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GaAs pseudomorphic HEMT structures grown by molecular beam epitaxy and hydrogen treatment
,. . . . . . . .
C R Y S T A L G R O W T H
ELSEVIER
Journal of Crystal Growth 169 (1996) 637-642
Characterization of deep centers in A1GaAs/InGaAs/GaAs pseudomorphic HEMT structures grown by molecular beam epitaxy and hydrogen treatment Liwu Lu
a,*,l, Songlin Feng a, Jiben Liang a, Zhanguo Wang a, j. Wang b, Y. Wang b, Weikun Ge b
Notional Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People's Republic of China b Department of Physics, The Hong Kong Universi~ of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 2 June 1996
Abstract
In AlGaAs/InGaAs/GaAs PM-HEMT structures, the characterization of deep centers, the degradation in electrical and optical properties and their effects on electrical performance of the PM-HEMTs have been investigated by DLTS, SIMS, PL and conventional van der Pauw techniques. The experimental results confirm that the deep level centers correlate strongly with the oxygen content in the A1GaAs layer, the PL response of PM-HEMTs, and the electrical performance of the PM-HEMTs. Hydrogen plasma treatment was used to passivate/annihilate these centers, and the effects of hydrogenation were examined.
1. Introduction
Pseudomorphic-high electron mobility transistors (PM-HEMTs) have been intensively studied as high speed and high frequency devices. The PM-HEMTs have shown improved performance compared to conventional A1GaAs/GaAs HEMTs. Unfortunately, phenomena such as large shift of threshold voltage and drain current collapse have been reported for PM-HEMTs operated at low temperature. They are
Corresponding author. Fax: +852 2358 1652: E-mail: phluliwu @usthk.est.hk. I Present address: Department of Physics, The Hong Kong University of Science and Technology, Kowloon, Hong Kong.
thought to be induced by deep centers in PM-HEMTs [1]. However, not much is known about the deep levels in the PM-HEMTs. Recently, there have been reports [2,3] related to deep centers lying near the midgap in addition to the well-known DX centers of A1GaAs alloys. The DX centers cause serious instability in modulation-doped heterostructure field effect transistors. Whereas near-midgap deep levels behave as recombination centers and affect the operating characteristics of minority-carder devices. However, neither the degradation induced by deep centers in electronic and optical properties of a strained A1GaAs/InGaAs/GaAs system nor their effect on the performance of PM-HEMTs have been clarified so far. It is thought that the lattice constant mismatch between the A1GaAs and InGaAs may
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Liwu Lu et al. / Journal of Crystal Growth 169 (1996) 637-642
produce deformation in PM-HEMTs. The purpose of this paper is to report a detailed characterization of deep centers, and their relationship to the degradation in electrical and optical properties of PMHEMTs. In the case of x = 0 of A1xGa l_xAs, i.e. GaAs, the dominant deep level is the well-known midgap state called EL2. Owing to scientific and applied interest, a great deal of experimental [4] and theoretical work [5] has been reported on this deep state, whose origin is not completely understood except for a consensus that the EL2 is associated with excess As. It is interesting that an EL2-1ike deep electron trap also exists in the GaAs-related alloy system such as AlxGa l_xAs. The activation energy of the EL2-1ike deep electron trap remains fixed with respect to the valence band, increasing with the A1 composition of AlxGal_xAs [6]. In this study particular attention is paid to the effects of EL2-1ike deep electron traps on the degradation in the electrical and optical performance of PM-HEMTs, the shift of the DX center in the strained A1GaAs layer of the A1GaAs/InGaAs/GaAs system for PM-HEMTs, and the possible origin as well as the passivation/annihilation of these deep electron traps.
2. E x p e r i m e n t a l p r o c e d u r e
The PM-HEMT samples CI034, CI030 and a HEMT reference sample CI032 used in this study were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates. The nominal layer thicknesses, aluminum concentrations and doping levels are given in Fig. la and lb, respectively. The growth temperature of HEMT samples was 600°C. The growth temperature of PM-HEMT samples was Table 1 Electrical parameters of PM-HEMTsamples Hall mobility(cm2/V. s) Carrier Run No. concentration 77 K 300 K (cm 3) CI034 CI030
8000 i 1000
1636 3445
4.6 × l0 II 1.7 N 1012
(a) 10nm n÷-GaAs 2xl0~cm"~ 50nm n-Alo.2Gao.sAs 1.5x101%m ~ 5nm un-Alo.2 Gao.sAs 15nm unlIno.lsGao.ssAs l~tm un-GaAs S. I. GaAs sub.
