Hydrogen passivation of InP layers grown on lattice-mismatched substrates can achieve thermally stable deactivation of dislocation-related deep levels, making this a promising process for improving the performance of heteroepitaxial InP space solar cells. However, in addition to dislocation-related defects, interstitial Zn (Zni) defects that are characteristic of Zn-doped InP and which form deep donor states within the InP band gap, are important considerations for optimizing the electronic quality of these layers. Here, we show that hydrogen forms complexes with and deactivates Zni donor states within Zn-doped InP grown by metalorganic chemical vapor deposition. A combination of photoluminescence (PL), electrochemical capacitance–voltage dopant profiling, secondary ion mass spectroscopy and current–voltage (I–V) measurements are applied to a set of samples receiving systematic hydrogenation and annealing treatments. We find that the deactivation of Zni deep donors, as detected by monitoring the evolution of the donor–acceptor transition using PL measurements, causes an increase of ∼50% in the net acceptor concentration of heavily Zn-doped heteroepitaxial InP by elimination of the acceptor compensation effect due to active Zni donors. Analysis of I–V characteristics indicates that Zni passivation sharply reduces depletion region recombination and shunt currents within heteroepitaxial diodes, causing an increase in the diode turn-on voltage from 680 to 960 mV. Subsequent annealing above 500 °C reactivates the Zni defects, resulting in a systematic increase in doping compensation as well as a decrease in VTO toward the original, as-grown value. A study of the reactivation kinetics for the H–Zni complex reveals a greater thermal stability than that of H–Zn acceptor complexes but less than that of H-dislocation complexes in InP, with an estimated dissociation energy for the H–Zni complex of 2.3 eV. While these effects are observed for both homoepitaxial and heteroepitaxial Zn-doped layers, the effect is far more pronounced for the heteroepitaxial layers due to the relatively high Zni concentration in the latter.

Hydrogen passivation of hetero-epitaxial InP solar cells is of interest for deactivation of dislocations and other defects caused by the cell/substrate lattice mismatch that currently limits the photovoltaic performance of these devices, Here we show that in addition to passivation of dislocations, hydrogen deactivates interstitial Zn donor , defects present within the Zn-doped emitter of metal organic chemical vapor deposition (MOC VD)-grown p(+)n hetero-epitaxial InP devices, Zn interstitial passivation increases the forward bias turn-on voltage of hetero-epitaxial InP diodes by as much as 280 m V over the non-hydrogenated value, reaching a value of 960 m V which is close to that obtained for homo-epitaxial diodes, The increase is reproducible and is not observed for either n(+)p structures or homo-epitaxial p(+)n structures. Through a combination of photoluminescence, C-V profiling, SIMS and I-V measurements we explain that the source of the voltage enhancement is a combination of decreased acceptor compensation ill the emitter and decreased current losses due to depletion region recombination and shunting paths associated with the high concentration of Zn interstitials ill Zn-doped hetero-epitaxial InP, (C) 1997 John Wiley & Sons, Ltd.

Hydrogen passivation of Zn interstitial defects (Zn-i) is shown to enhance the turn-on voltage (V-TO) of heteroepitaxial p(+)n InP/GaAs cells. By using a combination of photoluminescence (PL), electrochemical C-V dopant profiling, secondary ion mass spectroscopy and current-voltage (I-V) measurements we demonstrate that the mechanism for this improvement results from reduction in recombination-generation and shunt losses due to hydrogen deactivation of Zn-i defect complexes whose high concentration is due to the presence of dislocations. The deactivation of Zn-i deep donors also results in an increase in the effective emitter acceptor concentration by similar to 50% due to the elimination of the compensation effect introduced by active Zn-i donors.

Hydrogen passivation of Zn acceptors and Zn–H dissociation kinetics are compared for homoepitaxial and lattice-mismatched heteroepitaxial n+p InP structures. Doping profile measurements show a pronounced increase in the depth and degree of passivation in the p-type region of the heteroepitaxial samples indicating enhanced diffusion of hydrogen along dislocations, followed by additional Zn deactivation. Moreover, the strong affinity between hydrogen and extended defects is found to aid the subsequent dissociation of the Zn–H complexes as indicated by (i) reverse bias annealing (RBA) studies which show that the Zn–H dissociation energy decreases from 1.19 eV in homoepitaxial samples to 1.12 eV in heteroepitaxial samples, and (ii) enhanced passivation of extended defect-related traps by hydrogen that is liberated from Zn acceptors during the RBA process as determined by deep level transient spectroscopy.

