GaAsN Films Research Articles

Growth and evaluation of GaAsN films with different N distribution grown by atomic layer epitaxy method

GaAsN films with different N distributions have been grown using the atomic layer epitaxy method to evaluate the effects of N distribution on the electrical properties of GaAsN. Three films, which had the same N composition with different N distribution were fabricated by alternate stack of GaAsN 1 monolayer (ML) and GaAs (0, 3, 5) ML. According to the X-ray diffraction measurement, periodicity and small N interdiffusion between GaAs and GaAsN layers in grown GaAsN films were confirmed. Thus, N distribution in the films were successfully modified in the order of a few unit lattice. Contribution of each scattering mechanisms on carrier mobility and densities of scattering centers in grown GaAsN films were evaluated. Compared with the film with 0 ML of GaAs, the density of N induced scattering center decreased with insertion of GaAs 3 ML between GaAsN 1 ML. It again increased the film with insertion of GaAs 5 ML. These results suggest that controlling N distribution intentionally not only degraded but also improved the electrical properties of GaAsN films.

Assessment of nitrogen incorporation in dilute GaAsN films using isotopically enriched molecular beam epitaxy and resonant nuclear reaction analysis

This paper describes the development of a new materials analysis technique enabled by modifying standard crystal growth methods for dilute nitride III–V semiconductors. The key to this method is the use of nitrogen gas enriched with the 15N isotope during material growth via molecular beam epitaxy, which allows enhanced detection of nitrogen using resonant nuclear reaction analysis (RNRA). Films of GaAs15N have been synthesized and examined using x-ray diffraction (XRD) and secondary ion mass spectroscopy. Accelerator-based ion beam analysis techniques, both RNRA and Rutherford backscattering spectrometry (including ion beam channeling configurations), were used to examine the films to obtain depth profiles of nitrogen, to assess their crystallinity, and to determine whether nitrogen is incorporated into substitutional or interstitial lattice sites. This method avoids possible artifacts from surface contamination by N and has confirmed that for the growth parameters investigated, the incorporation of 15N into the material corresponds with the expected enrichment of the feed gas, that the films have excellent crystallinity and uniform composition throughout their thickness, and that the nitrogen in GaAsN is predominantly incorporated substitutionally, in agreement with the lattice distortion observed by XRD.

Double acceptor in p-type GaAsN grown by chemical beam epitaxy

The properties of the acceptor states in GaAsN grown by chemical beam epitaxy (CBE) are studied by analyzing their charges based on the Poole–Frenkel model. Deep level transient spectroscopy (DLTS) shows two acceptor levels at 0.11 and 0.19eV above the valence band maximum. The emission rates of carriers from these states are enhanced with increasing the electric field during the DLTS measurement, which indicates that the energies required for the emission are decreased. By analyzing this field-enhanced emission process, the polarizabilities of the levels at 0.11 and 0.19eV are found to be −1 (±0.1) and −2 (±0.1), respectively. In addition, these states have almost the same concentration. Therefore, we conclude that they originate from the same defect, acting as a double acceptor in GaAsN film grown by CBE.

Effect of gamma-ray irradiation on structural properties of GaAsN films grown by metal organic vapor phase epitaxy

The effects of gamma-ray irradiation on the structural properties of GaAs1−xNx films (N concentration=1.9 and 5.1 at%) grown by metal organic vapor phase epitaxy on GaAs (001) substrates were investigated. The GaAs1−xNx films were irradiated by gamma rays with irradiation strength of 0–2.0MGy. Scanning electron microscopy and atomic force microscopy results showed that a gamma ray with a strength of 0, 0.5, 1.0, 1.5, and 2.0MGy formed holes with a density of 0.0, 8.8, 9.4, 11.5, and 11.9μm−2, respectively, on the surface of a GaAs0.981N0.019 film with low N content. On the other hand, the irradiated high-N-content GaAs0.949N0.051 film exhibited a cross-hatch pattern, which was induced by partial strain relaxation at high N levels, with a line density of 0.0, 0.21, 0.37, 0.67, and 0.26μm−1 corresponding to an irradiation strength of 0, 0.5, 1.0, 1.5, and 2.0MGy, respectively. The high-resolution X-ray diffraction and Raman scattering results revealed an increase in N incorporation and strain relaxation after irradiation. In addition, the GaAs0.949N0.051 films exhibited phase separation, which took place via N out-diffusion across the interface when the irradiation strength exceeded 1.0MGy. Based on these results, the main cause of structural change was determined to be the irradiation effects including displacement damage and gamma-ray heating.

