Effect of electron and proton irradiation on recombination centers in GaAsN grown by chemical beam epitaxy

Effect of electron and proton irradiation on recombination centers in GaAsN grown by chemical beam epitaxy

Current Applied Physics 13 (2013) 1269e1274 Contents lists available at SciVerse ScienceDirect Current Applied Physics journal homepage: www.elsevie...

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Current Applied Physics 13 (2013) 1269e1274

Contents lists available at SciVerse ScienceDirect

Current Applied Physics journal homepage: www.elsevier.com/locate/cap

Effect of electron and proton irradiation on recombination centers in GaAsN grown by chemical beam epitaxy Boussairi Bouzazi*, Nobuaki Kojima, Yoshio Ohshita, Masafumi Yamaguchi Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 November 2012 Accepted 25 March 2013 Available online 11 April 2013

Deep level transient spectroscopy (DLTS) was deployed to study the evolution, upon electron irradiation and hydrogenation of GaAsN grown by chemical beam epitaxy, of the main nitrogen-related nonradiative recombination center (E1), localized at 0.33 eV below the bottom edge of the conduction band of the alloy. On one hand, the electron irradiation was found to enhance the density of E1 depending on the fluence dose. On the other hand, the hydrogenation was found to passivate completely E1. Furthermore, two new lattice defects were only observed in hydrogenated GaAsN films and were suggested to be in relationship with the origin of E1. The first defect was an electron trap at average thermal activation energy of 0.41 eV below the CBM of GaAsN and was identified to be the EL5-type native defect in GaAs, originating from interstitial arsenic (Asi). The second energy level was a hole trap, newly observed at average thermal activation energy of 0.11 eV above the valence band maximum of the alloy and its origin was tentatively suggested to be in relationship with the monohydrogenenitrogen (NeH) complex. As the possible origin of E1 was tentatively associated with the split interstitial formed from one N atom and one As atom in single V-site [(NeAs)As], we strongly suggested that the new hole trap took place after the dissociation of E1 and the formation of NeH complex. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Chemical beam epitaxy DLTS Irradiation Hydrogenation GaAsN

1. Introduction Ga1xInxAs1yNy (y ¼ 3x, x ¼ 0.03) alloy is a potential candidate for the lattice-matched tandem Ge/InGaAsN/GaAs/InGaP solar cell, which could achieve a conversion efficiency of more than 40% under AM0 spectrum radiation [1]. Meanwhile, adding a small atomic fraction of N into the lattice of GaAs markedly degrades the optoelectronic and mechanical properties of the new alloy [2e4]. This degradation was fundamentally associated with the nonradiative recombination process, since the incorporation of N into the alloy gives rise to the formation of high density electrically active N-related nonradiative recombination centers [5]. However, the origin of most energy states in GaAsN still remains elusive. Recently, we used single and double carrier pulse deep level transient spectroscopy (DLTS) to provide information about the distribution of electron and hole traps in the band gap energy of GaAsN grown by chemical beam epitaxy (CBE) [6]. As important result, it was confirmed with the formation of a nonradiative N-related recombination center, E1, with a localized thermal activation energy between 0.3 and 0.4 eV below the conduction band minimum * Corresponding author. E-mail addresses: [email protected], [email protected] (B. Bouzazi). 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.03.017

(CBM) [7]. The detail information about the electronic properties of E1 with our first trials to speculate its possible origin could be found elsewhere [8e10]. On the basis of the ShockleyeReadeHall model for trap-assisted recombination process, the lifetime of electrons from the CBM to E1 was evaluated to be approximately w0.2 ns, which is in accordance with the lifetime of electrons, measured by time-resolved photoluminescence (TRPL) [7]. Moreover, the origin of E1 was tentatively associated with the split interstitial (NeAs)As, formed from one N atom and one As atom in a single V-site, since its trapping density was found to be in relationship with the fluxes of N and As sources [10]. However, the information provided by DLTS method is not enough to confirm such origin. Therefore, it is worth relying our results to a large scope of both “direct” and “indirect” experiments to deeply investigate the origin of E1. Among the indirect methods, we cite the hydrogenation and electron irradiation methods. First, the hydrogen (H) atom, which has the smallest atomic size in nature and a high reaction with a wide variety of lattice defects, easily diffuses in semiconductor materials during growth and markedly affects their prevailing electrical properties [11,12]. Indeed, upon hydrogenation, H was found to neutralize the electrical activity of nonradiative recombination centers in several semiconductors, such as GaPN [13]. Furthermore, H was found to considerably affect the electrical properties of Ga(In)AsN [14e16]. During growth, H strongly binds to N to form different (NenH)