(b) 10nm n*-GaAs 2xl01~eln° 50nm n-A10.3Ga0.vAs 1.5xl01%m"3 5nm un-Al0.3Ga0.TAs lp.m un-GaAs
S. I. GaAs sub. Fig. 1. Schematic illustration of the layer structures for (a) PMHEMT and (b) HEMT samples grownby MBE.
520°C in order to prevent significant indium segregation. The electrical parameters of the PM-HEMT samples obtained by the conventional van der Pauw technique are shown in Table 1. The ohmic and Schottky contacts were made on the n+-GaAs top layer of the PM-HEMT and HEMT samples respectively for deep level transient spectroscopy (DLTS) measurements. The DLTS measurements in the temperature range from 77 to 380 K were carried out with a high sensitivity ( A C / C -~ 10 -5) lock-in-type spectrometer DLS-82E system from Semitrap, Hungary. All levels are confirmed by the temperature scan as well as frequency scan mode measurements [7]. The energy level positions are determined from Arrhenius plots, while capture cross sections are obtained directly from measurements of varying pulse width. The photoluminescence (PL) spectra were measured at 77 K. The photoexcitation was accomplished by an Ar + laser operating at 5145 A. The profiles of oxygen atomic concentration for PM-HEMT samples are characterized by secondary ion mass spectrometry (SIMS) on a Riber MIQ 156 SIMS system with secondary ion detection sensitivity of above 106 ions/s. To understand the effect of hydrogen on deep centers, hydrogen plasma treatment was used. Hydrogen plasma was produced by a 16.5 MHz rf oscillator with an output of 40 W. Hydrogen treatment was performed at 50 Torr, 50°C for 30 rain. o
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Liwu Lu et al. /Joutvzal of Co'stal Growth 169 (1996) 637-642
3. Results and discussion
EX
3.1. DLTS study
£t
Fig. 2 shows typical DLTS spectra obtained for sample CI034. Curve 1 (V~ = - 0 . 4 V) exhibits an E1 electron trap with an activation energy of 0.22 eV below E c and a capture cross section of 10 -18 cm 2 in the low temperature range. With increasing of reverse bias VR (VR = - 0 . 5 to - 2 . 0 V) applied to CI034, three entirely new electron traps, labeled E2, E3 and E4 with activation energies of 0.56, 0.64 and 0.79 eV below E~ and capture cross sections of 1 0 - 1 7 c m 2 (E2), 10 16 c m 2 (E3) and 1 0 - I 6 cm 2 (E4), respectively, emerge in curves 2 - 5 in addition to El. The doping and thickness of the nA l o 2 G a o s A s layer in CI034 were carefully controlled, so that the depletion regions at the 0.4-2.0 V reverse biases were inside the A10zGa08As layer. This indicates that El, E2, E3 and E4 traps are detected in the n-A102Ga08As layer. In comparison with the published data, E1 was assigned to the DX center associated with a column VI donor [8], the origin of the E2 trap is unknown but may be an intrinsic defect [9], the E3 trap may be related to oxygen [10], and the E3 and E4 traps are thought to
PM-HEMT CI034 f= 8.16Hz l
~
~E2~
!
100
I
150
I
200
I
250
E4 s
I
30~
I
350
Temperature (K) Fig. 2. DLTS spectra for CI034 with a 1 /zs pulse width and various biases VR: (1) -0.4, (2) -0.5. (3) - 1.0, (4) - 1.5, and (5) - 2 . 0 v.
HEMT f = 22.5 Hz CI032 E2
~.J
4
..~ El
EZ
9 I 100
I 150
I
200
I
I
I
I
250
300
350
400
Temperature (K) Fig. 3. DLTS spectra for CI032 and CI030 with a 1 /xs pulse width and various biases VR: (1) --1.0, (2) --5.0, (3) --2.0, (4) -3.0, (5) -4.0, (6) -5.0, (7) -5.5, (8) -6.0, and (9) -6.5 V.