Impact of growth temperature and substrate orientation on dilute-nitride-antimonide materials grown by MOVPE for multi-junction solar cell application

Nitrogen incorporation in bulk films of GaAsN, InGaAsN, and GaAsSbN films grown by metalorganic vapor phase epitaxy (MOVPE) on (100) and (311)B GaAs substrates was investigated. These films, nominally lattice-matched to a GaAs substrate, were deposited at relatively higher growth temperature (600°C) than typically used for MOVPE-grown dilute-nitride materials (~500–530°C), in order to reduce the background carbon impurity concentration. Even at these higher growth temperatures, sufficient N incorporation is achieved for targeting Eg~1eV InGaAsN and GaAsN with low background carrier concentration (1–2×1017cm−3). The presence of Sb is found to significantly inhibit N incorporation, making it challenging to achieve films of GaAsSbN grown at 600°C with a sufficient N concentration to achieve a 1eV band gap energy. For GaAsN and InGaAsN on (311)B GaAs substrates, increased N incorporation with lower background carbon concentration is observed, relative to films on (100) GaAs. By contrast, GaAsSbN on (311)B GaAs substrate exhibit lower-N incorporation relative to films on (100) GaAs, presumably due to surface site competition between Sb and N. The background hole carrier concentrations of thermally annealed InGaAsN and GaAsSbN on (311)B are about a factor of two lower than those on (100) GaAs substrate.

III-V Lattice-Mismatched and III-V-N Materials for Super High Efficiency Multi-Junction Solar Cells

ABSTRACTWe review recent progresses on in-situ observation of lattice relaxation of III-V lattice-mismatched system and analyses of defect properties in III-V-N solar cell materials. We found that there were five phases during the InGaAs growth on GaAs substrate. The transition point of the dominant dislocation behavior could be determined precisely. We also found that compositionally step-graded InGaAs/GaAs(001) buffers with overshooting (OS) layers were effective to control the strain of the top layer from tensile to compression. To understand the defect properties that dominate the electrical property of CBE-grown GaAsN films, we characterized deep levels in CBE-grown GaAsN films by DLTS. In this characterization, a well-known electron trap E1 (Ec-0.33eV) center in n-GaAsN and p-GaAsN was confirmed to be non-radiative recombination center by using double-carrier pulse DLTS.

Discrepancies in the nature of nitrogen incorporation in dilute-nitride GaSbN and GaAsN films

The authors studied the incorporation of dilute concentrations of nitrogen into GaSb and GaAs. There is a dissimilar interaction of N-Sb compared to N-As for films grown in the same manner, as indicated by discrepancies in x-ray diffraction and chemical analysis, and consistent with measured changes in the bandgap. The N in GaSb1−xNx resides in a quantifiable, “dark” phase correlating with the larger negative impact on the GaSb1−xNx optical properties relative to GaAs1−xNx. The authors hypothesize that the missing N resides in a phase that is optically inactive or have a much wider bandgap than the matrix material.

III–V–N Materials for Super-High Efficiency Multi Junction Solar Cells

The majority and minority carrier traps in GaAsN grown by chemical beam epitaxy (CBE) and their relationships with the electrical properties of the materials and solar cells are presented. By adopting a new flow-rate modulation CBE (FM-CBE) method, a higher mobility and a longer minority-carrier lifetime than those obtained by other growth methods have been achieved. We have characterized deep levels in grown GaAsN films by deep-level transient spectroscopy (DLTS). As a result, we found that 1) a hole trap H2 center (Ev + 0.15 eV) in p-GaAsN acts as an acceptor state and correlates with N concentration, 2) an electron trap E2 (Ec - 0.33 eV) center in n-GaAsN and p-GaAsN is a non-radiative recombination center and 3) a hole trap H1 center (Ev + 0.052 eV) newly observed in p-GaAsN acts as an acceptor state and a radiative recombination center. Although further analyses are required, it is very important to reduce the E1 defect density in (In)GaAsN to understand the degradation mechanism of the CBE-grown (In)GaAsN solar cell property and realize a higher efficiency.

Effects of gas-flow sequences on the self-limiting mechanisms of GaAsN films grown by atomic layer epitaxy

We adapted the atomic layer epitaxy (ALE) technique to grow GaAsN thin films. The effects of the gas flow sequences used on the self-limiting mechanism (SLM), the N incorporation ratios, and the concentrations of residual impurities were investigated. A pulse of the N precursor (monomethylhydrazine) was added after that of the Ga precursor (trimethylgallium) or the As (trisdimethylaminoarsenic) precursor, which were part of the conventional sequence used for the growth of GaAs films via ALE. If it was noticed that the SLM was in effect, the N precursor was supplied on the Ga- or As-terminated surface (the corresponding sequences and their resulting films are labeled as On-Ga and On-As, respectively). The On-As film exhibited rough surfaces, nonuniform N concentration, and disordered lattices owing to defects. On the other hand, in the case of the On-Ga sequence, GaAsN films with high-quality crystals were grown with the SLM in effect and the concentration of N being a few percent. The concentrations of the residual impurities (C and H) in the On-Ga film were low. This demonstrated that the On-Ga sequence was effective for growing GaAsN thin films on precisely controlled surfaces using the ALE technique.