Dislocations and related point defect complexes caused by lattice mismatch currently limit the performance of heteroepitaxial InP cells by introducing shunting paths across the active junction and by the formation of deep traps within the base region. We have previously demonstrated that plasma hydrogenation is an effective and stable means to passivate the electrical activity of such defects within heteroepitaxial InP layers. In this work, we present our first results on the hydrogen passivation of ac tual heteroepitaxial n+p and p+n InP cell structures grown on GaAs substrates by metal organic chemical vapor deposition (MOCVD). We have found that a 2-h exposure to a 13.56-MHz hydrogen plasma at 275°C reduces the deep level co ncentration in the base regions of both n+p and p+n heteroepitaxial InP cell structures from as-grown values of 5–7 × 1014 cm−3, down to 3–5 × 1012 cm−3. All dopants were successfully reactivated by a 400°C, 5-min anneal with no detectable activation of deep levels. Current-voltage (I-V) analysis indicated a subsequent ∼100-fold decrease in reverse leakage current at 1 V reverse bias, and an impro ved built-in voltage for the p+n structures. In addition to being passivated, dislocations are also shown to participate in secondary interactions during hydrogenation. We find that the presence of dislocations enhances hydrogen diffusion into the cell structure and lowers the apparent dissociation energy of Zn-H complexes from 1.19 eV for homoepitaxial Zn-doped InP to 1.12 eV for heteroepitaxial Zn-doped InP. This is explained by additional hydrogen trapping at dislocations subsequent to the r eactivation of Zn dopants after hydrogenation.

Hydrogen passivation of MOCVD-grown heteroepitaxial InP space solar cell structures with both n+p and p+n configurations are compared and the different passivation characteristics are investigated. A 2 hour exposure to a 13.56 MHz hydrogen plasma at 250°C reduces the deep level concentration in the base regions of both n+p and p+n heteroepitaxial InP cell structures from as-grown values near 1×10 cm-3, down to the 2-5×1012 cm-3 range, resulting in significant reductions in reverse leakage current for both configurations. All dopants were successfully reactivated by a 400°C, 5 minute anneal with no detectable activation of deep levels. Passivation of both structures are stable to >500°C. I-V characteristics demonstrate significant differences in the voltage behavior of the two cell configurations, with a ~280 mV increase in built-in voltage for p+n cells and no corresponding change for n+p cells. This enhancement in Vbi is attributed to complex interactions between Zn, H and extended defects within the heavily doped emitter

In previous work we reported on the stable passivation of dislocations in InP/GaAs heterostructures by plasma hydrogenation (Chatterjee et al., Appl. Phys. Lett. vol. 65, p. 58, 1994). In this article we investigate and compare the trapping kinetics and general trapping properties of dislocations in strain relaxed p-InP grown on GaAs by metalorganic chemical vapor deposition prior to and after hydrogen passivation using deep level transient spectroscopy (DLTS) and current-voltage-temperature (I-V/T) measurements to determine the complete role of hydrogen passivation in these heterostructures. Three hole traps, T1A, T1B, and T2, were detected and attributed to dislocations in heteroepitaxial p-InP which displayed the logarithmic capture kinetics, extended dependence on fill pulse time, and broadened DLTS features expected for dislocation related traps. Quantitative analysis of the DLTS characteristics revealed progressive asymmetry in DLTS peak shape, an increase in characteristic peak width, and a decrease in activation energy as fill pulse time is increased until saturation values were reached. These observations are explained on the basis of a distribution or band of energy states for each trap resulting from the interaction of electrically active sites either between closely spaced dislocations or along dislocation cores within the strain-relaxed InP. For fill pulse times increasing from 1 μs to 10 ms, activation energies for T1A decreased monotonically from 0.80 to 0.65 eV, for T1B from 0.56 to 0.45 eV, and for T2 from 0.45 to 0.35 eV, with saturation occurring at the upper and lower limits for each trap, which indicates a qualitative measure of the energy spread for each trap. Plasma hydrogenation was not only found to passivate dislocations by reducing the trap concentration from ∼6×1014 to ∼3×1012 cm−3 for a 2 h exposure, but also strikingly altered their basic trapping properties. The qualitative measure of energy spread for the T1A and T2 traps were narrowed from ∼100 to 150 meV to ∼20 to 30 meV after a 2 h hydrogen exposure, whereas T1B was not detected after passivation. In addition, a simultaneous reduction in fill pulse saturation time, DLTS peak broadening, and peak shift as a function of hydrogen exposure time were observed. These observations suggest that hydrogen passivation modifies the dislocation trapping characteristics toward a more point defectlike behavior due to an increase in the average spacing between electrically active dislocation sites. This in turn reduces the interactions between these sites and narrows the distribution of states within each defect band. Further, reverse bias I-V/T measurements revealed that the near midgap trap T1A, which was found to dominate the space charge generation current prior to passivation, is no longer dominant after hydrogen passivation. Instead a 2 h hydrogen treatment shifted the dominant center to an activation energy which more closely matches the shallow T2 level.

