temperature dependence of the hole concentration for (100) and (111)B GaAs substrates. It shows a strong dependence of the hole concentration on growth temperature. The hole concentration exponentially increases with increasing the inverse of growth ternperature. This tendency is well explained by the carbon incorporation mechanism proposed by us [6]. The sample grown on (100) GaAs at 450°C has a maximum hole concentration of 1.5 >< 1021 cm3. The hole mobility is 22 cm2/V s. corresponding to a resistivity of 1.9x104 ~2 cm. This is the lowest resistivity ever reported for p-GaAs epitaxial layers. The holes in the heavily C-doped p-GaAs do not freeze out even at 4.2 K, and its resistivity monotonically decreases with decreasing temperature. Since the temperature dependence of the resistivity is similar to that of metal resistivity, we call it metallic p-GaAs. .
Deposition chamber
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__________
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-
Such a lattice mismatch may generate misfit or
-
threading dislocations and degrade the crystallinity of p-GaAs. In order to reduce the lattice mismatch at high doping levels, we grew C-doped InGaAs layers, which are discussed later.
0
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700 600 500 400 Substrate temperature (°C) Fig. 2. Substrate temperature dependence of the hole concentration.
3.2. SIMS analysis
The carbon concentration of the p-GaAs with a hole concentration of 1 x 1021 cm was measured by SIMS. It agrees well with the hole concentration, which suggests that 100% of the incorporated carbon atoms act as an acceptor. This result also supports that carbon is an ideal p-type dopant for GaAs.
3.4. Bandgap narrowing Photoluminescence (PL) spectra of C-doped GaAs were measured at 4.2 K. The effective bandgap narrowing was estimated using the spectra in the same way as discussed in ref. [8] and the results are plotted in fig. 4. The measured bandgap with a hole concentration of 1020 cm ~ is about 100 meV lower than that of intrinsic GaAs. The effective bandgap narrowing is caused by heavy doping. We calculated the effective band~10’ a E
___________________________ ~ I p—GaAs
3.3. Lattice constant S
A double-crystal X-ray diffraction study of Cdoped GaAs was carried out to investigate the variation of the lattice constant at the various doping levels. Fig. 3 shows the hole concentration dependence of the lattice constant. The lattice constant decreases with increasing the carrier concentration, showing good agreement with the values calculated from Vegard’s law in the GaAs—C alloy system (solid line). The lattice mismatch between the GaAs layer with a hole concentration of 1 x 1021 cm3 and GaAs substrate is 0.5%.
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362
M. Konagai ci al.
/
Metallic p-type GaAs and InGaAs grown by MOMBE
gap narrowing as a function of an impurity concentration, using the functions of the density of states resulting from the work of Kane [9] and Morgan [10]. The screening length is a critical parameter in the calculations, and we adopted the screening length function in ref. [11]. Fig. 5 shows the values of the density of states as a function of the impurity concentration. The impurity band and band tailing shown in fig. 5 result in the effective bandgap narrowing. The solid line in fig. 4 is the theoretically calculated hole-concentration dependence of the effective bandgap narrowing. The theoretically calculated values of the effective bandgap narrowing show good agreement with the values determined from the PL measurements.
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Indium flux
4. C-doped p-InGaAs lattice-matched to a GaAs substrate The lattice constant of p-GaAs decreases with increasing the carrier concentration as discussed earlier. In order to match the lattice constant of the heavily C-doped layer with that of GaAs substrate, we have grown heavily C-doped InGaAs. Fig. 6 shows the dependence of the lattice constant and the hole concentration of the C-doped p-InGaAs on the indium flux. Heavily C-doped InGaAs with a hole concentration of 2.6 x 1020 cm3 and resistivity of 6.5 x i0~ ~2 cm, lattice-
00%
0
.