be identical to that observed in A1GaAs grown by metalorganic vapor phase epitaxy (MOVPE) and MBE [1]. Fig. 3 shows the typical DLTS spectra observed in samples CI030 and CI032. For CI030, four electron traps, labeled E l , E3, E4 and E5, were detected. The values of activation energy and capture cross section of these traps are 0.22, 0.64, 0.79 and 0.42 eV below Eo and 10 -18 cm 2 (El), 10 -16 c m 2 (E3), 1 0 - 1 6 c m 2 (E4) and 10 -18 cm 2 (E5) in curve 4. El, E3 and E4 are the identical traps that were detected in CI034, but E5 appears only in CI030. For the reference sample CI032, E1 ( E c - 0 . 2 2 eV), E2 (Ec - 0 . 5 6 eV), E4 ( E c - 0 . 7 9 eV) and E5 ( E c - 0 . 4 2 eV) were observed in curve 1. The E5 trap observed in CI030 and CI032 is related to a higher A1 mole fraction of Al~Gal_xAs (x > 0.22), and therefore assigned to the peak of the Si-related DX center. In comparison with CI032 (curves 1 and 2), it is obvious that the peak position of E5 (DX center) in CI030 (curves 3 - 9 ) shifted systematically toward higher temperature with increasing of VR (from - 2 to - 6 . 5 V). The peaks of E3, E4 also shifted to some extent, but E1 did not.
Liwu Lu et al. /Journal of Co'stal Growth 169 (1996) 637-642
640
The Arrhenius plots of emission time constants indicate that the increments of activation energy of E5 in CI030 have an amount of up to 40 meV ( E c - 0.42 eV for VR = - 2 V and Ec - 0.46 eV for VR = - 6 V, respectively). This suggests that some strains may exist in the A1GaAs layer. It can be expected that the lattice parameters mismatch between A1GaAs and InGaAs layers may produce deformation. Considering the magnitude of differences in their thermal expansion coefficient and lattice constant (the thermal expansion coefficient of 3.3 × 10 - 6 K -1 [1 1] for A10.2Ga0.sAs and 5.79 × 10 -6 K 1 [12] for In0.15Ga0.s5As; the lattice constant of 5.65 ,~ [13] for A10.2Ga0.sAs and 5.71 ,~ [13] for In0AsGa0.85As), the A1GaAs layer of the A1GaAs/InGaAs/GaAs system experiences a biaxial compression [13]. In the compression regime of A1GaAs the gap from a fundamental heavy-hole to the conduction band increases slowly with strain [13], resulting in the F conduction band minima of A1GaAs moving toward a higher energy position [14]. The shift of the activation energy for the electron thermal emission of the DX center (E5) in the strained A1GaA s layer of the A1GaAs/InGaAs/GaAs system can be measured at different reverse biases in DLTS measurements, although a more or less general agreement was reached assigning the L-band minima connection to the Si-related DX center [15]. At the same time, it is also noted that the E5 trap was turned into a broad and non-symmetrical peak, indicating that E5 might include certain defects induced by strains in A1GaAs in addition to the DX center. In comparison with CI030, one can find that the concentrations of E3 and FA in
" i 0 1~ ~10 16
E'~IO ts
[
'
-1
'
'
-2
'
'
-3
I
'
-4
'
'
-5
'
'
i
-6
Bias Voltage (V) Fig. 4. Comparison of E3 and FA for CI030 and CI034.
1022
1021 o .~
102~
10x9
[ I I 0.2 0.4 0.6 Depth (ktm)
0,8
Fig. 5. SIMS profiles of atomic oxygen content for CI030 CI034.
a n d
CI034 are higher (under the same reverse bias in DLTS measurements).
3.2. Comparison of deep traps Typical density distributions of deep electron traps for CI030 and CI034 obtained by DLTS measurements are shown in Fig. 4. It can be seen that the electrical parameters (cartier concentration and Hall mobility, see Table 1) of the PM-HEMTs associated strongly with the densities of the E3 and E4 traps, suggesting that the degradation in electrical properties is related to the densities of deep electron traps.