Acceptor Levels due to a Complex Including the Nitrogen–Hydrogen Bond in GaAsN Films Grown by Chemical Beam Epitaxy

The acceptor levels and their concentration in GaAsN films grown by the chemical beam epitaxy technique were investigated through detailed analysis of the temperature dependence of hole concentration. Two acceptor levels A1 and A2 were found, and their energy levels were fixed at 130 ±20 and 55 ±10 meV, respectively, from the valence band maximum. Both concentrations were on the order of 1017 cm-3 and compensated by the concentration of donor on the same order. The concentration of A1 had a linear relationship with the number of N–H bonds in films grown at the same growth temperature, while the slope decreased with growth temperature. These results strongly suggested that a complex defect including the N–H bond was the origin of the A1 acceptor level.

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

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 monohydrogen–nitrogen (N–H) 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 [(N–As)As], we strongly suggested that the new hole trap took place after the dissociation of E1 and the formation of N–H complex.

Nitrogen Related Deep Levels in GaAsN Films Investigated by a Temperature Dependence of Piezoelectric Photothermal Signal

The temperature dependences of the piezoelectric photo-thermal (PPT) signals from unintentionally doped p-type GaAsN films grown on semi-insulating GaAs substrate were measured from 80 to 300 K. From the theoretical analysis based on the rate equation for the recombination of photo exited carriers to the localized levels, we identified five majority hole traps, P1-P5 in GaAsN films. Among them, estimated concentrations of the P3 and P5 traps increased with the nitrogen contents. Therefore, we concluded that these two traps were due to nitrogen-related recombination centers in GaAsN.

Effect of Thermal Stress on a N-Related Recombination Center in GaAsN Grown by Chemical Beam Epitaxy

Isochronal and isothermal annealing treatments were carried out on GaAsN films grown by chemical beam epitaxy to clarify the evolution of a nonradiative recombination center, at an average energy level of 0.33 eV below the conduction band minimum of the alloy. This lattice defect showed a strong persistence during post-thermal annealing even under critical annealing conditions. By considering as-grown samples as references and after optimizing the temperature and time of annealing, the trapping density of this recombination center could be reduced to one-half. Since the origin of this N-related recombination center was tentatively considered as the split interstitial (N–As)As, we suggest that the evolution of its density could be explained in great part to the diffusion behavior of As atoms during thermal stress.

Spectroscopic ellipsometry studies of reactively sputtered nitrogen-rich GaAsN films

Nitrogen-rich GaAsN thin films have been deposited at 500°C by reactive rf sputtering of GaAs target in argon–nitrogen atmosphere. The arsenic content of the films was varied by changing the nitrogen percentage in the sputtering atmosphere and the As/Ga ratio in the films was estimated by X-ray fluorescence measurements. Spectroscopic ellipsometry measurements have been carried out on these films and the measured ellispometric spectra were fitted with theoretical spectra generated by using suitable model sample structures. From the best fit parameters of the dispersion model, band-gap values and variation of refractive index and extinction coefficient as a function of wavelength have been obtained for films deposited with different percentages of nitrogen in the sputtering atmosphere. The films deposited with 12% to 100% nitrogen in the sputtering atmosphere, which are of hexagonal GaN, exhibit GaN-like optical properties, though effects due to excess arsenic in amorphous phase are seen in the films deposited with less than 40% nitrogen. The films deposited with 5% to 12% nitrogen in sputtering atmosphere are dominantly polycrystalline GaAsxN1−x (x≈0.01 to 0.08) and exhibit variations in optical parameters, which are consistent with their structure and composition. The films deposited with less than 5% nitrogen in sputtering atmosphere are arsenic-rich and amorphous.

Improvements in Optoelectrical Properties of GaAsN by Controlling Step Density during Chemical Beam Epitaxy Growth

Improvements in optoelectrical properties of GaAsN are demonstrated by chemical beam epitaxy (CBE) growth on high-step-density GaAs substrates. The step density at the growing surface is controlled using 2, 4, and 10° off GaAs(001) wafers as substrates. The number of carrier scattering centers induced by N (SCN) in the grown GaAsN films is quantitatively evaluated from the temperature dependence of hole mobility and used as an indicator of film quality. In previous studies, SCN increased with increasing N composition independently of the growth technique used. By CBE with high-step-density substrates, the reduction in SCN is achieved. This method also improves the emission intensity of cathode luminescence.