4 8mbar)
(xlO
Fig. 6. The hole concentration and the lattice constants as a function of the indium flux.
matched with the GaAs substrate, was successfully obtained. The carrier concentration, however, decreases with increasing indium content. The carbon concentration, decreasing with increasing indium content, found in preliminary SIMS results, suggests that the decrease of the hole concentration may not be due to increased compensation, but to decreased carbon incorporation in InGaAs. Further verification is under way. 5. p—n Diodes with C-doped p-GaAs
GaAs:C 300K
A cross-section of theis fabricated with C-doped p-GaAs shown in p—n fig. 7.diodes The current at 0.8 V under forward bias is by five
p V
20
orders of magnitude larger than that under reverse bias, which clearly shows rectification, as seen in fig. 8. The forward I— V characteristics do not
3
N~=10
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10
depend on the hole concentration of p-GaAs and show the ideality factor of 1.8, which is attributed to surface leakage at the diode perimeter rather
0.2
1.0
than the recombination current in the depletion region caused by the lattice mismatch between
Fig. 5. Calculated density of states as a function of the carrier concentration,
nGaAs and p-GaAs. The reverse breakdown voltage decreases from 18 to 16 V with increasing the hole concentration from 3 x 1011 to 1 x 1020 cm
1O~-
-. -
io~
1010
- -
- -
-
0.8
0.6
0.4
0
0.2
0.4
0.6
0.8
Energy (eV)
M. Konagai et al. Device area
/
3OIjmX3Oiim
/
-- /
Si0 -..
2
-
Metallicp-type GaAs and InGaAs grown by MOMBE
/
,
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-.
heavily C-doped GaAs are under study.
Fig. 7. Cross-section of the fabricated p—n diodes with C-doped p-GaAs Layer
Carrier concentration (cmi) 20—3X1O18
Thickness (gum) 0.5
ptGaAs(C..doped) i-GaAs (undoped) n-GaAs (Si-doped) n~-GaAs(Si-doped)
2X1O -
0.04
8 X 1016 3x10’8
1.0 1.0
Since the reverse breakdown voltage becomes lower at 77 K for all the hole concentrations from 3 x 1018 to 1 >< 1020 cm the breakdown is considered to be caused by avalanche multiplication of carriers due to high electric field within the depletion region rather than by tunneling of carriers. In the doping range of p-GaAs studied for the p—n diodes, there seems to be no significant de~,
1
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-
psr2XlO20cm
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tics of an AlGaAs/GaAs HBT with a base doping of 1021 cm3 were calculated [12]. The transport of electrons in p-GaAs was estimated using a Monte Carlo method, taking account of the electron—hole, hole—plasmon and screened LO-phonon scatterings. The drift velocity of the electrons injected into the heavily doped p-GaAs as a function of the electric field is plotted in fig. 9. For hole concentrations between 1019 and 1020 cm ~ negative differential resistance is not observed at electric fields up to 10 kY/cm because of the hole—plasmon scattering. On the other hand, for a holeenergy concentration 1 x 1021 the plasmon becomes of larger than cm3, the F—L valley separation of 0.33 eV, and thus, even at lower electric fields, most electrons transfer to the 2 I
I
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/ FORWARD
-
-
p-GaAs K
NA=1017cm3
tRSE
C~~TO2~
Voltage (V)
I—V
6. Predictions of HBT performances with metallic p-GaAs base We have already reported that C is a more appropriate dopant than Be for thin heavily doped p-GaAs layers, particularly for the base layer of HBTs. The static and high-frequency characteris-
-
/
10
Fig. 8.
lattice mismatch between p- and n-GaAs. However, further increase of the hole concentration in p-GaAs of p—n diodesDiodes is expected degrademore the I—V characteristics. withtomuch
...-.
SI sub
10
363
gradation of the I—V characteristics due to the
AuGe/Nt/Au
P’~ -:
/
characteristics of the p—n diode with a hole concentration of 1020 cm3 in the p-GaAs.
10
Electric Field (kV/ cm)
Fig. 9. Drift velocity of minority electrons as a function of the electric field in p-type GaAs.