3.3. SIMS and PL studies The SIMS profiles for atomic concentration of oxygen in the PM-HEMTs are given in Fig. 5. Comparing CI030 with CI034, it is obvious that the oxygen content of the n-A1GaAs layer in CI034 is higher than that in CI030. This indicates that the oxygen content is directly related to the densities of the E3 and E4 traps. Bhattacharya et al. [10] detected an electron trap at 0.66 eV with a density of 8 × 1014 c m -3 for n-A10.3Ga0.7As, which was grown in an ambient known to contain a trace amount of oxygen. Hobson et al. [9] observed that the oxygen-related electron trap with 0.64 eV exists only at a very low concentration of 2 × 1012 cm 3 for a high-quality n-A1GaAs layer (i.e. low oxygen < 5 × 1016 cm -3 and low carbon < 3 X 1016 cm -3) grown by MOVPE. In this study we detected E3 ( E c - 0 . 6 4
Liwu Lu et al. / Journal of Co'stal Growth 169 (1996) 637-642
eV) and E4 (Ec - 0.79 eV) traps with higher densities (1015-1017 cm 3). SIMS results observed in CI030 and CI034 indicated that the oxygen level of the A1GaAs layer in our samples (oxygen content ~ 5 × 1 0 2 o cm 3 f o r C I 0 3 0 and ~ 3 × 1 0 2 1 cm -3 for CI034, respectively) is higher than the published value of A1GaAs grown by MBE where the oxygen level was 4 × 1017 cm -3 [16]. From the DLTS and SIMS results, one can draw a conclusion that the deep electron traps have a close dependence on the atomic oxygen content. PL spectra were measured at 77 K for CI030 and CI034. Typical PL spectra are shown in Fig. 6. The PL response peak observed is from the InGaAs layer of the PM-HEMT structure. There is non-remarkable change in line widths for PL peaks of the samples. The intensity, however, decreases rapidly with increasing densities of deep electron traps, implying that the amount of excited carriers drifted into the InGaAs layer during PL measurements are strongly affected by the deep electron traps. This indicates the degradation in optical properties of PM-HEMTs due to deep electron traps. The deep electron traps with larger concentration play an important role in the PL efficiency, although the PL efficiency might also be associated with the interface defects induced by the lattice mismatch between A1GaAs and InGaAs.
3.4. DLTS study after hydrogenation The CI030 and CI034 samples were processed at a low temperature of 50°C and an exchange current of 80 mA for 30 min in an atmosphere of hydrogen
=.
F~CI030
641
-g '(a) CI034 30n]in (1,2,3,4) 1 e~ r/3
C I 0 3 ~ 9
ibl ci030 30 min (SI9,10) f
100
150
200
250
300
350
Temperature (K) Fig. 7. DLTS spectra for CI030 and CI034 after hydrogenation.
plasma. The typical results of the hydrogen plasma effect on deep electron traps are shown in Fig. 7. It shows that deep electron traps can be passivated/annihilated when the samples were exposed to hydrogen plasma under special conductions.
4. Conclusions Based upon DLTS, SIMS, PL and van der Pauw measurements, we have shown that the deep electron traps with activation energies of E c - 0 . 6 4 eV and E c - 0 . 7 9 eV, having larger capture cross sections and densities, correlate strongly with the oxygen content and PL intensity of PM-HEMTs, and hence are responsible for the degradation in electrical and optical properties of PM-HEMTs. The observation on the shift of the DX center in the strained A1GaAs layer of the A1GaAs/InGaAs/GaAs system suggests that the DLTS technique can be used for optimizing designs of practical devices. The experimental results also show that hydrogen plasma can passivate/annihilate deep electron traps.
Acknowledgements B 1.1 112 1.3 1.4
Energy (eV) Fig. 6. PL spectra at 77 K for CI030 and CI034.
This work was supported by the Semiconductors Institute Region, National Integrated Optoelectronic Laboratory, Beijing, P.R. China. Work done at HKUST was supported by Grant MFG 9 4 / 9 5 SC03.
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Liwu Lu et al. / Journal of Crystal Growth 169 (1996) 637-642
References [1] A.J. Valois, G.Y. Robinson, K. Lee and M.S. Shur, J. Vac. Sci. Technol. B 1 (1983) 190. [2] T. Hashizume, H. Hasegawa and H. Ohno, J. Appl. Phys. 68 (1990) 3394. [3] L.W. Lu, J. Zhou, J.Y. Xu and Z.T. Zhong, Acta Phys. Sin. 42 (1993) 66. [4] For a review, see: Rev. Phys. Appl. 23 (1988) 727, Eds. H.J. yon Barceieben and B. Pajot, and references therein. [5] G.A. Baran and M.Schlater, Phys. Rev. B 35 (1987) 6154. [6] R.A. Puechner, D.A. Johnson and G.N. Maracas, Appl. Phys. Lett. 53 (1988) 1952. [7] G. Ferenczi, J. Boda and T. Pavelka, Phys. Status Solidi 94 (1986) Kl19. [8] O. Kumagai, H. Kawai, Y. Mori and K. Kanero, Appl. Phys. Lett. 45 (1984) 1322.