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complexes. These complexes essentially act as donors and/or acceptors and consequently increase the background doping in undoped GaAsN. This result was theoretically and experimentally studied, however, the exact structures of that complexes still remain elusive [15,17]. In addition, the pristine mechanical and electrical properties of N-free GaAs were restored upon hydrogenation. This result was deeply explained by the formation of NenH complexes, which passivate the role of single N atom in Ga(In)AsN [18e21]. Second, electron irradiation of semiconductors is an important experiment to investigate the origins of lattice defects because of the light mass of electrons, their high speed, and their high penetration power. For these reasons, we report in this research work the effect of electron irradiation hydrogenation on the distribution and evolution of lattice defects in GaAsN grown by CBE. 2. Experimental procedure One micron thick Si-doped GaAsN Schottky diode was grown by CBE on n-type GaAs (100), 2 off toward (110) substrates (2 cm  2 cm), under a growth temperature and a pressure of 460  C and w2  102 Pa, respectively. Triethylgallium (TEGa ¼ 0.1 sccm), tridimethylaminoarsenic (TDMAAs ¼ 1.0 sccm), and monomethylhydrazine (MMHy ¼ 9.0 sccm) were used as Ga, As, and N chemical compound sources, respectively. A silane (SiH4) source was used as n-type doping. The detail of growth conditions using CBE can be found elsewhere [4,6e10]. The GaAsN sample was sliced into several identical pieces to carry out hydrogenation and electron irradiation. For hydrogenation, H ions with multi-energy from 10 to 48 keV were implanted at room temperature into GaAsN films with peaks concentration of 5  1018 (HI1) and 1  1019 atom/cm3 (HI2), respectively. The depth of irradiation was thought to be distributed between 110 and 410 nm from the surface of GaAsN by calculating the SRIM 2003 simulation code [22]. For electron irradiation, two identical GaAsN pieces EI1 and EI2 were irradiated at room temperature with an energy of 2 MeV and with /cm2 and two different fluence doses of 4EI1 ¼ 9.0  1014 e 15  2 4EI2 ¼ 9.0  10 e/cm , respectively. Upon irradiations, all the samples were annealed at 500  C for 10 min under N2 gas and using GaAs cap-layers. The annealing temperature and time were judged sufficient to rearrange the atomic distribution in the crystal, since they are comparable to growth conditions. The N concentrations ([N]) of irradiated samples along with as grown sample were evaluated from the Bragg angles of the GaAs and GaAsN reflection obtained by high resolution X-ray diffraction (HRXRD) method. Al dots with a diameter of 1 mm were evaporated as Schottky contacts through a metal mask on the front side of the samples, at a vacuum pressure of 104 Pa, and a AueGe (88:12) alloy was deposited on the bottom surface as an ohmic contact. The ionized donor concentration (ND) at room temperature was calculated by the fitting of the MotteSchottky plot using the capacitanceevoltage method [23]. The DLTS spectra were collected using a BIO-RAD (DL8000) digital DLTS system. The activation energy Ea [EC  ET (eV)] or Ea [Ev þ ET (eV)] and the capture cross section sn (cm2) were determined from the slope and intercept values of the Arrhenius plot of the DLTS spectra, respectively [24]. The trapping density (NT) of each trap was evaluated by conventional DLTS method and was adjusted according to:

NTadj ¼ NT

WR2 L21

 L22

;

l ¼



 23 3 0 ðEF  ET Þ1=2 ; e2 ND

(2)

where e, 3 3 0,Wp, EF, and ET denote the elementary charge of an electron, the permittivity of GaAs, the depletion region at the pulse voltage VP, the Fermi level, and the energy level of each trap, respectively. In this adjustment, the value of ND was obtained from the CeV plot at the peak temperature of each energy level. 3. Results and discussion 3.1. Properties of GaAsN films upon irradiation Upon electron irradiation and hydrogenation, the crystal quality and the doping profile of GaAsN films were controlled by HRXRD and capacitanceevoltage measurements, respectively. The X-ray curves and MotteSchottky plots of irradiated films along with as grown sample are shown in Figs. 1 and 2, respectively. The full width at half maximum (FWHM) of GaAsN (004) peaks, summarized in Table 1, clearly indicates that the crystal quality was kept after irradiation and annealing. Furthermore, the straight lines of the MotteSchottky plots confirm that the doping profiles of irradiated samples were remained quasi-uniform over the depletion region that covers the reverse bias voltage range 0e3 V with interest for our DLTS measurements. The ionized donor concentration (ND) of irradiated samples along with as grown sample were calculated and plotted in Fig. 3. It is clear that ND increases with increasing the fluence dose of electron irradiation, whereas the opposite effect was observed with hydrogenation. This change in ND could not be easily explained, since several mechanisms may take place upon irradiation. Indeed, the ionization phenomenon, the formation of new donor and/or acceptor-like states or the change of the charge states of some energy levels. Upon hydrogenation, the main reason of dropping of ND could be expected to the formation of an acceptor-like state, since it was theoretically and experimentally confirmed that the H atom bounds strongly to N in

(1)

where WR is the total depletion region at the reverse bias voltage VR,L1 ¼ WR  l, L2 ¼ WP  l, and:

Fig. 1. HRXRD spectra of electron irradiated (EI1 and EI2), hydrogenated (HI1 and HI2), and as grown GaAsN film grown by CBE.

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Fig. 2. MotteSchottky plots of electron irradiated (EI1 and EI2), hydrogenated (HI1 and HI2), and as grown GaAsN film grown by CBE.

Fig. 3. Ionized donor concentration (ND) at room temperature in irradiated (EI1 and EI2), hydrogenated (HI1 and HI2), and as grown GaAsN film grown by CBE.

GaAsN to form a deep NeH related acceptor-like state, localized averagely at 0.15 eV above the valence band maximum (VBM) of the alloy. The detailed properties of this acceptor state could be found elsewhere [25,26].

E1 was observed in GaAsN or InGaAsN grown by CBE, metal-organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE) and exhibited approximately the same density level, despite the quite difference in the concentration of residual impurities between the three growth methods [8]. This implies that E1 is free from impurities, and could be a combination between the host atoms (N, As, and Ga). The origin of E1 was experimentally investigated using the dependence of NT(E1) to the fluxes of N and As sources as well as to the evolution of defects in hydrogenated n-type GaAsN. The results were correlated with theoretical calculation [10,27]. The split interstitial formed from one N and one As in a single As site (NeAs)As was tentatively suggested as a possible origin of E1 [10]. This expectation will be later considered to explain the evolution of the properties of E1 upon irradiations.

3.2. Nitrogen related recombination center in GaAsN As illustrated in Fig. 4, one dominant nitrogen-related majoritycarrier electron trap, E1, with activation energy of 0.331 eV below the CBM of Si-doped GaAsN Schottky device grown by CBE was observed in as grown sample with an adjusted trapping concentration and a thermal capture cross section of NT-adj(E1) ¼ 3.78  1016 cm3 and sn (E1) ¼ 5.18  1015 cm2, respectively. The thermal dependence of emission from E1 is plotted as Arrhenius plots in Fig. 5. Recently, E1 was directly confirmed to act as a nonradiative recombination center by single and double carrier pulse DLTS measurements and by investigating the temperature dependence of its capture cross section [7]. At room temperature, sn (E1) was evaluated to be around w1013 cm2, which gives a very large capture rate for electrons compared with that for native defects in GaAs [3e5]. Using the isothermal capacitance transient measurements, the density profiling of E1 was obtained and shown in Fig. 6. It is clear that E1 is quasi-uniformly distributed in the bulk of GaAsN. This result indeed implies that E1 was formed during the growth process. One of the reasons of the formation of E1 is the compensation for the tensile strain in the film caused by the small atomic size of N atom to that of As. Moreover,

