Oxygen-related deep level defects in solid-source MBE grown GaInP

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1 Journal of Crystal Growth (2001) Oxygen-related deep level defects in solid-source MBE grown GaInP N. Xiang a, *, A. Tukiainen a, J. Dekker b, J. Likonen c, M. Pessa a a Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FIN Tampere, Finland b Laboratory of Physics, Helsinki University of Technology, Espoo, Finland c Chemical Technology, Technical Research Centre of Finland, P.O. Box 1404, VTT, Finland Abstract We report the first observation of oxygen-related deep level defects in solid-source MBE-grown GaInP. Si-doped GaInP samples were studied by deep level transient spectroscopy (DLTS), secondary-ion mass spectrometry (SIMS), capacitance voltage (C2V) profiles, and photoluminescence (PL). Different amounts of oxygen impurities were introduced into GaInP epilayers by growing with different phosphorus cracking temperatures. Four traps were resolved by DLTS from the GaInP samples. Among them, two traps, with thermal activation energies of and ev, were found to be oxygen-related. # 2001 Elsevier Science B.V. All rights reserved. PACS: Hi; Eq; Cr Keywords: A1. Characterization; A1. Defects; A1. Impurities; A3. Molecular beam epitaxy; B2. Semiconducting indium gallium phosphide 1. Introduction Epitaxial Ga 0.5 In 0.5 P (hereafter written as GaInP), closely lattice matched to GaAs, has applications in optoelectronic devices such as semiconductor lasers, light emitting diodes and solar cells. The presence of deep level defects in GaInP can degrade device performance by creating non-radiative recombination centers and reducing the carrier lifetime [1]. Oxygen is a commonly suspected residual impurity in epi-layers grown by molecular beam epitaxy (MBE). It has been shown that oxygen impurities in MBE-grown Si-doped *Corresponding author. Tel.: ; fax: address: ning.xiang@orc.tut.fi (N. Xiang). GaInP compensate silicon doping and degrade the material optical properties [2]. Similar phenomenon was observed for MBE-grown InP [3]. Kwon et al. studied the effect of oxygen on GaInP grown by liquid phase epitaxy (LPE) [4] but did not find oxygen-related deep traps. Oxygen-related deep level defects have been reported in AlGaInP, InP, GaInAs, and AlGaAs [5 9] but, to our knowledge, not in MBE-grown GaInP. In this paper, we report the first systematic study on oxygen-related deep level defects in MBE-grown GaInP. 2. Experimental procedure Two sets of Si-doped GaInP samples were grown on n + -GaAs substrates using solid-source /01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S (01)