[9] W.S. Hobson, S.R. McAfee, K.S. Jones, N.G. Paroskevopoulos, C.R. Abernathy, S.K. Sputz, J.D. Harris, M. Lamont Schnoes and S.J. Pearton, Mater. Sci. Forum 83-87 (1992) 1063. [10] P.K. Bhattacharya, T. Matsumoto and S. Subramanian, J. Crystal Growth 68 (1984) 301. [11] M. Nenberger, III-V Ternary Semiconducting Compounds (1972) 25. [12] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1969) p. 376. [13] P. Voisin, SPIE 861 (1987) 89. [14] J.E. Dmochowski, P.D. Wang, R.A. Stradling and W. Trzeciakowski, Mater. Sci. Forum 83-87 (1992) 751. [15] E. Calleja, Solid State Phenom. 10 (1989) 73. [16] T. Achtnick, G. Burri and M. Ilegems, J. Vac. Sci. Technol. A 7 (1989) 2537.
C R Y S T A L G R O W T H
ELSEVIER
Journal of Crystal Growth 169 (1996) 637-642
Characterization of deep centers in A1GaAs/InGaAs/GaAs pseudomorphic HEMT structures grown by molecular beam epitaxy and hydrogen treatment Liwu Lu
a,*,l, Songlin Feng a, Jiben Liang a, Zhanguo Wang a, j. Wang b, Y. Wang b, Weikun Ge b
Notional Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People's Republic of China b Department of Physics, The Hong Kong Universi~ of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 2 June 1996
Abstract
In AlGaAs/InGaAs/GaAs PM-HEMT structures, the characterization of deep centers, the degradation in electrical and optical properties and their effects on electrical performance of the PM-HEMTs have been investigated by DLTS, SIMS, PL and conventional van der Pauw techniques. The experimental results confirm that the deep level centers correlate strongly with the oxygen content in the A1GaAs layer, the PL response of PM-HEMTs, and the electrical performance of the PM-HEMTs. Hydrogen plasma treatment was used to passivate/annihilate these centers, and the effects of hydrogenation were examined.
1. Introduction
Pseudomorphic-high electron mobility transistors (PM-HEMTs) have been intensively studied as high speed and high frequency devices. The PM-HEMTs have shown improved performance compared to conventional A1GaAs/GaAs HEMTs. Unfortunately, phenomena such as large shift of threshold voltage and drain current collapse have been reported for PM-HEMTs operated at low temperature. They are
Corresponding author. Fax: +852 2358 1652: E-mail: phluliwu @usthk.est.hk. I Present address: Department of Physics, The Hong Kong University of Science and Technology, Kowloon, Hong Kong.
thought to be induced by deep centers in PM-HEMTs [1]. However, not much is known about the deep levels in the PM-HEMTs. Recently, there have been reports [2,3] related to deep centers lying near the midgap in addition to the well-known DX centers of A1GaAs alloys. The DX centers cause serious instability in modulation-doped heterostructure field effect transistors. Whereas near-midgap deep levels behave as recombination centers and affect the operating characteristics of minority-carder devices. However, neither the degradation induced by deep centers in electronic and optical properties of a strained A1GaAs/InGaAs/GaAs system nor their effect on the performance of PM-HEMTs have been clarified so far. It is thought that the lattice constant mismatch between the A1GaAs and InGaAs may
0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PH S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 4 7 3 - 3
638
Liwu Lu et al. / Journal of Crystal Growth 169 (1996) 637-642
produce deformation in PM-HEMTs. The purpose of this paper is to report a detailed characterization of deep centers, and their relationship to the degradation in electrical and optical properties of PMHEMTs. In the case of x = 0 of A1xGa l_xAs, i.e. GaAs, the dominant deep level is the well-known midgap state called EL2. Owing to scientific and applied interest, a great deal of experimental [4] and theoretical work [5] has been reported on this deep state, whose origin is not completely understood except for a consensus that the EL2 is associated with excess As. It is interesting that an EL2-1ike deep electron trap also exists in the GaAs-related alloy system such as AlxGa l_xAs. The activation energy of the EL2-1ike deep electron trap remains fixed with respect to the valence band, increasing with the A1 composition of AlxGal_xAs [6]. In this study particular attention is paid to the effects of EL2-1ike deep electron traps on the degradation in the electrical and optical performance of PM-HEMTs, the shift of the DX center in the strained A1GaAs layer of the A1GaAs/InGaAs/GaAs system for PM-HEMTs, and the possible origin as well as the passivation/annihilation of these deep electron traps.