Heteroepitaxial InP films grown on GaAs substrates by metalorganic chemical vapor deposition are demonstrated to have a higher concentration of zinc occupying interstitial sites than do equivalent homoepitaxial InP layers. A zinc interstitial-related donor-to-acceptor photoluminescence peak is observed in heteroepitaxial InP films which is absent in the homoepitaxial sample spectra. Capacitance–voltage measurements yield a lower hole concentration in the heteroepitaxial layers versus the homoepitaxial layers, which is attributable to a higher fraction of zinc in interstitial sites within the heteroepitaxial layers. Additionally, the hole concentration of the heteroepitaxial layers is found to be lower near the heterojunction as compared with the film surface region, correlating with a higher dislocation density near the heterojunction as seen by transmission electron microscopy. We conclude that the increased zinc interstitial concentration and the reduced hole concentration are due to dislocation-zinc solute interactions.

The effects of hydrogenation on the properties of Zn-doped InP/GaAs heterostructures grown by metalorganic chemical vapor deposition were studied by current-voltage (I-V), deep level transient spectroscopy (DLTS), and photoluminescence. Significant improvements in leakage current and breakdown voltage in InP diodes on GaAs were observed after a 2 h hydrogen plasma exposure at 250 °C. DLTS indicated a corresponding reduction in total trap concentration from ∼6×1014 to ∼3×1012 cm−3 at a depth of ∼1.5 μm below the surface. The Zn dopants were completely reactivated by a subsequent 5 min 400 °C anneal without degradation of the reverse current or reactivation of the deep levels. Anneals in excess of 580 °C were necessary to reactivate the deep levels and degrade the leakage current to their original values, indicating the passivation of threading dislocations by hydrogen, and the existence of a wide temperature window for post-passivation processing.

Plasma hydrogenation has recently been demonstrated to be highly effective in passivating dislocations in heteroepitaxial InP and is a promising technique for achieving viable heteroepitaxial InP solar cells. In this paper, the effects of hydrogen on the fundamental properties of three dislocation related hole traps in heteroepitaxial InP/GaAs are presented. Hydrogen passivation significantly alters the dislocation trapping kinetics, causing point defect-like behavior consistent with a transformation from dislocation-related defect bands within the InP bandgap to a low concentration of individual deep levels after hydrogenation. Furthermore, hydrogen passivation is shown to shift the dominant space charge generation center from Ec-0.71 eV to Ec-0.92 eV, away from midgap. A model is proposed which explains these effects on the basis of decreased electronic interaction between dislocation sites. Finally, a comparison of InP material quality grown on GaAs, Ge and GaAs/Ge substrates is presented, and hydrogen passivation of InP/GaAs/Ge structures is reported

Deep levels in MOCVD grown p-InP on GaAs substrates have been investigated by Deep Level Transient Spectroscopy (DLTS). The effect of hydrogenation on the electrical activity of these levels has been studied through a combination of DLTS and Photolu-minescence (PL) measurements. DLTS measurements indicate a drop of trap density from ~ 5 × 1014 cm−3 to ~ 1 × 1012 cm−3 after hydrogenation. Annealing at 400°C reactivated only the dopants, while temperatures above 600°C were necessary for deep-level reactivation. This combined with a logarithmic dependence on fill pulse time, indicate that at least one broad DLTS peak is associated with dislocations. The PL the DLTS results show that the dislocation related traps are passivated by hydrogen, preferentially over the dopants and that a wide annealing window exists for dopant reactivation.