3.3. Evolution of defects in electron-irradiated GaAsN films DLTS spectra of irradiated samples are shown in Fig. 4. The N-related recombination center E1 was also observed in electron irradiated samples. Furthermore, a new energy level was partially recorded around 300 K. According to the classification of native defects in GaAs by Martin et al., this new energy level may be the donor-like native defect in GaAs, EL2 (þ/0), which origin was strongly associated with interstitial Asi [28]. In addition, with an /cm2 and given the accuracy irradiation fluence of 4EI1 ¼ 9.0  1014 e of measurements, no significant change of density of E1 was clearly observed. Nevertheless, increasing the fluence irradiance to

Table 1 Summary of [N], ND, activation energy Ea, capture cross section s, density NT, and possible origin of defects in as grown as well as in electron irradiated and hydrogenated GaAsN samples. Sample

FWHM

[N] (%)

ND (1017 cm-3)

Trap

Ea (eV)

NT (cm3)

Possible origin

15

8.43  1015

(NeAs)As

8.11  1015 1.42  1016

(NeAs)As

sn (cm2)

Ref. 0.0294 After electron irradiation EI1 0.0201 EI2 0.1997 After hydrogenation HI1 0.0231

0.308

6.73

E1

ECM-0.331

5.18  10

0.277 0.302

7.08 7.85

E1 E1

ECM-0.327 ECM-0.325

5.87  1015 6.36  1015

0.235

4.94

HI2

0.222

3.48

EH1 H1 EH1 H1

ECM-0.427 EVM-0.107 ECM-0.401 EVM-0.104

1.47 2.49 1.70 1.04

0.0244

   

1012 1019 1013 1017

1.13 4.34 1.81 4.87

   

1016 1015 1016 1015

EL5 in GaAs NeHeVGa e e

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Fig. 4. DLTS spectra recorded for electron irradiated, hydrogenated and as-grown Al/ GaAsN Schottky junctions with a reverse bias voltage (VR) of 3 V, a pulse voltage (Vp) of 0 V, a rate-window time (tw) of 100 ms, and a filling pulse width (tp) of 100 ms.

/cm2, the ratio of post- and preirradiation of 4EI2 ¼ 9.0  1015 e NT-adj(E1) reached a value of w2.93. This enhancement of E1 could be explained through two possible ways. First, the irradiation process activates the E1-related atoms, which are electrically inactive. If E1 is the split interstitial (NeAs)As, the N and/or As atoms migrate from their ideal sites to create interstitials Ni and Asi in the lower lattice and form new (NeAs)As centers. This scenario enhance the generation of N (VN) and As (VAs) vacancies. However, the

Fig. 5. Arrhenius plots of DLTS spectra recorded in as-grown as well as in electron irradiated and hydrogenated GaAsN samples.

Fig. 6. Depth profiling of the trapping density of the electron trap obtained in as grown GaAsN sample using isothermal transient capacitance method.

stability of N concentration upon irradiation, the smaller atomic size of N than As, and the formation of EL2-type defect in GaAsN make the transformation of As atoms more probable than of N atom. Second, E1 involves interstitial or substitutional atoms in its atomic structure or a combination with other point defects, which are activated with irradiation. 3.4. Evolution of defects in hydrogenated GaAsN films As shown in Fig. 4 and in contrast to the effect of electron irradiation, E1 was completely disappeared from the DLTS spectra of the two hydrogenated GaAsN samples. This result opens up interesting perspectives for the limitation of the recombination activity through E1. However, the hydrogenation introduces important damage to the semiconductor epilayer, which consequentially degrades the performances of GaAsN based solar cells. To understand the evolution of E1, a deep understanding of the interaction between H and N in presence of irradiation energy is required. For that, it is essential to analyze the formation of new defects to explain the dissociation of E1. As shown in Fig. 4, two new lattice defects were observed in the two hydrogenated samples and their thermal emission of carriers is plotted as Arrhenius plots in Fig. 5. The electronic signatures and adjusted densities of the new energy states are summarized in Table 1. The first energy level, labeled EH1, is an electron trap with average activation energy and average capture cross section of 0.41 eV below the CBM of GaAsN and 8.20  1013 cm2, respectively. The difference between the activation energies of EH1 in the two samples is evaluated to 26 meV. One obvious reason of this difference is the increase in trapping densities and broadness of EH1 peaks with increasing the fluence irradiation doses. Moreover, the change in ND upon hydrogenation modifies the magnitude of the electrical fields governing the depletion region of the Schottky devices, and therefore, affects the thermal emission of carriers from EH1 centers based on the PooleeFrenkel emission effect. EH1 is commonly observed in GaAs, and is identified as EL5-type defect on the basic of the classification by Martin et al. [28]. EL5 was observed in as grown and