2. E x p e r i m e n t a l p r o c e d u r e
The PM-HEMT samples CI034, CI030 and a HEMT reference sample CI032 used in this study were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates. The nominal layer thicknesses, aluminum concentrations and doping levels are given in Fig. la and lb, respectively. The growth temperature of HEMT samples was 600°C. The growth temperature of PM-HEMT samples was Table 1 Electrical parameters of PM-HEMTsamples Hall mobility(cm2/V. s) Carrier Run No. concentration 77 K 300 K (cm 3) CI034 CI030
8000 i 1000
1636 3445
4.6 × l0 II 1.7 N 1012
(a) 10nm n÷-GaAs 2xl0~cm"~ 50nm n-Alo.2Gao.sAs 1.5x101%m ~ 5nm un-Alo.2 Gao.sAs 15nm unlIno.lsGao.ssAs l~tm un-GaAs S. I. GaAs sub.
(b) 10nm n*-GaAs 2xl01~eln° 50nm n-A10.3Ga0.vAs 1.5xl01%m"3 5nm un-Al0.3Ga0.TAs lp.m un-GaAs
S. I. GaAs sub. Fig. 1. Schematic illustration of the layer structures for (a) PMHEMT and (b) HEMT samples grownby MBE.
520°C in order to prevent significant indium segregation. The electrical parameters of the PM-HEMT samples obtained by the conventional van der Pauw technique are shown in Table 1. The ohmic and Schottky contacts were made on the n+-GaAs top layer of the PM-HEMT and HEMT samples respectively for deep level transient spectroscopy (DLTS) measurements. The DLTS measurements in the temperature range from 77 to 380 K were carried out with a high sensitivity ( A C / C -~ 10 -5) lock-in-type spectrometer DLS-82E system from Semitrap, Hungary. All levels are confirmed by the temperature scan as well as frequency scan mode measurements [7]. The energy level positions are determined from Arrhenius plots, while capture cross sections are obtained directly from measurements of varying pulse width. The photoluminescence (PL) spectra were measured at 77 K. The photoexcitation was accomplished by an Ar + laser operating at 5145 A. The profiles of oxygen atomic concentration for PM-HEMT samples are characterized by secondary ion mass spectrometry (SIMS) on a Riber MIQ 156 SIMS system with secondary ion detection sensitivity of above 106 ions/s. To understand the effect of hydrogen on deep centers, hydrogen plasma treatment was used. Hydrogen plasma was produced by a 16.5 MHz rf oscillator with an output of 40 W. Hydrogen treatment was performed at 50 Torr, 50°C for 30 rain. o
639
Liwu Lu et al. /Joutvzal of Co'stal Growth 169 (1996) 637-642
3. Results and discussion
EX
3.1. DLTS study
£t
Fig. 2 shows typical DLTS spectra obtained for sample CI034. Curve 1 (V~ = - 0 . 4 V) exhibits an E1 electron trap with an activation energy of 0.22 eV below E c and a capture cross section of 10 -18 cm 2 in the low temperature range. With increasing of reverse bias VR (VR = - 0 . 5 to - 2 . 0 V) applied to CI034, three entirely new electron traps, labeled E2, E3 and E4 with activation energies of 0.56, 0.64 and 0.79 eV below E~ and capture cross sections of 1 0 - 1 7 c m 2 (E2), 10 16 c m 2 (E3) and 1 0 - I 6 cm 2 (E4), respectively, emerge in curves 2 - 5 in addition to El. The doping and thickness of the nA l o 2 G a o s A s layer in CI034 were carefully controlled, so that the depletion regions at the 0.4-2.0 V reverse biases were inside the A10zGa08As layer. This indicates that El, E2, E3 and E4 traps are detected in the n-A102Ga08As layer. In comparison with the published data, E1 was assigned to the DX center associated with a column VI donor [8], the origin of the E2 trap is unknown but may be an intrinsic defect [9], the E3 trap may be related to oxygen [10], and the E3 and E4 traps are thought to
PM-HEMT CI034 f= 8.16Hz l
~
~E2~
!