2 N. Xiang et al. / Journal of Crystal Growth (2001) MBE (SSMBE). Phosphorus was provided by a three-zone valved cracker cell. Gallium, indium and silicon were evaporated from the conventional effusion cells. The thickness of GaInP epilayers was 2 mm, the growth rate was kept at 1 mm/h, and growth temperature for GaInP was (495 5)8C (pyrometer reading). Samples in each set were grown with same Si dopant cell temperature but using different phosphorus cracking temperatures (T cr ). The incorporated oxygen and silicon impurity concentrations [N O ] and [N Si ], were determined by SIMS using ion-implanted standard samples. The oxygen impurity is thought to originate from phosphorus trioxide (P 4 O 6 ) in the phosphorus ingot [2]. More P 4 O 6 may be cracked at high T cr leading to more [N O ] in the epilayer. The P2/III ratio was 13 (within 10% error) for all the GaInP growths. Deep level transient spectroscopy (DLTS) was employed to detect the deep level defects. DLTS samples were made by evaporating Ni/Au/Ge/Au ohmic contacts at the bottom of n + -GaAs substrates and Au Schottky contacts on top of the GaInP epilayers. The diodes showed good current voltage (I2V) characteristics with ideality factors less than 1.2. Measurements were carried out using a Bio Rad DL8000 DLTFS system at 2 V reverse bias. The pulse voltages were 0 V for set 1 and 1 V for set 2, respectively. The pulse duration was 0.1 s and the period width was 0.2 s. Capacitance voltage (C2V) profile was utilized for checking the compensation. Photoluminescence (PL) and X-ray diffraction (XRD) were used to characterize the optical and crystalline properties of the grown samples. 3. Results and discussion All the GaInP samples were mirror-like and lattice-matched to GaAs within Da/a40.1%. Table 1 lists [N O ], [N Si ], and net doping levels [N net ] (measured by C2V) in each set of the samples grown with different T cr. It can also be seen in Table 1 that [N net ] decreases when [N O ] increases, while [N Si ] is not reduced. This may indicate that oxygen can compensate Si doping in GaInP. Room-temperature PL spectra from these samples are shown in Fig. 1. In both sets, PL intensity was found to decrease as [N O ] was increased. This can be an indication that oxygen impurities have created non-radiative recombination centers in GaInP. The DLTS spectra obtained from set 1 and 2 are both shown in Fig. 2. Four deep levels, marked as T1, T2, T3 and T4 are resolved. The Arrhenius plots are shown in the insets. The evaluation results are summarized in Table 2, where E a is the thermal activation energy and s the capture crosssection of a trap. [N T1 ], [N T2 ] and [N T3 ] are the concentrations of traps T1, T2 and T3. Of the four traps, T1 has been identified as either a DX center caused by Si doping [1,10 13] or a native defect caused by phosphorus vacancies [14,15]. T2 in samples D and E strongly overlaps with T3 so its accurate evaluation is difficult, while T3 has relatively high intensity in all the samples. T4 is located in the high temperature range and is not well resolved in any of the samples making accurate evaluation difficult. We have also observed peaks in this temperature range in other MBE-grown GaInP samples. This peak may in Table 1 SIMS and C V results from Si-doped GaInP samples Samples T cr (8C) [N O ] (cm 3 ) [N Si ] (cm 3 ) [N net ] (cm 3 ) (Set 1) A B (Set 2) D E F

3 246 N. Xiang et al. / Journal of Crystal Growth (2001) Fig. 1. Room-temperature PL spectra taken from Si-doped GaInP samples containing different oxygen impurity concentrations. Si concentrations are /cm 3 and /cm 3 in sets 1 and 2, respectively. Fig. 2. DLTS spectra measured from samples of set 1 (dashed lines), and set 2 (solid lines). Both sets were obtained using a 2 V reverse bias, a 0.1 s pulse, and a 0.2 s period. Pulse voltages were 0 and 1 V for sets 1 and 2, respectively. The Arrhenius plots used to calculate the activation energies for the trap emission processes are shown in the insets. N c is the density of states in the conduction band, v th is the thermal velocity of electrons, and t is the time constant of the capacitance transient. fact contain two or more peaks and at this stage the nature of T4 is unidentified. However, [N T2 ] and [N T3 ] both increase clearly with [N O ] indicating that T2 and T3 are oxygen-related. The difference in E a for T3 between set 1 and 2 can be due to the different doping levels and may indicate that T3 is a charged defect [16]. The electric field around a charged defect can be affected by doping so that E a changes. Note that the Arrhenius plots for T3 in set 1 are perfectly overlapping, indicating that the nature of T3 in samples A and B is same. The differences in T3 trap signatures among the samples in set 2 may be due to such effects as different configurations around the defects, peak broadening, or the influence of neighboring peaks. For example, in sample F, T3 is very broad and the neighboring peaks, T2 and T4, are stronger than in samples D and E which may explain why the value of s in sample F is much smaller than that in samples D and E. We also observed that the concentration of peak T3 increased following exposure to air, as shown in Fig. 3. A duplicate of sample B, labeled B*, was processed two weeks after the first. The DLTS spectra of the two samples were unchanged except for the amplitude of T3 which increased significantly. We attribute this to oxygen being incorporated at the surface. Another possible explanation might be phosphorous outgassing leading to phosphorous vacancies. Chae et al. has reported an interface state with E a ¼ 0:73 ev [17], possibly due to a reaction involving the phosphorus vacancy. Therefore, the increase in T3 may be due to the formation of phosphorus vacancies or the incorporation of additional oxygen, or a complex of the two. T3 might be expected to have higher concentration near the surface. This is different from traps due to oxygen incorporated during growth, which should be nearly uniform throughout the layer. It is interesting to compare our results of GaInP with that of AlGaInP. Kondo et al. discovered two oxygen-related traps, D2 and D3, in AlGaInP with E a 0.46 and 1.0 ev, respectively [6]. E a for D2 was almost the same as E a for T2 in our samples. E a for D3 was a bit higher than E a for T3. The trap concentrations of D2 ([N D2 ]) and D3 ([N D3 ]) were found to increase linearly with oxygen concentration, and [N D3 ]/[N D2 ] ratio was almost constant. It was suggested that D2 and D3 levels were a pair of charge-state dependent multiple levels caused by