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hydrogenated GaAs [29]. Its structure was extensively discussed and the common results deal with a complex defect free from impurities and dominated by Asi, such as AsGaeVAs, AsGaeVGaeVGa, AsGaeVGaeVAs, and AsGaeAsieVAseVGa [30e34]. This expectation provides a fundamental proof to discuss the passivation of E1 upon hydrogenation. The second energy level (H1) is a hole trap, newly observed at an average activation energy of 0.11 eV above the valence band maximum (VBM) of GaAsN. The capture cross section of H1 showed a difference of two orders in the two hydrogenated samples. This difference was expected to the shifting of H1 to lower temperature with the broadness of its peak and the increase of its trapping density with increasing the irradiation doses. In addition, it is worth mentioning the observation of minority carrier trap is not observed in quasi-ideal Schottky junction. This implies that the surfaces of the two hydrogenated samples were modified. A thin ptype layer was created with the formation of acceptor-like states, which allows the injection of minority carrier to the space charge region. These acceptor states are related to the interaction between H and N atoms to form NenH complexes. These results were theoretically and experimentally confirmed [14e17]. Indeed, using first-principles theoretical methods, Bonapasta and Philippines have proved the formation of NenH complexes in hydrogenated GaAsN alloys [35]. Experimentally, using X-ray absorption spectroscopy, Ciatto et al. have provided an evidence of the formation of abundant Ne2H complexes with C2y symmetry upon hydrogenation [36]. These complexes were commonly considered to be the main cause of H and N mutual passivation. Furthermore, NeHrelated complexes in GaAsN grown by CBE, were investigated using Hall Effect and Fourier transform infra-red (FTIR) absorption [14e 17]. In addition, a NeH-related hole trap and acceptor like-state (H2) was confirmed by DLTS measurements. The energy level of H2 was found to fluctuate between 0.1 and 0.2 eV above the VBM of the alloy. Its capture cross section was fitted to be between varying between w1016 to w1017 cm2 [25,26]. Given the accuracy of measurements, the electronic signature of H2 are practically identical to that of H1. For that, we strongly suggest that the hydrogenation completely dissociate the split interstitial (NeAs)As and leads to the formation of NeH bonds and more Asi, as confirmed by the increase of the density of EH1. 4. Conclusions In summary, the trapping density a N-related non-radiative recombination center E1 was found to increase with rising the fluence doses of electron irradiation, in contrast to the effect of hydrogenation, where E1 was completely passivated. Furthermore, two main defects were recorded upon hydrogenation and correlated with the possible structure of E1. Hence, the hydrogenation of GaAsN opens up interesting perspectives to limit the recombination activity of E1 and to recover the lifetime of minority carriers in the alloy. Acknowledgments Part of this work was supported by the New Energy Development Organization (NEDO) under the Ministry of Economy, Trade and Industry, Japan. References [1] S.R. Kurtz, D. Meyers, J.M. Olson, Projected performance of three- and Four-junction devices using GaAs and GaInP, in: Proceedings of the 26th IEEE Photovoltaic Specialists Conference, Anaheim, New York, 1997, pp. 875e878. [2] D.J. Friedman, J.F. Geisz, S.R. Kurtz, J.M. Olson, 1-eV solar cells with GaInNAs active layer, J. Crys. Growth 195 (1988) 409e415.

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