100
I
150
I
200
I
250
E4 s
I
30~
I
350
Temperature (K) Fig. 2. DLTS spectra for CI034 with a 1 /zs pulse width and various biases VR: (1) -0.4, (2) -0.5. (3) - 1.0, (4) - 1.5, and (5) - 2 . 0 v.
HEMT f = 22.5 Hz CI032 E2
~.J
4
..~ El
EZ
9 I 100
I 150
I
200
I
I
I
I
250
300
350
400
Temperature (K) Fig. 3. DLTS spectra for CI032 and CI030 with a 1 /xs pulse width and various biases VR: (1) --1.0, (2) --5.0, (3) --2.0, (4) -3.0, (5) -4.0, (6) -5.0, (7) -5.5, (8) -6.0, and (9) -6.5 V.
be identical to that observed in A1GaAs grown by metalorganic vapor phase epitaxy (MOVPE) and MBE [1]. Fig. 3 shows the typical DLTS spectra observed in samples CI030 and CI032. For CI030, four electron traps, labeled E l , E3, E4 and E5, were detected. The values of activation energy and capture cross section of these traps are 0.22, 0.64, 0.79 and 0.42 eV below Eo and 10 -18 cm 2 (El), 10 -16 c m 2 (E3), 1 0 - 1 6 c m 2 (E4) and 10 -18 cm 2 (E5) in curve 4. El, E3 and E4 are the identical traps that were detected in CI034, but E5 appears only in CI030. For the reference sample CI032, E1 ( E c - 0 . 2 2 eV), E2 (Ec - 0 . 5 6 eV), E4 ( E c - 0 . 7 9 eV) and E5 ( E c - 0 . 4 2 eV) were observed in curve 1. The E5 trap observed in CI030 and CI032 is related to a higher A1 mole fraction of Al~Gal_xAs (x > 0.22), and therefore assigned to the peak of the Si-related DX center. In comparison with CI032 (curves 1 and 2), it is obvious that the peak position of E5 (DX center) in CI030 (curves 3 - 9 ) shifted systematically toward higher temperature with increasing of VR (from - 2 to - 6 . 5 V). The peaks of E3, E4 also shifted to some extent, but E1 did not.
Liwu Lu et al. /Journal of Co'stal Growth 169 (1996) 637-642
640
The Arrhenius plots of emission time constants indicate that the increments of activation energy of E5 in CI030 have an amount of up to 40 meV ( E c - 0.42 eV for VR = - 2 V and Ec - 0.46 eV for VR = - 6 V, respectively). This suggests that some strains may exist in the A1GaAs layer. It can be expected that the lattice parameters mismatch between A1GaAs and InGaAs layers may produce deformation. Considering the magnitude of differences in their thermal expansion coefficient and lattice constant (the thermal expansion coefficient of 3.3 × 10 - 6 K -1 [1 1] for A10.2Ga0.sAs and 5.79 × 10 -6 K 1 [12] for In0.15Ga0.s5As; the lattice constant of 5.65 ,~ [13] for A10.2Ga0.sAs and 5.71 ,~ [13] for In0AsGa0.85As), the A1GaAs layer of the A1GaAs/InGaAs/GaAs system experiences a biaxial compression [13]. In the compression regime of A1GaAs the gap from a fundamental heavy-hole to the conduction band increases slowly with strain [13], resulting in the F conduction band minima of A1GaAs moving toward a higher energy position [14]. The shift of the activation energy for the electron thermal emission of the DX center (E5) in the strained A1GaA s layer of the A1GaAs/InGaAs/GaAs system can be measured at different reverse biases in DLTS measurements, although a more or less general agreement was reached assigning the L-band minima connection to the Si-related DX center [15]. At the same time, it is also noted that the E5 trap was turned into a broad and non-symmetrical peak, indicating that E5 might include certain defects induced by strains in A1GaAs in addition to the DX center. In comparison with CI030, one can find that the concentrations of E3 and FA in
" i 0 1~ ~10 16
E'~IO ts
[
'
-1
'
'
-2
'
'
-3
I
'
-4
'
'
-5
'
'
i
-6
Bias Voltage (V) Fig. 4. Comparison of E3 and FA for CI030 and CI034.
1022
1021 o .~
102~
10x9
[ I I 0.2 0.4 0.6 Depth (ktm)
0,8
Fig. 5. SIMS profiles of atomic oxygen content for CI030 CI034.
a n d
CI034 are higher (under the same reverse bias in DLTS measurements).