Relation between N–H complexes and electrical properties of GaAsN determined by H implantation

We investigated the relation between N–H complexes and the electrical properties of GaAsN, which is a potential material for fabricating super-high-efficiency multi-junction tandem solar cells. In order to separate the effect of other residual carrier such as carbon in a GaAsN film on the electrical properties, hydrogen (H) ions were implanted into GaAsN grown by chemical beam epitaxy (CBE) and then rapid thermal annealing from 250 to 650 °C was carried out. Two N–H complexes related to local vibrational modes (LVMs) in GaAsN were observed at 3098 and 3125 cm −1. With an increasing annealing temperature, the integrated peak intensity of the 3098 cm −1 peak ( I 3098) decreased, while that of the 3125 cm −1 peak ( I 3125) increased. This indicates that N–H complexes related to the 3125 cm −1 peak are thermally more stable than those related to the other peak. The hole concentrations and mobilities exhibited an increasing trend until an annealing temperature of 550 °C was reached. Their increases are attributed to the removal of donor-type defects. It is suggested that the N–H complexes related to the 3098 cm −1 peak are electrically active, while those giving the 3125 cm −1 peak are inactive.

Luminescence from the deep level N–N interstitials in GaAsN grown by metal organic chemical vapour deposition

Strong luminescence from the nitrogen–nitrogen (N–N) interstitials had been observed in GaAsN epilayers grown by metal organic chemical vapour deposition (MOCVD) using nitrogen as the carrier gas. These N–N interstitials in the GaAsN films were thoroughly studied through photoluminescence (PL) and secondary ion mass spectroscopy (SIMS) measurement combined with X-ray diffraction (XRD) measurement. The N–N interstitial emission peak resided at deep level of 0.2–0.23 eV below the inter-band gap transition energy of the GaAsN epilayer. The N–N interstitial PL intensity decreased after the GaAsN samples were thermal annealed. It had also been found that the N–N interstitials contribute to the substitutional nitrogen composition of the GaAsN epilayer after rapid thermal annealing were carried out on the GaAsN epilayer.

Local Vibration Modes of N-H Related Complexes in GaAsN Grown by Chemical Beam Epitaxy

The local vibration modes of N-H bonds Fourier transform infrared spectrometry for GaAsN films grown by chemical beam epitaxy are investigated. Four stretching modes of N-H bonds exist in the films. The absorption intensities of stretching modes at 3125, 3098 and 2952 cm-1 have no relationships, suggesting that at least three kinds of N-H related defects are formed in the GaAsN films. Major stretching modes are 3098 and 2952 cm-1. The integrated absorption (IA) of 3098 cm-1 is proportional to H concentration ([H]). The IA of 2952 cm-1 changes super linearly with [H]. A simple N-H bond and a complex defect including N-H are suggested as the origin of 3098 and 2952 cm-1, respectively.

Bandedge absorption of GaAsN films measured by the photothermal deflection spectroscopy

The optical absorption of GaAsN films grown by molecular beam epitaxy on GaAs substrates is measured using the mirage effect photothermal deflection spectroscopy (PDS). The PDS spectra were fitted with a modified Fernelius model, which takes into account multiple reflections within the GaAsN layer and GaAs substrate. This allowed the extraction of bandedge parameters for a series of GaAsN films with N content varying from 0.24% to 1.4% N. All films show a clear Urbach absorption edge with a composition-dependent bandgap consistent with literature and Urbach slope parameters roughly 3 times larger than GaAs values.

Effects of Residual Carbon and Hydrogen Atoms on Electrical Property of GaAsN Films Grown by Chemical Beam Epitaxy

The electrical behavior of nitrogen–hydrogen (N–H) complexes and carbon (C) atoms in GaAsN films grown by the chemical beam epitaxy (CBE) method have been studied by comparing H concentrations and hole concentrations. The contributions of H and C concentrations ([H] and [C]) to ionized impurity scattering have been also investigated. In the GaAsN films, there were three acceptor levels (A0, A1, and A2). The energy levels of A1 and A2 were 130 and 55 meV, respectively. The concentration of the deepest acceptor (A0) was more than 1019 cm-3, which was higher than [H] and [C]. A2 was observed only in films with a high [C]. The amount of A1 was proportional to [H]. Most of the residual H formed N–H complexes; thus, it is concluded that the N–H complex behaved as an acceptor in GaAsN films. At low temperatures, ionized impurity scattering limited the total hole mobility. The inverse numbers of the coefficients for the ionized scattering were consistent with [C] instead of [H]. This suggests that all the residual C atoms were ionized owing to compensation by the donors. A disagreement between N composition and alloy scattering has been also detected. This might indicate a fluctuation in the N composition in the GaAsN films.