4 N. Xiang et al. / Journal of Crystal Growth (2001) Table 2 Evaluation results of deep level traps from Si-doped GaInP samples Samples T1 T2 T3 E a (ev) [N T1 ] (cm 3 ) s (cm 2 ) E a (ev) [N T2 ] (cm 3 ) s (cm 2 ) E a (ev) [N T3 ] (cm 3 ) s (cm 2 ) (Set 1) A } } } B } } } (Set 2) D } } } } } } E } } } } } } F } } } Conclusions Fig. 3. Comparison of DLTS spectra from samples B and B*. These two samples were from the same grown wafer and were nominally processed with the same steps. However, sample B* was processed 2 weeks later than sample B. The measurement condition was the same as for set 1 mentioned in Fig. 2. the off-center substitutional oxygen defect (V P O). In our GaInP samples, [N T2 ] and [N T3 ] also increase with [N O ] but [N 3 ]/[N 2 ] ratio seems to change with [N O ]. Due to the difficulties in evaluation, precise [N T3 ]/[N T2 ] ratio is impossible to obtain. These results may also be compared with results obtained from As-based materials. For example, oxygen has been observed to give rise to a double acceptor in Al 0.10 Ga 0.90 As [18] while first principal calculations of GaAs have shown that oxygen interstitials or oxygen on As sites can give rise to defects with multiple charges, depending on the configuration of the defect [19]. More studies are needed to further clarify the roles that oxygen plays in the formation of deep levels in GaInP. Oxygen-related deep level defects in solid-source MBE grown Si-doped GaInP have been studied using DLTS, SIMS, C2V profile and PL. Four traps are resolved by DLTS. Among them, traps T2 and T3, with thermal activation energies of and ev, are found to be oxygen-related. T3 may also relate to phosphorus vacancy, or phosphorus vacancy oxygen complex. This is the first report to indicate the presence of oxygen-related deep level defects in MBE-grown GaInP. Acknowledgements This work is supported, partly, by the Academy of Finland within the EMMA MACOMIO Project no References [1] J. Dekker, A. Tukiainen, N. Xiang, S. Orsila, M. Saarinen, M. Toivonen, M. Pessa, N. Tkachenko, H. Lemmetyinen, J. Appl. Phys. 86 (1999) [2] W.E. Hoke, P.J. Lemonias, A. Torabi, J. Vac. Sci. Technol. B 16 (1998) [3] N. Xiang, J. Likonen, J. Turpeinen, M. Saarinen, M. Toivonen, M. Pessa, Fourth International Conference on Thin Film Physics and Applications, Shanghai, China, May 8 11, 2000, SPIE Proc. 4086, pp [4] H.K. Kwon, S.D. Kwon, I. Kim, J.B. Lee, B. Choe, H. Lim, J. Appl. Phys. 77 (1995) 512.

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