3.2. Comparison of deep traps Typical density distributions of deep electron traps for CI030 and CI034 obtained by DLTS measurements are shown in Fig. 4. It can be seen that the electrical parameters (cartier concentration and Hall mobility, see Table 1) of the PM-HEMTs associated strongly with the densities of the E3 and E4 traps, suggesting that the degradation in electrical properties is related to the densities of deep electron traps.
3.3. SIMS and PL studies The SIMS profiles for atomic concentration of oxygen in the PM-HEMTs are given in Fig. 5. Comparing CI030 with CI034, it is obvious that the oxygen content of the n-A1GaAs layer in CI034 is higher than that in CI030. This indicates that the oxygen content is directly related to the densities of the E3 and E4 traps. Bhattacharya et al. [10] detected an electron trap at 0.66 eV with a density of 8 × 1014 c m -3 for n-A10.3Ga0.7As, which was grown in an ambient known to contain a trace amount of oxygen. Hobson et al. [9] observed that the oxygen-related electron trap with 0.64 eV exists only at a very low concentration of 2 × 1012 cm 3 for a high-quality n-A1GaAs layer (i.e. low oxygen < 5 × 1016 cm -3 and low carbon < 3 X 1016 cm -3) grown by MOVPE. In this study we detected E3 ( E c - 0 . 6 4
Liwu Lu et al. / Journal of Co'stal Growth 169 (1996) 637-642
eV) and E4 (Ec - 0.79 eV) traps with higher densities (1015-1017 cm 3). SIMS results observed in CI030 and CI034 indicated that the oxygen level of the A1GaAs layer in our samples (oxygen content ~ 5 × 1 0 2 o cm 3 f o r C I 0 3 0 and ~ 3 × 1 0 2 1 cm -3 for CI034, respectively) is higher than the published value of A1GaAs grown by MBE where the oxygen level was 4 × 1017 cm -3 [16]. From the DLTS and SIMS results, one can draw a conclusion that the deep electron traps have a close dependence on the atomic oxygen content. PL spectra were measured at 77 K for CI030 and CI034. Typical PL spectra are shown in Fig. 6. The PL response peak observed is from the InGaAs layer of the PM-HEMT structure. There is non-remarkable change in line widths for PL peaks of the samples. The intensity, however, decreases rapidly with increasing densities of deep electron traps, implying that the amount of excited carriers drifted into the InGaAs layer during PL measurements are strongly affected by the deep electron traps. This indicates the degradation in optical properties of PM-HEMTs due to deep electron traps. The deep electron traps with larger concentration play an important role in the PL efficiency, although the PL efficiency might also be associated with the interface defects induced by the lattice mismatch between A1GaAs and InGaAs.
3.4. DLTS study after hydrogenation The CI030 and CI034 samples were processed at a low temperature of 50°C and an exchange current of 80 mA for 30 min in an atmosphere of hydrogen
=.
F~CI030
641
-g '(a) CI034 30n]in (1,2,3,4) 1 e~ r/3
C I 0 3 ~ 9
ibl ci030 30 min (SI9,10) f
100
150
200
250
300
350
Temperature (K) Fig. 7. DLTS spectra for CI030 and CI034 after hydrogenation.
plasma. The typical results of the hydrogen plasma effect on deep electron traps are shown in Fig. 7. It shows that deep electron traps can be passivated/annihilated when the samples were exposed to hydrogen plasma under special conductions.
4. Conclusions Based upon DLTS, SIMS, PL and van der Pauw measurements, we have shown that the deep electron traps with activation energies of E c - 0 . 6 4 eV and E c - 0 . 7 9 eV, having larger capture cross sections and densities, correlate strongly with the oxygen content and PL intensity of PM-HEMTs, and hence are responsible for the degradation in electrical and optical properties of PM-HEMTs. The observation on the shift of the DX center in the strained A1GaAs layer of the A1GaAs/InGaAs/GaAs system suggests that the DLTS technique can be used for optimizing designs of practical devices. The experimental results also show that hydrogen plasma can passivate/annihilate deep electron traps.
Acknowledgements B 1.1 112 1.3 1.4
Energy (eV) Fig. 6. PL spectra at 77 K for CI030 and CI034.
This work was supported by the Semiconductors Institute Region, National Integrated Optoelectronic Laboratory, Beijing, P.R. China. Work done at HKUST was supported by Grant MFG 9 4 / 9 5 SC03.
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Liwu Lu et al. / Journal of Crystal Growth 169 (1996) 637-642
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