364
/ Metallic p-type
M. Konagai ci a!.
cantly and decreases the current gain. In order to obtain a current gain of 10. it is necessary to reduce the base width to less than 300 A. Moreover, the cutoff frequencies of HBTs with the base doping of 1021 cm3 were estimated. The base transit time was calculated by Monte Carlo simulation, and for other delay times we used the
Table 1 Simulated HBT structure Layer
Thickness (A)
Type
Doping 3) (cm
AlAs fraction
Emitter
1200/300
n
5x1017
0.3/0.3—0
Base Collector Buffer
WB 5000 1500
p n n
1 x1021 5 X 10~ 5x1017
0 0 0
GaAs and InGaAs grown by MOMBE
_______________________________________________
parameters reported in ref. [2]. By reducing the base width to 200—300 A. fT 100 GHz and frnax 300 GHz are expected. =
upper valleys before they lose energy by the hole—plasmon scattering. The electron mobility is estimated to be 300 cm2/V. s for a hole concentration of 1 x 1021 cm3. Based on the calculated electron mobilities in p-GaAs, the static characteristics of AlGaAs/ GaAs HBTs with a base doping of 1021 cm3 were estimated, including the effects of bandgap narrowing. In addition to the Shockley—Read—Hall recombination, the Auger recombination was also taken into account. The calculation leads to an electron lifetime of 9.9 ps in the metallic p-type GaAs with a hole concentration of 1021 cm3. The HBT structure parameters used in the simulation are listed in table 1. Fig. 10 shows the common-emitter current gain versus collector current density for various base widths WB. Consideration of the effects of the bandgap narrowing in the base increases the current gain by a factor of 10 at this doping level. However, the Auger recombination increases the base current signifi-
=
7. Conclusions We have successfully grown metallic p-type GaAs by MOMBE. The maximum hole concentralion we achieved up to now is 1.5 x 1021 cm Even at a heavy doping level over 1021 cm ~ the electrical activation of the incorporated carbon as an acceptor is about 100%. The p—n diodes with C-doped p-GaAs show good rectification. However, a decrease in the lattice constant of GaAs with increasing hole concentration causes a lattice-mismatch problem. In order to solve the problem, we have also grown heavily C-doped InGaAs, lattice-matched with a GaAs substrate. for the first time. The bandgap narrowing effect due to heavy doping was also studied. Finally, we numerically simulated static and high-frequency characteristics of AlGaAs/GaAs HBTs with Cdoped metallic p-GaAs base layers. By reducing the base width to 200—300 A, fmax 300 GHz is expected. =
_______________________________________ 7C 60
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References
Wa 1 OOA 50
[1] J.L. Lievin and F. Alexandre, Electron. Letters 21(1985)
C
413. [2] R.A. Hanmi, M.B. Panish, RN. Nottenburg, Y.K. Chen
4~ a -
and D.R. Humphrey, AppI. Phys. Letters 54 (1989) 2586. [3] Y.C. Pao, T. Hierl and T. Cooper, J. AppI. Phys. 60 (1986)
30
0
200 20
~
_.__
300 10
(3 i02
~_.
I
I
1
i02 1O4 Collector current (A/cm0
I
10~
Fig. 10. Common-emitter current gain versus collector current density with the base width WB as a parameter.
201. [4] T. Yamada, E. Tokumitsu, K. Saito, T. Akatsuka, M. Miyauchi. M. Konagai and K. Takahashi, J. Crystal Growth 95 (1989) 145. [5] K. Saito, E. Tokumitsu. T. Akatsuka, M. Miyauchi, T. Yamada, M. Konagai and K. Takahashi, in: Proc. 15th Intern. Symp. on GaAs and Related Compounds, Atlanta, GA, 1988, Inst. Phys. Conf. Ser. 96, Ed. J.S. Harris (Inst. Phys., London—Bristol, 1989) P. 69.
M. Konagai et a!. / Metallicp-type GaAs and InGaAs grown by MOMBE [6] M. Konagai, T, Yamada, T. Akatsuka, K. Saito, E, Tokumitsu and K. Talcahashi, J. Crystal Growth 98 (1989) 167. [7] E. Tokumitsu, Y. Kudou, M. Konagai and K. Takahashi, Japan. J. AppI. Phys. 24 (1985) 292. [8] N.A. Titkov, E.I. Chaikina, EM. Komova and N.G. F.rmakova, Soviet Phys..Semicond. 15 (1981) 198.
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[91E.O. Kane, Phys. Rev. 131 (1963) 79. [101 TN. Morgan, Phys. Rev. 139 (1965) A343. [111 J.W. Slotboom, Solid-State Electron. 20 (1977) 279. [121 K. Saito, T. Yamada, T. Akatsuka, T. Fukamachi, E. Tokumitsu, M. Konagai and K. Takahashi, Japan. J. Appl. Phys. 28 (1989) L2081.